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2 PART Disorders Presenting in Skin and Mucous Membranes

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2

P A R T

Disorders Presenting in Skin and Mucous Membranes

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S E C T I O N

4

INFLAMMATORY DISORDERS BASED ON T-CELL REACTIVITY AND DYSREGULATION

C H A P T E R 1 0

Innate and Adaptive Immunity in the Skin

Robert L. ModlinJenny KimDieter MaurerChristine BangertGeorg Stingl

The human immune system is comprisedof two distinct functional parts: innateand adaptive. These two componentshave different types of recognition recep-tors and differ in the speed in which theyrespond to a potential threat to the host(Fig. 10-1). Cells of the innate immunesystem, including macrophages and den-dritic cells (DCs), use pattern recognitionreceptors encoded directly by the germ-line DNA, respond to biochemical struc-tures commonly shared by a variety ofdifferent pathogens, and elicit a rapid re-sponse against these pathogens, althoughno lasting immunity is generated. In con-trast, cells of the adaptive immune sys-tem, T and B lymphocytes, bear specificantigen receptors encoded by rearrangedgenes, and in comparison to the innate re-sponse, adaptive immunity developsmore slowly. A unique feature of theadaptive immune response is its ability togenerate and retain memory; thus, it hasthe capability of providing a more rapidresponse in the event of subsequent im-

munologic challenge. Although the innateand adaptive immune responses are dis-tinct, they interact and can each influencethe magnitude and type of their counter-part. Together, the innate and adaptiveimmune systems act in synergy to defendthe host against infection and cancer. Thischapter describes the roles of the innateand adaptive immune response in gener-ating host defense mechanisms in skin.

INNATE IMMUNE RESPONSE

Immune mechanisms that are used by thehost to immediately defend itself are re-ferred to as

innate immunity

. These includephysical barriers such as the skin and mu-cosal epithelium; soluble factors such ascomplement, antimicrobial peptides, che-mokines, and cytokines; and cells, includ-ing monocytes/macrophages, DCs, naturalkiller cells (NK cells), and polymorphonu-clear leukocytes (PMNs) (Fig. 10-2).

Physical and Chemical Barriers

1

Physical structures prevent most patho-gens and environmental toxins fromharming the host. The skin and the epithe-lial lining of the respiratory, gastrointesti-nal, and the genitourinary tracts providephysical barriers between the host and theexternal world. Skin, once thought to bean inert structure, plays a vital role in pro-tecting the individual from the external en-vironment. The epidermis impedes pene-tration of microbial organisms, chemicalirritants, and toxins; absorbs and blockssolar and ionized radiation; and inhibitswater loss (see Chap. 45).

Molecules of the Innate Immune System

COMPLEMENT

2

(See eFig. 10-2.1 in on-line edition; see also Chap. 36) One ofthe first innate defense mechanisms that

INNATE AND ADAPTIVE IMMUNITY

AT A GLANCE

Innate immune responses are

used by the host to immediately defend itself;

determine the quality and quantity of many adaptive immune responses;

are short lived;

have no memory;

include physical barriers (skin and mucosal epithelia);

include soluble factors such as com-plement, antimicrobial peptides, che-mokines, and cytokines;

include cells such as monocytes/mac-rophages, dendritic cells, natural killer cells, and polymorphonuclear leuko-cytes.

Adaptive immune responses

have memory;

have specificity;

are long-lasting;

in skin, are initiated by dendritic anti-gen-presenting cells in the epidermis (Langerhans cells) and by dermal den-dritic cells;

are executed by T lymphocytes and antibodies produced by B lympho-cytes.

Psoriasis vulgaris: chronic stable type. Multi-ple large scaling plaques on the trunk, arm, but-tocks, and abdomen. Lesions are polycyclic andconfluent and form geographic patterns. This pa-tient was cleared by acitretin/psoralen and ultravi-olet A light combination treatment within 4 weeks.

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awaits pathogens that overcome the ep-ithelial barrier is the alternative path-way of complement. Unlike the classiccomplement pathway that requires an-

tibody triggering, the lectin-dependentpathway as well as the alternative path-way of complement activation can bespontaneously activated by microbial

surfaces in the absence of specific anti-bodies (see eFig. 10-2.1 in on-line edi-tion). In this way, the host defensemechanism is activated immediately af-ter encountering the pathogen withoutthe 5 to 7 days required for antibodyproduction.

NEUROPEPTIDES

The skin is a richsource of neuropeptides, including neu-rotransmitters [e.g., calcitonin gene–re-lated peptide (CGRP), substance P, so-matostatin] and neurohormones (seeChap. 101). The inhibitory effects ofCGRP and substance P on Langerhanscell (LC) antigen presentation functionare discussed later. The neurohormoneproopiomelanocortin (POMC) is pro-duced by the pituitary gland as well asby a number of cell types, includingkeratinocytes.

ANTIMICROBIAL PEPTIDES

6

Antimicrobialpeptides are an important evolutionarilyconserved innate host defense mecha-nism in many organisms. Also, kerati-nocytes produce such peptides, includ-ing cathelicidins (LL-37) and

β

-defensins(BD-1, BD-2, BD-3). Their antimicrobialmechanism of action may relate to

FIGURE 10-2

The innate immune response in skin. In response to exogenous factors, such as foreign pathogens, ultraviolet (UV) radiation, and chemical ir-ritants, innate immune cells [granulocytes, mononuclear phagocytes, natural killer (NK) cells, keratinocytes] mount different types of responses including: (1) re-lease of antimicrobial agents; (2) induction of inflammatory mediators, such as cytokines, chemokines, neuropeptides, and eicosanoids; and (3) initiation andmodulation of the adaptive immune response. DDC = dermal dendritic cell; KC = keratinocyte; LC = Langerhans cell; MHC II = major histocompatibility complexclass II; Th1, Th2, 17 = T helper 1, 2, Th17; Treg = T regulatory cell.

1. Antimicrobial response: • defensins • cathelicidins/psoriasin • reactive oxygen intermediates

2. Inflammatory response: • cytokines • chemokines • neuropeptides • eicosanoids

UV radiationIrritants

Pathogens

MHC II

LC/DDCMacrophage

KC

NK cell

3. Influence adaptive immune response • activation of T cells

T cell response(Th1, Th2, Treg, Th17)

FIGURE 10-1

The immune system of higher vertebrates uses both innate and adaptive immune re-sponses. These immune responses differ in the way they recognize foreign antigens and the speed withwhich they respond; yet, they complement each other in eradicating foreign pathogens.

The immune response

Foreignpathogen

Innate response Adaptive response

• rapid response• pattern recognition receptors - germline encoded – CD14, mannose and scavenger• ↑cytokines, co-stimulatory molecules - instructive role for adaptive response• direct response for host defense – phagocytosis – antimicrobial activity

• slow response• recognition - initially low affinity receptors gene rearrangement clonal expansion

• response - T and B cells with receptors encoded by fully rearranged genes• memory

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membrane insertion and pore forma-tion. Adrenomedullin, members of theCGRP superfamily,

α

melanocyte-stim-ulating hormone, and secretory leuko-cyte protease inhibitor (antileukopro-tease, human seminal inhibitor I) areamong previously identified peptideswhose antimicrobial activities were dis-covered later.

β

-Defensins are cysteine-rich cationiclow-molecular-weight antimicrobial pep-tides. The first human

β

-defensin, HBD-1,was isolated from human hemofiltrateobtained from a patient with end-stage re-nal disease. It is constitutively expressedin the epidermis and is not transcription-ally regulated by inflammatory agents.HBD-1 has antimicrobial activity againstGram-negative bacteria and appears toplay a role in keratinocyte differentiation.A second human

β

-defensin, HBD-2, wasdiscovered in extracts of lesions from pso-riasis patients.

7

Unlike HBD-1 expression,HBD-2 expression is inducible by mi-crobes, including

Pseudomonas aeruginosa

,

Staphylococcus aureus

, and

Candida albi-cans

.

7

Not only can microbes stimulate ex-pression of HBD-2, but pro-inflammatorycytokines such as tumor necrosis factor-

α

(TNF-

α)

and interleukin 1 (IL-1) can alsoinduce HBD-2 transcription in keratino-cytes.

7

When tested for antimicrobial ac-tivity, HBD-2 showed effective activityagainst Gram-negative bacteria such as

Escherichia coli

and

P. aeruginosa

but notagainst Gram-positive bacteria such as

S.aureus

.

7

A third

β

-defensin, HBD-3, hasnow been isolated and characterized.Contact with TNF-

α

and with bacteriawas found to induce HBD-3 messengerRNA expression in keratinocytes. In addi-tion, HBD-3 demonstrated potent anti-microbial activity against

S. aureus

andvancomycin-resistant

Enterococcus faecium

.Therefore, HBD-3 is among the firsthuman

β

-defensins in skin to demon-strate effective antimicrobial activityagainst Gram-positive bacteria. The local-ization of human

β

-defensins to theouter layer of the skin and the fact the

β

-defensins have antimicrobial activityagainst a variety of microbes suggestthat human

β

-defensins are an essentialpart of cutaneous innate immunity. Fur-thermore, evidence indicating that hu-man

β

-defensins attract DCs and mem-ory T cells via CC chemokine receptor 6(CCR6)

8

provides a link between the in-nate and the adaptive immunity in skin.

Cathelicidins are cationic peptideswith a structurally variable antimicrobialdomain at the C-terminus. Whereas inmammals like pigs or cattle a variety ofcathelicidin genes exists, men (and mice)

possess only one gene. The human pre-cursor protein hCAP18 (human cathelici-din antimicrobial protein 18) is producedby skin cells, including keratinocytes,mast cells, neutrophils, and ductal cells ofeccrine glands. Neutrophil proteases (i.e.,proteinase 3) process hCAP18 into theeffector molecule LL-37, which plays animportant role in cutaneous host defensebecause of its pronounced antibacte-rial,

9,10

antifungal,

11

and antiviral

12,13

ac-tivities. LL-37 further contributes to in-nate immunity by attracting mast cellsand neutrophils via formyl peptide re-ceptor–like 1 and by inducing mediatorrelease from the latter cells via a G pro-tein–dependent, immunoglobulin E (IgE)–independent mechanism.

14

It has nowbeen shown that LL-37 is secreted intohuman sweat, where it is cleaved by aserine protease–dependent mechanisminto its peptides RK-31 or KS-30. Inter-estingly, these components display aneven more potent antimicrobial activitythan intact LL-37.

15

In atopic dermatitis (see Chap. 14),LL-37 is downregulated, probably dueto the effect of the T2 cytokines IL-4and IL-13, which renders atopic skinmore susceptible to skin infectionswith, for example,

S. aureus

, vaccinia vi-rus (eczema vaccinatum), or herpes sim-plex virus (eczema herpeticum).

10,12,13

Another important human antimicro-bial peptide has now been identified,psoriasin (S100A7).

16

It is secreted pre-dominantly by keratinocytes and playsa major role in killing the common gutbacterium

E. coli

. In fact, in vivo treat-ment of human skin with anti-psoriasinantibodies results in the massive growthof

E. coli

.

16

OTHER MEDIATORS

Other secreted pro-tein mediators that can be synthesizedand released from keratinocytes and thatmay play a role in host defense are thecomplement components C3 and factorB. Keratinocytes are among the cells thatsynthesize eicosanoids, an ensemble oflipid mediators regulating inflammatoryand immunologic reactions. They canproduce and release the cyclooxygenaseproduct prostaglandin E

2

, which hasboth pro-inflammatory and immunosup-pressive properties and, when acting onDCs, promotes the development of IL-4–dominated type 2 T-cell responses.

17

Other keratinocyte-derived eicosanoidsinclude the neutrophil chemoattractantleukotriene B

4

, the pro-inflammatory 12-lipoxygenase product 12(s)-hydroxyeico-satetraenoic acid, and 15-hydroxyeico-satetraenoic acid, an anti-inflammatory

and immunosuppressive metabolite ofthe 15-lipoxygenase pathway.

Another group of biologic responsemodifiers originating in keratinocytesand other epidermal cells is free radicalmolecules, now generally referred to as

reactive oxygen species

. These include thesuperoxide radical (O

2

˙

), hydrogen per-oxide (H

2

O

2

), the hydroxyl radical(OH˙), nitric oxide (NO), and others.These radicals are generally viewed asdangerously reactive entities threaten-ing the integrity of many tissues. Theskin is particularly at risk because it isexposed to oxygen from both inside andoutside and because of the activation ofoxygen by light (see Chaps. 88 and 89).Free radicals probably contribute to so-lar damage and photoaging of the skin.However, certain reactive oxygen spe-cies have potent inflammation-inducingproperties (e.g., free oxygen radicals) aswell as immunomodulatory properties(e.g., NO), and thus provide an impor-tant host defense mechanism againstmicrobial invasion. For discussion ofthese molecules, the reader is referred tothe review by Bickers and Athar.

18

PATTERN RECOGNITION RECEPTORS

Howdo the cells of the innate immune systemrecognize foreign pathogens? One waythat pathogens can be recognized anddestroyed by the innate immune systemis via receptors on phagocytic cells. Un-like adaptive immunity, the innate im-mune response relies on a relativelysmall set of germline-encoded receptorsthat recognize conserved molecular pat-terns that are shared by a large group ofpathogens. These are usually molecularstructures required for survival of the mi-crobes and therefore are not subject toselective pressure. In addition, pathogen-associated molecular patterns are specificto microbes and are not expressed in thehost system. Therefore, the innate im-mune system has mastered a clever wayto distinguish between self and nonselfand relays this message to the adaptiveimmune system.

Of key importance was the discoveryof the Toll-like receptors (TLRs), namedafter the

Drosophila

Toll gene whose pro-tein product, Toll, participates in innateimmunity and in dorsoventral develop-ment in the fruit fly.

19,20

The importanceof Toll signaling in mammalian cells wasconfirmed by the demonstration that thetransmembrane leucine-rich protein TLR4is involved in lipopolysaccharide (LPS)recognition.

21

In addition to TLRs, there exist a vari-ety of other transmembrane molecules

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that sense the presence of pathogens.These include the triggering receptorsexpressed on myeloid cells (TREM) pro-teins,

25

the family of Siglec molecules,

26

and a group of C-type lectin receptors.

27

The latter are prominently expressed onantigen-presenting cells (APCs) as, forinstance, dectin-1 and DC-SIGN [DC-specific intercellular adhesion molecule3 (ICAM-3) grabbing nonintegrin]. Theyare able to mediate efficient binding ofmicroorganisms such as yeast and my-cobacteria, respectively; achieve theirphagocytosis; and induce activation ofsignaling pathways that result in thematuration of phagosomes as well as inthe production of reactive oxygen andnitrogen derivatives.

Members of the TREM protein familyfunction as amplifiers of innate responses.Extreme examples of the consequences ofmicrobe activation of TREM proteins arelife-threatening septicemia and thedeadly hemorrhagic fevers caused byMarburg and Ebola virus infection.

28

TOLL-LIKE RECEPTORS

29

There is nowsubstantial evidence to support a rolefor mammalian TLRs in innate immu-nity (Fig. 10-3). First, TLRs recognize

pathogen-associated molecular patternspresent in a variety of bacteria, fungi,and viruses. Second, TLRs are expressedat sites that are exposed to microbialthreats. Third, the activation of TLRs in-duces signaling pathways that, on theone hand, stimulate the production ofeffector molecules (reactive oxygen spe-cies, NO), and, on the other, promotethe expression of co-stimulatory mole-cules and the release of cytokines and,as a result, the augmentation of theadaptive response. Fourth, TLRs directlyactivate host defense mechanisms thatthen combat the foreign invader.

TLRs were initially found to be ex-pressed in all lymphoid tissues but weremost highly expressed in peripheralblood leukocytes, including monocytes,B cells, T cells, granulocytes, and DCs.Certain TLRs (e.g., TLR2) are internal-ized after ligation. In such a situation,TLRs are recruited to the pathogen-containing phagosomes and discrimi-nate between Gram-positive and Gram-negative bacteria,

31

thus surveying theintracellular compartments of the cellsfor microbial invaders.

The expression of TLRs on cells of themonocyte/macrophage lineage is consis-

tent with the role of TLRs in modulatinginflammatory responses via cytokine re-lease. Because these cells migrate intosites that interface with the environ-ment—lung, skin, and gut—the locationof TLR-expressing cells would situatethem to defend against invading mi-crobes. TLR expression by adipocytes,intestinal epithelial cells, and dermal en-dothelial cells supports the notion thatTLRs serve a sentinel role with regard toinvading microorganisms. The regulationof TLR expression is critical to their rolein host defense, yet few factors havebeen identified that modulate this pro-cess. IL-4 acts to downregulate TLR ex-pression,

32

which suggests that T helper2 (Th2) adaptive immune responsesmight inhibit TLR activation.

In

Drosophila

, Toll is critical for hostdefense. The susceptibility of mice withspontaneous mutations in TLRs to bac-terial infection indicates that mamma-lian TLRs play a similar role. Activationof TLR2 by microbial lipoproteins in-duces activation of the inducible NOsynthase (NOS-II or iNOS) promoter,

37

which leads to the production of NO, aknown antimicrobial agent. There isstrong evidence that TLR2 activation

FIGURE 10-3

Toll-like receptors (TLRs) mediate innate immune response in host defense. Activation of TLRs by specific ligands induces (1) cytokine releaseand co-stimulatory molecules that instruct the type of adaptive immune response; (2) direct antimicrobial response; and (3) tissue injury. CpG DNA = immuno-stimulatory cytosine- and guanine-rich sequences of DNA; dsRNA = double-stranded RNA; LPS = lipopolysaccharide; NF-

κB = nuclear factor κB; ssRNA = sin-gle-stranded RNA; X = ligand unknown.

Toll-like receptors and host defense

Lipoproteins

Flagellin

CpG DNA ssRNALPS dsRNA

Profilin (?)

X?

TLR 1/2TLR 2/6

TLR5

TLR9 TLR7 TLR8 TLR4 TLR3TLR11

TLR10Transcription factors (e.g., NF-κB)

Immunomodulatory genes

Tissue injury• apoptosis• septic shockDirect antimicrobial response

• reactive oxygen intermediates

Influence adaptive response• cytokine production• co-stimulatory molecules

• cell-mediated immunity• humoral immunity

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leads to killing of intracellular Mycobac-terium tuberculosis in both mouse and hu-man macrophages.38 In mouse macro-phages, bacterial lipoprotein activationof TLR2 leads to a NO-dependent kill-ing of intracellular tubercle bacilli. In hu-man monocytes and alveolar macro-phages, bacterial lipoproteins similarlyactivate TLR2 to kill intracellular M. tu-berculosis; however, this occurs by anantimicrobial pathway that is NO inde-pendent, but dependent on the activationof the vitamin D receptor and poten-tially the induction of cathelicidin.39

These data provide evidence that mam-malian TLRs have retained not only thestructural features of Drosophila Toll thatallow them to respond to microbial li-gands but also the ability to directly ac-tivate antimicrobial effector pathwaysat the site of infection.

The activation of TLRs can also bedetrimental, leading to tissue injury. Theadministration of LPS to mice can resultin manifestations of septic shock, whichis dependent on TLR4.21 Evidence sug-gests that TLR2 activation by Propioni-bacterium acnes induces inflammatory re-sponses in acne vulgaris, which lead totissue injury.40 Aliprantis et al. demon-strated that microbial lipoproteins in-duce features of apoptosis via TLR2.41

Thus, microbial lipoproteins have theability to elicit both TLR-dependent acti-vation of host defense and tissue pathol-ogy. This dual signaling pathway is simi-lar to TNF receptor and CD40 signaling,which leads to both nuclear factor-κB ac-tivation and apoptosis.42,43 In this man-ner, it is possible for the immune systemto use the same molecules to activatehost defense mechanisms and then, byapoptosis, to downregulate the responsefrom causing tissue injury. Activation ofTLR can lead to the inhibition of the ma-jor histocompatibility complex (MHC)class II antigen presentation pathway,which can downregulate immune re-sponses leading to tissue injury but mayalso contribute to immunosuppression.44

Finally, Toll activation has been impli-cated in bone destruction.35

The critical biologic role of TLRs inhuman host defense can be deducedfrom the finding that TLR4 mutationsare associated with LPS hypo-respon-siveness in humans.45 By inference, onecan anticipate that humans with geneticalterations in TLR may have increasedsusceptibility to certain microbial infec-tions. Furthermore, it should be possibleto exploit the pathway of TLR activa-tion as a means to endorse immune re-sponses in vaccines and treatments for

infectious diseases as well as to abro-gate responses detrimental to the host.

Cells of the Innate Immune System

PHAGOCYTES Two key cells of the innateimmune system are characterized bytheir phagocytic function: macrophagesand PMNs. These cells have the capac-ity to take up pathogens, recognizethem, and destroy them. Some of thefunctions of these cells are regulated viaTLRs and complement receptors as out-lined earlier.

PMNs are normally not present inskin; however, during inflammatoryprocesses, these cells migrate to the siteof infection and inflammation, wherethey are the earliest phagocytic cells tobe recruited. These cells have receptorsthat recognize pathogens directly (seePattern Recognition Receptors), and dueto their expression of FcγRIII/CD16 andC3bR/CD35, can phagocytose microbescoated with antibody and with thecomplement component C3b. As a con-sequence, granules (containing myelo-peroxidase, elastase, lactoferrin, collage-nase, and other enzymes) are released,and microbicidal superoxide radicals(O2

–) are generated (see Chap. 30).

Effector Functions of Phagocytes. Activa-tion of phagocytes by pathogens in-duces several important effector mecha-nisms, for example, t r iggering ofcytokine production. A number of im-portant cytokines are secreted by mac-rophages in response to microbes, in-cluding IL-1, IL-6, TNF-α, IL-8, IL-12,and IL-10. IL-1, IL-6, and TNF-α play acritical role in inducing the acute-phaseresponse in the liver and in inducing fe-ver for effective host defense. TNF-α in-duces a potent inflammatory responseto contain infection. IL-8 is important asa mediator of PMN chemotaxis to thesite of infection (see also Chap. 11 oncytokines).

Another important defense mecha-nism triggered in phagocytes in re-sponse to pathogens is the induction ofdirect antimicrobial responses. Phago-cytic cells such as PMNs and macro-phages recognize pathogens, engulf them,and induce antimicrobial effector mech-anisms to kill the pathogens. PMNsgenerate oxygen-dependent or oxygen-independent killing. The release of toxicoxygen radicals, lysosomal enzymes,and antimicrobial peptides such as thehuman neutrophil defensins leads to di-rect killing of the microbial organisms.6

Similarly, activation of TLRs on macro-

phages by microbial ligands upregu-lates iNOS (NOS-II), which results inrapid generation of NO and powerfulmicrobicidal activity.37 Macrophagesuse this mechanism to contain some in-fectious organisms not susceptible toPMN attack, such as mycobacteria, cer-tain fungi, and parasites.

Phagocytic cells of the innate immunesystem can also be activated by cells ofthe adaptive immune system. CD40 is a50-kd glycoprotein present on the sur-face of B cells, monocytes, DCs, and en-dothelial cells. The ligand for CD40 isCD40L, a type II membrane protein of33 kd, preferentially expressed on acti-vated CD4+ T cells and mast cells.CD40-CD40L interaction plays a crucialrole in the development of effectorfunctions. CD4+ T cells activate macro-phages and monocytes to produce TNF-α, IL-1, IL-12, interferon-γ (IFN-γ), andNO via CD40-CD40L interaction.CD40L has also been shown to rescuecirculating monocytes from apoptoticdeath, thus prolonging their survival atthe site of inflammation. In addition,CD40-CD40L interaction during T-cellactivation by APCs results in IL-12 pro-duction. Therefore, it can be concludedthat CD40-CD40L interactions betweenT cells and macrophages play a role inmaintenance of Th1-type cellular re-sponses and mediation of inflammatoryresponses. Other studies have estab-lished a role for CD40-CD40L interac-tions in B-cell activation, differentiation,and Ig class switching.55 In addition,CD40-CD40L interaction leads to up-regulation of B7.1 (CD80) and B7.2(CD86) on B cells. This co-stimulatoryactivity induced on B cells then acts toamplify the response of T cells. Thesemechanisms underscore the impor-tance of the interplay between the in-nate and the adaptive immune systemin generating an effective host response.

EOSINOPHILS (See Chap. 30) Eosinophilsare a distinct class of bone marrow–derived granulocytes that normally con-stitute only a small fraction of peripheralblood leukocytes and occur in evensmaller numbers in peripheral tissues.The cytokines granulocyte-macrophagecolony-stimulating factor (GM-CSF), IL-3and, most importantly, IL-5 are critical fortheir development and maturation.

NATURAL KILLER CELLS58 NK cells appearas large granular lymphocytes. In hu-mans, the vast majority of these cells ex-hibit the CD3–, CD56+, CD16+, CD94+,CD161+ phenotype. Their function is to

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survey the body looking for altered cells,be they transformed or infected with vi-ruses (e.g., cytomegalovirus), bacteria(e.g., Listeria monocytogenes), or parasites(e.g., Toxoplasma gondii). These pathogensare then killed directly via perforin/granzyme- or Fas/FasL-dependent mech-anisms or indirectly via the secretion ofcytokines (e.g., IFN-γ).

How do NK cells discriminate be-tween normal and transformed orpathogen-infected tissue?

All nucleated cells express the MHCclass I molecules. NK cells have recep-tors, termed killer inhibitory receptors, thatrecognize the self MHC class I mole-cules. This recognition results in the de-livery of a negative signal to the NK cellthat paralyzes it. If a nucleated cell losesexpression of its MHC class I molecules,however, as often happens after malig-nant transformation or virus infection,the NK cell, on encountering it, will be-come activated and kill it.

In addition, NK cells have activatingreceptors that bind MHC-like ligands ontarget cells. One such receptor isNKGD2, which binds to the humannon-classic MHC class I chain-related Aand B molecules, MICA and MICB.59

MICA and MICB are not expressed insubstantial amounts on normal tissuesbut are overexpressed on carcinomas.60

NK cells are able to kill MICA/MICB-bearing tumors, which suggests a rolefor NKGD2 in immune surveillance.

Another cell type that, at least inmice, could serve a similar function isthe IFN-producing killer DC, whichshares several features with DCs andNK cells.61,62 Their human equivalenthas yet to be identified.

KERATINOCYTES Once thought to be in-ert, keratinocytes, the predominant cellsin the epidermis, can mount an immuneand/or inflammatory response throughsecretion of cytokines and chemokines,arachidonic acid metabolites, comple-ment components, and antimicrobialpeptides.

Keratinocytes of unperturbed skinproduce only a few of these mediators,such as the cytokines IL-1, IL-7, andtransforming growth factor-β (TGF-β),constitutively. Resident keratinocytescontain large quantities of pre-formedand biologically active IL-1α as well asimmature IL-1β in their cytoplasm.63

The likely in vivo role of this stored in-tracellular IL-1 is that of an immediateinitiator of inflammatory and repair pro-cesses after epidermal injury. IL-7 is animportant lymphocyte growth factor

that may have a role in the survival andproliferation of the T lymphocytes ofhuman skin. Some evidence exists forthe IL-7–driven propagation of lym-phoma cells in Sézary syndrome.

TGF-β, in addition to its growth-regu-lating effects on keratinocytes and fibro-blasts, modulates the inflammatory aswell as the immune response64 and is im-portant for LC development (see furtherin Development, Maintenance, and Fateof Skin Dendritic Cells under LangerhansCells and Other Dendritic Cells).65 Ondelivery of certain noxious, or at least po-tentially hazardous, stimuli (e.g., hy-poxia, trauma, non-ionizing radiation,haptens or other rapidly reactive chemi-cals like poison ivy catechols, silica, LPSs,and microbial toxins), the productionand/or release of many cytokines is oftendramatically enhanced. The biologic con-sequences of this event are manifold andinclude the initiation of inflammation(IL-1, TNF-α, IL-6, members of the che-mokine family), the modulation of LCphenotype and function (IL-1, GM-CSF,TNF-α, IL-10, IL-15), T-cell activation (IL-15, IL-18),66,67 T-cell inhibition (IL-10,TGF-β),68 and skewing of the lympho-cytic response in either the type 1 (IL-12,IL-18),69 type 2 (thymic stromal lym-phopoietin),70 or Th17 (IL-23) direction.71

In some cases, keratinocytes may alsoplay a role in amplifying inflammatorysignals in the epidermis originating fromnumerically minor epidermal cell sub-sets. One prominent example is the in-duction of pro-inflammatory cytokinessuch as TNF-α in keratinocytes by LC-derived IL-1β in the initiation phase ofallergic contact dermatitis.72 In the pres-ence of a robust stimulus, keratinocyte-derived cytokines may be released intothe circulation in quantities that causesystemic effects. During a severe sunburnreaction, for example, serum levels of IL-1, IL-6, and TNF-α are clearly elevatedand probably responsible for the systemicmanifestations of this reaction, such asfever, leukocytosis, and the production ofacute-phase proteins.73 There is also evi-dence that the ultraviolet (UV) radiation–inducible cytokines IL-6 and IL-10 can in-duce the production of autoantibodiesand thus be involved in the exacerbationof autoimmune diseases such as lupuserythematosus. The fact that secretedproducts of keratinocytes can reach thecirculation could conceivably also be usedfor therapeutic purposes. The demonstra-tion by Fenjves et al.74 that grafting ofapolipoprotein E gene–transfected hu-man keratinocytes onto mice results inthe detection of apolipoprotein E in the

circulation of the mouse supports the fea-sibility of such an approach.

Another important function of keratino-cytes is the production/secretion of factorsgoverning the influx and efflux of leuko-cytes into and out of the skin. Two goodexamples are the chemokines thymus andactivation-regulated chemokine (TARC;CC chemokine ligand 17, or CCL17) andcutaneous T cell–attracting chemokine(CTACK)/CCL27 and their correspondingreceptors CCR4 and CCR10, selectivelyexpressed on skin-homing T lymphocytes.Blocking of both chemokines drasticallyinhibits the migration of T cells to the skinin a murine model of contact hypersensi-tivity (CHS).75 Another more distinct func-tion of a keratinocyte-derived chemokinein the recruitment of leukocyte sub-popu-lations to the epidermis is suggested by theselective expression of the macrophage in-flammatory protein 3α (MIP-3α)/CCL20receptor, CCR6, on LCs and LCprecursors76 (see Development, Mainte-nance, and Fate of Skin Dendritic Cells andT Lymphocyte Sub-Populations).

The demonstration of cytokine recep-tors on and cytokine responsiveness bykeratinocytes established that the func-tional properties of these cells can be sub-ject to regulation by cells of the immunesystem. As a consequence, keratinocytesexpress or are induced to express immu-nologically relevant surface moieties thatcan be targeted by leukocytes for stimula-tory or inhibitory signal transduction.

In addition to cytokines, keratino-cytes secrete other factors such as neu-ropeptides, eicosanoids, and reactiveoxygen species. These mediators havepotent inflammatory and immunomod-ulatory properties and play an importantrole in the pathogenesis of cutaneous in-flammatory and infectious diseases aswell as in aging.

Keratinocytes can also synthesizecomplement and related receptors, in-cluding the C3b receptor [complementreceptor 1 (CR1), CD35], the Epstein-Barr virus receptor CR2 (C3d receptor,CD21), the C5a receptor (CD88), themembrane co-factor protein (CD46), thedecay-accelerating factor (CD55), andcomplement protectin (CD59). CD59may protect keratinocytes from attackby complement. Its engagement byCD2 stimulates the secretion of pro-inflammatory cytokines from keratino-cytes. Membrane co-factor (CD46) is re-ported to be a receptor for M protein ofgroup A streptococci and for measles vi-rus.80 Its ligation induces pro-inflamma-tory cytokines in keratinocytes such asIL-1α, IL-6, and GM-CSF.

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ADAPTIVE IMMUNE RESPONSEThe strength and the type of the innateresponse determines both the quantityand quality of an adaptive response ini-tiated by dendritic APCs in the epider-mis (LCs) and dermis (dermal DCs orDDCs) and executed by T lymphocytesand antibodies.

Lymphocytes

The adaptive immune response is medi-ated by T and B lymphocytes. Theunique role of these cells is the ability torecognize antigenic specificities in alltheir diversity. All lymphocytes derivefrom a common bone marrow stem cell.This finding has been exploited in variousclinical settings, with attempts to restorethe entire lymphocyte pool by bone mar-row or stem cell transplantation.

TYPES OF LYMPHOCYTES B cells mature inthe fetal liver and adult bone marrow.They produce antibodies—protein com-plexes that bind specifically to particularmolecules defined as antigens. As a con-sequence of recombinatorial events in

different Ig gene segments (V or variable;D or diversity; J or joining), each B cellproduces a different antibody molecule(Fig. 10-4). Some of this antibody ispresent on the surface of the B cell, con-ferring the unique ability of that B cell torecognize a specific antigen. B cells thendifferentiate into plasma cells, the actualantibody-producing and -secreting cells.The secreted antibody mediates humoralimmune responses. In skin, humoral im-munity contributes to the immune de-fense against extracellular pathogens.Antibodies bind to microbial agents andneutralize them or facilitate uptake ofthe pathogen by phagocytes that destroythem. Antibodies are also responsible formediating certain pathologic conditionsin skin. In particular, antibodies againstself-antigens lead to autoimmune dis-ease, typified in the pathogenesis ofpemphigus and bullous pemphigoid. Fur-thermore, IgE antibodies to foreign sub-stances elicit anaphylactic reactions (e.g.,penicillin urticaria).

T cells mature in the thymus, wherethey are selected to live or to die. ThoseT cells that will have the capacity to rec-ognize foreign antigens are positively se-

lected and can enter the circulation.Those T cells that react to self are nega-tively selected and destroyed. If the im-mune system is envisioned as a bureau-cracy, the T cell is the ideal bureaucrat. Tcells have the unique ability to directother cells of the immune system. Theydo this, in part, by releasing cytokines.For example, T cells contribute to cell-mediated immunity (CMI), required toeliminate intracellular pathogens, by re-leasing cytokines that activate macro-phages and other T cells. T cells releasecytokines that activate NK cells and alsorelease cytokines that permit the growth,differentiation, and activation of B cells.

During their maturation in the thy-mus, thymocytes start to express themolecules that allow T cells to displaytheir unique functional capacity, that is,to specifically recognize antigen in anMHC-restricted fashion (see GeneralPrinciples of Antigen Presentation).These are the T-cell antigen receptor(TCR) and the accessory molecules CD4and CD8. The vast majority of posi-tively selected mature thymocytes areeither CD4+/CD8– (single positive)MHC class II–restricted cells or CD8+/

� FIGURE 10-4 T-cell receptor (TCR) gene rearrangements. This diagram shows how diversity in TCRs and antibodies is generated by gene rearrangement. Forthe TCR, rearrangement of the β chain is shown, and for antibodies, that of immunoglobulin M heavy and light chains is depicted. The encoded antibody recog-nizes the nominal antigen per se, whereas the encoded TCR recognizes antigen in the context of an appropriate antigen-presenting molecule. Ag = antigen; APC= antigen-presenting cell; C = constant segment; D = diversity segment; J = joining segment; MHC = major histocompatibility complex; V = variable segment.

NH2

NH2

Light chain

Vκ1 Jκ1 Cκ VH1 DH1 JH1 Cµ

T cell

JβVβ Vβ Vβ Dβ Cβ

βα

TCR C C

J JV D V

MHC

APC

Ag

Heavy chain

Disulfide linkage

Ag-combining

site

Effector site

-s-s-

-s-s-

-s-s--s-s-

J HD H

V HAg-

combiningsite

JβVβ Dβ Cβ

Recombinationtranscription

Light chain Heavy chain

B cell

HOOC COOH

V J C V D J

C

C

V

C

C

VV V

J LV L

C

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CD4– (single positive) MHC class I–restricted cells, but some of them ex-press the TCR but no accessory mole-cules (double-negative thymocytes). Thesemature thymocytes leave the thymusand migrate to the peripheral lymphoidtissues (lymph nodes, spleen, Peyer’spatches, etc.). This process is most ac-tive in early infancy and childhood butcontinues with decreasing output wellinto adult life.

The TCR is a complex of moleculesconsisting of an antigen-binding het-erodimer (α/β or γ/δ chains) that is non-covalently linked with five CD3 sub-units (γ, δ, ε, ζ/η). TCR α/β or TCR γ/δmolecules must be paired with CD3molecules to be inserted into the T-cellsurface membrane81 (see Fig. 10-4).

The TCR chains form the actual anti-gen-binding unit, whereas the CD3complex mediates signal transduction,which results in either productive acti-vation or nonproductive silencing of theT lymphocyte.

The TCR chains have amino acid se-quence homology with and structuralsimilarities to Ig heavy and light chains.The genes encoding TCR molecules areencoded as clusters of gene segments (V,J, D, C or constant) that rearrange dur-ing T-cell maturation. Together with theaddition of nucleotides at the junctionof rearranged gene segments, this re-combinatorial process, which involvesthe enzymes recombinase activatinggene 1 and 2, results in a heterogeneityand diversity of the antigen recognitionunit that is broad enough to allow for asuccessful host defense.

The accessory molecules CD4 andCD8 stabilize the interaction of the TCRwith the MHC-linked peptide antigen.Although CD4 binds to MHC class IImolecules, CD8 acts as an adhesive bybinding to MHC class I molecules.

T-LYMPHOCYTE SUB-POPULATIONS T cellscan be classified and subdivided in dif-ferent ways: (1) on the basis of the ac-cessory molecules CD4 and CD8, (2) onthe basis of their activation status (na-ive, memory, effector T cells), and (3) onthe basis of their functional role in theimmune response, which is often linkedto the cytokine secretion property of therespective cell population.

As far as the activation status of Tcells is concerned, it appears that thestrength of the antigenic signal also de-termines the ultimate fate of a naive Tcell. On robust activation, these cellsdifferentiate into effector cells, whichare then selected to enter the memory

pool according to their capacity to ac-cess and use survival signals. Effector-memory cells home to peripheral tissuesand are responsible for immediate pro-tection against challenge. CCR7+ cen-tral-memory cells, on the other hand,home to secondary lymphoid organsand are responsible for secondary orlong-term responses to antigen andmight be involved in long-term mainte-nance of effector-memory cells.82

With regard to the functional capaci-ties of various T-cell subsets, it was orig-inally assumed that CD4+ cells predom-inantly subserve helper functions andthat CD8+ cells act as killer cells. Manyexceptions to this rule are now knownto exist; for example, both CD4+ andCD8+ regulatory cells are found, butCD4+ cells are still commonly referredto as helper T cells (Th cells) and CD8+

cells as cytotoxic T cells.Naive Th cells, so-called Th0 cells,

can differentiate into several func-tional classes of cells during an im-mune response: (1) Th1 cells (type 1 Tcells); (2) Th2 cells (type 2 T cells); (3) Th17cells; (4) regulatory T cells (Treg); and(5) natural killer T cells (NKT).

T HELPER 1/T HELPER 2 PARADIGM T cellsthat produce IL-2, IFN-γ, and TNF aretermed Th1 cells. They are the main car-riers of CMI. Other T cells produce IL-4,IL-5, IL-6, IL-13, and IL-15. These aretermed Th2 cells and are primarily re-sponsible for extracellular immunity(see later).83,84 Many factors influencewhether an uncommitted Th cell devel-ops into a mature Th1 or Th2 cell. Thecytokines IL-12 and IL-4, acting throughsignal transducer and activator of trans-cription (STAT) 4 and 6, respectively, arekey determinants of the outcome, as areantigen dose, level of co-stimulation,and genetic modifiers. Certain transcrip-tion factors have causal roles in thegene-expression programs of Th1 andTh2 cells. For example, the T-box trans-cription factor T-bet is centrally involvedin Th1 development, inducing both tran-scriptional competence of the IFN-γ lo-cus and selective responsiveness to thegrowth factor IL-12.85 By contrast, thezinc-finger transcription factor GATA-3seems to be crucial for inducing cer-tain key attributes of Th2 cells, suchas the transcriptional competence ofthe Th2 cytokine cluster, which in-cludes the genes encoding IL-4, IL-5,and IL-13.86,87

In murine models of intracellular in-fection, resistant versus susceptibleimmune responses appear to be regu-

lated by these two T-cell sub-popula-tions.50,51,88 Th1 cells, primarily by therelease of IFN-γ, activate macrophagesto kill or inhibit the growth of thepathogen and trigger cytotoxic T-cellresponses, which results in mild orself-curing disease. In contrast, Th2cells facilitate humoral responses andinhibit some cell-mediated immune re-sponses, which results in progressiveinfection. These cytokine patterns arecross-regulatory. The Th1 cytokineIFN-γ downregulates Th2 responses.The Th2 cytokines IL-4 and IL-10downregulate both Th1 responses andmacrophage function. The result isthat the host responds in an efficientmanner to a given pathogen by makingeither a Th1 or Th2 response. Some-times the host chooses an inappropri-ate cytokine pattern, which results inclinical disease.

The discovery that Th1/Th2 re-sponses could contribute to the out-come of human disease due to a singleantigen was first delineated by thestudy of leprosy. Because leprosy pre-sents as a spectrum of clinical manifes-tations that correlate with the immuneresponse to the pathogen, it provides anextraordinary window into immuneregulation in humans. At one end of thespectrum, patients with tuberculoid lep-rosy typify the resistant response thatrestricts the growth of the pathogen.The number of lesions is few, althoughtissue and nerve damage is frequent. Atthe opposite end of this spectrum, pa-tients with lepromatous leprosy repre-sent extreme susceptibility to M. lepraeinfection. In lepromatous leprosy, theskin lesions are numerous and growthof the pathogen is unabated, which re-sults in many viable M. leprae through-out the skin lesions. These clinical pre-sentations correlate with the level ofCMI against M. leprae. The standardmeasure of CMI to the pathogen is theMitsuda reaction. Patients are chal-lenged by intradermal injection of M.leprae, and induration is measured 3weeks later. The test result is positive intuberculoid patients and negative in lep-romatous patients. It is widely agreedthat T cells involved in CMI are pivotalin determining the outcome of infectionwith M. leprae, because, in correlationwith skin test results, lymphocyte reac-tivity is positive in tuberculoid patientsbut is negative in lepromatous patients.Yet, there is an interesting paradox inthat CMI and humoral responses ex-hibit an inverse relationship. Anti–M.leprae antibody levels are most elevated

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in patients with the lepromatous formof the disease and therefore are notthought to play a role in protection.

This paradox can best be explained interms of the patterns of cytokines in thelesions.52,89 The Th1 cytokines, princi-pally, IL-2 and IFN-γ, are more stronglyexpressed in tuberculoid lesions,whereas the Th2 cytokines, notably IL-4, IL-5, and IL-10, are characteristic oflepromatous lesions. These cytokinepatterns can be assigned to the major T-cell subsets observed in the lesions:CD4+ T cells predominate in tubercu-loid lesions and CD8+ T cells predomi-nate in lepromatous lesions. All of theM. leprae–specific CD4+ T cells derivedfrom tuberculoid patients produce IL-2and IFN-γ and are designated CD4+ type1 cells. The CD8+ T cells derived fromlepromatous lesions produce high lev-els of IL-4 and low levels of IFN-γ andare designated CD8+ type 2 cells.

In terms of the immunopathogenesisof leprosy (see Chap. 186), the abun-dance of IL-2 and IFN-γ in tuberculoidlesions is likely to contribute to the re-sistant state of immunity in these pa-tients. IL-2 may contribute to the hostdefense by inducing the clonal expan-sion of activated, cytokine-producing Tcells and augments the production ofIFN-γ. IFN-γ is well known to enhanceproduction of reactive oxygen and ni-

trogen intermediates by macrophagesand stimulates them to kill or restrictthe growth of intracellular pathogens.The cytokines found to be increased inlepromatous lesions might be expectedto contribute to the immune unrespon-siveness and failure of macrophage acti-vation in these individuals. IL-4 and IL-10 may contribute to the elevated anti–M. leprae antibodies in lepromatous pa-tients via their role in differentiationand Ig class switching of B cells, butthey also have a negative immunoregu-latory effect on CMI, downregulating T-cell and macrophage function.

Of particular interest to immunologistsis the delineation of factors that influencethe T-cell cytokine pattern. The innateimmune response is one important factorinvolved in determining the type of T-cellcytokine response (Fig. 10-5).

The ability of the innate immune re-sponse to induce the development of aTh1 response is mediated by release ofIL-12, a 70-kd heterodimeric protein.46

For example, in response to an intracel-lular pathogen, macrophages release IL-12, which acts on NK cells to releaseIFN-γ. The presence of IL-12, IL-2, andIFN-γ, with the relative lack of IL-4, fa-cilitates Th1 responses. In contrast, inresponse to allergens or extracellularpathogen, mast cells or basophils releaseIL-4, which in the absence of IFN-γ

leads to differentiation of T cells alongthe Th2 pathway. It is intriguing tospeculate that keratinocytes may alsoinfluence the nature of the T-cell cyto-kine response. Keratinocytes can pro-duce IL-10, particularly after exposureto UVB radiation.68 The released IL-10can specifically downregulate Th1 re-sponses, thus facilitating the develop-ment of Th2 responses.

T HELPER 17 CELLS Not every T cell–me-diated disease can be easily explainedby the Th1/Th2 paradigm. Some T-cellsub-populations are characterized bythe secretion of IL-17. These cells aretherefore termed Th17 cells. IL-23, amember of the IL-12 family, is appar-ently of key importance for the develop-ment of Th17 cells,90 which have beenlinked to a growing list of autoimmuneand inflammatory diseases such as neu-roinflammatory disorders, asthma, lu-pus erythematosus, rheumatoid arthri-tis, and, most notably, psoriasis.71 IL-17is believed to contribute to the patho-genesis of these diseases by acting as apotent pro-inflammatory mediator. Itwas originally assumed that Th1 andTh17 cells arise from a common Th1precursor, but it now appears that Th17cells are a completely separate and earlylineage of effector CD4+ Th cells pro-duced directly from naive CD4+ T cells.

� FIGURE 10-5 The role of innate immunity in determining the type of cytokine response. IFN = interferon; IL = interleukin; NK = natural killer; Th1, Th2 = Thelper 1, 2.

Mast cellother cells

IL-4

IL-4

Antigenpresenting cell

Virusesbacteria

Allergenshelminths

Humoralimmunity

Eosinophilresponses

Macrophagesuppression

B-cell stimulation

IL-12

IFN-γ

IFN-γ

IL-4

Th1 cell

IFN-γCell-mediated

immunity

Macrophage

Th2 cell

IL-4IL-10

IL-5

NK cell

IL-4

IFN-γ

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CYTOTOXIC T CELLS In responding to anintracellular pathogen (e.g., a virus) theT cell must lyse the infected cell. To doso, it must be able to recognize and re-spond to antigenic peptides encoded bythis pathogen and displayed on the cellsurface. For this to occur, antigens aris-ing in the cytosol are cleaved into smallpeptides by a complex of proteases,called the proteasome. The peptide frag-ments are then transported from the cy-tosol into the lumen of the endoplasmicreticulum, where they associate withMHC class I molecules. These peptide–class I complexes are exported to theGolgi apparatus and then to the cell sur-face (see General Principles of AntigenPresentation for more details). The mat-uration of a CD8+ T cell to a killer T cellrequires not only the display of the anti-genic signal but also the delivery ofhelper signals from CD4+ T cells, forwhich the functional interaction be-tween CD40 on the APC and CD40L onthe CD8+ T cell can substitute.

Two distinct subsets of cytotoxic Tcells have been identified and can be dif-ferentiated by the mechanism by whichthey kill targets,91 the end result beingthe induction of a programmed celldeath known as apoptosis.92,93 The firstmechanism of cytotoxicity involves theinteraction of two cell surface proteins,Fas ligand (CD95L) on the T cells andFas (CD95) on the target. Ligation ofthese molecules delivers a signalthrough Fas that induces the apoptosiscascade in the target. The second mech-anism involves the release of cytoplas-mic granules present in such T cells.These granules contain perforin, whichinduces a pore in the target, andgranzymes, serine esterases that, wheninjected into cells, trigger the apoptoticpathway. Such granules also containgranulysin, a protein with a broad spec-trum of antimicrobial activity againstbacteria, fungi, and parasites.91,94 In thismanner, cytotoxic T cells can directlykill microbial invaders. Besides contrib-uting to host defense against infectionand tumors, cytotoxic T cells can alsocontribute to tissue injury. For example,cytotoxic T cells recognize self-antigensof melanocytes and thus may contributeto the pathogenesis of vitiligo.95

REGULATORY T CELLS An important typeof immunomodulatory T cells that con-trols immune responses is the so-calledregulatory T cells (Treg cells), formerlyknown as T suppressor cells.96 Treg cellsare induced by immature APCs/DCsand play key roles in maintaining toler-

ance to self-antigens in the periphery.Loss of Treg cells is the cause of organ-specific autoimmunity in mice that re-sults in thyroiditis, adrenalitis, oophori-tis/orchitis, and so on. Treg cells are alsocritical for controlling the magnitudeand duration of immune responses tomicrobes. Under normal circumstances,the initial antimicrobial immune re-sponse results in the elimination of thepathogenic microorganism and is thenfollowed by an activation of Treg cellsto suppress the antimicrobe responseand prevent host injury. Some microor-ganisms (e.g., Leishmania parasites, my-cobacteria) have developed the capacityto induce an immune reaction in whichthe Treg component dominates the ef-fector response. This situation preventselimination of the microbe and resultsin chronic infection.

Regulatory functions are mediated bydistinct groups of CD4+, CD8+, andNKT cells. The best-characterized Tregsubset is the CD4+/CD25+/CTLA-4+/GITR (glucocorticoid-induced TNF re-ceptor family–related gene)+/FoxP3+

lymphocytes. The transcription factorFoxP3 is specifically linked to the sup-pressor function, as evidenced by thefindings that mutations in the FoxP3gene cause the fatal autoimmune andinflammatory disorder of scurfy in miceand IPEX (immune dysregulation, poly-endocrinopathy, enteropathy, X-linked)in humans. The cytokines TGF-β andIL-10 are thought to be the main media-tors of suppression.

CD8+ cells can be activated to be-come suppressor cells by antigenic pep-tides that are presented in the context ofan MHC class Ib molecule [Qa1 in mice;human leukocyte antigen E (HLA-E) inhumans]. CD8+ Treg cells suppress Tcells that have intermediate affinity forself or foreign antigens and are primarilyinvolved in self-nonself discrimination.

NKT cells are a distinctive populationof T cells. They have properties of NKcells but, at the same time, express TCRα/β that consists of an invariant α chain(Vα24-JαQ) paired with various Vβchains. These cells specifically recognizecertain tumor cell–associated or bacterialglycolipids in the context of CD1 mole-cules and are therefore implicated in tu-moricidal and bactericidal host responses(see CD1-Dependent Antigen Presenta-tion). On antigenic stimulation, NKTcells produce large quantities of cyto-kines, particularly IL-4 and IL-10, and canuse them to suppress Th1 responses. Thebiologic relevance of these in vitro datacan be deduced from the observation

that depletion of NKT cells can aggravateand accelerate Th1-mediated autoim-mune diseases in mice, such as insulin-dependent diabetes, multiple sclerosis,and inflammatory bowel disease.97

LYMPHOCYTES OF THE SKIN Normal skinis the prototype of a nonlymphoid or-gan, that is, an organ in which primarylymphocyte responses are not initiated.

As opposed to normal mouse skin, inwhich a resident population of dendriticepidermal T cells uniformly equippedwith a nonpolymorphic, canonical TCRγ/δ exists, normal human skin containsonly small numbers of lymphocytes, themajority of which are located in the der-mis and express TCR α/β rather than γ/δ.98,99 These T cells of normal human der-mis are preferentially clustered aroundpostcapillary venules of the superficialplexus high in the papillary dermis andare often situated just beneath the der-mal-epidermal junction and within, orin close proximity to, adnexal append-ages such as hair follicles and eccrinesweat ducts. Most of them belong to theCD45RO+ memory population—withthe CD4+/CD8– dominating over theCD4–/CD8+ phenotype—and express theskin-homing receptor cutaneous lympho-cyte–associated antigen (CLA).100

At perivascular sites, most T cellsstain positively for HLA-DR and CD25,which indicates that some of them rep-resent effector cells and others perhapsTreg cells.

Epidermal T cells account for approxi-mately 2 percent to 3 percent of allCD3+ cells in normal human skin. Theyreside primarily in the basal and supra-basal layers, often in close apposition toLCs. Most of them are CD8+/CD4– lym-phocytes that bear TCR α/β dimers.There also exists a minor subset of dou-ble-negative (CD4–/CD8–) intraepider-mal T cells with TCRs of either the α/βor the γ/δ phenotype. Their relation-ship, if any, to the murine dendritic epi-dermal T cell is not known.

The mechanism by which T lympho-cytes traffic into skin depends on achain of molecular events between cells.In skin-draining lymph nodes, the inter-action of naive T cells with antigen-bearing cutaneous DCs (LCs, DDCs) re-sults in the induction of the cell surfacemolecule CLA.101

CLA is a glycoprotein that defines asubset of memory T cells that home toskin. CLA is a glycosylated form of P-selectin glycoprotein ligand 1 that is ex-pressed constitutively on all human pe-ripheral blood T cells. The level of CLA

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on cells is regulated by an enzyme,α(1,3)-fucosyl transferase VII, that modi-fies P-selectin glycoprotein ligand 1. Inthis manner, CLA+ cells bind to both E-selectin and P-selectin, strengthening theinteraction between circulating T cellsand cutaneous endothelium, whereasCLA– cells bind P-selectin but do notbind E-selectin.102,103

In patients with allergic contact der-matitis (see Chap. 13), the CLA+ subset,but not the CLA– subset, contains the Tcells with the capacity to respond to theallergen.104 Furthermore, more than 90percent of T cells in inflammatory skindisease are CLA+. CLA facilitates theentry of T lymphocytes into skin bymediating tethering and rolling of Tcells on vascular endothelial cellsthrough binding to E-selectin. Chemo-kines released by the endothelial cellsincrease the binding affinity of T-cell ad-hesion molecules. T cells firmly adhereto endothelium by the interaction oflymphocyte function–associated anti-gen 1 (LFA-1) with ICAM-1 and verylate antigen 4 with vascular cell adhe-sion molecule. The interaction with theendothelial cells is now sufficientlystrong to permit transmigration of the Tcells into the skin and allow their partic-ipation in the inflammatory process.

A diversity of chemokines (see Chap.12) contributes to tissue-specific T-cellhoming. Leukocytes and nonleukocytesresiding in the skin can produce chemo-kines with T-cell chemotactic proper-ties, such as IL-8/CXC chemokine li-gand 8 (CXCL8), Gro α/CXCL1, IFN-γinducible protein-10/CXCL10, mono-kine induced by IFN-γ/CXCL9, macro-phage chemoattractant protein-1 (MCP-1)/CCL2, MCP-2/CCL8, MCP-3/CCL7,regulated on activation normal T-cell ex-pressed and secreted (RANTES)/CCL5,MIP-1α/CCL3, MIP-1β/CCL4, and lym-photactin/XCL1.

Of particular importance for skin hom-ing of memory T cells is the interactionof TARC/CCL17 and CTACK/CCL27with their corresponding chemokine re-ceptors on CLA+ T cells, CCR4 andCCR10, respectively.

The CC chemokine TARC/CCL17 isexpressed by vascular endothelial cellsof venules in normal and inflamed hu-man skin105 (see Chap. 163). CLA+

memory T cells in peripheral blood dis-playing CCR4 adhere to cutaneous ves-sels via TARC/CCL17-induced bindingto ICAM-1 and are thereby attracted tothe skin. In addition, the recruitment oftype 2 (Th2) T cells into diseased skincan be mediated by TARC/CCL17. In

atopic dermatitis, a prominent examplefor a Th2-mediated immune response,this chemokine is known to be upregu-lated in basal keratinocytes.106

CTACK/CCL27, another CC chemo-kine, is also critically involved in thehoming process under physiologic andinflammatory conditions.75 It is constitu-tively produced by basal keratinocytesand is also displayed on the surface ofdermal endothelial cells. Its expression isupregulated by IL-1β and TNF-α anddownregulated by glucocorticosteroids.The receptor for CCL27, CCR10, is ex-pressed on CLA+ T cells, and in vivo ex-periments have demonstrated a pivotalrole for CCL27-CCR10 interactions in Tcell–mediated skin inflammation.107

Adhesion molecule interactions thathelp to anchor T cells in the epidermis in-clude the attachment of LFA-1 (CD11a)–bearing T cells to ICAM-1+ (CD54) kerat-inocytes in inflamed skin and, morephysiologically, the αEβ7-E-cadherin–mediated binding of T cells to nonacti-vated keratinocytes.

The accumulation of T cells in skin isnot stochastic. It is abundantly clear thatspecific populations of T cells, identifiedby cell surface determinants and theircytokine profile, localize to the skin.Various cell surface determinants on Tcells allow detection of their presence.Initially, functional T-cell populationscould be delineated in skin according totheir expression of the CD4 and CD8molecules. In the majority of inflamma-tory conditions studied, including lichenplanus, psoriasis, and atopic dermatitis,CD4+ T cells outnumber CD8+ T cells,in proportions similar to or somewhatgreater than those seen in the peripheralblood. However, in the study of the skinlesions of human leprosy, CD4+ T cellswere found to be predominant in the tu-berculoid form of the disease, whereasCD8+ T cells were found to be predomi-nant in the lepromatous form of the dis-ease.108 Because all leprosy patientshave an excess of CD4+ T cells in theirblood, the abundance of CD8+ T cells inlepromatous skin lesions provides clearevidence for the specific accumulationof T-cell populations in skin.

A perhaps more relevant marker of T-cell populations is the diversity of theirTCRs. The clearest example is theclonality of the T-cell population in cu-taneous T-cell lymphoma, in which asingle V gene usage is found to predom-inate in different skin lesions from thesame individual109,110 (see Chap. 146).

The dominant expression of severalTCR V genes in an infiltrate is thought

to indicate that a small number of anti-gens drive the local inflammatory re-sponse. Unlike in normal human skinwhose TCR repertoire is rather diver-gent,111 a limited TCR V gene usage hasbeen reported to be present in the skinlesions of leprosy,112 psoriasis,113 basalcell carcinoma, and countless other reac-tions in which T cells are present. How-ever, in no instance has the limited setof antigens been defined and correlatedwith the TCR usage.

The most direct indication of relevantT-cell populations in skin is determina-tion of the number of T cells that recog-nize the antigen. It has been documentedthat 1 in 1000 to 1 in 10,000 T cells in theperipheral blood recognize a given anti-gen. In the skin, however, approximately1 in 50 to 1 in 100 T cells recognize theantigen causing the disease.114,115 Thusthere is as much as a 100-fold enrich-ment of antigen-reactive T cells at thesite of cutaneous inflammation.

With regard to survival and/or expan-sion of T cells of human skin/epidermis,it appears that IL-2, IL-7, and IL-15111

play important roles. The latter two T-cell growth factors can be produced byhuman epidermal cells, and all are over-expressed in T cell–rich skin lesions ofpatients with tuberculoid leprosy.

The Th1/Th2 paradigm provides in-sight into the pathogenesis of manyskin diseases in which T cells have animmunologic role. There is ample evi-dence that the Th1/Th2 paradigm is notrigid; there are situations in which amixture of cytokines is found and ex-amples of T-cell clones, known as Th0cells, that secrete a combination of Th1and Th2 cytokines. However, it hasbeen possible to find a number of der-matologic conditions in which either aTh1 or Th2 cytokine pattern predomi-nates. In the realm of cutaneous infec-tion, leprosy (see T Helper 1/T Helper 2Paradigm) and leishmaniasis are out-standing examples of diseases with aclinical spectrum in which Th1 and Th2cytokines appear to have a pathogenicrole. Leishmaniasis, like leprosy, is not asingle disease entity but a set of clinicalentities, each with a differing immuno-pathogenesis. As in leprosy, the type 1cytokine pattern is characteristic ofleishmaniasis lesions (see Chap. 206) inwhich CMI to the parasite is strong andthe lesions self-cure; the type 2 patterntypifies lesions in which immunity tothe parasite is weak and the cutaneouslesions are progressive.117,118 Studies inanimal models suggest that it may bepossible to induce effective CMI by vac-

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cination using a combination of parasiteantigens and recombinant IL-12.119 Thisimmunotherapeutic strategy engendersa Th1 cytokine response. Th1 responsesare involved in immunologic resistanceto Borrelia burgdorferi120 and Treponemapallidum,121 the causative agents ofLyme disease and syphilis, respectively.

Concepts of the pathogenesis ofatopic dermatitis (see Chap. 14) includea central role for allergen-specific T cellsthat produce Th2 or type 2 cytokines,including IL-4 and IL-5. Allergen-specificT cells with this cytokine profile havebeen demonstrated in the peripheralblood and skin lesions of subjects withactive disease.122–124 In addition, the le-sions of atopic dermatitis contain abun-dant expression of IL-10, although thesource of this cytokine is likely to be tis-sue macrophages and keratinocytes.125

The Th2 pattern of cytokines together isthought to induce increased Ig produc-tion, particularly of IgE, mast cellgrowth, and the infiltration of eosino-phils. These cytokines may also down-regulate Th1 responses, which wouldaccount for the increased susceptibilityto cutaneous bacterial infection.

Evidence suggests that pruritus, a keysymptom of atopic dermatitis, may alsobe linked to the Th2 response. The cy-tokine IL-31 induces severe pruritus anddermatitis in mice and is preferentiallyexpressed in Th2 cells. Human IL-31 issignificantly upregulated in pruriticforms of skin inflammation (atopic der-matitis, prurigo nodularis) but not innon-pruritic forms (psoriasis), and circu-lating CLA+ memory T cells of patientswith atopic dermatitis produce higherlevels of IL-31 that the T cells of donorswith psoriasis.126,127

Clinical trials attempting to alter theTh2 response in atopic dermatitis throughthe administration of IFN-γ have shownthat this treatment induces significantbut modest clinical improvement but noreduction in IgE levels in some pa-tients.128 This was noteworthy, becauseTh2 cytokines help B cells produce auto-antibodies in pemphigus vulgaris129 (seeChap. 52). The beneficial effect of IFN-γcame as somewhat of a surprise in that amediator shift has been described inatopic dermatitis, with Th2 cytokinesdominating in acute lesions and Th1 cy-tokines in chronic lesions.130

In allergic contact dermatitis (seeChap. 13), sensitization involves the de-velopment of a Th1 response, as evi-denced by the predominating IL-2 andIFN-γ production of murine T cells sen-sitized in vitro to haptenated APCs.131

The situation in the elicitation phase isless clear. In nickel contact dermatitis,antigen-specific Th1-type T-cell cloneswere described,132 as were Th2-type in-filtrating T cells in lesional skin.133

Th1/Th2 responses may be involvedin antitumor immunity. For example, IL-4 and IL-10 predominate in the lesionsof basal cell and squamous cell carci-noma, whereas the Th1 response ispresent in benign neoplasms54 (seeChaps. 114 and 115). The source of theIL-10 in these cutaneous carcinomas isthe tumor itself, a mechanism by whichthe cancer can downregulate antitumorT-cell responses. Within the spectrum ofcutaneous T-cell lymphoma, mycosisfungoides represents a Th1 cytokine re-sponse, whereas patients with the moreprogressive Sézary syndrome exhibit aTh2 cytokine response134 (see Chap.146). It was originally assumed that Th1cytokine responses predominate in in-volved and, to a lesser extent, unin-volved skin of patients with psoriasis.135

More recent evidence suggests that IL-23, rather than IL-12, is the key cytokinein this disease136 and that it exerts its ef-fects by triggering IL-22 production byTh17 cells, which results in dermal in-flammation and acanthosis.137 Althoughit is uncertain, these Th17 cells may beautoimmune, responding to self-anti-gens in the epidermis.

Whatever the role for the observed cy-tokine patterns in human disease, theTh1-Th2-Th17 paradigm exposes newtargets for therapy. Trials are under wayto exploit this knowledge through theuse of cytokine agonists and antagoniststo shift the balance between the differentTh patterns for the benefit of the patient.

Langerhans Cells and Other Dendritic Cells

DEFINITION In 1868, the medical studentPaul Langerhans, driven by his interestin the anatomy of skin nerves, identifieda population of dendritically shapedcells in the suprabasal regions of the epi-dermis after impregnating human skinwith gold salts.138 These cells, whichlater were found in virtually all stratifiedsquamous epithelia of mammals, arenow eponymously referred to as Langer-hans cells. There also exist substantialnumbers of dendritic leukocytes in thedermis. Although some of them repre-sent LCs on their way into or out of theepidermis, most of these cells are phe-notypically slightly different from LCsand are generally referred to as dermaldendritic cells.139 LCs and DDCs are lin-

eage-negative (Lin–), bone marrow–derived leukocytes endowed with ex-quisite migratory and antigen-present-ing properties. Thus, they phenotypi-cally and functionally resemble otherDCs present in most, if not all, lym-phoid and nonlymphoid tissues.140 Asthe gatekeepers of the immune system,they control the response to events per-turbing tissue homeostasis (Fig. 10-6A).

PHENOTYPIC PROPERTIES OF SKIN-BOUNDLANGERHANS CELLS AND DERMAL DEN-DRITIC CELLS The expression of theCa2+-dependent lectin Langerin (CD207)is currently the single best feature dis-criminating LCs from other cells. Lang-erin is a transmembrane molecule asso-ciated with and sufficient to formBirbeck granules, the prototypic and celltype–defining organelles of LCs (see Fig.10-6B). Birbeck granules are pentilami-nar cytoplasmic structures frequentlydisplaying a tennis racket shape at theultrastructural level. The additionalpresence of Langerin on the LC cell sur-face coupled with its binding specificityfor mannose suggests that Langerin isinvolved in the uptake of mannose-containing pathogens by LCs. However,the disruption of the Langerin gene inexperimental animals does not result ina marked loss in LC functionality.141

Notably, Langerin is expressed on virtu-ally all LCs in stratified epithelia as wellas on a major subset of DCs in thelung,142 which may or may not be di-rectly related to epidermal LCs.

The expression of additional mole-cules besides Langerin allows the identi-fication of LCs within normal unper-turbed epidermis. These include CD1a;the MHC class II antigens HLA-DR,HLA-DQ, and HLA-DP; and CD39, amembrane-bound, formalin-resistant, sulf-hydryl-dependent adenosine triphospha-tase (ATPase).

DDCs are phenotypically less wellcharacterized. Their best markers areprobably the molecules CD1b and CD1cas well as the subunit A of the clottingproenzyme factor XIII (factor XIIIa).DDCs can be distinguished from LCs bythe absence of Langerin expression andBirbeck granules, and from macrophagesby the abundant expression of MHCclass II molecules, DEC205/CD205, andthe absence of phagolysosomes at theultrastructural level.

TISSUE DISTRIBUTION OF LANGERHANSCELLS AND DERMAL DENDRITIC CELLS Inthe epidermis, the density of the LCpopulation varies regionally in human

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skin. On head, face, neck, trunk, andlimb skin, the LC density ranges be-tween 600 and 1000/mm2. Compara-tively low densities (approximately 200/mm2) are encountered in palms, soles,anogenital and sacrococcygeal skin, andthe buccal mucosa. The density of hu-man LCs decreases with age, and LCcounts in skin with chronic actinic dam-age are significantly lower than those inskin not exposed to UV light.

DDCs are located primarily in the vi-cinity of the superficial vascular plexus.

DEVELOPMENT, MAINTENANCE, AND FATE OFSKIN DENDRITIC CELLS (Fig. 10-7) HLA-DR+/ATPase+ DCs can be identified inthe human epidermis by 6 to 7 weeks ofestimated gestational age. These cellsmust originate from hemopoietic pro-genitor cells in the yolk sac or fetal liver,the primary sites of hemopoiesis duringthe embryonic period. Until the twelfthweek of pregnancy, these cells areCD1a– and lack Birbeck granules.Thereafter, and coinciding with the ini-tiation of bone marrow function, there

occurs a dramatic increase in CD1a ex-pression by epidermal DCs, which indi-cates the emergence of a true LC popu-lation. The relative numeric stability ofLC counts during later life must beachieved by a delicate balance of LCgeneration and immigration into theepidermis and LC death and emigrationfrom the epidermis.

Within the epidermis, LCs are an-chored to surrounding keratinocytes byE-cadherin–mediated homotypic adhe-sion.147 This anchoring and the displayof TGF-β1 also prevent terminal differ-entiation and migration (see later), thussecuring intraepidermal residence forthe cells under homeostatic conditions.

Two non–mutually exclusive path-ways of LC repopulation of the epider-mis may exist: LC division within theepidermis, and the differentiation ofLCs from skin-resident or blood-borneprecursors. Evidence for the first possi-bility is the demonstration of cycling/mitotic LCs in the epidermis,148 al-though it remains to be establishedwhether this cell division alone suffices

for maintaining the epidermal LC popu-lation. Notably, it has now been discov-ered that DDCs proliferate constitu-tively in situ in murine and humanquiescent dermis,149 which indicatesthat homeostatic cell division also con-tributes to the maintenance of this skinDC population.

The observation that the half-life ofLCs within unperturbed murine epider-mis is around 2 to 3 months150 suggestsa significant turnover of the epidermalLC population even under noninflam-matory conditions. In seeming contra-diction stands the observation that theLC population of human skin graftedonto a nude mouse remains rather con-stant for the life of the graft, despite epi-dermal proliferation and the absence ofcirculating precursors for human LCs.Moreover, epidermal LCs in micewhose bone marrow was lethally irradi-ated and subsequently transplanted areonly partially replaced by LCs of donororigin,151 whereas DCs in other organsare efficiently exchanged for donorDCs.152 Together, these observations

� FIGURE 10-6 A. Langerhans cells in a sheetpreparation of murine epidermis as revealed byanti–major histocompatibility complex class II (flu-orescein isothiocyanate) immunostaining. B. Elec-tron micrograph of a Langerhans cell in humanepidermis. Arrows denote Birbeck granules. N =nucleus. (From Stingl G: New aspects of Langer-hans cell functions. Int J Dermatol 19:189, 1980,with permission.) Inset: High-power electron mi-crograph of Birbeck granules. The curved arrowsindicate the zipper-like fusion of the fuzzy coats ofthe vesicular portion of the granule. The delimitingmembrane envelops two sheets of particles at-tached to it and a central lamella composed oftwo linear arrays of particles. (From Wolff K: Thefine structure of the Langerhans cell granule. JCell Biol 35:466, 1967, with permission.)

A

B

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suggest that a precursor cell populationresides in the dermis that is engagedconstantly in the self-renewal of the epi-dermal LC population under noninflam-matory conditions. The prime candidateLC precursors are dermal CD14+/CD11c+ cells that have the potential todifferentiate in vitro into LCs in a TGF-β1–dependent fashion.153

Under inflammatory conditions (e.g.,UV radiation exposure, graft-versus-hostdisease), an additional pathway of epi-dermal LC recruitment becomes opera-tive. In this situation, LC precursors en-ter the tissue, and their progenypopulate the epidermis in a fashion de-pendent on chemoattraction mediatedby LC-expressed chemokine receptorsCCR2 and CCR6,154 the ligands ofwhich are secreted by endothelial cellsand keratinocytes. Interestingly, a simi-lar pathway of inflammation-dependentprecursor recruitment exists for DDCs,which in contrast to that of LCs, relies

on CCR2– but not CCR6-dependent cellmigration.149 Thus, CCR6 and its ligandMIP-3α/CCL20 may be essential for epi-dermal LC localization in vivo, as postu-lated previously in studies of LCs differ-entiated from human progenitor cells invitro.76 The action of MIP-3α/CCL20may be assisted or replaced under non-inflammatory situations by the chemo-kine BRAK/CXCL14, which is constitu-tively produced by keratinocytes.155 Thedifferentiation stage of the biologicallyrelevant circulating LC precursors enter-ing inflamed skin in vivo remains to beresolved. However, evidence exists thatcommon myeloid progenitors, granulo-cyte-macrophage progenitors, mono-cytes, and even common lymphoid pro-genitors can give rise to the emergenceof an epidermal LC population in experi-mental animals.156,157

Under inflammatory conditions, DCtypes that are not residents of the nor-mal cutaneous environment appear in

the skin. These include plasmacytoidDCs (pDCs) and DCs that phenotypi-cally resemble myeloid DCs of the pe-ripheral blood. The pDCs are DC pre-cursors that are characterized by a highlydeveloped endoplasmic reticulum, whichresults in their plasma cell-like appear-ance.158 Functionally, pDCs display aunique ability to produce enormousamounts of natural IFNs in response toTLR ligands and thus were also namedprincipal type 1 IFN-producing cells.159 Un-der homeostatic conditions, pDCs arefound in peripheral blood and T cell–richareas of secondary lymphatic tissue. Incertain types of skin inflammation (e.g.,virus infection, lupus erythematosus,psoriasis, allergic contact dermatitis,atopic dermatitis), pDCs enter the skinin a fashion that engages the CXC che-mokine receptor CXCR3.160,161 Withinthe skin, pDCs localize in perivascularclusters with T cells and, on activationin situ, may contribute to antimicrobial

� FIGURE 10-7 Schematic diagram of the ontogeny, migration, and maturation pathways of cutaneous dendritic leukocytes. α6/β1,4 = α6/β1,4 integrins; B = Bcells; CCL = CC chemokine ligand; CCR = CC chemokine receptor; CD = cluster of differentiation-nomenclature of leukocyte antigens; CLA = cutaneous lympho-cyte–associated antigen; CLP = common lymphoid progenitor cell; CMP = common myeloid progenitor cell; CpG DNA = immunostimulatory cytosine- and guanine-rich sequences of DNA; DC = dendritic cell; DDC = dermal dendritic cell; dsRNA = double-stranded RNA; E-Cad = E-cadherin; GM-CSF = granulocyte-macrophagecolony-stimulating factor; IL = interleukin; LC = Langerhans cell; LPS = lipopolysaccharide; Mφ = macrophage; M-CSF = macrophage colony-stimulating factor;MMP = matrix metalloproteinase; T = T cells; TGF-β1 = transforming growth factor-β1; TLR = Toll-like receptor; TNF-α = tumor necrosis factor-α.

IL-1β

Danger signals

LPS CpG DNA

dsRNA Necrosis

AllergenTLR

LCAdhesion E-Cad

CCL27CCL20

TNF-αIL-16 IL-18

Activation

CCR7+

E-Cad+/-

IL-1α

α6/β1,4

MMP-2MMP-9

Osteopontin/CD44

Afferent lymphatics

Podoplanin +CCL21

BCD44

T

CCL19

CCL21

Lymph node

CD1a+

CD11c+

CCR6+

pre-LC

Progenitors

CMP

GM-CSFTNF-α

CLA+CD34+

TGF-β1

DDC

Stem cell

CLP CLA-

CD34+

Plasma-cytoid DC

DC2 DC1

CD123+

CD14-

CD11C-

pre-DC2

CD14+

monocyte/pre-DC

GM-C

SF

IL-3

IL-4 Mφ

TGF-β1

CD40

Lvi

rus

MigrationHom

ing

Derm

isEp

ider

mis

M-CSF

++++

+

+

++

++

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immune defense or (auto)immune-medi-ated tissue inflammation by the secre-tion of natural IFNs and other mecha-nisms that are still to be identified.

Another non-indigenous DC popula-tion originates from myeloid precursorsand has been detected in inflammatoryskin diseases such as atopic dermatitisand contact eczema. The so-called in-flammatory dendritic epidermal cells arecharacterized by the expression ofCD1a, CD1b, FcεRI, CD23, HLA-DR,and CD36.162 Evidence exists that animmune response triggered by thesecells is skewed in the Th1 direction.163

Finally, there remains the question asto the ultimate fate of the epidermal anddermal DC populations. Major pertur-bation of the cutaneous microenviron-ment (danger signal164) leads to their ac-tivation, which results either in theirelimination via the stratum corneum inthe case of LCs165 or, more importantly,in the migration of LCs/DDCs to lym-phoid tissues, where they initiate type 1(Th1/cytotoxic T cell 1)–dominated T-cell responses (see The Skin–InitiationSite and Target of Immune Response).By contrast, it is less clear what happensin normal skin. Does LC shedding occurunder physiologic (nondanger) condi-tions? Is there a natural flux of LCs/DDCs to the regional lymph nodes? Ifso, what are the functional conse-quences of such an occurrence? Evi-dence exists that melanin granules cap-tured in the skin accumulate in theregional lymph nodes but not in othertissues. The further observation of onlyvery few melanin granule–containingcells in TGF-β1–/– mice suggests that,under steady-state conditions, epider-mal and/or dermal antigens are carriedto the regional lymph nodes by TGF-β1–dependent cells (most likely LCs/DDCs) only. It appears that T lympho-cytes encountering such APCs in vivoare rendered unresponsive in an anti-gen-specific manner.166 It may thereforebe assumed that absence of pathogenicT-cell autoimmunity and/or lack of reac-tivity against seemingly innocuous envi-ronmental compounds (e.g., aeroaller-gens) in the periphery is primarily theconsequence of an active immune re-sponse rather than the result of its non-occurrence.

On receipt of danger signals (e.g., TLRligands, chemical haptens, hypoxia), thesituation is quite different. After a fewhours, LCs begin to enlarge, to display in-creased amounts of surface-bound MHCclass II molecules, and to migrate down-ward in the dermis, where they enter af-

ferent lymphatics and, finally, reach the T-cell zones of draining lymph nodes.167

During this process, LCs undergo pheno-typic changes similar to those that occurin single epidermal cell cultures168; that is,downregulation of molecules or struc-tures responsible for antigen uptake andprocessing as well as for LC attachmentto keratinocytes (e.g., Fc receptors, E-cad-herin) and upregulation of moieties re-quired for active migration and stimula-tion of robust responses of naive T cells(e.g., CD40, CD80, CD83, CD86). Themechanisms governing LC migration arebecoming increasingly clear. TNF-α andIL-1β (in a caspase 1–dependent fashion)are critical promoters of this process,whereas IL-10 inhibits its occurrence. In-creased cutaneous production and/or re-lease of the pro-inflammatory cytokinesis probably one of the mechanisms bywhich certain immunostimulatory com-pounds applied to or injected into the skin[e.g., imiquimod, unmethylated cytosine-phosphate-guanosine (CpG) oligonucle-otides] accelerate LC/DDC migration. In-terestingly enough, Cumberbatch et al.169

reported that, in psoriasis, LCs are im-paired in their migratory capacity. Thiswas somewhat unexpected in view of theremarkable overexpression of TNF-α inpsoriatic skin. These investigators alsofound that the failure of TNF-α and/or IL-1β to induce LC migration from unin-volved skin was not attributable to analtered expression of receptors for thesecytokines. The nature of this LC migra-tion inhibition factor is as yet unknown.

IL-16 also induces LC mobilization.This process could perhaps be operativein atopic dermatitis. In this disease, DCsof lesional skin exhibit surface IgEbound to high-affinity Fc receptors(FcεRI), and allergen-mediated receptorcross-linking results in enhanced IL-16production.

An important hurdle for emigratingLCs is the basement membrane. Duringtheir downward journey, LCs probablyattach to it via α6-containing integrin re-ceptors and produce proteolytic en-zymes such as type IV collagenase (ma-trix metalloproteinase 9) to penetrate itand to pave their way through thedense dermal network into the lym-phatic system. Evidence is accumulatingthat LC/DDC migration occurs in an ac-tive, directed fashion. Osteopontin is achemotactic protein that is essential inthis regard. It initiates LC emigrationfrom the epidermis and attracts LCs/DDCs to draining nodes by interactingwith an N-terminal epitope of the CD44molecule.170 The entry into and active

transport of cutaneous DCs within lym-phatic vessels appears to be mediatedby MCPs binding to CCR2 and bysecondary lymphoid-organ chemokine/CCL21 produced by lymphatic endo-thelial cells of the dermis and binding toCCR7 on maturing LCs and DDCs.171,172

Interestingly, CCL21 expression is up-regulated in irritant and allergic contactdermatitis, which implicates its regu-lated impact on DC emigration from theskin.173

FUNCTIONAL PROPERTIES OF DENDRITICCELLS Like the other members of theDC family, LCs and DDCs are “profes-sional” APCs, endowed with the uniquecapacity of stimulating antigen-specificresponses in naive, resting T cells. Toprovide a better understanding of thisfunctional property, the basic principlesof antigen uptake, processing, and pre-sentation are briefly reviewed.

GENERAL PRINCIPLES OF ANTIGEN PRESEN-TATION (Fig. 10-8) Unlike B cells, T cellscannot recognize soluble protein antigenper se; their antigen receptor TCR is de-signed to see antigen-derived peptidesbound to MHC locus–encoded mole-cules expressed by APCs.176 For the anti-gen-specific activation of Th cells, exog-enous antigen–derived peptides areusually presented in the context of MHCclass II molecules.177 In this situation,peptides are generated in the endocytic,endosomal/lysosomal pathway and arebound to MHC class II molecules. Theresulting MHC-peptide complex is ex-pressed at the APC surface for encounterby the TCR of CD4+ Th cells. In con-trast, most CD8+ T cells, destined to be-come cytotoxic T cells, recognize the en-dogenous antigen in association withMHC class I molecules.177 Because mostnucleated cells transcribe and expressMHC class I genes and gene products, itis evident that many cell types can serveas APCs for MHC class I–restricted anti-gen presentation and/or as targets forMHC class I–dependent attack by Tcells. In the MHC class II–dependent an-tigen-presentation pathway, DCs, in-cluding LCs and DDCs, B cells, andmonocytes/macrophages, are the majorAPC populations.

Major Histocompatibility Complex Class I–Restricted Antigen Presentation178. CLAS-SIC MAJOR HISTOCOMPATIBILITYCOMPLEX CLASS I PRESENTATIONPATHWAY. Immediately after their bio-synthesis, MHC class I heavy and light(β2-microglobulin) chains are inserted

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into the membranes of the endoplasmicreticulum. The third subunit of thefunctional MHC class I complex is thepeptide itself. The major sources of pep-tides for MHC class I loading are cyto-solic proteins, which can be targeted fortheir rapid destruction through the cata-lytic attachment of ubiquitin. Cytosolicproteinaceous material undergoes enzy-matic digestion by the proteasome toyield short peptide chains of 8 to 12amino acids, an appropriate length forMHC class I binding. In its basic confor-mation, the proteasome is a constitu-tively active “factory” for self-peptides.IFN-γ, by replacing or adding certainproteasomal subunits, induces “immu-noproteasomes,” presumably to fine-tune the degradation activity and speci-ficity to the demands of the immuneresponse. The processed peptides are

translocated to the endoplasmic reticu-lum by the transporter associated withantigen processing (TAP), an MHC-encoded dimeric peptide transporter.With the aid of chaperons (calnexin, cal-reticulin, tapasin), MHC class I mole-cules are loaded with peptides, releasedfrom the endoplasmic reticulum, andtransported to the cell surface. Severalinfectious agents with relevance to skinbiology have adopted strategies to sub-vert MHC class I presentation, and thusthe surveillance of cell integrity, by in-terfering with defined molecular targets.Important examples of such interferenceare the inhibition of proteasomal func-tion by the Epstein-Barr virus–encodedEBNA-1 protein, the competition forpeptide-TAP interactions by a herpessimplex virus protein, and the retentionor destruction of MHC class I molecules

by adenovirus- and human cytomegalo-virus-encoded products.

ALTERNATIVE MAJOR HISTOCOMPATI-BILITY COMPLEX CLASS I PRESENTATIONPATHWAYS (CROSS-PRESENTATION).Under certain conditions, exogenous an-tigen can reach the MHC class I presen-tation pathway. Significant evidence forthis cross-presentation first came fromin vivo experiments in mice demon-strating that viral, tumor, and MHC an-tigens can be transferred from MHC-mismatched donor cells to host bonemarrow–derived APCs to elicit antigen-specific cytotoxic T-cell responses thatare restricted to self MHC molecules.179

In vitro studies have now defined thatexosomes (i.e., small secretory vesiclesof approximately 100 nm in diametersecreted by various cell types, including

� FIGURE 10-8 Antigen-processing pathways. The intracellular antigen-processing pathways for major histocompatibility complex (MHC) class I, MHC classII, and CD1 presentation are shown. The MHC class I pathway involves the processing of cytoplasmic proteins, whereas the MHC class II pathway involves theprocessing of exogenous proteins. The CD1 pathway regulates the processing and presentation of self-glycosphingolipids and bacterial lipoglycans. DN T cell =double-negative (CD4–/CD8–) T cell; ER = endoplasmic reticulum; MIIC = MHC class II lysosomal peptide-loading compartment; NKT cell = natural killer T cell;TAP = transporter associated with antigen processing; TCR = T-cell receptor.

CD8+ T cell

NKT cell/DN T cell

CD4+T cell

TCRCD8

MHC class Ipathway

Proteasome

TAP

ER MHCClass I

MHCclass II

Golgi

MHC class IIpathway Endosome

pH<5

MIIC

EndosomeCD4

TCR

Birbeck granule

TCRVα24

CD1 p

athway

Lipid antigen

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tumor cells), heat shock proteins, im-mune complexes, and apoptotic cells(taken up via CD36 and ανβ3 or ανβ5integrins) can all serve as vehicles forthe delivery of antigen to DCs in a man-ner that permits the cross-presentationof antigen. In all in vitro systems inwhich a direct comparison has beenmade, DCs, including LCs, but notmonocytes/macrophages, were capableof cross-presentation.180,181 Three dis-tinct pathways are currently exploitedby which antigen can access MHC classI molecules of DCs: (1) a recycling path-way for MHC class I in which antigen isloaded in the endosome; (2) a pathwayby which retrograde transport of the an-tigen from the endosome to the endo-plasmic reticulum facilitates entry intothe classic MHC class I antigen presen-tation pathway; and (3) an endosome tothe cytosol transport pathway, whichagain allows antigen processing via theclassic MHC class I antigen presentationpathway.

Major Histocompatibility Complex Class II–Restricted Antigen Presentation177. MHCclass II molecules predominantly bindpeptides within endosomal/lysosomalcompartments. Sampling peptides inthese sub-cellular organelles allow classII molecules to associate with a broadarray of peptides derived from proteinstargeted for degradation after internal-ization by fluid phase or receptor-medi-ated endocytosis, macropinocytosis, orphagocytosis. One of the striking struc-tural differences between MHC class Iand class II molecules is the conforma-tion of their peptide-binding grooves.Whereas MHC class I molecules havebinding pockets to accommodate thecharged termini of peptides and thus se-lectively associate with short peptides,the binding sites of MHC class II mole-cules are open at both ends. Thus, MHCclass II molecules bind peptides withpreferred lengths of 15 to 22 amino ac-ids but can also associate with longermoieties.

Newly synthesized MHC class II α andβ subunits assemble in a stoichiometriccomplex with trimers of the type II trans-membrane glycoprotein invariant chain(Ii). The association with Ii contributes inat least three different ways to the func-tion of class II molecules: (1) Ii assemblypromotes the proper folding of class IImolecules in the endoplasmic reticulum;(2) the abluminal portion of Ii containssignal sequences that facilitate the exportof MHC class II–Ii complexes through theGolgi system to endosomes/lysosomes;

and (3) Ii prevents class II molecules frompremature loading by peptides intendedfor binding to MHC class I molecules inthe endoplasmic reticulum. The segmentof Ii functioning as a competitor for pep-tide binding to class II is termed class II–associated Ii peptide (CLIP; residues 81 to104 of Ii). Once the nascent MHC classIIα/β–Ii trimers arrive in the endosomal/lysosomal system, Ii is subject to proteol-ysis by acid hydrolases. The last pro-teolytic step, the generation of CLIP, iscatalyzed by cathepsin S in DCs, by ca-thepsin L in thymic epithelial cells, andby cathepsin F in macrophages. On HLA-DM–chaperoned exchange of CLIP forexogenous antigen-derived peptide, fullyassembled class II molecules are exportedto the cell surface and acquire a stableconformation. Depending on the cell typeand the activation status of a cell, thehalf-life of class II–peptide complexesvaries from a few hours to days. It is par-ticularly long (more than 100 hours) onDCs that have matured into potent im-munostimulatory cells of lymphoid or-gans on encounter with an inflammatorystimulus in nonlymphoid tissues. Thevery long retention of class II–peptidecomplexes on mature DCs ensures thatonly those peptides generated at sites ofinflammation will be displayed in lym-phoid organs for T-cell priming. Cyto-kines have long been known to regulateantigen presentation by DCs. In fact, pro-inflammatory (TNF-α, IL-1, IFN-γ) andanti-inflammatory (IL-10, TGF-β1) cyto-kines regulate presentation in MHC classII molecules in an antagonistic fashion.Mechanistically, regulatory effects in-clude the synthesis of MHC componentsand proteases, and the regulation of en-dolysosomal acidification.182,183

CD1-Dependent Antigen Presentation184.Besides peptides, self-glycosphingolipidsand bacterial lipoglycans may also act asT cell–stimulatory ligands. Moleculesthat bind and present these moieties be-long to the family of nonpolymorphic,MHC class I– and II–related CD1 pro-teins. In the skin, members of the CD1family are expressed mainly by LCs andDDCs (see Development, Maintenance,and Fate of Skin Dendritic Cells). TheCD1 isoforms CD1a, CD1b, CD1c, andCD1d sample both recycling endosomesof the early endocytic system and lateendosomes and lysosomes to which lipidantigens are delivered. Unlike in theMHC class II pathway, antigen loadingin the CD1 pathway occurs in a vacuolaracidification-independent fashion. T cellsexpressing a Vα24-containing canonic

TCR, NKT cells, and CD4–/CD8– T cellsinclude the most prominent subsets ofCD1-restricted T cells. CD1-restricted Tcells play important roles in host defenseagainst microbial infections. Accordingly,human subjects infected with M. tubercu-losis showed stronger responses to CD1c-mediated presentation of a microbiallipid antigen than control subjects, andactivation of CD1d-restricted NKT cellswith a synthetic glycolipid antigen re-sulted in improved immune responses toseveral infectious pathogens. Thus, theCD1 pathway of antigen presentationand glycolipid-specific T cells may pro-vide protection during bacterial and para-site infection, probably by the secretionof pro-inflammatory cytokines, the di-rect killing of infected target cells, and B-cell help for Ig production.

Compelling evidence exists that LCsand other skin DCs, as members of thefamily of professional APCs, play a piv-otal role in the induction of adaptive im-mune responses against pathogens andneoantigens introduced into and/or gen-erated in the skin (immunosurveillance).This is best illustrated by the early ob-servation that LC-containing, but notLC-depleted, epidermal cell suspensionspulse-exposed to either soluble proteinantigens or haptens elicit a geneticallyrestricted, antigen-specific, proliferativein vitro response in naive T cells.185 Al-though these observations imply thatthe LC/DC system is indispensable forthe occurrence of antigen-specific skinimmunity, it is equally clear that LCs/DCs as they occur in their tissue resi-dence are poorly, if at all, stimulatoryfor naive T cells. Inaba et al.186 foundthat freshly isolated LCs (“immature”LCs) can present soluble antigen toprimed MHC class II–restricted T cellsbut are only weak stimulators of naive,allogeneic T cells. In contrast, LCs puri-fied from epidermal cell suspensions af-ter a culture period of 72 hours or LCspurified from freshly isolated murineepidermal cells and cultured for 72hours in the presence of GM-CSF andIL-1 (“mature” LC) are extremely potentstimulators of primary T cell–prolifera-tive responses to alloantigens,186 solubleprotein antigens,187 and haptens.187 Thestrong immunostimulatory potential ofmature LCs for resting T cells does notmean that they are superior to freshlyisolated, immature LCs in every func-tional aspect. In fact, immature LCs farexcel cytokine-activated LCs in their ca-pacity to take up and process nativeprotein antigens.188 Accordingly, imma-ture rather than mature LCs/DCs ex-

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press antigen uptake receptors. MatureLCs, although fully capable of present-ing pre-processed peptides, have losttheir capacity to process and present na-tive protein antigens.188 This cytokine-induced/culture-induced switch fromthe processing/peptide loading to thepresentation mode is a highly regulatedLC/DC function attributable to funda-mental molecular changes in proteinsynthesis and vesicle trafficking. LCs intheir immature state display MHC classII antigens in lysosomal peptide-loadingcompartments (MIIC) and, to a muchlesser extent, on the cell surface. WhenLCs mature in situ, MHC class II mole-cules are transferred to the plasmamembrane. Likewise, MHC class I sur-face expression is upregulated duringthe process of DC maturation.

The display of MHC-peptide com-plexes on the DC surface delivers the“first signal” to T cells—that is, the trigger-ing of the TCR by the APC-bound pep-tide-MHC complex. Although this eventmay be sufficient to induce the prolifera-tion of primed T cells, it is insufficient forthe productive activation of naive T cells.The occurrence of the latter requires thereceipt of “second signals,” which can bedelivered by professional APCs. In fact,antigen-specific T cells that encounterMHC-expressing cells that cannot deliversecond signals (e.g., MHC class II–inducedkeratinocytes, endothelial cells, fibro-blasts) enter a state of anergy.189 Secondsignals also determine the magnitude andquality of primary and secondary T-cellresponses. In skin DCs, as well as in DCsfrom other locations, co-stimulatory mol-ecules that deliver second signals are up-regulated during maturation induced bysurface receptors triggered by ligands se-creted or presented by other somatic cellsor, alternatively, by microbial products(danger or competence signals). Secondsignals include secreted cytokines andmembrane-bound co-stimulators, the bestdefined of which are the two members ofthe B7 family, B7.1/CD80 and B7.2/CD86. LCs/DCs in situ do not express orexpress only minute amounts of these co-stimulatory molecules, but greatly upreg-ulate these moieties during maturation.Other co-stimulatory molecules includethe ICAM-1 that binds to LFA-1, andLFA-3, the ligand of T cell–expressedCD2. Other important ligand-receptorpairs that positively affect T-cell activa-tion by DCs include heat-stable antigen(CD24)/CD24L, CD40/CD40L, CD70/CD27L, OX40 (CD134)/OX40L, and re-ceptor activator of nuclear factor κB(RANK)/RANKL.

THE SKIN—INITIATION SITE AND TARGET OF IMMUNE RESPONSES (Fig. 10-9)

In 1983, Wayne Streilein coined the termskin-associated lymphoid tissues191 to de-scribe a functionally interactive circuit ofcells and tissues (dendritic APCs, cyto-kine-producing keratinocytes, and skin-homing T cells originating in skin-drain-ing peripheral lymph nodes) that providethe skin with unique immunosurveil-lance mechanisms for the successfulprevention of or combat against cancerand infectious diseases. These cells andtissues also secure the homeostasis of thehost by preventing the development ordownregulating the expression of exag-gerated tissue-destructive immune re-sponses against per se innocuous moietiessuch as autoantigens and certain allergens.

Critical to the understanding of thisyin-yang situation was the observationthat, under homeostatic conditions, theoverwhelming majority of antigen-presenting DCs are in an immaturestate that allows them to efficiently takeup antigen with the help of specific re-ceptor sites (e.g., Langerin, macrophagemannose receptor, C-type lectin recep-tor DEC-205, low-affinity IgG receptorCD32/FcγRII, high-affinity IgE recep-tor FcεRI, the thrombospondin receptorCD36, DC-SIGN), but does not endowthem with immunostimulatory proper-ties for naive resting T cells. On the de-livery of danger signals,164 however,DCs undergo a phenotypic and func-tional metamorphosis that enables themto elicit productive and, under optimalcircumstances, protective primary im-mune responses.

It was originally assumed that the in-duction of antigen-specific non-respon-siveness occurs when antigens are pre-sented in the context of non-dendriticAPCs (nonprofessional APCs). The earlyobservation that the application ofhapten to LC-deficient skin or mucosaresults in hapten-specific tolerance192

points in this direction.More recent evidence now suggests

that DCs/LCs themselves can actively in-duce immune tolerance. In vitro, imma-ture DCs preferentially activate Tregcells.193 In vivo, in the steady state, DCsinduce tolerance to specific antigens tar-geted to these cells.166,194,195 Mechanismsresponsible for the tolerance-inducingproperty of nonactivated DCs, althoughnot entirely understood, include (1) a re-duced expression of MHC-antigen com-plexes196 and co-stimulatory molecules197

on the cell surface; (2) the secretion of im-

munosuppressive cytokines such as IL-10,198 which fits well to the finding ofTreg induction by UV-irradiated, IL-10–producing Treg cells199; (3) the expres-sion of immunoinhibitory enzymes suchas indoleamine 2,3-dioxygenase200; (4) thereceipt of signals interfering with thematuration and migration of DCs, for ex-ample, neuropeptides such as CGRP201

and vasoactive intestinal peptide,202 theengagement of the CD47/SHPS-1 signaltransduction cascade,203 and others.

It appears that these different factorsare not equally operative in all situa-tions. LCs, for example, can activateself-antigen–specific CD8 T cells in thesteady state, which leads to chronic skindisease,204 and, at the same time, LCsare dispensable for205 or can evendownregulate206 the induction of CHS.

Perturbation of tissue homoeostasis(i.e., the delivery of a danger signal) ini-tiates a series of molecular events thatallow peripheral DCs to rapidly migrateto secondary lymphoid organs, a jour-ney during which they mature into po-tent immunostimulatory cells capable ofsensitizing unprimed lymphocytes forproductive responses. This can be wellexemplified by a rather simple manipu-lation: culture of explanted skin forwhich hypoxia apparently suffices totrigger migration and maturation of cu-taneous DCs.167 Another example is thetopical application of contact sensitizers(e.g., dinitrofluorobenzene), which leadsto the activation of certain protein ty-rosine kinases, the modification of cellu-lar content and structure of intracyto-plasmic organelles (increase in coatedpits and vesicles, endosomes and lyso-somes, Birbeck granules), and increasedin situ motility of these cells.207

It appears that cytokines, released asa consequence of physicochemical orinfection-associated tissue perturbation(e.g., keratinocyte-derived GM-CSF, TNF-α, IL-1) and/or ligation of CD40 mole-cules on DC/LC surfaces, provide thecritical signal for the induction of LCmaturation. But how do skin cells recog-nize a danger signal and translate it intoincreased cytokine production?

One biologically relevant pathway iscertainly the maturation signal that isdelivered to DCs/LCs by the uptake ofnecrotic cells. The fact that LCs are ca-pable of cross-presentation181,180 couldmake this an important mechanism inthe generation of a protective antitumorimmune response.

Successful defense against invadingmicroorganisms involves the recogni-tion of pathogen-associated molecular

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patterns through members of the TLRprotein family, 11 members of whichhave been classified so far (see earlier).Evidence now exists that human LCs ex-press messenger RNA encoding TLR1,TLR2, TLR3, TLR5, TLR6, and TLR10.208

Ligands of TLR1, TLR2, and TLR6 in-clude lipoproteins from M. tuberculosis, B.burgdorferi, T. pallidum, mycobacterial lipo-

arabinomannan, peptidoglycan, zymo-san, and glycophosphatidyl inositolanchors from T. pallidum and T. cruzilipoproteins. Whereas TLR5 detects bac-terial flagellin, TLR3 is associated withthe recognition of viral double-strandedRNA. LCs mount a particularly robustantiviral response to TLR3 agonists,209

which implies that natural or synthetic

ligands for TLR3 might prove useful inthe treatment of viral skin infections.

For immunotherapeutic purposes,particular attention has also focussed onTLR7, TLR8, and TLR9, which are intra-cellular receptors for nucleic acids. TLR7and TLR8 are engaged by viral single-stranded RNA and by synthetic smallmolecules mimicking features of nucleic

� FIGURE 10-9 The mechanisms operative in the initiation, expression, and downregulation of cutaneous immune responses. Induction of productive T-cellimmunity via the skin: The epicutaneous and/or intracutaneous de novo appearance of antigens (i.e., pathogens such as microorganisms and haptens) resultsin the elicitation of productive antigen-specific immunity when “danger signals” (i.e., bacterial DNA rich in unmethylated cytosine-phosphate-guanine repeatsand other Toll-like receptor ligands) are present at the time of antigenic exposure. The receipt of danger signals leads to tissue perturbation, as evidenced by theincreased secretion of granulocyte-macrophage colony-stimulating factor, tumor necrosis factor-α, and interleukin 1 (IL-1) by keratinocytes (KCs) and other skincells. Antigen-presenting cells (APCs) [Langerhans cells (LCs), dermal dendritic cells (DDCs)] that pick up the antigen, process it, and re-express it as a peptide–major histocompatibility complex (MHC) product on the surface are also profoundly affected by danger signals or danger signal–induced cytokines. The alter-ations of LCs/DDCs include the increased expression of MHC antigens, co-stimulatory molecules, and cytokines (IL-1β, IL-6, IL-12), as well as the enhancedemigration of these cells from the skin to the paracortical areas of the draining lymph nodes. At this site, the skin-derived dendritic cells (DCs) provide activationstimuli to the naive resting T cells surrounding them. This occurs in an antigen-specific fashion and thus results in the expansion of the respective clone(s).These primed T cells begin to express skin-homing receptors (e.g., CLA) as well as receptors for various chemoattractants that promote their attachment to der-mal microvascular endothelial cells of inflamed skin and, ultimately, their entry into this tissue. Elicitation of T cell–mediated tissue inflammation and pathogendefense: On receipt of a renewed antigenic stimulus by cutaneous APCs (LCs, DDCs), the skin-homing primed T cells expand locally and display the effectorfunctions needed for the elimination, or at least the attack, of the pathogen. Alternatively, primed T cells may encounter the antigen on the surface of nonprofes-sional APCs (e.g., MHC class II–bearing KCs), a situation that conceivably results in a state of clonal T-cell anergy. Downregulation and prevention of cutaneousT-cell immunity: In the absence of danger signals (tissue homeostasis), antigen-loaded LCs/DDCs also leave the cutaneous compartment and migrate towardthe draining lymph node. These cells or, alternatively, resident lymph node DCs that had picked up antigenic moieties from afferent lymphatics present this an-tigen in a nonproductive fashion—that is, they induce antigen-specific T-cell unresponsiveness or allow the responding T cell(s) to differentiate into immunosup-pressive T regulatory cells. The latter may limit antigen-driven clonal T-cell expansion during primary immune reactions in lymph nodes and during secondaryimmune reactions at the level of the peripheral tissue. Such events can result in the downregulation of both desired (antitumor, antimicrobial) and undesired(hapten-specific, autoreactive) immune responses. Ag = antigen; T = T naive cell; T* = anergic T cell; TCR = T-cell receptor.

Danger signals

T

TCR

Lymph node

Afferent phase Efferent phase

T*

T

T*

ImmatureLC/DDCMature

LC/DDC

DDC

Ag Ag

Ag Ag

LC

KCDDC

LC

DDC

Ag

LC

KC

AgCytokinesChemokines

Endothelial cells

Primed T cellT lymphocyte

Afferentlymphatic vessel

Clonalexpansion

Epidermis

Dermis

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acids, such as imiquimod and R-848.TLR9, on the other hand, recognizes oli-godeoxynucleotides (ODNs) containingunmethylated CpG motifs (CpG ODN),which are underrepresented in mamma-lian genomes but abundant in viral andbacterial DNA. Both TLR7 and TLR9 li-gands have profound effects on theskin’s immune system. Imiquimod andthe other imidazoquinolines inducestrong inflammation and, ultimately, re-gression of viral acanthomas and othersuperficial skin neoplasms.210 CpGODNs promote LC migration211 andpromote the development of Th1 re-sponses.212 It is still not clear how thetopical application of TLR7 and TLR8 li-gands could elicit such a robust inflam-matory response, because the respectivereceptors are predominantly expressedon pDCs, which are the main producersof type I IFNs and are virtually absent innormal human skin.213 The demonstra-tion of TLR7 on suprabasal keratino-cytes208 could indicate that these cells,rather than LCs or DDCs, are the primeskin targets of topical imidazoquino-lines and, by triggering the productionof pro-inflammatory cytokines and che-mokines in these cells, are responsiblefor the influx of different types of leuko-cytes, including plasmacytoid and mye-loid inflammatory-type DCs. The roleof these latter cells in the immune re-sponse has yet to be clarified. Evidenceexists that inflammatory-type myeloidDCs skew the immune response in aTh1 direction163 and that pDCs, de-pending on their state of activation, fa-vor the activation of Th2 and Treg cells,respectively. When topical imiquimodtreatment (see Chap. 221) results in theregression and, ultimately, resolution ofskin neoplasms, these inflammatory-type DCs are abundantly presentaround regressing tumor cell islands214

and can express molecules of the lyticmachinery such as perforin, granzymeB, and TNF-related apoptosis-inducingligand, which suggests their cytotoxicpotential.

The induction of skin cell injury and/or demise by cells of the innate immunesystem should not detract from the factthat adaptive mechanisms are responsi-ble for most of the desired immune reac-tions in the skin (i.e., the elimination ofpathogenic microorganisms and neo-plastic cells) as well as for immune-mediated injury of the skin. Examples ofthe latter are bullous diseases such as

pemphigus and bullous pemphigoid. Al-though these entities are mediated byautoantibodies, other skin diseases areapparently the result of exaggeratedand/or misdirected T-cell reactions.

The skin immune system is also af-fected by various immunomodulatingcompounds applied to or introducedinto the skin. The efficacy of imidazo-quinolines and CpG oligonucleotides incutaneous neoplasms has already beendiscussed, and one would predict that amore selective targeting of LCs/DDCswill increase their efficacy, tolerability,and safety.

Corticosteroids, the most frequentlyused immunoinhibitory and anti-inflam-matory substances in dermatology, havea profound influence on the phenotypeand function of cutaneous leukocytes atboth the topical and systemic levels. Af-ter topical application of betamethasonevalerate for only a few days, apoptoticevents are clearly visible in the epider-mal LC population, and on continuationof this treatment the epidermis can beessentially depleted of these cells. Theso-called inflammatory-type DCs (i.e., in-flammatory dendritic epidermal cellsand pDCs) are similarly susceptible tocorticosteroids, which is one of the rea-sons for the excellent efficacy of topicalcorticosteroids in treating acute derma-titis/eczema.220

The effects of the topical calcineurininhibitors tacrolimus and pimecrolimusare more selective. Their application tothe skin of patients with atopic dermati-tis leads to an apoptosis-induced deple-tion of T cells and to a gradual disap-pearance of inflammatory-type DCs butleaves the epidermal LC population es-sentially unaltered.220 Time will tellwhether the differential effects of topi-cal corticosteroids and calcineurin inhib-itors on the skin immune system willhave an influence on the long-termsafety of these compounds.

The entire field of topical immuno-modulation is now advancing rapidlybecause of (1) an increasingly better un-derstanding of the skin’s immune func-tion and, as a consequence, the identifi-cation of promising drug targets; (2) newcomputer-assisted methods of drug de-sign; and (3) new technologies that al-low for a better penetration of a givencompound into the skin and its guid-ance to the desired target. These devel-opments are heralding a new golden eraof skin-based immunotherapy, and the

skills of a well-trained dermatologist arerequired to use the therapy for the max-imum benefit of patients.

KEY REFERENCES

The full reference list for all chapters is available at www.digm7.com.

2. Gasque P: Complement: A unique innateimmune sensor for danger signals. MolImmunol 41:1089, 2004

6. Braff MH et al: Cutaneous defensemechanisms by antimicrobial peptides.J Invest Dermatol 125:9, 2005

29. Akira S, Uematsu S, Takeuchi O: Patho-gen recognition and innate immunity.Cell 124:783, 2006

55. van Kooten C, Banchereau J: CD40-CD40 ligand. J Leukoc Biol 67:2, 2000

71. Harrington LE, Mangan PR, Weaver CT:Expanding the effector CD4 T-cell rep-ertoire: The Th17 lineage. Curr OpinImmunol 18:349, 2006

81. von Boehmer H: Selection of the T-cellrepertoire: Receptor-controlled check-points in T-cell development. Adv Immu-nol 84:201, 2004

84. Abbas AK, Murphy KM, Sher A: Func-tional diversity of helper T lympho-cytes. Nature 383:787, 1996

92. Berke G: The binding and lysis of targetcells by cytotoxic lymphocytes: Molec-ular and cellular aspects. Annu RevImmunol 12:735, 1994

101. Butcher EC, Picker LJ: Lymphocyte hom-ing and homeostasis. Science 272:60, 1996

115. Modlin RL et al: Learning from lesions:Patterns of tissue inflammation in lep-rosy. Proc Natl Acad Sci U S A 85:1213,1988

138. Langerhans P: Über die Nerven der men-schlichen Haut. Virchows Arch 44:325, 1868

140. Banchereau J, Steinman RM: Dendriticcells and the control of immunity.Nature 392:245, 1998

159. Siegal FP et al: The nature of the princi-pal type 1 interferon-producing cells inhuman blood. Science 284:1835, 1999

164. Matzinger P: An innate sense of danger.Ann N Y Acad Sci 961:341, 2002

184. Porcelli SA, Modlin RL: The CD1 sys-tem: Antigen-presenting molecules forT cell recognition of lipids and glycolip-ids. Annu Rev Immunol 17:297, 1999

185. Stingl G, Tamaki K, Katz SI: Origin andfunction of epidermal Langerhans cells.Immunol Rev 53:149, 1980

191. Streilein JW: Skin-associated lymphoidtissues (SALT): Origins and functions. JInvest Dermatol 80 Suppl:12s, 1983

197. Lutz MB, Schuler G: Immature, semi-mature and fully mature dendritic cells:Which signals induce tolerance or immu-nity? Trends Immunol 23:445, 2002

202. Kodali S et al: Vasoactive intestinal pep-tide modulates Langerhans cell immunefunction. J Immunol 173:6082, 2004

210. Dockrell DH, Kinghorn GR: Imiquimodand resiquimod as novel immunomodu-lators. J Antimicrob Chemother 48:751,2001

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C H A P T E R 1 1

CytokinesIfor R. WilliamsBenjamin E. RichThomas S. Kupper

THE CONCEPT OF CYTOKINESWhen cells and tissues in complex organ-isms need to communicate over dis-tances greater than one cell diameter, sol-uble factors must be employed. A subsetof these factors is most important whenproduced or released transiently underemergent conditions. When faced withan infection- or injury-related challenge,the host must orchestrate a complex andcarefully choreographed series of steps. Itmust mobilize certain circulating whiteblood cells precisely to the relevant in-jured area (but not elsewhere) and guideother leukocytes involved in host de-fense, particularly T and B cells, to spe-cialized lymphatic tissue remote fromthe infectious lesion but sufficiently closeto contain antigens from the relevantpathogen. After a limited period of timein this setting (i.e., lymph node), antibod-ies produced by B cells, and effectormemory T cells, can be released into thecirculation and will localize at the site ofinfection. Soluble factors produced byresident tissue cells at the site of injury,by leukocytes and platelets that are re-cruited to the site of injury, and by mem-ory T cells ultimately recruited to thearea, all conspire to generate an evolvingand effective response to a challenge tohost defense. Most important, the levelof this response must be appropriate tothe challenge and the duration of the re-sponse must be transient; that is, longenough to decisively eliminate thepathogen, but short enough to minimizedamage to healthy host tissues. Much ofthe cell-to-cell communication involvedin the coordination of this response is ac-complished by cytokines.

General features of cytokines aretheir pleiotropism and redundancy. Be-fore the advent of a systematic nomen-clature for cytokines, most newly iden-tified cytokines were named accordingto the biologic assay that was beingused to isolate and characterize the ac-tive molecule (e.g., T-cell growth factorfor the molecule that was later renamedinterleukin 2, or IL-2). Very often, inde-pendent groups studying quite disparatebioactivities isolated the same molecule,

which revealed the pleiotropic effects ofthese cytokines. For example, before be-ing termed interleukin 1, this cytokinehad been variously known as endogenouspyrogen, lymphocyte-activating factor, andleukocytic endogenous mediator. Many cy-tokines have a wide range of activities,causing multiple effects in responsivecells and a different set of effects in eachtype of cell capable of responding. Theredundancy of cytokines typicallymeans that in any single bioassay (suchas induction of T-cell proliferation),multiple cytokines will display activity.In addition, the absence of a single cyto-kine (such as in mice with targeted mu-tations in cytokine genes) can often belargely or even completely compensatedfor by other cytokines with overlappingbiologic effects.

CLASSIFICATIONS OF CYTOKINES

Primary and Secondary Cytokines

A simple concept that continues to beextremely useful for discussion of cyto-kine function is the concept of “pri-mary” and “secondary” cytokines.5 Pri-mary cytokines are those cytokines thatcan, by themselves, initiate all theevents required to bring about leuko-cyte infiltration in tissues. IL-1 (both αand β forms) and tumor necrosis factor(TNF; includes both TNF-α and TNF-β)function as primary cytokines, as docertain other cytokines that signalthrough receptors that trigger the nu-clear factor κB (NF-κB) pathway. IL-1and TNF are able to induce cell adhe-sion molecule expression on endothelialcells [selectins as well as immunoglobu-lin superfamily members such as inter-cellular adhesion molecule 1 (ICAM-1)and vascular cellular adhesion molecule1 (VCAM-1)], to stimulate a variety ofcells to produce a host of additional cy-tokines, and to induce expression ofchemokines that provide a chemotacticgradient allowing the directed migra-tion of specific leukocyte subsets into asite of inflammation (see Chapter 12).Primary cytokines can be viewed aspart of the innate immune system (seeChap. 10), and in fact share signalingpathways with the so-called Toll-like re-ceptors (TLRs), a family of receptors thatrecognize molecular patterns character-istically associated with microbialproducts.6 Although other cytokinessometimes have potent inflammatoryactivity, they do not duplicate this fullrepertoire of activities. Many qualify as

secondary cytokines whose productionis induced after stimulation by IL-1 and/or TNF family molecules. The term sec-ondary does not imply that they are lessimportant or less active than primarycytokines; rather, it indicates that theirspectrum of activity is more restricted.

T-Cell Subsets Distinguished by Pattern of Cytokine Production

Another valuable concept that has with-stood the test of time is the assignmentof many T cell–derived cytokines intogroups based on the specific helper T-cell subsets that produce them (Fig. 11-1).The original two helper T-cell subsetswere termed Th1 and Th2. Commit-ment to one of these two patterns of cy-tokine secretion also occurs with CD8cytotoxic T cells and γ/δ T cells. Domi-nance of type 1 or type 2 cytokines in aT-cell immune response has profoundconsequences for the outcome of im-mune responses to certain pathogensand extrinsic proteins capable of servingas allergens.7 Nearly two decades afterthe original description of the Th1 andTh2 subsets, strong evidence hasemerged that there are other function-

CYTOKINESAT A GLANCE

■ Cytokines are polypeptide mediators that function in communication between hematopoietic cells and other cell types.

■ Cytokines often have multiple biologic activities (pleiotropism) and overlapping biologic effects (redundancy).

■ Primary cytokines, such as interleukin 1 and tumor necrosis factor-α, are suffi-cient on their own to trigger leukocyte influx into tissue.

■ Most cytokines signal through either the nuclear factor κB or the Jak/STAT signal-ing pathways.

■ Cytokine-based therapeutics now in use include recombinant cytokines, inhibitory monoclonal antibodies, fusion proteins composed of cytokine receptors and immunoglobulin chains, topical immuno-modulators such as imiquimod, and cyto-kine fusion toxins.

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ally significant patterns of cytokine se-cretion by T cells. The Th17 subset isdistinguished by production of a highlevel of IL-17, a member of a family ofrelated cytokines (the other membersare IL-17B through F) that had not yetbeen identified when the Th1 and Th2subsets were first described. Th17 cellspromote inflammation, and there is con-sistent evidence from human autoim-mune diseases and mouse models ofthese diseases that IL-17–producingcells are critical effectors in autoimmunedisease.8 Another subset of T cellsknown as regulatory T cells (or Treg cellsfor short) has emerged as a crucial sub-set involved in the maintenance of pe-ripheral self-tolerance.9 Two of the mostdistinctive features of Treg cells are theirexpression of the FoxP3 transcriptionfactor and production of transforminggrowth factor-β (TGF-β), a cytokinethat appears to be required for Tregcells to limit the excess activity of thepro-inflammatory T-cell subsets.10 IL-10may also be required for the activity ofTreg cells. Not only do each of these T-cell subsets exhibit distinctive patternsof cytokine production, cytokines arekey factors in influencing the differenti-ation of naive T cells into these subsets.IL-12 is the key Th1-promoting factor,IL-4 is required for Th2 differentiation,

and both IL-23 and TGF-β are involvedin promoting Th17 development.

Structural Classification of Cytokines

Not all useful classifications of cytokinesare based solely on analysis of cytokinefunction. Structural biologists, aided byimproved methods of generating homoge-nous preparations of proteins and estab-lishment of new analytical methods (e.g.,solution magnetic resonance spectros-copy) that complement the classical x-raycrystallography technique, have deter-mined the three-dimensional structure ofmany cytokines. These efforts have led tothe identification of groups of cytokinesthat fold to generate similar three-dimen-sional structures and bind to groups of cy-tokine receptors that also share similarstructural features. For example, most ofthe cytokine ligands that bind to receptorsof the hematopoietin cytokine receptorfamily are members of the four-helix bun-dle group of proteins. Four-helix bundleproteins have a shared tertiary architec-ture consisting of four antiparallel α-heli-cal stretches separated by short connect-ing loops. The normal existence of somecytokines as oligomers rather than mono-mers was discovered in part as the resultof structural investigations. For example,interferon-γ (IFN-γ) is a four-helix bundle

cytokine that exists naturally as a nonco-valent dimer. The bivalency of the dimerenables this ligand to bind and oligomer-ize two IFN-γ receptor complexes,thereby facilitating signal transduction.TNF-α and TNF-β are both trimers thatare composed almost exclusively of β-sheets folded into a “jelly roll” structuralmotif. Ligand-induced trimerization of re-ceptors in the TNF receptor family is in-volved in the initiation of signaling.

SIGNAL TRANSDUCTION PATHWAYS SHARED BY CYTOKINES

To accomplish their effects, cytokinesmust first bind with specificity and highaffinity to receptors on the cell surfaces ofresponding cells. Many aspects of thepleiotropism and redundancy manifestedby cytokines can be understood throughan appreciation of shared mechanisms ofsignal transduction mediated by cell sur-face receptors for cytokines. In the earlyyears of the cytokine biology era, the em-phasis of most investigative work wasthe purification and eventual cloning ofnew cytokines and a description of theirfunctional capabilities, both in vitro andin vivo. Most of the cytokine receptorshave now been cloned, and many of thesignaling cascades initiated by cytokineshave been described in great detail. Thevast majority of cytokine receptors can beclassified into a relatively small numberof families and superfamilies (Table 11-1),the members of which function in an ap-proximately similar fashion. Table 11-2lists the cytokines of particular relevancefor cutaneous biology, including the ma-jor sources, responsive cells, features ofinterest, and clinical relevance of each cy-tokine. Most cytokines send signals tocells through pathways that are very sim-ilar to those used by other cytokinesbinding to the same class of receptors. In-dividual cytokines often employ severaldownstream pathways of signal trans-duction, which accounts in part for thepleiotropic effects of these molecules.Nevertheless, we propose here that a fewmajor signaling pathways account formost effects attributable to cytokines. Ofparticularly central importance are theNF-κB pathway and the Jak/STAT path-way, described in the following sections.

Nuclear Factor κB, Inhibitor of κB, and Primary Cytokines

A major mechanism contributing to theextensive overlap between the biologic

� FIGURE 11-1 Cytokines control the development of specific CD4 helper T-cell subsets. The cytokinemilieu at the time of activation of naive undifferentiated CD4 T cells has a profound influence on the ultimatepattern of cytokine secretion adopted by fully differentiated T cells. Subsets of effector CD4 T cells with de-fined patterns of cytokine secretion include T helper 1 (Th1), Th2, and Th17 cells. Regulatory CD4 T cells(Treg cells) express the FoxP3 transcription factor, and their effects are mediated in part by their productionof transforming growth factor-β1 (TGF-β1) and/or interleukin 10 (IL-10). IFN = interferon; LT = lympho-toxin. (Adapted from Tato CM, O’Shea JJ: What does it mean to be just 17? Nature 441:166, 2006.)

Cytokines influencing CD4 development

Undifferentiatednaive CD4 T cell

IL-12

IL-4

TGF-β1IL-23IL-6

TGF-β1

Th1

Th2

Th17

Treg TGF-β1IL-10

IL-17

IFN-γ, LT-α

IL-4, IL-5, IL-13

Cytokines made bymature CD4 T cells

Foxp3

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activities of the primary cytokines IL-1and TNF is the shared use of the NF-κBsignal transduction pathway. IL-1 andTNF use completely distinct cell surfacereceptor and proximal signaling path-ways, but these pathways converge atthe activation of the NF-κB transcriptionfactor. NF-κB is of central importance inimmune and inflammatory processes be-cause a large number of genes that elicitor propagate inflammation have NF-κBrecognition sites in their promoters.11

NF-κB–regulated genes include cyto-kines, chemokines, adhesion molecules,nitric oxide synthase, cyclooxygenase,and phospholipase A2.

In unstimulated cells, NF-κB het-erodimers formed from p65 and p50subunits are inactive because they aresequestered in the cytoplasm as a resultof tight binding to inhibitor proteins inthe IκB family (Fig. 11-2). Signal trans-duction pathways that activate the NF-

TABLE 11-2Cytokines of Particular Relevance for Cutaneous Biology

CYTOKINE MAJOR SOURCES RESPONSIVE CELLS FEATURES OF INTEREST CLINICAL RELEVANCE

IL-1α Epithelial cells Infiltrating leukocytes Active form stored in keratinocytes IL-1Ra used to treat rheumatoid arthritis

IL-1β Myeloid cells Infiltrating leukocytes Caspase 1 cleavage required for activation

IL-1Ra used to treat rheumatoid arthritis

IL-2 Activated T cells Activated T cells, Treg cells Autocrine factor for activated T cells IL-2 fusion toxin targets CTCL

IL-4 Activated Th2 cells, NKT cells Lymphocytes, endothelial cells, keratinocytes

Causes B-cell class switching and Th2 differentiation

IL-5 Activated Th2 cells, mast cells B cells, eosinophils Regulates eosinophil response to parasites

Anti–IL-5 depletes eosinophils

IL-6 Activated myeloid cells, fibro-blasts, endothelial cells

B cells, myeloid cells, hepatocytes

Triggers acute-phase response, promotes immunoglobulin synthesis

IL-10 T cells, NK cells Myeloid and lymphoid cells Inhibits innate and acquired immune responses

IL-12 Activated APCs Th1 cells Promotes Th1 differentiation, shares p40 subunit with IL-23

Anti-p40 inhibits Crohn disease and psoriasis

IL-13 Activated Th2 cells Monocytes, keratinocytes, endothelial cells

Mediates tissue responses to parasites

IL-17 Activated Th17 cells Multiple cell types Mediates autoimmune diseases Potential drug target in autoim-mune disease

IL-23 Activated dendritic cells Memory T cells, Th17 cells Directs Th17 differentiation, medi-ates autoimmune disease

Anti-p40 inhibits Crohn disease and psoriasis

TNF-α Activated myeloid, lymphoid, and epithelial cells

Infiltrating leukocytes Mediates inflammation Anti–TNF-α effective in psoriasis

IFN-α and IFN-β

Plasmacytoid dendritic cells Most cell types Major part of antiviral response Elicited by topical imiquimod application

IFN-γ Activated Th1 cells, CD8 T cells, NK cells, dendritic cells

Macrophages, dendritic cells, naive T cells

Macrophage activation, specific iso-type switching

IFN-γ used to treat chronic granulomatous disease

APC = antigen-presenting cell; CTCL = cutaneous T-cell lymphoma; IFN = interferon; IL = interleukin; NK = natural killer; NKT = natural killer T cell; Th = T helper; TNF = tumor necrosis factor; Treg = T regulatory.

TABLE 11-1Major Families of Cytokine Receptors

RECEPTOR FAMILY EXAMPLEMAJOR SIGNAL TRANSDUCTION PATHWAY(S) LEADING TO BIOLOGIC EFFECTS

IL-1 receptor family IL-1R, type I NF-κB activation via TRAF6TNF receptor family TNFR1 NF-κB activation involving TRAF2 and TRAF5

Apoptosis induction via “death domain” proteins

Hematopoietin receptor family (class I receptors)

IL-2R Activation of Jak/STAT pathway

IFN/IL-10 receptor family (class II receptors)

IFN-γR Activation of Jak/STAT pathway

Immunoglobulin superfamily M-CSF R Activation of intrinsic tyrosine kinaseTGF-β receptor family TGF-βR, types

I and IIActivation of intrinsic serine/threonine kinase coupled to Smad proteins

Chemokine receptor family CCR5 Seven transmembrane receptors coupled to G proteins

CCR = CC chemokine receptor; IFN = interferon; IL = interleukin; Jak = Janus kinase; M-CSF = macro-phage colony-stimulating factor; NF-κB = nuclear factor κB; STAT = signal transducer and activator of trans-cription; TGF = transforming growth factor; TNF= tumor necrosis factor; TRAF = tumor necrosis factor receptor–associated factor.

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κB system do so through the activationof an IκB kinase (IKK) complex consist-ing of two kinase subunits (IKKα andIKKβ) and a regulatory subunit (IKKγ).The IKK complex phosphorylates IκBαand IκBβ on specific serine residues,yielding a target for recognition by anE3 ubiquitin ligase complex. The result-ing polyubiquitination marks this IκBfor rapid degradation by the 26S protea-some complex in the cytoplasm. OnceIκB has been degraded, the free NF-κB(which contains a nuclear localizationsignal) is able to pass into the nucleusand induce expression of NF-κB–sensi-tive genes. The presence of κB recogni-tion sites in cytokine promoters is very

common. Among the genes regulatedby NF-κB are IL-1β and TNF-α. This en-dows IL-1β and TNF-α with the capac-ity to establish a positive regulatoryloop that favors persistent inflamma-tion. Cytokines besides IL-1 and TNFthat activate the NF-κB pathway as partof their signal transduction mechanismsinclude IL-17 and IL-18.

Pro-inflammatory cytokines are notthe only stimuli that can activate the NF-κB pathway. Bacterial products (e.g., lipo-polysaccharide, or LPS), oxidants, activa-tors of protein kinase C (e.g., phorbol es-ters), viruses, and ultraviolet (UV)radiation are other stimuli that can stim-ulate NF-κB activity. TLR4 is a cell sur-

face receptor for the complex of LPS,LPS-binding protein, and CD14. The cy-toplasmic domain of TLR4 is similar tothat of the interleukin 1 receptor type 1(IL-1R1) and other IL-1R family membersand is known as the TIR domain (forToll/IL-1 receptor).12 When ligand isbound to a TIR domain–containing re-ceptor, one or more adapter proteins thatalso contain TIR domains are recruited tothe complex. MyD88 was the first ofthese adapters to be identified; the otherknown adapters are TIRAP (TIR do-main–containing adapter protein), TRIF(TIR domain–containing adapter induc-ing IFN-β), and TRAM (TRIF-relatedadapter molecule).13 Engagement of theadapter, in turn, activates one or more ofthe IL-1R–associated kinases (IRAK1 toIRAK4), which then signal throughTRAF6, a member of the TRAF (TNF re-ceptor–associated factor) family, andTAK1 (TGF-β–activated kinase) to acti-vate the IKK complex.14

Jak/STAT Pathway

A major breakthrough in the analysis ofcytokine-mediated signal transductionwas the identification of a common cellsurface to nucleus pathway used by themajority of cytokines. This Jak/STATpathway was first elucidated throughcareful analysis of signaling initiated byIFN receptors (Fig. 11-3) but was subse-quently shown to play a role in signal-ing by all cytokines that bind to mem-bers of the hematopoietin receptorfamily.15 The Jak/STAT pathway oper-ates through the sequential action of afamily of four nonreceptor tyrosine ki-nases (the Jaks or Janus family kinases)and a series of latent cytosolic transcrip-tion factors known as STATs (signaltransducers and activators of transcrip-tion). The cytoplasmic portions of manycytokine receptor chains are nonco-valently associated with one of the fourJaks [Jak1, Jak2, Jak3, and tyrosine ki-nase 2 (Tyk2)].

The activity of the Jak kinases is up-regulated after stimulation of the cyto-kine receptor. Ligand binding to the cy-tokine receptors leads to the associationof two or more distinct cytokine recep-tor subunits and brings the associatedJak kinases into close proximity witheach other. This promotes cross-phos-phorylation or autophosphorylation re-actions that in turn fully activate the ki-nases. Tyrosines in the cytoplasmic tailof the cytokine receptor as well as ty-rosines on other associated and newlyrecruited proteins are also phosphory-

� FIGURE 11-2 Activation of nuclear factor κB (NF-κB)–regulated genes after signaling by receptorsfor primary cytokines or by Toll-like receptors (TLRs) engaged by microbial products. Under resting con-ditions, NF-κB (a heterodimer of p50 and p65 subunits) is tightly bound to an inhibitor called IκB that se-questers NF-κB in the cytoplasm. Engagement of one of the TLRs or the signal transducing receptors forinterleukin 1 (IL-1) or tumor necrosis factor (TNF) family members leads to induction of IκB kinase activ-ity that phosphorylates IκB on critical serine residues. Phosphorylated IκB becomes a substrate for ubiq-uitination, which triggers degradation of IκB by the 26S proteasome. Loss of IκB results in release of NF-κB, which permits it to move to the nucleus and activate transcription of genes whose promoters containκB recognition sites. Ub = ubiquitin.

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lated. A subset of the newly phosphory-lated tyrosines can then serve as dock-ing points for attachment of additionalsignaling proteins bearing Src homology2 (SH2) domains. Cytoplasmic STATspossess SH2 domains and are recruitedto the phosphorylated cytokine recep-tors via this interaction. Homodimericor heterodimeric STAT proteins arephosphorylated by the Jak kinases andsubsequently translocate to the nu-cleus. In the nucleus they bind recogni-tion sequences in DNA and stimulatetranscription of specific genes, often incooperation with other transcriptionfactors. The same STAT molecules canbe involved in signaling by multiple dif-ferent cytokines. The specificity of theresponse in these instances may dependon the formation of complexes involv-ing STATs and other transcription fac-tors that then selectively act on a spe-cific set of genes.

INTERLEUKIN 1 FAMILY OF CYTOKINES (INTERLEUKINS 1α, 1β, 18, 33)

IL-1 is the prototype of a cytokine thathas been discovered many times inmany different biologic assays. Distinctgenes encode the α and β forms of hu-man IL-1, with only 26 percent homol-ogy at the amino acid level. Both IL-1sare translated as 31-kd molecules thatlack a signal peptide, and both reside inthe cytoplasm. This form of IL-1α is bi-ologically active, but 31-kd IL-1β mustbe cleaved by caspase 1 (initially termedinterleukin-1β–converting enzyme) to gen-erate an active molecule.

In general, IL-1β appears to be thedominant form of IL-1 produced bymonocytes, macrophages, Langerhanscells, and dendritic cells, whereas IL-1αpredominates in epithelial cells, includingkeratinocytes. This is likely to relate to

the fact that epithelial IL-1α is stored inthe cytoplasm of cells that comprise aninterface with the external environment.Such cells, when injured, release biologi-cally active 31-kd IL-1α and, by doing so,can initiate inflammation.5 If uninjured,however, these cells will differentiateand ultimately release their IL-1 contentsinto the environment. Leukocytes, in-cluding dendritic and Langerhans cells,carry their cargo of IL-1 inside the body,where its unregulated release could causesignificant tissue damage. Thus, biologi-cally active IL-1β release from cells iscontrolled at several levels: IL-1β genetranscription, caspase 1 gene transcrip-tion, and availability of the adapter pro-teins ASC and Ipaf that interact withcaspase 1 in the inflammasome to allowthe generation of mature IL-1β.16 Therole of IL-1β in the migration of Langer-hans cells from the epidermis during theinitiation of contact hypersensitivity is apivotal event in the egress of Langerhanscells from the epidermis and the genera-tion of successful sensitization. Studiesof mice deficient in IL-1α and IL-1β genessuggest that both molecules are impor-tant in contact hypersensitivity but thatIL-1α is more critical.

Active forms of IL-1 bind to the IL-1R1or type 1 IL-1 receptor.12 This is the solesignal-transducing receptor for IL-1, andits cytoplasmic domain has little homol-ogy with other cytokine receptors,showing greatest homology with the Tollgene product identified in Drosophila. Asecond cell surface protein, the IL-1R ac-cessory protein, or IL-1RAcP, must asso-ciate with IL-1R1 for signaling to occur.When IL-1 engages the IL-1R1/IL-1RAcPcomplex, recruitment of the MyD88adapter occurs, followed by interactionswith one or more of the IRAKs. These ki-nases in turn associate with TRAF6.Stepwise activation and recruitment ofadditional signaling molecules culminatein the induction of IKK activity. The netresult is the activation of a series of NF-κB–regulated genes.

A molecule known as the IL-1 receptorantagonist, or IL-1ra, can bind to IL-1R1but does not induce signaling throughthe receptor. This IL-1ra exists in threealternatively spliced forms, and an iso-form produced in monocytes is the onlyligand for the IL-1R1 that both containsa signal peptide and is secreted fromcells. Two other isoforms of IL-1ra, bothlacking signal peptides, are containedwithin epithelial cells. The function ofIL-1ra seems to be as a pure antagonistof IL-1 ligand binding to IL-1R1, andbinding of IL-1ra to IL-1R1 does not in-

� FIGURE 11-3 Participation of Jak (Janus kinase) and STAT (signal transducer and activator of trans-cription) proteins in interferon-γ (IFN-γ) signaling. Binding of human IFN-γ (a dimer) to its receptor bringsabout oligomerization of receptor complexes composed of α and β chains. The nonreceptor protein ty-rosine kinases Jak1 and Jak2 are activated and phosphorylate critical tyrosine residues in the receptorsuch as the tyrosine at position 440 of the α chain (Y440). STAT1α molecules are recruited to the IFN-γreceptor based on the affinity of their Src homology 2 (SH2) domains for the phosphopeptide sequencearound Y440. Receptor-associated STAT1α molecules then dimerize through reciprocal SH2-phosphoty-rosine interactions. The resulting STAT1α dimers translocate to the nucleus and stimulate transcriptionof IFN-γ–regulated genes.

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duce the mobilization of IL-1RAcP.Consequently, although both IL-1α/βand IL-1ra bind with equivalent affini-ties to IL-1R1, the association of IL-1R1with IL-1RAcP increases the affinity forIL-1α/β manyfold while not affectingthe affinity for IL-1ra. This is consistentwith the observation that a vast molarexcess of IL-1ra is required to fully an-tagonize the effects of IL-1. The biologicrole of IL-1ra is likely to be in thequenching of IL-1–mediated inflamma-tory responses, and mice deficient in IL-1ra show exaggerated and persistent in-flammatory responses.

A second means of antagonizing IL-1activity occurs via expression of a sec-ond receptor for IL-1, IL-1R2. This re-ceptor has a short cytoplasmic domainand serves to bind IL-1α/β efficiently,but not IL-1ra. This 68-kd receptor canbe cleaved from the cell surface by anunknown protease and released as a sta-ble, soluble 45-kd molecule that retainsavid IL-1–binding function. By bindingthe functional ligands for IL-1R1, IL-1R2serves to inhibit IL-1–mediated re-sponses. It is likely that IL-1R2 inhibitsIL-1 activity in another way, that is, byassociating with IL-1RAcP at the cellsurface and removing and sequesteringit from the pool available to associatewith IL-1R1. Thus, soluble IL-1R2 bindsto free IL-1, whereas cell surface IL-1R2sequesters IL-1RAcP. Expression of IL-1R2 can be upregulated by a number ofstimuli, including corticosteroids and IL-4. However, IL-1R2 can also be inducedby inflammatory cytokines, includingIFN-γ and IL-1, probably as a compensa-tory signal designed to limit the scaleand duration of the inflammatory re-sponse. Production of IL-1R2 serves tomake the producing cell and surround-ing cells resistant to IL-1–mediated acti-vation. Interestingly, some of the mostefficient IL-1–producing cells are alsothe best producers of the IL-1R2.

IL-18 was first identified based on itscapacity to induce IFN-γ. One name ini-tially proposed for this cytokine was IL-1γ, because of its homology with IL-1αand IL-1β. Like IL-1β, it is translated asan inactive precursor molecule of 23 kdand is cleaved to an active 18-kd speciesby caspase 1. It is produced by multiplecell types in skin, including keratino-cytes, Langerhans cells, and monocytes.IL-18 induces proliferation, cytotoxicity,and cytokine production by Th1 and nat-ural killer (NK) cells, mostly synergisti-cally with IL-12. The IL-18 receptor bearsstriking similarity to the IL-1 receptor.12

The binding chain (IL-18R) is an IL-1R1

homolog, originally cloned as IL-1Rrp1.IL-18R alone is a low-affinity receptorthat must recruit IL-18RAcP (a homologof IL-1RAcP). As for IL-1, both chains ofthe IL-18 receptor are required for signaltransduction. Although there is no IL-18homolog of IL-1ra, a molecule known asIL-18–binding protein binds to soluble ma-ture IL-18 and prevents it from bindingto the IL-18R complex.

More recently, it has become clearthat there is a family of receptors ho-mologous to the IL-1R1 and IL-18R mol-ecules,12 having in common a TIR motif(Fig. 11-4). All of these share analogoussignaling pathways initiated by theMyD88 adapter molecule. One of thesereceptors, originally known as ST2, wasinitially characterized as a gene ex-pressed by Th2 cells, but not by Th1cells. The description of a natural ligandfor ST2 designated IL-33 has added anew member to the IL-1 family thatshares characteristic features of othercytokines in the family, such as a re-quirement for processing by caspase 1

to release a mature form of the ligand.IL-33 stimulation of Th2 cells promotestheir production of the characteristicTh2 cytokines IL-4, IL-5, and IL-10. IL-1R1, IL-18R, IL-33R (ST2), the TLRs,and their ligands are all best viewed aselements of the innate immune systemthat signal the presence of danger or in-jury to the host.17

When IL-1 produced by epidermis wasoriginally identified, it was noted thatboth intact epidermis and stratum cor-neum contained significant IL-1 activity,which led to the concept that epidermiswas a shield of sequestered IL-1 sur-rounding the host, waiting to be releasedon injury. More recently, it was observedthat high levels of the IL-1ra co-existwithin keratinocytes; however, repeatedexperiments show that in virtually allcases, the amount of IL-1 present is suffi-cient to overcome any potential for inhi-bition mediated by IL-1ra. Studies havenow shown that mechanical stress to ke-ratinocytes permits the release of largeamounts of IL-1 in the absence of cell

� FIGURE 11-4 The interleukin 1 receptor (IL-1R) family and Toll-like receptors (TLRs) use a commonintracellular signaling pathway. Receptors for cytokines in the IL-1 family (typified by the IL-1 and IL-18receptors) share a common signaling domain with the TLRs (TLR1 to TLR11) called the Toll/IL-1 receptor(TIR) domain. The TIR domain receptors interact with TIR domain–containing adapter proteins such asMyD88 that couple ligand binding to activation of IL-1R–associated kinase (IRAK) and ultimately activa-tion of nuclear factor κB (NF-κB). IL-1RAcP = IL-1R accessory protein; TRAF = tumor necrosis factor re-ceptor–associated factor.

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death. Release of IL-1 induces expressionof endothelial adhesion molecules, in-cluding E-selectin, ICAM-1, and VCAM-1, as well as chemotactic and activatingchemokines. This attracts not onlymonocytes and granulocytes but a spe-cific sub-population of memory T cellsthat bear cutaneous lymphocyte antigenon their cell surface. Memory T cells pos-itive for cutaneous lymphocyte antigenare abundant in inflamed skin, compris-ing the majority of T cells present. There-fore, any injury to the skin, no matterhow trivial, releases IL-1 and attracts thispopulation of memory T cells. If they en-counter their antigen in this microenvi-ronment, their activation and subsequentcytokine production will amplify the in-flammatory response. This has been pro-posed as the basis of the clinical observa-tion of inflammation in response totrauma, known as the Koebner reaction.

TUMOR NECROSIS FACTOR: THE OTHER PRIMARY CYTOKINE

TNF-α is the prototype for a family of re-lated signaling molecules that mediatetheir biologic effects through a family ofrelated receptor molecules. TNF-α wasinitially cloned on the basis of its abilityto mediate two interesting biologic ef-fects: (1) hemorrhagic necrosis of malig-nant tumors, and (2) inflammation-asso-ciated cachexia. Although TNF-α exertsmany of its biologically important effectsas a soluble mediator, newly synthesizedTNF-α exists as a transmembrane pro-tein on the cell surface. A specific metal-loproteinase known as TNF-α–convertingenzyme (TACE) is responsible for mostTNF-α release by T cells and myeloidcells. The closest cousin of TNF-α isTNF-β, also known as lymphotoxin α (LT-α). Other related molecules in the TNFfamily include lymphotoxin β (LT-β),which combines with LT-α to form theLT-α1β2 heterotrimer; Fas ligand (FasL);TNF-related apoptosis-inducing ligand(TRAIL); TNF-related activation-inducedcytokine (TRANCE); and CD40 ligand(CD154). Although some of these otherTNF family members have not been tra-ditionally regarded as cytokines, theirstructure (all are type II membrane pro-teins with an intracellular N-terminusand an extracellular C-terminus) and sig-naling mechanisms are closely related tothose of TNF. The soluble forms of TNF-α, LT-α, and FasL are homotrimers, andthe predominant form of LT-β is themembrane-bound LT-α1β2 heterotrimer.Trimerization of TNF receptor familymembers by their trimeric ligands ap-

pears to be required for initiation of sig-naling and expression of biologic activity.

The initial characterization of TNF re-ceptors led to the discovery of two re-ceptor proteins capable of binding TNF-α with high affinity. The p55 receptorfor TNF (TNFR1) is responsible for mostbiologic activities of TNF, but the p75TNF receptor (TNFR2) is also capable oftransducing signals (unlike IL-1R2,which acts solely as a biologic sink forIL-1). TNFR1 and TNFR2 have substan-tial stretches of close homology and areboth present on most types of cells.Nevertheless, there are some notabledifferences between the two TNFRs.

Unlike cytokine receptors from sev-eral of the other large families, TNF sig-

naling does not involve the Jak/STATpathway. TNF-α evokes two types ofresponses in cells: (1) pro-inflammatoryeffects, and (2) induction of apoptoticcell death (Fig. 11-5). The pro-inflamma-tory effects of TNF-α, which includeupregulation of adhesion molecule ex-pression and induction of secondary cy-tokines and chemokines, stem in largepart from activation of NF-κB and canbe transduced through both TNFR1 andTNFR2. Induction of apoptosis by sig-naling through TNFR1 depends on a re-gion known as a death domain that is ab-sent in TNFR2, as well as interactionswith additional proteins with death do-mains within the TNFR1 signaling com-plex. Signaling initiated by ligand bind-

� FIGURE 11-5 Two contrasting outcomes of signaling through tumor necrosis factor receptor 1(TNFR1). Engagement of TNFR1 by trimeric tumor necrosis factor-α (TNF-α) can trigger apoptosis and/or nuclear factor κB (NF-κB) activation. Both processes involve the adapter protein TNFR-associateddeath domain (TRADD), which associates with TNFR1 via interactions between “death domains” (D.D.) onboth proteins. For NF-κB activation, TNFR–associated factor 2 (TRAF2) and receptor-interacting protein(RIP) are required. Induction of apoptosis occurs when the death domain–containing protein Fas-associ-ated death domain protein (FADD) associates with TRADD. FADD also contains a “death effector domain”(D.E.D.) that interacts with caspase 8 to initiate the apoptotic process. Cys = cysteine. (Adapted fromYuan J: Transducing signals of life and death. Curr Opin Cell Biol 9:247, 1997; and Nagata S: Apoptosisby death factor. Cell 88:355, 1997.)

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ing to TNFR1, Fas, or other deathdomain–containing receptors in theTNF family eventually leads to activa-tion of caspase 8 or 10 and the nuclearchanges and DNA fragmentation char-acteristic of apoptosis.

At least two TNFR family members(TNFR1 and the LT-β receptor) also con-tribute to the normal anatomic develop-ment of the lymphoid system. Mice de-ficient in TNF-α lack germinal centersand follicular dendritic cells. TNFR1 mu-tant mice show the same abnormalitiesplus an absence of Peyer’s patches. Micewith null mutations in LT-α or LT-βhave further abnormalities in lymphoidorganogenesis and fail to develop pe-ripheral lymph nodes.

TNF-α is an important mediator ofcutaneous inflammation, and its expres-sion is induced in the course of almostall inflammatory responses in skin. Nor-mal human keratinocytes and keratino-cyte cell lines produce substantialamounts of TNF-α after stimulationwith LPS or UV light. Cutaneous inflam-mation stimulated by irritants and con-tact sensitizers is associated with stronginduction of TNF-α production by kerat-inocytes. Exposure to TNF-α causesLangerhans cells to migrate to draininglymph nodes, which allows for sensiti-zation of naive T cells. One molecularmechanism that may contribute to TNF-α–induced migration of Langerhans cellstoward lymph nodes is reduced expres-sion of the E-cadherin adhesion mole-cule after exposure to TNF-α. Inductionof CC chemokine receptor 7 on bothepidermal and dermal antigen-present-ing cells correlates with movement intothe draining lymphatics. The predomi-nant TNFR expressed by keratinocytesis TNFR1. Autocrine signaling loops in-volving keratinocyte-derived TNF-αand TNFR1 lead to keratinocyte produc-tion of a variety of TNF-inducible sec-ondary cytokines.

The central role of TNF-α in inflam-matory diseases, including rheumatoidarthritis and psoriasis, has become evi-dent from clinical studies. Clinical drugsthat target the TNF pathway include thehumanized anti–TNF-α antibody inflix-imab, the human anti–TNF-α antibodyadalumimab, and the soluble TNF re-ceptor etanercept. Drugs in this class areU.S. Food and Drug Administration(FDA) approved for the treatment ofseveral autoimmune and inflammatorydiseases, including Crohn disease andrheumatoid arthritis. All of these anti-TNF drugs are also FDA approved forthe treatment of psoriatic arthritis, and

etanercept is approved for use in treat-ing chronic plaque psoriasis (see Chap.235). This class of drugs also has the po-tential to be valuable in the treatment ofother inflammatory dermatoses. Para-doxically, they are not effective againstall autoimmune diseases—multiple scle-rosis appears to worsen slightly aftertreatment with these agents. The TNFantagonists are powerful immunomod-ulating drugs, and appropriate caution isrequired in their use. Cases of cutane-ous T-cell lymphoma initially thoughtto represent psoriasis have rapidly pro-gressed to fulminant disease after treat-ment with TNF antagonists. TNF antag-onists can also allow the escape of latentmycobacterial infections from immunecontrol, with a potentially lethal out-come for the patient.

LIGANDS OF THE CLASS I (HEMATOPOIETIN RECEPTOR) FAMILY OF CYTOKINE RECEPTORS

The hematopoietin receptor family (alsoknown as the class I cytokine receptor fam-ily) is the largest of the cytokine receptorfamilies and comprises a number ofstructurally related type I membrane-bound glycoproteins. The cytoplasmicdomains of these receptors associatewith nonreceptor tyrosine kinase mole-cules, including the Jak kinases and srcfamily kinases. After ligand binding andreceptor oligomerization, these associ-ated nonreceptor tyrosine kinases phos-phorylate intracellular substrates, whichleads to signal transduction. Most of themultiple-chain receptors in the hemato-poietin receptor family consist of a cy-tokine-specific α chain subunit pairedwith one or more shared receptor sub-units. Five shared receptor subunitshave been described to date: the com-mon γ chain (γc), the common β chainshared between the IL-2 and IL-15 recep-tors; a distinct common β chain sharedbetween the granulocyte-macrophagecolony stimulating factor (GM-CSF), IL-3,and IL-5 receptors; the IL-12Rβ2 chainshared by the IL-12 and IL-23 receptors;and finally the glycoprotein 130 (gp130)molecule, which participates in signal-ing by IL-6 and related cytokines.

Cytokines with Receptors That Include the γc Chain

The receptor complexes using the γc chainare the IL-2, IL-4, IL-7, IL-9, IL-13, IL-15,and IL-21 receptors. Two of these recep-tors, IL-2R and IL-15R, also use the IL-

2Rβc chain. The γc chain is physically asso-ciated with Jak3, and activation of Jak3 iscritical to most signaling initiated throughthis subset of cytokine receptors.18

INTERLEUKIN 2 AND INTERLEUKIN 15 IL-2and IL-15 can each activate NK cells andstimulate proliferation of activated Tcells. IL-2 is a product of activated Tcells, and IL-2R is largely restricted tolymphoid cells. The IL-15 gene is ex-pressed by nonlymphoid tissues, andits transcription is induced by UVB ra-diation in keratinocytes and fibroblastsand by LPS in monocytes and dendriticcells. Multiple isoforms of IL-15Rα arefound in various hematopoietic andnon-hematopoietic cells. The IL-2R andIL-15R complexes of lymphocytes in-corporate up to three receptor chains,whereas most other cytokine receptorcomplexes have two. The affinities ofIL-2R and IL-15R for their respective li-gands can be regulated, and to some ex-tent, IL-2 and IL-15 compete with eachother. The highest-affinity receptorcomplexes for each ligand (approxi-mately 10–11 M) consist of the IL-2Rβcand γc chains, as well as their respectiveα chains (IL-2Rα, also known as CD25,and IL-15Rα). γc and IL-2Rβc withoutthe α chains form a functional lower-af-finity receptor for either ligand (10–8 to10–10 M). Although both ligands trans-mit signals through the γc chain, thosesignals elicit overlapping but distinct re-sponses in various cells. Activation ofnaive CD4 T cells by T-cell receptor andco-stimulatory molecules induces ex-pression of IL-2, IL-2Rα, and IL-2Rβc,which leads to vigorous proliferation.Prolonged stimulation of T-cell receptorand IL-2R leads to expression of FasLand activation-induced cell death. Al-though IL-2 signaling facilitates thedeath of CD4 T cells in response to sus-tained exposure to antigen, IL-15 inhib-its IL-2–mediated activation-inducedcell death as it stimulates growth. Simi-larly, IL-15 promotes proliferation ofmemory CD8 T cells, whereas IL-2 in-hibits it. IL-15 is also involved in the ho-meostatic survival of memory CD8 Tcells, NK cells, and NK T cells. Thesecontrasting biologic roles are illustratedby mice deficient in IL-2 or IL-2Rα,which develop autoimmune disorders,and mice deficient in IL-15 or IL-15Rα,which have lymphopenia and immunedeficiencies. Thus IL-15 appears to havean important role in promoting effectorfunctions of antigen-specific T cells,whereas IL-2 is involved in reining inautoreactive T cells.19

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INTERLEUKIN 4 AND INTERLEUKIN 13 IL-4and IL-13 are products of activated Th2cells that share limited structural homol-ogy (approximately 30 percent) and over-lapping but distinct biologic activities. Aspecific receptor for IL-4, which does notbind IL-13, is found on T cells and NKcells. It consists of IL-4Rα (CD124) and γcand transmits signals via Jak1 and Jak3. Asecond receptor complex that can bindeither IL-4 or IL-13 is found on keratino-cytes, endothelial cells, and other non-hematopoietic cells. It consists of IL-13Rα1 (CD213a1) and IL-4Rα and trans-mits signals via Jak1 and Jak2. These re-ceptors are expressed at low levels inresting cells, and their expression is in-creased by various activating signals. Cu-riously, exposure of monocytes to IL-4 orIL-13 suppresses expression of IL-4Rαand IL-13Rα1, whereas the opposite ef-fect is observed in keratinocytes. Bothsignal transduction pathways appear toconverge with the activation of STAT6,which is both necessary and sufficient todrive Th2 differentiation. Another cellsurface molecule homologous to IL-13Rα1, termed IL-13Rα2 (CD213a2),binds specifically to IL-13 but is notknown to transmit any signals.20

The biologic effects of engagement ofthe IL-4 receptor vary depending on thespecific cell type, but most pertain to itsprincipal role as a growth and differenti-ation factor for Th2 cells. Exposure ofnaive T cells to IL-4 stimulates them toproliferate and differentiate into Th2cells, which produce more IL-4, whichin turn leads to autocrine stimulationthat prolongs Th2 responses. Thus theexpression of IL-4 early in the immuneresponse can initiate a cascade of Th2cell development that results in a pre-dominately Th2 response. The genesencoding IL-4 and IL-13 are located in acluster with IL-5 which undergoes struc-tural changes during Th2 differentiationthat are associated with increased ex-pression. Although naive T cells canmake low levels of IL-4 when activated,IL-4 is also produced by activated NK Tcells. Mast cells and basophils also re-lease preformed IL-4 from secretorygranules in response to FcεRI-mediatedsignals. A prominent activity of IL-4 isthe stimulation of class switching of theimmunoglobulin genes of B cells. Ascritical factors in Th2 differentiation andeffector function, IL-4 and IL-13 are me-diators of atopic immunity. In additionto controlling the behavior of effectorcells they also act directly on residenttissue cells, such as in inflammatory air-way reactions.21

INTERLEUKIN 9 AND INTERLEUKIN 21 IL-9 isa product of activated Th2 cells that actsas an autocrine growth factor as well as amediator of inflammation.22 It is also pro-duced by mast cells in response to IL-10 orstem cell factor. It stimulates proliferationof T and B cells and promotes expressionof immunoglobulin E by B cells. It also ex-erts pro-inflammatory effects on mastcells and eosinophils. IL-9–deficient miceexhibit deficits in mast cell and goblet celldifferentiation. IL-21 is also a product ofTh2 T cells that signals through a receptorcomposed of a specific α chain (IL-21R)homologous to the IL-4R α chain and γc.

23

Absence of an intact IL-21 receptor is as-sociated with impaired Th2 responses.24

IL-9 and IL-21 can be grouped togetherwith IL-4 and IL-13 as cytokines that func-tion as effectors of allergic inflammatoryprocesses and may play an important rolein asthma and allergic disorders.

INTERLEUKIN 7 Mutations abrogating thefunction of IL-7, IL-7Rα (CD127), γc, orJak3 in mice or humans cause profoundimmunodeficiency as a result of T- andNK-cell depletion.18 This is principally dueto the indispensable role of IL-7 in pro-moting the expansion of lymphocytes andregulating the rearrangement of their anti-gen receptor genes. IL-7 is a potent mito-gen and survival factor for immature lym-phocytes in the bone marrow andthymus. The second function of IL-7 is asa modifier of effector cell functions in thereactive phase of certain immune re-sponses. IL-7 transmits activating signalsto mature T cells and certain activated Bcells. Like IL-2, IL-7 has been shown tostimulate proliferation of cytolytic T cellsand lymphokine-activated killer cells invitro and to enhance their activities invivo. Monocytes exposed to IL-7 releaseIL-6, IL-1α, IL-1β, and TNF-α and exhibitenhanced tumoricidal activity in vitro. IL-7 is a particularly significant cytokine forlymphocytes in the skin and other epithe-lial tissues. It is expressed by keratinocytesin a regulated fashion, and this expressionis thought to be part of a reciprocal signal-ing dialog between dendritic epidermal Tcells and keratinocytes in murine skin. Ke-ratinocytes release IL-7 in response to IFN-γ, and dendritic epidermal T cells secreteIFN-γ in response to IL-7.

An IL-7–related cytokine using onechain of the IL-7 receptor as part of its re-ceptor is thymic stromal lymphopoietin(TSLP). TSLP was originally identified as anovel cytokine produced by a thymic stro-mal cell line that could act as a growth fac-tor for B- and T-lineage cells. The TSLP re-ceptor consists of the IL-7 receptor α chain

and a second receptor chain homologousto but distinct from the γc chain. TSLP hasattracted interest because of its ability toprime dendritic cells to become strongerstimulators of Th2 cells. This activity maypermit TSLP to foster the development ofsome types of allergic diseases.25

Cytokines with Receptors Using the Interleukin 3 Receptor β Chain

The receptors for IL-3, IL-5, and GM-CSFconsist of unique cytokine-specific αchains paired with a common β chainknown as IL-3Rβ or βc (CD131). Each ofthese factors acts on subsets of early he-matopoietic cells.26 IL-3, which was previ-ously known as multi-lineage colony-stimu-lating factor, is principally a product ofCD4+ T cells and causes proliferation, dif-ferentiation, and colony formation of vari-ous myeloid cells from bone marrow. IL-5is a product of Th2 CD4+ cells and acti-vated mast cells that conveys signals to Bcells and eosinophils. IL-5 has a co-stimu-latory effect on B cells in that it enhancestheir proliferation and immunoglobulinexpression when they encounter theircognate antigen. In conjunction with aneosinophil-attracting chemokine knownas eotaxin, IL-5 plays a central role in theaccumulation of eosinophils that accom-panies parasitic infections and some cuta-neous inflammatory processes. IL-5 ap-pears to be required to generate a pool ofeosinophil precursors in bone marrowthat can be rapidly mobilized to theblood, whereas eotaxin’s role is focusedon recruitment of these eosinophils fromblood into specific tissue sites. GM-CSF isa growth factor for myeloid progenitorsproduced by activated T cells, phago-cytes, keratinocytes, fibroblasts, and vas-cular endothelial cells. In addition to itsrole in early hematopoiesis, GM-CSF haspotent effects on macrophages and den-dritic cells. In vitro culture of fresh Langer-hans cells in the presence of GM-CSF pro-motes their transformation into maturedendritic cells with maximal immuno-stimulatory potential for naive T cells.The effects of GM-CSF on dendritic cellsprobably account for the dramatic abilityof GM-CSF to evoke therapeutic antitu-mor immunity when tumor cells are engi-neered to express it.27

Interleukin 6 and Other Cytokines with Receptors Using Glycoprotein 130

Receptors for a group of cytokines in-cluding IL-6, IL-11, IL-27, leukemia in-hibitory factor, oncostatin M, ciliary

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neurotrophic factor, and cardiotrophin-1interact with a hematopoietin receptorfamily member, gp130, that does notappear to interact with any ligand by it-self. The gp130 molecule is recruitedinto signaling complexes with other re-ceptor chains when they engage theircognate ligands.

IL-6 is the most thoroughly character-ized of the cytokines that use gp130 forsignaling and serves as a paradigm fordiscussion of the biologic effects of thisfamily of cytokines. IL-6 is yet anotherexample of a highly pleiotropic cytokinewith multiple effects. A series of differ-ent names (including IFN-β2, B-cell stimu-latory factor 2, plasmacytoma growth factor,cytotoxic T cell differentiation factor, andhepatocyte-stimulating factor) were used forIL-6 before it was recognized that a sin-gle molecular species accounts for all ofthese activities. IL-6 acts on a wide vari-ety of cells of hematopoietic origin. IL-6stimulates immunoglobulin secretion byB cells and has mitogenic effects on B lin-eage cells and plasmacytomas. IL-6 alsopromotes maturation of megakaryo-cytes and differentiation of myeloid cells.Not only does it participate in hemato-poietic development and reactive im-mune responses, but IL-6 is also a centralmediator of the systemic acute-phase re-sponse. Increases in circulating IL-6 levelsstimulate hepatocytes to synthesize andrelease acute-phase proteins.

There are two distinct signal trans-duction pathways triggered by IL-6. Thefirst of these is mediated by the gp130molecule when it dimerizes on engage-ment by the complex of IL-6 and IL-6Rα. Homodimerization of gp130 andits associated Jak kinases (Jak1, Jak2,Tyk2) leads to activation of STAT3. Asecond pathway of gp130 signal trans-duction involves Ras and the mitogen-activated protein kinase cascade and re-sults in phosphorylation and activationof a transcription factor originally desig-nated nuclear factor of IL-6.

IL-6 is an important cytokine for skinand is subject to dysregulation in severalhuman diseases, including some withskin manifestations. IL-6 is produced ina regulated fashion by keratinocytes, fi-broblasts, and vascular endothelial cellsas well as by leukocytes infiltrating theskin. IL-6 can stimulate the proliferationof human keratinocytes under someconditions. Psoriasis is one of several in-flammatory skin diseases in which ele-vated expression of IL-6 has been de-scribed. Human herpesvirus 8 producesa viral homolog of IL-6 that may be in-volved in the pathogenesis of human

herpesvirus 8–associated diseases, in-cluding Kaposi sarcoma and body cav-ity–based lymphomas.

The other cytokines using gp130 as asignal transducer have diverse bioactivi-ties. IL-11 inhibits production of inflam-matory cytokines and has shown sometherapeutic activity in patients withpsoriasis. Exogenous IL-11 also stimu-lates platelet production and has beenused to treat thrombocytopenia occur-ring after chemotherapy. IL-27 is dis-cussed in the next section with the IL-12family of cytokines.

Interleukin 12, Interleukin 23, and Interleukin 27: Pivotal Cytokines for T Helper 1 and T Helper 17 Responses

IL-12 is different from most other cyto-kines in that its active form is a het-erodimer of two proteins, p35 and p40.IL-12 is principally a product of antigen-presenting cells such as dendritic cells,monocytes, macrophages, and certain Bcells in response to bacterial components,GM-CSF, and IFN-γ. Activated keratino-cytes are an additional source of IL-12 inskin. Human keratinocytes constitutivelymake the p35 subunit, whereas expres-sion of the p40 subunit can be inducedby stimuli including contact allergens,phorbol esters, and UV radiation.

IL-12 is a critical immunoregulatorycytokine that is central to the initiationand maintenance of Th1 responses. Th1responses that are dependent on IL-12provide protective immunity to intracel-lular bacterial pathogens. IL-12 also hasstimulatory effects on NK cells, promot-ing their proliferation, cytotoxic func-tion, and the production of cytokines,including IFN-γ. IL-12 has been shownto be active in stimulating protective an-titumor immunity in a number of ani-mal models.27

Two chains that are part of the cellsurface receptor for IL-12 have beencloned. Both are homologous to other βchains in the hematopoietin receptorfamily and are designated β1 and β2.The β1 chain is associated with Tyk2and the β2 chain interacts directly withJak2. The signaling component of theIL-12R is the β2 chain. The β2 chain isexpressed in Th1 but not Th2 cells andappears to be critical for commitment ofT cells to production of type 1 cyto-kines. IL-12 signaling induces the phos-phorylation of STAT1, STAT3, andSTAT4, but it is STAT4 that is essentialfor induction of a Th1 response.

IL-23 is a heterodimeric cytokine inthe IL-12 family that consists of the p40chain of IL-12 in association with a dis-tinct p19 chain. IL-23 has overlappingactivities with IL-12 but also inducesproliferation of memory T cells. Interestin IL-23 has been sparked by the obser-vation that IL-23 is involved in the in-duction of T cells producing IL-17 (Th17subset).28 The IL-23 receptor consists oftwo chains: the IL-12Rβ1 chain thatforms part of the IL-12 receptor and aspecific IL-23 receptor encoded by agene located near the IL-12Rβ2 gene.29

The newest member of the IL-12family is IL-27. IL-27 is also a het-erodimer and consists of a subunitcalled EBI3 that is homologous to IL-12p40 and a second subunit known as p28that is homologous to IL-12 p35. IL-27plays a role in the early induction of theTh1 response.30 The IL-27 receptor con-sists of a receptor called WSX-1 that as-sociates with the shared signal-trans-ducing molecule gp130.31

The IL-12 family of cytokines hasemerged as a promising new target foranti-cytokine pharmacotherapy. The ap-proach that has been developed the fur-thest to date is targeting both IL-12 andIL-23 with monoclonal antibodies di-rected against the common p40 subunit.An anti–human p40 monoclonal anti-body (CNTO 1275) was reported toshow beneficial effects in phase I trials inpsoriasis patients.32 A similar therapeuticantibody (ABT-784) has demonstratedefficacy in Crohn disease.33 The develop-ment of anti-p40 therapies is severalyears behind anti–TNF-α drugs, but p40is an attractive target for future drug de-velopment efforts for some types of im-mune-mediated diseases.

LIGANDS OF THE CLASS II FAMILY OF CYTOKINE RECEPTORS

A second major class of cytokine recep-tors with common features includestwo types of receptors for IFNs, IL-10R,and the receptors for additional IL-10–related cytokines including IL-19, IL-20,IL-22, and IL-24.

Interferons: Prototypes of Cytokines Signaling Through a Jak/STAT Pathway

IFNs were one of the first families of cy-tokines to be characterized in detail.The IFNs were initially subdivided intothree classes: IFN-α (the leukocyteIFNs), IFN-β (fibroblast IFN), and IFN-γ

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(immune IFN). The α and β IFNs are col-lectively called type I IFNs, and all ofthese molecules signal through the sametwo-chain receptor (the IFN-αβ recep-tor).34 The second IFN receptor is a dis-tinct two-chain receptor specific forIFN-γ. Both of these IFN receptors arepresent on many cell types within skinas well as in other tissues. Each of thechains comprising the two IFN recep-tors is associated with one of the Jak ki-nases (Tyk2 and Jak1 for the IFN-αβRand Jak1 and Jak2 for the IFN-γR). Onlyin the presence of both chains and twofunctional Jak kinases will effective sig-nal transduction occur after IFN binding.A new class of IFNs known as IFN-λ ortype III IFNs has now been identifiedthat has a low degree of homology withboth type I IFNs and IL-10.35 The cur-rent members of this class are IL-28A,IL-28B, and IL-29. Although the effectsof these cytokines are similar to those ofthe type I IFNs, they are less potent.These type III IFNs use a shared recep-tor that consists of the β chain of the IL-10 receptor associated with an IL-28 re-ceptor α chain.

Viruses, double-stranded RNA, andbacterial products are among the stimulithat elicit release of the type I IFNs fromcells. Plasmacytoid dendritic cells haveemerged as a particularly potent cellularsource of type I IFNs. Many of the ef-fects of the type I IFNs directly or indi-rectly increase host resistance to thespread of viral infection. Additional ef-fects mediated through IFN-αβR areincreased expression of major histo-compatibility complex (MHC) class Imolecules and stimulation of NK cell ac-tivity. Not only does it have well-known antiviral effects, but IFN-α alsocan modulate T-cell responses by favor-ing the development of a Th1 type of T-cell response. Finally, the type I IFNsalso inhibit the proliferation of a varietyof cell types, which provides a rationalefor their use in the treatment of sometypes of cancer. Forms of IFN-α enjoyconsiderable use clinically for indica-tions ranging from hairy cell leukemia,various cutaneous malignancies, andpapillomavirus infections (see Chap.196). Some of the same conditions thatrespond to therapy with type I IFNs alsorespond to topical immunomodulatoryagents like imiquimod. This syntheticimidazoquinoline drug is an agonist forthe TLR7 receptor, whose natural ligandis single-stranded RNA. Imiquimodstimulation of cells expressing TLR7elicits local release of large amounts oftype I IFNs from plasmacytoid dendritic

cells, which can trigger clinically usefulantiviral and tumor inhibitory effectsagainst genital warts, superficial basalcell carcinoma, and actinic keratoses.Resiquimod is a related synthetic com-pound that activates both TLR7 andTLR8, eliciting a slightly different spec-trum of cytokines.36

Production of IFN-γ is restricted to NKcells, CD8 T cells, and Th1 CD4 T cells.Th1 cells produce IFN-γ after engage-ment of the T-cell receptor, and IL-12can provide a strong co-stimulatory sig-nal for T-cell IFN-γ production. NK cellsproduce IFN-γ in response to cytokinesreleased by macrophages, includingTNF-α, IL-12, and IL-18. IFN-γ has anti-viral activity, but it is a less potent medi-ator than the type I IFNs for induction ofthese effects. The major physiologic roleof IFN-γ is its capacity to modulate im-mune responses. IFN-γ induces synthesisof multiple proteins that play essentialroles in antigen presentation to T cells,including MHC class I and class II glyco-proteins, invariant chain, the Lmp2 andLmp7 components of the proteasome,and the TAP1 and TAP2 intracellularpeptide transporters. These changes in-crease the efficiency of antigen presenta-tion to CD4 and CD8 T cells. IFN-γ isalso required for activation of macro-phages to their full antimicrobial poten-tial, enabling them to eliminate microor-ganisms capable of intracellular growth.Like type I IFNs, IFN-γ also has strongantiproliferative effects on some celltypes. Finally, IFN-γ is also an inducer ofselected chemokines (CXC chemokineligands 9 to 11) and an inducer of endo-thelial cell adhesion molecules (e.g.,ICAM-1 and VCAM-1). Because of thebreadth of IFN-γ’s activities, it comes theclosest of the T-cell cytokines to behav-ing as a primary cytokine.

Interleukin 10: An “Anti-Inflammatory” Cytokine

IL-10 is one of several cytokines thatprimarily exert regulatory rather thanstimulatory effects on immune re-sponses.37 IL-10 was first identified as acytokine produced by Th2 T cells thatinhibited cytokine production after acti-vation of T cells by antigen and antigen-presenting cells. IL-10 exerts its actionthrough a cell surface receptor found onmacrophages, dendritic cells, neutro-phils, B cells, T cells, and NK cells. Theligand-binding chain of the receptor ishomologous to the receptors for IFN-α/β and IFN-γ, and signaling events medi-

ated through the IL-10 receptor use aJak/STAT pathway. IL-10 binding to itsreceptor activates the Jak1 and Tyk2 ki-nases and leads to the activation ofSTAT1 and STAT3. The effects of IL-10on antigen-presenting cells such asmonocytes, macrophages, and den-dritic cells include inhibition of expres-sion of class II MHC and co-stimulatorymolecules (e.g., B7-1, B7-2) and de-creased production of T cell–stimulatingcytokines (e.g., IL-1, IL-6, and IL-12). Atleast four viral genomes harbor viral ho-mologues of IL-10 that transmit similarsignals by binding to the IL-10R.

A major source of IL-10 within skin isepidermal keratinocytes. KeratinocyteIL-10 production is upregulated after ac-tivation; one of the best-characterizedactivating stimuli for keratinocytes isUV irradiation. UV radiation–inducedkeratinocyte IL-10 production leads tolocal and systemic effects on immunity.Some of the well-documented immuno-suppressive effects that occur after UVlight exposure are the result of the liber-ation of keratinocyte-derived IL-10 intothe systemic circulation. IL-10 also playsa dampening role in other types of cuta-neous immune and inflammatory re-sponses, because the absence of IL-10predisposes mice to exaggerated irritantand contact sensitivity responses.

Novel Interleukin 10–Related Cytokines: Interleukins 19, 20, 22, and 24

A series of cytokines related to IL-10have been identified and shown to en-gage a number of receptor complexeswith shared chains.38 IL-19, IL-20, andIL-24 transmit signals via a complexconsisting of IL-20Rα and IL-20Rβ.Transgenic mice overexpressing IL-20develop severe cutaneous inflammationand altered epidermal proliferation anddifferentiation. Expression of the IL-20Rchains is strongly induced when theyare triggered, and they are only detectedon keratinocytes, endothelial cells, andcertain monocytes in association withinflammatory conditions such as psoria-sis.39 IL-22 activates a receptor consist-ing of IL-22R and IL-10Rβ, whereas IL-20 and IL-24 also engage a complex in-corporating IL-20Rβ and IL-22R.40,41

The profound effects of IL-20 expres-sion in transgenic mice and the associa-tion of IL-20R expression with psoriasispoint toward a significant role for thesecytokines in the epidermal changes as-sociated with cutaneous inflammation.

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TRANSFORMING GROWTH FACTOR-β FAMILY AND ITS RECEPTORS

TGF-β1 was first isolated as a secretedproduct of virally transformed tumor cellscapable of inducing normal cells in vitro toshow phenotypic characteristics associ-ated with transformation. Over 30 addi-tional members of the TGF-β family havenow been identified. They can be groupedinto several families: the prototypic TGF-βs (TGF-β1 to TGF-β3), the bone morpho-genetic proteins, the growth/differentia-tion factors, and the activins. The TGFname for this family of molecules is some-what of a misnomer, because TGF-β hasantiproliferative rather than proliferativeeffects on most cell types. Many of theTGF-β family members play an importantrole in development, influencing the dif-ferentiation of uncommitted cells into spe-cific lineages. TGF-β family members aremade as precursor proteins that are bio-logically inactive until a large pro-domainis cleaved. Monomers of the mature do-main of TGF-β family members are disul-fide linked to form dimers that strongly re-sist denaturation.

Participation of at least two cell surfacereceptors (type I and type II) with serine/threonine kinase activity is required forbiologic effects of TGF-β.42 Ligand bind-ing by the type II receptor (the true li-gand-binding receptor) is associated withthe formation of complexes of type I andtype II receptors. This allows the type IIreceptor to phosphorylate and activatethe type I receptor, a “transducer” mole-cule that is responsible for downstreamsignal transduction. Downstream signaltransmission from the membrane-boundreceptors in the TGF-β receptor family tothe nucleus is primarily mediated by afamily of cytoplasmic Smad proteins thattranslocate to the nucleus and regulatetranscription of target genes.

TGF-β has a profound influence on sev-eral types of immune and inflammatoryprocesses. An immunoregulatory role forTGF-β1 was identified in part throughanalysis of TGF-β1 knockout mice that de-velop a wasting disease at 20 days of ageassociated with a mixed inflammatory cellinfiltrate involving many internal organs.This phenotype is now appreciated to be aresult in part of the compromised develop-ment of regulatory T cells when TGF-β1 isnot available. Development of cells in thedendritic cell lineage is also perturbed inthe TGF-β1–deficient mice, as evidencedby an absence of epidermal Langerhanscells and specific sub-populations of lymphnode dendritic cells. A combination of ef-fects of TGF-β on fibroblast function make

it one of the most fibrogenic of all cyto-kines studied.43 TGF-β–treated fibroblastsdisplay enhanced production of collagenand other extracellular matrix molecules.In addition, TGF-β inhibits the productionof metalloproteinases by fibroblasts andstimulates the production of inhibitors ofthe same metalloproteinases (tissue inhibi-tors of metalloproteinase, or TIMPs). TGF-β effects on fibroblasts may be importantin promoting wound healing.

CHEMOKINES: SECONDARY CYTOKINES CENTRAL TO LEUKOCYTE MOBILIZATION

Chemokines are a large superfamily ofsmall cytokines that have two majorfunctions. First, they guide leukocytes viachemotactic gradients in tissue. Typi-cally, this is to bring an effector cell towhere its activities are required. Second,a subset of chemokines has the capacityto increase the binding of leukocytes viatheir integrins to ligands at the endothe-lial cell surface, which facilitates firm ad-hesion and extravasation of leukocytes intissue. The activities of this importantclass of cytokines are sufficiently com-plex that they are the subject of a sepa-rate chapter (Chap. 12).

CYTOKINE NETWORK—THERAPEUTIC IMPLICATIONS AND APPLICATIONS

This chapter has attempted to bring somedegree of order and logic to the analysisof a field of human biology that continuesto grow at a rapid rate. Although manythings may change in the world of cyto-kines, certain key concepts have stoodthe test of time. Principal among them isthe idea that cytokines are emergencymolecules, designed to be released locallyand transiently in tissue microenviron-ments. When cytokines are released per-sistently, the result is typically chronicdisease. One potential way to treat suchdiseases is with cytokine antagonists orother drugs that target cytokines or cyto-kine-mediated pathways.

Cytokines and cytokine antagonists arebeing used therapeutically by clinicians,and development of additional agents con-tinues. With certain notable exceptions,systemic cytokine therapy has been disap-pointing and is often accompanied by sub-stantial morbidity. In contrast, local andtransient administration of cytokines mayyield more promising results. An exampleof this approach is the transduction oftumor cells to express factors such asGM-CSF (GVAX vaccines) or IL-12 familymembers to enhance antitumor immune

responses.44 Conversely, agents that spe-cifically block cytokine activity are also be-ing developed. Antibodies and TNF recep-tor–Fc fusion proteins are FDA-approvedantagonists of TNF-α activity that arehighly effective at inducing durable remis-sions in psoriasis (see Chaps. 18, 235, and236). Antibodies against the p40 subunitshared by IL-12 and IL-23 are also active intreating psoriasis. Anakinra is a formula-tion of recombinant IL-1Ra approved bythe FDA as adjunct therapy or second-linemonotherapy for the treatment of adultrheumatoid arthritis and has been shownto be very effective in patients with neona-tal-onset multisystemic inflammatory dis-ease (see Chap. 134). Other cytokines thathave predominantly anti-inflammatory ef-fects, such as IL-10 and IL-11, show someinhibitory activity in psoriasis, but are notcurrently being developed further for thisindication. A class of pharmacologicagents that inhibits the production of mul-tiple T cell–derived cytokines is the cal-cineurin inhibitors. Tacrolimus and pime-crolimus both bind to the immunophilinFK-506 binding protein-12 (FKBP-12), pro-ducing complexes that bind to calcineurin,a calcium-dependent phosphatase thatacts on proteins in the nuclear factor ofactivated T cells’ family to promote theirnuclear translocation and activation of cy-tokine genes (including IL-2, IL-4, and IFN-γ)45 (see Chap. 221). Finally, fusion toxinslinked to cytokines, such as the IL-2 fusionprotein denileukin diftitox, exploit the cel-lular specificity of certain cytokine-recep-tor interactions to kill target cells (seeChap. 235). Denileukin diftitox is FDA ap-proved for the treatment of cutaneous T-cell lymphoma and has also shown thera-peutic activity in psoriasis.46 Each of theaforementioned approaches is still rela-tively new and open to considerable futuredevelopment. An understanding of cyto-kines by clinicians of the future is likely tobe central to effective patient care.

KEY REFERENCES

The full reference list for all chapters is available at www.digm7.com.

1. Oppenheim JJ: Cytokines: Past, present,and future. Int J Hematol 74:3, 2001

3. Luger TA et al: Epidermal cell (keratino-cyte)-derived thymocyte-activating fac-tor (ETAF). J Immunol 127:1493, 1981

4. Kupper TS: The activated keratinocyte: Amodel for inducible cytokine productionby non–bone marrow–derived cells incutaneous inflammatory and immuneresponses. J Invest Dermatol 94:146S, 1990

5. Kupper TS: Immune and inflammatoryprocesses in cutaneous tissues. Mecha-nisms and speculations. J Clin Invest86:1783, 1990

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C H A P T E R 1 2

Chemokines

Sam T. Hwang

The skin is an organ in which the migra-tion, influx, and egress of leukocytes oc-curs in both homeostatic and inflamma-tory processes. Chemokines and theirreceptors are accepted as vital mediatorsof cellular trafficking. Since the discoveryof the first chemoattractant cytokine, orchemokine, in 1977, 50 additional newchemokines and 17 chemokine recep-tors have been discovered. Most che-mokines are small proteins with molec-ular weights in the 8 to 10 kd range andare synthesized constitutively in somecells and can be induced in many celltypes by cytokines. Initially associatedonly with recruitment of leukocyte sub-sets to inflammatory sites,

1

it has be-come clear that chemokines play rolesin angiogenesis, neural development,cancer metastasis, hematopoiesis, andinfectious diseases. This chapter focusesprimarily on the function of chemokinesin inflammatory conditions, but alsotouches on the role of these moleculesin other settings as well.

STRUCTURE OF CHEMOKINES

Chemokines are grouped into four sub-families based on the spacing of amino ac-ids between the first two cysteines. TheCXC chemokines (also called

α

-chemo-kines

) show a C-X-C motif with one non-conserved amino acid between the twocysteines. The other major subfamily ofchemokines (called

β

-chemokines) lacksthe additional amino acid and is termedthe

CC subfamily

. The two remaining sub-families contain only one member each:the C subfamily is represented by lym-photactin, and fractalkine is the onlymember of the CXXXC (or CX3C) sub-family. Chemokines can also be assignedto one of two broad and, perhaps, over-lapping functional groups. One group[e.g., regulated on activation normal T-cell expressed and secreted (RANTES),macrophage inflammatory protein 1

α

/

β

,liver and activation-regulated chemokine(LARC)] mediates the attraction and re-cruitment of immune cells to sites of ac-tive inflammation, whereas others [e.g.,secondary lymphoid-organ chemokine(SLC) and stromal cell–derived factor-1(SDF-1)] appear to play a role in constitu-tive or homeostatic migration pathways.

2

The complexity and redundancy inthe nomenclature of chemokines haveled to the proposal for a systematic no-menclature for chemokines based onthe type of chemokine (C, CXC, CX3C,or CC) and a number based on the orderof discovery as proposed by Zlotnik andYoshie.

2

For example, SDF-1, a CXCchemokine, has the systematic nameCXCL12. Because both nomenclaturesare still in wide use, the original names(abbreviated in most cases) as well assystematic names are used interchange-ably throughout the chapter. Table 12-1provides a list of chemokine receptorsof interest in skin that are discussed inthis chapter as well as the major chemo-kine ligands that bind to them.

Chemokines are highly conserved andhave similar secondary and tertiary struc-ture. Based on crystallography studies, adisordered amino terminus followed bythree conserved antiparallel

β

-pleatedsheets is a common structural feature ofchemokines. Fractalkine is unique inthat the chemokine domain sits atop amucin-like stalk tethered to the plasmamembrane via a transmembrane do-main and short cytoplasmic tail.

3

Al-though CXC and CC chemokines formmultimeric structures under conditionsrequired for structural studies, these as-sociations may be relevant only whenchemokines associate with cell-surfacecomponents such as glycosaminogly-cans (GAGs) or proteoglycans. Becausemost chemokines have a net positivecharge, these proteins tend to bind tonegatively charged carbohydrates presenton GAGs. Indeed, the ability of posi-tively charged chemokines to bind toGAGs is thought to enable chemokinesto preferentially associate with the lu-menal surface of blood vessels despitethe presence of shear forces from theblood that would otherwise wash thechemokines away.

CHEMOKINE RECEPTORS AND SIGNAL TRANSDUCTION

Chemokine receptors are seven trans-membrane spanning membrane pro-teins that couple to intracellular hetero-trimeric G proteins containing

α

,

β

, and

γ

subunits.

2

They represent a part of alarge family of G protein coupled recep-tors (GPCRs), including rhodopsin, thathave critical biologic functions. Leuko-cytes express several G

α

protein sub-types: s, i, and q, whereas the

β

and

γ

subunits each have 5 and 11 knownsubtypes, respectively. This complexityin the formation of the heterotrimeric G

protein may account for specificity inthe action of certain chemokine recep-tors. Normally, G proteins are inactivewhen guanosine diphosphate (GDP) isbound, but they are activated when theGDP is exchanged for guanosine tri-phosphate (GTP) (Fig. 12-1). After bind-ing to ligand, chemokine receptors rap-idly associate with G proteins, which inturn increases the exchange of GTP forGDP. Pertussis toxin is a commonlyused inhibitor of GPCR that irreversiblyadenosine diphosphate-ribosylates G

α

subunits of the

α

i

class and subse-quently prevents most chemokine re-ceptor–mediated signaling.

Activation of G proteins leads to thedissociation of the G

α

and G

βγ

subunits(see Fig. 12-1). The G

α

subunit has beenobserved to activate protein tyrosine ki-nases and mitogen activated protein ki-nase, leading to cytoskeletal changes andgene transcription. The G

α

subunit re-tains GTP, which is slowly hydrolyzedby the guanosine triphosphatase (GTPase)activity of this subunit. This GTPase ac-tivity is both positively and negativelyregulated by GTPase-activating proteins

CHEMOKINES

AT A GLANCE

Chemokines and their receptors are vital mediators of cellular trafficking.

Most chemokines are small proteins with molecular weights in the 8- to 10-kd range.

Chemokines are synthesized constitu-tively in some cells and can be induced in many cell types by cytokines.

Chemokines play roles in inflammation, angiogenesis, neural development, can-cer metastasis, hematopoiesis, and infec-tious disease.

In skin, chemokines play important roles in atopic dermatitis, psoriasis, melanoma, melanoma metastasis, and some viral (including retroviral) infections.

Promising therapeutic applications of chemokines include the prevention of T-cell arrest on activated endothelium or blocking infection of T cells by human immunodeficiency virus 1 using CC che-mokine receptor 5 analogues.

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(also known as

regulator of G protein sig-naling proteins

). The G

βγ

dimer initiatescritical signaling events in regard to che-motaxis and cell adhesion. It activatesphospholipase C,

4

leading to formationof diacylglycerol and inositol triphos-phate [Ins(1,4,5)P

3

]. Ins(1,4,5)P

3

stimu-lates Ca

2+

entry into the cytosol, whichalong with diacylglycerol, activates pro-tein kinase C isoforms. Although the

G

βγ

subunits have been shown to becritical for chemotaxis, the G

α

i

subunithas no known role in chemotactic migra-tion. There is also evidence that bindingof chemokine receptors results in the ac-tivation of other intracellular effectors in-cluding Ras and Rho, phosphatidylinosi-tol-3-kinase.

5

RhoA and protein kinase C appear toplay a role in integrin affinity changes,

whereas phosphatidylinositol-3-kinasemay be critical for changes in the avid-ity state of lymphocyte function–associ-ated antigen 1. Other proteins havebeen found that regulate the synthesis,expression, or degradation of GPCRs.For example, receptor-activity-modify-ing proteins act as chaperones of seventransmembrane spanning receptors andregulate surface expression as well as

TABLE 12-1

Chemokine Receptors in Skin Biology

C

HEMOKINE

R

ECEPTOR

(CCR) C

HEMOKINE

L

IGAND

(CCL) E

XPRESSION

P

ATTERN

C

OMMENTS

R

EFERENCES

CCR1 MIP-1

α

(CCL3), RANTES (CCL5), MCP-3 (CCL7)

T, Mo, DCs, NK, B Migration of DCs and Mo; strongly upregulated in T by IL-2

82

CCR2 MCP-1 (CCL2), -3, -4 (CCL13) T, Mo Migration of T to inflamed sites; replenish Langer-hans cell precursors in epidermis; involved in skin fibrosis via MCP-1

32, 63, 82

CCR3 Eotaxin (CCL11) > RANTES, MCP-2 (CCL8), 3, 4

Eosinophils, basophils, Th2, NK

Migration of Th2, T, and “allergic” immune cells 22, 91

CCR4 TARC (CCL17), macrophage-derived chemokine (CCL22)

T (benign and malig-nant)

Expression in Th2 > Th1 cells; highly expressed on CLA+ memory T; TARC expression by keratinocytes may be important in atopic dermatitis; may guide trafficking of malignant as well as benign inflamma-tory T

10, 21, 44, 78, 92

CCR5 RANTES, MIP-1

α,β

(CCL3, 4) T, Mo, DCs Marker for Th1 cells; migration to acutely inflamed sites; may be involved in transmigration of T through endothelium; major HIV-1 fusion co-receptor

14, 82

CCR6 Liver and activation-regulated chemokine (CCL20)

T, DCs, B Expressed by memory, not naive, T; possibly involved in arrest of memory T to activated endothe-lium and recruitment of T to epidermis in psoriasis

54, 55, 93

CCR7 Secondary lymphoid-organ che-mokine (CCL21), Epstein-Barr virus–induced molecule-1 ligand chemokine (CCL19)

T, DCs, B, melanoma cells

Critical for migration of naive T and “central mem-ory” T to secondary lymphoid organs; required for mature DCs to enter lymphatics and localize to lymph nodes; facilitates nodal metastasis

15, 34, 38, 74, 94

CCR9 Thymus-expressed chemokine (CCL25)

T, melanoma cells Associated with melanoma small bowel metastases 75

CCR10 CTACK (CCL27) T (benign and malig-nant), melanoma cells

Preferential response of CLA

+

T to CTACK in vitro; may be involved in T (benign as well as malignant) homing to epidermis, where CTACK is expressed; survival of melanoma in skin

9, 27, 76, 80

CXCR1, 2 IL-8 (CXCL8), MGSA/GRO

α

(CXCL1), ENA-78 (CXCL5)

Neutrophils, NK, En, melanoma cells

Recruitment of neutrophils (e.g., epidermis in psori-asis); may be involved in angiogenesis; melanoma growth factor

61, 96, 97

CXCR3 Interferon-inducible protein 10 (CXCL10), monokine induced by interferon-

γ

(CXCL9), interferon-inducible T cell

α

chemoattrac-tant (CXCL11)

T Marker for Th1 cells and may be involved in T recruitment to epidermis in cutaneous T-cell lym-phoma; induces arrest of activated T on stimulated endothelium

18, 26

CXCR4 Stromal cell–derived factor-1

α

,

β

(CXCL12)T, DCs, En, melanoma cells

Major HIV-1 fusion co-receptor; involved in vascular formation; involved in melanoma metastasis

73, 82

CX3CR1 Fractalkine (CX3CL1) T, Mo, mast cells, NK May be involved in adhesion on activated T, Mo, and NK cells to activated endothelium

3, 97

B = B cells; CLA = cutaneous lymphocyte–associated antigen; CTACK = cutaneous T cell–attracting chemokine; DCs = dendritic cells; En = endothelial cells; HIV-1 = human immunodeficiency virus 1; IL = interleukin; MCP = macrophage chemoattractant protein; MIP = macrophage inflammatory protein; Mo = monocytes; NK = natural killer cells; RANTES = regulated on activation normal T-cell expressed and secreted; T = T cells; TARC = thymus and activation-regulated chemokine; Th = T helper.

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the ligand specificity of chemokine re-ceptors (see Fig. 12-1). Importantly, afterchemokine receptors are exposed to ap-propriate ligands, they are frequently in-ternalized, leading to an inability of thechemokine receptor to mediate furthersignaling. This downregulation of che-mokine function, which has beentermed

desensitization

, occurs because ofphosphorylation of Ser/Thr residues inthe C-terminal tail by proteins termed

GPCR kinases

and subsequent internal-ization of the receptor (see Fig. 12-1).Desensitization may be an importantmechanism for regulating the functionof chemokine receptors by inhibitingcell migration as leukocytes arrive at theprimary site of inflammation.

CHEMOKINES AND CUTANEOUS LEUKOCYTE TRAFFICKING

Generally speaking, chemokines arethought to play at least three differentroles in the recruitment of host defensecells, predominantly leukocytes, to sitesof inflammation.

6

First, they provide thesignal or signals required to cause leuko-cytes to come to a complete stop (i.e., ar-rest) in blood vessels at inflamed sitessuch as skin. Second, chemokines havebeen shown to have a role in the trans-migration of leukocytes from the lume-nal side of the blood vessel to the ablu-

menal side. Third, chemokines attractleukocytes to sites of inflammation inthe dermis or epidermis after transmi-gration. Keratinocytes and endothelialcells are a rich source of chemokineswhen stimulated by appropriate cyto-kines. In addition, chemokines and theirreceptors are known to play critical rolesin the emigration of resident skin den-dritic cells (DCs) [i.e., Langerhans cells(LCs) and dermal DCs] from the skin todraining lymph nodes (LNs) via afferentlymphatic vessels, a process that is es-sential for the development of acquiredimmune responses (see Chap. 10).

This section is divided into three sub-sections. The first introduces basic con-cepts of how all leukocytes arrest ininflamed blood vessels before transmi-gration by introducing the multistepmodel of leukocyte recruitment. Thesecond details mechanisms of T-cell mi-gration, and the final subsection focuseson the mechanisms by which chemo-kines mediate the physiologic migrationof DCs from the skin to regional LNs.

Multistep Model of Leukocyte Recruitment

For leukocytes to adhere and migrate toperipheral tissues, they must overcomethe pushing force of the vascular bloodstream as they bind to activated endo-

thelial cells at local sites of inflamma-tion. According to the multistep or cas-cade model of leukocyte recruitment(Fig. 12-2), one set of homologous adhe-sion molecules termed

selectins

mediatesthe transient attachment of leukocytesto endothelial cells while another set ofadhesion molecules termed

integrins

andtheir receptors (immunoglobulin super-family members) mediates strongerbinding (i.e., arrest) and transmigration.

7

The selectins (E-, L-, and P-selectin) aremembers of a larger family of carbohy-drate-binding proteins termed

lectins

.The selectins bind their respective car-bohydrate ligands located on proteinscaffolds and thus mediate the transientbinding or “rolling” of leukocytes on en-dothelial cells.

The skin-associated vascular selectinknown as

E-selectin

is upregulated on en-dothelial cells by inflammatory cyto-kines such as tumor necrosis factor(TNF)-

α

and binds to sialyl Lewis X–based carbohydrates. E-selectin ligandsform distinct epitopes known as the

cutaneous lymphocyte–associated antigen

(CLA). CLA is expressed by 10 percentto 40 percent of memory T cells and hasbeen suggested as a marker for skin-homing T cells.

8

At least two chemo-kine receptors [CC chemokine receptor10 (CCR10) and CCR4] show preferen-tial expression in CLA

+

memory T

FIGURE 12-1

Chemokine receptor–mediated signaling pathways. CK = chemokine; ER = endoplasmic reticulum; GDP = guanosine diphosphate; GRK = Gprotein coupled receptor kinase; GTP = guanosine triphosphate; MaPK = mitogen-activated protein kinase; PKC = protein kinase C; PLC = phospholipase C;PTK = protein tyrosine kinase(s); PTX = pertussis toxin; RAMP = receptor–activity-modifying protein; RGS = regulator of G protein signaling protein.

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cells.

9,10

Whereas E-selectin is likely tobe an important component of skin-selective homing, there is also evidenceto suggest that L-selectin is involved inT-cell migration to skin.

11,12

In the second phase of this model,

leukocyte integrins such as those of the

β

2

family must be “turned on” or acti-vated from their resting state to bind totheir counter-receptors such as intercel-lular adhesion molecule 1 that are ex-pressed by endothelial cells. A vast ar-ray of data suggest that the binding ofchemokines to leukocyte chemokine re-ceptors plays a critical role in activatingboth

β

1

and

β

2

integrins.

5,13

Activationof chemokine receptors leads to a com-plex signaling cascade (see Fig. 12-1)that causes a conformational change inindividual integrins that leads to in-creases in the affinity and avidity of in-dividual leukocyte integrins for their li-gands. Furthermore, later steps ofmigration (i.e., transmigration or diape-desis) have been shown to be depen-dent on chemokines as well in selectivecases.

14

In the case of neutrophils, theirability to roll on inflamed blood vesselslikely depends on their expression of L-selectin and E-selectin ligands whereas

their arrest on activated endothelialikely depends on their expression ofCXCR1 and CXCR2 as described belowfor wound healing (see Chap. 163). Inte-grin activation via chemokine-mediatedsignals appears to be more complex in Tcells, which appear to use multiple che-mokine receptors, and is described inmore detail in the following section.

Chemokine-Mediated Migration of T Cells

Antigen-inexperienced T cells are termed

naive

and can be identified by expres-sion three cell surface proteins: CD45RA(an isoform of the pan-leukocyte marker),L-selectin, and the chemokine receptorCCR7. These T cells migrate efficientlyto secondary LNs, where they maymake contact with antigen-bearing DCsfrom the periphery. Once activated byDCs presenting antigen, T cells then ex-press CD45RO, are termed

memory

Tcells, and appear to express a variety ofadhesion molecules and chemokine re-ceptors which facilitate their extrava-sation from blood vessels to inflamedperipheral tissue. A specific subset ofCCR7

, L-selectin

memory T cells, has

been proposed to represent an effectormemory T-cell subset that is ready forrapid deployment at peripheral sites interms of their cytotoxic activity andability to mobilize cytokines.

15

Although chemokines are both se-creted and soluble, the net positivecharge on most chemokines allowsthem to bind to negatively charged pro-teoglycans such as heparin sulfate thatare present on the lumenal surface ofendothelial cells, thus allowing them tobe presented to T cells as they roll alongthe lumenal surface (see Fig. 12-2). Afterligand binding, chemokine receptorssend intracellular signals that lead to in-creases in the affinity and avidity of T-cell integrins such as lymphocyte func-tion–associated antigen 1 and very lateantigen 4 for their endothelial receptorsintercellular adhesion molecule 1 andvascular cell adhesion molecule-1, re-spectively.

16

Only a few chemokine re-ceptors (CXCR4, CCR7, CCR4, andCCR6) are expressed at sufficient levelson resting peripheral blood T cells tomediate this transition. With activationand interleukin (IL)-2 stimulation, in-creased numbers of chemokine recep-tors (e.g., CXCR3) are expressed on acti-vated T cells, making them more likelyto respond to other chemokines. In sev-eral different systems, inhibition of spe-cific chemokines produced by endothe-lial cells or chemokine receptors foundon T cells dramatically influences T-cellarrest in vivo and in vitro.

17

CXCR3 serves as a receptor for che-mokine ligands Mig (monokine inducedby interferon-

γ

), interferon-inducible pro-tein 10 (IP-10), and interferon-inducibleT cell

α

chemoattractant. All three ofthese chemokines are distinguishedfrom other chemokines by being highlyupregulated by interferon-

γ

. Resting Tcells do not express functional levels ofCXCR3, but upregulate this receptorwith activation and cytokines such asIL-2. Once expressed on T cells, CXCR3is capable of mediating arrest of mem-ory T cells on activated endothelialcells.

18

The expression of its chemokineligands is strongly influenced by thecytokine interferon-

γ

, which synergisti-cally works with proinflammatory cyto-kines such as TNF-

α

to increase ex-pression of these ligands by activatedendothelial cells

18

and epithelial cells.In general, activation of T cells by cy-

tokines such as IL-2 is associated withthe enhanced expression of CCR1,CCR2, CCR5, and CXCR3. Just as Thelper 1 (Th1) and Th2 (T cell) subsetshave different functional roles, it might

FIGURE 12-2

Multistep model of leukocyte recruitment. Leukocytes, pushed by the blood stream,first transiently bind or “roll” on the surface of activated endothelial cells via rapid interactions with P-, E-, orL-selectin. Chemokines are secreted by endothelial cells and bind to proteoglycans that present the che-mokine molecules to chemokine receptors on the surface of the leukocyte. After chemokine receptor li-gation, intracellular signaling events lead to a change in the conformation of integrins and changes intheir distribution on the plasma membrane, resulting in integrin activation. These changes result in highaffinity/avidity binding of integrins to endothelial cell intercellular adhesion molecules (ICAMs) and vascu-lar cell adhesion molecule (VCAM)-1 in a step termed

firm adhesion

, which is then followed by transmi-gration of the leukocyte between endothelial cells and into tissue. CLA = cutaneous lymphocyte–associ-ated antigen; Ig = immunoglobulin; PSGL-1 = P-selectin glycoprotein ligand 1.

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have been predicted that these two sub-sets of T cells would express differentchemokine receptors. Indeed, CCR4

19–21

and CCR3

22

are associated with Th2cells in vitro, whereas Th1 cells are as-sociated with CCR5 and CXCR3.

23

In some instances, chemokine recep-tors may be regarded as functionalmarkers that identify Th1- versus Th2-type lymphocytes while also promotingtheir recruitment to inflammatory sitescharacterized by “allergic” or “cell-mediated” immunity, respectively. WhenT cells are activated in vitro in thepresence of Th1-promoting cytokines,CXCR3 and CCR5 appear to be highlyexpressed, whereas in the presence ofTh2-promoting cytokines, CCR4, CCR8,and CCR3 expression predominates. Inrheumatoid arthritis, a Th1-predomi-nant disease, many infiltrating T cellsexpress CCR5 and CXCR3

24

whereas,in atopic disease, CCR4 expressing Tcells may be more frequent.

21

There islikely to be overlap as demonstrated un-der some conditions in which both Th1and Th2 type T cells can expressCCR4.

20

The epidermis is a particularly richsource of chemokines, including RANTES,macrophage chemoattractant protein-1(MCP-1), IP-10, IL-8, LARC, and thy-mus and activation-regulated chemo-kine (TARC), which likely contributeto epidermal T-cell migration. Keratino-cytes from patients with distinctive skindiseases appear to express unique che-mokine expression profiles. For in-stance, keratinocytes derived from pa-tients with atopic dermatis (see Chap.14) synthesized messenger RNA forRANTES at considerably earlier timepoints in response to IL-4 and TNF-

α

incomparison to unaffected and psoriaticpatients.

25

Keratinocytes derived frompsoriatic patients (see Chap. 18) synthe-sized higher levels of IP-10 with cyto-kine stimulation as well as higher con-stitutive levels of IL-8,

25

a chemokineknown to recruit neutrophils. IL-8 maycontribute to the large numbers of neu-trophils that localize to the suprabasaland cornified layers of the epidermis inpsoriasis. IP-10 may serve to recruit acti-vated T cells of the Th1 helper pheno-type to the epidermis and has been pos-tulated to have a role in therecruitment of malignant T cells to theskin in cutaneous T-cell lymphomas.

26

Cutaneous T cell–attracting chemo-kine (CTACK)/CC chemokine ligand 27(CCL27) is selectively and constitutivelyexpressed in the epidermis, and its ex-pression is only marginally increased

under inflammatory conditions.

27

Inter-estingly, CTACK has been reported topreferentially attract CLA

+

memory Tcells in vitro

27

and has been demon-strated to play a role in the recruitmentand function of skin-homing T cells ininflammatory disease models.

28,29

Chemokines in the Trafficking of Dendritic Cells from Skin to Regional Lymph Nodes

Antigen-presenting cells, including DCsof the skin, are critical initiators of im-mune responses and their traffickingpatterns are thought to influence immu-nologic outcomes. Their mission in-cludes taking up antigen at sites of in-fection or injury and bringing theseantigens to regional LNs where theyboth present antigen and regulate theresponses of T and B cells. Skin-residentDCs are initially derived from hemato-poietic bone marrow progenitors

30

andmigrate to skin during the late prenataland newborn periods of life. Under rest-ing (steady state) conditions, homeo-static production by keratinocytes ofCXCL14 (receptor unknown) may beinvolved in attracting CD14

+

DC pre-cursors to the basal layer of the epider-mis.

31

Under inflammatory conditions,when skin-resident DC and LC leavethe skin in large numbers, keratinocytesrelease a variety of chemokines, includ-ing CCL2 and CCL7 (via CCR2)

32

andCCL20 (via CCR6),

33

which may attractmonocyte-like DC precursors to the epi-dermis to replenish the LC population.

When activated by inflammatory cy-tokines (e.g., TNF-

α

, and IL-1

β)

, lipo-polysaccharide, or injury, skin DCs, in-cluding LCs, leave the epidermis, enterafferent lymphatic vessels, and migrateto draining regional LNs where they en-counter both naive and memory T cells.Chemokines guide the DC on this jour-ney. Activated DC specifically upregu-late expression of CCR7, which binds tosecondary lymphoid tissue chemokine(SLC/CCL21), a chemokine expressedconstitutively by lymphatic endothelialcells (see eFig. 12-2.1 in on-line edi-tion).

34,35

SLC guides DCs into dermallymphatic vessels and helps retain themin SLC-rich regional draining LNs (Fig.12-3).36

Interestingly, naive T cells alsostrongly express CCR7 and use this re-ceptor to arrest on high endothelialvenules.37 The importance of the CCR7pathway is demonstrated by LCs fromCCR7 knockout mouse that demon-

strate poor migration from the skin toregional LNs38 and by the observationthat antibodies to SLC block migrationof DCs from the periphery to LNs.34

Thus, CCR7 and its ligands facilitate therecruitment of at least two differentkinds of cells—naive T cells and DCs—to the LNs through two different routesunder both inflammatory38 and restingconditions.36

After DCs reach the LN, they mustinteract with T cells to form a so-calledimmunologic synapse that is critical for T-cell activation. Activated DCs secrete anumber of chemokines, including mac-rophage-derived chemokine,39 whichattracts T cells to the vicinity of DCsand promotes adhesion between thetwo cell types.40,41 CCR5 (via CCL3/4)has also been identified as mediating re-cruitment of naive CD8+ T cells to ag-gregates of antigen-specific CD4+ T cellsand DCs.42 Therefore, chemokines or-chestrate a complex series of migrationpatterns, bringing both DCs and T cellsto the confines of the LN, where expres-sion of chemokines by DCs themselvesappears to be a direct signal for bindingof the T cell (see Fig. 12-3).

CHEMOKINES IN DISEASE

Atopic Dermatitis (See Chap. 14)

Atopic dermatitis is a prototypical Th2-mediated, allergic skin disease (see Chap.14) in which chemokines may playpathogenic roles.43 The mechanism oflymphocyte homing to skin in the set-ting of atopic dermatitis has been eluci-dated by clinical data from humans aswell as experimental data in the NC/Ngamouse model of atopic dermatitis,which suggest that the Th2-associatedchemokine receptor, CCR4, in conjunc-tion with its ligand, TARC/CCL17, mayplay a role in recruiting T cells to atopicskin. In human patients with atopic der-matitis, CLA+CCR4+ lymphocytes werefound to be increased in the peripheralblood of atopic dermatitis patients com-pared to controls.21 Moreover, serum lev-els of TARC in atopic dermatitis patientswere 10-fold higher than concentrationsfound in unaffected individuals and cor-related with disease severity, whereaspsoriatics showed only a minimal eleva-tion of TARC in the serum.44 Interest-ingly, another chemokine, CCL18, whosereceptor is currently unknown, is pro-duced by LC (as well as other antigen-presenting cells) and shows selectiveexpression in atopic skin relative to pso-riatic skin. Similar to TARC, CCL18 at-

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tracts CLA+ memory T cells.45 Perhapsof physiologic consequence, CCL18 ex-pression is elicited in volunteer skin aftertopical challenge with dust mite allergenand staphylococcal superantigen.46

The recruitment of eosinophils toskin is a frequently observed finding inallergic skin diseases, including atopicdermatitis and cutaneous drug reac-tions, and likely is mediated by chemo-kines. Eotaxin/CCL11 was initially iso-lated from the bronchoalveolar fluid ofguinea pigs after experimental allergicinflammation and binds primarily toCCR3, a receptor expressed by eosino-phils,47 basophils, and Th2 cells.22 Injec-tion of eotaxin into the skin promotesthe recruitment of eosinophils whereasanti-eotaxin antibodies delay the dermalrecruitment of eosinophils in the late-phase allergic reaction in mouse skin.48

Immunoreactivity and messenger RNAexpression of eotaxin and CCR3 areboth increased in lesional skin and se-rum of patients with atopic dermatitis,but not in nonatopic controls.49,50 Eo-taxin has also been shown to increaseproliferation of CCR3-expressing kerati-nocytes in vitro.51 Finally, expression ofeotaxin (and RANTES) by dermal endo-thelial cells has been correlated with theappearance of eosinophils in the dermis

in patients with onchocerciasis that ex-perience allergic reactions after treat-ment with ivermectin.52 These observa-tions suggest that production of eotaxinand CCR3 may contribute to the re-cruitment of eosinophils and Th2 lym-phocytes in addition to stimulating ke-ratinocyte proliferation.

Psoriasis (See Chap. 18)

Psoriasis, an inflammatory skin disor-der characterized by thickened, pruriticplaques, does not have a clear etiology,although it is considered a Th1-medi-ated, autoimmune disease. As shown inFig. 12-4 and reviewed by others,53

there are multiple potential traffickingpathways that may be mediated bychemokines in psoriasis. Chemokines,including LARC/CCL2054 and TARC/CCL17,10 that are expressed by vascularendothelial cells mediate the arrest ofeffector memory T cells on endothelialcells.55 In addition, both CCL17 andCCL20 can be synthesized by keratino-cytes, possibly contributing to T-cell mi-gration to the epidermis. Although theCCL17 receptor, CCR4, has been asso-ciated with Th2-type T cells,19 there isalso evidence suggesting that Th1-typeT cells can express this receptor.20

Neutrophils found in the epidermis ofpsoriatic skin are likely to be attractedthere by high levels of IL-8, whichwould act via CXCR1 and CXCR2. Inaddition to attracting neutrophils, IL-8 isan ELR+ CXC chemokine that is knownto be angiogenic, and it may also attractendothelial cells. This may lead to theformation of the long tortuous capillaryblood vessels in the papillary dermisthat are characteristic of psoriasis. More-over, keratinocytes also express CXCR2and thus may be auto-regulated by theexpression of CXCR2 ligands in theskin. Of note, an IL-8/CXCL8–produc-ing population of memory T cells thatexpress CCR6 has been isolated frompatients with acute generalized exan-thematous pustulosis, a condition in-duced most commonly by drugs (e.g.,aminopenicillins) and characterized bysmall intraepidermal or subcorneal ster-ile pustules (see Chap. 40).56 Similar Tcells have been isolated from patientswith Behçet disease and pustular psoria-sis.57 It is possible that this subpopula-tion of T cells contributes to neutrophilaccumulation in the stratum corneum(Munro abscesses) in psoriasis and otherinflammatory skin disorders character-ized by neutrophil-rich infiltrates in theabsence of frank infection.

� FIGURE 12-3 Trafficking of epi-dermal Langerhans cells (LCs) toregional lymph nodes. LCs are acti-vated by a variety of stimuli, includ-ing injury, infectious agents, and cy-tokines such as interleukin-1β andtumor necrosis factor-α. Havingsampled antigens, the activatedLCs downregulate E-cadherin andstrongly upregulate CC chemokinereceptor 7 (CCR7). Sensing theCCR7-ligand, secondary lymphoid-organ chemokine (SLC; ), pro-duced by lymphatic endothelialcells, the LCs migrate into lym-phatic vessels, passively flow to thelymph nodes, and stop in the T-cellzones (TCZs) that are rich in twoCCR7 ligands, SLC and Epstein-Barr virus–induced molecule-1 ligandchemokine (ELC). Note that chemo-kines also contribute to the recruit-ment of LCs under both resting andinflammatory conditions. BCZ = B-cell zone; CCL = CC chemokine li-gand; CXCL = CXC chemokine li-gand.

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Although the aforementioned chemo-kines have been shown to be expressedin psoriatic epidermis, they may also befound in a variety of skin diseases, in-cluding cutaneous T-cell lymphoma andatopic dermatitis. It is becoming appar-ent that multiple, rather than single,chemokines and their receptors likelycontribute to the fine-tuning of T-cellmigration in the skin.

Cancer

Chemokines may play a role in tumorformation and immunity in several dis-tinct ways, including the control of an-giogenesis and the induction of tumorimmune responses.58 CXC chemokinesthat express a three amino acid motifconsisting of glu-leu-arg (ELR) immedi-ately preceding the CXC signature areangiogenic, whereas most non-ELRCXC chemokines, except SDF-1, are an-giostatic. Interestingly, it is not clearthat ELR– chemokines actually bind tochemokine receptors to reduce angio-genesis. It has been proposed that theyact by displacing growth factors from

proteoglycans. In any event, the balancebetween ELR+ versus ELR– chemokinesis thought to contribute to the complexregulation of angiogenesis at tumorsites. IL-8, a prototypical ELR+ chemo-kine, can be secreted by melanoma cellsand has been detected in conjunctionwith metastatic dissemination of thiscancer.59 IL-8 may also act as an auto-crine growth factor for melanoma60 aswell as several other types of cancer. Al-though CXCR1 and CXCR2 bind IL-8 incommon, several other ELR+ CXC che-mokines, including growth regulatedoncogene α and epithelial-neutrophil–activating peptide-78, bind primarily toCXCR2. CXCR2 appears, in most in-stances, to be associated with both theangiogenic and growth regulatory prop-erties of tumors.61

Tumors, including melanoma, havelong been known to secrete chemokinesthat can attract a variety of leukocytes.The question arises as to why this is notdeleterious to the tumor itself. Breastcancers, for instance, are known to se-crete MCP-1, a chemokine that attractsmacrophages through CCR2. Higher tis-

sue levels of MCP-1 correlate with in-creasing numbers of macrophageswithin the tissue. Although chemokinessecreted by tumor cells do lead to re-cruitment of immune cells, this does notnecessarily lead to increased clearanceof the tumor.62

Inflammatory cells such as macro-phages may actually play a critical role incancer invasion and metastasis. First,MCP-1 may increase expression ofmacrophage IL-4 through an autocrinefeedback loop and possibly skew the im-mune response from Th1 to Th2. Inter-estingly, MCP-1–deficient mice showmarkedly reduced dermal fibrosis afterdermal challenge with bleomycin, a find-ing of possible relevance to the pathogen-esis of conditions such as scleroderma.63

Secondly, macrophages may promote tu-mor invasion and metastasis.64 The anti-tumor effects of specific chemokines mayoccur by a variety of mechanisms. ELR–

CXCR3 ligands such as IP-10 are potentlyanti-angiogenic and may act as down-stream effectors of IL-12–induced, naturalkiller cell–dependent angiostasis.65 Ofnote, some cancer cells can synthesize

� FIGURE 12-4 Possible sites of actions of chemokines in psoriasis. Chemokines attract both neutrophils (to form Munro abscesses) and lymphocytes via at-tachment to endothelial cells and then migrate to the epidermis (epidermotropism). Specific chemokines tend to attract either neutrophils or lymphocytes, but gen-erally not both. A newly identified subset of T cells can secrete interleukin-8 and attract neutrophils in conditions such as acute generalized exanthematous pustu-losis.58 Dendritic cells may also secrete chemokines, attract T cells, and stimulate conjugate formation that activates T cells. CCR = CC chemokine receptor;CTACK = cutaneous T cell–attracting chemokine; CXCR = CXC chemokine receptor; IL-8 = interleukin 8; I-TAC = interferon-inducible T cell α chemoattractant;LARC = liver and activation-regulated chemokine; MCP-1 = macrophage chemoattractant protein-1; MDC = macrophage-derived chemokine; Mig = monokineinduced by interferon-γ; RANTES = regulated on activation normal T-cell expressed and secreted; TARC = thymus and activation-regulated chemokine.

KeratinocytesMunro abscess

Activation

Endothelial cellsMigTARCMDCI-TACLARCCTACK

Initial adhesion of leukocytes to endothelium

GROαIL-8

CTACKRANTESLARCMCP-1

NeutrophilsMunro abscesses

T cellsepidermotropism

IL-8

Neutrophils (CXCR1/2) Lymphocytes (CCR2, CCR6, CXCR3, CCR10, CCR4)

Dendritic cells (with antigen)

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LARC, attracting immature DCs that ex-press CCR6.66 Experimentally, LARC hasbeen transduced into murine tumors,where it attracts DCs in mice and sup-presses tumor growth in experimentalsystems.67 Last, chemokines produced bytumor cells may attract CD4+CD25+ Tregulatory cells that suppress host anti-tu-mor cytolytic T cells.68

Tumor metastasis is the most com-mon source of mortality and morbidityin cancer. With skin cancers such asmelanoma, there is a propensity for spe-cific sites such as brain, lung, and liver,as well as distant skin sites. Cancersmay also metastasize via afferent lym-phatics and eventually reach regionaldraining LNs. The discovery of nodalmetastasis often portends a poor prog-nosis for the patient. In fact, the pres-ence of nodal metastases is one of themost powerful negative predictors ofsurvival in melanoma.69

Chemokines may play an importantrole in the site-specific metastases ofcancers of the breast and of melanoma(Fig. 12-5).70 Human breast cancer aswell as melanoma lines expressed thechemokine receptors CXCR4 and CCR7,whereas normal breast epithelial cellsand melanocytes do not appear to ex-press these receptors.71 CXCR4 is ex-pressed in over 23 different solid andhematopoietic cancers. Broad expres-sion of this receptor may be due to itsregulation by hypoxia, a condition com-mon to growing tumors, via the hy-poxia inducible factor-1α transcriptionfactor.72 In several different animals ofbreast cancer71 and melanoma metasta-sis,73 inhibition of CXCR4 with anti-bodies or peptides resulted in dramati-cally reduced metastases to distantorgans. Expression of CCR7 by cancercells, including gastric carcinoma andmelanoma, appears to be critical for in-vasion of afferent lymphatics and LNmetastasis. CCR7-transfected B16 mu-rine melanoma cells were found to me-tastasize with much higher efficiency toregional LNs compared to control B16cells after inoculation into the footpadof mice.74 CCR9 may also play a role inmelanoma metastasis to the smallbowel, which shows high expression ofthe CCR9 ligand, CCL25.75

CCR10 is highly expressed by mela-noma primary tumors76 and is corre-lated with nodal metastasis in mela-noma patients77 and in experimentalanimal models (see eFig. 12-5.1 in on-line edition).76 Engagement of CCR10by CTACK results in activation (viaphosphorylation) of the phosphatidyl-

inositol 3-kinase and Akt signaling path-ways, leading to anti-apoptotic effectsin melanoma cells.76 Because CTACK isconstitutively produced by keratino-cytes, it may act as a survival factor forboth primary as well as secondary(metastatic) melanoma tumors that ex-press CCR10. In fact, CCR10-activatedmelanoma cells become resistant tokilling by melanoma antigen-specificT cells.76 Interestingly, CCR478 andCCR1079,80 have been implicated in thetrafficking and/or survival of malignantT (lymphoma) cells to skin. Thus, a lim-ited number of specific chemokine re-ceptors appear to play distinct, non-redundant roles, in facilitating cancerprogression and metastasis (summa-rized in Fig. 12-5).

Infectious Diseases

Although chemokines and chemokinereceptors may have evolved as a host re-sponse to infectious agents, recent datasuggest infectious organisms may haveco-opted chemokine- or chemokine re-ceptor–like molecules to their own ad-vantage in selected instances. A varietyof microorganisms express chemokinereceptors, including US28 by cytomega-lovirus (see Chap. 193) and Kaposi sar-

coma herpesvirus (or human herpesvi-rus-8) GPCR. In the case of Kaposisarcoma herpesvirus GPCR, this recep-tor is able to promiscuously bind severalchemokines. More important, it is con-stitutively active and may work as agrowth promoter in Kaposi sarcoma(see Chap. 128).81

Human immunodeficiency virus 1(HIV-1), the causative agent of acquiredimmunodeficiency syndrome, is an en-veloped retrovirus that enters cells viareceptor-dependent membrane fusion(see Chap. 198). CD4 is the primary fu-sion receptor for all strains of HIV-1 andbinds to HIV-1 proteins, gp120 andgp41. However, different strains of HIV-1 have emerged that preferentially useCXCR4 (T-tropic) or CCR5 (M-tropic)or either chemokine receptor as a co-receptor for entry. Although other che-mokine co-receptors can potentially serveas co-receptors, most clinical HIV-1strains are primarily dual-tropic for ei-ther CCR5 or CXCR4.82

The discovery of a 32–base pair dele-tion (∆32) in CCR5 in some individualsthat leads to low levels of CCR5 expres-sion in T cells and DCs and correlateswith a dramatic resistance to HIV-1 in-fection demonstrated a clear role forCCR5 in the pathogenesis of HIV-1 in-

� FIGURE 12-5 Chemokine receptors in melanoma progression and metastasis. Chemokine receptorsplay distinct roles in melanoma metastasis.71 CC chemokine receptor 10 (CCR10) may enhance survivalof primary melanoma tumors and skin metastases. CCR7, CCR10, and, possibly, CXC chemokine recep-tor 4 (CXCR4) may contribute to lymph node metastasis. CXCR4 appears to be involved in primary tumordevelopment and metastasis at distant organ sites such as the lungs. CCR9 has been implicated in mel-anoma small bowel metastasis in patients.

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fection.83 Interestingly, the frequency of∆32 mutations in humans is surprisinglyhigh, and the complete absence ofCCR5 in homozygotes has only beenassociated with a more clinically severeform of sarcoidosis. Otherwise, theseindividuals are healthy. In fact, there isan association of less severe autoim-mune disease with this mutation.84

LCs reside in large numbers in the geni-tal mucosa and may be one of the first ini-tial targets of HIV-1 infection.85 Becauseinfected (activated) LCs likely enter der-mal lymphatic vessels and then localize toregional LNs, the physiologic migratorypathway of LCs may also coincidentallylead to the transmission of HIV-1 to Tcells within secondary lymphoid organs.CCR5 is expressed by immature or rest-ing LCs in the epidermis and is the targetof CCR5 analogues of RANTES that blockHIV infection,86 suggesting possible thera-peutic strategies in the treatment or pre-vention of HIV-1 disease.87 CXCR4 antag-onists may also be of clinical utility withT- or dual-tropic viruses.88

A newly described autosomal domi-nant genetic syndrome comprised ofwarts (human papilloma virus–associ-ated), hypogammaglobulinemia, infec-tions, and myelokathexis is the result ofan activating mutation (deletion) in the

cytoplasmic tail of the CXCR4 receptor orin yet unidentified downstream regulatorsof CXCR4 function.89,90 Bacterial infec-tions are common because myelokathexisis associated with neutropenia and abnor-mal neutrophil morphology. The nearlyuniversal presence of human papillomavirus infections associated with this syn-drome can involve multiple common, aswell as genital, wart subtypes (see eFig.12-5.2 in on-line edition) and suggest acritical role for normal CXCR4 function inimmunologic defense against this com-mon human pathogen.

Thus, the skin is rich in cells (kerati-nocytes, fibroblasts, endothelial cells,and immune cells) that are able to pro-duce chemokines. Chemokines not onlyorchestrate the migration of inflamma-tory cells, but also play roles in angio-genesis, cancer metastasis, and cellularproliferation. Other unanticipated bio-logic roles may ultimately be discov-ered. Just two of the promising thera-peutic applications of chemokines (ormolecules that mimic chemokines) maybe in (1) preventing undesirable migra-tion into the skin by preventing arrest ofT cells or other inflammatory cells onactivated endothelium and (2) blockingthe infection of DCs and T cells by HIV-1 virus using CCR5 analogues. Signaling

pathways are just beginning to be un-derstood, and further work needs to bedone to understand the regulation ofthese receptors, the specificity of intra-cellular activities, and the mechanismby which chemokine receptors work inthe face of multiple chemokines presentin many inflammatory sites.

KEY REFERENCES

The full reference list for all chapters is available at www.digm7.com.

1. Charo IF, Ransohoff RM: The manyroles of chemokines and chemokinereceptors in inflammation. N Engl J Med354:610, 2006

2. Zlotnik A, Yoshie O: Chemokines: Anew classification system and their rolein immunity. Immunity 12:121, 2000

6. Homey B: Chemokines and inflamma-tory skin diseases. Adv Dermatol 21:251,2005

36. Ohl L et al: CCR7 governs skin den-dritic cell migration under inflammatoryand steady-state conditions. Immunity21:279, 2004

77. Simonetti O et al: Potential role ofCCL27 and CCR10 expression in mela-noma progression and immune escape.Eur J Cancer 42:1181, 2006

90. Diaz GA, Gulino AV: WHIM syndrome:A defect in CXCR4 signaling. CurrAllergy Asthma Rep 5:350, 2005

C H A P T E R 1 3

Allergic Contact DermatitisDavid E. CohenSharon E. Jacob

As the primary interface with the envi-ronment, the skin is placed in the pre-carious position of routine exposure andassault from exogenous chemicals andphysical agents. Fortunately, most ofthese exposures result in no clinicallyapparent disease. However, in some cir-cumstances, a panoply of immunologicevents results in the sensitization andsubsequent elicitation of allergic contactdermatitis (ACD).

The classic interpretation of the skin asa simple barrier to penetration by exoge-nous agents underestimates the immu-nologic capacity of the integument.

Modern concepts have divulged thefinely orchestrated interplay of host de-fenses that cope with these onslaughts.In the 1950s, Landsteiner and Chase1

firmly established ACD as a form of cell-mediated hypersensitivity. It was not un-til the latter half of the twentieth century,however, that the fundamental role of in-tact lymphatic systems, cellular elements(Langerhans cells, keratinocytes, andlymphoid cells), and specific cytokines inthe sensitization and elicitation phases ofACD became recognized (see Chap. 10).1

Today we understand that these com-plex T-cell–mediated events are specifi-cally and sensitively targeted to one ormore chemical entities. When the levelof exposure exceeds the thresholds ofsensitization and elicitation, immuno-logic memory of the event is generated.That being said, the frequent lack of anobvious causative culprit or temporal re-lationship between the allergen and der-matitis leads to an intense detective andanalytic exercise of determining andsubsequently avoiding the offendingchemical entity.

EPIDEMIOLOGY

Much of the epidemiologic data regard-ing ACD has been extrapolated or in-ferred from government reports on theprevalence and economic impact of oc-cupational skin diseases. A basic as-sumption in many studies is that mostoccupational dermatitis is irritant in na-ture. More recent evidence, however,suggests that there is a larger proportionof allergic occupational dermatosis thanpreviously thought. Of the 5839 pa-tients patch-tested for contact dermati-tis by the North American Contact Der-matitis Group between 1998 and 2000,1097 (19 percent) were deemed to haveoccupationally related disease. In thisoccupational cohort, 60 percent of caseswere of allergic and 32 percent were ofirritant origin. Of note, the hands wereprimarily affected in two-thirds of aller-gic occupational cases and four-fifths ofirritant occupational cases2 (Fig. 13-1;see Chap. 211).

In 2001, 4714 cases of occupationaldermatitis were reported to the Bureau