Review

PPAR-g Activation As an Anti-inflammatory Therapyfor Respiratory Virus Infections

Josep Bassaganya-Riera,1 Rong Song,1 Paul C. Roberts,2 and Raquel Hontecillas1

Abstract

Newly emerged influenza viruses have attracted extensive attention due to their high infectivity, proin-flammatory actions, and potential to induce worldwide pandemics. Frequent mutations and gene reassortmentsbetween viruses complicate the development of protective vaccines and antiviral therapeutics. In contrast,targeting the host response for the development of novel cost-effective, broad-based prophylactic or therapeuticagents holds greater promise. Since inflammation often parallels the development of productive immune re-sponses, virus-induced pulmonary inflammation and injury represents an additional challenge to the devel-opment of broad-based immunotherapeutics. Excessive inflammatory responses to respiratory viruses, alsoknown as ‘‘cytokine storm,’’ are largely due to immune dysregulation that manifests as proinflammatory cy-tokine secretion. In addition to modulating lipid and glucose metabolism, peroxisome proliferator-activatedreceptors (PPAR) play important roles in antagonizing core inflammatory pathways such as NF-kB, AP1, andSTAT. Their role in regulating inflammatory responses caused by pulmonary pathogens is receiving increasingattention, setting the stage for the discovery of novel applications for anti-diabetic and lipid-lowering drugs. Thisreview focuses on the potential use of PPAR-g agonists to downregulate the inflammatory responses to respi-ratory virus-related pulmonary inflammation.

Key words: cytokines; inflammation; influenza virus; peroxisome proliferator-activated receptor-g

Introduction

Respiratory pathogens causing pneumonia are theleading cause of infectious disease-related death in in-

dustrialized countries. Although bacterial infections are morecommon in adults, respiratory viruses are the main etiologicagents of pneumonia in immunocompromised populationgroups, namely children and the elderly. Influenza, parain-fluenza, respiratory syncytial virus (RSV), and rhinovirusesare most frequently associated with respiratory infections inyoung infants and children (1). Influenza virus infectionsalone or complicated by secondary bacterial superinfectionskill more than 35,000 people in the United States every year(2). In addition, the yearly seasonal flu epidemics are aneconomic burden to the public health system, due to days ofrestricted activity, hospitalizations and visits to physicians (1).

Influenza viruses are constantly evolving through a pro-cess called ‘‘antigenic drift,’’ which typically refers to theaccumulation of mutations that alter the antigenicity of the

viral glycoproteins that cover the surface of the virus particle.These minor changes are constantly accumulating duringeach viral replication event, due to the error-prone nature ofthe viral RNA polymerase, which gives rise to an ever-changing pool of virus variants. Another process called‘‘antigenic shift,’’ or gene reassortment, is less common butintroduces more abrupt and significant genetic changes,leading to the introduction of a new subtype of virus withina population. These evolving antigenic drifts and shifts resultin new virus strains that may not be efficiently recognized bythe host’s immune system (3). The most recent example is the2009 swine-origin influenza virus H1N1 strain that has rap-idly spread, resulting in the WHO’s declaration of a newinfluenza pandemic in June of 2009 (4). Contrary to predic-tions of a devastating influenza pandemic, the rapid spreadof the virus has been associated with low mortality rates,comparable to those of currently circulating seasonal strains.However, this recent experience has confirmed that should ahighly pathogenic and highly transmissible strain emerge,

1Nutritional Immunology and Molecular Medicine Laboratory, Virginia Bioinformatics Institute, Virginia Polytechnic Institute and StateUniversity, Blacksburg, Virginia.

2Center for Molecular Medicine and Infectious Diseases, Department of Biomedical Sciences and Pathobiology, Virginia MarylandRegional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.

VIRAL IMMUNOLOGYVolume 23, Number 4, 2010ª Mary Ann Liebert, Inc.Pp. 343–352DOI: 10.1089/vim.2010.0016

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the lack of an adequate and timely supply of vaccines andantivirals would be a major obstacle to controlling virusspread (5).

Interventions based on vaccination, especially of high-riskgroups, and early administration of antiviral drugs, are thepreferred approaches for the prevention and treatment ofinfluenza infections, respectively. Currently, it takes about6 mo to develop a new vaccine (6). This creates a lag periodbetween the identification of a new strain and the adminis-tration of the first vaccine doses, leaving the population ex-posed to the virus. The immunopathology of influenza virusis characterized by the upregulation of cytokines and otherinflammatory mediators. For instance, mortality induced bythe highly pathogenic H5N1 strain correlated with highlevels of circulating cytokines and chemokines (7). Thus theuse of agents that target the host inflammatory responserather than the virus has been proposed for the treatment ofrespiratory infections in general, and influenza virus infec-tions in particular (8–12). They offer two major advantages:first, they would be efficacious regardless of changes in viralantigenicity; and second, their production is fast and rela-tively inexpensive, and in the event of a highly lethal pan-demic they would be readily available (13).

Targeting the Host Inflammatory Response to TreatInfluenza Virus Pneumonia

The newly-emerged H1N1 influenza strain has been foundto be more deadly to young individuals than the strains as-sociated with seasonal flu (14). It has been hypothesized thatthe immune systems of young people (18–30 y old) is moreprone to uncontrolled or hyper-immunoinflammatory re-sponses upon infection (15). This is in line with the ability ofhighly pathogenic strains to induce excessive proin-flammatory cytokine secretion. Influenza A (H5N1) and thereconstructed influenza A/1918 (H1N1) trigger an exacer-bated inflammatory response in humans that died from theinfection, and in mice inoculated experimentally (7,16). Pa-thological examination of lung tissue from fatal cases of the2009 swine-origin H1N1 strain support these findings (17). Ifthis hypothesis is correct, then the therapeutic interventionsto minimize influenza-related pathology and mortality re-quire suppression of the excessive inflammatory response,without hindering the induction of protective influenzavirus–specific immunity. However, knockout mice lackingkey cytokines or chemokines that participate in this response(i.e., MIP1-a, IL-6, and TNF-a), or their receptors (i.e., IL-1bR), were either unaffected or even more susceptible to thedisease. It has been argued that the existence of functionalredundancy between these inflammatory mediators andtheir signaling pathways could explain this apparent con-tradiction (18,19).

The use of the class of cholesterol-lowering drugs knownas statins as an anti-inflammatory intervention for influenzavirus–related lung pathology was first proposed in 2005 (9).However, published data on the putative benefits of statinsfor the treatment of viral pneumonia originated from ob-servational studies. Specifically, patients treated with statinswere less likely to be hospitalized or to die from influenzaviral infections (20,21). In contrast, other groups have shownonly marginal benefits associated with statin therapy (22). Inthis context, data from human clinical trials and mechanistic

studies in animals are required to investigate the therapeuticand prophylactic efficacy of statins against influenza.

Another class of drugs used to treat metabolic disorders isthe nuclear receptor peroxisome proliferator-activated re-ceptor (PPAR) agonists. Because PPAR activation antago-nizes inflammatory pathways, PPAR agonists could be usedto downregulate the host inflammatory response to influ-enza virus. Budd et al. reported in 2007 that the PPAR-gagonist gemfibrozil, a drug prescribed to lower plasma lipidsand cholesterol, increased survivability of mice infected withinfluenza virus A/Japan/H2N2 from 11% to 50% (23). Moreimportantly, gemfibrozil was able to ameliorate establishedviral disease, since the treatment was given 4 days after in-fection, when the mice already exhibited clinical signs.PPAR-g is another isoform that suppresses inflammation.Recently, Aldridge et al. demonstrated that the prophylacticadministration of pioglitazone, a synthetic PPAR-g ligand,reduced mortality of mice infected with influenza A/PR8/H1N1 virus from 92% to 50% (24). Other PPAR-g agonistshave shown potential beneficial effects for the treatment ofRSV infections. For instance, 15d-PGJ2, ciglitazone, and tro-glitazone decreased the production of the proinflammatorycytokines IL-1a and TNF-a, and the chemokines CXCL8and CCL5, as well as GM-CSF, IL-6, and intercellular adhe-sion molecule-1 of RSV-infected epithelial cells (25,26).Moreover, PPAR-g agonists inhibited viral G, F, and N pro-tein expression and RSV replication in lung epithelial cellsin vitro (27). It was also shown that late treatment with an-tivirals (i.e., 48 h post-infection) reduced mortality in miceinoculated with influenza A/H5N1 virus only when co-administered with the immunosuppressive drugs celecoxiband mesalamine (28). The improvement in survivability wasnot associated with decreased viral loads, but with inhibitionof cyclooxygenase-2. One of the immunomodulatory drugsused in this study, mesalamine, is also a PPAR-g agonist(11,29). Altogether, these data suggest that the suppressionof inflammatory responses by PPAR-g agonists could repre-sent a viable therapeutic or prophylactic strategy against fluand other respiratory infectious diseases.

PPAR-g Overview

Peroxisome proliferator-activated receptors (PPARs) be-long to the superfamily of nuclear hormone receptors with 48members identified in the human genome. There are threeknown PPAR isoforms: a, b or d, and g, which differ in theirtissue distribution and functional activity (30), and all threeare expressed in the lungs (31). PPAR-g is an endogenouslycontrolled molecular switch that regulates inflammation,immunity, and metabolism (32,33). This particular isoformwas initially studied as a key regulator of lipid and glucosehomeostasis by promoting adipocyte differentiation (34,35),and was identified as the molecular target for the insulinsensitizing thiazolidinediones (TZD) (36). However, in thelast 10 years, research has also focused on the anti-inflam-matory actions of PPAR-g. Administration of exogeneousligands of PPAR-g, including TZD, has proven to be benefi-cial in several animal models of autoimmune and inflam-matory diseases (37–40), and in human clinical trials withulcerative colitis patients (41).

PPAR-g activation and expression is also controlledby a diverse set of natural and synthetic molecules,

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including nutrients, non-nutrient endogenous ligands [i.e.,prostaglandins or hydroxy-containing PUFA such as 12/15-hydroxyeicosatetraenoic (12/15-HETE), 13-(S)-hydroxyocta-decadienoic (13-HODE), and conjugated linoleic acid (CLA)],and drugs [e.g., TZD and 5-aminosalicylic acid] (42–44). Inaddition, certain cytokines like IL-4 upregulate PPAR-g ex-pression (45), whereas others, like TNF-a, suppress it (46).This regulatory network provides a functional link betweenPPAR-g activity at the cell or tissue scales, and the state ofimmune activation or inflammation. PPAR-g exerts its func-tion mainly via a ligand-dependent activation. Ligandbinding induces PPAR and retinoid X receptor (RXR),forming a heterodimer (47). This activated PPAR-RXR het-erodimer translocates into the nucleus, where it binds per-oxisome-proliferator response elements (PPRE) located in theregulatory area of PPAR target genes. In this way ligand-activated PPAR-g regulates the transcription of a set of genesinvolved mainly in glucose homeostasis and lipid uptakeand metabolism (35,48). In addition, activated PPAR-g cansuppress the expression of specific subsets of proin-flammatory proteins by interfering with the transcriptionfactors that regulate their expression, such as NF-kB orMAPK (49,50).

In contrast to the induction of target genes, PPAR-gtransrepression activity does not require direct binding of theactivated form to the DNA in promoter regions of the af-fected genes. A well-characterized example is the suppres-sion of LPS-induced iNOS by rosiglitazone-activated PPAR-gin mouse macrophages. The ligand-dependent sumoylationof PPAR-g targets it to nuclear receptor co-repressor com-plexes on inflammatory gene promoters, which serves tostabilize and prevent signal-dependent removal of co-repressor complexes (51). Sumoylation of PPAR-g inducedby apoptotic cells has also been described in macrophages,which results in downregulation of TNF-a and IL-6 expres-sion (52). Graphical illustrations of the sumoylation of PPAR-g by rosiglitazone have been published by Glass et al. (51). Inaddition to ligand-dependent activation, PPAR-g activity canalso be modulated by phosphorylation, which may changethe affinity of PPAR-g for its ligands (53).

PPAR-g Expression and Function in the Lungs

PPAR-g is expressed in healthy lungs in the airway epi-thelium, bronchial submucosa, vascular smooth muscle cells,and endothelial cells (54). In addition, immune cells, in-cluding T cells, B cells, dendritic cells (DCs), neutrophils,eosinophils, and macrophages express PPAR-g during dif-ferentiation or following activation. As a general rule, PPAR-g expression increases as cells transition from an immature toa differentiated state. For instance, PPAR-g is expressed upondifferentiation of monocytes into macrophages (55,56). Inaddition, the healthy alveolar macrophage expresses morePPAR-g protein constitutively than other macrophage sub-sets (57). At the organ level, lung PPAR-g expression is up-regulated following allergen sensitization of mice, and in theairways of asthmatics (58). Moreover, treatment with ro-siglitazone caused a modest 15% improvement in the aller-gen challenge model in asthmatic patients (59). In contrast,endotoxin-induced lung injury was associated with lowerPPAR-g expression (54). As for other inflammatory condi-tions, several chronic lung pathologies including asthma,

chronic obstructive pulmonary disease (COPD), and lungfibrosis (60), are ameliorated by exogenous administration ofPPAR agonists. For instance, treatment of human lung fi-broblasts with PPAR-g agonists in vitro blocked their differ-entiation into myofibroblasts induced by exogenous TGF-b,and consequently diminished the collagen deposition thatleads to fibrosis (61). Also, the administration of the TZDpioglitazone via aerosol lowered antigen-induced hyperre-sponsiveness, BAL eosinophilia, type II cytokines, and IgElevels (61–63). Thus, PPAR-g is now being investigated as apotential target in a variety of lung-related chronic inflam-matory diseases. However, the role of PPAR-g and its ago-nists in regulating lung inflammation during respiratorymicrobial infections remains largely unknown.

Immune Cells Involved in Influenza-RelatedPulmonary Pathogenesis and the Potentialfor Regulation by PPAR-g

The host response induced by influenza virus is charac-terized by the activation of epithelial cells and alveolarmacrophages, followed by the recruitment of inflammatoryleukocytes into the pulmonary parenchyma (1,64). This re-sponse is associated with lung pathology, and in highlypathogenic influenza infections, with increased mortality(16–18). In parallel, the innate immune response is necessaryfor virus clearance and for the induction of productive CD8þ

T-cell responses. Thus the ideal immunomodulator shouldlower inflammation-induced pathology while still main-taining a microenvironment that is conducive for establish-ment of the virus-specific acquired immunity necessary tosuccessfully overcome the infection. Aldridge et al. showedthat while blockade of leukocyte recruitment via deletion ofCCR2 leads to increased mortality, moderate suppressionof MCP-1 (i.e., the ligand of CCR2) by exogenous activationof PPAR-g was beneficial. At the cellular level, the mecha-nism of protection depended on the decreased presence of asubset of DCs that secrete proinflammatory cytokines(tipDC) (65), and are also needed to induce/maintain virus-specific CD8 T cells in the lung (24). Thus the use of PPARagonists or other immune modulators could dampen thenegative consequences of the aggressive host response,without having a blunt effect in the induction of protectiveimmunity to the virus. The following section reviews the celltypes that contribute to the pulmonary inflammatory re-sponse and their regulation by PPAR-g and its agonists.

Respiratory epithelial cells

Respiratory epithelial cells, including alveolar and bron-chial epithelial cells, are the primary cellular targets and firstline of innate immune defense to influenza virus. Togetherwith alveolar macrophages and DCs, epithelial cells detectinfluenza viruses via PRRs (66). Infected epithelial cellsproduce several proinflammatory factors, which can modu-late the immune response as well as lead to tissue damage.Guillot et al. showed that influenza A virus activates toll-likereceptor 3 (TLR-3) signaling, leading to the secretion ofproinflammatory mediators such as IL-8, IL-6, regulated onactivation, normal T-cell expressed and secreted (RANTES),and IFN-b (67). Various strains of influenza viruses elicitdifferent profiles of cytokines and chemokines. For instance,compared to influenza A (H1N1), the H5N1 virus tends to

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elicit excessive production of IP-10, IFN-b, RANTES, and IL-6 in epithelial cells (68), whereas H3N2 strains exhibit milderinflammatory responses. The secretion of immune modula-tors and antigen presentation makes respiratory epithelialcells extremely important in immune defense following in-fluenza virus infection.

The death rate of alveolar epithelial cells can be as much as15% 24 h after infection (69). Inhibition of host mRNAtranslation in infected epithelial cells is one of the majorcauses of epithelial cell death. PB1-F2 protein of influenzavirus is demonstrated to induce cell death, including apo-ptosis of alveolar epithelial cells, via a mitochondria-relatedmechanism (70). TNF-related apoptosis-inducing ligand(TRAIL) expressed by macrophages also contributes to al-veolar epithelial cell apoptosis (71). Various cytokines alsoinduce apoptosis in alveolar epithelial cells. Production ofproinflammatory cytokines is reduced during epithelial cellapoptosis (72). Therefore, respiratory epithelial cell apoptosisis important to eliminate infected and damaged cells, as wellas to control inflammation and therefore maintain host re-sponses, while minimizing inflammation-induced tissuedamage.

Airway epithelial cells are the first barrier of the respira-tory system and the first to respond to infection. PPAR-gexpression in airway epithelial cells is constitutively highand will inhibit the expression of inflammatory factors suchas inducible nitric oxide synthase (iNOS) and IL-8. Agonistsof PPAR-g, such as the prostaglandin D2 metabolite 15-deoxy-D12,PGJ2 and ciglitazone, enhance this inhibition in adose-dependent manner, as do other PPAR g ligands (73).Therefore, the use of PPAR-g agonists to activate PPAR-g asan inflammatory modulator is prominent in the literature.However, the possible application of PPAR-g therapy tosuppress influenza-related inflammation is a novel and apromising therapy in the face of current and emergingpandemics.

Respiratory macrophages

Monocytes/macrophages play a central role in immuneregulation in response to pathogens, as well as maintainingpulmonary homeostasis. The alveolar macrophage is themost important subset of respiratory macrophages, ac-counting for the majority of immune cells in alveolar spaces.Infiltration of lung tissue by newly differentiated macro-phages is markedly increased in influenza virus–infectedhumans and mice, which suggests a possible role of thisimmune cell subset in inflammation and lung injury, as wellas in eliciting an adaptive immune response (74).

Although the alveolar macrophage contributes to lunghomeostatic regulation in part by suppressing the inductionof immune responses in healthy individuals, following in-fluenza virus infection they acquire a proinflammatoryphenotype. They respond to virus infection by producingproinflammatory cytokines such as TNF-a, IFN-b, IFN-l, IL-6, IL-1b, IP-10, CCL5/RANTES, CCL2/MCP-1, CCL3/MIP-1a, and CCL4/MIP1-b (74–76). IFN regulatory factor 3 (IRF3)and p38 kinase work separately to mediate H5N1-inducedTNF-a, IFN-b, IFN-l, and MCP-1 (77).

During the steady state, resident alveolar macrophages aremaintained at stable levels and are mainly renewed by pre-cursor cell proliferation. They function as phagocytes, but do

not migrate to regional lymph nodes, which constitute theprimary place for initiating adaptive immunity and the in-ductive sites of the respiratory mucosa (78). During viral-induced inflammation, recruited macrophages originatefrom peripheral blood monocytes, move across the alveolarepithelium, and infiltrate the alveolar spaces. Herold et al.demonstrated that it is the alveolar epithelial cell rather thanresident alveolar macrophages that release monocyte che-moattractants such as CCL2/MCP-1 and CCL5 (79).

Alveolar macrophages regulate the pulmonary immuneresponse (80), and can suppress APC function of pulmonaryDCs (81). The mechanism behind the suppression of alveolarmacrophages to reduce innate and adaptive immunity is notquite clear. Morris et al. found that it may be related to avb6

integrin expression in alveolar epithelial cells, a processregulated by TGF-b (82). In addition, high levels of PPAR-gexpression by alveolar macrophages had a suppressive rolein inflammatory cytokine secretion (83), as well as AP-1 andNF-kB activation (84), which suggests a possible PPAR-g-related mechanism.

Alveolar macrophages have also been reported to exert amodulatory role on DC migration (85). As noted previously,infiltrating exudate macrophages via release of TRAIL canalso potentiate respiratory epithelial cell death, leading toexacerbated lung injury and increased mortality (71). Lin et al.reported increased numbers of CCR2þ monocyte-derivedDCs and exudate macrophages in the lungs of influenza vi-rus–infected mice, which produce large amounts of cytokinesand could be the major cell types contributing to influenzavirus–induced pulmonary inflammation and pathology. Ofnote, CCR2-knockout mice have decreased macrophage in-filtration and pulmonary injury (86,87).

Alveolar macrophages highly express PPAR-g, which iseven further upregulated by IL-4 (88) and GM-CSF (89).Treatment with a PPAR-g agonist in vitro showed upregu-lated expression of PPAR-g, and suppressed production ofproinflammatory cytokines such as TNF-a and IL-12 (83), aswell as AP-1, and NF-kB (84). In addition, PPAR-g knockoutin macrophages leads to a significantly increased expressionof IFN-g, iNOS, IL-12, MIP-1a, and IP-10, which are Th-1immune mediators (90). PPAR-g has also been found toregulate macrophage colony-stimulating factor (M-CSF),which is a mediator of monocyte differentiation, activation,and survival, via transrepression of NF-kB (91). These ob-servations suggest that PPAR-g activation in alveolar mac-rophages elicits an anti-inflammatory effect, though themechanism is not yet fully understood. Of note, MCP-1production is reduced by administration of PPAR-g ligands(92). Thus PPAR-g may exert its anti-inflammatory functionpartially by indirectly regulating exudate macrophage infil-tration via its ability to limit MCP-1 production in alveolarmacrophages.

Dendritic cells

As professional antigen-presenting cells (APCs), DCs arefound in relatively small amounts at mucosal surfaces likeskin or the inner layer of the lungs and intestinal laminapropria. DCs directly interact with T and B lymphocytes atinductive sites (i.e., mediastinal lymph nodes) to initiateadaptive immune responses and serve as sentinels of theimmune system. In this regard, a newly characterized subset

346 BASSAGANYA-RIERA ET AL.

of DCs that produce TNF-a and iNOS, known as tipDCs (65),has been shown to contribute to both influenza virus–associated pulmonary inflammation, as well as to the inductionof protective immune responses (24). tipDCs accumulated inhigher numbers in the lungs of mice infected with highlypathogenic strains than in mice inoculated with strains thatinduce milder disease (24). Recruitment of monocytes anddifferentiation into tipDCs depends on CCR2 signaling (93).Very few tipDCs were found in lungs of CCR2–/– mice;however, this was paralleled by poor induction of influ-enza virus–specific CD8þ T cells and inefficient virus neu-tralization and clearance. The synthetic PPAR-g agonistpioglitazone suppressed without fully inhibiting tipDC ac-cumulation, thereby moderating the morbidity and mortalityof the influenza virus infection, while maintaining the abilityto mount adaptive immune responses (24). While the ex-pression of iNOS by murine macrophages is well docu-mented, the generation of NO by human macrophages hasbeen difficult to demonstrate, thereby limiting the transla-tional value of tipDCs.

Aldridge et al. found that the main difference betweenpioglitazone and vehicle-treated animals was the lower lev-els of MCP-2, which correlated with decreased presence oftipDCs. However, the lower numbers of these proin-flammatory cells was not associated with decreased overalliNOS expression in the lung (24). Moreover, although tipDCsare important for inducing and/or maintaining CD8þ T cellsin the lung parenchyma, there were no differences in viralloads, indicating that the enhanced CD8þ T-cell response didnot correlate with productive antiviral immunity. In additionto regulating the recruitment of DCs, PPAR-g ligands canalso modulate the induction of adaptive immune responsesby inhibiting DC maturation, the expression of co-stimulatorymolecules, and the secretion of IL-12 (94,95). During in vitrodifferentiation of human monocyte-derived DCs (moDCs),PPAR-g is expressed only briefly in the initial phases of thedifferentiation process. This window of PPAR-g activity co-incides with the upregulation of genes involved in lipiduptake and metabolism (56). Moreover, DC differentiation inthe presence of the PPAR-g agonist rosiglitazone suppressedthe expression of CD1a, CD1b, CD1c, and CD1e (i.e., group Ilipid-presenting molecules), whereas CD1d was significantlyupregulated. CD1d, which is normally present in monocytesbut not in DCs, presents a-galactosylceramide (a-GalCer)to suppressor natural killer T (NKT) cells, facilitating theirexpansion (96). Co-administration of a-GalCer as an adju-vant with inactivated influenza vaccine diminished theinitial CD8þ T-cell response, but in the long term en-hanced the generation of memory CTL and protective im-munity (97).

T lymphocytes

CD4þ T cells can promote the proliferation and differen-tiation of T cells, B cells, and other immune cells by secretingIL-2. CD4þ T cells are functionally heterogeneous, and in-clude Th-1, Th-2, and Th17, and naturally occurring as wellas induced regulatory T cells (Treg). PPAR-g activation hasbeen shown to downregulate the production of IFN-g, IL-17,and IL-2 by CD4þ T cells, thereby resulting in suppressedproliferation and Th-1 or Th-17 differentiation (98,99). Inaddition, the expression of functional PPAR-g in naturally-

occurring Treg is required for the prevention of effectorCD4þ T-cell-induced chronic colitis (99), and graft-versus-host disease (39). In the model of virus-induced respiratorydisease, IFN-g may contribute to both facilitating viralclearance, as well as augmenting the lung inflammatory re-sponse. Therefore, activation of PPAR-g would be beneficialin ameliorating CD4þ T-cell-driven pulmonary inflamma-tion, but could possibly impair Th1-associated antiviral im-munity. Therefore, treatment with PPAR-g agonists shouldbe considered as a prophylactic therapeutic for use againstthose viral strains that cause respiratory disease throughpulmonary inflammation.

CD8þ T cells include suppressor or regulatory T lym-phocyte (Ts) and cytolytic T lymphocyte (Tc) cells. There arerecent reports of Tc17 CD8þ T-cell effectors that contribute toprotective immunity against influenza (100). CD8þ T cellsplay an important role in viral clearance and recovery fromviral infection. CD8þ T-cell-deficient mice show impairedviral clearance and therefore higher mortality after influ-enza virus infection (101). Transfer of CD8þ T cells to B cell-deficient mice can facilitate the clearance of influenza virusfrom the lungs (102). IFN-g produced by CD8þ T cells isfound to play a critical role in the CD8þ T-cell response, sincetransfer of CD8þ T cells from IFN-g-deficient mice does notresult in immune protection (103). During influenza virusinfection, immediately after the induction of innate im-mune responses, activated antigen-presenting DCs migrateto the mediastinal lymph nodes and activate antigen-specificnaı̈ve CD4þ and CD8þ T cells (104). Migration of the virus-specific CD4þ and CD8þ T cells, which can express IL-2, IL-4,IL-10, and IFN-g (105), back to the lung is largely dependenton the chemokines expressed in the lung during the infection(106,107). In line with the concept that regulatory mecha-nisms parallel proinflammatory responses, Sun et al. foundthat during acute influenza virus infection, CD4þ and CD8þ

effector T cells produce high levels of IL-10, an importantanti-inflammatory cytokine that is reported to be producedby several immune cell types in response to inflammatorystimuli (108,109). Blockade of IL-10 leads to more severepulmonary inflammation during influenza virus infection. Inaddition, an increasing number of IL-10-producing CD8þ Tcells at the infection site indicates the possible modulatoryrole of CD8þ T cells during influenza virus infection. Alto-gether, virus-specific effector T cells play a significant role incontrolling influenza virus infection by inducing infected celllysis and the generation of inflammatory mediators, as wellas contributing to pulmonary homeostasis through the in-duction of regulatory responses. Even though no informationis available on the direct regulation of CD8þ T-cell responsesby PPAR-g compared to PPAR-g-mediated regulation ofCD4þ T-cell responses, a recent report suggests that PPAR-gactivation of DCs results indirectly in impaired activation ofantigen-specific CD8þ T cells by inducing the upregulation ofthe co-inhibitory molecule B7H1 (110). Further studies areneeded to examine the role of PPAR- g in CD8þ T cellsduring respiratory viral infection.

Neutrophils

Neutrophils constitute a large proportion of the inflam-matory leukocytes present in the lung following influ-enza virus infection. Although their role in influenza virus

THE ROLE OF PPAR-g IN RESPIRATORY VIRAL INFECTIONS 347

clearance is not yet well defined, it has been suggested thatthey may hinder influenza virus replication (111). Also, in-fluenza A virus infection can increase apoptosis of neutro-phils, which might contribute to the inflammatory response(112). Wareing et al. also showed that inhibition of neutrophilrecruitment did not have a significant effect on viral clear-ance (113). However, in a different study, neutrophil deple-tion aggravated lung pathology, leading to fatal pneumoniaand death in mice infected with a low dose of a sublethalH3N2 influenza virus (114). In the absence of neutrophils, thevirus spread to extrapulmonary locations. Thus, similarly tothe inflammatory tipDCs, modulation of neutrophil recruit-ment and function by PPAR-g could lead to diminishedpathology without having adverse effects on the control ofvirus spread and persistence. Human neutrophils expressmoderate levels of PPAR-g that can be increased by TNF-aand IL-4 (115). While the upregulation of mRNA and proteinby IL-4 is common to other cell types, the effect of TNF-aseems to be specific for neutrophils, as others have reportedan inhibitory effect on the expression and/or activation ofPPAR-g by this proinflammatory cytokine in other immunecells (116). In addition, PPAR-g-agonist treatment was shown

to inhibit neutrophil chemotaxis (115). Lipoxin A4 is an ei-cosanoid that plays a key role in the resolution of neutrophilicinflammation. However, decreased PPAR-g activity in neona-tal neutrophils correlated with reduced responsiveness to li-poxin A4 (117). Ciglitazone, another PPAR-g agonist, wasfound to significantly impair neutrophil infiltration into thelung via a NF-kB-related mechanism (118). Although thesefindings are not directly related to influenza-associated in-flammation, they do suggest that PPAR-g also modulates theanti-inflammatory responses in neutrophils, providing anadditional regulatory target for PPAR-g-based immuno-therapeutics.

Conclusion

Traditional antiviral therapeutics may offer limited effi-cacy due to the rapid evolution of emerging influenza virusstrains, and the emergence of strains resistant to antiviraltreatment. Since excessive inflammation has been linked toinfluenza virus-related mortality, immune modulation maybe required in addition to virus clearance to limit or preventsevere disease. Exogenous administration of PPAR-g ligands

FIG. 1. Epithelial cells are the first line of defense against respiratory viruses. They detect virus via pattern recognitionreceptors (PRRs) and produce proinflammatory factors. Following infection, bone marrow–derived macrophages infiltratethe lung area. Recruited and resident macrophages can regulate pulmonary inflammation. Although present in relativelysmall amounts in the respiratory mucosa, dendritic cells (DCs) migrate to the mediastinal lymph nodes and present antigen tonaive T and B cells, which in turn will migrate to the lungs as effector cells following a chemokine (CCL) gradient. tipDCsmigrate into the lung during influenza virus infection and mediate inflammatory responses. Epithelial cells, DCs, macro-phages, and T cells express peroxisome proliferator-activated receptor-g, and its activation results in amelioration of virus-induced pulmonary inflammation.

348 BASSAGANYA-RIERA ET AL.

can broadly target the majority of cells involved in virus-induced pulmonary pathology (Fig. 1), and it represents aneffective means of immune modulation. Its transrepressionof inflammatory genes significantly modulates the immu-nological networks at the respiratory mucosa, and thereforeexerts prophylactic and therapeutic effects during respira-tory virus infection. The specific mechanism by which PPAR-g agonism protects from influenza-induced lung pathologyremains unknown and additional studies are required. Forinstance, although pioglitazone diminished the recruitmentof TNF-a- and iNOS-producing DCs, the production of NOin human myeloid-derived cells has been difficult to dem-onstrate, with the exception of Mycobacterium tuberculosisinfection (119,120). The lack of parallelism between mice andhumans may call into question the translational value ofthese findings. However, PPAR-g is expressed in severalcompartments and cell types of the lung, and it is likely thatepithelial cells are also targeted by PPAR-g-based interven-tions. In non-human primates infected with avian-originH5N1 influenza A, induced virus replication was mainlylocalized to type II pneumocytes, and resulted in strong in-duction of IL-6 and TNF-a (121). Of note, both of theseproinflammatory cytokines are downregulated followingPPAR-g activation. Thus it is likely that the mechanism ofaction of PPAR-g agonists involves both the epithelial andimmune compartments. In addition, many agonists activatePPAR-g-independent pathways, thus the use of geneti-cally modified, conditional PPAR-g-deficient mice in im-mune, epithelial, or endothelial cells will aid in elucidatingthe cell specificity and mechanism of action of such putativetherapy.

Acknowledgments

We would like to thank Ms. Jennifer Wang for her assis-tance in the preparation of the figure.

Author Disclosure Statement

No competing financial interests exist.

References

1. Hippenstiel S, Opitz B, Schmeck B, and Suttorp N: Lungepithelium as a sentinel and effector system in pneumonia—molecular mechanisms of pathogen recognition and signaltransduction. Respir Res 2006;7:97.

2. Rothberg MB, Haessler SD, and Brown RB: Complicationsof viral influenza. Am J Med 2008;121:258–264.

3. Chen J, and Deng YM: Influenza virus antigenic variation,host antibody production and new approach to controlepidemics. Virol J 2009;6:30.

4. Cohen J, and Enserink M: After delays, WHO agrees: the2009 pandemia has begun. Science 2009;324:1496–1497.

5. Enserink M, Cohen J: Virus of the year. The novel H1N1influenza Science 2009;326:1607.

6. Quigley E: Influenza therapies: vaccines and antiviraldrugs. Drug Discov Today 2006;11:478–480.

7. De Jong MD, Simmons CP, Thanh TT, et al.: Fatal outcomeof human influenza A (H5N1) is associated with high viralload and hypercytokinemia. Nat Med 2006;12:1203–1207.

8. Fedson DS: Pandemic influenza: a potential role for statinsin treatment and prophylaxis. Clin Infect Dis 2006;43:199–205.

9. Fedson DS: Statin protection against influenza and COPDmortality. Chest 2007;132:1406; author reply 1406–1407.

10. Butler D: Cheaper approaches to flue divide researchers.Nature 2007;448:976–977.

11. Fedson DS: Confronting the next influenza pandemic withanti-inflammatory and immunomodulatory agents: whythey are needed and how they might work. Influenza OtherResp Viruses 2009;3:129–142.

12. Clark I, and Alleva L: Invited commentary on DavidFedson’s article. Influenza Other Resp Viruses 2009;3:199–201.

13. Fedson DS: Confronting an influenza pandemic with in-expensive generic agents: can it be done? Lancet Infect Dis2008;8:571–576.

14. Jain S, Kamimoto L, Bramley AM, et al.: Hospitalized pa-tients with 2009 H1N1 influenza in the United States,April–June 2009. N Engl J Med 2009;361:1935–1944.

15. Ahmed R, Oldstone MB, and Palese P: Protective immunityand susceptibility to infectious diseases: lessons from the1918 influenza pandemic. Nat Immunol 20078:1188–1193.

16. Kash JC, Tumpey TM, Proll SC, et al.: Genomic analysis ofincreased host immune and cell death responses inducedby 1918 influenza virus. Nature 2006;443:578–581.

17. Gill JR, Sheng ZM, Ely SF, et al.: Pulmonary pathologicfindings of fatal 2009 pandemic influenza A/H1N1 viralinfections. Arch Pathol Lab Med 2010;134:235–243.

18. Salomon R, Hoffmann E, and Webster RG: Inhibition of thecytokine response does not protect against lethal H5N1influenza infection. Proc Natl Acad Sci USA 2007;104:12479–12481.

19. Szretter KJ, Gangappa S, Lu X, et al.: Role of host cytokineresponses in the pathogenesis of avian H5N1 influenzaviruses in mice. J Virol 2007;81:2736–2744.

20. Almog Y, Novack V, Eisinger M, et al.: The effect of statintherapy on infection-related mortality in patients withatherosclerotic diseases. Crit Care Med 2007;35:372–378.

21. Kruger P, Fitzsimmons K, Cook D, et al.: Statin therapy isassociated with fewer deaths in patients with bacteraemia.Intensive Care Med 2006;32:75–79.

22. Kwong JC, Li P, and Redelmeier DA: Influenza morbidityand mortality in elderly patients receiving statins: a cohortstudy. PLoS One 2009;4:e8087.

23. Budd A, Alleva L, Alsharifi M, et al.: Increased survivalafter gemfibrozil treatment of severe mouse influenza.Antimicrob Agents Chemother 2007;51:2965–2968.

24. Aldridge JR Jr, Moseley CE, Boltz DA, et al.: TNF/iNOS-producing dendritic cells are the necessary evil of lethalinfluenza virus infection. Proc Natl Acad Sci USA2009;106:5306–5311.

25. Arnold R, and Konig W: Peroxisome-proliferator-activatedreceptor-gamma agonists inhibit the release of proin-flammatory cytokines from RSV-infected epithelial cells.Virology 2006;346:427–439.

26. Arnold R, Neumann M, and Konig W: Peroxisomeproliferator-activated receptor-gamma agonists inhibit re-spiratory syncytial virus-induced expression of intercellularadhesion molecule-1 in human lung epithelial cells. Im-munology 2007;121:71–81.

27. Arnold R, and Konig W: Peroxisome proliferator-activatedreceptor-gamma agonists inhibit the replication of respira-tory syncytial virus (RSV) in human lung epithelial cells.Virology 2006;350:335–346.

28. Zheng BJ, Chan KW, Lin YP, et al.: Delayed antiviral plusimmunomodulator treatment still reduces mortality in mice

THE ROLE OF PPAR-g IN RESPIRATORY VIRAL INFECTIONS 349

infected by high inoculum of influenza A/H5N1 virus.Proc Natl Acad Sci USA 2008;105:8091–8096.

29. Rousseaux C, Lefebvre B, Dubuquoy L, et al.: Intestinalantiinflammatory effect of 5-aminosalicylic acid is depen-dent on peroxisome proliferator-activated receptor-gamma.J Exp Med 2005;201:1205–1215.

30. Mangelsdorf DJ, Thummel C, Beato M, et al.: The nuclearreceptor superfamily: the second decade. Cell 1995;83:835–839.

31. Escher P, Braissant O, Basu-Modak S, et al.: Rat PPARs:quantitative analysis in adult rat tissues and regulation infasting and refeeding. Endocrinology 2001;142:4195–4202.

32. Bassaganya-Riera J, Guri A, King J, and Hontecillas R:Peroxisome proliferator-activated receptors: The nutrition-ally controlled molecular networks that integrate inflam-mation, immunity and metabolism. Curr Nutr Food Sci2005;1:179–187.

33. Guri AJ, Hontecillas R, and Bassaganya-Riera J: Peroxisomeproliferator-activated receptors: Bridging metabolic syn-drome with molecular nutrition. Clin Nutr 2006;25:871–885.

34. Graves RA, Tontonoz P, and Spiegelman BM: Analysis of atissue-specific enhancer: ARF6 regulates adipogenic geneexpression. Mol Cell Biol 1992;12:3313.

35. Tontonoz P, Hu E, Graves RA, et al.: mPPAR gamma 2:tissue-specific regulator of an adipocyte enhancer. GenesDev 1994;8:1224–1234.

36. Lehmann JM, Moore LB, Smith-Oliver TA, et al.: An anti-diabetic thiazolidinedione is a high affinity ligand for per-oxisome proliferator-activated receptor gamma (PPARgamma). J Biol Chem 1995;270:12953–12956.

37. Katayama K, Wada K, Nakajima A, et al.: A novel PPAR-gamma gene therapy to control inflammation associatedwith inflammatory bowel disease in a murine model.Gastroenterology 2003;124:1315–1324.

38. Lytle C, Tod TJ, Vo KT, et al.: The peroxisome proliferator-activated receptor gamma ligand rosiglitazone delaysthe onset of inflammatory bowel disease in mice with in-terleukin 10 deficiency. Inflamm Bowel Dis 2005;11:231–243.

39. Wohlfert EA, Nichols FC, Nevius E, and Clark RB:Peroxisome proliferator-activated receptor gamma (PPAR-gamma) and immunoregulation: enhancement of regula-tory T cells through PPARgamma-dependent and-independent mechanisms. J Immunol 2007;178:4129–4135.

40. Bassaganya-Riera J, Reynolds K, Martino-Catt S, et al.:Activation of PPAR gamma and delta by conjugated lino-leic acid mediates protection from experimental inflam-matory bowel disease. Gastroenterology 2004;127:777–791.

41. Lewis JD, Lichtenstein GR, Deren JJ, et al.: Rosiglitazone foractive ulcerative colitis: a randomized placebo-controlledtrial. Gastroenterology 2008;134:688–695.

42. Mokdad AH, Bowman BA, Ford ES, et al.: The continuingepidemics of obesity and diabetes in the United States.JAMA 2001;286:1195–1200.

43. Desvergne B, and Wahli W: Peroxisome proliferator-activated receptors: nuclear control of metabolism. EndocrRev 1999;20:649–688.

44. Nesto RW, Bell D, Bonow RO, et al.: Thiazolidinedione use,fluid retention, and congestive heart failure: a consensusstatement from the American Heart Association andAmerican Diabetes Association. October 7, 2003. Circula-tion 2003;108:2941–2948.

45. Huang JT, Welch JS, Ricote M, et al.: Interleukin-4-dependent production of PPAR-gamma ligands in macro-phages by 12/15-lipoxygenase. Nature 1999;400:378–382.

46. Zhou M, Wu R, Dong W, et al.: Endotoxin downregulatesperoxisome proliferator-activated receptor-gamma via theincrease in TNF-alpha release. Am J Physiol Regul IntegrComp Physiol 2008;294:R84–R92.

47. Gearing KL, Gottlicher M, Teboul M, et al.: Interaction ofthe peroxisome-proliferator-activated receptor and retinoidX receptor. Proc Natl Acad Sci USA 1993;90:1440–1444.

48. Tontonoz P, and Spiegelman BM: Fat and beyond: the di-verse biology of PPARgamma. Annu Rev Biochem 2008;77:289–312.

49. Ricote M, and Glass CK: PPARs and molecular mecha-nisms of transrepression. Biochim Biophys Acta 2007;1771:926–935.

50. Straus DS, and Glass CK: Anti-inflammatory actions ofPPAR ligands: new insights on cellular and molecularmechanisms. Trends Immunol 2007;28:551–558.

51. Pascual G, Fong AL, Ogawa S, et al.: A SUMOylation-dependent pathway mediates transrepression of inflam-matory response genes by PPAR-gamma. Nature 2005;437:759–763.

52. Jennewein C, Kuhn AM, Schmidt MV, et al.: Sumoylation ofperoxisome proliferator-activated receptor gamma by ap-optotic cells prevents lipopolysaccharide-induced NCoRremoval from kappaB binding sites mediating transre-pression of proinflammatory cytokines. J Immunol 2008;181:5646–5652.

53. Diradourian C, Girard J, and Pegorier JP: Phosphorylationof PPARs: from molecular characterization to physiologicalrelevance. Biochimie 2005;87:33–38.

54. Becker J, Delayre-Orthez C, Frossard N, and Pons F: Reg-ulation of inflammation by PPARs: a future approach totreat lung inflammatory diseases? Fundam Clin Pharmacol2006;20:429–447.

55. Moore KJ, Fitzgerald ML, and Freeman MW: Peroxisomeproliferator-activated receptors in macrophage biology:friend or foe? Curr Opin Lipidol 2001;12:519–527.

56. Szatmari I, Torocsik D, Agostini M, et al.: PPARgammaregulates the function of human dendritic cells primarily byaltering lipid metabolism. Blood 2007;110:3271–3280.

57. Smith MR, Standiford TJ, and Reddy RC: PPARs in alveolarmacrophage biology. PPAR Res 2007;23812.

58. Kobayashi M, Thomassen MJ, Rambasek T, et al.: Aninverse relationship between peroxisome proliferator-activated receptor gamma and allergic airway inflamma-tion in an allergen challenge model. Ann Allergy AsthmaImmunol 2005;95:468–473.

59. Richards DB, Bareille P, Lindo EL, et al.: Treatment with aperoxisomal proliferator activated receptor gamma agonisthas a modest effect in the allergen challenge model inasthma: A randomised controlled trial. Respir Med 2009;104:668–674.

60. Huang TH, Razmovski-Naumovski V, Kota BP, et al.: Thepathophysiological function of peroxisome proliferator-ac-tivated receptor-gamma in lung-related diseases. RespirRes 2005;6:102.

61. Burgess HA, Daugherty LE, Thatcher TH, et al.: PPAR-gamma agonists inhibit TGF-beta induced pulmonarymyofibroblast differentiation and collagen production: im-plications for therapy of lung fibrosis. Am J Physiol LungCell Mol Physiol 2005;288:L1146–1153.

350 BASSAGANYA-RIERA ET AL.

62. Woerly G, Honda K, Loyens M, et al.: Peroxisome pro-liferator-activated receptors alpha and gamma down-regulate allergic inflammation and eosinophil activation.J Exp Med 2003;198:411–421.

63. Trifilieff A, Bench A, Hanley M, et al.: PPAR-alpha and-gamma but not -delta agonists inhibit airway inflamma-tion in a murine model of asthma: in vitro evidence for anNF-kappaB-independent effect. Br J Pharmacol 2003;139:163–171.

64. McGill J, Heusel JW, and Legge KL: Innate immune controland regulation of influenza virus infections. J Leukoc Biol2009;86:803–812.

65. Serbina NV, Salazar-Mather TP, Biron CA, et al.: TNF/iNOS-producing dendritic cells mediate innate immune de-fense against bacterial infection. Immunity 2003;19:59–70.

66. Holt PG, Strickland DH, Wikstrom ME, and Jahnsen FL:Regulation of immunological homeostasis in the respira-tory tract. Nat Rev Immunol 2008;8:142–152.

67. Guillot L, Le Goffic R, Bloch S, et al.: Involvement of toll-like receptor 3 in the immune response of lung epithelialcells to double-stranded RNA and influenza A virus. J BiolChem 2005;280:5571–5580.

68. Chan MC, Cheung Cy, Chui WH, et al.: Proinflammatorycytokine responses induced by influenza A (H5N1) virusesin primary human alveolar and bronchial epithelial cells.Respir Res 2005;6:135.

69. Tesfaigzi Y: Roles of apoptosis in airway epithelia. Am JRespir Cell Mol Biol 2006;34:537–547.

70. Zamarin D, Garcia-Sastre A, Xiao X, et al.: Influenza virusPB1-F2 protein induces cell death through mitochondrialANT3 and VDAC1. PLoS Pathog 2005;1:e4.

71. Herold S, Steinmueller M, von Wulffen W, et al.: Lungepithelial apoptosis in influenza virus pneumonia: the roleof macrophage-expressed TNF-related apoptosis-inducingligand. J Exp Med 2008;205:3065–3077.

72. Brydon EW, Smith H, and Sweet C: Influenza A virus-induced apoptosis in bronchiolar epithelial (NCI-H292)cells limits pro-inflammatory cytokine release. J Gen Virol2003;84:2389–2400.

73. Wang AC, Dai X, Luu B, and Conrad DJ: Peroxisomeproliferator-activated receptor-gamma regulates airwayepithelial cell activation. Am J Respir Cell Mol Biol 2001;24:688–693.

74. Perrone LA, Plowden JK, Garcia-Sastre A, et al.: H5N1 and1918 pandemic influenza virus infection results in early andexcessive infiltration of macrophages and neutrophils in thelungs of mice. PLoS Pathog 2008;4:e1000115.

75. Cheung CY, Poon LL, Lau AS, et al.: Induction of proin-flammatory cytokines in human macrophages by influenzaA (H5N1) viruses: a mechanism for the unusual severity ofhuman disease? Lancet 2002;360:1831–1837.

76. Kaufmann A, Salentin R, Meyer RG, et al.: Defense againstinfluenza A virus infection: essential role of the chemokinesystem. Immunobiology 2001;204:603–613.

77. Hui KP, Lee SM, Cheung CY, et al.: Induction of proin-flammatory cytokines in primary human macrophages byinfluenza A virus (H5N1) is selectively regulated by IFNregulatory factor 3 and p38 MAPK. J Immunol 2009;182:1088–1098.

78. MacLean JA, Xia W, Pinto CE, et al.: Sequestration of in-haled particulate antigens by lung phagocytes. A mecha-nism for the effective inhibition of pulmonary cell-mediated immunity. Am J Pathol 1996;148:657–666.

79. Herold S, W von Wulffen, M Steinmueller, et al.: 2006Alveolar epithelial cells direct monocyte transepithelialmigration upon influenza virus infection: impact of che-mokines and adhesion molecules. J Immunol 1996;177:1817–1824.

80. Thepen T, Van Rooijen N, and Kraal G: Alveolar macro-phage elimination in vivo is associated with an increase inpulmonary immune response in mice. J Exp Med 1989;170:499–509.

81. Holt PG, Oliver J, Bilyk N, et al.: Downregulation of theantigen presenting cell function(s) of pulmonary dendriticcells in vivo by resident alveolar macrophages. J Exp Med1993;177:397–407.

82. Morris DG, Huang X, Kaminski N, et al.: Loss of integrinalpha(v)beta6-mediated TGF-beta activation causesMmp12-dependent emphysema. Nature 2003;422:169–173.

83. Asada K, Sasaki S, Suda T, et al.: Antiinflammatory roles ofperoxisome proliferator-activated receptor gamma in hu-man alveolar macrophages. Am J Respir Crit Care Med2004;169:195–200.

84. Ricote M, Valledor AF, and Glass CK: Decoding tran-scriptional programs regulated by PPARs and LXRs in themacrophage: effects on lipid homeostasis, inflammation,and atherosclerosis. Arterioscler Thromb Vasc Biol 2004;24:230–239.

85. Jakubzick C, Tacke F, Llodra J, et al.: Modulation of den-dritic cell trafficking to and from the airways. J Immunol2006;176:3578–3584.

86. Lin KL, Suzuki Y, Nakano H, et al.: CCR2þ monocyte-de-rived dendritic cells and exudate macrophages produceinfluenza-induced pulmonary immune pathology andmortality. J Immunol 2008;180:2562–2572.

87. Wareing MD, Lyon A, Inglis C, et al.: Chemokine regulationof the inflammatory response to a low-dose influenza in-fection in CCR2-/- mice. J Leukoc Biol 2007;81:793–801.

88. Reddy RC, Keshamouni VG, Jaigirdar SH, et al.: Deactiva-tion of murine alveolar macrophages by peroxisome pro-liferator-activated receptor-gamma ligands. Am J PhysiolLung Cell Mol Physiol 2004;286:L613–L619.

89. Bonfield TL, Farver CF, Barna BP, et al.: Peroxisome pro-liferator-activated receptor-gamma is deficient in alveolarmacrophages from patients with alveolar proteinosis. Am JRespir Cell Mol Biol 2003;29:677–682.

90. Malur A, McCoy AJ, Arce S, et al.: Deletion of PPARgamma in alveolar macrophages is associated with a Th-1pulmonary inflammatory response. J Immunol 2009;182:5816–5822.

91. Bonfield TL, Thomassen MJ, Farver CF, et al.: Peroxisomeproliferator-activated receptor-gamma regulates the ex-pression of alveolar macrophage macrophage colony-stimulating factor. J Immunol 2008;181:235–242.

92. Kintscher U, Goetze S, Wakino S, et al.: Peroxisomeproliferator-activated receptor and retinoid X receptorligands inhibit monocyte chemotactic protein-1-directedmigration of monocytes. Eur J Pharmacol 2000;401:259–270.

93. Serbina NV, and Pamer EG: Monocyte emigration frombone marrow during bacterial infection requires signalsmediated by chemokine receptor CCR2. Nat Immunol2006;7:311–317.

94. Klotz L, Dani I, Edenhofer F, et al.: Peroxisome proliferator-activated receptor gamma control of dendritic cell functioncontributes to development of CD4þ T cell anergy. J Im-munol 2007;178:2122–2131.

THE ROLE OF PPAR-g IN RESPIRATORY VIRAL INFECTIONS 351

95. Nencioni A, Grunebach F, Zobywlaski A, et al.: Dendriticcell immunogenicity is regulated by peroxisome pro-liferator-activated receptor gamma. J Immunol 2002;169:1228–1235.

96. Szatmari I, Gogolak P, Im JS, et al.: Activation of PPAR-gamma specifies a dendritic cell subtype capable of en-hanced induction of iNKT cell expansion. Immunity2004;21:95–106.

97. Guillonneau C, Mintern JD, Hubert FX, et al.: CombinedNKT cell activation and influenza virus vaccination boostsmemory CTL generation and protective immunity. ProcNatl Acad Sci USA 2009;106:3330–3335.

98. Clark RB, Bishop-Bailey D, Estrada-Hernandez T, et al.: Thenuclear receptor PPAR gamma and immunoregulation:PPAR gamma mediates inhibition of helper T cell re-sponses. J Immunol 2000;164:1364–1371.

99. Hontecillas R, and Bassaganya-Riera J: Peroxisomeproliferator-activated receptor gamma is required for reg-ulatory CD4þ T cell-mediated protection against colitis. JImmunol 2007;178:2940–2949.

100. Hamada H, Garcia-Hernandez Mde L, Reome JB, et al.:Tc17, a unique subset of CD8 T cells that can protect againstlethal influenza challenge. J Immunol 2009;182:3469–3481.

101. Bender BS, Croghan T, Zhang L, Small PA Jr: Transgenicmice lacking class I major histocompatibility complex-restricted T cells have delayed viral clearance and increasedmortality after influenza virus challenge. J Exp Med1992;175:1143–1145.

102. Graham MB and Braciale TJ: Resistance to and recoveryfrom lethal influenza virus infection in B lymphocyte-deficient mice. J Exp Med 1997;186:2063–2068.

103. You D, Ripple M, Balakrishna S, et al.: Inchoate CD8þ T cellresponses in neonatal mice permit influenza-induced per-sistent pulmonary dysfunction. J Immunol 2008;181:3486–3494.

104. Stoll S, Delon J, Brotz TM, and Germain RN: Dynamicimaging of T cell-dendritic cell interactions in lymph nodes.Science 2002;296:1873–1876.

105. Carding SR, Allan W, McMickle A, and Doherty PC: Ac-tivation of cytokine genes in T cells during primary andsecondary murine influenza pneumonia. J Exp Med 1993;177:475–482.

106. Lawrence CW, and Braciale TJ: Activation, differentiation,and migration of naive virus-specific CD8þ T cells duringpulmonary influenza virus infection. J Immunol 2004;173:1209–1218.

107. Roman E, Miller E, Harmsen A, et al.: CD4 effector T cellsubsets in the response to influenza: heterogeneity, migra-tion, and function. J Exp Med 2002;196:957–968.

108. Moore KW, de Waal Malefyt R, Coffman RL, and O’GarraA: Interleukin-10 and the interleukin-10 receptor. AnnuRev Immunol 2001;19:683–765.

109. Sun J, Madan R, Karp Cl, and Braciale TJ: Effector T cellscontrol lung inflammation during acute influenza virusinfection by producing IL-10. Nat Med 2009;15:277–284.

110. Klotz L, Hucke S, Thimm D, et al.: Increased antigen cross-presentation but impaired cross-priming after activation of

peroxisome proliferator-activated receptor gamma is me-diated by up-regulation of B7H1. J Immunol 2009;183:129–136.

111. Tate MD, Brooks AG, and Reading PC: The role of neu-trophils in the upper and lower respiratory tract duringinfluenza virus infection of mice. Respir Res 2008;9:57.

112. Colamussi ML, White MR, Crouch E, and Hartshorn KL:Influenza A virus accelerates neutrophil apoptosis andmarkedly potentiates apoptotic effects of bacteria. Blood1999;93:2395–2403.

113. Wareing MD, Shea AL, Inglis CA, et al.: CXCR2 is requiredfor neutrophil recruitment to the lung during influenzavirus infection, but is not essential for viral clearance. ViralImmunol 2007;20:369–378.

114. Tate MD, Deng YM, Jones JE, et al.: Neutrophils amelioratelung injury and the development of severe disease duringinfluenza infection. J Immunol 2009;183:7441–7450.

115. Reddy RC, Narala VR, Keshamouni VG, et al.: Sepsis-induced inhibition of neutrophil chemotaxis is mediated byactivation of peroxisome proliferator-activated receptor-{gamma}. Blood 2008;112:4250–4258.

116. Kim MS, Sweeney TR, Shigenaga JK, et al.: Tumor necrosisfactor and interleukin 1 decrease RXRalpha, PPARalpha,PPARgamma, LXRalpha, and the coactivators SRC-1, PGC-1alpha, and PGC-1beta in liver cells. Metabolism 2007;56:267–279.

117. Weinberger B, Quizon C, Vetrano AM, et al.: Mechanismsmediating reduced responsiveness of neonatal neutrophilsto lipoxin A4. Pediatr Res 2008;64:393–398.

118. Chima RS, Hake PW, Piraino G, et al.: Ciglitazone amelio-rates lung inflammation by modulating the inhibitor kap-paB protein kinase/nuclear factor-kappaB pathway afterhemorrhagic shock. Crit Care Med 2008;36:2849–2857.

119. Nicholson S, Bonecini-Almeida Mda G, Lapa e Silva JR,et al.: Inducible nitric oxide synthase in pulmonary alveolarmacrophages from patients with tuberculosis. J Exp Med1996;183:2293–2302.

120. Brightbill HD, Libraty DH, Krutzik SR, et al.: Host defensemechanisms triggered by microbial lipoproteins throughtoll-like receptors. Science 1999;285:732–736.

121. Baskin CR, Bielefeldt-Ohmann H, Tumpey TM, et al.: Earlyand sustained innate immune response defines pathologyand death in nonhuman primates infected by highlypathogenic influenza virus. Proc Natl Acad Sci USA 2009;106:3455–3460.

Address correspondence to:Dr. Raquel Hontecillas

Nutritional Immunology and Molecular Medicine LaboratoryVirginia Bioinformatics Institute

Virginia Polytechnic Institute and State UniversityBlacksburg, VA 24061

E-mail: rmagarzo@vt.edu

Received January 26, 2010; accepted May 5, 2010.

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