Platelets in rheumatoid arthritis:exploring the anti-inflammatory andantithrombotic potential of TNFinhibitorsArmen Yuri Gasparyan,1 George D Kitas1,2

Manfredi et al1 explore the relationshipbetween tumour necrosis factor α (TNFα),TNFα inhibition and platelet activation inrheumatoid arthritis (RA). Using elegantsystems they demonstrate that TNFα acti-vates platelets and promotes their proin-flammatory and procoagulant actions, whileTNF inhibition (TNFi) prevents plateletactivation. They suggest this is a potentialmechanism explaining why TNFi associateswith a reduction of cardiovascular (CV)events in patients with RA, as shown insome observational studies. Should we thenbe(come) interested in platelet biology inRA?The potential role of platelets in the

pathogenesis of RA and related comorbid-ities has, in fact, received much less attentionthan it deserves. Platelets are small (1–2 μm)anucleate cells that are intimately involvedin thrombosis, angiogenesis, bone remodel-ling, inflammation and autoimmunity. Theirbidirectional, damaging and protective rolesacross pathophysiological processes havebeen explored in several clinical and labora-tory studies. The discovery of specific plate-let markers and agonists, which target arange of membrane-bound receptors, hasresulted in better understanding of theirrole. It is now clear that platelet reactivitythroughout their short lifespan (8–10 days)is determined by megakaryopoiesis, which isregulated quantitatively by thrombopoietin;any chemical compound targeting the mat-uration of platelet precursors in the bonemarrow either directly or indirectly can alterthe thrombotic, immune and inflammatorypotential of circulating platelets.2 3

Activated platelets shed highly activemembranous structures—microparticles—and transform from disc-shaped toenlarged cells with pseudopods thatfacilitate their interaction with other

platelets, neutrophils, lymphocytes andother immune cells. Circulating neutro-phils, in turn, interact with activated plate-lets to boost their ‘thromboinflammatory’potential.4 Circulating cellular complexesof platelets are often found in the bloodstream of patients with inflammatory condi-tions, such as RA, and it is suggested thatplatelet-derived microparticles along withother biological agents play a more import-ant role in the development of synovitisthan activated platelets themselves.5 Indeed,laboratory studies have demonstrated thatplatelets exert arthritogenic propertiesthrough the activation of the collagenglycoprotein VI receptor and subsequentshedding of microparticles.6

As effector cells derived from megakaryo-cytes, platelets bear a wide variety of mem-brane receptors of their precursors,autonomous protein-synthesising platform(messenger RNA) and the granular systemof accumulated biologically active com-pounds (ie, α granules, dense granules, lyso-somes) facilitating their involvement indiverse immune reactions. α Granules areespecially rich in inflammatory cytokines,vasoactive substances, chemokines and sti-mulators of platelet aggregation, which areabundantly released upon platelet activa-tion. Biological agents of platelet granulespropagate inflammation, increase vascularpermeability and destroy cells at inflamedsites. Such processes could be involved inthe initiation and propagation of the inflam-matory response and tissue destruction bothin the rheumatoid joint and the vasculatureand merit further investigation in RA. Atexactly the opposite end, experimental andclinical studies have shown that some com-ponents of autologous platelet-rich plasma,and particularly ADP of dense granules, arecapable of stimulating proliferation of chon-drocytes and deposition of collagen I,thereby repairing damaged cartilage andtendons.7 8 There is, therefore, a distinctpossibility of regulating various patho-physiological processes in joints and sur-rounding tissues by suppressing andactivating platelets at different stages of thedisease.Avariety of specific markers of platelet acti-

vation have been tested across inflammatory

rheumatic diseases: platelet count, membrane-bound P-selectin and CD40 ligand (CD40L),β-thromboglobulin, platelet factor 4 (PF4),also known as chemokine (C-X-C motif)ligand 4 (CXCL4), platelet aggregates andplatelet–neutrophil complexes.5 Reactivethrombocytosis with elevation of plateletcount has been suggested to be of diagnosticimportance in the setting of antineutrophilcytoplasmic antibodies-associated vasculitis.9

Mildly elevated platelet count along withdecreased platelet size have been viewed as areflection of inflammatory megakaryopoiesisand activity of RA,10 while the ratio of plate-lets to neutrophils and lymphocytes has alsobeen suggested to reflect rheumatoid activ-ity.11 A recently emerged important piece ofevidence points to the fact that anticitrulli-nated protein antibodies (ACPA) directlystimulate the expression of P-selectin on plate-lets, secretion of soluble CD40L and forma-tion of platelet aggregates.12 With theincremental increase of ACPA preceding clin-ically manifest RA by months and years, it ispossible that insidious seropositivity andincrease of C-reactive protein determine plate-let activation and involvement from the earli-est stages of synovial inflammation onwards.

In the era of biological therapy severallines of evidence suggest that agents inhibit-ing TNFα and interleukin-6 suppress diseaseactivity and decrease platelet counts inRA.13 14 The effect of biologic disease-modifying antirheumatic drugs (bDMARDs)on platelet counts is not matched by non-biologic DMARDs, which appear to be lesseffective towards megakaryopoiesis. Theexception is bone marrow suppression andrelated thrombocytopenia as a rare occur-rence of sulfasalazine therapy,5 whichhowever is no longer a frontline choice inRA. Interestingly, the possible effects ofbDMARDs on megakaryopoiesis may differ,with reports suggesting that tocilizumab maydrop platelet counts in patients with RAmore potently than adalimumab.15 This isanother area that requires further investiga-tion, as a potential side effect and also as apossible desirable effect. IL-6 is known as aphysiological regulator of megakaryopoiesisacting likes thrombopoietin,16 17 and block-ing IL-6-related pathways may lead to anti-inflammatory and antithrombotic effects.

The present flow cytometric and electronmicroscopy study by Manfredi et al,1 shedslight on the molecular basis of modulationof platelet activity by inflammatory cyto-kines and their inhibitors. The authorsshould be congratulated for their extensivework characterising the effects of recombin-ant human TNFα and the TNFi, infliximab,on platelet-rich plasma and isolated plate-lets of patients with RA (box 1).

One of the important messages of thisstudy is that TNFi can only block the

1Departments of Rheumatology and Research andDevelopment, Dudley Group NHS Foundation Trust,Russells Hall Hospital, Dudley, UK; 2Arthritis ResearchUK Epidemiology Unit, University of Manchester,Manchester, UK

Correspondence to Professor George D Kitas,Research and Development Directorate, Dudley GroupNHS Foundation Trust, Russells Hall Hospital, DudleyDY1 2HQ, UK; kitas@dgh.nhs.uk

Gasparyan AY, Kitas GD. Ann Rheum Dis Month 2016 Vol 0 No 0 1

Editorial ARD Online First, published on April 4, 2016 as 10.1136/annrheumdis-2015-208720

Copyright Article author (or their employer) 2016. Produced by BMJ Publishing Group Ltd (& EULAR) under licence.

on 26 April 2018 by guest. P

rotected by copyright.http://ard.bm

j.com/

Ann R

heum D

is: first published as 10.1136/annrheumdis-2015-208720 on 4 A

pril 2016. Dow

nloaded from

TNFα-dependent activation pathways ofplatelets. Other pathways of platelet activa-tion, particularly those regulated by collagenand ADP, can still continue contributing toproinflammatory and prothromboticactions. Although platelet count was notanalysed in connection with flow cytometricmarkers, and this can be viewed as a limita-tion, it is plausible that TNFi exert theireffects on platelet receptors and also onplatelet precursors maturing throughoutmegakaryopoiesis.The authors have concentrated almost

exclusively on the concept of CV comorbid-ity and its reduction in RA. However, themechanisms they have described may be asimportant in the very essence of the disease,joint inflammation per se. It is, therefore,slightly disappointing not to see the wealthof data generated, analysed in the contextof disease activity, using well-establishedclinical and laboratory indices.To return to the original thesis: has this

study answered the question as to whetherinhibition of platelet activation by TNFitranslates into CV protection in RA? As theauthors themselves point out it has not, dueto evident design limitations such as itscross-sectional nature, small sample size,lack of long-term longitudinal follow-upand inability to control for multiple poten-tial confounders (box 1). The study is agood reference for future studies aiming toaddress CV risk reduction in RA. The CVimplications of antirheumatic therapies havegained almost as much importance as theirprimary efficacy and safety targets butstudies to address them are extremely diffi-cult to conduct.18 Very large numbers, lon-gitudinal design, the ability to control formultiple CV and other confounders thatmay be changing over time, and very

specific, ideally hard endpoints are requiredas a minimum. Potential difficulties shouldbe foreseen and accounted for. For example,longitudinal studies addressing plateletbiology may be limited by the lack of repro-ducibility of specific laboratory tests over along term (several months–years), the use ofdrugs required for other indications, andselection bias of the patients recruited com-pared with real-life patient groups (eg, TNFiare not used often in patients with heartfailure, the clinical setting where mostthrombotic events accumulate).In summary, the study by Manfredi et al,1

together with evidence from other sources,demonstrate clearly that the axis of megakar-yopoiesis, circulating platelets and plateletcellular complexes, and its modulation byantirheumatic therapies, may be very import-ant in the pathogenic mechanisms ofrheumatoid synovitis and related CVcomorbidity. Despite the many challenges, itmerits much more research attention than ithas been receiving thus far.

Contributors AYG wrote the first draft of themanuscript. GDK revised all parts, added severalsections. Both authors approved the final version.

Competing interests None declared.

Provenance and peer review Not commissioned;externally peer reviewed.

To cite Gasparyan AY, Kitas GD. Ann Rheum DisPublished Online First: [please include Day Month Year]doi:10.1136/annrheumdis-2015-208720

Received 24 February 2016Revised 10 March 2016Accepted 19 March 2016

▸ http://dx.doi.org/10.1136/annrheumdis-2015-208442

Ann Rheum Dis 2016;0:1–2.doi:10.1136/annrheumdis-2015-208720

REFERENCES1 Manfredi AA, Baldini M, Camera M, et al. Anti-TNFα

agents curb platelet activation in patients withrheumatoid arthritis. Ann Rheum Dis 2016;Published Online First 27 Jan 2016. doi:10.1136/annrheumdis-2015-208442

2 Liu J, DeNofrio J, Yuan W, et al. Geneticmanipulation of megakaryocytes to study plateletfunction. Curr Top Dev Biol 2008;80:311–35.

3 Lambert MP, Meng R, Xiao L, et al. Intramedullarymegakaryocytes internalize released platelet factor 4and store it in alpha granules. J Thromb Haemost2015;13:1888–99.

4 Sreeramkumar V, Adrover JM, Ballesteros I, et al.Neutrophils scan for activated platelets to initiateinflammation. Science 2014;346:1234–8.

5 Gasparyan AY, Stavropoulos-Kalinoglou A,Mikhailidis DP, et al. Platelet function in rheumatoidarthritis: arthritic and cardiovascular implications.Rheumatol Int 2011;31:153–64.

6 Boilard E, Nigrovic PA, Larabee K, et al. Plateletsamplify inflammation in arthritis via collagen-dependentmicroparticle production. Science 2010;327:580–3.

7 Zhou Q, Xu C, Cheng X, et al. Platelets promotecartilage repair and chondrocyte proliferation via ADPin a rodent model of osteoarthritis. Platelets 2015;Published Online First 1 Sep 2015. doi:10.3109/09537104.2015.1075493.

8 Alsousou J, Thompson M, Harrison P, et al. Effect ofplatelet-rich plasma on healing tissues in acute rupturedAchilles tendon: a human immunohistochemistry study.Lancet 2015;385(Suppl 1):S19.

9 Willeke P, Kümpers P, Schlüter B, et al. Plateletcounts as a biomarker in ANCA-associated vasculitis.Scand J Rheumatol 2015;44:302–8.

10 Gasparyan AY, Ayvazyan L, Mikhailidis DP, et al.Mean platelet volume: a link between thrombosisand inflammation? Curr Pharm Des 2011;17:47–58.

11 Fu H, Qin B, Hu Z, et al. Neutrophil- and platelet-to-lymphocyte ratios are correlated with disease activityin rheumatoid arthritis. Clin Lab 2015;61:269–73.

12 Habets KL, Trouw LA, Levarht EW, et al. Anti-citrullinated protein antibodies contribute to plateletactivation in rheumatoid arthritis. Arthritis Res Ther2015;17:209.

13 Weinblatt ME, Keystone EC, Furst DE, et al.Adalimumab, a fully human anti-tumor necrosisfactor alpha monoclonal antibody, for the treatmentof rheumatoid arthritis in patients taking concomitantmethotrexate: the ARMADA trial. Arthritis Rheum2003;48:35–45.

14 Matsuno H. Remarkable efficacy of tocilizumabfor treating rheumatoid arthritis in patients withhigh platelet counts. Mod Rheumatol 2015;25:38–42.

15 Gabay C, Emery P, van Vollenhoven R, et al.Tocilizumab monotherapy versus adalimumabmonotherapy for treatment of rheumatoid arthritis(ADACTA): a randomised, double-blind, controlledphase 4 trial. Lancet 2013;381:1541–50.

16 Zeidler C, Kanz L, Hurkuck F, et al. In vivo effectsof interleukin-6 on thrombopoiesis in healthyand irradiated primates. Blood 1992;80:2740–5.

17 Lemancewicz D, Bolkun L, Mantur M, et al. Bonemarrow megakaryocytes, soluble P-selectin andthrombopoietic cytokines in multiple myelomapatients. Platelets 2014;25:181–7.

18 Nurmohamed MT, Heslinga M, Kitas GD.Cardiovascular comorbidity in rheumaticdiseases. Nat Rev Rheumatol 2015;11:693–704.

Box 1 New knowledge and limitations of the study by Manfredi et al1

Important new data▸ Tumour necrosis factor α (TNFα) depletes α granules of rheumatoid platelets (TNFα

receptor 1-mediated effect).▸ TNFα leads to the expression of P-selectin, CD40L, tissue factor, fibrinogen binding

to rheumatoid platelets and forming platelet–neutrophil complexes (TNFα receptor2-mediated effect).

▸ Infliximab abrogates TNFα-specific platelet-activating effects but does not affectseveral other activation pathways in rheumatoid platelets.

▸ Platelets in rheumatoid arthritis (RA) are more activated than in coronary arterydisease and osteoarthritis.

Limitations▸ The relationships between platelet markers and cardiovascular risk/comorbidities in

RA were not explored.▸ Platelet count, a proxy of inflammatory megakaryopoiesis, was not analysed in

relation to flow cytometric markers of platelet activation in RA.

2 Gasparyan AY, Kitas GD. Ann Rheum Dis Month 2016 Vol 0 No 0

Editorial on 26 A

pril 2018 by guest. Protected by copyright.

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/annrheum

dis-2015-208720 on 4 April 2016. D

ownloaded from