COMMENTARY

Soothing touch of CD31 protectsendothelium during cellularimmune responsesOlivier Thaunata,b,1aInternational Center for Infectiology Research, INSERM Unit 1111, Claude Bernard Lyon1 University, Ecole Normale Supérieure de Lyon, CNRS UMR 5308, 69365 Lyon Cedex 07,France; and bTransplantation, Nephrology and Clinical Immunology Department, EdouardHerriot Hospital, 69003 Lyon, France

CD31 is a member of the Ig gene superfamilythat is present on the surface of plateletsand all leukocytes (1). CD31 is also highlyexpressed on endothelial cells, where it rep-resents a major constituent of the intercellular

junction (1). Based on this cellular distribu-tion and similarities in the cDNA sequencecoding for extracellular domains, CD31 hasinitially been considered as a member“among others” of the large family of cell

adhesion molecules (1). A striking illustrationof this lies in the first name that the moleculewas given: platelet endothelial cell adhesionmolecule-1 (PECAM-1). However, the dem-onstration that the CD31 cytoplasmic tailcontains consensus sequences typical ofimmunoreceptor tyrosine-based inhibitorymotifs (ITIMs) has led to a reconsiderationof CD31 biological functions (1). ITIM indeedcontains tyrosine residues, the phosphoryla-tion of which creates specific docking sitesfor Src-homology 2 domain-containing intra-cellular protein-tyrosine phosphatases. Thesecatalytic enzymes, once localized to their cy-toplasmic anchors and activated, are then ableto inhibit tyrosine kinase-mediated signaling,and thus cellular activation (1). Building onthese data, Cheung et al. (2) present in PNASanother advance in the understanding of theregulatory roles of CD31. Their study demon-strates that CD31 signaling through ITIM isnecessary to prevent inflammatory-inducedendothelial cell death.Maintenance of vascular integrity during

effector immune responses is a major chal-lenge in the cooperation between the immuneand the vascular systems. Endothelial cells areindeed essential to prevent the activation ofthe coagulation cascade, which would resultin ischemic necrosis of the tissue. However,immune effectors that are activated in second-ary lymphoid organs must interact with theendothelium to transit out of the bloodstreamand access the antigen-rich sites, where theimmune response is needed. Constitutive re-sistance of endothelial cells to cell deathinduced by inflammatory insults is thereforecritical for “healthy” immune responses. This“immune privilege” of endothelial cells is wellillustrated in transplantation. Because the do-nor is from the same species but geneticallydifferent, the recipient’s adaptive immunesystem recognizes donor-specific alloantigensexpressed by the graft. Immune response thatdevelops against alloantigens leads in turn to

Fig. 1. T-cell– and antibody-mediated rejection of kidney allograft. (A) Typical features of T-cell–mediated rejectioninclude infiltration of graft interstitium by recipient’s mononuclear cells, which attack donor’s tubular epithelial cells(black arrows). Despite the fact that trafficking of recipient’s mononuclear cells into the interstitium require interac-tions with graft endothelial cells, the latter remain unharmed both in the macrovessels and the microvessels.(B) Pattern of expression of CD31 in a kidney allograft undergoing T-cell–mediated rejection. CD31 is detected onrecipient’s mononuclear cells, and donor’s endothelial cells, from both the microvessels and the macrovessels.(C ) Typical features of vascular chronic antibody-mediated rejection, also known as “graft arteriosclerosis.” Black arrowshows the thickness of the neointima. (D) The binding of alloantibodies to graft endothelial cells triggers classicalcomplement pathway, which results in C4d deposition in graft microvessels and macrovessels. (Scale bar: 100 μm.)Abbreviations: a, artery; g, glomeruli; ptc, peritubular capillaries; t, tubules.

Author contributions: O.T. wrote the paper.

The author declares no conflict of interest.

See companion article on page E5815.

1Email: olivier.thaunat@chu-lyon.fr.

www.pnas.org/cgi/doi/10.1073/pnas.1517580112 PNAS | October 27, 2015 | vol. 112 | no. 43 | 13133–13134

COMMEN

TARY

the destruction of the transplanted organ, aprocess named “rejection.” Interestingly, dur-ing rejection, recipients’ effector T cells crossthe donor capillary endothelium without kill-ing the endothelial cells (Fig. 1A) (3). In con-trast, graft epithelial cells are highly susceptibleto T-cell–mediated destruction (Fig. 1A).One mechanism by which endothelial cells

may escape the destructive action of cytotoxicT cells is related to antigen presentation. Theoutcome of an interaction between cytotoxicT lymphocytes and target cells is partlydetermined by the amount of antigen pre-sented by the target cells. Although endothe-lial cells are endowed with antigen-presentingabilities, they express lower levels of immu-nodominant epitope–MHC complexes ontheir surface than leukocytes or epithelial cells(4). By presenting a different antigenic reper-toire compared with other cell types, endo-thelial cells may escape an attack by effectorcytotoxic T lymphocytes, which have beeneducated by professional antigen-presentingcells and are therefore specific for immuno-dominant epitopes. This mechanism is, how-ever, unlikely to be the only one involvedbecause several studies have previously docu-mented that triggering endothelial cell deathreceptors does not efficiently activate the ex-trinsic pathway of apoptosis (5).Combining different approaches, Cheung

et al. (2) demonstrate that exposure of endo-thelial cells to various inflammatory stimuliin vitro, including TNFα and cytotoxic Tlymphocytes, resulted in the phosphorylationof the ITIMs of CD31, which counteractedthe activation of the extrinsic pathway of ap-optosis via the activation of the Erk/Akt path-way. Akt activation by CD31 signaling inendothelial cells prevented the localizationof the forkhead transcription factor FoxO3to the nucleus, thus inhibiting the transcrip-tion of the proapoptotic genes and derepress-ing expression of the antiapoptotic gene cFlar(2). This study therefore provides importantnovel insights on the molecular mechanismswhereby the vascular endothelium remainsundamaged while interacting with effectorimmune cells migrating to the site of theresponse. Noteworthy, in contrast with en-dothelial cells, epithelial cells lack CD31expression (Fig. 1B), which could accountfor their high susceptibility to T-cell–medi-ated destruction (Fig. 1A). In line with thishypothesis, the authors showed that CD31gene transfer was sufficient to recapitulatethe cytoprotective mechanisms in CD31-neg-ative pancreatic beta cells, which becameresistant to T-cell–mediated rejection whengrafted in fully allogeneic recipients (2).CD31 immunoregulatory role is not limited

to endothelial cells. T cells and antigen-

presenting cells both express CD31, and trans-homophilic interactions between T cells anddendritic cells (DCs) play a crucial nonredun-dant role in defining the size of the ensuingimmune response. The same group demon-strated a few years ago that triggering CD31signaling in T cells during their priming,resulted in partial inhibition of proximal T-cellreceptor signaling, which regulated primaryclonal expansion, and acquisition of killingcapacity or regulatory functions (6). Further-more, an independent research group has re-cently reported that CD31 is also a keycoinhibitory receptor on DCs. Indeed, the dis-ruption of CD31 signaling favored the immu-nogenic maturation and migration of residentDCs to the draining lymph nodes (7). In con-trast, triggering CD31 signaling during DCmaturation resulted in reduced expression ofcostimulatory molecules and production of in-flammatory cytokines, whereas the expressionof regulatory cytokines TGF-β and IL-10 wasincreased (7).Based on these findings, which demonstrate

that CD31 signaling plays key nonredundantroles in pacifying cell–cell interactions at thevessel/blood interface under conditions of im-munological stress, it was tempting to specu-late that CD31 pathway could represent anattractive therapeutic target for immune-mediated diseases targeting the vasculature.Atherosclerosis, the principal cause of myocar-dial infarction and stroke, results from the in-filtration of lipoproteins in the intimal spaceand the development of a pathogenic cellularimmune response to modified lipoproteinswithin the arterial wall (8). Interestingly,it has been shown that genetic ablation ofCD31 enhanced lesion formation in ex-perimental atherosclerosis (9). Conversely,treating atherosclerosis-prone mice with afusion protein composed of the extracellularportion of CD31 fused to the constant partof IgG1 (CD31Rg), reduced the size ofatherosclerotic lesions (10). Furthermore,

CD31Rg treatment correlated with morecirculating regulatory T cells, blunted bloodT-cell activation, and reduced T-cell infil-tration within plaques (10).So, is CD31 signaling the panacea for en-

dothelial cells? Although graft endothelialcells remain relatively unharmed by recipi-ent’s cytotoxic T cells, most of transplantedorgans develop with time vascular chronicrejection lesions, also known as “graft arterio-sclerosis” (Fig. 1C). Recipient’s immune re-sponse against the graft is indeed not limitedto the generation of cellular effectors but alsocomprises the production of alloantibodies(11, 12). A seminal work by Russell, Colvin,and coworkers (13) demonstrated more than20 y ago that the passive transfer of alloanti-bodies was sufficient to promote the develop-ment of graft arteriosclerosis. Binding ofalloantibodies to directly accessible allogeneictargets expressed by graft endothelial cellstriggers the classical complement pathway,as witnessed by the deposition of C4d in graftmicrovasculature (Fig. 1D). This leads to therelease of adhesion molecules and cytokinesthat in turn recruit innate immune effectors(11, 12). Chronic vascular inflammation pro-motes the development of typical histologicallesions and irreversible loss of graft function(11, 12). Interestingly, Cheung et al. (2) showin their study that CD31-mediated cytopro-tection did not extend to mechanisms of celldeath other than apoptosis: CD31 KO andCD31 WT endothelial cells were equally sen-sitive to antibody-mediated cell death.Therefore, if the soothing touch of CD31

appears necessary to protect endothelial cellsduring their interaction with cellular effec-tors, the role of this pathway in the initiationand/or the effector phase of antibody-medi-ated vascular injuries is less evident and war-rants further investigations.

ACKNOWLEDGMENTS. I am grateful to Maud Rabeyrinfor providing the histological pictures.

1 Newman PJ, Newman DK (2003) Signal transduction pathwaysmediated by PECAM-1: New roles for an old molecule in plateletand vascular cell biology. Arterioscler Thromb Vasc Biol 23(6):953–964.2 Cheung K, et al. (2015) CD31 signals confer immune privilegeto the vascular endothelium. Proc Natl Acad Sci USA112:E5815–E5824.3 Halloran PF (2010) T cell-mediated rejection of kidneytransplants: A personal viewpoint. Am J Transplant 10(5):1126–1134.4 Kummer M, Lev A, Reiter Y, Biedermann BC (2005) Vascularendothelial cells have impaired capacity to present immunodominant,antigenic peptides: A mechanism of cell type-specific immuneescape. J Immunol 174(4):1947–1953.5 Richardson BC, Lalwani ND, Johnson KJ, Marks RM (1994) Fasligation triggers apoptosis in macrophages but not endothelial cells.Eur J Immunol 24(11):2640–2645.6 Ma L, et al. (2010) Ig gene-like molecule CD31 plays anonredundant role in the regulation of T-cell immunity and tolerance.Proc Natl Acad Sci USA 107(45):19461–19466.

7 Clement M, et al. (2014) CD31 is a key coinhibitory receptor in thedevelopment of immunogenic dendritic cells. Proc Natl Acad Sci USA111(12):E1101–E1110.8 Hansson GK (2005) Inflammation, atherosclerosis, and coronaryartery disease. N Engl J Med 352(16):1685–1695.9 Goel R, et al. (2008) Site-specific effects of PECAM-1 onatherosclerosis in LDL receptor-deficient mice. Arterioscler ThrombVasc Biol 28(11):1996–2002.10 Groyer E, et al. (2007) Atheroprotective effect of CD31 receptorglobulin through enrichment of circulating regulatory T-cells. J AmColl Cardiol 50(4):344–350.11 Pouliquen E, et al. (2015) Recent advances in renaltransplantation: Antibody-mediated rejection takes center stage.F1000Prime Rep 7:51.12 Thaunat O (2012) Humoral immunity in chronic allograftrejection: Puzzle pieces come together. Transpl Immunol 26(2-3):101–106.13 Russell PS, Chase CM, Winn HJ, Colvin RB (1994) Coronaryatherosclerosis in transplanted mouse hearts. II. Importance ofhumoral immunity. J Immunol 152(10):5135–5141.

13134 | www.pnas.org/cgi/doi/10.1073/pnas.1517580112 Thaunat