Physical proximity and functional interplay of PECAM-1 with the Fc receptor FcγRIIa on the platelet plasma membrane

We and others have recently defined that (P) under bar latelet (E) under bar ndothelial (C) under bar ell (A) under bar dhesion (M) under bar olecule-11 (PECAM-1/CD31) functions as a negative regulator of platelet-collagen interactions involving the glycoprotein VI/Fc: receptor gamma chain (GPVI/FcR-gamma chain) signaling pathway.(1,2) In this study, we hypothesized that PECAM-11 may be physically and functionally associated with FcgammaRIIa on the platelet membrane. The functional relationship between PECAM-1 and FcgammaRIIa was assessed by determining the effect of anti-PECAM-1 monoclonal antibody Fab fragments on FcgammaRlla-mediated platelet aggregation and heparin-induced thrombocytopenia (HITS)mediated platelet aggregation. Preincubation of washed platelets with monoclonal antibody fragments of 213134 directed against PECAM-1 and IV.3 directed against FcgammaRIIa completely blocked FcgammaRIIa-mediated platelet aggregation and HITS-mediated platelet aggregation, whereas anti-CD151 antibody had no blocking effect. Coengagement of FcgammaRIIa and PECAM-11 resulted in negative regulation of FcgammaRIIa-mediated phospholipase Cgamma2 activation, calcium mobilization, and phosphoinositide-3-kinase-dependent signaling pathways. In addition, the physical proximity of FcgammaRIIa and PECAM-1 was confirmed by using fluorescence resonance energy transfer and colmmunoprecipitation studies These results indicate that PECAM-1 and FcgammaRIIa are colocalized on the platelet membrane and PECAM-11 down-regulates FcgammaRIIa-medicated platelet responses.