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

We and others have recently defined that Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1/CD31) functions as a negative regulator of platelet-collagen interactions involving the glycoprotein VI/Fc receptor gamma chain (GPVI/FcR– chain) signaling pathway.1,2 In this study, we hypothesized that PECAM-1 may be physically and functionally associated with Fc RIIa on the platelet membrane. The functional relationship between PECAM-1 and Fc RIIa was assessed by determining the effect of anti–PECAM-1 monoclonal antibody Fab fragments on Fc RIIa-mediated platelet aggregation and heparin-induced thrombocytopenia (HITS)– mediated platelet aggregation. Preincubation of washed platelets with monoclonal antibody fragments of 2BD4 directed against PECAM-1 and IV.3 directed against Fc RIIa completely blocked Fc RIIa-mediated platelet aggregation and HITS-mediated platelet aggregation, whereas antiCD151 antibody had no blocking effect. Coengagement of Fc RIIa and PECAM-1 resulted in negative regulation of Fc RIIamediated phospholipase C 2 activation, calcium mobilization, and phosphoinositide 3-kinase–dependent signaling pathways. In addition, the physical proximity of Fc RIIa and PECAM-1 was confirmed by using fluorescence resonance energy transfer and coimmunoprecipitation studies. These results indicate that PECAM-1 and Fc RIIa are colocalized on the platelet membrane and PECAM-1 down-regulates Fc RIIa-mediated platelet responses. (Blood. 2003;102:3637-3645)