Platelet FcgammaRIIA His131Arg polymorphism and platelet function: antibodies to platelet-bound fibrinogen induce platelet activation.

Summary.  The His131Arg polymorphism of platelet FcγRIIA affects the binding affinity of certain IgG subclasses. The Arg131 allele has been associated with (auto)immune thrombocytopenia and heparin-induced thrombocytopenia in some studies. Because FcγRIIA can transmit platelet activation signals, we studied platelet responsiveness from 73 healthy donors to determine if this polymorphism modulated platelet function. Platelet function was studied by agonist and shear-induced activation, and standard aggregation. FcγRIIA was genotyped by allele-specific PCR. Compared with His131, the Arg131 allele was associated with significantly greater binding of activation-dependent antibodies. This effect was most prominent for the receptor-induced binding site (RIBS) antibodies F26 (P < 0.0001) and RIBS1 (P = 0.0057), and the ligand-induced binding site antibody LIBS1 (P = 0.0367). Unexpectedly, Arg131-positive platelets did not show greater fibrinogen binding, platelet aggregation or shear-induced platelet activation. We considered whether enhanced Fc binding and FcγRIIA cross-linking were responsible for those discrepancies. The increased binding of the two RIBS antibodies to the Arg131 isoform was abolished by blocking FcγRIIA, and the FcγRIIA genotype effect on F26 IgG binding was lost when F26 F(ab′)2 fragments were used. Furthermore, intact F26 and RIBS1 IgG directly and specifically induced P-selectin expression, and this effect was greatest in Arg131-positive platelets. We concluded that (a) the His131Arg polymorphism of FcγRIIA does not affect intrinsic platelet reactivity; (b) RIBS antibodies are able to cross-link FcγRIIA and activate platelets, and this activation has a modest effect on Arg131 platelets; and (c) flow cytometric based platelet assays may need to compensate for this FcγRIIA His131Arg effect on platelet activation.