Effect of the Pl(A2) alloantigen on the function of beta(3)-integrins in platelets.

The polymorphism responsible for the Pl(A2) alloantigen on the beta(3)-component of beta(3)-containing integrins is reported to be a risk factor for coronary thrombosis. This study examined the effect of Pl(A2) on the function of beta(3)-integrins using platelets from subjects homozygous and heterozygous for Pl(A1) and Pl(A2). There was overlap in the distribution of the dissociation constant (K(d)) and maximum fibrinogen binding (B(max)) values for fibrinogen binding to alpha(IIb)beta(3) on platelets from Pl(A1) and Pl(A2) homozygotes and Pl(A1)/Pl(A2) heterozygotes. However, whereas there was no statistical difference in these values for the Pl(A1) homozygotes and Pl(A2) heterozygotes, the K(d) for the Pl(A2) homozygotes was significantly lower than that for the Pl(A1)/Pl(A2) heterozygotes, but was not statistically different from that for the Pl(A1) homozygotes. No differences were detected in ADP sensitivity between platelets from Pl(A1) homozygotes and Pl(A1)/Pl(A2) heterozygotes, in the IC(50) for RGDS inhibition of fibrinogen binding to alpha(IIb)beta(3), in the alpha(v)beta(3)-mediated adhesion of platelets to osteopontin and vitronectin, and in the phorbol ester-stimulated adhesion to fibrinogen of B lymphocytes expressing alpha(IIb)beta(3) containing either the Pl(A1) or the Pl(A2) polymorphism. Finally, no differential effects of Pl(A2) on turbidometric platelet aggregation, platelet secretion, or platelet thrombus formation were found as measured in the PFA-100. Because no differences were detected in the ability of beta(3)-integrins to interact with ligands based on the presence or absence of the Pl(A2) polymorphism, the results suggest that factors unrelated to beta(3)-integrin function may account for the reported association of the Pl(A2) allele with coronary thrombosis.