Topographical Association of the Platelet Fc-receptor with the Glycoprotein IIb-IIIa Complex

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Abstract
In this study, we have examined whether the platelet Fc-receptor, FcγRII (CD32), is associated with either of the two major platelet membrane glycoproteins, the GPIb-IX complex and the GPIIb-IIIa complex. Monoclonal and polyclonal anti-GPIb-IX complex antibodies inhibited to only a moderate degree (< 40%) the binding of the anti-FcγRII monoclonal antibody, IV.3, to platelets. In contrast, 6 of 12 anti-GPIIb-IIIa monoclonal antibodies and a polyclonal, affinity-purified rabbit anti-GPIIb-IIIa antibody strongly cross-blocked the binding of IV.3 to platelets. This inhibition was dependent upon the Fab-mediated binding of these antibodies to the GPIIb-IIIa complex since they did not inhibit the binding of IV.3 to Glanzmann's thrombasthenic platelets which have normal levels of FcγRII but lack the GPIIb-IIIa complex. The anti-GPIIb-IIIa monoclonal antibodies, AP3 and VM16a, had no effect on platelet aggregation induced by ADP or thrombin but inhibited Fc-receptor-dependent platelet aggregation as induced by either acetone-aggregated human IgG or by activating monoclonal antibodies against GPIV, PTA1 or CD9. F(ab')2 fragments of these two anti-GPIIb-IIIa monoclonal antibodies also inhibited Fc-receptor-dependent platelet aggregation indicating that the observed interference by intact antibody was not due to the direct interaction of the Fc-portion of the antigen-antibody complex with FcγRII. In addition, the inhibitory anti-GPIIb-IIIa antibodies cross-blocked the binding of IV.3 to platelets at 0°C as well as at 22°C suggesting that the observed inhibition was not dependent on the lateral mobility of either GP IIb-IIIa or FcγRII in the platelet membrane. The combined results therefore strongly suggest that the platelet Fc-receptor, FcγRII, is topographically associated with the GPIIb-IIIa complex in the intact platelet membrane.