TOPOGRAPHICAL ASSOCIATION OF THE PLATELET FC-RECEPTOR WITH THE GLYCOPROTEIN IIB-IIIA COMPLEX

In this study, we have examined whether the platelet Fc-receptor, Fcga mmaRII (CD32), is associated with either of the two major platelet mem brane glycoproteins, the GPIb-IX complex and the GPIIb-IIIa complex. M onoclonal and polyclonal anti-GPIb-IX complex antibodies inhibited to only a moderate degree (< 40%) the binding of the anti-FcgammaRII mono clonal antibody, IV.3, to platelets. In contrast, 6 of 12 anti-GPIIb-I IIa monoclonal antibodies and a polyclonal, affinity-purified rabbit a nti-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 inhi bit the binding of IV.3 to Glanzmann's thrombasthenic platelets which have normal levels of FcgammaRII 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-rece ptor-dependent platelet aggregation as induced by either acetone-aggre gated human IgG or by activating monoclonal antibodies against GPIV, P TA1 or CD9. F(ab')2 fragments of these two anti-GPIIb-IIIa monoclonal antibodies also inhibited Fc-receptor-dependent platelet aggregation i ndicating that the observed interference by intact antibody was not du e to the direct interaction of the Fc-portion of the antigen-antibody complex with FcgammaRII. In addition, the inhibitory anti-GPIIb-IIIa a ntibodies cross-blocked the binding of IV.3 to platelets at 0-degrees- C as well as at 22-degrees-C suggesting that the observed inhibition w as not dependent on the lateral mobility of either GP IIb-IIIa or Fcga mmaRII in the platelet membrane. The combined results therefore strong ly suggest that the platelet Fc-receptor, FcgammaRII, is topographical ly associated with the GPIIb-IIIa complex in the intact platelet membr ane.