Exposure of platelet fibrinogen receptors by a monoclonal antibody to the GPIIb-IIIa complex, PMA4.

We found that a monoclonal antibody to the glycoprotein (GP) IIb-IIIa complex, PMA4, induces fibrinogen binding to platelets, and we examined the mechanism involved. Affinity chromatography and crossed immunoelectrophoresis showed that PMA4 recognized an epitope on the GPIIb-IIIa complex-specific domain. The binding of 125I-fibrinogen to platelets was induced by PMA4 in a concentration-dependent manner and was blocked by EDTA, RGDS peptides and an anti-GPIIb-IIIa monoclonal antibody, PMA1. Binding of the divalent antibody to the GPIIb-IIIa complex was necessary to induce fibrinogen binding and subsequent platelet aggregation, since Fab fragments, unlike PMA4 IgG and F(ab')2 fragments, did not induce fibrinogen binding or aggregation. The PMA4 IgG induced fibrinogen binding, serotonin secretion, and Ca2+ mobilization, whereas F(ab')2 induced fibrinogen binding only. In addition, F(ab')2-induced fibrinogen binding was not abolished in the presence of aspirin, H-7, a protein kinase C inhibitor, PGE1 or dibutyryl cyclic AMP. These results demonstrate that the binding of PMA4 divalent molecules to the GPIIb-IIIa complex can expose platelet fibrinogen receptors in the absence of the stimulatory effects of intracellular mediators on platelets. Thus, we conclude that the fibrinogen receptors on the GPIIb-IIIa complex can be exposed by direct action of the antibody on the complex molecules.