Antibodies against platelet membrane glycoproteins

Glycocalicin has been proposed as the common platelet receptor for both ristocetin-human VIIIR:WF and thrombin-induced platelet aggregation. Platelets which have lost glycocalicin do not respond to either ristocetin-human VIIIR:WF or bovine VIIIR:WF. Using antibodies to the platelet membrane glycoproteins Ia and Ib, IIb and IIIa, and glycocalicin we show that the Fab' fragments of anti-glycoproteins Ia and Ib and anti-glycocalicin IgG totally inhibited bovine VIIIR:WF-induced platelet aggregation, while those from anti-glycoproteins IIb and IIIa IgG were without effect. Thrombin-induced platelet aggregation was strongly inhibited by the Fab' fragments of anti-glycoproteins Ia and Ib IgG and anti-glycocalicin IgG, indicating that these glycoproteins play a major role in thrombin-platelet interaction. Fab' fragments of anti-glycoproteins IIb and IIIa IgG inhibited thrombin-induced platelet aggregation to a lesser extent implying that these glycoproteins are also somehow involved in the platelet response to thrombin perhaps as fibrinogen receptors. The data presented here give further support to the proposal that ristocetin-human VIIIR:WF and bovine VIIIR:WF share a common receptor on the platelet surface and indicate that this structure plays an important role in thrombin-induced platelet responses.