Characterization of an antiglycoprotein Ib monoclonal antibody that specifically inhibits platelet-thrombin interaction.

Platelet glycoprotein Ib (GPIb) acts as a high-affinity thrombin binding site and as a receptor for von Willebrand Factor (vWF). A new anti-GPIb monoclonal antibody (mAB) VM16d was produced that specifically inhibited platelet-thrombin but not platelet-vWF interaction. The epitope for VM16d was located within the 45 kDa N-terminal region of the alpha-chain of GPIb. VM16d inhibited platelet aggregation induced by low dose thrombin (0.05 U/ml) but did not affect platelet aggregation induced by ristocetin, bovine vWF, ADP or collagen. The same inhibitory effects on thrombin-induced platelet aggregation were observed with the whole IgG molecule of VM16d and its F(ab')2 and F(ab') fragments. VM16d also inhibited 14C-serotonin secretion induced by low dose thrombin and binding of 125I-thrombin but not ristocetin-dependent binding of 125I-vWF to platelets. These data indicate that the high-affinity thrombin binding site is located on the N-terminal 45 kDa domain of GPIb and that it is topographically separated from the vWF binding site.