Fibrin-dependent platelet procoagulant activity requires GPIb receptors and von Willebrand factor

Thrombin generation in platelet-rich plasma (PRP) involves complex interact ions between platelets and coagulation proteins. We previously reported tha t the addition of fibrin to PRP enhances tissue-factor initiated thrombin g eneration by approximate to 40%, and the current studies were designed to a ssess the mechanism(s) underlying thrombin generation in the absence and pr esence of fibrin. Blocking platelet GPIIb/IIIa + alpha v beta 3 receptors w ith a monoclonal antibody (MoAb) inhibited basal thrombin generation, but d id not affect the enhancement produced by fibrin. In contrast, blocking GPI b with any of three different MoAbs had no effect on basal thrombin generat ion, but essentially eliminated fibrin enhancement of thrombin generation. When thrombin generation was tested in PRP deficient in von Willebrand fact or (VWF), both basal and fibrin-enhanced thrombin generation were markedly reduced, and the addition of factor VIII did not normalize thrombin generat ion. Botrocetin, which induces the binding of vWF to GPIb, enhanced thrombi n generation. In all studies, the ability of PRP to support thrombin genera tion correlated with the production of platelet-derived microparticles and serum platelet-derived procoagulant activity. Thus, two separate mechanisms , both of which depend on vWF, appear to contribute to platelet-derived pro coagulant activity: one is independent of fibrin and relies primarily on GP IIb/IIIa, but with a minor contribution from alpha v beta 3; and the other is fibrin-dependent and relies on GPIb. These data may have implications fo r understanding the mechanisms of the abnormalities in serum prothrombin ti mes reported in Bernard-Soulier syndrome, hemorrhage in von Willebrand dise ase (vWD), and the increased risk of thrombosis associated with elevated VW F levels. (C) 1999 by The American Society of Hematology.