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

Thrombin generation in platelet-rich plasma (PRP) involves complex interactions between platelets and coagulation proteins. We previously reported that the addition of fibrin to PRP enhances tissue-factor initiated thrombin generation by κ 40%, and the current studies were designed to assess the mechanism(s) underlying thrombin generation in the absence and presence of fibrin. Blocking platelet GPIIb/IIIa + αvβ3 receptors with a monoclonal antibody (MoAb) inhibited basal thrombin generation, but did not affect the enhancement produced by fibrin. In contrast, blocking GPIb with any of three different MoAbs had no effect on basal thrombin generation, but essentially eliminated fibrin enhancement of thrombin generation. When thrombin generation was tested in PRP deficient in von Willebrand factor (vWF), both basal and fibrin-enhanced thrombin generation were markedly reduced, and the addition of factor VIII did not normalize thrombin generation. Botrocetin, which induces the binding of vWF to GPIb, enhanced thrombin generation. In all studies, the ability of PRP to support thrombin generation 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 procoagulant activity: one is independent of fibrin and relies primarily on GPIIb/IIIa, but with a minor contribution from αvβ3; and the other is fibrin-dependent and relies on GPIb. These data may have implications for understanding the mechanisms of the abnormalities in serum prothrombin times reported in Bernard-Soulier syndrome, hemorrhage in von Willebrand disease (vWD), and the increased risk of thrombosis associated with elevated vWF levels.