Thrombin overcomes the thrombosis defect associated with platelet GPVI/FcRγ deficiency

Fibrillar collagens are among the most potent activators of platelets and play an important role in the initiation of thrombosis. The glycoprotein VI (GPVI)/FcR gamma-chain complex is a central collagen receptor and inhibitors of GPVI produce a major defect in arterial thrombogenesis. In this study we have examined arterial thrombus formation in mice lacking the GPVI/FcR gamma-chain complex (FcR gamma(-/-)). Using 3 distinct arterial thrombosis models involving deep vascular injury, we demonstrate that deficiency of GPVI/FcR gamma is not associated with a major defect in arterial thrombus formation. In contrast, with milder vascular injury deficiency of GPVI/FcR gamma was associated with a 30% reduction in thrombus growth. Analysis of FcR gamma(-/-) platelets in vitro, using thrombin-dependent and -independent thrombosis models, demonstrated a major role for thrombin in overcoming the thrombosis defect associated with GPVI/FcR gamma deficiency. Inhibition of thrombin in vivo produced a much greater defect in thrombus formation in mice lacking GPVI/FcR gamma compared with normal controls. Similarly, thrombin inhibition produced a marked prolongation in bleeding time in FcR gamma(-/-) mice relative to wild-type mice. Our studies define an important role for thrombin in overcoming the hemostatic and thrombotic defect associated with GPVI/FcR gamma deficiency. Moreover, they raise the interesting possibility that the full antithrombotic potential of GPVI receptor antagonists may only be realized through the concurrent administration of anticoagulant agents.