At the site of vascular injury, platelet adhesion, activation, and aggregation culminate in thrombus information, the principle event underlying most acute arterial thromboocculsive disorders. Given that thrombosis is the leading cause of death world-wide, understanding the contribution of platelets to the process is of particular importance. Our novel observation of altered platelet thrombus formation in mice deficient in P-selectin serves as the basis of this proposal. Until recently, the primary function of P-selectin on activated platelets and endothelial cells was thought to be promotion of the initial interaction of these cells with leukocytes. However, P-selectin on activated endothelial cells was implicated in recruitment of platelets, and our observations suggest that the protein may also play a role in promoting platelet-platelet interactions. Thus, P-selectin and its ligand may be involved in promoting the interactions of platelets with leukocytes, endothelial cells, and with other platelets. This proposal will determine the role of platelet P-selectin in platelet-platelet interactions in vitro and platelet thrombus formation in vivo. This proposal tests the hypothesis that P-selectin on activated platelets interacts with a specific platelet counterreceptor and that this interaction affects platelet signaling and influences platelet-platelet interactions mediated by alpha Ilb,beta3. Complementary in vitro experiments will be conducted with human and mouse platelets, and mouse models of thrombosis will serve to define in vivo function of P-selectin. The Specific Aims are to: (1) establish an in vitro assay of platelet adhesion to Pselectin to be used to identify the platelet counter-receptor(s) for P-selectin, (2) determine the role of P-selectin in platelet-platelet interactions in vitro, (3) determine the role of P-selectin in models of thrombosis, and (4) to delineate the contribution of platelet and endothelial P-selectin to thrombosis and the arterial response to injury. This work will identify a novel target for future antithrombotic strategies and challenge the assumption that the beneficial effects of P-selectin antagonists are exclusive to leukocytes.