To date, all of the known activators of platelets and hemostasis are soluble and cell matrix molecules. The Kahn lab has demonstrated that interactions between podoplanin (PDPN), a transmembrane protein, and CLEC-2 receptors on the surface of platelets prevent blood-lymphatic mixing throughout life. This pathway requires platelet activation by CLEC2, but not platelet aggregation mediated by integrin activation. Preliminary studies from the Kahn lab reveal the basis for this phenotype to be a novel form of platelet-mediated hemostasis at the lympho-venous junction that is stimulated by PDPN- CLEC2 interaction. The Bergmeier lab has recently collaborated with the Kahn lab to demonstrate that platelet CLEC2 signaling is also required for hemostasis during vascular inflammation in the lung and skin. The proposed studies will address these new biological roles of PDPN-CLEC2 platelet activation, and investigate the mechanism by which CLEC2 signaling in platelets mediates such non-arterial, non-canonical hemostatic responses. Aim 1 will investigate the role of PDPN-CLEC2 signaling in preventing pulmonary and inflammatory hemorrhage, two very recently identified functions of this platelet activation pathway. Aim 2 will determine what effectors of CLEC2 signaling in platelets function during inflammation and in blood-lymphatic vascular separation. These studies will define a recently discovered platelet activation pathway that performs hemostatic roles not previously associated with platelet activation that are highly relevant to human disease.