The presence of circulating antibodies to anionic phospholipids (aPL) is associated with a syndrome of arterial and venous thrombosis, thrombocytopenia, neurologic disease, and recurrent fetal loss. The thrombotic disease can occur at an early age and can be associated with devastating clinical sequelae. Although there is evidence that aPL are pathogenic, the molecular basis of the pro-thrombotic phenotype associated with these antibodies is unknown. In addition, the clinical manifestations of aPL are heterogeneous, and there are no reliable predictors of thrombosis in the aPL syndrome. Activated endothelial cells express a thrombogenic phenotype which may play an important role in many acquired hypercoagulable states, including the aPL syndrome. aPL specifically activate vascular endothelial cells (EC), creating a thrombogenic surface on the vessel wall. The objectives of this proposal are to determine the molecular and cellular pathogenesis of the thrombotic disease in patients with aPL by studying the interaction of aPL with vascular EC, and to identify genetic and other determinants of thrombotic risk in patients with aPL related to these pathophysiologic processes. Risk factors for clinical thrombotic events in patients with aPL that will be studied will include endothelial cell activation by patient IgG, plasma homocystene level in the fasting state reductase genes. EC activation by aPL will be studied using patient samples as well as high affinity monoclonal aPL antibodies. Immunoprecipitation and affinity purification experiments will be performed to identify and characterize binding site(s) on EC for B2-glycoprotein-1, a co-factor necessary for aPL binding and EC activation. The role of oxidative stress and/or hyperhomocysteinemia in modulating EC activation by aPL will also be explored. These studies will broaden our understanding of the mechanisms of thrombosis, and may suggest novel preventative and therapeutic strategies for patients with aPL and vascular diseases.