: Lupus anticoagulants (LAs) and anti-phospholipid antibodies (APAs) are associated with an increased risk of thrombosis. It is our hypothesis that APA and/or LA subgroup(s) lead to thrombosis through the selective inhibition of the protein C anticoagulant pathway. We have found that the membrane requirements of the activated protein C (APC) complex are different from those of the procoagulant complexes. Specifically, the APC complex requires phosphatidylethanolamine (PE) for activity. Recently, we have also observed that phospholipid oxidation enhances APC activity specifically. These requirements mimic those of at least a subpopulation of autoantibodies found in lupus patients with thrombosis for the selective inhibition of the APC complex and thus may provide both the specificity and the link between the APC pathway, LA/APAs and thrombosis. It is the goal of this application to determine the relationship between the differential membrane structure requirements of the procoagulant and anticoagulant complexes and the specificity of pathogenic antibodies with emphasis on the role of oxidation. We will determine the mechanism of inhibition of the APC complex by prothrombotic antibodies (Abs) in terms of phospholipid requirements (presence of PE; role of oxidation), the target of the Abs (protein; membrane; protein/membrane) and possible role of other cofactor proteins. The utility of a chimeric form of APC to identify prothrombotic Abs and the prevalence of Abs to other members of the protein C pathway, TM and EPCR will also be investigated. Appreciation of the molecular mechanisms involved in the selective inhibition of the protein C pathway by identifiable subgroup(s) of LA/APAs should lead to better predictive and monitoring tests and potentially more specific and safer therapies in these difficult to manage patients.