Inhibition of APC anticoagulant activity on oxidized phospholipid by anti-β2-glycoprotein I monoclonal antibodies

Activated protein C (APC) anticoagulant activity and the ability to be inhibited by autoantibodies associated with thrombosis are strongly augmented by the presence of phosphatidlylethanolamine (PE) and phospholipid oxidation. beta(2)-glycoprotein I (beta(2)-GPI) is a major antigen for anti phospholipid antibodies present in patients with the antiphospholipid syndrome. We therefore investigated whether anti-beta(2)-GPI monoclonal antibodies (mAbs) could inhibit APC with similar membrane specificity. Five mouse mAbs that reacted with different epitopes on beta(2)-GPI were examined. Each inhibited the PE-, phospholipid oxidation-dependent enhancement of APC anticoagulant activity and required antibody divalency. A chimeric APC that retains anticoagulant activity but is relatively unaffected by protein S, PE, or oxidation was not inhibited by the antibodies. In purified systems, anti-beta(2)-GPI mAb inhibition of factor Va inactivation was greater in the presence of protein S and required beta(2)-GPI. Surprisingly, although the mAbs did increase beta(2)-GPI affinity for membranes, PE and oxidation had little influence on the affinity of the beta(2)-GPI antibody complex for the membrane vesicles. We conclude that antibodies to beta(2)-GPI inhibit APC function specifically and contribute to a hypercoaguable state by disrupting specific protein-protein interactions induced by oxidation of PE-containing membranes.