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

Abstract

AbstractActivated protein C (APC) anticoagulant activity and the ability to be inhibited by auto-antibodies associated with thrombosis are strongly augmented by the presence of phosphatidylethanolamine (PE) and phospholipid oxidation. β2-glycoprotein I (β2-GPI) is a major antigen for antiphospholipid antibodies present in patients with the antiphospholipid syndrome. We therefore investigated whether anti–β2-GPI monoclonal antibodies (mAbs) could inhibit APC with similar membrane specificity. Five mouse mAbs that reacted with different epitopes on β2-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–β2-GPI mAb inhibition of factor Va inactivation was greater in the presence of protein S and required β2-GPI. Surprisingly, although the mAbs did increase β2-GPI affinity for membranes, PE and oxidation had little influence on the affinity of the β2-GPI antibody complex for the membrane vesicles. We conclude that antibodies to β2-GPI inhibit APC function specifically and contribute to a hypercoaguable state by disrupting specific protein-protein interactions induced by oxidation of PE-containing membranes.