Binding of anticardiolipin antibodies to protein C via β2-glycoprotein I (β2-GPI): a possible mechanism in the inhibitory effect of antiphospholipid antibodies on the protein C system

SUMMARYIt is known that antiphospholipid antibodies (aPL) hamper the anticoagulant activity of the protein C system, but the mechanism is still obscure. In this study, we demonstrate that anticardiolipin antibodies (not anti-protein C autoantibodies) can bind protein C via β2-GPI, which bears their binding epitope, in a fashion dependent on negatively charged phospholipids. We studied the binding of IgG from aPL to protein C in the presence of β2-GPI by ELISA (anti-‘protein C’ antibody ELISA), and compared their binding with those obtained in the absence of β2-GPI. In the anti-‘protein C’ antibody ELISA system, 47% of 78 aPL+ patients had a positive titre in the presence of cardiolipin (CL) and β2-GPI, but binding was not found in the absence of β2-GPI. Highly significant correlations were found between the titre of anti-‘protein C’ antibody in the presence of β2-GPI and that of anti-β2-GPI antibody (r = 0·802, P = 0·0001). We further analysed the interaction between protein C, phospholipids, β2-GPI and human aCL MoAbs established from patients with antiphospholipid syndrome. In a first set of experiments, the binding of β2-GPI to protein C and its phospholipid dependency were investigated. β2-GPI bound to protein C in the presence of CL or phosphatidylserine, but not in the presence of phosphatidylcholine or phosphatidylethanolamine. In a second group of experiments, the binding of three human monoclonal aCL recognizing the cryptic epitope of β2-GPI (virtually anti-β2-GPI antibodies) was evaluated in the presence of cardiolipin and β2-GPI. All three human monoclonal aCL bound to protein C in the presence of CL and β2-GPI, whereas they did not in the absence of either β2-GPI or CL. These data suggest that protein C could be a target of aCL by making a complex with CL and β2-GPI, leading to protein C dysfunction.