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

Abstract

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 b 2 -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 b 2 -GPI by ELISA (anti-'protein C' antibody ELISA), and compared their binding with those obtained in the absence of b 2 -GPI. In the anti-'protein C' antibody ELISA system, 47% of 78 aPL þ patients had a positive titre in the presence of cardiolipin (CL) and b 2 -GPI, but binding was not found in the absence of b 2 -GPI. Highly significant correlations were found between the titre of anti-'protein C' antibody in the presence of b 2 -GPI and that of anti-b 2 -GPI antibody (r ¼ 0·802, P ¼ 0·0001). We further analysed the interaction between protein C, phospholipids, b 2 -GPI and human aCL MoAbs established from patients with antiphospholipid syndrome. In a first set of experiments, the binding of b 2 -GPI to protein C and its phospholipid dependency were investigated. b 2 -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 b 2 -GPI (virtually anti-b 2 -GPI antibodies) was evaluated in the presence of cardiolipin and b 2 -GPI. All three human monoclonal aCL bound to protein C in the presence of CL and b 2 -GPI, whereas they did not in the absence of either b 2 -GPI or CL. These data suggest that protein C could be a target of aCL by making a complex with CL and b 2 -GPI, leading to protein C dysfunction.

Basic mechanisms and pathogenesis of venous thrombosis

“…These inflammatory contributions may help to explain why some patients with lupus anticoagulants have increased risks of arterial and/or venous thrombosis. On the venous side, one frequently observed candidate is inhibition of the protein C anticoagulant pathway [67][68][69][70] . In addition, antibodies against thrombomodulin are often found in this patient group and in patients with idiopathic thrombosis, 71 potentially leading to impairment of the protein C anticoagulant pathway.…”

Molecular Pathogenesis of the Antiphospholipid Syndrome

“…Interestingly, protein C can be a target of aCL in the presence of cardiolipin and ␤ 2 GPI, leading to protein C dysfunction. 39 Also, antibodies of some aPL patients have been found to recognize heparin and inhibit the formation of antithrombin III-thrombin complexes. 40 Indeed, the remarkable heterogeneity of these aPL antibodies in even in a single patient, as recently demonstrated by clonal analysis, 41 adds to the difficulties in defining the pathogenic aPL response(s).…”

Association of Anticardiolipin Antibodies with Vascular Access Occlusion in Hemodialysis Patients: Cause or Effect?

“…β2GPI is a 50-kD protein with a natural anticoagulant activity. It inhibits surface-mediated activation of prekallikrein and factor XII, activated factor V, and platelet aggregation [12, 13, 14]. β2GPI may bind specifically to autoimmune aPL, and thus discriminate them from postinfectious alloimmune antibodies [13].…”