title: Binding of anticardiolipin antibodies to protein C via beta2-glycoprotein I (beta2-GPI): a possible mechanism in the inhibitory effect of antiphospholipid antibodies on the protein C system.
abstract: It 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 beta2-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 beta2-GPI by ELISA (anti-'protein C' antibody ELISA), and compared their binding with those obtained in the absence of beta2-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 beta2-GPI, but binding was not found in the absence of beta2-GPI. Highly significant correlations were found between the titre of anti-'protein C' antibody in the presence of beta2-GPI and that of anti-beta2-GPI antibody (r = 0.802, P = 0.0001). We further analysed the interaction between protein C, phospholipids, beta2-GPI and human aCL MoAbs established from patients with antiphospholipid syndrome. In a first set of experiments, the binding of beta2-GPI to protein C and its phospholipid dependency were investigated. Beta2-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 beta2-GPI (virtually anti-beta2-GPI antibodies) was evaluated in the presence of cardiolipin and beta2-GPI. All three human monoclonal aCL bound to protein C in the presence of CL and beta2-GPI, whereas they did not in the absence of either beta2-GPI or CL. These data suggest that protein C could be a target of aCL by making a complex with CL and beta2-GPI, leading to protein C dysfunction.
authors.affiliation: Lupus Research Unit, The Rayne Institute, St Thomas' Hospital, London, UK.
authors.forename: T
authors.initials: T
authors.lastname: Atsumi
authors.forename: M A
authors.initials: MA
authors.lastname: Khamashta
authors.forename: O
authors.initials: O
authors.lastname: Amengual
authors.forename: S
authors.initials: S
authors.lastname: Donohoe
authors.forename: I
authors.initials: I
authors.lastname: Mackie
authors.forename: K
authors.initials: K
authors.lastname: Ichikawa
authors.forename: T
authors.initials: T
authors.lastname: Koike
authors.forename: G R
authors.initials: GR
authors.lastname: Hughes
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D017153:Antibodies, Anticardiolipin / Q000276:immunology*; D017152:Antibodies, Antiphospholipid / Q000276:immunology*; D016736:Antiphospholipid Syndrome / Q000276:immunology; D001053:Apolipoproteins / Q000276:immunology*; D002648:Child; D005260:Female; D006023:Glycoproteins / Q000276:immunology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010314:Partial Thromboplastin Time; D011485:Protein Binding; D011486:Protein C / Q000276:immunology*; D053482:beta 2-Glycoprotein I
chemical_list: D017153:Antibodies, Anticardiolipin; D017152:Antibodies, Antiphospholipid; D001053:Apolipoproteins; D006023:Glycoproteins; D011486:Protein C; D053482:beta 2-Glycoprotein I