Effect of β2glycoprotein I and human monoclonal anticardiolipin antibody on the protein S/C4b-binding protein system

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Abstract
The effect of β2glycoprotein I (β2GPI) and human monoclonal anticardiolipin antibody (aCL) on the protein S/C4b-binding protein (C4BP) system was evaluated. The binding of C4BP to protein S was assessed by ELISA in the presence of β2GPI with/without human monoclonal aCL. β2GPI downregulated the binding between S and C4BP significantly. Human monoclonal aCL abolished the β2GPI inhibitory effect in a calcium (Ca++) independent fashion. In separate experiments, the reactivity of aCL towards protein S in the presence or absence of β2GPI and cardiolipin was investigated. Monoclonal aCL bound to protein S only in the presence of a combination of β2GPI and cardiolipin. This binding was Ca++ dependent. These findings suggest that human monoclonal aCL increases the affinity of C4BP for protein S, and that protein S may represent one of the targets for aCL when combined with β2GPI and cardiolipin. Both issues may explain acquired free protein S deficiency and the attendant risk of thrombosis in patients with aCL.