Opposite β2-glycoprotein I requirement for the binding of infectious and autoimmune antiphospholipid antibodies to cardiolipin liposomes is associated with antibody avidity

The aim of this study was to investigate the interaction of antiphospholipid antibodies (aPL) from two different populations (patients with autoimmune or infectious disorders) with cardiolipin (CL) arranged in a defined bilayer. beta(2)-Glycoprotein I (beta(2)GPI), an apolipoprotein that plays a critical role in the aPL binding to phospholipids, was quantified by dot blot in purified IgG-aPL samples, further classified according to apparent avidity to CL. In solid-phase assays, beta(2)GPI increased, preferentially, the binding of low-avidity autoimmune aPL to CL but inhibited the binding of low-avidity syphilitic aPL. In the absence of beta(2)GPI, both autoimmune and infectious aPL induced the leakage of the entrapped fluorescent probe, carboxyfluorescein (CF), from small unilamellar vesicles containing CL. aPL-induced probe leakage was protein concentration-dependent and characterized by a lag-phase onset of 100-120 min. beta(2)GPI increased the leakage rate induced by low-avidity autoimmune aPL only and inhibited the leakage induced by all syphilitic aPL. The following conclusions were provided: (1) in the absence of beta(2)GPI, autoimmune and infectious aPL bind to CL in a bilayer, inducing liposome leakage; (2) the leakage mechanism induced by aPL is suggested to be intravesicular; (3) beta(2)GPI requirement for phospholipid binding in both solid and fluid phase is associated to aPL avidity; (4) CL alone or the CL-beta(2)GPI complex are the most likely epitopes for autoimmune aPL; (5) aPL from syphilis patients can only form the CL-aPL complex, supporting that beta(2)GPI is not (part of) the target epitope. (C) 1999 Elsevier Science B.V. All rights reserved.