Autoimmune antiphospholipid antibodies are directed against a cryptic epitope expressed when beta 2-glycoprotein I is bound to a suitable surface

The antiphospholipid antibodies (aPL) present in autoimmune disorders are associated with thromboembolic episodes, and their binding to phospholipids (PL) is mediated by a plasma cofactor, beta 2-glycoprotein I (beta 2GPI). Both PL and beta 2GPI seem necessary for binding, thus indicating that the two components comprise the epitope against which aPL are directed. Using an anti-beta 2GPI antibody ELISA with the antigen adsorbed onto polyvinylchloride (PVC) plates, we detected high antibody titres in 12 out of 12 plasma from patients with the antiphospholipid syndrome. No or very low positivity was obtained when the same ELISA was carried out in polystyrene (PST) plates, while an increasing positivity was found when processed (i.e. more hydrophilic) or COOH-surface PST plates were used. When beta 2GPI dependent IgG-aPL were purified using agarose-immobilized cardiolipin, 4 out of 4 preparations were highly positive in anti-beta 2GPI antibody ELISA using PVC plates, while beta 2GPI was not fully recognized by aPL-IgG when adsorbed onto PST plates. These findings demonstrate that aPL are, in fact, anti-beta 2GPI antibodies directed against a cryptic epitope which is expressed when beta 2GPI is bound to anionic phospholipid, or another suitable surface.