AUTOIMMUNE ANTIPHOSPHOLIPID ANTIBODIES ARE DIRECTED AGAINST A CRYPTICEPITOPE 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 phos pholipids (PL) is mediated by a plasma cofactor, beta 2-glycoprotein I (beta 2GPI). Both PL and beta 2GPI seem necessary for binding, thus i ndicating that the two components comprise the epitope against which a PL are directed. Using an anti-beta 2GPI antibody ELISA with the antig en adsorbed onto polyvinylchloride (PVC) plates, we detected high anti body titres in 12 out of 12 plasmas from patients with the antiphospho lipid syndrome. No or very low positivity was obtained when the same E LISA was carried out in polystyrene (PST) plates, while an increasing positivity was found when processed (i.e. more hydrophilic) or COOH-su rface PST plates were used. When beta 2GPI dependent IgG-aPL were puri fied using agarose-immobilized cardiolipin, 4 out of 4 preparations we re 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-be ta 2GPI antibodies directed against a cryptic epitope which is express ed when beta 2GPI is bound to anionic phospholipid, or another suitabl e surface.