ANTI-BETA-2-GLYCOPROTEIN-I (BETA-2GPI) MONOCLONAL-ANTIBODIES WITH LUPUS ANTICOAGULANT-LIKE ACTIVITY ENHANCE THE BETA-2GPI BINDING TO PHOSPHOLIPIDS

beta 2-Glycoprotein I (beta 2GPI), a plasma glycoprotein with phosphol ipid-binding property, is known to be the actual target antigen for au toimmune type anticardiolipin antibodies (aCLs), Certain groups of aCL s (anti-beta GPI antibodies) exert lupus anticoagulant (LA) activity a nd perturb the function of vascular endothelial cells, This investigat ion aimed at highlighting some insights into the molecular basis by wh ich aCLs exert their biological effects by using anti-beta 2GPI mAbs w ith well-characterized epitopes from mice and from patients with antip hospholipid syndrome. Anti-beta 2GPI mAbs directed against the third d omain (Cof-20 and Cof-22) and fourth domain (Cof-21, EY1C8, and EY2C9) of beta 2GPI inhibited the thrombin generation induced by Russell's v iper venom in diluted plasma and that induced by the prothrombinase co mplex reconstituted with purified clotting factors, This anticoagulant activity was abrogated in the presence of an excess amount of phospho lipids, thus resembling the LA activity, In stark contrast, anti-beta 2GPI mAbs directed against the fifth domain and the carboxyterminal re gion of the fourth domain showed no LA-like activity. These findings s uggest that the LA activity of anti-beta 2GPi antibodies depends on th eir epitope specificity. Experiments carried out to clarify the mechan ism of the LA activity showed that anti-beta 2GPI mAbs with LA-like ac tivity, but not those without this effect, enhance the beta 2GPI bindi ng to phospholipids. In addition, the F(ab')(2) fragment, but not the Fab' fragment, of the anti-beta 2GPI mAbs was found to enhance the LA activity and the beta 2GPI binding to phospholipids, suggesting that a nti-beta 2GPI antibodies induce formation of multiple complexes of bet a 2GPI on the surface of phospholipids because of their bivalent prope rty. This clustering of P2GPI molecules induced by anti-beta 2GPI anti bodies, probably because of their multivalent property and epitope spe cificity, might hinder the lateral mobility and activation of clotting factors on the surface of phospholipids and thus exert LA activity, C lustering of beta 2GPI molecules may also explain the molecular mechan ism by which anti-beta 2GPI antibodies alter the function of leukocyte s and endothelial cells. The well-documented heterogeneous LA activity of aCLs (anti-beta 2GPI antibodies) may also be explained by their ep itope specificity.