BETA-2-GLYCOPROTEIN 1 (BETA-2GP1) ENHANCES CARDIOLIPIN BINDING-ACTIVITY BUT IS NOT THE ANTIGEN FOR ANTIPHOSPHOLIPID ANTIBODIES

Some investigators have reported that a serum protein, beta2-glycoprotein 1 (beta2GP1), either alone or in combination with negatively charged phospholipid, may be the antigen for anticardiolipin (aCL) antibodies. To examine these reports further, ELISA tests, inhibition experiments, Ouchterlony and Western blot techniques were used to examine anticardiolipin binding to beta2GP1. Sera from patients with the antiphospholipid syndrome (APS) and syphilis were studied, as well as whole IgG immunoglobulin and affinity purified (a.p.) IgG aCL antibodies. Results showed no binding of aCL antibodies to beta2GP1 in the absence of cardiolipin. beta2GP1 caused enhanced binding of aCL antibodies to cardiolipin, but this enhancement was not observed in inhibition experiments. Binding to cardiolipin occurred in the absence of beta2GP1. Enhancement of cardiolipin binding activity by beta2GP1 was observed for APS, but not for syphilis. We conclude that beta2GP1 is not the antigen for aCL antibodies, nor is it likely that the antibody recognizes shared beta2GP1-cardiolipin epitopes. Instead, this protein may make cardiolipin more available for aCL binding on solid surfaces by some yet undefined mechanism. This effect may not extend to aqueous suspensions.