REQUIREMENT FOR BETA(2) GLYCOPROTEIN-I AS COFACTOR IN THE BINDING OF IGM AND IGA ANTICARDIOLIPIN ANTIBODIES

Objective. To determine if IgM and IgA anticardiolipin (aCL) antibodie s require beta(2) glycoprotein I (beta(2)-GPI) as a cofactor for antib ody binding. Methods. Sera were selected from 7 patients with systemic lupus erythematosus (SLE), 6 of whom had high IgM and 6 high IgA aCL antibody binding. Control sera were obtained from 2 healthy individual s with no aCL antibodies. Serum proteins were initially separated by s epharose CL6B gel filtration chromatography, and IgM was further purif ied by affinity chromatography with mannan binding protein. IgA was is olated from the CL6B filtrate by jacalin lectin affinity chromatograph y. Levels of beta(2)-GPI in the immunoglobulin preparations were deter mined by antigen capture ELISA. Anticardiolipin antibody binding of Ig M and IgA was examined by ELISA with and without the addition of beta( 2)-GPI (10 mu g/ml) or 4% normal human serum and expressed in optical density units (OD). Results. beta(2)-GPI was required as a cofactor fo r IgM aCL antibody binding in 4 of 6 patients with SLE. In these, anti body binding to cardiolipin increased from (mean +/- SEM) 0.10 +/- 0.0 1 to 1.06 +/- 0.22 (p = 0.005) with the addition of beta(2)-GPI. For I gA, 5 of 6 patients with SLE demonstrated a requirement for beta(2)-GP I as a cofactor. Antibody binding increased from 0.27 +/- 0.05 to 1.77 +/- 0.35 (p = 0.003) with the addition of beta(2)-GPI. Conclusion. be ta(2)-GPI is required as a cofactor for IgM and IgA aCL antibody bindi ng.