STANDARDIZATION OF IMMUNOASSAYS FOR ANTIPHOSPHOLIPID ANTIBODIES WITH BETA-2GPI AND ROLE OF OTHER PHOSPHOLIPID COFACTORS

Presence of beta 2 Glycoprotein I (beta 2GPI), in addition to phosphol ipids, is an absolute requirement for binding APA. This binding is fre quently observed with beta 2GPI coated alone, however many APA react o nly with beta 2GPI complexed to phospholipids, but not with phospholip ids alone. We demonstrate that a subgroup of rabbit polyclonal antibod ies to human beta 2GPI binds to this protein only when it is coated on a solid surface, but not if it is in solution. In addition, beta 2GPI present in goat serum is strongly fixed by the coated phospholipids a nd the complexes formed bind as well APA as the rabbit antibodies to b eta 2GPI. The diluent used for testing APA, has a strong incidence on APA's reactivity as it can be a source of beta 2GPI. Antibody binding to beta 2GPI, Prothrombin, Protein S, and Annexin V, coated in the pre sence or in the absence of phospholipids, was tested in 55 patients wi th the antiphospholipid syndrome. The strongest binding of antibodies was observed in 39 plasma to a mixture of phospholipids and purified h uman beta 2GPI, however 17 samples also presented a significant reacti vity to beta 2GPI alone. Nine plasmas contained antibodies to Prothrom bin, 4 to Protein S, 3 to Annexin V, and 1 to Protein C. We conclude t hat most of the APA are directed to a complex of beta 2GPI and phospho lipids although in some patients antibodies to beta 2GPI alone or to o ther phosphoslipid binding proteins are present.