Anti- b 2 glycoprotein I ( b 2GPI) autoantibodies recognize an epitope on the first domain of b 2GPI

Anticardiolipin (aCL) autoantibodies are associated with thrombosis, recurrent fetal loss, and thrombo-cytopenia. Only aCL found in autoimmune disease require the participation of the phospholipid binding plasma protein b 2 glycoprotein I ( b 2GPI) for antibody binding and now are called anti- b 2GPI. The antigenic specificity of aCL affinity purified from 11 patients with high titers was evaluated in an effort to better understand the pathophysiology associated with aCL. Seven different recombinant domain-deleted mutants of human b 2GPI, and full length human b 2GPI (wild-type), were used in competition assays to inhibit the autoantibodies from binding to immobilized wild-type b 2GPI. Only those domain-deleted mutants that contained domain 1 inhibited the binding to immobilized wild-type b 2GPI from all of the patients. The domain-deleted mutants that contained domain 1 inhibited all aCL in a similar but not identical pattern, suggesting that these aCL recognize a similar, but distinguishable, epitope(s) present on domain 1. ‘‘Antiphospholipid antibodies’’ is the term generally given to describe autoantibodies that are associated with arterial and venous thromboses, recurrent fetal loss, thrombocytopenia, livedo reticularis, and a biological false positive VDRL test. They may occur alone, as in primary antiphospholipid