[Anti-β2 Glycoprotein I—β2 Glycoprotein I] immune complexes in patients with antiphospholipid syndrome and other autoimmune diseases

Abstract

Antiphospholipid syndrome (APS) is defined by the presence of aPL antibodies in patients with thromboembolic phenomena. Some antiphospholipid (aPL) antibodies, such as those directed against β₂-glycoprotein I (β₂GPI), are associated with thromboembolism, possess Lupus Anticoagulant (LA) activity and recognize their target antigen only when bound to specific surfaces or to phospholipids (PL). To ascertain whether both free and antibody-bound β₂GPI circulate in APS, we set up an ELISA to detect [IgG anti-β₂GPI–β₂GPI] immune complexes. In this system, rabbit anti-human β₂GPI antibodies were adsorbed onto plastic plates, incubated with patient plasma, and bound complexes were detected by means of alkaline phosphatase-labeled goat anti-human IgG; each assay was stopped when positive controls consisting of in vitro generated immune complexes reached an Optical Density (OD) of 0.5 at 405 nm. Plasma from 16 patients with APS showed a mean OD405 of 0.291 (range 0.115–0.558), not statistically different from the mean obtained for 15 age-and sex-matched healthy volunteers (mean OD405 = 0.169, range 0.066–0.264). Surprisingly, levels of immune complexes in 14 patients with other autoimmune diseases and no circulating anti-β₂GPI antibodies were statistically higher (mean OD405 = 0.552, range 0.204–0.991) than those of healthy subjects and patients with APS. These data indicate that while autoantibodies to β₂GPI are mainly unbound in plasma of patients with APS, they are complexed with their antigen in patients with other autoimmune diseases, possibly reflecting a higher binding affinity.

Keywords

anti-β2-glycoprotein I immune complexes antiphospholipid syndrome

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