Interactions of human monoclonal and polyclonal antiphospholipid antibodies with serine proteases involved in hemostasis

OBJECTIVE: To characterize the interaction between procoagulant and/or anticoagulant serine proteases and human monoclonal IgG antiphospholipid antibodies (aPL) and polyclonal IgG derived from patients with the antiphospholipid syndrome (APS). METHODS: Five human monoclonal IgG with small differences in their sequences were tested for binding to protein C, activated protein C, plasmin, factor VIIa (FVIIa), FIX, FIXa, and FXII. Serum levels of antithrombin and anti–activated protein C were compared in 32 patients with APS, 29 patients with systemic lupus erythematosus (SLE), and 22 healthy controls. Purified polyclonal IgG derived from APS patients with elevated levels of serum antithrombin antibodies was also tested for its functional effects on thrombin and antithrombin activity. RESULTS: Studies of monoclonal antibodies showed that sequence changes in human aPL are important in determining their ability to bind procoagulant and anticoagulant/fibrinolytic serine proteases. Mean IgG antithrombin levels were significantly elevated in patients with APS and in SLE patients with aPL but no APS (SLE/aPL+) compared to healthy controls, but anti–activated protein C levels were not increased in these patients. Moreover, IgG purified from patients with APS displayed higher avidity for thrombin and significantly inhibited antithrombin inactivation of thrombin compared with IgG from SLE/aPL+ patients. CONCLUSION: High-avidity antithrombin antibodies, which prevent antithrombin inactivation of thrombin, distinguish patients with APS from SLE/aPL+ patients, and thus may contribute to the pathogenesis of vascular thrombosis in APS.