Acquired activated protein C resistance associated with IgG antibodies against beta2-glycoprotein I and prothrombin as a strong risk factor for venous thromboembolism.

Venous thromboembolic events such as deep vein thrombosis and pulmonary embolism are common manifestations of antiphospholipid syndrome. Our aim was to clarify the roles of anti-phospholipid (aPL) antibodies in the pathogenesis of venous thromboembolism (VTE) in patients with systemic lupus erythematosus (SLE). We examined anti-cardiolipin/beta2-glycoprotein I (anti-CL/beta2-GPI) antibody concentrations, anti-phosphatidylserine/prothrombin (anti-PS/PT) antibody concentrations, and lupus anticoagulant (LA) activity in 87 patients with SLE (21 with VTE and 66 without thrombosis). Both anti-CL/beta2-GPI and anti-PS/PT antibodies strongly correlated with LA activity. Multivariate logistic analysis confirmed that both anti-CL/beta2-GPI and anti-PS/PT antibodies were significant independent risk factors for VTE (odds ratios = 4.98 and 7.54, respectively; 95% confidence intervals, 1.51-16.4 and 2.30-24.7, respectively). We therefore studied the in vitro effects of IgG fractions containing anti-CL/beta2-GPI or anti-PS/PT antibodies on the anticoagulant activity of activated protein C (APC) and found that purified IgG containing anti-CL/beta2-GPI or anti-PS/PT antibodies significantly hampered the anticoagulant activity of APC. We also studied the ability of IgG fractions to impede the anticoagulant activity of APC before and after complete removal of anti-CL/beta2-GPI or anti-PS/PT antibodies by adsorption. Removal of anti-CL/beta2-GPI or anti-PS/PT antibodies from all positive IgG samples clearly decreased the inhibitory effect of those samples on APC anticoagulant activity. Anti-CL/beta2-GPI and anti-PS/PT antibodies independently cause APC resistance, which may contribute to risk of VTE in patients with SLE.