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.
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