Effect of antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE) IgG on coagulant and enzymatic functions of factor Xa (FXa). (A) APS and SLE IgG significantly prolonged FXa-activated clotting time. IgG (200 μg/ml final concentration) were incubated with FXa (3.9 nM final concentration) for 10 minutes at 37°C followed by addition of phospholipid, calcium and fibrinogen mixture; bars represent mean ± standard error of the mean (SEM); P = 0.04; ****P <0.0001. (B) APS and SLE IgG inhibit FXa activity. FXa (2 nM) was incubated with IgG for 10 minutes at 37°C followed by the addition of chromogenic substrate; bars, mean ± SEM; ***P = 0.0002 for SLE versus healthy controls, P = 0.0008 for SLE versus APS; ****P <0.0001. (C) Inhibition of FXa by individual APS or SLE IgG samples compared to mixed APS/SLE IgG at equal final concentrations. Five APS-IgG and SLE-IgG samples with similar FXa binding were selected and equal concentrations mixed to compare to the ability of individual samples to bind FXa. Inhibitions observed by individual samples and their combinations were as follows (%): APS 1: 20.2, SLE 1: 7.1, APS 1 + SLE 1: 19.4; APS 2: 19.04, SLE 2: 17.9, APS 2 + SLE 2: 21.4; APS 3: 11.2, SLE 3: 11.4, APS 3 + SLE 3: 20.4; APS 4: 15.8, SLE 4:12.7, APS 4 + SLE 4: 19.7; APS 5: 10.4, SLE 5: 13.8, APS 5 + SLE 5: 15.2%. Experiments were performed in duplicate for each sample. Results are representative of at least three independent experiments.