aPLs induce TF activation and PS exposure. (A) CD115-selected spleen cells were stimulated for 15 minutes at 37°C with IgG (500 ng/mL), cofactor-independent aPLs HL5B, HL5B F(ab)₂ (100 ng/mL) or RR7F (500 ng/mL), or ATG (100 µg/mL). aPLs were either added alone or together with C3 inhibitor compstatin (50 µM), PDI inhibitor 16F16 (2 µM), anti-TF antibody 21E10, or isotype rat IgG2a control (5 µg/mL). Cell-associated PCA was subsequently measured by single-stage clotting assay. (B) CD115-selected cells from the indicated knockout strains were exposed to antibodies as in panel A in plasma from wt, C3^−/−, C5^−/−, or Lrp8^−/− mice as indicated and clotting times were determined. (C) Murine monocytes were stimulated with antibody and inhibitors in autologous plasma as indicated for 10 minutes. Samples were analyzed for PS exposure (Annexin V–FITC binding) using flow cytometry. (D) Murine monocytes were stimulated with antibody in plasma from wt, C3^−/−, C5^−/−, or Lrp8^−/− as indicated. PS exposure was determined as in panel C. Data in panels A and C are shown as mean ± standard deviation (SD). Data in panels B and D are shown as median, interquartile range, and range; n = 6; *P < .001. One-way ANOVA followed by the Dunnett multiple-comparison test. MFI, mean fluorescence intensity; unst., unstimulated.