Myeloid cell TFPI is required for aPL-induced thrombosis. (A-B) Murine CD115⁺ monocytes of control or TFPI^ΔK1 mice were stimulated for 10 minutes with aPLs (100 ng/mL) and then analyzed for PS exposure by Annexin V–FITC binding in flow cytometry (A) or clotting activity (B); mean ± SD. n = 6. *P < .001; 2-way ANOVA followed by the Sidak correction. (C) Induction of TNFα mRNA in CD115⁺ spleen monocytes stimulated for 1 hour with the indicated aPLs; mean ± SD. n = 6. *P < .001; 2-way ANOVA followed by the Sidak correction. (D) TNFα induction by IgG fractions isolated from 20 patients with APS in WT and TFPI^ΔK1 mice stimulated for 1 hour. APS patient IgG samples can be divided into 3 groups: patient IgG that binds only to cardiolipin in a cofactor-independent manner similar to HL5B (αCL; n = 11); patient IgG that binds to both antigens, similar to HL7G (αCL/β2GPI; n = 7); and patient IgG that binds only to β2GPI, similar to rJGG9 (αβ2GPI; n = 2). *P < .0001; 2-way ANOVA followed by the Sidak correction. (E) Ligation-induced arterial thrombosis in littermate TFPI^ΔK1 (n = 8) and TFPI^K1flfl control animals (n = 5). (F) Inferior vena cava ligation-induced venous thrombosis in littermate TFPI^ΔK1 (n = 9) and TFPI^K1flfl control animals (n = 9) 48 hours after flow restriction. No thrombus developed in 4 and 3 animals, respectively. (G) HL5B-amplified thrombosis analyzed in the flow-restricted vena cave inferior of the indicated mouse strains vs isotype control antibody; mean ± SD. *P = .007; t-test after Shapiro-Wilk test for normal distribution. HL5B: TFPI^K1flfl control mice, n = 8; TFPI^ΔK1 mice, n = 7; isotype control TFPI^K1flfl control mice, n = 5; TFPI^ΔK1 mice, n = 5. Scale bar = 100 µm.