Inhibition of TF expression by α-FcRn antibody. (A) HIT ICs. THP-1 cells were stimulated by HIT ICs as in Figure 1 in the presence of an IgG4 α-FcRn mAb (200, 100, or 50 µg/mL) or an IgG4 control immunoglobulin (Con Ab; 200 µg/mL), and TF activity was measured as in Figure 1A. Data from 3 to 6 independent experiments (mean ± standard error of the mean [SEM]) are shown relative to cells stimulated by PF4 (10 µg/mL) alone. (B) TF activity by cells stimulated with FcRn binding and nonbinding IgG ICs. To form ICs, OVA^NIP (200 µg/mL) was incubated with an α-NIP IgG antibody (10 µg/mL) that binds FcRn (IgG^WT IC) or an isotype-matched antibody with mutations that permit binding to classical IgG Fc receptors but not FcRn (IgG^IHH IC). The complexes were diluted 1:10 before addition to cells. Induction of TF activity in the absence or presence of α-FcRn antibody was determined as in panel A. Data from 3 independent experiments (mean ± SEM) are shown relative to cells stimulated with IgG^IHH IC and α-FcRn. ^%P = .0124, ^∧P = .0021, and ^‡P = .0006 by 1-way analysis of variance (ANOVA) with Fisher’s least significant difference test; **P < .001 by 1-way ANOVA with Sidak’s multiple comparison test.