FXIa potentiates fibrin formation on HUVECs. (A) HUVECs were pretreated with increasing concentrations of FXIa for 2 hours; reactions were stopped with aprotinin (50 µM). Subsequently, recalcified plasma was added to HUVECs, and fibrin generation was measured in plasma pretreated with 20 µg/mL 1A6. (B) HUVECs were pretreated with vehicle or 5 nM FXIa for 30, 60, or 120 minutes; reactions were stopped with aprotinin (50 µM). Subsequently, recalcified plasma was added to HUVECs, and fibrin generation was measured in plasma pretreated with 20 µg/mL 1A6. Selected experiments were performed in the presence of blocking anti-TFPI K1 and K2 antibodies (50 µg/mL) or a blocking anti-TF antibody (10 µg/mL). (C) HUVECs were pretreated with vehicle or 5 nM FXIa for 2 hours in the presence or absence of 25 µM Zn²⁺, 50 µg/mL 10C9, 10 µg/mL anti-TFPI K1 and K2 antibodies, 20 µg/mL 1A6, or 50 µM aprotinin; reactions were stopped with aprotinin (50 µM). Subsequently, recalcified plasma pretreated with 20 µg/mL 1A6 was added to HUVECs, and the time for fibrin generation to reach half-maximal (T_half-max) was measured. P < .05 with respect to vehicle in the presence of FXIa. Mann-Whitney U test was used for statistical comparisons.