HIT ULICs promote monocyte procoagulant activity and TF expression in a complement-dependent manner. (A) C1q binding to HIT ULICs enhances monocyte procoagulant activity. KKO/ISO ULICs and human C1q were added to monocytes in serum-free media for 5 hours at 37°C under 5% carbon dioxide. C1q was added at C1q:KKO molar ratios of 1:8 [C1q (hi)] and 1:800 [C1q (lo)]. Aliquots from the cell suspensions were analyzed for FXa activity by using a chromogenic assay. Results of 2 independent experiments, each done in quadruplicate (mean ± standard error of the mean), are shown as a fold increase in FXa generation relative to cells incubated with media alone. ***P < .0001 (compared with all other conditions using the 1-way analysis of variance with Tukey’s multiple comparison test). (B) HIT ULICs induce monocyte TF expression upstream of C5: WB blood was preincubated without or with complement inhibitors/controls (α-C1q ISO Ab/α-C1q Ab 300 μg/mL, CP40-CON/Cp40 20 μM, IV.3-ISO/IV.3 10 μg/mL, α-C5-ISO ab/α-C5 Ab 50 μg/mL) before adding PF4/heparin/KKO ULICs as shown on the x-axis. After 4 hours of incubation, RBCs were lysed, leukocytes were stained with α-CD14 and α-TF Ab, and the percentage of TF-expressing monocytes was measured. The graphic shows the results of 1 of 3 independent experiments tested in duplicate (mean ± SD). ***P < .0001 (1-way analysis of variance with the Tukey’s multiple comparison test). ns, not significant.