Opsonization by complement facilitates ULIC binding and thereby FcR effector functions. (A) Complement inhibition by α-C1qAb reduces deposition of KKO ULICs. WB was preincubated with buffer, α-C1q (300 µg/mL), or ISO control (300 µg/mL) before adding ULICs containing KKO or ISO IgG (50-200 µg/mL). Surface-bound mouse (m)IgG was measured by flow cytometry. α-C1q-Ab, but not its ISO control, prevented deposition of mIgG on the cell surface. Effects of complement inhibition could not be overcome with higher doses of ULIC-IgG. (B) Complement inhibition inhibits TF expression: Similar incubation conditions from panel A were used, followed by detection of monocyte TF by flow cytometry. Inhibition of complement reduced TF expression. The graphic shows results representative of 3 independent experiments tested in duplicate (mean ± SD). P < .0001 (1-way analysis of variance with the Tukey’s multiple comparison test for KKO+PF4/heparin [H] at each concentration vs each concentration of ISO controls and similarly for conditions using α-C1q at each concentration vs each concentration of ISO controls for α-C1q). *Buffer. **PF4/KKO (200 µg/mL). MFI, mean fluorescence intensity.