FH interacts with sialic acid on endothelial cells. (A) C3b was deposited on HUVECs by incubating cells in 25% NHS and using an anti-CD59 antibody to initiate complement activation. Deposition of C3b on cells was verified by showing increased binding of anti-C3c antibody by flow cytometry. (B-E) Binding of FH to C3b-bearing endothelial cells was competed with various proteins. Binding of fluorescently labeled FH was analyzed by flow cytometry. (B) A dose-response study of FH binding to TNF-α–activated C3b-bearing HUVECs in the presence of increasing concentrations of the lectin MAL II. (C) With 10 µM FH19-20, FH binding to TNF-α–activated C3b-bearing glomerular endothelial cells was inhibited, but it was not inhibited by 100 µM FH6-8. FH binding to (D) C3b-bearing HUVECs or to (E) TNF-α–activated C3b-bearing glomerular endothelial cells in the presence of wt and mutant FH19-20 fragments. Data on mutants of specific interest to this study are shown by dark gray–shaded bars, and data on controls are shown by light gray–shaded bars. Shown are average values (± SD) of experiments performed at least 3 times. One-way ANOVA with Tukey’s multiple comparison posttests was performed. **P < .01; ***P < .001.