CD20 antibodies of the IgA isotype differ in their capacity to trigger CDC. (A) The CDC activity of CD20 antibodies of the IgG1 (left panel) or IgA2 (right panel) isotype was analyzed in [⁵¹Cr] release assays against SU-DHL-4 cells using increasing concentrations of antibodies. (B) Comparison of the maximal CDC activity of CD20-IgG1 and CD20-IgA2 antibodies, as determined in (A). (C) CD20 antibody–triggered binding (C1q) and deposition of complement components (C4b/c, C3b/c) on SU-DHL-4 cells was analyzed by indirect flow cytometry. CD20 antibodies were added at 10 µg/mL for 60 minutes. Results are mean ± SEM from ≥3 experiments. *Significant difference specific antibody vs isotype control. ^§Significant difference between OBI-IgA2 and RTX-IgA2 or OFA-IgA2. ^#Significant difference lgG1 vs lgA2. Significant differences (P < .05) were calculated using ANOVA. conc., concentration; ctrl, control; MFI, mean fluorescence intensity; n.s., not significant.