FIP200 is required for plasma but not GC maintenance. (A) 5 WT and 10 Aicda-Cre^+/− FIP200-deficient mice were immunized with 50 μg NP₂₉-KLH with Imject Alum and sacrificed at day 11. Two replicates were performed; representative flow cytometry plots and frequency of Fas⁺CD38⁻ GC and CD138⁺ plasma B cells from one experiment shown. (B) WT and Aicda-Cre^+/− FIP200-deficient mice were immunized with 30 μg NP₂₉-KLH with Imject Alum. Mouse serum was collected at regular intervals, and anti-NP₂₉ (upper)– or anti-NP₇ (lower)–specific IgG antibodies were detected by ELISA. Plots show values for individual mice (symbols) and mean ± SD (bars). Top: *P = 0.13; bottom: *P = 0.043. (C) WT (n = 9) and Aicda-Cre^+/− Fip200-deficient (n = 8) mice were immunized with 30 μg NP₂₉-KLH with Alhydrogel and sacrificed at day 42. Representative flow cytometry plots and frequency of B220⁺CD38⁺IgD^loIgG1⁺NP⁺ memory B cells from one experiment. Top to bottom: *P = 0.0111, **P = 0.0025 (unpaired t test). (D) WT (n = 4) and Aicda-Cre^+/− FIP200-deficient (n = 4) mice were immunized with 30 μg NP₂₉-KLH with Imject Alum and boosted with 30 μg NP₂₉-KLH in PBS at day 42. Anti-NP₂₉– and anti-NP₇–specific IgG ASCs in BM at day 49 by ELISPOT. Plots show values for individual mice with triplicates. Significant (α = 0.05) P values were determined by an unpaired t test. Left to right: ***P = 0.0007, ***P = 0.0003.