B cell–specific FIP200-deficient mice showed an impaired humoral immune response. (A and B) Six WT and five B-Fip200^−/− mice were immunized with 30 μg NP₂₉-KLH with Imject Alum and boosted at day 85 with 30 μg NP₂₉-KLH in PBS. Mouse serum was collected throughout, and IC50 of anti-NP₂₉ (A)– or anti-NP₇ (B)–specific IgG antibodies was measured by ELISA. Serum collection days are marked on the x axes. (A) Left to right: *P = 0.0420, **P = 0.0098, **P = 0.0096, ***P = 0.0010 (unpaired t test). (B) Left to right; *P = 0.023, *P = 0.0102, **P = 0.0070, ***P = 0.0002 (unpaired t test). (C) Top: WT (n = 5) and B-Fip200^−/− (n = 3–7) mice were immunized with 50 μg NP₂₉-KLH with Imject Alum and sacrificed at day 11. Bottom: Representative flow cytometry plots and frequency of Fas⁺CD38⁻ GC and CD138⁺ plasma B cells from two experiments; frequencies from one representative experiment quantified. *P = 0.0237, **P = 0.0028 (unpaired t test). (D) WT (n = 9) and B-Fip200^−/− (n = 6) 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^loNP⁺ memory B cells from one experiment. Left to right: ***P = 0.0002, ***P = 0.0004 (unpaired t test). (E) At day 42, WT (n = 4) and B cell–specific FIP200-deficient mice (n = 4) were boosted with 30 μg NP₂₉-KLH in PBS. Anti-NP₂₉– and anti-NP₇–specific IgG ASCs in both spleen and BM at day 49 by ELISPOT. Plots show values for individual mice (symbols) and mean ± SD (bars). Top: *P = 0.0255; bottom: *P = 0.0272, **P = 0.0031 (unpaired t test). Performed twice, one replicate shown.