Cognate NKT cell help to B cells requires SAP-dependent signaling. (A) Outline of experiments in panels B through E. SAP^fl/fl.tgCreERT2 mice and B6 (WT) counterparts (9 to 10 weeks old) were treated with tamoxifen for 4 consecutive days and rested for an additional 5 days before immunization with 2 to 4 μg NP-αGalCer. Serum samples were analyzed on day 5 and 13, splenocytes on day 13. Antibody titers are combined from 2 independent experiments with 4 to 5 animals per group in each. Controls represent WT animals injected with αGalCer. (B-C) NP-specific IgM (B) and IgG (C) response 5 days after immunization with NP-αGalCer. (D) NP-specific serum IgG titers 13 days after immunization with NP-αGalCer. (E) Total numbers of B220⁺ GL7⁺ Fas⁺ NP-specific GC B cells in the spleen of NP-αGalCer (or αGalCer control) immunized mice. Representative result of 2 independent experiments.