DL1 signaling alters Ig switching and secretion. (A) FO B cells were labeled with CFSE and stimulated as indicated (anti-IgM at 12.5 μg/mL) for 72 hours. Results are representative of at least 3 independent experiments. (B) Histogram presents the frequency of IgG1⁺ B cells after treatment with anti-IgM + anti-CD40 for 72 hours. Error bars represent SEM (n = 6). (C) Flow cytometric analysis of supernatants from cultures stimulated as in Figure 3A using a CBA assay that identifies all 7 mouse Ig isotypes in the order indicated. Presence of a particular isotype is indicated by kappa fluorescence (with labels provided to indicate positive cultures) (ie, anti-IgM–treated cultures are negative for all isotypes, while the OP9-Ctrl anti-CD40 culture is positive for secreted IgG1 and IgM).