DL1 signals enhance MAPK activation during BCR- and CD40-mediated proliferation. (A) FO B cells were stimulated with anti-IgM (12.5 μg/mL) for the indicated times and then analyzed for expression of phosphorylated Erk (pErk) or p38 (pp38). Dashed lines indicate isotype controls for phosphostaining, while solid lines indicate phosphostaining in cells cultured on OP9-Ctrl (red) or OP9-DL1 (blue) cells. Results are representative of 3 independent experiments. (B) As in panel A, except that cells were stimulated with anti-CD40.