Serine phosphorylation of STAT1 regulates IgM production from MZ B cells. MZ B cells of WT mice were stimulated with 10 µg/ml CpG (A), 10 µg/ml LPS (B), or 1,000 U/ml IFN-α4 for the indicated times. Cell extracts were analyzed by immunoblotting with antibodies to serine- or tyrosine-phosphorylated STAT1 (pS-STAT1 or pY-STAT1) and STAT1. One experiment out of three is shown. (C) WT and Stat1^SA/SA mice were intravenously injected with 25 µg LPS and IgM in the serum was measured by ELISA at the indicated times (n = 6–11). Results represent three experiments. (D) MZ B cells of WT or Stat1^SA/SA mice (2 × 10⁶ each) were transferred into RBP-J CKO mice for 1 d, and then 25 µg LPS was intravenously injected. Total serum IgM of the mice treated at the indicated times was measured by ELISA (n = 3). Results represent three experiments. (E) MZ B cells of WT and Stat1^SA/SA mice were stimulated with the indicated doses of HK-S. pneumoniae in vitro for 4 d, and IgM in the culture supernatant was measured by ELISA (n = 4). Results represent four experiments. All values are shown as the means ± SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001, Student’s t test.