Impaired IgM response and increased susceptibility toS. pneumoniae infection in Stat1^−/− mice. (A) MZ B cells of WT and Stat1^−/− 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. Results represent three experiments. (B) WT and Stat1^−/− mice were primed with intravenous injection of 10⁵ CFU HK-S. pneumoniae for 24 h before being challenged with a lethal dose of 5 × 10⁴ live S. pneumoniae. Survival curves for WT and Stat1^−/− mice are shown. dpi, days postinfection (n = 7). Results represent two experiments. (C and D) Same as in B, except serum IgM (C) and anti-PC IgM (D) was measured by ELISA at the indicated times (n = 3). # mice died. Results represent two experiments. (E–G) Same as in B, except bacterial load in the blood (E), spleen (F), and liver (G) was measured (n = 3–5). Results represent three experiments. (H) Schematic diagram of serum transfer to Stat1^−/− mice from WT mice that had been primed with S. pneumoniae. (I) PBS or primed serum from WT mice was transferred into Stat1^−/− mice 1 d after infection with 5 × 10⁴ CFU of live S. pneumoniae (n = 4–6). Results represent two experiments. (J) Same as in I, except serum from primed WT mice were either untreated or PC-depleted first, and then transferred into Stat1^−/− mice. Survival curves are shown (n = 7). Results represent three experiments. All values are shown as the means ± SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001, Student’s t test (A and C–G) or Log-rank test (B, I, and J).