MZ B cells are the dominant B-cell population in the spleen of Notch2IC//CD19Cre^+/− mice. (A) Splenic weight of 8-week-old Notch2IC//CD19Cre^+/− mice and control littermates. Points represent data from individual mice, and horizontal bars mark the mean value. ***P < .001, as calculated by the 2-tailed Student t test. (B-C) Notch2IC expression strongly drives MZ B-cell differentiation. Lymphocytes of spleen (SP) and iLNs of the indicated genotype were analyzed by flow cytometry for the expression of IgM and IgD (B) or CD21 and CD23 (C). Plots B and C are gated on lymphocytes and B220⁺ cells. Numbers indicate mean percentages and SDs of lymphocyte-gated populations of B220⁺ B cells. The right panels are additionally gated on hCD2, displaying the distribution of Notch2IC-expressing cells. Fo B cells (IgM⁺IgD⁺), MZ B and transitional B cells (IgM⁺IgD^low; B); Fo B cells (CD21^intCD23⁺) and MZ B cells (CD21^highCD23^low; C). The calculation is based on 11 independent experiments. (D) B1a and B2 cells are decreased in the peritoneal cavity (PC) of Notch2IC//CD19Cre^+/− mice. Flow cytometric analysis of cells isolated from the PC. Cells are pregated on lymphocytes and IgM⁺ cells. Numbers display the percentages and SDs of lymphocyte-gated, IgM⁺ B1a (B220^lowCD5⁺), B1b (B220^lowCD5^low), and B2 (B220^highCD5⁻) cells. Values were calculated from 9 independent experiments. **P < .01 and ***P < .001, calculated by the 2-tailed Student t test, in comparison to control.