Mice lacking p100 have markedly increased numbers of MZ B cells. (A) Splenocytes from wild-type, p100^−/−, and nfkb2^−/− mice were stained for CD23 and CD21 and analyzed by flow cytometry (lymphocyte gate). Percentages indicate CD23⁻CD21⁺ MZ B cells. (B) CD23⁻ (top row) and CD23⁺ (bottom row) splenocyte subpopulations from wild-type, p100^−/−, and nfkb2^−/− mice were analyzed for CD21 and IgM expression. Percentages of newly formed T1 B cells (CD23⁻CD21⁻IgM^hi), T2 B cells (CD23⁺CD21⁺IgM^hi), MZ B cells (CD23⁻CD21^hiIgM^hi), and FO B cells (CD23⁺CD21^intIgM^lo) are shown (black regions). Among the T2 B-cell subpopulation, percentages of presumed MZ precursor B cells (MZPB; CD23⁺CD21^hiIgM^hi) are indicated (red regions). Representative data from 3 to 4 experiments are shown. (C) Absolute numbers of B-cell subpopulations per 10⁶ splenocytes from wild-type, p100^−/−, and nfkb2^−/− mice are shown. Error bars indicate standard deviation (SD) from at least 5 mice per genotype. (D) MZ B cells from p100^−/− mice show better survival than wild-type controls. Splenocytes were cultured for 48 hours in media or stimulated with LPS or anti-CD40 mAb. Percentages of Annexin-V⁺ apoptotic cells in MZ and FO B-cell subpopulations are shown. Error bars indicate SD from 3 experiments. Significant differences are indicated (Student t test, *P < .05; **P < .01; ***P < .002). (E) Spleens from p100^−/− mice exhibit an enlarged MZ B-cell population adjacent to MOMA-1⁺ macrophages. Spleen sections from age-matched wild-type and p100^−/− mice were stained with anti-IgM for B cells (Vector Blue) and MOMA-1 for MMMs (DAB brown). Objective: × 40. Images were acquired through an AX70 Olympus microscope (Olympus, Hamburg, Germany) with a 40×/0.75 objective by an Olympus DP70 digital camera running analySIS^B Soft Imaging System software (Olympus) and processed with Adobe Photoshop 8.0 software (Adobe, San Jose, CA).