Six- to 10-week-old CD19^+/CreSfpi1^lox/loxSpib^−/− mice have reduced follicular B cell frequencies. (A) Reduced B-cell frequencies in the spleen of CD19^+/CreSfpi1^lox/loxSpib^−/− mice compared with C57Bl/6 mice. Flow cytometric analysis was performed to determine the frequency of B cells expressing CD19 and B220 (top panels) or surface IgM and B220 (bottom panels) in the spleen of mice of the indicated genotypes. (B) Quantitation of the frequency of splenic B cells expressing CD19 and B220 for groups of 5 mice of the indicated genotypes. (C) Quantitation of the frequency of splenic B cells expressing surface IgM and B220 for groups of 5 mice of the indicated genotypes. (D) Reduction of FO B-cell frequency and increase of marginal zone (MZ) B-cell frequency in the spleen of CD19^+/CreSfpi1^lox/loxSpib^−/− mice. Flow cytometric analysis was performed to determine the relative frequency of FO B cells (IgM⁺CD23⁺, indicated by ovals in top panels) and MZ B cells (IgM^hiCD21^hi, indicated by ovals in bottom panels) in the spleen of mice of the indicated genotypes. Results are shown gated on mature B220⁺CD93⁻ B cells. (E) Quantitation of the frequency of splenic FO B cells (B220⁺CD93⁻IgM⁺CD23⁺) for groups of 5 mice of the indicated genotypes. (F) Quantitation of the frequency of splenic MZ B cells (B220⁺CD93⁻IgM^hiCD21^hi) for groups of 5 mice of the indicated genotypes.