Functional deletion of Mef2c/d in B-cell progenitors leads to a developmental block of B-cell differentiation. (A) Flow cytometric analysis to detect and enumerate pro-B cells (Lin⁻CD19⁺B220^medCD43^hiIgL⁻), pre-B cells (Lin⁻CD19⁺B220^medCD43^med/loIgL⁻), immature B cells (Lin⁻CD19⁺B220^medCD43^med/loIgL⁺), and mature (recirculating) B cells (Lin⁻CD19⁺B220^hiCD43⁻IgL⁺) in the BM of Mef2c^Δ/Δ (light blue), Mef2d^Δ/Δ (green), Mef2c/d^Δ/Δ (red), and control mice (Mef2c/d^+/+, dark blue). Pre-B cells were separated by forward light scatter (FSC) into large and small pre-B cells, as indicated. Representative flow cytometry plots of Mef2c/d^Δ/Δ and Mef2c/d^+/+ mice are shown (top). Graph (bottom) depicts the number of different B-cell subtypes within total BM. Each mouse is represented by a dot (n ≥ 8 per genotype). Horizontal lines indicate the median. **P < .01; ***P < .001; ns, not significant. (B-C) Evaluation of B cells in blood (n = 5 per cohort) (B) and spleen (n = 12 per cohort) (C) of Mef2c/d^Δ/Δ and control mice (Mef2c/d^+/+) using FACS analysis. Graph depicts the percentage of CD19⁺B220⁺ B cells. Dots represent individual mice and bars indicate the median. (D) B220/CD45R staining of B-cell zones (indicated by a star [★]) from lymph nodes of Mef2c/d^Δ/Δ and Mef2c/d^+/+ mice. B-cell zones are lacking in lymph nodes of Mef2c/d^Δ/Δ mice. Representative stainings from 2 experiments (I and II) using 2 mice per group are shown. Bar, 200 µm. (E) Schematic representation of the observed B-cell differentiation block at the pre-B-cell stage in BM of Mef2c/d^Δ/Δ mice. The stage in B-cell development at which Cd79a-hCre–mediated excision occurs is indicated. Receptors that are important for the proliferative expansion of the specific B-cell types are depicted.