Impaired B-cell development in the BM of IRF8^−/− mice. (A) BM cells from the indicated mice (n = 8) were stained for B220, IgM, CD43, BP-1, and HSA. The percentage of B cells falling within each gate is given. Hardy Frs A to F are indicated. L indicates large; S, small. (B) The frequency and absolute numbers of each B-cell subset. Data are mean plus or minus SEM of 6 mice. * indicates P value less than .001 compared with control groups; **, P value less than .05 compared with Fr A cells. (C) IL-7 cultured BM cells were stained with antibodies against B220, IgM, and pre-BCR. The anti–pre-BCR antibody (clone SL156) specifically recognizes an epitope composed of the μH chain and the surrogate L chain. The percentage of B cells falling within the gate is given. Data represent 1 of 3 mice with similar results. (D) Sorted HSCs from IRF8^+/+ and IRF8^−/− mice were injected into lethally irradiated CD45.2⁻ B6 mice. Two months later, BM cells were analyzed by FACS and the donor cells (CD45.2⁺) were identified by an anti-CD45.2 antibody. The frequency (left panel) and absolute cell numbers (right panel) of donor B cells were shown as mean plus or minus SEM of 6 mice per group. * P < .05; **P < .001 compared with the control group.