The bone marrow developmental issue is B cell–intrinsic. (A) Flow cytometric analysis of lethally irradiated CD45.1 recipient mice receiving equal numbers of bone marrow cells (1 × 10⁶ cells/genotype) from CD45.2 LMC and CD45.2 mb1^DPWY by IV injection. Bone marrow was analyzed by flow cytometry based on the Hardy classification system 8 weeks after transfer. Data are shown as a representative plot for n = 5 mice per group from a single experiment. Summary data for the percentage of B220⁺ in (B) total bone marrow, the percentage of B220⁺ cells maturing B cells as measured by CD43 expression is decreased in CD45.2 mb1^DPWY compared with (C) CD45.1 WT cotransferred cells, and the percentage of each (A-F) developmental subset in the total bone marrow of CD45.2 mb1^DPWY vs (D) CD45.1 WT cells showing the significant loss of B cells in fraction C onward. Data are shown as mean ± SEM. (E) Representative plots and (F) summary data of B-cell developmental stages in day 4 in vitro bone marrow. Equal numbers of total bone marrow cells from mb1^DPWY and LMC mice were cultured for 4 days in the presence of IL-7 and harvested for analysis. Cells were analyzed by flow cytometry based on the Hardy classification system. Data are representative of n = 1-3 mice/genotype from 2 independent experiments. *P < .05, ***P < .001.