Initiation of MHC II expression shapes B-cell development. (A) Flow cytometric identification of bone marrow Fr A-F B cells. (B-C) MHC II expression (B) and mean frequency (plus or minus SEM) of MHC II⁺ cells (C) in Fr A-F bone marrow B cells from mice of the indicated genotype (WT, n = 3; Itgax^creH2-Ab1^c, n = 5; Fcer2a^creH2-Ab1^c, n = 6). (D) MHC II expression (green), detected by immunofluorescence staining in WT Fr A-C bone marrow B cells (MHC II antibody clone M5/114.15.2 stain; scale bar, 5 μM). Itgax^creH2-Ab1^c Fr A bone marrow B cells are included as staining control. (E) Mean change (plus or minus SEM) from input frequency of CD45.2⁺ cells in Fr A-F bone marrow B cells from chimeras between CD45.1⁺ WT control and CD45.2⁺ test genotype donors (WT: WT, n = 9; WT: Itgax^creH2-Ab1^c, n = 6; WT: Fcer2a^creH2-Ab1^c, n = 5). *P < .001 between WT and other genotypes by Student t tests. (F-G) In vitro development of purified Fr A Itgax^creH2-Ab1^c B-cell precursors into pre-B (pre; B220⁺CD2⁺IgD⁻IgM⁻) and immature B cells (imm; B220⁺CD2⁺IgD⁻IgM⁺) (F), and mean change (plus or minus SEM) from input frequency of CD45.2⁺ cells in Fr A, pre-B cells, and immature B cells following in vitro development of purified Fr A cells in competitive cultures between CD45.1⁺ WT control and CD45.2⁺ test genotype input Fr A cells (G) (WT: WT, n = 6; WT: Itgax^creH2-Ab1^c, n = 2; WT: Fcer2a^creH2-Ab1^c, n = 8). *P < .02 between WT and other genotypes by Student t tests. FSC, forward scatter.