IL-7Rα cannot rescue B-cell development from c-Myb deficient fetal liver cells. (A) Generation of CD19⁺ B cells from retrovirally infected c-Myb^Plt4/Plt4 fetal liver cells. Fetal liver cells from e14.5 embryos were depleted of Ter119⁺ cells and transduced with control retrovirus or retrovirus encoding c-Myb, Ebf1, Bcl2, constitutively active STAT5A(1*6), or IL-7Rα; all retroviruses included an IRES GFP to identify infected cells. Retrovirally infected cells were grown on OP9 stromal cells for 8 days in the presence of IL-7 and Flt3L. Numbers above the boxes indicate percentages of B cells (CD19⁺) among GFP⁺ cells. Plots are representative of 6 independent experiments. (B) Quantitation of CD19⁺ B cells generated from c-Myb^Plt4/Plt4 fetal liver cells transduced with retrovirus as in panel A. The number of GFP⁺ CD19⁺ cells was counted after 4, 6, and 8 days of culture. Data represent the mean ± SEM of 3 independent experiments. (C) Flow cytometry of IL-7Rα surface expression on B cells generated from c-Myb^Plt4/Plt4 fetal liver cells transduced with control, c-Myb, or Ebf1 retrovirus. Plots are gated on GFP⁺ CD19⁺ cells. Data are representative of 6 independent experiments. (D-E) Generation of CD19⁺ B cells from retrovirally infected (D) c-Myb^+/+ or (E) c-Myb^Δmb1/Δmb1 CLPs. Cells were transduced and cultured as in panel A. Plots are representative of 2 independent experiments.