Proliferative response of BM B-lymphocyte subpopulations of C/EBPβ+/+ and C/EBPβ−/− mice to IL-7 stimulation. (A) FACS analysis of sorted B-cell subpopulations. BM cells from C/EBPβ+/+ and C/EBPβ−/− mice were stained with CD45/B220-FITC and with antimouse IgM-biotin followed by streptavidin-PE labeling. Cells were sorted in CD45/B220⁺/IgM⁻ and CD45/B220⁺/IgM⁺ cell subsets by FACS sorting, as detailed in Materials and Methods. A purity of greater than 99.5% was consistently achieved. (B) Response of BM B lymphocytes from C/EBPβ+/+ or C/EBPβ −/− mice to IL-7 stimulation. CD45/B220⁺/IgM⁻ and CD45/B220⁺/IgM⁺ cell subsets were isolated from BM cells of C/EBPβ+/+ and C/EBPβ −/− mice, cultured into 96-well plates at 2 × 10⁴/0.1 mL, and stimulated with the indicated amounts of IL-7. On day 7, cultures were obtained to determine [³H]-thymidine incorporation. The data are expressed as the mean ± the standard deviations of three independent experiments. Similar results were obtained in the case of several independent experiments with B-cell populations obtained from long-term B-cell cultures (not shown).