Lack of CD19 impairs B-cell maturation in BM cultures. (A) Cells grown in BM cultures were stained for the indicated surface markers and analyzed by FACS. Results from 1 of 4 representative experiments are shown. Numbers in graphs indicate cell percentages within the region. (B) Cells grown in BM cultures (□, wild type [wt];▵, CD19^-/-) were washed and recultured in the absence of IL-7 for 12 to 24 hours. Cells were then stained for B220 IgM and IgD and analyzed by FACS. The induction of differentiation was quantified by calculating the relative proportion of IgM⁺/IgD⁺ cells in the total IgM⁺ fraction. Results presented as mean ± SE of 4 different experiments. (C) Apoptosis rates in wt and CD19^-/- BM cultures were determined 12 to 24 hours after IL-7 withdrawal by TUNEL. Symbols indicate the same cell types as in panel B. Results presented as mean ± SE of 4 different experiments.