Enhanced apoptosis of PU.1^Δ/ΔCD19 B cells upon BCR cross-linking. (A) Reduced proliferation of PU.1^Δ/ΔCD19 B cells upon BCR cross-linking. Small resting naive B cells were isolated from the spleen and cultured in the presence of 10 μg/mL F(ab′)₂ anti-IgM. Cultures were pulsed with 1 μCi (37 Bq) [³H]thymidine for 6 hours and thymidine incorporation assessed. Mean counts per minute (CPM) ± standard deviation is shown. PU.1^+/+CD19 is shown in black circles and a solid line and PU.1^Δ/ΔCD19 in white with a dashed line. (B) Enhanced apoptosis in PU.1^Δ/ΔCD19 B cells upon BCR cross-linking. Cells were cultured in the presence of 10 μg/mL anti-IgM or IL-4 or in combination for the indicated time, the number of viable cells (annexin V negative, propidium iodide-negative) assessed by FACS, and the data converted to the percentage of the initial culture that remained viable at that time point. Numbers represent the mean ± standard deviation. (C) The expression of antiapoptotic genes in PU.1^Δ/ΔCD19 and PU.1^+/+CD19 BCR-stimulated B cells. Total RNA was isolated from cells cultured for 24 hours and assessed for the expression of antiapoptotic genes by RT-PCR.