CD27 signaling prevents IL-10–induced apoptosis of B cells. (A) B cells were cultured with medium alone, 100 ng/mL IL-10, IL-10 + 20% CD70 transfectants, or IL-10 + anti-CD40 MoAb (1 μg/mL). After 72 hours of incubation, the number of B cells undergoing apoptosis was determined by staining with propidium iodide, followed by analysis with flow cytometry. The percentage of apoptotic B cells is indicated. (B) DNA fragmentation was assessed by agarose gel electrophoresis. CD70 transfectants only (lane 1) and purified 2 × 10⁶ B cells (lanes 2 to 5) were incubated in the presence of medium alone (lane 2), 100 ng/mL IL-10 (lane 3), IL-10 + mock transfectants (lane 4), or IL-10 + CD70 transfectants (lane 5). After 3-day culture, the DNA fragmentation ladder was detected. A quantity of 4 × 10⁵ fixed transfectants were used for 1 line.