TRAIL kills EPO-responsive CD34 cells. (A) Flow cytometric analysis of CD34 cells at day 3 and day 18 of culture (EPO, SCF, and IL-3) treated with 50 ng/mL TRAIL for 48 hours; residual cell viability was evaluated by staining cells with annexin V–FITC and PI, while cell differentiation was monitored by glycophorin A–PE and CD71-CY5 expression. (B) CD34-derived erythroblast sensitivity to TRAIL-induced apoptosis. At the indicated time intervals, cells were treated for 48 hours with 50 ng/mL TRAIL, and the percentage of residual cell viability was monitored by staining with annexin V–FITC and PI. Each histogram is the mean of 3 independent experiments expressed as percentages of control (CD34 cells cultured without TRAIL), *P < .05. (C) Sensitivity to TRAIL of CD34 cells cultured 3 days with IL-3 and SCF in serum-free medium with (+EPO) or without (–EPO) EPO. Data are expressed as percentages of TRAIL-untreated cells (3 independent experiments, *P < .05).