Effect of TRAIL on erythroid cell survival/growth. CD34⁺ cells were cultured for 6 days in the presence of EPO + IL-3 + SCF. At this time point, cultures were preincubated with vehicle (0.1% dimethyl sulfoxide [DMSO]) or z-VAD-fmk (z-VAD, 20 μM) before treatment with TRAIL. After approximately 24 hours of treatment, apoptosis was assessed by annexin V–FITC binding and PI staining (A), whereas cell proliferation was assayed by measuring [³H]thymidine incorporation (B). In panel B, data represent the means ± SDs of 4 experiments; *P < .05. In panel C, representative cell morphology, examined on days 9 to 10 of culture by light microscopy after May-Grunwald-Giemsa staining, is shown. Most erythroblasts show an immature morphology in both TRAIL and TRAIL + z-VAD-fmk–treated cultures. Original magnification, × 40.