Role of Fas in homing of CD34⁺ cells. (A) Effect of cytokine exposure on the expression of surface receptors in MPB CD34⁺ cells. Freshly isolated CD34⁺ cells (□) or cultured cells (48-hour stimulation with SCF, FL [100 ng/mL] and IL-3, IL-6 [20 ng/mL]) (▪) were stained with FITC or PE-conjugated antihuman CD34, CD38, CXCR4, and Fas/CD95 mAb and were analyzed by flow cytometry. Each bar represents the mean ± SEM of 3 experiments (*P < .05 and **P < .01 compared with fresh cells; Student t test). (B) Blocking Fas ligation and effect on homing. Cultured CD34⁺ cells were pretreated with ZB4 (2 and 5 μg/mL) for 30 minutes (i) or were treated with sFasL at 10, 50, and 100 ng/mL (ii) before transplantation into animals (2 × 10⁶ cells/animal). Short-term BM homing was assayed as in Figure 2. Data are displayed as for Figure 2B.