Homing of freshly isolated and cytokine stimulated CD34⁺ cells. (A) Cell suspensions from the BM, spleen, and lungs of transplanted NOD/SCID mice recovered 1 hour and 24 hours after transplantation were assayed for colony formation using human-specific cytokines. Numbers of CFCs in lungs and spleen were corrected for the cell expansion after cytokine culture. Medians interquartiles, and ranges of 4 to 7 animals pooled from 3 experiments. (B) Time course of changes in BM homing after cytokine activation. CD34⁺ cells exposed to cytokines for 2, 4, or 5 days were infused into sublethally irradiated animals, and homing to BM was assessed 24 hours after infusion. Homing of fresh cells is shown for comparison. Data are from 4 independent experiments; each data point represents 1 animal, and the mean values are indicated. (C) Homing of freshly isolated CD34⁺ cells or cultured cells after 24 and 48 hours of culture to the spleen at 24 hours after transplantation. (D) CD34⁺ cells were cultured for 1, 4, 24, or 48 hours, and homing of infused cells to BM was assessed at 24 hours after transplantation; n = 2 experiments, data given for each animal (*P < .05; **P < .01; ***P < .001 compared with fresh cells; Mann-Whitney U test).