FTY720 up-regulates bone marrow homing of immature hematopoietic progenitors in vivo. PB CD34⁺ cells (2 × 10⁶) were injected into NOD/SCID mice, either untreated or after systemic pretreatment of animals and cells with FTY720 as indicated. (A) Representative flow cytometry analysis of cells recovered from the bone marrow of NOD/SCID mice 18 hours after transplantation. R1 = gate for human CD34⁺ cells according to the FSC/SSC characteristics; R2 = total bone marrow leukocytes. (B) Gated on R1; CD34/CD38 staining of cells recovered from the bone marrow of NOD/SCID mice that received transplantations of CD34⁺-enriched cells. (C) Homing of human progenitor cells in the bone marrow of NOD/SCID mice 18 hours after transplantation, depicted as human cells per 10⁴ total bone marrow leukocytes. Systemic pretreatment of animals and cells with FTY720 resulted in a significant increase in the number of more immature, CD34⁺/CD38^- cells recovered from the bone marrow, while the total number of human CD34⁺ HPCs was only moderately enhanced. ^**P < .01.