Flt3 Ligand Synergizes With Granulocyte-Macrophage Colony-Stimulating Factor or Granulocyte Colony-Stimulating Factor to Mobilize Hematopoietic Progenitor Cells Into the Peripheral Blood of Mice

Peripheral blood progenitor cells (PBPC) are increasingly being used in the clinic as a replacement for bone marrow (BM) in the transplantation setting. We investigated the capacity of several different growth factors, including human flt3 ligand (FL), alone and in combination with granulocyte-macrophage colony-stimulating factor (GM-CSF ) or granulocyte colony-stimulating factor (G-CSF ), to mobilize colony forming cells (CFU) into the peripheral blood (PB) of mice. Mice were injected subcutaneously (SC) with growth factors daily for up to 10 days. Comparing the single agents, we found that FL alone was superior to GM-CSF or G-CSF in mobilizing CFU into the PB. FL synergized with both GM-CSF or G-CSF to mobilize more CFU, and in a shorter period of time, than did any single agent. Administration of FL plus G-CSF for 6 days resulted in a 1,423-fold and 2,717-fold increase of colony-forming unit-granulocyte-macrophage (CFU-GM) and colony-forming unit granulocyte, erythroid, monocyte, megakaryocyte (CFU-GEMM) in PB, respectively, when compared with control mice. We also followed the kinetics of CFU numerical changes in the BM of mice treated with growth factors. While GM-CSF and G-CSF alone had little effect on BM CFU over time, FL alone increased CFU-GM and CFU-GEMM threefold and fivefold, respectively. Addition of GM-CSF or G-CSF to FL did not increase CFU in BM over levels seen with FL alone. However, after the initial increase in BM CFU after FL plus G-CSF treatment for 3 days, BM CFU returned to control levels after 5 days treatment, and CFU-GM were significantly reduced (65%) after 7 days treatment, when compared with control mice. Finally, we found that transplantation of FL or FL plus G-CSF-mobilized PB cells protected lethally irradiated mice and resulted in long-term multilineage hematopoietic reconstitution.