IN-VITRO EXPANSION OF HEMATOPOIETIC PROGENITORS AND MAINTENANCE OF STEM-CELLS - COMPARISON BETWEEN FLT3 FLK-2 LIGAND AND KIT-LIGAND/

The effects of FLT3/FLK-2 ligand (FL) and KIT ligand (KL) on in vitro expansion of hematopoietic stem cells were studied using lineage-negat ive (Lin(-))Sca-1-positive (Sca-1(+)) c-kit-positive (c-kit(+)) marrow cells from 5-fluorouracil (5-FU)-treated mice. As single agents, neit her FL nor KL could effectively support the proliferation of enriched cells in suspension culture. However, in combination with interleukin- ll (IL-ll), both FL and KL enhanced the production of nucleated cells and progenitors, The kinetics of stimulation by FL was different from that by KL in that the maximal expansion by FL of the nucleated cell a nd progenitor pools required a longer incubation than with KL. We then tested the reconstituting abilities of cells cultured for 1, 2, and 3 weeks by transplanting the expanded Ly5.1 cells together with ''compr omised'' marrow cells into lethally irradiated Ly 5.2 mice. Cells that had been expanded with either cytokine combination were able to maint ain the reconstituting ability of the original cells. Only cells that had been incubated with KL and IL-11 for 21 days had less reconstituti ng ability than fresh marrow cells. These results indicate that there can be significant expansion of progenitors in vitro without compromis ing the reconstituting ability of stem cells. Addition of IL-3 to perm issive cytokine combinations significantly reduced the ability of cult ured cells to reconstitute the hematopoiesis of irradiated hosts. Thes e observations should provide a basis for a rational approach to desig ning cytokine combinations for in vitro expansion of hematopoietic ste m cells. (C) 1997 by The American Society of Hematology.