The FLT3 ligand is a direct and potent stimulator of the growth of primitive and committed human CD34+ bone marrow progenitor cells in vitro.

The present studies investigated the effects of the recently cloned flt3 ligand (FL) on the in vitro growth and differentiation of primitive and committed subsets of human CD34+ bone marrow (BM) progenitor cells. FL alone was a weak growth stimulator of CD34+ BM cells, but synergistically and directly enhanced colony formation in combination with interleukin (IL) 3, granulocyte colony-stimulating factor (G-CSF), CSF-1, granulocyte macrophage (GM) CSF stem cell factor (SCF), and IL-6. FL and SCF were equally effective in stimulating colony formation in combination with IL-3. However, the tri-factor combination of FL + IL-3 + SCF stimulated 2.3-fold and 2.5-fold more colonies than FL + IL-3 and SCF + IL-3, respectively. These additional recruited progenitors appeared to be predominantly located in a primitive (CD71-) subset of the CD34+ progenitors, as 4.5-fold more colonies were formed by CD34+CD71- cells in response to FL + IL-3 + SCF than to FL + IL-3 or SCF + IL-3. Similar findings were observed in serum-containing and serum-deprived cultures. Whereas FL did not enhance burst-forming unit-erythroid (BFU-E) colony formation of CD34+ BM cells in the presence of serum, a low number of BFU-E colonies were formed in response to FL plus erythropoietin (Epo) under serum-deprived conditions. In addition, FL both in serum-containing and serum-deprived cultures stimulated colony formation of more committed myeloid progenitors in CD34+CD71+ BM cells. Thus, FL potently stimulates the growth of primitive and more committed human BM progenitor cells.