Regulation of colony forming cell generation by flt-3 ligand

The recently cloned ligand for the flt-3/flk-2 receptor was examined for its effect on colony formation by subpopulations of CD34⁺ cells including the least mature CD34⁺lin^?CD38^? small-medium lymphocyte-sized cell population. Flt-3 ligand (flt-3l) had little or no effect when added alone to cells. Isolated CD34⁺lin⁺ cells formed increased numbers of colony-forming cells (CFC) when flt-3l was added together with IL-3, IL-6, G-CSF, GM-CSF or c-kit ligand (KL), or with the combination of IL-3 and KL. Significant increases in CFC formation from CD34⁺lin^? cells were consistently seen when flt-3l was added to the IL-3 and KL combination, with variable effects observed when it was added to individual growth factors. Studies of the generation of CFC from CD34⁺lin^? cells in liquid cultures showed that cultures containing IL-3 and KL continued to produce CFC after 3 weeks of culture, whereas cultures with IL-3, KL and flt-3l produced few CFC past 2 weeks of culture. Flt-3l alone or the combination of IL-3 and KL did not stimulate significant growth of CD34⁺lin^?CD38^? small-medium lymphocyte-sized cells, although these cells reproducibly generated CFC when grown in the combination of IL-1β, IL-3, IL-6, G-CSF, GM-CSF and KL. Addition of flt-3l to either IL-3 and KL or to a combination of growth factors induced increased CFC in three of four experiments. These data therefore demonstrate a role for flt-3l in the induction of myelopoiesis by haemopoietic precursors, including the least mature subpopulation population of CD34⁺ cells.