Lymphoid-restricted development from multipotent candidate murine stem cells: distinct and complimentary functions of the c-kit and flt3-ligands.

The two tyrosine kinase receptors, c-kit and flt3, and their respective ligands KL and FL, have been demonstrated to play key and nonredundant roles in regulating the earliest events in hematopoiesis. However, their precise roles and potential interactions in promoting early lymphoid commitment and development remain unclear. Here we show that most if not all murine Lin(-/lo)Sca1(+)c-kit(+) bone marrow (BM) cells generating B220(+)CD19(+) proB-cells in response to FL and interleukin-7 (IL-7) also have a myeloid potential. In contrast to FL + IL-7, KL + IL-7 could not promote proB-cell formation from Lin(-/lo)Sca1(+)c-kit(+) cells. However, KL potently enhanced FL + IL-7-stimulated proB-cell formation, in part through enhanced recruitment of FL + IL-7-unresponsive Lin(-/lo)Sca1(+)c-kit(+) progenitors, and in part by enhancing the growth of proB-cells. The enhanced recruitment (4-fold) in response to KL occurred exclusively from the Lin(-/lo)Sca1(+)c-kit(+)flt3(-) long-term repopulating stem cell population, whereas KL had no effect on FL + IL-7-stimulated recruitment of Lin(-/lo)Sca1(+)c-kit(+)flt3(+) short-term repopulating cells. The progeny of FL + IL-7-stimulated Lin(-/lo)Sca1(+)c-kit(+) cells lacked in vitro and in vivo myeloid potential, but efficiently reconstituted both B and T lymphopoiesis. In agreement with this FL, but not KL, efficiently induced expression of B220 and IL-7 receptor-alpha on Lin(-/lo)Sca1(+)c-kit(+)flt3(+) cells. Thus, whereas KL appears crucial for recruitment of FL + IL-7-unresponsive candidate (c-kit(+)flt3(-)) murine stem cells, FL is essential and sufficient for development toward lymphoid restricted progenitors from a population of (c-kit(+)flt3(+)) multipotent short-term reconstituting progenitors.