Pluripotent hemopoietic stem cells in mice and humans.

Although it has been reported previously that pluripotent hemopoietic stem cells (P-HSCs) express c-kit, the receptor for stem cell factor (steel factor), we and other groups have recently shown that P-HSCs do not express c-kit. In this review, we provide evidence that c-kit<low P-HSCs in mice have long-term-reconstituting activity (LTRA > 2 years) and the capacity to form colony-forming units in spleen (CFU-S) on Day 16, although c-kit(low) HSCs or c-kit+ HSCs have LTRA less than 1.5 years and the capacity to form CFU-S on Day 14 or on Day 10, respectively. In addition, we have found that there is a major histocompatibility complex (MHC) restriction between P-HSCs and stromal cells; normal P-HSCs can proliferate and differentiate efficiently in collaboration with MHC class I-compatible (but not MHC class I-incompatible) stromal cells. In humans, we also show that c-kit<low P-HSCs can differentiate into c-kit(low) cells, then c-kit+ cells in vitro.