Functional characterization of a novel hematopoietic stem cell and its place in the c-kit maturation pathway in bone marrow cell development

While the majority of purified pluripotential hematopoietic stem cells (PHS C) express c-Kit, the receptor for steel factor, we have phenotypically and functionally separated a distinct class of PHSC that does not express c-Ki t. In contrast to c-Kit-postive (c-Kit(pos)) PHSC, the c-Kit-negative (C-Ki t(neg)) PHSC do not proliferate in response to multiple hematopoietic growt h factors in vitro and do not radioprotect or form macroscopic spleen colon ies (CFU-s) when transplanted into lethally irradiated recipients. However, the C-Kit(neg) PHSC show delayed or slow reconstitution kinetics when cotr ansplanted with radioprotective bone marrow cells. c-Kit(neg) PHSCs cells c an give rise to c-Kit(pos) cells with CFU-s activity, radioprotective activ ity, and PHSC activity. Thus, constituitive hematopoiesis is maintained by c-Kit(pos) PHSCS cells that are recruited from a more primitive quiescent c -Kit(neg) PHSC population, which represents a critical developmental stage in definitive hematopoiesis.