STUDIES OF W-MUTANT MICE PROVIDE EVIDENCE FOR ALTERNATE MECHANISMS CAPABLE OF ACTIVATING HEMATOPOIETIC STEM-CELLS

Previous studies have suggested that Steel factor (SF) can influence t he behavior of many types of hematopoietic progenitor cells both in vi vo and in vitro, although whether these may include the most primitive populations of totipotent repopulating cells remains controversial. T o approach this question, we measured the number of Scal(+)Lin(-)WGA() cells, the number of cells with demonstrable myeloid (long-term cult ure-initiating cell [LTC-IC]) or both myeloid and lymphoid (LTC-ICML) potential in 4- to 5-week-old long-term cultures containing irradiated primary marrow feeder layers, and the number of multilineage long-ter m in vivo repopulating cells (competitive repopulating unit [CRU]) pre sent in the marrow of W-42/+ or W-41/W-41 mice compared to +/+ control s. There was no significant effect of either of these W mutations on t he number of Sca1(+)Lin(-)WGA(+) cells and, in W-41/W-41 mice, neither LTC-IC nor LTC-ICML populations appeared to be affected. On the other hand, although W-41/W-41 and W-42/+ cells could both be detected in t he in vivo CRU assay, their numbers were markedly reduced (17- and sev en-fold, respectively) in spite of the fact that both of these W mutan t genotypes contained near normal numbers of day-9 and -12 colony-form ing units-spleen (CFU-S). In vitro quantitation of erythroid (burst-fo rming units-ery throid [BFU-E]), granulopoietic (CFU-granulocyte/macro phage [CFU-GM]), multilineage (CFU-granulocyte/erythrocyte/monocyte/ma crophage [CFU-GEMM]), and pre-B clonogenic progenitors (CFU-pre-B) als o revealed no differences in the numbers (or proliferative potential) of any of these cells when W-41 or W-41 or W-42/+ and normal mice were compared, although day 3 BFU-E from both types of W mutant mice showe d no response to the typical enhancing effect exerted by SF on their /+ counterparts. Taken together, these findings are consistent with th e view that sF activation of c-kit receptor-induced signaling events i s not a rate-limiting mechanism controlling red blood cell production during normal development until hematopoietic cells differentiate beyo nd the day-3 BFU-E stage. Nevertheless, normal hematopoietic stem cell s do appear to be responsive to SF, since their W mutant counterparts display a disadvantage in the in vivo setting which is exaggerated und er conditions of hematopoietic regeneration. On the other hand, altern ative mechanisms also appear to contribute to the regulation of hemato poietic stem cell numbers in vivo and to their detection as LTC-IC in vitro.