IMPAIRED STEEL FACTOR RESPONSIVENESS DIFFERENTIALLY AFFECTS THE DETECTION AND LONG-TERM MAINTENANCE OF FETAL LIVER HEMATOPOIETIC STEM-CELLSIN-VIVO

The results of previous studies have shown that the development of hem atopoiesis during fetal life can occur in the absence of Steel factor (SF) signaling. On the other hand, impairment of this mechanism can se verely compromise the ability of cells from adult bone marrow to regen erate hematopoiesis on their transplantation into myeloablated recipie nts. This apparent paradox could result from changes during ontogeny i n the responsiveness of hematopoietic stem cells to regulators that ma y substitute for SF as well as from differences in the availability of such factors during embryogenesis and in the myeloablated adult. To i nvestigate these possibilities, we studied the effect of W-41 and W-42 mutations on the numbers, phenotype, and posttransplant self-renewal behavior of primitive hematopoietic cells present in the fetal liver ( FL) of 14.5-day-old mouse embryos. In W-41/ W-41 FL, day-12 spleen col ony-forming units and long-term culture-initiating cells appeared both quantitatively and qualitatively similar to their counterparts in the FL of +/+ embryos, W-41/W-41 FL also contained near normal numbers (a pproximate to 50% of controls) of transplantable lymphomyeloid stem ce lls with competitive reconstituting ability in myeloablated adult +/recipients (as assessed for up to at least 16 weeks posttransplant). M oreover, both the original phenotype of these W-41/W-41 competitive re populating units (CRUs) and their clonal posttransplant output of matu re progeny were normal. Similarly, when myeloablated adult +/+ mice we re cotransplanted with 5 x 10(4) +/+ FL cells and a sevenfold to 70-fo ld excess of W-41/W-41 FL CRUs, the contribution of the +/+ FL CRUs to the circulating white blood cell count present 5 weeks later was mark edly reduced as compared with that of mice that received only +/+ FL c ells. However, over the next 3 months, the proportion of mature white blood cells that were derived from +/+ precursors increased significan tly (P < .002) in all groups (to greater than or equal to 30%), indica ting that the ability to sustain hematopoiesis beyond 5 weeks is more SF-dependent than the ability to initially reconstitute both lymphoid and myeloid compartments. Cells from individual FL of W-42/+ matings a lso showed an initial ability (at 7 to 8 weeks posttransplant) to comp etitively repopulate both lymphoid and myeloid compartments of myeloab lated +/+ adult recipients. However, in contrast to recipients of norm al or W-41/W-41 FL cells, the repopulation obtained with the W-42 muta nt stem cells was transient. Secondary transplants confirmed the inabi lity of the W-42 mutant cells to regenerate or even maintain a populat ion of transplantable stem cells. Taken together with previous results from studies of CRUs in adult W mice, these findings support the conc ept of changes in the way hematopoietic stem cells at different stages of development respond to the stimulatory conditions evoked in the my eloablated recipient. In addition, they provide the first definitive e vidence that SF is a limiting physiological regulator of sustained hem atopoietic stem cell self-renewal in vivo. (C) 1997 by The American So ciety of Hematology.