Phenotype of the engrafting stem cell in mice

The present data on engraftment into non‐myeloablated mice strongly suggest that engraftment is determined by host‐donor ratios as opposed to opening space. Theoretically, if the ratios of donor to host stem cells could be altered, especially without causing toxicity to the host animal, then the phenotypic readout could be increased in a clinically applicable manner. To research this further, we investigated low‐dose irradiation (100 cGy) for its effects on marrow, spleen and peripheral blood counts, as well as engrafting stem cell levels. We found a transient but significant depression in the white blood cell and platelet counts in the peripheral blood which returned to normal by two weeks, with no apparent deleterious effect on the animals. However, the same irradiation dose after two months impaired marrow repopulation and reduced engraftment potential to less than 20% capacity. These results suggested that we could obtain much higher phenotypic readouts after engraftment with this model; thus, we assessed the engraftment of 40 million male BALB/c marrow cells into female hosts exposed to 100 cGy at two, five and eight months after cell infusion. The resultant high levels of chimerism, reaching 100% in many cases, strongly suggest that the key to engraftment in these models is host‐donor stem cell ratios. One important issue relative to the above finding is whether cytokine‐stimulated proliferating stem cells have irreversibly lost engraftment capacity or whether changes in the engraftment capacity are of a plastic nature, possibly related to cell cycle transit. A number of experiments following engraftment have shown that the engraftment defect is reversible and can be repeatedly lost and regained during the initial portions of a cytokine‐stimulated culture. The above results suggest that, at least at the more primitive stem cell level, hematopoietic stem cell regulation may in part be based on a cell cycle model rather than a hierarchical system.