Engraftment of hematopoietic stem cells in nonmyeloablated and myeloablated hosts.

Hematopoietic stem cell engraftment has traditionally been assessed in irradiated murine hosts. More recently, we and others have shown that engraftment is virtually quantitative in host animals who have received no preconditioning myeloablation. It appears that at the stem cell level, engraftment may even be favored in the normal host. The final phenotypic readout in the engrafted animal is then determined by competition between the engrafted stem cells and the number of residual host stem cells. Further studies have indicated that with cytokine exposure in vitro, in vivo or 5-fluorouracil exposure and consequent stimulation of primitive hematopoietic stem cells to enter cell cycle, an engraftment defect relating to long-term engraftment occurs. This occurs in the face of an expansion of cycling surrogate stem cells as mirrored by the high proliferative potential colony-forming cell. These data indicate that the phenotype of the hematopoietic stem cell, as assessed in vitro, may not give insight into the phenotype of these cells in vivo.