Fms-related Tyrosine Kinase 3 Expression Discriminates Hematopoietic Stem Cells Subpopulations With Differing Engraftment-potential: Identifying the Most Potent Combination

Background. Fms-related tyrosine kinase 3 (Flt3)-ligand (FL) promotes the proliferation, differentiation, development, and mobilization ofhematopoietic cells. We previously found that FL-mobilized hematopoietic stem cells (HSC) engraft efficiently, whereas FL-expanded bone marrow HSC do not. The function of FL-mobilized c-Kit + Sca-1 + Lin - (KSL) subpopulations has not been systematically evaluated. A precise definition of the repopulating ability is needed to define which HSC subpopulations are critical for long-term chimerism and tolerance induction. FL significantly mobilized c-Kit hi and c-Kit lo Sca-1 + Lin - cells into peripheral blood (PB). Here, we evaluated the influence of Flt3 expression on long-term repopulating ability of HSC subpopulations. Methods. c-Kit hi or c-Kit lo KSL cells were sorted from PB of FL-treated green fluorescent protein-positive donors. The function of these cells was evaluated using competitive reconstitution assays, colony-forming units spleen, and colony forming cell assays. The function of c-Kit hi CD34 - Flt3 - KSL, c-Kit hi CD34 + Flt3 - KSL, c-Kit hi CD34 + Flt3 + KSL were investigated in an in vivo transplantation model. Results. Only FL-mobilized PB c-Kit hi KSL cells exhibited high spleen colony-forming unit activity, generated high numbers of both lymphoid and myeloid colonies in vitro, and rescued ablated recipients. FL-mobilization expanded both c-Kit hi CD34 + Flt3 - cells (short-term HSC) and c-Kit hi CD34-Flt3- KSL cells (long-term HSC). There was a significant decrease in c-Kit hi CD34 + Flt3 + KSL late multipotent progenitors in PB. A combination of c-Kit hi CD34 + Flt3 - and c-Kit hi CD34-Flt3- KSL cells offered the most effective rescue of ablated recipients. Conclusions. These data suggest that engraftment of purified HSC is influenced by both short- and long-term repopulating populations and that Flt3 expression may be useful for selecting the most critical HSC subpopulations for transplantation.