Stem Cell Factor Improves the Repopulating Ability of Primitive Hematopoietic Stem Cells after Sublethal Irradiation (and, to a Lesser Extent) after Bone Marrow Transplantation in Mice

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Abstract
Bone marrow transplantation (BMT) and sublethal irradiation (XRT) cause profound long-term damage to hematopoietic stem cells. We used the competitive repopulation assay in mice to test the ability of granulocyte-macrophage colony-stimulating factor (GM-CSF) and stem cell factor (SCF), cytokines given in clinical settings to enhance marrow recovery after XRT or BMT and to protect the marrow repopulating ability of primitive hematopoietic stem cells (PHSC) after these modalities. The repopulating ability of exhaustible multilineage progenitors (EMP) was also tested after these modalities, with or without cytokines. Repopulating abilities of EMP and PHSC were significantly reduced after XRT or BMT; PHSC were preferentially affected. Administration of SCF to C57B6/J mice after XRT resulted in improved EMP and PHSC repopulating ability, although progenitor numbers—repopulating units—were not completely returned to control levels. Whether given as a single dose or multiple doses, GM after XRT did protect PHSC function from the deleterious effects of XRT, but this was not a significant effect. SCF caused an increase in PHSC repopulating ability after BMT, but this too was not a significant difference. GM after BMT had little effect. SCF administration before XRT led to severe impairment of PHSC function with very little or no stem cell activity observed. Therefore, timing of its administration is an important consideration since preadministration of the cytokine before XRT can be extremely harmful to PHSC function.