Murine Haemopoietic Stem Cells with Long-term Engraftment and Marrow Repopulating Ability are More Resistant to Gamma-radiation than are Spleen Colony Forming Cells

Abstract

The radiation sensitivity of various subsets in the haemopoietic stem cell hierarchy was defined using a limiting dilution type long-term bone marrow culture technique that was previously shown to allow quantification of cells with spleen colony-forming potential (day-12 CFU-S) and in vivo marrow repopulating ability (MRA). Primitive stem cells that generate new in vitro clonable colony-forming cells (CFU-C) in the irradiated marrow (MRA) and have long-term repopulation ability (LTRA) in vitro (cobblestone area forming cell, CAFC day-28) had D₀ values of 1·25 and 1·38 Gy, respectively. A lower D₀ was found for the less primitive CFU-S day-12, CAFC day-12 and cells with erythroid repopulating ability (0·91, 1·08 and 0·97 Gy, respectively). CFU-S day-7 were the most radiosensitive (D₀ equalling 0·79 Gy), while CFU-C and CAFC day-5 were relatively resistant to irradiation (D₀ 1·33 and 1·77 Gy). Split-dose irradiation with a 6 h interval gave dose sparing for stem cells with MRA and even more with in vitro LTRA, less for CFU-S day-12 and CAFC day-10 and none for CFU-S day-7. The cell survival data of the specified stem cell populations were compared with the ability of a fixed number of B6-Gpi-1^a donor bone marrow cells to provide for short- and long-term engraftment in single- and split-dose irradiated cognenic B6-Gpi-1^b mice. Serial blood glucose phosphate isomerase (Gpi) phenotyping showed less chimerism in the split as compared to the single radiation dose groups beyond 4 weeks after transplant. Radiation dose-response curves corresponding to stable chimerism at 12 weeks for single and fractionated doses revealed appreciable split-dose recovery (D₂–D₁) in the order of 2 Gy. This was comparable to D₂–D₁ estimates for MRA and late-developing CAFC (1·27 and 1·43 Gy, respectively), but differed from the poor dose recovery in cells corresponding to the committed CFU-S day-7/12 and CAFC day-10 population (0·14–0·33 Gy). These data are together consistent with differential radiosensitivity and repair in the haemopoietic stem cell hierarchy, and provide a cellular basis for explaining the dose-sparing effect of fractionated total-body irradiation conditioning on long-term host marrow repopulation.