SURVIVAL OF SPLEEN COLONY-FORMING-UNITS (CFU-S) OF IRRADIATED BONE-MARROW CELLS IN MICE - EVIDENCE FOR THE EXISTENCE OF A RADIORESISTANT SUBFRACTION

Because of increasing evidence of heterogeneity in the hematopoietic s tem cell compartments, the radiosensitivity of spleen colony-forming u nits (CFU-S) was reevaluated to ascertain whether the classical single exponential curve for a graded dose of radiation is applicable at hig her doses of radiation, 400-600 cGy. Bone marrow cells (BMC) removed f rom mice immediately after death under anesthesia were irradiated in v itro. Great care was taken to exclude anoxic effects during irradiatio n and to avoid any possible effects in the recipient mice from injecti on of excessive numbers of BMC. By estimating the number of cells to b e injected to produce numbers of colonies within the evaluation range of the assay, we obtained a radiation survival curve that appeared to have a multiphasic concave shape; the D-0 value for the 400-600 cGy ra nge was estimated to be about 275 cGy, whereas the D, for the lower do ses was 95 cGy, the same value as previously reported. The reason a si ngle exponential survival curve was previously obtained after graded d oses of radiation is discussed, and a comparison of those results with the present data from in vitro radiation is made. Lacking experimenta l evidence, we speculate that the major factor that determines the slo pe of the survival curve is the degree to which the stem cells are in their normal hematopoietic environment during the irradiation. The pro bable existence of a fraction surviving after an exposure to 600 cGy, estimated by the Limiting dilution assay, was about 1 per 2x10(6) BMC. Such radio-insensitive CFU-S appear to be primitive CFU-S, which can contribute materially to the long-term survival of lethally irradiated bone marrow recipients.