ARE STEM-CELLS EXPOSED TO IONIZING-RADIATION IN-VIVO AS EFFECTIVE AS NONIRRADIATED TRANSFUSED STEM-CELLS IN RESTORING HEMATOPOIESIS

Are the stem cells that survive graded doses of ionizing radiation as effective in restoring hematopoiesis in irradiated mice as transfused nonirradiated stem cells? This question was addressed by determining a nimal dose mortality and 10-day colony-forming units (CFU-S) survival curves and then replotting the percent animal survival against the num ber of CFU-S surviving the different doses of radiation, and by determ ining the number of nonirradiated CFU-S injected into fatally irradiat ed mice that result in a CFU-S dose mortality response curve. The numb er of CFU-S surviving per mouse after doses of radiation resulting in 95, 50 and 5% animal survival were calculated to be 520, 300 and 153, respectively From the transfused CFU-S dose mortality curve of otherwi se fatally irradiated mice (8.5 Gy), the number of transfused normal C FU-S required for 95, 50 and 5% animal survival was estimated to be 15 3, 24 and 3, respectively. The ratios of surviving CFU-S to nonirradia ted, injected CFU-S are: at 95% survival. (6.3 Gy), 3.4; at 50% surviv al (6.88 Gy), 12.5; and at 5% survival (7.4 Gy), 51.0. These data show that, in addition to a reduced number of CFU-S, as radiation dose inc reases, the quality of surviving CFU-S responsible for 30-day survival decreases. By implication, the long-term repopulating cell (LTRC) tha t is now known not to be the 10-day CFU-S must also decrease.