FACTORS RELATED TO RESISTANCE TO HEMATOPOIETIC DEATH IN MICE

Mouse strain difference in the radiosensitivity to hematopoietic death is thought to be determined by several factors besides radiosensitivi ty and the initial number of hematopoietic stem cells. Factors related to the survival of mice exposed to X-irradiation were analyzed using BALB/cHeA and STS/A strains whose LD(50/30) values differ markedly (BA LB/cHeA, 5.55 Gy; STS/A, 8.45 Gy). STS/A mice exposed to 4 Gy of X-irr adiation showed a small reduction but rapid recovery of blood cells (l eukocytes, erythrocytes, and thrombocytes) when compared with BALB/cHe A mice. The survival of endogenous and exogenous CFU-S was much higher , by a magnitude of one log or more, in STS/A mice than those in BALB/ cHeA mice; whereas the initial numbers of femoral CFU-S were similar f or the two strains. The recovery of exogenous CFU-S was much more rapi d in STS/A mice than it was in BALB/cHeA mice after 4 Gy of X-irradiat ion. Furthermore, spleen colonies produced by the transfusion of STS/A marrow cells into syngeneic recipients were significantly larger than those produced by BALB/cHeA marrow cells, regardless of whether the m ice used for sources of marrow cells had been irradiated. But, there w as no such difference when unirradiated marrow cells from the two stra ins were transfused into (BALB/cHeA X STS/A) F-1 recipients. These res ults indicate the possible contribution of a host factor (s) that stim ulates the growth of spleen colonies after radiation to the radioresis tance of STS/A mice, in addition to the primary effect of higher numbe r of survivals of endogenous and exogenous CFU-S in STS/A mice.