Abstract
Previously, we have reported dominant growth of spleen colonies (colony forming unit in spleen: CFU-S), an index of radio-adaptive response. In the present study, the mechanism by which myeloid stem cells produce radio-adaptive responses was investigated. We used exogenous spleen colony formation to demonstrate that myeloid stem cells were halved by day 8 and failed to recover by day 14 after pre-irradiation at 0.5 Gy in mice. In addition, at 14 days, pre-irradiated myeloid stem cells showed 5-times higher resistance to in vitro irradiation at 5 Gy than the non-pre-irradiated controls. These findings suggest that the increased endogenous CFU-S associated with radio-adaptive response, as reported previously, is not attributable to overshoot of growth by low-dose stimulation but to newly acquired radio-resistance. Although similar enhanced resistance was also observed in the bone marrow stem cells at 8 days after pre-irradiation, survival rate after bone marrow transplantation was improved with myeloid cells at 14 days after pre-irradiation than those at 8 days. Furthermore, transplantation using myeloid stem cells of the same counts resulted in a high capability of only the stem cells at 14 days after priming irradiation to suppress bone marrow death. These findings suggest that radio-resistance associated with radio-adaptive response involves mechanisms at both cell and individual levels, which develop approx. 1 week and 2 weeks after priming exposure, respectively.
