Influence of very low doses of ionizing radiation on life span and immune system in mice

Epidemiological studies suggest a beneficial effect (hormesis) of ionizing radiation at low doses. Some experimental studies have confirmed this effect at doses of about 1 cGy/day, but no data concerning very low dose rates are available. In order to evaluate the effect of chronic very low dose rate on life span, six hundred one-month-old female C57B1/6 mice were exposed to chronic gamma irradiation of about 7 and 14 cGy/year. This is about 25 or 50 times higher than background but lower than doses used in previous experiments. Three hundred mice living in the same room were used as controls. Mean life span, determined by the survival time of 50% of each population, was increased in irradiated mice: 549 days in controls, 673 days of both irradiated groups. Differences were significant between the control and irradiated mice. Differences between mice irradiated with 7 or 14 cGy were not significant. No differences in weight curves were observed during the experiment. This effect on life span could be explained either by a stimulation of the immune system or a modulation of thr: oxidative status. In order to assess these hypothesis, an additional experiment was conducted. C57B1/6 female mice were irradiated at 10 cGy/year and the immune system and oxidative status explored at 2, 4, 6 ,8 and 12 months. Increased splenocyte counts in irradiated mice were observed 4 and 12 months after the beginning of irradiation while no difference in splenocyte response to mitogens was observed. A decrease of T suppressor lymphocytes in spleen was observed after 4 months of irradiation. Differences in IgG subclasses were observed at 2, 4 and 12 months between controls and irradiated mice. Decreased production of free radicals by peritoneal macrophages was observed 3 and 12 months after irradiation while significantly increased levels of liver glutathione were present in irradiated mice at 12 months. In conclusion, we did not record any deleterious effects for these low chronic doses until now. On the contrary, the results suggest a transient stimulation of the immune system associated with lower production of free radicals under very low doses of chronic irradiation, and are in good agreement with an increased life span.