Immune effects of low-dose radiation: Short-term induction of thymocyte apoptosis and long-term augmentation of T-cell-dependent immune responses

We and others have shown that low-dose X or gamma irradiation of mice leads to an increase in their survival after a subsequent lethal high-dose irrad iation. The greatest increase in radioresistance appears at a fixed window of dose and time, e.g. 8 weeks after 5-10 cGy or 2 weeks after 50 cGy preir radiation. We show that low-dose irradiation induces thymocyte apoptosis wi th a maximal level at 6 h postirradiation that returns to background levels after 24 h, At the same time, we observed no morphological alteration of s plenocytes and no early modification of the intensity of T-cell-dependent i mmune responses as measured by plaque-forming cell (PFC) counts. Neverthele ss, we found that PFCs were increased 2 weeks after 50 cGy irradiation, whi ch is the same time at which mice expressed the optimal increase in surviva l after a second lethal irradiation, We also examined thymocyte apoptosis a nd spleen PFCs in mice subjected to other stress-inducing pretreatments, Ou r results emphasize the existence of a lag time between the time of low-dos e irradiation in vivo and the appearance of radioresistance. A mechanism th at interconnects an environmental stimulus with the response of the animal is proposed based on the evidence presented here and reported in the litera ture. (C) 2000 by Radiation Research Society.