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 irradiation. 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 preirradiation. We show that low-dose irradiation induces thymocyte apoptosis with 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 splenocytes and no early modification of the intensity of T-cell-dependent immune responses as measured by plaque-forming cell (PFC) counts. Nevertheless, we found that PFCs were increased 2 weeks after 50 cGy irradiation, which is the same time at which mice expressed the optimal increase in survival after a second lethal irradiation. We also examined thymocyte apoptosis and spleen PFCs in mice subjected to other stress-inducing pretreatments. Our results emphasize the existence of a lag time between the time of low-dose irradiation in vivo and the appearance of radioresistance. A mechanism that interconnects an environmental stimulus with the response of the animal is proposed based on the evidence presented here and reported in the literature.
Matsubara
J
J
School of Medicine, Yokohama City University, Fukuura 3-9 Kanazawa-ku, Yokohama 236-0004, Japan.
Turcanu
V
V
Poindron
P
P
Ina
Y
Y
D000818:Animals; D017209:Apoptosis / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008813:Mice, Inbred ICR; D013601:T-Lymphocytes / Q000166:cytology / Q000276:immunology / Q000528:radiation effects*