ANS National
Meetings/Sessions

November 1995

Low-Level Radiation Effects on Immune Cells

Takashi Makinodan

(VA Medical Center, Los Angeles)

INTRODUCTION

                The purpose of this study was to characterize the effects of chronic low-dose ionizing radiation (LDR) on murine immune cells. Previously, it had been reported that LDR enhances the proliferative activity of T cells in vitro and delays the growth of transplantable immunogenic tumors in vivo. This suggests that LDR eliminates immune suppressor cells, which down regulates immune response and/or adaptively up regulates the responsiveness of immune effector cells. It had also been reported that human lymphocytes become refractive to high dose radiation-induced chromosomal aberrations by pre-treating mitotically active lymphocytes in vitro with very low doses of ionizing radiation, and the adaptive effect can be abrogated by cycloheximide. This suggests that protein synthesis is required for lymphocytes to respond adaptively to LDR.

METHODS

                Twelve-week-oldC57BL/6 mice were exposed to ionizing radiation (⁶⁰Co facility at doses ranging from 0.01 to 0.10 Gy/exposure, at a dose rate of 0.09 Gy/min, three to five times per week, for a period of 1 to 4 weeks). Three days after the last exposure, mice were killed and their spleens were analyzed for the immune cell composition, DNA strand breaks, of activity of a DNA repair-associated enzyme (poly [ADP- ribose] transferase), levels of heat shock protein and mRNA, and T-cell mitogen-induced proliferative response.

RESULTS

                The following results were obtained:

                1. An exposure dose of 0.04 Gy >0.10 Gy >0.01 Gy, and a total of 10 exposures > 15 exposures > 5 exposures for 2 weeks, 3 weeks, and 1 week, respectively, as judged by the proliferative response. These results suggest that the proliferation-enhancing effect of LDR is dose-dependent, the effective dosage of LDR is extremely low, and the effective dose range is very limited.

                2. T cells, and not B cells, are the primary cells in the spleen that are responsive to LDR, as judged by splenocyte reconstitution studies, and thymectomy abrogates the proliferation-enhancing effect of LDR. This suggests that the thymus, which generates new T cells and modulates the maturation of immature T cells extra thymically, is a primary target organ of LDR.

                3. The constitutive levels of DNA strand breaks and poly (ADP-ribose) transferase activity in splenocytes of mice exposed to LDR are lower than those of sham-irradiated control mice. This suggests that LDR promotes the repair of DNA strand breaks and/or eliminates cells with DNA damage.

                4. Splenocytes, whose proliferative activity had been enhanced by LDR, possess elevated constitutive levels of heat shock protein and mRNA; these splenocytes proliferate more I extensively in response to mitogenic stimulation than splenocytes of sham-irradiated control mice.

                These results suggest that LDR is modulating the expression of heat shock protein genes, which, in turn, suggests that the signal transduction process of T cells could be adaptively up-regulated by LDR.

                The physiologic outcomes of chronic LDR were assessed by determining the effects on autoimmune-susceptible C57BL/6 lpr/lpr mice and mammary tumor-susceptible C3H/He mice. With the former model, the results showed that LDR eliminates abnormal double-negative (CD4^-CD8^-) T cells, which are responsible for the characteristically enlarged lymph nodes and spleens and delays the onset of autoimmune disease manifestations. With the latter model, LDR was ineffective in down-regulating the growth of spontaneous mammary tumors in ad libitum-fed mice. In contrast, caloric restriction (calorically 70% of the food consumed by mice of ad libitum-fed diet) was effective in retarding tumor appearance; however, once the tumors appeared, they grew as rapidly as those inad libitum- fed mice. A striking effect was observed when chronic LDR was combined with caloric restriction [i.e., mice were adapted to a diet calorically equivalent to 70% of the ad libitum- fed diet in a stepwise manner (100% ® 90% ® 80% ® 70%) over a 3-week period, then exposed to chronic LDR (0.04 Gy/exposure on 3 alternating days per week for 4 weeks) and maintained on a calorically restricted diet throughout the study (44 weeks)]. The appearance of tumors was not only retarded, the growth rate approached 0,the incidence of rumors was drastically reduced, and the frequency of tumors undergoing regression was very high. Moreover, large clusters of CD8+ T cells were found infiltrating the regressing tumors but not in tumors of calorically restricted or ad libitum-fed mice.

CONCLUSION

                These results show that chronic LDR is effective in enhancing the immune responsiveness of T cells, and the enhanced immune responsiveness is associated with elevated constitutive levels of heat shock proteins and mRNA and reduced constitutive levels of DNA strand breaks. The biologic significance of chronic LDR is reflected by the retardation of disease in autoimmune-susceptible mice and by the down-regulation of spontaneous mammary tumors, when combined with calorie restriction. Further studies are in order to provide insight into the mechanism of adaptive up regulation of chronic LDR on immune-responsive T cells.