Long-term effects of low-dose proton radiation on immunity in mice: Shielded vs. unshielded

Background: Outside the protection of the terrestrial environment, astronauts on any long-term missions will unavoidably be exposed to fields of charged particle radiation dominated by protons. These fields and their biological risks are modified in complex ways by the presence of protective shielding. Methods: To examine the long-term effects of space-like proton exposures on immune status, we treated female C57BL/6 mice with 3 or 4 Gy of 250 MeV monoenergetic protons or the complex space-like radiation field produced after 250 MeV protons are transported through 15 g.cm(-2) aluminum shielding. The animals were euthanized 122 d post-irradiation and lymphocyte phenotypes, hematological parameters, and lymphocyte blastogenesis were characterized. Results: There were significant dose-dependent decreases in macrophage, CD3(+)/CD8(+) T, NK, platelet, and red blood cell populations, as well as low hematocrit and hemoglobin levels. In contrast, dose-dependent increases in spontaneous, but not mitogen-induced, blastogenesis were noted. The differences in dose composition between pristine and shielded proton fields did not lead to significant effects in most measures, but did result in significant changes in monocyte and macrophage populations and spontaneous blastogenesis in the spleen. Conclusions: The data indicate that whole body exposure to proton radiation at doses of the order of large solar particle events or clinical treatment fractions may have long-term effects on immune system status.