Variable Hematopoietic Responses to Acute Photons, Protons and Simulated Solar Particle Event Protons

Abstract

The goal of this study was to evaluate, for the first time, the response of bone marrow-derived cell populations to protons mimicking a space radiation environment. Materials and Methods: C57BL/6 mice were exposed to 2 Gray (Gy) simulated solar particle event protons (sSPE) over 36 h; energies ranged from 15 to 215 MeV/n and were administered in 10 MeV increments. Acute 2 Gy irradiation with photons (γ-rays) and protons were administered to different groups at 0.7 Gy/min and 0.9 Gy/min, respectively, for comparison with sSPE. The animals were euthanized on days 4 and 21 post-exposure for analyses. Results: Exposure to radiation, regardless of regimen, resulted in immune depression and other abnormalities in cell populations residing in the blood and spleen; the extent of the radiation damage was somewhat dependent upon body compartment and time post-exposure. However, variations were also noted among the three radiation regimens in a number of measurements: relative spleen mass, basal DNA synthesis by leukocytes, white blood cell counts and three-part differentials (lymphocytes, granulocytes, monocytes-macrophages), lymphocyte subpopulations (CD4⁺ T, CD8⁺T, B and NK cells) and erythrocyte and thrombocyte characteristics. Conclusion: The data demonstrate that exposure to proton radiation mimicking a solar explosion induces abnormalities in leukocytes, erythrocytes and platelets that may have adverse health consequences. However, the damaging effects of sSPE on leukocytes and platelets were generally less pronounced compared to the other radiation regimens. Results obtained with photons (γ-rays, X-rays) and monoenergetic protons at space-relevant total doses may not necessarily predict biological responses after exposure to a solar particle event.