Low-dose, low-dose-rate proton radiation modulates CD4 þ T cell gene expression DAILA S. GRIDLEY 1,2 , MICHAEL J. PECAUT 1,2 , ASMA RIZVI 2 , GEORGE B. COUTRAKON 3 , XIAN LUO-OWEN 1 , ADEOLA Y. MAKINDE 2 ,& JAMES M. SLATER 1 1 Department of Radiation Medicine, Radiation Research Laboratories, 2 Department of Basic Sciences, Division of Biochemistry and Microbiology, Loma Linda University and Medical Center, Loma Linda, California, and 3 Department of Physics, Northern Illinois University, DeKalb, Illinois, USA (Received 4 July 2008; revised 27 November 2008; accepted 4 December 2008) Abstract Purpose: To evaluate cluster of differentiation 4 þ (CD4 þ ) T cell gene expression and related parameters after whole-body exposure to proton radiation as it occurs in the spaceflight environment. Materials and methods: C57BL/6 mice were irradiated to total doses of 0, 0.01, 0.05, and 0.1 gray (Gy) at 0.1 cGy/h. On day 0 spleens were harvested from a subset in the 0, 0.01 and 0.1 Gy groups; (CD4 þ ) T cells were isolated; and expression of 84 genes relevant to T helper (Th) cell function was determined using reverse transcriptase-polymerase chain reaction (RT- PCR). Remaining mice were euthanized on days 0, 4, and 21 for additional analyses. Results: Genes with 42-fold difference and p 5 0.05 compared to 0 Gy were noted. After 0.01 Gy, five genes were up- regulated (Ccr5, Cd40, Cebpb, Igsf6, Tnfsf4) and three were down-regulated (Il4ra, Mapk8, Nfkb1). After 0.1 Gy there were nine up-regulated genes (Ccr4, Cd40, Cebpb, Cxcr3, Socs5, Stat4, Tbx21, Tnfrsf4, Tnfsf4); none were down-regulated. On day 0 after 0.01 Gy, CD4 þ T cell counts and CD4:CD8 ratio were low in the spleen (p 5 0.05). Spontaneous DNA synthesis in both spleen and blood was lowest in the 0.01 Gy group on day 0; on days 4 and 21 all p values were 40.1. Conclusion: The data show that the pattern of gene expression in CD4 þ T cells after protracted low-dose proton irradiation was significantly modified and highly dependent upon total dose. The findings also suggest that low-dose radiation, especially 0.01 Gy, may enhance CD4 þ T cell responsiveness. Keywords: Radiosensitivity, TBI, protons, lymphocytes, low dose rate, gene induction Introduction Immune dysfunction during extended voyages in space could have serious consequences on astronaut health. In cases of overwhelming infection that can kill in a matter of days and spread rapidly from one individual to another, short-term missions, which otherwise would have been successful, would have to be aborted, and such an infection would be catastrophic for long-term missions (e.g., Mars). Thus far, the duration of most missions has been within the range of days to a few months. The world record for total duration in space is approximately 2.2 years and was accrued by a cosmonaut in six missions conducted over the course of approximately 13 years (http://www.britannica.com/EBchecked/ topic/930424/Sergey-Konstantinovich-Krikalyov). With increasing interest in deep-space exploration, residence times beyond the Earth’s radioprotective magnetosphere will invariably become longer. Un- certainties in health risk estimates remain high for lunar and Mars missions that are likely to have durations of 90 days and approximately 1,000 days, respectively (Setlow et al. 1996, Simonsen et al. 2000, Cucinotta et al. 2005, Hellweg and Baum- stark-Khan 2007). In spite of relatively short residence times in the space environment to date, numerous immune aberrations have been reported in flight personnel, as well as animals carried onboard (Meehan et al. Correspondence: Daila S. Gridley, PhD, Loma Linda University; Chan Shun Pavilion, Room A-1010; 11175 Campus Street; Loma Linda, CA 92354, USA. Tel: þ1 (909) 558 8361. Fax: þ1 (909) 558 0825. E-mail: dgridley@dominion.llumc.edu Int. J. Radiat. Biol., Vol. 85, No. 3, March 2009, pp. 250–261 ISSN 0955-3002 print/ISSN 1362-3095 online Ó 2009 Informa Healthcare USA, Inc. DOI: 10.1080/09553000902748609