Abstract The immune system, especially the T cell component, is particularly sensitive to ionization radiation (IR), and epidemiologic data, such as the A-bomb cohort, have demonstrated dysfunction and perturbed T cell homeostasis even decades after exposure. While measurable acute effects of radiation on T cell populations and activation have been observed in vivo and in vitro, the long-term physiologic consequences on T cell immunity in acute-radiation-syndrome survivors and multi-organ injury mediated by immune dysfunction, as well as the underlying mechanisms, still have considerable uncertainty. Using a bacterial infection model, we have recently observed that the pathogen loads were higher in the irradiated mice at months after IR. We can connect this compromised protective immunity against pathogens with impaired T cell immunity, where we have reported abnormal immune metabolic reprogramming and perturbations in specific metabolic pathways in activated T cells after IR. With the growing appreciation of interactions between metabolism and immune cell function, it is increasingly appreciated that distinct metabolic needs in naïve, effector, and memory T cells require proper metabolic reprogramming to maintain effective T cell immunity after IR. One aim will be to dissect metabolic perturbations in T cells in various activation/differentiation stages as they contribute to pathogen immunity. A bacterial infection murine model will be used to assess the long-lasting effects of IR on changes on T cell immunity in a physiologic context. We will focus on CD8+ T cells in this study, because CD8+ subpopulations are more sensitive to IR-caused damage than CD4+, and secondly, they are critical for immune defense against intracellular pathogens, including viruses and bacteria. In addition to altering T cell metabolism, we expect that IR also induces distinct systemic metabolic changes, including those in lymphoid tissue and mucosal niche, as shown in our published metabolomic studies. Pro-inflammatory metabolites may synergize with IR-induced premature senescence and drive a positive feedback loop resulting in multi-organ injury. In this project we will examine the impact of the senescence-associated inflammatory phenotype on T cell metabolism and immune functions, and assess interventions using senescent cell ablation approach. Considering that in a real-life nuclear incident, an exposure to mixed fields of neutrons and photon is highly likely, we have already observed distinct differences between mixed field metabolic responses compared to single photon or neutron beams. So we will investigate the late effects of mixed neutron/photon radiation on T cells’ metabolism. We also plan to address the question how a radiomitigator modulates those IR-caused long- lasting effects including a senolytic approach. The successful completion of these aims will help to better understand how radiation causes persistent immune dysfunction in ARS survivors. The new concept of metabolic perturbations of T cells in specific activation/differentiation stages, e.g., naïve vs memory T cells, will lay the foundation for strategies for preventative intervention.

Project Narrative To fill the gap in mechanistic understanding of the delayed effects of acute radiation exposure (DEARE) on the immune system, this study will test our hypothesis that aberrant metabolic reprogramming in T cells is pivotal for mid to late immune dysfunction in survivors of the acute radiation syndrome. We will also investigate the late effects of mixed neutron/photon radiation on T cells’ metabolism, given that in a real-life nuclear incident, an exposure to mixed fields of neutrons and photon is highly likely. Additionally, interventions using senescent cell ablation approaches will be assessed for its feasibility to prevent adverse effects in T cells after radiation.