This proposal is designed to assess a long-acting interleukin 7 product, NT-I7, on phenotypic and functional T cell recovery after total body irradiation at radiation doses simulating exposures that would be expected during a radiation mass casualty incident. Clinical-grade NT-I7 is produced by NeoImmuneTech, Inc. (Rockville, Maryland), and is a recombinant human IL-7 immunoglobulin fusion protein that is more potent, stable, and longer acting than endogenous human IL-7. IL-7 is a cytokine involved in several areas of the adaptive immune system including thymopoiesis and NT-I7 acts as an exogenous form of IL-7 that exhibits beneficial effects on T cell recovery and increased antigen (Ag)-specific T cell response to viral infection. In preclinical studies NT-I7 has demonstrated beneficial preliminary results for T cell reconstitution after total body irradiation in mouse models. This is important because T lymphocytes are the most radiation sensitive blood cell type, a critical component of the immune response to infection – a significant risk following radiation injury – and there are no FDA approved drugs to support T cell recovery after radiation exposure.