Indiana University-Purdue University Indianapolis (IUPUI) / There is an urgent demand for effective medical countermeasures (MCM) in the event of high-dose radiation exposure ranging from nuclear plant disasters to potential nuclear warfare. Victims of lethal-dose radiation exposure face multi-organ injuries including the hematopoietic acute radiation syndrome (H-ARS) and the delayed effects of acute radiation exposure (DEARE) years after irradiation. Defective lymphocyte reconstitution and its subsequent immune insufficiency are some of the most serious consequences of H-ARS and DEARE. In order to investigate potential MCMs to protect or mitigate these radiation injuries, the prolonged tissue-specific immunosuppression at all levels of lymphocyte development in established murine H-ARS and DEARE models was defined, along with unique sex-related and age-related changes present in some tissues but not others. The “double hits” of irradiation and age-related stress on lymphopoiesis led to significant myeloid skew and long-term immune involution. Different kinds and different combinations of hematopoietic growth factors, some in combination with angiotensin converting enzyme inhibitor, were administered to lethally irradiated mice. These radiomitigators were found to significantly increase survival and enhance hematopoiesis in H-ARS, but they did little to alleviate the severity of DEARE including immune insufficiency. 16,16 dimethyl-prostaglandin E2 (dmPGE2), a long-acting formulation of PGE2 with similar biological effects as PGE2, was found to enhance survival and hematopoiesis in lethal-irradiated mice when used as radiomitigator or radioprotectant. The optimum time window for administration of radioprotectant and radiomitigator dmPGE2 was defined, which is -3hr to -15min prior to irradiation and +6hr to +30hr post irradiation. Significant survival efficacy of radioprotectant dmPGE2 was also demonstrated in pediatric and geriatric mice. Using specific PGE2 receptor (EP) agonists, the EP4 receptor was defined as the PGE2 receptor potentially responsible for dmPGE2 radioprotection. Radioprotectant dmPGE2 was also found to prevent radiation-induced thymic involution and to ameliorate the long-term immune suppression in radiation survivors in the DEARE phase via promoting hematopoietic stem cell differentiation towards to the lymphoid lineage. This is the first report of an effective MCM for H-ARS which also targets long-term thymic involution and lymphoid lineage reconstitution.