PROJECT SUMMARY/ABSTRACT Finding novel therapies for treatment of radiation-induced toxicities is of value to not only patients who are receiving radiotherapy for different conditions, but also to national security due to risk of terrorism attacks. There is urgent need to develop therapies than can be administered quickly after exposure to minimize the effects of radiation and enhance immune recovery. Preclinical models have informed a great deal of our current clinical practice in managing acute radiation syndrome (ARS). These models can be used to test and develop new cellular therapies. Choosing the proper cell subset, engineering it to produce the necessary cytokines, and understanding how the cells interact with other hematopoietic and immune cells in vivo after infusion are all critical factors in developing a proper cell-based therapy for ARS. Our group has previously characterized an alternatively activated, high IL-6 producing human macrophage subset called mesenchymal stem cell (MSC)-educated macrophages that can enhance survival from lethal ARS using a xenogeneic mouse model as compared to infusions of MSCs or macrophages alone. We have simplified generation of these cells by using exosomes from lipopolysaccharide-stimulated MSCs to educate monocytes into a radioprotective cell subset. The long-term objectives of this proposal are to use MSC-exosomes to improve the generation and efficacy of radioprotective cells and define their mechanism of radioprotection in preclinical models of ARS. We will test the hypotheses that: (1) LPS-high exosome-educated monocytes (LPS-high EEMos) can mediate radioprotection as an allogeneic cell therapy through production of IL-6; (2) LPS-high EEMos protect the host from ARS by trafficking to radiosensitive organs, like bone marrow, and can be tracked by magnetic resonance imaging (MRI); and (3) LPS-mimetics can be used to stimulate MSC exosome production that generate EEMos through let-7b microRNA secretion. Success of any of the individual aims will be a major advance in understanding how monocytes impact blood cell development after ARS. Translation of the entire proposal will lead to an innovative, mechanistic understanding of a new cellular therapy for treating ARS.