ABSTRACT The efficacy of cell-based immunotherapies in treating solid tumors is hampered due to challenges pertaining to infiltration, function, and persistence of CD8+ effector T cells. The tumor microenvironment (TME) is designed to favor tumor survival and depletes key nutrients from the extracellular space, creating a hostile environment unamenable to CD8+ effector function. Regulatory T (Tregs) cells can thrive in the glucose-low TME due to their adaptable metabolic re- wiring which promotes survival of Tregs and facilitates their suppression of effector immune cells. High frequency of Tregs in the TME is associated with reduced response to immunotherapies, increased malignancy and poor clinical outcome. Metabolic dysfunction in the tumor not only supports the survival of immunosuppressive cells but drives an accumulation of metabolic byproducts that are secreted into the interstitial fluid of the TME where they can act as signaling molecules communicating with neighboring cells, driving suppressive phenotypes. Succinate is a critical mitochondrial metabolite that is secreted from the tumor due to an intracellular buildup in the hypoxic conditions of the TME. We have identified that extracellular succinate promotes the induction of Tregs from CD4+ T cells and further enhances the Th1-suppressing phenotype of Tregs. We propose that by inhibiting the effects of succinate on Treg induction and phenotype, targeting succinate signaling - specifically the succinate receptor (SUCNR1) - in CD4+ T cells and Tregs, we can increase the number of tumor-specific TCR-transduced CD8+ T cells that infiltrate the solid tumor thereby enhancing the anti-tumor response and reducing tumor burden. We will test this using tumors that have modified succinate levels (using CRISPR/Cas9), small molecule antagonists for the SUCNR1 on CD4+ T cells and WT and SUCNR1-/- CD4+ T cells, and comprehensively assess the effects of tumor-derived succinate on Treg induction in the TME. Utilizing our in-house high avidity TCRs that recognize the tumor antigens MAGE-A1 and PRAME, will subsequently develop a therapeutic strategy targeting succinate signaling to attenuate the suppressive effects of Tregs on tumor-specific CD8+ effector T cell anti-tumor responses, using SUCNR1-/- T cells and a small molecule inhibitor targeting succinate. We will examine this in vivo using advanced humanized mouse models (MISTRG) that support myeloid cells thus creating an immune diverse TME. We will simultaneously use a murine melanoma immune competent model (B16OVA OT-1) to interrogate the effects of succinate on immunosuppression on the TME. These studies will provide a novel and innovative therapeutic strategy for combating the resident and infiltrating suppressive Tregs in the TME and increase the efficacy of TCR immunotherapy outcomes in the solid tumor.