Abstract: Non-small cell lung cancer, colorectal cancer, renal cell carcinoma, and melanoma represent four of the five most common forms of cancer in Veterans. In the metastatic setting, immunotherapy has emerged as a powerful oncologic treatment modality capable of producing durable control in all of these diseases. Unfortunately, the vast majority of Veterans do not benefit from this therapy, and thus, our long-term goal is to develop integrated treatment strategies with radiotherapy to enhance the efficacy of immunotherapy. The overall objective of this proposal is to define the importance of cell death induced by radiotherapy on immunotherapy efficacy. The central hypothesis of this proposal is that enhancing immunostimulatory radiation-induced ferroptosis will enhance immunotherapy efficacy. The rationale for this proposal comes from our published and unpublished data revealing that radiotherapy induces tumoral ferroptosis and the observation that radiotherapy and immunotherapy synergize to promote tumor control through ferroptosis. Beyond this, our results suggest that ferroptosis may be an immunogenic form of cell death. This hypothesis will be assessed and leveraged therapeutically to design rational combinatorial strategies using radiotherapy and immunotherapy in 3 aims. Aim 1 will define the molecular mediators (Aim 1A-C) and signaling pathways (Aim 1D) by which radiation-induced lipid oxidation and ferroptosis modulates anti-tumoral immunity. Aim 2 will determine the innate (Aim 2A, B) and adaptive (Aim 2C) cellular mediators by which radiation-induced lipid oxidation and ferroptosis regulate anti-tumoral immunity. Aim 3 will develop therapeutic strategies for modulating radiotherapy-induced ferroptosis to systemically augment immune checkpoint blockade efficacy (abscopal responses) in primary (Aim 3A) and metastatic (Aim 3B) melanoma tumor models. Furthermore, we will develop pharmacodynamic biomarkers (Aim 3C) of efficacy. The research proposed is innovative because the immunogenicity of ferroptosis has yet to be characterized. The completion of these aims is significant because it will establish a novel mechanistic link between ferroptosis, cytotoxic oncologic treatment modalities, and immune polarization, which can be leveraged therapeutically to improve immunotherapy efficacy in Veterans.