PROJECT SUMMARY/ABSTRACT Radiation therapy (RT) is a key component of standard of care treatments for glioblastoma (GBM), the most common and deadly primary brain malignancy in adults. Beyond the direct cytotoxic effect on tumor itself, RT- elicited anti-tumor immune responses have recently been appreciated as a key factor to the treatment outcomes. These responses are dependent on the functionality of myeloid cells, an essential component of the innate immune system. However, within tumor microenvironment, much of the myeloid compartment is programed to be immunosuppressive, which impairs the anti-tumor immune responses and thereby therapeutic effects of RT. The objective of this proposed work is to harness and reprogram immunosuppressive tumor-associated myeloid cells (TAMCs), the most abundant immune population in GBM, to amplify the RT-elicited anti-tumor immune responses. To enable a precise and efficient therapeutic targeting of TAMC, we propose the development of a bridge-lipid nanoparticle (B-LNP) platform with the ability to actively target the GBM-induced TAMC in-vivo. Our preliminary data suggest that B-LNP tethers TAMC to GBM through a “bridging” effect and concurrently blocks the anti-phagocytic effectors used by GBM to escape immune surveillance. This platform also enables TAMC- targeted delivery of an agonist for stimulator of interferon genes (STING), a key factor in bridging innate and adaptive anti-tumor immunity, resulting in the tumor displaying a pro-inflammatory phenotype that robustly stimulates effector T cell infiltration of tumor. In preclinical animal models, our TAMC-targeted reprogramming promotes brain tumor regression, and increases the anti-tumor activity of RT. The central hypothesis of this proposal is that nanoparticle therapies that simultaneously activate TAMC phagocytic activity and interferon pathway signaling will amplify the RT-stimulated anti-tumor immunity against GBM. We will focus on two different anti-GBM mechanisms of TAMC that our nanoparticle could harness: phagocytosis of GBM (Aim 1) and activation of effector T cell responses (Aim 2). Lastly, we will determine the effectiveness of TAMC-targeted therapy in the context of standard of care treatments for GBM (Aim 3). The feasibility for clinical translation will be thoroughly evaluated using preclinical animal models, including a unique humanized animal model of GBM, and clinical GBM samples, which will test the effectiveness of a humanized version of the therapeutics. Overall, our study provides a novel approach to reshape the immunosuppressive tumor microenvironment responsible for therapy resistance, and promote current standard of care therapies for GBM.