Project Summary Immunotherapy has shown profound benefit for adult patients but has yet to be fully unlocked for pediatric solid tumors such as osteosarcoma (OSA) where a significant percentage of children/adolescents succumb due to the presence of lung metastasis. OSA, like many poorly tumor immunogenic tumors, is defined by a lack of tumor specific targets and a regulatory tumor microenvironment (TME). Unleashing immunotherapy against poorly immunogenic cancers requires new technologies that activate the TME, while concomitantly engaging both innate and adaptive arms of the immune system to generate sustained cellular immunity. We have developed a novel (FDA approved) RNA-nanoparticle (RNA-NP) vaccine that simultaneously penetrates/reprograms the TME while inducing an OSA specific T cell response. This vaccine utilizes a novel engineering design that layers tumor derived mRNA into a lipid-nanoparticle (NP) “onion-like” package. We have shown that systemic administration of RNA-NPs safely mimics viremia, activating the quiescent immune system in only a few hours for induction of potent anti-tumor efficacy in several poorly immunogenic murine tumors resistant to immune checkpoint inhibitors. These RNA-NPs activate dendritic cells (DCs) that supplant regulatory intratumoral myeloid populations inducing antigen-recall response with long-term survivor benefits in murine metastatic pulmonary OSA models. We have established safety of RNA-NPs in acute/chronic murine toxicity studies, and launched a large animal canine OSA trial which demonstrated that RNA-NP administration is feasible, safe and immunologically active. While RNA-NPs mediate substantial anti-tumor activity, some animals suffer tumor outgrowth that warrant exploration of resistance mechanisms in our non-survivors. We have shown that RNA-NPs can be enriched for tumor specific antigens or configured with siRNAs to target pertinent regulatory axes (i.e. PD-L1), which can be studied in our murine/canine OSA models. The scientific premise for this work is that osteosarcoma is encased by a regulatory myeloid microenvironment that actively subverts adaptive immunity. We hypothesize that myeloid reprogramming of metastatic OSA will lead to safe eradication of disease. Our SPECIFIC AIMS are: 1. Establish mechanisms of OSA treatment resistance that can be overcome with adaptable RNA-NPs. 2. Identify correlates for vaccine response and escape in a comparative oncology canine OSA model. 3. Conduct a multi-institutional phase I/II study evaluating the safety and activity of the most promising RNA-NP formulation in recurrent OSA patients. Successful completion of this study will lead to a novel OSA therapy and a mechanistic understanding of its therapeutic effects that will be co-opted as biologic response correlates in a human clinical trial. .