Abstract 6336: Unleashing CAR T cell activity against osteosarcoma using tumor specific RNA-based vaccination

Abstract Background: Metastatic osteosarcoma (OSA) has a dismal prognosis despite novel targeted therapies, including CAR T cells. Unleashing CAR T cell therapy against poorly immunogenic cancers requires new technologies that activate the tumor microenvironment (TME), while inducing CAR T cell migration, proliferation, and persistence into tumor to sustain tumor-specific immunity. We have developed tumor RNA-loaded nanoparticle (CD70-encoding RNA-NP) vaccines to overcome CD70-specific CAR T cell limitations in OSA. Methods: Using two murine solid tumor models (K7M2 osteosarcoma and B16F0 melanoma, in Balb/c and C57Bl/6 background) highly metastatic to lungs, we implant K7M2 and B16F0 intravenously (i.v.). We subsequently generated NPs complexed with RNA encoding for CD70 antigen and control vaccine (GFP RNA). RNA-NPs are administered intravenously beginning on day 5 post-tumor implantation and day 7, 14 & 21. Mouse CD70-specific CAR T cells are manufactured after activation of T cells with CD3 and CD28 antibodies for 2 days. CAR T cells are ready for use in downstream experiments 2 days after pMSGV8-mCAR transduction and cells were injected (i.v.) on day 6. Results: In preliminary data, we show that intravenous administration of CD70-encoding RNA-NPs rapidly recruit CD70-directed CAR T cells into tumor stroma unlocking their activity in two immunocompetent aggressive lung metastatic tumor models. Interestingly, regression of CD70-expressing tumors associates with CAR T cell mobilization out of peripheral blood and into reticuloendothelial (RES) tissues (i.e., lung, and spleen), a phenomenon that correlates with upregulation of vaccine-induced type I interferon (IFN) and activation of CCR2 pathway. In addition to the CAR T-mediated antitumor killing, an antigen spreading phenomenon was observed. The long-term survival benefit (150 days) provided by this approach reached 70-80% in both models compared with 0-10% of control groups. Conclusions: Based on these findings, we hypothesize that tumor trafficking and persistence of CD70 CAR T cells depend on coordinated cytokine/chemokine signaling that can be ignited through intravenous RNA-NP vaccination. RNA-NPs in combination with CAR T cells prolong long-term survival of mice bearing these aggressive tumor models which was not only mediated by CAR T cells but also by activation of endogenous adaptive immunity. Future studies include the evaluation of this approach in large animal models prior to translation into first-in-human clinical trials. Citation Format: Arnav Barpujari, Rui Liu, Elizabeth Ogando-Rivas, Xiaojie Ma, Haipeng Taoa, Hector Gomez, Duane Mitchell, Jianping Huang, Elias Sayour, Paul Castillo. Unleashing CAR T cell activity against osteosarcoma using tumor specific RNA-based vaccination [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6336.