Abstract 4385: Engineering durable tumor immunity with a virus-tumor hybrid antigen dendritic cell vaccine

Abstract Introduction: Dendritic cell (DC)-based cancer vaccines have shown limited efficacy in clinical trials, a result that has been partly attributed to poorly immunogenic tumor antigens that fail to induce robust MHC class-II directed CD4+ T cell help. CD4+ T cells ‘license’ DCs to upregulate key costimulatory molecules (e.g. CD80/86) and cytokines (IL-12) that are essential for the generation of effective, long-lived tumor-directed CD8+ T cells. However, therapeutic approaches that effectively harness both T-cell subsets have been constrained by difficulties in identifying CD4+ T-cell-specific cancer neoepitopes. Here, we introduce , a novel DC-based cancer vaccine platform designed to co-present non-tumor-related CD4+ T helper epitopes from the SARS-CoV-2 Spike protein alongside tumor-specific CD8+ T-cell epitopes derived from tumor-associated antigens (TAAs), leveraging widespread Covid-19 immunity in the context of a cancer vaccine. Methods: In proof-of-concept studies, PROTEXI was generated by simultaneously loading mature bone marrow-derived DCs (BMDCs) with pMHC-II-OVA323-339 and pMHC-I-Trp2180-188 complexes. To model the prevalence of SARS-CoV-2 Spike-specific CD4+ T-cell memory, mice were either primed with DC-OVA323 or received adoptive transfer of OT-II CD4+ T cells prior to tumor challenge, followed by PROTEXI treatment (weekly for 2 weeks) or PROTEXI in combination with immune modulating agents. The therapeutic potential of PROTEXI in a human immune context was evaluated in humanized mice reconstituted with blood from COVID-19-vaccinated donors. The human version, PROTEXI-Spike/TAA, was designed to co-present CD4+ T-cell-restricted Spike epitopes together with CD8+ T-cell-restricted PRAME and MAGE-A3 antigens. Results: PROTEXI significantly improved survival and reduced tumor growth, correlating with enhanced T-cell infiltration into immune-cold tumors and induction of gene signatures associated with T-cell cytotoxicity, migration, and memory formation. Combination therapy with either anti-PD-1 immune checkpoint blockade or the ALK5 (TGF-β type I receptor) inhibitor Vactosertib further augmented therapeutic efficacy in therapy-resistant tumor models. Importantly, tumor-specific cytotoxic T-cell memory derived from PROTEXI mice was sufficient to mediate complete tumor rejection upon re-challenge. In humanized models, PROTEXI-Spike/TAA markedly expanded antigen-specific CD8+ T-cell populations and reduced tumor burden, demonstrating translational feasibility and potency. Conclusion: Collectively, these findings highlight the clinical potential of PROTEXI as an innovative DC vaccine platform that leverages the widespread prevalence of Spike-specific CD4+ T-cell immunity to enhance cancer vaccine efficacy—offering a promising therapeutic strategy for patients with advanced, immune-cold tumors. Citation Format: Jin Muk Kang, Eun Hyang Han, Jin-kyu Choi, Seunghee Youm, Tej Pareek, Seong-Jin Kim, John Letterio, Seunghwan Lim. Engineering durable tumor immunity with a virus-tumor hybrid antigen dendritic cell vaccine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4385.