Abstract 6738: Overlapping and distinct mechanisms of effective neoantigen cancer vaccines and immune checkpoint therapy

Abstract The goal of cancer vaccines and immune checkpoint therapy (ICT) is to expand and sustain T cells with enhanced anti-tumor capabilities. Here, we asked whether these distinct immunotherapies utilize similar cellular and functional mechanisms. We used multiple approaches to compare effective therapeutic mutant neoantigen (NeoAg) cancer vaccines with αPD-1, αCTLA-4, or αPD-1 and αCTLA-4 ICT in preclinical BrafV600EPten−/−Cdkn2a−/− melanoma models. Synthetic long peptide (SLP) MHC-I NeoAg vaccines induced a more than 3-fold expansion of proliferating intratumoral NeoAg-specific CD8 T cells that were functional despite high expression of PD-1 and TIM-3. NeoAg vaccines required not only CD8, but also CD4 T cells for efficacy. αCTLA-4 and/or αPD-1 ICT also increased the frequency and effector capabilities of CD8 T cells but to much less of an extent than NeoAg vaccines. These included NeoAg-specific CD8 T cells that displayed reduced expression of PD-1 and TIM-3. αCTLA-4 treated tumors displayed robust induction of ICOS+ Th1-like CD4 T cells expressing Bhlhe40 that resembled Th1-like CD4 T cells described in αCTLA-4-treated patients and when αCTLA-4 was combined with αPD-1, a small subset of Th2-like CD4 T cells was observed. Remarkably, we noted divergent effects on certain subsets of intratumoral macrophage populations induced by NeoAg vaccines as compared to ICT. Although effective NeoAg vaccines expanded M1-like iNOS+ macrophages with ICT also following this same pattern, NeoAg vaccines expanded rather than suppressed (as observed with ICT) distinct subpopulations of M2-like CX3CR1+ CD206+ macrophages. CODEX spatial imaging using a 46-marker panel revealed tumors from mice treated with NeoAg vaccines displayed concentrations of CD8 T cells expressing functional markers and the proliferation marker Ki67, with mice treated with αCTLA-4 or αPD-1 displaying both Ki67+ conventional CD4 and CD8 T cells within interior regions of tumor. Interestingly, these regions contained prominent blood vessels, along with MHC-II+ dendritic cells and iNOS+ macrophages. Our work shows that NeoAg vaccines lead to distinct, as well as overlapping alterations when compared to ICT. These distinct alterations provided a rationale for combination therapies that we validated in both our melanoma model, as well as the MC38 tumor model, where delayed treatment with NeoAg vaccines combined with αCTLA-4 or αPD-1 yielded anti-tumor immunity that was superior to combination αPD-1 and αCTLA-4. These findings suggest that NeoAg vaccines combined with single-agent ICT may be an effective therapy with potentially less toxicity than αPD-1/αCTLA-4 combination ICT. Further, we hypothesize that immunotherapies that deplete CX3CR1+ CD206+ macrophages may synergize with NeoAg cancer vaccines, which we are currently assessing. Citation Format: Sunita Keshari, Alexander S. Shavkunov, Qi Miao, Akata Saha, Charmelle D. Williams, Josué E. Pineda, Kenneth Hu, Kristen E. Pauken, Ken Chen, Matthew M. Gubin. Overlapping and distinct mechanisms of effective neoantigen cancer vaccines and immune checkpoint therapy [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 6738.