Abstract 6742: Nonsense-mediated mRNA decay inhibition augments <i>in vitro, in vivo</i> , and <i>ex vivo</i> anti-tumor immunity

Abstract Introduction: Neoantigens from somatic tumor mutations are essential for effective anti-tumor immune responses. Frameshift insertions and deletions (fs-indels) represent a rare but highly immunogenic mutation subtype, as they create novel open reading frames (neoORFs) that generate peptides that are significantly distinct from self-antigens. Nevertheless, fs-indels often introduce premature termination codons, leading to transcript degradation via the nonsense-mediated mRNA decay (NMD) pathway, leading to loss of immunogenic neoantigen. Approach: For the first time, we pharmacologically inhibited SMG1, a core component of the NMD pathway, across a range of preclinical models, including human and mouse cancer cell lines, patient-derived tumor organoids (PDTOs), patient-derived tumor fragments (PDTFs), and syngeneic mouse xenografts. We analyzed the changes in transcriptome, proteome, and immunopeptidome following SMG1 inhibition (SMG1i) and peptide reactivity in in vitro priming experiments. We then combined tumor-T cell co-cultures and PDTFs to assess the anti-tumor immunogenicity induced by SMG1i. Ex vivo and in vivo immunological responses were assessed by high-dimensional flow cytometry, cytometric bead array, and single-cell RNA- and TCR-sequencing. Results: Using multi-omic and checkpoint inhibitor (CPI) response data from over 1,000 patients, we show that decreased expression of the key NMD mediator, SMG1, correlates with improved CPI response. Inhibiting SMG1 ex vivo and in vivo activates and expands tumor-reactive T cells and sensitizes CPI efficacy. Mechanistically, SMG1 inhibition stabilizes frameshift-derived transcripts, increasing the abundance and surface presentation of immunogenic neoantigens. This results in an increase in neoepitope burden in tumors, similar to that seen in tumors with high tumor mutational burden (TMB), without inducing DNA damage. Co-culturing tumor cells and PDTOs with CD8+ T cells after SMG1i results in strong MHC class I antigen-dependent T cell activation and tumor cell killing. Conclusion: Our findings highlight SMG1 inhibition as a promising strategy to exploit an untapped source of highly immunogenic peptides. It enhances anti-tumor immunogenicity without introducing DNA mutations, regardless of tumor type or TMB status, providing translational evidence for sensitizing ICB responses. Citation Format: Hongchang Fu, Roberto Vendramin, Shanila Fernandez Patel, Yue Zhao, Danwen Qian, Lorena Ligammari, Osnat Bartok, Polina Greenberg, Ronen Levy, Andrea Castro, Krupa Thakkar, Jun Murai, Wei-ting Lu, Christopher C. Sng, Chen Weller, Gordon Beattie, Amandeep Bhamra, Roc Farriol-Duran, Despoina Karagianni, Marcellus Augustine, Krijn Djikstra, Christopher L. Pinder, Benjamin S. Simpson, Gordon Weng-Kit Cheung, TRACERx Consortium, Felipe Galvez Cancino, Petra Vlckova, Silvia Surinova, Manuel Rodriguez-Justo, Mansi Shah, Nicholas McGranahan, Jeremy G. Carlton, Eva Camilla Gronroos, Sergio Quezada, James Luke Reading, Samra Turajlic, Yardena Samuels, Charles Swanton, Kevin Litchfield. Nonsense-mediated mRNA decay inhibition augments in vitro, in vivo, and ex vivo anti-tumor immunity [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 6742.