Abstract A025: Noncanonical antigen T cell responses as a tool to measure DMG immune-driven evolution

Abstract Diffuse midline gliomas (DMG) are among the most devastating pediatric malignancies – it is almost universally fatal. Its location precludes surgery and current radio and chemotherapies have so far shown limited success. Immune cells have been shown to penetrate the blood brain barrier, and T cell-based immunotherapies that target peptides presented by the major histocompatibility complex (MHC) on brain tumors have shown therapeutic responses. Recently, T cells directed against the H3K27M mutation activated by peptide vaccines and T cells directed against GD2 through a chimeric antigen receptor have shown evidence of efficacy – speaking to the promise of using these immune cells after antigens have been identified. The ability of T cells to recognize tumor-associated changes that reflect the intracellular environment through T cell receptor (TCR) recognition of peptides on MHC (pMHC) offers more than just potential for therapeutic interventions. This property can also be harnessed as a highly sensitive tool to monitor the evolving dynamics between the tumor and the immune system. By assessing how T cells detect and respond to tumor-derived peptides presented on MHC molecules, researchers can gain valuable insights into how the tumor adapts and interacts with immune surveillance mechanisms over time. By integrating RNA sequencing with mass spectrometry-based peptide ligandomics across our bank of patient-derived DMG samples, we identified tumor-specific peptides translated from long non coding RNAs that are absent from healthy tissue. Functional interrogation using T cells from different human leukocyte antigen (HLA) allotypes uncovered subsets of lncRNA-derived peptides that elicit specific cytokine secretion, proliferation, and cytotoxicity against DMG cells. Preliminary in vivo assays further confirmed anti tumor activity. Analysis of T cell receptor repertoires showed convergence on several dominant clonotypes, suggesting constraints guiding TCR recognition of noncanonical pMHC complexes. Together, our findings provide the first direct evidence of immunogenic lncRNA-derived TCR epitopes on DMG, potentially providing us with a landscape with which to further track the evolution of T cell-mediated response to tumors. Citation Format: C. Russell Y. Cruz, Ashley Geiger, Palak Sekhri, Tima Shamekhi, Grace Lui, Sridevi Yadavilli, Eugene Hwang, Pouya Faridi, Javad Nazarian. Noncanonical antigen T cell responses as a tool to measure DMG immune-driven evolutione [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Cancer Evolution: The Dynamics of Progression and Persistence; 2025 Dec 4-6; Albuquerque, NM. Philadelphia (PA): AACR; Cancer Res 2025;85(23_Suppl):Abstract nr A025.