ID #1182 Reprogramming the Pre-Clinical Pediatric Glioblastoma Immune Microenvironment to Enhance Tumor-Infiltrating Lymphocyte Therapy

Abstract Glioblastoma (GBM) continues to have limited response to immunotherapy due to an immunosuppressive tumor microenvironment that limits T cell function. Tumor-infiltrating lymphocyte (TIL) therapy has proven effective in melanoma, but in GBM, poor TIL functionality has constrained translational impact. To address these barriers, we evaluated strategies to enhance TIL efficacy through immune preconditioning, checkpoint blockade, and optimized expansion in the SB28 murine glioblastoma model, a syngeneic system engineered to recapitulate key genetic and immunologic features of human adult and pediatric high-grade glioma. SB28 tumors are generated via Sleeping Beauty transposon–mediated insertion targeting TP53, NRAS, and PDGFB pathways, resulting in tumors with low mutational burden, limited immunogenicity, and histopathologic features that more closely resemble human adult and pediatric GBM than other commonly used murine models. TILs were harvested from orthotopic SB28 tumors following neoadjuvant anti-PD-1/anti-CTLA-4 therapy and expanded ex vivo with IL-2 and CD3/CD28 stimulation. Checkpoint-exposed TILs showed enhanced in vitro tumor cell killing and elevated IFN-γ production, with sustained cytotoxicity across multiple effector-to-target ratios. In vivo, systemic anti-Thy1.2 lymphodepletion reduced peripheral CD4+ and CD8+ T cells while preserving intratumoral populations. Adoptive transfer of TILs alone produced minimal survival benefit; however, when combined with post-transfer immune checkpoint blockade, survival significantly improved in the SB28 model. Checkpoint conditioning during ex vivo expansion alone was insufficient to drive these effects, indicating the crucial role of the immune milieu encountered after TIL transfer. These results demonstrate that TIL therapy efficacy in GBM depends on both intrinsic T cell fitness and the immune context into which they are introduced, especially within immunologically “cold” tumors like SB28. Our findings support combination strategies that pair TIL therapy with immune preconditioning and checkpoint blockade to overcome resistance mechanisms in GBM and advance adoptive cellular immunotherapy for high-grade gliomas.