Bispecific T Cell Engagers Sensitize the Glioblastoma Tumor Microenvironment to Immune Checkpoint Blockade Responsiveness 2261116

Abstract Introduction Glioblastoma (GBM) is the most aggressive brain tumor, accounting for ∼48% of all brain malignancies in children. Despite aggressive treatments, median survival remains only 12—15 months, with < 5% patients surviving beyond 5 years. Although several immunotherapies are currently in clinical trials, none have yet been approved. Locoregional treatment with personalized CAR-T cells has shown modest success. Moreover, immune checkpoint blockade (ICB) therapies are largely ineffective in GBM. Here, we explored Bispecific T cell engagers (BTE) — off-the-shelf therapeutics that connect pre-existing endogenous T cells with tumor targets — as a potential new immunotherapy to sensitize ICB responsiveness in GBM. Methods We used CD3:B7H3 BTEs to engage endogenous T cells through CD3 and tumor targets through B7H3, a tumor-associated antigen highly expressed on GBM. Preclinical immunocompetent murine models of syngeneic orthotopic immunologically distinct “hot” and “cold” GBM were treated intracranially with BTEs to bypass the challenge posed by blood-brain-barrier. Results BTE treatment led to robust expansion and effector differentiation of CD8 T cells, with increased effector T cell:Treg ratios and early tumor control. However, tumor control was not sustained, and correlated with loss of effector CD8 T cells and upregulation of PDL1 in the TME myeloid compartment. Converting this immune evasion mechanism into a tumor vulnerability, combination BTE therapy with ICB significantly enhanced tumor control, with improved 2X survival in 50% of the cohort. Tumor regression was associated with complete remodeling of the TME from immunosuppressive to more immunogenic with increased CD8 T cells, decreased Tregs, decreased myeloid-derived suppressor cells and increased activated microglia. Conclusion These studies set the groundwork for BTE immunotherapy in treatment refractory GBM and pose a unique window of opportunity for established ICB immunotherapies to synergize for long-term remission and overall survival. Funding Source This work was supported by the National Institutes of Health (National Cancer Institute), Pediatric Cancer Research Foundation and the Mark Foundation Topic Categories Tumor Immunology: Checkpoints, Prevention, and Treatment (TIPT)