Treatment of an Aggressive Orthotopic Murine Glioblastoma Model with Combination Checkpoint Blockade and a Multivalent Neoantigen Vaccine

BACKGROUND: Although clinical trials testing immunotherapies in glioblastoma (GBM) have yielded mixed results, new strategies targeting tumor-specific somatic coding mutations, termed neoantigens, represent promising therapeutic approaches. We characterized the microenvironment and neoantigen landscape of the aggressive CT2A GBM model in order to develop a platform to test combination checkpoint blockade and neoantigen vaccination. METHODS: Flow cytometric analysis was performed on intracranial CT2A and GL261 tumor-infiltrating lymphocytes (TIL). Whole exome DNA and RNA sequencing of the CT2A murine GBM was employed to identify expressed, somatic mutations. Predicted neoantigens were identified using the pVAC-seq software suite and top-ranking candidates were screened for reactivity by IFN-gamma enzyme linked immunospot (ELISPOT) assays. Survival analysis was performed comparing neoantigen vaccination, alphaPD-L1, or combination therapy. RESULTS: Compared to the GL261 model, CT2A exhibited immunologic features consistent with human GBM including reduced alphaPD-L1 sensitivity and hypofunctional TIL. Of the 29 CT2A neoantigens screened, we identified neoantigen-specific CD8+ T cell responses in the intracranial TIL and draining lymph nodes to two H2-Kb restricted, Epb4H471L and Pomgnt1R497L, and one H2-Db restricted neoantigen, Plin2G332R. Survival analysis showed that therapeutic neoantigen vaccination with Epb4H471L, Pomgnt1R497L, and Plin2G332R, in combination with alphaPD-L1 treatment was superior to alphaPD-L1 alone. CONCLUSIONS: We identified endogenous neoantigen specific CD8+ T cells within a alphaPD-L1 resistant murine GBM and show that neoantigen vaccination significantly augments survival benefit in combination with alphaPD-L1 treatment. These observations provide important preclinical correlates for GBM immunotherapy trials and support further investigation into the effects of multi-modal immunotherapeutic interventions on anti-glioma immunity.