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 (TILs). Whole-exome DNA and RNA sequencing of the CT2A murine GSM 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 interferon-gamma enzyme linked immunospot assays. Survival analysis was performed comparing neoantigen vaccination, anti-programmed cell death ligand 1 (alpha PD-L1), or combination therapy. Results. Compared with the GL261 model, CT2A exhibited immunologic features consistent with human GBM including reduced alpha PD-L1 sensitivity and hypofunctional TILs. Of the 29 CT2A neoantigens screened, we identified neoantigenspecific CD8+ T-cell responses in the intracranial TIL and draining lymph nodes to two H2-K-b restricted (Epb4(H471L) and Pomgnt1(R497L)) and one H2-D-b restricted neoantigen (Plin2(G332R)). Survival analysis showed that therapeutic neoantigen vaccination with Epb4(H471L), Pomgnt1(R497L), and Plin2(G332R), in combination with alpha PD-L1 treatment was superior to alpha PD-L1 alone. Conclusions. We identified endogenous neoantigen specific CD8+T cells within an alpha PD-L1 resistant murine GBM and show that neoantigen vaccination significantly augments survival benefit in combination with alpha PD-L1 treatment. These observations provide important preclinical correlates for GBM immunotherapy trials and support further investigation into the effects of multimodal immunotherapeutic interventions on antiglioma immunity.