IMMU-49. TARGETING ALTERNATIVE SPLICING VARIANTS TO STIMULATE ANTI-TUMOR IMMUNITY IN A NOVEL SYNGENEIC MOUSE MODEL OF DIFFUSE HEMISPHERIC GLIOMA, H3 G34-MUTANT

Abstract The prognosis for pediatric high-grade gliomas associated with mutations in the H3-A3 histone gene is very poor. To investigate whether peptide-pulsed dendritic cells (DC) together with checkpoint blockade might be a potential treatment modality for diffuse hemispheric glioma, H3 G34-mutant (DHG), we have developed a novel mouse model, and utilized a novel bioinformatics pipeline to identify tumor-associated peptide antigens (TAs). We used the RCAS/tv-A system to target the expression of H3/G34R and PDGFb, and to knock out p53 in neural progenitors in transgenic C57BL/6 neonatal mice. Three independent cell lines were obtained that expressed transcripts associated with oligodendrocyte and interneuron lineages, and formed lethal tumors after intracranial implantation. To identify TAs, we implemented a new computational workflow to identify nonapeptides generated from alternative splicing of mRNAs that were highly enriched in the tumor transcriptomes of two DHG patients, and predicted to bind the HLA-A*2:01 class I allele carried by these patients. Three of the candidate peptides were conserved in the mouse and also predicted to bind the H2-Kb MHC I allele carried by C57/BL6 mice. Using our syngeneic mouse model, we showed that DCs pulsed with these peptides together with anti-PD1 mAb provided a significant survival benefit compared with checkpoint inhibitor and DCs pulsed with peptides that were not predicted to bind the H2-Kb allele. Splenocytes isolated from successfully treated mice were analyzed through bulk TCR sequencing, and in comparison with control mice and publicly-accessible TCR libraries, 6 highly-represented TCR clonotypes were identified only in treated mice. In summary, we demonstrate that effective TAs can be extracted from RNAseq data, and can be targeted with DC vaccination to generate a systemic anti-tumor cytotoxic T cell response to eliminate DHG in a novel immunocompetent mouse model.