Utilizing 'viral mimicry' as a novel therapeutic approach in conjunction with anti-PD-1 immunotherapy increases immune activation and extends survival in a glioblastoma murine model

Abstract Therapeutic advances of glioblastoma (GBM) in the last two decades have only extended survival by a few months, maintaining a five-year survival rate of less than five percent. While immunotherapy has shown promise for other types of cancer, it has proved less efficacious as a monotherapy in GBM, partly due to a suppressive immune microenvironment. Being able to stimulate anti-tumor immune activity through alternative methods may allow for a more successful immunotherapeutic response. One such method is by disengaging epigenetic maintenance of repetitive element (RE) silencing. It was recently shown that the epigenetic pathway, FBXO44/SUV39H1, could be targeted specifically in cancer cells to release inhibition of REs such as endogenous retroviruses and retrotransposons. This effect resulted in increased activation of antiviral pathways and interferon signaling. We hypothesized using this treatment in conjunction with anti-PD-1 immunotherapy would allow for a synergistic effect against GBM tumor growth and increased survival. We therefore treated mice with intracranially implanted GL261 cells with a combined therapy of anti-PD-1 and SUV39H1 inhibition. One week after treatment started, we used single cell analysis using multiplexed flow cytometry to look at different immune populations. The largest percentage of immune cells comprised macrophages; with multiplex flow analysis, we were able to differentiate activated microglia (the brain resident immune cell) versus bone marrow derived macrophages (BMDMs; macrophages derived from the blood) based on CD49d expression. While no differences were found in overall macrophage numbers, the percentage of microglia was significantly higher than BMDMs. Strikingly, expression of PD-1 was significantly higher on microglia compared to BMDMs, while CD206, a marker of immune suppression, was significantly higher on the latter. T cells were also examined, with overall numbers again, not significantly different between treatment groups. However, those from mice treated with either the SUV39H1 inhibitor alone or in combination with anti-PD-1 had significantly higher levels of the co-stimulatory molecule CD40L on both CD4+ and CD8+ T cells. Lastly, survival analysis revealed a significant increase in survival with combinatorial treatment compared to either therapy alone or no treatment control. These results indicate the efficacy of an alternative treatment method working synergistically with anti-PD-1 immunotherapy to stimulate the immune system and slow GBM progression in a murine model. Citation Format: Katie B. Grausam, Jia Z. Shen, Joshua J. Breunig, Charles Spruck, Stephen L. Shiao. Utilizing ‘viral mimicry’ as a novel therapeutic approach in conjunction with anti-PD-1 immunotherapy increases immune activation and extends survival in a glioblastoma murine model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2057.