Mishra, D. K.; Morris, S. M.; Popovski, D.; Girard, E. J.; Bondoc, A.; Kumar, S. S.; Andrade, A. F.; Zhu, X.; Yao, F.; Brusniak, M.-Y.; Umaru, B.; Crotty, E. E.; Brasel, K.; Pakiam, F.; Russo, C.; Zeinieh, M.; Biery, M. C.; Coxon, M.; Conti, H.; Clarke, M.; Lu, M.; Rutka, J.; Llivichuzhca-Loja, D.; Konnikova, L.; Fouladi, M.; Jabado, N.; Huang, A.; Olson, J. M.; Drissi, R.

Background. Despite intensive therapies, outcomes for high-risk pediatric brain tumors (PBTs) remain dismal, prompting the search for novel treatments. DNA methyltransferase inhibitors (DNMTi) have been shown to prime tumors to improve response to checkpoint inhibition. The aim of this study was to investigate the potential of decitabine (DAC), in combination with a PD-1 inhibitor, to improve survival in pediatric high-risk brain tumor models. Methods. Analysis of human PBT datasets was performed to determine gene expression levels of immune cell associated markers. Tumor response to DAC, with or without a PD-1 inhibitor, was tested in murine models representing H3-wildtype diffuse intrinsic pontine glioma (DIPG), H3K27-mutant diffuse midline glioma (DMG), atypical teratoid rhabdoid tumor (ATRT), and medulloblastoma (MB). CyTOF analysis of allograft tumors was performed to characterize changes within the tumor microenvironment. Results. Analysis of PBT subtypes revealed heterogeneous expression of immune cell markers, checkpoint receptors, and MHC molecules. DAC treatment decreased DNA methylation and increased neoantigen expression in human and mouse tumor cells. DAC alone or in combination with a PD-1 inhibitor resulted in prolonged survival in syngeneic mouse models of DIPG and ATRT but not DMG and MB models. CyTOF analysis of mouse tumors revealed changes in local immune cell infiltration upon combination treatment. Conclusions. DAC in combination with a PD-1 inhibitor can alter the immune microenvironment in mouse tumor models. Changes were observed in H3-wildtype DIPG and ATRT models, suggesting that certain tumor subtypes may respond to checkpoint blockade after immune augmentation with DNMTi.