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Objective: Isocitrate dehydrogenase (IDH) mutations are found in more than 80% of low-grade gliomas and in the majority of secondary glioblastomas. IDH mutation (IDHmut) leads to aberrant production of an oncogenic metabolite that promotes epigenetic dysregulation by inducing hypermethylation to suppress transcription of various tumor suppressor genes. Hypermethylation in IDHmut gliomas leads to transcriptional repression of NKG2D ligands, especially UL16-binding protein (ULBP)-1 and ULBP-3, and subsequent evasion of natural killer (NK) cell-mediated lysis. The demethylating agent 5-aza-2'deoxycytodine (decitabine [DAC]) is a DNA methyltransferase 1 inhibitor that prevents hypermethylation and is capable of restoring NKG2D ligand expression in IDHmut gliomas to resensitize them to NK cells. Given its capacity for sustained epigenetic reprogramming, the authors hypothesized that DCA would be an effective immunotherapeutic agent in treating IDHmut gliomas in an NK cell-dependent manner by upregulating epigenetically repressed activating NKG2D ligands in IDHmut tumors. In this study, the authors sought to use a glioma stem cell, preclinical animal model to determine the efficacy of DAC in IDHmut and IDH wild-type (IDHwt) tumors, and to characterize whether the activity of DAC in gliomas is dependent on NK cell function.
Methods: Xenograft models of IDHwt and IDHmut gliomas were established in athymic-nude mice. When tumors were grossly visible and palpable, mice were treated with either DCA or dimethylsulfoxide intraperitoneally every 7 days. Tumor sizes were measured every 2 to 3 days. After the animals were euthanized, xenografts were harvested and analyzed for the following: tumor expression of NKG2D ligands, tumor susceptibility to human and murine NK cells, immunohistochemistry for NK infiltration, and tumor-infiltrating lymphocyte characterization.
Results: DAC significantly inhibited the growth of IDHmut xenografts in the athymic nude mice. This effect was abrogated with NK cell depletion. Ex vivo analysis of tumor cells from harvested xenografts confirmed that DAC increased NKG2D ligand ULBP-1 and ULBP-3 expressions, and enhanced susceptibility to lysis of both human and murine IDHmut glial cells with corresponding NK cells. Immunohistochemical analysis of the xenografts indicated that DCA-treated IDHmut gliomas had a greater level of NK infiltration into the tumor compared with the negative control. Finally, DCA radically altered the tumor-infiltrating lymphocyte landscape of IDHmut glioma xenografts by increasing NK cells, dendritic cells, and M1 macrophages, while decreasing suppressive monocyte infiltration.
Conclusions: DCA displayed novel immunotherapeutic functions in IDHmut gliomas. This effect was critically dependent on NK cells. Additionally, DCA significantly altered the tumor immune landscape in IDHmut gliomas from suppressive to proinflammatory.
