Ferroptosis has emerged as a cytotoxic T lymphocyte (CTL)-induced tumor cell death pathway. The regulation of tumor cell sensitivity to ferroptosis is incompletely understood. Here, we report that interferon regulatory factor 8 (IRF8) functions as a regulator of tumor cell intrinsic ferroptosis. Genome-wide gene expression profiling identified the ferroptosis pathway as an IRF8-regulated pathway in tumor cells. IRF8.KO tumor cells acquire resistance to intrinsic ferroptosis induction and IRF8-deficient tumor cells also exhibit decreased ferroptosis in response to tumor-specific CTLs. Irf8 deletion increased p53 expression in tumor cells and knocking out p53 in IRF8.KO tumor cells restored tumor cell sensitivity to intrinsic ferroptosis induction. Furthermore, IRF8.KO tumor cells grew significantly faster than WT tumor cells in immune-competent mice. To restore IRF8 expression in tumor cells, we designed and synthesized codon usage-optimized IRF8-encoding DNA to generate IRF8-encoding plasmid NTC9385R-mIRF8. Restoring IRF8 expression via a lipid nanoparticle-encapsulated NTC9385R-mIRF8 plasmid therapy suppressed established tumor growth in vivo. In human cancer patients, nivolumab responders have a significantly higher IRF8 expression level in their tumor cells as compared to the non-responders. Our data determine that IRF8 represses p53 expression to maintain tumor cell sensitivity to intrinsic ferroptosis.

Glioblastoma (GBM) standard of care includes surgical resection followed by ionizing radiation (IR) and Temozolomide, which induce DNA double-strand breaks. Homologous recombination (HR), a critical DNA double-strand break repair pathway, is augmented in GBM, contributing to resistance and poor patient outcomes. Here, we demonstrate that increasing deoxyribonucleoside triphosphate (dNTP) levels impairs HR-mediated double-strand break repair, rendering GBM cells sensitive to IR and Temozolomide. Interestingly, combining an elevated dNTP pool level with IR and/or Temozolomide promotes the recruitment of DNA polymerase-α/primase, which is typically involved in Okazaki fragment synthesis during DNA replication, to the DNA double-strand break site, thereby interfering with DNA end resection. Specifically, higher dNTP pool levels disrupted the recruitment of HR-associated proteins such as RPA70 and RAD51, an effect reversed by Aphidicolin, a DNA polymerase-α/primase inhibitor. Impaired HR delayed IR- and/or Temozolomide-induced DNA double-strand break repair, leading to growth arrest and apoptosis. Furthermore, higher dNTP pool levels led to downregulation of DNA replication and HR-associated genes, while upregulating several pro-apoptotic genes. Increased sensitivity to IR and Temozolomide was also observed in engineered IR-resistant GBM cell lines and in naturally recurrent patient-derived GBM cells that emerge post-therapy. These findings emphasize how dNTP pool levels regulate HR and uncover a promising vulnerability that could be exploited to overcome resistance to DNA-damaging treatments in GBM and beyond.