Autocrine IL6-Mediated Activation of the STAT3-DNMT Axis Silences the TNFα-RIP1 Necroptosis Pathway to Sustain Survival and Accumulation of Myeloid-Derived Suppressor Cells

Although accumulation of myeloid-derived suppressor cells (MDSC) is a hallmark of cancer, the underlying mechanism of this accumulation within the tumor microenvironment remains incompletely understood. We report here that TNF alpha-RIP1-mediated necroptosis regulates accumulation of MDSCs. In tumor-bearing mice, pharmacologic inhibition of DNMT with the DNA methyl-transferease inhibitor decitabine (DAC) decreased MDSC accumulation and increased activation of antigen-specific cytotoxic T lymphocytes. DAC-induced decreases in MDSC accumulation correlated with increased expression of the myeloid cell lineage-specific transcription factor IRF8 in MDSCs. However, DAC also suppressed MDSC-like cell accumulation in IRF8-deficient mice, indicating that DNA methylation may regulate MDSC survival through an IRF8-independent mechanism. Instead, DAC decreased MDSC accumulation by increasing cell death via disrupting DNA methylation of RIP1-dependent targets of necroptosis. Genome-wide DNA bisulfite sequencing revealed that the Tnf promoter was hypermethylated in tumor-induced MDSCs in vivo. DAC treatment dramatically increased TNF alpha levels in MDSC in vitro, and neutralizing TNF alpha significantly increased MDSC accumulation and tumor growth in tumor-bearing mice in vivo. Recombinant TNF alpha induced MDSC cell death in a dose- and RIP1-dependent manner. IL6 was abundantly expressed in MDSCs in tumor-bearing mice and patients with human colorectal cancer. In vitro, IL6 treatment of MDSC-like cells activated STAT3, increased expression of DNMT1 and DNMT3b, and enhanced survival. Overall, our findings reveal that MDSCs establish a STAT3-DNMT epigenetic axis, regulated by autocrine IL6, to silence TNF alpha expression. This results in decreased TNF alpha-induced and RIP1-dependent necroptosis to sustain survival and accumulation. Significance: These findings demonstrate that targeting IL6 expression or function represent potentially effective approaches to suppress MDSC survival and accumulation in the tumor microenvironment.