IRF8 represses GM-CSF expression in tumor cells to mediate myeloid-derived suppressor cell differentiation.

Abstract

Abstract Myeloid-derived suppressive cells (MDSCs) are immature myeloid cells that are induced by inflammatory mediators. Massive accumulation of MDSCs is a hallmark of cancer in mice and human patients. Proinflammatory factors are believed to induce MDSC differentiation. In an early study, we showed that T cell-produced GM-CSF, a proinflammatory cytokine, induces MDSC accumulation under physiological conditions. We further determined that IRF8 represses MDSC accumulation in vivo. Tumor cells are known to produce abundant GM-CSF; we therefore sought to test the hypothesis that IRF8 regulates GM-CSF expression to mediate MDSC accumulation in the tumor-bearing host. We observed that mouse sarcoma cells rapidly induce MDSC accumulation, which is positively correlated with tumor growth kinetics. Analysis of purified MDSCs from tumor-bearing mice indicated that GM-CSF mRNA levels increase and IRF8 expression decreases as tumor sizes increase. GM-CSF and IRF8 expression kinetics are therefore inversely correlated in MDSCs in vivo. Analysis of the csf2 gene promoter DNA sequence identified an IRF8-binding consensus element, and ChIP analysis determined that IRF8 binds to this DNA element in the csf2 promoter. Analysis of human sarcoma specimens revealed that the IRF8 promoter DNA is hypermethylated. Silencing IRF8 expression using IRF8-specific shRNA increased GM-CSF expression. In summary, we determine that tumor cells use GM-CSF to induce MDSC differentiation, and IRF8 functions as a transcriptional repressor of GM-CSF expression to suppress MDSC accumulation. Human sarcoma cells may use DNA methylation to silence IRF8 and increase GM-CSF expression to promote MDSC accumulation, thereby suppressing the anti-tumor immune response.