IRF7 regulates the development of granulocytic myeloid-derived suppressor cells through S100A9 transrepression in cancer.

Accumulation of myeloid-derived suppressor cells (MDSCs) is one of the major obstacles against achieving appropriate anti-tumor immune responses and successful tumor immunotherapy. Granulocytic MDSCs (G-MDSCs) are common in tumor-bearing hosts. However, the mechanisms regulating the development of MDSCs, especially G-MDSCs, remain poorly understood. In this report, we showed that interferon regulatory factor 7 (IRF7) plays an important role in the development of G-MDSCs, but not monocytic MDSCs. IRF7 deficiency caused significant elevation of G-MDSCs, and therefore enhanced tumor growth and metastasis in mice. IRF7 deletion did not affect the suppressive activity of G-MDSCs. Mechanistic studies showed that S100A9, a negative regulator of myeloid cell differentiation, was transrepressed by the IRF7 protein. S100A9 knockdown almost completely abrogated the effects of IRF7 deletion on G-MDSC development and tumor metastasis. Importantly, IRF7 expression levels negatively correlated with the G-MDSC frequency and tumor metastasis, as well as S100A9 expression, in cancer patients. In summary, our study demonstrated that IRF7 represents a novel regulator of G-MDSC development in cancer, which may have predictive value for tumor progression.