Myeloid-derived suppressor cell development is regulated by a STAT/IRF-8 axis

Interferon Regulatory Factor-8, an Integral Determinant of Myeloid-Derived Suppressor Cell Subset Development. Myeloid-derived suppressor cells (MDSC) are a major barrier to anticancer responses. Although much is known about how MDSC promote tumor progression, little is known about how they develop. We hypothesized that MDSC develop as a consequence of tumor-induced downregulation of interferon regulatory factor-8 (IRF-8), a key myeloid developmental transcription factor. We showed that: 1) IRF8-deficiency in mice generated myeloid populations highly homologous to tumor-induced MDSC; 2) IRF-8 overexpression in mice reduced MDSC accumulation and retarded tumor growth; 3) MDSC-inducing factors, G-CSF or GM-CSF, facilitated IRF-8 downregulation via STAT3- or STAT5-dependent pathways, respectively; and 4) IRF-8 levels in MDSC-like subsets of breast cancer patients were depressed compared to healthy donors. Altogether, our data implicate IRF-8 as a novel MDSC-dependent transcription factor. Splenic CD11b+Gr-1high cell populations from tumor-bearing mice, IRF8 knockout mice or non-tumor-bearing control mice were purified in two independent experiments by flow cytometry (> 97% purity) and subjected to whole genome expression profiling using Illumina microarrays.