Regulation of transcription factor IRF8 in myeloid progenitors is a critical checkpoint for formation of defective myeloid cells in cancer.

Abstract Defectivemyeloid cells are commonly observed across a large range of solid cancers. These defects contribute to immune suppression and tumor progression tometastatic disease. Early studies focused on characterizing such myeloid-derivedsuppressor cells (MDSC) in peripheral tissues and the tumor microenvironment, but molecular processes underlying their development remained unclear. Bone marrowis the major site for normal myeloid production and is known to respond to signalsfrom the periphery. Thus, we hypothesizedthat tumors perturb bone marrow myelopoiesis, resulting in production of MDSC. During myelopoiesis, transcription factor IRF8 expression within granulocyte/monocyteprogenitors (GMP) determines differentiation such that high IRF8 favorsmonocytes while low IRF8 favors granulocytes. We previously revealed that IRF8 suppressionby tumor-derived factors leads to accumulation of MDSC, but exactly when andwhere IRF8 is compromised was unknown. Here we utilize a novel mouse model expressingan IRF8-EGFP fusion protein to investigate specific changes in IRF8 during myelopoiesisin both orthotopic and spontaneous mammary tumor progression. Our results show that:1) GMP can be divided into IRF8hi and IRF8lo expressingpopulations corresponding to monocytic and granulocytic progenitor phenotypes,2) IRF8lo GMP preferentially expand with increasing tumor size andincreasing G-CSF serum concentration, and 3) enforced IRF8 expression restrainsthis expansion and reduces the frequency of myeloid suppressors in theperiphery. Altogether, these data identify modulation of IRF8 in myeloidprogenitors as a consequence of tumor development and potential target fortherapeutic intervention.