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Regulation of mononuclear phagocyte development by IRF8.

Mononuclear phagocytes, such as monocytes and dendritic cells (DCs), are essential for tissue homeostasis and immunity. In adults, these cells develop from hematopoietic stem cells via a common progenitor population. We have been investigating the mechanism underlying the development of mononuclear phagocytes from the viewpoint of gene expression control by transcription factors. Particularly, IRF8, the loss of which causes immunodeficiency and chronic myeloid leukemia-like neutrophilia in mice and humans, promotes the development of monocytes and DCs, while it limits neutrophil differentiation. IRF8 cooperates with the myeloid master transcription factor, PU.1, in mononuclear phagocyte progenitors. KLF4 and BATF3 serve as critical transcription factors downstream of IRF8 to induce the differentiation of monocytes and DCs, respectively. Conversely, IRF8 blocks the activity of the transcription factor C/EBPα to suppress the neutrophil differentiation program. Indeed, Irf8-/- mononuclear phagocyte progenitors do not efficiently generate monocytes and DCs and, instead, aberrantly give rise to a large number of neutrophils. Our recent data have begun to uncover the vital role of IRF8 in the establishment of distal enhancers in mononuclear phagocyte progenitors. These results place IRF8 as a central regulator of the development of monocytes and DCs.