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Open Access Research

Interferon regulatory factor 8/interferon consensus sequence binding protein is a critical transcription factor for the physiological phenotype of microglia

Makoto Horiuchi12, Kouji Wakayama3, Aki Itoh12, Kumi Kawai4, David Pleasure12, Keiko Ozato5 and Takayuki Itoh12*

Author Affiliations

1 Department of Neurology, University of California Davis, School of Medicine, 4860 Y Street, Sacramento, CA, 95817, USA

2 Institute for Pediatric Regenerative Medicine, 601A Shriners Hospitals for Children Northern California, 2425 Stockton Boulevard, Sacramento, CA, 95817, USA

3 Department of Advanced Clinical Science and Therapeutics, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8655, Japan

4 Department of Medical Technology, Graduate School of Medicine, Nagoya University, 1-1-20 Daikou-minami, Higashi-ku, Nagoya, 461-8673, Japan

5 Laboratory of Molecular Growth Regulation, Program in Genomics of Differentiation, National Institute of Child Health and Human Development, National Institutes of Health, 6 Center Drive 2753, Bethesda, MD, 20892, USA

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Journal of Neuroinflammation 2012, 9:227  doi:10.1186/1742-2094-9-227

Published: 28 September 2012

Abstract

Background

Recent fate-mapping studies establish that microglia, the resident mononuclear phagocytes of the CNS, are distinct in origin from the bone marrow-derived myeloid lineage. Interferon regulatory factor 8 (IRF8, also known as interferon consensus sequence binding protein) plays essential roles in development and function of the bone marrow-derived myeloid lineage. However, little is known about its roles in microglia.

Methods

The CNS tissues of IRF8-deficient mice were immunohistochemically analyzed. Pure microglia isolated from wild-type and IRF8-deficient mice were studied in vitro by proliferation, immunocytochemical and phagocytosis assays. Microglial response in vivo was compared between wild-type and IRF8-deficient mice in the cuprizon-induced demyelination model.

Results

Our analysis of IRF8-deficient mice revealed that, in contrast to compromised development of IRF8-deficient bone marrow myeloid lineage cells, development and colonization of microglia are not obviously affected by loss of IRF8. However, IRF8-deficient microglia demonstrate several defective phenotypes. In vivo, IRF8-deficient microglia have fewer elaborated processes with reduced expression of IBA1/AIF1 compared with wild-type microglia, suggesting a defective phenotype. IRF8-deficient microglia are significantly less proliferative in mixed glial cultures than wild-type microglia. Unlike IRF8-deficient bone marrow myeloid progenitors, exogenous macrophage colony stimulating factor (colony stimulating factor 1) (M-CSF (CSF1)) restores their proliferation in mixed glial cultures. In addition, IRF8-deficient microglia exhibit an exaggerated growth response to exogenous granulocyte-macrophage colony stimulating factor (colony stimulating factor 2) (GM-CSF (CSF2)) in the presence of other glial cells. IRF8-deficient microglia also demonstrate altered cytokine expressions in response to interferon-gamma and lipopolysaccharide in vitro. Moreover, the maximum phagocytic capacity of IRF8-deficient microglia is reduced, although their engulfment of zymosan particles is not overtly impaired. Defective scavenging activity of IRF8-deficient microglia was further confirmed in vivo in the cuprizone-induced demyelination model in mice.

Conclusions

This study is the first to demonstrate the essential contribution of IRF8-mediated transcription to a broad range of microglial phenotype. Microglia are distinct from the bone marrow myeloid lineage with respect to their dependence on IRF8-mediated transcription.

Keywords:
Microglia; Interferon regulatory factor; Phagocytosis; Cytokine; Cuprizone-induced demyelination