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      Macrophages in gastrointestinal homeostasis and inflammation

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          Abstract

          Monocyte-derived mononuclear phagocytes, particularly macrophages, are crucial to maintain gastrointestinal homeostasis in the steady state but are also important for protection against certain pathogens. However, when uncontrolled, they can promote immunopathology. Broadly two subsets of macrophages can be considered to perform the vast array of functions to complete these complex tasks: resident macrophages that dominate in the healthy gut and inflammation-elicited (inflammatory) macrophages that derive from circulating monocytes infiltrating inflamed tissue. Here, we discuss the features of resident and inflammatory intestinal macrophages, complexities in identifying and defining these populations and the mechanisms involved in their differentiation. In particular, focus will be placed on describing their unique ontogeny as well as local gastrointestinal signals that instruct specialisation of resident macrophages in healthy tissue. We then explore the very different roles of inflammatory macrophages and describe new data suggesting that they may be educated not only by the gut microenvironment but also by signals they receive during development in the bone marrow. Given the high degree of plasticity of gut macrophages and their multifaceted roles in both healthy and inflamed tissue, understanding the mechanisms controlling their differentiation could inform development of improved therapies for inflammatory diseases such as inflammatory bowel disease (IBD).

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          Most cited references 118

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          Induction of intestinal Th17 cells by segmented filamentous bacteria.

          The gastrointestinal tract of mammals is inhabited by hundreds of distinct species of commensal microorganisms that exist in a mutualistic relationship with the host. How commensal microbiota influence the host immune system is poorly understood. We show here that colonization of the small intestine of mice with a single commensal microbe, segmented filamentous bacterium (SFB), is sufficient to induce the appearance of CD4(+) T helper cells that produce IL-17 and IL-22 (Th17 cells) in the lamina propria. SFB adhere tightly to the surface of epithelial cells in the terminal ileum of mice with Th17 cells but are absent from mice that have few Th17 cells. Colonization with SFB was correlated with increased expression of genes associated with inflammation and antimicrobial defenses and resulted in enhanced resistance to the intestinal pathogen Citrobacter rodentium. Thus, manipulation of this commensal-regulated pathway may provide new opportunities for enhancing mucosal immunity and treating autoimmune disease.
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            Fate mapping analysis reveals that adult microglia derive from primitive macrophages.

            Microglia are the resident macrophages of the central nervous system and are associated with the pathogenesis of many neurodegenerative and brain inflammatory diseases; however, the origin of adult microglia remains controversial. We show that postnatal hematopoietic progenitors do not significantly contribute to microglia homeostasis in the adult brain. In contrast to many macrophage populations, we show that microglia develop in mice that lack colony stimulating factor-1 (CSF-1) but are absent in CSF-1 receptor-deficient mice. In vivo lineage tracing studies established that adult microglia derive from primitive myeloid progenitors that arise before embryonic day 8. These results identify microglia as an ontogenically distinct population in the mononuclear phagocyte system and have implications for the use of embryonically derived microglial progenitors for the treatment of various brain disorders.
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              Interleukin-10-deficient mice develop chronic enterocolitis.

              Interleukin-10 (IL-10) affects the growth and differentiation of many hemopoietic cells in vitro; in particular, it is a potent suppressor of macrophage and T cell functions. In IL-10-deficient mice, generated by gene targeting, lymphocyte development and antibody responses are normal, but most animals are growth retarded and anemic and suffer from chronic enterocolitis. Alterations in intestine include extensive mucosal hyperplasia, inflammatory reactions, and aberrant expression of major histocompatibility complex class II molecules on epithelia. In contrast, mutants kept under specific pathogen-free conditions develop only a local inflammation limited to the proximal colon. These results indicate that the bowel inflammation in the mutants originates from uncontrolled immune responses stimulated by enteric antigens and that IL-10 is an essential immunoregulator in the intestinal tract.
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                Author and article information

                Contributors
                john.grainger-2@manchester.ac.uk
                Journal
                Pflugers Arch
                Pflugers Arch
                Pflugers Archiv
                Springer Berlin Heidelberg (Berlin/Heidelberg )
                0031-6768
                1432-2013
                10 March 2017
                10 March 2017
                2017
                : 469
                : 3
                : 527-539
                Affiliations
                [1 ]ISNI 0000000121662407, GRID grid.5379.8, Manchester Collaborative Centre for Inflammation Research (MCCIR), , University of Manchester, ; Manchester, M13 9NT UK
                [2 ]ISNI 0000000121662407, GRID grid.5379.8, Faculty of Biological, Medical and Human Sciences (FBMH), , University of Manchester, ; Manchester, UK
                Article
                1958
                10.1007/s00424-017-1958-2
                5362667
                28283748
                © The Author(s) 2017

                Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

                Funding
                Funded by: FundRef http://dx.doi.org/10.13039/100004440, Wellcome Trust;
                Award ID: 104195/Z/14/Z
                Award Recipient :
                Funded by: FundRef http://dx.doi.org/10.13039/501100000268, Biotechnology and Biological Sciences Research Council;
                Award ID: BB/M025977/1
                Award Recipient :
                Categories
                Invited Review
                Custom metadata
                © Springer-Verlag Berlin Heidelberg 2017

                Anatomy & Physiology

                inflammatory bowel disease, commensal, gastrointestinal, mucosal, monocyte, macrophage

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