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      Inflammatory monocytes recruited after skeletal muscle injury switch into antiinflammatory macrophages to support myogenesis

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          Abstract

          Macrophages (MPs) are important for skeletal muscle regeneration in vivo and may exert beneficial effects on myogenic cell growth through mitogenic and antiapoptotic activities in vitro. However, MPs are highly versatile and may exert various, and even opposite, functions depending on their activation state. We studied monocyte (MO)/MP phenotypes and functions during skeletal muscle repair. Selective labeling of circulating MOs by latex beads in CX3CR1 GFP/+ mice showed that injured muscle recruited only CX3CR1 lo/Ly-6C + MOs from blood that exhibited a nondividing, F4/80 lo, proinflammatory profile. Then, within muscle, these cells switched their phenotype to become proliferating antiinflammatory CX3CR1 hi/Ly-6C cells that further differentiated into F4/80 hi MPs. In vitro, phagocytosis of muscle cell debris induced a switch of proinflammatory MPs toward an antiinflammatory phenotype releasing transforming growth factor β1. In co-cultures, inflammatory MPs stimulated myogenic cell proliferation, whereas antiinflammatory MPs exhibited differentiating activity, assessed by both myogenin expression and fusion into myotubes. Finally, depletion of circulating MOs in CD11b–diphtheria toxin receptor mice at the time of injury totally prevented muscle regeneration, whereas depletion of intramuscular F4/80 hi MPs at later stages reduced the diameter of regenerating fibers. In conclusion, injured skeletal muscle recruits MOs exhibiting inflammatory profiles that operate phagocytosis and rapidly convert to antiinflammatory MPs that stimulate myogenesis and fiber growth.

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          Alternative activation of macrophages.

           Siamon Gordon (2002)
          The classical pathway of interferon-gamma-dependent activation of macrophages by T helper 1 (T(H)1)-type responses is a well-established feature of cellular immunity to infection with intracellular pathogens, such as Mycobacterium tuberculosis and HIV. The concept of an alternative pathway of macrophage activation by the T(H)2-type cytokines interleukin-4 (IL-4) and IL-13 has gained credence in the past decade, to account for a distinctive macrophage phenotype that is consistent with a different role in humoral immunity and repair. In this review, I assess the evidence in favour of alternative macrophage activation in the light of macrophage heterogeneity, and define its limits and relevance to a range of immune and inflammatory conditions.
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            Obesity induces a phenotypic switch in adipose tissue macrophage polarization.

            Adipose tissue macrophages (ATMs) infiltrate adipose tissue during obesity and contribute to insulin resistance. We hypothesized that macrophages migrating to adipose tissue upon high-fat feeding may differ from those that reside there under normal diet conditions. To this end, we found a novel F4/80(+)CD11c(+) population of ATMs in adipose tissue of obese mice that was not seen in lean mice. ATMs from lean mice expressed many genes characteristic of M2 or "alternatively activated" macrophages, including Ym1, arginase 1, and Il10. Diet-induced obesity decreased expression of these genes in ATMs while increasing expression of genes such as those encoding TNF-alpha and iNOS that are characteristic of M1 or "classically activated" macrophages. Interestingly, ATMs from obese C-C motif chemokine receptor 2-KO (Ccr2-KO) mice express M2 markers at levels similar to those from lean mice. The antiinflammatory cytokine IL-10, which was overexpressed in ATMs from lean mice, protected adipocytes from TNF-alpha-induced insulin resistance. Thus, diet-induced obesity leads to a shift in the activation state of ATMs from an M2-polarized state in lean animals that may protect adipocytes from inflammation to an M1 proinflammatory state that contributes to insulin resistance.
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              Monocyte and macrophage heterogeneity.

              Heterogeneity of the macrophage lineage has long been recognized and, in part, is a result of the specialization of tissue macrophages in particular microenvironments. Circulating monocytes give rise to mature macrophages and are also heterogeneous themselves, although the physiological relevance of this is not completely understood. However, as we discuss here, recent studies have shown that monocyte heterogeneity is conserved in humans and mice, allowing dissection of its functional relevance: the different monocyte subsets seem to reflect developmental stages with distinct physiological roles, such as recruitment to inflammatory lesions or entry to normal tissues. These advances in our understanding have implications for the development of therapeutic strategies that are targeted to modify particular subpopulations of monocytes.
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                Author and article information

                Journal
                J Exp Med
                The Journal of Experimental Medicine
                The Rockefeller University Press
                0022-1007
                1540-9538
                14 May 2007
                : 204
                : 5
                : 1057-1069
                Affiliations
                [1 ]Institut National de la Santé et de la Recherche Médicale, Unité 841, Institut Mondor de Recherche Biomédicale, “Cell Interactions in the Neuromuscular System” Team, Université Paris 12 Val-de-Marne, 94000 Créteil, France
                [2 ]Plateforme Cytométrie Institut Mondor de Medecine Moleculaire, IFR10, Université Paris 12 Val-de-Marne, 94000 Créteil, France
                [3 ]Vrije Universiteit, Vrije Universiteit Medisch Centrum, Department of Molecular Cell Biology, Faculty of Medicine, 1081 BT Amsterdam, Netherlands
                [4 ]Assistance Publique-Hôpitaux de Paris, Groupe Henri Mondor-Albert Chenevier, Service d'immunologie-Biologie, 94000 Créteil, France
                Author notes

                CORRESPONDENCE Bénédicte Chazaud: benedicte.chazaud@ 123456creteil.inserm.fr

                Article
                20070075
                10.1084/jem.20070075
                2118577
                17485518
                Copyright © 2007, The Rockefeller University Press
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