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      Macrophage proliferation, provenance, and plasticity in macroparasite infection

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          Macrophages have long been center stage in the host response to microbial infection, but only in the past 10–15 years has there been a growing appreciation for their role in helminth infection and the associated type 2 response. Through the actions of the IL-4 receptor α (IL-4Rα), type 2 cytokines result in the accumulation of macrophages with a distinctive activation phenotype. Although our knowledge of IL-4Rα-induced genes is growing rapidly, the specific functions of these macrophages have yet to be established in most disease settings. Understanding the interplay between IL-4Rα-activated macrophages and the other cellular players is confounded by the enormous transcriptional heterogeneity within the macrophage population and by their highly plastic nature. Another level of complexity is added by the new knowledge that tissue macrophages can be derived either from a resident prenatal population or from blood monocyte recruitment and that IL-4 can increase macrophage numbers through proliferative expansion. Here, we review current knowledge on the contribution of macrophages to helminth killing and wound repair, with specific attention paid to distinct cellular origins and plasticity potential.

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          Most cited references 138

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          Macrophage-specific PPARgamma controls alternative activation and improves insulin resistance.

          Obesity and insulin resistance, the cardinal features of metabolic syndrome, are closely associated with a state of low-grade inflammation. In adipose tissue chronic overnutrition leads to macrophage infiltration, resulting in local inflammation that potentiates insulin resistance. For instance, transgenic expression of Mcp1 (also known as chemokine ligand 2, Ccl2) in adipose tissue increases macrophage infiltration, inflammation and insulin resistance. Conversely, disruption of Mcp1 or its receptor Ccr2 impairs migration of macrophages into adipose tissue, thereby lowering adipose tissue inflammation and improving insulin sensitivity. These findings together suggest a correlation between macrophage content in adipose tissue and insulin resistance. However, resident macrophages in tissues display tremendous heterogeneity in their activities and functions, primarily reflecting their local metabolic and immune microenvironment. While Mcp1 directs recruitment of pro-inflammatory classically activated macrophages to sites of tissue damage, resident macrophages, such as those present in the adipose tissue of lean mice, display the alternatively activated phenotype. Despite their higher capacity to repair tissue, the precise role of alternatively activated macrophages in obesity-induced insulin resistance remains unknown. Using mice with macrophage-specific deletion of the peroxisome proliferator activated receptor-gamma (PPARgamma), we show here that PPARgamma is required for maturation of alternatively activated macrophages. Disruption of PPARgamma in myeloid cells impairs alternative macrophage activation, and predisposes these animals to development of diet-induced obesity, insulin resistance, and glucose intolerance. Furthermore, gene expression profiling revealed that downregulation of oxidative phosphorylation gene expression in skeletal muscle and liver leads to decreased insulin sensitivity in these tissues. Together, our findings suggest that resident alternatively activated macrophages have a beneficial role in regulating nutrient homeostasis and suggest that macrophage polarization towards the alternative state might be a useful strategy for treating type 2 diabetes.
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            Transforming growth factor-beta regulation of immune responses.

            Transforming growth factor-beta (TGF-beta) is a potent regulatory cytokine with diverse effects on hemopoietic cells. The pivotal function of TGF-beta in the immune system is to maintain tolerance via the regulation of lymphocyte proliferation, differentiation, and survival. In addition, TGF-beta controls the initiation and resolution of inflammatory responses through the regulation of chemotaxis, activation, and survival of lymphocytes, natural killer cells, dendritic cells, macrophages, mast cells, and granulocytes. The regulatory activity of TGF-beta is modulated by the cell differentiation state and by the presence of inflammatory cytokines and costimulatory molecules. Collectively, TGF-beta inhibits the development of immunopathology to self or nonharmful antigens without compromising immune responses to pathogens. This review highlights the findings that have advanced our understanding of TGF-beta in the immune system and in disease.
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              Inflammation in wound repair: molecular and cellular mechanisms.

              In post-natal life the inflammatory response is an inevitable consequence of tissue injury. Experimental studies established the dogma that inflammation is essential to the establishment of cutaneous homeostasis following injury, and in recent years information about specific subsets of inflammatory cell lineages and the cytokine network orchestrating inflammation associated with tissue repair has increased. Recently, this dogma has been challenged, and reports have raised questions on the validity of the essential prerequisite of inflammation for efficient tissue repair. Indeed, in experimental models of repair, inflammation has been shown to delay healing and to result in increased scarring. Furthermore, chronic inflammation, a hallmark of the non-healing wound, predisposes tissue to cancer development. Thus, a more detailed understanding in mechanisms controlling the inflammatory response during repair and how inflammation directs the outcome of the healing process will serve as a significant milestone in the therapy of pathological tissue repair. In this paper, we review cellular and molecular mechanisms controlling inflammation in cutaneous tissue repair and provide a rationale for targeting the inflammatory phase in order to modulate the outcome of the healing response.

                Author and article information

                Immunol Rev
                Immunol. Rev
                Immunological Reviews
                BlackWell Publishing Ltd (Oxford, UK )
                November 2014
                15 October 2014
                : 262
                : 1
                : 113-133
                Institute for Immunology and Infection Research, School of Biological Sciences, University of Edinburgh Edinburgh, UK
                Author notes
                Correspondence to: Judith E. Allen, Institute for Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK, Tel.: +44 (0)131 650 7014, Fax: +44 (0)131 650 6564, e-mail: j.allen@ 123456ed.ac.uk
                © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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