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      The Benefits and Detriments of Macrophages/Microglia in Models of Multiple Sclerosis

      review-article
      1 , 2 , 1 , 2 , *
      Clinical and Developmental Immunology
      Hindawi Publishing Corporation

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          Abstract

          The central nervous system (CNS) is immune privileged with access to leukocytes being limited. In several neurological diseases, however, infiltration of immune cells from the periphery into the CNS is largely observed and accounts for the increased representation of macrophages within the CNS. In addition to extensive leukocyte infiltration, the activation of microglia is frequently observed. The functions of activated macrophages/microglia within the CNS are complex. In three animal models of multiple sclerosis (MS), namely, experimental autoimmune encephalomyelitis (EAE) and cuprizone- and lysolecithin-induced demyelination, there have been many reported detrimental roles associated with the involvement of macrophages and microglia. Such detriments include toxicity to neurons and oligodendrocyte precursor cells, release of proteases, release of inflammatory cytokines and free radicals, and recruitment and reactivation of T lymphocytes in the CNS. Many studies, however, have also reported beneficial roles of macrophages/microglia, including axon regenerative roles, assistance in promoting remyelination, clearance of inhibitory myelin debris, and the release of neurotrophic factors. This review will discuss the evidence supporting the detrimental and beneficial aspects of macrophages/microglia in models of MS, provide a discussion of the mechanisms underlying the dichotomous roles, and describe a few therapies in clinical use in MS that impinge on the activity of macrophages/microglia.

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          Most cited references140

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          Multiple Sclerosis

          New England Journal of Medicine, 343(13), 938-952
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            Local self-renewal can sustain CNS microglia maintenance and function throughout adult life.

            Microgliosis is a common response to multiple types of damage in the CNS. However, the origin of the cells involved in this process is still controversial and the relative importance of local expansion versus recruitment of microglia progenitors from the bloodstream is unclear. Here, we investigated the origin of microglia using chimeric animals obtained by parabiosis. We found no evidence of microglia progenitor recruitment from the circulation in denervation or CNS neurodegenerative disease, suggesting that maintenance and local expansion of microglia are solely dependent on the self-renewal of CNS resident cells in these models.
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              Infiltrating monocytes trigger EAE progression, but do not contribute to the resident microglia pool.

              In multiple sclerosis and the experimental autoimmune encephalitis (EAE) mouse model, two pools of morphologically indistinguishable phagocytic cells, microglia and inflammatory macrophages, accrue from proliferating resident precursors and recruitment of blood-borne progenitors, respectively. Whether these cell types are functionally equivalent is hotly debated, but is challenging to address experimentally. Using a combination of parabiosis and myeloablation to replace circulating progenitors without affecting CNS-resident microglia, we found a strong correlation between monocyte infiltration and progression to the paralytic stage of EAE. Inhibition of chemokine receptor-dependent recruitment of monocytes to the CNS blocked EAE progression, suggesting that these infiltrating cells are essential for pathogenesis. Finally, we found that, although microglia can enter the cell cycle and return to quiescence following remission, recruited monocytes vanish, and therefore do not ultimately contribute to the resident microglial pool. In conclusion, we identified two distinct subsets of myelomonocytic cells with distinct roles in neuroinflammation and disease progression.
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                Author and article information

                Journal
                Clin Dev Immunol
                Clin. Dev. Immunol
                CDI
                Clinical and Developmental Immunology
                Hindawi Publishing Corporation
                1740-2522
                1740-2530
                2013
                12 June 2013
                : 2013
                : 948976
                Affiliations
                1Hotchkiss Brain Institute, University of Calgary, Calgary, AB, Canada T2N 4N1
                2The Departments of Clinical Neurosciences and Oncology, University of Calgary, Calgary, AB, Canada T2N 4N1
                Author notes

                Academic Editor: Wolfgang J. Streit

                Article
                10.1155/2013/948976
                3694375
                23840244
                743e85fb-5374-4b38-96c7-1f3ab2b95d50
                Copyright © 2013 K. S. Rawji and V. W. Yong.

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

                History
                : 26 March 2013
                : 16 May 2013
                Categories
                Review Article

                Immunology
                Immunology

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