MicroRNAs are a family of regulatory molecules involved in many physiological processes, including differentiation and activation of cells of the immune system. We found that brainspecific miR-124 is expressed in microglia but not in peripheral monocytes or macrophages. When overexpressed in macrophages, miR-124 directly inhibited the transcription factor CCAAT/ enhancer-binding protein-α (C/EBP-α) and its downstream target PU.1, resulting in transformation of these cells from an activated phenotype into a quiescent CD45 low , major histocompatibility complex (MHC) class II low phenotype resembling resting microglia. During experimental autoimmune encephalomyelitis (EAE), miR-124 was downregulated in activated microglia. Peripheral administration of miR-124 in EAE caused systemic deactivation of macrophages, reduced activation of myelin-specific T cells and marked suppression of disease. Conversely, knockdown of miR-124 in microglia and macrophages resulted in activation of these cells in vitro and in vivo. These findings identify miR-124 both as a key regulator of microglia quiescence in the central nervous system and as a previously unknown modulator of monocyte and macrophage activation.MicroRNAs (miRNAs) belong to a family of small non-protein-coding RNAs that regulate expression of multiple target genes and are involved in many fundamental biological processes, such as embryonic development, cell proliferation, differentiation and apoptosis [1][2][3][4][5] . miRNAs promote degradation of mRNA or prevent translation of the target genes, and they can be viewed as endogenous mediators of RNA interference (RNAi) 1 . miRNAs have been identified as crucial regulators of differentiation of various cell types, © 2011 Nature America, Inc. All rights reserved. Correspondence should be addressed to H.L.W. (hweiner@rics.bwh.harvard.edu) and A.M.K. (akrichevsky@rics.bwh.harvard.edu).. 3 These authors contributed equally to this work. AUTHOR CONTRIBUTIONS E.D.P. performed all flow-cytometry assays, EAE experiments, experiments with chimeric, knockout and transgenic mice, in vivo injections of oligonucleotides, cell isolations, cell cultures, coculture assays and immunohistochemistry; collected and analyzed the data; and wrote the manuscript. T.V. performed in vitro transfections, miRNA and mRNA expression assays and data analysis, western blots, luciferase target validation assay, immunohistochemistry and in silico target prediction analysis and helped to write the manuscript. N.B. performed imaging cytometry. E.D.P. and A.M.K. conceived the project. E.D.P., T.V. and A.M.K. developed the hypothesis and designed the experiments. A.M.K. and H.L.W. discussed the hypothesis, helped with data interpretation, coordinated and directed the project and wrote the manuscript.Note: Supplementary information is available on the Nature Medicine website. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests. 2,6 . Furthermore, deregulated expression of specific miRNAs is associated with many patholo...

Monocytic cells exhibit a high level of heterogeneity and have two distinct modes of their activation: 1) classical M1 path associated with inflammation and tissue damage, and 2) alternative M2 path. Although it has been demonstrated that M2 macrophages play an important role in the regulation of the allergic immune responses, tissue maintenance and repair, little is known about the mechanisms that determine the M2 phenotype. We have previously shown that miR-124 is expressed in microglia that exhibit the M2 phenotype and overexpression of miR-124 in macrophages resulted in downregulation of a number of M1 markers (MHC class II, CD86) and up-regulation of several M2 markers (Fizz1, Arg1). We further investigated whether the polarization of macrophages towards the M2 phenotype induced miR-124 expression. We found that exposure of cells to IL-4 and IL-13 resulted in the upregulation of miR-124 in macrophages. We also demonstrated that IL-4 induced expression of three miR-124 precursor transcripts with predominant expression of pri-miR-124.3, suggesting regulation of miR-124 expression by IL-4 on a transcriptional level. Expression of miR-124 in microglia did not depend on IL-4 and/or IL-13, whereas expression of miR-124 in lung resident macrophages was IL-4 and IL-13-dependent and was upregulated by systemic administration of IL-4 or during allergic inflammation. Upregulation of several M2 markers (CD206, Ym1) and downregulation of the M1 markers (CD86, iNOS, TNF) in M2-polarized macrophages was abrogated by a miR-124 inhibitor, suggesting that this microRNA contributed to the M2 phenotype development and maintenance. Finally we showed that human CD14+CD16+ intermediate monocytes, which are found in increased numbers in patients with allergies and bronchial asthma, expressed high levels of miR-124 and exhibited other properties of M2-like cells. Thus, our study suggests that miR-124 serves as a regulator of the M2 polarization in various subsets of monocytic cells both in vitro and in vivo.

Rationale Platelets are known to participate in vascular pathologies; however, their role in neuroinflammatory diseases such as multiples sclerosis (MS) is unknown. Autoimmune CD4 T cells have been the main focus of studies of MS, although the factors that regulate T cell differentiation towards pathogenic Th1/Th17 phenotypes are not completely understood. Objectives We investigated the role of platelets in the modulation of CD4 T cell functions in MS patients and in mice with experimental autoimmune encephalitis (EAE), an animal model for MS. Methods and Results We found that early in MS and EAE platelets degranulated and produced a number of soluble factors serotonin (5HT), PF4 and PAF, which specifically stimulated differentiation of T cells towards pathogenic Th1, Th17 and IFN-γ/IL-17-producing CD4 T cells. At the later stages of MS and EAE platelets became exhausted in their ability to produce proinflammatory factors and stimulate CD4 T cells, but substantially increased their ability to form aggregates with CD4 T cells. Formation of platelet-CD4 T cell aggregates involved interaction of CD62P on activated platelets with adhesion molecule CD166 on activated CD4 T cells, contributing to downmodulation of CD4 T cell activation, proliferation and production of IFN-γ. Blocking of formation of platelet-CD4 T cell aggregates during progression of EAE substantially enhanced proliferation of CD4 T cell in the CNS and the periphery leading to exacerbation of the disease. Conclusion Our study indicates differential roles for platelets in the regulation of functions of pathogenic CD4 T cells during initiation and progression of CNS autoimmune inflammation.