MicroRNAs regulate gene expression posttranscriptionally and function within the cells in which they are transcribed. However, recent evidence suggests that microRNAs can be transferred between cells and mediate target gene repression. We find that endogenous miR-155 and miR-146a, two critical microRNAs that regulate inflammation, are released from dendritic cells within exosomes and are subsequently taken up by recipient dendritic cells. Following uptake, exogenous microRNAs mediate target gene repression and can reprogramme the cellular response to endotoxin, where exosome-delivered miR-155 enhances while miR-146a reduces inflammatory gene expression. We also find that miR-155 and miR-146a are present in exosomes and pass between immune cells in vivo, as well as demonstrate that exosomal miR-146a inhibits while miR-155 promotes endotoxin-induced inflammation in mice. Together, our findings provide strong evidence that endogenous microRNAs undergo a functional transfer between immune cells and constitute a mechanism of regulating the inflammatory response.

Summary Chronic inflammation is a contributing factor to most life shortening human diseases. However, the molecular and cellular mechanisms that sustain chronic inflammatory responses remain poorly understood making it difficult to treat this deleterious condition. Using a mouse model of age-dependent inflammation that results from a deficiency in miR-146a, we demonstrate that miR-155 contributed to the progressive inflammatory disease that emerged as Mir146a−/− mice grew older. Upon analyzing lymphocytes from inflamed versus healthy middle-aged mice we found elevated numbers of T follicular helper (Tfh) cells, germinal center (GC) B cells and autoantibodies, all occurring in a miR-155-dependent manner. Further, Cd4-cre Mir155fl/fl mice were generated and demonstrated that miR-155 functions in T cells, in addition to its established role in B cells, to promote humoral immunity in a variety of contexts. Taken together, our study discovers that miR-146a and miR-155 counter-regulate Tfh cell development that drives aberrant GC reactions during chronic inflammation.

Th17 cells are central to the pathogenesis of autoimmune disease, and recently specific noncoding microRNAs (miRNAs) have been shown to regulate their development. However, it remains unclear if miRNAs are also involved in modulating Th17 cell effector functions. Consequently, we examined the role of miR-155 in differentiated Th17 cells during their induction of Experimental Autoimmune Encephalomyelitis (EAE). Using adoptive transfer experiments, we found that highly purified, MOG antigen-specific Th17 cells lacking miR-155 were defective in their capacity to cause EAE. Gene expression profiling of purified miR-155−/− IL-17F+ Th17 cells identified a subset of effector genes that are dependent upon miR-155 for their proper expression through a mechanism involving repression of the transcription factor Ets1. Among the genes reduced in the absence of miR-155 was IL-23R, resulting in miR-155−/− Th17 cells being hypo-responsive to IL-23. Taken together, our study demonstrates a critical role for miR-155 in Th17 cells as they unleash autoimmune inflammation, and finds that this occurs through a signaling network involving miR-155, Ets1 and the clinically relevant IL-23-IL-23R pathway.

• miR-155 promotes myeloproliferation in the bone marrow, spleen, and blood of mice carrying the FLT3-ITD mutation.• miR-155 suppresses the IFN response in FLT3-ITD

Host-microbial interactions within the mammalian intestines must be properly regulated in order to promote host health and limit disease. Because the microbiota provide constant immunological signals to intestinal tissues, a variety of regulatory mechanisms have evolved to ensure proper immune responses to maintain homeostasis. However, many of the genes that comprise these regulatory pathways, including immune-modulating microRNAs (miRNAs), have not yet been identified or studied in the context of intestinal homeostasis. Here, we investigated the role of microRNA-146a (miR-146a) in regulating intestinal immunity and barrier function and found that this miRNA is expressed in a variety of gut tissues in adult mice. By comparing intestinal gene expression in WT and miR-146a−/− mice, we demonstrate that miR-146a represses a subset of gut barrier and inflammatory genes all within a network of immune-related signaling pathways. We also found that miR-146a restricts the expansion of intestinal T cell populations, including Th17, Tregs, and Tfh cells. GC B cells, Tfh ICOS expression, and the production of luminal IgA were also reduced by miR-146a in the gut. Consistent with an enhanced intestinal barrier, we found that miR-146a−/− mice are resistant to DSS-induced colitis, a model of Ulcerative Colitis (UC), and this correlated with elevated colonic miR-146a expression in human UC patients. Taken together, our data describe a role for miR-146a in constraining intestinal barrier function, a process that alters gut homeostasis and enhances at least some forms of intestinal disease in mice.