In fact, the gut microbiota exerts a wide range of effects on the intestinal mucosa . MVs from both strains and miRNAs which are differentially expressed in response to EcN or ECOR12 MVs. Based on the differential expression of the target genes and subsequent validation experiments, we correlated some of the selected miRNAs with the reported cytokine profile and specific T cell responses. As far as we know, this is the first study to analyze the regulation of miRNAs in DCs by MVs released by gut microbiota. Nissle 1917 1. Introduction The human intestine holds trillions of microbes that live in a symbiotic relationship with the host. This diverse, complex microbial community, known as the gut microbiota, is considered a hidden organ that performs an essential role in maintaining homeostasis and human health. In fact, the gut microbiota exerts a wide range of effects on the intestinal mucosa . Besides its contribution to food digestion and nutrient metabolism, the gut microbiota is essential for the hosts immune system development and for the modulation of the gut barrier and immune responses. Nowadays, microbiome research is essential to better understand human health, immunity and nutrition. The interconnection between nutrients, metabolites and microbes is a key factor governing the healthy/pathological status of an individual. The high plasticity of the human microbiome sets the basis GSK726701A for new therapeutic strategies aimed at restoring the altered gut microbiotas balance. The administration of prebiotics or probiotics are among these interventions, which basically try to exploit the beneficial effects of the commensal microbiota . The gut microbiota establishes dynamic and reciprocal interactions GSK726701A with the intestinal epithelium and the immune system. The detection of microbes by epithelial and gut-associated innate immune cells is mediated by pattern recognition receptors (PRRs) that specifically recognize conserved microbial-associated molecular patterns (MAMPs). The interactions of PRRs with their specific ligands activates signaling pathways that result in the secretion of chemokines, cytokines and antimicrobial peptides that help to control the guts microbial population. This feedback control is crucial in restricting immune activation and preserving mutualistic associations between the microbiota and the host. Immune homeostasis depends on the ability of intestinal cells to distinguish between pathogens and commensals. In addition to common MAMPs, pathogens express virulence factors that enable bacteria to infect host epithelial cells and redirect signal transduction pathways, shifting the outcomes of immune responses. The hosts response to pathogens leads to controlled inflammation that assists pathogen eradication. The response to symbiotic microbiota is known as tolerance, a state that depends on highly regulated innate and adaptive immune responses that contribute to basal immune training . The sampling of gut microbes is mainly mediated by cells of the mucosal innate immune system, which is comprised of resident macrophages and dendritic cells (DCs). DCs are central players in the immune system. These antigen-presenting cells can receive information from microbe-activated epithelial cells or connect the luminal environment by extending their dendrites through the inner mucosal lining. By this last mechanism, DCs directly sample fallotein gut microbes and orchestrate appropriate immune responses. In fact, DCs act as a link between the innate and the adaptive immune systems and can prime na?ve T cells through the release of immune mediators and antigen presentation, thus determining the fate of the immune response . Concerning the signaling pathways which are activated by the microbiota in order to modulate intestinal homeostasis, research was mainly focused on regulatory proteins and transcription factors. Now, the study of host microRNAs (miRNAs) as important regulators in the hostCmicrobiota interplay is receiving great interest. miRNAs are small non-coding RNAs (20C25 nucleotides) that, after maturation, associate with proteins to form a microRNA silencing complex that post-transcriptionally regulates the expression of target messenger RNAs (mRNAs) by binding to complementary sequences in the 3-UTR region . This interaction either represses translation or triggers mRNA degradation . Thus, miRNAs allow signaling pathways to be tightly regulated and are involved in the control of multiple cellular processes, including the GSK726701A immune response. Moreover, some studies reveal that both pathogens and gut bacteria greatly influence host miRNA expression [7,8,9]. The huge number of microbial cells represents a constant threat to the host. To maintain the spatial segregation of gut microbes and host tissues, the intestinal epithelium is covered by a mucus layer. The commensal microbiota resides in the outer mucus layer, while the inner layer is dense and.