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MicroRNA-155 (miR-155) has been involved in the response to inflammation in macrophages and lymphocytes. Here we show how miR-155 participates in the maturation of human dendritic cells (DC) and modulates pathogen binding by downregulating DC-specific intercellular adhesion molecule-3 grabbing non-integrin (DC-SIGN), after directly targeting the transcription factor PU.1. During the maturation of DCs, miR-155 increases up to 130-fold, whereas PU.1 protein levels decrease accordingly. We establish that human PU.1 is a direct target for miR-155 and localize the target sequence for miR-155 in the 3-untranslated region of PU.1. Also, overexpression of miR-155 in the THP1 monocytic cell line decreases PU.1 protein levels and DC-SIGN at both the mRNA and protein levels. We prove a link between the down-regulation of PU.1 and reduced transcriptional activity of the DC-SIGN promoter, which is likely to be the basis for its reduced mRNA expression, after miR-155 overexpression. Finally, we show that, by reducing DC-SIGN in the cellular membrane, miR-155 is involved in regulating pathogen binding as dendritic cells exhibited the lower binding capacity for fungi and HIV protein gp-120 when the levels of miR-155 were higher. Thus, our results suggest a mechanism by which miR-155 regulates proteins involved in the cellular immune response against pathogens that could have clinical implications in the way pathogens enter the human organism.MicroRNAs have emerged as important regulators of key cellular processes. They consist of endogenous small, non-coding RNA molecules of about 19 -22 nucleotides in length (1), which regulate mRNAs in a post-transcriptional manner. They bind to the 3Ј-untranslated regions of their target mRNAs and exert their function in two ways: mainly blocking the translation and also inducing their cleavage in a similar fashion to small interfering RNAs (2). MicroRNAs are initially expressed as long immature pri-microRNAs, which are processed in the nucleus into the precursor pre-microRNAs and finally matured by Dicer in the cytoplasm into the functional 19 -22-nucleotide long microRNAs, which are then incorporated into the RNAinduced silencing complex (1).The role of microRNAs is being intensively studied in many different fields such as fetal development and the immune system. One of the miRNAs that appears to play a particularly important role in the immune system is microRNA-155 (miR-155), 3 the expression of which is induced by inflammatory signals such as exposure to antigen, Toll-like receptor ligands, or interferon ␥ stimulation in T-cells, B-cells, and macrophages, respectively (3, 4). miR-155 knock-out mice show aberrant immune functions including defective B and T cell immunity and abnormal function of antigen-presenting cells (4, 5). These mutant mice exhibit an imbalance in the immune Th1/Th2 response, with the CD4ϩ T cells biased toward Th2 differentiation (4). A lack of miR-155 also leads to a failure in production of high-affinity IgG 1 antibodies by murine B-cells (28). This effect has b...

Transforming growth factor-beta (TGF-␤) is a pleiotropic cytokine with important effects on processes such as fibrosis, angiogenesis, and immunosupression. Using bioinformatics, we identified SMAD2, one of the mediators of TGF-␤ signaling, as a predicted target for a microRNA, microRNA-155 (miR-155). MicroRNAs are a class of small non-coding RNAs that have emerged as an important class of gene expression regulators. miR-155 has been found to be involved in the regulation of the immune response in myeloid cells. Here, we provide direct evidence of binding of miR-155 to a predicted binding site and the ability of miR-155 to repress SMAD2 protein expression. We employed a lentivirally transduced monocyte cell line (THP1-155) containing an inducible miR-155 transgene to show that endogenous levels of SMAD2 protein were decreased after sustained overexpression of miR-155. This decrease in SMAD2 led to a reduction in both TGF-␤-induced SMAD-2 phosphorylation and SMAD-2-dependent activation of the expression of the CAGA 12 LUC reporter plasmid. Overexpression of miR-155 altered the cellular responses to TGF-␤ by changing the expression of a set of genes that is involved in inflammation, fibrosis, and angiogenesis. Our study provides firm evidence of a role for miR-155 in directly repressing SMAD2 expression, and our results demonstrate the relevance of one of the two predicted target sites in SMAD2 3-UTR. Altogether, our data uncover an important role for miR-155 in modulating the cellular response to TGF-␤ with possible implications in several human diseases where homeostasis of TGF-␤ might be altered.Transforming growth factor-␤ (TGF-␤) is a pleiotropic cytokine with important effects on a wide range of cells and processes (1). TGF-␤ has major actions in wound healing/tissue remodeling, being pro-fibrotic, pro-angiogenic, and anti-proliferative (2-4). Importantly, TGF-␤ can also act as an immunosuppressive and anti-inflammatory cytokine with key roles in inflammation and in some types of cancer (4, 5). For example, some tumors have reduced responsiveness to the antiproliferative effects of TGF-␤ as a consequence of mutated TGF-␤ signaling pathways (6). Typically, TGF-␤ exerts its effect on gene expression through the transcription factors known as SMAD proteins (7). After binding of TGF-␤ by its heterodimeric receptor TGF-␤ receptor I and II, the receptor phosphorylates SMAD2 and SMAD3 (in the so-called canonical pathway), which then associate with SMAD4 and translocate to the nucleus. Here and in association with other transcription factors, SMADs induce or repress the expression of several genes (8).It is clearly vital that a major cytokine such as TGF-␤, which has such a wide range of effects in key physiological processes, must be under tight regulation to control its expression and activity. This regulation should include mechanisms that allow a variety of effects depending on different cellular and tissue contexts (1). In recent years, a new layer of general control of gene expression has been identified and sh...