MicroRNA-146a represses mycobacteria-induced inflammatory response and facilitates bacterial replication via targeting IRAK-1 and TRAF-6

MicroRNA-3570 Modulates the NF-κB Pathway in Teleost Fish by Targeting MyD88

The inflammatory response, a protective process to clear detrimental stimuli, constitutes the defense against infectious pathogens. However, excessive inflammation disrupts immune homeostasis, which may induce autoimmune and inflammatory diseases. In this study, we report that microRNA (miR)-3570 plays a negative role in the bacteria-induced inflammatory response of miiuy croaker. Upregulation of miR-3570 by Vibrio anguillarum and LPS inhibits LPS-induced inflammatory cytokine production, thus avoiding an excessive inflammation response. Evidence showed that miR-3570 targets MyD88 and posttranscriptionally downregulates its expression. Overexpression of miR-3570 in macrophages suppresses the expression of MyD88, as well as its downstream signaling of IL-1R-associated kinases 1 and 4 and TNFR-associated factor 6. These results suggest that miR-3570 plays a regulatory in the bacteria-induced inflammatory response through the MyD88-mediated NF-κB signaling pathway by targeting MyD88.

Small Non-Coding RNAs: New Insights in Modulation of Host Immune Response by Intracellular Bacterial Pathogens

Pathogenic bacteria possess intricate regulatory networks that temporally control the production of virulence factors and enable the bacteria to survive and proliferate within host cell. Small non-coding RNAs (sRNAs) have been identified as important regulators of gene expression in diverse biological contexts. Recent research has shown bacterial sRNAs involved in growth and development, cell proliferation, differentiation, metabolism, cell signaling, and immune response through regulating protein–protein interactions or via their ability to base pair with RNA and DNA. In this review, we provide a brief overview of mechanism of action employed by immune-related sRNAs, their known functions in immunity, and how they can be integrated into regulatory circuits that govern virulence, which will facilitate our understanding of pathogenesis and the development of novel, more effective therapeutic approaches to treat infections caused by intracellular bacterial pathogens.

MicroRNA-146a Contributes to SCI Recovery via Regulating TRAF6 and IRAK1 Expression

MicroRNA-146a participates in spinal cord injury (SCI) recovery. Until recently, how miRNA-146a participates in SCI remained unclear. In this study, we tried to explore the roles of miRNA-146a in the recovery of SCI using a rat model. The expression of the probable target genes of miRNA-146a (including IRAK1 and TARF6) as well as proinflammation cytokines were measured until 7 days after surgery in the three groups (sham group, SCI group, and miRNA-146a antagomir injection group). Also, the animals' motivations were estimated using Basso Beattie Bresnahan (BBB) during the whole experiment. A luciferase assay was performed to demonstrate that miRNA-146a could directly target the mRNAs of IRAK1 and TRAF6. Our experiments indicate that miRNA-146a inhibits proinflammatory cytokine secretion by suppressing IRAK1 and TRAF6 expression in the SCI model. In contrast, miRNA-146a may be upregulated by inflammatory mediators via the IRAK1/TRAF6 pathway in the spinal cord. As a negative feedback element, miRNA-146a could make sure that the expression of IRAK1- and TRAF6-mediated genes was under tight control. Thus, miRNA-146a may serve as a novel therapeutic target for SCI interventions.

MicroRNA expression signatures in lungs of mice infected with Mycobacterium tuberculosis

Tuberculosis (TB) is a major public health concern worldwide; however the factors that account for resistance or susceptibility to disease are not completely understood. Although some studies suggest that the differential expression of miRNAs in peripheral blood of TB patients could be useful as biomarkers of active disease, their involvement during the inflammatory process in lungs of infected individuals is unknown. Here, we evaluated the global expression of miRNAs in the lungs of mice experimentally infected with Mycobacterium tuberculosis on 30 and 60 days post-infection. We observed that several miRNAs were differentially expressed compared to uninfected mice. Furthermore, we verified that the expression of miR-135b, miR-21, miR-155, miR-146a, and miR-146b was significantly altered in distinct leukocyte subsets isolated from lungs of infected mice, while genes potentially targeted by those miRNAs were associated with a diversity of immune related molecular pathways. Importantly, we validated the inhibition of Pellino 1 expression by miR-135b in vitro. Overall, this study contributes to the understanding of the dynamics of miRNA expression in lungs during experimental TB and adds further perspectives into the role of miRNAs on the regulation of immune processes such as leukocyte activation.

Modulation of Host miRNAs by Intracellular Bacterial Pathogens

MicroRNAs (miRNAs) are short non-coding RNAs that regulate the expression of protein coding genes of viruses and eukaryotes at the post-transcriptional level. The eukaryotic genes regulated by miRNAs include those whose products are critical for biological processes such as cell proliferation, metabolic pathways, immune response, and development. It is now increasingly recognized that modulation of miRNAs associated with biological processes is one of the strategies adopted by bacterial pathogens to survive inside host cells. In this review, we present an overview of the recent findings on alterations of miRNAs in the host cells by facultative intracellular bacterial pathogens. In addition, we discuss how the altered miRNAs help in the survival of these pathogens in the intracellular environment.

Legionella pneumophila-Derived Outer Membrane Vesicles Promote Bacterial Replication in Macrophages

The formation and release of outer membrane vesicles (OMVs) is a phenomenon of Gram-negative bacteria. This includes Legionella pneumophila (L. pneumophila), a causative agent of severe pneumonia. Upon its transmission into the lung, L. pneumophila primarily infects and replicates within macrophages. Here, we analyzed the influence of L. pneumophila OMVs on macrophages. To this end, differentiated THP-1 cells were incubated with increasing doses of Legionella OMVs, leading to a TLR2-dependent classical activation of macrophages with the release of pro-inflammatory cytokines. Inhibition of TLR2 and NF-κB signaling reduced the induction of pro-inflammatory cytokines. Furthermore, treatment of THP-1 cells with OMVs prior to infection reduced replication of L. pneumophila in THP-1 cells. Blocking of TLR2 activation or heat denaturation of OMVs restored bacterial replication in the first 24 h of infection. With prolonged infection-time, OMV pre-treated macrophages became more permissive for bacterial replication than untreated cells and showed increased numbers of Legionella-containing vacuoles and reduced pro-inflammatory cytokine induction. Additionally, miRNA-146a was found to be transcriptionally induced by OMVs and to facilitate bacterial replication. Accordingly, IRAK-1, one of miRNA-146a's targets, showed prolonged activation-dependent degradation, which rendered THP-1 cells more permissive for Legionella replication. In conclusion, L. pneumophila OMVs are initially potent pro-inflammatory stimulators of macrophages, acting via TLR2, IRAK-1, and NF-κB, while at later time points, OMVs facilitate L. pneumophila replication by miR-146a-dependent IRAK-1 suppression. OMVs might thereby promote spreading of L. pneumophila in the host.

Pathogens Use and Abuse MicroRNAs to Deceive the Immune System

Emerging evidence has demonstrated that microRNAs (miRs) play a role in the survival and amplification of viruses, bacteria and other pathogens. There are various ways in which pathogens can benefit from miR-directed alterations in protein translation and signal transduction. Members of the herpesviridae family have previously been shown to encode multiple miRs, while the production of miRs by viruses like HIV-1 remained controversial. Recently, novel techniques have facilitated the elucidation of true miR targets by establishing miR-argonaute association and the subsequent interactions with their cognate cellular mRNAs. This, in combination with miR reporter assays, has generated physiologically relevant evidence that miRs from the herpesviridae family have the potential to downregulate multiple cellular targets, which are involved in immune activation, cytokine signaling and apoptosis. In addition, viruses and bacteria have also been linked to the induction of host cellular miRs, which have the capacity to mitigate immune activation, cytokine signaling and apoptosis. Interfering with miR expression may be clinically relevant. In the case of hepatitis C infection, the cellular miR-122 is already targeted therapeutically. This not only exemplifies how important miRs can be for the survival of specific viruses, but it also delineates the potential to use miRs as drug targets. In this paper we will review the latest reports on viruses and bacteria that abuse miR regulation for their benefit, which may be of interest in the development of miR-directed therapies.

The role of epigenetics in tuberculosis infection

Epigenetic mechanisms are pivotal in regulating gene expression during cellular response to extracellular stimuli. Bacterial infections have a profound effect on the host epigenome, which triggers susceptibility to diseases. Recent studies suggest that Mycobacterium tuberculosis (Mtb) can alter the host epigenome to modulate the transcriptional machinery and plays a major role in immunomodulation of the host immune response. However, the mechanism of epigenetic alterations during Mtb infection has not yet been fully understood. Thus, Mtb-induced epigenetic changes may affect the host cell by either activation or suppression of key immune genes involved in immune response or pathogen persistence. In this review, we discuss the principles of epigenetics, recent advances in Mtb-induced alterations in the host epigenetic landscape and their role in the host immune response.

microRNA-146a promotes mycobacterial survival in macrophages through suppressing nitric oxide production

Macrophages play a crucial role in host innate anti-mycobacterial defense, which is tightly regulated by multiple factors, including microRNAs. Our previous study showed that a panel of microRNAs was markedly up-regulated in macrophages upon mycobacterial infection. Here, we investigated the biological function of miR-146a during mycobacterial infection. miR-146a expression was induced both in vitro and in vivo after Mycobacterium bovis BCG infection. The inducible miR-146a could suppress the inducible nitric oxide (NO) synthase (iNOS) expression and NO generation, thus promoting mycobacterial survival in macrophages. Inhibition of endogenous miR-146a increased NO production and mycobacterial clearance. Moreover, miR-146a attenuated the activation of nuclear factor κB and mitogen-activated protein kinases signaling pathways during BCG infection, which in turn repressed iNOS expression. Mechanistically, miR-146a directly targeted tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) at post-transcriptional level. Silencing TRAF6 decreased iNOS expression and NO production in BCG-infected macrophages, while overexpression of TRAF6 reversed miR-146a-mediated inhibition of NO production and clearance of mycobacteria. Therefore, we demonstrated a novel role of miR-146a in the modulation of host defense against mycobacterial infection by repressing NO production via targeting TRAF6, which may provide a promising therapeutic target for tuberculosis.

The Defect in Autophagy Induction by Clinical Isolates of Mycobacterium Tuberculosis Is Correlated with Poor Tuberculosis Outcomes

Background

Tuberculosis (TB) represents a major global health problem. The prognosis of clinically active tuberculosis depends on the complex interactions between Mycobacterium tuberculosis (Mtb) and its host. In recent years, autophagy receives particular attention for its role in host defense against intracellular pathogens, including Mtb. In present study, we aim to investigate the relationship of autophagy induction by clinical isolates of Mtb with the clinical outcomes in patients with TB.

Methodology/Principal Findings

We collected 185 clinical isolates of Mtb, and determined the effect of these Mtb isolates on autophagy induction in macrophages. It was found that most of clinical isolates of Mtb were able to induce autophagosome formation in macrophages, however, the autophagy-inducing ability varied significantly among different isolates. Of importance, our results revealed that patients infected by Mtb with poor autophagy-inducing ability displayed more severe radiographic extent of disease (p<0.001), and were more likely to have unfavorable treatment outcomes (p<0.001). No significant association was observed between the extent of Mtb-induced autophagy with some socio-demographic characteristics (such as gender, age and tobacco consumption), and some laboratory tests (such as hemoglobin, leukocyte count and erythrocyte sedimentation rate). Furthermore, results from logistic regression analysis demonstrated that the defect in autophagy induction by clinical isolates of Mtb was an independent risk factor for far-advanced radiographic disease (aOR 4.710 [1.93–11.50]) and unfavorable treatment outcomes (aOR 8.309 [2.22–28.97]) in TB.

Conclusion/Significance

These data indicated that the defect in autophagy induction by Mtb isolates increased the risk of poor clinical outcomes in TB patients, and detection of clinical isolates-induced autophagosome formation might help evaluate the TB outcomes.

MiR-1303 Regulates Mycobacteria Induced Autophagy by Targeting Atg2B

MicroRNAs are emerging post-transcriptional regulators of gene expressions in both innate immunity and adaptive immunity. In mycobacteria infection, autophagy plays an important role in innate defense mechanism and is tightly regulated by the autophagy-related proteins. Here, we show that Atg2B is involved in the regulation of mycobacteria-induced autophagy. MiR-1303, which function is not defined yet, is found to negatively regulate mycobacteria-induced Atg2B protein production, ultimately down-regulate mycobacteria-induced autophagy. MiR-1303 production is shown to be upregulated during BCG infection and its production is regulated by PI3K and NFκB. It is also demonstrated that miR-1303 targets putative target sites on Atg2B and possibly represses its translation.

Immunomodulating microRNAs of mycobacterial infections

MicroRNAs are a class of small non-coding RNAs that have emerged as key regulators of gene expression at the post-transcriptional level by sequence-specific binding to target mRNAs. Some microRNAs block translation, while others promote mRNA degradation, leading to a reduction in protein availability. A single miRNA can potentially regulate the expression of multiple genes and their encoded proteins. Therefore, miRNAs can influence molecular signalling pathways and regulate many biological processes in health and disease. Upon infection, host cells rapidly change their transcriptional programs, including miRNA expression, as a response against the invading microorganism. Not surprisingly, pathogens can also alter the host miRNA profile to their own benefit, which is of major importance to scientists addressing high morbidity and mortality infectious diseases such as tuberculosis. In this review, we present recent findings on the miRNAs regulation of the host response against mycobacterial infections, providing new insights into host-pathogen interactions. Understanding these findings and its implications could reveal new opportunities for designing better diagnostic tools, therapies and more effective vaccines.

Epigenetics and miRNA during bacteria-induced host immune responses

Various cellular processes including the pathogen-specific immune responses, host-pathogen interactions and the related evasion mechanisms rely on the ability of the immune cells to be reprogrammed accurately and in many cases instantaneously. In this context, the exact functions of epigenetic and miRNA-mediated regulation of genes, coupled with recent advent in techniques that aid such studies, make it an attractive field for research. Here, we review examples that involve the epigenetic and miRNA control of the host immune system during infection with bacteria. Interestingly, many pathogens utilize the epigenetic and miRNA machinery to modify and evade the host immune responses. Thus, we believe that global epigenetic and miRNA mapping of such host-pathogen interactions would provide key insights into their cellular functions and help to identify various determinants for therapeutic value.

Association between microRNA polymorphisms and chronic pancreatitis

BACKGROUD

MicroRNAs play important roles in the development and progression of many human diseases. mir-146a could significantly suppress the induction of proinflammatory cytokines IL-1β, IL-6, TNF-α, NF-κB and chemokine MCP-1, which might play important roles in chronic pancreatitis. This study was conducted to evaluate the association between mir-146a rs2910164, a functional polymorphism in the pre-mir-146a, and chronic pancreatitis risk.

METHODS

The rs2910164 genotypes were determined in 165 patients with chronic pancreatitis and 200 healthy controls who were frequency matched for age and gender. One single nucleotide polymorphism (rs2910164) was genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RLFP).

RESULTS

The frequency of individuals who carried [G] allele was significantly higher in cases (62.7%) than in controls (53.7%, p = 0.015), which resulted in a statistically significant pathogenic effect associated with this variant allele (OR: 1.448, CI: 1.076-1.950; p = 0.015). The GC and GG genotypes showed strong and significant increased risk for complication of chronic pancreatitis (OR = 3.668, 95%CI = 1.233-10.916, p = 0.019; OR = 5.667, 95%CI = 1.852-17.336, p = 0.002). The individuals carrying G allele confer a lower expression level of mature mir-146a.

CONCLUSION

These findings suggest that the mir-146a rs2910164 may contribute to genetic susceptibility to chronic pancreatitis, and that mir-146a might be involved in chronic pancreatitis development.

Increased Expression of miR-146a in Children With Allergic Rhinitis After Allergen-Specific Immunotherapy

Purpose

MicroRNAs (miRs) were recently recognized to be important for immune cell differentiation and immune regulation. However, whether miRs were involved in allergen-specific immunotherapy (SIT) remains largely unknown. This study sought to examine changes in miR-146a and T regulatory cells in children with persistent allergic rhinitis (AR) after 3 months of subcutaneous immunotherapy (SCIT) and sublingual immunotherapy (SLIT).

Methods

Twenty-four HDM-sensitized children with persistent AR were enrolled and treated with SCIT (n=13) or SLIT (n=11) for 3 months. Relative miR-146a and Foxp3 mRNA expression, the TRAF6 protein level, and the ratio of post-treatment to baseline IL-10⁺CD4⁺ T cells between the SCIT and SLIT groups were examined in the peripheral blood mononuclear cells (PBMCs) of AR patients using quantitative reverse transcription polymerase chain reaction (qRT-PCR), flow cytometry, and Western blot analysis, respectively. Serum levels of IL-5 and IL-10 were determined using ELISA.

Results

After 3 months of SIT, both the TNSS and INSS scores were significantly decreased compared to the baseline value (P<0.01). The relative expression of miR-146a and Foxp3 mRNA was significantly increased after both SCIT and SLIT (P<0.01). The ratio of post-treatment to baseline IL-10⁺CD4⁺ T cells and the serum IL-10 level were significantly increased in both the SCIT and SLIT groups (P<0.01), whereas the TRAF6 protein level and serum IL-5 level were significantly decreased (P<0.01). No significant differences in these biomarkers were observed between the SCIT and SLIT groups.

Conclusions

Our findings suggest that miR-146a and its related biomarkers may be comparably modulated after both SCIT and SLIT, highlighting miR-146a as a potential therapeutic target for the improved management of AR.

MicroRNA-146a-5p attenuates neuropathic pain via suppressing TRAF6 signaling in the spinal cord

Glia-mediated neuroinflammation plays an important role in the pathogenesis of neuropathic pain. Our recent study demonstrated that TNF receptor associated factor-6 (TRAF6) is expressed in spinal astrocytes and contributes to the maintenance of spinal nerve ligation (SNL)-induced neuropathic pain. MicroRNA (miR)-146a is a key regulator of the innate immune response and was shown to target TRAF6 and reduce inflammation. In this study, we found that in cultured astrocytes, TNF-α, IL-1β, or lipopolysaccharide (LPS) induced rapid TRAF6 upregulation and delayed miR-146a-5p upregulation. In addition, miR-146a-5p mimic blocked LPS-induced TRAF6 upregulation, as well as LPS-induced c-Jun N-terminal kinase (JNK) activation and chemokine CCL2 expression in astrocytes. Notably, LPS incubation with astrocytes enhanced the DNA binding activity of AP-1 to the promoters of mir-146a and ccl2. TRAF6 siRNA or JNK inhibitor SP600125 significantly reduced LPS-induced miR-146a-5p increase in astrocytes. In vivo, intrathecal injection of TNF-α or LPS increased spinal TRAF6 expression. Pretreatment with miR-146a-5p mimic alleviated TNF-α- or LPS-induced mechanical allodynia and reduced TRAF6 expression. Finally, SNL induced miR-146a-5p upregulation in the spinal cord at 10 and 21days. Intrathecal injection of miR-146a-5p mimic attenuated SNL-induced mechanical allodynia and decreased spinal TRAF6 expression. Taken together, the results suggest that (1) miR-146a-5p attenuates neuropathic pain partly through inhibition of TRAF6 and its downstream JNK/CCL2 signaling, (2) miR-146a-5p is increased by the activation of TRAF6/JNK pathway. Hence, miR-146a-5p may be a novel treatment for chronic neuropathic pain.

Mycobacterium tuberculosis TlyA Protein Negatively Regulates T Helper (Th) 1 and Th17 Differentiation and Promotes Tuberculosis Pathogenesis

Mycobacterium tuberculosis, the causative agent of tuberculosis, is an ancient pathogen and a major cause of death worldwide. Although various virulence factors of M. tuberculosis have been identified, its pathogenesis remains incompletely understood. TlyA is a virulence factor in several bacterial infections and is evolutionarily conserved in many Gram-positive bacteria, but its function in M. tuberculosis pathogenesis has not been elucidated. Here, we report that TlyA significantly contributes to the pathogenesis of M. tuberculosis. We show that a TlyA mutant M. tuberculosis strain induces increased IL-12 and reduced IL-1β and IL-10 cytokine responses, which sharply contrasts with the immune responses induced by wild type M. tuberculosis. Furthermore, compared with wild type M. tuberculosis, TlyA-deficient M. tuberculosis bacteria are more susceptible to autophagy in macrophages. Consequently, animals infected with the TlyA mutant M. tuberculosis organisms exhibited increased host-protective immune responses, reduced bacillary load, and increased survival compared with animals infected with wild type M. tuberculosis. Thus, M. tuberculosis employs TlyA as a host evasion factor, thereby contributing to its virulence.

Association between ankylosing spondylitis and the miR-146a and miR-499 polymorphisms

miRNAs are small, non-coding RNAs that regulate the expression of multiple target genes at the post-transcriptional level. Single-nucleotide polymorphisms (SNPs) in miRNA sequences may alter miRNA expression and have been implicated in the pathogenesis of multiple forms of arthritis, including rheumatoid arthritis (RA) and osteoarthritis. The present study explored the association between ankylosing spondylitis (AS) and two single nucleotide polymorphisms (SNPs), miR-146a rs2910164G>C and miR-499 rs3746444T>C, in a Han Chinese population. A case-control study consisting of 102 subjects with AS and 105 healthy controls was designed. The two miRNA SNPs were identified by direct sequencing. Subsequently, their gene and genotype frequencies were compared with healthy controls. A significant difference was observed in the miR-146a rs2910164G>C SNP. The frequency of the G allele was markedly higher in the AS patients than in the healthy controls (P = 0.005, Pc = 0.01, OR = 1.787), and the frequency of the GG genotype was higher in AS patients than in controls (P = 0.014, Pc = 0.042, OR = 2.516). However, no significant association was found between the miR-499 rs3746444T>C variant and susceptibility to AS. This is the first study to address the association between the miR-146a rs2910164G>C and miR-499 rs3746444T>C polymorphisms and AS, and it suggests a potential pathogenic factor for AS. Further studies are needed to validate our findings in a larger series, as well as in other ethnic backgrounds.

Circulating miR-21, miR-146a and Fas ligand respond to postmenopausal estrogen-based hormone replacement therapy--a study with monozygotic twin pairs

Biological aging is associated with physiological deteriorations, which are partly due to changes in the hormonal profile. MicroRNAs regulate various processes associated with cell senescence; differentiation, replication and apoptosis. Serum microRNAs have potential to serve as noninvasive markers for diagnostics/prognostics and therapeutic targets. We analysed the association of estrogen-based hormone replacement therapy (HRT) with selected microRNAs and inflammation markers from the serum, leukocytes and muscle biopsy samples from 54 to 62 year-old postmenopausal monozygotic twins (n=11 pairs) discordant for HRT usage. Premenopausal 30-35 year-old women (n=8) were used as young controls. We focused on the hormonal aging and on the interaction between HRT use and the modulation of miR-21, miR-146a and classical inflammation markers. Fas-ligand was analysed since it functions in both apoptosis and inflammation. The inflammatory profile was healthier among the premenopausal women compared to the postmenopausal twins. Serum miR-21 and miR-146a levels and FasL concentrations were lower in HRT users compared to their non-using co-twins, demonstrating their responsiveness to HRT. Based on the pairwise FasL analysis, FasL concentration is likely to be genetically controlled. Overall, we suggest that postmenopausal estrogen deficiency sustains the development of "inflamm-aging". Estrogen sensitive, specific circulating microRNAs could be potential, early biomarkers for age-associated physiological deteriorations.

Dysregulation in microRNA expression in peripheral blood mononuclear cells of sepsis patients is associated with immunopathology

Sepsis is a major cause of death worldwide. It triggers systemic inflammation, the role of which remains unclear. In the current study, we investigated the induction of microRNA (miRNA) during sepsis and their role in the regulation of inflammation. Patients, on days 1 and 5 following sepsis diagnosis, had reduced T cells but elevated monocytes. Plasma levels of IL-6, IL-8, IL-10 and MCP-1 dramatically increased in sepsis patients on day 1. T cells from sepsis patients differentiated primarily into Th2 cells, whereas regulatory T cells decreased. Analysis of 1163 miRNAs from PBMCs revealed that miR-182, miR-143, miR-145, miR-146a, miR-150, and miR-155 were dysregulated in sepsis patients. miR-146a downregulation correlated with increased IL-6 expression and monocyte proliferation. Bioinformatics analysis uncovered the immunological associations of dysregulated miRNAs with clinical disease. The current study demonstrates that miRNA dysregulation correlates with clinical manifestations and inflammation, and therefore remains a potential therapeutic target against sepsis.

MicroRNAs: new regulators of Toll-like receptor signalling pathways

Toll-like receptors (TLRs), a critical family of pattern recognition receptors (PRRs), are responsible for the innate immune responses via signalling pathways to provide effective host defence against pathogen infections. However, TLR-signalling pathways are also likely to stringently regulate tissue maintenance and homeostasis by elaborate modulatory mechanisms. MicroRNAs (miRNAs) have emerged as key regulators and as an essential part of the networks involved in regulating TLR-signalling pathways. In this review, we highlight our understanding of the regulation of miRNA expression profiles by TLR-signalling pathways and the regulation of TLR-signalling pathways by miRNAs. We focus on the roles of miRNAs in regulating TLR-signalling pathways by targeting multiple molecules, including TLRs themselves, their associated signalling proteins and regulatory molecules, and transcription factors and functional cytokines induced by them, at multiple levels.