TLR13 recognizes bacterial 23S rRNA devoid of erythromycin resistance-forming modification

Milk-Derived Exosomes and Metabolic Regulation

Exosomes are natural nanoparticles that play an important role in cell-to-cell communication. Communication is achieved through the transfer of cargos, such as microRNAs, from donor to recipient cells and binding of exosomes to cell surface receptors. Exosomes and their cargos are also obtained from dietary sources, such as milk. Exosome and cell glycoproteins are crucial for intestinal uptake. A large fraction of milk exosomes accumulates in the brain, whereas the tissue distribution of microRNA cargos varies among distinct species of microRNA. The fraction of milk exosomes that escapes absorption elicits changes in microbial communities in the gut. Dietary depletion of exosomes and their cargos causes a loss of circulating microRNAs and elicits phenotypes such as loss of cognitive performance, increase in purine metabolites, loss of fecundity, and changes in the immune response. Milk exosomes meet the definition of bioactive food compounds.

The Toll Pathway in the Central Nervous System of Flies and Mammals

Toll receptors, first identified to regulate embryogenesis and immune responses in the adult fly and subsequently defined as the principal sensors of infection in mammals, are increasingly appreciated for their impact on the homeostasis of the central as well as the peripheral nervous systems. Whereas in the context of immunity, the fly Toll and the mammalian TLR pathways have been researched in parallel, the expression pattern and functionality have largely been researched disparately. Herein, we provide data on the expression pattern of the Toll homologues, signaling components, and downstream effectors in ten different cell populations of the adult fly central nervous system (CNS). We have compared the expression of the different Toll pathways in the fly to the expression of TLRs in the mouse brain and discussed the implications with respect to commonalities, differences, and future perspectives.

Detection of Microbial Infections Through Innate Immune Sensing of Nucleic Acids

Microbial infections are recognized by the innate immune system through germline-encoded pattern recognition receptors (PRRs). As most microbial pathogens contain DNA and/or RNA during their life cycle, nucleic acid sensing has evolved as an essential strategy for host innate immune defense. Pathogen-derived nucleic acids with distinct features are recognized by specific host PRRs localized in endolysosomes and the cytosol. Activation of these PRRs triggers signaling cascades that culminate in the production of type I interferons and proinflammatory cytokines, leading to induction of an antimicrobial state, activation of adaptive immunity, and eventual clearance of the infection. Here, we review recent progress in innate immune recognition of nucleic acids upon microbial infection, including pathways involving endosomal Toll-like receptors, cytosolic RNA sensors, and cytosolic DNA sensors. We also discuss the mechanisms by which infectious microbes counteract host nucleic acid sensing to evade immune surveillance.

Antibiotic treatment-induced secondary IgA deficiency enhances susceptibility to Pseudomonas aeruginosa pneumonia

Broad-spectrum antibiotics are widely used with patients in intensive care units (ICUs), many of whom develop hospital-acquired infections with Pseudomonas aeruginosa. Although preceding antimicrobial therapy is known as a major risk factor for P. aeruginosa-induced pneumonia, the underlying mechanisms remain incompletely understood. Here we demonstrate that depletion of the resident microbiota by broad-spectrum antibiotic treatment inhibited TLR-dependent production of a proliferation-inducing ligand (APRIL), resulting in a secondary IgA deficiency in the lung in mice and human ICU patients. Microbiota-dependent local IgA contributed to early antibacterial defense against P. aeruginosa. Consequently, P. aeruginosa-binding IgA purified from lamina propria culture or IgA hybridomas enhanced resistance of antibiotic-treated mice to P. aeruginosa infection after transnasal substitute. Our study provides a mechanistic explanation for the well-documented risk of P. aeruginosa infection following antimicrobial therapy, and we propose local administration of IgA as a novel prophylactic strategy.

Decoy peptides derived from the extracellular domain of toll-like receptor 2 (TLR2) show anti-inflammatory properties

Toll-like receptor 2 (TLR2) recognizes bacterial derived- and synthetic-lipopeptides after dimerization with TLR1 or TLR6. Hyper-activation of TLR2 has been described in several inflammatory diseases and the discovery of inhibitors of its pro-inflammatory activity represent potential starting points to develop therapeutics in such pathologies. We designed peptides derived from the TLR2 sequence comprising amino acid residues involved in ligand binding (Pam3CSK4) or heterodimerization (TLR2/TLR1) as pointed out by structural data.² We identified several peptides (P13, P13(LL), P16, P16(LL)) which inhibited TLR2/1 signaling in HEK293-TLR2 cells (MAPK activation and NF-kB activity). Moreover, P13L and P16L decreased TNFα release in human primary PBMCs and mouse macrophages. The peptides were selective for TLR2/1 as they did not inhibit the activity of other TLRs tested. P13L and P16L inhibited the internalization of Pam3CSK4 fluorescently labeled in macrophages and the heterodimerization of TLR2 with TLR1 as demonstrated by immunoprecipitation studies. Our data demonstrate that peptides derived from the region comprising the leucine-rich repeats (LRR) 11 and 13 in the extracellular domain of TLR2 are good starting points to develop more potent anti-inflammatory peptides with TLR2 inhibitory activity.

Delicate regulation of the cGAS-MITA-mediated innate immune response

Although it has long been demonstrated that cytosolic DNA is a potent immune stimulant, it is only in recent years that the molecular mechanisms of DNA-stimulated innate immune responses have emerged. Studies have established critical roles for the DNA sensor cyclic GMP-AMP synthase (cGAS) and the adapter protein MITA/STING in the innate immune response to cytosolic DNA or DNA viruses. Although the regulation of cGAS-MITA/STING-mediated signaling remains to be fully investigated, understanding the processes involved may help to explain the mechanisms of innate immune signaling events and perhaps autoinflammatory diseases and to provide potential therapeutic targets for drug intervention. In this review, we summarize recent progress on the regulation of the cGAS-MITA/STING-mediated innate immune response to DNA viruses at the organelle-trafficking, post-translational and transcriptional levels.

Twenty-seven-nucleotide repeat insertion in the rplV gene confers specific resistance to macrolide antibiotics in Staphylococcus aureus

Macrolide antibiotics are used for treatment of soft-tissue infection caused by Staphylococcus aureus in humans. However, infections with S. aureus are increasingly difficult to treat owing to the emergence and rapid spread of multiple-drug resistant S. aureus. Resistance to macrolide in S. aureus is mostly due to the modification of 23 S rRNA by methylases encoded by erm genes. Here, we have identified that a 27-nucleotide repeat sequence insertion in the rplV gene induced a specific resistance to macrolide antibiotics. An erythromycin-resistant strain, 8325^ER+, was screened by resistance to erythromycin from the macrolide-sensitive strain 8325-4. Comparative genome sequencing analysis showed that 8325^ER+ contained a 27-nt repeat sequence insertion in the rplV gene that encodes the ribosomal protein L22, when compared to its parent strain. The 27-nt repeat sequence led to an insertion of 9 amino acids in L22, which had been identified to reduce the sensitivity to erythromycin and other macrolide antibiotics. Moreover, we show that the ectopic expression of the mutated rplV gene containing the 27-nt repeat sequence insertion in several susceptible strains specifically conferred resistance to macrolide antibiotics. Our findings present a potential mechanism of resistance to macrolide antibiotics in S. aureus.

RIG-I and Other RNA Sensors in Antiviral Immunity

Pattern recognition receptors (PRRs) survey intra- and extracellular spaces for pathogen-associated molecular patterns (PAMPs) within microbial products of infection. Recognition and binding to cognate PAMP ligand by specific PRRs initiates signaling cascades that culminate in a coordinated intracellular innate immune response designed to control infection. In particular, our immune system has evolved specialized PRRs to discriminate viral nucleic acid from host. These are critical sensors of viral RNA to trigger innate immunity in the vertebrate host. Different families of PRRs of virus infection have been defined and reveal a diversity of PAMP specificity for wide viral pathogen coverage to recognize and extinguish virus infection. In this review, we discuss recent insights in pathogen recognition by the RIG-I-like receptors, related RNA helicases, Toll-like receptors, and other RNA sensor PRRs, to present emerging themes in innate immune signaling during virus infection.

Mechanism of pattern recognition receptors (PRRs) and host pathogen interplay in bovine mastitis

Bacterial infection in the mammary gland parenchyma induces local and subsequently systemic inflammation that results in a complex disease. Mastitis in bovine is the result of various factors which function together. This review is aimed to analyze the factors involved in the pathogenesis of common bacterial species for bovine mastitis. The bacterial growth patterns, signaling pathway and the pathogen-associated molecular patterns (PAMPs) which activate immune responses is discussed. Clear differences in bacterial infection pattern are shown between bacterial species and illustrated TLRs, NLRs and RLGs molecular mechanism for the initiation of intramammary infection. The underlying reasons for the differences and the resulting host response are analyzed. Understandings of the mechanisms that activate and regulate these responses are central to the development of efficient anticipatory and treatment management. The knowledge of bovine mammary gland to common mastitis causing pathogens with possible immune mechanism could be a new conceptual understanding for the prospect of mastitis control program.

Toll-like Receptor 9 Contains Two DNA Binding Sites that Function Cooperatively to Promote Receptor Dimerization and Activation

Toll-like receptor 9 (TLR9) recognizes DNA containing CpG motifs derived from bacteria and viruses and activates the innate immune response to eliminate them. TLR9 is known to bind to CpG DNA, and here, we identified another DNA binding site in TLR9 that binds DNA containing cytosine at the second position from the 5' end (5'-xCx DNA). 5'-xCx DNAs bound to TLR9 in the presence of CpG DNA and cooperatively promoted dimerization and activation of TLR9. Binding at both sites was important for efficient activation of TLR9. The 5'-xCx DNA bound the site corresponding to the nucleoside binding site in TLR7 and TLR8 as revealed by the structural analysis. This study revealed that TLR9 recognizes two types of DNA through its two binding sites for efficient activation. This information may contribute to the development of drugs that control the activity of TLR9.

MicroRNA regulation of Toll-like receptor signaling pathways in teleost fish

The innate immune system is the first line defense mechanism that recognizes, responds to, controls or eliminates invading pathogens. Toll-like receptors (TLRs) are a critical family of pattern recognition receptors (PRRs) tightly regulated by complex mechanisms involving many molecules to ensure a beneficial outcome in response to foreign invaders. MicroRNAs (miRNAs), a transcriptional and posttranscriptional regulator family in a wide range of biological processes, have been identified as new molecules related to the regulation of TLR-signaling pathways in immune responses. To date, at least 22 TLR types have been identified in more than a dozen different fish species. However, the functions and underlying mechanisms of miRNAs in the regulation of inflammatory responses related to the TLR-signaling pathway in fish is lacking. In this review, we summarize the regulation of miRNA expression profiles in the presence of TLR ligands or pathogen infections in teleost fish. We focus on the effects of miRNAs in regulating TLR-signaling pathways by targeting multiple molecules, including TLRs themselves, TLR-associated signaling proteins, and TLR-induced cytokines. An understanding of the relationship between the TLR-signaling pathways and miRNAs may provide new insights for drug intervention to manipulate immune responses in fish.

Unc93b1 is essential for cytokine activation of five PAMPs in the orange-spotted grouper (Epinephelus coioides)

Toll-like receptors (TLRs) are important innate immune receptors that recognize multiple pathogen-associated molecular patterns (PAMPs) and activate the immune responses to resist the invasion of pathogens. Many TLRs need assistance from trafficking chaperones to transport to the specific cell compartments and then are processed before they are activated. In this study, we identified an important trafficking chaperone, Unc-93 homolog B1 (unc93b1), from the Epinephelus coioides (orange-spotted grouper). The deduced protein sequence of Eco.unc93b1 was 632 amino acids, containing 12 transmembrane domains, consistent with other UNC93B1 proteins from other species. Phylogenetic analysis showed that Eco.Unc93b1 was clustered with teleost Unc93b1 and had the closest relationship with Larimichthys crocea (large yellow croaker) Unc93b1. Eco.unc93b1 was expressed the highest in the spleen, and its protein was co-localized with the endoplasmic reticulum and early endosomes in both human embryonic kidney 293T cells and grouper spleen cells (GS cells). Moreover, the stimulation of lipopolysaccharide (LPS), high-molecular-weight poly (I:C) (HMW), imidazoquinoline (R848), polyadenylic-polyuridylic acid (poly AU), and 19-mer Staphylococcus aureus 23S rRNA-derived oligoribonucleotide (ORN Sa 19) promoted the mRNA expression of unc93b1 in GS cells with different patterns. Furthermore, the cytokine expression induced by these PAMPs was suppressed, while Eco.unc93b1 was knocked down, by small interfering RNA. In conclusion, these results suggest that Eco.unc93b1 plays an essential role in several PAMP-induced immune responses.

Enteric Virome Sensing-Its Role in Intestinal Homeostasis and Immunity

Pattern recognition receptors (PRRs) sensing commensal microorganisms in the intestine induce tightly controlled tonic signaling in the intestinal mucosa, which is required to maintain intestinal barrier integrity and immune homeostasis. At the same time, PRR signaling pathways rapidly trigger the innate immune defense against invasive pathogens in the intestine. Intestinal epithelial cells and mononuclear phagocytes in the intestine and the gut-associated lymphoid tissues are critically involved in sensing components of the microbiome and regulating immune responses in the intestine to sustain immune tolerance against harmless antigens and to prevent inflammation. These processes have been mostly investigated in the context of the bacterial components of the microbiome so far. The impact of viruses residing in the intestine and the virus sensors, which are activated by these enteric viruses, on intestinal homeostasis and inflammation is just beginning to be unraveled. In this review, we will summarize recent findings indicating an important role of the enteric virome for intestinal homeostasis as well as pathology when the immune system fails to control the enteric virome. We will provide an overview of the virus sensors and signaling pathways, operative in the intestine and the mononuclear phagocyte subsets, which can sense viruses and shape the intestinal immune response. We will discuss how these might interact with resident enteric viruses directly or in context with the bacterial microbiome to affect intestinal homeostasis.

Recognition of Double-Stranded RNA and Regulation of Interferon Pathway by Toll-Like Receptor 10

Toll-like receptor (TLR)-10 remains an orphan receptor without well-characterized ligands or functions. Here, we reveal that TLR10 is predominantly localized to endosomes and binds dsRNA in vitro at endosomal pH, suggesting that dsRNA is a ligand of TLR10. Recognition of dsRNA by TLR10 activates recruitment of myeloid differentiation primary response gene 88 for signal transduction and suppression of interferon regulatory factor-7 dependent type I IFN production. We also demonstrate crosstalk between TLR10 and TLR3, as they compete with each other for dsRNA binding. Our results suggest for the first time that dsRNA is a ligand for TLR10 and propose novel dual functions of TLR10 in regulating IFN signaling: first, recognition of dsRNA as a nucleotide-sensing receptor and second, sequestration of dsRNA from TLR3 to inhibit TLR3 signaling in response to dsRNA stimulation.

Identification and functional characterization of Toll-like receptor 13 from orange-spotted grouper (Epinephelus coioides)

Toll-like receptors (TLRs) are one of the most important innate immune receptors, which recognize various pathogen-associated molecular patterns and activate the downstream immune response. Mouse TLR13 has been found to recognize a highly conserved sequence from bacterial or viral RNA and activate the myeloid differentiation primary response gene 88-dependent signaling response. The function of teleost tlr13 is still not fully understood, especially its relationship with bacterial RNA. In our study, we identified and characterized a tlr13 from Epinephelus coioides (orange-spotted grouper). The full-length cDNA of Eco. tlr13 contained a 2844 bp open reading frame, encoding 947 amino acids. The polypeptide was constitutive of a signal peptide, 13 leucine-rich repeats domains, a C-terminal leucine-rich repeats, a transmembrane domain and a conserved Toll/interleukin (IL)-1 receptor domain, indicating that Eco. Tlr13 exhibited a typical TLR structure. Multiple alignments showed that the Toll/IL-1 receptor domain of Eco. Tlr13 was identical with other homologues, and the phylogenetic tree suggested that Eco. Tlr13 was clustered with other TLR13s and had the closest relationship with predicted Lates calcarifer (sea bass) Tlr13. Subcellular localization analysis revealed that Eco. Tlr13 colocalized with the endoplasmic reticulum and early endosome. Moreover, Eco. tlr13 was broadly observed in all tested tissues with the relatively high expressions in the brain and immune-related tissues. After challenged with 19-mer Staphylococcus aureus 23S ribosomal RNA-derived oligoribonucleotide (ORN Sa19), the expression of Eco. tlr13 was significantly up-regulated in grouper spleen cells. Also, the luciferase assay further revealed that with the overexpression of Eco. Tlr13 in human embryonic kidney 293T cells, ORN Sa19 activated the promoter activity of interferon-β in a dose-dependent pattern. These results indicate that Eco. tlr13 may involve in the recognition of bacterial RNA.

Mudskippers and Their Genetic Adaptations to an Amphibious Lifestyle

Mudskippers are the largest group of amphibious teleost fish that are uniquely adapted to live on mudflats. During their successful transition from aqueous life to terrestrial living, these fish have evolved morphological and physiological modifications of aerial vision and olfaction, higher ammonia tolerance, aerial respiration, improved immunological defense against terrestrial pathogens, and terrestrial locomotion using protruded pectoral fins. Comparative genomic and transcriptomic data have been accumulated and analyzed for understanding molecular mechanisms of the terrestrial adaptations. Our current review provides a general introduction to mudskippers and recent research advances of their genetic adaptations to the amphibious lifestyle, which will be helpful for understanding the evolutionary transition of vertebrates from water to land. Our insights into the genomes and transcriptomes will also support molecular breeding, functional identification, and natural compound screening.

Biogenesis and iron-dependency of ribosomal RNA hydroxylation

Post-transcriptional modifications of ribosomal RNAs (rRNAs) are involved in ribosome biogenesis and fine-tuning of translation. 5-Hydroxycytidine (ho5C), a modification of unknown biogenesis and function, is present at position 2501 of Escherichia coli 23S rRNA. We conducted a genome-wide screen in E. coli to identify genes required for ho5C2501 formation, and found a previously-uncharacterized gene, ydcP (renamed rlhA), iron-sulfur cluster (isc) genes, and a series of genes responsible for prephenate biosynthesis, indicating that iron-sulfur clusters and prephenate are required for ho5C2501 formation. RlhA interacted with precursors of the 50S ribosomal subunit, suggesting that this protein is directly involved in formation of ho5C2501. RlhA belongs to a family of enzymes with an uncharacterized peptidase U32 motif and conserved Cys residues in the C-terminal region. These elements were essential for ho5C2501 formation. We also found that the frequency of ho5C2501 is modulated by environmental iron concentration. Together, our results reveal a novel biosynthetic pathway for RNA hydroxylation and its response to iron.

Non-mammalian Toll-like receptor 18 (Tlr18) recognizes bacterial pathogens in common carp (Cyprinus carpio L.): Indications for a role of participation in the NF-κB signaling pathway

Toll-like receptors are important pattern recognition receptors that can recognize pathogen-associated molecular patterns (PAMPs) and play a critical role in innate immunity. In the present study, tlr18 was identified from common carp (Cyprinus carpio L.) (named Cctlr18). The deduced amino acid sequence contained only a signal peptide, eight LRR (leucine-rich repeat) motifs, a transmembrane region and a TIR (Toll/IL-1 receptor) domain. Phylogenetic analysis showed that CcTlr18 was most closely related to Ctenopharyngodon idella Tlr18. Quantitative real-time PCR analysis showed that Cctlr18 was constitutively expressed in all investigated tissues with the highest expression level in the skin and lowest expression in the gonad. After injection with inactivated Aeromonas hydrophila, Cctlr18 expression was significantly up-regulated in the head kidney, foregut, hindgut and skin. Moreover, significant up-regulation of Cctlr8 was observed in the spleen, head kidney, hindgut and skin after immersion with live A. hydrophila. In addition, the expression of Cctlr18 was up-regulated in PGN or flagellin-stimulated HKLs. Luciferase reporter assays showed that Cctlr18 activated NF-κB in 293 T cells and that NF-κB activity was enhanced in Cctlr18 and Ccmyd88 co-transfected cells. Furthermore, Cctlr18 could induce the expression of cytokines genes, including ifn, il-1β and il-10, in EPC cells. The results suggested that Cctlr18 plays an important role in the immune response and provides basic information for investigating the mechanisms of fish tlr18.

Dendritic cell subsets

Dendritic cells (DC) are professional antigen presenting cells comprising a variety of subsets, as either resident or migrating cells, in lymphoid and non-lymphoid organs. In the steady state DC continually process and present antigens on MHCI and MHCII, processes that are highly upregulated upon activation. By expressing differential sets of pattern recognition receptors different DC subsets are able to respond to a range of pathogenic and danger stimuli, enabling functional specialisation of the DC. The knowledge of functional specialisation of DC subsets is key to efficient priming of T cells, to the design of effective vaccine adjuvants and to understanding the role of different DC in health and disease. This review outlines mouse and human steady state DC subsets and key attributes that define their distinct functions.

Group A Streptococcus encounters with host macrophages

Group A Streptococcus (GAS) is a leading human bacterial pathogen with diverse clinical manifestations. Macrophages constitute a critical first line of host defense against GAS infection, using numerous surface and intracellular receptors such as Toll-like receptors and inflammasomes for pathogen recognition and activation of inflammatory signaling pathways. Depending on the intensity of the GAS infection, activation of these signaling cascades may provide a beneficial early alarm for effective immune clearance, or conversely, may cause hyperinflammation and tissue injury during severe invasive infection. Although traditionally considered an extracellular pathogen, GAS can invade and replicate within macrophages using specific molecular mechanisms to resist phagolysosomal and xenophagic killing. Unraveling GAS-macrophage encounters may reveal new treatment options for this leading agent of infection-associated mortality. [Formula: see text].

The Human-Associated Archaeon Methanosphaera stadtmanae Is Recognized through Its RNA and Induces TLR8-Dependent NLRP3 Inflammasome Activation

The archaeon Methanosphaera stadtmanae is a member of the gut microbiota; yet, the molecular cross-talk between archaea and the human immune system and its potential contribution to inflammatory diseases has not been evaluated. Although archaea are as bacteria prokaryotes, they form a distinct domain having unique features such as different cell wall structures and membrane lipids. So far, no microbe-associated molecular patterns of archaea which activate innate immune receptors have been identified. By stimulating human myeloid cells with M. stadtmanae and purified archaeal nucleic acids, we identified both the microorganism and its RNA as potent stimuli for the innate immune system. To dissect the recognition and activation pathways induced by M. stadtmanae, human monocytic BLaER1 knockout cells were generated using the CRISPR/Cas9 system targeting components of TLR and inflammasome signaling. While the recognition of M. stadtmanae is mediated by TLR7 and TLR8, activation of the NLRP3 inflammasome depends solely on TLR8 engagement. Notably, this process resembles hallmarks of both the canonical and the recently described alternative inflammasome activation. Thus, we have demonstrated for the first time the specific recognition of and response to an archaeon by human cells at the molecular level.

Codevelopment of Microbiota and Innate Immunity and the Risk for Group B Streptococcal Disease

The pathogenesis of neonatal late-onset sepsis (LOD), which manifests between the third day and the third month of life, remains poorly understood. Group B Streptococcus (GBS) is the most important cause of LOD in infants without underlying diseases or prematurity and the third most frequent cause of meningitis in the Western world. On the other hand, GBS is a common intestinal colonizer in infants. Accordingly, despite its adaption to the human lower gastrointestinal tract, GBS has retained its potential virulence and its transition from a commensal to a dangerous pathogen is unpredictable in the individual. Several cellular innate immune mechanisms, in particular Toll-like receptors, the inflammasome and the cGAS pathway, are engaged by GBS effectors like nucleic acids. These are likely to impact on the GBS-specific host resistance. Given the long evolution of streptococci as a normal constituent of the human microbiota, the emergence of GBS as the dominant neonatal sepsis cause just about 50 years ago is remarkable. It appears that intensive usage of tetracycline starting in the 1940s has been a selection advantage for the currently dominant GBS clones with superior adhesive and invasive properties. The historical replacement of Group A by Group B streptococci as a leading neonatal pathogen and the higher frequency of other β-hemolytic streptococci in areas with low GBS prevalence suggests the existence of a confined streptococcal niche, where locally competing streptococcal species are subject to environmental and immunological selection pressure. Thus, it seems pivotal to resolve neonatal innate immunity at mucous surfaces and its impact on microbiome composition and quality, i.e., genetic heterogeneity and metabolism, at the microanatomical level. Then, designer pro- and prebiotics, such as attenuated strains of GBS, and oligonucleotide priming of mucosal immunity may unfold their potential and facilitate adaptation of potentially hazardous streptococci as part of a beneficial local microbiome, which is stabilized by mucocutaneous innate immunity.

DAMP-TLR-cytokine axis dictates the fate of tumor

Random mutations leading to loss of cell cycle control is not a rare occurrence in an organism but the mutated cells are recognized and eliminated preventing the development of a tumor. These potentially tumorigenic cells release damage-associated molecular patterns (DAMPs), which are recognized by toll-like receptors (TLRs) on macrophages and dendritic cells. The initial TLR-DAMP interactions lead to different responses such as altered antigen presentation and cytokine release that directly affect T cell activation and removal of the tumorigenic cells. The indirect effects of TLR-DAMP interaction include chemokine-directed altered T cell trafficking, angiogenesis for both T cell infiltration and tumor cell metastasis, and alteration of intra-tumoral milieu contributing to the development of tumor cells heterogeneity. Thus, the initial TLR-DAMP interaction has a set of local effects that modulate tumor cell growth and heterogeneity and a disseminating set of central effects that dynamically affect T cell trafficking and functions. Herein, we argue that the DAMP-TLR-cytokine axis in the tumor microenvironment serves as the mainstay that orchestrates and regulates the pro- and anti-tumor elements which dynamically interact between themselves eventuating in tumor regression or growth. The knowledge of this TLR-based immuno-surveillance framework is a key to developing a novel immunotherapy against cancer.

Toll-Like Receptor 8 Is a Major Sensor of Group B Streptococcus But Not Escherichia coli in Human Primary Monocytes and Macrophages

TLR8 is the major endosomal sensor of degraded RNA in human monocytes and macrophages. It has been implicated in the sensing of viruses and more recently also bacteria. We previously identified a TLR8-IFN regulatory factor 5 (IRF5) signaling pathway that mediates IFNβ and interleukin-12 (IL-12) induction by Staphylococcus aureus and is antagonized by TLR2. The relative importance of TLR8 for the sensing of various bacterial species is however still unclear. We here compared the role of TLR8 and IRF5 for the sensing of Group B Streptococcus (GBS), S. aureus, and Escherichia coli in human primary monocytes and monocyte-derived macrophages (MDM). GBS induced stronger IFNβ and TNF production as well as IRF5 nuclear translocation compared to S. aureus grown to the stationary phase, while S. aureus in exponential growth appeared similarly potent to GBS. Cytokine induction in primary human monocytes by GBS was not dependent on hemolysins, and induction of IFNβ and IL-12 as well as IRF5 activation were reduced with TLR2 ligand costimulation. Heat inactivation of GBS reduced IRF5 and NF-kB translocation, while only the viable E. coli activated IRF5. The attenuated stimulation correlated with loss of bacterial RNA integrity. The E. coli-induced IRF5 translocation was not inhibited by TLR2 costimulation, suggesting that IRF5 was activated via a TLR8-independent mechanism. Gene silencing of MDM using siRNA revealed that GBS-induced IFNβ, IL-12-p35, and TNF production was dependent on TLR8 and IRF5. In contrast, cytokine induction by E. coli was TLR8 independent but still partly dependent on IRF5. We conclude that TLR8-IRF5 signaling is more important for the sensing of GBS than for stationary grown S. aureus in human primary monocytes and MDM, likely due to reduced resistance of GBS to phagosomal degradation and to a lower production of TLR2 activating lipoproteins. TLR8 does not sense viable E. coli, while IRF5 still contributes to E. coli-induced cytokine production, possibly via a cytosolic nucleic acid sensing mechanism.

Toll-Like Receptor Function in Acute Wounds

Significance: Inflammation is an integral part of immune response and supports optimal wound healing in adults. Inflammatory cells such as neutrophils, macrophages, dendritic cells, lymphocytes, and mast cells produce important cytokines, chemokines, and growth factors. These immune cells interact with keratinocytes, fibroblasts, and endothelial cells (ECs), as well as the extracellular matrix within a complicated network that promotes and regulates wound healing. Aberrant and persistent inflammation may result in delayed wound healing, scar formation, or chronic wounds. Targeting the molecules involved in the inflammatory response may have great potential therapeutic value. Recent Advances and Critical Issues: Toll-like receptors (TLRs) are pattern recognition receptors that recognize pathogen-associated molecular patterns from microbes or danger-associated molecular patterns from damaged cells. The discovery of TLRs sheds new light on the mechanism by which the inflammatory or innate immune response is initiated in wound healing. Convincing evidence now shows that multiple types of cells, including infiltrating or resident inflammatory cells, keratinocytes, fibroblasts, and ECs, express specific types of TLRs. Experimental reduction of certain TLRs or treatment of wounds with TLR ligands has been shown to affect wound healing. A better understanding of the involvement of TLRs in the innate immune response during skin wound healing may suggest novel strategies to improve the quality of tissue repair. Future Directions: Despite the indisputable role of TLRs in regulating the immune response in acute wound healing, the functions of TLRs that are relevant to human wound healing and chronic wounds are poorly understood.

Conservation and Divergence of Ligand Recognition and Signal Transduction Mechanisms in Toll-Like Receptors

Toll-like receptors (TLRs) play a central role in innate immunity as pathogen sensors. During the last decade, structural analyses of TLRs have revealed the mechanisms of ligand recognition and signal transduction. Each TLR recognizes its cognate ligand in a different manner, whereas signal transduction is achieved by a common mechanism. In this review, the mechanisms of ligand recognition and signal transduction by TLRs are summarized based on recent structural information.

B. abortus RNA is the component involved in the down-modulation of MHC-I expression on human monocytes via TLR8 and the EGFR pathway

Despite eliciting a potent CD8⁺ T cell response, Brucella abortus is able to persist and establish a chronic infection inside its host. We have previously reported that the infection of human monocytes/macrophages with B. abortus inhibits the IFN-γ-induced MHC-I cell surface expression down-modulating cytotoxic CD8⁺ T cell responses. MHC-I down-modulation depends on bacterial viability and results from the capacity of B. abortus to retain the MHC-I molecules within the Golgi apparatus. Furthermore, we recently demonstrated that epidermal growth factor receptor (EGFR) pathway is involved in this phenomenon and that this is an early event during infection. However, the components and mechanisms whereby B. abortus is able to down-modulate MHC-I remained to be elucidated. In this study we demonstrated that the down-modulation of MHC-I expression is not mediated by well-known Brucella virulence factors but instead by B. abortus RNA, a PAMP associated to viability (vita-PAMP). Surprisingly, completely degraded RNA was also able to inhibit MHC-I expression to the same extent as intact RNA. Accordingly, B. abortus RNA and its degradation products were able to mimic the MHC-I intracellular retention within the Golgi apparatus observed upon infection. We further demonstrated that TLR8, a single-stranded RNA and RNA degradation products sensor, was involved in MHC-I inhibition. On the other hand, neutralization of the EGFR reversed the MHC-I inhibition, suggesting a connection between the TLR8 and EGFR pathways. Finally, B. abortus RNA-treated macrophages display diminished capacity of antigen presentation to CD8⁺ T cells. Overall, our results indicate that the vita-PAMP RNA as well as its degradation products constitute novel virulence factors whereby B. abortus, by a TLR8-dependent mechanism and through the EGFR pathway, inhibits the IFN-γ-induced MHC-I surface expression on human monocytes/macrophages. Thus, bacteria can hide within infected cells and avoid the immunological surveillance of cytotoxic CD8⁺ T cells.

Brucella abortus is one of the intracellular bacterial species that cause brucellosis, a zoonotic worldwide disease. An intricate enigma of Brucella immunity is its long-term persistence inside host despite a vigorous and specific immune response. Our study describes a novel immune evasion strategy exploited by B. abortus: the down-modulation of the IFN-γ-induced expression of the molecules responsible for antigen presentation to CD8⁺ T cells on the surface of monocytes, Major Histocompatibility Complex Class I (MHC-I) molecules. We found that the bacterial component responsible for this phenomenon is its RNA, a component associated with bacterial viability itself. Specifically, we demonstrated that not only does intact RNA down-modulate MHC-I but also the RNA degradation products. Bacterial RNA is sensed by pattern recognition receptors (PRRs), among which TLRs family has gained more attention. In this study, we demonstrated that the receptor involved in this phenomenon is TLR8, a single-stranded RNA and RNA degradation products sensor. By means of this mechanism, Brucella impairs antigen presentation to CD8⁺ T cells, hiding within infected cells and avoiding the immunological surveillance of cytotoxic cells. On balance, these results provide new evidence to understand how B. abortus can survive inside the host and persist chronically.

Toward a structural understanding of nucleic acid-sensing Toll-like receptors in the innate immune system

The history of mankind has been plagued by the tug of war with viral infections. Toll-like receptors (TLRs) and other receptors of the innate immune system constitute an early defense system against invading viruses by recognizing the viral genetic material, the nucleic acids (NAs). Agonistic ligands of NA-sensing TLRs play an emerging role in the treatment of viral diseases, demonstrating a crucial role of these receptors. Recently, crystal structures have afforded new insights into TLR recognition of NAs. An aberrant activation by self-NAs, which leads to the inflammation and autoimmunity, is avoided by strict regulation of NA-TLR interaction at multiple check-points. This Review summarizes the novel structural understanding of NA-sensing by TLRs and regulatory mechanisms of these receptors.

To Eat and to Be Eaten: Mutual Metabolic Adaptations of Immune Cells and Intracellular Bacterial Pathogens upon Infection

Intracellular bacterial pathogens (IBPs) invade and replicate in different cell types including immune cells, in particular of the innate immune system (IIS) during infection in the acute phase. However, immune cells primarily function as essential players in the highly effective and integrated host defense systems comprising the IIS and the adaptive immune system (AIS), which cooperatively protect the host against invading microbes including IBPs. As countermeasures, the bacterial pathogens (and in particular the IBPs) have developed strategies to evade or reprogram the IIS at various steps. The intracellular replication capacity and the anti-immune defense responses of the IBP's as well as the specific antimicrobial responses of the immune cells of the innate and the AIS depend on specific metabolic programs of the IBPs and their host cells. The metabolic programs of the immune cells supporting or counteracting replication of the IBPs appear to be mutually exclusive. Indeed, recent studies show that upon interaction of naïve, metabolically quiescent immune cells with IBPs, different metabolic activation processes occur which may result in the provision of a survival and replication niche for the pathogen or its eradication. It is therefore likely that within a possible host cell population subsets exist that are metabolically programmed for pro- or anti-microbial conditions. These metabolic programs may be triggered by the interactions between different bacterial agonistic components and host cell receptors. In this review, we summarize the current status in the field and discuss metabolic adaptation processes within immune cells of the IIS and the IBPs that support or restrict the intracellular replication of the pathogens.

Toll-Like Receptor Signaling and Its Inducible Proteins

Studies that bridge innate immunity and various diseases are making rapid advances. Macrophages and dendritic cells play a leading role in innate immunity, using pattern recognition receptors (PRRs) to sense both pathogen invasion and danger signals. Among these PRRs, the Toll-like receptors (TLRs) play a central role in the immune response by recognizing not only pathogen-associated molecular patterns from bacteria and viruses but also damage-associated molecular patterns from dying or injured cells. These TLR family molecules facilitate not only the elimination of pathogens but also the development of various disorders, including arteriosclerosis, cancer, and metabolic syndrome. TLRs activate signaling pathways that result in the production of cytokines, chemokines, and various inducible molecules associated with the immune response. Although most components of the innate immune signaling pathways, such as the TLR family and its downstream signaling, have been identified, the physiological roles of many TLR signal-inducible proteins remain unclear. Recent studies have shown that some TLR-inducible proteins are critical in the immune response and the development of various disorders. In this review, we focus on the TLR signaling pathways and the roles of some TLR-inducible proteins.

Small RNAs in Bacterial Virulence and Communication

Bacterial pathogens must endure or adapt to different environments and stresses during transmission and infection. Posttranscriptional gene expression control by regulatory RNAs, such as small RNAs and riboswitches, is now considered central to adaptation in many bacteria, including pathogens. The study of RNA-based regulation (riboregulation) in pathogenic species has provided novel insight into how these bacteria regulate virulence gene expression. It has also uncovered diverse mechanisms by which bacterial small RNAs, in general, globally control gene expression. Riboregulators as well as their targets may also prove to be alternative targets or provide new strategies for antimicrobials. In this article, we present an overview of the general mechanisms that bacteria use to regulate with RNA, focusing on examples from pathogens. In addition, we also briefly review how deep sequencing approaches have aided in opening new perspectives in small RNA identification and the study of their functions. Finally, we discuss examples of riboregulators in two model pathogens that control virulence factor expression or survival-associated phenotypes, such as stress tolerance, biofilm formation, or cell-cell communication, to illustrate how riboregulation factors into regulatory networks in bacterial pathogens.

miRNA-8159 is involved in TLR signaling pathway regulation after pathogen infection by direct targeting TLR13 in miiuy croaker

Toll-like receptors (TLRs) play a crucial role in the recognition of immune reactions against invading pathogens. The molecular regulation mechanisms of TLR expression in aquatic organisms remain unclear. MicroRNAs (miRNAs) are small non-coding RNAs that are critical adjustors of immune signaling pathway at the post-transcriptional level and play critical roles in intricate networks of host-pathogen interactions and innate immunity. The critical role of TLRs in host defense for discerning certain kinds of pathogen associated molecular patternsand striking a cascade immune response in fish have been demonstrated. Miiuy croaker TLR13 significantly increased after infection with Vibrio anguillarum, which suggests that mmiTLR13 plays an important role in innate immunity. In this study, the role of miR-8159 was explored in regulating TLR13, which is involved in inflammatory responses in miiuy croakers. Bioinformatics was used to predict miR-8159, which has a direct negative regulatory effect on TLR13 in miiuy croaker. Afterward, the dual luciferase reporter assay containing miRNA mimics or inhibitors and pre-miR-8159 showed that miR-8159 was the direct negative regulator of TLR13 in miiuy croaker. Moreover, miR-8159 downregulated the expression of TLR13 in the transcription level. The expression of miR-8159 could be upregulated by V. anguillarum challenged miiuy croaker and LPS exposure macrophages. Thus, miR-8159 could be induced by V. anguillarum and may function as a negative regulator of TLR13 in the immune response of miiuy croakers.

The islet-resident macrophage is in an inflammatory state and senses microbial products in blood

Ferris et al. show that macrophages in pancreatic islets express a gene signature of activation consistent with barrier macrophages. Macrophages are poised to react to blood inflammatory stimuli. In NOD mice, an additional immune activation signature is observed as early as 3 wk of age.

We examined the transcriptional profiles of macrophages that reside in the islets of Langerhans of 3-wk-old non-obese diabetic (NOD), NOD.Rag1^−/−, and B6.g7 mice. Islet macrophages expressed an activation signature with high expression of Tnf, Il1b, and MHC-II at both the transcript and protein levels. These features are common with barrier macrophages of the lung and gastrointestinal tract. Moreover, injection of lipopolysaccharide induced rapid inflammatory gene expression, indicating that blood stimulants are accessible to the macrophages and that these macrophages can sense them. In NOD mice, the autoimmune process imparted an increased inflammatory signature, including elevated expression of chemokines and chemokine receptors and an oxidative response. The elevated inflammatory signature indicates that the autoimmune program was active at the time of weaning. Thus, the macrophages of the islets of Langerhans are poised to mount an immune response even at steady state, while the presence of the adaptive immune system elevates their activation state.

Time Course of Inflammatory Gene Expression Following Crush Injury in Murine Skeletal Muscle

BACKGROUND

Early inflammation and secretion of proinflammatory cytokines such as IL-1β, IL-6, and TNF-α act as the key drivers to regulate inflammation after muscle injury. However, the effects of these key proinflammatory drivers in a noninvasive crush injury model are not well known. Understanding these effects is important for treating crush injuries that occur during natural disasters and military conflicts.

PURPOSE

We studied the timed mRNA expression of IL-1β, IL-6, and TNF-α in a noninvasive murine crush injury model to further understand their impact on proinflammatory cytokine pathways that are activated within the first 48 hours after a crush muscle injury.

METHODS

A total of 25 mice were anesthetized and placed on a crush injury apparatus platform with the apparatus piston situated in direct contact with intact skin overlying the right gastrocnemius muscle. Pressure at 45 psi was applied to the piston for 30 seconds for two applications. The mice recovered for either 4, 8, 24, or 48 hours postinjury, after which we harvested the gastrocnemius muscle of both legs. Microarray, confirmatory real-time polymerase chain reaction, and immunolabeling experiments were followed by a microarray time-course analysis.

RESULTS

Muscle IL-1β mRNA rose 270-fold within 4 hours and declined rapidly at 8 hours to 196-fold, 24 hours to 96-fold, and 48 hours to 10-fold. Muscle IL-6 followed the same pattern, with a 34-fold increase at 4 hours, 29-fold increase at 8 hours, 10-fold increase at 24 hours, and 5-fold increase at 48 hours. Ingenuity Pathway Analysis of IL-6 identified activation of two major downstream signaling pathways (IL-6/Stat3 and IL-1β/Egr1) as key activators of inflammation, regeneration, and fibrosis.

DISCUSSION

Closed crush muscle injury produced robust muscle cytokine expression levels, and the microarray findings allowed us to generate our most novel hypothesis: that high expression of IL-1β, IL-6, and TNF-α may be related to the downregulation of mitochondrial genes early after injury and triggers activation of genes in the repair and fibrosis machinery. The significance of these findings and the identified expression pathways of IL1-β, IL-6, and TNF-α and their downstream targets in skeletal muscle will allow us to further investigate targets for improved muscle recovery and limb-saving interventions.

Multi-receptor detection of individual bacterial products by the innate immune system

The receptors of the innate immune system detect specific microbial ligands to promote effective inflammatory and adaptive immune responses. Although this idea is well appreciated, studies in recent years have highlighted the complexity of innate immune detection, with multiple host receptors recognizing the same microbial ligand. Understanding the collective actions of diverse receptors that recognize common microbial signatures represents a new frontier in the study of innate immunity, and is the focus of this Review. Here, we discuss examples of individual bacterial cell wall components that are recognized by at least two and as many as four different receptors of the innate immune system. These receptors survey the extracellular or cytosolic spaces for their cognate ligands and operate in a complementary manner to induce distinct cellular responses. We further highlight that, despite this genetic diversity in receptors and pathways, common features exist to explain the operation of these receptors. These common features may help to provide unifying organizing principles associated with host defence.

Molecular basis of mycobacterial survival in macrophages

Macrophages play an essential role in the immune system by ingesting and degrading invading pathogens, initiating an inflammatory response and instructing adaptive immune cells, and resolving inflammation to restore homeostasis. More interesting is the fact that some bacteria have evolved to use macrophages as a natural habitat and tools of spread in the host, e.g., Mycobacterium tuberculosis (Mtb) and some non-tuberculous mycobacteria (NTM). Mtb is considered one of humanity's most successful pathogens and is the causal agent of tuberculosis, while NTMs cause opportunistic infections all of which are of significant public health concern. Here, we describe mechanisms by which intracellular pathogens, with an emphasis on mycobacteria, manipulate macrophage functions to circumvent killing and live inside these cells even under considerable immunological pressure. Such macrophage functions include the selective evasion or engagement of pattern recognition receptors, production of cytokines, reactive oxygen and nitrogen species, phagosome maturation, as well as other killing mechanisms like autophagy and cell death. A clear understanding of host responses elicited by a specific pathogen and strategies employed by the microbe to evade or exploit these is of significant importance for the development of effective vaccines and targeted immunotherapy against persistent intracellular infections like tuberculosis.

TLR sensing of bacterial spore-associated RNA triggers host immune responses with detrimental effects

The spores of pathogenic bacteria are involved in host entry and the initial encounter with the host immune system. How bacterial spores interact with host immunity, however, remains poorly understood. Here, we show that the spores of Bacillus anthracis (BA), the etiologic agent of anthrax, possess an intrinsic ability to induce host immune responses. This immunostimulatory activity is attributable to high amounts of RNA present in the spore surface layer. RNA-sensing TLRs, TLR7, and TLR13 in mice and their human counterparts, are responsible for detecting and triggering the host cell response to BA spores, whereas TLR2 mediates the sensing of vegetative BA. BA spores, but not vegetative BA, induce type I IFN (IFN-I) production. Although TLR signaling in itself affords protection against BA, spore RNA-induced IFN-I signaling is disruptive to BA clearance. Our study suggests a role for bacterial spore-associated RNA in microbial pathogenesis and illustrates a little known aspect of interactions between the host and spore-forming bacteria.

RNA sensors of the innate immune system and their detection of pathogens

The innate immune system plays a critical role in pathogen recognition and initiation of protective immune response through the recognition of pathogen associated molecular patterns (PAMPs) by its pattern recognition receptors (PRRs). Nucleic acids including RNA and DNA have been recognized as very important PAMPs of pathogens especially for viruses. RNA are the major PAMPs of RNA viruses, to which most severe disease causing viruses belong thus posing a tougher challenge to human and animal health. Therefore, the understanding of the immune biology of RNA PRRs is critical for control of pathogen infections especially for RNA virus infections. RNA PRRs are comprised of TLR3, TLR7, TLR8, RIG-I, MDA5, NLRP3, NOD2, and some other minorities. This review introduces these RNA PRRs by describing the cellular localizations, ligand recognitions, activation mechanisms, cell signaling pathways, and recognition of pathogens; the cross-talks between various RNA PRRs are also reviewed. The deep insights of these RNA PRRs can be utilized to improve anti-viral immune response. © 2017 IUBMB Life, 69(5):297-304, 2017.

Bioconjugation of Small Molecules to RNA Impedes Its Recognition by Toll-Like Receptor 7

A fundamental mechanism of the innate immune system is the recognition, via extra- and intracellular pattern-recognition receptors, of pathogen-associated molecular patterns. A prominent example is represented by foreign nucleic acids, triggering the activation of several signaling pathways. Among these, the endosomal toll-like receptor 7 (TLR7) is known to be activated by single-stranded RNA (ssRNA), which can be specifically influenced through elements of sequence structure and posttranscriptional modifications. Furthermore, small molecules TLR7 agonists (smTLRa) are applied as boosting adjuvants in vaccination processes. In this context, covalent conjugations between adjuvant and vaccines have been reported to exhibit synergistic effects. Here, we describe a concept to chemically combine three therapeutic functions in one RNA bioconjugate. This consists in the simultaneous TLR7 stimulation by ssRNA and smTLRa as well as the therapeutic function of the RNA itself, e.g., as a vaccinating or knockdown agent. We have hence synthesized bioconjugates of mRNA and siRNA containing covalently attached smTLRa and tested their function in TLR7 stimulation. Strikingly, the bioconjugates displayed decreased rather than synergistically increased stimulation. The decrease was distinct from the antagonistic action of an siRNA bearing a Gm motive, as observed by direct comparison of the effects in the presence of otherwise stimulatory RNA. In summary, these investigations showed that TRL7 activation can be impeded by bioconjugation of small molecules to RNA.

Next-Generation Sequencing-Based RiboMethSeq Protocol for Analysis of tRNA 2'-O-Methylation

Analysis of RNA modifications by traditional physico-chemical approaches is labor intensive, requires substantial amounts of input material and only allows site-by-site measurements. The recent development of qualitative and quantitative approaches based on next-generation sequencing (NGS) opens new perspectives for the analysis of various cellular RNA species. The Illumina sequencing-based RiboMethSeq protocol was initially developed and successfully applied for mapping of ribosomal RNA (rRNA) 2′-O-methylations. This method also gives excellent results in the quantitative analysis of rRNA modifications in different species and under varying growth conditions. However, until now, RiboMethSeq was only employed for rRNA, and the whole sequencing and analysis pipeline was only adapted to this long and rather conserved RNA species. A deep understanding of RNA modification functions requires large and global analysis datasets for other important RNA species, namely for transfer RNAs (tRNAs), which are well known to contain a great variety of functionally-important modified residues. Here, we evaluated the application of the RiboMethSeq protocol for the analysis of tRNA 2′-O-methylation in Escherichia coli and in Saccharomyces cerevisiae. After a careful optimization of the bioinformatic pipeline, RiboMethSeq proved to be suitable for relative quantification of methylation rates for known modified positions in different tRNA species.

Two SNPs in the promoter region of Toll-like receptor 4 gene are not associated with smoking in Saudi Arabia

Defects in the innate immune system, particularly in Toll-like receptors (TLRs), have been reported in several cigarette smoke-promoted diseases. The aim of this study was to examine the impact of tobacco smoke on allelic frequencies of TLR4 single-nucleotide polymorphisms (SNPs) and to compare the genotypic distribution of these SNPs in a Saudi Arabian population with that in previously studied populations. DNA was extracted from 303 saliva samples collected from smokers and nonsmokers. Two transitional SNPs in the promoter region of TLR4 were selected, rs2770150 (T/C) and rs10759931 (G/A). Genotype frequencies were determined using quantitative polymerase chain reaction. Our results showed a slight effect of smoking on the distribution of rs2770150 and rs10759931. However, the differences were not significant. Thus, we conclude that the SNPs selected for this study were independent of smoking and may not be related to smoking-induced diseases.

Selectivity of Human TLR9 for Double CpG Motifs and Implications for the Recognition of Genomic DNA

TLR9 acts as a first-line host defense against pathogens recognizing DNA comprising unmethylated CpG motifs present in bacteria and viruses. Species- and sequence-specific recognition differences were demonstrated for TLR9 receptors. Activation of human (h)TLR9 requires a pair of closely positioned CpG motifs within oligodeoxyribonucleotides (ODNs), whereas mouse TLR9 is effectively activated by an ODN with a single CpG motif. Molecular model-directed mutagenesis identified two regions, site A and site B, as important for receptor activation. Amino acid residues Gln³⁴⁶ and Arg³⁴⁸ within site A contribute to the sequence-specific recognition by hTLR9 in determining the bias for two appropriately spaced CpG motifs within immunostimulatory ODNs. Mutation of Gln⁵⁶² at site B, in combination with Gln³⁴⁶ and Arg³⁴⁸ mutations of mouse counterparts, increased activation of hTLR9 by mouse-specific ODN, mammalian genomic DNA, and bacterial DNA. We propose that the double CpG motif sequence-specificity of hTLR9 results in decreased activation by ODNs with a lower frequency of CpG motifs, such as from mammalian genomic DNA, which increases hTLR9 selectivity for pathogen versus host DNA.

TLR7 and TLR3 Sense Brucella abortus RNA to Induce Proinflammatory Cytokine Production but They Are Dispensable for Host Control of Infection

Brucella abortus is a Gram-negative, facultative intracellular bacterium that causes brucellosis, a worldwide zoonotic disease leading to undulant fever in humans and abortion in cattle. The immune response against this bacterium relies on the recognition of microbial pathogen-associated molecular patterns, such as lipoproteins, lipopolysaccharides, and DNA; however, the immunostimulatory potential of B. abortus RNA remains to be elucidated. Here, we show that dendritic cells (DCs) produce significant amounts of IL-12, IL-6, and IP-10/CXCL10, when stimulated with purified B. abortus RNA. IL-12 secretion by DCs stimulated with RNA depends on TLR7 while IL-6 depends on TLR7 and partially on TLR3. Further, only TLR7 plays a role in IL-12 production induced by B. abortus infection. Moreover, cytokine production in DCs infected with B. abortus or stimulated with bacterial RNA was reduced upon pretreatment with MAPK/NF-κB inhibitors. By confocal microscopy, we demonstrated that TLR7 is colocalized with B. abortus in LAMP-1⁺Brucella-containing vacuoles. Additionally, type I IFN expression and IP-10/CXCL10 secretion in DCs stimulated with bacterial RNA were dependent on TLR3 and TLR7. Our results suggest that TLR3 and TLR7 are not required to control Brucella infection in vivo, but they play an important role on sensing B. abortus RNA in vitro.

Importance of secreted bacterial RNA in bacterial-host interactions in the gut

Bacteria are ubiquitous in nature and found to be associated with human. Humans are benefited significantly from these associated bacteria. Our understanding is quite limited about these beneficial associations and how bacteria communicate with the host. It is assumed that secreted bacterial products contribute in bacterial-host signaling. A few studies have shown that bacteria secrete RNA into their extracellular medium, and these secreted RNA are capable of altering the host immune response. Multiple studies in eukaryotes have confirmed that secreted RNA can influence the functioning of other cells and their role in the development of several diseases, although not much is known about the composition of secreted bacterial RNA, how they are trafficked and how they impact on the functioning of host cells. By uncovering the beneficial role of secreted bacterial RNA, it would be possible to improve the human health.

Activation and pathogenic manipulation of the sensors of the innate immune system

The innate immune system detects the presence of microbes through different families of pattern-recognition receptors (PRRs). PRRs detect pathogens of all origins and trigger signaling events that activate innate and adaptive immunity. These events need to be tightly regulated in order to ensure optimal activation when required, and minimal signaling in the absence of microbial encounters. This regulation is achieved, at least in part, through the precise subcellular positioning of receptors and downstream signaling proteins. Consequently, mislocalization of these proteins inhibits innate immune pathways, and pathogens have evolved to alter host protein localization as a strategy to evade immune detection. This review describes the importance of subcellular localization of various PRR families and their adaptors, and highlights pathogenic immune evasion strategies that operate by altering immune protein localization.

Innate immunity against Legionella pneumophila during pulmonary infections in mice

Legionella pneumophila is an etiological agent of the severe pneumonia known as Legionnaires' disease (LD). This gram-negative bacterium is thought to replicate naturally in various freshwater amoebae, but also replicates in human alveolar macrophages. Inside host cells, legionella induce the production of non-endosomal replicative phagosomes by injecting effector proteins into the cytosol. Innate immune responses are first line defenses against legionella during early phases of infection, and distinguish between legionella and host cells using germline-encoded pattern recognition receptors such as Toll-like receptors , NOD-like receptors, and RIG-I-like receptors, which sense pathogen-associated molecular patterns that are absent in host cells. During pulmonary legionella infections, various inflammatory cells such as macrophages, neutrophils, natural killer (NK) cells, large mononuclear cells, B cells, and CD4+ and CD8+ T cells are recruited into infected lungs, and predominantly occupy interstitial areas to control legionella. During pulmonary legionella infections, the interplay between distinct cytokines and chemokines also modulates innate host responses to clear legionella from the lungs. Recognition by NK cell receptors triggers effector functions including secretion of cytokines and chemokines, and leads to lysis of target cells. Crosstalk between NK cells and dendritic cells, monocytes, and macrophages provides a major first-line defense against legionella infection, whereas activation of T and B cells resolves the infection and mounts legionella-specific memory in the host.