Single nucleotide polymorphisms of Toll-like receptors and susceptibility to infectious diseases

Toll-like receptors (TLRs) are the best-studied family of pattern-recognition receptors (PRRs), whose task is to rapidly recognize evolutionarily conserved structures on the invading microorganisms. Through binding to these patterns, TLRs trigger a number of proinflammatory and anti-microbial responses, playing a key role in the first line of defence against the pathogens also promoting adaptive immunity responses. Growing amounts of data suggest that single nucleotide polymorphisms (SNPs) on the various human TLR proteins are associated with altered susceptibility to infection. This review summarizes the role of TLRs in innate immunity, their ligands and signalling and focuses on the TLR SNPs which have been linked to infectious disease susceptibility.     

Toll样受体在结核病免疫应答中的作用

结核病(Tuberculosis)主要是由结核分枝杆菌(Mycobacterium tuberculosis,MTB)引起的重大传染性疾病。Toll样受体(Toll-like receptors,TLRs)在宿主的抗结核免疫应答中发挥重要作用。研究表明,TLR1、2、4、6和9与宿主抗MTB感染有关,其中TLR2的作用更为突出。免疫应答的早期阶段,TLR2介导了巨噬细胞的活化,通过产生具有直接杀伤效应的分子或者诱导宿主细胞的凋亡抑制MTB的增殖。然而TLR2介导的信号通路也可通过降低MHC-Ⅱ分子的表达来削弱抗原递呈的能力,促进MTB在宿主内的存活。近几年临床研究发现TLRs多态性位点与结核病易感有关也从侧面证实了TLRs在抗MTB感染中发挥重要作用。     

Lack of association of Toll-like receptor 9 gene polymorphism with Behçet's disease in Japanese patients

Toll-like receptors (TLRs) play an important role in the induction of defense mechanisms of the innate and adaptive immune responses to microbial pathogens. Genetic polymorphisms within the TLR9 gene have been reported to be associated with a variety of inflammatory and infectious diseases. Beh?et's disease (BD) is a chronic inflammatory disease, and the etiology of BD has yet to be fully elucidated. We investigated the potential association of the TLR9 gene with susceptibility to BD by analyzing the frequency of nine single nucleotide polymorphisms (SNPs) in a population of 200 Japanese BD patients and 102 randomized controls. Our results showed that SNPs in the TLR9 gene were not significantly associated with susceptibility to BD.     

Important aspects of Toll-like receptors, ligands and their signaling pathways

Due to the rapid increase of new information on the multiple roles of Toll-like receptors (TLRs), this paper reviews several main properties of TLRs and their ligands and signaling pathways. The investigation of pathogen infections in knockout mice suggests that specific TLRs play a key role in the activation of immune responses. Although the investigation of TLR biology is just beginning, a number of important findings are emerging. This review focuses on the following seven aspects of this emerging field: (a) a history of TLR and ligand studies; (b) the molecular basis of recognition by TLRs: TLR structures, pathogen-associated molecular pattern binding sites, TLR locations and functional responses; (c) cell types in TLR expression; (d) an overview of TLRs and their ligands: expression and ligands of cell-surface TLRs and of intracellular TLRs; (e) TLR-signaling pathways; (f) discussion: TLRs control of innate and adaptive systems; the trafficking of intracellular TLRs to endolysosomes; investigation of TLRs in regulating microRNA; investigation of crystal structure of TLRs with ligand binding; incidence of infectious diseases associated with single nucleotide polymorphisms (SNPs) in TLR genes; risk of cancer related to SNPs in TLR genes; TLR-ligand mediated anti-cancer effects; and TLR-ligand induced chronic inflammation and tumorigenesis; and (g) conclusions.     

Genetic variation in Toll-like receptors and disease susceptibility

Toll-like receptors (TLRs) are key initiators of the innate immune response and promote adaptive immunity. Much has been learned about the role of TLRs in human immunity from studies linking TLR genetic variation with disease. First, monogenic disorders associated with complete deficiency in certain TLR pathways, such as MyD88-IRAK4 or TLR3-Unc93b-TRIF-TRAF3, have demonstrated the specific roles of these pathways in host defense against pyogenic bacteria and herpesviruses, respectively. Second, common polymorphisms in genes encoding several TLRs and associated genes have been associated with both infectious and autoimmune diseases. The study of genetic variation in TLRs in various populations combined with information on infection has demonstrated complex interaction between genetic variation in TLRs and environmental factors. This interaction explains the differences in the effect of TLR polymorphisms on susceptibility to infection and autoimmune disease in various populations.     

Rapid and inexpensive real-time PCR for genotyping functional polymorphisms within the Toll-like receptor -2, -4, and -9 genes

The discovery of the human Toll-like receptors (TLRs) and the recognition of their pivotal role in sensing microbial pathogens during the last 5 years have resulted in a renewed appreciation of innate immunity. Due to their central role in both, triggering innate immunity as well as linking innate and adaptive immunity, genetic variations within the TLR genes, known to be associated with a variety of infectious diseases, are currently of great interest. Several single nucleotide polymorphisms (SNPs) within TLR genes have been described and seem to be associated with susceptibility to inflammatory diseases. However, methods for genotyping SNPs within the TLR genes, e.g. direct sequencing or polymerase chain reaction (PCR)-based restriction fragment length polymorphism (RFLP) analysis, are time-consuming. In this work, we report novel real-time PCR methods for genotyping five TLR SNPs within TLR-2, TLR-4 and TLR-9 that have been associated with various diseases using fluorescence labeled hybridization probes and the LightCycler instrument. In addition, we provide protocols employing a standard Taq polymerase in order to reduce substantially the costs for real-time PCR.     

Association analysis of Toll-like receptor 7 gene polymorphisms and Behçet's disease in Japanese patients

Action of Toll-like receptors (TLRs) is deeply associated with defense mechanisms of the innate and adaptive immune responses to microbial pathogens. There have been reports of genetic polymorphisms within the TLR7 gene being closely related to a variety of inflammatory and infectious diseases. Behçet's disease (BD) is an autoinflammatory disease, and the pathogenesis has yet to be fully discovered. We investigated whether polymorphisms of Toll-like receptor 7 (TLR7) are associated with BD by analyzing the frequency of eight single nucleotide polymorphisms (SNPs) within 200 Japanese BD patients and 102 randomized controls. We genotyped nine SNPs in the TLR7 gene and assessed the allele/genotype diversity between cases and controls for all SNPs. In all eight SNPs, statistically significant differences were not observed between cases and controls.     

Genetic polymorphisms of viral infection-associated Toll-like receptors in Chinese population

Toll-like receptors (TLRs) play a pivotal role in an innate immunity system, which controls inflammation responses and further instructs development of adaptive immunity. We enrolled 250 Han Chinese in Taiwan screening for the single nucleotide polymorphisms (SNPs) in TLRs associated with viral infection, including TLR2, TLR3, TLR4, TLR7, TLR8, and TLR9. The 6 SNPs not hitherto identified in Chinese populations, including TLR3 1377 C>T, TLR3 -7 C>A, TLR7 Gln11Leu, TLR7 IVS1+1817 G>T, TLR8 Met1Val, and TLR8 -129 G>C, had minor allele frequencies of 38%, 23%, 22.3%, 3%, 16.0%, and 16.0%, respectively. The frequencies of 2 common SNPs, TLR9, -1486 T>C and 2848 G>A, were 28% and 44%, respectively. As compared with other ethnic populations, Chinese displayed an opposite allele frequency of TLR8 Met1Val and TLR8 -129 G>C to Caucasians and African Americans. In addition, TLR2 Arg677Try, TLR2 Arg753Gln, TLR4 Asp299Gly, and TLR4 Thr399Ile that were apparent in approximately 10% of Caucasians were not detected in Chinese. In conclusion, obvious ethnic differences in TLR polymorphisms may in part reflect the ethnic diversity of host viral susceptibility.     

TLR4 single-nucleotide polymorphisms alter mucosal cytokine and chemokine patterns in Mexican patients with Helicobacter pylori-associated gastroduodenal diseases

Helicobacter pylori is associated with peptic ulcer and gastric adenocarcinoma. Toll-like receptors (TLRs) participate in H. pylori recognition, and single-nucleotide polymorphisms (SNPs) in TLRs are associated with impaired immune response. We aimed to evaluate the association of TLR2/R753Q and TLR4/D299G/T399I SNPs with gastroduodenal diseases; and study the effect of SNPs on cytokine and chemokine expression in the gastric mucosa. Study included 450 Mexican patients with gastroduodenal diseases. SNPs in TLRs 2 and 4 genes were analyzed by allele-specific PCR. Cytokines and chemokines were assessed by qRT-PCR and immunoassay. TLR4/D299G/T399I polymorphisms were more frequent in duodenal ulcer and showed a trend in gastric cancer, when compared with non-atrophic gastritis. Patients with TLR4 polymorphisms expressed significantly lower levels of IL-1beta, IL-6, IL-8 and GRO-alpha; and higher levels of TNF-alpha, IL-10, MCP-1 and MIP-1alpha . SNPs in TLR4 gene had an association with severe H. pylori-associated disease and with modified pattern of inflammatory cytokines and chemokines in the gastric mucosa. These results suggest that TLR4 SNPs contributes importantly to the clinical outcome of H. pylori infection.     

Insights into the toll‐like receptors in sexually transmitted infections

Toll‐like receptors (TLRs) are like soldiers of an innate immune system, which protects vital biological processes against invading pathogens. TLR signalling pathways help in the removal of pathogens and mediate well‐established inflammatory processes. However, these processes may also aid in the development or augmentation of an infection or an autoimmune disease. Recent studies have delineated TLR polymorphism's role in the loss of function, making hosts more resistant or vulnerable to the development of an infection. In this review, we have discussed the association of TLRs with sexually transmitted infections (STIs), especially to the pathogen‐specific ligands. We have also assessed the impact on TLR downstream signalling and the maintenance of cellular homeostasis during immune responses. Besides, we have discussed the role of TLRs single nucleotide polymorphisms in various STIs. Since TLRs are known to play a part in defence mechanisms and in aiding infections therefore, a thorough understanding of TLRs structure and molecular mechanisms is required to explain how they can influence the outcome of an STI. Such a strategy may lead to the development of novel and useful immunotherapeutic approaches to control pathogen progression and prevent transmission.     

Porcine Toll-like receptor 1, 6, and 10 genes: complete sequencing of genomic region and expression analysis

Toll-like receptors (TLRs) recognize various microbial components and play key roles in activating the innate immune system. Hence, their function is important in swine infectious diseases. We completely determined 173,804 bp of nucleotide sequence of a genomic region including porcine TLR6 and the newly identified porcine TLR homologues TLR1 and TLR10. The porcine genomic structure of these genes was highly conserved in comparison with the corresponding region in humans. Analysis of their expression in porcine tissues showed differences in expression patterns between porcine TLR10 and TLR1 or TLR6. Moreover, phylogenetic analysis of the cytoplasmic regions of TLR genes suggested that the signal transduction pathway of TLR10 was different from those of TLR1 and TLR6. We also developed six polymorphic microsatellite markers within this genomic region; these markers will be valuable for association studies between TLR genes and resistance or susceptibility to infectious diseases in swine.     

Influence of Toll‐Like Receptor Gene Polymorphisms to Tuberculosis Susceptibility in Humans

Tuberculosis (TB) is caused by Mycobacterium tuberculosis (M. tb), and it remains one of the major bacterial infections worldwide. Innate immunity is an important arm of antimycobacterial host defence mechanism that senses various pathogen‐associated molecular patterns (PAMP) of microbes by a variety of pattern recognition receptors (PRRs). As per the recent discovery, Toll‐like receptors (TLRs) play a crucial role in the recognition of M. tb, this immune activation occurs only in the presence of functional TLRs. Variants of TLRs may influence their expression, function and alters the recognition or signalling mechanism, which leads to the disease susceptibility. Hence, the identification of mutations in these receptors could be used as a marker to screen the individuals who are at risk. In this review, we discuss TLR SNPs and their signalling mechanism to understand the susceptibility to TB for better therapeutic approaches.     

TLR9 gene polymorphism (rs187084, rs352140): association with acute rejection and estimated glomerular filtration rate in renal transplant recipients

The Toll‐like receptors (TLRs) are related to innate immunity. TLR9, a member of TLRs, is expressed in immune cell–rich tissues and mediates cellular response. We investigated the association between TLR9 polymorphisms and kidney allograft outcomes. To investigate whether TLR9 polymorphisms are associated with acute rejection after renal transplantation, two single nucleotide polymorphisms (SNPs) of TLR9 gene (rs187084 ‐1486; rs352140, G2848A) were selected and genotyped by direct sequencing in 342 renal transplant recipients. SNPStats, SNPAnalyzer, Helixtree and Haploview version 4.2 were used to analyse genetic data. Multiple logistic regression models (codominant, dominant, recessive and log‐additive) were used to evaluate odds ratios (ORs), 95% confidence intervals (CIs) and P values. Both SNPs, TLR9 rs187084 ‐1486 and rs352140 G2848A, of recipients were associated with the risk of acute rejection in renal transplantation. C allele of rs187084 ‐1486 and A allele of rs352140 G2848A were protective genotype for acute rejection (OR 0.6, 95% CI 0.40–0.92; P = 0.018, OR 0.64, 95% CI 0.42–0.98; P = 0.04, respectively). rs187084 ‐1486 CT and rs352140 G2848A GA genotype were associated with a lower eGFR after a year of renal transplantation. TLR9 polymorphisms, rs187084 and rs352140, of recipients were associated with the risk of acute rejection in renal transplantation. The patients with rs187084 ‐1486 CT and rs352140 G2848A GA genotype showed a lower eGFR after a year of renal transplantation.     

The Role of Toll-Like Receptor Signaling in Human Immunodeficiencies

Through the last decade, clinical immunology has witnessed a considerable progress in understanding the role of the innate immunity in human host defense, with Toll-like receptors (TLRs) being the most extensively innate immune receptors investigated. Growing literature documents the relevance of TLR signaling pathways to human disease, revealing a small, but expanding, group of new monogenic primary immunodeficiencies, in patients with various infectious diseases, previously considered as of unexplained “idiopathic” origin. Herein, we review these recently described deficiencies. Autosomal recessive IRAK-4 and myeloid differentiation factor 88 deficiencies were reported in 2003 and 2008, respectively, conferring predisposition to pyogenic bacterial infections, and autosomal recessive UNC93B1 and autosomal dominant TLR3 deficiencies were reported in 2006 and 2007, respectively, conferring predisposition to herpes simplex encephalitis. Furthermore, we highlight the published data associating TLR polymorphism with an altered susceptibility to infectious diseases.     

Toll-like receptor 3 in viral pathogenesis: friend or foe?

Viral infections frequently induce acute and chronic inflammatory diseases, yet the contribution of the innate immune response to a detrimental host response remains poorly understood. In virus‐infected cells, double‐stranded RNA (dsRNA) is generated as an intermediate during viral replication. Cell necrosis (and the release of endogenous dsRNA) is a common event during both sterile and infectious inflammatory processes. The discovery of Toll‐like receptor 3 (TLR3) as an interferon‐inducing dsRNA sensor led to the assumption that TLR3 was the master sentinel against viral infections. This simplistic view has been challenged by the discovery of at least three members of the DExd/H‐box helicase cytosolic sensors of dsRNA that share with TLR3 the Toll–interleukin‐1 receptor (TIR) ‐adapter molecule TIR domain‐containing adaptor protein interferon‐β (TRIF) for downstream type I interferon signalling. Data are conflicting on the role of TLR3 in protective immunity against viruses in the mouse model. Varying susceptibility to infection and disease outcomes have been reported in TLR3‐immunodeficient mice. Surprisingly, the susceptibility to develop herpes simplex virus‐1 encephalitis in humans with inborn defects of the TLR3 pathway varies, and TLR3‐deficient humans do not show increased susceptibility to other viral infections. Therefore, a current challenge is to understand the protective versus pathogenic contribution of TLR3 in viral infections. We review recent advances in the identification of TLR3‐signalling pathways, endogenous and virus‐induced negative regulators of the TLR3 cascade, and discuss the protective versus pathogenic role of TLR3 in viral pathogenesis.     

Association of TLR polymorphisms with development of tuberculosis in Indonesian females

Tuberculosis (TB) is caused by Mycobacterium tuberculosis and is a major cause of morbidity and mortality worldwide. Many candidate genes have been investigated for a possible association with TB. Toll-like receptors (TLRs) are known to play important roles in human innate immune systems. Polymorphisms in and functions of TLRs have been investigated to identify associations with specific infectious diseases, including TB. Here, we examined whether single-nucleotide polymorphisms (SNPs) in TLRs and genes in TLR signaling were associated with TB susceptibility in Indonesian and Vietnamese populations. A statistically significant association was observed between TB susceptibility in a classified Indonesian female group and rs352139, an SNP located in the intron of TLR9, using the genotype (P = 2.76E-04) and recessive (AA vs AG+GG, P = 2.48E-04, odds ratio = 1.827, 95% confidence interval = 1.321-2.526) models. Meta-analysis of the Indonesian and Vietnamese populations showed that rs352139 was significantly associated with TB in the recessive model. This finding indicated that a TLR9 polymorphism might have an important role in the susceptibility to M. tuberculosis in Asian populations.     

Genetic variation within TLR10 is associated with Crohn's disease in a New Zealand population

Toll-like receptors (TLRs) play an important role in the induction and regulation of the innate immune system and have been implicated in both infectious and inflammatory diseases. Recently the first association of TLR10 with Crohn's disease (CD) was reported. Here, we attempted to validate this association, using a candidate gene single nucleotide polymorphism (SNP) study of TLR10 in CD. We identified tagging SNPs, and genotyped these SNPs in a Caucasian New Zealand dataset consisting of 406 CD patients and 638 controls. In this sample, we were able to demonstrate an association between CD and several different TLR10 SNPs and haplotypes. Phenotypic analysis showed an association with early age at first diagnosis, inflammatory and ileocolonic CD behavior, requirement of bowel resection, and extra intestinal manifestations. This study provides evidence to suggest that genetic variation in TLR10 plays a role in interindividual differences in CD susceptibility and clinical outcome.     

No associations established between single nucleotide polymorphisms in human Toll‐like receptor 2 and Toll‐interacting protein and Staphylococcus aureus bloodstream infections

Staphylococcus aureus bloodstream infections (SABSI) are associated with high morbidity and mortality. The Toll‐like receptor 2 (TLR2) and Toll‐interacting protein (TOLLIP) are important in recognition and regulation of human innate immunity response to S. aureus. Single nucleotide polymorphisms (SNPs) in the TLR2 and TOLLIP encoding genes have been associated with disease, including BSI. The aim of this study was to examine potential associations between a selection of SNPs in the genes encoding TLR2 and TOLLIP, and predisposition, severity, and outcome of SABSI. All patients ≥18 years of age with at least one S. aureus positive blood culture collected from March 2011 through February 2014 at Akershus University Hospital, Lørenskog, Norway, were considered for inclusion. Patients attending elective orthopaedic surgery (total hip and knee replacements, lumbar surgery) served as a control group. The TLR2 Arg753Gln, TLR2 Pro631His, TOLLIP rs5743942, and rs5743867 polymorphisms were analysed using TaqMan SNP Genotyping Assays. A total of 209 SABSI patients and 295 controls were included. The TLR2 Arg753Gln and TLR2 Pro631His polymorphisms were infrequent with no homozygotes and <10% heterozygotes. The included TLR2 and TOLLIP polymorphisms were not associated with susceptibility to SABSI, severity, 30‐day all‐cause mortality, or SABSI caused by the clonal complex 30 (CC30) genotype.     

Toll‐like receptors as novel therapeutic targets for herpes simplex virus infection

Seropositivity for HSV reaches more than 70% within the world population, and yet no approved vaccine exists. While HSV1 is responsible for keratitis, encephalitis, and labialis, HSV2 carriers have a high susceptibility to other STD infections, such as HIV. Induction of antiviral innate immune responses upon infection depends on a family of pattern recognition receptors called Toll‐like receptors (TLR). TLRs bridge innate and adaptive immunity by sensing virus infection and activating antiviral immune responses. HSV adopts smart tricks to evade innate immunity and can also manipulate TLR signaling to evade the immune system or even confer destructive effects in favor of virus replication. Here, we review mechanisms by which HSV can trick TLR signaling to impair innate immunity. Then, we analyze the role of HSV‐mediated molecular cues, in particular, NF‐κB signaling, in promoting protective versus destructive effects of TLRs. Finally, TLR‐based therapeutic opportunities with the goal of preventing or treating HSV infection will be discussed.