Epigenetic regulation of Toll-like receptors 2 and 4 in kidney disease

Journal of Molecular Medicine - Kidney disease affects more than 10% of the worldwide population and causes significant morbidity and mortality. Epigenetic mechanisms such as DNA methylation,...     

Critical roles of Toll-like receptors in host defense

Drosophila Toll is involved not only in dorsoventral patterning of embryos but also in immune responses to microbial infection. Several Toll-like receptors (TLRs) have also been identified in mammals. They are expressed on macrophages or dendritic cells (DCs), which are essential sentinels for innate immunity. These cells utilize TLRs as a recognition and signal transducing receptor for microbial molecular components. The most characterized mammalian TLR, TLR4, is a receptor for lipopolysaccharides (LPS). TLR2 recognizes other components, such as peptideglycans (PGN). This recognition, called pattern recognition, is essential for the establishment of innate immunity, which is the basis for host defense. In this article, we review recent findings about this expanding receptor family.     

Toll样受体在机体抗病毒免疫反应中的作用

哺乳动物的Toll样受体(TLR)家族具有模式识别受体的功能,其可以识别微生物的保守分子成分,启动机体的固有免疫系统,从而帮助机体清除病原体.利用TLR敲除的动物或细胞模型进行的研究使人们认识到TLR在机体抗病毒免疫反应中发挥着重要作用.病毒与宿主细胞的TLR结合后,通过NF-κB或IRF-3的信号路径激活细胞因子的表达,从而激发免疫应答.研究TLR如何与病原体结合及如何激活下游基因对深入认识病原体所致相关疾病的发病机制、免疫应答及病理生理具有重要的意义,并为病毒性疾病的临床治疗或免疫预防提供新的思路.     

SNPs in the exons of Toll-like receptors are associated with susceptibility to type 1 diabetes in Chinese population

Objective

Toll-like receptors (TLRs) recognize a wide range of pathogen-associated molecular patterns (PAMP) and mount the initiation of immune response. Single nucleotide polymorphisms (SNPs) in exons of genes encoding TLRs might be responsible for the generation of an abnormal immune response which could lead to autoimmune diseases. In this study, we investigated the SNPs in TLRs in a Chinese population, and we hypothesized that SNPs in TLRs are associated with type 1 diabetes (T1D), an autoimmune disease caused by destruction of insulin producing pancreatic β-cells, in the studied population.

Research design and methods

We selected 28 SNPs in exons of TLRs with an aim to identify those that might have a direct correlation with T1D etiology and many have not been included in previous GWAS studies. Genotyping of those SNPs in TLRs was performed in 429 T1D patients and 300 age and gender-matched healthy controls in Chinese Han population which was not included in the earlier GWAS studies.

Results

Among the SNPs genotyped, the T allele of TLR1–626 was positively associated with T1D (OR = 1.98, Pc = 0.01). We identified another T1D association locus in TLR6: the homozygous AA genotype of TLR6-1329 was negatively and heterozygous GA was positively associated with T1D (OR = 0.54, Pc = 0.02 and OR = 1.70, Pc = 0.03, respectively). We also identified the haplotype T-G-A in TLR1 gene to be positively associated with T1D (OR = 2.22, Pc = 0.03). Additional haplotypes in TLR-6 also showed significant positive and negative association. In addition, our haplotype analysis and conditional analysis showed that these two SNPs are the primary T1D associated loci among the SNPs tested in our cohort in each TLR gene.

Conclusion

SNPs and haplotypes in TLR1 and TLR6 gene were associated with T1D in Chinese Han population. Our study, for the first time, indicates that TLR1 and TLR6 gene might play important roles in the etiology of T1D.     

Respiratory infections and type 1 diabetes: Potential roles in pathogenesis

Among the environmental factors associated with type 1 diabetes (T1D), viral infections of the gut and pancreas has been investigated most intensely, identifying enterovirus infections as the prime candidate trigger of islet autoimmunity (IA) and T1D development. However, the association between respiratory tract infections (RTI) and IA/T1D is comparatively less known. While there are significant amounts of epidemiological evidence supporting the role of respiratory infections in T1D, there remains a paucity of data characterising infectious agents at the molecular level. This gap in the literature precludes the identification of the specific infectious agents driving the association between RTI and T1D. Furthermore, the effect of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections on the development of IA/T1D remains undeciphered. Here, we provide a comprehensive overview of the evidence to date, implicating RTIs (viral and non-viral) as potential risk factors for IA/T1D.     

Epigenetic mechanisms of macrophage activation in type 2 diabetes

The alarming rise of obesity and type 2 diabetes (T2D) has put a tremendous strain on global healthcare systems. Over the past decade extensive research has focused on the role of macrophages as key mediators of inflammation in T2D. The inflammatory environment in the obese adipose tissue and pancreatic β-cell islets creates and perpetuates imbalanced inflammatory macrophage activation. Consequences of this chronic low-grade inflammation include insulin resistance in the adipose tissue and pancreatic β-cell dysfunction. Recently, the emerging field of epigenetics has provided new insights into the pathogenesis of T2D, while also affording potential new opportunities for treatment. In macrophages, epigenetic mechanisms are increasingly being recognized as crucial controllers of their phenotype. Here, we first describe the role of macrophages in T2D. Then we elaborate on epigenetic mechanisms that regulate macrophage activation, thereby focusing on T2D. Next, we highlight how diabetic conditions such as hyperlipidemia and hyperglycemia could induce epigenetic changes that promote an inflammatory macrophage phenotype. In conclusion we discuss possible therapeutic interventions by targeting macrophage epigenetics and speculate on future research directions.     

Toll-like receptor 3 gene polymorphisms in South African Blacks with type 1 diabetes

Type 1 diabetes is the consequence of exposure of genetically susceptible individuals to specific environmental precipitants. The innate immune system provides the initial response to exogenous antigen and links with the adaptive immune system. The aim of this study was to assess the role of polymorphisms occurring in the cytoplasmic region of toll-like receptor (TLR) 3 gene and immediate 5' sequence, in subjects of Zulu descent with type 1 diabetes in KwaZulu-Natal, South Africa. Seventy-nine subjects with type 1 diabetes and 74 healthy normal glucose tolerant gender-matched control subjects were studied. Parts of exon 4 and exon 3/intron 3 of the TLR3 gene were studied by polymerase chain reaction, direct sequencing and restriction enzyme digestion with Bts 1. Of the nine polymorphisms studied, a significant association with type 1 diabetes was found for the major allele in the 2593 C/T polymorphism and for the minor alleles in the 2642 C/A and 2690 A/G polymorphisms, which were found to be in complete linkage disequilibrium. Correction of the P-values for the number of alleles studied, however, rendered the results no longer significant. These results suggest that polymorphisms in the TLR3 gene, which is part of the innate immune system, may be associated with type 1 diabetes in this population.     

Toll-like receptors, cytokines and HIV-1

The present investigation examined variations in cytokine and Toll-like receptor expression by T-lymphocytes in HIV infection. 50 HIV cases and 10 normal controls were evaluated for TLR3, TLR4, TLR8, TLR9, IL-2, IL-4, IL-6, IL-10, IL-12, TNF-alpha, and IFN-gamma expression and staining intensities. TLR3 was expressed in 37 HIV (74%) cases, and TLR4 was brightly expressed in all (100%) HIV cases. However, TLR3 and TLR4 remained negative in all normal controls. Results reveal that TLR4 up-regulation is not limited to gram-negative bacterial infection. No statistical difference was detected when TLR8 and TLR9 expression were compared between the two test populations. A statistically significant difference was detected when IL-2, IL-4, IL-6, IL-10, IL-12, and IFN-gamma expression were analyzed in HIV cases and normal controls. When cytokine expression was compared with stage of disease, results indicated that a Th(1) to Th(2) cytokine switch did not occur. Only TNF-alpha expression decreased as infection progressed from the chronic phase into AIDS. When TLR expression was compared to cytokine expression, no statistical differences were detected. These data point to the need for a more in-depth study to determine whether or not the up-regulation of Toll-like receptor expression increases cytokine expression via the NFkappaB pathway.     

The expression and roles of Toll-like receptors in the biology of the human neutrophil

Neutrophils are amongst the first immune cells to arrive at sites of infection, where they initiate antimicrobial and proinflammatory functions, which serve to contain infection. Sensing and defeating microbial infections are daunting tasks as a result of their molecular heterogeneity; however, Toll-like receptors (TLRs) have emerged as key components of the innate-immune system, activating multiple steps in the inflammatory reaction, eliminating invading pathogens, and coordinating systemic defenses. Activated neutrophils limit infection via the phagocytosis of pathogens and by releasing antimicrobial peptides and proinflammatory cytokines and generating reactive oxygen intermediates. Through the production of chemokines, they additionally recruit and activate other immune cells to aid the clearance of the microbes and infected cells and ultimately, mount an adaptive immune response. In acute inflammation, influx of neutrophils from the circulation leads to extremely high cell numbers within tissues, which is exacerbated by their delayed, constitutive apoptosis caused by local inflammatory mediators, potentially including TLR agonists. Neutrophil apoptosis and safe removal by phagocytic cells limit tissue damage caused by release of neutrophil cytotoxic granule contents. This review addresses what is currently known about the function of TLRs in the biology of the human neutrophil, including the regulation of TLR expression, their roles in cellular recruitment and activation, and their ability to delay apoptotic cell death.     

Low frequency of Toll-like receptors 2 and 4 gene polymorphisms in Mexican patients and their association with type 2 diabetes

Type 2 diabetes (T2D) is characterized by a chronic low-grade inflammatory state. SNP in Toll-like receptor (TLR) genes has been associated with impaired inflammatory response. We genotyped the TLR4/D299G, TLR4/T399I and TLR2/R753Q polymorphisms. Low frequency was found with no association with T2D, nevertheless the TLR2 SNP was associated with lower levels in HDL-cholesterol values.     

Epigenetic regulation of B cells and its role in autoimmune pathogenesis

B cells play a pivotal role in the pathogenesis of autoimmune diseases. Although previous studies have shown many genetic polymorphisms associated with B-cell activation in patients with various autoimmune disorders, progress in epigenetic research has revealed new mechanisms leading to B-cell hyperactivation. Epigenetic mechanisms, including those involving histone modifications, DNA methylation, and noncoding RNAs, regulate B-cell responses, and their dysregulation can contribute to the pathogenesis of autoimmune diseases. Patients with autoimmune diseases show epigenetic alterations that lead to the initiation and perpetuation of autoimmune inflammation. Moreover, many clinical and animal model studies have shown the promising potential of epigenetic therapies for patients. In this review, we present an up-to-date overview of epigenetic mechanisms with a focus on their roles in regulating functional B-cell subsets. Furthermore, we discuss epigenetic dysregulation in B cells and highlight its contribution to the development of autoimmune diseases. Based on clinical and preclinical evidence, we discuss novel epigenetic biomarkers and therapies for patients with autoimmune disorders.     

Toll-like receptors on regulatory T cells: expanding immune regulation

Regulatory T (Treg) cells maintain peripheral tolerance and limit effector responses to prevent excessive immune-mediated tissue damage. However, recent research reveals that Treg cells also dampen the induction of immune responses and, thus, must be controlled to enable the effective protection against infections and cancer. Until now, this control of Treg-cell function has been believed to be by communication through cytokines or by stimulation through co-stimulatory molecules on antigen-presenting cells. However, new evidence has demonstrated that Treg cells can also sense pathogens directly through Toll-like receptors (TLRs) and, consequently, modify their behaviour. This review examines the ramifications of TLR engagement on Treg cells and conventional T cells, and discusses the potential role of TLRs on Treg cells and the consequences for disease therapy.     

Toll-like receptor 7 deficiency suppresses type 1 diabetes development by modulating B-cell differentiation and function

Innate immunity mediated by Toll-like receptors (TLRs), which can recognize pathogen molecular patterns, plays a critical role in type 1 diabetes development. TLR7 is a pattern recognition receptor that senses single-stranded RNAs from viruses and host tissue cells; however, its role in type 1 diabetes development remains unclear. In our study, we discovered that Tlr7-deficient (Tlr7^−/−) nonobese diabetic (NOD) mice, a model of human type 1 diabetes, exhibited a significantly delayed onset and reduced incidence of type 1 diabetes compared with Tlr7-sufficient (Tlr7^+/+) NOD mice. Mechanistic investigations showed that Tlr7 deficiency significantly altered B-cell differentiation and immunoglobulin production. Moreover, Tlr7^−/− NOD B cells were found to suppress diabetogenic CD4⁺ T-cell responses and protect immunodeficient NOD mice from developing diabetes induced by diabetogenic T cells. In addition, we found that Tlr7 deficiency suppressed the antigen-presenting functions of B cells and inhibited cytotoxic CD8⁺ T-cell activation by downregulating the expression of both nonclassical and classical MHC class I (MHC-I) molecules on B cells. Our data suggest that TLR7 contributes to type 1 diabetes development by regulating B-cell functions and subsequent interactions with T cells. Therefore, therapeutically targeting TLR7 may prove beneficial for disease protection.     

Distinct roles of pattern recognition receptors CD14 and Toll-like receptor 4 in acute lung injury

Acute lung injury (ALI) induced by lipopolysaccharide (LPS) is a major cause of mortality among humans. ALI is characterized by microvascular protein leakage, neutrophil influx, and expression of proinflammatory mediators, followed by severe lung damage. LPS binding to its receptors is the crucial step in the causation of these multistep events. LPS binding and signaling involves CD14 and Toll-like receptor 4 (TLR4). However, the relative contributions of CD14 and TLR4 in the induction of ALI and their therapeutic potentials are not clear in vivo. Therefore, the aim of the present study was to compare the roles of CD14 and TLR4 in LPS-induced ALI to determine which of these molecules is the more critical target for attenuating ALI in a mouse model. Our results show that CD14 and TLR4 are necessary for low-dose (300-microg/ml) LPS-induced microvascular leakage, NF-kappaB activation, neutrophil influx, cytokine and chemokine (KC, macrophage inflammatory protein 2, tumor necrosis factor alpha, interleukin-6) expression, and subsequent lung damage. On the other hand, when a 10-fold-higher dose of LPS (3 mg/ml) was used, these responses were only partially dependent on CD14 and they were totally dependent on TLR4. The CD14-independent LPS response was dependent on CD11b. A TLR4 blocking antibody abolished microvascular leakage, neutrophil accumulation, cytokine responses, and lung pathology with a low dose of LPS but only attenuated the responses with a high dose of LPS. These data are the first to demonstrate that LPS-induced CD14-dependent and -independent (CD11b-dependent) signaling pathways in the lung are entirely dependent on TLR4 and that blocking TLR4 might be beneficial in lung diseases caused by LPS from gram-negative pathogens.     

Toll-like receptors and autoimmunity

The understanding of autoimmune diseases experienced an impressive boost since the Toll-like receptors (TLRs) have been identified as possible key players in autoimmune pathophysiology. Although these receptors recognize a variety of structures derived from viruses, bacteria and fungi leading to subsequent initiation of the relevant immune responses recent data support the idea that TLRs are crucial in the induction and perpetuation of certain autoimmune diseases, especially the systemic lupus erythematosus (SLE). In this review we will summarize recent data on involvement of TLRs in the development of autoimmune diseases. This review will focus on TLRs 7, 8 and 9 which were originally identified as receptors specific for bacterial and viral RNA/DNA, but more recent in vitro and in vivo studies have linked these receptors to the detection of host RNA, DNA, and RNA- or DNA-associated proteins in the context of autoimmunity.