Development of TLR inhibitors for the treatment of autoimmune diseases

Summary The innate immune system is a critical element of protection from invading pathogens. The specific receptors that recognize various components of the pathogens trigger signals that result in the production of proinflammatory cytokines as well as the activation of antigen-presenting cells, which activate the adaptive immune system. The discovery of the Toll-like receptors (TLRs) as important components of pathogen recognition has brought new understanding of the key signaling molecules involves in innate immune activation. Interestingly, it appears that most TLRs can recognize self-ligands as well and that mechanisms are required to discriminate between self and non-self ligands. One of these mechanisms is the expression of all the nucleic acid-specific TLR in endosomal compartments and not on the cell surface. Inappropriate activation of TLRs by self-components can result in sterile inflammation or autoimmunity. For example, TLR7 and TLR9 activation by endogenous RNA and DNA, transported to the endosomes in the form of immune complexes or non-covalently associated with cationic peptides, could be an important mechanism involved in promoting diseases such as systemic lupus erythematosus and psoriasis. In this review, we discuss the rationale for self-recognition by TLR7 and TLR9 as an important part of the development of lupus and other autoimmune diseases. We describe novel inhibitors of these receptors and provide evidence to support their use as novel therapeutic agents for autoimmunity.     

Toll样受体7作为新靶点在过敏性及自身免疫性疾病中的研究进展

Toll样受体(Toll-like receptors,TLRs)属于模式识别受体(pattern recognition receptors,PRRs)家族,通过迅速识别入侵微生物病原相关分子模式(pathogen-associated molecular patterns,PAMP)激活下游信号传导途径,引发机体的免疫反应,是连接固有免疫与适应性免疫的桥梁.目前在人类发现10种不同TLRs,分别命名为TLR1 ～TLR10.近年的研究显示,TLR7与过敏性疾病、自身免疫性疾病—过敏性哮喘、系统性红斑狼疮(systemic lupus erythematosus,SLE)等有密切的联系.TLR7的激活状态影响上述疾病的发生、发展和预后.这些证据预示TLR7可能作为潜在靶点为哮喘和SLE的治疗提供新的方向.     

The role of toll-like receptors in systemic lupus erythematosus

Systemic lupus erythematosus is an autoimmune disease characterized by the production of autoantibodies against a relatively limited range of nuclear antigens. These autoantibodies result in the formation of immune complexes that deposit in tissues and induce inflammation, thereby contributing to disease pathology. Growing evidence suggests that recognition of nucleic acid motifs by Toll-like receptors may play a role in both the activation of antinuclear B cells and in the subsequent disease progression after immune complex formation. The endosomal localization of the nucleic acid-sensing Toll-like receptors (TLRs), TLR3, 7, and 9, is believed to contribute to the distinction between endogenous nucleic acids and those of foreign origin. In this article we review recent work that suggests a role for the B-cell receptor and Fcγ receptors in delivering nuclear antigens to intracellular compartments allowing TLR activation by endogenous nucleic acids. A number of in vitro studies have presented evidence supporting a role for TLRs in SLE pathology. However, recent studies that have examined the contributions of individual TLRs to SLE by using TLR-deficient mice suggest that the situation is far more complicated in vivo. These studies show that under different circumstances TLR signaling may either exacerbate or protect against SLE-associated pathology. Further understanding of the role of TLRs in pathological autoreactivity of the adaptive immune system will likely lead to important insights into the etiopathogenesis of SLE and potential targets for novel therapies.     

Polymorphisms of toll-like receptor 2 and 4 genes in rheumatoid arthritis and systemic lupus erythematosus

Human toll-like receptors (TLRs) participate in the innate response and signal the activation of adaptive immunity. Therefore, these TLRs may be important in autoimmune diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). We investigated, by using a polymerase chain reaction restriction-fragment length polymorphism method, the possible association between the polymorphisms of TLR2 (Arg677Trp and Arg753Gln) and TLR4 (Asp299Gly and Thr399Ile) genes with the susceptibility or severity of RA and SLE. Our study population consisted of 122 patients with SLE, 224 patients with RA, and a control group of 199 healthy individuals. The TLR2 polymorphisms were very rare in our population; no individual carrying the TLR2-Arg677Trp polymorphism was observed, whereas the TLR2-Arg753Gln polymorphism was present in only 1% of the total population. We found no statistically significant differences in the TLR4-Asp299Gly and the TLR4-Thr399Ile genotype or allele distribution between SLE patients, RA patients, and control individuals. Similarly, no association was found with any of the demographic and clinical parameters tested either in RA or in SLE patients. In conclusion, a case-control study was used to analyze, for the first time, the influence of TLR2 and TLR4 gene polymorphism on the predisposition and clinical characteristics of SLE and RA but provided no evidence for association of TLR2 or TLR4 gene polymorphism with either disease in the population under study.     

Recognition of nucleic acid and nucleic acid analogs by Toll-like receptors 7, 8 and 9

The mammalian immune system senses pathogens through pattern recognition receptors (PRR) and responds with activation. Toll-like receptors (TLRs) that are expressed on immune and non-immune cells play a critical role in this process. As part of the innate immune response, TLRs lead to cellular activation and cytokine production with subsequent initiation of an adaptive immune response. TLR7-9 recognize single-stranded RNA, nucleoside analogs and single-stranded CpG-DNA, respectively, and their activation initiates the immune response against viruses and bacteria. Furthermore, the stimulation of these TLRs may be exploited for adjuvant therapy, vaccination and anti-tumor responses. However, a role in the generation or perpetuation of autoimmune diseases such as systemic lupus erythematosus (SLE) has also been suggested.     

Toll-like receptors 7, 8, and 9: linking innate immunity to autoimmunity

Summary: Toll-like receptors (TLRs) detect infections by highly conserved components of pathogens that are either not present in our own cells or are normally sequestered in cellular compartments that are inaccessible to the TLRs. Most TLRs are expressed on the cell surface, where they have been shown to detect pathogen-expressed molecules such as lipopolysaccharides and lipopeptides. A subset of TLRs, including TLR3, TLR7, TLR8, and TLR9, are expressed intracellularly within one or more endosomal compartments and detect nucleic acids. Because pathogen and host nucleic acids have very similar structures, these endosomal TLRs may face an extra challenge to induce anti-pathogen immune responses while avoiding the induction of autoimmune diseases. With the rapid growth in understanding of the biology of the TLRs has come an increasing awareness of their effects on autoimmunity, several aspects of which are the focus of this review. First, recent studies have revealed an inappropriate activation of TLR7, TLR8, and TLR9 in systemic lupus erythematosus and several other autoimmune diseases. Secondly, the potential for therapeutic development of TLR antagonists is considered. Finally, with the rapid progress in the development of therapeutic agonists for the TLRs, there is accompanying attention to the theoretical possibility that such therapy may induce autoimmunity or autoimmune diseases.     

Toll-like receptor gene polymorphisms are associated with susceptibility to graves' ophthalmopathy in Taiwan males

Background  

Toll-like receptors (TLRs) are a family of pattern-recognition receptors, which plays a role in eliciting innate/adaptive immune responses and developing chronic inflammation. The polymorphisms of TLRs have been associated with the risk of various autoimmune diseases, including systemic lupus erythematosus (SLE), multiple sclerosis and rheumatorid arthritis. The aim of this study was to evaluate whether TLR genes could be used as genetic markers for the development of Graves' ophthalmopathy (GO).     

Nucleic acid sensing Toll-like receptors in autoimmunity

Trafficking and activation of the nucleic acid sensing TLRs is subject to unique regulatory requirements imposed by the risk of self-recognition. Like all TLRs these receptors traffick through the Golgi, however, access to the secretory pathway is controlled by a binding partner present in the ER. Receptor activation in the endolysosome is regulated through a proteolytic mechanism that requires activity of compartment-resident proteases, thereby preventing activation in other regions of the cell. Advances in our understanding of the cell biology of these receptors have been paralleled by efforts to understand their precise roles in autoimmunity. Mouse models have revealed that TLR7 and TLR9 make unique contributions to the types of self-molecules recognized in disease and possibly disease severity. Currently, methods of inhibiting TLR7 and TLR9 are being tested in clinical trials for systemic lupus erythamatosus.     

Possible evidence of systemic lupus erythematosus and periodontal disease association mediated by Toll‐like receptors 2 and 4

Toll‐like receptors (TLRs) participate in the innate immune response and trigger the immune responses of the body. Systemic lupus erythematosus (SLE) is an autoimmune disease of unknown aetiology, characterized by an excessive autoimmune response in the body affecting the connective tissues. The disease is possibly triggered by both environmental aetiological factors and pathological organic processes such as exposure to sunlight, chronic infectious processes and genetic factors. Conversely, periodontal disease is an infectious disease caused by microorganisms in the oral cavity, resulting in a chronic inflammatory process which continuously stimulates the immune response, thus causing damage to the periodontal tissues. The expression of both TLR‐2 and TLR‐4 receptors are increased in both SLE and periodontal disease. Periodontitis might trigger excessive activation of immune response occurring in SLE by maintaining a high expression of TLRs, leading in turn to the acceleration of the onset and progression of autoimmune reactions. In addition, periodontal treatment is able to reduce the expression of these receptors and therefore the symptoms of SLE. Here we discuss the possible interaction between SLE and periodontitis, and suggest further studies evaluating common features in both factors that could explored, due to morbidity and mortality of SLE and the high incidence of periodontal infections around the world.     

Activation profile of Toll‐like receptors of peripheral blood lymphocytes in patients with systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease associated with aberrant activation of T and B lymphocytes for the production of inflammatory cytokines and autoreactive antibodies. Animal studies of SLE have indicated that Toll‐like receptors (TLR) are important in the pathogenesis of murine lupus. In the present clinical study, differential protein expressions of TLR‐1–9 of monocytes and different lymphocyte subsets from patients with SLE and normal control subjects were determined by flow cytometry. Results showed that the expression of intracellular TLRs (TLR‐3, ‐8, ‐9) and extracellular TLRs (TLR‐1, ‐2, ‐4, ‐5, ‐6) were elevated in monocytes, CD4⁺ T lymphocytes, CD8⁺ T lymphocytes and B lymphocytes of SLE patients compared to control subjects (all P < 0·001). Moreover, cell surface expression of TLR‐4 on CD4⁺ T lymphocytes and CD8⁺ T lymphocytes, and TLR‐6 on B lymphocytes, were correlated positively with SLE disease activity index (SLEDAI) (TLR‐4 on CD4⁺ T lymphocytes and CD8⁺ T lymphocytes: r = 0·536, P = 0·04; r = 0·713, P = 0·003; TLR‐6 in B lymphocytes: r = 0·572, P = 0·026). In concordance with the above results, there is an observable increased relative induction (%) of inflammatory cytokine interleukin (IL)‐1β, IL‐6, IL‐10 and IL‐12, chemokines CCL2, CXCL8, CCL5 and CXCL10 from peripheral blood mononuclear cells (PBMC) upon differential stimulation by PolyIC (TLR‐3 ligand), lipopolysaccharide (TLR‐4 ligand), peptidoglycan (TLR‐2 ligand), flagellin (TLR‐5 ligand), R837 (TLR‐7 ligand) and CpG DNA (TLR‐9 ligand) in SLE patients compared to controls. These results suggest that the innate immune response for extracellular pathogens and self‐originated DNA plays immunopathological roles via TLR activation in SLE.     

Emerging roles of TLR7 and TLR9 in murine SLE

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by B cell hyperactivity leading to the production of various autoantibodies and subsequent development of glomerulonephritis, i.e. lupus nephritis. Among the principal targets of autoantibodies produced in murine SLE are nucleic acid–protein complexes, such as chromatin and ribonucleoproteins, and the envelope glycoprotein gp70 of endogenous retroviruses. The preferential production of these autoantibodies is apparently promoted by the presence of genetic abnormalities leading to defects in the elimination of apoptotic cells and to an enhanced expression of endogenous retroviruses. Moreover, recent studies revealed that the innate receptors TLR7 and TLR9 are critically involved in the activation of dendritic cells and autoreactive B cells through the recognition of endogenous DNA- or RNA-containing antigens and subsequent development of autoimmune responses against nuclear autoantigens. Furthermore, the regulation of autoimmune responses against endogenous retroviral gp70 by TLR7 suggested the implication of endogenous retroviruses in this autoimmune response. Clearly, further elucidation of the precise molecular role of TLR7 and TLR9 in the development of autoimmune responses will help to develop novel therapeutic strategies and targets for SLE.     

Toll like receptors and autoimmunity: a critical appraisal

There is a constant interplay between the innate and adaptive immune systems, which leads to a protective immune response against pathogens and contributes effectively to self-non-self discrimination. Toll-like receptors (TLRs) are key components of the innate immune system, which activate multiple inflammatory pathways and coordinate systemic defense against pathogens. In addition to recognizing unique molecular patterns associated with different classes of pathogens, TLRs may also recognize a number of self proteins and endogenous nucleic acids. Data originating predominantly from animal models of autoimmune disease and circumstantial data from human patients suggest that inappropriate activation of TLR pathways by endogenous or exogenous ligands may lead to the initiation and/or perpetuation of autoimmune responses and tissue injury.     

Bruton's tyrosine kinase regulates TLR9 but not TLR7 signaling in human plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) represent a key cell type for both innate and adaptive immunity. PDCs express both TLR7 and TLR9 and the recognition of nucleic acids by these two receptors triggers the production of a large amount of type‐I IFN and the induction of PDC maturation into APCs. This unique feature of PDCs is at the basis of clinical development of both TLR7 and TLR9 agonists for infectious diseases, allergy, cancer, and asthma. However, TLR7 and TLR9 recognition of self‐nucleic acids is linked to many autoimmune diseases including lupus, and a better understanding of the signaling pathways of these two receptors in PDCs is thus important. We have identified Bruton's tyrosine kinase (Btk) as an important player for TLR9 but not TLR7 signaling in human PDCs. Blocking Btk using a specific inhibitor leads to the reduction of all TLR9‐induced responses in PDCs, including cytokine production and expression of costimulatory molecules, while this has no impact on the TLR7 response. This identifies Btk as a key molecule in TLR9 signaling in PDCs and is the first demonstration that the TLR7 and TLR9 pathways can be dissociated in human PDCs.     

The role of innate immunity in the induction of autoimmunity

The autoimmune diseases are a diverse group of conditions characterized by abnormal B and T cell reactivity in association with autoantibody production. Among these diseases, systemic lupus erythematosus (SLE) is notable for the expression of antibodies to DNA, with these antibodies representing diagnostic markers. While mammalian DNA is immunologically inert, DNA from bacteria can potently stimulate the innate immune system, activating both toll-like receptors (TLRs) as well as non-TLR internal receptors. Since the sera of normal humans contain antibodies specific for bacterial DNA, this molecule appears to be immunogenic during infection. In pre-autoimmune mice, immunization with bacterial DNA can induce anti-DNA autoantibody production, suggesting a role in initiating this response. The immune properties of DNA are mutable, however, since mammalian DNA can acquire immunological activity when bound to certain proteins or anti-DNA antibodies to form immune complexes. In SLE, these immune complexes can drive the production of interferon by plasmacytoid dendritic cells, thereby intensifying autoimmunity. Together, these observations suggest that DNA can induce innate as well as adaptive immune responses and promote the pathogenesis of SLE because of its intrinsic immunostimulatory activity.     

Ⅰ型干扰素参与调节SLE外周T细胞活化和TLR表达谱的研究

血清高水平Ⅰ型干扰素(typeⅠinterferon)是系统性红斑狼疮(SLE)患者重要的病理特征之一,外周高水平IFN-α对T细胞的免疫调节作用值得深入探讨。本研究首先比较分析了SLE患者外周血T细胞活化和TLR分子表达格局,结果显示SLE患者外周血CD4~+或CD8~+T细胞和正常人相比呈现出更加活化的表型变化,其表面活化标志CD69和HLA-DR表达阳性率均高于正常人;分析T细胞表达TLR分子格局发现,SLE患者较正常人T细胞中TLR分子的表达有明显升高,其中CD4~+T细胞中TLR8和TLR9的升高明显,而在CD8~+T细胞中明显升高的TLR分子有TLR3和TLR8;结合SLE病理状态下外周高水平IFN-α的持续存在,我们进一步分析了IFN-α对正常T细胞活化和TLR分子表达谱的影响,结果显示IFN-α协同TCR信号可以促进T细胞的活化,并上调T细胞中部分TLR分子的表达,其中CD4~+和CD8~+T细胞中TLR8的表达均明显上升。综合分析SLE患者和经IFN-α活化的正常T细胞的活化和TLR分子表达谱的变化格局,提示SLE病理状态下高水平的Ⅰ型干扰素可以与T细胞的持续活化有关,其对T细胞表达TLR分子格局的影响,特别是与核酸类分子配体相关的TLR分子的表达增高为内源性核酸类配体参与T细胞的活化提供了分子基础。     

Toll-like receptor function and signaling

Mammals sense pathogen invasion through pattern-recognition receptors. A group of transmembrane proteins, Toll-like receptors (TLRs), play critical roles as pattern-recognition receptors. They are mainly expressed on antigen-presenting cells, such as macrophages or dendritic cells, and their signaling activates antigen-presenting cells to provoke innate immunity and to establish adaptive immunity. Each TLR has common effects, such as inflammatory cytokine induction or upregulation of costimulatory molecule expression, but also has its specific function, exemplified by type I IFN-inducing ability. These immunoadjuvant effects are not only critical in antimicrobial immunity but are also involved in manifestations of autoimmunity. Furthermore, some TLR agonists are now promising therapeutic tools for various immune disorders, including allergy. Therefore understanding molecular mechanisms on TLRs should be quite useful in the development of therapeutic maneuvers against allergy and autoimmune diseases.     

Toll样受体9与红斑狼疮易感性关系的研究进展

Toll样受体（Toll-like receptors, TLRs）作为连接人体天然免疫与获得性免 疫的桥梁,在自身免疫性疾病的发病中起着重要作用。其中TLR9与其特异性配体 CpGDNA结合,然后通过信号转导激活B细胞、树突状细胞,可产生各种细胞因子和自 身抗体。近年来研究发现,系统性红斑狼疮（Systemic lupus erythematosus ,SLE） 的易感性可能与TLR9基因变异有关,目前研究结论不一。本文将从鼠和人两个研究 层次来阐述TLR9与SLE易感性的关系,揭示TLR9在SLE发病中的作用机理。     

SLE-associated risk factors affect DC function

Numerous risk alleles for systemic lupus erythematosus (SLE) have now been identified. Analysis of the expression of genes with risk alleles in cells of hematopoietic origin demonstrates them to be most abundantly expressed in B cells and dendritic cells (DCs), suggesting that these cell types may be the drivers of the inflammatory changes seen in SLE. DCs are of particular interest as they act to connect the innate and the adaptive immune response. Thus, DCs can transform inflammation into autoimmunity, and autoantibodies are the hallmark of SLE. In this review, we focus on mechanisms of tolerance that maintain DCs in a non-activated, non-immunogenic state. We demonstrate, using examples from our own studies, how alterations in DC function stemming from either DC-intrinsic abnormalities or DC-extrinsic regulators of function can predispose to autoimmunity.     

Emerging View of Autophagy in Systemic Lupus Erythematosus

Aberrations of both innate immunity and adaptive immunity in genetically predisposed individuals evoked by environmental factors are suggested to be implicated in pathophysiological processes of systemic lupus erythematosus (SLE). Autophagy, a degradation pathway in which cytoplasmic content is engulfed and degraded by the lysosome, has been recently demonstrated to be involved in multiple cytoplasmic homeostatic progresses and interact with nearly all parts of the innate and adaptive immune system. More recently, some lines of evidence from genetic, cell biology and model animal studies also suggests a pivotal role of autophagy in mediating the occurrence and development of SLE. We discuss and synthesize studies that have begun to demonstrate how autophagy cause and/or promote autoimmunity in SLE.