Regulation of mammalian defensin expression by Toll-like receptor-dependent and independent signalling pathways

The immune system consists of innate and adaptive immune responses. The innate immune system confers non-specific protection against a large number of pathogens, hence, serving as the first line of defence. The innate immune system utilizes Toll-like receptors (TLRs) to recognize and bind pathogen-associated molecular patterns (PAMPs). Binding of PAMPs leads to TLR activation, which, in turn, initiates MAPK- or NF-kappaB-dependent cascades that culminate in a proinflammatory response. This response involves the secretion of cytokines, chemokines and broad-spectrum antibacterial substances, such as defensins. Increased defensin synthesis is also mediated by the activation of receptors other than TLRs, such as NOD2, IL-17R and PAR-2. This review summarizes the recently characterized signalling pathways leading to increased defensin synthesis as well as the pathway by which defensins activate TLRs on immature dendritic and memory T cells. Thus, not only do defensins eliminate pathogens, but they also recruit the adaptive immune system in instances of infection and/or inflammation.     

Toll-dependent and toll-independent innate antiviral immunity

Until recently, adaptive immunity and cytotoxic T cells were considered as the only essential components of the antiviral defence arsenal. Additional data that do not rule out the crucial role of these cells in the clearance of viral pathogens have, however, recently emerged. They indicate that innate immune cells such as macrophages, dendritic cells, gammadelta T cells as well as natural killer (NK) cells play a primordial role in this mechanism. It is now well established that innate immune cells can detect various pathogens (bacteria, viruses, fungi or parasites) very rapidly and respond to their presence through the activation of specific receptors. Once activated, these molecules trigger several signalling cascades that culminate in the establishment of very potent defence mechanisms. In addition, cytokines produced during this initial response are essential for the activation of the adaptive immune response which will add specificity and memory to the system. Among the innate immune receptors, attention has focused on the Toll-like receptors (TLR) and many reports indicate that some of the TLRs are clearly involved in defence against viral pathogens. However, new molecules, acting independently from any TLR, have recently been discovered. They define a second antiviral pathway which is presently the subject of intense research. In this article, we will review the role of the different molecules involved in each pathway within the framework of innate antiviral defence.     

TLR family and viral infection

The immune system has been divided into innate and adaptive component, each of which has different roles and functions in defending the organism against foreign agents, such as bacteria and viruses. An important advance in our understanding of early events in microbial recognition and subsequent development of immune responses was the identification of Toll-like receptors (TLRs) as key molecules of the innate immune systems. The family of TLRs in vertebrates detects conserved structures found in a broad range of pathogens and triggers innate immune responses. At present, 11 members of the TLR family have been identified. A subset of TLRs recognize viral components and induce antiviral responses by producing type I interferons. Recent accumulating evidence has clarified signaling pathways triggered by TLRs in viral infection.     

Structural aspects of nucleic acid-sensing Toll-like receptors

Invading pathogens elicit potent immune responses in cells through interactions between structurally conserved molecules derived from the pathogens and specialized innate immune receptors such as the Toll-like receptors (TLRs). Nucleic acid is one of the principal TLR ligands. Nucleic acid-sensing TLRs recognize an array of nucleic acids, including double-stranded RNA, single-stranded RNA, and DNAs with specific sequence motifs. Although ligand-induced dimerization is commonly observed followed by TLR activation, both the specific recognition mechanisms and the ligand–receptor interactions vary among different TLRs. In this review, we highlight our current understanding of how these receptors recognize their cognate ligands based on the recent advances in structural biology.     

Modulation of TNFSF expression in lymphoid tissue inducer cells by dendritic cells activated with Toll-like receptor ligands

Toll-like receptors (TLRs), which recognize structurally conserved components among pathogens, are mainly expressed by antigen-presenting cells such as dendritic cells (DCs), B cells, and macrophages. Recognition through TLRs triggers innate immune responses and influences antigen-specific adaptive immune responses. Although studies on the expression and functions of TLRs in antigen-presenting cells have been extensively reported, studies in lymphoid tissue inducer (LTi) cells have been limited. In this study, we observed that LTi cells expressed TLR2 and TLR4 mRNA as well as TLR2 protein and upregulated OX40L, CD30L, and TRANCE expression after stimulation with the TLR2 ligand zymosan or TLR4 ligand LPS. The expression of tumor necrosis factor superfamily (TNFSF) members was significantly upregulated when cells were cocultured with DCs, suggesting that upregulated TNFSF expression may contribute to antigen-specific adaptive immune responses.     

Group A streptococcus activates type I interferon production and MyD88-dependent signaling without involvement of TLR2, TLR4, and TLR9

Bacterial pathogens are recognized by the innate immune system through pattern recognition receptors, such as Toll-like receptors (TLRs). Engagement of TLRs triggers signaling cascades that launch innate immune responses. Activation of MAPKs and NF-kappaB, elements of the major signaling pathways induced by TLRs, depends in most cases on the adaptor molecule MyD88. In addition, Gram-negative or intracellular bacteria elicit MyD88-independent signaling that results in production of type I interferon (IFN). Here we show that in mouse macrophages, the activation of MyD88-dependent signaling by the extracellular Gram-positive human pathogen group A streptococcus (GAS; Streptococcus pyogenes) does not require TLR2, a receptor implicated in sensing of Gram-positive bacteria, or TLR4 and TLR9. Redundant engagement of either of these TLR molecules was excluded by using TLR2/4/9 triple-deficient macrophages. We further demonstrate that infection of macrophages by GAS causes IRF3 (interferon-regulatory factor 3)-dependent, MyD88-independent production of IFN. Surprisingly, IFN is induced also by GAS lacking slo and sagA, the genes encoding cytolysins that were shown to be required for IFN production in response to other Gram-positive bacteria. Our data indicate that (i) GAS is recognized by a MyD88-dependent receptor other than any of those typically used by bacteria, and (ii) GAS as well as GAS mutants lacking cytolysin genes induce type I IFN production by similar mechanisms as bacteria requiring cytoplasmic escape and the function of cytolysins.     

Toll样受体识别机制及其生物学意义

Toll样受体介导的信号转导通路在对抗外来病原体的天然免疫应答中起重要作用。Toll样受体是一个天然模板识别受体家族,能识别固有性模板(微生物和哺乳动物所共有的病原相联的分子模板PAMPs)。Toll样受体通过巨噬细胞和其他免疫细胞来识别,其中TLR4识别内毒素、TLR2识别肽聚糖、TLR9识别细菌DNA、TLR5识别鞭毛蛋白、TLR3识别双链RNA等。本探讨了多种Toll受体家族成员在动物体内识别机理及功能,概述了其应用研究进展。     

Toll-like receptors and corneal innate immunity

The ocular surface is constantly exposed to a wide array of microorganisms. The ability of the cornea to recognize pathogens as foreign and eliminate them is critical to retain its transparency, hence preservation of sight. In the eye, as in other parts of the body, the early response against invading pathogens is provided by innate immunity. Corneal innate immune system uses a series of pattern recognition receptors to detect the presence of pathogens thus allowing for rapid host defense responses to invading microbes. A key component of such receptors is the "Toll-like receptors" (TLRs), which have come to occupy the center stage in innate immunity against invading pathogens. An increasing number of studies have shown that TLRs are expressed by a variety of tissues and cells of the eye and play an important role in ocular defense against microbial infection. Here in this review we summarize the current knowledge about TLR expression in human eye with main emphasis on the cornea, and discuss the future directions of the field.     

Innate immunity and adjuvants

Innate immunity was for a long time considered to be non-specific because the major function of this system is to digest pathogens and present antigens to the cells involved in acquired immunity. However, recent studies have shown that innate immunity is not non-specific, but is instead sufficiently specific to discriminate self from pathogens through evolutionarily conserved receptors, designated Toll-like receptors (TLRs). Indeed, innate immunity has a crucial role in early host defence against invading pathogens. Furthermore, TLRs were found to act as adjuvant receptors that create a bridge between innate and adaptive immunity, and to have important roles in the induction of adaptive immunity. This paradigm shift is now changing our thinking on the pathogenesis and treatment of infectious, immune and allergic diseases, as well as cancers. Besides TLRs, recent findings have revealed the presence of a cytosolic detector system for invading pathogens. I will review the mechanisms of pathogen recognition by TLRs and cytoplasmic receptors, and then discuss the roles of these receptors in the development of adaptive immunity in response to viral infection.     

B细胞Toll样受体的表达和功能

Toll样受体(TLRs)广泛表达于固有免疫和获得性免疫系统.它们通过识别内外源性致病原含有的保守病原体相关模式分子,启动宿主防卫反应.TLRs也是沟通固有免疫和获得性免疫反应,尤其是T细胞介导的细胞免疫反应的重要桥梁.新近研究表明,几乎所有亚型TLR均表达在B淋巴细胞,不仅参与B细胞增殖、成熟和功能调节,而且在系统性红斑狼疮和慢性淋巴细胞白血病等疾病发生过程中发挥重要调节作用.以TLRs为靶点,调节B细胞介导的免疫反应,可能成为具有崭新应用前景的免疫治疗途径和方法.     

PRRs in pathogen recognition

The innate immune system provides the first line of host defense against invading microorganisms before the development of adaptive immune responses. Innate immune responses are initiated by germline-encoded pattern recognition receptors (PRRs), which recognize specific structures of microorganisms. Toll-like receptors (TLRs) are pattern-recognition receptors that sense a wide range of microorganisms, including bacteria, fungi, protozoa and viruses. TLRs exist either on the cell surface or in the lysosome/endosome compartment and induce innate immune responses. Recently, cytoplasmic PRRs have been identified which detect pathogens that have invaded the cytosol. This review focuses on the pathogen recognition of PRRs in innate immunity.     

Role of toll-like receptors gene polymorphism in hepatocellular carcinoma

Abstract

Toll-like receptors (TLRs), evolutionarily conserved innate, play important roles in the development of autoimmunity. TLRs proteins are localized on the cell surface or in endosomes and play critical roles in innate immune responses against different pathogens. Aberrant stimulation of the innate immune system through intracellular TLRs may lead to hyperactive immune responses and contribute to the pathogenesis of hepatocellular carcinoma (HCC). HCC is the seventh most common cancer and the third leading cause of cancer deaths worldwide, and innate immune takes a most important role in HCC. There was no review to sum up the role of TLRs gene polymorphism in HCC. This review was performed to sum up the role of TLRs gene polymorphism in HCC.     

Toll样受体(TLRs)的信号转导与免疫调节

Toll样受体(Toll-like receptors,TLRs)是进化中比较保守的一个受体家族,至少包括10个成员.TLRs能特异地识别病原相关的分子模式(PAMPs),不仅在激活天然免疫中发挥重要的作用,而且还调节获得性免疫,是连接天然免疫和获得性免疫的桥梁.近年来,TLRs信号转导的研究,特别是在负调控研究领域,进展非常迅速.对TLRs信号通路新进展以及TLRs在抗感染免疫中的作用进行了综述.     

Antiviral signaling through pattern recognition receptors

Viral infection is detected by the host innate immune system. Innate immune cells such as dendritic cells and macrophages detect nucleic acids derived from viruses through pattern recognition receptors (PRRs). Viral recognition by PRRs initiates the activation of signaling pathways that lead to production of type I interferon and inflammatory cytokines, which are important for the elimination of viruses. Two types of PRRs that recognize viral nucleic acids, Toll-like receptors (TLR) and RIG-I-like RNA helicases (RLH), have been identified. Of the TLRs, TLR3 recognizes viral double-stranded (ds) RNA, TLR7 and human TLR8 identify viral single-stranded (ss) RNA and TLR9 detects viral DNA. TLRs are located in endosomal compartments, whereas RLH are present in the cytoplasm where they detect viral dsRNA or ssRNA. Here we review the role of TLRs and RLHs in the antiviral innate immune response.     

Proteome analysis of human macrophages reveals the upregulation of manganese-containing superoxide dismutase after toll-like receptor activation

Macrophages are essential for the development of innate immune responses against a variety of infectious factors. They detect invading pathogens via their pattern recognition receptors such as toll-like receptors (TLRs). TLR7/8 recognizes ssRNA from various viruses. In the present study, we have used 2-DE gel-based proteomics to find novel TLR7/8 target proteins in human monocyte-derived macrophages in order to improve our understanding of the virus recognition by this TLR. A total of 27 protein spots were found to be reproducibly differentially expressed between control and TLR7/8 activated 2-DE gel pairs, 18 spots being more than two-fold upregulated and nine spots being at least two-fold downregulated. Several proteins involved in defense against toxic superoxide (O2-) and other reactive oxygen species, such as manganese-containing superoxide dismutase (SOD2), glutathione peroxidase, and peroxiredoxins 1 and 6 were highly upregulated after TLR7/8 activation. Western blot analysis showed that activation of macrophages with TLR2, TLR3, TLR4, and TLR7/8 ligands also strongly upregulated SOD2 protein expression. In conclusion, our results show that the activation of pattern recognition receptors of the innate immune system results in strong upregulation of SOD2 gene expression suggesting that SOD2 protects macrophages from oxidative stress during microbial infection.     

Ligand-regulated chimeric receptor approach reveals distinctive subcellular localization and signaling properties of the Toll-like receptors

Toll-like receptors (TLRs) are sensors for the detection of invading infectious agents and can initiate innate immune responses. Because the innate immune system induces an appropriate defense against different pathogens, different TLR signaling domains may have unique properties that are responsible for eliciting distinctive responses to different types of pathogens. To test this hypothesis, we created ligand-regulated TLR chimeric receptors composed of the extracellular region of TLR4 and the transmembrane and cytoplasmic regions of other TLRs and expressed these chimeras in macrophages lacking endogenous TLR4. Interestingly, the chimeras between TLR4 and either TLR3, TLR7, or TLR9 were localized completely intracellularly whereas other chimeras were expressed on the cell surface. Lipopolysaccharide (LPS), a ligand for these chimeras, induced the activation of nuclear factor kappa B and mitogen-activated protein kinases and the subsequent production of pro-inflammatory cytokines in macrophages expressing TLR4, TLR4/TLR5, or TLR4/TLR8 chimeras but not in macrophages expressing TLR4/TLR1, TLR4/TLR2, or TLR4/TLR6 chimeras. Co-expression of unresponsive chimeras in some combinations (chimeras with TLR1+TLR2 or TLR2+TLR6 but not TLR1+TLR6) resulted in LPS responsiveness, indicating functional complementarity. Furthermore, the pair of TLR2+TLR6 chimera required approximately 10-fold less LPS to induce the same responses compared with the TLR1+TLR2 pair. Finally, LPS induced effective interferon-beta production and subsequent Stat1 phosphorylation in macrophages expressing full-length TLR4 but not other cell surface TLR chimeras. These results suggest that the functions of TLRs are diversified not only in their extracellular regions for ligand recognition but also in their transmembrane and cytoplasmic regions for subcellular localization and signaling properties.     

Autophagy in regulation of Toll-like receptor signaling

Toll-like receptors (TLRs) serve as the major innate immune sensors for detection of specific molecular patterns on various pathogens. TLRs activate signaling events mainly by utilizing ubiquitin-dependent mechanisms. Recent research advances have provided evidence that TLR signaling is linked to induction of autophagy. Autophagy is currently known to affect both of the immune defense and suppression of inflammatory responses. In TLR-associated immune responses, autophagic lysis of intracellular microbes (called xenophagy) contributes to the former mechanism, while the latter seems to be mediated by the control of the mitochondrial integrity or selective autophagic clearance of aggregated signaling proteins (called aggrephagy). Several autophagy-related ubiquitin-binding proteins, such as SQSTM1/p62 and NDP52, mediate xenophagy and aggrephagy. In this review, we summarize the expanded knowledge regarding TLR signaling and autophagy signaling. After that, we will focus on autophagy-associated signaling downstream of TLRs and the effect of autophagy on TLR signaling, thus highlighting the signaling crosstalk between the TLR-associated innate immune responses and the regulation of innate immunity by xenophagy and aggrephagy.     

TLR3 and TLR4 expression in healthy and diseased human endometrium

Background  

Toll-like receptors (TLRs) play an essential role in the innate immune system by initiating and directing immune response to pathogens. TLRs are expressed in the human endometrium and their regulation might be crucial for the pathogenesis of endometrial diseases.