Innate immunity: structure and function of TLRs

The innate immune system provides the first line of defence against infection. Through a limited number of germline-encoded receptors called pattern recognition receptors (PRRs), innate cells recognize and are activated by highly conserved structures expressed by large group of microorganisms called pathogen-associated molecular patterns (PAMPs). PRRs are involved either in recognition (scavenger receptors, C-type lectins) or in cell activation (Toll-like receptors or TLR, helicases and NOD molecules). TLRs play a pivotal role in cell activation in response to PAMPs. TLR are type I transmembrane proteins characterized by an intracellular Toll/IL 1 receptor homology domain that are expressed by innate immune cells (dendritic cells, macrophages, NK cells), cells of the adaptive immunity (T and B lymphocytes) and non immune cells (epithelial and endothelial cells, fibroblasts). In all the cell types analyzed, TLR agonists, alone or in combination with costimulatory molecules, induce cell activation. The crucial role played by TLR in immune cell activation has been detailed in dendritic cells. A TLR-dependent activation of dendritic cells is required to induce their maturation and migration to regional lymph nodes and to activate na?ve T cells. The ability of different cell types to respond to TLR agonists is related to the pattern of expression of the TLRs and its regulation as well as their intracellular localization. Recent studies suggest that the nature of the endocytic and signaling receptors engaged by PAMPs may determine the nature of the immune response generated against the microbial molecules, highlighting the role of TLRs as molecular interfaces between innate and adaptive immunity. In this review are summarized the main biological properties of the TLR molecules.     

树突细胞上Toll样受体的介导作用

树突状细胞是体内最重要的抗原提呈细胞,它表面表达多种Toll样受体。Toll样受体可通过多条途径来激活树突细胞,介导树突细胞对抗原的摄取,递呈及生存与凋亡,促进T细胞增值和分化并参与免疫反应。     

TLRs and chronic inflammation

After the discovery of Toll-like receptors (TLRs), innate immune mechanisms came back in the focus of scientific research. With more and more mechanisms of TLR biology known, it has become clear that these and also other innate immune receptors are not only of crucial importance in the immune response to invading pathogens, but also play a role in the homeostasis of commensal flora and in the response to stress and danger signals. In this respect, increasing evidence is found that inappropriate quantity or quality of TLR ligands or aberrant response to TLR activation plays a role in a variety of chronic inflammatory diseases. In this review, an overview of the currently known TLRs and their signaling pathways is given and reports about their expression and activation in chronic inflammatory diseases are recapitulated.     

TLRs mediate IFN-gamma production by human uterine NK cells in endometrium

The human endometrium (EM) contains macrophages, NK cells, T cells, B cells, and neutrophils in contact with a variety of stromal and epithelial cells. The interplay between these different cell types and their roles in defense against pathogen invasion in this specialized tissue are important for controlling infection and reproduction. TLRs are a family of receptors able to recognize conserved pathogen-associated molecular patterns. In this study, we determined the expression of TLRs on uterine NK (uNK) cells from the human EM and the extent to which uNK cells responded to TLR agonist stimulation. uNK cells expressed TLRs 2, 3, and 4, and produced IFN-gamma when total human endometrial cells were stimulated with agonists to TLR2 or TLR3 (peptidoglycan or poly(I:C), respectively). Activated uNK cell clones produced IFN-gamma upon stimulation with peptidoglycan or poly(I:C). However, purified uNK cells did not respond directly to TLR agonists, but IFN-gamma was produced by uNK cells in response to TLR stimulation when cocultured with APCs. These data indicate that uNK cells express TLRs and that they can respond to TLR agonists within EM by producing IFN-gamma. These data also indicate that the uNK cells do not respond directly to TLR stimulation, but rather their production of IFN-gamma is dependent upon interactions with other cells within EM.     

Toll-like receptors (TLRs): An old family of immune receptors with a new face in cancer pathogenesis

In the dark path of tumorigenesis, the more carefully the cancer biology is studied, the more brilliant answers could be given to the countless questions about its orchestrating derivers. The identification of the correlation between Toll-like receptors (TLRs) and different processes involved in carcinogenesis was one of the single points of blinding light highlighting the interconnection between the immune system and cancer. TLRs are a wide family of single-pass membrane-spanning receptors that have developed through the evolution to recognize the structurally conserved molecules derived from microorganisms or damaged cells. But this is not everything about these receptors as they could orchestrate several downstream signalling pathways leading to the formation or suppression of cancer cells. The present review is tempted to provide a concise schematic about the biology and the characters of TLRs and also summarize the major findings of the regulatory role of TLRs and their associated signalling in the pathogenesis of human cancers.     

Roles of NOD1 (NLRC1) and NOD2 (NLRC2) in innate immunity and inflammatory diseases

NOD1 {nucleotide-binding oligomerization domain 1; NLRC [NOD-LRR (leucine-rich repeat) family with CARD (caspase recruitment domain) 1]} and NOD2 (NLRC2) are among the most prominent members of the NLR (NOD-LRR) family –proteins that contain nucleotide-binding NACHT domains and receptor-like LRR domains. With over 20 members identified in humans, NLRs represent important components of the mammalian innate immune system, serving as intracellular receptors for pathogens and for endogenous molecules elaborated by tissue injury. NOD1 and NOD2 proteins operate as microbial sensors through the recognition of specific PG (peptidoglycan) constituents of bacteria. Upon activation, these NLR family members initiate signal transduction mechanisms that include stimulation of NF-κB (nuclear factor-κB), stress kinases, IRFs (interferon regulatory factors) and autophagy. Hereditary polymorphisms in the genes encoding NOD1 and NOD2 have been associated with an increasing number of chronic inflammatory diseases. In fact, potential roles for NOD1 and NOD2 in inflammatory disorders have been revealed by investigations using a series of animal models. In the present review, we describe recent experimental findings associating NOD1 and NOD2 with various autoimmune and chronic inflammatory disorders, and we discuss prospects for development of novel therapeutics targeting these NLR family proteins.     

Toll-like receptors: a family of pattern-recognition receptors in mammals

The innate immune system uses a variety of germline-encoded pattern-recognition receptors that recognize conserved microbial structures or pathogen-associated molecular patterns, such as those that occur in the bacterial cell-wall components peptidoglycan and lipopolysaccharide. Recent studies have highlighted the importance of Toll-like receptors (TLRs) as a family of pattern-recognition receptors in mammals that can discriminate between chemically diverse classes of microbial products. First identified on the basis of sequence similarity with the Drosophila protein Toll, TLRs are members of an ancient superfamily of proteins, which includes related proteins in invertebrates and plants. TLRs activate innate immune defense reactions, such as the release of inflammatory cytokines, but increasing evidence supports an additional critical role for TLRs in orchestrating the development of adaptive immune responses. The sequence similarity between the intracellular domains of the TLRs and the mammalian interleukin-1 and interleukin-18 cytokine receptors reflects the use of a common intracellular signal-transduction cascade triggered by these receptor classes. But more recent findings have demonstrated that there are in fact TLR-specific signaling pathways and cellular responses. Thus, TLRs function as sentinels of the mammalian immune system that can discriminate between diverse pathogen-associated molecular patterns and then elicit pathogen-specific cellular immune responses.     

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.     

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.     

Nucleic acid-sensing TLRs as modifiers of autoimmunity

The immune system requires precise regulation of activating and inhibitory signals so that it can mount effective responses against pathogens while ensuring tolerance to self-components. Some of the most potent activation signals are triggered by innate immune molecules, particularly those in the TLR family. Recent studies have shown that engagement of TLRs plays a significant role in both innate and adaptive immunity. This review focuses on the ways that TLR function might contribute to the etiology of lupus-like syndromes in the context of an autoimmune-prone environment. By considering the sources, localization, and expression of both nucleic acids and the molecules that bind them, we discuss several ways that innate immunity can play a role in the development of systemic autoimmunity.     

Microglia initiate central nervous system innate and adaptive immune responses through multiple TLRs

Microglia are the resident macrophage-like population in the CNS. Microglia remain quiescent until injury or infection activates the cells to perform effector inflammatory and APC functions. Our previous studies have shown that microglia infected with a neurotropic strain of Theiler's murine encephalomyelitis virus secreted innate immune cytokines and up-regulated costimulatory molecules and MHC class II, enabling the cells to present viral and myelin Ags to CD4+ T cells. Recently, TLRs have been shown to recognize pathogen-associated molecular patterns and initiate innate immune responses upon interaction with infectious agents. We examined TLR expression on brain microglia and their functional responses upon stimulation with various TLR agonists. We report that mouse microglia express mRNA for all of the recently identified TLRs, TLR1-9, used for recognition of bacterial and viral molecular patterns. Furthermore, stimulation of quiescent microglia with various TLR agonists, including LPS (TLR4), peptidoglycan (TLR2), polyinosinic-polycytidylic acid (TLR3), CpG DNA (TLR9), and infection with viable Theiler's murine encephalomyelitis virus, activated the cells to up-regulate unique patterns of innate and effector immune cytokines and chemokines at the mRNA and protein levels. In addition, TLR stimulation activated up-regulation of MHC class II and costimulatory molecules, enabling the microglia to efficiently present myelin Ags to CD4+ T cells. Thus, microglia appear to be a unique and important component of both the innate and adaptive immune response, providing the CNS with a means to rapidly and efficiently respond to a wide variety of pathogens.     

Disifin (Sodium tosylchloramide) and Toll-like receptors (TLRs): evolving importance in health and diseases

Disifin has emerged as a unique and very effective agent used in disinfection of wounds, disinfection of surfaces, materials and water, and other substances contaminated with almost every type of pathogenic microorganism ranging from viruses, bacteria, fungi and yeast, and, very possibly, protozoan parasites, as well. The major active component of Disifin is tosylchloramide sodium (chloramine T). However, the mechanism by which Disifin suppresses the activities of pathogenic microbial agents remains enigmatic. The molecular mechanisms, and the receptors and the signal transducing pathways responsible for the biological effects of Disifin are largely unknown. Despite considerable advances, enormous investigative efforts and large resources invested in the research on infectious diseases, microbial infection still remains a public health problem in many parts of the world. The exact nature of the pathogenic agents responsible for many infectious diseases, and the nature of the receptors mediating the associated inflammatory events are incompletely understood. Recent advances in understanding the molecular basis for mammalian host immune responses to microbial invasion suggest that the first line of defense against microbes is the recognition of pathogen-associated molecular patterns (PAMPs) by a family of transmembrane pattern-recognizing and signal transducing receptor proteins called Toll-like receptors (TLRs). The TLR family plays an instructive role in innate immune responses against microbial pathogens, as well as the subsequent induction of adaptive immune responses. TLRs mediate recognition and inflammatory responses to a wide range of microbial products and are crucial for effective host defense by eradication of the invading pathogens. Now, recent updates demonstrated the ability of Disifin-derived products, Disifin-Animal and Disifin-Pressant to effectively suppress the progression and activities of Chikungunya fever and that of avian influenza A virus [A/cardialis/Germany/72, H7N1: the agent of a highly pathogenic avian influenza (HPAI)] infection, respectively. Overall, the above findings led me to suggest that Disifin and TLRs may mechanistically overlap in the processes of executing their functions against pathogenic microbial organisms. Thus, elucidating and better understanding of the molecular underpinnings responsible for the biochemical effects of Disifin-products, and the nature and mode of the interaction(s) of Disifin with TLRs in the process of exerting their biological effects may open a novel dimension in the research of infectious diseases, which may provide novel therapeutic targets for the prevention and treatment of a wide range of infectious diseases.     

Expression of toll-like receptors on B lymphocytes

Toll-like receptors (TLRs) are a family of trans-membrane receptors that play an important role in the innate immune system. Most studies examining the cellular expression of TLRs on immune cells have focussed on neutrophils, monocytes and dendritic cells, but there is little evidence of TLRs being expressed on lymphocytes. Using 3-colour flow cytometry, expression of TLR-1, TLR-2, TLR-3, TLR-4, and TLR-9 on peripheral blood lymphocyte populations was determined. Further examination of TLRs on CD5- and CD5+ CD19+ B cell subsets was performed. The binding of TLR1 and TLR9 antibodies was detected on 15-90% of resting B cells, but not on resting T-cells. The higher expression of TLR1 and TLR9 on CD5+ B cells compared to CD5- B cells may reflect the role of B1 cells in more primitive, less specific antibody responses.     

Vasoactive intestinal peptide downregulates proinflammatory TLRs while upregulating anti-inflammatory TLRs in the infected cornea

TLRs recognize microbial pathogens and trigger an immune response, but their regulation by neuropeptides, such as vasoactive intestinal peptide (VIP), during Pseudomonas aeruginosa corneal infection remains unexplored. Therefore, C57BL/6 (B6) mice were injected i.p. with VIP, and mRNA, protein, and immunostaining assays were performed. After VIP treatment, PCR array and real-time RT-PCR demonstrated that proinflammatory TLRs (conserved helix-loop-helix ubiquitous kinase, IRAK1, TLR1, TLR4, TLR6, TLR8, TLR9, and TNFR-associated factor 6) were downregulated, whereas anti-inflammatory TLRs (single Ig IL-1-related receptor [SIGIRR] and ST2) were upregulated. ELISA showed that VIP modestly downregulated phosphorylated inhibitor of NF-κB kinase subunit α but upregulated ST2 ~2-fold. SIGIRR was also upregulated, whereas TLR4 immunostaining was reduced in cornea; all confirmed the mRNA data. To determine whether VIP effects were cAMP dependent, mice were injected with small interfering RNA for type 7 adenylate cyclase (AC7), with or without VIP treatment. After silencing AC7, changes in mRNA levels of TLR1, TNFR-associated factor 6, and ST2 were seen and unchanged with addition of VIP, indicating that their regulation was cAMP dependent. In contrast, changes were seen in mRNA levels of conserved helix-loop-helix ubiquitous kinase, IRAK1, 2, TLR4, 9 and SIGIRR following AC7 silencing alone; these were modified by VIP addition, indicating their cAMP independence. In vitro studies assessed the effects of VIP on TLR regulation in macrophages and Langerhans cells. VIP downregulated mRNA expression of proinflammatory TLRs while upregulating anti-inflammatory TLRs in both cell types. Collectively, the data provide evidence that VIP downregulates proinflammatory TLRs and upregulates anti-inflammatory TLRs and that this regulation is both cAMP dependent and independent and involves immune cell types found in the infected cornea.     

TLRs: Linking inflammation and breast cancer

Breast cancer is one of the leading causes of mortality in the females. Intensive efforts have been made to understand the molecular mechanisms of pathogenesis of breast cancer. The physiological conditions that lead to tumorigenesis including breast cancer are not well understood. Toll like receptors (TLRs) are essential components of innate immune system that protect the host against bacterial and viral infection. The emerging evidences suggest that TLRs are activated through pathogen associated molecular patterns (PAMPs) as well as endogenous molecules, which lead to the activation of inflammatory pathways. This leads to increased levels of several pro-inflammatory cytokines and chemokines mounting inflammation. Several evidences support the view that chronic inflammation can lead to cancerous condition. Inflammation aids in tumor progression and metastasis. Association of inflammation with breast cancer is emerging. TLR mediated activation of NF-κB and IRF is an essential link connecting inflammation to cancer. The recent reports provide several evidences, which suggest the important role of TLRs in breast cancer pathogenesis and recurrence. The current review focuses on emerging studies suggesting the strong linkages of TLR mediated regulation of inflammation during breast cancer and its metastasis emphasizing the initiation of the systematic study.     

Human lymphatic endothelial cells express multiple functional TLRs

The lymphatic endothelium is the preferred route for the drainage of interstitial fluid from tissues and also serves as a conduit for peripheral dendritic cells (DCs) to reach draining lymph nodes. Lymphatic endothelial cells (LECs) are known to produce chemokines that recruit Ag-loaded DCs to lymphatic vessels and therefore are likely to regulate the migration of DCs to lymph nodes. TLRs are immune receptors that recognize pathogen associated molecular patterns and then signal and stimulate production of inflammatory chemokines and cytokines that contribute to innate and adaptive immune responses. TLRs are known to be expressed by a wide variety of cell types including leukocytes, epithelial cells, and endothelial cells. Because the TLR expression profile of LECs remains largely unexamined, we have undertaken a comprehensive study of the expression of TLR1-10 mRNAs and protein in primary human dermal (HD) and lung LECs as well as in htert-HDLECs, which display a longer life-span than HDLECs. We found that all three cell types expressed TLR1-6 and TLR9. The responsiveness of these LECs to a panel of ligands for TLR1-9 was measured by real-time RT-PCR, ELISA, and flow cytometry, and revealed that the LECs responded to most but not all TLR ligands by increasing expression of inflammatory chemokines, cytokines, and adhesion molecules. These findings provide insight into the ability of cells of the lymphatic vasculature to respond to pathogens and potential vaccine adjuvants and shape peripheral environments in which DCs will acquire Ag and environmental cues.     

Toll样受体对T细胞功能及代谢的影响

Toll样受体（Toll-like receptors, TLRs）在先天免疫系统中广泛表达,可通过促进抗原提呈细胞（antigen presenting cells,APC）共刺激分子的表达从而间接导致T细胞活化。然而研究发现,TLR也可在T细胞中表达,并可在没有APC的情况下直接调节T细胞的代谢与功能。本文综述了TLR信号对不同T细胞亚群代谢和免疫功能的直接调控作用,为T细胞介导的癌症及自身免疫病等疾病的预防和治疗提供了新的思路。     

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.     

Toll-like receptors: linking innate and adaptive immunity

Detection of and response to microbial infections by the immune system depends largely on a family of pattern-recognition receptors called Toll-like receptors (TLRs). These receptors recognize conserved molecular products derived from various classes of pathogens, including Gram-positive and -negative bacteria, DNA and RNA viruses, fungi and protozoa. Recognition of ligands by TLRs leads to a series of signaling events resulting in induction of acute responses necessary to kill the pathogen. TLRs are also responsible for the induction of dendritic cell maturation, which is responsible and necessary for initiation of adaptive immune responses. Although TLRs control induction of adaptive immunity, it is not clear at this point how responses are appropriately tailored by individual TLRs to the advantage of the host.