Toll-like receptors (TLRs) play a crucial role in innate immune response in mammals. Individual TLRs recognize microbial components that are conserved among pathogens and activate their signaling pathways. Each TLR has its own cascade of signaling pathway for exhibiting its specific responses through selective utilization of TIR domain-containing adaptors. Increasing evidence for roles of TLRs in various diseases provides us new insights for a basis of new therapies. In this review, we discuss the possibilities of therapeutics targeting TLRs in various diseases and explain potential problems associated with such approaches. Martin Braddock – AstraZeneca R&D Charnwood, Loughborough, UK Ian Adcock – Department of Thoracic Medicine, National Heart and Lung Institute, Imperial College of Science, Technology and Medicine, London, UK Immune-modulation represents an exciting a potential very rich area for the development of pharmacological agents in a wide variety of immune-mediated diseases. The Toll receptor family recognise specific ligands which may function as agonists or antagonists and which serve to modulate receptor signalling via a complex intra-cellular pathway. This pathway involves many mediators, some of which are targets for drug therapy such as members of the MAP kinase family. A detailed understanding of the role of these receptors and their ligands in the innate and acquired immune response will demonstrate their true value as promising drug targets for future therapies.
Gene expression is controlled at multiple points, including signal transduction, transcription and mRNA stability. So far, transcriptional regulation has been extensively studied. However, recent studies have revealed that control of gene expression at the mRNA level is as important as transcriptional control in the immune response. The innate immune system is an evolutionally conserved host defense mechanism against pathogens. Innate immune responses are initiated by activation of pattern recognition receptors (PRRs), which recognize microbial components. Among them, Toll-like receptors (TLRs) are capable of sensing organisms ranging from bacteria to fungi, protozoa and viruses, and play a major role in innate immunity. Individual TLRs recognize different microbial components, activate different signaling pathways via selective usage of adaptor molecules, and give rise to different patterns in gene expression. We are now focusing on the role of genes induced in response to TLR stimulation, particularly the genes that are rapidly induced in a MyD88-dependent manner within 30min after LPS stimulation. Among them, we have recently identified a novel gene named Zc3h12a which has a CCCH-type zinc finger domain. The knockout mice developed spontaneous autoimmune diseases accompanied by splenomegaly and lymphadenopathy. Subsequent studies showed that Zc3h12a is a nuclease involved in destabilization of IL-6 and IL-12mRNA via the stem loop structure present in the 3’UTR of these genes. We renamed it Regulatory RNase-1 (Regnase-1) based on the function. I would like to discuss the role of Regnase-1 in the immune response.
Toll-like receptor (TLR) ligands that signal via TIR-domain-containing adapter-inducing IFNβ (TRIF) activate the IκB kinase (IKK)-related kinases, TRAF associated NFκB activator (TANK)-binding kinase-1 (TBK1) and IKKε, which then phosphorylate IRF3 and induce the production of IFNβ. Here we show that TBK1 and IKKε are also activated by TLR ligands that signal via MyD88. Notably, the activation of IKKε is rapid, transient, and it precedes a more prolonged activation of TBK1. The MyD88- and TRIF-dependent signaling pathways activate the IKK-related kinases by two signaling pathways. One is mediated by the canonical IKKs, whereas the other culminates in the autoactivation of the IKK-related kinases. Once activated, TBK1/IKKε then phosphorylate and inhibit the canonical IKKs. The negative regulation of the canonical IKKs by the IKK-related kinases occurs in both the TRIF- and MyD88-dependent TLR pathways, whereas IRF3 phosphorylation is restricted to the TRIF-dependent signaling pathway. We have discovered that the activation of IKKε is abolished, the activation of TBK1 is reduced, and the interaction between the IKK-related kinases and the canonical IKKs is suppressed in TANK −/− macrophages, preventing the IKK-related kinases from negatively regulating the canonical IKKs. In contrast, IRF3 phosphorylation and IFNβ production was normal in TANK −/− macrophages. Our results demonstrate a key role for TANK in enabling the canonical IKKs and the IKK-related kinases to regulate each other, which is required to limit the strength of TLR signaling and ultimately, prevent autoimmunity.
Toll-like receptors (TLRs) are essential in the host defense against microbial pathogens. Individual TLRs recognize distinct structural components of pathogens and evoke inflammatory responses. Recent evidence indicates that TLRs recognize not only bacteria and fungi but also viruses. The molecular mechanisms by which TLRs induce differential gene expression are now beginning to be clarified.
STAT (signal transducers and activators of transcription) proteins are activated in response to a large number of cytokines, growth factors, and hormones. Upon activation following the binding of ligands to their receptors, STAT proteins dimerize, translocate to the nucleus, and bind to the promoters of specific target genes. To date, seven mammalian members of the STAT family have been identified. Although some cytokines and growth factors can activate multiple STAT proteins, some STATs are activated with considerable specificity. The physiological role of each individual STAT protein is now being examined through the study of "knockout" mice, harboring a null allele for the particular gene. STAT1-deficient mice exhibit a selective signaling defect in response to interferons. STAT4 and STAT6 are essential for Th1-and Th2-responses, respectively. STAT5a-deficient mice exhibit defective mammary gland development. A study of STAT5b-deficient mice indicates that STAT5b mediates the sexually dimorphic effects of growth hormone in the liver. STAT5a and 5b also play different biological roles in the immune system. STAT3-deficient mice die during early embryogenesis, but the role of STAT3 in adult tissues can be assessed by utilizing the Cre-loxP recombination system to ablate the gene later in life. Analyses of tissue-specific STAT3-deficient mice indicate that STAT3 plays a crucial role in a variety of biological functions, including cell growth, suppression of apoptosis, and cell motility.
Small intestinal innate lymphoid cells (mainly NKp46+ILCs) are known to have a protective role from infection of pathogenic bacteria through the production of IL-22. The proportions of ILCs localized intestinal lamina propria are higher than those in the others lymphoid tissues, such as mesenteric lymph node, spleen, liver, and bone marrow. In this study, we did a microarray experiment to study whether small intestinal ILCs (Lin-c-Kit+Sca-1- cells) highly express which genes as compared with non-ILCs (Lin-c-Kit-Sca-1- cells). We selected 251 up-regulated genes and 219 down-regulated genes in Lin-c-Kit+Sca-1- cells. Especially, Lin-c-Kit+Sca-1- cells highly expressed genes coding to Il22, Csf2rb2, mast cell proteases (Mcpts), and Rorc. To address whether which ILC populations are main source of IL-22, we more detailed divided Lin-c-Kit+Sca-1- cells into the two groups [Lin-c-Kit+NKp46+ (NKp46+ILCs) and Lin-c-Kit+NKp46-CD4-]. Only Lin-c-Kit+NKp46-CD4- cells expressed Csf2rb2 and Mcpts transcripts. These cells expressed higher level of Il22 mRNA, IL-22 protein, and IL-17F protein in physiological condition and in IL-23 stimulated condition compared with NKp46+ILCs. Moreover, these cells could differentiate into mast cells in the presence of mIL-3 and mSCF. Taken together, Lin-c-Kit+NKp46-CD4- cells as mast cell progenitors may regulate small intestinal homeostasis through the production of Mcpts, IL-17F, and IL-22 in physiological condition and in pathophysiological condition.
Recognition of pathogens by Toll-like receptors (TLRs) triggers innate immune responses via signaling pathways mediated by several Toll/IL-1R (TIR) domain-containing adaptors such as MyD88, TIRAP, and TRIF. MyD88 is a common adaptor that is essential for proinflammatory cytokine production, whereas TRIF mediates the MyD88-independent pathway from TLR3 and TLR4 that is responsible for type I interferon production in response to double-stranded RNA and LPS, respectively. TIRAP specifically participates in the MyD88-dependent pathways shared by TLR2 and TLR4, and TRAM is essential for the TLR4-mediated MyD88-independent pathway. Thus, TIR domain-containing adaptors play an important role in the TLR mediated signaling pathways.
