Innate immune recognition of viral infection

Toll-like receptors (TLRs) are key molecules of the innate immune systems, which detect conserved structures found in a broad range of pathogens and triggers innate immune responses. A subset of TLRs recognize viral components and induce antiviral responses by producing type I interferons. Whereas TLR2 and TLR4 recognize viral components at the cell surface, TLR3, TLR7, TLR8 and TLR9 are exclusively expressed in endosomal compartments. After phagocytes internalize viruses or virus-infected apoptotic cells, viral nucleic acids are released in phagolysosomes and are recognized by these TLRs. Recent reports have shown that hosts also have a mechanism to detect replicating viruses in the cytoplasm in a TLR-independent manner. In this review, we focus on the viral recognition by innate immunity and the signaling pathways.     

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.     

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.     

Differential roles for RIG-I-like receptors and nucleic acid-sensing TLR pathways in controlling a chronic viral infection

The necessity for pathogen recognition of viral infection by the innate immune system in initiating early innate and adaptive host defenses is well documented. However, little is known about the role these receptors play in the maintenance of adaptive immune responses and their contribution to resolution of persistent viral infections. In this study, we demonstrate a nonredundant functional requirement for both nucleic acid-sensing TLRs and RIG-I-like receptors in the control of a mouse model of chronic viral infection. Whereas the RIG-I-like receptor pathway was important for production of type I IFNs and optimal CD8(+) T cell responses, nucleic acid-sensing TLRs were largely dispensable. In contrast, optimal anti-viral Ab responses required intact signaling through nucleic acid-sensing TLRs, and the absence of this pathway correlated with less virus-specific Ab and deficient long-term virus control of a chronic infection. Surprisingly, absence of the TLR pathway had only modest effects on Ab production in an acute infection with a closely related virus strain, suggesting that persistent TLR stimulation may be necessary for optimal Ab responses in a chronic infection. These results indicate that innate virus recognition pathways may play critical roles in the outcome of chronic viral infections through distinct mechanisms.     

How Location Governs Toll-Like Receptor Signaling

Toll-like receptors (TLRs) are a family of innate immune system receptors responsible for recognizing conserved pathogen-associated molecular patterns (PAMPs). PAMP binding to TLRs initiates intracellular signaling pathways that lead to the upregulation of a variety of costimulatory molecules and the synthesis and secretion of various cytokines and interferons by cells of the innate immune system. TLR-induced innate immune responses are a prerequisite for the generation of most adaptive immune responses, and in the case of B cells, TLRs directly regulate signaling from the antigen-specific B-cell receptor. The outcome of TLR signaling is determined, in part, by the cells in which they are expressed and by the selective use of signaling adaptors. Recent studies suggest that, in addition, both the ligand recognition by TLRs and the functional outcome of ligand binding are governed by the subcellular location of the TLRs and their signaling adaptors. In this review we describe what is known about the intracellular trafficking and compartmentalization of TLRs in innate system's dendritic cells and macrophages and in adaptive system's B cells, highlighting how location regulates TLR function.     

Toll-like receptors are key participants in innate immune responses

During an infection, one of the principal challenges for the host is to detect the pathogen and activate a rapid defensive response. The Toll-like family of receptors (TLRs), among other pattern recognition receptors (PRR), performs this detection process in vertebrate and invertebrate organisms. These type I transmembrane receptors identify microbial conserved structures or pathogen-associated molecular patterns (PAMPs). Recognition of microbial components by TLRs initiates signaling transduction pathways that induce gene expression. These gene products regulate innate immune responses and further develop an antigen-specific acquired immunity. TLR signaling pathways are regulated by intracellular adaptor molecules, such as MyD88, TIRAP/Mal, between others that provide specificity of individual TLR- mediated signaling pathways. TLR-mediated activation of innate immunity is involved not only in host defense against pathogens but also in immune disorders. The involvement of TLR-mediated pathways in auto-immune and inflammatory diseases is described in this review article.     

The role of innate immune receptors in the control of Brucella abortus infection: toll-like receptors and beyond

Research into intracellular sensing of microbial products is an up and coming field in innate immunity. Toll-like receptors (TLRs) recognize Brucella spp. and bacterial components and initiate mononuclear phagocyte responses that influence both innate and adaptive immunity. Recent studies have revealed the intracellular signaling cascades involved in the TLR-initiated immune response to Brucella infection. TLR2, TLR4 and TLR9 have been implicated in host interactions with Brucella; however, TLR9 has the most prominent role. Further, the relationship between specific Brucella molecules and various signal transduction pathways needs to be better understood. MyD88-dependent and TRIF-independent signaling pathways are involved in Brucella activation of innate immune cells through TLRs. We have recently reported the critical role of MyD88 molecule in dendritic cell maturation and interleukin-12 production during B. abortus infection. This article discusses recent studies on TLR signaling and also highlights the contribution of NOD and type I IFN receptors during Brucella infection. The better understanding of the role by such innate immune receptors in bacterial infection is critical in host-pathogen interactions.     

EBV lytic-phase protein BGLF5 contributes to TLR9 downregulation during productive infection

Viruses use a wide range of strategies to modulate the host immune response. The human gammaherpesvirus EBV, causative agent of infectious mononucleosis and several malignant tumors, encodes proteins that subvert immune responses, notably those mediated by T cells. Less is known about EBV interference with innate immunity, more specifically at the level of TLR-mediated pathogen recognition. The viral dsDNA sensor TLR9 is expressed on B cells, a natural target of EBV infection. Here, we show that EBV particles trigger innate immune signaling pathways through TLR9. Furthermore, using an in vitro system for productive EBV infection, it has now been possible to compare the expression of TLRs by EBV(-) and EBV(+) human B cells during the latent and lytic phases of infection. Several TLRs were found to be differentially expressed either in latently EBV-infected cells or after induction of the lytic cycle. In particular, TLR9 expression was profoundly decreased at both the RNA and protein levels during productive EBV infection. We identified the EBV lytic-phase protein BGLF5 as a protein that contributes to downregulating TLR9 levels through RNA degradation. Reducing the levels of a pattern-recognition receptor capable of sensing the presence of EBV provides a mechanism by which the virus could obstruct host innate antiviral responses.     

Polyinosinic acid is a ligand for toll-like receptor 3

Innate immune responses are critical in controlling viral infections. Viral proteins and nucleic acids have been shown to be recognized by pattern recognition receptors of the Toll-like receptor (TLR) family, triggering downstream signaling cascades that lead to cellular activation and cytokine production. Viral DNA is sensed by TLR9, and TLRs 3, 7, and 8 have been implicated in innate responses to RNA viruses by virtue of their ability to sense double-stranded (ds) RNA (TLR3) or single-stranded RNA (murine TLR7 and human TLR8). Viral and synthetic dsRNAs have also been shown to be a potent adjuvant, promoting enhanced adaptive immune responses, and this property is also dependent on their recognition by TLR3. It has recently been shown that mRNA that is largely single-stranded is a ligand for TLR3. Here we have investigated the ability of single-stranded homopolymeric nucleic acids to induce innate responses by murine immune cells. We show for the first time that polyinosinic acid (poly(I)) activates B lymphocytes, dendritic cells, and macrophages and that these responses are dependent on the expression of both TLR3 and the adaptor molecule, Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta (TRIF). We therefore conclude that TLR3 is able to sense both single-stranded RNA and dsRNA.     

The Adjuvant Activity of Alphavirus Replicons Is Enhanced by Incorporating the Microbial Molecule Flagellin into the Replicon

Ligands of pattern recognition receptors (PRRs) including Toll-like receptors (TLRs) stimulate innate and adaptive immune responses and are considered as potent adjuvants. Combinations of ligands might act in synergy to induce stronger and broader immune responses compared to stand-alone ligands. Alphaviruses stimulate endosomal TLRs 3, 7 and 8 as well as the cytoplasmic PRR MDA-5, resulting in induction of a strong type I interferon (IFN) response. Bacterial flagellin stimulates TLR5 and when delivered intracellularly the cytosolic PRR NLRC4, leading to secretion of proinflammatory cytokines. Both alphaviruses and flagellin have independently been shown to act as adjuvants for antigen-specific antibody responses. Here, we hypothesized that alphavirus and flagellin would act in synergy when combined. We therefore cloned the Salmonella Typhimurium flagellin (FliC) gene into an alphavirus replicon and assessed its adjuvant activity on the antibody response against co-administered antigen. In mice immunized with recombinant alphavirus, antibody responses were greatly enhanced compared to soluble FliC or control alphavirus. Both IgG1 and IgG2a/c responses were increased, indicating an enhancement of both Th1 and Th2 type responses. The adjuvant activity of FliC-expressing alphavirus was diminished but not abolished in the absence of TLR5 or type I IFN signaling, suggesting the contribution of several signaling pathways and some synergistic and redundant activity of its components. Thus, we have created a recombinant adjuvant that stimulates multiple signaling pathways of innate immunity resulting in a strong and broad antibody response.     

Inhibition of homodimerization of toll-like receptor 4 by 6-shogaol

Toll-like receptors (TLRs) play a critical role in sensing microbial components and inducing innate immune and inflammatory responses by recognizing invading microbial pathogens. Lipopolysaccharide-induced dimerization of TLR4 is required for the activation of downstream signaling pathways including nuclear factor-kappa B (NF-κB). Therefore, TLR4 dimerization may be an early regulatory event in activating ligand-induced signaling pathways and induction of subsequent immune responses. Here, we report biochemical evidence that 6-shogaol, the most bioactive component of ginger, inhibits lipopolysaccharide-induced dimerization of TLR4 resulting in the inhibition of NF-κB activation and the expression of cyclooxygenase-2. Furthermore, we demonstrate that 6-shogaol can directly inhibit TLR-mediated signaling pathways at the receptor level. These results suggest that 6-shogaol can modulate TLR-mediated inflammatory responses, which may influence the risk of chronic inflammatory diseases.     

Negative regulation of Toll-like receptor signaling pathway

TLRs are primary sensors of invading pathogens, recognizing conserved microbial molecules and activating signaling pathways that are pivotal to innate and adaptive immune responses. However, a TLR signaling pathway must be tightly controlled because its excessive activation can contribute to the pathogenesis of many human diseases. This review provides a summary of the different mechanisms that are involved in the negative regulation of TLR signaling pathways.     

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.     

Toll-like receptors and innate immunity

Toll-like receptors (TLRs) are evolutionarily conserved innate receptors expressed in various immune and non-immune cells of the mammalian host. TLRs play a crucial role in defending against pathogenic microbial infection through the induction of inflammatory cytokines and type I interferons. Furthermore, TLRs also play roles in shaping pathogen-specific humoral and cellular adaptive immune responses. In this review, we describe the recent advances in pathogen recognition by TLRs and TLR signaling.     

Toll-like receptors in autoimmunity with special reference to systemic lupus erythematosus

The Toll-like receptor (TLR) family plays a fundamental role in host innate immunity by mounting a rapid and potent inflammatory response to pathogen infection. TLRs recognize distinct microbial components and activate intracellular signaling pathways that induce expression of host inflammatory genes. Several studies have indicated that TLRs are implicated in many inflammatory and immune disorders. Extensive research in the past decade to understand TLR-mediated mechanisms of innate immunity has enabled pharmaceutical companies to begin to develop novel therapeutics for the purpose of controlling an inflammatory disease. The roles of TLRs in the development of autoimmune diseases have been studied. TLR7 and TLR9 have key roles in production of autoantibodies and/or in development of systemic autoimmune disease. It remains to be determined their role in apoptosis, in the pathogenesis of RNA containing immune complexes, differential expression of TLRs by T regulatory cells.     

Thematic review series: The immune system and atherogenesis. Paying the price for pathogen protection: toll receptors in atherogenesis

Atherosclerosis is a chronic inflammatory response characterized by the accumulation of cells of innate and acquired immune systems within the intima of the arterial wall. Macrophages are the predominant participant in innate immune responses in atherosclerosis. Protein receptors expressed by macrophages and endothelial cells recognize components and products of microorganisms and play a vital role in innate immunity. In particular, the members of the toll-like receptor (TLR) family play a critical role in the inflammatory components of atherosclerosis. Both exogenous ligands involved in microbial recognition as well as endogenous ligands involved in sterile inflammation pathways are implicated in the pathology of atherosclerosis. In this review, we discuss our current understanding of the role of TLRs and their coactivators in atherosclerosis, with particular emphasis on studies in atherosclerosis-prone hypercholesterolemic mice.     

Dendritic cell subtypes as primary targets of vaccines: the emerging role and cross-talk of pattern recognition receptors

Preventive vaccination is the most successful approach against infectious diseases and has a great impact on world health. Vaccines operate through the activation of innate immunity that helps to stimulate antigen-specific T- and B-lymphocytes. These events are orchestrated by dendritic cells (DCs) that are able to sample foreign structures and concomitantly sense 'danger signals'. Thus, DCs provide a functional link between innate and acquired immunity, and due to their regulatory potential are referred to as natural adjuvants. Human conventional and plasmacytoid DCs express different sets of well-characterized Toll-like membrane receptors (TLRs) that recognize a broad range of conserved molecular patterns of pathogens. The recently discovered cytosolic Nod-like receptors (NLRs) and RIG-like helicases (RLHs) also turned out to participate in pathogen recognition and modulation of immune responses through interacting signaling pathways. As a result of their collaboration, the TLR, NLR and RLH recognition systems induce the secretion of different combinations of cytokines that play a fundamental role in T-cell activation and instruction. Ligands of the innate recognition systems emerge as new adjuvants for vaccine design, whereas manipulation of the signaling pathways mediated by these receptors offers new avenues for fine tuning immune responses and optimizing immunotherapies.     

TLR8: An Innate Immune Receptor in Brain,Neurons and Axons

Toll-like receptors (TLRs) play essential roles in generating innate immune responses, and are evolutionarily conserved across species. In mammals, TLRs specifically recognize the conserved microbial structural motifs referred to as pathogen-associated molecular patterns (PAMPs). Ligand recognition by TLRs activates signaling cascades that culminate in proinflammatory cytokine production and eventual elimination of invading pathogens. Although TLRs in mammals are expressed predominantly in the immune system, certain TLRs with poorly characterized function are also found in neurons. We recently profiled TLR8 expression during mouse brain development and established its localization in neurons and axons. We uncovered a novel role for TLR8 as a suppressor of neurite outgrowth as well as an inducer of neuronal apoptosis, and found that TLR8 functions in neurons through an NF-κB-independent mechanism. These findings add a new layer of complexity for TLR signaling, and expand the realm of mammalian TLR function to the central nervous system (CNS) beyond the originally discovered immune context. Herein, we complement our earlier report with additional data, discuss their biological and mechanistic implications in CNS developmental and pathological processes, and thus further our perspective on TLR signaling and potential physiological roles in mammals.