Critical role of Toll-like receptor signaling in NK cell activation

Toll-like receptors (TLRs) and NK cell receptors are the most important receptor superfamilies in innate immunity. TLRs act as the sensor of external pathogens, while NK cells detect alterations in endogenous protein expression on target cells through activating and inhibitory receptors. Accumulating data has demonstrated that TLRs and NK cell receptors can coordinate and regulate each other during immune responses, which contributes to the initiation of innate response and the priming of adaptive responses. TLRs can activate NK cell function directly or with the help of accessory cells in a cytokine or cell-to-cell contact dependent manner. More understanding of the recognition of innate receptors and interactions between them may provide important insights into the design of effective strategies to combat tumor and microbial infections. In this review, we summarize how TLRs and NK cells discriminate the self or non-self components respectively. And importantly, we pay more attention to the role of TLR sig-naling in induction of NK cell activation, responses and the crosstalk between them.     

Control of B-cell responses by Toll-like receptors

Toll-like receptors (TLRs) detect microbial infection and have an essential role in the induction of immune responses. TLRs can directly induce innate host defence responses, but the mechanisms of TLR-mediated control of adaptive immunity are not fully understood. Although TLR-induced dendritic cell maturation is required for activation of T-helper (T(H)) cells, the role of TLRs in B-cell activation and antibody production in vivo is not yet known. Here we show that activation and differentiation of T(H) cells is not sufficient for the induction of T-dependent B-cell responses. We find that, in addition to CD4+ T-cell help, generation of T-dependent antigen-specific antibody responses requires activation of TLRs in B cells.     

RICK/Rip2/CARDIAK mediates signalling for receptors of the innate and adaptive immune systems

The immune system consists of two evolutionarily different but closely related responses, innate immunity and adaptive immunity. Each of these responses has characteristic receptors-Toll-like receptors (TLRs) for innate immunity and antigen-specific receptors for adaptive immunity. Here we show that the caspase recruitment domain (CARD)-containing serine/threonine kinase Rip2 (also known as RICK, CARDIAK, CCK and Ripk2) transduces signals from receptors of both immune responses. Rip2 was recruited to TLR2 signalling complexes after ligand stimulation. Moreover, cytokine production in Rip2-deficient cells was reduced on stimulation of TLRs with lipopolysaccharide, peptidoglycan and double-stranded RNA, but not with bacterial DNA, indicating that Rip2 is downstream of TLR2/3/4 but not TLR9. Rip2-deficient cells were also hyporesponsive to signalling through interleukin (IL)-1 and IL-18 receptors, and deficient for signalling through Nod proteins-molecules also implicated in the innate immune response. Furthermore, Rip2-deficient T cells showed severely reduced NF-kappaB activation, IL-2 production and proliferation on T-cell-receptor (TCR) engagement, and impaired differentiation to T-helper subtype 1 (TH1) cells, indicating that Rip2 is required for optimal TCR signalling and T-cell differentiation. Rip2 is therefore a signal transducer and integrator of signals for both the innate and adaptive immune systems.     

UNC93B1 delivers nucleotide-sensing toll-like receptors to endolysosomes

Signalling by means of toll-like receptors (TLRs) is essential for the development of innate and adaptive immune responses. UNC93B1, essential for signalling of TLR3, TLR7 and TLR9 in both humans and mice, physically interacts with these TLRs in the endoplasmic reticulum (ER). Here we show that the function of the polytopic membrane protein UNC93B1 is to deliver the nucleotide-sensing receptors TLR7 and TLR9 from the ER to endolysosomes. In dendritic cells of 3d mice, which express an UNC93B1 missense mutant (H412R) incapable of TLR binding, neither TLR7 nor TLR9 exits the ER. Furthermore, the trafficking and signalling defects of the nucleotide-sensing TLRs in 3d dendritic cells are corrected by expression of wild-type UNC93B1. However, UNC93B1 is dispensable for ligand recognition and signal initiation by TLRs. To our knowledge, UNC93B1 is the first protein to be identified as a molecule specifically involved in trafficking of nucleotide-sensing TLRs. By inhibiting the interaction between UNC93B1 and TLRs it should be possible to achieve specific regulation of the nucleotide-sensing TLRs without compromising signalling via the cell-surface-disposed TLRs.     

Programming the magnitude and persistence of antibody responses with innate immunity

Many successful vaccines induce persistent antibody responses that can last a lifetime. The mechanisms by which they do so remain unclear, but emerging evidence indicates that they activate dendritic cells via Toll-like receptors (TLRs). For example, the yellow fever vaccine YF-17D, one of the most successful empiric vaccines ever developed, activates dendritic cells via multiple TLRs to stimulate proinflammatory cytokines. Triggering specific combinations of TLRs in dendritic cells can induce synergistic production of cytokines, which results in enhanced T-cell responses, but its impact on antibody responses remain unknown. Learning the critical parameters of innate immunity that program such antibody responses remains a major challenge in vaccinology. Here we demonstrate that immunization of mice with synthetic nanoparticles containing antigens plus ligands that signal through TLR4 and TLR7 induces synergistic increases in antigen-specific, neutralizing antibodies compared to immunization with nanoparticles containing antigens plus a single TLR ligand. Consistent with this there was enhanced persistence of germinal centres and of plasma-cell responses, which persisted in the lymph nodes for >1.5 years. Surprisingly, there was no enhancement of the early short-lived plasma-cell response relative to that observed with single TLR ligands. Molecular profiling of activated B cells, isolated 7 days after immunization, indicated that there was early programming towards B-cell memory. Antibody responses were dependent on direct triggering of both TLRs on B cells and dendritic cells, as well as on T-cell help. Immunization protected completely against lethal avian and swine influenza virus strains in mice, and induced robust immunity against pandemic H1N1 influenza in rhesus macaques.     

Immunology: Toll-like receptors and antibody responses

Microbial components, such as lipopolysaccharides, augment immune responses by activating Toll-like receptors (TLRs). Some have interpreted this to mean that TLR signalling might not only help to initiate the adaptive immune response, but may also be required for it. The expanded view is shared by Pasare and Medzhitov, who conclude from an analysis of mice deficient in MyD88 (a TLR-signalling adaptor protein) that the generation of T-dependent antigen-specific antibody responses requires activation of TLRs in B cells. However, we show here that robust antibody responses can be elicited even in the absence of TLR signals. This appreciable TLR-independence of immune responses should be taken into account in the rational design of immunogenic and toleragenic vaccines.     

Card9 controls a non-TLR signalling pathway for innate anti-fungal immunity

Fungal infections are increasing worldwide due to the marked rise in immunodeficiencies including AIDS; however, immune responses to fungi are poorly understood. Dectin-1 is the major mammalian pattern recognition receptor for the fungal component zymosan. Dectin-1 represents the prototype of innate non-Toll-like receptors (TLRs) containing immunoreceptor tyrosine-based activation motifs (ITAMs) related to those of adaptive antigen receptors. Here we identify Card9 as a key transducer of Dectin-1 signalling. Although being dispensable for TLR/MyD88-induced responses, Card9 controls Dectin-1-mediated myeloid cell activation, cytokine production and innate anti-fungal immunity. Card9 couples to Bcl10 and regulates Bcl10-Malt1-mediated NF-kappaB activation induced by zymosan. Yet, Card9 is dispensable for antigen receptor signalling that uses Carma1 as a link to Bcl10-Malt1. Thus, our results define a novel innate immune pathway and indicate that evolutionarily distinct ITAM receptors in innate and adaptive immune cells use diverse adaptor proteins to engage selectively the conserved Bcl10-Malt1 module.     

The innate immune response to bacterial flagellin is mediated by Toll-like receptor 5

The innate immune system recognizes pathogen-associated molecular patterns (PAMPs) that are expressed on infectious agents, but not on the host. Toll-like receptors (TLRs) recognize PAMPs and mediate the production of cytokines necessary for the development of effective immunity. Flagellin, a principal component of bacterial flagella, is a virulence factor that is recognized by the innate immune system in organisms as diverse as flies, plants and mammals. Here we report that mammalian TLR5 recognizes bacterial flagellin from both Gram-positive and Gram-negative bacteria, and that activation of the receptor mobilizes the nuclear factor NF-kappaB and stimulates tumour necrosis factor-alpha production. TLR5-stimulating activity was purified from Listeria monocytogenes culture supernatants and identified as flagellin by tandem mass spectrometry. Expression of L. monocytogenes flagellin in non-flagellated Escherichia coli conferred on the bacterium the ability to activate TLR5, whereas deletion of the flagellin genes from Salmonella typhimurium abrogated TLR5-stimulating activity. All known TLRs signal through the adaptor protein MyD88. Mice challenged with bacterial flagellin rapidly produced systemic interleukin-6, whereas MyD88-null mice did not respond to flagellin. Our data suggest that TLR5, a member of the evolutionarily conserved Toll-like receptor family, has evolved to permit mammals specifically to detect flagellated bacterial pathogens.     

Toll样受体2的研究进展

Toll样受体家族（TOU likereceptors,TLRs）是先天性免疫系统进化过程中形成的非常保守的模式识别受体家族,Toll样受体2（T011-likereceptors2,TLR2）是已经克隆的Toll样受体家族中表达范围最广,识别病原微生物种类最多的成员。它可单独或协同其他Toll样受体家族成员完成对病原体相关分子模式的识别,触发机体对致病微生物的级联免疫应答,尤其是针对细胞毒素的抗炎症反应具有重要的作用,已经成为多种疾病治疗的新靶点。文章对N-SL动物TLR2的分布,结构特征,配体识别,信号转导及其生物学功能的最新研究进展进行了综述。     

Structural basis for signal transduction by the Toll/interleukin-1 receptor domains

Toll-like receptors (TLRs) and the interleukin-1 receptor superfamily (IL-1Rs) are integral to both innate and adaptive immunity for host defence. These receptors share a conserved cytoplasmic domain, known as the TIR domain. A single-point mutation in the TIR domain of murine TLR4 (Pro712His, the Lps(d) mutation) abolishes the host immune response to lipopolysaccharide (LPS), and mutation of the equivalent residue in TLR2, Pro681His, disrupts signal transduction in response to stimulation by yeast and gram-positive bacteria. Here we report the crystal structures of the TIR domains of human TLR1 and TLR2 and of the Pro681His mutant of TLR2. The structures have a large conserved surface patch that also contains the site of the Lps(d) mutation. Mutagenesis and functional studies confirm that residues in this surface patch are crucial for receptor signalling. The Lps(d) mutation does not disturb the structure of the TIR domain itself. Instead, structural and functional studies indicate that the conserved surface patch may mediate interactions with the down-stream MyD88 adapter molecule, and that the Lps(d) mutation may abolish receptor signalling by disrupting this recruitment.     

The adaptor molecule TIRAP provides signalling specificity for Toll-like receptors

Mammalian Toll-like receptors (TLRs) function as sensors of infection and induce the activation of innate and adaptive immune responses. Upon recognizing conserved pathogen-associated molecular products, TLRs activate host defence responses through their intracellular signalling domain, the Toll/interleukin-1 receptor (TIR) domain, and the downstream adaptor protein MyD88 (refs 1-3). Although members of the TLR and the interleukin-1 (IL-1) receptor families all signal through MyD88, the signalling pathways induced by individual receptors differ. TIRAP, an adaptor protein in the TLR signalling pathway, has been identified and shown to function downstream of TLR4 (refs 4, 5). Here we report the generation of mice deficient in the Tirap gene. TIRAP-deficient mice respond normally to the TLR5, TLR7 and TLR9 ligands, as well as to IL-1 and IL-18, but have defects in cytokine production and in activation of the nuclear factor NF-kappaB and mitogen-activated protein kinases in response to lipopolysaccharide, a ligand for TLR4. In addition, TIRAP-deficient mice are also impaired in their responses to ligands for TLR2, TLR1 and TLR6. Thus, TIRAP is differentially involved in signalling by members of the TLR family and may account for specificity in the downstream signalling of individual TLRs.     

Innate immune recognition and regulation in liver injury: A brief report from a series of studies

The discovery of innate immune receptors and the emergence of liver immunology （high content of NK and NKT cells in liver） led to the second research summit in innate immunity since the finding of NK cells in the middle 1970s. Liver disease is one of the most dangerous threats to humans, and the progress in innate immunology and liver immunology made it possible to re-explain the cellular and mo- lecular immune mechanisms of liver disease. In the past ten years, we have found that innate recognition of hepatic NK and NKT subsets were involved in murine liver injury. We established a novel NK cell-dependent acute murine hepatitis model by activating Toll-like receptor-3 （TLR-3） with an injection of poly I：C, which may mimic mild viral hepatitis （such as Chronic Hepatitis B）. We observed that a network of innate immune cells including NK, NKT and Kupffer cells is involved in liver immune injury in our established NK cell-dependent murine,model. We noted that TLR-3 on Kupffer cells activated by pretreatment with poly I： C might protect against bacterial toxin （LPS）-induced fulminant hepatitis by down-regulating TLR-4 function, while TLR-3 pre-activation of NK cells might reduce Con A-induced NKT cell-mediated fulminant hepatitis by blocking NKT cell recruitment to the liver. We also found that the oversensitivity to injury by immune stimulation in HBV （hepatitis B virus） transgenic mice （full HBV gene-tg or HBs-tg） correlated to the over-expression of Real, an NKG2D （natural killer cell group 2D） ligand of NK cells or CDld, a ligand of TCR-V14 of NKT cells, on HBV＋ hepatocytes, which leads to an innate immune response against hepatocytes and is critical in liver immune injury and regeneration.     

Critical role of TRAF3 in the Toll-like receptor-dependent and -independent antiviral response

Type I interferon (IFN) production is a critical component of the innate defence against viral infections. Viral products induce strong type I IFN responses through the activation of Toll-like receptors (TLRs) and intracellular cytoplasmic receptors such as protein kinase R (PKR). Here we demonstrate that cells lacking TRAF3, a member of the TNF receptor-associated factor family, are defective in type I IFN responses activated by several different TLRs. Furthermore, we show that TRAF3 associates with the TLR adaptors TRIF and IRAK1, as well as downstream IRF3/7 kinases TBK1 and IKK-epsilon, suggesting that TRAF3 serves as a critical link between TLR adaptors and downstream regulatory kinases important for IRF activation. In addition to TLR stimulation, we also show that TRAF3-deficient fibroblasts are defective in their type I IFN response to direct infection with vesicular stomatitis virus, indicating that TRAF3 is also an important component of TLR-independent viral recognition pathways. Our data demonstrate that TRAF3 is a major regulator of type I IFN production and the innate antiviral response.     

Recognition of double-stranded RNA and activation of NF-kappaB by Toll-like receptor 3

Toll-like receptors (TLRs) are a family of innate immune-recognition receptors that recognize molecular patterns associated with microbial pathogens, and induce antimicrobial immune responses. Double-stranded RNA (dsRNA) is a molecular pattern associated with viral infection, because it is produced by most viruses at some point during their replication. Here we show that mammalian TLR3 recognizes dsRNA, and that activation of the receptor induces the activation of NF-kappaB and the production of type I interferons (IFNs). TLR3-deficient (TLR3-/-) mice showed reduced responses to polyinosine-polycytidylic acid (poly(I:C)), resistance to the lethal effect of poly(I:C) when sensitized with d-galactosamine (d-GalN), and reduced production of inflammatory cytokines. MyD88 is an adaptor protein that is shared by all the known TLRs. When activated by poly(I:C), TLR3 induces cytokine production through a signalling pathway dependent on MyD88. Moreover, poly(I:C) can induce activation of NF-kappaB and mitogen-activated protein (MAP) kinases independently of MyD88, and cause dendritic cells to mature.     

Helper T cells regulate type-2 innate immunity in vivo

Type-2 immunity requires orchestration of innate and adaptive immune responses to protect mucosal sites from pathogens. Dysregulated type-2 responses result in allergy or asthma. T helper 2 (T(H)2) cells elaborate cytokines, such as interleukin (IL)-4, IL-5, IL-9 and IL-13, which work with toxic mediators of innate immune cells to establish environments that are inhospitable to helminth or arthropod invaders. The importance of T(H)2 cells in coordinating innate immune cells at sites of inflammation is not known. Here we show that polarized type-2 immune responses are initiated independently of adaptive immunity. In the absence of B and T cells, IL-4-expressing eosinophils were recruited to tissues of mice infected with the helminth Nippostrongylus brasiliensis, but eosinophils failed to degranulate. Reconstitution with CD4 T cells promoted accumulation of degranulated IL-4-expressing cells, but only if T cells were stimulated with cognate antigen. Degranulation correlated with tissue destruction, which was attenuated if eosinophils were depleted. Helper T cells confer antigen specificity on eosinophil cytotoxicity, but not cytokine responses, so defining a novel mechanism that focuses tissue injury at sites of immune challenge.     

Distinct molecular mechanism for initiating TRAF6 signalling

Tumour-necrosis factor (TNF) receptor-associated factor 6 (TRAF6) is the only TRAF family member that participates in signal transduction of both the TNF receptor (TNFR) superfamily and the interleukin-1 receptor (IL-1R)/Toll-like receptor (TLR) superfamily; it is important for adaptive immunity, innate immunity and bone homeostasis. Here we report crystal structures of TRAF6, alone and in complex with TRAF6-binding peptides from CD40 and TRANCE-R (also known as RANK), members of the TNFR superfamily, to gain insight into the mechanism by which TRAF6 mediates several signalling cascades. A 40 degrees difference in the directions of the bound peptides in TRAF6 and TRAF2 shows that there are marked structural differences between receptor recognition by TRAF6 and other TRAFs. The structural determinant of the petide TRAF6 interaction reveals a Pro-X-Glu-X-X-(aromatic/acidic residue) TRAF6-binding motif, which is present not only in CD40 and TRANCE-R but also in the three IRAK adapter kinases for IL-1R/TLR signalling. Cell-permeable peptides with the TRAF6-binding motif inhibit TRAF6 signalling, which indicates their potential as therapeutic modulators. Our studies identify a universal mechanism by which TRAF6 regulates several signalling cascades in adaptive immunity, innate immunity and bone homeostasis.     

Activation of the innate immune receptor Dectin-1 upon formation of a 'phagocytic synapse'

Innate immune cells must be able to distinguish between direct binding to microbes and detection of components shed from the surface of microbes located at a distance. Dectin-1 (also known as CLEC7A) is a pattern-recognition receptor expressed by myeloid phagocytes (macrophages, dendritic cells and neutrophils) that detects β-glucans in fungal cell walls and triggers direct cellular antimicrobial activity, including phagocytosis and production of reactive oxygen species (ROS). In contrast to inflammatory responses stimulated upon detection of soluble ligands by other pattern-recognition receptors, such as Toll-like receptors (TLRs), these responses are only useful when a cell comes into direct contact with a microbe and must not be spuriously activated by soluble stimuli. In this study we show that, despite its ability to bind both soluble and particulate β-glucan polymers, Dectin-1 signalling is only activated by particulate β-glucans, which cluster the receptor in synapse-like structures from which regulatory tyrosine phosphatases CD45 and CD148 (also known as PTPRC and PTPRJ, respectively) are excluded (Supplementary Fig. 1). The 'phagocytic synapse' now provides a model mechanism by which innate immune receptors can distinguish direct microbial contact from detection of microbes at a distance, thereby initiating direct cellular antimicrobial responses only when they are required.     

Chromatin-IgG complexes activate B cells by dual engagement of IgM and Toll-like receptors

Autoreactive B cells are present in the lymphoid tissues of healthy individuals, but typically remain quiescent. When this homeostasis is perturbed, the formation of self-reactive antibodies can have serious pathological consequences. B cells expressing an antigen receptor specific for self-immunoglobulin-gamma (IgG) make a class of autoantibodies known as rheumatoid factor (RF). Here we show that effective activation of RF+ B cells is mediated by IgG2a-chromatin immune complexes and requires the synergistic engagement of the antigen receptor and a member of the MyD88-dependent Toll-like receptor (TLR) family. Inhibitor studies implicate TLR9. These data establish a critical link between the innate and adaptive immune systems in the development of systemic autoimmune disease and explain the preponderance of autoantibodies reactive with nucleic acid-protein particles. The unique features of this dual-engagement pathway should facilitate the development of therapies that specifically target autoreactive B cells.