Identification of Lps2 as a key transducer of MyD88-independent TIR signalling

p38 MAPK activation controls the TLR3-mediated up-regulation of cytotoxicity and cytokine production in human NK cells

Natural killer (NK) cells are a component of the innate immunity against viral infections through their rapid cytotoxic activity and cytokine production. Although the synthetic double-stranded (ds) RNA polyinosinic-polycytidylic acid (poly I:C), a mimic of a common product of viral infections, is known to rapidly up-regulate their in vivo functions, NK cell ability to directly respond to dsRNA is still mostly unknown. Our results show that treatment with poly I:C significantly up-regulates both natural and CD16-mediated cytotoxicity of highly purified human NK cells. Poly I:C also induces the novel capability of producing CXCL10 chemokine in human NK cells and synergistically enhances interferon-gamma (IFN-gamma) production induced by either adaptive or innate cytokines. In accordance with the expression of Toll-like receptor-3 (TLR3) and of TRIF/TICAM-1 adaptor, poly I:C stimulation induces the activation of interferon regulatory factor-3 (IRF-3) transcription factor and of p38 mitogen-activated protein kinase (MAPK) in human NK cells. Finally, we demonstrate that p38 MAPK activity is required for the dsRNA-dependent enhancement of cytotoxicity and CXCL10 production. The occurrence of dsRNA-induced signaling and functional events closely correlates with the TLR3 mRNAprofile in different NK cell populations. Taken together, these data identify p38 as a central component of NK cell ability to directly respond to dsRNA pathogen-associated molecular pattern (PAMP).

SHIP, TGF-β, and Endotoxin Tolerance

Research into the biology of endotoxin (aka lipopolysaccharide; LPS) began well over 100 years ago and has had many unexpected turns. The phenomenon of endotoxin tolerance was among these and has engendered immense curiosity over the years. Sly and colleagues (this issue of Immunity) have taken an important step toward understanding the phenomenon of endotoxin tolerance and have enhanced our comprehension of LPS signaling as a whole by demonstrating that the cytosolic phosphatase SHIP is required to permit the development of an LPS-refractory state in macrophages and mice.

IL-4-dependent Th2 collateral priming to inhaled antigens independent of Toll-like receptor 4 and myeloid differentiation factor 88

Allergic asthma is an inflammatory lung disease thought to be initiated and directed by type 2 helper T cells responding to environmental Ags. The mechanisms by which allergens induce Th2-adaptive immune responses are not well understood, although it is now clear that innate immune signals are required to promote DC activation and Th2 sensitization to inhaled proteins. However, the effect of ongoing Th2 inflammation, as seen in chronic asthma, on naive lymphocyte activation has not been explored. It has been noted that patients with atopic disorders demonstrate an increased risk of developing sensitivities to new allergens. This suggests that signals from an adaptive immune response may facilitate sensitization to new Ags. We used a Th2-adoptive transfer murine model of asthma to identify a novel mechanism, termed "collateral priming," in which naive CD4(+) T cells are activated by adaptive rather than innate immune signals. Th2 priming to newly encountered Ags was dependent on the production of IL-4 by the transferred Th2 population but was independent of Toll-like receptor 4 signaling and the myeloid differentiation factor 88 Toll-like receptor signaling pathway. These results identify a novel mechanism of T cell priming in which an Ag-specific adaptive immune response initiates distinct Ag-specific T cell responses in the absence of classical innate immune system triggering signals.

The role of interleukin 1 receptor-associated kinase-4 (IRAK-4) kinase activity in IRAK-4-mediated signaling

Interleukin 1 receptor (IL-1R)-associated kinase-4 (IRAK-4) is required for various responses induced by IL-1R and Toll-like receptor signals. However, the molecular mechanism of IRAK-4 signaling and the role of its kinase activity have remained elusive. In this report, we demonstrate that IRAK-4 is recruited to the IL-1R complex upon IL-1 stimulation and is required for the recruitment of IRAK-1 and its subsequent activation/degradation. By reconstituting IRAK-4-deficient cells with wild type or kinase-inactive IRAK-4, we show that the kinase activity of IRAK-4 is required for the optimal transduction of IL-1-induced signals, including the activation of IRAK-1, NF-kappaB, and JNK, and the maximal induction of inflammatory cytokines. Interestingly, we also discover that the IRAK-4 kinase-inactive mutant is still capable of mediating some signals. These results suggest that IRAK-4 is an integral part of the IL-1R signaling cascade and is capable of transmitting signals both dependent on and independent of its kinase activity.

Inferences, questions and possibilities in Toll-like receptor signalling

The Toll-like receptors (TLRs) are the key proteins that allow mammals--whether immunologically naive or experienced--to detect microbes. They lie at the core of our inherited resistance to disease, initiating most of the phenomena that occur in the course of infection. Quasi-infectious stimuli that have been used for decades to study inflammatory mechanisms can activate the TLR family of proteins. And it now seems that many inflammatory processes, both sterile and infectious, may depend on TLR signalling. We are in a good position to apply our understanding of TLR signalling to a range of challenges in immunology and medicine.

A Toll-like receptor 2 ligand stimulates Th2 responses in vivo, via induction of extracellular signal-regulated kinase mitogen-activated protein kinase and c-Fos in dendritic cells

The adaptive immune system can generate distinct classes of responses, but the mechanisms that determine this are poorly understood. In this study, we demonstrate that different Toll-like receptor (TLR) ligands induce distinct dendritic cell (DC) activation and immune responses in vivo. Thus, Escherichia coli LPS (TLR-4 stimulus), activates DCs to produce abundant IL-12(p70), but little IL-10, and stimulates Th1 and Tc1 responses. In contrast, Pam-3-cys (TLR-2 stimulus) elicits less IL-12(p70), but abundant IL-10, and favors Th2 and T cytotoxic 2 (Tc2) responses. These distinct responses likely occur via differences in extracellular signal-regulated kinase signaling in DCs. Thus, Pam-3-cys induces enhanced extracellular signal-regulated kinase signaling, compared with LPS, resulting in suppressed IL-12(p70) and enhanced IL-10 production, as well as enhanced induction of the transcription factor, c-Fos. Interestingly, DCs from c-fos(-/-) mice produce more IL-12(p70), but less IL-10, compared with control DCs. Therefore, different TLR ligands induce distinct cytokines and signaling in DCs, and differentially bias Th responses in vivo.

Toll-like receptor 3: a link between toll-like receptor, interferon and viruses

Production of type I interferon (IFN-alpha/beta) by virus-infected cells is the central event in their antiviral immune responses. In mammalian cells, IFN-alpha/beta gene transcription is induced through distinct signaling pathways by viral infection or by treatment with double-stranded (ds) RNA, which is an intermediate of virus replication. Toll-like receptor 3 (TLR3) was found to recognize dsRNA and transmit signals to activate NF-kappaB and the IFN-beta promoter. Recent identification of the TLR3-adaptor protein and its downstream signaling molecules, which are involved in IFN-alpha/beta production, revealed a novel IFN-inducing pathway for an anti-viral immune response. Here, we summarize the current knowledge of TLR3-mediated immune responses.

Poxvirus protein N1L targets the I-kappaB kinase complex, inhibits signaling to NF-kappaB by the tumor necrosis factor superfamily of receptors, and inhibits NF-kappaB and IRF3 signaling by toll-like receptors

Poxviruses encode proteins that suppress host immune responses, including secreted decoy receptors for pro-inflammatory cytokines such as interleukin-1 (IL-1) and the vaccinia virus proteins A46R and A52R that inhibit intracellular signaling by members of the IL-1 receptor (IL-1R) and Toll-like receptor (TLR) family. In vivo, the TLRs mediate the innate immune response by serving as pathogen recognition receptors, whose oligomerized intracellular Toll/IL-1 receptor (TIR) domains can initiate innate immune signaling. A family of TIR domain-containing adapter molecules transduces signals from engaged receptors that ultimately activate NF-kappaB and/or interferon regulatory factor 3 (IRF3) to induce pro-inflammatory cytokines. Data base searches detected a significant similarity between the N1L protein of vaccinia virus and A52R, a poxvirus inhibitor of TIR signaling. Compared with other poxvirus virulence factors, the poxvirus N1L protein strongly affects virulence in vivo; however, the precise target of N1L was previously unknown. Here we show that N1L suppresses NF-kappaB activation following engagement of Toll/IL-1 receptors, tumor necrosis factor receptors, and lymphotoxin receptors. N1L inhibited receptor-, adapter-, TRAF-, and IKK-alpha and IKK-beta-dependent signaling to NF-kappaB. N1L associated with several components of the multisubunit I-kappaB kinase complex, most strongly associating with the kinase, TANK-binding kinase 1 (TBK1). Together these findings are consistent with the hypothesis that N1L disrupts signaling to NF-kappaB by Toll/IL-1Rs and TNF superfamily receptors by targeting the IKK complex for inhibition. Furthermore, N1L inhibited IRF3 signaling, which is also regulated by TBK1. These studies define a role for N1L as an immunomodulator of innate immunity by targeting components of NF-kappaB and IRF3 signaling pathways.

The roles of two IkappaB kinase-related kinases in lipopolysaccharide and double stranded RNA signaling and viral infection

Viral infection and stimulation with lipopolysaccharide (LPS) or double stranded RNA (dsRNA) induce phosphorylation of interferon (IFN) regulatory factor (IRF)-3 and its translocation to the nucleus, thereby leading to the IFN-beta gene induction. Recently, two IkappaB kinase (IKK)-related kinases, inducible IkappaB kinase (IKK-i) and TANK-binding kinase 1 (TBK1), were suggested to act as IRF-3 kinases and be involved in IFN-beta production in Toll-like receptor (TLR) signaling and viral infection. In this work, we investigated the physiological roles of these kinases by gene targeting. TBK1-deficient embryonic fibroblasts (EFs) showed dramatic decrease in induction of IFN-beta and IFN-inducible genes in response to LPS or dsRNA as well as after viral infection. However, dsRNA-induced expression of these genes was residually detected in TBK1-deficient cells and intact in IKK-i-deficient cells, but completely abolished in IKK-i/TBK1 doubly deficient cells. IRF-3 activation, in response not only to dsRNA but also to viral infection, was impaired in TBK1-deficient cells. Together, these results demonstrate that TBK1 as well as, albeit to a lesser extent, IKK-i play a crucial role in the induction of IFN-beta and IFN-inducible genes in both TLR-stimulated and virus-infected EFs.

The RNA helicase RIG-I has an essential function in double-stranded RNA-induced innate antiviral responses

Intracellular double-stranded RNA (dsRNA) is a chief sign of replication for many viruses. Host mechanisms detect the dsRNA and initiate antiviral responses. In this report, we identify retinoic acid inducible gene I (RIG-I), which encodes a DExD/H box RNA helicase that contains a caspase recruitment domain, as an essential regulator for dsRNA-induced signaling, as assessed by functional screening and assays. A helicase domain with intact ATPase activity was responsible for the dsRNA-mediated signaling. The caspase recruitment domain transmitted 'downstream' signals, resulting in the activation of transcription factors NF-kappaB and IRF-3. Subsequent gene activation by these factors induced antiviral functions, including type I interferon production. Thus, RIG-I is key in the detection and subsequent eradication of the replicating viral genomes.

Does Toll-like receptor 3 play a biological role in virus infections?

The Toll-like receptor (TLR) family functions to recognize conserved microbial and viral structures with the purpose of activating signal pathways to instigate immune responses against infections by these organisms. For example, in vitro studies reveal that the TLR3 ligand is a double-stranded RNA (dsRNA), a product of viral infections. From this observation, it has been proposed that TLR3 is likely an important first signal for virus infections. We approached this issue by investigating the role of TLR3 in four different infectious viral models (lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus (VSV), murine cytomegalovirus (MCMV), and reovirus) and in TLR3 genetically deficient ((-/-)) mice. Our results indicate that TLR3 is not universally required for the generation of effective antiviral responses because the absence of TLR3 does not alter either viral pathogenesis or impair host's generation of adaptive antiviral responses to these viruses.

Lipopolysaccharide and double-stranded RNA up-regulate toll-like receptor 2 independently of myeloid differentiation factor 88

Toll-like receptor 2 (TLR2) is a signaling receptor for a variety of microbial products, including bacterial lipoproteins and peptidoglycan, and is central in initiating immune responses toward Gram-positive bacteria, spirochetes, and mycobacteria. The mechanisms behind regulation of TLR2 protein expression are still not well understood. By using a newly developed monoclonal antibody against mouse TLR2, we detected TLR2 protein expression on macrophages, neutrophils, and dendritic cells. Endogenous macrophage TLR2 localized mostly to the cell membrane, with particular accumulation around phagosomes containing zymosan. Treatment of macrophages with the TLR2 antibody diminished cellular response to lipoproteins and down-regulated membrane TLR2. Marked up-regulation of surface TLR2 was observed on macrophages in response to whole bacteria, lipoproteins, lipopolysaccharide, poly(I-C) (double-stranded RNA), R848, and CpG DNA, and this up-regulation appeared to be a very sensitive marker for the presence of microbial products. Up-regulation of TLR2 in response to stimuli correlated with an increased response to secondary lipoprotein exposure following a low concentration of primary lipoprotein challenge. By comparison, exposure to a larger primary challenge induced a hyporeactive state. Most interestingly, lipopolysaccharide- and double-stranded RNA-induced up-regulation of surface TLR2 in macrophages was found to be MyD88-independent, whereas the up-regulation in response to lipoproteins, R848, and CpG DNA was absent in MyD88-deficient cells. We conclude that complex mechanisms regulate expression and signaling via TLR2. Up-regulation of TLR2 in the presence of low, yet clinically relevant amounts of microbial products may be an important mechanism by which the immune system boosts its response to a beginning infection.

Forward genetic dissection of afferent immunity: the role of TIR adapter proteins in innate and adaptive immune responses

The innate immune system senses pathogens largely through signals initiated by proteins known as 'Toll-like receptors' (TLRs), of which ten representatives are known to be encoded in the human genome. The understanding of the biochemical circuitry that maintains the innate capacity for immune recognition and response has loomed as a major hurdle in immunology. A total of five adapter proteins with cytoplasmic domain homology to the TLRs are known to exist in mammals. These proteins show preferential association with individual TLR family members, giving a particular character to the signals that distinct microorganisms initiate, and also initiate the adaptive immune response. The adaptive immune response is dependent upon upregulation of costimulatory molecules (UCM) such as CD80 and CD86. Forward genetic analysis has revealed that this upregulation depends upon an adapter encoded by a locus known as Lps2, and upon type I interferon receptor signaling.

Modulating vaccine responses with dendritic cells and Toll-like receptors

The immune system is ignorant or even unresponsive to most foreign proteins that are injected in a soluble, deaggregated form, but when injected together with an immune-stimulating agent (i.e. an adjuvant, such as CpG-rich DNA), these foreign proteins can generate robust immunity and long-lived memory to the antigen. In fact, the nature of the adjuvant is what determines the particular type of immune response that follows, which may be biased towards cytotoxic T-cell responses, antibody responses, particular classes of T-helper responses, or antibody isotypes. Clearly, the ability of a vaccine to skew the response toward a particular type is of paramount importance, because different pathogens require distinct types of protective immunities. Therefore, the quest to manipulate the immune system to generate optimally effective immunity against different pathogens can justifiably be considered the 'grand challenge' of modern immunology. Central to this issue is a rare but widely distributed network of cells known as dendritic cells (DCs). DCs, which have been called 'Nature's adjuvants,' express pathogen recognition receptors, such as the Toll-like receptors (TLRs) and C-type lectins, which enable them to sense and respond to microbes or vaccines. Research in the last decade has demonstrated a fundamental role for DCs in initiating and controlling the quality and strength of the immune response. As such, DCs and TLRs represent attractive immune modulatory targets for vaccinologists. The present review provides a summary of emerging themes in the biology DCs and TLRs, with a particular focus on relevance for vaccine development.

Toll-like receptors: function and roles in lung disease

Toll-like receptor (TLR) proteins have been shown to play a pivotal role in both innate and adaptive immune responses in higher vertebrates. TLR proteins enable the host to recognize a large number of pathogen-associated molecular patterns such as bacterial lipopolysaccharides, viral RNA, CpG-containing DNA, and flagellin, among others. Engagement of TLR proteins leads to the upregulation of costimulatory molecules and proinflammatory cytokines, as well as reactive nitrogen and oxygen products. The role of TLR proteins in lung-associated pathologies such as airway hyperreactivity, allergic asthma, and tuberculosis is being intensively studied. This review summarizes many of the findings made to date on the roles of TLR proteins in a variety of lung diseases. Generally, TLR proteins serve a protective role in infectious diseases, such as tuberculosis. The progression of chronic inflammatory lung diseases, such as allergic asthma, can also be influenced by TLR-dependent responses.

Requirement for a conserved Toll/interleukin-1 resistance domain protein in the Caenorhabditis elegans immune response

The p38 mitogen-activated protein kinase pathway regulates innate immune responses in evolutionarily diverse species. We have previously shown that the Caenorhabditis elegans p38 mitogen-activated protein kinase, PMK-1, functions in an innate immune response pathway that mediates resistance to a variety of microbial pathogens. Here, we show that tir-1, a gene encoding a highly conserved Toll/IL-1 resistance (TIR) domain protein, is also required for C. elegans resistance to microbial pathogens. RNA interference inactivation of tir-1 resulted in enhanced susceptibility to killing by pathogens and correspondingly diminished PMK-1 phosphorylation. Unlike all known TIR-domain adapter proteins, overexpression of the human TIR-1 homologue, SARM, in mammalian cells was not sufficient to induce expression of NF-kappaB or IRF3-dependent reporter genes that are activated by Toll-like receptor signaling. These data reveal the involvement of a previously uncharacterized, evolutionarily conserved TIR domain protein in innate immunity that is functionally distinct from other known TIR domain signaling adapters.

TIR domain-containing adaptors define the specificity of TLR signaling

The concept that Toll-like receptors (TLRs) recognize specific molecular patterns in various pathogens has been established. In signal transduction via TLRs, MyD88, which harbors a Toll/IL-1 receptor (TIR)-domain and a death domain, has been shown to link between TLRs and MyD88-dependent downstream events leading to proinflammatory cytokine production and splenocyte proliferation. However, recent studies using MyD88-deficient mice have revealed that some TLRs possess a MyD88-independent pathway, which is represented by interferon (IFN)-beta production induced by LPS stimulation. This indicates that additional signaling molecules other than MyD88 exist in the TLR signaling pathway. Indeed, two additional TIR domain-containing adaptors, TIRAP/Mal and TRIF, have recently been identified. Both define the specific biological responses of each TLR.

RIP1 is an essential mediator of Toll-like receptor 3-induced NF-kappa B activation

Stimulation of Toll-like receptors (TLRs) initiates potent innate immune responses through Toll-interleukin 1 receptor (TIR) domain-containing adaptors such as MyD88 and Trif. Analysis of Trif-deficient mice has shown that TLR3-dependent activation of the transcription factor NF-kappa B by the TLR3 ligand double-stranded RNA is Trif dependent. Here we investigated the 'downstream' signaling events that regulate TLR3-dependent Trif-induced NF-kappa B activation. Trif recruited the kinases receptor interacting protein (RIP)-1 and RIP3 through its RIP homotypic interaction motif. In the absence of RIP1, TLR3-mediated signals activating NF-kappa B, but not the kinase JNK or interferon-beta, were abolished, suggesting that RIP1 mediates Trif-induced NF-kappa B activation. In contrast, the presence of RIP3 negatively regulated the Trif-RIP1-induced NF-kappa B pathway. Therefore, in contrast to other TLRs, which use interleukin 1 receptor-associated kinase (IRAK) proteins to activate NF-kappa B, TLR 3-induced NF-kappa B activation is dependent on RIP kinases.

Microbial recognition by Toll-like receptors

Toll-like receptors (TLRs) sense invasion of microorganisms by detecting microbial components that are conserved among pathogens. Recognition of microbial components by TLRs triggers activation of the innate immune system. Signaling pathways via TLRs originate from conserved cytoplasmic Toll/IL-1 receptor (TIR) domains. Recent accumulating evidence demonstrates that TIR domain-containing adaptors, such as MyD88, TIRAP/Mal, TRIF, and TRAM, regulate TLR-mediated signaling pathways. MyD88 is common to all TLR-mediated pathways, which lead to the production of inflammatory cytokines, whereas TRIF mediates induction of IFN-beta in TLR3 and TLR4 signaling pathways. TIRAP/Mal is implicated in the TLR2- and TLR4-mediated MyD88-dependent signaling pathway. TRAM is specifically involved in the TLR4-mediated TRIF-dependent pathway. Thus, TIR domain-containing adaptors play a pivotal role in TLR signaling pathways, which culminate in pathogen-specific immune responses.

Recognition of single-stranded RNA viruses by Toll-like receptor 7

Viral infection of mammalian host results in the activation of innate immune responses. Toll-like receptors (TLRs) have been shown to mediate the recognition of many types of pathogens, including viruses. The genomes of viruses possess unique characteristics that are not found in mammalian genomes, such as high CpG content and double-stranded RNA. These genomic nucleic acids serve as molecular signatures associated with viral infections. Here we show that TLR7 recognizes the single-stranded RNA viruses, vesicular stomatitis virus and influenza virus. The recognition of these viruses by plasmacytoid dendritic cells and B cells through TLR7 results in their activation of costimulatory molecules and production of cytokines. Moreover, this recognition required intact endocytic pathways. Mice deficient in either the TLR7 or the TLR adaptor protein MyD88 demonstrated reduced responses to in vivo infection with vesicular stomatitis virus. These results demonstrate microbial ligand recognition by TLR7 and provide insights into the pathways used by the innate immune cells in the recognition of viral pathogens.

Toll-Like Receptors Are Temporally Involved in Host Defense

Toll-like receptors (TLRs) are evolutionarily conserved proteins that recognize microbial molecules and initiate host defense. To investigate how TLRs work together to fight infections, we tested the role of TLRs in host defense against the Gram-negative bacterial pathogen, Salmonella. We show that TLR4 is critical for early cytokine production and killing of bacteria by murine macrophages. Interestingly, later on, TLR2, but not TLR4, is required for macrophage responses. Myeloid differentiation factor 88, an adaptor protein directly downstream of TLRs, is required for both early and late responses. TLR4, TLR2, and myeloid differentiation factor 88 are involved in murine host defense against Salmonella in vivo, which correlates with the defects in host defense observed in vitro. We propose a model where the sequential activation of TLRs tailors the immune response to different microbes.

Distinct pathways of LPS-induced NF-κB activation and cytokine production in human myeloid and nonmyeloid cells defined by selective utilization of MyD88 and Mal/TIRAP

How lipopolysaccharide (LPS) signals through toll-like receptors (TLRs) to induce nuclear factor (NF)–κB and inflammatory cytokines in sepsis remains unclear. Major candidates for that process are myeloid differentiation protein 88 (MyD88) and MyD88 adaptor-like/TIR domain-containing adaptor protein (Mal/TIRAP) but their role needs to be further defined. Here, we have examined the role of MyD88 and Mal/TIRAP in primary human cells of nonmyeloid and myeloid origin as physiologically relevant systems. We found that MyD88 and Mal/TIRAP are essential for LPS-induced IκBα phosphorylation, NF-κB activation, and interleukin 6 (IL-6) or IL-8 production in fibroblasts and endothelial cells in a pathway that also requires IKK2. In contrast, in macrophages neither MyD88, Mal/TIRAP, nor IκB kinase 2 (IKK2) are required for NF-κB activation or tumor necrosis factor α (TNFα), IL-6, or IL-8 production, although Mal/TIRAP is still involved in the production of interferon β (IFNβ). Differential usage of TLRs may account for that, as in macrophages but not fibroblasts or endothelial cells, TLR4 is expressed in high levels at the cell surface, and neutralization of TLR4 but not TLR2 blocks LPS signaling. These observations demonstrate for the first time the existence of 2 distinct pathways of LPS-induced NF-κB activation and cytokine production in human myeloid and nonmyeloid cells defined by selective utilization of TLR4, MyD88, Mal/TIRAP, and IKK2, and reveal a layer of complexity not previously expected.

The role of Toll-like receptor 4 in environmental airway injury in mice

Inhalation of toxins commonly found in air pollution contributes to the development and progression of asthma and environmental airway injury. In this study, we investigated the requirement of toll-like receptor 4 (TLR4) in mice for pulmonary responses to three environmental toxins: aerosolized lipopolysaccharide, particulate matter (residual oil fly ash), and ozone. The physiologic and biologic responses to these toxins were evaluated by the extent of airway responsiveness, neutrophil recruitment to the lower respiratory tract, changes in inflammatory cytokines, and the concentration of protein in the lavage fluid. Genetically engineered, TLR4-deficient mice (C57BL/6(TLR4-/-)) were unresponsive to inhaled lipopolysaccharide, except for minimal increases in some inflammatory cytokines. In contrast, C57BL/6(TLR4-/-) mice did not differ from wild-type mice in their airway response to instilled residual oil fly ash or acute ozone exposure; however, we found that, despite a robust inflammatory response, C57BL/6(TLR4-/-) mice are protected against the development of airway hyperresponsiveness after subchronic ozone exposure. These data demonstrate in the mouse that the requirement of TLR4 for pulmonary inflammation depends on the nature of the toxin and appears specific to toxin and exposure conditions.

ST2 is an inhibitor of interleukin 1 receptor and Toll-like receptor 4 signaling and maintains endotoxin tolerance

The Toll-interleukin 1 receptor (TIR) superfamily, defined by the presence of an intracellular TIR domain, initiates innate immunity through activation of the transcription factor NF-kappa B, leading to the production of proinflammatory cytokines. ST2 is a member of the TIR family that does not activate NF-kappa B and has been suggested as an important effector molecule of T helper type 2 (T(H)2) responses. We show here that the membrane-bound form of ST2 negatively regulated type I interleukin 1 receptor (IL-1RI) and Toll-like receptor 4 (TLR4) but not TLR3 signaling by sequestrating the adaptors MyD88 and Mal. In contrast to wild-type mice, ST2-deficient mice failed to develop endotoxin tolerance. Thus, these results provide a molecular explanation for the function of ST2 in T(H)2 responses, as inhibition of TLRs promotes a T(H)2 response, and also identify ST2 as a key regulator of endotoxin tolerance.

Myeloid Differentiation Factor 88 Is Required for Cross-Priming In Vivo

We describe a role for myeloid differentiation factor 88 (MyD88) in the induction of functional CTLs in vivo, in response to exogenously administered Ag, using a heat shock fusion protein, hsp65-P1, as a model Ag. CD8 T cells transferred into MyD88-deficient animals produce normal numbers of CD8 effector cells that have normal activation marker profiles after immunization with hsp65-P1. However, these CD8 T cells produced significantly less IFN-gamma and showed reduced killing activity. This reduction in activation of functional CTLs appears to be unrelated to Toll-like receptor 4 function, because in vitro hsp65-P1-experienced Toll-like receptor 4-deficient dendritic cells (DCs), but not MyD88-deficient DCs, activated CD8 T cells to a similar extent to wild-type DCs. We identify a cross-presentation defect in MyD88-deficient DCs that, when treated with hsp65-P1 fusion protein, results in surface display of fewer SIYRYYGL/class I MHC complexes. Thus, MyD88 plays a role in the developmental maturation of DCs that allows them to prime CD8 T cells through cross-presentation.

Molecular aspects of intestinal epithelial cell-bacterial interactions that determine the development of intestinal inflammation

The mechanisms by which intestinal epithelial cells perceive and respond to microbes, both pathogenic and commensal, is important to understand the pathogenesis of inflammatory diseases. Recent work has established that most eukaryotic cells possess families of receptors that can detect the structural signatures of prokaryotic life. Cells respond to the detection of microbes by activation of distinct cytoplasmic signaling cascades that ultimately result in the transcriptional activation of programs of genes with proinflammatory and anti-apoptotic function. These responses generally suffice to eliminate microbial threats. Also important are potential mechanisms by which microbes can influence the intestinal epithelial responses, influences with significant implications for the normal function of the intestine and inflammatory diseases.

Toll-like receptors 9 and 3 as essential components of innate immune defense against mouse cytomegalovirus infection

Several subsets of dendritic cells have been shown to produce type I IFN in response to viral infections, thereby assisting the natural killer cell-dependent response that eliminates the pathogen. Type I IFN production can be induced both by unmethylated CpG-oligodeoxynucleotide and by double-stranded RNA. Here, we describe a codominant CpG-ODN unresponsive phenotype that results from an N-ethyl-N-nitrosourea-induced missense mutation in the Tlr9 gene (Tlr9(CpG1)). Mice homozygous for the Tlr9(CpG1) allele are highly susceptible to mouse cytomegalovirus infection and show impaired infection-induced secretion of IFN-alpha/beta and natural killer cell activation. We also demonstrate that both the Toll-like receptor (TLR) 9 --> MyD88 and TLR3 --> Trif signaling pathways are activated in vivo on viral inoculation, and that each pathway contributes to innate defense against systemic viral infection. Whereas both pathways lead to type I IFN production, neither pathway offers full protection against mouse cytomegalovirus infection in the absence of the other. The Tlr9(CpG1) mutation alters a leucine-rich repeat motif and lies within a receptor domain that is conserved within the evolutionary cluster encompassing TLRs 7, 8, and 9. In other TLRs, including three mouse-specific TLRs described in this paper, the affected region is not represented. The phenotypic effect of the Tlr9(CpG1) allele thus points to a critical role for TLR9 in viral sensing and identifies a vulnerable amino acid within the ectodomain of three TLR proteins, essential for a ligand response.

The protein kinase PKR is required for macrophage apoptosis after activation of Toll-like receptor 4

Macrophages are pivotal constituents of the innate immune system, vital for recognition and elimination of microbial pathogens. Macrophages use Toll-like receptors (TLRs) to detect pathogen-associated molecular patterns--including bacterial cell wall components, such as lipopolysaccharide or lipoteichoic acid, and viral nucleic acids, such as double-stranded (ds)RNA--and in turn activate effector functions, including anti-apoptotic signalling pathways. Certain pathogens, however, such as Salmonella spp., Shigellae spp. and Yersiniae spp., use specialized virulence factors to overcome these protective responses and induce macrophage apoptosis. We found that the anthrax bacterium, Bacillus anthracis, selectively induces apoptosis of activated macrophages through its lethal toxin, which prevents activation of the anti-apoptotic p38 mitogen-activated protein kinase. We now demonstrate that macrophage apoptosis by three different bacterial pathogens depends on activation of TLR4. Dissection of anti- and pro-apoptotic signalling events triggered by TLR4 identified the dsRNA responsive protein kinase PKR as a critical mediator of pathogen-induced macrophage apoptosis. The pro-apoptotic actions of PKR are mediated both through inhibition of protein synthesis and activation of interferon response factor 3.

Toll-like receptor 3-mediated activation of NF-kappaB and IRF3 diverges at Toll-IL-1 receptor domain-containing adapter inducing IFN-beta

We have previously shown that double-stranded RNA-triggered, Toll-like receptor 3 (TLR3)-mediated signaling is independent of MyD88, IRAK4, and IRAK. Instead, TRAF6, TAK1, and TAB2 are recruited to TLR3 on poly(I.C) stimulation. TRAF6-TAK1-TAB2 are then translocated to the cytosol where TAK1 is phosphorylated and activated, leading to the activation of IkappaB kinase and NFkappaB. The present study addressed two important questions: (i) How are TRAF6, TAK1, and TAB2 recruited to TLR3? (ii) Are TRAF6, TAK1, and TAB2 also required for TLR3-mediated IRF3 activation? Recently, a novel Toll-IL-1 receptor (TIR)-containing adapter, TIR domain-containing adapter inducing IFN-beta (TRIF), was shown to play a critical role in TLR3-mediated activation of NF-kappaB and IRF3. We found that TLR3 recruits TRAF6 via adapter TRIF through a TRAF6-binding sequence in TRIF (PEEMSW, amino acids 250-255). Mutation of this TRAF6-binding sequence abolished the interaction of TRIF with TRAF6, but not with TLR3. Interestingly, mutation of the TRAF6-binding site of TRIF only abolished its ability to activate NF-kappaB but not IRF3, suggesting that TLR3-mediated activation of NF-kappaB and IRF3 might bifurcate at TRIF. In support of this finding, we showed that DN-TRAF6 and DN-TAK1 blocked poly(I.C)-induced NF-kappaB but not IRF3 activation. Furthermore, whereas poly(I.C)-induced NF-kappaB activation is completely abolished inTRAF6-/- MEFs, the signal-induced activation of IRF3 is TRAF6 independent. In conclusion, TRIF recruits TRAF6-TAK1-TAB2 to TLR3 through its TRAF6-binding site, which is required for NF-kappaB but not IRF3 activation. Therefore, double-stranded RNA-induced TLR3/TRIF-mediated NF-kappaB and IRF3 activation diverge at TRIF.

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.

Herpes simplex virus type 1 activates murine natural interferon-producing cells through toll-like receptor 9

Natural interferon-producing cells (IPCs) specialize in the production of high levels of type 1 interferons (IFNs) in response to encapsulated DNA and RNA viruses. Here we demonstrate that the secretion of type 1 IFN in response to herpes simplex virus type 1 (HSV-1) in vitro is mediated by the toll-like receptor 9 (TLR9)/MyD88 pathway. Moreover, IPCs produce interleukin-12 (IL-12) in response to HSV-1 in vitro, which is also dependent on TLR9/ MyD88 signaling. Remarkably, though TLR9/MyD88-deficiency abrogates IPC responses to HSV-1 in vitro, mice lacking either MyD88 or TLR9 are capable of controlling HSV-1 replication in vivo after local infection, demonstrating that TLR9- and MyD88-independent pathways in cells other than IPCs can effectively compensate for defective IPC responses to HSV-1.

Signaling in transitional type 2 B cells is critical for peripheral B-cell development

Splenic peripheral B-cell development and the events regulating this functionally significant but relatively poorly defined developmental process have become a major focus in recent studies in B-cell immunology. Following the exit from the bone marrow, peripheral B cells develop through transitional type 1 (T1) and transitional type 2 (T2) B-cell stages. Emerging data suggest that the T2 subset is the immediate precursor of the mature B-cell populations present in the spleen. In this review, we first elaborate on the evidence describing the unique properties of CD21hiCD24hiCD23hiIgMhiIgDhi T2 B cells. T2 cells uniquely activate a proliferative, pro-survival, and differentiation program in response to B-cell antigen receptor (BCR) engagement. The potential mechanisms leading to the differential BCR responsiveness of T1 versus T2 B cells are discussed. We also review evidence that distinguishes key BCR-dependent signaling pathways operative in T2 and mature B cells. These signaling cascades include a protein kinase Cbeta (PKCbeta)-dependent cell-survival pathway and a second PKCbeta-independent pathway essential for BCR-driven differentiation. Finally, we discuss recent intriguing results suggesting that the type of signal(s) encountered by T2 cells leads to their differential maturation toward the follicular mature versus marginal zone mature B-cell populations. These combined observations suggest important implications with regard to B-cell selection and tolerance, potential novel therapeutic targets for B-cell lymphomas, and how the intricate balance of commensal organisms and other microenvironmental signals interact to promote the generation of 'innate-like' versus adaptive effector B-cell populations.

Mechanisms of the TRIF-induced interferon-stimulated response element and NF-kappaB activation and apoptosis pathways

Toll-like receptor-3 is critically involved in host defense against viruses through induction of type I interferons (IFNs). Recent studies suggest that a Toll/interleukin-1 receptor domain-containing adapter protein (TRIF) and two protein kinases (TANK-binding kinase-1 (TBK1) and IkappaB kinase (IKK)-epsilon) are critically involved in Toll-like receptor-3-mediated IFN-beta production through activation of IFN regulatory factor (IRF)-3 and IRF-7. In this study, we demonstrate that TRIF interacts with both IRF-7 and IRF-3. In addition to TBK1 and IKKepsilon, our results indicate that IKKbeta can also phosphorylate IRF-3 and activate the IFN-stimulated response element. TRIF-induced IRF-3 and IRF-7 activation was mediated by TBK1 and its downstream kinases IKKbeta and IKKepsilon. TRIF induced NF-kappaB activation through an IKKbeta- and tumor necrosis factor receptor-associated factor-6-dependent (but not TBK1- and IKKepsilon-dependent) pathway. In addition, TRIF also induced apoptosis through a RIP/FADD/caspase-8-dependent and mitochondrion-independent pathway. Furthermore, our results suggest that the TRIF-induced IFN-stimulated response element and NF-kappaB activation and apoptosis pathways are uncoupled and provide a molecular explanation for the divergent effects induced by the adapter protein TRIF.

Toll-like receptors and the host defense against microbial pathogens: bringing specificity to the innate-immune system

Toll-like receptors (TLRs) have been identified as a major class of pattern-recognition receptors. Recognition of pathogen-associated molecular patterns (PAMPs) by TLRs, alone or in heterodimerization with other TLR or non-TLR receptors, induces signals responsible for the activation of genes important for an effective host defense, especially proinflammatory cytokines. Although a certain degree of redundancy exists between signals induced by the various TLRs, recent studies have identified intracellular pathways specific for individual TLRs. This leads to the release of cytokine profiles specific for particular PAMPs, and thus, TLRs confer a certain degree of specificity to the innate-immune response. In addition to the activation of the innate-immune response, TLR-mediated recognition represents a link between the innate- and acquired-immune systems, by inducing the maturation of dendritic cells and directing the T helper responses. Alternatively, recent data have also suggested TLR-mediated escape mechanisms used by certain pathogenic microorganisms, especially through TLR2 induction of anti-inflammatory cytokines. Finally, the crucial role of TLRs for the host defense against infections has been strengthened recently by the description of patients partially defective in the TLR-activation pathways.

Controlling the Toll road to dendritic cell polarization

The activation of dendritic cells (DC) via Toll-like receptors (TLRs) plays a decisive role in shaping the outcome of primary immune responses. Following TLR engagement by microbial products, DC migrate from peripheral tissues to lymphoid organs and up-regulate major histocompatibility complex and costimulatory molecules, acquiring the unique capacity to prime pathogen-specific, naïve T cells. In addition, DC determine the character of the ensuing immune response by secreting cytokines that drive the development of T cells into T helper cell type 1 (Th1), Th2, or T regulatory effector cells. Three major factors influence the pattern of cytokines released by DC and accordingly, the Th balance: the lineage to which DC belong; the maturation stimulus; and inflammatory mediators present at the site of infection. A major focus of this review is the capacity of DC to integrate these factors and elicit distinct classes of immune responses.

The interferon in TLR signaling: more than just antiviral

The Toll-like receptor (TLR) system is responsible for the recognition of infectious agents leading to initiation of the primary innate, and later adaptive, immune response. Genetic technologies have enabled the discovery of new factors involved in these systems, their genetic manipulation and the global analyses of their effects on gene expression. Furthermore, this increased understanding has resulted in the need to reassess our preconceptions about the functions of well-known molecules. For example, type I interferons (IFNs), which were discovered as antiviral proteins, are now known to be produced in response to TLR activation by many pathogens, including bacteria. Should we be surprised? Has the inflammatory response unexpectedly highjacked the body's antiviral system? Or are we too easily blinkered by preconceptions from how a compound was discovered?

Lps2 and signal transduction in sepsis: at the intersection of host responses to bacteria and viruses

A phenotype-driven approach led to the first understanding of precisely what the Toll-like receptors (TLR) did, when it was determined that the mammalian endotoxin (lipopolysaccharide; LPS) receptor is encoded by TLR4. The TLRs are the primary sensors of the innate immune system, and without them, small inocula of microorganisms pose a major threat to the host, growing unchecked for a long period before they are recognized. Mutations that affect innate immune sensing may account for a substantial fraction of sepsis, and a highly significant excess of mutations in TLR4 has been identified in patients with systemic meningococcal disease. As such, it is important to understand the pathways that are responsible for innate immune sensing, including the signaling intermediates utilized by the TLRs. Random germline mutagenesis identified a locus, Lps2, which is required for normal responses to double-stranded RNA and LPS. Hence, a single transducer was found to serve both the TLR3 and TLR4 response pathways. The Lps2 mutation was found to ablate entirely the MyD88-independent pathway for LPS sensing, indicating that two and only two branches of the LPS sensing pathway exist in macrophages, and homozygotes for the mutation were resistant to LPS, but markedly susceptible to infection with mouse cytomegalovirus. Remarkably, Lps2 mutant mice entirely failed to produce type I interferons in response to a viral infection. It would appear that Lps2 is the most proximal component of a signal integration system required for innate immune responses to both viral and bacterial diseases. Positional cloning revealed that the TIR adapter protein Trif/Ticam-1 is structurally altered by the Lps2 mutation. This adapter is responsible for shared effects of responses to viral and bacterial pathogens.

IFN-regulatory factor 3-dependent gene expression is defective in Tbk1-deficient mouse embryonic fibroblasts

Virus infection, double-stranded RNA, and lipopolysaccharide each induce the expression of genes encoding IFN-alpha and -beta and chemokines, such as RANTES (regulated on activation, normal T cell expressed and secreted) and IP-10 (IFN-gamma inducible protein 10). This induction requires the coordinate activation of several transcription factors, including IFN-regulatory factor 3 (IRF3). The signaling pathways leading to IRF3 activation are triggered by the binding of pathogen-specific products to Toll-like receptors and culminate in the phosphorylation of specific serine residues in the C terminus of IRF3. Recent studies of human cell lines in culture have implicated two noncanonical IkappaB kinase (IKK)-related kinases, IKK-epsilon and Traf family member-associated NF-kappaB activator (TANK)-binding kinase 1 (TBK1), in the phosphorylation of IRF3. Here, we show that purified recombinant IKK-epsilon and TBK1 directly phosphorylate the critical serine residues in IRF3. We have also examined the expression of IRF3-dependent genes in mouse embryonic fibroblasts (MEFs) derived from Tbk1(-/-) mice, and we show that TBK1 is required for the activation and nuclear translocation of IRF3 in these cells. Moreover, Tbk1(-/-) MEFs show marked defects in IFN-alpha and -beta, IP-10, and RANTES gene expression after infection with either Sendai or Newcastle disease viruses or after engagement of the Toll-like receptors 3 and 4 by double-stranded RNA and lipopolysaccharide, respectively. Finally, TRIF (TIR domain-containing adapter-inducing IFN-beta), fails to activate IRF3-dependent genes in Tbk1(-/-) MEFs. We conclude that TBK1 is essential for IRF3-dependent antiviral gene expression.

Differential regulation of interleukin 1 receptor and Toll-like receptor signaling by MEKK3

Interleukin 1 receptor (IL-1R) and Toll-like receptors (TLRs) induce inflammatory genes through the complex of MyD88, IL-1R-associated protein kinase (IRAK) and tumor necrosis factor receptor-associated factor 6 (TRAF6), which is believed to function 'upstream' of the cascades of IkappaB kinase (IKK) and nuclear factor-kappaB (NF-kappaB); extracellular signal-regulated protein kinase (ERK); c-Jun N-terminal kinase (JNK); and p38 mitogen-activated protein kinase (MAPK). Here we show that MAPK-ERK kinase kinase (MEKK3) is an essential signal transducer of the MyD88-IRAK-TRAF6 complex in IL-1R-TLR4 signaling. MEKK3 forms a complex with TRAF6 in response to IL-1 and lipopolysaccharide (LPS) but not CpG, and is required for IL-1R- and TLR4-induced IL-6 production. Furthermore, MEKK3 is crucial for IL-1- and LPS-induced activation of NF-kappaB and JNK-p38 but not ERK, indicating that MAPKs are differentially activated during IL-1R-TLR4 signaling. These data demonstrate that MEKK3 is crucial for IL-1R and TLR4 signaling through the IKK-NF-kappaB and JNK-p38 MAPK pathways.*Note: In the version of this article originally published online, the third author's name was incorrect. The correct author name should be Yong Lin. This error has been corrected for the HTML and print versions of this article.

Interferon Regulatory Factor-3-mediated Activation of the Interferon-sensitive Response Element by Toll-like receptor (TLR) 4 but Not TLR3 Requires the p65 Subunit of NF-κ

Interferon regulatory factor (IRF) 3 is a transcription factor that binds the interferon-sensitive response element (ISRE) and is activated by Toll-like receptor 3 (TLR3) and TLR4. We have found that a dominant negative form of I kappa B kinase 2 and a mutant form of I kappa B, which acts as a super-repressor of NF-kappa B, blocked activation of the ISRE by the TLR4 ligand lipopolysaccharide but not the TLR3 ligand poly(I-C). TLR4 failed to activate the ISRE in mouse embryonic fibroblasts bearing a targeted deletion of p65, whereas the response to TLR3 in these cells was normal. The p65 subunit of NF-kappa B was detected in the lipopolysaccharide-activated but not poly(I-C)-activated ISRE-binding complex. Finally, p65 promoted transactivation of gene expression by IRF-3. These results therefore indicate that IRF-3-mediated activation of the ISRE by TLR4 but not TLR3 requires the p65 subunit of NF-kappa B.