Sensing of microbial molecular patterns by Toll-like receptors

Toll-like receptors (TLRs) sense structural patterns in microbial molecules and initiate immune defense mechanisms. The structures of many extracellular and intracellular domains of TLRs have been studied in the last 10 years. These structures reveal the extraordinary diversity of TLR-ligand interactions. Some TLRs use internal hydrophobic pockets to bind bacterial ligands and others use solvent-exposed surfaces to bind hydrophilic ligands. The structures suggest a common activation mechanism for TLRs: ligand binding to extracellular domains induces dimerization of the intracellular domains and so activates intracellular signaling pathways. Recently, the structure of the death domain complex of one of the signaling adapters, myeloid differentiation factor 88 (MyD88), has been determined. This structure shows how aggregation of signaling adapters recruits downstream kinases. However, we are still far from a complete understanding of TLR activation. We need to study the structures of TLR7-10 in complex with their ligands. We also need to determine the structures of TLR-adapter aggregates to understand activation mechanisms and the specificity of the signaling pathways. Ultimately, we will have to study the structures of the complete TLR signaling complexes containing full-length receptors, ligands, signaling, and bridging adapters, and some of the downstream kinases to understand how TLRs sense microbial infections and activate immune responses against them.

Inhibition of TLR4 signaling by TRAM-derived decoy peptides in vitro and in vivo

Toll/IL-1R (TIR) domain-containing adapter-inducing IFN-β (TRIF)-related adapter molecule (TRAM) serves as a bridging adapter that enables recruitment of TRIF to activated TLR4 and thereby mediates the induction of TRIF-dependent cytokines. A library of cell-permeating decoy peptides derived from TRAM TIR domain has been screened for the ability of individual peptides to inhibit TLR4 signaling in primary murine macrophages. Peptides derived from TRAM TIR BB loop (TM4) and C helix (TM6) inhibited the LPS-induced activation of MyD88-dependent and TRIF-dependent cytokines, as well as MAPK activation. TM4 and TM6 did not block macrophage activation induced by TLR2, TLR9, or retinoic acid-inducible gene 1-like receptor agonists. Both TM4 and TM6 blocked coimmunoprecipitation of TRAM and TLR4 ectopically expressed in HEK293T cells. Both peptides also blocked the LPS-induced recruitment of MyD88 to TLR4 in primary murine macrophages. In vivo examination of TRAM-derived peptides demonstrated that all peptides that were inhibitory in vitro profoundly suppressed systemic inflammatory response elicited in mice by a sublethal LPS dose, and protected mice against a lethal LPS challenge. This research identifies novel TLR inhibitors effective in vitro and in vivo and validates the approach taken in this study as a rational way for development of signaling inhibitors and lead therapeutics.

Evolutionary analysis of Antarctic teleost Toll-like receptor 2

In the present study we address the investigation of TLR2 evolutionary selection in two Antarctic teleosts, Trematomus bernacchii (Nototheniidae) and Chionodraco hamatus (Channichthyidae). The nucleotide sequence of TLR2 has been determined in both species, encoding 20 leucine-rich repeats (LRRs) in the extracellular region and a classical Toll/IL-1R (TIR) domain in the intracellular region. High expression level of T. bernacchii TLR2 was found in spleen and skin. Using different methods we identified six codons that underwent Darwinian selection while 20 were found to be negatively selected. Molecular models of C. hamatus and T. bernacchii TLR2 ectodomain as well as of the TIR domain were built by Homology Modeling. Molecular Dynamics simulations were performed in water for 15 ns. The sites under positive selection were residing on the convex side of the solenoid, four out of six were in a 35-residue-long region including the central/N-terminal domain boundary: two in the external loop of LRR11 and the other two in the LRR12 loop. This region has been demonstrated to be the functional site of ligand interaction in human TLR2 structure. Antarctic TLR2 models showed more flexibility than TLR2 from the temperate species Gasterosteus aculeatus. These results suggest that the selective pressure has shaped TLR2 molecule in such a way that increased its activity under the peculiar Antarctic environmental conditions.

Therapeutic inhibition of pro-inflammatory signaling and toxicity to staphylococcal enterotoxin B by a synthetic dimeric BB-loop mimetic of MyD88

Staphylococcal enterotoxin B (SEB) exposure triggers an exaggerated pro-inflammatory cytokine response that often leads to toxic shock syndrome (TSS) associated with organ failure and death. MyD88 mediates pro-inflammatory cytokine signaling induced by SEB exposure and MyD88(-/-) mice are resistant to SEB intoxication, suggesting that MyD88 may be a potential target for therapeutic intervention. We targeted the BB loop region of the Toll/IL-1 receptor (TIR) domain of MyD88 to develop small-molecule therapeutics. Here, we report that a synthetic compound (EM-163), mimic to dimeric form of BB-loop of MyD88 attenuated tumor necrosis factor (TNF)- α, interferon (IFN)-γ, interleukin (IL)-1β, IL-2 and IL-6 production in human primary cells, whether administered pre- or post-SEB exposure. Results from a direct binding assay, and from MyD88 co-transfection/co-immunoprecipitation experiments, suggest that EM-163 inhibits TIR-TIR domain interaction. Additional results indicate that EM-163 prevents MyD88 from mediating downstream signaling. In an NF-kB-driven reporter assay of lipopolysaccharide-stimulated MyD88 signaling, EM-163 demonstrated a dose-dependent inhibition of reporter activity as well as TNF-α and IL-1β production. Importantly, administration of EM-163 pre- or post exposure to a lethal dose of SEB abrogated pro-inflammatory cytokine responses and protected mice from toxic shock-induced death. Taken together, our results suggest that EM-163 exhibits a potential for therapeutic use against SEB intoxication.

Adaptive evolution of Toll-like receptor 5 in domesticated mammals

Background

Previous studies have proposed that mammalian toll like receptors (TLRs) have evolved under diversifying selection due to their role in pathogen detection. To determine if this is the case, we examined the extent of adaptive evolution in the TLR5 gene in both individual species and defined clades of the mammalia.

Results

In support of previous studies, we find evidence of adaptive evolution of mammalian TLR5. However, we also show that TLR5 genes of domestic livestock have a concentration of single nucleotide polymorphisms suggesting a specific signature of adaptation. Using codon models of evolution we have identified a concentration of rapidly evolving codons within the TLR5 extracellular domain a site of interaction between host and the bacterial surface protein flagellin.

Conclusions

The results suggest that interactions between pathogen and host may be driving adaptive change in TLR5 by competition between species. In support of this, we have identified single nucleotide polymorphisms (SNP) in sheep and cattle TLR5 genes that are co-localised and co-incident with the predicted adaptive codons suggesting that adaptation in this region of the TLR5 gene is on-going in domestic species.

Toll/interleukin-1 receptor domain dimers as the platform for activation and enhanced inhibition of Toll-like receptor signaling

TIR (Toll/IL-1 receptor) domains mediate interactions between TLR (Toll-like) or IL-1 family receptors and signaling adapters. While homotypic TIR domain interactions mediate receptor activation they are also usurped by microbial TIR domain containing proteins for immunosuppression. Here we show the role of a dimerized TIR domain platform for the suppression as well as for the activation of MyD88 signaling pathway. Coiled-coil dimerization domain, present in many bacterial TCPs, potently augments suppression of TLR/IL-1R signaling. The addition of a strong coiled-coil dimerization domain conferred the superior inhibition against the wide spectrum of TLRs and prevented the constitutive activation by a dimeric TIR platform. We propose a molecular model of MyD88-mediated signaling based on the dimerization of TIR domains as the limiting step.

Bacterial TIR-containing proteins and host innate immune system evasion

The innate immune system provides the first line of host defence against invading pathogens. Key to upregulation of the innate immune response are Toll-like receptors (TLRs), which recognize pathogen-associated molecular patterns (PAMPs) and trigger a signaling pathway culminating in the production of inflammatory mediators. Central to this TLR signaling pathway are heterotypic protein-protein interactions mediated through Toll/interleukin-1 receptor (TIR) domains found in both the cytoplasmic regions of TLRs and adaptor proteins. Pathogenic bacteria have developed a range of ingenuous strategies to evade the host immune mechanisms. Recent work has identified a potentially novel evasion mechanism involving bacterial TIR domain proteins. Such domains have been identified in a wide range of pathogenic bacteria, and there is evidence to suggest that they interfere directly with the TLR signaling pathway and thus inhibit the activation of NF-κB. The individual TIR domains from the pathogenic bacteria Salmonella enterica serovar Enteritidis, Brucella sp, uropathogenic E. coli and Yersinia pestis have been analyzed in detail. The individual bacterial TIR domains from these pathogenic bacteria seem to differ in their modes of action and their roles in virulence. Here, we review the current state of knowledge on the possible roles and mechanisms of action of the bacterial TIR domains.

Systems biology approaches to toll-like receptor signaling

Toll-like receptor (TLR) signaling pathways constitute an evolutionarily conserved host defense system that protects against a broad range of infectious agents. Modeling of TLR signaling has been carried out at several levels. Structural models of TLRs and their adaptors, which utilize a small number of structural domains to recognize a diverse range of pathogens, provide a starting point for understanding how pathogens are recognized and signaling events initiated. Various experimental and computational techniques have been used to construct models of downstream signal transduction networks from the measurements of gene expression and chromatin structure under resting and perturbed conditions along with predicted regulatory sequence motifs. Although a complete and accurate mathematical model of all TLR signaling pathways has yet to be derived, many important modules have been identified and investigated, enhancing our understanding of innate immune responses. Extensions of these models based on emerging experimental techniques are discussed. © 2012 Wiley Periodicals, Inc.

Molecular analysis of the binding mode of Toll/interleukin-1 receptor (TIR) domain proteins during TLR2 signaling

Toll-like receptor (TLR) signaling is initiated by the binding of various adaptor proteins through ligand-induced oligomerization of the Toll/interleukin-1 receptor (TIR) domains of the TLRs. TLR2, which recognizes peptidoglycans, lipoproteins or lipopeptides derived from Gram-positive bacteria, is known to use the TIR domain-containing adaptor proteins myeloid differentiating factor 88 (MyD88) and MyD88 adaptor-like (Mal). Molecular analyses of the binding specificity of MyD88, Mal, and TLR2 are important for understanding the initial defenses mounted against Gram-positive bacterial infections such as Streptococcus pneumoniae. However, the detailed molecular mechanisms involved in the multiple interactions of these TIR domains remain unclear. Our study demonstrates that the TIR domain proteins MyD88, Mal, TLR1, and TLR2 directly bind to each other in vitro. We have also identified two binding interfaces of the MyD88 TIR domain for the TLR2 TIR domain. A residue at these interfaces has recently been found to be mutated in innate immune deficiency patients. These novel insights into the binding mode of TIR proteins will contribute to elucidation of the mechanisms underlying innate immune deficiency diseases, and to future structural studies of hetero-oligomeric TIR-TIR complexes.

Targeting Toll-like receptor (TLR) signaling by Toll/interleukin-1 receptor (TIR) domain-containing adapter protein/MyD88 adapter-like (TIRAP/Mal)-derived decoy peptides

Toll/interleukin-1 receptor (TIR) domain-containing adapter protein/MyD88 adapter-like (TIRAP/Mal) is an adapter protein that facilitates recruitment of MyD88 to TLR4 and TLR2 signaling complexes. We previously generated a library of cell-permeating TLR4 TIR-derived decoy peptides fused to the translocating segment of the Drosophila Antennapedia homeodomain and examined each peptide for the ability to inhibit TLR4 signaling (Toshchakov, V. Y., Szmacinski, H., Couture, L. A., Lakowicz, J. R., and Vogel, S. N. (2011) J. Immunol. 186, 4819–4827). We have now expanded this study to test TIRAP decoy peptides. Five TIRAP peptides, TR3 (for TIRAP region 3), TR5, TR6, TR9, and TR11, inhibited LPS-induced cytokine mRNA expression and MAPK activation. Inhibition was confirmed at the protein level; select peptides abolished the LPS-induced cytokine production measured in cell culture 24 h after a single treatment. Two of the TLR4 inhibitory peptides, TR3 and TR6, also inhibited cytokine production induced by a TLR2/TLR1 agonist, S-(2,3-bis(palmitoyloxy)-(2R,2S)-propyl)-N-palmitoyl-(R)-Cys-Ser-Lys₄-OH; however, a higher peptide concentration was required to achieve comparable inhibition of TLR2 versus TLR4 signaling. Two TLR4 inhibitory peptides, TR5 and TR6, were examined for the ability to inhibit TLR4-driven cytokine induction in mice. Pretreatment with either peptide significantly reduced circulating TNF-α and IL-6 in mice following LPS injection. This study has identified novel TLR inhibitory peptides that block cellular signaling at low micromolar concentrations in vitro and in vivo. Comparison of TLR4 inhibition by TLR4 and TIRAP TIR-derived peptides supports the view that structurally diverse regions mediate functional interactions of TIR domains.

Structural insights into the assembly of large oligomeric signalosomes in the Toll-like receptor-interleukin-1 receptor superfamily

The Toll-like receptor (TLR)-interleukin 1 receptor (IL-1R) superfamily plays fundamentally important roles in innate immune and inflammatory responses. Structural studies have begun to show that upon ligand stimulation, TLRs and IL-1Rs assemble large oligomeric intracellular signaling complexes, or "signalosomes," to induce the activation of kinases and E3 ubiquitin ligases, leading eventually to the activation of the transcription factors that are responsible for the expression of genes whose products mediate immune and inflammatory responses. The different scaffolds identified by these structural studies provide a molecular foundation for understanding the formation of microscopically visible signaling clusters that have long been known to cell biologists. Here, we illustrate the potential mechanisms of step-by-step assembly from the membrane-proximal interactions to the more downstream events. Formation of large oligomeric signalosomes may help to establish a digital threshold response in TLR and IL-1R signaling.

Poxviral protein A46 antagonizes Toll-like receptor 4 signaling by targeting BB loop motifs in Toll-IL-1 receptor adaptor proteins to disrupt receptor:adaptor interactions

Toll-like receptors (TLRs) have an anti-viral role in that they detect viruses, leading to cytokine and IFN induction, and as such are targeted by viruses for immune evasion. TLR4, although best known for its role in recognizing bacterial LPS, is also strongly implicated in the immune response to viruses. We previously showed that the poxviral protein A46 inhibits TLR4 signaling and interacts with Toll-IL-1 receptor (TIR) domain-containing proteins of the receptor complex. However the exact molecular mechanism whereby A46 disrupts TLR4 signaling remains to be established, and may yield insight into how the TLR4 complex functions, since viruses often optimally target key residues and motifs on host proteins for maximal efficiency. Here we show that A46 targets the BB loop motif of TIR proteins and thereby disrupts receptor:adaptor (TLR4:Mal and TLR4:TRAM), but not receptor:receptor (TLR4:TLR4) nor adaptor:adaptor (Mal:MyD88, TRAM:TRIF, and Mal:Mal) TIR interactions. The requirement for an intact BB loop for TIR adaptor interactions correlated with the protein:protein interfaces antagonized by A46. We previously discovered a peptide fragment derived from A46 termed VIPER (Viral Inhibitory Peptide of TLR4), which specifically inhibits TLR4 responses. Here we demonstrate that the region of A46 from which VIPER is derived represents the TLR4-specific inhibitory motif of the intact protein, and is essential for A46:TRAM interactions. This study provides the molecular basis for pathogen subversion of TLR4 signaling and clarifies the importance of TIR motif BB loops, which have been selected for viral antagonism, in the formation of the TLR4 complex.

Structural basis of species-specific endotoxin sensing by innate immune receptor TLR4/MD-2

Lipopolysaccharide (LPS), also known as endotoxin, activates the innate immune response through toll-like receptor 4 (TLR4) and its coreceptor, MD-2. MD-2 has a unique hydrophobic cavity that directly binds to lipid A, the active center of LPS. Tetraacylated lipid IVa, a synthetic lipid A precursor, acts as a weak agonist to mouse TLR4/MD-2, but as an antagonist to human TLR4/MD-2. However, it remains unclear as to how LPS and lipid IVa show agonistic or antagonistic activities in a species-specific manner. The present study reports the crystal structures of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa complexes at 2.5 and 2.7 Å resolutions, respectively. Mouse TLR4/MD-2/LPS exhibited an agonistic "m"-shaped 2:2:2 complex similar to the human TLR4/MD-2/LPS complex. Mouse TLR4/MD-2/lipid IVa complex also showed an agonistic structural feature, exhibiting architecture similar to the 2:2:2 complex. Remarkably, lipid IVa in the mouse TLR4/MD-2 complex occupied nearly the same space as LPS, although lipid IVa lacked the two acyl chains. Human MD-2 binds lipid IVa in an antagonistic manner completely differently from the way mouse MD-2 does. Together, the results provide structural evidence of the agonistic property of lipid IVa on mouse TLR4/MD-2 and deepen understanding of the ligand binding and dimerization mechanism by the structurally diverse LPS variants.

Structural insights into TIR domain specificity of the bridging adaptor Mal in TLR4 signaling

MyD88 adaptor-like protein (Mal) is a crucial adaptor that acts as a bridge to recruit the MyD88 molecule to activated TLR4 receptors in response to invading pathogens. The specific assembly of the Toll/interleukin-1 receptor (TIR) domains of TLR4, Mal and MyD88 is responsible for proper signal transduction in the TLR4 signaling pathway. However, the molecular mechanism for the specificity of these TIR domains remains unclear. Here, we present the crystal structure of the TIR domain of the human Mal molecule (Mal-TIR) at a resolution of 2.4 Å. Unexpectedly, Mal-TIR exhibits an extraordinarily long AB loop, but no αB helix or BB loop, distinguishing it from other TIR domains. More importantly, the Mal-TIR AB loop is capable of mediating direct binding to the TIR domains of TLR4 and MyD88 simultaneously. We also found that Mal-TIR can form a back-to-back dimer that may resemble the dimeric assembly of the entire Mal molecule. Our data demonstrate the bridge role of the Mal-TIR domain and provide important information about TIR domain specificity.

Structure of the activating IL-1 receptor signaling complex

Interleukin-1 (IL-1)-family cytokines are mediators of innate and adaptive immunity. They exert proinflammatory effects by binding a primary receptor that recruits a receptor accessory protein to form a signaling-competent heterotrimeric complex. Here we present the crystal structure of IL-1β bound to its primary receptor IL-1RI and its receptor accessory protein IL-1RAcP, providing insight into how IL-1-type cytokines initiate signaling and revealing an evolutionary relationship with the fibroblast growth factor receptor family.

A Toll/interleukin (IL)-1 receptor domain protein from Yersinia pestis interacts with mammalian IL-1/Toll-like receptor pathways but does not play a central role in the virulence of Y. pestis in a mouse model of bubonic plague

The Toll/interleukin (IL)-1 receptor (TIR) domain is an essential component of eukaryotic innate immune signalling pathways. Interaction between TIR domains present in Toll-like receptors and associated adaptors initiates and propagates an immune signalling cascade. Proteins containing TIR domains have also been discovered in bacteria. Studies have subsequently shown that these proteins are able to modulate mammalian immune signalling pathways dependent on TIR interactions and that this may represent an evasion strategy for bacterial pathogens. Here, we investigate a TIR domain protein from the highly virulent bacterium Yersinia pestis, the causative agent of plague. When overexpressed in vitro this protein is able to downregulate IL-1β- and LPS-dependent signalling to NFκB and to interact with the TIR adaptor protein MyD88. This interaction is dependent on a single proline residue. However, a Y. pestis knockout mutant lacking the TIR domain protein was not attenuated in virulence in a mouse model of bubonic plague. Minor alterations in the host cytokine response to the mutant were indicated, suggesting a potential subtle role in pathogenesis. The Y. pestis mutant also showed increased auto-aggregation and reduced survival in high-salinity conditions, phenotypes which may contribute to pathogenesis or survival.

Recognition of nucleic acids by pattern-recognition receptors and its relevance in autoimmunity

Summary:  Host cells trigger signals for innate immune responses upon recognition of conserved structures in microbial pathogens. Nucleic acids, which are critical components for inheriting genetic information in all species including pathogens, are key structures sensed by the innate immune system. The corresponding receptors for foreign nucleic acids include members of Toll‐like receptors, RIG‐I‐like receptors, and intracellular DNA sensors. While nucleic acid recognition by these receptors is required for host defense against the pathogen, there is a potential risk to the host of self‐nucleic acids recognition, thus precipitating autoimmune and autoinflammatory diseases. In this review, we discuss the roles of nucleic acid‐sensing receptors in guarding against pathogen invasion, discriminating between self and non‐self, and contributing to autoimmunity and autoinflammatory diseases.

Identification of interaction sites for dimerization and adapter recruitment in Toll/interleukin-1 receptor (TIR) domain of Toll-like receptor 4

Toll-like receptor signaling requires interactions of the Toll/IL-1 receptor (TIR) domains of the receptor and adapter proteins. Using the mammalian protein-protein interaction trap strategy, homology modeling, and site-directed mutagenesis, we identify the interaction surfaces in the TLR4 TIR domain for the TLR4-TLR4, TLR4-MyD88 adapter-like (MAL), and TLR4-TRIF-related adapter molecule (TRAM) interaction. Two binding sites are equally important for TLR4 dimerization and adapter recruitment. In a model based on the crystal structure of the dimeric TLR10 TIR domain, the first binding site mediates TLR4-TLR4 TIR-TIR interaction. Upon dimerization, two identical second binding sites of the TLR4 TIR domain are juxtaposed and form an extended binding platform for both MAL and TRAM. In our mammalian protein-protein interaction trap assay, MAL and TRAM compete for binding to this platform. Our data suggest that adapter binding can stabilize the TLR4 TIR dimerization.

Extensive polymorphism in the porcine Toll-like receptor 10 gene

The great importance of the Toll-like receptors (TLRs) in innate immunity is well established, but one family member--TLR10--remains elusive. TLR10 is expressed in various tissues in several species, but its ligand is not known and its function is still poorly understood. The open reading frame of TLR10 was sequenced in 15 wild boars, representing three populations, and in 15 unrelated domestic pigs of Hampshire, Landrace and Large White origin. Amino acid positions corresponding to detected nonsynonymous single nucleotide polymorphisms (SNPs) were analysed in the crystal structures determined for the human TLR1-TLR2-lipopeptide complex and the human TLR10 Toll/Interleukin 1 receptor (TIR) dimer. SNP occurrence in wild boars and domestic pigs was compared, and haplotypes for the TLR10 gene and the TLR6-1-10 gene cluster were reconstructed. Despite the limited number of animals sequenced in the present study (N = 30), a larger number of SNPs were found in TLR10 than recently reported for TLR1, TLR6 and TLR2. Thirty-three SNPs were detected, of which 20 were nonsynonymous. The relative frequency of nonsynonymous (d(N) ) and synonymous (d(S) ) SNPs between wild boars and domestic pigs was higher in TLR10 than recently reported for TLR1, TLR6 and TLR2. However, the polymorphism reported in the present study seems to leave the function of the TLR10 molecule unaffected. Furthermore, no nonsynonymous SNPs were detected in the part of the gene corresponding to the hinge region of the receptor, probably reflecting rigorously acting functional constraint. The total number of SNPs and the number of nonsynonymous SNPs were significantly lower (P < 0.05) in the wild boars than in the domestic pigs, and fewer TLR10 haplotypes were present in the wild boars. The majority of the TLR6-1-10 haplotypes were specific for either wild boars or domestic pigs, probably reflecting differences in microbial environment and population history.

Structural biology of the Toll-like receptor family

Innate immune receptors respond to common structural patterns in microbial molecules and are called pattern recognition receptors. Toll-like receptors (TLRs) play critical roles in the innate immune system by recognizing microbial lipids, carbohydrates, nucleic acids, and proteins. Precise definition of the ligand "pattern" of TLRs has been difficult to determine primarily owing to a lack of high-resolution structures. Recently, the structures of several TLR-ligand complexes and the intracellular signaling domains have been determined by X-ray crystallography. This new structural information, combined with extensive biochemical and immunological data accumulated over decades, sheds new light on ligand-recognition and -activation mechanisms. In this review, we summarize the TLR structures and discuss proposed ligand-recognition and -activation mechanisms.

Comparative analysis of species-specific ligand recognition in Toll-like receptor 8 signaling: a hypothesis

Toll-like receptors (TLRs) play a central role in the innate immune response by recognizing conserved structural patterns in a variety of microbes. TLRs are classified into six families, of which TLR7 family members include TLR7, 8, and 9, which are localized to endolysosomal compartments recognizing viral infection in the form of foreign nucleic acids. In our current study, we focused on TLR8, which has been shown to recognize different types of ligands such as viral or bacterial ssRNA as well as small synthetic molecules. The primary sequences of rodent and non-rodent TLR8s are similar, but the antiviral compound (R848) that activates the TLR8 pathway is species-specific. Moreover, the factors underlying the receptor's species-specificity remain unknown. To this end, comparative homology modeling, molecular dynamics simulations refinement, automated docking and computational mutagenesis studies were employed to probe the intermolecular interactions between this anti-viral compound and TLR8. Furthermore, comparative analyses of modeled TLR8 (rodent and non-rodent) structures have shown that the variation mainly occurs at LRR14-15 (undefined region); hence, we hypothesized that this variation may be the primary reason for the exhibited species-specificity. Our hypothesis was further bolstered by our docking studies, which clearly showed that this undefined region was in close proximity to the ligand-binding site and thus may play a key role in ligand recognition. In addition, the interface between the ligand and TLR8s varied depending upon the amino acid charges, free energy of binding, and interaction surface. Therefore, our current work provides a hypothesis for previous in vivo studies in the context of TLR signaling.

Anti-CD180 (RP105) activates B cells to rapidly produce polyclonal Ig via a T cell and MyD88-independent pathway

CD180 is homologous to TLR4 and regulates TLR4 signaling, yet its function is unclear. We report that injection of anti-CD180 mAb into mice induced rapid Ig production of all classes and subclasses, with the exception of IgA and IgG2b, with up to 50-fold increases in serum IgG1 and IgG3. IgG production after anti-CD180 injection was not due to reactivation of memory B cells and was retained in T cell-deficient (TCR knockout [KO]), CD40 KO, IL-4 KO, and MyD88 KO mice. Anti-CD180 rapidly increased both transitional and mature B cells, with especially robust increases in transitional B cell number, marginal zone B cell proliferation, and CD86, but not CD80, expression. In contrast, anti-CD40 induced primarily follicular B cell and myeloid expansion, with increases in expression of CD80 and CD95 but not CD86. The expansion of splenic B cells was due, in part, to proliferation and occurred in wild-type and TCR KO mice, whereas T cell expansion occurred in wild-type, but not in B cell-deficient, mice, indicating a direct role for B cells in CD180 stimulation in vivo. Combination of anti-CD180 with various MyD88-dependent TLR ligands biased B cell fate because coinjection diminished Ig production, but purified B cells exhibited synergistic proliferation. Anti-CD180 had no effect on cytokine production from B cells, but it increased IL-6, IL-10, and TNF-α production in combination with LPS or CpG. Thus, CD180 stimulation induces intrinsic B cell proliferation and differentiation, causing rapid increases in IgG, and integrates MyD88-dependent TLR signals to regulate proliferation, cytokine production, and differentiation.

Crystal structure of Toll-like receptor adaptor MAL/TIRAP reveals the molecular basis for signal transduction and disease protection

Initiation of the innate immune response requires agonist recognition by pathogen-recognition receptors such as the Toll-like receptors (TLRs). Toll/interleukin-1 receptor (TIR) domain-containing adaptors are critical in orchestrating the signal transduction pathways after TLR and interleukin-1 receptor activation. Myeloid differentiation primary response gene 88 (MyD88) adaptor-like (MAL)/TIR domain-containing adaptor protein (TIRAP) is involved in bridging MyD88 to TLR2 and TLR4 in response to bacterial infection. Genetic studies have associated a number of unique single-nucleotide polymorphisms in MAL with protection against invasive microbial infection, but a molecular understanding has been hampered by a lack of structural information. The present study describes the crystal structure of MAL TIR domain. Significant structural differences exist in the overall fold of MAL compared with other TIR domain structures: A sequence motif comprising a β-strand in other TIR domains instead corresponds to a long loop, placing the functionally important "BB loop" proline motif in a unique surface position in MAL. The structure suggests possible dimerization and MyD88-interacting interfaces, and we confirm the key interface residues by coimmunoprecipitation using site-directed mutants. Jointly, our results provide a molecular and structural basis for the role of MAL in TLR signaling and disease protection.

In silico approach to inhibition of signaling pathways of Toll-like receptors 2 and 4 by ST2L

Toll-like receptors (TLRs) activate a potent immunostimulatory response. There is clear evidence that overactivation of TLRs leads to infectious and inflammatory diseases. Recent biochemical studies have shown that the membrane-bound form of ST2 (ST2L), a member of the Toll-like/IL-1 receptor superfamily, negatively regulates MyD88-dependent TLR signaling pathways by sequestrating the adapters MyD88 and Mal (TIRAP). Specifically, ST2L attenuates the recruitment of Mal and MyD88 adapters to their receptors through its intracellular TIR domain. Thus, ST2L is a potent molecule that acts as a key regulator of endotoxin tolerance and modulates innate immunity. So far, the inhibitory mechanism of ST2L at the molecular level remains elusive. To develop a working hypothesis for the interactions between ST2L, TLRs (TLR1, 2, 4, and 6), and adapter molecules (MyD88 and Mal), we constructed three-dimensional models of the TIR domains of TLR4, 6, Mal, and ST2L based on homology modeling. Since the crystal structures of the TIR domains of TLR1, 2 as well as the NMR solution structure of MyD88 are known, we utilized these structures in our analysis. The TIR domains of TLR1, 2, 4, 6, MyD88, Mal and ST2L were subjected to molecular dynamics (MD) simulations in an explicit solvent environment. The refined structures obtained from the MD simulations were subsequently used in molecular docking studies to probe for potential sites of interactions. Through protein-protein docking analysis, models of the essential complexes involved in TLR2 and 4 signaling and ST2L inhibiting processes were developed. Our results suggest that ST2L may exert its inhibitory effect by blocking the molecular interface of Mal and MyD88 adapters mainly through its BB-loop region. Our predicted oligomeric signaling models may provide a basis for the understanding of the assembly process of TIR domain interactions, which has thus far proven to be difficult via in vivo studies.

The structural biology of Toll-like receptors

The membrane-bound Toll-like receptors (TLRs) trigger innate immune responses after recognition of a wide variety of pathogen-derived compounds. Despite the wide range of ligands recognized by TLRs, the receptors share a common structural framework in their extracellular, ligand-binding domains. These domains all adopt horseshoe-shaped structures built from leucine-rich repeat motifs. Typically, on ligand binding, two extracellular domains form an "m"-shaped dimer sandwiching the ligand molecule bringing the transmembrane and cytoplasmic domains in close proximity and triggering a downstream signaling cascade. Although the ligand-induced dimerization of these receptors has many common features, the nature of the interactions of the TLR extracellular domains with their ligands varies markedly between TLR paralogs.

Similar Structures but Different Roles - An Updated Perspective on TLR Structures

Toll-like receptors (TLRs) are pattern recognition receptors that recognize conserved structures in pathogens, trigger innate immune responses, and prime antigen-specific adaptive immunity. Elucidation of crystal structures of TLRs interacting with their ligands such as TLR1-2 with triacylated lipopeptide, TLR2-6 with diacylated lipopeptide, TLR4-MD-2 with LPS, and TLR3 with double-stranded RNA (dsRNA) have enabled an understanding of the initiation of TLR signaling. Agonistic ligands such as LPS, dsRNA, and lipopeptides induce "m" shaped TLR dimers in which C-termini converge at the center. Such central convergence is necessary to bring the two intracellular receptor TIR domains closer together and promote their dimerization, which serves as an essential step in downstream signaling. In this review, we summarize TLR ECD structures that have been reported to date with special emphasis on ligand recognition and activation mechanism.

TLR2: A Crossroads between Infections and Autoimmunity?

Environment has both pathogenic and protective roles in the determination of autoimmune disease development, possibly through infectious agents. TLR2 has the capability to recognize the widest range of PAMPs, and it is important for the recognition of mycobacteria and gram-positive bacteria. Here we review recent information showing that TLR2 ligands, its signaling machinery and the effects of its engagement on T cell polarization and differentiation, all play a decisive role in experimental models of autoimmunity. Thus, we propose that engagement of TLR2 is an important crossroads between encounter with bacteria and development of self-reactive diseases.

Structural and functional analysis of a plant resistance protein TIR domain reveals interfaces for self-association, signaling, and autoregulation

The Toll/interleukin-1 receptor (TIR) domain occurs in animal and plant immune receptors. In the animal Toll-like receptors, homodimerization of the intracellular TIR domain is required for initiation of signaling cascades leading to innate immunity. By contrast, the role of the TIR domain in cytoplasmic nucleotide-binding/leucine-rich repeat (NB-LRR) plant immune resistance proteins is poorly understood. L6 is a TIR-NB-LRR resistance protein from flax (Linum usitatissimum) that confers resistance to the flax rust phytopathogenic fungus (Melampsora lini). We determine the crystal structure of the L6 TIR domain and show that, although dispensable for pathogenic effector protein recognition, the TIR domain alone is both necessary and sufficient for L6 immune signaling. We demonstrate that the L6 TIR domain self-associates, most likely forming a homodimer. Analysis of the structure combined with site-directed mutagenesis suggests that self-association is a requirement for immune signaling and reveals distinct surface regions involved in self-association, signaling, and autoregulation.

Location, location, location: is membrane partitioning everything when it comes to innate immune activation?

In the last twenty years, the general view of the plasma membrane has changed from a homogeneous arrangement of lipids to a mosaic of microdomains. It is currently thought that islands of highly ordered saturated lipids and cholesterol, which are laterally mobile, exist in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place, such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs), in order to detect pathogens. One family of such receptors are the Toll-like receptors (TLRs), which are the central “sensing” apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands. These nanoscale assemblies of sphingolipid, cholesterol, and TLRs stabilize and coalesce, forming signalling platforms, which transduce signals that lead to innate immune activation. In the current paper, we will investigate all past and current literature concerning recruitment of extracellular and intracellular TLRs into lipid rafts and how this membrane organization modulates innate immune responses.

Yersinia pestis TIR-domain protein forms dimers that interact with the human adaptor protein MyD88

Recent research has highlighted the presence of Toll/Interleukin 1 receptor (TIR)-domain proteins (Tdps) in a range of bacteria, suggested to form interactions with the human adaptor protein MyD88 and inhibit intracellular signaling from Toll-like receptors (TLRs). A Tdp has been identified in Yersinia pestis (YpTdp), a highly pathogenic bacterium responsible for plague. Expression of a number of YpTIR constructs of differing lengths (YpTIR1, S130-A285; YpTIR2, I137-I273; YpTIR3, I137-246; YpTIR4, D107-S281) as fusions with an N-terminal GB1 tag (the B1 immunoglobulin domain of Streptococcal protein G) yielded high levels of soluble protein. Subsequent purification yielded 4-6 mg/L pure, folded protein. Thrombin cleavage allowed separation of the GB1 tag from YpTIR4 resulting in folded protein after cleavage. Nuclear magnetic resonance spectroscopy, size exclusion chromatography, SDS-PAGE analysis and static light scattering all indicate that the YpTIR forms dimers. Generation of a double Cys-less mutant resulted in an unstable protein containing mainly monomers indicating the importance of disulphide bonds in dimer formation. In addition, the YpTIR constructs have been shown to interact with the human adaptor protein MyD88 using 2D NMR and GST pull down. YpTIR is an excellent candidate for further study of the mechanism of action of pathogenic bacterial Tdps.

TIR domain-containing adaptor SARM is a late addition to the ongoing microbe-host dialog

Toll/interleukin-1 receptor (TIR) domain-containing proteins play important roles in defense against pathogens in both animals and plants, connecting the immunity signaling pathways via a chain of specific protein-protein interactions. Among them is SARM, the only TIR domain-containing adaptor that can negatively regulate TLR signaling. By extensive phylogenetic analysis, we show here that SARM is closely related to bacterial proteins with TIR domains, suggesting that this family has a different evolutionary history from other animal TIR-containing adaptors, possibly emerging via a lateral gene transfer from bacteria to animals. We also show evidence of several similar, independent transfer events, none of which, however, survived in vertebrates. An evolutionary relationship between the animal SARM adaptor and bacterial proteins with TIR domains illustrates the possible role that bacterial TIR-containing proteins play in regulating eukaryotic immune responses and how this mechanism was possibly adapted by the eukaryotes themselves.

Porcine Toll-like receptors: recognition of Salmonella enterica serovar Choleraesuis and influence of polymorphisms

Salmonella enterica serovar Choleraesuis (SC) is a highly invasive pathogen that causes enteric and septicemic diseases in pigs. Although there have been some reports on gene expression profiles in the course of infection with SC in pigs, little is known about the genes involved in the infection. By measuring activation, as represented by nuclear factor-κB activity, after stimulation by the pathogen, we showed the involvement of Toll-like receptor (TLR) 5 and the TLR2-TLR1 heterodimer in the recognition of SC. We previously found single nucleotide polymorphisms (SNPs) in the TLRs of various pig populations. Here we demonstrated that the polymorphisms resulting in amino acid changes TLR5(R148L), TLR5(P402L), and TLR2(V703M) attenuated the responses to SC by the cells transfected with the TLR genes. Each of these three SNPs was differently restricted in distribution among breeds. These results suggest that there are differences in resistance to salmonellosis among breeds; these differences may be of great importance for the pig industry in terms of breeding and vaccine development.

Targeting TLR4 signaling by TLR4 Toll/IL-1 receptor domain-derived decoy peptides: identification of the TLR4 Toll/IL-1 receptor domain dimerization interface

Agonist-induced dimerization of TLR4 Toll/IL-1R (TIR) domains initiates intracellular signaling. Therefore, identification of the TLR4-TIR dimerization interface is one key to the rational design of therapeutics that block TLR4 signaling. A library of cell-permeating decoy peptides, each of which represents a nonfragmented patch of the TLR4 TIR surface, was designed such that the peptides entirely encompass the TLR4 TIR surface. Each peptide was synthesized in tandem with a cell-permeating Antennapedia homeodomain sequence and tested for the ability to inhibit early cytokine mRNA expression and MAPK activation in LPS-stimulated primary murine macrophages. Five peptides--4R1, 4R3, 4BB, 4R9, and 4αE--potently inhibited all manifestations of TLR4, but not TLR2 signaling. When tested for their ability to bind directly to TLR4 TIR by Förster resonance energy transfer using time-resolved fluorescence spectroscopy, Bodipy-TMR-X-labeled 4R1, 4BB, and 4αE quenched fluorescence of TLR4-Cerulean expressed in HeLa or HEK293T cells, whereas 4R3 was partially active, and 4R9 was least active. These findings suggest that the area between the BB loop of TLR4 and its fifth helical region mediates TLR4 TIR dimerization. Moreover, our data provide direct evidence for the utility of the decoy peptide approach, in which peptides representing various surface-exposed segments of a protein are initially probed for the ability to inhibit protein function, and then their specific targets are identified by Förster resonance energy transfer to define recognition sites in signaling proteins that may be targeted therapeutically to disrupt functional transient protein interactions.

Crystallization, X-ray diffraction analysis and preliminary structure determination of the TIR domain from the flax resistance protein L6

The Toll/interleukin-1 receptor (TIR) domain is a protein-protein interaction domain that is found in both animal and plant immune receptors. In animal Toll-like receptor signalling, both homotypic TIR-domain interactions between two receptor molecules and heterotypic interactions between receptors and TIR-domain-containing adaptors are required for initiation of an innate immune response. The TIR domains in cytoplasmic nucleotide-binding/leucine-rich repeat (NB-LRR) plant disease-resistance proteins are not as well characterized, but recent studies have suggested a role in defence signalling. In this study, the crystallization, X-ray diffraction analysis and preliminary structure determination of the TIR domain from the flax resistance protein L6 (L6TIR) are reported. Plate-like crystals of L6TIR were obtained using PEG 200 as a precipitant and diffracted X-rays to 2.3 Å resolution. Pseudo-translation complicated the initial assignment of the crystal symmetry, which was ultimately found to correspond to space group P2(1)2(1)2 with two molecules per asymmetric unit. The structure of L6TIR was solved by molecular replacement using the structure of the TIR-domain-containing protein AT1G72930 from Arabidopsis as a template.

Modeling of the Toll-like receptor 3 and a putative Toll-like receptor 3 antagonist encoded by the African swine fever virus

African swine fever virus (ASFV) is a large double-stranded DNA virus responsible for a lethal pig disease, to which no vaccine has ever been obtained. Its genome encodes a number of proteins involved in virus survival and transmission in its hosts, in particular proteins that inhibit signaling pathways in infected macrophages and, thus, interfere with the host's innate immune response. A recently identified novel ASFV viral protein (pI329L) was found to inhibit the Toll-like receptor 3 (TLR3) signaling pathway, TLR3 being a crucial "danger detector." pI329L has been predicted to be a transmembrane protein containing extracellular putative leucine-rich repeats similar to TLR3, suggesting that pI329L might act as a TLR3 decoy. To explore this idea, we used comparative modeling and other structure prediction protocols to propose (a) a model for the TLR3-Toll-interleukin-1 receptor homodimer and (b) a structural fold for pI329L, detailed at atomistic level for its cytoplasmic domain. As this later domain shares only remote sequence relationships with the available TLR3 templates, a more complex modeling strategy was employed that combines the iterative implementation of (multi)threading/assembly/refinement (I-TASSER) structural prediction with expertise-guided posterior refinement. The final pI329L model presents a plausible fold, good structural quality, is consistent with the available experimental data, and it corroborates our hypothesis of pI329L being a TLR3 antagonist.

Neural injury following stroke: are Toll-like receptors the link between the immune system and the CNS?

The CNS can exhibit features of inflammation in response to injury, infection or disease, whereby resident cells generate inflammatory mediators, including cytokines, prostaglandins, free radicals and complement, chemokines and adhesion molecules that recruit immune cells, and activate glia and microglia. Cerebral ischaemia triggers acute inflammation, which exacerbates primary brain damage. The regulation of inflammation after stroke is multifaceted and comprises vascular effects, distinct cellular responses, apoptosis and chemotaxis. There are many cell types that are affected including neurons, astrocytes, microglia and endothelial cells, all responding to the resultant neuroinflammation in different ways. Over the past 20 years, researchers examining brain tissue at various time intervals after stroke observed the presence of inflammatory cells, neutrophils and monocytes at the site of injury, as well as the activation of endogenous glia and microglia. This review examines the involvement of these cells in the progression of neural injury and proposes that the Toll-like receptors (TLRs) are likely to be an integral component in the communication between the CNS and the periphery. This receptor system is the archetypal pathogen sensing receptor system and its presence and signalling in the brain following neural injury suggests a more diverse role. We propose that the TLR system presents excellent pharmacological targets for the design of a new generation of therapeutic agents to modulate the inflammation that accompanies neural injury.

1H, 13C, and 15N resonance assignment of the TIR domain of human MyD88

Myeloid differentiating factor 88 (MyD88) is one of a critical adaptor molecule in the Toll-like receptor (TLR) signaling pathway. The TIR domain of MyD88 serves as a protein-protein interaction module and interacts with other TIR-containing proteins such as Mal (MyD88 adaptor-like) and Toll-like receptor 4 to form signal initiation complexes. Here we report the (15)N, (13)C, and (1)H chemical shift assignments of the TIR domain of MyD88. The resonance assignments obtained in this work will contribute to the study of heteromeric TIR-TIR interactions between MyD88 and TIR-containing receptors or adaptors.

TAK-242 (resatorvid), a small-molecule inhibitor of Toll-like receptor (TLR) 4 signaling, binds selectively to TLR4 and interferes with interactions between TLR4 and its adaptor molecules

TAK-242 (resatorvid), a small-molecule-specific inhibitor of Toll-like receptor (TLR) 4 signaling, inhibits the production of lipopolysaccharide-induced inflammatory mediators by binding to the intracellular domain of TLR4. Cys747 in TLR4 has been identified previously as the binding site of TAK-242. However, the mechanism by which TAK-242 inhibits TLR4 signaling after binding to TLR4 remains unknown. The present study demonstrated, using coimmunoprecipitation, that TAK-242 interferes with protein-protein interactions between TLR4 and its adaptor molecules. Among 10 different human TLRs, TAK-242 selectively bound to TLR4. The time course of the inhibitory effect of TAK-242 on inflammatory mediator production corresponded to that of the binding of TAK-242 to TLR4. TAK-242 inhibited the association of TLR4 with Toll/interleukin-1 receptor domain-containing adaptor protein (TIRAP) or Toll/interleukin-1 receptor domain-containing adaptor protein inducing interferon-β-related adaptor molecule (TRAM) in human embryonic kidney (HEK) 293 cells overexpressing TLR4, MD-2, and TIRAP or TRAM, respectively. TAK-242 inhibited the TIRAP-mediated activation of nuclear factor κB (NF-κB) and the TRAM-mediated activation of NF-κB and interferon-sensitive response element in HEK293 cells stably expressing TLR4, MD-2, and CD14. The activation of endogenous interleukin-1 receptor-associated kinase in RAW264.7 cells was also inhibited by TAK-242 treatment. These findings suggest that TAK-242 binds selectively to TLR4 and subsequently disrupts the interaction of TLR4 with adaptor molecules, thereby inhibiting TLR4 signal transduction and its downstream signaling events. This work proposes a novel paradigm of a small molecule capable of disrupting protein-protein interactions.

Reactive oxygen species enhance TLR10 expression in the human monocytic cell line THP-1

We investigated TLR10 expression in human monocytes, THP-1 cells, cultured in hypoxia (3% O₂). Levels of both TLR10 mRNA and protein in THP-1 cells cultured in hypoxia were significantly higher than those cultured in normoxia (20% O₂). We examined intracellular reactive oxygen species (ROS) content in hypoxic cells, and TLR10 expression in cells treated with hydrogen peroxide (H₂O₂), to determine whether the increase in TLR10 expression observed with hypoxia was due to an increase in intracellular ROS levels. We found that the level of intracellular ROS in cells subject to hypoxia was significantly higher than in normoxia. Experiments with ROS synthesis inhibitors revealed that hypoxia induced ROS production is mainly due to NADPH oxidase activity. TLR10 mRNA expression was increased by treatment with H₂O₂ at concentrations ranging from 50 to 250 μM. We screened the TLR10 promoter and found putative binding sites for transcription factors (TFs), such as NF-κB, NF-AT and AP-1. Next, we examined TF activities using a luciferase reporter assay. Activities of NF-κB, NF-AT and AP-1 in the cells treated with H₂O₂ were significantly higher than in untreated cells. The experiment with TF inhibitors revealed that ROS-induced upregulation of TLR10 expression is mainly due to NF-κB activation. Overall, our results suggest that hypoxia or ROS increase TLR10 expression in human monocytes and the transcriptional activities of NF-κB are involved in this process. Therefore, it is suggested that ROS produced by various exogenous stimuli may play a crucial role in the regulation of expression and function of TLR10 as second messengers.

Subversion of innate immune signaling through molecular mimicry

Innate immune signaling is mediated by a number of membrane-anchored or cytosolic receptor or sensor molecules. Several receptor families utilize conserved signaling domains such as the Toll/interleukin-1 receptor (TIR) domain and Pyrin domain (PYD) to link microbe recognition to induction of proinflammatory cytokines and interferons. Recent studies have identified a number of bacterial and viral TIR domains and PYD domains that directly target the signaling function of their host homologues. Emerging biochemical and structural studies of these microbial TIR and PYD domains suggest that they are mimics of their host counterparts at the sequence and structure levels. Unraveling the mechanisms of such molecular mimicry is crucial to our understanding and clinical intervention of infectious diseases and inflammatory disorders.

Targeting TLR/IL-1R signalling in human diseases

The members of Toll-like receptor/Interleukin (IL)-1 receptor (TLR/IL-1R) superfamily play a fundamental role in the immune response. These receptors detect microbial components and trigger complex signalling pathways that result in increased expression of multiple inflammatory genes. On the other hand, an aberrant activation of TLR/IL-1R signalling can promote the onset of inflammatory and autoimmune diseases, raising the interest in the development of therapeutic strategies for the control of their function. In this review, we illustrate the structural and functional features of TLR/IL-1R proteins and discuss some recent advances in the approaches undertaken to develop anti-inflammatory therapeutic drugs. In particular, we will focus on inhibitors, such as decoy peptides and synthetic mimetics, that interfere with protein-protein interactions between signalling molecules of the TLR/IL-1R superfamily. Given their central role in innate and adaptive immune responses, it is foreseen that pharmaceutical modulation of TLR/IL-1R signalling pathways by these drugs might yield clinical benefits in the treatment of inflammatory and autoimmune diseases.

The crystal structure of a TIR domain from Arabidopsis thaliana reveals a conserved helical region unique to plants

Plants use a highly evolved immune system to exhibit defense response against microbial infections. The plant TIR domain, together with the nucleotide-binding (NB) domain and/or a LRR region, forms a type of molecule, named resistance (R) proteins, that interact with microbial effector proteins and elicit hypersensitive responses against infection. Here, we report the first crystal structure of a plant TIR domain from Arabidopsis thaliana (AtTIR) solved at a resolution of 2.0 A. The structure consists of five beta-strands forming a parallel beta-sheet at the core of the protein. The beta-strands are connected by a series of alpha-helices and the overall fold mimics closely that of other mammalian and bacterial TIR domains. However, the region of the alphaD-helix reveals significant differences when compared with other TIR structures, especially the alphaD3-helix that corresponds to an insertion only present in plant TIR domains. Available mutagenesis data suggest that several conserved and exposed residues in this region are involved in the plant TIR signaling function.

Human TLRs 10 and 1 share common mechanisms of innate immune sensing but not signaling

TLRs are central receptors of the innate immune system that drive host inflammation and adaptive immune responses in response to invading microbes. Among human TLRs, TLR10 is the only family member without a defined agonist or function. Phylogenetic analysis reveals that TLR10 is most related to TLR1 and TLR6, both of which mediate immune responses to a variety of microbial and fungal components in cooperation with TLR2. The generation and analysis of chimeric receptors containing the extracellular recognition domain of TLR10 and the intracellular signaling domain of TLR1, revealed that TLR10 senses triacylated lipopeptides and a wide variety of other microbial-derived agonists shared by TLR1, but not TLR6. TLR10 requires TLR2 for innate immune recognition, and these receptors colocalize in the phagosome and physically interact in an agonist-dependent fashion. Computational modeling and mutational analysis of TLR10 showed preservation of the essential TLR2 dimer interface and lipopeptide-binding channel found in TLR1. Coimmunoprecipitation experiments indicate that, similar to TLR2/1, TLR2/10 complexes recruit the proximal adaptor MyD88 to the activated receptor complex. However, TLR10, alone or in cooperation with TLR2, fails to activate typical TLR-induced signaling, including NF-kappaB-, IL-8-, or IFN-beta-driven reporters. We conclude that human TLR10 cooperates with TLR2 in the sensing of microbes and fungi but possesses a signaling function distinct from that of other TLR2 subfamily members.

Caspase-1 targets the TLR adaptor Mal at a crucial TIR-domain interaction site

Toll-like receptors (TLRs) are crucial components of innate immunity, ensuring efficient responses against invading pathogens. After ligand binding, TLR signaling is initiated by recruitment of adaptor molecules, a step mediated by homotypic Toll-IL-1 receptor (TIR) domain interactions. Four TIR-containing TLR adaptor molecules are described, all of which are susceptible to modification and strict regulation. For example, caspase-1 is reported to cleave the TLR adaptor Mal at position D198, an event that is indispensible for Mal function. In this report, we use the mammalian two-hybrid technique MAPPIT to study the implications of Mal cleavage. We show that a Mal mutant, which mimics caspase-1 cleavage and a caspase-1-uncleavable MalD198A mutant, are abrogated in their bridging function and lose the ability to activate NF-κB. A MalD198E mutant is still fully functional, suggesting that caspase-1 cleavage of Mal is not necessary for Mal-mediated signaling. D198 of Mal is conserved in MyD88 and TLR4 TIR domains and the negatively charged amino acid at this position is crucial for the interactions and function of Mal, MyD88 and TLR4 TIR. Our data suggest an inhibitory, rather than an activating role for caspase-1 in Mal regulation, and show that the caspase-1 cleavage site in Mal is part of a TIR-domain interaction site.

Membrane partitioning: is location everything when it comes to endotoxin recognition?

Lipid rafts are envisaged as islands of highly ordered saturated lipids and cholesterol that are laterally mobile in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place- such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs) in order to "sense" pathogens. One family of such receptors are the Toll like receptors (TLRs), which are the central "sensing" apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands and these constitute signalling platforms, which transducer singals that lead to innate immune activation. In this chapter will review all past and current literature concerning recruitment of TLRs into lipid rafts and how this membrane compartmentalization is crucial for innate immune responses.

Activating immunity: lessons from the TLRs and NLRs

The Toll-like receptors and NOD-like receptors are key families in the innate immune response. The specific detection of activating ligand facilitates receptor interactions, the formation of multiprotein signalling complexes and initiation of signal transduction cascades. This process can trigger the upregulation of proinflammatory mediators, apoptosis, and modulation of other immune defences. Recently, significant advances have been made in the identification of new activating ligands and the determination of the molecular basis of ligand recognition within these receptor families. Understanding these processes provides information essential to the development of new vaccine adjuvants and the treatment of infectious diseases, inflammatory disorders and, potentially, cancer.