Structure and function of Toll receptors and their ligands

MyD88 adaptor-like is not essential for TLR2 signaling and inhibits signaling by TLR3

Although a clear role for the adaptor protein myeloid differentiation factor-88 (MyD88) adaptor-like (Mal, or TIRAP) in TLR4 signaling has been demonstrated, there is limited information on its role in TLR2 signaling. Here we have systematically analyzed the role of Mal in signaling by TLR2, TLR4, and as a control TLR3 in murine macrophages and dendritic cells. Mal was not required for the induction of IL-6 or NFkappaB activation at high concentrations of the TLR1/2 ligand Pam(3)Cys-Ser-(Lys)(4) or the TLR2/6 ligand macrophage-activating lipopeptide-2 and was required for these responses only at low ligand concentrations. Similarly, induction of IL-6 by Salmonella typhimurium, which is sensed by TLR2, required Mal only at low levels of bacteria. Mal was required for IL-6 induction at all concentrations of the TLR4 ligand LPS. Mal deficiency boosted IL-6 induction by the TLR3 ligand polyinosinic-polycytidylic acid. Activation of JNK, but not p38 or IkappaB degradation, was similarly potentiated in response to polyinosinic-polycytidylic acid in Mal-deficient macrophages. MyD88 was vital for all TLR2 and TLR4 responses and, similar to Mal, was also inhibitory for TLR3-dependent IL-6 and JNK induction. MyD88 interacted with the Toll/IL-1R domains of TLR1, TLR2, TLR4, and TLR6. Mal interacted with the Toll/Il-1R domains of TLR1, TLR2, and TLR4 but not with TLR6. Our study, therefore, reveals that Mal is dispensable in TLR2 signaling at high ligand concentrations in macrophages and dendritic cells, with MyD88 probably coupling to the TLR2 receptor complex at sufficient levels to allow activation. An inhibitory role for Mal in TLR3 signaling to JNK was also demonstrated.

Loss of Kupffer cells in diet-induced obesity is associated with increased hepatic steatosis, STAT3 signaling, and further decreases in insulin signaling

While adipose tissue-associated macrophages contribute to development of chronic inflammation and insulin resistance of obesity, little is known about the role of hepatic Kupffer cells in this environment. Here we address the impact of Kupffer cell ablation using clodronate-encapsulated liposome depletion in a diet-induced obese (DIO) and insulin resistant mouse model. Hepatic expression of macrophage markers measured by realtime RT-PCR remained unaltered in DIO mice despite characteristic expansion of adipose tissue-associated macrophages. DIO mouse livers displayed increased expression of alternative activation markers but unaltered proinflammatory cytokine expression when compared to lean mice. Kupffer cell ablation reduced hepatic anti-inflammatory cytokine IL-10 mRNA expression in lean and DIO mice by 95% and 84%, respectively. Despite decreased hepatic IL-6 gene expression after ablation in lean and DIO mice, hepatic STAT3 phosphorylation, Socs3 and acute phase protein mRNA expression increased. Kupffer cell ablation in DIO mice resulted in additional hepatic triglyceride accumulation and a 30-40% reduction in hepatic insulin receptor autophosphorylation and Akt activation. Implicating systemic loss of IL-10, high-fat-fed IL-10 knockout mice also displayed increased hepatic STAT3 signaling and hepatic triglyceride accumulation. Insulin signaling was not altered, however. In conclusion, Kupffer cells are a major source of hepatic IL-10 expression, the loss of which is associated with increased STAT3-dependent signaling and steatosis. One or more additional factors appear to be required, however, for the Kupffer cell-dependent protective effect on insulin receptor signaling in DIO mice.

LRRC19, a novel member of the leucine-rich repeat protein family, activates NF-kappaB and induces expression of proinflammatory cytokines

We have identified a new functional transmembrane receptor, LRRC19 (leucine-rich repeat containing 19), that belongs to the LRR protein family. LRRC19's central core has four analogous LRR repeating modules in a juxtaposed array and a casein kinase (CK2) phosphorylation site in the cytoplasmic domain. LRRC19 mRNA was found in the kidney, spleen and intestine of adult mice using both RT-PCR and in situ hybridization. LRRC19 does not contain a cytoplasmic Toll/IL-1 receptor (TIR) domain but was able to activate NF-kappaB and induce production of proinflammatory cytokines. LRRC19 shares a close evolutionary relationship with multiple Toll-like receptors (TLRs), especially TLR3. Importantly, the TLR3 ligand, as well as other TLR ligands, significantly promoted the expression of proinflammatory cytokines and the activation of NF-kappaB by LRRC19. Thus, LRRC19 may play an important role in inducing innate immune responses in certain tissues such as the kidney.

Structure and regulation of cytoplasmic adapter proteins involved in innate immune signaling

Initiation of the innate immune response requires agonist recognition by a pathogen recognition receptor. Following ligand binding, conformational rearrangement of the receptor creates a molecular scaffold from which signal transduction is propagated via complex cellular signaling pathways. This in turn leads to the induction of a pro-inflammatory immune response. A critical component of these signaling pathways is the homotypic interaction of receptor and adapter proteins via specific protein interaction domains. Within the innate immune signaling cascade, homotypic interactions between members of the death domain family and the Toll/interleukin-1 receptor domain are particularly important. Here we discuss the current understanding of the molecular basis of these homotypic receptor:adapter interactions and their role in innate immune signal transduction.

Proteolytic cascade for the activation of the insect toll pathway induced by the fungal cell wall component

The insect Toll signaling pathway is activated upon recognition of Gram-positive bacteria and fungi, resulting in the expression of antimicrobial peptides via NF-kappaB-like transcription factor. This activation is mediated by a serine protease cascade leading to the processing of Spätzle, which generates the functional ligand of the Toll receptor. Recently, we identified three serine proteases mediating Toll pathway activation induced by lysine-type peptidoglycan of Gram-positive bacteria. However, the identities of the downstream serine protease components of Gram-negative-binding protein 3 (GNBP3), a receptor for a major cell wall component beta-1,3-glucan of fungi, and their order of activation have not been characterized yet. Here, we identified three serine proteases that are required for Toll activation by beta-1,3-glucan in the larvae of a large beetle, Tenebrio molitor. The first one is a modular serine protease functioning immediately downstream of GNBP3 that proteolytically activates the second one, a Spätzle-processing enzyme-activating enzyme that in turn activates the third serine protease, a Spätzle-processing enzyme. The active form of Spätzle-processing enzyme then cleaves Spätzle into the processed Spätzle as Toll ligand. In addition, we show that injection of beta-1,3-glucan into Tenebrio larvae induces production of two antimicrobial peptides, Tenecin 1 and Tenecin 2, which are also inducible by injection of the active form of Spätzle-processing enzyme-activating enzyme or processed Spätzle. These results demonstrate a three-step proteolytic cascade essential for the Toll pathway activation by fungal beta-1,3-glucan in Tenebrio larvae, which is shared with lysine-type peptidoglycan-induced Toll pathway activation.

An oligomeric signaling platform formed by the Toll-like receptor signal transducers MyD88 and IRAK-4

Toll-like receptors (TLRs) mediate responses to pathogen-associated molecules as part of the vertebrate innate immune response to infection. Receptor dimerization is coupled to downstream signal transduction by the recruitment of a post-receptor complex containing the adaptor protein MyD88 and the IRAK protein kinases. In this work, we show that the death domains of human MyD88 and IRAK-4 assemble into closed complexes having unusual stoichiometries of 7:4 and 8:4, the Myddosome. Formation of the Myddosome is likely to be a key event for TLR4 signaling in vivo as we show here that pathway activation requires that the receptors cluster into lipid rafts. Taken together, these findings indicate that TLR activation causes the formation of a highly oligomeric signaling platform analogous to the death-inducing signaling complex of the Fas receptor pathway.

Identification of an N-terminal recognition site in TLR9 that contributes to CpG-DNA-mediated receptor activation

Although it is well established that TLR9 recognizes CpG-DNA, the structural details of ligand-receptor interaction are still mostly unknown. The extracellular domain of TLR9 is composed of 25 leucine-rich repeat (LRR) motifs, 5 of which bear inserting sequences that do not conform to the LRR consensus motif. In this study, we show that the functional integrity of the extracellular domain of murine TLR9 is lost by deletion of individual LRR motifs. When deleting only the inserting sequences, we observed that LRR2, 5, and 8 contribute to receptor activation by CpG-DNA. The latter deletions did not affect receptor dimerization but inhibited CpG-DNA binding. On the basis of a homology modeling approach, we furthermore identify a positively charged region in the N terminus that is essential for CpG-DNA-induced TLR9 activation. This interaction site mirrors findings previously shown for the structural recognition of dsRNA by TLR3 and hints toward a general principle of nucleic acid recognition by the respective TLR.

Complement promotes the development of inflammatory T-helper 17 cells through synergistic interaction with Toll-like receptor signaling and interleukin-6 production

Toll-like receptors (TLRs) and complement are 2 major components of innate immunity that provide a first-line host defense and shape the adaptive immune responses. We show here that coincidental activation of complement and several TLRs in mice led to the synergistic production of serum factors that promoted T-helper cell 17 (Th17) differentiation from anti-CD3/CD28 or antigen-stimulated T cells. Although multiple TLR-triggered cytokines were regulated by complement, Th17 cell-promoting activity in the serum was correlated with interleukin (IL)-6 induction, and antibody neutralization of IL-6 abrogated the complement effect. By using both in vitro and in vivo approaches, we examined in more detail the mechanism and physiologic implication of complement/TLR4 interaction on Th17-cell differentiation. We found that the complement effect required C5a receptor, was evident at physiologically relevant levels of C5a, and could be demonstrated in cultured peritoneal macrophages as well as in the setting of antigen immunization. Importantly, despite an inhibitory effect of complement on IL-23 production, complement-promoted Th17 cells were functionally competent in causing autoimmunity in an adoptive transfer model of experimental autoimmune encephalomyelitis. Collectively, these data establish a link between complement/TLR interaction and Th17-cell differentiation and provide new insight into the mechanism of action of complement in autoimmunity.

Therapeutic targeting of Toll-like receptors for infectious and inflammatory diseases and cancer

Since first being described in the fruit fly Drosophila melanogaster, Toll-like receptors (TLRs) have proven to be of great interest to immunologists and investigators interested in the molecular basis to inflammation. They recognize pathogen-derived factors and also products of inflamed tissue, and trigger signaling pathways that lead to activation of transcription factors such as nuclear factor-kappaB and the interferon regulatory factors. These in turn lead to induction of immune and inflammatory genes, including such important cytokines as tumor necrosis factor-alpha and type I interferon. Much evidence points to a role for TLRs in immune and inflammatory diseases and increasingly in cancer. Examples include clear roles for TLR4 in sepsis, rheumatoid arthritis, ischemia/reperfusion injury, and allergy. TLR2 has been implicated in similar pathologic conditions and also in systemic lupus erythematosus (SLE) and tumor metastasis. TLR7 has also been shown to be important in SLE. TLR5 has been shown to be radioprotective. Recent advances in our understanding of signaling pathways activated by TLRs, structural insights into TLRs bound to their ligands and antagonists, and approaches to inhibit TLRs (including antibodies, peptides, and small molecules) are providing possiblemeans by which to interfere with TLRs clinically. Here we review these recent advances and speculate about whether manipulating TLRs is likely to be successful in fighting off different diseases.

Novel mutations in TLR genes cause hyporesponsiveness to Mycobacterium avium subsp. paratuberculosis infection

BACKGROUND

Toll like receptors (TLR) play the central role in the recognition of pathogen associated molecular patterns (PAMPs). Mutations in the TLR1, TLR2 and TLR4 genes may change the ability to recognize PAMPs and cause altered responsiveness to the bacterial pathogens.

RESULTS

The study presents association between TLR gene mutations and increased susceptibility to Mycobacterium avium subsp. paratuberculosis (MAP) infection. Novel mutations in TLR genes (TLR1- Ser150Gly and Val220Met; TLR2 - Phe670Leu) were statistically correlated with the hindrance in recognition of MAP legends. This correlation was confirmed subsequently by measuring the expression levels of cytokines (IL-4, IL-8, IL-10, IL-12 and IFN-gamma) in the mutant and wild type moDCs (mocyte derived dendritic cells) after challenge with MAP cell lysate or LPS. Further in silico analysis of the TLR1 and TLR4 ectodomains (ECD) revealed the polymorphic nature of the central ECD and irregularities in the central LRR (leucine rich repeat) motifs.

CONCLUSION

The most critical positions that may alter the pathogen recognition ability of TLR were: the 9th amino acid position in LRR motif (TLR1-LRR10) and 4th residue downstream to LRR domain (exta-LRR region of TLR4). The study describes novel mutations in the TLRs and presents their association with the MAP infection.

The inflammasomes: guardians of the body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1beta and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

In the trenches of plant pathogen recognition: Role of NB-LRR proteins

As in nearly every discipline of plant biology, new insights are constantly changing our understanding of plant immunity. It is now clear that plant immunity is controlled by two layers of inducible responses: basal responses triggered by conserved microbial features and specific responses triggered by gene-for-gene recognition of pathogen effector proteins by host resistance (R) proteins. The nucleotide-binding domain leucine-rich repeat (NB-LRR) class of R proteins plays a major role in the combat against a wide range of plant pathogens. The variation that has been generated and is maintained within these conserved proteins has diversified their specificity, subcellular localisations, activation and recognition mechanisms, allowing them to specifically adapt to different plant-pathogen interaction systems. This review addresses recent advances in the molecular role of NB-LRR proteins in pathogen recognition and activation of plant defence responses.

Bioinformatic analysis of Toll-like receptor sequences and structures

Continual advancements in computing power and sophistication, coupled with rapid increases in protein sequence and structural information, have made bioinformatic tools an invaluable resource for the molecular and structural biologists. With the degree of sequence information continuing to expand at an almost exponential rate, it is essential that scientists today have a basic understanding of how to utilise, manipulate, and analyse this information for the benefit of their own experiments. In the context of Toll-interleukin-1 receptor (TIR) domain containing proteins, we describe here a series of the more common and user-friendly bioinformatic tools available as internet-based resources. These will enable the identification and alignment of protein sequences, the identification of functional motifs, the characterisation of protein secondary structure, the identification of protein structural folds and distantly homologous proteins, and the validation of the structural geometry of modelled protein structures.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

Mycoplasma lipoproteins and Toll-like receptors

Mycoplasmas, the smallest free-living, self-replicating bacteria with diameters of 200 to 800 nm, have been reported to be associated with human diseases. It is well known that the mycoplasma lipoprotein/peptide is able to modulate the host immune system, whose N-terminal structure is an important factor in inducing immunity and distinguishing Toll-like receptors (TLRs). However, there is still no clear elucidation about the pathogenic mechanism of mycoplasma lipoprotein/peptide and the signaling pathway. Some researchers have focused on understanding the structures of these proteins and the relationships between their structure and biological function. This review provides an update on the research in this field.

GPR109A is a G-protein-coupled receptor for the bacterial fermentation product butyrate and functions as a tumor suppressor in colon

Short-chain fatty acids, generated in colon by bacterial fermentation of dietary fiber, protect against colorectal cancer and inflammatory bowel disease. Among these bacterial metabolites, butyrate is biologically most relevant. GPR109A is a G-protein-coupled receptor for nicotinate but recognizes butyrate with low affinity. Millimolar concentrations of butyrate are needed to activate the receptor. Although concentrations of butyrate in colonic lumen are sufficient to activate the receptor maximally, there have been no reports on the expression/function of GPR109A in this tissue. Here we show that GPR109A is expressed in the lumen-facing apical membrane of colonic and intestinal epithelial cells and that the receptor recognizes butyrate as a ligand. The expression of GPR109A is silenced in colon cancer in humans, in a mouse model of intestinal/colon cancer, and in colon cancer cell lines. The tumor-associated silencing of GPR109A involves DNA methylation directly or indirectly. Reexpression of GPR109A in colon cancer cells induces apoptosis, but only in the presence of its ligands butyrate and nicotinate. Butyrate is an inhibitor of histone deacetylases, but apoptosis induced by activation of GPR109A with its ligands in colon cancer cells does not involve inhibition of histone deacetylation. The primary changes in this apoptotic process include down-regulation of Bcl-2, Bcl-xL, and cyclin D1 and up-regulation of death receptor pathway. In addition, GPR109A/butyrate suppresses nuclear factor-kappaB activation in normal and cancer colon cell lines as well as in normal mouse colon. These studies show that GPR109A mediates the tumor-suppressive effects of the bacterial fermentation product butyrate in colon.

Different functional role of domain boundaries of Toll-like receptor 4

Toll-like receptors (TLRs) recognize molecules representing danger signals via their ectodomain, while signal transduction is provided by the cytosolic TIR domain that recruits adapter proteins upon dimerization. Since in crystal structures both domains dimerize as rigid bodies, any structural adjustment must be provided by the intermediate segments between the domains. We investigated domain coupling by inserting flexible linkers between the structural domains of TLR4. Insertion of linkers between the transmembrane and cytosolic TIR domain did not affect activation, indicating that TIR domain dimerization is triggered by proximity. In contrast, insertion of a linker between the transmembrane and ectodomain or within the ectodomain decreased activation proportionally with the length of the linker. This suggests the requirement for tight coupling of the ectodomain to the membrane, which may facilitate its interaction with ligand, promote dimerization and prevent interaction with the cell-membrane surface. Native linker sizes of TLR4 orthologs support these conclusions.

Variation matters: TLR structure and species-specific pathogen recognition

Toll-like receptors (TLRs) are a family of pattern recognition receptors that are an important link between innate and adaptive immunity. Many vaccines incorporate ligands for TLRs as an adjuvant and are developed in rodent models, with the resulting data transferred to other species. Vaccine features can be improved markedly by emphasizing the biological relevance when evaluating other animal models for host-pathogen interaction and by taking greater advantage of the unique experimental opportunities that are offered by large animal, non-rodent models. Here, we aim to summarize our current knowledge of species-specific TLR responses and briefly discuss that vaccine efficacy in relevant host species might be improved by considering the species-specific TLR responses.

Ligands, cell-based models, and readouts required for Toll-like receptor action

This chapter details the tools that are available to study Toll-like receptor (TLR) biology in vitro. This includes ligands, host cells, and readouts. The use of modified TLRs to circumvent some technical problems is also discussed.

Predicting Toll-like receptor structures and characterizing ligand binding

Toll-like receptor (TLR) ligand-binding domains comprise 18-25 tandem copies of a 24-residue motif known as the leucine-rich repeat (LRR). Unlike other LRR proteins, TLRs contain significant numbers of non-consensus LRR sequences, which makes their identification by computer domain search programs problematic. Here, we provide methods for identifying non-consensus LRRs. Using the location of these LRRs, hypothetical models are constructed based on the known molecular structures of homologous LRR proteins. However, when a hypothetical model for TLR3 is compared with the molecular structure solved by x-ray crystallography, the solenoid curvature, planarity, and conformations of the LRR insertions are incorrectly predicted. These differences illustrate how non-consensus LRR motifs influence TLR structure. Since the determination of molecular structures by crystallography requires substantial amounts of protein, we describe methods for producing milligram amounts of TLR3 extracellular domain (ECD) protein. The recombinant TLR3-ECD previously used to solve the molecular structure of TLR3-ECD has also been used to study the binding of TLR3-ECD to its ligand, double-stranded RNA (dsRNA). In the last section, we describe the preparation of defined TLR3 ligands and present methods for characterizing their interaction with TLR3-ECD.

Use of primary human cells in high-throughput screens

Traditionally, the objective of high-throughput screening (HTS) has been to identify compounds that interact with a defined target protein as the starting point for a chemistry lead optimisation programme. To enable this it has become commonplace to express the drug target in a recombinant expression system and use this reagent as the source of the biological material to support the HTS campaign. In this chapter we describe an alternative HTS methodology with the objective of identifying compounds that mediate a change in a defined physiological end point as a consequence of compound activity in human primary cells. Rather than screening at a defined molecular target, such "phenotypic" screens permit the identification of compounds that act at any target protein within the cell to regulate the end point under study. As an example of such a screen we will describe an HTS campaign to identify compounds that promote the production of the cytokine interferon-alpha from human blood peripheral mononuclear cells (PBMCs) isolated from whole blood. We describe the procedures required to obtain and purify human PBMCs and the electrochemiluminescence-based assay technology used to detect interferon-alpha and highlight the challenges associated with this screening paradigm.

Analysis of the functional role of Toll-like receptor-4 tyrosine phosphorylation

Toll-like receptors (TLRs) are principal innate immune sensors critically involved in the recognition of evolutionary conserved microbial and viral structures called "pathogen-associated molecular patterns" (PAMPs). Although recognition patterns of many TLRs have been characterized, molecular mechanisms that initiate TLR signaling are poorly understood. Since posttranslational modifications of many receptor systems are important in initiating signaling, we studied whether tyrosine phosphorylation of TLR4, the principal sensor of Gram-negative bacterial lipopolysaccharide (LPS) plays a role in TLR4 signal-transducing functions. We found that LPS induced TLR4 tyrosine phosphorylation and mutations of tyrosine residues in the Toll-IL-1R signaling domain markedly suppressed TLR4-mediated activation of JNK and p38 MAP kinases and transcription factors NF-kappaB, RANTES, and IFN-beta. This chapter summarizes a combination of methodological approaches that can be used to demonstrate an indispensable role of TLR4 tyrosine phosphorylation in receptor signaling, including transient transfections, site-directed mutagenesis, immunoprecipitation and immunoblot analyses, and analyses of transcription factor activation in reporter assays.

LRRCE: a leucine-rich repeat cysteine capping motif unique to the chordate lineage

Background

The small leucine-rich repeat proteins and proteoglycans (SLRPs) form an important family of regulatory molecules that participate in many essential functions. They typically control the correct assembly of collagen fibrils, regulate mineral deposition in bone, and modulate the activity of potent cellular growth factors through many signalling cascades. SLRPs belong to the group of extracellular leucine-rich repeat proteins that are flanked at both ends by disulphide-bonded caps that protect the hydrophobic core of the terminal repeats. A capping motif specific to SLRPs has been recently described in the crystal structures of the core proteins of decorin and biglycan. This motif, designated as LRRCE, differs in both sequence and structure from other, more widespread leucine-rich capping motifs. To investigate if the LRRCE motif is a common structural feature found in other leucine-rich repeat proteins, we have defined characteristic sequence patterns and used them in genome-wide searches.

Results

The LRRCE motif is a structural element exclusive to the main group of SLRPs. It appears to have evolved during early chordate evolution and is not found in protein sequences from non-chordate genomes. Our search has expanded the family of SLRPs to include new predicted protein sequences, mainly in fishes but with intriguing putative orthologs in mammals. The chromosomal locations of the newly predicted SLRP genes would support the large-scale genome or gene duplications that are thought to have occurred during vertebrate evolution. From this expanded list we describe a new class of SLRP sequences that could be representative of an ancestral SLRP gene.

Conclusion

Given its exclusivity the LRRCE motif is a useful annotation tool for the identification and classification of new SLRP sequences in genome databases. The expanded list of members of the SLRP family offers interesting insights into early vertebrate evolution and suggests an early chordate evolutionary origin for the LRRCE capping motif.

Inhibition of complement and CD14 attenuates the Escherichia coli-induced inflammatory response in porcine whole blood

The innate immune response is a double-edged sword in systemic inflammation and sepsis. Uncontrolled or inappropriate activation can damage and be lethal to the host. Several studies have investigated inhibition of downstream mediators, including tumor necrosis factor alpha (TNF-alpha) and interleukin-1beta (IL-1beta). Emerging evidence indicates that upstream inhibition is a better therapeutic approach for attenuating damaging immune activation. Therefore, we investigated inhibition of two central innate immune pathways, those of complement and CD14/Toll-like receptor 4 (TLR4)/myeloid differentiation protein 2 (MD-2), in a porcine in vitro model of Escherichia coli-induced inflammation. Porcine whole blood anticoagulated with lepuridin, which did not interfere with the complement system, was incubated with E. coli lipopolysaccharide (LPS) or whole bacteria. Inhibitors of complement and CD14 and thus the LPS CD14/TLR4/MD-2 receptor complex were tested to investigate the effect on the inflammatory response. A broad range of inflammatory readouts were used to monitor the effect. Anti-CD14 was found to saturate the CD14 molecule on granulocytes and completely inhibited LPS-induced proinflammatory cytokines in a dose-dependent manner. Anti-CD14 significantly reduced the levels of the E. coli-induced proinflammatory cytokines TNF-alpha and IL-1beta, but not IL-8, in a dose-dependent manner. No effect on bacterial clearance was seen. Vaccinia complement control protein and smallpox inhibitor of complement enzymes, two Orthopoxvirus-encoded complement inhibitors, completely inhibited complement activation. Furthermore, these agents almost completely inhibited the expression of wCD11R3, which is associated with CD18 as a beta2 integrin, on porcine granulocytes and decreased IL-8 levels significantly in a dose-dependent manner. As expected, complement inhibition reduced bacterial clearance. We conclude that inhibition of complement and CD14 attenuates E. coli-induced inflammation and might be used as a therapeutic regimen in gram-negative sepsis along with appropriate treatment with antibiotics.

Highlights of a workshop to discuss targeting inflammation in cystic fibrosis

A workshop to discuss anti-inflammatory approaches in the treatment of CF was held at Novartis Institutes for Biomedical Research (NIBR, Horsham, UK) in March 2008. Key opinion leaders in the field (Hugo De Jonge, Stuart Elborn, Erich Gulbins, Mike Konstan, Rick Moss, Scott Randell and Adriano Rossi), and NIBR scientists were brought together to collectively address three main aims: (i) to identify anti-inflammatory targets in CF, (ii) to evaluate the pros and cons of targeting specific cell types and (iii) to discuss model systems to profile potential therapeutic agents. The highlights of the workshop are captured in this review.

Structures of the toll-like receptor family and its ligand complexes

Toll-like receptors (TLRs) play central roles in the innate immune response by recognizing conserved structural patterns in diverse microbial molecules. Here, we discuss ligand binding and activation mechanisms of the TLR family. Hydrophobic ligands of TLR1, TLR2, and TLR4 interact with internal protein pockets. In contrast, dsRNA, a hydrophilic ligand, interacts with the solvent-exposed surface of TLR3. Binding of agonistic ligands, lipopeptides or dsRNA, induces dimerization of the ectodomains of the various TLRs, forming dimers that are strikingly similar in shape. In these "m"-shaped complexes, the C termini of the extracellular domains of the TLRs converge in the middle. This observation suggests the hypothesis that dimerization of the extracellular domains forces the intracellular TIR domains to dimerize, and this initiates signaling by recruiting intracellular adaptor proteins.

Age-related macular degeneration: activation of innate immunity system via pattern recognition receptors

Age-related macular degeneration (AMD) is the most common cause of irreversible loss of central vision. Histopathological studies have demonstrated that inflammation is the key player in the pathogenesis of AMD. Genetic studies have revealed that complement factor H is a strong risk factor for the development of AMD. However, innate immunity defence involves several other pattern recognition receptors (PRRs) which can trigger inflammatory responses. Retinal pigment epithelial (RPE) cells have the main role in the immune defence in macula. In this study, we examine in detail the endogenous danger signals which can activate different PRRs in RPE cells, such as Toll-like, NOD-like and scavenger receptors along with complement system. We also characterise the signalling pathways triggered by PRRs in evoking inflammatory responses. In addition, we will discuss whether AMD pathology could represent the outcome of chronic activation of the innate immunity defence in human macula.

Enhanced B-cell activation mediated by TLR4 and BCR crosstalk

Despite the important role of B lymphocytes as a bridge between the innate and the adaptive immune system, little is known regarding lipopolysaccharide (LPS) recognition, activation of signalling networks or conceivable cooperation between LPS and the B-cell antigen receptor (BCR). Here, we show that primary B cells can efficiently discriminate between different LPS chemotypes, responding with at least 100-fold higher sensitivity to rough-form LPS compared with smooth-form LPS. Using genetically modified mice, we demonstrate that B lymphocytes recognize all LPS chemotypes via Toll-like receptor 4 (TLR4). In addition, we dissect the signalling pathways that lead to CD69 upregulation upon TLR4 and BCR activation in primary B cells. Our data suggest that TLR4 and BCR induce CD69 transcription via two distinct sets of signalling molecules, exerting quantitative and qualitative differences in B-cell activation. Finally, we show that simultaneous stimulation of TLR4 and BCR additively elevates B-cell activation. In contrast, co-engagement of TLR4 and BCR by antigen-coupled LPS synergistically enhances activation of B cells, pointing out attractive targets for signalling crosstalk in B lymphocytes.

Toll-like receptors: their roles in bacterial recognition and respiratory infections

Although respiratory infections cause significant morbidity and mortality throughout the world, the immunologic factors that mediate host susceptibility to these infections remain poorly understood. The lung contains a vast surface at the host-environment interface and acts as a crucial barrier to invading pathogens. The lung is equipped with specialized epithelial and hematopoietic cells, which express pattern recognition receptors that act as both sentinels and mediators of pulmonary innate immunity. Toll-like receptors (TLRs) mediate a particularly critical role in pathogen recognition and subsequent initiation of the host immune response. In this review, we will summarize current knowledge of TLRs and their bacterial ligands and explore their role in respiratory infections. Moreover, we will highlight recent advances in the role of TLRs in pulmonary infections from a human immunogenetics perspective.

The adjuvanticity of a mannosylated antigen reveals TLR4 functionality essential for subset specialization and functional maturation of mouse dendritic cells

The evidence that dendritic cell (DC) subsets produce differential cytokines in response to specific TLR stimulation is robust. However, the role of TLR stimulation in Ag presentation and phenotypic maturation among DC subsets is not clear. Through the adjuvanticity of a novel mannosylated Ag, mannosylated dendrimer OVA (MDO), as a pathogen-associated molecular pattern Ag, we characterized the functionality of GM-CSF/IL-4-cultured bone marrow DC and Flt3 ligand (Flt3-L) DC subsets by Ag presentation and maturation assays. It was demonstrated that both bone marrow DCs and Flt3-L DCs bound, processed, and presented MDO effectively. However, while Flt3-L CD24(high) (conventional CD8(+) equivalent) and CD11b(high) (CD8(-) equivalent) DCs were adept at MDO processing by MHC class I and II pathways, respectively, CD45RA(+) plasmacytoid DCs presented MDO poorly to T cells. Successful MDO presentation was largely dependent on competent TLR4 for Ag localization and morphological/phenotypic maturation of DC subsets, despite the indirect interaction of MDO with TLR4. Furthermore, Toll/IL-1 receptor-domain-containing adaptor-inducing IFN-beta, but not MyD88, as a TLR4 signaling modulator was indispensable for MDO-induced DC maturation and Ag presentation. Taken together, our findings suggest that DC subsets differentially respond to a pathogen-associated molecular pattern-associated Ag depending on the intrinsic programming and TLRs expressed. Optimal functionality of DC subsets in Ag presentation necessitates concomitant TLR signaling critical for efficient Ag localization and processing.

Assembly of oligomeric death domain complexes during Toll receptor signaling

The Drosophila Toll receptor is activated by the endogenous protein ligand Spätzle in response to microbial stimuli in immunity and spatial cues during embryonic development. Downstream signaling is mediated by the adaptor proteins Tube, the kinase Pelle, and the Drosophila homologue of myeloid differentiation primary response protein (dMyD88). Here we have characterized heterodimeric (dMyD88-Tube) and heterotrimeric (dMyD88-Tube-Pelle) death domain complexes. We show that both the heterodimeric and heterotrimeric complexes form kidney-shaped structures and that Tube is bivalent and has separate high affinity binding sites for dMyD88 and Pelle. Additionally we found no interaction between the isolated death domains of Pelle and dMyD88. These results indicate that the mode of assembly of the heterotrimeric dMyD88-Tube-Pelle complex downstream of the activated Toll receptor is unique. The measured dissociation constants for the interaction between the death domains of dMyD88 and Tube and of Pelle and a preformed dMyD88-Tube complex are used to propose a model of the early postreceptor events in Drosophila Toll receptor signaling.

Mycobacterium avium glycopeptidolipids require specific acetylation and methylation patterns for signaling through toll-like receptor 2

Toll-like receptors (TLRs) recognize pathogen-associated molecules and play a vital role in promoting an immune response against invading microbes. TLR2, one of the key members of the TLR family, recognizes a wide variety of microbial products, including lipoproteins and lipopeptides, from a number of pathogens. Recent studies from our laboratory indicate that glycopeptidolipids (GPLs), a major surface component of Mycobacterium avium and other non-tuberculosis mycobacteria, are ligands for TLR2. However, the molecular requirements necessary for the GPL-TLR2 interaction were not defined in this report. In the present study we isolated different GPL species from M. avium, and using mass spectrometry and NMR analyses, characterized the molecular requirements of the GPL-TLR2 interaction. Interestingly, the extent of the respective acetylation and methylation of the 6-deoxytalose and rhamnose contained within the core GPL structure dictated whether the GPL signaled through TLR2. These experiments illustrate how subtle changes in a complex TLR2 ligand can alter its affinity for this important receptor, and suggest that M. avium could potentially modify its GPL structure to limit its interaction with TLR2.

Points of control exerted along the macrophage-endothelial cell-polymorphonuclear neutrophil axis by PECAM-1 in the innate immune response of acute colonic inflammation

PECAM-1 is expressed on endothelial cells and leukocytes. Its extracellular domain has been implicated in leukocyte diapedesis. In this study, we used PECAM-1(-/-) mice and relevant cells derived from them to assess the role of PECAM-1 in an experimental model of acute colonic inflammation with a predominant innate immune response, i.e., 2,4,6-trinitrobenzine sulfonic acid (TNBS). Using chimeric approaches, we addressed the points of control exerted by PECAM-1 along the macrophage-endothelial cell-polymorphonuclear neutrophil (PMN) axis. In vivo, TNBS-induced colitis was ameliorated in PECAM-1(-/-) mice, an event attributed to PECAM-1 on hematopoietic cells rather than to PECAM-1 on endothelial cells. The in vivo innate immune response was mimicked in vitro by using a construct of the vascular-interstitial interface, i.e., PMN transendothelial migration was induced by colonic lavage fluid (CLF) from TNBS mice or macrophages (MPhi) challenged with CLF. Using the construct, we confirmed that endothelial cell PECAM-1 does not play a role in PMN transendothelial migration. Although MPhi activation (NF-kappaB nuclear binding) and function (keratinocyte-derived chemokine production) induced by CLF was diminished in PECAM-1(-/-) MPhi, this did not affect their ability to promote PMN transendothelial migration. By contrast, PECAM-1(-/-) PMN did not adhere to or migrate across endothelial cell monolayers in response to CLF. Further, as compared with PECAM-1(+/+) PMN, PECAM-1(-/-) PMN were less effective in orientating their CXCR2 receptors (polarization) in the direction of a chemotactic gradient. Collectively, our findings indicate that PECAM-1 modulation of PMN function (at a step before diapedesis) most likely contributes to the inflammation in a colitis model with a strong innate immune component.

Using innate immunity to characterize the host response to microbial invasion in severe sepsis

Severe sepsis is a life-threatening systemic inflammatory response to microbial infection. Although mortality from severe sepsis has decreased modestly over the past three decades, its incidence is increasing dramatically. Current treatment of severe sepsis relies primarily on identification and appropriate antibiotic coverage of the invading pathogen, with supportive therapies aimed primarily at reducing the risk of associated organ injury. Current efforts to identify the invading pathogens are successful in approximately only 50% of patients with severe sepsis. In this review, we examine host-pathogen interactions through the Toll-like receptors and Nod signaling systems as principal mechanisms of the innate immune response, and explore the application of high-throughput genomic technologies that have permitted a more complete dissection of the innate immune response during severe sepsis. Early controlled studies suggest that these genome-wide tools can readily identify pathogen-specific host responses in macrophages and dendritic cells. However, despite considerable progress, clinical application of these approaches to pathogen recognition in hospitalized patients with severe sepsis is still years away.

In vitro differentiation of dendritic cells in the presence of prostaglandin E2 alters the IL-12/IL-23 balance and promotes differentiation of Th17 cells

PGE2, an endogenous lipid mediator released in inflammatory conditions, affects both dendritic cell (DC) differentiation and maturation. Whereas the effect of PGE2 on fully differentiated DC was studied extensively, little is known about its effects on DC differentiation. In this study, we show that bone marrow-derived DC generated in the presence of PGE2 (DCp) acquire a proinflammatory profile; produce higher levels of proinflammatory cytokines/chemokines; express higher levels of MHC class II, costimulatory molecules, and TLRs; and exhibit increased activation of the NF-kappaB-signaling pathway. In addition, DCp exhibit a different IL-12/IL-23 profile than DC generated in the absence of PGE2. The low IL-12 and high IL-23 production in LPS-stimulated DCp is associated with the down-regulation of p35 and the up-regulation of p19 expression, respectively. In agreement with the DCp proinflammatory phenotype and especially with the altered IL-12/IL-23 balance which strongly favors IL-23, DCp also affect T cell differentiation. In contrast to DC which favor Th1 differentiation, DCp promote Th17 and inhibit Th1/Th2 differentiation, in vitro and in vivo. Previous in vivo studies indicated that PGE2 had a proinflammatory effect, especially in models of autoimmune diseases. Our results suggest that the proinflammatory effects of PGE2 could be mediated, at least partially, through effects on differentiating DC and subsequent alterations in CD4+ T cell differentiation, resulting in the preferential development of pathogenic autoimmune Th17 cells.

MAST3: a novel IBD risk factor that modulates TLR4 signaling

Inflammatory bowel disease (IBD) is a chronic disorder caused by multiple factors in a genetically susceptible host. Significant advances in the study of genetic susceptibility have highlighted the importance of the innate immune system in this disease. We previously completed a genome-wide linkage study and found a significant locus (IBD6) on chromosome 19p. We were interested in identifying the causal variant in IBD6. We performed a two-stage association mapping study. In stage 1, 1530 single-nucleotide polymorphisms (SNPs) were selected from the HapMap database and genotyped in 761 patients with IBD. Among the SNPs that passed the threshold for replication, 26 were successfully genotyped in 754 additional patients (stage 2). One intronic variant, rs273506, located in the microtubule-associated serine/threonine-protein kinase gene-3 (MAST3), was found to be associated in both stages (pooled P=1.8 x 10(-4)). We identified four MAST3 coding variants, including a non-synonymous SNP rs8108738, correlated to rs273506 and associated with IBD. To test whether MAST3 was expressed in cells of interest, we performed expression assays, which showed abundant expression of MAST3 in antigen-presenting cells and in lymphocytes. The knockdown of MAST3 specifically decreased Toll-like receptor-4-dependent NF-kappaB activity. Our findings are additional proofs of the pivotal role played by modulators of NF-kappaB activity in IBD pathogenesis.

Ligand binding by repeat proteins: natural and designed

Repeat proteins contain tandem arrays of small structural motifs. As a consequence of this architecture, they adopt non-globular, extended structures that present large, highly specific surfaces for ligand binding. Here we discuss recent advances toward understanding the functional role of this unique modular architecture. We showcase specific examples of natural repeat proteins interacting with diverse ligands and also present examples of designed repeat protein-ligand interactions.

Lipoteichoic acid synergizes with glycosphingolipids to potently stimulate secretion of interleukin-6 from human blood cells

In the present study, we found that lipoteichoic acid (LTA) synergizes with glycosphingolipids to stimulate human blood cells to secrete cytokines. We employed globoside, kerasin, and lactosylceramide as representative neutral glycosphingolipids and mixed gangliosides GM(2) and GM(3) as representative acidic glycosphingolipids. LTA and the glycosphingolipids enhanced cytokine secretion by human whole blood, peripheral blood mononuclear cells, and purified monocytes in a dose-dependent manner. The level of synergy ranged up to approximately 10-fold greater than the additive stimulation caused by LTA and glycosphingolipid alone. The greatest synergy was observed with GM(3). We also found that LTA synergizes with the synthetic bacterial lipopeptide mimic Pam3CysK4. In contrast, the glycosphingolipids suppressed the stimulation caused by Pam3CysK4. The stimulation of human cells requires the simultaneous presence of LTA and the glycosphingolipids and probably requires their physical interactions, as shown by dot blotting and nondenaturing polyacrylamide gel electrophoresis experiments. We hypothesize that the enhanced stimulation is due to heterooligomers that form between LTA and glycosphingolipids at the subcritical micelle concentrations used in these experiments. Previous studies showed that LTA also synergizes with hemoglobin. The data taken together suggest that LTA may be a pathogen-associated molecular pattern, although its full activity requires the presence of a synergistic partner(s).