Specific High Affinity Interactions of Monomeric Endotoxin·Protein Complexes with Toll-like Receptor 4 Ectodomain

Exploration of gut microbiome and inflammation: A review on key signalling pathways

The gut microbiome, a crucial component of the human system, is a diverse collection of microbes that belong to the gut of human beings as well as other animals. These microbial communities continue to coexist harmoniously with their host organisms and perform various functions that affect the host's general health. Each person's gut microbiota has a unique makeup. The gut microbiota is well acknowledged to have a part in the local as well as systemic inflammation that underlies a number of inflammatory disorders (e.g., atherosclerosis, diabetes mellitus, obesity, and inflammatory bowel disease).The gut microbiota's metabolic products, such as short-chain fatty acids (butyrate, propionate, and acetate) inhibit inflammation by preventing immune system cells like macrophages and neutrophils from producing pro-inflammatory factors, which are triggered by the structural elements of bacteria (like lipopolysaccharide). The review's primary goal is to provide comprehensive and compiled data regarding the contribution of gut microbiota to inflammation and the associated signalling pathways.

Activation of Toll-like receptor 4 by Ebola virus-shed glycoprotein is direct and requires the internal fusion loop but not glycosylation

Infection by the Ebola virus, a member of the Filoviridae family of RNA viruses, leads to acute viral hemorrhagic fever. End-stage Ebola virus disease is characterized by a cytokine storm that causes tissue damage, vascular disintegration, and multi-organ failure. Previous studies showed that a shed form of the viral spike glycoprotein (sGP1,2) drives this hyperinflammatory response by activating Toll-like receptor 4 (TLR4). Here, we find that glycosylation is not required for activation of TLR4 by sGP1,2 and identify the internal fusion loop (IFL) as essential for inflammatory signaling. sGP1,2 competes with lipid antagonists of TLR4, and the IFL interacts directly with TLR4 and co-receptor MD2. Together, these findings indicate that sGP1,2 activates TLR4 analogously to bacterial agonist lipopolysaccharide (LPS) by binding into a hydrophobic pocket in MD2 and promoting the formation of an active heterotetramer. This conclusion is supported by docking studies that predict binding sites for sGP1,2 on TLR4 and MD2.

Consensus Enolase of Trypanosoma Cruzi: Evaluation of Their Immunogenic Properties Using a Bioinformatics Approach

There is currently no vaccine against American trypanosomiasis, caused by the parasite Trypanosoma cruzi. This is due to the genomic variation observed in the six DTUs of T. cruzi. This work aims to propose a consensus sequence of the enolase protein from different strains of T. cruzi and mainly evaluate its immunogenic properties at the bioinformatic level. From specialized databases, 15 sequences of the enolase gene were aligned to obtain a consensus sequence, where this sequence was modeled and then evaluated and validated through different bioinformatic programs to learn their immunogenic potential. Finally, chimeric peptides were designed with the most representative epitopes. The results showed high immunogenic potential with six epitopes for MHC-I, and seven epitopes for MHC-II, all of which were highly representative of the enolase present in strains from the American continent as well as five epitopes for B cells. Regarding the computational modeling, molecular docking with Toll-like receptors showed a high affinity and low constant of dissociation, which could lead to an innate-type immune response that helps to eliminate the parasite. In conclusion, the consensus sequence proposed for enolase is capable of providing an ideal immune response; however, the experimental evaluation of this enolase consensus and their chimeric peptides should be a high priority to develop a vaccine against Chagas disease.

PI3Kγ Mediates Microglial Proliferation and Cell Viability via ROS

(1) Background: Rapid microglial proliferation contributes to the complex responses of the innate immune system in the brain to various neuroinflammatory stimuli. Here, we investigated the regulatory function of phosphoinositide 3-kinase γ (PI3Kγ) and reactive oxygen species (ROS) for rapid proliferation of murine microglia induced by LPS and ATP. (2) Methods: PI3Kγ knockout mice (PI3Kγ KO), mice expressing catalytically inactive PI3Kγ (PI3Kγ KD) and wild-type mice were assessed for microglial proliferation using an in vivo wound healing assay. Additionally, primary microglia derived from newborn wild-type, PI3Kγ KO and PI3Kγ KD mice were used to analyze PI3Kγ effects on proliferation and cell viability, senescence and cellular and mitochondrial ROS production; the consequences of ROS production for proliferation and cell viability after LPS or ATP stimulation were studied using genetic and pharmacologic approaches. (3) Results: Mice with a loss of lipid kinase activity showed impaired proliferation of microglia. The prerequisite of induced microglial proliferation and cell viability appeared to be PI3Kγ-mediated induction of ROS production. (4) Conclusions: The lipid kinase activity of PI3Kγ plays a crucial role for microglial proliferation and cell viability after acute inflammatory activation.

Ovine CD14- an Immune Response Gene Has a Role Against Gastrointestinal Nematode Haemonchus contortus-A Novel Report

CD14 (also known as the monocyte differentiation antigen) is an important immune response gene known to be primarily responsible for innate immunity against bacterial pathogens, and as a pattern recognition receptor (PRR), binds with LPS (endotoxin), lipoproteins, and lipotechoic acid of bacteria. So far very limited work has been conducted in parasitic immunology. In the current study, we reported the role of CD14 in parasitic immunology in livestock species (sheep) for the first time. Ovine CD14 is characterized as a horse-shoe shaped bent solenoid with a hydrophobic amino-terminal pocket for CD14 along with domains. High mutation frequency was observed, out of total 41 mutations identified, 23 mutations were observed to be thermodynamically unstable and 11 mutations were deleterious in nature, causing major functional alteration of important domains of CD14, an indication of variations in individual susceptibility for sheep against Haemonchus contortus infestations. In silico studies with molecular docking reveal a role of immune response against Haemonchus contortus in sheep, which is later confirmed with experimental evidence through differential mRNA expression analysis for sheep, which revealed better expression of CD14 in Haemonchus contortus infected sheep compared to that of non-infected sheep. We confirmed the above findings with supportive evidence through haematological and biochemical analyses. Phylogenetic analysis was conducted to assess the evolutionary relationship with respect to humans and it was observed that sheep may well be used as model organisms due to better genetic closeness compared to that of mice.

Lipopolysaccharide Recognition in the Crossroads of TLR4 and Caspase-4/11 Mediated Inflammatory Pathways

The innate immune response to lipopolysaccharide is essential for host defense against Gram-negative bacteria. In response to bacterial infection, the TLR4/MD-2 complex that is expressed on the surface of macrophages, monocytes, dendritic, and epithelial cells senses picomolar concentrations of endotoxic LPS and triggers the production of various pro-inflammatory mediators. In addition, LPS from extracellular bacteria which is either endocytosed or transfected into the cytosol of host cells or cytosolic LPS produced by intracellular bacteria is recognized by cytosolic proteases caspase-4/11 and hosts guanylate binding proteins that are involved in the assembly and activation of the NLRP3 inflammasome. All these events result in the initiation of pro-inflammatory signaling cascades directed at bacterial eradication. However, TLR4-mediated signaling and caspase-4/11-induced pyroptosis are largely involved in the pathogenesis of chronic and acute inflammation. Both extra- and intracellular LPS receptors-TLR4/MD-2 complex and caspase-4/11, respectively-are able to directly bind the lipid A motif of LPS. Whereas the structural basis of lipid A recognition by the TLR4 complex is profoundly studied and well understood, the atomic mechanism of LPS/lipid A interaction with caspase-4/11 is largely unknown. Here we describe the LPS-induced TLR4 and caspase-4/11 mediated signaling pathways and their cross-talk and scrutinize specific structural features of the lipid A motif of diverse LPS variants that have been reported to activate caspase-4/11 or to induce caspase-4/11 mediated activation of NLRP3 inflammasome (either upon transfection of LPS in vitro or upon infection of cell cultures with intracellular bacteria or by LPS as a component of the outer membrane vesicles). Generally, inflammatory caspases show rather similar structural requirements as the TLR4/MD-2 complex, so that a "basic" hexaacylated bisphosphorylated lipid A architecture is sufficient for activation. However, caspase-4/11 can sense and respond to much broader variety of lipid A variants compared to the very "narrow" specificity of TLR4/MD-2 complex as far as the number and the length of lipid chains attached at the diglucosamine backbone of lipid A is concerned. Besides, modification of the lipid A phosphate groups with positively charged appendages such as phosphoethanolamine or aminoarabinose could be essential for the interaction of lipid A/LPS with inflammatory caspases and related proteins.

Memory-Like Responses of Brain Microglia Are Controlled by Developmental State and Pathogen Dose

Microglia, the innate immune cells of the central nervous system, feature adaptive immune memory with implications for brain homeostasis and pathologies. However, factors involved in the emergence and regulation of these opposing responses in microglia have not been fully addressed. Recently, we showed that microglia from the newborn brain display features of trained immunity and immune tolerance after repeated contact with pathogens in a dose-dependent manner. Here, we evaluate the impact of developmental stage on adaptive immune responses of brain microglia after repeated challenge with ultra-low (1 fg/ml) and high (100 ng/ml) doses of the endotoxin LPS in vitro. We find that priming of naïve microglia derived from newborn but not mature and aged murine brain with ultra-low LPS significantly increased levels of pro-inflammatory mediators TNF-α, IL-6, IL-1β, MMP-9, and iNOS as well as neurotrophic factors indicating induction of trained immunity (p < 0.05). In contrast, stimulation with high doses of LPS led to a robust downregulation of pro-inflammatory cytokines and iNOS independent of the developmental state, indicating induced immune tolerance. Furthermore, high-dose priming with LPS upregulated anti-inflammatory mediators IL-10, Arg-1, TGF- β, MSR1, and IL-4 in newborn microglia (p < 0.05). Our data indicate pronounced plasticity of the immune response of neonate microglia compared with microglia derived from mature and aged mouse brain. Induced trained immunity after priming with ultra-low LPS doses may be responsible for enhanced neuro-inflammatory susceptibility of immature brain. In contrast, the immunosuppressed phenotype following high-dose LPS priming might be prone to attenuate excessive damage after recurrent systemic inflammation.

Memory-Like Inflammatory Responses of Microglia to Rising Doses of LPS: Key Role of PI3Kγ

Trained immunity and immune tolerance have been identified as long-term response patterns of the innate immune system. The causes of these opposing reactions remain elusive. Here, we report about differential inflammatory responses of microglial cells derived from neonatal mouse brain to increasing doses of the endotoxin LPS. Prolonged priming with ultra-low LPS doses provokes trained immunity, i.e., increased production of pro-inflammatory mediators in comparison to the unprimed control. In contrast, priming with high doses of LPS induces immune tolerance, implying decreased production of inflammatory mediators and pronounced release of anti-inflammatory cytokines. Investigation of the signaling processes and cell functions involved in these memory-like immune responses reveals the essential role of phosphoinositide 3-kinase γ (PI3Kγ), one of the phosphoinositide 3-kinase species highly expressed in innate immune cells. Together, our data suggest profound influence of preceding contacts with pathogens on the immune response of microglia. The impact of these interactions—trained immunity or immune tolerance—appears to be shaped by pathogen dose.

Detecting lipopolysaccharide in the cytosol of mammalian cells: Lessons from MD-2/TLR4

Proinflammatory immune responses to Gram-negative bacterial lipopolysaccharides (LPS) are crucial to innate host defenses but can also contribute to pathology. How host cells sensitively detect structural features of LPS was a mystery for years, especially given that a portion of the molecule essential for its potent proinflammatory properties-lipid A-is buried in the bacterial membrane. Studies of responses to extracellular and vacuolar LPS revealed a crucial role for accessory proteins that specifically bind LPS-rich membranes and extract LPS monomers to generate a complex of LPS, MD-2, and TLR4. These insights provided means to understand better both the remarkable host sensitivity to LPS and the means whereby specific LPS structural features are discerned. More recently, the noncanonical inflammasome, consisting of caspases-4/5 in humans and caspase-11 in mice, has been demonstrated to mediate responses to LPS that has reached the host cytosol. Precisely how LPS gains access to cytosolic caspases-and in what form-is not well characterized, and understanding this process will provide crucial insights into how the noncanonical inflammasome is regulated during infection. Herein, we briefly review what is known about LPS detection by cytosolic caspases-4/5/11, focusing on lessons derived from studies of the better-characterized TLR4 system that might direct future mechanistic questions.

Human αS1-casein induces IL-8 secretion by binding to the ecto-domain of the TLR4/MD2 receptor complex

BACKGROUND

The milk protein α_S1-casein was recently reported to induce secretion of proinflammatory cytokines via Toll-like receptor 4 (TLR4). In this study, α_S1-casein was identified as binder of theTLR4 ecto domain.

METHODS

IL-8 secretion after stimulation of TLR4/MD2 (myeloid differentiation factor 2)/CD14 (cluster of differentiation 14)-transfected HEK293 cells (TLR4⁺) and Mono Mac 6 cells (MM6) with recombinant α_S1-casein, or LPS as control was monitored. Binding of α_S1-casein to TLR4 was quantified by microscale thermophoresis (MST).

RESULTS

α_S1-casein induced secretion of IL-8 in TLR4⁺ cells and in MM6 cells with a six-times higher final IL-8 concentration in supernatants. IL-8 secretion was inhibited by intracellular TLR4-domain antagonist TAK-242 with an IC₅₀-value of 259.6 nM, by ecto-domain TLR4 antagonistic mianserin with 10-51 μM and by anti-CD14-IgA. The binding constants (K_D) of α_S1-casein to the TLR4, MD2, and CD14 were 2.8 μM, 0.3 μM and 2.7 μM, respectively. Finally, α_S1-casein showed a higher affinity to TLR4/MD2 (K_D: 2.2 μM) compared to LPS (K_D: 8.2 μM).

CONCLUSION

Human α_S1-casein induced proinflammatory effects are dependent upon binding to the TLR4 ectodomain and the presence of CD14. α_S1-casein displayed stronger TLR4 agonistic activity than LPS via a different mode of action.

GENERAL SIGNIFICANCE

Breast milk protein α_S1-casein is a proinflammatory cytokine.

Diverse pro-inflammatory endotoxin recognition systems of mammalian innate immunity

In humans and other mammals, recognition of endotoxins—abundant surface lipopolysaccharides (LPS) of Gram-negative bacteria—provides a potent stimulus for induction of inflammation and mobilization of host defenses. The structurally unique lipid A region of LPS is the principal determinant of this pro-inflammatory activity. This region of LPS is normally buried within the bacterial outer membrane and aggregates of purified LPS, making even more remarkable its picomolar potency and the ability of discrete variations in lipid A structure to markedly alter the pro-inflammatory activity of LPS. Two recognition systems—MD-2/TLR4 and “LPS-sensing” cytosolic caspases—together confer LPS responsiveness at the host cell surface, within endosomes, and at sites physically accessible to the cytosol. Understanding how the lipid A of LPS is delivered and recognized at these diverse sites is crucial to understanding how the magnitude and character of the inflammatory responses are regulated.

Aminosugar-based immunomodulator lipid A: synthetic approaches

The immediate immune response to infection by Gram-negative bacteria depends on the structure of a lipopolysaccharide (LPS, also known as endotoxin), a complex glycolipid constituting the outer leaflet of the bacterial outer membrane. Recognition of picomolar quantities of pathogenic LPS by the germ-line encoded Toll-like Receptor 4 (TLR4) complex triggers the intracellular pro-inflammatory signaling cascade leading to the expression of cytokines, chemokines, prostaglandins and reactive oxygen species which manifest an acute inflammatory response to infection. The "endotoxic principle" of LPS resides in its amphiphilic membrane-bound fragment glycophospholipid lipid A which directly binds to the TLR4·MD-2 receptor complex. The lipid A content of LPS comprises a complex mixture of structural homologs varying in the acylation pattern, the length of the (R)-3-hydroxyacyl- and (R)-3-acyloxyacyl long-chain residues and in the phosphorylation status of the β(1→6)-linked diglucosamine backbone. The structural heterogeneity of the lipid A isolates obtained from bacterial cultures as well as possible contamination with other pro-inflammatory bacterial components makes it difficult to obtain unambiguous immunobiological data correlating specific structural features of lipid A with its endotoxic activity. Advanced understanding of the therapeutic significance of the TLR4-mediated modulation of the innate immune signaling and the central role of lipid A in the recognition of LPS by the innate immune system has led to a demand for well-defined materials for biological studies. Since effective synthetic chemistry is a prerequisite for the availability of homogeneous structurally distinct lipid A, the development of divergent and reproducible approaches for the synthesis of various types of lipid A has become a subject of considerable importance. This review focuses on recent advances in synthetic methodologies toward LPS substructures comprising lipid A and describes the synthesis and immunobiological properties of representative lipid A variants corresponding to different bacterial species. The main criteria for the choice of orthogonal protecting groups for hydroxyl and amino functions of synthetically assembled β(1→6)-linked diglucosamine backbone of lipid A which allows for a stepwise introduction of multiple functional groups into the molecule are discussed. Thorough consideration is also given to the synthesis of 1,1'-glycosyl phosphodiesters comprising partial structures of 4-amino-4-deoxy-β-L-arabinose modified Burkholderia lipid A and galactosamine-modified Francisella lipid A. Particular emphasis is put on the stereoselective construction of binary glycosyl phosphodiester fragments connecting the anomeric centers of two aminosugars as well as on the advanced P(III)-phosphorus chemistry behind the assembly of zwitterionic double glycosyl phosphodiesters.

High-affinity caspase-4 binding to LPS presented as high molecular mass aggregates or in outer membrane vesicles

Caspases of the non-canonical inflammasome (caspases -4, -5, and -11) directly bind endotoxin (LOS/LPS) and can be activated in the absence of any co-factors. Models of LPS-induced caspase activation have postulated that 1:1 binding of endotoxin monomers to caspase trigger caspase oligomerization and activation, analogous to that established for endotoxin-induced activation of MD-2/TLR4. However, using metabolically radiolabeled LOS and LPS, we now show high affinity and selective binding of caspase-4 to high molecular mass aggregates of purified endotoxin and to endotoxin-rich outer membrane vesicles without formation of 1:1 endotoxin:caspase complexes. Thus, our findings demonstrate markedly different endotoxin recognition properties of caspase-4 from that of MD-2/TLR4 and strongly suggest that activation of caspase-4 (and presumably caspase-5 and caspase-11) are mediated by interactions with activating endotoxin-rich membrane interfaces rather than by endotoxin monomers.

Molecular Basis of the Functional Differences between Soluble Human Versus Murine MD-2: Role of Val135 in Transfer of Lipopolysaccharide from CD14 to MD-2

Myeloid differentiation factor 2 (MD-2) is an extracellular protein, associated with the ectodomain of TLR4, that plays a critical role in the recognition of bacterial LPS. Despite high overall structural and functional similarity, human (h) and murine (m) MD-2 exhibit several species-related differences. hMD-2 is capable of binding LPS in the absence of TLR4, whereas mMD-2 supports LPS responsiveness only when mMD-2 and mTLR4 are coexpressed in the same cell. Previously, charged residues at the edge of the LPS binding pocket have been attributed to this difference. In this study, site-directed mutagenesis was used to explore the hydrophobic residues within the MD-2 binding pocket as the source of functional differences between hMD-2 and mMD-2. Whereas decreased hydrophobicity of residues 61 and 63 in the hMD-2 binding pocket retained the characteristics of wild-type hMD-2, a relatively minor change of valine to alanine at position 135 completely abolished the binding of LPS to the hMD-2 mutant. The mutant, however, retained the LPS binding in complex with TLR4 and also cell activation, resulting in a murine-like phenotype. These results were supported by the molecular dynamics simulation. We propose that the residue at position 135 of MD-2 governs the dynamics of the binding pocket and its ability to accommodate lipid A, which is allosterically affected by bound TLR4.

Energetics of Endotoxin Recognition in the Toll-Like Receptor 4 Innate Immune Response

Bacterial outer membrane lipopolysaccharide (LPS) potently stimulates the mammalian innate immune system, and can lead to sepsis, the primary cause of death from infections. LPS is sensed by Toll-like receptor 4 (TLR4) in complex with its lipid-binding coreceptor MD-2, but subtle structural variations in LPS can profoundly modulate the response. To better understand the mechanism of LPS-induced stimulation and bacterial evasion, we have calculated the binding affinity to MD-2 of agonistic and antagonistic LPS variants including lipid A, lipid IVa, and synthetic antagonist Eritoran, and provide evidence that the coreceptor is a molecular switch that undergoes ligand-induced conformational changes to appropriately activate or inhibit the receptor complex. The plasticity of the coreceptor binding cavity is shown to be essential for distinguishing between ligands, whilst similar calculations for a model bacterial LPS bilayer reveal the "membrane-like" nature of the protein cavity. The ability to predict the activity of LPS variants should facilitate the rational design of TLR4 therapeutics.

Studies of the TLR4-associated protein MD-2 using yeast-display and mutational analyses

Bacterial lipopolysaccharide (LPS) activates the innate immune system by forming a complex with myeloid differentiation factor 2 (MD-2) and Toll-like receptor 4 (TLR4), which is present on antigen presenting cells. MD-2 plays an essential role in this activation of the innate immune system as a member of the ternary complex, TLR4:MD-2:LPS. With the goal of further understanding the molecular details of the interaction of MD-2 with LPS and TLR4, and possibly toward engineering dominant negative regulators of the MD-2 protein, here we subjected MD-2 to a mutational analysis using yeast display. The approach included generation of site-directed alanine mutants, and ligand-driven selections of MD-2 mutant libraries. Our findings showed that: (1) proline mutations in the F119-K132 loop that binds LPS were strongly selected for enhanced yeast surface stability, (2) there was a preference for positive-charged side chains (R/K) at residue 120 for LPS binding, and negative-charged side chains (D/E) for TLR4 binding, (3) aromatic residues were strongly preferred at F119 and F121 for LPS binding, and (4) an MD-2 mutant (T84N/D101A/S118A/S120D/K122P) exhibited increased binding to TLR4 but decreased binding to LPS. These studies revealed the impact of specific residues and regions of MD-2 on the binding of LPS and TLR4, and they provide a framework for further directed evolution of the MD-2 protein.

Purified monomeric ligand.MD-2 complexes reveal molecular and structural requirements for activation and antagonism of TLR4 by Gram-negative bacterial endotoxins

A major focus of work in our laboratory concerns the molecular mechanisms and structural bases of Gram-negative bacterial endotoxin recognition by host (e.g., human) endotoxin-recognition proteins that mediate and/or regulate activation of Toll-like receptor (TLR) 4. Here, we review studies of wild-type and variant monomeric endotoxin.MD-2 complexes first produced and characterized in our laboratories. These purified complexes have provided unique experimental reagents, revealing both quantitative and qualitative determinants of TLR4 activation and antagonism. This review is dedicated to the memory of Dr. Theresa L. Gioannini (1949-2014) who played a central role in many of the studies and discoveries that are reviewed.

Emerging principles governing signal transduction by pattern-recognition receptors

The problem of recognizing and disposing of non-self-organisms, whether for nutrients or defense, predates the evolution of multicellularity. Accordingly, the function of the innate immune system is often intimately associated with fundamental aspects of cell biology. Here, we review our current understanding of the links between cell biology and pattern-recognition receptors of the innate immune system. We highlight the importance of receptor localization for the detection of microbes and for the initiation of antimicrobial signaling pathways. We discuss examples that illustrate how pattern-recognition receptors influence, and are influenced by, the general membrane trafficking machinery of mammalian cells. In the future, cell biological analysis likely will rival pure genetic analysis as a tool to uncover fundamental principles that govern host-microbe interactions.

A cross-disciplinary perspective on the innate immune responses to bacterial lipopolysaccharide

The study of innate immunity to bacteria has focused heavily on the mechanisms by which mammalian cells detect lipopolysaccharide (LPS), the conserved surface component of Gram-negative bacteria. While Toll-like receptor 4 (TLR4) is responsible for all the host transcriptional responses to LPS, recent discoveries have revealed the existence of several TLR4-independent responses to LPS. These discoveries not only broaden our view of the means by which mammalian cells interact with bacteria, but they also highlight new selective pressures that may have promoted the evolution of bacterial immune evasion strategies. In this review, we highlight past and recent discoveries on host LPS sensing mechanisms and discuss bacterial countermeasures that promote infection. By looking at both sides of the host-pathogen interaction equation, we hope to provide comprehensive insights into host defense mechanisms and bacterial pathogenesis.

Identification of key residues that confer Rhodobacter sphaeroides LPS activity at horse TLR4/MD-2

The molecular determinants underpinning how hexaacylated lipid A and tetraacylated precursor lipid IVa activate Toll-like receptor 4 (TLR4) are well understood, but how activation is induced by other lipid A species is less clear. Species specificity studies have clarified how TLR4/MD-2 recognises different lipid A structures, for example tetraacylated lipid IVa requires direct electrostatic interactions for agonism. In this study, we examine how pentaacylated lipopolysaccharide from Rhodobacter sphaeroides (RSLPS) antagonises human TLR4/MD-2 and activates the horse receptor complex using a computational approach and cross-species mutagenesis. At a functional level, we show that RSLPS is a partial agonist at horse TLR4/MD-2 with greater efficacy than lipid IVa. These data suggest the importance of the additional acyl chain in RSLPS signalling. Based on docking analysis, we propose a model for positioning of the RSLPS lipid A moiety (RSLA) within the MD-2 cavity at the TLR4 dimer interface, which allows activity at the horse receptor complex. As for lipid IVa, RSLPS agonism requires species-specific contacts with MD-2 and TLR4, but the R2 chain of RSLA protrudes from the MD-2 pocket to contact the TLR4 dimer in the vicinity of proline 442. Our model explains why RSLPS is only partially dependent on horse TLR4 residue R385, unlike lipid IVa. Mutagenesis of proline 442 into a serine residue, as found in human TLR4, uncovers the importance of this site in RSLPS signalling; horse TLR4 R385G/P442S double mutation completely abolishes RSLPS activity without its counterpart, human TLR4 G384R/S441P, being able to restore it. Our data highlight the importance of subtle changes in ligand positioning, and suggest that TLR4 and MD-2 residues that may not participate directly in ligand binding can determine the signalling outcome of a given ligand. This indicates a cooperative binding mechanism within the receptor complex, which is becoming increasingly important in TLR signalling.

Tetraacylated lipid A and paclitaxel-selective activation of TLR4/MD-2 conferred through hydrophobic interactions

LPS exerts potent immunostimulatory effects through activation of the TLR4/MD-2 receptor complex. The hexaacylated lipid A is an agonist of mouse (mTLR4) and human TLR4/MD-2, whereas the tetraacylated lipid IVa and paclitaxel activate only mTLR4/MD-2 and antagonize activation of the human receptor complex. Hydrophobic mutants of TLR4 or MD-2 were used to investigate activation of human embryonic kidney 293 cells by different TLR4 agonists. We show that each of the hydrophobic residues F438 and F461, which are located on the convex face of leucine-rich repeats 16 and 17 of the mTLR4 ectodomain, are essential for activation of with lipid IVa and paclitaxel, which, although not a structural analog of LPS, activates cells expressing mTLR4/MD-2. Both TLR4 mutants were inactive when stimulated with lipid IVa or paclitaxel, but retained significant activation when stimulated with LPS or hexaacylated lipid A. We show that the phenylalanine residue at position 126 of mouse MD-2 is indispensable only for activation with paclitaxel. Its replacement with leucine or valine completely abolished activation with paclitaxel while preserving the responsiveness to lipid IVa and lipid A. This suggests specific interaction of paclitaxel with F126 because its replacement with leucine even augmented activation by lipid A. These results provide an insight into the molecular mechanism of TLR4 activation by two structurally very different agonists.

Sequence context induced antimicrobial activity: insight into lipopolysaccharide permeabilization

Mechanistic insights into the permeabilization of the outer membrane of Gram negative bacteria by an antimicrobial peptide lactoferrampin, a 17 residue peptide, using high and low resolution spectroscopy in conjunction with MD simulation.

Conformationally constrained lipid A mimetics for exploration of structural basis of TLR4/MD-2 activation by lipopolysaccharide

Recognition of the lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria, by the Toll-like receptor 4 (TLR4)-myeloid differentiation factor 2 (MD-2) complex is essential for the control of bacterial infection. A pro-inflammatory signaling cascade is initiated upon binding of membrane-associated portion of LPS, a glycophospholipid Lipid A, by a coreceptor protein MD-2, which results in a protective host innate immune response. However, activation of TLR4 signaling by LPS may lead to the dysregulated immune response resulting in a variety of inflammatory conditions including sepsis syndrome. Understanding of structural requirements for Lipid A endotoxicity would ensure the development of effective anti-inflammatory medications. Herein, we report on design, synthesis, and biological activities of a series of conformationally confined Lipid A mimetics based on β,α-trehalose-type scaffold. Replacement of the flexible three-bond β(1→6) linkage in diglucosamine backbone of Lipid A by a two-bond β,α(1↔1) glycosidic linkage afforded novel potent TLR4 antagonists. Synthetic tetraacylated bisphosphorylated Lipid A mimetics based on a β-GlcN(1↔1)α-GlcN scaffold selectively block the LPS binding site on both human and murine MD-2 and completely abolish lipopolysaccharide-induced pro-inflammatory signaling, thereby serving as antisepsis drug candidates. In contrast to their natural counterpart lipid IVa, conformationally constrained Lipid A mimetics do not activate mouse TLR4. The structural basis for high antagonistic activity of novel Lipid A mimetics was confirmed by molecular dynamics simulation. Our findings suggest that besides the chemical structure, also the three-dimensional arrangement of the diglucosamine backbone of MD-2-bound Lipid A determines endotoxic effects on TLR4.

Identification of single nucleotide polymorphisms in hematopoietic cell transplant patients affecting early recognition of, and response to, endotoxin

Hematopoietic cell transplant (HCT) is a life-saving therapy for many malignant and non-malignant bone marrow diseases. Associated morbidities are often due to transplant-related toxicities and infections, exacerbated by regimen-induced immune suppression and systemic incursion of bacterial products. Patients undergoing myeloablative conditioning for HCT become endotoxemic and display blood/plasma changes consistent with lipopolysaccharide (LPS)-induced systemic innate immune activation. Herein, we addressed whether patients scheduled for HCT display differences in recognition/response to LPS ex vivo traceable to specific single nucleotide polymorphisms (SNPs). Two SNPs of LPS binding protein (LBP) were associated with changes in plasma LBP levels, with one LBP SNP also associating with differences in efficiency of extraction and transfer of endotoxin to myeloid differentiation factor-2 (MD-2), a step needed for activation of TLR4. None of the examined SNPs of CD14, bactericidal/permeability-increasing protein (BPI), TLR4 or MD-2 were associated with corresponding protein plasma levels or endotoxin delivery to MD-2, but CD14 and BPI SNPs significantly associated with differences in LPS-induced TNF-α release ex vivo and infection frequency, respectively. These findings suggest that specific LBP, CD14 and BPI SNPs might be contributory assessments in studies where clinical outcome may be affected by host response to endotoxin and bacterial infection.

Inefficient TLR4/MD-2 heterotetramerization by monophosphoryl lipid A

Synthetic forms of E. coli monophosphoryl lipid A (sMLA) weakly activate the MyD88 (myeloid differentiation primary response protein) branch of the bifurcated TLR4 (Toll-like receptor 4) signaling pathway, in contrast to diphosphoryl lipid A (sDLA), which is a strong activator of both branches of TLR4. sMLA's weak MyD88 signaling activity is apparent downstream of TLR4/MyD88 signaling as we show that sMLA, unlike sDLA, is unable to efficiently recruit the TNF receptor-associated factor 6 (TRAF6) to the Interleukin-1 receptor-associated kinase 1 (IRAK1). This reduced recruitment of TRAF6 explains MLA's lower MAPK (Mitogen Activated Protein Kinase) and NF-κB activity. As further tests of sMLA's ability to activate TLR4/Myeloid differentiation factor 2 (MD-2), we used the antibody MTS510 as an indicator for TLR4/MD-2 heterotetramer formation. Staining patterns with this antibody indicated that sMLA does not effectively drive heterotetramerization of TLR4/MD-2 when compared to sDLA. However, a F126A mutant of MD-2, which allows lipid A binding but interferes with TLR4/MD-2 heterotetramerization, revealed that while sMLA is unable to efficiently form TLR4/MD-2 heterotetramers, it still needs heterotetramer formation for the full extent of signaling it is able to achieve. Monophosphoryl lipid A's weak ability to form TLR4/MD-2 heterotetramers was not restricted to synthetic E. coli type because cells exposed to a biological preparation of S. minnesota monophosphoryl lipid A (MPLA) also showed reduced TLR4/MD-2 heterotetramer formation. The low potency with which sMLA and MPLA drive heterotetramerization of TLR4/MD-2 contributes to their weak MyD88 signaling activities.

Reviewing and identifying amino acids of human, murine, canine and equine TLR4 / MD-2 receptor complexes conferring endotoxic innate immunity activation by LPS/lipid A, or antagonistic effects by Eritoran, in contrast to species-dependent modulation by lipid IVa

There is literature evidence gathered throughout the last two decades reflecting unexpected species differences concerning the immune response to lipid IVa which provides the opportunity to gain more detailed insight by the molecular modeling approach described in this study. Lipid IVa is a tetra-acylated precursor of lipid A in the biosynthesis of lipopolysaccharide (LPS) in Gram-negative bacteria. Lipid A of the prototypic E. coli-type is a hexa-acylated structure that acts as an agonist in all tested mammalian species by innate immunorecognition via the Toll-like receptor 4 (TLR4)/myeloid differentiation factor 2 (MD-2) receptor complex. In contrast, lipid IVa is proinflammatory in mouse cells (agonism) but it remains inactive to human macrophages and even antagonizes the action of potent agonists like E. coli-type lipid A. This particular ambivalent activity profile of lipid IVa has been confirmed in other mammalian species: in equine cells Lipid IVa also acts in a weak agonistic manner, whereas being inactive and antagonizing the lipid A-induced activation of canine TLR4/MD-2. Intriguingly, the respective TLR4 amino acid sequences of the latter species are more identical to the human (67%, 68%) than to the murine (62%, 58%) ortholog. In order to address the unpaired activity-sequence dualism for human, murine, canine and equine species regarding the activity of lipid IVa as compared to LPS and lipid A and, we review the literature and computationally pinpoint the differential biological effects of lipid IVa versus LPS and lipid A to specific amino acid residues. In contrast to lipid IVa the structurally related synthetic compound Eritoran (E5564) acts consistently in an antagonistic manner in these mammalian species and serves as a reference ligand for molecular modeling in this study. The combined evaluation of data sets provided by prior studies and in silico homology mapping of differential residues of TLR4/MD-2 complexes lends detailed insight into the driving forces of the characteristic binding modes of the lipid A domain in LPS and the precursor structure lipid IVa to the receptor complex in individual mammalian species.

Radioiodination of an endotoxin·MD-2 complex generates a novel sensitive, high-affinity ligand for TLR4

A purified complex of metabolically labeled [(3)H]lipooligosaccharide (LOS) and recombinant human myeloid differentiation factor 2 (MD-2), [(3)H]LOS·MD-2, has been used to demonstrate pM affinity binding interactions with soluble TLR4 ectodomain (TLR4ecd). For measurement of the binding parameters of membrane-bound TLR4, we took advantage of the stability of endotoxin·MD-2 and tyrosine(s) present on the surface of MD-2 to radioiodinate LOS·MD-2. Radioiodinated LOS·MD-2 generated a reagent with an estimated 1:1 molar ratio of [(125)I] to sMD-2 with 20-fold higher specific radioactivity and TLR4-activating properties comparable to metabolically-labeled LOS·MD-2. LOS·MD-2[(125)I] and [(3)H]LOS·MD-2 have similar affinities for soluble (FLAG) TLR4ecd and for membrane-bound TLR4 in HEK293T/TLR4 cells. In a similar dose-dependent manner, sMD-2 and LOS·MD-2 inhibit LOS·MD-2[(125)I] binding to TLR4 indicating the pM affinity binding of LOS·MD-2[(125)I] is agonist-independent. LOS·MD-2[(125)I] allowed measurement of low levels of cell-surface human or murine TLR4 expressed by stable cell lines (2000-3000 sites/cell) and quantitatively measures low levels of 'MD-2-free' TLR4 (est. 250 molecules/cell) in cells co-expressing TLR4 and MD-2. Occupation of 50-100 TLR4/cell by LOS·MD-2 is sufficient to trigger measurable TLR4-dependent cell activation. LOS·MD-2[(125)I] provides a powerful reagent to measure quantitatively functional human and murine cell-surface TLR4, including in cells where surface TLR4 is potentially functionally significant but not detectable by other methods.

The crystal structure of human soluble CD14 reveals a bent solenoid with a hydrophobic amino-terminal pocket

Human monocyte differentiation Ag CD14 is a pattern recognition receptor that enhances innate immune responses to infection by sensitizing host cells to bacterial LPS (endotoxin), lipoproteins, lipoteichoic acid, and other acylated microbial products. CD14 physically delivers these lipidated microbial products to various TLR signaling complexes that subsequently induce intracellular proinflammatory signaling cascades upon ligand binding. The ensuing cellular responses are usually protective to the host but can also result in host fatality through sepsis. In this work, we have determined the x-ray crystal structure of human CD14. The structure reveals a bent solenoid typical of leucine-rich repeat proteins with an amino-terminal pocket that presumably binds acylated ligands including LPS. Comparison of human and mouse CD14 structures shows great similarity in overall protein fold. However, compared with mouse CD14, human CD14 contains an expanded pocket and alternative rim residues that are likely to be important for LPS binding and cell activation. The x-ray crystal structure of human CD14 presented in this article may foster additional ligand-bound structural studies, virtual docking studies, and drug design efforts to mitigate LPS-induced sepsis and other inflammatory diseases.

Structural analyses of human Toll-like receptor 4 polymorphisms D299G and T399I

BACKGROUND

TLR4 polymorphism replacing Asp-299 with Gly and Thr-399 with Ile (D299G/T399I) causes LPS hyporesponsiveness.

RESULTS

TLR4(SNPs)·MD-2·LPS exhibits an agonistic 2:2:2 architecture. Local structural differences were observed around D299G, but not around T399I, SNP site.

CONCLUSION

These local differences cause the modulation of surface properties of TLR4, which may affect ligand binding.

SIGNIFICANCE

This study provides structural evidence of the functionality of the mutant TLR4 carrying the SNPs. Toll-like receptor 4 (TLR4) and its coreceptor MD-2 recognize bacterial lipopolysaccharide (LPS) and signal the innate immune response. Two single nucleotide polymorphisms (SNPs) of human TLR4, D299G and T399I, have been identified and suggested to be associated with LPS hyporesponsiveness. Moreover, the SNPs have been proposed to be associated with a variety of infectious and noninfectious diseases. However, how the SNPs affect the function of TLR4 remains largely unknown. Here, we report the crystal structure of the human TLR4 (D299G/T399I)·MD-2·LPS complex at 2.4 Å resolution. The ternary complex exhibited an agonistic "m"-shaped 2:2:2 architecture that was similar to that of the human wild type TLR4·MD-2·LPS complex. Local structural differences that might affect the binding of the ligands were observed around D299G, but not around T399I, SNP site.

Respiratory syncytial virus fusion protein-induced toll-like receptor 4 (TLR4) signaling is inhibited by the TLR4 antagonists Rhodobacter sphaeroides lipopolysaccharide and eritoran (E5564) and requires direct interaction with MD-2

Respiratory syncytial virus (RSV) is a leading cause of infant mortality worldwide. Toll-like receptor 4 (TLR4), a signaling receptor for structurally diverse microbe-associated molecular patterns, is activated by the RSV fusion (F) protein and by bacterial lipopolysaccharide (LPS) in a CD14-dependent manner. TLR4 signaling by LPS also requires the presence of an additional protein, MD-2. Thus, it is possible that F protein-mediated TLR4 activation relies on MD-2 as well, although this hypothesis has not been formally tested. LPS-free RSV F protein was found to activate NF-κB in HEK293T transfectants that express wild-type (WT) TLR4 and CD14, but only when MD-2 was coexpressed. These findings were confirmed by measuring F-protein-induced interleukin 1β (IL-1β) mRNA in WT versus MD-2(-/-) macrophages, where MD-2(-/-) macrophages failed to show IL-1β expression upon F-protein treatment, in contrast to the WT. Both Rhodobacter sphaeroides LPS and synthetic E5564 (eritoran), LPS antagonists that inhibit TLR4 signaling by binding a hydrophobic pocket in MD-2, significantly reduced RSV F-protein-mediated TLR4 activity in HEK293T-TLR4-CD14-MD-2 transfectants in a dose-dependent manner, while TLR4-independent NF-κB activation by tumor necrosis factor alpha (TNF-α) was unaffected. In vitro coimmunoprecipitation studies confirmed a physical interaction between native RSV F protein and MD-2. Further, we demonstrated that the N-terminal domain of the F1 segment of RSV F protein interacts with MD-2. These data provide new insights into the importance of MD-2 in RSV F-protein-mediated TLR4 activation. Thus, targeting the interaction between MD-2 and RSV F protein may potentially lead to novel therapeutic approaches to help control RSV-induced inflammation and pathology.

IMPORTANCE

This study shows for the first time that the fusion (F) protein of respiratory syncytial virus (RSV), a major cause of bronchiolitis and death, particularly in infants and young children, physically interacts with the Toll-like receptor 4 (TLR4) coreceptor, MD-2, through its N-terminal domain. We show that F protein-induced TLR4 activation can be blocked by lipid A analog antagonists. This observation provides a strong experimental rationale for testing such antagonists in animal models of RSV infection for potential use in people.

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.

Toll-like receptor 2 is required for LPS-induced Toll-like receptor 4 signaling and inhibition of ion transport in renal thick ascending limb

Previously we demonstrated that basolateral LPS inhibits HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through TLR4-dependent ERK activation. Here we report that the response of the MTAL to basolateral LPS requires TLR2 in addition to TLR4. The basolateral addition of LPS (ultrapure Escherichia coli K12) decreased HCO(3)(-) absorption in isolated, perfused MTALs from wild-type mice but had no effect in MTALs from TLR2(-/-) mice. In contrast, inhibition of HCO(3)(-) absorption by lumen LPS was preserved in TLR2(-/-) MTALs, indicating that TLR2 is involved specifically in mediating the basolateral LPS response. LPS also did not increase ERK phosphorylation in MTALs from TLR2(-/-) mice. TLR2 deficiency had no effect on expression of TLR4, MD-2, or MyD88. However, LPS-induced recruitment of MyD88 to the basolateral membrane was impaired in TLR2(-/-) MTALs. Inhibition of HCO(3)(-) absorption by LPS did not require CD14. Co-immunoprecipitation studies demonstrated an association between TLR4 and TLR2. Inhibition of HCO(3)(-) absorption by TLR2-specific ligands was preserved in MTALs from TLR4(-/-) mice. These results indicate that the effect of basolateral LPS to inhibit HCO(3)(-) absorption in the MTAL through MyD88-dependent ERK activation depends on a novel interaction between TLR4 and TLR2. TLR2 plays a dual role in the induction of intracellular signals that impair MTAL function, both through cooperation with TLR4 to mediate ERK signaling by LPS and through a TLR4-independent signaling pathway activated by Gram-positive bacterial ligands. Regulation of TLR2 expression and its interaction with TLR4 may provide new mechanisms for controlling and therapeutic targeting of TLR4-mediated LPS responses.

Lipopolysaccharide neutralization by antimicrobial peptides: a gambit in the innate host defense strategy

Antimicrobial peptides (AMPs) are nowadays understood as broad multifunctional tools of the innate immune system to fight microbial infections. In addition to its direct antimicrobial action, AMPs can modulate the host immune response by promoting or restraining the recruitment of cells and chemicals to the infection focus. Binding of AMPs to lipopolysaccharide is a critical step for both their antimicrobial action and their immunomodulatory properties. On the one hand, removal of Gram-negative bacteria by AMPs can be an effective strategy to prevent a worsened inflammatory response that may lead to septic shock. On the other hand, by neutralizing circulating endotoxins, AMPs can successfully reduce nitric oxide and tumor necrosis factor-α production, hence preventing severe tissue damage. Furthermore, AMPs can also interfere with the Toll-like receptor 4 recognition system, suppressing cytokine production and contributing to modulate the inflammatory response. Here, we review the immune system strategies devised by AMPs to avoid an exacerbated inflammatory response and thus prevent a fatal end to the host.

NMR studies of hexaacylated endotoxin bound to wild-type and F126A mutant MD-2 and MD-2·TLR4 ectodomain complexes

Host response to invasion by many gram-negative bacteria depends upon activation of Toll-like receptor 4 (TLR4) by endotoxin presented as a monomer bound to myeloid differentiation factor 2 (MD-2). Metabolic labeling of hexaacylated endotoxin (LOS) from Neisseria meningitidis with [(13)C]acetate allowed the use of NMR to examine structural properties of the fatty acyl chains of LOS present in TLR4-agonistic and -antagonistic binary and ternary complexes with, respectively, wild-type or mutant (F126A) MD-2 ± TLR4 ectodomain. Chemical shift perturbation indicates that Phe(126) affects the environment and/or position of each of the bound fatty acyl chains both in the binary LOS·MD-2 complex and in the ternary LOS·MD-2·TLR4 ectodomain complex. In both wild-type and mutant LOS·MD-2 complexes, one of the six fatty acyl chains of LOS is more susceptible to paramagnetic attenuation, suggesting protrusion of that fatty acyl chain from the hydrophobic pocket of MD-2, independent of association with TLR4. These findings indicate that re-orientation of the aromatic side chain of Phe(126) is induced by binding of hexaacylated E, preceding interaction with TLR4. This re-arrangement of Phe(126) may act as a "hydrophobic switch," driving agonist-dependent contacts needed for TLR4 dimerization and activation.

IL-27 enhances LPS-induced proinflammatory cytokine production via upregulation of TLR4 expression and signaling in human monocytes

IL-27, which is produced by activated APCs, bridges innate and adaptive immunity by regulating the development of Th cells. Recent evidence supports a role for IL-27 in the activation of monocytic cells in terms of inflammatory responses. Indeed, proinflammatory and anti-inflammatory activities are attributed to IL-27, and IL-27 production itself is modulated by inflammatory agents such as LPS. IL-27 primes LPS responses in monocytes; however, the molecular mechanism behind this phenomenon is not understood. In this study, we demonstrate that IL-27 priming results in enhanced LPS-induced IL-6, TNF-α, MIP-1α, and MIP-1β expression in human primary monocytes. To elucidate the molecular mechanisms responsible for IL-27 priming, we measured levels of CD14 and TLR4 required for LPS binding. We determined that IL-27 upregulates TLR4 in a STAT3- and NF-κB-dependent manner. Immunofluorescence microscopy revealed enhanced membrane expression of TLR4 and more distinct colocalization of CD14 and TLR4 upon IL-27 priming. Furthermore, IL-27 priming enhanced LPS-induced activation of NF-κB family members. To our knowledge, this study is the first to show a role for IL-27 in regulating TLR4 expression and function. This work is significant as it reveals new mechanisms by which IL-27 can enhance proinflammatory responses that can occur during bacterial infections.

Endotoxin{middle dot}albumin complexes transfer endotoxin monomers to MD-2 resulting in activation of TLR4

Response to Gram-negative bacteria (GNB) is partially mediated by the recognition of GNB-derived endotoxin by host cells. Potent host response to endotoxin depends on the sequential interaction of endotoxin with lipopolysaccharide binding protein (LBP), CD14, MD-2 and TLR4. While CD14 facilitates the efficient transfer of endotoxin monomers to MD-2 and MD-2·TLR4, activation of MD-2·TLR4 can occur in the absence of CD14 through an unknown mechanism. Here, we show that incubation of purified endotoxin (E) aggregates (E(agg), M ( r ) ≥ 20 million) in PBS with ≥ 0.1% albumin in the absence of divalent cations Ca(2+) and Mg(2+), yields E·albumin complexes (M ( r ) ∼70,000). E·albumin transfers E monomers to sMD-2 or sMD-2·TLR4 ectodomain (TLR4(ecd)) with a 'K (d)' of ∼4 nM and induces MD-2·TLR4-dependent, CD14-independent cell activation with a potency only 10-fold less than that of monomeric E·CD14 complexes. Our findings demonstrate, for the first time, a mechanistic basis for delivery of endotoxin monomers to MD-2 and for activation of TLR4 that is independent of CD14.

The ectodomain of the Toll-like receptor 4 prevents constitutive receptor activation

Toll-like receptor 4 (TLR4) is involved in activation of the innate immune response in a large number of different diseases. Despite numerous studies, the role of separate domains of TLR4 in the regulation of receptor activation is poorly understood. Replacement of the TLR4 ectodomain with LPS-binding proteins MD-2 or CD14 resulted in a robust ligand-independent constitutive activation comparable with the maximal stimulation of the receptor with LPS. The same effect was achieved by the replacement of the ectodomain with a monomeric fluorescent protein or a 24-kDa gyrase B fragment. This demonstrates an intrinsic dimerization propensity of the transmembrane and cytoplasmic domains of TLR4 and reveals a previously unknown function of the ectodomain in inhibiting spontaneous receptor dimerization. Constitutive activation was abolished by the replacement of the ectodomain by a bulkier protein ovalbumin. N-terminal deletion variants of TLR4 revealed that the smallest segment of the ectodomain that already prevents constitutive activity comprises only 90 residues (542 to 631) of the total 608 residues. We conclude that TLR4 represents a receptor with a low threshold of activation that can be rapidly activated by the release of inhibition exerted by its ectodomain. This is important for the sensitivity of TLR4 to activation by different agonists. The TLR4 ectodomain has multiple roles in enabling ligand regulated activation, providing proper localization while serving as an inhibitor to prevent spontaneous, ligand-independent dimerization.

Exploring the LPS/TLR4 signal pathway with small molecules

The identification of the bacterial endotoxin receptors for innate immunity, most notably TLR4 (Toll-like receptor 4), has sparked great interest in therapeutic manipulation of the innate immune system. In the present mini-review, several natural and synthetic molecules that modulate the TLR4-mediated LPS (lipopolysaccharide) signalling in animals and humans are considered, and their mechanisms of action are discussed. The process of LPS sensing and signal amplification in humans is based on the sequential action of specific receptors situated in the extracellular side of the innate immunity cells, which bind and transfer LPS to TLR4: LBP (LPS-binding protein), CD14, MD-2 (myeloid differentiation protein 2). We classified the compounds active on TLR4 pathway depending on the specific molecular targets (LPS, LBP, CD14, MD-2 or TLR4). Small molecules developed by our group are described that inhibit LPS-stimulated TLR4 activation by selectively targeting the LPS-CD14 interaction. These compounds have an interesting antiseptic shock, anti-inflammatory and anti-neuropathic pain activity in vivo.

Neisseria meningitidis lipid A mutant LPSs function as LPS antagonists in humans by inhibiting TLR 4-dependent cytokine production

Lipopolysaccharide is a major constituent of the outer membrane of Gram-negative bacteria and important in the induction of pro-inflammatory responses. Recently, novel LPS species derived from Neisseria meningitidis H44/76 by insertional inactivation of the lpxL1 and lpxL2 genes have been created with a lipid A portion consisting of five (penta-acylated lpxL1) or four (tetra-acylated lpxL2) fatty acids connected to the glucosamine backbone instead of six fatty acids in the wild-type LPS. We show that these mutant LPS-types are poor inducers of cytokines (tumor-necrosis factor-α, IL-1β, IL-10, IL-RA) in human mononuclear cells. Both penta- and tetra-acylated meningococcal LPSs were able to inhibit cytokine production by wild-type Escherichia coli or meningococcal LPS. Binding of FITC-labelled E. coli LPS TLR4 transfected Chinese hamster ovary (CHO) cells was inhibited by both mutant LPS-types. Experiments with CHO fibroblasts transfected with human CD14 and TLR4 showed that the antagonizing effect was dependent on the expression of human TLR4. In contrast to the situation in humans, lpxL1 LPS has agonistic activity for cytokine production in peritoneal macrophages of DBA mice, and exacerbated arthritis in murine collagen induced arthritis model. N. meningitidis lipid A mutant LPSs lpxL1 and lpxL2 function as LPS antagonists in humans by inhibiting TLR4-dependent cytokine production but have agonistic activity in mice.

The cationic amphiphile 3,4-bis(tetradecyloxy)benzylamine inhibits LPS signaling by competing with endotoxin for CD14 binding

The identification of the bacterial endotoxin receptors for innate immunity, most notably the Toll-like receptor 4 (TLR4), has sparked great interest in therapeutic manipulation of innate immune system. We have recently developed synthetic molecules that have been shown to inhibit TLR4 activation in vitro and in vivo. Here we present the synthesis and the biological characterization of a new molecule, the cationic amphiphile 3,4-bis(tetradecyloxy)benzylamine, with a structure strictly related to the previously developed TLR4 modulators. This compound is able to inhibit in a dose-dependent manner the LPS-stimulated TLR4 activation in HEK cells. In order to characterize the mechanism of action of this compound, we investigated possible interactions with the extracellular components that bind and shuttle LPS to TLR4, namely LBP, CD14, and MD-2. This compound inhibited LBP/CD14-dependent LPS transfer to MD-2.TLR4, resulting in reduced formation of a (LPS-MD-2-TLR4)(2) complex. This effect was due to inhibition of the transfer of LPS from aggregates in solution to sCD14 with little or no effect on LPS shuttling from LPS/CD14 to MD-2. This compound also inhibited transfer of LPS monomer from full-length CD14 to a truncated, polyhistidine tagged CD14. Taken together, our findings strongly suggest that this compound inhibits LPS-stimulated TLR4 activation by competitively occupying CD14 and thereby reducing the delivery of activating endotoxin to MD-2.TLR4.

Hemin and a metabolic derivative coprohemin modulate the TLR4 pathway differently through different molecular targets

Heme is a prosthetic group in a large number of essential proteins that have a pivotal role in oxygen transport, storage and electron shuttling. High amounts of free heme are associated with pathological states. Recently, it has been suggested that activation of Toll-like receptor 4 (TLR4) is one of the ways in which the 'danger signal' of free heme is detected. Here, we examine the biochemical basis of the modulation of the TLR4 pathway by hemin (iron(III)-protoporphyrin IX) and its metabolic, oxidated derivative coprohemin (iron(III)-coproporphyrin I). High concentrations of hemin (50 μM) triggered TLR4-mediated IL-8 production in the human HEK293/TLR4 cell line in the absence of the co-receptors CD14 and MD-2; the latter an essential co-receptor for TLR4 activation by endotoxin. Hemin and endotoxin have additive effects when co-administrated to HEK/TLR4 cells, suggesting that hemin and endotoxin activate TLR4 by different mechanisms. Coprohemin, in contrast to hemin, is unable to trigger TLR4-dependent activation of HEK/TLR4 cells, but instead causes dose-dependent inhibition of endotoxin-stimulated IL-8 production. The inhibitory effect of coprohemin is paralleled by reduced delivery of endotoxin to MD-2 (-TLR4) that is necessary for activation of TLR4 by endotoxin. Thus, despite their similar chemical structure, hemin and coprohemin have very different effects on the TLR4 pathway, the former acting as a mild agonist of TLR4, the latter as an antagonist selectively targeting the endotoxin-MD-2 interaction.

An apple oligogalactan prevents against inflammation and carcinogenesis by targeting LPS/TLR4/NF-κB pathway in a mouse model of colitis-associated colon cancer

Evidence strongly supported a link between inflammation and cancer. Patients with colitis have high risk for development of colon cancer. Nuclear factor-kappa B (NF-κB), partially induced by lipopolysaccharide (LPS) binding to Toll-like receptor (TLR) 4, is a vital molecule in supervising the transformation of colitis to colon cancer. It could be a good strategy to prevent colitis carcinogenesis for targeting LPS/TLR4/NF-κB pathway. In the present study, we obtained an oligogalactan composed of five galacturonic acids from apple pectin and evaluated its protective efficacy on intestinal toxicities and carcinogenesis in a mouse model of colitis-associated colon cancer induced by 1,2-dimethylhydrazine and dextran sodium sulfate (DSS). The apple oligogalactan (AOG) was highly effective against intestinal toxicities and carcinogenesis and decreased the elevated levels of TLR4 and tumor necrosis factor-α (TNF-α) induced by inflammation in vivo in this model system. In vitro studies, AOG alone only slightly increased the levels of protein expression and messenger RNA of TLR4, phosphorylation of IκBα and production of TNF-α in HT-29 cells. However, AOG significantly decreased the elevation of all the biomarkers induced by LPS when it was combined with LPS. The effect of AOG may be related to membrane internalization and redistribution of TLR4 from cell membrane to cytoplasm. AOG is active against inflammation and carcinogenesis through targeting LPS/TLR4/NF-κB pathway. Both AOG and LPS are agonists of TLR4 for sharing the same ligand but AOG has a much lower intrinsic activity than that of LPS. AOG may be useful for treatment of colitis and prevention of carcinogenesis in the clinics.

Toll-like receptor 4 in CNS pathologies

The responses of the brain to infection, ischemia and trauma share remarkable similarities. These and other conditions of the CNS coordinate an innate immune response marked by activation of microglia, the macrophage-like cells of the nervous system. An important contributor to microglial activation is toll-like receptor 4, a pathogen-associated molecular pattern receptor known to initiate an inflammatory cascade in response to various CNS stimuli. The present review traces new efforts to characterize and control toll-like receptor 4 in inflammatory etiologies of the nervous system.

Expression of functional D299G.T399I polymorphic variant of TLR4 depends more on coexpression of MD-2 than does wild-type TLR4

Two missense variants (D299G and T399I) of TLR4 are cosegregated in individuals of European descent and, in a number of test systems, result in reduced responsiveness to endotoxin. How these changes within the ectodomain (ecd) of TLR4 affect TLR4 function is unclear. For both wild-type and D299G.T399I TLR4, we used endotoxinCD14 and endotoxinMD-2 complexes of high specific radioactivity to measure: 1) interaction of recombinant MD-2TLR4 with endotoxinCD14 and TLR4 with endotoxinMD-2; 2) expression of functional MD-2TLR4 and TLR4; and 3) MD-2TLR4 and TLR4-dependent cellular endotoxin responsiveness. Both wild-type and D299G.T399I TLR4(ecd) demonstrated high affinity (K(d) approximately 200 pM) interaction of endotoxinCD14 with MD-2TLR4(ecd) and endotoxinMD-2 with TLR4(ecd). However, levels of functional TLR4 were reduced up to 2-fold when D299G.T399I TLR4 was coexpressed with MD-2 and >10-fold when expressed without MD-2, paralleling differences in cellular endotoxin responsiveness. The dramatic effect of the D299G.T399I haplotype on expression of functional TLR4 without MD-2 suggests that cells expressing TLR4 without MD-2 are most affected by these polymorphisms.

Evidence of a specific interaction between new synthetic antisepsis agents and CD14

Synthetic molecules derived from natural sugars with a positively charged amino group or ammonium salt and two lipophilic chains have been shown to inhibit TLR4 activation in vitro and in vivo. To characterize the mechanism of action of this class of molecules, we investigated possible interactions with the extracellular components that bind and shuttle endotoxin [lipopolysaccharide (LPS)] to TLR4, namely, LBP, CD14, and MD-2. Molecules that inhibited TLR4 activation inhibited LBP.CD14-dependent transfer of endotoxin monomers derived from aggregates of tritiated lipooligosaccharide ([(3)H]LOS) from Neisseria meninigitidis to MD-2.TLR4, resulting in a reduced level of formation of a ([(3)H]LOS.MD-2.TLR4(ECD))(2) (M(r) approximately 190000) complex. This effect was due to inhibition of the transfer of [(3)H]LOS from aggregates in solution to sCD14 with little or no effect on [(3)H]LOS shuttling from [(3)H]LOS.sCD14 to MD-2. These compounds also inhibited transfer of the [(3)H]LOS monomer from full-length CD14 to a truncated, polyhistidine-tagged CD14. Dose-dependent inhibition of the transfer of [(3)H]LOS between the two forms of CD14 was observed with each of three different synthetic compounds that inhibited TLR4 activation but not by another structurally related analogue that lacked TLR4 antagonistic activity. Saturation transfer difference (STD) NMR data showed direct binding to CD14 by the synthetic TLR4 antagonist mediated principally through the lipid chains of the synthetic compound. Taken together, our findings strongly suggest that these compounds inhibit TLR4 activation by endotoxin by competitively occupying CD14 and thereby reducing the level of delivery of activating endotoxin to MD-2.TLR4.

MD-2-mediated ionic interactions between lipid A and TLR4 are essential for receptor activation

Lipopolysaccharide (LPS) activates innate immune responses through TLR4.MD-2. LPS binds to the MD-2 hydrophobic pocket and bridges the dimerization of two TLR4.MD-2 complexes to activate intracellular signaling. However, exactly how lipid A, the endotoxic moiety of LPS, activates myeloid lineage cells remains unknown. Lipid IV(A), a tetra-acylated lipid A precursor, has been used widely as a model for lipid A activation. For unknown reasons, lipid IV(A) activates proinflammatory responses in rodent cells but inhibits the activity of LPS in human cells. Using stable TLR4-expressing cell lines and purified monomeric MD-2, as well as MD-2-deficient bone marrow-derived macrophages, we found that both mouse TLR4 and mouse MD-2 are required for lipid IV(A) activation. Computational studies suggested that unique ionic interactions exist between lipid IV(A) and TLR4 at the dimerization interface in the mouse complex only. The negatively charged 4'-phosphate on lipid IV(A) interacts with two positively charged residues on the opposing mouse, but not human, TLR4 (Lys(367) and Arg(434)) at the dimerization interface. When replaced with their negatively charged human counterparts Glu(369) and Gln(436), mouse TLR4 was no longer responsive to lipid IV(A). In contrast, human TLR4 gained lipid IV(A) responsiveness when ionic interactions were enabled by charge reversal at the dimerization interface, defining the basis of lipid IV(A) species specificity. Thus, using lipid IV(A) as a selective lipid A agonist, we successfully decoupled and coupled two sequential events required for intracellular signaling: receptor engagement and dimerization, underscoring the functional role of ionic interactions in receptor activation.

Novel roles of lysines 122, 125, and 58 in functional differences between human and murine MD-2

The MD-2/TLR4 complex provides a highly robust mechanism for recognition and response of mammalian innate immunity to Gram-negative bacterial endotoxins. Despite overall close structural and functional similarity, human (h) and murine (m) MD-2 show several species-related differences, including the ability of hMD-2, but not mMD-2, to bind endotoxin (E) in the absence of TLR4. Wild-type mMD-2 can support TLR4-dependent cell activation by E only when mMD-2 and mTLR4 are coexpressed in the same cell. However, replacement of Glu122, Leu125, and/or Asn58 of mMD-2 with the corresponding residues (lysines) of hMD-2 was sufficient to yield soluble extracellular MD-2 that reacted with monomeric E . sCD14 complex to form extracellular monomeric E . MD-2 that activated cells expressing TLR4 without MD-2. Moreover, in contrast to wild-type mMD-2, double and triple mMD-2 mutants also supported E-triggered signaling in combination with human TLR4. Conversely, a K125L mutant of hMD-2 reacted with E . CD14 and activated TLR4 only when coexpressed with TLR4, and not when secreted without TLR4. These findings reveal novel roles of lysines 122, 125, and 58 in human MD-2 that contribute to the functional differences between human and murine MD-2 and, potentially, to differences in the sensitivity of humans and mice to endotoxin.