Dehydrocostus lactone alleviates irinotecan-induced intestinal mucositis by blocking TLR4/MD2 complex formation

BACKGROUND

Irinotecan (CPT-11) is used as chemotherapeutic drug for treatment of colorectal cancer. However, without satisfactory treatments, its gastrointestinal toxicities such as diarrhea and intestinal inflammation severely restrained its clinical application. Roots of Aucklandia lappa Decne. are used as traditional Chinese medicine to relieve gastrointestinal dysfunction and dehydrocostus lactone (DHL) is one of its main active components. Nevertheless, the efficacy and mechanism of DHL against intestinal mucositis remains unclear.

PURPOSE

The present study aimed to investigate the protective effects of DHL on CPT-11-induced intestinal mucositis and its underlying mechanisms.

METHODS

The protective effect of DHL was investigated in CPT-11-induced mice and lipopolysaccharide (LPS)+CPT-11 induced THP-1 macrophages. Body weight, diarrhea score, survival rate, colon length, and histopathological changes in mice colon and jejunum were analyzed to evaluate the protective effect of DHL in vivo. And DHL on reducing inflammatory response and regulating TLR4/NF-κB/NLRP3 pathway in vivo and in vitro were explored. Moreover, DHL on the interaction between TLR4 and MD2 was investigated. And silencing TLR4 targeted by siRNA was performed to validate the mechanisms of DHL on regulating the inflammation.

RESULTS

DHL prevented CPT-11-induced intestinal damage, represented by reducing weight loss, diarrhea score, mortality rate and the shortening of the colon. Histological analysis confirmed that DHL prevented intestinal epithelial injury and improved the intestinal barrier function in CPT-11 induced mice. Besides, DHL significantly downregulated the level of inflammatory cytokines by inhibiting TLR4/NF-κB/NLRP3 signaling pathway in CPT-11-induced mice and LPS+CPT-11-induced THP-1 macrophages. In addition, DHL blocked TLR4/MD2 complex formation. Molecular docking combined with SIP and DARTS assay showed that DHL could bind to TLR4/MD2 and occludes the hydrophobic pocket of MD2. Furthermore, Silencing TLR4 abrogated the effect of DHL on LPS+CPT-11 induced inflammatory response in THP-1 macrophages. Additionally, DHL ameliorate the CPT-11-induced intestinal mucositis without affecting the anti-tumor efficacy of CPT-11 in the tumor xenograft mice.

CONCLUSION

This study found that DHL exhibited the anti-inflammatory effects in CPT-11-induced intestinal mucositis by inhibiting the formation of TLR4/MD2 complex and then regulation of NF-κB/NLRP3 signaling pathway. DHL is potentially served as a novel strategy of combined medication with CPT-11.

Dried tangerine peel polysaccharide (DTPP) alleviates hepatic steatosis by suppressing TLR4/MD-2-mediated inflammation and endoplasmic reticulum stress

Non-alcoholic fatty liver disease (NAFLD) is a complex pathogenic metabolic syndrome characterized by increased inflammation and endoplasmic reticulum stress. In recent years, natural polysaccharides derived from traditional Chinese medicine have shown significant anti-inflammatory effects, making them an attractive therapeutic option. However, little research has been conducted on the therapeutic potential of dried tangerine peel polysaccharide (DTPP) - one of the most important medicinal resources in China. The results of the present study showed that DTPP substantially reduced macrophage infiltration in vivo and suppressed the expression of pro-inflammatory factors and endoplasmic reticulum stress-related genes. Additionally, surface plasmon resonance analysis revealed that DTPP had a specific affinity to myeloid differentiation factor 2, which consequently suppressed lipopolysaccharide-induced inflammation via interaction with the toll-like receptor 4 signaling pathway. This study provides a potential molecular mechanism underlying the anti-inflammatory effects of DTPP on NAFLD and suggests DTPP as a promising therapeutic strategy for NAFLD treatment.

In Silico Analyses Indicate a Lower Potency for Dimerization of TLR4/MD-2 as the Reason for the Lower Pathogenicity of Omicron Compared to Wild-Type Virus and Earlier SARS-CoV-2 Variants

The SARS-CoV-2 Omicron variants have replaced all earlier variants, due to increased infectivity and effective evasion from infection- and vaccination-induced neutralizing antibodies. Compared to earlier variants of concern (VoCs), the Omicron variants show high TMPRSS2-independent replication in the upper airway organs, but lower replication in the lungs and lower mortality rates. The shift in cellular tropism and towards lower pathogenicity of Omicron was hypothesized to correlate with a lower toll-like receptor (TLR) activation, although the underlying molecular mechanisms remained undefined. In silico analyses presented here indicate that the Omicron spike protein has a lower potency to induce dimerization of TLR4/MD-2 compared to wild type virus despite a comparable binding activity to TLR4. A model illustrating the molecular consequences of the different potencies of the Omicron spike protein vs. wild-type spike protein for TLR4 activation is presented. Further analyses indicate a clear tendency for decreasing TLR4 dimerization potential during SARS-CoV-2 evolution via Alpha to Gamma to Delta to Omicron variants.

Insights into the molecular basis and mechanism of heme-triggered TLR4 signalling: The role of heme-binding motifs in TLR4 and MD2

Haemolytic disorders, such as sickle cell disease, are accompanied by the release of high amounts of labile heme into the intravascular compartment resulting in the induction of proinflammatory and prothrombotic complications in affected patients. In addition to the relevance of heme-regulated proteins from the complement and blood coagulation systems, activation of the TLR4 signalling pathway by heme was ascribed a crucial role in the progression of these pathological processes. Heme binding to the TLR4-MD2 complex has been proposed recently, however, essential mechanistic information of the processes at the molecular level, such as heme-binding kinetics, the heme-binding capacity and the respective heme-binding sites (HBMs) is still missing. We report the interaction of TLR4, MD2 and the TLR4-MD2 complex with heme and the consequences thereof by employing biochemical, spectroscopic, bioinformatic and physiologically relevant approaches. Heme binding occurs transiently through interaction with up to four HBMs in TLR4, two HBMs in MD2 and at least four HBMs in their complex. Functional studies highlight that mutations of individual HBMs in TLR4 preserve full receptor activation by heme, suggesting that heme interacts with TLR4 through different binding sites independently of MD2. Furthermore, we confirm and extend the major role of TLR4 for heme-mediated cytokine responses in human immune cells.

Radix Glycyrrhizae extract and licochalcone a exert an anti-inflammatory action by direct suppression of toll like receptor 4

ETHNOPHARMACOLOGICAL RELEVANCE

Radix Glycyrrhizae (GL), a herbal medicine that is widely available, has shown advantages for a variety of inflammatory diseases. Toll like receptor 4 (TLR4) pathway has been shown to play a key role in the progression of inflammation.

AIM OF THE STUDY

The purpose of this study was to investigate the involvement of TLR4 in the anti-inflammatory mechanism of GL extract and its active constituent on acute lung injury (ALI).

MATERIALS AND METHODS

A model of inflammation produced by lipopolysaccharide (LPS) was established in C57BL/6 mice and macrophages derived from THP-1. To screen the active components of GL, molecular docking was used. Molecular dynamics and surface plasmon resonance imaging (SPRi) were used to study the interaction of a specific drug with the TLR4-MD2 complex. TLR4 was overexpressed by adenovirus to confirm TLR4 involvement in the anti-inflammatory activities of GL and the chosen chemical.

RESULTS

We observed that GL extract significantly reduced both LPS-induced ALI and the production of pro-inflammatory factors including TNF-α, IL-6 and IL-1β. Additionally, GL inhibited the binding of Alexa 488-labeled LPS (LPS-488) to the membrane of THP-1 derived macrophages. GL drastically reduce on the expression of TLR4 and the activation of mitogen-activated protein kinases (MAPKs) and nuclear factor-B (NF-κB). Furthermore, molecular docking revealed that Licochalcone A (LicoA) docked into the LPS binding site of TLR4-MD2 complex. MD2-LicoA binding conformation was found to be stable using molecular dynamic simulations. SPRi indicated that LicoA bound to TLR4-MD2 recombinant protein with a KD of 3.87 × 10^-7 M. LicoA dose-dependently reduced LPS-488 binding to the cell membrane. LicoA was found to significantly inhibit LPS-induced lung damage and inflammation. Furthermore, LicoA inhibited TLR4 expression, MAPK and NF-κB activation in a dose-dependent manner. The inhibitory effects of GL and LicoA on LPS-induced inflammation and TLR4 signaling activation were partly eliminated by TLR4 overexpression.

CONCLUSION

Our findings imply that GL and LicoA exert inhibitory effects on inflammation by targeting the TLR4 directly.

Exercise-Induced Irisin Decreases Inflammation and Improves NAFLD by Competitive Binding with MD2

Non-alcoholic fatty liver disease (NAFLD) is a global clinical problem. The MD2-TLR4 pathway exacerbates NAFLD progression by promoting inflammation. Long-term exercise is considered to improve NAFLD but the underlying mechanism is still unclear. In this study, we examined the protective effect and molecular mechanism of exercise on high-fat diet (HFD)-induced liver injury. In an HFD-induced NAFLD mouse model, exercise training significantly decreased hepatic steatosis and fibrosis. Interestingly, exercise training blocked the binding of MD2-TLR4 and decreased the downstream inflammatory response. Irisin is a myokine that is highly expressed in response to exercise and exerts anti-inflammatory effects. We found that circulating irisin levels and muscle irisin expression were significantly increased in exercised mice, suggesting that irisin could mediate the effect of exercise on NAFLD. In vitro studies showed that irisin improved lipid metabolism, fibrosis, and inflammation in palmitic acid (PA)-stimulated AML12 cells. Moreover, binding assay results showed that irisin disturbed MD2-TLR4 complex formation by directly binding with MD2 but not TLR4, and interfered with the recognition of stimuli such as PA and lipopolysaccharide with MD2. Our study provides novel evidence that exercise-induced irisin inhibits inflammation via competitive binding with MD2 to improve NAFLD. Thus, irisin could be considered a potential therapy for NAFLD.

Interaction between polysaccharides and toll-like receptor 4: Primary structural role, immune balance perspective, and 3D interaction model hypothesis

Various structural types of polysaccharides are recognized by toll-like receptor 4 (TLR4). However, the mechanism of interaction between the polysaccharides with different structures and TLR4 is unclarified. This review summarized the primary structure of polysaccharides related to TLR4, mainly including molecular weight, monosaccharide composition, glycosidic bonds, functional groups, and branched-chain structure. The optimal primary structure for interacting with TLR4 was obtained by the statistical analysis. Besides, the dual-directional regulation of TLR4 signaling cascade by polysaccharides was also elucidated from an immune balance perspective. Finally, the 3D interaction model of polysaccharides to TLR4-myeloid differentiation factor 2 (MD2) complex was hypothesized according to the LPS-TLR4-MD2 dimerization model and the polysaccharides solution conformation. The essence of polysaccharides binding to TLR4-MD2 complex is a multivalent non-covalent bond interaction. All the arguments summarized in this review are intended to provide some new insights into the interaction between polysaccharides and TLR4.

Full-Atom Model of the Agonist LPS-Bound Toll-like Receptor 4 Dimer in a Membrane Environment

The Toll-like receptor 4 (TLR4)/myeloid differentiation factor 2 (MD-2) innate immunity system is a membrane receptor of paramount importance as therapeutic target. Its assembly, upon binding of Gram-negative bacteria lipopolysaccharide (LPS), and also dependent on the membrane composition, finally triggers the immune response cascade. We have combined ab-initio calculations, molecular docking, all-atom molecular dynamics simulations, and thermodynamics calculations to provide the most realistic and complete 3D models of the active full TLR4 complex embedded into a realistic membrane to date. Our studies give functional and structural insights into the transmembrane domain behavior in different membrane environments, the ectodomain bouncing movement, and the dimerization patterns of the intracellular Toll/Interleukin-1 receptor domain. Our work provides TLR4 models as reasonable 3D structures for the (TLR4/MD-2/LPS)2 architecture accounting for the active (agonist) state of the TLR4, and pointing to a signal transduction mechanism across cell membrane. These observations unveil relevant molecular aspects involved in the TLR4 innate immune pathways and will promote the discovery of new TLR4 modulators.

Synthesis and evaluation of the anti-inflammatory activity of novel 8-quinolinesulfonamide derivatives as TLR4/MD-2 inhibitors with efficacy in adjuvant-induced arthritis

In this study, a series of 8-quinolinesulfonamidederivatives was synthesized, and their anti-inflammatory activity was evaluated. Among them, compound 3l was found to be the best anti-inflammatory agent, with IC50 values of 2.61 ± 0.39, 9.74 ± 0.85, and 12.71 ± 1.34 μM against NO, TNF-α and IL-1β production respectively. And 3l could significantly prevent lipopolysaccharide (LPS)-induced expression of inflammatory mediators (iNOS and COX-2). Molecule docking results showed that 3l could bind to the LPS binding site of toll-like receptor 4 (TLR4)/MD-2, and 3l was then identified as TLR4/MD-2 inhibitor by co-immunoprecipitation (co-IP) and cellular thermal shift assay (CTESA). Preliminary mechanism studies indicated that 3l could prevent TLR4 from being activated by disrupting TLR4/MD-2 heterodimerization and TLR4 homodimerization, thereby blocking the activation of the NF-κB/MAPK signaling pathway. Furthermore, observation of rat foot swelling, joint pathology and serum inflammatory cytokine levels proved that compound 3l had a significant therapeutic effect on adjuvant-induced arthritis (AIA) in rats in vivo. These results indicated that compound 3l is a potential anti-inflammatory agent, from which more effective anti-inflammatory drugs could be developed.

Immune Effect Regulated by the Chain Length: Interaction between Immune Cell Surface Receptors and Synthetic Glycopolymers

Glycopolymer-based drugs for immunotherapy have attracted increasing attention because the affinity between glycans and proteins plays an important role in immune responses. Previous studies indicate that the polymer chain length influences the affinity. In the studies on enhancing the immune response by glycans, it is found that both oligosaccharides and long-chain glycopolymers work well. However, there is a lack of systematic studies on the immune enhancement effect and the binding ability of oligomers and polymers to immune-related proteins. In this paper, to study the influence of the chain length, glycopolymers based on N-acetylglucosamine with different chain lengths were synthesized, and their interaction with immune-related proteins and their effect on dendritic cell maturation were evaluated. It was proved that compared with l-glycopolymers (degree of polymerization (DP) > 20), s-glycopolymers (DP < 20) showed better binding ability to the dendritic cell-specific ICAM-3-grabbing nonintegrin protein and the toll-like receptor 4 and myeloid differentiation factor 2 complex protein by quartz crystal microbalance and molecular docking simulation. When the total sugar unit amounts are equal, s-glycopolymers are proved to be superior in promoting dendritic cell maturation by detecting the expression level of CD80 and CD86 on the surface of dendritic cells. Through the combination of experimental characterization and theoretical simulation, a deep look into the interaction between immune-related proteins and glycopolymers with different chain lengths is helpful to improve the understanding of the immune-related interactions and provides a good theoretical basis for the design of new glycopolymer-based immune drugs.

Aureusidin derivative CNQX inhibits chronic colitis inflammation and mucosal barrier damage by targeting myeloid differentiation 2 protein

Our previous study has found that aureusidin can inhibit inflammation by targeting myeloid differentiation 2 (MD2) protein. Structural optimization of aureusidin gave rise to a derivative named CNQX. LPS was used to induce inflammation in intestinal macrophages; flow cytometry, PI staining and Hoechst 33342 staining were used to detect the apoptotic level of macrophages; enzyme-linked immunosorbent assay (ELISA) was utilized to detect the expression level of inflammatory factors (including IL-1β, IL-18 and TNF-α); immunofluorescence staining was used to investigate the expression of MD2; Western blot was employed to measure the protein level of TLR4, MD2, MyD88 and p-P65. As a result, CNQX with IC50 of 2.5 μM can significantly inhibit the inflammatory damage of macrophages, decrease apoptotic level, reduce the expression level of inflammatory factors and simultaneously decrease the expression level of TLR4, MD2, MyD88 as well as p-P65. Caco-2 cell line was used to simulate the intestinal mucosal barrier in vitro, LPS was employed to induce cell injury in Caco-2 (to up-regulate barrier permeability), and CNQX with IC50 of 2.5 μl was used for intervention. Flow cytometry was used to detect the apoptotic level of Caco-2 cells, trans-epithelial electric resistance (TEER) was measured, FITC-D was used to detect the permeability of the intestinal mucosa, and Western blot was used to detect the expression levels of tight junction proteins (including occludin, claudin-1, MyD88, TLR4 and MD2). As a result, CNQX decreased the apoptotic level of Caco-2 cells, increased TEER value, decreased the expression levels of MyD88, TLR4 and MD2, and increased the protein levels of tight junction proteins (including occludin and claudin-1). C57BL/6 wild-type mice were treated with drinking water containing Dextran sulphate sodium (DSS) to establish murine chronic colitis model. After CQNX intervention, we detected the bodyweight, DAI score and H&E tissue staining to evaluate the life status and pathological changes. Immunohistochemistry (IHC) staining was used to detect the expression of MD2 protein, tight junction protein (including occludin and claudin-1). Transmission electron microscopy and FITC-D were used to detect intestinal mucosal permeability. Western blot was used to detect the expression levels of tight junction proteins (including occludin, claudin-1, MyD88, TLR4 and MD2) in the intestinal mucosa tissue. Consequently, CNQX can inhibit the intestinal inflammatory response in mice with colitis, inhibit the mucosal barrier injury, increase the expression of tight junction proteins (including occludin and claudin-1) and decrease the expression levels of MyD88, TLR4 and MD2. Mechanistically, pull-down and immunoprecipitation assays showed that CNQX can inhibit the activation of TLR4/MD2-NF-κB by binding to MD2 protein. Collectively, in this study, we found that CNQX can suppress the activation of TLR4 signals by targeting MD2 protein, thereby inhibiting inflammation and mucosal barrier damage of chronic colitis.

Paeoniflorin exerts antidepressant-like effects through enhancing neuronal FGF-2 by microglial inactivation

Ethnopharmacological relevance Paeonia lactiflora is a famous Traditional Chinese medicine widely used for immunological regulation. Paeoniflorin, the main component of Paeonia lactiflora, exerts neuroprotective and antidepressant-like effects in rodents.

AIM OF THE STUDY

Fibroblast growth factor 2 (FGF-2) is essentially required in the central nervous system as it acts as both a neurotrophic factor and an anti-inflammatory factor participating in the regulation of proliferation, differentiation and apoptosis of neurons in the brain. However, it is unclear whether paeoniflorin could exert antidepressant effects via regulating FGF-2.

MATERIALS AND METHODS

In the present study, the effects of paeoniflorin were evaluated in depressive mice induced by the endotoxin lipopolysaccharide (LPS) injection.

RESULTS

The results showed that paeoniflorin markedly increased sucrose preference and reduced immobility time in LPS mice, indicating antidepressant effects. Consistent with the results from molecular docking showing paeoniflorin antagonizes TLR4, NF-κB and NLRP3, the biochemical analysis also indicated paeoniflorin inhibited TLR4/NF-κB/NLRP3 signaling, decreased proinflammatory cytokine levels and microglial activation in the hippocampus of LPS induced mice. In addition, the levels of neuronal FGF-2 and the density of dendritic spine were improved by paeoniflorin. More importantly, the FGFR1 inhibitor SU5402 prevented the antidepressant effects of paeoniflorin and blocked the neuroinflammatory and neurogenic regulatory effects of paeoniflorin, indicating that FGF-2/FGFR1 activation was required for the effects of paeoniflorin.

CONCLUSION

Taken together, the results demonstrate that paeoniflorin exhibits neuroprotective and antidepressant effects in mice, which may be mediated by activating neuronal FGF-2/FGFR1 signaling via the inhibition of microglial activation in the hippocampus.

Ergosta-7, 9 (11), 22-trien-3β-ol Interferes with LPS Docking to LBP, CD14, and TLR4/MD-2 Co-Receptors to Attenuate the NF-κB Inflammatory Pathway In Vitro and Drosophila

Ergosta-7, 9 (11), 22-trien-3β-ol (EK100) was isolated from Cordyceps militaris, which has been used as a traditional anti-inflammatory medicine. EK100 has been reported to attenuate inflammatory diseases, but its anti-inflammatory mechanism is still unclear. We were the first to investigate the effect of EK100 on the Toll-like receptor 4 (TLR4)/nuclear factor of the κ light chain enhancer of B cells (NF-κB) signaling in the lipopolysaccharide (LPS)-stimulated RAW264.7 cells and the green fluorescent protein (GFP)-labeled NF-κB reporter gene of Drosophila. EK100 suppressed the release of the cytokine and attenuated the mRNA and protein expression of pro-inflammatory mediators. EK100 inhibited the inhibitor kappa B (IκB)/NF-κB signaling pathway. EK100 also inhibited phosphatidylinositol-3-kinase (PI3K)/Protein kinase B (Akt) signal transduction. Moreover, EK100 interfered with LPS docking to the LPS-binding protein (LBP), transferred to the cluster of differentiation 14 (CD14), and bonded to TLR4/myeloid differentiation-2 (MD-2) co-receptors. Compared with the TLR4 antagonist, resatorvid (CLI-095), and dexamethasone (Dexa), EK100 suppressed the TLR4/AKT signaling pathway. In addition, we also confirmed that EK100 attenuated the GFP-labeled NF-κB reporter gene expression in Drosophila. In summary, EK100 might alter LPS docking to LBP, CD14, and TLR4/MD-2 co-receptors, and then it suppresses the TLR4/NF-κB inflammatory pathway in LPS-stimulated RAW264.7 cells and Drosophila.

Myeloid differentiation protein 2 mediates angiotensin II-induced inflammation and mesenchymal transition in vascular endothelium

Angiotensin II (Ang II)-induced vascular inflammation and injury entails endothelial to mesenchymal transition (EndMT). Recent studies have shown that Ang II engages toll-like receptor 4 (TLR4) in the vasculature to mediate adverse effects. Here, we aimed to investigate whether myeloid differentiation protein 2 (MD2), an extracellular molecule indispensable for TLR4 activation, mediates Ang II-induced vascular injury and EndMT. We utilized MD2 knockout mice and wildtype mice treated with a specific MD2 inhibitor to decipher its role in aortas of Ang II-challenged mice. To confirm our results and to provide mechanistic insights, we exposed cultured endothelial cells to Ang II, with or without MD2 silencing. We show that Ang II causes deleterious remodeling and EndMT in aortas of mice within two weeks. These Ang II effects were largely absent in MD2 knockout mice and in wildtype mice treated with a MD2 inhibitor. MD2 silencing in cultured endothelial cells confirmed the essential role of MD2 in Ang II-induced inflammatory factor induction, and EndMT-associated phenotypic change. We also found that Ang II-MD2-EndMT axis involves the activation of nuclear factor-κB. Our studies highlight an essential role of MD2 in Ang II-induced vascular inflammation and EndMT contributing to vascular injury. These results also imply that MD2 may be targeted to dampen inflammatory cardiovascular and EndMT-associated diseases.

N-palmitoyl-D-glucosamine, a Natural Monosaccharide-Based Glycolipid, Inhibits TLR4 and Prevents LPS-Induced Inflammation and Neuropathic Pain in Mice

Toll-like receptors (TLRs) are key receptors through which infectious and non-infectious challenges act with consequent activation of the inflammatory cascade that plays a critical function in various acute and chronic diseases, behaving as amplification and chronicization factors of the inflammatory response. Previous studies have shown that synthetic analogues of lipid A based on glucosamine with few chains of unsaturated and saturated fatty acids, bind MD-2 and inhibit TLR4 receptors. These synthetic compounds showed antagonistic activity against TLR4 activation in vitro by LPS, but little or no activity in vivo. This study aimed to show the potential use of N-palmitoyl-D-glucosamine (PGA), a bacterial molecule with structural similarity to the lipid A component of LPS, which could be useful for preventing LPS-induced tissue damage or even peripheral neuropathies. Molecular docking and molecular dynamics simulations showed that PGA stably binds MD-2 with a MD-2/(PGA)3 stoichiometry. Treatment with PGA resulted in the following effects: (i) it prevented the NF-kB activation in LPS stimulated RAW264.7 cells; (ii) it decreased LPS-induced keratitis and corneal pro-inflammatory cytokines, whilst increasing anti-inflammatory cytokines; (iii) it normalized LPS-induced miR-20a-5p and miR-106a-5p upregulation and increased miR-27a-3p levels in the inflamed corneas; (iv) it decreased allodynia in peripheral neuropathy induced by oxaliplatin or formalin, but not following spared nerve injury of the sciatic nerve (SNI); (v) it prevented the formalin- or oxaliplatin-induced myelino-axonal degeneration of sciatic nerve. SIGNIFICANCE STATEMENT We report that PGA acts as a TLR4 antagonist and this may be the basis of its potent anti-inflammatory activity. Being unique because of its potency and stability, as compared to other similar congeners, PGA can represent a tool for the optimization of new TLR4 modulating drugs directed against the cytokine storm and the chronization of inflammation.

Immunoinformatics approach to understand molecular interaction between multi-epitopic regions of SARS-CoV-2 spike-protein with TLR4/MD-2 complex

BACKGROUND

The coronavirus (CoV) spike (S) protein is critical for receptor binding, membrane fusion and internalization of the virus into the human cells. We have tried to search the epitopic component of the S-protein that might be served as crucial targets for the vaccine development and also tried to understand the molecular mechanism of epitopes and TLR4/MD-2 complex for adaptive immunity.

MATERIAL AND METHODS

Here we identified the antigenicity and the epitopic divergence of S-protein via immunoinformatics approach. The study was performed to identify the epitopes, composition of amino acids and its distribution in epitopic regions, composition of amino acid between the identified epitopes, secondary structure architecture of epitopes, physicochemical and biochemical parameters and molecular interaction between the identified epitope and TLR4/MD-2 complex. The SARS-CoV-2 can be possibly recognised by TLR4 of host immune cells that are responsible for the adaptive immune response.

RESULTS

We identified four SARS-CoV-2 S-protein 9mer antigenic epitopes and observed that they bind with the TLR4/MD-2 complex by varied stable molecular bonding interactions. Molecular interaction between these characterized epitopes with TLR4/MD-2 complex might be indicated the binding affinity and downstream signalling of adaptive immune response. Different physicochemical and biochemical parameters such as O-glycosylation and N-glycosylation, Hydrophobicity, GRAVY were identified within epitopic regions of S-protein. These parameters help to understand the protein-protein interaction between epitopes and TLR4/MD-2 complex. The study also revealed different epitopic binding pockets of TLR4/MD-2 complex.

CONCLUSIONS

The identified epitopes impart suitable prospects for the development of novel peptide-based epitopic vaccine for the control of COVID-19 infection.

Octominin Inhibits LPS-Induced Chemokine and Pro-inflammatory Cytokine Secretion from RAW 264.7 Macrophages via Blocking TLRs/NF-κB Signal Transduction

Inflammation is a well-organized innate immune response that plays an important role during the pathogen attacks and mechanical injuries. The Toll-like receptors (TLR)/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a major signal transduction pathway observed in RAW 264.7 macrophages during the inflammatory responses. Here, we investigated the anti-inflammatory effects of Octominin; a bio-active peptide developed from Octopus minor in RAW 264.7 macrophages in vitro. Octominin was found to inhibit lipopolysaccharides (LPS)-stimulated transcriptional activation of NF-κB in RAW 264.7 cells and dose-dependently decreased the mRNA expression levels of TLR4. Specifically, in silico docking results demonstrated that Octominin has a potential to inhibit TLR4 mediated inflammatory responses via blocking formation of TLR4/MD-2/LPS complex. We also demonstrated that Octominin could significantly inhibit LPS-induced secretion of pro-inflammatory cytokine (interleukin-β; IL-1β, IL-6, and tumor necrosis factor-α) and chemokines (CCL3, CCL4, CCL5, and CXCL10) from RAW 264.7 cells. Additionally, Octominin repressed the LPS-induced pro-inflammatory mediators including nitric oxide (NO), prostaglandin E2, inducible NO synthase, and cyclooxygenase 2 in macrophages. These results suggest that Octominin is a potential inhibitor of TLRs/NF-κB signal transduction pathway and is a potential candidate for the treatment of inflammatory diseases.

Macrophage-derived myeloid differentiation protein 2 plays an essential role in ox-LDL-induced inflammation and atherosclerosis

Background

Atherosclerosis is a chronic inflammatory disease. Although Toll-like receptor 4 (TLR4) has been involved in inflammatory atherosclerosis, the exact mechanisms by which oxidized-low-density lipoproteins (ox-LDL) activates TLR4 and elicits inflammatory genesis are not fully known. Myeloid differentiation factor 2 (MD2) is an extracellular molecule indispensable for lipopolysaccharide recognition of TLR4.

Method

Apoe^−/−Md2^−/− mice and pharmacological inhibitor of MD2 were used in this study. We also reconstituted Apoe^−/− mice with either Apoe^−/− or Apoe^−/−Md2^−/− marrow-derived cells. Mechanistic studies were performed in primary macrophages, HEK-293T cells, and cell-free system.

Finding

MD2 levels are elevated in atherosclerotic lesion macrophages, and MD2 deficiency or pharmacological inhibition in mice reduces the inflammation and stunts the development of atherosclerotic lesions in Apoe^−/− mice fed with high-fat diet. Transfer of marrow-derived cells from Apoe-Md2 double knockout mice to Apoe knockout mice confirmed the critical role of bone marrow-derived MD2 in inflammatory factor induction and atherosclerosis development. Mechanistically, we show that MD2 does not alter ox-LDL uptake by macrophages but is required for TLR4 activation and inflammation via directly binding to ox-LDL, which triggers MD2/TLR4 complex formation and TLR4-MyD88-NFκB pro-inflammatory cascade.

Interpretation

We provide a mechanistic basis of ox-LDL-induced macrophage inflammation, illustrate the role of macrophage-derived MD2 in atherosclerosis, and support the therapeutic potential of MD2 targeting in atherosclerosis-driven cardiovascular diseases.

Funding

This work was supported by the National Key Research Project of China (2017YFA0506000), National Natural Science Foundation of China (21961142009, 81930108, 81670244, and 81700402), and Natural Science Foundation of Zhejiang Province (LY19H020004).

Myeloid Differentiation Protein 2 Mediates Angiotensin II-Induced Liver Inflammation and Fibrosis in Mice

Angiotensin II (Ang II) participates in the pathogenesis of liver injury. Our previous publications reported that myeloid differentiation protein 2 (MD2) mediates Ang II-induced cardiac and kidney inflammation by directly binding to Ang II. Thus, we hypothesize that MD2 is critical to Ang II-induced liver injury. Subcutaneous injections of Ang II for 8 weeks were adopted to build the liver injury model. With a specific MD2 inhibitor L6H21 and MD2 knockout mice, we reported that MD2 inhibition and knockout significantly mitigate liver inflammation and fibrosis in mice injected with Ang II. To be more specific, the functional and pathological damages induced by Ang II were mitigated by L6H21 or MD2 knockout. MD2 knockout or L6H21 administration inhibited the Ang II-induced upregulation of fibrosis markers, inflammatory cytokines, and adhesion molecules in gene or protein levels. The activation of NF-κB and Extracellular signal-regulated kinases (ERK) induced by Ang II was also reversed by L6H21 treatment or MD2 deficiency. Note that the co-immunoprecipitation study showed that L6H21 downregulated the ANG II-induced toll-like receptor 4 (TLR4)/MD2 complex in liver tissues while having no effects on MD2 expression. Our results reported the critical role of MD2 in the progress of liver injury and suggested that MD2 is a potential therapeutic target for liver injury.

Expression of human TLR4/myeloid differentiation factor 2 directs an early innate immune response associated with modest increases in bacterial burden during Coxiella burnetii infection

Human TLR4 (hTLR4) and mouse TLR4 molecules respond differently to hypo-acylated LPS. The LPS of Coxiella burnetii is hypo-acylated and heavily glycosylated and causes a minimal response by human cells. Thus, we hypothesized that mice expressing hTLR4 molecules would be more susceptible to C. burnetii infection. Our results show that transgenic mice expressing hTLR4 and the human myeloid differentiation factor 2 (MD-2) adaptor protein (hTLR4/MD-2) respond similarly to wild type mice with respect to overall disease course. However, differences in bacterial burdens in tissues were noted, and lung pathology was increased in hTLR4/MD2 compared to wild type mice. Surprisingly, bone marrow chimera experiments indicated that hTLR4/MD-2 expression on non-hematopoietic cells, rather than the target cells for C. burnetii infection, accounted for increased bacterial burden. Early during infection, cytokines involved in myeloid cell recruitment were detected in the plasma of hTLR4/MD2 mice but not wild type mice. This restricted cytokine response was accompanied by neutrophil recruitment to the lung in hTLR4/MD2 mice. These data suggest that hTLR4/MD-2 alters early responses during C. burnetii infection. These early responses are precursors to later increased bacterial burdens and exacerbated pathology in the lung. Our data suggest an unexpected role for hTLR4/MD-2 in non-hematopoietic cells during C. burnetii infection.

Inhibition of myeloid differentiation factor 2 by baicalein protects against acute lung injury

BACKGROUND

ALI/ARDS is characterized by severe hypoxemic respiratory failure attributed to inflammatory tissue injury. There are no treatment modalities able to prevent/reverse the dire pathological sequelae in these patients. Evidence links the inflammatory lung injury to uncontrolled activation of the immune signaling complex, TLR4-MD2 (Toll-like receptor-myeloid differentiation factor 2). Baicalein, a natural flavonoid, is reported to have robust anti-inflammatory properties, but its inhibition mechanism remains unclear.

HYPOTHESIS/PURPOSE

This study investigated the protective mechanisms of baicalein on ALI/ARDS.

METHODS

We used two experimental mouse models of LPS-induced ALI, pulmonary infection model (intratracheal LPS), and systemic infection model (intravenous LPS). Blood, BALF, lung and liver tissues were analyzed using routine methods. In vitro studies using peritoneal mouse macrophages or recombinant proteins were designed to elucidate inhibition mechanisms of baicalein.

RESULTS

Our critical new findings revealed that Baicalein was an MD2 inhibitor, directly bound to MD2, effectively suppressing TLR4-MD2 activation and the subsequent MAPK and NF-κB signaling. The inhibited MD2 prevented development of inflammatory tissue injury and improved survival. The importance of MD2 in the inflammatory injury in ALI was corroborated by data obtained from MD2^-/- mice, which did not develop the characteristic LPS-induced lung tissue damage. Thus, the findings indicated that MD2 was critical for development of ALI, functioning as an early upstream signal driving the progression of inflammatory injury.

CONCLUSION

Baicalein, as a direct and selective MD2 inhibitor, inhibited the early upstream TLR4-MD2 signaling and is a promising therapeutic agent for the treatment of ALI/ARDS.

Saturation of acyl chains converts cardiolipin from an antagonist to an activator of Toll-like receptor-4

Cardiolipins (CLs) are tetra-acylated diphosphatidylglycerols found in bacteria, yeast, plants, and animals. In healthy mammals, CLs are unsaturated, whereas saturated CLs are found in blood cells from Barth syndrome patients and in some Gram-positive bacteria. Here, we show that unsaturated but not saturated CLs block LPS-induced NF-κB activation, TNF-α and IP-10 secretion in human and murine macrophages, as well as LPS-induced TNF-α and IL-1β release in human blood mononuclear cells. Using HEK293 cells transfected with Toll-like receptor 4 (TLR4) and its co-receptor Myeloid Differentiation 2 (MD2), we demonstrate that unsaturated CLs compete with LPS for binding TLR4/MD2 preventing its activation, whereas saturated CLs are TLR4/MD2 agonists. As a consequence, saturated CLs induce a pro-inflammatory response in macrophages characterized by TNF-α and IP-10 secretion, and activate the alternative NLRP3 inflammasome pathway in human blood-derived monocytes. Thus, we identify that double bonds discriminate between anti- and pro-inflammatory properties of tetra-acylated molecules, providing a rationale for the development of TLR4 activators and inhibitors for use as vaccine adjuvants or in the treatment of TLR4-related diseases.

Stereochemistry and innate immune recognition: (+)-norbinaltorphimine targets myeloid differentiation protein 2 and inhibits toll-like receptor 4 signaling

Deregulation of innate immune TLR4 signaling contributes to various diseases including neuropathic pain and drug addiction. Naltrexone is one of the rare TLR4 antagonists with good blood-brain barrier permeability and showing no stereoselectivity for TLR4. By linking 2 naltrexone units through a rigid pyrrole spacer, the bivalent ligand norbinaltorphimine was formed. Interestingly, (+)-norbinaltorphimine [(+)-1] showed ∼25 times better TLR4 antagonist activity than naltrexone in microglial BV-2 cell line, whereas (-)-norbinaltorphimine [(-)-1] lost TLR4 activity. The enantioselectivity of norbinaltorphimine was further confirmed in primary microglia, astrocytes, and macrophages. The activities of meso isomer of norbinaltorphimine and the molecular dynamic simulation results demonstrate that the stereochemistry of (+)-1 is derived from the (+)-naltrexone pharmacophore. Moreover, (+)-1 significantly increased and prolonged morphine analgesia in vivo. The efficacy of (+)-1 is long lasting. This is the first report showing enantioselective modulation of the innate immune TLR signaling.-Zhang, X., Peng, Y., Grace, P. M., Metcalf, M. D., Kwilasz, A. J., Wang, Y., Zhang, T., Wu, S., Selfridge, B. R., Portoghese, P. S., Rice, K. C., Watkins, L. R., Hutchinson, M. R., Wang, X. Stereochemistry and innate immune recognition: (+)-norbinaltorphimine targets myeloid differentiation protein 2 and inhibits toll-like receptor 4 signaling.

Unveiling the Interplay between the TLR4/MD2 Complex and HSP70 in the Human Cardiovascular System: A Computational Approach

While precise mechanisms underlying cardiovascular diseases (CVDs) are still not fully understood, previous studies suggest that the innate immune system, through Toll-like receptor 4 (TLR4), plays a crucial part in the pathways leading to these diseases, mainly because of its interplay with endogenous molecules. The Heat-shock protein 70 family (HSP70-70kDa) is of particular interest in cardiovascular tissues as it may have dual effects when interacting with TLR4 pathways. Although the hypothesis of the HSP70 family members acting as TLR4 ligands is becoming widely accepted, to date no co-crystal structure of this complex is available and it is still unknown whether this process requires the co-adaptor MD2. In this study, we aimed at investigating the interplay between the TLR4/MD2 complex and HSP70 family members in the human cardiovascular system through transcriptomic data analysis and at proposing a putative interaction model between these proteins. We report compelling evidence of correlated expression levels between TLR4 and MD2 with HSP70 cognate family members, especially in heart tissue. In our molecular docking simulations, we found that HSP70 in the ATP-bound state presents a better docking score towards the TLR4/MD2 complex compared to the ADP-bound state (-22.60 vs. -10.29 kcal/mol, respectively). Additionally, we show via a proximity ligation assay for HSP70 and TLR4, that cells stimulated with ATP have higher formation of fluorescent spots and that MD2 might be required for the complexation of these proteins. The insights provided by our computational approach are potential scaffolds for future in vivo studies investigating the interplay between the TLR4/MD2 complex and HSP70 family members in the cardiovascular system.

An MD2-derived peptide promotes LPS aggregation, facilitates its internalization in THP-1 cells, and inhibits LPS-induced pro-inflammatory responses

MD2, a 160-residue accessory glycoprotein, is responsible for the recognition and binding of Gram-negative bacterial membrane component, lipopolysaccharide (LPS). Internalization of pathogen inside the mononuclear phagocytes has also been attributed to MD2 which leads to the clearance of pathogens from the host. However, not much is known about the segments in MD2 that are responsible for LPS interaction or internalization of pathogen inside the defense cells. A 16-residue stretch (MD54) from MD2 protein has been identified that possesses a short heptad repeat sequence and four cationic residues enabling it to participate in both hydrophobic and electrostatic interactions with LPS. An MD54 analog of the same size was also designed in which a leucine residue at a heptadic position was replaced with an alanine residue. MD54 but not its analog, MMD54 induced aggregation of LPS and aided in its internalization within THP-1 monocytes. Furthermore, MD54 inhibited LPS-induced nuclear translocation of NF-κB in PMA-treated THP-1 and TLR4/MD2/CD14-transfected HEK-293T cells and the production of pro-inflammatory cytokines. In addition, in in vivo experiments, MD54 showed marked protection and survival of mice against LPS-induced inflammation and death. Overall, we have identified a short peptide with heptad repeat sequence from MD2 that can cause aggregation of LPS and abet in its internalization within THP-1 cells, resulting in attenuation of LPS-induced pro-inflammatory responses in vitro and in vivo.

Candida albicans-derived mannoproteins activate NF-κB in reporter cells expressing TLR4, MD2 and CD14

The ability of soluble C. albicans 20A (serotype A) mannoprotein (CMP) to serve as a ligand for toll-like receptor 4 (TLR4) and its co-receptors was examined using commercially available and stably-transfected HEK293 cells that express human TLR4, MD2 and CD14, but not MR. These TLR4 reporter cells also express an NF-κB-dependent, secreted embryonic alkaline phosphatase (SEAP) reporter gene. TLR4-reporter cells exhibited a dose-dependent SEAP response to both LPS and CMP, wherein peak activation was achieved after stimulation with 40–50 μg/mL of CMP. Incubation on polymyxin B resin had no effect on CMP’s ligand activity, but neutralized LPS-spiked controls. HEK293 Null cells lacking TLR4 and possessing the same SEAP reporter failed to respond to LPS or CMP, but produced SEAP when activated with TNFα. Reporter cell NF-κB responses were accompanied by transcription of IL-8, TNFα, and COX-2 genes. Celecoxib inhibited LPS-, CMP-, and TNFα-dependent NF-κB responses; whereas, indomethacin had limited effect on LPS and CMP responses. SEAP production in response to C. albicans A9 mnn4Δ mutant CMP, lacking phosphomannosylations on N-linked glycans, was significantly greater (p ≤ 0.005) than SEAP responses to CMP derived from parental A9 (both serotype B). These data confirm that engineered human cells expressing TLR4, MD2 and CD14 can respond to CMP with NF-κB activation and the response can be influenced by variations in CMP-mannosylation. Future characterizations of CMPs from other sources and their application in this model may provide further insight into variations observed with TLR4 dependent innate immune responses targeting different C. albicans strains.

Palmitic acid is a toll-like receptor 4 ligand that induces human dendritic cell secretion of IL-1β

Palmitic acid (PA) and other saturated fatty acids are known to stimulate pro-inflammatory responses in human immune cells via Toll-like receptor 4 (TLR4). However, the molecular mechanism responsible for fatty acid stimulation of TLR4 remains unknown. Here, we demonstrate that PA functions as a ligand for TLR4 on human monocyte derived dendritic cells (MoDCs). Hydrophobicity protein modeling indicated PA can associate with the hydrophobic binding pocket of TLR4 adaptor protein MD-2. Isothermal titration calorimetry quantified heat absorption that occurred during PA titration into TLR4/MD2, indicating that PA binds to TLR4/MD2. Treatment of human MoDCs with PA resulted in endocytosis of TLR4, further supporting the function of PA as a TLR4 agonist. In addition, PA stimulated DC maturation and activation based on the upregulation of DC costimulatory factors CD86 and CD83. Further experiments showed that PA induced TLR4 dependent secretion of the pro-inflammatory cytokine IL-1β. Lastly, our experimental data show that PA stimulation of NF-κB canonical pathway activation is regulated by TLR4 signaling and that reactive oxygen species may be important in upregulating this pro-inflammatory response. Our experiments demonstrate for the first time that PA activation of TLR4 occurs in response to direct molecular interactions between PA and MD-2. In summary, our findings suggest a likely molecular mechanism for PA induction of pro-inflammatory immune responses in human dendritic cells expressing TLR4.

Modification of sample processing for the Limulus amebocyte lysate assay enhances detection of inflammogenic endotoxin in intact bacteria and organic dust

The pro-inflammatory potency and causal relationship with asthma of inhaled endotoxins have underscored the importance of accurately assessing the endotoxin content of organic dusts. The Limulus amebocyte lysate (LAL) assay has emerged as the preferred assay, but its ability to measure endotoxin in intact bacteria and organic dusts with similar sensitivity as purified endotoxin is unknown. We used metabolically radiolabeled Neisseria meningitidis and both rough and smooth Escherichia coli to compare dose-dependent activation in the LAL with purified endotoxin from these bacteria and shed outer membrane (OM) blebs. Labeled [14C]-3-OH-fatty acids were used to quantify the endotoxin content of the samples. Purified meningococcal and E. coli endotoxins and OM blebs displayed similar specific activity in the LAL assay to the purified LPS standard. In contrast, intact bacteria exhibited fivefold lower specific activity in the LAL assay but showed similar MD-2-dependent potency as purified endotoxin in inducing acute airway inflammation in mice. Pre-treatment of intact bacteria and organic dusts with 0.1 M Tris-HCl/10 mM EDTA increased by fivefold the release of endotoxin. These findings demonstrate that house dust and other organic dusts should be extracted with Tris/EDTA to more accurately assess the endotoxin content and pro-inflammatory potential of these environmental samples.

Soluble CD83 Inhibits T Cell Activation by Binding to the TLR4/MD-2 Complex on CD14+ Monocytes

The transmembrane protein CD83, expressed on APCs, B cells, and T cells, can be expressed as a soluble form generated by alternative splice variants and/or by shedding. Soluble CD83 (sCD83) was shown to be involved in negatively regulating the immune response. sCD83 inhibits T cell proliferation in vitro, supports allograft survival in vivo, prevents corneal transplant rejection, and attenuates the progression and severity of autoimmune diseases and experimental colitis. Although sCD83 binds to human PBMCs, the specific molecules that bind sCD83 have not been identified. In this article, we identify myeloid differentiation factor-2 (MD-2), the coreceptor within the TLR4/MD-2 receptor complex, as the high-affinity sCD83 binding partner. TLR4/MD-2 mediates proinflammatory signal delivery following recognition of bacterial LPSs. However, altering TLR4 signaling can attenuate the proinflammatory cascade, leading to LPS tolerance. Our data show that binding of sCD83 to MD-2 alters this signaling cascade by rapidly degrading IL-1R-associated kinase-1, leading to induction of the anti-inflammatory mediators IDO, IL-10, and PGE2 in a COX-2-dependent manner. sCD83 inhibited T cell proliferation, blocked IL-2 secretion, and rendered T cells unresponsive to further downstream differentiation signals mediated by IL-2. Therefore, we propose the tolerogenic mechanism of action of sCD83 to be dependent on initial interaction with APCs, altering early cytokine signal pathways and leading to T cell unresponsiveness.

A Novel Class of Small Molecule Agonists with Preference for Human over Mouse TLR4 Activation

The best-characterized Toll-like receptor 4 (TLR4) ligands are lipopolysaccharide (LPS) and its chemically modified and detoxified variant, monophosphoryl lipid A (MPL). Although both molecules are active for human TLR4, they demonstrate a potency preference for mouse TLR4 based on data from transfected cell lines and primary cells of both species. After a high throughput screening process of small molecule libraries, we have discovered a new class of TLR4 agonist with a species preference profile differing from MPL. Products of the 4-component Ugi synthesis reaction were demonstrated to potently trigger human TLR4-transfected HEK cells but not mouse TLR4, although inclusion of the human MD2 with mTLR4 was able to partially recover activity. Co-expression of CD14 was not required for optimal activity of Ugi compounds on transfected cells, as it is for LPS. The species preference profile for the panel of Ugi compounds was found to be strongly active for human and cynomolgus monkey primary cells, with reduced but still substantial activity for most Ugi compounds on guinea pig cells. Mouse, rat, rabbit, ferret, and cotton rat cells displayed little or no activity when exposed to Ugi compounds. However, engineering the human versions of TLR4 and MD2 to be expressed in mTLR4/MD2 deficient mice allowed for robust activity by Ugi compounds both in vitro and in vivo. These findings extend the range of compounds available for development as agonists of TLR4 and identify novel molecules which reverse the TLR4 triggering preference of MPL for mouse TLR4 over human TLR4. Such compounds may be amenable to formulation as more potent human-specific TLR4L-based adjuvants than typical MPL-based adjuvants.

Investigation on the agonistic and antagonistic biological activities of synthetic Chlamydia lipid A and its use in in vitro enzymatic assays

The synthetic 1,4′-bisphosphorylated penta-acyl and tetra-acyl lipid A structures representing the major molecular species of natural chlamydial lipid A were tested for their endotoxic activities as measured by interleukin-8 release from human embryonic kidney (HEK) 293 cells expressing Toll-like receptor (TLR) 2 or TLR4. Both compounds were unable to activate HEK293 cells transiently transfected with TLR2. The penta-acyl lipid A was a weak activator of HEK293 cells expressing TLR4/MD-2/CD14 whereas tetra-acyl lipid A was inactive even at high concentrations. The weak activity of the penta-acyl lipid A could be antagonized by the tetra-acyl derivative of Escherichia coli lipid A (compound 406) or the anti-CD14 monoclonal antibody MEM-18. Both, tetra- and pentaacyl lipid A were unable to antagonize the activity of synthetic E. coli-type lipid A (compound 506) or smooth lipopolysaccharide of Salmonella enterica serovar Friedenau. Tetra- and penta-acyl lipid A served as acceptors for Kdo transferases from E. coli, Chlamydia trachomatis and Chlamydophila psittaci as shown by in vitro assays and detection of the products by thin layer chromatography and immune staining with monoclonal antibody.

Computational Approaches to Toll-Like Receptor 4 Modulation

Toll-like receptor 4 (TLR4), along with its accessory protein myeloid differentiation factor 2 (MD-2), builds a heterodimeric complex that specifically recognizes lipopolysaccharides (LPS), which are present on the cell wall of Gram-negative bacteria, activating the innate immune response. Some TLR4 modulators are undergoing preclinical and clinical evaluation for the treatment of sepsis, inflammatory diseases, cancer and rheumatoid arthritis. Since the relatively recent elucidation of the X-ray crystallographic structure of the extracellular domain of TLR4, research around this fascinating receptor has risen to a new level, and thus, new perspectives have been opened. In particular, diverse computational techniques have been applied to decipher some of the basis at the atomic level regarding the mechanism of functioning and the ligand recognition processes involving the TLR4/MD-2 system at the atomic level. This review summarizes the reported molecular modeling and computational studies that have recently provided insights into the mechanism regulating the activation/inactivation of the TLR4/MD-2 system receptor and the key interactions modulating the molecular recognition process by agonist and antagonist ligands. These studies have contributed to the design and the discovery of novel small molecules with promising activity as TLR4 modulators.

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.

Critical residues involved in Toll-like receptor 4 activation by cationic lipid nanocarriers are not located at the lipopolysaccharide-binding interface

DiC14-amidine is a cationic lipid that was originally designed as a lipid nanocarrier for nucleic acid transport, and turned out to be a Toll-like receptor 4 (TLR4) agonist as well. We found that while E. coli lipopolysaccharide (LPS) is a TLR4 agonist in all species, diC14-amidine nanoliposomes are full agonists for human, mouse and cat receptors but weak horse agonists. Taking advantage of this unusual species specificity, we used chimeric constructs based on the human and horse sequences and identified two regions in the human TLR4 that modulate the agonist activity of diC14-amidine. Interestingly, these regions lie outside the known LPS-binding domain. Competition experiments also support our hypothesis that diC14-amidine interacts primarily with TLR4 hydrophobic crevices located at the edges of the TLR4/TLR4* dimerization interface. We have characterized potential binding modes using molecular docking analysis and suggest that diC14-amidine nanoliposomes activate TLR4 by facilitating its dimerization in a process that is myeloid differentiation 2 (MD-2)-dependent and cluster of differentiation 14 (CD14)-independent. Our data suggest that TLR4 may be activated through binding at different anchoring points, expanding the repertoire of TLR4 ligands to non-MD-2-binding lipids.

Myeloid differentiation-2 is a potential biomarker for the amplification process of allergic airway sensitization in mice

BACKGROUND

Allergic sensitization is a key step in the pathogenesis of asthma. However, little is known about the molecules that are critical regulators for establishing allergic sensitization of the airway. Thus, we conducted global gene expression profiling to identify candidate genes and signaling pathways involved in house dust mite (HDM)-induced allergic sensitization in the murine airway.

METHODS

We sensitized and challenged mice with HDM or saline as a control through the airway on days 1 and 8. We evaluated eosinophilia in bronchoalveolar lavage fluid (BALF), airway inflammation, and mucus production on days 7 and 14. We extracted total RNA from lung tissues of HDM- and saline-sensitized mice on days 7 and 14. Microarray analyses were performed to identify up-regulated genes in the lungs of HDM-sensitized mice compared to the control mice. Data analyses were performed using GeneSpring software and gene networks were generated using Ingenuity Pathways Analysis (IPA).

RESULTS

We identified 50 HDM-mediated, stepwise up-regulated genes in response to allergic sensitization and amplification of allergic airway inflammation. The highest expressed gene was myeloid differentiation-2 (MD-2), a lipopolysaccharide (LPS)-binding component of Toll-like receptor (TLR) 4 signaling complex. MD-2 protein was expressed in lung vascular endothelial cells and was increased in the serum of HDM-sensitized mice, but not in the control mice.

CONCLUSIONS

Our data suggest MD-2 is a critical regulator of the establishment of allergic airway sensitization to HDM in mice. Serum MD-2 may represent a potential biomarker for the amplification of allergic sensitization and allergic inflammation.

Celastrol blocks binding of lipopolysaccharides to a Toll-like receptor4/myeloid differentiation factor2 complex in a thiol-dependent manner

ETHNOPHARMACOLOGICAL RELEVANCE

Tripterygium wilfordii (lei gong teng; Thunder of God Vine), which belongs to the Celastraceae family, has long been used in traditional Chinese medicine to treat inflammation and rheumatoid arthritis. Celastrol is a bioactive compound isolated from T. wilfordii.

AIM OF THE STUDY

We investigated whether celastrol suppressed binding of lipopolysaccharides (LPS) to myeloid differentiation factor 2 (MD2), thereby downregulating Toll-like receptor4 (TLR4) activation in mouse primary macrophages.

MATERIALS AND METHODS

Cytokine expression was determined by polymerase chain reaction analysis and enzyme-linked immunosorbent assay in bone marrow-derived primary macrophages (BMDMs). The kinase activity of tank-binding kinase 1 (TBK1) was examined by a luciferase reporter assay and an in vitro kinase assay. LPS binding to MD2 was examined by an in vitro binding assay and confocal microscopy analysis.

RESULTS

Celastrol reduced LPS-induced expression of inflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-12, and IL-1β, at both the mRNA and protein levels in BMDMs. Celastrol suppressed LPS binding to MD2, as shown by the in vitro binding assay, whereas it did not inhibit TBK1. In addition, co-localization of LPS with MD2 in BMDMs was blocked by celastrol. The inhibitory effects of celastrol on LPS binding to MD2 were reversed by thiol donors (N-acetyl-L-cysteine and dithiothreitol), suggesting that the thiol reactivity of celastrol contributes to its inhibitory effects on TLR4 activation in macrophages.

CONCLUSION

Our results demonstrate that celastrol suppresses TLR4 activation through the inhibition of LPS binding to the TLR4/MD2 complex. These results provide a novel mechanism of action by which celastrol contributes to the anti-inflammatory activity of T. wilfordii.

HIV-1 Tat Protein Induces Production of Proinflammatory Cytokines by Human Dendritic Cells and Monocytes/Macrophages through Engagement of TLR4-MD2-CD14 Complex and Activation of NF-κB Pathway

We recently reported that the human immunodeficiency virus type-1 (HIV-1) Tat protein induced the expression of programmed death ligand-1 (PD-L1) on dendritic cells (DCs) through a TLR4 pathway. However, the underlying mechanisms by which HIV-1 Tat protein induces the abnormal hyper-activation of the immune system seen in HIV-1 infected patients remain to be fully elucidated. In the present study, we report that HIV-1 Tat protein induced the production of significant amounts of the pro-inflammatory IL-6 and IL-8 cytokines by DCs and monocytes from both healthy and HIV-1 infected patients. Such production was abrogated in the presence of anti-TLR4 blocking antibodies or soluble recombinant TLR4-MD2 as a decoy receptor, suggesting TLR4 was recruited by Tat protein. Tat-induced murine IL-6 and CXCL1/KC a functional homologue of human IL-8 was abolished in peritoneal macrophages derived from TLR4 KO but not from Wt mice, confirming the involvement of the TLR4 pathway. Furthermore, the recruitment of TLR4-MD2-CD14 complex by Tat protein was demonstrated by the activation of TLR4 downstream pathways including NF-κB and SOCS-1 and by down-modulation of cell surface TLR4 by endocytosis in dynamin and lipid-raft-dependent manners. Collectively, these findings demonstrate, for the first time, that HIV-1 Tat interacts with TLR4-MD2-CD14 complex and activates the NF-κB pathway, leading to overproduction of IL-6 and IL-8 pro-inflammatory cytokines by myeloid cells from both healthy and HIV-1 infected patients. This study reveals a novel mechanism by which HIV-1, via its early expressed Tat protein, hijacks the TLR4 pathway, hence establishing abnormal hyper-activation of the immune system.

Insights into the species-specific TLR4 signaling mechanism in response to Rhodobacter sphaeroides lipid A detection

TLR4 in complex with MD2 senses the presence of lipid A (LA) and initiates a signaling cascade that curb the infection. This complex is evolutionarily conserved and can initiate the immune system in response to a variety of LAs. In this study, molecular dynamics simulation (25 ns) was performed to elucidate the differential behavior of TLR4/MD2 complex in response to Rhodobacter sphaeroides lipid A (RsLA). Penta-acyl chain-containing RsLA is at the verge of agonist (6 acyl-chains) and antagonist (4 acyl-chains) structure, and activates the TLR4 pathway in horses and hamsters, while inhibiting in humans and murine. In the time-evolved coordinates, the promising factors that dictated the differential response included the local and global mobility pattern of complexes, solvent-accessible surface area of ligand, and surface charge distributions of TLR4 and MD2. We showed that the GlcN1-GlcN2 backbone acquires agonist (3FXI)-like configurations in horses and hamsters, while acquiring antagonist (2E59)-like configurations in humans and murine systems. Moreover, analysis of F126 behavior in the MD2 F126 loop (amino acids 123-129) and loop EF (81-89) suggested that certain sequence variations also contribute to species-specific response. This study underlines the TLR4 signaling mechanism and provides new therapeutic opportunities.

Emodin ameliorated lipopolysaccharide-induced fulminant hepatic failure by blockade of TLR4/MD2 complex expression in D-galactosamine-sensitized mice

Emodin has been reported to possess anti-inflammatory and anti-oxidant activities. The aim of this study was to explore the effect and mechanism of emodin on lipopolysaccharide (LPS)-induced fulminant hepatic failure (FHF) in D-galactosamine (D-GalN)-sensitized mice. Our results showed that pretreatment with emodin inhibited the elevation of plasma aminotransferases, alleviated the hepatic histopathological abnormalities and improved the survival rate of LPS/D-GalN-primed mice. Moreover, emodin markedly attenuated the increased serum and hepatic tumor necrosis factor-α (TNF-α) production, and activated hepatic p38 mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-κB) signal pathways in LPS/D-GalN-challenged mice. Furthermore, using an in vitro experiment, we found that emodin dose-dependently suppressed TNF-α production, dampened AP-1 and NF-κB activation, and blocked toll-like receptor (TLR) 4/myeloid differentiation factor (MD) 2 complex expression in LPS-elicited RAW264.7 mouse macrophage cells. Taken together, these data suggested that emodin could effectively prevent LPS-induced FHF, which might be mediated by inhibition of TNF-α production, deactivation of MAPKs and NF-κB, and blockade of TLR4/MD2 complex expression.

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.

Low-intensity pulsed ultrasound (LIPUS) inhibits LPS-induced inflammatory responses of osteoblasts through TLR4-MyD88 dissociation

Previous reports have shown that osteoblasts are mechano-sensitive. Low-intensity pulsed ultrasound (LIPUS) induces osteoblast differentiation and is an established therapy for bone fracture. Here we have examined how LIPUS affects inflammatory responses of osteoblasts to LPS. LPS rapidly induced mRNA expression of several chemokines including CCL2, CXCL1, and CXCL10 in both mouse osteoblast cell line and calvaria-derived osteoblasts. Simultaneous treatment by LIPUS significantly inhibited mRNA induction of CXCL1 and CXCL10 by LPS. LPS-induced phosphorylation of ERKs, p38 kinases, MEK1/2, MKK3/6, IKKs, TBK1, and Akt was decreased in LIPUS-treated osteoblasts. Furthermore, LIPUS inhibited the transcriptional activation of NF-κB responsive element and Interferon-sensitive response element (ISRE) by LPS. In a transient transfection experiment, LIPUS significantly inhibited TLR4-MyD88 complex formation. Thus LIPUS exerts anti-inflammatory effects on LPS-stimulated osteoblasts by inhibiting TLR4 signal transduction.

The attenuated inflammation of MPL is due to the lack of CD14-dependent tight dimerization of the TLR4/MD2 complex at the plasma membrane

TLR4/MD-2 senses lipid A, activating the MyD88-signaling pathway on the plasma membrane and the TRIF-signaling pathway after CD14-mediated TLR4/MD-2 internalization into endosomes. Monophosphoryl lipid A (MPL), a detoxified derivative of lipid A, is weaker than lipid A in activating the MyD88-dependent pathway. Little is known, however, about mechanisms underlying the attenuated activation of MyD88-dependent pathways. We here show that MPL was impaired in induction of CD14-dependent TLR4/MD-2 dimerization compared with lipid A. Impaired TLR4/MD-2 dimerization decreased CD14-mediated TNFα production. In contrast, MPL was comparable to lipid A in CD14-independent MyD88-dependent TNFα production and TRIF-dependent responses including cell surface CD86 up-regulation and IFNβ induction. Although CD86 up-regulation is dependent on TRIF signaling, it was induced by TLR4/MD-2 at the plasma membrane. These results revealed that the attenuated MPL responses were due to CD14-initiated responses at the plasma membrane, but not just to responses initiated by MyD88, that is, MPL was specifically unable to induce CD14-dependent TLR4/MD-2 dimerization that selectively enhances MyD88-mediated responses at the plasma membrane.

Polyoxygenated cholesterol ester hydroperoxide activates TLR4 and SYK dependent signaling in macrophages

Oxidation of low-density lipoprotein (LDL) is one of the major causative mechanisms in the development of atherosclerosis. In previous studies, we showed that minimally oxidized LDL (mmLDL) induced inflammatory responses in macrophages, macropinocytosis and intracellular lipid accumulation and that oxidized cholesterol esters (OxCEs) were biologically active components of mmLDL. Here we identified a specific OxCE molecule responsible for the biological activity of mmLDL and characterized signaling pathways in macrophages in response to this OxCE. Using liquid chromatography – tandem mass spectrometry and biological assays, we identified an oxidized cholesteryl arachidonate with bicyclic endoperoxide and hydroperoxide groups (BEP-CE) as a specific OxCE that activates macrophages in a TLR4/MD-2-dependent manner. BEP-CE induced TLR4/MD-2 binding and TLR4 dimerization, phosphorylation of SYK, ERK1/2, JNK and c-Jun, cell spreading and uptake of dextran and native LDL by macrophages. The enhanced macropinocytosis resulted in intracellular lipid accumulation and macrophage foam cell formation. Bone marrow-derived macrophages isolated from TLR4 and SYK knockout mice did not respond to BEP-CE. The presence of BEP-CE was demonstrated in human plasma and in the human plaque material captured in distal protection devices during percutaneous intervention. Our results suggest that BEP-CE is an endogenous ligand that activates the TLR4/SYK signaling pathway. Because BEP-CE is present in human plasma and human atherosclerotic lesions, BEP-CE-induced and TLR4/SYK-mediated macrophage responses may contribute to chronic inflammation in human atherosclerosis.

Carbon monoxide-based therapy ameliorates acute pancreatitis via TLR4 inhibition

The protective role of hemeoxygenase-1 (HO-1) in various inflammatory conditions is mediated in part by its products, carbon monoxide (CO) and biliverdin. Here we investigated a therapeutic role for CO and CO-primed cells in acute pancreatitis (AP). In a mouse model of AP, treatment with CO-releasing molecule-2 (CORM-2) decreased mortality, pancreatic damage, and lung injury. CORM-2 decreased systemic inflammatory cytokines, suppressed systemic and pancreatic macrophage TNF-α secretion, and inhibited macrophage TLR4 receptor complex expression. In both human and mouse cells, CORM-2 inhibited endogenous and exogenous ligand-dependent TLR4 activation, which indicates that CORM-2 could be therapeutic for both early and late stages of AP, which involve sterile- and endotoxin-mediated inflammation, respectively. Mice engrafted with TLR4-deficient hematopoietic cells were protected against caerulein-induced AP. In the absence of leukocyte TLR4 expression, CORM-2 did not confer additional protection, which indicates that CORM-2-dependent effects are mediated via suppression of macrophage TLR4 activation. We determined that CO was directly responsible for the protective effects of CORM-2 in AP, as inactive forms of CORM-2 were ineffective. Importantly, adoptive transfer of CORM-2-primed cells reduced AP. Such a therapeutic approach would translate the beneficial effects of CO-based therapies, avoiding CO- or CO-RM-mediated toxicities in AP and a wide range of diseases.

Eritoran attenuates tissue damage and inflammation in hemorrhagic shock/trauma

BACKGROUND

Severe injury and associated hemorrhagic shock lead to an inflammatory response and subsequent increased tissue damage. Numerous reports have shown that injury-induced inflammation and the associated end-organ damage is driven by Toll-like receptor 4 (TLR4) activation via damage-associated molecular patterns. We examined the effectiveness of Eritoran tetrasodium (E5564), an inhibitor of TLR4 function, in reducing inflammation induced during hemorrhagic shock with resuscitation (HS/R) or after peripheral tissue injury (bilateral femur fracture, BFF).

MATERIAL AND METHODS

Mice underwent HS/R or BFF with or without injection of Eritoran (5 mg/kg body weight) or vehicle control given before, both before and after, or only after HS/R or BFF. Mice were sacrificed after 6 h and plasma and tissue cytokines, liver damage (histology; aspartate aminotransferase/alanine aminotransferase), and inflammation (NF-κB) and gut permeability were assessed.

RESULTS

In HS/R Eritoran significantly reduced liver damage (values ± SEM: alanine aminotransferase 9910 ± 3680 U/L versus 1239 ± 327 U/L and aspartate aminotransferase 5863 ± 2000 U/L versus 1246 ± 243 U/L, P < 0.01) at 6 h compared with control when given just before HS and again just prior to resuscitation. Eritoran administration also led to lower IL-6 levels in plasma and liver and less NF-κB activation in liver. Increases in gut barrier permeability induced by HS/R were also prevented with Eritoran. Eritoran similarly diminished BFF-mediated systemic inflammatory responses.

CONCLUSION

These data suggest Eritoran can inhibit tissue damage and inflammation induced via TLR4/myeloid differentiation factor 2 signaling from damage-associated molecular patterns released during HS/R or BFF. Eritoran may represent a promising therapeutic for trauma patients to prevent multiple organ failure.

MD-2-dependent human Toll-like receptor 4 monoclonal antibodies detect extracellular association of Toll-like receptor 4 with extrinsic soluble MD-2 on the cell surface

MD-2 is essential for lipopolysaccharide (LPS) recognition of Toll-like receptor 4 (TLR4) but not for cell surface expression. The TLR4/MD-2 complex is formed intracellularly through co-expression. Extracellular complex formation remains a matter for debate because of the aggregative nature of secreted MD-2 in the absence of TLR4 co-expression. We demonstrated extracellular complex formation using three independent monoclonal antibodies (mAbs), all of which are specific for complexed TLR4 but unreactive with free TLR4 and MD-2. These mAbs bound to TLR4-expressing Ba/F3 cells only when co-cultured with MD-2-secreting Chinese hamster ovary cells or incubated with conditioned medium from these cells. All three mAbs bound the extracellularly formed complex indistinguishably from the intracellularly formed complex in titration studies. In addition, we demonstrated that two mAbs lost their affinity for TLR4/MD-2 on LPS stimulation, suggesting that these mAbs bound to conformation-sensitive epitopes. This was also found when the extracellularly formed complex was stimulated with LPS. Additionally, we showed that cell surface TLR4 and extrinsically secreted MD-2 are capable of forming the functional complex extracellularly, indicating an additional or alternative pathway for the complex formation.

Amelioration in wound healing in diabetic toll-like receptor-4 knockout mice

Toll-like receptor-4 (TLR4) is a sentinel pathogen recognition receptor with a pivotal role in inflammation, tissue injury, diabetes and its complications. The aim of the study was to examine the contribution of TLR4 expression and activation to the prolonged inflammation observed in diabetic wounds. Diabetes was induced in male C57BL/6J and TLR4 knockout (KO) mice using streptozotocin (STZ) with matching non-diabetic mice as control. After 2weeks of persistent hyperglycemia in the mice, full-thickness excision wounds were made on the backs aseptically. Total RNA and protein were subjected to real-time PCR and western blot analyses. Wound sizes were measured using digital planimetry. TLR4 mRNA and protein expression increased significantly in wounds of diabetic mice compared with non-diabetic mice (P<0.05). IL-6, TNF-α concentration and nuclear factor-κB (NF-κB) activation were increased in diabetic wounds compared to non-diabetic wounds and knockout of TLR4 alleviates wound healing and decreases inflammation in diabetic TLR4 KO mice. Collectively, our findings show that increased TLR4 mRNA and protein expression and activation contribute to the prolonged inflammation in the diabetic wounds and that absence of TLR4 may result in decreased inflammation and improved wound healing.

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.

The effects of oral and enteric Campylobacter concisus strains on expression of TLR4, MD-2, TLR2, TLR5 and COX-2 in HT-29 cells

Campylobacter concisus, a Gram-negative bacterium that colonizes the human oral cavity, has been shown to be associated with inflammatory bowel diseases (IBD). The effects of different C. concisus strains on intestinal epithelial expression of Toll like receptors (TLR) have not been investigated. This study examined the effects of C. concisus strains isolated from patients with IBD and controls on expression of TLR4, its co-receptor myeloid differentiation factor (MD)-2; TLR2, TLR5, cyclooxygenase-2 (COX-2) and interleukin (IL)-8 in HT-29 cells.Fourteen oral and enteric C. concisus strains isolated from patients with IBD and healthy controls were co-incubated with HT-29 cells. Expression of TLR4, MD-2, TLR2, TLR5 and COX-2 in HT-29 cells in response to C. concisus infection was examined by Western blot, flow cytometry analysis and immunofluorescent staining visualized by confocal microscope. Production of IL-8 was evaluated by enzyme-linked immunosorbent assay.Both oral and enteric C. concisus strains upregulated expression of TLR4 in HT-29 cells. The levels of glycosylated TLR4 (Gly-TLR4) and surface TLR4 induced by C. concisus strains isolated from patients with IBD were significantly higher than those induced by C. concisus strains isolated from the healthy controls. Four C. concisus strains isolated from patients with IBD induced more than two-fold increase of surface expression of MD-2. C. concisus did not affect expression of TLR2 and TLR5. All C. concisus strains induced production of IL-8 and COX-2 in HT-29 cells.This study shows that some C. concisus strains, most from patients with IBD, upregulate surface expression of TLR4 and MD-2 in HT-29 cells. These data suggest that a potential role of specific C. concisus strains in modulating the intestinal epithelial responses to bacterial LPS needs to be investigated.

Toll-like receptor 4 activation reduces adrenal chromaffin cell excitability through a nuclear factor-κB-dependent pathway

The adrenal medulla contains fenestrated capillaries that allow catecholamines and neuropeptides secreted by adrenal chromaffin cells (ACCs) to readily access the circulation. These capillaries may also allow bacterial products to enter the adrenal medulla and interact with ACCs during infection. One potential mediator of this interaction is toll-like receptor 4 (TLR-4), a pattern-recognition receptor that detects lipopolysaccharide (LPS) from Gram-negative bacteria. Evidence suggests that excitable cells can express TLR-4 and that LPS can modulate important neuronal and endocrine functions. The present study was therefore performed to test the hypothesis that TLR-4 activation by LPS affects ACC excitability and secretory output. RT-PCR revealed that TLR-4, cluster of differentiation 14, myeloid differentiation protein-2, and myeloid-derived factor 88 are expressed within mouse adrenal medullae. TLR-4 immunoreactivity was observed within all tyrosine hydroxylase immunoreactive ACCs. Incubation of isolated ACCs in LPS dose dependently hyperpolarized the resting membrane potential and enhanced large conductance (BK) Ca(2+)-activated K(+) currents. LPS (10 μg/ml) also increased rheobase, decreased the number of action potentials fired at rheobase, and reduced the percentage of ACCs exhibiting spontaneous and anodal break action potentials. Although catecholamine release was unaltered, LPS significantly reduced high-K(+)-stimulated neuropeptide Y release from isolated ACCs. LPS did not alter the excitability of ACCs from TLR-4(-/-) mice. Inhibition of nuclear factor-κB signaling with SC-514 (20 μm) abolished the effects of LPS on ACC excitability. Our findings suggest that LPS acts at TLR-4 to reduce ACC excitability and neuropeptide Y release through an nuclear factor-κB-dependent pathway.