N-linked glycosylations at Asn(26) and Asn(114) of human MD-2 are required for toll-like receptor 4-mediated activation of NF-kappaB by lipopolysaccharide

In silico Designing of an Epitope-Based Vaccine Against Common E. coli Pathotypes

Escherichia coli (E. coli) is a Gram-negative bacterium that belongs to the family Enterobacteriaceae. While E. coli can stay as an innocuous resident in the digestive tract, it can cause a group of symptoms ranging from diarrhea to live threatening complications. Due to the increased rate of antibiotic resistance worldwide, the development of an effective vaccine against E. coli pathotypes is a major health priority. In this study, a reverse vaccinology approach along with immunoinformatics has been applied for the detection of potential antigens to develop an effective vaccine. Based on our screening of 5,155 proteins, we identified lipopolysaccharide assembly protein (LptD) and outer membrane protein assembly factor (BamA) as vaccine candidates for the current study. The conservancy of these proteins in the main E. coli pathotypes was assessed through BLASTp to make sure that the designed vaccine will be protective against major E. coli pathotypes. The multitope vaccine was constructed using cytotoxic T lymphocyte (CTL), helper T lymphocyte (HTL), and B cell lymphocyte (BCL) epitopes with suitable linkers and adjuvant. Following that, it was analyzed computationally where it was found to be antigenic, soluble, stable, and non-allergen. Additionally, the adopted docking study, as well as all-atom molecular dynamics simulation, illustrated the promising predicted affinity and free binding energy of this constructed vaccine against the human Toll-like receptor-4 (hTLR-4) dimeric state. In this regard, wet lab studies are required to prove the efficacy of the potential vaccine construct that demonstrated promising results through computational validation.

The role of neuraminidase in TLR4-MAPK signalling and the release of cytokines by lupus serum-stimulated mesangial cells

Previously, we demonstrated neuraminidase (NEU) activity or NEU1 expression, specifically, is increased in the kidneys of lupus mice and urine of human patients with nephritis. Additionally, NEU activity mediates IL-6 secretion from lupus-prone MRL/lpr primary mouse mesangial cells (MCs) in response to an IgG mimic. IL-6 mediates glomerular inflammation and promotes tissue damage in patients and mouse strains with lupus nephritis. This study further elucidates the mechanisms by which NEU activity and NEU1 specifically mediates the release of IL-6 and other cytokines from lupus-prone MCs. We demonstrate significantly increased release of multiple cytokines and NEU activity in MRL/lpr MCs in response to serum from MRL/lpr mice (lupus serum). Inhibiting NEU activity significantly reduced secretion of three of those cytokines: IL-6, GM-CSF and MIP1α. Message levels of Il-6 and Gm-csf were also increased in response to lupus serum and reduced when NEU activity was inhibited. Neutralizing antibodies to cell-surface receptors and MAPK inhibitors in lupus serum- or LPS-stimulated MCs indicate TLR4 and p38 or ERK MAP kinase signalling play key roles in the NEU-mediated secretion of IL-6. Significantly reduced IL-6 release was observed in C57BL/6 (B6) Neu1+/+ primary MCs compared with wild-type (Neu1+/+) B6 MCs in response to lupus serum. Additional results show inhibiting NEU activity significantly increases sialic acid-containing N-glycan levels. Together, our novel observations support a role for NEU activity, and specifically NEU1, in mediating release of IL-6 from lupus-prone MCs in response to lupus serum through a TLR4-p38/ERK MAPK signalling pathway that likely includes desialylation of glycoproteins.

Identification of a Heme Activation Site on the MD-2/TLR4 Complex

Myeloid differentiation factor-2 (MD-2) binds lipopolysaccharide (LPS) and initiates toll-like receptor-4 (TLR4) pro-inflammatory signaling. Heme also activates TLR4 signaling, but it is unknown if heme interacts with MD-2. Therefore, we examined MD-2 for a potential heme activation site. Heme-agarose and biotin-heme/streptavidin-agarose pulled down recombinant MD-2, which was inhibited by excess free heme. UV/visible spectroscopy confirmed MD-2-heme binding. To determine whether MD-2 was required for heme-mediated TLR4 signaling, HEK293 cells were transfected with MD-2, TLR4, CD14, and an NF-κB luciferase reporter, and then stimulated with heme or LPS. Heme or LPS treatment elicited robust reporter activity. Absence of MD-2, TLR4 or CD14 plasmid abolished NF-κB reporter responses to heme or LPS. In silico analysis identified two potential heme docking sites on MD-2 near conserved amino acids W23/S33/Y34 and Y36/C37/I44. Heme-induced NF-κB activity was reduced by 39 and 78% in HEK293 cells transfected with MD-2 mutants W23A and Y34A, respectively, compared to WT-MD-2. NF-κB activation by LPS was not affected by the same mutants. Biotinyl-heme/streptavidin-agarose pulled down 68% less W23A and 80% less W23A/S33A/Y34A mutant MD-2 than WT-MD-2. In contrast, at the Y36/C37/I44 MD-2 site, heme-induced NF-κB activity was significantly increased by mutants Y36A (191% of WT-MD-2) and unchanged by mutants C37A and I44A (95 and 92%, respectively, of WT-MD-2). In conclusion, these data suggest that heme binds and activates TLR4 signaling at amino acids W23 and Y34 on MD-2.

Ganglioside Synthesis by Plasma Membrane-Associated Sialyltransferase in Macrophages

Gangliosides are constituents of the mammalian cell membranes and participate in the inflammatory response. However, little is known about the presence and enzymatic activity of ganglioside sialyltransferases at the cell surface of macrophages, one of the most important immune cells involved in the innate inflammatory process. In the present study, using biochemical and fluorescent microscopy approaches, we found that endogenous ST8Sia-I is present at the plasma membrane (ecto-ST8Sia-I) of murine macrophage RAW264.7 cells. Moreover, ecto-ST8Sia-I can synthetize GD3 ganglioside at the cell surface in lipopolysaccharide (LPS)-stimulated macrophages even when LPS-stimulated macrophages reduced the total ST8Sia-I expression levels. Besides, cotreatment of LPS with an inhibitor of nitric oxide (NO) synthase recovered the ecto-ST8Sia-I expression, suggesting that NO production is involved in the reduction of ST8Sia-I expression. The diminution of ST8Sia-I expression in LPS-stimulated macrophages correlated with a reduction of GD3 and GM1 gangliosides and with an increment of GD1a. Taken together, the data supports the presence and activity of sialyltransferases at the plasma membrane of RAW264.7 cells. The variations of ecto-ST8Sia-I and ganglioside levels in stimulated macrophages constitutes a promissory pathway to further explore the physiological role of this and others ganglioside metabolism-related enzymes at the cell surface during the immune response.

Distinct evolution of toll-like receptor signaling pathway genes in cetaceans

BACKGROUND

The relatively rapid spread and diversity of marine pathogens posed an initial and ongoing challenge for cetaceans (whales, dolphins, and porpoises), descendants of terrestrial mammals that transitioned from land to sea approximately 56 million years ago. Toll-like receptors (TLRs) play important roles in regulating immunity against pathogen infections by detecting specific molecular patterns and activating a wide range of downstream signaling pathways. The ever-increasing catalogue of mammalian genomes offers unprecedented opportunities to reveal genetic changes associated with evolutionary and ecological processes.

OBJECTIVE

This study aimed to explore the molecular evolution of TLR signaling pathway genes in cetaceans.

METHODS

Genes involved in the TLR signaling pathway were retrieved by BLAST searches using human coding sequences as queries. We tested each gene for positive selection along the cetacean branches using PAML and Hyphy. Physicochemical property changes of amino acids at all positively selected residues were assessed by TreeSAAP and visualized with WebLogo. Bovine and dolphin TLR4 was assessed using human embryonic kidney cell line HEK293, which lacks TLR4 and its co-receptor MD-2.

RESULTS

We demonstrate that eight TLR signaling pathway genes are under positive selection in cetaceans. These include key genes in the response to Gram-negative bacteria: TLR4, CD14, and LY96 (MD-2). Moreover, 41 out of 65 positively selected sites were inferred to harbor substitution that dramatically changes the physicochemical properties of amino acids, with most of them situated in or adjacent to functional regions. We also found strong evidence that positive selection occurred in the lineage of the Yangtze finless porpoise, likely reflecting relatively recent adaptions to a freshwater milieu. Species-specific differences in TLR4 response were observed between cetacean and terrestrial species. Cetacean TLR4 was significantly less responsive to lipopolysaccharides from a terrestrial E. coli strain, possibly a reflection of the arms race of host-pathogen co-evolution faced by cetaceans in an aquatic environment.

CONCLUSION

This study provides further impetus for studies on the evolution and function of the cetacean immune system.

Mass Spectrometry-based Structural Analysis and Systems Immunoproteomics Strategies for Deciphering the Host Response to Endotoxin

One cause of sepsis is systemic maladaptive immune response of the host to bacteria and specifically, to Gram-negative bacterial outer-membrane glycolipid lipopolysaccharide (LPS). On the host myeloid cell surface, proinflammatory LPS activates the innate immune system via Toll-like receptor-4/myeloid differentiation factor-2 complex. Intracellularly, LPS is also sensed by the noncanonical inflammasome through caspase-11 in mice and 4/5 in humans. The minimal functional determinant for innate immune activation is the membrane anchor of LPS called lipid A. Even subtle modifications to the lipid A scaffold can enable, diminish, or abolish immune activation. Bacteria are known to modify their LPS structure during environmental stress and infection of hosts to alter cellular immune phenotypes. In this review, we describe how mass spectrometry-based structural analysis of endotoxin helped uncover major determinations of molecular pathogenesis. Through characterization of LPS modifications, we now better understand resistance to antibiotics and cationic antimicrobial peptides, as well as how the environment impacts overall endotoxin structure. In addition, mass spectrometry-based systems immunoproteomics approaches can assist in elucidating the immune response against LPS. Many regulatory proteins have been characterized through proteomics and global/targeted analysis of protein modifications, enabling the discovery and characterization of novel endotoxin-mediated protein translational modifications.

Structure-Activity Relationship in Monosaccharide-Based Toll-Like Receptor 4 (TLR4) Antagonists

The structure-activity relationship was investigated in a series of synthetic TLR4 antagonists formed by a glucosamine core linked to two phosphate esters and two linear carbon chains. Molecular modeling showed that the compounds with 10, 12, and 14 carbons chains are associated with higher stabilization of the MD-2/TLR4 antagonist conformation than in the case of the C16 variant. Binding experiments with human MD-2 showed that the C12 and C14 variants have higher affinity than C10, while the C16 variant did not interact with the protein. The molecules, with the exception of the C16 variant, inhibited the LPS-stimulated TLR4 signal in human and murine cells, and the antagonist potency mirrored the MD-2 affinity calculated from in vitro binding experiments. Fourier-transform infrared, nuclear magnetic resonance, and small angle X-ray scattering measurements suggested that the aggregation state in aqueous solution depends on fatty acid chain lengths and that this property can influence TLR4 activity in this series of compounds.

Essential role of MD-2 in B-cell responses to lipopolysaccharide and Toll-like receptor 4 distribution

Toll-like receptor 4 (TLR4) mediates lipopolysaccharide (LPS) signaling in a variety of cell types. MD-2 is associated with the extracellular domain of TLR4 and augments TLR4-dependent LPS responses in vitro. Moreover, mice lacking MD-2 (MD-2— /—) do not respond to LPS, survive endotoxin shock, and are susceptible to Salmonella typhimurium infection. Here, we further show that B cells lacking MD-2 do not up-regulate CD23 in response to LPS. TLR4 predominantly resides in the Golgi apparatus without MD-2. MD-2 is essential for LPS responses in vivo.

Molecular mechanisms of regulation of Toll-like receptor signaling

TLRs play a critical role in the detection of microbes and endogenous "alarmins" to initiate host defense, yet they can also contribute to the development and progression of inflammatory and autoimmune diseases. To avoid pathogenic inflammation, TLR signaling is subject to multilayer regulatory control mechanisms, including cooperation with coreceptors, post-translational modifications, cleavage, cellular trafficking, and interactions with negative regulators. Nucleic acid-sensing TLRs are particularly interesting in this regard, as they can both recognize host-derived structures and require internalization of their ligand as a result of intracellular sequestration of the nucleic acid-sensing TLRs. This review summarizes the regulatory mechanisms of TLRs, including regulation of their access to ligands, receptor folding, intracellular trafficking, and post-translational modifications, as well as how altered control mechanism could contribute to inflammatory and autoimmune disorders.

Modulating LPS signal transduction at the LPS receptor complex with synthetic Lipid A analogues

Sepsis, defined as a clinical syndrome brought about by an amplified and dysregulated inflammatory response to infections, is one of the leading causes of death worldwide. Despite persistent attempts to develop treatment strategies to manage sepsis in the clinical setting, the basic elements of treatment have not changed since the 1960s. As such, the development of effective therapies for reducing inflammatory reactions and end-organ dysfunction in critically ill patients with sepsis remains a global priority. Advances in understanding of the immune response to sepsis provide the opportunity to develop more effective pharmaceuticals. This article details current information on the modulation of the lipopolysaccharide (LPS) receptor complex with synthetic Lipid A mimetics. As the initial and most critical event in sepsis pathophysiology, the LPS receptor provides an attractive target for antisepsis agents. One of the well-studied approaches to sepsis therapy involves the use of derivatives of Lipid A, the membrane-anchor portion of an LPS, which is largely responsible for its endotoxic activity. This article describes the structural and conformational requirements influencing the ability of Lipid A analogues to compete with LPS for binding to the LPS receptor complex and to inhibit the induction of the signal transduction pathway by impairing LPS-initiated receptor dimerization.

MD2 expression is reduced in large airways of smokers and COPD smokers

Toll-like receptor 4 (TLR4) signaling requires a number of accessory proteins to initiate a signal. MD-2 is one of the accessory proteins with a relevant role in lipopolysaccharide responses. Although cigarette smoke increases TLR4 expression, TLR4 signaling is altered in smokers and in smokers COPD patients. The main aims of this study were to explore whether MD2 is altered in large and small airways of COPD and of smokers without COPD. The expression of MD2 ex vivo was assessed by immunohistochemistry in surgical specimens from current smokers COPD (s-COPD; n = 14), smokers without COPD (S; n = 7), and from non-smoker non-COPD subjects (C; n = 11. The in vitro effects of cigarette smoke extracts on the MD2 expression in a human bronchial epithelial cell line (16-HBE) were also assessed by flow cytometry. MD2 is reduced in the epithelium and in the submucosa in large airways but not in the epithelium and in the submucosa in small airways of smokers and of s-COPD. The expression of MD2 in the submucosa of the large airways is significantly higher in comparison to the submucosa of the small airways in all the studied groups. In vitro, cigarette smoke is able to increase TLR4 but it reduces MD2 in a dose-dependent manner in bronchial epithelial cells. Cigarette smoke may alter innate immune responses reducing the expression of the MD2, a molecule with an important role in TLR4 signaling.

Tunicamycin inhibits Toll-like receptor-activated inflammation in RAW264.7 cells by suppression of NF-κB and c-Jun activity via a mechanism that is independent of ER-stress and N-glycosylation

In this study, we investigated the effect of tunicamycin on the production of pro-inflammatory molecules in RAW264.7 macrophage cells in response to lipopolysaccharide (LPS) and Toll-like receptor (TLR) agonists. Tunicamycin caused a reduction in LPS-induced nitric oxide (NO) production and expression of inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2), interleukin-1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α). In contrast, other ER stress-inducing chemicals, such as A23187 and thapsigargin (TG), increased LPS-induced COX-2 expression and had no effect on LPS-induced iNOS, TNF-α or IL-1β expression. Furthermore, the inhibitory effect of tunicamycin on LPS-induced inflammation was not influenced by salubrinal, an ER stress inhibitor, suggesting that the anti-inflammatory effect of tunicamycin is independent of ER stress. Tunicamycin also inhibited the expression of inflammatory molecule mRNAs induced by stimulation of TLR2 (with lipoteichoic acid) or TLR3 (with polyinosinic:polycytidylic acid), which do not require myeloid differentiation protein-2 (MD2) for their activation. Moreover, inhibition of LPS-induced iNOS expression was not inhibited by castanospermine, another N-glycosylation inhibitor, suggesting that the inhibitory effect of tunicamycin on LPS-induced iNOS induction is likely independent of MD2 N-glycosylation. Tunicamycin inhibited nuclear factor-kappaB (NF-κB) activity by suppressing LPS-induced nuclear translocation of p50 and subsequent DNA binding of p50 and p65 to the NF-κB site of the iNOS promoter. Tunicamycin also inhibited the transcriptional activity of a cAMP-response element (CRE) reporter, possibly by inhibiting c-Jun activation. Therefore, we conclude that tunicamycin represses TLR-induced inflammation through suppression of NF-κB and CRE activity via a mechanism that is independent of ER-stress and N-glycosylation.

The immunomodulatory effect of plant lectins: a review with emphasis on ArtinM properties

Advances in the glycobiology and immunology fields have provided many insights into the role of carbohydrate-protein interactions in the immune system. We aim to present a comprehensive review of the effects that some plant lectins exert as immunomodulatory agents, showing that they are able to positively modify the immune response to certain pathological conditions, such as cancer and infections. The present review comprises four main themes: (1) an overview of plant lectins that exert immunomodulatory effects and the mechanisms accounting for these activities; (2) general characteristics of the immunomodulatory lectin ArtinM from the seeds of Artocarpus heterophyllus; (3) activation of innate immunity cells by ArtinM and consequent induction of Th1 immunity; (4) resistance conferred by ArtinM administration in infections with intracellular pathogens, such as Leishmania (Leishmania) major, Leishmania (Leishmania) amazonensis, and Paracoccidioides brasiliensis. We believe that this review will be a valuable resource for more studies in this relatively neglected area of research, which has the potential to reveal carbohydrate targets for novel prophylactic and therapeutic strategies.

ArtinM offers new perspectives in the development of antifungal therapy

The thermally dimorphic fungus Paracoccidioides brasiliensis is the causative agent of paracoccidioidomycosis (PCM), the most frequent systemic mycosis that affects the rural populations in Latin America. Despite significant developments in antifungal chemotherapy, its efficacy remains limited since drug therapy is prolonged and associated with toxic side effects and relapses. In response to these challenges, it is now recognized that several aspects of antifungal immunity can be modulated to better deal with fungal infections. A common idea for halting fungal infections has been the need to activate a cell-based, pro-inflammatory Th1 immune response to improve the fungal elimination. ArtinM, a D-mannose binding lectin from Artocarpus heterophyllus, has the property of modulating immunity against several intracellular pathogens. Here, we review the immunomodulatory activity of ArtinM during experimental PCM in mice. Both prophylactic and therapeutic protocols of ArtinM administration promotes a Th1 immune response balanced by IL-10, which outstandingly reduces the fungal load in organs of the treated mice while maintaining a controlled inflammation at the site of infection. A carbohydrate recognition-based interaction of ArtinM with Toll-like receptor 2 (TLR2) accounts for initiating the immunomodulatory effect of the lectin. The precise identification of the TLR2 N-glycan(s) targeted by ArtinM may support novel basis for the development of antifungal therapy.

Proteome-wide analysis of single-nucleotide variations in the N-glycosylation sequon of human genes

N-linked glycosylation is one of the most frequent post-translational modifications of proteins with a profound impact on their biological function. Besides other functions, N-linked glycosylation assists in protein folding, determines protein orientation at the cell surface, or protects proteins from proteases. The N-linked glycans attach to asparagines in the sequence context Asn-X-Ser/Thr, where X is any amino acid except proline. Any variation (e.g. non-synonymous single nucleotide polymorphism or mutation) that abolishes the N-glycosylation sequence motif will lead to the loss of a glycosylation site. On the other hand, variations causing a substitution that creates a new N-glycosylation sequence motif can result in the gain of glycosylation. Although the general importance of glycosylation is well known and acknowledged, the effect of variation on the actual glycoproteome of an organism is still mostly unknown. In this study, we focus on a comprehensive analysis of non-synonymous single nucleotide variations (nsSNV) that lead to either loss or gain of the N-glycosylation motif. We find that 1091 proteins have modified N-glycosylation sequons due to nsSNVs in the genome. Based on analysis of proteins that have a solved 3D structure at the site of variation, we find that 48% of the variations that lead to changes in glycosylation sites occur at the loop and bend regions of the proteins. Pathway and function enrichment analysis show that a significant number of proteins that gained or lost the glycosylation motif are involved in kinase activity, immune response, and blood coagulation. A structure-function analysis of a blood coagulation protein, antithrombin III and a protease, cathepsin D, showcases how a comprehensive study followed by structural analysis can help better understand the functional impact of the nsSNVs.

Sialyl residues modulate LPS-mediated signaling through the Toll-like receptor 4 complex

We previously reported that neuraminidase (NA) pretreatment of human PBMCs markedly increased their cytokine response to lipopolysaccharide (LPS). To study the mechanisms by which this occurs, we transfected HEK293T cells with plasmids encoding TLR4, CD14, and MD2 (three components of the LPS receptor complex), as well as a NFκB luciferase reporting system. Both TLR4 and MD2 encoded by the plasmids are α-2,6 sialylated. HEK293T cells transfected with TLR4/MD2/CD14 responded robustly to the addition of LPS; however, omission of the MD2 plasmid abrogated this response. Addition of culture supernatants from MD2 (sMD2)-transfected HEK293T cells, but not recombinant, non-glycosylated MD2 reconstituted this response. NA treatment of sMD2 enhanced the LPS response as did NA treatment of the TLR4/CD14-transfected cell supplemented with untreated sMD2, but optimal LPS-initiated responses were observed with NA-treated TLR4/CD14-transfected cells supplemented with NA-treated sMD2. We hypothesized that removal of negatively charged sialyl residues from glycans on the TLR4 complex would hasten the dimerization of TLR4 monomers required for signaling. Co-transfection of HEK293T cells with separate plasmids encoding either YFP- or FLAG-tagged TLR4, followed by treatment with NA and stimulation with LPS, led to an earlier and more robust time-dependent dimerization of TLR4 monomers on co-immunoprecipitation, compared to untreated cells. These findings were confirmed by fluorescence resonance energy transfer (FRET) analysis. Overexpression of human Neu1 increased LPS-initiated TLR4-mediated NFκB activation and a NA inhibitor suppressed its activation. We conclude that (1) sialyl residues on TLR4 modulate LPS responsiveness, perhaps by facilitating clustering of the homodimers, and that (2) sialic acid, and perhaps other glycosyl species, regulate MD2 activity required for LPS-mediated signaling. We speculate that endogenous sialidase activity mobilized during cell activation may play a role in this regulation.

Differential effect of inhibiting MD-2 and CD14 on LPS- versus whole E. coli bacteria-induced cytokine responses in human blood

BACKGROUND

Sepsis is a major world-wide medical problem with high morbidity and mortality. Gram-negative bacteria are among the most important pathogens of sepsis and their LPS content is regarded to be important for the systemic inflammatory reaction. The CD14/myeloid differentiation factor 2 (MD-2)/TLR4 complex plays a major role in the immune response to LPS . The aim of this study was to compare the effects of inhibiting MD-2 and CD14 on ultra-pure LPS - versus whole E. coli bacteria-induced responses.

METHODS

Fresh human whole blood was incubated with upLPS or whole E. coli bacteria in the presence of MD-2 or CD14 neutralizing monoclonal antibodies, or their respective controls, and/or the specific complement-inhibitor compstatin. Cytokines were measured by a multiplex (n = 27) assay. NFκB activity was examined in cells transfected with CD14, MD-2 and/or Toll-like receptors.

RESULTS

LPS-induced cytokine response was efficiently and equally abolished by MD-2 and CD14 neutralization. In contrast, the response induced by whole E. coli bacteria was only modestly reduced by MD-2 neutralization, whereas CD14 neutralization was more efficient. Combination with compstatin enhanced the effect of MD-2 neutralization slightly. When compstatin was combined with CD14 neutralization, however, the response was virtually abolished for all cytokines, including IL-17, which was only inhibited by this combination. The MD-2-independent effect observed for CD14 could not be explained by TLR2 signaling.

CONCLUSION

Inhibition of CD14 is more efficient than inhibition of MD-2 on whole E. coli-induced cytokine response, suggesting CD14 to be a better target for intervention in Gram-negative sepsis, in particular when combined with complement inhibition.

CNL, a ricin B-like lectin from mushroom Clitocybe nebularis, induces maturation and activation of dendritic cells via the toll-like receptor 4 pathway

A novel lectin, isolated from the basidiomycete mushroom Clitocybe nebularis and termed C. nebularis lectin (CNL), exhibits an immunostimulatory effect on the most potent antigen-presenting cells, the dendritic cells (DCs). Treatment of human monocyte-derived DCs with CNL in doses from 1 to 10 μg/ml resulted in a dose-dependent induction of overall DC maturation characteristics. Exposure of DCs to CNL for 48 hr resulted in extensive up-regulation of co-stimulatory molecules CD80 and CD86, as well as of the maturation marker CD83 and HLA-DR molecules. Such CNL-matured DCs (CNL-DCs) were capable of inducing a T helper type 1-polarized response in naive CD4+  CD45RA+ T cells in 5-day allogeneic co-cultures. The allostimulatory potential of CNL-DCs was significantly increased relative to untreated controls, as was their capacity to produce several pro-inflammatory cytokines such as interleukin-6, interleukin-8 and tumour necrosis factor-α. By using a specific Toll-like receptor 4 (TLR4) signalling inhibitor, CLI-095, as well as Myd88 inhibitory peptide, we have shown that DC activation by CNL is completely dependent on the TLR4 activation pathway. Furthermore, activation of TLR4 by CNL was confirmed via TLR4 reporter assay. Measurement of p65 nuclear factor-κB and p38 mitogen-activated protein kinase (MAPK) phosphorylation levels following CNL stimulation of DCs revealed primarily an increase in nuclear factor-κB activity, with less effect on the induction of p38 MAPK signalling than of lipopolysaccharide-matured DCs. The CNL had the ability to activate human DCs in such a way as to subsequently direct T helper type 1 T-cell responses. Our results encourage the use of mushroom-derived lectins for use in therapeutic strategies with aims such as to strengthen anti-tumour immune responses.

Serum soluble MD-1 levels increase with disease progression in autoimmune prone MRL(lpr/lpr) mice

MD-1 is a secreted protein that forms a complex with radioprotective 105 (RP105) and this complex plays a crucial role in lipopolysaccharide (LPS) recognition by B cells. Disease progression is known to improve in RP105-deficient lupus-prone MRL(lpr/lpr) mice. Furthermore, a soluble form of the homologous MD-2 protein is present in the plasma of septic patients and can opsonize gram-negative bacteria in cooperation with Toll-like receptor (TLR) 4. We have now established a flow cytometry-based assay to detect the soluble form of murine MD-1 (sMD-1) and explored potential roles in autoimmunity. The assay was quantitative and validated with sera from MD-1-deficient mice. Interestingly, heat-inactivated murine serum diminished the ability of sMD-1 to bind RP105. The sMD-1 was secreted by bone marrow-derived macrophages from C57BL/6 mice. Autoimmune prone MRL(lpr/lpr) mice had higher levels of sMD-1 than control MRL(+/+) mice, and levels markedly increased with disease progression. Expression of MD-1 but not MD-2 mRNA increased with age in the liver and kidney of MRL(lpr/lpr) mice. Finally, immunohistochemical analyses revealed that MD-1 was present in infiltrated macrophages within perivascular lesions of the MRL(lpr/lpr) kidney. This correlation suggests that sMD-1 may contribute to pathogenesis in this autoimmune disease model.

Significance of MD-2 and MD-2B expression in rat liver during acute cholangitis

AIM

To investigate the expression of myeloid differentiation protein-2 (MD-2), MD-2B (a splicing isoform of MD-2 that can block Toll-like receptor 4 (TLR4)/MD-2 LPS-mediated signal transduction) and TLR4 in the liver of acute cholangitis rats.

METHODS

Male Sprague-Dawley rats (SPF level) were randomly divided into four groups: (A) sham-operated group; (B) simple common bile duct ligation group; (C) acute cholangitis group; and (D) acute cholangitis anti-TLR4 intervention group (n = 25 per group). Rat liver tissue samples were used to detect TLR4, MD-2 and MD-2B mRNA expression by fluorescence quantitative PCR in parallel with pathological changes.

RESULTS

In acute cholangitis, liver TLR4 and MD-2 mRNA expression levels at 6, 12, 24, 48 and 72 h were gradually up-regulated but MD-2B mRNA expression gradually down-regulated (P < 0.05). After TLR4 antibody treatment, TLR4 and MD-2 mRNA expression were lower compared with the acute cholangitis group (P < 0.05). However, MD-2B mRNA expression was higher than in the acute cholangitis group (P < 0.05). MD-2 and TLR4 mRNA expressions were positively correlated (r = 0.94981, P < 0.05) and MD-2B mRNA expression was negatively correlated with MD-2 and TLR4 mRNA (r = -0.89031, -0.88997, P < 0.05).

CONCLUSION

In acute cholangitis, MD-2 plays an important role in the process of TLR4- mediated inflammatory response to liver injury while MD-2B plays a negative regulatory role.

Inhibitory effects of soluble MD-2 and soluble CD14 on bacterial growth

The effects of the soluble forms of the endotoxin receptor molecules sMD-2 and sCD14 on bacterial growth were studied. When Escherichia coli and Bacillus subtilis were incubated at 37 degrees C for 18 hr with either sMD-2 or sCD14, growth of these bacteria was significantly inhibited as evaluated by viable cell counts and NADPH/NADH activity. A mutant of sCD14 (sCD14d57-64) lacking a region essential for LPS binding did not inhibit the growth of E. coli, whereas this mutant did inhibit the growth of B. subtilis. Addition of excess PG to the bacterial culture reversed the inhibitory effect of sMD-2 on the growth of B. subtilis, but not on the growth of E. coli. Furthermore, when evaluated by ELISA, both sMD-2 and sCD14 bound specifically to PG. Taken together, these results indicate that sMD-2 and sCD14 inhibit the growth of both Gram-positive and Gram-negative bacteria and further suggest that binding to PG and LPS is involved in the inhibitory effect of sMD-2 on Gram-positive bacteria and of sCD14 on Gram-negative bacteria, respectively.

Identification of a novel human MD-2 splice variant that negatively regulates Lipopolysaccharide-induced TLR4 signaling

Myeloid differentiation factor 2 (MD-2) is a secreted gp that assembles with TLR4 to form a functional signaling receptor for bacterial LPS. In this study, we have identified a novel alternatively spliced isoform of human MD-2, termed MD-2 short (MD-2s), which lacks the region encoded by exon 2 of the MD-2 gene. Similar to MD-2, MD-2s is glycosylated and secreted. MD-2s also interacted with LPS and TLR4, but failed to mediate LPS-induced NF-kappaB activation and IL-8 production. We show that MD-2s is upregulated upon IFN-gamma, IL-6, and TLR4 stimulation and negatively regulates LPS-mediated TLR4 signaling. Furthermore, MD-2s competitively inhibited binding of MD-2 to TLR4. Our study pinpoints a mechanism that may be used to regulate TLR4 activation at the onset of signaling and identifies MD-2s as a potential therapeutic candidate to treat human diseases characterized by an overly exuberant or chronic immune response to LPS.

Dependence of pathogen molecule-induced toll-like receptor activation and cell function on Neu1 sialidase

The signaling pathways of mammalian Toll-like receptors (TLR) are well characterized, but the initial molecular mechanisms activated following ligand interactions with the receptors remain poorly defined. Here, we show a membrane controlling mechanism that is initiated by ligand binding to TLR-2, -3 and-4 to induce Neu1 sialidase activity within minutes in live primary bone marrow (BM) macrophage cells and macrophage and dendritic cell lines. Central to this process is that Neu1 and not Neu2,-3 and-4 forms a complex with TLR-2,-3 and-4 on the cell surface of naïve macrophage cells. Neuraminidase inhibitors BCX1827, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (DANA), zanamivir and oseltamivir carboxylate have a limited significant inhibition of the LPS-induced sialidase activity in live BMC-2 macrophage cells but Tamiflu (oseltamivir phosphate) completely blocks this activity. Tamiflu inhibits LPS-induced sialidase activity in live BMC-2 cells with an IC(50) of 1.2 microM compared to an IC(50) of 1015 microM for its hydrolytic metabolite oseltamivir carboxylate. Tamiflu blockage of LPS-induced Neu1 sialidase activity is not affected in BMC-2 cells pretreated with anticarboxylesterase agent clopidogrel. Endotoxin LPS binding to TLR4 induces Neu1 with subsequent activation of NFkappaB and the production of nitric oxide and pro-inflammatory IL-6 and TNFalpha cytokines in primary and macrophage cell lines. Hypomorphic cathepsin A mice with a secondary Neu1 deficiency respond poorly to LPS-induced pro-inflammatory cytokines compared to the wild-type or hypomorphic cathepsin A with normal Neu1 mice. Our findings establish an unprecedented mechanism for pathogen molecule-induced TLR activation and cell function, which is critically dependent on Neu1 sialidase activity associated with TLR ligand treated live primary macrophage cells and macrophage and dendritic cell lines.

CD14+ monocytes are vulnerable and functionally impaired under endoplasmic reticulum stress in patients with type 2 diabetes

OBJECTIVE

Although patients with diabetes suffer from increased infections and a higher incidence of cancer due to impaired immune function, details on diabetes-induced decrease in immunity are lacking. We assessed how immune-mediating peripheral blood mononuclear cells (PBMCs) are affected in diabetes.

RESEARCH DESIGN AND METHODS

From 33 patients with type 2 diabetes and 28 healthy volunteers, we obtained PBMCs and investigated their susceptibility to apoptosis and functional alteration.

RESULTS

In a subpopulation of PBMCs, monocytes derived from patients with diabetes were more susceptible to apoptosis than monocytes from healthy volunteers. Monocytes from patients with diabetes had decreased phagocytotic activity and were less responsive to Toll-like receptor (TLR) ligands, although the expression of TLRs did not differ significantly between the two groups. Furthermore, monocytes from patients with diabetes had a distinctly different gene expression profile compared with monocytes from normal volunteers as assessed with DNA microarray analysis. Specifically, quantitative real-time detection PCR measurements showed an elevated expression of the markers of endoplasmic reticulum (ER) stress in diabetic monocytes, and electron microscopic examination of monocytes revealed morphologic alterations in the ER of cells derived from patients with diabetes. Consistently, the ER stress inducer tunicamycin increased apoptosis of otherwise healthy monocytes and attenuated the proinflammatory responses to TLR ligands.

CONCLUSIONS

These data suggest that monocytes comprise a substantially impaired subpopulation of PBMCs in patients with diabetes and that ER stress is involved in these pathologic changes mechanistically. This implies that the affected monocytes should be investigated further to better understand diabetic immunity.

Neutrophil activation induced by ArtinM: release of inflammatory mediators and enhancement of effector functions

The D-mannose binding lectin ArtinM from Artocarpus integrifolia, previously known as KM+ and artocarpin, is considered a stimulant of Th1-type immunity, which is able to confer resistance to some intracellular pathogens. In addition, ArtinM induces neutrophil migration by haptotaxis through simultaneous interactions of its carbohydrate recognition domains (CRDs) with glycans expressed on the extracellular matrix and the neutrophil surface. In the present study, we have expanded the characterization of ArtinM as a neutrophil activator. Exposure of neutrophils to ArtinM for 15 min resulted in tyrosine phosphorylation of intracellular proteins, a process that was selectively inhibited by d-mannose or mannotriose. Shortly after stimulation, neutrophils secreted high levels of LTB(4) and underwent shedding of L-selectin from their surface. Exposure to ArtinM enhanced neutrophil functions, such as respiratory burst and zymozan and Listeria monocytogenes phagocytosis. In addition, ArtinM-stimulated neutrophils displayed increased CXCL-8 secretion and TLR2 gene transcription. These results demonstrate that ArtinM is able to induce potent neutrophil activation, a feature that should be strongly considered in the assessment of the lectin capacity to confer resistance against infections.

Paclitaxel binding to human and murine MD-2

Paclitaxel (PTX) is an important cancer chemotherapeutic agent that binds to beta-tubulin and prevents mitosis through microtubule overstabilization. Recent evidence also implicates PTX in the induction of apoptosis of cancer cells via the TLR4 innate immune pathway. The TLR4 accessory protein, MD-2, is an essential component for the species-specific proinflammatory activity of PTX on murine cells. However, whether PTX binds to human MD-2 and how MD-2 and TLR4 interact with PTX are not well defined. Recombinant human MD-2 (rhMD-2) was produced in a Pichia pastoris expression system, and the interaction between rhMD-2 and PTX was assessed by an enzyme-linked immunosorbent assay to show that PTX binds rhMD-2. Formation of the latter complex was found to be dose-dependent and inhibited by anti-MD-2 antibody but not by an isotype control antibody. As measured by human tumor necrosis factor alpha production, human THP-1 monocytes expressing TLR4 and MD-2 were poorly responsive to the addition of PTX, but murine macrophages expressing TLR4 and MD-2 responded in a dose-dependent manner. Human embryonic kidney (HEK293) cells transfected with both human TLR4 and human MD-2 or human MD-2 and murine TLR4 were also poorly responsive to PTX (10 microm). However, HEK293 cells transfected with murine MD-2 and human TLR4 or murine MD-2 and murine TLR4 were highly responsive to PTX (10 microm), indicating that the murine MD-2/PTX interaction is required for TLR4 activation. To further define the structural differences for MD-2/TLR4 activation, crystal structures of both murine and human MD-2 were subjected to PTX docking by computational methods. These models indicate that PTX binds in the pocket of both human and mouse MD-2 structures. The species-specific difference between human and murine MD-2 activation of TLR4 by PTX can be explained by alterations of surface charge distribution (i.e. electrostatic potential), binding pocket size, and the locus of PTX binding within the MD-2 pocket, which results in reorganization of the 123-130 amino acid loop. In particular, Phe(126) appears to operate as a bridge for TLR4.MD-2 dimerization in the mouse but not the human protein.

Folimycin (concanamycin A) inhibits LPS-induced nitric oxide production and reduces surface localization of TLR4 in murine macrophages

Lipopolysaccharide (LPS) is a major cell wall component of Gram-negative bacteria and signals through a receptor complex which consists of TLR4, MD-2 and CD14. LPS signaling in macrophages induces the production of many pro-inflammatory molecules, including nitric oxide (NO). In this study, we have shown that folimycin, a macrolide antibiotic and a specific inhibitor of vacuolar ATPase (V-ATPase), inhibits LPS-induced NO production, but not TNFalpha production, in murine elicited peritoneal macrophages. However, folimycin did not affect interferon-gamma induced NO production. LPS-induced iNOS mRNA and protein expression and NF-kappaB activation were also inhibited by folimycin. Interestingly, folimycin-treated cells showed reduced surface expression of TLR4 molecules and dilated Golgi apparatus. These findings suggest that folimycin, by inhibiting V-ATPases, alters intra-Golgi pH, which in turn causes defective processing and reduced surface expression of TLR4 reducing the strength of LPS signaling in murine macrophages.

Therapeutic administration of KM+ lectin protects mice against Paracoccidioides brasiliensis infection via interleukin-12 production in a toll-like receptor 2-dependent mechanism

KM(+) is a mannose-binding lectin from Artocarpus integrifolia that induces interleukin (IL)-12 production by macrophages and protective T helper 1 immune response against Leishmania major infection. In this study, we performed experiments to evaluate the therapeutic activity of jackfruit KM(+) (jfKM(+)) and its recombinant counterpart (rKM(+)) in experimental paracoccidioidomycosis. To this end, jfKM(+) or rKM(+) was administered to BALB/c mice 10 days after infection with Paracoccidiodes brasiliensis. Thirty days postinfection, lungs from the KM(+)-treated mice contained significantly fewer colony-forming units and little to no organized granulomas compared to the controls. In addition, lung homogenates from the KM(+)-treated mice presented higher levels of nitric oxide, IL-12, interferon-gamma, and tumor necrosis factor-alpha, whereas higher levels of IL-4 and IL-10 were detected in the control group. With mice deficient in IL-12, Toll-like receptor (TLR) 2, TLR4, or TLR adaptor molecule MyD88, we demonstrated that KM(+) led to protection against P. brasiliensis infection through IL-12 production, which was dependent on TLR2. These results demonstrated a beneficial effect of KM(+) on the severity of P. brasiliensis infection and may expand its potential use as a novel immunotherapeutic molecule.

Both ERK and JNK are required for enhancement of MD-2 gene expression during differentiation of HL-60 cells

BACKGROUND INFORMATION

MD-2 is associated with the extracellular domain of TLR4 (Toll-like receptor 4) and augments TLR4-dependent LPS (lipopolysaccharide) responses in vitro. Our previous investigation found that PMA-induced HL-60 cell differentiation to macrophages is associated largely with TLR2 and CD14 and, to a much lesser extent, with TLR4.

RESULTS

We studied the MD-2 expression during differentiation of HL-60 cells induced by PMA. The results showed that PMA, but not VitD(3) (1alpha,25-dihydroxy-vitamin D(3)), strongly induces MD-2 gene expression by HL-60 cells in a time- and dose-dependent manner. Treatment with an MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] inhibitor (PD98059) and a JNK (c-Jun N-terminal kinase) inhibitor (SP600125) suppresses PMA-induced MD-2 gene expression, whereas impairment of p38 function by treatment with the inhibitor SB203580 has no effect on MD-2 mRNA. In order to reveal the possible molecular mechanism for such a regulation of MD-2 gene expression, we cloned and analysed the putative MD-2 gene promoter. Transient transfection of different deletion mutants demonstrated that the region -185/-171 (5'-TCCTTTACAGGAAGT-3') of the MD-2 gene promoter is closely related to gene transcription in response to PMA. Additionally, the transcription factor Elk-1 has been found to bind this specific motif.

CONCLUSIONS

These results suggest that ERK and JNK pathways are involved in PMA-mediated MD-2 gene expression during HL-60 cell differentiation, and the activation of the MEK/possible ERK/Elk signal pathway is the mechanism responsible for PMA-induced MD-2 gene expression in differentiated HL-60 cells.

Isolation of monomeric and dimeric secreted MD-2. Endotoxin.sCD14 and Toll-like receptor 4 ectodomain selectively react with the monomeric form of secreted MD-2

Potent cell activation by endotoxin requires sequential protein-endotoxin and protein-protein interactions involving lipopolysaccharide-binding protein, CD14, MD-2, and Toll-like receptor 4 (TLR4). MD-2 plays an essential role by bridging endotoxin (E) recognition initiated by lipopolysaccharide-binding protein and CD14 to TLR4 activation by presenting endotoxin as a monomeric E.MD-2 complex that directly and potently activates TLR4. Secreted MD-2 (sMD-2) exists as a mixture of monomers and multimers. Published data suggest that only MD-2 monomer can interact with endotoxin and TLR4 and support cell activation, but the apparent instability of MD-2 has thwarted efforts to more fully separate and characterize the individual species of sMD-2. We have taken advantage of the much greater stability of sMD-2 in insect culture medium to fully separate sMD-2 monomer from dimer by gel sieving chromatography. At low nanomolar concentrations, the sMD-2 monomer, but not dimer, reacted with a monomeric complex of E.sCD14 to form monomeric E.MD-2 and activate HEK293/TLR4 cells. The monomer, but not dimer, also reacted with the ectodomain of TLR4 with an affinity comparable with the picomolar affinity of E.MD-2. These findings demonstrate directly that the monomeric form of sMD-2 is the active species both for reaction with E.CD14 and TLR4, as needed for potent endotoxin-induced TLR4 activation.

Increased release of sMD-2 during human endotoxemia and sepsis: a role for endothelial cells

MD-2 is the crucial cofactor of TLR4 in the detection of LPS. Here, we show that soluble MD-2 (sMD-2) circulates in plasma of healthy individuals as a polymeric protein. The total amount of sMD-2 in septic plasma was strongly elevated and contained both sMD-2 polymers and monomers, the latter representing the putative biologically active form of MD-2. Moreover, during experimental human endotoxemia, the monomeric and total sMD-2 content in plasma increased with the kinetics of an acute phase protein. The increase in sMD-2 monomers was paralleled by enhanced TLR4 costimulatory activity. The presence of functional sMD-2 during endotoxemia and sepsis was confirmed by immunodepletion. Immunohistochemistry revealed that MD-2 expression in septic patients was strongly enhanced on endothelium and multiple inflammatory cells in lung and liver. In vitro studies showed that sMD-2 release appears to be restricted to endothelial cells and dendritic cells. Release of sMD-2 by endothelial cells was strongly enhanced by LPS and TNF-alpha stimulation. Taken together, this study demonstrates the increase of both circulating polymeric and functional monomeric sMD-2 during endotoxemia and sepsis, and evidence is provided that the endothelium is involved in this process.

The role of TLR activation in inflammation

The Toll-like receptor family was originally identified in Drosophila, where it provides important developmental and immunological signalling. In mammals, the developmental signal appears to have been lost, but the immunological defence role of these receptors has been expanded to provide broad recognition of bacterial, fungal, viral and parasitic pathogens. There is increasing evidence that these receptors go beyond the recognition of microbial molecules to sense host tissue damage. Recognition of host molecules and commensal microbes is also involved in the restoration of normal tissue architecture after injury and in maintenance of epithelial health. Recent developments in the TLR field highlight the importance of these molecules to human health and disease and demonstrate that their targeting, to boost immunity or inhibit inflammation, is both feasible and also potentially challenging.

Soluble MD-2 is an acute-phase protein and an opsonin for Gram-negative bacteria

Myeloid differentiation factor-2 (MD-2) is a lipopolysaccharide (LPS)-binding protein usually coexpressed with and binding to Toll-like receptor 4 (TLR4), conferring LPS responsiveness of immune cells. MD-2 is also found as a soluble protein. Soluble MD-2 (sMD-2) levels are markedly elevated in plasma from patients with severe infections, and in other fluids from inflamed tissues. We show that sMD-2 is a type II acute-phase protein. Soluble MD-2 mRNA and protein levels are up-regulated in mouse liver after the induction of an acute-phase response. It is secreted by human hepatocytic cells and up-regulated by interleukin-6. Soluble MD-2 binds to Gram-negative but not Gram-positive bacteria, and sMD-2 secreted by hepatocytic cells is an essential cofactor for the activation of TLR4-expressing cells by Gram-negative bacteria. Soluble MD-2 opsonization of Gram-negative bacteria accelerates and enhances phagocytosis, principally by polymorphonuclear neutrophils. In summary, our results demonstrate that sMD-2 is a newly recognized type II acute-phase reactant, an opsonin for Gram-negative bacteria, and a cofactor essential for the activation of TLR4-expressing cells. This suggests that sMD-2 plays a key role in the host innate immune response to Gram-negative infections.

Human MD-2 discrimination of meningococcal lipid A structures and activation of TLR4

MD-2, a eukaryotic accessory protein, is an essential component for the molecular pattern recognition of bacterial endotoxins. MD-2 interacts with lipid A of endotoxins [lipopolysaccharide (LPS) or lipooligosaccharide (LOS)] to activate human toll-like receptor (TLR) 4. The structure of lipid A influences the subsequent activation of human TLR4 and the immune response, but the basis for the discrimination of lipid A structures is unclear. A recombinant human MD-2 (rMD-2) protein was produced in the Pichia pastoris yeast expression system. Human embryonic kidney (HEK293) cells were transfected with human TLR4 and were stimulated with highly purified LOS (0.56 pmol) from Neisseria meningitidis or LPS from other structurally defined bacterial endotoxins in the presence or absence of human rMD-2. Human rMD-2 restored, in a dose-dependent manner, interleukin (IL-8) responsiveness to LOS or LPS in TLR4-transfected HEK293 cells. The interaction of endotoxin with human rMD-2 was then assessed by enzyme-linked immunosorbent assays. Wild-type meningococcal LOS (Wt m LOS) bound human rMD-2, and binding was inhibited by an anti-MD-2 antibody to MD-2 dose-dependently (P < 0.005). Wt m LOS or meningococcal KDO(2)-lipid A had the highest binding affinity for human rMD-2; unglycosylated meningococcal lipid A produced by meningococci with defects in the 3-deoxy-d-manno-2-octulosonic acid (KDO) biosynthesis pathway did not appear to bind human rMD-2 (P < 0.005). The affinity of meningococcal LOS with a penta-acylated lipid A for human rMD-2 was significantly less than that for hexa-acylated LOS (P < 0.05). The hierarchy in the binding affinity of different lipid A structures for human rMD-2 was directly correlated with differences in TLR4 pathway activation and cytokine production by human macrophages.

Roles of N-linked glycans in the recognition of microbial lipopeptides and lipoproteins by TLR2

Details of roles of carbohydrates attached to Toll-like receptors (TLRs) in the recognition of pathogen-associated molecular patterns and in the formation of the functional receptor complex still remain unknown. This study was designed to determine whether the glycans linked at Asn114, Asn199, Asn414 and Asn442 residues of TLR2 ectodomain were involved in the recognition of diacylated lipopeptide and lipoprotein. Single and multiple mutants were transfected into human embryonic kidney (HEK) 293 cells together with a NF-kappaB luciferase reporter plasmid. All of these mutants were expressed on the surface. SDS-PAGE of the transfectants demonstrated that these mutants migrated lower than wild-type TLR2 and their molecular masses decreased as the number of mutated Asn residues increased. TLR2(N114A), TLR2(N199A) and TLR2(N414A) as well as wild-type TLR2 induced NF-kappaB activation when stimulated with these ligands, whereas TLR2(N442A) failed to induce NF-kappaB activation. All of triple and quadruple mutants failed to induce NF-kappaB activation, but were associated with both wild-type TLR2 and TLR6 in the transfectants. TLR2(N114A,N199A), TLR2(N114A,N414A) and, to a lesser extent, TLR2(N114A,N442A), in which two N-linked glycans are speculated to be exposed to the concave surface of TLR2 solenoid, not only induce NF-kappaB activation but also are associated with wild-type TLR2 and TLR6. These results suggest that the glycan at Asn442 and at least two N-linked glycans speculated to be exposed to the concave surface of TLR2 solenoid are involved in the recognition of ligands by TLR2 and/or in formation or maturation of a functional TLR2 receptor complex.

MD-2

Toll-like receptors (TLRs) are a small family of type-I glycoproteins that bind to and are activated by conserved non-self molecular signatures carried by microorganisms. Toll-like receptor 4 is triggered by most lipopolysaccharides (LPS). LPS is a complex amphipathic saccharolipidic glycan derived from Gram-negative bacteria. Unique among TLRs, TLR4 activity and interaction with its natural ligand(s) strictly depends on the presence of the extracellular adaptor MD-2. MD-2 is a small secreted glycoprotein that binds with cytokine-like affinities to both the hydrophobic portion of LPS and to the extracellular domain of TLR4. The interaction between MD-2 and LPS induces a triggering event on TLR4, which involves the molecular rearrangement of the receptor complex and its homotypic aggregation. In silico analysis suggests that MD-2 and MD-1 are paralogs derived from a common predecessor at the level of early vertebrates. In this review, we summarize the current state of knowledge concerning MD-2.

Trypsin-sensitive modulation of intestinal epithelial MD-2 as mechanism of lipopolysaccharide tolerance

Intestinal epithelial cells (IEC) are constantly exposed to both high concentrations of the bacterial ligand LPS and the serine protease trypsin. MD-2, which contains multiple trypsin cleavage sites, is an essential accessory glycoprotein required for LPS recognition and signaling through TLR4. The aim of this study was to characterize the expression and subcellular distribution of intestinal epithelial MD-2 and to delineate potential functional interactions with trypsin and then alteration in inflammatory bowel disease (IBD). Although MD-2 protein expression was minimal in primary IEC of normal colonic or ileal mucosa, expression was significantly increased in IEC from patients with active IBD colitis, but not in ileal areas from patients with severe Crohn's disease. Endogenous MD-2 was predominantly retained in the calnexin-calreticulin cycle of the endoplasmic reticulum; only a small fraction was exported to the Golgi. MD-2 expression correlated inversely with trypsin activity. Biochemical evidence and in vitro experiments demonstrated that trypsin exposure resulted in extensive proteolysis of endogenous and soluble MD-2 protein, but not of TLR4 in IEC, and was associated with desensitization of IEC to LPS. In conclusion, the present study suggests that endoplasmic reticulum-associated MD-2 expression in IBD may be altered by ileal protease in inflammation, leading to impaired LPS recognition and hyporesponsiveness through MD-2 proteolysis in IEC, thus implying a physiologic mechanism that helps maintain LPS tolerance in the intestine.

Mutational analysis of membrane and soluble forms of human MD-2

Toll-like receptor 4 and MD-2 form a receptor for lipopolysaccharide (LPS), a major constituent of Gram-negative bacteria. MD-2 is a 20-25-kDa extracellular glycoprotein that binds to Tolllike receptor 4 (TLR4) and LPS and is a critical part of the LPS receptor. Here we have shown that the level of MD-2 expression regulates TLR4 activation by LPS. Using site-directed mutagenesis, we have found that glycosylation has no effect on MD-2 function as a membrane receptor for LPS. We used alanine-scanning mutagenesis to identify regions of human MD-2 that are important for TLR4 and LPS binding. We found that mutation in the N-terminal 46 amino acids of MD-2 did not substantially diminish LPS activation of Chinese hamster ovary (CHO) cells co-transfected with TLR4 and mutant MD-2. The residues 46-50 were important for LPS activation but not LPS binding. The residues 79-83, 121-124, and 125-129 are identified as important in LPS activation but not surface expression of membrane MD-2. The function of soluble MD-2 is somewhat more sensitive to mutation than membrane MD-2. Our results suggest that the 46-50 and 127-131 regions of soluble MD-2 bind to TLR4. The region 79-120 is not involved in LPS binding but affects monomerization of soluble MD-2 as well as TLR4 binding. We define the LPS binding region of monomeric soluble MD-2 as a cluster of basic residues 125-131. Studies on both membrane and soluble MD-2 suggest that domains of MD-2 for TLR4 and LPS binding are separate as well as overlapping. By mapping these regions on a three-dimensional model, we show the likely binding regions of MD-2 to TLR4 and LPS.

The etiology of sepsis: turned inside out

The sepsis syndrome is thought to occur when microbial products activate Toll-like receptors stimulating widespread inflammation, in turn causing organ failure, shock and death. However, recent discoveries reveal that: (i) not only microbial substances but also endogenous molecules can trigger Toll-like receptors; (ii) Toll-like receptor-4, the endotoxin receptor, is constitutively suppressed; and (iii) the first step in sepsis could be the release of Toll-like receptor-4 from suppression. These discoveries suggest that endotoxin might not always initiate the sepsis syndrome and they explain why anti-endotoxin therapies fail. The discoveries also suggest new therapeutic targets - endogenous agonists and Toll-like receptor regulators - for treatment of sepsis.

Effects of Possible Endocrine Disrupting Chemicals on Bacterial Component-Induced Activation of NF-.KAPPA.B

Endocrine disrupting chemicals (EDCs) have a possibility to exacerbate infectious diseases because EDCs disturb the human immune system by interfering with endocrine balance. To assess the influence of EDCs on the innate immune function of macrophages, we investigated the effects of thirty-seven possible endocrine disruptors on lipopolysaccharide (LPS)- or bacterial lipopeptide (Pam3CSK4)-induced activation of nuclear factor kappa B (NF-kappaB). Alachlor, benomyl, bisphenol A, carbaryl, kelthane, kepone, octachlorostyrene, pentachlorophenol, nonyl phenol, p-octylphenol and ziram inhibited both LPS- and Pam3CSK4-induced activation of NF-kappaB. Simazine inhibited only LPS-induced activation. A strong inhibitory effect was observed with ziram and benomyl. On the other hand, diethylhexyl adipate and 4-nitrotoluene tended to enhance the activation induced by Pam3CSK4 and LPS, respectively. Aldicarb, amitrole, atrazine, benzophenone, butyl benzyl phthalate, 2,4-dichlorophenoxy acetic acid, dibutyl phthalate, 2,4-dichlorophenol, dicyclohexyl phthalate, diethylhexyl phthalate, diethyl phthalate, dihexyl phthalate, di-n-pentyl phthalate, dipropyl phthalate, malathion, methomyl, methoxychlor, metribuzin, nitrofen, permethrin, trifluralin, 2,4,5-trichlorophenoxyacetic acid and vinclozolin had no significant effects at 100 microM. These results indicate that some agrochemicals have the potential to inhibit macrophage function and suggest that endocrine disruptors may influence the development of bacterial infections.

N-linked glycosylation at Asn3 and the positively charged residues within the amino-terminal domain of the c1 inhibitor are required for interaction of the C1 Inhibitor with Salmonella enterica serovar typhimurium lipopolysaccharide and lipid A

The C1 inhibitor (C1INH), a plasma complement regulatory protein, prevents endotoxin shock, at least partially via the direct interaction of its amino-terminal heavily glycosylated nonserpin region with gram-negative bacterial lipopolysaccharide (LPS). To further characterize the potential LPS-binding site(s) within the amino-terminal domain, mutations were introduced into C1INH at the three N-linked glycosylation sites and at the four positively charged amino acid residues. A mutant in which Asn(3) was replaced with Ala was markedly less effective in its binding to LPS, while substitution of Asn(47) or Asn(59) had little effect on binding. The mutation of C1INH at all four positively charged amino acid residues (Arg(18), Lys(22), Lys(30), and Lys(55)) resulted in near-complete failure to interact with LPS. The C1INH mutants that did not bind to LPS also did not suppress LPS binding or LPS-induced up-regulation of tumor necrosis factor alpha mRNA expression in RAW 264.7 macrophages. In addition, the binding of C1INH mutants to diphosphoryl lipid A was decreased in comparison with that of recombinant wild-type C1INH. Therefore, the interaction of C1INH with gram-negative bacterial LPS is dependent both on the N-linked carbohydrate at Asn(3) and on the positively charged residues within the amino-terminal domain.

Herpes simplex virus 1 infected cell protein 0 forms a complex with CIN85 and Cbl and mediates the degradation of EGF receptor from cell surfaces

Infected cell protein 0 (ICP0) is a 775-residue multifunctional herpes simplex virus protein associated with numerous functions related to transactivation of gene expression and repression of host defenses to infection. We report that an uncharted domain of ICP0 located between residues 245 and 510 contains multiple SH3 domain binding motifs similar to those required for binding to CIN85, the M(r) 85,000 protein that interacts with Cbl. CIN85 and Cbl are involved in endocytosis and negative regulation of numerous receptor tyrosine kinases. We report that ICP0 binds CIN85 in a reciprocal manner and that the complexes pulled down by ICP0 also contain Cbl. We tested the role of ICP0 in the down-regulation of receptor tyrosine kinases by using epidermal growth factor receptor (EGFR) as a prototypic receptor. In transfection assays, ICP0, in the absence of other viral genes, down-regulated EGF-dependent expression of a reporter gene (luciferase). ICP0 also down-regulated both total and cell surface levels of EGFR in EGF-independent manner. In wild-type virus-infected cells, the surface levels of EGFR were also decreased in the absence of EGF stimulation. Stimulation by EGF enhanced the decrease in surface EGFR. We conclude that ICP0 encodes SH3 domain binding sites that function to down-regulate signaling pathways associated with receptor tyrosine kinases. The results suggest that ICP0 precludes signaling to the infected cells through the receptor tyrosine kinases.

Human TLR10 Is a Functional Receptor, Expressed by B Cells and Plasmacytoid Dendritic Cells, Which Activates Gene Transcription through MyD88

Human TLR10 is an orphan member of the TLR family. Genomic studies indicate that TLR10 is in a locus that also contains TLR1 and TLR6, two receptors known to function as coreceptors for TLR2. We have shown that TLR10 was not only able to homodimerize but also heterodimerized with TLRs 1 and 2. In addition, unlike TLR1 and TLR6, TLR10 was expressed in a highly restricted fashion as a highly N-glycosylated protein, which we detected in B cell lines, B cells from peripheral blood, and plasmacytoid dendritic cells from tonsil. We were also able to detect TLR10 in a CD1a(+) DC subset derived from CD34(+) progenitor cells which resemble Langerhans cells in the epidermis. Although we were unable to identify a specific ligand for TLR10, by using a recombinant CD4TLR10 molecule we also demonstrated that TLR10 directly associates with MyD88, the common Toll IL-1 receptor domain adapter. Additionally, we have characterized regions in the Toll IL-1 receptor domain of TLR10 that are essential in the activation of promoters from certain inflammatory cytokines. Even though TLR10 expression has not been detected in mice, we have identified a partial genomic sequence of the TLR10 gene that was present but nonfunctional and disrupted by a retroviral insertion in all mouse strains tested. However, a complete TLR10 sequence could be detected in the rat genome, indicating that a functional copy may be preserved in this species.

The functional and structural properties of MD-2 required for lipopolysaccharide binding are absent in MD-1

MD-1 and MD-2 are secretory glycoproteins that exist on the cell surface in complexes with transmembrane proteins. MD-1 is anchored by radioprotective 105 (RP105), and MD-2 is associated with TLR4. In vivo studies revealed that MD-1 and MD-2 have roles in responses to LPS. Although the direct binding function of MD-2 to LPS has been observed, the physiological function of MD-1 remains unknown. In this study, we compared the LPS-binding functions of MD-1 and MD-2. LPS binding to cell surface complexes was detected for cells transfected with TLR4/MD-2. In contrast, binding was not observed for RP105/MD-1-transfected cells. When rMD-2 protein was expressed in Escherichia coli, it was purified in complexes containing LPS. In contrast, preparations of MD-1 did not contain LPS. When rMD-2 protein was prepared in a mutant strain lacking the lpxM gene, LPS binding disappeared. Therefore, the secondary myristoyl chain attached to the (R)-3-hydroxymyristoyl chain added by LpxM is required for LPS recognition by MD-2, under these conditions. An amphipathic cluster composed of basic and hydrophobic residues in MD-2 has been suggested to be the LPS-binding site. We specifically focused on two Phe residues (119 and 121), which can associate with fatty acids. A mutation at Phe(191) or Phe(121) strongly reduced binding activity, and a double mutation at these residues prevented any binding from occurring. The Phe residues are present in MD-2 and absent in MD-1. Therefore, the LPS recognition mechanism by RP105/MD-1 is distinct from that of TLR4/MD-2.

Identification of a novel isoform of MD-2 that downregulates lipopolysaccharide signaling

MD-2 is an association molecule of Toll-like receptor 4 and is indispensable for the recognition of lipopolysaccharide. Here we report the identification of mRNA for an alternatively spliced form of MD-2, named MD-2B, which lacks the first 54 bases of exon 3. When overexpressed with MD-2, MD-2B competitively suppressed NF-kappaB activity induced by LPS. Regardless of the truncation, however, MD-2B still bound to TLR4 as efficiently as MD-2. Flow cytometric analyses revealed that MD-2B inhibited TLR4 from being expressed on the cell surface. Our data indicate that MD-2B may compete with MD-2 for binding to TLR4 and decrease the number of TLR4/MD-2 complexes on the cell surface, resulting in the inhibition of LPS signaling.

The potential for Toll-like receptors to collaborate with other innate immune receptors

Cells of the innate immune system express a large repertoire of germ-line encoded cell-surface glycoprotein receptors including Toll-like receptors (TLRs). TLRs recognize conserved motifs on microbes and induce inflammatory signals. Evidence suggests that individual members of the TLR family or other non-TLR surface antigens either physically or functionally interact with each other and cumulative effects of these interactions instruct the nature and outcome of the immune response to a particular pathogen.