Crystal structure of CD14 and its implications for lipopolysaccharide signaling

Antibody-Based Fusion Proteins Allow Ca2+ Rewiring to Most Extracellular Ligands

The Ca2+ signaling toolkit is the set of proteins used by living systems to generate and respond to Ca2+ signals. The selective expression of these proteins in particular tissues, cell types and subcellular locations allows the Ca2+ signal to regulate a diverse set of cellular processes. Through synthetic biology, the Ca2+ signaling toolkit can be expanded beyond the natural repertoire to potentially allow a non-natural ligand to control downstream cellular processes. To realize this potential, we exploited the ability of an antibody to bind its antigen exclusively in combination with the ability of the cytoplasmic domain of vascular endothelial growth factor receptor 2 (VEGFR2) to generate a Ca2+ signal upon oligomerization. Using protein fusions between antibody variants (i.e., nanobody, single-chain antibody and the monoclonal antibody) and the VEGFR2 cytoplasmic domain, Ca2+ signals were generated in response to extracellular stimulation with green fluorescent protein, mCherry, tumor necrosis factor alpha and soluble CD14. The Ca2+ signal generation by the stimulus did not require a stringent transition from monomer to oligomer state, but instead only required an increase in the oligomeric state. The Ca2+ signal generated by these classes of antibody-based fusion proteins can be rewired with a Ca2+ indicator or with an engineered Ca2+ activated RhoA to allow for antigen screening or migration to most extracellular ligands, respectively.

Recent progress in the discovery of myeloid differentiation 2 (MD2) modulators for inflammatory diseases

Myeloid differentiation protein 2 (MD2), together with Toll-like receptor 4 (TLR4), binds lipopolysaccharide (LPS) with high affinity, inducing the formation of the activated homodimer LPS-MD2-TLR4. MD2 directly recognizes the Lipid A domain of LPS, leading to the activation of downstream signaling of cytokine and chemokine production, and initiation of inflammatory and immune responses. However, excessive activation and potent host responses generate severe inflammatory syndromes such as acute sepsis and septic shock. MD2 is increasingly being considered as an attractive pharmacological target for the development of potent anti-inflammatory agents. In this Keynote review, we provide a comprehensive overview of the recent advances in the structure and biology of MD2, and present MD2 modulators as promising agents for anti-inflammatory intervention.

Heparan Sulfate Proteoglycans as Relays of Neuroinflammation

Heparan sulfate proteoglycans (HSPGs) are implicated as inflammatory mediators in a variety of settings, including chemokine activation, which is required to recruit circulating leukocytes to infection sites. Heparan sulfate (HS) polysaccharide chains are highly interactive and serve co-receptor roles in multiple ligand:receptor interactions. HS may also serve as a storage depot, sequestering ligands such as cytokines and restricting their access to binding partners. Heparanase, through its ability to fragment HS chains, is a key regulator of HS function and has featured prominently in studies of HS's involvement in inflammatory processes. This review focuses on recent discoveries regarding the role of HSPGs, HS, and heparanase during inflammation, with particular focus on the brain. HS chains emerge as critical go-betweens in multiple aspects of the inflammatory response-relaying signals between receptors and cells. The molecular interactions proposed to occur between HSPGs and the pathogen receptor toll-like receptor 4 (TLR4) are discussed, and we summarize some of the contrasting roles that HS and heparanase have been assigned in diseases associated with chronic inflammatory states, including Alzheimer's disease (AD). We conclude by briefly discussing how current knowledge could potentially be applied to augment HS-mediated events during sustained neuroinflammation, which contributes to neurodegeneration in AD.

Lipopolysaccharide Upregulates Palmitoylated Enzymes of the Phosphatidylinositol Cycle: An Insight from Proteomic Studies

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria that induces strong proinflammatory reactions of mammals. These processes are triggered upon sequential binding of LPS to CD14, a GPI-linked plasma membrane raft protein, and to the TLR4/MD2 receptor complex. We have found earlier that upon LPS binding, CD14 triggers generation of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂], a lipid controlling subsequent proinflammatory cytokine production. Here we show that stimulation of RAW264 macrophage-like cells with LPS induces global changes of the level of fatty-acylated, most likely palmitoylated, proteins. Among the acylated proteins that were up-regulated in those conditions were several enzymes of the phosphatidylinositol cycle. Global profiling of acylated proteins was performed by metabolic labeling of RAW264 cells with 17ODYA, an analogue of palmitic acid functionalized with an alkyne group, followed by detection and enrichment of labeled proteins using biotin-azide/streptavidin and their identification with mass spectrometry. This proteomic approach revealed that 154 fatty-acylated proteins were up-regulated, 186 downregulated, and 306 not affected in cells stimulated with 100 ng/ml LPS for 60 min. The acylated proteins affected by LPS were involved in diverse biological functions, as found by Ingenuity Pathway Analysis. Detailed studies of 17ODYA-labeled and immunoprecipitated proteins revealed that LPS induces S-palmitoylation, hence activation, of type II phosphatidylinositol 4-kinase (PI4KII) β, which phosphorylates phosphatidylinositol to phosphatidylinositol 4-monophosphate, a PI(4,5)P₂ precursor. Silencing of PI4KIIβ and PI4KIIα inhibited LPS-induced expression and production of proinflammatory cytokines, especially in the TRIF-dependent signaling pathway of TLR4. Reciprocally, this LPS-induced signaling pathway was significantly enhanced after overexpression of PI4KIIβ or PI4KIIα; this was dependent on palmitoylation of the kinases. However, the S-palmitoylation of PI4KIIα, hence its activity, was constitutive in RAW264 cells. Taken together the data indicate that LPS triggers S-palmitoylation and activation of PI4KIIβ, which generates PI(4)P involved in signaling pathways controlling production of proinflammatory cytokines.

Discovery of caffeic acid phenethyl ester derivatives as novel myeloid differentiation protein 2 inhibitors for treatment of acute lung injury

Myeloid differentiation protein 2 (MD2) is an essential molecule which recognizes lipopolysaccharide (LPS), leading to initiation of inflammation through the activation of Toll-like receptor 4 (TLR4) signaling. Caffeic acid phenethyl ester (CAPE) from propolis of honeybee hives could interfere interactions between LPS and the TLR4/MD2 complex, and thereby has promising anti-inflammatory properties. In this study, we designed and synthesized 48 CAPE derivatives and evaluated their anti-inflammatory activities in mouse primary peritoneal macrophages (MPMs) activated by LPS. The most active compound, 10s, was found to bind with MD2 with high affinity, which prevented formation of the LPS/MD2/TLR4 complex. The binding mode of 10s revealed that the major interactions with MD2 were established via two key hydrogen bonds and hydrophobic interactions. Furthermore, 10s showed remarkable protective effects against LPS-caused ALI (acute lung injury) in vivo. Taken together, this work provides new lead structures and candidates as MD2 inhibitors for the development of anti-inflammatory drugs.

By Capturing Inflammatory Lipids Released from Dying Cells, the Receptor CD14 Induces Inflammasome-Dependent Phagocyte Hyperactivation

A heterogeneous mixture of lipids called oxPAPC, derived from dying cells, can hyperactivate dendritic cells (DCs) but not macrophages. Hyperactive DCs are defined by their ability to release interleukin-1 (IL-1) while maintaining cell viability, endowing these cells with potent aptitude to stimulate adaptive immunity. Herein, we found that the bacterial lipopolysaccharide receptor CD14 captured extracellular oxPAPC and delivered these lipids into the cell to promote inflammasome-dependent DC hyperactivation. Notably, we identified two specific components within the oxPAPC mixture that hyperactivated macrophages, allowing these cells to release IL-1 for several days, by a CD14-dependent process. In murine models of sepsis, conditions that promoted cell hyperactivation resulted in inflammation but not lethality. Thus, multiple phagocytes are capable of hyperactivation in response to oxPAPC, with CD14 acting as the earliest regulator in this process, serving to capture and transport these lipids to promote inflammatory cell fate decisions.

Funiculosin variants and phosphorylated derivatives promote innate immune responses via the Toll-like receptor 4/myeloid differentiation factor-2 complex

The Toll-like receptor 4 (TLR4)/myeloid differentiation factor-2 (MD-2) complex is essential for LPS recognition and induces innate immune responses against Gram-negative bacteria. As activation of TLR4/MD-2 is also critical for the induction of adaptive immune responses, TLR4/MD-2 agonists have been developed as vaccine adjuvants, but their efficacy has not yet been ascertained. Here, we demonstrate that a funiculosin (FNC) variant, FNC-RED, and FNC-RED and FNC derivatives are agonists for both murine and human TLR4/MD-2. FNC-RED induced nuclear factor-κB (NF-κB) activation via murine TLR4/MD-2, whereas FNC had no TLR4/MD-2 stimulatory activity. Biacore analysis revealed that FNC-RED binds to murine TLR4/MD-2 but not murine radioprotective 105 (RP105)/myeloid differentiation factor-1 (MD-1), another LPS sensor. FNC-RED induced CD14-independent expressions of pro-inflammatory cytokines and co-stimulatory molecules in murine macrophages and dendritic cells. In contrast, FNC-RED stimulation was reduced in CD14-dependent LPS responses, including dimerization and internalization of TLR4/MD-2 and IFN-β expression. FNC-RED-induced IL-12p40 production from murine dendritic cells was dependent on NF-κB but not MAPK pathway. In addition, fetal bovine serum augmented lipid A-induced NF-κB activation but blocked FNC-RED-mediated responses. Two synthetic phosphate group-containing FNC-RED and FNC derivatives, FNC-RED-P01 and FNC-P01, respectively, activated human TLR4/MD-2, unlike FNC-RED. Finally, computational analysis revealed that this species-specific activation by FNC-RED and FNC-RED-P01 resulted from differences in electrostatic surface potentials between murine and human TLR4/MD-2. We conclude that FNC-RED and its synthetic derivative represent a novel category of murine and human TLR4/MD-2 agonist.

Spotted Gar and the Evolution of Innate Immune Receptors

The resolution of the gar genome affords an opportunity to examine the diversification and functional specialization of immune effector molecules at a distant and potentially informative point in phylogenetic development. Although innate immunity is effected by a particularly large number of different families of molecules, the focus here is to provide detailed characterization of several families of innate receptors that are encoded in large multigene families, for which orthologous forms can be identified in other species of bony fish but not in other vertebrate groups as well as those for which orthologs are present in other vertebrate species. The results indicate that although teleost fish and the gar, as a holostean reference species, share gene families thought previously to be restricted to the teleost fish, the manner in which the members of the multigene families of innate immune receptors have undergone diversification is different in these two major phylogenetic radiations. It appears that both the total genome duplication and different patterns of genetic selection have influenced the derivation and stabilization of innate immune genes in a substantial manner during the course of vertebrate evolution.

The interaction between farming/rural environment and TLR2, TLR4, TLR6 and CD14 genetic polymorphisms in relation to early- and late-onset asthma

Asthma phenotypes based on age-of-onset may be differently influenced by the interaction between variation in toll-like receptor (TLR)/CD14 genes and environmental microbes. We examined the associations between single-nucleotide polymorphisms (SNP) in the TLR/CD14 genes and asthma, and their interaction with proxies of microbial exposure (childhood farm exposure and childhood rural environment). Ten SNPs in four genes (TLR2, TLR4, TLR6, CD14) were genotyped for 1,116 participants from the Tasmanian Longitudinal Health Study (TAHS). Using prospectively collected information, asthma was classified as never, early- (before 13 years) or late-onset (after 13 years). Information on childhood farm exposure/childhood rural environment was collected at baseline. Those with early-onset asthma were more likely to be males, had a family history of allergy and a personal history of childhood atopy. We found significant interaction between TLR6 SNPs and childhood farm exposure. For those with childhood farm exposure, carriers of the TLR6-rs1039559 T-allele (p-interaction = 0.009) and TLR6-rs5743810 C-allele (p-interaction = 0.02) were associated with lower risk of early-onset asthma. We suggest the findings to be interpreted as hypothesis-generating as the interaction effect did not withstand correction for multiple testing. In this large, population-based longitudinal study, we found that the risk of early- and late-onset asthma is differently influenced by the interaction between childhood farming exposure and genetic variations.

Microbe-inducible trafficking pathways that control Toll-like receptor signaling

The receptors of the mammalian innate immune system are designed for rapid microbial detection, and are located in organelles that are conducive to serve these needs. However, emerging evidence indicates that the sites of microbial detection are not the sites of innate immune signal transduction. Rather, microbial detection triggers the movement of receptors to regions of the cell where factors called sorting adaptors detect active receptors and promote downstream inflammatory responses. These findings highlight the critical role that membrane trafficking pathways play in the initiation of innate immunity to infection. In this review, we describe pathways that promote the microbe-inducible endocytosis of Toll-like receptors (TLRs), and the microbe-inducible movement of TLRs between intracellular compartments. We highlight a new class of proteins called Transporters Associated with the eXecution of Inflammation (TAXI), which have the unique ability to transport TLRs and their microbial ligands to signaling-competent regions of the cell, and we discuss the means by which the subcellular sites of signal transduction are defined.

Reconstruction of LPS Transfer Cascade Reveals Structural Determinants within LBP, CD14, and TLR4-MD2 for Efficient LPS Recognition and Transfer

Lipopolysaccharide (LPS), the major component of the outer membrane of Gram-negative bacteria, binds Toll-like receptor 4 (TLR4)-MD2 complex and activates innate immune responses. LPS transfer to TLR4-MD2 is catalyzed by both LPS binding protein (LBP) and CD14. To define the sequential molecular interactions underlying this transfer, we reconstituted in vitro the entire LPS transfer process from LPS micelles to TLR4-MD2. Using electron microscopy and single-molecule approaches, we characterized the dynamic intermediate complexes for LPS transfer: LBP-LPS micelles, CD14-LBP-LPS micelle, and CD14-LPS-TLR4-MD2 complex. A single LBP molecule bound longitudinally to LPS micelles catalyzed multi-rounds of LPS transfer to CD14s that rapidly dissociated from LPB-LPS complex upon LPS transfer via electrostatic interactions. Subsequently, the single LPS molecule bound to CD14 was transferred to TLR4-MD2 in a TLR4-dependent manner. The definition of the structural determinants of the LPS transfer cascade to TLR4 may enable the development of targeted therapeutics for intervention in LPS-induced sepsis.

Small leucine-rich repeat proteoglycans in corneal inflammation and wound healing

The small leucine rich repeat proteoglycans are major components of the cornea. Lumican, keratocan, decorin, biglycan and osteoglycin are present throughout the adult corneal stroma, and fibromodulin in the peripheral limbal area. In the cornea literature these proteoglycan have been reviewed as structural, collagen fibril-regulating proteins of the cornea. However, these proteoglycans are members of the leucine-rich-repeat superfamily, and share structural similarities with pathogen recognition toll-like receptors. Emerging studies are showing that these have a range of interactions with cell surface receptors, chemokines, growth factors and pathogen associated molecular patterns and are able to regulate host immune response, inflammation and wound healing. This review discusses what is known about their innate immune-related role directly in the cornea, and studies outside the field that find interesting links with innate immune and wound healing responses that are likely to be relevant to the ocular surface. In addition, the review discusses phenotypes of mice with targeted deletion of proteoglycan genes and genetic variants associated with human pathologies.

CD36 Differently Regulates Macrophage Responses to Smooth and Rough Lipopolysaccharide

Lipopolysaccharide (LPS) is the major pathogen-associated molecular pattern of Gram-negative bacterial infections, and includes smooth (S-LPS) and rough (R-LPS) chemotypes. Upon activation by LPS through CD14, TLR4/MD-2 heterodimers sequentially induce two waves of intracellular signaling for macrophage activation: the MyD88-dependent pathway from the plasma membrane and, following internalization, the TRIF-dependent pathway from endosomes. We sought to better define the role of scavenger receptors CD36 and CD204/SR-A as accessory LPS receptors that can contribute to pro-inflammatory and microbicidal activation of macrophages. We have found that CD36 differently regulates activation of mouse macrophages by S-LPS versus R-LPS. The ability of CD36 to substitute for CD14 in loading R-LPS, but not S-LPS onto TLR4/MD-2 allows CD14-independent macrophage responses to R-LPS. Conversely, S-LPS, but not R-LPS effectively stimulates CD14 binding to CD36, which favors S-LPS transfer from CD14 onto TLR4/MD-2 under conditions of low CD14 occupancy with S-LPS in serum-free medium. In contrast, in the presence of serum, CD36 reduces S-LPS binding to TLR4/MD-2 and the subsequent MyD88-dependent signaling, by mediating internalization of S-LPS/CD14 complexes. Additionally, CD36 positively regulates activation of TRIF-dependent signaling by both S-LPS and R-LPS, by promoting TLR4/MD-2 endocytosis. In contrast, we have found that SR-A does not function as a S-LPS receptor. Thus, by co-operating with CD14 in both R- and S-LPS loading onto TLR4/MD-2, CD36 can enhance the sensitivity of tissue-resident macrophages in detecting infections by Gram-negative bacteria. However, in later phases, following influx of serum to the infection site, the CD36-mediated negative regulation of MyD88-dependent branch of S-LPS-induced TLR4 signaling might constitute a mechanism to prevent an excessive inflammatory response, while preserving the adjuvant effect of S-LPS for adaptive immunity.

TLR4/MD-2 activation by a synthetic agonist with no similarity to LPS

Structurally disparate molecules reportedly engage and activate Toll-like receptor (TLR) 4 and other TLRs, yet the interactions that mediate binding and activation by dissimilar ligands remain unknown. We describe Neoseptins, chemically synthesized peptidomimetics that bear no structural similarity to the established TLR4 ligand, lipopolysaccharide (LPS), but productively engage the mouse TLR4 (mTLR4)/myeloid differentiation factor 2 (MD-2) complex. Neoseptin-3 activates mTLR4/MD-2 independently of CD14 and triggers canonical myeloid differentiation primary response gene 88 (MyD88)- and Toll-interleukin 1 receptor (TIR) domain-containing adaptor inducing IFN-beta (TRIF)-dependent signaling. The crystal structure mTLR4/MD-2/Neoseptin-3 at 2.57-Å resolution reveals that Neoseptin-3 binds as an asymmetrical dimer within the hydrophobic pocket of MD-2, inducing an active receptor complex similar to that induced by lipid A. However, Neoseptin-3 and lipid A form dissimilar molecular contacts to achieve receptor activation; hence strong TLR4/MD-2 agonists need not mimic LPS.

Therapeutic Developments Targeting Toll-like Receptor-4-Mediated Neuroinflammation

Toll-like receptors (TLRs) have been shown to play an important role in the immune system, which warrants study of their remarkable potential as pharmacological targets. Activation of TLRs requires participation from specific pathogen-associated molecular patterns (PAMPs) and accessory proteins such as myeloid differentiation protein 2 (MD2), lipopolysaccharide binding protein (LBP), and cluster differentiation antigen 14 (CD14). Assembly of the TLR4-MD2-LPS complex is essential in TLR4 activation. Recent studies have revealed that TLR4 activation is a significant trigger of signal transmission pathways in the nervous system, which could result in chronic pain as well as opioid tolerance and dependence. Researchers of the molecular structure of TLRs and their accessory proteins have opened a door to syntheses of TLRs agonists and antagonists, such as eritoran. Small-molecule modulators of TLR4, such as MD2-I and tricyclic antidepressants, offer more promising prospects than peptides, given their convenience in oral administration and lower cost. Herein we mainly discuss the mechanisms and clinical prospects of TLR4 agonists and antagonists.

Mechanisms of Toll-like Receptor 4 Endocytosis Reveal a Common Immune-Evasion Strategy Used by Pathogenic and Commensal Bacteria

Microbe-induced receptor trafficking has emerged as an essential means to promote innate immune signal transduction. Upon detection of bacterial lipopolysaccharides (LPS), CD14 induces an inflammatory endocytosis pathway that delivers Toll-like receptor 4 (TLR4) to endosomes. Although several regulators of CD14-dependent TLR4 endocytosis have been identified, the cargo-selection mechanism during this process remains unknown. We reveal that, in contrast to classic cytosolic interactions that promoted the endocytosis of transmembrane receptors, TLR4 was selected as cargo for inflammatory endocytosis entirely through extracellular interactions. Mechanistically, the extracellular protein MD-2 bound to and dimerized TLR4 in order to promote this endocytic event. Our analysis of LPS variants from human pathogens and gut commensals revealed a common mechanism by which bacteria prevent inflammatory endocytosis. We suggest that evasion of CD14-dependent endocytosis is an attribute that transcends the concept of pathogenesis and might be a fundamental feature of bacteria that inhabit eukaryotic hosts.

The small leucine-rich repeat proteoglycans in tissue repair and atherosclerosis

Proteoglycans consist of a protein core with one or more covalently attached glycosaminoglycan (GAG) side chains and have multiple roles in the initiation and progression of atherosclerosis. Here we discuss the potential and known functions of a group of small leucine-rich repeat proteoglycans (SLRPs) in atherosclerosis. We focus on five SLRPs, decorin, biglycan, lumican, fibromodulin and PRELP, because these have been detected in atherosclerotic plaques or demonstrated to have a role in animal models of atherosclerosis. Decorin and biglycan are modified post-translationally by substitution with chondroitin/dermatan sulphate GAGs, whereas lumican, fibromodulin and PRELP have keratan sulphate side chains, and the core proteins have leucine-rich repeat (LRR) motifs that are characteristic of the LRR superfamily. The chondroitin/dermatan sulphate GAG side chains have been implicated in lipid retention in atherosclerosis. The core proteins are discussed here in the context of (i) interactions with collagens and their implications in tissue integrity, fibrosis and wound repair and (ii) interactions with growth factors, cytokines, pathogen-associated molecular patterns and cell surface receptors that impact normal physiology and disease processes such as inflammation, innate immune responses and wound healing (i.e. processes that are all important in plaque development and progression). Thus, studies of these SLRPs in the context of wound healing are providing clues about their functions in early stages of atherosclerosis to plaque vulnerability and cardiovascular disease at later stages. Understanding of signal transduction pathways regulated by the core protein interactions is leading to novel roles and therapeutic potential for these proteins in wound repair and atherosclerosis.

Comparative Geometrical Analysis of Leucine-Rich Repeat Structures in the Nod-Like and Toll-Like Receptors in Vertebrate Innate Immunity

The NOD-like receptors (NLRs) and Toll-like receptors (TLRs) are pattern recognition receptors that are involved in the innate, pathogen pattern recognition system. The TLR and NLR receptors contain leucine-rich repeats (LRRs) that are responsible for ligand interactions. In LRRs short β-strands stack parallel and then the LRRs form a super helical arrangement of repeating structural units (called a coil of solenoids). The structures of the LRR domains of NLRC4, NLRP1, and NLRX1 in NLRs and of TLR1-5, TLR6, TLR8, TLR9 in TLRs have been determined. Here we report nine geometrical parameters that characterize the LRR domains; these include four helical parameters from HELFIT analysis. These nine parameters characterize well the LRR structures in NLRs and TLRs; the LRRs of NLR adopts a right-handed helix. In contrast, the TLR LRRs adopt either a left-handed helix or are nearly flat; RP105 and CD14 also adopt a left-handed helix. This geometrical analysis subdivides TLRs into four groups consisting of TLR3/TLR8/TLR9, TLR1/TLR2/TRR6, TLR4, and TLR5; these correspond to the phylogenetic tree based on amino acid sequences. In the TLRs an ascending lateral surface that consists of loops connecting the β-strand at the C-terminal side is involved in protein, protein/ligand interactions, but not the descending lateral surface on the opposite side.

Unique Toll-Like Receptor 4 Activation by NAMPT/PBEF Induces NFκB Signaling and Inflammatory Lung Injury

Ventilator-induced inflammatory lung injury (VILI) is mechanistically linked to increased NAMPT transcription and circulating levels of nicotinamide phosphoribosyl-transferase (NAMPT/PBEF). Although VILI severity is attenuated by reduced NAMPT/PBEF bioavailability, the precise contribution of NAMPT/PBEF and excessive mechanical stress to VILI pathobiology is unknown. We now report that NAMPT/PBEF induces lung NFκB transcriptional activities and inflammatory injury via direct ligation of Toll-like receptor 4 (TLR4). Computational analysis demonstrated that NAMPT/PBEF and MD-2, a TLR4-binding protein essential for LPS-induced TLR4 activation, share ~30% sequence identity and exhibit striking structural similarity in loop regions critical for MD-2-TLR4 binding. Unlike MD-2, whose TLR4 binding alone is insufficient to initiate TLR4 signaling, NAMPT/PBEF alone produces robust TLR4 activation, likely via a protruding region of NAMPT/PBEF (S402-N412) with structural similarity to LPS. The identification of this unique mode of TLR4 activation by NAMPT/PBEF advances the understanding of innate immunity responses as well as the untoward events associated with mechanical stress-induced lung inflammation.

Outer membrane vesicles of Tannerella forsythia: biogenesis, composition, and virulence

Tannerella forsythia is the only ‘red‐complex’ bacterium covered by an S‐layer, which has been shown to affect virulence. Here, outer membrane vesicles (OMVs) enriched with putative glycoproteins are described as a new addition to the virulence repertoire of T. forsythia. Investigations of this bacterium are hampered by its fastidious growth requirements and the recently discovered mismatch of the available genome sequence (92A2 = ATCC BAA‐2717) and the widely used T. forsythia strain (ATCC 43037). T. forsythia was grown anaerobically in serum‐free medium and biogenesis of OMVs was analyzed by electron and atomic force microscopy. This revealed OMVs with a mean diameter of ~100 nm budding off from the outer membrane while retaining the S‐layer. An LC‐ESI‐TOF/TOF proteomic analysis of OMVs from three independent biological replicates identified 175 proteins. Of these, 14 exhibited a C‐terminal outer membrane translocation signal that directs them to the cell/vesicle surface, 61 and 53 were localized to the outer membrane and periplasm, respectively, 22 were predicted to be extracellular, and 39 to originate from the cytoplasm. Eighty proteins contained the Bacteroidales O‐glycosylation motif, 18 of which were confirmed as glycoproteins. Release of pro‐inflammatory mediators from the human monocytic cell line U937 and periodontal ligament fibroblasts upon stimulation with OMVs followed a concentration‐dependent increase that was more pronounced in the presence of soluble CD14 in conditioned media. The inflammatory response was significantly higher than that caused by whole T. forsythia cells. Our study represents the first characterization of T. forsythia OMVs, their proteomic composition and immunogenic potential.

Vasopressin inhibits endotoxin binding in activated macrophages

BACKGROUND

Vasopressin possesses potent anti-inflammatory effects. Endotoxin recognition (mediated by cluster of differentiation 14 [CD14]), endotoxin binding, and subsequent nuclear factor-κB (NF-κB) activation are essential mechanisms for initiation of the inflammatory response. We elucidated the effects of vasopressin on these essential mechanisms of inflammation with the hypothesis that vasopressin could inhibit CD14 expression, endotoxin binding, and NF-κB activation in activated macrophages.

METHODS

Murine macrophage-like cell line RAW264.7 cells were stimulated with endotoxin (lipopolysaccharide [LPS]; 100 ng/mL) or LPS plus vasopressin (1000 pg/mL; designated as the LPS and the LPS + V groups, respectively). After reaction, between-group differences in inflammatory molecule concentrations and levels of NF-κB activation, endotoxin-macrophages binding, and CD14 expression were compared. Analysis of variance was performed for statistical analysis.

RESULTS

The concentrations of chemokine macrophage inflammatory protein 2 and cytokine interleukin 6 of the LPS + V group were significantly lower than those of the LPS group (P = 0.004 and P < 0.001). The nuclear concentration of phosphorylated NF-κB p65 and cytosolic concentration of phosphorylated inhibitor-κBα of the LPS + V group were significantly lower than those of the LPS group (all P < 0.05). In addition, the level of endotoxin-macrophages binding of the LPS + V group was significantly lower than that of the LPS group (P < 0.001). The level of surface CD14 expression of the LPS + V group was also significantly lower than that of the LPS group (P = 0.019).

CONCLUSIONS

This study confirmed the potent anti-inflammatory effects of vasopressin. The mechanisms underlying the anti-inflammatory effects of vasopressin may involve its effects on inhibiting CD14 expression, endotoxin binding, and subsequent NF-κB activation.

Microglial Heparan Sulfate Proteoglycans Facilitate the Cluster-of-Differentiation 14 (CD14)/Toll-like Receptor 4 (TLR4)-Dependent Inflammatory Response

Microglia rapidly mount an inflammatory response to pathogens in the central nervous system (CNS). Heparan sulfate proteoglycans (HSPGs) have been attributed various roles in inflammation. To elucidate the relevance of microglial HSPGs in a pro-inflammatory response we isolated microglia from mice overexpressing heparanase (Hpa-tg), the HS-degrading endoglucuronidase, and challenged them with lipopolysaccharide (LPS), a bacterial endotoxin. Prior to LPS-stimulation, the LPS-receptor cluster-of-differentiation 14 (CD14) and Toll-like receptor 4 (TLR4; essential for the LPS response) were similarly expressed in Ctrl and Hpa-tg microglia. However, compared with Ctrl microglia, Hpa-tg cells released significantly less tumor necrosis factor-α (TNFα), essentially failed to up-regulate interleukin-1β (IL1β) and did not initiate synthesis of proCD14. Isolated primary astroyctes expressed TLR4, but notably lacked CD14 and in contrast to microglia, LPS challenge induced a similar TNFα response in Ctrl and Hpa-tg astrocytes, while neither released IL1β. The astrocyte TNFα-induction was thus attributed to CD14-independent TLR4 activation and was unaffected by the cells HS status. Equally, the suppressed LPS-response in Hpa-tg microglia indicated a loss of CD14-dependent TLR4 activation, suggesting that microglial HSPGs facilitate this process. Indeed, confocal microscopy confirmed interactions between microglial HS and CD14 in LPS-stimulated microglia and a potential HS-binding motif in CD14 was identified. We conclude that microglial HSPGs facilitate CD14-dependent TLR4 activation and that heparanase can modulate this mechanism.

Structural biology of innate immunity

Innate immune responses depend on timely recognition of pathogenic or danger signals by multiple cell surface or cytoplasmic receptors and transmission of signals for proper counteractions through adaptor and effector molecules. At the forefront of innate immunity are four major signaling pathways, including those elicited by Toll-like receptors, RIG-I-like receptors, inflammasomes, or cGAS, each with its own cellular localization, ligand specificity, and signal relay mechanism. They collectively engage a number of overlapping signaling outcomes, such as NF-κB activation, interferon response, cytokine maturation, and cell death. Several proteins often assemble into a supramolecular complex to enable signal transduction and amplification. In this article, we review the recent progress in mechanistic delineation of proteins in these pathways, their structural features, modes of ligand recognition, conformational changes, and homo- and hetero-oligomeric interactions within the supramolecular complexes. Regardless of seemingly distinct interactions and mechanisms, the recurring themes appear to consist of autoinhibited resting-state receptors, ligand-induced conformational changes, and higher-order assemblies of activated receptors, adaptors, and signaling enzymes through conserved protein-protein interactions.

Innate immune pattern recognition: a cell biological perspective

Receptors of the innate immune system detect conserved determinants of microbial and viral origin. Activation of these receptors initiates signaling events that culminate in an effective immune response. Recently, the view that innate immune signaling events rely on and operate within a complex cellular infrastructure has become an important framework for understanding the regulation of innate immunity. Compartmentalization within this infrastructure provides the cell with the ability to assign spatial information to microbial detection and regulate immune responses. Several cell biological processes play a role in the regulation of innate signaling responses; at the same time, innate signaling can engage cellular processes as a form of defense or to promote immunological memory. In this review, we highlight these aspects of cell biology in pattern-recognition receptor signaling by focusing on signals that originate from the cell surface, from endosomal compartments, and from within the cytosol.

Immunity genes and susceptibility to otitis media: a comprehensive review

Otitis media (OM) is a middle ear infection associated with inflammation and pain. This disease frequently afflicts humans and is the major cause of hearing loss worldwide. OM continues to be one of the most challenging diseases in the medical field due to its diverse host targets and wide range of clinical manifestations. Substantial morbidity associated with OM is further exacerbated by high frequency of recurrent infections leading to chronic suppurative otitis media (CSOM). Children have greater susceptibility to, and thus, suffer most frequently from OM, which can cause significant deterioration in quality of life. Genetic factors have been demonstrated, in large part by twin and family studies, to be key determinants of OM susceptibility. In this review, we summarize the current knowledge on immunity genes and selected variants that have been associated with predisposition to OM. In particular, polymorphisms in innate immunity and cytokine genes have been strongly linked with the risk of developing OM. Future studies employing state-of-the-art technologies, including next-generation sequencing (NGS), will aid in the identification of novel genes associated with susceptibility to OM. This, in turn, will open up avenues for identifying high-risk individuals and designing novel therapeutic strategies based on precise targeting of these genes.

Co-operation of TLR4 and raft proteins in LPS-induced pro-inflammatory signaling

Toll-like receptor 4 (TLR4) is activated by lipopolysaccharide (LPS), a component of Gram-negative bacteria to induce production of pro-inflammatory mediators aiming at eradication of the bacteria. Dysregulation of the host responses to LPS can lead to a systemic inflammatory condition named sepsis. In a typical scenario, activation of TLR4 is preceded by binding of LPS to CD14 protein anchored in cholesterol- and sphingolipid-rich microdomains of the plasma membrane called rafts. CD14 then transfers the LPS to the TLR4/MD-2 complex which dimerizes and triggers MyD88- and TRIF-dependent production of pro-inflammatory cytokines and type I interferons. The TRIF-dependent signaling is linked with endocytosis of the activated TLR4, which is controlled by CD14. In addition to CD14, other raft proteins like Lyn tyrosine kinase of the Src family, acid sphingomyelinase, CD44, Hsp70, and CD36 participate in the TLR4 signaling triggered by LPS and non-microbial endogenous ligands. In this review, we summarize the current state of the knowledge on the involvement of rafts in TLR4 signaling, with an emphasis on how the raft proteins regulate the TLR4 signaling pathways. CD14-bearing rafts, and possibly CD36-rich rafts, are believed to be preferred sites of the assembly of a multimolecular complex which mediates the endocytosis of activated TLR4.

Trehalose- and glucose-derived glycoamphiphiles: small-molecule and nanoparticle Toll-like receptor 4 (TLR4) modulators

An increasing number of pathologies have been linked to Toll-like receptor 4 (TLR4) activation and signaling, therefore new hit and lead compounds targeting this receptor activation process are urgently needed. We report on the synthesis and biological properties of glycolipids based on glucose and trehalose scaffolds which potently inhibit TLR4 activation and signaling in vitro and in vivo. Structure-activity relationship studies on these compounds indicate that the presence of fatty ester chains in the molecule is a primary prerequisite for biological activity and point to facial amphiphilicity as a preferred architecture for TLR4 antagonism. The cationic glycolipids here presented can be considered as new lead compounds for the development of drugs targeting TLR4 activation and signaling in infectious, inflammatory, and autoimmune diseases. Interestingly, the biological activity of the best drug candidate was retained after adsorption at the surface of colloidal gold nanoparticles, broadening the options for clinical development.

Fungal glycans and the innate immune recognition

Polysaccharides such as α- and β-glucans, chitin, and glycoproteins extensively modified with both N- and O-linked carbohydrates are the major components of fungal surfaces. The fungal cell wall is an excellent target for the action of antifungal agents, since most of its components are absent from mammalian cells. Recognition of these carbohydrate-containing molecules by the innate immune system triggers inflammatory responses and activation of microbicidal mechanisms by leukocytes. This review will discuss the structure of surface fungal glycoconjugates and polysaccharides and their recognition by innate immune receptors.

Pterostilbene attenuates lipopolysaccharide-induced learning and memory impairment possibly via inhibiting microglia activation and protecting neuronal injury in mice

The present study aims to evaluate the effects of pterostilbene on lipopolysaccharide (LPS)-induced learning and memory impairment as well as the possible changes of microglia and neurons. Firstly, learning and memory function was investigated by behavioral tests. Pterostilbene attenuated LPS-induced learning and memory impairment tested by Y-maze and Morris water maze. Secondly, immunohistochemical method was used to study the changes of microglia and neurons. The results showed that pterostilbene produced a significant decrease in the number of Iba-1 and Doublecortin (DCX) positive cells and a significant increase in neuronal nuclear antigen (NeuN)-stained area of neurons in mouse hippocampal compared to the LPS group. Finally, an in vitro study was performed to further confirm the inhibitory effect on microglia activation and protective effect on neurons exerted by pterostilbene. The results demonstrated that pterostilbene significantly inhibited microglia activation, showing the obvious decrease of LPS-induced production of NO, TNF-α and IL-6 in N9 microglial cells. In addition, the viability of SH-SY5Y cells decreased by conditioned media of LPS-activated N9 microglial cells was remarkably recovered by pterostilbene. In summary, the present study demonstrated for the first time that pterostilbene attenuated LPS-induced learning and memory impairment, which may be associated with its inhibitory effect on microglia activation and protective effect on neuronal injury.

NMR-based structural analysis of the complete rough-type lipopolysaccharide isolated from Capnocytophaga canimorsus

We here describe the NMR analysis of an intact lipopolysaccharide (LPS, endotoxin) in water with 1,2-dihexanoyl-sn-glycero-3-phosphocholine as detergent. When HPLC-purified rough-type LPS of Capnocytophaga canimorsus was prepared, (13)C,(15)N labeling could be avoided. The intact LPS was analyzed by homonuclear ((1)H) and heteronuclear ((1)H,(13)C, and (1)H,(31)P) correlated one- and two-dimensional NMR techniques as well as by mass spectrometry. It consists of a penta-acylated lipid A with an α-linked phosphoethanolamine attached to C-1 of GlcN (I) in the hybrid backbone, lacking the 4'-phosphate. The hydrophilic core oligosaccharide was found to be a complex hexasaccharide with two mannose (Man) and one each of 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo), Gal, GalN, and l-rhamnose residues. Position 4 of Kdo is substituted by phosphoethanolamine, also present in position 6 of the branched Man(I) residue. This rough-type LPS is exceptional in that all three negative phosphate residues are "masked" by positively charged ethanolamine substituents, leading to an overall zero net charge, which has so far not been observed for any other LPS. In biological assays, the corresponding isolated lipid A was found to be endotoxically almost inactive. By contrast, the intact rough-type LPS described here expressed a 20,000-fold increased endotoxicity, indicating that the core oligosaccharide significantly contributes to the endotoxic potency of the whole rough-type C. canimorsus LPS molecule. Based on these findings, the strict view that lipid A alone represents the toxic center of LPS needs to be reassessed.

VB-201, an oxidized phospholipid small molecule, inhibits CD14- and Toll-like receptor-2-dependent innate cell activation and constrains atherosclerosis

Atherosclerosis is an inflammatory disease of the vascular wall. Activated monocytes and dendritic cells (DC) in the intima layer of the vasculature promote atherogenesis. Toll-like receptor (TLR)-2 and TLR-4, which are predominantly expressed on these cells and mediate their activation, are essential for atherosclerosis development. In this study we demonstrate that VB-201, an oxidized phospholipid (Ox-PL) small molecule, inhibits TLR signalling restricted to TLR-2 and TLR-4 in human and mouse monocytes and DC. Mechanistically, we show that VB-201 binds directly to TLR-2 and CD14, the TLR-4 co-receptor, to impair downstream cues and cytokine production. In a rabbit model, oral administration of VB-201 constrained atherosclerosis progression. This effect was not due to reduced cholesterol abundance, as hyperlipidaemia was sustained. We suggest that VB-201 may counter inflammation where TLR-2 and/or CD14 complicity is essential, and is therefore beneficial for the treatment of atherosclerosis.

Ubiquitin ligase Cbl-b and obesity-induced insulin resistance

Obesity causes type 2 diabetes, atherosclerosis and cardiovascular diseases by inducing systemic insulin resistance. It is now recognized that obesity is related to chronic low-grade inflammation in adipose tissue. Specifically, activated immune cells infiltrate adipose tissue and cause inflammation. There is increasing evidence that activated macrophages accumulate in the hypertrophied adipose tissue of rodents and humans and induce systemic insulin resistance by secreting inflammatory cytokines. Accordingly, a better understanding of the molecular mechanisms underlying macrophage activation in adipose tissue will facilitate the development of new therapeutic strategies. Currently, little is known about the regulation of macrophage activation, although E3 ubiquitin ligase Casitas B-lineage lymphoma (Cbl)-b was identified recently as a novel negative regulator of macrophage activation in adipose tissue. Cbl-b, which is a suppressor of T- and B-cell activation, inhibits intracellular signal transduction by targeting some tyrosine kinases. Notably, preventing Cbl-b-mediated macrophage activation improves obesity-induced insulin resistance in mice. c-Cbl is another member of the Cbl family that is associated with insulin resistance in obesity. These reports suggest that Cbl-b and c-Cbl are potential therapeutic targets for treating obesity-induced insulin resistance. In this review, we focus on the importance of Cbl-b in macrophage activation in aging-induced and high-fat diet-induced obesity.

CD14 mediates binding of high doses of LPS but is dispensable for TNF-α production

Activation of macrophages with lipopolysaccharide (LPS) involves a sequential engagement of serum LPS-binding protein (LBP), plasma membrane CD14, and TLR4/MD-2 signaling complex. We analyzed participation of CD14 in TNF-α production stimulated with 1-1000 ng/mL of smooth or rough LPS (sLPS or rLPS) and in sLPS binding to RAW264 and J744 cells. CD14 was indispensable for TNF-α generation induced by a low concentration, 1 ng/mL, of sLPS and rLPS. At higher doses of both LPS forms (100-1000 ng/mL), TNF-α release required CD14 to much lower extent. Among the two forms of LPS, rLPS-induced TNF-α production was less CD14-dependent and could proceed in the absence of serum as an LBP source. On the other hand, the involvement of CD14 was crucial for the binding of 1000 ng/mL of sLPS judging from an inhibitory effect of the anti-CD14 antibody. The binding of sLPS was also strongly inhibited by dextran sulfate, a competitive ligand of scavenger receptors (SR). In the presence of dextran sulfate, sLPS-induced production of TNF-α was upregulated about 1.6-fold. The data indicate that CD14 together with SR participates in the binding of high doses of sLPS. However, CD14 contribution to TNF α production induced by high concentrations of sLPS and rLPS can be limited.

Recognition of lipopolysaccharide pattern by TLR4 complexes

Lipopolysaccharide (LPS) is a major component of the outer membrane of Gram-negative bacteria. Minute amounts of LPS released from infecting pathogens can initiate potent innate immune responses that prime the immune system against further infection. However, when the LPS response is not properly controlled it can lead to fatal septic shock syndrome. The common structural pattern of LPS in diverse bacterial species is recognized by a cascade of LPS receptors and accessory proteins, LPS binding protein (LBP), CD14 and the Toll-like receptor4 (TLR4)–MD-2 complex. The structures of these proteins account for how our immune system differentiates LPS molecules from structurally similar host molecules. They also provide insights useful for discovery of anti-sepsis drugs. In this review, we summarize these structures and describe the structural basis of LPS recognition by LPS receptors and accessory proteins.

Chimeric anti-CD14 IGG2/4 Hybrid antibodies for therapeutic intervention in pig and human models of inflammation

CD14 is a key recognition molecule of innate immune responses, interacting with several TLRs. TLR signaling cross-talks extensively with the complement system, and combined CD14 and complement inhibition has been proved effective in attenuating inflammatory responses. Pig models of human diseases have emerged as valuable tools to study therapeutic intervention, but suitable neutralizing Abs are rare. Undesired Fc-mediated functions, such as platelet activation and IL-8 release induced by the porcine CD14-specific clone Mil2, limit further studies. Therefore, an inert human IgG2/IgG4 hybrid C region was chosen for an rMil2. As revealed in ex vivo and in vivo pig experiments, rMil2 inhibited the CD14-mediated proinflammatory cytokine response similar to the original clone, but lacked the undesired Fc-effects, and inflammation was attenuated further by simultaneous complement inhibition. Moreover, rMil2 bound porcine FcRn, a regulator of t1/2 and biodistribution. Thus, rMil2, particularly combined with complement inhibitors, should be well suited for in vivo studies using porcine models of diseases, such as sepsis and ischemia-reperfusion injury. Similarly, the recombinant anti-human CD14 IgG2/4 Ab, r18D11, was generated with greatly reduced Fc-mediated effects and preserved inhibitory function ex vivo. Such Abs might be drug candidates for the treatment of innate immunity-mediated human diseases.

The antiviral activity of poly-γ-glutamic acid, a polypeptide secreted by Bacillus sp., through induction of CD14-dependent type I interferon responses

Poly-γ-glutamic acid (γ-PGA) is an anionic polypeptide secreted by Bacillus sp. that has been shown to activate immune cells through interactions with toll-like receptor 4 (TLR4). However, its ability to induce the type I interferon (IFN) response has not yet been characterized. Here, we demonstrate that γ-PGA induces type I IFN signaling pathway via the TLR4 signaling pathway. The induction required both myeloid differentiation factor 2 (MD2) and the pattern-recognition receptor CD14, which are two TLR4-associated accessory proteins. The γ-PGA with high molecular weights (2000 and 5000 kDa) was able to activate the subsequent signals through TLR4/MD2 to result in dimerization of IRF-3, a transcription factor required for IFN gene expression, leading to increases in mRNA levels of the type I IFN-response genes, 2′–5′ OAS and ISG56. Moreover, γ-PGA (2000 kDa) displayed an antiviral activity against SARS coronavirus and hepatitis C virus. Our results identify high-molecular weight γ-PGA as a TLR4 ligand and demonstrate that γ-PGA requires both CD14 and MD2 for the activation of type I IFN responses. Our results suggest that the microbial biopolymer γ-PGA may have therapeutic potential against a broad range of viruses sensitive to type I IFNs.

Evolutionary analysis of TLR9 genes reveals the positive selection of extant teleosts in Perciformes

The innate immune system can recognize non-self through pattern recognition receptors. Toll-like receptors were the best-known members of these receptors, and they could sense, recognize, and bind pathogen-associated molecular patterns. TLRs played an important role in innate immune system and were conserved in both invertebrate and vertebrate lineages. Thereinto, TLR9 could detect unmethylated CpG motifs in dsDNA and was expected to undergo coevolution with its microbial ligands. It was known that aquatic and terrestrial organisms dwelled in different environments which contained different pathogens, and they had to adapt to their local environmental conditions. Therefore, we collected TLR9 genes from invertebrate to vertebrate to further explore whether the huge differences between aquatic and terrestrial environments affected the TLR9s evolution between aquatic and terrestrial organisms. Molecular evolution analysis detected positively selected sites in the ancestral lineages of vertebrates, teleosts, and Perciformes but not in the ancestral lineage of mammals. In PAML, site model revealed that extant mammalian TLR9 genes underwent positive selection. However, the positive selection of extant teleosts appeared primarily in Perciformes in which there were 14 positively selected sites. Among these sites, two of them were located on the amino acid insertions of the leucine-rich repeats which could create DNA binding sites, three were found on the convex surface which might possibly affect the flexibility of the TLR solenoids, and six were located on the β-face of concave surface which contained the ligand-binding sites of the TLR solenoids. In other ML methods, we also found three sites under selection that coincided with the codons identified by M8 and these sites were all located in LRRs. The diverse aquatic and terrestrial environments might possess different pathogens to make the living organisms adapt to their local environmental conditions. The positive selection on LRRs in TLR9s of Perciformes might be associated with the adaptation to the rapidly evolving pathogens in the water.

The N-terminus of CD14 acts to bind apoptotic cells and confers rapid-tethering capabilities on non-myeloid cells

Cell death and removal of cell corpses in a timely manner is a key event in both physiological and pathological situations including tissue homeostasis and the resolution of inflammation. Phagocytic clearance of cells dying by apoptosis is a complex sequential process comprising attraction, recognition, tethering, signalling and ultimately phagocytosis and degradation of cell corpses. A wide range of molecules acting as apoptotic cell-associated ligands, phagocyte-associated receptors or soluble bridging molecules have been implicated within this process. The role of myeloid cell CD14 in mediating apoptotic cell interactions with macrophages has long been known though key molecules and residues involved have not been defined. Here we sought to further dissect the function of CD14 in apoptotic cell clearance. A novel panel of THP-1 cell-derived phagocytes was employed to demonstrate that CD14 mediates effective apoptotic cell interactions with macrophages in the absence of detectable TLR4 whilst binding and responsiveness to LPS requires TLR4. Using a targeted series of CD14 point mutants expressed in non-myeloid cells we reveal CD14 residue 11 as key in the binding of apoptotic cells whilst other residues are reported as key for LPS binding. Importantly we note that expression of CD14 in non-myeloid cells confers the ability to bind rapidly to apoptotic cells. Analysis of a panel of epithelial cells reveals that a number naturally express CD14 and that this is competent to mediate apoptotic cell clearance. Taken together these data suggest that CD14 relies on residue 11 for apoptotic cell tethering and it may be an important tethering molecule on so called 'non-professional' phagocytes thus contributing to apoptotic cell clearance in a non-myeloid setting. Furthermore these data establish CD14 as a rapid-acting tethering molecule, expressed in monocytes, which may thus confer responsiveness of circulating monocytes to apoptotic cell derived material.

Role of CD14 in host protection against infections and in metabolism regulation

CD14 is a glycosylphosphatidylinositol (GPI)-anchored receptor known to serve as a co-receptor for several Toll-like Receptors (TLRs) both at the cell surface and in the endosomal compartment. CD14 can be expressed by cells of both hematopoietic and non-hematopoietic origin as a cell membrane or secreted protein. Although CD14 was discovered more than 20 years ago, its activities remain largely to be defined. Most of the information available concerns CD14's role as a co-receptor working with TLR4 and facilitating cellular responses to low doses of lipopolysaccharide (LPS). Recent studies have highlighted and molecularly defined many other functions of this pattern recognition receptor (PRR). These functions include the mechanisms through which CD14 allows the activation of the TLR4-TRAM-TRIF pathway upon LPS stimulation; the capacity of CD14 to transduce a TLR4-independent signaling pathway leading to the activation of NFAT transcription factor family members with important consequences in myeloid cells; the CD14 influence on cell metabolism in conditions predisposing to obesity. In this review, we summarize recent progresses toward the molecular definition of the multiple roles exerted by CD14 in innate immune cells in response to LPS and the consequences of CD14 activation in physiologic and pathologic conditions.

Signaling of high mobility group box 1 (HMGB1) through toll-like receptor 4 in macrophages requires CD14

High mobility group box 1 (HMGB1) is a DNA-binding protein that possesses cytokinelike, proinflammatory properties when released extracellularly in the C23-C45 disulfide form. HMGB1 also plays a key role as a mediator of acute and chronic inflammation in models of sterile injury. Although HMGB1 interacts with multiple pattern recognition receptors (PRRs), many of its effects in injury models occur through an interaction with toll-like receptor 4 (TLR4). HMGB1 interacts directly with the TLR4/myeloid differentiation protein 2 (MD2) complex, although the nature of this interaction remains unclear. We demonstrate that optimal HMGB1-dependent TLR4 activation in vitro requires the coreceptor CD14. TLR4 and MD2 are recruited into CD14-containing lipid rafts of RAW264.7 macrophages after stimulation with HMGB1, and TLR4 interacts closely with the lipid raft protein GM1. Furthermore, we show that HMGB1 stimulates tumor necrosis factor (TNF)-α release in WT but not in TLR4(-/-), CD14(-/-), TIR domain-containing adapter-inducing interferon-β (TRIF)(-/-) or myeloid differentiation primary response protein 88 (MyD88)(-/-) macrophages. HMGB1 induces the release of monocyte chemotactic protein 1 (MCP-1), interferon gamma-induced protein 10 (IP-10) and macrophage inflammatory protein 1α (MIP-1α) in a TLR4- and CD14-dependent manner. Thus, efficient recognition of HMGB1 by the TLR4/MD2 complex requires CD14.

CD14 protein acts as an adaptor molecule for the immune recognition of Salmonella curli fibers

Amyloids, protein aggregates with a cross β-sheet structure, contribute to inflammation in debilitating disorders, including Alzheimer's disease. Enteric bacteria also produce amyloids, termed curli, contributing to inflammation during infection. It has been demonstrated that curli and β-amyloid are recognized by the immune system via the Toll-like receptor (TLR) 2/TLR1 complex. Here we investigated the role of CD14 in the immune recognition of bacterial amyloids. We used HeLa 57A cells, a human cervical cancer cell line containing a luciferase reporter gene under the control of an NF-κB promoter. When HeLa 57A cells were transiently transfected with combinations of human expression vectors containing genes for TLR2, TLR1, and CD14, membrane-bound CD14 enhanced NF-κB activation through the TLR2/TLR1 complex stimulated with curli fibers or recombinant CsgA, the curli major subunit. Similarly, soluble CD14 augmented the TLR2/TLR1 response to curli fibers in the absence of membrane-bound CD14. We further revealed that IL-6 and nitric oxide production were significantly higher by wild-type (C57BL/6) bone marrow-derived macrophages compared with TLR2-deficient or CD14-deficient bone marrow-derived macrophages when stimulated with curli fibers, recombinant CsgA, or synthetic CsgA peptide, CsgA-R4-5. Binding assays demonstrated that recombinant TLR2, TLR1, and CD14 bound purified curli fibers. Interestingly, CD14-curli interaction was specific to the fibrillar form of the amyloid, as demonstrated by using synthetic CsgA peptides proficient and deficient in fiber formation, respectively. Activation of the TLR2/TLR1/CD14 trimolecular complex by amyloids provides novel insights for innate immunity with implications for amyloid-associated diseases.

Liposomal lipopolysaccharide initiates TRIF-dependent signaling pathway independent of CD14

Lipopolysaccharide (LPS) is recognized by CD14 with Toll-like receptor 4 (TLR4), and initiates 2 major pathways of TLR4 signaling, the MyD88-dependent and TRIF-dependent signaling pathways. The MyD88-dependent pathway induces inflammatory responses such as the production of TNF-α, IL-6, and IL-12 via the activation of NFκB and MAPK. The TRIF-dependent pathway induces the production of type-I IFN, and RANTES via the activation of IRF-3 and NFκB, and is also important for the induction of adaptive immune responses. CD14 plays a critical role in initiating the TRIF-dependent signaling pathway response to LPS, to support the internalization of LPS via endocytosis. Here, we clearly demonstrate that intracellular delivery of LPS by LPS-formulated liposomes (LPS-liposomes) initiate only TRIF-dependent signaling via clathrin-mediated endocytosis, independent of CD14. In fact, LPS-liposomes do not induce the production of TNF-α and IL-6 but induce RANTES production in peritoneal macrophages. Additionally, LPS-liposomes could induce adaptive immune responses effectively in CD14-deficient mice. Collectively, our results strongly suggest that LPS-liposomes are useful as a TRIF-dependent signaling-based immune adjuvant without inducing unnecessary inflammation.

Sensing of microbial molecular patterns by Toll-like receptors

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

Recognition of lipid A variants by the TLR4-MD-2 receptor complex

Lipopolysaccharide (LPS) is a component of the outer membrane of almost all Gram-negative bacteria and consists of lipid A, core sugars, and O-antigen. LPS is recognized by Toll-like receptor 4 (TLR4) and MD-2 on host innate immune cells and can signal to activate the transcription factor NFκB, leading to the production of pro-inflammatory cytokines that initiate and shape the adaptive immune response. Most of what is known about how LPS is recognized by the TLR4-MD-2 receptor complex on animal cells has been studied using Escherichia coli lipid A, which is a strong agonist of TLR4 signaling. Recent work from several groups, including our own, has shown that several important pathogenic bacteria can modify their LPS or lipid A molecules in ways that significantly alter TLR4 signaling to NFκB. Thus, it has been hypothesized that expression of lipid A variants is one mechanism by which pathogens modulate or evade the host immune response. Additionally, several key differences in the amino acid sequences of human and mouse TLR4-MD-2 receptors have been shown to alter the ability to recognize these variations in lipid A, suggesting a host-specific effect on the immune response to these pathogens. In this review, we provide an overview of lipid A variants from several human pathogens, how the basic structure of lipid A is recognized by mouse and human TLR4-MD-2 receptor complexes, as well as how alteration of this pattern affects its recognition by TLR4 and impacts the downstream immune response.