Lipoteichoic Acid and Toll-like Receptor 2 Internalization and Targeting to the Golgi Are Lipid Raft-dependent

IFITM3 restricts virus-induced inflammatory cytokine production by limiting Nogo-B mediated TLR responses

Interferon-induced transmembrane protein 3 (IFITM3) is a restriction factor that limits viral pathogenesis and exerts poorly understood immunoregulatory functions. Here, using human and mouse models, we demonstrate that IFITM3 promotes MyD88-dependent, TLR-mediated IL-6 production following exposure to cytomegalovirus (CMV). IFITM3 also restricts IL-6 production in response to influenza and SARS-CoV-2. In dendritic cells, IFITM3 binds to the reticulon 4 isoform Nogo-B and promotes its proteasomal degradation. We reveal that Nogo-B mediates TLR-dependent pro-inflammatory cytokine production and promotes viral pathogenesis in vivo, and in the case of TLR2 responses, this process involves alteration of TLR2 cellular localization. Nogo-B deletion abrogates inflammatory cytokine responses and associated disease in virus-infected IFITM3-deficient mice. Thus, we uncover Nogo-B as a driver of viral pathogenesis and highlight an immunoregulatory pathway in which IFITM3 fine-tunes the responsiveness of myeloid cells to viral stimulation.

Role of Lipid Rafts in Pathogen-Host Interaction - A Mini Review

Lipid rafts, also known as microdomains, are important components of cell membranes and are enriched in cholesterol, glycophospholipids and receptors. They are involved in various essential cellular processes, including endocytosis, exocytosis and cellular signaling. Receptors are concentrated at lipid rafts, through which cellular signaling can be transmitted. Pathogens exploit these signaling mechanisms to enter cells, proliferate and egress. However, lipid rafts also play an important role in initiating antimicrobial responses by sensing pathogens via clustered pathogen-sensing receptors and triggering downstream signaling events such as programmed cell death or cytokine production for pathogen clearance. In this review, we discuss how both host and pathogens use lipid rafts and associated proteins in an arms race to survive. Special attention is given to the involvement of the major vault protein, the main constituent of a ribonucleoprotein complex, which is enriched in lipid rafts upon infection with vaccinia virus.

Exosomes and biomimetic nanovesicles-mediated anti-glioblastoma therapy: A head-to-head comparison

Exosomes (Exos) are promising vehicles for brain drug delivery due to nanosize and the ability to breach the blood-brain barrier (BBB). But the low yield of natural exosomes limits its application for nanomedicine. The generation of bioinspired nanovesicles (BNVs) that mimicking Exos is attractive, but there is a lack of comparative evaluation of Exos and BNVs. Here, we perform the first head-to-head comparison study of Exos and BNVs for brain tumor drug delivery. We show that BNVs derived from brain-derived endothelial cells are competent alternative nanocarrier to natural exosomes. The drug-loading capacity of Exos and BNVs are similar, but the yield of BNVs is substantially higher (500-fold) than Exos. Doxorubicin (DOX)-loaded BNVs (BNV/DOX) and DOX-loaded Exos (Exo/DOX) showed similar pharmacokinetic profiles and prolonged circulation od DOX. Despite inconsistent mechanisms, BNV/DOX can across the BBB, and exhibit suppression effects similar to Exo/DOX on the progress of glioblastoma (GBM) in zebrafish and in vivo subcutaneous and orthotopic xenografts mice models, with minimal systemic toxicity. Findings from this head-to-head comparison study indicate that autologous BNVs is a effective alternative of Exos for brain tumor nanomedicine.

What We Know So Far about the Metabolite-Mediated Microbiota-Intestinal Immunity Dialogue and How to Hear the Sound of This Crosstalk

Trillions of microorganisms, termed the "microbiota", reside in the mammalian gastrointestinal tract, and collectively participate in regulating the host phenotype. It is now clear that the gut microbiota, metabolites, and intestinal immune function are correlated, and that alterations of the complex and dynamic host-microbiota interactions can have deep consequences for host health. However, the mechanisms by which the immune system regulates the microbiota and by which the microbiota shapes host immunity are still not fully understood. This article discusses the contribution of metabolites in the crosstalk between gut microbiota and immune cells. The identification of key metabolites having a causal effect on immune responses and of the mechanisms involved can contribute to a deeper insight into host-microorganism relationships. This will allow a better understanding of the correlation between dysbiosis, microbial-based dysmetabolism, and pathogenesis, thus creating opportunities to develop microbiota-based therapeutics to improve human health. In particular, we systematically review the role of soluble and membrane-bound microbial metabolites in modulating host immunity in the gut, and of immune cells-derived metabolites affecting the microbiota, while discussing evidence of the bidirectional impact of this crosstalk. Furthermore, we discuss the potential strategies to hear the sound of such metabolite-mediated crosstalk.

Perinatal Infection: A Major Contributor to Efficacy of Cooling in Newborns Following Birth Asphyxia

Neonatal encephalopathy (NE) is a global burden, as more than 90% of NE occurs in low- and middle-income countries (LMICs). Perinatal infection seems to limit the neuroprotective efficacy of therapeutic hypothermia. Efforts made to use therapeutic hypothermia in LMICs treating NE has led to increased neonatal mortality rates. The heat shock and cold shock protein responses are essential for survival against a wide range of stressors during which organisms raise their core body temperature and temporarily subject themselves to thermal and cold stress in the face of infection. The characteristic increase and decrease in core body temperature activates and utilizes elements of the heat shock and cold shock response pathways to modify cytokine and chemokine gene expression, cellular signaling, and immune cell mobilization to sites of inflammation, infection, and injury. Hypothermia stimulates microglia to secret cold-inducible RNA-binding protein (CIRP), which triggers NF-κB, controlling multiple inflammatory pathways, including nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasomes and cyclooxygenase-2 (COX-2) signaling. Brain responses through changes in heat shock protein and cold shock protein transcription and gene-expression following fever range and hyperthermia may be new promising potential therapeutic targets.

Ocular Microbiota and Intraocular Inflammation

The term ocular microbiota refers to all types of commensal and pathogenic microorganisms present on or in the eye. The ocular surface is continuously exposed to the environment and harbors various commensals. Commensal microbes have been demonstrated to regulate host metabolism, development of immune system, and host defense against pathogen invasion. An unbalanced microbiota could lead to pathogenic microbial overgrowth and cause local or systemic inflammation. The specific antigens that irritate the deleterious immune responses in various inflammatory eye diseases remain obscure, while recent evidence implies a microbial etiology of these illnesses. The purpose of this review is to provide an overview of the literature on ocular microbiota and the role of commensal microbes in several eye diseases. In addition, this review will also discuss the interaction between microbial pathogens and host factors involved in intraocular inflammation, and evaluate therapeutic potential of targeting ocular microbiota to treat intraocular inflammation.

TLR2 Agonistic Small Molecules: Detailed Structure-Activity Relationship, Applications, and Future Prospects

Toll-like receptors (TLRs) are the pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) in microbial species. Among the various TLRs, TLR2 has a special place due to its ability to sense the widest repertoire of PAMPs owing to its heterodimerization with either TLR1 or TLR6, broadening its ligand diversity against pathogens. Various scaffolds are reported to activate TLR2, which include naturally occurring lipoproteins, synthetic lipopeptides, and small heterocyclic molecules. We described a detailed SAR in TLR2 agonistic scaffolds and also covered the design and chemistry for the conjugation of TLR2 agonists to antigens, carbohydrates, polymers, and fluorophores. The approaches involved in delivery of TLR2 agonists such as lipidation of antigen, conjugation to polymers, phosphonic acids, and other linkers to achieve surface adsorption, liposomal formulation, and encapsulating nanoparticles are elaborated. The crystal structure analysis and computational modeling are also included with the structural features that facilitate TLR2 activation.

Intestinal Bacteria Maintain Adult Enteric Nervous System and Nitrergic Neurons via Toll-like Receptor 2-induced Neurogenesis in Mice

BACKGROUND & AIMS

The enteric nervous system (ENS) exists in close proximity to luminal bacteria. Intestinal microbes regulate ENS development, but little is known about their effects on adult enteric neurons. We investigated whether intestinal bacteria or their products affect the adult ENS via toll-like receptors (TLRs) in mice.

METHODS

We performed studies with conventional C57/BL6, germ-free C57/BL6, Nestin-creER^T2:tdTomato, Nestin-GFP, and ChAT-cre:tdTomato. Mice were given drinking water with ampicillin or without (controls). Germ-free mice were given drinking water with TLR2 agonist or without (controls). Some mice were given a blocking antibody against TLR2 or a TLR4 inhibitor. We performed whole gut transit, bead latency, and geometric center studies. Feces were collected and analyzed by 16S ribosomal RNA gene sequencing. Longitudinal muscle myenteric plexus (LMMP) tissues were collected, analyzed by immunohistochemistry, and levels of nitric oxide were measured. Cells were isolated from colonic LMMP of Nestin-creER^T2:tdTomato mice and incubated with agonists of TLR2 (receptor for gram-positive bacteria), TLR4 (receptor for gram-negative bacteria), or distilled water (control) and analyzed by flow cytometry.

RESULTS

Stool from mice given ampicillin had altered composition of gut microbiota with reduced abundance of gram-positive bacteria and increased abundance of gram-negative bacteria, compared with mice given only water. Mice given ampicillin had reduced colon motility compared with mice given only water, and their colonic LMMP had reduced numbers of nitrergic neurons, reduced neuronal nitric oxide synthase production, and reduced colonic neurogenesis. Numbers of colonic myenteric neurons increased after mice were switched from ampicillin to plain water, with increased markers of neurogenesis. Nestin-positive enteric neural precursor cells expressed TLR2 and TLR4. In cells isolated from the colonic LMMP, incubation with the TLR2 agonist increased the percentage of neurons originating from enteric neural precursor cells to approximately 10%, compared with approximately 0.01% in cells incubated with the TLR4 agonist or distilled water. Mice given an antibody against TLR2 had prolonged whole gut transit times; their colonic LMMP had reduced total neurons and a smaller proportion of nitrergic neurons per ganglion, and reduced markers of neurogenesis compared with mice given saline. Colonic LMMP of mice given the TLR4 inhibitor did not have reduced markers of neurogenesis. Colonic LMMP of germ-free mice given TLR2 agonist had increased neuronal numbers compared with control germ-free mice.

CONCLUSIONS

In the adult mouse colon, TLR2 promotes colonic neurogenesis, regulated by intestinal bacteria. Our findings indicate that colonic microbiota help maintain the adult ENS via a specific signaling pathway. Pharmacologic and probiotic approaches directed towards specific TLR2 signaling processes might be developed for treatment of colonic motility disorders related to use of antibiotics or other factors.

Therapeutic Effect of Bifidobacterium Administration on Experimental Autoimmune Myasthenia Gravis in Lewis Rats

Beneficial effects of probiotics on gut microbiota homeostasis and inflammatory immune responses suggested the investigation of their potential clinical efficacy in experimental models of autoimmune diseases. Indeed, administration of two bifidobacteria and lactobacilli probiotic strains prevented disease manifestations in the Lewis rat model of Myasthenia Gravis (EAMG). Here, we demonstrate the clinical efficacy of therapeutic administration of vital bifidobacteria (i.e., from EAMG onset). The mechanisms involved in immunomodulation were investigated with ex vivo and in vitro experiments. Improvement of EAMG symptoms was associated to decreased anti-rat AChR antibody levels, and differential expression of TGFβ and FoxP3 immunoregulatory transcripts in draining lymph nodes and spleen of treated-EAMG rats. Exposure of rat bone marrow-derived dendritic cells to bifidobacteria or lactobacilli strains upregulated toll-like receptor 2 mRNA expression, a key molecule involved in bacterium recognition via lipotheicoic acid. Live imaging experiments of AChR-specific effector T cells, co-cultured with BMDCs pre-exposed to bifidobacteria, demonstrated increased percentages of motile effector T cells, suggesting a hindered formation of TCR-peptide-MHC complex. Composition of gut microbiota was studied by 16S rRNA gene sequencing, and α and β diversity were determined in probiotic treated EAMG rats, with altered ratios between Tenericutes and Verrucomicrobia (phylum level), and Ruminococcaceae and Lachnospiraceae (family level). Moreover, the relative abundance of Akkermansia genus was found increased compared to healthy and probiotic treated EAMG rats. In conclusion, our findings confirms that the administration of vital bifidobacteria at EAMG onset has beneficial effects on disease progression; this study further supports preclinical research in human MG to evaluate probiotic efficacy as supplementary therapy in MG.

Receptor Heterodimerization and Co-Receptor Engagement in TLR2 Activation Induced by MIC1 and MIC4 from Toxoplasma gondii

The microneme organelles of Toxoplasma gondii tachyzoites release protein complexes (MICs), including one composed of the transmembrane protein MIC6 plus MIC1 and MIC4. In this complex, carbohydrate recognition domains of MIC1 and MIC4 are exposed and interact with terminal sialic acid and galactose residues, respectively, of host cell glycans. Recently, we demonstrated that MIC1 and MIC4 binding to the N-glycans of Toll-like receptor (TLR) 2 and TLR4 on phagocytes triggers cell activation and pro-inflammatory cytokine production. Herein, we investigated the requirement for TLR2 heterodimerization and co-receptors in MIC-induced responses, as well as the signaling molecules involved. We used MICs to stimulate macrophages and HEK293T cells transfected with TLR2 and TLR1 or TLR6, both with or without the co-receptors CD14 and CD36. Then, the cell responses were analyzed, including nuclear factor-kappa B (NF-κB) activation and cytokine production, which showed that (1) only TLR2, among the studied factors, is crucial for MIC-induced cell activation; (2) TLR2 heterodimerization augments, but is not critical for, activation; (3) CD14 and CD36 enhance the response to MIC stimulus; and (4) MICs activate cells through a transforming growth factor beta-activated kinase 1 (TAK1)-, mammalian p38 mitogen-activated protein kinase (p38)-, and NF-κB-dependent pathway. Remarkably, among the studied factors, the interaction of MIC1 and MIC4 with TLR2 N-glycans is sufficient to induce cell activation, which promotes host protection against T. gondii infection.

CD14: Biology and role in the pathogenesis of disease

Human monocyte differentiation antigen CD14 is a pattern recognition receptor (PRR) that enhances innate immune responses. CD14 was first identified as a marker of monocytes to signal intracellular responses upon bacterial encounters. Given the absence of an intracellular tail, CD14 was doubted to have the signaling capacities. Later CD14 was confirmed as the TLR co-receptor for the detection of pathogen-associated molecular patterns. However, CD14 has been revealed as a multi-talented receptor. In last decade, CD14 was identified to activate NFAT to regulate the life cycle of myeloid cells in a TLR4-independent manner and to transport inflammatory lipids to induce phagocyte hyperactivation. And its influences on multiple related diseases have been further considered. In this review, we summarize advancements in the basic biology of the CD14 including its structure, binding ligands, signaling pathways, and its roles in the pathogenesis of inflammation, atherosclerosis, tumor and metabolic diseases. We also discuss the therapeutic potential of targeting the CD14 in related diseases.

The medicinal leech as a valuable model for better understanding the role of a TLR4-like receptor in the inflammatory process

Despite extensive investigation focused on both the molecular characteristics and the expression level of Toll-like receptors (TLRs) during the inflammatory response in vertebrates, few data are available in the literature on the role of these proteins in invertebrate's immune response. Here, we propose the medicinal leech as a valuable model to better elucidate the role of TLR4 and its related products, such as tumor necrosis factor (TNF-α), after activation of the leech peripheral immune system with the endogenous medicinal leech recombinant allograft inflammatory factor-1 (rHmAIF-1) or with an exogenous stimulus, such as lipopolysaccharide (LPS). Our results indicate that activated macrophages (HmAIF-1⁺) and granulocytes (CD11b⁺) express both TLR4 and its coreceptor CD14. Moreover, functional studies performed by injecting a cyanobacterium selective TLR4 antagonist CyP demonstrated that only the TLR4 pathway was blocked, while the immune response caused by lipoteichoic acid (LTA) treatment is not affected. These results are consistent with literature on vertebrates, indicating that TLR4 functions as a LPS receptor while the recognition of LTA may involve other pathways.

Myristoylated rhinovirus VP4 protein activates TLR2-dependent proinflammatory gene expression

Asthma exacerbations are often caused by rhinovirus (RV). We and others have shown that Toll-like receptor 2 (TLR2), a membrane surface receptor that recognizes bacterial lipopeptides and lipoteichoic acid, is required and sufficient for RV-induced proinflammatory responses in vitro and in vivo. We hypothesized that viral protein-4 (VP4), an internal capsid protein that is myristoylated upon viral replication and externalized upon viral binding, is a ligand for TLR2. Recombinant VP4 and myristoylated VP4 (MyrVP4) were purified by Ni-affinity chromatography. MyrVP4 was also purified from RV-A1B-infected HeLa cells by urea solubilization and anti-VP4 affinity chromatography. Finally, synthetic MyrVP4 was produced by chemical peptide synthesis. MyrVP4-TLR2 interactions were assessed by confocal fluorescence microscopy, fluorescence resonance energy transfer (FRET), and monitoring VP4-induced cytokine mRNA expression in the presence of anti-TLR2 and anti-VP4. MyrVP4 and TLR2 colocalized in TLR2-expressing HEK-293 cells, mouse bone marrow-derived macrophages, human bronchoalveolar macrophages, and human airway epithelial cells. Colocalization was absent in TLR2-null HEK-293 cells and blocked by anti-TLR2 and anti-VP4. Cy3-labeled MyrVP4 and Cy5-labeled anti-TLR2 showed an average fractional FRET efficiency of 0.24 ± 0.05, and Cy5-labeled anti-TLR2 increased and unlabeled MyrVP4 decreased FRET efficiency. MyrVP4-induced chemokine mRNA expression was higher than that elicited by VP4 alone and was attenuated by anti-TLR2 and anti-VP4. Cytokine expression was similarly increased by MyrVP4 purified from RV-infected HeLa cells and synthetic MyrVP4. We conclude that, during RV infection, MyrVP4 and TLR2 interact to generate a proinflammatory response.

Curli-Containing Enteric Biofilms Inside and Out: Matrix Composition, Immune Recognition, and Disease Implications

Biofilms of enteric bacteria are highly complex, with multiple components that interact to fortify the biofilm matrix. Within biofilms of enteric bacteria such as Escherichia coli and Salmonella species, the main component of the biofilm is amyloid curli. Other constituents include cellulose, extracellular DNA, O antigen, and various surface proteins, including BapA. Only recently, the roles of these components in the formation of the enteric biofilm individually and in consortium have been evaluated. In addition to enhancing the stability and strength of the matrix, the components of the enteric biofilm influence bacterial virulence and transmission. Most notably, certain components of the matrix are recognized as pathogen-associated molecular patterns. Systemic recognition of enteric biofilms leads to the activation of several proinflammatory innate immune receptors, including the Toll-like receptor 2 (TLR2)/TLR1/CD14 heterocomplex, TLR9, and NLRP3. In the model of Salmonella enterica serovar Typhimurium, the immune response to curli is site specific. Although a proinflammatory response is generated upon systemic presentation of curli, oral administration of curli ameliorates the damaged intestinal epithelial barrier and reduces the severity of colitis. Furthermore, curli (and extracellular DNA) of enteric biofilms potentiate the autoimmune disease systemic lupus erythematosus (SLE) and promote the fibrillization of the pathogenic amyloid α-synuclein, which is implicated in Parkinson's disease. Homologues of curli-encoding genes are found in four additional bacterial phyla, suggesting that the biomedical implications involved with enteric biofilms are applicable to numerous bacterial species.

Understanding early TLR signaling through the Myddosome

TLRs are expressed on the plasma and endosomal membranes of innate immune cells acting as sensors of foreign and inherent danger signals that threaten the host. Upon activation, TLRs facilitate the assembly of large intracellular oligomeric signaling complexes, termed Myddosomes, which initiate key signal transduction pathways to elicit critical inflammatory immune responses. The formation of the Myddosome is integral for TLR signaling; however, the molecular mechanisms controlling its formation, disassembly, and the subsequent proximal signaling events remain to be clearly defined. In this review, we present a brief overview of TLR signal transduction pathways, summarize the current understanding of the Myddosome and the proteins that comprise its structure, including MyD88 and members of the IL-1 receptor-associated kinase (IRAK) family. Finally, we will discuss recent advances and open questions regarding early TLR signaling in the context of the Myddosome complex.

Presentation matters: Impact of association of amphiphilic LPS with serum carrier proteins on innate immune signaling

Recognition of Pathogen-associated Molecular Patterns (PAMPs) by Toll-like receptors is central to innate immunity. Many bacterial PAMPs such as lipopolysaccharide (LPS) and lipoteichoic acid have amphiphilic properties. The hydrophobicity of amphiphilic PAMPs contributes to increasing entropy and causes these molecules to self-aggregate or bind host carrier proteins in aqueous physiological environments. The goal of this work was to determine how innate immune signaling is impacted by physical presentation and association of amphiphilic PAMPs with serum carrier proteins, using LPS as an example molecule. Specifically, we measured LPS-induced cytokine profiles in murine macrophages when the antigen was presented associated with the various serum carrier proteins in serum versus a serum-depleted system. Our study demonstrates that the observed cytokine profiles are dramatically different when LPS is presented in buffer, versus in serum when it is associated with proteins, specifically with respect to inhibition of pro-inflammatory cytokines in the latter. These studies suggest that LPS-mediated cytokine expression is dependent on its presentation in physiological systems. The amphiphilicity of bacterial PAMPs and consequent association with lipoproteins is a feature, which should be taken into account in the design of in vitro experiments. Further studies of the interdependencies of different serum carriers can identify pathways for drug delivery and diagnostics.

Time-lapse monitoring of TLR2 ligand internalization with newly developed fluorescent probes

As a mammalian toll-like receptor family member protein, TLR2 recognizes lipoproteins from bacteria and modulates the immune response by inducing the expression of various cytokines. We have developed fluorescence-labeled TLR2 ligands with either hydrophilic or hydrophobic fluorescence groups. The labeled ligands maintained the inflammatory IL-6 induction activity and enabled us to observe the internalization and colocalization of the TLR2 ligands using live-cell imaging. The time-lapse monitoring in the live-cell imaging of the fluorescence-labeled TLR2 ligand showed that TLR2/CD14 expression in the host cells enhanced the internalization of TLR2 ligand molecules.

Protein Palmitoylation and Its Role in Bacterial and Viral Infections

S-palmitoylation is a reversible, enzymatic posttranslational modification of proteins in which palmitoyl chain is attached to a cysteine residue via a thioester linkage. S-palmitoylation determines the functioning of proteins by affecting their association with membranes, compartmentalization in membrane domains, trafficking, and stability. In this review, we focus on S-palmitoylation of proteins, which are crucial for the interactions of pathogenic bacteria and viruses with the host. We discuss the role of palmitoylated proteins in the invasion of host cells by bacteria and viruses, and those involved in the host responses to the infection. We highlight recent data on protein S-palmitoylation in pathogens and their hosts obtained owing to the development of methods based on click chemistry and acyl-biotin exchange allowing proteomic analysis of protein lipidation. The role of the palmitoyl moiety present in bacterial lipopolysaccharide and lipoproteins, contributing to infectivity and affecting recognition of bacteria by innate immune receptors, is also discussed.

KATP channel block inhibits the Toll-like receptor 2-mediated stimulation of NF-κB by suppressing the activation of Akt, mTOR, JNK and p38-MAPK

Changes in the K_ATP channel activity have been shown to regulate inflammation and immune responses. Using human keratinocytes, we investigated the effect of K_ATP channel inhibition on inflammatory mediator production in relation to the Toll like receptor-2-mediated-Akt, mTOR and NF-κB pathways, as well as JNK and p38-MAPK, which regulate the transcription genes involved in immune and inflammatory responses. 5-Hydroxydecanoate (a selective K_ATP channel blocker), glibenclamide (a cell surface and mitochondrial K_ATP channel inhibitor), the Akt inhibitor, rapamycin, Bay 11-7085 and N-acetylcysteine reduced the lipoteichoic acid- or peptidoglycan-induced production of cytokines and chemokines, and production of reactive oxygen species and increased the levels and activities of Kir 6.2, NF-κB, phosphorylated-Akt and mTOR, and the activation of JNK and p38-MAPK in keratinocytes. Inhibitors of c-JNK (SP600125) and p38-MAPK (SB203580) attenuated the lipoteichoic acid- or peptidoglycan-induced production of inflammatory mediators, the activation of the JNK and p38-MAPK, and the production of reactive oxygen species in keratinocytes. The results show that K_ATP channel blockers may reduce the bacterial component-stimulated production of inflammatory mediators in keratinocytes by suppressing the Toll-like receptor-2-mediated activation of the Akt, mTOR and NF-κB pathways, as well as JNK and p38-MAPK. The suppressive effect of K_ATP channel blockers appears to be achieved by the inhibition of reactive oxygen species production.

Dendritic Cell Sensing of Hydrophobic Di- and Triacylated Lipopeptides Self-Assembled within Synthetic Virus-like Particles

Dendritic cells (DCs) play critical roles in developing immune defenses. One important aspect is interaction with pathogen-associated molecular patterns (PAMPs)/danger-associated molecular patterns, including di- and triacylated lipopeptides. Isolated or synthetic lipopeptides are potent vaccine adjuvants, interacting with cell surface TLR2 heterodimers. In contrast, deep embedment within bacteria cell walls would impair lipopeptide interaction with cell surface TLR2, requiring degradation for PAMP recognition. Accordingly, DC processing in the absence of surface TLR2 ligation was defined using synthetic virus-like particles (SVLPs) carrying hydrophobic TLR2 PAMPs within di- and triacylated lipopeptide cores (P2Cys-SVLPs and P3Cys-SVLPs) compared with SVLPs lacking immunomodulatory lipopeptides. DCs rapidly and efficiently internalized SVLPs, which was dominated by slow endocytic processing via macropinocytosis, although some caveolar endocytosis was implicated. This delivered SVLPs primarily into macropinosomes often interacting with EEA-1⁺ early endosomes. Although endoplasmic reticulum association was occasionally noted, association with recycling/sorting structures was not observed. Involvement of LysoTracker⁺ structures slowly increased with time, with SVLPs present in such structures ultimately dominating. Only SVLPs carrying di- and triacylated lipopeptide cores induced DC activation and maturation independently of surface TLR2 ligation. Intracellular recognition of SVLP TLR2 ligands was confirmed by observing SVLPs' association with internal TLR2, which had similar kinetics to SVLP association with LysoTracker. This related to inflammatory cytokine induction by SVLP⁺ DCs, with adaptive immune response activation ex vivo/in vivo. Importantly, particular DCs, not monocytes, recognized intracellular exposure of the TLR2 PAMPs carried by di- and triacylated SVLP cores, which indicates subset-distinct recognition of functional internal TLR2 ligands. Thus, vaccines carrying hydrophobic TLR2 ligands would interact with particular DCs for efficient induction of specific immunity in the absence of additional adjuvant.

Multi-receptor detection of individual bacterial products by the innate immune system

The receptors of the innate immune system detect specific microbial ligands to promote effective inflammatory and adaptive immune responses. Although this idea is well appreciated, studies in recent years have highlighted the complexity of innate immune detection, with multiple host receptors recognizing the same microbial ligand. Understanding the collective actions of diverse receptors that recognize common microbial signatures represents a new frontier in the study of innate immunity, and is the focus of this Review. Here, we discuss examples of individual bacterial cell wall components that are recognized by at least two and as many as four different receptors of the innate immune system. These receptors survey the extracellular or cytosolic spaces for their cognate ligands and operate in a complementary manner to induce distinct cellular responses. We further highlight that, despite this genetic diversity in receptors and pathways, common features exist to explain the operation of these receptors. These common features may help to provide unifying organizing principles associated with host defence.

Cutting Edge: A Dual TLR2 and TLR7 Ligand Induces Highly Potent Humoral and Cell-Mediated Immune Responses

TLR agonists are currently being developed and tested as adjuvants in various formulations to optimize the immunogenicity and efficacy of vaccines. The aim of this study was to evaluate the immunostimulatory properties of a novel compound incorporating covalently linked moieties designed to stimulate both TLR2 and TLR7. This dual TLR2/TLR7 agonist induced the maturation of dendritic cells and primed substantial populations of cytolytic and highly polyfunctional effector CD8⁺ T cells in vitro, and safely potentiated the immunogenic properties of a nanoparticulate Ag in vivo, eliciting humoral responses with a balanced T_H1/T_H2 profile in mice. Collectively, these data reveal the potential utility of chimeric adjuvants with synergistic activities mediated via TLRs.

Bacterial amyloid curli acts as a carrier for DNA to elicit an autoimmune response via TLR2 and TLR9

Bacterial biofilms are associated with numerous human infections. The predominant protein expressed in enteric biofilms is the amyloid curli, which forms highly immunogenic complexes with DNA. Infection with curli-expressing bacteria or systemic exposure to purified curli-DNA complexes triggers autoimmunity via the generation of type I interferons (IFNs) and anti-double-stranded DNA antibodies. Here, we show that DNA complexed with amyloid curli powerfully stimulates Toll-like receptor 9 (TLR9) through a two-step mechanism. First, the cross beta-sheet structure of curli is bound by cell-surface Toll-like receptor 2 (TLR2), enabling internalization of the complex into endosomes. After internalization, the curli-DNA immune complex binds strongly to endosomal TLR9, inducing production of type I IFNs. Analysis of wild-type and TLR2-deficient macrophages showed that TLR2 is the major receptor that drives the internalization of curli-DNA complexes. Suppression of TLR2 internalization via endocytosis inhibitors led to a significant decrease in Ifnβ expression. Confocal microscopy analysis confirmed that the TLR2-bound curli was required for shuttling of DNA to endosomal TLR9. Structural analysis using small-angle X-ray scattering revealed that incorporation of DNA into curli fibrils resulted in the formation of ordered curli-DNA immune complexes. Curli organizes parallel, double-stranded DNA rods at an inter-DNA spacing that matches up well with the steric size of TLR9. We also found that production of anti-double-stranded DNA autoantibodies in response to curli-DNA was attenuated in TLR2- and TLR9-deficient mice and in mice deficient in both TLR2 and TLR9 compared to wild-type mice, suggesting that both innate immune receptors are critical for shaping the autoimmune adaptive immune response. We also detected significantly lower levels of interferon-stimulated gene expression in response to purified curli-DNA in TLR2 and TLR9 deficient mice compared to wild-type mice, confirming that TLR2 and TLR9 are required for the induction of type I IFNs. Finally, we showed that curli-DNA complexes, but not cellulose, were responsible elicitation of the immune responses to bacterial biofilms. This study defines the series of events that lead to the severe pro-autoimmune effects of amyloid-expressing bacteria and suggest a mechanism by which amyloid curli acts as a carrier to break immune tolerance to DNA, leading to the activation of TLR9, production of type I IFNs, and subsequent production of autoantibodies.

Mechanisms for the activation of Toll-like receptor 2/4 by saturated fatty acids and inhibition by docosahexaenoic acid

Saturated fatty acids can activate Toll-like receptor 2 (TLR2) and TLR4 but polyunsaturated fatty acids, particularly docosahexaenoic acid (DHA) inhibit the activation. Lipopolysaccharides (LPS) and lipopetides, ligands for TLR4 and TLR2, respectively, are acylated by saturated fatty acids. Removal of these fatty acids results in loss of their ligand activity suggesting that the saturated fatty acyl moieties are required for the receptor activation. X-ray crystallographic studies revealed that these saturated fatty acyl groups of the ligands directly occupy hydrophobic lipid binding domains of the receptors (or co-receptor) and induce the dimerization which is prerequisite for the receptor activation. Saturated fatty acids also induce the dimerization and translocation of TLR4 and TLR2 into lipid rafts in plasma membrane and this process is inhibited by DHA. Whether saturated fatty acids induce the dimerization of the receptors by interacting with these lipid binding domains is not known. Many experimental results suggest that saturated fatty acids promote the formation of lipid rafts and recruitment of TLRs into lipid rafts leading to ligand independent dimerization of the receptors. Such a mode of ligand independent receptor activation defies the conventional concept of ligand induced receptor activation; however, this may enable diverse non-microbial molecules with endogenous and dietary origins to modulate TLR-mediated immune responses. Emerging experimental evidence reveals that TLRs play a key role in bridging diet-induced endocrine and metabolic changes to immune responses.

Structurally diverse genes encode Tlr2 in rainbow trout: The conserved receptor cannot be stimulated by classical ligands to activate NF-κB in vitro

The mammalian toll-like receptor 2 (TLR2) is a dominant receptor for the recognition of Gram-positive bacteria. Its structure and functional properties were unknown in salmonid fish. In RT-PCR and RACE experiments, we obtained the full-length cDNA sequence encoding Tlr2 from rainbow trout (Oncorhynchus mykiss) as well as a copy of an unspliced nonsense message from a highly segmented gene. The primary structure of the encoded receptor complies with the domain structure and ligand-binding sites known from mammals and other fish species and sorts well into the evolutionary tree of teleostean Tlr2s. We retrieved a gene version encoding the receptor on a single exon (tlr2a) and also a partial sequence of a second gene variant being segmented into multiple exons (tlr2b). Surprisingly, the abundances of both transcript variants accounted only for ∼10% of all Tlr2-encoding transcripts in various tissues and cell types of healthy fish. This suggests the expression of several distinct tlr2 gene variants in rainbow trout. We expressed tlr2a in HEK-293 cells, but were unable to demonstrate its functionality through NF-κB activation. Neither synthetic lipopeptides known to stimulate mammalian TLR2 nor different bacterial challenges induced OmTLR2-mediated NF-κB activation, not in HEK-293 or in salmon CHSE-214 cells. Positive demonstration of TLR2-MYD88 interaction excluded that its functional impairment caused the failure of NF-κB activation. We discuss impaired heterodimerization with a necessary Tlr partner as one from among several alternatives to explain the dysfunction of Tlr2a in the interspecies reconstitution system of TLR signaling.

Src-family-tyrosine kinase Lyn is critical for TLR2-mediated NF-κB activation through the PI 3-kinase signaling pathway

TLR2 has a prominent role in host defense against a wide variety of pathogens. Stimulation of TLR2 triggers MyD88-dependent signaling to induce NF-κB translocation, and activates a Rac1-PI 3-kinase dependent pathway that leads to transactivation of NF-κB through phosphorylation of the P65 NF-κB subunit. This transactivation pathway involves tyrosine phosphorylations. The role of the tyrosine kinases in TLR signaling is controversial, with discrepancies between studies using only chemical inhibitors and knockout mice. Here, we show the involvement of the tyrosine-kinase Lyn in TLR2-dependent activation of NF-κB in human cellular models, by using complementary inhibition strategies. Stimulation of TLR2 induces the formation of an activation cluster involving TLR2, CD14, PI 3-kinase and Lyn, and leads to the activation of AKT. Lyn-dependent phosphorylation of the p110 catalytic subunit of PI 3-kinase is essential to the control of PI 3-kinase biological activity upstream of AKT and thereby to the transactivation of NF-κB. Thus, Lyn kinase activity is crucial in TLR2-mediated activation of the innate immune response in human mononuclear cells.

Role of lipid microdomains in TLR-mediated signalling

Over the last twenty years, evidence has been provided that the plasma membrane is partitioned with microdomains, laterally mobile in the bilayer, providing the necessary microenvironment to specific membrane proteins for signalling pathways to be initiated. We discuss here the importance of such microdomains for Toll-like receptors (TLR) localization and function. First, lipid microdomains favour recruitment and clustering of the TLR machinery partners, i.e. receptors and co-receptors previously identified to be required for ligand recognition and signal transmission. Further, the presence of the so-called Cholesterol Recognition Amino-Acid Consensus (CRAC) sequences in the intracellular juxtamembrane domain of several Toll-like receptors suggests a direct role of cholesterol in the activation process. This article is part of a Special Issue entitled: Lipid-protein interactions.

Interferon γ-induced GTPase promotes invasion of Listeria monocytogenes into trophoblast giant cells

Listeria monocytogenes is well known for having the ability to cross the placental barrier, leading to fetal infections and abortion. However, the mechanisms leading to infectious abortion are poorly understood. In this study, we demonstrate that interferon γ-induced GTPase (IGTP) contributes to the invasion of L. monocytogenes into trophoblast giant (TG) cells, which are placental immune cells. Knockdown of IGTP in TG cells decreased the relative efficiencies of L. monocytogenes invasion. Moreover, IGTP accumulated around infected L. monocytogenes in TG cells. Treatment of TG cells with phosphatidylinositol 3-kinase (PI3K)/Akt inhibitors also reduced bacterial invasion. PI3K/Akt inhibitor or IGTP knockdown reduced the amount of phosphorylated Akt. Monosialotetrahexosylganglioside (GM1) gangliosides, lipid raft markers, accumulated in the membrane of L. monocytogenes-containing vacuoles in TG cells. Furthermore, treatment with a lipid raft inhibitor reduced bacterial invasion. These results suggest that IGTP-induced activation of the PI3K/Akt signaling pathway promotes bacterial invasion into TG cells.

Skin fibroblasts from individuals with Chediak-Higashi Syndrome (CHS) exhibit hyposensitive immunogenic response

Background

Chediak-Higashi Syndrome (CHS) is a rare autosomal recessive disease characterized by immunodeficiency, oculocutaneous albinism, neurological dysfunction, and early death. Individuals with CHS present with increased susceptibility to infections of the skin, upper-respiratory tract, gastrointestinal tract, and oral tissues. Classical CHS is caused by mutations in the gene encoding lysosomal trafficking regulator (LYST). Although defects in cytotoxic T cell lytic secretory granule secretion and neutrophil phagocytosis are suggested to contribute to the immunodeficiency in CHS, the underlying molecular mechanisms are unknown. We hypothesized that skin fibroblasts from CHS subjects exhibit impaired immune response due to defective trafficking of inflammatory factors.

Methods and results

Primary skin fibroblasts from CHS subjects or healthy controls were assessed for genes encoding inflammatory response factors using PCR array. At baseline, we found CD14, IL1R1 and TLR-1 were down-regulated significantly (≥2 fold change) and the genes encoding TLR-3, IL-1β and IL-6 were up-regulated in CHS cells compared to control cells. When challenged with E. coli lipopolysaccharide (LPS), CHS cells were less responsive than control cells, with only 8 genes significantly up-regulated (3–68 fold change) compared to baseline values, whereas 28 genes in control cells were significantly up-regulated at a much higher magnitude (3–4,629 fold change). In addition, 50% of the genes significantly up-regulated in LPS-treated control cells were significantly lower in LPS-treated CHS cells. IL-6, a fibroblast-derived proinflammatory cytokine essential for fighting infections was significantly lower in culture media of CHS cells with or without LPS. Furthermore, Western blot and immunofluorescent staining revealed that TLR-2 and TLR-4 were diminished on cell membranes of CHS cells and dissociated from Rab11a.

Conclusions

For the first time, results from our study indicate defective trafficking of TLR-2 and TLR-4 contributes to the hyposensitive response of CHS skin fibroblasts to immunogenic challenge, providing a potential therapeutic target for clinical intervention in CHS.

Cutting edge: New chimeric NOD2/TLR2 adjuvant drastically increases vaccine immunogenicity

TLR ligands are critical activators of innate immunity and are being developed as vaccine adjuvants. However, their usefulness in conjunction with NOD-like receptor agonists remains poorly studied. In this study, we evaluated a new ligand that targets both TLR2 and NOD2 receptors. We assessed its ability to enhance dendritic cell maturation in vitro in addition to improving systemic and mucosal immune responses in mice. The chimeric NOD2/TLR2 ligand induced synergistic upregulation of dendritic cell maturation markers, costimulatory molecules, and secretion of proinflammatory cytokines compared with combinations of separate ligands. Furthermore, when coadministered with biodegradable nanoparticles carrying a model Ag, the ligand was able to induce high Ag-specific IgA and IgG titers at both systemic and mucosal sites after parenteral immunizations. These findings point out the potential utility of chimeric molecules TLR/NOD as adjuvants for vaccines to induce systemic and mucosal immune responses.

Differential role for p120-catenin in regulation of TLR4 signaling in macrophages

Activation of TLR signaling through recognition of pathogen-associated molecular patterns is essential for the innate immune response against bacterial and viral infections. We have shown that p120-catenin (p120) suppresses TLR4-mediated NF-кB signaling in LPS-challenged endothelial cells. In this article, we report that p120 differentially regulates LPS/TLR4 signaling in mouse bone marrow-derived macrophages. We observed that p120 inhibited MyD88-dependent NF-κB activation and release of TNF-α and IL-6, but enhanced TIR domain-containing adapter-inducing IFN-β-dependent IFN regulatory factor 3 activation and release of IFN-β upon LPS exposure. p120 silencing diminished LPS-induced TLR4 internalization, whereas genetic and pharmacological inhibition of RhoA GTPase rescued the decrease in endocytosis of TLR4 and TLR4-MyD88 signaling, and reversed the increase in TLR4-TIR domain-containing adapter-inducing IFN-β signaling induced by p120 depletion. Furthermore, we demonstrated that altered p120 expression in macrophages regulates the inflammatory phenotype of LPS-induced acute lung injury. These results indicate that p120 functions as a differential regulator of TLR4 signaling pathways by facilitating TLR4 endocytic trafficking in macrophages, and support a novel role for p120 in influencing the macrophages in the lung inflammatory response to endotoxin.

The same and not the same: heterogeneous functional activation of prostate tumor cells by TLR ligation

Background

Many types of tumors are organized in a hierarchy of heterogeneous cell populations with different molecular signature. Such heterogeneity may be associated with different responsiveness to microenvironment stimuli. In the present study, the effects of lipopolysaccharide (LPS) and lipoteichoic acid (LTA), as well-known mediators of inflammation, on cancerous behavior of three prostate tumor cells, LNCaP, PC3 and DU145, were investigated.

Methods

Expression of TLR1-10, CD14 and MyD88 transcripts was investigated by RT-PCR. Protein expression of TLR2 and 4 was scrutinized by flow cytometry, immunofluorescent staining and Western blotting. Experiments were set up to assess the effects of LPS and LTA at different concentrations and times on cell proliferation, extracellular matrix invasion, adhesion and cytokine production.

Results

We showed that prostate cancer cell lines differentially express TLR1-10, MyD88 and CD14 transcripts. DU145 failed to express TLR4 gene. Positively-identified TLR2 protein in all prostate cancer cells and TLR4 protein in PC3 and LNCaP by Western blotting was not accompanied by cell surface expression, as judged by flow cytometry. Immunofluorescent staining clearly demonstrated predominantly perinuclear localization of TLR2 and TLR4. LTA activation of all prostate cancer cells significantly increased cell proliferation. Regardless of lacking TLR4, DU145 cells proliferated in response to LPS treatment. While LPS caused increased invasiveness of LNCaP, invasive capacity of PC3 was significantly reduced after LPS or LTA stimulation. Stimulation of all prostate tumor cells with LTA was associated with increased cell adhesion and IL-8 production. IL-6 production, however, was differentially regulated by LPS stimulation in prostate tumor cells.

Conclusion

The data shows that cancer cells originated from the same histologically origin exhibit heterogeneous response to the same TLR ligand. Therefore, a thorough and comprehensive judgment on how and to what extent a particular cancer is affected by TLR agonist could not be inferred by studying an individual cell line.

Molecular mechanisms of TLR2-mediated antigen cross-presentation in dendritic cells

Cross-presentation is a key function of dendritic cells (DCs), which present exogenous Ags on MHC class I molecules to prime CTL responses. The effects of TLR triggering on the cross-presentation of exogenous Ags by DCs remain unclear. In this study, we used synthetic dipalmitoylated peptides and TLR2 agonist-conjugated peptides as models to elucidate the mechanisms of TLR2-mediated cross-presentation. We observed that the internalization of dipalmitoylated peptides by bone marrow-derived DCs was facilitated by TLR2 via clathrin-mediated endocytosis. The administration of these dipalmitoylated peptide-pulsed bone marrow-derived DCs eliminated established tumors through TLR2 signaling. We further demonstrated that the induction of Ag-specific CTL responses and tumor regression by dipalmitoylated peptides was TAP independent. In addition, presentation of dipalmitoylated peptides by MHC class I molecules was blocked in the presence of an endosomal acidification inhibitor (chloroquine) or a lysosomal degradation inhibitor (Z-FL-COCHO). The endocytosed dipalmitoylated peptide also passed rapidly from early endosome Ag-1-positive endosomes to RAS-related GTP-binding protein 7 (Rab7)-associated late endosomes compared with their nonlipidated counterparts. Furthermore, we found that dipalmitoylated peptide-upregulated Rab7 expression correlated with Ag presentation via the TLR2/MyD88 pathway. Both JNK and ERK signaling pathways are required for upregulation of Rab7. In summary, our data suggest that TLR2-mediated cross-presentation occurs through the upregulation of Rab7 and a TAP-independent pathway that prime CTL responses.

TLR2 ligands induce NF-κB activation from endosomal compartments of human monocytes

Localization of Toll-like receptors (TLR) in subcellular organelles is a major strategy to regulate innate immune responses. While TLR4, a cell-surface receptor, signals from both the plasma membrane and endosomal compartments, less is known about the functional role of endosomal trafficking upon TLR2 signaling. Here we show that the bacterial TLR2 ligands Pam₃CSK₄ and LTA activate NF-κB-dependent signaling from endosomal compartments in human monocytes and in a NF-κB sensitive reporter cell line, despite the expression of TLR2 at the cell surface. Further analyses indicate that TLR2-induced NF-κB activation is controlled by a clathrin/dynamin-dependent endocytosis mechanism, in which CD14 serves as an important upstream regulator. These findings establish that internalization of cell-surface TLR2 into endosomal compartments is required for NF-κB activation. These observations further demonstrate the need of endocytosis in the activation and regulation of TLR2-dependent signaling pathways.

Low pH environmental stress inhibits LPS and LTA-stimulated proinflammatory cytokine production in rat alveolar macrophages

Gastric aspiration increases the risks for developing secondary bacterial pneumonia. Cytokine elaboration through pathogen recognition receptors (PRRs) is an important mechanism in initiating innate immune host response. Effects of low pH stress, a critical component of aspiration pathogenesis, on the PRR pathways were examined, specifically toll-like receptor-2 (TLR2) and TLR4, using isolated rat alveolar macrophages (aMØs). We assessed the ability of aMØs after brief exposure to acidified saline to elaborate proinflammatory cytokines in response to lipopolysaccharide (LPS) and lipoteichoic acid (LTA) stimulation, known ligands of TLR4 and TLR2, respectively. Low pH stress reduced LPS- and LTA-mediated cytokine release (CINC-1, MIP-2, TNF-α, MCP-1, and IFN-β). LPS and LTA increased intracellular Ca²⁺ concentrations while Ca²⁺ chelation by BAPTA decreased LPS- and LTA-mediated cytokine responses. BAPTA blocked the effects of low pH stress on most of LPS-stimulated cytokines but not of LTA-stimulated responses. In vivo mouse model demonstrates suppressed E. coli and S. pneumoniae clearance following acid aspiration. In conclusion, low pH stress inhibits antibacterial cytokine response of aMØs due to impaired TLR2 (MyD88 pathway) and TLR4 signaling (MyD88 and TRIF pathways). The role of Ca²⁺ in low pH stress-induced signaling is complex but appears to be distinct between LPS- and LTA-mediated responses.

Type I interferon and NF-κB activation elicited by herpes simplex virus gH/gL via αvβ3 integrin in epithelial and neuronal cell lines

αvβ3 integrin represents a novel sensing system which detects herpes simplex virus (HSV) and bacterial constituents. In cooperation with Toll-like receptor 2 (TLR2), it elicits an innate response that leads to activation of type I interferon (IFN), NF-κB, and a specific set of cytokines. We report that this defensive branch is functional in cells which represent experimental models of epithelial, including keratinocytic, and neuronal cells. These are the major targets of HSV in vivo. HSV entered the three cell lines via distinct routes. Hence, the defensive response was independent of the route of virus entry. Soluble gH/gL sufficed to elicit type I IFN and NF-κB activation and represents the viral pathogen-associated molecular pattern (PAMP) of this defense system.

Immunostimulation by synthetic lipopeptide-based vaccine candidates: structure-activity relationships

Peptide-based vaccines offer several advantages over conventional whole organism or protein approaches by offering improved purity and specificity in inducing immune response. However, peptides alone are generally non-immunogenic. Concerns remain about the toxicity of adjuvants which are critical for immunogenicity of synthetic peptides. The use of lipopeptides in peptide vaccines is currently under intensive investigation because potent immune responses can be generated without the use of adjuvant (thus are self-adjuvanting). Several lipopeptides derived from microbial origin, and their synthetic versions or simpler fatty acid moieties impart this self-adjuvanting activity by signaling via Toll-like receptor 2 (TLR2). Engagement of this innate immune receptor on antigen-presenting cell leads to the initiation and development of potent immune responses. Therefore optimization of lipopeptides to enhance TLR2-mediated activation is a promising strategy for vaccine development. Considerable structure-activity relationships that determine TLR2 binding and consequent stimulation of innate immune responses have been investigated for a range of lipopeptides. In this mini review we address the development of lipopeptide vaccines, mechanism of TLR2 recognition, and immune activation. An overview is provided of the best studied lipopeptide vaccine systems.

Inflammasome-mediated secretion of IL-1β in human monocytes through TLR2 activation; modulation by dietary fatty acids

Many studies have shown that TLR4- and TLR2-deficient mice are protected from high-fat diet-induced inflammation and insulin resistance, suggesting that saturated fatty acids derived from the high-fat diet activate TLR-mediated proinflammatory signaling pathways and induce insulin resistance. However, evidence that palmitic acid, the major dietary saturated fatty acid, can directly activate TLR has not been demonstrated. In this article, we present multiple lines of evidence showing that palmitic acid directly activates TLR2, a major TLR expressed on human monocytes, by inducing heterodimerization with TLR1 in an NADPH oxidase-dependent manner. Dimerization of TLR2 with TLR1 was inhibited by the n-3 fatty acid docosahexaenoic acid. Activation of TLR2 by palmitic acid leads to expression of pro-IL-1β that is cleaved by caspase-1, which is constitutively present in monocytes, to release mature IL-1β. Our results reveal mechanistic insight about how palmitic acid activates TLR2, upregulates NALP3 expression, and induces inflammasome-mediated IL-1β production in human monocytes, which can trigger enhanced inflammation in peripheral tissues, and suggest that these processes are dynamically modulated by the types of dietary fat we consume.

Image thresholding techniques for localization of sub-resolution fluorescent biomarkers

In this article, we explore adaptive global and local segmentation techniques for a lab-on-chip nutrition monitoring system (NutriChip). The experimental setup consists of Caco-2 intestinal cells that can be artificially stimulated to trigger an immune response. The eventual response is optically monitored using immunofluoresence techniques targeting toll-like receptor 2 (TLR2). Two problems of interest need to be addressed by means of image processing. First, a new cell sample must be properly classified as stimulated or not. Second, the location of the stained TLR2 must be recovered in case the sample has been stimulated. The algorithmic approach to solving these problems is based on the ability of a segmentation technique to properly segment fluorescent spots. The sample classification is based on the amount and intensity of the segmented pixels, while the various segmenting blobs provide an approximate localization of TLR2. A novel local thresholding algorithm and three well-known spot segmentation techniques are compared in this study. Quantitative assessment of these techniques based on real and synthesized data demonstrates the improved segmentation capabilities of the proposed algorithm.

Sweeten PAMPs: Role of Sugar Complexed PAMPs in Innate Immunity and Vaccine Biology

Innate sensors play a critical role in the early innate immune responses to invading pathogens through sensing of diverse biochemical signatures also known as pathogen associated molecular patterns (PAMPs). These biochemical signatures primarily consist of a major family of biomolecules such as proteins, lipids, nitrogen bases, and sugar and its complexes, which are distinct from host molecules and exclusively expressed in pathogens and essential to their survival. The family of sensors known as pattern recognition receptors (PRRs) are germ-line encoded, evolutionarily conserved molecules, and consist of Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), NOD-like receptors (NLRs), C-type lectin-like receptors (CLRs), and DNA sensors. Sensing of PAMP by PRR initiates the cascade of signaling leading to the activation of transcription factors, such as NF-κB and interferon regulatory factors (IRFs), resulting in a variety of cellular responses, including the production of interferons (IFNs) and pro-inflammatory cytokines. In this review, we discuss sensing of different types of glycosylated PAMPs such as β-glucan (a polymeric sugar) or lipopolysaccharides, nucleic acid, and so on (sugar complex PAMPs) by different families of sensors, its role in pathogenesis, and its application in development of potential vaccine and vaccine adjuvants.

TLR2 & Co: a critical analysis of the complex interactions between TLR2 and coreceptors

TLRs play a major role in microbe-host interactions and innate immunity. Of the 10 functional TLRs described in humans, TLR2 is unique in its requirement to form heterodimers with TLR1 or TLR6 for the initiation of signaling and cellular activation. The ligand specificity of TLR2 heterodimers has been studied extensively, using specific bacterial and synthetic lipoproteins to gain insight into the structure-function relationship, the minimal active motifs, and the critical dependence on TLR1 or TLR6 for activation. Different from that for specific well-defined TLR2 agonists, recognition of more complex ligands like intact microbes or molecules from endogenous origin requires TLR2 to interact with additional coreceptors. A breadth of data has been published on ligand-induced interactions of TLR2 with additional pattern recognition receptors such as CD14, scavenger receptors, integrins, and a range of other receptors, all of them important factors in TLR2 function. This review summarizes the roles of TLR2 in vivo and in specific immune cell types and integrates this information with a detailed review of our current understanding of the roles of specific coreceptors and ligands in regulating TLR2 functions. Understanding how these processes affect intracellular signaling and drive functional immune responses will lead to a better understanding of host-microbe interactions and will aid in the design of new agents to target TLR2 function in health and disease.

Perioperative Dynamics of TLR2, TLR4, and TREM-1 Expression in Monocyte Subpopulations in the Setting of On-Pump Coronary Artery Bypass Surgery

Hypercytokinemia plays a key role in the pathogenesis of systemic inflammatory response syndrome (SIRS). Monocytes are the main source of cytokines in the early inflammatory phase. Simultaneous stimulation of toll-like receptors (TLRs) and triggering receptor expressed on myeloid cells (TREM-1) activating receptor on monocytes results in the amplification of the inflammatory signal and multiple increase in proinflammatory cytokine production. The dynamics of those receptors expression on monocyte surface of patients with uncomplicated SIRS course followed coronary artery bypass surgery (CABG) was studied. The increase in TLR2 and TREM-1 expression on the first day after CABG induces proinflammatory and amplification potentials of monocytes in that period. The decrease in TLR2 surface expression on the seventh day compared to the preoperative values can be regarded as a mechanism limiting inflammatory response. The highest level of TLR2, TLR4, and TREM-1 surface expression was observed in CD14^hiCD16⁺ monocyte subpopulation, confirming its proinflammatory profile.

Toll-like receptor 4 is not targeted to the lysosome in cystic fibrosis airway epithelial cells

The innate immune response to bacterial infection is mediated through Toll-like receptors (TLRs), which trigger tightly regulated signaling cascades through transcription factors including NF-κB. LPS activation of TLR4 triggers internalization of the receptor-ligand complex which is directed toward lysosomal degradation or endocytic recycling. Cystic fibrosis (CF) patients display a robust and uncontrolled inflammatory response to bacterial infection, suggesting a defect in regulation. This study examined the intracellular trafficking of TLR4 in CF and non-CF airway epithelial cells following stimulation with LPS. We employed cells lines [16hBE14o-, CFBE41o- (CF), and CFTR-complemented CFBE41o-] and confirmed selected experiments in primary nasal epithelial cells from non-CF controls and CF patients (F508del homozygous). In control cells, TLR4 expression (surface and cytoplasmic) was reduced after LPS stimulation but remained unchanged in CF cells and was accompanied by a heightened inflammatory response 24 h after stimulation. All cells expressed markers of the early (EEA1) and late (Rab7b) endosomes at basal levels. However, only CF cells displayed persistent expression of Rab7b following LPS stimulation. Rab7 variants may directly internalize bacteria to the Golgi for recycling or to the lysosome for degradation. TLR4 colocalized with the lysosomal marker LAMP1 in 16 hBE14o- cells, suggesting that TLR4 is targeted for lysosomal degradation in these cells. However, this colocalization was not observed in CFBE41o- cells, where persistent expression of Rab7 and release of proinflammatory cytokines was detected. Consistent with the apparent inability of CF cells to target TLR4 toward the lysosome for degradation, we observed persistent surface and cytoplasmic expression of this pathogen recognition receptor. This defect may account for the prolonged cycle of chronic inflammation associated with CF.

Regulation of pattern recognition receptors by the apolipoprotein A-I mimetic peptide 4F

OBJECTIVE

The apolipoprotein A-I (apoA-I) mimetic peptide 4F favors the differentiation of human monocytes to an anti-inflammatory phenotype and attenuates lipopolysaccharide (LPS)-induced inflammatory responses. We investigated the effects of LPS on the Toll-like receptor (TLR) signaling pathway in 4F-differentiated monocyte-derived macrophages.

METHODS AND RESULTS

Monocyte-derived macrophages were pretreated with 4F or vehicle for 7 days. 4F downregulated cell-surface TLRs (4, 5, and 6) as determined by flow cytometry. 4F attenuated the LPS-dependent upregulation of genes encoding TLR1, 2, and 6 and genes of the MyD88-dependent (CD14, MyD88, TRAF6, interleukin-1 receptor-associated kinase 4, and inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase beta) and MyD88-independent (interferon regulatory factor 3, TANK-binding kinase 1, and Toll-interleukin 1 receptor domain-containing adaptor-inducing interferon-β) pathways as determined by microarray analysis and quantitative reverse transcriptase polymerase chain reaction. Functional analyses of monocyte-derived macrophages showed that 4F reduced LPS-dependent TLR4 recycling, phosphorylation of nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha, activation and translocation of nuclear factor-κB and inhibited the secretion of tumor necrosis factor-α and interleukin-6 induced by LPS or lipoteichoic acid. These changes were associated with depletion of cellular cholesterol and caveolin, components of membrane lipid rafts.

CONCLUSIONS

These data suggest that disruption of rafts by 4F alters the assembly of TLR-ligand complexes in cell membranes and inhibits proinflammatory gene expression in monocyte-derived macrophages, thus attenuating the responsiveness of macrophages to LPS.

Soluble glucan is internalized and trafficked to the Golgi apparatus in macrophages via a clathrin-mediated, lipid raft-regulated mechanism

Glucans are natural product carbohydrates that stimulate immunity. Glucans are internalized by the pattern recognition receptor, Dectin-1. Glucans were thought to be trafficked to phagolysosomes, but this is unproven. We examined the internalization and trafficking of soluble glucans in macrophages. Incubation of macrophages with glucan resulted in internalization of Dectin-1 and glucan. Inhibition of clathrin blocked internalization of the Dectin-1/glucan complex. Lipid raft depletion resulted in decreased Dectin levels and glucan uptake. Once internalized, glucans colocalized with early endosomes at 0 to 15 min, with the Golgi apparatus at 15 min to 24 h, and with Dectin-1 immediately (0 h) and again later (15 min-24 h). Glucans did not colocalize with lysosomes at any time interval examined. We conclude that the internalization of Dectin-1/glucan complexes in macrophages is mediated by clathrin and negatively regulated by lipid rafts and/or caveolin-1. Upon internalization, soluble glucans are trafficked via endosomes to the Golgi apparatus, not lysosomes.

Recent advances in Toll-like receptors and anterior uveitis

Uveitis involves acute, recurrent or chronic inflammation of the uvea, and occurs when the normal state of ocular immune privilege has broken down. Accumulating evidence implicates the role of microbial triggers in the development of various forms of immune-mediated uveitis in addition to its causative role in infectious uveitis. Toll-like receptors (TLRs) are the most important pattern-recognition receptors of the innate immune system that recognize pathogen-associated molecular patterns of microbes. Activation of TLRs by pathogen-associated molecular patterns leads to the induction of an inflammatory cascade and activation of both innate and adaptive arms of the immune response. TLRs have been implicated in the pathogenesis of various inflammatory diseases, including uveitis. This review provides an update on recent progress in TLR research and uveitis, specifically summarizing new evidence for the role of TLRs in anterior uveitis. There have been important observations from studies involving human ocular tissue, clinical uveitis and from experimental animal models of uveitis, such as endotoxin-induced uveitis. The 'Toll rush' has certainly gained momentum, and future advances in this field have the potential for selectively targeting the TLR pathway and ultimately translating into better therapies for patients with sight-threatening uveitis.

Lactobacillus plantarum lipoteichoic acid alleviates TNF-α-induced inflammation in the HT-29 intestinal epithelial cell line

We recently observed that lipoteichoic acid (LTA) isolated from Lactobacillus plantarum inhibited endotoxin-mediated inflammation of the immune cells and septic shock in a mouse model. Here, we examined the inhibitory role of L. plantarum LTA (pLTA) on the inflammatory responses of intestinal epithelial cells (IEC). The human colon cell line, HT-29, increased interleukin (IL)-8 expression in response to recombinant human tumor necrosis factor (TNF)-alpha, but not in response to bacterial ligands and interferon (IFN)-gamma. TNF-α also increased the production of inducible nitric oxide synthase (iNOS), nitric oxide (NO), and intercellular adhesion molecule 1 (ICAM-1) through activation of p38 mitogen-activated protein kinase (MAPK) from HT-29 cells. However, the inflammatory response of HT-29 on TNF-α stimulation was significantly inhibited by pLTA treatment. This pLTA-mediated inhibition accompanied the inhibition of nuclear factor (NF)-kappa B and MAPKs. Our data suggest that pLTA regulates cytokine-mediated immune responses and may be a good candidate for maintaining intestinal homeostasis against excessive inflammation.