Binding of bacterial peptidoglycan to CD14

Porcine Macrophage Markers and Populations: An Update

Besides its importance as a livestock species, pig is increasingly being used as an animal model for biomedical research. Macrophages play critical roles in immunity to pathogens, tissue development, homeostasis and tissue repair. These cells are also primary targets for replication of viruses such as African swine fever virus, classical swine fever virus, and porcine respiratory and reproductive syndrome virus, which can cause huge economic losses to the pig industry. In this article, we review the current status of knowledge on porcine macrophages, starting by reviewing the markers available for their phenotypical characterization and following with the characteristics of the main macrophage populations described in different organs, as well as the effect of polarization conditions on their phenotype and function. We will also review available cell lines suitable for studies on the biology of porcine macrophages and their interaction with pathogens.

Internalization of Polymeric Bacterial Peptidoglycan Occurs through Either Actin or Dynamin Dependent Pathways

Peptidoglycan (PGN), a polymeric glycan macromolecule, is a major constituent of the bacterial cell wall and a conserved pathogen-associated molecular pattern (PAMP) that triggers immune responses through cytosolic sensors. Immune cells encounter both PGN polymers and hydrolyzed muropeptides during infections, and primary human innate immune cells respond better to polymeric PGN than the minimal bioactive subunit muramyl dipeptide (MDP). While MDP is internalized through macropinocytosis and/or clathrin-mediated endocytosis, the internalization of particulate polymeric PGN is unresolved. We show here that PGN macromolecules isolated from Bacillus anthracis display a broad range of sizes, making them amenable for multiple internalization pathways. Pharmacologic inhibition indicates that PGN primarily, but not exclusively, is internalized by actin-dependent endocytosis. An alternate clathrin-independent but dynamin dependent pathway supports 20–30% of PGN uptake. In primary monocytes, this alternate pathway does not require activities of RhoA, Cdc42 or Arf6 small GTPases. Selective inhibition of PGN uptake shows that phagolysosomal trafficking, processing and downstream immune responses are drastically affected by actin depolymerization, while dynamin inhibition has a smaller effect. Overall, we show that polymeric PGN internalization occurs through two endocytic pathways with distinct potentials to trigger immune responses.

E-cigarette and food flavoring diacetyl alters airway cell morphology, inflammatory and antiviral response, and susceptibility to SARS-CoV-2

Diacetyl (DA) is an α-diketone that is used to flavor microwave popcorn, coffee, and e-cigarettes. Occupational exposure to high levels of DA causes impaired lung function and obstructive airway disease. Additionally, lower levels of DA exposure dampen host defenses in vitro. Understanding DA’s impact on lung epithelium is important for delineating exposure risk on lung health. In this study, we assessed the impact of DA on normal human bronchial epithelial cell (NHBEC) morphology, transcriptional profiles, and susceptibility to SARS-CoV-2 infection. Transcriptomic analysis demonstrated cilia dysregulation, an increase in hypoxia and sterile inflammation associated pathways, and decreased expression of interferon-stimulated genes after DA exposure. Additionally, DA exposure resulted in cilia loss and increased hyaluronan production. After SARS-CoV-2 infection, both genomic and subgenomic SARS-CoV-2 RNA were increased in DA vapor- compared to vehicle-exposed NHBECs. This work suggests that transcriptomic and physiologic changes induced by DA vapor exposure damage cilia and increase host susceptibility to SARS-CoV-2.

Uptake, recognition and responses to peptidoglycan in the mammalian host

Microbiota, and the plethora of signalling molecules that they generate, are a major driving force that underlies a striking range of inter-individual physioanatomic and behavioural consequences for the host organism. Among the bacterial effectors, one finds peptidoglycan, the major constituent of the bacterial cell surface. In the steady-state, fragments of peptidoglycan are constitutively liberated from bacterial members of the gut microbiota, cross the gut epithelial barrier and enter the host system. The fate of these peptidoglycan fragments, and the outcome for the host, depends on the molecular nature of the peptidoglycan, as well the cellular profile of the recipient tissue, mechanism of cell entry, the expression of specific processing and recognition mechanisms by the cell, and the local immune context. At the target level, physiological processes modulated by peptidoglycan are extremely diverse, ranging from immune activation to small molecule metabolism, autophagy and apoptosis. In this review, we bring together a fragmented body of literature on the kinetics and dynamics of peptidoglycan interactions with the mammalian host, explaining how peptidoglycan functions as a signalling molecule in the host under physiological conditions, how it disseminates within the host, and the cellular responses to peptidoglycan.

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

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

Toll-Like Receptors: General Molecular and Structural Biology

Background/Aim

Toll-like receptors (TLRs) are pivotal biomolecules in the immune system. Today, we are all aware of the importance of TLRs in bridging innate and adaptive immune system to each other. The TLRs are activated through binding to damage/danger-associated molecular patterns (DAMPs), microbial/microbe-associated molecular patterns (MAMPs), pathogen-associated molecular patterns (PAMPs), and xenobiotic-associated molecular patterns (XAMPs). The immunogenetic molecules of TLRs have their own functions, structures, coreceptors, and ligands which make them unique. These properties of TLRs give us an opportunity to find out how we can employ this knowledge for ligand-drug discovery strategies to control TLRs functions and contribution, signaling pathways, and indirect activities. Hence, the authors of this paper have a deep observation on the molecular and structural biology of human TLRs (hTLRs).

Methods and Materials

To prepare this paper and fulfill our goals, different search engines (e.g., GOOGLE SCHOLAR), Databases (e.g., MEDLINE), and websites (e.g., SCOPUS) were recruited to search and find effective papers and investigations. To reach this purpose, we tried with papers published in the English language with no limitation in time. The iCite bibliometrics was exploited to check the quality of the collected publications.

Results

Each TLR molecule has its own molecular and structural biology, coreceptor(s), and abilities which make them unique or a complementary portion of the others. These immunogenetic molecules have remarkable roles and are much more important in different sections of immune and nonimmune systems rather than that we understand to date.

Conclusion

TLRs are suitable targets for ligand-drug discovery strategies to establish new therapeutics in the fields of infectious and autoimmune diseases, cancers, and other inflammatory diseases and disorders.

Serum-borne lipids amplify TLR-activated inflammatory responses

TLRs recognize conserved pathogen associated molecular patterns and generate innate immune responses. Several circulating and cell membrane associated proteins have been shown to collaborate with TLRs in sensing microbial ligands and promoting inflammatory responses. Here, we show that serum and serum-borne lipids including lysophosphatidylcholine (LPC) amplify inflammatory responses from intestinal epithelial cells and mononuclear phagocytes primed with microbial TLR ligands. Treatment with the inhibitors of G protein-coupled receptor (GPCR) signaling, suramin, or pertussis toxin (PT), the inhibitor of JNK-MAPK, or knockdown of LPC response-regulating GPCR, G2A, decreases the augmentation brought about by serum or LPC in TLR-induced inflammatory response. In vivo administration of PT or anti-G2A antibody reduces TLR2-activated cytokine secretion. The ability of host lipids to costimulate TLR-generated cellular responses represents a novel pathway for the amplification of innate immunity and inflammation.

Mini-Review: Bioactivities of Bacterial Cell Envelopes in the Central Nervous System

During acute bacterial meningitis, recognition of the bacterial envelope by immune cells of the central nervous system (CNS) generates a robust response that is essential to clear bacteria. This response is further amplified during treatment when lytic antibiotics, required for cure, also generate a burst of highly inflammatory cell envelope debris. Different peptidoglycan (PG) subcomponents interact with neurons, glia, and the blood brain barrier resulting in the entire symptom complex of meningitis. Recently, this CNS-cell envelope signaling axis has been extended to non-inflammatory recognition of cell wall components circulating from endogenous bacteria to the brain resulting in both benefit and chronic damage. This review will describe the molecular details of a broad array of cell envelope-induced responses in the CNS and what current strategies can be implemented to improve clinical outcome.

Antiviral efficacy of orally delivered neoagarohexaose, a nonconventional TLR4 agonist, against norovirus infection in mice

The neoagarohexaose (NA6) is an oligosaccharide that is derived from agarose, the major component of red algae cell walls, by enzymatic hydrolysis. Here we show that NA6 is a noncanonical Toll-like receptor 4 (TLR4) agonist with antiviral activity against norovirus. Its TLR4 activation was dependent on myeloid differentiation factor 2 (MD2) and cluster of differentiation 14 (CD14), leading to interferon-β (IFN-β) and tumor necrosis factor-α (TNF-α) production. This effect was abolished by TLR4 knockdown or knockout in murine macrophages. NA6 inhibited murine norovirus (MNV) replication with an EC₅₀ of 1.5 μM in RAW264.7 cells. It also lowered viral RNA titer in a human hepatocellular carcinoma Huh7-derived cell line harboring a human norovirus subgenomic replicon. The antiviral activity of NA6 was mainly attributed to IFN-β produced through the TLR4-TRIF signaling pathway. NA6-induced TNF-α, which had little effect on norovirus replication per se, primed macrophages to mount greater antiviral innate immune responses when IFN signaling was activated. NA6 boosted the induction of IFN-β in MNV-infected RAW264.7 cells and upregulated IFN-regulatory factor-1, an IFN-stimulated gene. NA6 induced IFN-β expression in the distal ileum with Peyer's patches and oral administration of NA6 reduced MNV loads through activation of TLR4 signaling, highlighting its potential contribution to protective antiviral innate immunity against norovirus.

The Role of Presepsin in Patients with Acute Surgical Diseases

AIM:

The purpose of this study was to determine the level of significance of markers in the development of intra-abdominal hypertension in patients with acute surgical diseases of the abdominal cavity.

METHODS:

The authors surveyed 100 patients who were monitored at the Regional Clinical Hospital, Karaganda. The criterion for inclusion in the study was the informed consent of patients to participate in the study, the presence of acute surgical pathology, and the monitoring of intra-abdominal pressure over time. The exclusion criteria for patients from the study is the presence of sub and decompensation of associated diseases: trauma (hematoma of the bladder), bladder tumour and impaired integrity of the pelvic ring. The design of the study was by the legislation of the Republic of Kazakhstan, international ethical norms and normative documents of research organizations, approved by the ethics committee of the Karaganda State Medical University.

RESULTS:

According to the world scientific literature, there are 4 indicators that change their value in response to increases in pressure in the abdominal cavity: fibrinogen and prothrombin index (the main indicators of the coagulogram); marker of blood clots D-dimer; early marker of translocation of bacterial flora into the bloodstream sCD14 (presepsin).

CONCLUSION:

The authors concluded that the obtained data indicate that an increase in intra-abdominal pressure in acute surgical diseases of the abdominal cavity causes hypercoagulation and an increase in presepsin. Monitoring IAP with simultaneous measurement of the level of presepsin significantly improves the stratification of critical patients in need of emergency surgery.

Impact of initial plasma presepsin level for clinical outcome in hospitalized patients with pneumonia

Background

Presepsin, the soluble CD14 subtype, is known as a sepsis biomarker. However, its clinical significance in pneumonia is unclear. We investigated the effects of plasma presepsin level on clinical outcomes in patients with pneumonia.

Methods

Patients over 18 years old admitted to our hospital due to pneumonia from May 2016 through November 2017 were reviewed using electronic medical records. One hundred and seventy-two patients who underwent measurement of plasma presepsin levels on admission were enrolled. Median age of enrolled patients was 81 years [interquartile range (IQR), 68-86 years]. Pneumonia severity index (PSI) class and A-DROP score on admission were calculated. The receiver operating characteristic (ROC) curve analysis was performed to assess the prognostic value of 30-day mortality and to identify the optimal cut-off value of plasma presepsin level. Correlations between plasma presepsin level and other factors were assessed using the Spearman's test. The Kaplan-Meier survival analysis and the log-rank test were performed to assess the two curves differentiated with the optimal cut-off value of plasma presepsin level.

Results

Seventeen patients (9.9%) died within 30 days of admission. The deceased patients had higher value of plasma presepsin on admission (539 pg/mL; IQR, 414-832 pg/mL) compared with the survivors (334 pg/mL; IQR, 223-484 pg/mL) (P=0.001). The areas under ROC curve for predicting 30-day mortality were 0.742 for plasma presepsin, 0.755 for A-DROP score, and 0.774 for PSI class. Plasma presepsin level was not associated with etiology of pneumonia. However, it was moderately correlated with serum creatinine level (rs =0.524, P<0.001). The ROC curve analysis derived 470 pg/mL of plasma presepsin level as the optimal cut-off value for predicting 30-day mortality. The Kaplan-Meier survival analysis showed that patients with plasma presepsin level ≥470 pg/mL on admission had significantly higher 30-day mortality than those with plasma presepsin level <470 pg/mL (P<0.001). Among patients with A-DROP score ≥3, those with plasma presepsin level ≥470 mg on admission had significantly higher 30-day mortality (P=0.013). Similarly, among patients with PSI class ≥4, those with plasma presepsin level ≥470 mg on admission had significantly higher 30-day mortality (P=0.005).

Conclusions

In hospitalized pneumonia patients, plasma presepsin level on admission could be a useful predictor of 30-day mortality and an additional prognostic biomarker on existing severity assessment scales.

Conserved Bacterial-Binding Peptides of the Scavenger-Like Human Lymphocyte Receptor CD6 Protect From Mouse Experimental Sepsis

Sepsis is an unmet clinical need constituting one of the most important causes of death worldwide, a fact aggravated by the appearance of multidrug resistant strains due to indiscriminate use of antibiotics. Host innate immune receptors involved in pathogen-associated molecular patterns (PAMPs) recognition represent a source of broad-spectrum therapies alternative or adjunctive to antibiotics. Among the few members of the ancient and highly conserved scavenger receptor cysteine-rich superfamily (SRCR-SF) sharing bacterial-binding properties there is CD6, a lymphocyte-specific surface receptor. Here, we analyze the bacterial-binding properties of three conserved short peptides (11-mer) mapping at extracellular SRCR domains of human CD6 (CD6.PD1, GTVEVRLEASW; CD6.PD2 GRVEMLEHGEW; and CD6.PD3, GQVEVHFRGVW). All peptides show high binding affinity for PAMPs from Gram-negative (lipopolysaccharide; K_d from 3.5 to 3,000 nM) and Gram-positive (lipoteichoic acid; K_d from 36 to 680 nM) bacteria. The CD6.PD3 peptide possesses broad bacterial-agglutination properties and improved survival of mice undergoing polymicrobial sepsis in a dose- and time-dependent manner. Accordingly, CD6.PD3 triggers a decrease in serum levels of both pro-inflammatory cytokines and bacterial load. Interestingly, CD6.PD3 shows additive survival effects on septic mice when combined with Imipenem/Cilastatin. These results illustrate the therapeutic potential of peptides retaining the bacterial-binding properties of native CD6.

Association of CD14 rs2569190 polymorphism with mortality in shock septic patients who underwent major cardiac or abdominal surgery: A retrospective study

The aim of this study was to investigate the relationship between the CD14 rs2569190 polymorphism and death related to septic shock in white European patients who underwent major cardiac or abdominal surgery. We carried out a retrospective study in 205 septic shock patients. The septic shock diagnosis was established by international consensus definitions. The outcome variable was the death within 28, 60 and 90 days after septic shock diagnosis. The CD14 rs2569190 polymorphism was analyzed by Agena Bioscience’s MassARRAY platform. For the genetic association analysis with survival was selected a recessive inheritance model (GG vs. AA/AG). One hundred thirteen out of 205 patients (55.1%) died with a survival median of 39 days (95%CI = 30.6; 47.4). Patients with rs2569190 GG genotype had shorter survival probability than rs2569190 AA/AG genotype at 60 days (62.3% vs 50%; p = 0.035), and 90 days (62.3% vs 52.6%; p = 0.046). The rs2569190 GG genotype was associated with increased risk of septic shock-related death in the first 60 days (adjusted hazard ratio (aHR) = 1.67; p = 0.016) and 90 days (aHR = 1.64; p = 0.020) compared to rs2569190 AA/AG genotype. In conclusion, the presence of CD14 rs2569190 GG genotype was associated with death in shock septic patients who underwent major surgery. Further studies with bigger sample size are required to verify this relationship.

Extending Immunological Profiling in the Gilthead Sea Bream, Sparus aurata, by Enriched cDNA Library Analysis, Microarray Design and Initial Studies upon the Inflammatory Response to PAMPs

This study describes the development and validation of an enriched oligonucleotide-microarray platform for Sparus aurata (SAQ) to provide a platform for transcriptomic studies in this species. A transcriptome database was constructed by assembly of gilthead sea bream sequences derived from public repositories of mRNA together with reads from a large collection of expressed sequence tags (EST) from two extensive targeted cDNA libraries characterizing mRNA transcripts regulated by both bacterial and viral challenge. The developed microarray was further validated by analysing monocyte/macrophage activation profiles after challenge with two Gram-negative bacterial pathogen-associated molecular patterns (PAMPs; lipopolysaccharide (LPS) and peptidoglycan (PGN)). Of the approximately 10,000 EST sequenced, we obtained a total of 6837 EST longer than 100 nt, with 3778 and 3059 EST obtained from the bacterial-primed and from the viral-primed cDNA libraries, respectively. Functional classification of contigs from the bacterial- and viral-primed cDNA libraries by Gene Ontology (GO) showed that the top five represented categories were equally represented in the two libraries: metabolism (approximately 24% of the total number of contigs), carrier proteins/membrane transport (approximately 15%), effectors/modulators and cell communication (approximately 11%), nucleoside, nucleotide and nucleic acid metabolism (approximately 7.5%) and intracellular transducers/signal transduction (approximately 5%). Transcriptome analyses using this enriched oligonucleotide platform identified differential shifts in the response to PGN and LPS in macrophage-like cells, highlighting responsive gene-cassettes tightly related to PAMP host recognition. As observed in other fish species, PGN is a powerful activator of the inflammatory response in S. aurata macrophage-like cells. We have developed and validated an oligonucleotide microarray (SAQ) that provides a platform enriched for the study of gene expression in S. aurata with an emphasis upon immunity and the immune response.

Protective Effects of Human and Mouse Soluble Scavenger-Like CD6 Lymphocyte Receptor in a Lethal Model of Polymicrobial Sepsis

Sepsis still constitutes an unmet clinical need, which could benefit from novel adjunctive strategies to conventional antibiotic therapy. The soluble form of the scavenger-like human CD6 lymphocyte receptor (shCD6) binds to key pathogenic components from Gram-positive and -negative bacteria and shows time- and dose-dependent efficacy in mouse models of monobacterial sepsis. The objective of the present work was to demonstrate the effectiveness of infusing mouse and human sCD6 by different systemic routes, either alone or as adjunctive therapy to gold standard antibiotics, in a lethal model of polymicrobial sepsis. To this end, C57BL/6 mice undergoing high-grade septic shock induced by cecal ligation and puncture (CLP; ≥90% lethality) were infused via the intraperitoneal (i.p.) or intravenous (i.v.) route with shCD6 at different doses and time points, either alone or in combination with imipenem/cilastatin (I/C) at a dose of 33 mg/kg of body weight every 8 h. Significantly reduced mortality and proinflammatory cytokine levels were observed by i.p. infusion of a single shCD6 dose (1.25 mg/kg) 1 h pre- or post-CLP. When using the i.v. route, mice survival was significantly extended by starting shCD6 infusion at later time points post-CLP (up to 6 h after CLP). Significant adjunctive effects on mouse survival were observed by i.p. or i.v. infusion of shCD6 in combination with i.p. I/C post-CLP. Similar results were obtained in mice expressing high sustained levels (5 to 10 μg/ml) of mouse sCD6 in serum by means of transduction with hepatotropic adeno-associated virus (AAV). Taken together, the data support the conserved antibacterial effects of human and mouse sCD6 and their use as adjunctive therapy in experimental models of complex and severe polymicrobial sepsis.

Role of MD-2 in TLR2- and TLR4-mediated recognition of Gram-negative and Gram-positive bacteria and activation of chemokine genes

MD-2 is associated with TLR4 on the cell surface and enables TLR4 to respond to LPS. TLR2 without MD-2 does not respond to pure protein-free endotoxic LPS, ReLPS, and lipid A. MD-2 enables TLR2 to respond to non-activating LPS, ReLPS, and lipid A, and enhances TLR2-mediated responses to Gram-negative and Gram-positive bacteria, protein-containing LPS, peptidoglycan, and lipoteichoic acid. MD-2 enables TLR4 to respond to a wide variety of endotoxic LPS partial structures, Gram-negative bacteria, and Gram-positive lipoteichoic acid, but not to Gram-positive bacteria, peptidoglycan, and lipopeptide. MD-2 physically associates with both TLR4 and TLR2, but the association with TLR2 is weaker than with TLR4. Also, MD-2 and TLR2 and TLR4 enhance each other's expression. The highest induced genes in human monocytes stimulated with Gram-positive and Gram-negative bacterial cell wall components are chemokine genes, and IL-8 is the highest induced chemokine. Both Gram-positive and Gram-negative bacteria activate TLR2→ MyD88→IRAK→TRAF→NIK→IKK→NF-κB signal transduction pathway that induces transcription of the IL-8 gene. Therefore, TLR2 is a functional receptor for both Gram-positive and Gram-negative bacteria and it induces activation of IL-8.

Peptidoglycan recognition in innate immunity

The innate immune system recognizes micro-organisms through a series of pattern recognition receptors that are highly conserved in evolution. Peptidoglycan (PGN) is a unique and essential component of the cell wall of virtually all bacteria, is not present in eukaryotes, and is an excellent target for the innate immune system. Indeed, higher eukaryotes, including mammals, have several PGN recognition molecules, including CD14, Toll-like receptor 2 (TLR2), nucleotide oligomerization domain (Nod)-containing proteins, a family of peptidoglycan recognition proteins (PGRPs), and PGN-lytic enzymes (lysozyme and amidase). These molecules induce host responses to micro-organisms, degrade PGN, or have direct antimicrobial effects.

Structural requirements of muramylpeptides for induction of Toll-like receptor 2-mediated NF-κB activation in CHO cells

We previously demonstrated that Gram-positive bacteria activated immune cells via CD14 and Toll-like receptor 2 (TLR2). Although peptidoglycan, a major constituent of the bacterial cell wall, substituted for whole organisms, the essential structure of muramylpeptides required to stimulate the cells is not clear. We further investigated the critical determinant for recognition by CD14 and TLR2. Chinese hamster ovary (CHO) fibroblasts, which do not express a functional TLR2 transcript, were transfected with TLR2 or TLR4. These cells were exposed to freeze-dried Staphylococcus epidermidis and were subsequently subjected to the pro-inflammatory transcription factor nuclear factor-κB (NF-κB)-dependent CD25 expression assay. Heterologous expression of human TLR2, but not TLR4, in CHO cells conferred immune responsiveness to freeze-dried S. epidermidis. A preparation of peptidoglycan from S. epidermidis substituted for whole organisms. Staphylococcus aureus lytic enzyme-digested product (SEPS) from peptidoglycan retained the activity, but hydrolysis of the glycan backbone in SEPS by M-1 endo- N-acetylmuramidase resulted in loss of the activity. These findings showed that cellular activation by Gram-positive cell wall components was mediated by TLR2, but not TLR4, and indicated that the glycan backbone of peptidoglycan is critical for TLR2-mediated NF-κB activation.

Function of CD14 as a peptidoglycan receptor: differences and similarities with LPS

Peptidoglycan (PGN) is a macrophage activator from Gram-positive bacteria. PGN activates cells of hemopoietic origin through CD14 since: (i) PGN-unresponsive CD14-negative cells become PGNresponsive after transfection with CD14 and expression of membrane CD14; (ii) PGN binds to CD14 with high affinity; and (iii) anti-CD14 mAbs inhibit both binding of PGN to CD14 and activation of CD14-positive cells by PGN. However, there are several differences in the function of CD14 as PGN and LPS receptor: (i) the kinetics of binding are different; (ii) the affinity of binding in the absence of LPS-binding protein (LBP) is higher for PGN than LPS; (iii) LBP does not increase the affinity of binding of PGN to CD14 and does not enhance cell activation by PGN (in contrast to LPS); (iv) the regions of CD14 needed for binding and activation are partially similar and partially different for PGN and LPS; (v) sCD14:PGN complexes, in contrast to sCD14:LPS complexes, do not activate CD14-negative cells; (vi) PGN, in contrast to LPS, does not activate CHO cells expressing mCD14; and (vii) PGN and LPS induce differential activation of MAP kinases, but activate similar transcription factors (NF-κB, ATF1/CREB, and AP-1).

Differential roles of TLR2 and TLR4 in the host response to Gram-negative bacteria: lessons from a lipopolysaccharide-deficient mutant of Neisseria meningitidis

The inflammatory response to bacterial infections plays an important role in the detection and elimination of invading micro-organisms. Various components of the bacterial cell wall are capable of activating this pro-inflammatory response. In the case of Gram-negative bacteria, lipopolysaccharide (LPS) is the dominant trigger, although other bacterial factors are also capable of activating this systemic inflammatory response. Recently, Toll-like receptors (TLRs) have been implicated in host responses to bacterial pathogens. Specifically, TLR4 mediates LPS responses while TLR2 plays a broader role in the recognition of a variety of bacteria and bacterial antigens. The experiments in this study were designed to examine the role of Gram-negative cell wall components, other than LPS, and their cellular receptors in the host response to infection using an LPS-deficient mutant of Neisseria meningitidis. Although less potent than the parental strain, we found the LPS-deficient mutant to be a capable inducer of the inflammatory response in a variety of cell types. Moreover, cellular activation by this mutant required expression of CD14 and TLR2.

Gastrointestinal and hepatic mechanisms limiting entry and dissemination of lipopolysaccharide into the systemic circulation

The human microbiota consists of 100 trillion microorganisms that provide important metabolic and biological functions benefiting the host. However, the presence in host plasma of a gut-derived bacteria component, the lipopolysaccharide (LPS), has been identified as a causal or complicating factor in multiple serious diseases such as sepsis and septic shock and, more recently, obesity-associated metabolic disorders. Understanding the precise mechanisms by which gut-derived LPS is transported from the gut lumen to the systemic circulation is crucial to advance our knowledge of LPS-associated diseases and elaborate targeted strategies for their prevention. The aim of this review is to synthetize current knowledge on the host mechanisms limiting the entry and dissemination of LPS into the systemic circulation. To prevent bacterial colonization and penetration, the intestinal epithelium harbors multiple defense mechanisms including the secretion of antimicrobial peptides and mucins as well as detoxification enzymes. Despite this first line of defense, LPS can reach the apical site of intestinal epithelial cells (IECs) and, because of its large size, likely crosses IECs via transcellular transport, either lipid raft- or clathrin-mediated endocytosis or goblet cell-associated passage. However, the precise pathway remains poorly described. Finally, if LPS crosses the gut mucosa, it is directed via the portal vein to the liver, where major detoxification processes occur by deacetylation and excretion through the bile. If this disposal process is not sufficient, LPS enters the systemic circulation, where it is handled by numerous transport proteins that clear it back to the liver for further excretion.

Eritoran Suppresses Colon Cancer by Altering a Functional Balance in Toll-like Receptors That Bind Lipopolysaccharide

Colorectal carcinogenesis is affected by overexpression of the lipopolysaccharide (LPS) receptors CD14 and TLR4, which antagonize each other by affecting epithelial cell proliferation and apoptosis. Eritoran is an investigational drug for sepsis treatment that resembles the lipid A moiety of LPS and therefore acts as a TLR4 inhibitor. In the present study, we explored the potential therapeutic uses and mechanisms of action of eritoran in reducing colon cancer progression. Eritoran administration via intracolonic, intragastric, or intravenous routes significantly reduced tumor burden in a chemically induced mouse model of colorectal carcinoma. Decreased proliferation and increased apoptosis were observed in mouse tumor cells after eritoran treatment. In vitro cultures of mouse primary tumor spheroids and human cancer cell lines displayed increased cell proliferation and cell-cycle progression following LPS challenge. This effect was inhibited by eritoran and by silencing CD14 or TLR4. In contrast, apoptosis induced by eritoran was eliminated by silencing CD14 or protein kinase Cζ (PKCζ) but not TLR4. Lastly, LPS and eritoran caused hyperphosphorylation of PKCζ in a CD14-dependent and TLR4-independent manner. Blocking PKCζ activation by a Src kinase inhibitor and a PKCζ-pseudosubstrate prevented eritoran-induced apoptosis. In summary, our work offers a preclinical proof of concept for the exploration of eritoran as a clinical treatment, with a mechanistic rationale to reposition this drug to improve the management of colorectal cancer. Cancer Res; 76(16); 4684-95. ©2016 AACR.

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.

Static magnetic field attenuates lipopolysaccharide-induced inflammation in pulp cells by affecting cell membrane stability

One of the causes of dental pulpitis is lipopolysaccharide- (LPS-) induced inflammatory response. Following pulp tissue inflammation, odontoblasts, dental pulp cells (DPCs), and dental pulp stem cells (DPSCs) will activate and repair damaged tissue to maintain homeostasis. However, when LPS infection is too serious, dental repair is impossible and disease may progress to irreversible pulpitis. Therefore, the aim of this study was to examine whether static magnetic field (SMF) can attenuate inflammatory response of dental pulp cells challenged with LPS. In methodology, dental pulp cells were isolated from extracted teeth. The population of DPSCs in the cultured DPCs was identified by phenotypes and multilineage differentiation. The effects of 0.4 T SMF on DPCs were observed through MTT assay and fluorescent anisotropy assay. Our results showed that the SMF exposure had no effect on surface markers or multilineage differentiation capability. However, SMF exposure increases cell viability by 15%. In addition, SMF increased cell membrane rigidity which is directly related to higher fluorescent anisotropy. In the LPS-challenged condition, DPCs treated with SMF demonstrated a higher tolerance to LPS-induced inflammatory response when compared to untreated controls. According to these results, we suggest that 0.4 T SMF attenuates LPS-induced inflammatory response to DPCs by changing cell membrane stability.

Construction and expression of a novel anti-CD14 human-mouse chimeric antibody Hm2F9

Anti-CD14 antibody can inhibit the lipopolysaccharide (LPS)-induced systemic inflammatory response syndrome in case of bacteremia or endotoxemia. To obtain chimeric anti-CD14 antibody, we constructed and expressed a novel chimeric antibody Hm2F9 composed of anti-CD14 single-chain fragment variable (scFv) and the Fc region (the hinge, CH2, and CH3 domains) of human IgG1 in Chinese hamster ovary (CHO) cells based on our previous study of scFv2F9. The Hm2F9 antibody, sized 150 kDa, retained the strong specific antigen-binding ability to the CD14 antigen with a comparable activity (the percentage of positive cells 99.07%) to its parental murine antibody 2F9 (the percentage of positive cells 98.86%). At the same time, Hm2F9 could manifestly block the binding of LPS to CD14, whose positive-cell percentage drops significantly with percentage of 98.63% (from 98.37% to 1.35%). The chimeric antibody Hm2F9 expressed in CHO cells retained high affinity to human CD14 and biological function to LPS.

Evolving Bacterial Envelopes and Plasticity of TLR2-Dependent Responses: Basic Research and Translational Opportunities

Innate immune mechanisms that follow early recognition of microbes influence the nature and magnitude of subsequent adaptive immune responses. Early detection of microbes depends on pattern recognition receptors that sense pathogen-associated molecular patterns or microbial-associated molecular patterns (PAMPS or MAMPs, respectively). The bacterial envelope contains MAMPs that include membrane proteins, lipopeptides, glycopolymers, and other pro-inflammatory molecules. Bacteria are selected by environmental pressures resulting in quantitative or qualitative changes in their envelope structures that often promote evasion of host immune responses and therefore, infection. However, recent studies have shown that slight, adaptive changes in MAMPs on the bacterial cell wall may result in their ability to induce the secretion not only of pro-inflammatory cytokines but also of anti-inflammatory cytokines. This effect can fine-tune the subsequent response to microbes expressing these MAMPs and lead to the establishment of a commensal state within the host rather than infectious disease. In this review, we will examine the plasticity of Toll-like receptor (TLR) 2 signaling as evidence of evolving MAMPs, using the better-characterized TLR4 as a template. We will review the role of differential dimerization of TLR2 and the arrangement of signaling complexes and co-receptors in determining the capacity of the host to recognize an array of TLR2 ligands and generate different immune responses to these ligands. Last, we will assess briefly how this plasticity may expand the array of interactions between microbes and immune systems beyond the traditional disease-causing paradigm.

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.

A static magnetic field attenuates lipopolysaccharide-induced neuro-inflammatory response via IL-6-mediated pathway

An effective method for controlling brain damage and neurodegeneration caused by inflammation remains elusive. Down-expression of the lipopolysaccharide (LPS)-induced inflammatory cytokines resulting in endotoxin tolerance is reported as an alternative anti-infection treatment. Nonetheless, because the dosage and action site are hard to control, endotoxin tolerance caused by low-dose LPS injection in brain tissue may induce side effects. The aim of this study was to test the hypothesis that static magnetic fields (SMF) stimulate endotoxin tolerance in brain tissue. In this study, survival rate and pathological changes in brain tissues of LPS-challenged mice were examined with and without SMF treatment. In addition, the effects of SMF exposure on growth rate and cytokine expression of LPS-challenged BV-2 microglia cells were monitored. Our results showed that SMF pre-exposure had positive effects on the survival rate and histological outcomes of LPS-treated mice. Furthermore, SMF exposure significantly decreased IL-6 expression in BV-2 cells (p < 0.05) by a phenomenon similar to endotoxin tolerance. We suggest that SMF has potential as an alternative simulation source for controlling LPS-induced excess neuro-inflammatory response.

CD14 and TLR4 mediate cytokine release promoted by electronegative LDL in monocytes

AIMS

Electronegative LDL (LDL(-)), a minor modified LDL present in the circulation, induces cytokine release in monocytes. We aimed to determine the role of the receptor CD14 and toll-like receptors 2 and 4 (TLR2, TLR4) in the inflammatory action promoted by LDL(-) in human monocytes.

METHODS AND RESULTS

Monocytes were preincubated with antibodies to neutralize CD14, TLR2 and TLR4. The release of monocyte chemoattractant protein 1 (MCP1), and interleukin 6 and 10 (IL6 and IL10) promoted by LDL(-) was inhibited 70-80% by antiCD14 and antiTLR4, and 15-25% by antiTLR2. The involvement of CD14 and TLR4 was confirmed by gene silencing experiments. The human monocytic THP1 cell line overexpressing CD14 released more cytokines in response to LDL(-) than the same THP1 cell line without expressing CD14. VIPER, a specific inhibitor of the TLR4 signaling pathway, blocked 75-90% the cytokine release promoted by LDL(-). Cell binding experiments showed that monocytes preincubated with neutralizing antibodies presented lesser LDL(-) binding than non-preincubated monocytes The inhibitory capacity was antiCD14>antiTLR4>>antiTLR2. Cell-free experiments performed in CD14-coated microtiter wells confirmed that CD14 was involved in LDL(-) binding. When LDL(-) and lipopolysaccharide (LPS) were added simultaneously to monocytes, cytokine release was similar to that promoted by LDL(-) alone. Binding experiments showed that LDL(-) and LPS competed for binding to monocytes and to CD14 coated-wells.

CONCLUSIONS

CD14 and TLR4 mediate cytokine release induced by LDL(-) in human monocytes. The cross-competition between LPS and LDL(-) for the same receptors could be a counteracting action of LDL(-) in inflammatory situations.

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

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

Regulation of TLR2-mediated tolerance and cross-tolerance through IRAK4 modulation by miR-132 and miR-212

Innate immune response is the first defense against pathogens via recognition by various conserved pattern recognition receptors, such as TLRs, to initiate a rapid and strong cytokine alarm. TLR signaling-mediated cytokine production must be properly regulated to prevent pathological conditions deriving from overproduction of cytokines. In this study, the role of specific microRNAs in TLR-signaling pathway was investigated to reveal the cross-interaction and -regulation in the MyD88 pathway. In peptidoglycan (PGN)/TLR2-stimulated THP-1 monocytes, PBMCs, and primary macrophages showed rapid and dramatic miR-132 and miR-212 (miR-132/-212) upregulation. This newly identified response appeared earlier in time than the characteristic miR-146a response in LPS-TLR4 stimulation. The rapid induction of miR-132/-212 was transcription factor CREB dependent, and the sustained expression of miR-132/-212 was responsible for inducing tolerance to subsequent PGN challenge. Cross-tolerance was observed by TLR5 ligand flagellin and heat-killed or live bacteria resulting from miR-132/-212 upregulation. Mechanistically, IRAK4 was identified and validated as a target of miR-132/-212 by luciferase reporter assay and seed-sequence mutagenesis of the reporter. Transfection of miR-132 or miR-212 alone mimicked PGN tolerance in monocytes, whereas transfected specific miRNA inhibitors tampered the tolerance effect. During bacterial infection, PGN-mediated TLR2 signaling induces miR-132/-212 to downregulate IRAK4, an early component in the MyD88-dependent pathway, whereas LPS/TLR4-induced miR-146a downregulates downstream components of the same MyD88-dependent pathway. The identification of miR-132/-212 and miR-146a together to prevent damaging consequences from the overproduction of proinflammatory cytokines by targeting a common signaling pathway is significant and will provide insights into future design and development of therapeutics.

Effects of bacterial toxins on endothelial tight junction in vitro: a mechanism-based investigation

ABSTRACT Lipopolysaccharide (LPS) and lipoteichoic acid (LTA), principal cell wall components of Gram-negative and Gram-positive bacteria, respectively, play a central role in altering the blood-brain barrier and facilitate bacterial infection of the host brain. Despite the significance of bacterial toxins in disease pathogenesis, mechanisms by which toxins impair the barrier are not yet known. This study, using an in vitro cell culture model, showed that LPS and LTA interacted with the endothelial cells and disrupted the tight junction between the cells that increased the barrier's permeability. Both toxins increased inducible nitric oxide synthase (iNOS) mRNA that is indicative of an increase in intracellular NO release, disrupted architecture of the tight junction proteins, suppressed zonula occludens-1 (ZO-1) and occludin (OCL) and junctional adhesive molecules (JAM) mRNA levels, and increased tumor necrosis factor alpha (TNFalpha) and interleukin-1 beta (IL-1beta) mRNA levels. Anti-CD14 antibodies blocked the increase in TNFalpha and IL-1beta mRNA levels but did not affect either changes in the tight junction or iNOS, ZO-1, OCL, and JAM mRNA levels in endothelial cells and astrocytes. Although both toxins did not cross the endothelial barrier, the abluminal neurons exhibited high inflammatory activity characterized by a sequential increase in TNFalpha, IL-1beta, external receptor kinase (ERK), and RelA-p50 that induced inflammation, followed by an increase in anti-inflammatory/apoptotic factors including IL-10 and cysteine-aspartic acid protease-8 (CASPASE-8), which resolve inflammation and induce apoptosis. Anti-CD14 antibodies in luminal buffer blocked the pro- and anti-inflammatory effects of the toxins in neurons. Thus, the CD14-TLR cascade that participates in the inflammatory effects of toxins may not participate in the toxin-induced barrier disruption in vitro. Since the toxins did not cross the endothelial barrier, induction of inflammation in neurons was due to a release of proinflammatory cytokines in the abluminal fluid.

Herpesviral infection and Toll-like receptor 2

In the last decade, substantial progress has been made in understanding the molecular mechanisms involved in the initial host responses to viral infections. Herpesviral infections can provoke an inflammatory cytokine response, however, the innate pathogen-sensing mechanisms that transduce the signal for this response are poorly understood. In recent years, it has become increasingly evident that the Toll-like receptors (TLRs), which are germline-encoded pattern recognition receptors (PRRs), function as potent sensors for infection. TLRs can induce the activation of the innate immunity by recruiting specific intracellular adaptor proteins to initiate signaling pathways, which then culminating in activation of the nuclear factor kappa B (NF-κB) and interferon-regulatory factors (IRFs) that control the transcription of genes encoding type I interferon (IFN I) and other inflammatory cytokines. Furthermore, activation of innate immunity is critical for mounting adaptive immune responses. In parallel, common mechanisms used by viruses to counteract TLR-mediated responses or to actively subvert these pathways that block recognition and signaling through TLRs for their own benefit are emerging. Recent findings have demonstrated that TLR2 plays a crucial role in initiating the inflammatory process, and surprisingly that the response TLR2 triggers might be overzealous in its attempt to counter the attack by the virus. In this review, we summarize and discuss the recent advances about the specific role of TLR2 in triggering inflammatory responses in herpesvirus infection and the consequences of the alarms raised in the host that they are assigned to protect.

Accessory molecules for Toll-like receptors and their function. Nat Rev Immunol. 2012;12:168-179. [PMID: 22301850 DOI: 10.1038/nri3151]

Toll-like receptors (TLRs) are essential components of the innate immune system. Accessory proteins are required for the biosynthesis and activation of TLRs. Here, we summarize recent findings on TLR accessory proteins that are required for cell-surface and endosomal TLR function, and we classify these proteins based on their function as ligand-recognition and delivery cofactors, chaperones and trafficking proteins. Because of their essential roles in TLR function, targeting of such accessory proteins may benefit strategies aimed at manipulating TLR activation for therapeutic applications.

The role of lipopolysaccharide and peptidoglycan, two glycosylated bacterial microbe-associated molecular patterns (MAMPs), in plant innate immunity

In an environment that is rich in potentially pathogenic microorganisms, the survival of higher eukaryotic organisms depends on efficient pathogen sensing and rapidly mounted defence responses. Such protective mechanisms are found in all multicellular organisms, and are collectively referred to as 'innate immunity'. Innate immunity is the first line of defence against invading microorganisms in vertebrates and the only line of defence in invertebrates and plants. Bacterial glycoconjugates, such as lipopolysaccharides (LPSs) from the outer membrane of Gram-negative bacteria and peptidoglycan (PGN) from the cell walls of both Gram-positive and Gram-negative bacteria, have been found to act as elicitors of plant innate immunity. These conserved, indispensable, microbe-specific molecules are also referred to as 'microbe-associated molecular patterns' (MAMPs). MAMPs are recognized by the plant innate immune system through the action of pattern recognition receptors (PRRs). A greater insight into the mechanisms of MAMP recognition and the description of PRRs for different microbial glycoconjugates will have considerable impact on the improvement of plant health and disease resistance. Here, the current knowledge about LPS and PGN as MAMPs is reviewed.

Therapeutic targeting of innate immunity with Toll-like receptor 4 (TLR4) antagonists

Early recognition of invading bacteria by the innate immune system has a crucial function in antibacterial defense by triggering inflammatory responses that prevent the spread of infection and suppress bacterial growth. Toll-like receptor 4 (TLR4), the innate immunity receptor of bacterial endotoxins, plays a pivotal role in the induction of inflammatory responses. TLR4 activation by bacterial lipopolysaccharide (LPS) is achieved by the coordinate and sequential action of three other proteins, LBP, CD14 and MD-2 receptors, that bind lipopolysaccharide (LPS) and present it to TLR4 by forming the activated (TLR4-MD-2-LPS)(2) complex. Small molecules active in modulating the TLR4 activation process have great pharmacological interest as vaccine adjuvants, immunotherapeutics or antisepsis and anti-inflammatory agents. In this review we present natural and synthetic molecules active in inhibiting TLR4-mediated LPS signalling in humans and their therapeutic potential. New pharmacological applications of TLR4 antagonists will be also presented related to the recently discovered role of TLR4 in the insurgence and progression of neuropathic pain and sterile inflammations.

Interaction of the HIV-1 gp120 viral protein V3 loop with bacterial lipopolysaccharide: a pattern recognition inhibition

HIV-1 represents an elusive target for therapeutic compounds due to its high rate of mutation. Targeting structural patterns instead of a constantly changing specific three-dimensional structure may represent an approach that is less sensitive to viral mutations. The V3 loop of gp120 of HIV-1, which is responsible for binding of viral gp120 to CCR5 or CXCR4 coreceptors, has already been identified as an effective target for the inhibition of viral entry. The peptide derived from the V3 loop of gp120 specifically interacts with the lipid A moiety of LPS, as does the full gp120 protein. NMR analysis of V3 in complex with LPS shows formation of an amphipathic turn. The interaction between LPS and V3 relies on the structural pattern, comprising a combination of hydrophobic and charge interactions, similar to the interaction between antimicrobial peptides and LPS. LPS inhibited binding of gp120 to the surface of target T cells. Nonendotoxic LPS antagonists inhibited viral infection, demonstrating the possibility for the development of an inhibitor of HIV-1 attachment to T cells based on the recognition of a conserved structural pattern.

Recognition of pathogenic microbes by the Drosophila phagocytic pattern recognition receptor Eater

Non-opsonic phagocytosis is a primordial form of pathogen recognition that is mediated by the direct interaction of phagocytic receptors with microbial surfaces. In the fruit fly Drosophila melanogaster, the EGF-like repeat containing scavenger receptor Eater is expressed by phagocytes and is required to survive infections with gram-positive and gram-negative bacteria. However, the mechanisms by which this receptor recognizes different types of bacteria are poorly understood. To address this problem, we generated a soluble, Fc-tagged receptor variant of Eater comprising the N-terminal 199 amino acids including four EGF-like repeats. We first established that Eater-Fc displayed specific binding to broad yet distinct classes of heat- or ethanol-inactivated microbes and behaved similarly to the membrane-bound, full-length Eater receptor. We then used Eater-Fc as a tool to probe Eater binding to the surface of live bacteria. Eater-Fc bound equally well to naive or inactivated Staphylococcus aureus or Enterococcus faecalis, suggesting that in vivo, Eater directly targets live gram-positive bacteria, enabling their phagocytic clearance and destruction. By contrast, Eater-Fc was unable to interact with live, naive gram-negative bacteria (Escherichia coli, Serratia marcescens, and Pseudomonas aeruginosa). For these bacteria, Eater-Fc binding required membrane-disrupting treatments. Furthermore, we found that cecropin A, a cationic, membrane-disrupting antimicrobial peptide, could promote Eater-Fc binding to live E. coli, even at sublethal concentrations. These results suggest a previously unrecognized mechanism by which antimicrobial peptides cooperate with phagocytic receptors to extend the range of microbes that can be targeted by a single, germline-encoded receptor.

Green tea polyphenol epigallocatechin-3-gallate inhibits TLR2 signaling induced by peptidoglycan through the polyphenol sensing molecule 67-kDa laminin receptor

Here we show the molecular basis for the inhibition of peptidoglycan (PGN)-induced TLR2 signaling by a major green tea polyphenol epigallocatechin-3-gallate (EGCG). Recently, we identified the 67-kDa laminin receptor (67LR) as the cell-surface EGCG receptor. Anti-67LR antibody treatment or silencing of 67LR resulted in abrogation of the inhibitory action of EGCG on PGN-induced production of pro-inflammatory mediators and activation of mitogen-activated protein kinases. Silencing of Toll-interacting protein (Tollip), a negative regulator of TLR signaling impaired the TLR2 signaling inhibitory activity of EGCG, suggesting that TLR2 response could be inhibited by EGCG via 67LR and Tollip.

Divergent responses to peptidoglycans derived from different E. coli serotypes influence inflammatory outcome in trout, Oncorhynchus mykiss, macrophages

Background

Pathogen-associated molecular patterns (PAMPs) are structural components of pathogens such as lipopolysaccharide (LPS) and peptidoglycan (PGN) from bacterial cell walls. PAMP-recognition by the host results in an induction of defence-related genes and often the generation of an inflammatory response. We evaluated both the transcriptomic and inflammatory response in trout (O. mykiss) macrophages in primary cell culture stimulated with DAP-PGN (DAP; meso-diaminopimelic acid, PGN; peptidoglycan) from two strains of Escherichia coli (PGN-K12 and PGN-O111:B4) over time.

Results

Transcript profiling was assessed using function-targeted cDNA microarray hybridisation (n = 36) and results show differential responses to both PGNs that are both time and treatment dependent. Wild type E. coli (K12) generated an increase in transcript number/diversity over time whereas PGN-O111:B4 stimulation resulted in a more specific and intense response. In line with this, Gene Ontology analysis (GO) highlights a specific transcriptomic remodelling for PGN-O111:B4 whereas results obtained for PGN-K12 show a high similarity to a generalised inflammatory priming response where multiple functional classes are related to ribosome biogenesis or cellular metabolism. Prostaglandin release was induced by both PGNs and macrophages were significantly more sensitive to PGN-O111:B4 as suggested from microarray data.

Conclusion

Responses at the level of the transcriptome and the inflammatory outcome (prostaglandin synthesis) highlight the different sensitivity of the macrophage to slight differences (serotype) in peptidoglycan structure. Such divergent responses are likely to involve differential receptor sensitivity to ligands or indeed different receptor types. Such changes in biological response will likely reflect upon pathogenicity of certain serotypes and the development of disease.