Structural characterization of the lipid A component of Helicobacter pylori rough- and smooth-form lipopolysaccharides

Pathogenicity and virulence of Helicobacter pylori: A paradigm of chronic infection

Infection with Helicobacter pylori is one of the most common infections of mankind. Infection typically occurs in childhood and persists for the lifetime of the host unless eradicated with antimicrobials. The organism colonizes the stomach and causes gastritis. Most infected individuals are asymptomatic, but infection also causes gastric and duodenal ulceration, and gastric cancer. H. pylori possesses an arsenal of virulence factors, including a potent urease enzyme for protection from acid, flagella that mediate motility, an abundance of outer membrane proteins that can mediate attachment, several immunomodulatory proteins, and an ability to adapt to specific conditions in individual human stomachs. The presence of a type 4 secretion system that injects effector molecules into gastric cells and subverts host cell signalling is associated with virulence. In this review we discuss the interplay of H. pylori colonization and virulence factors with host and environmental factors to determine disease outcome in infected individuals.

Comparison of gastric inflammation and metaplasia induced by Helicobacter pylori or Helicobacter felis colonization in mice

Gastric cancer is the fifth most diagnosed cancer in the world. Infection by the bacteria Helicobacter pylori (HP) is associated with approximately 75% of gastric cancer cases. HP infection induces chronic gastric inflammation, damaging the stomach and fostering carcinogenesis. Most mechanistic studies on gastric cancer initiation are performed in mice and utilize either mouse-adapted strains of HP or the natural mouse pathogen Helicobacter felis (HF). Here, we identified the differences in gastric inflammation, atrophy, and metaplasia associated with HP and HF infection in mice. PMSS1 HP strain or the CS1 HF strain were co-cultured with mouse peritoneal macrophages to assess their immunostimulatory effects. HP and HF induced similar cytokine production from cultured mouse peritoneal macrophages revealing that both bacteria exhibit similar immunostimulatory effects in vitro. Next, C57BL/6J mice were infected with HP or HF and were assessed 2 months post-infection. HP-infected mice caused modest inflammation within both the gastric corpus and antrum, and did not induce significant atrophy within the gastric corpus. In contrast, HF induced significant inflammation throughout the gastric corpus and antrum. Moreover, HF infection was associated with significant atrophy of the chief and parietal cell compartments and induced the expression of pyloric metaplasia (PM) markers. HP is poorly immunogenic compared to HF. HF induces dramatic CD4+ T cell activation, which is associated with increased gastric cancer risk in humans. Thus, HP studies in mice are better suited for studies on colonization, while HF is more strongly suited for studies on the effects of gastric inflammation on tumorigenesis.  .

IMPORTANCE

Mouse infection models with Helicobacter species are widely used to study Helicobacter pathogenesis and gastric cancer initiation. However, Helicobacter pylori is not a natural mouse pathogen, and mouse-adapted H. pylori strains are poorly immunogenic. In contrast, Helicobacter felis is a natural mouse pathogen that induces robust gastric inflammation and is often used in mice to investigate gastric cancer initiation. Although both bacterial strains are widely used, their disease pathogenesis in mice differs dramatically. However, few studies have directly compared the pathogenesis of these bacterial species in mice, and the contrasting features of these two models are not clearly defined. This study directly compares the gastric inflammation, atrophy, and metaplasia development triggered by the widely used PMSS1 H. pylori and CS1 H. felis strains in mice. It serves as a useful resource for researchers to select the experimental model best suited for their studies.

Helicobacter pylori enhances HLA-C expression in the human gastric adenocarcinoma cells AGS and can protect them from the cytotoxicity of natural killer cells

Helicobacter pylori (H. pylori) seems to play causative roles in gastric cancers. H. pylori has also been detected in established gastric cancers. How the presence of H. pylori modulates immune response to the cancer is unclear. The cytotoxicity of natural killer (NK) cells, toward infected or malignant cells, is controlled by the repertoire of activating and inhibitory receptors expressed on their surface. Here, we studied H. pylori-induced changes in the expression of ligands, of activating and inhibitory receptors of NK cells, in the gastric adenocarcinoma AGS cells, and their impacts on NK cell responses. AGS cells lacked or had low surface expression of the class I major histocompatibility complex (MHC-I) molecules HLA-E and HLA-C-ligands of the major NK cell inhibitory receptors NKG2A and killer-cell Ig-like receptor (KIR), respectively. However, AGS cells had high surface expression of ligands of activating receptors DNAM-1 and CD2, and of the adhesion molecules LFA-1. Consistently, AGS cells were sensitive to killing by NK cells despite the expression of inhibitory KIR on NK cells. Furthermore, H. pylori enhanced HLA-C surface expression on AGS cells. H. pylori infection enhanced HLA-C protein synthesis, which could explain H. pylori-induced HLA-C surface expression. H. pylori infection enhanced HLA-C surface expression also in the hepatoma Huh7 and HepG2 cells. Furthermore, H. pylori-induced HLA-C surface expression on AGS cells promoted inhibition of NK cells by KIR, and thereby protected AGS cells from NK cell cytotoxicity. These results suggest that H. pylori enhances HLA-C expression in host cells and protects them from the cytotoxic attack of NK cells expressing HLA-C-specific inhibitory receptors.

Comparison of gastric inflammation and metaplasia induced by Helicobacter pylori or Helicobacter felis colonization in mice

Background

Gastric cancer is the fifth most diagnosed cancer in the world. Infection by the bacteria Helicobacter pylori (HP) is associated with approximately 75% of gastric cancer cases. HP infection induces chronic gastric inflammation, damaging the stomach and fostering carcinogenesis. Most mechanistic studies on Helicobacter- induced gastric cancer initiation are performed in mice and utilize either mouse-adapted strains of HP or the natural mouse pathogen Helicobacter felis (HF). Each of these infection models is associated with strengths and weaknesses. Here, we identified the differences in immunogenicity and gastric pathological changes associated with HP and HF infection in mice.

Material and Methods

PMSS1 HP strain or with the CS1 HF strain were co-cultured with mouse peritoneal macrophages to assess their immunostimulatory effects. C57BL/6J mice were infected with HP or HF, and gastric inflammation, atrophy, and metaplasia development were assessed 2 months post-infection.

Results

HP and HF induced similar cytokine production from cultured mouse peritoneal macrophages. HP-infected mice caused modest inflammation within both the gastric corpus and antrum and did not induce significant atrophy within the gastric corpus. In contrast, HF induced significant inflammation throughout the gastric corpus and antrum. Moreover, HF infection was associated with significant atrophy of the chief and parietal cell compartments and induced expression of pyloric metaplasia markers.

Conclusions

HP is poorly immunogenic compared to HF. HF induces dramatic CD4+ T cell activation, which is associated with increased gastric cancer risk in humans. Thus, HP studies in mice are better suited for studies on colonization, while HF is more strongly suited for pathogenesis and cancer initiation studies.

Immune Biology and Persistence of Helicobacter pylori in Gastric Diseases

Helicobacter pylori is a prevalent pathogen, which affects more than 40% of the global population. It colonizes the human stomach and persists in its host for several decades or even a lifetime, if left untreated. The persistent infection has been linked to various gastric diseases, including gastritis, peptic ulcers, and an increased risk for gastric cancer. H. pylori infection triggers a strong immune response directed against the bacterium associated with the infiltration of innate phagocytotic immune cells and the induction of a Th1/Th17 response. Even though certain immune cells seem to be capable of controlling the infection, the host is unable to eliminate the bacteria as H. pylori has developed remarkable immune evasion strategies. The bacterium avoids its killing through innate recognition mechanisms and manipulates gastric epithelial cells and immune cells to support its persistence. This chapter focuses on the innate and adaptive immune response induced by H. pylori infection, and immune evasion strategies employed by the bacterium to enable persistent infection.

A Sensitive GC-MS Method for Quantitation of Lipid A Backbone Components and Terminal Phosphate Modifications

Lipid A, the hydrophobic anchor of lipopolysaccharide (LPS) present in the outer membrane of Gram-negative bacteria, serves as a target for cationic antimicrobial peptides, such as polymyxins. Membrane stress from polymyxins results in activation of two-component regulatory systems that produce lipid A modifying enzymes. These enzymes add neutral moieties, such as aminoarabinose (AraN) and ethanolamine (EtN) to lipid A terminal phosphates that mask the phosphate's negative charge and inhibit electrostatic interaction with the cationic polymyxins. Currently, these modifications may be detected by MALDI-TOF MS; however, this analysis is only semiquantitative. Herein we describe a GC-MS method to quantitate lipid A backbone components, glucosamine (GlcN) and inorganic phosphate (Pi), along with terminal phosphate modifications AraN and EtN. In this assay, lipid A is isolated from Gram-negative bacterial samples, hydrolyzed into its individual moieties, and derivatized via methoximation followed by silylation prior to analysis via GC-MS. Changes in AraN and EtN quantity were characterized using a variety of regulatory mutants of Salmonella, revealing differences that were not detected using MALDI-TOF MS analysis. Additionally, an increase in the abundance of AraN and EtN modifications were observed when resistant Enterobacter and Escherichia coli strains were grown in the presence of colistin (polymyxin E). Lastly, increased levels of Pi were found in bisphosphorylated lipid A compared to monophosphorylated lipid A samples. Because lipid A modifications serve as indicators of polymyxin resistance in Gram-negative bacteria, this method provides the capacity to monitor polymyxin resistance by quantification of lipid A modification using GC-MS.

Variation, Modification and Engineering of Lipid A in Endotoxin of Gram-Negative Bacteria

Lipid A of Gram-negative bacteria is known to represent a central role for the immunological activity of endotoxin. Chemical structure and biosynthetic pathways as well as specific receptors on phagocytic cells had been clarified by the beginning of the 21st century. Although the lipid A of enterobacteria including Escherichia coli share a common structure, other Gram-negative bacteria belonging to various classes of the phylum Proteobacteria and other taxonomical groups show wide variety of lipid A structure with relatively decreased endotoxic activity compared to that of E. coli. The structural diversity is produced from the difference of chain length of 3-hydroxy fatty acids and non-hydroxy fatty acids linked to their hydroxyl groups. In some bacteria, glucosamine in the backbone is substituted by another amino sugar, or phosphate groups bound to the backbone are modified. The variation of structure is also introduced by the enzymes that can modify electrostatic charges or acylation profiles of lipid A during or after its synthesis. Furthermore, lipid A structure can be artificially modified or engineered by the disruption and introduction of biosynthetic genes especially those of acyltransferases. These technologies may produce novel vaccine adjuvants or antagonistic drugs derived from endotoxin in the future.

Mucoid and coccoid Helicobacter pylori with fast growth and antibiotic resistance

BACKGROUND

In this study, one Helicobacter pylori isolate, from gastric biopsy of a dyspeptic patient that turned into mucoid-coccoid (MC) form upon consecutive subcultures, was identified. The culturability, antibiotic resistance, and lipid contents of MC were compared with those of non-mucoid (NM) spiral H pylori.

MATERIALS AND METHODS

Mucoid-coccoid and NM H pylori were subcultured on Brucella blood agar (BBA) and incubated under aerobic and microaerobic atmospheres at 37°C. Cultures were examined for colony characteristics and bacterial morphology after 1-3 days. The isolates were identified by biochemical tests and detection of H pylori-16S rDNA. Antibiogram was performed with currently used antibiotics for H pylori eradication. Cellular lipid contents were extracted and analyzed by gas chromatography.

RESULTS

Compared with pin-pointed and glistening colonies of NM H pylori that appeared under microaerobic conditions, MC H pylori grew well in consecutive subcultures under aerobic and microaerobic atmospheres and produced white patches of mucoid colonies. MC exhibited coccoid and NM spiral morphology. Both isolates were catalase, oxidase, and urease positive and contained 16S rDNA. Compared with NM that was susceptible to almost all the antibiotics, MC was resistant to all the antibiotics. Lipid analyses showed high frequency of unsaturated fatty acids and cholesterol in MC.

CONCLUSIONS

Coccoid forms with high fatty acid and cholesterol contents that show resistance to antibiotics might resist against other stressful conditions such as gastric acidity and immune response. Moreover, mucoid property may enhance resistance of coccoids to stresses. With mucoid-coccoid lifestyle, H pylori may establish a chronic infection refractory to antimicrobial therapy.

Variations in the Expression of Terminal Oligosaccharide Units and Glycosylation of Poly(N-acetyllactosamine) Chain in the Helicobacter pylori Lipopolysaccharide upon Colonization of Rhesus Macaques

Helicobacter pylori is an important human pathogen that causes gastritis, gastric and duodenal ulcers, and gastric cancer. O-polysaccharides of H. pylori lipopolysaccharide (LPS) are composed of (β1→3)-poly(N-acetyllactosamine) (polyLacNAc) decorated with multiple α-L-fucose residues. In many strains, their terminal LacNAc units are mono- or di-fucosylated to mimic Lewis X (Le^x) and/or Lewis Y (Le^y) oligosaccharides. The studies in rhesus macaques as a model of human infection by H. pylori showed that this bacterium adapts to the host during colonization by expressing host Lewis antigens. Here, we characterized LPS from H. pylori strains used in the previous study, including the parental J166 strain and the three derivatives (98-149, 98-169, and 98-181) isolated from rhesus macaques after long-term colonization. Chemical and NMR spectroscopic analyses of the LPS showed that the parent strain expressed Le^x, Le^y, and H type 1 terminal oligosaccharide units. The daughter strains were similar to the parental one in the presence of the same LPS core and fucosylated polyLacNAc chain of the same length but differed in the terminal oligosaccharide units. These were Le^x in the isolates 98-149 and 98-169, which corresponded to the Le^a phenotype of the host animals, and Le^y was found in the 98-181 isolate from the macaque characterized by the Le^b phenotype. As Le^a and Le^b are isomers of Le^x and Le^y, respectively, the observed correlation confirmed adaptation of the expression of terminal oligosaccharide units in H. pylori strains to the properties of the host gastric mucosa. The 98-181 strain also acquired glucosylation of the polyLacNAc chain and was distinguished by a lower expression of fucosylated internal LacNAc units (internal Le^x) as a result of decoration of polyLacNAc with β-glucopyranose, which may also play a role in the bacterial adaptation.

Prokaryotic and Mitochondrial Lipids: A Survey of Evolutionary Origins

Mitochondria and bacteria share a myriad of properties since it is believed that the powerhouses of the eukaryotic cell have evolved from a prokaryotic origin. Ribosomal RNA sequences, DNA architecture and metabolism are strikingly similar in these two entities. Proteins and nucleic acids have been a hallmark for comparison between mitochondria and prokaryotes. In this chapter, similarities (and differences) between mitochondrial and prokaryotic membranes are addressed with a focus on structure-function relationship of different lipid classes. In order to be suitable for the theme of the book, a special emphasis is reserved to the effects of bioactive sphingolipids, mainly ceramide, on mitochondrial membranes and their roles in initiating programmed cell death.

The Interaction of Helicobacter pylori with TFF1 and Its Role in Mediating the Tropism of the Bacteria Within the Stomach

Helicobacter pylori colonises the human stomach and has tropism for the gastric mucin, MUC5AC. The majority of organisms live in the adherent mucus layer within their preferred location, close to the epithelial surface where the pH is near neutral. Trefoil factor 1 (TFF1) is a small trefoil protein co-expressed with the gastric mucin MUC5AC in surface foveolar cells and co-secreted with MUC5AC into gastric mucus. Helicobacter pylori binds with greater avidity to TFF1 dimer, which is present in gastric mucus, than to TFF1 monomer. Binding of H. pylori to TFF1 is mediated by the core oligosaccharide subunit of H. pylori lipopolysaccharide at pH 5.0–6.0. Treatment of H. pylori lipopolysaccharide with mannosidase or glucosidase inhibits its interaction with TFF1. Both TFF1 and H. pylori have a propensity for binding to mucins with terminal non-reducing α- or β-linked N-acetyl-d-glucosamine or α-(2,3) linked sialic acid or Gal-3-SO₄²⁻. These findings are strong evidence that TFF1 has carbohydrate-binding properties that may involve a conserved patch of aromatic hydrophobic residues on the surface of its trefoil domain. The pH-dependent lectin properties of TFF1 may serve to locate H. pylori deep in the gastric mucus layer close to the epithelium rather than at the epithelial surface. This restricted localisation could limit the interaction of H. pylori with epithelial cells and the subsequent host signalling events that promote inflammation.

Mechanisms of Inflammasome Signaling, microRNA Induction and Resolution of Inflammation by Helicobacter pylori

Inflammasome-controlled transcription and subsequent cleavage-mediated activation of mature IL-1β and IL-18 cytokines exemplify a crucial innate immune mechanism to combat intruding pathogens. Helicobacter pylori represents a predominant persistent infection in humans, affecting approximately half of the population worldwide, and is associated with the development of chronic gastritis, peptic ulcer disease, and gastric cancer. Studies in knockout mice have demonstrated that the pro-inflammatory cytokine IL-1β plays a central role in gastric tumorigenesis. Infection by H. pylori was recently reported to stimulate the inflammasome both in cells of the mouse and human immune systems. Using mouse models and in vitro cultured cell systems, the bacterial pathogenicity factors and molecular mechanisms of inflammasome activation have been analyzed. On the one hand, it appears that H. pylori-stimulated IL-1β production is triggered by engagement of the immune receptors TLR2 and NLRP3, and caspase-1. On the other hand, microRNA hsa-miR-223-3p is induced by the bacteria, which controls the expression of NLRP3. This regulating effect by H. pylori on microRNA expression was also described for more than 60 additionally identified microRNAs, indicating a prominent role for inflammatory and other responses. Besides TLR2, TLR9 becomes activated by H. pylori DNA and further TLR10 stimulated by the bacteria induce the secretion of IL-8 and TNF, respectively. Interestingly, TLR-dependent pathways can accelerate both pro- and anti-inflammatory responses during H. pylori infection. Balancing from a pro-inflammation to anti-inflammation phenotype results in a reduction in immune attack, allowing H. pylori to persistently colonize and to survive in the gastric niche. In this chapter, we will pinpoint the role of H. pylori in TLR- and NLRP3 inflammasome-dependent signaling together with the differential functions of pro- and anti-inflammatory cytokines. Moreover, the impact of microRNAs on H. pylori-host interaction will be discussed, and its role in resolution of infection versus chronic infection, as well as in gastric disease development.

Progress in the synthesis and biological evaluation of lipid A and its derivatives

Lipid A is one of the core structures of bacterial lipopolysaccharides (LPSs), and it is mainly responsible for the strong immunostimulatory activities of LPS through interactions with the Toll-like receptors and other molecules in the human immune system. To obtain structurally homogeneous and well-defined lipid As and its derivatives in quantities meaningful for various biological studies and applications, their chemical synthesis has become a focal point. This review has provided a survey of significant progresses made in the synthesis of lipid A, and its derivatives that carry diverse saturated and unsaturated lipids, have the phosphate group at its reducing end replaced with a more stable phosphate or carboxyl group, or lack the reducing end phosphate or both phosphate groups, as well as progresses in the synthesis of LPS analogs and other lipid A conjugates. These synthetic molecules have facilitated the elucidation of the structure-activity relationships of lipid A useful for the design and development of lipid A based therapeutics, such as those utilized to treat sepsis, and other medical applications, for example the use of monophosphoryl lipid A as a carrier molecule for the study of fully synthetic self-adjuvanting conjugate vaccines. These topics are also briefly covered in the current review.

Aminosugar-based immunomodulator lipid A: synthetic approaches

The immediate immune response to infection by Gram-negative bacteria depends on the structure of a lipopolysaccharide (LPS, also known as endotoxin), a complex glycolipid constituting the outer leaflet of the bacterial outer membrane. Recognition of picomolar quantities of pathogenic LPS by the germ-line encoded Toll-like Receptor 4 (TLR4) complex triggers the intracellular pro-inflammatory signaling cascade leading to the expression of cytokines, chemokines, prostaglandins and reactive oxygen species which manifest an acute inflammatory response to infection. The "endotoxic principle" of LPS resides in its amphiphilic membrane-bound fragment glycophospholipid lipid A which directly binds to the TLR4·MD-2 receptor complex. The lipid A content of LPS comprises a complex mixture of structural homologs varying in the acylation pattern, the length of the (R)-3-hydroxyacyl- and (R)-3-acyloxyacyl long-chain residues and in the phosphorylation status of the β(1→6)-linked diglucosamine backbone. The structural heterogeneity of the lipid A isolates obtained from bacterial cultures as well as possible contamination with other pro-inflammatory bacterial components makes it difficult to obtain unambiguous immunobiological data correlating specific structural features of lipid A with its endotoxic activity. Advanced understanding of the therapeutic significance of the TLR4-mediated modulation of the innate immune signaling and the central role of lipid A in the recognition of LPS by the innate immune system has led to a demand for well-defined materials for biological studies. Since effective synthetic chemistry is a prerequisite for the availability of homogeneous structurally distinct lipid A, the development of divergent and reproducible approaches for the synthesis of various types of lipid A has become a subject of considerable importance. This review focuses on recent advances in synthetic methodologies toward LPS substructures comprising lipid A and describes the synthesis and immunobiological properties of representative lipid A variants corresponding to different bacterial species. The main criteria for the choice of orthogonal protecting groups for hydroxyl and amino functions of synthetically assembled β(1→6)-linked diglucosamine backbone of lipid A which allows for a stepwise introduction of multiple functional groups into the molecule are discussed. Thorough consideration is also given to the synthesis of 1,1'-glycosyl phosphodiesters comprising partial structures of 4-amino-4-deoxy-β-L-arabinose modified Burkholderia lipid A and galactosamine-modified Francisella lipid A. Particular emphasis is put on the stereoselective construction of binary glycosyl phosphodiester fragments connecting the anomeric centers of two aminosugars as well as on the advanced P(III)-phosphorus chemistry behind the assembly of zwitterionic double glycosyl phosphodiesters.

Comparison of lipopolysaccharides composition of two different strains of Helicobacter pylori

BACKGROUND

Helicobacter pylori (H. pylori) is a Gram-negative, microaerophilic bacterium that is recognized as a major cause of chronic gastritis, peptic ulcers, and gastric cancer. Comparable to other Gram-negative bacteria, lipopolysaccharides (LPS) are an important cellular component of the outer membrane of H. pylori. The LPS of this organism plays a key role in its colonization and persistence in the stomach. In addition, H. pylori LPS modulates pathogen-induced host inflammatory responses resulting in chronic inflammation within the gastrointestinal tract. Very little is known about the comparative LPS compositions of different strains of H. pylori with varied degree of virulence in human. Therefore, LPS was analyzed from two strains of H. pylori with differing potency in inducing inflammatory responses (SS1 and G27). LPS were extracted from aqueous and phenol layer of hot-phenol water extraction method and subjected for composition analysis by gas chromatography - mass spectrometry (GC-MS) to sugar and fatty acid compositions.

RESULTS

The major difference between the two strains of H. pylori is the presence of Rhamnose, Fucose and GalNAc in the SS1 strain, which was either not found or with low abundance in the G27 strain. On the other hand, high amount of Mannose was present in G27 in comparison to SS1. Fatty acid composition of lipid-A portion also showed considerable amount of differences between the two strains, phenol layer of SS1 had enhanced amount of 3 hydroxy decanoic acid (3-OH-C10:0) and 3-hydroxy dodecanoic acid (3-OH-C12:0) which were not present in G27, whereas myristic acid (C14:0) was present in G27 in relatively high amount.

CONCLUSION

The composition analysis of H. pylori LPS, revealed differences in sugars and fatty acids composition between a mouse adapted strain SS1 and G27. This knowledge provides a novel way to dissect out their importance in host-pathogen interaction in further studies.

TIFA Signaling in Gastric Epithelial Cells Initiates the cag Type 4 Secretion System-Dependent Innate Immune Response to Helicobacter pylori Infection

Helicobacter pylori is a bacterial pathogen that colonizes the human stomach, causing inflammation which, in some cases, leads to gastric ulcers and cancer. The clinical outcome of infection depends on a complex interplay of bacterial, host genetic, and environmental factors. Although H. pylori is recognized by both the innate and adaptive immune systems, this rarely results in bacterial clearance. Gastric epithelial cells are the first line of defense against H. pylori and alert the immune system to bacterial presence. Cytosolic delivery of proinflammatory bacterial factors through the cag type 4 secretion system (cag-T4SS) has long been appreciated as the major mechanism by which gastric epithelial cells detect H. pylori Classically attributed to the peptidoglycan sensor NOD1, recent work has highlighted the role of NOD1-independent pathways in detecting H. pylori; however, the bacterial and host factors involved have remained unknown. Here, we show that bacterially derived heptose-1,7-bisphosphate (HBP), a metabolic precursor in lipopolysaccharide (LPS) biosynthesis, is delivered to the host cytosol through the cag-T4SS, where it activates the host tumor necrosis factor receptor-associated factor (TRAF)-interacting protein with forkhead-associated domain (TIFA)-dependent cytosolic surveillance pathway. This response, which is independent of NOD1, drives robust NF-κB-dependent inflammation within hours of infection and precedes NOD1 activation. We also found that the CagA toxin contributes to the NF-κB-driven response subsequent to TIFA and NOD1 activation. Taken together, our results indicate that the sequential activation of TIFA, NOD1, and CagA delivery drives the initial inflammatory response in gastric epithelial cells, orchestrating the subsequent recruitment of immune cells and leading to chronic gastritis.IMPORTANCEH. pylori is a globally prevalent cause of gastric and duodenal ulcers and cancer. H. pylori antibiotic resistance is rapidly increasing, and a vaccine remains elusive. The earliest immune response to H. pylori is initiated by gastric epithelial cells and sets the stage for the subsequent immunopathogenesis. This study revealed that host TIFA and H. pylori-derived HBP are critical effectors of innate immune signaling that account for much of the inflammatory response to H. pylori in gastric epithelial cells. HBP is delivered to the host cell via the cag-T4SS at a time point that precedes activation of the previously described NOD1 and CagA inflammatory pathways. Manipulation of the TIFA-driven immune response in the host and/or targeting of ADP-heptose biosynthesis enzymes in H. pylori may therefore provide novel strategies that may be therapeutically harnessed to achieve bacterial clearance.

The redefinition of Helicobacter pylori lipopolysaccharide O-antigen and core-oligosaccharide domains

Helicobacter pylori lipopolysaccharide promotes chronic gastric colonisation through O-antigen host mimicry and resistance to mucosal antimicrobial peptides mediated primarily by modifications of the lipid A. The structural organisation of the core and O-antigen domains of H. pylori lipopolysaccharide remains unclear, as the O-antigen attachment site has still to be identified experimentally. Here, structural investigations of lipopolysaccharides purified from two wild-type strains and the O-antigen ligase mutant revealed that the H. pylori core-oligosaccharide domain is a short conserved hexasaccharide (Glc-Gal-DD-Hep-LD-Hep-LD-Hep-KDO) decorated with the O-antigen domain encompassing a conserved trisaccharide (-DD-Hep-Fuc-GlcNAc-) and variable glucan, heptan and Lewis antigens. Furthermore, the putative heptosyltransferase HP1284 was found to be required for the transfer of the third heptose residue to the core-oligosaccharide. Interestingly, mutation of HP1284 did not affect the ligation of the O-antigen and resulted in the attachment of the O-antigen onto an incomplete core-oligosaccharide missing the third heptose and the adjoining Glc-Gal residues. Mutants deficient in either HP1284 or O-antigen ligase displayed a moderate increase in susceptibility to polymyxin B but were unable to colonise the mouse gastric mucosa. Finally, mapping mutagenesis and colonisation data of previous studies onto the redefined organisation of H. pylori lipopolysaccharide revealed that only the conserved motifs were essential for colonisation. In conclusion, H. pylori lipopolysaccharide is missing the canonical inner and outer core organisation. Instead it displays a short core and a longer O-antigen encompassing residues previously assigned as the outer core domain. The redefinition of H. pylori lipopolysaccharide domains warrants future studies to dissect the role of each domain in host-pathogen interactions. Also enzymes involved in the assembly of the conserved core structure, such as HP1284, could be attractive targets for the design of new therapeutic agents for managing persistent H. pylori infection causing peptic ulcers and gastric cancer.

Microbiota, Immune Subversion, and Chronic Inflammation

Several host-adapted pathogens and commensals have evolved mechanisms to evade the host innate immune system inducing a state of low-grade inflammation. Epidemiological studies have also documented the association of a subset of these microorganisms with chronic inflammatory disorders. In this review, we summarize recent studies demonstrating the role of the microbiota in chronic inflammatory diseases and discuss how specific microorganisms subvert or inhibit protective signaling normally induced by toll-like receptors (TLRs). We highlight our work on the oral pathogen Porphyromonas gingivalis and discuss the role of microbial modulation of lipid A structures in evasion of TLR4 signaling and resulting systemic immunopathology associated with atherosclerosis. P. gingivalis intrinsically expresses underacylated lipid A moieties and can modify the phosphorylation of lipid A, leading to altered TLR4 signaling. Using P. gingivalis mutant strains expressing distinct lipid A moieties, we demonstrated that expression of antagonist lipid A was associated with P. gingivalis-mediated systemic inflammation and immunopathology, whereas strains expressing agonist lipid A exhibited modest systemic inflammation. Likewise, mice deficient in TLR4 were more susceptible to vascular inflammation after oral infection with P. gingivalis wild-type strain compared to mice possessing functional TLR4. Collectively, our studies support a role for P. gingivalis-mediated dysregulation of innate and adaptive responses resulting in immunopathology and systemic inflammation. We propose that anti-TLR4 interventions must be designed with caution, given the balance between the protective and destructive roles of TLR signaling in response to microbiota and associated immunopathologies.

The Importance of Toll-like Receptors in NF-κB Signaling Pathway Activation by Helicobacter pylori Infection and the Regulators of this Response

Helicobacter pylori (H. pylori) is a common pathogenic bacterium in the stomach that infects almost half of the population worldwide and is closely related to gastric diseases and some extragastric diseases, including iron-deficiency anemia and idiopathic thrombocytopenic purpura. Both the Maastricht IV/Florence consensus report and the Kyoto global consensus report have proposed the eradication of H. pylori to prevent gastric cancer as H.pylori has been shown to be a major cause of gastric carcinogenesis. The interactions between H. pylori and host receptors induce the release of the proinflammatory cytokines by activating proinflammatory signaling pathways such as nuclear factor kappa B (NF-κB), which plays a central role in inflammation, immune response, and carcinogenesis. Among these receptors, Toll-like receptors (TLRs) are classical pattern recognition receptors in the recognition of H. pylori and the mediation of the host inflammatory and immune responses to H. pylori. TLR polymorphisms also contribute to the clinical consequences of H. pylori infection. In this review, we focus on the functions of TLRs in the NF-κB signaling pathway activated by H. pylori, the regulators modulating this response, and the functions of TLR polymorphisms in H.pylori-related diseases.

Structural and biological characterisation of a novel tetra-acyl lipid A from Escherichia coli F515 lipopolysaccharide acting as endotoxin antagonist in human monocytes

We here report on the structural analysis of a novel tetra-acyl lipid A (LAtetra) isolated from Escherichia coli deep rough (Re)-mutant strain F515. In addition to the biologically active hexa-acyl E. coli-type lipid A (compound 506), this incompletely acylated lipid A was found to be also present in the native LPS. Its structure was studied without further derivatisation by chemical analysis, matrix-assisted laser desorption/ionization mass spectrometry, and one- and two-dimensional 1H- and 13C-NMR spectroscopy. It was found to be structurally distinct from the tetraacyl lipid A biosynthetic precursor Ia (compound 406) in lacking the primary ( R)-3-hydroxytetradecanoic acid 14:0(3-OH) in position 3′ ester-linked to the `non-reducing' glucosamine (GlcN II). The hydroxyl group at the ( R)-3-hydroxytetradecanoic acid attached to position 2′ of GlcN II was found to be substituted by dodecanoic acid (12:0), thus forming a dodecanoyloxytetradecanoyl residue 14:0[3- O(12:0)]. The acylation pattern at the `reducing' GlcN I was identical to that of compound 406 in having two primary ( R)-3-hydroxy tetradecanoic acid residues [14:0(3-OH)] attached to positions 3 (ester-linked) and 2 (amide-linked), respectively. In human mononuclear cells (hMNC) the new LAtetra antagonized LPS-induced release of interleukine-1 (IL-1), interleukine-6 (IL-6), and tumor necrosis factor (TNF) in a dose-dependant manner with identical antagonistic potency as compared with compound 406. Also like compound 406, it was found to be an agonist in murine macrophage-like J774.1 cells.

Invited review: Diversity of endotoxin and its impact on pathogenesis

Lipopolysaccharide or LPS is localized to the outer leaflet of the outer membrane and serves as the major surface component of the bacterial cell envelope. This remarkable glycolipid is essential for virtually all Gram-negative organisms and represents one of the conserved microbial structures responsible for activation of the innate immune system. For these reasons, the structure, function, and biosynthesis of LPS has been an area of intense research. The LPS of a number of bacteria is composed of three distinct regions — lipid A, a short core oligosaccharide, and the O-antigen polysaccharide. The lipid A domain, also known as endotoxin, anchors the molecule in the outer membrane and is the bioactive component recognized by TLR4 during human infection. Overall, the biochemical synthesis of lipid A is a highly conserved process; however, investigation of the lipid A structures of various organisms shows an impressive amount of diversity. These differences can be attributed to the action of latent enzymes that modify the canonical lipid A molecule. Variation of the lipid A domain of LPS serves as one strategy utilized by Gram-negative bacteria to promote survival by providing resistance to components of the innate immune system and helping to evade recognition by TLR4. This review summarizes the biochemical machinery required for the production of diverse lipid A structures of human pathogens and how structural modification of endotoxin impacts pathogenesis.

Serum antibody response against Helicobacter pylori NCTC 11637 smooth- and rough-lipopolysaccharide phenotypes in patients with H. pylori-related gastropathy

The antigenicity of the Helicobacter pylori lipopolysaccharide (LPS) molecule during the course of natural H. pylori infection in humans was investigated. The IgG and IgA responses against smooth (S)- and rough (R)-form LPS were evaluated in H. pylori positive patients with chronic gastritis (CG, n = 30) and duodenal ulcer disease (DU, n = 16), and in 15 H. pylori-negative dyspeptic subjects. The results demonstrated that anti H. pylori LPS IgG and IgA antibody levels were significantly enhanced in both groups of H. pylori-positive patients compared with H. pylori-negative subjects, thus confirming that H. pylori LPS is part of the immunogenic antigen profile of the bacterium. In addition, a marked response against R-LPS, which significantly correlated with that observed against S-LPS, was found for both IgG and IgA, thus indicating that core oligosaccharide plays a powerful immunogenic role. Since the O-side chain of LPS from H. pylori NCTC 11637 contains epitopes which mimic Lewis x (Lex) antigens, the presence of antibodies to monomeric, trimeric, and polymeric Lex was also investigated. Antibodies against polymeric Lex were detected in two patients suffering from chronic atrophic gastritis and active chronic gastritis, respectively.

Remodeling of Helicobacter pylori lipopolysaccharide

Modification of the lipid A domain of lipopolysaccharide (LPS) has been reported to contribute to the virulence and pathogenesis of various Gram-negative bacteria. The Kdo (3-deoxy-D- manno-octulosonic acid)—lipid A domain of Helicobacter pylori LPS shows several differences to that of Escherichia coli. It has fewer acyl chains, a reduced number of phosphate groups, much lower immunobiological activity, and only a single Kdo sugar is attached to the disaccharide backbone. However, H. pylori synthesizes a minor lipid A species resembling that of E. coli, which is both bis-phosphorylated and hexa-acylated suggesting that the major species results from the action of specific modifying enzymes. This work describes two enzymes, a lipid A phosphatase and a phosphoethanolamine transferase, involved in the periplasmic modification of the 1-position of H. pylori lipid A. Furthermore, we report a novel Kdo trimming enzyme that requires prior removal of the 1-phosphate group for enzymatic activity. Discovery of the enzymatic machinery involved in the remodeling of H. pylori LPS will help unravel the importance of these modifications in H. pylori pathogenesis.

Apoptosis induction in gastric mucous cells in vitro: lesser potency of Helicobacter pylori than Escherichia coli lipopolysaccharide, but positive interaction with ibuprofen

Non-steroidal anti-inflammatory drugs (NSAIDs) cause peptic ulcer disease, but whether they interact with Helicobacter pylori to promote damage is controversial. Moreover, the reported induction of apoptosis in gastric cells by H. pylori lipopolysaccharide (LPS) (10—9 g/ml) contrasts with studies showing low immunological potency of this LPS. Therefore, the effects of LPS from H. pylori NCTC 11637 and Escherichia coli O111:B4 on apoptosis in a primary culture of guinea-pig gastric mucous cells were investigated in the presence and absence of the NSAID, ibuprofen. Cell loss was estimated by a crystal violet assay, and apoptosis determined from caspase activity and from condensation and fragmentation of nuclei. Exposure to E. coli LPS for 24 h caused cell loss and enhanced apoptotic activity at concentrations ≥ 10—9 g/ml, but similar effects were only obtained with H. pylori LPS at concentrations ≥ 10— 6 g/ml. Although ibuprofen (250 µM) caused cell loss and apoptosis, addition of either E. coli or H. pylori LPSs further enhanced these effects. In conclusion, LPS and ibuprofen interact to enhance gastric cell loss and apoptosis. In such interactions, E. coli LPS is more potent than that of H. pylori. The low potency of H. pylori LPS may contribute to a chronic low-grade gastritis that can be enhanced by the use of NSAIDs.

Lipopolysaccharide modification in Gram-negative bacteria during chronic infection

The Gram-negative bacterial lipopolysaccharide (LPS) is a major component of the outer membrane that plays a key role in host-pathogen interactions with the innate immune system. During infection, bacteria are exposed to a host environment that is typically dominated by inflammatory cells and soluble factors, including antibiotics, which provide cues about regulation of gene expression. Bacterial adaptive changes including modulation of LPS synthesis and structure are a conserved theme in infections, irrespective of the type or bacteria or the site of infection. In general, these changes result in immune system evasion, persisting inflammation and increased antimicrobial resistance. Here, we review the modifications of LPS structure and biosynthetic pathways that occur upon adaptation of model opportunistic pathogens (Pseudomonas aeruginosa, Burkholderia cepacia complex bacteria, Helicobacter pylori and Salmonella enterica) to chronic infection in respiratory and gastrointestinal sites. We also discuss the molecular mechanisms of these variations and their role in the host-pathogen interaction.

The contribution of toll-like receptor 2 on Helicobacter pylori activation of the nuclear factor-kappa B signaling pathway in gastric epithelial cells

Helicobacter pylori (H. pylori) is a spiral shaped gram-negative bacterium that induces immune responses in the gastric mucosa. Toll-like receptors (TLRs) play important roles in mediating inflammatory cytokines by recognition of conserved molecular patterns on bacteria. Changes in the expression of toll-like receptor (TLR) 2, TLR4 and the relative inflammatory cytokines were analyzed in normal gastric epithelial GES-1 cells following treatment with H. pylori or Escherichia coli lipopolysaccharide (E. coli LPS) in order to investigate the contribution of TLRs in recognizing and mediating the inflammatory response to H. pylori, and study the differences in TLRs' performance between H. pylori and E. coli. Specific inhibitors for the declines in TLR2 and TLR4 were also employed. The results showed that H. pylori infection increased TLR2 expression in GES-1 cells, but TLR4 remained unchanged regardless of H. pylori or TLR2 small interfering RNA treatment. Furthermore, the secretion of cyclooxygenase-2 (COX-2) induced by H. pylori was inhibited by declines in TLR2, but not in TLR4. In conclusion, TLR2 plays an even more important role than TLR4 not only in recognizing H. pylori, but also in the induction of inflammatory cytokines initiated by H. pylori. However, both TLR2 and TLR4 are necessary in mediating the inflammatory response to E. coli LPS.

Structural characterization of the lipid A from the LPS of the haloalkaliphilic bacterium Halomonas pantelleriensis

Halomonas pantelleriensis DSM9661(Τ) is a Gram-negative haloalkaliphilic bacterium isolated from the sand of the volcanic Venus mirror lake, closed to seashore in the Pantelleria Island in the south of Italy. It is able to optimally grow in media containing 3-15 % (w/v) total salt and at pH between 9 and 10. To survive in these harsh conditions, the bacterium has developed several strategies that probably concern the bacteria outer membrane, a barrier regulating the exchange with the environment. In such a context, the lipopolysaccharides (LPSs), which are among the major constituent of the Gram-negative outer membrane, are thought to contribute to the restrictive membrane permeability properties. The structure of the lipid A family derived from the LPS of Halomonas pantelleriensis DSM 9661(T) is reported herein. The lipid A was obtained from the purified LPS by mild acid hydrolysis. The lipid A, which contains different numbers of fatty acids residues, and its partially deacylated derivatives were completely characterized by means of ESI FT-ICR mass spectrometry and chemical analysis. Preliminary immunological assays were performed, and a comparison with the lipid A structure of the phylogenetic proximal Halomonas magadiensis is also reported.

Caspase-11: arming the guards against bacterial infection

As a front line of defense against pathogenic microbes, our body employs a primitive, yet highly sophisticated and potent innate immune response pathway collectively referred to as the inflammasome. Innate immune cells, epithelial cells, and many other cell types are capable of detecting infection or tissue injury and mounting a coordinated molecular defense. For example, Gram-negative bacteria are specifically detected via a surveillance mechanism that involves activation of extracellular receptors such as Toll-like receptors (TLRs) followed by intracellular recognition and activation of pathways such as caspase-11 (caspase-4/5 in humans). Importantly, lipopolysaccharide (LPS), the major component of the outer membrane of Gram-negative bacteria, is a strong trigger of these pathways. Extracellular LPS primarily stimulates TLR4, which can serve as a priming signal for expression of inflammasome components. Intracellular LPS can then trigger caspase-11-dependent inflammasome activation in the cytoplasm. Here, we briefly review the burgeoning caspase-11-dependent non-canonical inflammasome field, focusing mainly on the innate sensing of LPS.

Helicobacter pylori activates the TLR2/NLRP3/caspase-1/IL-18 axis to induce regulatory T-cells, establish persistent infection and promote tolerance to allergens

The Gram-negative bacterium Helicobacter pylori is both a normal constituent of the human gastric microbiota as well as a pathogen tightly associated with severe gastric disorders. The ability of H. pylori to activate the inflammasome and caspase-1 in antigen-presenting and other cells, and the resulting processing and release of caspase-1-dependent cytokines, impacts both the immunomodulatory and pathogenic activities of H. pylori. This article summarizes recent insights by us and others on the bacterial and host prerequisites of inflammasome activation. H. pylori predominantly activates the NLRP3 inflammasome through a process that requires TLR2-dependent licensing. We identified the urease enzyme, a colonization determinant known to be required for acid adaptation, as critically required for activation of the TLR2/NLRP3/caspase-1 axis. The phenotypes of urease mutants, as well as mouse strains defective for TLR2 or NLRP3, are discussed with respect to their ability to support persistent colonization, immune tolerance and immunity to H. pylori.

Macrophages recognize the Helicobacter pylori type IV secretion system in the absence of toll-like receptor signalling

Helicobacter pylori strains carrying the cag pathogenicity island (cagPAI) provoke an increased inflammatory response, conferring an increased risk of ulcer formation and carcinogenesis. How the immune system recognizes the presence of cagPAI positive strains is yet unclear. By comparing the transcriptional response of wild type and MyD88/Trif(-/-) bone marrow macrophages to infection with H. pylori, we found that the majority of regulated genes were dependent on toll-like receptor (TLR) signalling. To determine the role of TLR-independent responses, we analysed the transcriptome of MyD88/Trif(-/-) bone marrow macrophages at different time points after infection with cagPAI positive versus negative strains. We identified a group of genes that exhibited different kinetic behaviour depending on whether cagPAI was present. Analysis of their gene expression kinetics demonstrated that this responsiveness to cagPAI was observed only in MyD88/Trif(-/-) macrophages. This group of cagPAI-sensing genes was enriched for AU-rich element containing early response genes involved in immune regulation, including interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). Recognition of cagPAI positive strains was found to be mediated by the type IV secretion system (cagT4SS), rather than its effector protein CagA. We hypothesize that anergic macrophages of the gastric mucosa initiate an innate immune response following detection of the T4SS of H. pylori.

Toll-like Receptor 10 in Helicobacter pylori Infection

Innate immunity plays important roles in the primary defense against pathogens, and epidemiological studies have suggested a role for Toll-like receptor 1 (TLR1) in Helicobacter pylori susceptibility. Microarray analysis of gastric biopsy specimens from H. pylori-positive and uninfected subjects showed that TLR10 messenger RNA (mRNA) levels were upregulated approximately 15-fold in infected subjects; these findings were confirmed by real-time quantitative polymerase chain reaction analysis. Immunohistochemical investigation showed increased TLR10 expression in the gastric epithelial cells of infected individuals. When H. pylori was cocultured with NCI-N87 gastric cells, both TLR10 and TLR2 mRNA levels were upregulated. We compared the ability of TLR combinations to mediate nuclear factor-κB (NF-κB) activation. Compared with other TLR2 subfamily heterodimers, the TLR2/TLR10 heterodimer mediated the greatest NF-κB activation following exposure to heat-killed H. pylori or H. pylori lipopolysaccharide. We conclude that TLR10 is a functional receptor involved in the innate immune response to H. pylori infection and that the TLR2/TLR10 heterodimer functions in H. pylori lipopolysaccharide recognition.

Interplay of the Gastric Pathogen Helicobacter pylori with Toll-Like Receptors

Toll-like receptors (TLRs) are crucial for pathogen recognition and downstream signaling to induce effective immunity. The gastric pathogen Helicobacter pylori is a paradigm of persistent bacterial infections and chronic inflammation in humans. The chronicity of inflammation during H. pylori infection is related to the manipulation of regulatory cytokines. In general, the early detection of H. pylori by TLRs and other pattern recognition receptors (PRRs) is believed to induce a regulatory cytokine or chemokine profile that eventually blocks the resolution of inflammation. H. pylori factors such as LPS, HSP-60, NapA, DNA, and RNA are reported in various studies to be recognized by specific TLRs. However, H. pylori flagellin evades the recognition of TLR5 by possessing a conserved N-terminal motif. Activation of TLRs and resulting signal transduction events lead to the production of pro- and anti-inflammatory mediators through activation of NF-κB, MAP kinases, and IRF signaling pathways. The genetic polymorphisms of these important PRRs are also implicated in the varied outcome and disease progression. Hence, the interplay of TLRs and bacterial factors highlight the complexity of innate immune recognition and immune evasion as well as regulated processes in the progression of associated pathologies. Here we will review this important aspect of H. pylori infection.

Helicobacter pylori: bacterial strategy for incipient stage and persistent colonization in human gastric niches

Helicobacter pylori (H. pylori) undergoes decades long colonization of the gastric mucosa of half the population in the world to produce acute and chronic gastritis at the beginning of infection, progressing to more severe disorders, including peptic ulcer disease and gastric cancer. Prolonged carriage of H. pylori is the most crucial factor for the pathogenesis of gastric maladies. Bacterial persistence in the gastric mucosa depends on bacterial factors as well as host factors. Herein, the host and bacterial components responsible for the incipient stages of H. pylori infection are reviewed and discussed. Bacterial adhesion and adaptation is presented to explain the persistence of H. pylori colonization in the gastric mucosa, in which bacterial evasion of host defense systems and genomic diversity are included.

Chemistry of lipid A: at the heart of innate immunity

In many Gram-negative bacteria, lipopolysaccharide (LPS) and its lipid A moiety are pivotal for bacterial survival. Depending on its structure, lipid A carries the toxic properties of the LPS and acts as a potent elicitor of the host innate immune system via the Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) receptor complex. It often causes a wide variety of biological effects ranging from a remarkable enhancement of the resistance to the infection to an uncontrolled and massive immune response resulting in sepsis and septic shock. Since the bioactivity of lipid A is strongly influenced by its primary structure, a broad range of chemical syntheses of lipid A derivatives have made an enormous contribution to the characterization of lipid A bioactivity, providing novel pharmacological targets for the development of new biomedical therapies. Here, we describe and discuss the chemical aspects regarding lipid A and its role in innate immunity, from the (bio)synthesis, isolation and characterization to the molecular recognition at the atomic level.

Species-specific activation of TLR4 by hypoacylated endotoxins governed by residues 82 and 122 of MD-2

The Toll-like receptor 4/MD-2 receptor complex recognizes endotoxin, a Gram-negative bacterial cell envelope component. Recognition of the most potent hexaacylated form of endotoxin is mediated by the sixth acyl chain that protrudes from the MD-2 hydrophobic pocket and bridges TLR4/MD-2 to the neighboring TLR4 ectodomain, driving receptor dimerization via hydrophobic interactions. In hypoacylated endotoxins all acyl chains could be accommodated within the binding pocket of the human hMD-2. Nevertheless, tetra- and pentaacylated endotoxins activate the TLR4/MD-2 receptor of several species. We observed that amino acid residues 82 and 122, located at the entrance to the endotoxin binding site of MD-2, have major influence on the species-specific endotoxin recognition. We show that substitution of hMD-2 residue V82 with an amino acid residue with a bulkier hydrophobic side chain enables activation of TLR4/MD-2 by pentaacylated and tetraacylated endotoxins. Interaction of the lipid A phosphate group with the amino acid residue 122 of MD-2 facilitates the appropriate positioning of the hypoacylated endotoxin. Moreover, mouse TLR4 contributes to the agonistic effect of pentaacylated msbB endotoxin. We propose a molecular model that explains how the molecular differences between the murine or equine MD-2, which both have sufficiently large hydrophobic pockets to accommodate all five or four acyl chains, influence the positioning of endotoxin so that one of the acyl chains remains outside the pocket and enables hydrophobic interactions with TLR4, leading to receptor activation.

At the bedside: Helicobacter pylori, dysregulated host responses, DNA damage, and gastric cancer

Clinical trials performed in populations at high GC risk have demonstrated that eradication of Helicobacter pylori from the stomach with a course of combination antimicrobials helps prevent gastric carcinogenesis when they are administered before precancerous lesions have yet developed. In this review, we consider the insights into H. pylori-associated gastric carcinogenesis that have been gained from these and many other clinical studies in the field to highlight priority areas for basic research and clinical investigation. Among these are defining the magnitude of the risk reduction that may be achieved in clinical practice and at a population level by H. pylori eradication and investigating when, during the slow multistep progression to GC, intervention will be of the most benefit. Additional strategies to prevent GC induced by H. pylori, including chemoprevention, dietary modification, and close endoscopic surveillance, may also have value in augmenting the risk reduction. Why only a small subpopulation of those infected by H. pylori go on to develop GC may be partially explained by genetic susceptibility related to SNPs in several genes regulating the intensity of the gastric inflammatory response to H. pylori. Investigation of the basic mechanisms underlying the promotion of GC by H. pylori and the associated inflammatory response will likely continue to improve clinical strategies for the prevention of one of the most common causes of cancer death globally. See related review, At the Bench: H. pylori, dysregulated host responses, DNA damage, and gastric cancer.

Pattern-recognition receptors and gastric cancer

Chronic inflammation has been associated with an increased risk of several human malignancies, a classic example being gastric adenocarcinoma (GC). Development of GC is known to result from infection of the gastric mucosa by Helicobacter pylori, which initially induces acute inflammation and, in a subset of patients, progresses over time to chronic inflammation, gastric atrophy, intestinal metaplasia, dysplasia, and finally intestinal-type GC. Germ-line encoded receptors known as pattern-recognition receptors (PRRs) are critical for generating mature pro-inflammatory cytokines that are crucial for both Th1 and Th2 responses. Given that H. pylori is initially targeted by PRRs, it is conceivable that dysfunction within genes of this arm of the immune system could modulate the host response against H. pylori infection, and subsequently influence the emergence of GC. Current evidence suggests that Toll-like receptors (TLRs) (TLR2, TLR3, TLR4, TLR5, and TLR9), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) (NOD1, NOD2, and NLRP3), a C-type lectin receptor (DC-SIGN), and retinoic acid-inducible gene (RIG)-I-like receptors (RIG-I and MDA-5), are involved in both the recognition of H. pylori and gastric carcinogenesis. In addition, polymorphisms in genes involved in the TLR (TLR1, TLR2, TLR4, TLR5, TLR9, and CD14) and NLR (NOD1, NOD2, NLRP3, NLRP12, NLRX1, CASP1, ASC, and CARD8) signaling pathways have been shown to modulate the risk of H. pylori infection, gastric precancerous lesions, and/or GC. Further, the modulation of PRRs has been suggested to suppress H. pylori-induced inflammation and enhance GC cell apoptosis, highlighting their potential relevance in GC therapeutics. In this review, we present current advances in our understanding of the role of the TLR and NLR signaling pathways in the pathogenesis of GC, address the involvement of other recently identified PRRs in GC, and discuss the potential implications of PRRs in GC immunotherapy.

The molecular mechanism of species-specific recognition of lipopolysaccharides by the MD-2/TLR4 receptor complex

Lipid A, a component of bacterial lipopolysaccharide, is a conserved microbe-associated molecular pattern that activates the MD-2/TLR4 receptor complex. Nevertheless, bacteria produce lipid A molecules of considerable structural diversity. The human MD-2/TLR4 receptor most efficiently recognizes hexaacylated bisphosphorylated lipid A produced by enterobacteria, but in some animal species the immune response can be elicited also by alternative lipid A varieties, such as tetraacylated lipid IVa or pentaacylated lipid A of Rhodobacter spheroides. Several crystal structures revealed that hexaacylated lipid A and tetraacylated lipid IVa activate the murine MD-2/TLR4 in a similar manner, but failed to explain the antagonistic vs. agonistic activity of lipid IVa in the human vs. equine receptor, respectively. Targeted mutagenesis studies of the receptor complex revealed intricate combination of electrostatic and hydrophobic interactions primarily within the MD-2 co-receptor, but with a contribution of TLR4 as well, that contribute to species-specific recognition of lipid A. We will review current knowledge regarding lipid A diversity and species-specific activation of the MD-2/TLR4 receptor complex in different species (e.g. human, mouse or equine) by lipid A varieties.

A cross-disciplinary perspective on the innate immune responses to bacterial lipopolysaccharide

The study of innate immunity to bacteria has focused heavily on the mechanisms by which mammalian cells detect lipopolysaccharide (LPS), the conserved surface component of Gram-negative bacteria. While Toll-like receptor 4 (TLR4) is responsible for all the host transcriptional responses to LPS, recent discoveries have revealed the existence of several TLR4-independent responses to LPS. These discoveries not only broaden our view of the means by which mammalian cells interact with bacteria, but they also highlight new selective pressures that may have promoted the evolution of bacterial immune evasion strategies. In this review, we highlight past and recent discoveries on host LPS sensing mechanisms and discuss bacterial countermeasures that promote infection. By looking at both sides of the host-pathogen interaction equation, we hope to provide comprehensive insights into host defense mechanisms and bacterial pathogenesis.

Human immune responses to H. pylori HLA Class II epitopes identified by immunoinformatic methods

H. pylori persists in the human stomach over decades and promotes several adverse clinical sequelae including gastritis, peptic ulcers and gastric cancer that are linked to the induction and subsequent evasion of chronic gastric inflammation. Emerging evidence indicates that H. pylori infection may also protect against asthma and some other immune-mediated conditions through regulatory T cell effects outside the stomach. To characterize the complexity of the CD4+ T cell response generated during H. pylori infection, computational methods were previously used to generate a panel of 90 predicted epitopes conserved among H. pylori genomes that broadly cover HLA Class II diversity for maximum population coverage. Here, these sequences were tested individually for their ability to induce in vitro responses in peripheral blood mononuclear cells by interferon-γ ELISpot assay. The average number of spot-forming cells/million PBMCs was significantly elevated in H. pylori-infected subjects over uninfected persons. Ten of the 90 peptides stimulated IFN-γ secretion in the H. pylori-infected group only, whereas two out of the 90 peptides elicited a detectable IFN-γ response in the H. pylori-uninfected subjects but no response in the H. pylori-infected group. Cytokine ELISA measurements performed using in vitro PBMC culture supernatants demonstrated significantly higher levels of TNF-α, IL-2, IL-4, IL-6, IL-10, and TGF-β1 in the H. pylori-infected subjects, whereas IL-17A expression was not related to the subjects H. pylori-infection status. Our results indicate that the human T cell responses to these 90 peptides are generally increased in actively H. pylori-infected, compared with H. pylori-naïve, subjects. This information will improve understanding of the complex immune response to H. pylori, aiding rational epitope-driven vaccine design as well as helping identify other H. pylori epitopes with potentially immunoregulatory effects.

Biosynthesis and export of bacterial lipopolysaccharides

Lipopolysaccharide molecules represent a unique family of glycolipids based on a highly conserved lipid moiety known as lipid A. These molecules are produced by most gram-negative bacteria, in which they play important roles in the integrity of the outer-membrane permeability barrier and participate extensively in host-pathogen interplay. Few bacteria contain lipopolysaccharide molecules composed only of lipid A. In most forms, lipid A is glycosylated by addition of the core oligosaccharide that, in some bacteria, provides an attachment site for a long-chain O-antigenic polysaccharide. The complexity of lipopolysaccharide structures is reflected in the processes used for their biosynthesis and export. Rapid growth and cell division depend on the bacterial cell's capacity to synthesize and export lipopolysaccharide efficiently and in large amounts. We review recent advances in those processes, emphasizing the reactions that are essential for viability.

Identification of a broad family of lipid A late acyltransferases with non-canonical substrate specificity

Most Gram-negative organisms produce lipopolysaccharide (LPS), a complex macromolecule anchored to the bacterial membrane by the lipid A moiety. Lipid A is synthesized via the Raetz pathway, a conserved nine-step enzymatic process first characterized in Escherichia coli. The Epsilonproteobacterium Helicobacter pylori uses the Raetz pathway to synthesize lipid A; however, only eight of nine enzymes in the pathway have been identified in this organism. Here, we identify the missing acyltransferase, Jhp0255, which transfers a secondary acyl chain to the 3'-linked primary acyl chain of lipid A, an activity similar to that of E. coli LpxM. This enzyme, reannotated as LpxJ due to limited sequence similarity with LpxM, catalyses addition of a C12:0 or C14:0 acyl chain to the 3'-linked primary acyl chain of lipid A, complementing an E. coli LpxM mutant. Enzymatic assays demonstrate that LpxJ and homologues in Campylobacter jejuni and Wolinella succinogenes can act before the 2' secondary acyltransferase, LpxL, as well as the 3-deoxy-d-manno-octulosonic acid (Kdo) transferase, KdtA. Ultimately, LpxJ is one member of a large class of acyltransferases found in a diverse range of organisms that lack an E. coli LpxM homologue, suggesting that LpxJ participates in lipid A biosynthesis in place of an LpxM homologue.

Gram-positive and gram-negative bacterial toxins in sepsis: a brief review

Bacterial sepsis is a major cause of fatality worldwide. Sepsis is a multi-step process that involves an uncontrolled inflammatory response by the host cells that may result in multi organ failure and death. Both gram-negative and gram-positive bacteria play a major role in causing sepsis. These bacteria produce a range of virulence factors that enable them to escape the immune defenses and disseminate to remote organs, and toxins that interact with host cells via specific receptors on the cell surface and trigger a dysregulated immune response. Over the past decade, our understanding of toxins has markedly improved, allowing for new therapeutic strategies to be developed. This review summarizes some of these toxins and their role in sepsis.

Role of childhood infection in the sequelae of H. pylori disease

The persistence of Helicobacter pylori infection plays a fundamental role in the development of H. pylori-associated complications. Since the majority of infected persons acquire the bacteria during early childhood, an examination of the immunobiology of H. pylori infection in children compared with that of adults may help identify host factors that contribute to persistent infection. Therefore, we begin our review of the role of persistence in H. pylori disease with an assessment of the clinical features of H. pylori infection in children. We next review the bacterial factors that promote colonization and evasion of host defense mechanisms. We then focus our attention on the early host immunological factors that promote persistence of the infection and its complications in humans and mouse models. We also highlight topics in which further research is needed. An examination of how immunological factors cause divergent manifestations of H. pylori infection in children compared with adults may provide new insight for therapeutic modification or prevention of persistent H. pylori infection and its complications.

Innate immunomodulation by lipophilic termini of lipopolysaccharide; synthesis of lipid As from Porphyromonas gingivalis and other bacteria and their immunomodulative responses

Synthetic studies of lipid A and LPS partial structures have been performed to investigate the relationship between structures and functions of LPS. Recent studies have suggested several pathological implications of LPS from parasitic bacteria due to its influence on the host immune responses. To address this issue, we established an efficient synthetic strategy that is widely applicable to the synthesis of various lipid As by using a key disaccharide intermediate with selectively cleavable protecting groups. Porphyromonas gingivalis and Helicobacter pylori lipid As were synthesized and their biological activities were evaluated. All synthetic lipid As did not induce strong inflammatory responses: some are very weak cytokine inducers and others are antagonistic in IL-6 and IL-8 induction with E. coli LPS. On the other hand, P. gingivalis lipid As showed potent IL-18 inducing activity. Since IL-18 has been shown to correlate with chronic inflammation, P. gingivalis LPS may be implicated in the chronic inflammatory responses.