Chemical structure and biological activity of a lipid A component from Helicobacter pylori strain 206

Helicobacter pylori Recombinant CagA Regulates Th1/Th2 Balance in a BALB/c Murine Model

Purpose:Helicobacter pylori is recognized as one of the prevalent causes of human gastricinfection. In the present study, the role of mixed immunization with H. pylori lipopolysaccharide(LPS) and recombinant cytotoxin-associated gene A (rCagA) as a stimulator of host immuneresponses was determined.

Methods: BALB/c mice were immunized with different formulations by the systemic administrationat 14-day intervals. The effects of the formulations plus CpG adjuvants were assessed before andpost-immunization in separated studies. Moreover, the expression of Th1/Th2 cytokines wasquantified in sera of immunized mice using reverse transcription polymerase chain reaction (RTPCR)test and the protein levels confirmed with enzyme linked immunosorbent assay (ELISA).Finally, the specific antibody levels in sera were studied by ELISA and the tendency of cellularresponse was examined by IgG1/IgG2a ratio.

Results: Data of Western blotting verified the presence of constructed protein. Analysisof lymphocyte proliferation showed that CpG-conjugated rCagA increases lymphocytesproliferation compared to the control group. Also, it was shown that formulations containing LPSand rCagA promote a Th1 response indicated by interferon-gamma expression and induced Th1/Th2 balance. Additionally, the specific IgG1, total IgG and IgG2a levels elevated in response toall treatments. Ultimately, the IgG2a/IgG1 ratio in the mice immunized with rCagA-containingformulations increased.

Conclusion: These results indicated that rCagA protein carried with CpG adjuvant not onlymaintained its antigenicity throughout the experiment but also induced robust Th1-biasedimmune responses. Therefore, it holds promise for the production of an efficient vaccine against H. pylori infection.

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.

Review: D-Galactosamine lethality model: scope and limitations

D-Galactosamine (D-galN) is well established as sensitizing mice and other animals to the lethal effects of TNF, specifically, and by several orders of magnitude. Protection by anti-TNF neutralizing antibody is complete, as is (metabolically-based) protection by uridine. Sensitization occurs regardless of the origin of the released TNF, whether it is released from macrophages and/or T-cells. The same is true for the challenging agent which leads to the release of TNF, whether it is endotoxin, a superantigen, lipoprotein, bacterial DNA, or bacteria, either killed or proliferating. Most studies have utilized endotoxin as the challenging agent, and more than 70 agents have been reported to confer protection against LPS and/or TNF challenge in the model. The model has provided new insight regarding modes of protection, including from dexamethasone, which protects against challenge from LPS but not from challenge by TNF. The D-galN lethality model has also been used to test for synergistic behavior between different bacterial components, and to test for lethality when only small amounts of the challenging agent are available (lipid A chemistry).

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.

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.

Inflammation, immunity, and vaccines for Helicobacter pylori infection

The tight control of the innate and adaptive immune responses in the stomach mucosa during chronic Helicobacter pylori infection is of prime importance for the bacteria to persist and for the host to prevent inflammation-driven diseases. This review summarizes recent data on the roles of innate and adaptive immune responses during H. pylori/host interactions. In addition, the latest preclinical developments of H. pylori vaccines are discussed with a special focus on the clinical trial reported by Zeng et al., who provided evidence that oral vaccination significantly reduces the acquisition of natural H. pylori infection in children.

Kdo2 -lipid A: structural diversity and impact on immunopharmacology

3-deoxy-d-manno-octulosonic acid-lipid A (Kdo₂-lipid A) is the essential component of lipopolysaccharide in most Gram-negative bacteria and the minimal structural component to sustain bacterial viability. It serves as the active component of lipopolysaccharide to stimulate potent host immune responses through the complex of Toll-like-receptor 4 (TLR4) and myeloid differentiation protein 2. The entire biosynthetic pathway of Escherichia coli Kdo₂-lipid A has been elucidated and the nine enzymes of the pathway are shared by most Gram-negative bacteria, indicating conserved Kdo₂-lipid A structure across different species. Yet many bacteria can modify the structure of their Kdo₂-lipid A which serves as a strategy to modulate bacterial virulence and adapt to different growth environments as well as to avoid recognition by the mammalian innate immune systems. Key enzymes and receptors involved in Kdo₂-lipid A biosynthesis, structural modification and its interaction with the TLR4 pathway represent a clear opportunity for immunopharmacological exploitation. These include the development of novel antibiotics targeting key biosynthetic enzymes and utilization of structurally modified Kdo₂-lipid A or correspondingly engineered live bacteria as vaccines and adjuvants. Kdo₂-lipid A/TLR4 antagonists can also be applied in anti-inflammatory interventions. This review summarizes recent knowledge on both the fundamental processes of Kdo₂-lipid A biosynthesis, structural modification and immune stimulation, and applied research on pharmacological exploitations of these processes for therapeutic development.

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.

Endotoxins: lipopolysaccharides of gram-negative bacteria

Endotoxin refers lipopolysaccharide that constitutes the outer leaflet of the outer membrane of most Gram-negative bacteria. Lipopolysaccharide is comprised of a hydrophilic polysaccharide and a hydrophobic component known as lipid A which is responsible for the major bioactivity of endotoxin. Lipopolysaccharide can be recognized by immune cells as a pathogen-associated molecule through Toll-like receptor 4. Most enzymes and genes related to the biosynthesis and export of lipopolysaccharide have been identified in Escherichia coli, and they are shared by most Gram-negative bacteria based on available genetic information. However, the detailed structure of lipopolysaccharide differs from one bacterium to another, suggesting that additional enzymes that can modify the basic structure of lipopolysaccharide exist in bacteria, especially some pathogens. These structural modifications of lipopolysaccharide are sometimes tightly regulated. They are not required for survival but closely related to the virulence of bacteria. In this chapter we will focus on the mechanism of biosynthesis and export of lipopolysaccharide in bacteria.

Comprehensive structure characterization of lipid A extracted from Yersinia pestis for determination of its phosphorylation configuration

We report on comprehensive structure characterization of lipid A extracted from Yersinia pestis (Yp) for determination of its phosphorylation configuration that was achieved by combining the methods of molecular biology with high-resolution tandem mass spectrometry. The phosphorylation pattern of diphosphorylated lipid A extracted from Yp has recently been found to be a heterogeneous mixture of C-1 and C-4' bisphosphate, C-1 pyrophosphate, and C-4' pyrophosphate (Proc. Natl. Acad. Sci. 2008, 105, 12742). To reduce the inherent phosphate heterogeneity of diphosphorylated lipid A extracted from Yp, we incorporated specific C-1 and C-4' position phosphatases into wild type KIM6+ Yp grown at 37 degrees C. Comprehensive high-resolution tandem mass spectrometric analyses of lipid A extracted from Yp modified with either the C-1 or C-4' phosphatase allowed for unambiguous structure assignment of monophosphorylated and diphosphorylated lipid A and distinction of isomeric bisphosphate and pyrophosphate forms. The prevalent aminoarabinose modification was determined to be exclusively attached to the lipid A disaccharide via a phospho-diester linkage at either or both the C-1 and C-4' positions.

Advances in vaccination against Helicobacter pylori

A vaccination against Helicobacter pylori may represent both prophylactic and therapeutic approaches to the control of H. pylori infection. Different protective H. pylori-derived antigens, such as urease, vacuolating cytotoxin A, cytotoxin-associated antigen, neutrophil-activating protein and others can be produced at low cost in prokaryote expression systems and most of these antigens have already been administered to humans and shown to be safe. The recent development by Graham et al. of the model of H. pylori challenge in humans, the recent published clinical trials and the last insight generated in animal models of H. pylori infection regarding the immune mechanisms leading to vaccine-induced Helicobacter clearance will facilitate the evaluation of immunogenicity and efficacy of H. pylori vaccine candidates in Phase II and III clinical trials.

The role of endotoxin in infection: Helicobacter pylori and Campylobacter jejuni

Both Helicobacter pylori and Campylobacter jejuni are highly prevalent Gram-negative microaerophilic bacteria which are gastrointestinal pathogens of humans; H. pylori colonizes the gastroduodenal compartment and C. jejuni the intestinal mucosa. Although H. pylori causes chronic gastric infection leading to gastritis, peptic ulcers and eventually gastric cancer while C. jejuni causes acute infection inducing diarrhoeal disease, the endotoxin molecules of both bacterial species contrastingly contribute to their pathogenesis and the autoimmune sequelae each induces. Compared with enterobacterial endotoxin, that of H. pylori has significantly lower endotoxic and immuno-activities, the molecular basis for which is the underphosphorylation and underacylation of the lipid A component that interacts with immune receptors. This induction of low immunological responsiveness by endotoxin may aid the prolongation of H. pylori infection and therefore infection chronicity. On the other hand, this contrasts with acute infection-causing C. jejuni where overt inflammation contributes to pathology and diarrhoea production, and whose endotoxin is immunologically and endotoxically active. Futhermore, both H. pylori and C. jejuni exhibit molecular mimicry in the saccharide components of their endotoxins which can induce autoreactive antibodies; H. pylori expresses mimicry of Lewis and some ABO blood group antigens, C. jejuni mimicry of gangliosides. The former has been implicated in influencing the development of inflammation and gastric atrophy (a precursor of gastic cancer), the latter is central to the development of the neurological disorder Guillain-Barré syndrome. Both diseases raise important questions concerning infection-induced autoimmunity awaiting to be addressed.

Lipopolysaccharide: Biosynthetic pathway and structure modification

Lipopolysaccharide that constitutes the outer leaflet of the outer membrane of most Gram-negative bacteria is referred to as an endotoxin. It is comprised of a hydrophilic polysaccharide and a hydrophobic component referred to as lipid A. Lipid A is responsible for the major bioactivity of endotoxin, and is recognized by immune cells as a pathogen-associated molecule. Most enzymes and genes coding for proteins responsible for the biosynthesis and export of lipopolysaccharide in Escherichia coli have been identified, and they are shared by most Gram-negative bacteria based on genetic information. The detailed structure of lipopolysaccharide differs from one bacterium to another, consistent with the recent discovery of additional enzymes and gene products that can modify the basic structure of lipopolysaccharide in some bacteria, especially pathogens. These modifications are not required for survival, but are tightly regulated in the cell and closely related to the virulence of bacteria. In this review we discuss recent studies of the biosynthesis and export of lipopolysaccharide, and the relationship between the structure of lipopolysaccharide and the virulence of bacteria.

Reduction of Helicobacter infection in IL-10-/- mice is dependent on CD4+ T cells but not on mast cells

BACKGROUND

In contrast to wild type, interleukin-10-deficient (IL-10(-/-)) mice are able to clear Helicobacter infection. In this study, we investigated the immune response of IL-10(-/-) mice leading to the reduction of Helicobacter infection.

MATERIALS AND METHODS

We characterized the immune responses of Helicobacter felis-infected IL-10(-/-) mice by studying the systemic antibody and cellular responses toward Helicobacter. We investigated the role of CD4(+) T cells in the Helicobacter clearance by injecting H. felis-infected IL-10(-/-) mice with anti-CD4 depleting antibodies. To examine the role of mast cells in Helicobacter clearance, we constructed and infected mast cells and IL-10 double-deficient mice.

RESULTS

Reduction of Helicobacter infection in IL-10(-/-) mice is associated with strong humoral (fivefold higher serum antiurease antibody titers were measured in IL-10(-/-) in comparison to wild-type mice, p < .008) and cellular (urease-stimulated splenic CD4(+) T cells isolated from infected IL-10(-/-) mice produce 150-fold more interferon-gamma in comparison to wild-type counterparts, p < .008) immune responses directed toward Helicobacter. Depletion of CD4(+) cells from Helicobacter-infected IL-10(-/-) mice lead to the loss of bacterial clearance (rapid urease tests are threefold higher in CD4(+) depleted IL-10(-/-) in comparison to nondepleted IL-10(-/-) mice, p < .02). Mast cell IL-10(-/-) double-deficient mice clear H. felis infection, indicating that mast cells are unnecessary for the bacterial eradication in IL-10(-/-) mice.

CONCLUSION

Taken together, these results suggest that CD4(+) cells are required for Helicobacter clearance in IL-10(-/-) mice. This reduction of Helicobacter infection is, however, not dependent on the mast cell population.

The lipid A 1-phosphatase of Helicobacter pylori is required for resistance to the antimicrobial peptide polymyxin

Modification of the phosphate groups of lipid A with amine-containing substituents, such as phosphoethanolamine, reduces the overall net negative charge of gram-negative bacterial lipopolysaccharide, thereby lowering its affinity to cationic antimicrobial peptides. Modification of the 1 position of Helicobacter pylori lipid A is a two-step process involving the removal of the 1-phosphate group by a lipid A phosphatase, LpxEHP (Hp0021), followed by the addition of a phosphoethanolamine residue catalyzed by EptAHP (Hp0022). To demonstrate the importance of modifying the 1 position of H. pylori lipid A, we generated LpxEHP-deficient mutants in various H. pylori strains by insertion of a chloramphenicol resistance cassette into lpxEHP and examined the significance of LpxE with respect to cationic antimicrobial peptide resistance. Using both mass spectrometry analysis and an in vitro assay system, we showed that the loss of LpxEHP activity in various H. pylori strains resulted in the loss of modification of the 1 position of H. pylori lipid A, thus confirming the function of LpxEHP. Due to its unique lipid A structure, H. pylori is highly resistant to the antimicrobial peptide polymyxin (MIC > 250 microg/ml). However, disruption of lpxEHP in H. pylori results in a dramatic decrease in polymyxin resistance (MIC, 10 microg/ml). In conclusion, we have characterized the first gram-negative LpxE-deficient mutant and have shown the importance of modifying the 1 position of H. pylori lipid A for resistance to polymyxin.

An outer membrane enzyme encoded by Salmonella typhimurium lpxR that removes the 3'-acyloxyacyl moiety of lipid A

The Salmonella and related bacteria modify the structure of the lipid A portion of their lipopolysaccharide in response to environmental stimuli. Some lipid A modifications are required for virulence and resistance to cationic antimicrobial peptides. We now demonstrate that membranes of Salmonella typhimurium contain a novel hydrolase that removes the 3'-acyloxyacyl residue of lipid A in the presence of 5 mM Ca2+. We have identified the gene encoding the S. typhimurium lipid A 3'-O-deacylase, designated lpxR, by screening an ordered S. typhimurium genomic DNA library, harbored in Escherichia coli K-12, for expression of Ca2+-dependent 3'-O-deacylase activity in membranes. LpxR is synthesized with an N-terminal type I signal peptide and is localized to the outer membrane. Mass spectrometry was used to confirm the position of lipid A deacylation in vitro and the release of the intact 3'-acyloxyacyl group. Heterologous expression of lpxR in the E. coli K-12 W3110, which lacks lpxR, resulted in production of significant amounts of 3'-O-deacylated lipid A in growing cultures. Orthologues of LpxR are present in the genomes of E. coli O157:H7, Yersinia enterocolitica, Helicobacter pylori, and Vibrio cholerae. The function of LpxR is unknown, but it could play a role in pathogenesis because it might modulate the cytokine response of an infected animal.

A novel Helicobacter pylori cell-surface polysaccharide

Helicobacter pylori bacteria colonize the gastric mucosa of more than half of the world's human population and its infection may instigate a wide spectrum of gastric diseases in the host. At the moment, there is no vaccine against H. pylori, a microorganism recognized as a category 1 human carcinogen, and treatment is limited to antibiotic management. Pioneering antigenic studies carried out by Penner and co-workers, which employed homologous H. pylori antisera specific for cell-surface lipopolysaccharide (LPS), revealed the presence of six distinct H. pylori serotypes (O1 to O6). Subsequent studies have shown that H. pylori serotype O1 expressed LPS with lengthy O-chain polysaccharide (PS) composed of Lewis blood-group structures ('Lewis O-chains'), serotype O3 LPS produced 'Lewis O-chains' attached to a heptoglycan domain, serotype O4 LPS possessed LPS with glucosylated 'Lewis O-chains' and serotype O6 LPS expressed the heptoglycan domain capped by a short 'Lewis O-chain'. These LPSs were terminated at the reducing-end by a core oligosaccharide and lipid A of conserved structures. With the intent of formulating a multivalent H. pylori LPS-based vaccine, we are studying the structural variability of H. pylori cell-surface glycans. Here, we describe the novel LPS structure produced by H. pylori serotype O2 that differed markedly from the typical H. pylori 'Lewis O-chain' structures, in that its main component was an elongated PS composed of alternating 2-, and 3-monosubstituted alpha-D-Glcp residues [-->2)-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->]n. These findings revealed the bio-molecular basis for the observed serospecificity of H. pylori serotype O2, and that this unique bacterial PS must be included in the formulation of a multivalent LPS H. pylori vaccine.

A novel 3-deoxy-D-manno-octulosonic acid (Kdo) hydrolase that removes the outer Kdo sugar of Helicobacter pylori lipopolysaccharide

The lipid A domain anchors lipopolysaccharide (LPS) to the outer membrane and is typically a disaccharide of glucosamine that is both acylated and phosphorylated. The core and O-antigen carbohydrate domains are linked to the lipid A moiety through the eight-carbon sugar 3-deoxy-D-manno-octulosonic acid known as Kdo. Helicobacter pylori LPS has been characterized as having a single Kdo residue attached to lipid A, predicting in vivo a monofunctional Kdo transferase (WaaA). However, using an in vitro assay system we demonstrate that H. pylori WaaA is a bifunctional enzyme transferring two Kdo sugars to the tetra-acylated lipid A precursor lipid IV(A). In the present work we report the discovery of a Kdo hydrolase in membranes of H. pylori capable of removing the outer Kdo sugar from Kdo2-lipid A. Enzymatic removal of the Kdo group was dependent upon prior removal of the 1-phosphate group from the lipid A domain, and mass spectrometric analysis of the reaction product confirmed the enzymatic removal of a single Kdo residue by the Kdo-trimming enzyme. This is the first characterization of a Kdo hydrolase involved in the modification of gram-negative bacterial LPS.

Novel biosynthetic functions of lipopolysaccharide rfaJ homologs from Helicobacter pylori

Activity screening and insertional inactivation of lipopolysaccharide (LPS) biosynthetic genes in Helicobacter pylori have led to the successful characterization of two key enzymes encoded by HP0159 (JHP0147) and HP1105 (JHP1032) open reading frames (ORFs) which are members of the large and diverse carbohydrate active enzymes (CAZY) GT-8 (rfaJ) family of glycosyltransferases. Activity screening of a genomic library led to the identification of the enzyme involved in the biosynthesis of the type 2 N-acetyl-lactosamine O-chain backbone, the beta-1,3-N-acetyl-glucosaminyl transferase. In addition, the activity screening approach led to the identification and characterization of a key core biosynthetic enzyme responsible for the biosynthesis of the alpha-1,6-glucan polymer. This alpha-1,6-glucosyltransferase protein is encoded by the HP0159 ORF. Both enzymes play an integral part in the biosynthesis of LPS, and insertional inactivation leads to the production of a truncated LPS molecule on the bacterial cell surface. The LPS structures were determined by mass spectrometry and chemical analyses. The linkage specificity of each glycosyltransferase was determined by nuclear magnetic resonance (NMR) analysis of model compounds synthesized in vitro. A cryogenic probe was used to structurally characterize nanomole amounts of the product of the HP1105 (JHP1032) enzyme. In contrast to the HP0159 enzyme, which displays the GT-8-predicted retaining stereochemistry for the reaction product, HP1105 (JHP1032) is the first member of this GT-8 family to have been shown to have an inverting stereochemistry in its reaction products.

Biosynthesis, transport, and modification of lipid A

Lipopolysaccharide (LPS) is the major surface molecule of Gram-negative bacteria and consists of three distinct structural domains: O-antigen, core, and lipid A. The lipid A (endotoxin) domain of LPS is a unique, glucosamine-based phospholipid that serves as the hydrophobic anchor of LPS and is the bioactive component of the molecule that is associated with Gram-negative septic shock. The structural genes encoding the enzymes required for the biosynthesis of Escherchia coli lipid A have been identified and characterized. Lipid A is often viewed as a constitutively synthesized structural molecule. However, determination of the exact chemical structures of lipid A from diverse Gram-negative bacteria shows that the molecule can be further modified in response to environmental stimuli. These modifications have been implicated in virulence of pathogenic Gram-negative bacteria and represent one of the molecular mechanisms of microbial surface remodeling used by bacteria to help evade the innate immune response. The intent of this review is to discuss the enzymatic machinery involved in the biosynthesis of lipid A, transport of the molecule, and finally, those enzymes involved in the modification of its structure in response to environmental stimuli.

Periplasmic cleavage and modification of the 1-phosphate group of Helicobacter pylori lipid A

Pathogenic bacteria modify the lipid A portion of their lipopolysaccharide to help evade the host innate immune response. Modification of the negatively charged phosphate groups of lipid A aids in resistance to cationic antimicrobial peptides targeting the bacterial cell surface. The lipid A of Helicobacter pylori contains a phosphoethanolamine (pEtN) unit directly linked to the 1-position of the disaccharide backbone. This is in contrast to the pEtN units found in other pathogenic Gram-negative bacteria, which are attached to the lipid A phosphate group to form a pyrophosphate linkage. This study describes two enzymes involved in the periplasmic modification of the 1-phosphate group of H. pylori lipid A. By using an in vitro assay system, we demonstrate the presence of lipid A 1-phosphatase activity in membranes of H. pylori. In an attempt to identify genes encoding possible lipid A phosphatases, we cloned four putative orthologs of Escherichia coli pgpB, the phosphatidylglycerol-phosphate phosphatase, from H. pylori 26695. One of these orthologs, Hp0021, is the structural gene for the lipid A 1-phosphatase and is required for removal of the 1-phosphate group from mature lipid A in an in vitro assay system. Heterologous expression of Hp0021 in E. coli resulted in the highly selective removal of the 1-phosphate group from E. coli lipid A, as demonstrated by mass spectrometry. We also identified the structural gene for the H. pylori lipid A pEtN transferase (Hp0022). Mass spectrometric analysis of the lipid A isolated from E. coli expressing Hp0021 and Hp0022 shows the addition of a single pEtN group at the 1-position, confirming that Hp0022 is responsible for the addition of a pEtN unit at the 1-position in H. pylori lipid A. In summary, we demonstrate that modification of the 1-phosphate group of H. pylori lipid A requires two enzymatic steps.

Helicobacter pylori lipopolysaccharide: Biological activities in vitro and in vivo, pathological correlation to human chronic gastritis and peptic ulcer

AIM: To determine the biological activity of Helicobacter pylori (H pylori) lipopolysaccharide (H-LPS) and understand pathological correlation between H-LPS and human chronic gastritis and peptic ulcer.

METHODS: H-LPS of a clinical H pylori strain and LPS of Escherichia coli strain O55:B5 (E-LPS) were extracted by phenol-water method. Biological activities of H-LPS and E-LPS were detected by limulus lysate assay, pyrogen assay, blood pressure test and PBMC induction test in rabbits, cytotoxicity test in NIH 3T3 fibroblast cells and lethality test in NIH mice. By using self-prepared rabbit anti-H-LPS serum as the first antibody and commercial HRP-labeled sheep anti-rabbit sera as the second antibody, H-LPS in biopsy specimens from 126 patients with chronic gastritis (68 cases) or gastric ulcer (58 cases) were examined by immunohistochemistry.

RESULTS: Fibroblast cytotoxicity and mouse lethality of H-LPS were weaker than those of E-LPS. But the ability of coagulating limulus lysate of the two LPSs was similar (+/0.5 ng/mL). At 0.5 h after H-LPS injection, the blood pressures of the 3 rabbits rapidly declined. At 1.0 h after H-LPS injection, the blood pressures in 2 of the 3 rabbits fell to zero causing death of the 2 animals. For the other one rabbit in the same group, its blood pressure gradually elevated. At 0.5 h after E-LPS injection, the blood pressures of the three rabbits also quickly declined and then maintained at low level for approximately 1.0 h. At 0.5 h after injection with H-LPS or E-LPS, PBMC numbers of the rabbits showed a remarkable increase. The total positivity rate of H-LPS from 126 biopsy specimens was 60.3% (76/126). H-LPS positivity rate in the biopsy specimens from chronic gastritis (50/68, 73.5%) was significantly higher than that from gastric ulcer (26/58, 44.8%) (χ² = 10.77, P < 0.01). H-LPS positivity rates in biopsy specimens from chronic superficial gastritis (38/48, 79.2%) and chronic active gastritis (9/10, 90.0%) were significantly higher than that of the patients with atrophic gastritis (3/10, 30.0%) (χ² = 7.50-9.66, P < 0.01).

CONCLUSION: The biological activities of H-LPS were weaker than those of E-LPS, the activities of H-LPS of lowering rabbit blood pressure and inducing rabbit PBMC were relatively stronger. H-LPS may play a critical role in inducing inflammatory reaction in human gastritis.

Neisseria meningitidis lipooligosaccharide structure-dependent activation of the macrophage CD14/Toll-like receptor 4 pathway

Meningococcal lipopoly(oligo)saccharide (LOS) is a major inflammatory mediator of fulminant meningococcal sepsis and meningitis. Highly purified wild-type meningococcal LOS and LOS from genetically defined mutants of Neisseria meningitidis that contained specific mutations in LOS biosynthesis pathways were used to confirm that meningococcal LOS activation of macrophages was CD14/Toll-like receptor 4 (TLR4)-MD-2 dependent and to elucidate the LOS structural requirement for TLR4 activation. Expression of TLR4 but not TLR2 was required, and antibodies to both TLR4 and CD14 blocked meningococcal LOS activation of macrophages. Meningococcal LOS alpha or beta chain oligosaccharide structure did not influence CD14/TLR4-MD-2 activation. However, meningococcal lipid A, expressed by meningococci with defects in 3-deoxy-D-manno-octulosonic acid (KDO) biosynthesis or transfer, resulted in an approximately 10-fold (P < 0.0001) reduction in biologic activity compared to KDO2-containing meningococcal LOS. Removal of KDO2 from LOS by acid hydrolysis also dramatically attenuated cellular responses. Competitive inhibition assays showed similar binding of glycosylated and unglycosylated lipid A to CD14/TLR4-MD-2. A decrease in the number of lipid A phosphate head groups or penta-acylated meningococcal LOS modestly attenuated biologic activity. Meningococcal endotoxin is a potent agonist of the macrophage CD14/TLR4-MD-2 receptor, helping explain the fulminant presentation of meningococcal sepsis and meningitis. KDO2 linked to meningococcal lipid A was structurally required for maximal activation of the human macrophage TLR4 pathway and indicates an important role for KDO-lipid A in endotoxin biologic activity.

Endotoxic and immunobiological activities of a chemically synthesized lipid A of Helicobacter pylori strain 206-1

A synthetic lipid A of Helicobacter pylori strain 206-1 (compound HP206-1), which is similar to its natural lipid A, exhibited no or very low endotoxic activities as compared to Escherichia coli-type synthetic lipid A (compound 506). Furthermore, compound HP206-1 as well as its natural lipid A demonstrated no or very low mitogenic responses in murine spleen cell. On the other hand, compound HP206-1 showed a weaker but significant production of interleukin-8 in a gastric cancer cell line, MKN-1, in comparison with compound 506. Furthermore, compound HP206-1 exhibited induction of tumor necrosis factor-alpha production in human peripheral blood mononuclear cells and the cytokine production was clearly inhibited by mouse anti-human Toll-like receptor (TLR) 4 monoclonal antibody HTA125. Our findings indicate that the chemically synthesized lipid A, mimicking the natural lipid A portion of lipopolysaccharide from H. pylori strain 206-1, has a low endotoxic potency and immunobiological activities, and is recognized by TLR4.

The basis of persistent bacterial infections

Selected bacterial pathogens, such as Helicobacter pylori and Mycobacterium tuberculosis, establish persistent infections in mammalian hosts despite activating inflammatory and antimicrobial responses. The strategies used to overcome host defense responses vary with the anatomical location of the infection but often rely on deliberate manipulations of the host cell responses. Phylogenetically unrelated bacteria can share similar strategies for the establishment of persistence and, in selected examples, one even can define homologous "persistence" genes. Such observations suggest that persistent infection is a specific phase in infection pathogenesis rather than a fortuitous imbalance in the host-pathogen interaction.