Virulent strains of Helicobacter pylori demonstrate delayed phagocytosis and stimulate homotypic phagosome fusion in macrophages

Specific entry of Helicobacter pylori into cultured gastric epithelial cells via a zipper-like mechanism

Although Helicobacter pylori has generally been considered an extracellular pathogen, a number of in vitro infection experiments and biopsy examinations have shown that it is capable of occasionally entering mammalian host cells. Here, we characterized this entry process by using AGS cells as a host cell model. In gentamicin protection-invasion assays, the number of H. pylori colonies recovered was lower than that for Salmonella enterica serovar Typhimurium X22, Escherichia coli expressing InvA, and Yersinia enterocolitica YO:9 grown at 25 degrees C but higher than that for Neisseria gonorrhoeae VP1 and Y. enterocolitica YO:9 grown at 37 degrees C. At the ultrastructural level, the entry process was observed to occur via a zipper-like mechanism. Internalized H. pylori was bound in tight LAMP-1-containing vacuoles in close association with condensed filamentous actin and tyrosine phosphorylation signals. Wortmannin, a potent inhibitor of phosphatidylinositol 3-kinase, and calphostin C, an inhibitor of protein kinase C, both inhibited the entry of H. pylori in a sensitive and dose-dependent manner; however, the level of entry was enhanced by sodium vanadate, an inhibitor of tyrosine phosphatases and ATPases. Furthermore, the cytokine tumor necrosis factor alpha antagonized the entry of H. pylori into AGS cells. Collectively, these results demonstrate that the entry of H. pylori into AGS cells occurs via a zipper-like mechanism which involves various host signal transduction events.

Gene expression profiling in AGS cells stimulated with Helicobacter pylori isogenic strains (cagA positive or cagA negative)

To study host response to CagA, human gastric cancer cell line AGS was infected with a Helicobacter pylori type I wild-type or isogenic cagA-negative mutant. Differentially expressed genes were identified using cDNA array technology. By Northern blotting, downregulation of focal adhesion kinase and upregulation of LIM kinase mRNA in the presence of CagA were clearly verified. Furthermore, upregulation of LIM kinase, macrophage inflammatory protein-2, c-myc, and bone morphogenetic protein-1 and downregulation of transcription factor Y-box binding protein-1 and focal adhesion kinase mRNA in response to H. pylori type I infection compared to the uninfected control could be shown by Northern blotting. Hence, these findings identified new targets for further functional studies on H. pylori-associated pathogenesis.

Icm/dot-dependent upregulation of phagocytosis by Legionella pneumophila

Legionella pneumophila is the causative agent of Legionnaires' disease, a severe pneumonia. Dependent on the icm/dot loci, L. pneumophila survives and replicates in macrophages and amoebae within a specialized phagosome that does not fuse with lysosomes. Here, we report that phagocytosis of wild-type L. pneumophila is more efficient than uptake of icm/dot mutants. Compared with the wild-type strain JR32, about 10 times fewer icm/dot mutant bacteria were recovered from HL-60 macrophages in a gentamicin protection assay. The defect in phagocytosis of the mutants could be complemented by supplying the corresponding genes on a plasmid. Using fluorescence microscopy and green fluorescent protein (GFP)-expressing strains, 10-20 times fewer icm/dot mutant bacteria were found to be internalized by HL-60 cells and human monocyte-derived macrophages (HMMPhi). Compared with icm/dot mutants, wild-type L. pneumophila infected two to three times more macrophages and yielded a population of highly infected host cells (15-70 bacteria per macrophage) that was not observed with icm/dot mutant strains. Wild-type and icmT mutant bacteria were found to adhere similarly and compete for binding to HMMPhi. In addition, wild-type L. pneumophila was also phagocytosed more efficiently by Acanthamoeba castellanii, indicating that the process is independent of adherence receptor(s). Wild-type L. pneumophila enhanced phagocytosis of an icmT mutant strain in a synchronous co-infection, suggesting that increased phagocytosis results from (a) secreted effector(s) acting in trans.

Host cell signaling in Helicobacter pylori infection

Helicobacter pylori represents a highly successful human microbial pathogen that has infected approximately half of the world's population. This gram-negative microorganism colonizes the human epithelial layer in the stomach and induces a state of chronic inflammation that does not resolve the underlying infection and often leads to gastric or duodenal ulcers, or more rarely to gastric cancer. Among the reactions in H. pylori-infected epithelial cells the induction of proinflammatory cytokines, cell spreading and movement, as well as a scattered phenotype appear strictly dependent on the expression of pathogenicity island-encoded proteins in H. pylori. This review will discuss the features of the H. pylori-induced signal transduction leading to changes in host cellular function. Topics discussed comprise the signaling and the phenotypes associated with the type IV secretion system, the activation of target genes involved in gastric physiology, and putative mechanisms leading to the development of gastric cancer.

Pathogenic trickery: deception of host cell processes

Microbial pathogens cause a spectrum of diseases in humans. Although the disease mechanisms vary considerably, most pathogens have developed virulence factors that interact with host molecules, often usurping normal cellular processes, including cytoskeletal dynamics and vesicle targeting. These virulence factors often mimic host molecules, and mediate events as diverse as bacterial invasion, antiphagocytosis, and intracellular parastism.

The role of the neutrophil and phagocytosis in infection caused by Helicobacter pylori

Recent advances in our understanding of Helicobacter pylori-phagocyte interactions indicate that these organisms actively modulate phagocyte function in order to retard phagocytosis, while simultaneously inducing a strong respiratory burst. The central players in this dynamic include H. pylori neutrophil activating protein and factors that are associated with the cag pathogenicity island type IV secretion apparatus. Additionally, catalase, alkyl hydroperoxide reductase, and factors that are unique to type I strains allow bacteria to resist phagocytic killing.

Helicobacter pylori resists phagocytosis by macrophages: quantitative assessment by confocal microscopy and fluorescence-activated cell sorting

Helicobacter pylori infection of the stomach epithelium is characterized by an infiltration of polymorphonuclear and mononuclear cells. These immune cells contribute to mucosal damage which may eventually lead to gastritis, peptic ulcer, gastric cancer, and/or MALT-associated gastric lymphoma. Here we show that H. pylori inhibits its own uptake, as well as in trans the phagocytosis of Neisseria gonorrhoeae, by human and murine macrophages. This antiphagocytic activity is dependent on the presence of the cag pathogenicity island in the H. pylori genome. We demonstrate that H. pylori also expresses its antiphagocytic activity towards the myelomonocytic cell line JOSKM, thus providing a potent model for the study of the interaction between H. pylori and phagocytes. Our data were obtained using laser confocal microscopy and flow cytometry after quenching the fluorescence of labeled extracellular bacteria. The antiphagocytic activity of H. pylori may explain the persistence of H. pylori and its pathological consequences. The use of cell lines and flow cytometry will hopefully facilitate progress in our understanding of the immune escape of these persistent bacteria.

VAMP3 null mice display normal constitutive, insulin- and exercise-regulated vesicle trafficking

To investigate the physiological function of the VAMP3 vesicle SNARE (v-SNARE) isoform in the regulation of GLUT4 vesicle trafficking, we generated homozygotic VAMP3 null mice by targeted gene disruption. The VAMP3 null mice had typical growth rate and weight gain, with normal maintenance of fasting serum glucose and insulin levels. Analysis of glucose disposal and insulin sensitivity demonstrated normal insulin and glucose tolerance, with no evidence for insulin resistance. Insulin stimulation of glucose uptake in isolated primary adipocytes was essentially the same for the wild-type and VAMP3 null mice. Similarly, insulin-, hypoxia-, and exercise-stimulated glucose uptake in isolated skeletal muscle did not differ significantly. In addition, other general membrane trafficking events including phagocytosis, pinocytosis, and transferrin receptor recycling were also found to be unaffected in the VAMP3 null mice. Taken together, these data demonstrate that VAMP3 function is not necessary for either regulated GLUT4 translocation or general constitutive membrane recycling.

Virulence factors of Helicobacter pylori

To date a number of virulence factors have been identified and characterised from the gastric pathogen Helicobacter pylori. The vacuolating toxin (VacA) is a major determinant of H. pylori-associated gastric disease. In non-polarised cells, VacA alters the endocytic pathway, resulting in the release of acid hydrolases and the reduction of both extracellular ligand degradation and antigen processing. The toxin forms trans-membrane anion-specific channels and reduces the transepithelial electrical resistance of polarized monolayers. Localization of the VacA channels in acidic intracellular compartments causes osmotic swelling which, together with membrane fusion, leads to vacuole formation. The neutrophil-activating protein of H. pylori (HP-NAP) induces the production of oxygen radicals in human neutrophils via a cascade of intracellular activation events which may contribute to the damage of the stomach mucosa. This protein has recently been shown to be an important antigen in the human immune response to H. pylori infection. In addition, mice vaccinated with recombinant HP-NAP were protected against H. pylori challenge. H. pylori strains that are associated with severe tissue damage and inflammation possess the cag pathogenicity island that contains several genes encoding factors involved in the induction of proinflammatory cytokines/chemokines and of a type IV secretion system involved in the delivery of a highly immunogenic protein, CagA, into eukaryotic cells. Recent advances in our understanding of the involvement of VacA, HP-NAP and the CagA/Type IV secretion system in the H. pylori-associated disease process are discussed in this review.

Cellular responses induced after contact with Helicobacter pylori

Contact-dependent activation of the cag organelle, a type IV secretion system of Helicobacter pylori, promotes translocation of CagA into the host cell. CagA is an immunodominant antigen of H. pylori, encoded by cag. It is thought to be associated with severe clinical outcomes, but has an unclear role in pathogenesis. Now we know that CagA is injected into the host and is tyrosine-phosphorylated by a membrane-associated eukaryotic tyrosine kinase. After activation, CagA induces morphological changes in the host, as well as actin reorganization, variations in the cell cycle and autocrine effects. Subversion of cell control may ultimately lead to cellular damage and to increased risks for gastric cancer development. cag instability contributes to long-term persistence within the host by attenuating bacterial virulence. We still do not know if additional factors are co-translocated with CagA and we do not know their specific mechanisms of action, but there is a strong experimental evidence that indicates that cag is the major player in the host-pathogen relationship.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori infects over half of the world population. Infection with the bacterium causes gastritis and peptic ulcer disease and is associated with the development of gastric cancers. However, only a small proportion of individuals develop these complications of infection. Therefore, identification of both host and bacterial factors that mediate disease is an intense area of current research interest. This review highlights recent advances in understanding of the mechanisms underlying disease pathophysiology following infection with H. pylori.

Interaction of Helicobacter pylori with professional phagocytes: role of the cag pathogenicity island and translocation, phosphorylation and processing of CagA

Chronic infection of the human gastric mucosa with Helicobacter pylori is a major cause of gastroduodenal pathologies, including peptic ulcerations, mucosa-associated lymphoid tissue (MALT) lymphoma and adenocarcinoma. Helicobacter pylori strains carrying the cag pathogenicity island, which encodes an active type IV protein secretion system (cag+ or type I strains), are preferentially associated with strong gastric inflammation and severe disease. We show here that cag+ H. pylori strains use the type IV secretion system to inject the bacterial protein CagA into various types of professional phagocytes, including human polymorphonuclear leucocytes (PMNs) and the human and murine macrophage cell lines THP-1 and J774A.1 CagA is rapidly tyrosine phosphorylated and proteolytically processed to generate a stable 35-45 kDa C-terminally tyrosine-phosphorylated protein fragment. H. pylori was efficiently ingested by the different types of phagocytic cells. A chromosomal deletion of the complete pathogenicity island had no significant effect on the rate of ingestion. Furthermore, the survival rate of H. pylori in the phagosome was unchanged between the wild type and a deletion mutant lacking the type IV secretion system. Thus, the type IV secretion system seems to be involved neither in active phagocytosis resistance nor in prolonged survival of the bacteria in phagocytic cells.

Identification of differentially expressed genes in normal and tumor human gastric tissue

The search for differentially expressed genes in gastric cancer may help define molecular alterations and molecular diagnosis of gastric cancer. Using the differential display PCR technique, we identified 18 genes that are differentially expressed between normal and tumor human gastric tissues. Their expressions were verified with reverse Northern blot analysis and Northern blot analysis. Oxidative phosphorylation-related genes, antizyme inhibitor of ornithine decarboxylase, protein phosphatase-1beta, 35-kDa peroxisomal membrane protein, and cystic fibrosis transmembrane conductance receptor were highly expressed in tumor tissue, whereas pepsinogen A, Na-K ATPase alpha subunit, nerve growth factor receptor, and alpha-tropomyosin were highly expressed in normal tissue. In addition, 3 unknown genes were found to be differentially expressed in paired gastric tissues. These differentially expressed genes may provide significant opportunities for further understanding of gastric carcinogenesis and the molecular diagnosis of gastric cancer.

Bacterial inhibition of phagocytosis

The concerted study of molecular mechanisms of phagocytosis and the inhibition of phagocytosis by specific products of extracellular bacterial pathogens has borne considerable fruit. The importance of tyrosine phosphorylation and of the Rho family of GTPases has become clear to cell biologists, but pathogenic bacteria recognized the importance of these signalling pathways in phagocytic cells long ago. The discoveries described in this review are only the beginning. The simultaneous pursuit of the mechanisms and molecules involved in the initiation and regulation of phagocytosis and that pathogenic bacteria use to inhibit phagocytosis will surely identify more interesting pathways on each side of the contest. Are there any obvious possibilities? There are several bacterial factors that have the potential to inhibit known mechanisms of phagocytosis. Clostridium species, for example, make a number of exotoxins of interest. Clostridium botulinum and Clostridium tetani neurotoxins inactivate the regulated secretory machinery by proteolytic cleavage of SNARE proteins, and targets of tetanus toxin and botulinum b toxin inhibit the exocytotic delivery of membrane vesicles needed for phagocytosis of large particles (Hackam et al., 1998). Moreover, the C3 exotoxin of C. botulinum catalyses ADP ribosylation and inactivation of rho family GTPases (Wiegers et al., 1991), and toxins A and B of C. difficile UDP-glucosylate and inactivate rho GTPases and thereby disrupt the actin cytoskeleton (Just et al., 1995a,b). However, as Clostridia lack the machinery for type III secretion, these proteins are not rapidly targeted to the phagocyte cytoplasm. More searching may reveal a pathogen that has combined the type III secretory machinery with clostridia toxin-like substrates. A potentially unique strategy for remaining outside phagocytes is exhibited by Helicobacter pylori, which contain a type IV secretion system. Unopsonized virulent strains of H. pylori bind readily to macrophages but are only internalized after a delay of several minutes. Such a delay appears to be sufficient for the bacteria to remain extracellular (Allen et al., 2000). Elucidation of the mechanism used by H. pylori to delay phagocytosis may reveal one or more novel virulence factors as well as one or more novel targets in the phagocyte that will add to the understanding of a fundamental process in host defence. Another field ripe for further mechanistic investigation is complement receptor-mediated phagocytosis. Dedicated study of the molecular events and molecular mediators of phagocytosis downstream of CR3 is likely to reveal interesting differences from FcgammaR phagocytosis and is just as likely to reveal that microbes have discovered unique mechanisms for circumventing them. Study of extracellular pathogens and the mechanisms that they use to remain outside phagocytic cells has revealed a great deal about the initial encounter between pathogen and phagocyte. We can look forward to additional discoveries about the host-pathogen interactions and the mechanisms and factors that each side uses to battle against the other.