Helicobacter pylori is the first formally recognized bacterial carcinogen and is one of the most successful human pathogens, as over half of the world's population is colonized with this gram-negative bacterium. Unless treated, colonization usually persists lifelong. H. pylori infection represents a key factor in the etiology of various gastrointestinal diseases, ranging from chronic active gastritis without clinical symptoms to peptic ulceration, gastric adenocarcinoma, and gastric mucosa-associated lymphoid tissue lymphoma. Disease outcome is the result of the complex interplay between the host and the bacterium. Host immune gene polymorphisms and gastric acid secretion largely determine the bacterium's ability to colonize a specific gastric niche. Bacterial virulence factors such as the cytotoxin-associated gene pathogenicity island-encoded protein CagA and the vacuolating cytotoxin VacA aid in this colonization of the gastric mucosa and subsequently seem to modulate the host's immune system. This review focuses on the microbiological, clinical, immunological, and biochemical aspects of the pathogenesis of H. pylori.

Pathogenesis of Helicobacter pylori Infection

Helicobacter pylori is a gastric pathogen that colonizes approximately 50% of the world's population. Infection with H. pylori causes chronic inflammation and significantly increases the risk of developing duodenal and gastric ulcer disease and gastric cancer. Infection with H. pylori is the strongest known risk factor for gastric cancer, which is the second leading cause of cancer-related deaths worldwide. Once H. pylori colonizes the gastric environment, it persists for the lifetime of the host, suggesting that the host immune response is ineffective in clearing this bacterium. In this review, we discuss the host immune response and examine other host factors that increase the pathogenic potential of this bacterium, including host polymorphisms, alterations to the apical-junctional complex, and the effects of environmental factors. In addition to host effects and responses, H. pylori strains are genetically diverse. We discuss the main virulence determinants in H. pylori strains and the correlation between these and the diverse clinical outcomes following H. pylori infection. Since H. pylori inhibits the gastric epithelium of half of the world, it is crucial that we continue to gain understanding of host and microbial factors that increase the risk of developing more severe clinical outcomes.

Helicobacter pylori and Gastric Cancer: Factors That Modulate Disease Risk

Outer membrane (OM) vesicles are ubiquitously produced by Gram-negative bacteria during all stages of bacterial growth. OM vesicles are naturally secreted by both pathogenic and nonpathogenic bacteria. Strong experimental evidence exists to categorize OM vesicle production as a type of Gram-negative bacterial virulence factor. A growing body of data demonstrates an association of active virulence factors and toxins with vesicles, suggesting that they play a role in pathogenesis. One of the most popular and best-studied pathogenic functions for membrane vesicles is to serve as natural vehicles for the intercellular transport of virulence factors and other materials directly into host cells. The production of OM vesicles has been identified as an independent bacterial stress response pathway that is activated when bacteria encounter environmental stress, such as what might be experienced during the colonization of host tissues. Their detection in infected human tissues reinforces this theory. Various other virulence factors are also associated with OM vesicles, including adhesins and degradative enzymes. As a result, OM vesicles are heavily laden with pathogen-associated molecular patterns (PAMPs), virulence factors, and other OM components that can impact the course of infection by having toxigenic effects or by the activation of the innate immune response. However, infected hosts can also benefit from OM vesicle production by stimulating their ability to mount an effective defense. Vesicles display antigens and can elicit potent inflammatory and immune responses. In sum, OM vesicles are likely to play a significant role in the virulence of Gram-negative bacterial pathogens.

Virulence and Immunomodulatory Roles of Bacterial Outer Membrane Vesicles

Helicobacter pylori in health and disease.

Host-Bacterial Interactions in Helicobacter pylori Infection

Helicobacter pylori toxin, VacA, damages the gastric epithelium by erosion and loosening of tight junctions. Here we report that VacA also interferes with T cell activation by two different mechanisms. Formation of anion-specific channels by VacA prevents calcium influx from the extracellular milieu. The transcription factor NF-AT thus fails to translocate to the nucleus and activate key cytokine genes. A second, channel-independent mechanism involves activation of intracellular signaling through the mitogen-activated protein kinases MKK3/6 and p38 and the Rac-specific nucleotide exchange factor, Vav. As a consequence of aberrant Rac activation, disordered actin polymerization is stimulated. The resulting defects in T cell activation may help H. pylori to prevent an effective immune response leading to chronic colonization of its gastric niche.

https://rupress.org/jem/article-pdf/198/12/1887/1146612/jem198121887.pdf

The Helicobacter pylori Vacuolating Toxin Inhibits T Cell Activation by Two Independent Mechanisms

Summary


Helicobacter pylori is the causative agent of peptic ulcer disease. A major virulence factor of H. pylori is VacA, a toxin that causes massive vacuolization of epithelial cell lines in vitro and gastric epithelial erosion in vivo. Although VacA is exported over the outer membrane and is released from the bacteria, a portion of the toxin remains associated with the bacterial surface. We have found surface-associated toxin to be biologically active and spatially organized into distinct toxin-rich domains on the bacterial surface. Upon bacterial contact with host cells, toxin clusters are transferred directly from the bacterial surface to the host cell surface at the bacteria–cell interface, followed by uptake and intoxication. This contact-dependent transfer of VacA represents a cost-efficient route for delivery of VacA and potentially other bacterial effector molecules to target cells.

Helicobacter pylori toxin VacA is transferred to host cells via a novel contact-dependent mechanism.

Most Helicobacter pylori strains secrete a toxin (VacA) that causes massive vacuolization of target cells and which is a major virulence factor of H. pylori. The VacA amino-terminal region is required for the induction of vacuolization. The aim of the present study was a deeper understanding of the critical role of the N-terminal regions that are protected from proteolysis when VacA interacts with artificial membranes. Using a counterselection system, we constructed an H. pylori strain, SPM 326-Δ49-57, that produces a mutant toxin with a deletion of eight amino acids in one of these protected regions. VacA Δ49-57 was correctly secreted by H. pylori but failed to oligomerize and did not have any detectable vacuolating cytotoxic activity. However, the mutant toxin was internalized normally and stained the perinuclear region of HeLa cells. Moreover, the mutant toxin exhibited a dominant negative effect, completely inhibiting the vacuolating activity of wild-type VacA. This loss of activity was correlated with the disappearance of oligomers in electron microscopy. These findings indicate that the deletion in VacA Δ49-57 disrupts the intermolecular interactions required for the oligomerization of the toxin.

A Helicobacter pylori Vacuolating Toxin Mutant That Fails To Oligomerize Has a Dominant Negative Phenotype

Human gastric glycosphingolipids recognized by Helicobacter pylori vacuolating cytotoxin VacA.

There are two alleles, m1 and m2, of the midregion of the vacuolating cytotoxin gene (vacA) of Helicobacter pylori which code for toxins with different cell specificities. Here we describe the construction of five chimeric strains in which regions of vacA were exchanged between the two genotypes. By analyzing the toxicity of these strains for HeLa and RK13 cells we have confirmed that a 148-amino-acid region determines the phenotypic differences between the two forms of the protein and that this entire region is important for cytotoxicity. Furthermore, we have used our chimeric strains to investigate whether variations in the midregion of VacA have an effect on phorbol 12-myristate 13-acetate (PMA)-induced VacA sensitivity in HL-60 cells. The PMA-induced VacA sensitivity of HL-60 cells has been previously associated with the appearance of the cell surface receptor protein tyrosine phosphatase beta (RPTPβ). Our data indicate that both the m1 and m2 forms of VacA are able to utilize RPTPβ, and the cell-specific phenotype of the midregion is independent of the presence of RPTPβ. It appears that another as-yet-unidentified receptor exists in HL-60 cells that accounts for the m2 phenotype in this cell line. Also, by studying the effect of PMA on levels of RPTPβ in other cell lines and toxicity of VacA in these cell lines we have shown that RPTPβ does not play a major role in the vacuolation of HeLa cells.

https://iai.asm.org/content/iai/74/1/49.full.pdf

The Cell-Specific Phenotype of the Polymorphic vacA Midregion Is Independent of the Appearance of the Cell Surface Receptor Protein Tyrosine Phosphatase β

Silvia Barone

Papers

The Helicobacter pylori Vacuolating Toxin Inhibits T Cell Activation by Two Independent Mechanisms

Helicobacter pylori toxin VacA is transferred to host cells via a novel contact-dependent mechanism.

A Helicobacter pylori Vacuolating Toxin Mutant That Fails To Oligomerize Has a Dominant Negative Phenotype

Human gastric glycosphingolipids recognized by Helicobacter pylori vacuolating cytotoxin VacA.

The Cell-Specific Phenotype of the Polymorphic vacA Midregion Is Independent of the Appearance of the Cell Surface Receptor Protein Tyrosine Phosphatase β