Structural Insights into Helicobacter pylori Cag Protein Interactions with Host Cell Factors

Role of Helicobacter pylori virulence factors and alteration of the Tumor Immune Microenvironment: challenges and opportunities for Cancer Immunotherapy

Various forms of malignancies have been linked to Helicobacter pylori. Despite advancements in chemotherapeutic and surgical approaches, the management of cancer, particularly at advanced stages, increasingly relies on the integration of immunotherapy. As a novel, safe therapeutic modality, immunotherapy harnesses the immune system of the patient to treat cancer, thereby broadening treatment options. However, there is evidence that H. pylori infection may influence the effectiveness of immunotherapy in various types of cancer. This association is related to H. pylori virulence factors and the tumor microenvironment. This review discusses the influence of H. pylori infection on immunotherapy in non-gastrointestinal and gastrointestinal tumors, the mechanisms underlying this relationship, and directions for the development of improved immunotherapy strategies.

Helicobacter pylori Virulence Factors-Mechanisms of Bacterial Pathogenicity in the Gastric Microenvironment

Gastric cancer constitutes one of the most prevalent malignancies in both sexes; it is currently the fourth major cause of cancer-related deaths worldwide. The pathogenesis of gastric cancer is associated with the interaction between genetic and environmental factors, among which infection by Helicobacter pylori (H. pylori) is of major importance. The invasion, survival, colonization, and stimulation of further inflammation within the gastric mucosa are possible due to several evasive mechanisms induced by the virulence factors that are expressed by the bacterium. The knowledge concerning the mechanisms of H. pylori pathogenicity is crucial to ameliorate eradication strategies preventing the possible induction of carcinogenesis. This review highlights the current state of knowledge and the most recent findings regarding H. pylori virulence factors and their relationship with gastric premalignant lesions and further carcinogenesis.

Activity and Functional Importance of Helicobacter pylori Virulence Factors

Helicobacter pylori is a very successful Gram-negative pathogen colonizing the stomach of humans worldwide. Infections with this bacterium can generate pathologies ranging from chronic gastritis and peptic ulceration to gastric cancer. The best characterized H. pylori virulence factors that cause direct cell damage include an effector protein encoded by the cytotoxin-associated gene A (CagA), a type IV secretion system (T4SS) encoded in the cag-pathogenicity island (cag PAI), vacuolating cytotoxin A (VacA), γ-glutamyl transpeptidase (GGT), high temperature requirement A (HtrA, a serine protease) and cholesterol glycosyl-transferase (CGT). Since these H. pylori factors are either surface-exposed, secreted or translocated, they can directly interact with host cell molecules and are able to hijack cellular functions. Studies on these bacterial factors have progressed substantially in recent years. Here, we review the current status in the characterization of signaling cascades by these factors in vivo and in vitro, which comprise the disruption of cell-to-cell junctions, induction of membrane rearrangements, cytoskeletal dynamics, proliferative, pro-inflammatory, as well as, pro-apoptotic and anti-apoptotic responses or immune evasion. The impact of these signal transduction modules in the pathogenesis of H. pylori infections is discussed.

Structural and Molecular Biology of Type IV Secretion Systems

Type IV secretion systems (T4SSs) are nanomachines that Gram-negative, Gram-positive bacteria, and some archaea use to transport macromolecules across their membranes into bacterial or eukaryotic host targets or into the extracellular milieu. They are the most versatile secretion systems, being able to deliver both proteins and nucleoprotein complexes into targeted cells. By mediating conjugation and/or competence, T4SSs play important roles in determining bacterial genome plasticity and diversity; they also play a pivotal role in the spread of antibiotic resistance within bacterial populations. T4SSs are also used by human pathogens such as Legionella pneumophila, Bordetella pertussis, Brucella sp., or Helicobacter pylori to sustain infection. Since they are essential virulence factors for these important pathogens, T4SSs might represent attractive targets for vaccines and therapeutics. The best-characterized conjugative T4SSs of Gram-negative bacteria are composed of twelve components that are conserved across many T4SSs. In this chapter, we will review our current structural knowledge on the T4SSs by describing the structures of the individual components and how they assemble into large macromolecular assemblies. With the combined efforts of X-ray crystallography, nuclear magnetic resonance (NMR), and more recently electron microscopy, structural biology of the T4SS has made spectacular progress during the past fifteen years and has unraveled the properties of unique proteins and complexes that assemble dynamically in a highly sophisticated manner.

Association of a single nucleotide polymorphism in the TLR2 gene (rs3804099), but not in the TLR4 gene (rs4986790), with Helicobacter pylori infection and peptic ulcer

BACKGROUND/AIMS

Toll-like receptors (TLRs), particularly TLR2 and TLR4, take part in elicitation of immune responses against Helicobacter pylori (H. pylori). This study aimed to investigate the relationship between single nucleotide polymorphisms (SNP) rs3804099 in the TLR2 gene and rs4986790 in the TLR4 gene with H. pylori infection and peptic ulcer (PU).

MATERIALS AND METHODS

Blood specimens were obtained from 350 individuals, including 100 H. pylori-infected patients with PU, 125 H. pylori-infected asymptomatic subjects (AS), and 125 non-infected healthy subjects (NHS). The DNA was extracted, and the SNPs were determined using ARMS-PCR method.

RESULTS

The frequency of CT genotype at TLR2 SNP rs3804099 in both the PU and AS groups was significantly higher than in the NHS group (p<0.05). In total H. pylori-infected individuals (PU+AS), the frequency of the CT genotype at rs3804099 was also significantly higher than in the NHS group (p<0.005). The frequency of the CC genotype at rs3804099 in PU+AS was markedly lower than in the NHS group (p=0.066). PU patients carried CT genotype more frequently than total healthy individuals (AS+NHS) (p<0.03). The distribution of the TT genotype was lower, whereas the frequency of the CT genotype was higher in AS individuals infected with CagA+ strains than those infected with CagA- strains (p<0.03). No significant differences were found among the PU, AS, and NHS groups regarding the genetic differences at rs4986790 in the TLR4 gene.

CONCLUSION

These results provide evidence regarding the association of the rs3804099 in the TLR2 gene with H. pylori infection and PU. The rs3804099 may affect vulnerability to H. pylori infection, particularly to CagA+ strains of bacteria.

Integrin but not CEACAM receptors are dispensable for Helicobacter pylori CagA translocation

Translocation of the Helicobacter pylori (Hp) cytotoxin-associated gene A (CagA) effector protein via the cag-Type IV Secretion System (cag-T4SS) into host cells is a hallmark of infection with Hp and a major risk factor for severe gastric diseases, including gastric cancer. To mediate the injection of CagA, Hp uses a membrane-embedded syringe-like molecular apparatus extended by an external pilus-like rod structure that binds host cell surface integrin heterodimers. It is still largely unclear how the interaction of the cag-T4SS finally mediates translocation of the CagA protein into the cell cytoplasm. Recently certain carcinoembryonic antigen-related cell adhesion molecules (CEACAMs), acting as receptor for the Hp outer membrane adhesin HopQ, have been identified to be involved in the process of CagA host cell injection. Here, we applied the CRISPR/Cas9-knockout technology to generate defined human gastric AGS and KatoIII integrin knockout cell lines. Although confocal laser scanning microscopy revealed a co-localization of Hp and β1 integrin heterodimers on gastric epithelial cells, Hp infection studies using the quantitative and highly sensitive Hp β-lactamase reporter system clearly show that neither β1 integrin heterodimers (α1β1, α2β1 or α5β1), nor any other αβ integrin heterodimers on the cell surface are essential for CagA translocation. In contrast, deletion of the HopQ adhesin in Hp, or the simultaneous knockout of the receptors CEACAM1, CEACAM5 and CEACAM6 in KatoIII cells abolished CagA injection nearly completely, although bacterial binding was only reduced to 50%. These data provide genetic evidence that the cag-T4SS-mediated interaction of Hp with cell surface integrins on human gastric epithelial cells is not essential for CagA translocation, but interaction of Hp with CEACAM receptors is facilitating CagA translocation by the cag-T4SS of this important microbe.

Overexpression of serine protease HtrA enhances disruption of adherens junctions, paracellular transmigration and type IV secretion of CagA by Helicobacter pylori

BACKGROUND

The serine protease HtrA is an important factor for regulating stress responses and protein quality control in bacteria. In recent studies, we have demonstrated that the gastric pathogen Helicobacter pylori can secrete HtrA into the extracellular environment, where it cleaves-off the ectodomain of the tumor suppressor and adherens junction protein E-cadherin on gastric epithelial cells.

RESULTS

E-cadherin cleavage opens cell-to-cell junctions, allowing paracellular transmigration of the bacteria across polarized monolayers of MKN-28 and Caco-2 epithelial cells. However, rapid research progress on HtrA function is mainly hampered by the lack of ΔhtrA knockout mutants, suggesting that htrA may represent an essential gene in H. pylori. To circumvent this major handicap and to investigate the role of HtrA further, we overexpressed HtrA by introducing a second functional htrA gene copy in the chromosome and studied various virulence properties of the bacteria. The resulting data demonstrate that overexpression of HtrA in H. pylori gives rise to elevated rates of HtrA secretion, cleavage of E-cadherin, bacterial transmigration and delivery of the type IV secretion system (T4SS) effector protein CagA into polarized epithelial cells, but did not affect IL-8 chemokine production or the secretion of vacuolating cytotoxin VacA and γ-glutamyl-transpeptidase GGT.

CONCLUSIONS

These data provide for the first time genetic evidence in H. pylori that HtrA is a novel major virulence factor controlling multiple pathogenic activities of this important microbe.