Helicobacter pylori

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Helicobacter pylori

  • Spiral shaped
  • Gram negative bacteria
  • Has polar flagella
  • Lives near the surface of the human gastric mucosa
  • Avoids the bactericidal activity of stomach acid
  • Attaches and communicates with the gastric epithelium and the immune system
  • H.pylori infection is the strongest known risk factor for the development of gastroduodenal ulcers
  • Infection --> 60-80% gastric ulcers, 95% duodenal ulcers
  • H.pylori is classed as a carcinogen - there is an epidemiological relationship to gastric adenocarcinoma and gastric mucosa-associated lymphoid tissue lymphoma
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Epidemiology

  • Infection is acquired during childhood and lasts a lifetime. Transmission is rare in adults.
  • The majority of cases (80-90%) carry and transmit the bacteria without any symptoms of disease.
  • Infection is found in 50% of the population in developed nations, and in 80% of the population in the developing world.
  •  It is classed as a commensal organism
  •  Rates of acquisition have decreased in industrialised countries
  • Density of housing, overcrowding, number of siblings, sharing beds and a lack of running water are all linked to higher acquistion levels
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Transmission and entry

  • H.pylori is restricted to humans and closely related primates
  • The majority of infections occur in families through close person-to-person contact
  • Most transmission occurs in childhood, in some countries 90% of children are infected by 10yrs old
  • Maternal-to-child and sibling-sibling transmission is most likely
  • H.pylori is fragile outside the stomach - it is rapidly killed by high oxygen lvels and even light
  • Gastric-oral transmission occurs - e.g. during gastroenteritis with vomiting
  • Faecal-oral transmision is possible if H.pylori survives transit through the lower GI tract, which is uncommon in healthy individuals.
  • The bacteria is not typically culturable from human faeces, patients with diarrhoea --> 20% of stool samples had culturable bacteria
  • Epidemiological studies have shown that transmission correlates with exposure to an infected family member with gastroenteritis.
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Colonisation of Mucous Layer

  • H.pylori avoids the bactericidal activity of stomach acid by generating large quanitites of cytosolic and cell surface urease.
  • Urease breaks down urea to generate ammonia and CO2 --> it acts as a buffer
  • Urease is not produced by human cells, so urease detection is a common diagnostic tool
  • Urease is expressed by all strains
  • NH3 can affect the tight junctions of epithelial cells - affects epithelial barrier

Urease breath test

  • Urea is labelled iwith C13 or C14
  • Labelled urea diffuses through the mucosal gel layer
  • H.pylori splits urea into ammonia and labelled CO2
  • CO2 is exhaled and collected
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Colonisation of Mucous Layer Cont.

  • However, H.pylori can only survive for a few minutes in stomach acid, so it actively swims towards the mucosal surface
  • It remains in the mucous layer which is contantly renewed
  • It uses polar flagella for motility and swims away from acidic environments
  • Mutant bacteria which lack flagella have a reduced ability to colonise the mucosa
  • Flagellin of H.pylori doesn't bind or activtae TLR5 - evasion of the immune system
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Interaction with Gastric Epithelium

  • About 20% of the H.pylori in the stomach is attached to the surfaces of epithelial cells.
  • H.pylori also binds to intercellular junctions, which causes cytoskeletal changes deep in the intercellular spaces
  • It apears that H.pylori live predominantly in the mucous layer and that contact with the gastric epithelia is generally limited
  • Intact epithelial barriers are important structures in antimicrobial defence - H.pylori has strategies to alter cell-cell and cell matrix adhesion.
  • H.pylori preferentially adheres to the apical-junctional complex of epithelial cells. It alters the localisation of apical-junctional component proteins, disrupts epithelial barrier function, cell adhesion, and cell polarity, and induces an invasive phenotype.
  • Translocated CagA interacts with PAR1, which results in the disruption of tight junctions
  • Urease activity can also disrupt the tight junction
  • VacA can increase the tight junction permeability to low-molecular weight molecules and ions.
  • Despite a relatively small genome, 30 genes are dedicated to the epxression of outer membrane proteins - several adhesins
  • Multiple outer membrane proteins bind to specific receptors on gastric epithelial cells
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Membrane proteins

  • BabA is a highly conserved adhesin which binds to the Lewis Blood group antigen. It is encoded by a strain-specific babA2 gene which is associated with an increased risk of gastric cancer
  • SabA is an adhesin which binds sialyl-Lewis^X antigen
  • Lewis antigens are also encoded in Helicobacter genome - mimicry
  • OipA is a cellular receptor that isn't yet determined, but influences the host's pro-inflammatory cytokine release
  • H.pylori upregulates the expression of and binding to DAF
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Why does H.pylori bind to cell surfaces?

1. Adehsion causes cellular damage and inflammation

  • SabA binds to the inflamed mucosa and activates neutrophils
  • Adhesion for the delivery of toxins (CagA and VacA)

2. Adhesion to avoid mechanical clearance and to promote invasion and persistence

  • Mucous secretion, gastric juics and peristalis
  • Requires adhesion to prevent bacterial removal

3. Use of cell surface as a site of replication

  • Unclear, but evidence supports this
  • H.pylori removes cholesterol from the host epithelial cell and incorporates and modifies it within its own cell membrane
  • Glycosylates cholesterol to escape macrophage phagocytosis - mimicry and evasion
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VacA - H.pylori toxin

  • Vacuolating toxin
  • A pore-forming cytotoxin
  • All strains have VacA but there is genetic diverisity
  • Secreted by bacteria to host cell membranes
  • Forms pores which allow the leakage of Cl-
  • Affects epithelial and immune cells
  • Is linked to gastric malignancy
  • VacA is composed of two domains (p33 and p55) when secreted from the bacteria
  • p33 - pore formation, p55 - cell binding
  • Oligomer disassembles in acidic conditions
  • Monomers then insert into the plasma membrane - act as chloride channels --> membrane depolarisation
  • Also, endocytosis of VacA/receptor complex
  • Results in osmotic sweeling, may affect epithelia
  • VacA can also induce channels in mitochondrial membranes --> cell death by apoptosis
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VacA - H.pylori toxin Cont.

  • VacA binds various epithelial cell components: fibronectin, epidermal growth factor receptor (EGF-R), sphingomyelin.
  • VacA induces mast cells to produce TNFa and IL6 --> proinflammatory molecules
  • Also prevents the fusion of lysosomes and phagosomes --> affects phagocytosis
  • VacA also inhibits antigen processing and presentation. It affects T-lymphocytes response, which inhibits the host's immune adaptive response
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CagA - H.pylori toxin

  • Cytotoxin-associated gene A
  • Its exact role in disease and its function is now becoming clearer
  • CagA can activate a number of signalling pathways
  • Affects the epithelial junctions - affects tight junctions and cellular differentiation
  • Pathogenic strains of H.pylori have a Cag pathogenicity island
  • CagA is an oncoprotein --> associated with inflammation, peptic ulcers and gastric cancer
  • CagA is phosphorylated by Src and Abl kinases when it enters host cells
  • A number of sites in CagA can be phosphorylated
  • The number of sites varies with each strain, so the degree of phosphorylation varies with the strain
  • Therefore, different strains cause different levels of changes in cell morphology
  • Phosphorylated CagA results in actin-cytoskeletal rearrangments. In cellular culture, the phenotypic changes resemble malignant cellualr transformation
  • CagA remains near the plasma membrane after being phosphorylated. It can trigger a number of growth factoe pathways. Affects proliferation, adhesion and cytoskeletal organisation of epithelial cells. In this way, H.pylori contributes to peptic ulcer disease and cancer.
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CagA - H.pylori toxin Cont.

  • Therefore CagA is a major virulence factor for Helicobacter pylori
  • However not all CagA becomes phosphorylated
  • Non-phosphorylated CagA induces host cell proliferation and immune repsonse in gastric epithelial cells
  • This promotes the survival and multiplication of epithelial cells, which allows for long-term H.pylori colonisation.

Another function of the Cag secretion system is to introduce peptidoglycan into the host cell. Peptidoglycan is recognised by NOD1-->IL8 --> which recruits neutophils and monocytes to the gastric mucosa. NOD1 activation --> release of peptidoglycan --> drives inflammation.

CagA+ strains activate inflammatory pathways and oncogenic cellular responses.

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Cag pathogenicity island

  • CagA is in a large pathogenicity island
  • Horizontal transfer occurs to more virulent strains, but it is not known where it came from
  • CagA has no homology to other known proteins
  • Other genes in the pathogenicity island have homology to type IV secretion systems (Type IV transfer of bacterial products into host cells)
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Integrins

  • The receptor for the pilus is a5b1 integrins
  • CagA is mostly transferred via integrins
  • Integrins are prodominantly expressed on the basal surface of the host gastric epithelia, whilst adhesion via inegrins is though to be on the apical surface --> redistribution of integrins to apical surface?
  • Integrins are cell adhesion receptors which mediate cell-cell, cell-extracellular matrix and cell-pathogen interactions
  • Heterodimers - a and b subunit. In mammals there are 18 different a and 8 different b subunits --> 24 distinct heterodimers.
  • A number of different viruses and bacteria bind integrins for adhesion and invasion of host cells.
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Type IV secretion system

A wide range of Gram-negative bacteria transfer virulence factors into host target cells using a type IV secretion system.

  • Type IV - membrane spanning secretion channel and an extracellular pilus
  • Acts as a molecular syringe
  • The needle-like pili appear within about 30min of contact with a cell
  • The secretion system injects CagA into host cells which activates NF-kB. This induces pro-inflammatory cytokines such as IL-8

The pilus of the type IV system contains CagL

  • This protein is highly conserved among pathogenic H.pylori strains
  • Has an RGD motif, arginine-glycine-aspartate
  • This motif binds to integrins
  • CagL is encoded by the pathogenicity island
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Apical junctional complex

  • H,pylori also affects the apical junctional complex
  • Apical junctions form a barrier between the lumen and intersitial spaces
  • They also regulate cell proliferation, cell-cell adhesion and cell movement
  • CagA targets thses epithelial juncations and affects their function, leading to dysregulated cell-cell adhesion and a loss of polarity. Cells have an invasive phenotype similar to carcinoma cells
  • However, how and if these all occur in vivo is unclear
  • Phosphorylated CagA can be found in a patient's gastric mucosal cells on biopsy
  • However CagA can also be present without any signs of clinical disease
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Persistence

  • To colonise the stomach for extended periods, H.pylori must overcome all the physical and cellular barriers
  • It must also avoid the innate and adaptive immune responses, which are triggered in the stomach due to the presence of the bacteria
  • A number of factors from H.pylori appear to decrease inflammation or recognition by the immune system e.g.flagellar proteins are not recognised by TLR, LPS of H.pylori is not particularly toxic and is modified so that it resembles human glycans (minicry) in order to avoid immune recognition.
  • Although, H. pylori induces a strong antibody response these antibodies are not able to clear the infection
  • In 20-30% of individuals pathogenic autoantibodies are produced (often against gastric proton pumps). This contributes to achlorhydria or an autoimmune gastritis
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Neutrophils

  • H.pylori induces the migration of neutophils to the gastric mucosa (nuerrophil activating protein)

How does H.pylori survive?

  • H.pylori product inhibits nuetrophil phagocytosis
  • Neutrophils engulf H,pylori but the lysosome doesn’t fuse with phagosome – intracellular survival
  • Reactive oxygen species produced by neutrophil but only released outside cell -->local inflammation. Similar with macrophages
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H.pylori infection

Infection with H. pylori is associated with 3 potential outcomes

Basic process of damage – gastritis. Associated with immune mechanisms

Extent and distribution of gastritis determines clinical outcome

Simple gastritis phenotype

  • Majority of infected subjects
  • Mild mixed gastritis/pangastritis
  • High gastrin but normal acid secretion
  • No gastric atrophy
  • Commonly seen in asymptomatic patients
  • No significant clinical outcome - usually develop no clinical disease

Duodenal ulcer phenotype

  • Around 10-15% of infected subjects, especially in Western countries
  • Antral predominant gastritis
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H.pylori infection Cont.

  • High gastrin and acid sceretion
  • Impaired inhibitory control of acid secretion
  • Protection from gastric cancer - mutually exclusive

Gastric cancer phenotype

  • Around 1% of infected subjects
  • Most serious phenotype
  • Corpus-predominant gastritis
  • Multi-focal atrophic gastritis
  • High gastrin
  • Hyo/achlorhydria
  • Low pepsingoen I and pepsinogen I/II ratio
  • Increased risk of gastric cancer
  • Chronic infection --> inflammation and increased risk of gastric cancer, especially in certain parts of Asia
  • Inflammation induced by H.pylori  is the initial stage of carcinogenesis
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Factors for chronic H.pylori infection

Host genetic factors:

  • IL-1B-511*T
  • IL-1-RN*2*2
  • IL-10 ATA haplotype
  • TNF-A-308*A
  • Il-8-251*A
  • TLR4+896*G
  • MBL2 HYD haplotype

Bacterial virulence factors:

  • CagA PAI
  • VacA s1/m1

Environmental factors

  • Smoking
  • Poor diet
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Treatment of H.pylori infection

  • Eradicating H.pylori is of significant benefit to patients with dyspesia
  • It is not known if this si the same for reducing the risk of the development of gastric cancer
  • Proton-pump inhibitor, clarithromycin and either amoxicillin or metronidazole
  • European guidelines --> 14 days of treament
  • Treatment may be affected by antibiotic-resistant strains (main factor- usually clarithromycin resistance). Also by patients adherence to treatment
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H.pylori genetics

Helicobacter  species are mostly host-specific --> so they have coevolved with their host

By comparing the nucleotide sequence of different strains and measuring the maximal rate of nucleotide mutation within the host, it is possible to calculate when Helicobacter pylori and its host shared a common ancestor i.e. the "original" Helicobacter pylori in which there would have been no mutation.

Genetic diversity among different strains of H.pylori decreases with distance from East Africa; in the same way that genetic diversity decreases among humans decreases from East Africa

Conclusion: H.pylori has coevolved with humans, at least since the exodus of humans from Africa 60,000 years ago ("Out of Africa" hypothesis)

  • Although there is a high level of genetic variability in H pylori, the common virulence factors are present in all - Urease, VacA, CagA. However is the variability within each of the genes.
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