Pathogenesis of Helicobacter pylori Infection

1. The Organism

2. Colonization and Survival in the Gastric Niche

• Robbins, Cotran & Kumar Pathologic Basis of Disease; Yamada's Textbook of Gastroenterology, 7th ed.

3. Key Virulence Factors

A. CagA (Cytotoxin-Associated Gene A) - the "oncoprotein"

• Encoded within the cag pathogenicity island (cag PAI), a ~40-kb genomic segment
• CagA is injected directly into gastric epithelial cells via a type IV secretion system (T4SS) - essentially a molecular syringe
• Once inside host cells, CagA is tyrosine-phosphorylated by Src and Abl kinases, and activates multiple growth factor signaling pathways (Ras-ERK, PI3K, SHP-2)
• This stimulates abnormal cell proliferation, disrupts tight junctions, and contributes to the carcinogenic cascade
• CagA-positive strains cause more severe inflammation and are strongly associated with gastric adenocarcinoma and duodenal ulcer
• The cag T4SS also translocates heptose bisphosphate (HBP), a metabolite that activates NF-κB via TIFA, amplifying inflammatory cytokine production (IL-8, TNF-α, IL-1β)
• H. pylori DNA itself is translocated into host cells via T4SS, activating intracellular TLR9

B. VacA (Vacuolating Cytotoxin A)

• A secreted pore-forming toxin present in virtually all H. pylori strains, but with major functional variation based on allele type
• The vacA gene has polymorphic regions: the signal (s) region (s1 or s2) and the middle (m) region (m1 or m2)
• vacA s1/m1 strains induce strongest vacuolation and are most strongly linked to ulcer disease and gastric cancer
• s2/m2 strains show no vacuolating activity
• VacA effects on host cells:
  • Induces large cytoplasmic vacuoles by disrupting endosomal/lysosomal trafficking
  • Triggers apoptosis of epithelial cells
  • Suppresses T-cell responses - contributing to the longevity of infection by impairing immune clearance

C. Other Virulence Factors

• OipA (outer inflammatory protein A): induces IL-8 production
• DupA: associated with duodenal ulcer risk
• Lipopolysaccharide (LPS): weakly immunogenic compared to other gram-negatives; contains Lewis antigens (Le^x, Le^y) that mimic host antigens - a mechanism of molecular mimicry allowing immune evasion
• Mulholland and Greenfield's Surgery, 7th ed.; Yamada's Textbook of Gastroenterology, 7th ed.; Sherris & Ryan's Medical Microbiology, 8th ed.

4. Host Inflammatory Response

1. Bacterial adhesion and virulence factor injection trigger epithelial cells to secrete IL-8 and other chemokines
2. Neutrophil and lymphocyte recruitment → mucosal infiltration
3. Activated macrophages and T cells release TNF-α, IL-1β, IL-6, IL-12
4. Reactive oxygen species (ROS) from neutrophils cause direct epithelial DNA damage
5. Chronic inflammation drives polyclonal B-cell proliferation → can give rise to gastric MALToma if driver mutations are acquired

5. Disease Outcomes - Determined by Gastritis Pattern

A. Antral-Predominant Gastritis → Duodenal Ulcer (DU)

1. H. pylori colonizes predominantly the antrum, leaving the acid-producing corpus relatively spared
2. Antral inflammation suppresses somatostatin production by D cells
3. Loss of somatostatin removes the normal negative feedback on G cells → hypergastrinemia
4. Excess gastrin increases parietal cell mass and stimulates ECL cells → ↑ histamine → ↑ acid output from the healthy corpus
5. The increased acid load in the duodenum promotes gastric metaplasia of the duodenal bulb (protective response to excess acid)
6. H. pylori can only colonize gastric-type mucosa - gastric metaplasia now allows duodenal colonization → local inflammation and ulceration
7. Bicarbonate secretion by the duodenum is also reduced, removing a key mucosal defense

B. Pangastritis → Gastric Ulcer (GU) and Gastric Adenocarcinoma

1. Inflammation involving both antrum and corpus (pangastritis)
2. Corpus inflammation damages parietal cells → normal or reduced acid (so DU does not develop)
3. Progressive atrophy of parietal glands → hypochlorhydria → continued hypergastrinemia
4. Reduced acid creates a permissive environment for further bacterial colonization and mucosal damage
5. If inflammation is severe, particularly at transitional zones of mucosal weakness (lesser curvature junction of antral and oxyntic mucosa), gastric ulcers form
6. Progressive atrophy → intestinal metaplasia → dysplasia → gastric adenocarcinoma (the Correa cascade)

6. Carcinogenesis - The Correa Cascade

• CagA activates growth factor signaling (SHP-2, Ras-ERK, PI3K) mimicking sustained growth factor stimulation
• Chronic inflammation produces mutagenic reactive oxygen/nitrogen species causing DNA strand breaks
• Hypochlorhydria allows overgrowth of other bacteria that produce carcinogenic N-nitroso compounds
• Sustained hypergastrinemia is trophic for gastric epithelium
• H. pylori also causes polyclonal B-cell proliferation that can acquire driver mutations → gastric B-cell MALToma (MALT lymphoma) (eradication of H. pylori can cause MALToma regression)
• Robbins, Cotran & Kumar Pathologic Basis of Disease; Yamada's Textbook of Gastroenterology, 7th ed.

7. Host Genetic Factors Modifying Disease

• IL-1β gene polymorphisms (particularly IL-1β -511 and +31): IL-1β is both a pro-inflammatory cytokine and a potent acid-suppressant. Polymorphisms causing ↑ IL-1β production → more atrophy/cancer risk rather than ulcer
• IL-1RA, TNF-α, IL-10 polymorphisms: modify mucosal inflammatory intensity
• Blood group O: increases H. pylori adhesion (H. pylori expresses adhesins that bind blood group antigens) → higher risk of PUD; blood groups A/B/AB have relative risk 0.75-0.91
• HLA-DQA1 genotype: associated with H. pylori-negative ulcer disease

Summary Table

• Yamada's Textbook of Gastroenterology, 7th ed. - Chapters 49 & 10 (Pathogenesis of H. pylori-induced PUD and virulence factors)
• Robbins, Cotran & Kumar Pathologic Basis of Disease - Chapter 17 (Pathogenesis section)
• Harrison's Principles of Internal Medicine, 22nd ed. - Chapter 335
• Medical Microbiology, 9th ed. - Chapter 28
• Mulholland and Greenfield's Surgery, 7th ed. - Helicobacter pylori virulence factors