Helicobacter pylori A model organism for understanding bacterial pathogenesis

1. Helicobacter pylori A model organism for understanding bacterial pathogenesis

2. Objectives To understand the molecular mechansms by which Helicobacter pylori causes human disease To use Helicobacter pylori as a model for understanding bacterial-host interactions

3. Discovery of H. pylori Early 1900’s - bacteria were detected by microscopy in human gastric tissue 1983: successful culture of a previously unrecognized bacterial organism from human gastric tissue Initial name: Campylobacter pyloridis Revised name: Helicobacter pylori

5. Characteristics of H. pylori Gram-negative curved rod Microaerophilic Colonies visible after 48-72 hours Strong enzymatic activities: urease, oxidase, catalase

7. Representative Helicobacter species Organism H. pylori H. heilmannii H. felis H. mustelae H. nemestrinae H. acinonyx H. cinaedi H. fennelliae H. muridarum H. Canis H. pullorum H. bilis H. hepaticus Host and site Human stomach Many mammals, stomach Cat or dog stomach Ferret stomach Macaque stomach Cheetah stomach Rodent or human intestine Human intestine Rodent intestine Dog intestine Chicken intestine Mouse intestine or liver Mouse intestine or liver

8. Whole genome analysis of two different H. pylori strains Two H. pylori strains selected for genomic sequence analysis (strains 26695 and J99) Number of predicted ORFs is about 1500 Number of ORFs present only in strain 26695 = 117 Number of ORFs present only in strain J99 = 89 Nature 1999;397:176-180

11. H. pylori epidemiology The human stomach is the main reservoir. H. pylori is found in humans throughout the world. Infection is typically acquired early in life. Infection usually persists for decades if not treated. Person-to-person transmission is likely. H. pylori is present in about 30% of the U.S. population.

12. Helicobacter pylori on a global scale Global human population: 6 billion Proportion infected with H. pylori: 60% Global number of H. pylori-infected humans: about 3.6 billion

14. Localization of H. pylori in the human stomach H. pylori lives in the mucus layer overlying gastric mucosa (non-invasive bacterium). H. pylori only colonizes the mucus layer overlying gastric-type epithelium. H. pylori can colonize the duodenum in regions of gastric metaplasia.

15. Ingestion of H. pylori by a human volunteer Baseline: normal gastric histology Day 0: ingestion of organism Day 2: Epigastric pain, nausea, vomiting Day 5: Neutrophilic antral gastritis, gastric pH 1.2 Day 8: Gastric pH 7.6 Day 10: Resolution of symptoms Day 30: Persistence of gastritis and H. pylori

17. H. pylori and gastric inflammation Inflammation always accompanies H. pylori infection. Termed “chronic superficial gastritis” Lymphocytes, monocytes, neutrophils in lamina propria Usually asymptomatic Resolves following eradication of H. pylori

19. H. pylori infection is a risk factor for duodenal ulcer disease >90% of patients with “idiopathic” duodenal ulcers are infected with H. pylori Prior H. pylori infection is associated with an increased risk for duodenal ulcer H. pylori infection causes gastric damage in animal models Eradication of H. pylori results in decreased rates of recurrence

21. H. pylori infection is a risk factor for gastric carcinoma Case-control studies based on serologic analysis of stored sera Experimental H. pylori infection of Mongolian gerbils results in gastric tumors Prospective study of patients who have a high risk for developing gastric cancer

23. Prospective study of the development of gastric cancer Population: 1526 Japanese patients 1246 H. pylori-positive 445 with non-ulcer dyspepsia 297 with gastric ulcer 229 with gastric hyperplastic polyps 275 with duodenal ulcer 280 H. pylori-negative Mean followup period: 7.8 years NEJM 2001;345:784-789

24. Development of gastric cancer related to endoscopic findings No. (%) with gastric cancer H. pylori-positive (n=1246) 36 (2.9) NUD (n=445) 21 (4.7) Gastric ulcer (n=297) 10 (3.4) Gastric polyps (n=229) 5 (2.2) Duodenal ulcer (n=275) 0 H. pylori-negative (n=280) 0

26. Gastric non-Hodgkins lymphoma Monoclonal B-cell proliferation Spectrum of severity MALT: mucosal-associated lymphoid tissue Eradication of H. pylori is associated with tumor regression

27. Infectious diseases with carcinogenic potential Viral: Hepatitis viruses Papilloma virus Bacterial: H. pylori Parasite: Clonorchis sinensis Schistosomiasis

28. Pathology or disease states associated with H. pylori infection Chronic superficial gastritis Gastric ulcer Duodenal ulcer Atrophic gastritis Gastric adenocarcinoma Gastric MALT lymphoma Gastric non-Hodgkins lymphoma

30. PATHOGENESIS OF H. pylori INFECTION

31. Stages in a “typical” bacterial infection Bacterial residence in a natural reservoir Bacterial encounter with the host Attachment to cells (adherence) Entry into tissue (invasion) Bacterial replication Evasion of host defenses Return to original reservoir or transmission to new hosts

32. Stages in a “typical” bacterial infection Bacterial residence in a natural reservoir Bacterial encounter with the host Attachment to cells (adherence) Entry into tissue (invasion) Bacterial replication Evasion of host defenses Return to original reservoir or transmission to new hosts

33. Bacterial virulence determinants Specific bacterial components (virulence determinants) may be required for various stages of the infectious process. Adhesins mediate bacterial adherence to host cells. Bacterial surface proteins (e.g. invasins) mediate entry into host cells. Bacterial secreted toxins contribute to tissue damage or modulation of host cell function. Bacterial surface components or secreted factors mediate resistance to host defenses.

34. Important questions relevant to H. pylori What are the mechanisms that allow H. pylori to colonize the human stomach, whereas other bacteria cannot? How does H. pylori interact with the gastric mucosa? How does H. pylori persistently colonize the stomach without being eradicated by host defenses? Why does H. pylori specifically colonize the human stomach? Why are there multiple possible clinical outcomes of H. pylori infection, and what are the factors that determine clinical outcome?

35. How does H. pylori survive in the acidic gastric environment?

36. H. pylori adaptations for life in an acidic environment Motility (flagella) Localization in the gastric mucus layer Urease activity Acid-induced changes in H. pylori gene expression Modulation of gastric acid physiology

37. How does H. pylori interact with gastric epithelial cells?

40. Binding of H. pylori to gastric epithelial cells via multiple adhesin-receptor interactions BabA adhesin binds to Lewis b on surface of cells SabA adhesin binds to sialyl-dimeric Lewis x on surface of cells

48. Multiple effects of VacA on gastric epithelial cells Cell vacuolation Formation of anion-selective membrane channels Alterations in endocytic trafficking Alterations in antigen presentation Extracellular release of lysosomal proteases Inhibition of cell proliferation Release of cytochrome c from mitochondria Increased permeability of epithelial monolayers Apoptosis Inhibition of T cell activation

50. THE H. pylori cag PATHOGENICITY ISLAND ~40 kb chromosomal region present in some H. pylori strains but not others

54. Intracellular activities of H. pylori CagA Translocated CagA protein undergoes phosphorylation on tyrosine residues, via eukaryotic cell kinases Phosphorylated CagA associates with SHP-2 (a tyrosine phosphatase) and stimulates phosphatase activity; changes in cell morphology Non-phosphorylated Caga interacts with Grb2, leading to activation of Ras/MEK/ERK pathway; cell scattering and proliferation

59. Functions attributed to the cag pathogenicity island Encodes CagA, a high-molecular-mass antigen that is translocated into epithelial cells and undergoes tyrosine phosphorylation Type IV secretion system for translocation of CagA into host cells Induction of cytokine expression in epithelial cells

60. Alterations that occur following binding of H. pylori to gastric epithelial cells Changes in cell shape and morphology (vacuolation, hummingbird phenotype) Activation of signal transduction pathways, leading to expression of cytokines Apoptosis

61. WHY ARE THERE MULTIPLE POSSIBLE CLINICAL OUTCOMES OF H. pylori INFECTION?

62. Why are there multiple possible clinical outcomes of H. pylori infection? Heterogeneity among H. pylori strains Heterogeneity among humans Heterogeneity in environmental influences

63. H. pylori factors that contribute to heterogeneity in clinical outcomes Presence/absence of cag pathogenicity island Allelic variation in vacA Type s2 vacA alleles encode a VacA protein that is less cytotoxic than type s1 alleles Strains containing the cag pathogenicity island and type s1 vacA alleles are associated with increased risk for peptic ulcer disease and gastric cancer.

64. Host factors that contribute to heterogeneity in clinical outcomes Variations in acid-secretory capacity of the stomach are relevant to clinical outcome Interleukin-1-beta is a strong inhibitor of gastric acid secretion Certain IL-1-beta genetic polymorphisms are associated with an increased risk for hypochlorhydria and gastric cancer.

65. Factors influencing development of clinical disease Bacterial factors cag pathogenicity island vacuolating toxin Host factors Gastric acidity Immune responses Environmental factors Gender Smoking

66. Comparisons between H. pylori and other bacterial pathogens Non-invasive versus invasive organisms Organisms that cause extensive tissue damage versus those that cause minimal damage Organisms that cause an acute transient infection versus those that establish chronic colonization

67. Key points H. pylori represents a model for understanding the process by which bacteria colonize humans and cause disease Striking features of H. pylori include its capacity to establish persistent infection and its role in the development of gastric carcinoma. Each bacterial pathogen has its own unique set of virulence factors. However, there are many recurrent themes in the pathogenic mechanisms used by different pathogens.