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The problem
Background
Helicobacter pylori are helix-shaped, microaerophilic bacteria with polar flagella that live near the surface of the human gastric mucosa. Approximately half of the world`s population harbours H. pylori in their upper gastrointestinal tract, with 3rd world countries being most affected. There is evidence of co-evolution of human and H. pylori, tracing the migration of human race in the last tens of thousands of years (Covacci et al., 1999).
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Helicobacter pylori Global prevalence of H.pylori infection
Infection with H. pylori is usually acquired early in childhood and lasts for a lifetime, and the majority of those infected (80%–90%) remain asymptomatic. However, infection with H. pylori is known to be the strongest risk factor for the development of peptic ulcers. Furthermore, H. pylori was the first bacterium to be classified as a definite carcinogen by the WHO. Thus, from the medical point of view, H. pylori represents a challenging pathogen responsible for much mortality worldwide.
The generally accepted route of transmission for H. pylori is from person to person, e.g. by oral-oral transmission, since H. pylori can only survive for short periods of time because it is rapidly killed by higher oxygen tension and light. Therefore, infection with H.pylori is considered as a "kissing disease".
Infection with H. pylori can lead to several clinical outcomes. By far the most common gastric phenotype is the »simple gastritis« which is characterized by mild inflammation of the stomach lining and little disruption of gastric acid secretion. The second main phenotype is the duodenal ulcer phenotype (duodenum is the beggining portion of the small intestine) which accounts for up to 15 % in infected subjects. Subjects with this phenotype have defective inhibitory control of gastric acid secretion, which leads to very high acid output and consequently the development of peptic ulcers.
If untreated, peptic ulcers can commonly result in haemorrhage into the stomach or
the abdominal cavity which often leads to severe blood loss and even death.
Image from http://medicalimages.allrefer.com/large/ulcer-emergencies. jpg
The third and most serious phenotype is the »gastric cancer phenotype«, which is, in contrast to the previous two phenotypes, characterized with hypo- or achlorhydria (low acid secretion) and affects up to 5% of infected subjects. Gastric cancer is a formidable disorder, primarily because most patients are diagnozed at an advanced stage of disease. Despite the curative surgical resection many affected individuals often die of recurrent disease.
Interestingly, H. pylori is the only known organism capable of colonizing the severe environment of the human stomach, but even this bacterium can withstand low pH for only short periods of time. H. pylori has developed several mechanisms to avoid bactericidal activity of HCl. It uses its polar flagella for motility in chemotactic response to pH gradient, by which it stays in close proximity to the surface of the epithelium, where pH is near neutral. Besides that, H. pylori was shown to be capable of adhering to the surface and even invading the epithelial cells so as to protect itself from gastric contents and mechanical clearance to promote perseverance. H. pylori also posesses artificial means of increasing pH in its direct proximity by generating large quantities of cytosolic and cell surface-associated urease, an enzyme capable of the breakdown of urea into ammonia and carbon dioxide.
Alongside the capability of breaching the cellular and physical barriers, H. pylori has additionally developed ways of avoiding both the innate and the adaptive immune response of the host organism. To date, several bacterial strategies have been described (some of them are illustrated in table and figure below). For example, H. pylori flagellar proteins have developed in a way that masks them from the Toll-like receptor 5 (an innate immune receptor, that recognizes various other bacterial flagellins). LPS from H. pylori is 1000 times less pyrogenic and 500-fold less toxic than that of gram-negative enteric bacteria due to modifications that mimic human glycans and therefore induces less inflammation. H. pylori is also capable of obtaining cholesterol directly from epithelial cell membranes and incorporating it into its own lipid layer. To escape macrophage phagocytosis it modifies obtained cholesterol by adding a glucosyl group (a reaction catalysed by cholesterol-α glucosyltransferase). Additionally, H. pylori reduces generation of bactericidal nitric oxide by the macrophage. Its enzyme arginase competes with macrophages inducible nitric oxide synthase for substrate L-arginine that is used by the bacteria for the synthesis of urea (a substrate for urease). Moreover, exposure to H. pylori products upregulates transcription of arginase II in macrophages, which catalyses breakdown of L-arginine into urea and L-ornithine that is subsequently metabolized into polyamines that trigger apoptosis in macrophages.
Helicobacter pylori modifies its LPS and flagellin so that they do not activate Toll-like receptors 4 and 5 (opposite to LPS and flagellin of Escherichia coli), which results in no activation of the immune system cells.
Summary of strategies that H. pylori employs to avoid detection of the immune system
| MECHANISM |
EXAMPLE |
| Bypassing the TLR system |
- Flagellin fails to activate TLR5 (Gewirtz et al., 2004)- Tetraacylated LPS poorly activates TLR4 and may actually work as an antagonist (Moran and Aspinall, 1998)- DNA is rich in A/T nucleotides and frequently methylated, making TLR9 response more unprobable (Blaser and Atherton, 2004) |
| Minimization of innate immunity |
- Modification of LPS (long 3-hydroxy fatty acids of lipid A, unusual phosphorylation pattern…) (Moran et al., 1997, 1998) |
| Mimicry of host antigens |
- Lewis expression (O antigen region of H. pylori LPS) (Wirth et al., 1997) |
| Antigenic variation |
CagY (Aras et al., 2003) |
| Host gene expression modulation |
- Upregulation of specific inflammatory and immune mediators including β-defensin, protease inhibitor, chemokine receptor, interleukin-1β, tumor necrosis factor-α-inducible protein… (Israel and Peek, 2006) |
| Downregulation of immune effectors |
- Blocking the proliferation of T cell through VacA and B cell proliferation through CagA-induced supression- Interference with phagocytosis (Baldari et al., 2005) |
| Avoidance of attack by ROIs and RNIs |
- Enzymes involved in ROI scavenging, such as catalase and superoxide dismutase - Arginase regulates NO synthesis (Baldari et al., 2005) |
| Ability to colonize gastric environment |
- Neutralizing pH around the organism with urease enzyme- Interaction between adhesins and local cell receptors- Expressing mucolytic molecules- Relative absence of immune cells in gastric mucosa and rare competition with commensal bacteria |
| High mutational and recombinational frequency, high diversity |
- Rapid development in the bacterial population of high-level resistance to commonly used antibiotics- High competence for uptake of DNA from other H. pylori strains (Sansonetti and Di Santo, 2007) |
There is also experimental evidence linking bacterial adhesion and disease progression as an outcome of delivery of toxins, for instance VacA (pore-forming cytotoxin) that is characterized as a potent inhibitor of T-cell function and believed to be capable of inserting into the membranes of late endosomal vesicles to perturb antigen presentation. VacA can also disrupt the barrier function of tight junctions and thereby causes leakage of ions and small molecules from host cells, so that H. pylori can acquire nutrients across an intact epithelial barrier.
Current medical treatment of peptic ulcers and H. pylori infection consists mainly of three- and four-drug regimes, which combine two antibiotics with either proton pump inhibitors to reduce the production of gastric acid and/or cytoprotective agents that protect cells from toxic chemicals or other stimuli. Although these regimes are very effective at eradicating H. pylori with cure rates higher than 90%, there remains a serious problem of increasing antibiotic resistance. In addition, re-infection with H. pylori after antimicrobal therapy occurs frequently in countries where infection rates are high. Therefore, the only way to effectively prevent H. pylori (re)infection seems to be through long-term vaccination programs.
In the past 25 years since H. pylori has been first described and cultured, more than 20.000 publications have appeared on the subject. Understanding how this organism interacts with its host is of fundamental importance for preparation an intelligent strategy to prevent and cure H. pylori infections. To date, some promising approaches in vaccine development have been tested but still an effective vaccine against H. pylori remains to be demonstrated. Current knowledge of immunology, especially TLR activation and subsequent signaling, has enabled our team to design and test out some innovative ideas in vaccine development.
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