Persistent Helicobacter pylori infection and host response

Abstract

Helicobacter pylori persistently colonizes the gastric mucosa of approximately one half of the world's population. Colonization always leads to chronic gastric inflammation, which may progress to peptic ulcer disease, gastric cancer or MALT lymphoma. The nature of H. pylori infection is determined by bacterial, host and environmental factors. Variability in strain virulence may be explained by the high genetic diversity of the species. The work presented in this thesis focuses on diverse bacterial factors and mechanisms that modulate persistent H. pylori infection.

In paper I, we characterized the effects of clinical H. pylori isolates with diverse inflammatory background on human gastric adenocarcinoma cells (AGS) and studied their capacity to cause maturation and activation of monocyte-derived dendritic cells (DCs). The inflammatory background of the bacteria was defined by the infiltration of lymphocytes and granulocytes at the site of infection. We found that the expression of the cytokines interleukin (IL)-6 IL-12, TNF-alpha and IL-1beta was higher when DCs were infected with H. pylori of a higher inflammatory background. Such a correlation could not be described when analyzing IL-8 induction in AGS cells. We suggest, that DCs play a major role in H. pylori infection and that the bacterium can influence the outcome of the interaction.

In papers II and III, we investigated the prevalence of H. pylori virulence factors, the combination of virulence factors and their association with disease development in three major ethnicities in Malaysia and Singapore. The genes encoding an intact cag PAI, babA, oipA ON and vacA s1 and i1 were present in more than 85% of the isolates, irrespective of the disease state or ethnicity. This demonstrates that these factors are not reliable predictors of disease in these populations. In contrast, the prevalence of dupA, hp0521 alleles and EPIYA motifs varied with ethnicity. We observe an association between EPIYA motifs and hp0521 alleles and suggest a novel function for HP0521, probably correlated to CagA. The prevalence of dupA was further investigated in Swedish and Australian isolates. It varied between the geographic groups and could not be consistently associated with duodenal ulcer or gastric cancer across the investigated ethnic groups. The dupA gene was highly conserved in H. pylori strains with different geographic and ethnic background. A role in IL-8 induction in AGS cells could not be confirmed.

In paper IV, we examined the genetic diversity of type I restriction-modification (R-M) systems in clinical isolates of H. pylori. Due to the high sequence variability observed, we focused on the HsdS subunit, which is responsible for sequence specificity of the enzyme. A high number of novel allelic hsdS variants could be described. To investigate intra-strain diversity, we analyzed isolates obtained from six individuals at two different time points, four years apart. Most sequence variation could be observed for two isolates from patients with ongoing atrophy development. We therefore propose that H. pylori alters the specificity of type I R-M systems in response to a changing gastric environment, which may lead to adaptation.

H. pylori is a genetically diverse species. The work presented in this thesis provides an insight into the effects of this high diversity on immune response to the bacterium, on variation in H. pylori virulence as well as on a possible bacterial adaptation process.