Neutrophil recruitment in H. pylori induced inflammation: characterization of regulatory factors

Abstract: Helicobacter pylori, a bacterium colonizing the human gastric mucosa, is strongly associatedwith the development of gastric and duodenal ulcers, as well as gastric, adenocarcinoma.Virtually all infected individuals develop active chronic gastritis characterized by a continousinfiltration of neutrophils as well as mononuclear cells. This thesis focuses on the mechanismsresponsible for continous recruitment of neutrophils to the infected gastric mucosa.To evaluate the role of H. pylori lipopolysaccharides (LPS) in the recruitment of leukocytes tothe gastric mucosa, we examined the cytokine and chemokine production from humanmonocytes stimulated with LPS isolated from different H. pylori strains, as well as fromseveral other gram-negative bacteria. H. pylori LPS induced a large production of neutrophilrecruiting CXC-chemokines from human monocytes, but was a weak inducer of otherproinflammatory and immunoregulatory cytokines. There were no differences between LPSpreparations from different H. pylori strains in their ability to induce cytokines andchemokines.We also studied how human endothelial cells respond to stimulation with H. pylori strainsexpressing different putative virulence factors, with regard to production of neutrophilrecruiting chemokines as well as adhesion molecule expression. Some strains activated theendothelial cells to upregulate adhesion molecule expression, and to induce production ofCXC-chemokines. However, this effect was not dependent on whether the strain was isolatedfrom a duodenal ulcer patient or an asymptomatic carrier, and the expression of knownputative virulence factors could not completely explain the capacity of the strains to activatethe endothelial cells. Release of as yet unidentified factors also seems to influence the straincapacity.H. pylori can secrete neutrophil chemotactic components, but it is not known if H. pylori caninduce neutrophil transendothelial migration. We examined neutrophil migration throughhuman endothelial cells, using H. pylori or H. pylori components as stimuli. All tested H.pylori strains induced neutrophil transendothelial migration, and by use of H. pylori culturefiltrates and purified proteins we found that the single most important factor was the H. pylorineutrophil-activating protein (HP-NAP). Presumably, HP-NAP acts directly on neutrophils,which in turn influence endothelial cells to support their transmigration.In conclusion, our results show that H. pylori has the ability to directly recruit neutrophils tothe site of infection by secretion of neutrophil chemotactic factors. H. pylori also stimulatesendothelial cells to express adhesion molecules and to produce neutrophil recruitingchemokines. In addition, H. pylori LPS has the ability to induce monocytes to selectivelyproduce neutrophil recruiting chemokines. Therefore, several bacterial factors, acting ondifferent cell types in the host, probably contribute to the continous recruitment of neutrophilsto the H. pylori infected gastric mucosa, and thereby also to the tissue damage caused byneutrophil products.