The damaged mucosal barrier in intestinal inflammation: interaction with therapeutic bacteria and roles for metallothioneins as immunomodulators

Inflammatory bowel diseases (IBD) are a group of chronic, relapsing, immunologically-mediated disorders of the intestine, the main forms being Crohn’s disease and ulcerative colitis. Recent studies underscore the importance of the damaged epithelial barrier and the dysregulated innate immune system in the pathogenesis of IBD. These two components cause defects in the interaction of the host with the bacterial flora. The epithelium must maintain a physical barrier between the lumen and the underlying mucosa. Defects in the mucus layer, integrity, and permeability are found in IBD and are thought to have direct implications on disease initiation and/or progress. Furthermore, it has become clear that the intestinal epithelial cell lining exerts an important role in the identification of bacteria and the appropriate counteraction. The involvement of the innate immune system can be explained by two theories. The first one states that an exaggerated innate response and a loss of tolerance for the commensal intestinal microflora are caused by defective down-regulation of pattern recognition receptor signalling. The second theory encompasses a defective innate immunity originating from failure of recognition of bacterial threats or an ineffective response, which results in persistence of the exogenous stimulus and an exaggerated secondary immune response. The first part of this thesis focuses on the interaction of therapeutic bacteria with the damaged epithelial barrier. Genetically modified Lactococcus lactis bacteria were developed in order to provide a local mucosal delivery of the anti-inflammatory cytokine interleukin (IL)-10. Since local delivery of the cytokine in the tissue seems primordial, we focused on the in vivo interaction of the bacteria with intestinal murine mucosa and observed differences between an intact and an inflamed (damaged) epithelial barrier. In healthy intestinal tissue, we found an occasional transcellular uptake in follicle-associated epithelium. Our data were suggestive of dendritic cell sampling, but not of sampling by M cells. In inflamed mucosal tissue, both confocal and electron microscopic analysis suggested an enhanced uptake of lactococci through the paracellular route, which is likely to be a result of the damaged mucosal barrier. We were able to show the presence of viable and IL-10-producing lactococci in ileal and colonic lamina propria. The absence of lactococci in mesenteric lymph nodes or in the spleen excluded a systemic circulation of the bacteria, being safe for administration in patients. In the second part, the role of metallothioneins (MTs) as immunomodulators in intestinal inflammation is investigated. We found a significantly lower mRNA and protein expression in the epithelium of unaffected tissue of Crohn’s disease patients. In vitro studies with an MT-knockdown intestinal epithelial cell line showed a defective secretion of the neutrophil chemokine IL-8 upon bacterial challenge. A time course study of dextran sodium sulphate-induced colitis revealed that MTnull mice exhibited a significantly more severe histological inflammation in the early phase of colitis compared with wild type mice, a difference which was due to epithelial MTs. Both these in vitro and in vivo studies point to an immunomodulatory role for epithelial MTs in the acute intestinal immune response. In ulcerative colitis, we did not find this impaired basal epithelial MT expression. Here, active inflammation caused a down-regulation of epithelial MT and an up-regulation of non-epithelial MT which was also observed in other forms of intestinal inflammation. The inflammation-dependent down-regulation in the epithelium is in contrast with the known induction of MTs by proinflammatory cytokines and reactive oxygen intermediates, which are produced during inflammation. We found that transforming growth factor (TGF)-β, one of the few inhibitors identified for MT mRNA production, could decrease epithelial MT expression in ex vivo experiments. Contrary to the decrease in the epithelium, an increase in MT-expressing cells occurs in the lamina propria during active inflammation, predominantly caused by strongly MT-expressing fibroblasts of the granulation tissue. Since we did not find a correlation between MT expression and proliferation (a correlation which has been reported in vitro in the literature), we suggest that the role of MTs in these cells is a cytoprotective one, which would be beneficial for host cells in the inflammatory environment. Finally, we reviewed the MT regulatory mechanisms in health and in disease.

Grantor dc:publisher

Ghent University. Faculty of Medicine and Health Sciences

Year dc:date

2009

Author dc:creator

• Waeytens, Anouk

Contributors dc:contributor

• Cuvelier, Claude
• De Vos, Martine

• Medicine and Health Sciences
• lactococcus lactis
• inflammatory bowel diseases
• metallothionein
• intestinal mucosa

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://biblio.ugent.be/publication/686780
https://biblio.ugent.be/publication/686780/file/4334926

OAI identifier oai:identifier

oai:archive.ugent.be:686780