Protecting intestinal epithelial integrity by galacto-oligosaccharides: Keeping it tight

Abstract

The intestinal barrier serves as a first line of host defense against potentially harmful stressors from the environment ingested with food, and is primarily formed by epithelial cells connected by tight junctions. Oligosaccharides have been identified as components in milk, particularly in colostrum, that support the development of intestinal microbiota in the early phase of life and contribute to the maturation of the immune system in infants. Currently, galacto-oligosaccharides (GOS) are used in infant formulas to improve and maintain intestinal health. The aim of the research project described in this thesis was to identify possible non-microbiota dependent effects of GOS. To this end, various in vitro and in vivo models were developed, in which the epithelial barrier function was experimentally disrupted by the mycotoxin deoxynivalenol (DON). DON is regularly occurring in human food commodities, particular in wheat and wheat based products, but also in breast milk, and is of increasing concern in human and animal health. Using an in vitro model with Caco-2 cell monolayers (a human colonic epithelial cell line) grown on inserts, it could be demonstrated that exposure to DON resulted in a loss of barrier integrity, a derangement of tight junction proteins and an increase in the paracellular transport of marker molecules. Comparative in vivo experiments in mice confirmed the derangement of tight junction proteins, revealed changes in the intestinal architecture and again demonstrated an increased paracellular transport of marker molecules indicating the loss of barrier integrity. With these models at hand, the protective effects of GOS (oligosaccharides based on the milk sugar lactose) were studied. Both, in vitro as well in vivo experiments indeed demonstrated a significant protective effect of GOS when applied prior to a DON challenge. Next to the protective effect on tight junction assembly and cellular localization, GOS also prevented the pro-inflammatory effects of DON, measured by a suppression of the synthesis and release of interleukine-8 (CXCL8) (and its murine homolog CXCL1) in both experimental models. Even more interesting was the protective effect of GOS on the release of IL-33, a more recently detected cytokine that together with its receptor ST2 plays a pivotal role in the induction of Th2-cell mediated immune responses in the course of various inflammatory reactions, allergies and auto-immune diseases. Additional experiments were devoted to the identification of the possible structure-activity relationship of various galacto- and fructo-oligosaccharides and revealed that in addition to concentration-dependent effects, particularly the degree of polymerization seems to be a key parameter determining the biological activity of individual oligosaccharides. In summary, the work described in this thesis demonstrates for the first time a number of non-microbiota dependent, direct effects of oligosaccharides on the intestinal immune system as well as on the tight junction network that guarantees the intestinal barrier integrity.