Effect of Intestinal Microbiota on the Immune System Preclinical Studies

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There is accumulating evidence that the interaction between the intestinal micro-biota and the gut plays an important role for the postnatal development of the immune system. However, the interactions between the intestinal epithelial and immune cells and the different species of the intestinal microbiota are very complex and not fully understood. The complexity of these interactions is based on the fact that on the one hand the human defense system consists of several layers, for example, of mechanical and chemical barriers (first line of defence) as well as innate and adaptive immunity (67) all of which can be influenced by microbiota (68).

Following the PASSCLAIM recommendation (69), studies in mice were recommended to substantiate conclusions related to immunological effects of dietary compounds. As for the prebiotic function, the most systematic studies were performed with a mixture of GOS/lcFOS. The experimental data have been intensively reviewed recently by Vos et al. (70).

In mice, it could be shown that GOS/lcFOS was bifidogenic in a dose-dependent manner, resulting in a reduction of the fecal pH and in a fecal SCFA pattern as found in human infants, thus, supporting the relevance of the animal data for the human situation (71).

In a mouse vaccination model adapted to study the effect of prebiotics, the animals were vaccinated twice with the Influvac (Orthomyxovirus influenza) vaccine (booster vaccination after 21 days). The response to the vaccination was measured at day 30 after the first vaccination. Parameters used to identify the response to vaccination were DTH response (skin response after local subcutaneous vaccine injection as an in vivo measure for Th1-mediated immunity), plasma titers of specific antibodies, ex vivo lymphocyte stimulation, T-cell proliferation, cytokine production, and natural killer cell activity. A specific prebiotic mixture (GOS/lcFOS) significantly stimulated the vaccination response in a dose-dependent manner and increased the DTH response indicating a modulation of the immune system toward a Th1-dominated immune response. This effect only occurred if the intervention with prebiotic nutrition started before the first vaccination. However, it was not observed if the prebiotics were fed after the first vaccination (71). This influence of treatment timing indicated that the prebiotic function was mainly mediated by the developing intestinal microbiota and probably not just due to direct interactions with the gut luminal and mesenterial immune cells. It might also indicate that the use of prebiotics for prevention was more relevant than any treatment approach.

In the same experiments, different classical fiber mixtures in a similar dose to the GOS/lcFOS mixture were tested. There was no effect of these fibers on the measured parameter of the immune system, indicating that different nondigestible carbohydrates react differently with respect to intestinal microbiota and immune function (71).

The same group studied the effects of a specific prebiotic mixture on the effect of allergic reaction in a mouse model using ovalbumin as antigen. The animals were sensitized by 10 g ovalbumin in alum and boosted 7 days later. The allergic reaction was measured before and after inhalation challenge (10 mg/ml ovalbumin; 20 min duration). The animals were challenged at 21, 24, and 27 days after first sensitization. Parameters used to identify the response to allergen exposure were airway responsiveness, bronchial lavage inflammatory cells, and antibody levels in plasma. Feeding the GOS/lcFOS mixture reduced significantly the allergic reaction against ovalbumin as demonstrated by reduction of bronchial constriction after metacholine application, reduction in inflammatory cells in the bronchial lavage fluid, and reduction in the IgE concentration in plasma (72,73).

In summary, the animal data allow the conclusion that prebiotics, like the mixture of GOS/lcFOS, modulate the immune system and provide a preventive effect with regard to the development of allergic diseases. This effect is mainly mediated by modulation of the intestinal microbiota.

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