Insights on the impact of diet-mediated microbiota alterations on immunity and diseases

The intestinal tract is inhabited by a large and diverse community of bacteria collectively referred to as the gut microbiota. The intestinal microbiota is composed by 500-1000 distinct species, and alterations in its composition are associated with a variety of diseases including obesity, diabetes, and inflammatory bowel disease (IBD). Importantly, microbiota transplantation from diseased patients or mice (IBD, metabolic syndrome, etc.) to germ-free mice was found to be sufficient to transfer some aspects of disease phenotypes, indicating that altered microbiota is playing a direct role in those particular conditions. Moreover, it is now well admitted that the intestinal microbiota is involved in shaping and maturating the immune system, with for example the observation that germ-free animals harbor a poorly developed intestinal immune system and that some single bacteria species, such as segmented filamentous bacteria (SFB), are sufficient to induce the expansion of Th17 cells (CD4⁺ T helper cells producing IL-17). We will present herein an overview of the interactions occurring between the intestinal microbiota and the immune system, and we will discuss how a dietary-induced disruption of the intestinal environment may influence transplantation outcomes.