Microbiome and immunity, from homeostasis to pathology

Host-microbe coevolution has established an extremely robust symbiotic equilibrium, probably in response to the need to maintain a mutualistic relationship in which each "party" finds an indisputable benefit. This lesson showed that the mucosal immune system controlled the composition of the gut microbiota, and that the gut microbiota in turn controlled the immune system. The molecules that regulate this subtle balance are beginning to be understood. These include antimicrobial peptides such as cationic peptides, cathelicidin and Reg3gamma. These molecules, produced constitutively or inducibly, ensure the spatial segregation of the microbiota from the mucosa, particularly through their ability to impregnate the mucus layer covering the epithelium, creating a microbial exclusion zone. Other players have recently been identified, in particular innate lymphocytes or ILCs. The lesson concluded with a demonstration of the extraordinary contribution of the axenic (flora-free) mouse model to the analysis of the immunoregulatory functions of the microbiota.