Regulation of innate and adaptive immunity by the commensal microbiota

The microbial communities that inhabit the intestinal tract are essential for mammalian health. Communication between the microbiota and the host establishes and maintains immune homeostasis, enabling protective immune responses against pathogens while preventing adverse inflammatory responses to harmless commensal microbes. Specific bacteria, such as segmented filamentous bacteria, Clostridium species, and Bacteroides fragilis, are key contributors to immune homeostasis in the gut. The cellular and molecular interactions between intestinal microbes and the immune system are rapidly being elucidated. Here, we review advances in our understanding of the microbial populations that shape the mucosal immune system and create a protective defense that prevents infection while tolerating friendly commensals.

► The microbiota contributes to the maintenance of immune homeostasis in the intestine.
► The host must remain tolerant of commensals and able to mount protective immune responses against pathogenic microbes.
► Specific bacteria promote lymphocyte differentiation and help establish innate immune defense.
► Some members of the intestinal microbiota can provoke aberrant immune responses that extend beyond the gut.