New friendships and old feuds: relationships between innate lymphoid cells and microbial communities

Mammalian barrier surfaces, including the skin, lung and intestine, are constantly exposed to environmental stimuli, including beneficial and pathogenic microbes, dietary substances and non-organic materials. At these anatomical sites it is essential to maintain barrier integrity to promote tissue homeostasis and prevent local and systemic inflammation. As such, changes in the composition and localization of commensal bacteria are closely associated with inflammatory, metabolic and infectious disease in mammals. Cells of the innate and adaptive immune systems have a crucial role in the tight regulation of host-commensal relationships. A recently described family of immune cells, termed innate lymphoid cells (ILCs), contributes to inflammation, modulates adaptive immunity and regulates wound healing and tissue regeneration. ILCs are present at barrier surfaces, and thus are in close proximity to environmental antigens, including commensal bacteria. The composition and localization of microbial communities have a profound impact on immunity at barrier surfaces as well as at distant sites. This review will summarize the phenotypic characteristics of ILC family members and discuss recent findings about the interactions between ILCs and the microbiota in the contexts of homeostasis, immunity, inflammation and tissue organization and repair.