MAIT cells, surveyors of a new class of antigen: development and functions.

Mucosal associated invariant T (MAIT) cells are evolutionarily conserved T cells that are restricted by the non-classical MHC-1b molecule, MR1. MAIT cells are selected on hematopoietic cells, and exit the thymus with a naïve phenotype before expanding in the periphery and attaining a memory phenotype. MAIT cells represent an abundant oligoclonal population in human blood and liver. MAIT cells react against a newly identified class of antigens: vitamin B metabolites, which are found in most bacteria and yeasts. MAIT cells secrete IFN-γ and IL-17 and their frequencies are modified in several diseases. The specificity, evolutionary conservation and unique features of MAIT cells indicate important functions, either against a ubiquitous pathogen or in gut immune/epithelial homeostasis.