Mucosal-associated invariant T (MAIT) cells: an evolutionarily conserved T cell subset

Besides mainstream TCRαβ T cells harboring a very diverse repertoire, two subsets display an evolutionarily conserved invariant repertoire. This striking conservation indicates important and unique functions. CD1d-restricted NK-T cells expressing an invariant Vα14 TCRα chain have been implicated in microbial and tumor responses as well as in auto-immunity. In this review, we describe the other subset, which bears the canonical hVα7.2/mVα19-Jα33 TCRα chain paired with a restricted set of Vβ segments. These invariant T cells are present in mice, humans and cattle. They are preferentially located in the gut lamina propria (LP) of humans and mice and are therefore called mucosal-associated invariant T (MAIT) cells. Selection/expansion of this population requires B lymphocytes expressing MR1, a monomorphic major histocompatibility complex class I-related molecule that is also strikingly conserved in diverse mammalian species. MAIT cells are not present in germ-free mice, indicating that commensal flora is required for their expansion in the gut LP. The nature of the ligand and the putative functions of these MAIT cells are discussed.