MR1 antigen presentation to MAIT cells: new ligands, diverse pathways?

The major histocompatibility complex class I-related molecule MR1 is the only antigen (Ag) presenting molecule that captures and displays vitamin B-derived metabolites that are unique to a wide array of microbes. Presentation of these metabolite Ag at the cell surface activates mucosal-associated invariant T (MAIT) cells, a highly abundant innate-like T cell population, and represents a recently-described mechanism used by the mammalian immune system to sense pathogenic or commensal microbes. Our understanding of the cell biology of how MR1 accomplishes this unique task is still evolving, but recent advances are allowing a general picture to emerge. Further, the list of metabolites presented by MR1 is expanding both by identification of natural metabolites and the design of synthetic ligands. Here we review the latest studies contributing to our growing understanding of this rapidly expanding field.