MAIT cells and viruses.

Mucosal associated invariant T cells (MAIT cells) bear a T cell receptor (TCR) that specifically targets microbially derived metabolites. Functionally, they respond to bacteria and yeasts, which possess the riboflavin pathway, essential for production of such metabolites and which are presented on MR1. Viruses cannot generate these ligands, so a priori, they should not be recognized by MAIT cells and indeed this is true when considering recognition through the TCR. However, MAIT cells are distinctive in another respect, since they respond quite sensitively to non-TCR signals, especially in the form of inflammatory cytokines. Thus, a number of groups have shown that virus infection can be "sensed" by MAIT cells and a functional response invoked. Since MAIT cells are abundant in humans, especially in tissues such as the liver, the question has arisen as to whether this TCR-independent MAIT cell triggering by viruses plays any role in vivo. In this review, we will discuss the evidence for this phenomenon and some common features which emerge across different recent studies in this area.