Patterns of nonclassical MHC antigen presentation

The identification of pattern-recognition receptors that selectively respond to evolutionarily conserved chemical (often pathogen-derived) moieties has provided key insight into how innate immune cells facilitate rapid and relatively specific antimicrobial immune activity. In contrast, relatively slower adaptive immune responses rely on T cell clonal expansion that develops in response to variable peptides bound to the groove of classical major histocompatibility complex (MHC) proteins. For certain nonclassical 'MHC-like' class Ib proteins, such as H2-M3 and CD1d, their respective binding grooves seem to have been adapted to present to T cells unique molecular patterns analogous to those involved in innate signaling. Here we propose that another MHC class Ib protein, MR1, which is required for the gut flora–dependent development of mucosa-associated invariant T cells, presents either a microbe-produced or a microbe-induced pattern.