Major histocompatibility complex class I-related protein 1 molecule (MR1) presents bacteria-derived vitamin B metabolites that are synthesised by wide range of microbes. Antigen presentation of MR1 is critical for the establishment, development and activation of highly abundant innate-like T cells, mucosal associated invariant T (MAIT) cells. Activated MAIT cells secrete inflammatory cytokines and acquire cytotoxic activity to clear the infection. Recent study also showed that MR1 presents tumour associated antigen (TAA) that expressed by wide range of tumour cells and promotes tumour killing. Hence, understanding of the regulatory machinery MR1 antigen presentation is important for the development of potential therapy for bacterial infection as well as cancer.
MR1 is maintained intracellularly as an endoplasmic reticulum (ER)-resident pool in the absence of infection, but encounter of metabolic ligands induces the trafficking of MR1 to the cell surface. MR1-ligand complexes stay on the cell surface for several hours after which they are internalised and mostly degraded. Elimination of surface MR1 is a requirement to terminate MAIT cell responses, so the cellular machinery that controls the internalisation of MR1 plays a critical role in the regulation of the MR1-MAIT cell axis.
With genome-wide CRISPR-Cas9 library screen, we have identified adaptor protein complex 2 alpha subunit (AP2A1) as the regulator of MR1 internalisation. Interaction of AP2A1 and MR1 was observed via proximity ligation assay. When AP2A1 is depleted in the cells, MR1 is internalised at much lower rate and presents antigen for prolong time. MR1 cytoplasmic tail consists of tyrosine-based motif which is highly conserved across mammalian species. However, this tyrosine-based motif is not a canonical sorting motif. In conclusion, MR1 internalisation is regulated by AP2 using a novel recognition motif. These results open the possibility of manipulating the internalisation of MR1 without affecting the trafficking properties of most AP2-regulated membrane proteins.