MR1 is the only antigen presenting molecule that displays a pathogen metabolic signature, the Vitamin B-related antigens (VitBAg). This allows the MR1-restricted MAIT cells to detect infection even if the antigen presenting cell has not physically encountered the pathogen. Given the distinct nature of MR1 ligands and their role, the MR1 presentation pathway is expected to possess unique features absent in other pathways of antigen presentation. Indeed, we recently showed that MR1 is the only human antigen presenting molecule that does not constitutively present self-ligands. Instead the majority of MR1 accumulates in the endoplasmic reticulum (ER) as a pool of empty molecules until a VitBAg forms a covalent bond (Schiff base) with a Lys residue in the antigen binding site of MR1. This acts as a “molecular switch” that triggers complete folding and egress of MR1 molecules from the ER to the cell surface (1). We hypothesised that MR1 requires molecular machinery along this pathway, therefore we aimed to characterise these accessory proteins that enable MR1 presentation of VBAg. Using genome-wide CRISPR-Cas9 screens and proteomics approaches we identified chaperones required for MR1 stabilisation, folding, and trafficking to and from the cell surface. We found that tapasin and tapasin-related protein both bound to ER-resident MR1, and the deletion of these resulted in a loss of the majority of the ligand-receptive MR1 pool due to faster degradation of MR1. While nascent molecules could still load and present VitBAg without these chaperones, the loss of the MR1 pool resulted in a significantly reduced presentation of VitBAg. Therefore, these chaperones play a key role in stabilising empty MR1 in the ER for the rapid presentation of bacterial metabolites, and illustrates a role beyond facilitating MHC class I peptide loading.

(1) McWilliam et al, Nat Immunol 2016