Cytosolic delivery of bacterial metabolites by riboflavin transporters promotes MR1 antigen presentation and MAIT cell recognition.

Major histocompatibility complex class I-related protein 1 (MR1) presents microbial Vitamin B-related metabolite antigens (VitBAg) at the cell surface to activate mucosal-associated invariant T (MAIT) cells. Precisely how antigen-presenting cells capture these MR1 ligands is not known. Here, we show that the most effective route for presentation of bacterial VitBAg involves passage through the cytosol. Consistent with structural similarities with riboflavin, we find that VitBAg presentation is inhibited by riboflavin. We further show that riboflavin carriers transport VitBAg into cells and enhance MR1 antigen presentation to MAIT cells. However, elimination of specific riboflavin carriers does not ablate VitBAg presentation, indicating cells possess redundant mechanisms to internalize this family of MR1 ligands. Our findings provide new insights into the intracellular pathway used by VitBAg to bind MR1 molecules and identify potential approaches to boost MR1-mediated MAIT cell responses for therapeutic benefits.