MAIT cells are characterized by two conserved features: the expression of invariant TCR α/β chains and restriction by the MHC-like protein MR1. Unlike traditional MHC’s, MR1 does not present peptide antigens but in fact binds a family of bacterial vitamin metabolites.

Structural characterisation and mass spectrometry was used to confirm the identity of MR1-bound 6-formyl pterin (6FP) and riboflavin metabolites. Although 6-formyl pterin (6-FP), a folic acid metabolite (also known as vitamin B9), can bind MR1, it cannot stimulate MAIT cells. Instead, a distinct riboflavin metabolite induces MAIT cell activation. Structurally, this metabolite (6-hydroxymethyl-8-d-ribityllumazine) and others that stimulate MAIT cells are distinct from 6-FP by virtue of an extra ribityl moiety that might interact with the TCR.

MR1 has a small Ag-binding cleft that is lined with basic and hydrophobic residues and this providing an environment suited to small molecule binding. This also raises the question of molecules binding to the F’ pocket. All of this data suggests that wide range of ligands can bind MR1. The recent studies have provided important insights into the identity of MAIT exogenous ligands, characterization of endogenous ligand(s) remains unclear.

Here we present data on the identification of a variety of antigens by surface MR1 in a cellular model using mass spectrometry. .