MR1 presents vitamin B6-related compounds for recognition by MR1-reactive T cells

The major histocompatibility complex class I related protein (MR1) presents microbially derived vitamin B2 precursors to mucosal- associated invariant T (MAIT) cells. MR1 can also present other metabolites to activate MR1- restricted T cells expressing more diverse T cell receptors (TCRs), some with anti- tumor reactivity. However, knowledge of the range of the antigen(s) that can activate diverse MR1- reactive T cells remains incomplete. Here, we identify pyridoxal (vitamin B6) as a naturally presented MR1 ligand using unbiased mass spectrometry analyses of MR1-bound metabolites. Pyridoxal, and the related compound, pyridoxal 5- phosphate bound to MR1 and enabled cell surface upregulation of wild type MR1*01 and MR1 expressing the Arg9His polymorphism associated with the MR1*04 allotype in a manner dependent on Lys43- mediated Schiff- base formation. Crystal structures of MR1*01 in complex with pyridoxal and pyridoxal 5- phosphate showed how these ligands were accommodated within the A- pocket of MR1. T cell lines transduced with the 7.G5 TCR, which has reported "pan- cancer" specificity, were specifically activated by pyridoxal presented by antigen- presenting cells expressing MR1*01 and MR1 allotypes bearing the less common Arg9His polymorphism. 7.G5 T cells also recognized, to a lesser extent, pyridoxal 5- phosphate and, importantly, recognition of both vitamers was blocked by an anti-MR1 antibody. 7.G5 TCR reactivity toward pyridoxal was enhanced when presented by the Arg9His polymorphism- bearing MR1 allotypes. Vitamin B6, and vitamers thereof, have been associated with various cancers, and here we describe a link between this ligand, MR1, and its allomorphs, and the pan- cancer 7.G5 TCR. This work identifies an MR1 ligand that can activate a diverse MR1- restricted TCR.