Structure-function relationships of a ribityl uracil T-cell antigen

Abstract

A newly characterised T-lymphocyte is the mucosal associated invariant T (MAIT) cell. In contrast to other classes of T-cells that recognise peptide and lipid-based antigens, MAIT cells are the only class so far known to be activated by heteroaromatic molecules. The most potent activator is a ribityl uracil (5-OP-RU) side-product of the biosynthesis of riboflavin. MAIT cells are abundant in blood and mucosal tissues but their roles in human health and disease are still being elucidated. Ternary crystal structures show 5-OP-RU sandwiched between the MHC-like protein (MR1) and the MAIT T-cell receptor (TCR), with specific hydrogen bonds between the ribityl hydroxyls of 5-OP-RU and both TCR and MR1 proteins. A series of deoxygenated and monohydroxylated analogues of 5-OP-RU were designed and synthesized to dissect the specific contributions from each hydroxyl to MAIT cell activation. Synthetic routes to each of the C-3, C-4 and C-5 positions of d-Ribose were investigated and the Barton-McCombie deoxygenation was used as a key step in the synthesis of the C-3 and C-4 deoxyribitylamines. The uracil core of the analogues and the iminocarbonyl motif were installed with established methodology. MAIT cells activation assays of the compounds showed that the C-2’ and C-3’ hydroxyl groups played a key role in MAIT cell activation. Loss of either of these hydroxyls decreased the potency and the maximum MAIT cell activation level of the compounds particularly in the case of 2-deoxy-5-OP-RU. Conversely the hydroxy ethyl and hydroxy propyl derivatives were the only monohydroxylated derivatives which showed activity. Additionally, a hydrophobic pocket which could be accessed by derivatising the C-5’ position was found. These findings highlight key areas of sensitivity for the ligand-MR1-MAIT TCR axis which can be used in the development of new chemical probes for this system.