Poster Presentation CD1-MR1 2024

MR1 senses the natural riboflavin degradation metabolites (#109)

Mohamed R. Abdelaal 1 , Patricia T. Illing 1 , Emi Ito 2 , Jeffrey Y W Mak 3 4 , David P Fairlie 3 4 , Sho Yamasaki 2 5 , Anthony W. Purcell 1 , Jamie Rossjohn 1 6 , Wael Awad 1
  1. Infection and Immunity Program and Department of Biochemistry and Molecular Biology, Biomedical Discovery Institute, Monash University, Melbourne, Victoria, Australia
  2. Research Institute for Microbial Diseases, Osaka University, Suita 565-0871, Japan
  3. Institute for Molecular Bioscience, The University of Queensland, Brisbane, Queensland, Australia
  4. ARC Centre of Excellence in Advanced Molecular Imaging, University of Queensland, Brisbane, Queensland, Australia
  5. Immunology Frontier Research Center (IFReC), Osaka University, Suita 565-0871, Japan
  6. Institute of Infection and Immunity, Cardiff University School of Medicine, Heath Park, Cardiff, United Kingdom

Major-histocompatibility-complex (MHC) class 1-related (MR1) protein presents small-molecule metabolites to Mucosal-Associated Invariant T (MAIT) cells. The egress of MR1 to the cell surface is ligand-dependent; albeit the ability of small-molecule ligands to impact on MR1 cellular trafficking is the least understood. Riboflavin precursors have been extensively studied as ligands for MR1, including 5-OP-RU, modulating the activity of MAIT cells. Here, we report that the natural riboflavin degradation products are sensed by MR1. In contrast to the MAIT-activating riboflavin precursors like 5-OP-RU, we found that the riboflavin degradation metabolites reduce MR1 level on the surface of antigen-presenting cells and compete with 5-OP-RU for MAIT cell activation. Biochemical assays revealed that these compounds are moderate binders for MR1, albeit the cellular assays showed that they interfere with the MR1 presentation reducing its level on the cell surface. Lower MR1 exposure on the cell surface interrupted the MR1-MAIT signalling leading to reduced 5-OP-RU-dependent MAIT cell activation. We solved the crystal structures for riboflavin metabolites bound to MR1 and characterised the network of interactions stabilising the ligands in the MR1 ligand-binding cleft. Overall, we characterised the first natural metabolites of riboflavin that could reduce cell surface MR1 and subsequently suppress MAIT cell activity implicating a potential role in regulating the MAIT cell immune homeostasis.