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The antigen-presenting molecule MR1 binds host-generated riboflavin catabolites.

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Area of Science:

  • Immunology
  • Structural Biology

Background:

  • Mucosal-associated invariant T (MAIT) cells are crucial immune sentinels.
  • MHC class I-related protein (MR1) presents vitamin B-based antigens to MAIT cells.
  • Microbial riboflavin precursors are known MR1 ligands, but host-derived catabolites' roles are unclear.

Purpose of the Study:

  • To investigate the binding of host-generated riboflavin catabolites to MR1.
  • To determine the impact of these catabolites on MR1 surface levels and MAIT cell activation.
  • To elucidate the structural basis of MR1-catabolite interactions.

Main Methods:

  • Surface plasmon resonance (SPR) for binding affinity.
  • Flow cytometry for MR1 surface levels.
  • Crystallography for structural analysis of MR1-ligand complexes.
  • MAIT cell activation assays.

Main Results:

  • Riboflavin catabolites (FMF, lumichrome, lumiflavin, alloxazine) bind MR1 moderately; riboflavin binds weakly.
  • Catabolites reduce MR1 surface expression by ER retention, unlike RF precursors.
  • Catabolites weakly compete with antigens, selectively inhibiting MAIT activation.
  • Crystal structures reveal binding in the MR1 A eal-pocket; lumichrome forms a unique covalent bond with MR1-Lys43.

Conclusions:

  • Host-derived three-ringed isoalloxazines bind MR1 and reduce its surface levels.
  • These catabolites can dampen MAIT cell immunity by inhibiting MR1 presentation.
  • Structural insights reveal novel MR1-ligand interactions, including a unique covalent bond.