The antigen presenting molecule MR1 binds riboflavin catabolites

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• Major histocompatibility-complex (MHC) class I-related (MR1) protein presents vitamin B-derived antigens to Mucosal-Associated Invariant T (MAIT) cells.
• While microbial riboflavin precursors are known MR1 ligands, the role of host-generated riboflavin catabolites in MR1-mediated immunity is unclear.

Purpose of the Study:

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

Main Methods:

• Ligand binding assays to assess affinity of riboflavin catabolites for MR1.
• Cell surface expression analysis of MR1.
• MAIT cell activation assays.
• X-ray crystallography to determine the structure of MR1-ligand complexes.

Main Results:

• Riboflavin catabolites (FMF, lumichrome, lumiflavin, alloxazine) bind MR1 with moderate affinity, while riboflavin binds weakly.
• These catabolites reduce MR1 cell surface expression by retaining MR1 in the endoplasmic reticulum (ER).
• Crystal structures reveal binding in the A eal-pocket, with lumichrome forming a covalent bond with MR1-Lys43.
• Catabolites weakly compete with vitamin B antigens, inhibiting MAIT cell activation.

Conclusions:

• Host-generated three-ring isoalloxazines can bind MR1 and downregulate its cell surface expression.
• This interaction potentially dampens MAIT cell immunity.
• Identified a novel mechanism of immune regulation involving MR1 and endogenous metabolites.