The interplay between citrullination and HLA-DRB1 polymorphism in shaping peptide binding hierarchies in rheumatoid...

Area of Science:

• Immunogenetics
• Molecular Immunology
• Rheumatology

Background:

• The HLA-DRB1 locus is a major genetic factor in rheumatoid arthritis (RA) susceptibility.
• Citrullinated self-peptides bind to HLA-DR molecules with the shared epitope (SE) motif, but binding preferences vary.
• The precise molecular mechanisms dictating differential binding affinities remain incompletely understood.

Purpose of the Study:

• To investigate the binding affinities of citrullinated and non-citrullinated self-peptides to different HLA-DRB1 allomorphs.
• To elucidate the structural basis for preferential binding of citrullinated peptides to SE-positive HLA-DRB1 variants.

Main Methods:

• Fluorescence polarization assay to quantify peptide-HLA-DRB1 binding affinities.
• Analysis of 34 self-peptides implicated in RA against three HLA-DRB1*04 allomorphs (HLA-DRB1*04:01, *04:04, *04:05).
• Structural determination of eight HLA-DR4-self-epitope complexes.

Main Results:

• A strong correlation between peptide binding affinity and citrullination at the P4 position was observed for all tested HLA-DRB1 allomorphs.
• Peptide-binding hierarchies differed across HLA-DRB1 allomorphs due to polymorphisms outside the SE motif.
• Structural analysis revealed conformational convergence at P4-Cit and identified polymorphic residues in P1 and P9 pockets influencing binding.

Conclusions:

• Citrullination of self-peptides significantly enhances binding to SE-positive HLA-DRB1 allomorphs.
• Polymorphisms in HLA-DRB1 outside the SE motif contribute to differential peptide binding specificities.
• This study provides a molecular framework for understanding the interaction between self-antigen citrullination and HLA polymorphisms in RA pathogenesis.