Citrullination modulates antigen processing and presentation by revealing cryptic epitopes in rheumatoid arthritis

Area of Science:

• Immunology
• Autoimmunity
• Molecular Biology

Background:

• Autoimmune diseases like rheumatoid arthritis (RA) involve immune responses to self-antigens.
• Citrullination, the conversion of arginine to citrulline, is a key feature in RA pathogenesis.
• Cryptic peptides, normally hidden from the immune system, are implicated in autoimmunity.

Purpose of the Study:

• To investigate if protein citrullination exposes cryptic peptides by altering protein structure and proteolytic cleavage.
• To determine the role of citrullination in the generation of autoantigen repertoires in RA.
• To elucidate the mechanism by which citrullination contributes to the loss of immune tolerance in RA.

Main Methods:

• Analysis of major histocompatibility complex (MHC) class II processing pathways.
• Proteolytic cleavage assays to study antigen processing.
• T cell stimulation assays using peptides from citrullinated and non-citrullinated autoantigens.
• Comparison of T cell responses in RA patients and healthy controls.

Main Results:

• Citrullination alters the processing and presentation of autoantigens.
• A unique citrullination-dependent peptide repertoire, primarily of native sequences, is generated.
• This repertoire elicits a more robust T cell response in RA patients with anti-citrullinated protein antibodies.
• Altered protease cleavage and protein destabilization, not direct citrulline epitope presentation, drive repertoire generation.

Conclusions:

• Protein citrullination in RA leads to the generation of a distinct self-peptide repertoire through modified antigen processing.
• This citrullination-induced peptide repertoire enhances T cell stimulation in RA patients.
• The findings suggest a novel mechanism for how protein citrullination breaches immune tolerance in rheumatoid arthritis.