Uncovering a Shared Epitope-Activated Protein Citrullination Pathway

Area of Science:

• Immunology
• Rheumatology
• Molecular Biology

Background:

• Rheumatoid arthritis (RA) is linked to shared epitope (SE)-coding HLA-DRB1 alleles and anticitrullinated protein antibodies (ACPA).
• The pathogenic roles of SE and ACPA in RA and their association mechanisms remain unclear.
• Previous research indicated SE acts as a signal transduction ligand activating a proarthritogenic, bone-erosive pathway mediated by cell surface calreticulin.

Purpose of the Study:

• To elucidate the molecular mechanisms linking SE, protein citrullination, ACPA production, and bone destruction in a mouse model of RA.
• To investigate the role of calreticulin in SE-mediated signaling and its impact on protein citrullination.

Main Methods:

• Stimulation of murine macrophages with lipopolysaccharide (LPS) or dithiothreitol (DTT).
• Assessment of cell surface calreticulin translocation.
• Measurement of SE-activated calcium signaling and peptidylarginine deiminase (PAD) activity.
• Analysis of citrullinated protein levels in cells.
• Administration of LPS to transgenic mice carrying a human SE-coding HLA-DRB1 allele.
• Measurement of serum TNF-α and ACPA levels, and assessment of bone destruction.

Main Results:

• LPS or DTT stimulation induced calreticulin translocation to the cell surface in macrophages.
• Cell surface calreticulin enhanced SE-activated calcium signaling and PAD activation, increasing citrullinated proteins.
• LPS administration in SE-HLA-DRB1 transgenic mice elevated serum TNF-α and ACPA levels.
• Significant bone destruction in the terminal phalanges was observed in these mice.

Conclusions:

• A novel signal transduction pathway initiated by the SE has been identified.
• This pathway involves calreticulin, leading to protein citrullination, ACPA generation, and subsequent bone destruction in RA.
• These findings provide critical insights into the pathogenesis of RA and the roles of SE and ACPA.