Synovial fluid is a site of citrullination of autoantigens in inflammatory arthritis.

Abstract

OBJECTIVE:

To examine synovial fluid as a site for generating citrullinated antigens, including the candidate autoantigen citrullinated alpha-enolase, in rheumatoid arthritis (RA).

METHODS:

Synovial fluid was obtained from 20 patients with RA, 20 patients with spondylarthritides (SpA), and 20 patients with osteoarthritis (OA). Samples were resolved using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by staining with Coomassie blue and immunoblotting for citrullinated proteins, alpha-enolase, and the deiminating enzymes peptidylarginine deiminase type 2 (PAD-2) and PAD-4. Proteins from an RA synovial fluid sample were separated by 2-dimensional electrophoresis, and each protein was identified by immunoblotting and mass spectrometry. Antibodies to citrullinated alpha-enolase peptide 1 (CEP-1) and cyclic citrullinated peptide 2 were measured by enzyme-linked immunosorbent assay.

RESULTS:

Citrullinated polypeptides were detected in the synovial fluid from patients with RA and patients with SpA, but not in OA samples. Alpha-enolase was detected in all of the samples, with mean levels of 6.4 ng/microl in RA samples, 4.3 ng/microl in SpA samples, and <0.9 ng/microl in OA samples. Two-dimensional electrophoresis provided evidence that the alpha-enolase was citrullinated in RA synovial fluid. The citrullinating enzyme PAD-4 was detected in samples from all 3 disease groups. PAD-2 was detected in 18 of the RA samples, in 16 of the SpA samples, and in none of the OA samples. Antibodies to CEP-1 were found in 12 of the RA samples (60%), in none of the SpA samples, and in 1 OA sample.

CONCLUSION:

These results highlight the importance of synovial fluid for the expression of citrullinated autoantigens in inflammatory arthritis. Whereas the expression of citrullinated proteins is a product of inflammation, the antibody response remains specific for RA.

• Authors: Andrew Kinloch, Karin Lundberg, Robin Wait, Natalia Wegner, Ngee Han Lim, Albert J W Zendman, Tore Saxne, Vivianne Malmström, Patrick J Venables
• Journal: Arthritis and rheumatism (Arthritis Rheum) Vol. 58 Issue 8 Pg. 2287-95 (Aug 2008) ISSN: 0004-3591 [Print] United States
• PMID: 18668562 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Autoantigens
• Biomarkers, Tumor
• DNA-Binding Proteins
• Peptides, Cyclic
• Tumor Suppressor Proteins
• cyclic citrullinated peptide
• Citrulline
• Hydrolases
• PADI4 protein, human
• Protein-Arginine Deiminase Type 4
• Protein-Arginine Deiminases
• ENO1 protein, human
• Phosphopyruvate Hydratase

Topics

• Arthritis, Rheumatoid (immunology, metabolism)
• Autoantigens (metabolism)
• Biomarkers, Tumor (metabolism)
• Case-Control Studies
• Citrulline (metabolism)
• DNA-Binding Proteins (metabolism)
• Humans
• Hydrolases (metabolism)
• Osteoarthritis (immunology, metabolism)
• Peptides, Cyclic (metabolism)
• Phosphopyruvate Hydratase (metabolism)
• Protein-Arginine Deiminase Type 4
• Protein-Arginine Deiminases
• Spondylarthritis (immunology, metabolism)
• Synovial Fluid (metabolism)
• Tumor Suppressor Proteins (metabolism)