Circulating immune complexes contain citrullinated fibrinogen in rheumatoid arthritis

Xiaoyan Zhao¹^,2 , Nwora Lance Okeke¹^,2 , Orr Sharpe¹^,2 , Franak M Batliwalla³ , Annette T Lee³ , Peggy P Ho⁴ , Beren H Tomooka¹^,2 , Peter K Gregersen³ and William H Robinson¹^,2

¹GRECC, VA Palo Alto Health Care System, 3801 Miranda Avenue, Palo Alto, CA 94304, USA

²Department of Medicine, Division of Immunology and Rheumatology, Stanford University School of Medicine, 269 Campus Drive West, Stanford, CA 94305, USA

³Robert S. Boas Center for Genomics and Human Genetics, Feinstein Institute for Medical Research, 350 Community Drive, Manhasset, NY 11030, USA

⁴Department of Neurology and Neurological Sciences, Stanford University School of Medicine, 269 Campus Drive West, Stanford, CA 94305, USA

author email corresponding author email

Arthritis Research & Therapy 2008, 10:R94doi:10.1186/ar2478


Published:	18 August 2008

See related editorial by van Venrooij and Pruijn, http://arthritis-research.com/content/10/5/117

Abstract

Introduction

There is increasing evidence that autoantibodies and immune complexes (ICs) contribute to synovitis in rheumatoid arthritis (RA), yet the autoantigens incorporated in ICs in RA remain incompletely characterised.

Methods

We used the C1q protein to capture ICs from plasma derived from human RA and control patients. Antibodies specific for immunoglobulin were used to detect ICs, and fibrinogen antibodies were used to detect fibrinogen-containing ICs. RA and control plasma were separated by liquid chromatography, and fractions then characterised by ELISA, immunoblotting and mass spectrometry. Immunohistochemical staining was performed on rheumatoid synovial tissue.

Results

C1q-immunoassays demonstrated increased levels of IgG (p = 0.01) and IgM (p = 0.0002) ICs in plasma derived from RA patients possessing anti-cyclic citrullinated peptide (CCP+) autoantibodies as compared with healthy controls. About one-half of the anti-CCP+ RA possessed circulating ICs containing fibrinogen (p = 0.0004). Fractionation of whole RA plasma revealed citrullinated fibrinogen in the high molecular weight fractions that contained ICs. Positive correlations were observed between fibrinogen-containing ICs and anti-citrullinated fibrinogen autoantibodies, anti-CCP antibody, rheumatoid factor and certain clinical characteristics. Immunohistochemical staining demonstrated co-localisation of fibrinogen, immunoglobulin and complement component C3 in RA pannus tissue. Mass spectrometry analysis of immune complexes immunoprecipitated from RA pannus tissue lysates demonstrated the presence of citrullinated fibrinogen.

Conclusion

Circulating ICs containing citrullinated fibrinogen are present in one-half of anti-CCP+ RA patients, and these ICs co-localise with C3 in the rheumatoid synovium suggesting that they contribute to synovitis in a subset of RA patients.