Circulating immune complexes contain citrullinated fibrinogen in rheumatoid arthritis.
BACKGROUND
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.


CONCLUSIONS
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.
Zhao
Xiaoyan
X
GRECC, VA Palo Alto Health Care System, Palo Alto, CA 94304, USA.
Okeke
Nwora Lance
NL
Sharpe
Orr
O
Batliwalla
Franak M
FM
Lee
Annette T
AT
Ho
Peggy P
PP
Tomooka
Beren H
BH
Gregersen
Peter K
PK
Robinson
William H
WH
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000936:Antigen-Antibody Complex / Q000097:blood*; D001171:Arthritis, Juvenile / Q000378:metabolism; D001172:Arthritis, Rheumatoid / Q000378:metabolism*; D015551:Autoimmunity; D016022:Case-Control Studies; D002648:Child; D002956:Citrulline / Q000378:metabolism*; D015922:Complement C1q; D003176:Complement C3 / Q000378:metabolism; D005260:Female; D005340:Fibrinogen / Q000378:metabolism*; D006801:Humans; D007118:Immunoassay; D008297:Male; D008875:Middle Aged; D013583:Synovial Membrane / Q000378:metabolism*; D055815:Young Adult
D000936:Antigen-Antibody Complex; D003176:Complement C3; D002956:Citrulline; D015922:Complement C1q; D005340:Fibrinogen