Demonstration of an unidentified 48 kD polypeptide in circulating immune complexes in rheumatoid arthritis

Circulating immune complexes (CIC) were isolated from 25 patients with rheumatoid arthritis (RA) by anti-C1q affinity chromatography. The components were detected by silver stained sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) gels and identified by the Western blot. The CIC were composed of 20 different polypeptides, including albumin, immunoglobulin, complement, and acute phase reactants. Two components (molecular weight 48 kD and 45 kD respectively) remained unidentified. The 48 kD polypeptide was found in CIC from six out of 14 patients (43%) with extra-articular RA, but from none of eight patients with vasculitic complications of other connective tissue diseases. All immunoreactants were more frequently found in the patients with extra-articular features of RA. Although these results emphasise that most CIC in RA are composed of endogenous proteins, the 48 kD polypeptide is a candidate for an extrinsic antigen in RA.