Immunoglobulin isotype composition of circulating and intra-articular immune complexes in patients with inflammatory joint disease.

The immunoglobulin (Ig) heavy chain isotype composition of intra-articular and circulating immune complexes (ICs) were determined by a Raji cell flow cytometric assay in paired serum and synovial fluid samples from 15 patients with rheumatoid arthritis (RA) and 15 patients with other articular diseases (osteoarthritis, ankylosing spondylitis, gout, psoriatic arthritis, Reiter's disease). ICs were most prevalent in synovial fluid samples of patients with RA but were infrequently detected in serum and synovial fluid samples from the non-RA patients. ICs in patients with RA were heterogeneous both in the prevalence of Ig subclasses identified and in the distribution of the respective Ig isotypes within the complexes. Furthermore, differences were observed in the Ig isotype composition of ICs in paired serum and synovial fluid samples indicating that circulating ICs may not always arise simply by spill-over from articular sites. The possible mechanisms for IC formation in RA are discussed with reference to four patients who displayed features of extra-articular disease.