Oligoclonal immunoglobulins and smooth muscle antibodies in arthritic joints

In twelve synovial fluid/serum pairs from patients with various types of seronegative polyarthritis, homogeneous gamma-bands by agarose gel electrophoresis were found in seven of the synovial fluids and in only one of the sera. In six of the fluids with gamma-bands, smooth muscle antibodies (SMA) were also present, usually in a titre identical to that in serum. In fluids with no gamma-bands, no SMA were detected. In forty synovial fluid/serum pairs from paitients with seropositive rheumatoid arthritis, no gamma-bands were detected in the synovial fluids, and SMA were present in only three pairs. Absorption and inhibition experiments did not give evidence that the SMA activity in seronegative polyarthritis was confined to the gamma-bands in the synovial fluids. The SMA activity in the fluids seemed to be directed against both actin and 'non-actin' muscular antigens. The association between locally produced oligoclonal immunoglobulins and possible locally produced SMA with differnet electrophoretic mobility suggests that in some of thes patients there is a local synovial production of oligoclonal antibodies with different specificities. Thus, even if the results may indicate a local virus infection in some arthritic joints, they may also be dur to an unspecific local stimulation of B cells or to a specific antigen stimulation combined with an unspecific co-activation of other antibody-producing cells.