IgG with a deviant conformation in serum and synovial fluid from rheumatoid arthritis patients

Specific rabbit antisera were prepared against an IgG with a special conformation (IgG spec.) previously detected in some sera from patients with rheumatoid arthritis. The antibodies had no affinity to normal human IgG and were not anti-idiotypic to human rheumatoid factor. The affinity of IgG spec. to the antibodies could not be explained by an antiglobulin activity to rabbit IgG. The amount of protein with affinity to immobilized specific IgG F(ab')2 of the antibodies was determined in serum and synovial fluid from patients with various joint diseases. A relationship between the content of IgG spec. and the diagnosis of seropositive rheumatoid arthritis was found on analysis of serum samples. IgG spec. also occurred in synovial fluid from some individuals with seropositive rheumatoid arthritis. Differences in the serum content of IgG spec. could not be explained by differences in the normal IgG content. Circular dichroism analysis of isolated IgG spec. showed that in the region(s) close to tyrosine residue(s) this polyclonal protein had similarities to heat-aggregated IgG.