IgM-rheumatoid factors cross-reactive with IgG and a cell nuclear antigen: apparent 'masking' in original serum.

Fourteen out of fifteen IgM-rheumatoid factor (IgM-RF) preparations isolated from high-titred seropositive rheumatoid sera by affinity chromatography on immobilized human IgG have been shown to cross-react with a cell nuclear antigen, as manifest by positive immunofluorescent anti-nuclear antibody (ANA) staining on rat liver and kidney tissue sections. The cross-reacting ANA activity within isolated IgM-RF preparations was inhibitable by both human and rabbit IgG, as well as by isolated rat liver chromatin, and could be isolated from seropositive ANA-negative sera. It would appear that this cross-reacting ANA activity can be masked in the original seropositive serum and may often require purification of RF from serum for its manifestation.