Cross‐reactive antiidiotypic antibodies against human rheumatoid factors from patients with juvenile rheumatoid arthritis

We prepared antiidiotypic (anti‐Id) antibody to 2 polyclonal IgM rheumatoid factors (IgM‐RF) and 2 polyclonal “hidden” IgM‐RF. The anti‐Id antibodies were isolated by chromatography on Sepharose 4B, to which was bound rabbit anti‐human IgG Fc fragments. F(ab′)₂ fragments from the anti‐Id antibodies were generated by pepsin digestion and isolated by gel filtration. The anti‐Id antibodies directed against RF from 4 patients with juvenile rheumatoid arthritis (JRA) were tested by an inhibition hemolytic assay for crossreactivity with IgM‐RF from 4 adult patients with rheumatoid arthritis, 6 patients with JRA, and 13 JRA patients with hidden RF. The 4 anti‐Id antibodies had variable cross‐reactivity with the isolated adult RA RF, JRA RF, and JRA hidden RF. Similar results were obtained by a direct‐binding enzyme‐linked immunosorbent assay for the anti‐Id antibodies. The broad pattern of cross‐reactivity was apparently unrelated to a particular amino acid sequence, but was associated with the antigen‐binding site of IgM‐RF. These results suggest the possibility that the anti‐Id antibodies prepared against isolated RF obtained from JRA patients bear the “internal image” of antigen; that is, the Fc region of human IgG. These anti‐Id antibodies may be generated in JRA patients and may possess specific immunomodulatory properties.