Expression of the major rheumatoid factor cross‐reactive idiotype in juvenile rheumatoid arthritis

The major rheumatoid factor cross‐reactive idiotype (RF‐CRI), defined by prototypic monoclonal IgM rheumatoid factors, is expressed in high frequency by pokeweed mitogen–derived plasma cells from patients with rheumatoid arthritis who express RF in their sera. Unlike adults with rheumatoid arthritis, most patients with juvenile rheumatoid arthritis are seronegative for RF, as detected by classic IgG binding assays. We report that ∼50% of seronegative patients with juvenile rheumatoid arthritis express the RF‐CRI in high frequency among their pokeweed mitogen–derived plasma cells, and that ∼33% of patients express the RF‐CRI in high titer in their sera. The possible mechanisms for expression of an idiotypic marker of RF without expression of IgG binding activity by classic assays are discussed.