The effect of antiidiotypic antibody on the in vitro production of rheumatoid factor was studied in rheumatoid arthritis patients with cross‐reactive idiotypic determinants. Antiidiotypic antibodies (ascites IgG) were developed against monoclonal rheumatoid factor (Ka m‐RF) by a cell fusion procedure. These antibodies were idiotype‐specific, since: 1) they reacted only with F(ab')2 fragment of Ka m‐RF; 2) they failed to react with normal IgM without rheumatoid factor activity; and 3) their F(ab')2 fragment inhibited the rheumatoid factor activity of Ka m‐RF. The antiidiotypic antibody strongly suppressed the in vitro production of rheumatoid factor by lymphocytes from unrelated rheumatoid arthritis patients with cross‐reactive idiotypes. The suppression was specific, since ascites IgG failed to suppress in vitro anti‐keyhole limpet hemocyanin antibody production by the lymphocytes. These results indicate that antiidiotypic antibody may influence the regulation of rheumatoid factor production in patients with rheumatoid arthritis.
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