British Journal of Rheumatology 1993;32:862-869 ORIGINAL PAPERS A QUANTITATIVE ELISA FOR MEASUREMENT OF RHEUMATOID FACTOR ASSOCIATED CROSS-REACTIVE IDIOTYPES IN SERUM FROM PATIENTS WITH RHEUMATIC DISEASES

A sensitive ELISA for the quantitation of RF associated cross-reactive idiotypes (CRI) has been developed. CRI bearing IgM was captured by immobilized F(ab')2 fragments of monoclonal anti-CRI antibodies and revealed with isotype specific conjugates. Factors affecting sensitivity, specificity and reproducibility of the assay were assessed. The sensitivity limit was less than 10 ng/ml and the linear assay range for the CRI standard curve was from 80 to 500 ng/ml. Using this ELISA system we have quantitated serum levels of the CRI in different groups of patients with RA, early synovitis and normal individuals. Our results indicate a significant increase in the levels of the CRI in the patient groups compared to the normal subjects tested, and suggest involvement of a diverse repertoire of Ig variable region germline genes and/or somatic muta-tion in production of polyclonal RF in RA. KEY WORDS: ELISA, Rheumatoid factor, Rheumatoid arthritis, Cross-reactive idiotypes. IDIOTOPES (Id) are serologically defined antigenic determinants expressed within the variable region of immunoglobulin (Ig) molecules [1]. Private Id are unique to an antibody molecule, while cross-reactive Id (CRI) are expressed on different antibodies of the same or different specificities [2, 3]. Analysis of severa