Characterization of monoclonal IgG antibodies produced by hybridomas derived from rheumatoid synovial cells

IgM rheumatoid factors (RF) are the predominant autoantibody found in rheumatoid arthritis. They are polyclonal, fix complement, and are directed against epitopes in the Fc portion of IgG. One hypothesis regarding the induction and persistence of RF production in rheumatoid arthritis is that the Fc of IgG is somehow altered, rendering it antigenic. In this study, to better understand the derivation and pathogenicity of RF in rheumatoid arthritis, monoclonal IgG (mIgG) constitutively secreting hybridomas were established by fusing rheumatoid synovial mononuclear cells (RSC) from patients with a mouse/human heteromyeloma cell line, F3B6. To clarify the primary structure of IgG Fc constant regions produced locally by RSC, we amplified the cDNA corresponding to the CH2 and CH3 domains of an IgG1-, IgG2-, and an IgG3-producing hybridoma derived from RSC. The amplified DNA segments were cloned in M13 vectors and sequenced. Interestingly, very few differences in the nucleotide sequences were observed, and the deduced amino acid sequences were identical, except for the allotype, with those encoded by the human germline genes GEA and CL. Thus, the primary structure of the IgG1, IgG2, and IgG3 Fc regions produced by RSC were not altered when compared with those encoded by the unmutated human germline gene. These results suggest that factors other than altered IgG induce and sustain high avidity RF production in rheumatoid arthritis.