Human monoclonal rheumatoid factor derived from rheumatoid synovial cells monospecific for rabbit IgG

Rheumatoid factors (RF) in rheumatoid arthritis (RA) are polyclonal autoantibodies directed against antigenic epitopes located in the Fc portion of the IgG molecule. Hybridoma technology has overcome the difficulty of their polyclonality, so that monoclonal RF (mRF) can be examined for their individual binding specificities and genetics. We isolated a monoclonal IgM RF secreting hybridoma (designated H4) from the rheumatoid synovial cells (RSC) of a patient with RA. H4 bound specifically with rabbit IgG (RIgG) and had no human IgG (HIgG) reactivity. By direct binding ELISA and absorption experiments, 6% of the RIgG reactive RSC RF in this patient with RA was monospecific for RIgG. H4 was tested against RIgG F(ab')2 and pFc' fragments, and bound only to the pFc' fragment (CH3 domain). Moreover, H4 mRF had high avidity for RIgG in a capture ELISA. Total RNA was extracted and the variable region heavy (VH) and light (VL) chain cDNA were amplified using polymerase chain reaction technology. Sequence analysis of the IgM RF VH and VL chain genes indicated usage of the VH26 germline gene (VhIII gene family) and a new V lambda germline gene. Our results suggest preferential use of restricted germline genes in the formation of autoantibodies in human autoimmune diseases. The pathological significance of RIgG specific RF is still unclear. However, this finding suggests that all RSC RF production may not necessarily be induced by autologous IgG.