Heteroclitic polyclonal and monoclonal aNTI-Gm(a) and aNTI-Gm(g) human rheumatoid factors react with epitopes induced in Gm(a-), Gm(g-) IgG by interaction with antigen or by nonspecific aggregation: A possible mechanism for the in vivo generation of rheumatoid factors

Heteroclitic rheumatoid factors (RF) are specific for allotypic determinants, e.g., Gm(a) or Gm(g) on allogeneic, but not autologous IgG. All polyclonal RF we isolated from nine rheumatoid arthritis pa-tients with circulating Gm(a-), (b+), (g-), (f+) IgG displayed dual heteroclitic activity for the Gm(a) and Gm(g) allotypes, as shown by using appropriate RBC agglutination assays and affinity columns bearing Gm(a+) or Gm(g+) IgG. To investigate possible mechanisms underlying the in vivo generation of heteroclitic RF, we tested the ability of non-specifically and immune-specifically aggregated Gm(a-), (g-) IgG to function as targets for RF from Gm(a-), (g-) patients with rheumatoid arthritis. Heat aggregation (63°C for 20 min) or binding to Ag (as in tetanus toxoid-antitetanus toxoid complexes) induced a "functional" Gm(a+) and/or (g+) phenotype in Gm(a-), (g-) IgG from five healthy subjects and five rheumatoid patients, as suggested by the ability of these altered IgG to function as efficient targets for six heteroclitic RF in direct binding and competitive inhibition experiments. That heterocliticity and dual Gm(a), Gm(g) specificity can be features of a single antibody molecule was formally demonstrated by analysis of a monoclonal RF (IgM mAb 61) generated from a Gm(a-), (g-) rheumatoid patient. RF mAb 61 displayed a high affinity (Kd, 10^-7 M) for IgG Fc fragment of Gm(a+) and (g+) IgG or aggregated autologous Gm(a-), (g-) IgG but did not bind to native autologous IgG. To investigate the molecular basis of the acquired Gm(a) phenotype, PBMC from five Gm(a-) patients with rheumatoid arthritis and two Gm(a-) normal subjects were cultured in vitro after activation with PWM. In most instances, these PBMC produced IgG that behaved as Gm(a+) in sensitive ELISA. Application of the polymerase chain reaction (PCR), using probes specific for the nucleotide sequence coding for the Gm(a) tetrapeptide, to the amplification of DNA from the in vitro-stimulated Gm(a-) normals or rheumatoid patients' PBMC provided no evidence for Gm(a) nucleotide sequences. The present data suggest that acquisition of the Gm(a) determinant by Gm(a-) IgG may result from subtle changes in the CH2-CH3 RF-binding region. Such changes would occur when Gm(a) IgG are complexed with Ag or nonspecifically altered, thereby providing a possible explanation for the induction of heteroclitic RF in Gm(a-) rheumatoid arthritis patients.