IgG Subclass Specificity Discriminates Restricted IgM Rheumatoid Factor Responses From More Mature Anti-Citrullinated Protein Antibody-Associated or Isotype-Switched IgA Responses.

To investigate the presence and patterns of specific IgG subclass recognition by IgM rheumatoid factor (IgM-RF) and IgA-RF with a newly developed enzyme-linked immunosorbent assay (ELISA), which can discriminate between polyspecific and restricted RF responses. Polyspecific and restricted RF responses were determined with our ELISA, which uses individually coated recombinant IgG subclasses instead of polyclonal IgG as target antibodies. Fine specificity was determined using target antibodies with single amino acid mutations in the Fc region. In a screening panel of 93 sera that were previously found to be IgM-RF positive in a conventional RF assay, we were able to discriminate between sera with polyspecific IgM-RF responses (i.e., RF responses directed against all 4 IgG subclasses) and those with restricted IgM-RF responses, with low or absent relative reactivity against IgG2, IgG3, or IgG4. We found the largest variation for anti-IgG3 reactivity. Samples without detectable anti-IgG4 reactivity formed an independent group from the other restricted RF responses and the polyspecific RF responses. The specificity of these anti-IgG4-negative sera could be pinpointed to single amino acid differences between IgG1 and IgG4. Polyspecific RF responses more often showed signs of RF response maturation, with more isotype switching to IgA-RF, as compared to restricted RF responses. In a cohort of IgM-RF+ and/or anti-citrullinated protein antibody (ACPA)-positive arthralgia patients, we found restricted RF responses in 35% (49 of 140) of RF+/ACPA- patients, while RF+/ACPA+ patients, who have a much higher risk of developing rheumatoid arthritis, virtually always (123 of 128 [96%]) showed a polyspecific RF response. IgG subclass-specific RF distinguishes between immature restricted RF responses and potentially more pathogenic, ACPA-associated polyspecific responses.