Background Although anti-citrullinated protein antibodies (ACPAs) appear to be more specific for diagnosing rheumatoid arthritis (RA), rheumatoid factors (RF), defined as autoantibodies binding the Fc portion of IgG, are still a hallmark of diagnosis and severity of RA. Evaluation of the RF response is often limited to measurement of IgM-RF recognizing polyclonal human or rabbit IgG. This leads to an oversimplification of the multi-specificity of RF and makes it difficult to distinguish pathological RA-associated RF from RF present in other (rheumatic) diseases or in healthy individuals and may limit the value of RF testing in prediction of arthritis development and disease course.
By determining reactivity of RF with the four individual IgG subclasses instead of polyclonal IgG it should be possible to classify the RF response not only in a quantitative (titer), but also in a qualitative way i.e. polyspecificity for all subclasses versus restricted specificity for only some subclasses. It has been suggested that polyspecificity of RF is a specific feature of an RA-associated RF response and that restricted RF responses are not only present in RA, but also in individuals without arthritis. Distinguishing between polyspecific and restricted RF responses could be useful to discriminate between pathological and non-pathological RF responses and could have value in predicting onset and disease course of RA.
Objectives Set up an ELISA system capable of screening large cohorts for presence and patterns of specific IgG subclass recognition by IgM-RF and IgA-RF, thereby discriminating polyspecific and restricted RF responses.
Methods Polyspecific and restricted RF responses were determined with our ELISA system that measures anti-IgG subclass specific RF by using 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.
Results In a panel of 93 sera previously tested IgM-RF positive in a conventional RF assay we could discriminate between sera containing polyspecific IgM-RF responses, i.e. RF responses directed against all four IgG subclasses, and restricted IgM-RF responses with low/absent reactivity against IgG2, IgG3 or IgG4. Subclass specific IgM-RF reactivity varied most for anti-IgG3 reactivity. Samples lacking a-IgG4 reactivity formed an independent group from other restricted RF responses and polyspecific RF responses. The specificity of these a-IgG4neg sera could be pinpointed to various 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, compared to restricted RF responses. Also in a validation cohort of RF+ACPA+/- arthralgia patients we found restricted RF responses in 35% (49/140) of RF+ACPA- patients, while RF+ACPA+ patients, associated with a higher risk of future RA, virtually always (123/128 (96%)) showed a polyspecific RF response.
Conclusions Anti-IgG subclass specific RF distinguishes between immature restricted RF responses and potentially more pathogenic, ACPA associated polyspecific responses.
Disclosure of Interest None declared