A new assay for IgG rheumatoid factor activity and its use to analyse rheumatoid factor reactivity with human IgG isotypes

Identification of Clinically and Pathophysiologically Relevant Rheumatoid Factor Epitopes by Engineered IgG Targets

OBJECTIVE

Rheumatoid factors (RFs), which are anti-IgG autoantibodies strongly associated with rheumatoid arthritis (RA), are also found in other diseases and in healthy individuals. RFs bind to various epitopes in the constant (Fc-) domain of IgG. Therefore, disease-specific reactivity patterns may exist. This study was undertaken in order to develop a new approach to dissecting RF epitope binding patterns across different diseases.

METHODS

We analyzed RF reactivity patterns in serum from patients with seropositive arthralgia, patients with RA, and patients with primary Sjögren's syndrome (SS) using bioengineered, natively folded IgG-Fc targets that demonstrated selective RF binding toward several distinct regions of the IgG-Fc domain.

RESULTS

Rheumatoid factor responses primarily bound the Fc Elbow region, with a smaller number of RFs binding the Fc Tail region, while the Fc receptor binding region was hardly targeted. A restricted reactivity against the IgG-Fc Tail region was associated with less positivity for anti-citrullinated protein antibodies (ACPAs) and less arthritis development in arthralgia, whereas combined reactivity toward IgG-Fc Tail and Elbow regions was associated with more arthritis development. Reactivity toward the IgG-Fc Tail region was observed far more frequently in RA than in primary SS.

CONCLUSION

Bioengineered IgG targets enable serologic characterization of RF reactivity patterns, and use of this approach appears to reveal patterns associated with ACPA detection and arthritis development in patients with arthralgia. These patterns are able to distinguish RA patients from primary SS patients. This new methodology improves the clinical value of RFs and our understanding of their pathophysiologic processes.

Rheumatoid factor, anti-heat shock protein (65 kDa) antibodies and anti-nuclear antibodies in equine joint diseases

To consider the hypothesis that autoimmune mechanisms may contribute to the pathology of equine joint diseases, 3 autoimmune responses were assayed in sera and synovial fluids. IgM-rheumatoid factor and antibodies to heat shock protein 65 kDa were determined by ELISA; anti-nuclear antibodies were assayed by indirect immunofluorescence to whole cell nuclear components. All parameters showed only modest increases, if any and not in a pattern related to disease, although some statistically significant increases were detected. Group analysis showed significantly elevated synovial fluid IgM-rheumatoid factor (IgM-RF) in horses with OA (P < 0.01), traumatised joints (P < 0.01) and articular fractured bones (P < 0.001). There was no significantly increased IgM-RF in the sera of horses with joint disorders compared to control horses. Significantly raised anti-heat shock protein molecular weight 65 kDa (HSP65) antibodies were only found in the synovial fluids of the horses with traumatised joints (P < 0.001). No correlations were found between the synovial fluid and sera levels of IgM-RF or anti-HSP65 antibodies. Synovial fluid anti-HSP65 antibody and IgM-RF levels positively correlated in the OCD (P < 0.001), fracture (P < 0.01) and synovitis (P < 0.05) groups. As antibodies to HSP65 correlated with IgG concentrations in synovial fluids, it is not possible to draw conclusions on HSP roles in joint disease pathogenesis. No serum anti-nuclear antibodies (ANA) were detected by immunofluorescence using rat liver and a human epithelial cell line (HEp-2) as substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical value of ELISA assays for IgM and IgG rheumatoid factors

The clinical value of enzyme linked immunosorbent assays (ELISA) assays for IgM and IgG rheumatoid factors was assessed in a series of studies using rabbit IgG as antigen. The tests were reproducible with intra-assay coefficients of variation of 6% and could be simply and rapidly performed. Normal ranges were established using 106 sera from healthy controls. In a cross sectional study of 208 rheumatoid patients these assays were compared with the Rose-Waaler and laser nephelometric assessments of rheumatoid factor. In some patients there were discrepancies between rheumatoid factor positivity determined by one method or another. IgM ELISA and Rose-Waaler titres showed a significant correlation (r = +0.58; p less than 0.001), but there was a low correlation between IgM and IgG ELISA (r = +0.27; p less than 0.001). There was no evidence to show that the measurement of IgM or IgG rheumatoid factor gave significantly more clinical information than traditional tests such as the Rose-Waaler or latex agglutination tests.

Isolation and analysis of complement activating aggregates from synovial fluid of patients with rheumatoid arthritis using monoclonal anti-C3d antibodies

The complement activating aggregates in synovial fluids of patients with rheumatoid arthritis (RA) have been isolated using monoclonal IgM anti-C3d antibodies attached to solid phases, and the content of the material bound has been analysed. High levels of aggregated IgG bearing C3d were found in RA synovial fluids, and IgG was the major immunoglobulin bound from such synovial fluids by anti-C3d Sepharose. A strong correlation was shown between levels of aggregated IgG bearing C3d and complement activation, as judged by C3d levels. Significant (but less strong) relationships were also observed between C3d levels and both complement consuming and C1q binding activity. C3d levels and levels of aggregated IgG bearing C3d were both significantly associated with the numbers of polymorphonuclear leucocytes (PMNs) found in RA synovial fluids. From these results it is concluded that the aggregated immunoglobulins bearing C3d (particularly IgG) isolated from RA synovial fluids are responsible for activating complement and attracting PMNs into the joint space. Radioimmunoassay showed no correlation, however, between levels of aggregated IgG (or IgM) bearing C3d and rheumatoid factor (RF) activity bound by anti-C3d. In addition, the material bound by anti-C3d Sepharose from most synovial fluid polyethylene glycol precipitates did not contain either IgM or IgG RF. Thus both techniques show that the majority of complexes bearing C3d do not contain RF. As the complement fixing aggregates apparently contain only immunoglobulin and complement components the results raise the problem of how the aggregates are formed. It is suggested that RA IgG may remain aggregated after either antigen or antibody (RF) has dissociated from the complex.