Rheumatoid factors (RF) are antibodies that are found in the majority of patients with rheumatoid arthritis (RA). They are routinely detected by their reactivity with the heavy chains of IgG immunoglobulins. This reactivity with IgG is enhanced when the IgG is modified, such as in the form of an immune complex, or the IgG is heat aggregated or the IgG is allowed to adhere to a solid surface. Most RF's react with IgG from many mammalian sources but in routine testing in the United States human IgG is used.

Rheumatoid factors are found most prevalently in the IgM class of immunoglobulins and it is this class of RF that is detected in routine testing. The presence of RF is a diagnostic criterion for RA and the titer of these antibodies is roughly correlated with progressive and erosive disease. Titers of RF frequently fall in response to successful treatment with disease modifying therapy and during prolonged remissions.

The RF Test Deficiencies

While RF is a relatively simple, inexpensive and reproducible laboratory test, there are a number of deficiencies in its use as a diagnostic and prognostic test for RA:

1. The specificity of RF is approximately 70-80% depending on the population examined. Moderate to high titers of RF are frequently encountered in patients with other forms of connective tissue disease, especially Sjogren's syndrome and mixed connective tissue disease (MCTD), as well as in patients with lymphoma and monoclonal gammopathies. Rheumatoid factors are a regular feature of essential mixed cryoglobulinemia.
2. The prevalence of RF in patients meeting the criteria of the American College of Rheumatology for RA is approximately 80% and this number is even lower early in the course of the disease when a diagnostic test is most helpful.
3. Most children with juvenile RA are RF negative.

In addition to these clinical deficiencies, the study of RF has not, thus far, provided any insight into the pathogenesis of RA. Given these deficiencies, there is considerable interest in finding additional serological markers for RA.

Two Other Antibodies

Two types of autoantibodies have been described in the past 20 years that appear to supplement RF as diagnostic and prognostic tests. One group of antibodies has been referred to as the anti-perinuclear antibodies and are detected, using immunofluorescent microscopy, by their reactivity with keratohyaline granules in human buccal mucosal cells. A second group of antibodies are detected by their reactivity with keratin in stratified epithelial tissues - usually sections of rat esophagus.

Both of these antibody systems appear to have a sensitivity for RA of approximately 60-70% and a specificity greater than 90% in a rheumatoid disease population. Both appear to be independent of RF and are present early in the course of RA. However, neither of these assays has been adapted for routine clinical use because of technical difficulties.

The Anti-CCP Test

Biochemical studies have indicated that the antigens in both assays are forms of an intracellular molecule known as fillagrin. More recently, the actual peptides in fillagrin responsible for antigenic reactivity with these antibodies have been shown to be cyclic peptides containing the unusual amino acid citrulline. Citrulline is produced by posttranslational modification of arginine through the action of an enzyme arginine peptidyl deimminase. Synthetic models of these peptides have been prepared and applied as antigenic substrates in an ELISA immunoassay (the anti-cyclic citrullinated peptide assay, i.e., anti-CCP antibody test). Like most ELISA assays, the test is easy to perform and is reproducible.

In one recent study, the anti-CCP antibody ELISA assay had a specificity of 98% and a sensitivity of 68%. In our laboratory, we found that the specificity of the anti-CCP test was closer to 90% when compared to patients with other active rheumatic diseases.

The presence of anti-CCP antibodies appears to be independent of RF and has been found with the same sensitivity in patients with early RA as well as in patients with established disease. This means that physicians now have a diagnostic serological marker for approximately half of the RA patients who were previously seronegative. Even more importantly from a clinical viewpoint, anti-CCP antibodies seem to be associated with erosive and progressive disease.

An antigen which reacts with anti-CCP antibodies appears to be present in the inflamed rheumatoid synovial membrane; and B cells spontaneously producing IgM anti-CCP antibodies are found in the synovial fluid of patients with active RA and not in other inflamed synovial fluids. These latter observations suggest that the anti-CCP antibodies may lead to the discovery of one of the elusive antigens responsible for eliciting an immunoinflammatory response in RA.

Anti-gp 130-RAPS

Anti-fillagrin and anti-CCP antibodies, along with rheumatoid factors, do not, undoubtedly, represent the complete picture of autoantibodies associated with RA. Recently, IgG autoantibodies to a soluble receptor for interleukin 6 (gp 130-RAPS) have been reported to be present in 73% of patients with RA and were very specific for this disease even when compared to patients with other connective tissue diseases (specificity 96%). Furthermore, the titers of this antibody correlated with RA disease activity. While gp 130-RAPS was identified in synovial tissue, it was not specific for RA synovium.

Testing for anti-gp 130-RAPS is not yet available from any routine laboratories. Other autoantibodies have also been described in RA but their significance and practical application have yet to be explored fully. For a list of some of these antibodies, see Table 1 below.

Summary

The anti-CCP antibody assay appears be a new serological test for RA. It is a useful diagnostic and prognostic test that supplements RF. The anti-CCP assay takes on additional clinical significance given the current trend to treat RA early and aggressively with disease modifying agents. Further study of this antibody system may lead to a better understanding of the pathogenesis of RA.