An inception cohort of 238 patients having peripheral joint synovitis of less than 12 months duration was evaluated clinically and followed prospectively for 1 year to determine the clinical significance of a number of rheumatoid arthritis (RA) associated autoantibodies. Serum samples collected at the time of the initial evaluation were tested for rheumatoid factor (RF) and antibodies to Sa (anti-Sa), RA-33, (pro)filaggrin [antifilaggrin antibody (AFA)], cyclic citrullinated peptide (anti-CCP), calpastatin, and keratin [antikeratin antibody (AKA)]. RF had a sensitivity of 66% and a specificity of 87% for RA. Anti-Sa, AFA, and anti-CCP all had a specificity of more than 90%, but a sensitivity of less than 50% for this diagnosis. Overall, there was a high degree of correlation between AFA, AKA, anti-Sa or anti-CCP, this being highest between anti-Sa and anti-CCP (odds ratio, 13.3; P < 0.001). Of the 101 patients who were positive for at least one of these four autoantibodies, 57% were positive for only one. Finally, anti-SA identified a subset of predominantly male RA patients with severe, erosive disease. Anti-SA, AFA and anti-CCP are all specific for early RA but, overall, have little additional diagnostic value over RF alone. Although these antibodies may preferentially recognize citrullinated antigens, the modest degree of concordance between them in individual patient sera suggests that it is unlikely a single antigen is involved in generating these responses.
A spectrum of autoantibodies is now known to be specifically associated with RA. There continues to be uncertainty as to what stage of the disease each of these autoantibodies develop, and whether they are associated with unique clinical features.
To help address these questions, a spectrum of autoantibodies known to be associated with RA in a cohort of patients with early synovitis was evaluated.
An inception cohort of 238 patients having peripheral joint synovitis of less than 12 months duration was evaluated clinicially then followed prospectively for 1 year. Patients were classified as having RA on the basis of fulfilling the 1987 criteria. Serum samples collected at the time of the initial evaluation were tested for anti-Sa and anti-RA-33 using immunoblotting, and to (pro)filaggrin (AFA), anti-CCP, and calpastatin (anti-RA-1) using enzyme-linked immunosorbent assay techniques. AKA were detected using immunoflurescence on human epidermal tissue. RF was tested by nephelometry. HLA-DRB1 alleles were determined using sequence specific primers. Initial and 1 year radiographs were evaluated for the presence of erosions.
Of the 238 patients with synovitis of recent onset in the cohort, 106 (45%) met RA criteria, 102 (96%) of whom met the criteria on their initial visit. Diagnoses in the remaining patients included 22 (9%) with reactive arthritis, 14 (6%) with psoriatic arthritis or another form of spondylarthropathy, 11 (5%) with another well-defined rheumatic diagnosis, and 85 (36%) with undifferentiated arthritis. The RA patients were significantly older than the nonRA patients (46 ± 13 versus 39 ± 13; P < 0.001), had higher mean swollen joint count (13.8 ± 9.7 versus 2.3 ± 2.3; P < 0.001), and higher C-reactive protein (CRP) level (1.9 ± 1.9 versus 1.6 ± 2.4; P < 0.01). Table 1 summarizes the prevalence of the various RA associated antibodies in patients diagnosed as having RF-positive (RF+) RA, RF-negative (RF-) RA, and nonRA. Regarding the characteristics of these tests, RF had the highest sensitivity at 66%, and all the other antibodies individually were less than 50% sensitive. AFA, anti-Sa, anti-CCP were greater than 90% specific for RA, while RF and AKA were 80-90% specific, and anti-RA-33 and anti-RA-1 was not specific for this diagnosis. The data further indicate that adding any one of AFA, AKA, anti-Sa, or anti-CCP to RF increases the specificity for RA from 80 to 90%. In the absence of RF, the presence of one or more of these antibodies carried a sensitivity of only 31% for RF- RA, with anti-Sa being the most specific at 98%. Overall, there was a high degree of correlation between AFA, AKA, anti-Sa or anti-CCP, this being highest between anti-Sa and anti-CCP (odds ratio, 13.3; P < 0.001). Despite this high level of correlation, of the 101 patients who were positive for at least one of these four autoantibodies, 57% were positive for only one, suggesting considerable variability in individual reactivity patterns.
RA has been shown in multiple populations to be associated with HLA-DRB1 alleles encoding for the shared epitope (SE). In this study, as illustrated in Table 2, the presence of each of these autoantibodies was significantly associated with having two shared epitope alleles, even when only the RA patients were considered.
Patients with anti-Sa antibodies were predominantly male (61% versus 28%; P<0.01), had significantly higher swollen joint counts (18 ± 12 versus 13 ± 9; P=0.02), and higher CRP levels (2.6 ± 3 mg/dl versus 1.6 ± 1.4 mg/dl; P=0.03) at the initial visit. Despite subsequently begin treated with significantly higher doses of prednisone (4.8 ± 6.0 mg/day versus 1.8 ± 3.3 mg/day; P<0.01), and more disease modifying antirheumatic drug therapy (1.4 ± 0.8 versus 0.9 ± 0.7 disease modifying antirheumatic drugs; P<0.01), the anti-Sa-positive RA patients had a higher frequency of erosions than the rest of the RA patients (60% versus 33%; P=0.03). Neither RF nor SE were associated with the disease severity measures, and analyses evaluating all the other autoantibodies failed to reveal a similar trend.
Despite a well-documented lack of specificity, RF continues to be a central part of the definition of RA, primarily because of its favourable sensitivity profile. In our cohort, RF had a sensitivity of 66%, a specificity of 87%, and an overall accuracy of 78% for the diagnosis of RA. AFA, anti-Sa, anti-CCP were all highly specific for this diagnosis, and when any of them were present in conjunction with RF, the specificity for RA approached 100%. Potentially of more importance to the clinician is the diagnostic value of these antibodies when RF is not detectable. Our data indicate that only 31% of RF- RA patients had any of AKA, AFA, anti-Sa or anti-CCP, and that anti-Sa was the most specific for this diagnosis. This modest level of sensitivity suggests that testing for this spectrum of autoantibodies carries little advantage over RF alone in diagnosing early RA.
AFA, AKA, and antiperinuclear factor (APF) have all been proposed to identify a common antigen present in the skin protein (pro)filaggrin. It has continued to be puzzling why a skin antigen would be targeted relatively specifically in a disorder that is primarily articular. A potential explanation for this may relate to the demonstration that citrulline appears to be an essential constituent of the antigenic determinants recognized by AKA, APF, and AFA. The citrulline rich (pro)filaggrin molecule makes an ideal substrate for detecting this reactivity. Moreover, the SA antigen, which, unlike (pro)filaggrin, is detectable in rheumatoid synovium, has recently been shown to also be citrullinated. It is thus possible that AKA, AFA, APE, and anti-Sa all recognize one or more citrullinated antigens. Despite this possibility, the modest degree of concordance between them in individual patient sera suggests that it is unlikely that a single antigen is involved in generating these responses.
This study provides evidence suggesting that anti-Sa antibodies appear to be a marker for a subset of early RA patients whose disease may be more severe and erosive. Moreover, it was determined that anti-Sa, AFA, and anti-CCP were all highly associated with SE, particularly two copies. We examined a spectrum of potential RA severity indicators including the number of swollen joints, CRP level, and presence of early radiographic erosions. Our data indicate that anti-Sa was more highly associated with these measures of RA severity than any other parameter, including the most accepted prognostic indicators, RF and SE.
In conclusion, it is demonstrated that antibodies directed against putatively citrullinated antigens including SA, filaggrin, keratin, and CCP are the most specific for RA, and are detectable early in the disease course. It will be of interest to find out whether the cumulative prevalence of specific autoantibody subsets tends to increase over time, as this would suggest that the mechanisms underlying the development of these reactivities continue to evolve over the course of the arthropathy.