The HLA–DRB1 shared epitope alleles are primarily a risk factor for anti–cyclic citrullinated peptide antibodies and are not an independent risk factor for development of rheumatoid arthritis

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Abstract
Objective The shared epitope (SE)–containing HLA–DRB1 alleles represent the most significant genetic risk factor for rheumatoid arthritis (RA). Recent studies indicate that the SE alleles are associated with only RA that is characterized by the presence of anti–cyclic citrullinated peptide (anti-CCP) antibodies, and not with anti-CCP–negative disease. In this study we investigated whether the SE alleles contribute to the development of anti-CCP–positive RA, or whether they are associated solely with the presence of anti-CCP antibodies. We therefore determined the influence of the SE alleles and anti-CCP antibodies on the progression from recent-onset undifferentiated arthritis (UA) to RA. Methods Patients with recent-onset UA at the 2-week visit (n = 570) were selected from the Leiden Early Arthritis Cohort. SE alleles, rheumatoid factor (RF) status, and anti-CCP antibody levels were determined. Progression to RA or other diagnoses was monitored. Results One hundred seventy-seven patients with UA developed RA during the 1-year followup, whereas the disease in 393 patients remained unclassified or was given other diagnoses. The SE alleles correlated with the presence of anti-CCP antibodies, but not with the presence of RF. Both in SE-positive and in SE-negative patients with UA, the presence of anti-CCP antibodies was significantly associated with the development of RA. More intriguingly, however, no apparent contribution of the SE alleles to the progression to RA was found when analyses were stratified according to the presence of anti-CCP antibodies. In patients with anti-CCP–positive disease, the presence of SE alleles was associated with significantly higher levels of anti-CCP antibodies, suggesting that the SE alleles act as classic immune response genes. Conclusion The SE alleles do not independently contribute to the progression to RA from UA, but rather contribute to the development of anti-CCP antibodies.