Production of anti-citrullinated protein antibodies (ACPAs) is an important biomarker for rheumatoid arthritis (RA). We undertook this study to determine whether genetic factors (HLA-DRB1 alleles) are associated with extreme ACPA levels in individuals with ACPA-positive RA, and to ascertain whether there are any phenotypic characteristics associated with these subgroups of RA.
HLA-DRB1 allelic groups were genotyped in 1,073 ACPA-positive RA patients from the Swedish Epidemiological Investigation of Rheumatoid Arthritis study. We found that 283 patients (26.4%) had high ACPA levels (defined as >1,500 units/ml using the Euro-Diagnostica anti-CCP2 test), while the rest of the patients had moderate ACPA levels and served as the comparison group. A replication group consisted of 235 RA patients.
No significant differences in baseline disease activity were observed between patients with high and those with moderate ACPA levels. However, the HLA-DRB1*15 allele was associated with high ACPA levels (P=0.0002). A similar trend was detected in HLA-DRB1*15-positive patients in the replication cohort, with meta-analysis of the discovery and replication cohorts demonstrating an overall effect of HLA-DRB1*15 on development of high ACPA levels in both the discovery and replication cohorts (P<0.0001 by Mantel-Haenszel test with a fixed-effects model).
Our data indicate that HLA-DRB1*15 may promote the production of high ACPA levels. Due to the high value of ACPA level scores in the 2010 American College of Rheumatology/European League Against Rheumatism classification criteria for RA, the presence of HLA-DRB1*15 may, at least in part, contribute to fulfilling the criteria for RA. This illustrates the complex nature of the genetic regulation of ACPA levels. Additional mechanistic studies of the regulation of ACPAs and ACPA-positive RA are pending.