ACPA characterize the immune response to citrullinated peptide antigens that are a hallmark of seropositive adult RA. ACPA can precede the development of RA by several years and also demonstrate a high specificity for RA. The current generation of ACPA test kits also approach the sensitivity of RF, which generally have a lower specificity for RA. This has resulted in the incorporation of the ACPA status in clinical practice. Additionally, ACPA identify a distinct subset of RA highlighted by several genetic association studies. Smoking has been implicated to be involved in triggering the process of citrullination that eventually leads to auto-immune activation in susceptible hosts. Other factors involved in citrullination include periodontal disease and infections[24–27]. It is likely that the mechanism of citrullination is different in children than in adults, with infections playing a greater role than smoking. In a recent study, Gilliam et al., suggested that in children with IgM-RF positive polyarticular JIA, fibrinogen is the target of citrullination.
Our analysis of published studies indicate that in children with JIA who do not have the RF-positive polyarticular subtype, the prevalence of ACPA is ~6%. In our study we observed a significant number of children who were negative for RF-IgM but positive for ACPA. These children phenotypically differed from those who were positive for both RF-IgM and ACPA and are less likely to carry shared epitope encoding HLA DRB1. It should be noted that most of the 23 children who were RF-IgM negative and ACPA positive had low titers of ACPA (range 21–75, median 27). These observations suggest that a low titer of ACPA in the absence of RF-IgM, identifies a subset of JIA that is different from the RF-positive polyarticular JIA subset. Lack of follow -up-studies to determine if these children subsequently developed RF-IgM is a limitation of our study. We observed that children who were positive for both RF-IgM and ACPA phenotypically resemble adults with RA, and also encompass the typical child with polyarticular “RF positive JIA.” We propose that in children who are RF-IgM negative, a higher threshold for ACPA-positivity might be prudent in order to identify those children who have a phenotype compatible with RA. It should be noted that higher titers of ACPA are given more weight in the revised criteria for RA.
Our study also highlights some limitations of the ILAR classification criteria. For instance, we identified 3 children who phenotypically resembled seropositive RA, (two positive tests for RF-IgM, and positive for ACPA, carriage of the HLA DRB1 shared epitope) but had to be classified as “undifferentiated JIA” due to the specific requirements of the ILAR criteria. Future revisions of ILAR criteria should take into consideration inclusion of children with fewer than 5 joints who otherwise have biomarkers such as RF-IgM/ACPA, as well as to limit some of the exclusion criteria such as family history of psoriasis or positive HLA B27, if both RF-IgM and ACPA are positive. Presence of both these biomarkers clearly identifies a phenotype akin to adult seropositive RA.
Although our study included a relatively large cohort of children with JIA tested for ACPA, it did have some limitations. A larger cohort of healthy control children would have been ideal. However, the frequency of ACPA in our controls was comparable to other published cohorts. Several of the cases with only positive ACPA had low titers. Our study was also cross-sectional and, therefore, did not address the relationship between the various biomarkers and treatment, disease activity and long term outcome. We have shown that children with RF/ACPA-positive JIA demonstrate similar HLA-DRB1 genotypes as adults with RA. In the present study, the objective was to investigate if children who were ACPA+/RF- differed compared to ACPA+/RF+. Therefore, we limited HLA typing to RF+/ACPA+ and RF-/ACPA + subjects.
Another notable finding from this study is that anti-RA33 antibodies do not appear to be associated with the JIA phenotype. Anti-RA33 antibodies are directed against a nuclear protein antigen identical to the A2 protein of the heterogeneous nuclear ribonucleoprotein. Anti-RA33 antibodies were first described to be prevalent in sera from adults with RA. There have been three prior studies of RA33 in small JIA cohorts. Gabay et al., reported that anti-RA33 antibodies were detected in 11% of sera from 124 children with JIA, and none of the controls. Children with RF + polyarticular JIA had a higher prevalence of antri-RA33 antibodies (57%), although only 7 children with RF + polyarticular JIA were included in this analysis. Nesher et al., also reported finding RA33 antibodies in sera from children with polyarticular as well as oligoarticular JIA. In a recent study of 42 Egyptian patients with JIA, anti-RA33 antibodies were detected in 83% of JIA patients with polyarticular onset, 33% with oligoarticular onset and 57% with systemic onset disease according to a study-specific cut off. By contrast in our study, the prevalence of anti-RA33 antibodies was low and was not significantly different between cases and controls. We also did not observe a higher prevalence of this specific antibody maker namely; among those with polyarticular JIA. The reasons for the different results observed could be due to a combination of factors, different methodologies cutoffs, sample sizes, composition of JIA subtypes, effects of storage, as well as true biologic differences.
Children with positive ACPA but negative RF are frequent and may define a distinct subset of children with JIA. If validated in other large JIA cohorts, ACPA testing should be included in the classification of JIA.