Anticarbamylated protein (anti-CarP) antibodies are present in sera of juvenile idiopathic arthritis (JIA) patients

In juvenile idiopathic arthritis (JIA) patients there is a lack of markers that predict severe disease. Although anticitrullinated protein antibodies (ACPA) have contributed substantially to the understanding of rheumatoid arthritis (RA),1 their detection in JIA has not been equally useful as incidence rates in JIA patients are low2 and merely confined to the polyarticular immunoglobulin (Ig)M-rheumatoid factor (RF)-positive category resembling RA. Recently, anticarbamylated protein (anti-CarP) antibodies were detected in 45% of RA patients and importantly also in 16%–20% ACPA-negative patients.3–5 Within the ACPA-negative patients, anti-CarP antibodies were associated with more severe radiographic progression.3 Since most JIA patients are ACPA-negative we investigated whether anti-CarP antibodies are present in the sera of JIA patients and how they are related to ACPA and IgM-RF. JIA patients from three Dutch sources were included: the BeSt for Kids trial (NTR 1574, a treatment strategy study) (n=33), a previously described cohort6 (n=48) and the Arthritis and Biologicals in Children (ABC) register7 (n=153). Healthy controls (n=107) (mean age/range 11/(2–20)) are stem-cell graft …