Secretory antibodies to citrullinated peptides in plasma and saliva from rheumatoid arthritis patients and their unaffected first‐degree relatives

The aim of this study was to evaluate secretory antibodies to citrullinated proteins (ACPA) in plasma and immunoglobulin (Ig)A ACPA in saliva from patients with rheumatoid arthritis (RA) and their unaffected first‐degree relatives (FDRs). Patients with RA (n = 194) and first‐degree relatives unaffected by RA (n = 191) were recruited for analysis of secretory antibodies to second‐generation cyclic citrullinated peptides (anti‐CCP) in plasma. From a subpopulation (25 RA patients, 21 first‐degree relatives and 11 controls), saliva samples were obtained for IgA anti‐CCP analysis. The presence of secretory ACPA was compared between subject categories, and related to genetic and environmental risk factors. Secretory ACPA occurred in 37 (19%) plasma samples from patients with RA, but only in two (1%) of FDRs. IgA ACPA in saliva was found in three of 25 (12%) patients with RA, but not in any of the 21 FDRs (< 5%). No significant associations were seen between the presence of secretory ACPA and SE or smoking, either among RA patients or among FDRs. Despite occurring in 19% of RA plasma, secretory ACPA was rare in both saliva and plasma among FDRs, even among those positive for conventional ACPA of non‐mucosal origin. Longitudinal studies are warranted to determine whether circulating secretory ACPA occurs before or in parallel with the development of clinical arthritis.