Autoantibodies recognizing citrullinated rat filaggrin in an ELISA using citrullinated and non-citrullinated recombinant proteins as antigens are highly diagnostic for rheumatoid arthritis

The objective of the study was to determine the diagnostic value for rheumatoid arthritis (RA) of anti-filaggrin autoantibodies (autoAb) recognizing citrullinated recombinant rat filaggrin (ACRF) in community cases of very early arthritis. To evaluate the diagnostic value of ACRF, were studied sera from patients with different classified rheumatic diseases and healthy subjects (group 1, n= 422) and 314 community cases of very early arthritis (group 2) that were classified as RA (n = 176), non-RA (n = 63) and undifferentiated (n = 75) arthritides after 1 years of follow-up. ACRF were measured using a new ELISA, with results expressed as the difference between the OD value obtained on citrullinated minus that on noncitrullinated rat filaggrin (differential ACRF; dACRF). For both groups, rheumatoid factors (RF), anti-keratin autoAb (AKA) and anti-perinuclear factor (APF) were tested; for group 2, anti-CCP autoAb were also tested. Different reactivity patterns against citrullinated and noncitrullinated filaggrin were observed. Almost all sera reacting with citrullinated but not noncitrullinated filaggrin were from RA patients. Among RA and non-RA sera that recognized both forms of filaggrin, a positive result was obtained only with RA sera. For groups 1 and 2, dACRF sensitivity was 58.4% and 30.7%, and specificity for RA was 99.5% and 98.4%, respectively. In group 2, dACRF specificity for RA was better than that of RF (92.1%), APF (95.2%), AKA (96.8%) and anti-CCP (95.2%). dACRF positive predictive value was high (98.2) and close to that given by the concomitant positivity of RF and anti-CCP autoAb. Despite a high positive correlation between AKA, APF, anti-CCP and dACRF test results, they were complementary since some sera were positive for only one test. Thus, in a community setting, anti-citrullinated rat filaggrin reactivity detected by a new ELISA, whose originality is based on the difference between serum's reactivities on the citrullinated and native forms of filaggrin, had a higher diagnostic value for RA than other autoAb.