Evaluation of anti-citrullinated filaggrin antibodies as hallmarks for the diagnosis of rheumatic diseases

Abstract

Background: Anti-filaggrin antibodies (AFA) are among the most specific antibodies for rheumatoid arthritis, so procedures for their detection should be included in early biological diagnoses. AFA can be detected by indirect immunofluorescence (anti-keratin antibodies, AKA) or by new enzyme immunoassays (EIA). Their comparative performance needs to be established.

Objective: To compare these technical procedures to optimise the serological diagnosis of rheumatoid arthritis.

Methods: Results obtained using AKA and EIA were compared in 271 sera from 140 patients with rheumatoid arthritis at various stages, 98 patients with other autoimmune diseases, and 33 healthy subjects. EIA were successively undertaken with citrullinated linear filaggrin peptide (home made EIA) or cyclic citrullinated peptide (CCP2, commercial kits). Rheumatoid factor (RF) was assessed by EIA in all patients.

Results: Anti-CCP2 kits showed the best sensitivity and specificity (65% and 96%, respectively). Among the 140 patients with rheumatoid arthritis, those with very recent disease (less than six months' duration, n = 21) were studied as a separate group. In this group, the sensitivity of anti-CCP2 kits decreased to approximately 50%. Nevertheless this assay remained the most accurate when compared with AKA or home made EIA using linear filaggrin peptides. The combination of anti-CCP2 and RF only slightly increased the sensitivity of the diagnosis of very early rheumatoid arthritis.

Conclusions: Kits using citrullinated cyclic peptides (CCP2) were more suitable than either AKA or EIA using linear filaggrin peptides for the diagnosis of early rheumatoid disease.

Figure 1

(A) Analysis of the serum IgG response to the citrullinated filaggrin peptide (cfc6) in different populations of subjects. Patients with rheumatoid arthritis had a significantly higher serum IgG response to cfc6 than those with other connective diseases or the healthy controls. However, a cut off value established at OD 140 (mean optical density value of the healthy subjects +3 SD) did not accurately discriminate the patients with rheumatoid arthritis from those with the other connective tissue diseases. IgG anti-filaggrin peptide was detected by this procedure in more than 30% of the cases of Sjogren's syndrome and SLE. Results are expressed as mean optical densities in the reactive well subtracted from the OD in blank well. (B) Evaluation of the serum IgG response related to the citrullinated form of cfc6 in different populations. Subtraction of the optical density of the IgG response to cf0 from that of the IgG reactivity to cfc6 discriminated the IgG response of the rheumatoid arthritis group to filaggrin peptide. A receiver operating characteristic (ROC) curve (insert) established a cut off value at 100 arbitrary units that discriminated rheumatoid arthritis patients with a sensitivity of 50% and a specificity of 98%. The boundaries of the boxes indicate the 25th and 75th centiles; whiskers above and below the boxes indicate the 90th and 10th centiles. RA, rheumatoid arthritis; SLE, systemic lupus erythematosus; SS, Sjogren's syndrome.