We developped an ELISA using a deiminated recombinant rat filaggrin (ArFA-ELISA) and assessed its diagnostic value for Rheumatoid Arthritis (RA). 714 sera from well characterised rheumatic patients, including 241 RA, were analysed. The results were compared to those obtained with another ELISA using a recombinant filaggrin of human origin and with those of two reference tests.
Recombinant rat filaggrin was obtained by PCR amplification of Wistar rat genomic DNA, cloning, production in E. Coli and purification by metal chelate chromatography. The affinity-purified filaggrin was deiminated with type II rabbit skeletal muscle peptidylarginine deiminase. Deiminated and non-deiminated filaggrin were used as immunosorbents and the difference between optical densities on the two antigens were considered as the titer. The other tests were performed following previously described methods : 'AKA' were assayed by semiquantitative indirect immunofluorescence, antibodies to human epidermis filaggrin by immunoblotting (AhFA-IB) and by a recently described ELISA, using a deiminated recombinant human filaggrin (AhFA-ELISA).
Whatever the chosen specificity threshold, the diagnostic sensitivity of ArFA-ELISA was significantly higher than that of the three other tests.
As expected, the titres of ArFA-ELISA, 'AKA', AhFA-IB and AhFA-ELISA were closely correlated (P < 10-5). However, among RA sera, only 53% were concordant for the four tests, 25% being positive with only one test. Consequently, the successive use of ArFA-ELISA, then 'AKA' detection only when the first test is negative, would allow a diagnostic sensitivity of 0.67 to be reached, keeping a specificity close to 0.99.
This ArFA-ELISA appears as one of the most efficient among the tests previously described for detection of antifilaggrin/anti-citrullinated peptides autoantibodies, in terms of diagnostic accuracy for RA.
Its diagnostic performance in early RA and its prognostic value are currently under evaluation.