Antifilaggrin autoantibodies, previously known as 'antikeratin' antibodies or antiperinuclear factor, are serum IgG that constitute the most specific diagnostic markers of rheumatoid arthritis (RA). We showed that they specifically recognise deiminated forms of the α and β chains of fibrin in the rheumatoid synovium. Subsequently, we developed a new ELISA for these autoantibodies, using in vitro deiminated human fibrinogen as immunosorbent (AhFibA-ELISA). We evaluated its diagnostic performance in a cohort of 617 patients with well-characterised rheumatic diseases, including 181 patients with established RA: at a diagnostic specificity of 98.5%, the ELISA presents a diagnostic sensitivity of 76%. It is to date the most efficient test for the diagnosis of RA.
On the basis of this test, we undertook to determine the subclass distribution of AhFibA.
From the AhFibA-ELISA, four ELISAs using monoclonal antibodies to each IgG subclass (IgG1, IgG2, IgG3 and IgG4) were developed. The ELISAs were adjusted to allow the respective proportions of each AhFibA subclass to be determined in each serum sample tested. 141 RA patients positive for AhFibA were analysed.
For each IgG subclass, the titres (optical density [OD] values) in the whole population of patients were found to be significantly correlated with those obtained with the AhFibA-ELISA. IgG1 AhFibA reached the highest OD values (range 0.137–3.028, median: 1.125), followed by IgG4 AhFibA (range 0–2.846, median 0.043), IgG3 AhFibA (range 0–1.448, median 0.034) and lastly IgG2 AhFibA (range 0–0.695, median 0.040). The predominance of IgG1 AhFibA was also observed at the individual level since, among the 141 AhFibA-positive sera, all but one contained at least 40% of IgG1 AhFibA. In 39.7% of the sera, one or several other subclasses accounted for more than 10% of total AhFibA. IgG4 AhFibA was the most frequently associated subclass, 25% of the sera containing more than 10% of these antibodies. Only 10.7 and 7.9% of the sera contained more than 10% of IgG2 and IgG3 AhFibA, respectively.
These results confirm and extend those previously obtained by indirect immunofluorescence for 'antikeratin' antibodies. The predominance of IgG1 AhFibA, and their frequent association with IgG4 AhFibA, raises the question of the Th1/Th2 balance in RA. Moreover, the predominance of IgG1 AhFibA is compatible with effector mechanisms involving complement activation and/or the engagement of Fc gamma receptors.