Immunoblotting detection of so-called 'antikeratin antibodies': a new assay for the diagnosis of rheumatoid arthritis.

OBJECTIVES To assess the diagnostic value for rheumatoid arthritis (RA), of an immunoblotting assay based on the rat oesophagus epithelium antigens recognised by the so-called 'antikeratin antibodies' ('AKA'), antigens that have been identified as three non-cytokeratin proteins (referred to as A, B and C proteins). METHODS After polyacrylamide gel electrophoresis in non-denaturing conditions and electrotransfer of an epithelial extract, the immunoreactivities to the A, B and C proteins of a series of serum samples from 88 patients with RA and 100 patients with non-rheumatoid rheumatic diseases, were semiquantitatively evaluated. RESULTS A total of 81.8% of RA serum samples recognised the three proteins, while 91% of non-RA serum samples only weakly recognised the A and B proteins but not the C protein. Only in the group of RA patients, were the titres of the antibodies to the A, B and C proteins found to be significantly correlated with each other and with the titres of 'AKA' detected by the standard indirect immunofluorescence (IIF) method. For a diagnostic specificity of 99%, the diagnostic sensitivities of the detection of the A and B proteins were 50% and 43.2%, respectively, when those of the detection of 'AKA' by IIF and of IgM-rheumatoid factor by enzyme-linked immunosorbent assay were 42% and 54%, respectively. In contrast, at a same specificity of 99%, the diagnostic sensitivity of the detection of the C protein was significantly higher with a value of 70.5%. CONCLUSION This immunoblotting assay which is the first immunochemical method proposed for the detection of 'AKA, should be validated on larger series of patients but can already be considered as a very powerful test for the serological diagnosis of RA.