Subclass distribution of IgG antibodies to the rat oesophagus stratum corneum (so‐called anti‐keratin antibodies) in rheumatoid arthritis

SUMMARY Serum IgG, labelling the stratum corneum of the rat oesophagus epithelium, so‐called anti‐keratin antibodies (AKA) constitute the most specific marker for the diagnosis of rheumatoid arthritis. In this study, we investigated 31 IgG AKA‐positive rheumatoid sera and 21 control sera from patients with non‐rheumatoid inflammatory rheumatic diseases. The serum level of IgG 1 ,2,3 and 4 was determined by radial immunodiffusion and the subclass distribution of IgG AKA by a three‐step semi‐quantitative immunofluorescence assay using standard monoclonal antibodies specific for each of the four human IgG subclasses. In the rheumatoid sera, the serum level of IgGl was found to be significantly increased and the level of IgG2 significantly decreased with regard to the control sera, while the levels of IgG3 and 4 as well as total IgG were in the normal range. IgGl,2,3, and 4 AKA were detected in 27 (87%), 6 (19%), 4 (13%) and II (35%) of the 31 rheumatoid sera, respectively, and were found to be independent of the clinical and biological indices of the disease. In spite of inter‐individual heterogeneity, two predominant profiles were distinguished: IgGI (alone) and lgG(I +4), which together represented 18 sera (58%). The large predominance of IgGI AKA and the quasiabsence of IgG2 AKA suggest that the recognized antigen may be partly comprised of protein. Moreover, the high frequency of occurrence of IgG4 AKA might result from chronic exposure to the eliciting antigen, which could be a genuine autoantigen since we demonstrated that it is also present in the stratum corneum of human epidermis.