Autoantibodies to inner ear and endothelial antigens in Cogan's syndrome

Cogan's syndrome is a chronic inflammatory disease of unknown origin, characterised by sensorineural hearing loss, episcleritis, and vasculitis. An autoimmune origin has been suggested but not proven. Our aim was to establish whether or not an autoimmune process is the cause of the disease. We used pooled IgG immunoglobulins derived from eight patients with Cogan's syndrome to screen a random peptide library to identify disease relevant autoantigen peptides. Among the identified peptides, one was recognised by all the patients’ sera. Antibodies against peptides were affinity purified from patients’ sera and used to characterise the autoantigen, to stain human cochlea, and to transfer the features of Cogan's disease into animals. We identified an immunodominant peptide that shows similarity with autoantigens such as SSA/Ro and with the reovirus III major core protein lambda 1. The peptide sequence shows similarity also with the cell-density enhanced protein tyrosine phosphatase-1 (DEP-1/CD148), which is expressed on the sensory epithelia of the inner ear and on endothelial cells. IgG antibodies against the peptide, purified from the patients’ sera, recognised autoantigens and DEP-1/CD148 protein, bound human cochlea, and inhibited proliferation of cells expressing DEP-1/CD148. The same antibodies bound connexin 26, gene mutations of which lead to congenital inner-ear deafness. Furthermore, these antibodies were able to induce the features of Cogan's disease in mice. Our results indicate that Cogan's syndrome is an autoimmune disease, characterised by the presence of autoantibodies able to induce tissue damage on binding of cell-surface molecules present on the sensory epithelia of the inner ear and on endothelial cells.