GAD AUTOANTIBODIES IN IDDM, STIFF-MAN SYNDROME, AND AUTOIMMUNE POLYENDOCRINE SYNDROME TYPE-I RECOGNIZE DIFFERENT EPITOPES

Glutamic acid decarboxylase (GAD) is a major islet cell autoantigen in insulin-dependent diabetes mellitus (IDDM), and autoantibodies are fo und in high frequencies in patients with recent-onset IDDM, stiff-man syndrome (SMS), and autoimmune polyendocrine syndrome type I (APS I). Antigens in autoimmune disorders are often enzymes, and autoantibody b inding frequently inhibit their activity. In this study, we examined t he reactivity of anti-GAD-containing sera from 7 patients with IDDM, 4 patients with SMS, and 5 patients with APS I. All sera immunoprecipit ated GAD from [S-35]methionine-labeled rat islet lysates and the sera from patients with SMS and APS I, but none of the IDDM patients' sera, identified the GAD protein in Western blots. Two of four SMS patients ' sera and 5 of 5 APS I patients' sera, in contrast to 0 of 7 IDDM pat ients' sera, inhibited the enzymatic activity of GAD. When the various sera were tested with the GAD(65) and GAD(67) isoforms, produced sepa rately by transient expression in COS cells, the enzymatic activity of GAD(65) was inhibited by sera from patients with SMS and APS I, where as no effect on the GAD(67) activity was observed. Taken together, the results demonstrate that the GAD autoantibodies in these three disord ers display marked differences in epitope recognition and indicate tha t, during the development of the diseases, the autoantigen is being pr esented to the immune system through separate pathogenetic mechanisms.