Summary Background The immune response to an isoform of glutamic acid decarboxylase (GAD), GAD65, is associated with two clinically distinct diseases, stiff-man syndrome (SMS) and type 1 (insulin-dependent) diabetes mellitus. We sought to identify differences in the cellular and humoral immune responses to GAD in these two diseases. Methods We compared T-cell responses in 14 SMS patients with axial disease and 17 patients with type 1 diabetes. Findings Peripheral blood T cells of eight SMS patients recognised different immunodominant epitopes of GAD65 compared with T cells from 17 patients with type 1 diabetes. GAD regions 81–171 and 313–403 induced a dominant T-cell response in six of eight patients with SMS but in only one of 17 patients with type 1 diabetes (p=0·001). No SMS patients responded dominantly to GAD fragments 161–243 and 473–555 compared with ten patients with type 1 diabetes (p=0·008). GAD antibodies were detected in 11 of 14 SMS patients (seven with diabetes) and 11 of 17 patients with type 1 diabetes; IgG1 was dominant in both groups. SMS patients, however, were more likely than patients with diabetes to have isotypes other than IgG1 (p=0·03), in particular, IgG4 or IgE isotypes, which were not detected in patients with type 1 diabetes (p=0·012). Interpretation Our findings indicate differences between patients with SMS and type 1 diabetes in cellular (epitope recognition) and humoral (isotype pattern) responses to GAD65. Thus the same autoantigen can elicit distinct immune responses in patients with SMS, even when associated with diabetes, compared with patients with type 1 diabetes.
