Glutamic acid decarboxylase autoantibodies in preclinical insulin-dependent diabetes

Insulin-dependent diabetes mellitus (IDDM) is associated with serum antibodies that precipitate a 64-kDa pancreatic islet cell protein reported to be glutamic acid decarboxylase (GAD; glutamate decarboxylase, EC 4.1.1.15). Previously, antibodies to GAD were found in the rare neurological disorder stiff man syndrome. To demonstrate directly antibodies to GAD, enzymatically active GAD was first purified from fresh human cerebellum. Brain GAD activity was precipitated by noninhibitory antibodies in the sera of 16/26 (62%) subjects defined as having preclinical IDDM (islet cell antibody-positive first-degree relatives of a person with IDDM), 3/13 (23%) with recent-onset IDDM, and 3/3 with the stiff man syndrome. In addition, sera of 5/26 (19%) preclinical and 2/13 (15%) recent-onset IDDM subjects contained antibodies that precipitated GAD but inhibited its activity. Thus, overall, 21/26 (81%) preclinical and 5/13 (38%) recent-onset IDDM subjects had antibodies that precipitated GAD activity. Antibodies to GAD were not detected in sera from subjects with other autoimmune diseases (n = 29) or healthy controls (n = 14). GAD affinity-purified to homogeneity (specific activity, 58 units/mg) was specifically immunoprecipitated as a single 60-kDa species by the IDDM sera. In an ELISA incorporating whole mouse brain GAD captured by the GAD-6 monoclonal antibody the frequencies of GAD antibodies for all subject groups were indistinguishable from those found by precipitation of human brain enzymatic activity. We conclude that (i) GAD is an (auto)antigen in a majority of subjects operationally defined as having preclinical IDDM, (ii) pancreatic islet and brain GAD are likely to be cross-reactive, and (iii) the majority of GAD antibodies are directed away from the catalytic site of the brain enzyme. The lower frequency of GAD antibodies in recent-onset IDDM subjects indicates either that immunoreactivity is lost with near-total beta-cell destruction or that GAD antibodies denote a low risk of progression to clinical disease.