Antibodies to GAD65 epitopes at diagnosis and over the first 10 years of clinical type 1 diabetes mellitus.

Antibodies to glutamate decarboxylase (GAD65Ab) may persist, and their titres even increase after the clinical onset of type 1 diabetes. To characterize this phenomenon in detail, we analysed sequentially antibodies to GAD65 epitope clusters in a radio-binding assay in patients with type 1 diabetes. Serum samples were taken at diagnosis and 2, 5 and 10 years later from 50 young patients who had tested positive for GAD65Ab at least once during observation. The levels of GAD65Ab peaked in 21 patients after diagnosis. Antibodies to the middle region of GAD65 (GAD65-M-Ab, 88%) were more common at diagnosis than antibodies to the C-terminal (GAD65-C-Ab, 68%, P < 0.05) or N-terminal region (4%, P < 0.001). Antibodies to middle and especially to C-terminal epitopes decreased in those with decreasing levels of GAD65Ab (P < 0.001), whereas the frequencies of GAD65-M-Ab and GAD65-C-Ab remained quite stable among the subjects with increasing levels. Lower exogenous insulin dose and HbA(1) levels and stronger humoral immune response to islet cells were observed in those with increasing levels of GAD65-M-Ab than in those with decreasing levels (P < 0.05). The present observation supports the view that the middle region of GAD65 comprises immunodominant epitopes. An enhanced humoral immune response to GAD65 after diagnosis is related to persistent immune reactivity to the middle and C-terminal regions.