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OBJECTIVE--To study how tolerance to GAD65 affects the development of autoimmunity to other β-cell autoantigens (β-CAAs) in GAD654ransgenic (GAD-tg) NOD mice. RESEARCH DESIGN AND METHODS--We used ELISPOT to characterize the frequency and functional phenotype of T-cell responses to GAD65 and other β-CAAs at different ages in GAD-tg mice and their NOD mouse littermates. RESULTS--In young GAD-tg mice, Th1 responses to GAD65's dominant determinants were 13-18% of those in young NOD mice. This coincided with a great reduction in Th1 responses to other β-CAAs. Evidently, GAD65-reactive T-cells are important for activating and/or expanding early autoreactivities in NOD mice. As GAD-tg mice aged, their T-cell responses to GAD65 remained low, but they developed supernormal splenic and pancreatic lymph node T-cell autoimmunity to other β-CAAs. Apparently, the elimination/impairment of many GAD65-reactive T-cells allowed other β-CAA-reactive T-cells to eventually expand to a greater extent, perhaps by reducing competition for antigen-presenting cells, or homeostatic proliferation in the target tissue, which may explain the GAD-tg mouse's usual disease incidence. CONCLUSIONS--Transgenically induced reduction of GAD65 autoreactivity curtailed the development of early T-cell responses to other β-CAAs. However, later in life, β-CAA-reactive T-cells expanded to supernormal levels. These data suggest that early p-cell autoreactivities are mutually dependent for support to activate and expand, while later in the disease process, autoantigen-specific T-cell pools can expand autonomously. These findings have implications for understanding type 1 diabetes immunopathogenesis and for designing antigen-based immunotherapeutics., The role of T-cell autoreactivity to the 65-kDa form of GAD (GAD65) in the etiology of type 1 diabetes in nonobese diabetic (NOD) mice has been much debated, in large [...]