The role of GAD in the pathogenesis of type I diabetes

Insulin-dependent diabetes mellitus results from the cell-mediated autoimmune destruction of insulin producing pancreatic β cells. The involvement of a variety of β cell antigens has been reported in humans and NOD mice. Among them, glutamic acid decarboxylase (GAD) appears to play a critical role in the initiation and development of disease.; In the present study, the H-AS-GAD mouse in which GAD expression was suppressed was used to develop transgenic mice for the investigation of the role of GAD in the pathogenesis of IDDM. First, H-AS-GAD mice were continuously backcrossed with NOD mice for 12 generations to obtain an NOD genotype. Diabetes did not occur in H-AS-GAD mice in any generation and the prevention of diabetes seemed to be correlated with the inhibition of splenic T cell responses against GAD as well as other β autoantigens such as HSP60 and insulin. Second, to investigate the effect of the suppression of GAD expression on the fully established diabetogenic splenocytes derived from acutely diabetic NOD mice; H-AS-GAD SLID mice were produced by backcrossing H-AS-GAD mice with NOD SCID mice. Diabetogenic splenocytes did not transfer disease into H-AS-GAD SCID mice, while transgene-negative SCID mice became diabetic within 4 weeks after adoptive transfer. These imply that GAD-reactive immune responses may be critical for disease progression. Finally, the significance of GAD was examined in a Super-GAD65 transgenic mouse carrying a sense-GAD65 transgene under the control of the MHC class I promoter for systemic expression of GAD65. To suppress GAD expression in the Super-GAD65 transgenic mouse in a β cell specific manner, H-AS-GAD mice were mated with Super-GAD65 mice and the incidence of diabetes was monitored in Double-transgenic mice expressing both sense GAD65 and anti-sense GAD65/67 transgenes. Immunohistochemical staining showed that the anti-sense-GAD65/67 transgene effectively suppressed GAD expression in the Double-transgenic mice. None of the Double transgenic mice developed diabetes. However, approximately 80% of transgene-negative mice developed diabetes. Thus, β cell-specific GAD expression is necessary for the development of IDDM and GAD-related autoimmunity may be involved in the pathogenic disease process.