ABSENCE OF AUTOANTIBODIES AGAINST GLUTAMATE-DECARBOXYLASE (GAD) IN THE NONOBESE DIABETIC (NOD) MOUSE AND LOW EXPRESSION OF THE ENZYME IN MOUSE ISLETS

GAD is a major islet cell autoantigen in human type 1 diabetes mellitu s. Autoantibodies are preferentially directed against the 65-kD isofor m of the enzyme which is the only form expressed in human islets of La ngerhans. The NOD mouse is a spontaneous model of type 1 diabetes, fre quently employed in studies dealing with the immunopathogenesis of the disease. In the present study the reactivity of sera from 34 prediabe tic and 15 diabetic NOD mice was tested against GAD protein present in islets of Langerhans and cerebellum, and against recombinant, semi-pu rified GAD-65 and GAD-67. A rabbit antiserum (K2) raised against GAD-6 7 could readily recognize the recombinant GAD-67 and the isoform prese nt in rat and mouse islets and mouse brain. A MoAb (GAD-6) specific fo r the GAD-65 isoform reacted against the recombinant GAD-65 and the is oform present in rat islets and mouse brain, whereas no reactivity was observed when using mouse islets. However, when testing the NOD mice sera by immunohistochemistry, immunoprecipitation and Western blot, no reactivity against any of the isoforms of GAD could be detected. Usin g reverse transcription polymerase chain reaction (PCR), GAD-67 mRNA c ould be detected in mouse and rat islets and in mouse brain. GAD-65 mR NA could also be detected in rat islets and mouse brain, but apparentl y a much lower copy number is present in mouse islets. These findings stress important differences in the immune response occurring in the a nimal model NOD mouse compared with human type 1 diabetes, and emphasi ze that human and animal type 1 diabetes possibly represent the final outcome of several different etiological factors.