BRAIN-REACTIVE AUTOANTIBODIES IN BB D RATS DO NOT RECOGNIZE GLUTAMIC-ACID DECARBOXYLASE/

The BB rat spontaneously develops insulin-dependent diabetes mellitus (IDDM) similar to that in humans. The most practical markers of beta c ell autoimmunity are circulating antibodies to islet cell components. In particular autoantibodies to the enzyme glutamic acid decarboxylase (GAD) are a common feature of IDDM development in humans. This study aims at investigating the prevalence and levels of autoantibodies in B E rats to antigens in a semipurified, GAD-enriched preparation from ra t brain. Eighteen diabetes-prone BB/d rats (10 male and eight female) were tail bled weekly from age 28 days to 113 days and antibodies dete cted on the rat brain preparation by ELISA. Antibody levels were expre ssed as arbitrary units relative to a standard positive serum. Individ ual rats varied in the time and order of antibody appearance and IDDM onset, with the earliest occurrence being 42 days and 69 days, respect ively. In some rats antibody production was maintained but declined in others. By 113 days 85% of diabetic rats had at some time been positi ve for autoantibodies to brain components, compared with 25% of non-di abetics (P=0.09 by Fisher's exact test). Immunoabsorption studies usin g recombinant rat GAD-65 or recombinant human GAD-67 failed to inhibit the binding of BB rat sera to the original rat brain preparation. A c apture ELISA using GAD-6 MoAb to capture GAD-65 from rat brain prepara tion or from a preparation of recombinant rat GAD-65, failed to detect anti-GAD antibodies in BE rats. Immunofluorescent staining of tissue sections showed the autoantibodies to be brain-specific, but having di stinct staining patterns to the anti-GAD antibodies of Stiff Man Syndr ome serum. In conclusion, BB rats possess autoantibodies reactive with rat brain antigens which may be associated with IDDM. However, these are not directed against GAD.