Comparative analysis of epitope recognition of glutamic acid decarboxylase(GAD) by autoantibodies from different autoimmune disorders

Autoantibodies to GAD, an important marker of the autoimmune process in typ e I or insulin-dependent diabetes mellitus (IDDM), are also found in non-di abetic individuals with autoimmune polyendocrine syndrome type 1 (APS1), AP S2, and stiff man syndrome (SMS). Most IDDM sera contain two distinct GAD a ntibody specificities, one of which targets an epitope region in the middle -third of GAD65 (IDDM-E1; amino acids 221-359) and one of which targets the carboxy-third of GAD65 (IDDM-E2; amino acids 453-569). Using 11 chimeric G AD65/GAD67 proteins to maintain conformation-dependent epitopes of GAD65, w e compared the humoral repertoire of IgG antibodies from an individual with APS2-like disease (b35, b78, and b96) and MoAbs from an IDDM patient (MICA -2, MICA-3, and MICA-4). Neither the APS2 IgG antibodies nor the IDDM MoAbs bind the amino-terminal third of GAD65, but instead target the carboxy-ter minal two-thirds of GAD65. Amino acids 270-359 (IDDM-E1) are targeted by on e APS2 IgG antibody and MICA-4, while two other APS2 IgG antibodies, MICA-2 and MICA-3, target amino acids 443-585 (IDDM-E2). Using GAD65/67 chimera t hat span the IDDM-E2 region, we found that MICA-2 binds amino acids 514-528 of GAD65, but two APS2 IgG antibodies require this region and amino acids 529-570. In contrast, the binding of MICA-3 requires two discontinuous amin o acid segments of GAD65 (452-513 and 528-569), but not amino acids 514-528 . These results indicate that there are both similarities and differences i n the humoral response to GAD65 in APS2 and IDDM.