Insulin-dependent (type 1) diabetes mellitus is an organ-specific autoimmune disease frequently associated with an islet-specific humoral autoimmune response. The role of islet cell autoantibodies in the disease process is unclear; in particular, it is not known whether they are a non-specific side effect of islet cell destruction or play a role in the autoimmune network leading to type 1 diabetes. Here we report the immunoglobulin gene usage and somatic mutation rates of a panel of seven human monoclonal islet cell autoantibodies (MICA 1-7) directed towards the major islet cell autoantigen glutamate decarboxylase (GAD). These autoantibodies were produced from cells from two patients with newly diagnosed type 1 diabetes. VH1, VH4 and V lambda 2 gene segments were frequently used in the MICA, but no correlation between V gene usage and epitope recognition was found. The nonrandom ratio of replacement versus silent mutations in the variable gene region, an accumulation of replacement mutations in the complementarity determining regions, which confer antigen binding, and the high relative avidity for GAD observed for MICA 1, 3, 4, and 6, suggested that the immune response to GAD is driven by the antigen. In contrast, MICA 2, 5, and 7, revealing a lower affinity for antigen, have accumulated a large number of silent mutations. These latter antibodies may, therefore, be characteristic for later stages of the chronic autoimmune disease. Our results argue in favor of an antigen-driven autoantibody response to islets in human type 1 diabetes. They suggest that GAD is an important target of autoimmunity associated with type 1 diabetes.
