The processes that lead to the production of islet cell autoantibodies in insulin-dependent (type 1) diabetes mellitus (IDDM) are largely unknown. Humoral autoimmunity may be the result of an antigen-independent polyclonal B cell activation, or a consequence of an antigen driven B cell activation and selection for the antigen. We have analysed the gene elements encoding the immunoglobulin variable regions of seven human monoclonal islet cell antibodies (MICA) 1-7 directed to the major islet autoantigen glutamate decarboxylase (GAD65). These autoantibodies were derived from two patients with newly diagnosed IDDM. The variable gene regions of the MICA revealed different sequences, and no relation between V gene usage and shared epitope recognition of the MICA was evident. An elevated usage of VH 1, VH 4 and Vlambda 2 gene segments was observed. The underrepresentation of VH 3 family members in the MICA discriminated them from most autoantibodies. The high relative avidities for GAD65 of MICA 1, 3, 4 and 6 and their high, nonrandom ratio of replacement versus silent mutations in the antigen binding regions indicated that the humoral response to GAD65 is driven by the antigen. MICA 2, 5 and 7 showed as well an excess of replacement mutations in the antigen binding regions, but revealed lower relative avidities for their antigen. Since these clones accumulated many somatic mutations in their variable gene regions, they may be characteristic for later stages of the autoimmune disease. The results suggest that, in humans, an antigen driven B cell activation and affinity maturation process may contribute to the production of GAD65-autoantibodies found in patients with IDDM.
