A MONOCLONAL ANTIBODY-BASED CHARACTERIZATION OF AUTOANTIBODIES AGAINST GLUTAMIC-ACID DECARBOXYLASE IN ADULTS WITH LATENT AUTOIMMUNE DIABETES

Autoantibodies to glutamic acid decarboxylase (GAD) are an important m arker of the autoimmune-mediated beta-cell destruction in insulin-depe ndent (Type I) diabetes. However, these autoantibodies are also found in patients with Stiff-man syndrome (SMS) without onset of diabetes an d some diabetic patients who initially present as non-insulin dependen t (Type II) diabetes later becoming insulin-dependent, called as laten t autoimmune diabetes in adults (LADA), To study the immune response t o GAD in these LADA patients a competitive radiobinding assay based on murine monoclonal antibodies recognizing three different GAD regions was performed. The monoclonal antibodies against GAD recognize two dif ferent linear epitopes localized at the N-(amino acids 4-17) and C-ter minus (amino acids 572-585) and one conformation-dependent epitope reg ion (amino acids 221-442 IDDM-E1) known to be immunodominant for diabe tes-associated autoantibodies. All LADA sera (20/20) reduced substanti ally the I-125-GAD binding of the monoclonal antibodies reactive with the conformation-dependent epitope region IDDM-E1 and only 20% of thes e sera additionally diminished the I-125-GAD65 binding by those monocl onals reactive with the both linear epitopes, The SMS sera completely abolished the GAD binding of all three monoclonals, reflecting a broad er repertoire including an immune response against the IDDM-E1, a conf ormation-dependent GAD65 epitope region, also revealed if the SMS sera are diluted to equivalent antibody concentrations. In summary, our re sults show that diabetes-associated GAD autoantibodies even in adult p atients with a late autoimmune process preferentially recognize a conf ormation-dependent middle GAD65 region. An immune response to all thre e GAD epitope regions is seldom in these LADA patients and only detect able in association with high antibody titres.