Autoantibodies to glutamic acid decarboxylase (GAD) are an important marker of 
the autoimmune-mediated beta-cell destruction in insulin-dependent (Type I) 
diabetes. However, these autoantibodies are also found in patients with 
Stiff-man syndrome (SMS) without onset of diabetes and some diabetic patients 
who initially present as non-insulin dependent (Type II) diabetes later becoming 
insulin-dependent, called as latent autoimmune diabetes in adults (LADA). To 
study the immune response to 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 different linear epitopes localized at the N- (amino acids 4-17) 
and C-terminus (amino acids 572-585) and one conformation-dependent epitope 
region (amino acids 221-442 IDDM-E1) known to be immunodominant for 
diabetes-associated autoantibodies. All LADA sera (20/20) reduced substantially 
the 125I-GAD binding of the monoclonal antibodies reactive with the 
conformation-dependent epitope region IDDM-E1 and only 20% of these sera 
additionally diminished the 125I-GAD65 binding by those monoclonals reactive 
with the both linear epitopes. The SMS sera completely abolished the GAD binding 
of all three monoclonals, reflecting a broader repertoire including an immune 
response against the IDDM-E1, a conformation-dependent GAD65 epitope region, 
also revealed if the SMS sera are diluted to equivalent antibody concentrations. 
In summary, our results show that diabetes-associated GAD autoantibodies even in 
adult patients with a late autoimmune process preferentially recognize a 
conformation-dependent middle GAD65 region. An immune response to all three GAD 
epitope regions is seldom in these LADA patients and only detectable in 
association with high antibody titres.