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

Clin Exp Immunol. 1999 Dec;118(3):349-56. doi: 10.1046/j.1365-2249.1999.01030.x.


Autoantibodies to GAD, an important marker of the autoimmune process in type I or insulin-dependent diabetes mellitus (IDDM), are also found in non-diabetic individuals with autoimmune polyendocrine syndrome type 1 (APS1), APS2, and stiff man syndrome (SMS). Most IDDM sera contain two distinct GAD antibody 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 GAD65/GAD67 proteins to maintain conformation-dependent epitopes of GAD65, we 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-terminal two-thirds of GAD65. Amino acids 270-359 (IDDM-E1) are targeted by one 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 that 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 amino 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 in the humoral response to GAD65 in APS2 and IDDM.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Antibodies, Monoclonal / immunology
  • Antibody Specificity / immunology
  • Autoantibodies / immunology*
  • Diabetes Mellitus, Type 1 / immunology*
  • Epitopes / immunology*
  • Glutamate Decarboxylase / genetics
  • Glutamate Decarboxylase / immunology*
  • Humans
  • Immunoglobulin G / immunology
  • Isoenzymes / genetics
  • Isoenzymes / immunology*
  • Molecular Sequence Data
  • Polyendocrinopathies, Autoimmune / immunology*
  • Precipitin Tests
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / immunology
  • Sequence Homology, Amino Acid


  • Antibodies, Monoclonal
  • Autoantibodies
  • Epitopes
  • Immunoglobulin G
  • Isoenzymes
  • Recombinant Fusion Proteins
  • Glutamate Decarboxylase
  • glutamate decarboxylase 1
  • glutamate decarboxylase 2