GAD65 antibody isotypes and epitope recognition during the prediabetic process in siblings of children with type I diabetes
We observed 42 initially non‐diabetic siblings of affected children to characterize the humoral immune response to the 65 kDa isoform of glutamic acid decarboxylase (GAD65) in preclinical type I diabetes. During the observation period with a mean duration of 9·6 years 21 siblings progressed to type I diabetes. The humoral immune response to GAD65 was observed initially as a simultaneous response to the middle (M) and carboxy (C)‐terminal regions of the GAD65 molecule in most cases, and if the response was restricted initially to the middle region, it spread rapidly to the C‐terminal domain and in a few cases later to the amino (N)‐terminal domain. There was some heterogeneity in the GAD65 isotype response, but it was composed mainly of antibodies of immunoglobulin (Ig) G1 subclass. Responses of IgG2‐, IgG4‐, IgM‐ and IgA‐GAD65Ab were observed frequently, whereas IgE‐ and IgG3‐GAD65Ab responses were seen more rarely. Initially, the non‐progressors tended more often to have IgG2‐ and IgG4‐GAD65Ab than the progressors. As a sign of a dynamic process a significant isotype spreading was seen for IgG2‐GAD65Ab (P < 0·05) and close to significant for IgM (P = 0·06) among progressors and for IgM‐GAD65Ab (P < 0·05) among non‐progressors during the observation period. This study failed to identify any GAD65 epitope‐ or isotype‐specific antibody reactivity that could be used as a marker for progression to disease, as such progression was not associated with any specific changes in reactivity over time. Our findings indicate that epitope‐ and isotype‐specific GAD65 antibodies are hardly capable of separating progressors from non‐progressors among GAD65Ab‐positive first‐degree relatives of children with type I diabetes.