CONTRIBUTION OF GLUTAMATE-DECARBOXYLASE ANTIBODIES TO THE REACTIVITY OF ISLET-CELL CYTOPLASMIC ANTIBODIES

The contribution of glutamate decarboxylase (Mr 65000) antibodies to t he reactivity of islet cell cytoplasmic antibodies with the 'whole' is let staining pattern from patients with newly diagnosed Type I diabete s was investigated. Diluted sera (n=10) were preincubated with increas ing concentrations of purified recombinant human islet glutamate decar boxylase (Mr 65000) and the change in islet cell cytoplasmic antibody binding was evaluated by quantitative immunocytochemistry. Binding to islet cells was partially blocked by glutamate decarboxylase in 9/10 d iluted sera; the maximum blocking obtained at high concentrations of g lutamate decarboxylase (5 mug/ml) was 36% (median, range 24-61%). In c ontrast, binding to islet cells in three diluted sera (two polyendocri ne patients without Type I diabetes and one patient with newly diagnos ed Type I diabetes) with the 'selective' islet staining pattern was to tally blocked by glutamate decarboxylase. The concentration of glutama te decarboxylase required to achieve maximum blocking was less for the 'whole' islet (0.4-8.0 mug/mul undiluted serum) compared to the 'sele ctive' islet (20-645 mug/mul undiluted serum) positive sera. All sera were positive for glutamate decarboxylase antibodies in an immunopreci pitation assay using S-35-methionine labelled extract of baby hamster kidney cells transfected with glutamate decarboxylase. However, the bi nding activity of these antibodies was less in the sera positive for t he 'whole' islet compared to the 'selective' islet staining pattern. I n conclusion, glutamate decarboxylase antibodies contribute partially to the reactivity of islet cell cytoplasmic antibodies of the 'whole' islet staining pattern in the sera of newly diagnosed patients with Ty pe I diabetes, and totally to the reactivity of the 'selective' islet staining pattern. The antigens recognized by the other antibodies cont ributing to the 'whole' islet reactivity remain to be defined.