HUMAN AUTOANTIBODIES REACT WITH GLUTAMIC-ACID DECARBOXYLASE ANTIGEN IN HUMAN AND RAT BUT NOT IN MOUSE PANCREATIC-ISLETS

The presence of one of the major targets for autoantibodies in Type 1 (insulin-dependent) diabetes mellitus, the enzyme glutamic acid decarboxylase, was studied in human, rat and mouse pancreatic tissue using immunoprecipitation and immunohistochemical techniques. Immunoprecipitation of glutamic acid decarboxylase was attempted with lysates of [S-35]-methionine-labelled rat or mouse pancreatic islets using two different glutamic acid decarboxylase antisera, one mouse monoclonal antibody raised against the 65 kDa isoform of the enzyme, sera from six patients with Type 1 diabetes, one patient with stiff-man syndrome and sera from 19 non-obese diabetic mice. The same sera were used for immunoperoxidase staining of cryosections of human, rat or mouse pancreas. Using patient sera glutamic acid decarboxylase was detected by immunoprecipitations from isolated rat islets but not from islets of five different mouse strains tested, including the non-obese diabetic mouse. When using the non-obese diabetic mouse sera, glutamic acid decarboxylase could not be detected in either rat or mouse tissue. Immunoperoxidase staining demonstrated high levels of glutamic acid decarboxylase in human and rat pancreatic islets but low levels in mouse islets. Direct measurements of enzyme activity showed glutamic acid decarboxylase to be present in mouse islets at a level of about 40 % of that in rat islets, and subsequent Western blot analyses indicated that mouse islets express the 67 kDa isoform, whereas in rat islets both the 67 and 65 kDa isoforms are present. The species difference at the level of one of the major islet cell autoantigens in Type 1 diabetes thus indicates that human or rat and mouse islets, respectively, express immunologically distinct forms of glutamic acid decarboxylase and differ in their way of regulating the enzyme production.