Islet cell antibodies: variable immunostaining of pancreatic islet cells and carcinoid tissue

Islet cell antibodies (ICA) in sera of patients with autoimmune diabetes mellitus generally stain not only the insulin-producing beta cells but also the non-beta cells of the islets of Langerhans. The antibodies have been reported to react also with the chromaffin cells of carcinoid tissue. In the present study, we examined in detail the reactivity of 10 ICA-positive sera of patients with new onset insulin-dependent diabetes mellitus (IDDM) and two sera of patients with stiff-man syndrome.

The sera were analysed by immunofluorescence and by immunoperoxidase staining of human islets as well as by immunoprecipitations using 35S-methionine labelled rat islet lysates. In addition, immunofluorescence analyses of carcinoid tissues were carried out.

Eight of the 10 IDDM-positive sera reacted with all islet endocrine cells, whereas two sera showed staining restricted to the beta cells, as did the two sera of the patients with stiff-man syndrome. All beta cell 'selective' sera, but only 6 of 8 'whole' islet positive sera, immunoprecipitated the 64.kDa glutamic acid decarboxylase (GAD) antigen. The staining of carcinoid tissue was variable and did not correlate with the 'whole' or 'selective' staining pattern of islets.

The data underline a heterogeneity of ICA and indicate the presence of a separate, non-GAD antigen in islet cells. It is possible that in future studies, a resolution of ICA titres with respect to different types of islet cellular reactivity might provide insights into the pathogenesis of IDDM and improve the prognostic implications of antibody determinations.