We investigated the morphology of mouse islets 5 days after completion of low-dose streptozocin treatment of C57BL/6 mice by electron microscopy. At this stage, mice were still normoglycemic and light microscopy did not reveal massive islet infiltration. The electron-microscopic investigation revealed two characteristics indicative of ongoing islet cell destruction. In all islets investigated, lysed islet beta-cells were recognized by disrupted plasma membranes and concomitantly decreased plasma contrast. Many of the lysed islet beta-cells still contained numerous insulin granules. We also found immunocytes scattered throughout the islets, most of which could be identified as macrophages. Some were found engaged in phagocytosis of islet beta-cell debris. This early stage of islet lesion termed single-cell insulitis is followed by the well-known later stage of massive infiltration easily recognized in light microscopy. Administration of silica particles to mice treated with low-dose streptozocin inhibited macrophage infiltration of islets as shown by immunocytochemistry with macrophage-specific monoclonal antibody F4/80. In parallel, the development of hyperglycemia was suppressed. The observations favor a pathogenic role of macrophages in islet destruction.

Administration of silica to young BB-rats almost completely prevented the development of spontaneous diabetes. Only 1 of 31 silica-treated rats developed hyperglycemia, whereas 9 of 31 in the untreated group did so. Since silica is highly specific in its action against macrophages, our observations indicate an important role of these cells in the pathogenesis of the disease.

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