Low-Dose Streptozocin-Induced Diabetes in Mice: Electron Microscopy Reveals Single-Cell Insulitis Before Diabetes Onset

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 β-cells were recognized by disrupted plasma membranes and concomitantly decreased plasma contrast. Many of the lysed islet β-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 β-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.