Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease whose notorious pathologic feature is insulitis accompanied by destruction of β-cells. In this morphological study, we examined the pancreatic events during the onset of diabetes in spontaneously diabetic BB/Organon rats. Dendritic cells were the first cells to accumulate around the islets, followed by lymphocytes. Scavenger macrophages and MHC class II-positive β-cells were only seen late in the disease. These observations suggest a role for antigen-presenting dendritic cells in the onset of the β-cell-specific autoimmune reaction and emphasize the necessity to distinguish between the several monocyte-macrophage subtypes in studies on the pathogenesis of IDDM.

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