The NOD/Lt mouse, a widely used model of human autoimmune IDDM, was used to establish the mode of β-cell death responsible for the development of IDDM. Apoptotic cells were present within the islets of Langerhans in hematoxylin and eosin–stained sections of pancreases harvested from 3- to 18-week-old female NOD/Lt mice (a range of 11–50 apoptotic cells per 100 islets). Immunohistochemical localization of insulin to the dying cells confirmed the β-cell origin of the apoptosis. Although some islets from age-matched control female NOD/scid mice contained apoptotic cells, virtually all of these cells were insulin negative as determined by immunohistochemistry. The small number of apoptotic insulin-positive cells identified in islets from NOD/scid mice (a range of 0–1 apoptotic cells per 100 islets) was not statistically significant, compared with the numbers recorded in NOD/Lt mice. All dying cells showed the morphological changes characteristic of cell death by apoptosis and stained positively with the TUNEL method for end-labeling DNA strand breaks. The maximum mean amount of β-cell apoptosis occurring in NOD/Lt mice was at week 15 (50 apoptotic cells per 100 islets), which coincided with the earliest onset of diabetes as determined by blood glucose, urine glucose, and pancreatic immunoreactive insulin measurements. While there was no peak incidence of β-cell apoptosis throughout the time period studied (weeks 3–18), the incidence of apoptosis decreased at week 18, by which time 50% of the animals had overt diabetes. The low levels of β-cell apoptosis observed is indicative of a gradual deletion of the β-cell population throughout the extensive preclinical period seen in this model and would be sufficient to account for the β-cell loss resulting in IDDM. Apoptosis of β-cells preceded the appearance of T-cells (CD3-positive by immunohistochemistry) in islets. Lymphocytic infiltration of islets (insulitis) was not detected until week 6. The results show that β-cell apoptosis is responsible for the development of IDDM in the NOD/Lt mouse and that its onset precedes lymphocytic infiltration of the islets.

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