Insulitis occurs by 5 wk of age in all NOD mice. However, diabetes is detectable only after 3–5 mo of age and only in ∼50% of females and 10% of males in our colony. Therefore, it is predictable that changes in the T-lymphocyte repertoire of diabetes-prone mice occur and predispose them to disease. We demonstrate here that an altered (with respect to control BALB/cJ mice) thymic T-lymphocyte maturation reflected by a depletion (∼12%) of CD4+CD8+ T lymphocytes and a reciprocal increase in CD4CD8 T lymphocytes precedes the onset of diabetes. This depletion was detected only ∼3 mo after insulitis and is manifested by a specific loss (∼3%) of immature T lymphocytes bearing Vβ8lo (lo is a relative level of expression) T-lymphocyte receptor. By onset of diabetes, an even greater decrease (∼35%) of CD4+CD8+ and a reciprocal increase of CD4CD8 T lymphocytes were apparent and accompanied by the same depletion (3%) of Vβ8lo T lymphocytes. Administration of cyclophosphamide (CY), which accelerates the appearance of diabetes in NOD mice, caused similar depletions of CD4+CD8+ and Vβ8lo thymic T lymphocytes. The same alterations in the distribution of these thymic T-lymphocyte subsets were evident even earlier in insulitis- and diabetes-free NON mice, indicating that these changes in thymic T-lymphocyte development may be necessary but not sufficient to give rise to diabetes. Despite the common genetic origin of NOD and NON mice, differences at their MHC-linked and -unlinked loci may account for their differential susceptibility to diabetes. Analyses of peripheral lymph node (LN) T lymphocytes showed a decrease (6–10%) in the frequency of the CD4+ T-lymphocyte subset and a concomitant reduction (3–4%) in CD4+Vβ8+ T lymphocytes in spontaneously and CY-induced diabetic NOD mice. Interestingly, the latter reduction resulted primarily from a depletion of CD4+Vβ8.1+ LN T lymphocytes in diabetic mice and was not detectable either in prediabetic NOD mice at 16 wk of age or in nondiabetic NON mice. These data suggest that depletion of CD4+ regulatory T lymphocytes and/or the rerouting of CD4+Vβ8.1+ effector T lymphocytes from the peripheral LN to the pancreas during progression to disease onset mediate the pathogenesis of diabetes.

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