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Exercise interventions alone may not reverse the long-term sequelae of breast cancer treatment and allow young adult patients to return to work.

The thymus expresses proinsulin, among many other tissue-specific antigens, and the inheritance of genetically determined low thymic proinsulin expression has been associated with impaired proinsulin-specific autoreactive T-cell tolerance and type 1 diabetes susceptibility. The cellular and molecular biology of proinsulin expression in the thymus remains unknown, and contradictory reports exist regarding the identity of proinsulin-producing cells. Using knock-in mice expressing ␤-galactosidase (␤-Gal) under the control of an endogenous insulin promoter, we found that thymic proinsulin and ␤-Gal transcripts were detectable at high levels in purified thymic epithelial cells. Immunohistochemical analysis of ␤-Gal activity showed that most proinsulin expression can be accounted for by rare medullary epithelial cells of the Hassall's corpuscles. Moreover, flow cytometry analyses of ␤-Gal-positive cells showed that only 1-3% of all epithelial cells express proinsulin, and this technique will now provide us with a method for isolating the proinsulin-producing cells in mouse thymus. Diabetes 53: 354 -359, 2004 T ype 1 diabetes results from autoimmune destruction of the insulin-producing pancreatic ␤-cells by autoreactive T-cells (1). The inherited immunologic defects responsible for this disease are polygenic in nature. Besides the major histocompatibility complex (HLA) on chromosome 6p21 (2-4), one of the few genetic loci that have been confirmed and functionally studied maps to the insulin gene regulatory region, containing a variable number of tandem repeats polymorphism. Variable number of tandem repeats alleles have little effect on pancreatic insulin expression, but in the thymus, the predisposing class I alleles correlate with low and the protective class III alleles with high levels of insulin expression (5,6). Low expression of insulin in the thymus of NOD mice has been reported (7) and may play a role in diabetes susceptibility. More important, we recently showed that mice with thymus-restricted insulin deficiency showed a strong peripheral proinsulin-specific T-cell reactivity when compared with control animals (8). This corroborates our hypothesis (5) that genetically determined low thymic insulin expression levels predispose to human type 1 diabetes through less efficient insulinspecific autoreactive T-cell selection. In addition, it has recently been shown that NOD mice with drastically reduced insulin expression in the thymus present an accelerated type 1 diabetes development (9).The thymus is a central lymphoid organ responsible for the generation of the T-cell repertoire, during which those T-cells that recognize self-antigens must be eliminated during T-cell development in the thymus by negative selection through clonal deletion (10 -13). The importance of negative selection for tolerance to tissue-specific autoantigens such as insulin had, until recently, been in doubt. Such antigens were traditionally thought to be unavailable for presentation in the thymus, and tolerance to them was believed to o...