CD4 ؉ CD25 ؉ regulatory T cells are essential in the protection from organ-specific autoimmune diseases. In the pancreas, they inhibit actions of autoreactive T cells and thereby prevent diabetes progression. The signals that control the generation, the maintenance, or the expansion of regulatory T cell pool in vivo remain poorly understood. Here we show that a transient pulse of transforming growth factor ␤ (TGF-␤) in the islets during the priming phase of diabetes is sufficient to inhibit disease onset by promoting the expansion of intraislet CD4 ؉ CD25 ؉ T cell pool. Approximately 40 -50% of intraislet CD4 ؉ T cells expressed the CD25 marker and exhibited characteristics of regulatory T cells including small size, high level of intracellular CTLA-4, expression of Foxp3, and transfer of protection against diabetes. Results from in vivo incorporation of BrdUrd revealed that the generation of a high frequency of regulatory T cells in the islets is due to in situ expansion upon TGF-␤ expression. Thus, these findings demonstrate a previously uncharacterized mechanism by which TGF-␤ inhibits autoimmune diseases via regulation of the size of the CD4 ؉ CD25 ؉ regulatory T cell pool in vivo.T ype I diabetes is an autoimmune disease that results from the failure of tolerance to beta-cell antigens (1). The mechanisms that have evolved to ensure discrimination between self and nonself are highly complex and not foolproof. Models of passive tolerance, such as thymic deletion of autoreactive T cells or nonreponsiveness in the periphery because of anergy or ignorance, cannot account for the presence of autoreactive T cells in healthy individuals despite the absence of the development of organ-specific autoimmune diseases. T cells endowed with suppressive function to control actions of autoreactive T cells were described decades ago and were thought originally to be a specialized T cell population the effect of which would be mediated by secreted antigen-specific factors (2). Nowadays, suppressor T cells (also referred to as regulatory T cells) are delineated into two cell subsets of natural regulatory (CD4 ϩ CD25 ϩ ) cells that emerge from the thymus (3, 4) and adaptive regulatory (CD4 ϩ CD25 Ϫ ) cells induced in the periphery to develop suppressive activity (5-7). However, this concept of dichotomous thymic CD25 ϩ versus adaptive CD25 Ϫ regulatory T cells has been challenged by several reports, supporting evidence for the peripheral generation of CD25 ϩ regulatory T cells in vivo and in vitro (8)(9)(10)(11)(12). The finding that suppressive functions are instructively programmed by the expression of Foxp3 finally provided the basis for integrating a unified model of regulatory T cell diversity (13-15). Forced expression of Foxp3 in CD4 ϩ CD25 Ϫ nonregulatory T cells, either by retroviral expression or in transgenic mice, showed acquisition of suppressive activity in vitro and inhibition of disease in vivo, inducing in a substantial proportion of Foxp3-bearing cells the expression of CD25 and GITR markers indicating that ex...

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