Functional roles of Fas and Bcl-2-regulated apoptosis of T lymphocytes.

Apoptotic cell death is an important mechanism for maintaining homeostasis in the immune system and for regulating the fates of lymphocytes following encounters with self and foreign Ags. To study the physiologic roles of the proapoptotic Fas pathway and the antiapoptotic protein, Bcl-2, in T cell maturation and homeostasis, a TCR transgene has been bred into mice lacking functional Fas and mice that express Bcl-2 constitutively. In vitro, Fas-deficient T cells are resistant to activation-induced cell death, whereas Bcl-2-overexpressing T cells are resistant to death induced by withdrawal of growth factors. In vivo, Bcl-2-overexpressing mice accumulate T cells in the thymus and peripheral lymphoid tissues in the absence of Ag, but these cells are deleted normally after Ag administration. In contrast, Fas-deficient mature T cells are present in normal numbers in the absence of Ag, but are resistant to Ag-induced deletion. Both Fas-deficient and Bcl-2 overexpressing thymocytes are deleted when exposed to transgene-encoded circulating self Ag, indicating that the pathways of apoptosis controlled by these proteins are not critical for negative selection of developing thymocytes. Moreover, deficiency of Fas, but not Bcl-2 overexpression, results in the accumulation of autoreactive T cells in peripheral lymphoid tissues. These results demonstrate that Fas and Bcl-2 regulate different pathways of apoptosis that may serve distinct functions in lymphocyte homeostasis and in the maintenance of T cell tolerance.