Transforming growth factor (TGF)-beta1, a potent immunoregulatory molecule,
was found to control the life and death decisions of T lymphocytes. Both t
hymic and peripheral T cell apoptosis was increased in mice lacking TGF-bet
a1 (TGF-beta1(-/-)) compared with wild-type littermates. Engagement of the
T cell receptor enhanced this aberrant T cell apoptosis, as did signaling t
hrough either the death receptor Fas or the tumor necrosis factor alpha rec
eptor in peripheral T cells. Strikingly, TGF-beta was localized within the
mitochondria of normal T cells, and the absence of TGF-beta1 resulted in di
sruption of mitochondrial membrane potential (Delta Psi (m)), which marks t
he point of no return in a cell condemned to die. This TGF-beta -dependent
regulation of viability appears dissociable from the TGF-beta1 membrane rec
eptor-Smad3 signaling pathway, but associated with a mitochondrial antiapop
totic protein Bcl-XL. Thus, TGF-beta1 may protect T cells at multiple sites
in the death pathway, particularly by maintaining the essential integrity
of mitochondria. These findings may have broad implications not only for T
cell selection and death in immune responses and in the generation of toler
ance, but also for defining the mechanisms of programmed cell death in gene
ral.