Role of T-bet in Commitment of T H 1 Cells Before IL-12-Dependent Selection

How cytokines control differentiation of helper T (T H ) cells is controversial. We show that T-bet, without apparent assistance from interleukin 12 (IL-12)/STAT4, specifies T H 1 effector fate by targeting chromatin remodeling to individual interferon-γ (IFN-γ) alleles and by inducing IL-12 receptor β2 expression. Subsequently, it appears that IL-12/STAT4 serves two essential functions in the development of T H 1 cells: as growth signal, inducing survival and cell division; and as trans-activator, prolonging IFN-γ synthesis through a genetic interaction with the coactivator, CREB-binding protein. These results suggest that a cytokine does not simply induce T H fate choice but instead may act as an essential secondary stimulus that mediates selective survival of a lineage.