How cytokines control differentiation of helper T (TH) cells is controversial. We show that T-bet, without apparent assistance from interleukin 12 (IL-12)/STAT4, specifies TH1 effector fate by targeting chromatin remodeling to individual interferon-gamma (IFN-gamma) alleles and by inducing IL-12 receptor beta2 expression. Subsequently, it appears that IL-12/STAT4 serves two essential functions in the development of TH1 cells: as growth signal, inducing survival and cell division; and as trans-activator, prolonging IFN-gamma synthesis through a genetic interaction with the coactivator, CREB-binding protein. These results suggest that a cytokine does not simply induce TH fate choice but instead may act as an essential secondary stimulus that mediates selective survival of a lineage.

Type 1 helper T (T(H)1) cells are essential for cellular immunity, but their ontogeny, maturation and durability remain poorly understood. By constructing a dominant-negative form of T-bet, we were able to determine the role played by this lineage-inducing trans-activator in the establishment and maintenance of heritable T(H)1 gene expression. Optimal induction of interferon-gamma (IFN-gamma) expression required genetic interaction between T-bet and its target, the homeoprotein Hlx. In fully mature T(H)1 cells, reiteration of IFN-gamma expression and stable chromatin remodeling became relatively independent of T-bet activity and coincided with demethylation of DNA. In contrast, some lineage attributes, such as expression of IL-12R beta 2 (interleukin 12 receptor beta 2), required ongoing T-bet activity in mature T(H)1 cells and their progeny. These findings suggest that heritable states of gene expression might be maintained by continued expression of the inducing factor or by a mechanism that confers a stable imprint of the induced state.