The present of GATA-3 mRNA in naive T cells and induction of T-bet mRNA after cell activation, even in the absence of Stat1-mediated signals, are consistent with the early transcription of IFNy and Il4 after stimulation. Cells activated in the absence of either IL-12 or IL4 express both transcriptional activators.
Although T-bet and GATA-3 were identified on the basis of their capacity to activate cytokines in developing Th subsets, in each case these transcription factors powerfully silence opposing cytokines. Importantly, the cytokine-silencing capacity of the regulatory transcripiton factor is cell-intrinsic. For example, expression of T-bet in IFNyR1 deficient Th2 cells caused comparable suppression of IL-4 production as in WT cells. Thus despite the role of IFNy/Stat1-signaling in T-bet induction, T-bet mediated silencing of IL-4 expression was IFNy independent. Similarly, GATA-3 suppresses IFNy production in the absence of IL-4/STAT6-meidated signaling by a cell intrinsic mechanisms.
Since T-bet and GATA-3 operate intrinsically to suppress opposing cytokines, these transcription factors themselves represent critical targets of the cytokine milieu. Accordingly, the Th2-inducing factor GATA-3 is extinguished in Th1 cells by IL-12/State4-mediated signaling. Conversely, the Th1-inducing factor T-bet is extinguished in Th2 cells by IL-4/Stat6 mediated signaling. With the loss of one of these master regulators, the capacity to sustain downstream cytokine gene expression at the discrete gene becomes lost and lineages become established in response to the remaining regulatory, resulting in terminal differentiation into polarized T helper subsets.
It is believe that T-bet and Gata-3 exert their governance of cell fate through alternations of chromatin structure.
T-bet is a key regulator (defining factor) of Th1 differentiation. T-bet is a Th1 specific transcription factor that is necessary for IFN-y expression in CD4+ Th1 cells. T-bet is expressed exclusively in Th1 cells and its expression is mediated mainly by a STAT-4 dependent pathway. The Th1-restricted transcription factor T-bet directly trans-activates the Ifng promoter and induces remodeling of the endogenous Ifng locus. T cells of T-bet-deficient mice do not produce IFN-y. Transient expression of T-bet in polarized Th2 cells partially reconstitutes Th1 differentiation, enabling IFN-y expression in response to polyclonal stimulation.
GATA-3 is a key regulator (definining factor) of Th2 differentiation. GATA-3 is expressed during Th2 differentiation via pathways that probably involve the IL-4 dependent activation of the signal transducer and activator of transcription (STAT)-6. However, ectopic GATA-3 expression by naive CD4+ T cells can compensate for the lack of STAT6 expression and lead to Th2 development. Like STAT6, it is not known how GATA-3 exerts its influecne. GATA-3 is critical for Th2 development. GATA-3 was reported to suppress Th1 development by suppressing the expression of STAT-4. In addition, GATA-3 transactivates the expression of genes for IL-5 and IL-13 directly in Th2 cells.
GATA-3 is responsible for the induction of chromatin remodeling at the Th2 cytokine locus by acting at several control sequences, including the recently identified Il4 enhancer VA and the intergenic conserved region CNS-1. Ectopic expression of GATA-3 induced Th2-specific DNase I hypersensitive site(s) in Th1 cells and in STAT6-deficient cells. It is possible that GATA-3 acts through enhancer regions to induce chromatin remodeling. GATA3 is a downstream effector of STAT6.