Although IFN-γ alone does not prime type I T helper cell (Th1) differentiation, the loss of IFN-γ signaling leads to impaired Th1 phenotype: IFN-γ receptor-deficient (Ifngr-/-) Th1 cells fail to permanently repress IL-4 expression. They can differentiate into IL-4-producing cells under Th2-inducing conditions. These observations suggest that IFN-γ signaling plays a critical role in si- lencing Il4 gene in Th1 cells and stabilizing Th1 phenotype. IFN-γ signaling has been further shown to inhibit IL-4 expression by inhibiting STAT6 phosphorylation. This work aims to study the mechanism by which STAT1, the downstream molecule that transduces IFN-γ signaling, may mediate suppression of IL-4 expression in Th1 cells. The results show that STAT1-deficient naive CD4+ T cells express a reduced level of IFN-γ as well as an elevated level of IL-4. These cells exhibit bias to differentiate into Th2 cells under unpolarized conditions. Under Th1-inducing conditions, STAT1-deficient naive CD4+ T cells show impaired Th1 differentiation: Stat1-/- Th1 cells express reduced levels of IFN-γ and T-bet. These cells also fail to repress the expression of IL-4 and GATA-3 and retain STAT6 signaling. More importantly, Stat1-/- Th1 cells can be effectively induced to differentiate into IL-4-producing cells under Th2-inducing conditions. Ectopic expression of T-bet in Stat1-/- Th1 cells dramatically represses their ability to do so and drastically restores IFN-γ expression, suggesting that STAT1 may inhibit IL-4 expression through T-bet. Finally, histone H3 acetylation (H3AC) and histone H3 K4 dimethylation (H3K4dim) were observed in two enhancer regions of Il4 gene locus in Stat1-/- Th1 cells, suggesting a permissive status of Il4 gene locus in these cells. Thus, this study reveals new mechanisms by which STAT1 signaling may mediate repression of Il4 gene in Th1 cells: upregulating T-bet that subsequently represses GATA3 and IL-4 expression, antagonizing STAT6 signaling, and inhibiting epigenetic modifications in Il4 gene locus.
