T-bet Down-Modulation in Tolerized Th1 Effector CD4 Cells Confers a TCR-Distal Signaling Defect That Selectively Impairs IFN- γ Expression.

When Th1 effector CD4 cells encounter tolerizing antigen in vivo their capacity to express the effector cytokines IFN-γ and TNF-α is lost more rapidly than non-effector functions such as IL-2 production and proliferation. To localize the relevant intracellular signaling defects, cytokine expression was compared following restimulation with antigen vs agents that bypass TCR-proximal signaling. IFN-γ and TNF-α expression were both partially rescued when TCR-proximal signaling was bypassed, indicating that both TCR-proximal and distal signaling defects impair the expression of these two effector cytokines. In contrast, bypassing TCR-proximal signaling fully rescued IL-2 expression. T-bet, a transcription and chromatin remodeling factor that is required to direct the differentiation of naive CD4 cells into IFN-γ-expressing Th1 effectors, was partially down-modulated in tolerized Th1 effectors. Enforcing T-bet expression during tolerization selectively rescued the ability to express IFN-γ, but not TNF-α. Conversely, expression of a dominant-negative T-bet in Th1 effectors selectively impaired the ability to express IFN-γ, but not TNF-α. Analysis of histone acetylation at the IFN-γ promoter further suggests that down-modulation of T-bet expression during Th1 effector CD4 cell tolerization does not impair IFN γ expression potential through alterations in chromatin structure.