Epigenetic and Transcriptional Programs Lead to Default IFN-γ Production by γδ T Cells1

γδ T cells have unique features and functions compared with αβ T cells and have been proposed to bridge the innate and adaptive immune responses. Our earlier studies demonstrated that splenic γδ T cells predominantly produce IFN-γ upon activation in vitro, which is partially due to the expression of the Th1-specific transcription factor T-bet. In this study we have explored the epigenetic and transcriptional programs that underlie default IFN-γ production by γδ T cells. We show that the kinetics of IFN-γ transcription is faster in γδ T cells compared with CD4+ and CD8+ T cells and that γδ T cells produce significantly greater amounts of IFN-γ in a proliferation-independent manner when compared with other T cell subsets. By analyzing the methylation pattern of intron 1 of the ifn-γ locus, we demonstrate that this region in naive γδ T cells is hypomethylated relative to the same element in naive CD4+ and CD8+ T cells. Furthermore, naive γδ T cells constitutively express eomesodermin (Eomes), a transcription factor important for IFN-γ production in CD8+ T cells, and Eomes expression levels are enhanced upon activation. Retroviral transduction of activated γδ T cells from both wild-type and T-bet-deficient mice with a dominant negative form of Eomes significantly reduced IFN-γ production, indicating a critical role for this transcription factor in mediating IFN-γ production by γδ T cells in a T-bet-independent manner. Our results demonstrate that both epigenetic and transcriptional programs contribute to the early vigorous IFN-γ production by γδ T cells.