T cell exhaustion plays a major role in failure to control chronic infections. High expression of inhibitory receptors, including PD-1, and the inability to sustain functional T cell responses contribute to exhaustion. However, the transcriptional control of these processes remains unclear. Here we demonstrate that the transcription factor T-bet regulates CD8+ T cell exhaustion and inhibitory receptor expression. T-bet directly repressed Pdcd1 transcription and decreased the expression of other inhibitory receptors. While elevated T-bet promoted terminal differentiation following acute infection, high T-bet expression sustained exhausted CD8+ T cells and repressed inhibitory receptor expression during chronic viral infection. Persisting antigenic stimulation caused T-bet downregulation, which resulted in more severe exhaustion of CD8+ T cells. These observations suggest therapeutic opportunities involving increasing T-bet expression during chronic infection.

SUMMARYT-bet and Bcl-6 are required to establish TH1 or TFH gene expression profiles, respectively. Here, we demonstrated that high interleukin 2 (IL-2) concentrations inhibited Bcl-6 expression in polarized TH1 cells. Mechanistically, the low amounts of Bcl-6 normally found in effector TH1 cells could not repress its target genes because a T-bet-Bcl-6 complex masked the Bcl-6 DNA-binding domain. TH1 cells increased their Bcl-6/T-bet ratio in response to limiting IL-2 conditions, allowing excess Bcl-6 to repress its direct target Prdm1 (which encodes Blimp-1). The Bcl-6-dependent repression of Blimp-1 effectively induced a partial TFH-profile because Blimp-1 directly repressed a subset of TFH-signature genes, including Cxcr5. Taken together, IL-2-signaling regulates the Bcl-6-Blimp-1 axis in TH1 cells to maintain flexibility with a TFH-like gene profile.

SUMMARY Functionally exhausted T cells express high levels of the PD-1 inhibitory receptor, and therapies that block PD-1 signaling show promise for resolving chronic viral infections and cancer. Using human and murine systems of acute and chronic viral infections we analyzed epigenetic regulation of PD-1 expression during CD8 T cell differentiation. During acute infection, naïve to effector CD8 T cell differentiation was accompanied by a transient loss of DNA methylation of the Pdcd1 locus that was directly coupled to the duration and strength of TCR signaling. Further differentiation into functional memory cells coincided with Pdcd1 remethylation providing an adapted program for regulation of PD-1 expression. In contrast, the Pdcd1 regulatory region was completely demethylated in exhausted CD8 T cells and remained unmethylated even when virus titers decreased. This lack of DNA remethylation leaves the Pdcd1 locus poised for rapid expression, potentially providing a signal for premature termination of antiviral functions.