Demethylation of the PD-1 Promoter Is Imprinted during the Effector Phase of CD8 T Cell Exhaustion

PD-1 is an inhibitory receptor that has a major role in T cell dysfunction during chronic infections and cancer. While demethylation of the PD-1 promoter DNA is observed in exhausted T cells isolated from chronically infected individuals, little is known about when this stable demethylation of PD-1 promoter DNA is programmed during the course of a chronic infection. To assess if PD-1 promoter DNA demethylation is impacted by prolonged stimulation during effector phase of chronic infection, we adoptively transferred virus-specific day 8 effector CD8 T cells from mice infected with lymphocytic choriomeningitis virus (LCMV) clone 13 into recipient mice that had cleared an acute infection. We observed that LCMV-specific CD8 T cells from chronically infected mice maintained their surface expression of PD-1 even after transfer into acute immune mice until day 45 posttransfer. Interestingly, the PD-1 transcriptional regulatory region continued to remain unmethylated in these donor CD8 T cells generated from a chronic infection. The observed maintenance of PD-1 surface expression and the demethylated PD-1 promoter were not a result of residual antigen in the recipient mice, because similar results were seen when chronic infection-induced effector cells were transferred into mice infected with a variant strain of LCMV (LCMV V35A) bearing a mutation in the cognate major histocompatibility complex class I (MHC-I) epitope that is recognized by the donor CD8 T cells. Importantly, the maintenance of PD-1 promoter demethylation in memory CD8 T cells was coupled with impaired clonal expansion and higher PD-1 re-expression upon secondary challenge. These data show that the imprinting of the epigenetic program of the inhibitory receptor PD-1 occurs during the effector phase of chronic viral infection. Since PD-1 is a major inhibitory receptor regulating T cell dysfunction during chronic viral infection and cancers, a better understanding of the mechanisms that regulate PD-1 expression is important. In this work, we demonstrate that the PD-1 epigenetic program in antigen-specific CD8 T cells is fixed during the priming phase of chronic infection.