PD-1 Expression during Acute Infection Is Repressed through an LSD1–Blimp-1 Axis

During prolonged exposure to Ags, such as chronic viral infections, sustained TCR signaling can result in T cell exhaustion mediated in part by expression of programmed cell death-1 (PD-1) encoded by the Pdcd1 gene. In this study, dynamic changes in histone H3K4 modifications at the Pdcd1 locus during ex vivo and in vivo activation of CD8 T cells suggested a potential role for the histone H3 lysine 4 demethylase LSD1 in regulating PD-1 expression. CD8 T cells lacking LSD1 expressed higher levels of Pdcd1 mRNA following ex vivo stimulation as well as increased surface levels of PD-1 during acute, but not chronic, infection with lymphocytic choriomeningitis virus (LCMV). Blimp-1, a known repressor of PD-1, recruited LSD1 to the Pdcd1 gene during acute, but not chronic, LCMV infection. Loss of DNA methylation at Pdcd1's promoter-proximal regulatory regions is highly correlated with its expression. However, following acute LCMV infection, in which PD-1 expression levels return to near baseline, LSD1-deficient CD8 T cells failed to remethylate the Pdcd1 locus to the levels of wild-type cells. Finally, in a murine melanoma model, the frequency of PD-1-expressing tumor-infiltrating LSD1-deficient CD8 T cells was greater than in wild type. Thus, LSD1 is recruited to the Pdcd1 locus by Blimp-1, downregulates PD-1 expression by facilitating the removal of activating histone marks, and is important for remethylation of the locus. Together, these data provide insight into the complex regulatory mechanisms governing T cell immunity and regulation of a critical T cell checkpoint gene.