Programmed cell death protein (PD)-1, a molecule upregulated by activated/exhausted CD8+ T cells during viral infection (28–30), IFN-γ, which we detected by both intracellular staining and analysis of the cell culture SP and the degranulation marker CD107a were higher expressed by NK cells after encounter of HDV-infected cells (Figure 1D). This evidence concerns the gene PDCD1 and viral infectious disease.