Typically, CD4+ T cells secrete IL-2 and IFN-γ but chronic viral infections can significantly alter their cytokine profile to resemble those of their CD8+ counterparts with upregulation of programmed cell death protein 1 (PD-1) and downregulation of IL-2 and IFN-γ [90,91,92]. This evidence concerns the gene PDCD1 and viral infectious disease.