PDCD1 and HIV infectious disease: It was shown more than 15 years ago that during chronic HIV infection, increased expression of PD-1 on HIV-specific CD8+ T cells is associated with reduced effector functions and that blocking PD-1 could enhance HIV-specific CD8+ T-cell survival, proliferation, and reinvigorate effector functions upon T cell receptor (TCR) stimulation with cognate antigen.5