Phenotypic and functional studies of the programmed death-1 (PD-1) molecule on CD4(+) and CD8(+) T cells were performed on peripheral blood mononuclear cells from uninfected and simian immunodeficiency virus (SIV)-infected rhesus macaques. These data demonstrated a rapid upregulation of PD-1 expression on tetramer-positive CD8(+) T cells from MamuA.01(+) SIV-infected macaques upon infection. Upregulation of PD-1 on total CD8(+) T cells was not detectable. In contrast, CD4(+) T-cell PD-1 expression was markedly higher in total CD4(+) T cells during chronic, but not acute, infection and there was a correlation between the level of PD-1 expression on naive and central memory CD4(+) T cells and the levels of viral loads. Such association was emphasized further by a marked decrease of PD-1 expression on tetramer-positive CD8 T cells as well as on CD4(+) T cells on longitudinal samples collected before and after the initiation of antiretroviral therapy and downregulation of viral replication in vivo. Cloning of PD-1 and its two ligands from several non-human primate species demonstrated > 95% conservation for PD-1 and PD-L2 and only about 91% homology for PD-L1. Functional studies using soluble recombinant PD-1 protein or PD-1-immunoglobulin G fusion proteins induced marked increases in the SIV-specific proliferative responses of both CD4(+) and CD8(+) T cells from rhesus macaques. The results of these studies serve as a foundation for future in vivo trials of the use of rMamu-PD-1 to potentially enhance and/or restore antiviral immune responses in vivo.