Background:  A role for PD-1 (programmed death-1) in functional exhaustion of virus-specific CD8⁺ T cells was recently shown. Here, we examined the expression of PD-1 on simian immunodeficiency virus (SIV)-specific CD8⁺ T cells and its possible involvement in regulation of cytokine production, proliferation, and survival of these cells.

Methods:  The expression of PD-1 was examined by polychromatic flow cytometry. SIV-specific CD8⁺ T cells were detected by CM-9 and TL-8 MHC-I tetramers in SIV-infected MamuA*01⁺ rhesus monkeys. Memory CD8⁺ T cell populations were identified by using CD28 and CD95 markers. Production of interferon-gamma (IFN-g), tumor necrosis factor-alpha (TNF-a), and interleukin-2 (IL-2) by CM-9-specific CD8⁺ T cells was measured in relation to PD-1 expression after in vitro stimulation with the CM-9 peptide. CFSE-dilution was used to measure proliferation and apoptosis was evaluated by annexinV binding.

Results:  The majority of CM-9-specific CD8⁺ T cells was found to express high levels of PD-1 in all tissues tested, independent of their memory differentiation status. Lower expression was observed on CM-9-specific CD8⁺ T cells located at mucosal sites. PD-1 expression increased rapidly on CM-9- and TL-8-specific CD8⁺ T cells after acute SIV infection. The expression of PD-1 remained high on CM-9-specific CD8⁺ T cells, while it decreased on TL-8-specific CD8⁺ T cells concomitantly with viral escape at that epitope. Stimulation in vitro with CM-9 peptide resulted in production of IFN-γ, TNF-α, and IL-2 by PD-1^hi CM-9-specific CD8⁺ T cells. The percentage of functional cells, however, was found to be higher in PD-1^low cells. Additionally, PD-1^hi CM-9-specific CD8⁺ T cells were more sensitive to spontaneous apoptosis than the PD-1^low cells. Furthermore, treatment with CM-9 peptide resulted in rapid loss of PD-1^hi CM-9-specific CD8⁺ T cells, likely through activation-induced cell death leading to poor CM-9-specific proliferation. Therefore, loss of survival likely governs the reduced proliferation capacity of PD-1^hi compared with PD-1^low CM-9-specific CD8⁺ T cells.

Conclusions:  Our data point to a negative role for PD-1 in SIV-specific CD8⁺ T-cell function. Manipulation of the interaction of PD-1 with its ligands could potentially benefit the restoration of cytotoxic T lymphocyte (CTL) responses in SIV infection, probably by affecting their survival ability.