Background:  In the setting of HIV/simian immunodeficiency virus (SIV) infection, PD-1 expression on antigen-specific T cells has been reported to correlate with markers of disease progression, to elevate susceptibility to apoptosis, and to inhibit proliferative capacity. This leads to the conclusion that PD-1 is a marker of T cell exhaustion. However, PD-1 is also up-regulated in the setting of acute viral infection not characterized by T cell exhaustion, and the extent to which PD-1 represents a specific marker for dysfunctional T cells or an activation marker is still controversial.

Methods:  Using polychromatic flow cytometry, we characterized PD-1⁺ CD8⁺ T cells in the blood of SIV_mac239-infected rhesus macaques, to assess proliferation (Ki-67), activation (HLA-DR), and expression of effector molecules (perforin, granzyme B). SIV-specific cells were identified using Gag CM9 or Tat SL8 MHC-class I tetramers. We longitudinally and cross-sectionally analyzed CD28 expression in animals infected with wild-type (wt) SIV_mac239 or attenuated SIV_mac239∆3. Viral loads were determined by quantitative real-time polymerase chain reaction (PCR). Statistical significance was calculated using Mann-Whitney nonparametric test, Spearman-Rank or linear regression.

Results:  Longitudinal analysis of PD-1 expression in SIV_mac239∆3-infected macaques revealed persistent expression of PD-1 on Gag CM9-specific T cells, >10 weeks after clearance of plasma viremia. The frequency of Ki-67⁺ Tat SL8-specific cells in chronically wild-type SIV_mac239-infected macaques was significantly higher in PD-1⁺ vs PD-1^- cells (p = 0.0002). PD-1⁺ Tat SL8-specific cells expressed elevated levels of HLA-DR compared to PD-1^- cells (p = 0.02). Comparison of PD-1⁺ and PD-1^- Tat SL8 specific cells showed equal expression levels of granzyme B and perforin in both populations. In contrast to cells from wild-type SIV-infected animals, PD-1⁺ Gag CM9-specific cells from animals infected with SIV_mac239∆3 exhibited high levels of CD28 expression (p = 0.009). We found a significant down-regulation of CD28-expression on PD-1⁺ Gag CM9-specific cells with progression from acute into chronic wild-type SIV_mac239 infection (p <0.0001), and CD28 expression was negatively correlated with viral load (p = 0.05).

Conclusions:  These data suggest that PD-1 might serve a dual function in chronic retroviral infections, being up-regulated on activated, functional cells as well as dysfunctional cells. PD-1 expression should be evaluated in the context of other markers to specifically identify dysfunctional cells.