Background:  Ligation of the programmed death (PD)-1 molecule by its ligands PD-L1 and PD-L2 can lead to anergy and exhaustion of virus-specific CD4 and CD8 T cells, which may result in an inhibition of function. In this cross-sectional study we assessed the expression of PD-1 and its associated molecule PD-L1 in a cohort of treated chronically HIV-1-infected individuals, as compared with levels observed in uninfected healthy individuals.

Methods:  Freshly separated peripheral blood mononuclear cells (PBMC) from 23 chronically HIV-1-infected patients on effective HAART and 10 seronegative controls were assessed for percentage and mean fluorescence intensity (MFI) of PD-1 and PD-L1 on T cells. Further analysis of PD-L1 on the total PBMC population was performed. Drugs used in the HAART regimens of the patients were recorded. Statistical analysis of variance (ANOVA) and Mann Whitney U tests were performed where p <0.05 was the cut off for significance.

Results:  Patients plasma HIV-1 RNA load was below detection limit (50 copies/mL) and median CD4 T-cell count was 500 µL/blood (range 241 to 997). The percentage of PD-L1 expressing cells was slightly elevated in HIV-1-infected patients compared to uninfected controls, however this difference did not reach statistical significance (p = 0.7898). The percentage of T cells expressing PD-1 seen in patients on boosted-protease inhibitor (PI) regimen (n = 12) was slightly lower to that obtained for non-nucleoside reverse transcriptase inhibitor (NNRTI)-treated patients (n = 11), (p = 0.0789). MFI for PD-L1 was increased in HIV-1-infected patients on both PBMC (p = 0.0004) and gated T cells (p = 0.0023).

Conclusions:  No statistically significant differences in the percentage of cells expressing either PD-1 or PD-L1 were observed between the infected and uninfected cohort. The higher relative MFI of PD-L1 observed in HIV-1-infected individuals compared to uninfected controls may be a signature of a persistent anergic state. The small differences in the numbers of PD-1 expressing cells observed between the 2-drug regimens may reflect a more potent action of the boosted-PI treatment. Previous reports show that untreated viremic patients have higher expression of PD-1, our data suggest that the observed similarities in percentage expression between controls and HIV-1⁺ treated patients are indicative of normalization of PD-1 expression after initiation of HAART, however PD-L1 MFI levels remain higher in HIV-1⁺ patients.