One of the major obstacles to HIV cure resides in the capacity of HIV to establish a transcriptionally silent reservoir, which is not targeted by the immune system or by ART. However, under certain circumstances, HIV transcription might be reactivated. In this context, we recently demonstrated that the levels of HIV transcription in PD-1+/Tfh cells were significantly higher as compared to any other blood and lymph node (LN) memory CD4 T-cell populations. On the basis of this observation, we hypothesized that increased HIV transcription in Tfh cells may be facilitated by the reduction in the inhibitory signals delivered through PD-1/PD-L1 interaction in the germinal centers (GCs).
To test this hypothesis, we assessed 1) the expression of PD-1 and PD-L1 by mass cytometry and immunohistochemistry (IHC) and 2) the impact of PD-1/PD-L1 interaction on HIV production in blood and LN memory CD4 T cells from viremic and aviremic, ART-treated HIV-infected subjects.
We show that PD-1 was highly expressed in blood and LN memory CD4 T-cell populations and particularly in Tfh cells. In contrast, PD-L1 expression was predominant in extra-follicular areas and restricted to macrophages and dendritic cells (P<0.05). Interestingly, IHC revealed that the PD-1/PD-L1 ratio was significantly higher in GCs as compared to extra-follicular areas of aviremic, ART-treated HIV-1 infected subjects (P<0.05). In addition, PD-1/PD-L1 ratio in GCs was about 20-fold higher in ART-treated than viremic, HIV-infected subjects (P<0.05), suggesting that PD-1/PD-L1 interactions might be altered in GCs of ART-treated HIV-infected subjects. We then showed that TCR-mediated HIV production from latently infected memory CD4 T cells was significantly inhibited in presence of PD-L1 recombinant protein (P<0.05), indicating that PD-1/PD-L interactions substantially reduced TCR-induced HIV production. Finally, we demonstrated that the anti-PD-1 mAb, pembrolizumab efficiently reactivated HIV replication from latently infected blood memory CD4 T cells (P<0.05).
This study suggests that an imbalance in the PD-1/PDL-1 signaling may contribute to the persistence of HIV in memory CD4 T cells and to the presence of active HIV transcription in LN Tfh cells. Taken together, these data provide the rationale for the development of intervention strategies targeting PD-1 and/or PD-L1 to reactivate HIV replication.