T follicular helper (Tfh) cells expressing high levels of PD-1 were recently shown to serve as a major site of active and persistent HIV transcription despite prolonged ART. The present study aimed to determine the potential role of immune checkpoint (IC)/IC-Ligand (IC-L) interactions on HIV transcription in lymph node (LN) microenvironment.
To address this issue, we assessed the expression of ICs and IC-Ls on LN cell populations and the impact of IC/IC-L interactions on T-cell proliferation, reactivation of HIV production and HIV transcription in LN memory CD4 T-cell populations from viremic and aviremic ART-treated HIV-infected subjects (N=47).
We showed that PD-1 and TIGIT are the two major ICs expressed on Tfh cells of healthy, viremic and ART treated HIV-infected subjects ex vivo. We subsequently showed that PD-L1 and to a lower extent CD155 (TIGIT ligand) recombinant proteins significantly reduced TCR-mediated T-cell proliferation and reactivation of HIV production in vitro (P<0.05), demonstrating that PD-1 and TIGIT signaling pathways were 1) functionally active on PD-1+/Tfh cells and 2) regulate TCR-mediated HIV transcription and production. We therefore explored the phenotype, the frequency and the tissue distribution of IC-L expressing cells and showed that PD-L1 and CD155 were predominantly co-expressed on LN HLA-DRhighCD1chigh dendritic cells (DCs). The frequencies of PD-L1+ DCs in viremic HIV-infected subjects directly correlated with HIV viral load (r=0.93 P=0.0002) and significantly dropped after prolonged ART (P<0.05). Interestingly, PD-L1 expressing cells were detected in both extra-follicular and germinal center (GC) areas of viremic HIV-infected subjects, but were barely detectable in GCs of ART treated subjects, suggesting that ART initiation had a profound impact on IC-L tissue distribution and that PD-1/PD-L1 interactions might be selectively reduced in GCs of ART-treated subjects. Finally, the frequencies of LN PD-L1+ DCs inversely correlated with HIV transcription (r=-0.89; P<0.05) in LN memory CD4 T cells, indicating that PD-L1+ DCs contribute to control HIV transcription in vivo.
HIV exploits the IC regulatory mechanism of T cell activation and function to favor persistence of HIV transcription/production in treated aviremic HIV-infected subjects. It also indicates that an imbalance in IC/IC-L interactions is a novel mechanism contributing to HIV persistence in germinal centers.