Date Published: January 25, 2019
Publisher: Public Library of Science
Author(s): John K. Bui, Joshua C. Cyktor, Elizabeth Fyne, Shalyn Campellone, Stephen W. Mason, John W. Mellors, Roberto F. Speck.
Blockade of the programmed cell death protein/ligand 1 (PD-1/PD-L1) pathway with monoclonal antibodies (mAb) is now commonly used for cancer immunotherapy and has therapeutic potential in chronic viral infections including HIV-1. PD-1/PD-L1 blockade could augment HIV-1-specific immune responses and reverse HIV-1 latency, but the latter effect has not been clearly shown. We tested the ability of the human anti-PD-L1 mAb BMS-936559 and the human anti-PD-1 mAb nivolumab to increase HIV-1 virion production ex vivo from different peripheral blood mononuclear cell populations obtained from donors on suppressive antiretroviral therapy (ART). Fresh peripheral blood mononuclear cells (PBMC), CD8-depleted PBMC, total CD4+ T cells, and resting CD4+ T cells were purified from whole blood of HIV-1-infected donors and cultured in varying concentrations of BMS-936559 (20, 5, or 1.25μg/mL) or nivolumab (5 or 1.25μg/mL), with or without anti-CD3/CD28 stimulatory antibodies. Culture supernatants were assayed for virion HIV-1 RNA by qRT-PCR. Ex vivo exposure to BMS-936559 or nivolumab, with or without anti-CD3/CD28 stimulation, did not consistently increase HIV-1 virion production from blood mononuclear cell populations. Modest (2-fold) increases in virus production were observed in a subset of donors and in some cell types but were not reproducible in longitudinal samples. Cell surface expression of PD-1 and PD-L1 were not associated with changes in virus production. Ex vivo blockade of the PD-1 axis alone has limited effects on HIV-1 latency.
Antiretroviral therapy (ART) does not cure HIV-1 infection because of a persistent reservoir of cells carrying intact proviruses that are capable of infectious virus production, leading to virus replication, spread and rebound viremia if ART is stopped [1–8]. The “shock and kill” strategy for an HIV-1 cure aims to deplete the HIV-1 reservoir by reversing latency and promoting the death of infected cells, either by viral cytopathic effect or by immune-mediated killing . Immune checkpoint blockade is a strategy that has been investigated for its potential to enhance HIV-1-specific immunity , and promote proviral expression (i.e., provide a “kick”) by activation of infected CD4+ T cells. Generally, immune checkpoints regulate the immune system to promote self-tolerance and limit inflammation to minimize collateral tissue damage [10,11]. In chronic HIV-1 infection, immune checkpoint expression is increased both in individuals with uncontrolled viremia and in those on ART with suppression of viremia [12,13], and is associated with more rapid HIV-1 disease progression  and shorter time to viral rebound following ART cessation . This important role of immune checkpoint expression is further supported by ex vivo studies using human cells and in vivo studies in both animal models and humans demonstrating that HIV-1-specific immune function is augmented by blockade of immune checkpoints [10,16–19].
Immune checkpoint blockade can promote anti-HIV-1 immunity [10,16–19] but may also possess the desired property of proviral activation (i.e., latency reversal). CD4+ T cells expressing immune checkpoint markers (PD-1, TIGIT, LAG3) are enriched for HIV-1-infected cells and correlate with the number of HIV-1 infected cells in individuals receiving ART [13,44,45,50–52]. Therefore, immune checkpoint blockade could impact the HIV-1 reservoir by destabilizing proviral latency and enhancing proviral expression. This concept was demonstrated in viremic and aviremic SIV-infected rhesus macaques wherein in vivo checkpoint blockade with anti-PD-1 antibody induced transient increases in SIV viremia [53,54]. Observed responses were associated with CD4+ T cell proliferation , suggesting that PD-1 blockade promoted T cell activation that led to latency reversal. By contrast, other in vivo macaque studies showed no change in viremia when disrupting the PD-1 axis [55,56].