The Effect of Latency Reversal Agents on Primary CD8+ T Cells: Implications for Shock and Kill Strategies for Human Immunodeficiency Virus Eradication

Implications of HIV-1 Nef for "Shock and Kill" Strategies to Eliminate Latent Viral Reservoirs

Finding a cure for HIV is challenging because the virus is able to integrate itself into the host cell genome and establish a silent state, called latency, allowing it to evade antiviral drugs and the immune system. Various “shock and kill” strategies are being explored in attempts to eliminate latent HIV reservoirs. The goal of these approaches is to reactivate latent viruses (“shock”), thereby exposing them to clearance by viral cytopathic effects or immune-mediated responses (“kill”). To date, there has been limited clinical success using these methods. In this review, we highlight various functions of the HIV accessory protein Nef and discuss their double-edged effects that may contribute to the limited effectiveness of current “shock and kill” methods to eradicate latent HIV reservoirs in treated individuals.

SMAC Mimetics Induce Autophagy-Dependent Apoptosis of HIV-1-Infected Resting Memory CD4+ T Cells

Long-lived resting memory CD4+ T cells (TCM) are a major reservoir of latent HIV infection. We hypothesized that latent HIV-TCM cells are maintained by aberrant expression of cell survival factors, including XIAP, BIRC2/cIAP1, and beclin-1. DIABLO/SMAC mimetics are therapeutic agents that compromise cell survival by hijacking host apoptotic machinery. We found that DIABLO/SMAC mimetics (birinapant, GDC-0152, and embelin) selectively kill HIV-TCM without increasing virus production or targeting uninfected TCM. Treatment of HIV-TCM with DIABLO/SMAC mimetics promoted XIAP and BIRC2 degradation, which triggered autophagy and the formation of a cell death complex consisting of pro-apoptotic (FADD, RIPK1, RIPK3, and caspase 8) and autophagy (ATG5, ATG7, and SQSTM1) proteins. Genetic or pharmacological inhibition of autophagy induction, but not autophagy-mediated degradation, abrogated this interaction and subsequent cell death. Our findings identify a mechanism whereby DIABLO/SMAC mimetics exploit autophagy and apoptotic machinery to selectively induce killing of HIV-TCM without viral reactivation while sparing uninfected cells.

T cell toxicity of HIV latency reversing agents

Combination antiretroviral therapy reduces morbidity and mortality in HIV infected patients. However, the cure of HIV infection is hindered by the persistence of the latent HIV reservoir. Latency reversing agents (LRAs) are developed to target the HIV latently infected cells for HIV reactivation. In addition to reversal of HIV latency, the eradication of HIV latently infected cells will require effector HIV-specific CD8+ T cells. Therefore it is imperative we understand how LRAs affect immune cells. We have performed a comparative in depth analysis of the cytotoxicity of several compounds belonging to four LRA classes on T cells, B cells, and NK cells. In addition, the effect of these LRAs on activation and inhibitory receptor expression of CD8+ T cells was examined. We show that the HDAC inhibitors romidepsin and panobinostat are highly cytotoxic for CD4+ and CD8+ T cells, whereas the PKC agonists bryostatin and prostratin and BET inhibitors JQ1 and OXT-015 were less cytotoxic. The BAF inhibitors CAPE and pyrimethamine exhibit no cytotoxicity. Drug-specific cytotoxicity on CD8+ T cells was comparable between healthy controls and cART-treated HIV-infected patients. Bryostatin and both BET inhibitors downregulated the expression of CD279 on CD8+ T cells without affecting their activation. Our comparison of LRAs identified differences in cytotoxicity between LRA classes and members within a class and suggests that some LRAs such as bryostatin and BET inhibitors may also downregulate inhibitory receptors on activated HIV-specific CD8+ T cells. These findings may guide the use of LRAs that have the capacity to preserve or restore CD8+ T cell immunity.

Determinants of the efficacy of HIV latency-reversing agents and implications for drug and treatment design

HIV eradication studies have focused on developing latency-reversing agents (LRAs). However, it is not understood how the rate of latent reservoir reduction is affected by different steps in the process of latency reversal. Furthermore, as current LRAs are host-directed, LRA treatment is likely to be intermittent to avoid host toxicities. Few careful studies of the serial effects of pulsatile LRA treatment have yet been done. This lack of clarity makes it difficult to evaluate the efficacy of candidate LRAs or predict long-term treatment outcomes. We constructed a mathematical model that describes the dynamics of latently infected cells under LRA treatment. Model analysis showed that, in addition to increasing the immune recognition and clearance of infected cells, the duration of HIV antigen expression (i.e., the period of vulnerability) plays an important role in determining the efficacy of LRAs, especially if effective clearance is achieved. Patients may benefit from pulsatile LRA exposures compared with continuous LRA exposures if the period of vulnerability is long and the clearance rate is high, both in the presence and absence of an LRA. Overall, the model framework serves as a useful tool to evaluate the efficacy and the rational design of LRAs and combination strategies.

TLR3 agonist and CD40-targeting vaccination induces immune responses and reduces HIV-1 reservoirs

Activation of HIV-1 reservoirs and induction of anti-HIV-1 T cells are critical to control HIV-1 rebound after combined antiretroviral therapy (cART). Here we evaluated in humanized mice (hu-mice) with persistent HIV-1 infection the therapeutic effect of TLR3 agonist and a CD40-targeting HIV-1 vaccine, which consists of a string of 5 highly conserved CD4+ and CD8+ T cell epitope-rich regions of HIV-1 Gag, Nef, and Pol fused to the C-terminus of a recombinant anti-human CD40 antibody (αCD40.HIV5pep). We show that αCD40.HIV5pep vaccination coadministered with poly(I:C) adjuvant induced HIV-1-specific human CD8+ and CD4+ T cell responses in hu-mice. Interestingly, poly(I:C) treatment also reactivated HIV-1 reservoirs. When administrated in therapeutic settings in HIV-1-infected hu-mice under effective cART, αCD40.HIV5pep with poly(I:C) vaccination induced HIV-1-specific CD8+ T cells and reduced the level of cell-associated HIV-1 DNA (or HIV-1 reservoirs) in lymphoid tissues. Most strikingly, the vaccination significantly delayed HIV-1 rebound after cART cessation. In summary, the αCD40.HIV5pep with poly(I:C) vaccination approach both activates replication of HIV-1 reservoirs and enhances the anti-HIV-1 T cell response, leading to a reduced level of cell-associated HIV-1 DNA or reservoirs. Our proof-of-concept study has significant implication for the development of CD40-targeting HIV-1 vaccine to enhance anti-HIV-1 immunity and reduce HIV-1 reservoirs in patients with suppressive cART.

Barriers for HIV Cure: The Latent Reservoir

Thirty-five years after the identification of HIV-1 as the causative agent of AIDS, we are still in search of vaccines and treatments to eradicate this devastating infectious disease. Progress has been made in understanding the molecular pathogenesis of this infection, which has been crucial for the development of the current therapy regimens. However, despite their efficacy at limiting active viral replication, these drugs are unable to purge the latent reservoir: a pool of cells that harbor transcriptionally inactive, but replication-competent HIV-1 proviruses, and that represent the main barrier to eradicate HIV-1 from affected individuals. In this review, we discuss advances in the field that have allowed a better understanding of HIV-1 latency, including the diverse cell types that constitute the latent reservoir, factors influencing latency, tools to study HIV-1 latency, as well as current and prospective therapeutic approaches to target these latently infected cells, so a functional cure for HIV/AIDS can become a reality.

Evaluation of resistance to HIV-1 infection ex vivo of PBMCs isolated from patients with chronic myeloid leukemia treated with different tyrosine kinase inhibitors

Current antiretroviral treatment (ART) may control HIV-1 replication but it cannot cure the infection due to the formation of a reservoir of latently infected cells. CD4+ T cell activation during HIV-1 infection eliminates the antiviral function of the restriction factor SAMHD1, allowing proviral integration and the reservoir establishment. The role of tyrosine kinases during T-cell activation is essential for these processes. Therefore, the inhibition of tyrosine kinases could control HIV-1 infection and restrict the formation of the reservoir. A family of tyrosine kinase inhibitors (TKIs) is successfully used in clinic for treating chronic myeloid leukemia (CML). The safety and efficacy against HIV-1 infection of five TKIs was assayed in PBMCs isolated from CML patients on prolonged treatment with these drugs that were infected ex vivo with HIV-1. We determined that the most potent and safe TKI against HIV-1 infection was dasatinib, which preserved SAMHD1 antiviral function and avoid T-cell activation through TCR engagement and homeostatic cytokines. Imatinib and nilotinib showed lower potency and bosutinib was quite toxic in vitro. Ponatinib presented similar profile to dasatinib but as it has been associated with higher incidence of arterial ischemic events, dasatinib would be the better choice of TKI to be used as adjuvant of ART in order to avoid the establishment and replenishment of HIV-1 reservoir and move forward towards an HIV cure.

In Vitro Exposure to Prostratin but Not Bryostatin-1 Improves Natural Killer Cell Functions Including Killing of CD4+ T Cells Harboring Reactivated Human Immunodeficiency Virus

In the attempt of purging the HIV-1 reservoir through the “shock-and-kill” strategy, it is important to select latency-reversing agents (LRAs) devoid of deleterious effects on the antiviral function of immune effector cells. Here, we investigated two LRAs with PKC agonist activity, prostratin (PRO) and bryostatin-1 (BRY), for their impact on the function of natural killer (NK) cells, the major effectors of innate immunity whose potential in HIV-1 eradication has emerged in recent clinical trials. Using NK cells of healthy donors, we found that exposure to either PRO or BRY potently activated NK cells, resulting in upmodulation of NKG2D and NKp44 activating receptors and matrix metalloprotease-mediated shedding of CD16 receptor. Despite PRO and BRY affected NK cell phenotype in the same manner, their impact on NK cell function was diverse and showed considerable donor-to-donor variation. Altogether, in most tested donors, the natural cytotoxicity and antibody-dependent cellular cytotoxicity (ADCC) of NK cells were either improved or maintained by PRO, while both activities were impaired by BRY. Moreover, we analyzed the effect of these drugs on the capacity of treated NK cells to kill autologous latently infected CD4⁺ T cells reactivated via the same treatment. First, we found that PRO but not BRY increased upmodulation of the ULBP2 ligand for NKG2D on reactivated p24⁺ cells. Importantly, we showed that clearance of reactivated p24⁺ cells by NK cells was enhanced when both targets and effectors were exposed to PRO but not to BRY. Overall, PRO had a superior potential compared with BRY as to the impact on key NK cell functions and on NK-cell-mediated clearance of the HIV-1 reservoir. Our results emphasize the importance of considering the effects on NK cells of candidate “shock-and-kill” interventions. With respect to combinative approaches, the impact on NK cells of each LRA should be re-evaluated upon combination with a second LRA, which may have analogous or opposite effects, or with immunotherapy targeting NK cells. In addition, avoiding co-administration of LRAs that negatively impact ADCC activity by NK cells might be essential for successful application of antibodies or vaccination to “shock-and-kill” strategies.

Diverse Impacts of HIV Latency-Reversing Agents on CD8+ T-Cell Function: Implications for HIV Cure

Antiretroviral therapy regimens durably suppress HIV replication, but do not cure infection. This is partially attributable to the persistence of long-lived pools of resting CD4+ T-cells harboring latent replication-competent virus. Substantial clinical and pre-clinical research is currently being directed at purging this viral reservoir by combining pharmacological latency reversal with immune effectors, such as HIV-specific CD8+ T-cells, capable of eliminating reactivated targets-the so-called "shock-and-kill" approach. However, several studies indicate that the latency-reversing agents (LRAs) may affect CD8+ T-cell function. The current review aims to frame recent advances, and ongoing challenges, in implementing "shock-and-kill" strategies from the perspective of effectively harnessing CD8+ T-cells. We review and contextualize findings indicating that LRAs often have unintended impacts on CD8+ T-cell function, both detrimental and beneficial. We identify and attempt to bridge the gap between viral reactivation, as measured by the detection of RNA or protein, and bona fide presentation of viral antigens to CD8+ T-cells. Finally, we highlight factors on the effector (CD8+) and target (CD4+) cell sides that contribute to whether or not infected-cell recognition results in killing/elimination. These perspectives may contribute to an integrated view of "shock-and-kill," with implications for therapeutic development.

Targeting the Latent Reservoir for HIV-1

Antiretroviral therapy can effectively block HIV-1 replication and prevent or reverse immunodeficiency in HIV-1-infected individuals. However, viral replication resumes within weeks of treatment interruption. The major barrier to a cure is a small pool of resting memory CD4+ T cells that harbor latent HIV-1 proviruses. This latent reservoir is now the focus of an intense international research effort. We describe how the reservoir is established, challenges involved in eliminating it, and pharmacologic and immunologic strategies for targeting this reservoir. The development of a successful cure strategy will most likely require understanding the mechanisms that maintain HIV-1 proviruses in a latent state and pathways that drive the proliferation of infected cells, which slows reservoir decay. In addition, a cure will require the development of effective immunologic approaches to eliminating infected cells. There is renewed optimism about the prospect of a cure, and the interventions discussed here could pave the way.

HIV-Specific T Cells Generated from Naive T Cells Suppress HIV In Vitro and Recognize Wide Epitope Breadths

The Berlin Patient represents the first and only functional HIV cure achieved by hematopoietic stem cell transplant (HSCT). In subsequent efforts to replicate this result, HIV rebounded post-HSCT after withdrawal of antiretroviral therapy. Providing HIV-specific immunity through adoptive T cell therapy may prevent HIV rebound post-HSCT by eliminating newly infected cells before they can seed systemic infection. Adoptive T cell therapy has demonstrated success in boosting Epstein-Barr virus and cytomegalovirus-specific immunity post-HSCT, controlling viral reactivation. However, T cell immunotherapies to boost HIV-specific immunity have been limited by single-epitope specificity and minimal persistence or efficacy in vivo. To improve this strategy, we sought to generate allogeneic HIV-specific T cells from human leukocyte antigen (HLA)-A02+ HIV-negative adult or cord blood donors. We focused on HLA-A02+ donors due to well-characterized epitope restrictions observed in HIV+ populations. We show that multi-antigen HIV-specific T cells can be generated from naive T cells of both cord blood and adults using a reproducible good manufacturing practice (GMP)-grade protocol. This product lysed antigen-pulsed targets and suppressed active HIV in vitro. Interestingly, these cells displayed broad epitope recognition despite lacking recognition of the common HLA-A02-restricted HIV epitope Gag SL9. This first demonstration of functional multi-antigen HIV-specific T cells has implications for improving treatment of HIV through allogeneic HSCT.

Tackling HIV Persistence: Pharmacological versus CRISPR-Based Shock Strategies

Jan Svoboda studied aspects of viral latency, in particular with respect to disease induction by avian RNA tumor viruses, which were later renamed as part of the extended retrovirus family. The course of retroviral pathogenesis is intrinsically linked to their unique property of integrating the DNA copy of the retroviral genome into that of the host cell, thus forming the provirus. Retroviral latency has recently become of major clinical interest to allow a better understanding of why we can effectively block the human immunodeficiency virus type 1 (HIV-1) in infected individuals with antiviral drugs, yet never reach a cure. We will discuss HIV-1 latency and its direct consequence—the formation of long-lasting HIV-1 reservoirs. We next focus on one of the most explored strategies in tackling HIV-1 reservoirs—the “shock and kill” strategy—which describes the broadly explored pharmacological way of kicking the latent provirus, with subsequent killing of the virus-producing cell by the immune system. We furthermore present how the clustered regularly interspaced palindromic repeats (CRISPR) and associated protein (Cas) system can be harnessed to reach the same objective by reactivating HIV-1 gene expression from latency. We will review the benefits and drawbacks of these different cure strategies.

Elimination of HIV-1 Latently Infected Cells by Gnidimacrin and a Selective HDAC Inhibitor

We have previously reported gnidimacrin (GM), a protein kinase C (PKC) agonist, significantly reduces the frequency of HIV-1 latently infected cells in peripheral blood mononuclear cells (PBMCs) from patients undergoing successful antiretroviral therapy at low picomolar concentrations ex vivo, which is distinct from other latency reversing agents. In this study, we demonstrate that strong viral reactivation by GM is a mechanism for elimination of latently infected cells, and a histone deacetylase inhibitor (HDACI), a thiophenyl benzamide (TPB), further potentiated the efficacy of GM against latent HIV-1. The effect of GM on latent HIV-1 activation was potentiated by TPB in cell models by 2-3-fold. The GM/TPB combination further decreased the frequency of HIV-infected cells in latently infected patient PBMCs over 3-fold when compared with GM alone, which caused a 5-fold reduction compared with the solvent control. Thus, GM/TPB is a unique combination that may reduce latent HIV-1 reservoirs at nontoxic concentrations.

Latent HIV reservoirs exhibit inherent resistance to elimination by CD8+ T cells

The presence of persistent, latent HIV reservoirs in CD4+ T cells obstructs current efforts to cure infection. The so-called kick-and-kill paradigm proposes to purge these reservoirs by combining latency-reversing agents with immune effectors such as cytotoxic T lymphocytes. Support for this approach is largely based on success in latency models, which do not fully reflect the makeup of latent reservoirs in individuals on long-term antiretroviral therapy (ART). Recent studies have shown that CD8+ T cells have the potential to recognize defective proviruses, which comprise the vast majority of all infected cells, and that the proviral landscape can be shaped over time due to in vivo clonal expansion of infected CD4+ T cells. Here, we have shown that treating CD4+ T cells from ART-treated individuals with combinations of potent latency-reversing agents and autologous CD8+ T cells consistently reduced cell-associated HIV DNA, but failed to deplete replication-competent virus. These CD8+ T cells recognized and potently eliminated CD4+ T cells that were newly infected with autologous reservoir virus, ruling out a role for both immune escape and CD8+ T cell dysfunction. Thus, our results suggest that cells harboring replication-competent HIV possess an inherent resistance to CD8+ T cells that may need to be addressed to cure infection.

Effects of exosome on the activation of CD4+ T cells in rhesus macaques: a potential application for HIV latency reactivation

Exosomes are small extracellular vesicles (EVs), released by a wide variety of cell types, carry donor origin-proteins, cytokines, and nucleic acids, transport these cargos to adjacent or distant specific recipient cells, and thereby regulate gene expression and activation of target cells. In this study, we isolated and identified exosomes in rhesus macaques, and investigated their effects on cell tropism and activation, especially their potential to reactivate HIV latency. The results indicated that plasma-derived exosomes preferentially fuse to TCR-activated T cells and autologous parent cells. Importantly, the uptake of exosomes, derived from IL-2 stimulated CD4+ T cells, effectively promoted reactivation of resting CD4+ T-cell, as indicated by an increased viral transcription rate in these cells. These findings provide premise for the potential application of exosome in the reactivation of HIV latency, in combination its use as functional delivery vehicles with antiretroviral therapy (ART).

Novel Latency Reversal Agents for HIV-1 Cure

Antiretroviral therapy (ART) has rendered HIV-1 infection a treatable illness; however, ART is not curative owing to the persistence of replication-competent, latent proviruses in long-lived resting T cells. Strategies that target these latently infected cells and allow immune recognition and clearance of this reservoir will be necessary to eradicate HIV-1 in infected individuals. This review describes current pharmacologic approaches to reactivate the latent reservoir so that infected cells can be recognized and targeted, with the ultimate goal of achieving an HIV-1 cure.

Curaxin CBL0100 Blocks HIV-1 Replication and Reactivation through Inhibition of Viral Transcriptional Elongation

Despite combination antiretroviral therapy (cART), acquired immunodeficiency syndrome (AIDS), predominantly caused by the human immunodeficiency virus type 1 (HIV-1), remains incurable. The barrier to a cure lies in the virus' ability to establish a latent infection in HIV/AIDS patients. Unsurprisingly, efforts for a sterilizing cure have focused on the “shock and kill” strategy using latency-reversing agents (LRAs) to complement cART in order to eliminate these latent reservoirs. However, this method faces numerous challenges. Recently, the “block and lock” strategy has been proposed. It aims to reinforce a deep state of latency and prevent sporadic reactivation (“blip”) of HIV-1 using latency-promoting agents (LPAs) for a functional cure. Our studies of curaxin 100 (CBL0100), a small-molecule targeting the facilitates chromatin transcription (FACT) complex, show that it blocks both HIV-1 replication and reactivation in in vitro and ex vivo models of HIV-1. Mechanistic investigation elucidated that CBL0100 preferentially targets HIV-1 transcriptional elongation and decreases the occupancy of RNA Polymerase II (Pol II) and FACT at the HIV-1 promoter region. In conclusion, CBL0100 is a newly identified inhibitor of HIV-1 transcription that can be used as an LPA in the “block and lock” cure strategy.

Sequential Vacc-4x and romidepsin during combination antiretroviral therapy (cART): Immune responses to Vacc-4x regions on p24 and changes in HIV reservoirs

OBJECTIVES

The REDUC clinical study Part B investigated Vacc-4x/rhuGM-CSF therapeutic vaccination prior to HIV latency reversal using romidepsin. The main finding was a statistically significant reduction from baseline in viral reservoir measurements. Here we evaluated HIV-specific functional T-cell responses following Vacc-4x/rhuGM-CSF immunotherapy in relation to virological outcomes on the HIV reservoir.

METHODS

This study, conducted in Aarhus, Denmark, enrolled participants (n = 20) with CD4>500 cells/mm³ on cART. Six Vacc-4x (1.2 mg) intradermal immunizations using rhuGM-CSF (60 μg) as adjuvant were followed by 3 weekly intravenous infusions of romidepsin (5 mg/m²). Immune responses were determined by IFN-γ ELISpot, T-cell proliferation to p24 15-mer peptides covering the Vacc-4x region, intracellular cytokine staining (ICS) to the entire HIV^Gag and viral inhibition.

RESULTS

The frequency of participants with CD8+ T-cell proliferation assay positivity was 8/16 (50%) at baseline, 11/15 (73%) post-vaccination, 6/14 (43%) during romidepsin, and 9/15 (60%)post-romidepsin. Participants with CD8+ T-cell proliferation assay positivity post-vaccination showed reductions in total HIV DNA post-vaccination (p = 0.006; q = 0.183), post-latency reversal (p = 0.005; q = 0.183), and CA-RNA reductions post-vaccination (p = 0.015; q = 0.254). Participants (40%) were defined as proliferation 'Responders' having ≥2-fold increase in assay positivity post-baseline. Robust ELISpot baseline responses were found in 87.5% participants. No significant changes were observed in the proportion of polyfunctional CD8+ T-cells to HIV^Gag by ICS. There was a trend towards increased viral inhibition from baseline to post-vaccination (p = 0.08).

CONCLUSIONS

In this 'shock and kill' approach supported by therapeutic vaccination, CD8+ T-cell proliferation represents a valuable means to monitor functional immune responses as part of the path towards functional HIV cure.

The histone deacetylase inhibitor SAHA simultaneously reactivates HIV-1 from latency and up-regulates NKG2D ligands sensitizing for natural killer cell cytotoxicity

In pilot HIV-1 eradication studies, patients' immune responses were ineffective at killing viral reservoirs reactivated through latency reversing agents (LRAs) like suberoylanilide hydroxamic acid (SAHA). We hypothesized that T cells harboring reactivated HIV-1 express MIC and ULBP ligands for the activating NKG2D receptor of natural killer (NK) cells. Here, we demonstrated that MICA/B and ULBP2 are induced by SAHA on primary T cells harboring reactivated virus. Using latently HIV-1-infected J-Lat 6.3/8.4/9.2 and J1.1 cell lines, we showed that SAHA reverts latency and, simultaneously, up-regulates MICA/B and ULBP2 acting at the transcriptional level and through ATR activation, thus sensitizing T cells with reactivated virus to NKG2D-mediated killing by NK cells. Moreover, IL-2 and IL-15 potently boosted NKG2D expression and cytotoxicity of NK cells against SAHA-reactivated p24⁺ target cells. Therefore, immunotherapy with cytokines enhancing NKG2D-mediated NK-cell cytotoxicity combined with administration of LRAs up-modulating NKG2D ligands, represents a promising approach towards HIV-1 eradication.

The effect of Ingenol-B on the suppressive capacity of elite suppressor HIV-specific CD8+ T cells

Background

Some latency-reversing agents (LRAs) inhibit HIV-specific CD8+ T cell responses. In a prior study of protein kinase C (PKC) agonists, we found that bryostatin-1 inhibited elite controller/suppressor (ES) CD8+ T cell suppressive activity whereas prostratin had no effect. Ingenol-B is another PKC agonist with potent LRA activity both by itself and in combination with the bromodomain inhibitor JQ1; however its effect on CD8+ T cell mediated control of HIV-1 replication is unknown.

Methods

CD8+ T cells were isolated from ES and treated with bryostatin-1, prostratin, ingenol-B, and JQ1 as well as a combination of each PKC-agonist with JQ1. The cells were then tested in the viral suppression assay. To assess possible mechanisms of inhibition, CD8+ T cells were treated with the LRAs and analyzed for the expression of various immune cell markers.

Results

Ingenol-B had no effect on the ability of ES CD8+ T cells to suppress viral replication, however, the combination of ingenol-B and JQ1 caused a modest, but significant decrease in this suppressive capacity. The mechanism of the inhibitory effect of the JQ1 and ingenol-B combination relative to ingenol-B alone was unclear but the effect appeared to be dose dependent.

Conclusions

Ingenol-B does not inhibit HIV-specific CD8+ T cell responses in vitro. These responses are however modestly inhibited when 100 nMingenol-B is combined with JQ1. Since HIV-specific CD8+ T cell activity may be essential for the eradication of reactivated latently infected cells, the potency of latency-reversal activity of drug combinations must be balanced against the effects of the combinations on HIV-specific CD8+ T cell responses.

Toll-Like Receptor 7 Agonist GS-9620 Induces HIV Expression and HIV-Specific Immunity in Cells from HIV-Infected Individuals on Suppressive Antiretroviral Therapy

Antiretroviral therapy can suppress HIV replication to undetectable levels but does not eliminate latent HIV, thus necessitating lifelong therapy. Recent efforts to target this persistent reservoir have focused on inducing the expression of latent HIV so that infected cells may be recognized and eliminated by the immune system. Toll-like receptor (TLR) activation stimulates antiviral immunity and has been shown to induce HIV from latently infected cells. Activation of TLR7 leads to the production of several stimulatory cytokines, including type I interferons (IFNs). In this study, we show that the selective TLR7 agonist GS-9620 induced HIV in peripheral blood mononuclear cells (PBMCs) from HIV-infected individuals on suppressive antiretroviral therapy. GS-9620 increased extracellular HIV RNA 1.5- to 2-fold through a mechanism that required type I IFN signaling. GS-9620 also activated HIV-specific T cells and enhanced antibody-mediated clearance of HIV-infected cells. Activation by GS-9620 in combination with HIV peptide stimulation increased CD8 T cell degranulation, production of intracellular cytokines, and cytolytic activity. T cell activation was again dependent on type I IFNs produced by plasmacytoid dendritic cells. GS-9620 induced phagocytic cell maturation and improved effector-mediated killing of HIV-infected CD4 T cells by the HIV envelope-specific broadly neutralizing antibody PGT121. Collectively, these data show that GS-9620 can activate HIV production and improve the effector functions that target latently infected cells. GS-9620 may effectively complement orthogonal therapies designed to stimulate antiviral immunity, such as therapeutic vaccines or broadly neutralizing antibodies. Clinical studies are under way to determine if GS-9620 can target HIV reservoirs.

IMPORTANCE Though antiretroviral therapies effectively suppress viral replication, they do not eliminate integrated proviral DNA. This stable intermediate of viral infection is persistently maintained in reservoirs of latently infected cells. Consequently, lifelong therapy is required to maintain viral suppression. Ultimately, new therapies that specifically target and eliminate the latent HIV reservoir are needed. Toll-like receptor agonists are potent enhancers of innate antiviral immunity that can also improve the adaptive immune response. Here, we show that a highly selective TLR7 agonist, GS-9620, activated HIV from peripheral blood mononuclear cells isolated from HIV-infected individuals with suppressed infection. GS-9620 also improved immune effector functions that specifically targeted HIV-infected cells. Previously published studies on the compound in other chronic viral infections show that it can effectively induce immune activation at safe and tolerable clinical doses. Together, the results of these studies suggest that GS-9620 may be useful for treating HIV-infected individuals on suppressive antiretroviral therapy.

The Antiviral Immune Response and Its Impact on the HIV-1 Reservoir

Latently infected resting memory CD4⁺ T cells represent a major barrier to HIV-1 eradication. Studies have shown that it will not be possible to cure HIV-1 infection unless these cells are eliminated. Latently infected cells probably do not express viral antigens and thus may not be susceptible to the HIV-1 specific immune response, nevertheless the size and composition of the reservoir is influenced by the immune system. In this chapter, we review the different components of the HIV-1 specific immune response and discuss how the immune system can be harnessed to eradicate the virus.

In vitro effects of the small-molecule protein kinase C agonists on HIV latency reactivation

The persistence of latently HIV-infected cellular reservoirs represents the major obstacle to virus eradication in patients under antiretroviral therapy (ART). Cure strategies to eliminate these reservoirs are thus needed to reactivate proviral gene expression in latently infected cells. In this study, we tested optimal concentrations of PKC agonist candidates (PEP005/Ingenol-3-angelate, prostratin, bryostatin-1, and JQ1) to reactivate HIV latency in vitro, and examined their effects on cell survival, activation and epigenetic histone methylation after treatment alone or in combination in cell line and isolated CD4 T cells from SIV-infected macaques. The results showed that PKC agonists increased cell activation with different degrees of latency reactivation, concomitant with reduced levels of histone methylation. With increasing concentrations, prostratin and byrostain-1 treatment rapidly reduced cell survival and cell activation. The PKC agonist combinations, or in combination with JQ1, led to modest levels of synergistic reactivation of HIV. Remarkably, PEP005 treatment alone caused marked reactivation of HIV latency, similar to PMA stimulation. These findings suggested that PEP005 alone, as indicated its lower cytotoxicity and lower effective dose inducing maximal reactivation, might be a candidate for effectively reactivating HIV latency as part of a therapeutic strategy for HIV infection.