Ex Vivo Bioactivity and HIV-1 Latency Reversal by Ingenol Dibenzoate and Panobinostat in Resting CD4(+) T Cells from Aviremic Patients

Effective and targeted latency reversal in CD4+ T cells from individuals on long term combined antiretroviral therapy initiated during chronic HIV-1 infection

To date, an affordable, effective treatment for an HIV-1 cure remains only a concept with most "latency reversal" agents (LRAs) lacking specificity for the latent HIV-1 reservoir and failing in early clinical trials. We assessed HIV-1 latency reversal using a multivalent HIV-1-derived virus-like particle (HLP) to treat samples from 32 people living with HIV-1 (PLWH) in Uganda, US and Canada who initiated combined antiretroviral therapy (cART) during chronic infection. Even after 5-20 years on stable cART, HLP could target CD4+ T cells harbouring latent HIV-1 reservoir resulting in 100-fold more HIV-1 release into culture supernatant than by common recall antigens, and 1000-fold more than by chemotherapeutic LRAs. HLP induced release of a divergent and replication-competent HIV-1 population from PLWH on cART. These findings suggest HLP provides a targeted approach to reactivate the majority of latent HIV-1 proviruses among individuals infected with HIV-1.

Single-molecule RNA-FISH analysis reveals stochasticity in reactivation of latent HIV-1 regulated by Nuclear Orphan Receptors NR4A and cMYC

HIV-1 eradication strategies require complete reactivation of HIV-1 latent cells by Latency Reversing Agents (LRA). Current methods lack effectiveness due to incomplete proviral reactivation. We employed a single-molecule RNA-FISH (smRNA-FISH) and FISH-Quant analysis and found that proviral reactivation is highly variable from cell-to-cell, stochastic, and occurs in bursts and waves, with different kinetics in response to diverse LRAs. Approximately 1-5% of latent cells exhibited stochastic reactivation without LRAs. Through single-cell RNA-seq analysis, we identified NR4A3 and cMYC as extrinsic factors associated with stochastic HIV-1 reactivation. Concomitant with HIV-1 reactivation cMYC was downregulated and NR4A3 was upregulated in both latent cell lines and primary CD4 + T-cells from aviremic patients. By inhibiting cMYC using SN-38, an active metabolite of irinotecan, we induced NR4A3 and HIV-1 expression. Our results suggest that inherent stochasticity in proviral reactivation contributes to cell-to-cell variability, which could potentially be modulated by drugs targeting cMYC and NR4A3.

Mapping the interplay between NK cells and HIV: therapeutic implications

Although highly effective at durably suppressing plasma HIV-1 viremia, combination antiretroviral therapy (ART) treatment regimens do not eradicate the virus, which persists in long-lived CD4+ T cells. This latent viral reservoir serves as a source of plasma viral rebound following treatment interruption, thus requiring lifelong adherence to ART. Additionally, challenges remain related not only to access to therapy but also to a higher prevalence of comorbidities with an inflammatory etiology in treated HIV-1+ individuals, underscoring the need to explore therapeutic alternatives that achieve sustained virologic remission in the absence of ART. Natural killer (NK) cells are uniquely positioned to positively impact antiviral immunity, in part due to the pleiotropic nature of their effector functions, including the acquisition of memory-like features, and, therefore, hold great promise for transforming HIV-1 therapeutic modalities. In addition to defining the ability of NK cells to contribute to HIV-1 control, this review provides a basic immunologic understanding of the impact of HIV-1 infection and ART on the phenotypic and functional character of NK cells. We further delineate the qualities of "memory" NK cell populations, as well as the impact of HCMV on their induction and subsequent expansion in HIV-1 infection. We conclude by highlighting promising avenues for optimizing NK cell responses to improve HIV-1 control and effect a functional cure, including blockade of inhibitory NK receptors, TLR agonists to promote latency reversal and NK cell activation, CAR NK cells, BiKEs/TriKEs, and the role of HIV-1-specific bNAbs in NK cell-mediated ADCC activity against HIV-1-infected cells.

Host Cell Redox Alterations Promote Latent HIV-1 Reactivation through Atypical Transcription Factor Cooperativity

Immune cell state alterations rewire HIV-1 gene expression, thereby influencing viral latency and reactivation, but the mechanisms are still unfolding. Here, using a screen approach on CD4⁺ T cell models of HIV-1 latency, we revealed Small Molecule Reactivators (SMOREs) with unique chemistries altering the CD4⁺ T cell state and consequently promoting latent HIV-1 transcription and reactivation through an unprecedented mechanism of action. SMOREs triggered rapid oxidative stress and activated a redox-responsive program composed of cell-signaling kinases (MEK-ERK axis) and atypical transcription factor (AP-1 and HIF-1α) cooperativity. SMOREs induced an unusual AP-1 phosphorylation signature to promote AP-1/HIF-1α binding to the latent HIV-1 proviral genome for its activation. Consistently, latent HIV-1 reactivation was compromised with pharmacologic inhibition of oxidative stress sensing or of cell-signaling kinases, and transcription factor’s loss of expression, thus functionally linking the host redox-responsive program to viral transcriptional rewiring. Notably, SMOREs induced the redox program in primary CD4⁺ T cells and reactivated latent HIV-1 in aviremic patient samples alone and in combination with known latency-reversing agents, thus providing physiological relevance. Our findings suggest that manipulation of redox-sensitive pathways could be exploited to alter the course of HIV-1 latency, thus rendering host cells responsive to help achieve a sterilizing cure.

Inhibition of a Chromatin and Transcription Modulator, SLTM, Increases HIV-1 Reactivation Identified by a CRISPR Inhibition Screen

Despite effective antiretroviral therapy, HIV-1 persistence in latent reservoirs remains a major obstacle to a cure. We postulate that HIV-1 silencing factors suppress HIV-1 reactivation and that inhibition of these factors will increase HIV-1 reactivation. To identify HIV-1 silencing factors, we conducted a genome-wide CRISPR inhibition (CRISPRi) screen using four CRISPRi-ready, HIV-1-d6-GFP-infected Jurkat T cell clones with distinct integration sites. We sorted cells with increased green fluorescent protein (GFP) expression and captured single guide RNAs (sgRNAs) via targeted deep sequencing. We identified 18 HIV-1 silencing factors that were significantly enriched in HIV-1-d6-GFPhigh cells. Among them, SLTM (scaffold attachment factor B-like transcription modulator) is an epigenetic and transcriptional modulator having both DNA and RNA binding capacities not previously known to affect HIV-1 transcription. Knocking down SLTM by CRISPRi significantly increased HIV-1-d6-GFP expression (by 1.9- to 4.2-fold) in three HIV-1-d6-GFP-Jurkat T cell clones. Furthermore, SLTM knockdown increased the chromatin accessibility of HIV-1 and the gene in which HIV-1 is integrated but not the housekeeping gene POLR2A. To test whether SLTM inhibition can reactivate HIV-1 and further induce cell death of HIV-1-infected cells ex vivo, we established a small interfering RNA (siRNA) knockdown method that reduced SLTM expression in CD4+ T cells from 10 antiretroviral therapy (ART)-treated, virally suppressed, HIV-1-infected individuals ex vivo. Using limiting dilution culture, we found that SLTM knockdown significantly reduced the frequency of HIV-1-infected cells harboring inducible HIV-1 by 62.2% (0.56/106 versus 1.48/106 CD4+ T cells [P = 0.029]). Overall, our study indicates that SLTM inhibition reactivates HIV-1 in vitro and induces cell death of HIV-1-infected cells ex vivo. Our study identified SLTM as a novel therapeutic target. IMPORTANCE HIV-1-infected cells, which can survive drug treatment and immune cell killing, prevent an HIV-1 cure. Immune recognition of infected cells requires HIV-1 protein expression; however, HIV-1 protein expression is limited in infected cells after long-term therapy. The ways in which the HIV-1 provirus is blocked from producing protein are unknown. We identified a new host protein that regulates HIV-1 gene expression. We also provided a new method of studying HIV-1-host factor interactions in cells from infected individuals. These improvements may enable future strategies to reactivate HIV-1 in infected individuals so that infected cells can be killed by immune cells, drug treatment, or the virus itself.

The Role of Ingenane Diterpenes in Cancer Therapy: From Bioactive Secondary Compounds to Small Molecules

Diterpenes are a class of critical taxonomic markers of the Euphorbiaceae family, representing small compounds (eg, molecules) with a wide range of biological activities and multi-target therapeutic potential. Diterpenes can exert different activities, including antitumor and multi-drug resistance-reversing activities, and antiviral, immunomodulatory, and anti-inflammatory effects, mainly due to their great structural diversity. In particular, one polycyclic skeleton has been highlighted: ingenane. Besides this natural diterpene, promising polycyclic skeletons may be submitted to chemical modification—by in silico approaches, chemical reactions, or biotransformation—putatively providing more active analogs (eg, ingenol derivatives), which are currently under pre-clinical investigation. This review outlines the current mechanisms of action and potential therapeutic implications of ingenol diterpenes as small cancer molecules.

Changes to the Simian Immunodeficiency Virus (SIV) Reservoir and Enhanced SIV-Specific Responses in a Rhesus Macaque Model of Functional Cure after Serial Rounds of Romidepsin Administrations

HIV persistence requires lifelong antiretroviral therapy (ART), calling for a cure. The histone deacetylase inhibitor, romidepsin, is used in the "shock and kill" approach with the goal of reactivating virus and subsequently clearing infected cells through cell-mediated immune responses. We tested serial and double infusions of romidepsin in a rhesus macaque (RM) model of SIV functional cure, which controls virus without ART. Off ART, romidepsin reactivated SIV in all RMs. Subsequent infusions resulted in diminished reactivation, and two RMs did not reactivate the virus after the second or third infusions. Therefore, those two RMs received CD8-depleting antibody to assess the replication competence of the residual reservoir. The remaining RMs received double infusions, i.e., two doses separated by 48-h. Double infusions were well tolerated, induced immune activation, and effectively reactivated SIV. Although reactivation was gradually diminished, cell-associated viral DNA was minimally changed, and viral outgrowth occurred in 4/5 RMs. In the RM which did not reactivate after CD8 depletion, viral outgrowth was not detected in peripheral blood mononuclear cells (PBMC)-derived CD4+ cells. The frequency of SIV-specific CD8+ T cells increased after romidepsin administration, and the increased SIV-specific immune responses were associated, although not statistically, with the diminished reactivation. Thus, our data showing sequential decreases in viral reactivation with repeated romidepsin administrations with all RMs and absence of viral reactivation after CD8+ T-cell depletion in one animal suggest that, in the context of healthy immune responses, romidepsin affected the inducible viral reservoir and gradually increased immune-mediated viral control. Given the disparities between the results of romidepsin administration to ART-suppressed SIVmac239-infected RMs and HIV-infected normal progressors compared to our immune-healthy model, our data suggest that improving immune function for greater SIV-specific responses should be the starting point of HIV cure strategies. IMPORTANCE HIV cure is sought after due to the prevalence of comorbidities that occur in persons with HIV. One of the most investigated HIV cure strategies is the "shock and kill" approach. Our study investigated the use of romidepsin, a histone deacetylase (HDAC) inhibitor, in our rhesus macaque model of functional cure, which allows for better resolution of viral reactivation due to the lack of antiretroviral therapy. We found that repeated rounds of romidepsin resulted in gradually diminished viral reactivation. One animal inevitably lacked replication-competent virus in the blood. With the accompanying enhancement of the SIV-specific immune response, our data suggest that there is a reduction of the viral reservoir in one animal by the cell-mediated immune response. With the differences observed between our model and persons living with HIV (PWH) treated with romidepsin, specifically in the context of a healthy immune system in our model, our data thereby indicate the importance of restoring the immune system for cure strategies.

Insights Into Persistent HIV-1 Infection and Functional Cure: Novel Capabilities and Strategies

Although HIV-1 replication can be efficiently suppressed to undetectable levels in peripheral blood by combination antiretroviral therapy (cART), lifelong medication is still required in people living with HIV (PLWH). Life expectancies have been extended by cART, but age-related comorbidities have increased which are associated with heavy physiological and economic burdens on PLWH. The obstacle to a functional HIV cure can be ascribed to the formation of latent reservoir establishment at the time of acute infection that persists during cART. Recent studies suggest that some HIV reservoirs are established in the early acute stages of HIV infection within multiple immune cells that are gradually shaped by various host and viral mechanisms and may undergo clonal expansion. Early cART initiation has been shown to reduce the reservoir size in HIV-infected individuals. Memory CD4+ T cell subsets are regarded as the predominant cellular compartment of the HIV reservoir, but monocytes and derivative macrophages or dendritic cells also play a role in the persistent virus infection. HIV latency is regulated at multiple molecular levels in transcriptional and post-transcriptional processes. Epigenetic regulation of the proviral promoter can profoundly regulate the viral transcription. In addition, transcriptional elongation, RNA splicing, and nuclear export pathways are also involved in maintaining HIV latency. Although most proviruses contain large internal deletions, some defective proviruses may induce immune activation by expressing viral proteins or producing replication-defective viral-like particles. In this review article, we discuss the state of the art on mechanisms of virus persistence in the periphery and tissue and summarize interdisciplinary approaches toward a functional HIV cure, including novel capabilities and strategies to measure and eliminate the infected reservoirs and induce immune control.

The ingenol-based protein kinase C agonist GSK445A is a potent inducer of HIV and SIV RNA transcription

Activation of the NF-κB signaling pathway by Protein Kinase C (PKC) agonists is a potent mechanism for human immunodeficiency virus (HIV) latency disruption in vitro. However, significant toxicity risks and the lack of evidence supporting their activity in vivo have limited further evaluation of PKC agonists as HIV latency-reversing agents (LRA) in cure strategies. Here we evaluated whether GSK445A, a stabilized ingenol-B derivative, can induce HIV/simian immunodeficiency virus (SIV) transcription and virus production in vitro and demonstrate pharmacological activity in nonhuman primates (NHP). CD4⁺ T cells from people living with HIV and from SIV⁺ rhesus macaques (RM) on antiretroviral therapy (ART) exposed in vitro to 25 nM of GSK445A produced cell-associated viral transcripts as well as viral particles at levels similar to those induced by PMA/Ionomycin, indicating that GSK445A can potently reverse HIV/SIV latency. Importantly, these concentrations of GSK445A did not impair the proliferation or survival of HIV-specific CD8⁺ T cells, but instead, increased their numbers and enhanced IFN-γ production in response to HIV peptides. In vivo, GSK445A tolerability was established in SIV-naïve RM at 15 μg/kg although tolerability was reduced in SIV-infected RM on ART. Increases in plasma viremia following GSK445A administration were suggestive of increased SIV transcription in vivo. Collectively, these results indicate that GSK445A is a potent HIV/SIV LRA in vitro and has a tolerable safety profile amenable for further evaluation in vivo in NHP models of HIV cure/remission.

Antiretroviral therapy (ART) is not a definitive cure for HIV infection, in part, because the virus is able to integrate its genetic material in the host cell and remain in a dormant but fully replication-competent form during ART. These latently-infected cells can persist for long periods of time and remain hidden from the host’s immune system. If ART is stopped, the virus can reactivate from this pool of infected cells and resume HIV replication and disease progression. As such, finding and eliminating cells with latent HIV infection is priority for HIV cure research. One approach is to use compounds referred to as latency-reversing agents, that can induce HIV reactivation during ART. The goal of this approach is to facilitate elimination of infected cells by the virus itself once it reactivates or by the host’s immune system, once virus induction renders the cells detectable by the immune system, while also preventing the virus from infecting new cells due to the continued presence of ART. In this study we report on the activity of a novel latency-reversing agent called GSK445A, a potent activator of the enzyme protein kinase C (PKC). We show that GSK445A can induce HIV and simian immunodeficiency virus (SIV) latency reversal in vitro and has a tolerable saftey profile in nonhuman primates that should permit further testing of this PKC-agonist in strategies to cure HIV.

So Pathogenic or So What?-A Brief Overview of SIV Pathogenesis with an Emphasis on Cure Research

HIV infection requires lifelong antiretroviral therapy (ART) to control disease progression. Although ART has greatly extended the life expectancy of persons living with HIV (PWH), PWH nonetheless suffer from an increase in AIDS-related and non-AIDS related comorbidities resulting from HIV pathogenesis. Thus, an HIV cure is imperative to improve the quality of life of PWH. In this review, we discuss the origins of various SIV strains utilized in cure and comorbidity research as well as their respective animal species used. We briefly detail the life cycle of HIV and describe the pathogenesis of HIV/SIV and the integral role of chronic immune activation and inflammation on disease progression and comorbidities, with comparisons between pathogenic infections and nonpathogenic infections that occur in natural hosts of SIVs. We further discuss the various HIV cure strategies being explored with an emphasis on immunological therapies and "shock and kill".

Histone deacetylase inhibition reduces deleterious cytokine release induced by ingenol stimulation

Latency reversing agents (LRAs), such as protein kinase C (PKC) agonists, constitute a promising strategy for exposing and eliminating the HIV-1 latent reservoir. PKC agonists activate NF-κB and induce deleterious pro-inflammatory cytokine production. Adjuvant pharmacological agents, such as ruxolitinib, a JAK inhibitor, have previously been combined with LRAs to reduce deleterious pro-inflammatory cytokine secretion without inhibiting HIV-1 reactivation in vitro. Histone deacetylase inhibitors (HDACi) are known to dampen pro-inflammatory cytokine secretion in the context of other diseases and synergize with LRAs to reactivate latent HIV-1. This study investigates whether a panel of epigenetic modifiers, including HDACi, could dampen PKC-induced pro-inflammatory cytokine secretion during latency reversal. We screened an epigenetic modifier library for compounds that reduced intracellular IL-6 production induced by the PKC agonist Ingenol-3,20-dibenzoate. We further tested the most promising epigenetic inhibitor class, HDACi, for their ability to reduce pro-inflammatory cytokines and reactivate latent HIV-1 ex vivo. We identified nine epigenetic modulators that reduced PKC-induced intracellular IL-6. In cells from aviremic individuals living with HIV-1, the HDAC1-3 inhibitor, suberohydroxamic acid (SBHA), reduced secretion of pro-inflammatory cytokines TNF-α, IL-5, IL-2r, and IL-17 but did not significantly reactivate latent HIV-1 when combined with Ingenol-3,20-dibenzoate. Combining SBHA and Ingenol-3,20-dibenzoate reduces deleterious cytokine production during latency reversal but does not induce significant viral reactivation in aviremic donor PBMCs. The ability of SBHA to reduce PKC-induced pro-inflammatory cytokines when combined with Ingenol-3,20-dibenzoate suggests SBHA can be used to reduced PKC induced pro-inflammatory cytokines but not to achieve latency reversal in the context of HIV-1.

Bryostatin-1 decreases HIV-1 infection and viral production in human primary macrophages

While combination antiretroviral therapy maintains undetectable viremia in People Living With HIV (PLWH), a life-long treatment is necessary to prevent viremic rebound after therapy cessation. This rebound seemed mainly caused by long lived HIV-1 latently infected cells reversing to a viral productive status. Reversing latency and elimination of these cells by the so-called shock and kill strategy is one of the main investigated leads to achieve an HIV-1 cure. Small molecules referred as latency reversal agents (LRAs) proved to efficiently reactivate latent CD4⁺ T cells. However, LRAs impact on de novo infection or HIV-1 production in productively infected macrophages remain elusive. Nontoxic doses of bryostatin-1, JQ1 and romidepsin were investigated in human monocyte-derived macrophages (MDMs). Treatment with bryostatin-1 or romidepsin resulted in a downregulation of CD4 and CCR5 receptors respectively, accompanied by a reduction of R5 tropic virus infection. HIV-1 replication was mainly regulated by receptor modulation for bryostatin-1, while romidepsin effect rely on upregulation of SAMHD1 activity. LRA stimulation of chronically infected cells did not enhance neither HIV-1 production nor gene expression. Surprisingly, bryostatin-1 caused a major decrease in viral production. This effect was not viral strain specific but appears to occur only in myeloid cells. Bryostatin-1 treatment of infected MDMs led to decreased amounts of capsid and matrix mature proteins with little to no modulation of precursors. Our observations revealed that bryostatin-1-treated myeloid and CD4⁺ T cells are responding differently upon HIV-1 infection. Therefore, additional studies are warranted to more fully assess the efficiency of HIV-1 eradicating strategies. Importance HIV-1 persists in a cellular latent form despite therapy that quickly propagates infection upon treatment interruption. Reversing latency would contribute to eradicate these cells, closing a gap to a cure. Macrophages are an acknowledged HIV-1 reservoir during therapy and are suspected to harbor latency establishment in vivo. Yet, the impact of latency reversal agents (LRAs) on HIV-1 infection and viral production in human macrophages is poorly known but nonetheless crucial to probe the safety of this strategy. In this in vitro study, we discovered encouraging anti-replicative features of distinct LRAs in human macrophages. We also described a new viral production inhibition mechanism by protein kinase C agonists which is specific to myeloid cells. This study provides new insights on HIV-1 propagation restriction potentials by LRAs in human macrophages and underline the importance of assessing latency reversal strategy on all HIV-1 targeted cells.

Addressing an HIV cure in LMIC

HIV-1 persists in infected individuals despite years of antiretroviral therapy (ART), due to the formation of a stable and long-lived latent viral reservoir. Early ART can reduce the latent reservoir and is associated with post-treatment control in people living with HIV (PLWH). However, even in post-treatment controllers, ART cessation after a period of time inevitably results in rebound of plasma viraemia, thus lifelong treatment for viral suppression is indicated. Due to the difficulties of sustained life-long treatment in the millions of PLWH worldwide, a cure is undeniably necessary. This requires an in-depth understanding of reservoir formation and dynamics. Differences exist in treatment guidelines and accessibility to treatment as well as social stigma between low- and-middle income countries (LMICs) and high-income countries. In addition, demographic differences exist in PLWH from different geographical regions such as infecting viral subtype and host genetics, which can contribute to differences in the viral reservoir between different populations. Here, we review topics relevant to HIV-1 cure research in LMICs, with a focus on sub-Saharan Africa, the region of the world bearing the greatest burden of HIV-1. We present a summary of ART in LMICs, highlighting challenges that may be experienced in implementing a HIV-1 cure therapeutic. Furthermore, we discuss current research on the HIV-1 latent reservoir in different populations, highlighting research in LMIC and gaps in the research that may facilitate a global cure. Finally, we discuss current experimental cure strategies in the context of their potential application in LMICs.

Potential Utility of Natural Killer Cells for Eliminating Cells Harboring Reactivated Latent HIV-1 Following the Removal of CD8+ T Cell-Mediated Pro-Latency Effect(s)

An impediment to curing HIV-1 infection is the persistence of latently infected cells in ART-treated people living with HIV (PLWH). A key strategy for curing HIV-1 infection is to activate transcription and translation of latent virus using latency reversing agents (LRAs) and eliminate cells harboring reactivated virus via viral cytopathic effect or immune clearance. In this review, we provide an overview of available LRAs and their use in clinical trials. Furthermore, we describe recent data suggesting that CD8⁺ T cells promote HIV-1 latency in the context of ART, even in the presence of LRAs, which might at least partially explain the clinical inefficiency of previous “shock and kill” trials. Here, we propose a novel cure strategy called “unlock, shock, disarm, and kill”. The general premise of this strategy is to shut down the pro-latency function(s) of CD8⁺ T cells, use LRAs to reverse HIV-1 latency, counteract anti-apoptotic molecules, and engage natural killer (NK) cells to mediate the killing of cells harboring reactivated latent HIV-1.

Activation of the Anti-Oxidative Stress Response Reactivates Latent HIV-1 Through the Mitochondrial Antiviral Signaling Protein Isoform MiniMAVS

The mitochondrial antiviral signaling protein (MAVS) is part of the cell's innate immune mechanism of defense. MAVS mRNA is bicistronic and can give rise to a full length-MAVS and a shorter isoform termed miniMAVS. In response to viral infections, viral RNA can be sensed by the cytosolic RNA sensors retinoic acid-inducible gene I (RIG-I) and/or melanoma differentiation-associated protein 5 (MDA5) and activate NF-κB through interaction with MAVS. MAVS can also sense cellular stress and activate an anti-oxidative stress (AOS) response through the activation of NF-κB. Because NF-κB is a main cellular transcription factor for HIV-1, we wanted to address what role MAVS plays in HIV-1 reactivation from latency in CD4 T cells. Our results indicate that RIG-I agonists required full length-MAVS whereas the AOS response induced by Dynasore through its catechol group can reactivate latent HIV-1 in a MAVS dependent manner through miniMAVS isoform. Furthermore, we uncover that PKC agonists, a class of latency-reversing agents, induce an AOS response in CD4 T cells and require miniMAVS to fully reactivate latent HIV-1. Our results indicate that the AOS response, through miniMAVS, can induce HIV-1 transcription in response to cellular stress and targeting this pathway adds to the repertoire of approaches to reactivate latent HIV-1 in 'shock-and-kill' strategies.

Diverse Mucosal-Associated Invariant TCR Usage in HIV Infection

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that specifically target bacterial metabolites but are also identified as innate-like sensors of viral infection. Individuals with chronic HIV-1 infection have lower numbers of circulating MAIT cells compared with healthy individuals, yet the features of the MAIT TCR repertoire are not well known. We isolated and stimulated human PBMCs from healthy non-HIV-infected donors (HD), HIV-infected progressors on antiretroviral therapy, and HIV-infected elite controllers (EC). We sorted MAIT cells using flow cytometry and used a high-throughput sequencing method with bar coding to link the expression of TCRα, TCRβ, and functional genes of interest at the single-cell level. We show differential patterns of MAIT TCR usage among the groups. We observed expansions of certain dominant MAIT clones in HIV-infected individuals upon Escherichia coli stimulation, which was not observed in clones of HD. We also found different patterns of CDR3 amino acid distributions among the three groups. Furthermore, we found blunted expression of phenotypic genes in HIV individuals; most notably, HD mounted a robust IFNG response to stimulation, whereas both HIV-infected progressors and EC did not. In conclusion, our study describes the diverse MAIT TCR repertoire of persons with chronic HIV-1 infection and suggest that MAIT clones of HIV-infected persons may be primed for expansion more than that of noninfected persons. Further studies are needed to examine the functional significance of unique MAIT cell TCR usage in EC.

Humanized Mice for the Evaluation of Novel HIV-1 Therapies

With the discovery of antiretroviral therapy, HIV-1 infection has transitioned into a manageable but chronic illness, which requires lifelong treatment. Nevertheless, complete eradication of the virus has still eluded us. This is partly due to the virus’s ability to remain in a dormant state in tissue reservoirs, ‘hidden’ from the host’s immune system. Also, the high mutation rate of HIV-1 results in escape mutations in response to many therapeutics. Regardless, the development of novel cures for HIV-1 continues to move forward with a range of approaches from immunotherapy to gene editing. However, to evaluate in vivo pathogenesis and the efficacy and safety of therapeutic approaches, a suitable animal model is necessary. To this end, the humanized mouse was developed by McCune in 1988 and has continued to be improved on over the past 30 years. Here, we review the variety of humanized mouse models that have been utilized through the years and describe their specific contribution in translating HIV-1 cure strategies to the clinic.

The Intact Non-Inducible Latent HIV-1 Reservoir is Established In an In Vitro Primary TCM Cell Model of Latency

The establishment of HIV-1 latency has hindered an HIV-1 cure. "Shock and Kill" strategies to target this reservoir aim to induce the latent provirus with latency reversing agents (LRAs). However, recent studies have shown that the majority of the intact HIV-1 viral reservoir found in ART-suppressed HIV infected individuals is not inducible. We sought to understand whether this non-inducible reservoir is established, and thus able to be studied, in an in vitro primary T_CM model of latency. Furthermore, we wanted to expand this model system to include R5-tropic and non-B subtype viruses. To that end, we generated our T_CM model of latency with an R5 subtype B virus, AD8 and an R5 subtype C virus, MJ4. Our results demonstrate that both intact and defective proviruses are generated in this model. Less than 50% of intact proviruses are inducible regardless of viral strain in the context of maximal stimulation through the TCR or with different clinically relevant LRAs including the HDAC inhibitors SAHA and MS-275, the PKC agonist Ingenol 3,20-dibenzoate or the SMAC mimetic AZD-5582. Our findings suggest that current LRA strategies are insufficient to effectively reactivate intact latent HIV-1 proviruses in primary CD4 T_CM cells and that the mechanisms involved in the generation of the non-inducible HIV-1 reservoir can be studied using this primary in vitro model.Importance: HIV-1 establishes a latent reservoir that persists under antiretroviral therapy. Antiretroviral therapy is able to stop the spread of the virus and the progression of the disease but does not target this latent reservoir. If antiretroviral therapy is stopped, the virus is able to resume replication and the disease progresses. Recently, it has been demonstrated that most of the latent reservoir capable of generating replication competent virus cannot be induced in the laboratory setting. However, the mechanisms that influence the generation of this intact and non-inducible latent reservoir are still under investigation. Here we demonstrate the generation of defective, intact and intact non-inducible latent HIV-1 in a T_CM model of latency using different HIV-1 strains. Thus, the mechanisms which control inducibility can be studied using this primary cell model of latency, which may accelerate our understanding of the latent reservoir and the development of curative strategies.

HIV latency reversal agents: A potential path for functional cure?

Despite the advances in Human Immunodeficiency Virus (HIV) treatment, the cure for all HIV patients still poses a major challenge, which needs to be surpassed in the coming years. Among the strategies pursuing this aim, the 'kick-and-kill' approach, which involves the reactivation and elimination of a latent HIV reservoir that resides in some CD4⁺ T cells, appears promising. The first step of this approach requires the use of latency reversal agents (LRAs) that induce the reactivation of the latent virus. Although several classes of LRAs have been reported so far, some limitations of these compounds still need to be overcome before their clinical use. The complete exhaustion of the reservoir of latent virus will contribute to promote the second step of this approach, facilitating the elimination of the reactivated HIV. Therefore, potent, safe, and non-toxic LRAs are necessary to promote efficient elimination of the HIV-1 virus from its reservoir. In this review article, we focus on the promising LRAs that have been described in the literature over the past few years, highlighting the advantages and disadvantages of their use in the 'kick and kill' approach, thus opening a new avenue in the development of a potential cure.

Block-And-Lock: New Horizons for a Cure for HIV-1

HIV-1/AIDS remains a global public health problem. The world health organization (WHO) reported at the end of 2019 that 38 million people were living with HIV-1 worldwide, of which only 67% were accessing antiretroviral therapy (ART). Despite great success in the clinical management of HIV-1 infection, ART does not eliminate the virus from the host genome. Instead, HIV-1 remains latent as a viral reservoir in any tissue containing resting memory CD4⁺ T cells. The elimination of these residual proviruses that can reseed full-blown infection upon treatment interruption remains the major barrier towards curing HIV-1. Novel approaches have recently been developed to excise or disrupt the virus from the host cells (e.g., gene editing with the CRISPR-Cas system) to permanently shut off transcription of the virus (block-and-lock and RNA interference strategies), or to reactivate the virus from cell reservoirs so that it can be eliminated by the immune system or cytopathic effects (shock-and-kill strategy). Here, we will review each of these approaches, with the major focus placed on the block-and-lock strategy.

Latency-Reversing Agents Induce Differential Responses in Distinct Memory CD4 T Cell Subsets in Individuals on Antiretroviral Therapy

Latent proviruses persist in central (T_CM), transitional (T_TM), and effector (T_EM) memory cells. We measured the levels of cellular factors involved in HIV gene expression in these subsets. The highest levels of acetylated H4, active nuclear factor κB (NF-κB), and active positive transcription elongation factor b (P-TEFb) were measured in T_EM, T_CM, and T_TM cells, respectively. Vorinostat and romidepsin display opposite abilities to induce H4 acetylation across subsets. Protein kinase C (PKC) agonists are more efficient at inducing NF-κB phosphorylation in T_CM cells but more potent at activating PTEF-b in the T_EM subset. We selected the most efficient latency-reversing agents (LRAs) and measured their ability to reverse latency in each subset. While ingenol alone has modest activities in the three subsets, its combination with a histone deacetylase inhibitor (HDACi) dramatically increases latency reversal in T_CM cells. Altogether, these results indicate that cellular HIV reservoirs are differentially responsive to common LRAs and suggest that combination of compounds will be required to achieve latency reversal in all subsets.

Proteasomal Inhibition Potentiates Latent HIV Reactivation

Despite the success of antiretroviral therapy (ART), ART fails to eradicate the virus and HIV cure has remained beyond the reach of current treatments. ART targets replicating virally infected but not latently infected cells, which have limited expression of factors important for proliferation and cellular activity, including positive transcription elongation factor b (P-TEFb) and nuclear factor κB (NF-κB). Levels of the cyclin T1 (CycT1) subunit of P-TEFb are low to absent in resting T cells, and treatment with proteasome inhibitors (PIs) increases CycT1 protein levels to those of proliferating T cells. In this study, the clinically approved PI bortezomib reactivated latent HIV in latently infected primary CD4+ T cells. Bortezomib not only increased levels of CycT1 but also activated NF-κB. Strikingly, as opposed to most currently researched latency reversing agents (LRAs), bortezomib did not require a second LRA to potently reactivate latent HIV. Effects of bortezomib on resting T cells and reactivation of HIV suggest a possible direction for future attempts to diminish the viral reservoir in HIV+ individuals.

Maraviroc reactivates HIV with potency similar to that of other latency reversing drugs without inducing toxicity in CD8 T cells

Human immunodeficiency virus (HIV) remains incurable due to latent reservoirs established in non-activated CD4 T cells. Current efforts to achieve a functional cure rely on immunomodulatory strategies focused on enhancing the functions of cytotoxic cells. Implementation of these actions requires a coordinated activation of the viral transcription in latently infected cells so that the reservoir became visible and accessible to cytotoxic cells. As no latency reversing agent (LRA) has been shown to be completely effective, new combinations are of increasing importance. Recent data have shown that maraviroc is a new LRA. In this work, we have explored how the combination of maraviroc with other LRAs influences on HIV reactivation using in vitro latency models as well as on the cell viability of CD8 T cells from ART-treated patients. Maraviroc reactivated HIV with a potency similar to other LRAs. Triple combinations resulted toxic and were rejected. No dual combination was synergistic. The combination with panobinostat or disulfiram maintained the effect of both drugs without inducing cell proliferation or toxicity. Maraviroc does not alter the viability of CD8 T cells isolated from patients under antiretroviral treatment. This finding enhances the properties of maraviroc as a LRA.

Naive CD4+ T Cells Harbor a Large Inducible Reservoir of Latent, Replication-competent Human Immunodeficiency Virus Type 1

BACKGROUND

The latent human immunodeficiency virus type 1 (HIV-1) reservoir represents a major barrier to a cure. Based on the levels of HIV-1 DNA in naive (TN) vs resting memory CD4+ T cells, it is widely hypothesized that this reservoir resides primarily within memory cells. Here, we compared virus production from TN and central memory (TCM) CD4+ T cells isolated from HIV-1-infected individuals on suppressive therapy.

METHODS

CD4+ TN and TCM cells were purified from the blood of 7 HIV-1-infected individuals. We quantified total HIV-1 DNA in the CD4+ TN and TCM cells. Extracellular virion-associated HIV-1 RNA or viral outgrowth assays were used to assess latency reversal following treatment with anti-CD3/CD28 monoclonal antibodies (mAbs), phytohaemagglutinin/interleukin-2, phorbol 12-myristate 13-acetate/ionomycin, prostratin, panobinostat, or romidepsin.

RESULTS

HIV-1 DNA was significantly higher in TCM compared to TN cells (2179 vs 684 copies/106 cells, respectively). Following exposure to anti-CD3/CD28 mAbs, virion-associated HIV-1 RNA levels were similar between TCM and TN cells (15 135 vs 18 290 copies/mL, respectively). In 4/7 donors, virus production was higher for TN cells independent of the latency reversing agent used. Replication-competent virus was recovered from both TN and TCM cells.

CONCLUSIONS

Although the frequency of HIV-1 infection is lower in TN compared to TCM cells, as much virus is produced from the TN population after latency reversal. This finding suggests that quantifying HIV-1 DNA alone may not predict the size of the inducible latent reservoir and that TN cells may be an important reservoir of latent HIV-1.

Fingolimod inhibits multiple stages of the HIV-1 life cycle

Antiretroviral drugs that target various stages of the Human Immunodeficiency Virus (HIV) life cycle have been effective in curbing the AIDS epidemic. However, drug resistance, off-target effects of antiretroviral therapy (ART), and varying efficacy in prevention underscore the need to develop novel and alternative therapeutics. In this study, we investigated whether targeting the signaling molecule Sphingosine-1-phosphate (S1P) would inhibit HIV-1 infection and generation of the latent reservoir in primary CD4 T cells. We show that FTY720 (Fingolimod), an FDA-approved functional antagonist of S1P receptors, blocks cell-free and cell-to-cell transmission of HIV and consequently reduces detectable latent virus. Mechanistically, FTY720 impacts the HIV-1 life cycle at two levels. Firstly, FTY720 reduces the surface density of CD4, thereby inhibiting viral binding and fusion. Secondly, FTY720 decreases the phosphorylation of the innate HIV restriction factor SAMHD1 which is associated with reduced levels of total and integrated HIV, while reducing the expression of Cyclin D3. In conclusion, targeting the S1P pathway with FTY720 could be a novel strategy to inhibit HIV replication and reduce the seeding of the latent reservoir.

Structure-Activity Relationship Analysis of Benzotriazine Analogues as HIV-1 Latency-Reversing Agents

“Shock and kill” therapeutic strategies toward HIV eradication are based on the transcriptional activation of latent HIV with a latency-reversing agent (LRA) and the consequent killing of the reactivated cell by either the cytopathic effect of HIV or an arm of the immune system. We have recently found several benzotriazole and benzotriazine analogues that have the ability to reactivate latent HIV by inhibiting signal transducer and activator of transcription 5 (STAT5) SUMOylation and promoting STAT5 binding to the HIV long terminal repeat and increasing its transcriptional activity.

“Shock and kill” therapeutic strategies toward HIV eradication are based on the transcriptional activation of latent HIV with a latency-reversing agent (LRA) and the consequent killing of the reactivated cell by either the cytopathic effect of HIV or an arm of the immune system. We have recently found several benzotriazole and benzotriazine analogues that have the ability to reactivate latent HIV by inhibiting signal transducer and activator of transcription 5 (STAT5) SUMOylation and promoting STAT5 binding to the HIV long terminal repeat and increasing its transcriptional activity. To understand the essential structural groups required for biological activity of these molecules, we performed a systematic analysis of >40 analogues. First, we characterized the essential motifs within these molecules that are required for their biological activity. Second, we identified three benzotriazine analogues with similar activity. We demonstrated that these three compounds are able to increase STAT5 phosphorylation and transcriptional activity. All active analogues reactivate latent HIV in a primary cell model of latency and enhance the ability of interleukin-15 to reactivate latent HIV in cells isolated from aviremic participants. Third, this family of compounds also promote immune effector functions in vitro in the absence of toxicity or global immune activation. Finally, initial studies in mice suggest lack of acute toxicity in vivo. A better understanding of the biological activity of these compounds will help in the design of improved LRAs that work via inhibition of STAT5 SUMOylation.

Pharmacological Activation of Non-canonical NF-κB Signaling Activates Latent HIV-1 Reservoirs In Vivo

"Shock and kill" strategies focus on purging the latent HIV-1 reservoir by treating infected individuals with therapeutics that activate the latent virus and subsequently eliminating infected cells. We have previously reported that induction of non-canonical nuclear factor κB (NF-κB) signaling through a class of small-molecule antagonists known as Smac mimetics can reverse HIV-1 latency. Here, we describe the development of Ciapavir (SBI-0953294), a molecule specifically optimized for HIV-1 latency reversal that was found to be more efficacious as a latency-reversing agent than other Smac mimetics under clinical development for cancer. Critically, this molecule induced activation of HIV-1 reservoirs in vivo in a bone marrow, liver, thymus (BLT) humanized mouse model without mediating systemic T cell activation. This study provides proof of concept for the in vivo efficacy and safety of Ciapavir and indicates that Smac mimetics can constitute a critical component of a safe and efficacious treatment strategy to eliminate the latent HIV-1 reservoir.

Prodrugs of PKC modulators show enhanced HIV latency reversal and an expanded therapeutic window

AIDS is a pandemic disease caused by HIV that affects 37 million people worldwide. Current antiretroviral therapy slows disease progression but does not eliminate latently infected cells, which resupply active virus, thus necessitating lifelong treatment with associated compliance, cost, and chemoexposure issues. Latency-reversing agents (LRAs) activate these cells, allowing for their potential clearance, thus presenting a strategy to eradicate the infection. Protein kinase C (PKC) modulators-including prostratin, ingenol esters, bryostatin, and their analogs-are potent LRAs in various stages of development for several clinical indications. While LRAs are promising, a major challenge associated with their clinical use is sustaining therapeutically meaningful levels of the active agent while minimizing side effects. Here we describe a strategy to address this problem based on LRA prodrugs, designed for controllable release of the active LRA after a single injection. As intended, these prodrugs exhibit comparable or superior in vitro activity relative to the parent compounds. Selected compounds induced higher in vivo expression of CD69, an activation biomarker, and, by releasing free agent over time, significantly improved tolerability when compared to the parent LRAs. More generally, selected prodrugs of PKC modulators avoid the bolus toxicities of the parent drug and exhibit greater efficacy and expanded tolerability, thereby addressing a longstanding objective for many clinical applications.

Nicotinamide activates latent HIV-1 ex vivo in ART suppressed individuals, revealing higher potency than the association of two methyltransferase inhibitors, chaetocin and BIX01294

Background

Latent HIV-1 is a major hurdle in obtaining HIV-1 sustained virological remission (SVR). Here we explored histone deacetylation inhibition property of nicotinamide (NAM; n = 17) for the first time in comparison to a combination of methyltransferase inhibitors (MTIs; Chaetocin and BIX01294; n = 25) to reactivate latent HIV ex vivo in CD8-depleted PBMCs from antiretroviral treated aviremic individuals.

Results

NAM reactivated HIV-1 from 13/17 (76.4%) samples compared to 20/25 (80.0%) using MTIs with mean viral load (VLs) of 4.32 and 3.22 log₁₀ RNA copies/mL, respectively (p = 0.004). Mean purging time after NAM and MTIs stimulation was 5.1 and 6.75 days, respectively (p = 0.73). Viral purging in autologous cultures exhibited blunted HIV recovery with fluctuating VLs followed by a complete viral extinction when expanded in allogenic system. Electron microscopy from five supernatants revealed anomalous viral particles, with lack of complete viral genomes when characterized by ultradeep sequencing through metagenomics approach (n = 4).

Conclusion

NAM alone was more potent HIV-1 activator than combination of MTIs, with potential of clinical use.

Current Status of Latency Reversing Agents Facing the Heterogeneity of HIV-1 Cellular and Tissue Reservoirs

One of the most explored therapeutic approaches aimed at eradicating HIV-1 reservoirs is the "shock and kill" strategy which is based on HIV-1 reactivation in latently-infected cells ("shock" phase) while maintaining antiretroviral therapy (ART) in order to prevent spreading of the infection by the neosynthesized virus. This kind of strategy allows for the "kill" phase, during which latently-infected cells die from viral cytopathic effects or from host cytolytic effector mechanisms following viral reactivation. Several latency reversing agents (LRAs) with distinct mechanistic classes have been characterized to reactivate HIV-1 viral gene expression. Some LRAs have been tested in terms of their potential to purge latent HIV-1 in vivo in clinical trials, showing that reversing HIV-1 latency is possible. However, LRAs alone have failed to reduce the size of the viral reservoirs. Together with the inability of the immune system to clear the LRA-activated reservoirs and the lack of specificity of these LRAs, the heterogeneity of the reservoirs largely contributes to the limited success of clinical trials using LRAs. Indeed, HIV-1 latency is established in numerous cell types that are characterized by distinct phenotypes and metabolic properties, and these are influenced by patient history. Hence, the silencing mechanisms of HIV-1 gene expression in these cellular and tissue reservoirs need to be better understood to rationally improve this cure strategy and hopefully reach clinical success.

New Frontiers in Measuring and Characterizing the HIV Reservoir

A cure for HIV infection remains elusive due to the persistence of replication-competent HIV proviral DNA during suppressive antiretroviral therapy (ART). With the exception of rare elite or post-treatment controllers of viremia, withdrawal of ART invariably results in the rebound of viremia and progression of HIV disease. A thorough understanding of the reservoir is necessary to develop new strategies in order to reduce or eliminate the reservoir. However, there is significant heterogeneity in the sequence composition, genomic location, stability, and expression of the HIV reservoir both within and across individuals, and a majority of proviral sequences are replication-defective. These factors, and the low frequency of persistently infected cells in individuals on suppressive ART, make understanding the reservoir and its response to experimental reservoir reduction interventions challenging. Here, we review the characteristics of the HIV reservoir, state-of-the-art assays to measure and characterize the reservoir, and how these assays can be applied to accurately detect reductions in reservoir during efforts to develop a cure for HIV infection. In particular, we highlight recent advances in the development of direct measures of provirus, including intact proviral DNA assays and full-length HIV DNA sequencing with integration site analysis. We also focus on novel techniques to quantitate persistent and inducible HIV, including RNA sequencing and RNA/gag protein staining techniques, as well as modified viral outgrowth methods that seek to improve upon throughput, sensitivity and dynamic range.

HIV-1 Latency and Latency Reversal: Does Subtype Matter?

Cells that are latently infected with HIV-1 preclude an HIV-1 cure, as antiretroviral therapy does not target this latent population. HIV-1 is highly genetically diverse, with over 10 subtypes and numerous recombinant forms circulating worldwide. In spite of this vast diversity, much of our understanding of latency and latency reversal is largely based on subtype B viruses. As such, most of the development of cure strategies targeting HIV-1 are solely based on subtype B. It is currently assumed that subtype does not influence the establishment or reactivation of latent viruses. However, this has not been conclusively proven one way or the other. A better understanding of the factors that influence HIV-1 latency in all viral subtypes will help develop therapeutic strategies that can be applied worldwide. Here, we review the latest literature on subtype-specific factors that affect viral replication, pathogenesis, and, most importantly, latency and its reversal.

Dual effects of the novel ingenol derivatives on the acute and latent HIV-1 infections

The latent reservoir of HIV-1 in resting memory CD4⁺ T cells serves as a major barrier to curing HIV-1 infection. Reactivation of latent HIV-1 is proposed as a promising strategy for the clearance of the viral reservoirs. Because of the limitations of current latency reversal agents (LRAs), identification of new LRAs is urgently required. Here, we analyzed Euphorbia kansui extracts and obtained three ingenol derivative compounds named EK-1A, EK-5A and EK-15A. We found that ingenol derivatives can effectively reactivate latent HIV-1 at very low (nanomolar) concentrations in HIV latency model in vitro. Furthermore, ingenol derivatives exhibited synergy with other LRAs in reactivating latent HIV-1. We verified that EK-15A can promote latent HIV-1 reactivation in the ex vivo resting CD4⁺ T cells isolated from the peripheral blood of HIV-infected individuals on suppressive antiretroviral therapy. In addition, ingenol derivatives down-regulated the expression of cell surface HIV co-receptors CCR5 and CXCR4, therefore potentially preventing new infection of HIV-1. Our results indicated that the ingenol derivatives extracted from Euphorbia kansui have dual functions: reactivation of latent HIV-1 and inhibition of HIV-1 infection.

Dual effect of the broad spectrum kinase inhibitor midostaurin in acute and latent HIV-1 infection

Midostaurin is a multi-kinase inhibitor with antineoplastic activity. We assessed the capacity of midostaurin to affect early and late steps of HIV-1 infection and to reactivate HIV-1 latently infected cells, alone or in combination with histone deacetylase inhibitors (HDACi) known to act as latency-reversing agents (LRA). Acute HIV-1 infection was assessed by flow cytometry in three cell types treated with midostaurin in the presence or absence of SAMHD1. Non-infected cells were treated with midostaurin and harvested for Western blot analysis. Macrophage infections were also measured by quantitative RT-PCR. HIV-1 latency reactivation was assessed in several latency models. Midostaurin induced G2/M arrest and inhibited CDK2, preventing the phosphorylation of SAMHD1 associated to inhibition of its dNTPase activity. In the presence of SAMHD1, midostaurin blocked HIV-1 DNA formation and viral replication. However, following Vpx-mediated SAMHD1 degradation, midostaurin increased viral transcripts and virus replication. In three out of four HIV-1 latency models, including primary CD4⁺ T cells, midostaurin effectively reversed HIV-1 latency and was synergistic in combination with LRA vorinostat and panobinostat. Our study describes a dual effect for midostaurin in HIV-1 infection, antiviral or proviral depending on SAMHD1 activation, and highlights a role for active SAMHD1 in regulating the activity of potential HIV-1 latency reversal agents.

Chemokine receptor CCR5 correlates with functional CD8+ T cells in SIV-infected macaques and the potential effects of maraviroc on T-cell activation

C-C chemokine receptor 5 (CCR5) plays an essential role in HIV pathogenesis as the major coreceptor on CD4⁺ T cells used by HIV, yet the function of CCR5 on CD8 T cells is not well understood. Furthermore, the immunologic effects of the CCR5 inhibitor maraviroc (MVC), despite approval for clinical use, have not yet been well evaluated for their potential effects on cytotoxic T-cell responses. In this study, we characterized the development and function of CCR5⁺CD8⁺ T cells in rhesus macaques with or without Simian immunodeficiency virus (SIV) infection. We also investigated the effects of the CCR5 antagonist MVC on functional CCR5⁺CD8⁺ T-cell responses in vitro. The data show that CCR5⁺CD8⁺ T cells have an effector memory phenotype and increase with age in systemic and mucosal lymphoid tissues as a heterogeneous population of polyfunctional CD8 T cells. In addition, CCR5 is highly expressed on SIV gag-specific (CM9⁺) CD8⁺ T cells in SIV-infected macaques, yet CCR5⁺CD8⁺ T cells are significantly reduced in mucosal lymphoid tissues with disease progression. Furthermore, in vitro MVC treatment reduced activation and cytokine secretion of CD8⁺ T cells via a CCR5-independent pathway. These findings suggest that surface CCR5 protein plays an important role in differentiation and activation of CD8⁺ T cells. Although MVC may be helpful in reducing chronic inflammation and activation, it may also inhibit virus-specific CD8⁺ T-cell responses. Thus optimal use of CCR5 antagonists either alone or in combination with other drugs should be defined by further investigation.-Wang, X., Russell-Lodrigue, K. E., Ratterree, M. S., Veazey, R. S., Xu, H. Chemokine receptor CCR5 correlates with functional CD8⁺ T cells in SIV-infected macaques and the potential effects of maraviroc on T-cell activation.

Between a shock and a hard place: challenges and developments in HIV latency reversal

Latently infected cells that persist in HIV-infected individuals on antiretroviral therapy (ART) are a major barrier to cure. One strategy to eliminate latency is by activating viral transcription, commonly called latency reversal. Several small non-randomised clinical trials of latency reversing agents (LRAs) in HIV-infected individuals on ART increased viral production, but disappointingly did not reduce the number of latently infected cells or delay time to viral rebound following cessation of ART. More recent approaches aimed at reversing latency include compounds that both activate virus and also modulate immunity to enhance clearance of infected cells. These immunomodulatory LRAs include toll-like receptor agonists, immune checkpoint inhibitors and some cytokines. Here, we provide a brief review of the rationale for transcription-activating and immunomodulatory LRAs, discuss recent clinical trials and some suggestions for combination approaches and research priorities for the future.

HUSH, a Link Between Intrinsic Immunity and HIV Latency

A prominent obstacle to HIV eradication in seropositive individuals is the viral persistence in latent reservoir cells, which constitute an HIV sanctuary out of reach of highly active antiretroviral therapies. Thus, the study of molecular mechanisms governing latency is a very active field that aims at providing solutions to face the reservoirs issue. Since the past 15 years, another major field in HIV biology focused on the discovery and study of restriction factors that shape intrinsic immunity, while engaging in a molecular battle against HIV. Some of these restrictions factors act at early stages of the virus life cycle, alike SAMHD1 antagonized by the viral protein Vpx, while others are late actors. Until recently, no such factor was identified in the nucleus and found active at the level of provirus expression, a crucial step where latency may take place. Today, two studies highlight Human Silencing Hub (HUSH) as a potential restriction factor that controls viral expression and is antagonized by Vpx. This Review discusses HUSH restriction in the light of the actual knowledge of intrinsic immunity and HIV latency.

Roles of CDKs in RNA polymerase II transcription of the HIV-1 genome

Studies of RNA Polymerase II (Pol II) transcription of the HIV-1 genome are of clinical interest, as the insight gained may lead to strategies to selectively reactivate latent viruses in patients in whom viral replication is suppressed by antiviral drugs. Such a targeted reactivation may contribute to a functional cure of infection. This review discusses five Cyclin-dependent kinases - CDK7, CDK9, CDK11, CDK2, and CDK8 - involved in transcription and processing of HIV-1 RNA. CDK7 is required for Pol II promoter clearance of reactivated viruses; CDK7 also functions as an activating kinase for CDK9 when resting CD4⁺ T cells harboring latent HIV-1 are activated. CDK9 is targeted by the viral Tat protein and is essential for productive Pol II elongation of the HIV-1 genome. CDK11 is associated with the TREX/THOC complex and it functions in the 3' end processing and polyadenylation of HIV-1 transcripts. CDK2 phosphorylates Tat and CDK9 and this stimulates Tat activation of Pol II transcription. CDK8 may stimulate Pol II transcription of the HIV-1 genome through co-recruitment with NF-κB to the viral promoter. Some notable open questions are discussed concerning the roles of these CDKs in HIV-1 replication and viral latency.

Synthetic Ingenols Maximize Protein Kinase C-Induced HIV-1 Latency Reversal

Antiretroviral therapy (ART) does not cure HIV-1 infection due to the persistence of proviruses in long-lived resting T cells. Strategies targeting these latently infected cells will be necessary to eradicate HIV-1 in infected individuals. Protein kinase C (PKC) activation is an effective mechanism to reactivate latent proviruses and allows for recognition and clearance of infected cells by the immune system. Several ingenol compounds, naturally occurring PKC agonists, have been described to have potent latency reversal activity. We sought to optimize this activity by synthesizing a library of novel ingenols via esterification of the C-3 hydroxyl group of the ingenol core, which itself is inactive for latency reversal. Newly synthesized ingenol derivatives were evaluated for latency reversal activity, cellular activation, and cytotoxicity alongside commercially available ingenols (ingenol-3,20-dibenzoate, ingenol 3-hexanoate, and ingenol-3-angelate) in HIV latency cell lines and resting CD4⁺ T cells from aviremic participants. Among the synthetic ingenols that we produced, we identified several compounds that demonstrate high efficacy and represent promising leads as latency reversal agents for HIV-1 eradication.

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.

Combination of a Latency-Reversing Agent With a Smac Mimetic Minimizes Secondary HIV-1 Infection in vitro

Latency-reversing agents (LRAs) are considered a potential tool to cure human immunodeficiency virus type 1 (HIV-1) infection, but when they are taken alone, virus production by reactivated cells and subsequent infection will occur. Hence, it is crucial to simultaneously take appropriate measures to prevent such secondary HIV-1 infection. In this regard, a strategy to minimize the production of infectious viruses from LRA-reactivated cells is worth pursuing. Here, we focused on a second mitochondria-derived activator of caspases (Smac) mimetic, birinapant, to induce apoptosis in latent HIV-1-infected cells. When birinapant was administered alone, it only slightly increased the expression of caspase-3. However, in combination with an LRA (e.g., PEP005), it strongly induced the expression of caspase-3 followed by enhanced apoptosis. Importantly, the combination eliminated reactivated cells and drastically reduced HIV-1 production. Finally, we found that birinapant decreased the mRNA expression of HIV-1 that was induced by PEP005 in the primary CD4⁺ T-cells from HIV-1-carrying patients as well. These results suggest that the combination of an LRA and an “apoptosis-inducing” agent, such as a Smac mimetic, is a possible treatment option to decrease HIV-1 reservoirs without the occurrence of HIV-1 production by reactivated cells.

Natural product-derived compounds in HIV suppression, remission, and eradication strategies

While combination antiretroviral therapy (cART) has successfully converted HIV to a chronic but manageable infection in many parts of the world, HIV continues to persist within latent cellular reservoirs, which can become reactivated at any time to produce infectious virus. New therapies are therefore needed not only for HIV suppression but also for containing or eliminating HIV reservoirs. Compounds derived from plant, marine, and other natural products have been found to combat HIV infection and/or target HIV reservoirs, and these discoveries have substantially guided current HIV therapy-based studies. Here we summarize the role of natural product-derived compounds in current HIV suppression, remission, and cure strategies.

The HIV-1 Tat Protein: Mechanism of Action and Target for HIV-1 Cure Strategies

The general mechanism involved in Tat activation of RNA Polymerase II (RNAP II) elongation of the integrated HIV-1 was elucidated over 20 years ago. This mechanism involves Tat binding to the TAR RNA element that forms at the 5' end of viral transcripts and recruiting a general RNAP II elongation factor termed as PTEFb. This elongation factor consists of CDK9 and Cyclin T1, and when recruited by Tat to TAR RNA, CDK9 was proposed to phosphorylate the carboxyl terminal domain of RNAP II and thereby activate elongation. Research in the past two decades has shown that the mechanism of Tat action is considerably more complicated than this simple model. In metabolically active cells, CDK9 and Cyclin T1 are now known to be largely sequestered in a RNA-protein complex termed the 7SK RNP. CDK9 and Cyclin T1 are released from the 7SK RNP by mechanisms not yet fully elucidated and along with Tat, bind to TAR RNA and orchestrate the assembly of a Super Elongation Complex (SEC) containing several additional proteins. CDK9 in the SEC then phosphorylates multiple substrates in the RNAP II complex to activate elongation. Importantly for therapeutic strategies, CDK9 and Cyclin T1 functions are down-regulated in resting CD4+ T cells that harbor latent HIV-1, and their up-regulation is required for reactivation of latent virus. Current strategies for a functional cure of HIV-1 infection therefore are likely to require development of latency reversal agents that up-regulate CDK9 and Cyclin T1 function in resting CD4+ T cells.

Peering into the HIV reservoir

The main obstacle to HIV eradication is the establishment of a long-term persistent HIV reservoir. Although several therapeutic approaches have been developed to reduce and eventually eliminate the HIV reservoir, only a few have achieved promising results. A better knowledge of the mechanisms involved in the establishment and maintenance of HIV reservoir is of utmost relevance for the design of new therapeutic strategies aimed at purging it with the ultimate goal of achieving HIV eradication or alternatively a functional cure. In this regard, it is also important to take a close look into the cellular HIV reservoirs other than resting memory CD4 T-cells with key roles in reservoir maintenance that have been recently described. Unraveling the special characteristics of these HIV cellular compartments could aid us in designing new therapeutic strategies to deplete the latent HIV reservoir.

HIV Tat/P-TEFb Interaction: A Potential Target for Novel Anti-HIV Therapies

Transcription is a crucial step in the life cycle of the human immunodeficiency virus type 1 (HIV 1) and is primarily involved in the maintenance of viral latency. Both viral and cellular transcription factors, including transcriptional activators, suppressor proteins and epigenetic factors, are involved in HIV transcription from the proviral DNA integrated within the host cell genome. Among them, the virus-encoded transcriptional activator Tat is the master regulator of HIV transcription. Interestingly, unlike other known transcriptional activators, Tat primarily activates transcriptional elongation and initiation by interacting with the cellular positive transcriptional elongation factor b (P-TEFb). In this review, we describe the molecular mechanism underlying how Tat activates viral transcription through interaction with P-TEFb. We propose a novel therapeutic strategy against HIV replication through blocking Tat action.

Natural Products and HIV/AIDS

The study of natural products in biomedical research is not a modern concept. Many of the most successful medical therapeutics are derived from natural products, including those studied in the field of HIV/AIDS. Biomedical research has a rich history of discovery based on screens of medicinal herbs and traditional medicine practices. Compounds derived from natural products, which repress HIV and those that activate latent HIV, have been reported. It is important to remember the tradition in medical research to derive therapies based on these natural products and to overcome the negative perception of natural products as an "alternative medicine."

Benzotriazoles Reactivate Latent HIV-1 through Inactivation of STAT5 SUMOylation

The presence of latent HIV-1 in infected individuals represents a major barrier preventingviral eradication. For that reason, reactivation of latent viruses in the presence of antiretroviral regimens has been proposed as a therapeutic strategy to achieve remission. We screened for small molecules and identified several benzotriazole derivatives with the ability to reactivate latent HIV-1. In the presence of IL-2, benzotriazoles reactivated and reduced the latent reservoir in primary cells, and, remarkably, viral reactivation was achieved without inducing cell proliferation, T cell activation, or cytokine release. Mechanistic studies showed that benzotriazoles block SUMOylation of phosphorylated STAT5, increasing STAT5’s activity and occupancy of the HIV-1 LTR. Our results identify benzotriazoles as latency reversing agents and STAT5 signaling and SUMOylation as targets for HIV-1 eradication strategies. These compounds represent a different direction in the search for “shock and kill” therapies.

FISHing Out the Hidden Enemy: Advances in Detecting and Measuring Latent HIV-Infected Cells

The indomitable aspect of HIV-1 infection is not that HIV-1 proviral DNA is integrated into host DNA but that it can also turn itself off, remaining invisible to drug or immune surveillance. Thus, the goals of eradication include ways to precisely excise HIV-1 DNA or wake up the silent HIV-1 provirus and eliminate the infected cells thus identified. Methods to identify and fish out the latently infected cells or to delineate their characteristics are being rapidly developed. In 2016, Baxter et al. (A. E. Baxter, J. Niessl, R. Fromentin, J. Richard, F. Porichis, R. Charlebois, M. Massanella, N. Brassard, N. Alsahafi, G. G. Delgado, J. P. Routy, B. D. Walker, A. Finzi, N. Chomont, and D. E. Kaufmann, Cell Host Microbe 20:368–380, 2016, https://doi.org/10.1016/j.chom.2016.07.015) and Martrus et al. (G. Martrus, A. Niehrs, R. Cornelis, A. Rechtien, W. García-Beltran, M. Lütgehetmann, C. Hoffmann, and M. Altfeld, J Virol 90:9018–9028, 2016, https://doi.org/10.1128/JVI.01448-16) reported using the fluorescence in situ hybridization-flow cytometry technique to identify and quantify cells expressing HIV-1 RNA and Gag protein, as well as bearing unique cell surface markers. In a recent article in mBio, Grau-Expósito et al. (J. Grau-Expósito, C. Serra-Peinado, L. Miguel, J. Navarro, A. Curran, J. Burgos, I. Ocaña, E. Ribera, A. Torrella, B. Planas, R. Badía, J. Castellví, V. Falcó, M. Crespo, and M. J. Buzon, mBio 8:e00876-17, 2017, https://doi.org/10.1128/mBio.00876-17) reported a similar method that they claim to be more sensitive. With these methods, researchers are one step closer to measuring latent reservoirs and eliminating critical barriers to HIV eradication.

Shocking HIV out of hiding: where are we with clinical trials of latency reversing agents?

PURPOSE OF REVIEW

To provide an overview of the initial experiences with the use of latency-reversing agents (LRAs) in clinical trials in HIV and to discuss and contrast results arising from these studies.

RECENT FINDINGS

Although the clinical administration of histone deacetylase inhibitors (HDACis) and disulfiram to HIV-infected individuals on antiretroviral therapy significantly increased cell-associated HIV RNA in CD4 T cells and in some cases plasma HIV RNA, this did not reduce the frequency of latently infected cells in blood. Potential reasons for this include insufficient potency in latency reversal, lack of virus or immune-mediated cytolysis of virus-expressing cells and/or a high frequency of immune escape mutations in the recently activated virus. Analyses of HIV-specific T-cell responses in vivo did not demonstrate that HDACis impair immune cell effector functions.

SUMMARY

More effective latency-reversing interventions and additional strategies to eliminate virus-expressing cells are needed. Key challenges include testing combinations of LRAs and/or LRAs with immune modulation to optimize potency in the absence of adverse events. A better understanding of the mechanisms of action of LRAs as well as strategies to enhance potency and penetration in tissue are key challenges for future studies.

Proviral Latency, Persistent Human Immunodeficiency Virus Infection, and the Development of Latency Reversing Agents

Quiescent proviral genomes that persist during human immunodeficiency virus type 1 (HIV-1) infection despite effective antiretroviral therapy (ART) can fuel rebound viremia after ART interruption and is a central obstacle to the cure of HIV infection. The induction of quiescent provirus is the goal of a new class of potential therapeutics, latency reversing agents (LRAs). The discovery, development, and testing of HIV LRAs is a key part of current efforts to develop latency reversal and viral clearance strategies to eradicate established HIV infection. The development of LRAs is burdened by many uncertainties that make drug discovery difficult. The biology of HIV latency is complex and incompletely understood. Potential targets for LRAs are host factors, and the potential toxicities of host-directed therapies in individuals that are otherwise clinically stable may be unacceptable. Assays to measure latency reversal and assess the effectiveness of potential therapeutics are complex and incompletely validated. Despite these obstacles, novel LRAs are under development and beginning to enter combination testing with viral clearance strategies. It is hoped that the steady advances in the development of LRAs now being paired with emerging immunotherapeutics to clear persistently infected cells will soon allow measurable clinical advances toward an HIV cure.