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

Harnessing antiviral RNAi therapeutics for pandemic viruses: SARS-CoV-2 and HIV

Using the knowledge from decades of research into RNA-based therapies, the COVID-19 pandemic response saw the rapid design, testing and production of the first ever mRNA vaccines approved for human use in the clinic. This breakthrough has been a significant milestone for RNA therapeutics and vaccines, driving an exponential growth of research into the field. The development of novel RNA therapeutics targeting high-threat pathogens, that pose a substantial risk to global health, could transform the future of health delivery. In this review, we provide a detailed overview of the two RNA interference (RNAi) pathways and how antiviral RNAi therapies can be used to treat acute or chronic diseases caused by the pandemic viruses SARS-CoV-2 and HIV, respectively. We also provide insights into short-interfering RNA (siRNA) delivery systems, with a focus on how lipid nanoparticles can be functionalized to achieve targeted delivery to specific sites of disease. This review will provide the current developments of SARS-CoV-2 and HIV targeted siRNAs, highlighting strategies to advance the progression of antiviral siRNA along the clinical development pathway.

Interventions during Early Infection: Opening a Window for an HIV Cure?

Although combination antiretroviral therapy (ART) has been a landmark achievement for the treatment of human immunodeficiency virus (HIV), an HIV cure has remained elusive. Elimination of latent HIV reservoirs that persist throughout HIV infection is the most challenging barrier to an HIV cure. The progressive HIV infection is marked by the increasing size and diversity of latent HIV reservoirs until an effective immune response is mobilized, which can control but not eliminate HIV infection. The stalemate between HIV replication and the immune response is manifested by the establishment of a viral set point. ART initiation during the early stage limits HIV reservoir development, preserves immune function, improves the quality of life, and may lead to ART-free viral remission in a few people living with HIV (PLWH). However, for the overwhelming majority of PLWH, early ART initiation alone does not cure HIV, and lifelong ART is needed to sustain viral suppression. A critical area of research is focused on determining whether HIV could be functionally cured if additional treatments are provided alongside early ART. Several HIV interventions including Block and Lock, Shock and Kill, broadly neutralizing antibody (bNAb) therapy, adoptive CD8+ T cell therapy, and gene therapy have demonstrated delayed viral rebound and/or viral remission in animal models and/or some PLWH. Whether or not their application during early infection can improve the success of HIV remission is less studied. Herein, we review the current state of clinical and investigative HIV interventions and discuss their potential to improve the likelihood of post-treatment remission if initiated during early infection.

Mechanisms and efficacy of small molecule latency-promoting agents to inhibit HIV reactivation ex vivo

Drugs that inhibit HIV transcription and/or reactivation of latent HIV have been proposed as a strategy to reduce HIV-associated immune activation or to achieve a functional cure, yet comparative studies are lacking. We evaluated 26 drugs, including drugs previously reported to inhibit HIV transcription (inhibitors of Tat-dependent HIV transcription, Rev, HSF-1/PTEF-b, HSP90, Jak/Stat, or SIRT1/Tat deacetylation) and other agents that were not tested before (inhibitors of PKC, NF-κB, SP-1, or histone acetyltransferase; NR2F1 agonists), elongation (inhibitors of CDK9/ PTEF-b), completion (inhibitors of PolyA-polymerase), or splicing (inhibitors of human splice factors). To investigate if those drugs would vary in their ability to affect different blocks to HIV transcription, we measured levels of initiated, elongated, midtranscribed, completed, and multiply spliced HIV RNA in PBMCs from antiretroviral therapy-suppressed individuals following ex vivo treatment with each drug and subsequent T cell activation. We identified new drugs that prevent HIV reactivation, including CDK and splicing inhibitors. While some drugs inhibited 1 or 2 steps, other drugs (CDK inhibitors, splicing inhibitors, tanespimycin, and triptolide) inhibited multiple stages of HIV transcription and blocked the production of supernatant viral RNA. These drugs and targets deserve further study in strategies aimed at reducing HIV-associated immune activation or achieving a functional cure.

Impact of the interaction between herpes simplex virus 1 ICP22 and FACT on viral gene expression and pathogenesis

Facilitates chromatin transcription (FACT) interacts with nucleosomes to promote gene transcription by regulating the dissociation and reassembly of nucleosomes downstream and upstream of RNA polymerase II (Pol II). A previous study reported that herpes simplex virus 1 (HSV-1) regulatory protein ICP22 interacted with FACT and was required for its recruitment to the viral DNA genome in HSV-1-infected cells. However, the biological importance of interactions between ICP22 and FACT in relation to HSV-1 infection is unclear. Here, we mapped the minimal domain of ICP22 required for its efficient interaction with FACT to a cluster of five basic amino acids in ICP22. A recombinant virus harboring alanine substitutions in this identified cluster led to the decreased accumulation of viral mRNAs from UL54, UL38, and UL44 genes, reduced Pol II occupancy of these genes in MRC-5 cells, and impaired HSV-1 virulence in mice following ocular or intracranial infection. Furthermore, the treatment of mice infected with wild-type HSV-1 with CBL0137, a FACT inhibitor currently being investigated in clinical trials, significantly improved the survival rate of mice. These results suggested that the interaction between ICP22 and FACT was required for efficient HSV-1 gene expression and pathogenicity. Therefore, FACT might be a potential therapeutic target for HSV-1 infection.IMPORTANCEICP22 is a well-known regulatory factor of HSV-1 gene expression, but its mechanism(s) are poorly understood. Although the interaction of FACT with ICP22 was reported previously, its significance in HSV-1 infection is unknown. Given that FACT is involved in gene transcription, it is of interest to investigate this interaction as it relates to HSV-1 gene expression. To determine a direct link between the interaction and HSV-1 infection, we mapped a minimal domain of ICP22 required for its efficient interaction with FACT and generated a recombinant virus carrying mutations in the identified domain. Using the recombinant virus, we obtained evidence suggesting that the interaction between ICP22 and FACT promoted Pol II transcription from HSV-1 genes and viral virulence in mice. In addition, CBL0137, an inhibitor of FACT, effectively protected mice from lethal HSV-1 infection, suggesting FACT might be a potential target for the development of novel anti-HSV drugs.

KDM5A/B contribute to HIV-1 latent infection and survival of HIV-1 infected cells

Combinational antiretroviral therapy (cART) suppresses human immunodeficiency virus type 1 (HIV-1) viral replication and pathogenesis in acquired immunodeficiency syndrome (AIDS) patients. However, HIV-1 remains in the latent stage of infection by suppressing viral transcription, which hinders an HIV-1 cure. One approach for an HIV-1 cure is the "shock and kill" strategy. The strategy focuses on reactivating latent HIV-1, inducing the viral cytopathic effect and facilitating the immune clearance for the elimination of latent HIV-1 reservoirs. Here, we reported that the H3K4 trimethylation (H3K4me3)-specific demethylase KDM5A/B play a role in suppressing HIV-1 Tat/LTR-mediated viral transcription in HIV-1 latent cells. Furthermore, we evaluated the potential of KDM5-specific inhibitor JQKD82 as an HIV-1 "shock and kill" agent. Our results showed that JQKD82 increases the H3K4me3 level at HIV-1 5' LTR promoter regions, HIV-1 reactivation, and the cytopathic effects in an HIV-1-latent T cell model. In addition, we identified that the combination of JQKD82 and AZD5582, a non-canonical NF-κB activator, generates a synergistic impact on inducing HIV-1 lytic reactivation and cell death in the T cell. The latency-reversing potency of the JQKD82 and AZD5582 pair was also confirmed in peripheral blood mononuclear cells (PBMCs) isolated from HIV-1 aviremic patients and in an HIV-1 latent monocyte. In latently infected microglia (HC69) of the brain, either deletion or inhibition of KDM5A/B results in a reversal of the HIV-1 latency. Overall, we concluded that KDM5A/B function as a host repressor of the HIV-1 lytic reactivation and thus promote the latency and the survival of HIV-1 infected reservoirs.

Eliminating the HIV tissue reservoir: current strategies and challenges

BACKGROUND

Acquired immunodeficiency syndrome (AIDS) is still one of the most widespread and harmful infectious diseases in the world. The presence of reservoirs housing the human immunodeficiency virus (HIV) represents a significant impediment to the development of clinically applicable treatments on a large scale. The viral load in the blood can be effectively reduced to undetectable levels through antiretroviral therapy (ART), and a higher concentration of HIV is sequestered in various tissues throughout the body, forming the tissue reservoir - the source of viremia after interruption treatment.

METHODS

We take the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) as a guideline for this review. In June 2023, we used the Pubmed, Embase, and Scopus databases to search the relevant literature published in the last decade.

RESULTS

Here we review the current strategies and treatments for eliminating the HIV tissue reservoirs: early and intensive therapy, gene therapy (including ribozyme, RNA interference, RNA aptamer, zinc finger enzyme, transcriptional activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats/associated nuclease 9 (CRISPR/Cas9)), 'Shock and Kill', 'Block and lock', immunotherapy (including therapeutic vaccines, broadly neutralising antibodies (bNAbs), chimeric antigen receptor T-cell immunotherapy (CAR-T)), and haematopoietic stem cell transplantation (HSCT).

CONCLUSION

The existence of an HIV reservoir is the main obstacle to the complete cure of AIDS. Choosing the appropriate strategy to deplete the HIV reservoir and achieve a functional cure for AIDS is the focus and difficulty of current research. So far, there has been a lot of research and progress in reducing the HIV reservoir, but in general, the current research is still very preliminary. Much research is still needed to properly assess the reliability, effectiveness, and necessity of these strategies.

A Camptothetin Analog, Topotecan, Promotes HIV Latency via Interference with HIV Transcription and RNA Splicing

Low level HIV transcription during modern antiretroviral therapy (ART) in persons with HIV is linked to residual inflammation and associated diseases, like cardiovascular disease and cancer. The "block and lock" approach to hold HIV in a state of deep latency may help decrease residual inflammation in a person with HIV on ART and thus improve health. A camptothecin analog topotecan (TPT) was previously implicated as an inhibitor of active HIV replication. Using an in vitro primary T cell model of HIV latency, we demonstrated that (i) TPT reduces HIV transcriptional activity in latently infected cells; (ii) downregulation of HIV RNA by TPT cannot be reversed by latency reversing agents; (iii) several primary and secondary mechanism of action of TPT may be involved in control of HIV replication; (iv) regulation of HIV RNA by TPT is dependent on splicing complexity; (v) increase in proportion of unspliced HIV transcripts was facilitated by intron retention and upregulation of splicing factors, specifically SRSF6, by TPT. Although high TPT dosing (10 μM) was needed to achieve the observed effects, viability of primary CD4+ T cells was not greatly affected. Because toxicity can be observed with TPT in persons with cancer, TPT is unlikely to be used as an anti-HIV agent in clinic, but our study provides proof that camptothetin has "block and lock" activity. Other camptothetin analogs, which are less toxic than TPT, should be designed and tested as HIV "block and lock" agents. IMPORTANCE HIV survives in a state of very low activity, called latency, for long periods in persons with HIV on antiretroviral therapy. This low activity of HIV is linked to residual inflammation and associated diseases, such as heart disease and cancer. New strategies are being explored to further silence the HIV provirus and suppress residual inflammation. This study provides strong evidence that the camptothetin analog, Topotecan, can reduce residual activity of HIV in an experimental model of HIV latency. While Topotecan itself is likely not suitable for use in the clinic due to its toxicity, other camptothetin analogs should be designed and investigated as "block and lock" agents.

Determinants of Retroviral Integration and Implications for Gene Therapeutic MLV-Based Vectors and for a Cure for HIV-1 Infection

To complete their replication cycle, retroviruses need to integrate a DNA copy of their RNA genome into a host chromosome. Integration site selection is not random and is driven by multiple viral and cellular host factors specific to different classes of retroviruses. Today, overwhelming evidence from cell culture, animal experiments and clinical data suggests that integration sites are important for retroviral replication, oncogenesis and/or latency. In this review, we will summarize the increasing knowledge of the mechanisms underlying the integration site selection of the gammaretrovirus MLV and the lentivirus HIV-1. We will discuss how host factors of the integration site selection of retroviruses may steer the development of safer viral vectors for gene therapy. Next, we will discuss how altering the integration site preference of HIV-1 using small molecules could lead to a cure for HIV-1 infection.

The reservoir of latent HIV

The persistence of latent reservoir of the human immunodeficiency virus (HIV) is currently the major challenge in curing HIV infection. After HIV infects the human body, the latent HIV is unable to be recognized by the body’s immune system. Currently, the widely adopted antiretroviral therapy (ART) is also unble to eliminate it, thus hindering the progress of HIV treatment. This review discusses the existence of latent HIV vault for HIV treatment, its formation and factors affecting its formation, cell, and tissue localization, methods for detection and removing latent reservoir, to provide a comprehensive understanding of latent HIV vault, in order to assist in the future research and play a potential role in achieving HIV treatment.

Cure and Long-Term Remission Strategies

The majority of virally suppressed individuals will experience rapid viral rebound upon antiretroviral therapy (ART) interruption, providing a strong rationale for the development of cure strategies. Moreover, despite ART virological control, HIV infection is still associated with chronic immune activation, inflammation, comorbidities, and accelerated aging. These effects are believed to be due, in part, to low-grade persistent transcription and trickling production of viral proteins from the pool of latent proviruses constituting the viral reservoir. In recent years there has been an increasing interest in developing what has been termed a functional cure for HIV. This approach entails the long-term, durable control of viral expression in the absence of therapy, preventing disease progression and transmission, despite the presence of detectable integrated proviruses. One such strategy, the block-and-lock approach for a functional cure, proposes the epigenetic silencing of proviral expression, locking the virus in a profound latent state, from which reactivation is very unlikely. The proof-of-concept for this approach was demonstrated with the use of a specific small molecule targeting HIV transcription. Here we review the principles behind the block-and-lock approach and some of the additional strategies proposed to silence HIV expression.

Fighting HIV-1 Persistence: At the Crossroads of "Shoc-K and B-Lock"

Despite the success of highly active antiretroviral therapy (HAART), integrated HIV-1 proviral DNA cannot be eradicated from an infected individual. HAART is not able to eliminate latently infected cells that remain invisible to the immune system. Viral sanctuaries in specific tissues and immune-privileged sites may cause residual viral replication that contributes to HIV-1 persistence. The “Shock or Kick, and Kill” approach uses latency reversing agents (LRAs) in the presence of HAART, followed by cell-killing due to viral cytopathic effects and immune-mediated clearance. Different LRAs may be required for the in vivo reactivation of HIV-1 in different CD4⁺ T cell reservoirs, leading to the activation of cellular transcription factors acting on the integrated proviral HIV-1 LTR. An important requirement for LRA drugs is the reactivation of viral transcription and replication without causing a generalized immune activation. Toll-like receptors, RIG-I like receptors, and STING agonists have emerged recently as a new class of LRAs that augment selective apoptosis in reactivated T lymphocytes. The challenge is to extend in vitro observations to HIV-1 positive patients. Further studies are also needed to overcome the mechanisms that protect latently infected cells from reactivation and/or elimination by the immune system. The Block and Lock alternative strategy aims at using latency promoting/inducing agents (LPAs/LIAs) to block the ability of latent proviruses to reactivate transcription in order to achieve a long term lock down of potential residual virus replication. The Shock and Kill and the Block and Lock approaches may not be only alternative to each other, but, if combined together (one after the other), or given all at once [namely “Shoc-K(kill) and B(block)-Lock”], they may represent a better approach to a functional cure.

A new small-molecule compound, Q308, silences latent HIV-1 provirus by suppressing Tat- and FACT-mediated transcription

Eliminating the latent HIV reservoir remains a difficult problem for creating an HIV functional cure or achieving remission. The "block-and-lock" strategy aims to steadily suppress transcription of the viral reservoir and lock the HIV promoter in deep latency using latency-promoting agents (LPAs). However, to date, most of the investigated LPA candidates are not available for clinical trials, and some of them exhibit immune-related adverse reactions. The discovery and development of new, active, and safe LPA candidates for an HIV cure are necessary to eliminate residual HIV-1 viremia through the "block-and-lock" strategy. In this study, we demonstrated that a new small-molecule compound, Q308, silenced the HIV-1 provirus by inhibiting Tat-mediated gene transcription and selectively downregulating the expression levels of the facilitated chromatin transcription (FACT) complex. Strikingly, Q308 induced the preferential apoptosis in HIV-1 latently infected cells, indicating that Q308 may reduce the size of the viral reservoir and thus further prevent viral rebound. These findings highlight that Q308 is a novel and safe anti-HIV-1 inhibitor candidate for a functional 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.

Inhibition of Polo-like kinase 1 (PLK1) facilitates the elimination of HIV-1 viral reservoirs in CD4+ T cells ex vivo

Although combination antiretroviral therapy is effective in controlling HIV-1 infection, latent HIV-1 proviruses cannot be eliminated. HIV-1 reactivation induced by the mere use of latency-reversing agents is insufficient to render death of reservoir cells, indicating that certain intrinsic survival mechanisms exist. We report that Polo-like kinase 1 (PLK1) plays a critical role in survival of CD4+ T cells that undergo HIV-1 reactivation from latency or de novo infection. PLK1 is elevated in both scenarios, which requires HIV-1 Nef. HIV-1 enhances PLK1 SUMOylation, causing its nuclear translocation and protein stabilization. Inhibition or knockdown of PLK1 markedly facilitates death of HIV-1-infected CD4+ T cells. Furthermore, PLK1 inhibitors strikingly reduce the size of HIV-1 latent reservoirs in primary CD4+ T cells. Our findings demonstrate that HIV-1 infection hijacks PLK1 to prevent cell death induced by viral cytopathic effects, and that PLK1 is a promising target for chemical "killing" of HIV-1 reservoir cells.

Novel Approaches Towards a Functional Cure of HIV/AIDS

Therapeutic approaches towards a functional cure or eradication of HIV have gained renewed momentum upon encouraging data emerging from studies in SIV monkey models and recent results from human clinical studies. However, a multitude of questions remain to be addressed, including how to deal with the latent viral reservoir, how to boost the host immune response to the virus and what the hurdles are to reach relevant viral compartments in the body. Advances have been made especially with regard to identifying agents that can reactivate the latent virus in vivo and boost the cellular and humoral immunity, but it remains largely unclear whether any of these strategies can awaken a sufficiently large fraction of the viral reservoir and whether the boosted immunity can prevent rapid viral replication once antiretroviral treatments are stopped.

Epigenetic Suppression of HIV in Myeloid Cells by the BRD4-Selective Small Molecule Modulator ZL0580

Brain-resident microglia and myeloid cells (perivascular macrophages) are important HIV reservoirs in vivo, especially in the central nervous system (CNS). Despite antiretroviral therapy (ART), low-level persistent HIV replication in these reservoirs remains detectable, which contributes to neuroinflammation and neurological disorders in HIV-infected patients. New approaches complementary to ART to repress residual HIV replication in CNS reservoirs are needed. Our group has recently identified a BRD4-selective small molecule modulator (ZL0580) that induces the epigenetic suppression of HIV. Here, we examined the effects of this compound on HIV in human myeloid cells. We found that ZL0580 induces potent and durable suppression of both induced and basal HIV transcription in microglial cells (HC69) and monocytic cell lines (U1 and OM10.1). Pretreatment of microglia with ZL0580 renders them more refractory to latent HIV reactivation, indicating an epigenetic reprogramming effect of ZL0580 on HIV long terminal repeat (LTR) in microglia. We also demonstrate that ZL0580 induces repressive effect on HIV in human primary monocyte-derived macrophages (MDMs) by promoting HIV suppression during ART treatment. Mechanistically, ZL0580 inhibits Tat transactivation in microglia by disrupting binding of Tat to CDK9, a process key to HIV transcription elongation. High-resolution micrococcal nuclease mapping showed that ZL0580 induces a repressive chromatin structure at the HIV LTR. Taken together, our data suggest that ZL0580 represents a potential approach that could be used in combination with ART to suppress residual HIV replication and/or latent HIV reactivation in CNS reservoirs, thereby reducing HIV-associated neuroinflammation.IMPORTANCE Brain-resident microglia and perivascular macrophages are important HIV reservoirs in the CNS. Persistent viral replication and latent HIV reactivation in the CNS, even under ART, are believed to occur, causing neuroinflammation and neurological disorders in HIV-infected patients. It is critical to identify new approaches that can control residual HIV replication and/or latent HIV reactivation in these reservoirs. We here report that the BRD4-selective small molecule modulator, ZL0580, induces potent and durable suppression of HIV in human microglial and monocytic cell lines. Using an in vitro HIV-infected, ART-treated MDM model, we show that ZL0580 also induces suppressive effect on HIV in human primary macrophages. The significance of our research is that it suggests a potential new approach that has utility in combination with ART to suppress residual HIV replication and/or HIV reactivation in CNS reservoirs, thereby reducing neuroinflammation and neurological disorders in HIV-infected individuals.

Key Players in HIV-1 Transcriptional Regulation: Targets for a Functional Cure

HIV-1 establishes a life-long infection when proviral DNA integrates into the host genome. The provirus can then either actively transcribe RNA or enter a latent state, without viral production. The switch between these two states is governed in great part by the viral protein, Tat, which promotes RNA transcript elongation. Latency is also influenced by the availability of host transcription factors, integration site, and the surrounding chromatin environment. The latent reservoir is established in the first few days of infection and serves as the source of viral rebound upon treatment interruption. Despite effective suppression of HIV-1 replication by antiretroviral therapy (ART), to below the detection limit, ART is ineffective at reducing the latent reservoir size. Elimination of this reservoir has become a major goal of the HIV-1 cure field. However, aside from the ideal total HIV-1 eradication from the host genome, an HIV-1 remission or functional cure is probably more realistic. The "block-and-lock" approach aims at the transcriptional silencing of the viral reservoir, to render suppressed HIV-1 promoters extremely difficult to reactivate from latency. There are unfortunately no clinically available HIV-1 specific transcriptional inhibitors. Understanding the mechanisms that regulate latency is expected to provide novel targets to be explored in cure approaches.

Tat-Based Therapies as an Adjuvant for an HIV-1 Functional Cure

The human immunodeficiency virus type 1 (HIV) establishes a chronic infection that can be well controlled, but not cured, by combined antiretroviral therapy (cART). Interventions have been explored to accomplish a functional cure, meaning that a patient remains infected but HIV is undetectable in the blood, with the aim of allowing patients to live without cART. Tat, the viral transactivator of transcription protein, plays a critical role in controlling HIV transcription, latency, and viral rebound following the interruption of cART treatment. Therefore, a logical approach for controlling HIV would be to block Tat. Tackling Tat with inhibitors has been a difficult task, but some recent discoveries hold promise. Two anti-HIV proteins, Nullbasic (a mutant of Tat) and HT1 (a fusion of HEXIM1 and Tat functional domains) inhibit viral transcription by interfering with the interaction of Tat and cellular factors. Two small molecules, didehydro-cortistatin A (dCA) and triptolide, inhibit Tat by different mechanisms: dCA through direct binding and triptolide through enhanced proteasomal degradation. Finally, two Tat-based vaccines under development elicit Tat-neutralizing antibodies. These vaccines have increased the levels of CD4⁺ cells and reduced viral loads in HIV-infected people, suggesting that the new vaccines are therapeutic. This review summarizes recent developments of anti-Tat agents and how they could contribute to a functional cure for HIV.

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.

Block-And-Lock Strategies to Cure HIV Infection

Today HIV infection cannot be cured due to the presence of a reservoir of latently infected cells inducing a viral rebound upon treatment interruption. Hence, the latent reservoir is considered as the major barrier for an HIV cure. So far, efforts to completely eradicate the reservoir via a shock-and-kill approach have proven difficult and unsuccessful. Therefore, more research has been done recently on an alternative block-and-lock functional cure strategy. In contrast to the shock-and-kill strategy that aims to eradicate the entire reservoir, block-and-lock aims to permanently silence all proviruses, even after treatment interruption. HIV silencing can be achieved by targeting different factors of the transcription machinery. In this review, we first describe the underlying mechanisms of HIV transcription and silencing. Next, we give an overview of the different block-and-lock strategies under investigation.

Deep latency: A new insight into a functional HIV cure

Latent HIV reservoir is the main obstacle that prevents a cure for HIV-1 (HIV). While antiretroviral therapy is effective in controlling viral replication, it cannot eliminate latent HIV reservoirs in patients. Several strategies have been proposed to combat HIV latency, including bone marrow transplantation to replace blood cells with CCR5-mutated stem cells, gene editing to disrupt the HIV genome, and “Shock and Kill” to reactivate latent HIV followed by an immune clearance. However, high risks and limitations to scale-up in clinics, off-target effects in human genomes or failure to reduce reservoir sizes in patients hampered our current efforts to achieve an HIV cure. This necessitates alternative strategies to control the latent HIV reservoirs. This review will discuss an emerging strategy aimed to deeply silence HIV reservoirs, the development of this concept, its potential and caveats for HIV remission/cure, and prospective directions for silencing the latent HIV, thereby preventing viruses from rebound.

Curaxin CBL0137 has the potential to reverse HIV-1 latency

A cure for human immunodeficiency virus type-1 (HIV-1) has been hampered by the limitation of current combination antiretroviral therapy (cART) to address the latent reservoirs in HIV-1 patients. One strategy proposed to eradicate these reservoirs is the "shock and kill" approach, where latency-reversing agents (LRAs) are used to reactivate and promote viral cell death and/or immune killing of reactivated cells. Here, we report that curaxin CBL0137, an antitumor compound, can potentiate tumor necrosis factor-α-mediated reactivation of latently infected HIV-1cell lines. Additionally, the single use of CBL0137 is sufficient to reactivate HIV-1 latent reservoirs in peripheral mononuclear cells (PBMCs) isolated from HIV-1 positive, cART-treated, aviremic patients. Thus, CBL0137 possesses capabilities as a LRA and could be considered for the "shock and kill" approach.

Impact of LEDGIN treatment during virus production on residual HIV-1 transcription

Background

Persistence of latent, replication-competent provirus is the main impediment towards the cure of HIV infection. One of the critical questions concerning HIV latency is the role of integration site selection in HIV expression. Inhibition of the interaction between HIV integrase and its chromatin tethering cofactor LEDGF/p75 is known to reduce integration and to retarget residual provirus to regions resistant to reactivation. LEDGINs, small molecule inhibitors of the interaction between HIV integrase and LEDGF/p75, provide an interesting tool to study the underlying mechanisms. During early infection, LEDGINs block the interaction with LEDGF/p75 and allosterically inhibit the catalytic activity of IN (i.e. the early effect). When present during virus production, LEDGINs interfere with proper maturation due to enhanced IN oligomerization in the progeny virions (i.e. the late effect).

Results

We studied the effect of LEDGINs present during virus production on the transcriptional state of the residual virus. Infection of cells with viruses produced in the presence of LEDGINs resulted in a residual reservoir that was refractory to activation. Integration of residual provirus was less favored near epigenetic markers associated with active transcription. However, integration near H3K36me3 and active genes, both targeted by LEDGF/p75, was not affected. Also in primary cells, LEDGIN treatment induced a reservoir resistant to activation due to a combined early and late effect.

Conclusion

LEDGINs present a research tool to study the link between integration and transcription, an essential question in retrovirology. LEDGIN treatment during virus production altered integration of residual provirus in a LEDGF/p75-independent manner, resulting in a reservoir that is refractory to activation.

Electronic supplementary material

The online version of this article (10.1186/s12977-019-0472-3) contains supplementary material, which is available to authorized users.

Cat and Mouse: HIV Transcription in Latency, Immune Evasion and Cure/Remission Strategies

There is broad scientific and societal consensus that finding a cure for HIV infection must be pursued. The major barrier to achieving a cure for HIV/AIDS is the capacity of the HIV virus to avoid both immune surveillance and current antiretroviral therapy (ART) by rapidly establishing latently infected cell populations, termed latent reservoirs. Here, we provide an overview of the rapidly evolving field of HIV cure/remission research, highlighting recent progress and ongoing challenges in the understanding of HIV reservoirs, the role of HIV transcription in latency and immune evasion. We review the major approaches towards a cure that are currently being explored and further argue that small molecules that inhibit HIV transcription, and therefore uncouple HIV gene expression from signals sent by the host immune response, might be a particularly promising approach to attain a cure or remission. We emphasize that a better understanding of the game of "cat and mouse" between the host immune system and the HIV virus is a crucial knowledge gap to be filled in both cure and vaccine research.

Current Strategies for Elimination of HIV-1 Latent Reservoirs Using Chemical Compounds Targeting Host and Viral Factors

Since the implementation of combination antiretroviral therapy (cART), rates of HIV type 1 (HIV-1) mortality, morbidity, and newly acquired infections have decreased dramatically. In fact, HIV-1-infected individuals under effective suppressive cART approach normal life span and quality of life. However, long-term therapy is required because the virus establish a reversible state of latency in memory CD4⁺ T cells. Two principle strategies, namely "shock and kill" approach and "block and lock" approach, are currently being investigated for the eradication of these HIV-1 latent reservoirs. Actually, both of these contrasting approaches are based on the use of small-molecule compounds to achieve the cure for HIV-1. In this review, we discuss the recent progress that has been made in designing and developing small-molecule compounds for both strategies.

HEXIM1-Tat chimera inhibits HIV-1 replication

Transcription of HIV provirus is a key step of the viral cycle, and depends on the recruitment of the cellular positive transcription elongation factor b (P-TEFb) to the HIV promoter. The viral transactivator Tat can displace P-TEFb from the 7SK small nuclear ribonucleoprotein, where it is bound and inactivated by HEXIM1, and bring it to TAR, which allows the stalled RNA polymerase II to transition to successful transcription elongation. In this study, we designed a chimeric inhibitor of HIV transcription by combining functional domains from HEXIM1 and Tat. The chimera (HT1) potently inhibited gene expression from the HIV promoter, by competing with Tat for TAR and P-TEFb binding, while keeping the latter inactive. HT1 inhibited spreading infection as well as viral reactivation in lymphocyte T cell line models of HIV latency, with little effect on cellular transcription and metabolism. This proof-of-concept study validates an innovative approach to interfering with HIV transcription via peptide mimicry and competition for RNA-protein interactions. HT1 represents a new candidate for HIV therapy, or HIV cure via the proposed block and lock strategy.

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.

HIV LTR-Driven Antisense RNA by Itself Has Regulatory Function and May Curtail Virus Reactivation From Latency

Latently infected T lymphocytes are an important barrier toward eliminating a persistent HIV infection. Here we describe an HIV-based recombinant fluorescent-lentivirus referred to as “rfl-HIV” that enables to analyze sense and antisense transcription by means of fluorescence reporter genes. This model virus exhibited similar transcriptional and functional properties of the antisense transcript as observed with a wild type HIV, and largely facilitated the generation of latently-infected T cells clones. We show that latently-infected cells can be divided into two types, those with and those without antisense transcription. Upon addition of latency reversal agents, only the cells that lack antisense transcripts are readily reactivated to transcribe HIV. Thus, antisense transcripts may exhibit a dominant suppressor activity and can lock an integrated provirus into a non-reactivatable state. These findings could have important implications for the development of strategies to eradicate HIV from infected individuals.