New latency-promoting agents for a block-and-lock functional cure strategy

PURPOSE OF REVIEW

Currently, HIV-infected patients are treated with antiretroviral therapy. However, when the treatment is interrupted, viral rebound occurs from latently infected cells. Therefore, scientists aim to develop an HIV-1 cure which eradicates or permanently silences the latent reservoir.

RECENT FINDINGS

Previously, scientists focused on the shock-and-kill cure strategy, which aims to eradicate the latent reservoir using latency-reactivating agents. Limited success shifts the interest towards the block-and-lock cure approach, which aims to achieve a functional cure by "blocking" HIV-1 transcription and "locking" the provirus in a deep latent state, resistant to treatment-interruption. In this strategy, latency promoting agents are used to induce transcriptional silencing and alter the epigenetics environment at the HIV promotor.

SUMMARY

For the block-and-lock cure strategy to succeed more investigation into the transcriptional and epigenetic regulation of HIV-1 gene expression is necessary to design optimal latency-promoting agents. In this review, we will discuss the latency promoting agents that have been described in literature during the past 2 years (2022-2023).

HIV-1 transcriptional modulation: novel host factors and prospective therapeutic strategies

PURPOSE OF REVIEW

This review highlights advances in HIV transcription and epigenetic latency mechanisms and outlines current therapeutic approaches to eliminate or block the HIV-1 latent reservoir.

RECENT FINDINGS

Novel host factors have been reported to modulate HIV-1 transcription and latency. Chromatin affinity purification strategies followed by mass spectrometry (ChAP-MS) identified the chaperone protein p32 to play an important role in HIV-1 transcriptional regulation via interactions with the viral transcriptional activator Tat. Similarly, an shRNA screen identified the methyltransferase SMYD5 contributing to HIV-1 transcriptional activation also by modulating Tat activity. These new factors, among others, represent potential druggable targets that could be explored in the 'block-and-lock' or 'shock-and-kill' approaches.

SUMMARY

The HIV-1 latent reservoir is established early after infection, persists during antiretroviral therapy, and is the source of viral rebound after treatment interruption. An HIV cure requires either eliminating this reservoir or blocking latent proviral reactivation in the absence of antiretroviral therapy (ART). Understanding the mechanisms and key-players modulating HIV transcriptional and reactivation may facilitate therapeutic advancements. Here we summarize, the latest findings on host factors' roles in HIV transcriptional regulation.

Forging a Functional Cure for HIV: Transcription Regulators and Inhibitors

Current antiretroviral therapy (ART) increases the survival of HIV-infected individuals, yet it is not curative. The major barrier to finding a definitive cure for HIV is our inability to identify and eliminate long-lived cells containing the dormant provirus, termed viral reservoir. When ART is interrupted, the viral reservoir ensures heterogenous and stochastic HIV viral gene expression, which can reseed infection back to pre-ART levels. While strategies to permanently eradicate the virus have not yet provided significant success, recent work has focused on the management of this residual viral reservoir to effectively limit comorbidities associated with the ongoing viral transcription still observed during suppressive ART, as well as limit the need for daily ART. Our group has been at the forefront of exploring the viability of the block-and-lock remission approach, focused on the long-lasting epigenetic block of viral transcription such that without daily ART, there is no risk of viral rebound, transmission, or progression to AIDS. Numerous studies have reported inhibitors of both viral and host factors required for HIV transcriptional activation. Here, we highlight and review some of the latest HIV transcriptional inhibitor discoveries that may be leveraged for the clinical exploration of block-and-lock and revolutionize the way we treat HIV infections.

Towards a Functional Cure of HIV-1: Insight Into the Chromatin Landscape of the Provirus

Despite potent combination antiretroviral therapy, HIV-1 infection persists due to irreversible integration of the virus in long-living cells of the immune system. The main focus of HIV-1 cure strategies has been on HIV-1 eradication, yet without great success so far. Therefore, HIV-1 remission or a functional cure, whereby the virus is silenced rather than eradicated, is considered as an alternative strategy. Elite controllers, individuals who spontaneously control HIV-1, may point us the way toward a functional HIV-1 cure. In order to achieve such a cure, a profound understanding of the mechanisms controlling HIV-1 expression and silencing is needed. In recent years, evidence has grown that the site of integration as well as the chromatin landscape surrounding the integration site affects the transcriptional state of the provirus. Still, at present, the impact of integration site selection on the establishment and maintenance of the HIV-1 reservoirs remains poorly understood. The discovery of LEDGF/p75 as a binding partner of HIV-1 integrase has led to a better understanding of integration site selection. LEDGF/p75 is one of the important determinants of integration site selection and targets integration toward active genes. In this review, we will provide an overview of the most important determinants of integration site selection. Secondly, we will discuss the chromatin landscape at the integration site and its implications on HIV-1 gene expression and silencing. Finally, we will discuss how interventions that affect integration site selection or modifications of the chromatin could yield a functional cure of HIV-1 infection.

The Block-and-Lock Strategy for Human Immunodeficiency Virus Cure: Lessons Learned from Didehydro-Cortistatin A

Antiretroviral therapy effectively controls human immunodeficiency virus (HIV) infection. However, a reservoir of latently infected cells persists under suppressive therapy, constituting a major barrier to an HIV cure. The block-and-lock approach to a functional cure aims at the transcriptional and epigenetic silencing of proviruses, blocking viral reactivation in the absence of therapy, preventing disease progression and transmission, despite the presence of detectable integrated proviruses. This approach has been put forward for exploration based on the activity of didehydro-cortistatin A, an inhibitor of the HIV transcriptional activator Tat. Here we review the mechanisms by which didehydro-cortistatin A inhibition of Tat's feedback loop transcriptional amplification results in epigenetic silencing of the HIV promoter, and we discuss the benefits and limitations of the block-and-lock approach for an HIV cure.

The XPB Subunit of the TFIIH Complex Plays a Critical Role in HIV-1 Transcription and XPB Inhibition by Spironolactone Prevents HIV-1 Reactivation from Latency

HIV transcription requires assembly of cellular transcription factors at the HIV-1promoter. The TFIIH general transcription factor facilitates transcription initiation by opening the DNA strands around the transcription start site and phosphorylating the C-terminal domain for RNA polymerase II (RNAPII) for activation. Spironolactone (SP), an FDA approved aldosterone antagonist, triggers the proteasomal degradation of the XPB subunit of TFIIH, and concurrently suppresses acute HIV infection in vitro Here we investigated SP as a possible block-and-lock agent for a functional cure aimed at the transcriptional silencing of the viral reservoir. The long-term activity of SP was investigated in primary and cell line models of HIV-1 latency and reactivation. We show that SP rapidly inhibits HIV-1 transcription by reducing RNAPII recruitment to the HIV-1 genome. shRNA knockdown of XPB confirmed XPB degradation as the mechanism of action. Unfortunately, long-term pre-treatment with SP does not result in epigenetic suppression of HIV upon SP treatment interruption, since virus rapidly rebounds when XPB reemerges; however, SP alone without ART maintains the transcriptional suppression. Importantly, SP inhibits HIV reactivation from latency in both cell line models and resting CD4⁺T cells isolated from aviremic infected individuals upon cell stimulation with latency reversing agents. Furthermore, long-term treatment with concentrations of SP that potently degrade XPB does not lead to global dysregulation of cellular mRNA expression. Overall, these results suggest that XPB plays a key role in HIV transcriptional regulation and XPB degradation by SP strengthens the potential of HIV transcriptional inhibitors in block-and-lock HIV cure approaches.IMPORTANCE Antiretroviral therapy (ART) effectively reduces an individual's HIV loads to below the detection limit, nevertheless rapid viral rebound immediately ensues upon treatment interruption. Furthermore, virally suppressed individuals experience chronic immune activation from ongoing low-level virus expression. Thus, the importance of identifying novel therapeutics to explore in block-and-lock HIV functional cure approaches, aimed at the transcriptional and epigenetic silencing of the viral reservoir to block reactivation from latency. We investigated the potential of repurposing the FDA-approved spironolactone (SP), as one such drug. SP treatment rapidly degrades a host transcription factor subunit, XPB, inhibiting HIV transcription and blocking reactivation from latency. Long-term SP treatment does not affect cellular viability, cell cycle progression or global cellular transcription. SP alone blocks HIV transcription in the absence of ART but does not delay rebound upon drug removal as XPB rapidly reemerges. This study highlights XPB as a novel drug target in block-and-lock therapeutic approaches.

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.

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.

The Tat inhibitor didehydro-cortistatin A suppresses SIV replication and reactivation

The HIV-1 transactivation protein (Tat) binds the HIV mRNA transactivation responsive element (TAR), regulating transcription and reactivation from latency. Drugs against Tat are unfortunately not clinically available. We reported that didehydro-cortistatin A (dCA) inhibits HIV-1 Tat activity. In human CD4⁺ T cells isolated from aviremic individuals and in the humanized mouse model of latency, combining dCA with antiretroviral therapy accelerates HIV-1 suppression and delays viral rebound upon treatment interruption. This drug class is amenable to block-and-lock functional cure approaches, aimed at a durable state of latency. Simian immunodeficiency virus (SIV) infection of rhesus macaques (RhMs) is the best-characterized model for AIDS research. Here, we demonstrate, using in vitro and cell-based assays, that dCA directly binds to SIV Tat's basic domain. dCA specifically inhibits SIV Tat binding to TAR, but not a Tat-Rev fusion protein, which activates transcription when Rev binds to its cognate RNA binding site replacing the apical region of TAR. Tat-TAR inhibition results in loss of RNA polymerase II recruitment to the SIV promoter. Importantly, dCA potently inhibits SIV reactivation from latently infected Hut78 cells and from primary CD4⁺ T cells explanted from SIV_mac239-infected RhMs. In sum, dCA's remarkable breadth of activity encourages SIV-infected RhM use for dCA preclinical evaluation.-Mediouni, S., Kessing, C. F., Jablonski, J. A., Thenin-Houssier, S., Clementz, M., Kovach, M. D., Mousseau, G., de Vera, I.M.S., Li, C., Kojetin, D. J., Evans, D. T., Valente, S. T. The Tat inhibitor didehydro-cortistatin A suppresses SIV replication and reactivation.

Insight in HIV Integration Site Selection Provides a Block-and-Lock Strategy for a Functional Cure of HIV Infection

Despite significant improvements in therapy, the HIV/AIDS pandemic remains an important threat to public health. Current treatments fail to eradicate HIV as proviral DNA persists in long-living cellular reservoirs, leading to viral rebound whenever treatment is discontinued. Hence, a better understanding of viral reservoir establishment and maintenance is required to develop novel strategies to destroy latently infected cells, and/or to durably silence the latent provirus in infected cells. Whereas the mechanism of integration has been well studied from a catalytic point of view, it remains unknown how integration site selection and transcription are linked. In recent years, evidence has grown that lens epithelium-derived growth factor p75 (LEDGF/p75) is the main determinant of HIV integration site selection and that the integration site affects the transcriptional state of the provirus. LEDGINs have been developed as small molecule inhibitors of the interaction between LEDGF/p75 and integrase. Recently, it was shown that LEDGIN treatment in cell culture shifts the residual integrated provirus towards the inner nuclear compartment and out of transcription units in a dose dependent manner. This LEDGIN-mediated retargeting increased the proportion of provirus with a transcriptionally silent phenotype and the residual reservoir proved refractory to reactivation in vitro. LEDGINs provide us with a research tool to study the link between integration and transcription, a quintessential question in retrovirology. LEDGIN-mediated retargeting of the residual reservoirs provides a novel potential "block-and-lock" strategy as a functional cure of HIV infection.

In Vivo Suppression of HIV Rebound by Didehydro-Cortistatin A, a "Block-and-Lock" Strategy for HIV-1 Treatment

HIV-1 Tat activates viral transcription and limited Tat transactivation correlates with latency establishment. We postulated a "block-and-lock" functional cure approach based on properties of the Tat inhibitor didehydro-Cortistatin A (dCA). HIV-1 transcriptional inhibitors could block ongoing viremia during antiretroviral therapy (ART), locking the HIV promoter in persistent latency. We investigated this hypothesis in human CD4⁺ T cells isolated from aviremic individuals. Combining dCA with ART accelerates HIV-1 suppression and prevents viral rebound after treatment interruption, even during strong cellular activation. We show that dCA mediates epigenetic silencing by increasing nucleosomal occupancy at Nucleosome-1, restricting RNAPII recruitment to the HIV-1 promoter. The efficacy of dCA was studied in the bone marrow-liver-thymus (BLT) mouse model of HIV latency and persistence. Adding dCA to ART-suppressed mice systemically reduces viral mRNA in tissues. Moreover, dCA significantly delays and reduces viral rebound levels upon treatment interruption. Altogether, this work demonstrates the potential of block-and-lock cure strategies.