Regulation of HIV-1 latency by T-cell activation

The immunosuppressive tuberculosis-associated microenvironment inhibits viral replication and promotes HIV-1 latency in CD4+ T cells

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is the most common coinfection among people living with HIV-1. This coinfection is associated with accelerated HIV-1 disease progression and reduced survival. However, the impact of the HIV-1/TB coinfection on HIV-1 replication and latency in CD4+ T cells remains poorly studied. Using the acellular fraction of tuberculous pleural effusion (TB-PE), we investigated whether viral replication and HIV-1 latency in CD4+ T cells are affected by a TB-associated microenvironment. Our results revealed that TB-PE impaired T cell receptor-dependent cell activation and decreased HIV-1 replication in CD4+ T cells. Moreover, this immunosuppressive TB microenvironment promoted viral latency and inhibited HIV-1 reactivation. This study indicates that the TB-induced immune response may contribute to the persistence of the viral reservoir by silencing HIV-1 expression, allowing the virus to persist undetected by the immune system, and increasing the size of the latent HIV-1 reservoir.

The histone methyltransferase SETD2 regulates HIV expression and latency

Understanding the mechanisms that drive HIV expression and latency is a key goal for achieving an HIV cure. Here we investigate the role of the SETD2 histone methyltransferase, which deposits H3K36 trimethylation (H3K36me3), in HIV infection. We show that prevention of H3K36me3 by a potent and selective inhibitor of SETD2 (EPZ-719) leads to reduced post-integration viral gene expression and accelerated emergence of latently infected cells. CRISPR/Cas9-mediated knockout of SETD2 in primary CD4 T cells confirmed the role of SETD2 in HIV expression. Transcriptomic profiling of EPZ-719-exposed HIV-infected cells identified numerous pathways impacted by EPZ-719. Notably, depletion of H3K36me3 prior to infection did not prevent HIV integration but resulted in a shift of integration sites from highly transcribed genes to quiescent chromatin regions and to polycomb repressed regions. We also observed that SETD2 inhibition did not apparently affect HIV RNA levels, indicating a post-transcriptional mechanism affecting HIV expression. Viral RNA splicing was modestly reduced in the presence of EPZ-719. Intriguingly, EPZ-719 exposure enhanced responsiveness of latent HIV to the HDAC inhibitor vorinostat, suggesting that H3K36me3 can contribute to a repressive chromatin state at the HIV locus. These results identify SETD2 and H3K36me3 as novel regulators of HIV integration, expression and latency.

Integrated Single-cell Multiomic Analysis of HIV Latency Reversal Reveals Novel Regulators of Viral Reactivation

Despite the success of antiretroviral therapy, human immunodeficiency virus (HIV) cannot be cured because of a reservoir of latently infected cells that evades therapy. To understand the mechanisms of HIV latency, we employed an integrated single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin with sequencing (scATAC-seq) approach to simultaneously profile the transcriptomic and epigenomic characteristics of ∼ 125,000 latently infected primary CD4+ T cells after reactivation using three different latency reversing agents. Differentially expressed genes and differentially accessible motifs were used to examine transcriptional pathways and transcription factor (TF) activities across the cell population. We identified cellular transcripts and TFs whose expression/activity was correlated with viral reactivation and demonstrated that a machine learning model trained on these data was 75%-79% accurate at predicting viral reactivation. Finally, we validated the role of two candidate HIV-regulating factors, FOXP1 and GATA3, in viral transcription. These data demonstrate the power of integrated multimodal single-cell analysis to uncover novel relationships between host cell factors and HIV latency.

The spleen is the graveyard of CD4+ cells in patients with immunological failure of visceral leishmaniasis and AIDS

BACKGROUND

Visceral leishmaniasis (VL), or kala-azar, is a common comorbidity in patients with AIDS in endemic areas. Many patients continue to experiences relapses of VL despite virological control, but with immunological failure. These patients remain chronically symptomatic with hypersplenism, for example with anemia, leukopenia, and thrombocytopenia, and are at risk of severe co-infection due to low CD4+ count. Therefore, in this study, splenectomized patients with VL and HIV infection were investigated to understand why the CD4+ count fails to recover in these patients, evaluating the importance of spleen mass for hypersplenism and immunological failure.

METHODS

From a retrospective open cohort of 13 patients who had previously undergone splenectomy as salvage therapy for relapsing VL, 11 patients with HIV infection were investigated. This study compared the patients' complete blood cell count (CBC) and CD4+ and CD8+ cell counts before and after splenectomy with respect to spleen weight.

RESULTS

CBC was substantially improved after splenectomy, indicating hypersplenism. However, to the best of our knowledge, this is the first study to show that spleen mass is strongly and negatively correlated with CD4+ cell count (ρ = -0.71, P = 0.015).

CONCLUSIONS

This finding was unexpected, as the spleen is the most extensive lymphoid tissue and T-lymphocyte source. After reviewing the literature and reasoning, we hypothesized that the immunological failure was secondary to CD4+ loss initially by apoptosis in the spleen induced by productive HIV infection and, subsequently, by pyroptosis sustained by parasitic infection in spleen macrophages.

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.

Bimodal Expression Patterns, and Not Viral Burst Sizes, Predict the Effects of Vpr on HIV-1 Proviral Populations in Jurkat Cells

Integration site landscapes, clonal dynamics, and latency reversal with or without vpr were compared in HIV-1-infected Jurkat cell populations, and the properties of individual clones were defined. Clones differed in fractions of long terminal repeat (LTR)-active daughter cells, with some clones containing few to no LTR-active cells, while almost all cells were LTR active for others. Clones varied over 4 orders of magnitude in virus release per active cell. Proviruses in largely LTR-active clones were closer to preexisting enhancers and promoters than low-LTR-active clones. Unsurprisingly, major vpr+ clones contained fewer LTR-active cells than vpr- clones, and predominant vpr+ proviruses were farther from enhancers and promoters than those in vpr- pools. Distances to these marks among intact proviruses previously reported for antiretroviral therapy (ART)-suppressed patients revealed that patient integration sites were more similar to those in the vpr+ pool than to vpr- integrants. Complementing vpr-defective proviruses with vpr led to the rapid loss of highly LTR-active clones, indicating that the effect of Vpr on proviral populations occurred after integration. However, major clones in the complemented pool and its vpr- parent population did not differ in burst sizes. When the latency reactivation agents prostratin and JQ1 were applied separately or in combination, vpr+ and vpr- population-wide trends were similar, with dual-treatment enhancement being due in part to reactivated clones that did not respond to either drug applied separately. However, the expression signatures of individual clones differed between populations. These observations highlight how Vpr, exerting selective pressure on proviral epigenetic variation, can shape integration site landscapes, proviral expression patterns, and reactivation properties. IMPORTANCE A bedrock assumption in HIV-1 population modeling is that all active cells release the same amount of virus. However, the findings here revealed that when HIV-infected cells expand into clones, each clone differs in virus production. Reasoning that this variation in expression patterns constituted a population of clones from which differing subsets would prevail under differing environmental conditions, the cytotoxic HIV-1 protein Vpr was introduced, and population dynamics and expression properties were compared in the presence and absence of Vpr. The results showed that whereas most clones produced fairly continuous levels of virus in the absence of Vpr, its presence selected for a distinct subset of clones with properties reminiscent of persistent populations in patients, suggesting the possibility that the interclonal variation in expression patterns observed in culture may contribute to proviral persistence in vivo.

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.

Combinatorial Use of Both Epigenetic and Non-Epigenetic Mechanisms to Efficiently Reactivate HIV Latency

The persistence of latent HIV provirus pools in different resting CD4+ cell subsets remains the greatest obstacle in the current efforts to treat and cure HIV infection. Recent efforts to purge out latently infected memory CD4+ T-cells using latency-reversing agents have failed in clinical trials. This review discusses the epigenetic and non-epigenetic mechanisms of HIV latency control, major limitations of the current approaches of using latency-reversing agents to reactivate HIV latency in resting CD4+ T-cells, and potential solutions to these limitations.

ZBTB2 represses HIV-1 transcription and is regulated by HIV-1 Vpr and cellular DNA damage responses

Previously, we reported that cellular transcription factor ZASC1 facilitates DNA-dependent/RNA-independent recruitment of HIV-1 TAT and the cellular elongation factor P-TEFb to the HIV-1 promoter and is a critical factor in regulating HIV-1 transcriptional elongation (PLoS Path e1003712). Here we report that cellular transcription factor ZBTB2 is a novel repressor of HIV-1 gene expression. ZBTB2 strongly co-immunoprecipitated with ZASC1 and was dramatically relocalized by ZASC1 from the cytoplasm to the nucleus. Mutations abolishing ZASC1/ZBTB2 interaction prevented ZBTB2 nuclear relocalization. We show that ZBTB2-induced repression depends on interaction of cellular histone deacetylases (HDACs) with the ZBTB2 POZ domain. Further, ZASC1 interaction specifically recruited ZBTB2 to the HIV-1 promoter, resulting in histone deacetylation and transcription repression. Depleting ZBTB2 by siRNA knockdown or CRISPR/CAS9 knockout in T cell lines enhanced transcription from HIV-1 vectors lacking Vpr, but not from these vectors expressing Vpr. Since HIV-1 Vpr activates the viral LTR by inducing the ATR kinase/DNA damage response pathway, we investigated ZBTB2 response to Vpr and DNA damaging agents. Expressing Vpr or stimulating the ATR pathway with DNA damaging agents impaired ZASC1’s ability to localize ZBTB2 to the nucleus. Moreover, the effects of DNA damaging agents and Vpr on ZBTB2 localization could be blocked by ATR kinase inhibitors. Critically, Vpr and DNA damaging agents decreased ZBTB2 binding to the HIV-1 promoter and increased promoter histone acetylation. Thus, ZBTB2 is recruited to the HIV-1 promoter by ZASC1 and represses transcription, but ATR pathway activation leads to ZBTB2 removal from the promoter, cytoplasmic sequestration and activation of viral transcription. Together, our data show that ZASC1/ZBTB2 integrate the functions of TAT and Vpr to maximize HIV-1 gene expression.

The Human immunodeficiency virus 1 (HIV-1) TAT and VPR proteins, in combination with cellular transcription factors, regulate the switch between transcriptionally active productive infection and the transcriptionally inactive latent state. Previously we reported that ZASC1, a cellular transcription factor linked to multiple squamous cell carcinomas and inherited ataxias, contributes to an RNA-independent, DNA-dependent step in recruiting the TAT/P-TEFb complex that is critical for HIV-1 transcription elongation to the HIV-1 promoter. Here we show ZASC1 interacts with ZBTB2, another cellular transcription factor with strong links to cancer. ZASC1 interaction relocalizes ZBTB2 from the cytoplasm to the HIV-1 promoter in the nucleus where ZBTB2 interacts with cellular HDACs, increases HIV-1 promoter histone deacetylation and represses viral transcription. We show that Vpr-mediated activation of the ATR/DNA damage pathway regulates ZBTB2 relocalization by ZASC1. Thus, the cellular transcription factors ZASC1 and ZBTB2 regulate the transcription elongation activities of HIV-1 TAT and the Vpr activation of the cellular DNA damage response pathway to determine the transcriptional fate of the HIV-1 provirus. These results also have strong implications for the role of ZASC1/ZBTB2 and the DNA damage response in cancer and inherited ataxias.

AP-1 and NF-κB synergize to transcriptionally activate latent HIV upon T-cell receptor activation

Latent HIV-1 proviruses are capable of reactivating productive lytic infection, but the precise molecular mechanisms underlying emergence from latency are poorly understood. In this study, we determined the contribution of the transcription factors NF-κB, NFAT, and AP-1 in the reactivation of latent HIV following T-cell receptor (TCR) activation using Jurkat T-cell clones harboring single latent HIV proviruses. Our findings demonstrate that during reactivation from latency, NF-κB enhances HIV transcription while NFAT inhibits it by competing with NF-κB for overlapping binding sites on the HIV long terminal repeat (LTR). We have also demonstrated for the first time the molecular contribution of AP-1 in the reactivation of HIV from latency, whereby AP-1 synergizes with NF-κB to regulate HIV transcriptional elongation following TCR activation.

Alterations of redox and iron metabolism accompany the development of HIV latency

HIV-1 persists in a latent form during antiretroviral therapy, mainly in CD4+ T cells, thus hampering efforts for a cure. HIV-1 infection is accompanied by metabolic alterations, such as oxidative stress, but the effect of cellular antioxidant responses on viral replication and latency is unknown. Here, we show that cells survive retroviral replication, both in vitro and in vivo in SIVmac-infected macaques, by upregulating antioxidant pathways and the intertwined iron import pathway. These changes are associated with remodeling of promyelocytic leukemia protein nuclear bodies (PML NBs), an important constituent of nuclear architecture and a marker of HIV-1 latency. We found that PML NBs are hyper-SUMOylated and that PML protein is degraded via the ubiquitin-proteasome pathway in productively infected cells, before latency establishment and after reactivation. Conversely, normal numbers of PML NBs were restored upon transition to latency or by decreasing oxidative stress or iron content. Our results highlight antioxidant and iron import pathways as determinants of HIV-1 latency and support their pharmacologic inhibition as tools to regulate PML stability and impair latency establishment.

HIV Reactivation in Latently Infected Cells With Virological Synapse-Like Cell Contact

HIV reactivation from latency is induced by cytokines but also by cell contact with other cells. To better understand this, J1.1 cells, a latent HIV-1-infected Jurkat derivative, were cocultured with its parental Jurkat. J1.1 cells became p17MA-positive and produced a high level of HIV p24CA antigen, only when they were cocultured with stimulated Jurkat with cell-to-cell contact. In contrast, very little p24CA was produced when they were cocultured without cell contact. Similar results were obtained when latent ACH-2 and its parental A3.01 cells were cocultured. Confocal microscopy revealed that not only HIV-1 p17MA and gp120Env but also LFA-1, CD81, CD59, and TCR CD3 accumulated at the cell contact site, suggesting formation of the virological synapse-like structure. LFA-1–ICAM-1 interaction was involved in the cell-to-cell contact. When J1.1 was cocultured with TCR-deficient Jurkat, the p17MA-positive rate was significantly lower, although the cell-to-cell contact was not impaired. Quantitative proteomics identified 54 membrane molecules, one of which was MHC class I, that accumulated at the cell contact site. Reactivation from latency was also influenced by the presence of stromal cells. Our study indicated that latent HIV-1 in J1.1/ACH-2 cells was efficiently reactivated by cell-to-cell contact with stimulated parental cells, accompanying the virological synapse-like structure.

Single-Cell Analysis of Quiescent HIV Infection Reveals Host Transcriptional Profiles that Regulate Proviral Latency

A detailed understanding of the mechanisms that establish or maintain the latent reservoir of HIV will guide approaches to eliminate persistent infection. We used a cell line and primary cell models of HIV latency to investigate viral RNA (vRNA) expression and the role of the host transcriptome using single-cell approaches. Single-cell vRNA quantitation identified distinct populations of cells expressing various levels of vRNA, including completely silent populations. Strikingly, single-cell RNA-seq of latently infected primary cells demonstrated that HIV downregulation occurred in diverse transcriptomic environments but was significantly associated with expression of a specific set of cellular genes. In particular, latency was more frequent in cells expressing a transcriptional signature that included markers of naive and central memory T cells. These data reveal that expression of HIV proviruses within the latent reservoir are influenced by the host cell transcriptional program. Therapeutic modulation of these programs may reverse or enforce HIV latency.

Nuclear Transit and HIV LTR Binding of NF-κB Subunits Held by IκB Proteins: Implications for HIV-1 Activation

No effective therapy to eliminate the HIV latently infected cell reservoir has been developed. One approach, “shock and kill”, employs agents that activate HIV, subsequently killing the activated infected cells and/or virus. Shock and kill requires agents that safely and effectively activate HIV. One class of activation agents works through classical NF-κB pathways, but global NF-κB activators are non-specific and toxic. There exist two major IκBs: IκBα, and IκBε, which hold activating NF-κB subunits in the cytoplasm, releasing them for nuclear transit upon cell stimulation. IκBα was considered the main IκB responsible for gene expression regulation, including HIV activation. IκBε is expressed in cells constituting much of the latent HIV reservoir, and IκBε knockout mice have a minimal phenotype, suggesting that IκBε could be a valuable target for HIV activation and reservoir depletion. We previously showed that targeting IκBε yields substantial increases in HIV expression. Here, we show that IκBε holds c-Rel and p65 activating NF-κB subunits in the cytoplasm, and that targeting IκBε with siRNA produces a strong increase in HIV expression associated with enhanced c-Rel and p65 transit to the nucleus and binding to the HIV LTR of the activating NF-κBs, demonstrating a mechanism through which targeting IκBε increases HIV expression. The findings suggest that it may be helpful to develop HIV activation approaches, acting specifically to target IκBε and its interactions with the NF-κBs.

Lathyrane, Premyrsinane, and Related Diterpenes from Euphorbia boetica: Effect on in Vitro Neural Progenitor Cell Proliferation

Lathyrane-type diterpenes previously have been proven to promote proliferation of neural precursor cells (NPCs) by targeting and activating one or more protein kinase C (PKC) isozymes. Aiming to find new drug candidates with a lathyrane skeleton to modulate adult neurogenesis through PKC activation, a phytochemical study of a methanol extract of the aerial parts of Euphorbia boetica was carried out. Seven new diterpenes, representing the premyrsinane (1-3), myrsinane (4, 5), and cyclomyrsinane types (6, 7), along with three known diterpenes, belonging to the cyclomyrsinane (8) and lathyrane types (9, 10), were isolated. The chemical structures and relative configurations of the new compounds were determined by extensive NMR spectroscopic studies and comparison with known compounds. The absolute configurations for compounds 2, 3, 6, and 7 were proposed, based on a comparison of the experimental ECD spectra of compounds 2 and 7 with those of known related compounds. The activity of lathyrane compounds 9 and 10 as promoters of NPC proliferation was evaluated using a neurosphere assay. Both compounds increased the size of neurospheres in a dose-dependent manner when proliferation was stimulated by the epidermal growth factor and the basic fibroblast growth factor.

Insights into the HIV Latency and the Role of Cytokines

Human immunodeficiency virus-1 (HIV-1) has the ability to infect latently at the level of individual CD4+ cells. Latent HIV-1 proviruses are transcriptionally silent and immunologically inert, but are still capable of reactivating productive lytic infection following cellular activation. These latent viruses are the main obstacle in the eradication of HIV-1, because current HIV-1 treatment regimens are ineffective against them. Normal immunological response against an antigen activates CD4+ naïve T cells. The activated CD4+ naïve T cells undergo cell cycle, resulting in further transformation and profound proliferation to form effector CD4+ T-cells. Notably, in HIV-1 infected individuals, some of the effector CD4+ T cells get infected with HIV-1. Upon fulfillment of their effector functions, almost all activated CD4+ T cells are committed to apoptosis or programmed cell death, but a miniscule fraction revert to quiescence and become resting memory CD4+ T cells to mediate a rapid immunological response against the same antigen in the future. However, due to the quiescent nature of the resting memory T cells, the integrated HIV-1 becomes transcriptionally silent and acquires a latent phenotype. Following re-exposure to the same antigen, memory cells and integrated HIV-1 are stimulated. The reactivated latent HIV provirus subsequently proceeds through its life cycle and eventually leads to the production of new viral progeny. Recently, many strategies against HIV-1 latency have been developed and some of them have even matured to the clinical level, but none can yet effectively eliminate the latent HIV reservoir, which remains a barrier to HIV-1 cure. Therefore, alternative strategies to eradicate latent HIV need to be considered. This review provides vital knowledge on HIV latency and on strategies to supplement highly active anti-retroviral therapy (HAART) with cytokine-mediated therapeutics for dislodging the latent HIV reservoirs in order to open up new avenues for curing HIV.

Apobec3A maintains HIV-1 latency through recruitment of epigenetic silencing machinery to the long terminal repeat

HIV-1 integrates into the genome of target cells and establishes latency indefinitely. Understanding the molecular mechanism of HIV-1 latency maintenance is needed for therapeutic strategies to combat existing infection. In this study, we found an unexpected role for Apobec3A (apolipoprotein B MRNA editing enzyme catalytic subunit 3A, abbreviated "A3A") in maintaining the latency state within HIV-1-infected cells. Overexpression of A3A in latently infected cell lines led to lower reactivation, while knockdown or knockout of A3A led to increased spontaneous and inducible HIV-1 reactivation. A3A maintains HIV-1 latency by associating with proviral DNA at the 5' long terminal repeat region, recruiting KAP1 and HP1, and imposing repressive histone marks. We show that knockdown of A3A in latently infected human primary CD4 T cells enhanced HIV-1 reactivation. Collectively, we provide evidence and a mechanism by which A3A reinforces HIV-1 latency in infected CD4 T cells.

HIV and the Macrophage: From Cell Reservoirs to Drug Delivery to Viral Eradication

Macrophages serve as host cells, inflammatory disease drivers and drug runners for human immunodeficiency virus infection and treatments. Low-level viral persistence continues in these cells in the absence of macrophage death. However, the cellular microenvironment changes as a consequence of viral infection with aberrant production of pro-inflammatory factors and promotion of oxidative stress. These herald viral spread from macrophages to neighboring CD4⁺ T cells and end organ damage. Virus replicates in tissue reservoir sites that include the nervous, pulmonary, cardiovascular, gut, and renal organs. However, each of these events are held in check by antiretroviral therapy. A hidden and often overlooked resource of the macrophage rests in its high cytoplasmic nuclear ratios that allow the cell to sense its environment and rid it of the cellular waste products and microbial pathogens it encounters. These phagocytic and intracellular killing sensing mechanisms can also be used in service as macrophages serve as cellular carriage depots for antiretroviral nanoparticles and are able to deliver medicines to infectious disease sites with improved therapeutic outcomes. These undiscovered cellular functions can lead to reductions in persistent infection and may potentially facilitate the eradication of residual virus to eliminate disease.

Systems analysis of latent HIV reversal reveals altered stress kinase signaling and increased cell death in infected T cells

Viral latency remains the most significant obstacle to HIV eradication. Clinical strategies aim to purge the latent CD4+ T cell reservoir by activating viral expression to induce death, but are undercut by the inability to target latently infected cells. Here we explored the acute signaling response of latent HIV-infected CD4+ T cells to identify dynamic phosphorylation signatures that could be targeted for therapy. Stimulation with CD3/CD28, PMA/ionomycin, or latency reversing agents prostratin and SAHA, yielded increased phosphorylation of IκBα, ERK, p38, and JNK in HIV-infected cells across two in vitro latency models. Both latent infection and viral protein expression contributed to changes in perturbation-induced signaling. Data-driven statistical models calculated from the phosphorylation signatures successfully classified infected and uninfected cells and further identified signals that were functionally important for regulating cell death. Specifically, the stress kinase pathways p38 and JNK were modified in latently infected cells, and activation of p38 and JNK signaling by anisomycin resulted in increased cell death independent of HIV reactivation. Our findings suggest that altered phosphorylation signatures in infected T cells provide a novel strategy to more selectively target the latent reservoir to enhance eradication efforts.

FoxO4 negatively controls Tat-mediated HIV-1 transcription through the post-transcriptional suppression of Tat encoding mRNA

The connection between the repression of human immunodeficiency virus type 1(HIV-1) transcription and the resting CD4+ T cell state suggests that the host transcription factors involved in the active maintenance of lymphocyte quiescence are likely to repress the viral transactivator, Tat, thereby restricting HIV-1 transcription. In this study, we analysed the interplay between Tat and the forkhead box transcription factors, FoxO1 and FoxO4. We show that FoxO1 and FoxO4 antagonize Tat-mediated transactivation of HIV-1 promoter through the repression of Tat protein expression. No effect was observed on the expression of two HIV-1 accessory proteins, Vif and Vpr. Unexpectedly, we found that FoxO1 and FoxO4 expression causes a strong dose-dependent post-transcriptional suppression of Tat mRNA, indicating that FoxO should effectively inhibit HIV-1 replication by destabilizing Tat mRNA and suppressing Tat-mediated HIV-1 transcription. In accordance with this, we observed that the Tat mRNA half-life is reduced by FoxO4 expression. The physiological relevance of our findings was validated using the J-Lat 10.6 model of latently infected cells. We demonstrated that the overexpression of a constitutively active FoxO4-TM mutant antagonized HIV-1 transcription reactivation in response to T cell activators, such as TNF-α or PMA. Altogether, our findings demonstrate that FoxO factors can control HIV-1 transcription and provide new insights into their potential role during the establishment of HIV-1 latency.

Identification of benzazole compounds that induce HIV-1 transcription

Despite advances in antiretroviral therapy, HIV-1 infection remains incurable in patients and continues to present a significant public health burden worldwide. While a number of factors contribute to persistent HIV-1 infection in patients, the presence of a stable, long-lived reservoir of latent provirus represents a significant hurdle in realizing an effective cure. One potential strategy to eliminate HIV-1 reservoirs in patients is reactivation of latent provirus with latency reversing agents in combination with antiretroviral therapy, a strategy termed "shock and kill". This strategy has shown limited clinical effectiveness thus far, potentially due to limitations of the few therapeutics currently available. We have identified a novel class of benzazole compounds effective at inducing HIV-1 expression in several cellular models. These compounds do not act via histone deacetylase inhibition or T cell activation, and show specificity in activating HIV-1 in vitro. Initial exploration of structure-activity relationships and pharmaceutical properties indicates that these compounds represent a potential scaffold for development of more potent HIV-1 latency reversing agents.

SUN2 Silencing Impairs CD4 T Cell Proliferation and Alters Sensitivity to HIV-1 Infection Independently of Cyclophilin A

Linker of nucleoskeleton and cytoskeleton (LINC) complexes connect the nucleus to the cytoskeleton in eukaryotic cells. We previously reported that the overexpression of SUN2, an inner nuclear membrane protein and LINC complex component, inhibits HIV infection between the steps of reverse transcription and nuclear import in a capsid-specific manner. We also reported that SUN2 silencing does not modulate HIV infection in several cell lines. Silencing of SUN2 was recently reported to decrease HIV infection of CD4 T cells, an effect which was suggested to result from modulation of cyclophilin A (CypA)-dependent steps of HIV infection. We confirm here that HIV infection of primary CD4 T cells is compromised in the absence of endogenous SUN2, and we extend these findings to additional viral strains. However, we find that CypA is not required for the decreased infection observed in SUN2-silenced cells and, conversely, that endogenous SUN2 is not required for the well-documented positive modulation of HIV infection by CypA. In contrast, CD4 T cells lacking SUN2 exhibit a considerable defect in proliferative capacity and display reduced levels of activation markers and decreased viability. Additionally, SUN2-silenced CD4 T cells that become infected support reduced levels of viral protein expression. Our results demonstrate that SUN2 is required for the optimal activation and proliferation of primary CD4 T cells and suggest that the disruption of these processes explains the contribution of endogenous SUN2 to HIV infection in primary lymphocytes.IMPORTANCE Linker of nucleoskeleton and cytoskeleton (LINC) complexes connect the nucleus to the cytoskeleton. We previously reported that the overexpression of the LINC complex protein SUN2 inhibits HIV infection by targeting the viral capsid and blocking infection before the virus enters the nucleus. A recent report showed that the depletion of endogenous SUN2 in primary CD4 T cells results in decreased HIV infection and that this involves cyclophilin A (CypA), a host protein that interacts with the capsid of HIV to promote infection. We confirm that HIV infection is reduced in CD4 T cells lacking SUN2, but we find no role for CypA. Instead, SUN2 silencing results in CD4 T cells with decreased viability and much lower proliferation rates. Our results show that SUN2 is required for optimal CD4 T cell activation and proliferation and explain the reduced level of HIV infection in the absence of SUN2.

Rapamycin-mediated mTOR inhibition uncouples HIV-1 latency reversal from cytokine-associated toxicity

Current strategies for HIV-1 eradication require the reactivation of latent HIV-1 in resting CD4+ T cells (rCD4s). Global T cell activation is a well-characterized means of inducing HIV-1 transcription, but is considered too toxic for clinical applications. Here, we have explored a strategy that involves a combination of immune activation and the immunosuppressive mTOR inhibitor rapamycin. In purified rCD4s from HIV-1-infected individuals on antiretroviral therapy, rapamycin treatment downregulated markers of toxicity, including proinflammatory cytokine release and cellular proliferation that were induced after potent T cell activation using αCD3/αCD28 antibodies. Using an ex vivo assay for HIV-1 mRNA, we demonstrated that despite this immunomodulatory effect, rapamycin did not affect HIV-1 gene expression induced by T cell activation in these rCD4s. In contrast, treating activated rCD4s with the immunosuppressant cyclosporin, a calcineurin inhibitor, robustly inhibited HIV-1 reactivation. Importantly, rapamycin treatment did not impair cytotoxic T lymphocyte (CTL) recognition and killing of infected cells. These findings raise the possibility of using rapamycin in conjunction with T cell-activating agents in HIV-1 cure strategies.

HIV-Induced Epigenetic Alterations in Host Cells

Human immunodeficiency virus (HIV), a member of the Retroviridae family, is a positive-sense, enveloped RNA virus. HIV, the causative agent of acquired immunodeficiency syndrome (AIDS) has two major types, HIV-1 and HIV-2 In HIV-infected cells the single stranded viral RNA genome is reverse transcribed and the double-stranded viral DNA integrates into the cellular DNA, forming a provirus. The proviral HIV genome is controlled by the host epigenetic regulatory machinery. Cellular epigenetic regulators control HIV latency and reactivation by affecting the chromatin state in the vicinity of the viral promoter located to the 5' long terminal repeat (LTR) sequence. In turn, distinct HIV proteins affect the epigenotype and gene expression pattern of the host cells. HIV-1 infection of CD4(+) T cells in vitro upregulated DNMT activity and induced hypermethylation of distinct cellular promoters. In contrast, in the colon mucosa and peripheral blood mononuclear cells from HIV-infected patients demethylation of the FOXP3 promoter was observed, possibly due to the downregulation of DNA methyltransferase 1. For a curative therapy of HIV infected individuals and AIDS patients, a combination of antiretroviral drugs with epigenetic modifying compounds have been suggested for the reactivation of latent HIV-1 genomes. These epigenetic drugs include histone deacetylase inhibitors (HDACI), histone methyltransferase inhibitors (HMTI), histone demethylase inhibitors, and DNA methyltransferase inhibitors (DNMTI).

Reactivation of Latent HIV-1 Expression by Engineered TALE Transcription Factors

The presence of replication-competent HIV-1 -which resides mainly in resting CD4+ T cells--is a major hurdle to its eradication. While pharmacological approaches have been useful for inducing the expression of this latent population of virus, they have been unable to purge HIV-1 from all its reservoirs. Additionally, many of these strategies have been associated with adverse effects, underscoring the need for alternative approaches capable of reactivating viral expression. Here we show that engineered transcriptional modulators based on customizable transcription activator-like effector (TALE) proteins can induce gene expression from the HIV-1 long terminal repeat promoter, and that combinations of TALE transcription factors can synergistically reactivate latent viral expression in cell line models of HIV-1 latency. We further show that complementing TALE transcription factors with Vorinostat, a histone deacetylase inhibitor, enhances HIV-1 expression in latency models. Collectively, these findings demonstrate that TALE transcription factors are a potentially effective alternative to current pharmacological routes for reactivating latent virus and that combining synthetic transcriptional activators with histone deacetylase inhibitors could lead to the development of improved therapies for latent HIV-1 infection.

Using animal models to overcome temporal, spatial and combinatorial challenges in HIV persistence research

Research challenges associated with understanding HIV persistence during antiretroviral therapy can be categorized as temporal, spatial and combinatorial. Temporal research challenges relate to the timing of events during establishment and maintenance of HIV persistence. Spatial research challenges regard the anatomical locations and cell subsets that harbor persistent HIV. Combinatorial research challenges pertain to the order of administration, timing of administration and specific combinations of compounds to be administered during HIV eradication therapy. Overcoming these challenges will improve our understanding of HIV persistence and move the field closer to achieving eradication of persistent HIV. Given that humanized mice and non-human primate HIV models permit rigorous control of experimental conditions, these models have been used extensively as in vivo research platforms for directly addressing these research challenges. The aim of this manuscript is to provide a comprehensive review of these recent translational advances made in animal models of HIV persistence.

HSP70 binding protein 1 (HspBP1) suppresses HIV-1 replication by inhibiting NF-κB mediated activation of viral gene expression

HIV-1 efficiently hijacks host cellular machinery and exploits a plethora of host–viral interactions for its successful survival. Identifying host factors that affect susceptibility or resistance to HIV-1 may offer a promising therapeutic strategy against HIV-1. Previously, we have reported that heat shock proteins, HSP40 and HSP70 reciprocally regulate HIV-1 gene-expression and replication. In the present study, we have identified HSP70 binding protein 1 (HspBP1) as a host-intrinsic inhibitor of HIV-1. HspBP1 level was found to be significantly down modulated during HIV-1 infection and virus production inversely co-related with HspBP1 expression. Our results further demonstrate that HspBP1 inhibits HIV-1 long terminal repeat (LTR) promoter activity. Gel shift and chromatin immunoprecipitation assays revealed that HspBP1 was recruited on HIV-1 LTR at NF-κB enhancer region (κB sites). The binding of HspBP1 to κB sites obliterates the binding of NF-κB hetero-dimer (p50/p65) to the same region, leading to repression in NF-κB mediated activation of LTR-driven gene-expression. HspBP1 also plays an inhibitory role in the reactivation of latently infected cells, corroborating its repressive effect on NF-κB pathway. Thus, our results clearly show that HspBP1 acts as an endogenous negative regulator of HIV-1 gene-expression and replication by suppressing NF-κB-mediated activation of viral transcription.

Patient-adapted, specific activation of HIV-1 by customized TAL effectors (TALEs), a proof of principle study

The major obstacle to cure infections with human immunodeficiency virus (HIV-1) is integrated proviral genomes, which are not eliminated by antiretroviral therapies (ART). Treatment approaches with latency-reversing agents (LRAs) aim at inducing provirus expression to tag latently-infected cells for clearance through viral cytopathic effects or cytotoxic T cell (CTL) responses. However, the currently tested LRAs reveal evident drawbacks as gene expression is globally induced and viral outgrowth is insecure. Here, we present transcription activator-like effector (TALE) proteins as potent tools to activate HIV-1 specifically. The large variety of circulating HIV-1 strains and, accordingly, integrated proviruses was addressed by the programmable DNA-specificity of TALEs. Using customized engineered TALEs, a substantial transcription activation and viral outgrowth was achieved with cells obtained from different HIV-1 patients. Our data suggest that TALEs may be useful tools in future strategies aimed at removing HIV-1 reservoirs.

Control of FoxO4 Activity and Cell Survival by TRIM22 Directs TLR3-Stimulated Cells Toward IFN Type I Gene Induction or Apoptosis

Activation of innate immune response, induced after the recognition of double-stranded RNA (dsRNA), formed during replication of most viruses, results in intracellular signaling cascades ultimately culminating in the expression of type I interferon (IFN). In this study, we provide the first evidence that FoxO4 triggers the activation of the innate immune signaling pathway in coupling stimulation of TLR3 and RIG-like receptors by the synthetic dsRNA analog, poly(I:C), to IFN-β and IFN-induced gene induction, whereas knockdown of FoxO4 had opposite effects. Similar effects of FoxO4 were observed during paramyxovirus-mediated IFN-β transcriptional induction. We further found that knockdown of FoxO4 did not affect IRF3 and NF-κB activation by poly(I:C), suggesting that FoxO4 would act downstream in the signaling pathway. In addition, we show that the IFN-induced TRIM22 ubiquitin ligase targets FoxO4 and antagonizes its activity through an unrelated ubiquitin/autophagosomic-lysosomal pathway. Unexpectedly, TRIM22 knockdown strongly sensitizes cells to dsRNA-induced caspase-dependent apoptosis, as early as 2 h after poly(I:C) stimulation, concomitantly to the inhibition of the expression of the antiapoptotic protein, Bcl-2, indicating that TRIM22 might be a key factor for controlling the cell survival after TLR3 stimulation. Taken together, our data demonstrate that the regulation of FoxO4 protein expression and cell survival by TRIM22 controls TLR3-mediated IFN type I gene induction, preventing excessive antiviral response through dsRNA-induced apoptosis.

A new model for post-integration latency in macroglial cells to study HIV-1 reservoirs of the brain

OBJECTIVE

Macroglial cells like astrocytes are key targets for the formation of HIV-1 reservoirs in the brain. The 'shock-and-kill' HIV-1 cure strategy proposes eradication of reservoirs by clinical treatment with latency reversing agents (LRAs). However, virus activation may endanger the brain, due to limited cell turnover, viral neurotoxicity and poor penetration of antiretroviral drugs. Since the brain is not accessible to clinical sampling, we established an experimental model to investigate the LRA effects on HIV-1 latency in macroglial reservoirs.

DESIGN

Human neural stem cells (HNSC.100) were used to generate a system that models HIV-1 transcriptional latency in proliferating progenitor, as well as differentiated macroglial cell populations and latency-modulating effects of LRAs and compounds targeting HIV-1 transcription were analysed.

METHODS

HNSCs were infected with pseudotyped Env-defective HIV-1 viruses. HIV-1 DNA and RNA levels were quantified by qPCR. Expression of latent GFP-reporter viruses was analysed by confocal microscopy and flow cytometry. NF-κB signalling was investigated by confocal microscopy and chromatin immunoprecipitation.

RESULTS

Two of the eight well known LRAs (tumour necrosis factor-alpha, suberoylanilide hydroxamic acid) reactivated HIV-1 in latently infected HNSCs. Tumour necrosis factor-alpha reactivated HIV-1 in progenitor and differentiated populations, whereas suberoylanilide hydroxamic acid was more potent in progenitors. Pre-treatment with inhibitors of key HIV-1 transcription factors (NF-κB, Cdk9) suppressed HIV-1 reactivation.

CONCLUSION

We conclude that latent HIV-1 in macroglial reservoirs can be activated by selected LRAs. Identification of small molecules that suppress HIV-1 reactivation supports functional cure strategies. We propose using the HNSC model to develop novel strategies to enforce provirus quiescence in the brain.

Effect of suberoylanilide hydroxamic acid (SAHA) administration on the residual virus pool in a model of combination antiretroviral therapy-mediated suppression in SIVmac239-infected indian rhesus macaques

Nonhuman primate models are needed for evaluations of proposed strategies targeting residual virus that persists in HIV-1-infected individuals receiving suppressive combination antiretroviral therapy (cART). However, relevant nonhuman primate (NHP) models of cART-mediated suppression have proven challenging to develop. We used a novel three-class, six-drug cART regimen to achieve durable 4.0- to 5.5-log reductions in plasma viremia levels and declines in cell-associated viral RNA and DNA in blood and tissues of simian immunodeficiency virus SIVmac239-infected Indian-origin rhesus macaques, then evaluated the impact of treatment with the histone deacetylase inhibitor (HDACi) suberoylanilide hydroxamic acid (SAHA; Vorinostat) on the residual virus pool. Ex vivo SAHA treatment of CD4(+) T cells obtained from cART-suppressed animals increased histone acetylation and viral RNA levels in culture supernatants. cART-suppressed animals each received 84 total doses of oral SAHA. We observed SAHA dose-dependent increases in acetylated histones with evidence for sustained modulation as well as refractoriness following prolonged administration. In vivo virologic activity was demonstrated based on the ratio of viral RNA to viral DNA in peripheral blood mononuclear cells, a presumptive measure of viral transcription, which significantly increased in SAHA-treated animals. However, residual virus was readily detected at the end of treatment, suggesting that SAHA alone may be insufficient for viral eradication in the setting of suppressive cART. The effects observed were similar to emerging data for repeat-dose SAHA treatment of HIV-infected individuals on cART, demonstrating the feasibility, utility, and relevance of NHP models of cART-mediated suppression for in vivo assessments of AIDS virus functional cure/eradication approaches.

Sulfonation pathway inhibitors block reactivation of latent HIV-1

Long-lived pools of latently infected cells are a significant barrier to the development of a cure for HIV-1 infection. A better understanding of the mechanisms of reactivation from latency is needed to facilitate the development of novel therapies that address this problem. Here we show that chemical inhibitors of the sulfonation pathway prevent virus reactivation, both in latently infected J-Lat and U1 cell lines and in a primary human CD4+ T cell model of latency. In each of these models, sulfonation inhibitors decreased transcription initiation from the HIV-1 promoter. These inhibitors block transcription initiation at a step that lies downstream of nucleosome remodeling and affects RNA polymerase II recruitment to the viral promoter. These results suggest that the sulfonation pathway acts by a novel mechanism to regulate efficient virus transcription initiation during reactivation from latency, and further that augmentation of this pathway could be therapeutically useful.

Tumor suppressor cylindromatosis (CYLD) controls HIV transcription in an NF-κB-dependent manner

Characterizing the cellular factors that play a role in the HIV replication cycle is fundamental to fully understanding mechanisms of viral replication and pathogenesis. Whole-genome small interfering RNA (siRNA) screens have identified positive and negative regulators of HIV replication, providing starting points for investigating new cellular factors. We report here that silencing of the deubiquitinase cylindromatosis protein (CYLD), increases HIV infection by enhancing HIV long terminal repeat (LTR)-driven transcription via the NF-κB pathway. CYLD is highly expressed in CD4(+) T lymphocytes, monocyte-derived macrophages, and dendritic cells. We found that CYLD silencing increases HIV replication in T cell lines. We confirmed the positive role of CYLD silencing in HIV infection in primary human CD4(+) T cells, in which CYLD protein was partially processed upon activation. Lastly, Jurkat T cells latently infected with HIV (JLat cells) were more responsive to phorbol 12-myristate 13-acetate (PMA) reactivation in the absence of CYLD, indicating that CYLD activity could play a role in HIV reactivation from latency. In summary, we show that CYLD acts as a potent negative regulator of HIV mRNA expression by specifically inhibiting NF-κB-driven transcription. These findings suggest a function for this protein in modulating productive viral replication as well as in viral reactivation.

IMPORTANCE

HIV transcription is regulated by a number of host cell factors. Here we report that silencing of the lysine 63 deubiquitinase CYLD increases HIV transcription in an NF-κB-dependent manner. We show that CYLD is expressed in HIV target cells and that its silencing increases HIV infection in transformed T cell lines as well as primary CD4(+) T cells. Similarly, reactivation of latent provirus was facilitated in the absence of CYLD. These data suggest that CYLD, which is highly expressed in CD4(+) T cells, can control HIV transcription in productive infection as well as during reactivation from latency.

HIV-1 eradication strategies: design and assessment

PURPOSE OF REVIEW

Recent developments have generated renewed interest in the possibility of curing HIV-1 infection. This review describes some of the practical challenges that will need to be overcome if curative strategies are to be successful.

RECENT FINDINGS

The latent reservoir for HIV-1 in resting memory CD4 T cells is the major barrier to curing the infection. The most widely discussed approach to curing the infection involves finding agents that reverse latency in resting CD4 T cells, with the assumption that the cells will then die from viral cytopathic effects or be lysed by host cytolytic T lymphocytes (CTLs). A major challenge is the development of in-vitro models that can be used to explore mechanisms and identify latency-reversing agents (LRAs). Although several models have been developed, including primary cell models, none of them may fully capture the quiescent state of the cells that harbour latent HIV-1 in vivo. An additional problem is that LRAs that do not cause T-cell activation may not lead to the death of infected cells. Finally, measuring the effects of LRAs in vivo is complicated by the lack of correlation between different assays for the latent reservoir.

SUMMARY

Progress on these practical issues is essential to finding a cure.

Genetically modified hematopoietic stem cell transplantation for HIV-1-infected patients: can we achieve a cure?

The cure of a human immunodeficiency virus (HIV)-1-infected patient following allogeneic transplantation from a CCR5-null donor and potential cure of two patients transplanted with CCR5 wild-type hematopoietic stem cells (HSC) have provided renewed optimism that a potential alternative to conventional antiretroviral therapy (ART) is forthcoming. While allogeneic grafts have thus far suggested complete eradication of viral reservoirs, it has yet to be observed following autologous HSC transplantation. Development of curative autologous transplantation strategies would significantly increase the number of treatable patients, eliminating the need for matched donors and reducing the risks of adverse events. Recent studies suggest gene therapy may provide a mechanism for developing curative therapies. Expression of cellular/artificial restriction factors or disruption of CCR5 has been shown to limit viral replication and provide protection of genetically modified cells. However, significant obstacles remain with regards to the depletion of established viral reservoirs in an autologous transplantation setting devoid of the "allo-effect". Here, we discuss results from early-stage clinical trials and recent findings in animal models of gene modified HSC transplantation. Finally, we propose innovative combination therapies that may aid in the reduction and/or elimination of viral reservoirs in HIV-1-infected patients and promote the artificial development of a natural controller phenotype.

Replication-competent noninduced proviruses in the latent reservoir increase barrier to HIV-1 cure

Antiretroviral therapy fails to cure HIV-1 infection because latent proviruses persist in resting CD4(+) T cells. T cell activation reverses latency, but <1% of proviruses are induced to release infectious virus after maximum in vitro activation. The noninduced proviruses are generally considered defective but have not been characterized. Analysis of 213 noninduced proviral clones from treated patients showed 88.3% with identifiable defects but 11.7% with intact genomes and normal long terminal repeat (LTR) function. Using direct sequencing and genome synthesis, we reconstructed full-length intact noninduced proviral clones and demonstrated growth kinetics comparable to reconstructed induced proviruses from the same patients. Noninduced proviruses have unmethylated promoters and are integrated into active transcription units. Thus, it cannot be excluded that they may become activated in vivo. The identification of replication-competent noninduced proviruses indicates that the size of the latent reservoir-and, hence, the barrier to cure-may be up to 60-fold greater than previously estimated.

ZASC1 stimulates HIV-1 transcription elongation by recruiting P-TEFb and TAT to the LTR promoter

Transcription from the HIV-1 LTR promoter efficiently initiates but rapidly terminates because of a non-processive form of RNA polymerase II. This premature termination is overcome by assembly of an HIV-1 TAT/P-TEFb complex at the transactivation response region (TAR), a structured RNA element encoded by the first 59 nt of HIV-1 mRNA. Here we have identified a conserved DNA-binding element for the cellular transcription factor, ZASC1, in the HIV-1 core promoter immediately upstream of TAR. We show that ZASC1 interacts with TAT and P-TEFb, co-operating with TAT to regulate HIV-1 gene expression, and promoting HIV-1 transcriptional elongation. The importance of ZASC1 to HIV-1 transcription elongation was confirmed through mutagenesis of the ZASC1 binding sites in the LTR promoter, shRNAs targeting ZASC1 and expression of dominant negative ZASC1. Chromatin immunoprecipitation analysis revealed that ZASC1 recruits Tat and P-TEFb to the HIV-1 core promoter in a TAR-independent manner. Thus, we have identified ZASC1 as novel regulator of HIV-1 gene expression that functions through the DNA-dependent, RNA-independent recruitment of TAT/P-TEFb to the HIV-1 promoter.

Establishment and molecular mechanisms of HIV-1 latency in T cells

Treatment of an HIV infected individual with antiretroviral drugs is a successful way to suppress the plasma viral RNA load below the limit of detection (50 copies HIV RNA/ml plasma). This can provide lifelong protection against virus-induced pathogenesis in drug-adherent patients. Unfortunately, even after many years of continuous treatment, the virus persists and the plasma viral load will rebound rapidly when therapy is interrupted. The reason for this rapid rebound is the presence of a long-lived reservoir of latent HIV-1 proviruses that can be reactivated in resting memory T cells. Attempts to eliminate these proviruses have thus far not been successful and this long-lived latent reservoir is therefore considered a major obstacle toward a cure for HIV-1. A detailed understanding of the molecular mechanisms causing HIV latency and knowledge on the establishment of this reservoir may give us clues for future strategies aiming at the eradication of this reservoir.

Feline immunodeficiency virus latency

Despite highly effective anti-retroviral therapy, HIV is thought to persist in patients within long-lived cellular reservoirs in the form of a transcriptionally inactive (latent) integrated provirus. Lentiviral latency has therefore come to the forefront of the discussion on the possibility of a cure for HIV infection in humans. Animal models of lentiviral latency provide an essential tool to study mechanisms of latency and therapeutic manipulation. Of the three animal models that have been described, the feline immunodeficiency virus (FIV)-infected cat is the most recent and least characterized. However, several aspects of this model make it attractive for latency research, and it may be complementary to other model systems. This article reviews what is known about FIV latency and chronic FIV infection and how it compares with that of other lentiviruses. It thereby offers a framework for the usefulness of this model in future research aimed at lentiviral eradication.

Endothelial cell stimulation overcomes restriction and promotes productive and latent HIV-1 infection of resting CD4+ T cells

Highly active antiretroviral therapy (HAART) is able to suppress human immunodeficiency virus type 1 (HIV-1) to undetectable levels in the majority of patients, but eradication has not been achieved because latent viral reservoirs persist, particularly in resting CD4(+) T lymphocytes. It is generally understood that HIV-1 does not efficiently infect resting CD4(+) T cells, and latent infection in those cells may arise when infected CD4(+) T lymphoblasts return to resting state. In this study, we found that stimulation by endothelial cells can render resting CD4(+) T cells permissible for direct HIV infection, including both productive and latent infection. These stimulated T cells remain largely phenotypically unactivated and show a lower death rate than activated T cells, which promotes the survival of infected cells. The stimulation by endothelial cells does not involve interleukin 7 (IL-7), IL-15, CCL19, or CCL21. Endothelial cells line the lymphatic vessels in the lymphoid tissues and have frequent interactions with T cells in vivo. Our study proposes a new mechanism for infection of resting CD4(+) T cells in vivo and a new mechanism for latent infection in resting CD4(+) T cells.

From reactivation of latent HIV-1 to elimination of the latent reservoir: the presence of multiple barriers to viral eradication

The discovery of a stable latent reservoir for HIV-1 in resting memory CD4(+) T cells provides a mechanism for lifelong persistence of HIV-1. The long-lived latently infected cells persist in spite of prolonged highly active antiretroviral therapy and present a major barrier to a cure of HIV-1 infection. In this review, we discuss the current understanding of HIV-1 persistence and latent viral infection in the context of effective antiretroviral therapy and the recent progress in purging latent viral reservoirs. Recent studies demonstrate that reactivation of latent HIV-1 is a promising strategy for the depletion of these viral reservoirs. A thorough evaluation of the anti-latency activity of drug candidates should include the measurement of changes in intracellular viral RNA, plasma virus levels, and the size of latent viral reservoirs, as well as potential adverse effects. Currently, there are several technical barriers to the evaluation of anti-latency drugs in vivo. We also discuss these challenging issues that remain unresolved.

Reactivation of latent HIV by histone deacetylase inhibitors

Latent HIV persists in CD4(+) T cells in infected patients under antiretroviral therapy (ART). Latency is associated with transcriptional silencing of the integrated provirus and driven, at least in part, by histone deacetylases (HDACs), a family of chromatin-associated proteins that regulate histone acetylation and the accessibility of DNA to transcription factors. Remarkably, inhibition of HDACs is sufficient to reactivate a fraction of latent HIV in a variety of experimental systems. This basic observation led to the shock and kill idea that forcing the transcriptional activation of HIV might lead to virus expression, to virus- or host-induced cell death of the reactivated cells, and to the eradication of the pool of latently infected cells. Such intervention might possibly lead to a cure for HIV-infected patients. Here, we review the basic biology of HDACs and their inhibitors, the role of HDACs in HIV latency, and recent efforts to use HDAC inhibitors to reactivate latent HIV in vitro and in vivo.

Fluctuations in Tat copy number when it counts the most: a possible mechanism to battle the HIV latency

The HIV-1 virus can enter a dormant state and become inactive, which reduces accessibility by antiviral drugs. We approach this latency problem from an unconventional point of view, with the focus on understanding how intrinsic chemical noise (copy number fluctuations of the Tat protein) can be used to assist the activation process of the latent virus. Several phase diagrams have been constructed in order to visualize in which regions of the parameter space noise can drive the activation process. Essential to the study is the use of a hyperbolic coordinate system, which greatly facilitates quantification of how the various reaction rate combinations shape the noise behavior of the Tat protein feedback system. We have designed a mathematical manual of how to approach the problem of activation quantitatively, and introduce the notion of an “operating point” of the virus. For both noise-free and noise-based strategies we show how operating point off-sets induce changes in the number of Tat molecules. The major result of the analysis is that for every noise-free strategy there is a noise-based strategy that requires lower dosage, but achieves the same anti-latency effect. It appears that the noise-based activation is advantageous for every operating point.

Recent developments in human immunodeficiency virus-1 latency research

Almost 30 years after its initial discovery, infection with the human immunodeficiency virus-1 (HIV-1) remains incurable and the virus persists due to reservoirs of latently infected CD4(+) memory T-cells and sanctuary sites within the infected individual where drug penetration is poor. Reactivating latent viruses has been a key strategy to completely eliminate the virus from the host, but many difficulties and unanswered questions remain. In this review, the latest developments in HIV-persistence and latency research are presented.

Targeting IκB proteins for HIV latency activation: the role of individual IκB and NF-κB proteins

Latently infected cell reservoirs represent the main barrier to HIV eradication. Combination antiretroviral therapy (cART) effectively blocks viral replication but cannot purge latent provirus. One approach to HIV eradication could include cART to block new infections plus an agent to activate latent provirus. NF-κB activation induces HIV expression, ending latency. Before activation, IκB proteins sequester NF-κB dimers in the cytoplasm. Three canonical IκBs, IκBα, IκBβ, and IκBε, exist, but the IκB proteins' role in HIV activation regulation is not fully understood. We studied the effects on HIV activation of targeting IκBs by single and pairwise small interfering RNA (siRNA) knockdown. After determining the relative abundance of the IκBs, the relative abundance of NF-κB subunits held by the IκBs, and the kinetics of IκB degradation and resynthesis following knockdown, we studied HIV activation by IκB knockdown, in comparison with those of known HIV activators, tumor necrosis factor alpha (TNF-α), tetradecanoyl phorbol acetate (TPA), and trichostatin A (TSA), in U1 monocytic and J-Lat 10.6 lymphocytic latently infected cells. We found that IκBα knockdown activated HIV in both U1 and J-Lat 10.6 cells, IκBβ knockdown did not activate HIV, and, surprisingly, IκBε knockdown produced the most HIV activation, comparable to TSA activation. Our data show that HIV reactivation can be triggered by targeting two different IκB proteins and that IκBε may be an effective target for HIV latency reactivation in T-cell and macrophage lineages. IκBε knockdown may offer attractive therapeutic advantages for HIV activation because it is not essential for mammalian growth and development and because new siRNA delivery strategies may target siRNAs to HIV latently infected cells.

Is specific HIV eradication from the brain possible or needed?

INTRODUCTION

There is increasing interest in the possibility of eradication of HIV, given the recent case reports. However, it is not clear to what extent brain involvement by HIV poses a challenge to systemic eradication strategies.

AREAS COVERED

This review will outline the mechanisms of HIV latency, the various eradication strategies presently under consideration followed by a discussion of the issue of the frequency and severity of brain involvement by HIV. In those patients with HIV brain disease the challenges will be delineated as well as potential approaches to circumvent or minimise them.

EXPERT OPINION

Eradication of HIV from the brain using specific methodologies is likely only needed in some patients. However, both the identification of such patients and the details of the necessary methodologies require much more research.

HIV-1 Nef: a multifaceted modulator of T cell receptor signaling

Nef, an accessory protein of the Human Immunodeficiency Virus type 1 (HIV-1), is dispensable for viral replication in cell culture, but promotes virus replication and pathogenesis in the infected host. Acting as protein-interaction adaptor, HIV-1 Nef modulates numerous target cell activities including cell surface receptor expression, cytoskeletal remodeling, vesicular transport, and signal transduction. In infected T-lymphocytes, altering T-cell antigen receptor (TCR) signaling has long been recognized as one key function of the viral protein. However, reported effects of Nef range from inhibition to activation of this cascade. Recent advances in the field begin to explain these seemingly contradictory observations and suggest that Nef alters intracellular trafficking of TCR proximal machinery to disrupt plasma membrane bound TCR signaling while at the same time, the viral protein induces localized signal transduction at the trans-Golgi network. This review summarizes these new findings on how HIV-1 Nef reprograms TCR signalling output from a broad response to selective activation of the RAS-Erk pathway. We also discuss the implications of these alterations in the context of HIV-1 infection and in light of current concepts of TCR signal transduction.

Lentiviral latency in peripheral CD4+ T cells isolated from feline immunodeficiency virus-infected cats during the asymptomatic phase is not associated with hypermethylation of the proviral promoter

Lentiviral latency remains a principal obstacle to curative AIDS therapy. Transcriptional repression and latency permits lentiviruses to evade host immune responses and antiretroviral drugs. We have established a model of peripheral CD4+ T cell lentiviral latency in cats experimentally infected with feline immunodeficiency virus (FIV). Multiple mechanisms of lentiviral transcriptional repression have been proposed including epigenetic mechanisms resulting in promoter hypermethylation and/or chromatin condensation. Methylation of promoter-associated cytosines in the cytosine-guanine dinucleotide (CpG) has been associated with transcriptional repression in both eukaryotic promoters and integrated retroviral genomes. Using methylcytosine mapping, we examined the CpG methylation patterns in both the 5' and 3' long terminal repeats (LTR) of the FIV provirus in peripheral blood mononuclear cells, monocytes and CD4+ T cells isolated during the acute and asymptomatic phases of infection. Here we report no evidence that proviral promoter hypermethylation is associated with lentiviral latency in peripheral CD4+ T cells and monocytes obtained from experimentally FIV-infected cats.

HIV reservoirs: pathogenesis and obstacles to viral eradication and cure

Plasma HIV viremia can be suppressed and maintained below the limits of detection for prolonged periods of time in the vast majority of HIV-infected individuals who receive antiretroviral therapy (ART). Thus, the clinical outcome for HIV-infected individuals who have access to these drugs is dramatically improved. However, ART alone cannot eradicate HIV in infected individuals and this impediment is likely in part due to the persistence of viral reservoirs in the peripheral blood and lymphoid tissues of infected individuals despite the suppression of plasma viremia. In recent years, major research efforts have been dedicated to a better understanding of the pathogenesis of persistent HIV infection and to the development of therapeutic strategies aimed at eradicating virus in infected individuals receiving ART. In this review, we discuss the pathophysiology of CD4 T-cell HIV reservoirs, including recent advances in our understanding of the mechanisms of persistent viral infection and perspectives for eradication of HIV in infected individuals.