Promoter Targeting RNAs: Unexpected Contributors to the Control of HIV-1 Transcription

In spite of prolonged and intensive treatment with combined antiretroviral therapy (cART), which efficiently suppresses plasma viremia, the integrated provirus of HIV-1 persists in resting memory CD4⁺ T cells as latent infection. Treatment with cART does not substantially reduce the burden of latent infection. Once cART is ceased, HIV-1 replication recrudesces from these reservoirs in the overwhelming majority of patients. There is increasing evidence supporting a role for noncoding RNAs (ncRNA), including microRNAs (miRNAs), antisense (as)RNAs, and short interfering (si)RNA in the regulation of HIV-1 transcription. This appears to be mediated by interaction with the HIV-1 promoter region. Viral miRNAs have the potential to act as positive or negative regulators of HIV transcription. Moreover, inhibition of virally encoded long-asRNA can induce positive transcriptional regulation, while antisense strands of siRNA targeting the NF-κB region suppress viral transcription. An in-depth understanding of the interaction between ncRNAs and the HIV-1 U3 promoter region may lead to new approaches for the control of HIV reservoirs. This review focuses on promoter associated ncRNAs, with particular emphasis on their role in determining whether HIV-1 establishes active or latent infection.

Plasmodium infection reduces the volume of the viral reservoir in SIV-infected rhesus macaques receiving antiretroviral therapy

Background

Previous studies indicated that Plasmodium infection activates the immune system, including memory CD4+ T cells, which constitute the reservoir of human immunodeficiency virus type-1 (HIV-1). Therefore, we postulated that co-infection with malaria might activate the reservoir of HIV-1. To test this hypothesis, we used a rhesus macaque model of co-infection with malaria and simian immunodeficiency virus (SIV), along with antiretroviral therapy (ART).

Results

Our results showed that Plasmodium infection reduced both the replication-competent virus pool in resting CD4+ T cells and the integrated virus DNA (iDNA) load in peripheral blood mononuclear cells in the monkeys. This reduction might be attributable to malaria-mediated activation and apoptotic induction of memory CD4+ T cells. Further studies indicated that histone acetylation and NF-kappaB (NF-κB) activation in resting CD4+ T cells may also play an important role in this reduction.

Conclusions

The findings of this work expand our knowledge of the interaction between these two diseases. As more HIV-1-infected individuals in malaria-endemic areas receive ART, we should explore whether any of the patients co-infected with Plasmodium experience virologic benefits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0112-x) contains supplementary material, which is available to authorized users.

Interleukin 7 up-regulates CD95 protein on CD4+ T cells by affecting mRNA alternative splicing: priming for a synergistic effect on HIV-1 reservoir maintenance

Interleukin-7 (IL-7) has been used as an immunoregulatory and latency-reversing agent in human immunodeficiency virus type 1 (HIV-1) infection. Although IL-7 can restore circulating CD4(+) T cell counts in HIV-1-infected patients, the anti-apoptotic and proliferative effects of IL-7 appear to benefit survival and expansion of HIV-1-latently infected memory CD4(+) T lymphocytes. IL-7 has been shown to elevate CD95 on CD4(+) T cells in HIV-1-infected individuals and prime CD4(+) T lymphocytes to CD95-mediated proliferative or apoptotic signals. Here we observed that through increasing microRNA-124, IL-7 down-regulates the splicing regulator polypyrimidine tract binding protein (PTB), leading to inclusion of the transmembrane domain-encoding exon 6 of CD95 mRNA and, subsequently, elevation of CD95 on memory CD4(+) T cells. Moreover, IL-7 up-regulates cellular FLICE-like inhibitory protein (c-FLIP) and stimulates c-Jun N-terminal kinase (JNK) phosphorylation, which switches CD95 signaling to survival mode in memory CD4(+) T lymphocytes. As a result, co-stimulation through IL-7/IL-7R and FasL/CD95 signal pathways augments IL-7-mediated survival and expansion of HIV-1-latently infected memory CD4(+) T lymphocytes. Collectively, we have demonstrated a novel mechanism for IL-7-mediated maintenance of HIV-1 reservoir.

Targeting HIV latency: resting memory T cells, hematopoietic progenitor cells and future directions

Current therapy for HIV effectively suppresses viral replication and prolongs life, but the infection persists due, at least in part, to latent infection of long-lived cells. One favored strategy toward a cure targets latent virus in resting memory CD4(+) T cells by stimulating viral production. However, the existence of an additional reservoir in bone marrow hematopoietic progenitor cells has been detected in some treated HIV-infected people. This review describes approaches investigators have used to reactivate latent proviral genomes in resting CD4(+) T cells and hematopoietic progenitor cells. In addition, the authors review approaches for clearance of these reservoirs along with other important topics related to HIV eradication.

[CTIP2, a multifunctional protein: cellular physiopathology and therapeutic implications]

The transcription factor CTIP2 (BCL11B) is a multifunctional protein involved in numerous cell physiological processes. To date, many molecular mechanisms underlying this process have been discovered, which highlighted the importance of the epigenetic regulation of genes and the regulation of the elongation factor P-TEFb. Furthermore studies of the deregulation of CTIP2 showed the association of CTIP2 to numerous pathologies including cancer and cardiac hypertrophy. A better comprehension of the physiopathology of these diseases might lead to the design of therapeutical strategies intending to prevent CTIP2 deregulation. Moreover, CTIP2 and its associated proteins constitute potential targets in strategies aiming to reduce and/or purge HIV-1 cell reservoirs.

HIV-1 transcriptional regulation in the central nervous system and implications for HIV cure research

Human immunodeficiency virus type-1 (HIV-1) invades the central nervous system (CNS) during acute infection which can result in HIV-associated neurocognitive disorders in up to 50% of patients, even in the presence of combination antiretroviral therapy (cART). Within the CNS, productive HIV-1 infection occurs in the perivascular macrophages and microglia. Astrocytes also become infected, although their infection is restricted and does not give rise to new viral particles. The major barrier to the elimination of HIV-1 is the establishment of viral reservoirs in different anatomical sites throughout the body and viral persistence during long-term treatment with cART. While the predominant viral reservoir is believed to be resting CD4(+) T cells in the blood, other anatomical compartments including the CNS, gut-associated lymphoid tissue, bone marrow, and genital tract can also harbour persistently infected cellular reservoirs of HIV-1. Viral latency is predominantly responsible for HIV-1 persistence and is most likely governed at the transcriptional level. Current clinical trials are testing transcriptional activators, in the background of cART, in an attempt to purge these viral reservoirs and reverse viral latency. These strategies aim to activate viral transcription in cells constituting the viral reservoir, so they can be recognised and cleared by the immune system, while new rounds of infection are blocked by co-administration of cART. The CNS has several unique characteristics that may result in differences in viral transcription and in the way latency is established. These include CNS-specific cell types, different transcription factors, altered immune surveillance, and reduced antiretroviral drug bioavailability. A comprehensive understanding of viral transcription and latency in the CNS is required in order to determine treatment outcomes when using transcriptional activators within the CNS.

Immunologic strategies for HIV-1 remission and eradication

Antiretroviral therapy (ART) is able to suppress HIV-1 replication indefinitely in individuals who have access to these medications, are able to tolerate these drugs, and are motivated to take them daily for life. However, ART is not curative. HIV-1 persists indefinitely during ART as quiescent integrated DNA within memory CD4(+) T cells and perhaps other long-lived cellular reservoirs. In this Review, we discuss the role of the immune system in the establishment and maintenance of the latent HIV-1 reservoir. A detailed understanding of how the host immune system shapes the size and distribution of the viral reservoir should lead to the development of a new generation of immune-based therapeutics, which may eventually contribute to a curative intervention.

A cure for HIV: is it in sight?

HIV is a devastating disease affecting millions of people worldwide despite the advent of successful antiretroviral therapy (ART). However, ART does not result in a cure and has to be taken for life. Accordingly, researchers are turning towards cure efforts, particularly in the light of two patients whose HIV has been seemingly eradicated. Numerous approaches and strategies have been considered for curing HIV, but no scalable and safe solution has yet been reached. With newly discovered difficulties in measuring the HIV reservoir, the main barrier to a cure, the only true test of cure is to stop ART and see whether the virus becomes detectable. However, it is possible that this treatment interruption may be associated with certain risks for patients. Here, we compare the current major approaches and recent advances for curing HIV, as well as discuss ways of evaluating HIV cure and the safety concerns involved.

HIV-1 latency: an update of molecular mechanisms and therapeutic strategies

The major obstacle towards HIV-1 eradication is the life-long persistence of the virus in reservoirs of latently infected cells. In these cells the proviral DNA is integrated in the host’s genome but it does not actively replicate, becoming invisible to the host immune system and unaffected by existing antiviral drugs. Rebound of viremia and recovery of systemic infection that follows interruption of therapy, necessitates life-long treatments with problems of compliance, toxicity, and untenable costs, especially in developing countries where the infection hits worst. Extensive research efforts have led to the proposal and preliminary testing of several anti-latency compounds, however, overall, eradication strategies have had, so far, limited clinical success while posing several risks for patients. This review will briefly summarize the more recent advances in the elucidation of mechanisms that regulates the establishment/maintenance of latency and therapeutic strategies currently under evaluation in order to eradicate HIV persistence.

As2O3 synergistically reactivate latent HIV-1 by induction of NF-κB

None of the current agents can safely and effectively eliminate latent HIV-1 reservoirs, meaning that there is a major barrier to the final cure of AIDS. Arsenic trioxide (As2O3), a drug used to treat acute promyelocytic leukemia (APL), was reported to affect the transcription factors and pathways involved in modulating HIV-1 expression. However, little is known about the effect and molecular basis of As2O3 in inducing HIV-1 expression in latently infected cells. Using the Jurkat T cell model of HIV-1 latency, we found that As2O3 activated latent HIV-1 replication with a similar potency to valproic acid (VPA) and did so in a dose- and time-dependent manner. We also found that As2O3 synergistically reactivated latent HIV-1 transcription with prostratin, tumor necrosis factor alpha (TNF-α) or VPA. Moreover, we provide evidence indicating that As2O3-induced HIV-1 activation involves the nuclear factor kappa B (NF-κB) signaling pathway. In conclusion, we have found that As2O3 can synergistically reactivate latent HIV-1 with other activators, which may provide a new tool to unravel the mechanisms of virus latency and reactivation.

Immune interventions in HIV infection

Immune-based therapy (IBT) interventions have found a window of opportunity within some limitations of the otherwise successful combined antiretroviral therapy (cART). Two major paradigms drove immunotherapeutic research to combat human immunodeficiency virus (HIV) infection. First, IBTs were proposed either to help restore CD4(+) T-cell counts in cases of therapeutic failures with cytokines, interleukin-2 (IL-2) or IL-7, or to better control HIV and disease progression during treatment interruptions with anti-HIV therapeutic candidate vaccines. The most widely used candidates were HIV-recombinant live vector-based alone or combined with other vaccine compounds and dendritic cell (DC) therapies. A more recent and current paradigm aims at achieving HIV cure by combining IBT with cART using either cytokines to reactivate virus production in latently infected cells and/or therapeutic immunization to boost HIV-specific immunity in a 'shock and kill' strategy. This review summarizes the rationale, hopes, and mechanisms of successes and failures of these cytokine-based and vaccine-based immune interventions. Results from these first series of IBTs have been so far somewhat disappointing in terms of clinical relevance, but have provided lessons that are discussed in light of the future combined strategies to be developed toward an HIV cure.

Immune activation and HIV persistence: implications for curative approaches to HIV infection

Despite complete or near-complete suppression of human immunodeficiency virus (HIV) replication with combination antiretroviral therapy, both HIV and chronic inflammation/immune dysfunction persist indefinitely. Untangling the association between the virus and the host immune environment during therapy might lead to novel interventions aimed at either curing the infection or preventing the development of inflammation-associated end-organ disease. Chronic inflammation and immune dysfunction might lead to HIV persistence by causing virus production, generating new target cells, enabling infecting of activated and resting target cells, altering the migration patterns of susceptible target cells, increasing the proliferation of infected cells, and preventing normal HIV-specific clearance mechanisms from function. Chronic HIV production or replication might contribute to persistent inflammation and immune dysfunction. The rapidly evolving data on these issues strongly suggest that a vicious cycle might exist in which HIV persistence causes inflammation that in turn contributes to HIV persistence.

The immunological synapse: the gateway to the HIV reservoir

A major challenge in the development of a cure for human immunodeficiency virus (HIV) has been the incomplete understanding of the basic mechanisms underlying HIV persistence during antiretroviral therapy. It is now realized that the establishment of a latently infected reservoir refractory to immune system recognition has thus far hindered eradication efforts. Recent investigation into the innate immune response has shed light on signaling pathways downstream of the immunological synapse critical for T-cell activation and establishment of T-cell memory. This has led to the understanding that the cell-to-cell contacts observed in an immunological synapse that involve the CD4⁺ T cell and antigen-presenting cell or T-cell–T-cell interactions enhance efficient viral spread and facilitate the induction and maintenance of latency in HIV-infected memory T cells. This review focuses on recent work characterizing the immunological synapse and the signaling pathways involved in T-cell activation and gene regulation in the context of HIV persistence.

An in-depth comparison of latent HIV-1 reactivation in multiple cell model systems and resting CD4+ T cells from aviremic patients

The possibility of HIV-1 eradication has been limited by the existence of latently infected cellular reservoirs. Studies to examine control of HIV latency and potential reactivation have been hindered by the small numbers of latently infected cells found in vivo. Major conceptual leaps have been facilitated by the use of latently infected T cell lines and primary cells. However, notable differences exist among cell model systems. Furthermore, screening efforts in specific cell models have identified drug candidates for "anti-latency" therapy, which often fail to reactivate HIV uniformly across different models. Therefore, the activity of a given drug candidate, demonstrated in a particular cellular model, cannot reliably predict its activity in other cell model systems or in infected patient cells, tested ex vivo. This situation represents a critical knowledge gap that adversely affects our ability to identify promising treatment compounds and hinders the advancement of drug testing into relevant animal models and clinical trials. To begin to understand the biological characteristics that are inherent to each HIV-1 latency model, we compared the response properties of five primary T cell models, four J-Lat cell models and those obtained with a viral outgrowth assay using patient-derived infected cells. A panel of thirteen stimuli that are known to reactivate HIV by defined mechanisms of action was selected and tested in parallel in all models. Our results indicate that no single in vitro cell model alone is able to capture accurately the ex vivo response characteristics of latently infected T cells from patients. Most cell models demonstrated that sensitivity to HIV reactivation was skewed toward or against specific drug classes. Protein kinase C agonists and PHA reactivated latent HIV uniformly across models, although drugs in most other classes did not.

A pilot study assessing the safety and latency-reversing activity of disulfiram in HIV-1-infected adults on antiretroviral therapy

BACKGROUND

Transcriptionally silent human immunodeficiency virus type 1 (HIV-1) DNA persists in resting memory CD4(+) T cells despite antiretroviral therapy. In a primary cell model, the antialcoholism drug disulfiram has been shown to induce HIV-1 transcription in latently infected resting memory CD4(+) T cells at concentrations achieved in vivo.

METHODS

We conducted a single-arm pilot study to evaluate whether 500 mg of disulfiram administered daily for 14 days to HIV-1-infected individuals on stable suppressive antiretroviral therapy would result in reversal of HIV-1 latency with a concomitant transient increase in residual viremia or depletion of the latent reservoir in resting memory CD4(+) T cells.

RESULTS

Disulfiram was safe and well tolerated. There was a high level of subject-to-subject variability in plasma disulfiram levels. The latent reservoir did not change significantly (1.16-fold change; 95% confidence interval [CI], .70- to 1.92-fold; P = .56). During disulfiram administration, residual viremia did not change significantly compared to baseline (1.53-fold; 95% CI, .88- to 2.69-fold; P = .13), although residual viremia was estimated to increase by 1.88-fold compared to baseline during the postdosing period (95% CI, 1.03- to 3.43-fold; P = .04). In a post hoc analysis, a rapid and transient increase in viremia was noted in a subset of individuals (n = 6) with immediate postdose sampling (HIV-1 RNA increase, 2.96-fold; 95% CI, 1.29- to 6.81-fold; P = .01).

CONCLUSIONS

Administration of disulfiram to patients on antiretroviral therapy does not reduce the size of the latent reservoir. A possible dose-related effect on residual viremia supports future studies assessing the impact of higher doses on HIV-1 production. Disulfiram affects relevant signaling pathways and can be safely administered, supporting future studies of this drug.

Entinostat is a histone deacetylase inhibitor selective for class 1 histone deacetylases and activates HIV production from latently infected primary T cells

OBJECTIVES

To compare the potency, toxicity and mechanism of action of multiple histone deacetylase inhibitors (HDACi) in activating HIV production from latency.

DESIGN

In-vitro analysis of HDACi in a primary T-cell model of HIV latency and latently infected cell lines.

METHODS

Latently infected chemokine ligand 19 (CCL19)-treated CD4⁺ T cells and the latently infected cell lines ACH2 and J-Lat were treated with a panel of HDACi, including entinostat, vorinostat, panonbinostat and MCT3. Viral production and cell viability were compared. Expression of cellular HDACs was measured by western blot and PCR. Association of HDACs with the HIV long-terminal repeat (LTR) using latently infected CCL19-treated primary CD4⁺ T cells in the presence and absence of specific HDACi was determined by chromatin immunoprecipitation (ChIP).

RESULTS

We demonstrated considerable variation in the potency and toxicity of HDACi in latently infected primary CD4⁺ T cells and cell lines. All HDACi tested activated HIV production in latently infected primary T cells with greatest potency demonstrated with entinostat and vorinostat and greatest toxicity with panobinostat. Following the addition of HDACi in vitro, there were no changes in markers of T-cell activation or expression of the HIV coreceptors chemokine (C-X-C motif) receptor 4 (CXCR4) or chemokine (C-C motif) receptor type 5 (CCR5). ChIP analysis of latently infected CCL19-treated primary CD4⁺ T cells showed binding by HDAC1, HDAC2 and HDAC3 to the LTR with removal of HDAC1 and HDAC2 following treatment with the HDACi vorinostat and HDAC1 only following treatment with entinostat.

CONCLUSION

The HDACi entinostat, selective for inhibition of class I HDACs, induced virus expression in latently infected primary CD4⁺ T cells making this compound an attractive novel option for future clinical trials.

Towards an HIV cure: science and debate from the International AIDS Society 2013 symposium

The International AIDS Society convened the multi-stakeholder “Towards an HIV Cure” symposium in Kuala Lumpur, Malaysia in 2013 to address the significant research challenges posed by the search for a cure for HIV infection. Current antiretroviral regimens select for a small reservoir of cells that harbour latent HIV provirus, produce few or no HIV virions, and resist detection or clearance by host immunity. The symposium examined basic molecular science and animal model data, and emerging and ongoing clinical trial results to prioritise strategies and determine the viral and immune responses that could lead to HIV remission without ART. Here we review the presentations that scrutinized the molecular mechanisms controlling virus expression from proviral DNA, and the intrinsic cellular restriction and immune mechanisms preventing viral production. Insights from the basic science have translated into new therapeutic strategies seeking HIV remission without ongoing therapy, and much interest was focused on these ongoing trials. We also summarise the emerging ethical issues and patient expectations as concepts move into the clinic.

Developing Combined HIV Vaccine Strategies for a Functional Cure

Increasing numbers of HIV-infected individuals have access to potent antiretroviral drugs that control viral replication and decrease the risk of transmission. However, there is no cure for HIV and new strategies have to be developed to reach an eradication of the virus or a natural control of viral replication in the absence of drugs (functional cure). Therapeutic vaccines against HIV have been evaluated in many trials over the last 20 years and important knowledge has been gained from these trials. However, the major obstacle to HIV eradication is the persistence of latent proviral reservoirs. Different molecules are currently tested in ART-treated subjects to reactivate these latent reservoirs. Such anti-latency agents should be combined with a vaccination regimen in order to control or eradicate reactivated latently-infected cells. New in vitro assays should also be developed to assess the success of tested therapeutic vaccines by measuring the immune-mediated killing of replication-competent HIV reservoir cells. This review provides an overview of the current strategies to combine HIV vaccines with anti-latency agents that could act as adjuvant on the vaccine-induced immune response as well as new tools to assess the efficacy of these approaches.

Immune control of HIV-1 reservoirs

PURPOSE OF REVIEW

To discuss the recent major advances in the understanding of how host immune defenses contribute to HIV reservoir control.

RECENT FINDINGS

Immune control of HIV-1 reservoirs is a two-step process: viral replication activation from latent reservoirs followed by elimination of virus-expressing cells by the host. Environmental factors, such as pro-inflammatory type-I interferon, chemokines or cytokines, can facilitate HIV-1 replication, confer dormancy in CD4 cells or confer resistance to cytopathogenic effects of cytotoxic CD8 T cells. Therefore, they constitute a double-edged sword for immune control of HIV reservoirs. Concomitantly, adaptive immunity takes advantage of CD4 T-cell homeostatic mechanisms and can expose HIV-1 antigen-expressing cells to HIV-specific cytotoxic CD8 T cells, and limit virus spreading. These highly interconnected phenomena can lead to quasi-equilibrium between the HIV-1 reservoirs and host immune control that can serve as a model for the 'shock and kill' immune-based therapeutic strategies in play in the course of finding an HIV cure.

SUMMARY

Immune control of HIV reservoirs in CD4 T cells involves modulation of both HIV-1 latency and the continuous reseeding of the reservoir offering conceptual models that may advance HIV cure strategies.

An HIV-1 replication pathway utilizing reverse transcription products that fail to integrate

Integration is a central event in the replication of retroviruses, yet ≥ 90% of HIV-1 reverse transcripts fail to integrate, resulting in accumulation of unintegrated viral DNA in cells. However, understanding what role, if any, unintegrated viral DNA plays in the natural history of HIV-1 has remained elusive. Unintegrated HIV-1 DNA is reported to possess a limited capacity for gene expression restricted to early gene products and is considered a replicative dead end. Although the majority of peripheral blood CD4(+) T cells are refractory to infection, nonactivated CD4 T cells present in lymphoid and mucosal tissues are major targets for infection. Treatment with cytokine interleukin-2 (IL-2), IL-4, IL-7, or IL-15 renders CD4(+) T cells permissive to HIV-1 infection in the absence of cell activation and proliferation and provides a useful model for infection of resting CD4(+) T cells. We found that infection of cytokine-treated resting CD4(+) T cells in the presence of raltegravir or with integrase active-site mutant HIV-1 yielded de novo virus production following subsequent T cell activation. Infection with integration-competent HIV-1 naturally generated a population of cells generating virus from unintegrated DNA. Latent infection persisted for several weeks and could be activated to virus production by a combination of a histone deacetylase inhibitor and a protein kinase C activator or by T cell activation. HIV-1 Vpr was essential for unintegrated HIV-1 gene expression and de novo virus production in this system. Bypassing integration by this mechanism may allow the preservation of genetic information that otherwise would be lost.

Advances in detection and monitoring of plasma viremia in HIV-infected individuals receiving antiretroviral therapy

PURPOSE OF REVIEW

This review will describe advances in detection and results of monitoring persistent viremia in patients on long-term suppressive therapy. In addition, the review explores the usefulness of these methods in determining the effectiveness of new HIV-1 eradication strategies in purging persistent HIV-1 reservoirs.

RECENT FINDINGS

Quantification of plasma HIV-1 RNA levels remains essential for determining the success of combination antiretroviral therapy (cART) in treated patients. Recently, several new platforms with improved sensitivity for quantifying HIV-1 RNA have been developed and the application of these assays has revealed that low-level viremia persists in patients on suppressive therapy. In addition, new technological advances such as digital PCR have been proposed to increase the sensitivity of measuring and characterizing persistent HIV-1 viremia. The application of these assays will be important in determining the effectiveness of future HIV-1 eradication strategies.

SUMMARY

The level of HIV-1 RNA in patient plasma remains an important marker for determining the success of cART. New sensitive assays have found that HIV-1 persists in the plasma of patients on suppressive therapy that may have implications for the clinical management of this disease and strategies for eliminating HIV-1 infection.

Altering cell death pathways as an approach to cure HIV infection

Recent cases of successful control of human immunodeficiency virus (HIV) by bone marrow transplant in combination with suppressive antiretroviral therapy (ART) and very early initiation of ART have provided proof of concept that HIV infection might now be cured. Current efforts focusing on gene therapy, boosting HIV-specific immunity, reducing inflammation and activation of latency have all been the subject of recent excellent reviews. We now propose an additional avenue of research towards a cure for HIV: targeting HIV apoptosis regulatory pathways. The central enigma of HIV disease is that HIV infection kills most of the CD4 T cells that it infects, but those cells that are spared subsequently become a latent reservoir for HIV against which current medications are ineffective. We propose that if strategies could be devised which would favor the death of all cells which HIV infects, or if all latently infected cells that release HIV would succumb to viral-induced cytotoxicity, then these approaches combined with effective ART to prevent spreading infection, would together result in a cure for HIV. This premise is supported by observations in other viral systems where the relationship between productive infection, apoptosis resistance, and the development of latency or persistence has been established. Therefore we propose that research focused at understanding the mechanisms by which HIV induces apoptosis of infected cells, and ways that some cells escape the pro-apoptotic effects of productive HIV infection are critical to devising novel and rational approaches to cure HIV infection.

HIV-1 transcription and latency: an update

Combination antiretroviral therapy, despite being potent and life-prolonging, is not curative and does not eradicate HIV-1 infection since interruption of treatment inevitably results in a rapid rebound of viremia. Reactivation of latently infected cells harboring transcriptionally silent but replication-competent proviruses is a potential source of persistent residual viremia in cART-treated patients. Although multiple reservoirs may exist, the persistence of resting CD4+ T cells carrying a latent infection represents a major barrier to eradication. In this review, we will discuss the latest reports on the molecular mechanisms that may regulate HIV-1 latency at the transcriptional level, including transcriptional interference, the role of cellular factors, chromatin organization and epigenetic modifications, the viral Tat trans-activator and its cellular cofactors. Since latency mechanisms may also operate at the post-transcriptional level, we will consider inhibition of nuclear RNA export and inhibition of translation by microRNAs as potential barriers to HIV-1 gene expression. Finally, we will review the therapeutic approaches and clinical studies aimed at achieving either a sterilizing cure or a functional cure of HIV-1 infection, with a special emphasis on the most recent pharmacological strategies to reactivate the latent viruses and decrease the pool of viral reservoirs.

Barriers to a cure for HIV: new ways to target and eradicate HIV-1 reservoirs

Antiretroviral therapy for HIV infection needs lifelong access and strict adherence to regimens that are both expensive and associated with toxic effects. A curative intervention will be needed to fully stop the epidemic. The failure to eradicate HIV infection during long-term antiretroviral therapy shows the intrinsic stability of the viral genome in latently infected CD4T cells and other cells, and possibly a sustained low-level viral replication. Heterogeneity in latently infected cell populations and homoeostatic proliferation of infected cells might affect the dynamics of virus production and persistence. Despite potent antiretroviral therapy, chronic immune activation, inflammation, and immune dysfunction persist, and are likely to have important effects on the size and distribution of the viral reservoir. The inability of the immune system to recognise cells harbouring latent virus and to eliminate cells actively producing virus is the biggest challenge to finding a cure. We look at new approaches to unravelling the complex virus-host interactions that lead to persistent infection and latency, and discuss the rationale for combination of novel treatment strategies with available antiretroviral treatment options to cure HIV.

Promoting immunity during chronic infection--the therapeutic potential of common gamma-chain cytokines

The continued global burden wrought by chronic infectious disease is unrelenting. Current therapies have curbed the severity of disease for patients, but Human Immunodeficiency Virus (HIV) and Hepatitis B (HBV) infection remain incurable and Mycobacterium tuberculosis (MTB) is rapidly becoming resistant to our existing antibiotics. Much attention has been given to enhancing T cell immunity through the use of certain common gamma-chain cytokines, which have proven to be essential and necessary for T cell survival and function. This article reviews the pre-clinical and clinical literature surrounding IL-2, IL-7, IL-15 and IL-21 and we comment on the potential therapeutic promise of these cytokines as adjuvant treatments for chronic infectious diseases.

Administration of a Toll-like receptor 9 agonist decreases the proviral reservoir in virologically suppressed HIV-infected patients

Toll-like receptor (TLR) agonists can reactivate HIV from latently infected cells in vitro. We aimed to investigate the TLR-9 agonist, CPG 7909's in vivo effect on the proviral HIV reservoir and HIV-specific immunity. This was a post-hoc analysis of a double-blind randomized controlled vaccine trial. HIV-infected adults were randomized 1∶1 to receive pneumococcal vaccines with or without 1 mg CPG 7909 as adjuvant at 0, 3 and 9 months. In patients on suppressive antiretroviral therapy we quantified proviral DNA at 0, 3, 4, 9, and 10 months (31 subjects in the CPG group and 37 in the placebo-adjuvant group). Furthermore, we measured HIV-specific antibodies, characterized T cell phenotypes and HIV-specific T cell immunity. We observed a mean reduction in proviral DNA in the CPG group of 12.6% (95% CI: −23.6–0.0) following each immunization whereas proviral DNA in the placebo-adjuvant group remained largely unchanged (6.7% increase; 95% CI: −4.2–19.0 after each immunization, p = 0.02). Among participants with additional cryo-preserved PBMCs, HIV-specific CD8+ T cell immunity as indicated by increased expression of degranulation marker CD107a and macrophage inflammatory protein 1β (MIP1β) tended to be up-regulated following immunization with CPG 7909 compared with placebo as adjuvant. Further, increasing proportion of HIV-specific CD107a and MIP1β-expressing CD8+ T cells were strongly correlated with decreasing proviral load. No changes were observed in T cell phenotype distribution, HIV-specific CD4+ T cell immunity, or HIV-specific antibodies. TLR9-adjuvanted pneumococcal vaccination decreased proviral load. Reductions in proviral load correlated with increasing levels of HIV specific CD8+ T cells. Further investigation into the potential effect of TLR9 agonists on HIV latency is warranted.

Interleukin-7 promotes HIV persistence during antiretroviral therapy

HIV persists in latently infected memory CD4(+) T cells during antiretroviral therapy (ART). When administered to HIV-infected subjects receiving suppressive ART, interleukin-7 (IL-7) increases the number of CD4(+) T cells by promoting their survival and proliferation. However, little is known about the impact of IL-7 on HIV persistence during ART. By isolating large numbers of CD4(+) T cells from HIV-infected subjects, we demonstrate that IL-7 enhances viral production in productively infected cells but does not disrupt viral latency in latently infected cells. When administered to virally suppressed subjects, IL-7 led to the rapid proliferation of memory CD4(+) T cells, which resulted in a 70% increase in the absolute number of circulating CD4(+) T cells harboring integrated HIV DNA 4 weeks after therapy. The genetic diversity of the viral reservoir increased transiently in the majority of the subjects studied before returning to baseline values. Altogether, our results indicate that IL-7 promotes the mechanisms of HIV persistence during ART by enhancing residual levels of viral production and inducing proliferation of latently infected cells, and suggest that IL-7 does not represent a suitable candidate therapeutic strategy for HIV eradication. This trial was registered at www.clinicaltrials.gov as #NCT00099671 (AIDS Clinical Trials Group protocol 5214).

Eliminating the latent HIV reservoir by reactivation strategies: advancing to clinical trials

Combination antiretroviral therapy (cART) has transformed HIV from a deadly to a chronic disease, but HIV patients are still burdened with excess morbidity and mortality, long-term toxicities from cART, stigmatization, and insufficient access to cART worldwide. Thus, a cure for HIV would have enormous impact on society as well as the individual. As the complexity and mechanisms of HIV persistence during therapy are being unraveled, new therapeutic targets for HIV eradication are discovered. Substances that activate HIV production in the latently infected cells have recently received much attention. By turning on expression of latent HIV proviruses, reactivation strategies could contribute to the eradication HIV infection. Compounds that are currently being or soon to be tested in clinical trials are emphasized. The results from these trials will provide important clues as to whether or not reactivating strategies could become significant components of a cure for HIV.

Lost in transcription: molecular mechanisms that control HIV latency

Highly active antiretroviral therapy (HAART) has limited the replication and spread of the human immunodeficiency virus (HIV). However, despite treatment, HIV infection persists in latently infected reservoirs, and once therapy is interrupted, viral replication rebounds quickly. Extensive efforts are being directed at eliminating these cell reservoirs. This feat can be achieved by reactivating latent HIV while administering drugs that prevent new rounds of infection and allow the immune system to clear the virus. However, current approaches to HIV eradication have not been effective. Moreover, as HIV latency is multifactorial, the significance of each of its molecular mechanisms is still under debate. Among these, transcriptional repression as a result of reduced levels and activity of the positive transcription elongation factor b (P-TEFb: CDK9/cyclin T) plays a significant role. Therefore, increasing levels of P-TEFb expression and activity is an excellent strategy to stimulate viral gene expression. This review summarizes the multiple steps that cause HIV to enter into latency. It positions the interplay between transcriptionally active and inactive host transcriptional activators and their viral partner Tat as valid targets for the development of new strategies to reactivate latent viral gene expression and eradicate HIV.

The search for an HIV cure: tackling latent infection

Strategies to eliminate infectious HIV that persists despite present treatments and with the potential to cure HIV infection are of great interest. One patient seems to have been cured of HIV infection after receiving a bone marrow transplant with cells resistant to the virus, although this strategy is not viable for large numbers of infected people. Several clinical trials are underway in which drugs are being used to activate cells that harbour latent HIV. In a recent study, investigators showed that activation of latent HIV infection in patients on antiretroviral therapy could be achieved with a single dose of vorinostat, a licensed anticancer drug that inhibits histone deacetylase. Although far from a cure, such studies provide some guidance towards the logical next steps for research. Clinical studies that use a longer duration of drug dosing, alternative agents, combination approaches, gene therapy, and immune-modulation approaches are all underway.

Comparative analysis of measures of viral reservoirs in HIV-1 eradication studies

HIV-1 reservoirs preclude virus eradication in patients receiving highly active antiretroviral therapy (HAART). The best characterized reservoir is a small, difficult-to-quantify pool of resting memory CD4⁺ T cells carrying latent but replication-competent viral genomes. Because strategies targeting this latent reservoir are now being tested in clinical trials, well-validated high-throughput assays that quantify this reservoir are urgently needed. Here we compare eleven different approaches for quantitating persistent HIV-1 in 30 patients on HAART, using the original viral outgrowth assay for resting CD4⁺ T cells carrying inducible, replication-competent viral genomes as a standard for comparison. PCR-based assays for cells containing HIV-1 DNA gave infected cell frequencies at least 2 logs higher than the viral outgrowth assay, even in subjects who started HAART during acute/early infection. This difference may reflect defective viral genomes. The ratio of infected cell frequencies determined by viral outgrowth and PCR-based assays varied dramatically between patients. Although strong correlations with the viral outgrowth assay could not be formally excluded for most assays, correlations achieved statistical significance only for integrated HIV-1 DNA in peripheral blood mononuclear cells and HIV-1 RNA/DNA ratio in rectal CD4⁺ T cells. Residual viremia was below the limit of detection in many subjects and did not correlate with the viral outgrowth assays. The dramatic differences in infected cell frequencies and the lack of a precise correlation between culture and PCR-based assays raise the possibility that the successful clearance of latently infected cells may be masked by a larger and variable pool of cells with defective proviruses. These defective proviruses are detected by PCR but may not be affected by reactivation strategies and may not require eradication to accomplish an effective cure. A molecular understanding of the discrepancy between infected cell frequencies measured by viral outgrowth versus PCR assays is an urgent priority in HIV-1 cure research.

Efforts to cure HIV-1 infection have focused on a small pool of CD4⁺ T cells that carry viral genetic information in a latent form. These cells persist even in patients on optimal antiretroviral therapy. Novel therapeutic strategies targeting latently infected cells are being developed, and therefore practical assays for measuring latently infected cells are urgently needed. These cells were discovered using a virus culture assay in which the cells are induced to release virus particles that are then expanded in culture. This assay is difficult, time-consuming, and expensive. Here we evaluate alternative approaches for measuring persistent HIV-1, all of which rely on the detection of viral genetic information using the polymerase chain reaction (PCR). None of the PCR-based assays correlated precisely with the virus culture assay. The fundamental problem is that infected cell frequencies determined by PCR are at least 2 logs higher than frequencies determined by the culture assay. Much of this difference may be due to cells carrying defective forms of the virus. These cells may not be eliminated by strategies designed to target latently infected cells. In this situation, successful clearance of latently infected cells might be masked by a large unchanging pool of cells carrying defective HIV-1.

Interleukin-7 facilitates HIV-1 transmission to cervico-vaginal tissue ex vivo

The majority of HIV-1 infections in women occur through vaginal intercourse, in which virus-containing semen is deposited on the cervico-vaginal mucosa. Semen is more than a mere carrier of HIV-1, since it contains many biological factors, in particular cytokines, that may affect HIV-1 transmission. The concentration of interleukin (IL)-7, one of the most prominent cytokines in semen of healthy individuals, is further increased in semen of HIV-1-infected men. Here, we investigated the potential role of IL-7 in HIV-1 vaginal transmission in an ex vivo system of human cervico-vaginal tissue. We simulated an in vivo situation by depositing HIV-1 on cervico-vaginal tissue in combination with IL-7 at concentrations comparable with those measured in semen of HIV-1-infected individuals. We found that IL-7 significantly enhanced virus replication in ex vivo infected cervico-vaginal tissue. Similarly, we observed an enhancement of HIV-1 replication in lymphoid tissue explants. Analysis of T cells isolated from infected tissues showed that IL-7 reduced CD4⁺ T cell depletion preventing apoptosis, as shown by the decrease in the number of cells expressing the apoptotic marker APO2.7 and the increase in the expression of the anti-apoptotic protein B-cell lymphoma (Bcl)-2. Also, IL-7 increased the fraction of cycling CD4⁺ T cells, as evidenced by staining for the nuclear factor Ki-67. High levels of seminal IL-7 in vivo may be relevant to the survival of the founder pool of HIV-1-infected cells in the cervico-vaginal mucosa at the initial stage of infection, promoting local expansion and dissemination of HIV infection.

Male-to-female HIV-1 transmission occurs predominantly through vaginal intercourse when the virus is transmitted with seminal fluid. The identification of the determinants of HIV-1 transmission to the female lower genital tract is of pivotal importance for understanding the basic mechanisms of HIV-1 infection. This understanding is necessary for the development of new strategies to prevent or contain this infection. Semen of HIV-1-infected individuals is highly enriched with IL-7, a crucial cytokine for the life cycle of CD4⁺ T cells, the primary target of HIV-1. Here, we utilized a system of human cervico-vaginal and lymphoid tissues ex vivo to study the effect of IL-7 on HIV-1 transmission and dissemination. Our results show that IL-7 at concentrations comparable to those found in semen of HIV-1-infected individuals enhanced HIV-1 replication by preventing the death and by stimulating the proliferation of CD4⁺ T cells. This allows sustained viral production by infected cells and provides new cellular targets for propagation of infection. The concentration of IL-7 in semen of HIV-1-infected men may be a key determinant of the efficiency of HIV-1 transmission to an uninfected female partner through vaginal intercourse.

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.

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.

Treating HIV-1 Infection: What Might the Future Hold?

Advances in antiretroviral combination therapy lasting the past two decades have transformed HIV-1 infection from a fatal disease into a chronic medical condition that in many cases does not compromise life quality. There are 25 different antiretroviral agents available currently, allowing for patient-centered, individualized management of HIV-1 infection, and ongoing progress in HIV-1 virology and antiretroviral pharmacology is likely to expand treatment options further in the future. Nevertheless, antiretroviral therapy continues to have limitations, including insufficient immunological reconstitution, selection of drug resistance, ongoing abnormal immune activation despite effective suppression of HIV-1 viremia, and the inability to target latently infected cells that are responsible for long-term viral persistence. Owing to these shortcomings, the theoretical ability of antiretroviral therapy to extend life expectancy to normal levels is not realized in many cases. Strategies to address these limitations are a matter of active ongoing research and will be summarized in this article.

Selective histonedeacetylase inhibitor M344 intervenes in HIV-1 latency through increasing histone acetylation and activation of NF-kappaB

BACKGROUND

Histone deacetylase (HDAC) inhibitors present an exciting new approach to activate HIV production from latently infected cells to potentially enhance elimination of these cells and achieve a cure. M344, a novel HDAC inhibitor, shows robust activity in a variety of cancer cells and relatively low toxicity compared to trichostatin A (TSA). However, little is known about the effects and action mechanism of M344 in inducing HIV expression in latently infected cells.

METHODOLOGY/PRINCIPAL FINDINGS

Using the Jurkat T cell model of HIV latency, we demonstrate that M344 effectively reactivates HIV-1 gene expression in latently infected cells. Moreover, M344-mediated activation of the latent HIV LTR can be strongly inhibited by a NF-κB inhibitor aspirin. We further show that M344 acts by increasing the acetylation of histone H3 and histone H4 at the nucleosome 1 (nuc-1) site of the HIV-1 long terminal repeat (LTR) and by inducing NF-κB p65 nuclear translocation and direct RelA DNA binding at the nuc-1 region of the HIV-1 LTR. We also found that M344 synergized with prostratin to activate the HIV-1 LTR promoter in latently infected cells.

CONCLUSIONS/SIGNIFICANCE

These results suggest the potential of M344 in anti-latency therapies and an important role for histone modifications and NF-κB transcription factors in regulating HIV-1 LTR gene expression.

Perspectives on interleukin-7 therapy in HIV infection

PURPOSE OF REVIEW

HIV infection is characterized by depletion of CD4 T cells and altered immune function, leading to severe immune deficiency. Mechanisms leading to this T-cell depletion are not completely understood. Potent antiretroviral therapy restores T-cell counts and improves prognosis. Apart from antiviral therapy for the infection, immunotherapies such as interleukin-7 that influence T-cell homeostatic mechanisms are undergoing clinical evaluation. Because of its pleiotropic effects on developing and mature T cells, interleukin-7 may help to restore immune function during HIV infection.

RECENT FINDINGS

Recent studies explored the therapeutic use of interleukin-7 in simian immunodeficiency virus models and in HIV-infected patients. Interleukin-7 can help to restore CD4 T-cell number and function.

SUMMARY

Numerous recent findings highlight the importance of interleukin-7 pathway impairment in the pathogenesis of HIV infection. Notably, interleukin-7 levels increased with advancing CD4 T-cell lymphopenia, whereas interleukin-7 receptor expression is downregulated mainly on CD8 T cells. Therapeutic trials conducted in monkeys and in humans (phase I) have provided evidence on the role of interleukin-7 in thymopoiesis and in restoration of T-cell functions. Interleukin-7 appeared to be well tolerated and to have no deleterious effects on viral load. These results should be confirmed in larger phase I/II studies.

GB virus C infection is associated with a reduced rate of reactivation of latent HIV and protection against activation-induced T-cell death

BACKGROUND

GB virus C (GBV-C) coinfection is associated with reduced immune activation and a block in CD4(+) T-cell proliferation following interleukin-2 (IL-2) therapy in HIV-infected individuals. We examined peripheral blood mononuclear cells (PBMCs) from HIV-infected subjects with and without GBV-C viraemia to determine if GBV-C correlated with reactivation of latent HIV, T-cell proliferation or T-cell survival following in vitro activation with phytohaemagglutinin A and IL-2 (PHA/IL-2).

METHODS

HIV-infected subjects whose HIV viral load was suppressed on combination antiretroviral therapy (cART) for >6 months were studied. PBMCs were cultured with and without PHA/IL-2 and monitored for HIV reactivation, proliferation and survival. GBV-C viraemia and in vitro replication were detected by real-time RT-PCR. HIV reactivation was determined by measuring HIV p24 antigen in culture supernatants. Proliferation was measured by counting viable cells and survival measured by flow cytometry.

RESULTS

Of 49 HIV-infected individuals, 26 had GBV-C viraemia. Significantly less HIV reactivation and PBMC proliferation following in vitro activation with PHA/IL-2 was observed in samples from GBV-C viraemic subjects compared with non-viraemic controls. Following 5 weeks in culture, GBV-C replication was associated with preservation of CD4(+) and CD8(+) T-cells compared with non-viraemic controls.

CONCLUSIONS

GBV-C appears to inhibit immune activation and IL-2 signalling pathways, which might contribute to a reduction in reactivation of latent HIV from cellular reservoirs. In addition, GBV-C viraemia was associated with a reduction in activation-induced T-cell death. GBV-C-associated T-cell effects could contribute to the observed protective effect of GBV-C coinfection in HIV-infected individuals.

Modulation of HIV reservoirs by host HLA: bridging the gap between vaccine and cure

Latent HIV reservoirs are the greatest challenge facing an HIV cure. Here, we review recent evidence supporting an important role for the host immune response, in particular HLA class I-restricted CD8+ T lymphocytes, in modulating HIV reservoirs during natural infection. These observations indicate that factors governing immune-mediated control of HIV may also contribute to the clearance of viral reservoirs. As such, critical gaps in our understanding of HIV immunology hinder efforts to develop both an effective HIV vaccine as well as novel therapies that may lead to a cure. The importance of elucidating correlates of protective cellular immunity should be recognized during research to develop and test potential HIV elimination strategies.

Soluble mediators of inflammation in HIV and their implications for therapeutics and vaccine development

From early in the HIV epidemic it was appreciated that many inflammatory markers such as neopterin and TNF-α were elevated in patients with AIDS. With the advent of modern technology able to measure a broad array of cytokines, we now know that from the earliest points of infection HIV induces a cytokine storm. This review will focus on how cytokines are disturbed in HIV infection and will explore potential therapeutic uses of cytokines. These factors can be used directly as therapy during HIV infection, either to suppress viral replication or prevent deleterious immune effects of infection, such as CD4+ T cell depletion. Cytokines also show great promise as adjuvants in the development of HIV vaccines, which would be critical for the eventual control of the epidemic.

The role of cytokines in the establishment, persistence and eradication of the HIV reservoir

HIV persists in cellular and anatomical reservoirs during Highly Active Antiretroviral Therapy (HAART). In vitro studies as well as in vivo observations have identified cytokines as important factors regulating the immunological and virological mechanisms involved in HIV persistence. Immunosuppressive cytokines might contribute to the establishment of viral latency by dampening T cell activation and HIV production, thereby creating the necessary immuno-virological condition for the establishment of a pool of latently infected cells. Other cytokines that are involved in the maintenance of memory CD4(+) T cells promote the persistence of these cells during HAART. Conversely, proinflammatory cytokines may favor HIV persistence by exacerbating low levels of ongoing viral replication in lymphoid tissues even after prolonged therapy. The ability of several cytokines to interfere with the molecular mechanisms responsible for HIV latency makes them attractive candidates for therapeutic strategies aimed at reducing the pool of latently infected cells. In this article, we review the role of cytokines in HIV persistence during HAART and discuss their role as potential eradicating agents.

HIV latency: experimental systems and molecular models

Highly active antiretroviral therapy (HAART) has shown great efficacy in increasing the survival of HIV infected individuals. However, HAART does not lead to the full eradication of infection and therefore has to be continued for life. HIV persists in a transcriptionally inactive form in resting T cells in HAART-treated patients and can be reactivated following T-cell activation. These latently infected cells allow the virus to persist in the presence of HAART. Here, we review recent advances in the study of the molecular mechanisms of HIV latency. We also review experimental models in which latency is currently studied. We focus on the epigenetic mechanisms controlling HIV transcription and on the role of chromatin and its post-translational modifications. We discuss how small molecule inhibitors that target epigenetic regulators, such as HDAC (histone deacetylase) inhibitors, are being tested for their ability to reactivate latent HIV. Finally, we discuss the clinical potential of these drugs to flush out latently infected cells from HIV-infected patients and to eradicate the virus.

Eliminating the HIV reservoir

In the past few years, major advances have been achieved in understanding the nature and the maintenance mechanisms of the HIV reservoir. Although antiretroviral therapy works well in a majority of patients, it faces problems of compliance, resistance, toxicity, and cost. In most cases, the remaining HIV reservoir precluding antiretroviral cessation consists of a tiny cell pool that is long-lived and inaccessible to current therapies. New strategies are therefore needed to either purge or control this residual reservoir and finally stop antiretroviral drugs. Both ways leading to a functional or a sterilizing cure are currently pursued. Several molecules have been identified to achieve these goals and some of them have already entered clinical testing in humans. In this article, we review recent findings on the biology of HIV persistence and detail how HIV eradication trials should be designed in the near future.

Achieving a cure for HIV infection: do we have reasons to be optimistic?

The introduction of highly active antiretroviral therapy (HAART) in 1996 has transformed a lethal disease to a chronic pathology with a dramatic decrease in mortality and morbidity of AIDS-related symptoms in infected patients. However, HAART has not allowed the cure of HIV infection, the main obstacle to HIV eradication being the existence of quiescent reservoirs. Several other problems have been encountered with HAART (such as side effects, adherence to medication, emergence of resistance and cost of treatment), and these motivate the search for new ways to treat these patients. Recent advances hold promise for the ultimate cure of HIV infection, which is the topic of this review. Besides these new strategies aiming to eliminate the virus, efforts must be made to improve current HAART. We believe that the cure of HIV infection will not be attained in the short term and that a strategy based on purging the reservoirs has to be associated with an aggressive HAART strategy.

Molecular Understanding of HIV-1 Latency

The introduction of highly active antiretroviral therapy (HAART) has been an important breakthrough in the treatment of HIV-1 infection and has also a powerful tool to upset the equilibrium of viral production and HIV-1 pathogenesis. Despite the advent of potent combinations of this therapy, the long-lived HIV-1 reservoirs like cells from monocyte-macrophage lineage and resting memory CD4+ T cells which are established early during primary infection constitute a major obstacle to virus eradication. Further HAART interruption leads to immediate rebound viremia from latent reservoirs. This paper focuses on the essentials of the molecular mechanisms for the establishment of HIV-1 latency with special concern to present and future possible treatment strategies to completely purge and target viral persistence in the reservoirs.

HIV reservoirs and strategies for eradication

Combination antiretroviral therapy (cART) has led to a reduction in morbidity and mortality in HIV-infected patients but therapy is lifelong and there is no cure for HIV. The major barriers to cure include HIV latency, which has been identified in different T-cell subsets, as well as persistence of HIV in anatomical reservoirs. We review recent developments in our understanding of the major reservoirs of HIV in patients on cART as well as how latency is established and maintained in T cells. Finally, we review the scientific rationale of and clinical experience with pharmacotherapeutic strategies aimed at eliminating latently infected cells.

HIV RNA suppression and immune restoration: can we do better?

HAART has significantly changed the natural history of HIV infection: patients receiving antiretrovirals are usually able to control viremia, even though not all virological responders adequately recover their CD4+ count. The reasons for poor immune restoration are only partially known and they include genetic, demographic and immunologic factors. A crucial element affecting immune recovery is immune activation, related to residual viremia; indeed, a suboptimal virological control (i.e., low levels of plasma HIV RNA) has been related with higher levels of chronic inflammation and all-cause mortality. The sources of residual viremia are not yet completely known, even though the most important one is represented by latently infected cells. Several methods, including 2-LTR HIV DNA and unspliced HIV RNA measurement, have been developed to estimate residual viremia and predict the outcome of antiretroviral therapy. Considering that poor immunologic responders are exposed to a higher risk of both AIDS-related and non-AIDS-related diseases, there is a need of new therapeutic strategies, including immunomodulators and drugs targeting the latent viral reservoirs, in order to face residual viremia but also to “drive” the host immunologic responses.

Protein Kinase C: One Pathway towards the Eradication of Latent HIV-1 Reservoirs

An effective means to eradicate latent reservoirs in HIV-1-infected individuals remains elusive. Attempts to purge these reservoirs were undertaken over a decade ago without success. The subsequent lapse in further clinical attempts since may have been justified as our knowledge of the mechanisms which underpin the latent state still evolves. Although additional novel molecular antagonists of HIV-1 latency have subsequently been reported, these candidate agents have not been tested in human trials for reservoir ablation. This review provides an overview of the protein kinase C (PKC) pathway which can be modulated by small molecular agents to induce the expression of latent HIV-1 from within infected reservoir cells. Some of these agents have been tested against select cancers with seemingly tolerable side effects. As such, modulation of the PKC pathway may yet be a viable mechanism toward HIV-1 reservoir eradication.