HIV-1 latency in actively dividing human T cell lines

An HIV-1 CRISPR-Cas9 membrane trafficking screen reveals a role for PICALM intersecting endolysosomes and immunity

HIV-1 hijacks host proteins involved in membrane trafficking, endocytosis, and autophagy that are critical for virus replication. Molecular details are lacking but are essential to inform on the development of alternative antiviral strategies. Despite their potential as clinical targets, only a few membrane trafficking proteins have been functionally characterized in HIV-1 replication. To further elucidate roles in HIV-1 replication, we performed a CRISPR-Cas9 screen on 140 membrane trafficking proteins. We identified phosphatidylinositol-binding clathrin assembly protein (PICALM) that influences not only infection dynamics but also CD4+ SupT1 biology. The knockout (KO) of PICALM inhibited viral entry. In CD4+ SupT1 T cells, KO cells exhibited defects in intracellular trafficking and increased abundance of intracellular Gag and significant alterations in autophagy, immune checkpoint PD-1 levels, and differentiation markers. Thus, PICALM modulates a variety of pathways that ultimately affect HIV-1 replication, underscoring the potential of PICALM as a future target to control HIV-1.

Latency Reversing Agents: Kick and Kill of HTLV-1?

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia/lymphoma (ATLL), is a retrovirus, which integrates into the host genome and persistently infects CD4⁺ T-cells. Virus propagation is stimulated by (1) clonal expansion of infected cells and (2) de novo infection. Viral gene expression is induced by the transactivator protein Tax, which recruits host factors like positive transcription elongation factor b (P-TEFb) to the viral promoter. Since HTLV-1 gene expression is repressed in vivo by viral, cellular, and epigenetic mechanisms in late phases of infection, HTLV-1 avoids an efficient CD8⁺ cytotoxic T-cell (CTL) response directed against the immunodominant viral Tax antigen. Hence, therapeutic strategies using latency reversing agents (LRAs) sought to transiently activate viral gene expression and antigen presentation of Tax to enhance CTL responses towards HTLV-1, and thus, to expose the latent HTLV-1 reservoir to immune destruction. Here, we review strategies that aimed at enhancing Tax expression and Tax-specific CTL responses to interfere with HTLV-1 latency. Further, we provide an overview of LRAs including (1) histone deacetylase inhibitors (HDACi) and (2) activators of P-TEFb, that have mainly been studied in context of human immunodeficiency virus (HIV), but which may also be powerful in the context of HTLV-1.

Interferon-inducible TRIM22 contributes to maintenance of HIV-1 proviral latency in T cell lines

The human immunodeficiency virus type-1 (HIV-1) establishes a state of latent infection in a small number of CD4⁺ T lymphocytes that, nonetheless, represent a major obstacle to viral eradication. We here show that Tripartite Motif-containing protein 22 (TRIM22), an epigenetic inhibitor of Specificity protein 1 (Sp1)-dependent HIV-1 transcription, is a relevant factor in maintaining a state of repressed HIV-1 expression at least in CD4⁺ T cell lines. By knocking-down (KD) TRIM22 expression, we observed an accelerated reactivation of a doxycycline (Dox)-controlled HIV-1 replication in the T lymphocytic SupT1 cell line. Furthermore, we here report for the first time that TRIM22 is a crucial factor for maintaining a state of HIV-1 quiescence in chronically infected ACH2 -T cell line while its KD potentiated HIV-1 expression in both ACH-2 and J-Lat 10.6 cell lines upon cell stimulation with either tumor necrosis factor-α (TNF-α) or histone deacetylase inhibitors (HDACi). In conclusion, TRIM22 is a novel determinant of HIV-1 latency, at least in T cell lines, thus representing a potential pharmacological target for strategies aiming at curtailing or silencing the pool of latently infected CD4⁺ T lymphocytes constituting the HIV-1 reservoir in individuals receiving combination antiretroviral therapy.

EcoHIV infection of mice establishes latent viral reservoirs in T cells and active viral reservoirs in macrophages that are sufficient for induction of neurocognitive impairment

Suppression of HIV replication by antiretroviral therapy (ART) or host immunity can prevent AIDS but not other HIV-associated conditions including neurocognitive impairment (HIV-NCI). Pathogenesis in HIV-suppressed individuals has been attributed to reservoirs of latent-inducible virus in resting CD4+ T cells. Macrophages are persistently infected with HIV but their role as HIV reservoirs in vivo has not been fully explored. Here we show that infection of conventional mice with chimeric HIV, EcoHIV, reproduces physiological conditions for development of disease in people on ART including immunocompetence, stable suppression of HIV replication, persistence of integrated, replication-competent HIV in T cells and macrophages, and manifestation of learning and memory deficits in behavioral tests, termed here murine HIV-NCI. EcoHIV established latent reservoirs in CD4+ T lymphocytes in chronically-infected mice but could be induced by epigenetic modulators ex vivo and in mice. In contrast, macrophages expressed EcoHIV constitutively in mice for up to 16 months; murine leukemia virus (MLV), the donor of gp80 envelope in EcoHIV, did not infect macrophages. Both EcoHIV and MLV were found in brain tissue of infected mice but only EcoHIV induced NCI. Murine HIV-NCI was prevented by antiretroviral prophylaxis but once established neither persistent EcoHIV infection in mice nor NCI could be reversed by long-acting antiretroviral therapy. EcoHIV-infected, athymic mice were more permissive to virus replication in macrophages than were wild-type mice, suffered cognitive dysfunction, as well as increased numbers of monocytes and macrophages infiltrating the brain. Our results suggest an important role of HIV expressing macrophages in HIV neuropathogenesis in hosts with suppressed HIV replication.

Retroviral DNA Integration

The integration of a DNA copy of the viral RNA genome into host chromatin is the defining step of retroviral replication. This enzymatic process is catalyzed by the virus-encoded integrase protein, which is conserved among retroviruses and LTR-retrotransposons. Retroviral integration proceeds via two integrase activities: 3'-processing of the viral DNA ends, followed by the strand transfer of the processed ends into host cell chromosomal DNA. Herein we review the molecular mechanism of retroviral DNA integration, with an emphasis on reaction chemistries and architectures of the nucleoprotein complexes involved. We additionally discuss the latest advances on anti-integrase drug development for the treatment of AIDS and the utility of integrating retroviral vectors in gene therapy applications.

Potent and Targeted Activation of Latent HIV-1 Using the CRISPR/dCas9 Activator Complex

HIV-1 provirus integration results in a persistent latently infected reservoir that is recalcitrant to combined antiretroviral therapy (cART) with lifelong treatment being the only option. The "shock and kill" strategy aims to eradicate latent HIV by reactivating proviral gene expression in the context of cART treatment. Gene-specific transcriptional activation can be achieved using the RNA-guided CRISPR-Cas9 system comprising single guide RNAs (sgRNAs) with a nuclease-deficient Cas9 mutant (dCas9) fused to the VP64 transactivation domain (dCas9-VP64). We engineered this system to target 23 sites within the long terminal repeat promoter of HIV-1 and identified a "hotspot" for activation within the viral enhancer sequence. Activating sgRNAs transcriptionally modulated the latent proviral genome across multiple different in vitro latency cell models including T cells comprising a clonally integrated mCherry-IRES-Tat (LChIT) latency system. We detected consistent and effective activation of latent virus mediated by activator sgRNAs, whereas latency reversal agents produced variable activation responses. Transcriptomic analysis revealed dCas9-VP64/sgRNAs to be highly specific, while the well-characterized chemical activator TNFα induced widespread gene dysregulation. CRISPR-mediated gene activation represents a novel system which provides enhanced efficiency and specificity in a targeted latency reactivation strategy and represents a promising approach to a "functional cure" of HIV/AIDS.

A minimal fate-selection switch

To preserve fitness in unpredictable, fluctuating environments, a range of biological systems probabilistically generate variant phenotypes--a process often referred to as 'bet-hedging', after the financial practice of diversifying assets to minimize risk in volatile markets. The molecular mechanisms enabling bet-hedging have remained elusive. Here, we review how HIV makes a bet-hedging decision between active replication and proviral latency, a long-lived dormant state that is the chief barrier to an HIV cure. The discovery of a virus-encoded bet-hedging circuit in HIV revealed an ancient evolutionary role for latency and identified core regulatory principles, such as feedback and stochastic 'noise', that enable cell-fate decisions. These core principles were later extended to fate selection in stem cells and cancer, exposed new therapeutic targets for HIV, and led to a potentially broad strategy of using 'noise modulation' to redirect cell fate.

The integrase cofactor LEDGF/p75 associates with Iws1 and Spt6 for postintegration silencing of HIV-1 gene expression in latently infected cells

The persistence of a latent reservoir containing transcriptionally silent, but replication-competent, integrated provirus is a serious challenge to HIV eradication. HIV integration is under the control of LEDGF/p75, the cellular cofactor of viral integrase. Investigating possible postintegration roles for LEDGF/p75, we find that LEDGF/p75 represses HIV expression in latently infected cells. LEDGF/p75 associated with two proteins involved in the control of gene expression and chromatin structure, Spt6 and Iws1, to form a stable complex. Iws1 plays a role in the establishment of latent infection, whereas Spt6 functions to recruit Iws1 and LEDGF/p75 to the silenced provirus and maintains histone occupancy at the HIV promoter. In latently infected cells, depletion of the complex results in reactivation of HIV expression Altogether, our results indicate that a complex containing LEDGF/p75, Iws1, and Spt6 participates in regulating postintegration steps of HIV latency.

Progress and challenges in the use of latent HIV-1 reactivating agents

Highly active antiretroviral therapy (HAART) can effectively suppress the replication of human immunodeficiency virus-1 (HIV-1) and block disease progression. However, chronic HIV-1 infection remains incurable due to the persistence of a viral reservoir, including the transcriptionally silent provirus in CD4(+) memory T cells and the sanctuary sites that are inaccessible to drugs. Reactivation and the subsequent elimination of latent virus through virus-specific cytotoxic effects or host immune responses are critical strategies for combating the disease. Indeed, a number of latency reactivating reagents have been identified through mechanism-directed approaches and large-scale screening, including: (1) histone deacetylase inhibitors (HDACi); (2) cytokines and chemokines; (3) DNA methyltransferase inhibitors (DNMTI); (4) histone methyltransferase inhibitors (HMTI); (5) protein kinase C (PKC) activators; (6) P-TEFb activators; and (7) unclassified agents, such as disulfram. They have proved to be efficacious in latent cell line models and CD4(+) T lymphocytes from HIV-1-infected patients. This review comprehensively summarizes the recent progress and relative challenges in this field.

Dendritic cell type-specific HIV-1 activation in effector T cells: implications for latent HIV-1 reservoir establishment

BACKGROUND

Latent HIV type I (HIV-1) infections can frequently occur in short-lived proliferating effector T lymphocytes. These latently infected cells could revert into resting T lymphocytes and thereby contribute to the establishment of the long-lived viral reservoir. Monocyte-derived dendritic cells can revert latency in effector T cells in vitro.

METHODS

Here we investigated the latency activation properties of tissue-specific immune cells, including a large panel of dendritic cell subsets, to explore in which body compartments effector T cells are most likely to maintain latent HIV-1 provirus and thus potentially contribute to the long-lived reservoir.

RESULTS

Our results demonstrate that blood or genital tract dendritic cells do not activate latent provirus in effector T cells, whereas gut or lymphoid dendritic cells induce virus production from latently infected effector T cells in our in-vitro model for latency. Toll-like receptor 3-induced interferon production by myeloid dendritic cells abolished the dendritic cells' ability to induce viral gene expression.

CONCLUSIONS

In this study, we show that HIV-1 provirus residing in effector T cells is activated from latency by tissue-specific dendritic cell subsets and other immune cells with remarkably different efficiencies.Our new assay system points to an important, neglected aspect of HIV-1 research: the ability of other immune cells, especially dendritic cells, to differentially affect latency establishment as well as virus reactivation.

An evolutionary role for HIV latency in enhancing viral transmission

HIV latency is the chief obstacle to eradicating HIV but is widely believed to be an evolutionary accident providing no lentiviral fitness advantage. However, findings of latency being "hardwired" into HIV's gene-regulatory circuitry appear inconsistent with latency being an evolutionary accident, given HIV's rapid mutation rate. Here, we propose that latency is an evolutionary "bet-hedging" strategy whose frequency has been optimized to maximize lentiviral transmission by reducing viral extinction during mucosal infections. The model quantitatively fits the available patient data, matches observations of high-frequency latency establishment in cell culture and primates, and generates two counterintuitive but testable predictions. The first prediction is that conventional CD8-depletion experiments in SIV-infected macaques increase latent cells more than viremia. The second prediction is that strains engineered to have higher replicative fitness—via reduced latency—will exhibit lower infectivity in animal-model mucosal inoculations. Therapeutically, the theory predicts treatment approaches that may substantially enhance "activate-and-kill" HIV-cure strategies.

A hardwired HIV latency program

Biological circuits can be controlled by two general schemes: environmental sensing or autonomous programs. For viruses such as HIV, the prevailing hypothesis is that latent infection is controlled by cellular state (i.e., environment), with latency simply an epiphenomenon of infected cells transitioning from an activated to resting state. However, we find that HIV expression persists despite the activated-to-resting cellular transition. Mathematical modeling indicates that HIV's Tat positive-feedback circuitry enables this persistence and strongly controls latency. To overcome the inherent crosstalk between viral circuitry and cellular activation and to directly test this hypothesis, we synthetically decouple viral dependence on cellular environment from viral transcription. These circuits enable control of viral transcription without cellular activation and show that Tat feedback is sufficient to regulate latency independent of cellular activation. Overall, synthetic reconstruction demonstrates that a largely autonomous, viral-encoded program underlies HIV latency—potentially explaining why cell-targeted latency-reversing agents exhibit incomplete penetrance.

Visualization of positive transcription elongation factor b (P-TEFb) activation in living cells

Regulation of transcription elongation by positive transcription elongation factor b (P-TEFb) plays a central role in determining the state of cell activation, proliferation, and differentiation. In cells, P-TEFb exists in active and inactive forms. Its release from the inactive 7SK small nuclear ribonucleoprotein complex is a critical step for P-TEFb to activate transcription elongation. However, no good method exists to analyze this P-TEFb equilibrium in living cells. Only inaccurate and labor-intensive cell-free biochemical assays are currently available. In this study, we present the first experimental system to monitor P-TEFb activation in living cells. We created a bimolecular fluorescence complementation assay to detect interactions between P-TEFb and its substrate, the C-terminal domain of RNA polymerase II. When cells were treated with suberoylanilide hydroxamic acid, which releases P-TEFb from the 7SK small nuclear ribonucleoprotein, they turned green. Other known P-TEFb-releasing agents, including histone deacetylase inhibitors, bromodomain and extraterminal bromodomain inhibitors, and protein kinase C agonists, also scored positive in this assay. Finally, we identified 5'-azacytidine as a new P-TEFb-releasing agent. This release of P-TEFb correlated directly with activation of human HIV and HEXIM1 transcription. Thus, our visualization of P-TEFb activation by fluorescent complementation assay could be used to find new P-TEFb-releasing agents, compare different classes of agents, and assess their efficacy singly and/or in combination.

HIV eradication: combinatorial approaches to activate latent viruses

The concept of eradication of the Human Immune Deficiency Virus (HIV) from infected patients has gained much attention in the last few years. While combination Anti-Retroviral Therapy (c-ART) has been extremely effective in suppressing viral replication, it is not curative. This is due to the presence of a reservoir of latent HIV infected cells, which persist in the presence of c-ART. Recently, pharmaceutical approaches have focused on the development of molecules able to induce HIV-1 replication from latently infected cells in order to render them susceptible to viral cytopathic effects and host immune responses. Alternative pathways and transcription complexes function to regulate the activity of the HIV promoter and might serve as molecular targets for compounds to activate latent HIV. A combined therapy coupling various depressors and activators will likely be the most effective in promoting HIV replication while avoiding pleiotropic effects at the cellular level. Moreover, in light of differences among HIV subtypes and variability in integration sites, the combination of multiple agents targeting multiple pathways will increase likelihood of therapeutic effectiveness and prevent mutational escape. This review provides an overview of the mechanisms that can be targeted to induce HIV activation focusing on potential combinatorial approaches.

Interplay between viral Tat protein and c-Jun transcription factor in controlling LTR promoter activity in different human immunodeficiency virus type I subtypes

HIV-1 transcription depends on cellular transcription factors that bind to sequences in the long-terminal repeat (LTR) promoter. Each HIV-1 subtype has a specific LTR promoter configuration, and minor sequence changes in transcription factor binding sites (TFBSs) or their arrangement can influence transcriptional activity, virus replication and latency properties. Previously, we investigated the proviral latency properties of different HIV-1 subtypes in the SupT1 T cell line. Here, subtype-specific latency and replication properties were studied in primary PHA-activated T lymphocytes. No major differences in latency and replication capacity were measured among the HIV-1 subtypes. Subtype B and AE LTRs were studied in more detail with regard to a putative AP-1 binding site using luciferase reporter constructs. c-Jun, a member of the AP-1 transcription factor family, can activate both subtype B and AE LTRs, but the latter showed a stronger response, reflecting a closer match with the consensus AP-1 binding site. c-Jun overexpression enhanced Tat-mediated transcription of the viral LTR, but in the absence of Tat inhibited basal promoter activity. Thus, c-Jun can exert a positive or negative effect via the AP-1 binding site in the HIV-1 LTR promoter, depending on the presence or absence of Tat.

Epigenetic analysis of HIV-1 proviral genomes from infected individuals: predominance of unmethylated CpG's

Efforts to cure HIV-1 infections aim at eliminating proviral DNA. Integrated DNA from various viruses often becomes methylated de novo and transcriptionally inactivated. We therefore investigated CpG methylation profiles of 55 of 94 CpG's (58.5%) in HIV-1 proviral genomes including ten CpG's in each LTR and additional CpG's in portions of gag, env, nef, rev, and tat genes. We analyzed 33 DNA samples from PBMC's of 23 subjects representing a broad spectrum of HIV-1 disease. In 22 of 23 HIV-1-infected individuals, there were only unmethylated CpG's regardless of infection status. In one long term nonprogressor, however, methylation of proviral DNA varied between 0 and 75% over an 11-year period although the CD4+ counts remained stable. Hence levels of proviral DNA methylation can fluctuate. The preponderance of unmethylated CpG's suggests that proviral methylation is not a major factor in regulating HIV-1 proviral activity in PBMC's. Unmethylated CpG's may play a role in HIV-1 immunopathogenesis.

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.

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.

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.

Dendritic cell-induced activation of latent HIV-1 provirus in actively proliferating primary T lymphocytes

HIV-1 latency remains a formidable barrier towards virus eradication as therapeutic attempts to purge these reservoirs are so far unsuccessful. The pool of transcriptionally silent proviruses is established early in infection and persists for a lifetime, even when viral loads are suppressed below detection levels using anti-retroviral therapy. Upon therapy interruption the reservoir can re-establish systemic infection. Different cellular reservoirs that harbor latent provirus have been described. In this study we demonstrate that HIV-1 can also establish a silent integration in actively proliferating primary T lymphocytes. Co-culturing of these proliferating T lymphocytes with dendritic cells (DCs) activated the provirus from latency. Activation did not involve DC-mediated C-type lectin DC-SIGN signaling or TCR-stimulation but was mediated by DC-secreted component(s) and cell-cell interaction between DC and T lymphocyte that could be inhibited by blocking ICAM-1 dependent adhesion. These results imply that circulating DCs could purge HIV-1 from latency and re-initiate virus replication. Moreover, our data show that viral latency can be established early after infection and supports the idea that actively proliferating T lymphocytes with an effector phenotype contribute to the latent viral reservoir. Unraveling this physiologically relevant purging mechanism could provide useful information for the development of new therapeutic strategies that aim at the eradication of HIV-1 reservoirs.

Cellular and molecular mechanisms involved in the establishment of HIV-1 latency

Latently infected cells represent the major barrier to either a sterilizing or a functional HIV-1 cure. Multiple approaches to reactivation and depletion of the latent reservoir have been attempted clinically, but full depletion of this compartment remains a long-term goal. Compared to the mechanisms involved in the maintenance of HIV-1 latency and the pathways leading to viral reactivation, less is known about the establishment of latent infection. This review focuses on how HIV-1 latency is established at the cellular and molecular levels. We first discuss how latent infection can be established following infection of an activated CD4 T-cell that undergoes a transition to a resting memory state and also how direct infection of a resting CD4 T-cell can lead to latency. Various animal, primary cell, and cell line models also provide insights into this process and are discussed with respect to the routes of infection that result in latency. A number of molecular mechanisms that are active at both transcriptional and post-transcriptional levels have been associated with HIV-1 latency. Many, but not all of these, help to drive the establishment of latent infection, and we review the evidence in favor of or against each mechanism specifically with regard to the establishment of latency. We also discuss the role of immediate silent integration of viral DNA versus silencing of initially active infections. Finally, we discuss potential approaches aimed at limiting the establishment of latent infection.

Quantitation of HIV-1 DNA with a sensitive TaqMan assay that has broad subtype specificity

The increasing diversity of HIV-1 isolates makes virus quantitation challenging, especially when diverse isolates co-circulate in a geographical area. Measuring the HIV-1 DNA levels in cells has become a valuable practical tool for fundamental and clinical research. A quantitative HIV-1 DNA assay was developed based on TaqMan(®) technology. Primers that target the highly conserved LTR region were designed to detect a broad array of HIV-1 variants, including viral isolates from many subtypes, with high sensitivity. Introduction of a pre-amplification step prior to the TaqMan(®) reaction allowed the specific amplification of fully reverse transcribed viral DNA. Execution of the pre-amplification step with a second primer set enables for the exclusive quantitation of the 2-LTR circular HIV-1 DNA form.

Proteomic analysis of HIV-T cell interaction: an update

This mini-review summarizes techniques applied in, and results obtained with, proteomic studies of human immunodeficiency virus type 1 (HIV-1)–T cell interaction. Our group previously reported on the use of two-dimensional differential gel electrophoresis (2D-DIGE) coupled to matrix assisted laser-desorption time of flight peptide mass fingerprint analysis, to study T cell responses upon HIV-1 infection. Only one in three differentially expressed proteins could be identified using this experimental setup. Here we report on our latest efforts to test models generated by this data set and extend its analysis by using novel bioinformatic algorithms. The 2D-DIGE results are compared with other studies including a pilot study using one-dimensional peptide separation coupled to MS^E, a novel mass spectrometric approach. It can be concluded that although the latter method detects fewer proteins, it is much faster and less labor intensive. Last but not least, recent developments and remaining challenges in the field of proteomic studies of HIV-1 infection and proteomics in general are discussed.

A doxycycline-dependent human immunodeficiency virus type 1 replicates in vivo without inducing CD4+ T-cell depletion

A novel genetic approach for the control of virus replication was used for the design of a conditionally replicating human immunodeficiency virus (HIV) variant, HIV-rtTA. HIV-rtTA gene expression and virus replication are strictly dependent on the presence of a non-toxic effector molecule, doxycycline (dox), and thus can be turned on and off at will in a graded and reversible manner. The in vivo replication capacity, pathogenicity and genetic stability of this HIV-rtTA variant were evaluated in a humanized mouse model of haematopoiesis that harbours lymphoid and myeloid components of the human immune system (HIS). Infection of dox-fed BALB Rag/γc HIS (BRG-HIS) mice with HIV-rtTA led to the establishment of a productive infection without CD4(+) T-cell depletion. The virus did not show any sign of escape from dox control for up to 10 weeks after the onset of infection. No reversion towards a functional Tat-transactivating responsive (TAR) RNA element axis was observed, confirming the genetic stability of the HIV-rtTA variant in vivo. These results demonstrate the proof of concept that HIV-rtTA replicates efficiently in vivo. HIV-rtTA is a promising tool for fundamental research to study virus-host interactions in vivo in a controlled fashion.

Latency profiles of full length HIV-1 molecular clone variants with a subtype specific promoter

Background

HIV-1 transcription initiation depends on cellular transcription factors that bind to promoter sequences in the Long Terminal Repeat (LTR). Each HIV-1 subtype has a specific LTR promoter configuration and even minor sequence changes in the transcription factor binding sites (TFBS) or their arrangement can impact transcriptional activity. Most latency studies have focused on HIV-1 subtype B strains, and the degree to which LTR promoter variation contributes to differences in proviral latency is therefore largely unknown. Latency differences may influence establishment and size of viral reservoirs as well as the possibility to clear the virus by therapeutic intervention.

Results

We investigated the proviral transcriptional latency properties of different HIV-1 subtypes as their LTRs have unique assemblies of transcription factor binding sites. We constructed recombinant viral genomes with the subtype-specific promoters inserted in the common backbone of the subtype B LAI isolate. The recombinant viruses are isogenic, except for the core promoter region that encodes all major TFBS, including NFκB and Sp1 sites. We developed and optimized an assay to investigate HIV-1 proviral latency in T cell lines. Our data show that the majority of HIV-1 infected T cells only start viral gene expression after TNFα activation.

Conclusions

There were no gross differences among the subtypes, both in the initial latency level and the activation response, except for subtype AE that combines an increased level of basal transcription with a reduced TNFα response. This subtype AE property is related to the presence of a GABP instead of NFκB binding site in the LTR.

Maintenance of CD4+ T-cell memory and HIV persistence: keeping memory, keeping HIV

PURPOSE OF REVIEW

The present review summarizes the current challenges for the design of new therapeutic strategies toward HIV eradication in individuals receiving suppressive highly active antiretroviral therapy (HAART). We will focus on the experimental evidence suggesting that immunological mechanisms involved in the generation and maintenance of memory CD4+ T cells are also responsible for the establishment and persistence of a stable reservoir for HIV.

RECENT FINDINGS

Recent studies performed on clinical samples obtained from virally suppressed HIV-infected individuals indicate that T-cell survival and homeostatic proliferation, two major mechanisms involved in the maintenance of immunological memory, contribute to the persistence of latently infected memory CD4+ T cells. Thus, the long lifespan characteristic of the HIV reservoir is likely a consequence of the capacity of the immune system to generate and maintain memory CD4+ T cells for a long period.

SUMMARY

These findings suggest that strategies aimed at reducing the pool of latently infected cells should interfere with the survival pathways responsible for the long-term maintenance of memory CD4+ T cells. Because memory CD4+ T cells are critical for appropriate immune defense, targeted approaches are needed to interfere only with the long-term survival of discrete fractions of memory T cells carrying proviral DNA.

Control of HIV replication in astrocytes by a family of highly conserved host proteins with a common Rev-interacting domain (Risp)

OBJECTIVE

In human astrocytes, restriction of HIV replication involves inhibition of HIV Rev activity. We previously identified a Rev-interacting human protein fragment (16.4.1) that can reduce Rev activity. The 16.4.1 sequence is contained in a group of highly similar host cell proteins, which we call the Risp family. Here we investigate whether the Risp family is connected to HIV replication in astrocytes.

METHODS

Cell/tissue lysates were analyzed for Risp expression by western blot with various anti-Risp antibodies. The interaction of astrocytic Risp members with Rev was investigated by affinity chromatography. Astrocytes were transfected with expression plasmids containing cDNAs encoding full-length Risp or the isolated 16.4.1 region for Risp overexpression or with siRNAs designed for Risp knock-down. Rev activity was investigated with a Rev-reporter assay. RNA levels were quantified by real-time RT-PCR, HIV Gag levels by p24ELISA.

RESULTS

Expression of the Risp family was demonstrated in human brain tissues and astrocytes. Astrocytes were shown to produce Risp family members that interact with Rev. Production of HIV Gag proteins and Rev-dependent RNAs in persistently infected astrocytes increased upon Risp knock-down and decreased upon Risp overexpression. Risp knock-down increased Rev activity and raised proportions of Rev proteins in the nucleus of astrocytes.

CONCLUSION

Our results link the Risp family to restriction of HIV production and inhibition of Rev activity in astrocytes. We conclude that the Risp family represents a novel family of host factors that can control HIV replication and may be important for the containment of HIV infection in brain reservoirs.

Cell line-dependent variability in HIV activation employing DNMT inhibitors

Long-lived reservoirs of Human Immunodeficiency Virus (HIV) latently infected cells present the main barrier to a cure for HIV infection. Much interest has focused on identifying strategies to activate HIV, which would be used together with antiretrovirals to attack reservoirs. Several HIV activating agents, including Tumor Necrosis Factor alpha (TNFα) and other agents that activate via NF-kB are not fully effective in all latent infection models due to epigenetic restrictions, such as DNA methylation and the state of histone acetylation. DNA methyltransferases (DNMT) inhibitors like 5-aza-2'deoxycytidine (Aza-CdR) and histone deacetylase (HDAC) inhibitors like Trichostatin A (TSA) have been proposed as agents to enhance reactivation and have shown activity in model systems. However, it is not clear how the activities of DNMT and HDAC inhibitors range across different latently infected cell lines, potential models for the many different latently infected cells within an HIV patient. We determined HIV activation following treatment with TNFα, TSA and Aza-CdR across a range of well known latently infected cell lines. We assessed the activity of these compounds in four different Jurkat T cell-derived J-Lat cell lines (6.3, 8.4, 9.2 and 10.6), which have a latent HIV provirus in which GFP replaces Nef coding sequence, and ACH-2 and J1.1 (T cell-derived), and U1 (promonocyte-derived) cell lines with full-length provirus. We found that Aza-CdR plus TNFα activated HIV at least twice as well as TNFα alone for almost all J-Lat cells, as previously described, but not for J-Lat 10.6, in which TNFα plus Aza-CdR moderately decreased activation compared to TNFα alone. Surprisingly, a much greater reduction of TNFα-stimulated activation with Aza-CdR was detected for ACH-2, J1.1 and U1 cells. Reaching the highest reduction in U1 cells with a 75% reduction. Interestingly, Aza-CdR not only decreased TNFα induction of HIV expression in certain cell lines, but also decreased activation by TSA. Since DNMT inhibitors reduce the activity of provirus activators in some HIV latently infected cell lines the use of epigenetic modifying agents may need to be carefully optimized if they are to find clinical utility in therapies aimed at attacking latent HIV reservoirs.

A lentiviral vector that activates latent human immunodeficiency virus-1 proviruses by the overexpression of tat and that kills the infected cells

Despite the efficient HIV-1 replication blockage achieved with current highly active antiretroviral therapy (HAART) therapies, HIV-1 persists in the body and survives in a latent state that can last for the entire life of the patient. A long-lived reservoir of latently infected CD4(+) memory T cells represents the most important sanctuary for the virus and the greatest obstacle for viral eradication. In this work, we present an initial step toward a gene therapy approach aimed at the activation of latent provirus to induce the death of latently infected T cells. Latent HIV-1 infection is characterized by the failure of viral gene expression as a consequence of uninitiated or aborted transcription. We have constructed an HIV-1-based lentiviral vector (p5p53RTAT3) that expresses the viral trans-activating protein Tat in a drug-regulated manner and p53 in a Rev-dependent manner. We have demonstrated that the Tat-expressed protein from p5p53RTAT3 vector reactivates latent HIV-1 proviruses in J1.1 and ACH-2 cell lines and promotes p53-induced apoptosis in the presence of Rev. Our system was able to trigger the trans-activation of the provirus 5' long terminal repeat (LTR), stimulate the expression of the Rev protein from a tat-defective provirus, and provoke apoptosis selectively in the cells transfected with a tat-defective HIV-1 provirus in contrast to those with no HIV-1 provirus. However, the Rev-dependent p53 killing of latently infected cells was not effective enough for complete elimination of the awakened HIV-1 viruses. In summary, we have developed a vector system that is efficient in activating latent HIV-1 proviruses but that needs further improvement to kill infected cells.

Combinatorial latency reactivation for HIV-1 subtypes and variants

The eradication of HIV-1 will likely require novel clinical approaches to purge the reservoir of latently infected cells from a patient. We hypothesize that this therapy should target a wide range of latent integration sites, act effectively against viral variants that have acquired mutations in their promoter regions, and function across multiple HIV-1 subtypes. By using primary CD4(+) and Jurkat cell-based in vitro HIV-1 latency models, we observe that single-agent latency reactivation therapy is ineffective against most HIV-1 subtypes. However, we demonstrate that the combination of two clinically promising drugs-namely, prostratin and suberoylanilide hydroxamic acid (SAHA)-overcomes the limitations of single-agent approaches and can act synergistically for many HIV-1 subtypes, including A, B, C, D, and F. Finally, by identifying the proviral integration position of latent Jurkat cell clones, we demonstrate that this drug combination does not significantly enhance the expression of endogenous genes nearest to the proviral integration site, indicating that its effects may be selective.

Intracellular transactivation of HIV can account for the decelerating decay of virus load during drug therapy

Linking the intracellular transactivation circuit of HIV into a virus dynamics model can account for activation of infected cells and reversion into latency.

We hypothesize that the activation of latently infected cells is governed by the basal transcription rate of the integrated provirus rather than through extracellular stimuli.

This systems approach to modelling virus dynamics offers a promising framework to infer the extracellular dynamics of cell populations from their intracellular reaction networks.

The viral reservoir of latently infected cells is considered to be one of the major barriers for eradicating the virus from patients infected with HIV. During prolonged antiretroviral therapy, it has been shown that the pool of latently infected cells decays very slowly and at a decreasing rate. The underlying mechanisms causing this decelerating decay are still unclear (Lassen et al, 2004a, 2004b; Han et al, 2007). A recent study has shown that HIV can exhibit a switch-like behavior where infected cells can either be activated or become resting in a latent state (Weinberger et al, 2005). To investigate the effect of this switch-like behavior on the viral infection dynamics, we devise a new model that links the intracellular transactivation of the virus with the extracellular virus dynamics (Box 1). The model can explain the typical decelerating decay of HIV that is observed during antiretroviral therapy. We find that the activation of latently infected cells is governed by the basal transcription rate of the inserted provirus. Therefore, our analysis suggests that increasing the basal transcription rate of the HIV provirus could serve as a new therapeutic intervention for eradicating the pool of latently infected cells. In addition, our systems approach to modeling virus dynamics offers a promising framework for inferring the extracellular dynamics of cell populations from their intracellular reaction networks.

Basic virus dynamics models have been essential in understanding quantitative issues of HIV replication. However, several parts of the viral life cycle remain elusive. One of the most critical steps is the start of viral transcription, which is governed by the regulatory protein trans-activator of transcription (Tat) that induces a positive feedback loop. It has been shown that this feedback loop can alternate between two states leading to a transient activation of viral transcription. Using Monte Carlo simulations, we integrate the transactivation circuit into a new virus dynamics model having an age-dependent transactivation rate and reversion into latency. The cycling of infected cells between an activated and latent state results in the typical decelerating decay of virus load following therapy. Further, we hypothesize that the activation of latently infected cells is governed by the basal transcription rate of the integrated provirus rather than the intra- or extracellular environment. Finally, our systems approach to modeling virus dynamics offers a promising framework to infer the extracellular dynamics of cell populations from their intracellular reaction networks.

Cell-type-specific proteome and interactome: using HIV-1 Tat as a test case

HIV-1 is a small retrovirus that wreaks havoc on the human immune system. It is a puzzle to the scientific community how a virus that encodes only nine proteins can take complete control of its host and redirect the cell to complete replication or maintain latency when necessary. One way to explain the control elicited by HIV-1 is through numerous protein partners that exist between viral and host proteins, allowing HIV-1 to be intimately involved in virtually every aspect of cellular biology. In addition, we postulate that the complexity exerted by HIV-1 can not merely be explained by the large number of protein-protein interactions documented in the literature but, rather, cell-type-specific interactions and post-translational modifications of viral proteins must be taken into account. We use HIV-1 Tat and its influence on viral transcription as an example of cell-type-specific complexity. The influence of post-translational modifications (acetylation and methylation), as well as subcellular localization on Tat binding partners, is also discussed.

T-cell dysfunction in HIV-1 infection: targeting the inhibitors

Since AIDS emerged almost three decades ago, there have been considerable advances in the field of antiretroviral chemotherapy for those chronically infected with HIV-1. However, this therapy is noncurative and as our understanding of HIV-1 immunopathogenesis increases, it is becoming apparent that further therapeutic interventions are required to reverse the devastating effects of HIV-1 infection worldwide. While viral clearance remains the principle goal of HIV-1 treatment, this article describes immunotherapeutic options that target the immunological effects of the virus, to reduce its presence in the body and counteract viral-induced T-cell dysfunction and inhibition. Such approaches may augment existing antiretroviral therapy to overturn virus-induced T-cell anergy in the infected host, improving levels of immune control that reduce viremia and decrease the rate of transmission.

Molecular control of HIV-1 postintegration latency: implications for the development of new therapeutic strategies

The persistence of HIV-1 latent reservoirs represents a major barrier to virus eradication in infected patients under HAART since interruption of the treatment inevitably leads to a rebound of plasma viremia. Latency establishes early after infection notably (but not only) in resting memory CD4⁺ T cells and involves numerous host and viral trans-acting proteins, as well as processes such as transcriptional interference, RNA silencing, epigenetic modifications and chromatin organization. In order to eliminate latent reservoirs, new strategies are envisaged and consist of reactivating HIV-1 transcription in latently-infected cells, while maintaining HAART in order to prevent de novo infection. The difficulty lies in the fact that a single residual latently-infected cell can in theory rekindle the infection. Here, we review our current understanding of the molecular mechanisms involved in the establishment and maintenance of HIV-1 latency and in the transcriptional reactivation from latency. We highlight the potential of new therapeutic strategies based on this understanding of latency. Combinations of various compounds used simultaneously allow for the targeting of transcriptional repression at multiple levels and can facilitate the escape from latency and the clearance of viral reservoirs. We describe the current advantages and limitations of immune T-cell activators, inducers of the NF-κB signaling pathway, and inhibitors of deacetylases and histone- and DNA- methyltransferases, used alone or in combinations. While a solution will not be achieved by tomorrow, the battle against HIV-1 latent reservoirs is well- underway.

Regulation of HIV-1 transcription at 3% versus 21% oxygen concentration

HIV transcription is induced by the HIV-1 Tat protein, in concert with cellular co-factors including CDK9, CDK2, NF-kappaB, and others. The cells of most of the body's organs are exposed to approximately 3-6% oxygen, but most in vitro studies of HIV replication are conducted at 21% oxygen. We hypothesized that activities of host cell factors involved in HIV-1 replication may differ at 3% versus 21% O(2), and that such differences may affect HIV-1 replication. Here we show that Tat-induced HIV-1 transcription was reduced at 3% O(2) compared to 21% O(2). HIV-1 replication was also reduced in acutely or chronically infected cells cultured at 3% O(2) compared to 21% O(2). This reduction was not due the decreased cell growth or increased cellular toxicity and also not due to the induction of hypoxic response. At 3% O(2), the activity of CDK9/cyclin T1 was inhibited and Sp1 activity was reduced, whereas the activity of other host cell factors such as CDK2 or NF-kappaB was not affected. CDK9-specific inhibitor ARC was much less efficient at 3% compared to 21% O(2) and also expression of CDK9/cyclin T1-dependent IkappaB inhibitor alpha was repressed. Our results suggest that lower HIV-1 transcription at 3% O(2) compared to 21% O(2) may be mediated by lower activity of CDK9/cyclin T1 and Sp1 at 3% O(2) and that additional host cell factors such as CDK2 and NF-kappaB might be major regulators of HIV-1 transcription at low O(2) concentrations.

The utilization of humanized mouse models for the study of human retroviral infections

The development of novel techniques and systems to study human infectious diseases in both an in vitro and in vivo settings is always in high demand. Ideally, small animal models are the most efficient method of studying human afflictions. This is especially evident in the study of the human retroviruses, HIV-1 and HTLV-1, in that current simian animal models, though robust, are often expensive and difficult to maintain. Over the past two decades, the construction of humanized animal models through the transplantation and engraftment of human tissues or progenitor cells into immunocompromised mouse strains has allowed for the development of a reconstituted human tissue scaffold in a small animal system. The utilization of small animal models for retroviral studies required expansion of the early CB-17 scid/scid mouse resulting in animals demonstrating improved engraftment efficiency and infectivity. The implantation of uneducated human immune cells and associated tissue provided the basis for the SCID-hu Thy/Liv and hu-PBL-SCID models. Engraftment efficiency of these tissues was further improved through the integration of the non-obese diabetic (NOD) mutation leading to the creation of NODSCID, NOD/Shi-scid IL2rγ^-/-, and NOD/SCID β2-microglobulin^null animals. Further efforts at minimizing the response of the innate murine immune system produced the Rag2^-/-γ_c^-/- model which marked an important advancement in the use of human CD34+ hematopoietic stem cells. Together, these animal models have revolutionized the investigation of retroviral infections in vivo.

Epigenetic regulation of HIV-1 latency by cytosine methylation

Human immunodeficiency virus type 1 (HIV-1) persists in a latent state within resting CD4⁺ T cells of infected persons treated with highly active antiretroviral therapy (HAART). This reservoir must be eliminated for the clearance of infection. Using a cDNA library screen, we have identified methyl-CpG binding domain protein 2 (MBD2) as a regulator of HIV-1 latency. Two CpG islands flank the HIV-1 transcription start site and are methylated in latently infected Jurkat cells and primary CD4⁺ T cells. MBD2 and histone deacetylase 2 (HDAC2) are found at one of these CpG islands during latency. Inhibition of cytosine methylation with 5-aza-2′deoxycytidine (aza-CdR) abrogates recruitment of MBD2 and HDAC2. Furthermore, aza-CdR potently synergizes with the NF-κB activators prostratin or TNF-α to reactivate latent HIV-1. These observations confirm that cytosine methylation and MBD2 are epigenetic regulators of HIV-1 latency. Clearance of HIV-1 from infected persons may be enhanced by inclusion of DNA methylation inhibitors, such as aza-CdR, and NF-κB activators into current antiviral therapies.

Current drug therapies inhibit replication of the human immunodeficiency virus (HIV). In patients undergoing these therapies, the amount of HIV is reduced to an undetectable level and HIV-related disease subsides. However, stopping antiviral drug therapy results in the quick return of HIV and of disease. One reason for this is latently infected cells, in which virus replication is temporarily halted. When drug therapy is stopped, virus from these latently infected cells can resume infection and spread to other cells in the patient, resulting in the return of disease. Here, we demonstrate that one mechanism of latency is DNA methylation, in which chemical groups called methyl groups are added to HIV DNA. We also identify a host protein called methyl-CpG binding domain protein 2 (MBD2) that binds methylated HIV DNA and is an important mediator of latency. Furthermore, we demonstrate that a drug that inhibits DNA methylation potently reactivates latent HIV. Novel strategies to eliminate or reduce the latent reservoir are necessary. Our findings may prove useful in the development of novel therapies to efficiently reactivate latent HIV-1, thus making it susceptible to current drug therapies.

"Shock and kill" effects of class I-selective histone deacetylase inhibitors in combination with the glutathione synthesis inhibitor buthionine sulfoximine in cell line models for HIV-1 quiescence

Latently infected, resting memory CD4+ T cells and macrophages represent a major obstacle to the eradication of HIV-1. For this purpose, "shock and kill" strategies have been proposed (activation of HIV-1 followed by stimuli leading to cell death). Histone deacetylase inhibitors (HDACIs) induce HIV-1 activation from quiescence, yet class/isoform-selective HDACIs are needed to specifically target HIV-1 latency. We tested 32 small molecule HDACIs for their ability to induce HIV-1 activation in the ACH-2 and U1 cell line models. In general, potent activators of HIV-1 replication were found among non-class selective and class I-selective HDACIs. However, class I selectivity did not reduce the toxicity of most of the molecules for uninfected cells, which is a major concern for possible HDACI-based therapies. To overcome this problem, complementary strategies using lower HDACI concentrations have been explored. We added to class I HDACIs the glutathione-synthesis inhibitor buthionine sulfoximine (BSO), in an attempt to create an intracellular environment that would facilitate HIV-1 activation. The basis for this strategy was that HIV-1 replication decreases the intracellular levels of reduced glutathione, creating a pro-oxidant environment which in turn stimulates HIV-1 transcription. We found that BSO increased the ability of class I HDACIs to activate HIV-1. This interaction allowed the use of both types of drugs at concentrations that were non-toxic for uninfected cells, whereas the infected cell cultures succumbed more readily to the drug combination. These effects were associated with BSO-induced recruitment of HDACI-insensitive cells into the responding cell population, as shown in Jurkat cell models for HIV-1 quiescence. The results of the present study may contribute to the future design of class I HDACIs for treating HIV-1. Moreover, the combined effects of class I-selective HDACIs and the glutathione synthesis inhibitor BSO suggest the existence of an Achilles' heel that could be manipulated in order to facilitate the "kill" phase of experimental HIV-1 eradication strategies.

HIV interactions with monocytes and dendritic cells: viral latency and reservoirs

HIV is a devastating human pathogen that causes serious immunological diseases in humans around the world. The virus is able to remain latent in an infected host for many years, allowing for the long-term survival of the virus and inevitably prolonging the infection process. The location and mechanisms of HIV latency are under investigation and remain important topics in the study of viral pathogenesis. Given that HIV is a blood-borne pathogen, a number of cell types have been proposed to be the sites of latency, including resting memory CD4⁺ T cells, peripheral blood monocytes, dendritic cells and macrophages in the lymph nodes, and haematopoietic stem cells in the bone marrow. This review updates the latest advances in the study of HIV interactions with monocytes and dendritic cells, and highlights the potential role of these cells as viral reservoirs and the effects of the HIV-host-cell interactions on viral pathogenesis.

Oral complications of HIV disease

Oral lesions are among the early signs of HIV infection and can predict its progression to acquired immunodeficiency syndrome (AIDS). A better understanding of the oral manifestations of AIDS in both adults and children has implications for all health care professionals. The knowledge of such alterations would allow for early recognition of HIV-infected patients. The present paper reviews epidemiology, relevant aspects of HIV infection related to the mouth in both adults and children, as well as current trends in antiretroviral therapy and its connection with orofacial manifestations related to AIDS.

The 2008 Retrovirology Prize: Ben Berkhout and his RNA world

Ben Berkhout wins the 2008 Retrovirology Prize.

The aftermath of the Merck's HIV vaccine trial

The recently released results of the Merck's Phase IIb "test-of concept" vaccine trials have shown no protection from HIV-1 infection in the vaccinated group compared with a control group vaccinated with placebo. The study was designed to test the Merck's MRKAd5 trivalent candidate vaccine. The vaccine formulation was expected to stimulate a HIV-specific T cell immune response and to either prevent infection, or to reduce the levels of the viral load in vaccinated subjects. Upon the first evaluation of the interim data, the independent Data and Safety Monitoring Board (DSMB) underscored no protection from HIV-1 infection in the vaccine-inoculated volunteers compared with the control group; accordingly, the vaccine trial was stopped. This disappointing outcome warrants a critical analysis of the current vaccine studies and calls for a renewed effort toward a rational design of novel immunogens to be tested in large primate trials.