Nuclear factor of activated T cells is a driving force for preferential productive HIV-1 infection of CD45RO-expressing CD4+ T cells

Balance between Retroviral Latency and Transcription: Based on HIV Model

The representative of the Lentivirus genus is the human immunodeficiency virus type 1 (HIV-1), the causative agent of acquired immunodeficiency syndrome (AIDS). To date, there is no cure for AIDS because of the existence of the HIV-1 reservoir. HIV-1 infection can persist for decades despite effective antiretroviral therapy (ART), due to the persistence of infectious latent viruses in long-lived resting memory CD4+ T cells, macrophages, monocytes, microglial cells, and other cell types. However, the biology of HIV-1 latency remains incompletely understood. Retroviral long terminal repeat region (LTR) plays an indispensable role in controlling viral gene expression. Regulation of the transcription initiation plays a crucial role in establishing and maintaining a retrovirus latency. Whether and how retroviruses establish latency and reactivate remains unclear. In this article, we describe what is known about the regulation of LTR-driven transcription in HIV-1, that is, the cis-elements present in the LTR, the role of LTR transcription factor binding sites in LTR-driven transcription, the role of HIV-1-encoded transactivator protein, hormonal effects on virus transcription, impact of LTR variability on transcription, and epigenetic control of retrovirus LTR. Finally, we focus on a novel clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/dCas9)-based strategy for HIV-1 reservoir purging.

Ets-2 Acts As a Transcriptional Repressor of the Human Immunodeficiency Virus Type 1 through Binding to a Repressor-Activator Target Sequence of 5'-LTR

HIV-1 is transcriptionally active in activated T helper (Th)-cells and inactive in naive or resting memory Th-cells. Ets-2 is a preinduction transcriptional repressor of the IL-2 gene in naive Th-cells and a candidate transcriptional repressor of HIV-1 in the same cells, because the −279 to −250 upstream region of HIV-1-LTR [repressor–activator target sequence (RATS)], that participates in HIV-1-LTR transcriptional silencing, encompasses the AAGGAG Ets-2 binding site. In this proof of concept study, we investigated whether Ets-2 represses the expression of HIV-1. To assess whether Ets-2 can repress HIV-1 transcriptional activation acting through RATS, we transfected Jurkat cells with an Ets-2 overexpression plasmid (pCDNA3-ets-2) or Ets-2 silencing plasmids (ets-2-shRNA) and, as target genes, plasmids carrying the whole HIV-1-LTR sequence (HIV-1-LTR-CAT) or two copies of the RATS sequence (2× RATS-CAT) or a point mutation in the Ets-2 binding site (2× mutantRATS-CAT) or CMV-CAT (control). Ets-2 overexpression resulted in a significant reduction of HIV-1-LTR-CAT and 2× RATS-CAT activities in stimulated cells, but not of the 2× mutantRATS-CAT or CMV-CAT. Ets-2 silencing led to increased activities of HIV-1-LTR-CAT and 2× RATS-CAT in unstimulated cells, but had no effect on the activities of 2× mutantRATS-CAT and CMV-CAT. To assess Ets-2 binding to HIV-1-LTR–RATS in naive Th-cells, we isolated naive Th-cell nuclear proteins and passed them through an Ets-2 antibody column; electrophoretic mobility shift assays were performed using an RATS probe mixed with consecutive protein eluates. Ets-2 bound to the HIV-1-LTR–RATS in a dose-dependent manner. To assess Ets-2 binding to RATS in vivo, Jurkat cells were transfected with 2× RATS-CAT and stained for the Ets-2 protein and the RATS sequence by combining immunofluorescence and fluorescence in situ hybridization techniques. In unstimulated cells, Ets-2 bound to RATS, whereas no binding was observed in stimulated cells. To test for RATS specificity, the same experiments were performed with 2× mutantRATS-CAT, and no binding of Ets-2 was observed. The results were corroborated by chromatin immunoprecipitation assays performed with the same cells. Our results show that Ets-2 is a transcriptional repressor of HIV-1. Repression of HIV-LTR-RATS mediated by Ets-2 may account for the low-level transcription and replication of HIV-1 in naive Th-cells, and contribute to the viral latency and maintenance of viral reservoirs in patients, despite long-term therapy.

The Immune Interaction between HIV-1 Infection and Mycobacterium tuberculosis

The modulation of tuberculosis (TB)-induced immunopathology caused by human immunodeficiency virus (HIV)-1 coinfection remains incompletely understood but underlies the change seen in the natural history, presentation, and prognosis of TB in such patients. The deleterious combination of these two pathogens has been dubbed a "deadly syndemic," with each favoring the replication of the other and thereby contributing to accelerated disease morbidity and mortality. HIV-1 is the best-recognized risk factor for the development of active TB and accounts for 13% of cases globally. The advent of combination antiretroviral therapy (ART) has considerably mitigated this risk. Rapid roll-out of ART globally and the recent recommendation by the World Health Organization (WHO) to initiate ART for everyone living with HIV at any CD4 cell count should lead to further reductions in HIV-1-associated TB incidence because susceptibility to TB is inversely proportional to CD4 count. However, it is important to note that even after successful ART, patients with HIV-1 are still at increased risk for TB. Indeed, in settings of high TB incidence, the occurrence of TB often remains the first presentation of, and thereby the entry into, HIV care. As advantageous as ART-induced immune recovery is, it may also give rise to immunopathology, especially in the lower-CD4-count strata in the form of the immune reconstitution inflammatory syndrome. TB-immune reconstitution inflammatory syndrome will continue to impact the HIV-TB syndemic.

A novel HIV-1-encoded microRNA enhances its viral replication by targeting the TATA box region

Background

A lot of microRNAs (miRNAs) derived from viral genomes have been identified. Many of them play various important roles in virus replication and virus-host interaction. Cellular miRNAs have been shown to participate in the regulation of HIV-1 viral replication, while the role of viral-encoded miRNAs in this process is largely unknown.

Results

In this report, through a strategy combining computational prediction and deep sequencing, we identified a novel HIV-1-encoded miRNA, miR-H3. MiR-H3 locates in the mRNA region encoding the active center of reverse transcriptase (RT) and exhibits high sequence conservation among different subtypes of HIV-1 viruses. Overexpression of miR-H3 increases viral production and the mutations in miR-H3 sequence significantly impair the viral replication of wildtype HIV-1 viruses, suggesting that it is a replication-enhancing miRNA. MiR-H3 upregulates HIV-1 RNA transcription and protein expression. A serial deletion assay suggests that miR-H3 targets HIV-1 5′ LTR and upregulates the promoter activity. It interacts with the TATA box in HIV-1 5′ LTR and sequence-specifically activates the viral transcription. In addition, chemically-synthesized small RNAs targeting HIV-1 TATA box activate HIV-1 production from resting CD4⁺ T cells isolated from HIV-1-infected patients on suppressive highly active antiretroviral therapy (HAART).

Conclusions

We have identified a novel HIV-1-encoded miRNA which specifically enhances viral production and provide a specific method to activate HIV-1 latency.

T Cell Transcription Factors and Their Impact on HIV Expression

By targeting CD4⁺ effector T cells, HIV has a dramatic impact on the depletion, expansion and function of the different polarized T cell subsets. The maturation of T cell lineages is in part driven by intrinsic transcription factors that potentially influence how efficiently HIV replicates. In this review, we explore whether transcription factors that are required for polarizing T cells influence HIV replication. In particular, we examine provirus transcription as well as the establishment and maintenance of HIV latency. Furthermore, it is suggested these factors may provide novel cell-specific therapeutic strategies for targeting the HIV latent reservoir.

Analysis of multiple cell reservoirs expressing unspliced HIV-1 gag-pol mRNA in patients on antiretroviral therapy

AIMS: Longitudinal percentage change of eight HIV-1 gag-pol mRNA cellular reservoirs from HIV-infected subjects on antiretroviral therapy was ascertained by simultaneous ultrasensitive subpopulation staining/hybridization in situ (SUSHI). MATERIALS #ENTITYSTARTX00026; METHODS: Serial peripheral blood mononuclear cells were taken from three subjects with treatment success, limited response and viral breakthrough plasma viral load (PVL) profiles. SUSHI was carried out on monocytes, macrophages, CD4(+) cells and naive, memory and activated T-cell reservoirs followed with broad light scatter flow cytometry. RESULTS: All gag-pol(+) reservoirs declined in the treatment success patient and similar to PVL. Only some gag-pol(+) reservoirs responded similarly to PVL for the limited treatment patient, and most gag-pol(+) reservoirs increased 16 weeks prior to PVL breakthrough in the viral breakthrough patient. CONCLUSION: SUSHI measures changes in a wide range of gag-pol(+) reservoirs in response to antiretroviral therapy.

Control of HIV latency by epigenetic and non-epigenetic mechanisms

Intensive antiretroviral therapy successfully suppresses viral replication but is unable to eradicate the virus. HIV persists in a small number of resting memory T cells where HIV has been transcriptionally silenced. This review will focus on recent insights into the HIV transcriptional control mechanisms that provide the biochemical basis for understanding latency. There are no specific repressors of HIV transcription encoded by the virus, instead latency arises when the regulatory feedback mechanism driven by HIV Tat expression is disrupted. Small changes in transcriptional initiation, induced by epigenetic silencing, lead to profound restrictions in Tat levels and force the entry of proviruses into latency. In resting memory T cells, which carry the bulk of the latent viral pool, additional restrictions, especially the limiting cellular levels of the essential Tat cofactor P-TEFb and the transcription initiation factors NF-κB and NFAT ensure that the provirus remains silenced unless the host cell is activated. The detailed understanding of HIV transcription is providing a framework for devising new therapeutic strategies designed to purge the latent viral pool. Importantly, the recognition that there are multiple restrictions imposed on latent proviruses suggest that proviral reactivation will not be achieved when only a single reactivation step is targeted and that any optimal activation strategy will require both removal of epigenetic blocks and the activation of P-TEFb.

Host factors mediating HIV-1 replication

Human immunodeficiency virus type 1(HIV-1) infection is the leading cause of death worldwide in adults attributable to infectious diseases. Although the majority of infections are in sub-Saharan Africa and Southeast Asia, HIV-1 is also a major health concern in most countries throughout the globe. While current antiretroviral treatments are generally effective, particularly in combination therapy, limitations exist due to drug resistance occurring among the drug classes. Traditionally, HIV-1 drugs have targeted viral proteins, which are mutable targets. As cellular genes mutate relatively infrequently, host proteins may prove to be more durable targets than viral proteins. HIV-1 replication is dependent upon cellular proteins that perform essential roles during the viral life cycle. Maraviroc is the first FDA-approved antiretroviral drug to target a cellular factor, HIV-1 coreceptor CCR5, and serves to intercept viral-host protein-protein interactions mediating entry. Recent large-scale siRNA and shRNA screens have revealed over 1000 candidate host factors that potentially support HIV-1 replication, and have implicated new pathways in the viral life cycle. These host proteins and cellular pathways may represent important targets for future therapeutic discoveries. This review discusses critical cellular factors that facilitate the successive steps in HIV-1 replication.

Identification and functional analysis of a second RBF-2 binding site within the HIV-1 promoter

Transcription from the HIV-1 long terminal repeat (LTR) is mediated by numerous host transcription factors. In this study we characterized an E-box motif (RBE1) within the core promoter that was previously implicated in both transcriptional activation and repression. We show that RBE1 is a binding site for the RBF-2 transcription factor complex (USF1, USF2, and TFII-I), previously shown to bind an upstream viral element, RBE3. The RBE1 and RBE3 elements formed complexes of identical mobility and protein constituents in gel shift assays, both with Jurkat T-cell nuclear extracts and recombinant USF/TFII-I. Furthermore, both elements are regulators of HIV-1 expression; mutations in LTR-luciferase reporters and in HIV-1 molecular clones resulted in decreased transcription, virion production, and proviral expression in infected cells. Collectively, our data indicate that RBE1 is a bona fide RBF-2 binding site and that the RBE1 and RBE3 elements are necessary for mediating proper transcription from the HIV-1 LTR.

The molecular biology of HIV latency: breaking and restoring the Tat-dependent transcriptional circuit

PURPOSE OF REVIEW

Despite the remarkable success of intensive antiretroviral drug therapy in blocking the HIV replication, the virus persists in a small number of cells in which HIV has been transcriptionally silenced. This review will focus on recent insights into the HIV transcriptional control mechanisms that provide the biochemical basis for understanding latency.

RECENT FINDINGS

Latency arises when the regulatory feedback mechanism driven by HIV Tat expression is disrupted. Small changes in transcriptional initiation, induced by epigenetic silencing, can lead to restrictions in Tat levels and entry of proviruses into latency. In resting memory T-cells, which carry the bulk of the latent viral pool, additional restrictions limiting cellular levels of the essential Tat cofactor P-TEFb and the transcription initiation factors nuclear factor kappa B and nuclear factor of activated T cells ensure that the provirus remains silenced unless the host cell is activated.

SUMMARY

Strategies to purge the latent proviral pool require nontoxic activator molecules. The multiple restrictions imposed on latent proviruses that need to be overcome suggest that proviral reactivation will not be achieved when only a single reactivation step is targeted but will require both removal of epigenetic blocks and the activation of P-TEFb. Alternatively, new inhibitors that block proviral reactivation could be developed.

Minocycline suppresses activation of nuclear factor of activated T cells 1 (NFAT1) in human CD4+ T cells

Minocycline is a tetracycline family antibiotic that has anti-inflammatory and immunomodulatory properties. These properties have shown promise in the treatment of conditions such as rheumatoid arthritis, Huntington disease, and multiple sclerosis. As lymphocyte activation is involved in the pathogenesis of many of these diseases, T cells are postulated to be a primary target in minocycline therapy. Previous studies have demonstrated attenuation of CD4(+) T cell activation by minocycline, but a specific mechanism has not been elucidated. In this study, we investigated the effect of minocycline on the activity of three key transcription factors regulating CD4(+) T cell activation: NF-κB, AP-1 (activator protein 1), and NFAT (nuclear factor of activated T) cells. Our data demonstrate that minocycline selectively impairs NFAT-mediated transcriptional activation, a result of increased phosphorylation and reduced nuclear translocation of the isoform NFAT1. Minocycline increased the activity of the NFAT kinase GSK3 and decreased intracellular Ca(2+) flux, both of which facilitate NFAT1 phosphorylation. These findings provide a novel mechanism for minocycline induced suppression of CD4(+) T cell activation and may better inform the application of minocycline as an immunomodulatory agent.

The gastrointestinal tract and AIDS pathogenesis

Gastrointestinal disease has been recognized as a major manifestation of human immunodeficiency virus infection since the earliest recognition of acquired immunodeficiency syndrome (AIDS). Originally, these disease manifestations were considered to be sequelae of the immune destruction that characterizes AIDS rather than being central to the pathogenesis of AIDS. Over time, it has become clear that the mucosal immune system in general and the intestinal immune system in particular are central to the pathogenesis of AIDS, with most of the critical events (eg, transmission, viral amplification, CD4+ T-cell destruction) occurring in the gastrointestinal tract. Compared with peripheral blood, these tissues are not easily accessible for analysis and have only begun to be examined in detail recently. In addition, although the resulting disease can progress over years, many critical events happen within the first few weeks of infection, when most patients are unaware that they are infected. Moreover, breakdown of the mucosal barrier and resulting microbial translocation are believed to be major drivers of AIDS progression. In this review, we focus on the interaction between primate lentiviruses and the gastrointestinal tract and discuss how this interaction promotes the pathogenesis of AIDS and drives immune dysfunction and progression to AIDS. This article draws extensively on work done in the nonhuman primate model of AIDS to fill gaps in our understanding of AIDS in humans.

FOXP3 inhibits HIV-1 infection of CD4 T-cells via inhibition of LTR transcriptional activity

FOXP3 is a necessary transcription factor for the development and function of CD4+ regulatory T-cells (Tregs). The role of Tregs in HIV-1 infection remains unclear. Here, we show that expression of FOXP3 in primary human CD4 T-cells significantly inhibits HIV-1 infection. Since FOXP3 inhibits NFAT activity, and NFAT proteins contribute to HIV-1 transcription, we explore a transcriptional repressive function of HIV-1 LTR by FOXP3. Over-expression of FOXP3 in primary CD4 T-cells inhibits wild-type HIV-1 LTR reporter activity, and truncation mutants demonstrate that repression of the LTR by FOXP3 requires the dual proximal NF kappaB/NFAT binding sites. Interestingly, FOXP3 decreases binding of NFAT2 to the HIV-1 LTR in vivo. Furthermore, FOXP3 does not inhibit infection of HIV-1 NL4-3 which is mutated to disrupt transcription factor binding at either proximal NFAT or NF kappaB binding sites. These data suggest that resistance of Tregs to HIV-1 infection is due to inhibition of HIV-1 LTR transcription by FOXP3.

Crystal structure of NFAT bound to the HIV-1 LTR tandem kappaB enhancer element

The host factor, nuclear factor of activated T-cells (NFAT), regulates the transcription and replication of HIV-1. Here, we have determined the crystal structure of the DNA binding domain of NFAT bound to the HIV-1 long terminal repeat (LTR) tandem kappaB enhancer element at 3.05 A resolution. NFAT binds as a dimer to the upstream kappaB site (Core II), but as a monomer to the 3' end of the downstream kappaB site (Core I). The DNA shows a significant bend near the 5' end of Core I, where a lysine residue from NFAT bound to the 3' end of Core II inserts into the minor groove and seems to cause DNA bases to flip out. Consistent with this structural feature, the 5' end of Core I become hypersensitive to dimethylsulfate in the in vivo footprinting upon transcriptional activation of the HIV-1 LTR. Our studies provide a basis for further investigating the functional mechanisms of NFAT in HIV-1 transcription and replication.

WFDC1/ps20 is a novel innate immunomodulatory signature protein of human immunodeficiency virus (HIV)-permissive CD4+ CD45RO+ memory T cells that promotes infection by upregulating CD54 integrin expression and is elevated in HIV type 1 infection

Understanding why human immunodeficiency virus (HIV) preferentially infects some CD4(+) CD45RO(+) memory T cells has implications for antiviral immunity and pathogenesis. We report that differential expression of a novel secreted factor, ps20, previously implicated in tissue remodeling, may underlie why some CD4 T cells are preferentially targeted. We show that (i) there is a significant positive correlation between endogenous ps20 mRNA in diverse CD4 T-cell populations and in vitro infection, (ii) a ps20(+) permissive cell can be made less permissive by antibody blockade- or small-interference RNA-mediated knockdown of endogenous ps20, and (iii) conversely, a ps20(low) cell can be more permissive by adding ps20 exogenously or engineering stable ps20 expression by retroviral transduction. ps20 expression is normally detectable in CD4 T cells after in vitro activation and interleukin-2 expansion, and such oligoclonal populations comprise ps20(positive) and ps20(low/negative) isogenic clones at an early differentiation stage (CD45RO(+)/CD25(+)/CD28(+)/CD57(-)). This pattern is altered in chronic HIV infection, where ex vivo CD4(+) CD45RO(+) T cells express elevated ps20. ps20 promoted HIV entry via fusion and augmented CD54 integrin expression; both of these effects were reversed by anti-ps20 antibody. We therefore propose ps20 to be a novel signature of HIV-permissive CD4 T cells that promotes infection in an autocrine and paracrine manner and that HIV has coopted a fundamental role of ps20 in promoting cell adhesion for its benefit. Disrupting the ps20 pathway may therefore provide a novel anti-HIV strategy.

TLR2 Signaling Renders Quiescent Naive and Memory CD4+T Cells More Susceptible to Productive Infection with X4 and R5 HIV-Type 1

It has been recently demonstrated that circulating microbial products are responsible for a systemic immune activation in individuals infected with HIV-type 1. Bacterial products carry structural conserved motifs recognized by TLRs. Some TLR members are expressed in primary human CD4+ T cells but the precise functional role played by these pattern recognition receptors is still imprecise. In this study, we report that engagement of TLR2 in quiescent naive and memory CD4+ T cells leads to the acquisition of an effector-like phenotype. Interestingly, engagement of TLR2 renders both cell subsets more susceptible to productive infection with X4 virions and a higher virus production was seen with R5 viruses. It can be proposed that exposure of resting CD4+ T cells to pathogen-derived products that can engage TLR2 induces the acquisition of an effector-like phenotype in naive and memory CD4+ T lymphocytes, a phenomenon that might result in an acceleration of virus replication, immune dysregulation, and HIV-type 1-mediated disease progression.

Regulation of HIV-1 latency by T-cell activation

HIV-infected patients harbor approximately 10(5)-10(6) memory CD4 T-cells that contain fully integrated but transcriptionally silent HIV proviruses. While small in number, these latently infected cells form a drug-insensitive reservoir that importantly contributes to the life-long persistence of HIV despite highly effective antiviral therapy. In tissue culture, latent HIV proviruses can be activated when their cellular hosts are exposed to select proinflammatory cytokines or their T-cell receptors are ligated. However, due to a lack of potency and/or dose-limiting toxicity, attempts to purge virus from this latent reservoir in vivo with immune-activating agents, such as anti-CD3 antibodies and IL-2, have failed. A deeper understanding of the molecular underpinnings of HIV latency is clearly required, including determining whether viral latency is actively reinforced by transcriptional repressors, defining which inducible host transcription factors most effectively antagonize latency, and elucidating the role of chromatin in viral latency. Only through such an improved understanding will it be possible to identify combination therapies that might allow complete purging of the latent reservoir and to realize the difficult and elusive goal of complete eradication of HIV in infected patients.

Anti-CD45RO suppresses human immunodeficiency virus type 1 replication in microglia: role of Hck tyrosine kinase and implications for AIDS dementia

Macrophages and microglia are productively infected by HIV-1 and play a pivotal role in the pathogenesis of AIDS dementia. Although macrophages and microglia express CD45, a transmembrane protein tyrosine phosphatase, whether modulation of its activity affects human immunodeficiency virus type 1 (HIV-1) replication is unknown. Here, we report that of the five human CD45 isoforms, microglia express CD45RB and CD45RO (RB > RO) and treatment of microglia with a CD45 agonist antibody alphaCD45RO (UCHL-1) inhibits HIV-1 replication. alphaCD45RO prevented HIV-1 negative factor (Nef)-induced autophosphorylation of hematopoietic cell kinase (Hck), a myeloid lineage-specific Src kinase. Recombinant CD45 protein also inhibited HIV-1-induced Hck phosphorylation in microglia. Antennapedia-mediated delivery of Hck Src homology domain 3 (SH3), a domain that binds to the Nef PxxP motif with high affinity, reduced HIV-1-induced Hck phosphorylation and HIV-1 production in microglia. HIV-1-induced LTR transactivation was observed in U38 cells stably overexpressing wild-type Hck but not kinase-inactive Hck. In microglia, alphaCD45RO reduced activation of transcription factors (NF-kappaB and CCAAT enhancer binding protein) necessary for LTR transactivation in macrophages. These results establish that in myeloid lineage cells, Nef interacts with the Hck SH3 domain, resulting in autophosphorylation of Hck and an increase in HIV-1 transcription. alphaCD45RO-mediated inhibition of HIV-1 replication in microglia identifies the CD45 protein tyrosine phosphatase as a potential therapeutic target for HIV-1 infection/AIDS dementia.

NFAT5 regulates HIV-1 in primary monocytes via a highly conserved long terminal repeat site

To replicate, HIV-1 capitalizes on endogenous cellular activation pathways resulting in recruitment of key host transcription factors to its viral enhancer. RNA interference has been a powerful tool for blocking key checkpoints in HIV-1 entry into cells. Here we apply RNA interference to HIV-1 transcription in primary macrophages, a major reservoir of the virus, and specifically target the transcription factor NFAT5 (nuclear factor of activated T cells 5), which is the most evolutionarily divergent NFAT protein. By molecularly cloning and sequencing isolates from multiple viral subtypes, and performing DNase I footprinting, electrophoretic mobility shift, and promoter mutagenesis transfection assays, we demonstrate that NFAT5 functionally interacts with a specific enhancer binding site conserved in HIV-1, HIV-2, and multiple simian immunodeficiency viruses. Using small interfering RNA to ablate expression of endogenous NFAT5 protein, we show that the replication of three major HIV-1 viral subtypes (B, C, and E) is dependent upon NFAT5 in human primary differentiated macrophages. Our results define a novel host factor-viral enhancer interaction that reveals a new regulatory role for NFAT5 and defines a functional DNA motif conserved across HIV-1 subtypes and representative simian immunodeficiency viruses. Inhibition of the NFAT5-LTR interaction may thus present a novel therapeutic target to suppress HIV-1 replication and progression of AIDS.

Interleukin-7 induces HIV replication in primary naive T cells through a nuclear factor of activated T cell (NFAT)-dependent pathway

Interleukin (IL)-7 plays several roles critical to T cell maturation, survival, and homeostasis. Because of these functions, IL-7 is under investigation as an immune-modulator for therapeutic use in lymphopenic clinical conditions, including HIV. We reported that naive T cells, typically not permissive to HIV, can be productively infected when pre-treated with IL-7. We evaluated the mechanism by which IL-7-mediates this effect. IL-7 potently up-regulated the transcriptional factor NFAT, but had no effect on NFkappaB. Blocking NFAT activity using a number of reagents, such as Cyclosporin A, FK-506, or the NFAT-specific inhibitor known as VIVIT peptide, all markedly reduced IL-7-mediated induction of HIV replication in naive T cells. Additional neutralization of cytokines present in IL-7-treated cultures and/or those that have NFAT-binding sequences within their promotors indicated that IL-10, IL-4, and most significantly IFNgamma, all contribute to IL-7-induction of HIV productive replication in naive T cells. These data clarify the mechanism by which IL-7 can overcome the block to HIV productive infection in naive T cells, despite their quiescent cell status. These findings are relevant to the treatment of HIV disease and understanding HIV pathogenesis in the naive CD4+ T cell compartment, especially in light of the vigorous pursuit of IL-7 as an in vivo immune modulator.

LFA-1 is a key determinant for preferential infection of memory CD4+ T cells by human immunodeficiency virus type 1

Memory CD4+ T cells are considered a stable latent reservoir for human immunodeficiency virus type 1 (HIV-1) and a barrier to eradication of this retroviral infection in patients under therapy. It has been shown that memory CD4+ T cells are preferentially infected with HIV-1, but the exact mechanism(s) responsible for this higher susceptibility remains obscure. Previous findings indicate that incorporation of host-derived intercellular adhesion molecule 1 (ICAM-1) in HIV-1 increases virus infectivity. To measure the putative involvement of virus-anchored ICAM-1 in the preferential infection of memory cells by HIV-1, quiescent and activated naive and memory T-cell subsets were exposed to isogenic virions either lacking or bearing ICAM-1. Memory CD4+ T cells were found to be more susceptible than naive CD4+ T cells to infection with ICAM-1-bearing virions, as exemplified by a more important virus replication, an increase in integrated viral DNA copies, and a more efficient entry process. Interactions between virus-associated host ICAM-1 and cell surface LFA-1 under a cluster formation seem to be responsible for the preferential HIV-1 infection of the memory cell subset. Altogether, these data shed light on a potential mechanism by which HIV-1 preferentially targets long-lived memory CD4+ T cells.

Endothelial cells promote human immunodeficiency virus replication in nondividing memory T cells via Nef-, Vpr-, and T-cell receptor-dependent activation of NFAT

Human endothelial cells (ECs) enhance human immunodeficiency virus (HIV) replication within CD4(+) memory T cells by 50,000-fold in a Nef-dependent manner. Here, we report that EC-mediated HIV type 1 replication is also dependent on an intact vpr gene. Moreover, we demonstrate that despite a requirement for engaging major histocompatibility complex (MHC) class II molecules and costimulators, EC-stimulated virus-producing cells (p24(high) T cells) do not proliferate, nor are they arrested in the cell cycle. Rather, they are minimally activated, sometimes expressing CD69 but not CD25, HLA-DR, VLA-1, or effector cytokines. Blocking antibodies to interleukin 2 (IL-2), IL-6, IL-7, or tumor necrosis factor do not inhibit viral replication. Cyclosporine effectively inhibits viral replication, as does disruption of the NFAT binding site in the viral long terminal repeat. Furthermore, in the presence of ECs, suboptimal T-cell receptor (TCR) stimulation with phytohemagglutinin L supports efficient viral replication, and suboptimal stimulation with toxic shock syndrome toxin 1 leads to viral replication selectively in the TCR-stimulated, Vbeta2-expressing T cells. Collectively, these data indicate that ECs provide signals that promote Nef- and Vpr-dependent HIV replication in memory T cells that have been minimally activated through their TCRs. Our studies suggest a mechanism for HIV replication in vivo within the reservoir of circulating memory CD4(+) T cells that persist despite antiretroviral therapy and further suggest that maintenance of immunological memory by MHC class II-expressing ECs via TCR signaling may contribute to HIV rebound following cessation of antiretroviral therapy.

T-cell activation leads to poor activation of the HIV-1 clade E long terminal repeat and weak association of nuclear factor-kappaB and NFAT with its enhancer region

The enhancer region in the human immunodeficiency virus type 1 (HIV-1) 5'-long terminal repeat (LTR) is very important for viral transcription. This promoter sequence binds both nuclear factor-kappaB and NFAT, two important modulators of HIV-1 gene expression. Previous studies have indicated that the enhancer regions of the different HIV-1 clade LTRs differ in their number of NF-kappaB-binding sites. In this study, we have compared the activation potential of the different HIV-1 clade and HIV-2 LTRs and assessed their interaction with NFAT and NF-kappaB. In T-cell lines and primary CD4(+) T-cells, the results showed that the HIV-1 clade E LTR (with a single NF-kappaB-binding site) was the weakest LTR regardless of the tested activators, whereas the HIV-2 LTR was the most responsive LTR. The clade E enhancer region was also demonstrated to be the weakest enhancer region in transfection experiments with luciferase reporter-based vectors. Electrophoretic mobility shift assays with extracts from activated CD4(+) T-cells indicated that, although NF-kappaB and NFAT bound all enhancers, HIV-1 clade E and HIV-2 LTR enhancers were poor binding targets for these two factors. Weak NFAT binding to clade E enhancers was also confirmed using NFAT1-expressing 293T cells in competition experiments. We have also shown the absence of interaction of NF-kappaB or NFAT with the third NF-kappaB repeat present in clade C. However, the clade C enhancer bound NFAT more efficiently than all other enhancer regions tested. Our results hence demonstrate for the first time that differences in the binding of NF-kappaB and NFAT to the enhancer regions could be responsible for some of the observed variation in HIV-1 clade LTR activation, whereas HIV-2 LTR activation seems mostly independent of these interactions.

Prostratin antagonizes HIV latency by activating NF-kappaB

A subset of quiescent memory CD4 T cells harboring integrated but transcriptionally silent proviruses poses a currently insurmountable barrier to the eradication of the human immunodeficiency virus (HIV) in infected patients. Induction of HIV gene expression in these latently infected cells by immune activating agents has been proposed as one approach to confer sensitivity to antiretroviral therapy. Interest has recently focused on the non-tumor-promoting phorbol ester, prostratin, as a potential agent to activate latent HIV proviruses. Using multiple Jurkat T cell lines containing integrated but transcriptionally latent HIV proviruses (J-Lat cells), we now demonstrate that prostratin effectively activates HIV gene expression in these latently infected cells. We further show that prostratin acts by stimulating IKK-dependent phosphorylation and degradation of IkappaBalpha, leading to the rapid nuclear translocation of NF-kappaB and activation of the HIV-1 long terminal repeat in a kappaB enhancer-dependent manner. In contrast, NFAT and AP-1 are not induced by prostratin. Using chromatin immunoprecipitation assays to identify host transcription factors recruited to the latent HIV-1 promoter in living cells, we find that prostratin induces RelA binding. Analysis of potential upstream signal transducers demonstrates that prostratin stimulates membrane translocation of classical, novel, and atypical protein kinase C (PKC) isoforms. Studies with isoform-specific PKC inhibitors suggest that the novel PKCs play a particularly prominent role in the prostratin response. These findings provide new insights into the molecular pathway through which prostratin antagonizes HIV latency highlighting a central role for the action of NF-kappaB.

Reciprocal regulation of the nuclear factor of activated T cells and HIV-1

The human immunodeficiency virus type 1 (HIV-1) has evolved to coordinate its replication with the activation state of the host CD4T cell. To this end, it taps into major host cell signaling pathways and their associated transcription factors. Of these, T-cell activation and the transcription factor NF-kappaB, respectively, have become the best-studied examples. The past several years have revealed compelling evidence that another transcription factor family involved in T-cell activation, the nuclear factor of activated T cells (NFAT), plays an important role in the regulation of HIV-1. Major advances have been made in our understanding of the interaction of HIV-1 with this intriguing transcription factor. The duplicated NF-kappaB binding sites in the HIV-1 enhancer surprisingly also bind NFAT proteins and appear to be the most important targets for NFAT transactivation of the HIV-1 long terminal repeat. The crystal structure of NFAT1 bound to one of these duplicated sites was solved recently. Interestingly, it showed that NFAT1 binds to this site as a homodimer and occupies the core of the NF-kappaB site, suggesting mutually exclusive binding and alternate transactivation by these two factors. NFAT also regulates HIV-1 infection indirectly, as it can relieve a block to reverse transcription in quiescent T cells. In turn, HIV-1, and particularly its Tat and Nef gene products, can upregulate NFAT expression and activity. This reciprocal regulation between virus and transcription factor potentially creates a positive feedback loop, which may facilitate the establishment of early HIV-1 infection and, later, the transition from latent to productive infection. The immunosuppressive drug cyclosporin A (CsA) inhibits NFAT activity and thus represents a potential treatment for HIV-1 infection. Recent small-scale clinical trials have yielded optimistic results, suggesting roles for CsA after organ transplantation in HIV-1+ individuals and as adjunct treatment in stable early HIV-1 infection.

Protein tyrosyl phosphatases in T cell activation: implication for human immunodeficiency virus transcriptional activity

The protein tyrosine phosphatases (PTPs) superfamily is a large group of enzymes showing a wide diversity of structure and biological functions. Their implication in the regulation of signal transduction processes is critical for homeostasis and efficient cellular activation. Disturbance of the delicate balance between protein tyrosine kinase and protein tyrosine phosphatase activities is at the heart of a large number of diseases. Control of cellular activation is especially important for human immunodeficiency virus type 1 (HIV-1) since this retrovirus requires activated T cells in order to replicate efficiently. Identification of PTPs implicated in signaling pathways leading to upregulation of HIV-1 gene transcription therefore contributes to the general understanding of cellular factors needed for strong HIV-1 replication and progression to AIDS. The use of bisperoxovanadium compounds as potent, specific, and highly purified PTP inhibitors releases HIV-1 from PTP control and strongly increases HIV-1 gene expression. These inhibitors can thus be used to study signal transduction mechanisms regulated by PTP activity that are important for HIV-1 replication and provide new and interesting therapeutic avenues for the efficient control of this debilitating retroviral infection.

Regulation of HIV-1 gene transcription: from lymphocytes to microglial cells

Transcription is a crucial step for human immunodeficiency virus type 1 (HIV-1) expression in all infected host cells, from T lymphocytes, thymocytes, monocytes, macrophages, and dendritic cells in the immune system up to microglial cells in the central nervous system. To maximize its replication, HIV-1 adapts transcription of its integrated proviral genome by ideally exploiting the specific cellular environment and by forcing cellular stimulatory events and impairing transcriptional inhibition. Multiple cell type-specific interplays between cellular and viral factors perform the challenge for the virus to leave latency and actively replicate in a great diversity of cells, despite the variability of its long terminal repeat region in different HIV strains. Knowledge about the molecular mechanisms underlying transcriptional regulatory events helps in the search for therapeutic agents that target the step of transcription in anti-HIV strategies.

Structure of NFAT1 bound as a dimer to the HIV-1 LTR kappa B element

DNA binding by NFAT1 as a dimer has been implicated in the activation of host and viral genes. Here we report a crystal structure of NFAT1 bound cooperatively as a dimer to the highly conserved kappa B site from the human immunodeficiency virus 1 (HIV-1) long terminal repeat (LTR). This structure reveals a new mode of dimerization and protein-DNA recognition by the Rel homology region (RHR) of NFAT1. The two NFAT1 monomers form a complete circle around the kappa B DNA through protein-protein interactions mediated by both their N- and C-terminal subdomains. The major dimer interface, formed by the C-terminal domain, is asymmetric and substantially different from the symmetric dimer interface seen in other Rel family proteins. Comparison to other NFAT structures, including NFAT5 and the NFAT1-Fos-Jun-ARRE2 complex, reveals that NFAT1 adopts different conformations and its protein surfaces mediate distinct protein-protein interactions in the context of different DNA sites.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

Engagement of CD43 enhances human immunodeficiency virus type 1 transcriptional activity and virus production that is induced upon TCR/CD3 stimulation

Human immunodeficiency virus type 1 (HIV-1) transcriptional activity is regulated by several cytokines and T cell activators. CD43 (sialophorin) is a sialoglycoprotein expressed on the surface of a wide variety of blood cells including T lymphocytes. Several studies have shown that CD43 ligation induces proliferation and activation of human T lymphocytes. We were thus interested in defining whether CD43-mediated signaling events can modulate the life cycle of HIV-1. We demonstrate here that CD43 cross-linking potentiates HIV-1 promoter-driven activity and virus production that is seen following the engagement of the T-cell receptor (TCR).CD3 complex. This effect is independent of the CD28 co-stimulatory molecule and is mediated by both NF-kappaB and NFAT transcription factors. A number of signal transducers known to be involved in the TCR/CD3-dependent signal transduction pathway, including p56(lck), p36(lat), and SLP-76, as well as capacitative entry of calcium, are crucial for the noticed CD43 co-stimulatory effect. Calcium mobilization studies indicate that a synergy is occurring between CD43- and TCR/CD3-mediated signaling events leading to an augmented calcium release. These data suggest that CD43 can be seen as a co-stimulatory cell surface constituent that can modulate HIV-1 expression in T lymphocytes.