Human immunodeficiency virus Tat induces functional unresponsiveness in T cells.

Soluble proteins of the human immunodeficiency virus (HIV) might play a significant role in the pathogenesis of HIV infection. The addition of synthetic Tat peptides, but not that of the recombinant Nef or Vif protein, inhibited proliferative responses of CD4+ tetanus antigen-specific, exogenous interleukin-2 (IL-2)-independent T-cell clones in a dose-dependent manner. In addition, Tat peptides inhibited the anti-CD3 monoclonal antibody-induced proliferative responses of both purified CD4+ and CD8+ T cells. Tat did not affect proliferative responses induced by phorbol myristate acetate plus ionomycin. The Tat peptides at the concentrations used (0.1 to 3 micrograms/ml) did not affect the viability of the cells as determined by trypan blue exclusion. Treatment of Tat peptides with polyclonal Tat antibodies abrogated the inhibitory effect of Tat. Soluble Tat proteins secreted by HeLa cells transfected with the tat gene also inhibited antigen-induced proliferation of the T-cell clones. Tat inhibited the anti-CD3 monoclonal antibody-induced IL-2 mRNA expression and IL-2 secretion but did not affect IL-2 receptor alpha-chain mRNA or protein expression on peripheral blood T cells. Finally, treatment of T-cell clones with the Tat peptide did not affect the antigen-induced increase in intracellular calcium, hydrolysis of phosphatidyl inositol to inositol trisphosphate, or translocation of protein kinase C from the cytosol to the membrane. These studies demonstrate that the mechanism of the Tat-mediated inhibition of T-cell functions involves a phospholipase C gamma 1-independent pathway.     

Human immunodeficiency virus type 1 Tat protein modulates cell cycle and apoptosis in Epstein-Barr virus-immortalized B cells

Patients infected with human immunodeficiency virus type 1 (HIV-1) develop a spectrum of B cell lymphoproliferative disorders ranging from polyclonal B cell activation to B cell lymphomas. While a direct role of Epstein-Barr virus (EBV) is well recognized for most of these lesions, recent findings have suggested that transactivator HIV-1 Tat protein might be involved in the pathogenesis of B cell lymphomas. Tat-expressing EBV-positive B cells were generated by transduction with a retroviral Tat-encoding vector. B(Tat+) cells expressed lower levels of anti-apoptotic protein Bcl-2 than parental and control B(Tat-) cells, generated by transduction with an empty retroviral vector, and were more prone to apoptosis upon serum withdrawal, as assessed by analysis of annexin V-stained cells and cleavage of poly-ADP-ribose-polymerase by caspase 3. Nevertheless, in serum starvation, B(Tat-) cells mainly exhibited the Rb hypo-phosphorylated form, underwent cell cycle arrest, and grew in single cell suspension, while B(Tat+) cells displayed the Rb hyper-phoshorylated form, progressed throughout the cell cycle, and retained the ability to grow in small clumps. Finding that B(Tat+) cells maintained proliferative capacity upon serum withdrawal suggests that cells expressing Tat have growth advantages among the EBV-driven cell proliferations and may originate B cell clones with more oncogenic potential.     

Human immunodeficiency virus type 1 Tat induces B7-H1 expression via ERK/MAPK signaling pathway

In HIV-infected subjects, B7-H1 synthesis and expression are up-regulated, and the degree of dysregulation correlates with the severity of disease. HIV-1 Tat protein, the viral transactivating factor, represents a key target for the host immune response. However, the relationship between B7-H1 and Tat protein has not been addressed. Here, we chose human endothelial cells which provide costimulatory signals sufficiently to influence T cells. We used recombinant pcDNA3.1(+)–Tat plasmid to transfect human endothelial cells ECV304 to establish stable Tat-expressed cell strain, and found that HIV-1 Tat was able to induce B7-H1 expression in ECV304 cells by Real-time PCR and flow cytometry analysis, and inhibited lymphocyte proliferation in co-culture system. Moreover, by using pharmacological inhibitor of ERK pathway, HIV-1 Tat induces B7-H1 expression via ERK/MAPK signaling pathway was corroborated. In summary, our results indicate that HIV-1 Tat could induce B7-H1 synthesis in ECV304 cells through ERK/MAPK signaling pathway.     

Human immunodeficiency virus type 1 Vpr induces apoptosis following cell cycle arrest.

The human immunodeficiency virus type 1 (HIV-1) vpr gene encodes a protein which induces arrest of cells in the G2 phase of the cell cycle. Here, we demonstrate that following the arrest of cells in G2, Vpr induces apoptosis in human fibroblasts, T cells, and primary peripheral blood lymphocytes. Analysis of various mutations in the vpr gene revealed that the extent of Vpr-induced G2 arrest correlated with the levels of apoptosis. However, the alleviation of Vpr-induced G2 arrest by treatment with the drug pentoxifylline did not abrogate apoptosis. Together these studies indicate that induction of G2 arrest, but not necessarily continued arrest in G2, was required for Vpr-induced apoptosis to occur. Finally, Vpr-induced G2 arrest has previously been correlated with inactivation of the Cdc2 kinase. Some models of apoptosis have demonstrated a requirement for active Cdc2 kinase for apoptosis to occur. Here we show that accumulation of the hypophosphorylated or active form of the Cdc2 kinase is not required for Vpr-induced apoptosis. These studies indicate that Vpr is capable of inducing apoptosis, and we propose that both the initial arrest of cells and subsequent apoptosis may contribute to CD4 cell depletion in HIV-1 disease.     

Human immunodeficiency virus type 1 Vpu protein induces rapid degradation of CD4.

CD4 is an integral membrane glycoprotein which is known as the human immunodeficiency virus (HIV) receptor for infection of human cells. The protein is synthesized in the endoplasmic reticulum (ER) and subsequently transported to the cell surface via the Golgi complex. HIV infection of CD4+ cells leads to downmodulation of cell surface CD4, due at least in part to the formation of stable intracellular complexes between CD4 and the HIV type 1 (HIV-1) Env precursor polyprotein gp160. This process "traps" both proteins in the ER, leading to reduced surface expression of CD4 and reduced processing of gp160 to gp120 and gp41. We have recently demonstrated that the presence of the HIV-1-encoded integral membrane protein Vpu can reduce the formation of Env-CD4 complexes, resulting in increased gp160 processing and decreased CD4 stability. We have studied the effect of Vpu on CD4 stability and found that Vpu induces rapid degradation of CD4, reducing the half-life of CD4 from 6 h to 12 min. By using a CD4-binding mutant of gp160, we were able to show that this Vpu-induced degradation of CD4 requires retention of CD4 in the ER, which is normally accomplished through its binding to gp160. The involvement of gp160 in the induction of CD4 degradation is restricted to its function as a CD4 trap, since, in the absence of Env, an ER retention mutant of CD4, as well as wild-type CD4 in cultures treated with brefeldin A, a drug that blocks transport of proteins from the ER, is degraded in the presence of Vpu.     

Human immunodeficiency virus type 1 Tat regulates endothelial cell actin cytoskeletal dynamics through PAK1 activation and oxidant production

Human immunodeficiency virus type 1 Tat exerts prominent angiogenic effects which may lead to a variety of vasculopathic conditions in AIDS patients. Because endothelial cells undergo prominent cytoskeletal rearrangement during angiogenesis, we investigated the specific effects of Tat on the endothelial cell actin cytoskeleton. Glutathione S-transferase (GST)-Tat, at a level of 200 ng/ml (equivalent to 52 ng of Tat/ml), caused stress fiber disassembly, peripheral retraction, and ruffle formation in human umbilical vein endothelial cells (HUVEC) and human lung microvascular endothelial cells. At 600 ng of GST-Tat/ml (157 ng of Tat/ml), actin structures were lost, and severe cytoskeletal collapse occurred. In contrast, GST-Tat harboring mutations within either the cysteine-rich or basic domains exerted minimal effects on the endothelial cytoskeleton. HUVEC expressing a DsRed-Tat fusion protein displayed similar actin rearrangements, followed by actin collapse, whereas neighboring nontransfected cells retained normal actin structures. Because active mutants of p21-activated kinase 1 (PAK1) induce identical changes in actin dynamics, we hypothesized that Tat exerts its cytoskeletal effects through PAK1. GST-Tat activated PAK1 within 5 min, and adenovirus delivery of a kinase-dead PAK1 [PAK1(K298A)] completely prevented cytoskeletal collapse induced by GST-Tat or DsRed-Tat and also blocked downstream activation of c-Jun N-terminal kinase. Further, GST-Tat increased phosphorylation of the NADPH oxidase subunit p47(phox) and caused its rapid redistribution to membrane ruffles. PAK1(K298A) blocked p47(phox) phosphorylation, and interference with NADPH oxidase function through superoxide scavenging or through expression of a transdominant inhibitor, p67(V204A), prevented GST-Tat-induced alterations in the actin cytoskeleton. We conclude that Tat induces actin cytoskeletal rearrangements through PAK1 and downstream activation of the endothelial NADPH oxidase.     

Human immunodeficiency virus type 1 induces 1-beta-D-arabinofuranosylcytosine resistance in human H9 cell line.

We have found that chronically HIV-1(IIIB)-infected H9 cells showed 21-fold resistance to 1-beta-D-arabinofuranosylcytosine (ARA-C) compared with uninfected H9 cells. In the infected H9 cells, a 37% increase of dCTP pool and a 34% increase of dATP were observed, and no alteration of dTTP and dGTP was observed, compared with the uninfected H9 cells. A marked decrease of ARA-CTP generation was observed in the infected H9 cells after 3-h incubation with 0.1-10 microM ARA-C. The level of deoxycytidine kinase activity with ARA-C as substrate was similar in both the infected and the uninfected cells; however, a 37-fold increase of cytidine deaminase activity was observed in the infected H9 cells. These results indicate that the induction of cytidine deaminase activity by HIV-1(IIIB) infection conferred ARA-C resistance to H9 cells. This conclusion was supported by the observation that a marked reversal of ARA-C resistance in the infected H9 cells occurred after treatment with the inhibitor of cytidine deaminase, 3,4,5,6-tetrahydrouridine. The understanding of these cellular alterations in drug sensitivity may facilitate the development of effective therapeutic strategies against HIV-1-infected cells.     

Human immunodeficiency virus type 1 vpr induces apoptosis through caspase activation

Human immunodeficiency virus type 1 (HIV-1) Vpr is a 96-amino-acid protein that is found associated with the HIV-1 virion. Vpr induces cell cycle arrest at the G(2)/M phase of the cell cycle, and this arrest is followed by apoptosis. We examined the mechanism of Vpr-induced apoptosis and found that HIV-1 Vpr-induced apoptosis requires the activation of a number of cellular cysteinyl aspartate-specific proteases (caspases). We demonstrate that ectopic expression of anti-apoptotic viral proteins, which inhibit caspase activity, and addition of synthetic peptides, which represent caspase cleavage sites, can inhibit Vpr-induced apoptosis. Finally, inhibition of caspase activity and subsequent inhibition of apoptosis results in increased viral expression, suggesting that therapeutic strategies aimed at reducing Vpr-induced apoptosis in vivo require careful consideration.     

Effects of the Tat basic domain on human immunodeficiency virus type 1 transactivation, using chemically synthesized Tat protein and Tat peptides.

To study the structure relationship of different Tat domains, the full-length Tat protein Tat1-86, the gene product of the first exon Tat1-72 which retains full activity of the protein, and a panel of shorter peptides mimicking different regions of the primary structure of the Tat protein were chemically synthesized by the solid-phase method, using an efficient protocol. Synthetic Tat1-86 and Tat1-72 transactivated beta-galactosidase activity in HeLa cells containing the lacZ gene under the control of the human immunodeficiency virus type 1 long terminal repeat. Analyses of the activity of Tat1-86 and Tat1-72 with the sulfhydryl of cysteine residues free or protected by the acetamidomethyl group showed that only the Tat fragments with deprotected cysteine residues retain transactivation ability. In contrast, peptide Tat1-48 was inactive, with cysteine residues either free or protected. Similarly, other shorter synthetic peptides covering the different Tat domains were inactive. Interestingly, when peptides Tat1-48 and Tat38-60 were used simultaneously, a significant transactivation was obtained. This result suggests that both peptide domains are implicated in transactivation, probably by acting at two different sites. This permits us to propose a fundamentally new step in the understanding of the molecular mechanism of Tat transactivation.     

Human immunodeficiency virus type 1 Vpr induces apoptosis in human neuronal cells

Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system (CNS) causes AIDS dementia complex (ADC) in certain infected individuals. Recent studies have suggested that patients with ADC have an increased incidence of neuronal apoptosis leading to neuronal dropout. Of note, a higher level of the HIV-1 accessory protein Vpr has been detected in the cerebrospinal fluid of AIDS patients with neurological disorders. Moreover, extracellular Vpr has been shown to form ion channels, leading to cell death of cultured rat hippocampal neurons. Based on these previous findings, we first investigated the apoptotic effects of the HIV-1 Vpr protein on the human neuronal precursor NT2 cell line at a range of concentrations. These studies demonstrated that apoptosis induced by both Vpr and the envelope glycoprotein, gp120, occurred in a dose-dependent manner compared to protein treatment with HIV-1 integrase, maltose binding protein (MBP), and MBP-Vpr in the undifferentiated NT2 cells. For mature, differentiated neurons, apoptosis was also induced in a dose-dependent manner by both Vpr and gp120 at concentrations ranging from 1 to 100 ng/ml, as demonstrated by both the terminal deoxynucleotidyltransferase (Tdt)-mediated dUTP-biotin nick end labeling and Annexin V assays for apoptotic cell death. In order to clarify the intracellular pathways and molecular mechanisms involved in Vpr- and gp120-induced apoptosis in the NT2 cell line and differentiated mature human neurons, we then examined the cellular lysates for caspase-8 activity in these studies. Vpr and gp120 treatments exhibited a potent increase in activation of caspase-8 in both mature neurons and undifferentiated NT2 cells. This suggests that Vpr may be exerting selective cytotoxicity in a neuronal precursor cell line and in mature human neurons through the activation of caspase-8. These data represent a characterization of Vpr-induced apoptosis in human neuronal cells, and suggest that extracellular Vpr, along with other lentiviral proteins, may increase neuronal apoptosis in the CNS. Also, identification of the intracellular activation of caspase-8 in Vpr-induced apoptosis of human neuronal cells may lead to therapeutic approaches which can be used to combat HIV-1-induced neuronal apoptosis in AIDS patients with ADC.     

Human immunodeficiency virus type 1 induces persistent changes in mucosal and blood gammadelta T cells despite suppressive therapy

Gammadelta T cells are primarily found in the gastrointestinal mucosa and play an important role in the first line of defense against viral, bacterial, and fungal pathogens. We sought to examine the impact of human immunodeficiency virus type 1 (HIV-1) infection on mucosal as well as peripheral blood gammadelta T-cell populations. Our results demonstrate that HIV-1 infection is associated with significant expansion of Vdelta1 and contraction of Vdelta2 cell populations in both the mucosa and peripheral blood. Such changes were observed during acute HIV-1 infection and persisted throughout the chronic phase, without apparent reversion after treatment with highly active antiretroviral therapy (HAART). Despite an increase in the expression of CCR9 and CD103 mucosal homing receptors on peripheral blood gammadelta T cells in infected individuals, mucosal and peripheral blood gammadelta T cells appeared to be distinct populations, as reflected by distinct CDR3 length polymorphisms and sequences in the two compartments. Although the underlying mechanism responsible for triggering the expansion of Vdelta1 gammadelta T cells remains unknown, HIV-1 infection appears to have a dramatic impact on gammadelta T cells, which could have important implications for HIV-1 pathogenesis.     

Inhibition of human immunodeficiency virus type 1 and type 2 Tat function by transdominant Tat protein localized to both the nucleus and cytoplasm.

We introduced various mutations into the activation and RNA binding domains of human immunodeficiency virus type 1 (HIV-1) Tat in order to develop a novel and potent transdominant Tat protein and to characterize its mechanism of action. The different mutant Tat proteins were characterized for their abilities to activate the HIV LTR and inhibit the function of wild-type Tat in trans. A Tat protein containing a deletion of the basic domain (Tat(delta)49-57) localized exclusively to the cytoplasm of transfected human cells was nonfunctional and inhibited both HIV-1 and HIV-2 Tat function in a transdominant manner. Tat proteins containing mutations in the cysteine-rich and core domains were nonfunctional but failed to inhibit Tat function in trans. When Tat nuclear or nucleolar localization signals were fused to the carboxy terminus of Tat(delta)49-57, the chimeric proteins localized to the nucleus or nucleolus, respectively, and remained capable of acting in a transdominant manner. Introduction of secondary mutations in the cysteine-rich and core domains of the various transdominant Tat proteins completely eliminated their abilities to act in a transdominant fashion. Our data best support a mechanism in which these transdominant Tat proteins squelch a cellular factor or factors that interact with the Tat activation domain and are required for Tat to function.     

Tat protein of human immunodeficiency virus type 1 subtype C strains is a defective chemokine

Human immunodeficiency virus type 1 (HIV-1)-associated dementia (HAD) is correlated with increased monocyte migration to the brain, and the incidence of HAD among otherwise asymptomatic subjects appears to be lower in India than in the United States and Europe (1 to 2% versus 15 to 30%). Because of the genetic differences between HIV-1 strains circulating in these regions, we sought to identify viral determinants associated with this difference. We targeted Tat protein for these studies in view of its association with monocyte chemotactic function. Analyses of Tat sequences representing nine subtypes revealed that at least six amino acid residues are differentially conserved in subtype C Tat (C-Tat). Of these, cysteine (at position 31) was highly (>99%) conserved in non-subtype C viruses and more than 90% of subtype C viruses encoded a serine. We hypothesized a compromised chemotactic function of C-Tat due to the disruption of CC motif and tested it with the wild type C-Tat (CS) and its two isogenic variants (CC and SC) derived by site-directed mutagenesis. We found that the CS natural variant was defective for monocyte chemotactic activity without a loss in the transactivation property. While the CC mutant is functionally competent for both the functions, in contrast, the SC mutant was defective in both. Therefore, the loss of the C-Tat chemotactic property may underlie the reduced incidence of HAD; although not presenting conclusive evidence, this study provides the first evidence for a potential epidemiologic phenomenon associated with biological differences in the subtype C viruses.