vpr deletion mutant of simian immunodeficiency virus induces AIDS in rhesus monkeys

Vpr attenuates antiviral immune responses and is critical for full pathogenicity of SIVmac239 in rhesus macaques

The accessory viral protein R (Vpr) is encoded by all primate lentiviruses. Vpr counteracts DNA repair pathways, modulates viral immune sensing, and induces cell-cycle arrest in cell culture. However, its impact in vivo is controversial. Here, we show that deletion of vpr is associated with delayed viral replication kinetics, rapid innate immune activation, development and maintenance of strong B and T cell responses, and increased neutralizing activity against SIVmac239 in rhesus macaques. All wild-type SIVmac239-infected animals maintained high viral loads, and five of six developed fatal immunodeficiency during ∼80 weeks of follow-up. Lack of Vpr was associated with better preservation of CD4+ T cells, lower viral loads, and an attenuated clinical course of infection in most animals. Our results show that Vpr contributes to efficient viral immune evasion and the full pathogenic potential of SIVmacin vivo. Inhibition of Vpr may improve humoral immune control of viral replication.

In vivo Serial Passaging of Human-Simian Immunodeficiency Virus Clones Identifies Characteristics for Persistent Viral Replication

We previously reported that a human immunodeficiency virus type 1 with a simian immunodeficiency virus vif substitution (HSIV-vif_NL4-3) could replicate in pigtailed macaques (PTMs), demonstrating that Vif is a species-specific tropism factor of primate lentiviruses. However, infections did not result in high-peak viremia or setpoint plasma viral loads, as observed during simian immunodeficiency virus (SIV) infection of PTMs. Here, we characterized variants isolated from one of the original infected animals with CD4 depletion after nearly 4years of infection to identify determinants of increased replication fitness. In our studies, we found that the HSIV-vif clones did not express the HIV-1 Vpr protein due to interference from the vpx open reading frame (ORF) in singly spliced vpr mRNA. To examine whether these viral genes contribute to persistent viral replication, we generated infectious HSIV-vif clones expressing either the HIV-1 Vpr or SIV Vpx protein. And then to determine viral fitness determinants of HSIV-vif, we conducted three rounds of serial in vivo passaging in PTMs, starting with an initial inoculum containing a mixture of CXCR4-tropic [Vpr-HSIV-vif_NL4-3 isolated at 196 (C/196) and 200 (C/200) weeks post-infection from a PTM with depressed CD4 counts] and CCR5-tropic HSIV (Vpr⁺ HSIV-vif derivatives based NL-AD8 and Bru-Yu2 and a Vpx expressing HSIV-vif_Yu2). Interestingly, all infected PTMs showed peak plasma viremia close to or above 10⁵ copies/ml and persistent viral replication for more than 20weeks. Infectious molecular clones (IMCs) recovered from the passage 3 PTM (HSIV-P3 IMCs) included mutations required for HIV-1 Vpr expression and those mutations encoded by the CXCR4-tropic HSIV-vif_NL4-3 isolate C/196. The data indicate that the viruses selected during long-term infection acquired HIV-1 Vpr expression, suggesting the importance of Vpr for in vivo pathogenesis. Further passaging of HSIV-P3 IMCs in vivo may generate pathogenic variants with higher replication capacity, which will be a valuable resource as challenge virus in vaccine and cure studies.

HIV-1 Vpr induces cell cycle arrest and enhances viral gene expression by depleting CCDC137

The HIV-1 Vpr accessory protein induces ubiquitin/proteasome-dependent degradation of many cellular proteins by recruiting them to a cullin4A-DDB1-DCAF1 complex. In so doing, Vpr enhances HIV-1 gene expression and induces (G2/M) cell cycle arrest. However, the identities of Vpr target proteins through which these biological effects are exerted are unknown. We show that a chromosome periphery protein, CCDC137/cPERP-B, is targeted for depletion by HIV-1 Vpr, in a cullin4A-DDB1-DCAF1 dependent manner. CCDC137 depletion caused G2/M cellcycle arrest, while Vpr-resistant CCDC137 mutants conferred resistance to Vpr-induced G2/M arrest. CCDC137 depletion also recapitulated the ability of Vpr to enhance HIV-1 gene expression, particularly in macrophages. Our findings indicate that Vpr promotes cell-cycle arrest and HIV-1 gene expression through depletion of CCDC137.

Mannose receptor is an HIV restriction factor counteracted by Vpr in macrophages

HIV-1 Vpr is necessary for maximal HIV infection and spread in macrophages. Evolutionary conservation of Vpr suggests an important yet poorly understood role for macrophages in HIV pathogenesis. Vpr counteracts a previously unknown macrophage-specific restriction factor that targets and reduces the expression of HIV Env. Here, we report that the macrophage mannose receptor (MR), is a restriction factor targeting Env in primary human monocyte-derived macrophages. Vpr acts synergistically with HIV Nef to target distinct stages of the MR biosynthetic pathway and dramatically reduce MR expression. Silencing MR or deleting mannose residues on Env rescues Env expression in HIV-1-infected macrophages lacking Vpr. However, we also show that disrupting interactions between Env and MR reduces initial infection of macrophages by cell-free virus. Together these results reveal a Vpr-Nef-Env axis that hijacks a host mannose-MR response system to facilitate infection while evading MR's normal role, which is to trap and destroy mannose-expressing pathogens.

Illuminating the Role of Vpr in HIV Infection of Myeloid Cells

Vpr is a 14 kDa accessory protein conserved amongst extant primate lentiviruses that is required for virus replication in vivo. Although many functions have been attributed to Vpr, its primary role, and the function under selective pressure in vivo, remains elusive. The minimal importance of Vpr in infection of activated CD4+ T cells in vitro suggests that its major importance lies in overcoming restriction to virus replication in non-cycling myeloid cell populations, such as macrophages and dendritic cells. HIV-1 replication is attenuated in the absence of Vpr in myeloid cells such as monocyte-derived dendritic cells (MDDCs) and macrophages, and is correlated with the ability of Vpr to overcome a post-integration transcriptional defect in these cells. Intriguingly, recent identification of the human hub silencing (HUSH) complex as a target for DCAF^CRL4-mediated degradation by numerous ancestral SIV Vpr alleles, and the Vpr paralog Vpx, signifies the potential function of HIV-1 Vpr to alter yet-to-be identified chromatin remodeling complexes and prevent host-mediated transcriptional repression of both invading viral genomes and pro-inflammatory responses. Myeloid cells constitute an important bridge between innate and adaptive immune responses to invading pathogens. Here, we seek to illustrate the numerous means by which Vpr manipulates the myeloid cellular environment and facilitates virus replication, myeloid cell-dependent HIV transmission, and systemic virus dissemination.

Retroviral restriction: nature's own solution

PURPOSE OF REVIEW

The present review will discuss recent advances in the development of anti-HIV therapies inspired by studies of the mechanisms of host restriction factor-mediated resistance to HIV infection.

RECENT FINDINGS

Manipulating the interplay between host cell restriction factors and viral accessory factors that overcome them can potentially be therapeutically useful. Preliminarily successful therapies - some of which are entering clinical trials - either inhibit the ability of virus to evade restriction factor-mediated immunity, or promote intracellular levels of restriction factors. These aims are achieved by multiple means, which are discussed.

SUMMARY

Many restriction factors appear to provide potentially useful targets for anti-HIV therapies, so time and interest should be invested in investigating ways to successfully therapeutically manipulate restriction factor-mediated immunity.

Comparative Analysis of Tat-Dependent and Tat-Deficient Natural Lentiviruses

The emergence of human immunodeficiency virus (HIV) causing acquired immunodeficiency syndrome (AIDS) in infected humans has resulted in a global pandemic that has killed millions. HIV-1 and HIV-2 belong to the lentivirus genus of the Retroviridae family. This genus also includes viruses that infect other vertebrate animals, among them caprine arthritis-encephalitis virus (CAEV) and Maedi-Visna virus (MVV), the prototypes of a heterogeneous group of viruses known as small ruminant lentiviruses (SRLVs), affecting both goat and sheep worldwide. Despite their long host-SRLV natural history, SRLVs were never found to be responsible for immunodeficiency in contrast to primate lentiviruses. SRLVs only replicate productively in monocytes/macrophages in infected animals but not in CD4+ T cells. The focus of this review is to examine and compare the biological and pathological properties of SRLVs as prototypic Tat-independent lentiviruses with HIV-1 as prototypic Tat-dependent lentiviruses. Results from this analysis will help to improve the understanding of why and how these two prototypic lentiviruses evolved in opposite directions in term of virulence and pathogenicity. Results may also help develop new strategies based on the attenuation of SRLVs to control the highly pathogenic HIV-1 in humans.

Vpr Promotes Macrophage-Dependent HIV-1 Infection of CD4+ T Lymphocytes

Vpr is a conserved primate lentiviral protein that promotes infection of T lymphocytes in vivo by an unknown mechanism. Here we demonstrate that Vpr and its cellular co-factor, DCAF1, are necessary for efficient cell-to-cell spread of HIV-1 from macrophages to CD4+ T lymphocytes when there is inadequate cell-free virus to support direct T lymphocyte infection. Remarkably, Vpr functioned to counteract a macrophage-specific intrinsic antiviral pathway that targeted Env-containing virions to LAMP1+ lysosomal compartments. This restriction of Env also impaired virological synapses formed through interactions between HIV-1 Env on infected macrophages and CD4 on T lymphocytes. Treatment of infected macrophages with exogenous interferon-alpha induced virion degradation and blocked synapse formation, overcoming the effects of Vpr. These results provide a mechanism that helps explain the in vivo requirement for Vpr and suggests that a macrophage-dependent stage of HIV-1 infection drives the evolutionary conservation of Vpr.

Genetic characterization of near full length SIVdrl genomes from four captive drills (Mandrillus leucophaeus)

We sequenced near full length SIVdrl genomes from four captive drills (Mandrillus leucophaeus). All four animals were born in captivity in German zoos. Although serologically SIV negative before acquisition in zoo A in 2008 and 2009, during a routine analysis all four animals were determined to be SIV antibody positive in 2011. Comparisons of the four new SIVdrl sequences showed high identity among each other (90.7-97.7% in env) and to the only published full length sequence SIVdrl FAO (90.5-92.8% in env), which is also derived from a captive drill. SIVdrl infections seem to be highly prevalent in captive drills, probably resulting from frequent animal transfers between the zoos in an effort to maintain this highly endangered species and its genetic diversity. This should be kept in mind as SIVdrl may be transmitted to uninfected animals in open groups and potentially also to animal keepers having contact with these nonhuman primates.

HIV-1 Vpr suppresses the cytomegalovirus promoter in a CRL4(DCAF1) E3 ligase independent manner

Although the Vpr protein of human immunodeficiency virus type 1 (HIV-1) has been shown to act as a transcriptional activator of the HIV-1 LTR and certain host genes, the current study demonstrates that it can also function as a potent inhibitor of the cytomegalovirus (CMV) promoter. Previous studies have shown that the cell cycle arrest and apoptotic functions of Vpr required recruitment of the CRL4(DCAF1) E3 ligase, but this complex is shown not to be required for inhibition of the CMV promoter. We identified conserved sites (A30/V31) from diverse Vpr from HIV/SIV that were critical for blocking the CMV promoter activity. Interestingly, the Vpr mutant A30S/V31S protein also impaired the ability of Vpr to down-regulate transcription of the host UNG2 gene. Our findings shed light on the dual functions of Vpr on the transcription of HIV-1, other viruses and host genes which may contribute to viral replication and disease progression in vivo.

The HIV-1 accessory protein Vpr induces the degradation of the anti-HIV-1 agent APOBEC3G through a VprBP-mediated proteasomal pathway

The host anti-HIV-1 factor APOBEC3G (A3G) plays a potential role in restricting HIV-1 replication, although this antagonist can be encountered and disarmed by the Vif protein. In this paper, we report that another HIV-1 accessory protein, viral protein R (Vpr), can interact with A3G and intervene in its antiviral behavior. The interaction of Vpr and A3G was predicted by computer-based screen and confirmed by a co-immunoprecipitation (Co-IP) approach. We found that Vpr could reduce the virion encapsidation of A3G to enhance viral replication. Subsequent experiments showed that Vpr downregulated A3G through Vpr-binding protein (VprBP)-mediated proteasomal degradation, and further confirmed that the reduction of A3G encapsidation associated with Vpr was due to Vpr's degradation-inducing activity. Our findings highlight the versatility of Vpr by unveiling the hostile relationship between Vpr and A3G. In addition, the observation that A3G is targeted to the proteasomal degradation pathway by Vpr in addition to Vif implicates the existence of crosstalk between different HIV-1-host ubiquitin ligase complex systems.

HIV-1 Vpr-a still "enigmatic multitasker"

Like other HIV-1 auxiliary proteins, Vpr is conserved within all the human (HIV-1, HIV-2) and simian (SIV) immunodeficiency viruses. However, Vpr and homologous HIV-2, and SIV Vpx are the only viral auxiliary proteins specifically incorporated into virus particles through direct interaction with the Gag precursor, indicating that this presence in the core of the mature virions is mainly required for optimal establishment of the early steps of the virus life cycle in the newly infected cell. In spite of its small size, a plethora of effects and functions have been attributed to Vpr, including induction of cell cycle arrest and apoptosis, modulation of the fidelity of reverse transcription, nuclear import of viral DNA in macrophages and other non-dividing cells, and transcriptional modulation of viral and host cell genes. Even if some more recent studies identified a few cellular targets that HIV-1 Vpr may utilize in order to perform its different tasks, the real role and functions of Vpr during the course of natural infection are still enigmatic. In this review, we will summarize the main reported functions of HIV-1 Vpr and their significance in the context of the viral life cycle.

HIV-1 Vpr accelerates viral replication during acute infection by exploitation of proliferating CD4+ T cells in vivo

The precise role of viral protein R (Vpr), an HIV-1-encoded protein, during HIV-1 infection and its contribution to the development of AIDS remain unclear. Previous reports have shown that Vpr has the ability to cause G₂ cell cycle arrest and apoptosis in HIV-1-infected cells in vitro. In addition, vpr is highly conserved in transmitted/founder HIV-1s and in all primate lentiviruses, which are evolutionarily related to HIV-1. Although these findings suggest an important role of Vpr in HIV-1 pathogenesis, its direct evidence in vivo has not been shown. Here, by using a human hematopoietic stem cell-transplanted humanized mouse model, we demonstrated that Vpr causes G₂ cell cycle arrest and apoptosis predominantly in proliferating CCR5⁺ CD4⁺ T cells, which mainly consist of regulatory CD4⁺ T cells (Tregs), resulting in Treg depletion and enhanced virus production during acute infection. The Vpr-dependent enhancement of virus replication and Treg depletion is observed in CCR5-tropic but not CXCR4-tropic HIV-1-infected mice, suggesting that these effects are dependent on the coreceptor usage by HIV-1. Immune activation was observed in CCR5-tropic wild-type but not in vpr-deficient HIV-1-infected humanized mice. When humanized mice were treated with denileukin diftitox (DD), to deplete Tregs, DD-treated humanized mice showed massive activation/proliferation of memory T cells compared to the untreated group. This activation/proliferation enhanced CCR5 expression in memory CD4⁺ T cells and rendered them more susceptible to CCR5-tropic wild-type HIV-1 infection than to vpr-deficient virus. Taken together, these results suggest that Vpr takes advantage of proliferating CCR5⁺ CD4⁺ T cells for enhancing viremia of CCR5-tropic HIV-1. Because Tregs exist in a higher cycling state than other T cell subsets, Tregs appear to be more vulnerable to exploitation by Vpr during acute HIV-1 infection.

HIV-1 encodes nine genes, five of which (gag, pol, env, tat, and rev) are essential for viral replication, and four, termed accessory genes (vif, vpu, nef, and vpr), appear to aid virus infection. Of the four accessory proteins, Vpr is the most enigmatic. It is well known that Vpr has the potential to cause G₂ cell cycle arrest and apoptosis in vitro. Moreover, it has been reported that Vpr-mediated G₂ arrest increases HIV-1 production in vitro. However, the role of Vpr in HIV-1 propagation in vivo remains unclear. Here, by using a humanized mouse model, we demonstrate that Vpr enhances CCR5-tropic but not CXCR4-tropic HIV-1 replication in vivo by exploiting Tregs during acute infection. In CCR5-tropic HIV-1-infected humanized mice, Vpr-dependent G₂ cell cycle arrest and apoptosis are predominantly observed in infected Tregs, and wild-type but not vpr-deficient HIV-1-infected mice displayed acute Treg depletion. This Vpr-dependent Treg depletion may lead to immune activation and provide a pool of activated/proliferating CD4⁺ T cells, which supports subsequent HIV-1 expansion in vivo. This is the first report demonstrating the role of Vpr in HIV-1 infection in vivo.

Identification and molecular characterization of SIV Vpr R50G mutation associated with long term survival in SIV-infected morphine dependent and control macaques

Viral protein R (Vpr) is an accessory protein of HIV and SIV involved in the pathogenesis of viral infection. In this study, we monitored SIV evolution in the central nervous system and other organs from morphine-dependent and control animals by sequencing vpr in an attempt to understand the relationship between drug abuse, disease progression, and compartmentalization of viral evolution. Animals in the morphine group developed accelerated disease and died within twenty weeks post-infection. A unique mutation, R50G, was identified in the macaques that survived regardless of morphine exposure. Functional studies revealed that the R50G mutation exhibited altered cellular localization and decreased the expression levels of both IL-6 and IL-8. Our results, therefore, suggest that sequence changes within the SIV/17E-Fr vpr occur regardless of drug abuse but correlate with survival, and that they alter disease progression rates by affecting Vpr functions.

Molecular mechanisms of HIV immune evasion of the innate immune response in myeloid cells

The expression of intrinsic antiviral factors by myeloid cells is a recently recognized mechanism of restricting lentiviral replication. Viruses that enter these cells must develop strategies to evade cellular antiviral factors to establish a productive infection. By studying the cellular targets of virally encoded proteins that are necessary to infect myeloid cells, a better understanding of cellular intrinsic antiviral strategies has now been achieved. Recent findings have provided insight into how the lentiviral accessory proteins, Vpx, Vpr and Vif counteract antiviral factors found in myeloid cells including SAMHD1, APOBEC3G, APOBEC3A, UNG2 and uracil. Here we review our current understanding of the molecular basis of how cellular antiviral factors function and the viral countermeasures that antagonize them to promote viral transmission and spread.

Evidence for Vpr-dependent HIV-1 replication in human CD4+ CEM.NKR T-cells

BACKGROUND

Vpr is exclusively expressed in primate lentiviruses and contributes to viral replication and disease progression in vivo. HIV-1 Vpr has two major activities in vitro: arrest of cell cycle in the G2 phase (G2 arrest), and enhancement of viral replication in macrophages. Previously, we reported a potent HIV-1 restriction in the human CD4+ CEM.NKR (NKR) T cells, where wild-type (WT) HIV-1 replication was inhibited by almost 1,000-fold. From the parental NKR cells, we isolated eight clones by limiting dilution. These clones showed three levels of resistance to the WT HIV-1 infection: non-permissive (NP), semi-permissive (SP), and permissive (P). Here, we compared the replication of WT, Vif-defective, Vpr-defective, and Vpu-defective viruses in these cells.

RESULTS

Although both WT and Vpu-defective viruses could replicate in the permissive and semi-permissive clones, the replication of Vif-defective and Vpr-defective viruses was completely restricted. The expression of APOBEC3G (A3G) cytidine deaminase in NKR cells explains why Vif, but not Vpr, was required for HIV-1 replication. When the Vpr-defective virus life cycle was compared with the WT virus life cycle in the semi-permissive cells, it was found that the Vpr-defective virus could enter the cell and produce virions containing properly processed Gag and Env proteins, but these virions showed much less efficiency for reverse transcription during the next-round of infection. In addition, although viral replication was restricted in the non-permissive cells, treatment with arsenic trioxide (As2O3) could completely restore WT, but not Vpr-defective virus replication. Moreover, disruption of Vpr binding to its cofactor DCAF1 and/or induction of G2 arrest activity did not disrupt the Vpr activity in enhancing HIV-1 replication in NKR cells.

CONCLUSIONS

These results demonstrate that HIV-1 replication in NKR cells is Vpr-dependent. Vpr promotes HIV-1 replication from the 2nd cycle likely by overcoming a block at early stage of viral replication; and this activity does not require DCAF1 and G2 arrest. Further studies of this mechanism should provide new understanding of Vpr function in the HIV-1 life cycle.

Tat has a dual role in simian immunodeficiency virus transcription

Tat has a pivotal role in human and simian immunodeficiency virus (HIV and SIV) replication because it stimulates transcription by binding to the trans-activator response (TAR) element. In addition, several other Tat functions have been proposed. Most studies have focused on HIV-1 Tat and much less is known about SIV Tat. An SIVmac239 variant was constructed previously in which the Tat-TAR transcription mechanism is functionally replaced by the doxycycline-inducible Tet-On gene expression mechanism (SIV-rtTA). In this study, SIV-rtTA variants were used to analyse the functions of SIV Tat. It was shown that Tat-minus SIV-rtTA variants replicated efficiently in PM1 T-cells, ruling out an additional essential Tat function. Nevertheless, replication was suboptimal in other cells, and evolutionary pressure to repair Tat expression was documented. It was demonstrated that SIV-rtTA required Tat for optimal gene expression, despite the absence of the Tat-TAR axis. This Tat effect was lost upon replacement of the long terminal repeat promoter region by a non-related promoter. These results indicate that Tat can activate SIV transcription via TAR RNA and U3 DNA elements but has no other essential function in replication in cultured cells. The experiments were limited to cell lines and PBMCs, and did not exclude an accessory Tat function under specific conditions or in vivo.

Vif substitution enables persistent infection of pig-tailed macaques by human immunodeficiency virus type 1

Among Old World monkeys, pig-tailed macaques (Pt) are uniquely susceptible to human immunodeficiency virus type 1 (HIV-1), although the infection does not persist. We demonstrate that the susceptibility of Pt T cells to HIV-1 infection is due to the absence of postentry inhibition by a TRIM5 isoform. Notably, substitution of the viral infectivity factor protein, Vif, with that from pathogenic SIVmne enabled replication of HIV-1 in Pt T cells in vitro. When inoculated into juvenile pig-tailed macaques, the Pt-tropic HIV-1 persistently replicated for more than 1.5 to 2 years, producing low but measurable plasma viral loads and persistent proviral DNA in peripheral blood mononuclear cells. It also elicited strong antibody responses. However, there was no decline in CD4(+) T cells or evidence of disease. Surprisingly, the Pt-tropic HIV-1 was rapidly controlled when inoculated into newborn Pt macaques, although it transiently rebounded after 6 months. We identified two notable differences between the Pt-tropic HIV-1 and SIVmne. First, SIV Vif does not associate with Pt-tropic HIV-1 viral particles. Second, while Pt-tropic HIV-1 degrades both Pt APOBEC3G and APOBEC3F, it prevents their inclusion in virions to a lesser extent than pathogenic SIVmne. Thus, while SIV Vif is necessary for persistent infection by Pt-tropic HIV-1, improved expression and inhibition of APOBEC3 proteins may be required for robust viral replication in vivo. Additional adaptation of the virus may also be necessary to enhance viral replication. Nevertheless, our data suggest the potential for the pig-tailed macaque to be developed as an animal model of HIV-1 infection and disease.

Vpr is required for efficient Nef expression from unintegrated human immunodeficiency virus type 1 DNA

Unintegrated human immunodeficiency virus (HIV) DNA are viral DNA products formed naturally during HIV replication. While the integrated proviral DNA form is transcriptionally active and results in productive infection, unintegrated DNA is also capable of expression of viral RNA and proteins. Previously, we showed that HIV Vpr enhances expression from integrase-defective HIV. Here we show that Vpr activation of expression is partially dependent upon the presence of a transcriptionally active HIV promoter and results in increased transcription of unspliced gag and spliced nef viral RNA. While Tat is detectable during infection with integrase-defective HIV, Tat levels are not affected by the presence of Vpr. Mutation studies reveal that Tat is dispensable for the Vpr-mediated enhancement of expression from unintegrated DNA. We find that virion-associated Vpr is sufficient for Nef expression from unintegrated viral DNA, resulting in the efficient downregulation of CD4 from the surface of infected cells. These results provide a mechanism by which Nef expression from unintegrated HIV type 1 DNA expression occurs.

Inhibition of human immunodeficiency virus envelope glycoprotein- mediated single cell lysis by low-molecular-weight antagonists of viral entry

The coexpression of human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins and receptors leads to the lysis of single cells by a process that is dependent upon membrane fusion. This cell lysis was inhibited by low-molecular-weight compounds that interfere with receptor binding or with receptor-induced conformational transitions in the envelope glycoproteins. A peptide, T20, potently inhibited cell-cell fusion but had no effect on single cell lysis mediated by the HIV-1 envelope glycoproteins. Thus, critical events in the lysis of single cells by the HIV-1 envelope glycoproteins occur in intracellular compartments accessible only to small inhibitory compounds.

The Vpr protein from HIV-1: distinct roles along the viral life cycle

The genomes of human and simian immunodeficiency viruses (HIV and SIV) encode the gag, pol and env genes and contain at least six supplementary open reading frames termed tat, rev, nef, vif, vpr, vpx and vpu. While the tat and rev genes encode regulatory proteins absolutely required for virus replication, nef, vif, vpr, vpx and vpu encode for small proteins referred to "auxiliary" (or "accessory"), since their expression is usually dispensable for virus growth in many in vitro systems. However, these auxiliary proteins are essential for viral replication and pathogenesis in vivo. The two vpr- and vpx-related genes are found only in members of the HIV-2/SIVsm/SIVmac group, whereas primate lentiviruses from other lineages (HIV-1, SIVcpz, SIVagm, SIVmnd and SIVsyk) contain a single vpr gene. In this review, we will mainly focus on vpr from HIV-1 and discuss the most recent developments in our understanding of Vpr functions and its role during the virus replication cycle.

Single-cycle immunodeficiency viruses provide strategies for uncoupling in vivo expression levels from viral replicative capacity and for mimicking live-attenuated SIV vaccines

To reduce the risks associated with live-attenuated immunodeficiency virus vaccines, single-cycle immunodeficiency viruses (SCIVs) were developed by primer complementation and production of the vaccine in the absence of vif in a vif-independent cell line. After a single intravenous injection of SCIVs into rhesus monkeys, peak viral RNA levels of 10(3) to 10(4) copies/ml plasma were observed, indicating efficient expression of SCIV in the vaccinee. After booster immunizations with SCIVs, SIV-specific humoral and cellular immune responses were observed. Although the vaccine doses used in this pilot study could not protect vaccinees from subsequent intravenous challenge with pathogenic SIVmac239, our results demonstrate that the novel SCIV approach allows us to uncouple in vivo expression levels from the viral replicative capacity facilitating the analysis of the relationship between viral expression levels or viral genes and immune responses induced by SIV.

Cytolysis by CCR5-using human immunodeficiency virus type 1 envelope glycoproteins is dependent on membrane fusion and can be inhibited by high levels of CD4 expression

T-tropic (X4) and dualtropic (R5X4) human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins kill primary and immortalized CD4(+) CXCR4(+) T cells by mechanisms involving membrane fusion. However, because much of HIV-1 infection in vivo is mediated by M-tropic (R5) viruses whose envelope glycoproteins use CCR5 as a coreceptor, we tested a panel of R5 and R5X4 envelope glycoproteins for their ability to lyse CCR5(+) target cells. As is the case for CXCR4(+) target cells, HIV-1 envelope glycoproteins expressed by single-round HIV-1 vectors killed transduced CD4(+) CCR5(+) cells in a membrane fusion-dependent manner. Furthermore, a CD4-independent R5 HIV-1 envelope glycoprotein was able to kill CD4-negative target cells expressing CCR5, demonstrating that CD4 is not intrinsically required for the induction of death. Interestingly, high levels of CD4 expression protected cells from lysis and syncytium formation mediated by the HIV-1 envelope glycoproteins. Immunoprecipitation experiments showed that high levels of CD4 coexpression inhibited proteolytic processing of the HIV-1 envelope glycoprotein precursor gp160. This inhibition could be overcome by decreasing the CD4 binding ability of gp120. Studies were also undertaken to investigate the ability of virion-bound HIV-1 envelope glycoproteins to kill primary CD4(+) T cells. However, neither X4 nor R5X4 envelope glycoproteins on noninfectious virions caused death in primary CD4(+) T cells. These results demonstrate that the interaction of CCR5 with R5 HIV-1 envelope glycoproteins capable of inducing membrane fusion leads to cell lysis; overexpression of CD4 can inhibit cell killing by limiting envelope glycoprotein processing.

Specific G2 arrest of caprine cells infected with a caprine arthritis encephalitis virus expressing vpr and vpx genes from simian immunodeficiency virus

Primate lentivirus (HIV and SIV) vpr accessory genes encode 12- to 14-kDa proteins which induce cell cycle arrest at the G2 phase of infected cells, preventing them from going through mitosis. Members of the HIV-2/SIVmac/SIVsmm group also encode a second closely related accessory protein called Vpx. Vpx and HIV Vpr are critical for virus replication in nondividing cells due to their participation in nuclear import of the preintegration complex. Caprine arthritis encephalitis virus (CAEV) and maedi visna virus are the natural lentiviruses of domestic goat and sheep, respectively, and their genomes do not carry vpr and vpx genes. In this study, we generated chimeric CAEV-based genomes carrying vpr and vpx genes from SIVmac239 and tested their ability to induce G2 cell cycle arrest in infected caprine cells. CAEV-pBSCAvpxvpr is the chimeric genome that was shown to be infectious and replication competent. Our data demonstrated that CAEV-pBSCAvpxvpr-infected goat synovial membrane cell monolayer developed more cytopathic effects and a high proportion of cells remained in the G2 phase of cell cycle. This G2 arrest was observed both at the early and at the late stages of infection, while minimal effect was observed with the parental CAEV-pBSCA. These results, described for the first time in mammalian cells other than those of primates, indicate that Vpr-induced G2 cell cycle arrest is not restricted to only primate cells. Thus, conservation of Vpx/Vpr protein functions in caprine cells suggests a possible role for these proteins in the virus life cycle and its ability to adapt to new hosts. The data presented here thus raise a pertinent question about the biological significance of the conservation of Vpr and Vpx functions in caprine cells despite the high phylogenic distance between primates and small ruminants.

Human immunodeficiency virus type 1 (HIV-1) Vpr enhances expression from unintegrated HIV-1 DNA

Retroviral DNA synthesized prior to integration, termed unintegrated viral DNA, is classically believed to be transcriptionally inert and to serve only as a precursor to the transcriptionally active integrated proviral DNA form. However, it has recently been found to be expressed under some circumstances during human immunodeficiency virus type 1 (HIV-1) replication and may play a significant role in HIV-1 pathogenesis. HIV-1 Vpr is a virion-associated accessory protein that is critical for HIV-1 replication in nondividing cells and induces cell cycle arrest and apoptosis. We find that Vpr, either expressed de novo or released from virions following viral entry, is essential for unintegrated viral DNA expression. HIV-1 mutants defective for integration in either the integrase catalytic domain or the cis-acting att sites can express unintegrated viral DNA at levels similar to that of wild-type HIV-1, but only in the presence of Vpr. In the absence of Vpr, the expression of unintegrated viral DNA decreases 10- to 20-fold. Vpr does not affect the efficiency of integration from integrase-defective HIV-1. Vpr-mediated enhancement of expression from integrase-defective HIV-1 requires that the viral DNA be generated in cells through infection and is mediated via a template that declines over time. Vpr activation of expression does not require exclusive nuclear localization of Vpr nor does it correlate with Vpr-mediated cell cycle arrest. These results attribute a new function to HIV-1 Vpr and implicate Vpr as a critical component in expression from unintegrated HIV-1 DNA.

Replication, immunogenicity, and protective properties of live-attenuated simian immunodeficiency viruses expressing interleukin-4 or interferon-gamma

Nef deletion mutants of SIV-expressing interleukin-4 (SIV-IL4) or interferon-gamma (SIV-IFN) were constructed to study the effect of interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) on viral load, immunogenicity, and protective properties. Four rhesus monkeys were infected with SIV-IL4 and four were infected with SIV-IFN. During the acute phase of infection, the cell-associated viral load, but not the plasma viral RNA load, was approximately 10-fold lower in SIV-IFN-infected macaques than in SIV-IL4-infected rhesus monkeys. The viral load declined to hardly detectable levels 4 months postinfection in all animals. SIV antibody titers and the affinity of these antibodies were higher in SIV-IL4-infected macaques than in SIV-IFN-infected animals, consistent with a stimulation of T helper cell type 2 immune responses by IL-4. At peak viremia, there was a trend to higher interleukin-12 and perforin mRNA levels of the lymph nodes in the SIV-IFN-infected macaques than in the SIV-IL4-infected monkeys. Deletion of the viral IFN gene, but not the viral IL-4 gene, after the development of antiviral immune responses suggests a repressive effect of IFN, but not IL-4, on virus spread in vivo. A trend to higher set point viral RNA levels in SIV-IL4-infected monkeys in comparison to monkeys infected with the parental nef deletion mutant and similar viral RNA levels during the acute phase of infection suggest that IL-4 expression leads to a slight reduction in the control of virus replication by host immune responses. However, SIV-IL4 and SIV-IFN induced protection against a homologous challenge virus. Subsequent challenge with an SIV-HIV-1 hybrid virus (SHIV) also revealed protection in the absence of neutralizing antibodies.

The first HxRxG motif in simian immunodeficiency virus mac239 Vpr is crucial for G(2)/M cell cycle arrest

The highly conserved Vpr protein mediates cell cycle arrest, transcriptional transactivation, and nuclear import of the preintegration complex in human immunodeficiency virus type 1. To identify functional domains in simian immunodeficiency virus (SIV) mac239 Vpr, we mutagenized selected motifs within an alpha-helical region and two C-terminal HxRxG motifs. All Vpr mutants located to the nucleus. Substitution of four amino acids in the alpha-helical domain did not interfere with cell cycle arrest, while a single substitution abolished cell cycle arrest function. Mutation of the first HxRxG motif to AxAxA also resulted in loss of cell cycle arrest, while mutation of the second motif had no effect. Interestingly, both Vpr mutants impaired in cell cycle arrest function also showed reduced transactivation of the SIV long terminal repeat, suggesting that arrest of cells at G(2)/M mediates or contributes to transactivation by Vpr.

Insights into the biology of HIV-1 viral protein R

HIV-1 viral protein R (Vpr) is a small, highly conserved accessory protein encoded by the HIV genome that serves many functions in the viral life cycle. Vpr induces G2 cell cycle arrest, which is thought to indirectly enhance viral replication by increasing transcription from the LTR. Vpr has also been implicated in facilitating infection of nondividing cells, most notably macrophages. Because Vpr is a nucleo-cytoplasmic shuttling protein, its role in enhancing viral replication in macrophages may be mediated through enhanced entry of the HIV preintegration complex through the limiting nuclear pore. Free Vpr is detectable in the serum of patients, and in vitro studies implicate extracellular forms of Vpr as an effector of cellular responses mediated through its ability to transduce through intact cytoplasmic membranes. We review the biologic properties of Vpr, focusing on its mechanism of action, role in HIV replication, and significance for host pathogenesis.

HIV-1 replication cycle

The HIV-1 is a formidable pathogen with establishment of a persistent infection based on the ability to integrate the proviral genome into chronically infected cells, and by the rapid evolution made possible by a high mutation rate and frequent recombination during the viral replication. HIV-1 has a variety of novel genes that facilitate viral persistence and regulation of HIV replication, but this virus also usurps cellular machinery for HIV replication, particularly during gene expression and virion assembly and budding. Recent success with antiretroviral therapy may be limited by the emergence HIV drug resistance and by toxicities and other requirements for successful long-term therapy. Further investigation of HIV-1 replication may allow identification of novel targets of antiretroviral therapy that may allow continued virus suppression in patients of failing current regiments, particularly drugs that target HIV-1 entry and HIV-1 integration.

Structure of human immunodeficiency virus type 1 Vpr(34-51) peptide in micelle containing aqueous solution

Human immunodeficiency virus type 1 protein R (HIV-1 Vpr) promotes nuclear entry of viral nucleic acids in nondividing cells, causes G(2) cell cycle arrest and is involved in cellular differentiation and cell death. Vpr subcellular localization is as variable as its functions. It is known, that consistent with its role in nuclear transport, Vpr localizes to the nuclear envelope of human cells. Further, a reported ion channel activity of Vpr is clearly dependent on its localization in or at membranes. We focused our structural studies on the secondary structure of a peptide consisting of residues 34-51 of HIV-1 Vpr. This part of Vpr plays an important role in Vpr oligomerization, contributes to cell cycle arrest activity, and is essential for virion incorporation and binding to HHR23A, a protein involved in DNA repair. Employing NMR spectroscopy we found this part of Vpr to be almost completely alpha helical in the presence of micelles, as well as in trifluoroethanol containing and methanol/chloroform solvent. Our results provide structural data suggesting residues 34-51 of Vpr to contain an amphipathic, leucine-zipper-like alpha helix, which serves as a basis for oligomerization of Vpr and its interactions with cellular and viral factors involved in subcellular localization and virion incorporation of Vpr.

Analysis of apoptosis induced by HIV-1 Vpr and examination of the possible role of the hHR23A protein

The HIV-1 Vpr protein induces apoptosis of cells, the mechanism of which is unknown. To clarify how this function may be related to other Vpr functions, we simultaneously assessed the effects of multiple point mutations upon various Vpr properties. Our data suggest that induction of arrest by Vpr may be unnecessary for induction of apoptosis. This is exemplified by a C-terminal mutant, R80A, that does not arrest cells, yet induces low but significant levels of apoptosis. We also show that mutation of Vpr at both of its nuclear localization sequences (within its alpha-helices and the overlapping leucine zipper-like domain) does not affect induction of either apoptosis or cell cycle arrest. This indicates that neither sequence is essential for these two functions of Vpr. It further suggests that multimerization of Vpr, which maps to residues 60 and 67 within the leucine-rich region, is unnecessary for initiation of apoptosis and arrest. We previously found that the Vpr-binding protein, hHR23A, can partially alleviate induction of arrest. We now show that overexpression of hHR23A itself causes apoptosis of cells. Mutation of its C-terminal UBA( 2 ) domain that is responsible for binding Vpr disrupts the apoptotic effect. This suggests that Vpr may induce apoptosis through a pathway involving hHR23A.

T cell apoptosis causes peripheral T cell depletion in mice transgenic for the HIV-1 vpr gene

Vpr, an accessory protein of HIV, is known to affect viral replication as well as cell growth, differentiation, and apoptosis in vitro. To investigate its pathogenicity in vivo, we have produced mice transgenic for the HIV-1 vpr gene with the CD4 enhancer/promoter. Interestingly, apoptotic death of T lymphocytes was enhanced in those mice, causing marked reduction of T cells in lymphatic organs and peripheral blood. Involvement of Bcl-x, Bax, and Caspase-1, but not of the Fas-Fas ligand system, was suggested in the apoptotic processes. These observations suggest that Vpr is involved in the pathogenesis of T cell depletion in HIV-infected people.

Membrane-fusing capacity of the human immunodeficiency virus envelope proteins determines the efficiency of CD+ T-cell depletion in macaques infected by a simian-human immunodeficiency virus

The mechanism of the progressive loss of CD4+ T lymphocytes, which underlies the development of AIDS in human immunodeficiency virus (HIV-1)-infected individuals, is unknown. Animal models, such as the infection of Old World monkeys by simian-human immunodeficiency virus (SHIV) chimerae, can assist studies of HIV-1 pathogenesis. Serial in vivo passage of the nonpathogenic SHIV-89.6 generated a virus, SHIV-89.6P, that causes rapid depletion of CD4+ T lymphocytes and AIDS-like illness in monkeys. SHIV-KB9, a molecularly cloned virus derived from SHIV-89.6P, also caused CD4+ T-cell decline and AIDS in inoculated monkeys. It has been demonstrated that changes in the envelope glycoproteins of SHIV-89.6 and SHIV-KB9 determine the degree of CD4+ T-cell loss that accompanies a given level of virus replication in the host animals (G. B. Karlsson et. al., J. Exp. Med. 188:1159-1171, 1998). The envelope glycoproteins of the pathogenic SHIV mediated membrane fusion more efficiently than those of the parental, nonpathogenic virus. Here we show that the minimal envelope glycoprotein region that specifies this increase in membrane-fusing capacity is sufficient to convert SHIV-89.6 into a virus that causes profound CD4+ T-lymphocyte depletion in monkeys. We also studied two single amino acid changes that decrease the membrane-fusing ability of the SHIV-KB9 envelope glycoproteins by different mechanisms. Each of these changes attenuated the CD4+ T-cell destruction that accompanied a given level of virus replication in SHIV-infected monkeys. Thus, the ability of the HIV-1 envelope glycoproteins to fuse membranes, which has been implicated in the induction of viral cytopathic effects in vitro, contributes to the capacity of the pathogenic SHIV to deplete CD4+ T lymphocytes in vivo.

Solution structure of human immunodeficiency virus type 1 Vpr(13-33) peptide in micelles

Human immunodeficiency virus type 1 protein R (HIV-1 Vpr) promotes nuclear entry of viral nucleic acids in nondividing cells, causes G2 cell cycle arrest and is involved in cellular differentiation and cell death. Also, Vpr subcellular localization is as variable as its functions. It is known that, consistent with its role in nuclear transport, Vpr localizes to the nuclear envelope of human cells. Further, a reported ion channel activity of Vpr obviously is dependent on its localization in or at membranes. We focused our structural studies on the secondary structure of a peptide consisting of residues 13-33 of HIV-1 Vpr in micelles. Employing nuclear magnetic resonance and circular dichroism spectroscopy we found this part of Vpr, known to be essential for nuclear localization, to be almost completely alpha helical. Our results provide structural data suggesting residues 13-33 of Vpr to form an amphipathic, leucine-zipper-like alpha helix that serves as a basis for interactions with a variety of viral and cellular factors.

Positive and negative aspects of the human immunodeficiency virus protease: development of inhibitors versus its role in AIDS pathogenesis

In this review we summarize multiple aspects of the human immunodeficiency virus (HIV) protease from both structural and functional viewpoints. After an introductory overview, we provide an up-to-date status report on protease inhibitors (PI). This proceeds from a discussion of PI structural design, to how PI are optimally utilized in highly active antiretroviral triple therapy (one PI along with two reverse transcriptase inhibitors), the emergence of PI resistance, and the natural role of secretory leukocyte PI. Then we switch to another focus: the interaction of HIV protease with other genes in acute and persistent infection, which in turn may have an effect on AIDS pathogenesis. We conclude with a discussion on future directions in HIV treatment, involving multiple-target anti-HIV therapy, vaccine development, and novel reactivation-inhibitory reagents.

Simian immunodeficiency virus containing mutations in N-terminal tyrosine residues and in the PxxP motif in Nef replicates efficiently in rhesus macaques

SIVmac Nef contains two N-terminal tyrosines that were proposed to be part of an SH2-ligand domain and/or a tyrosine-based endocytosis signal and a putative SH3-ligand domain (P(104)xxP(107)). In the present study, we investigated the effects of combined mutations in these tyrosine and proline residues on simian immunodeficiency virus (SIV) Nef interactions with the cellular signal transduction and endocytic machinery. We found that mutation of Y(28)F, Y(39)F, P(104)A, and P(107)A (FFAA-Nef) had little effect on Nef functions such as the association with the cellular tyrosine kinase Src, downregulation of cell surface expression of CD4 and class I major histocompatibility complex, and enhancement of virion infectivity. However, mutations in the PxxP sequence reduced the ability of Nef to stimulate viral replication in primary lymphocytes. Three macaques infected with the SIVmac239 FFAA-Nef variant showed high viral loads during the acute phase of infection. Reversions in the mutated prolines were observed between 12 and 20 weeks postinfection. Importantly, reversion of A(107)-->P, which restored the ability of Nef to coprecipitate a 62-kDa phosphoprotein in in vitro kinase assays, did not precede the development of a high viral load. The Y(28)/Y(39)-->F(28)/F(39) substitutions did not revert. In conclusion, mutations in both the tyrosine residues and the putative SH3 ligand domain apparently do not disrupt major aspects of SIV Nef function in vivo.

Evidence for recombination of live, attenuated immunodeficiency virus vaccine with challenge virus to a more virulent strain

Live, attenuated immunodeficiency virus vaccines, such as nef deletion mutants, are the most effective vaccines tested in the simian immunodeficiency virus (SIV) macaque model. In two independent studies designed to determine the breadth of protection induced by live, attenuated SIV vaccines, we noticed that three of the vaccinated macaques developed higher set point viral load levels than unvaccinated control monkeys. Two of these vaccinated monkeys developed AIDS, while the control monkeys infected in parallel remained asymptomatic. Concomitant with an increase in viral load, a recombinant of the vaccine virus and the challenge virus could be detected. Therefore, the emergence of more-virulent recombinants of live, attenuated immunodeficiency viruses and less-aggressive wild-type viruses seems to be an additional risk of live, attenuated immunodeficiency virus vaccines.

Antiretroviral therapy during primary immunodeficiency virus infection can induce persistent suppression of virus load and protection from heterologous challenge in rhesus macaques

A limited period of chemotherapy during primary immunodeficiency virus infection might provide a long-term clinical benefit even if treatment is initiated at a time point when virus is already detectable in plasma. To evaluate this strategy, we infected rhesus macaques with the pathogenic simian/human immunodeficiency virus RT-SHIV and treated them with the antiretroviral drug (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for 8 weeks starting 7 or 14 days postinfection. PMPA treatment suppressed viral replication efficiently in all of the monkeys. After chemotherapy ended, virus replication rebounded and viral RNA in plasma reached levels comparable to that of the controls in four of the six monkeys. However, in the other two animals, virus loads peaked only moderately after withdrawal of the drug and then declined to low or even undetectable levels. These low levels of viremia remained stable for at least 31 weeks after cessation of therapy. At this time point, these two monkeys were challenged with SIV(8980) to evaluate whether the host responses which were able to keep RT-SHIV replication under control were also sufficient to protect against infection with a highly pathogenic heterologous virus. Both monkeys proved to be protected against the heterologous virus. In one of the two animals, low levels of SIV(8980) replication were detected. Thus, by chemotherapy during the acute phase of pathogenic virus replication, we could achieve not only persistent virus load suppression in two out of six monkeys but also protection from subsequent heterologous challenge. By this chemotherapeutic attenuation, the replication kinetics of attenuated viruses could be mimicked and a vaccination effect similar to that induced by live attenuated simian immunodeficiency virus vaccines was achieved.

Mutational analysis of Vpr-induced G2 arrest, nuclear localization, and cell death in fission yeast

Cell cycle G2 arrest, nuclear localization, and cell death induced by human immunodeficiency virus type 1 Vpr were examined in fission yeast by using a panel of Vpr mutations that have been studied previously in human cells. The effects of the mutations on Vpr functions were highly similar between fission yeast and human cells. Consistent with mammalian cell studies, induction of cell cycle G2 arrest by Vpr was found to be independent of nuclear localization. In addition, G2 arrest was also shown to be independent of cell killing, which only occurred when the mutant Vpr localized to the nucleus. The C-terminal end of Vpr is crucial for G2 arrest, the N-terminal alpha-helix is important for nuclear localization, and a large part of the Vpr protein is responsible for cell killing. It is evident that the overall structure of Vpr is essential for these cellular effects, as N- and C-terminal deletions affected all three cellular functions. Furthermore, two single point mutations (H33R and H71R), both of which reside at the end of each alpha-helix, disrupted all three Vpr functions, indicating that these two mutations may have strong effects on the overall Vpr structure. The similarity of the mutant effects on Vpr function in fission yeast and human cells suggests that fission yeast can be used as a model system to evaluate these Vpr functions in naturally occurring viral isolates.

Genetic instability of live, attenuated human immunodeficiency virus type 1 vaccine strains

Live, attenuated viruses have been the most successful vaccines in monkey models of human immunodeficiency virus type 1 (HIV-1) infection. However, there are several safety concerns about using such an anti-HIV vaccine in humans, including reversion of the vaccine strain to virulence and recombination with endogenous retroviral sequences to produce new infectious and potentially pathogenic viruses. Because testing in humans would inevitably carry a substantial risk, we set out to test the genetic stability of multiply deleted HIV constructs in perpetuated tissue culture infections. The Delta3 candidate vaccine strain of HIV-1 contains deletions in the viral long terminal repeat (LTR) promoter and the vpr and nef genes. This virus replicates with delayed kinetics, but a profound enhancement of virus replication was observed after approximately 2 months of culturing. Analysis of the revertant viral genome indicated that the three introduced deletions were maintained but a 39-nucleotide sequence was inserted in the LTR promoter region. This insert was formed by duplication of the region encoding three binding sites for the Sp1 transcription factor. The duplicated Sp1 region was demonstrated to increase the LTR promoter activity, and a concomitant increase in the virus replication rate was measured. In fact, duplication of the Sp1 sites increased the fitness of the Delta3 virus (Vpr/Nef/U3) to levels higher than that of the singly deleted DeltaVpr virus. These results indicate that deleted HIV-1 vaccine strains can evolve into fast-replicating variants by multiplication of remaining sequence motifs, and their safety is therefore not guaranteed. This insight may guide future efforts to develop more stable anti-HIV vaccines.

Env-independent protection induced by live, attenuated simian immunodeficiency virus vaccines

Live attenuated simian immunodeficiency viruses (SIV), such as nef deletion mutants, are the most effective vaccines tested in the SIV-macaque model so far. To modulate the antiviral immune response induced by live attenuated SIV vaccines, we had previously infected rhesus monkeys with a nef deletion mutant of SIV expressing interleukin 2 (SIV-IL2) (B. R. Gundlach, H. Linhart, U. Dittmer, S. Sopper, S. Reiprich, D. Fuchs, B. Fleckenstein, G. Hunsmann, S. Stahl-Hennig, and K. Uberla, J. Virol. 71:2225-2232, 1997). In the present study, SIV-IL2-infected macaques and macaques infected with the nef deletion mutant SIVDeltaNU were challenged with pathogenic SIV 9 to 11 months postvaccination. In contrast to the results with naive control monkeys, no challenge virus could be isolated from the SIV-IL2- and SIVDeltaNU-infected macaques. However, challenge virus sequences could be detected by nested PCR in some of the vaccinated macaques. To determine the role of immune responses directed against Env of SIV, four vaccinated macaques were rechallenged with an SIV-murine leukemia virus (MLV) hybrid in which the env gene of SIV had been functionally replaced by the env gene of amphotropic MLV. All vaccinated macaques were protected from productive infection with the SIV-MLV hybrid in the absence of measurable neutralizing antibodies, while two naive control monkeys were readily infected. Since the SIV-MLV hybrid uses the MLV Env receptor Pit2 and not CD4 and a coreceptor for virus entry, chemokine inhibition and receptor interference phenomena were not involved in protection. These results indicate that the protective responses induced by live attenuated SIV vaccines can be independent of host immune reactions directed against Env.

In vivo pathogenesis of a human immunodeficiency virus type 1 reporter virus

Our understanding of human immunodeficiency virus type 1 (HIV-1)-induced pathogenesis is hampered by the inability to detect HIV-1 gene expression in infected viable cells. In this report, we describe two HIV-1 reporter constructs that are replication competent and cytopathic in vivo. These constructs contain DNA regions of two different lengths that bear the cDNA for the murine heat-stable antigen in the vpr region of a CXCR4-tropic virus. We used the SCID-hu mouse model and these reporter viruses to perform detailed kinetic studies of HIV-1 infection of human thymocytes in vivo. We document that the CD4(+)/CD8(+) thymocytes are the first to express virus and that this subset demonstrates the most rapid and extensive HIV-1-induced cell depletion. Following depletion of this subset, subsequent virus expression occurs predominantly in phenotypically CD4(-) cells, suggesting that CD4 down-regulation occurs in HIV-1-infected thymocytes in vivo. These results demonstrate the utility of these HIV-1 reporter constructs to monitor HIV pathogenesis in vitro and in vivo.

Pleiotropic effects of HIV-1 protein R (Vpr) on morphogenesis and cell survival in fission yeast and antagonism by pentoxifylline

Expression of HIV-1 Vpr causes cell cycle G2 arrest, change in cell shape, and cell death over a large evolutionary distance ranging from human to yeast cells. As a step toward understanding these highly conserved Vpr functions, we have examined the effect of Vpr on cytoskeletal elements and the viability of fission yeast. We demonstrate that the changes in cell morphology induced by Vpr in fission yeast are caused by several underlying cellular abnormalities, including increased biosynthesis of chitin in the cell wall, disruption of the actin cytoskeleton, and altered polarity for cell growth. The extent of these cellular alterations and cell survival correlates with the level of vpr expression. Accompanying cell death, Vpr induces aberrant nuclear morphologies in fission yeast which are similar to those found during the apoptosis induced by Vpr in mammalian cells. The Vpr-induced cytopathic effects and cell death can be suppressed by treatment with pentoxifylline, a compound that inhibits HIV-1 viral replication and suppresses Vpr-induced cell cycle G2 arrest in human and fission yeast cells. The results presented here suggest that pentoxifylline suppresses the effects of Vpr by blocking interactions of Vpr with cellular proteins. Given that pentoxifylline has potential therapeutic value in blocking the effects of Vpr in HIV-infected patients, understanding the molecular mechanisms by which pentoxifylline antagonizes Vpr may have general implications for HIV therapy.

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) interacts with Lys-tRNA synthetase: implications for priming of HIV-1 reverse transcription

The vpr gene of human immunodeficiency virus type 1 (HIV-1) encodes a 96-amino-acid 14-kDa protein (viral protein R [Vpr]), which is produced late in the viral life cycle and is incorporated into the virion. Although Vpr is not required for viral replication in transformed cell lines and primary T lymphocytes, it is essential for productive infection of macrophages and monocytes and appears to be important for pathogenesis in vivo. To establish the role of Vpr in HIV-1 replication and pathogenesis, we have isolated cellular proteins with which Vpr interacts. By using the yeast two-hybrid system, Lys-tRNA synthetase (LysRS) was identified as a Vpr-interacting protein. The interaction between Vpr and LysRS was characterized both in vitro and in vivo, and the domains of Vpr required for the interaction were defined. In the presence of Vpr, LysRS-mediated amino-acylation of tRNA(Lys) is inhibited. Since tRNA(Lys) is the primer for reverse transcription of the HIV-1 genome, this suggests that the interaction between Vpr and LysRS may influence the initiation of HIV-1 reverse transcription.

Arginine residues in the C-terminus of HIV-1 Vpr are important for nuclear localization and cell cycle arrest

HIV-1 viral protein R (Vpr) is predominantly localized to the nucleus and plays an important role for viral preintegration complex import into the nucleus. In this study, we investigated the influence on subcellular localization of Arg residues in the C-terminus of Vpr. Consistent with previous studies, about 90% of the cells manifested diffuse nuclear staining in the Vpr-expressed cells. Besides diffuse nuclear staining, punctate perinuclear staining, and punctate cytoplasmic staining were also observed in the immunofluorescence studies. Deletion of the Ser-Arg-lle-Gly residues (amino acids 79-82; SRIG) had no effect on the Vpr localization. However, deletion of the Arg-Gln-Arg-Arg residues (amino acids 85-88; RQRR) resulted in a smooth perinuclear staining pattern. Substitution of five Arg residues with Asn (amino acids 80, 85, 87, 88, and 90; R-->N5) resulted in a diffuse cytoplasmic staining. Subcellular fractionation analyses support the immunofluorescence staining results. These findings indicate that the C-terminal Arg residues of HIV-1 Vpr play an important role for Vpr nuclear localization. All the Vpr mutants were appropriately expressed, exhibited no significant defect on the protein stability, and were incorporated efficiently into virus-like particles. Both SRIG and R-->N5 mutants lost their cell cycle arrest activities and the RQRR deletion only exhibited a low level of cell arrest activity. Therefore, the Arg residues in the HIV-1 Vpr C-terminus are important for Vpr nuclear localization and cell cycle arrest, but had no effect on protein stability or Vpr incorporation into virus-like particles.

Human immunodeficiency virus type 1 Vpr interacts with HHR23A, a cellular protein implicated in nucleotide excision DNA repair

The human immunodeficiency virus type 1 (HIV-1) vpr gene is an evolutionarily conserved gene among the primate lentiviruses HIV-1, HIV-2, and simian immunodeficiency viruses. One of the unique functions attributed to the vpr gene product is the arrest of cells in the G2 phase of the cell cycle. Here we demonstrate that Vpr interacts physically with HHR23A, one member of an evolutionarily conserved gene family involved in nucleotide excision repair. Interaction of Vpr with HHR23A was initially identified through a yeast two-hybrid screen and was confirmed by the demonstration of direct binding between bacterially expressed recombinant and transiently expressed or chemically synthesized protein products. Visualization of HHR23A and Vpr by indirect immunofluorescence and confocal microscopy indicates that the two proteins colocalize at or about the nuclear membrane. We also map the Vpr-binding domain in HHR23A to a C-terminal 45-amino-acid region of the protein previously shown to have homology to members of the ubiquitination pathway. Overexpression of HHR23A and a truncated derivative which includes the Vpr-binding domain results in a partial alleviation of the G2 arrest induced by Vpr, suggesting that the interaction between Vpr and HHR23A is critical for cell cycle arrest induced by Vpr. These results provide further support for the hypothesis that Vpr interferes with the normal function of a protein or proteins involved in the DNA repair process and, thus, in the transmission of signals that allow cells to transit from the G2 to the M phase of the cell cycle.

AIDS pathogenesis: the role of accessory gene mutations, leading to formation of long-lived persistently infected cells and/or apoptosis-inducing HIV-1 particles

Human immunodeficiency virus type 1 (HIV-1) infection indirectly induces activation-dependent apoptosis in bystander immune CD4+ T-cells, a hallmark of AIDS pathogenesis. It is well known that this pathogenetic event is significantly correlated with a high virus load. Active viral replication occurs in HIV-1 asymptomatic carriers throughout all stages of clinical disease. Most of the HIV-1 in plasma is derived from short-lived infected cells with a half life of a few days; however, a minor population of virus is derived from long-lived persistently and latently infected cells. Recently, the importance of such latent reservoirs for HIV-1 has come to the forefront because of studies with potent antiretroviral inhibitors that block only new rounds of infection. An initial large drop in viral load occurs within two weeks as noted by a decrease in plasma viremia. This is then followed by a slower second-phase decay, since only a small fraction of latently infected resting CD4+ T-cells carry replication-competent, integrated provirus. This review highlights the mechanisms of apoptosis induction in bystander immune cells by both protease-defective, gp120-containing HIV-1 particles, as well as by wild-type virus that appears to be derived predominantly from long-lived infected cells. A model involving the NH2-terminal Nef domain (p7) in this 'bystander apoptosis' event is also presented.

Protection of SIVmac-infected macaque monkeys against superinfection by a simian immunodeficiency virus expressing envelope glycoproteins of HIV type 1

The infection of macaque monkeys by attenuated simian immunodeficiency virus can vaccinate against pathogenic molecular clones and isolates of the same virus. The correlates of this potent protective immunity are not fully understood but may be the key to an effective AIDS vaccine for humans. Aiming to determine whether host immune responses to envelope glycoprotein are an essential component of the immunity to primate lentiviruses, we have tried to superinfect SIVmac-infected macaque monkeys with SHIVsbg, a chimeric primate lentivirus constructed from the SIVmac239 genome with the env, rev, tat, and vpu genes from HIV-1 Lai. After inoculation of a large dose of SHIVsbg, the chimeric virus was isolated by coculture of mononuclear blood cells from four of five SIV-infected monkeys, but three animals were protected from extracellular SHIV viremia and did not seroconvert to HIV-1 glycoproteins. In the two SIV-infected monkeys that did develop SHIV viremia, cell-associated viral load was reduced at least 100-fold. These data indicate that an antiviral response capable of effectively controlling primate lentivirus replication might not necessarily involve the envelope glycoprotein.