Compatibility of rev gene activity in the four groups of primate lentiviruses

Suppression of viral rebound by a Rev-dependent lentiviral particle in SIV-infected rhesus macaques

Persistence of human immunodeficiency virus (HIV) reservoirs prevents viral eradication, and consequently HIV-infected patients require lifetime treatment with antiretroviral therapy (ART) [1-5]. Currently, there are no effective therapeutics to prevent HIV rebound upon ART cessation. Here we describe an HIV/SIV Rev-dependent lentiviral particle that can be administered to inhibit viral rebound [6-9]. Using simian immunodeficiency virus (SIV)-infected rhesus macaques as a model, we demonstrate that the administration of pre-assembled SIV Rev-dependent lentiviral particles into SIVmac239-infected Indian rhesus macaques can lead to reduction of viral rebound upon ART termination. One of the injected animals, KC50, controlled plasma and CNS viremia to an undetectable level most of the time for over two years after ART termination. Surprisingly, detailed molecular and immunological characterization revealed that viremia control was concomitant with the induction of neutralizing antibodies (nAbs) following the administration of the Rev-dependent vectors. This study emphasizes the importance of neutralizing antibodies (nAbs) for viremia control [10-15], and also provides proof of concept that the Rev-dependent vector can be used to target viral reservoirs, including the CNS reservoirs, in vivo. However, future large-scale in vivo studies are needed to understand the potential mechanisms of viremia control induced by the Rev-dependent vector.

Rev: beyond nuclear export

Rev remains a hot topic. In this review, we revisit the insights that have been gained into the control of gene expression by the retroviral protein Rev and speculate on where current research is leading. We outline what is known about the role of Rev in translation and encapsidation and how these are linked to its more traditional role of nuclear export, underlining the multifaceted nature of this small viral protein. We discuss what more is to be learned in these fields and why continuing research on these 116 amino acids and understanding their function is still important in devising methods to combat AIDS.

Establishment of a biological assay system for human retroviral protease activity

In order to obtain indicator cell lines that are exquisitely susceptible to human T-lymphotropic virus type 1 (HTLV-1), luciferase gene driven by HTLV-1 long terminal repeat (LTR) was transfected into lymphocytic H9 cells with neo gene, and cell lines were selected by G418. A cell line (H9/K30luc) was found to produce an extremely high level of luciferase only when co-cultured with HTLV-1 producer MT-2 cells. Both in the absence and presence of a reverse transcriptase (RT) inhibitor azidothymidine, H9/K30luc cells generated similarly high luciferase activity upon co-cultivation with MT-2 cells. To develop an equivalent system for human immunodeficiency virus type 1 (HIV-1), H9/NL432 cells, which are stably infected with HIV-1 and producing a low level of the virus-like MT-2 cells for HTLV-1, were generated. Together with the indicator cell line H9/H1luc for HIV-1 already reported, antiviral effects of some agents on HTLV-1 and HIV-1 could be readily and sensitively evaluated by similar methods. In fact, by using our system, an HIV-1 protease inhibitor, saquinavir, was demonstrated to be highly effective against HIV-1 but not against HTLV-1.

A new approach to AIDS research and prevention: the use of gene-mutated HIV-1/SIV chimeric viruses for anti-HIV-1 live-attenuated vaccines

The lack of a suitable animal model is a major obstacle to developing anti-HIV-1 vaccines. We successfully generated an SIVmac/HIV-1 chimeric virus (SHIV) (designated as NM-3rN) that contains the HIV-1 env gene and is infectious to macaque monkeys. Challenging the vaccinated macaque monkeys with NM-3rN, we developed an evaluation system for anti-HIV-1 Env-targeted vaccines. For the purpose of making the vaccine, a series of gene-mutated SHIVs were constructed. The monkeys vaccinated with these SHIVs had long-term anti-virus immunities without manifesting the disease, and became resistant to a challenge inoculation with NM-3rN. The sera from a monkey showed that, after the vaccination, the neutralizing antibodies not only against the parental HIV-1 but also against an antigenically different HIV-1 were raised. In vivo experiments confirmed that the vaccinated monkeys were protected from the challenge inoculum of an antigenically different SHIV-MN. Vaccination of monkeys with the attenuated SHIVs showed that further gene-deletion of the SHIV resulted in less immunogenicity. Nevertheless, the attenuated SHIVs had a vaccine effect against the challenge inoculation. In addition to specific immunities including neutralizing antibodies and cytotoxic T cells, a more complicated immune mechanism induced by live vaccine appears to play a role in this protection. Our data suggest that the live vaccine can induce strong and wide-range immunity against HIV-1. These SHIVs should contribute to understanding the pathogenicity of AIDS and to the development of future anti-HIV-1 live vaccines for humans.

The HIV-1 Rev protein

The nuclear export of intron-containing HIV-1 RNA is critically dependent on the activity of Rev, a virally encoded sequence-specific RNA-binding protein. Rev shuttles between the nucleus and the cytoplasm and harbors both a nuclear localization signal and a nuclear export signal. These essential peptide motifs have now been shown to function by accessing cellular signal-mediated pathways for nuclear import and nuclear export. HIV-1 Rev therefore represents an excellent system with which to study aspects of transport across the nuclear envelope.

HIV-1 capsid mutants inhibit the replication of wild-type virus at both early and late infection phases

In-frame mutations were introduced into various portions of the human immunodeficiency virus type 1 (HIV-1) gag gene, and potentials of the mutants to suppress the replication of wild-type HIV-1 were monitored. In contrast to results obtained with matrix and nucleocapsid mutants, almost all capsid mutants blocked HIV-1 replication completely in single-round replication assays. A capsid mutant designated C6b was demonstrated to be one of the most efficient inhibitors for HIV-1 reported to date, and to be effective at both early and late viral replication phases. T-cells, which are engineered to express the C6b Gag in response to HIV-1 infection, were perfectly resistant to HIV-1.

Genetic complementation between replication-defective mutants of HIV-1 and SIVagm

To investigate the functional complementation of essential genes for virus growth between HIV-1 and SIVagm derived from African green monkeys, we co-transfected replication-defective molecular clones containing mutations in gag, pol, env, tat or rev, and monitored transient complementation by reverse transcriptase assay (RT), cytopathic effect (CPE) and immunofluorescence assay (IFA). The following results were obtained: 1) No complementation was observed in combinations of the gag and pol mutants. 2) The rev mutant of HIV-1 was minimally complemented by other SIVagm mutants, although the rev mutant of SIVagm was significantly complemented by other HIV-1 mutants. 3) Among all combinations tested, the env mutant of HIV-1 was the most effectively complemented by SIVagm mutants. 4) CPE was mostly absent in combinations of the env mutant of SIVagm and the gag, pol, or tat mutants of HIV-1, although there were significant positive results in RT and IFA assays. These findings provided basic information about the functional compatibility of pathogenic HIV-1 and nonpathogenic SIVagm which will be useful for generating chimeras of these two viruses.

Rev-dependency of expression of human immunodeficiency virus type 1 gag and env genes

Structural gene expression of human immunodeficiency virus type 1 (HIV-1) requires a viral regulatory protein, Rev transactivator. We investigated Rev-dependency of HIV-1 gene expression by various reporter systems. Expression of unspliced and single-spliced viral mRNAs was demonstrated to be differentially dependent on the Rev function. This difference of Rev-dependency was found not to be determined by cis-elements in gag, pol, and env coding sequences reported so far, and was lost when the reporter constructs containing minimum elements for Rev-responsiveness such as splice signals and rev responsive element were used for experiments. These findings indicated that the fundamental structure of HIV-1 mRNA was critical for the differential regulation of gene expression by Rev transactivator.

Functional analysis of human spuma retrovirus genome

Human spuma retrovirus (HSRV) belongs to retroviruses that possess a complex genome organization. HSRV carries at least three extra genes in the region between env and the 3' long terminal repeat, which are not found in simple retroviruses. Via alternative splicing, these HSRV genes can encode several proteins. To genetically study the requirements of these viral proteins for viral replication in tissue cultures, a number of mutant viruses were constructed from an infectious molecular clone of HSRV. All mutants grew normally in the cell lines tested, except for those lacking transcriptional transactivator activity. By reporter-based transient assay systems, no Rev/Rex equivalent was detected in the HSRV proteins.

Functionality of chimeric Rev proteins of HIV/SIV

Studies on functional compatibility of various Rev proteins derived from all known human and simian immunodeficiency virus subgroups have shown that this essential gene product is not always exchangeable among the viruses. In an attempt to map the region of Rev proteins responsible for the observed nonreciprocal complementation, hybrid genomic Rev expression vectors were constructed by exchanging the first and second exons of rev genes, and were examined for their abilities to activate reporter clones by transfection. With one exception, the second coding exon of rev gene determined the functional specificity of Rev proteins.

The genome of feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a member of the genus Lentivirus of the family Retroviridae. FIV can infect T lymphocytes and monocytes/macrophages in vitro and in vivo, and causes an acquired immunodeficiency syndrome-like disease in cats. Several isolates of FIV from geographically distant countries have been molecularly cloned. There is considerable heterogeneity especially in Env gene among the FIV isolates and they can be divided into two or more subgroups. Like other lentiviruses, FIV has a complex genome structure. Gag gene encodes matrix, capsid and nucleocapsid proteins, and Pol gene encodes protease, reverse transcriptase, dUTPase and integrase. The dUTPase is not present in the primate lentiviruses but present in the non-primate lentiviruses. Env gene encodes surface and transmembrane envelope glycoproteins. In addition to the structural and enzymatic proteins, at least three more genes (Vif, ORF A, Rev) are present in FIV. Vif is related to the infectivity of the cell-free viruses. Rev functions in the stability and transport of incompletely spliced viral RNAs from the nucleus to cytoplasm and is indispensable for virus replication. Although the Tat protein of the primate lentiviruses is essential for virus replication, ORF A (putative Tat gene) of FIV is not essential for virus replication in established feline T lymphoblastoid cell lines. However, the ORF A gene product is related to the efficient replication of the virus in primary peripheral blood lymphocytes. In the long terminal repeat (LTR) of FIV, there are many putative binding sites for enhancer/promoter proteins. Among these binding sites, the putative AP-1 site is important for basal promoter activity of the LTR and responsible for the T cell activation signal through protein kinase C, however the site is not required for the virus replication in established feline T lymphoblastoid cell lines. Comparative study of the molecular biology of lentiviruses revealed that the genome structure, splicing pattern and functional enhancer protein-binding sites of FIV are more similar to those of the ruminant lentiviruses than those of the primate lentiviruses.

Compatibility of Tat and Rev transactivators in the primate lentiviruses

Primate immunodeficiency viruses carry a unique set of transacting regulator genes, which are essential for viral replication. The exchangeability of these Tat and Rev transactivators derived from viruses of the four major subgroups identified to date was assessed in transient transfection and infection assay systems. The human immunodeficiency virus type 1 (HIV-1), a major causative virus of human AIDS, efficiently activated the other viruses. In contrast, the tat and rev gene products of HIV-2, SIVAGM (virus of the African green monkey), and SIVMND (virus of the mandrill) did not fully transactivate the HIV-1. In particular, the rev of HIV-1 was not substantially replaced by those of the other viruses. The result that HIV-1 is distinct from the other immunodeficiency viruses with respect to the compatibility of two transactivators gives a firm functional basis for the unique phylogenetic position of HIV-1.

Mechanism of action of regulatory proteins encoded by complex retroviruses

Complex retroviruses are distinguished by their ability to control the expression of their gene products through the action of virally encoded regulatory proteins. These viral gene products modulate both the quantity and the quality of viral gene expression through regulation at both the transcriptional and posttranscriptional levels. The most intensely studied retroviral regulatory proteins, termed Tat and Rev, are encoded by the prototypic complex retrovirus human immunodeficiency virus type 1. However, considerable information also exists on regulatory proteins encoded by human T-cell leukemia virus type I, as well as several other human and animal complex retroviruses. In general, these data demonstrate that retrovirally encoded transcriptional trans-activators can exert a similar effect by several very different mechanisms. In contrast, posttranscriptional regulation of retroviral gene expression appears to occur via a single pathway that is probably dependent on the recruitment of a highly conserved cellular cofactor. These two shared regulatory pathways are proposed to be critical to the ability of complex retroviruses to establish chronic infections in the face of an ongoing host immune response.

Comparative analysis of Rev function in human immunodeficiency virus types 1 and 2

The Rev proteins of the related but distinct human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) display incomplete functional reciprocity. One possible explanation for this observation is that HIV-2 Rev is unable to interact with the HIV-1 Rev-response element (RRE1). However, an analysis of the biological activity of chimeric proteins derived from HIV-1 and HIV-2 Rev reveals that this target specificity does not map to the Rev RNA binding domain but is instead primarily determined by sequences known to mediate Rev multimerization. Both HIV-1 and HIV-2 Rev are shown to bind the RRE1 in vitro with identical RNA sequence specificity. The observation that HIV-2 Rev can inhibit RRE1-dependent HIV-1 Rev function in trans indicates that the direct interaction of HIV-2 Rev with the RRE1 also occurs in vivo. These data suggest that HIV-2 Rev forms a protein-RNA complex with the RRE1 that leads to only minimal Rev activity. It is hypothesized that this low level of Rev function results from the incomplete and/or aberrant multimerization of HIV-2 Rev on this heterologous RNA target sequence.

Functional analysis of biologically distinct genetic variants of simian immunodeficiency virus isolated from a mandrill

We examined the biological properties of two infectious clones of a simian immunodeficiency virus, SIVMND, which were designated as pMD121 and pMD122. Upon transfection into CD4-negative cells, pMD122 generated virions much less efficiently than pMD121. Likewise, the growth kinetics in CD4-positive cells of virus derived from pMD122 were remarkably delayed relative to those of virus from pMD121. The cytocidal activity of the MD122 virus was also low. A series of recombinant clones were constructed from pMD121 and pMD122 to determine the sequence responsible for the low virulence of the MD122 virus. The genetic determinant in pMD122 responsible for its properties was mapped to within a region (316 base pairs) encompassing the tat, rev, and env coding sequences. Sequence analysis revealed that the two clones differed by only one nucleotide in this region. A nucleotide substitution G (pMD121) to T (pMD122) altered an arginine codon to a serine codon in the first tat coding exon. Transient transfection experiments showed that the tat activity of pMD122 was about twofold less than that of pMD121. These findings indicate that small differences in tat activity can have a dramatic effect on the biological behavior of SIVMND.

Functional classification of simian immunodeficiency virus isolated from a chimpanzee by transactivators

In reporter-based transient expression systems, we characterized simian immunodeficiency virus from a chimpanzee (SIVCPZ), with special reference to the human immunodeficiency virus type 1 (HIV-1). SIVCPZ was not equally activated by tat and rev transactivators derived from representative primate lentiviruses. HIV-1 alone activated SIVCPZ to the full extent in both tat and rev assays. The tat and rev gene products of SIVCPZ, as well as those of HIV-1, efficiently transactivated the other viruses. These results indicate that SIVCPZ is identical to HIV-1 with regard to the compatibility of tat and rev gene activities among four subgroups of primate lentiviruses.

Isolation and characterization of a highly divergent HIV-2[GH-2]: generation of an infectious molecular clone and functional analysis of its rev-responsive element in response to primate retrovirus transactivators (Rev and Rex)

A highly divergent HIV-2 designated as HIV-2[GH-2] was obtained from an AIDS-related complex (ARC) patient in Ghana. A full-length molecular clone of this isolate was obtained and a biologically active clone was constructed. Its restriction pattern differed from that of prototype HIV-2[GH-1] in 25 of 35 restriction sites, but was strikingly similar to a previously characterized HIV-2 isolate from a Ghanaian (HIV-2ALT). The conserved integrase region (288-bp fragment) previously displayed 95% identity with that of ALT but 17-20% divergence from the HIV-2 prototype member, and a new distinct subgroup (HIV-2b) of HIV-2 consisting of GH-2 and ALT was postulated (Miura et al. 1991.) These isolates, however, were biologically distinguishable from each other by its replication capacity in a monocyte line, U937, in which GH-2 could not grow but ALT grew well. In addition, the nucleotide sequence of the LTR of this new isolate displays 21% divergence from that of prototype HIV-2[GH-1], but the core enhancer, Sp1 binding sites and TATA box were conserved. Although the 3' half of the env gene sequence which is deleted in HIV-2ALT clone showed 27% diversity from the prototype, functional differences in the rev-responsive element were not observed.

Embryonal long terminal repeat-binding protein is a murine homolog of FTZ-F1, a member of the steroid receptor superfamily

The embryonal long terminal repeat-binding protein, ELP, is present in undifferentiated mouse embryonal carcinoma cells. It binds to and suppresses transcription of the Moloney leukemia virus long terminal repeat in undifferentiated murine embryonal carcinoma cells. We report here that ELP is a mouse homolog of Drosophila FTZ-F1, which positively regulates transcription of the fushi tarazu gene in blastoderm-stage embryos of the fly. As members of the steroid receptor superfamily, ELP and FTZ-F1 have both DNA binding and putative ligand binding domains which are well conserved between the two. ELP and FTZ-F1 function in cells in the extremely early stage of development. A high degree of conservation between the two transcription factors during the evolution of these species indicates the importance of their functions in early-stage embryogenesis. In addition, the sequence elements they recognize do not contain repeat units, in contrast to other steroid receptors, which usually bind to either palindromic or direct repeat sequences.

The emergence of simian/human immunodeficiency viruses

Molecular evolutionary analyses strongly support the hypothesis that human immunodeficiency viruses have recently arisen from a diverse pool of nonhuman primate immunodeficiency viruses. Our understanding of the molecular phylogenetic relationships between primate and nonprimate lentiviruses is less certain, partly because key intermediate forms are still to be discovered. DNA and protein sequence comparisons reveal uncanny dissimilarities, as well as similarities, among the genetic sequences of these complex retroviruses, thereby giving rise to the notion that primate lentiviruses are participants in "fast-forward" evolution.

Embryonal long terminal repeat-binding protein is a murine homolog of FTZ-F1, a member of the steroid receptor superfamily

The embryonal long terminal repeat-binding protein, ELP, is present in undifferentiated mouse embryonal carcinoma cells. It binds to and suppresses transcription of the Moloney leukemia virus long terminal repeat in undifferentiated murine embryonal carcinoma cells. We report here that ELP is a mouse homolog of Drosophila FTZ-F1, which positively regulates transcription of the fushi tarazu gene in blastoderm-stage embryos of the fly. As members of the steroid receptor superfamily, ELP and FTZ-F1 have both DNA binding and putative ligand binding domains which are well conserved between the two. ELP and FTZ-F1 function in cells in the extremely early stage of development. A high degree of conservation between the two transcription factors during the evolution of these species indicates the importance of their functions in early-stage embryogenesis. In addition, the sequence elements they recognize do not contain repeat units, in contrast to other steroid receptors, which usually bind to either palindromic or direct repeat sequences.

A comparison of regulatory features in primate lentiviruses

Historically, research into the regulation of gene expression in primate lentiviruses has focused on human immunodeficiency virus type 1 (HIV-1), the primary cause of acquired immunodeficiency syndrome (AIDS) in humans. The increasing emergence of HIV-2 as a human pathogen, and the importance of the various simian immunodeficiency viruses (SIV) as models for the treatment and prevention of HIV-1-induced disease, suggest that an understanding of gene regulation in these related viruses will become increasingly important. Here, the present state of knowledge in this latter field is reviewed. In general, while the data support the hypothesis that viral gene expression is regulated by very similar mechanisms in all primate lentiviruses, it also is clear that differences in detail do exist. These differences may influence the pathogenic potential of the different strains of primate lentiviruses and must be considered in evaluating SIV as an appropriate in vivo model for HIV-1.

Genetic characterization of simian immunodeficiency virus isolated from an African mandrill

We constructed an infectious molecular clone of simian immunodeficiency virus from an African mandrill (SIVMND). Upon transfection, this clone directed the production of progeny virus particles infectious to and cytopathic for CD4+ human leukemia cells. Thirteen frameshift proviral mutants with an alteration in the eight open reading frames of SIVMND were generated by recombinant DNA techniques, and were analyzed biologically and biochemically. While mutations in the structural genes gag, pol, and env abolished viral growth and induction of cytopathology, mutants of the vif, vpr, and nef genes were fully biologically active. Of the tat and rev mutants, only one rev mutant grew in CD4+ cells with delayed kinetics. In reporter-based transient expression systems, transactivation potentials of the tat and rev mutants were evaluated. A mutant lacking 2nd coding exon of tat gene exhibited tat activity similar to that of the wild type clone. The infectious rev mutant was partially defective for rev gene activity.

SIV/HIV recombinants and their use in studying biological properties

A series of chimeric clones between human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency viruses (SIV) were constructed. Viability of the recombinant viruses was dependent on the position of recombination. Infectious chimeric viruses between HIV-1 and SIVAGM (isolated from an African green monkey) and those between HIV-1 and SIVMAC (isolated from a rhesus monkey) were examined for host cell tropism. Viral determinants that restrict the replication of SIVAGM in human MT-4 cells and that of HIV-1 in macaque monkey peripheral blood mononuclear cells (PBMC) mapped to the 5' half of the virus genome. One HIV-1/SIVMAC chimera which contained the HIV-1 env gene was shown to replicate in macaque PBMC in vitro and to infect macaque monkeys in vivo. This HIV-1/SIVMAC chimera will be useful for a variety of AIDS pathogenesis and vaccine studies.