The same inducible nuclear proteins regulates mitogen activation of both the interleukin-2 receptor-alpha gene and type 1 HIV

Both the interleukin-2 receptor-alpha (Tac, p55, IL-2R alpha) gene and the long terminal repeat (LTR) of type 1 HIV are activated by various T cell mitogens. We now demonstrate that an inducible 86 kd nuclear protein termed HIVEN86A specifically binds to both the enhancer element of the HIV-1 LTR and a closely related 12 bp sequence present in the 5' regulatory region (-267 to -256) of the IL-2R alpha gene. The interaction of these binding sites with HIVEN86A and perhaps other cellular proteins such as NF-kappa B appears importantly involved in mitogen activation since the isolated IL-2R alpha promoter binding site or single elements of the normally duplicated HIV-1 enhancer alone are sufficient to confer mitogen inducibility on an unresponsive heterologous promoter. Together these findings suggest that the normal action of an inducible nuclear DNA binding protein(s) involved in the regulation of IL-2R alpha gene expression can be subverted by the HIV-1 provirus to promote activation of retroviral gene transcription.

Kappa B-specific DNA binding proteins: role in the regulation of human interleukin-2 gene expression

Transcriptional activation of the human interleukin-2 (IL-2) gene, like induction of the IL-2 receptor alpha (IL-2R alpha) gene and the type 1 human immunodeficiency virus (HIV-1), is shown to be modulated by a kappa B-like enhancer element. Mutation of a kappa B core sequence identified in the IL-2 promoter (-206 to -195) partially inhibits both mitogen- and HTLV-I Tax-mediated activation of this transcription unit and blocks the specific binding of two inducible cellular factors. These kappa B-specific proteins (80 to 90 and 50 to 55 kilodaltons) similarly interact with the functional kappa B enhancer present in the IL-2R alpha promoter. These data suggest that these kappa B-specific proteins have a role in the coordinate regulation of this growth factor-growth factor receptor gene system that controls T cell proliferation.

Tumor necrosis factor alpha induces proteins that bind specifically to kappa B-like enhancer elements and regulate interleukin 2 receptor alpha-chain gene expression in primary human T lymphocytes

We have investigated the biochemical basis for the activation of interleukin 2 receptor alpha-subunit (IL-2R alpha) gene expression in primary human T lymphocytes by a cytokine (tumor necrosis factor alpha), a T-cell mitogen (phorbol 12-myristate 13-acetate), and the transactivator protein (Tax) from the type I human T-cell leukemia virus. Using in vivo transfection techniques specificially designed for these primary T cells in conjunction with in vitro gel retardation and DNA footprinting assays, we found that activation of the IL-2R alpha promoter by each of these agents involves the induction of nuclear proteins that specifically interact with a kappa B-like enhancer element (i.e., an element resembling the immunoglobulin kappa-chain enhancer sequence recognized by transcription factor NF-kappa B). DNA-protein crosslinking studies revealed that primary T cells express at least three different inducible DNA-binding proteins (50-55, 70-75, and 80-90 kDa) that specifically interact with this IL-2R alpha kappa B element.

Human immunodeficiency virus type 1 vectors efficiently transduce human hematopoietic stem cells

Lentiviruses are potentially advantageous compared to oncoretroviruses as gene transfer agents because they can infect nondividing cells. We demonstrate here that human immunodeficiency virus type 1 (HIV-1)-based vectors were highly efficient in transducing purified human hematopoietic stem cells. Transduction rates, measured by marker gene expression or by PCR of the integrated provirus, exceeded 50%, and transduction appeared to be independent of mitosis. Derivatives of HIV-1 were constructed to optimize the vector, and a deletion of most of Vif and Vpr was required to ensure the long-term persistence of transduced cells with relatively stable expression of the marker gene product. These results extend the utility of this lentivirus vector system.

HIV-1, HTLV-1 and normal T-cell growth: transcriptional strategies and surprises

In this review, Warner Greene and colleagues discuss recent studies that have revealed an intriguing molecular interplay between two pathogenic human retroviruses, HIV-1 and HTLV-1, and certain cellular genes that normally control T-cell growth. Activation of T cells during an immune response results in the induction of select transcription factors that bind specifically to kappa B enhancer elements present in both the IL-2R alpha and IL-2 genes. Normal T-cell growth is in part regulated by the transient expression of these genes. The Tax protein of HTLV-1 induces these same kappa B-specific proteins, but in contrast to immune stimulation, HTLV-1 infection of T cells leads to constitutive IL-2R alpha gene expression and immortalization. A second human retrovirus, HIV-1, can subvert the normal action of the kappa B-binding factors induced by these immune stimuli. Rather than promoting T-cell growth, these factors may augment viral replication and promote T-cell death.

Inhibition of human immunodeficiency virus type 1 replication in human T cells by retroviral-mediated gene transfer of a dominant-negative Rev trans-activator

Human immunodeficiency virus type 1 (HIV-1) is the causative agent of the acquired immunodeficiency syndrome (AIDS). Currently, no satisfactory treatment for this viral disease is available. Somatic gene therapy has been proposed as an alternative to conventional therapies. Several antiviral gene therapy approaches including ribozymes, antisense inhibition, and RNA-decoy strategies, as well as dominant-negative mutants of HIV-1 proteins (Gag, Tat, and Rev) have been suggested. To prove the concept of trans-dominant inhibition of HIV-1 replication, we transduced CEM cells with a retroviral vector encoding a dominant-negative rev gene. Amplification of integrase-specific proviral sequences from high molecular weight DNA indicated successful HIV-1 human T-lymphotropic virus type IIIB (HTLV-IIIB) infection of all cells. In contrast to CEM cells and CEM cells expressing the rev wild-type (wt) gene, infection of two CEM-RevM10 clones with HIV-1 did not result in the release of significant levels of p24 Gag antigen as measured by antigen capture assay, indicating a block in HIV-1 replication due to the presence of the trans-dominant Rev protein. Furthermore, the parental CEM cells as well as CEM cells expressing the Rev wt protein were effectively killed in the course of the HIV-1 infection, whereas all CEM cells expressing the RevM10 protein were unaffected in their growth rate.

Mesenchymal stem cells as vehicles for gene delivery

Mesenchymal stem cells contribute to the regeneration of mesenchymal tissues such as bone, cartilage, muscle, ligament, tendon, adipose, and marrow stroma. Transduction of mesenchymal stem cells from species other than humans is required for the development of disease models in which mesenchymal stem cells-based gene delivery is evaluated. Attempts to transduce mesenchymal stem cells from some species with amphotropic retroviral vectors were unsuccessful, leading to comparative mesenchymal stem cells transductions with xenotropic and gibbon-ape leukemia virus envelope-pseudotyped retroviral vectors. Human, baboon, canine, and rat mesenchymal stem cells were transduced optimally with amphotropic vector supernatants. In contrast, sheep, goat, and pig mesenchymal stem cells showed highest transduction levels with xenotropic retroviral vector supernatant, and rabbit mesenchymal stem cells were transduced optimally with gibbon-ape-enveloped vectors. Using a myeloablative canine transplantation model and gene-marked canine mesenchymal stem cells, the biodistribution of infused and ex vivo expanded mesenchymal stem cells were examined. The majority of transduced canine mesenchymal stem cells were found in the bone marrow samples. The current study shows the use of mesenchymal stem cells as a delivery vehicle for gene transfer studies, and validates the feasibility of delivering mesenchymal stem cells to the marrow compartment for stromal regeneration after cancer-associated cytotoxic therapies.

RevM10-expressing T cells derived in vivo from transduced human hematopoietic stem-progenitor cells inhibit human immunodeficiency virus replication

A key feature of the pathogenesis of human immunodeficiency virus type 1 (HIV-1) infection is the gradual loss of CD4-positive T cells. A number of gene therapy strategies have been designed with the intent of inhibiting HIV replication in mature T cells. As T cells are products of hematolymphoid differentiation, insertion of antiviral genes into hematopoietic stem cells could serve as a vehicle to confer long-term protection in progeny T cells derived from transduced stem cells. One such "cellular immunization" strategy utilizes the gene coding for the HIV-1 rev trans-dominant mutant protein RevM10 which has been demonstrated to inhibit HIV-1 replication in T-cell lines and in primary T cells. In this study, we used a Moloney murine leukemia virus-based retrovirus encoding a bicistronic message coexpressing RevM10 and the murine CD8-alpha' chain (Lyt2). This vector allows rapid selection of transgene-expressing cells as well as quantitation of transgene expression. We demonstrate that RevM10-transduced CD34-enriched hematopoietic progenitor-stem cells (HPSC) isolated from human umbilical cord blood or from granulocyte colony-stimulating factor-mobilized peripheral blood can give rise to mature thymocytes in the SCID-hu thymus/liver mouse model. The phenotypic distribution of HPSC-derived thymocytes is normal, and expression of the transgene can be detected by flow cytometric analysis. Moreover, we demonstrate that RevM10 can inhibit HIV replication in T cells derived from transduced HPSC after expansion in vitro. This is the first demonstration of anti-HIV efficacy in T cells derived from transduced human HPSC.

Stimulation of the human immunodeficiency virus type 1 enhancer by the human T-cell leukemia virus type I tax gene product involves the action of inducible cellular proteins

The human immunodeficiency virus type 1 (HIV-1) preferentially infects CD4+ T lymphocytes and may exist as a latent provirus within these cells for extended periods. The transition to a productive retroviral infection results in T-cell death and clinically may lead to the acquired immune deficiency syndrome. Accelerated production of infectious HIV-1 virions appears to be closely linked to a heightened state of T-cell activation. The transactivator (Tax) protein of the type I human T-cell leukemia virus (HTLV-I) can produce such an activated T-cell phenotype and augments activity of the HIV-1 long terminal repeat. One Tax-responsive region within the HIV-1 long terminal repeat has been mapped to a locus composed of two 10-base-pair direct repeats sharing homology with the binding site for the eucaryotic transcription factor NF-kappaB (GGGACTTTCC). Tax-expressing Jurkat T cells contain one or more inducible cellular proteins that specifically associate with the HIV-1 enhancer at these binding sites. Microscale DNA affinity precipitation assays identified a Tax-inducible 86-kilodalton protein, HIVEN86A, as one of these HIV-1 enhancer-binding factors. The interaction of HIVEN86A, and presumably other cellular proteins, with the HIV-1 enhancer appears functionally important as oligonucleotides corresponding to this enhancer were sufficient to impart Tax inducibility to an unresponsive heterologous promoter. These findings suggest that the Tax-inducible cellular protein HIVEN86A plays an important role in the transcriptional activation of the HIV-1 enhancer. These specific protein-DNA interactions may also be important for the transition of HIV-1 from a latent to a productive mode of infection. Furthermore, these findings highlight an intriguing biological interplay between HTLV-1 and HIV-1 through a cellular transcriptional pathway that is normally involved in T-cell activation and growth.

A novel human amphotropic packaging cell line: high titer, complement resistance, and improved safety

Successful retroviral-mediated gene therapy will depend on safe, efficient packaging cell lines for vector particle production. Existing packaging lines for murine leukemia virus (MLV)-based vectors are predominantly derived from NIH/3T3 cells which carry endogenous MLV sequences that could participate in recombination to form replication-competent retrovirus (RCR). To identify cells devoid of such sequences, we screened genomic DNA from eight cell lines. DNA from the human 293 cell line did not cross-hybridize with MLV sequences, and these cells were able to secrete Gag particles after transfection. We derived a stable amphotropic packaging cell line (called ProPak-A) in 293 cells in which the Gag-Pol and Env (packaging) functions are expressed separately from a heterologous (non-MLV) promoter, to maximally reduce homology between packaging and vector sequences. ProPak-A-based producer cells are efficient, yielding higher stable titers than PA317-based producers. In addition, a vector that consistently gave rise to RCR in PA317 cells never resulted in detectable RCR in ProPak-A-based producer cultures. We have also shown that ProPak-A-packaged particles are not inactivated by human serum. Thus, the packaging cells we describe are as efficient and safer than the amphotropic packaging cells most commonly used in clinical gene therapy work and are also more appropriate for in vivo gene delivery.

Scaffold attachment region-mediated enhancement of retroviral vector expression in primary T cells

We have studied retroviral transgene expression in primary human lymphocytes. Our data demonstrate that transgene expression is high in activated primary CD4+ T cells but significantly decreased in mitotically quiescent cells. Incorporation of a DNA fragment from the scaffold attachment region (SAR) of the human beta interferon gene into the vector improved transgene expression, particularly in quiescent cells. The SAR element functioned in an orientation-dependent manner and enhanced expression of Moloney murine leukemia virus- and murine embryonic stem cell-based vectors. Clonal analysis of transduced T cells showed that the SAR sequence did not confer position-independent expression on a transgene but rather prevented the decrease of expression when cells became quiescent. The SAR sequence also enhanced transgene expression in T cells generated from retrovirally transduced CD34-enriched hematopoietic progenitor-stem cells in a SCID-hu thymus-liver mouse model. We have used the SAR-containing retroviral vector to express the RevM10 gene, a trans-dominant mutant of the human immunodeficiency virus type 1 (HIV-1) Rev gene. Compared to a standard retroviral vector, the SAR-containing vector was up to 2 orders of magnitude more efficient in inhibiting replication of the HIV-1 virus in infected CD4+ peripheral blood lymphocyte populations in vitro. This is the first demonstration that SAR elements can be used to improve retroviral vector expression in human primary T cells.

Functional analysis of human T-cell leukemia virus type I rex-response element: direct RNA binding of Rex protein correlates with in vivo activity

The human T-cell leukemia virus type I rex gene product plays a critical role in the expression of the retroviral structural proteins Gag and Env from incompletely spliced mRNAs. Rex protein acts through a cis element (rex-response element [RxRE]) which is located in the U3/R region of the 3' long terminal repeat and is present on all human T-cell leukemia virus type I-specific mRNAs. Two domains of the predicted secondary structure of the RxRE are crucially important for Rex action in vivo as measured by two assay systems. In vitro studies using highly purified recombinant Rex protein revealed a specific and direct interaction with radiolabeled RxRE sequences. The correlation between our in vivo results and the direct binding of Rex protein to mutant and wild-type RxRE sequences supports both the existence of the predicted secondary structure and the importance of this direct interaction with the cis-acting RNA sequence for Rex function in vivo.

Functional mapping of the human immunodeficiency virus type 1 Rev RNA binding domain: new insights into the domain structure of Rev and Rex

Expression of human immunodeficiency virus type 1 (HIV-1) structural proteins requires the direct interaction of the viral trans-activator protein Rev with its cis-acting RNA sequence (Rev-response element [RRE]). A stretch of 14 amino acid residues of the 116-amino-acid Rev protein is sufficient to impose nucleolar localization onto a heterologous protein. Our results demonstrated that these same amino acid residues confer Rev-specific RRE binding to the heterologous human T-cell leukemia virus type I Rex protein. In addition, our results indicated that amino acids distinct from the nuclear localization signal are important for Rex-specific RRE RNA binding.

Regulation of interleukin 2 receptor alpha subunit (Tac or CD25 antigen) gene expression: binding of inducible nuclear proteins to discrete promoter sequences correlates with transcriptional activation

Transfection of deleted forms of the human interleukin 2 receptor alpha subunit (IL-2R alpha; also called CD25 or Tac antigen) gene (IL2RA) promoter revealed a requirement for sequences 3' of base -317 for phytohemagglutinin- and phorbol 12-myristate 13-acetate (PMA)-induced promoter activation in CD4+ Jurkat T cells. In contrast, sequences 3' of base -271 were sufficient for promoter induction in CD4-/CD8- YT-1 T cells or Jurkat cells expressing the transactivator protein (tat-I) of human T-cell lymphotropic virus type I (HTLV-I). Gel retardation assays revealed that nuclear extracts from induced, but not uninduced, Jurkat and YT-1 cells mediated the formation of two specific DNA-protein complexes with oligonucleotides spanning the region of the IL2RA promoter from position -291 to -245, which contains two imperfect direct repeats (IDRs). Consistent with the different 5' sequence requirements for promoter activation in Jurkat and YT-1 cells, oligonucleotides corresponding to the region from -267 to -243 (downstream IDR and flanking region) formed only one complex with induced Jurkat extracts but two complexes with induced YT-1 extracts. Oligonucleotides containing the region of the IL2RA promoter from -293 to -270 (upstream IDR and flanking region) failed to bind protein in either cell type. In further support of the biological significance of these DNA-protein interactions, the IL2RA oligonucleotide from -291 to -245 proved to be sufficient in either orientation to confer PMA inducibility to the mitogen-insensitive thymidine kinase gene promoter in Jurkat cells. Together, these findings suggest that the interaction of inducible DNA binding proteins with the IL2RA promoter between bases -291 and -245 plays an important role in mitogen-induced changes in the transcriptional activity of this receptor gene. Furthermore, the requisite 5' sequences appear to differ in T cells depending upon the nature of the activation signal and perhaps the stage of cellular maturation.

Definition of the human immunodeficiency virus type 1 Rev and human T-cell leukemia virus type I Rex protein activation domain by functional exchange

The human retroviruses human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type I (HTLV-I) are characterized by complex regulation of gene expression. Each virus encodes a posttranscriptional regulator, the 19-kDa HIV-1 Rev protein and the 27-kDa HTLV-I Rex protein, which is required for viral replication. Expression of these trans activators results in the cytoplasmic accumulation of unspliced or singly spliced viral mRNA which encode the gag, pol, and env gene products. The finding that the HTLV-I Rex protein is able to functionally substitute for the Rev protein of HIV-1 indicates that HIV-1 Rev and HTLV-I Rex may interact with the same component of a cellular pathway involved in either mRNA splicing or transport. In this study, we have generated functional Rev/Rex hybrid proteins by domain exchange. We have defined, using in vivo and in vitro analyses, the activation domains of Rev and Rex which are the putative targets of a common host cell factor(s) required for Rev and Rex function.

HTLV-I tax induces cellular proteins that activate the kappa B element in the IL-2 receptor alpha gene

Jurkat T cell lines constitutively expressing Tax, the 40-kilodalton transactivator protein of human T lymphotropic virus type I (HTLV-I), were used to investigate the mechanism by which this viral product deregulates the expression of the interleukin-2 receptor alpha gene (IL-2R alpha, Tac). Transfection of deleted forms of the IL-2R alpha promoter and in vitro DNA-binding studies revealed that a 12-base pair promoter segment, which has homology with the binding site for NF-kappa B, was required for Tax-induced activation of the IL-2R alpha promoter in vivo. An 18-base pair oligonucleotide containing this kappa B-like regulatory element proved sufficient to confer Tax inducibility upon a heterologous promoter. DNA affinity precipitation assays showed that Tax, like mitogenic stimuli, induced the expression of the 86-kilodalton cellular protein HIVEN86A, which specifically binds to the IL-2R alpha kappa B element in vitro. Furthermore, DNA/protein cross-linking studies revealed that several polypeptides interact with this sequence motif. Thus, the deregulation of IL-2R alpha gene expression encountered in HTLV-I leukemias appears to involve Tax activation of one or more cellular proteins that are normally induced by mitogens and that directly contribute to transcriptional activation of this receptor gene.

High transdominant RevM10 protein levels are required to inhibit HIV-1 replication in cell lines and primary T cells: implication for gene therapy of AIDS

Expression of antiviral genes in CD4+ T cells has been proposed as a strategy for gene therapy of AIDS. Over the past years, we and others have developed retroviral vectors encoding the RevM10 protein, a dominant-negative mutant of the HIV-1 Rev trans-activator protein. We could demonstrate gene transfer and inhibition of HIV-1 replication in cultured T cell lines and primary T cells. However, little is known about the levels of the antiviral protein required to achieve a therapeutic effect, particularly in primary cells. In this report, we compare different vector designs with regard to expression of the antiviral gene to develop an optimal vector for clinical applications. Our results demonstrate that intracellular steady-state RevM10 protein levels expressed from the Moloney murine leukemia virus (MoMLV), myeloproliferative sarcoma virus (MPSV) or mouse embryonic stem cell virus (MESV) promoters located in the long terminal repeat (LTR) were uniformly higher than from internal promoters (eg CMV, PGK). Analysis of selected vectors in acutely and chronically HIV-infected cell lines suggested that threshold levels of RevM10 expression are required to achieve inhibition of HIV replication. LTR-driven RevM10 expression also yielded high steady-state protein levels in activated primary T cells resulting in inhibition of HIV replication, and there was no apparent difference between the MoMLV, MPSV and MESV-LTR vectors. However, RevM10 expression was down-regulated in resting primary cells and consequently anti-HIV efficacy was significantly reduced. Taken together, the data suggest that relatively high steady-state levels of RevM10 protein are required to achieve inhibition of HIV replication and that the MPSV- and MESV-derived retroviral vectors show no advantage over the MoMLV-based vectors for expression of anti-HIV genes in human T cells.

Improved detection of replication-competent retrovirus

Recombinant retroviral vectors are the predominant delivery system in human gene therapy protocols. Since contaminating replication-competent retrovirus (RCR) can arise during the production of retroviral vector supernatants, sensitive assays for the screening of supernatants are necessary. In this study, we present a marker rescue assay based upon a Mus dunni cell line stably transduced with a lacZ gene. We show that detection of RCR in vector supernatants by the M. dunni lacZ marker rescue assay or PG-4 S+ L- focus-forming assay is equally sensitive. By inoculating test supernatants under centrifugation (which we term spinoculation), we increased the sensitivity of detection of RCR 10 to 100-fold with the PG-4 S+ L- and lacZ marker rescue assays. While the spinoculation protocol had no adverse effects on cells, spinoculation of high titer vector supernatants onto PG-4 cells resulted in some cytotoxicity, making identification of RCR positive cultures difficult. However, spinoculation of vector supernatants onto M. dunni lacZ cells resulted in no cytotoxicity, and also partially overcame inhibition of detection of low levels of RCR due to the presence of high titer replication-incompetent vector.

RevM10-mediated inhibition of HIV-1 replication in chronically infected T cells

Two clinical regimens have been proposed for gene therapies of acquired immunodeficiency syndrome (AIDS): (i) Genetic modification of differentiated peripheral mononuclear cells ex vivo and (ii) gene delivery into hematopoietic stem/progenitor cells ex vivo. Various antiviral strategies targeted at different molecular processes in the human immunodeficiency virus type 1 (HIV-1) life cycle are currently being pursued, all with the goal of reducing HIV-1 replication. Until now, all successful studies have reported inhibition in acutely HIV-infected cells that had been genetically modified prior to infection. These promising results do not address a clinically relevant question: What is the contribution of already infected peripheral mononuclear and hematopoietic stem/progenitor cells to disease progression? In this report, we demonstrate inhibition of both HIV-1 replication and production of infectious particles in chronically infected human T leukemia cell lines. The antiviral effect on the transduced cell population correlates with the expression of the dominant-negative RevM10 protein. This is the first demonstration that a gene therapy-based treatment can achieve antiviral efficacy in human T leukemia cells chronically infected with HIV-1.

Mutational analysis of functional domains in the HIV-1 Rev trans-regulatory protein

HIV-1 replication depends on the expression of trans-regulatory genes (tat, rev) encoded in the 3' part of the retroviral genome. HIV-1 Rev trans-activator protein allows the cytoplasmic translocation of incompletely spliced retroviral mRNA which is required for the translational switch from regulatory (Tat, Rev, Nef) to structural proteins (Gag, Pol, Env). The HIV-1 Rev regulatory protein comprises an activation domain (RAD) and a RNA binding domain (RBD). Both functional domains are not well defined and the RBD appears to overlap with the nuclear localization signal (NLS). Our mutational analysis localized the Rev protein domain important for RRE (nucleotide 7781 to 8000) binding in vitro to amino acid residues 31 to 50. Mutations in this domain always resulted in exclusion from the nucleoli. Furthermore, these mutants did not support Rev-dependent p24 Gag production in vivo. Sequences immediately upstream of this domain (RevM4, RevM19) were attenuated in their in vivo activity possibly indicating a role in Rev protein oligomerization. The observed tight correlation between subcellular localization and RNA binding in vitro indicates that this short stretch of amino acids supports two essential functions required for HIV-1 replication.

Retroviral vectors designed for targeted expression of RNA polymerase III-driven transcripts: a comparative study

Retroviral gene delivery systems for RNA polymerase II (RNA pol II)-based promoters have been developed and are widely used in gene transfer studies. In contrast, gene delivery systems with RNA pol III-based expression cassettes have not been studied comprehensively, although therapeutic applications (e.g., ribozymes, antisense, triplex RNA and RNA decoys) have been proposed. In this report, we describe retroviral vectors designed to optimize expression of short chimeric RNAs transcribed from a number of RNA pol III promoters. Our results show that all analysed RNA pol III expression cassettes (tRNA, U6, Ad VA1), regardless of orientation, do not transcribe efficiently when located between the retroviral long terminal repeats (LTRs). In contrast, high steady-state expression levels can be achieved by inserting the RNA pol III expression cassette into the U3 region of the LTR (double-copy design). Compared to human tRNA gene promoters (tRNA(Met), tRNA(Val)), the human small nuclear RNA U6 gene (U6) and the adenovirus virus-associated RNA 1 (Ad VA1) gene promoters yielded higher expression levels. The majority of the chimeric U6-derived transcripts were detected in the nuclear RNA fraction, and the VA1 and tRNA-driven transcripts were predominantly detected in the cytoplasmic compartments. This report is the first comparative study of RNA pol III-driven promoters expressing short chimeric transcripts leading to an optimized retroviral-vector design.

Interaction of distinct nuclear proteins with sequences controlling the expression of polyomavirus early genes

The interaction between cellular factors and polyoma virus (Py) DNA was investigated by using a gel retention assay. Nuclear extracts from various cell lines (NIH 3T3, NIH 3T6, LTK-, F9) contained proteins that formed specific and distinct complexes with Py B enhancer fragments of either wild-type or F9-1 mutant origin. The presence of an excess amount of other well-characterized DNA sequences, including the Py A enhancer, the murine sarcoma virus enhancer, and the simian virus 40 enhancer-promoter region, did not interfere with this protein-DNA interaction. However, a fragment previously defined as containing the lymphotropic papovavirus enhancer shares the binding of some common factor. This observation, in combination with the results of retention gel assays at different Mg2+ concentrations, indicates the interaction of several nuclear factors and Py DNA. The assay systems that were used allowed a distinction between some factors on the basis of their different biochemical and sequence requirements. The contact sites of these complexes were mapped to the B enhancer region of Py with Bal 31-derived mutant restriction fragments and ExoIII nuclease and are compatible with the functional domains determined in vivo.

Functional characterization of a complex protein-DNA-binding domain located within the human immunodeficiency virus type 1 long terminal repeat leader region

Transcriptional trans activation of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) by the viral tat trans activator is mediated by an LTR-specific sequence located immediately 3' to the start of transcription initiation. We have used a range of molecular techniques to examine DNA-protein interactions that occur in the vicinity of this cis-acting sequence. Our results demonstrate the existence of a sequence-specific DNA-protein interaction involving the HIV-1 leader DNA and map this binding event to between -2 and +21 base pairs relative to the HIV-1 LTR transcription start site. Evidence suggesting that this interaction involves three distinct protein-DNA contact sites extending along one side of the DNA helix is presented. Mutation of these sites was found to ablate protein-DNA binding yet was observed to have no effect on either the basal or tat trans-activated level of HIV-1 LTR-specific gene expression. We therefore conclude that this DNA-protein interaction has a function distinct from the regulation of HIV-1 LTR-specific gene expression.

Scanning mutagenesis of the arginine-rich region of the human immunodeficiency virus type 1 Rev trans activator

The structural proteins of human immunodeficiency virus type 1, for example, Gag and Env, are encoded by unspliced and incompletely spliced viral transcripts. The expression of these mRNAs in the cytoplasm, along with their commensurate translation, is absolutely dependent on the virally encoded Rev trans activator. Previous studies have demonstrated that Rev binds directly to its substrate mRNAs via an arginine-rich element that also serves as its nuclear localization sequence. In an attempt to define the specific amino acid residues that are important for in vivo activity, we have constructed a series of missense mutations that scan across this region. Our data demonstrate that all eight arginine residues within this element can, individually, be substituted for either leucine or lysine with no apparent loss of function. Importantly, these findings suggest that no single amino acid within the arginine-rich domain of Rev is, by itself, essential for activity and that considerable functional redundancy is therefore likely to exist within this region. Interestingly, one mutant in which a tryptophan had been substituted for a serine failed to accumulate exclusively in the nucleus but still bound RNA in a manner that was indistinguishable from that of the wild-type protein. This observation indicates that features of the arginine-rich region that are additional to those required for RNA binding are important for Rev's correct accumulation in the nucleus.

Novel retroviral packaging cell lines: complementary tropisms and improved vector production for efficient gene transfer

We report increased transduction of human hematopoietic progenitor cells through a combination of novel retroviral vector packaging cell lines, and improved vector supernatant production. The new ProPak packaging cell lines produce either murine leukemia virus (MLV) xenotropic (ProPak-X cells) or amphotropic particles (ProPak-A cells), and ProPak-based producer cells were demonstrated to be free of replication-competent retrovirus (RCR) by stringent testing. Vector supernatants from ProPak or existing packaging cell lines producing different pseudotyped particles (amphotropic MLV, xenotropic MLV or gibbon ape leukemia virus) were compared for the ability to transduce clinically relevant human hematopoietic cells. All vector types transduced primary human CD34-positive or CD4-positive cells, regardless of tropism. However, consistently higher transduction of target cells was achieved with ProPak-derived amphotropic vector than with PA317-packaged amphotropic vector. The highest transduction of human hematopoietic progenitor cells was achieved with vector supernatant generated from a coculture of the ProPak-X and ProPak-A cell lines. This ping-pong amplification yielded supernatant containing vector targeted to two distinct receptors present on human cells, and did not result in detectable RCR formation. In addition, we describe conditions for improved vector supernatant production in a packed-bed bioreactor.