Expression of genes introduced into cells by retroviral infection is more efficient than that of genes introduced into cells by DNA transfection

Generation of a stable packaging cell line producing high-titer PPT-deleted integration-deficient lentiviral vectors

The risk of insertional mutagenesis inherent to all integrating exogenous expression cassettes was the impetus for the development of various integration-defective lentiviral vector (IDLV) systems. These systems were successfully employed in a plethora of preclinical applications, underscoring their clinical potential. However, current production of IDLVs by transient plasmid transfection is not optimal for large-scale production of clinical grade vectors. Here, we describe the development of the first tetracycline-inducible stable IDLV packaging cell line comprising the D64E integrase mutant and the VSV-G envelope protein. A conditional self-inactivating (cSIN) vector and a novel polypurine tract (PPT)-deleted vector were incorporated into the newly developed stable packaging cell line by transduction and stable transfection, respectively. High-titer (~10(7) infectious units (IU)/ml) cSIN vectors were routinely generated. Furthermore, screening of single-cell clones stably transfected with PPT-deleted vector DNA resulted in the identification of highly efficient producer cell lines generating IDLV titers higher than 10(8) IU/mL, which upon concentration increased to 10(10) IU/ml. IDLVs generated by stable producer lines efficiently transduce CNS tissues of rodents. Overall, the availability of high-titer IDLV lentivirus packaging cell line described here will significantly facilitate IDLV-based basic science research, as well as preclinical and clinical applications.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

Genetic modification of hematopoietic stem cells with nonviral systems: past progress and future prospects

Serious unwanted complications provoked by retroviral gene transfer into hematopoietic stem cells (HSCs) have recently raised the need for the development and assessment of alternative gene transfer vectors. Within this context, nonviral gene transfer systems are attracting increasing interest. Their main advantages include low cost, ease of handling and large-scale production, large packaging capacity and, most importantly, biosafety. While nonviral gene transfer into HSCs has been restricted in the past by poor transfection efficiency and transient maintenance, in recent years, biotechnological developments are converting nonviral transfer into a realistic approach for genetic modification of cells of hematopoietic origin. Herein we provide an overview of past accomplishments in the field of nonviral gene transfer into hematopoietic progenitor/stem cells and we point at future challenges. We argue that episomally maintained self-replicating vectors combined with physical methods of delivery show the greatest promise among nonviral gene transfer strategies for the treatment of disorders of the hematopoietic system.

Retroviral vectors for human gene delivery

The potential for gene therapy to cure a wide range of diseases has lead to high expectations and a great increase in research efforts in this area. At present, viral vectors are the most efficient means of delivering a corrective gene into human cells. While a number of different viral vectors are under development, retroviral vectors are currently the most common type used in clinical trials today. However, the production of retroviral vectors for gene therapy applications faces a number of challenges. Of primary concern is the low titre of vector stocks produced by packaging cells in culture and the inherent instability of retroviral vector activity. The problems facing large-scale retroviral vector production are outlined in this review and the research efforts by a number of groups who have attempted to optimise production methods are presented.

Rescue of retroviral envelope fusion deficiencies by cationic liposomes

BACKGROUND

Retroviral particles that are inappropriately enveloped can transduce target cells if pre-associated with cationic liposomes. This study optimises and addresses the mechanism of liposome-enhanced gene delivery, and explores the potential for such agents to compensate for fusion deficiency associated with chimaeric envelope proteins.

METHODS

Particles bearing wild-type, chimaeric or no envelope proteins were complexed with DOTAP or DC-Chol/DOPE cationic liposomes and added to target cells for various times. Particle binding was determined by detection of cell-associated capsid protein and infectivity was measured histochemically.

RESULTS

Stable association of cationic liposomes with retrovirus particles significantly enhanced their binding rate to target cells in proportion to the increase of transduction kinetics for infectious virus. Binding of virus was equivalent with or without envelope protein and/or virus receptor, indicating that a non-specific interaction precedes receptor recognition. Non-infectious combinations were rescued by the intrinsic fusogenicity of the cationic liposomes, which enabled entry of the viral core, but left subsequent events unaltered. The optimised transduction rate with non-enveloped particles and DOTAP approached that of amphotropic-enveloped virus in some cases, although the effect was target-cell-dependent. DC-Chol/DOPE was less potent at direct fusion but was able to enhance 600-fold the receptor-dependent action of chimaeric envelopes that were deficient in fusion by virtue of the addition of targeting domains.

CONCLUSIONS

These data have implications for the development of retroviral vector targeting strategies from the perspectives of the specificity of target cell interaction and compensating for chimaeric envelope fusion deficiency.

In vivo analysis of retroviral enhancer mutations in hematopoietic cells: SP1/EGR1 and ETS/GATA motifs contribute to long terminal repeat specificity

The objective of this work was to identify, in the context of chromosomally integrated DNA, the contribution of defined transcription factor binding motifs to the function of a complex retrovirus enhancer in hematopoietic cells in vivo. Repopulating murine hematopoietic cells were transduced with equal gene dosages of replication-incompetent retrovirus vectors encoding enhanced green fluorescent protein. Enhancer sequences were derived from mouse spleen focus-forming virus. Destruction of GC-rich sites representing overlapping targets for SP1 or EGR1 uniformly attenuated gene expression (approximately 25 to 70% of wild-type levels) in all hematopoietic lineages, as shown by multicolor flow cytometry of peripheral blood and bone marrow cells at various time points posttransplantation. In contrast, a point mutation within a dual ETS/GATA motif that abolished transactivation by ETS factors but not by GATA-1 slightly increased activity in erythroid cells and significantly attenuated enhancer function in T lymphocytes. This study shows that controlled gene transfer in transplantable hematopoietic cells allows a functional analysis of distinct cis elements within a complex retrovirus enhancer, as required for the characterization and engineering of various cellular and viral regulatory sequences in basic research and gene therapy.

Generation of replication-defective helper-free vectors based on simian immunodeficiency virus

A systematic study on generating simian immunodeficiency virus (SIV)-based vectors was carried out. The goal was to generate helper-free, replication-defective SIVmac-based vectors at high titers. The general approach was to cotransfect into human 293T cells a plasmid carrying the vector construct along with two helper plasmids that together expressed the SIVmac virion proteins. Initial vectors carried the bacterial beta-galactosidase gene (beta-gal). These vectors had a technical difficulty: "pseudotransduction" of beta-gal protein produced during the 293T cell transfections. As a result, infection of cultures with these vector stocks also resulted in passive transfer into, and X-gal staining of, cells that had not actually been infected by the vector. A second generation of vectors expressing the enhanced jellyfish green fluorescence protein (EGFP) was not subject to this artifact. A systematic study of the SIVmac-based EGFP vectors was carried out. Helper-free vector stocks were obtained when helper plasmids lacking the SIVmac packaging signals were used. By employing envelope helper plasmids derived from different SIVmac isolates, it was possible to generate SIVmac-based vectors pseudotyped with envelope proteins of different cell tropism. Optimization of vector and helper plasmid structures, transfection conditions, and infection procedures ultimately yielded vector titers in excess of 10(6)/ml.

Transgene expression in zebrafish: A comparison of retroviral-vector and DNA-injection approaches

To assess alternative methods for introducing expressing transgenes into the germ line of zebrafish, transgenic fish that express a nuclear-targeted, enhanced, green fluorescent protein (eGFP) gene were produced using both pseudotyped retroviral vector infection and DNA microinjection of embryos. Germ-line transgenic founders were identified and the embryonic progeny of these founders were evaluated for the extent and pattern of eGFP expression. To compare the two modes of transgenesis, both vectors used the Xenopus translational elongation factor 1-alpha enhancer/promoter regulatory cassette. Several transgenic founder fish which transferred eGFP expression to their progeny were identified. The gene expression patterns are described and compared for the two modes of gene transfer. Transient expression of eGFP was detected 1 day after introducing the transgenes via either DNA microinjection or retroviral vector infection. In both cases of gene transfer, transgenic females produced eGFP-positive progeny even before the zygotic genome was turned on. Therefore, GFP was being provided by the oocyte before fertilization. A transgenic female revealed eGFP expression in her ovarian follicles. The qualitative patterns of gene expression in the transgenic progeny embryos after zygotic induction of gene expression were similar and independent of the mode of transgenesis. The appearance of newly synthesized GFP is detectable within 5-7 h after fertilization. The variability of the extent of eGFP expression from transgenic founder to transgenic founder was wider for the DNA-injection transgenics than for the retroviral vector-produced transgenics. The ability to provide expressing germ-line transgenic progeny via retroviral vector infection provides both an alternative mode of transgenesis for zebrafish work and a possible means of easily assessing the insertional mutagenesis frequency of retroviral vector infection of zebrafish embryos. However, because of the transfer of GFP from oocyte to embryo, the stability of GFP may create problems of analysis in embryos which develop as quickly as those of zebrafish.

Threshold number of provirus copies required per cell for efficient virus production and interference in moloney murine leukemia virus-infected NIH 3T3 cells

The gag-pol readthrough mutant of Moloney murine leukemia virus, MLV-B(CAG) (T. Odawara, H. Yoshikura, M. Oshima, T. Tanaka, D. S. Jones, F. Nemoto, Y. Kuchino, and A. Iwamoto, J. Virol. 65:6376-6379, 1991), was poorly complemented by a mutant encoding only Gag. This is because with all the genetic elements necessary for env expression present in MLV-B(CAG), insufficient Env protein was produced by the cells expressing MLV-B(CAG) for efficient virus production. Since the env mRNA expression per provirus in the MLV-B(CAG)- and wild-type-MLV-producing cells were the same and since the cells expressing the former contained eightfold fewer proviral copies, the insufficient Env expression by the former was found to be due to insufficient proviral copies in the cells. Examination of the cell clones having various proviral copies of Deltawt MLV (M. Oshima, T. Odawara, T. Matano, H. Sakahira, Y. Kuchino, A. Iwamoto, and H. Yoshikura, J. Virol. 70:2286-2295, 1996) showed that mRNA level was proportional to the number of proviral copies while interference and virus production followed a sigmoid curve with a sharp rise at the threshold number of proviral copies of around four per cell. Multicycle infection probably continues until the threshold level of proviral copies is attained in natural infection too.

Excision of an integrated provirus by the action of FLP recombinase

Retroviral vectors can be used to insert a single, intact copy of a transgene into a chromosome. If the duplication of the LTR (long-terminal repeat) that naturally occurs during reverse transcription of the retroviral genome is exploited to introduce two equally oriented FLP recognition target (FRT) sites, a substrate for FLP recombinase is created. A pulse of FLP recombinase activity can then be applied to excise the intervening sequences with the retention of a single LTR. This procedure is of potential use for manipulating an integration site after a period of expression enabling a variety of critical controls. We describe the properties of such a retroviral vector containing a dicistronic expression cassette with a reporter gene in the first and a positive/negative selection marker in the second cistron. This vector permits the selection and control of each step during the sequence of genomic manipulations enabled by site-specific recombination events.

Establishment of parameters for optimal transduction efficiency and antitumor effects with purified high-titer HSV-TK retroviral vector in established solid tumors

Suicide gene therapy using the herpes simplex thymidine kinase gene and ganciclovir is an attractive strategy for solid tumors. Early animal studies involved intratumoral injection of retroviral producer cells or unprocessed supernatant to generate an antitumor effect. Xenotransplantation of producer cells proved effective in several models, but the crude supernatants from the same cells were of insufficient titer to produce antitumor effects. We have developed new non-murine producer lines that yield replication-defective retroviral vectors encoding thymidine kinase at high titer which are then further purified and processed, resulting in pharmaceutical grade retroviral vectors with titers of up to 10(8) cfu/ml. Purified, high-titer retroviral preparations were injected directly into solid tumors in two syngeneic mouse tumor models. Significant antitumor responses and some cures were observed following systemic ganciclovir therapy. Assays using monoclonal antibodies to measure thymidine kinase protein expression at the single cell level in vitro and in vivo were developed so that therapeutic transgene expression could be quantified. Intralesional delivery resulted in transduction of over 20% of tumor cells in a protocol designed to maximize transduction on the basis of separate analyses of route, dosage, and schedule of vector administration. A consensus strategy evolved in which the combined effects of increased titer and a longer duration of retroviral vector administration interact to maximize transduction efficiency. These results indicate that purified high-titer retroviral vectors have the potential to transfer effective quantities of therapeutic genes into solid tumors in human subjects and highlight some pharmacologic factors that could be valuable in the design of clinical gene therapy protocols.

3'-end processing and kinetics of 5'-end joining during retroviral integration in vivo

Retroviral replication depends on integration of viral DNA into a host cell chromosome. Integration proceeds in three steps: 3'-end processing, the endonucleolytic removal of the two terminal nucleotides from each 3' end of the viral DNA; strand transfer, the joining of the 3' ends of viral DNA to host DNA; and 5'-end joining (or gap repair), the joining of the 5' ends of viral DNA to host DNA. The 5'-end joining step has never been investigated, either for retroviral integration or for any other transposition process. We have developed an assay for 5'-end joining in vivo and have examined the kinetics of 5'-end joining for Moloney murine leukemia virus (MLV). The interval between 3'-end and 5'-end joining is estimated to be less than 1 h. This assay will be a useful tool for examining whether viral or host components mediate 5'-end joining. MLV integrates its DNA only after its host cell has completed mitosis. We show that the extent of 3'-end processing is the same in unsynchronized and aphidicolin-arrested cells. 3'-end processing therefore does not depend on mitosis.

Excision of specific DNA-sequences from integrated retroviral vectors via site-specific recombination

Vectors for gene transfer and gene therapy were developed which combine the advantages of the integrase and recombinase systems. This was achieved by inserting two loxP sites for specific DNA excision into an MESV based retroviral vector. We show that this 'retroviral lox system' allows the infection of cells and the expression of transferred genes. In addition, we constructed an efficient retrovirus-based expression system for a modified Cre recombinase. Functional tests for DNA excision from integrated retroviral lox vectors were performed by the use of a negative selectable marker gene (thymidine kinase). Cre expression in cells infected with retroviral lox vectors and subsequent BrdU selection for cells in which site-specific recombination has occurred results in large numbers of independent cell clones. These results were confirmed by detailed molecular analysis. In addition we developed retroviral suicide vectors in which the enhancer/promoter elements of both LTRs were replaced by lox sequences. We show that lox-sequences located in the LTRs of retroviral vectors are stable during retroviral replication. Potential applications of this system would be the establishment of revertants of retrovirus-infected cells by controlled excision of nearly the complete proviral DNA.

A simple and efficient procedure for generating stable expression libraries by cDNA cloning in a retroviral vector

cDNA expression cloning is a powerful method for the rescue and identification of genes that are able to confer a readily identifiable phenotype on specific cell types. Retroviral vectors provide several advantages over DNA-mediated gene transfer for the introduction of expression libraries into eukaryotic cells since they can be used to express genes in a wide range of cell types, including those that form important experimental systems such as the hemopoietic system. We describe here a straightforward and efficient method for generating expression libraries by using a murine retroviral vector. Essentially, the method involves the directional cloning of cDNA into the retroviral vector and the generation of pools of stable ecotropic virus producing cells from this DNA. The cells so derived constitute the library, and the virus they yield is used to infect appropriate target cells for subsequent functional screening. We have demonstrated the feasibility of this procedure by constructing several large retroviral libraries (10(5) to 10(6) individual clones) and then using one of these libraries to isolate cDNAs for interleukin-3 and granulocyte-macrophage colony-stimulating factor on the basis of the ability of these factors to confer autonomous growth on the factor-dependent hemopoietic cell line FDC-P1. Moreover, the frequency at which these factor-independent clones were isolated approximated the frequency at which they were represented in the original plasmid library. These results suggest that expression cloning with retroviruses is a practical and efficient procedure and should be a valuable method for the isolation of important regulatory genes.

A simple and efficient procedure for generating stable expression libraries by cDNA cloning in a retroviral vector

cDNA expression cloning is a powerful method for the rescue and identification of genes that are able to confer a readily identifiable phenotype on specific cell types. Retroviral vectors provide several advantages over DNA-mediated gene transfer for the introduction of expression libraries into eukaryotic cells since they can be used to express genes in a wide range of cell types, including those that form important experimental systems such as the hemopoietic system. We describe here a straightforward and efficient method for generating expression libraries by using a murine retroviral vector. Essentially, the method involves the directional cloning of cDNA into the retroviral vector and the generation of pools of stable ecotropic virus producing cells from this DNA. The cells so derived constitute the library, and the virus they yield is used to infect appropriate target cells for subsequent functional screening. We have demonstrated the feasibility of this procedure by constructing several large retroviral libraries (10(5) to 10(6) individual clones) and then using one of these libraries to isolate cDNAs for interleukin-3 and granulocyte-macrophage colony-stimulating factor on the basis of the ability of these factors to confer autonomous growth on the factor-dependent hemopoietic cell line FDC-P1. Moreover, the frequency at which these factor-independent clones were isolated approximated the frequency at which they were represented in the original plasmid library. These results suggest that expression cloning with retroviruses is a practical and efficient procedure and should be a valuable method for the isolation of important regulatory genes.

Generation of human monoclonal antibodies by transformation of lymphoblastoid B cells with ras oncogene

Human monoclonal antibodies (hu-mAbs) of predetermined specificity and isotype are potentially important for a variety of applications, including therapy and diagnosis. Their efficient generation, however, is still hampered by technical difficulties. Even the most established approaches to the generation of hu-mAbs, i.e., B cell immortalization by Epstein-Barr virus (EBV) and/or fusion with appropriate myeloma cell lines, are characterized by a relatively low efficiency. It has been shown that expression of activated Ha- or N-ras oncogenes causes the malignant transformation and plasmacytoid differentiation of EBV-immortalized lymphoblastoid cell (LC) lines, suggesting that activated ras oncogenes can convert LC lines into effective hu-mAb producers. We have used retroviral vector-mediated gene transfer to introduce an activated Ha-ras (v-ras) oncogene into four distinct LC lines producing hu-mAbs of different classes (IgM and IgG) and specificities (to human insulin, human thyroglobulin and rabies virus glycoprotein). The cloning efficiency and antibody secretion of these ras-transformed LC (ras-LC) lines were compared with those of the hybrid LC (hyb-LC) lines generated by fusing the same parental LC lines with the Ig non-secretor F3B6 human-mouse hybrid cells. ras-LC lines were comparable to their hybrid counterparts in either parameter tested. This, together with the relatively higher efficiency of the method, suggests that ras transformation may constitute a valid alternative to the currently available technologies for hu-mAbs production from LC lines.

Retroviral vectors for persistent expression in vivo

Retroviral vectors provide a safe and efficient method of introducing genes of therapeutic interest into dividing cells. The principle limitation of these vectors in the past has been poor gene expression in vivo. This problem has been overcome recently through the use of tissue-specific enhancers in commonly used retroviral vectors. In this review we discuss both the relevant biology and some of the practical applications of retroviral vectors in gene therapy.

Moloney murine sarcoma virus MuSVts110 DNA: cloning, nucleotide sequence, and gene expression

We have cloned Moloney murine sarcoma virus (MuSV) MuSVts110 DNA by assembly of polymerase chain reaction (PCR)-amplified segments of integrated viral DNA from infected NRK cells (6m2 cells) and determined its complete sequence. Previously, by direct sequencing of MuSVts110 RNA transcribed in 6m2 cells, we established that the thermosensitive RNA splicing phenotype uniquely characteristic of MuSVts110 results from a deletion of 1,487 nucleotides of progenitor MuSV-124 sequences. As anticipated, the sequence obtained in this study contained precisely this same deletion. In addition, several other unexpected sequence differences were found between MuSVts110 and MuSV-124. For example, in the noncoding region upstream of the gag gene, MuSVts110 DNA contained a 52-nucleotide tract typical of murine leukemia virus rather than MuSV-124, suggesting that MuSVts110 originated as a MuSV-helper murine leukemia virus recombinant during reverse transcription rather than from a straightforward deletion within MuSV-124. In addition, both MuSVts110 long terminal repeats contained head-to-tail duplications of eight nucleotides in the U3 region. Finally, seven single-nucleotide substitutions were found scattered throughout MuSVts110 DNA. Three of the nucleotide substitutions were in the gag gene, resulting in one coding change in p15 and one in p30. All of the remaining nucleotide changes were found in the noncoding region between the 5' long terminal repeat and the gag gene. In NIH 3T3 cells transfected with the cloned MuSVts110 DNA, the pattern of viral RNA expression conformed with that observed in cells infected with authentic MuSVts110 virus in that viral RNA splicing was 30 to 40% efficient at growth temperatures between 28 and 33 degrees C but reduced to trace levels above 37 degrees C.

Matrix protein of Akv murine leukemia virus: genetic mapping of regions essential for particle formation

Type C retroviruses assemble at the plasma membrane of the infected cell. Attachment of myristic acid to the N terminus of the Gag precursor polyprotein has been shown to be essential for membrane localization and virus morphogenesis. Here, we report that the matrix (MA) protein contains regions that in conjunction with myristylation are important for Gag protein stability and the assembly of murine leukemia viruses. We identified these domains by generating a series of Akv murine leukemia virus mutants carrying small in-frame deletions within the coding region of the MA protein encompassing 129 amino acids. Studies show that mutants with deletions within the segment encoding the first 102 amino acids were all replication defective, whereas the C-terminal residues 103 to 124 seem not to have any critical function in virus maturation. Cells expressing the replication-defective genomes did not release any detectable Gag proteins. In one mutant, deletion of 3 amino acids in the N terminus resulted in an inefficiently myristylated, stable Gag polyprotein. The remaining defect genomes encoded unstable Gag proteins, although they were modified with myristic acid. The results suggest that the matrix domain plays an important role in stabilizing the Gag polyprotein.

A 36-base-pair core sequence of locus control region enhances retrovirally transferred human beta-globin gene expression

The locus control region of the human beta-globin cluster consists of four major DNase I hypersensitive sites (HS). When linked to globin genes, the locus control region confers a high level of erythroid-specific expression of these genes in transgenic mice or transfected erythroid cell lines. We have examined the effect of one of these sites, HS2, on human beta-globin gene expression in a murine erythroleukemia cell line (MEL) after retrovirus-mediated gene transfer. We incorporated a 732- or 412-base-pair (bp) segment of HS2 in the retroviral construct carrying the human beta-globin gene. These fragments rendered the viruses unstable as the human beta-globin gene was rearranged or deleted in all the packaging cell lines examined. On the other hand, when a 36-bp fragment containing the NFE-2/AP-1 binding consensus in this region was inserted into the retroviral construct, we recovered 6 stable packaging cell lines of 12 examined, similar in percentage to the construct with the beta-globin gene alone. The virus titers of the packaging cell lines from these two constructs were similar. We infected MEL cells with viruses produced from three packaging cell lines of each of the two constructs and measured the ratio of human beta-globin to mouse alpha-globin mRNA after hexamethylenebisacetamide induction. The overall level of expression increased 2-fold from 6.0% to 12.7% with the addition of this 36-bp enhancer.

Functional characterization of a U5 ribozyme: intracellular suppression of human immunodeficiency virus type 1 expression

We have designed a ribozyme that cleaves human immunodeficiency virus type 1 (HIV-1) RNA in U5 (at nucleotide +115). This ribozyme was tested in vitro and was found to give efficient and specific digestion of RNA containing the HIV-1 U5 sequence. When the U5 ribozyme was placed into the HIV-1 genome, virus replication was suppressed in tissue culture. Introduction of this ribozyme into cells by using an amphotropic retrovirus vector significantly reduced expression of U5-containing RNA in cells chronically infected with HIV-1. Naive T cells were cocultivated with packaging cells that produce defective amphotropic retroviruses containing the U5 ribozyme. These lymphocytes were found to be partially protected from HIV-1 infection.

Acquisition of responsiveness to chemical carcinogens by rodent embryo fibroblasts expressing high levels of the c-myc proto-oncogene

We investigated the ability of overexpression of the c-myc proto-oncogene to potentiate in vitro transformation by model chemical carcinogens. A mouse c-myc gene was introduced to C3H 10T1/2 and Rat 6 embryo fibroblast cell lines via a retroviral vector containing the gene for neomycin resistance. Our present work extends previous findings by showing that individual vectored C3H 10T1/2 clones have enhanced (two-fold to sevenfold) sensitivity to benzo[a]pyrene (BP) and N-methyl-N-nitro-N'-nitrosoguanidine (MNNG). Rat 6 clones acquiring the c-myc gene display various degrees of altered morphology. They form orderly but densely packed cells, grow to higher saturation density, and yield microcolonies in soft agar. The degree of altered growth properties is directly correlated with the level of c-myc expression. Transient exposure of c-myc-expressing clones to BP and MNNG induced the formation of distinct, large colonies in soft agar, whereas the untreated cells formed microcolonies and the parental Rat 6 cells remained single cells in soft agar. We also demonstrated that the degree of responsiveness to chemical carcinogens of the clones correlates with their ability to form microcolonies in soft agar. These cells overexpressing c-myc may be used as a model system to study the interaction between oncogenes and chemical carcinogens in the process of multistage carcinogenesis.

Inhibition of proliferation of primary avian fibroblasts through expression of histone H5 depends on the degree of phosphorylation of the protein

To obtain stable and constitutive expression of histone H5 at levels comparable to those observed in normal chicken erythrocytes, an avian self-inactivating retroviral vector was used to transfer the H5 gene into cells which do not express this protein. The vector, pDAH5, was obtained by removing the CAAT and TATA boxes of the 3'LTR of the avian leukosis virus RAV-2 and inserting the H5 sequence. Infection of QT6 quail cells with the recombinant virus (DAH5) led to the stable integration of the foreign H5 gene at low copy number, to the formation of correctly initiated mRNA transcripts and to the production of H5 protein. The amount of H5 expressed was equivalent to that of a mature chicken erythrocyte. Expression of histone H5 in DAH5 transformed cells, such as QT6 or AEV-ES4, transformed chicken embryo fibroblasts had only slight effects on the growth rate and did not inhibit cell replication. Conversely, the effect of H5 expression on normal quail and chicken fibroblasts was dramatic: cells acquired the aspect of quiescent fibroblasts, grew very slowly, and nuclei looked compacted, often extruded from the cell. The H5 histone produced in QT6-transformed cells was found to be phosphorylated while in normal chicken fibroblasts the protein lacked this posttranslational modification. It is proposed that the chromatin-condensing role of histone H5 is inhibited by its phosphorylation.

Characterization and comparison of avian and murine helper cell lines for production of replication-defective retroviruses for avian transformation

Several approaches were taken to identify improved helper cell lines for the production of replication-defective avian retroviral vectors for avian transformation. Both QT6 and D17 cells were engineered to become helper cell lines for the production of reticuloendotheliosis virus vectors. The results showed that the majority of lines from the D17, QT6, and D17C3 cells produced titers in the 10(2) to 10(3) cfu/mL range, with one QT6 line producing 10(5) cfu/mL. This high producer line was relatively free of helper virus when restricted to low passage. An amphotropic murine cell line produced a 6- to 10-fold higher amount of virus and had a comparable higher titer on chicken cells, suggesting possible application to avian transformation.

Retroviral vectors related to the myeloproliferative sarcoma virus allow efficient expression in hematopoietic stem and precursor cell lines, but retroviral infection is reduced in more primitive cells

Retroviral vectors are considered to be the most suited vehicles for somatic gene therapy with hematopoietic stem cells as targets. Retrovirus-mediated gene transfer into differentiation-restricted hematopoietic precursor (FDC-P1, FDC-P2) and multipotent progenitor (stem) cell lines (FDC-Pmix) is inefficient. Two cellular restrictions are involved. One is specific for stem but not precursor cells and is at the level of transcription. Due to a unique property of the transcriptional control region of the myeloproliferative sarcoma virus (MPSV), vectors derived from MPSV are not affected by this block. The second restriction occurs before proviral DNA synthesis and integration. This inhibition of effective viral infection depends on the state of differentiation, being more pronounced in multipotent clonogenic blast cells. This block to retroviral infection affects all retroviral vectors tested.

Gene expression from heterologous promoters in a replication-defective avian retrovirus vector in quail cells

Avian retrovirus vectors, with potential for use in avian transformation, were constructed to evaluate the relative efficiency of promoters placed internal to the viral long terminal repeats (LTR). The vectors are replication-defective reticuloendotheliosis plasmids that contain the neomycin phosphotransferase gene under control of the 5' LTR and an internal promoter that directs expression of the chloramphenicol acetyltransferase gene. The internal promoters were the SV40 early, the mouse metallothionein I, and the human cytomegalovirus immediate early (HCMV-IE) promoters. Under transient conditions in QT6 cells, the HCMV-IE promoter construct was by far the strongest. However, expression dropped greatly from the HCMV-IE promoter after integration into the quail cell genome. Evidence suggests that the HCMV-IE promoter is selectively suppressed by methylation after stable transfection but not after infection.

A novel technique for the production of hybrid antibodies

Chemically linked bifunctional antibodies (heteroconjugates) composed of one antibody specific for the TcR/CD3 complex on cytotoxic T cells and another specific for viral antigens expressed on the surface of infected cells have been shown to redirect CTL to lyse virus-infected cells. Hybrid antibodies are bifunctional antibodies produced by the fusion of two hybridomas. As a result of their native dimeric immunoglobulin structure, hybrid antibodies may be more effective than heteroconjugates in vivo. We have developed a unique method for production of hybrid antibodies by infecting each hybridoma with a different retrovirus vector which confers resistance to either G418 or methotrexate. The hybridomas are fused and selected in medium containing both inhibitors. Using this technique, we have produced hybrid antibodies made up of one antibody combining site which binds to the TcR and a second specific for the hemagglutinin of X-31 influenza virus. We show that this hybrid antibody effectively mediates the lysis of virus-infected cells in the presence of appropriate CTL. Thus hybrid antibodies as well as heteroconjugates can redirect CTL to lyse virus-infected targets.

The chicken lysozyme 5' matrix attachment region increases transcription from a heterologous promoter in heterologous cells and dampens position effects on the expression of transfected genes

Matrix attachment regions (MARs) are DNA elements that dissect the genome into topologically separated domains by binding to a chromosomal skeleton. This study explored the putative influence of the MAR located 5' of the chicken lysozyme gene on expression of heterologous genes in heterologous cell systems. Expression of a construct with the chloramphenicol acetyltransferase (CAT) indicator gene controlled by the herpes simplex virus thymidine kinase promoter (TC) and a construct in which the same transcriptional unit is flanked by chicken lysozyme 5' MARs (MTCM) was assayed after stable transfection into rat fibroblasts. Median CAT activity per copy number in MTCM transfectants was elevated approximately 10-fold relative to that in TC transfectants. Total variation in normalized CAT activity decreased from more than 100-fold among TC transfectants to nearly 6-fold among MTCM transfectants. The steady-state level of transcripts and the relative rate of transcription were increased in MTCM transfectants, as shown by S1 nuclease and run-on transcription assays, respectively. The chicken lysozyme 5' MAR thus can confer elevated, less position-dependent expression on a heterologous promoter in cells of a different species by increasing the density of transcribing RNA polymerase molecules. MAR-mediated transcriptional enhancement suggests that MARs are important for gene expression and not just for DNA packaging.

The chicken lysozyme 5' matrix attachment region increases transcription from a heterologous promoter in heterologous cells and dampens position effects on the expression of transfected genes

Matrix attachment regions (MARs) are DNA elements that dissect the genome into topologically separated domains by binding to a chromosomal skeleton. This study explored the putative influence of the MAR located 5' of the chicken lysozyme gene on expression of heterologous genes in heterologous cell systems. Expression of a construct with the chloramphenicol acetyltransferase (CAT) indicator gene controlled by the herpes simplex virus thymidine kinase promoter (TC) and a construct in which the same transcriptional unit is flanked by chicken lysozyme 5' MARs (MTCM) was assayed after stable transfection into rat fibroblasts. Median CAT activity per copy number in MTCM transfectants was elevated approximately 10-fold relative to that in TC transfectants. Total variation in normalized CAT activity decreased from more than 100-fold among TC transfectants to nearly 6-fold among MTCM transfectants. The steady-state level of transcripts and the relative rate of transcription were increased in MTCM transfectants, as shown by S1 nuclease and run-on transcription assays, respectively. The chicken lysozyme 5' MAR thus can confer elevated, less position-dependent expression on a heterologous promoter in cells of a different species by increasing the density of transcribing RNA polymerase molecules. MAR-mediated transcriptional enhancement suggests that MARs are important for gene expression and not just for DNA packaging.

Different relative expression from two murine leukemia virus long terminal repeats in unintegrated transfected DNA and in integrated retroviral vector proviruses

Results of transient-expression studies have suggested a correlation between tissue-specific pathogenicity of murine leukemia viruses and the relative transcriptional activities of their long terminal repeats in various cell types. To test whether transient-expression ratios are representative of those of integrated proviruses, we developed a system for generation of retroviral transmission vectors differing only in U3. Vectors with the long terminal repeats of leukemogenic SL3-3 and nonleukemogenic Akv viruses were used for infection of a lymphoid cell line. We then compared expression in infected cells with transient expression after DNA transfection. In contrast to a high SL3-3/Akv reporter gene expression ratio in the transient assays, the ratio in stably infected populations was low. Sets of random cell clones from the two infected populations showed wide variation, with a mean value ratio identical to the population ratio but a considerably higher ratio between lowest values. We suggest that the lower expression levels, like transient expression, reflect inherent enhancer strength and that the higher levels represent chromosomal influence. The different pathogenicity, despite the moderate difference in average expression, may then relate to a different capacity for insertional oncogene activation owing to the different inherent enhancer strengths revealed by the transient-expression assays and the least active proviruses.

Gene transfer into hemopoietic stem cells using retroviral vectors

Gene transfer to hemopoietic cells offers a variety of new approaches to the experimental hematologist as well as potentially providing a means for correcting a number of genetic disorders of humans. From the experimental viewpoint, gene transfer utilizing retroviral vectors introduces new methods for analyzing hemopoietic cell lineages, and the effects of over-expression of genes affecting hemopoietic cell proliferation and differentiation. The unique properties of retroviral vectors and the optimized methods currently in use to infect hemopoietic cells are represented as a brief review of a rapidly expanding new field of experimental hematology.

Oncogenicity of human N-ras oncogene and proto-oncogene introduced into retroviral vectors

The N-ras gene is the only member of the ras family which has never been naturally transduced into a retrovirus. In order to study the in vitro and in vivo oncogenicity of N-ras and to compare its pathogenicity to that of H-ras, we have inserted an activated or a normal form of human N-ras cDNA into a slightly modified Harvey murine sarcoma virus-derived vector in which the H-ras p21 coding region had been deleted. The resulting constructions were transfected into NIH 3T3 cells. The activated N-ras-containing construct (HSN) induced 10(4) foci per microgram of DNA and was found to be as transforming as H-ras was. After infection of the transfected cells by either the ecotropic Moloney murine leukemia virus or the amphotropic 4070A helper viruses, rescued transforming viruses were injected into newborn mice. Both pseudotypes of HSN virus containing activated N-ras induced the typical Harvey disease with similar latency. However, we found that the virus which contained normal N-ras p21 (HSn) was also pathogenic and induced splenomegaly, lymphadenopathies, and sarcoma in mice after a latency of 3 to 7 weeks. In addition, Moloney murine leukemia virus pseudotypes of N-ras caused neurological disorders in 30% of the infected animals. These results differed markedly from those of previous experiments in which we had inserted the activated form of N-ras in the pSV(X) vector: the resulting SVN-ras virus was transforming on NIH 3T3 cells but was poorly oncogenic in vivo (M. Souyri, C. F. Koehne, P. V. O'Donnel, T. H. Aldrich, M. E. Furth, and E. Fleissner, Virology 158:69-78). However, similarly poor oncogenicity was also observed when the v-H-ras coding sequence was inserted in pSV(X) vector, which indicated that the vector sequences play a crucial role in the pathogenicity of a given oncogene. Altogether, these data demonstrated unequivocally that N-ras is potentially as oncogenic as H-ras and that such oncogenic effect could depend on the vector environment.

Avian retroviral vectors derived from avian defective leukemia virus: role of the translational context of the inserted gene on efficiency of the vectors

We have constructed retroviral vectors derived from the genome of avian erythroblastosis virus ES4 (AEV ES4). The neo selectable gene was substituted for the original v-erbA or v-erbB oncogenes of AEV, either in the same or in a different reading frames. Recombinant retrovirus were rescued and used to infect chicken embryo fibroblasts or quail QT6 cells. When the neo gene was inserted in the same reading frame as the original oncogene, we obtained (1) a high level of expression of the neo gene, (2) a balanced ration of both genomic and subgenomic RNAs, and (3) high titer recombinant viruses. Conversely, when the neo gene was inserted in a reading frame different from that of the original oncogene, we observed (1) a very low level of expression of the neo protein, (2) a predominance of the viral transcript used as translational template for the neo protein synthesis, and (3) low titer recombinant viruses. One of the vectors was used to transfer a human delta-globin gene into avian cells in culture without detectable rearrangement of this gene, but exhibited a deletion within the conserved noncoding region located between the two original oncogenes. Our data provide information for further construction of double expression vectors. Furthermore, three of the vectors would provide helpful tools to identify genetic elements of the virus genome involved in splicing regulation.

Generation of a helper cell line for packaging avian leukosis virus-based vectors

We constructed an avian leukosis virus-based packaging cell line, pHF-g, containing Rous-associated virus DNA with several alterations to abolish RNA packaging. One of them is a 52-base-pair deletion encompassing the putative encapsidation signal in the leader region. The 3' long terminal repeat was also removed and replaced by the polyadenylation sequence from the herpes simplex virus thymidine kinase gene. When pHF-g cells were transfected by an avian leukosis virus-based vector, they produced replication-defective virus at high titer but they did not release any replication-competent particles. Proviral DNA was shown to be correctly integrated as well as correctly expressed. Viral RNAs were shown to be correctly translated into gag-related polypeptides.

Construction and analysis of deletion mutations in the U5 region of Moloney murine leukemia virus: effects on RNA packaging and reverse transcription

A collection of deletion mutations was generated in the U5 region of cloned DNA copies of Moloney murine leukemia virus or a related retrovirus. Cell lines expressing the mutant DNAs were generated by cotransformation, and the virions released were characterized biochemically. Deletions in the 5' part of U5 profoundly reduced packaging of the viral RNA into virions; one deletion in the 3' part of U5 did not block packaging but affected reverse transcription. One mutant with a deletion in the central part of U5 was fully viable and served to separate the two functional parts of U5.