METHOD FOR OBTAINING RETROVIRAL PACKAGING CELL LINES PRODUCING HIGH TRANSDUCING EFFICIENCY RETROVIRAL SUPERNATANT

Abstract: This invention provides a method for obtaining a recombinant retroviral packaging cell capable of producing retroviral vectors as well as the recombinant packaging cell obtained by the method. Also provided is a method of producing recombinant retroviral particles obtained by introducing into the packaging cells obtained according to the methods disclosed herein, a recombinant retroviral vector and propagating the resulting producer cells under conditions favorable for the production and secretion of retroviral vector supernatant. The retroviral supernatants produced by these methods also is claimed herein. This invention further provides a method for screening retroviral vector supernatant for high transduction efficiency and methods for producing retroviral vector supernatant for transducing cells with high efficiency in gene therapy applications.

Description: METHOD FOR OBTAINING RETROVIRAL PACKAGING CELL LINES PRODUCING HIGH TRANSDUCING EFFICIENCY RETROVIRAL SUPERNATANT This application is a continuation-in-part of United States patent application Serial No. 08/572,959, filed December 15, 1995. TECHNICAL FIELD OF THE INVENTION This invention generally relates to the derivation and use of packaging cell lines for the production of retroviral transducing supernatant. BACKGROUND OF THE INVENTION Human gene transfer involves the transfer of one or more therapeutic genes and the sequences controlling their expression to appropriate target cells. A number of vector systems have been developed for the transfer of the therapeutic genes for various clinical indications. In vivo gene transfer involves the direct administration of vector to the target cells within a patient. Ex vivo gene transfer entails removing target cells from an individual, modifying them ex vivo and returning the modified cells to the patient. The majority of gene therapy protocols approved for clinical trials by the NIH Recombinant DNA Advisory Committee (RAC) have used amphotropic retroviral vectors (ORDA Reports Recombinant DNA Advisory Committee (RAC) Data Management Report, June 1994, (1994) Human Gene Therapy 5: 1295-1302). Retroviral vectors are the vehicle of choice primarily due to the generally high rate of gene transfer obtained in experiments with cell lines and the ability to obtain stable integration of the genetic material, ensuring that the progeny of the modified cell will contain the transferred genetic material. For a review of retroviral vectors and their use in the transfer and expression of foreign genes, see Gilboa (1988) Adv. Exp. Med. Biol 241:29; Luskey et al. (1990) Ann. N. Y. Acad. Sci. 612:398; and Smith (1992) J Hematother. 1: 155-166. Many retroviral vectors currently in use are derived from the Moloney murine leukemia virus (MMLV). In most cases, the viral gag, pol and env sequences are removed from the virus, allowing for insertion of foreign DNA sequences. Genes encoded by the foreign DNA are often expressed under the control of the strong viral promoter in the LTR. Such a construct can be packaged into vector particles efficiently if the gag, pol and env functions are provided in trans by a packaging cell line. Thus, when the vector construct is introduced into the packaging cell, the Gag-Pol and Env proteins produced by the cell assemble with the vector RNA to produce replication-defective or transducing virions that are secreted into the culture medium. The vector particles thus produced can infect and integrate into the DNA ofthe target cell, but generally will not produce infectious virus since it is lacking essential viral sequences. Most ofthe packaging cell lines currently in use have been transfected with separate plasmids encoding Gag-Pol and Env, so that multiple recombination events are necessary before a replication-competent retrovirus (RCR) can be produced. Commonly used retroviral vector packaging cell lines are based on the murine NIH/3T3 cell line and include PA317 (Miller & Buttimore (1986) Mol. CeU Biol. 6:2895; Miller & Rosman (1989) BioTechniques 7:980), CRIP (Danos & Mulligan (1988) Proc. Nat! Acad Sci USA 85:6460), and gp + aml2 (Markowitz et al. (1988) Virology 167:400). Allhough splitting the gag-pol and env genes within the packaging cell genome decreases the incidence of RCR, RCR is occasionally observed in clinical-scale productions of retroviral vector preparations and is a major safety concern. This is likely due, at least in part, to the fact that NIH/3T3 cells contain endogenous MLV sequences (Irving et al. (1993) Bio/Technol. 11:1042-1046) which could participate in recombination to form RCR (Cosset et al. (1993) Virology 193:385-395 and Vanin et al. (1994) J. Virology 68:4241-4250), particularly in mass culture during large-scale clinical vector production. The range of host cells that may be infected by a retrovirus or transduced by a retroviral vector is determined by the viral Env protein. The recombinant virus can be used to infect virtually any cell type recognized by the Env protein provided by the packaging cell, resulting in the integration of the viral genome in the transduced cell and the stable production of the foreign gene product. The efficiency of infection is also related t ie level of expression of the receptor on the target cell. In general, murine ecotropic Env of MMLV allows infection of rodent cells, whereas amphotropic Env allows infection of rodent, avian and some primate cells, including human cells. Xenotropic vector systems utilizing murine xenotropic Env would also allow transduction of human cells. The host range of retroviral vectors has been altered by substituting the Env protein of the base virus with that of a second virus. The resulting, "pseudotyped" vector particle has the host range of the virus donating the envelope protein and expressed by the packaging cell line. For example, the G-glycoprotein from vesicular stomatitis virus (VSV-G) has been substituted for the MMLV Env protein, thereby broadening the host range. See, e.g., Burns et al. (1993) Proc. Natl. Acad. Sci USA 90:8033-8037 and International PCT patent application Publication No. WO 92/14829. Inconsistent results and inefficient gene transfer to some target cell types are two additional problems associated with current retroviral vector systems. For example, hematopoietic stem cells are an attractive target cell type for gene therapy because of their self-renewal capacity and their ability to differentiate into all hematopoietic lineages, thereby repopulating a patient with the modified cells. Yet retroviral gene transfer into hematopoietic stem cells has been inconsistent and disappointingly inefficient. Kantoff et al. (1987) J. Exp. Med. 166:219-234; Miller, A.D. (1990) Blood ' 76:271 -278; and Xu et al. (1994) J. Virol. 68:7634. Efforts to increase gene transfer efficiency include producing higher end-point -titer retroviral vector supematants. End-point titer is a measure of the number of functional vector particles in a preparation which, when increased, should theoretically increase transduction efficiency by increasing the ratio of functional vector to target cells, i.e. increasing multiplicity of infection (m.o.i.). Despite increased end-point titers, however, retroviral gene transfer efficiency (transduction efficiency) has not increased correspondingly (Xu et al. (1994), supra; Paul (1993) Hum. Gene Therapy 4:609-615; Fraes-Lutz et al. (1994) 22:857-865). Efforts to increase end-point titer have included improving production of retroviral vector supematants (see Kotani et al. (1994) Human Gene Therapy, 5: 19-28) and physical concentration of vector particles by ultrafiltration (Paul, et al. (1993), supra, and Kotani. et al. (1994) supra). It was shown that incubation of producer cells at 32°C rather than at 37°C yielded supematants with higher end-point titers, but transduction efficiencies were not compared (See Kotani, et al. (1994) supra). The authors of Kotani et al. (1994) supra, postulated that the higher titers were due to a lower rate of inactivation combined with a faster rate of virion production at 32°C. In another study, transduction efficiency was measured before and after concentration of three supematants with similar end-point titers Paul et al. (1993) supra. In each case, concentration increased end-point titer and modestly improved the transduction efficiency. However, the transduction efficiency achieved with one of the unconcentrated supematants was significantly higher than that achieved with the other concentrates (Paul et al. (1993) supra). For in vivo gene therapy applications, it is important that the retroviral vector not be inactivated by human serum before transducing the target cells. Reports show that human serum inactivates a number of recombinant retroviruses, apparently via a complement pathway. Both viral envelope and producer cell components have been reported to be responsible for viral sensitivity to human complement (Takeuchi et al. (1994) J. Virol. 68(12):8001). Thus, a need exists for methods of reproducibly increasing transduction efficiency and for providing stable, safe packaging cell lines for producing high transduction efficiency retroviral preparations. This invention satisfies these needs and provides related advantages as well. SUMMARY OF THE INVENTION This invention provides, inter alia, a method for obtaining a recombinant retroviral packaging cell capable of producing retroviral vectors. In one embodiment, the method comprises the steps of selecting a retrovirus and obtaining a cell free of endogenous retroviral nucleic acid. These steps are interchangeably performed. However, after selection ofthe retrovirus, a minimal gag-pol open reading frame (ORF) insert is isolated from the retrovirus. Alternatively, a nucleic acid molecule coding for functionally equivalent retroviral minimal ORF can be isolated and used in the methods disclosed herein. In the same manner, a minimal env ORF is isolated from wild type retrovirus or an equivalent nucleic acid molecule is obtained. The minimal ORF nucleic ac:.d molecules are then amplified, either by insertion into a suitable replication vector or plasmid and replication of a host cell containing the vector and/or plasmid or by other non-biological methods (PCR). After amplification, and consistent with the method of am Dlification, the minimal ORF nucleic acid molecules are inserted into a cell pre-selected to be devoid of endogenous retroviral nucleic acid. The transformed cells are then propagated under conditions favorable for expression of the minimal retroviral gag-pol and env ORF. Suitable candidate packaging cell lines include, but are not limited to mammalian cells such as COS, Vero, HT-1080, D17 MRC-5, FS-4, TE671, human embryonic kidney (293), and HeLa. The invention also provides the use of an ELISA method to screen for production of retroviral structural proteins to identify a retroviral packaging cell capable of producing recombinant, transducing retroviral vector particles. In further embodiment of this method, the cells that produce high levels ofthe retroviral Gag-Pol protein and the retroviral Env protein are identified or selected for by assaying for gag-pol and/or env ORF translation products in the supematant. This invention also provides the recombinant packaging cell obtained by the methods in all various embodiments described herein, including the amphotropic cell line designated ProPak-A and the xenotropic packaging cell line designated ProPak-X. Other embodiments ofthe packaging cell lines produced by the methods disclosed herein are packaging cell lines characterized by having the ability to produce transducing supematant that is: resistant to human complement; has a transducing efficiency of greater than or equal to 50% when assayed on NIH/3T3 cells, or greater than that achieved with supematant from PA317-based cells, or has a transducing efficiency of greater than or equal to 20% when assayed on 293 cells; and substantially free of RCR after interaction of retroviral vector sequences and continuous culture of more than 2 weeks with an indicator cell line. Further provided by this invention is a method of producing recombinant retroviral particles obtained by introducing into the packaging cells obtained according to the methods disclosed herein, a recombinant retroviral vector and propagating the resulting producer cells under conditions favorable for the production and secretion of retroviral vector supematant. With respect to the propagation ofthe producer cells, methods are provided for obtaining a retroviral vector supernatant having high transduction efficiency by culturing the producer cells in a packed-bed bioreactor and harvesting the resultant supematant. In one method, the packed-bed bioreactor has a surface to volume ratio of 5 to 50 cm 2 /ml, and the producer cells are cultured at a temperature of 30°C to about 37°C The retroviral supematants produced by these methods are also claimed herein. This invention also provides a method for screening retroviral vector supematant for high transduction efficiency and methods for producing retroviral vector supematant for transducing cells with high efficiency in gene therapy applications. Yet another embodiment ofthe invention is a method of increasing the transduction efficiency of a cell by transducing a cell with a retroviral vector supernatant obtained from the culture of one or more recombinant packaging cells produced by the methods ofthe invention. Vector supematant containing particles of more than one tropism can be produced by the co-culture of two or more producer cells of complementary tropisms. The present invention provides a method of producing viral vector supematant of high transduction efficiency. The prevailing view in the art is that increasing the viral titer will result in an increase in the transduction efficiency. As shown herein, this is not the case. Rather, higher transduction efficiency is correlated with increased numbers of functional vector particles. Vector particles are not all produced identicall and high titer supematants can contain many non-transducing particles. DESCRIPTION OF THE FIGURES Figure 1 schematically shows the plasmid constructs for expression of MLV structural genes, that were used as insertion plasmids for construction of the packaging cell lines ProPak -A and Pro-Pak-X. The top three constructs are for expression of Gag-Pol and the bottom three are Env expression constructs. Minimal MLV sequences are a common feature among these plasmids. CMV-IE denotes cytomegalovirus immediate early promoter. SD/SA denotes splice donor/splice acceptor site. MLV is the murine leukemia viral promoter present in the viral LTR. RSV LTR is the LTR promoter of Rous Sarcoma Virus. SV40 denotes the simian virus 40 early promoter. pA is the poly-a.denylation site. Ea refers to the amphotropic envelope gene, Ex refers to the xenotropic envelope gene, and Eax denotes the chimeric ampho/xenotropic envelope gene. Figure 2 shows the structure of several of the retroviral vectors used in this The name of each vector is indicated on the left. Open boxes represent the MLV LTR elements; RevMlO, trans-dominant mutant of the HIV Rev protein; TK, Heφes Simplex virus- 1 thymidine kinase promoter; Neo, neomycin phosphotransferase; SV, simian virus 40 early promoter; nls, nuclear localization signal; IRES, internal ribosomal entry site element; NGFR, nerve growth factor receptor; puro, puromycin resistance gene; CMV, CMV-IE promoter. LMiLy and LLySN encode the Lyt2 surface antigen expressed either directly from the retroviral LTR promoter in LLySN, or via an IRES element (ires) in LMiLy. LLySN contains the neomycin phosphotransferase gene (Neo) expressed from an internal SV40 promoter (SV). Figure 3 shows the result of exposing to human serum, supematants containing lacZ- encoding vector prepared from stable producer cells (PA317; PE501), by transient transfection of vector into packaging cells (ProPak-A) or by co-transfection of packaging and vector constructs into 293 cells (293). Supematants were mixed with an equal volume of a pool of human serum from 4 healthy donors, incubated for 1 hour at 37°C, and the residual titer determined on NIH/3T3. The serum was either untreated (HS) or had been heat-inactivated for 30 min at 56°C (HI-HS). The human serum pool had a hemolytic titer (CH 50 ; EZ Complement Assay, Diamedix, Miami, Florida) of 1 17 to 244 before, and < 8 after heat-inactivation. End-point titers (cfu x 10-5/ml) of supematants treated with heat- inactivated serum (100 %) were: PA317, 5.0; ProPak-A, 1.0; PE501 , 1.4, and 293, 1.1. The bars indicate the range for duplicate samples. HI-HS denotes heat inactivated human serum. HS denotes human serum. Figures 4A, 4B and 4C show transduction efficiencies of vector supematants. Figure 4 A is a comparison ofthe transduction efficiency of Lyt-2-encoding (LMiLy vector) viral supematants produced from ProPak-A and PA317 cells cultured in a packed-bed bioreactor, as assayed on NIH/3T3 cells. Figure 4B is a comparison of the transduction efficiency of vector supematants (LMiLy vector) produced from ProPak-A and PA 317 cells cultured in an aerated packed-bed bioreactor, as assayed on 293 cells. Figure 4C is a comparison ofthe transduction efficiencies achieved with Lyt2-encoding (LLySN) vector supematants produced from PA317 or ProPak-A-based producer cells, as the proportion (%) of NIH/3T3 cells that stained with anti-Lyt2 antibody (Pharmingen, San Diego, CA) 2 days after inoculation ofthe NIH/3T3 cells with the dilutions of vector supematant shown. Supematants were prepared from confluent producer cell cultures after culturing for 12 hours at 32°C. Figures 5A and 5B show the results of comparison of transduction mediated by various viral vector supematants. 5A is a comparison of end-point titer and transduction efficiency for PA.SVNLZ supematants measured on NIH/3T3 cells. Figure 5B is a comparison of transduction efficiencies measured on NIH/3T3, HeLa or Jurkat cells for three PA.LMTNL supematants with the end-point titer measured on NIH/3T3 cells. IE + 05 means an end- point titer of 1x10 ' cfu/ml on NIH/3T3 cells. Error bars show the standard error for three determinations. Figure 6 shows the transduction efficiency of PA.SVNLZ supematants before or after concentration. The original supematant and the concentrate (see Example 2 and Table 4) were diluted with medium and inoculated onto NIH/3T3 cells. Figures 7A and 7B show measurement of transduction efficiency (Figure 7A) or end¬ point titer of PA.SVNLZ retroviral vector supematant on NIH/3T3 cells (Figure 7B), showing the inactivation of vector incubated at 37°C, 32°C or 0°C. Figures 8A and 8B show the time-course of production of PA.SVNLZ retroviral vector in a 75 cm " tissue culture flask at 32°C or 37°C measured as either transduction efficiency (Figure 8A) or end-point titer on NIH/3T3 cells (Figure 8B) or. Figure 9 is a time-course of PA.LMTNL vector production from a confluent roller bottle culture at 32°C as measured by transduction efficiency or end-point titer. Figures 10A and 10B show a time-course of vector production for PA.SVNLZ producer cultures in a roller bottle (900 cm 2 ) or packed bed-bioreactor (12,000 cm 2 ) shown by transduction efficiency (Figure 10A) or end-point titer (Figure 10B). IE -+ 05 means an end-point titer of lxl 0 5 cfu/ml on NIH/3T3 cells. Figure 11 illustrates a bioreactor that is suitable for the production of retroviral vector supematant. The numbers indicate the following: 1 : medium inlet from 10 L vessel; 2: air inlet medipure air/5% C0 2 ; 3: Fibercell discs lOg; 4: agitation rate 80 m seeding, 250 φm production; 5: medium outlet: to 2 L vessels on ice; and 6: supematant samples. Figure 12 illustrates the set up of a continuous perfusion packed-bed bioreactor that is suitable for the production of retroviral vector supematant. The parts of the set-up are indicate by numbers as follows: 1 , peristaltic pump; 2, medium feed tank (10 L); 3, packed bed bioreactor (500 ml); 4, magnetic stirrer; 5, air/CO 2 mix; 6, 2 L vessels for daily supematant collection maintained at 0°C. Incubator temperatures are at 37°C for growth of the producer cells and at 32°-37°C for vector production. Figure 13 show the results of the comparison of vector production from ProPak-A.855 producer cells in T -flasks versus aerated packed-bed bioreactor, and as measured on 293 cells. Time (h) is in hours. Transduction efficiency is presented as the percentage of cells expressing the vector-encoded surface marker protein. See Figure 2 for structure of vector PG855. Figure 14 shows transduction of 293 cells with ProPak-X-derived supematants harvested at different times post seeding of cells in either a T-flask or the packed-bed bioreactor. Figure 15 is a comparison of transduction efficiency achieved with supematants harvested from ProPak-A. LMiLy cells cultured in different vessels, as shown. Figures 16A and 16B shows transduction of bone marrow (CD34 + ) with LMiLy packed- bed supematants. Figure 16A shows transduction after spinoculation with single supematants. Figure 16B shows transduction after spinoculation with single or mixed (ampho + xeno) supematant. Figures 17Aand 17B show transduction of cell lines with Lyt2-encoding vector preparations with various tropisms. Average values for duplicate samples are plotted. In Figure 17A, cell lines were inoculated at unit gravity with LLySN vector from ProPak-X (PP-X), ProPak-A (PP-A), or PGl 3 (PG)-based producer populations. MLV(V-G). LMiLy is an MLV(VSV-G) pseudotype supematant prepared by transient transfection. In Figure 17B, Quail cells (Qcl. 3; Cullen et al., 1983) were inoculated with LMiLy vector particles bearing the chimeric envelope (Eax; prepared by transient transfection), the xenotropic (ProPak-X) or the amphotropic (ProPak-A) envelope. Figure 18. Complement resistance of vector packaged in ProPak-X, ProPak-A, or PA317 packaging cell lines. Supematants were incubated for 30 min at 37°C with an equal volume of human serum. The residual transducing activity relative to samples incubated in medium was determined on 293 (ProPak-X) or NIH/3T3 cells (ProPak-A, PA317). The human serum, a pool from 4 healthy donors, had a hemolytic titer (CH 50 ) of 150 to 313 (EZ Complement Assay, Diamedix, Miami, FL). Bars represent the range for duplicate samples. Figure 19 shows a comparison of vector production methods. ProPak-A.52. LMiLy supematants were harvested one day after confluence (visual monolayer in T- flask and roller bottle cultures), diluted 1 in 8 and inoculated onto NIH/3T3 cells. Lyt2 expression was analyzed 3 days later by FACS. Figure 20 shows phenotypic analysis 3 days after spinoculation of CD34- positive cells. Cells were stained with anti-Lyt2-phycoerythrin antibody, and either anti-CD34-sulphur rhodamine (panels A and B) or a panel of fluorescein isothiocyanate-labeled, lineage- specific antibodies (panels C and D). The panel of lineage-specific antibodies contained antibodies to the following hematopoietic cell lineages: thymocytes (CD2); granulocytes, monocytes and macrophages (CD 14, CD 15); natural killer cells (CD 16); B lymphocytes (CD 19); and erythrocytes (glycophorin). Samples were analyzed on a Bee ton-Dickinson Vantage flow cytometer. Panels A and C are from uninoculated, and panels B and D from inoculated cells. The values represent the proportion of the cell population in the respective quadrants. MODEfS) FOR CARRYING OUT THE INVENTION Definitions Unless otherwise specified herein, common definitions are intended by the words and terms used herein. For example, "retrovirus" denotes a class of viruses which use RNA-directed DNA polymerase, or "reverse transcriptase" to copy a viral RNA genome into a double-stranded DNA intermediate which can be incoφorated into chromosomal DNA (a "provirus") of an avian or mammalian host cell. Retrovirus also exist as free virions, that contain the structural and enzymatic proteins of the retrovirus (including reverse transcriptase), two copies ofthe viral genome, and portions of the host cell's plasma membrane in which is embedded the viral envelope glycoprotein. Many such retroviruses are known to those skilled in the art and are described, for example, in Weiss et al., eds, RNA Tumor Viruses. 2nd ed., Cold Spring Harbor Press, Cold Spring Harbor. New York (1984 and 1985). Plasmids containing retroviral genomes also are widely available from the American Type Culture Collection (ATCC), and other sources as described in Gacesa and Ramii. Vectors: Essential Data. John Wiley & Sons. New York (1994). The nucleic acid sequences of a large number of these viruses are known and are generally available from databases such as GENBANK. for example. The complete nucleic acid sequence of the MoMLV and other MLVs is known in the art. "Packaging cell line" is a recombinant cell line containing nucleic acid expressing retroviral Gag, Pol and Env structural proteins. Because the packaging cell line lacks the retroviral nucleic acid coding for packaging signal and other cis-acting elements, infectious virions cannot be produced. A "producer cell" is a packaging cell as defined above which also contains a replication-defective retroviral vector which is packaged into the vector particle. The producer cell produces transducing retroviral-based particles containing "foreign" (i.e., non-retroviral) genes, such as therapeutic or marker genes. A "target cell" is a cell to be transduced with a recombinant retroviral vector. Thus, target cells may be, for example, a cell line used for assessing the quality of a retroviral vector preparation, a primary cell for genetic modification ex vivo, or a cell within a patient that will be modified by in vivo introduction of a retroviral vector. The terms "polynucleotide", "oligonucleotide", "nucleic acids" and "nucleic acid molecules" are used interchangeably, and refer to a polymeric form of nuc ieotides of any length, either deoxyribonucleotides or ribonucleotides, or analogs thereof. Polynucleotides may have any three-dimensional structure, and may perform any function, known or unknown. The following are non-limiting examples of polynucleotides: a gene or gene fragment, exons, introns, messenger RNA (mRNA), transfer RNA, ribosomal RNA, ribozymes, cDNA, recombinant polynucleotides, branched polynucleotides, plasmids, vectors, isolated DNA of any sequence, isolated RNA of any sequence, nucleic acid probes, and primers. A polynucleotide may comprise modified nucleotides, such as methylated nucleotides and nucleotide analogs. If present, modifications to the nucleotide structure may be imparted before or after assembly of the polymer. The sequence of nucleotides may be interrupted by non-nucleotide components. A polynucleotide may be further modified after polymerization, such as by conjugation with a labeling component. The term polynucleotide, as used herein, refers interchangeably to double- and single-stranded molecules. Unless otherwise specified or required, any embodiment of the invention described herein that is a polynucleotide encompasses both the double-stranded form and each of two complementary single-stranded forms known or predicted to make up the double-stranded form. A "gene" can refer to a polynucleotide or a portion of a polynucleotide comprising a sequence that encodes a protein. It is often desirable for the gene also to comprise a promoter operatively iinked to the coding sequence in order to effectively promote transcription. Enhancers, repressors and other regulatory sequences also cε.n be included in order to modulate activity of the gene, as is well known in the art (see, e.g.. the references cited herein). A "detectable marker" gene is a gene that allows cells carrying the gene to be specifically detected (i.e., to be distinguished from cells which do not carry the marker gene). A large variety of such marker genes are known in the art. Preferred examples of such marker genes encode proteins appearing on cellular surfaces, thereby facilitating simplified and rapid detection and/or cellular sorting. A "selectable marker" gene is a gene that allows cells carrying the gene to be specifically selected for or against, in the presence of a corresponding selective agent. By way of illustration, an antibiotic resistance gene can be used as a positive selectable marker gene that allows a host cell to be positively selected for in the presence of the corresponding antibiotic. A variety of positive and negative selectable markers are known in the art, some of which are described herein. In the context of polynucleotides, a "linear sequence" or a "sequence" is an order of nucleotides in a polynucleotide in a 5' to 3' direction in which residues that neighbor each other in the sequence are contiguous in the primary stmcture of the polynucleotide. A "partial sequence" is a linear sequence of part of a polynucleotide which is known to comprise additional residues in one or both directions. "Hybridization" refers to a reaction in which one or more polynucleotides react to form a complex that is stabilized via hydrogen bonding between the bases of the nucleotide residues. The hydrogen bonding may occur by Watson-Crick base pairing. Hoogsteen binding, or in any other sequence-specific manner. The complex may comprise two strands forming a duplex structure, three or more strands forming a multi-stranded complex, a single self-hybridizing strand, or any combination of these. A hybridization reaction may constitute a step in a more extensive process, such as the initiation of a PCR. or the enzymatic cleavage of a polynucleotide by a ribozyme. Hybridization reactions can be performed under conditions of different "stringency". Conditions that increase the stringency of a hybridization reaction are widely known and published in the art: See, for example, Sambrook et al. (1989) infra "T m " is the temperature in degrees Centigrade at which 50% of a polynucleotide duplex made of complementary strands hydrogen bonded in an antiparallel direction by Watson-Crick base paring dissociates into single strands under the conditions of the experiment. T m may be predicted according to standard formula; for example: T m = 81.5 + 16.6 log [Na + ] + 0.41 (%G/C) - 0.61 (%F) - 600/L where Na is the cation concentration (usually sodium ion) in mol/L; (%G/C) is the number of G and C residues as a percentage of total residues in the duplex; (%F) is the percent formamide in solution (wt/vol); and L is the number of nucleotides in each strand ofthe duplex. A linear sequence of nucleotides is "identical" to another linear sequence, if the order of nucleotides in each sequence is the same, and occurs without substitution, deletion, or material substitution. It is understood that purine and pyrimidine nitrogenous bases with similar structures can be functionally equivalent in terms of Watson-Crick base-pairing; and the inter-substitution of like nitrogenous bases, particularly uracil and thymine, or the modification of nitrogenous bases, such as by methylation, does not constitute a material substitution. An RNA and a DNA polynucleotide have identical sequences when the sequence for the RNA reflects the order of nitrogenous bases in the polyribonucleotide, the sequence for the DNA reflects the order of nitrogenous bases in the polydeoxyribonucleotide, and the two sequences satisfy the other requirements of this definition. Where at least one of the sequences is a degenerate oligonucleotide comprising an ambiguous residue, the two sequences are identical if at least one of the alternative forms ofthe degenerate oligonucleotide is identical to the sequence with which it is being compared. For example, AYAAA is identical to ATAAA, if AYAAA is a mixture of ATAAA and ACAAA. When comparison is made between polynucleotides, it is implicitly understood that complementary strands are easily generated, and the sense or antisense strand is selected or predicted that maximizes the degree of identity between the polynucleotides being compared. For example, where one or both ofthe polynucleotides being compared is double-stranded, the sequences are identical if one strand of the first polynucleotide is identical with one strand of the second polynucleotide. Similarly, when a polynucleotide probe is described as identical to its target, it is understood that it is the complementary strand ofthe target that participates in the hybridization reaction between the probe and the target. A linear sequence of nucleotides is "essentially identical" to another linear sequence, if both sequences are capable of hybridizing to form duplexes with the same complementary polynucleotide. Sequences that hybridize under conditions of greater stringency are more preferred. It is understood that hybridization reactions can accommodate insertions, deletions, and substitutions in the nucleotide sequence. Thus, linear sequences of nucleotides can be essentially identical even if some of the nucleotide residues do not precisely correspond or align. Sequences that correspond or align more closely to the invention disclosed herein are comparably more preferred. Generally, a polynucleotide region of about 25 residues is essentially identical to another region, if the sequences are at least about 80% identical; more preferably, they are at least about 90% identical; more preferably, they are at least about 95% identical; still more preferably, the sequences are 100% identical. A polynucleotide region of 40 residues or more will be essentially identical to another region, after alignment of homologous portions if the sequences are at least about 75% identical; more preferably, they are at least about 80% identical; more preferably, they are at least about 85% identical; even more preferably, they are at least about 90% identical; still more preferably, the sequences are 100% identical. In determining whether polynucleotide sequences are essentially identical, a sequence that preserves the functionality ofthe polynucleotide with which it is being compared is particularly preferred. Functionality can be determined by different parameters. For example, if the polynucleotide is to be used in reactions that involve hybridizing with another polynucleotide, then preferred sequences are those which hybridize to the same target under similar conditions. In general, the T m of a DNA duplex decreases by about 10°C for even' 1% decrease in sequence identity for duplexes of 200 or more residues; or by about 50°C for duplexes of less than 40 residues, depending on the position of the mismatched residues. Essentially identical sequences of about 100 residues will generally form a stable duplex with each other"s respective complementary sequence at about 20°C less than T m ; preferably, they will form a stable duplex at about 15°C less; more preferably, they will form a stable duplex at about 10°C less; even more preferably, they will form a stable duplex at about 50°C less; still more preferably, they will form a stable duplex at about T m . In another example, if the polypeptide encoded by the polynucleotide is an important part of its functionality, then preferred sequences are those which encode identical or essentially identical polypeptides. Thus, nucleotide differences which cause a conservative amino acid substitution are preferred over those which cause a non-conservative substitution, nucleotide differences which do not alter the amino acid sequence are more preferred, while identical nucleotides are even more preferred. Insertions or deletions in the polynucleotide that result in insertions or deletions in the polypeptide are preferred over those that result in the down-stream coding region being rendered out of phase; polynucleotide sequences comprising no insertions or deletions are even more preferred. The relative importance of hybridization properties and the encoded polypeptide sequence of a polynucleotide depends on the application of the invention. A polynucleotide has the same "characteristics" of another polynucleotide if both are capable of forming a stable duplex with a particular third polynucleotide under similar conditions of maximal stringency. Preferably, in addition to similar hybridization properties, the polynucleotides also encode essentially identical polypeptides. "Conserved" residues of a polynucleotide sequence are those residues which occur unaltered in the same position of two or more related sequences being compared. Residues that are relatively conserved are those that are conserved amongst more related sequences than residues appearing elsewhere in the sequences. "Related" polynucleotides are polynucleotides that share a significant proportion of identical residues. A "probe" when used in the context of polynucleotide manipulation refers to an oligonucleotide which is provided as a reagent to detect a target potentially present in a sample of interest by hybridizing with the target. Usually, a probe will comprise a label or a means by which a label can be attached, either before or subsequent to the hybridization reaction. Suitable labels include, but are not limited to radioisotopes, fluorochromes, chemiluminescent compounds, dyes, and proteins, including enzymes. A "primer" is an oligonucleotide, generally with a free 3' -OH group, that binds to a target potentially present in a sample of interest by hybridizing with the target, and thereafter promotes polymerization of a polynucleotide complementary to the target. Processes of producing replicate copies ofthe same polynucleotide, such as PCR or gene cloning, are collectively referred to herein as "amplification" or "reputation". For example, single or double-stranded DNA may be replicated to form another DNA with the same sequence. RNA may be replicated, for example, by an RNA-directed RNA polymerase, or by reverse-transcribing the DNA and then performing a PCR. In the latter case, the amplified copy of the RNA is a DNA with the identical sequence. A "polymerase chain reaction" ("PCR") is a reaction in which replicate copies are made of a target polynucleotide using one or more primers, and a catalyst of polymerization, such as a reverse transcriptase or a DNA polymerase, and particularly a thermally stable polymerase enzyme. Generally, a PCR involves reiteratively fcπriing three steps: "annealing", in which the temperature is adjusted such that oligonucleotide primers are permitted to form a duplex with the polynucleotide to be amplified; "elongating", in which the temperature is adjusted such that oligonucleotides that have formed a duplex are elongated with a DNA polymerase, using the polynucleotide to which they have formed the duplex as a template; and "melting", in which the temperature is adjusted such that the polynucleotide and elongated oligonucleotides dissociate. The cycle is then repeated until the desired amount of amplified polynucleotide is obtained. Methods for PCR are taught in U.S. Patent Nos. 4,683,195 (Mullis) and 4,683,202 (Mullis et al.). A "control element" or "control sequence" is a nucleotide sequence involved in an interaction of molecules that contributes to the functional regulation of a polynucleotide, including replication, duplication, transcription, splicing, translation, or degradation of the polynucleotide. The regulation may affect the frequency, speed, or specificity of the process, and may be enhancing or inhibitory in nature. Control elements are known in the art. For example, a "promoter" is an example of a control element. A promoter is a DNA region capable under certain conditions of binding RNA polymerase and initiating transcription of a coding region located downstream (in the 3' direction) from the promoter. Retroviral long terminal repeat sequences (LTR) contain strong promoters that are suitably used in the inventions described herein. "Operatively linked" refers to a juxtaposition of genetic elements, wherein the elements are in a relationship permitting them to operate in the expected manner. For instance, a promoter is operatively linked to a coding region if the promoter helps initiate transcription of the coding sequence. There may be intervening residues between the promoter and coding region so long as this functional relationship is maintained. The "gag" gene of a retrovirus refers to the 5' gene on retrovirus genomes and is an abbreviation for group-specific antigens. It is translated to give a precursor polyprotein which is subsequently cleaved to yield three to five capsid proteins. The "poE gene refers to a gene encoding a polymerase. Thus, the pol gene encodes for a retrovirus reverse transcriptase and also encodes the IN protein needed for viral integration into cell DNA. The "em'" or envelope region of a retrovirus genome codes for the envelope proteins. For the puφose of this invention, the "env " gene is to include not only the naturally occurring env sequence from a virus, but also modifications to the env gene , such as env genes that are modified to alter target specificity of retrovirus or alternative env genes that are used to generate "pseudotyped" retrovirus. Preferred env genes for use in this invention include, but are not limited to amphotropic env, murine xenotropic env, Gibbon Ape Leukemia virus (GaLV) env and the VSV-G protein-encoding gene. The terms "polypeptide", "peptide" and "protein" are used interchangeably herein to refer to polymers of amino acids of any length. The polymer may be linear or branched, it may comprise modified amino acids, and it may be interrupted by non-amino acids. The terms also encompass an amino acid polymer that has been modified; for example, disulfide bond formation, glycosylation, lipidation, acetylation, phosphorylation, or any other manipulation, such as conjugation with a labeling component. The "biochemical function" or "biological activity" of a polypeptide includes any feature of the polypeptide detectable by suitable experimental investigation. "Altered" biochemical function can refer to a change in the primary, secondary, tertiary, or quaternary structure of the polypeptide; detectable, for example, by molecular weight determination, circular dichroism, antibody binding, difference spectroscopy, or nuclear magnetic resonance. It can also refer to a change in reactivity, such as the ability to catalyze a certain reaction, or the ability to bind a cofactor, substrate, inhibitor, drug, hapten. or other polypeptide. A substance may be said to "interfere" with the biochemical function of a polypeptide if it alters the biochemical function of the polypeptide in any of these ways. A "fusion polypeptide" is a polypeptide comprising regions in a di fferent position in the sequence than occurs in nature. The regions may normally exist in separate proteins and are brought together in the fusion polypeptide; or they may normally exist in the same protein but are placed in a new arrangement in the fusion polypeptide. A fusion polypeptide may be created, for example, by chemical synthesis, or by creating and translating a polynucleotide in which the peptide regions are encoded in the desired relationship. An "isolated" polynucleotide, polypeptide, protein, antibody, nucleic acid, oracid,or other substance refers to a preparation ofthe substance devoid of at least some ofthe other components that may also be present where the substance or a similar substance naturally occurs or is a purification technique to enrich it from a source mixture. Enrichment can be measured on an absolute basis, such as weight per volume of solution, or it can be measured in relation to a second, potentially interfering substance present in the source mixture. A substance can also be provided in an isolated state by a process of artificial assembly, such as by chemical synthesis or recombinant expression. A polynucleotide used in a reaction, such as a probe used in a hybridization reaction, a primer used in a PCR, or a polynucleotide present in a pharmaceutical preparation, is referred to as "specific" or "selective" if it hybridizes or reacts with the intended target more frequently, more rapidly, or with greater duration than it does with alternative substances. Similarly, a polypeptide is referred to as "specific" or "selective" if it binds an intended target, such as a ligand, hapten, substrate, antibody, or other polypeptide more frequently, more rapidly, or with greater duration than it does to alternative substances. An antibody is referred to as "specific" or "selective" if it binds via at least one antigen recognition site to the intended target more frequently, more rapidly, or with greater duration than it does to alternative substances. A polynucleotide, polypeptide, or antibody is said to "selectively inhibit" or "selectively interfere with" a reaction if it inhibits or interferes with the reaction between particular substrates to a greater degree or for a greater duration than it does with the reaction between alternative substrates. A "cell line" or "cell culture" denotes higher eukaryotic cells grown or maintained in vitro. It is understood that the descendants of a cell may not be completely identical (either moφhologically, genotypically, or phenotypically) to the parent cell. The term "primate" as used herein refers to any member of the highest order of mammalian species. This includes (but is not limited to) prosimians, such as lemurs and lorises; tarsioids, such as tarsiers; new-world monkeys, such as squirrel monkeys (Saimiri sciureus) and tamarins; old-world monkeys such as macaques (including Macaca nemestrina, Macaca fascicularis, and Macaca fuscata); hylobatids, such as gibbons and siamangs; pongids, such as orangutans, gorillas, and chimpanzees; and hominids, including humans. "Mean residence time" is the average amount of time the culture medium remains in contact with the producer cells during the production phase. The optimal mean residence time is determined by the following considerations: 1) cell specific vector production rate; 2) rate of vector inactivation; 3) cell specific nutrient uptake rates; 4) cell specific metabolite production rates; 5) temperature; 6) volumetric cell density; and 7) the target cells for which the vector supematant is intended. Given a volumetric producer cell density of > lxlO 6 celLs/ml, the optimal mean residence time ranges from 3 to 6 hours with PA317-based cultures; from 6 to 12 hours with ProPak-A-based cultures; and 12 to 24 hours with ProPak-X or PG13-based cultures. These mean residence times are based upon maximal transduction of cell lines. DESCRIPTION OF EMBODIMENTS This invention provides a method for obtaining a safe recombinant retroviral packaging cell capable of producing retroviral-based vectors and retroviral vector supematant. The method comprises the steps of selecting a retrovirus that will provide the retroviral env and gag-pol oligonucleotide sequences for the recombinant production of retroviral env and gag-pol gene products and obtaining a eukaryotic cell free of endogenous related retroviral nucleic acids or oligonucleotides of the same retroviral type. Although any retrovirus can be suitably used in the method of this invention. the use of the murine leukemia virus (MLV) is presently described. Thus, if any of gag, pol or env genes are to be derived from a MLV, the candidate packaging cell should be screened for absence of endogenous MLV retroviral nucleic acid and those sequences closely rel ted to MLV which, by recombination, would produce replication competent retrovirus. The env gene product determines the target cell specificity ofthe recombinant retrovirus particle and will, therefore, be selected to provide optimal transduction of the target cells of interest. Env may be the MLV amphotropic Env or any envelope capable of combining to form infectious "pseudotyped" retrovirus particles. For example, MLV amphotropic and murine xenotropic retroviral vectors are known to transduce human cells. Other env genes of interest include those from Gibbon Ape Leukemia Virus (GaLV), RD1 14, FeLV-C, FeLV-B, BLV, and HIV-1. See PCT Publication No. WO 92/14829 (page 25, line 1 through 1 through page 34, line 1). In addition, the env gene can be modified to more specifically target the recombinant retrovirus to the target cell of interest. For, example, the Env protein may be modified by combination with an antibody binding site specific for a cell surface antigen on the target cells of interest, e.g. an anti-CD34 antibody for targeting to hematopoietic stem and progenitor cells (Cossett et al. (1995) J. Virol. 69:6314-6322 and Kasahara, et al. (1994) Science 266: 1373-1376). The env gene can also be modified so as to broaden the cell tropism of the virus, for e.g. by constructing a chimeric env that can bind to both amphotropic and xenotropic receptors or to a unique receptor on cells. In one embodiment, a chimeric amphotropic/xenotropic envelope gene. Eax, was constructed as described in Examples 6 and 7. Vector particles with a Eax envelope displayed dual cell tropism and will therefore transduce a broader range of target cells. For clinical gene therapy applications it is important that the retroviral vector sequences and structural gene sequences be designed to minimize recombination to form replication competent retrovirus (RCR). By introducing the gag-pol and env gene sequences into the packaging cell separately so that they integrate in different areas of he packaging cell genome, the rate of RCR formation is decreased since multiple recombination events are required to generate RCR. Nevertheless, RCR are sometimes found in recombinant retroviral vector preparations. One possible reason is the presence of endogenous retroviral sequences in the murine cells (NIH/3T3) that the most commonly used packaging cell lines are based on, which may recombine with the introduced retroviral sequences. Therefore, the safe packaging cells ofthe present invention are generated from a eukaryotic cell line lacking endogenous retroviral sequences which would be capable of producing RCR by recombination with introduced retroviral sequences. Thus, in one embodiment, the packaging cells ofthe present invention are derived from a cell line having no detectable endogenous retroviral sequences related to MLV. In addition, as described in the Examples herein, the cell line is preferably screened for the ability to stably secrete Gag-Pol and Env proteins and to efficiently transduce target cells rather than to produce high end-point titers alone. Preferably the eukaryotic cell line will be a non-murine cell line, more preferably a primate cell line, and most preferably a human cell line. The inventors have found that human 293 cells are free of retroviral sequences related to MLV, and when used as the basis for stable packaging cells, are able to produce high transduction efficiency retroviral vectors. A cell free of related retroviral nucleic acid is obtained by screening a candidate cell for endogenous retroviral nucleic acid using methods well known to those of skill in the art and exemplified below. For example, several available cell lines such as the Mus dunni tail fibroblasts (see Lander and Chattopadhyay (1984) J. Virol. 52:695-698) are reportedly free of endogenous retroviral nucleic acid and thus, are suitably used in the methods disclosed herein. Alternatively, one of skill in the art can determine if the cell line contains endogenous retroviral nucleic acid by isolating a nucleic acid sample from the candidate cell line and probing for the endogenous DNA or RNA using methods such as traditional Southern and Northern hybridization analysis or the polymerase chain reaction ("PCR"), using retroviral specific probes, and when available, commercially available PCR kits (Invitrogen, San Diego, CA). Southern and Northern hybridization analyses are described, for example in Sambrook et al. (1989) infra. PCR methods are described in Gene Expression Technology. Goeddel, et al. eds., Academic Press, Inc. New York (1991). A cell is free of endogenous related retroviral nucleic acid if no hybridization is detected, even at low stringency conditions of 500mM sodium ions, or if the primer used for the PCR analysis does not provide amplified nucleic acid. Preferably highly conserved sequences spanning viral LTR, packaging sequence and gag-pol gene regions are used as probes. The candidate eukaryotic cell is of any suitable type, i.e., murine, non-murine, mammalian, primate, canine and human, provided that the cell line lacks endogenous retroviral sequences, grows well in culture, can be transfected or transduced with the appropriate gag-pol and env expression constructs and can express the viral proteins. The candidate cell line is preferably primate, and most preferably human. It has been found that primate and preferably human-based packaging cells can be used to produce retroviral vectors resistant to human complement. In addition, the method further comprises using minimal gag-pol and env sequences to further decrease the chances of recombination to produce RCR. A minimal gag-pol open reading frame (ORF) and minimal env ORF are obtained from the selected retroviruses. The minimal ORF ofthe retroviral sequences are defined to include only those retroviral sequences from the ATG through the stop codon ofthe gene with no flanking sequences. Fragments ofthe gene as well as biological equivalents thereof also can be used provided that functional protein is produced when introduced into the candidate packaging cell line. In one embodiment, isolated retroviral nucleic acid coding for the minimal gag-pol and env ORF is selected for use. In a preferred embodiment, the nucleic acid is selected from MLV and the minimal sequences are determined to consist of nucleotides from about 621 to 5837 (gag- pol) (numbering from Shinnick et al. (1981)) and about nucleotides 37 to 2000 (env) (numbering from Ott et al. (1990). These nucleotide positions will vary with different MLVs. It should be understood, although not always explicitly stated, that nucleic acid sequences or molecules that are "equivalent" are determined to produce the same phenotypic effect as the isolated minimal ORF described herein, can be utilized as the minimal ORF sequences in the methods described herein. For example, altered, but phenotypically equivalent nucleic acid molecules are referred to as "equivalent nucleic acids". The minimal gag-pol and env ORF nucleic acid molecules can be isolated using the technique described in the experimental section described below or replicated using PCR (Perkin-Elmer) and published sequence information. For example, the sequence can be replicated by PCR (Perkin-Elmer) which, in combination with the synthesis of oligonucleotides, allows easy reproduction of DNA sequences. The PCR technology is tlie subject matter of United States Patent Nos. 4,683,195, 4,800,159, 4,754,065, and 4,683,202 and described in PCR: The Polymerase Chain Reaction Mullis et al. eds., Birkhauser Press, Boston (1994) and references cited therein. As is apparent to those of skill in the art, modifications and/or additions to the viral sequences are made to facilitate isolation and expression ofthe amplified DNA. It is conceived that amphotropic and xenotropic cell lines are produced by this method and is determined by the selection ofthe env gene. Thus, the selection ofthe gag-pol and env gene are not restricted to isolation from the same vims, or the same vims type. An example of an amphotropic-producing packaging cell line produced by this method is ProPak-A and an example of a xenotropic packaging cell line, also produced by this method, is ProPak-X. Thus, the invention further provides the isolated genes operatively linked to a promoter of RNA transcription, as well as other regulatory sequences for replication and/or transient or stable expression ofthe DNA. To minimize the chance of RCR it is preferred to avoid using extraneous viral sequences. The term "operatively linked" is defined above. The promoter may be a minimal MLV-LTR, and is preferably "heterologous" to the retroviral gene. Suitable promoters are those that drive stable, high-level expression of gag- pol and env. Examples of suitable promoters include, but are not limited to, the cytomegalovirus immediate early (CMV) promoter, Rous Sarcoma Vims (RSV) LTR, murine moloney leukemia vims (MMLV) LTR, or other viral LTR sequences. Vectors and plasmids which contain a promoter or a promoter/enhancer, with termination codons and selectable marker sequences, as well as a cloning site into which an inserted piece of DNA can be operatively linked to that promoter, are well known in the art and commercially available. To minimize the chance of RCR formation, gag-pol and env genes are preferably incoφorated into separate expression plasmids and the plasmids introduced sequentially into the eukaryotic host cells. In plasmid amplification, the bacterial host cells are propagated at temperatures in the range from about 28°C to about 32°C and more preferably at about 30°C. to prevent recombination of viral sequences carried on the plasmids and to maintain the integrity ofthe plasmids. The separate expression plasmids containing the retroviral gag-pol and env genes are then introduced in separate, sequential steps into the candidate packaging cells by techniques well known to those of skill in the art, such as calcium phosphate precipitation, electroporation and lipofection (Sambrook et al. (1989) supra). The insertion technique also can involve the use of a modified integrase enzyme that will recognize a specific site on the target cell genome. Such-site specific insertion allows the genes to be inserted at sites on host cells' DNA that will minimize the chances of insertional mutagenesis, minimize interference from other host cellular sequences, and allow insertion of sequences at specific target sites so as to reduce or eliminate the expression of undesirable genes. Introduction of gag-pol and env genes can be done in either order. After insertion and integration ofthe gag, pol and env genes, the cells are screened for retroviral gene expression. If a selectable marker gene, such as antibiotic resistance, was used in combination with the retroviral sequences, the population can be enriched for cells for expressing the selectable marker (and therefore the retroviral genes) by growing the candidate cells in the presence ofthe antibiotic. Cells which survive and propagate will contain both the antibiotic resistance gene and the retroviral sequences. Still fiirther, an ELISA with the appropriate antibody to a product ofthe expressed sequences will be : simple and quick assay to determine whether and to what extent the cells contain and cwpress the retroviral genes. Thus, the invention also provides an ELISA method to screen for expression of retroviral genes in candidate packaging cells to identify a retroviral packaging cell capable of producing recombinant, transducing retroviral vector particles. In particular, the ELISA will be used to screen for production of retroviral structural proteins such as Gag, Pol and Env. The ELISA method ofthe invention can also have diagnostic use in patients who have undergone gene therapy. In one specific embodiment, Env protein is detected in a sandwich ELISA assay using an antibody from hybridoma 83A25 as primary antibody, to capture the protein. The captured protein is then detected using as secondary antibody, antisera 79S-334, followed by an enzyme-conjugated antispecies antibody and the substrate for the enzyme. In a similar manner, Gag is detected separately using as primary antibody, an antibody from hybridoma R187 followed by antisera 77S-227, enzyme-conjugated antispecies antibody and enzyme substrate. The primary antibody can be provided in the form of hybridoma culture supematant, ascites or purified antibody. The anti-species antibody is preferably conjugated to a label such as an enzyme. Suitable enzymes include horeseradish peroxidase and alkaline phosphatase. The appropriate substrate to the particular enzyme is used. This particular embodiment ofthe ELISA assay is described in detail in the Examples below. Preferably, the candidate packaging cell lines are further screened for the ability to produce vector supematant having high transduction efficiency. Transduction efficiency is measured by the ability of the vector supematant to transduce a target cell population. The target cell population can be human 293 cells, NIH/3T3 cell or primary cells. This invention also provides recombinant packaging cells obtained by the method described above. The recombinant packaging cell lines are an improvement over prior art packaging cell lines because, when transduced with a suitable retroviral vector and propagated, the cell lines of this invention produce a retroviral titer having high transduction efficiency. Thus, the cell line of this invention is characterized by producing viral supematant that is resistant to human complement; has a high transduction efficiency; and is substantially free of RCR after continuous culture of more than 2 weeks and up to at least 12 weeks. As used herein, "transduction efficiency" refers to the percentage ofthe inoculated target cell population which has been marked with the vector. Gene marking can be measured by determining the fraction of cells which have integrated proviral vector (eg. by PCR) or by determining the fraction of cells expressing the transgene (eg. by FACS analysis). A recombinant retrovims preparation (e.g., supematant or supematant concentrate) that has high transduction efficiency will produce an increased percentage of inoculated target cells that have been marked with the viral vector. Packaging cells ofthe present invention are able to produce recombinant retroviral supematants capable of higher transduction efficiency than standard murine, PA317 cells as assayed on NIH/ T3 cells, and particularly as assayed on human 293 or primary cells. Suitable reagents and methods for performing this analysis are provided in the experimental section below. Also provided herein is a method of producing retroviral vector supernatant comprising transducing the novel packaging cells produced according to the method described above with a suitable retroviral vector to generate producer cells. This invention further provides the retroviral supematant so produced. In addition to the gene of interest to be delivered to the host cell, the retroviral vector will contain the "packaging signal" that allows the retroviral vector nucleic acid to be packaged in the vector particle in the recombinant packaging cell line, and the long terminal repeat that allow the vector nucleic acid to become effectively integrated into the target cell genome. The LTRs are positioned at either end ofthe proviral nucleic acid and also generally contain regulatory sequences such as promoter/enhancers that affect expression of the therapeutic or marker gene of interest. The gene or genes of interest also can be operably linked to a suitable promoter which can be constitutive, cell-type specific, stage-specific and /or modulable. Enhancers, such as those from other vims, can also be included. In a separate embodiment, the retroviral vectors can contain genes coding for selectable and/or detectable markers that facilitate isolation of transduced eel s. In other embodiments, it may be desirable for the vector to include a "suicide gene" that allows recipient cells to be selectively eliminated at will. Isolation, insertion and use: of such markers and suicide genes are well known to those of skill in the art as exemplified in PCT Publication Nos. WO 92/08796 and WO 94/28143. Prior art methods to improve gene transfer (transduction) efficiency with retroviral vectors often focused on increasing the end-point titer. The evidence presented herein does not support a correlation between end-point titer and transduction efficiency. The evidence presented herein also does not support the conclusion that increasing end-point titer necessarily increases transduction efficiency because non-transducing particles interfere with transducing virions and reduce transduction efficiency without reducing-end point titer. Indeed, the evidence shows that packaging cell line clones should be screened for ability to generate supematant based on transduction efficiency rather than on end-point titers. Thus, the invention further provides a method of producing retroviral vector supematant having higher transduction efficiency which is more suitable for gene therapy, comprising culturing producer cells in a packed-bed bioreactor having a surface to volume ratio of about 5 to 50 cm /ml, culturing the producer cells population at a temperature of around 30°C to about 37°C and harvesting supematant produced by said producer cells at such time as the transduction efficiency ofthe supematant is optimal for a target cell, thereby obtaining a high transduction efficiency supematant. As used herein, a packed-bed bioreactor refers to a cell culture vessel comprising a bed matrix in which cells grow in a confined three-dimensional space. Reactor volume size. and volume of packed-bed matrix can vary depending on the quantity of retroviral supematant required. The matrix which comprises the packed-bed has the following properties: 1) The matrix is made of a material which allows for the attachment and growth of anchorage-dependent cells. The material can also be surface-coated or surface-treated to achieve this quality; 2) The matrix can entrap cells normally cultured in suspension so as to confine them to a three-dimensional space; 3) The matrix has a large surface area to volume ratio (5 to 50 cm 2 /ml), enabling cells to grow to high volumetric densities (10 6 to 10 8 cells/ml). This property ensures a high volumetric vector production. In a specific embodiment, the surface to volume ratio is from about 20 to about 30 cm /ml and preferably is about 24 cm /ml; 4) The matrix has a large void volume when packed to minimize the pressure-drop across the bed during operation. A negligible pressure drop ensures an even supply of nutrients to cells throughout the bed. The bioreactor is available commercially and the bed matrix can be packed separately. Preferably, the bed matrix is constructed such that the pressure drop across the bed matrix is in the range of 0-0.25 mbar/cm. Ideally, the pressure drop is negligible. It is intended that any retroviral producer cell line can be cultured in the packed-bed bioreactor. However, the producer cells derived from the packaging cell lines produced according to the above method are particularly well suited for use in this method to produce retroviral vector supematants. Such cells include, but are not limited to, producer cells derived from 3T3 or 293 cells. Examples of 3T3 and 293-based producer cell lines are those based on the PA317, GP-Aml2, CRIP, PGl 3, ProPak-A and ProPak-X packaging cell lines. After seeding of producer cells into the packed-bed reactor, cells can be cultured at temperatures ranging from 30° to 37°C. Preferably, the cells will be initially cultured at 37°C to allow for maximal cell growth. Once the culture achieves a high cell density (10 6 cells/ml or greater), the temperature can be lowered to 32° during the production phase to minimize the loss of transducing vector due to thermal inactivation. The temperature selected for the production phase will depend on the following variables: temperature differences in the cell specific vector production rate; the rate of vector inactivation at a given temperature; cell density; and the rate at which the vessel is perfused with fresh medium. The time to harvest the retroviral supematant to obtain optimal transduction efficiency will vary with the parental cell type ofthe producer cells. "Mean residence time" and "transduction efficiency" are defined above. Optimal transduction efficiency for a retroviral supematant refers to the highest percentage ofthe inoculated target cell population that is marked with the viral vector, obtained under the culture conditions tested. If the producer cells are derived from PA317 cells, the supematant can be harvested after a mean residence time of from 2 to 12 hours starting from the time the producer cell;; reach a cell density of at least 10 cells per ml. For human 293-derived producer cells, the supematant is harvested after a mean residence time of from 1 to 24 hours, usually from 6 to 24 hours. Mean residence time can be decreased as cell density increases. The packed-bed bioreactor can be operated in a fed-batch or perfusion mode. Fed- batch operation refers to the changing of a part or all ofthe culture medium at discrete time intervals. Perfusion mode refers to the continual supply of fresh medium at a specified perfusion rate which is equivalent to the rate at which vector supematant is continually harvested. The perfusion mode of operation is preferred as it maintains a more stable culture environment, yielding less stress on the producer cells. Depending on the producer cell line and the volumetric cell density, the optima! perfusion rate can vary from 1 reactor volume per day to 24 reactor volumes per day. Preferably, the perfusion rate is from 2- 12 reactor volumes per day, even more preferably at 2-6 reactor volumes per day, and most preferably, at a constant rate of about 4 reactor volumes per day. In the most preferred embodiment, the perfusion rate increases proportionally to the cell density. The cell density can be monitored indirectly by nutrient consumption, metabolite production, or preferably, by oxygen uptake. The optimal perfusion rate, for a given producer cell line at a given volumetrc cell density, is established by determining the mean residence time ofthe culture medium required to produce a vector supematant which yields the maximum transduction efficiency of the target cell population. The cell culture conditions and parameters described above are also applied to a method of producing retroviral supematant having high transduction efficiency, from non- murine derived producer cells. After seeding in the packed-bed bioreactor, the cells can be continuously cultured at 37°C or 32°C. Alternatively, the cells are initially cultured at 37°C until the cells produce a confluent monolayer, and are thereafter cultured at 32°C. The producer cells derived from the non-murine packaging cell lines produced according to the above method are particularly suited for use in this method for obtaining vector supematant. Such cells include, but are not limited to producer cells derived from 293 cells, HT1080 cells and Dl 7cells. In one particular embodiment, the non-murine producer cells are derived from 293 cells and the retroviral supematant is harvested after a mean residence time of from 6 to 24 hours, starting from the time the producer cells reach a cell density of at least 10 6 cells per ml. To achieve maximal producer cell densities in the packed-bed bioreactor, proper aeration ofthe vessel is important. It has been noted that with sufficient oxygen transfer, supematants with higher transduction efficiencies are obtained. Aeration conditions providing air saturation greater than 20%, preferably between 20% and about 60% are necessary to achieve efficient vector production. Most preferred are aeration conditions providing greater than 40%o of air saturation. One of skill in the art can easily determine the most suitable culture medium for the cells using commercially available basal medium and supplementing the medium as provided by the art. Cells can be cultured in the medium supplemented with fetal bovine serum (FBS), and vector supematants produced in the presence of FBS. However, for clinical use, serum- free medium is preferred. To achieve this, cells can be initially expanded and grown in the packed-bed bioreactor in medium supplemented with FBS, and switched to a serum-free medium formulation for the production of serum-free vector supematants. Most preferably, cells will be cultured and vector supematants produced in serum-free media starting with cells derived from a serum-free master cell bank. The disclosed method also can be utilized to produce retroviral vector supematant having high transduction efficiency by seeding primate producer cells in a packed-bed bioreactor. Suitable primate producer cells include those derived from 293 cells, ProPak-A and ProPak-X cells. When the producer cell is derived from a 293 cell, it is preferable to harvest the retroviral supematant when the culture medium's mean residence time in the reactor is about 6-24 hours, starting from the time the producer cells reached a cell density of at least 10 6 cell per ml. The invention further provides a method of obtaining high transduction efficiency MLV-based retroviral supematant suitable for gene therapy, comprising culturing human- derived producer cells under the conditions described above. Human-derived producer cells are seeded in a packed-bed bioreactor having a surface area to volume ratio cf about 5 to 50 cm /ml , at a concentration starting from about 2 to 3 x 10 cells/cm ' , under constant perfusion, with aeration sufficient to maintain the culture medium at about 20% to 60% > air saturation, preferably at or greater than 40% air saturation. The cells are cultured at a temperature of about 37°C until they grow to a cell density of at least 10 eel Is per ml whereupon the temperature is then lowered to 32°C. Retroviral supematant is then collected after an additional 6 to about 24 hours of incubation time. Human cells useful in this method include, but are not limited to those derived from 293 cells, HT 1080 cells, ProPak-A cells and ProPak-X cells. The present method of producing retroviral vector supematants also encompasses co- culturing of two or more producer cell lines under the conditions described above. In this circumstance, the packed-bed bioreactor is seeded with two or more different producer cell lines, preferably containing the same vector and having complementary tropisms, e.g. producer cells derived from human amphotropic and xenotropic packaging cell lines. This co-culture technique results in an amplification of vector copy number within the producer cells, and yields vector supematants which contain a mixture of retroviral vector particles targeted to distinct receptors expressed on most human cells. The culture supematant produced by the methods described herein is also within the scope of this invention. The supematant has been shown to transduce cells, and in particular, stem cells very efficiently. Prior to transduction, the stem cells are isolated and selected. Methods of isolating and selecting cells are well known in the art. For example, sorted CD34 Thy 1 Lin " cells from either adult bone marrow (ABM) or mobilized peripheral blood (MPB) are used. CD34 + Thy 1 + Lin " are highly enriched in human hematopoietic stem cells. See. U.S. Patent No. 5,061,620. Various methods can be used to isolate stem cells. For the puφose o:~ illustrating one method of preparing stem cells from ABM, 20 mL of fresh bone marrow can be isolated by aspiration ofthe iliac crest from normal human volunteers. Marrow is separated by taking the mononuclear cell fraction following a Ficoll-Perque separation, positive-selected for CD34 + cells according to the method described by Sutherland et al. (1992) Exp. Hematol. 20:590. Briefly, cells are resuspended in staining buffer (SB) (HBSS containing 10 mM HEPES, 2% heat-inactivated FCS) at 5 x 10 7 cells/mL. QBEND10 (anti-CD34) (Amac, Westbrook, ME) is added at 1/100 dilution, and then the cells are incubated on ice for 30 min. Cells are then washed in SB with a FCS underlay, and resuspended at 4 x 10 /mL in SB. An equal volume of washed Dynal sheep anti-mouse IgG,Fc magnetic beads (Dynal, Oslo, Norway), is added at a 1 : 1 bead to cell ratio, to give a final cell concentration of 2 x 10 7 cells/mL. After 30 min incubation on ice, with gentle inversion, the tube was placed against a Dynal magnet (Dynal) for 2 minutes, and CD34 " cells removed. Following two washes, 20 mL of 'glycoprotease' (O-sialoglycoprotein endopeptidase, Accurate Chemical, Westbury. New York) plus 180 mL of RPMI (JRH Biosciences)/20% FCS is added and the beads incubated at 37°C for 30 min to cleave the QBEND10 epitope, and release CD34 + cells from the beads. Beads are then washed three times to maximize cell recovery. The glycoprotease used for the release step in the positive selection procedure has been shown not to effect subsequent ex vivo expansion of progenitors (Marsh et al. (1992) Leukemia 6:926). Mobilized peripheral blood (MPB) samples can be obtained with informed consent from multiple myeloma patients. Patients are treated on day 1 with cyclophosphamide at 6 g/m (1.5 g/m every 3 hours x 4 doses). From day 1 until the start of leukopheresis (usually 10-28 days), granulocyte macrophage colony stimulating factor (GM-CSF) is given at 0.25 mg/m /day. Apheresis for total white cells is started when the peripheral blood white cell count is greater than 500 cells/ml and the platelet count is greater than 50,000 cells/ml. Patients are apheresed daily until from 6 x 10 mononuclear cells (MNC) are collected. Fresh MPB samples are then elutriated with a JE5.0 Beckman counterflow elutriator equipped with a Sanderson chamber (Beckman, Palo Alto, CA). Cells are resuspended in elutriation medium (Biowhittaker, Walkersville, MD) at pH 7.2, supplemented with 0.5% human serum albumin (HSA). The rotor speed is set at 2000 RPM, the cells are introduced. and the first fraction collected at a flow rate of 9.6 ml/min. Fractions 2 and 3 are collected at the respective flow rates of 14 and 16 ml/min. The larger cells remaining in the chamber are collected after stopping the rotor. Cells are resuspended in RPMI supplemented with 5% HSA, 10 mg/ml DNAse I and penicillin/streptomycin at 50 U/ml and 50 mg/ml. respectively. Fractions 2 and 3 are pooled and incubated with 1 mg/ml heat-inactivated human gamma- globulin to block non-specific Fc binding. Granulocytes are further depleted by incubation with CDI 5 conjugated to magnetic beads (Dynal M450, Oslo, Norway) followed by magnetic selection. CD34 + Thyl + Lin " cells are isolated from ABM and MPB by flow cytometry as follows. Antibodies to CD 14 and CDI 5 were obtained as FITC conjugates from Becton- Dickinson. Antibodies to Thy-1 (GM201) can be obtained from a commercial source or prepared and detected with anti-IgGl-PE conjugate from Caltag. Antibody 10 CD34 (Tuk 3) also can be commercially obtained and detected with an anti-IGg3-Texas Red conjugate (Southern Biotechnologies). Anti-CD34 antibody or an IgG3 isotype matched control is added to cells in staining buffer (HBSS, 2% FCS, 10 mM HEPES) for 20 minutes on ice, together with anti-Thy-1 antibody at 5 μg/ml. Cells are washed with a FCS underlay, and then incubated with Texas Red conjugated goat anti-mouse IgG3 antibody and phycoerythrin-conjugated goat anti- mouse IgGl antibody for 20 minutes on ice. Blocking IgGl is then added for 10 minutes. After blocking, the FITC-conjugated lineage antibody panel (CD14 and CDI 5) is added, and incubated for another 20 minutes on ice. After a final washing, cells are resuspended in staining buffer containing propidium iodide (PI). Cells are sorted on a Vantage cell sorter (Becton Dickinson) equipped with dual argon ion lasers, the primary laser emitting at 488 nm and a dye laser (Rhodamine 6G) emitting at 600 nm (Coherent Innova 90, Santa Cruz, CA). Residual erythrocytes, debris and dead cells are excluded by light scatter gating plus an FL3 (PI) low gate. The sorted cell population is diluted 1 :1 in HBSS, pelleted, and resuspended in HBSS for hemocytometer counting. For transduction, the sorted cells are suspended in fresh or freshly thawed retroviral supematant diluted in appropriate media containing cytokines. Cells and vector are then centrifuged and resuspended and cultured in cytokine-enriched media for about three days. For culturing CD34+ hematopoietic stem or progenitor cells, the cytokines are preferably a combination including, but not exclusively, IL-3, IL-6, LIF and SCF. After tliree days, the cells are harvested and used to determine bulk transduction frequency as described below. Alternatively, the cells can be introduced into a patient by methods well known to those of skill in the art for gene therapy. Further provided by the invention is a method of increasing the gene transduction efficiency of a cell by transducing target cells with a retroviral vector supematant derived from one or more recombinant packaging cells produced by the methods ofthe present invention. Preferably, the target cell is a primary hematopoietic cell such as a stem cell, ln a specific embodiment, the retroviral supematant is derived from the culture of ProPak-A.6 or ProPak-A.52 and ProPak-X. The transduction efficiency of a cell population can also be increased by inoculating or transducing a target cell population with vector particles of more than one tropism. This is achieved by transducing the cells with a vector particle having a modified envelope protein as described above, wherein the envelope protein can bind to more than one receptor. Examples 6 and 7 below provide transduction of target cells with a vector particle having a chimeric amphotropic/xenotropic envelope encoded by the Eax env gene. Alternatively, the cells are simultaneously transduced with vector supematants containing particles of complementary tropisms, eg. amphotropic and xenotropic vector supematants, mixed into a single inoculum. The inoculum containing vector supematants of different tropisms can be prepared by mixing the two supematants obtained from their respective, separately cultured producer cells, or the two complementary producer cell lines can be co- cultured to produce a single supematant containing the two types of vector particles. In a co- culture, a production vessel, preferably a packed-bed bioreactor, is seeded with a mixture of producer cells with complementary tropisms that produce vector particles which are capable of transducing human cells. In one embodiment ofthe present invention, cells are transduced with a retroviral supematant produced from the co-culture of a ProPak-A and a ProPak-X producer cell line, specifically, ProPak-A.52.LMiLy and ProPak-X.LMiLy. The co-culture of ProPak-A and ProPak-X producer cells have the advantage that they do not produce RCR. Close to 100% gene transfer was achieved using such co-culture supematants on MPB-derived CD34+ cells. Yet another aspect ofthe invention is a method of producing a packaging cell line capable of expressing more than one envelope protein with tropism for human cells, e.g. amphotropic and xenotropic envelope proteins. This packaging cell line is derived by introduction of expression plasmids for other envelope genes into an existing packaging cell line. For example, ProPak-A.6 cells can be transfected with the expression plasmid pCI-Ex and cell clones isolated which secrete vector particles that transduce quail cells or other cells which are transduced by xenotropic particles but not by amphotropic particles. Such multi- tropism particles would be expected to transduce target cells with higher efficiency in the situation where the target cells express the individual receptors but at concentrations too low to allow stable vector-cell complex formation by the pure amphotropic or xenotropic particles. The following examples are intended to illustrate, not limit the scope ofthe invention disclosed herein. Experimental As noted above, successful retroviral-mediated gene therapy requires safe and efficient packaging cell lines for vector particle production. Existing packaging lines for murine leukemia vims-based vectors are predominantly derived from NIH/3T3 cells which carry endogenous murine viral sequences which could participate in recombination to form replication competent retrovims (RCR), thereby rendering them unsafe. Provided herein are methods for constmcting safe and efficient retroviral packaging cell lines for vector particle production as well as the cell lines so obtained. Materials and Methods Unless otherwise stated, materials and reagents are prepared and methods are conducted, as set forth below. Producer cell lines are named by noting the packaging cell line and inserted vector. For example, PA.SVNLZ denotes a producer cell line derived from packaging cell line PA317 carrying the vector SVNLZ. PP-A and PP-X denote ProPak-A and ProPak-X, respectively. PA317-Based Cell Lines and Retroviral Vectors The retroviral vector producer cell lines disclosed herein were generated from the amphotropic PA317 packaging cell line produced according to the methods of Miller and Buttimore, (1986) Mol. CeU Biol. 6:2895-2902. Producer cell lines, the GP+E86 packaging cell line produced according to the method of Markowitz, et al. (1 988) J. Virol 62:1 120-1 124, and NIH/3T3 cells (ATCC CRL1658) were cultured in Dulbecco"s modified Eagle medium (DMEM) supplemented with 4.5 g/L of glucose, 4 mM L- glutamine and 5% Cosmic Calf Serum (CCS, Hyclone, UT). The human epithelial carcinoma line HeLa (ATCC CCL2) was cultured in DMEM supplemented with 4.5 g/L of glucose, 4 mM L-glutamine and 10% fetal bovine serum (FBS, Hyclone, UT). The human lymphoid cell line Jurkat was cultured in RPMI supplemented with 4.5 g/L of glucose, 2 mM L-glutamine, and 10%> FBS. All cell lines were maintained in an incubator at 37°C under 5% C0 2 unless otherwise stated. The SVNLZ (see Bonnerot, et al. (1987) Proc. Natl. Acad. Sci. U.S.A. 84:6795- 6799) and LMTNL vectors are known in the art (See for example, Escaich et al. (1995) Human Gene Therapy 6:625-634). Figure 2 sets forth a map of these vectors. The SVNLZ vector encodes the LacZ gene with a nuclear localization signal expressed from the simian vims 40 early promoter (Figure 2). The LMTNL vector encodes a trans- dominant mutant ofthe HIV Rev protein (RevMlO) expressed from the MMLV-LTR, and the neomycin phosphotransferase (neo) gene expressed from the thymidine kinase promoter (Figure 2). PA317 Culture Conditions For vector production, PA317-based producer cells were seeded into culture vessels at a density of 3x10 cells/cm unless otherwise noted. Once the cells had formed a confluent monolayer (approximately 3 days), the medium was changed and the temperature lowered from 37 β C to 32°C. Three types of culture vessels were used: (1) 75- cm tissue culture flasks with filter caps (Costar, Cambridge, MA); (2) 900-cm roller bottles with filter caps (Costar, Cambridge, MA); or (3) a 500 ml bench scale packed-bed bioreactor (New Brunswick Scientific, Edison, NJ) with 10 g of Fibra Cell discs (12,000 cm ). Samples from the bioreactor (1 ml) were removed with a sterile syringe through a sampling port. Supematant samples were cleared of cellular debris by filtration through 0.45 μm nitrocellulose filters (Nalgene, Rochester, NY), snap-frozen in methanol/dry-ice. and stored at -70°C. Frozen supematants were thawed at 37°C and kept on ice until assayed. All supematants used in this study were free of replication competent retrovims as determined by the S+L- assay on PG4 cells as described in Cornetta, et al. (1993) Human Gene Therapy 4:579-588 and Forestell et al. (1996) J. Virological Methods 60: 171-178. Assays for Retroviral Vectors LacZ end-point titration NIH/3T3 cells were plated into 24 well plates at 5x10 ,4 cells per well, and inoculated the next day with 0.2 ml of diluted vector supematant (10 " ' to 10 *6 dilution series) in the presence of polybrene (8 μg/ml). After three hours, the inoculum was aspirated and 0.5 ml of fresh medium added. Three days post inoculation, the cells were fixed with 0.5% glutaraldehyde and stained with X-gal using the protocol of Bagnis et al. (1993) Oncogene 8:737-743. The end-point titer (cfu ml) was calculated by counting a statistically representative number of blue colonies in a well, multiplied by the dilution factor, and divided by the volume of the inoculum. All end-point titrations were performed in duplicate. For the xenotropic vectors prepared using ProPak-X, titrations were performed using the same procedure except that human 293 cells were used as the target cells. LacZ transduction efficiency Cells were plated into 24 well plates at 5x10 cells per well, and inoculated the next day with 0.2 ml of a 1 : 1 dilution of vector supematant with medium in the presence of polybrene (8 μg/ml). Three hours later, the inoculum was aspirated and 0.5 ml of fresh medium added. Transduction efficiencies were determined three days post-inoculation by flow cytometry using the FluoReporter lacZ detection kit (Molecular Probes, Inc., Eugene, OR). For NIH/3T3 or 293 cells, the reaction with fluorescein di-β-D-galactopyranoside (FDG) was quenched after one minute with ImM phenethylthio-β-D-galactopyranoside (PETG). For Jurkat and HeLa cells, PETG was added after a one hour incubation on ice. Acquisition and analysis were performed using a Becton Dickinson FACScan and the LYSYS software package. Transduction efficiency was determined as the percentage of cells expressing the lacZ gene (green fluorescence intensity) above the basal fluorescence levels defined as that of non-transduced control cells. G418 resistance end-point titer Titrations were performed in 6 well plates seeded with 2.5x10 -.4 NIH/3T3 cells per well, and inoculated the next day with 1.0 ml of diluted vector supematant (1 " ' to 10 "6 dilution series). Three hours later, the inoculum was aspirated and 2.0 ml of fresh medium was added. One day after inoculation, G418 was added to each well at a final concentration of 0.7 mg/ml, and the medium was changed as required until individual colonies could be seen (typically 10 to 14 days). The end-point titer (cfu/ml), was calculated by counting a statistically representative number of G418 resistant colonies in a well and multiplied by the dilution factor. For the xenotropic vectors prepared using ProPak-X, titrations were performed using the same procedure except that human 293 cells were used as the target cells. G418 resistance transduction efficiency Cells were seeded into 6 well plates at 1x10 cells per well. The next day, cells were inoculated with 1.0 ml of a 1 :1 dilution of vector supematant with medium in the presence of polybrene (8 μg/ml). Three hours later, the inoculum was aspirated and 2.0 ml of fresh medium was added. Three days post-inoculation, adherent cells were trypsinized and seeded into 6 well plates at a 10-fold dilution series from 10 cells/well to 1 cell/well. Jurkat cells were plated in methylcellulose medium to allow individual colonies to be scored. Duplicate plates were seeded, and incubated in the presence or absence of G418. The percentage transduction efficiency was calculated as the number of G418-resistant colonies divided by the number of colonies that formed in the absence of G418, multiplied by 100. The following methods relate to Examples 6-12. Retroviral packaging cell culture Packaging and producer cell lines were grown in Dulbecco's modified Eagle's medium (DMEM: JRH Biosciences, Lenexa, KS) supplemented with fetal bovine serum (FBS: HyClone Laboratories Inc.. Logan, UT) at either 5% (PA 317 (Miller and Buttimore, (1986) Mol. CeU. Biol. 6: 2895-2902), PG13 (Miller et al., (1991 ) J. Virol. 65: 2220-2224)) or 10% (ProPak). Expression Constructs and Retroviral Vectors Gag— Pol and Env proteins were expressed from separate plasmids which, along with the retroviral vectors used in this study, are described in Figures 1 and 2. Methods are described herein for derivation and analysis of cells expressing MLV proteins, analysis of Gag-Pol and Env proteins in tissue culture supematants, and titration of retroviral vectors encoding the Lyt2 antigen (Rigg et al. (1995) J. Immunol. Meth. 188: 501 -509). A producer cell line and the respective retroviral vector supematant are indicated by the cell name followed by a period and the name of the vector, e.g., ProPak-A.LLySN (Amphotropic), ProPak-X.LLySN (Xenotropic), PA317.LMiLy or PG13.LMiLy. Co-cultures of ProPak-X and ProPak-A-based producer cell lines are indicated with a slash, i.e. ProPak-A/X.LMiLy. Vector Supematant Production Producer cells were seeded into culture vessels at a density of 3x 10 cells/cm 2 or greater. Once the cells had formed a confluent monolayer (approximately 3 days), the medium was changed and the temperature lowered from 37°C to 32°C. Thereafter, supematant was harvested at 12 hour intervals. For supematant production in T-flasks from pools of LLySN-carrying producer cells, medium was repeatedly harvested for up to 10 days at 32°C. Supematants were then compared for their ability to transduce NIH/3T3 or 293 cell lines, and those supematants that yielded the highest transduction were used in the studies reported here. Three types of culture vessels were used: (1) 75-, 162— or 225-cm 2 tissue culture flasks with filter caps (Costar, Cambridge, MA); (2) 900-cm 2 roller bottles with filter caps (Costar, Cambridge, MA); and (3) a 500 ml packed-bed bioreactor (New Brunswick Scientific, Edison, NJ) with 10 g of Fibra Cell discs (12,000 cm 2 ). The bioreactor was seeded with 5x10 cells/ml and the medium was circulated by a marine-type impeller. Supplemental aeration was achieved by direct micro-sparging of a medical grade mix of air with 5% CO 2 , and 0.01% Pluronic F-68 (Sigma, St. Louis, MO) was added to prevent cell damage from shear. Vector supematants were cleared of cellular debris by filtration through 0.45 μm nitrocellulose filters (Nalgene, Rochester, NY), snap-frozen in methanol/dry-ice, and stored at -70°C. Aliquots were thawed at 37°C and kept on ice until use. All supematants used for gene transfer experiments were free of replication- competent retrovims (RCR) as determined by the extended S+L- assay on PG4 cells (Forestell et al., (1996) J. Virol. Meth. supra). Inoculation Procedures Unless otherwise stated, cells were inoculated with vector once at unit gravity for 3 h at 37°C. Inoculation under centrifugation, termed spinoculation, (Kotani et al., (1994) Hum. Gene Ther.S: 19-28; Bahnson et al., (1995) J. Virol. Meth. 54: 131-143; Forestell et al., (1996) supra) was at 2550g for 3 or 4 h. Polybrene (8 μg/ml) or protamine sulfate (4 μg/ml) was added to inocula for cells lines or primary cells, respectively. Primary Hematopoietic Cell Purification PBL were isolated from peripheral blood mononuclear cells as described (Rigg et al., (1995) supra) and depleted of CD8-positive cells. The CD34-positive (CD34+) fraction was isolated from mobilized peripheral blood (MPB) or adult bone marrow (ABM) with the Baxter Isolex affinity column (Baxter Healthcare, Deerfield, IL). Further purification to obtain the fraction carrying the Thy-1 antigen (CD34+/Thy+) was achieved by high-speed flow cytometric sorting, as described by Sasaki et al, (1995) J. Hemat. 4: 503-514). Hematopoietic stem and progenitor cells (HSPC) were cultured in a 1 : 1 mixture of IMDM and RPMI media containing 10% FBS, IL-3 and IL-6 (20 ng/ml each; Sandoz Pharma, Basel, Switzerland), and stem cell factor (SCF; Amgen Inc., Thousand Oaks, CA) or leukemia inhibitory factor (LIF; Sandoz) (100 ng/ml). Transduction Efficiency Assays Transduction efficiencies achieved with Lyt2-encoding vector supematants were quantitated as the proportion of target cells that expressed Lyt2 antigen 2 or 3 days following inoculation (Rigg et al., (1995) supra). Integration of the RevMlO gene into HSPC clonogenic progeny was determined by PCR. Methylcellulose (Stem Cell Technologies, Vancouver, Canada) was supplemented with IL-3, IL-6, SCF, erythropoietin (Amgen, Thousand Oaks, CA) and granulocyte-macrophage colony stimulating factor (Immunex, Seattle, WA). Individual colonies of all lineages (CFU-C) were picked after 12 to 14 days culture in methylcellulose culture, and assayed for the revMIO gene in cellular DNA using a PCR assay as described in Plavec et al. (1996) Gene Therapy 3:723. Colonies were scored positive if both revMIO and β-globin sequences were detected. Example 1 : Constmction of Packaging Cells To minimize the chance of recombination with endogenous viral sequences to form RCR, candidate cell lines were screened for endogenous retroviral, and in particular, endogenous MMLV nucleic acids. Genomic DNA from a variety of cell lines was analyzed by Southern blot hybridization by the method generally disclosed in Sambrook et al., (1989) Molecular Cloning: A Laboratory Manual 2nd ed., Cold Spring Harbor Laboratory, New York, using probes specific to retrovirus MMLV long terminal repeat (LTR) or MMLV gag-pol sequences. The following cell lines were screened: CHO cells which are used to produce recombinant proteins; Vero and MRC-5 cells in which vaccines are produced (WHO (1989) "Technical Report Series No. 786" WHO, Geneva), and human embryonic kidney 293 cells (ATCC CRL 1573) that are used to produce adenovirus vectors for clinical gene therapy applications and retroviral vectors (Pear et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90:8392-8396; Finer et al. (1994) Blood 83:43-50). Mus dunni tail fibroblasts (NIH) also were included since these cells are reportedly free of endogenous MMLV sequences (Lander and Chattopadhyay (1984) J. Virol. 53:695-698). Genomic DNA from NIH/3T3 cells, the basis for the majority of existing packaging cell lines, hybridized very strongly with both MMLV-specific probes at low or high stringency. Hybridization conditions are provided in the key below Table 1. In contrast, neither probe cross-hybridized with genomic DNA from 293 or MRC-5 cells, even at low stringency (Table 1). In addition, no cross-hybridization was seen with genomic DNA from Mus dunni, MDCK, Vero or Fox Lung cells at high-stringency (see Table 1 , below). Table 1 Screening of cell lines for cross-hybridization to MMLV LTR ov gag-pol sequences. Endogenous MMLV Sequences ^Hybridization) Hybridization Probe: LTR gag/pol Wash Stringency: Low High Low High Cell Lines Tested 293 (ATCC CRL 1573) MDCK (ATCC CCL 34) ± ± Mus dunni tail fibroblasts ± ± Vero (ATCC CCL 81) ± Fox Lung (ATCC CCL 168) ± MRC-5 (ATCC CCL 171) NIH 3T3 (ATCC CRL 1658) ++ CHO-K1 (ATCC 61) Keyi Hybridization signal strength: -, none; ±, weak; +, moderate; ++, strong. Probes: LTR (positions relative to cap site of genomic RNA): nucleotides -232 (Eco RV) to 563 (Pst I of 5' leader sequence); gag-pol : nucleotides 739 (Pst I in gag) to 3705 (Sal I in pol). Stringency (65°C in all instances): Low, 500 mM Na+; High, 50 mM Na+. ATCC: American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland. 20852, U.S.A. Fox Lung and MRC-5 were discounted due to poor growth or limited cell division potential respectively, which would preclude sub-cloning of stably-transfected cells. Thus, 293, MDCK, Mus dunni and Vero cells were identified as candidate cell lines from which to derive packaging cell lines. To decrease the probability of RCR formation, separate expression plasmids were constmcted for gag-pol and env, as disclosed in Danos (1991) Proc. Natl. Acad. Sci. U.S.A. 85:6460-6464; Danos and Mulligan, (1988) Proc. Natl. Acad. Sci. U.S.A. 85:6460-6464; Markovitz et al. (1988) J. Virol. 62: 1 120-1 124; Miller (1990) Human Gene Therapy 1:5- 14; Morgenstem and Land (1990) Nucl. Acids Res. 18:3587-3596. In contrast to existing packaging cells, however, only the minimum genetic information required to encode Gag- Pol and Env proteins was included. The structural gene sequences were amplified by polymerase chain reaction (PCR) using the primers shown below to obtain the open reading frames (ORFs) from the initiation to the termination codons ofthe gag-pol or env genes flanked by Not I restriction sites, and the fragments were subcloned into pBluescript SK+ (Stratagene, La Jolla, California). Oligonucleotide primers (Genosys Biotechnologies, Woodlands, Texas) corresponding to the N-terminus of the genes also placed the AUG in the ideal context for translation (Kozak (1987)) J. Mol. Biol. 196:947- 950), and those corresponding to the C-terminus encoded a second in-frame stop codon. The integrity of PCR products was verified by DNA sequencing. The gag-pol ORF was amplified from the plasmid pVH-2, which carries the infectious Moloney MLV sequence (Miller and Verma (1984) J. Virol. 49:214-222). using the primer pair: 5'-AAAAAAAAGCGGCCGCGCCGCCACCATϋGGCCAGACTGTTACCAC-3' (SEC- ID NO: 1), and 5'-AAAAAAAAGCGGCCGCTCAttaGGGGGCCTCGCGGG-3' (SEQ ID NO: 2). The underlined ATG is that ofthe pl5G g (bases 621 to 623) and the codon in lower case corresponds to the pol stop codon (bases 5835 to 5837). The expression plasmid pCMV-gp with the human cytomegalovirus immediate early (CMV) promoter was constmcted by inserting the gag-pol fragment into the pcDNA3 plasmid (Invitrogen, San Diego, CA) from which the neomycin resistance expression cassette (Dralll to BsmI) had been deleted. Figure 1 schematically shows the plasmid constructs used for expression of MMLV structural genes. Plasmids carrying the gag-pol ORF were propagated at 30°C to prevent recombination under the conditions set forth in Joshi and Jeang (1993) BioTechniques 14:883-886. To amplify the env gene, a contiguous amphotropic envelope sequence was constmcted from p4070A constmcted according to the method of Ott et al. (1990) J. Virol 64:757-766, and amplified with the primer pair: 5'-TAATCTACGCGGCCGCCACCAIQGCGCGTTCAACGCTC-3' (SEQ ID NO: 3) and 5'-AATGTGATGCGGCCGCtcaTGGCTCGTACTCTATGG-3' (SEQ ID NO: 4). The underlined ATG corresponds to bases 37 to 39, and the stop codon (lower case), bases 1998 to 2000 (Ott et al. (1990) supra. The CMV promoter-env expression plasmid pCMV*Ea was created by insertion of the env ORF in place ofthe beta- galactosidase gene of pCMVβ (Clontech, Palo Alto, CA) modified by mutation of an extraneous ATG in the SV40 intron to ACG (SD/SA*). The integrity of the PCR products was verified by DNA sequencing. To further identify the optimal cell line, sandwich ELISA assays were developed to detect Gag and Env proteins in transfected cells and particularly in supematants. Plates were coated with hybridoma culture supematants from either 83A25 (available from the NIH) as disclosed in Evans et al. (1990) J. Virol. 64:6176-6183, for Env, or Rl 87 (ATCC CRL 1912) for Gag. Captured proteins were detected with 79S-834 and 77S-227 anti-sera (Quality Biotech, Camden, New Jersey), respectively, and horseradish peroxidase- conjugated anti-species antibodies and substrate 2,2-Azinobis (3-ethylbenzothiazoiine-6- sulfonic acid) (Pierce, Rockford, Illinois). The ability to produce vector particles was assessed by transfection of a gag-pol expression plasmid into candidate cell lines and selection of drug-resistant pools. Only the supematants from gag-pol -transfected 293 or Mus dunni cells contained Gag protein. No Gag was secreted by transfected Vero or MDCK cells, although Gag was present in the cell lysates. Vero and MDCK cells were discounted as candidate cell lines, and human 293 cells were elected to derive packaging cells. Constmction of ProPak-A To derive amphotropic packaging cells, the pCMV*Ea plasmid was introduced into 293 cells (ATCC CRL 1573) by co-transfection (Profection Kit, Promega, Madison, Wisconsin) at a ratio of 15:1 with the pHA58 plasmid (as disclosed in Riele et al. (1990) Nature 348:649-651) conferring resistance to hygromycin B (250 mg/ml; Boehringer, Indianapolis, Indiana). Stably-selected populations were stained with anti-Env antibody (83A25, available from the NIH) and individual Env-positive cells were isolated by automatic cell deposition on a FACStar Plus (Becton Dickinson, San Jose, CA). Three clones with the highest fluorescence intensity were further characterized. All tiiree yielded equivalent titers upon transient co-transfection with gag-pol and vector plasmids. Next, the pCMV-gp constmct was stably transfected into one ofthe three 293-Env clones by cotransfection with the plasmid pSV2pac (as disclosed in Vara et al. (1986) Nucl. Acids Res. 14:4617-4624). Puromycin-resistant (1 mg/ml; Sigma, St. Louis. Missouri) clones were grown to confluence, medium was exchanged and supematants were collected 16 hours later, filtered, and analyzed for Gag and Env production by using the sandwich ELISA as described in the previous page. Sixteen clones were identified (16/37) that secreted high levels of Gag and Env antigens. Of these, six clones produced vims in transient transfections at titers within 2 to 3-fold of Oz 2 cells (Table 2A), the amphotropic equivalent of BOSC 23 cells (see Pear et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90:8392- 8396). Table 2A Comparison of end-point titers from transiently-transfected ProPak-A cell clones Transient Titers - MFG lac Z Vector Cell Line or End-Point Titer Clone # (cfu x 10 "5 /ml) Oz2 13.8 ± 0.3 ProPak.12 8.8 ± 0.8 ProPak.31 8.0 ± 0.5 ProPak.6 7.5 ± 0.0 ProPak.27 6.8 + 1.2 ProPak.21 6.0 ± 1.0 ProPak.5 6.0 ± 2.0 Supematants were collected after 16 hours at 32°C, and end-point titers determined on NIH/3T3 cells using the materials and methods disclosed above. cfu: colony forming units. Cells were seeded at 2 xl O ' cells/cm in 6-well plates, and transfected 16 hours later with 2.5 mg/well MFG-lacZ DNA (Dranoff et al. (1993) supra and described herein) was in the presence of 25 mM chloroquine (Pear et al. (1993), supra). Titers are the average and range for duplicate transfections. Oz 2 cells also are called Bing cells and are the amphotropic equivalent of BOSC23 cells (Pear et al., 1993). Table 2B Comparison of end-point titers from stable producer cell clones with the LMTNL Vector End-Point Titer Producer Clone (G418 r cfu x 10 "6 /ml PA317.LMTNL 1.7 ± 0.7 ProPak-A.6.LMTNL.6 2.1 ± 0.3 ProPak-A.6.LMTNL.7 2.2 ± 0.8 Supematants were collected after 16 hours at 32 C C, and end-point titers determined on NIH 3T3 cells cfu: colony forming units. Supematants were harvested from confluent cultures of producer cell clones in T-75 flasks. The average and range for triplicate samples is given. One clone with high transient transfection titers was selected and designated ProPak-A.6. ProPak-A.6 was deposited with the American Type Culture Collection (ATCC), 12301 Parklawn Drive, Rockville, MD 20852 U.S.A. on December 1 5, 1995, under the provisions for the Budapest Treaty for the Deposit of Microorganisms for the Puφoses of Patent Procedure, and was accorded ATCC A ssion No. CRL 12306. Transient titers reflect the efficiency of transient transfectic and the titers obtained with ProPak-A cells are lower than those achieved with Oz 2 cells, possibly because Oz 2 is based on a 293T cell clone selected for high transient transfection efficiency (Pear et al. (1993) supra). Constmction of ProPak-X A xenotropic packaging cell line, designated ProPak-X, was constructed as follows. The ATG in the splice donor/splice acceptor of pCMV plasmid was mutated to ACG as described above. The CMV promoter was excised (EcoRl/XhoI, blunt-ended), and replaced with the MoMLV LTR (Asp 718/HindIII, blunt-ended) from plasmid pVH2. The β-galactosidase gene was replaced by the gag-pol ORF (Notl fragment) to generate pMoMLVgp. pMoMLVgp was co-transfected with pHA58 into 293 cells (ATCC CRL 1573) by calcium phosphate co-precipitation and hygromycin B-resistant cells were selected. Clones were screened for the level of Gag secretion and one clone secreting high levels of Gag was selected (designated ProGag); this clone yielded high viral titers in transient transfection. The plasmid pNZBxeno, containing the murine xenotropic env gene, was obtained from Christine Kozak (NIH). The non-contiguous env sequences in pNZBxeno were made contiguous by digestion with Sail and EcoRI and ligation into the Sail site of pBluescript (Stratagene). The xeno env ORF was amplified by PCR and cloned into Xhol/Xbal digested pCI (Promega) to generate the expression plasmid pCI*Ex. pCI*Ex was co- transfected with pSV2pac into the cell line selected as above (ProGag) by calcium phosphate precipitation and puromycin-resistant cells were selected. The resulting cells were screened for Env expression by flow cytometry and clones designated ProPak-X, expressing high levels of Env were screened for ability to produce transducing vector. Samples of one clone, clone 36 designated ProPak-X.36, have been deposited with the American Type Culture Collection (ATCC), 12301 Parklawn Drive, Rockville, MD 20852 U.S.A. on December 15, 1995, under the provisions for the Budapest Treaty for the Deposit of Microorganisms for the Puφoses of Patent Procedure, and the deposit was accorded ATCC Accession No.CRL 12007. Supematant produced by ProPak-X based producer cells were tested for end-point titer and transduction efficiency as described above, except that human 293 cells were used as the target cells. In a similar manner to ProPak-X, amphotropic cell lines were also derived by transfection ofthe amphotropic envelope-encoding plasmid, pCMV*Ea, into ProGag cells and isolation of clones yielding high transduction efficiencies. One of these, clone number 52 (ProPak-A.52), is used extensively in experiments described herein. Lack of RCR The safety ofthe ProPak-A cells was determined by stringent testing for the ability to recombine to generate RCR. In previous work it was found that the vector BC140revM10 (Bevec et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89:9870-9874) reproducibly gave rise to RCR in PA317 cells. BC140revM10 carries the extended packaging sequence, including the ATG ofthe gag ORF. The LMTNL vector (constructed as described in Escaich et al. (1995) supra), in contrast, lacks part of the 5' untranslated region and contains no gag sequences and is therefore less likely to recombine and form RCR. (See Figure 2). The BC140revM10 or LMTNL vectors were each introduced into PA317 or ProPak-A cells and culture supematants were tested for RCR (using the methods disclosed in Haapala et al. (1985J J. Virol. 53:827-833 and Printz et al. (1995; Gene Therapy 2: 143-150) at weekly intervals. The PA317/BC140revM10 combination (transfected or transduced) gave rise to RCR detectable by direct inoculation of culture supematant onto PG4 cells at 4 weeks (Table 3, below). Cultures were maintained for 4 more weeks, and also tested by co-culture of producer cells with M. dunni cells to amplify any RCR in the culture, followed by S+L- assay on PG4 cells. Even by this stringent assay for RCR, the ProPak-A-based producer pools were all free of RCR (Table 3). Table 3 Assay for presence of RCR in cultures carrying the BCHOrevMlO or LMTNL vectors Packaging Transfected Supematant Co-culture Cell Line with: Transduced with: RCR (wk) RCR (wk 8) ProPak-A pBC140revM10 N/A (>8) Negative ProPak-A pLMTNL N/A (>8) Negative ProPak-A N/A M(G).BC140revM10 (>8) Negative ProPak-A N/A M(G).LMTNL (>8) Negative PA317 pBC140revM10 N/A 4 Not tested PA317 pLMTNL N/A (>8) Negative PA317 N/A M(G).BC140revM10 4 Positive PA317 N/A M(G).LMTNL (>8) Negative RCR detected by S+L- assay on PG4 cells (ATCC CRL 2032) by inoculation with supematant from producer cell cultures, or after 3 passages of co-culture with Mus dunni cells. N/A: not applicable. (>8): no RCR detected 8 weeks after G418-resistant pools established. M(G).: transient MMLV(VSV-G) pseudotype (Yee et a/. (1994) Proc. Natl. Acad. Sci. 91:9564-9568) used as inoculum. Resistance to inactivation by human serum Recently, interest has arisen in the in vivo application of retroviral gene transfer by direct administration of vector particles into human beings. In addition, targeting of particles bearing hybrid ligand-ecotropic env genes to specific receptors has been reported (Kasahara et al. (1994); Science 266: 1373-1376 Somia et al. (1995); Proc. Natl. Acad. Sci. U.S.A. 92:1510-1514; Cosset et al. (1995) J. Virol. 69:6314-6322). A pre-requisite is that the particles are not inactivated by human serum. Therefore, ProPak-A.6 or A.52, ProPak- X or PA317-packaged vector particles were analyzed for susceptibility to inactivation by human semm. In addition ecotropic supematants packaged in either PE501 cells (NIH 3T3-based; see Miller and Rosman, (1989) BioTechniques 7:980-990) or in 293 cells were analyzed. Vector particles with the various envelopes produced from 293 cells were resistant, while supematants packaged in NIH/3T3 cells were inactivated by incubation with human serum (Figure 3). Takeuchi et al. (1994) supra, concluded that resistance of vector particles to human semm was determined by both the host cell type and the viral envelope. The data submitted herein shows that packaging of amphotropic, xenotropic and ecotropic vectors in 293-based cells is sufficient to confer resistance to human complement. For the data presented in Examples 2 and 3, herein, the parental packaging cell line was PA317, unless otherwise noted. Example 2: Comparison of End-Point Titer and Transduction Efficiency For gene therapy applications, it is necessary to generate large volumes of characterized supematants, which cannot be easily prepared by transient transfection. It was therefore necessary to determine the stable end-point titers and the transduction efficiencies. End-point titers were determined for supematants from producer cell clones which had been transduced with the PA.LMTNL vector, in which an internal thymidine kinase promoter (T in vector name) drives the neomycin phosphotransferase gene (N). As shown in Table 2B, end-point titers from ProPak-A-based producer cells were marginally higher than those for the best PA317-based producer clone. In addition. the titers from ProPak-A.LMTNL producer pools were stable when passaged for 3 months in the absence of drug selection. As shown herein, while end-point titers are broadly used, transduction efficiency is a better measure of gene transfer potency. However, the assay can be laborious with vectors encoding drug resistance genes. Therefore PA317- or ProPak-A-based producer cell populations carrying a vector (LLySN, Figure 2) derived from the LXSN vector (Miller and Rosman (1989) supra) were prepared by insertion of the Lyt2 surface marker gene (Tagawa et al. (1986) supra). Surface expression of the Lyt2 antigen allows simple, quantitative determination of transduction efficiency by FACS. Higher transduction efficiencies were achieved with supematants from two independently-derived ProPak-A- LLySN populations than with supematants from three PA317. LLySN pools assayed on NIH/3T3 cells (see Figure 4C). Suφrisingly, when the same supematants were assayed for transduction efficiency on human 293 cells, the superiority of ProPak-A is even greater (Figure 4B). To determine the relationship of end-point titer to transduction efficiency, supematants were harvested from producer cells cultured under a variety of conditions to optimize production of retroviral vector supematants. In addition to assaying end-point titers, the proportion of cells transduced after a single inoculation (i.e. transduction efficiency) also was determined. Figure 5A shows the end-point titers and transduction efficiencies obtained with 70 different PA317-derived -β-galactosidase-encoding SVNLZ (PA.SVNLZ; Figure 2) vector supematants. No direct correlation between the end-point titer and transduction efficiency ofthe supematants was found (correlation factor, r = 0.07). To confirm that the lack of correlation between transduction efficiency and end¬ point titer was not specific to the NIH/3T3 cells or the flow cytometry method used to quantitate the proportion of cells transduced with the PA317-derived SVNLZ vector, supematants containing an amphotropic retrovirus encoding the neomycin phosphotransferase gene (LMTNL, Figure 2) were tested on NIH/3T3 as well as Jurkat and HeLa cells (Figure 5B). The results demonstrate that the supernatant which yielded the highest transduction efficiency on NIH/3T3 cells also gave the highest transduction efficiency on both Jurkat and HeLa cells (Figure 5B). Furthermore, the supematant which yielded the highest transduction efficiency did not have the highest end-point titer, again distinguishing between these two functional measurements. Similar results also were obtained with the PA.SVNLZ vector on several different cell lines suggesting that the lack of correlation between end-point titer and transduction efficiency is neither specific to the indicator gene nor to the target cell species. Concentration of Vector Supematants The lack of correlation between end-point titer and transduction efficiency also was apparent when the effect of physically concentrating retroviral vector supematants by ultrafiltration was examined. Vector supematants were concentrated using three different ultrafiltration systems. The Sartocon Mini cross flow ultrafiltration system (Sartorius, Bohemia, NY) was used with a 77.4 cm 2 , 100,000 kDa molecular weight cut-off (MWCO) polysulfone module at a feed pressure of 3 pounds per square inch (psi). concentration was achieved within one hour at room temperature. An Amicon Stirred Cell model 8050 (Amicon, Beverly, MA) was used with a YM100 ultrafilter (3.4 cm 2 , 100,000 kDa MWCO). Positive pressure was maintained with sterile, filtered regulated air, and concentration achieved in 30 minutes at room temperature. Small volumes (5 to 20 ml), were concentrated using Filtron 300,000 kDa MWCO centrifugal concentrators (Filtron Tech. Coφ., Northborough, MA). Concentration was achieved within 45 minutes by centrifugation at 3,000 g in a Beckman GS-6KR centrifuge (Beckman, Palo Alto. CA) at 4°C. Table 4 summarizes data from five independent experiments using two different retroviral vectors and the three different ultrafiltration systems. End-point titers increased in proportion to the volume reduction (up to 19-fold), indicating that the vector was not inactivated by this procedure. However, higher transduction efficiencies were not achieved. Equivalent transduction efficiencies were achieved with supematants produced at 32°C before and after concentration (Table 4). In contrast, retroviral vector supematants produced at 37°C had lower transduction efficiencies following ultrafiltration (Table 4). Interestingly, the lower transduction efficiencies could be restored to the level of the original supematant by diluting the concentrates (see Figure 6), suggesting that an inhibiting agent had been co-concentrated with the transducing particles. In principle, the inhibitor could be non-transducing retroviral particles, non-vector-associated envelope protein, or a non-viral component of the tissue culture supematant. Envelope-Specific Inhibition of Retroviral Vector Transduction To address the nature of the inhibiting agent, tissue culture supernatant from different packaging cell lines or NIH/3T3 cells was tested to determine whether it could inhibit transduction. Supematants from packaging cell lines contain all the necessary vector particle proteins, but lack vector genomes and hence are unable to transduce cells. Ecotropic vectors use different receptors to enter cells than amphotropic vectors and competition for binding to receptors can occur only between viral particles with the same tropism. PA.SVNLZ supematant was mixed with supematant from the following: parental amphotropic PA317 packaging cells; supematant from the ecotropic GP+E86 packaging cell line; PA.SVNLZ concentrate from above or supematant from NIH/3T3 cells, and the transduction efficiency and end-point titers were measured. Addition of supematant from NIH/3T3 cells or the GPE+86 packaging cell line had no effect on transduction efficiency or end-point titer (Tabie 5). In contrast, addition of either parental PA317 or concentrated vector supematant derived from producer cell line PA.SVNLZ reduced transduction efficiency, even though in the latter case, the end-point titer was increased by addition ofthe concentrated PA.SVNLZ supematant (Table 5). These data indicate that inhibition of amphotropic vector transduction by amphotropic envelope protein in particulate or free form. Table 5 Comparison of the transduction of NIH/3T3 cells achieved with PA.SVNLZ supernatant diluted 1:1 with other supematants. Supernatant Transduction f End-Point f Efficiency (%) Titer (cfu/ml x 10 "6 ) NIH/3T3 22.9 ± 0.5 1.4 ± 0.5 GP+E86 (no vector) 24.0 ± 0.5 2.0 ± 0.3 PA317 (no vector) 12.1 ± 0.4 2.5 ± 0.3 Cone. PA.SVNLZ* 12.7 ± 0.4 3.7 ± 0.1 * 8.4-fold concentrate with Sartocon 100 kD ultrafilter (Experiment 1. Table 4) f values given are the average of two samples, and the range. Stability of Retroviral Vector Particles at Different Temperatures Concentration of supernatant produced at 37°C reduced transduction efficiency relative to the original supematant (Table 4). It is possible that these supematants contain a higher proportion of inactivated vector than supematants produced at 32°C. To test vector particle stability at different temperatures, SVNLZ supematant produced at 32°C was incubated at either 37°C, 32°C, or 0°C for various lengths of time. The original supematant had a transduction efficiency of 25% (see Figure 7A), and after incubation for 24 hours at 37°C, 32°C or 0°C the transduction efficiencies were reduced to 2. 14 and 22% respectively. Incubation at 0°C for 4 days further reduced the transduction efficiency to 12%, while the end-point titer remained relatively stable (see Figure 6B). At all temperatures examined, transduction efficiency declined more rapidly than end-point titer. The half-life of vector particles at 37°C was calculated as 4.1 hour based on the end-point titer, similar to previous reports. The half-life of vector at 32°C and 0°C was 12.0 hours and 123.4 hours, respectively. It is interesting to note that at all temperatures examined, end-point titer remained relatively stable (2.5 ± 0.4 x 10 1 cfu/ml) for at least the first eight hours. This initial stability in end-point titer, which was not observed for transduction efficiency, has been seen in three separate experiments using either the PA.S NLZ or PA.LMTNL vector. Retroviral Production Kinetics Given the greater stability of retroviral vector particles at 32°C than at 37°C, experiments were performed to study the kinetics of vector production at these temperatures. SVNLZ producer cells were grown to approximately 80% confluency at 37°C, at which time the medium was changed and the cells placed at either 37°C or 32°C. Supematant samples were collected at different time points, snap-frozen, and assayed for both end-point titer (Figure 8 A) and transduction efficiency (Figure 8B). Over the first 6 hours, similar amounts of transducing vector accumulated in cultures at 37°C or 32°C (Figures 7A and 7B). Calculations incoφorating the inactivation rate of the vector at 37°C or 32°C show that the rate of vector production is slightly higher at 37°C than at 32°C (0.031 cfu/cell hour compared to 0.027 cfu cell/hour). Consistent with this, the amount of p30 capsid or gp70 envelope protein present in the samples measured by ELI 3 A demonstrated that vector secretion is also marginally higher at 37°C than at 32°C. Similar results were also found with the LMTNL vector. While the rate of virion inactivation is lower at 32°C than 37°C, the inactivation of vector particles at 32 D C is still significant (Figure 7A), and the time that supematants remain at this temperature should therefore be minimized. Experiments were performed to determine the minimum time required to produce supematants with maximal transduction efficiency. Figure 9 gives the time-course of vector production from a confluent PA.LMTNL producer cell culture in a roller bottle at 32°C, and shows that the transduction efficiency reaches a plateau 3 hours after medium exchange. Example 3: Comparison of Production Methods The kinetics of vector production from PA317-based producer cell lines suggest that once producer cells reach confluency, supematants should be collected every 3 to 5 hours to obtain supematant with a high transduction efficiency. To achieve these conditions, a bench scale packed-bed bioreactor was operated under either fed-batch (periodic medium exchange) or perfusion mode (continual medium perfusion). In the latter, supematant leaving the bioreactor was collected into a container at 0°C to minimize inactivation. Initial experiments with the PA.SVNLZ producer cell line cultured in the packed-bed bioreactor operated in fed-batch mode showed comparable vector production by end-point titer and transduction efficiency to that in tissue culture flasks. Next, the packed-bed bioreactor was operated in perfusion mode and compared to a roller bottle operated in fed-batch mode (Table 6). Sampling of supematants from the bioreactor or roller bottles began one day after cells were seeded. In both cultures, incubation was initially at 37°C and lowered to 32°C after 50 hours. The cells in both cultivation systems reached confluency after approximately 100 hours of incubation. Transduction efficiencies achieved with supematants produced in the packed-bed bioreactor were greater than those from roller bottle cultures (Figure 10A). However, supematants produced in the packed- bed bioreactor had lower end-point titers than supematants from the roller bottle (Figure 10B) possibly due to the shorter residence time ofthe medium in the reactor (5.75 hours compared to 24 hours for the roller bottle). Over a 5 day period a total of 10L of supematant was collected using the packed-bed bioreactor, compared to IL from the roller bottle. Table 6 Operating Parameters of Different Production Systems Parameter Packed Bed Bioreactor * Roller Bottle + Surface Area (cm 2 ) 12.000 1 ,700 Volume (ml) 500 200 Mode of Operation perfusion fed-batch Dilution Rate (Vol./day) 4.2 1.0 Total Production Vol. (ml) 10.000 1 ,000 Seeding Density (cells/ml) 6.5 x 10 5 2.3 x 10 5 Final Density (cells/ml) 5.91 x 10 6 9.06 x 10 6 End-Point Titer (cfu/ml) 1.45 x 10 6 2.40 x 10 7 Transduction Efficiency (%) 36.0 29.5 * lOg of Fibercell discs in bench-top New Brunswick Scientific bioreactor + Coming expanded surface area roller bottle Example 4: Comparison of Vector Production from ProPak-A and PA317 Ce' ls Supematant Production from Bioreactor 10 g of the New Brunswick Scientific Fiber Cell disks (catalog #M1 1 76-9984) were placed into the spinner basket ofthe packed-bed bioreactor (catalog #M 1222-9990) and washed several times with PBS before autoclaving. Prior to seeding, the bioreactor and Fibercell discs were washed twice in medium containing 5% FBS. The reactor was seeded with 2.4 to 3.6x10 producer cells (same for ProPak and PA317) in a total volume of 500 ml of medium (i.e. 2 to 3x10 4 cells/cm 2 ) (DV1EM with 5% to 10% FBS). The agitation rate upon seeding was set at approximately 80 rpm. A minimum of 3 hours was required for all of the cells to become attached to the Fibercell discs. Approximately 4 to 18 hours after seeding, 50% of the medium was exchanged, and 5ml of Pluronic F68 (Sigma cat# P5556) was added to the bioreactor to a final concentration of 0.1%. At this time aeration was initiated by gently sparging medical grade air/5% CO 2 mix (Altair cat#39222) into the reactor through a 0.2 mm filter. Agitation was increased to approximately 250 φm. Cells were allowed to grow for three days at 37°C and then the temperature was lowered to 32°C. Medium exchanges should be performed on the cells every 4 to 12 hours, or the reactor operated in perfusion-mode. The perfusion rate was 2 to 6 reactor volumes per day. Perfused supematant harvested from the bioreactor was collected into 2L glass vessels (vented cap) kept on ice. The harvested supematant was filtered through a 0.45 μm filter, aliquoted, and snap frozen in MeOH/dry ice. Snap-freezing can be performed in other ways such as in liquid nitrogen. The bioreactor and set-up ofthe continuous perfusion operation are shown in Figures 1 1 and 12. Supematant Preparation from Roller Bottles Producer cells were seeded at 3xl0 4 cells/cm 2 in 0.25 ml of DMEM plus 5% FBS per cm 2 of surface area. The rotational speed was set at 0.6 φm. Cells were grown at 37°C for two days. On day three the temperature was decreased to 32°C. The medium was aspirated and replaced with fresh DMEM medium plus 5% FCS (0.2 ml/cm of surface area). Medium was added with a pipette to the bottom of the flask to prevent cells from peeling off and to avoid bubble formation and cell contact. The semm level may be lowered to 2% FBS. For PA317-based producers, two batches of medium were collected every day (every 12 hours) and pooled to make a single lot. For ProPak-A based producers, supematants were collected every 12 to 24 hours. Each batch was filtered through a 0.45μm filter immediately, and kept at 4°C until batches were pooled (preferably no longer than about 12 to 24 hours). The pooled batches were aliquoted, snap frozen, and stored at - 80°C. Supematant collection can continue as long as producer cells are healthy. Figure 13 shows the comparison of vector production from ProPak-A cells culture in T-flasks and in the packed-bed bioreactor with aeration as assayed on 293 cells. The supematant produced using the packed bed bio-reactor mediates considerably higher transduction than that produced in T-flasks. Figure 14 shows that ProPak-X cells also produced higher transduction efficiency supematant when cultured in the packed-bed bioreactor compared to T-flasks. Dilution of vector supematant produced by ProPak-A under different culture conditions is shown in Figure 15. Again, the aerated packed-bed bioreactor produced higher quality vector supematants as determined by transduction efficiency. Example 5: Primary Cell Transductions Cell selection and analysis Apheresed samples were obtained with informed consent from multiple myeloma or breast cancer patients. Stem cells were mobilized into the peripheral blood by treatment with cyclophosphamide (Cytoxan) and GM-CSF (multiple myeloma), G-CSF above (breast cancer), or Cytoxan + VP-16 + CDDP + G-CSF (breast cancer). Apheresis for total white cells was started when the peripheral blood white cell count was greater than 500 cells/ml and the platelet count was greater than 50,000 cells/ml. Patients were apheresed daily until 6 x 10 mononuclear cells (MNC) were collected. The cells were washed twice in PBS (partially depleting platelets), and CD34+ cells were positively selected using a Baxter Isolex™ cell selector. The recovered cells averaged 85% to 99% CD34+ purity as determined by FACS analysis. Transduction CD34 + cells (0.5 x 10 6 ) were suspended in 0.5 ml of freshly thawed r≥troviral supematant and diluted 1 : 1 in Whitlock/Witte medium (50% IMDM, 50% RPMI 1640, 10% FCS, 4xl0 "5 M 2-mercaptoethanol, lOmM HEPES, 100 μ/ml penicillin, 100 mg/ml streptomycin, and 4mM glutamine) containing cytokines at the following final concentrations: c-kit ligand (Amgen) 100 ng/ml or leukemia inhibiting factor (LIF, Sandoz); IL-3 (Sandoz) 20 ng/ml; IL-6 (Sandoz) 20 ng/ml. Protamine sulfate was added at a final concentration of 4 μg/ml or polybrene was added at a final concentration of 8 μg/ml. The cells and vector were C :,trifuged at 2800 x g, 33°C to 35°C, for three hours. The cells were resuspended in medium with cytokines and cultured for three days. After three days, cells were harvested and approximately 4 x 10 cells used to determine bulk iransduction efficiency, and plated in methylcellulose to determine progenitor cell transduction efficiency (see below). For the LLySN and LMily vectors, which contain the murine Lyt-2 marker gene, bulk transduction efficiency was determined by staining with APC or PE co jugated anti- Lyt-2 (Pharmingen) and sulfurhodamine conjugated anti-CD34. The results are shown in Figures 16A and 16B. Vector supematant from either ProPak-A or ProPak-X transduced primary human cells with greater efficiency than that from PA317 cells. Furthermore, the combination of amphotropic and xenotropic vector can be useful to deliver two vectors to the same cell since they do not compete for the same receptor. Methylcellulose assay Cells from each transduction (2.5 x 10 " to 10 x 10 ) were added to 4 ml of methylcellulose medium (Stem Cell Technologies) plus 1 ml IMDM containing the following cytokines (final concentration): c-kit ligand 100 ng/ml; GM-CSF (Amgen) 10 ng/ml; IL-3 10 ng/ml; IL-6 10 ng/ml; rhEPO (Amgen) 2 units/ml. 1.0 ml of the cell/cytokine methylcellulose mixture was plated onto five 35 mm plates using a 5 ml syringe and 16 gauge needle, and the plates were placed in a 37°C incubator for 2 weeks. After 14 days, single methylcellulose colonies were picked and analyzed for the presence of neo or RevMIO by PCR. Analysis Individual colonies were picked by aspiration in 5μl and transferred to 25 μl to 50 μl of PCR lysis buffer. PCR lysis buffer is a l .T mixture of buffer A (100 mM KCl, 10 mM Tris pH 8.2, 2.5 mM MgCl 2 ) and buffer B (10 mM Tris pH 8.3, 2.5 mM MgCl 2 , 1 % Tween 20, 1% NP40, 100 μl/ml proteinase K). The mixture was allowed to incubate ovemight at 37°C or for 2 h at 56°C. The proteinase K was inactivated by heating at 94°C for 10 to 30 minutes, and 5-10 μl ofthe lysate was used for the PCR reaction. The PCR reaction amplified a lOObp fragment ofthe β-globin gene and either a 240bp fragment of the neo gene or a 180bp fragment of RevMIO, depending on the vector used. The primers used were as follows. Rev: 5' TCgATTAgTgAACggATCCTT 3' (SEQ ID NO: 5) 5' CTCCtgACTCCAATATTgCAg 3' (SEQ ID NO: 6) Neo: 5' TCgACgTTgTCACTgAAgCg 3' (SEQ ID NO: 7) 5' gCTCTTCgTCCAgATCATCC 3' (SEQ ID NO: 8) Beta-globin: 5' ACACAACTgTgTTCACTAgC 3' (SEQ ID NO: 9) 5' CAACTTCATCCACgTTCACC 3' (SEQ ID NO: 10) The reactions were performed in a 40 μl final volume in a Perkin Elmer thermal cycler 9600 as follows: 5 min denaturation at 94°C; 40 cycles of 30 sec at 94°C. 30 sec at 62°C, 1 min at 72°C; and 10 min at 72°C. PCR products are visualized by ethidium bromide agarose gel electrophoresis (Sambrook et al. ( 1989) supra) and the PCR products confirmed by Southern blot hybridization. A sample was considered positive if both the Rev or neo band and the β-globin band were present. The results, summarized in Table 7, show that for two different tissues (MPB and ABM), the ProPak supematants performed better than the PA317 supematants. The results in Figure 16A show that both the xenotropic ProPak-X (X.36) and the amphotropic ProPak-A.6 were able to produce retroviral vector preparations which transduced human hematopoietic stem/progenitor cells with higher efficiency than that produced from PA317 cells. 202962003640 Table 7 METHYL CELLULOSE DATA — CD34+ CELLS 10 (Number of Colonies) TISSUE LABEL PACKAGING VECTOR TESTED POSITIVE TRANSDUCTION PCR TARGET CELLS EFFICIENCY (%) SEQUENCE MPB Ampho PA 317 LLySN 24 4 17 neo MPB Xeno PP-X.36 LLlySN 24 7 29 neo ro MPB Ampho PP-A.6 LLySN 24 1 1 46 neo ABM Ampho PP-A.6 LMiLy 56 13 23 rev ABM Ampho PA 317 LMiLy 56 4 7 rev Key: MPB: Mobilized peripheral blood ABM: Adult bone marrow from a cadaver 15 neo: Neomycin rev: Rev MIO The following series of examples describe the generation of high quality vector supematants with MLV-xenotropic, MLV-amphotropic or GaLV envelope and the ability of these different enveloped vectors from human or murine-based packaging cells to transduce primary human hematopoietic cells. The complementary tropisms and safety of the ProPak-A and ProPak-X cell lines were also exploited to produce vector supematants by co-culture. It was demonstrated that after inoculation of hematopoietic stem and progenitor cells (HSPC) with co-culture supematants containing both amphotropic and xenotropic vector particles, 100% ofthe colony-forming progeny contained transgene. Expression of transgene in cells with a phenotype characteristic of hematopoietic progenitor cells was also demonstrated. Example 6: Derivation of Packaging Cells To facilitate derivation of packaging cell lines with a variety of envelopes, a cell line expressing the MLV Gag-Pol functions was first constructed. The gag-pol gene was expressed from the MMLV-LTR promoter, the CMV-IE promoter, or the Rous Sarcoma Virus LTR (Figure 1 ). The gag-pol open reading frame (ORF) (Rigg et al., ( 1996) supra) was sub-cloned into the expression vector pMLV*. This plasmid was derived from pCMV* (Rigg et al. (1996) supra) by replacement of the CMV promoter with the MLV LTR (M-LTR) promoter. The envelope protein ORF ' s were sub-cloned into the restriction sites shown in plasmid pCI (Promega, Madison, WI) which contains the cytomegalovirus immediate early promoter (CMV-IE), a chimeric intron (SD/SA), and the simian virus 40 late polyadenylation sequence (pA). The xenotropic envelope (Ex) gene was obtained by PCR from a linear template derived from plasmid pXeno (O'Neill et al. (1985) J Virol. 53: 100-106) using the following oligonucleotide primer pair: 5'-ACCTCGAGCCGCCAGCCΔIQGAAGGTTCAGCGTTCTC-3' (SEQ ID NO: 1 1) and 5 , -AATCTAGACttaTTCACGCGATTCTACTTC-3' (SEQ ID NO: 12). The underlined ATG corresponds to nucleotides 291 to 293, and lower case letters denote the stop codon, nucleotides 223 to 225 (O'Neill et al. (1985) supra). The chimeric envelope (Eax) gene was constmcted by replacing the sequence between the Apa I and Bgl II sites of the amphotropic envelope gene (nucleotides 772 to 963; Ott et al. (1990) J Virol. 64: 757-766) with a synthetic DNA corresponding to the sequence of the xenotropic envelope gene from 10A1 (nucleotides 801 to 965 ; Ott et al., (1990) supra). Plasmids encoding Gag-Pol were cotransfected into 293 cells which are free of MLV-like sequences, which are free of endogenous MLV-like sequences (Rigg et al. (1996) supra). Transient Gag expression was detected by ELISA in all cases, but it was only possible to isolate clones stably expressing Gag at levels equivalent to the ProPak-A.6 cells (Rigg et al. (1996) supra) with the MLV-LTR driven Gag. Transient cotransfection with vector and envelope plasmids of the 293/Gag-Pol cells (called ProGag) yielded higher titer supematants than cotransfection of Anjou cells (Pear et al. (1993) Proc. Natl. Acad. Sci. c7&4 90: 8392-8396). Next, expression plasmids bearing the genes for the xenotropic (Ex, Figure 1) or amphotropic (Rigg et al., (1996) supra) envelope proteins were stably introduced into ProGag cells, and clones expressing either xenotropic (called ProPak-X) or amphotropic (ProPak-A.52) envelope proteins were isolated by flow cytometry. In turn, the vectors LLySN and LMiLy (Figure 2) were transduced into these cells, and the resulting producer cells were used to generate the supematants used in this study. Retroviral vectors LMiLy and LLySN have been described (Rigg et al., (1995) supra and Rigg et al, (1996) Virol. 218: 290-295). LMiLy and LLySN encode the Lyt2 surface antigen (Ly) (Tagawa et al., (1986) Proc. Natl. Acad. Sci. USA 83: 3*22-3426) expressed either directly from the retroviral LTR (L) promoter in LLySN, or via an internal ribosomal entry site (i) in LMiLy. The packaging (psi) sequence in LMiLy is to nucleotide 566 (Shinnick et al., (1981) Nature 293: 543-548), and LLySN has the longer psi sequence and non-functional ATG (Bender et al., (1987) J. Virol. 64: 1639-1646; Miller and Rosman, (1989) Biotechniques 7: 980-990), and also contains the neomycin phosphotransferase gene (N) expressed from an internal SV40 promoter (S). Unless indicated otherwise, LLySN supematants were prepared in T-flasks from G4 18 resistant cell populations, and LMiLy supematants were prepared from producer cell clones in the sparged packed-bed bioreactor. In contrast to Ex and Ea envelopes (Figure 1), no cells stably expressing either the chimeric Eax (Figure 1) or GaLV envelope proteins could be isolated in repeated attempts since cell lysis resulted. While transient supematants could be prepared by cotransfection of ProGag cells with the Eax and vector expression plasmids, no stable expression ofthe Eax envelope was achieved in 293, ProGag or HT1080 cells. Most dramatic was transfection of the GALV envelope-encoding plasmid which resulted in total lysis of ProGag or 293 cell cultures within 24 hours. Transient GaLV supematants could not be produced with cotransfected ProGag cells although the GaLV envelope gene was functional, since vector particles were released if murine cells were cotransfected. Example 7: Determination of Tropism To confirm the phenotype of vector particles bearing the amphotropic, xenotropic or chimeric ampho/xeno envelope, supematants were inoculated onto cell lines from different species chosen to distinguish between the various envelope tropisms. Cells were inoculated with vector particles packaged in the new ProPak-X (xenotropic) or ProPak-A.52 (amphotropic) cell lines, or with vector supematants from cotransfection of vector and the expression plasmid encoding the Eax envelope. For comparison. MLV particles pseudotyped with the GaLV (from PGl 3 cells) glycoprotein or the G protein of vesicular stomatitis vims (VSV-G, Yee et al., (1994) Proc. Natl. Acad. Sci. USA 91: 9564- 9568) were also included. The tropism observed for the xenotropic, amphotropic and GaLV envelopes (Fig. 20A) corresponded with those established for retrovimses (Teich, ( 1984) Cold Spring Harbor Laboratory). By analogy to the genotype and receptor tropism of MLV-10A1 (Ott et al., (1990) supra; Wilson et al., (1994) J. Virol. 68: 7697-7703: and Wilson et al. (1995) J. Virol. 69: 534-537), the Eax envelope should mediate retroviral vector binding to both the amphotropic and xenotropic receptors. Vector particles bearing the Eax envelope transduced quail cells, which are resistant to amphotropic particles but susceptible to xenotropic vector (Figure 17B), and also transduced murine NIH/3T3 cells which are resistant to xenotropic vector (Figure 17A). Therefore, this modification ofthe amphotropic envelope protein conferred specificity for a cell line that cannot be transduced with amphotropic vector, establishing the extended tropism. As expected, all cell types were transduced with the pantropic MLV(VSV-G) vector particles. Lower transduction efficiencies were achieved with this pseudotype, possibly a result of having to prepare supematants by transient cotransfection because ofthe toxicity ofthe VSV-G protein. Example 8: Resistance to Human Semm It was shown above that amphotropic and ecotropic particles generated from 293 cells were not inactivated by treatment with human serum under conditions which inactivate amphotropic vector packaged in murine PA317 cells. The xenotropic vector was similarly tested for resistance and the xenotropic particles were found to not be inactivated by treatment with human semm (Figure 18). Vector particles from the ProPak-A.52 packaging clone were also resistant, while the PA317-packaged vector was almost completely inactivated (Figure 18), as has been previously demonstrated (Takeuchi et al., (1994) J. Virol. 68: 8001-8007; Rigg et al., (1996) supra). Example 9: RCR Generation Test The Gag-Pol packaging functions in ProPak-X and ProPak-A.52 cell hnes are expressed from the MLV-LTR promoter and these cells therefore carry more MLV-derived sequences than the ProPak-A.6 cells. However, generation of RCR due to recombination of MLV-derived sequences is unlikely since a minimum of three recombination events would still be required. Only 32 nucleotides ofthe R sequence are retained in the LTR promoter used, a relatively short sequence homology with vectors. Nevertheless, cultures and supematants were stringently tested for RCR. Co-cultures of ProPak-A and ProPak-X cells carrying the vector BCHOrevMlO were maintained. This vector, as before (Rigg et al., (1 96) supra), rapidly generated RCR in PA317 cells (Table 8). Although cultures were maintained for up to 3 months, in no case was RCR detectable either in supematants or cells from cultures containing ProPak cells (Table 8). Table 8. Tests for RCR in producer cell cultures or co-cultures Packaging Cell Supematant Co-culture Expt. Line(s) Vector Carried RCR (wk) RCR (wk l2) 1 PA317 LMTNL (>12) ND PA317 BCHOrevMlO 6 ND Pro-Pak-A.52 LMTNL (>12) ND ProPak-A.52 BCHOrevMlO (>12) ND ProPak-X LMTNL (>12) ND ProPak-X BCHOrevMlO (>12) ND PP-A.52/PP-X* LMTNL (>12) negative PP-A.52/PP-X* BCHOrevMlO (>12) negative 2 PA317 BCHOrevMlO 6 ND ProPak-A.52 BCHOrevMlO (>12) negative ProPak-X BCHOrevMlO (>12) ND ProPak-A.6 BCHOrevMlO (>12) negative PP-A.52/PP-X* BCHOrevMlO (>12) negative ND: not determined (>12): no RCR detected 12 weeks after G418 -resistant pools established. *Co-culrure of PP-X and PP-A producer cell populations. Vectors were introduced into the packaging cells by spinoculation at HOOg with MLV(VSV- G) pseudotype supematants produced by transient transfection of ProGag cells with vector and VSV-G expression plasmids (Yee et al., (1994) supra). Populations of producer cells were selected for G418 resistance and passaged every 3 or 4 days. RCR was detected by S+L- assay on PG4 cells (ATCC CRL 2032) by inoculation with supematant from producer cell cultures, or after 3 passages of co-culture with Mus dunni cells (Forestell et al., (1995) supra; Printz et al., (1995) Gene Ther. 2: 143-150) in the presence of 2 μg/ml polybrene. Example 10: Improved Retroviral Vector Supematant Production in a Bioreactor Applying principles of retroviral vector production defined with PA317-based producer cell lines (Forestell et al., (1995) supra), vector production from ProPak cell lines in a packed-bed bioreactor was investigated. Using the PP-A.52. LMiLy producer cell line, the packed-bed bioreactor operated in fed-batch mode was compared with a roller bottle and T-fiask for production of vector supematant. The effect of supplemental sparging of an air/C0 2 mix into the bioreactor was also examined to determine if oxygen transfer from the bioreactor head-space was limiting cell growth and vector production. Supplemental aeration was achieved by direct micro-sparging of a medical grade mix of air with 5% C0 2 , and 0.01% Pluronic F-68 (Sigma, St. Louis, MO) was added to preven: cell damage from shear. Vector supematants harvested from the different production vessels were compared for their ability to transduce NIH 3T3 cells (Figure 19). Supematants harvested from the T-flask and roller bottle cultures yielded transduction efficiencies of 62% and 63% respectively, while supematant from the non-sparged packed-bed bioreactor yielded a transduction efficiency of 78%. Gene transfer was greatest (100%) with vector produced in the sparged packed-bed bioreactor indicating that head-space oxygen transfer alone was limiting vector production and cell growth in the non-sparged bioreactor. Measurements of the glucose and glutamine concentrations indicated that these key nutrients were not iimiting in any ofthe cultures. In this particular experiment, vector production was also analyzed by measurement of the viral envelope and Gag proteins by ELISA :o determine if the improved transduction efficiency was due to increased vector production. Although the final volumetric cell density in the sparged packed-bed bioreactor was slightly lower than in the T-flask (4.0x10 6 and 4.7x10 6 cells/ml respectively), the vector supematants from the sparged packed-bed bioreactor yielded higher levels of viral protein as well as higher transduction than T-flask supematants. These results indicated that the improved production in the sparged packed-bed bioreactor was due to increased cell-specific vector productivity. PA317, PGl 3, ProPak-A.6, ProPak-A.52, and ProPak-X based produced cell lines were cultured in the packed-bed bioreactor under sparged conditions. In every case, a 2- to 20-fold improvement in vector production was achieved in comparison to T-flask cultures. In particular, it was determined if the increased gene transfer into cell lines could also be achieved within primary cells. In addition to the higher transduction of NIH/3T3 cells (Figure 19), ProPak supematants produced in the sparged packed-bed bioreactor yielded approximately 3-fold higher transduction efficiencies with CD34-positive cells than vector preparations from T-flask cultures (Table 9). Using two different assays (Lyt2 expression in bulk cultures or PCR analysis of CFU-C), different estimates for gene transfer efficiency were obtained; however, the relative numbers were consistent. Table 9. Transduction of CD34-positive cells from MPB(normal donor): comparison of T- flask and packed-bed bioreactor supematants. Transduction Efficiency (%) T-Flask Packed-Bed Lyt2 CFU-C Lyt2 CFU-C Producer Cell Line Expression Marking Expression Marking ProPak-A.LMiLy 1.3 5.3 (7/132) 3.9 15.0 (22/147) ProPak-X.LMiLy 3.8 18.1 (27/149) 9.9 43.3 (49/1 13) Cells were inoculated once, and Lyt2 expression was measured 3 days later. Gene marking of CFU-C, determined by PCR, is expressed as a percentage and values in brackets are the number of RevMlO-positive colonies divided by the total number that yielded β-globin signals, as described above. Example 1 1 : Primary Cell Transduction with Different Vector Tropisms This Example compared transduction of CD34-positive hematopoietic progenitor cells or CD4-positive PBL with vectors of different tropisms (amphotropic, xenotropic, GaLV) prepared from stable producer cell lines. Cells were exposed to vector once to directly compare transduction efficiencies which are presented as Lyt2 surface marker expression relative to the expression achieved with PA317-packaged vector preparations (Table 10). Regardless of tropism, all vector types successfully transduced CD34-positive cells isolated from MPB or ABM, or CD4-positive PBL (Table 10). This implies that the receptors for all three vector types are expressed on these cells. While no single vector tropism appeared to mediate significantly higher levels of transduction than any other, the highest transduction efficiencies were achieved with ProPak-A or ProPak-X supematants (Table 10). Although derived by different means, comparable transduction was achieved with vector from either ProPak-A.6 or ProPak-A.52 clones (Table 10). Also in these comparisons, amphotropic vector supematants from the human ProPak-A cell lines consistently transduced a higher proportion of target cells than amphotropic vector prepared from PA317-based producer cells (Table 10). The phenotype of the transduced progenitor cells was determined by flow cytometry. The majority of cells expressing the Lyt 2 gene exhibited a phenotype characteristic of early hematopoietic progenitor cells, that is, CD34 antigen was expressed (Figure 20, panel B) in the absence of antigens specific for differentiated hematopoietic cell lineages (Figure 20, panel D). Since no single tropism appeared to preferentially transduce primary cells, cells were also inoculated with a mixture of vector packaged separately in ProPak- A or ProPak-X cells, to test whether higher transduction could be achieved by simultaneous inoculation with vectors targeting distinct receptors. In all but one of five cases (PP-A & PP-X mix; Table 3) slightly higher transduction was achieved with the mixture (Table 10). It is possible that in these experiments the concentration of vector in the supematants may be limiting, since individual supematants were diluted 4-fold in the mixture, compared with 2-fold dilutions where single inocula were used. Table 10. Transduction of primary human hematopoietic cells with vector supematants of different tropisms from T-flasks (A, LLySN vector) or the sparged packed-bed bioreactor (B, LMiLy vector). A. T-Flask/LLySN Relative Lyt2 Expression CD4+ CD34+ CD34+ CD34+ Producer Cell Line Base PBL MPB (a) MPB (b) MPB (c) PA317 1.0 1.0 1.0 1.0 ProPak-A.52. 2.4 1.6 1.8 1.6 ProPak-A.6 2.7 2.0 2.4 1.7 ProPak-X 2.7 1.5 1.9 1.2 PG13 2.6 1.1 1.3 ND ProPak-A.52 & X mix ND 2.0 ND ND B. Bioreactor/LMiLy Relative Lyt2 Expression CD4+ CD34+ CD34+ 34+/Thy-l + Producer Cell Line Base PBL ABM MPB (b) MPB (d) PA317 1.0 1.0 1.0 1.0 ProPak-A.5.2 3.9 ND 2.5 ND ProPak-A.6 3.0 2.5 3.2 1.5 ProPak-X 5.8 2.1 2.0 1.9 PG13 1.2 0.7 0.9 ND ProPak-A.52 & X mix 8.1 2.8 3.7 2.8 Transduction of different primary cell types achieved after a single inoculation with supematants containing the LLySN vector (A), or the LMiLy vector (B). Transduction efficiencies were measured as the proportion of Lyt2-expressing cells, and values have been normalized to that achieved using PA317-based supematants. Hematopoietic cell populations isolated from six different tissues were inoculated, specifically, CD4+ PBL. or CD34+ cells selected from: ABM or MPB from 2 breast cancer patients (a and c), a multiple myeloma patient (b), or a normal donor (d). ND=not determined. Example 12: Vector Production in Producer Cell Co-Cultures In attempts to increase the transduction efficiency, vector supematants were produced from a co-culture ofthe complementary ProPak-X. LMiLy and ProPak-A.52.LMiLy producer cells in the packed-bed bioreactor. This techn.que, known as ping-pong amplification, results in higher titers with murine and avian producer cell lines, probably as a result of increased vector copy number (Bodine et al., (1990) Proc. Natl. Acad. Sci. USA 87: 3738-3742; Cosset et al., (1993) Virol. 193: 385-395; Hoatlin et al., (1995) J. Mol. Med. 73: 1 13-120). The problem that has arisen in the pasi with co-culture production is the generation of RCR (Bodine et al., (1990) supra; Cosset et al., (1993) supra; Muenchau et al., (1990) Virol. 176: 262-265). However, it was already shown herein that no RCR arises during extended co-culture of ProPak-A and ProPak-X cells carrying the BCHOrevMlO vector (Table 8), an event that is even less likely with the LMiLy vector which lacks sequences that overlap with the ProPak packaging constmcts. Nevertheless, supematants and end-of-production cells were exhaustively tested and found to be free of RCR by stringent assays (incubation or co-culture with Mus dunni and S+L- assay on PG-4 cells). In addition, the presence of both amphotropic and xenotropic vector particles was confirmed by transduction of permissive and restrictive cell lines. The ability of PA317, ProPak- A, ProPak-X or ProPak- A/X. LMiLy vector supematants to transduce CD34-positive cells upon a single spinoculation was first compared. Cell samples were analyzed for Lyt2 expression, and the presence of the revMIO transgene in committed progenitor cells was determined by PCR. Consistent with Lyt2 expression assays (Table 10), supematants from the human-based ProPak-A or -X producers transduced cells more efficiently than PA317-packaged vector, and the highest level of transduction was achieved with the ProPak- A/X supematant (Table 1 ] ). Using the ProPak-A/X. LMiLy supematant, different cytokine combinations and multiple rounds of inoculation were investigated in an effort to maximize gene transfer efficiency. CD34-positive cells were spinoculated with PP-A/X.LMiLy vector once, twice on the same day, or once a day on two consecutive days in the presence of IL-3, IL-6, and either LIF or SCF. Prior to the first spinoculation, cells were incubated for 1 clay in 20ng/ml each of IL-3 and IL-6, and 50ng/ml SCF or LIF as described above. The results in Table 12 show that while gene transfer was higher in cultures treated with SCF after a single inoculation, gene transfer into CFU-C was 100% for cultures inoculated twice on consecutive days in either LIF or SCF. Furthermore, in both cytokine cocktails the Lyt2 transgene was expressed in up to 40% ofthe total cell population, and in 35% of the cells which were also CD34-positive. Table 11. Transduction of CD34+/Thy+ cells from MPB: comparison of vector supematants from different producer cell lines. Transduction Efficiency (%) Producer Cell Line Lyt2 Expression CFU-C PA317.LMiLy 2.8 12 (27/219) ProPak-A.ό.LMiLy 4.1 24 (49/206) ProPak-X.LMiLy 5.2 25 (35/141) ProPak-A.52/X.LMiLy 7.9 36 (65/182) Transduction efficiencies achieved with vector supematants prepared in the sparged packed-bed bioreactor from the producer cell lines shown. CD34+/Thy+ cells from normal MPB were inoculated at unit gravity for 1 hour followed by a 3 hour spinoculation. Transduction efficiency is given as the expression of the Lyt2 surface antigen or the marking frequency of individual methyl cellulose colonies (CFU-C; Table 9). Table 12. Transduction of CD34+ cells from MPB: comparison of inoculaticm protocol and cytokine cocktail. Transduction Efficiency (%) IL-3, IL-6, LIF IL-3, IL-6, SCF Inoculation Lyt2 CFU-C Lyt2 CFU-C Protocol Expression PCR Expression PCR once 13.3 51 (47/92) 14.5 74 (68/92) twice, same day 22.7 86 (79/92) 22.3 100 (92/92) twice, consecutive days 40.4 100 (92/92) 40.4 100 (92/92) Transduction of CD34+ cells (MPB) with packed-bed bioreactor supematant from a co- culture of ProPak amphotropic and xenotropic LMiLy producer cells. Cells were spinoculated and incubated as shown in the presence of IL-3, IL-6, and LIF or SCF. Expression of Lyt2 was determined 2 days after the last inoculation, and the mεjking frequency of individual methyl cellulose colonies (CFU-C) was determined (Tε.ble 9). Table 13. Characteristics ofthe ProPak Cell Lines Packaging Transduction Human Expression Plasmids Cell Line Tropism Efficiency Serum RCR Gag-Poi Env ProPak-X xenotropic >PG 13; PA317 Resistant Negative pMLV*gp pCI-Ex ProPak-A.52 amphotropic >PA317 Resistant Negative p LV'gp pCMV*Ea ProPak-A.6' amphotropic ≥PA317 Resistant Negative pCMV-gp pCMV*Ea Rigg et al., (1996) supra. Described above are safe, new retroviral vector packaging cell lines which can target distinct receptors on human cells and provide efficient gene transfer to human cells for gene therapy applications. The new lines described in Example 6 differ from the ProPak-A clone 6 ofExample 1 since the Gag-Pol ORFs were expressed from the MLV-LTR promoter (Table 13) instead ofthe CMV promoter in ProPak-A.6. Despite carrying additional MLV-specific sequences, no RCR was detectable in supematants or cells using a stringent test, namely extended co-culture of ProPak-X and ProPak-A.52 cells carrying the BCHOrevMlO vector. These ProPak vector particles are resistant to human serum, and should therefore remain functional if administered in vivo to humans. The ProPak-X and ProPak-A cell lines produce particles which utilize distinct receptors on human cells, and the tropisms were confirmed by determining the ability to transduce cell lines from various species. Both receptors can be targeted by producing supematants from a co-culture of ProPak- A and ProPak-X producer cell lines. The safety of the ProPak cell lines allows for efficient ping-pong amplification ofthe vector without the danger of RCR formation. Using ProPak-A X vector, 100%) marking of CFU-C derived from CD34-positive cells purified from MPB was achieved. Significantly, the present experiments demonstrated expression ofthe Lyt2 transgene in 40% ofthe inoculated CD34-positive cells derived from MPB, 2 days post-inoculation. The present experiments also indicated that high-efficiency gene transfer is best achieved with supematants prepared from stable producer cell cultures. The approach described above is applicable to production from stable cells, existing and future vector systems. As is apparent to those of skill in the art, various modifications and alterations to the above can be made without departing from the spirit and scope ofthe invention disclosed herein. Accordingly, the invention is limited only by the following claims. SEQUENCE LISTING (1) GENERAL INFORMATION: (l) APPLICANT: RIGG, RICHARD J. DANDO, JONATHAN S. CHEN, JINGYI FORESTELL, SEAN P. PLAVEC, IVAN BOHNLEIN, ERNST (ll) TITLE OF INVENTION: METHOD FOR OBTAINING RETROVIRAL PACKAGING CELL LINES PRODUCING HIGH TRANSDUCING EFFICIENCY RETROVIRAL SUPERNATANT (ill) NUMBER OF SEQUENCES: 12 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: MGF^ISON & FOERSTER (B) STREET. 755 PAGE MILL ROAD (C) CITY- PALO ALTO (D) STATE: CA (E) COUNTRY: USA (F) ZIP: 93304-1018 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentin Release #1.0, Version #1.30 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER. PCT/US96/ (B) FILING DATE: (C) CLASSIFICATION: (vill) ATTORNEY/AGENT INFORMATION- (A) NAME: KONSKI, ANTOINETTE F (B) REGISTRATION NUMBER: 34,202 (C) REFERENCE/DOCKET NUMBER 20296-20036.40 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (415) 813-5600 (B) TELEFAX: (415) 494-0792 (C) TELEX: 706141 (2) INFORMATION FOR SEQ ID NO: 1 : (l) SEQUENCE CHARACTERISTICS: (A) LENGTH: 45 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS. single (D) TOPOLOGY: linear (xi) SEQUENCE DESCRIPTION SEQ ID NO:l AAAAAAAAGC GGCCGCGCCG CCACCATGGG CCAGACTGTT ACCAC 45 (2) INFORMATION FOR SEQ ID NO.2 : (l) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE- nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY, linear (x ) SEQUENCE DESCRIPTION SEQ ID NO:2 : AAAAAAAAGC GGCCGCTCAT TAGGGGGCCT CGCGGG 36 (2) INFORMATION FOR SEQ ID NO•3. (i) SEQUENCE CHARACTERISTICS (A) LENGTH 38 base pairs (B) TYPE nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY, linear (xi) SEQUENCE DESCRIPTION SEQ ID NO:3 TAATCTACGC GGCCGCCACC ATGGCGCGTT CAACGCTC (2) INFORMATION FOR SEQ ID NO.4 : (l) SEQUENCE CHARACTERISTICS: (A) LENGTH 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY, linear (xi) SEQUENCE DESCRIPTION. SEQ ID NO:4 AATGTGATGC GGCCGCTCAT GGCTCGTACT CTATGG 36 (2) INFORMATION FOR SEQ ID NO.5 : (i) SEQUENCE CHARACTERISTICS (A) LENGTH 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY linear (xi) SEQUENCE DESCRIPTION SEQ ID NO:5 : TCGATTAGTG AACGGATCCT T 2 1 (2) INFORMATION FOR SEQ ID NO:6 : (l) SEQUENCE CHARACTERISTICS. (A) LENGTH. 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS. single (D) TOPOLOGY, linear (xi) SEQUENCE DESCRIPTION- SEQ ID NO 6. CTCCTGACTC CAATATTGCA G 21 (2) INFORMATION FOR SEQ ID NO 7- (l) SEQUENCE CHARACTERISTICS (A) LENGTH 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY, linear (xi) SEQUENCE DESCRIPTION SEQ ID NO 7 TCGACGTTGT CACTGAAGCG 20 (2) INFORMATION FOR SEQ ID NO • 8 • (l) SEQUENCE CHARACTERISTICS (A) LENGTH. 20 base pairs (B) TYPE nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY linear (χι) SEQUENCE DESCRIPTION SEQ ID NO 8 GCTCTTCGTC CAGATCATCC 20 (2) INFORMATION FOR SEQ ID NO ■ 9 • (l) SEQUENCE CHARACTERISTICS (A) LENGTH 20 base pairs (B) TYPE nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY linear (xi) SEQUENCE DESCRIPTION SEQ ID NO 9 ACACAACTGT GTTCACTAGC 20 (2) INFORMATION FOR SEQ ID NO 10 (l) SEQUENCE CHARACTERISTICS- (A) LENGTH 20 base pairs (B) TYPE nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY: linear (xi) SEQUENCE DESCRIPTION SEQ ID NO 10 CAACTTCATC CACGTTCACC 20 (2) INFORMATION FOR SEQ ID NO 11 (ι) SEQUENCE CHARACTERISTICS (A) LENGTH 37 base pairs (B) TYPE nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (xi) SEQUENCE DESCRIPTION SEQ ID NO 11 ACCTCGAGCC GCCAGCCATG GAAGGTTCAG CGTTCTC 37 (2) INFORMATION FOR SEQ ID NO:12: (l) SEQUENCE CHARACTERISTICS: (A) LENGTH: 30 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY, linear (xi) SEQUENCE DESCRIPTION. SEQ ID NO 12- AATCTAGACT TATTCACGCG ATTCTACTTC 30

Claims: What is claimed is: 1. A method for obtaining a recombinant retroviral packaging cell capable of producing retroviral vectors comprising the steps: a. selecting a retrovims; b. obtaining a eukaryotic cell free of endogenous retroviral nucleic acid of the same type as the retrovims of step (a); c. preparing a minimal gag-pol open reading frame (orf) insert from the retrovirus or its equivalent; d. inserting the minimal gag-pol ORF prepared from step (c) into an appropriate expression plasmid; e. preparing a minimal env open reading frame (orf) insert from the retrovirus or its equivalent; f. inserting the minimal env orf prepared from step (e) into an appropriate expression plasmid; g. inserting the expression plasmids of steps (d) and (f) into tie cell of step (b); h. propagating the cell obtained from step (g) under conditions favorable for expression of the minimal retroviral gag-pol and env orf; and i. screening the cell of step (h) for retroviral Gag, Pol and Env production, thereby obtaining a retroviral packaging cell capable of packaging recombinant retroviral vector sequences to produce recombinant, transducing retroviral vector particles. 2. The method of claim 1 , wherein the retrovirus is a murine leukemia vims. 3. The method of claim 1, wherein the cell is a non-murine cell. 4. The method of claim 3, wherein the non-murine cell is a primε.te cell. 5. The method of claim 4, wherein the primate cell is a human cell. 6. The method of claim 3, wherein the non-murine cell is selected from the group consisting of Vero, HT-1080, D17 MRC-5, FS-4. TE671 , human embryonic kidney (293), and HeLa. 7. The method of claim 6, wherein the human embryonic kidney cells are 293 cells (ATCC CRL 1573). 8. The method of claim 1 , wherein the gag-pol ORF is a moloney murine leukemia virus gag-pol gene . 9. The method of claim 1 , wherein the env ORF is a murine leukemia virus env gene. 10. The method of claim 1, wherein the plasmid of steps (d) or (f) comprises a selectable or detectable marker gene. 1 1. The method of ciaim 1 , wherein in step (i), Gag, Pol and Env production is screened by a sandwich ELISA assay. 12. The method of claim 1 1 , wherein Env is detected using a primary antibody from hybridoma 83A25 followed by antisera 79S-834, enzyme-conjugated antispecies antibody and enzyme substrate; and Gag is detected separately using a primary antibody from hybridoma R187 followed by antisera 77S-227, enzyme-conjugated antispecies antibody and enzyme substrate. 13. The method of claim 1 , further comprising screening the cell after step (i), for the ability to produce a vector supematant having high transduction efficiency, comprising measuring the ability ofthe vector supematant to transduce a target cell population. 14. The method of claim 13, wherein the target cell population is human 293 cells. 15. The method of claim 1 wherein the cell is propagated in a sparged packed- bed bioreactor. 16. The method of claim 8 wherein the gag-pol gene is expressed from the MMLV-LTR promoter. 17. The method of claim 8, wherein the gag-pol gene is expressed from the CMV-IE promoter or the RSV-LTR promoter. 18. The recombinant retroviral packaging cell obtained by the method of claim 1. 19. The retroviral packaging cell of claim 18, wherein the cell produces an amphotropic env. 20. The retroviral packaging cell of claim 18, wherein the cell produces a xenotropic env. 21. The retroviral packaging cell of claim 18, wherein the cell produces a chimeric amphotropic/xenotropic env. 22. A retroviral packaging cell line capable of packaging retrovirai vector sequences to produce a recombinant, transducing retroviral vector particle, the recombinant vector particle characterized by: a. being resistant to human complement; b. having a high transduction efficiency; and c. being incapable of generating RCR after continuous culture ofthe packaging cell line for more than 2 weeks. 23. The retroviral packaging cell line of claim 22 wherein the cell line is designated ProPak-A.6 and has ATCC Accesssion No. CRL 12006. 24. The retroviral packaging cell line of claim 22 wherein the cell line is designated ProPak A.52. 25. The retroviral packaging cell line of claim 22 wherein the cell line is designated ProPak-X.36 and has ATCC Accession No.CRL 12007. 26. A method of producing a retroviral vector producer cell which comprises transducing the cells of claim 16 with a retroviral-based vector and subsequently propagating the cell under conditions favorable for the production and secretion of retroviral vector supematant. 27. The retrovirai vector producer cell produced by the method of claim 26. 28. A method of increasing the gene transduction efficiency of a cell, comprising transducing the cell with a retroviral vector supematant produced from the culture of at least one recombinant packaging cell of claim 18. 29. The method of claim 28 wherein the packaging cell is selected from group consisting of ProPak-A.6, ProPak-A.52 or ProPak-X.36. 30. The method of claim 28 wherein the the retrovirus supematant is produced from the co-culture of a first and a second complementary packaging cell. 31. The method of claim 30 wherein the first packaging cell is amphotropic and the second packaging cell is xenotropic. 32. The method of claim 30 wherein the first packaging cell is ProPak-X and the second packaging cell is ProPak-A. 33. The method of claim 32 wherein the first packaging cell is PraPak-X.LMily and the second packaging cell is ProPak-A.52.LMily. 34. The method of claim 29 wherein the cell is a primary human hematopoietic cell. 35. The method of claim 29 wherein the cell is a human hematopoietic stem cell. 36. The method of claim 34 wherein the hematopoietic cell is a CD34+Thyl + cell from mobilized peripheral blood or a CD4+ PBL . 37. The method of claim 36 wherein the packaging cell is propagated in a sparged packed-bed bioreactor and the hematopoieitic cell is transduced by two or more rounds of spinoculation with retroviral vector supernatant in the presence of ιL3, IL6 and either LIF or SCF. 38. Use of an ELISA method to screen for production of retroviral structural proteins, to identify a retroviral packaging cell capable of producing recombinant, transducing vector particles.

Inventors: BOHNLEIN ERNST, CHEN JINGYI, DANDO JONATHAN S, FORESTELL SEAN P, PLAVEC IVAN, RIGG RICHARD J, BOHNLEIN, ERNST, CHEN, JINGYI, DANDO, JONATHAN, S., FORESTELL, SEAN, P., PLAVEC, IVAN, RIGG, RICHARD, J.
Applicants: BOHNLEIN ERNST, CHEN JINGYI, DANDO JONATHAN S, FORESTELL SEAN P, PLAVEC IVAN, RIGG RICHARD J, SYSTEMIX INC, BOHNLEIN, ERNST, CHEN, JINGYI, DANDO, JONATHAN, S., FORESTELL, SEAN, P., PLAVEC, IVAN, RIGG, RICHARD, J., SYSTEMIX, INC.
Application year: 1996
Main classification: C12N15
IPC codes: A61K48, C12M3, C12N5, C12N7, C12N15