TOXIC-PROTEIN EXPRESSING VIRUSES AND PRODUCER CELL LINES

Abstract: Expression vectors are provided which have a tightly controllable promoter which directs the expression of a fusogenic protein. Also provided are nucleic acid molecules which encode toxic of fusogenic proteins, wherein one or more cysteine residues in the nucleic acid molecule has been altered to an opal termination codon.

Description: TOXIC-PROTEIN EXPRESSING VIRUSES AND PRODUCER CELL LINES Technical Field The present invention relates generally to compositions and methods for producing recombinant viruses, and more specifically, to producer cell lines and methods related thereto which allow production of recombinant viruses which encode or contain toxic or fusogenic proteins. Background Of The Invention Since the discovery of DNA in the 1940s and continuing through the most recent era of recombinant DNA technology, substantial research has been undertaken in order to realize the possibility that the course of disease may be affected through interaction with the nucleic acids of living organisms. Most recently, a wide variety of methods have been described for altering or affecting genes, including for example, viral vectors derived from retroviruses, adenoviruses, vaccinia viruses, herpes viruses, and adeno-associated viruses (see Jolly, Cancer Gem Therapy 7(l):51-64, 1994). Recombinant retroviruses and various uses thereof have been described in numerous references including, for example, Mann et al., Cell 33: 153, 1983, Cane and Mulligan, Proc. Nat'l. Acad. Sci. USA 81:6349, 1984, Miller et al., Human Gene Therapy 7:5-14, 1990, U.S. Patent Nos. 4,405,712, 4,861,719, 4,980,289 and PCT Application Nos. WO 89/02,468, WO 89/05,349 and WO 90/02,806. Briefly, a foreign gene of interest may be incorporated into the retrovirus in place of the normal retroviral RNA. When the retrovirus injects its RNA into a cell, the foreign gene is also introduced into the cell, and may then be integrated into the host's cellular DNA as if it were the retrovirus itself. Expression of this foreign gene within the host results in expression of the foreign protein by the host cell. Currently, certain retroviral vectors (such as those that have an amphotropic envelope) offer a broad host range, but fail to compare to the host range offered by the VSV-G protein (Marsh and Helenius, Advances in Virus Research 36: 107-151, 1989). Briefly, Vesicular Stomatitis Virus (VSV) is known to infect various types of tissues including muscle, liver, neuronal, fibroblast and non-replicating hematopoietic cells. VSV-G has an extended tropism compared to amphotropic envelope which enables entry into man, lower primates, rodents, amphibians, fish and insects. A recent report has indicated that VSV G-pseudotyped retroviral vectors can transduce human peripheral hematopoietic stem/ progenitor cells much more efficiently than amphotropic vectors (Kerr et al., Blood 84 (Suppl. l):402a, 1994). However, because of the exquisite fusogenic properties of VSV-G protein, a stable cell line has been very difficult to produce, because of cell fusion throughout the entire culture (Yee et al., Proc. Natl. Acad. Sci. USA P7:9564-9568, 1994). Some expression of VSV-G has been reported in murine C127 fibroblasts (Florkiewicz et al., J. of Cell Biol. 97:1381-1388, 1983) and MDCK (Roman et al., Experimental Cell Research 775:376-387, 1988) cells, although levels of cell surface expression were low. The present invention provides compositions and methods which allow the production of recombinant viral particles with fusogenic envelopes such as VSV-G, or which express other proteins which are toxic to cells. The present invention provides these, as well as other related advantages. Summary of the Invention Briefly stated, the present invention provides compositions and methods for producing recombinant retroviruses which have toxic and/or fusogenic proteins. For example, within one aspect of the present invention expression vectors are provided comprising a murine CMV IE promoter operably linked to a nucleic acid sequence which encodes a toxic and/or fusogenic protein. Within another aspect, expression vectors are provided comprising a tightly controllable promoter that directs the expression of a nucleic acid sequence which encodes a fusogenic protein. Representative examples of fusogenic proteins include Vesicular Stomatitis Virus-G protein, and Rabies virus G protein. Representative examples of toxic proteins include γ-IFN, IL-2, and TNF-β. Representative examples of tightly controllable promoters include tTA and LAP promoter systems. Within related aspects, such expression vectors may further comprise a selenium responsive element, as well as the aforementioned nucleic acid sequence wherein one or more cysteine residues encoded by the nucleic acid sequence has been altered to an opal termination codon. Within other aspects, expression vectors are provided comprising a promoter operably linked to a nucleic acid sequence encoding a fusogenic protein and/or a toxic protein, and a selenium responsive element, wherein one or more cysteine residues in the sequence encoding the toxic or fusogenic protein has been altered to an opal termination codon. Also provided are retroviral vectors which contain one of the above noted expression vectors, packaging cells which comprise a gag/pol expression cassette and an expression vector as described above, as well as producer cell lines which comprise a gag/pol expression cassette, an expression vector as described above, and a retroviral vector construct. Within other aspects of the present invention, mutant nucleic acid molecules are provided which encode a fusogenic protein, or a toxic protein, wherein one or more cysteine residues in the nucleic acid molecule have been altered to an opal termination codon. Within yet another aspect of the present invention, methods are provided for transiently producing retrovector particles, comprising the steps of (a) incubating between 10 and 10 cells which contain a gag/pol expression cassette and retroviral vector construct in a culture having a surface area greater than 225 cm 2 , under conditions and for a time sufficient to allow the cells to enter the log phase of growth, and (b) introducing an expression vector as described above into the culture, such that retrovector particles may be transiently produced from the cells. Representative examples of methods for introducing the expression vector into the culture of cells such that entry of the expression vector into the cells occurs, include lipofection, electroporation, or transfection utilizing a source of Ca -1" * " ions and a source of PO " . These and other aspects of the present invention will become evident upon reference to the following detailed description and attached drawings. In addition, various references are set forth below which describe in more detail certain procedures or compositions {e.g., plasmids, etc.), and are therefore incorporated by reference in their entirety as if each were individually noted for incorporation. Brief Description of the Drawings Figure 1 is a schematic illustration of a VSV-G/SRE expression construct. Figure 2 is a schematic illustration of the distribution of cysteine residues in a VSV-G gene. Figure 3 is a schematic illustration of the Hindϋl "L" fragment of CMV. Figure 4A is a Southern blot analysis of three isolates of RM-CMVG. Figure 4B is a schematic illustration of the probe used in the analysis provided in Figure 4A. Figures 5A and 5B are two photographs (200X) of RCMVG and wt MCMV NIH3T3 infected cells. Figure 6 is a Western blot of VSV-G expressing isolates. Figure 7 is a Western blot of VSV-G expression isolates. Detailed Description of the Invention Prior to setting forth the invention, it may be helpful to an understanding thereof to set forth definitions of certain terms that will be used hereinafter. "Expression vector" and "expression cassette" refers to an assembly which is capable of directing the expression of a sequence or gene of interest. The nucleic acid expression vector must include a promoter which, when transcribed, is operably linked to the sequence(s) or gene(s) of interest, as well as a polyadenylation sequence. Within certain embodiments of the invention, both the promoter and the polyadenylation sequence are from a source which is heterologous to the helper elements, gag/pol and env. Within other embodiments of the invention, the expression vectors described herein may be contained within a plasmid construct. "Retroyiral vector," "retroviral vector construct" "RETROVECTOR CONSTRUCT ™" and "recombinant retroviral vector." which generally is a subset of 'expression vectors' as discussed above, refers to a nucleic acid construct which carries, and within certain embodiments, is capable of directing the expression of a nucleic acid molecule of interest. The retroviral vector must include at least one transcriptional promoter/enhancer or locus defining element(s), or other elements which control gene expression by other means such as alternate splicing, nuclear RNA export, post- translational modification of messenger, or post-transcriptional modification of protein. Such retroviral vectors must also include a packaging signal, long terminal repeats (LTRs) or portion thereof, and positive and negative strand primer binding sites appropriate to the retrovirus used (if these are not already present in the retroviral vector). Optionally, the recombinant retroviral vector may also include a signal which directs polyadenylation, selectable markers such as Neo, TK, hygromycin, phleomycin, histidinol, or DHFR, as well as one or more restriction sites and a translation termination sequence. By way of example, such vectors typically include a 5' LTR, a tRNA binding site, a packaging signal, an origin of second strand DNA synthesis, and a 3' LTR or a portion thereof. "Recombinant retrovirus." "retroviral gene delivery vehicle" and "retroviral vector particle" as utilized within the present invention refers to a retrovirus which carries at least one gene of interest. The retrovirus may also contain a selectable marker. The recombinant retrovirus is capable of reverse transcribing its genetic material into DNA and incoφorating this genetic material into a host cell's DNA upon infection. "Packaging cell" refers to a cell which contains those elements necessary for production of infectious recombinant virus which are lacking in a recombinant viral vector. Typically, such packaging cells contain one or more expression cassettes which are capable of expressing proteins which encode gag, pol and env proteins. "Producer cell" or "Vector producing cell" refers to a cell which contains all elements necessary for production of recombinant viral vector particles. "Fusogenic protein" refers to glycoproteins which cause cells within a culture to fuse into a multinucleate syncytia. Representative examples of fusogenic proteins include VSV-G and Rabies G protein. "Toxic-protein" refers to a protein which exhibits cytotoxic or cytostatic behavior in cells. ). For purposes of the present invention, a protein is deemed to be cytotoxic if, when expressed in a proliferating culture, greater than 20% cell death occurs within a 48 hour time period (as compared to the same culture which does not express the protein). Similarly, a protein is deemed to be cytostatic if, when expressed in a proliferating culture, a decrease in cell doubling time of greater than 20% occurs within a 48 hour time period (as compared to the same culture which does not express the protein). Preferred assays for the measurement of cytotoxicity include the MTT assay which uses an non-radioactive format to quantitate viable cell number in proliferation assays (e.g., Promega Corp., Madison, WI), or 3 H incoφoration into nascent DNA. A number of other assay formats however, may likewise be utilized to evaluate various parameters associated with cell toxicity, including for example, cell viability (dye exclusion, cell counting) and cell metabolism (dye reduction) (See Cell Biology, A laboratory Handbook, ed. J.E. Celis, Academic Press for further details and methods. Representative examples of toxic proteins include γ interferon, interleukin-2, TNF α and TNF β. "Tightly controllable" refers to promoters (including promoter systems) or other elements which maintain a strong restraint over transcription of a gene. More specifically, as utilized within the context of the present invention it should be understood that tightly controllable promoters induce greater than about 10 fold, and preferably greater than about 50 to about 100 fold level of induction a gene of interest (e.g., a nucleic acid molecule encoding a toxic or fusogenic protein). As noted above, the present invention provides expression vectors which have a tightly controllable promoter that directs the expression of a nucleic sequence encoding a toxic and/or fusogenic protein, as well as recombinant retroviral particles which contain on their surface fusogenic proteins. Generally, as discussed in more detail below, such retroviral vector particles may be made by controlling the expression of a toxic or fusogenic protein either through transcriptional or translational regulatory mechanisms, so that large quantities of the desired retroviral vector particle may be produced. A. PREPARATION OF RECOMBINANT RETROVIRAL VECTORS. PACKAGING CELLS, PRODUCER CELLS AND RECOMBINANT RETROVIRUSES As noted above, the present invention provides recombinant retroviruses which are constructed to carry or express a selected nucleic acid molecule of interest. Retroviral gene delivery vehicles of the present invention may be readily constructed from a wide variety of retroviruses, including for example, B, C, and D type retroviruses as well as spumaviruses and lentiviruses (see RNA Tumor Viruses, Second Edition, Cold Spring Harbor Laboratory, 1985). Such retroviruses may be readily utilized in order to assemble or construct retroviral gene delivery vehicles given the disclosure provided herein, and standard recombinant techniques {e.g., Sambrook et al, Molecular Cloning: A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, 1989; Kunkle, PNAS 52:488, 1985). Representative examples of retroviral gene delivery vehicles that may be utilized within the context of the present invention include, for example, those described in EP 0,415,731; WO 90/07936; WO 94/03622; WO 93/25698; WO 93/25234; U.S. Patent No. 5,219,740; WO 93/11230; WO 93/10218; Vile and Hart, Cancer Res. 55:3860-3864, 1993; Vile and Hart, Cancer Res. 55:962-967, 1993; Ram et al., Cancer Res. 55:83-88, 1993; Takamiya et al., J. Neurosci. Res. 55:493-503, 1992; Baba et al., J. Neurosurg. 79:729-735, 1993 (U.S. Patent No. 4,777, ,127, GB 2,200,651, EP 0,345,242 and WO 91/02805). Particularly preferred recombinant retroviruses include those described in WO 91/02805 and US 95/05789. Packaging cell lines suitable for use with the above-described vector constructs may be readily prepared (see also WO 92/05266), and utilized to create producer cell lines (also termed vector cell lines or "VCLs") for the production of recombinant vector particles, given the disclosure provided herein. Within other aspects of the present invention, methods are provided for transiently producing retrovector particles, comprising the general steps of (a) incubating between 10 and 10 s cells which contain a gag/pol expression cassette and retroviral vector construct in a culture having a surface area greater than 225 cm 2 , under conditions and for a time sufficient to allow the cells to enter log phase of growth, and (b) introducing an expression vector as described above into the culture, such that retrovector particles may be transiently produced from the cells. Representative examples of methods for introducing the expression vector into the culture of cells such that entry of the expression vector into the cells occurs, include lipofection, electroporation, or transfection utilizing a source of Ca ++ ions and a source of PO 4 ~ (e.g., CaCl and HBS, Promega, Madison WI). Within certain embodiments, the surface area of the culture may be greater than about 1200 cm 2 , or even greater than about 6,000 cm . Utilizing the methods of the present invention as disclosed herein, packaging cell lines that produce recombinant retroviral particles at titers greater than 10^ or 10? cfu/ml (in crude supernatant) may readily be obtained. In addition, it should be noted that such titers are generally obtained from titer assays on HT1080 cells, which produce a three-fold lower titer than titers obtained on murine 3T3 cells. B. REGULATION OF GENE EXPRESSION As noted above, the present invention provides tightly controllable or regulated expression vectors (including retroviral vectors, as well as retroviral vector particles) which, through transcriptional and/or translational control mechanisms, allow the production of highly toxic or fusogenic proteins. Such vectors, in contrast to other inducible promoter systems, do not exhibit pleiotropic effects on eukaryotic cells. For example, in one aspect of the present invention expression vectors are provided wherein a LAP (Lac Activator Protein; Levine and Shenk, Molec. Cell. Biol. 70:3343- 3356, 1990; PNAS 55:5072-5076, 1991) promoter system is operably linked to and capable of expressing a sequence encoding one of the aforementioned toxic or fusogenic proteins. Briefly, LAPs are potent activators of several promoters (SV40, vaccinia virus promoter, T3 bacteriophage) containing lac operator sequences positioned either upstream or downstream of the transcription unit. A single lac operator allows for transactivation, multiple operators act synergistically. Binding affinity and specificity of lac repressor to operator sequences are very high with a binding affinity of 10"^ M. The LAP fusion protein has three components: (1) lac repressor; (2) SV40 nuclear localization signal; and (3) VP16 (transcription activation domain of HSV). One representative illustration of such a system is provided in Example 4 below. Within another aspect of the present invention, expression vectors are provided which comprise a tet (tetracycline-controlled) promoter system which is operably linked to and capable of expressing one of the above-mentioned toxic or fusogenic proteins. Briefly, the tet promoter is negatively regulated by the presence of tetracycline. Within one embodiment, a constitutively expressed tetracycline-controlled transactivator (tTA) may be generated by fusing the tet repressor with the activating domain of VP16 from Heφes simplex (Gossen and Bujard, PNAS 59:5547-5551, 1992). In the absence of tetracycline, this transactivator stimulates transcription from a minimal CMV-derived promoter sequence which also contains tet operator sequences located upstream of the promoter. Integration of a sequence encoding a fusogenic protein such as VSV-G downstream from the tTA-dependent promoter leads to high level expression of VSV-G in the absence of tetracycline. Such vectors are particularly useful for the production of recombinant retroviral proteins, since tetracycline may be maintained in the medium while packaging, and subsequently, producer cell lines are made. After a sufficient number of producer cell lines are grown up, tetracycline may then be removed from the medium, thus allowing the production of VSV-G containing retroviral vector particles. One representative illustration of such a system is provided below in Example 3. Other examples of tightly controllable promoter or promoter systems which may be utilized within the present invention include GAL1 (J. Biotech. 52(2): 179-89, 1994); Gene, 75 (l):19-25, 1995; P.N.A.S. 59(23): 11589-93, 1992); steroids such as Dexamethasone (Biochim Biophys Acta, 279(3):653-9, 1994; Nawa et al., J Biol. Chem. 2<57(36):16883-16888, 1986; J Vir., 64(IQ):5132-42. 1990); heavy metal regulated systems such as Metallothionein (J. Vir., <55(9):5602-12, 1994; Care, 74(8):1643-9, 1993; J Vir. Methods, 55(1 -2): 135-47, 1993); T7/lac (J Biol Chem, 269(16): 12106-10, 1994; Biochem Biophys Res Comm., 755(3):972-81, 1992); Tat and or tat/rev inducible lentiviral promoter systems, e.g., HIV-1 or 2 (see e.g., Leukemia, 7Suppl 2:S61-5, 1993; P.N.A.S. 59(l):182-6, 1992); Heat shock (J. Biol Chem, 265(9):6708-13, 1993); NF-KB responsive (Mol Aspects Med, J4(3):ll\-90, 1993); Cyclic AMP (Roesler et al., J. Biol. Chem. 263( 19):9063-9066, 1988) As noted above, within other aspects of the present invention expression vectors are provided which are tightly regulated through translational mechanisms. For example, within one aspect of the present invention expression vectors are provided comprising a promoter (e.g., one of the promoters discussed above) operably linked to a nucleic acid molecule encoding a toxic or fusogenic protein, and a selenium responsive element, wherein one or more cysteine residues in the nucleic acid molecule have been altered to an opal termination codon. Briefly, selenoproteins constitute a unique group of prokaryotic and eukaryotic polypeptides, all of which incoφorate the sulfur-like element selenium as selenocysteine. Selenoproteins include bacterial formate dehydrogenases, the mammalian glutathione peroxidase (GPx) family, mammalian type I iodothyronine 5' deiodinase, and mammalian selenoprotein P (Bock et al., Trends Biochem. Sci. 76:463, 1991). All of these proteins incoφorate selenocysteine cotranslationally at a UGA codon (Bock et al., Supra), formerly known only as a "stop" (or opal) codon, and utilize a unique selenocysteine-charged tRNA^eCys containing the appropriate UGA anticodon (Lee et al., J. Biol. Chem. 264:9724-9121, 1989). For example, within one embodiment of the invention cells are grown with the toxic or fusogenic gene in the repressed state, i.e., in the absence of selenium or with greatly reduced selenium levels compared to normal growth media (e.g., DMEM supplemented with 10% FBS). This may be achieved by adapting cells to growth in a defined media, such as IT (Insulin, transferrin; Collaborative Bioproducts, Waltham MA). Cells can be grown in the selenium-deficient media and when near confluency, the media is changed to either FBS containing media (FBS usually contains high quantities of selenium) or the cells moved in ITS media (Insulin, transferrin, selenium; Collaborative Bioproducts, Waltham MA). Upon selenium repletion, the mutated TGA codon is read as a selenocysteine codon and the selenocysteine incoφorated into the toxic or fusogenic protein. Although selenium responsive elements are provided herein as a representative example of translational regulatory mechanisms, it should be understood that the present invention is not so limited. In particular, a variety of other translational regulatory elements may be utilized either alone, or in combination with the above noted transcriptional elements in order control expression of a given sequence. For example, Iron response elements (IREs) occur naturally in critical proteins involved in iron metabolism. They are moderately stable stem-loop structures of approximately 26-35 nucleotides found as single copies in the 5' or 3' untranslated regions of these proteins (Hentze et. al., Science 255:1570-1573, 1987). IREs provide the mRNA binding site for a cytosolic protein called the IRE-binding protein (IRE-BP) (Leibold and Munro, P.N.A.S. 55:2171-2175, 1988). This protein is regulated by iron availability (Haile et. al., PNAS 89:7536-7540. 1992). Under conditions of iron excess, cytosolic IRE-BP do not bind IREs. Under conditions of iron deprivation, IRE-BP exhibit high affinity binding to IREs (Kd<100pM). The switch between high and low affinity binding does not require protein synthesis and represents a posttranslational modification of the IRE-BP. The inactive protein can be activated to bind RNA with high affinity in the presence of either high concentrations of iron chelators or reducing agents, thus it appears to have a redox- sensitive sulfhydryl switch distinguishing the two binding states. Incoφoration of IREs into the 5' or 3' untranslated regions of genes of interest (GOI) such as fusogenic or toxic proteins can result in the ability to repress translation of these genes under conditions of iron deprivation. For example, the following oligonucleotide sequence or other closely related sequences (and their complements) may be synthesized with flanking restriction enzyme sites (RES) for incoφoration into the 5' untranslated region of a toxic gene or fusogenic gene such as the VSV-G gene at approximately -150 bp. IRE sequence oligonucleotide 3 (Sequence D. No. 1): 5'(RES)-CGTCGGGGTTTCCTGCTTCAACAGTGCTTGGACGGAACCCGG CG-(RES)3' IRE sequence oligonucleotide 4 (Sequence LD. No. 2): 5'(RES)-CGCCGGGTTCCGTCCAAGCACTGTTGAAGCAGGAAACCCCG TCG-(RES)3' Packaging cell lines which contain IRE- VSV-G constructs may be maintained in media with low iron concentrations, with high concentrations of reducing agents such as dithiothreitol (DTT; e.g., at lOOmM), or with the iron chelator desferrioxamine at 50- 1 OOmM. Upon addition of hemin at 50-1 OOmM as an iron source for 20 hours, the affinity of the IRE-BP drops and translation of VSV-G proceeds, generating up to 100-fold increase in protein expression. This therefore permits a regulatable burst of high production of VSV-G pseudotyped retroviral vector. Within other aspects of the invention, regulation of gene expression may be accomplished by utilizing viral vectors that can replicate in one cell type, but abortively infect another (i.e., are replication competent in their native species and replication incompetent in other species). Such a system may be used for the production of many types of viral vectors, where production of viral proteins is toxic (e.g. , AAV and rep proteins, Adenovirus vectors, Heφes Viral vectors, Sindbis and other alpha viruses). For example, within one embodiment of the invention, expression vectors are provided comprising a murine CMV IE promoter operably linked to a nucleic acid sequence that encodes a toxic or fusogenic protein. Briefly MCMV is a member of the beta family of Heφes viruses. These viruses contain within their virion a transactivator protein responsible for initiating transcription from the immediate early genes. MCMV abortively infects human cells, i.e., the virus enters the target human cells, some "early gene" expression occurs, but no viral particles are produced. This allows the use of MCMV to express otherwise toxic proteins in a "burst" fashion in human cells, followed by harvest of the product without further production of, and contamination with, MCMV. Therefore the product incoφorating the toxic or fusogenic protein can be harvested and used on target cells or tissues without significant MCMV toxicity. This trans-activation phenomenon may be used to regulate expression of a wide variety of toxic and or fusogenic genes, such as the VSV-G gene, in at least two distinct methods: a) The VSV-G gene may be placed under the control of a MCMV immediate early (IE) gene such as iel or ie3, by fusing the promoter regions from an IE gene to the VSV-G gene. The modified VSV-G gene is then reintroduced into the MCMV genome by homologous recombination. As VSVg protein is not immediately toxic, and the life cycle of MCMV in mouse cells is two or three days, stocks of MCMV can be produced at conventional titers (10 7 -10 9 pfu/ml). Recombinant virus stocks are obtained by sequential rounds of plaque purification and may be used to infect human pre-producer cells (cells containing a retroviral vector, but lacking an envelope protein). Because MCMV causes an abortive infection in human cells, VSV-G pseudotyped retroviral vectors will be released from the infected cells with no contamination by MCMV. b) The VSV-G gene can be placed under the control of the ie gene regulatory sequences from MCMV. This construct can be transfected into a pre-producer cell line (described above). In the absence of the ie transactivators, constitutive transcription from the MCMV IE promoter is very low, resulting in minimal expression of the toxic VSV-G gene protein. If the pre-producer cell is infected with wild type MCMV, the transactivator causes very rapid and strong expression of the VSV-G gene leading to secretion of VSV-G pseudotyped retroviral vectors. Because MCMV causes an abortive infection in human cells, VSV-G pseudotyped retroviral vectors will be released from the infected cells with no contamination by MCMV. In the event that low level MCMV replication does occur in the human pre-producer cells, the MCMV infected cultures may be treated with gancyclovir to prevent synthesis of late MCMV proteins. Alternatively, the transactivating MCMV need not be functional in terms of replicative capacity and a recombinant MCMV may be engineered which lacks the early or late genes, or both. Such a recombinant MCMV may be propagated in cells stably containing a single copy of the deleted genes (i.e., early or late) to trans-complement and allow MCMV virus formation. Although MCMV is provided herein as an example, it should be understood that the present invention is not so limited. In particular, a variety of other viruses, which are replication competent in their native species, and replication incoml4 petent in other species, may likewise be utilized in the present invention. Representative examples include animal viruses such as mouse adapted influenza virus and rhesus rotavirus, as well as other which are set forth below in Table 1. TABLE 1 VIRUS PERMISSIVE HOST ABORTIVE INFECTION mCMV mouse human fowlpox birds human swinepox pigs human pseudorabies pigs, cows. farm human animals polyoma mouse human SV40 African Green Monkey human temperature-sensitive cells at permissive cells at nonpermissive viruses of all kinds temperature (usually 32C) temp, (usually 37-39C) C. TOXIC AND FUSOGENIC PROTEINS As discussed above, a wide variety of either naturally occurring, or synthetically produced toxic or fusogenic proteins may be expressed by the expression vectors and/or viral vectors of the present invention. Representative examples of fusogenic proteins (which mediate a fusogenic state) include VSV-G (Lazarini et al., J. Vir. 45:113-181, 1983), Rabies virus-G (Gaudin, et al., J. Virol. 7:1365-1372, 1993), Semliki Forest virus, Sindbis, and other alpha viruses which have El and E2 glycoproteins (Whitt, M. et al., Virol. 755:681, 1991). Within other aspects of the invention, synthetic chimeras of these glycoproteins may be produced which have altered activities. For example, a synthetic chimera of a rabies extracellular domain fused to VSV-G transmembrane and cytoplasmic domain may be produced, which paradoxically retains infectivity but lacks fusion activity. (Whitt, M. et al., Virol. 755:681, 1991). Additionally, construction of extracellular-transmembrane domain only glycoproteins may be useful for pseudotyping. Representative examples of viruses that may be pseudotyped in this manner include HIV (Virology 209(2): 696-700, 1995; J Vir. 69(3): 1984-9, 1995; AIDS Res Hum Retroviruses 5(9):1669-77, 1992); SIV (J. Med. Primatol 21 (2-3):69-73, 1992); HTLV (Int. J. Cancer 60(4):561-10, 1995; Int. J. Cancer 59(5):655-60, 1994; Int. J. Cancer 59(3):416-21, 1994; Int. J. Cancer 5<5(l):100-5, 1994; J. Vir. 65(l):162-9, 1991 ; Virology 150(1) :420-4, 1991); MuLV (J. Vir. 67(8):4712- 21, 1993; J. Vir. 66(3): 1468-75, 1992); FIV (J. Vir. 67(7):4142-53, 1993; P.N.A.S. 59(18):8457-61, 1992); RSV (J. Gen. Vir. 75 ( Pt 11):2995-7, 1992); AKV (Virology 75<5(l):161-6, 1992); Cocal (J. Vir. Methods 44(2-3):2Sl-304, 1993); VSV-G (Virology 775(2):373-83, 1990; J Vir. 5(3):1202-7, 1991); HSV (Vaccine 77(6):675-8, 1993); and LDV (J. Vir. 69(l):4231-44, 1995). The present invention may also be useful in the production of recombinant viruses which express toxic proteins. Representative examples of proteins which are toxic to cells include γ-interferon, IL-2, and TNF-β, as well as other toxic molecules (discussed in more detail below) that may be expressed by the recombinant virus, including for example, ricin, abrin, diphtheria toxin, cholera toxin, antiviral protein, tritin, Shigella toxin, and Pseudomonas exotoxin A D. OTHER HETEROLOGOUS NUCLEIC ACID MOLECULES A wide variety of nucleic acid molecules may be carried and/or expressed by the expression vectors or recombinant retroviruses of the present invention. Generally, the nucleic acid molecules which are described herein do not occur naturally in the expression vector or recombinant retrovirus that carries it, and provides some desirable benefit, typically an ability to fight a disease, or other pathogenic agent or condition. Substances which may be encoded by the nucleic acid molecules described herein include proteins (e.g., antibodies including single chain molecules), immunostimulatory molecules (such as antigens) immunosuppressive molecules, blocking agents, palliatives (such as toxins, antisense ribonucleic acids, ribozymes, enzymes, and other material capable of inhibiting a function of a pathogenic agent) cytokines, various polypeptides or peptide hormones, their agonists or antagonists, where these hormones can be derived from tissues such as the pituitary, hypothalamus, kidney, endothelial cells, liver, pancreas, bone, hemopoetic marrow, and adrenal. Such polypeptides can be used for induction of growth, regression of tissue, suppression of immune responses, apoptosis, gene expression, blocking receptor-ligand interaction, immune responses and can be treatment for certain anemias, diabetes, infections, high blood pressure, abnormal blood chemistry or chemistries (e.g., elevated blood cholesterol, deficiency of blood clotting factors, elevated LDL with lowered HDL), levels of Alzheimer associated amyloid protein, bone erosion/calcium deposition, and controlling levels of various metabolites such as steroid hormones, purines, and pyrimidines. Within one aspect of the present invention, methods are provided for administration of an expression vector or a recombinant retrovirus which directs the expression of a palliative. Representative examples of palliatives that act directly to inhibit the growth of cells include toxins such as ricin (Lamb et al., Eur. J. Biochem. 148:265-210, 1985). Within another aspect of the invention, the expression vector or recombinant retrovirus directs the expression of a substance capable of activating an otherwise inactive precursor into an active inhibitor of a pathogenic agent, or a conditional toxic palliative, which are palliatives that are toxic for the cell expressing the pathogenic condition. As should be evident given the disclosure provided herein, a wide variety of inactive precursors may be converted into active inhibitors of a pathogenic agent. For example, expression vectors or recombinant retroviruses which direct the expression of a gene product (e.g. , a protein) such as Heφes Simplex Virus Thymidine Kinase (HSVTK) or Varicella Zoster Virus Thymidine Kinase (VZVTK) which assists in metabolizing antiviral nucleoside analogues to their active form are therefore useful in activating nucleoside analogue precursors (e.g., AZT or ddC) into their active form. AZT or ddl therapy will thereby be more effective, allowing lower doses, less generalized toxicity, and higher potency against productive infection. Additional nucleoside analogues whose nucleotide triphosphate forms show selectivity for retroviral reverse transcriptase but, as a result of the substrate specificity of cellular nucleoside and nucleotide kinases are not phosphorylated, will be made more efficacious. In yet another aspect, expression vectors and recombinant retroviruses are provided which have a therapeutic effect by encoding one or more ribozymes (RNA enzymes) (Haseloff and Gerlach, Nature 554:585, 1989) which will cleave, and hence inactivate, RNA molecules corresponding to a pathogenic function (see also, Foster and Symons, Cell 48: 211-220, 1987; Haseloff and Gerlach, Nature 328: 596-600, 1988; Ruffher et al., Biochem 29: 10,695-10,702, 1990; U.S. Patent Nos. 5,254,678 and 4,987,071 ; and PCT Publication No. WO93/23569). In another aspect, expression vectors and recombinant retroviruses are provided comprising a biologically active nucleic acid molecule that is an antisense sequence (an antisense sequence may also be encoded by a nucleic acid sequence and then produced within a host cell via transcription). In preferred embodiments, the antisense sequence is selected from the group consisting of sequences which encode influenza virus, HIV, HSV, HPV, CMV, and HBV. The antisense sequence may also be an antisense RNA complementary to RNA sequences necessary for pathogenicity. Alternatively, the biologically active nucleic acid molecule may be a sense RNA (or DNA) complementary to RNA sequences necessary for pathogenicity. Still further aspects of the present invention relate to recombinant retroviruses capable of immunostimulation. Briefly, the ability to recognize and defend against foreign pathogens is essential to the function of the immune system. In particular, the immune system must be capable of distinguishing "self from "nonself (i.e., foreign), so that the defensive mechanisms of the host are directed toward invading entities instead of against host tissues. Cytolytic T lymphocytes (CTLs) are typically induced, or stimulated, by the display of a cell surface recognition structure, such as a processed, pathogen-specific peptide, in conjunction with a MHC class I or class II cell surface protein. In one embodiment, the invention provides methods for stimulating a specific immune response and inhibiting viral spread by using an expression vector or recombinant retroviruses that direct the expression of an antigen or modified form thereof in susceptible target cells, wherein the antigen is capable of either (1) initiating an immune response to the viral antigen or (2) preventing the viral spread by occupying cellular receptors required for viral interactions. In another aspect of the invention, the expression vectors or recombinant retroviruses of the present invention may be constructed to express "immunomodulatory factors." Immunomodulatory factors refer to factors that, when manufactured by one or more of the cells involved in an immune response, or, which when added exogenously to the cells, causes the immune response to be different in quality or potency from that which would have occurred in the absence of the factor. The quality or potency of a response may be measured by a variety of assays known to one of skill in the art including, for example, in vitro assays which measure cellular proliferation (e.g., ^H thymidine uptake), and in vitro cytotoxic assays (e.g. , which measure 51 Cr release) (see, Warner et al., AIDS Res. and Human Retroviruses 7:645-655, 1991). Immunomodulatory factors may be active both in vivo and ex vivo. Representative examples of such immunomodulatory factors include cytokines or lymhokines, interferons (e.g., γ-IFN), tumor necrosis factors (TNFs) (Jayaraman et al., J. Immunology 144:942-951 , 1990), CD3 (Krissanen et al., Immunogenetics 26:258-266, 1987), ICAM-1 (Altaian et al., Nature 555:512-514, 1989; Simmons et al., Nature 331:624-621, 1988), ICAM-2, LFA-l, LFA-3 (Wallner et al., J. Exp. Med. 166(4):923- 932, 1987), MHC class I molecules, MHC class II molecules, B7.1-.3, β2-microglobulin (Pames et al., PNAS 75:2253-2257, 1981), chaperones such as calnexin and MHC linked transporter proteins or analogs thereof (Powis et al., Nature 554:528-531, 1991). The present invention also includes expression vectors and recombinant retroviruses which encode immunogenic portions of desired antigens including, for example, viral, bacterial or parasite antigens. Representative examples include hepatitis Band C viral antigens (e.g., WO 93/15207), feline leukemia virus and/or immonudeficiency virus antigens (e.g., WO 94/06921). Still other examples include expression vectors or recombinant retroviruses which direct the expression of a non- tumorigenic, altered genes such as the ras (ras*) gene and the p53 gene (see WO 93/10814). E. CELL CULTURE CONCENTRATION AND PURIFICATION OF RECOMBINANT RETROVIRAL PARTICLES As noted above, the present invention provides recombinant retroviral preparations suitable for administration to humans and other warm-blooded animals. In particular, a wide variety of methods may be utilized in order to produce recombinant retroviruses suitable for administration, including for example, the use of fermenters or bioreactors, roller bottles, cell hotels or cell factories, and hollow fiber culture. Briefly, for bioreactors or fermenters, cells are preferably grown on microcarriers (i.e., Cytodex 1 or Cytodex 2; Pharmacia, Piscataway, NJ. at concentrations ranging from 3 to 15 g/L microcarrier. For roller bottles, suitable conditions include those described above for bioreactors, with the exception that microcarrier beads are generally not preferred. Representative examples of such methods, as well as other methods such as cell holds or cell factories and hollow linker culture, are described within WO 96/0926. Subsequent to culturing, a wide variety of methods may be utilized for increasing viral concentration and purity, including for example, precipitation of recombinant retroviruses with ammonium sulfate, polyethylene glycol ("PEG") concentration, concentration by centrifugation (either with or without gradients such as PERCOLL, or "cushions" such as sucrose, use of concentration filters (e.g., Amicon filtration), and 2- phase separations. Representative examples of concentrating and purifying and recombinant retroviral particles are described within WO 96/0926. F. ADMINISTRATION As noted above, high titer recombinant retroviral particles of the present invention may be administered to a wide variety of locations including, for example, into sites such as the cerebral spinal fluid, bone marrow, joints, arterial endothelial cells, rectum, buccal/sublingual, vagina, the lymph system, to an organ selected from the group consisting of lung, liver, spleen, skin, blood and brain, or to a site selected from the group consisting of tumors and interstitial spaces. Within other embodiments, the recombinant retrovirus may be administered intraocularly, intranasally, sublinually, orally, topically, intravesically, intrathecally, topically, intravenously, intraperitoneally, intracranially, intramuscularly, or subcutaneously. Other representative routes of administration include gastroscopy, ECRP and colonoscopy, which do not require full operating procedures and hospitalization, but may require the presence of medical personnel. The following examples are offered by way of illustration, and not by way of limitation. EXAMPLES EXAMPLE 1 PREPARATION OF RETROVIRAL VECTOR BACKBONES Retroviral backbones suitable for use with the present invention may be readily prepared by one of skill in the art. Representative examples of such backbones, such as KT-1 and KT-3, are described in WO 91/02805 and WO 95/05789. EXAMPLE 2 RECOVERY OF THE VSV-G GENE FROM THE PLASMID MLP-G The VSV-G gene (Lazarini et al., J. Vir. 45:773-781, 1983; see also, Sequence LD. Nos. 1 and 2) is isolated from a plasmid containing VSV-G (e.g., pMLP-G, which has the adenoviral major late promoter and the tri-partite leader sequence (Kaufman, P.N.A.S. 52:689-693, 1985; Ballay et al., in Hepadna Viruses, W. Robinson, K. Koike and H. Will (eds.), pp. 173-187, Liss, New York, 1987; see also WO 92/05266) as a 1.6 kb PCR product using primers #5 and #2, which also introduce a Sac II site at the 5' and an EcoR I site at the 3' end, respectively. Primer-2: (Sequence ID No. 3) 5'- GCC GAA TTC AAT TGA GGC CTC TTT G- 3' (25-mer) Primer-5: (Sequence ID No. 4) 5'- GAT CCG CGG AGA ATC GAT CTG TTT CCT TGA CAC TAT GAA GTG CCT TTT G- 3 * (49-mer) The PCR product is cloned into the TA cloning vector pCR™II (INVITROGEN CORP., San Diego, CA), and checked for the correct sequence via partial sequencing and correct orientation of the VSV-G gene. The VSV-G gene is then recovered from the TA cloning vector and ligated into the eukaryotic expression vector pcDNAIII (INVITROGEN). The PCR clone is then checked for functionality by co-transfection of 293(2-3) cells with the VSV-G expressing pcDNAIII and pCB/β-gal at a ratio of 1 : 1 using the profectin kit from PROMEGA. As a positive control, pMLP-G is cotransfected with the pCB/β-gal plasmid in parallel. After the cotransfection, fresh media is added to the cells, supernatant collected and sterile-filtered (0.45 mm) 24 hours later. HT1080 target cells are then transduced with both supematants at various dilutions as follows. On day one, three wells of a 6-well plate are seeded with 2xl0 5 HT1080 in 2 ml media (DMEM high glucose with 10% FBS, nonessential amino acids and sodium pyruvate) per well. Cells are incubated for 24 hours at 37°C and 10% CO2 before transduction the next day. On day two the media is removed, and one ml media containing 8 μg polybrene/ml final is added per well. Cells are incubated for 30 minutes at 37°C and 10% CO2. The supematant is diluted by gentle pipetting of vector containing media to an approximate concentration of 5 cfu/ml. Five, 20 and 50 ul of the diluted supematant is added into wells 1 to 3, respectively, the media swirled and incubated at 37°C and 10% CO2 for 24 hours. On day three one ml of media is added per well and cells are incubated at 37°C and 10% CO2 for 24 hours. On day four an X-gal staining procedure on the transduced cells is carried out. The total number of blue cells is counted per well and the titer determined. Briefly, the media is drained, and 0.5 ml of fixing solution (PBS with 2% (v/v) formaldehyde and 0.2% (v/v) glutaraldehyde) is added per well. The cells are incubated for 5 minutes at room temperature, the fixing solution drained and the cells washed with 2 ml PBS per well. The wells are drained and 0.5 ml of freshly mixed X-gal stain (PBS with 5 mM potassium ferrocyanide, 5 mM potassium ferricyanide, 2 mM MgCl2, and 1 mg/ml X-gal) is added per well. Cells are incubated from 2 hours to overnight at 37°C and blue cells counted. If the titer between the pMLP-G and the PCR-based VSV-G gene product in the expression vector is comparable, the PCR-based VSV-G gene is sequenced by Lark Sequencing Technologies and compared to the VSV-G sequence in pMLP-G. If the titer is much lower or zero with the VSV-G gene retrieved by PCR, other plasmid clones with VSV-G ligated onto PCR II are screened. The functional VSV-G gene can be recovered from either the prokaryotic TA cloning vector pCRTMj or from the eukaryotic expression vector pcDNAIII. EXAMPLE 3 USE OF THE TETRACYCLINE-INDUCIBLE PROMOTER SYSTEM TO PRODUCE A STABLE, VSV-G EXPRESSING PACKAGING CELL LINE A. Introduction As described in more detail below, the tetracycline transactivator (tTA) is introduced into the 293(2-3) PCL and constitutively expressed. A second transfection of the 293(2-3)tTA cell line introduces the tet operator sequences with the VSV-G gene fused behind the tet operator sequences. B. Plasmid Preparations Plasmid preparations of each of the following plasmids are carried out using the QIAGEN plasmid kits (QIAGEN Inc., Chatsworth, CA) essentially as described by the manufacturer. Briefly, the plasmids are electroporated into E. coli DH 12S (Life Technologies, Inc., Gaithersburg, MD), and cultures are grown in LB broth complemented with ampicillin at 100 μg/ml final concentration for all the plasmids except for pMLP-G, which requires tetracycline at 5 μg/ml final concentration. Electroporated bacteria are plated as single colonies on LB agar plates containing the respective antibiotics. Single colonies are selected, mini plasmid preparations carried out and a characteristic restriction enzyme digest performed to verify identity of each plasmid. Once a single clone is identified, glycerol cultures (1 :1 dilution of 40% glycerol and bacterial suspension) are stored at -80°C. The table below lists the available plasmids, describes the coding regions and the source of the plasmids PLASMIDS FOR THF. PRODUCTION OF A STABLE. TETRACYCLINE-ΪNDUCIBLE VSV-G PCL Name Coding Regions Reference pUHD 15-20 tTA under HSV-Tk promoter see, e.g., U.S. Patent No. 5,464,758 pUHD 15-1 tTA under hCMV U.S. Patent No. 5,464,758; promoter/enhancer Gossen M. and Bujard H., PNAS 59:5547-5551 (1992) pUHD 10-3 hCMV promoter/enhancer with Gossen and Bujard, PNAS heptamerized upstream tet- 89:5541-5551 (1992) operators as described in ref. above, followed by a multiple cloning site Name Coding Regions Reference pMLP-G VSV-G gene under the MLP Lazzarini et al., J. Virol. 45:113- promoter 781 (1983) pT3/T7-Luc Luciferase expression vector Clontech (Clontech Inc., Palo Alto, CA) pUT 507 Phleomycin/Zeocin resistance Tiraby et al., Somatic Cell and plasmid Molecular Genetics 74:243-252 (1988) pHIS Histidinol marker plasmid (see below) pCB/β-gal β-gal coding plasmid Irwin et al., J. Vir. 68(8): 5036- 5044, 1994 The eukaryotic expression vector, pREP8 (Invitrogen, San Diego, CA), which contains the histidinol resistance gene, as well as the Epstein Barr virus origin of replication and EBNA, is digested with EcoRI and a 5.8 Kb fragment isolated, religated and transfected into E. coli. This vector, pHIS, contains only the plasmid origin of replication, ampicillin resistance gene and the histidinol resistance gene with the SV40 promoter and polyadenylation sequences. C. Constmction of the Luciferase Reporter Plasmid pUHD 10-3/luc Tetracycline transactivator function is first examined in a transient transfection assay using a tTA-controlled luciferase reporter unit. Both regulator plasmids, pUHD 15- 20 as well as pUHD 15-1 are analyzed in the transient assays to determine which plasmid gives a better system in terms of leakiness and level of activation after tetracycline withdrawal. The cotransfection experiments are carried out in 293(2-3) cells and D-17 gag/pol cells, as well as HeLa cells as a positive control. 1. Cloning of the Luciferase Gene into p UHD 10-3. Plasmid pUHD 10-3 is linearized with EcoR I (New England Biolabs, Inc., Beverly, MA), the 5' overhang filled in with nucleotides and the Klenow fragment in the restriction enzyme buffer. The vector is then treated with PCI (phenol :chloroform:isoamylalcohol, Amresco Inc., Solon, Ohio) to remove proteins, and ethanol-precipitated. Hind III linkers are ligated to the plasmid, the plasmid restricted with Hind III and dephosphorylated with CIP (Calf Intestinal Phosphatase). After dephosphorylation, the vector is PCI-extracted and gel-purified using the Qiaex gel purification kit from Qiagen, essentially according to manufacturer's procedures. The luciferase gene is isolated from pT3/T7-Luc (Clontech) as a 1.9 kb fragment after a Hind III digest (New England Biolabs), the two fragments purified over an agarose gel to separate the 1.9 kb luciferase-coding fragment from the 3.15 kb vector and the luciferase gene ligated into the Hindlll restricted pUHD 10-3 vector to produce pUHD 10- 3/luc. An aliquot of the ligation is electroporated into E. coli DH 12S (Life Technologies), the cultures grown in LB broth for one hour at 37°C, an aliquot plated onto LB agar plates complemented with ampicillin at 100 μg/ml final concentration and incubated at 37°C overnight. Single colonies are selected, grown in 2 ml LB broth supplemented with ampicillin to 100 μg/ml final concentration overnight and a plasmid mini preparation using the Qiagen miniprep kit (Qiagen Inc., Chatsworth, CA) according to manufacturer's instmctions. In order to identify clones carrying the luciferase gene, a restriction enzyme double digest with Sal I and Sac I (New England Laboratories) or a single digest with Hind III (New England Laboratories) is carried out. The electrophoresis of the digest on a 1% analytical agarose gel shows the 3.15 kb linearized vector and a 1.9 kb fragment for the luciferase gene. Clones with both fragments are analyzed further with respect to the orientation of the luciferase gene in the vector. A Xba I digest (New England Laboratories) is performed and the electrophoresed fragments examined on a 1% agarose gel. The fragment pattem for the correct orientation is 0.14, 1.76 and 3.15 kb and for the inverse orientation 0.14 and 5.0 kb. A plasmid preparation of a clone with the correct luciferase orientation (pUHD 10- 3/luc) is carried out using the Qiagen plasmid kit. 2. Transient Expression of Luciferase in 293(2-3), D-l 7 gag/pol and HeLa Cells. 293(2-3) and D-l 7 packaging cells (WO 92/05266) (as well as HeLa cells as a positive control), are cotransfected with pUHD 10-3/luc and either pUHD 15-20 or pUHD 15-1 at ratios of 1 :20, 1 :50 and 1 :100. The plasmids are introduced via calcium phosphate precipitation using the profection kit from Promega (Promega Coφ., Madison, WI) as recommended by manufacturer. Two tissue culture dishes of each of the various combinations of plasmids, ratios and cell lines are set up. Tetracycline, anhydrocycline or deoxycycline is added to one dish but not to the other. One of the tetracyclines is added at 1 μg/ml final concentration several hours before the cotransfection. The cells of both dishes per combination are analyzed 24 hours post transfection for luciferase activity. 3. Analysis of Luciferase Activity. The luciferase assay is carried out according to manufacturer's instmctions (Promega). The tTA regulator plasmid (pUHD 15-20 or pUHD 15-1) that gives the lowest basal activity and the highest degree of activation of luciferase under tetracycline-free culture is utilized in order to establish a stable VSV-G producing 293(2-3) and or D-l 7 cell line. 4. Construction of a Stable 293(2-3)tTA and D-l 7 gag/pol tTA Cell Lines The regulatory system in 293(2-3) and D-l 7 gag/pol cells is established in two steps. First, 293(2-3) and D-l 7 gag/pol cells are cotransfected with a tTA regulator plasmid and pUT 507 at a ratio of 10: 1. The plasmids are introduced via calcium phosphate precipitation using the profection kit from PROMEGA. The cotransfection is carried out as recommended by manufacturer. Two days after the cotransfection the cells are selected with zeocin for 10 days until all the non-resistant cells are dead. The cells are then taken off the selection and a limiting dilution is carried out in several 96-well plates such that one third of the wells shows growth. Each clone is transiently transfected with pUHD 10-3/luc and screened for luciferase activity. Clones with the high levels of luciferase activity and low basal levels in the presence of tetracycline are used to introduce the VSV-G gene cloned into pUHD 10-3. The cells are then taken off the selection and a limiting dilution is carried out in several 96-well plates such that one third of the wells shows growth. Each clone is transiently transfected with pUHD 10-3/luc and screened for luciferase activity. Clones with the high levels of luciferase activity and low basal levels in the presence of tetracycline are used to introduce the VSV-G gene cloned into pUHD 10-3. D. Construction of the VSV-G Expression Plasmid pUHD 1 -3 VSV-G The VSV-G gene is recovered from pCR™II TA cloning vector with an Eco-RI- SacII double digest. The 1.6 kb VSV-G gene is isolated and purified by electrophoresis over a 1% agarose gel using the QIAEX kit as described above. The plasmid pUHD 10-3 is restricted with SacII and EcoRI and the VSV-G gene ligated into the backbone. An aliquot of the ligation is electroporated into E. coli DH 12S (Life Technologies), the cultures grown in LB broth for one hour at 37°C, an aliquot plated onto LB agar plates complemented with ampicillin to 100 μg/ml final concentration and incubated at 37°C overnight. Single colonies are picked, grown in 2 ml LB broth complemented with ampicillin to 100 μg/ml final concentration overnight and a plasmid mini preparation carried out as described above. In order to identify clones carrying the VSV-G gene, a restriction enzyme double digest with Sac II and Xba I (New England Laboratories) is carried out. The electrophoresis of the digest on a 1% analytical agarose gel shows the 3.15 kb linearized vector and a 1.6 kb fragment for the VSV-G gene. A plasmid preparation of a clone with the correct VSV-G orientation (pUHD 10- 3/VSV-G) is carried out using the Qiagen plasmid kit. E. Constmction of a Stable 293(2-3)tTA/VSV-G and/or D-l 7 gag/pol tTA VSV-G Cell Lines The VSV-G gene is introduced into the 293(2-3)tTA and/or D-l 7 tTA-VSV-G clone by cotransfection of pUHD 10-3/VSV-G and pUT 507, followed by selection with phleomycin or zeocin. Both plasmids are introduced into 293(2-3)tTA and D-l 7 gag/pol cells at a ratio of 10:1 via calcium phosphate precipitation using the profection kit from Promega as recommended by manufacturer. From the time of cotransfection, the cells are kept in tetracycline containing media (1 μg/ml final concentration). Two days after the cotransfection the cells are selected with histidinol for 10 days until all the non-resistant cells are dead. The cells are then taken off the selection and a limiting dilution is carried out in several 96-well plates such that one third of the wells shows growth. Each clone is split into two tissue culture dishes, one dish containing media without tetracycline to induce the production of VSV-G. One day later, the cells in media without tetracycline are transfected with the β-gal gene (from pCB/β-gal) via calcium phosphate transfection in order to determine the titering potential of each of the PCL clones. Two days after the transfection, the supematant is harvested and HT1080 target cells transduced with the viral particles carrying the β-gal gene as follows. On day one, per supematant three wells of a 6-well plate are seeded with 2 x 10^ HT1080 in 2 ml media (DMEM high glucose with 10% FBS, nonessential amino acids and sodium pymvate) per well. Cells are incubated exactly for 24 hours at 37°C and 10% CO2 before transduction the next day. On day two the media is removed, and one ml media containing 8 mg polybrene/ml is added per well. Cells are incubated for 30 minutes at 37°C and 10% CO2. Dilute the supematant by gentle pipetting in media to an approximate concentration of 5 CFU/ml. Add 5, 20 and 50 ml of the diluted supematant into wells 1 to 3, respectively, swirl media and incubate at 37°C and 10% CO2 for 24 hours. On day three one ml of media is added per well and cells incubated at 37°C and 10% CO2 for 24 hours. On day four an X-gal staining procedure on the transduced cells is carried out. The total number of blue cells is counted per well and the titer determined. For the X-gal staining the media is drained, 0.5 ml of fixing solution (PBS with 2% (v/v) formaldehyde and 0.2% (v/v) glutaraldehyde) added per well of a 12-well plate, the cells incubated for 5 minutes at room temperature, the fixing solution drained, cells washed with 2 ml PBS per well, wells drained and 0.5 ml of freshly mixed X-gal stain (PBS with 5 mM potassium ferrocyanide, 5 mM potassium ferricyanide, 2 mM MgCl2, and 1 mg/ml X-gal) added per well, cells incubated from 2 hours to ovemight at 37°C and blue cells counted. Once the titering potential is determined, four clones with the highest capacity to produce infectious viral particles are kept in culture for several weeks in media containing tetracycline in order to determine the basal levels of VSV-G expression. Only clones that do not show effects of VSV-G toxicity are useful as stable VSV-G PCLs. EXAMPLE 4 USE OF IPTG/TEMPERATURE-INDUCIBLE LAP PROMOTER SYSTEMS TO PRODUCE A STABLE, VSV-G EXPRESSING PACKAGING CELL LINE As described in more detail below, each of the LAPs driven by a strong promoter is introduced into the 293(2-3) packaging cell line and constitutively expressed. A second transfection of the 293(2-3)LAP cell line introduces the LAP binding domain (lac operator sequences) driven by a weak promoter with the VSV-G gene fused behind the lac operator sequences. A. Description of the LAP267 and LAP348 Promoter Systems LAP348: This promoter system is negatively regulated by IPTG, therefore the expression of the VSV-G envelope is repressed after addition of IPTG. LAP267: In contrast, LAP267 allows expression of the VSV-G envelope after addition of IPTG and/or down-shift of temperature to 32°C. LAP267 contains an insertion of the transcriptional activation domain of HSV VP16 within the inducer-binding and dimerization domain of the lac repressor protein which makes the LAP267 protein temperature-sensitive. IPTG has no effect on LAP267 activity at the permissive temperature (32°C). LAP267 activity is rescued by IPTG at 39.5°C. B. Plasmid Preparations Plasmid preparations of each of the following plasmids are carried out using the Qiagen plasmid kits (Qiagen Inc., Chatsworth, CA) according to manufacturer's suggestions. Briefly, the plasmids are electroporated into E. coli DH 12S (Life Technologies, Inc., Gaithersburg, MD), the cultures grown in LB broth complemented with ampicillin at 100 μg/ml final concentration for all the plasmids except for pMLP-G which needs tetracycline at 5 μg/ml final concentration. Electroporated bacteria are plated as single colonies on LB agar plates containing the respective antibiotic, single colonies picked, mini plasmid preparations carried out and a characteristic restriction enzyme digest performed to verify identity of each plasmid. Once a single clone is identified, glycerol cultures (1:1 dilution of 40% glycerol and bacterial suspension) are stored at -80°C. The table below lists the available plasmids, describes the coding regions and the source of the plasmids. PLASMIDS FOR THE PRODUCTION OF A STABLE, IPTG-INDUCIBLE VSV-G PCL Name Coding Regions Reference pCMV267 LAP267 under minimal CMV PNAS 88:5012-5016, 1991 promoter pHCMVLAP348 LAP348 under minimal CMV Molec. Cell. Biol. 70:3343-3356, promoter 1990 pL7CAT One set of tandem A and B lac Molec. Cell. Biol. 70:3343-3356, operator repeats followed by 1990 SV40 promoter and CAT gene pL14CAT Two sets of tandem A and B lac Molec. Cell. Biol. 70:3343-3356, operator repeats followed by 1990 SV40 promoter and CAT gene pMLP-G VSV-G gene under the MLP J. Vir. 45:773-781, 1983 promoter pT3/T7-Luc Luciferase expression vector Clontech (Clontech Inc., Palo Alto, CA) pUT 507 Phleomycin/Zeocin resistance Somatic Cell and Molecular plasmid Genetics 74:243-252, 1988 pHIS Histidinol marker plasmid (see above) pCB/β-gal β-gal coding plasmid Irwin et al., J. Vir. <5S(8):5036- 5044, 1994 C. Constmction of the Luciferase Reporter Plasmids pL7/luc and pLl 4/luc The LAP functions are first examined in a transient transfection assay using a LAP267 or LAP348-controlled luciferase reporter unit. Combinations of both regulator plasmids, pCMV267 and pHCMVLAP348, together with either of the two reporter plasmids pL7/luc and pLl 4/luc are analyzed in the transient assays to determine which plasmid combination gives a better system in terms of leakiness and level of activation after IPTG/temperature induction. The cotransfection experiments are carried out in 293(2-3) and D-l 7 gag/pol cells, as well as HeLa cells as a positive control. D. Cloning of the Luciferase Gene into pL7CAT and pL14CAT The Stu l-Bal I fragment from pL7CAT and pL14CAT coding for the CAT gene is deleted using a double digest of each of the two reporter plasmids with Stu I and Bal I. The two resulting blunt-ended fragments for each plasmid are separated in a 1 % agarose gel and the pL7 and pL14 backbone are cut out, gel purified using the Qiaex gel purification kit from Qiagen according to manufacturer's procedures. This step eliminates the CAT gene from the pL7CAT and pL14CAT plasmids. Hindlll linkers are ligated to each of the backbones, the backbones restricted with Hindlll and treated with calf intestinal phosphatase to decrease the chance of religation. The luciferase gene is isolated from the pT3/T7-Luc plasmid by Hindlll digest, the 1.9 kb luciferase gene isolated and purified over a 1 % agarose gel as described above and the luciferase fragment ligated into the Hindlll sites of each of the plasmid backbones. An aliquot of the ligation is electroporated into E. coli DH 12S (Life Technologies), the cultures grown in LB broth for one hour at 37°C, an aliquot plated onto LB agar plates complemented with ampicillin at 100 μg/ml final concentration and incubated at 37°C overnight. Single colonies are picked, grown in 2 ml LB broth supplemented with ampicillin to 100 μg/ml final concentration overnight and a plasmid mini preparation using the Qiagen miniprep kit (Qiagen Inc., Chatsworth, CA) according to manufacturer's instmctions. In order to identify clones carrying the luciferase gene, an analytical restriction enzyme digest is carried out. A plasmid preparation of one clone each with the correct luciferase orientation (pL7/luc and pLl 4/luc) is carried out using the Qiagen plasmid kit. Ε. Transient Expression of Luciferase in 293(7-3 ^ ) D-l 7 eae/vol and Hela Cells 293(2-3), D-l 7 gag/pol and HeLa cells as positive control are cotransfected with either pL7/luc or pLl 4/luc and either pCMV267 or pHCMVLAP348 at ratios of 1 :20, 1 :50 and 1 : 100. The plasmids are introduced via calcium phosphate precipitation using the profection kit from Promega (Promega Coφ., Madison, WI) as recommended by manufacturer. Two tissue culture dishes of each of the various combinations of plasmids, ratios and cell lines are set up. One of the two dishes is induced with IPTG at 5 mM and/or temperature shift for the LAP267 containing combinations and IPTG withdrawal for the LAP348 containing combinations. Cells are induced several hours before the cotransfection. The cells of both dishes per combination are analyzed 24 hour post transfection for luciferase activity. F. Analysis of Luciferase Activity The luciferase assay is carried out according to manufacturer's instmctions (Promega). The LAP regulator plasmid (pCMV267 or pHCMVLAP348) that gives the lowest basal activity and the highest degree of activation of luciferase under non-inducing culture conditions is utilized in order to establish a stable VSV-G producing 293(2-3) and/or D-17 gag/pol cell line. G. Constmction of a Stable 293f2-3^LAP267. D-17 gag/pol LAP267. D-17 gag/pol LAP348 and/or 293(2-3 LAP348 Cell Line The regulatory system in 293(2-3) and D-17 gag/pol cells is established in two steps. First, 293(2-3) and D-17 gag/pol cells are cotransfected with a LAP regulator plasmid and pUT507 at a ratio of 10: 1. The plasmids are introduced via calcium phosphate precipitation using the profection kit from Promega, essentially according to the manufacturer's instmctions. The cotransfection is carried out as recommended by manufacturer. Two days after the cotransfection the cells are selected with G418 at 0.8 mg/ml for 10 days until all the non-resistant cells are dead. The cells are then taken off the selection and a limiting dilution is carried out in several 96-well plates such that one third of the wells shows growth. Each clone is transiently transfected with pL7/luc or pLl 4/luc and screened for luciferase activity. Clones with high levels of luciferase activity and low basal levels under non-inducing culture conditions are used to introduce the VSV-G gene cloned into pL7 or pLl 4. H. Constmction of the VSV-G Expression Plasmids pL7/VSV-G and pL14/VSV-G The VSV-G gene is recovered from the pCR™II TA cloning vector with an EcoRI digest. The 1.6 kb VSV-G gene is isolated and purified by electrophoresis over a 1% agarose gel using the QIAEX kit as described above. The plasmids pL7CAT and pL14CAT are digested with Stul and Ball which deleted the CAT gene. EcoRI linkers are ligated to the blunt ends, the backbones restricted with EcoRI and treated with calf intestinal phosphatase. Then the 1.6 kb VSV-G is ligated into the pL7 and pL14 backbones. An aliquot of the ligation is electroporated into E. coli DH 12S (LIFE TECHNOLOGIES), the cultures grown in LB broth for one hour at 37°C, an aliquot plated onto LB agar plates complemented with ampicillin to 100 μg/ml final concentration and incubated at 37°C ovemight. Single colonies are picked, grown in 2 ml LB broth complemented with ampicillin to 100 μg/ml final concentration ovemight and a plasmid mini preparation carried out as described above. An analytical restriction enzyme digest is carried out for several clones of each of the ligations in order to identify the correct ligation. A plasmid preparation of a clone for each ligation with the correct VSV-G orientation (pL7/VSV-G and pL14/VSV-G) is carried out using the Qiagen plasmid kit. I. Constmction of a Stable 293f2-3^LAP267/VSV-G D-17 gag/pol LAP267/VSrG and/or 293f2-3^LAP348/VSV-G. D-17 gag/pol LAP 348/VSV-G Cell Lines The VSV-G gene is introduced into the 293(2-3)LAP and D-17 gag/pol LAP clones in a cotransfection of pL7/VSV-G or pL14/VSV-G and pHIS for selection with histidinol. Both plasmids are introduced into 293(2-3)LAP and D-17 gag/pol LAP cells at a ratio of 10:1 (VSV-G:selection marker) via calcium phosphate precipitation using the profection kit from PROMEGA as recommended by manufacturer. From the time of cotransfection, the cells are kept in non-inducing media. Two days after the cotransfection the cells are selected with histidinol for 10 days until all the non-resistant cells are dead. The cells are then taken off the selection and a limiting dilution is carried out in several 96-well plates such that one third of the wells shows growth. Each clone is split into two tissue culture dishes, one dish is cultured under VSV-G inducing conditions and one dish is cultured under VSV-G repressing conditions. One day later, the induced cells are transfected with the β-gal gene (from pCB/β-gal) via calcium phosphate transfection in order to determine the titering potential of each of the PCL clones. The non-induced cells are transfected with the β-gal gene as well in order to analyze leakiness of VSV-G expression. Two days after the transfection, the supematant is harvested and HT1080 target cells transduced with the viral particles carrying the β-gal gene as follows. On day one, per supematant three wells of a 6- well plate are seeded with 2x10^ HT1080 in 2 ml media (DMEM high glucose with 10% FBS, nonessential amino acids and sodium pymvate) per well. Cells are incubated exactly for 24 hours at 37°C and 10% CO2 before transduction the next day. On day two the media is removed, and one ml media containing 8 μg polybrene/ml is added per well. Cells are incubated for 30 minutes at 37°C and 10% CO2. Dilute the supematant by gentle pipetting in media to an approximate concentration of 5 CFU/μl. Five, 20 and 50 ml of the diluted supematant is added into wells 1 to 3, respectively, the media swirled and incubated at 37°C and 10% CO2 for 24 hours. On day three one ml of media is added per well and cells incubated at 37°C and 10% CO2 for 24 hours. On day four an X-gal staining procedure on the transduced cells is carried out. Briefly, the media is drained, and 0.5 ml of fixing solution (PBS with 2% (v/v) formaldehyde and 0.2% (v/v) glutaraldehyde) is added per well of a 12-well plate. The cells are incubated for 5 minutes at room temperature, the fixing solution drained and cells washed with 2 ml PBS per well. The wells are then drained and 0.5 ml of freshly mixed X-gal stain (PBS with 5 mM potassium ferrocyanide, 5 M potassium ferricyanide, 2 mM MgCl2, and 1 mg/ml X-gal) is added per well. Cells are incubated from 2 hours to ovemight at 37°C and blue cells counted. Once the titering potential is determined, the four clones with the highest capacity to produce infectious viral particles are kept in culture for several weeks in non-inducing media in order to determine the basal levels of VSV-G expression. Only clones that do not show effects of VSV-G toxicity are useful as stable VSV-G PCLs. EXAMPLE 5 ALTERNATE GLYCOPROTEIN METABOLISM FOR PRODUCTION OF STABLE PSEUDOTYPING CELLS As noted above, the present invention also provides cell lines which have an altered or mutated glycosylation pathway, which carry or contain an expression vector that directs the expression of an otherwise toxic glycoslylated protein, or fusogenic protein. For example, Lee 1, 2, and 8 cells (ATCC CRL 1735, 1736, 1737), which are known to have altered glycosylation phenotypes, may be screened by transfection with pMLP-G essentially and selected for antibiotic resistance with a suitable selection marker. Isolated colonies are analyzed by flow cytometry using a monoclonal antibody to VSV-G (e.g., from Sigma Chemical Coφ., St. Louis, MO). Stable cell clones are used for the generation of packaging cell lines. EXAMPLE 6 RABIES G PROTEIN PACKAGING CELL LINES A plasmid vector containing attenuated rabies vims strain SAD B19 G protein (pSAD61 EMBL/GenBank Ace. No. M31046, Conzelmann, FRCVDA Tubingen, D) is used as a template to PCR amplify the G protein sequence for use in pseudotyping retroviral vectors. Briefly, the sequence encoding G protein is amplified according to the manufacturer's instmctions (Perkin Elmer) utilizing the primers set forth below: Sequence ID No. 5 5'-3'GTCAAGCTTAACATCCCTCAAAAGACTCAAGGAAAGATGG This primer is the sense sequence of the G protein containing the ATG start site, with a Hind III site at the 5' end. Sequence ID No. 6 5*-3' GAT ATC TCG AGC TTA CAG TCT GGT CTC ACC CCC ACT CTT This primer is the anti-sense sequence of the G protein at the termination of the protein, with a Xho I site at the 5i end. Both the 5' and 3' primers have restriction sites compatible with the 5' Hind III and 3' Xho I sites of the multicloning site of pcDNA3 (Invitrogen SD, CA). The resultant PCR product is isolated, digested with Hind III and Xho I and ligated to Hind III and Xho I digested pcDNA3 to generate the pcDNA3/Rab-G plasmid. Gag/pol pre-packaging lines are transfected with pcDNA3/Rab-G and selected for Neo r resistance. Stable cell lines are selected using the appropriate dmg, and the pool subcloned. EXAMPLE 7 ALPHA VIRUS CORE, El, E2 PACKAGING CELL LINES The VSV-G protein equivalent in alphavirases such as the Sindbis vims is a heteroprotein of three proteins: core, El and E2. As described in more detail below, a plasmid vector containing Sindbis DNA (ATCC 75891) may be utilized as a template to amplify the core and El and E2 glycoproteins for use in pseudotyping retroviral vectors. Briefly, similar to that described above in Example 6, core, El and E2 protein is amplified according to the manufacturer's instmctions (Perkin Elmer) utilizing the following primers: Sequence ID No. 7 5'-3'GTCAAGCTTGCTAGCTACAACACCACCACCATGAATACAG This primer is the sense sequence of the core protein containing the ATG start site, with a Hind III site at the 5' end. Sequence ID No. 8 5'-3' GAT ATC TCG AGC GGC CGC TCA TCT TCG TGT GCT AGT CAG This primer is the anti-sense sequence of the E2 protein at the termination of the protein, with a Xho I site at the 5i end. Both the 5' and 3' primers have restriction sites compatible with the 5' Hind III and 3' Xho I sites of the multicloning site of pcDNA3 (Invitrogen SD, CA). The resultant PCR product is isolated, digested with Hind III and Xho I and ligated to Hind III and Xho I digested pcDNA3 to generate the pcDNA3/coreElE2G plasmid. Gag/pol pre-packaging lines are transfected with pcDNA3/coreElE2G and selected for Neo r resistance. Stable cell lines are selected using the appropriate dmg, and the pool subcloned. EXAMPLE 8 SYNTHETIC G PROTEIN PACKAGING CELL LINES An example of a synthetic G protein is the ectodomain of rabies G, which has been truncated at the anchor residues on the cytoplasmic end of the transmembrane region. This truncation is made using the reagents from Example 5, except that the following 3' primer is utilized: Sequence ID No. 9 5'-3' GAT ATC TCG AGT TAG ACT CTT CTA CAA CAT GTC ATC AGG This primer is the anti-sense sequence of the G protein at the anchor region of the protein, with a Xho I site at the 5 i end. The resultant PCR product is isolated, digested with Hind III and Xho I and ligated to H d III and Xho I digested pcDNA3 to generate the pcDNA3/ΔG plasmid. Gag/pol pre-packaging lines are transfected with pcDNA3/ΔG and selected for Neo r resistance. Stable cell lines are selected using the appropriate dmg, and the pool subcloned. EXAMPLE 9 GENERATION OF OPAL MUTATED SELENIUM-REGULATED VSV-G EXPRESSION VECTORS As discussed above, VSV-G gene mutations may be created in which selected cysteine codons are converted to the selenocysteine isoacceptor TGA. As described in more detail below, constructs containing one, two, four or more mutagenized cysteine codons may be generated by fusing opal mutant coding regions to 3' untranslated regions containing the GPX1 SRE (see Figure 1 for a schematic illustration of the constmcts). Thus, in the absence of selenium, the UGA (opal) codon will result in termination; but upon selenium replenishment, a selenocysteine residue will be incoφorated into the polypeptide. For the VSV-G protein, structurally important cysteine residues are particularly preferred for replacement, so that selenocysteine will maintain the normal architecture, but any read-through by suppressor tRNAs in selenium-deprived cells should result in a destabilizing amino acid substitution. More importantly however, mutagenesis should be avoided in the trimerization domain: even though selenium is nearly identical to sulfur chemically, avoiding mutations in this region will minimize any chance that the mutation could alter the trimerization capacity and thus the function of the engineered VSV-G protein. A. Mutagenesis of the VSV-G Gene Sequence analysis of the VSV-G cDNA demonstrates 15 cysteine residues concentrated in the amino-terminal portion of the coding sequence (see Figure 2), with 11 cysteines encoded by UGC and 4 by UGU codons (Rose, J. Virol. 39:519-528, 1981). The mature protein, of approximately 67 kD contains only 13 cysteines as two are contained within the NH2-terminal signal sequence (16 amino-acids) which is cleaved during VSV- G protein transport to the cell surface. The protein exists as non-covalently bound homotrimers on the cell surface. Analysis of the coding region of VSV-G indicates that convenient restriction sites exist to allow the isolation of four restriction fragments which can be used as substrates for PCR-based mutagenesis. The restriction fragments are as follows: Fragment Size Number of cvsteines Cla l-Nde I 540 bp 8 Nde l-Sty I 120 bp 2 Sty l-Pst I 150 bp 2 Pst l-Kpn I 220 bp 2 Three pairs of cysteine residues have been identified which are separated by less than 30 nucleotides and may be mutagenized as pairs to opal terminator codons. PCR-based mutagenesis may be performed on individual restriction fragments of the VSV-G coding sequence and examine the effect of one, two or four opal mutations on VSV-G expression in a transient system. The Cla l-Sty I fragment (660 bp) contains 10 cysteines thereby permitting mutagenesis of two pairs of cysteines simultaneously using nested primers. The mutagenesis scheme is preferably performed with different combinations of mutagenic primers to allow mutagenesis of individual or multiple cysteines in a restriction fragment. This method provides a "library" of wild type and mutagenized fragments which can be easily recombined in a combinatorial manner to generate full length VSV-G genes containing different opal mutations. The PCR products are then cloned and sequenced to check for fidelity of mutagenesis. The fragments are then recombined initially to generate full length VSV-G mutants containing one, two, or four or more cys- >opal mutations. B. Fusion of mutant VSV-G genes to the Selenium Response Element fSRF Mutagenized VSV-G genes may be fused to the 3' untranslated region from the human GPX1 gene in a manner similar to that described by Shen et al., (J. Biol. Chem. 268:1 1463-11469, 1993). More specifically, the VSV-G gene may be inserted into the polylinker of pcDNA3 (Invitrogen, San Diego, CA), and the SRE from GPX1 (cDNA nucleotides 920-1070) inserted 3' to the VSV-G gene. In addition, the neomycin selection marker may be substituted for the hygromycin resistance marker. EXAMPLE 10 GENERATION OF PRE-PRODUCER CELL LINES A. Generation of a Pre-Producer Cell Line Pre-producer cell lines are cells containing gag/pol genes and a retroviral vector, but lacking an envelope gene. Such cell lines may be produced essentially as follows. Briefly, N2 βgal is used to transduce 293 2-3 cells (which contain only a gag/pol expression cassette), and G418 resistant clones are isolated. Clones are then tested for βgal expression, and a stable positive clone isolated. B. Testing Pre-Producer Cell Lines for Growth in Selenium Deficient Media 293 2-3/β gal pre-producer cells are thawed into DMEM-10% FBS and systematically adapted to selenium deficient media, such as DMEM supplemented with insulin/transferrin (IT, Collaborative Research) (see generally, Speier et al., supra). This protocol involves initial growth in FBS-containing media and sequential depletion of FBS from the media with concomitant increase in IT-containing media. The adaptation procedure to a selenium-deficient state is usually complete within 2-3 weeks and adapted cells can often be maintained in a selenium-deficient state for more than three months. EXAMPLE 11 GENERATION AND TESTING OF PRODUCER CELL LINES FOR SELENIUM-REGULATED EXPRESSION OF VSV-G PROTEIN A. Generation of VSV-G/SRE stably transfected 293 2-3/β-gal cells VSV-G/SRE expression constructs are transfected into 293 2-3/β-gal cells using either electroporation or the CaPO4 procedure. All transfection and dilution cloning procedures are performed in selenium-deficient media. The VSV-G/SRE transfected 293 2-3/β-gal cells are selected in hygromycin and resistant cells are cloned by limiting dilution to generate single cell clones. B. Initial screen for selenium-regulated VSV-G expression During expansion of single cell clones, the antibiotic resistant pools of cells are subjected to a preliminary screen for selenium-regulated expression of VSV-G/SRE. Cells are grown to 30% confluency (it will take approx. 3 days until confluence is reached) and selenium, as the bioavailable form sodium selenite, is added to the media at 25 ng/ml. VSV-G protein is turned on rapidly and the cells initiate fusion within 48 hours in response to selenium repletion. EXAMPLE 12 PRODUCTION OF RETROVIRAL VECTOR BY ULTRA-LARGE SCALE TRANSIENT TRANSFECTION A. Introduction High level expression of a newly introduced protein from a cell line is often desired, i.e., for expressing gag-pol or an envelope protein in a retroviral vector packaging cell line. However, in some cases cytopathic or cytostatic effects of toxic proteins such as VSV G, can severely limit their maximum stable expression level so as to severely limit or eliminate their utility in a stably transfected and stably expressing cell line. A useful altemative to stable expression is transient expression following transient transfection of an expression plasmid. Such transfections may be accomplished utilizing either DEAE Dextran, electroporation, liposomes, or CaPO precipitation (see Current Protocols in Molecular Biology or Molecular Cloning, a Laboratory Manual for typical protocols). Following the transient transfection, a percentage of the cells will take up and express reasonably high levels of even a toxic gene for at least a few days. For production of retroviral vectors, transient transfection with CaPO 4 has been the transient transfection method most often chosen for reasons including simplicity, high DNA copy number and expression per cell, and most importantly, high titer vector production. It has been used for small scale vector production with either typical retroviral envelope proteins (Finer et al., Blood 55:43-50, 1994; Pear et al., PNAS USA 90:8392, 1993) or for pseudotyping with VSV G protein (Bums et al., PNAS USA 90:8033-8037, 1993). While this methodology is well adapted to make small or trial quantities of a variety of different vectors, it is much less suitable for medium or larger scale production. For example, CaPO 4 transfection efficiency is known to vary widely depending on exact pH of solutions and a number of poorly understood other parameters. It is generally considered to be intolerant of more than small protocol variations and published protocols (i.e., Current Protocols in Molecular Biology or Molecular Cloning, a Laboratory Manual), specify transfection of cells in 10 cm or smaller plates. Accordingly, published accounts of CaPO 4 transfection typically specify 10 cm or smaller plates for free gas exchange and CO 2 equilibration. Any scale up is then done by working with large numbers of these plates, as for example in the recent paper by J. Yee et al (Yee et al., "A general method for the generation of high-titer, pantropic retroviral vectors: Highly efficient infection of primary hepatocytes," PNAS USA 97:9564-9568, 1994). The scale up potential of small plate transfections is clearly limited. As described in more detail below, utilizing culture factories (1200 to 6,000 cm 2 each) retroviral vector can be produced with crude titers equal or greater than that obtained with transfection in traditional small plates. The protocol is simple, uses common inexpensive reagents to transfect the desired plasmids, and it greatly facilitates the production of much larger quantities of a retroviral vector than are practical with previously reported transient transfection methods. Table one shows the results of one infection of 293 2-3 cells in a 2 layer, 1200 cm cell factory to produce G-pseudotyped N2 β-gal retroviral vector. The simple protocol described here requires only addition of the CaPO 4 precipitated DNA to the media over the cells, and changing to fresh media after 24 hours, and so is readily scalable to large numbers of cell factories. This makes feasible the production of tens or hundreds of liters of high titer G-pseudotyped vector. Time Post Titer (BCFU/M1) Total Volume Total retroviral Transfection Harvested vector particles 48 hours 9.2 x lO 5 200 mis 1.8 X lO 8 72 hours 1.6 x lO 6 200 mis 3.2 X lO 8 The cell line 293 2-3 is a MLV gag-pol expressing clone derived from human embryonic kidney 293 cells. The plasmid pMLP-G expresses VSV G from an adenovirus MLP promoter. The plasmid containing the test retroviral vector, pN2 B-gal neo, is co¬ transfected along with the pMLP-G plasmid. Similar titers may be obtained utilizing either this co-transfection method, or by transfection of pMLP-G alone into a 293 2-3 / B- gal cell line (a 293 2-3 clone, transduced with the B-gal vector). B. Production of VSV G-Pseudotvped Retroviral Vector by CaPQ4 Bulk Transfection. 1. Preparation of cells for transfection. The 293 cells (ATCC CRL 1573) expressing MLV gag-pol, called 293 2-3 are passed the day prior to the transfection to a 2-layer cell factory (Nunc) of -1200 cm cell culture area. This gag pol expressing clone of the cell line 293 is made by standard methods as have been described elsewhere. Between 4 and 48 hours later, preferably 18-24 hours, the cells are used for transfection having reached a level of confluence preferably between 30% and 90%, most preferably 60-80%. 2. Preparation of the DNA CaP0 4 precipitate. Two plasmids are co-transfected in one precipitate. The plasmid MLP-G expresses the VSV G gene, and the plasmid pCBβ-gal contains a retroviral vector expressing β-gal and the neomycin resistance gene. For every 100 cm of culture area 10-40 μg of each plasmid, preferably -20 μg, is used. The plasmids are prepared by Quiagen kit standard methods. Optionally they are sterilized prior to transfection by ethanol precipitation or chloroform extraction. For the cell factory -240 μg of each plasmid in TE (10 mM. Tris, pH 7.4, 1 mM EDTA) are mixed in a 50 ml polypropylene conical tube (Coming or Falcon), and then diluted to 10.8 ml (.9 ml/100 cm 2 ) with water and TE (1/10 TE) such that the final concentration of Tris is 2 mM or less. Then 1.2 mis (.1 ml/100 cm 2 ) of 2.5 M Cacl 2 are mixed in. To a separate 50 ml tube, 12 ml of Hepes buffered saline (2x HeBS) pH -7.08 is added (see solution recipes below). In a sterile tissue culture hood the 2x HeBS is continually vortexed while the DNA mixture is added by slow dropwise addition from a pipette over a period of ~30 seconds. The resulting precipitate is allowed to sit at room temperature for 15 minutes. 3. Transfection. The standard growth media is removed from the cells, the DNA precipitate is added to it, and the media plus DNA is gently replaced into the cell factory. After 4 to 24 hours, preferably 16-24 hours, the media is removed and the cells are rinsed 2x with -50 ml of PBS. Fresh media is then replaced at daily interval for as many days of harvesting as desired but at least two days, day 2 and day 3 post transfection are always collected as the typically optimal days for best titer. 4. Solutions 2.5 M CaCl 2 : 367.4 g CaCl 2 dihydride per liter, filter sterilized. 2x HeBS: 0.21 g NA 2 HPO 4ι 16.4 g NaCl, 11.9 g HEPES acid, -800 ml H 2 O Adjust pH to 7.08 with 10 N NaOH, and H 2 O to 1 liter. Filter sterilize, and store at -20°C in aliquots. EXAMPLE 13 A VSV-G FUSION PROTEIN WHICH PREVENTS INDUCTION OF SYNCYTIA IN PRODUCER CELLS A. Background As discussed in more detail below, the last 49 amino acids of the C terminus of VSV-G, which includes the transmembrane domain (TM), can be replaced with the last 54 amino acids of the MoMLV env. This chimeric protein includes several MLV components: the trans membrane domain, the amphipathic loop and the R peptide. Other chimeric proteins that can similarly be made include fusions of the two envelope proteins within or outside the TM domains, e.g., the entire or part of the VSV-G TM domain is fused to MLV sequences within the C terminus. As utilized herein, it should be understood that envelope proteins and sequence combinations derived from fusion of the two protein sequences will lead to attenuated fusogenicity of the resulting chimera. B. Construction of a VSV-G Fusion Protein As an example the following describes the replacement of the last 49 amino acids of the C-terminus of VSV-G with the equivalent C-terminus 54 amino acids of MLV. The resulting amino acid sequence is shown below. The junction is denoted (::): (VSV-G Indiana) ...GWFSSWK:: IMGPLIVLLLFGP... - (Ampho env-4070A)... 1. Materials: MoMLV 4070A Ampho envelope expression vector pEnvAm-Dra which includes the SV40 poly A. The VSV (Indiana strain) envelope protein G is derived from pMLP-G. pSC6 is a CMV promoter expression cassette (see WO 91/02805). The constmction of the chimeric protein utilizes the overlapping PCR method, and the following primers: Primer I: (VSV-G: 803 - 825 bp; including a Pst I site) (Sequence ID No. 10) Pstl 5' GG ATC TCT TTG CTG CAG CCA GAT Primer II: (Junction sequence: 1398-1415 of G and 7616-7634 of Ampho) (Sequence ID No. 11) 5 ' TGG TTC AGT AGT TGG AAA / ATC ATG GGA CCT CTA ATA G Primer III: (Complimentary to primer II) (Sequence ID No. 12) 5'CTATTAGAGGTCCCATGAT/TTTCCAACTACTGAACCA Primer IV: (SV40 poly A from pEnvAm-Dra; 161-185 bp; Bam HI site; negative strand) (Sequence ID No. 13) BamHI 5' C ATG TCT GGA TCC CCA GGA AGC TCC 2. Construction of the expression vector: PCR amplified DNAs using the two sets of primers (I plus II and III plus IV) are annealed and PCR amplified using primers I and IV. The resulting DNA is digested with the restriction enzymes Pst I and Bam HI. The resulting fragment of about 900 bp of hybrid G/A is cloned into pSC6 (EcoRV and BamHl digested) together with the Cla I (blunted) to Pst I (800 bp) of VSV-G DNA. This plasmid is designated pG/A. 3. Cell culture/titer assay: (a) The hybrid env plasmid DNA (pG/A) is transfected using the CaPO4 precipitate method into the gag-pol expressing 293/2-3 cells as described above. Formation of syncytia is assessed visually after 1, 2 and 3 days. (b) In parallel, pG/A is cotransfected with pKT-1 into 293/2-3 and SCV21 or DA gag-pol cells to assess transient titers and envelope functionality. This transient (48 hours) expression method is described above. The cotransfection of pMLP-G plus N2 βgal into the above-mentioned cells is used as a control in this experiment. (c) In order to assess stable expression of the hybrid envelope in producer cells: forty eight hours post transfection the cells (from (b) above) are subjected to G418 selection for 10 days and stable pools are tested for vector production, infectivity and any tendency to form syncytia. EXAMPLE 14 USE OF RECOMBINANT CYTOMEGALOVIRUS EXPRESSING VESICULAR STOMATITIS VIRUS-G PROTEIN TO PSEUDOTYPE RETROVIRAL VECTORS A. Materials and Methods 1. Viruses and cell lines. Wild type MCMV (Smith strain K181+), RM427, and RMCMVG are grown and titered on NIH3T3 cells as described in Manning et al., J. Vir. 66: 794-3802, 1992; Stoddart, J Vir. 68(10): 6243 -6253, 1994. NIH3T3, HeLa, 293, and 293 gag /pol(a 293 line containing gag and pol but lacking an env gene) cells are grown in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum. 2. Plasmids and preparation of recombinant virus. Plasmid cloning is by standard technique (Sambrook et al, Molecular Cloning: A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, 1989). pON401 contains the Hind III L fragment of MCMV (Manning et al, Vir. 66:3794-3802, 1992). pCMV-G contains the VSV-G protein under transcriptional control of the HCMV ie\ promoter-enhancer. To construct pON401CMVG, the 2665 -bp Hinc II fragment of pCMV-G containing VSV-G is cloned between the Hpa I sites of pON401 (Figure 3). This results in a deletion of the 79-bp Hpa I fragment in pON401. pMLP-G contains the VSVG protein under transcriptional control of the adenovims major late promoter. Protocols for the production and isolation of recombinant MCMV have been described (Manning et al., Vir. 767:477-484, 1988). Briefly, 15 μg of linearized pON401CMVG is co-transfected with RM427 viral DNA using calcium phosphate precipitation. The amount of RM427 DNA used in the transfection is determined empirically for each viral DNA preparation. The co-transfections are harvested when cytopathic effect (cpe) is complete, approximately 5 days post transfection. To isolate β-gal- recombinants, a limiting dilution assay is done. Briefly, 96-well tissue culture plates are seeded with NIH3T3 cells (IO 4 cells/well). Recombinant vims pools arising from the co-transfection are titered, and then used to infect the 96-well plates at a dilution which would contain approximately 75 plaque forming units (pfu) per 96 well-plate. When cytopathic effect (cpe) is evident (about 7 days after infection) methyl umberryliferone (MUG) is added to all wells at a final concentration of 150 μg/ml. At various time points (3-24 h) the plate is put on a UV light box and photographed. Wells containing parental -gal+ vims (RM427) would fluoresce. Uninfected wells, or wells infected by a β-gal- recombinant, would not fluoresce. Vims from wells with cpe and no fluorescence is grown up and analyzed by DNA blot analysis. 3. DNA analysis. Viral DNA is prepared from purified virions (Vieira et al., J Virol. 65:4837-4846, 1994) and subjected to DNA blot analysis as previously described (Manning et al., Vir. 767:477-484, 1988). 4. Isolation of infected-cell proteins and immunoblotting. To prepare total cell protein, cells are infected with vims at a multiplicity of infection (MOI) of 10. At the appropriate time point, cells are rinsed with phosphate buffered saline and then lysed in NP40 lysis buffer (100 mM NaCl, 20 mM Tris pH 7.5, 1 mM EDTA, 0.5% NP40, 0.5% deoxycholate) supplemented with aprotinin (2 μg/ml), pepstatin (1 μg/ml), leupeptin (2 μg-ml), and Peflabloc SC (1 mg/ml). All protease inhibitors are purchased from Boehringer Mannheim. Plates are incubated on ice for 5 minutes. Cells are harvested by scraping, placed in 1.5 ml microcentrifuge tubes, and spun in a microfiige at high speed for 10 minutes at 4°C. Supematants are collected and stored at -80°C until analyzed. Infected-cell proteins are separated on 10% SDS-polyacrylamide gels (Novex), transferred to nitrocellulose, and subjected to Western blot analysis. The primary antibody is a mouse monoclonal to VSV-G glycoprotein (Sigma ImmunoChemicals) used at a dilution of 1: 100,000. Peroxidase-conjugated goat anti-mouse (Boehringer Mannheim) at a dilution of 1 :30,000) is used as the secondary antibody. Blots are developed using ECL (Amersham) following manufacturer's directions. 5. Producer cells, retroviral vector production, and titration. To make the cell line for use in the production of pseudotyped vector, 293 gag /pol cells are infected with N2 β-gal, a VSV-G pseudotyped retroviral vector. N2 βgal contains the genes for neomycin phosphotransferase (neor) and β-galactosidase (β-gal). G418 resistant colonies are selected. Staining of the resulting cell line with Bluogal (GIBCO BRL) shows that all cells are blue. To produce VSV-G pseudotyped retrovirus, 10 cm plates of 293 gag /pol (N2 β-gal) cells are infected with RMCMVG at the desired MOI in a total volume of 2 ml. Two hours after infection, the cells are washed twice, and then incubated in 2 ml medium until harvested. Twenty four or 48 hours after infection, supematants are filtered (0.45um pore) and stored at -80°C until titered. Supematants are titered on HeLa cells. The day before infection 10 cm plates are seeded with 2 X IO 6 cells. On day 1, plates are infected with lOOμl of supematant in 2 ml of media containing Polybrene (Sigma; 8 μg/ml). After a 2 hour incubation, the plates are washed, and incubated ovemight with fresh media. On day 2, plates infected with supematants from RMCMVG-infected cells are split 1 : 10 ant 1 : 100. Those from mock or wt MCMV-infected supematants are split 1 :2. On day three, G418 (Sigma; 1 mg/ml) is added to the media. Ten to fourteen days later, colonies are stained and counted. B. Results 1. Construction of RMCMVG. RMCMVG is isolated following co-transfection of NIH3T3 cells with RM427 DNA and pON401 -CMVG (Fig 3). After screening eight 96-well plates for β-gal- recombinants, six wells are identified that contained β-gal vims. Three of these are grown up and plaque purified an additional time. These represent recombinants arising from two independent co-transfections. Viral DNA is prepared from these three isolates and subjected to DNA blot analysis. As shown in Figure 4, all three recombinant viruses contain the 5.7 kbp and 4.3 kbp Hindlll bands predicted for a VSV-G recombinant. Isolate D9 is grown up and stock titers of approximately 1 x IO 8 plaque forming units (PFU)/ml are obtained. This is identical to stock titers of wild type MCMV indicating that VSV-G expression is not toxic to MCMV replication in cell culture. In contrast to wild type MCMV, RMCMVG plaques demonstrated a marked syncytium phenotype (Figure 5A). 2. RMCMVG expresses VSV G glycoprotein in permissive and non- permissive cells. NIH3T3 cells are infected with the three isolates of RMCMVG and subjected to Western blot analysis. As demonstrated in Figure 6, NIH 3T3 cells infected with all three isolates expressed abundant amounts of G protein at 24 hours after infection. Cells infected with RM427 do not show any VSV-G expression. As a positive control, a lane containing G glycoprotein produced after transfection of 293 cells with pMLP-G is included. To determine whether RMCMVG expresses G protein in nonpermissive cells, 293 cells are infected with RMCMVG and protein is harvested at 6, 24, and 48 hour post infection. As shown in Figure 7, VSV-G protein expression is detected as early as 6 hour post infection, but peaked at 24 hour post infection. 3. RMCMVG infection can be used to pseudotype retroviral vectors. To investigate the ability of RMCMVG to pseudotype retroviral vectors, 293 gag /pol (N2 β-gal) cells are infected with either wild type MCMV or RMCMVG. Supematants are collected from cells at 24 hour and 48 hour after infection and titered. As shown in Table 1, peak neo titers of approximately 1 x IO 4 colony forming units (CFU)/ml are found in the supematants of cells 24 hour after infection by RMCMVG at a MOI of 10 or 20. No retroviral titers are found in the supematants of mock infected cells or cells infected with wild type MCMV. Table l VSV G-pseudotvped vector production from 293 gag /pol fN2 β-gaU cells following infection with RMCMVG or wild type MCMV Titer (CFU / ml) a Vims MOI b 24 h 48 h RMCMVG 1 9.0 x lO 3 2.0 x IO 2 10 2.3 x IO 4 4.0 x lO 3 20 4.2 x lO 4 1.2 x lO 4 wt MCMV 1 nd c nd 10 nd nd 20 nd nd Mock nd nd Since 293 cells are not permissive for MCMV replication there should be no RMCMVG in the supematants. When supematants are titered for RMCMVG, titers of 200 plaque forming units (PFU) /ml are found (data not shown). This small amount of MCMV is probably vims remaining from the initial infection. The large size of MCMV (300 nm) relative to retroviral vector should allow for simple removal of this residual MCMV from the supematant. From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

Claims: aims We claim: 1. An expression vector, comprising a murine CMV IE promoter operably linked to a nucleic acid sequence which encodes a toxic or fusogenic protein. 2. An expression vector, comprising a tightly controllable promoter that directs the expression of a nucleic acid sequence which encodes a fusogenic protein. 3. The expression vector according to claims 1 or 2 wherein said fusogenic protein is Vesicular Stomatitis Vims -G protein. 4. The expression vector according to claim 2 wherein said tightly controllable promoter is tTA promoter system. 5. The expression vector according to claim 2 wherein said tightly controllable promoter is a LAP promoter system. 6. An expression vector according to claims 1 or 2, further comprising a selenium responsive element. 7. An expression vector, comprising a promoter operably linked to a nucleic acid sequence encoding a fusogenic protein, and a selenium responsive element, wherein one or more cysteine residues in said sequence encoding a fusogenic protein have been altered to an opal termination codon. 8. An expression vector, comprising a promoter operably linked to a nucleic acid sequence encoding a protein toxic to cells, and a selenium responsive element, wherein one or more cysteine residues encoded by said sequence encoding a toxic protein have been altered to an opal termination codon. 9. A retroviral vector which contains an expression vector according to any one of claims 1-8. 10. A packaging cell line comprising a gag/pol expression cassette and an expression vector according to any one of claims 1 to 8. 11. A producer cell line comprising a gag/pol expression cassette, an expression vector according to any one of claims 1 to 7, and a retroviral vector constmct. 12. A mutant nucleic acid molecule encoding a fusogenic protein, wherein one or more cysteine residues in said nucleic acid molecule have been altered to an opal termination codon. 13. A mutant nucleic acid molecule encoding a protein toxic to cells, wherein one or more cysteine residues in said nucleic acid molecule have been altered to an opal termination codon. 14. A method for transiently producing retrovector particles, comprising: (a) incubating between 10 and 10 cells which contain a gag/pol expression cassette and retroviral vector constmct in a culture having a surface area greater than 225 cm 2 , under conditions and for a time sufficient to allow said cells to enter log phase of growth; and (b) introducing an expression vector according to any one of claims 1 to 8 into said culture, such that retrovector particles may be transiently produced from said cells. 15. A recombinant vims which directs the expression of a toxic or fusogenic protein, which is replication competent in its native species, and replication incompetent in other species. 16. A cell line with an altered or mutated glycosylation pathway, which carries an expression vector that directs the expression of a glycosylated toxic protein or fusogenic protein.

Inventors: CHADA SUNIL, DEPOLO NICHOLAS J, JOLLY DOUGLAS J, MANNING WILLIAM C, MURPHY JOHN E, RALSTON ROBERT O II, ROBBINS JOAN, SAUTER SYBILLE, CHADA, SUNIL, DEPOLO, NICHOLAS, J., JOLLY, DOUGLAS, J., MANNING, WILLIAM, C., MURPHY, JOHN, E., RALSTON, ROBERT, O., II, ROBBINS, JOAN, SAUTER, SYBILLE
Applicants: VIAGENE INC, CHIRON VIAGENE, INC.
Application year: 1996
Main classification: C12N15
IPC codes: C07K14, C12N5, C12N7, C12N15