Patent Publication Number: US-2009227031-A1

Title: Cells for adenovirus vector and protein production

Description:
INCORPORATION BY REFERENCE 
     This application claims priority to U.S. provisional patent application Ser. No. 60/894,018 filed Mar. 9, 2007. 
     The foregoing applications, and all documents cited therein or during their prosecution (“appln cited documents”) and all documents cited or referenced in the appln cited documents, and all documents cited or referenced herein (“herein cited documents”), and all documents cited or referenced in herein cited documents, together with any manufacturer&#39;s instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a novel cell line for adenovirus (Ad) and protein production that does not eliminate the overlap between the cell-line and adenovirus-derived vector sequences, but represses the frequency of undesirable homologous recombination events by the use of a large non-homologous spacer element(s). 
     BACKGROUND OF THE INVENTION 
     Adenovirus vectors have attracted considerable interest over the past decade, with ongoing clinical development programs for applications ranging from replacement therapy for protein deficiencies to cancer therapeutics to prophylactic vaccines (reviewed by Altaras et al., Adv Biochem Eng Biotechnol. 2005; 99:193-260). Consequently, considerable product, process, analytical, and formulation development has been undertaken to support these programs. For example, “gutless” vectors have been developed in order to improve gene transfer capacity and durability of expression, new cell lines have been developed to minimize undesirable recombination events, production conditions have been optimized to improve volumetric productivities, analytical techniques and scaleable purification processes have advanced towards the goal of purified adenovirus becoming a “well-characterized biological”, and liquid formulations have been developed which maintain virus infectivity at 2-8 degrees C. for over 18 months. 
     Cell lines for adenovirus (“Ad”) production include such cell lines as human embryonic kidney (“HEK”) 293 cells, 293-ORF6 cells and human embryonic retinoblasts (“HER”) PER.C6 cells. A problem with these cell lines is either homologous recombination leading to the generation of undesirable replication competent adenovirus (RCA), such as is found with HEK 293 cells, or the incorporation of specific Ad genes that may impact on virus productivity or limit the type of vector whose replication it supports. 
     Most researchers use the HEK 293 cell-line when replication-deficient vectors are produced. It incorporates Ad sequences 1-4344 nucleotides (nt) consisting of the Ad inverted terminal repeat (“ITR”), early region 1A (“E1A”), early region 1B (“E1B”) and protein IX (“pIX”) genes. E1A and E1B are essential regulatory genes, pIX codes for a non-essential capsid protein. This cell-line is able to complement Ad vectors deficient in E1A and E1B. Although pIX is expressed at a low level, it is not sufficient to complement for pIX deficiency. The most serious drawback of this cell-line is the production of replication competent adenovirus (“RCA”) through homologous recombination between the overlapping Ad sequences in the cells and the vector. RCA is a safety issue for the FDA and other regulatory authorities. See, e.g.,  FIG. 1 . 
     The cell line 293-ORF6 is based on 293 cells. This cell-line which also incorporates the E4-ORF6 Ad gene and has been specifically engineered to support the production of Ad vectors deficient in the E1A, E1B and E4 genes. Although, E1A and E1B deleted vectors can grow in this cell-line, only E1A, E1B and E4 deleted vectors can be grown without the occurrence of RCA. See, e.g., U.S. Pat. Nos. 6,974,695, 6,913,922, 6,869,794, 6,579,522, 6,492,169 and 6,291,214. 
     PER.C6 cells are based on primary retinoid cells. This cell-line incorporates Ad sequences 459-3510 nt, comprising the Ad E1A and E1B genes, but not the pIX gene or the Ad ITR. The important aspect of this cell-line that E1A and E1B deleted Ad vectors may be produced without the production of RCA generated via homologous recombination since the overlap between the producer cells and many vector constructs has been eliminated. See, e.g., U.S. Pat. Nos. 5,994,128, 6,265,212, 6,492,169, 6,602,706, 6,670,188, 6,692,966, 6,783,980, 6,855,544, 6,869,794 and 6,974,695 and  FIG. 2 . Ad vector yields from this cell line are typically reduced relative to their yield from HEK 293. 
     There remains a need for cell lines for Ad production that avoid the production of replication competent adenovirus while maintaining high vector yields. 
     Citation or identification of any document in this application is not an admission that such a document is available as prior art to the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention is based, in part, upon a novel cell-line based on 293 cells which represses undesirable homologous recombination events or their effects by the use of a large non-homologous spacer element(s). 
     The present invention relates to a mammalian packaging or production cell which may comprise a non-homologous spacer element inserted into an adenovirus nucleotide sequence, wherein the spacer element represses undesirable homologous recombination to reduce the frequency of production of replication-competent adenovirus when adenovirus vectors are propagated in the cell. Advantageously, the cell is a human embryonic kidney cell or 293 cell which can complement grows of adenovirus vectors deficient in the E1A and E1B genes. The spacer element may be inserted by site directed integration at a specific site in the adenovirus nucleotide sequence present in the packaging cell. Further spacer(s) might also be inserted into the already integrated spacer element by the use of specific site specific recombination element(s) such as, but not limited to, the lox, frt and attB sites used by the CRE, FLPe and phiC31 systems, respectively. Insertion of spacer element(s) can be also accomplished by the use of transposable element(s) as such as, but not limited to, Sleeping Beauty (SB) transposase. The specific site in the adenovirus nucleotide sequence may be after the end of the early region 1B (“E1B”) transcription unit but in front of the protein IX (“pIX”) transcription unit, after the inverted terminal repeat (“ITR”) and packaging sequences but before the early region 1A (“E1A”) transcription start site or after the E1A sequences but before the E1B sequences, or in other regions that do not interfere with their function. 
     There are several embodiments of the spacer element. In a first embodiment, the spacer element may comprise one or more regulatory elements such as, but not limited to, promoters, enhancers, insulators, polyadenylation and termination signals. In a second embodiment, the spacer element may be of varying sizes, such as, but not limited to, about 2000 to about 3000 base pairs, at least about 2000 base pairs, at least about 4000 base pairs or at least about 6000 base pairs. In a third embodiment, the spacer element may not express any genes. 
     In a fourth embodiment, the spacer element may express one or more genes advantageous for adenovirus or protein production such as, but not limited to, anti-apoptotic genes, growth promoting genes, kinases and selectable markers. Anti-apoptotic genes include, but are not limited to, CrmA and Bcl-2. Growth promoting genes include, but are not limited to, dominant negative double-stranded RNA-dependent protein kinase (PKR), adenoviral VA gene, SV40 T-antigen gene and cytokines. In an advantageous embodiment, the gene is dominant negative double-stranded RNA-dependent protein kinase (PKR) or adenoviral VA gene. More preferably the adenoviral VA gene comprises a mutated internal promoter, such that the transcription specificity is changed from polymerase III to polymerase II. The expression level of the modified VA gene can be modulated by the use of an inducible polymerase II promoter system such as, but not limited to the tetracycline repressor or metallothionein promoter systems. Selectable markers include, but are not limited to, neomycin, puromycin, zeomycin, bleomycin and ABC transporter (such as ABCG2, see, e.g., International Patent Publication WO 03/035685). The one or more genes may be inserted into the spacer element randomly or may be inserted into the spacer element by site directed integration. 
     There are several embodiments of the promoters. In a first embodiment, the expression of one or more genes may be driven by a constitutive promoter, an inducible promoter or a regulatable promoter. A constitutive promoter may be selected from the group consisting of CMV, RSV, SV40, PKG and TK. An inducible promoter or regulatable promoter may be selected from the group consisting of a metallothionein promoter or a tetracycline repressor (“tetR”) system. Advantageously, the promoter is a metallothionein promoter, preferably a sheep metallothionein promoter 1a gene promoter. 
     The present invention encompasses methods of repressing undesirable homologous recombination or their effects in a mammalian cell comprising inserting a non-homologous spacer element into an adenovirus nucleotide sequence. 
     It is noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. Patent law; e.g., they can mean “includes”, “included”, “including”, and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. Patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the invention. 
     These and other embodiments are disclosed or are obvious from and encompassed by, the following Detailed Description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description, given by way of example, but not intended to limit the invention solely to the specific embodiments described, may best be understood in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts RCA generation in 293 cells, 
         FIG. 2  depicts RCA free vector production in PER.C6 cell line, 
         FIG. 3  depicts VLI-293 cells, 
         FIG. 4  depicts potential spacer configurations, including several expression cassettes, regulatory elements (poly A, enhancers, promoters, insulators), regulatable promoters (e.g. sMT promoter, tetR), anti-apoptotic genes (e.g. CrmA, Bcl-2), growth promoting genes (e.g. dominant negative PKR, cytokines), selectable markers (e.g. neomycin, puromycin, ABCG2) and secondary integration target sites (e.g. lox, frt, attB phiC31 sequences) 
         FIG. 5  depicts an E1B/pIX promoter region, 
         FIG. 6  depicts a spacer configuration, 
         FIG. 7  depicts RT-PCR analysis of DHFR expression where lane 1 is the 1 kb+marker, lane 2 is mock transfected 293 cells−zn, lane 3 is mock transfected 293 cells+zn, lane 4 is pMT010/a+transfected 293 cells−zn, lane 5 is pMT010/a+transfected 293 cells−zn, lane 6 is pGL3-TKpr-DHFR-TKpA transfected 293 cells, lane 7 is pRc-Ad5-ec1-3-lox transfected 293 cells and lane 8 is a dH2O control, 
         FIG. 8  depicts generation of test vector for pRcCMV-IVS-Cre, 
         FIG. 9  depicts an analysis of Cre recombinase activity with the top row demonstrating cells under visible light, and the bottom row demonstrating cells under UV for fluorescence for detection of eGFP, 
         FIG. 10  depicts analysis of TK functionality and 
         FIG. 11  depicts generation of the pRc-Spacer circular plasmid. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is based, in part, upon a novel cell-line based on 293 cells which represses undesirable homologous recombination events or their effects by the use of a large non-homologous spacer element(s). See, e.g.,  FIG. 3 . 
     In accordance with the present invention, there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Maniatis, Fritsch &amp; Sambrook, “Molecular Cloning: A Laboratory Manual (1982); “DNA Cloning: A Practical Approach,” Volumes I and II (D. N. Glover ed. 1985); “Oligonucleotide Synthesis” (M. J. Gait ed. 1984); “Nucleic Acid Hybridization” [B. D. Hames &amp; S. J. Higgins eds. (1985)]; “Transcription and Translation” [B. D. Hames &amp; S. J. Higgins eds. (1984)]; “Animal Cell Culture” [R. I. Freshney, ed. (1986)]; “Immobilized Cells And Enzymes” [IRL Press, (1986)]; B. Perbal, “A Practical Guide To Molecular Cloning” (1984). Therefore, if appearing herein, the following terms shall have the terminology set out below. 
     A “DNA molecule” refers to the polymeric form of deoxyribonucleotides (adenine, guanine, thymine, or cytosine) in its either single stranded form, or a double-stranded helix. This term refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear DNA molecules (e.g., restriction fragments), viruses, plasmids, and chromosomes. In discussing the structure herein according to the normal convention of giving only the sequence in the 5′ to 3′ direction along the nontranscribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). 
     A “vector” is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A “replicon” is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo; i.e., capable of replication under its own control. An “origin of replication” refers to those DNA sequences that participate in DNA synthesis. An “expression control sequence” is a DNA sequence that controls and regulates the transcription and translation of another DNA sequence. A coding sequence is “operably linked” and “under the control” of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then translated into the protein encoded by the coding sequence. 
     In general, expression vectors containing promoter sequences which facilitate the efficient transcription and translation of the inserted DNA fragment are used in connection with the host. The expression vector typically contains an origin of replication, promoter(s), terminator(s), as well as specific genes which are capable of providing phenotypic selection in transformed cells. The transformed hosts can be fermented and cultured according to means known in the art to achieve optimal cell growth. 
     A DNA “coding sequence” is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic (e.g., mammalian) DNA, and even synthetic DNA sequences. A polyadenylation signal and transcription termination sequence will usually be located 3′ to the coding sequence. A “cDNA” is defined as copy-DNA or complementary-DNA, and is a product of a reverse transcription reaction from an mRNA transcript. 
     Transcriptional and translational control sequences are DNA or RNA regulatory sequences, such as promoters, enhancers, polyadenylation signals, terminators, and the like, that provide for the expression of a coding sequence in a host cell. A “cis-element” is a nucleotide sequence, also termed a “consensus sequence” or “motif”, that interacts with other proteins which can upregulate or downregulate expression of a specific gene locus. A “signal sequence” can also be included with the coding sequence. This sequence encodes a signal peptide, N-terminal to the polypeptide, that communicates to the host cell and directs the polypeptide to the appropriate cellular location. Signal sequences can be found associated with a variety of proteins native to prokaryotes and eukaryotes. 
     A “promoter sequence” is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3′ direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3′ terminus by the transcription initiation site and extends upstream (5′ direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence is a transcription initiation site, as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Eukaryotic promoters often, but not always, contain “TATA” boxes and “CAT” boxes. Prokaryotic promoters contain Shine-Dalgarno sequences in addition to the -10 and -35 consensus sequences. 
     The term “oligonucleotide” is defined as a molecule comprised of two or more deoxyribonucleotides, preferably more than three. Its exact size will depend upon many factors which, in turn, depend upon the ultimate function and use of the oligonucleotide. The term “primer” as used herein refers to an oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product, which is complementary to a nucleic acid strand, is induced, i.e., in the presence of nucleotides and an inducing agent such as a DNA polymerase and at a suitable temperature and pH. The primer may be either single-stranded or double-stranded and must be sufficiently long to prime the synthesis of the desired extension product in the presence of the inducing agent. The exact length of the primer will depend upon many factors, including temperature, source of primer and use for the method. For example, for diagnostic applications, depending on the complexity of the target sequence, the oligonucleotide primer typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. 
     The primers herein are selected to be “substantially” complementary to different strands of a particular target DNA or RNA sequence. This means that the primers must be sufficiently complementary to hybridize with their respective strands. Therefore, the primer sequence need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5′ end of the primer, with the remainder of the primer sequence being complementary to the strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the primer, provided that the primer sequence has sufficient complementarity with the sequence to hybridize therewith and thereby form the template for the synthesis of the extension product. 
     As used herein, the terms “restriction endonucleases” and “restriction enzymes” refer to enzymes which cut double-stranded DNA at or near a specific nucleotide sequence. 
     “Recombinant DNA technology” refers to techniques for uniting two heterologous DNA molecules, usually as a result of in vitro ligation of DNAs from different organisms. Recombinant DNA molecules are commonly produced by experiments in genetic engineering. Synonymous terms include “gene splicing”, “molecular cloning” and “genetic engineering”. The product of these manipulations results in a “recombinant” or “recombinant molecule”. 
     A cell has been “transformed” or “transfected” with exogenous or heterologous DNA when such DNA has been introduced inside the cell. The transforming DNA may or may not be integrated (covalently linked) into the genome of the cell. In prokaryotes, yeast, and mammalian cells for example, the transforming DNA may be maintained on an episomal element such as a vector or plasmid. With respect to eukaryotic cells, a stably transformed cell is one in which the transforming DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing the transforming DNA. A “clone” is a population of cells derived from a single cell or ancestor by mitosis. A “cell line” is a clone of a primary cell that is capable of stable growth in vitro for many generations. An organism, such as a plant or animal, that has been transformed with exogenous DNA is termed “transgenic”. 
     Two DNA sequences are “substantially homologous” when at least about 75% (preferably at least about 80%, and most preferably at least about 90% or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantially homologous can be identified by comparing the sequences using standard software available in sequence data banks, or in a Southern hybridization experiment under, for example, stringent conditions as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Maniatis et al., supra; DNA Cloning, Vols. I &amp; II, supra; Nucleic Acid Hybridization, supra. 
     A “heterologous” region of the DNA construct is an identifiable segment of DNA within a larger DNA molecule that is not found in association with the larger molecule in nature. Thus, when the heterologous region encodes a mammalian gene, the gene will usually be flanked by DNA that does not flank the mammalian genomic DNA in the genome of the source organism. In another example, the coding sequence is a construct where the coding sequence itself is not found in nature (e.g., a cDNA where the genomic coding sequence contains introns, or synthetic sequences having codons different than the native gene). Allelic variations or naturally-occurring mutational events do not give rise to a heterologous region of DNA as defined herein. For example, a polynucleotide may be placed by genetic engineering techniques into a plasmid or vector derived from a different source and be a heterologous polynucleotide. A promoter removed from its native coding sequence and operatively linked to a coding sequence other than the native sequence is a heterologous promoter. 
     As used herein, “fragment” or “portion” as applied to a gene or a polypeptide, will ordinarily be at least 10 residues, more typically at least 20 residues, and preferably at least 30 (e.g., 50) residues in length, but less than the entire, intact sequence. Fragments of these genes can be generated by methods known to those skilled in the art, e.g., by restriction digestion of naturally occurring or recombinant genes, by recombinant DNA techniques using a vector that encodes a defined fragment or gene, or by chemical synthesis. 
     A standard northern blot assay can be used to ascertain the relative amounts of mRNA in a cell or tissue in accordance with conventional northern hybridization techniques known to those persons of ordinary skill in the art. Alternatively, a standard Southern blot assay may be used to confirm the presence and the copy number of the gene of interest in accordance with conventional Southern hybridization techniques known to those of ordinary skill in the art. Both the northern blot and Southern blot use a hybridization probe, e.g. radiolabelled cDNA or oligonucleotide of at least 20 (preferably at least 30, more preferably at least 50, and most preferably at least 100 consecutive nucleotides in length). The DNA hybridization probe can be labelled by any of the many different methods known to those skilled in this art. 
     Hybridization reactions can be performed under conditions of different “stringency.” Conditions that increase stringency of a hybridization reaction are well known. See for examples, “Molecular Cloning: A Laboratory Manual”, second edition (Sambrook et al. 1989). Examples of relevant conditions include (in order of increasing stringency): incubation temperatures of 25° C., 37° C., 50° C., and 68° C.; buffer concentrations of 10×SSC, 6×SSC, 1×SSC, 0.1×SSC (where SSC is 0.15 M NaCl and 15 mM citrate buffer) and their equivalent using other buffer systems; formamide concentrations of 0%, 25%, 50%, and 75%; incubation times from 5 minutes to 24 hours; 1, 2 or more washing steps; wash incubation times of 1, 2, or 15 minutes; and wash solutions of 6×SSC, 1×SSC, 0.1×SSC, or deionized water. 
     The labels most commonly employed for these studies are radioactive elements, enzymes, chemicals which fluoresce when exposed to ultraviolet light, and others. A number of fluorescent materials are known and can be utilized as labels. These include, for example, fluorescein, rhodamine, auramine, Texas Red, AMCA blue and Lucifer Yellow. A particular detecting material is anti-rabbit antibody prepared in goats and conjugated with fluorescein through an isothiocyanate. Proteins can also be labeled with a radioactive element or with an enzyme. The radioactive label can be detected by any of the currently available counting procedures. The preferred isotope may be selected from  3 H,  14 C,  32 P,  35 S,  36 Cl,  51 Cr,  57 Co,  58 Co,  59 Fe,  90 Y,  125 I,  131 I, and  186 Re. 
     Enzyme labels are likewise useful, and can be detected by any of the presently utilized calorimetric, spectrophotometric, fluorospectrophotometric, amperometric or gasometric techniques. The enzyme is conjugated to the selected particle by reaction with bridging molecules such as carbodiimides, diisocyanates, glutaraldehyde and the like. Many enzymes which can be used in these procedures are known and can be utilized. The preferred are peroxidase, β-glucuronidase, β-D-glucosidase, β-D-galactosidase, urease, glucose oxidase plus peroxidase and alkaline phosphatase. U.S. Pat. Nos. 3,654,090, 3,850,752, and 4,016,043 are referred to by way of example for their disclosure of alternate labeling material and methods. 
     The term “exogenous gene,” as it is used herein, refers to any gene in the spacer element within the adenovirus sequence. The gene includes a nucleic acid sequence encoding a gene product operably linked to a promoter. For example, the gene can comprise a non-native nucleic acid sequence encoding a gene product operably linked to a native promoter, or a native nucleic acid sequence encoding a gene product operably linked to a non-native promoter or in a non-native location. It should be appreciated that the exogenous gene can be any gene encoding an RNA or protein of interest to the skilled artisan. Therapeutic genes, genes encoding a protein that is to be studied in vitro and/or in vivo, antisense nucleic acids, and modified viral genes are illustrative of possible exogenous genes. 
     The novel cell-line may be based on 293 cells and is referred to as “293-VLI cells”. 293-VLI cells do not eliminate the overlap between the cell-line and adenoviral vector sequences, but repress undesirable homologous recombination events or their effects by the use of a large non-homologous spacer element(s). In a preferred embodiment the spacer is inserted by site directed integration at a specific site into the already existing Ad sequence in 293 cells. 
     The specific site in the adenovirus nucleotide sequence may be after the end of the early region 1B (“E1B”) transcription unit but in front of the protein IX (“pIX”) transcription unit, after the inverted terminal repeat (“ITR”) and packaging sequences but before the early region 1A (“E1A”) transcription start site or after the E1A sequences but before the E1B sequences, or at other regions that do not disrupt their function. 
     Potential spacer configurations are depicted in  FIG. 4 . 
     The spacer element may comprise one or more regulatory elements such as, but not limited to, promoters, enhancers, insulators, polyadenylation and termination signals. 
     The spacer element may comprise site specific recombination element(s) such as, but not limited to, the lox, frt and attB sites used by the CRE, FLPe and phiC31 systems respectively. Insertion of spacer element(s) can be also accomplished by the use of transposable element(s) as such as, but not limited to, Sleeping Beauty transposase. 
     A preferred site of insertion is after nucleotide 3510 at the end of the E1B transcription unit, but in front of the pIX transcription unit. Additional sites of insertions could create further reduction in the probability of RCA generation. One site of insertion is after the ITR and packaging sequences but before the E1A transcriptional start site. Furthermore, another site of insertion is after the E1A sequences, but before the E1B sequences. In these insertions some of the regulatory elements (promoters, enhancers, poly A or termination signals) may be provided by the spacer element. 
     The spacer may serve two functions. First, the size of a non-homologous spacer radically represses homologous recombination between direct repeats as the size increases to ˜2-3000 base pairs (bp) (see, e.g., Perez et al., 2005, Biotechniques 39:109-15). Second, the size limit on adenovirus packaging is 105% (see, e.g., Bett et al., 1993, J Virol 67:5911-21), therefore even in the rare event of homologous recombination between the overlapping Ad sequences, the produced Ad would be too large to be efficiently packaged and propagated. Taking these points into consideration and knowing that the Ad genome is ˜36,000 bp, the preferred size of the spacer should not be smaller than 2000 bp, but preferably should be larger than 4000 bp and even more preferably should be larger than 6000 bp. 
     In one embodiment, the spacer sequence is inert (i.e. not expressing any transcripts). 
     In a preferred embodiment the spacer sequence contain several expression cassettes expressing different genes that are advantageous for Ad or protein production. The spacer element expresses one or more genes advantageous for adenovirus or protein production such as, but not limited to, anti-apoptotic genes, growth promoting genes, kinases and selectable markers. Anti-apoptotic genes include, but are not limited to, CrmA and Bcl-2. Growth promoting genes include, but are not limited to, dominant negative double-stranded RNA-dependent protein kinase (PKR), adenoviral VA gene, SV40 T-antigen gene and cytokines. In an advantageous embodiment, the gene is dominant negative double-stranded RNA-dependent protein kinase (PKR) or modified adenoviral VA gene. 
     The double-stranded (ds) RNA-dependent protein kinase (PKR), also termed eukaryotic translation initiation factor 2-alpha kinase 2 (EIF2AK2), (also termed DAI, p68, DsI, P1/eIF-2, and PRKR); GenBenk Accession: NM — 002759, has a central role in the mechanisms employed by the cell to counteract virus attacks (see, e.g., Proud, 1995, Trends Biochem Sci 20:241-6). PKR phosphorylates the translation initiation factor eIF-2 and inhibits its activity and so shutting down the translational machinery of the cell. However, PKR plays a role in normal control of cell growth and differentiation and so the amounts of phosphorylated and un-phosphorylated eIF-2 are in equilibrium. By shifting this equilibrium towards the un-phosphorylated form, the maximum translational capacity (i.e. protein production) of the cell could be harvested. There are several trans-dominant negative mutants of PKR (dnPKR) has been identified. It has been also demonstrated, that expression of dnPKR in a cell could increase protein production transiently (see, e.g., Donze et al., 1999, 322-9. Virology 256). It also has been demonstrated that retrovirus production can be increased by the use of PKR inhibitors (see, e.g., Pernod et al., 2004, Biotechniques 36:576-8, 580). 
     It has been also demonstrated, that transient expression of the adenoviral VA gene in a cell could increase protein production transiently (Akusjarvi et. al. (1987) Mol. Cel. Biol. 7, 549; Kaufman and Murtha (1987) Mol. Cel. Biol. 7, 1568). The permanent expression of VA gene in a cell line could be deleterious, and might need to be regulated. The adenoviral VA gene is transcribed by polymerase III through internal promoter sequences. Mutating these internal promoter sequences, such that the transcription specificity is changed from polymerase III to polymerase II allows for the operatively linking a regulatable polymerase II promoter. The expression level of the modified VA gene can be modulated by the use of an inducible polymerase II promoter system such as, but not limited to the tetracycline repressor or metallothionein promoter systems. 
     Selectable markers include, but are not limited to, neomycin, puromycin, zeomycin, bleomycin and ABC transporter. The one or more genes may be inserted into the spacer element randomly or may be inserted into the spacer element by site directed integration. 
     These advantageous gene sequences could be inserted into the cell-line genome randomly without the use of site directed integration. However, the preferred method would be site directed integration through the incorporation of these gene sequences into the spacer element. This method has the great advantage of predetermining the exact configuration, position and number of transcription elements. This greatly simplifies the later characterization of the cell-line according to regulatory requirements. 
     The invention also provides a system comprising the inventive cell and a replication-deficient adenoviral vector comprising an adenoviral genome deficient in the at least one essential gene function of the one or more regions (i.e., a replication-deficient adenoviral vector comprising the deficiencies complemented for by the inventive cell). The inventive cell complements the E1A and E1B deficiencies; other adenovirus vector deficiencies can be complemented or supplemented through the incorporation of the adenovirus genes into the cell genome. In the most preferred embodiment the adenovirus complementing genes that are missing or poorly expressed in the vector are incorporated into the spacer element(s) (e.g., E4, E4-ORF6, E2A, E2B, pIX, fiber, penton base, hexon). The invention further provides a method of propagating a replication-deficient adenoviral vector. The method comprises providing a cell of the invention, introducing the replication-deficient adenoviral vector into the cell, wherein the replication-deficient adenoviral vector comprises an adenoviral genome deficient in at least one essential gene function of one or more regions, and maintaining the cell (e.g., under conditions suitable for adenoviral propagation) to propagate the adenoviral vector. 
     The adenoviral vector is deficient in at least one gene function (of the adenoviral genome) required for viral propagation (i.e., an adenoviral essential gene function), thereby resulting in a “replication-deficient” adenoviral vector. The adenoviral vector is deficient in the one or more adenoviral essential gene functions complemented for by the inventive cell to allow for propagation of the replication-deficient adenoviral vector when present in the cell. 
     Preferably, the adenoviral vector is deficient in at least one essential gene function of the E1 region, e.g., the E1a region and/or the E 1b region, of the adenoviral genome that is required for viral replication. The recombinant adenovirus also can have a mutation in the major late promoter (MLP), as discussed in International Patent Application WO 00/00628. More preferably, the vector is deficient in at least one essential gene function of the E1 region and at least part of the nonessential E3 region (e.g., an Xba I deletion of the E3 region). The adenoviral vector can be “multiply deficient,” meaning that the adenoviral vector is deficient in one or more essential gene functions in each of two or more regions of the adenoviral genome. For example, the aforementioned E1-deficient or E1-, E3-deficient adenoviral vector can be further deficient in at least one essential gene of the E4 region and/or at least one essential gene of the E2 region (e.g., the E2A region and/or E2B region). Adenoviral vectors deleted of the entire E4 region can elicit lower host immune responses. Examples of suitable adenoviral vectors include adenoviral vectors that lack (a) all or part of the E1 region and all or part of the E2 region, (b) all or part of the E1 region, all or part of the E2 region, and all or part of the E3 region, (c) all or part of the E1 region, all or part of the E2 region, all or part of the E3 region, and all or part of the E4 region, (d) at least part of the E1a region, at least part of the E1b region, at least part of the E2a region, and at least part of the E3 region, (e) at least part of the E1 region, at least part of the E3 region, and at least part of the E4 region, and (f) all essential adenoviral gene products (e.g., adenoviral amplicons comprising ITRs and the packaging signal only). The adenoviral vector can contain a wild type pIX gene. Alternatively, although not preferably, the adenoviral vector also can contain a pIX gene that has been modified by mutation, deletion, or any suitable DNA modification procedure. In any of these embodiments the adenoviral sequences may lie within their normal context or be relocated to other regions of the vector or be in an alternative orientation. They may have been genetically modified to exploit codon degeneracy while maintaining the function of one or more encoded viral proteins. 
     The replication-deficient adenoviral vector can be generated by using any species, strain, subtype, or mixture of species, strains, or subtypes, of an adenovirus or a chimeric adenovirus as the source of vector DNA. The adenoviral vector can be any adenoviral vector capable of growth in a cell, which is in some significant part (although not necessarily substantially) derived from or based upon the genome of an adenovirus. The adenoviral vector preferably comprises an adenoviral genome of a wild-type adenovirus of group C, especially of serotype (i.e., Ad5). Adenoviral vectors are well known in the art and are described in, for example, U.S. Pat. Nos. 5,559,099, 5,712,136, 5,731,190, 5,837,511, 5,846,782, 5,851,806, 5,962,311, 5,965,541, 5,981,225, 5,994,106, 6,020,191, and 6,113,913, International Patent Applications WO 95/34671, WO 97/21826, and WO 00/00628, and Thomas Shenk, “Adenoviridae and their Replication,” and M. S. Horwitz, “Adenoviruses,” Chapters 67 and 68, respectively, in Virology, B. N. Fields et al., eds., 3d ed., Raven Press, Ltd., New York (1996). 
     The construction of adenoviral vectors is well understood in the art and involves the use of standard molecular biological techniques, such as those described in, for example, Sambrook et al., supra, Watson et al., supra, Ausubel et al., supra, and other references mentioned herein. Moreover, adenoviral vectors can be constructed and/or purified using the methods set forth, for example, in U.S. Pat. No. 5,965,358 and International Patent Applications WO 98/56937, WO 99/15686, and WO 99/54441. 
     When a cell is used to propagate a replication-deficient adenoviral vector, it is desirable to avoid a recombination event between the cellular genome (of the cell) and the adenoviral genome (of the adenoviral vector) that would result in the generation of a replication-competent adenovirus (RCA). As such, there is preferably insufficient overlap between the genome of the cell and the replication-deficient adenoviral vector genome to mediate a recombination event sufficient to result in a replication-competent adenovirus. If overlap exists, the overlapping sequences desirably are predominantly located in the nucleic acid flanking the coding region of the complementation factor (the “trans-complementing region”) in the cellular genome and the nucleotide sequences adjacent to the missing region(s) of the adenoviral genome. Ideally, there is no sequence overlap between the cellular genome and the adenoviral vector genome. However, it is acceptable that partial overlap exists between the sequences of the cellular genome and the adenoviral vector genome on one side of the trans-complementing region. In such an event, the region of homology preferably is contiguous with the trans-complementing region. The generation of RCA desirably is diminished such that (a) the cell produces less than about one replication-competent adenoviral vector for at least about 20 passages after infection with the adenoviral vector, (b) the cell produces less than about one replication-competent adenoviral vector in a period of about 36 hours post-infection, (c) the cell produces less than about one replication-competent adenoviral vector per 1×10 10  total viral particles (preferably 1×10 11  total viral particles, more preferably 1×10 12  total viral particles, and most preferably 1×10 13  total viral particles), or any combination of (a)-(c). Optimally, the amount of overlap between the cellular genome and the adenoviral genome (i.e., the genome of the adenoviral vector being propagated in the cell) is insufficient to mediate a homologous recombination event that results in a replication-competent adenoviral vector such that replication-competent adenoviruses are eliminated from the vector stocks resulting from propagation of the replication-deficient adenoviral vector in the cell. Virus growth yield and virus plaque formation have been previously described (see, e.g., Burlseson et al., Virology: a Laboratory Manual, Academic Press Inc. (1992)), and measuring RCA as a function of plaque forming units is described in U.S. Pat. No. 5,994,106. 
     Alternatively, the adenoviral vector is preferably conditionally replication deficient in at least one gene function required for viral replication in specific cells or tissues. Preferably, the adenoviral vector is deleted in at least one essential gene of the E1 region of the adenoviral genome, particularly the E1a region, more preferably, the vector is deficient in the retinoblastoma (Rb) binding site as described in U.S. Pat. No. 6,824,771. 
     It should be appreciated that the deletion of different regions of the adenoviral gene transfer vector can alter the immune response of the mammal, in particular, deletion of different regions can reduce the inflammatory response generated by the adenoviral gene transfer vector. Furthermore, the adenoviral gene transfer vector&#39;s coat protein can be modified so as to decrease the adenoviral gene transfer vector&#39;s ability or inability to be recognized by a neutralizing antibody directed against the wild-type coat protein, as described in International Patent Application WO 98/40509. Other suitable modifications to the adenoviral gene transfer vector are described in U.S. Pat. Nos. 5,559,099; 5,731,190; 5,712,136; and 5,846,782 and International Patent Applications WO 97/20051, WO 98/07877, and WO 98/54346. 
     Methods for making and/or administering a vector or recombinants or plasmid for expression of gene products of genes of the invention either in vivo or in vitro can be any desired method, e.g., a method which is by or analogous to the methods disclosed in, or disclosed in documents cited in: U.S. Pat. Nos. 4,603,112; 4,769,330; 4,394,448; 4,722,848; 4,745,051; 4,769,331; 4,945,050; 5,494,807; 5,514,375; 5,744,140; 5,744,141; 5,756,103; 5,762,938; 5,766,599; 5,990,091; 5,174,993; 5,505,941; 5,338,683; 5,494,807; 5,591,639; 5,589,466; 5,677,178; 5,591,439; 5,552,143; 5,580,859; 6,130,066; 6,004,777; 6,130,066; 6,497,883; 6,464,984; 6,451,770; 6,391,314; 6,387,376; 6,376,473; 6,368,603; 6,348,196; 6,306,400; 6,228,846; 6,221,362; 6,217,883; 6,207,166; 6,207,165; 6,159,477; 6,153,199; 6,090,393; 6,074,649; 6,045,803; 6,033,670; 6,485,729; 6,103,526; 6,224,882; 6,312,682; 6,348,450 and 6,312,683; U.S. patent application Serial No. 920,197, filed Oct. 16, 1986; WO 90/01543; WO91/11525; WO 94/16716; WO 96/39491; WO 98/33510; EP 265785; EP 0 370 573; Andreansky et al., Proc. Natl. Acad. Sci. USA 1996; 93:11313-11318; Ballay et al., EMBO J. 1993; 4:3861-65; Felgner et al., J. Biol. Chem. 1994; 269:2550-2561; Frolov et al., Proc. Natl. Acad. Sci. USA 1996; 93:11371-11377; Graham, Tibtech 1990; 8:85-87; Grunhaus et al., Sem. Virol. 1992; 3:237-52; Ju et al., Diabetologia 1998; 41:736-739; Kitson et al., J. Virol. 1991; 65:3068-3075; McClements et al., Proc. Natl. Acad. Sci. USA 1996; 93:11414-11420; Moss, Proc. Natl. Acad. Sci. USA 1996; 93:11341-11348; Paoletti, Proc. Natl. Acad. Sci. USA 1996; 93:11349-11353; Pennock et al., Mol. Cell. Biol. 1984; 4:399-406; Richardson (Ed), Methods in Molecular Biology 1995; 39, “Baculovirus Expression Protocols,” Humana Press Inc.; Smith et al. (1983) Mol. Cell. Biol. 1983; 3:2156-2165; Robertson et al., Proc. Natl. Acad. Sci. USA 1996; 93:11334-11340; Robinson et al., Sem. Immunol. 1997; 9:271; and Roizman, Proc. Natl. Acad. Sci. USA 1996; 93:11307-11312. 
     There are several embodiments of the promoters. In a first embodiment, the expression of one or more genes is driven by a constitutive promoter, an inducible promoter or a regulatable promoter. A constitutive promoter is selected from the group consisting of CMV, RSV, SV40, PKG and TK. An inducible promoter or regulatable promoter is selected from the group consisting of a metallothionein promoter or a tetracycline repressor (“tetR”) system. Advantageously, the promoter is a metallothionein promoter, preferably a sheep metallothionein promoter 1a gene promoter. 
       FIG. 5  depicts an E1B/pIX promoter region. 
     Permanent incorporation of a nucleic acid sequence into the cell genome, which upon expression produces dnPKR protein or adenovirus VA RNA can have beneficial effects on the production of a specific protein or adenovirus vector. It is possible that dnPKR or VA transcripts are produced from a constitutive promoter persistently without causing any deleterious effects to the cell line, however this is unlikely. Constitutive expression of dnPKR or VA is likely impossible as it would be deleterious to the cell-line stability and production features. Therefore, the incorporation of a regulatable system is preferred. 
     There have been many systems described in the prior art to regulate gene expression of a polymerase II transcribed gene. One of the possibilities is to use a repressible system like the use of the tetracyclin repressor (tetR) system described in U.S. Pat. No. 5,972,650. However, the preferred configuration is a simple inducible system. A preferred system is the sheep metallothionein 1a gene (sMT-Ia) promoter, which has a very low basal level and is inducible with zinc (see, e.g., Peterson et al., 1986, Eur J Biochem 160:579-85 and U.S. Pat. Nos. 5,851,806, 5,994,106 and 6,482,616). To use these polymerase II systems with the adenoviral VA gene its internal promoter sequences would need to be mutated to inactivate polymerase III transcription. 
     Polynucleotides comprising a desired sequence can be inserted into a suitable cloning or expression vector, and the vector in turn can be introduced into a suitable host cell for replication and amplification. Polynucleotides can be introduced into host cells by any means known in the art. The vectors containing the polynucleotides of interest can be introduced into the host cell by any of a number of appropriate means, including direct uptake, endocytosis, transfection, f-mating, electroporation, transfection employing calcium chloride, rubidium chloride, calcium phosphate, DEAE-dextran, or other substances; microprojectile bombardment; lipofection; and infection (where the vector is infectious, for instance, a retroviral vector). The choice of introducing vectors or polynucleotides will often depend on features of the host cell. 
     In view of the above, the method can further comprise subsequently repeating the administration of an adenoviral gene transfer vector comprising the exogenous gene encoding the gene product and/or a replication competent Ad vector with or without vector comprising the exogenous gene encoding the gene product to the appropriate tissue of the animal. 
     Thus, the inventive virions can be targeted to cells within any organ or system, including, for example, respiratory system (e.g., trachea, upper airways, lower airways, alveoli), nervous system and sensory organs (e.g., skin, ear, nasal, tongue, eye), digestive system (e.g., oral epithelium and sensory organs, salivary glands, stomach, small intestines/duodenum, colon, gall bladder, pancreas, rectum), muscular system (e.g., skeletal muscle, connective tissue, tendons), skeletal system (e.g., joints (synovial cells), osteoclasts, osteoblasts, etc.), immune system (e.g., bone marrow, stem cells, spleen, thymus, lymphatic system, etc.), circulatory system (e.g., muscles, connective tissue, and/or endothelia of the arteries, veins, capillaries, etc.), reproductive system (e.g., testes, prostate, uterus, ovaries), urinary system (e.g., bladder, kidney, urethra), endocrine or exocrine glands (e.g., breasts, adrenal glands, pituitary glands), etc or delivered systemically. These adenoviral vectors are capable of delivering gene products with high efficiency and specificity to cells expressing receptors which recognize the ligand component of the fiber-fibritin-ligand chimera. A person having ordinary skill in this art would recognize that one may exploit a wide variety of genes encoding e.g. receptor ligands or antibody fragments which specifically recognize cell surface proteins unique to a particular cell type to be targeted. 
     The invention further encompasses a method for administrating the adenovirus of the present invention propagated in the cell-line of the present invention to a subject in need thereof which may comprise administering to the subject in need thereof a therapeutically effective amount of the adenovirus described herein wherein the non-native amino acid targets the tumor cell such that the adenovirus infects the target cells. 
     The present invention can be practiced with any suitable animal, preferably the present invention is practiced with a mammal, more preferably, a human. Additionally, the adenoviral vector can be a gene transfer vector or a replication competent vector and can be administered, e.g., once, twice, or more, to any suitable tissue or delivered systemically to the animal. Systemic administration can be accomplished through intravenous injection, either bolus or continuous, or any other suitable method. 
     After subsequent administration of the adenoviral gene transfer vector comprising an exogenous gene, production of the gene product in the tissue of the animal is desirably at least 1% of (such as at least 10% of, preferably at least 50% of, more preferably at least 80% of, and most preferably, the same as or substantially the same as) production of the gene product after initial administration of the same adenoviral gene transfer vector containing the exogenous gene. Methods for comparing the amount of gene product produced in the tissue of administration are known in the art. The comparison can be made at the same time after the initial and subsequent administrations of the adenoviral gene transfer vector. 
     After subsequent administration of a replication competent adenoviral vector, replication of the vector in the tissue of the animal is desirably at least 1% of (such as at least 10% of, preferably at least 50% of, more preferably at least 80% of, and most preferably, the same as or substantially the same as) replication of the vector after initial administration. Methods for comparing the amount of adenovirus replication in the tissue of administration are known in the art. The comparison can be made at the same time after the initial and subsequent administrations of the adenoviral vector. 
     To facilitate the administration of adenoviral vectors, they can be formulated into suitable pharmaceutical compositions. Generally, such compositions include the active ingredient (i.e., the adenoviral vector) and a pharmacologically acceptable carrier. Such compositions can be suitable for delivery of the active ingredient to a patient for medical application, and can be manufactured in a manner that is itself known, e.g., by means of conventional mixing, dissolving, granulating, dragee-making, levigating, emulsifying, encapsulating, entrapping or lyophilizing processes. 
     Pharmaceutical compositions for use in accordance with the present invention can be formulated in a conventional manner using one or more pharmacologically or physiologically acceptable carriers comprising excipients, as well as optional auxiliaries, which facilitate processing of the active compounds into preparations, which can be used pharmaceutically. Proper formulation is dependent upon the route of administration chosen. Thus, for injection, the active ingredient can be formulated in aqueous solutions, preferably in physiologically compatible buffers. For transmucosal administration, penetrants appropriate to the barrier to be permeated are used in the formulation. Such penetrants are generally known in the art. For oral administration, the active ingredient can be combined with carriers suitable for inclusion into tablets, pills, dragees, capsules, liquids, gels, syrups, slurries, suspensions and the like. For administration by inhalation, the active ingredient is conveniently delivered in the form of an aerosol spray presentation from pressurized packs or a nebuliser, with the use of a suitable propellant. The active ingredient can be formulated for parenteral administration by injection, e.g., by bolus injection or continuous infusion. Such compositions can take such forms as suspensions, solutions or emulsions in oily or aqueous vehicles, and can contain formulatory agents such as suspending, stabilizing and/or dispersing agents. Other pharmacological excipients are known in the art. 
     Those of ordinary skill in the art can easily make a determination of the proper dosage of the adenoviral gene transfer vector. Generally, certain factors will impact the dosage that is administered; although the proper dosage is such that, in one context, the exogenous gene is expressed and the gene product is produced in the particular muscle of the mammal. Preferably, the dosage is sufficient to have a therapeutic and/or prophylactic effect on the animal. The dosage also will vary depending upon the exogenous gene to be administered. Specifically, the dosage will vary depending upon the particular muscle of administration, including the specific adenoviral vector, exogenous gene and/or promoter utilized. For purposes of considering the dose in terms of particle units (pu), also referred to as viral particles, it can be assumed that there are 100 particles per particle forming unit (pfu) (e.g., 1×10 12  pfu is equivalent to 1×10 14  pu). 
     The present invention also encompasses methods of repressing homologous recombination in a mammalian cell comprising inserting a non-homologous spacer element into an adenovirus nucleotide sequence. The methods of repressing undesirable homologous recombination events in a mammalian cell or their effects, advantageously a human embryonic kidney or 293 cell, encompasses all embodiments of the novel cell line described herein. 
     The invention will now be further described by way of the following non-limiting examples. 
     EXAMPLE 
     A highly efficient “base” expression cassette was designed and its level of expression tested with a luciferase gene for protein production capacity. 
     The spacer design is illustrated in  FIG. 6 . The spacer design consists of four expression cassettes (plus an extra poly A sequence terminating E1B) that inserted into the Ad chromosomal DNA sequence at nucleotide location 3511. The recombination plasmid includes the Ad homologous sequences and a TK expression cassette for negative selection. A short description of these cassettes is in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Expression cassettes 
               
            
           
           
               
               
               
               
               
            
               
                 Expression cassette 
                 # 
                 Description 
                 Purpose 
                 ~Size (bp) 
               
               
                   
               
               
                 sMTpr-IVS-mPKR-BGH 
                 1 
                 Regulatable promoter driving mPKR 
                 Up regulation 
                 2000 
               
               
                 SVpr-puro-bGlo 
                 2 
                 SV40 promoter driving puromyocin 
                 Selection 
                 1800 
               
               
                 TKpr-DHFR-TK 
                 3 
                 SV40 promoter driving DHFR 
                 Amplification 
                 1500 
               
               
                 EFpr-CrmA-CSF 
                 4 
                 EFa promoter driving CrmA 
                 Stabilization 
                 2000 
               
               
                 PKGpr-TK-TK 
                 5 
                 PKG promoter driving TK 
                 Recombination selection 
                 2000 
               
               
                 Total 
                   
                 Spacer only 
                 Minimum size 
                 7300 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Overview of cloning strategy 
               
            
           
           
               
               
               
               
               
               
            
               
                 Spacer Element 
                 Backbone 
                 Promoter 
                 CDS 
                 PolyA 
                 pRc cloning 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ad5 sequence 
                 pRcsMT-IVS- 
                 Generate PCR fragment from Ad5wt300 with blunt 
                 To be cloned 
               
               
                 2011-3510 
                 puro 
                 ends to clone into NruI site and hGH poly A to be 
                 into NruI site 
               
               
                   
                   
                 incorporated at the end of E1B 
               
            
           
           
               
               
               
               
               
               
            
               
                 sMT-PKR- 
                 pRcCMV-IVS- 
                 sMT 
                 PKR-BD 
                 BGH 
                 N/A 
               
               
                 BGHpolyA 
                 puro 
                 From pMT010/A 
                 From Origene 
                 Already 
               
               
                   
                   
                 replace CMV 
                 PCR 1-243aa 
                 present 
               
               
                   
                   
                 Insert NruI/BstXI 
                 frag; 
               
               
                   
                   
                   
                 Insert NotI/XbaI 
               
               
                 SVpr-puro- 
                 pSelect-puro 
                 SV40 
                 Puromycin 
                 bGlo 
                 To replace the 
               
               
                 bGlopolyA 
                   
                 To insert into 
                 Already present 
                 Already 
                 BamHI region - 
               
               
                   
                   
                 BspLU11I/AseI 
                   
                 present 
                 the inclusion 
               
               
                   
                   
                 sites 
                   
                   
                 of a multi- 
               
            
           
           
               
               
               
               
            
               
                 Lox::frt::attB:: 
                 pSelect-puro 
                 Generate fragment with Lox::frt::attB::phiC31::mcs 
                 cloning site 
               
               
                 phiC31::mcs 
                   
                 Mcs = multicloning site 
                 allows for 
               
               
                   
                   
                 Fragment generated with BamHI/EcorI ends to insert 
                 further cloning 
               
               
                   
                   
                 at 3′ end of BGlo poly A 
                 in this plasmid 
               
            
           
           
               
               
               
               
               
               
            
               
                 TKpr-DHFR- 
                 pGL3-basic 
                 TK 
                 mDHFR 
                 TK 
                 Cassette with 
               
               
                 TKpolyA 
                   
                 From pMEP4 
                 From pMT010A 
                 From pMEP4 
                 AvrII/NheI 
               
               
                   
                   
                   
                 HindIII/BglII 
                   
                 ends inserted 
               
               
                   
                   
                   
                   
                   
                 into NheI site 
               
               
                 hEF1pr-CrmA- 
                 pEF-BOS 
                 hEF1 
                 crmA 
                 G-CSF 
                 Cassette with 
               
               
                 G-CSFpolyA 
                 On order 
                 Already present 
                 Order from 
                 Already 
                 AvrII/NheI 
               
               
                   
                   
                   
                 Planetgene 
                 present 
                 ends inserted 
               
               
                   
                   
                   
                 Insert BstXI 
                   
                 into NheI site 
               
            
           
           
               
               
               
               
            
               
                 Ad5 sequence 
                 pRcsMT-IVS- 
                 Generate PCR fragment from Ad5wt300 with 
                 Cassette 
               
               
                 3511-5000 
                 puro 
                 AvrII/NheI ends for pRc cloning 
                 inserted into 
               
               
                   
                   
                   
                 NheI site 
               
            
           
           
               
               
               
               
               
               
            
               
                 PKGpr-TK- 
                 pNTK 
                 PKG 
                 TK 
                 TK 
                 Cassette 
               
               
                 TKpolyA 
                   
                 Already present 
                 Already present 
                 Already 
                 inserted into 
               
               
                   
                   
                   
                   
                 present 
                 AscI site 
               
               
                   
               
            
           
         
       
     
     Recombination sequences for the potential simple insertion of further expression cassettes for protein production are incorporated at a site between the puro and DHFR cassettes. This design allows for the efficient amplification of genomic sequences using methotrexate and puromycin. The insert incorporates the integration sequence elements: lox-frt-attB-phiC31. 
     Generation of the Spacer Cassette in pRc Backbone. 
     The pRc-Spacer contains all the cassettes described in Table 1 and illustrated in  FIG. 6 . pRcCMVpuro is starting backbone, and a synthetic intron (IVS) was inserted to improve expression of genes. In a sequential manner, various cassettes and additional sequences were generated. Thus far, three cassettes (1. sMTpr-IVS-mPKR-BGH, 2. SVpr-puro-bGlo and 3. TKpr-DHFR-TK), the 5′ Ad5 sequence and the Lox region were inserted. 
     Briefly, for the first cassette a truncated version of the eukaryotic translation initiation factor 2-alpha kinase 2 (EIF2AK2), PKR-BD, which is the 1-243 aa portion of PKR, performed the best in the pRcCMV-IVS-puro backbone. At this point, the CMV promoter in pRcCMV-IVS-puro was replaced with the sheep metallothionein promoter from the pMT010/A+vector which is induced in the presence of Zn. In front of this cassette, an Ad5 sequence from 2011-3510 a hGH polyA was inserted next to Ad5 nucleotide 3510 to prevent E1B read through. 
     A new puromycin cassette in pSelect-puro plasmid was created to include a SV40 promoter infront of the puromycin gene and utilized the plasmid BGlo polyA already present. The lox-phi sequence with a multi-cloning site was inserted following the BGlo polyA. This entire cassette was excised with BglII/BamHI and replaced the BamHI region in the pRc-IVS-puro backbone. The backbone containing both the Ad5 sequence and new puromycin cassette was named pRc-Ad5-ec1-2-lox. The puromycin cassette was confirmed to be functional by transforming  E. coli  and seeding onto agar-puro plates. 
     In addition the insertion of the TKpr-DHFR-TK cassette was completed, which was originally generated in the pGL3-basic backbone. Once all the pieces had been cloned into the pGL3 backbone, the TK promoter, murine DHFR fragment and TK polyA, and inserted an AvrII site, the cassette was excised from pGL3 with NheI/AvrII and inserted into the unique NheI site in the Lox-Phi-MCS fragment. This version of the plasmid is known as pRc-Ad5-ec1-3-lox. DHFR was determined to be expressed from the pGL3-TKpr-DHFR-TKpolyA plasmid through western blot analysis with the reagents. Although alternate western blot reagents were optimized for hCrmA expression, there were problems with the DHFR detection. However, in the meantime, RT-PCR analysis confirmed DHFR expression from the pRcAd5-ec1-3-lox plasmid ( FIG. 7 ). In this analysis, 293 cells were transfected with 2 ug of the appropriate DHFR expressing plasmid, pMT010/a+, pGL3-TKpr-DHFR-TKpA or pRcAd5-ec1-3-lox or mock transfected. After 48 hours cells were harvested and 1 ug of RNA used in a one-step RT-PCR reaction with mDHFR specific primers. Faint bands in lanes 2 &amp; 3 do indicate these primers can also detect hDHFR but is clear all plasmid transfected cells expressed DHFR. 
     Analysis of the Lox Region 
     While the puromycin cassette functionality was confirmed by transforming  E. coli  and plating onto agar-puro plates, the Lox region was also analyzed, initially in pRc-Ad5-ec1-2-lox, and then the final pRc-Spacer plasmid. To do so, a Cre recombinase expressing plasmid and a plasmid containing lox sites flanking a gene of interest was needed. 
     The generation of the Cre recombinase expressing plasmid, pRcCMV-IVS-Cre-puro was completed by inserting a PCR fragment of Cre from the AdCMVCre obtained from GTC. Cre expression and functionality were tested by co-transfecting 293 cells with pRcCMV-IVS-Cre-puro and pBluescript-LL-eGFP-CMV (illustrated in  FIG. 8 ). The construction of pBluescript-LL-eGPF-CMV was completed through blunt end cloning of the CMV promoter into the EcoRV site and eGPF-polyA into the SmaI site of pBluescript-LL. If the Cre recombinase works then the eGFP-CMV sequence will rearrange bringing eGFP under control of CMV and cells are fluorescent. pRcCMV-IVS-Cre-puro works as illustrated in  FIG. 9 . When cells were transfected with 2 ug of pBluescript-LL-eGFP-CMV (shortened name, pBS-LL-eGFP-CMV) there was no fluorescense. However when they were co-transfected with 0.5 ug of pRcCMV-IVS-Cre-puro (pRc-Cre shortened name) there were cells fluoresced indicating that pRc-Cre expressed functional Cre recombinase. 
     With respect to point a plasmid containing lox sites flanking a gene of interest, human growth factor was used as a gene of interest. pBluescript II KS (+) was the backbone plasmid for generating the test vector, pBS-Lox-RSVpr-GH-Lox, and controls, pBS-Lox-OGH-Lox and pBluescript-LL. Furthermore, pBS-Lox-OGH-Lox can be used to compare different promoters for expression of hGH, while pBluescript-LL is flexible to allow different promoter/transgene expression. Southern blotting is used for detecting successful cre/lox mechanics. 
     CrmA Cassette 
     A CrmA cassette was previously generated using the pEF-Bos plasmid, which contains the hEF1 promoter and a 3′UTR with GM-CSF polyA. Expression of CrmA was confirmed through western blot analysis using the BD Biosciences anti-CrmA antibody as the probe with the new western blot gels and reagents. The pEF-hCrmA plasmid was altered to contain a 5′ AvrII site, and 3′ NheI site so that the entire cassette could be excised through AvrII/NheI. This fragment was inserted into NheI linearized pRc-Ad5-ec1-3-lox. After determining it was necessary to switch to another type of electrocompetent bacteria, it was confirmed through enzyme digests and sequencing that pRc-Ad5-ec1-4-lox was generated. In this instance, STBL2 electrocompetent bacteria was used, but STBL4 may be used for future cloning. 
     Ad5 Sequence 3511-5000 
     A second piece of the Ad5 sequence, Ad5seq3511-5000, was generated flanked by AvrII and NheI ends, using PCR methods and ligated the fragment into NheI linearized pRc-Ad5-ec1-4-lox. This plasmid is now known as pRc-Adrec-ec1-4-lox (see  FIG. 11 ) and is ready for insertion of the final cassette. 
     PGK-TK Cassette 
     With respect to the final piece, the PGK-TK cassette, a plasmid called pNTK was obtained and TK expression was confirmed in 293 cells transfected with 2 ug of pNTK using western blot analysis. The functionality of TK expressed from pNTK was confirmed in a cytoxicity assay (see  FIG. 10 ). 
     In this analysis 293 cells plated in a 96 well plate were mock transfected or transfected with 0.25 ug, 0.5 ug or 1 ug of pNTK plasmid. After 5 hours, cells were refeed with normal media, and then 16 hours later were treated with ganciclovir at the doses indicated. Cytoxicity was measured at 48 hours following ganciclovir treatment using the WST-1 reagent. As can be seen in  FIG. 5 , at the 5 ug/ml and 50 ug/ml doses, there was a large difference between mock transfected cells and those treated with pNTK plasmid indicating that TK is functional. 
     To generate the final pRc-Sapcer plasmid (as shown in  FIG. 11 ), the initial plan was to excise the cassette with XbaI from pNTK and in the first instant use adaptor oligos to generate AscI ends on this cassette for cloning into the AscI site in the Lox multi-cloning site. However, due to various difficulties, the cassette was amplified from pNTK and cloned into NheI/PacI in pRc-Adrec-ec1-4-lox. As the plasmid has been sequenced sequentially throughout the cloning period, the sequence of pRc-Spacer is confirmed with a series of digests and PCR on the various cassettes. 
     Generation of Test Virus 
     To analyze the properties of the proposed 293 cell line, a test virus is required. Ad5.eGFP is synthesized by recombining a shuttle vector containing eGFP (enhanced GFP) and pAdEasy vector. The resulting recombinant genome is initially rescued on standard 293 cells, and then the primary lysate stock stored until a comparison can be made on the new cells line versus a standard 293 cell line for RCA breakthrough. 
     The invention is further described by the following numbered paragraphs: 
     1. A mammalian cell comprising a non-homologous spacer element inserted into an adenovirus nucleotide sequence, wherein the spacer element represses undesirable homologous recombination. 
     2. The mammalian cell of paragraph 1 wherein the cell is a human embryonic kidney cell. 
     3. The mammalian cell of paragraph 1 or 2 wherein the spacer element is inserted by site directed integration at a specific site in the adenovirus nucleotide sequence. 
     4. The mammalian cell of paragraph 3 wherein the site is after the end of the E1B transcription unit but in front of the pIX transcription unit. 
     5. The mammalian cell of paragraph 3 wherein the site is after the ITR and packaging sequences but before the E1A transcription start site. 
     6. The mammalian cell of paragraph 3 wherein the site is after the E1A sequences but before the E1B sequences. 
     7. The mammalian cell of any one of paragraphs 1 to 6 wherein the spacer element comprises one or more regulatory elements. 
     8. The mammalian cell of paragraph 7 wherein the one or more regulatory elements are selected from the group consisting of promoters, enhancers, insulators, polyadenylation and termination signals. 
     9. The mammalian cell of any one of paragraphs 1 to 8 wherein the spacer element is about 2000 to about 3000 base pairs. 
     10. The mammalian cell of any one of paragraphs 1 to 8 wherein the spacer element is at least about 2000 base pairs. 
     11. The mammalian cell of any one of paragraphs 1 to 8 wherein the spacer element is at least about 4000 base pairs. 
     12. The mammalian cell of any one of paragraphs 1 to 8 wherein the spacer element is at least about 6000 base pairs. 
     13. The mammalian cell of any one of paragraphs 1 to 12 wherein the spacer element comprises one or more integration sequence elements. 
     14. The mammalian cell of paragraph 13 wherein the one or more integration sequence elements is selected from the group consisting of lox, frt, attB phiC31 integration site sequences. 
     15. The mammalian cell of any one of paragraphs 1 to 14 wherein the spacer element does not express any genes. 
     16. The mammalian cell of any one of paragraphs 1 to 14 wherein the spacer element expresses one or more genes advantageous for adenovirus or protein production. 
     17. The mammalian cell of paragraph 16 wherein the one or more genes is selected from the group consisting of anti-apoptotic genes, growth promoting genes, kinases and selectable markers. 
     18. The mammalian cell of paragraph 17 wherein the anti-apoptotic genes are selected from the group consisting of CrmA and Bcl-2. 
     19. The mammalian cell of paragraph 17 wherein the growth promoting genes are selected from the group consisting of dominant negative double-stranded RNA-dependent protein kinase (PKR), adenoviral VA gene, SV40 T-antigen gene and cytokines. 
     20. The mammalian cell of paragraph 17 wherein the selectable markers are selected from the group consisting of neomycin, puromycin, zeomycin, bleomycin and ABC transporter. 
     21. The mammalian cell of any one of paragraphs 16 to 20 wherein the expression of one or more genes is driven by a constitutive promoter, an inducible promoter or a regulatable promoter. 
     22. The mammalian cell of paragraph 21 wherein the constitutive promoter is selected from the group consisting of CMV, RSV, SV40, PKG and TK. 
     23. The mammalian cell of paragraph 21 wherein the inducible promoter or regulatable promoter is selected from the group consisting of a metallothionein promoter or a tetR system. 
     24. The mammalian cell of any one of paragraphs 16 to 23 wherein the one or more genes are inserted into the spacer element randomly. 
     25. The mammalian cell of any one of paragraphs 16 to 23 wherein the one or more genes are inserted into the spacer element by site directed integration. 
     26. The mammalian cell of any one of paragraphs 16 to 25 wherein the one or more genes is a dominant negative double-stranded RNA-dependent protein kinase. 
     27. The mammalian cell of any one of paragraphs 15 to 26 wherein the one or more genes is a mutant adenoviral VA gene. 
     28. The mammalian cell of paragraph 26 or 27 wherein the expression of a dominant negative double-stranded RNA-dependent protein kinase or mutant adenoviral VA gene is driven by a tetracycline repressor system. 
     29. The mammalian cell of paragraph 26 or 27 wherein the expression of a dominant negative double-stranded RNA-dependent protein kinase or mutant adenoviral VA gene is driven by a metallothionein promoter. 
     30. The mammalian cell of paragraph 29 wherein the metallothionein promoter is sheep metallothionein promoter 1a gene promoter. 
     31. The mammalian cell of any one of paragraphs 1 to 30 comprising at two or more non-homologous spacer elements inserted into the adenovirus nucleotide sequence. 
     32. The mammalian cell of paragraph 31 wherein the two or more non-homologous spacer elements are identical. 
     33. The mammalian cell of paragraph 32 wherein the two or more non-homologous spacer elements are different. 
     33. A method of repressing homologous recombination events in a mammalian cell comprising inserting a non-homologous spacer element into an adenovirus nucleotide sequence. 
     34. The method of paragraph 33 comprising the mammalian cells of any one of paragraphs 1 to 32. 
     35. A mammalian cell comprising a non-homologous spacer element inserted into an adenovirus nucleotide sequence, wherein the spacer element reduces the presence of replication-competent adenovirus (RCA) in adenovirus vector production. 
     36. The mammalian cell of paragraph 35 wherein the frequency of RCA generated in adenovirus vector production is reduced by at least ten fold compared to production in the parental mammalian cell without a non-homologous spacer element. 
     37. The mammalian cell of paragraph 35 wherein the frequency of RCA generated in adenovirus vector production is reduced by at least hundred fold compared to production in the parental mammalian cell without a non-homologous spacer element. 
     38. The mammalian cell of paragraph 35 wherein the cell is a human embryonic kidney cell. 
     39. The mammalian cell of any of the paragraph 35 to 38 wherein the spacer element is inserted by site directed integration at a specific site in the adenovirus nucleotide sequence. 
     40. The mammalian cell of paragraph 39 wherein the site is after the end of the E1B transcription unit but in front of the pIX transcription unit. 
     41. The mammalian cell of paragraph 39 wherein the site is after the ITR and packaging sequences but before the E1A transcription start site. 
     42. The mammalian cell of paragraph 39 wherein the site is after the E1A sequences but before the E1B sequences. 
     43. The mammalian cell of any one of paragraphs 35 to 42 wherein the spacer element comprises one or more regulatory elements. 
     44. The mammalian cell of paragraph 43 wherein the one or more regulatory elements are selected from the group consisting of promoters, enhancers, insulators, polyadenylation and termination signals. 
     45. The mammalian cell of any one of paragraphs 35 to 44 wherein the spacer element is about 2000 to about 3000 base pairs. 
     46. The mammalian cell of any one of paragraphs 35 to 44 wherein the spacer element is at least about 2000 base pairs. 
     47. The mammalian cell of any one of paragraphs 35 to 44 wherein the spacer element is at least about 4000 base pairs. 
     48. The mammalian cell of any one of paragraphs 35 to 44 wherein the spacer element is at least about 6000 base pairs. 
     49. The mammalian cell of any one of paragraphs 35 to 48 wherein the spacer element comprises one or more integration sequence elements. 
     50. The mammalian cell of paragraph 49 wherein the one or more integration sequence elements is selected from the group consisting of lox, frt, attB phiC31 integration site sequences. 
     51. The mammalian cell of any one of paragraphs 35 to 50 wherein the spacer element does not express any genes. 
     52. The mammalian cell of any one of paragraphs 35 to 50 wherein the spacer element expresses one or more genes advantageous for adenovirus or protein production. 
     53. The mammalian cell of paragraph 52 wherein the one or more genes is selected from the group consisting of anti-apoptotic genes, growth promoting genes, kinases and selectable markers. 
     54. The mammalian cell of paragraph 53 wherein the anti-apoptotic genes are selected from the group consisting of CrmA and Bcl-2. 
     55. The mammalian cell of paragraph 52 wherein the growth promoting genes are selected from the group consisting of dominant negative double-stranded RNA-dependent protein kinase (PKR), adenoviral VA gene, SV40 T-antigen gene and cytokines. 
     56. The mammalian cell of paragraph 52 wherein the selectable markers are selected from the group consisting of neomycin, puromycin, zeomycin, bleomycin and ABC transporter. 
     57. The mammalian cell of any one of paragraphs 52 to 56 wherein the expression of one or more genes is driven by a constitutive promoter, an inducible promoter or a regulatable promoter. 
     58. The mammalian cell of paragraph 57 wherein the constitutive promoter is selected from the group consisting of CMV, RSV, SV40, PKG and TK. 
     59. The mammalian cell of paragraph 57 wherein the inducible promoter or regulatable promoter is selected from the group consisting of a metallothionein promoter or a tetR system. 
     60. The mammalian cell of any one of paragraphs 52 to 59 wherein the one or more genes are inserted into the spacer element randomly. 
     61. The mammalian cell of any one of paragraphs 52 to 59 wherein the one or more genes are inserted into the spacer element by site directed integration. 
     62. The mammalian cell of any one of paragraphs 52 to 61 wherein the one or more genes is a dominant negative double-stranded RNA-dependent protein kinase. 
     63. The mammalian cell of any one of paragraphs 51 to 62 wherein the one or more genes is a mutant adenoviral VA gene. 
     64. The mammalian cell of paragraph 62 or 63 wherein the expression of a dominant negative double-stranded RNA-dependent protein kinase or mutant adenoviral VA gene is driven by a tetracycline repressor system. 
     65. The mammalian cell of paragraph 62 or 63 wherein the expression of a dominant negative double-stranded RNA-dependent protein kinase or mutant adenoviral VA gene is driven by a metallothionein promoter. 
     66. The mammalian cell of paragraph 65 wherein the metallothionein promoter is sheep metallothionein promoter 1a gene promoter. 
     67. The mammalian cell of any one of paragraphs 35 to 66 comprising at two or more non-homologous spacer elements inserted into the adenovirus nucleotide sequence. 
     68. The mammalian cell of paragraph 67 wherein the two or more non-homologous spacer elements are identical. 
     69. The mammalian cell of paragraph 68 wherein the two or more non-homologous spacer elements are different. 
     70. The mammalian cell of any one of paragraphs 35 to 69 wherein is the reduced frequency is due to inefficient packaging and propagation. 
     71. The mammalian cell of paragraph 70 wherein the inefficient packaging and propation is due to the resulting size of the recombinant non-homologous spacer element inserted into the adenovirus nucleotide sequence. 
     72. A method of reducing the frequency of generation of RCA in a mammalian cell comprising inserting a non-homologous spacer element into an adenovirus nucleotide sequence. 
     73. The method of paragraph 72 comprising the mammalian cells of any one of paragraphs 35 to 71. 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the above paragraphs is not to be limited to particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope of the present invention.