METHODS AND MEANS FOR TARGETED GENE DELIVERY

Abstract: The present invention provides a method of producing viral gene delivery vehicles which can be transferred to preselected cell types by using targeting conjugates. The gene delivery vehicules comprise: 1) the gene of interest (to be delivered); 2) a viral capsid or envelope carrying a member of a specific binding pair, the counterpart of which is not directly associated with the surface of the target cell. These vehicles can be made unable to bind to their natural receptor on the cell. The targeting conjugates are composed of the counterpart member of the specific binding pair linked to a targeting moiety which is a cell-type specific ligand (or fragments thereof). The number of the specific binding pair present on the viral vehicles can be for example an immunoglobulin binding moiety (e.g. capable of binding to a Fc fragment, protein A, protein G, FcR or an anti-Ig antibody), or biotin, avidin or streptavidin. The outer membrane or capsid of the virus may contain a substance which mediates entrance of the gene delivery vehicle into the target cell. Due to the specificity of the ligand, the high affinity of the binding pair and to the inability of the gene delivery vehicle to be targeted when used alone, the universality of the method for gene delivery, together with its high cell type selectively can easily be achievied by the use of various adequate targeting conjugates.

Description: Title: Methods and means for targeted gene delivery. Field of the Invention; The present invention relates to the field of providing cells with additional genetic information through recombinant DNA techniques. It especially relates to methods and means of providing specific groups of cells with additional genetic information, in particular in the context of gene therapy. Gene therapy is a recently developed concept which has a very broad range of applications, ranging from supplementing deficiencies in a mammals' set of proteins, usually resulting from genetic disorders, to the treatment of cancer, (auto-)immune diseases or (viral) infections, usually by eliminating or suppressing the responsible set of cells or organisms. One of the main problems in gene therapy is delivering the genetic material to the target cells and not to other cells. Another problem in gene therapy is that certain cell types are extremely refractory to current gene transfer techniques. The present invention relates generally to gene delivery vehicles and their use in gene therapy, and more particularly, to recombinant viruses which can be targeted to susceptible target cells. Retroviruses are RNA viruses which efficiently integrate their genetic information into the genomic DNA of infected cells via a reverse-transcribed DNA intermediate. This property of their life-cycle and the fact that parts of their genetic material can be replaced by foreign DNA sequences make retroviruses one of the most promising vectors for the delivery of genes in human gene therapy procedures, most notably for gene therapies which rely on gene transfer into dividing tissues. Most retroviral vector systems are based on mouse retroviruses and consist of two components, i.e. (i) the recombinant retroviral vector carrying the foreign sequences of interest, and (ii) so-called packaging cells expressing the structural viral proteins of which the encoding sequences are lacking in the retroviral vector. Expression of (i) in (ii) results in the production of recombinant retroviral particles capable of transducing susceptible target cells. The infectivity and host cell range of the retrovirus particle is conferred by an envelope glycoprotein which specifically binds to a receptor molecule on the target cell membrane. The envelope glycoprotein of all known retroviruses consists of two associated peptides, which are derived by proteolytic cleavage from the same precursor protein encoded by the retroviral env gene (Dickson et al. in Weiss et al. (ed. ) Molecular biology of tumor viruses (1984), Cold Spring Harbor Press, pp.513-648) . The amino terminal peptide encompasses the specific binding site for its receptor on the target cell membrane, thus determining the virus host range (Hunter and Swanstrom, Curr. Top. Microbiol. Immunol. 157(1990) :187) . The carboxy terminal peptide, which contains trans-membrane anchor sequences, is assumed to account for the selective uptake of the envelope glycoprotein in the virus particle and to mediate fusion between the virus membrane and - depending on the type of virus - the plasma membrane or intracellular vesicle membrane of the target cell. Several envelope glycoprotein variants with different infection spectra for mammalian cells have been identified. All known env variants have a rather broad infection spectrum in common. Here lies one of the major shortcomings of current recombinant retrovirus technology. In numerous gene therapy applications targeted delivery of genes into defined cells would be desired, most notably in the case of in vitro gene transfer into cell types present with low abundance in cell mixtures and in approaches for in vivo gene transfer into cells in a living mammalian body. Conventional gene transfer techniques have the disadvantage in such applications of low efficiency of gene transfer to RECTIFIED SHEET (RULE 91) ISA/EP desired target cells, because the gene transfer vehicles are taken up by other cells as well. In addition, the cell mixture or living mammalian body may contain cells to which gene transfer is absolutely undesired. E.g., genes providing protection against chemotherapeutic drugs should not be transferred into malignant cells. Therefore, increasing attention is focused on devising procedures to limit the retrovirus infection spectrum. By employing particular env variants the transduction spectrum can be limited to some extend, but true specificity for most target cells of interest can not be obtained this way. On the other hand, on the surface of some specific cell types the expression of receptors for retroviral entry is extremely low. An important example of a cell type with low retrovirus receptor expression is the pluripotent stem cell of the hematopoietic system (Orlic et al. , Blood 86 suppl 1 (1995) :628a) . A preferred strategy to accomplish targeted delivery is to direct the retrovirus particle to cell membrane molecules differing from the natural envelope glycoprotein receptor, said molecule being specifically expressed on the membrane of the desired target cell . Present ideas about how this could be done include: 1. direct chemical coupling of a ligand for a target cell molecule to the viral envelope glycoprotein (Neda et al, J. Biol. Chem. 266 (1991) : 14143) , 2. bridging the viral envelope glycoprotein to a molecule on the target cell through a complex of two antibodies, one directed against the viral envelope glycoprotein and the other against the molecule on the target cell (Goud et al., Virology 163 (1988) :251; Roux et al. , Proc. Natl. Acad. Sci. USA 86(1989) : 9079; Etienne-Julan et al., J. Gen. Virol. 73(1992) : 3251) , 3. bridging the viral envelope glycoprotein to a molecule on the target cell through a complex of an antibody directed against the viral envelope glycoprotein and a peptide ligand for the molecule on the target cell (Etienne-Julan et al., J. Gen. Virol. 73 (1992): 3251) RECTIFIED SHEET (RULE 91) ISA/EP 4. replacing the specific binding site of the viral envelope glycoprotein for its receptor by a peptide ligand for a target cell surface molecule, 5. co-expression on the virus membrane of the natural viral envelope glycoprotein and a ligand for a target cell surface molecule (Young et al., Science 250(1990) :142l) , and 6. co-expression on the virus membrane of the natural viral envelope glycoprotein and an altered viral envelope glycoprotein in which the specific binding site for its receptor has been replaced by a peptide ligand for a target cell surface molecule (Young et al., Science 250(1990) :1421; Russell et al., ucl. Acid Res. 5(1993) :1081; Chu et al.. Gene Ther. 1(1994) :292; Kasahara et al., Science 266(1994) -1373; Chu and Domburg, J. Virol. 69(1995) :2659) . Monoclonal antibodies or fragments thereof, exhibiting high specificity and affinity for the target cell specific molecule, are amongst the preferred ligands for targeted delivery. Approaches nos. 2 and 3 mentioned above rely on antibodies and promising tools for use in the approaches nos. 4 and 6 are chimaeric molecules between viral envelope glycoproteins and single-chain antibody fragments of the variable antigen-binding domain of immunoglobulins (scFv) (Russell et al., Νucl. Acid Res. 5 (1993) :1081; Chu et al., Gene Ther. 1(1994) :292; Chu and Domburg, J. Virol. 69 (1995) :2659) . A chimaeric molecule of such an scFv fragment and a different membrane anchoring protein than the viral envelope glycoprotein could be used for approach no. 5. An important limitation of all these previously invented approaches is that a new virus with a specific targeting ligand (chemically or genetically modified envelope glycoprotein, or co-expressed ligand) or a new specific dual-antibody complex has to be made for each target cell type. The present invention bypasses this limitation. Adenovirus capsids are regular icosahedrons composed of 252 subunits, of which 240 are so-called hexons and 12 are so-called pentons. The pentons are located at the vertices of the icosahedron. They contain a penton base on the surface of the capsid which is composed of five molecules of a 85 kD polypeptide. A fiber composed of a homotrimer of 62 kD polypeptides projects from the penton base outward. The fiber protein is responsible for attachment of the adenovirus to its receptor (reviewed by Horwitz, in: Virology, 2nd edition (Fields et al, ed) , Raven Press, New York, 1990, pp.1679-1721) . By exchanging fiber protein domains from two adenoviruses of different serotype, Stevenson et al. (J. Virol. 69 (1995) :2850) have shown that the receptor specificity is determined by the head domain of the fiber protein. The adeno-associated virus (AAV) capsid is comprised of three viral proteins (VP) : VP-1, VP-2, and VP-3. These proteins have molecular masses of 87 kD, 73 kD, and 62 kD, respectively. In mature virions VP-l, VP-2 and VP-3 are found at relative abundance of approximately 1 : 1 : 10. In vitro,, the three proteins assemble spontaneously into virion-like structures. It appears, therefore, that capsid formation in infected cells proceeds independent of viral DNA synthesis (reviewed by Kotin, Hum. Gene Ther. 5 (1994) :793) . It has been shown possible to insert sequences into the genes encoding the capsid proteins, resulting in the exposure of His-residues on the surface of intact AAV capsids. Consequently, these altered AAV virions were able to bind to a nickel-column (unpublished reεults from the group of Dr. R. Samulski, Univ. of North Carolina, Chapel Hill, NC) . The present invention makes use of the possibility to insert sequences in said capsid genes. The present invention thus provides a gene delivery vehicle for delivering genetic material to a target cell, comprising an expressable recombinant nucleic acid molecule encoding a gene product of interest, a capsid or an envelope derived from a virus, and a member of a specific binding pair, the counterpart of which is not directly associated with the surface of the target cell. This gene delivery vehicle itself is preferably incapable of specifically binding to a target cell, meaning that it is no longer an infectious virus particle. Instead of having its regular infectivity it is provided with a member of a specific binding pair, either as a part of its envelope or as a part of its capsid. This gene delivery vehicle in itself cannot be targeted to a target cell, but it is a novel and inventive intermediate in the process of preparing a targeting gene delivery vehicle, which together with a targeting component is capable of delivering a gene to target cells only, or at least of delivering genes to the target cells in a far greater amount than to non-target cells (or other targets, such as infectious organisms) , or of delivering a gene to cells previously difficult to transduce as a result of low viral receptor expression. The invention thus also provides a targeting component which is a conjugate for use in targeting a gene delivery vehicle as disclosed above to a target cell, which conjugate comprises the counterpart of the member of a specific binding pair (which member is present on or in the gene delivery vehicle, which counterpart is coupled to a targeting moiety being capable of binding to a target molecule associated with the surface of a target cell. Thus the conjugate has dual specificity. On the one side the counterpart recognizes the member of a specific binding pair present on or in the gene delivery vehicle and on the other side it recognizes a target molecule associated with the surface of the target cell. When such a conjugate is coupled to a gene delivery vehcle a targeting gene delivery vehicle is obtained. Thus the invention also provides a targeting gene delivery vehicle, comprising a gene delivery vehicle according as disclosed hereinabove, coupled to a conjugate according also described above. The main advantage of the gene delivery vehicles according to the invention is that they have a recognizable moiety, which is independent from the target cell the genes have to be delivered to. This means that they can be targeted to many different target cells, by using different conjugates which all recognize the vehicle (a constant part of the conjugate) and which recognize a number of different targets associated with target cells on the other side. Thus the gene of interest can be delivered to many different cells, without having to prepare a new vehicle. A similar approach has been disclosed and is described above as approach no.3. In this approach however, the vehicle has not been provided with an additional member of a specific binding pair, but a viral antigen (the envelope glycoprotein) itself is the member of a specific binding pair being recognized by an antibody. Apart from the drawback of having to make a new specific dual-antibody complex for every delivery system, an even more important drawback is that all antigens have to be bound to an antibody, because otherwise the vehicle will retain its capability of infecting its regular host cells, whereas for the gene delivery vehicles according to the invention these glycoproteins are preferably (if not necessarily) not present or altered to impair their normal function. In the following it will become clear that preferred embodiments of the present invention are so called kits (kits of parts) which provide a gene delivery vehicle as one part and at leaεt one targeting moiety as another part. Another part of the invention therefor is a kit of parts for delivering genetic material to a target cell, compriεing a gene delivery vehicle according to the invention and at leaεt one conjugate of the counterpart member of the specific binding pair coupled to a targeting moiety, said targeting moiety being capable of binding to a target molecule associated with the surface of a target cell. The embodiments given above are the general formats of the invention. Thus it is clear that as well a gene delivery vehicle itself, as well as a conjugate, the delivery vehicle coupled to a conjugate and the kit in which both conjugate and delivery vehicle all can be adjusted to the specific embodiments which are recited for the kits only. Thus the specific conjugates without the delivery vehicle and the specific vehicles without the conjugates as well as the specific targeting vehicles are part of this invention as well. For example where a kit of parts is disclosed comprising as a member an immunoglobulin binding protein and as its counterpart an immunoglobulin, this means that the conjugate comprising the immunoglobulin is part of the invention as is the gene delivery vehicle comprising the immunoglobulin binding protein. The definitions in the claims are themselves clearly understood in the art. Specific binding pairs for instance are considered to be any two molecules which have a high affinity specific interaction between the two members (herein often referred to as the member and its counterpart) . These may be antibody (fragments or derivatives) and the corresponding antigen, receptors and their ligands (in this case especially proteinaceous ligands), well known binding pairs such as avidin/biotin or streptavidin/biotin, peptide structures that can specifically interact in solution and the like. In many instances members of these binding pairs will not be normally present in the envelope or the capsid of a virus and will thus also not be normally present on the gene delivery vehicles according to the invention, For that purpose they may have to be modified in a suitable manner, for instance chemically, or they may need to be present as fusion molecules or hybrid molecules linked to a component which ensures their presence in or on the capsid or envelope. As long as they still have the same function (in kind not in amount) as the original member of the binding pair (i.e. they still bind the counterpart) they should be RECTIFIED SHEET (RULE 91) ISA/EP considered as being a derivative or equivalent of the original member of the binding pair and are thus paret of the present invention. The εame goeε for the counterpart of the member of the εpecific binding pair and alεo for the targeting moiety, which iε the other side of the conjugate. Hereinunder a number of poεεibilitieε of providing a capsid or an envelope with the function of a member of a specific binding pair are given, exemplified for the preferred vehicles, but skilled artisianε will be able to tranεfer theεe teachingε to other vehicleε and/or other memberε of εpecific binding pairε aε well as other methods of providing these memberε on envelopeε and/or capsids without departing from the present invention. A conjugate is defined for the purposes of the present invention as Any molecule which has at least two different specific recognition εiteε, which are εomehow linked. Thiε means that it does not necessarily include chemical coupling or fusion proteins, although these are conjugates according to the invention, but that it also includes molecules which normally have this double specific recognition, such as for instance antibodies which recognize an antigen specifically and of which the constant region is specifically recognized by for instance Fc receptors. A nucleic acid encoding a gene of interest should be interpreted as not only encoding proteins, but also antisense nucleic acids and other useful nucleic acids, although these are not uεually conεidered to be genes. When the nucleic acids do encode proteins there are many known geneε of intereεt in the field, particularly in the field of gene therapy. All these genes can be delivered to target cellε uεing the methodε and meanε of the present invention. Of particular interest are suicide genes, such as Herpeε Simplex Viruε Thymidine Kinaεe and others disclosed in WO 93/07281. The target cells to which the genes of interest can be delivered are basically any target cells that have a target molecule asεociated with their surface with which it iε poεεible to diεtinguiεh them from other cellε uεing a targeting moiety. The ability to distinguish may lie in the abundance of a certain target molecule on a certain subset of cells. Furthermore, it is not necessary for the application of the invention that the nature of the target molecule is known. Targeting moieties can be selected from combinatorial peptide libraries on the basis of differential binding to molecules expressed on the surface of different cell types . Useful combinatorial peptide libraries for the invention include those in which a large variety of peptides is displayed on the surface of filamentous bacteriophages (e.g. Scott and Smith, Science 249(1990) : 386) . Screening for individual library members that interact with desired target cells allows the isolation of the nucleotide sequences encoding suitable peptide structures to be used as a targeting moiety in the invention. For this purpose, libraries displaying scFv variants are particularly useful. Moreover, methods that increase the combinatorial diversity of said libraries make the number of targeting moieties that can be generated for application in the invention almost limitless . Said methods include PCR-based random mutagenesis techniques (Stemmer, Proc. Natl. Acad. Sci. USA 91(1994):10747; Crameri and Stemmer, Biotechniques 18(1995) :194) . Thus it is understood from the above that the invention discloses methods for specific gene delivery into any target cell that can be phenotypicaUy distinguished from other cells, also when the nature of this distinction has not been revealed. Important target cells are tumor cells, cells of the hematopoietic system, hepatocytes, endothelial cells, lung cells, cells of the central nervous system, muscle cells and cells of the gastro-intestinal tract. Usual target molecules are receptors, surface antigens and the like. As indicated above the invention also includes a kit of parts wherein the member of a specific binding pair is an immunoglobulin binding moiety and its counterpart is an immunoglobulin or a derivative or a fragment thereof. RECTIFIED SHEET (RULE 91) ISA/EP Preferably the immunoglobulin binding moiety iε capable of binding to a constant region of an immunoglobulin. A much preferred immunoglobulin binding moiety is protein A, protein G, or an Fc receptor. One of the most commonly used immunoglobulin-binding proteins is Staphylococcus aureus protein A. Thiε 42 kD polypeptide exhibits strong binding to the Fc region of many IgG molecules, including human IgGl, lgG2 and IgG4 and mouse IgG2a and IgG2b, without interfering with the antigen binding εite (Surolia et al., Trends Biochem. Sci. 7(1981):74; Lindmark et al, J. Immunol. Meth. 62(1983) :1). A wider range of mammalian immunoglobulins is bound by protein G, which is isolated from a group C Streptococcus species. It has strong binding capacity for all human and mouse IgG subclaεεeε (Bjork and Kronvall, J. Immunol. 133 (1984) :969; Sjδbring et al., J. Biol. Chem. 266(1991) :399) . Protein A and G do not intrinsically bind to mammalian cell surfaces or virus membranes or capsids. Expresεion of (parts of) these molecules on the surface of gene delivery vehicles will therefore require the generation of hybrid molecules. A hybrid between streptavidin and two of the IgG binding domains of protein A has been made and bound one IgG molecule per hybrid (Sano and Cantor, Bio/Technol. 9 (1991) :1378) . The mammalian receptors for the Fc domain of immunoglobulins (FcR) , however, are trans-membrane molecules (reviewed by Van de inkel and Capel, Immunol. Today 14 (1993) :215) . These receptorε provide a feedback between the humoral and cellular immune reεponεeε. Their interaction with immunoglobulinε triggerε immune functions such as phagocytosis, cytotoxicity, cytokine release and enhancement of antigen preεentation. FcR are memberε of the immunoglobulin εuperfamily and in humanε three classes of receptors for the Fc domain of IgG (FcgR) are recognized (hFcgRl, hFcgRII, and hFcgRIII) . hFcgRI iε unique in itε capacity to bind with high affinity to monomeric IgG (Ka = 10 8 -10 9 M" 1 ) . Itε binding is strong to human IgG3, IgGl, and IgG4 (with decreasing affinity) , and to mouse IgG2a and IgG3, whereas human lgG2 and mouse IgGl and IgG2b are bound much weaker. hFcgRI is constitutively expresεed on monocyteε and macrophageε and itε expression can be induced on neutrophils and eosinophils. Three highly homologous genes for hFcgRI have been identified. Of these, hFcgRIA encodes a trans-membrane molecule, hFcgRIa (a-chain) , conεisting of three extracellular immunoglobulin-like domainε (one of which iε not found among other memberε of the hFcgR-family) , a 21 amino acid tranε-membrane region, and a charged cytoplasmic tail of 61 amino acids. On the membrane of some cell types the hFcgRIa a-chain is found asεociated with a diεulfide-linked homodimer of g-chainε, which are alεo componentε of other members of the FcR family (Scholl and Geha, Proc. Νatl. Acad. Sci. USA 90(1993) :8847) . There exiεts some controversy whether the presence of g-chains is absolutely essential for membrane expresεion and ligand binding of the hFcgRIa a-chain (Takai et al, Cell 76 (1994) :519) . The signal transduction after immunoglobulin binding to the a-chain into the interior of the cell is mediated by the g-chainε; the cytoplasmic domain of the a- chain does not seem to play a role in signal transduction (Indik et al, Exp. Hematol. 22 (1994) :599) . Because the constant regions of immunoglobulins differ between species, anti-immunoglobulin antibodies can be generated by cross-εpecies immunization. For several specieε antibodies have been raised which εhow εpecificity to individual immunoglobulin chains or even subclasεeε. Molecular cloning of the sequences encoding these antibodies offers the posεibility to conεtruct εcFv, which compriεe of the variable regionε of the heavy and light chainε of the immunoglobulin molecule linked by a flexible peptide bridge. Such εcFv have been shown to be as potent in antigen binding as are immunoglobulin Fab fragments (Bird et al, Science 242(1988) :423) . Hybrid moleculeε between εcFv and viruε envelope moleculeε have been made and expreεεed on the εurface of mammalian cells and were shown to be capable of antigen binding and targeted gene delivery (Chu et al, Gene Ther. 1(1994) :292; Chu and Do burg, J. Virol. 69(1995) :2659) . When the gene delivery vehicle is provided with an immunoglobulin binding moiety, only a suitable antibody (or a derivative or a fragment(retaining the binding site for the immunoglobulin binding protein) ) is needed as the entire conjugate. Thus, when an antibody is available which can distinguiεh a target cell from other cellε, the gene delivery vehicle iε ready to be uεed to target the gene of intereεt to εaid cell uεing εaid antibody. Thuε for any cell which can be diεtinguiεhed from other cellε uεing antibodies, a targeted gene delivery syεtem is provided. The same is true for another embodiment of the present invention, in which the member of a specific binding pair iε biotin and itε counterpart iε avidin or εtreptavidin, or in which the member of a εpecific binding pair iε avidin or strepatavidin and its counterpart is biotin. In this embodiment the delivery vehicle iε provided with the one member of the (strept)avidin/biotin couple and thus the conjugate compriseε the other member. Thuε any targeting moiety that can be biotinylated or can be coupled to (εtrept)avidin iε now uεable with the gene delivery vehicle according to the preεent invention. Thiε makeε the number of targeting moietieε that can be applied almoεt limitleεε. Any molecule aεεociated with the εurface of a target cell for which a εpecific binding partner iε known or can be produced iε in principle uεeful aε a target for the preεently invented gene delivery εyεtem, if in preεence or abundance it differε from one εubεet of cellε to another. Antibodies of course are a good example of suitable targeting moietieε of thiε laεt embodiment, aε they were for the embodiment before that. Thuε a preferred embodiment of the preεent invention encompaεses a kit of partε wherein the targeting moiety iε an antibody or a fragment or a derivative thereof, recognizing the target molecule asεociated with the εurface of the target cell. Another group of suitable targeting moietieε are ligandε wherein the target molecule is a receptor (for which the targeting moiety is a ligand) asεociated with the εurface of the cell. For many cell type-εpecific antigenε protein ligandε have been identified which bind with high specificity and/or affinity, e.g. cytokines binding to their cellular receptors. Most proteins can be labeled with the water¬ soluble vitamin biotin, using simple procedures. Most biotinylations are performed uεing εuccinimide eεterε of biotin. Binding to the protein proceedε through free amino groupε, normally of lyεyl residues. Biotinylated molecules are bound with exceptionally high affinity (Ka=l0 14 -10 15 M " ---) by both Avidin and Streptavidin ( ilchek and Bayer, Immunol. Today 5(1984):39). Avidin is a 68 kD glycoprotein isolated from egg white and Streptavidin is a 60 kD protein from Streptomyces avidinii . Both molecules are homotetramers; each subunit contains a single biotin binding-site. The high affinity of their binding makes the biotin-avidin or biotin-streptavidin interactions eεεentially irreversible. A functional streptavidin gene has been cloned (Sano and Cantor, Proc. Νatl. Acad. Sci. USA 87 (1990) :142) and a streptavidin mutant has been generated (Sano and Cantor, Proc. Νatl. Acad. Sci. USA 92 (1995) :3180) with reduced biotin-binding affinity (approx. Ka=10 8 M "1 ) providing specific and tight, yet reversible biotin-binding. As for Protein A and G, avidin and εtreptavidin do not intrinεically bind to mammalian cell εurfaces or virus membranes or capεids. Expreεεion of (partε of) theεe molecules on the surface of gene delivery vehicles will therefore also require the generation of hybrid moleculeε. Hybridε of εtreptavidin and heterologouε proteins have been made and retained full biotin binding capacity (Sano and Cantor, Bio/Technol. 9 (1991) :1378; Sano et al., Proc. Νatl. Acad. Sci. USA 89 (1992) :1534) . In yet another embodiment of the invention the member of the specific binding pair and its counterpart are both peptides forming three-dimensional structures that can interact in solution. Peptides useful in this aspect of the invention include but are not restricted to dimerization motifs that are identified within proteins known to form dimers, such as the yeast transcription factor GCN4, the mammalian transcription factor C/EBP, and the nuclear transforming oncogene products fos, jun, and myc. Alternatively, synthetic peptides can be used that are designed on the basis of the knowledge of inter and intra protein interactions . The paradigm of peptide motifs known to dimerize is the coiled-coil structure, a subset of which is the so called leucine zipper (Landschultz et al. , Science 240(1988) :1759; 0'Shea et al. , Science 243(1989):538; O'Shea et al., Science 254 (1991) : 539) . Several proteins containing a leucine zipper motif can not only form homodimers but also heterodimers (Hai et al. , Genes & Dev. 3 (1989):2083; Roman et al., Genes & Dev. 4(1990) : 1404) . The leucine zipper dimerization region itself contains all of the structural information determining the dimerization specificity (Agre et al., Science 246(1989) : 922; Kouzarides and Ziff, Nature 340(1989) ;568; O'Shea et al . , Science 245(1989):646) . For the design of the member of the specific binding pair and its counterpart in the present invention it is preferred that the peptides used preferentially, if not exclusively, form heterodimers and not homodimers. Vinson et al (Genes & Dev. 7(1993): 1047) have proposed and verified a rule for the rational design of leucine zipper motifs at preferentially form heterodimers. Therefore, it is preferred that the peptides used in this aspect of the invention are designed according to this so called "i+5 salt bridge" rule. Expression of a dimerizing peptide motif on the surface of the delivery vehicle as well as conjugation of its dimerizing counterpart to the targeting moiety will require the generation of hybrid molecules. Many different hybrid proteins with functional leucine zipper domains have already been produced, including dimeric antibodies based on scFv (Pack and Pluckthun, Biochemistry 31(1992) :1579) , dimeric RECTIFIED SHEET (RULE 91) ISA/EP antibodies based on monoclonal Fab' fragments (Kostelny et al., J. Immunol. 148(1992) :1547) , and a set of heterodimerizing proteins with leucine zippers, one linked to phagemid pill coat protein and the other linked to the products from a cDΝA library, that can be used for the production of a combinatorial phagemid cDΝA library (Crameri and Suter, Gene 137(1994) : 69) . When the gene delivery vehicle is provided with a peptide structure capable of dimerization with a counterpart in solution it can be used in combination with any targeting moiety that can be coupled to said counterpart. Using standard techniques known in the art expression constructs can be made for insertion of foreign sequences resulting in the translation of fusion proteins containing a leucine zipper terminal domain. Said coupling may either be direct or via an intervening linker sequence separating the individual folding domains of said targeting moiety and counterpart. Linker sequences known in the art that provide the peptide main chain with conformational freedom, thereby separating individual folding domains, are usually rich in glycine residues. Examples of linker peptide sequences useful in the invention include but are not restricted to (Gly 4 -Ser) 3 (Batra et al. , J. Biol. Chem., 265(1990) : 15198) and (Gly 4 -Thr) 3 (Bird et al. , Science 242(1988): 423) . It is thus understood from the above that also in this aspect of the invention the number of targeting moieties that can be applied is almost limitless. As stated above it is possible to target the gene delivery vehicles to many different cells using different conjugates (antibodies, biotinylated targeting moieties or targeting moieties conjugated to a peptide structure that can interact in solution) . It is thus preferred to provide a kit of parts according to the invention comprising a multitude of different conjugates, comprising the same counterpart member but a number of different targeting moieties. RECTIFIED SHEET (RULE 91) ISA/EP Although as stated above the skilled artiεan will be able to apply the teaching of the preεent invention to other viruεeε than thoεe exemplified herein, the exemplified viruεeε are preferred embodimentε, becauεe of the experience with the viruεeε in gene therapy conceptε. Retroviruseε are eεpecially εuitable for gene tranεfer into replicating cellε. Foreign geneε introduced by retroviruεeε become εtable componentε of the genome of the target cell and itε progeny. Adenoviruses, on the other hand, efficiently introduce foreign genetic material into non-dividing target cells. Adeno-asεociated virus is the only currently known non-pathogenic DΝA-virus. It could provide an alternative for both retroviruεes and adenoviruses in certain gene transfer applications. Thus it iε preferred that the gene delivery vehicle is derived from an adenoviruε, an adeno associated virus or a retrovirus. Brief Description of the Drawings: Figure 1 depictε the εpecific binding of biotinylated G-CSF to mouse fibroblasts expressing DG-CSFR on their membrane. FACS histograms of biotinylated G-CSF binding (log scale) .are given. Solid lines represent measurement after incubation in biotinylated G-CSF, PE-Streptavidin, biotinylated goat-anti-Streptavidin, and PE-Streptavidin. Broken lines represent results of control incubations without biotinylated G-CSF. The percentage of cells showing εpecific binding of biotinylated G-CSF is shown in the upper right corner of each panel. Panel A: unmanipulated mouse fibroblastε, panel B: mouse fibroblastε tranεduced with LΝCX-GRDcyt viruε and selected with G418, panel C: mouse fibroblastε tranεduced with LΝCX-GRDcyt viruε, selected with G418, FACS sorted for GRDcyt expression and cloned by limiting dilution. Figure 2 depicts the levels of hFcgRIa expression on the membrane of PA317 cells during in vitro culture for up to eight weeks. Unmanipulated PA317 cells, and two mixed populations of transfected PA317 cells which were FACS sorted for hFcgRIa- expression (PAFcR sorted-315 and PAFcR sorted-216) were stained with the anti-FcgRI MoAb 22 or with an irrelevant anti-KLH isotype control. The ratios between the median signal intensities after staining with MoAb 22 over the control are given. Figure 3 depicts the structure of the two hybrid hFcgRIa/Mo-MuLV env molecules and their parental hFcgRIa and Mo-MuLV env proteins. The Mo-MuLV env peptides gp70 and pl5E which are formed by proteolytic cleavage are shown separately and their physical connection on the precursor peptide is delineated by a broken line. Numbers above different segments indicate number of aminoacids . Asterisks indicate position of Cys-residues . Arrows represent primers used for PCR amplification. Areas in FcRenv-15 and FcRenv-70 indicated with striped boxes underneath were made by PCR amplification. Positions of relevant restriction enzyme cleavage sites are indicated. Mo-MuLV env sequences are shown in grey, hFcgRIa sequences in white. Black boxes represent non-coding Mo-MuLV sequences. S, signal peptide; Pro, proline-rich hinge-like region; TM, trans-membrane spanning domain; C, cytoplasmic domain; EC1, EC2, EC3, extracellular domains 1, 2, and 3, each forming one immunoglobulin-like loop. Figure 4 shows in vitro immunoglobulin binding by hybrid hFcgRIa/Mo-MuLV env molecules FcRenv-15 and FcRenv- 70. Positive and negative protein controls are wild type hFcgRIa (wt-FcR) and irrelevant firefly luciferase proteins, respectively. After in vitro coupled transcription and translation of the DNA sequences encoding these proteins, or a control reaction without DNA template (no DNA) in the presence of 3 H-labelled leucine, the formed proteins were precipitated with Sepharose beads loaded with IgG2a antibodies. The total reaction product (T) , precipitated material (P) and precipitation supernatant (S) were separated through 10% SDS-polyacrylamide gel. The signal in RECTIFIED SHEET (RULE 91) ISA/EP the gel was amplified and exposed to an x-ray film. Extended exposureε are indicated with a - above the bandε. The molecular weightε (MW) of a marker protein mixture iε indicated at the right side of the gel. Detailed Description of the Invention: The preεent invention provides a method to universally exploit a recombinant virus preparation for the targeted delivery of genetic material to many different cell types. To this end, the invention describes methods for the production of recombinant viruseε carrying on their εurface moleculeε (memberε of a εpecific binding pair) which can specifically interact with a set of secondary moleculeε (conjugateε compriεing the counterpartε of the members of a specific binding pair) . Said secondary molecules posseεε the ligand εpecificity for target cell-εpecific antigenε. The indirect binding of the recombinant viruε to the target cell via a εingle εecondary molecule, which allowε redirection of the εame recombinant virus preparation to different target cells depending on the added secondary molecule, iε a very important aspect of the invention. Useful secondary molecules to be employed in the invention include immunoglobulins or fragments thereof, chemically modified immunoglobulins or fragments thereof, chemically modified natural protein ligands for target cell antigens, fragmentε thereof, or recombinant derivativeε thereof, and immunoglobulins, fragments thereof, recombinant derivativeε thereof, natural protein ligandε for target cell antigenε, fragmentε thereof, or recombinant derivatives thereof that are coupled to a peptide motif that can interact with a specific counterpart in suspension. Biotinylation iε the paradigm for the chemical modificationε which could be employed. Typically, the specificity of the secondary molecules for the target cell antigen iε high, and preferably the affinity of the εecondary moleculeε for the target cell antigen iε high. Example l deεcribeε the specific interaction between a cytokine receptor molecule expressed on a mamalian cell surface and a biotinylated cytokine ligand. The invention is not restricted in the range of target cell antigens which could be employed to mediate targeted gene delivery. In principle, any target cell antigen for which a specific secondary molecule is available or can be made available is eligible for application of the invention. It is recognized, however, that not every target cell-specific molecule might serve as an internalization site for viruses bound to it. Furthermore, the efficiency of gene delivery may depend on the composition of the molecular bridge between the virus and the target cell-specific molecule (Etienne-Julan et al. , J. Gen. Virol. 73(1992) :3251) . Hence, attempts to target gene delivery vehicles to specific cell types relying on methods in which a specific target cell ligand is expressed on the surface of the virus particle, either as a wild-type molecule or as a hybrid with a heterologous protein, can be extremely costly and time-consuming since certain chosen approaches may be destined to fail. The procedures disclosed in this invention, however, allow for comprehensive screening of useful target molecules by employing many different secondary molecules added to the same gene delivery vehicle preparation. Therefore, application of the invention will tremendously increase the chances for successful targeted gene delivery and allow optimization of the procedures at relatively low costs. In addition, standardization and validation of the gene delivery preparations will be simplified as compared to strategies using antibod -antibody or antibody-ligand complexes or methods relying on direct chemical modification of the virus envelope or capsid. The binding of the secondary molecules to the recombinant viruses is mediated by substances which are exposed on the virus surface and have specific binding capacity for the secondary molecules. Preferably, said RECTIFIED SHEET (RULE 91) ISA/EP substances bind to the secondary molecules with high affinity. Said substances are selected from or contain regions from proteins including but not restricted to Protein A, Protein G, FcR, anti-immunoglobulin scFv, Avidin, Streptavidin, proteins containing structural peptide domains that can specifically interact with a peptide domain counterpart, and recombinant derivatives from the above. Their exposure on the surface of recombinant viruses is envisaged in several ways, which are listed below: A. for recombinant viruses with an envelope: Recombinant retroviruses are described in the invention as the paradigm for recombinant enveloped viruses . The invention is, however, not restricted to retroviruses. The invention also applies to other enveloped viruses, such as e.g. rhabdoviruses or herpes viruses. 1. If the binding substance is itself a trans-membrane molecule, e.g. FcR, the complete wild-type binding substance can be expressed on the surface of the retrovirus producing cell line. Alternatively, retrovirus packaging constructs can be expressed in cells naturally expressing the binding substance. Whether these approaches will result in inclusion of the binding substance in the retrovirus particle depends at least in part on the nature of the substance. During the assembly of murine retroviruses at the cell surface cellular membrane molecules are merely excluded from the virus particle, although exceptions have been observed (Zavada, J. Gen. Virol. 63(1982) : 15; Calafat et al. , J. Gen. Virol. 64 (1983) :1241) . In addition, exclusion of the binding substance from the virus membrane may be overcome by expressing it at very high density on the membrane of the retrovirus producing cell line (Suomalainen and Garoff, J. Virol. 68(1994) :4879) . RECTIFIED SHEET (RULE 91) ISA/EP 2. In the application of method no. 1 it is preferred that the trans-membrane binding substance lacks sequences which might elicit intracellular procesεeε in the retroviruε producing cell line or intraviral proceεses in the viruε particle. Such could perhapε occur upon binding of the εecondary targeting molecule. Therefore, the binding εubεtance can also be a modified derivative of the wild¬ type molecule. Modifications include any mutation preventing εignal proceεεing through the binding substance. The use of such truncated binding εubεtanceε iε furthermore preferred aε to prevent interactionε with cellular proteinε in the retroviruε producing cell line. It has been suggeεted that interactions with cytoplasmic proteins might be partially responεible for the excluεion of hoεt cell εurface proteinε from retroviruε particleε (Young et al., Science 250(1990) :1421) . 3. Since the fuεion proceεs between an infective retrovirus particle and a target cell is assumed to be mediated by an as yet undefined region in the carboxy terminal envelope peptide it can be envisaged that target cell binding according to method no. 1 or 2 might be insufficient to obtain internalization of the gene delivery vehicle into the target cell. Therefore, it may be necesεary to co-expreεs the trans-membrane binding εubεtances on the retrovirus surface with wild-type retroviral envelope molecules or segmentε thereof, which provide the retroviruε with fuεion capability. Exampleε 2 and 3 deεcribe the co-expreεεion of hFcgRIa and wild-type retroviral envelope molecules on the surface of recombinant retrovirus producing cell lines and functional IgG Fc-binding by the exposed hFcgRIa molecules. It iε preferred that the co-expressed envelope molecule is incapable of recognizing receptor molecules on cells present in the mixture of cells or in the living animal body subjected to the gene delivery preparation. Thiε can be achieved by employing truncated envelope moleculeε lacking their receptor recognition εite or wild-type envelope moleculeε from a retroviruε εpecies with a tropism restricted to cells not present in the mixture of cells or in the living animal body subjected to the gene delivery preparation. 4. Hybrid molecules can be made containing (segmentε of) a binding εubεtance and (εegmentε of) a heterologouε protein which anchorε the hybrid molecule in the membrane of the retroviruε producing cell line. Thiε method alεo applieε to binding εubεtanceε which are not themεelveε trans- membrane proteins. Molecules serving as a membrane anchor can be derived from natural trans-membrane proteins including but not restricted to cytokine or hormone receptors, cell adhesion or interaction moleculeε, complement receptorε, and immunoglobulin receptorε. Alternatively, the hybrid molecule could compriεe a membrane attachment region from a glycosylphoεphatidylinositol-anchored protein. The invention is not restricted to a defined junction site between the heterologous molecules. 5. In the application of method no. 4 it iε preferred that the membrane anchor molecule lackε εequenceε which might elicit intracellular proceεεes in the retrovirus producing cell line or intraviral procesεeε in the virus particle, or interact with cellular proteins in the retrovirus producing cell line. It is, therefore, preferred that εequenceε conferring the above be deleted from the hybrid binding εubεtance. 6. Becauεe the retroviral envelope molecule is selectively incorporated in retroviral particles and is assumed to mediate fusion between the virus and the target cell, it iε preferred over methodε nos. 4 and 5 to construct hybrid molecules containing the retroviral envelope peptide sequences responsible for the above processeε and the peptide εequences from the binding substance responsible for binding to the secondary target molecule. Also this method applies to binding substances which are not themselves trans-membrane proteins. The invention is not restricted to a defined junction site between the heterologous molecules. Example 4 describes the construction of two such hybrid binding subεtances and Example 5 showε that said hybrid binding εubεtances retain functional binding capacity for secondary target molecules. 7. Hybrid molecules can be made containing (segmentε of) a binding εubstance and (segmentε of) a heterologous protein which specifically interacts with extracellular regions of membrane molecules on the retrovirus producing cell surface. Preferably, the hybrid molecule interacts with membrane molecules which are εpecifically incorporated into retrovirus particles. Typically, these membrane molecules are (εegmentε of) retroviral envelope proteinε. Thiε method alεo applieε to binding εubstances which are not themselveε tranε-membrane proteins. 8. The expression of viral membrane proteins on the cell surface requires correct folding and aεεembly in the endoplaεmatic reticulum (ER) of the viruε producing cell line (reviewed by Doms et al., Virology 193 (1993) :545) . Retroviral envelope molecules asεemble into dimerε, trimers, or tetramers, depending on the retroviruε species. Incorrect oligomerization may prevent proper transport to the Golgi apparatuε, leading to degradation of the molecule. It can be anticipated that thiε would be the faith of εome hybrid εubεtances produced according to methods nos. 4 to 7. This could possibly be prevented by co-expresεing the hybrid εubεtance with wild- ype envelope molecules or segmentε thereof. The formation of hetero- oligomers might ensure correct procesεing in the ER. The preferred requirements for co-expressed envelope molecules described under no. 3 alεo apply here. The principleε of methodε A 1-8 alεo apply to pseudotyped viruses in which the envelope glycoproteins from one virus specieε are carried on the membrane of a heterologous virus εpecieε. Such phenotypic mixing of envelope glycoproteins occurs between a variety of different virus families (reviewed by Zavada, J. gen. Virol. 63 (1982) :15) . The invention also applies to enveloped viruses with hybrid envelope proteins, comprising fragments of related or even unrelated viruses. It has, e.g., been shown possible to generate infectious recombinant retroviruses carrying hybrid influenza hemaglutinin/retrovirus envelope proteins (Dong et al. , J. Virol. 66(1992) :7374) . B. for viruses with a capsid: The invention is described for, but not restricted to recombinant adenoviruses and adeno-associated viruses. Hybrid molecules can be made containing (segments of) a binding substance and (segments of) virus capsid proteins. For adenoviruses, it is preferred to locate the binding substance in the head domain of the fiber protein. This domain carries the receptor recognition site and it has been shown possible to change the adenovirus receptor specificity by manipulating this segment (Stevenson et al . , J. Virol. 69(1995) : 2850) . For certain hybrid molecules it may be necessary to co-express the hybrid molecule with wild-type fiber protein to ensure proper fiber trimerization and association with the penton base complex. Alternatively, other capsid proteins of adenovirus may be used to in¬ corporate the binding substance. E.g., Curiel et al . (Hum. Gene Ther. 3(1992): 147) have generated a chimeric serotype 5 adenovirus that contains a Mycoplasma pneumoniae Pl protein epitope as part of the hexon protein. It is furthermore preferred that the natural adenovirus receptor binding sequences be deleted from the hybrid molecule. Hybrid AAV capsid proteins with altered binding specificity are preferably made using (segments of) VP3, since the AAV capsid mainly comprises of this molecule. Alternatively, VPI or VP2 could be used. The AAV viral proteins allow insertions which are exposed on the capsid RECTIFIED SHEET (RULE 91) ISA/EP surface (unpublished resultε from the group of Dr. R. Samulski, Univ. of North Carolina, Chapel Hill, NC) . For certain hybrid molecules it will be necesεary to co-express the hybrid molecule with wild-type capsid protein. The invention is not reεtricted to εpecific junction sites between the heterologous proteins. Since many virus capsidε self-asεemble independent of the presence of viral DNA, empty pseudocapsids can be generated in vitro. Aεsembly of empty AAV-like particles does not even require all three capsid proteins (Ruffing et al., J. Virol. 66 (1992) :6922) . Thus, there is significant freedom to design novel pseudocapεidε. Empty viruε-like particles can be used to package and transfer exogenous non¬ viral DNA. This has been shown for Polyoma Virus pseudocapεidε (Forstova et al., Hum. Gene Ther. 6 (1995) :297) . Furthermore, exogenous non-viral DNA can be chemically coupled onto the exterior of virus capεidε (Curiel et al., Hum. Gene Ther. 3(1992):147; Cotten et al., Proc. Natl. Acad. Sci. USA 89 (1992) :6099) . The invention emphatically alεo applieε to the latter two approacheε and their combination, i.e. chemical coupling of exogenous DNA to the outer surface of empty capsids. EXAMPLES Example l Specific binding of biotinylated G-CSF to G-CSFR molecules expressed on the surface of mouse fibroblasts. To express receptors for granulocyte-colony stimulating factor (G-CSF) on the cell membrane of murine fibroblastε, PA317 cellε (Miller and Buttimore, Mol. Cell. Biol. 6 (1986) :2895) were tranεduced with ecotropic LΝCX/GRDcyt virus. The construct pLNCX/GRDcyt (generated in the laboratory of Dr. I. Touw, Erasmus University Rotterdam, The Netherlands) was made by inserting the cDNA εequence for a mutant G-CSF receptor (DG-CSFR) into pLNCX (Miller and Rosman, BioTechniques 7 (1989) :980) . DG-CSFR differs from the wild-type molecule by an almost complete deletion of the intracellular domain, rendering the receptor incapable of signal-tranεduction. The extracellular and tranε-membrane domainε of DG-CSFR are of wild-type structure. DG-CSFR exhibits, therefore, normal binding capacity for its cytokine ligand. Tranεduced PA317 cellε were iεolated by εelecting for resistance to 1 mg/ml G418 (Geneticin; Gibco, Paisley, UK) , conferred by expression of the neo r gene in LNCX/GRDcyt. Resistant PA317 cells were incubated with biotinylated G-CSF. Bound G-CSF was viεualiεed on a Fluoreεcence Activated Cell Sorter (FACS) after incubation with PE-conjugated Streptavidin (Molecular Probeε, Eugene, OR) , biotinylated goat-anti-Streptavidin antibodies (Vector Laboratories, Burlingame, CA) , and a second incubation with PE-Streptavidin. As can be seen in figure 1, untransduced PA317 cells did not bind any biotinylated G-CSF, whereas approximately 27% of the transduced cells bound G-CSF. After FACS sorting of positive cells and cloning by limiting dilution, a clone was isolated which εtrongly bound biotinylated G-CSF (figure IC) . Thiε example εhowε that a biotinylated cytokine εpecifically bindε to its natural receptor expresεed on the membrane of a mammalian cell line. Example 2 Generation of recombinant retrovirus packaging cell lines expressing hFcgRIa proteins on their cell membrane. For expreεεion of functional hFcgRIa on the εurface of retroviruε packaging cell lineε the conεtruct pRc/CMV- hFcgRIa was used. This construct waε generated by inserting a 1.3 kb Hindlll-ΝotI fragment including the full-length pl35 hFcgRIa-cDΝA sequence (Allen and Seed, Science 243 (1989) :378; GenBank accession number M21090) from clone CDM into the polylinker of pRc/CMV. Twenty microgram pRc/CMV-hFcgRIa was transfected onto the ecotropic packaging cell line GP+E-86 (Markowitz et al., J. Virol. 62 (1988) :1120) and onto the amphotropic packaging cell line PA317, according to the method described by Chen and Okayama (Mol. Cell. Biol. 7 (1987) :2745) . Transfectants were selected for resistance to 1 mg/ml G418, conferred by expression of the SV4Opr- neo r -p(A) casette on the pRc/CMV-hFcgRIa conεtruct. Poolε of reεiεtant cellε were analyεed for hFcgRIa expression on a FACS, after staining with the anti- FcgRI monoclonal antibody (MoAb) 22 (Guyre et al., J. Immunol. 143 (1989) :1650) and FITC-conjugated goat-anti-mouse IgG antibodies (GaM-FITC; Becton Dickinson Immunocytometry Systems, San Jose, CA [BDIS] ) . Table 1 shows that only few G4l8-resistant GP+E-86 cellε (E86FCR) and G418-reεiεtant PA317 cellε (PAFcR) were found to expreεs FcgRl on their surface. FcgRI-expressing cells were sorted using the FACS and expanded. After two weeks of culture, the εorted populationε were analyεed for FcgRI-expression as before. As can be seen in table 1, the sorting procedure resulted in εignificantly enriched FcgRI-expressing E86FcR and PAFcR cell populations. Table 1. FcgRI Expression on pRc/CMV-hFcgRIa Transfected and FACS Sorted Ecotropic and Amphotropic Recombinant Retrovirus Packaging Cell Lines. Packaging Cellε FcRI Positive Average FcgRI Cells (%) 1 Expresεion 2 GP+E-86 0.1 N.D. PA317 0.2 0.98 E86FCR 0.9 3 N.D. E86FcR εorted 16.4 1.16 PAFCR 7.63 N.D. PAFcR sorted- 10.5 1.43 315 4 PAFcR sorted- 20.5 2.37 216 4 - 1 Percentage cells exhibiting FITC fluorescence following incubation with MoAb 22 and GaM-FITC. Controls incubated with GaM-FITC only were set to ≤ 0.1 %. 2 Relative median FITC fluorescence after incubation with MoAb 22 and GaM-FITC as compared to incubation with irrelevant anti-KLH isotype control MoAb (BDIS) and GaM- FITC. 3 Average of two independent meaεurements. 4 PAFcR cellε were εubjected to two independent εorting experimentε. N.D., not done. To study whether hFcgRIa remained stably expresεed on the packaging cell εurface in the abεence of g-chain co- expreεsion, two εorted cell populationε expreεεing different levelε of hFcgRIa aε deduced from their median FITC-εignalε after staining with MoAb 22 aε compared to εtaining with irrelevant isotype controls, were cultured for up to eight weeks in culture medium containing G418. Regular reanalysiε showed rather stable median fluorescence εignalε (approx. 1.5 and 2.3 for the two εorted populationε, as compared to l.O for untransfected PA317 cells; figure 2) throughout the observation period. Immunophenotypic analysiε of wild-type hFcgRIa revealed three diεtinct epitopeε, one of which compriεeε the Fc- binding domain (Guyre et al., J. Immunol. 143(1989) :1650) . One of the sorted cell populations was analyεed uεing the directly FlTC-εtained MoAbε 22 and 32.2 (Medarex, Annandale, ΝJ) , which each define one of the two non-Fc-binding epitopeε. Table 2 εhowε that both MoAbε recognized the hFcgRIa molecule expoεed on the cell εurface. The increaεe in fluoreεcence which was observed when both MoAbs were combined demonstrate that they bind to distinct epitopes on the packaging cell εurface. Pre-incubation of the cellε with irrelevant unεtained mouεe immunoglobulinε of IgGl or lgG2a iεotype (BDIS) did not influence the binding of MoAbε 22 and 32.2 (not εhown) . These reεultε demonεtrate that hFcgRIa of correct immunophenotypic εtructure waε expreεεed on the cell εurface of a retroviruε packaging cell line. Table 2. Immunophenotype Analyεiε of Amphotropic Packaging Cellε Expreεsing hFcgRIa. Mean Fluoreεcence FITC-labeled MoAb % Positive Intenεity (a.u.) Cells anti-KLH (irrelevant) 0.9 1 15.0 anti-hFcgRIa MoAb 22 13.0 64.6 anti-hFcgRIa MoAb 8.1 46.3 32.2 anti-hFcgRla MoAbs 22 21.1 105.8 + 32.2 PAFcR εorted-216 cells were incubated with the indicated FITC-labeled MoAbs and analysed on a FACS. - 1 - The treshold for a poεitive εcore waε εet at > 99 % negative cells on the anti-KLH-FITC stained sample. Ecotropic and amphotropic hFcgRIa-expresεing populations were cloned by limiting dilution. Twelve individual E86FcR and eleven individual PAFcR clones were expanded and analyεed for hFcgRIa-expression. The two clones expressing the highest level of hFcgRIa from each population were used to investigate whether hFcgRIa-expression on the cell surface of packaging cells influences the retrovirus production. To this end, cell-free supernatant was harvested from these four clones as well as from the parental packaging cell lines under standard conditions (72 hours at 32°C in 10 ml alpha-modified DMEM with 10% BCS per 80 cm 2 confluent cell monolayer) . The supernatantε were analyεed for reverse transcriptaεe activity (Goff et al., J. Virol. 38(1981) :339) , in a two-fold dilution titration. Comparison of the radiation intensity of incorporated 32 P-dTTPε by expoεure to X-ray film revealed that expreεεion of hFcgRIa did not influence the εhedding of viruε particleε into the culture medium more than a factor 2.3 (table 3) . Table 3. Retroviruε Particle Production by Packaging Cell Lineε Expreεεing hFcgRIa. 1 Relative median fluorescence signal after staining with FITC-labeled MoAbs 22+32.2 as compared to anti-KLH-FITC. 2 The RT-activity in the culture supernatant is given relative to the RT-activity shed by PA317 cells. N.D. , not done. Example 3 Immunoglobulin binding bv hFcgRIa proteins expressed on the cell membrane of recombinant retrovirus packaging cell lines. To test whether hFcgRIa expresεed on the surface of retrovirus packaging cells is capable of specific immunoglobulin-Fc binding, PAFcR sorted-315 cells were incubated with irrelevant mouse immunoglobulins of IgGl or IgG2a isotype. FcgRI molecules should bind lgG2a molecules with high affinity and IgGl molecules with low affinity. As can be εeen in table 4, approximately one third of the sorted cells were expreεεing low amountε of hFcgRIa, aε indicated by MoAb22-εtaining with low relative median εignal. All hFcgRIa moleculeε on PAFcR cellε alεo bound mouse IgG2a (similar percentage positive cells and signal intensity) , whereas mouse IgGl was hardly bound. On human primary monocyteε, which were uεed aε a poεitive control, hFcgRIa moleculeε were εtrongly expressed and specifically bound mouse IgG2a immunoglobulinε. The capacity of hFcgRIa moleculeε on monocyteε to bind immunoglobulinε, however, waε lower than meaεured on pRc/CMV-hFcgRIa tranεfected packaging cellε (55% of MoAb22-positive monocytes bound IgG2a, with a much lower signal intensity) . In conclusion, all hFcgRIa molecules characterised by MoAb22-binding expresεed on the εurface of PAFcR cellε exhibit a correct εtructure for specific high affinity binding of immunoglobulins. Table 4. Immunoglobulin-Fc Binding by FcgRI Expresεed on PA317 Cellε. Primary Human Monocytes PA317 PAFcR sorted- MoAb 315 % Pos. Rel. % POS. Rel. % Pos. Rel. Med 1 Med. Med. none 0.8 (1) 0.9 (1) 0.8 (1) irr- 0.7 1.84 0.6 0.96 6.2 1.15 IgGl irr- 35.1 3.69 0.4 0.95 30.6 1.61 IgG2a MoAb22 64.2 8.82 0.5 0.94 33.0 1.65 Human primary monocytes, PA317 cells, or a pool of pRc/CMV- hFcgRIa tranεfected PA317 cellε partially expreεεing hFcgRIa (PAFcR εorted-315) were first incubated with MoAb22 (IgGl isotype), anti-KLH IgGl, anti-KLH IgG2a, or PBS/1% BSA ("none"), followed by an incubation with GaM-FITC. The percentage positive cellε and the median fluoreεcence intenεities were determined on a FACS. 1 Relative median fluoreεcence intenεity of cellε incubated with indicated primary MoAb and GaM-FITC aε compared to cellε incubated with GaM-FITC alone. Example 4 Construction of hybrid hFcgRIa/Mo-MuLV envelope genes. For the conεtruction of hybrid moleculeε with immunoglobulin-binding propertieε of hFcgRIa and viruε- incorporation and membrane-fuεion propertieε of envelope glycoprotein, pRc/CMV-hFcgRIa (εee above) waε uεed in combination with Mo-MuLV env εequenceε (Shinnick et al., Nature 293 (1981) :543) . Two different hybrid hFcgRIa/Mo-MuLV- eπv conεtructε were made (figure 3). In both conεtructε, 3* hFcgRIa εequenceε in the pRc/CMV-hFcgRIa conεtruct were replaced by 3* Mo-MuLV env εequenceε. Both conεtructs contain pl35 sequenceε from the leader εequence to the HinPll-εite at position 849-852. This fragment compriseε the three extracellular domains of hFcgRIa, excluding the 61 carboxy terminal nucleotides. In hybrid construct pRc/CMV- FcRenv-15, the pl35-fragment is coupled to pl5E env sequences encoded by a fragment starting from the Hpal-εite at position 7195-7200 of Mo-MuLV until position 7823 in the IR sequence of the 3'LTR. To thiε end, the Hpal-site was converted into a HinPlI-site by PCR, and a Notl-site was introduced in the 3*LTR by PCR. An internal Spel-Pvull fragment (nt 7487-7747) was exchanged for cloned sequences. The correct sequence of the hFcgRIa/Mo-MuLV-env junction and of all sequenceε generated by PCR waε confirmed by εequencing. Hybrid gene FcRenv-15 encodeε an in-frame fuεion protein compriεing the extracellular domainε of hFcgRIa until an Arg-reεidue 12 aminoacidε downεtream from the 3' Cys-residue forming the third immunoglobulin-like extracellular loop and all but the five most 5' pl5E aminoacids. To generate hybrid construct pRc/CMV-FcRenv-70, the HinPll-site from the 5' pl35-fragment was made blunt-end and waε ligated to a BamHI (poεition 6537-6542) -Pvull (poεition 7745-7750) fragment of Mo-MuLV, after the BamHI- εite waε made blunt-end, and the 3' PvuII-ΝotI PCR-fragment from pRc/CMV-FcRenv-15. Ligation of the blunt-end HinPll and BamHI-εites restored the BamHI-site. The correct structure of the hFcgRIa/Mo-MuLV-env junction was confirmed by sequencing. Hybrid gene FcRenv-70 comprises the same hFcgRIa-fragment as described for pRc/CMV-FcRenv-15 coupled to the Mo-MuLV env εequenceε starting from an lie-residue at the BamHI-site in the amino terminal domain of gp70 and including the complete proline-rich hinge-like region and carboxy terminal domain of gp70 and the complete pl5E peptide. On July 24, 1996, the applicants have deposited with the European Collection of Cell Cultures (ECACC) , Saliεbury, Wiltshire, U.K., the plasmid pRc/CMV-FcRenv-15, given ECACC accession number P96072515, and plasmid pRc/CMV-FcRenv-70, given ECACC acceεεion number P96072514. These deposits were made under the provisions of the Budapest Treaty on the International Recognition of the Depoεit of Microorganiεms for the purposeε of patent procedure and the regulationε thereunder. Example 5 Hybrid hFcgRIa MQ-MuLV envelope genes FcRenv-15 and FcRenv- 70 encode proteins of correct molecular weight that have mmunoglobulin binding capacity To test whether the fusion genes FcRenv-15 and FcRenv-70 encode functional hybrid molecules with immunoglobulin binding properties, the pRc/CMV-FcRenv-15 and pRc/CMV-FcRenv-70 constructε (Example 4) were uεed aε templateε for coupled in vitro tranεcription and tranεlation. Tranεcription waε initiated at the T7 promoter upεtream of the gene inεertion in the pRc/CMV vector. One microgram pRc/CMV-FcRenv-15 or pRc/CMV-FcRenv-70 DΝA waε added to a 25 microliter leucine-free 17 TnT coupled reticulocyte lyεate reaction mixture (Promega, Madiεon, WI) with 20 units RΝasin ribonucleaεe inhibitor (Promega) and 20 microCi 3 H-leucine- (Amerεham, Buckinghamεhire, UK) . The reaction waε allowed to proceed for 120 minuteε at 30°C according to the guidelineε provided by the manufacturer. Control reactions were performed without DΝA template, with an irrelevant control DΝA template (firefly luciferase control DΝA provided with the T7 TnT coupled reticulocyte lysate syεtem) , and with poεitive control pRc/CMV-hFcgRIa encoding wild-type hFCgRIa. To 5 microliter of the reaction mixture, 20 microliter SDS-PAGE loading buffer (to a final concentration of 62.5 mM Triε pH 6.8, 10% glycerol, 2% beta- mercaptoethanol, 0.5% SDS, 0,0025% bromo phenol blue) waε added. The remaining 20 microliter waε diluted with 180 microliterε of precipitation buffer (10 mM Triε-HCl pH 7.5, 150 mM ΝaCl, 5 mM EDTA, 1% Triton-100 with proteaεe inhibitors PMSF at 0.1 mM, aprotinin at 10 microgram/ml, pepstatin at 5 microgram/ml, trypsin inhibitor at 10 microgram/ml, and leupeptin at 10 microgram/ml) . Next, 10 microliter KLH-aKLH-IgG2a-Sepharose slurry was added and mixed by tilted rotation for 90 minutes at room temperature. Said slurry was prepared according to the following procedure. First, 16.4 mg Keyhole Limpet Hemocyanin (KLH; Sigma, . St. Louis, Mo) was coupled at 1.8 mg/ml to 1 gram CNBr-activated Sepharose 4B (Pharmacia Biotech, Uppsala, Sweden) according to the manufacturers instructions. The KLH-Sepharose product waε εtored in PBS with 0.002% εodium azide at 4°C. Next, KLH-Sepharoεe was mixed 1:1 with 50 microgram/ml anti-KLH-IgG2a monoclonal antibody (Beckton- Dickinεon Immunocytometry Systems, San Jose, CA) and incubated for 30 minutes at room temperature. Finally, the KLH-aKLH-lgG2a-Sepharose beads were washed two times in 20 volumes PBS. Thiε procedure should result in Sepharose beads coated with immunoglobulins of the IgG2a isotype that have their Fc domain projecting outwardε. After incubation of the in vitro tranεcribed and translated reaction product with KLH-aKLH-IgG2a-Sepharose, the KLH-aKLH-IgG2a-Sepharose beads were spun down and the precipitation supernatant was removed. To 5 microliter precipitation supernatant 20 microliter SDS-PAGE loading buffer was added as above. The pelleted KLH-aKLH-lgG2a-Sepharose beads were washed 6- timeε in 1 ml precipitation buffer. After a final spin, 25 microliter SDS-PAGE loading buffer was added to the pellet. The above described procedure resulted in three different sampleε of each coupled in vitro tranεcription- tranεlation reaction, i.e., the total reaction product, the immunoglobulin bound precipitated product, and the remaining material in the precipitation εupernatant. Ten microliterε of these sampleε were heated at 85°C for 2 minuteε and subsequently separated by SDS-PAGE through a 10% gel according to standard procedures (Laemmli, Nature 227 (1970) :680) . After separation, the proteinε in the gel were fixed in water/iεopropanol/acetic acid (65%/25%/10% vol.) for 30 minuteε at room temperature and thereafter incubated for 15 minutes at room temperature in Amplify (Amersham) for fluorographic enhancement of the signal. Finally, the gel waε dried for 90 minuteε at 80°C under vacuum and expoεed to X-ray film with an enhancing εcreen. Figure no. 5 shows the result of this experiment. As can be seen, the conεtructε with geneε encoding wild-type hFcgRIa or hFcgRIa/Mo-MuLV envelope fuεion moleculeε all expreεεed proteinε exhibiting a migration pattern correεponding to their predicted molecular weights, i.e., 42.5 kD for hFcgRIa, 52 kD for FcRenv-15, and 75,6 kD for FcRenv-70. The latter gene furthermore encodes a smaller protein product. This molecule perhaps represents the approximately 60 kD proteolytically procesεed protein after releaεe of the pl5E peptide. As can furthermore be seen in Figure no. 5, all these protein products were precipitated by KLH-aKLH-IgG2a- Sepharoεe beadε, in contraεt to the irrelevant control protein luciferaεe, demonεtrating that the immunoglobulin binding property of wild-type hFcgRIa is retained after fusion of the three extracellular domainε of hFcgRIa to Mo¬ MuLV envelope εequenceε.

Claims: CLAIMS 1. A method for delivering genetic material to a target cell, comprising preparing a gene delivery vehicle comprising a molecule encoding a gene of interest, a capsid or an envelope derived from a virus and a first member of a specific binding pair, preparing a conjugate comprising a part which is a complementary member of said specific binding. pair and a part which has affinity for a molecule associated with the surface of the target cell, contacting the gene delivery vehicle and the conjugate, so that the members of the specific binding pair can bind and delivering the complex of conjugate and gene delivery vehicle to said target cell. 2. A method according to claim 1, whereby said first member of a specific binding pair has no specific affinity for molecules associated with the surface of the target cell. 3. A gene delivery vehicle for use in a method according to claim 1 or 2, comprising an expressible recombinant nucleic acid molecule encoding a gene product of interest, a capsid or an envelope derived from a virus, and a member of a specific binding pair, the counterpart of which is not directly associated with the surface of the target cell. 4. A gene delivery vehicle according to claim 3, whereby the envelope or the capsid material is incapable of specifically binding to a target cell. 5. A conjugate for use in a method according to claim 1 or 2 targeting a gene delivery vehicle according to claim 3 or 4 to a target cell comprising the counterpart of the member of a specific binding pair coupled to a targeting moiety being capable of binding to a target molecule associated with the surface of a target cell. RECTIFIED SHEET (RULE 91) ISA/EP 6. A targeting gene delivery vehicle complex, comprising a gene delivery vehicle according to claim 3 or 4, coupled to a conjugate according to claim 5. 7. A kit of parts for delivering genetic material to a target cell, comprising a gene delivery vehicle according to any one of claims 3-4 and at least one conjugate of the counterpart member of the specific binding pair coupled to a targeting moiety, said targeting moiety being capable of binding to a target molecule associated with the surface of a target cell. 8. A kit of parts according to claim 7, wherein the member of a specific binding pair is an immunoglobulin binding moiety and its counterpart is an immunoglobulin or a derivative or a fragment thereof. 9. A kit of parts according to claim 8, whereby the immunoglobulin binding moiety is capable of binding to a constant region of an immunoglobulin. 10. A kit of parts according to claim 9, whereby the immunoglobulin binding moiety is protein A, protein G, or an Fc receptor. 11. A kit of parts according to claim 7 wherein the member of a specific binding pair is biotin and its counterpart is avidin or streptavidin. 12. A kit of parts according to claim 7 wherein the member of a specific binding pair is avidin or strepatavidin and its counterpart is biotin. 13. A kit of parts according to anyone of claims 7-12, wherein the targeting moiety is an antibody or a fragment or a derivative thereof, recognizing the target molecule associated with the surface of the target cell. 14. A kit of parts according to anyone of claims 7-12, wherein the target molecule is a receptor for which the targeting moiety is a ligand. 15. A kit of parts according to anyone of claims 7-14, wherein the gene delivery vehicle is derived from an adenovirus, an adeno associated virus or a retrovirus. RECTIFIED SHEET (RULE 91) ISA/EP 16. A kit of parts according to anyone of claims 7-15, comprising a multitude of different conjugates, comprising the same counterpart member but a number of different targeting moieties.

Inventors: VALERIO DOMENICO, VAN BEUSECHEM VICTOR WILLEM, VALERIO, DOMENICO, VAN BEUSECHEM, VICTOR, WILLEM
Applicants: BEUSECHEM VICTOR WILLEM VAN, INTROGENE BV, VALERIO DOMENICO, INTROGENE B.V., VALERIO, DOMENICO, VAN BEUSECHEM, VICTOR, WILLEM
Application year: 1996
Main classification: C12N15
IPC codes: A61K35, A61K47, A61K48, C07K14, C12N5, C12N15