FR2716461A1 - Host-vector system usable in gene therapy. - Google Patents

Abstract

The invention relates to a host-vector system for expressing a transgene in a target cell or a human or animal tissue, characterized in that it consists of a eukaryotic cell and established in line, in which have been transfected: a ) a recombinant pseudo-retroviral sequence in which the env gene has been deleted in whole or in part and substituted by said transgene at the level of the env gene is shown in FIG. 1a; b) a nucleic acid sequence including a sequence encoding an envelope protein, which sequence is under the control of a promoter and is associated, if appropriate, with said transgene, and flanked at its 3 'end by a polyadenylation site and shown in Figure 1b; said recombinant viral genome and said sequence being capable of complementing in trans and enabling said cell to be eager to produce defective infectious viruses lacking the env gene.

Description

BOTg-VECTOR SYSTEM FOR USE IN GENE TBERAPY
The present invention relates to a new approach for gene therapy applied in particular to the elimination of certain categories of cells, such as tumor cells or certain cells infected with viruses, by the production of recombinant retroviruses from new genetic constructs.

 The concept of gene therapy using killer or suicide genes has been developed since 1986 by numerous approaches. It involves the expression of a gene, this expression allowing the transformation of a non-toxic substance or toxic substance by the cell. This concept of gene therapy is applicable and transposable to any gene whose expression in a target cell makes it possible to transform an inactive substance into an active substance, or vice versa, leading either to destruction (example of the suicide gene), or to restoration certain functions.

 In all that follows, the application of these concepts to suicide genes is more particularly developed taking into account the accessibility of its experimental implementation. The transposition of the means of the invention to any type of transgene, the expression of which leads to modifying the effect of a substance in a sense of activation or inactivation, is nevertheless within the reach of the skilled person.

 The herpes simplex virus type I thymidine kinase (HSV1-TK) is the enzyme that has been the subject of the greatest number of investigations relating to suicide genes.

 This enzyme, non-toxic to eukaryotic cells, has the characteristic of being able to transform certain nucleoside analogs such as aciclovir (ACV) or ganciclovir (GCV) into monophosphated molecules, which cell kinases are normally incapable of (GB Elion, J. Antimicrob .

Chemother. 12: 9-17 (1983)). These monophosphate nucleosides are then converted by cellular enzymes into nucleotide triphosphates which can be used during DNA synthesis and block the elongation process, thereby causing cell death. The nucleoside triphosphate analog is therefore only toxic to dividing cells.

 We therefore understand all the advantages that can be drawn from the use of this type of conditional toxins applied to gene therapy, in particular anticancer therapy.

 Firstly, only conditional toxicity makes it possible to generate stable cell clones producing viral pseudo-particles capable of producing such a transgene and transferring the suicide gene into the target cells. It is indeed sufficient to cultivate these cells in the absence of ACV or GCV since the thymidine kinase of HSV1 is not toxic to the cell in the absence of these drugs.

 Then, in the event of a side effect of treatment, stopping the administration of ACV or GCV immediately ceases the toxicity due to the transgene; in addition, an adjustment of the doses of the nucleoside analog makes it possible to selectively destroy the cells which express the transgene strongly while preserving the cells in which the gene is weakly expressed; this toxicity restricted to dividing cells is an important advantage, in particular for the treatment of cancer cells.

Finally, in vitro and in vivo experimental data have shown that cells not expressing HSV1
TK, but in contact with these were also destroyed by ACV treatment ("Metabolic Cooperation" or "bystander effect") (Moolten Fil., 1986,
Cancer Research, 46: 5276-5281 and Culver KW et al., 1992, Science 256: 1550-1552).

 The mechanism of this effect is still not understood, but it is possible that triphosphated nucleoside analogs can pass from one cell to another via the "gap junctions".

 Retroviruses appear to be the vectors of choice for transferring exogenous genes into eukaryotic cells, in particular human cells.

 However, an essential prerequisite for the use of retroviruses for therapeutic purposes and in particular in gene therapy is to ensure the safety of their use.

 The main danger of using retroviruses in gene therapy is the possibility of dissemination of a reconstituted wild retrovirus in the cell population or the tissue considered.

 Such proliferation could lead to multiple integrations of the retrovirus genome into the genome of infected cells, which could lead to any type of genetic disorder.

 This type of approach therefore poses the problem of reaching as many target cells as possible, and therefore the ability of this system to reach as many tumor cells as possible, while ensuring the control of the proliferation of viruses.

 Many approaches have been developed so far to develop packaging cells ("packaging cell lines") which produce only defective retroviruses, and carriers of the transgene of interest; the development of such cell lines has considerably increased the use of retroviruses in gene therapy, thanks to the security conferred by these systems.

The first vectors used were retroviral vectors carried by pseudoretroviral particles having an amphotropic envelope to allow their use in different species, and defective to control their dissemination. The natural starting virus is generally the murine virus of
Moloney, from which the elements acting in cis and the elements acting in trans have been separated to form two defective genomes.

 The viral vector proper has retained the essential cis sequences: the LTRs for the control of transcription and integration, the psi sequence necessary for packaging, the PB sequence necessary for viral replication. The viral genes (gag, pol, env) are deleted and replaced by the transgene to be expressed in a target cell placed in principle under its own promoter or under a promoter considered more powerful or regulable.

 The retroviral genes gag, pol, env are often integrated into another vector, sometimes called a helper, which is defective for the LTR and + sequences. Their expression allows the packaging of the transgene, to the exclusion of the genes necessary for the multiplication of the viral genome and for the formation of complete viral particles.

 The proviral form of the Helper is generally integrated into the genome of a murine cell line (for example fibroblast NIH / 3T3) which plays both the role of host for the vector and of Helper for the functions which it lacks. After transfection of the vector, the cell strain becomes capable of producing defective infectious viral particles. However, these particles contain only the gene to be transferred (transgene) but do not contain the information necessary for the reconstitution of complete viral particles in the target cells. This system is therefore designed to prevent any further spread of virus after the first infection, that is to say if the infectious virus carrying the transgene enters a cell lacking information of the Helper type (gag, pol and env). , its production is stopped.

 So, conventionally, the packaging line ("packaging cell line") is a cell line capable of providing Helper information in trans with respect to the viral genome present in the particle, in such a way that defective infectious viral particles can to reconstitute.

Different packaging systems allowing the production of defective recombinant viruses have been described in the following documents
- Patent application EP 0 243 204 describes the use of retroviruses as a means of packaging any substance to cross the membrane of eukaryotic target cells
- Patent application WO 89/07150 describes a packaging cell for a retrovirus, the two plasmids expressing in trans the various viral genes (env, gag, pol), but neither of them comprising the + packaging sequence; virus production in these lines is between 104 and 106 CFU / ml
- patent applications WO 90/02806, WO 90/12087, EP 0 476 953, WO 93/04167 and WO 93/10218 are publications describing gene transfers in vivo by the use of retroviral vectors, themselves produced by packaging cells.

 The numerous references of constructions and applications cited in these patents are included by way of references cited in the present application.

 Recent developments have led to the use of gene transfer in vivo by directly transferring vector-producing packaging cells for the treatment of tumors; the elimination, for example, of microscopic experimental brain tumors by stereotaxic injection of cells producing TK-HSV1 retroviruses, then treatment with GCV, has been reported by Culver et al. (Science 256: 1550-1552, 1992).

Similarly, some of the authors of the present application have shown that, in rats, the in vivo transfer of the HSV1-TK gene by direct injection of murine fibroblasts producing recombinant retroviral particles, leads to a considerable reduction of hepatic tumors in the rat (Caruso
M. et al., Proc. Natl. Acad. Sci. USA, 90: 7024-7028, 1993 and Yves Panis et al., CR Acad. SC. Paris ,, Tome 315, Series III, p. 541-544); these articles also demonstrate a very significant decrease in the number of cancer cells at the tumor level, knowing that, surprisingly, the number of cells transduced in vivo by this type of technique is undoubtedly less than 10% for TK- HSV1.

 This effect could be explained by the metabolic cooperation mechanism already mentioned above.

 This system of packaging lines making it possible to produce viral particles carrying a transgene is therefore extremely promising for use in gene therapy and in particular for the expression of conditional genes.

These systems developed and described in the numerous references cited above however have certain limits which are
1) the low productivity of these lines
cell (infectious titer less than or equal to
106 PFU / ml), the normal Mo MuLV retrovirus
producing around 108 PFU / ml and a virus like
adenovirus about 109;
2) a second limitation of this type of technique is
that it implements fibroblastic lines
NIH / 3T3 which are adherent lines; this is
an advantage when using the supernatant of
cells containing the viral particles but
by cons is a disadvantage when you want
use the transfected lines themselves at
drug title, lines injected in vivo
then producing the viral particles
recombined in situ
3) a third drawback, which is the
consideration of the security advantage of the system,
is that if the viral particles expressed in
located by the packaging line can transfer
the transgene of interest in cells in
division, any further dissemination is
blocked at this level. This limits the effectiveness of the
transgene transfer due to blockage of the
dissemination.

 It was therefore important to design a system which could possibly self-maintain the production of recombinant viral particles carrying the gene of interest while being sure to control any subsequent propagation at the chosen time in order to preserve the required safety conditions, namely total control of the dissemination of the virus in its wild form.

 In what follows, the term "transgene" designates the gene to be expressed in the target cell, a gene which, in the examples is of the suicide gene type.

 The expression "recombinant pseudo-retroviral sequence" means that the sequence contains all of the genes necessary for the expression of a pseudo-retroviral particle with the exception of the envelope (env) gene and optionally comprising one or more transgenes.

 The expression "host cell" means the cell containing the two recombinant nucleic acid sequences, which can be used either as a medicament or to produce defective recombinant viruses.

 The expression "target cell" means the cell which it is sought to treat by introduction of the transgene.

 The present invention responds and makes it possible to overcome the limits of the existing systems mentioned above by the construction of a host-vector system making it possible to express a transgene in a target cell or a human or animal tissue, characterized in that it consists of a eukaryotic cell established in line, in which have been transfected a) a recombinant pseudo-retroviral sequence in which the env gene has been deleted in whole or in part and substituted by said transgene at the level of the env gene as shown in Figure la, so that the transgene is expressed on spliced transcripts similar to those of the env gene in wild Moloney virus; b) a nucleic acid sequence including a sequence coding for an envelope or membrane protein, which sequence is under the dependence of a promoter and is associated if appropriate with said transgene, and flanked at its 3 ′ end a site of polyadenylation and shown in Figure lb; said recombinant viral genome and said sequence being capable of complementing each other in trans and allowing said host cell to produce defective infectious viruses lacking the env gene.

 It appears clearly in this system that the host cell harboring the two recombinant sequences above cannot be assimilated to a packaging cell insofar as the first recombinant pseudo-retroviral sequence carries the psi sequence and allows, when it is transfected alone, independently of the second sequence containing the env gene, the production by the host cell of viral particles carrying the entire starting sequence, but which are devoid of envelope and therefore non-infectious.

The cell therefore behaves well as the host of a normal or recombinant viral vector authorizing the production of viral particles without the addition of
Helper or co-expression in trans as described above.

 The addition in this host cell of a gene coding for the viral envelope protein under the dependence of a promoter and followed by a polyadenylation sequence allows the transcription, then the translation of this gene into protein in the host cell. and, subsequently, the reconstitution of viral particles carrying the same genome as the previous ones (and represented in FIG. 1 a) except that they now have an envelope which enables them to infect target cells. These infectious viral particles are only, after infection of a target cell, capable of producing infectious viral particles if the env gene is added again and is expressed.

 This system is particularly advantageous as we will see below when the transgene to be expressed is a suicide gene, in particular the thymidine kinase of the Herpes Simplex virus (HSV1-TK).

In the host-vector system of the invention, the recombinant pseudo-retroviral sequence is derived from the genome of the Moloney MuLV virus, the 5 'or 3' LTR sequences being of wild origin or derived from different mutants or combined. Mention may be made, for example, of constructions of the LTR type derived from the mutants mov3, mov9 and movl3 as described in
Caruso M. et al. in Proc. Natl. Acad. Sci. USA, 90: 7024-7028, 1993.

 This nucleic acid sequence (FIG. 1 a), which will be encapsulated in the viral particle produced by the host cell, is extremely close to the "wild" retroviral genome, which has the advantage of obtaining higher titers than those obtained in the systems described above in the state of the art with the packaging lines and the trans complementations of the viral genes.

 By way of example, and as we will see below, while in prior art systems the maximum titer with the packaging cells is of the order of 106, this system allows titles between 106 and 109 and preferably between 107 and 108 PFU / ml.

 A second characteristic of the host vector system of the invention is that the host cell is transferred simultaneously with a nucleic acid sequence including a sequence coding for an envelope protein, this protein being of viral or even retroviral origin, or membrane protein. , or even cellular.

 "Simultaneously" means that the two sequences, the pseudo-retroviral sequence and the nucleic acid sequence coding for the envelope gene, are expressed simultaneously in the host cell, the transfections being able to be carried out either simultaneously or successively.

The envelope gene chosen may be homologous to the pseudo-retroviral sequence, that is to say be derived for example from Mo-MuLV, allowing the reconstitution of homologous pseudo-viral particles or be derived from another virus, for example the vesicular stomatitis virus (VSV) or that of HIV, which gene when it is transcribed and translated into envelope protein allows the reconstitution of a viral pseudo-type whose envelope is an envelope of VSV (or of
HIV) and the genome comprising the LTR, psi, PB, gag and pol of MuLV, as well as the transgene.

Finally, the env genome can be of cellular origin and code for a membrane protein allowing targeting of the viral particle on a specific ligand, in particular for a type receptor.
CD4.

 In addition, the envelope can be a chimeric protein whose carboxyterminal end is derived from the intramembrane sequences of the Moloney envelope. There are indeed experimental data which show that these membrane sequences can be involved in the concentration of these envelope proteins on the surface of the viral particle. With these chimeric molecules, the efficiency of expression of the chimeric env protein on the surface of virions will be increased.

 A second advantage of this host-vector system allowing the expression of these heterologous viral pseudo-types is the possible possibility of purifying these viral pseudo-particles by ultracentrifugation without losing infectious power. This makes it possible to obtain viral suspensions whose titer can reach 109 PFU / ml.

The nucleic acid sequence containing the env gene as described in Figure 1b below, can be transfected by any type of means, whether physical means or using viral or retroviral vectors. Among the known physical means, mention may be made of micro-injection, liposomes or even so-called biolistic or bombardment methods as described in Kriegler, M. Gene
Transfer and Expression; A Laboratory Manual; MacMillan
Publishers Ltd .; 1990.

 The transfection can also take place by integration of this nucleic acid sequence into a viral vector and in particular of the adenovirus.

 The advantage of this last method is the possibility of setting up a self-sustaining system in vivo while keeping full control of viral proliferation.

 Indeed, the host-vector system comprising as transgene a suicide gene and injected for example into a tumor will express a recombinant viral particle which, itself, will reinfect dividing cells in situ.

 These cells can in turn reexpress a viral particle, but the latter will be non-infectious since the viral genome will not have been complemented in trans by an envelope gene.

 If, at this stage, a recombinant expression vector containing the env gene is injected, there may again be trans-complementation and the cycle reproduced once more; the security of the system is, in this way, preserved since a double transfection is necessary, one by the viral pseudo-particle MuLV capable of infecting only the dividing cells and the other by the adenovirus capable of infecting all of the cells. Therefore, only dividing cells are likely to be co-transfected, to express viral genes in trans and to produce viral pseudoparticles capable of infecting target cells.

 This system therefore makes it particularly elegant to transfect into a target cell, for example the suicide gene TK, allowing the expression of the latter and conferring on the cell which contains it the sensitivity to nucleoside analogs; the simultaneous transfection of an envelope gene allows the production by this same target cell of infectious particles comprising the same recombinant pseudo-viral genome defective in the envelope gene.

 This cycle can therefore be repeated the number of times desired as long as the carrier sequence for the env gene is provided exogenously; the system stops after a replicative cycle of the virus in the target cell, production of infectious particles, reinfection and production of non-infectious particles.

 The whole of this cycle is described in Figure 2.

 A third advantage of the host-vector system according to the invention is that any type of eukaryotic cells established in lines can be chosen from the moment when this line is capable of producing this type of amphotropic viral particles.

 In addition to the NIH / 3T3 lines conventionally used to produce recombinant viral particles of Moloney virus, the host-vector system of the invention allows the use of cultivable lines in suspension, such as for example myelomas of mice or cells of insects.

 The obvious advantage of using cells in suspension is the industrial application for the preparation of drugs usable in gene therapy, large quantities of these cells being necessary for such preparations which are obviously much easier to obtain with cells in suspension only with adherent cells.

 This new host-vector system described above can also be advantageously implemented by using lepidopteran cells or more generally insect cells (with the exception of Diptera) as host cells.

These cells indeed have numerous advantages for building host-vector systems usable as drugs in gene therapy: - they are capable of producing proteins
virus envelopes infecting
eukaryotes but themselves are not
infectable with these viruses - insect cells do not produce viruses
of mammals, viral safety tests
concerning are therefore reduced;
these cells are not infected with viruses
potentially pathogenic to humans; he is
so much safer to use these cells
for use in human therapy on the plan
viral safety testing - insect cells grow in suspension and
therefore can be cultivated on a large scale
insect cells are able to
intratissular movements; also, in the case
where they would be injected into a tumor,
they can therefore move around and deliver their
pseudo-viral particles distant from the place
injection-it is possible to use promoters
specific to insect cells which are
strong promoters: for example, it is possible
to use baculo virus promoters which
give high expression rates and therefore a
many increased pseudo-viral particles
since there are several strong promoters
from baculo virus, so it's possible
perform complex constructions in order to
limit the risks of recombination between
different transfected genes.

 All these advantages make insect cells, possibly genetically modified in order to be able to produce amphotropic pseudoviral particles, a tool of choice for the construction of the host-vector system of the invention.

 A person skilled in the art will know, on a case-by-case basis, according to the transgene he wishes to express in a target cell, to carry out the constructions using the appropriate expression vectors and the appropriate host cells allowing the transcomplementation of the envelope gene with the genome retroviral carrying the psi sequence, gag, pol genes and transgenes, and allowing dissemination by the addition of the env gene, controlled in time and space.

 Another advantageous improvement of the host-vector system of the invention consists in integrating, into the nucleic acid sequence carrying the env gene, a sequence of the HSV1 thymidine kinase gene or another conditional suicide gene; this improvement is a second key making it possible to increase the efficiency of the treatment, as well as the security of the system, since, in this way, the thymidine kinase gene is expressed in the target cell under the dependence of promoters of different types and, by Consequently, this makes it possible to increase the sensitivity to nucleoside analogs conferred on the cells by the expression of this thymidine kinase.

Another particularly advantageous improvement for viral safety problems in therapeutic use in humans is the use of a host-vector system in which - the host cells are insect cells,
non-mammalian virus producing - the vector carrying the env gene used under the
dependence of a promoter includes upstream of
env gene a homologous retrovirus sequence
defective recombinant used in the system
host-vector, for example all or part of a gene
gag or all or part of a pol gene.

 This system is interesting because, if homologous recombination with a retrovirus occurs in a system where the host contains endogenous retroviruses, for example in mouse cells, then it will lead to the production of non-functional retroviruses. This recombination will take place so that the resulting recombinant genome will also be defective. Injecting this type of cell into humans will be safe from a viral point of view; similarly, the use with this system of an insect cell in the host-vector system cannot lead to homologous recombinations, the insect cells not carrying a mammalian retrovirus and, consequently, the injection for therapeutic purposes, it will only be possible to produce the defective recombinant viral particles of the invention, excluding viral particles resulting from homologous recombination and which could then be infectious.

 The present invention also relates to a method of expression of a transgene for gene therapy which consists in the implementation of the following steps a) construction of a host-vector system obtained by transfection in a eukaryotic host cell, d firstly, a pseudo-retroviral sequence in which the env gene is deleted in its entirety and replaced by the transgene, for example at the ATG of said env gene and, secondly, an acid sequence nucleic acids comprising in its structure a sequence coding for an envelope protein under the control of a promoter, if appropriate associated with the transgene and flanked at its 3 ′ end with a polyadenylation sequence, b) bringing said system into contact with the cells in which the transgene must be expressed, c) if necessary, transfer again into the target cells of the above sequence containing the env gene.

 In the process of the invention, the transgene can be a gene of therapeutic interest and in particular a suicide gene such as the thymidine kinase gene from HSV1 conferring, as was said above, the sensitivity of the cells expressing this gene to nucleoside analogs.

In the method of the invention, the pseudo-retroviral sequence is directly derived from the
Moloney MuLV, the 5 'or 3' LTR sequences being derived directly from particular viral species such as mov9, mov3 or movl3, they can be of wild, mutant or combined origin.

 In the method of the invention, the sequence containing the env gene is chosen in particular from the retrovirus sequences coding for said gene and in particular the env gene of MuLV or the env gene of VSV or HIV. It goes without saying that the reconstituted viral particle is a hybrid particle, the envelope of which consists of the envelope protein of the virus from which the sequence of viral particles has been derived, can only be produced in dividing cells and by transcomplementation of the viral pseudo-particle derived from the Moloney virus and from the envelope gene derived from the adenoviral vector.

 More generally, the invention relates to a method allowing the expression of a transgene for gene therapy characterized by the simultaneous transfection into target cells of a retroviral genome of general structure represented in FIG. 1a, which genome has all the sequences necessary for the expression of a viral particle, but lacks the envelope gene conferring on said viral particle its infectious nature and, on the other hand, a nucleic acid sequence containing in its structure an envelope gene of virus or cell, the co-expression in trans of the two sequences thus transfected allowing the expression of infectious recombinant viral particles but lacking in their genome of said env gene.

 This co-expression in trans of the two types of nucleic acid sequences can be carried out either in a host cell thus allowing the production of a host-vector system usable as such in gene therapy, or by simultaneous injection at the level of target cells to be treated.

 I1 clearly appears to those skilled in the art the iterative nature of the system used by adding as necessary the second nucleic acid sequence carrying the env gene and capable of being expressed under the control of a promoter.

 The invention also relates to the use in gene therapy of the host-vector system described above and obtained by simultaneous transfection into a eukaryotic cell of two nucleic acid sequences whose expression in trans allows said eukaryotic cell to produce infectious viral particles carrying the transgene, this use being advantageously applicable to the expression of a suicide gene in target cells to be destroyed.

 Finally, the invention relates to drugs which can be used in gene therapy and which are characterized in that they contain, as active principle, eukaryotic cells which have undergone double transfection with the nucleic acid sequences as described in FIGS. 1a and 1b; the active principle of a medicament according to the present invention may also consist, on the one hand, of infectious recombinant viral particles carrying a viral genome in which the env gene has been substituted, for example at the level of its initiation codon by the transgene to be expressed and, on the other hand, by a nucleic acid sequence comprising in its structure the envelope gene, under the dependence of a promoter and, where appropriate, flanked by a sequence coding for example for the gene thymidine kinase.

 Those skilled in the art will know how to implement all the possible variants of this type of host-vector system, by routine experimentation.

 Such variants, or equivalents, form part of the invention as claimed below.

 The figures and examples which follow will illustrate the invention more precisely.

- Figure 1 shows the construction of the two nucleic acid sequences to be transfected simultaneously.

In this figure
figure la represents the recombinant virus
defective Moloney in which the gray area
represents sequences derived from the
Moloney and the X gene on a black background represents the
transgene
FIG. 1b represents a Y gene coding for
a viral envelope or for a protein
membrane flanked in 5 'by a promoter and
optionally a retrovirus sequence, and
flanked in 3 'of a polyadenylation sequence
figure lc, for the record, indicates the sequence
wild Moloney virus.

- Figure 2 shows the principle of coexpress ion in trans and the production of defective and infectious viral particles allowing the reintroduction into a target cell of the defective recombinant retroviral genome.

 In the first part of the diagram, a vector of the type represented in FIG. 1a is introduced into the host cell. (It should be noted that this introduction can be carried out either by transfection or by infection with a defective retroviral particle as described at the end of this diagram). The result of this expression of the vector represented in FIG. 1a is the production of viral particles whose genome is derived from this vector but which do not carry an envelope protein on their surface and which are therefore non-infectious (as represented in the second part of the diagram).

 A vector of the type represented in FIG. 1b, carrying the env gene, is then expressed by transfection in this cell. The complementation in trans then allows the expression on the surface of the viral particles of the env protein, these viral particles remaining devoid of the env gene.

- Figure 3 shows the therapeutic scheme from the generation of cells producing infectious viral particles carrying the transgene to their injection in situ into the tumor as well as the perpetuation of the infectious cycle by in situ complementation with an envelope gene. The left column shows the production of defective recombinant viral particles as already represented in FIG. 2, which cells are injected in vivo into a tumor. The defective recombinant viral particles produced in situ are then capable of infecting tumor cells the therapeutic effect of the transgene can then be expressed in these cells and the transduced tumor cells themselves produce viral particles carrying the transgene and devoid of envelope. The cycle stops at this level, unless the vector is added later according to FIG. 1b.

EXAMPLE I Construction of a Moloney virus carrying a therapeutic gene and defective for the viral envelope
The vector used is derived from the retroviral vector pMA 245, which is derived from the virus of
Moloney MuLV and was built from fragments
MuLV cloned from different Mov species (Mov-3, -9, and -13) as established by Jaenisch et al., 1981, Cell 24: 519-529.

 Genetic construction is easily carried out by a person skilled in the art. We start from a plasmid carrying an infectious Moloney provirus. Such a plasmid, when transfected into a suitable cell line (for example mouse NIH-3T3 cell), gives rise to infectious viral particles carrying the wild Moloney genome derived from the transfected genetic construct. In this genome, the gene coding for the envelope is replaced by a gene coding for the protein of therapeutic interest. This construction is carried out so that this gene is synthesized from the same spliced transcripts as the "wild" envelope gene.

For the sake of efficiency of expression of the gene of therapeutic interest, and also efficiency of production of retroviral particles, it is even possible to substitute the gene of therapeutic interest so that the ATG codon thereof is found exactly at the position of the ATG of the Moloney env gene. The gene of therapeutic interest is more readily derived from complementary DNA and therefore does not contain an intron and is also devoid of polyadenylation sequence. The genetic construction is such that it therefore respects the natural splicing of the
Moloney, that it respects the framework for reading the gag and pol genes and that it respects the integrity of the
Moloney virus LTR3 '. The entire genetic construction is carried out according to the rules of the art by enzymatic digestion of DNA then ligation of the appropriate fragments; these techniques are described in Maniatis et al., 1989, Molecular Cloning, A
Laboratory Manual. If necessary, it uses PCR to generate genetic fragments at the ends of which are the restriction sites suitable for cloning.

If necessary also, the different genetic fragments making up the infectious Moloney provirus can be broken down into shorter fragments distributed over the separate plasmids, so that they are more simply manipulated and then reassembled in an appropriate manner. If necessary also, site-directed mutagenesis can be used to introduce either the restriction sites necessary for carrying out the genetic construction, or to substitute for specific positions the various genes. At the end of this genetic construction, there is a plasmid which, when transfected into a cell (for example NIH-3T3), is expressed as a wild Moloney virus and therefore results in the generation of particles. viruses whose genome is made up of the provirus itself. Due to the homology between the infectious provirus and the newly constructed defective provirus, the synthesis of the different RNAs is carried out with efficiency in optimized proportions which make the number of viral particles which are produced by the cell very close to that of a wild virus. This number of viral particles is therefore much higher than that produced by conventional packaging lines in which the different structural genes of the Moloney virus are carried by different genetic constructs, and in which the retroviral vector which constitutes the genome of the viral particle is on a different plasmid and has a very different structure and size than the
Wild Moloney. It should be noted that the viral particles thus produced are not infectious because they lack an envelope. This envelope protein can be brought independently into this cell using a vector, the construction of which is carried out below.

EXAMPLE II Construction of the nucleic acid sequence carrying the env gene
It is indeed possible to transfect into this cell a new simple genetic construction containing a gene with membrane expression under the control of a conventional promoter (such as for example the promoter of the SV40 virus, a promoter of a housekeeping gene such as PGK, or else even a specific promoter of a given cell type and which will therefore offer additional security of use). This expression vector also comprises polyadenylation sequences originating from different genetic constructions (polyadenylation sequences of SV40, of Globin, of growth hormone, etc.). The membrane gene can either be an envelope of the Moloney virus itself, or an envelope of other virus which can be picked up by the viral particle and expressed on the surface thereof, or even a membrane cell protein which can also be present on the surface of the virus particle. This is particularly the case for the molecule
CD4 which has been shown to be able to be incorporated by different retroviruses.

 Another embodiment is the construction of chimeric membrane proteins, such that their intracellular part is derived from the intracellular part of the envelope of the Moloney virus, and their extracellular part is a membrane protein allowing better targeting of the viral particle. The presence of the intracellular regions of the Moloney envelope probably ensures better efficiency of expression of this chimeric envelope on the surface of the viral particle.

 This second genetic construction is also carried out so that it does not contain any sequence derived from a Moloney virus other than the envelope gene itself. More precisely, it cannot contain a sequence which has not previously been deleted on the preceding genetic construction. In this way, the possibility of recombination between this expression vector and the defective retrovirus as previously described is greatly minimized, a recombination which could lead to the production of an infectious Moloney retrovirus this time. . These genetic constructions are also carried out according to the rules of the art as indicated above.

EXAMPLE III Expression of viral particles by transcomplementation of the two sequences constructed respectively in Examples I and II
The constructs are used to infect p3X63Ag8 myeloma cells.

- EXAMPLE IV: Experimental results obtained on established tumors of the host-vector system obtained with the constructions described in Examples I, II and
III
I1 was found that this host-vector system thus constructed produces infectious retroviral particles at a titer of 108 particles / ml.

Titer was checked by cell infection
L- of thymidine kinase deficient mice and infected according to the method described in Caruso M. et al., Proc. Natl. Acad. Sci. USA, 90: 7024-7028, 1993.

In conclusion, the method of treating cells in vivo by gene therapy using the host-vector system and the drugs containing as active principle this system appears to be particularly advantageous with regard to the preexisting techniques on the various points mentioned more high, namely
- the productivity of the emitted viral particles
(at least 1 to 2 logs above productivity
classic packaging cells),
- the operational nature at the level of a stake
in industrial production of a
drug using this system since this system
host-vector can use host cells
cultivable in suspension and therefore at large
ladder,
- an increase in the efficiency of the system
target cell level since production
of infectious viral particles carrying the transgene is an iterative process which can be repeated in vivo by adding, using a vector or any suitable means, a nucleic acid sequence carrying the env gene and dependent on its promoter; the system can be blocked making it possible to stop any dissemination of recombinant viral particles by stopping the addition of the env gene; this makes it possible to control, in the case of suicide genes for example, the number of cells destroyed by adding nucleoside analogs as a function of the size of a possible tumor or of the number of cells to be destroyed, - finally, this system allows great flexibility of use while maintaining viral safety and can be applied to any other conditional gene expression system.

Claims (22)

 1. Host-vector system for expressing a transgene in a target cell or a human or animal tissue characterized in that it consists of a eukaryotic cell and established in line, in which have been transfected a) a pseudo sequence -recombinant retroviral in which the env gene has been deleted in whole or in part and substituted by said transgene at the level of the env gene is represented in FIG. b) a nucleic acid sequence including a sequence encoding an envelope protein, which sequence is dependent on a promoter and is associated, where appropriate, with said transgene, and flanked at its 3 ′ end with a polyadenylation site and represented in FIG. 1b, said recombinant viral genome and said sequence being capable of complementing in trans and allowing said host cell to produce defective infectious viruses lacking the env gene.  2. Host-vector system according to claim 1, characterized in that the nucleic acid sequence including a sequence coding for an envelope protein further comprises, upstream of said sequence, a sequence homologous to the defective recombinant retrovirus used in the system host vector chosen in particular from all or part of the gag gene or all or part of the pol gene.  3. System according to claims 1 and 2, characterized in that the transgene to be expressed is a suicide gene, in particular the thymidine kinase of the virus Herpes Simplex (HSV1-TK).  4. System according to claims 1 and 2, characterized in that the recombinant pseudoviral sequence is derived from the Moloney MuLV genome, the 5 'or 3' LTR sequences being of wild, mutant or combined origin.  5. System according to claim 1 or 2, characterized in that the nucleic acid sequence coding for the env gene is of viral origin and chosen in particular from the sequences coding for the env gene of MuLV or VSV, HIV.  6. System according to claim 1 or 2, characterized in that the nucleic acid sequence coding for the env gene is of cellular origin and consists of a sequence coding for a membrane protein, allowing targeting of the viral particle on a specific ligand, in particular a protein coding for a CD4 type receptor.  7. System according to one of claims 1 or 2, characterized in that the env gene is a chimeric protein in which 1 the carboxyterminal end is derived from the intramembrane sequences of the Moloney envelope.  8. System according to claim 1 to 7, characterized in that the sequence containing the env gene can be introduced by infection with a viral vector, in particular the adenovirus, or by a physical method.  9. System according to one of claims 1 to 8, characterized in that the eukaryotic cell is a cell established in line and can be chosen in particular from fibroblastic lines such as line 3T3, or lines capable of being cultivated in suspension ion such as mouse myeloma cells.  10. System according to one of claims 1 to 8, characterized in that the cells used are lepidopteran cells.  11. A method of expression of a transgene for gene therapy consisting in the implementation of the following steps a) the construction of a host-vector system obtained by transfection into a eukaryotic host cell, on the one hand, a pseudoretroviral sequence in which the env gene is deleted in its entirety and replaced by the transgene, in particular at the ATG of said env gene and, on the other hand, a nucleic acid sequence comprising in its structure a sequence coding for an envelope protein under the control of a promoter, if appropriate associated with the transgene and flanked at its 3 ′ end of a polyadenylation sequence, b) bringing said system into contact with the cells in which the transgene must be expressed, c) if necessary, the transfer again into the target cells of the above sequence containing the env gene.  12. Method according to claim 11, characterized in that the transgene is a suicide gene, in particular the thymidine kinase gene of HSV1, and in that the cells in which said transgene is integrated are destroyed by the addition of an analogue of nucleotide, including ganciclovir or aciclovir.  13. Method according to claim 11, characterized in that the pseudoretroviral sequence is directly derived from the Moloney MuLV virus, the 5 'or 3' LTR sequences being of wild, mutant or combined origin.  14. Method according to one of claims 11 to 13, characterized in that the sequence coding for the env gene is chosen in particular from the sequences of viruses coding for said gene, in particular MuLV, or the VSV, HIV, or sequences encoding cell membrane proteins, including CD4.  15. Method according to one of claims 11 to 13, characterized in that the sequence containing the env gene is introduced into the target cell by a viral vector, in particular the adenovirus.  16. Method according to one of claims 11 to 13, characterized in that the sequence containing the env gene is introduced into the target cells by physical methods, in particular by bombardment, fusion of liposomes, micro-injection.  17. Method according to one of claims 11 to 13, characterized in that the cell used is chosen from established lines not expressing the transgene of interest considered, in particular mouse 3T3 fibroblastic lines, mouse myelomas or insect cells.  18. A method allowing the expression of a transgene for gene therapy which consists in a double transfection of the target cells, on the one hand, by defective infectious viruses obtained by the host-vector system of claim 1 and, on the other hand, by a nucleic acid sequence containing in its structure a retrovirus envelope gene, the coexpress ion in vivo in trans of the two sequences allowing the expression of infectious recombinant viral particles but lacking in their genome of said env gene.  19. Use in the manufacture of a medicament for the gene therapy of a host vector system obtained by the succession of the following stages a) simultaneous transfection into a eukaryotic cell, on the one hand, of a pseudoretroviral sequence in which the gene env is deleted in its entirety and replaced by the transgene and, on the other hand, a nucleic acid sequence comprising in its structure a sequence coding for an envelope protein under the control of a promoter, optionally associated with the transgene and flanked at its 3 'end with a polyadenylation sequence, and, if appropriate, associated at its 3' end with a sequence homologous to the defective recombinant retrovirus used in the host-vector system of a gene coding for a retroviral protein , b) bringing said system into contact with the cells in which the transgene must be expressed, c) if necessary, transferring again into the cells uthe targets of the above sequence containing the env gene.  20. Use according to claim 19, characterized in that the gene to be expressed is a suicide gene.  21. Use according to claim 20, characterized in that the suicide gene is the thymidine kinase gene, the target cells in which the gene is expressed being destroyed by addition of a nucleoside analog, in particular aciclovir or ganciclovir .  22. Medicinal product usable in gene therapy characterized in that it contains as active principle eukaryotic cells having undergone a double transfection and represented by the host-vector system according to claim 1.