WO1995003399A2

WO1995003399A2 - Production method for preparation of disabled viruses

Info

Publication number: WO1995003399A2
Application number: PCT/GB1994/001559
Authority: WO
Inventors: Stephen Charles Inglis; Michael Edward Griffith Boursnell; Claire Entwisle
Original assignee: Cantab Pharmaceuticals Research Ltd
Current assignee: Cantab Pharmaceuticals Research Ltd
Priority date: 1993-07-19
Filing date: 1994-07-19
Publication date: 1995-02-02
Anticipated expiration: 1996-01-19
Also published as: WO1995003399A3; AU7192394A

Abstract

A transgenic species whose genome comprises a heterologous gene which is functionally equivalent to a non-retroviral virus gene which is essential for the production of normal infectious non-retroviral virus is taught. A method for manufacturing a mutant virus whose genome is disabled in respect of at least one virus gene essential for the production of infectious virus, such that the virus can infect normal cells and undergo replication and expression of viral antigen gene in those cells, but cannot produce normal infectious virus, which method comprises following the production of a transgenic animal whose genome comprises at least one heterologous gene which is functionally equivalent to the at least one virus gene, infecting at least one cell derived from the animal which contains the at least one heterologous gene with mutant virus; causing the at least one cell to express the heterologous gene so as to allow production of infectious virus particles containing the disabled gene; and harvesting infectious virus particles containing the disabled genome.

Description

PRODUCTION METHOD FOR PREPARATION OF DISABLED VIRUSES

The present invention relates to a method for preparing disabled viruses. In particular, the method relates to preparing disabled viruses for prophylactic and therapeutic use against human and animal disease. In particular it relates to the use of transgenic animals and cell lines prepared therefrom that have been engineered to contain virus genes to propagate viruses from whose genome the said genes have been removed.

Background to the invention

The use of genetically disabled viruses has been suggested for several clinical applications. One of these is immunisation in which a disabled virus may be administered to a human or animal in order to generate protective responses against antigens of the disabled virus itself, or against the products of heterologous genes incorporated into the disabled virus genome (Eloit et al., J. Gen.

Virol., 71, 2425, 1990; Nguyen et al. , J. Virol., 66, 7067, 1992; Farrell et al., J. Virol 68, 927-932; 1994; and O92/05263) . A second application for genetically disabled viruses is for gene therapy in which the virus is used to introduce heterologous DNA, either temporarily or permanently into the genome of cells within a whole animal in order to achieve a beneficial effect from expression of the inserted sequences (Fink et al. , Hum. Gene. Ther., 3, 11, 1992; Federoff et al. , Proc. Natl. Acad. Sci., 89, 1636-1640, 1992; Culver et al. , Science, 256, 1550, 1992). Disabled viruses could also be used to destroy specific target cells to achieve a therapeutic benefit.

In each case, the success of the application depends on the capacity of the disabled virus to infect host cells. Thus the genome sequences contained within the virus will be introduced into a host cell and may be expressed to achieve a therapeutic or prophylactic effect. This effect may arise from immunological recognition of the encoded proteins (immunisation) or from direct expression of a beneficial or deleterious gene sequence (gene therapy) . Since the disabled virus will, however, be unable to complete its replication cycle and so will not have the capacity to spread from cell to cell within the infected host, it should not have the capacity to cause general damage to the host organism. Indeed for gene therapy applications, it may be important that the replication cycle of the disabled virus is blocked at a very early stage, so that the cell infected with the disabled virus is not killed by the virus and retains the capacity to express the gene for some time.

Currently the most appropriate strategy for creating a genetically disabled virus is to engineer the virus genome so that at least one essential component of the genome is lost or disabled, resulting in a virus which cannot complete its replication cycle in a target cell. In order to generate a virus with these characteristics, however, it is necessary to develop a system in which the disabled virus can be propagated. This may be done by creating a cell or cell line in which the missing or disabled virus gene or genes are present and expressed. Thus the cell should be able to complement the defect within the virus genome, and allow virus replication. There have been some experimental studies concerning the generation of such complementing cells and of their use to produce genetically disabled viruses of a variety of different types (Harrison, Graham & Williams, Virology 77, 319-329, 1977; Culver et al 1992; ibid; Gluzman, Cell, 23 182-195, 1981; Cai et al, J. Virol. 62, 714-721, 1987; Ligas and Johnson, J. Virol. 62, 1486, 1988; Deluca et al. , J. Virol., 56, 558, 1985) . Furthermore, the general principles and methodologies relating to the production of a genetically disabled virus in complementing cells, is disclosed by the present applicants in WO92/05263. Reference may be made to that patent publication for technical information needed to implement the present invention.

The present invention provides materials and methods relating to the preparation of disabled viruses for which in vitro cell culture systems are unavailable or unacceptable.

For example, in the preparation of material for administration to humans, strategies involving the use of continuous cell lines grown in the laboratory may prove unacceptable. Such cell lines, which have been the basis for the production of all complementing cells described so far, are generally immortal. This means that they may carry oncogenic mutations which could increase their tumorigenic potential. A production method using cells of this type may therefore not be appropriate for the manufacture of commercial vaccines and therapeutics based on disabled viruses. In which case, the use of non- immortal complementing cell lines such as the human diploid fibroblast line MRC5 might be considered as an alternative. The problem here is that non-immortal cells can divide only a limited number of times before they die. To date, complementing cell lines have generally been prepared by transfection of a cell population with the appropriate gene or genes, and selection of a single cell clone for propagation. The number of cell divisions required to produce sufficient cells from the original transfected cell, for large scale manufacture, may thus exceed the useful life span of the cells for the production of disabled virus.

The present invention may also be usefully employed where the virus in which a disabling mutation is desired, grows poorly, if at all, in continuous cell lines, but can be grown effectively in primary cell cultures or tissue derived from a suitable animal, or in the whole animal itself. Examples of viruses for which propagation systems based on continuous laboratory cell lines are unavailable or inadequate include hepatitis virus, papillomavirus, and certain members of the adenovirus, astrovirus, coronavirus, picornavirus and herpesvirus groups.

The availability of animals transgenic for the complementing gene or genes and tissue or cells derived from those animals, provides a means of propagating genetically disabled versions of such viruses which ameliorates one or more of the above and other problems.

The present invention provides a method for manufacturing a mutant virus with a genome disabled in respect of at least one virus gene essential for the production of infectious virus, which mutant virus can infect normal cells and undergo replication and expression of viral antigen gene in those cells, but cannot produce normal infectious virus, which method comprises following the production of a transgenic animal whose genome comprises at least one heterologous gene functionally equivalent to the at least one virus gene; infecting at least one cell derived from the animal with the mutant virus which cell contains the at least one heterologous gene, causing the infected cell to express the at least one heterologous gene so as to allow production of infectious virus particles containing the disabled genome; and harvesting infectious virus particles containing the disabled genome. Also provided is a transgenic animal whose genome comprises at least one heterologous gene functionally equivalent to a virus gene which is essential for the production of normal infectious virus. A gene which is essential for the production of normal infectious virus may be any gene, the functional disablement of, or the absence of which from the viral genome, prevents the virus from completing its infectious cycle. The heterologous gene may comprise the virus essential gene itself, or an analogue thereof which encodes a protein substantially equivalent in function to the protein encoded by the essential gene. A transgenic animal as above is provided for use in a method for manufacturing a mutant virus, particularly use in a method for manufacturing a mutant virus for prophylactic or therapeutic use. Also provided, is use of a transgenic animal in a method for manufacturing a mutant virus as above, particularly use of a transgenic animal in a method for manufacturing a mutant virus for prophylactic or therapeutic use.

It is to be stated that the term 'animal' as used herein should be interpreted as including any species of the kingdom Animalia.

The virus gene may be derived from a DNA or an RNA virus. The virus gene may be of an RNA virus. The RNA virus may be a retrovirus. The virus gene may be of a DNA virus . The DNA virus may be a herpes virus, an adenovirus or a pox virus. The herpesvirus may be a human herpesvirus. The human herpes virus may be herpes si plex virus type 1, herpes simplex virus type 2, varicella zoster virus, cytomegalovirus, Epstein Barr virus, human herpesvirus type 6 or human herpes virus type 7.

The herpes virus may be an animal herpesvirus. The animal herpes virus may be feline herpesviruses, porcine pseudorabiesvirus, bovines herpesviruses, equine herpesviruses, avian herpesviruses, canine herpesviruses, caprine herpesviruses, piscine herpesviruses.

The essential virus gene may encode a structural protein. The structural protein may be an envelope-associated glycoprotein or a capsid protein. Alternatively the essential virus gene may encode a virus-specific non- structural protein which may be an enzyme or any other non- structural protein involved in genome replication. Where the transgenic animal comprises more than one heterologous genes in order to complement more than one disabled or missing virus genes essential for the production of normal infectious virus, the genes may code for a mixture of structural and non-structural proteins.

In the case of herpesvirus, the virus gene may be one encoding the HSV glycoproteins gH and gL, or one encoding the HSV regulatory virus proteins ICPO (RL2) , ICP4 (RSI) , ICP27 (UL54), ICP34.5 (RL1) VP16 (UL48) . The heterologous gene may be provided by functional homologues of these genes which exist in other herpes viruses. The functional homologues of the said HSV glycoproteins may be gpIII or gene 60 of Varicella zoster virus, gH(UL75) or a gL(UL115) of cytomegalovirus or gH(gp85) or gL(BKRF2) of Epstein Barr virus.

In the case of retroviruses, the virus gene may be one encoding the gag, env and pol proteins. In the case of adenoviruses, the gene may be one encoding the Ela and Elb proteins. Where appropriate, any heterologous gene may be provided by functional homologues of the above mentioned genes which exist in other viruses.

A functional homologue is a gene which encodes a protein which has an ability to function in a manner similar to another protein eg those proteins referred to.

The virus gene maybe of a double-stranded DNA virus. The double-stranded DNA virus may be a pox virus. The pox virus maybe a vaccinia virus.

The heterologous gene maybe in said genome under the control of a promoter sequence which functions to permit expression of said heterologous gene in said animal . The promoter sequence may be the HSV-1 gD promoter sequence. The transgenic animal may be mammalian or avian or piscine. The present invention also provides a tissue or cell sample from a transgenic animal as above. The present invention also provides an egg of a transgenic species as above. The egg may be a fertilised egg.

An intact individual animal may be infected and the infectious virus particles harvested therefrom after an appropriate interval by removal of a cell or tissue sample.

Alternatively, a cell, cell sample or portion of tissue for infecting with the mutant virus maybe obtained from an individual animal, the cell or cell sample then cultured in vitro and infectious virus particles harvested by recovery from the culture. The cell sample or tissue may be an intact egg. The egg may be fertilised.

The genetically disabled but infectious particles may be useful for introducing one or more genes, or other desirable nucleic acid sequences, into cells of a human or animal subject infected therewith, in order to generate a useful prophylactic or therapeutic immune response, or to produce some other prophylactically or therapeutically beneficial effect such as gene therapy. Thus the infectious particles may be designed act as a vector. The genes may be heterologous to the infectious particle and/or the human or animal subject infected therewith. Alternatively the genes may simply be missing or disabled in the particular human or animal subject being treated by infection with the disabled virus, but it may be that these genes would be present in a normal healthy individual.

The infectious particles may also be used to introduce eg (i) genes encoding antigenic proteins from other pathogens, where the intention is to raise a useful immune response against those proteins; (ii) genes encoding immunoregulatory molecules such as cytokines or molecules involved in lymphocyte signalling where the intention is to increase or decrease the immune response raised in the infected subject against the proteins encoded by the disabled genome; or (iii) genes encoding enzymes or other proteins that may have direct therapeutic benefit. The above are illustrative examples and the list is not intended to be exhaustive.

The infectious particles may be administered by direct inoculation of a human or animal subject, or by in vitro infection of the cells taken from the subject. The cells infected may be subsequently reintroduced into the subject for culture or alternatively cultured in vitro.

Selected virus genes which are essential for the production of normal infectious virus, may be inactivated within the chosen virus genome, usually by creating specific deletions within, or encompassing the genes. Where administration of the disabled virus is designed to stimulate a useful immune response in the treated human or animal subject, preferably a single essential virus gene will be deleted, and this gene will be selected, such that replication of the viral genome will not be prevented.

Thus cells of the treated subject which are infected with the disabled virus will be able to produce more proteins encoded by the replicated genetic material deriving from the virus, leading to an enhanced immune response. The essential gene deleted, may in some cases encode a protein which instead of being needed for assembly of new virus particles inside the cells of the treated host, is required to confer infectivity on the assembled particles. In this case, infection of the host cells will result in the production of new virus particles which are not infectious, and which therefore cannot multiply further within the treated host. The applicants have described an example of this kind of approach in WO92/05263. WO92/05263 describes the creation of a mutant virus in which the glycoprotein H gene is deleted from HSV in order to create a disabled virus for prophylactic and therapeutic vaccination against herpes simplex virus. This disabled virus can be propagated in complementing cells carrying and expressing a functional gH gene, and the virus produced from these cells used to successfully vaccinate both prophylactically and therapeutically, against herpes simplex virus-induced disease in mouse and guinea pig models. (WO92/05263; Farrell et al., J. Virol. 68, 927-932, 1994) . To date however, the cells available for complementing the disabled virus have been based on continuous laboratory cell lines. The present application describes as an example illustrative of the general teaching herein, the generation of gH-complementing cells through creation of an animal transgenic for the gH gene.

Where administration of the disabled virus is designed to carry useful genes into the treated host cells in order to achieve long term expression of those genes for therapeutic benefit, it may be preferable to select for deletion one or more genes that act early in the virus replication cycle, and are needed to allow virus genome replication. Thus the disabled virus genome may be introduced into cells in a way that leads to survival of the infected cell and long term expression of the genes carried by the virus genome.

An example of this kind of strategy has been described for herpes simplex virus. A mutant virus with a deletion in the essential ICP4 gene, which could be propagated on a complementing cell expressing the ICP4 gene, was engineered to contain a marker gene (encoding 3-galactosidase) under the control of a retrovirus promoter. This disabled virus was then inoculated into the sciatic nerve of a mouse, leading to the expression of β-galactosidase in a stable number of sensory neurons in the dorsal root ganglian for up to 24 weeks (Dobson et al. , Neuron, 5, 353-360, 1990) . Thus genetically disabled HSV viruses can be used as vectors to induce long term expression of therapeutically useful gene products in neuronal cells.

A further example of this approach is the use of genetically disabled retroviruses to deliver a variety of heterologous genes or nucleic acid sequences into host cells for therapeutic or prophylactic effect (Culver et al., 1992, ibid) . In this case the disabled genome may lack all of the virus protein coding sequences, in particular those encoding gag, pol and env genes, retaining only those sequences required for packaging of the disabled genome into virus particles and integration of the disabled genome into the genome of the host cell. In order to achieve successful packaging of the disabled genome, the virus must be propagated in a cell which can provide each of the missing gene products.

A virus with one or more essential genes deleted (or functionally destroyed) from its genome has to be grown in a cell (the complementary cell) which provides the virus with the product of the deleted gene. Hence although the virus lacks a functional gene encoding an essential protein, if it is grown in the appropriate host cell, it will multiply and produce complete virus particles which are to outward appearances indistinguishable from the original virus. The mutant virus preparation is inactive in the sense that it has a defective genome and cannot produce new infectious virus in a normal host, and so may be administered safely in the quantity required to generate a beneficial effect in the host.

However, since the mutant virus grown in the complementing cell should have all of the virus proteins normally present in the non-mutant virus, it will still, itself, be infectious, in the sense that it can bind to a cell, enter it, and initiate the viral replication cycle. It can therefore provide the opportunity for expression, within the cells of the treated subject, of genes encoded by the mutant virus genome.

This invention can be applied to any virus where one or more essential gene(s) can be identified and deleted from or inactivated within the virus genome. For DNA viruses, such as Adeno, Pox, Herpes, Papova, Hepadna, Papilloma and Parvo viruses and for Retro viruses, this can be achieved directly by (i) the in vitro manipulation of cloned DNA copies of the selected essential gene to create specific DNA changes; and (ii) re-introduction of the altered version into the virus genome through standard procedures of recombination and marker rescue. The invention however, is also applicable to RNA viruses such as those from the Myxo, Paramyxo, Rhabdo, Picorna, Alpha, Flavi, Bunya, Arena and Filoviruses. Techniques are now available which allow complementary DNA copies of a RNA virus genome to be manipulated in vitro by standard genetic techniques, and then converted to RNA by in vitro transcription. The resulting RNAs may then be re-introduced into the virus genome. The technique has been used to create specific changes in the genome of both positive and negative stranded RNA viruses, e.g. poliovirus (Racaniello and Baltimore, Science, 214, 916-919, 1981) and influenza virus (Lutyes et al. , Cell, 59, 1107-1113, 1989).

As stated earlier, as well as using such an inactivated virus/transgenic species (or transgenic cell) combination to produce safe vaccines against the wild-type virus, this invention also deals with the use of the same system to produce safe viral vectors for use as vaccines against foreign pathogens, or for delivery of useful genes into host cells for direct therapeutic benefit (gene therapy) .

An example of such a vector is one based on HSV. The HSV genome is large enough to accomr late considerable additional genetic information and several examples of recombinant HSV viruses carrying and expressing foreign genetic material have been described (e.g. Ligas and Johnson, J. Virol. 1988, supra) . Thus a virus with a deletion in an essential virus gene as described above, and also carrying and expressing a defined foreign gene, could be used as a safe vector for vaccination to generate an immune response against the foreign protein.

A particular characteristic of HSV is that it may become latent in neurones of infected individuals, and occasionally reactivate leading to a local lesion. Thus an HSV with a deletion in an essential virus gene and expressing a foreign gene could be used to produce deliberately latent infection of neurones in the treated individual. In the case where the deleted essential gene is not required early in the infectious cycle, reactivation of such a latent infection would not lead to the production of a lesion, since the virus vector would be unable to replicate fully, but would result in the onset of the initial part of the virus replication cycle. During this time, expression of the foreign antigen could occur, leading to the generation of immune response. In a situation where the deleted HSV gene specified a protein which was not needed for virus assembly, but only for infectivity or assembled virions, such a foreign antigen might be incorporated into the assembled virus particles, leading to enhancement of its immunogenic effect. This expression of the foreign gene and incorporation of its protein in a viral particle could of course also occur at the stage where the mutant virus is first produced in its transgenic complementing host. In which case, the mutant virus when used as a vaccine could present immediately the foreign protein to the species being treated. Alternatively, in the case where the deleted essential gene (or genes) were those required early in the infectious cycle, the replication cycle of the virus might be arrested before reactivation. In this situation, long term expression of a desired heterologous gene or nucleic acid sequence might be achieved in neuronal tissue, by incorporating the heterologous sequence into the HSV genome under the control of a promoter known to operate in latently infected neuronal cells, for example the HSV specific LAT promoter.

In another example, vaccinia virus, a poxvirus, can carry and express genes from various pathogens, and it has been demonstrated that these form effective vaccines when used in animal experimental systems. The potential for use in humans is vast, but because of the known side effects associated with the widespread use of vaccinia as a vaccine against smallpox, there is reluctance to use an unmodified vaccinia virus on a large scale in humans. There have been attempts to attenuate vaccinia virus by deleting non- essential genes such as the vaccinia growth factor gene (Buller, Chakrabarti, Cooper, Twardzik & Moss, J. Virology 62, 866-874, 1988) . However, such attenuated viruses can still replicate in vivo, albeit at a reduced level.

A vaccinia virus with a deletion in an essential gene, together with an appropriate complementing cell line has recently been described (Sutter et al, J. Virol 68, 4109- 4116, 1994) . It has been suggested that this virus could provide a safer alternative to replication-competent vaccinia virus for vaccination purposes.

In another example, disabled retroviruses have been used to introduce foreign nucleic acid sequences into target cells with the aim of achieving long term gene expression. Notably, the mouse retrovirus murine leukemia virus has been used to deliver the gene encoding the human adenosine deaminase (ADA) gene into human bone marrow cells in an attempt to control severe immunodeficiency (Culver et al., 1992, supra) . This gene delivery system involves the use of a disabled MuLV genome which lacks the genes encoding the virus gag, pol and env proteins, but which contains instead, the ADA gene under the control of a suitable promoter sequence to allow its expression in infected cells. The disabled genome also contains a sequence required for packaging of the DNA into MuLV virus particles. Consequently the disabled genome can be incorporated into infectious retrovirus particles when placed in a cell engineered to express the gag, pol and env proteins (the packaging cell) . These infectious particles can then be used to infect suitable target cells, either in vivo, or in vitro to achieve the desired gene transfer. Once inside the target cell, the disabled genome can become integrated into the target cell genome, and expression of the heterologous gene can ensue. As yet, the packaging cells used for this purpose have been based on continuous laboratory cell lines, some of which clearly have tumorigenic potential. Generation of packaging cell lines through production of animals transgenic for the complementing cell genes would present an attractive and much safer alternative. In another example, genetically disabled adenoviruses have been used to introduce foreign nucleic acid sequences into target cells with the aim of achieving long term gene expression.

Disabled adenovirus carrying deletions in the essential genes encoding virus early proteins such as Ela, can be propagated on complementing cells expressing the corresponding protein (Harrison et al., 1977, ibid) . These viruses can then be manipulated to contain heterologous genes, where expression is desired in an appropriate target tissue. This approach has been used to create disabled viruses carrying the human cystic fibrosis transmembrane conductance regulator (CFTR) gene which can be used to transfer the gene for CFTR to the airway epithelium of experimental animals (Rosenfeld et al., Cell, 68 143-155, 1992) and human cystic fibrosis patients (Zabner et al. , Cell, 75, 206-216, 1993) .

Although we have referred above to a mutant virus being disabled in an essential gene, and optionally containing a gene for an immunogenic pathogen protein, the mutant could be disabled in more than one essential gene, and/or contain more than one immunogenic pathogen protein gene. Thus, the mutant virus might include the gene for HIV env gene 120, to act as a vaccine in the manner suggested above, but also the genes encoding the HIV gag and pol proteins to be expressed within the cells of the vaccinated host. The present invention provides a novel and useful method by which complementing cells and disabled viruses may be generated without the need for the use of laboratory cell lines, in perpetuo. In this process, a gene or set of genes which are functionally equivalent to the gene or genes deleted from, or disabled in, the virus genome, is introduced into the genome of a suitable animal. Thus tissues from the animal in which the complementing gene or genes is expressed can be used for virus propagation. These tissues may be used in a number of ways. Most preferably, they could be used to prepare primary cultures of cells which can then be used in conventional ways to propagate the disabled virus. However there are other ways in which they may be used. For example specific tissues could be removed and used to set up organ cultures which could then be infected with the disabled virus. For avian species, the disabled virus could be propagated in embryonated eggs and the replicated virus harvested directly from the embryos or fluid surrounding them. Finally the disabled virus could be grown in the whole animal by direct infection of the whole animal and harvested by removal of specific tissues.

The invention can be applied to any virus where one or more essential genes can be identified and deleted from or inactivated within the virus genome, (see earlier for examples) . These genes can be prepared as cloned copies, by conventional procedures, placed under the control of suitable promoter sequences that will allow their expression in one or more tissues of an appropriate animal, and introduced into the animal by standard techniques for transgenesis.

The invention can be applied to any animal species whose cells are capable of supporting the replication of the disabled virus, and for which techniques are available to prepare transgenic animals. The latter species currently include mice, cattle, sheep, pigs, goats, fish and birds, but it is anticipated that in future, existing technology will be extended to include many other species, and new techniques will be developed for those animals for which current methods are unsuitable.

For avian species the disabled virus may be propagated in embryonated eggs and the replicated virus harvested directly from the embryos or fluid surrounding them. The disabled virus may also be grown in the whole animal by direction infection of the animal and harvested by removal of specific tissues.

A whole animal may also be used to propagate a disabled virus expressing a heterologous gene product, as in viruses prepared for gene therapy, and to study the function and consequences of expression of the heterologous gene product within the animal. In such systems the gene product may be of a beneficial or deleterious gene sequence. In order that the invention may be clearly understood, it will be further described by way of example only, and not by way of limitation, with reference to the following figures in which:

Fig. 1 illustrates a method for preparing a DNA fragment containing the HSV-1 gH gene under the control of the HSV-1 gD promoter for creation of a transgenic animal;

Fig. 2 illustrates PCR analysis of DNA obtained from tail biopsies of transgenic mice showing the presence of the HSV gH transgene;

Fig. 3 illustrates growth of the gH-deleted virus in primary cells from transgenic and non-transgenic control mice; and

Creation of a mouse transgenic for the HSV gH gene

Techniques for the generation of transgenic animals are well established. There are a number of different strategies available for achieving this end, any of which would be suitable for generation of animals capable of providing complementing cell lines (Manipulating the Mouse Embryo - A Laboratory Manual, Cold Spring Harbor Laboratory Press, USA, 1986) . In the following example, the technique of microinjection of DNA into the pronucleus of the developing embryo is used to generate animals in which the required DNA is inserted into the germ line of the mouse.

A variety of different eukaryotic promoters could be used to drive expression of the transgene. Some would be expected to operate constitutively in each cell of the transgenic animal, while others offer the possibility of more restricted expression, such as those which are tissue specific, or which are only activated in the presence of an inducer. For the invention described here, each type of promoter would be suitable, provided that expression of the transgene can be achieved in the appropriate cell type for growth of the disabled virus. In some cases, however, it may be that long term expression of a virus gene product could be damaging to the host cell, and so the third option, i.e. the choice of an inducible promoter is preferred.

In the following example, the HSV gH transgene is placed under the control of the HSV-1 glycoprotein D promoter. It is known that this promoter is only functional in the presence of certain other HSV-specific proteins (Forrester et al., J. Virol., 66, 341, 1992), and so expression of the gH gene will only be activated in cells from the transgenic animal following infection by the HSV virus.

All cloning steps are carried out using standard procedures (Molecular Cloning - A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory Press, USA, 1989) . a) Preparation of the HSV-l gH gene under the control of the gD promoter.

The plasmid pgDBrgH (Forrester et al. , J. Virol., 66, 341, 1992) contains the HSV-l gene under the control of the HSV- 1 gD promoter sequence (Figure 1) . This plasmid is digested with the restriction enzyme Sstl to release a fragment of 4.3kb including the gH gene and gD promoter, which is purified by gel electrophoresis and cloned into the Sstl site of the plasmid pUC119 (Viera and Messing, Methods Enzymol. , 153, 3, 1987) to generate the plasmid pIMC02 (Figure 1) . A Notl restriction site was then introduced into this plasmid downstream of the gH gene stop codon, by oligonucleotide-directed mutagenesis (Brierley et al., Cell, 57, 537, 1989) using the synthetic oligonucleotide 5' CCCGGTGGTGCCGCGGCCGCAGCCCCTCTTTG 3' .

The resulting plasmid pIMC03 (Figure 1) was digested with Sstl, treated with DNA polymerase (Klenow fragment) to create blunt ends and then further digested with Notl to release a 3kb fragment containing the gD promoter and gH glycoprotein DNA sequences. An additional enzyme, Seal is included in this final digest, in order to facilitate gel purification of the relevant 3kb fragment.

The gH gene-containing fragment is then cloned into the vector pRC/CMV (British Biotechnology Group, Oxford, U.K.) which had been previously digested with Notl and Nrul, in order to create plasmid pIMC05 (Figure 1) . The plasmid pIMC05 is digested with the restriction enzymes BspHI and Sfil to release a 4.4kb fragment including the HSV-l gD virus specific promoter upstream of the HSV-l gH gene. This fragment is isolated by electroelution from an agarose gel, further purified by ELUTIP-D chromatography as per the manufacturers instructions (Schleicher & Schuell, Dassel, W. Germany) , precipitated with ethanol precipitation, and finally dissolved in water at an approximate concentration of 200ng/μl.

b) Introduction of the gH gene into the mouse genome

The procedure for introduction of the DNA fragment containing the HSV gH gene into the genome of mouse embryos involves three stages:

1. the isolation of pre-implantation fertilised single cell mouse embryos;

2. microinjection of the HSV DNA into the pronucleus;

3. transfer of microinjected embryos into pseudopregnant recipient females.

This technique is well established and in described in detail elsewhere (Manipulating the Mouse Embryo - A

Laboratory Manual, Cold Spring Harbor Laboratory Press, USA, 1986) . Animals resulting from gestation of the injected embryos are then screened for the presence of the appropriate transgene.

c) Screening mice for the presence of the gH gene

This is carried out by PCR analysis of DNA extracted from tissue derived from the mouse tail as follows.

A 2cm section of the tail is excised with scissors, and incubated overnight at 56°C in 750μl TDB buffer (5mM EDTA pH 8.0; 200mM NaCl; lOOmM Tris pH8.0; 0.2% SDS) with proteinase K (250μg/ml) . The digested sample is then extracted twice with 500μl phenol:chloroform (1:1), and the aqueous phase removed. DNA is precipitated by the addition of 750μl isopropanol, recovered by centrifugation, washed in 70% ethanol and dissolved in lOOμl TE buffer (lOOmM Tris pH8.0; ImM EDTA) . The sample is heated for 2h to destroy DNase activity.

PCR analysis is then carried out on the DNA by standard procedures (Molecular Cloning - A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, USA, 1989) using the following oligonucleotides:

1. 5' TATGGTCGACTCTCAGTACAATCTGCTCTG 3'

2. 5' GGCGTACATGACCGCGCTGTGCTTGTGGCC 3' The result of this analysis is shown in Figure 2. PCR amplification using the primers detailed above is expected to yield a fragment of size 1278 base pairs. The presence of this fragment is evident in samples from mouse numbers 601 and 608.

Positive results from the PCR analysis are confirmed by Southern blot hybridisation of the same DNA samples after digestion with the enzyme Bglll, using radio-labelled gH fragment as probe (Vogelstein and Feinberg, Analytical Biochem. , 132, 6, 1983) .

Breeding of mice to generate homozvσotes

Mice with evidence of the presence of the desired transgene, or those of their progeny to whom the transgene has been passed, may be used directly to prepare cells for propagation of the disabled virus. However it will usually be necessary to establish a breeding colony of animals homozygous for the transgene.

For this purpose male animals carrying the transgene will be mated with females, and the offspring tested as before for the presence of the transgene. Male and female animals carrying the transgene are identified and mated in order to generate homozygotes, which are then identified in accordance with standard procedures by Southern blotting and conventional genetic backcross analysis. Propagation of a gH-deleted virus in cell cultures derived from foetal mice

a) Preparation of primary cell cultures from foetal mice

Pregnant female mice at 2 weeks gestation are anaesthetised and sacrificed. The skin is swabbed with 70% ethanol/water and an incision made in the abdomen to reveal the internal organs. The foetal mice are removed in utero, individual mice are dissected free of maternal and placental tissue and placed in sterile 35mm petri dishes containing 1ml of growth media (Dulbecco's Modified Media containing 1% glutamine, 2% penicillin/streptomycin, 1% amphotericin B, 20% foetal bovine serum) . Tissue is homogenised by sucking up and down in a 1ml syringe, transferred to a 25cm² flask and incubated to generate a monolayer culture of embryonic fibroblasts.

b) Growth of gH-deleted virus in transgenic mouse cell cultures.

Primary cultures of mouse embryo fibroblasts were cultured from transgenic 2nd generation heterozygote offspring and from control non-transgenic litter mates. Cells are seeded into 24 well plates, and infected with either gH-deleted virus (Forrester et al 1992) or wild type virus SC16, at a multiplicity of 5pfu/cell. An engineered Vero cell line transfected with plasmid pIMC05 (Fig. 1) and selected for the ability to complement the growth of gH-deleted virus (termed CR1 cells) was included as a control in the assay. At selected times following infection, cells are harvested, sonicated to release intracellular virus, and assayed by standard plaque assay. Results are expressed as log pfu/ml (Fig. 3) . gH-deleted virus grows to equivalent titre as wild type HSV virus SC16 in complementary gH expressing lines, and fails to grow in non-expressing lines.

Replication of a gH-deleted virus in a transgenic mouse

Transgenic 2nd generation offspring, non-transgenic siblings and control C57BL/6 mice (Charles River, UK) were infected by scarification of the left ear pinna with an inoculum of either wild type HSV SC16 or gH-deleted virus at 10⁶ pfu/dose. 5 days post infection, animals were killed, the left ear pinnae and pooled innervating sensory ganglia (CII, CIII, CIV) were dissected and stored at - 70°C. Samples were later homogenised and assayed by standard plaque assay for the presence of virus by titration on Vero and CR1 cells. Results are expressed as total pfu recovered from the selected tissue (Table 1) . In the gH transgenic mice infectious gH-deleted virus is recovered at 5 days post inoculation from the infection site (the ear) , demonstrating in vivo replication and effective in vivo complementation of the disabled virus. gH-deleted virus was not detected in the ganglia of these animals, suggesting that the disabled virus was not able to transfer into neuronal tissue. No gH deleted virus is recovered from the non-transgenic host, either in the ear or the sensory ganglia.

Table 1

Mice n= Virus Ear virus titre on Ganglia virus titre on inoculation gH - cells gH + cells gH - cells gH + cell

Transgenic 4 SC16ΔgH 0.00 3.92 *0.00 *0.00 (gH+) +/- 0.0 +/- 1.8 +/- 0.0 +/- 0.0

10 Transgenic 4 WT 5.52 5.57 3.56 3.59 (gH+) +/- 0.4 +/- 0.3 +/- 0.3 +/- 0.3

Non-transgenic 2 WT 5.33 5.28 2.35 2.30 siblings (gH-) +/- 0.3 +/- 0.4 +/- 0.8 +/- 0.9

15

Non-transgenic 4 SClδΔgH 0.00 0.00 *0.00 *0.00 control +/- 0.0 +/- 0.0 +/- 0.0 +/- 0.0

(gH-)

20

Results are expressed as log 10 of total virus at 5 days, zero values are below the limit of detection of this assay, less than log 1.4, * less than log 0.6.

Claims

1. A method for manufacturing a mutant virus with a genome disabled in respect of at least one virus gene essential for the production of infectious virus, which mutant virus can infect normal cells and undergo replication and expression of viral antigen gene in those cells, but cannot produce normal infectious virus which method comprises following the production of a transgenic animal whose genome comprises at least one heterologous gene functionally equivalent to said at least one virus gene infecting at least one cell derived from said animal with said mutant virus which cell contains said at least one heterologous gene, causing the infected cell to express said at least one heterologous gene so as to allow production of infectious virus particles containing said disabled genome; and harvesting infectious virus particles containing said disabled genome.

2. A method according to claim 1 wherein said at least one heterologous gene is in said genome under the control of a promoter sequence which functions to permit expression of said at least one heterologous gene in said infected cell.

3. A method according to claim 1 or claim 2 wherein said at least one virus gene is of an RNA virus.

4. A method according to claim 3 wherein the RNA virus is a retrovirus.

5. A method according to claim 1 or claim 2 wherein said at least one virus gene is of a DNA virus.

6. A method according to claim 5 wherein the DNA virus is a herpes virus.

7. A method according to claim 6 wherein the herpes virus is a human herpes virus.

8. A method according to claim 7 wherein the human herpes virus is one of: herpes simplex virus type 1 herpes simplex virus type 2 varicella zoster virus cytomegalovirus

Epstein Barr virus herpes virus type 6 herpes virus type 7

9. A method according to claim 6 wherein said herpes virus is a herpes simplex virus (HSV) .

10. A method according to claim 6 wherein said herpes virus is a herpes simplex virus (HSV) and a said at least one virus gene is a glycoprotein gene.

11. A method according to claim 9 or 10 wherein said at least one virus gene is the glycoprotein gH gene.

12. A method according to claim 11 wherein the promoter sequence is the HSV-l gH promoter sequence.

13. A method according to claim 6 wherein the herpes virus is a non-human herpes virus.

14. A method according to claim 13 wherein the non-human herpes virus is a feline herpesvirus; or porcine pseudorabiesvirus; or a bovine herpesvirus; or an equine herpesvirus; or an avian herpesvirus; or a canine herpesvirus; or a caprine herpesvirus; or a piscine herpesvirus.

15. A method according to claim 5 wherein the DNA virus is an adenovirus.

16. A method according to claim 5 wherein the DNA is a pox virus.

17. A method according to claim 16 wherein the pox virus is a vaccinia virus.

18. A method according to any one of the preceding claims wherein the transgenic animal is avian.

19. A method according to any one of claims 1 to 17 wherein the transgenic animal is mammalian.

20. A method according to any one of claims 1 to 17 wherein the transgenic animal is piscine.

21. A method according to any one of the preceding claims which comprises infecting the transgenic animal, and harvesting infectious virus particles therefrom after an appropriate interval to allow growth of said particle,s by removing from said animal a suitable cell or tissue sample.

22. A method according to claim 21 wherein the particles are harvested by extracting them from said cell or tissue sample.

23. A method according to any one of claims 1 to 20 which comprises: isolating from said transgenic animal a cell or tissue sample; infecting at least one cell of the sample with said mutant virus; culturing the infected cell to express the at least one heterologous gene so as to produce infectious virus particles containing the disabled genome; and harvesting infectious virus particles by recovery from the culture.

24. A method according to claim 23 wherein the cell is an egg.

25. A method according to claim 24 wherein the cell is a fertilised egg.