DNA EXPRESSION SYSTEMS BASED ON ALPHAVIRUSES

Abstract: Efficient protein production from cloned DNA in animal cells has been hampered by the lack of suitable expression systems. The requirements of such an expression system are (1) to produce functional or immunogenic forms of protein molecules in a wide variety of animal cells, (2) high efficiency and (3) technical simplicity. The present invention is related to a technical solution to this problem. A DNA molecule encoding protein sequences is inserted into engineered variants of the cDNA of a positive stranded RNA virus genome from alphavirus which then, via RNA transcription and transfection into tissue culture cells, is used to produce either chimaeric virus particles for immunization or recombinant virus for protein production. Because of optimized conditions of transfection and the nature of the virus replication the present system combines both simplicity and safety in terms of handling, efficiency in terms of level of protein and RNA production, as well as broad host range.

Description: DNAEXPRESSIONSYSTEMS BASEDONALPHAVIRUSES The present invention is related to DNA expression systems based on alphaviruses, which systems can be used to transform animal cells for use in the pro¬ duction of desired products, such as proteins and vaccines, in high yields. The rapid development of biotechnology is to a iarge extent due to the introduction of recombinant DNA technique, which has revolutionized cellbiological and medical research by opening new approaches to elucidate the molecular mechanisms of the cell. With the aid of the techniques of cDNA cloning, large numbers of interesting protein molecules are characterized each year. Therefore, a lot of research activity is today directed to elucidate the relationship between struc- ture and function of these molecules. Eventually this knowledge will increase our possibilities to preserve healthiness and combat diseases in both humans and animals. Indeed, there is today a growing list of new "cloned" protein products that are already used as pharmaceuticals or diagnostics. In the recombinant DNA approaches to study biological questions, DNA expression systems are crucial elements. Thus, efficient DNA expression systems, which are simple and safe to use, give high yields of the desired product and can be used in a variety of host cells, especially also in mammalian cells, are in great demand. Many attempts have been made to develop DNA expres¬ sion systems, which fulfill these requirements. Often, viruses have been used as a source of such systems. However, up to date none of the existing viral expres¬ sion systems fulfill all these requirements in a satis¬ fying way. For instance, the Baculovirus expression system for cDNA is extremely efficient but can be used only in insect cells (see Reference l of the list of cited references; for the sake of convenience, in the following the cited references are only identified by the number they have on said list) . As many important molecules will have to be produced and processed in cells of mammalian origin in order for them to become active, this system cannot be used in such cases. Furthermore, the Baculovirus cDNA expression system is not practically convenient for analysis of the relationship between structure and function of a pro¬ tein because this involves in general the analysis of whole series of mutant variants. Today it takes about 6-8 weeks to construct a single Baculo recombinant virus for phenotype analyses. This latter problem is also true for the rather efficient Vaccinia recombinant virus and other contemporary recombinant virus cDNA expression systems (2,3). The procedure to establish stably transformed cell lines is also a very laborious procedure, and in addition, often combined with very low levels of protein expression. Hitherto, most attempts to develop viral DNA expres¬ sion systems have been based on viruses having DNA genomes or retroviruses, the replicative intermediate of the latter being double stranded DNA. Recently, however, also viruses comprising RNA genomes have been used to develop DNA expression systems. In EP 0 194 809 RNA transformation vectors derived from (+) strand RNA viruses are disclosed which comprise capped viral RNA that has been modified by insertion of exogenous RNA into a region non-essential for replication of said virus RNA genome. These vectors are used for expression of the function of said exo¬ genous RNA in cells transformed therewith. The RNA can be used in solution or packaged into capsids. Further¬ more, this RNA can be used to generate new cells having new functions, i.e. protein expression. The invention of said reference is generally claimed as regards host cells, (+) strand RNA viruses and the like. Neverthe- less, it is obvious from the experimental support provided therein that only plant cells have been trans¬ formed and in addition only Bromo Mosaic virus, a plant virus, has been used as transformation vector. Although it is stated in said reference that it is readily apparent to those skilled in the art to convert any RNA virus-cell system to a useful expression system for exogenous DNA using principals described in the reference, this has not been proven to be true in at least the case of animal cell RNA viruses. The reasons for this seem to be several. These include: 1) Inefficiencies in transfecting animal cells with in vitro transcribed RNA; 2) Inefficiency of apparently replication com¬ petent RNA transcripts to start RNA replication after commonly used transfection procedures; 3) The inability to produce high titre stocks of recombinant virus that does not contain any helper virus; 4) The inability to establish stable traits of transformed cells expressing the function of the exogenous RNA. In Proc. Natl. Acad. Sci. USA, Vol 84, 1987, pp 4811- 4815 a gene expression system based on a member of the Alphavirus genus, viz. Sindbis virus, is disclosed which is used to express the bacterial CAT (chlor- amphenicol acetyltransferase) gene in avian cells, such as chicken embryo fibroblasts. Xiong et al., Science, Vol 243, 1989, 1188-1191 also disclose a gene expression system based on sindbis virus. This system is said to be efficient in a broad range of animal cells. Expression of the bacterial CAT gene in insect, avian and mammalian cells inclusive of human cells is disclosed therein. Even though it is known from prior art that one member of the Alphavirus genus, the Sindbis virus, can tolerate insertion and direct the expression of at least one foreign gene, the bacterial chloramfenicol acetyl transferase (CAT) gene, it is evident from the results described that both systems described above are both ineffective in terms of exogenous gene expression and also very cumbersome to use. Hence, neither system has found any usage in the field of DNA expression in animal cells today. In the first example a cDNA copy of a defective interfering (DI) virus variant of Sindbis virus was used to carry the CAT gene. RNA was transcribed in vitro and used to transfeet avian cells and some CAT protein production could be demonstrated after in- fecting cells with wild-type Sindbis virus. The latter virus provided the viral replicase for expression of the CAT construct. The inefficiency of this system depends on 1) low level of initial DI-CAT RNA transfec¬ tion (0.05-0.5 % of cells) and 2) inefficient usage of the DI-CAT RNA for protein translation because of unnatural and suboptimal protein intitation translation signals. This same system also results in packaging of some of the recombinant DI-CAT genomes into virus particles. However, this occurs simultaneously with a very large excess of wild-type Sindbis virus produc¬ tion. Therefore, the usage of this mixed virus stock for CAT expression will be much hampered by the fact that most of the replication and translation activity of the cells infected with such a stock will deal with the wild-type and not with recombinant gene expression. Much of the same problems are inherent to the other Sindbis expression system described. In this an RNA replication competent Sindbis DNA vector is used to carry the CAT gene. RNA produced in vitro is shown to replicate in animal cells and CAT activity is found. However, as only a very low number of cells are trans- fected the overall CAT production remains low. Another possible explanation for this is that the Sindbis con¬ struct used is not optimal for replication. Wild-type Sindbis virus can be used to rescue the recombinant genome into particles together with an excess of wild- type genomes and this mixed stock can then be used to express a CAT protein via infection. However, this stock has the same problems as described above for the recombinant DI system. The latter paper shows also that if virus is amplified by several passages increased titres of the recombinant virus particles can be obtained. However, one should remember that the titre of the wild-type virus will increase correspondingly and the original problem of mostly wild-type virus production remains. There are also several potential problems when using several passages to produce a mixed virus stock. As there is no selected pressure for pre¬ servation of the recombinant genomes these might easily 1) undergo rearrangements and 2) become outnumbered by wild-type genomes as a consequence of less efficient replication and/or packaging properties. Another important aspect of viral DNA expression vectors is use thereof to express antigens of unrelated pathogens and thus they can be used as vaccines against such pathogens. Development of safe and effective vaccines against viral diseases has proven to be quite a difficult task. Although many existing vaccines have helped to combat the worldwide spread of many infectious diseases, there is still a large number of infectious agents against which effective vaccines are missing. The current pro¬ cedures of preparing vaccines present several problems: (1) it is often difficult to prepare sufficiently large amounts of antigenic material; (2) In many cases there is the additional hazard that the vaccine preparation is not killed or sufficiently attenuated; (3) Effective vaccines are often hard to produce since there is a major difficulty in presenting the antigenic epitope in an immunologically active form; (4) In the case of many viruses, genetic variations in the antigenic components results in the evolution of new strains with new sero- logical specificities, which again creates a need for the development of new vaccines. Two types of viral DNA vectors have been developed in order to overcome many of these problems in vaccine production. These either provide recombinant viruses or provide chimaeric viruses. The recombinant viruses contain a wild-type virus package around a recombinant genome. These particles can be used to infect cells which then produce the antigenic protein from the re¬ combinant genome. The chimaeric viruses also contain a recombinant genome but this speci ies the production of an antigen, usually as part of a normal virus struc¬ tural protein, which then will be packaged in progeny particles and e.g. exposed on the surface of the viral spike proteins. The major advantages of these kind of virus preparations for the purpose of being used as a vaccine are 1) that they can be produced in large scale and 2) that they provide antigen in a natural form to the immunological system of the organism. Cells, which have been infected with recombinant viruses, will synthesize the exogenous antigen product, process it into peptides that then present them to T cells in the normal way. In the case of the chimaeric virus there is, in addition, an exposition of the antigen in the context of the subunits of the virus particle itself. Therefore, the chimaeric virus is also called an epitope carrier. The major difficulty with these kind of vaccine preparations are, how to ensure a safe and limited replication of the particles in the host without side effects. So far, some success has been obtained with vaccinia virus as an example of the recombinant virus approach (69) and of polio virus as an example of a chimaeric particle (70-72) . As both virus variants are based on commonly used vaccine strains one might argue that they could be useful vaccine candidates also as recombinant respectively chimaeric particles (69-72) . However, both virus vaccines are combined with the risk for side effects, even severe ones, and in addition these virus strains have already been used as vaccines in large parts of the population in many countries. As is clear from the afore mentioned discussion there is much need to develop improved DNA expression systems both for an easy production of important proteins or polypeptides in high yields in various kinds of animal cells and for the production of recombinant viruses or chimaeric viruses to be used as safe and efficient vaccines against various pathogenes. Thus, an object of the present invention is to provide an improved DNA expression system based on virus vectors which can be used both to produce proteins and polypeptides and as recombinant virus or chimaeric virus, which system offers many advantages over prior art. To that end, according to the present invention there is provided an RNA molecule derived from an alphavirus RNA genome and capable of efficient infection of animal host cells, which RNA molecule comprises the complete alphavirus RNA genome regions, which are essential to replication of the said alphavirus RNA, and further comprises an exogenous RNA sequence capable of expres¬ sing its function in said host cell, said exogenous RNA sequence being inserted into a region of the RNA mole- cule which is non-essential to replication thereof. Alphavirus is a genus belonging to the family Togaviridae -having single stranded RNA genomes of positive polarity enclosed in a nucleocapsid surrounded by an evelope containing viral spike proteins. The Alphavirus genus comprises among others the Sindbis virus, the Semliki Forest virus (SFV) and the Ross River virus, which are all closely related. According to a preferred embodiment of the invention, the Semliki Forest virus (SFV) is used as the basis of the DNA expression system. The exogenous RNA sequence encodes a desired genetic trait, which is to be conferred on the virus or the host cell, and said sequence is usually complementary to a DNA or cDNA sequence encoding said genetic trait. Said DNA sequence may be comprised of an isolated natural gene, such as a bacterial or mammalian gene, or may constitute a synthetic DNA sequence coding for the desired genetic trait i.e. expression of a desired product, such as an enzyme, hormone, etc. or expression of a peptide sequence defining an exogenous antigenic epitope or determinant. If the exogenous RNA sequence codes for a product, such as a protein or polypeptide, it is inserted into the viral RNA genome replacing deleted structural pro¬ tein encoding region(s) thereof, whereas a viral epi¬ tope encoding RNA sequence may be inserted into structural protein encoding regions of the viral RNA genome, which essentially do not comprise deletions or only have a few nucleosides deleted. The RNA molecule can be used per se, e.g. in solution to transform animal cells by conventional transfection, e.g. the DEAE-Dextran method or the calcium phosphate precipitation method. However, the rate of transforma¬ tion of cells, and, thus the expression rate can be expected to increase substantially if the cells are transformed by infection with infectious viral par- tides. Thus, a suitable embodiment of the invention is related to an RNA virus expression vector comprising the RNA molecule of this invention packaged into infec¬ tious particles comprising the said RNA within the alphavirus nucleocapsid and surrounded by the membrane including the alphavirus spike proteins. The RNA molecule of the present invention can be packaged into such particles without restraints pro- vided that it has a total size corresponding to the wild type alphavirus RNA genome or deviating therefrom to an extent compatible with package of the said RNA into the said infectious particles. These infectious particles, which include recombinant genomes packaged to produce a pure, high titre recombi¬ nant virus stock, provides a means for exogenous genes or DNA sequences to be expressed by normal virus par¬ ticle infection, which as regards transformation degree, is much more efficient than RNA transfection. According to a suitable embodiment of the invention such infectious particles are produced by cotrans- fection of animal host cells with the present RNA which lacks part of or the complete region(s) encoding the structural viral proteins together with a helper RNA molecule transcribed in vitro from a helper DNA vector comprising the SP6 promoter region, those 5' and 3' regions of the alphavirus cDNA which encode cis acting signals needed for RNA replication and the region encoding the viral structural proteins but lacking essentially all of the nonstructural virus proteins encoding regions including sequenses encoding RNA signals for packaging of RNA into nucleocapsid par¬ ticles, and culturing the host cells. According to another aspect of the invention effi¬ cient introduction of the present RNA into animal host cells can be achieved by electroporation. For example, in the case of Baby Hamster Kidney (BHK) cells a trans¬ formation degree of almost 100 % has been obtained for the introduction of an RNA transcript derived from SFV cDNA of the present invention. This makes it possible to reach so'high levels of exogenous protein production in every cell that the proteins can be followed in total cell lysates without the need of prior concentra- tion by antibody precipitation. By electroporation, it is also possible to obtain a high degree of cotransfection in the above process for production of infectious particles comprising packaged RNA of the present invention. Essentially all animal cells will contain both the present RNA molecule and the helper RNA molecule, which leads to a very effi- cient trans complementation and formation of infectious partcles. A pure recombinant virus stock, consisting of up to 10 9 -10 10 infectious particles, can be obtained from 5 x 10 6 cotransfected cells after only a 24 h incubation. Furthermore, the so obtained virus stock is very safe to use, since it is comprised of viruses containing only the desired recombinant genome, which can infect host cells but can not produce new progeny virus. Theoretically, a regeneration of a wild-type virus genome could take place when producing the recombinant virus in the contransfected cells. However, the possi¬ bility to avoid spread of such virus can be eliminated by incorporating a conditionally lethal mutation into the structural part of the helper genome. Such-a muta- tion is described in the experimental part of this application. Thus, the virus produced with such a helper will be noninfectious if not treated in vitro under special conditions. The technique of electroporation is well known within the field of biotechnology and optimal conditions can be established by the man skilled in the art. For instance, a BioRad Gene pulser apparatus (BioRad, Richmond, CA, USA) can be used to perform said process. The RNA molecule of the present invention is derived by in vivo or in vitro transcription of a cDNA clone, originally produced from an alphavirus RNA and com¬ prising an inserted exogenous DNA fragment encoding a desired genetic trait. Accordingly, the present invention is also related to a DNA expression vector comprising a full-length or partial cDNA complementary to alphavirus RNA or parts thereof and located immediately downstream of the SP6 RNA polymerase promoter and having a 5'ATGG, a 5'GATGG or any other 5' terminus and a TTTCCA 69 ACTAGT or any other 3 ' terminus. According to one aspect of the present invention portions of the viral cDNA are deleted, the deletions comprising the complete or part of the region(s) en¬ coding the virus structural proteins, and the vector further comprises an integrated polylinker region, which may correspond to BamHI-Smal-Xmal, inserted at a location which enables an exogenous DNA fragment en¬ coding a foreign polypeptide or protein to be inserted into the vector cDNA for subsequent expression in an animal host cell. According to another aspect of this invention, the vector is comprised of full-length cDNA wherein an exogenous DNA fragment encoding a foreign epitopic peptide sequence can be inserted into a region coding for the viral structural proteins. It is appreciated that this cDNA clone with its exogenous DNA insert is very efficiently replicated after having been introduced into animal cells by transfection. A very important aspect of the present invention is that it is applicable to a broad range of host cells of animal origin. These host cells can be selected from avian, mammalian, reptilian, amphibian, insect and fish cells. Illustrative of mammalian cells are human, monkey, hamster, mouse and porcine cells. Suitable avian cells are chicken cells, and as reptilian cells viper cells can be used. Cells from frogs and from mosquitoes and flies (Drosophila) are illustrative of amphibian and insecticidal cells, respectively. A very efficient virus vector/host cell system according to the invention is based on SFV/BHK cells, which will be discussed more in detail further below. However, even though a very important advantage of the present DNA expression vector is that it is very efficient in a broad variety of animal cells it can also be used in other eucaryotic cells and in pro- caryotic cells. The present invention is also related to a method to produce transformed animal host cells comprising trans¬ fection of the cells with the present RNA molecule or with the present transcription vector comprised of cDNA and carrying an exogenous DNA fragment. According to a suitable embodiment of the invention, transfection is produced by the above mentioned electroporation method, a very high transfection rate being obtained. A further suitable transformation process is based on infection of the animal host cells with the above mentioned infectious viral particles comprising the present RNA molecule. The transformed cells of the present invention can be used for different purposes. One important aspect of the invention is related to use of the present transformed cells to produce a poly- peptide or a protein by culturing the transformed cells to express the exogenous RNA and subsequent isolation and purification of the product formed by said exepres- sion. The transformed cells can be produced by infec¬ tion with the present viral particles comprising exo- genous RNA encoding the polypeptide or protein as men¬ tioned above, or by transfection with an RNA transcript obtained by in vitro transcription of the present DNA vector comprised of cDNA and carrying an exogenous DNA fragment coding for the polypeptide or the protein. Another important aspect of the invention is related to use of the present transformed cells for the produc¬ tion of antigens comprised of chimaeric virus particles for use as immunizing component in vaccines or for immunization purposes for in vivo production of immunizing components for antisera production. Accordingly, the present invention is also related to an antigen consisting of a chimaeric alphavirus having an exogenous epitopic peptide sequence inserted into its structural proteins. Preferably, the chimaeric alphavirus is derived from SFV. According to a suitable embodiment, the exogenous epitopic peptide sequence is comprised of an epitopic peptide sequence derived from a structural protein of a virus belonging to the immunodeficiency virus class inclusive of the human immunodeficiency virus types. A further aspect of the invention is related to a vaccine preparation comprising the said antigen as immunizing component. In said vaccine the chimaeric alphavirus is suitably attenuated by comprising mutations, such as the condi- tionally lethal SFV-mutation described before, amber (stop codon) or temperature sensitive mutations, in its genome. For instance, if the chimaeric virus particles con¬ taining the afore mentioned conditional lethal mutation in its s tructural proteins (a defect to undergo a certain proteolytical cleavage in host cell during morphogenesis) is used as a vaccine then this is first activated by limited proteolytic treatment before given to the organism so that it can infect recipient cells. New chimaeric particles will be formed in cells infected with the activated virus but these will again be of the lethal phenotype and further spread of infec¬ tion is not possible. The invention is also concerned with a method for the production of the present antigen comprising a) in vitro transcription of the cDNA of the present DNA vector carrying an exogenous DNA fragment encoding the foreign epitopic peptide sequence and transfection of animal host cells with the produced RNA transcript, or b) transfection of animal host cells with the said cDNA of the above step a) , culturing the transfected cells and recovering the chimaeric alphavirus antigen. Preferably, transfection is produced by electroporation. Still another aspect of the invention is to use a recombinant virus containing exogenous RNA encoding a polypeptide antigen for vaccination purpose or to pro¬ duce antisera. In this case the recombinant virus or the conditionally lethal variant of it is used to infect cells in vivo and antigen production will take place in the infectious cells and used for antigen presentation to the immunological system. According to another embodiment of the invention, the present antigen is produced in an organism by using in vivo infection with the present infectious particles containing exogenous RNA encoding an exogenous epitopic peptide sequence. In the following, the present invention will be illustrated more in detail with reference to the Semliki Forest virus (SFV) , which is representative for the alphaviruses. This description can be more fully understood in conjunction with the accompanying drawings in which: Fig. 1 is a schematic view over the main assembly and disassembly events involved in the life cycle of the Semliki Forest virus, and also shows regulation of the activation of SFV entry functions by p62 cleavage and pH; Fig. 2 illustrates the use of translocation signals during synthesis of the structural proteins of SFV; top, the gene map of the 26S subgenomic RNA; middle, the processOf membrane translocation of the p62, 6K and El proteins; small arrows on the lumenal side denote signal peptidase cleavages; at the bottom, the characteristics of the three signal peptides are listed; Fig. 3 shows features that make SFV an excellent choice as an expression vector; Fig. 4 A-C show the construction of full-length infectious clones of SFV; Fig. 4A shows a schematic restriction map of the SFV genome; primers used for initiating cDNA synthesis are indicated as arrows, and the cDNA inserts used to assemble the final clone are showed as bars; Fig. 4B shows plasmid pPLH211, i.e. the SP6 expression vector used as carrier for the full- length infectious clone of SFV, and the resulting plasmid pSP6-SFV4; Fig. 4C shows the structure of the SP6 promoter area of the SFV clone; the stippled bars indicate the SP6 promoter sequence, and the first necleotide to be transcribed is marked by an asterisk; underlined regions denote authentic SFV sequences; Fig. 5 shows the complete nucleotide sequence of the pSP6-SFV4 RNA transcript as DNA (U = T) and underneath the DNA sequence, the amino acid sequence of the non- structural polyprotein and the structural polyprotein; Fig. 6 shows an SFV cDNA expression system for the production of virus after transfection of in vitro made RNA into cells; Fig. 7 shows the construction of the SFV expression vectors pSFVl-3 and of the Helper 1; Fig. 8 shows the polylinker region of SFV vector plas ids pSFVl-3; the position of the promoter for the subgenomic 26S RNA is boxed, and the first nucleotide to be transcribed is indicated by an asterisk; Fig. 9 is a schematic presentation of in vivo packaging of pSFVl-dhfr RNA into infectious particles using helper trans complementation; (dhfr means di- hydrofolate reductase) Fig. 10 shows the use of trypsin to convert p62- containing noninfectious virus particles to infectious particles by cleavage of p62 to E2 and E3; Fig. 11 shows the expression of heterologous proteins in BHK cells upon RNA transfection by electroporation; and Fig. 12 shows in its upper part sequences encompas¬ sing the major antigenic site of SFV and the in vitro made substitutions leading to a BamHI restriction endo- nuclease site, sequences spanning the principal neutralizing domain of the HIV gpl20 protein, and the HIV domain inserted into the SFV carrier protein E2 as a BamHI oligonucleotide; and its lower part is a schematic presentation of the SFV spike structure with blow-ups of domain 246-251 in either wild type or chimaeric form. The alphavirus Semliki Forest virus (abbreviated SFV in the following text) has for some 20 years been used as model system in both virology and cell biology to study membrane biosynthesis, membrane structure and membrane function as well as protein-RNA interactions (4, 5) . The major reason for the use of SFV as such a model is due to its simple structure and efficient replication. With reference to Fig. 1-3, in the following the SFV and its replication are explained more in detail. In essential parts, this disclosure is true also for the other alphaviruses, such as the Sindbis virus, and many of the references cited in this connection are indeed directed to the Sindbis virus. SFV consists of an RNA- containing nucleocapsid and a surrounding membrane composed of a lipid bilayer and proteins, a regularly arranged icosahedral shell of a protein called C pro¬ tein forming the capsid inside which the genomic RNA is packaged. The capsid is surrounded by the lipid bilayer that contains three proteins called El, E2, and E3. These so-called envelope proteins are glycoproteins and their glycosylated portions are on the outside of the lipid bilayer, complexes of these proteins forming the "spikes" that can be seen in electron micrographs to project outward from the surface of the virus. The SFV genome is a single-stranded 5'-capped and 3'- polyadenylated RNA molecule of 11422 nucleotides (6,7) . It has positive polarity, i.e. it functions as an mRNA, and naked RNA is able to start an infection when in¬ troduced into the cytoplasm of a cell. Infection is initiated when the virus binds to protein receptors on the host cell plasma membrane, whereby the virions become selectively incorporated into "coated pits" on the surface of the plasma membrane, which invaginate to form coated vesicles inside the cell, whereafter said vesicles bearing endocytosed virions rapidly fuse with organelles called endosomes. From the endosome, the virus escapes into the cell cytosol as the bare nucleo¬ capsid, the viral envelope remaining in the endosome. Thereafter, the nucleocapsid is "uncoated" and, thus, the genomic RNA is released. Referring now to Fig. 1, infection then proceeds with the translation of the 5* two-thirds of the genome into a polyprotein which by self-cleavage is processed to the four nonstructural proteins nsPl-4 (8) . Protein nsPl encodes a methyl transferase which is responsible for virus-specific capping activity as well as initiation of minus strand synthesis (9, 10); nsP2 is the protease that cleaves the polyprotein into its four subcomponents (11, 12) ; nsP3 is a phosphoprotein (13, 14) of as yet unknown function, and nsP4 contains the SFV RNA polymerase activity (15, 16) . Once the nsP proteins have been synthesized they are responsible for the replication of the plus strand (42S) genome into full-length minus strands. These molecules then serve as templates for the production of new 42S genomic RNAs. They also serve as templates for the synthesis of subgenomic (26S) RNA. This 4073 nucleotides long RNA is colinear with the last one-third of the genome, and its synthesis is internally initiated at the 26S promoter on the 42S minus strands (17, 18). The capsid and envelope proteins are synthesized in different compartments, and they follow separate path¬ ways through the cytoplasm, viz. the envelope proteins are synthesized by membrane-bound ribosomes attached to the rough endoplasmic reticulum, and the capsid protein is synthesized by free ribosomes in the cytosol. How¬ ever, the 26S RNA codes for all the structural proteins of the virus, and these are synthesized as a poly¬ protein precursor in the order C-E3-E2-6K-E1 (19) . Once the capsid (C) protein has been synthesized it folds to act as a protease cleaving itself off the nascent chain (20, 21) . The synthesized C proteins bind to the re- cently replicated genomic RNA to form new nucleocapsid structures in the cell cytoplasm. The said cleavage reveals an N-terminal signal sequ¬ ence in the nascent chain which is recognized by the signal recognition particle targeting the nascent chain - ribosome complex to the endoplasmic reticulum (ER) membrane (22, 23), where it is cotranslationally translocated and cleaved by signal peptidase to the three structural membrane proteins p62 (precursor form of E3/E2) , 6K and El (24, 25). The translocational signals used during the synthesis of the structural proteins are illustrated in Fig. 2. The membrane pro¬ teins undergo extensive posttranslational modifications within the biosynthetic transport pathway of the cell. The p62 protein forms a heterodimer with El via its E3 domain in the endoplasmic reticulum (26) . This dimer is transported out to the plasma membrane, where virus budding occurs through spike nucleocapsid interactions. At a very late (post-Golgi) stage of transport the p62 protein is cleaved to E3 and E2 (27) , the forms that are found in mature virions. This cleavage activates the host cell binding function of the virion as well as the membrane fusion potential of El. The latter activi¬ ty is expressed by a second, low-pH activation step after the virus enters the endosomes of a new host cell and is responsible for the release of the viral nucleo¬ capsid into the cell cytoplasm (28-32) . The mature virus particles contain one single copy of the RNA genome encapsidated within 180 copies of the capsid protein in a T=3 symmetry, and is surrounded by a lipid bilayer carrying 240 copies of the spike trimer protein consisting of E1+E2+E3 arranged in groups of three in a T=4 symmetry (33) . The SFV entry functions are activated and regulated by p62 cleavage and pH. More specifically, the p62-El heterodi ers formed in the ER are acid resistant. When these heterodimers are transported to the plasma mem- brane via the Golgi complex the El fusogen cannot be activated in spite of the mildly acidic environment, since activation requires dissociation of the complex. As is illustrated in Fig. 1, the released virus particles contain E2E1 complexes. Since the association between E2 and El is sensitive to acidic pH, during entry of the virus into a host cell through endocytosis the acidic milieu of the endosome triggers the dis¬ sociation of the spike complex (El E2 E3) resulting in free El. The latter can be activated for the catalysis of the fusion process between the viral and endosomal membranes in the infection process as disclosed above. As indicated in the preceding parts of the dis¬ closure, the alphavirus system, and especially the SFV system, has several unique features which are to advantage in DNA expression systems. These are summarized below with reference to Fig. 3. 1. Genome of positive polarity. The SFV RNA genome is of positive polarity, i.e. it functions directly as mRNA, and infectious RNA molecules can thus be obtained by transcription from a full-length cDNA copy of the genome. 2. Efficient replication. The infecting RNA molecule codes for its own RNA replicase, which in turn drives an efficient RNA replication. Indeed, SFV is one of the most efficiently replicating viruses known. Within a few hours up to 200.000 copies of the plus-RNAs are made in a single cell. Because of the abundance of these molecules practically all ribosomes of the in¬ fected cell will be enrolled in the synthesis of the virus encoded proteins, thus overtaking host protein synthesis (34) , and pulse-labelling of infected cells results in almost exclusive labelling of viral proteins. During a normal infection 10 5 new virus particles are produced from one single cell, which calculates to at least 10 8 protein molecules encoded by the viral genome (5) . 3. Cytoplasmic replication. SFV replication occurs in the cell cytoplasm, where the virus replicase trans¬ cribes and caps the subgenomes for production of the structural proteins (19) . It would obviously be very valuable to include this feature in a cDNA expression system to eliminate the many problems that are encount¬ ered in the conventional "nuclear" DNA expression systems, such as mRNA splicing, limitations in trans¬ cription factors, problems with capping efficiency and mRNA transport. 4. Late onset of cytopathic effects. The cytopathic effects in the infected cells appear rather late during infection. Thus, there is an extensive time window from about 4 hours after infection to up to 24 hours after infection during which a very high expression level of the structural proteins is combined with negligible morphological change. 5. Broad host range. This phenomenon is probably a consequence of the normal life cycle which includes transmission through arthropod vectors to wild rodents and birds in nature. Under laboratory conditions, SFV infects cultured mammalian, avian, reptilian and insect cells (35) (Xiong, et al, loc. cit.) 6. In nature SFV is of very low pathogenicity for humans. In addition, the stock virus produced in tissue culture cells is apparently apathogenic. By means of specific mutations it is possible to create condi¬ tionally lethal mutations of SFV, a feature that is of great use to uphold safety when massproduction of virus stocks is necessary. In the nucleotide and amino acid sequences the following abbreviations have been used in this specifi- cation: Ala, alanine; lie, isoleucine; leu, leucine; Met, methionine; Phe, phenylalanine; Pro, proline; Trp, tryptophan; Val, valine; Asn, asparagine; Cys, cysteine; Gin, glutamine; Gly, glycine; Ser, serine; Thr, threonine; Tys, tyrosine; Arg, arginine; His, histidine; Lys, lysine; Asp, aspartic acid; Glu, glutamic acid; A, adenine; C, cytosine; G, guanine; T, thymine; U, uracil. The materials and the general methodology used in the following examples are disclosed below. 1. Materials. Most restriction enzymes, DNA Polymerase I, Klenow fragment, calf intestinal phos- phatase, T4 DNA ligase and T4 Polynucleotide kinase were from Boehringer (Mannheim, FRG) . SphI, StuI and Kpnl together with RNase inhibitor (RNasin) and SP6 Polymerase were from Promega Biotec (Madison, WI) . Sequenase (Modified T7 polymerase) was from United States Biochemical (Cleveland, Ohio) . Proteinase K was from Merck (Darmstadt, FRG) . Ribonucleotides, deoxy- ribonucleotides, dideoxyribonucleotides and the cap analogue m 7 G(5')ppp(5')G were from Pharmacia (Sweden). Oligonucleotides were produced using an Applied Bio- systems synthesizer 380B followed by HPLC and NAP-5 (Pharmacia) purification. Spermidine, phenylmethylsul- fonyl fluoride (PMSF) , diethylpyrocarbonate (DEPC) , bovine serum albumin (BSA) , creatine phosphate and creatine phdsphokinase were from Sigma (St. Louis, Mo) . Pansorbin was from CalBiochem (La Jolla, CA) . Agarose was purchased from FMC BioProducts (Rockland, Maine) , and acrylamide from BioRad (Richmond, CA) . L-[ 35 S]- methionine and α-[ 35 S]-dATP-α-S were from Amersham. 2. Virus growth and purification: BHK-21 cells were grown in BHK medium (Gibco Life Technologies, Inc., New York) supplemented with 5 % fetal calf serum, 10 % tryptose phosphate broth, 10 mM HEPES (N-2-hydroxy- ethylpiperazine-N'-2-ethanesulfonic acid) and 2 mM glutamine. 90 % confluent monolayers were washed once with PBS and infected with SFV in MEM containing 0.2 % bovine serum albumin (BSA) , 10 mM HEPES and 2 mM gluta¬ mine at a multiplicity of 0.1. Twenty-four hours post infection (p.i.) the medium was collected and cell debris removed by centrifugation at 8,000 xg for 20 min at 4°C. The virus was pelleted from the medium by centrifugation at 26,000 rpm for 1.5 h in an SW28 rotor at 4°C. The virus was resuspended in TN containing 0.5 mM EDTA. 3. Metabolic labeling and immunoprecipitation. Con¬ fluent monolayers of BHK cells grown in MEM supplement¬ ed with 10 mM HEPES, 2 mM glutamine, 0.2 % BSA, 100 IU/mol of penicillin and 100 μg/ml streptomycin, were infected at a multiplicity of 50 at 37°C. After 1 h p.i. the medium was replaced with fresh and growth continued for 3.5 h. The medium was removed and cells washed once with PBS and overlayed with methionine-free MEM containing 10 mM HEPES and 2 mM glutamine. After 30 min at 37°C the medium was replaced with the same con- taining 100 μCi/ml of [ 35 S]methionine (Amersham) and the plates incubated for 10 min at 37°C. The cells were washed twice with labeling medium containing 10X excess ethionine and then incubated in same medium for various times. The plates were put on ice, cells washed once with ice-cold PBS and finally lysis buffer (l % NP-40 - 50 mM Tris-HCl, pH 7.6 - 150 mM NaCl - 2 mM EDTA) containing 10 μg/ml PMSF (phenylmethylsulfonyl fluoride) was added. Cells were scraped off the plates, and nuclei removed by centrifugation at 6,000 rpm for 5 min at 4°C in an Eppendorf centrifuge. Immunoprecipita- tions of proteins was performed as described (31) . Briefly, antibody was added to lysate and the mixture kept on ice for 30 min. Complexes were recovered by binding to Pansorbin for 30 min on ice. Complexes were washed once with low salt buffer, once with high salt buffer, and once with 10 mM Tris-HCl, pH 7.5, before heating with gel loading buffer. To precipitate dhfr, SDS was added to 0.1 % and the mixture heated to 95°C for 2 min followed by addition of 10 volumes of lysis buffer. Anti-El [8.139], anti-E2 [5.1] (36), and anti-C [12/2] (37) monoclonals have been described. The human transferrin receptor was precipitated with the mono¬ clonal antibody OKT-9 in ascites fluid. This prepara¬ tion was provided by Thomas Ebel at our laboratory using a corresponding hybridoma cell line obtained from ATCC (American Typ Culture Collection) No CRL 8021. Polyclonal rabbit anti-mouse dhfr was a kind gift from E. Hurt (European Molecular Biology Laboratory, Heidel¬ berg, FRG) and rabbit anti-lysozyme has been described (38). 4. Immunofluorescence. To perform indirect im uno- fluorescence, infected cell monolayers on glass cover- slips were rinsed twice with phosphate-buffered saline (PBS) and fixed in -20°C methanol for 6 min. After fixation, the methanol was removed and the coverslip washed 3 times with PBS. Unspecific antibody binding was blocked by incubation at room temperature with PBS containing 0.5 % gelatin and 0.25 % BSA. The blocking buffer was removed and replaced with same buffer con¬ taining primary antibody. After 30 min at room tempera¬ ture the reaction was stopped by washing 3 times with PBS. Binding of secondary antibody (FITC-conjugated sheep anti-mouse [BioSys, Compiegne, France]) was done as for the primary antibody. After 3 washes with PBS and one rinse with water the coverslip was allowed to dry before mounting in Moviol 4-88 (Hoechst, Frankfurt am Main, FRG) containing 2.5 % DABCO (1,4-diazobicyclo- [2.2.2]-octane) . 5. DNA procedures. Plasmids were grown in Escherichia coli DH5α (Bethesda Research Laboratories) [recA endAl gyrA96 thil hsdR17 supE44 relAl Δ(lacZYA-argF)U169 φ80dlacZΔ(M15)] . All basic DNA procedures were done essentially as described (39). DNA fragments were ^ iso- lated from agarose gels by the freeze-thaw method (40) including 3 volumes of phenol during the freezing step to increase yield and purity. Fragments were purified by benzoyl-naphthoyl-DEAE (BND) cellulose (Serva Fein- biochemica, Heidelberg, FRG) chromatography (41) . Plasmids used for production of infectious RNA were purified by sedimentation through 1 M NaCl followed by banding in CsCl (39) . In some cases plasmids were puri¬ fied by Qiagen chromatography (Diagen Gmbh, Dϋsseldorf, FRG) . 6. Site-directed oligonucleotide mutagenesis. For oligonucleotide mutagenesis, relevant fragments of the SFV cDNA clone were subcloned into M13mpl8 or mp 19 (42) and transformed (43) into DH5αFIQ [endAl hsdRl suρE44 thil recAl gyrA96 relAl ø80dlacΔ(M15) Δ(lacZYA- argF)U169/F'proAB lacl q lacZΔ(M15) Tn 5] (Bethesda Research Laboratories) . RF DNA from these constructs was transformed into RZ1032 (44) [Hfr KL16 dutl ungl thil relAl supE44 zbd279:TnlO.], and virus grown in the presence of uridine to incorporate uracil residues into the viral genome. Single stranded DNA was isolated by phenol extraction from PEG precipitated phage. Oligo- nucleotides were synthesized on an Applied Biosystems 38OB synthesizer and purified by gel filtration over NAP-5 columns (Pharmacia) . The oligonucleotides 5•-CGGCCAGTGAATTCTGATTGGATCCCGGGTAATTAATTGAATTACATCCC- TACGCAAACG , 5 • -GCGCACTATTATAGCACCGGCTCCCGGGTAATTAATT- GACGCAAACGTTTTACGGCCGCCGG and 5•-GCGCACTATTATAGCACCATG- GATCCGGGTAATTAATTGACGTTTTACGGCCGCCGGTGGCG were used to insert the new linker sites [BamHI-Smal-Xmal] into the SFV cDNA clone. The oligonucleotides 5'-CGGCGGTCCTA- GATTGGTGCG and 5"-CGCGGGCGCCACCGGCGGCCG were used as sequencing primers (SP1 and SP2) up- and downstream of the polylinker site. Phosphorylated oligonucleotides were used in mutagenesis with Sequenase (Unites States Biochemicals, Cleveland, Ohio) as described earlier (44, 45). In vitro made RF forms were transformed.into DH5αF*IQ and the resulting phage isolates analyzed for the presence of correct mutations by dideoxy sequencing according to the USB protocol for using Sequenase. Finally, mutant fragments were reinserted into the full-length SFV cDNA clone. Again, the presence of the appropriate mutations was verified by sequencing from the plasmid DNA. Deletion of the 6K region has been described elsewhere. 7. In vitro transcription. Spel linearized plasmid DNA was used as template for in vitro transcription. RNA was synthesized at 37°C for 1 h in 10-50 μl reac¬ tions containing 40 mM Tris-HCl (pH 7.6), 6 mM spermidine-HCl, 5 mM dithiothreitol (DTT) , 100 μg/ml of nuclease free BSA, 1 mM each of ATP, CTP and UTP, 500 μM of GTP, 1 unit/μl of RNasin and 100-500 units/ml of SP6 RNA polymerase. For production of capped trans¬ cripts (46), the analogs m 7 G(5')ppp(5')G or m 7 G(5')ppp(5')A were included in the reaction at 1 mM. For quantitation of RNA production, trace amounts of [α- 32 P]-UTP (Amersham) was included in the reactions and incorporation measured from trichloroacetic acid preci¬ pitates. When required, DNA or RNA was digested at 37°C for 10 min by adding DNase 1 or RNase A at 10 units/μg template or 20 μg/ml respectively. 8. RNA transfection. Transfection of BHK monolayer cells by the DEAE-Dextran method was done as described previously (47) . For transfection by electroporation, RNA was added either directly from the in vitro trans¬ cription reaction or diluted with transcription buffer containing 5 mM DTT and 1 unit/μl of RNasin. Cells were trypsinized, washed once with complete BHK-cell medium and once with ice-cold PBS (without MgCl 2 and CaCl 2 ) and finally resuspended in PBS to give 10 7 cells/ml. Cells were either used directly or stored (in BHK medium) on ice over night. For electroporation, 0.5 ml of cells were transferred to a 0.2 cm cuvette (BioRad), 10-50 μl of RNA solution added and the solution mixed by invert- ing the cuvette. Electroporation was performed at room temperature by two consecutive pulses at 1.5 kV/25 μF using a BioRad Gene Pulser apparatus with its pulse controller unit set at maximum resistance. After in¬ cubation for 10 min, the cells were diluted 1:20 in complete BHK-cell medium and transferred onto tissue culture plates. For plaque assays, the electroporated cells were plated together with about 3xl0 5 fresh cells per ml and incubated at 37°C for 2 h, then overlayed with 1.8 % low melting point agarose in complete BHK- cell medium. After incubation at 37°C for 48 h, plaques were visualized by staining with neutral red. 9. Gel electrophoresis. Samples for sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE) were prepared and run on 12 % separating gels with a 5 % stacking gel as previously described (48) . For re¬ solving the 6K peptide, a 10 % - 20 % linear acrylamide gradient gel was used. Gels were fixed in 10 % acetic acid - 30 % methanol for 30 min before exposing to Kodak XAR-5 film. When a gel was prepared for fluoro- graphy (49) , it was washed after fixation for 30 min in 30 % methanol and then soaked in 1M sodium salicylate - 30 % methanol for 30 min before drying. Nucleic acids were run on agarose gels using 50 mM Tris-borate - 2.5 mM Na 2 EDTA as buffer. For staining 0.2 μg/ml of ethidium bromide was included in the buffer and gel during the run. Example 1 In this example a full-length SFV cDNA clone is prepared and placed in a plasmid containing the SP6 RNA polymerase promoter to allow in vitro trancription of full-length and infectious transcripts. This plasmid which is designated pSP6-SFV4 has been deposited on 28 NOV 1991 at PHLS Centre for Applied Microbiology & Research European Collection of Animal Cell Cultures, Porton Down, Salisbury, Wiltshire, U.K:, and given the pro- visional accession number 91112826. As illustrated in Fig. 4A-C the strategy for con¬ struction the SFV clone was to prime cDNA synthesis on several positions along the template RNA downstream of suitable restriction endonuclease sites defined by the known nucleotide sequence of the SFV RNA molecule. Virus RNA was isolated by phenol-chloroform extraction from purified virus (obtainable among others from the Arbovirus collection in Yale University, New Haven, USA) and used as template for cDNA synthesis as previously described (50) . First strand synthesis was primed at three positions, using 5•-TTTCTCGTAGTTCTCCTC- GTC as primer-1 (SFV coordinate 2042-2062) and 5'-GTTA- TCCCAGTGGTTGTTCTCGTAATA as primer-2 (SFV coordinate 3323-3349) and an oligo-dT 12 -i 8 as Primer -3 (3' end of SFV) Fig. 4A) . Second strand synthesis was preceded by hybridization Of the Oligonucleotide 5•-ATGGCGGATGTGTGACATACACGACGCC (identical to the 28 first bases of the genome sequence of SFV) to the first strand cDNA. After completion of second strand synthesis cDNA was trimmed and in all cases except in the case of the primer-1 reaction, the double-stranded adaptor 5•-AATTCAAGCTTGCGGCCGCACTAGT / GTTCGAACGCCGGCGTGATCA-3' (5'-sticky-EcoRI-Hindlll-Notl- XmaIII-SpeI-blunt-3') was added and the cDNa cloned into EcoRl cleaved pTZ18R (Pharmacia, Sweden) as described (51). The cloning of the 5 1 end region was done in a different way. Since SFV contains a Hindlll site at position 1947, cDNA primed with primer-1 should contain this area and therefore Hindlll could be used to define the 3 1 end of that cDNA. To obtain a restric¬ tion site at the very 5' end of the SFV, cDNA was cloned into Smal-Hindlll cut pGEMl (Pro ega Biotec, Madison, Wl) . Since the SFV genome starts with the sequence 5*-ATGG, ligation of this onto the blunt Gee¬ s' end of the S al site created an Ncol site C'CATGG. Although the SFV sequence contains 3 Ncol sites, none of these are within the region preceding the Hindlll site, and thus these 5 1 end clones could be further subcloned as Ncol-Hindlll fragments into a vector especially designed for this purpose (see below) . The original cDNA clones in pGEMl were screened by restric- tion analysis and all containing inserts bigger than 1500 bp were selected for further characterization by sequencing directly from the plasmid into both ends of the insert, using SP6 or T7 sequencing primers. The SFV 5'-end clones in pTZ18R were sequenced using lac sequencing primers. To drive in vitro synthesis of SFV RNA the SP6 promoter was used. Cloning of the SFV 5' end in front of this promoter without adding too many foreign nucleotides required that a derivative of pGEMl had to be constructed. Hence, pGEMl was opened " at EcoRl and Bal31 deletions were created, the DNA blunted with T4 DNA polymerase and an Ncol oligonucleotide (5 1 - GCCATGGC) added. The clones obtained were screened by colony hybridization (39) with the oligonucleotide 5 1 - GGTGACACTATAGCCATGGC designed to pick up (at suitable stringency) the variants that had the Ncol sequence immediately at the transcription initiation site of the SP6 promoter (G underlined) . Since the Bal31 deletion had removed all restriction sites of the multicloning site of the original plasmid, these were restored by cloning a Pvul-Ncol fragment from the new variant into another variant of pGEMl (pDHlOl) that had an Ncol site inserted at' its Hindlll position in the polylinker. This created the plasmid pDH20l. Finally, the adaptor used for cloning the SFV cDNA was inserted into pDH201 between the EcoRI and PvuII sites to create plasmid pPLH211 (Fig. 4B) . This plasmid was then used as recipient for SFV cDNA fragments in the assembly of the full-length clone by combining independent overlapping subclones using these sites. The fragments and the relevant restriction sites used to assemble the full- length clone, pSP6-SFV4, are depicted in (Fig. 4A} . For the 5'-end, the selected fragment contained the proper SFV sequence 5'-ATGG, with one additional G-residue in front. When this G-residue was removed it reduced transcription efficiency from SP6 but did not affect infectivity of the in vitro made RNA. Thus, the clone used for all subsequent work contains the G-residue at the 5' end. For the 3'-end of the clone, a cDNA fragment containing 69 A-residues was selected. By inclusion of the unique Spel site at the 3 '-end of the cDNa, the plasmid can be linearized to allow for runoff transcription in vitro giving RNA-carrying 70 A- residues. Fig. 4C shows the 5' and 3" border sequences of the SFV cDNA clone. The general outline how to obtain and demonstrate infectivity of the full-length SFV RNA is depicted in Fig. 6. The complete nucleotide sequence of the pSP6-SFV4 SP6 transcript together with the amino acid sequences of the nonstructural and the structural polyproteins is shown in Fig. 5. Typically, about 5 μg of RNA per 100 ng of template was obtained using 10 units of polymerase, but the yield could be increased considerably by the use of more enzyme. The conditions slightly differ from those reported earlier for the production of infectious tran¬ scripts of alphaviruses (52) (47) . A maximum production of RNA was obtained with rNTP concentrations at 1 mM. However, since infectivity also is dependent on the presence of a 5'cap structure optimal infectivity was obtained when the GTP concentration in the transcrip¬ tion reaction was halved. This drop had only a marginal effect on the amounts of RNA produced but raised the specific infectivity by a factor of 3 (data not shown) . The cDNA sequence shown in Fig. 5 has been used in the following examples. However, sequences having one or a few nucleotides, which differ from those shown in Fig. 5, could also be useful as vectors, even if these might be less efficient as illustrated above with the SFV cDNA sequence lacking the first 5'-G nucleotide in Fig. 5. Example 2. In this example the construction of SFV DNA expres¬ sion vectors is disclosed. The cDNA clone coding for the complete genome of SFV obtained in Example 1 was used to construct a SFV DNA expression vector by deletion of the coding region of the 26S structural genes to make way for heterologous inserts. However, the nonstructural coding region, which is required for the production of the nsPl-4 replicase complex is preserved. RNA replication is dependent on short 5' (nt 1-247) (53, 54, 55) and 3 » (nt 11423-11441) sequence elements (56, 57) , and there¬ fore, also these had to be included in the vector con¬ struct, as had the 26S promoter just upstream of the C gene (17, 18) . As is shown in Fig. 7, first, the Xbal (6640)-Nsil (8927) fragment from the SFV cDNA clone pSP6-SFV4 from Example 1 was cloned into pGEM7Zf(+) (Pro ega Corp., Wl, USA) (Step A). From the resulting plasmid, pGEM7Zf(+)- SFV, the EcoRI fragment (SFV coordinates 7391 and 88746) was cloned into M13mpl9 to insert a BamHI - Xmal - Smal polylinker sequence immediately downstream from the 26S promoter site using site-directed mutagenesis (step B) . Once the correct mutants had been verfied by sequencing from M13 ssDNA (single stranded) , the EcoRI fragments were reinserted into pGEM7Zf(+)-SFV (step C) and then cloned back as Xbal-Nsλ fragments into pSP6- SFV4 (step D) . To delete the major part of the cDNA region coding for the structural proteins of SFV, these plasmids were then cut with AsuII (7783) and Ndel (11033) , blunted using Klenow fragment in the presence of all four nucleotides, and religated to create the final vectors designated pSFVl, pSFV2 and pSFV3, respectively (step E) . The vectors retain the promoter region of the 26S subgenomic RNA and the last 49 amino acids of the El protein as well as the complete non- coding 3' end of the SFV genome. In the vectors the subgenomic (26S) protein coding portion has been replaced with a polylinker sequence allowing the insertional cloning of foreign cDNA sequences under the 26S promoter. As is shown in Fig. 8 these three vectors have the same basic cassette in¬ serted downstream from the 26S promoter, i.e. a poly¬ linker (BamHI-Smal-Xmal) followed by a translational stop-codons in all three reading frames. The vectors differ as to the position where the polylinker cassette has been inserted. In pSFVl the cassette is situated 31 bases downstream of the 26S transcription initiation site. The initiation motive of the capsid gene transla¬ tion is identical to the consensus sequence (58) . Therefore, this motive has been provided for in pSFV2, where it is placed immediately after the motive of the capsid gene. Finally, pSFV3 has the cassette placed immediately after the initiation codon (AUG) of the capsid gene. Sequencing primers (SP) needed for checking both ends of an insert have been designed to hybridize either to the 26S promoter region (SP1) , or to the region following the stop codon cassette (SP2) . Note that the 26S promoter overlaps with the 3'-end of the nsP4 coding region. For pSFV2, the cloning site is positioned immediately after the translation initia¬ tion site of the SFV capsid gene. For pSFV3, the cloning site is positioned three nucleotides further downstream, i.e. immediately following to the initial AUG codon of the SFV capsid gene. The three translation stop codons following the polylinker are boxed. The downstream sequencing primer (SP1) overlaps with the 26S promoter, and the upstream sequencing primer (Sp2) overlaps the Xmalll site. Example 3 In this example an in vivo packaging system encompas¬ sing helper virus vector constructs is prepared. The system allows SFV variants defective in struc¬ tural protein functions, or recombinant RNAs derived from the expression vector construct obtained in Example 2, to be packaged into infectious virus particles. Thus, this system allows recombinant RNAs to be introduced into cells by normal infection. The help¬ er vector, called pSFV-Helperl, is constructed by deleting the region between the restriction endo- nuclease sites AccI (308) and AccI (6399) of pSP6-SFV4 obtained in Example 1 by cutting and religation as shown in Fig. 7, step F. The vector retains the 5 1 and 3* signals needed for RNA replication. Since almost the complete nsP region of the Helper vector is deleted, RNA produced from this construct will not replicate in the cell due to the lack of a functional replicase complex. As is shown in Fig. 9, after transcription in vitro of pSFVl-recombinant and helper cDNAs, helper RNA is cotransfected with the pSFVl - recombinant deriva¬ tive, the helper construct providing the structural proteins needed to assemble new virus particles, and the recombinant providing the nonstructural proteins needed for RNA replication, SFV particles comprising recombinant genomes being produced. The cotransfection is preferably produced by electroporation as is dis¬ closed in Example 6 and preferably BHK cells are used as host cells. To package the RNA a region at the end of nsPl is required, ah area which has been shown to bind capsid protein (57, 59) . Since the Helper lacks this region, RNA derived from this vector will not be packaged and hence, transfections with recombinant and Helper pro¬ duces only virus particles that carry recombinant- derived RNA. It follows that these viruses cannot be passaged further and thus provide a one-step virus stock. The advantage is that infections with these particles will not produce any viral proteins. Example 4 This example illustrates the construction of variants of the full-length SFV cDNA clone from Example 1 that allow insertion of foreign DNA sequences encoding foreign epitopes, and the production of recombinant (chimaeric) virus carrying said foreign epitopes as integral parts of the p62, E2 or El spike proteins. To this end, a thorough knowledge of the function, topology and antigenic structure of the E2 and El envelope proteins has been of the essence. Earlier studies on the pathogenicity of alphaviruses have shown that antibodies against E2 are type-specific and have good neutralizing activity while those against El generally are group-specific and are nonneutralizing (5) . However, not until recently have antigenic sites of the closely related alphaviruses SFV, Sindbis, and Ross River been mapped and correlated to the level of amino acid sequence (60, 61, 62, 63) . These studies have shown that the most dominant sites in question are at amino acid positions 216, 234 and 246-251 of the SFV E2 spike protein. Interestingly, these three sites are exactly the same as the ones predicted by computer analysis. In the present example domain 246-251 was used, since this area has a highly conserved structure and hydropathy profile within the group of alpha¬ viruses. Insertion of a gene encoding a foreign epitope into the 246-251 region of the pSP6-SFV4 p62 protein yields particles with one new epitope on each hetero- dimer, i.e. '240 copies. To create a unique restriction endonuclease site that would allow specific insertion of foreign epitopes into the E2 portion of the SFV genome, a BamHI site was inserted by site directed mutagenesis using the oligo¬ nucleotide 5 » -GATCGGCCTAGGAGCCGAGAGCCC. Example 5 In this example a conditionally lethal variant of SFV is constructed from the SFV cDNA obtained in Example 1, which variant carries a mutation in the p62 protein resulting in a noncleavable from of said protein, with the result that this variant as such cannot infect new host cells, unless first cleaved with exogenously added protease. As illustrated in Fig. 10, this construct can be advantageously used as a vaccine carrier for foreign epitopes, since this form of the virus cannot enter new host cells although assembled with wild type efficiency in transfected cells. The block can be overcome by trypsin treatment of inactive virus particles. This converts the particle into a fully entry-competent form which can be used for amplification of this virus variant stock. Once activated the SFV variant " will enter cells normally through the endocytic pathway and start infec- tion. Viral proteins will be made and budding takes place at the plasma membrane. However, all virus particles produced will be of inactive form and the infection will thus cease after one round of replica¬ tion. The reason for the block in infection proficiency is a mutation which has been introduced by site directed mutagenesis into the cleavage site of p62. This arginine to leucine substitution (at amino acid postion 66 of the E3 portion of the p62 protein) changes the consensus features of the cleavage site so that it will not be recognized by the host cell pro- teinase that normally cleaves the p62 protein to the E2 and E3 polypeptides during transport to the cell surface. Instead, only exogenously added trypsin will be able to perform this cleavage, which in this case occurs at the arginine residue 65 immediately preceding the original cleavage site. As this cleavage regulates the activation of the entry function potential of the virus by controlling the binding of the entry spike subunit, the virus particle carrying only uncleaved p62 will be completely unable to enter new host cells. The creation of the cleavage deficient mutation E2 has been described earlier (29) . An Asull - Nsλ fragment spanning this region was then isolated and cloned into the full-length cDNA clonepSP6-SFV4. Example 6 In this example transfection of BHK cells with SFV RNA molecules transcribed in vitro from full-length cDNA from Example 1 or variants thereof or the SFV vectors from Example 2, which comprise exogenous DNA, is disclosed. The transfection is carried out by electroporation which is shown to be very efficient at optimized conditions. BHK cells were transfected with the above SFV RNA molecules by electroporation and optimal conditions were determined by varying parameters like temperature, voltage, capacitance, and number of pulses. Optimal transfection was obtained by 2 consecutive pulses of 1.5 kV at 25 μF, under which negligible amounts of cells were killed. It was found that it was better to keep the cells at room tempeature than at 0°C during the whole procedure. Transfection by electroporation was also measured as a function of input RNA. As expected, an increase in transfection frequency was not linearly dependent on RNA concentration, and about 2 μg of cRNA were needed to obtain 100 % transfection. On comparison with conventional transfection, this is a great improvement. For example, with DEAE-Dextran transfection optimally, only 0.2 % of the cells were transfected." Example 7 This example illustrates heterologous gene expression driven by the SFV vector, pSFVl from Example 2, for genes encoding the 21 kD cytoplasmic mouse dihydro- folate reductase (dhfr) , the 90 kD membrane protein human transferrin receptor (TR) , and finally the 14 kD secretory protein chicken lysozyme. The dhfr gene was isolated from pGEM2-dhfr (64) as a BamHI-Hindlll fragment blunted with Klenow fragment and inserted into Smal-cut pSFVl. The transferrin receptor gene was first cloned from pGEMl-TR (64, 65) as an Xbal-EcoRI fragment into pGEM7ZF(+) and subsequently from there as a BamHI fragment into pSFVl. Finally, a BamHI fragment from pGEM2 carrying the lysozyme gene (21) was cloned into pSFVl. To study the expression of the heterologous proteins, in vitro-made RNA of the dhfr and TR constructs was electroporated into BHK cells. RNA of wild type SFV was used as control. At different time points post electro- poration (p.e.) cells were pulse-labeled for 10 min followed by a 10 min chase, whereafter the lysates were analyzed by gel electrophoresis and autoradiography. The results are shown in Figure 11. More specifically, BHK cells were transfected with RNAs of wild type SFV, pSFVl-dhfr, and pSFVl-TR, pulse-labeled at 3, 6, 9, 12, 15 and 24 h p.e. Equal amounts of lysate were run on a 12 % gel. The 9 h sample was also used in immunopreci- pitation (IP) of the SFV, the dhfr and the transferrin receptor proteins. Cells transfected with pSFVl- lysozyme were pulse-labeled at 9 h p.e. and then chased for the times (hours) indicated. An equal portion of lysate or medium was loaded on the 13,5 % gel. IP represents immunoprecipitation from the 1 h chase lysate sample. The U-lane is lysate of labeled but untransfected cells. At 3 h p.e.hardly any exogenous proteins were made, since the incoming RNA starts with minus strand synthesis which does not peak until about 4-5 h p.e. (5) . At this time point, almost all labeled proteins were of hos origin. In contrast, at 6 h p.e. the exogenous proteins were synthesized with great efficiency, and severe inhibition of host protein syn¬ thesis was evident. This was even more striking at 9 h p.e., when maximum shut down had been reached. Efficient production of the heteroloqous proteins con¬ tinued up to 24 h p.e., after which production slowed down (data not shown) , indicating that the cells had entered a stationary phase. Since chicken lysozyme is a secretory protein, its expression was analyzed both from cell lysates and from the growth medium. Cells were pulse-labeled at 9 h p.e. and then chased up to 8 h. The results are shown in Fig. 11. Although lysozyme was slowly secreted, almost all labeled material was secreted to the medium during the chase. Example 8 This example illustrates the present in vivo packaging system. In vitro-made RNA of pSFVl-TR was mixed with Helper RNA at different ratios and these mixtures were co- transfected into BHK cells. Cells were grown for 24 h after which the culture medium was collected and the virus particles pelleted by ultracentrifugation. The number of infectious units (i.u.) was determined by i munofluorescence. It was found that a 1:1 ratio of Helper and recombinant most efficiently produced in¬ fectious particles, and on the average 5 x 10 6 cells yielded 2.5 x 10 9 i.u. The infectivity of the virus stock was tested by infecting BHK cells at different multiplicities of infection ( .o.i.). In Fig. 11 the results for expression of human transferrin receptor in BHK cells after infection by such in vivo packaged particles carrying pSFVl-TR recombinant RNA is shown to the lower right. 200 μl of virus diluted in MEM (in¬ cluding 0,5'% BAS and 2 mM glutamine) was overlaid on cells to give m.o.i. values ranging from 5 to 0.005. After 1 h at 37°C, complete BHK medium was added and growth continued for 9 h, at which time a 10 min pulse (100 μCi 35 S-methionine/ml) and 10 min chase was performed, and the cells dissolved in lysis buffer. 10 μl out of the 300 μl lysate (corresponding to 30,000 cells) was run on the 10 % gel, and the dried gel was exposed for 2 h at -70°C. Due to the high expression level, only 3,000 cells are needed to obtain a distinct band on the autoradiograph with an over night exposure. Thus, it was found that efficient protein production and concomitant hos protein shut-off occurred at about 1 i.u. per cell. Since one SFV infected cell produces on the average 10 8 capsid protein molecules, it follows that a virus stock produced from a single electropora¬ tion can be used to produce 10 17 protein molecules equaling about 50 mg of protein. From the foregoing experimental results it is obvious that the present invention is related to very useful and efficient expression system which lacks several of the disadvantages of the hitherto existing expression system. The major advantages of the present system are shortly summarized as follows: (1) High titre recombinant virus stocks can be produced in one day by one transfection experiment. There is no need for selection/screening, plaque purifica¬ tion and amplification steps. This is appreciated since an easy production of recombinant virus is especially important in experiments where the phenotypes of large series of mutants have to be characterized. (2) The recombinant virus stock is free from helper virus since only the recombinant genome but not the helper genome contains a packaging signal. (3) The recombinant virus can be used to infect the recombinant genome in a "natural" and nonleakey way into a large variety of cells including insect and most higher euoaryotic cell types. Such a wide host range is very useful for an expressions system especially when cell-type-specific posttranslatio- nal modification reactions are required for the activity of the expressed protein. (4) The level of protein expression obtained is extremely high, the level corresponding to those of the viral proteins during infection. There is also a host cell protein shut-off which makes it possible to follow the foreign proteins clearly in cell lysates without the need for antibody mediated antigen concentration. This will facilitate DNA expression experiments in cell biology considerab¬ ly. Furthermore, problems of interference by the endogenous counter part to an expressed protein (i.e. homo-oligomerization reactions) can be avoided. Example 9 This example illustrates epitope carriers. A very important example where vaccine development is of the utmost importance concerns the acquired immuno¬ deficiency syndrome (AIDS) caused by the human immuno¬ deficiency virus HIV-1 (66, 67) . Sofar, all attempts to produce an efficient vaccine against HIV-1 have failed, although there was a very recent report that vaccina- tion with disrupted SIV-1 (Simian immunodeficiency virus) to a certain extent may give protection against infections of that virus (68) . However, development of safe and effective vaccine against HIV-1 will be very difficult due to the biological properties of the virus. In the present exampel one epitope of HIV-1 was inserted into an antigenic domain of the E2 protein of SFV. The epitope used is located in glycoprotein gpl20 of HIV-1, spanning amino acids 309-325. This forms the variable loop of HIV-1 and is situated immediately after an N-glycosylated site. A chimaera was constructed where the 309-325 epitope of HIV was inserted into the BamHI site using cassette insertion of ready-made oligonucleotides encoding the HIV epitope. The required base substitutions at the BamHI site did not lead to any amino acid changes in the vector, although two amino acids (Asp and Glu) changed places. This change did not have any deleterious effect since in vitro made vector RNA induced cell infection with wild type efficiency. Fig. 12 shows the sequences in the area of interest in the epitope carrier. In preliminary experiments, it has been shown that chimaeric proteins were produced. The proteins can be immunoprecipitated with anti-HIV anti¬ bodies. It is to be expected that these are also used for production of chimaeric virus particles that can be used for vaccine preparation against HIV. Such par- tides are shown in Fig. 12, lower part. List of references 1) Bishop, D.H.L. (1990) . Gene expression using insect cells and viruses. In current Opinion in Biotechnology, Vol. 1, Rosenberg, M. , and Moss, B. , eds. (London: Current Opinion Ltd.), pp. 62-67. 2) Moss, B. (1990) . Regulation of Vaccinia virus transcription. Ann. Rev. Biochem. 59, 661-688. 3) Moss, B, and Flexner, C. (1989) . Vaccinia virus expression vectors. Ann. N.Y. Acad Sci. 569, 86-103. 4) Garoff, H. , Kondor-Koch, C. , and Riedel, H. (1982). Structure and assembly of alphaviruses. Curr. Top. Microbiol. Immunol. 99, 1-50. 5) Strauss, E.G., and Strauss, J.H. (1986). Structure and replication of the alphavirus genome. 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Claims: Claims 1. An RNA molecule derived from an alphavirus RNA genome and capable of efficient infection of animal host cells, which RNA molecule comprises the complete alphavirus RNA genome regions, which are essential to replication of the said alpha¬ virus RNA, and further comprises an exogenous RNA sequence capable of expressing its function in said host celϊ, said exogenous RNA sequence being inserted into a region of the RNA molecule which is non-essential to replication thereof. 2. The RNA of claim 1, wherein the said alphavirus is Semliki Forest virus (SFV) . 3. The RNA of claim 1 or 2, wherein the exogenous RNA sequence encodes a protein, a polypeptide or a peptide sequence defining an exogenous antigenic epitope or determinant. 4. The RNA of claim 3 wherein the exogenous RNA sequence encodes an epitope sequence of a structural protein of an immunodeficiency virus inclusive of the human immunodeficiency virus (HIV) types. 5. The RNA of any preceding claim, wherein the alphavirus derived RNA molecule regions comprise a 5* terminal portion, the coding region(s) for non structural proteins required for RNA replication, the subgenome promoter region and a 3* terminal portion of said viral RNA. 6. The RNA of claim 2, 3 or 5, wherein the exogenous RNA sequence encodes a foreign polypeptide or protein and is integrated into the SFV subgenomic 26S RNA substituting deleted parts thereof. 7. The RNA of claim 2, 3, 4 or 5, wherein the exogenous RNA sequence encodes a foreign viral epitopic peptide sequence and is located in a region of the RNA coding for structural alphavirus proteins enabling the exogenous RNA to be expressed as said viral epitope as part of the matured virus particle. 8. The RNA of claim 2, 3, 4 or 5, wherein the exogenous RNA sequence encodes a foreign viral epitopic peptide sequence inserted into the p62 spike precursor subunit encoding region of the SFV genome. 9. An RNA expression vector comprising the RNA of any preceding claim packaged into infectious particles comprising the RNA within the alphavirus nucleocapsid and surrounded by membrane with alphavirus spike proteins. 10. The vector of claim 9, wherein the RNA has a total size corresponding to the wild type alphavirus RNA genome or deviating therefrom to an extent compatible with package of the RNA into the infectious particles. 11. DNA transcription vector comprising a cDNA having one strand complementary to the RNA of any of claims 1 to 8. 12. A DNA expression vector comprising a full-length or partial cDNA complementary to alphavirus RNA or parts thereof and located immediately downstream of the SP6 RNA polymerase promoter and having a 5'ATGG or 5'GATGG or any other 5' terminus and a TTTCCA 69 ACTAGT or any other 3' terminus. 13. The vector of claim 12 having portions of the viral cDNA deleted, the deletions comprising the complete or part of the region(s) encoding the virus structural proteins, and further comprising an integrated polylinker region, which may correspond to BamHI-Smal-Xmal, inserted at a location which enables an exogenous DNA fragment encoding a foreign poly¬ peptide or protein to be inserted into the vector cDNA for subsequent expression in an animal host cell. 14. The vector of claim 12 or 13 wherein the alphavirus is SFV. 15. The vector of claim 12 or 14 comprising full-length cDNA and further comprising an exogenous DNA fragment encoding a foreign epitopic peptide sequence or antigenic determinant inserted into a region of the viral structural proteins. 16. The vector of claim 15 wherein the exogenous DNA fragment is inserted into the p62 spike precursor subunit encoding region of the SFV cDNA. 17. The vector of any preceding claim comprising an SFV derived cDNA which carries a conditionally lethal SFV mutation in the region encoding the p62 cleavage site, a cellularly uncleavable but extracellularly cleavable form of p62 being expressed. 18. The vector of claim 13 comprising SFV-derived cDNA, the vector being pSFVl, pSFV2 or pSFV3 having a structure as shown in Fig. 8. 19. An RNA transcript derived from transcription of the DNA-vector of any of claims 12-18 carrying an exogenous DNA fragment. 20. A method to produce the vector of claim 9 or 10 wherein the alphavirus derived RNA lacks part of or the complete region(s) encoding the structural viral proteins, the method comprising cotransfection of animal host cells with the RNA transcript of claim 19, wherein the alphavirus RNA lacks part(s) of or the complete region(s) encoding the viral struc¬ tural proteins, with helper RNA transcribed in vitro from a helper DNA vector and culturing the host cells. 21. The method of claim 20 wherein the cotransfection is produced by electroporation of the host cells. 22. Helper vector for use in the method according to claim 20 or 21, said vector being comprised of the DNA vector of claim 12 wherein the regions encoding non structural virus proteins are almost completely deleted, including sequences encoding RNA signals for packaging of RNA into nucleocapsid particles, but the 5 1 and 3' signals needed for RNA replication and the region encoding the promoter for the structural sub- genome are in addition to those encoding the structural region preserved. 23. Helper vector of claim 22 wherein the cDNA has its origin from SFV and the deletion extends from the AccI (308) to the AccI (6399) restriction endonuclease site of the full- length cDNA vector of claim 12. 24. Helper vector of claim 22 and 23 where the structural region contains the mutation described in claim 17 or another conditionally lethal mutation. 25. The method of claim 20 wherein cells transformed to produce helper RNA according to claims 20, 22 or 23 are trans¬ fected with RNA transcript of claim 19. 26. A host cell of animal origin transformed with the RNA of any of claims 1-8, the DNA transcription vector of claims 11 or the DNA vector of any of claims 12-18 carrying an exogenous DNA fragment. 27. The host cell of claim 26 wherein the cell is an avian, a mammalian, a reptilian, an amphibian, an insecticidal or a fish cell. 28. The host cell of claim 27 which is the hamster BHK cell. 29. A method to produce the transformed host cell of claim 26, 27 or 28 comprising transfection of the cell with the RNA of any of claim 1-8, with the cDNA of claim 11 or of any of claims 12-18 carrying an exogenous DNA fragment or infection of the cell with the infectious viral particles of claim 9 or 10. 30. The method of claim 29 wherein the transfection is produced by electroporation of the host cell. 31. A method for the production of a polypeptide or protein comprising infection of animal host cells with infec¬ tious particles according to claim 9 or 10, containing exo¬ genous RNA encoding said polypeptide or protein and produced according to method of claim 20 or 21, culturing the said transformed cells to express the exogenous RNA and isolation and purification of the product formed by said expression. 32. A method for the production of a polypeptide or protein comprising in vitro transcription of the cDNA of the vector of any of claims 11-18 carrying an exogenous DNA frag- ment coding for the polypeptide or protein, transfection of animal host cells with the produced RNA transcript, transformed animal host cells being obtained harbouring the RNA transcript, culturing the said transformed cells to express the exogenous RNA and isolation and purification of the product formed by said expression. 33. The metod of claim 32 wherein the vector cDNA is comprised of the cDNA of the vector of claim 17 carrying the exogenous DNA fragment. 34. An antigen consisting of a chimaeric alphavirus having an exogenous epitopic peptide sequence or antigenic determinant inserted into its structural proteins. 35. The antigen of claim 34 wherein the chimaeric alpha- virus is derived from SFV. 36. The antigen of claim 34 or 35, wherein the exogenous epitopic peptide sequence is comprised of an epitopic peptide sequence derived from a structural protein of a virus belonging to the immunodeficiency virus class inclusive of the human immunodeficiency virus types. 37. Vaccine preparation comprising the antigen of claim 34, 35 or 36 as immunizing component. 38. Vaccine of claim 37 wherein the chimaeric alphavirus is attenuated by comprising the conditionally lethal SFV mutation of claim 17, an amber (stop codon) a temperature sensitive mutation or other mutation in its genome. 39. A method for the production of an antigen of claim 34, 35 or 36 comprising a) in vitro transcription of the cDNA of the vector of any of claims 11-18 carrying an exogenous DNA fragment encoding the foreign epitopic peptide sequence or antigenic determinant and transfection of animal host cells with the produced RNA transcript, or b) transfection of animal host cells with the said cϋNA of the above step a) , culturing the transfected cells and recovering the chimaeric alphavirus antigen. 40. The method of claim 32, 33 or 39 wherein the trans¬ fection is produced by electroporation of the host cell. 41. A method for the production of an antigen in an organism by using in vivo infection with infectious particles according to claim 9 or 10 containing exogenous RNA encoding an exogenous epitopic peptide sequence or antigenic determinant, and produced according the claim 20 or 21.

Inventors: GAROFF HENRIK, LILJESTROEM PETER, GAROFF, HENRIK, LILJESTROEM, PETER
Applicants: BIOPTION AB
Application year: 1991
Main classification: A61K39
IPC codes: A61K35, A61K39, C07K14, C12N5, C12N7, C12N15, C12P21, C12R1