ALPHAVIRUS VECTORS

Abstract: A modified alphavirus expression vector is provided wherein at least one optimal heterologous splice site is introduced to the alphavirus replicon to prevent aberrant splicing of the alphavirus, which may be Semliki Forest virus following administration of the vector to a host.

Description: TITLE OF INVENTION ALPHAVIRUS VECTORS FIELD OF INVENTION The present invention relates to the field of DNA vaccines and is particularly concerned with modified alpha virus vectors for use in such vaccines . BACKGROUND OF THE INVENTION Semliki Forest virus (SFV) is a member of the Alphavirus genus in the Togaviridae family. The mature virus particle contains a single copy of a ssRNA genome with a positive polarity that is 5* -capped and 3'- polyadenylated. It functions as an mRNA and naked RNA can start an infection when introduced into cells. Upon infection/transfection, the 5' two-thirds of the genome is translated into a polyprotein that is processed into the four nonstructural proteins (nsPl to 4) by self cleavage. Once the ns proteins have been synthesized they are responsible for replicating the plus-strand (42S) genome into full-length minus strands (ref . 14) . These minus-strands then serve as templates for the synthesis of new plus-strand (42S) genomes and the 26S subgenomic mRNA (ref. 1 - Throughout this application, various references are cited in parentheses to describe more fully the state of the art to which this invention pertains. Full bibliographic information for each citation is found at the end of the specification. The disclosures of these references are hereby incorporated by reference into the present disclosure) . This subgenomic mRNA, which is colinear with the last one- third of the genome, encodes the SFV structural proteins. In 1991 Liljestrom and Garoff (ref. 2) designed a series of expression vectors based on the SFV CDNA replicon. These vectors had the virus structural protein genes deleted to make the way for heterologous inserts, but preserved the nonstructural coding region for production of the nsPl to 4 replicase complex. Short 5 ' and 3 ' sequence elements required for RNA replication were also preserved. A polylinker site was inserted downstream from the 26S promoter followed by translation stop sites in all three frames. An Spel site was inserted just after the 3 ' end of the SFV CDNA for linearization of the plasmid for use in vi tro transcription reactions. Injection of SFV RNA encoding a heterologous protein have been shown to result in the expression of the foreign protein and the induction of antibody in a number of studies (refs. 3,4) . The use of SFV RNA inoculation to express foreign proteins for the purpose of immunization would have several of the advantages associated with plasmid DNA immunization. For example, SFV RNA encoding a viral antigen may be introduced in the presence of antibody to that virus without a loss in potency due to neutralization by antibodies to the virus. Also, because the protein is expressed in vivo the protein should have the same conformation as the protein expressed by the virus itself. Therefore, concerns about conformational changes which could occur during protein purification leading to a loss in immunogenicity, protective epitopes and possibly immunopotentiation, could be avoided by plasmid DNA immunization. In WO95/27044, the disclosure of which is incorporated herein by reference, there is described the use of alphavirus cDNA vectors based on cDNA complementary to the alphavirus RNA sequence . Once transcribed from the cDNA under transceptional control of a heterologous promoter, the alphavirus RNA is able to self-replicate by means of its own replicase and thereby amplify the copy number of the transcribed recombinant RNA molecules . SUMMARY OF THE INVENTION The present invention is concerned with modifications to the alphavirus cDNA vectors described in the aforementioned WO 95/27044 to permit enhanced replication of the alphavirus. In the present invention, a heterologous splice site is introduced into the alphavirus replicon sequence, particularly that of Semliki Forest virus (SFV) . Accordingly, in one aspect, the present invention provides an expression vector comprising a DNA molecule complementary to at least part of an alphavirus RNA genome, which DNA molecule comprises the complement of the complete alphavirus RNA genome regions which are essential for replication of the said alphavirus RNA, and further comprises a heterologous DNA sequence capable of expression ' in a suitable host, such as a human or animal host, said heterologous DNA sequence being inserted into a region of the DNA molecule which is non-essential to replication thereof, and the DNA molecule being placed under transcriptional control of a promoter sequence functional in said animal or human host, wherein at least one heterologous splice site is provided in the DNA molecule to prevent aberrant RNA splicing of the alphavirus. The alphavirus molecule is a large molecule and, accordingly, there is a high probability of cryptic splice sites, thereby impairing the replication of the alphavirus and hence its ability to express the heterlogous DNA is impaired. By introducing the at least one optimal heterologous splice site in accordance with the present invention into the alphavirus replicon sequence, any splicing is likely to be directed at the heterologous splice site rather than any cryptic splice sites, restores the function of the SFV replicon when removed, and may improve transport of RNA from the nucleus (ref. 6) . In the constructs provided herein, the promoter is placed upstream of the 5' -end of the alphavirus sequence, such that the resultant transcript has an authentic 5 ' -end, which is required for the efficient replication of the alphavirus RNA replicon. In addition, there may be provided at the 3 ' end of the Semliki Forest virus segment, a hepatitis delta virus ribozyme sequence to ensure proper in vivo cleavage at the 3 '-end of the sequence. Any other convenient sequence may be employed to achieve this effect. The heterologous splice site sequence may be provided by the nucleotide sequence of the rabbit β- globin intron II, as described in reference 5. Such heterologous splice site sequence may be inserted into the complement sequence at any convenient location which generates perfect splice junctions. This precludes replication of the alphavirus, unless it is authentically removed by splicing.. I have identified five suitable sites in the SFV replicon, which are contained within an EcoRV-Spel fragment of the replicon which is 8010 bp in length (Fig. 3). The first such site is a Ppu-MI site, at position 2719 within the EcoRV-Spel fragment. In constructing the modified vectors provided herein, the EcoRV-Spel fragment is cut with Ppu-MI at position 2719 and made blunt-ended with Mung Bean nuclease, which removes three bases from the SFV sequence. A blunt-ended β-globin II intron, which is 536 bp long, is ligated into the site and replaces the missing three bases with sequence added to the 3' -end of the β-globin intron sequence (Fig. 1) . The other four suitable sites for insertion of the Intron are the PvuII sites at bp 2518, 3113, 6498 and 6872 of the EcoRV-Spel fragment. Insertion of the Intron is achieved by cutting with PvuII (a blunt end cutter) and the blunt-ended β-globin II intron sequence (Fig. 2) is ligated into one or more of these sites. In a further aspect of the present invention, there is provided a cloning vector suitable for expression in a host cell of an heterologous DNA sequence, which comprises a DNA molecule complementing to at least part of an alphavirus RNA genome, which DNA molecule comprises the complement of the complete alphavirus RNA genome regions and has a cloning site for insertion therein of a heterologous DNA sequence capable of expression in a host cell, said cloning site being located in a region of the DNA molecule which is non-essential to replication thereof; a promoter sequence functional in said host cell and transcriptionally controlling said DNA molecule, said promoter sequence being placed upstream of the 5 ' -end of the DNA molecule such that the resultant transcript had an authentic 5' end; at least one heterologous splice set provided in the complement of the DNA molecule to generate perfect splice junctions in the alphavirus in order to prevent aberrant splicing and an additional DNA sequence at the 3 ' -end of the DNA molecule to direct proper in vivo cleavage at the 3 ' - end of the reactant mRNA transcript . BRIEF DESCRIPTION OF DRAWINGS Figure 1 shows the DNA sequence of the β-globin intron II including three additional nucleotides at the 3'-end thereof (SEQ ID No:l); Figure 2 shows the DNA sequence of the β-globin intron II (SEQ ID No:2); Figures 3A to 3C show the DNA sequence of the EcoRV-Spel fragment of Semliki Forest virus replicon (SEQ ID No: 3) ; Figures 4A to 4D show the DNA sequence of the pSFV link (SEQ ID no : 4) prepared as illustrated in Figure 5; Figure 5 shows construction of pSFVlink (11060 bp) from pSFVl using a linker sequence (SEQ ID nos: 5,6); Figures 6A to 6D show the nucleotide sequence of plasmid pMP76 (SEQ ID no: 11, prepared as illustrated in Figures 8A to 8D; Figure 7 illustrates subsections of plasmid pSFV link (see Figure 5) ; Figure 8A to 8D show the construction of plasmid pMP76 from plasmids pMP53, pMP70, pMP47, pMP55 and pMP71; Figures 9A to 9B show the construction of plasmids pMP53, pMP54 and pMP55 from plasmid pMP52; Figure 10 shows the construction of plasmid MP52 from pUC19 using a linker sequence (SEQ ID no: 7,8); Figures 11A to 11B show the construction of plasmids pMP46, pMP47 and pMP70 from pUC19 and fragment from pSFV link, prepared as seen in Figure 7; and Figures 12A to 12B show the construction of plasmid pMP71 from plasmid pCMV3. GENERAL DESCRIPTION OF INVENTION As discussed above, the present invention provides a modified alphavirus DNA. The alphavirus preferably is Semliki Forest virus. In particular, the present invention provides a cloning vector for heterologous gene expression in a host, such as an animal or human. The promoter sequence may comprise a promoter of eukaryotic or prokaryotic origin. Suitable promoters are the cytomegalovirus immediate early promoter (pCMV) , although other promoters, such as the Rous sarcoma virus long-terminal repeat promoter (pRSV) , since, in the case of these and similar promoters, transcription is performed by the DNA-dependent RNA polymerase of the host cell. Additionally, the SP6, T3 or T7 promoters can be used, provided that the cell has first been transformed with genes encoding SP6, T3 or T7 RNA polymerase molecules which are either inserted into the chromosome or remain episomal . Expression of these (SP6, T3 , T7) RNA polymerase-encoding genes is dependent on the host cell DNA-dependent RNA polymerase . The heterologous DNA insert may comprise the coding sequence for a desired product, which may be a biologically active protein or polypeptide, for example, the heterologous DNA insert may code for HIV sequences, e.g., an immunogenic or antigenic protein or polypeptide, or a therapeutically active protein or polypeptide. The heterologous DNA may also comprise additional sequences, such as a sequence complementary to an RNA sequence which is a self-cleaving ribozyme sequence . The DNA vectors provided herein may be administered to a host, including a human host, for in vivo expression of the heterologous DNA sequence, in accordance with a further aspect of the invention, in order to generate an immune response in the host, which may be a protective immune response. The DNA vectors may be further formulated into immunogenic compositions for such administration. BIOLOGICAL DEPOSITS Certain vectors that contain the Semliki Forest virus replicon and referred to herein have been deposited with the American Type Culture Collection (ATCC) located at 10801 University Boulevard, Manassas, VA 20110-2209, U.S.A., pursuant to the Budapest Treaty and prior to the filing of this application. Samples of the deposited plasmids will become available to the public upon grant of a patent based upon this United States patent application and all restrictions on access to the deposits will be removed at that time. Non-viable deposits will be replaced. The invention described and claimed herein is not to be limited in scope by plasmids deposited, since the deposited embodiment is intended only as an illustration of the invention. Deposit Summary Plasmid ATCC Designation Date Deposited pMP76 EXAMPLES The above disclosure generally describes the present invention. A more complete understanding can be obtained by reference to the following specific Examples. These Examples are described solely for purposes of illustration and are not intended to limit the scope of the invention. Changes in form and substitution of equivalents are contemplated as circumstances may suggest or render expedient. Although specific terms have been employed herein, such terms are intended in a descriptive sense and not for purposes of limitations. Methods of molecular genetics, protein biochemistry and immunology used but not explicitly described in this disclosure and these Examples are amply reported in the scientific literature and are well within the ability of those skilled in the art. EXAMPLE 1 This Example describes the construction of plasmid pMP7β as outlined in Figures 5, 7, 8A, 8B, 8C, 8D, 9A, 9B, 10, 11A, 11B, 12A and 12B. Plasmid pSFV link was created by restricting plasmid pSFVl (Gibco) with BamHl. This plasmid was then ligated with a linker (SEQ ID no: 5 and 6) to produce plasmid pSFV link (Figures 4A to 4D, Figure 5) . Some of the SFV replicon fragments were subcloned by restricting pSFVlink with EcoRV and Spel and isolating the 890bp EcoRV-Spel fragment. This fragment was then restricted with EcoRI and the 1906bp EcoRV- EcoRI, the 1578bp and 3627bp EcoRI-EcoRI and the 899bp EcoRI-Spel fragments isolated (Fig.7). The 1909bp EcoRV-EcoRI SFV fragment was cloned into EcoRV-EcoRI restricted plasmid pMP52 to produce plasmid pMP53 (Fig.9A) . The 899bp EcoRI -Spel SFV fragment was cloned into EcoRI -Spel restricted pMP52 to produce pMP54 (Fig.9A). Plasmid pMP54 was then restricted with Spel and made blunt-ended with Mung Bean nuclease. The plasmid was then restricted with Bglll, dephosphorylated and ligated to the hepatitis delta virus ribozyme linker (SEQ ID nos. 9 and 10), that had been phosphorylated, to produce pMP55 (Fig. 9B) . Plasmid pMP52 was created by ligating a linker (SEQ ID nos: 7, 8), into the EcoRI site of pUC19 (Fig.10) . The I578bp EcoRI -SFV fragment ws cloned into the EcoRI site of pUC19, to produce pMP46 (Fig.llA) . This plasmid was then restricted with PpuMl and made blunt-ended with Mung Bean nuclease. The rabbit β- globin intron II PCR fragment (Fig.l) was made blunt- ended with Mung Bean nuclease, phosphorylated and ligated to the PpuMI restricted pMP46 to produce plasmid pMP70 (Fig.llB) . The 3627bp EcoRI SFV fragment was cloned into the EcoRI site of pUC19 to produce pMP47 (Fig.llA) . Plasmid pCMV3 , which contains the CMV promoter, Intron A sequence, BGH poly A sequence and SU40 poly A sequence, was restricted with Ndel and EcoRV. The 3191bp Ndel-EcoRV fragment was isolated and dephosphorylated. The 1321bp Ndel -EcoRV fragment was isolated and restricted with Sacl . The Ndel-Sacl fragment of 334bp was isolated (Fig.l2A). The isolated SacI-EcoRV PCR fragment containing the 5 ' -end of SFV was ligated to the previously isolated 334bp Ndel -Sacl fragment and the 3191bp Ndel -EcoRV fragment to produce pMP71 (Fig.l2A and 12B) . Plasmid pMP53 was then restricted with EcoRI and BamHl and ligated to the isolated and dephosphorylated 2151bp EcoRI fragment from pMP70 (Fig.δA) . This ligation was then restricted with EcoRV and the 4057bp EcoRV-EcoRI fragment purified (Fig.8A) . Plasmid pMP47 was restricted with EcoRI and the 3627bp EcoRI fragment isolated and dephosphorylated (Fig.8B) . Plasmid pMP55 was then restricted with Bglll, dephosphorylated and restricted with EcoRI. The 985bp EcoRI-Bglll fragment was isolated and ligated to the previously isolated EcoRI fragment from pMP47 (Fig.δB) . The ligation reaction was then phosphorylated and the 4612bp EcoRI-Bglll fragment isolated. Plasmid pMP71 was restricted with EcoRV and BamHl then dephosphorylated. This fragment was used in a 3- way ligation with the previously isolated 4612bp EcoRI - Bglll fragment from pMP47 and pMP55, and the 4057bp EcoRV-EcoRI fragment from pMP53 and pMP70, to produce pMP76 (Figs.δB and 8C) . The 5 ' end of the SFV replicon was produced by PCR amplification of pSFVl using primers SFV-5'-3 having the sequence 5 ' -ATCTATGAGCTCGTTTAGTGAACCGTATGGCGGATGTGTGACATACA-3 ' and EcoR-SPE having the sequence 5' -TCCACCTCCAAGGATATCCAAGATGAGTGTG-S 1 (SEQ ID no: 9 and SEQ ID no: 10 respectively) between the CMV promoter and the 5' end of the SFV replicon. The resulting PCR fragment was restricted with Sacl and EcoRV (Fig. 13; SEQ ID no: 11) and the fragment isolated. SUMMARY OF DISCLOSURE In summary of this disclosure, the present invention provides a modified alphavirus-based expression vector wherein at least one optimal splice site is introduced to the alphavirus replicon to prevent aberrant splicing of the alphavirus genome; and improve transport of RNA out of the nucleus. Modifications are possible within the scope of the invention. REFERENCES 1. Fulginiti, V.A. , Eller, J.J., Sieber, O.F., Joyner, J.W., Minamitani, M. and Meiklejohn, G. , (1969) Am. J. Epidemiol. 89 (4), 435-448. 2. Chin, J., Magoffin, R.L., Shearer, L.A., Schieble, J.H. and Lennette, E.H. (1969) Am. J. Epidemiol. 89 (4) , 449-463. 3. Jensen, K.E., Peeler, B.E. and Dulworth, W.G. (1962) J. Immunol. 89, 216-226. 4. Murphy, B.R., Prince, G.A. , Collins, P.L., Van Wyke-Coelingh, K. , Olmstead, R.A. , Spriggs, M.K., Parrott, R.H. , Kim, H.-Y., Brandt, CD. and Chanock, R.N. (1988) Vir. Res. 11, 1-15. 5. Chapman, B.S.; Thayer, R.M. ; Vincent, K.A. and Haigwood, N.L., Nucl. Acids. Res. 1991, 19: 3979- 3986. Huang, Zhi-ming and Yen, T. S. Benedict, Molecular and Cell Biology, July 1995, p.3864-3869.

Claims: CLAIMS 1. An expression vector, comprising a DNA molecule complementary to at least part of an alphavirus RNA genome, which DNA molecule comprises the complement of the complete alphavirus RNA genome regions which are essential for replication of the said alphavirus RNA and further comprises a heterologous DNA seuence capable of expression in a host, said heterologous DNA sequence being inserted into a region of the DNA molecule which is non-essential to replication thereof, and the DNA molecule being placed under transcriptional control of a promoter sequence functional in said host, wherein at least one heterologous splice site is provided in the DNA molecule to prevent aberrant RNA splicing of the alphavirus. 2. The vector of claim 1 wherein said promoter is placed upstream of the 5 '-end of the DNA molecule such that the resultant transcript has an authentic 5' -end. 3. The vector of claim 2 wherein said promoter is the cytomegalovirus immediate early promoter. 4. The vector of claim 1 which further comprises an additional DNA sequence at the 3 ' -end of the DNA molecule to direct proper in vivo cleavage at the 3 ' - end of the DNA molecule. 5. The vector of claim 4 wherein said additional DNA sequence comprises a hepatitis delta ribozyme sequence. 6. The vector of claim 1 wherein the heterologous splice site sequence is provided by the DNA sequence of the rabbit β-globin intron II. 7. The vector of claim 6 wherein the heterologous splice site sequence is inserted into the DNA molecule at a location which generates perfect splice junctions and restores the function of the SFV replicon when removed. 8. The vector of claim 1 wherein the alphavirus is a Simliki Forest virus. 9. A cloning vector suitable for expression in a host cell of an heterologous DNA sequence, which comprises: a DNA molecule complementing to at least part of an alphavirus RNA genome, which DNA molecule comprises the complement of the complete alphavirus RNA genome regions and has a cloning site for insertion therein of a heterologous DNA sequence capable of expression in a host cell, said cloning site being located in a region of the DNA molecule which is non-essential to replication thereof; a promoter sequence functional in said host cell and transcriptionally controlling said DNA molecule, said promoter sequence being placed upstream of the 5 ' - end of the DNA molecule such that the resultant transcript had an authentic 5' end; at least one heterologous splice set provided in the complement of the DNA molecule to permit aberrant RNA splicing of one to generate perfect splice junctions in the alphavirus; and an additional DNA sequence at the 3 ' -end of the DNA molecule to direct proper in vivo cleavage at the 3 '-end of the reactant RNA molecule. 10. The cloning vector of claim 9 wherein said heterologous splice set is provided by the DNA sequence of the rabbit β-globin intron II. 11. The cloning vector of claim 9 wherein said additional sequence comprises a hepatitis delta ribozyme sequence . 12. The cloning vector of claim 8 wherein the alphavirus is a Semliki Forest virus. 13. The cloning vector of claim 8 which has the identifying characteristics of plasmid pMP76 shown in Figure 8D. 14. The cloning vector of claim 8 having SEQ ID no: 11.

Inventors: PARRINGTON MARK, PARRINGTON, MARK
Applicants: CONNAUGHT LAB, PARRINGTON MARK, CONNAUGHT LABORATORIES LIMITED, PARRINGTON, MARK
Application year: 1998
Main classification: C12N15
IPC codes: C12N15, C12R1