Chimeric and/or growth-restricted flaviviruses

Abstract: The invention includes a chimeric virus for use in a vaccine preparation having a genome comprising nucleic acid sequences encoding at least one structural protein from one flavivirus and nucleic acid sequences encoding nonstructural protein from another flavivirus. The genome preferably includes mutations within the viral genome that reduce virus virulence and in a particularly preferred embodiment these vaccines are directed to flaviviruses such as dengue virus, tick-borne encephalitis virus and Japanese encephalitis virus.

Description: Field of the Invention The present invention relates, in general, to recombinant viable chimeric flaviviruses , and to vaccines for dengue virus and other flaviviruses, including tick-borne encephalitis virus (TBEV). The invention further relates to cDNA sequences encoding the RNA transcripts to direct the production of recombinant dengue type 4 virus, recombinant mutant dengue type 4 virus chimeric dengue viruses, chimeric dengue viruses incorporating mutations to recombinant DNA fragments generated therefrom, and the cells transformed therewith. Background Information The family Flaviviridae includes approximately 60 enveloped, positive strand RNA viruses, most of which are transmitted by an insect vector. Many members of this family cause significant public health problems in different regions of the world (Monath, T.P. (1986) In: The Togaviridae and Flaviviridae . S. Schlesinger et al., eds. pp. 375-440. Plenum Press, New York). The genome of all flaviviruses sequenced thus far has the same gene order: 5'-C-preM-E-NS1-NS2A-NS2B-NS3-NS4A-NS4B-NS5-3' in which the first three genes code for the structural proteins the capsid (C), the premembrane protein (pre M) and the envelope protein (E). Dengue is a mosquito-borne viral disease which occurs in tropical and sub-tropical regions throughout the world. The dengue virus subgroup causes more human disease than any other member of the flavivirus family. Dengue is characterized by fever, rash, severe headache and joint pain. Its mortality rate is low. However, over the past few decades, a more severe form of dengue, characterized by hemorrhage and shock (dengue hemorrhagic fever/dengue shock syndrome; DHF/DSS) has been observed with increasing frequency in children and young adults. DHF/DSS occurs most often during dengue virus infection in individuals previously infected with another dengue virus serotype. This has led to the suggestion that immune enhancement of viral replication plays a role in the pathogenesis of the more severe form of disease (Halstead, S.B. (1988) Science 239, 476-481). Dengue epidemics are a major public health problem in many tropical and subtropical areas where the vector mosquito species are abundant. Despite 40 years of intensive research, safe and effective vaccines for dengue virus disease are not available. The WHO has assigned dengue virus as a high priority target for accelerated research and vaccine development. Soon after their isolation in 1944, dengue viruses were passaged repeatedly in mouse brain, resulting in the selection of mouse neurovirulent mutants (Sabin, A.B. (1952) Amer . J. Trop. Med. Hyg. 1:30-50). Interestingly, studies performed in volunteers showed that mouse brain-adapted neurovirulent mutants of three strains of type 1 or type 2 dengue virus were attenuated, but still immunogenic for humans (Sabin, A.B. (1952) Amer. J. Trop. Med . Hyg. 1:30-50; Sabin, A.B. (1955) Amer. J. Trop. Med . Hyg. 4:198-207; Sabin, A.B. (1955) Amer. J. Trop. Med . Hyg. 4:198-207; Schlesinger, R.W. et al. (1956) J. Immunol. 77:352-364; Wisseman, C.L. et al. (1963) Amer. J. Trop. Med . 12:620-623). However, the mutants were not developed further as candidate vaccine strains because of concern for mouse brain antigens in the vaccine preparations. Since that time, virus mutants that: (i) exhibited the small plaque size phenotype, and/or (ii) were temperature sensitive, and/or (iii) were adapted to cell cultures derived from an unnatural host (i.e., host range mutants), have been selected and evaluated as candidates for inclusion in a live attenuated virus vaccine (Harrison, V.R. et al. (1977) Infec. Immun. 18:151-156; Hoke, C.H. et al. (1990) Am . J. Trop. Med . Hyg. 43:219-226; Bhamarapravati, N. et al. (1987) Bull. WHO. 65:189-195). However, despite 25 years of such efforts, safe, effective dengue vaccines are still not available for general use. Inactivated whole dengue virus vaccines have been shown to be insufficiently immunogenic. Live virus vaccines attenuated by serial passage in cell culture have suffered from genetic instability under attenuation or poor immunogenicity. The present invention represents a technical breakthrough by providing chimeric dengue and flavivirus vaccines. These four serotypes of dengue viruses (type 1 to type 4) are distinguishable by plaque reduction neutralization using serotype-specific monoclonal antibodies and by less specific tests using polyclonal sera (Bankcroft, W.M. et al. (1979) Pan Am. Hlth. Org. Sci. Publ. 375:175-178; Henchal, E.A. et al. (1982) Am . J. Trop. Med . Hyg. 31:548-555). The existence of serotypes was first discovered during early studies in human volunteers, which showed that infection with one dengue serotype induced durable homotypic immunity, whereas heterotypic immunity lasted only 3 to 5 months (Sabin, A.B. (1952) Amer. J. Trop. Med . Hyg. 1:30-50). An effective dengue vaccine that contains all four serotypes in order to induce broad immunity to dengue viruses in general would help to preclude the occurrence of DHF/DSS. The complete nucleotide sequences have been determined for dengue virus types 3 and 4 and several strains of type 2 virus including the mouse-neurovirulent New Guinea C, however, only the 5' portion of the type 1 virus genome has been sequenced (Mackow, E. et al. (1987) Virology 159:217-228; Zhao, B. et al. (1986) Virology 155:77-88; Osatomi, K. & Sumiyoshi, H. (1990) Virology 176:643-647; Irie, A. et al. (1989) Gene 75:197-211; Mason, P.W. et al. (1987) Virology 161:262-267; Hahn, Y.S. et al. (1988) Virology 162:167-180). The results of these studies indicate that the four dengue virus serotypes share a common genome organization. The genome of the dengue type 4 Caribbean strain 814669 was found to contain 10646 nucleotides (Mackow, E. et al. (1987) Virology 159:217-228; Zhao, B. et al. (1986) Virology 155:77-88). The first 101 nucleotides at the 5' end and the last 384 at the 3' end are non-coding. The remaining sequence codes for a 3386 amino-acid polyprotein which includes the three structural proteins, namely, capsid (C), premembrane (pre-M), and envelope (E), at its N-terminus, followed by seven non-structural proteins in the order, provided above, that is consistent with all Flavivirus genomes identified thus far. The polyprotein is processed to generate 11 or more viral proteins by cell signal peptidase(s) and by viral proteases (Markoff, L. (1989) J. Virol, 63:3345-3352; Falgout, B. et al. (1989) J . Virol, 63:1852-1860; Falgout, B. et al. (1991) J . Virol. 65:2467-2476; Hori, H. & Lai, CJ. (1990) J. Virol . 64:4573-4577). Previously we constructed a full-length dengue virus cDNA that could serve as the template for transcription of infectious RNA. We have obtained stably cloned full-length dengue virus cDNA and in vitro RNA transcripts derived from the DNA template were shown to be infectious for cells in culture. However, this infectious construct and infectious RNA transcripts generated therefrom are pathogenic. Moreover, the attenuated dengue viruses generated thus far are genetically unstable and have the potential to revert back to a pathogenic form overtime. Yet, attenuated viruses are desirable since they are generally known to provided long-lasting immunity. Therefore, modifications to this construct or to chimeric constructs that then direct the production of a less pathogenic virus would be a considerable advance to attenuated flavivirus vaccine technology. Accordingly, we have constructed a series of deletions in the 3' non-coding region of cDNA, as disclosed herein, and have recovered viable dengue virus mutants for analysis of growth characteristics. Other members of the Flavivirus family are also pathogenic. Examples include tick-borne encephalitis virus and Japanese Encephalitis Virus. Like attenuated dengue virus vaccines, attenuated tick-borne encephalitis virus (TBEV) virus has tended to be genetically unstable and poorly immunogenic. Therefore, other attenuated flavivirus vaccines would also be a considerable advance in the art. Thus, this invention additionally employs modified full-length recombinant cDNA constructs of dengue virus or another flavivirus as a framework for gene manipulation and chimeric virus development for the production of vaccines to other Flaviviruses. Tick-borne encephalitis virus (TBEV) is transmitted exclusively by ticks and can be divided into two serologically distinguishable subtypes: the Eastern subtype (prototype strain Sofjin), prevalent in Siberian and Far Eastern regions of Russia, and the Western subtype (prototype strain Neudorfl), common in eastern and central Europe. TBEV causes a serious encephalitic illness with a mortality rate ranging from 1 to 30%. For a review of TBEV see Calisher, et al. ( J. Gen. Virol. 70:37-43). Currently, an experimental TBE vaccine produced by formalin inactivation of TBEV is available, but this vaccine has several limitations. For example, the vaccine is not sufficiently immunogenic, therefore repeated vaccinations are required to generate a protective immune response. Even when antibody responses to the vaccine are present, the vaccine fails to provide protective responses to the virus in 20% of the population. Therefore, there remains a need for an improved TBEV vaccine. Summary of the Invention In one aspect, the present invention provides a recombinant DNA construct containing nucleic acid derived from at least two flaviviruses. The construct includes a region of nucleic acid operably encoding no more than two structural proteins from tick-borne encephalitis virus (TBEV). This region is operably linked to a region of nucleic acid encoding structural proteins from a flavivirus other than TBEV. Preferably, the construct includes a region of nucleic acid operably encoding flavivirus capsid protein operably linked to the region of nucleic acid operably encoding no more than two structural proteins from tick-borne encephalitis virus. The regions of nucleic acid encoding capsid protein and non-structural proteins are preferably from dengue virus, such as dengue type 4 virus. In a preferred form, the construct contains at least one mutation in the nucleic acid derived from at least two flaviviruses. In one embodiment of this preferred form, the mutation ablates NS1(1) protein glycosylation. In another embodiment of this form, the mutation prevents production of the mature flavivirus membrane protein. The mutation preferably affects viral growth rate. The invention also includes RNA transcripts corresponding to these recombinant DNA constructs are also included within the present invetnion. In another aspect of the present invention, there is provided a chimeric virus having a genome derived from a flavivirus. This chimeric virus include a region of nucleic acid encoding no more than two structural proteins from tick-borne encephalitis virus, anda region of nucleic acid encoding nonstructural proteins from another flavivirus. Preferably, the virus includes a region of nucleic acid encoding capsid protein from the other flavivirus The nucleic acid from tick-borne encephalitis virus can encode any of a number of proteins, such as pre-membrane protein and/or the envelope protein. The nucleic acid encoding structural proteins from another flavivirus is preferably from dengue virus, such as type 4 dengue virus. In one embodiment, the chimeric virus of this aspect of the invention contains at least one mutation in the nucleic acid. This mutation can take any of a number of forms, such as a mutation that ablates NS1(1) protein glycosylation, or one that prevents production of the mature flavivirus membrane protein. The mutation preferably affects viral growth rate. Another aspect of the present invention provides a chimeric virus that includes region of nucleic acid encoding the premembrane and envelope proteins of tick-borne encephalitis virus, and a region of nucleic acid encoding the capsid protein from another flavivirus, and a region of nucleic acid encoding non-structural proteins from the same flavivirus. The flavivirus can be, for example, a dengue virus, such as dengue virus type 4. In this aspect of the invention, a preferred form of the chimeric virus includes at least one mutation in the region of nucleic acid encoding premembrane and envelope protein of tick-borne encephalitis virus. The mutation can be one that prevents production of the mature flavivirus membrane protein. In another preferred form, the chimeric virus includes at least one mutation in the regions of nucleic acid from the flavivirus. This mutation can be one that ablates NS1(1) protein glycosylation. In another aspect, the invention provides a vaccine for humans against tick-borne encephalitis comprising a chimeric virus of the present invention that is capable of generating a protective immune response in a vertebrate. Thus, the invention also includes a method for preparing a vaccine for humans against a flavivirus comprising. This method includes preparing a DNA construct operably encoding the premembrane and envelope protein from the flavivirus and the capsid and nonstructural protein from dengue virus, generating infectious RNA transcripts from the DNA construct, introducing the RNA transcripts into a cell, expressing the RNA transcripts in the cell, harvesting the virus from the cells, testing the virus in a vertebrate, and inoculating the humans with the virus. The preparing step can include introducing mutations into the DNA construct. A preferred aspect of the present invention provides a chimeric virus that includes region of nucleic acid encoding the premembrane and envelope proteins of Japanese encephalitis virus, and a region of nucleic acid encoding the capsid protein from another flavivirus, and a region of nucleic acid encoding non-structural proteins from the same flavivirus, such as dengue virus. Another aspect of the present invention provides a chimeric virus having an RNA genome. This genome includes a region of nucleic acid operatively encoding non-structural protein of type 4 dengue virus and a region of nucleic acid operatively encoding structural protein of a type 1 dengue virus, type 2 dengue virus, type 3 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, or another flavivirus. In one embodiment of the invention, the genome of the chimeric virus is substantially free from nucleic acid operatively encoding type 4 dengue virus structural protein. Preferred embodiments of the chimeric virus can include p2A(D1 WP) or p2A(D2 NGC) RNA. Preferably, the chimeric virus includes a region of nucleic acid operatively encoding non-structural protein of type 1 dengue virus, type 2 dengue virus, or type 3 dengue virus, and also includes a region of nucleic acid operatively encoding structural protein of type 1 dengue virus, type 2 dengue virus, type 3 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, or another flavivirus. The nucleic acid operatively encoding non-structural protein is preferably from a different virus than the region of nucleic acid operatively encoding non-structural protein. Another aspect of the invention provides a chimeric virus having an RNA genome. This genome includes a region of nucleic acid operatively encoding non-structural protein of yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus or another flavivirus, and a region of nucleic acid operatively encoding structural protein of type 1 dengue virus, type 2 dengue virus, type 3 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, or another flavivirus. The region of nucleic acid operatively encoding structural protein is preferably from a different virus than the region of nucleic acid operatively encoding non-structural protein. Yet another aspect of the invention provides a vaccine, capable of generating a protective immune response to a virus in a vertebrate. This vaccine includes a safe and immunologically effective amount of any of the viruses of the present invention and a pharmaceutically and immunologically acceptable carrier. Still another aspect of the invention provides another vaccine that is capable of generating a protective immune response to a virus in a vertebrate. In this aspect of the invention, the vaccine includes a safe and immunologically effective amount of the following viruses: a) a chimeric virus wherein the genome of the virus includes a region of nucleic acid encoding non-structural protein of type 4 dengue virus and a region of nucleic acid encoding structural protein of type 1 dengue virus, b) a chimeric virus, wherein the genome of the virus includes a region of nucleic acid encoding non-structural protein of type 4 dengue virus and a region of nucleic acid encoding structural protein of type 2 dengue virus, c) a chimeric virus, wherein the genome of the virus includes a region of nucleic acid encoding non-structural protein of type 4 dengue virus and a region of nucleic acid encoding structural protein of type 3 dengue virus, and d) an attenuated type 4 dengue virus. A further aspect of the present invention provides a DNA segment that includes a non-structural region of type 4 dengue virus, and a structural region from one of the following viruses: type 1 dengue virus, type 2 dengue virus, type 3 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and another flavivirus. In a preferred embodiment of the invention, the DNA segment contains a promoter operably linked to the structural and nonstructural regions. In another preferred embodiment, the DNA segment contains a promoter which is a SP6 or T7 promoter. In yet another preferred embodiment, the DNA segment is p2A(D1 WP) or p2A(D2 NGC). Other aspects of the invention provide other chimeric DNA segments. The DNA segments of these aspects of the invention include a non-structural region of type 1 dengue virus, type 2 dengue virus or type 3 dengue virus. These DNA segments further include a structural region from one of the following viruses: type 1 dengue virus, type 2 dengue virus, type 3 dengue virus type, 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and a flavivirus, with the proviso that the structural region and the non-structural region are not from the same virus. A further aspect of the invention provides a DNA segment that includes a non-structural region from one of the following viruses: yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and another flavivirus, and a structural region from one of the following viruses: type 1 dengue virus, type 2 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and another flavivirus wherein the structural region is from a different virus than the non-structural region. Another aspect of the invention provides a DNA segment comprising a non-structural region or a portion thereof of a flavivirus, and a structural region or a portion thereof from another flavivirus. Still another aspect of the invention provides a vaccine, capable of generating a protective immune response to a virus in a vertebrate. This vaccine includes a safe and immunologically effective amount of a chimeric virus. The genome of the virus includes nucleic acid operably encoding the structural proteins from a tick-borne encephalitis virus and nucleic acid operably encoding the non-structural protein from a dengue virus. Yet another aspect of the invention provides another chimeric virus. In this aspect, the chimeric virus has a genome of the virus operably encodes tick-borne encephalitis structural protein and dengue type 4 virus non-structural protein. In a preferred embodiment, the structural proteins of the virus are derived from tick-borne encephalitis virus. Still another aspect of the invention provides a segment of DNA operably encoding tick-borne encephalitis structural protein and dengue type 4 virus non-structural protein. A further aspect of the invention provides an isolated DNA fragment that encodes infectious dengue type 4 viral RNA. Still another aspect of the invention provides a recombinant DNA construct comprising the DNA fragment of the present invention, and a vector. In a preferred embodiment, the vector is a plasmid. A further aspect of the invention includes a host cell stably transformed with a recombinant DNA construct according to the present invention in a manner allowing expression of the DNA fragment. In a preferred embodiment, the host cell is a prokaryotic cell. Another aspect of the invention provides a method for producing mutants of dengue type 4 virus. This method includes the steps of (i) introducing mutations into the genome of dengue type 4 virus by site-directed mutagenesis,(ii) recovering infectious dengue type 4 viruses harboring the mutations, and (iii) evaluating recovered viruses. In one embodiment of this method, the recovered viruses are evaluated for attenuated or avirulent phenotype. Still another aspect of the invention provides a vaccine for humans against dengue type 4 virus. This vaccine includes an avirulent dengue type 4 virus, in an amount sufficient to induce immunization against the disease, and a pharmaceutically acceptable carrier. In a preferred embodiment, the avirulent dengue type 4 virus is obtained by engineering mutations at strategic regions in the viral genome. Yet another aspect of the present invention provides a DNA fragment that encodes chimeric flaviviral RNA. The chimeric RNA includes, for example, a member selected from the group consisting of dengue type 1, dengue type 2 and dengue type 3. Still another aspect of the invention provides a method for construction of chimeric dengue viruses. This method includes replacing DNA fragments of the dengue type 4 virus DNA according to the method of the present invention with corresponding genes en bloc, or a fraction thereof, from a flavivirus. In a preferred embodiment, the flavivirus is one of the following viruses: dengue type 1, dengue type 2 and dengue type 3. In another preferred embodiment, the dengue type 4 virus contains at least one mutation. The present invention includes another aspect which provides a vaccine for humans against dengue virus. In this aspect, the invention includes an infectious chimeric virus, in an amount sufficient to induce immunization against the disease, and a pharmaceutically acceptable carrier. The chimeric virus is, for example, dengue type 1, dengue type 2, dengue type 3 or dengue type 4 virus. Yet another aspect of the invention provides a recombinant DNA construct comprising the DNA fragment of the present invention, and a vector. In one preferred embodiment, the vector is a plasmid. Still another aspect of the present invention provides a host cell stably transformed with the recombinant DNA construct according to the method of the present invention, in a manner allowing expression of the DNA. In a preferred embodiment, the host cell is a prokaryotic cell. Still another aspect of the invention provides a DNA fragment that encodes a dengue type 4 viral RNA, wherein the DNA fragment contains a substitution mutation in the sequence encoding one or more of eight amino acids at the C terminus of NS1 of the cleavage site of the non-structural protein NS1-NS2A. A further aspect of the invention provides a DNA fragment that encodes a dengue type 4 viral RNA, wherein the DNA fragment contains a substitution at the site encoding glycine, which site is at position + 1 following the cleavage site of the non-structural protein NS1-NS2A. A still further aspect of the invention provides a DNA fragment that encodes a dengue type 4 viral RNA, wherein the DNA fragment contains a deletion in the 3'-noncoding region. In a preferred embodiment, the deletion is between 30 and 202 nucleotides in length. Another aspect of the invention provides a recombinant DNA construct including any of the DNA fragments of the present invention and a vector. Still another aspect of the invention provides an infectious RNA transcript of a DNA fragment of the present invention. A further aspect of the invention provides a host cell transfected with the DNA constructs of the present invention in a manner allowing expression of the DNA fragment. Such a host cell can be a mammalian or an insect cell. Still another aspect of the invention provides a vaccine for humans against dengue type 4 virus. In this aspect of the invention, the vaccine includes a mutant dengue type 4 virus exhibiting reduced virulence, in an amount sufficient to induce immunization against the disease. In a preferred embodiment, the mutant dengue type 4 virus is obtained by engineering deletion mutation in the 3'-noncoding region in the viral genome. In another preferred embodiment, the mutant dengue type 4 virus is obtained by engineering substitution mutation in the viral DNA sequence encoding one or more of eight amino acids at the C terminus of NS1 of the cleavage site of the non-structural protein NS1-NS2A. In yet another preferred embodiment, the mutant dengue type 4 virus is obtained by engineering substitution at the viral DNA site encoding glycine, which site is at position + 1 following the cleavage site of the non-structural protein NS1-NS2A. Still another aspect of the invention provides a method for construction of chimeric dengue viruses comprising replacing DNA fragments of the dengue type 4 virus DNA with corresponding genes en bloc, or a fraction thereof, from a flavivirus. In a preferred embodiment, the flavivirus is one of: dengue type 1, dengue type 2, dengue type 3, Japanese encephalitis and tick-borne encephalitis virus. In another preferred embodiment, the method for construction of chimeric dengue viruses includes replacing DNA fragments of the dengue type 4 virus DNA according to the present invention, with corresponding genes en bloc, or a fraction thereof, from a flavivirus. Preferably, the flavivirus is selected from the group consisting of dengue type 1, dengue type 2, dengue type 3 and Japanese encephalitis. Also preferably, the method for construction of chimeric dengue viruses includes replacing DNA fragments of the dengue type 4 virus DNA according to the present invention, with corresponding genes en bloc, or a fraction thereof, from a flavivirus. In the preferred method, the flavivirus is selected from the group consisting of dengue type 1, dengue type 2, dengue type 3, Japanese encephalitis and tick-borne encephalitis virus. One additional aspect of the invention provides a vaccine against dengue virus that is administered in an amount sufficient to induce immunization against the disease. The vaccine includes a chimeric virus exhibiting reduced virulence, wherein the chimeric virus contains a DNA fragment that encodes a dengue type 4 viral RNA containing a deletion in the 3'-non-encoding region. In a preferred embodiment, the chimeric virus is selected from the group consisting of dengue type 2, dengue type 3 and dengue type 4 virus. Further aspects of the present invention will become apparent to those of ordinary skill in the art upon reference to the ensuing description of the invention. Description of the Figures Figure 1. Structure of full-length dengue virus cDNAs used for preparation of chimeric dengue viruses. Figure 2. Percentage of virus infected cells and titer of virus following transfection with RNA transcripts. Figure 3. Serotype analysis of chimeric dengue viruses. Figure 4. Polyacrylamide gel analysis of dengue type 1, type 2, or type 4 viral proteins produced by parental or chimeric dengue viruses. Figure 5. Morphology of viral plaques on LLC-MK 2 cells. Figure 6. Mouse neurovirulence of parental dengue type 2 NGC or dengue type 4 virus or type 2/type 4 chimeric virus. Figure 7 provides the terminal sequences of cloned full-length dengue virus cDNA used for transcription of infectious RNA The full-length dengue virus cDNA totaling 10646 nucleotides in length was cloned adjacent to the promoter sequence for SP6 polymerase and the unique Asp 718 cleavage site as diagrammed. Note that there is an additional C residue at the position between 10448 and 10449 and an additional G residue between 10470 and 10471, which were missing in the original dengue type 4 virus sequence. The sequences near the predicted 5'-terminus and 3'-terminus of the RNA transcript are also shown. Transcription initiating G was placed preceding the 3'-dengue sequence as shown in bold letters. The Asp 718 cleavage sequence GGTACC was positioned immediately following the 3'-dengue sequence as shown in bold letters. Figure 8 diagrams the full-length dengue cDNA clones and the restriction endonuclease digestion pattern. The diagram depicts the full-length dengue cDNA clone containing the SP6 promoter sequence at the 5'-end and an Asp 718 cleavage site at the 3'-end. The Bst B1 cleavage site at nucleotide 5069 separates the dengue genome into the 5'- and 3'-fragments. Replacement of these fragments in the full-length clone 1A with the corresponding 5'-2, 3-B, on 3'-C fragments gave rise to other full-length combinations as shown. Figure 9 provides the restriction endonuclease pattern of the full length dengue cDNA clone digested with Bgl II, Nsi I, and Asp 718 and the digests were analyzed by separation on an agarose gel. M shows λ Hind III DNA fragments as molecular size markers in kilobase pairs. Figure 10 provides a demonstration that dengue virus recovered from infections RNA contains the genomic sequence of the template DNA. Progeny dengue virus was recovered from LLC-MK 2 , cells transfected with RNA transcribed from clone 2A DNA (wild type control), or from clone 2A (P) DNA containing a Pst I site at nucleotide 3473 of the dengue sequence. Genomic RNA extracted from the recovered virus was used for reverse transcription and the cDNA product was used as the template to produce the 1343 bp DNA fragment (nucleotides 3193-4536) by PCR using the appropriate primers. Pst I digestion of the PCR product is shown. Lane M shows the 123 DNA ladders as size markers. Figure 11 provides a comparison of dengue virus proteins from recovered virus and parental virus infected cells. 35 S-methionine labelled lysates from LLC-MK 2 , cells infected with recovered dengue virus or parental wild type were prepared for immunoprecipitation with dengue hyperimmune mouse ascetic fluid. The immunoprecipitates were separated by electrophoresis on SDS-12% polyacrylamide gel. Gel lanes shown contain lysates from cells infected with: (1) parental dengue virus; (2) dengue virus recovered from clone 2A DNA; (3) dengue virus recovered from clone 2A (P) DNA; lane 4 is from mock infected cell lysate; and lane M contains protein size markers in kilo daltons shown on the left. Labelled bands that correspond to dengue virus proteins including PreM, E, NS, and NS3 are indicated. The identities of other labelled bands have not been assigned. Figure 12 provides the plaque morphology of wild-type dengue type 4 virus and its derived amino acid substitution mutant D4 (G 1126 →E) Confluent mosquito C6/36 cells in a 76-cm 2 flask were infected with dengue virus and an agarose overlay was subsequently added. Six days after infection, cells were strained with neutral red. The plaque size was measured the next day. The average plaque size of wild-type dengue 4 virus and the substitution mutant was calculated from 10 individual plaques. The mutant virus shown contained Glu (E) substituting for Gly (G) at position 1126 of the dengue type 4 polypeptide sequence. This position corresponds to position + 1 at the NS1-NS2A cleavage sequence. Figure 13 provides the genome structure and properties of dengue type 4 virus mutants containing deletions in the 3' noncoding sequence. The linear map shown depicts the 384-nucleotide 3' noncoding sequence of dengue type 4 virus. In this region at least three sequence elements are shown: the conserved sequence 2 (CS-2) at nt 234 to nt 211 and at nt 143 to nt 120 as indicated by the filled boxes. The conserved sequence 1 (CS-1) at nt 105 to nt 82 as indicated by the open box, and the stem-and-loop structure within the last 84 nucleotides. Seven cDNA clones, each containing a deletion mutation ranging from 30 to 202 nucleotides in length at positions specified, were constructed and the derived RNA transcripts used for transfection of permissive LLC-MK 2 cells. Mutant D4 (Δ3', 172-83) cDNA which lacks the entire CS-1 sequence was apparently lethal since a viable virus was not detected after repeated attempts. Viable deletion mutants were recovered from the remaining six RNA transcripts. The growth properties of wild type virus and its derived deletion mutants were compared by plaque assay on LLC-MK 2 , cell monolayers stained six days after infection. Under the same conditions, wild type virus showed plaques of approximately 0.3 cm in size. The reverse numbering system was used to assign nucleotide positions of dengue type 4 virus. The last nucleotide at 10646 is assigned nt 1 using this numbering system. Figure 14 illustrates the plaque morphology of wild type dengue 4 virus and the derived mutants containing deletions in the 3' noncoding region. Plaque assay were performed on mosquito C6/36 cells. Confluent mosquito C6/36 cells in a 75-cm 2 flask were infected with wild type dengue type 4 virus, or viable deletion mutant viruses at an appropriate multiplicity. As described in the examples, infected cells were stained with neutral red six days after infection. The plaque size was measured the next day. The average plaque size of 10 individual plaques was calculated for each mutant. Panel A shows uninfected cell monolayer (Mock), dengue type 4 virus deletion mutants, (Δ3', 172-113), (Δ3', 172-143), and the parental wild type virus. Figure 15 shows wild type virus and several deletion mutants including (Δ3', 243-183), (Δ3' 303-183). and (Δ3' 384-183). The reverse numbering system was used to assign the deletion positions of the dengue type 4 sequence. Figure 16 provides a table of NS1-NS2A junctions. Figure 17 provides the intergenic junctions in chimeric TBEV/DEN4 constructs. Restriction enzyme cleaved TBEV cDNA fragments were inserted into DEN4 cDNA at appropriate sites as indicated by the underlined sequence. The amino acids and the encoding nucleotide sequences of TBEV are in bold letters. The nucleotide numbering system is that disclosed by Pletnev et al., Virology 174:250-263 (1990). Figure 18 is a photograph of polyacrylamide gel separation of viral proteins produced by parental DEN4 and chimeric TBE(ME)/DEN4 viruses. [ 35 S]Methionine-labeled lysates of vTBE(ME)/DEN4 or DEN4 virus-infected, or uninfected simian cells were immunoprecipitated using TBEV HMAF (1), or DEN4 HMAF (2), or rabbit serum specific to NS3 (3), or NS5 (4), or preM (5), or E (6) of DEN4 and analyzed on an SDS-12% polyacrylamide gel. The molecular weights of protein markers are in kilodaltons. The locations of DEN4 proteins are indicated along the right margin. Figure 19 provides the growth curves for vTBE(ME)/DEN4 and DEN4 on LLC-MK 2 and C6/36 cells. Cells were harvested at indicated times (days after infection) at an MOI of 0.01 pfu/cell, and the virus titer was determined by a plaque assay. Figure 20 is a copy of a photograph of a nitrocellulose filter blotted with samples of RNA isolated from infected tissue culture cells. The filters were probed with [ 32 P]-labeled pTBE(ME)/DEN4 nick-translated DNA. Figure 21 is a photograph of polyacrylamide gel separation of cell lysate protein from vTBE(ME)/DEN4 and DEN4-Infected LLC-MK 2 or C6/36 cells at various times after infection. Figure 22 provides the results of a study to determine the protective efficacy and neurovirulence of vTBE(ME)/DEN4 in mice. Figure 23 provides a listing of several exemplary mutations incorporated into the TBE(ME)/DEN4 construct to assess the effect of the mutations on neurovirulence. Figure 24 summarizes the results of the neurovirulence studies using recovered TBE(ME)/DEN4 chimeras from constructs containing mutations provided in Figure 23. Detailed Description of the Invention GENERATION OF CHIMERIC FLAVIVIRUSES Dengue virus contains an approximately 11-kilobase positive strand RNA genome, which codes in one open reading frame for three structural proteins (capsid (C), premembrane (preM) and envelope (E)), followed by seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5). The present invention relates to a chimeric dengue virus containing non-structural proteins from one "type" of dengue viruses or a flavivirus and structural proteins from a different "type" of dengue viruses or another flavivirus. In one embodiment, the present invention relates to a chimeric virus comprising: 1) a non-structural region of type 1, 2, 3 or 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, or a flavivirus and 2) a structural region selected from type 1 dengue virus, type 2 dengue virus, type 3 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus or a flavivirus wherein the structural region is from a different "type" dengue virus or flavivirus than the non-structural region. In another embodiment, the present invention relates to a chimeric virus comprising: 1) a non-structural region of type 4 dengue virus (DEN4), and 2) a structural region selected from type 1 dengue virus, type 2 dengue virus, type 3 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus and a flavivirus. In one preferred embodiment, the virus is substantially free of type 4 dengue virus structural region. In a preferred embodiment, the virus comprises p2A(D1 WP) or p2A(D2 NGC) RNA. In a further embodiment, the present invention relates to a chimeric virus comprising: 1) a non-structural region of type 1 dengue virus, and 2) a structural region selected from type 2 dengue virus, type 3 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus and a flavivirus. In a further embodiment, the present invention relates to a chimeric virus comprising: 1) a non-structural region of type 2 dengue virus, and 2) a structural region selected from type 1 dengue virus, type 3 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus and a flavivirus. In another embodiment, the present invention relates to a chimeric virus comprising: 1) a non-structural region of type 3 dengue virus, and 2) a structural region selected from type 1 dengue virus, type 2 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus and a flavivirus. In another embodiment, the present invention relates to a chimeric virus comprising: 1) a non-structural region selected from yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and a flavivirus and 2) a structural region selected from type 1 dengue virus, type 2 dengue virus, type 4 dengue virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and a flavivirus wherein said structural region is from a different virus than the non-structural region. In a further embodiment, the present invention relates to a vaccine comprising at least one of the above-described chimeric viruses and a pharmaceutically and immunologically acceptable carrier. The vaccine includes virus in a quantity selected depending on the route of administration. Although subcutaneous routes of administration are preferred, the above-described vaccine could also be administered by an intradermal route. One skilled in the art will appreciate that the amounts to be administered for any particular treatment protocol can be readily determined. In yet another embodiment, the present invention relates to a vaccine comprising: 1) a chimeric virus comprising a non-structural region of type 4 dengue virus and a structural region of type 1 dengue; 2) a chimeric virus comprising a non-structural region of type 4 dengue virus and a structural region of type dengue; 3) a chimeric virus comprising a non-structural region of type 4 dengue virus and a structural region of type 3 dengue; and 4) an attenuated type 4 dengue virus. In one preferred embodiment, the attenuated virus contains a mutation (point mutation, deletion, addition, or a combination of the above) in a non-structural protein region (for example, an altered NS1-NS2 cleavage site sequence). In another preferred embodiment, the attenuated virus contains a mutation (point mutation, deletion, addition, or a combination of above) in the 3' non-coding region. In a further preferred embodiment, the attenuated virus contains a mutation (point mutation, deletion, addition, or a combination of above) in the 5' non-coding region. In another preferred embodiment, the attenuated virus is naturally derived or laboratory engineered. In another embodiment, the present invention relates to a DNA segment which encodes at least one of the above-described viruses. In one preferred embodiment, the DNA segment contains a promoter (preferably, a eukaryotic, procaryotic, or viral promoter; more preferably, an SP6 or T7 promoter) operably linked to the structural and nonstructural regions. In another preferred embodiment the DNA segment comprises p2A(D1 WP) or p2A(D2 NGC). Thus, using the methods provided in the examples below, it is contemplated that one with skill in the art can generate chimeric dengue virus vaccines comprising intertypic recombinants that combine structural regions of one member of the flavivirus family with nonstructural regions from another flavivirus family. In a preferred embodiment, chimeric dengue viruses are produced that incorporate the nonstructural proteins of dengue type 4 (DEN4) with the structural proteins of dengue type 1. For a further review of these methods see Bray, et al.( Proc. Natl. Acad Sci. 88: 1042-1046, 1991. Example 17, below, discloses the production of a chimeric virus that incorporates the nonstructural regions of dengue type 4 virus with the structural regions of tick-borne encephalitis virus (TBEV(CME)/DEN4) following the methods provided for dengue chimeric virus production in Example 1. Virus produced from these methods replicated in culture and was used to infect tissue culture cells or laboratory animals. MUTATIONS IN THE DENGUE GENOME AFFECTING NEUROVIRULENCE We have also engineered a series of viable dengue virus mutants containing deletions in the 3' noncoding region which exhibit altered growth properties and plaque morphology in cell culture. Similarly, we have engineered another series of growth-restricted dengue virus mutants that contain amino acid substitutions in a novel cleavage site sequence in the non-structural protein region of the viral polyprotein. Dengue virus mutants with reduced replicative capacity are evaluated for reduced virulence in experimental animals to determine their suitability for use in a live virus vaccine. In the present invention, the construction of growth-restricted dengue type 4 virus mutants containing deletions in the 3'-noncoding region was effected by using a full-length dengue type 4 virus cDNA clone for engineering deletions into the strategic regions. Deletion mutants can offer the benefit of being less subject to reversion of phenotype than amino acid substitution mutants. Like other mosquito-borne flaviviruses, dengue type 4 virus has a 3'-noncoding sequence that contains conserved regions, designated conserved sequence 1 (CS-1) and conserved sequence 2 (CS-2), and a potential terminal stem-and-loop structure. Deletions ranging from 3-202 nucleotides can be introduced into various locations in the 384 nucleotide 3'-noncoding sequence of dengue type 4 cDNA. RNA transcripts prepared from a cDNA clone lacking sequence of the CS-1 region, but retaining the stem-and-loop structure that do not produce progeny virus indicate that the deletion is lethal. On the other hand, infectious viruses can be recovered from most other 3' non-coding region deletion constructs. Plaque assay of these mutants and the wild-type virus can be performed on mosquito cells (C6/36). The results of these analyses show that most of the deletion mutants form plaques that are reduced in size, ranging from 1.0 to 0.3cm, dependent on the location and the extent of deletion. This form of altered plaque morphology indicates that these deletion mutants exhibit a growth-restriction phenotype. This panel of dengue virus mutants containing deletions in the 3' non-coding region can be evaluated for their infectivity and immunogenicity in experimental animals. Mutants which exhibit reduced virulence for experimental primates can be selected as candidate vaccine viruses for evaluation in humans. The present invention also relates to a recombinant DNA construct comprising the DNA fragment containing a deletion in the 3' noncoding region, and a vector. The invention also relates to host cells transfected with the DNA construct. The invention also relates to a method of producing mutants of dengue type 4 virus, comprising the steps of introducing deletions mutations into the 3' noncoding region of the viral genome, and recovering infectious viruses harboring the deletion mutations. In another embodiment, the present invention relates to a DNA fragment that encodes a dengue type 4 viral RNA, where the fragment contains a substitution in the sequence encoding one or more of eight amino acids at the C terminus of NS1 of the cleavage site of the non-structural protein NS1-NS2A. A summary of the substitutions is provided in Figure 16. The present invention also relates to a DNA fragment that encodes a dengue type 4 viral RNA, where the fragment contains a substitution at the position coding for glycine, that is, the + 1 position following the cleavage site of the non-structural protein NS1-NS2A. The invention further relates to an infectious RNA transcript of the DNA fragments described herein. The construction of growth-restricted dengue type 4 virus mutants containing mutations in the nonstructural proteins can be effected by using the full-length dengue type 4 virus cDNA clone to engineer such strategic mutations. Mutations affecting the functional activity of dengue virus nonstructural proteins can cause growth restriction. For example, modification of a cleavage sequence or a viral protease moiety can result in suboptimal cleavage of the viral polyprotein, thus restricting viral growth. We have selected the polyprotein NS1-NS2A cleavage site as a target for constructing dengue virus mutants that are growth restricted because of inefficient cleavage. We have shown that dengue type 4 virus NS1-NS2A cleavage requires an 8 amino acid domain at the C terminus of NS1. We have further demonstrated the effect of substitutions at each of these positions and Gly at position + 1 following the cleavage site. A panel of mutations of amino acid substitution in this domain have been evaluated for their effect on cleavage (see Figure 16). Mutations that produced a wide range of effects on cleavage were identified and a number of such mutations were selected for incorporation into the full-length dengue type 4 virus cDNA clone. RNA transcripts derived from mutant cDNA have been used to transfect cells which produced viable dengue virus mutants bearing a defined mutation in the NS1-NS2A cleavage region. These mutants were first characterized by plaque assay on mosquito C6/36 cells. Several mutants produce plaques of reduced size indicating that these mutant viruses exhibits growth restriction in cell culture. The present invention also relates to a recombinant DNA construct comprising the DNA fragment containing a substitution mutation in the sequence encoding for one or more of eight amino acids at the C terminus of NS1 of the cleavage site of the non-structural protein NS1-NS2A, and a vector. The invention also relates to host cells transfected with the DNA construct. In addition, the present invention relates to a recombinant DNA construct comprising the DNA fragment containing a substitution at the position coding for glycine, that is, the + 1 position following the cleavage site of the non-structural protein NS1-NS2A, and a vector. The invention also relates to host cells transfected with the DNA construct. In another embodiment, the present invention relates to a vaccine for humans against dengue type 4, comprising a mutant dengue type 4 virus, as described above, exhibiting reduced virulence, in an amount sufficient to induce immunization against the disease. The mutants of the invention can be evaluated in primates for: (1) virulence as measured by the duration of viremia and (2) immunogenicity as indicated by the type and magnitude of antibody response following viral infection. Dengue virus mutants that show reduced virulence but retain sufficient immunogenicity in monkeys can be evaluated during clinical trials in humans. In a further embodiment, the present invention relates to the construction of dengue type 4 viruses containing mutations in the dengue type 4 3' noncoding region and/or nonstructural protein genes that confer satisfactory attenuation, and the use of each dengue type 4 virus mutants to construct intertypic chimeric viruses with the antigenic specificity of other dengue virus serotypes thereby creating attenuated viruses that might be used for prevention of disease caused by dengue type 1, type 2 or type 3 virus. Chimeric viruses can also be constructed with antigenic specificity for other flaviviruses, such as Japanese encephalitis and tick-borne encephalitis. The current strategy for immunization against dengue favors the use of a vaccine preparation containing all four dengue serotypes. This would protect individuals in endemic areas from the risk of severe dengue Hemorrhagic Shock Syndrome resulting from subsequent infection with a different dengue serotype. Currently, candidate attenuated dengue vaccines are being prepared by a standardized technique that involves serial passage in cells of an unnatural host. Only a limited success has been achieved with these host range mutants. For example, while a dengue 2 vaccine was found to be satisfactorily attenuated, a dengue 3 vaccine prepared by this approach caused dengue fever in human volunteers. The present invention provides a method for constructing viable chimeric dengue viruses. The present invention also provides methods of producing a vaccine effective against all four dengue serotypes using chimeric viruses that share a common dengue type 4 virus genetic background, that includes the type 4 virus 5' and 3' noncoding sequences as well as the sequence coding for all of its 7 nonstructural proteins. For this purpose, we have constructed a type 4 dengue virus that contains deletions in the 3' noncoding system, or in one of the nonstructural protein genes and have shown that these dengue virus mutants exhibit a significant restriction of replicative capacity. Accordingly, mutations conferring satisfactory attenuation can be engineered within these common regions, resulting in a set of four cloned, attenuated dengue viruses that would separately express the four dengue serotype specificities. These viruses can include the parental type 4 virus, a type 1 (structural protein gene region) -type 4 chimera, a similar type 2-type 4 chimers and a similar type 3-type 4 chimera, as well as chimeras of Japanese encephalitis and tick-borne encephalitis. Inactivated whole dengue virus vaccines have been shown to lack sufficient immunogenicity. Live virus vaccines attenuated by serial passage in cell culture have suffered from insufficient attenuation, genetic instability or over attenuation. A dengue type 2 vaccine is still in an early developmental stage. Live vaccines of the remaining three serotypes are not available. The present invention represents a technical breakthrough in the ability to construct dengue viruses with defined mutations in the viral genome. CHIMERIC TICK-BORNE ENCEPHALITIS VIRUS VACCINE Tick-borne encephalitis chimeric virus prepared by incorporating the three structural protein sequences: capsid, membrane and envelope into a dengue virus construct containing sequences encoding nonstructural dengue virus protein. It is likely that dengue/tick-borne encephalitis virus (TBEV) combination requires the cooperation of dengue virus and TBEV viral protein and nucleic acid sequences. The construct generated viruses that were not infectious as either native counterpart. DEN4 and TBEV have the same genome organization and share the same strategy of gene expression, but a comparison of sequences between the two viruses indicates that the sequence homology is relatively low (Pletnev, et al. Virology 174: 250-263 (1990) hereby incorporated by reference). For example, the amino acid identity between the two viruses is 15.4% for capsid protein (C), 15.9% for membrane protein (M), 36.5% for envelope glycoprotein (E) and 39.1% for nonstructural protein (NS1). The following embodiment discloses an improved chimeric TBEV vaccine. We have previously described cloned full-length dengue virus cDNA that can be used as a template for in vitro transcription of infectious RNA. This was done by cloning stable full-length dengue cDNA copies in a strain of E. coli using the pBR322 vector. Dengue virus was recovered from permissive cells transfected with the in vitro RNA transcripts of the cDNA. The properties of the virus produced by cells transfected with infectious RNA transcripts of dengue cDNA were identical to the properties of the virus from which the cDNA clone was derived. In addition, we have disclosed in this invention the production of a chimeric dengue virus having a genome containing nonstructural protein genes of one serotype of dengue virus and structural protein genes from another serotype of dengue virus. As one example of a chimeric dengue virus, the capsid, preM (membrane), and envelope gene sequences from one dengue virus were used to replace the corresponding genes of dengue type 4 virus in a recombinant cDNA construct. Viruses produced in cells transfected with this construct are useful in the preparation of vaccines against homotypic dengue viruses. As disclosed supra, this application additionally discloses the construction of chimeric virus containing gene sequences corresponding to a nonstructural region of dengue virus and a structural region of another flavivirus. In particular, a chimeric tick-borne encephalitis virus is disclosed. The methods of this invention as outlined in the section entitled "Generation of Chimeric Flaviviruses", disclose the production of a recombinant dengue virus construct incorporating three structural proteins from TBEV. Virus produced from cells transfected with this construct are candidate virus for live virus vaccines. The invention associated with this application provides a significant and unexpected improvement to the art of chimeric flavivirus production as well as compelling evidence for the utility of this novel vaccine strategy. One with skill in the art could expect that a chimeric flavivirus combination incorporating all of the structural proteins of one virus with the nonstructural elements of a second virus would be readily recoverable and efficiently replicate in cultured cells because of the possible requirement of functional cooperation of structural proteins derived from distantly related flaviviruses. However, unexpectedly, the present invention identifies a chimeric TBEV virus (TBEV/DEN4) containing two structural proteins (i.e. M and E) from the TBEV genome that assemble with a third structural protein (i.e. C) from dengue virus. PRODUCTION OF CHIMERIC TBEV/DEN4 CONSTRUCTS Since the amino acid sequence homology between DEN4 and TBEV is low, a variety of TBEV and DEN4 chimeric cDNA constructs were made by replacing the corresponding DEN4 sequences with TBEV sequences. These constructs were prepared using techniques well known in the art of molecular biology. The constructs were tested to determine if any of the different gene combinations would yield a viable chimeric virus following in vitro transcription of the cDNA construct and transfection of the RNA product into susceptible cells. Previously, subgenomic cDNA fragments of TBEV (strain Sofjin) were cloned and the nucleotide sequences were determined using techniques well known in the art (see Pletnev, et al., Virology , supra. and Pletnev, et al. FEBS Lett . 200: 317-321, 1986). These plasmids provide overlapping gene sequences that together include the entire TBEV genome sequence. Using plasmids pGEM2-CME or pGEM2-ENS1 as the template and appropriate oligonucleotide primers (see Example 17), a series of TBEV cDNA fragments were prepared by polymerase chain reaction (PCR), each fragment defining one or more specific genes flanked by restriction enzyme cleavage sites. The fragments are provided in Figure 17 and the sequence of the fragments denoted by the nucleotide positions provided in the right margin are available in the publication by Pletnev, et al. Virology 174:250-263, 1990). Figure 17 shows seven such TBEV cDNA fragments and the modified termini suitable for joining to the appropriate sites similarly introduced into full-length DEN4 cDNA RNA transcripts by site-directed mutagenesis. Plasmids pGEM2-CME containing nucleotides (nt) 76-1977 and pGEM2-ENS1 containing nucleotides 966-3672 of the TBEV sequence were constructed from plasmids p10, p4, p18, p2, p11 (see Pletnev, et al. Virology, 1990 supra .) by ligation of the TBEV fragments at shared restriction enzyme sites. Plasmids DEN4 p5'-2 and p5'-2(ΔPstI,XhoI) (Bray, et al., Proc. Natl. Acad Sci, 1991 supra ), and a derivative, p5'-2 (ΔPstI,XhoI,ΔHindIII) were used for substitution of one or more TBEV genes in place of the corresponding DEN4 genes. Those with skill in the art of molecular biology will be able to use the primer sequences SEQ ID NOS:21-31 as identified in Example 17 to generate the chimeric TBEV constructs. The sequences at the junctions of the chimeric constructs were verified by nucleic acid sequencing. All of the resulting chimeric plasmids contained the SP6 promoter positioned upstream of a transcription initiating G followed by an A residue representing the first DEN4 nucleotide. Prior to in vitro transcription, the plasmids were linearized at the unique Asp718 cleavage site immediately following the 3' end of the DEN4 sequence. The constructs were transcribed by in vitro transcription using techniques disclosed by Lai, et al. ( Proc. Natl. Acad Sci. 88: 5139-5143, 1991). The transcription products from the chimeric constructs of Figure 17 were tested for infectivity by transfecting the RNA into simian LLC-MK 2 cells. Transfection procedures used either Lipofectin™ (Bethesda Research Laboratories, Inc., Gaithersberg, Maryland) or DOTAP (N-[1-(2, 3-dioleoyloxy)propyl]-N, N, N-trimethylammoniummethylsulfate, Boehringer Mannheim, Indianapolis, Indiana) to introduce the chimeric RNA transcripts into permissive cells. While cultures of simian LLC-MK 2 cells were used in these procedures, it is contemplated that any permissive cell type for TBEV replication could alternatively be used. The transfected cultures were assessed for the presence of virus over time by immunofluorescence using TBEV-specific rabbit serum or hyperimmune mouse ascitic fluid. Cells transfected with RNA transcripts of pTBE(ME)/DEN4 (including sequences from the premembrane and envelope proteins of TBEV linked to capsid protein and nonstructural sequences of DEN4) stained positively with TBEV-specific rabbit serum, TBEV-specific hyperimmune mouse ascitic fluid (HMAF) and DEN4-specific HMAF. Control cultures infected with DEN4 alone were negative for immunofluorescently labelled TBEV-specific serum. This indicated that chimeric vTBE(ME)/DEN4 expressed both TBEV and DEN4 specific antigens. Chimeric virus isolated from the transfections vTBEV and is designated TBE(CME)/DEN4 (genomic construct includes the capsid, membrane and envelope genes of TBEV and nonstructural sequences of DEN4)and vTBE(ME)/DEN4. As disclosed in Example 17, sixteen days after transfection with TBE(CME)/DEN4 RNA, approximately 1% of cells stained positively for TBEV and DEN4 specific antigens. The percentage of positive cells increased over time to provide a peak titer of 6x10 5 pfu/ml at day 26 post transfection, with 80% of the cells transfected. In contrast, cells transfected with TBE(ME)/DEN4 RNA produced virus more quickly and at day 16 100% of the culture was infected. In contrast, the titer of TBE(ME)/DEN4 virus (denoted v TBE(ME)(DEN4) present in transfected cells was 4x10 6 pfu/ml at day 26 post infection. In addition, the progeny virus produced from cells infected with the chimeric constructs was sequenced to confirm that the DEN4 and TBEV junctions of the genomic fragments from the chimeric virions still matched the junctions of the chimeric constructs as illustrated in Figure 17. CHARACTERIZATION OF TBEV/DEN4 VIRUSES Chimeric virus was characterized using the methods provided in Example 17 and all work with the chimeric TBEV/DEN4 viruses was conducted in a BL-3 containment facility. The proteins produced from the chimeric viruses were compared to proteins produced from dengue 4 virus. Proteins of dengue 4 virus recovered from the full-length DEN4 cDNA clone were identified in the above section entitled "Generation of Chimeric Flaviviruses", illustrated in Figure 4 and disclosed by Bray et al. ( Proc. Natl. Acad Sci. USA (1991), supra.). The pattern of protein separation of dengue virus protein was compared to chimeric viral proteins produced from vTBE(ME)DEN4 infected cells. In this assay (see Figure 18 and Example 18) confluent LLC-MK 2 cells were infected at a multiplicity of infection (MOI) of 0.01. Infected cultures were incubated with 35 S-methionine using techniques disclosed by Lai et al., Proc Natl . Acad . Sci. USA (1991), supra. Infected cell lysates were immunoprecipitated with Lane:1) TBEV-specific HMAF; 2) DEN4-specific HMAF; 3) rabbit serum raised against DEN NS3, 4) rabbit antiserum specific to DEN4 NS5, 5) rabbit antiserum specific to DEN4 preM, or 6) rabbit antiserum specific to DEN4 E protein. Immunoprecipitates were analyzed by polyacrylamide gel electrophoresis (PAGE) under denaturation conditions, as illustrated in Figure 18, using techniques disclosed by Laemmli ( Nature 227: 680-685, 1970). Both chimeric and parental DEN4 viruses produced protein bands identified as DEN4 NS3 and NS5 (lanes 3 and 4). DEN4 preM or E proteins were not identified in cells infected with the chimeric virus but were identified in DEN4-infected cells (see lanes 5 and 6). DEN4 specific HMAF precipitated only protein bands from vTBE(ME)/DEN4-infected cell lysate that comigrated with DEN4 NS1 and NS3 nonstructural proteins (lane 2). TBEV HMAF immunoprecipitated a protein having the correct size for TBEV E protein (55kDa) from the lysate of cells infected with chimeric vTBE(ME)/DEN4. The chimeric virus-infected cell protein migrated faster than DEN4 E (lanes 1 and 6). In previous experiments TBEV HMAF failed to precipitate native TBEV preM and C proteins from TBEV infected cells. Therefore, identification of TBEV preM and C proteins was not expected. The profile of protein bands produced in LLC-MK 2 cells by vTBE(CME)/DEN4 was identical to that produced by vTBE(ME)/DEN4. Thus, the chimeric viruses produced the expected proteins in LLC-MK 2 cells. The plaque morphology of the viruses was assessed by comparing the viral plaques produced by DEN4 with those of plaques produced from the chimeric constructs. DEN4 viral plaques had an average size of 12.1 mm on mosquito C6/36 cells, whereas vTBE(ME)/DEN4 produced plaques averaging 6.5 mm. In contrast, vTBE(ME)/DEN4 produced plaques that were 5-fold larger than those produced by DEN4 on simian LLC-MK 2 cells. This suggested that the chimeric virus replicated more efficiently in LLC-MK 2 cells than did DEN4. These unexpected results were confirmed by analysis of the growth rate and the viral yield of vTBE(ME)/DEN4 in infected LLC-MK 2 cells (see Figure 19). The chimeric virus reached a titer of 10 8 pfu/ml which is approximately 1000-fold higher than that attained by parental DEN4, under the same conditions. Chimeric vTBE(ME)/DEN4 virus grew slowly on mosquito C6/36 cells and reached a titer 100-times lower than that produced by parental DEN4. The plaque size of chimeric vTBE(CME)/DEN4 did not differ appreciably from that of DEN4 on LLC-MK 2 cells. In these studies, virus was quantitated by plaque assay using techniques disclosed by Bancroft, et al. ( Pan Am. Health Organ. Sci. Publ. 375: 175-178, 1979). The difference in plaque size may reflect an incompatibility of TBEV C protein with DEN4 viral RNA which prevents efficient interaction of the proteins during viral RNA packaging and viral maturation. It is also possible that the difference in plaque size is the result of a substitution of the six nucleotides upstream of the AUG codon in the DEN4 5' noncoding region. This sequence change could influence the efficiency of viral protein translation. Analysis of viral RNA production (Figure 20) from the chimeric constructs together with the protein data of Figure 21 provide an explanation for the small-plaque phenotype and the reduced growth rate of vTBE(ME)/DEN4 in infected C6/36 cells. Cultures of LLC-MK 2 or C6/36 cells (approximately 10 6 cells) were collected at various times following virus adsorption, and lysed in buffer containing sodium dodecyl sulphate (SDS). Total RNA was isolated from the cell lysate and the medium by phenol extraction using techniques provided by Maniatis, et al. ( Molecular Cloning, A Laboratory Manual . 1982 Cold Spring Harbor Laboratory, New York). RNA samples were denatured in formaldehyde and applied to a nitrocellulose filter (BA85, Schleicher and Schuell. The filters were hybridized with a nick-translated, [ 32 P]-labeled pTBE(ME)/DEN4 DNA probe. RNA transcripts made in vitro from pTBE(ME)/DEN4 were used as a positive control. Viral proteins from DEN4-infected LLC-MK 2 or C6/36 cells were compared to viral proteins from vTBE(ME)/DEN4-infected LLC-MK 2 or C6/36 cells. DEN4 viral proteins including E, NS1 and NS3 accumulated to a high level in DEN4-infected LLC-MK 2 or C6/36 cells by 48 hr after infection (Figure 21). However, the kinetics of viral protein synthesis differed between vTBE(ME)/DEN4-infected LLC-MK 2 and C6/36 cells. Viral proteins were detected as early as 8 hr post postinfection in LLC-MK 2 cells, whereas viral proteins were not detected in C6/36 cells until 48 hr after infection. Approximately 70% of the vTBE(ME)/DEN4 virions remained in the medium following inoculation onto C6/36 cells, whereas only a small fraction of the inoculated virus was found in medium of LLC-MK 2 cell cultures. This finding suggests that entry of chimeric vTBE(ME)/DEN4 into LLC-MK 2 cells was more efficient than was entry into C6/36 cells and possibly as a consequence, the virus grew more slowly and to a lower titer in these cells as compared with DEN4 virus. Following entry into the LLC-MK 2 cells, replication of chimeric viral RNA was more rapid than that of DEN4 viral RNA. On the other hand, RNA synthesis of chimeric virus was slower than that of DEN4 in infected C6/36 cells. Thus, vTBE(ME)/DEN4 exhibited reduced efficiency of entry into mosquito cells and this was associated with a reduced production of viral RNA and proteins. These observations are consistent with the low efficiency of transfection of TBEV viral RNA into C6/36 cells. (Mandl, et al. J. Virol 65: 4070-4077, 1991). It will be understood by those of skill in the art, that there are a variety of methods that could be used to incorporate the nucleic acid sequences from one virus with the nucleic acid sequences of another virus. Such ligation schemes and cloning strategies are known in the art. Therefore it is contemplated that other techniques could be used and the employment of these techniques would not detract from the claimed invention. EFFICACY OF THE CHIMERIC VACCINE AND DETERMINATION OF NEUROVIRULENCE vTBE(ME)/DEN4 was assessed for protective efficacy as a vaccine by introducing the virus preparation into test animals through various routes of administration. The neurovirulence of vTBE(ME)/DEN4 as compared to parental dengue virus was analyzed by inoculating mice intracerebrally (IC). Other administration routes included intradermal (ID) or intraperitoneal (IP) injections. In one experiment, three-day-old suckling BALB/c mice were injected IC with a dose of 10 2 pfu of virus in 0.02 ml of MEM/0.25% human serum albumin and in another set of experiments, six-week-old BALB/c female mice were inoculated IC, ID or IP. Mice were observed for 21 days for symptoms of encephalitis or death, and surviving adult mice were bled 20 days after infection to evaluate the antibody response to TBEV. Surviving mice were challenged IP at 21 days postinfection with 10 3 LD 50 TBEV (strain Sofjin) in a BL-4 containment facility to determine the protective efficacy of the vaccine. vTBE(ME)/DEN4 retained the neurovirulence of its TBEV parent when inoculated directly into the brain (IC) of suckling or adult mice (Figure 22 and Example 21). As a positive control, comparisons were made to a representative strain of TBEV (strain Sofjin). TBEV is highly neurovirulent and 0.1 pfu readily caused fatal encephalitis in 50% of suckling mice inoculated IC. Similarly, TBEV was also highly neurovirulent for adult mice inoculated by IP. In this case, the ID 50 was 14.2 pfu. In contrast, vTBE(MB)/DEN4 did not cause encephalitis when inoculated by a peripheral route, either ID or IP (See Figure 22) indicating that were safe potentially safe routes for vaccination. The immunogenicity of vTBE(ME)/DEN4 was analyzed by immunoprecipitation of antibody to the virus using [ 35 S]-labeled antigens using serum antibody of surviving mice. Analysis of the immunoprecipitates by polyacrylamide gel electrophoresis revealed that antibodies specific to DEN4 NS1 (a protein encoded by the chimeric virus) were readily detected, but antibodies to TBEV E were either of low titer or were not detectable. Presumably since mice are not a natural host for TBEV, peripheral inoculation of mice permitted only low level viral replication. Mice that survived IP or ID inoculation of vTBE(ME)/DEN4 were studied for evidence of resistance to subsequent lethal TBEV challenge. Twenty-one days after inoculation with vTBE(ME)/DEN4 or DEN4 virus, mice were challenged IP with 10 3 LD 50 of the highly neurovirulent Sofjin strain of TBEV. The mice that survived IP or ID inoculation with chimeric vTBE(ME)/DEN4 were protected against subsequent challenge, whereas all three groups of mice previously immunized with DEN4 died between day 11 and day 20 (see Figure 22). These results indicated that the virus as a vaccine is protective and specific for TBEV infection. Nonimmunized control mice died of encephalitis between day 10 and day 16 following TBEV infection. TBE(ME)/DEN4 virus uniformly caused encephalitis in both suckling and adult mice following intracerebral inoculation, whereas mice inoculated with DEN4 developed dengue virus-associated disease with low frequency. Thus, the chimeric virus retains the mouse neurovirulence of TBEV from which its preM and E genes were derived. This indicates that most, if not all, of the genetic determinants of TBEV mouse neurovirulence map within these two structural protein genes. However, unlike parental TBEV, vTBE(ME)/DEN4 was not pathogenic when adult mice were inoculated peripherally, indicating a loss of neuroinvasiveness by peripheral inoculation. These findings suggest that a region of the TBEV genome other than the preM and E genes is required for TBEV to invade the CNS and produce encephalitis. Mice inoculated peripherally with vTBE(ME)/DEN4 were protected against subsequent intraperitoneal challenge with a lethal dose of TBEV, whereas mice similarly inoculated with DEN4 were not. These findings indicate that the preM (precursor of M), M and/or E proteins of vTBE(ME)/DEN4 contain major antigenic determinants for protective immunity against TBEV encephalitis in mice. MODIFYING NEUROVIRULENCE OF CHIMERIC VTBE(ME)/DEN4 As described in the section above entitled "Mutations in the Dengue Genome Affecting Neurovirulence", the recombinant dengue virus construct was modified by site-directed mutagenesis or mutagenesis by PCR or the like to produce a modified virus suitable for vaccination because of its reduced neurovirulence characteristics. The vTBEV/DEN4 genome can be modified using similar techniques, as disclosed herein, to obtain reduced neurovirulence and thereby improving the safety of an attenuated virus vaccine for TBEV. Success in constructing a viable TBEV/DEN4 chimera that retains the protective antigens of TBEV, but lacks the peripheral invasiveness of TBEV, provides the basis for pursuing a novel strategy for immunoprophylaxis of this important pathogen, namely the development of an attenuated TBEV vaccine. However, before this goal can be realized, an additional modification of the chimera must be achieved, namely ablation of neurovirulence for the CNS as measured by direct inoculation of virus into the brain. Since the TBEV/DEN4 chimera retains the neurovirulence of its TBEV parent, it was necessary to abolish this property by engineering strategic mutations in the DEN4 or TBEV portion of the chimeric genome and evaluating their effect on mouse neurovirulence. These mutants can potentially include mutations introduced at any number of locations in the viral genome. Initial studies have assessed chimeric viruses with the following mutations: (1) deletions in the 5' noncoding region, (2) mutations that ablate the preM-to-M cleavage site or the glycosylation sites of preM, or E, or NS1 protein, or (3) point mutations in the E gene. Mutations tested to date are included in Figure 23. As part of a study to systematically modify the TBE(ME)/DEN4 construct and test for the effect of the genetic alterations on neurovirulence, six different constructs containing one or more changes in the protein sequence were prepared see Example 22. It will be understood that using the techniques provided in this invention, one with skill in the art can create any number of substitutions, additions or deletions to the TBE(ME)/DEN4 construct and test for changes in neurovirulence. Similarly, one with skill in the art can readily combine the mutations provided in Figure 23 with these or other mutations to test the chimeric construct having preferably at least one alteration in the genetic composition as compared with the chimeric construct combining native sequences. In studies to assess the ability of the mutant constructs to direct the production of mutant chimeric virus, six mutant chimeras were recovered from the transfected LLC-MK 2 cells and the viruses were analyzed for changes in plaque morphology. The progeny of mutant TBE(ME)/DEN4 viruses were amplified by passage in LLC-MK 2 cells and analyzed by plaque assay on LLC-MK 2 cells or on mosquito C6/36 cells. Chimera TBE(CME)/DEN4, which included all three structural protein genes of TBEV, was similarly analyzed. Wild type DEN4 was also used as a control. As shown in Figure 24, the plaque size of most mutants on either cell line was reduced as compared to the viral plaques of parental TBE(ME)/DEN4 suggesting that these mutants were restricted for viral replication. Mutants which contained a defective glycosylation site in E or NS1 produced plaques smallest in size among the series of mutants tested. The mutant chimeric constructs were additionally tested for neurovirulence in mice. Mice were inoculated with the virus as described in Example 21 and described in Example 23. Infected mice were observed for signs of encephalitis and death over 31 days. Constructs containing the *NS1(1)-Glc - and *PreM/M - mutations exhibited an LD50 greater than 100 pfu indicating that these mutants exhibited a greater than 100-fold reduction in mouse neurovirulence. This finding indicates that the NS1(1)-Glc - , PreM/M - or both mutations could be used to confer attenuation of mouse neurovirulence. This findings suggest that similar mutations may also be employed to confer attenuation of other encephalitic flaviviruses including Japanese Encephalitis Virus. It is further contemplated that those chimeric constructs showing reducing neurovirulence in mice as, evidenced by increased LD 50 values, are next tested in primates. Example 24 provides a testing scheme for vaccine efficacy in primates. Following successful primate studies the vaccines of this invention are tested in clinical trials in humans. Those with skill in the art can modify the primate studies to make them suitable for human study. Therefore, the inventions of this application provide a number of vaccines and teach one with skill in the art to prepare chimeric dengue virus vaccines to generate protective immune responses to the serotypes of dengue virus or to other flaviviruses. Such a vaccine preparation can preferably contain a pool of chimeric viruses each expressing structural protein for at least one dengue virus. Using the techniques disclosed herein, one with skill in the art of virology and molecular biology can generate chimeric viruses that incorporate structural regions of one flavivirus with nonstructural regions of a second virus. Preferably, this second virus in a dengue virus and still more preferably, this second virus is dengue type 4 virus. In addition, this invention teaches one with skill in the art to modify the dengue virus construct using site-directed mutagenesis or the like to produce an improved dengue virus vaccine and it is contemplated that these modifications can similarly be incorporated into the chimeric virus construct. The invention also teaches the preparation and testing of chimeric viruses that include non-structural regions and at least one structural region of a dengue virus in combination with structural gene regions from another flavivirus. In particular a vaccine for TBEV is disclosed that is a chimeric virus containing one structural region together with nonstructural regions from dengue 4 and sequences encoding two structural proteins from TBEV. The encouraging results observed thus far with the TBEV(ME)/DEN4 chimera suggest that these techniques are also useful to produce a vaccine for viruses more closely related to dengue virus than TBEV. One such example is Japanese encephalitis virus which continues to be a major public health problem in the Far East. Thus, in a further embodiment, the techniques of this invention are used to combine the DEN4 cDNA nonstructural regions with at least one structural gene from Japanese encephalitis virus and the remaining structural gene regions of DEN4. It is further contemplated that other combinations of nonstructural regions and structural regions of different flaviviruses could similarly be prepared and tested using the techniques of this invention. For example, a recombinant construct of Japanese encephalitis virus could be modified to replace one or more structural regions with corresponding gene sequences from tick-borne encephalitis virus while continuing to supply the viral nonstructural regions. Similarly one could generate a recombinant cDNA encoding tick-borne encephalitis virus and replace the structural regions with at least one gene encoding dengue virus structural protein. All of the publications cited here and in the Examples below are incorporated by reference. Particular embodiments of the invention will be discussed in detail and reference will be made to possible variations within the scope of the invention. There are a variety of alternative techniques and procedures available to those of skill in the art which would similarly permit one to successfully perform the intended invention. EXAMPLES AND GENERAL INFORMATION RELATING THERETO Viruses : Dengue type 4 virus strain 814669 was used for construction of full-length cDNA that served as the source of full-length infectious RNA transcripts (Mackow, E. et al. (1987) Virology 159, 217-228; Zhao, B. et al. (1986) Virology 155, 77-88). A preparation of dengue type 1 virus Western Pacific strain (D1 WP), a fetal rhesus lung cell passage level 9, was kindly provided by Dr. K. Eckels (WRAIR, Washington, DC) (McKee, K.T. et al. (1987) Am. J. Trop. Med . Hyg. 36, 435-442). A mouse brain preparation of mouse neurovirulent dengue type 2 virus New Guinea C strain (D2 NGC), mouse brain passage level 38, was kindly provided by Dr. D. Dubois (WRAIR, Washington, DC) (Sabin, A.B. (1952) Amer. J. Trop. Med Hyg. 1, 30-50). Dengue 1 and dengue 2 viruses were amplified by one passage in C6/36 mosquito cells. Each of these three viruses was then passaged once in LLC-MK 2 simian kidney cells, and the resulting virus suspension was used to study plaque morphology and mouse neurovirulence. Cloning Vectors : Plasmid p5'-2, which contains the 5'-half of the dengue 4 genome cDNA, was modified to facilitate replacement of the three structural protein genes (Lai, C.J. et al. (1991) Proc. Natl . Acad Sci. USA 88, 5139-5143). First, a unique Xho I site was introduced through site-directed mutagenesis at nucleotide 2342 (A-G) of the dengue 4 sequence, near the 3' end of E, creating p5'-2 (Xho I). This nucleotide change did not alter the amino acid sequence. This vector was then digested at the unique Bst BI site within the dengue sequence and at the unique Asp 718 site which immediately follows the dengue sequence in p5'-2, and the fragment was ligated to the 3' half of the dengue 4 genome, creating full-length p2A (Xho I). Second, the Bgl II site at nucleotide 88, which is conserved among flaviviruses, was made unique by removing three other Bgl II sites in p5'-2. The Bal II site at the junction between pBR322 and the SP6 promoter was removed by inserting a Not I linker. Two other sites at 4128 and 4277 were removed by shortening the vector to the recently introduced Pst I site at 3473 (McKee, K.T. et al. (1987) Am . J . Trop. Med . Hyg. 36, 435-442). Plasmid p5'-2 (Xho I, Pst I) was subsequently used for fragment exchange to create chimeric cDNA. Figure 1 presents the following plasmids containing full-length dengue cDNA which were constructed: p2A (Xho I), a complete cDNA copy of the dengue 4 genome, in which a unique Xho I site was created near the 3' end of the E gene; p2A (D1 WP), derived from p2A (Xho I) by replacing the sequence between the Bgl II site in the 5' non-coding region and the unique Xho I site with the corresponding cDNA of dengue 1 Western Pacific strain; p2A (D2 NGC), prepared in the same manner by replacing the Bgl II-Xho I fragment with the corresponding cDNA from dengue 2 New Guinea C strain. B: Bgl II; X: Xho I; A: Asp 718 restriction enzyme sites. Chimeric cDNA : Dengue type 1 or type 2 virus grown in C6/36 mosquito cells was purified and virion RNA extracted according to the procedure described by (Zhao, B. et al. (1986) ( Virology 155, 77-88). Dengue I first-strand cDNA was synthesized by reverse transcription using a negative-sense oligonucleotide which hybridized to nucleotides 2306-2338 of the dengue 1 sequence. This primer sequence contained a silent third base change (A-G) at nucleotide 2316 to create an Xho I site at the position corresponding to the site in the dengue 4 E gene in p5-2 (Xho I, Pst I) described above. The dengue 1 cDNA was then used as the template to synthesize double-stranded DNA by PCR, using the negative-sense oligonucleotide and a positive-sense primer which hybridized to dengue 1 nucleotides 51-70, and contained the conserved Bgl II site. The PCR product was digested with Bgl II and Xho I, and then cloned into p5'-2 (Xho I, Pst I), replacing the corresponding dengue 4 sequence. The Cla I-Xho I fragment containing the dengue 1 sequence was then joined with the remaining dengue 4 cDNA from p2A (Xho I) to create full-length chimera p2A (D1 WP). Similarly, dengue 2 first-strand cDNA was synthesized using a negative sense primer that hybridized to dengue 2 nucleotides 2310-2364. This primer contains three base changes: T-C at 2333, A-G at 2336, and C-A at 2337. These changes create an Xho I site at the position corresponding to the Xho I site in the dengue 4 E gene described above. These changes did not alter the amino acid sequence. For double strand DNA synthesis by PCR, the negative-sense primer was used, and the positive-sense primer was the same used for dengue 1. The PCR product was digested with Bgl II and Xho I, cloned into p5'-2 (Xho I), and the Cla I-Xho 1 fragment was used to replace the corresponding fragment of p2A (Xho I) to create p2A (D2 NGC). RNA Transcription, Transfection and Recovery of Virus : Transfection of cells with full-length RNA transcripts was performed as described (Lai, C.J. et al. (1991) Proc. Natl. Acad. Sci. USA 88, 5139-5143). Ten days after transfection, cells were trypsinized and transferred to a a 6-well plate and to a chamber slide. Two days later, cells on the slide were tested by immunofluorescence (IFA) for evidence of dengue viral antigens. If IFA showed that most cells was infected, the cells in the 6-well plate were trypsinized, mixed with a 6-fold excess of uninfected cells, and incubated until cytopathic effects (appearance of numerous dead cells in the medium) became evidence, usually after 6-7 days. The infected cells were then harvested by removing the medium, scraping the cells, resuspending them in a standard volume of 50% Eagle's minimal essential medium/50% serum, and then freezing. On the other hand, if a small percentage of cells was positive, the cells were trypsinized, diluted 1:3 in fresh medium, and allowed to grow without addition of uninfected cells. The percentage of infected cells was then estimated on a weekly basis, and cell lysates were harvested and titered for virus at intervals. Detection of Dengue Virus Antigens : Infected cells were analyzed by IFA using serotype-specific monoclonal antibodies (mab) 1F1 (dengue 1), 3M5 (dengue 2), 5D4 (dengue 3) and 1H10 (dengue 4) and NS1-specific mab 1G6 (dengue 4), originally produced by Dr. M.K. Gentry and Dr. E. Henchal (WRAIR, Washington DC) (Henchal, E.A. et al. (1982) Am. J. Trop. Med . Hyg. 31, 548-555). Monoclonal antibodies were used at a dilution of 1:50-1:300, while fluorescein-conjugated anti-mouse antiserum (Kierkegaard-Perry Laboratories, Gaithersburg, MD) was used at 1:100. Results were recorded photographically; for each mab, infected cells yielding a negative result were photographed for the same exposure time as positive cells. To analyze proteins of parental (wild-type) dengue viruses and of progeny virus v2A (Xho I), confluent LLC-MK 2 cells in a 6-well plate were infected with virus at an MOI of 0.2. Six days after infection, cells were labeled with 50µCi/well of S-methionine-free medium for 6 hours and cells were then lysed with RIPA buffer. For chimeric virus v2A (D1 WP), labeling was performed when approximately 100% of the cells were infected, while v2A (D2 NGC), labeling was performed when about 30% were infected. Lysates were precipitated with the appropriate homotypic or heterotypic hyperimmune mouse ascitic fluid (HMAF) and analyzed by electrophoresis on SDS-12% polyacrylamide gels. Plaque Morphology : Viruses were characterized for plaque morphology on LLC-MK 2 cells (Bancroft, W. H. et al. (1979) Pan Am. Hlth. Org. Sci. Publ. 375, 175-178). Each parental virus was passaged once in LLC-MK 2 cells prior to analysis of plaque morphology. Neutral red overlay was added to cultures after 6-9 days of incubation. Evaluation of Chimeric Viruses in Suckling Mice : Three-day old Swiss mice were inoculated intracerebrally with 2000 pfu of parental or chimeric dengue virus, in the form of a diluted lysate of virus-infected LLC-MK 2 cells. Negative control mice were injected with a lysate of uninfected cells. One liter was inoculated with parental dengue type 4 virus (814669), dengue type 1 virus WP, or dengue type 2 virus NGC. Two liters were inoculated with v2A (Xho I), v2A (D1 WP), or v2A (D2 NGC). Inoculated mice were monitored for 21 days for signs of encephalitis and for death. Example 1 Chimeric cDNA & Viruses The full-length dengue type 4 virus cDNA (p2A) was used to construct chimeric viruses in which its structural gene sequences were replaced by the corresponding sequences of a dengue type 1 or type 2 virus. The type 1 virus (WP) chosen for this purpose was a low tissue culture passage strain that had not been adapted to growth in mouse brain. The type 2 strain (NGC) was a highly neurovirulent mutant that had been selected during serial intracerebral passage in mice. The type 2 mutant was chosen for this study because it offered the possibility of mapping the genetic determinants of mouse neurovirulence. Transformation of E. coli strain HD1528 bacteria with plasmid constructs p2A (Xho I), p2A (D1 WP) or p2A (D2 NGC) produced a stable plasmid population. The predicted structure of these plasmids is shown in Figure 1. Their structure was verified by restriction enzyme mapping. Approximately one-half of the LLC-MK 2 cells transfected with RNA transcripts from p2A (Xho I) were positive by immunofluorescence staining for dengue virus antigens on day 12 (Figure 2A). Figure 2 presents (A) LLC-MK 2 cells transfected with the same concentration of RNA transcripts from p2A (Xho I), p2A (D1 W), or p2A (D2 NGC). Percentage of virus-infected cells following transfection was estimated by IFA after replating in fresh medium. (B) Infected cells were harvested when a majority of cells was positive by IFA. Titration of virus was performed by plaque assay (Bancroft, W.H. et al. (1979) Pan Am. Hlth. Org. Sci. Publ. 375, 175-178). In a previous study, a similar proportion of antigen positive cells, in the range of 20-50%, was observed 12 days after transfection with transcripts from p2A and p2A (Pst I), which in both instances yielded virus with wild-type phenotype (Lai, C.J. et al. (1991) Proc. Natl. Acad Sci. USA 88, 5139-5143). In contrast, on day 12 less than 1% of cells transfected with transcripts from p2A (D1 WP) or p2A (D2 NGC) were positive. The percentage of cells infected with v2A (D1 WP) increased to approximately 5% by day 19, 30% by day 26, and 80% by day 33, at which time the titer of virus present in transfected cells was 5 x 10 6 pfu/ml. (Figures 2A and 2B). In contrast, v2A (D2 NGC) grew more slowly: we estimated that 1% of cells were positive on day 19, but less than a third of the cells had become infected by day 54. The virus titer was only 1.5 x 10 2 pfu/ml at 30 days, 2.5 x 10 2 at 44 days, and it did not reach 10 4 pfu/ml until day 58. At 72 days after transfection, a majority of cells were infected, and the titer of the cell suspension was found to be 10 2 pfu/ml. The titer of virus produced by cells transfected with p2A (Xho I) RNA transcripts (10 6 pfu/ml) was the same as that observed when cell cultures were infected with type 4 virus at an MOI of 0.1. In addition, the highest titer of virus produced by cultures transfected with the type 1/type 4 chimera (5 x 10 6 pfu/ml) was similar to that observed following infection of cell cultures with type 1 virus at an MOI of 0.1 (1.5 x 10 7 pfu/ml). The highest titer of the type 2/type 4 chimera produced following transfection (10 2 pfu/ml) was similar to that produced by cell cultures infected with its type 2 parent at an MOI of 0.5. Example 2 Characterization of Chimeric Structural Proteins Indirect immunofluorescence was performed to characterize progeny viruses, as shown in Figure 3. Figure 3 presents immunofluorescence assay performed on LLC-MK 2 cells infected with v2A (Xho I), v2A (D1 WP), or v2A (D2 NGC), or on uninfected cells. Monoclonal antibodies were used at dilutions of 1:50-1:300, while fluorescein-conjugated anti-mouse antibody was used at 1:100. The serotype specificity of the monoclonal antibodies is shown at left. Cells infected with v2A (Xho I) were stained with dengue 4 specific mab 1H10 by IFA, but did not bind dengue 1 specific mab 1F1 or dengue 2 specific mab 3M5. As predicted, cells infected with v2A (D1 WP) reacted only with 1F1, and those infected with v2A (D2 NGC) were stained only with 3H5. Mab 5D4, specific for dengue 3 virus, did not react with any of the infected cells. These results indicated that both chimeric viruses exhibited the antigenicity specified by their structural protein genes. As expected, Mab 1G5, specific for dengue 4 non-structural protein NSI, stained cells infected by progeny of the dengue type 4 cDNA, i.e., 2A (Xho I), as well as either of the chimeric viruses, 2A (D1 WP) or 2A (D2 NGC) derived from the type 4 virus. The proteins of parental dengue type 4 virus and of its cDNA-derived progeny v2A (Xho I) appeared to be identical when analyzed by immunoprecipitation with homotypic HMAF followed by polyacrylamide gel electrophoresis (Figure 4). Figure 4 presents 35 S-methionine-labeled lysates of virus-infected LLC-MK 2 cells or uninfected control cells were immunoprecipitated with hyperimmune mouse ascitic fluid (HMAF) raised against dengue 1, 2 or 4, and analyzed by electrophoresis on an SDS-12% polyacrylamide gel (acrylamide:bis = 60:1.6). At the top of each lane, virus is indicated vertically, while the HMAF with which the lysate was immunoprecipitated is indicated horizontally. Markers: protein molecular size markers are in kilodaltons. The locations of the E and pre-M glycoproteins of dengue 1, 2 and 4 are indicated (Figure 4). The proteins of parental dengue type 1 virus (WP) and of dengue type 2 virus (NGC) were also analyzed. The E glycoproteins of dengue 1 and dengue 2 co-migrated, but they migrated more slowly than the E protein of dengue 4. Similarly, the pre-M glycoproteins of dengue 1 and 2 comigrated, but they also migrated more slowly than the pre-M of dengue 4. The difference in relative molecular size of the E proteins was approximately 4 kd, while that of the pre-M proteins was about 1 kd. These differences in molecular size of E and pre-M were probably due to variation in the extent of glycosylation or conformational differences. The immunoprecipitated proteins of viruses v2A (D1 WP) and v2A (D2 NGC) showed a hybrid pattern. Pre-M and E of v2A (D1 WP), immunoprecipitated with dengue 1 HMAF, co-migrated with pre-M and E of parental dengue 1, but dengue 4 HMAF precipitated only bands which co-migrated with dengue 4 non-structural proteins. Similarly, pre-M and E of parental dengue 2, while dengue 4 HMAF precipitated non-structural proteins resembling those of type 4 virus. Parental dengue type 4 virus and its cDNA-derived progeny v2A (Xho I) produced similar plaques on LLC-MK 2 cells (not shown). The plaques of parental viruses were clearly visible when stained with neutral red six days after infection, but chimera v2A (D2 NGC) failed to produce detectable plaques at that time. If staining was delayed until day 9, however, very small v2A (D2 NGC) plaques were detected, much smaller than those of v2A (Xho I) or wild-type dengue 2 NGC (Figure 5). Plaques of dengue 4 814669 did not differ from those of progeny virus v2A (Xho I). Plaques of v2A (D1 WP) were basically similar to those of v2A (Xho I). Figure 5 presents monolayers of LLC-MK 2 cells infected differ from those of progeny virus v2A (Xho I). Plaques of v2A (D1 WP) were basically similar to those of v22A (Xho I). Figure 5 presents monolayers of LLC-MK 2 cells infected with dengue virus, followed by an agarose overlay (Bancroft, W.H. et al. (1979) Pan Am. Hlth. Org. Sci. Publ. 375, 175-178). Neutral red overlay was applied 9 days after infection of the cell monolayer, and plaques were photographed the next day. A: 2A (Xho I); B: 2A (D1 NGC); C: wild-type NGC dengue 3. Example 3 Virulence of Viruses for Suckling Mice Chimera v2A (D2 NGC) was compared with its parental viruses, mouse-brain adapted dengue 2 NGC and v2A (Xho I), for mouse neurovirulence. Dengue 2 NGC proved to be the most rapidly lethal: each of 10 mice died of encephalitis on day 5 or day 6 post-inoculation, with a mean survival time of 5.1 days, while each of the 15 mice inoculated with chimera v2A (D2 NGC) survived 8-11 days before dying, with a mean survival time of 9.1 days (Figure 6). Figure 6 presents three-day-old Swiss mice inoculated intracerebrally with 2000 pfu of either parental or chimeric dengue virus, in the form of a diluted lysate of infected LLC-MK 2 cells. The number of mice injected was: dengue 2 (NGC), 10 mice; v2A (D2 NGC), 15; dengue 4 814669, 12; v2A (Xho I), 18. Negative controls (11 mice) were injected with a diluted lysate of uninfected cells. Mice were then monitored daily for signs of encephalitis and for death. The difference in survival distributions is significant (p = 00001, Smirnov two-point statistic). Parental virus dengue 4 814669 were also compared to its progeny v2A (Xho I). Five of 12 mice inoculated with the parental virus died of encephalitis, the first death occurring on day 11, while only one of 18 mice inoculated with v2A (Xho I) died, 16 days after inoculation; the rest remained healthy. Although the survival distributions do not differ significantly according to the Smirnov statistic (p>.14), the percent survival of the two groups differs by Fisher's exact test (p=.0256). This suggests that dengue 4 814669, or a subset of virions in the virus preparation, possesses a degree of neurovirulence, and that progeny virus v2A (Xho I) may represent a non-neurovirulent sub-population, or may contain nucleotide changes which arose during cloning or virus propagation. None of 12 mice inoculated with parental dengue type 1 virus (WP), or 18 injected with its chimeric progeny v2A (D1 WP), or 11 which received a lysate of uninfected cells developed encephalitis or died. MUTATIONAL ANALYSIS We have cloned a series of dengue cDNA inserts that spanned the entire length of the dengue type 4 virus genome. These were used in an initial attempt to clone a full-length dengue DNA, as in Example 4. Example 4 Initial Attempt to Clone a Full-Length dengue DNA in Plasmid pBR322 Containing the SP6 Promoter The various dengue type 4 cDNA inserts were joined at shared restriction enzyme sites to form a full-length dengue DNA copy using the same Pst I cloning site of PBR322. For in vitro transcription, the SP6 polymerase promoter sequence was placed at the 5'-end preceding the dengue sequence. The predicted 5'-sequence of the RNA transcripts is shown in Figure 7. The A residue of the first dengue nucleotide is positioned immediately following the normal SP6 polymerase transcription initiating G. The m7G cap structure that is present at the 5'-end of the genomic RNA was provided by incorporation of a m7G pppG analog in the transcription reaction. Such a DNA structure would generate dengue RNA containing an additional nucleotide at the 5'-terminus. To produce run-off transcripts, a unique Asp 718 cleavage sequence was introduced at the 3'-end of the dengue sequence. As shown in Fig. 7, in the template strand 5 additional nucleotides are present preceding the Asp 718 cleavage site. If transcription proceeds to the last nucleotide, the RNA transcripts would contain these 5 additional residues at their 3'-terminus. During this study employing E. coli strain HB101 as the host for transformation, it was noticed that plasmid containing the full-length dengue DNA were often unstable as plasmid in many transformants underwent rearrangement and many colonies had to be screened in order to isolate a clone DNA with the predicted restriction enzyme pattern. We sought to examine the stability of plasmid produced by different strains of E. coli. The highly transformation-competent, commercially available E. coli strain DH5α and E. coli strain DB1528 used earlier in the laboratory were compared with E. coli strain HB101. It was found that strain DB1528 produced transformants exhibiting a colony size 3-4 times larger than BH101 transformants. Above all, transformants of DB1528 generally yielded plasmid with the predicted restriction enzyme pattern suggesting that E. coli DB1528 is the strain of choice to produce stably cloned dengue full-length DNA. However, the in vitro RNA transcripts made from this first full-length dengue DNA clone failed to produce dengue virus when tested on transfected cells where virion RNA as the positive control at 100-times less concentration yielded dengue virus as detected by an indicated immunofluorescence assay. Example 5 Replacement of dengue DNA Segments in the Full-Length Construct with Independently Derived DNA Clones It was reasoned that the failure to produce infectious RNA transcripts was due to the presence of defective mutations in the full-length clone. Such mutations could presumably arise from cloning of a defective population in the virus stock or a coupling error during cloning or propagation of plasmid. We decided to carry out replacement of dengue DNA segments that might contain one or more defective mutations with the corresponding segments from independently cloned dengue DNA constructs. In order to provide a framework for a systematic replacement experiment the 5' and 3' fragments of the dengue sequence were cloned separately. The unique Bst B1 site at nucleotide 5069 was used to separate the full-length dengue sequence into two fragments each representing approximately 50% of the genome. The 5'-fragment of the first full-length clone containing the SP6 promoter was designated 5'-1 and the remaining 3' sequence between Bst B1 and Asp 718 designated 3'-A. A plasmid containing the second 5'-fragment, 5'-2, was constructed from an independently derived set of dengue cDNA inserts. A unique Asp 718 site was introduced at the Pst I site of pBR 322 downstream of the Bst B1 site of the dengue sequence. Thus, this plasmid was also suitable for use as a cloning vector for insertion of the 3' fragment in the full-length DNA construction. Two additional 3'-fragments, 3'-B and 3'-C were also constructed from an independent series of dengue cDNA inserts (Fig. 8). Replacement of the 3' fragment in the first full-length clone 1A with 3'-B or 3'-C fragments produced two other full-length clones, 1B and 1C. Similarly, substitution of the 5'-fragment in all three full-length DNA constructs with the 5'-2 fragment yielded three other full-length DNA clones, i.e., 2A, 2B and 2C. Digestion of these plasmids with Bgl II. Nsi I, and Asp 718 showed a pattern of predicted DNA fragments that was indistinguishable among all six full-length clones as shown in Figure 9. Thus, the successful propagation of plasmid containing apparently full-length dengue DNA sequence in E. coli train BD1528 allowed the production of RNA transcripts in vitro for evaluation of their infectivity in cultured cells. Example 6 Initial Evidence for the Infectivity of RNA Transcripts Made In Vitro RNA transcripts produced from the constructed full-length dengue DNA templates were each tested for infectivity by transfection of LLC-MK 2 cells. Dengue infected cells were readily observed 10 days following transfection with 2A RNA as detected by an indirect immunofluorescence assay. RNA transcripts from other four full-length DNA clones were negative by this assay indicating that these DNA clones, like clone 1A, contained lethal mutations that were not detected by restriction enzyme analysis. In addition to positive identification of dengue virus infected cells as described above, infectious dengue virus was recovered from the medium or the cell lysate of 2A RNA transfected cells. The titer of dengue virus present in the transfected cell lysate was 10 1 pfu/ml. Treatment of 2A RNA transcripts with DNase I did not affect their infectivity while RNase treatment completely abolished their infectivity. Since plaquing of dengue virus was not carried out following transfection of cell monolayers with RNA, production of virus was identified following an extended incubation for virus amplification. Using this indirect immunofluorescence assay procedure, infectivity was detected at a minimum concentration with 1 ng genomic RNA or with 10 ng 2A RNA transcripts. These results of this test indicate that the RNA transcripts made from clone 2A were infectious following transfection of permissive cultured cells. Example 7 Other Evidence for Infectivity of RNA Transcripts In order to provide a formal proof that infectious dengue virus was produced by cells transfected with clone 3A RNA transcripts, two mutations (G3473--> Tand C3476--> T) that created a new Pst I site at nucleotide 3473 in the dengue genome but did not affect amino acid sequence, were introduced into the full-length dengue clone 2A DNA. The RNA transcripts prepared from this mutant DNA, after complete removal of the DNA template by exhaustic digestion with DNase I, were used for transfection of cells. The transfected cells produced infectious dengue virus. Genomic RNA extracted from the progeny virus was reverse transcribed and the cDNA product was used as a template for PCR to generate a DNA fragment between nucleotides 3193-4536. As shown in Figure 10, Pst I digestion of the PCR DNA product yielded two fragments 280 and 1063 base pairs in length as predicted by the presence of the Pst I cleavage sequence. The control PCR DNA product of virus derived from 2A RNA was insensitive to Pst I digestion. This observation provided evidence that the progeny virus derived from the RNA transcripts of the mutant 2A DNA containing the Pst I site. Example 8 Dengue Virus Recovered from Infectious RNA Transcribed In Vitro The progeny dengue virus was isolated from the lysate of cells transfected with RNA transcripts made from the clone 2A or clone 2A (Pst) template, and compared with the parental wild type virus for their ability to plaque on LLC-MK 2 cell monolayers. Six days after infectious both progeny virus and parental virus produced characteristic dengue plaques of variable sizes. Although the parental virus grown by passages in mosquito cells showed predominantly small plaques, whereas progeny virus mixed but yielded mostly large plaques, suggesting that recovered virus may represent a cloned population. dengue specific proteins produced in progeny virus infected cells and in parental virus infected cells were also compared. As shown in Figure 11, the profile of protein bands including PreM, E, NS1, NS2, and other unassigned dengue protein bands precipitated by dengue hyperimmune ascetic fluid appeared to be indistinguishable for both progeny virus and for parental virus. This result indicates that the recovered dengue virus exhibited the same genotype and phenotype of virus from which the cDNA clone was derived. ENGINEERING VIABLE DENGUE VIRUS BY AMINO ACID SUBSTITUTION AT THE NS1-NS2A CLEAVAGE JUNCTION Example 9 Construction of NS1-NS2A DNA Specifying Amino Acid Substitutions at the Cleavage Site Sequence Intermediate recombinant pSC11-NS1-NS2A DNA constructed for expression of authentic NS1 was used in this study. The dengue virus DNA contained the coding sequence for the 24-amino-acid N-terminal signal and the entire polypeptide sequences of NS1 and NS2A. Initially, this plasmid DNA was modified to facilitate replacement of DNA segments containing mutations. Two silent mutations were introduced into NS1-NS2A DNA (G 3473 → T and C 3476 → A) by oligonucleotide directed mutagenesis. These changes created a new Pstl site at nucleotide (nt) 3476. The recombinant plasmid contained a unique NcoI site at nt 3320 within the dengue NS1 coding sequence. The NS1-NS2A cleavage junction is located at nt 3477. In order to introduce amino acid substitutions at the cleavage site sequence, nucleotide changes were created by oligonucleotide directed mutagenesis. A series of oligonucleotide were synthesized and used as negative strand primers in a polymerase chain reaction (PCR). The positive strand primer was 5' AAGGCTATGCCACGCAAA 3' (SEQ ID NO: 1) which is located upstream of the NcoI site. For example, at cleavage position -2 (Thr 1124 ) oligo GCC CTG GCC GGC CTT CAC CTG TGA TTT GAC CAT (SEQ ID NO: 2) was used to substitute Lys for Thr. In the same manner, oligo GCC CTG GCC GGC CTG CAC CTG TGA TTT GAC CAT (SEQ ID NO: 3) was used to substitute Gln for Thr, and oligo GCC CTG GCC GGC CAG CAC CTG TGA TTT GAC CAT (SEQ ID NO: 4) was used to substitute Leu for Thr. Similarly, oligo TTC TGA TGT GCC CTG TTC GGC CGT CAC CTG TGA (SEQ ID NO: 5) was used to substitute Glu for Gly 1120 at cleavage position + 1, or oligo TTC TGA TGT GCC CTG CCA GGC CGT CAC CTG TGA (SEQ ID NO: 6) was used to substitute Trp for the same Gly at cleavage position + 1. For construction of intermediate recombinant DNA containing the specified mutation, the DNA fragment between the newly created PstI site and the unique NcoI site was replaced with the series of PCR DNA products that were cleaved with NcoI and PstI. Recombinant vaccinia viruses expressing these mutant NS1-NS2A sequences were made according to the procedure previously described (Zhao, B., Prince, G., Horswood, R., Eckels, K., Summers, P., Chanock, R. and Lai, C.J. (1987). Expression of dengue virus structural proteins and nonstructural protein NS1 by a recombinant vaccinia virus. J. Virol. 61:4019-4022). Example 10 Analysis of Cleavage at the NS1-NS2A Junction Confluent CV-1 cells in a 6-well plate were infected with recombinant vaccinia virus at 5 PFU per cell and maintained in minimum essential medium plus 2% fetal calf serum (MEM2). AT 16-20 hours after infection, the medium was removed and placed with methionine-free MEM2 and after 1 hour of incubation, this medium was replaced with 0.7 ml of methionine-free MEM2 containing 100 µci of 35 S-methionine (specify activity > 800 µci/mol). After a two-hour labelling period, cells were lysed in RIPA buffer (1% sodium deoxycholate, 1% Nonidot P-40, 0.1% sodium dodecyl sulfate [SDS]), 0.1 M-Tris-HCl, pH 7.5, 0.15 M NaCl) and the lysate centrifuged to remove cell debris. The clear supernatant of the lysate was used for analysis of dengue virus proteins by immunoprecipitation using dengue hyper immune mouse ascitic fluid (HMAF). The immunoprecipitates were analyzed by electrophoretic separation on an SDS-12% polyacrylamide gel (acrylamide/bis ration-60:1.6). Labelled protein bands were visualized by fluorgraphy. The gel bands corresponding to the uncleaved NS1-NS2A precursor and the cleaved NS1 product were excised and the radio-activity counted in a liquid scintillation counter. To determine the extent of cleavage at the NS1-NS2A junction, the differential immunoprecipitability between NS1-NS2A and NS1 was corrected on the assumption that the total label of the expressed NS1-NS2A was the same in cells infected with each mutant at the same pfu input. Wild-type NS1-NS2A expressed predominantly cleaved NS1 and the NS1-NS2A precursor was not detected. The extent of cleavage for wild-type virus was assigned 100%. Example 11 Construction of Full-Length Dengue Type 4 cDNA Specifying Amino Acid Substitutions at the NS1-NS2A Cleavage Sequence Mutants of NS1-NS2A DNA that showed a range of reduced cleavage at the NS1-NS2A junction as constructed and analyzed in the previous section were selected for construction of dengue virus mutants containing such amino acid substitutions. Four such mutations established earlier were employed: (1) Gly at cleavage positive + 1 Glu, (2) Thr at position -2 to Lys, (3) Thr at position -2 to Gln, and (4) Thr at position -2 to Leu. Mutant psc11-NS1-NS2A DNA was digested with SpeI at nt 3338 and StuI at nt 3616 to isolate 278 at fragment. This DNA fragment containing the mutation was used for replacement of the corresponding wild-type DNA fragment in plasmid p5'-2 which contains the 5'-half of the dengue cDNA sequence. The remaining 3' dengue cDNA fragment between BstEl (nt 5069) and Asp718 at the 3' end was joined to the p5'-2 DNA at the shared restriction enzyme cleavage sites. In this manner, full-length cDNA was created and cloned using the pBR 322 vector. This series of cDNA contained mutated sequences coding for amino acid substitutions at the NS1-NS2A cleavage junction. Example 12 Dengue Virus Type 4 Virus Mutants Exhibiting Suboptimal Cleavage at the NS1-NS2A Junction A major emphasis is now being placed on the application of molecular understanding of dengue virus to the development of safe and effective, attenuated live virus vaccines against dengue disease. Restriction of dengue virus replication should cause attenuation. Modification of a cleavage sequence or viral protease components may result in suboptimal cleavage of the viral polyprotein thus restricting viral growth. The polyprotein NS1-NS2A cleavage site has been chosen as the first target for constructing dengue virus mutants that are growth-restricted because of inefficient cleavage. We have shown that NSI-NS2A cleavage of dengue type 4 virus requires an 8 amino-acid sequence at the C-terminus of NS1 preceding the cleavage junction (Hori, H. and Lai, C.J. (1990). Cleavage of dengue virus NS1-NS2A requires an octapeptide sequence at the C terminus of NS1. J. Virol. 64:4573-4577). Comparison of the 8 amino-acid sequence among flaviviruses revealed an interesting motif in which Ala(-1), Val(-3), Ser(-5), Val(-7), and Met/Leu-8) are strictly conserved, whereas Thr(-2), Gln(-4), and Lys(-6) vary. The effect of amino-acid substitutions of Ala at position -1, Thr at position -2, Val at position -3, and Gly at position + 1 has been analyzed. The result of this analysis is shown in Figure 16. Amino acid substitution at conserved position -1, or -3 yielded a low level of cleavage. Substitutions of amino acids at non-conserved position -2, or + 1 showed no effect or only moderate reduction of cleavage. A panel of amino acid substitutions that produced a range of cleavage efficiencies were selected for incorporation into the full-length cDNA and the in vitro derived RNA transcripts were used for the construction of dengue virus mutants. One dengue virus mutant designated DEN4(Gly 1120 → Glu) that contained Glu substituting for Gly (+1), and three other mutants designated DEN4 (Thr 1124 → Lys), DEN4 (Thr 1124 → Gln), and DEN4 (Thr 1124 → Leu), containing substitutions of Thr(2), were recovered from transfected LLC-MK 2 cells. The growth properties of these mutants were characterized by plaque assay on mosquito C6/36 cells, as described below. Figure 12 shows the result of this assay DEN4 (Gly 1124 → Glu) and the control parent virus. The plaque size of the mutant virus measured approximately 0.1 cm in diameter was greatly reduced compared to the plaque size of 1.1 cm for the parent virus: other dengue virus mutants also showed reduced plaque size suggesting that these viruses exhibited growth restriction in cultured cells. It is likely that the growth restriction resulting from suboptimal cleavage may have a profound effect on viral virulence in the infected host. ENGINEERING VIABLE DENGUE VIRUS 3' NONCODING REGION DELETION MUTANTS Example 13 Construction of dengue cDNA Containing Deletions in the 3' Noncoding Region To facilitate introduction of deletion mutations into the 384-nucleotide 3' non-coding region, the 540nt subfragment of dengue 4 cDNA between the unique BamHI site at nt 10104 and the Asp718 site at the 3' end was initially inserted into the pGEM3 cloning vector. In this recombinant plasmid the unique Apal site at nt 10470 of the dengue 4 sequence is located approximately at the midpoint of the 3' non-coding sequence. Using the conveniently located Apal site two series of deletion mutations were constructed: one series of the constructs contained deletions downstream of the Apal site and the other series of deletions was placed upstream of the Apal site. Oligonucleotide-directed mutagenesis was performed to engineer the first series of deletion mutations ranging from 30 to 90 nucleotides in length. The following oligonucleotide that contain the Apal cleavage site, the appropriate deletions as indicated, followed by the downstream sequence were used as positive strand primers in a PCR. The negative strand primer was oligo GAG CTG GTA CCA GAA CCT GTT GGA TCA A (SEQ ID NO: 10) which contains the 3' dengue sequence followed by the Asp718 cleavage sequence. The full-length dengue 4 cDNA (clone 2A) was used as the template. To introduce these deletion mutations into the dengue 4 sequence, the wild-type virus DNA fragment between the Apal and the Asp718 sites in the pGEM3 recombinant plasmid was replaced with the PCR DNA products which were cleaved with Apal and Asp718. Similarly, oligonucleotide-directed mutagenesis was also performed to engineer the second series of deletion mutations ranging from 61 to 202 nucleotides in length and place upstream of the Apal site. The following oligonucleotide were synthesized and uses as negative strand primers in a PCR. The positive strand primer was oligo GCC TCC ATG GCC ATA TGC TCA GC (SEQ ID NO: IS) which is located between nucleotides 9885-9907 upstream of the BamHI site. The full length dengue 4 cDNA was used as the template. As was described earlier, to introduce these deletion mutations into the dengue 4 sequence the wild-type virus DNA fragment between the BamH1 and the Apal sites in the intermediate pGEM3 plasmid was replaced with the PCR DNA products which were cleaved with BamH1 and Apal. In the last stage of engineering the cDNA template, the BamH1-Asp718 fragments from both series of constructs were isolated and cloned with the remaining dengue 4 sequence. Example 14 RNA Transcription Transfection and Recovery of Virus Dengue virus cDNA clones were linearized by digestion with Asp718 at the 3' end and the linear DNA was used as templates for in vitro transcription by SP6 polymerase. Transfection of cells with RNA transcripts was performed as described (Lai, et al., Proc. Natl. Acad. Sci. USA 88:5139-5143 (1991)). Ten days after transfection, cells were trypsinized and transferred to a 6-well plate and to a chamber slide. Two days later, cells on the slide were tested by immunofluorescence (IFA) for evidence of dengue virus antigens. If IFA showed most cells were infected, the infected cells in the 6-well plate were harvested by removing the medium, scraping the cells, resuspending them in a standard volume of 50% Eagle's minimal essential medium/50% sera, and then freezing. On the other hand, if a small percentage of cells were positive, the cells were trypsinized, diluted in 1:3 in fresh medium, and allowed to grow. The percentage of infected cells were then estimated on a weekly basis, and cell lysates were harvested. Example 15 Plaque Morphology Viruses were characterized for plaque morphology on primate-derived LLC-MK 2 cells or on mosquito C6/36 cells (Bankcroft, et al., Pan Am. Health Organ. Sci. Publ. 375:175-178 (1979); Hoke, et al., Am. J. Trop. Med . Hyg. 43:219-226 (1990)). Parental wild-type virus was passaged once in LLC-MK 2 cells prior to analysis of plaque morphology. Specifically, LLC-MK 2 cells in a 25-cm 2 flask were injected with 0.2 ml of dengue virus to be tested in a serial 10-fold dilutions in medium 199 containing 2% fetal bovine serum. After absorption for 1 hour at 35° C, each monolayer was added with 7 ml of an agarose overlay containing 1x medium 199, 0.3% sodium bicarbonate ½x, Eagle basal medium vitamins and ½x Eagle basal medium amino acids 10% fetal bovine serum, 0.5% agarose. After incubation for six days at 35° C, the flasks were overlaid a second time with 4.0 ml of normal saline containing 0.5% ME agarose and 1:7,500 neutral red. Virus plaques appeared within 24 hours after staining. Plaque morphology on C6/36 cells was carried out to characterize all viruses mutants isolated. For this purpose, confluent C6/36 cells in a 75 - cm 2 flask were inoculated with 0.5 ml of virus serially diluted in MEM plus 2% fetal bovine serum, allowed to absorb at 35°C for 1-2 hours. The infected cells were then added with 20 ml/flask of an agarose overlay (1x Hanks' balanced salt solution, 0.5% lactalbumin hydrolysate, 10% fetal bovine serum, 0.12% sodium bicarbonate, 0.75% Seakem™ GTG agarose). Cultures were incubated for 7 days at 35° C. Cultures were then stained with 5ml of a liquid neutral red stain solution consisting of NaCI 8.0 g/l, KCI 0.4 g/l, glucose 1.0 g/l, Na 2 HCO2, 22.5 mg/l, and neutral red 3.3 mg/l. After 3-5 hours incubation at 35°C the excess stain was removed and the cultures returned to the incubator. Plaques were generally visible following 24-36 hours in the incubator. Example 16 Dengue Type 4 Virus Mutants Containing Deletion in the 3' Noncoding Region The dengue recombinant DNA system now also allows us to construct dengue virus mutants containing deletions in the 5' and 3' noncoding regions of the viral RNA as well as the coding region. Deletion mutants would offer the advantage of being less subject to reversion of phenotype than amino acid substitution mutants. Progress has been made in engineering deletion mutations in the 3' noncoding region of the dengue type 4 virus genome. Like other mosquito-borne flaviviruses, dengue type 4 virus contains stretches of conserved sequences, termed conserved sequence 1 (CS-1) and conserved sequence 2 (CS-2), and a potential terminal stem-and-loop structure in the 3' noncoding region. Deletions ranging from 30-120 nucleotides have been produced in various locations in the 385 nucleotide 3' non-coding sequence of dengue type 4 virus RNA. Mutant RNA transcribed from a cDNA clone lacking sequences in the CS-1 region, but retaining the stem-and-loop structure did not produce progeny virus suggesting that the deletion virus has been recovered from most other deletion constructs (Fig. 13). Plaque assay of these mutants and the wild type virus control was performed on C6/36 cells. The results of this analysis showed that most of these mutants formed plaques that were reduced in size, ranging from 1.0 to 0.3 cm, dependent on the location and the extent of deletion (Fig. 14 and 15). The altered plaque morphology indicates that these deletion mutants exhibited a growth-restriction phenotype. This panel of deletion mutants may exhibit other interesting and potentially important properties such as reduced virulence for experimental animals and humans. Example 17 Full-Length Chimeric TBEV/DEN4 cDNA and Production of Chimeric Viruses Previously, subgenomic cDNA fragments of the TBEV (strain Sofjin) were cloned and the nucleotide sequence determined (see Pletnev et al., Virology (1990), supra .). Plasmids pGEM2-CME containing nucleotides (nt) 76-1977 and pGEM2-ENS1 containing nucleotides 966-3672 of the TBEV sequence were provided by Dr. E. Yu. Dobricova (Novosibirsk Institute of Bioorganic Chemistry, Russia) from plasmids p10, p4, p18, p2, p11 (Pletnev et al., supra ) by joining the TBEV segments at shared restriction enzyme sites. Plasmids DEN4 p5'-2 and p5'-2(ΔPstI,XhoI), and a derivative p5'-2 (ΔPstI,Xhol,ΔHindIII) (Bray et al ., Proc. Natl. Acad . Sci. USA, (1991) supra ) were used as the recombinant cDNA construct for the substitution of one or more TBEV genes in place of the corresponding DEN4 genes. Synthetic oligonucleotides (oligos) were used as negative or positive-strand primers to generate double-stranded DNAs by polymerase chain reaction (PCR). Restriction enzyme cleavage sequences were introduced at or near the TBEV and DEN4 intergenic junctions by PCR-directed site-directed mutagenesis (Figure 17). Seven intermediate chimeric plasmids containing the indicated TBEV genes were initially constructed and stable full-length TBEV/DEN4 cDNAs were identified after transforming E. coli strain BD1528 (described by Lai et al., (1991) supra ). The sequences surrounding the junctions between TBEV and DEN4 genes in each plasmid were confirmed by nucleic acid sequencing using techniques well known in the art of molecular biology. All chimeric plasmids contained the SP6 promoter positioned upstream of a transcription initiating G followed by an A residue representing the first DEN4 nucleotide. Prior to in vitro transcription, plasmids were linearized at the unique Asp718 cleavage site immediately following the 3' end of the DEN4 sequence (using techniques provided by Lai et al., Proc. Natl. Acad. Sci. USA (1991) supra.). Recovery of chimeric viruses and subsequent study of their properties was conducted in a BL-3 containment facility. Transcription reactions were performed essentially as described by Lai et al., Proc. Nal. Acad Sci . USA (1991) supra. Prior to transfection, the transcription mixture was treated with 40 u/ml DNase I at 37°C for 15 min. The RNA transcripts were then used to transfect semiconfluent simian LLC-MK 2 cells in the presence of (i) Lipofectin™ (Bethesda Research Laboratories, Inc.) as described previously by Lai et al. supra or (ii) N-[1-(2, 3-dioleoyloxy)propyl]-N, N, N-trimethylammoniummethylsulfate (DOTAP) (Boehringer Manneheim). Under DOTAP transfection conditions, the transfection mixture contained RNA transcripts (2 µg) in 40 µl of 0.02 mM Hepes buffer, pH 7.05, and 12 µl DOTAP in 30 µl Hepes buffer. After incubation at room temperature for 10 min., 2 ml medium 199 containing 10% fetal bovine serum (FBS) was added; 0.5 ml of the mixture was distributed to subconfluent cells in 4 wells of a 24-well plate. Ten days later, cells were trypsinized and transferred to a 6-well plate and to chamber slides for an additional 2 days of incubation in growth medium. Cells on the slides were tested by immunofluorescence assay (IFA) to detect the presence of DEN4 or TBEV antigens using a 1:100 dilution of DEN4-specific hyperimmune mouse ascitic fluid (HMAF), TBEV-specific HMAF, or TBEV-specific rabbit serum (kindly supplied by Dr. A. S. Karavanov, Institute of Poliomyelitis, Moscow, Russia). Fluorescein-conjugated anti-mouse or anti-rabbit serum (Kirkegaard-Perry, Gatithersberg, Maryland) was used at the same dilution. When IFA indicated that 50-80% of cells were infected, cells in the 6-well plate were trypsinized, mixed with 2-fold excess uninfected cells, and grown in a T 75 flask for 6 days. The infected cells were then harvested together with the medium, mixed with an equal volume of fetal bovine serum, and frozen as a cell suspension. The thawed cell lysate was used as the source of progeny chimeric virus. Only TBE(ME)/DEN4 and TBE(CME)/DEN4 were identified by transfection experiments using Lipofectin™ or DOTAP. Progeny virus was identified by immunofluorescent assay. To verify the genomic structure of progeny chimeric TBE(ME)/DEN4 virus designated vTBEV(ME)/DEN4, the total cellular RNA was isolated from infected LLC-MK 2 cells by phenol extraction using techniques provided by Mackow et al., Virology 159:217-228, 1987. Oligo 5'GCTCCGGGGTGTAAGTCCATT3' (SEQ ID NO: 16) complementary to nucleotide (nt) positions 5090-5110 (see Mackow, et al., Virology 159:217-228, 1987) of the DENS sequence was used as a primer for reverse transcription. Single-strand cDNA was used as a template and oligo 5'GACCGACAAGGACAGTTCCAAATCGGA3' (SEQ ID NO: 17) at nt positions 18-44 of the DEN4 sequence served as the positive strand primer, and oligo 5'CTTTTTGGAACCATTGGTGACT3' (SEQ ID NO: 18)at nt positions 2130-2152 of the TBEV sequence as the negative strand primer for PCR. For nucleotide positions relating to dengue virus see Zhao, et al., Virology 155:77-88, 1986 and for nucleotide positions relating to tick-borne encephalitis virus see Pletnev et al., Virology, (1990), supra. The PCR DNA product of 2118 base-pairs (bp) was digested with PstI, BgIII or EcoRI to verify the presence of restriction enzyme sites. Similarly, oligo 5'GTCCGGCCGTCACCTGTGATT3' (SEQ ID NO: 19) and oligo 5'TCACGGTGCACACATTTGGAAAAC3' (SEQ ID NO: 20) (complementary to DEN4 genome at 3459-3480 nt and complementary to the negative strand of TBEV genome at 967-985 nt, respectively) were also used in PCR. The PCR product was digested with EcoRl, PstI, BamHI, XhoI or SphI to verify the DNA sequence. The PCR DNA product (2118 bp) from the vTBE(ME)/DEN4 genome was digested with HindIII and BamHI restriction endonuclease enzymes. The HindIII-BamHI-subfragment (1922 bp) was purified by PAGE and cloned into the pGEM3 vector at the complementary restriction sites. The sequence of the junction between the C gene of DEN4 (300-398 nt) and the preM gene of TBEV (418-460 nt) was confirmed by nucleic acid sequencing Oligonucleotides used for the construction of TBE/DEN4 chimeric DNA included: TBEV-specific oligos: DEN4-specific oligos: ID SEQ NO:23 was used to prepare the DEN4 5' noncoding region/TBEV capsid junction to form a Bg1 II/Bam H1 site. ID SEQ NOS: 30 and 24 were used to prepare the DEN4 C/TBEV PreM junction to form Pst1/Pst1 sites. SEQ ID NOS: 28 and 21 were used to prepare the DEN4 Pre M/TBEV E junction to form Bam H1/Bg1 II sites. The DEN4 NS1/TBEV NS1 junction was prepared using SEQ ID NO: 26 to form Xho1/Xho1 sites. The TBEV E/DEN4 NS1 junction was prepared using SEQ ID NO:25 to form Xbo1/Xbo1 sites. The TBEV C/DEN4 Pre M junction was prepared using SEQ ID NOS: 27 and 31 to generate PST1/Pst1 sites and the TBEV NS1/DEN4 NS2A junction was prepared using SEQ ID NOS: 22 and 29 to generate Sph1/Sph1 sites. The + and - signs associated with the sequences identify upstream and downstream primers respectively. Example 18 Protein Analysis of Chimeric Virions To analyze proteins produced by DEN4 v2A(XhoI), the full length dengue 4 virus cDNA construct, or vTBE(ME)/DEN4, confluent LLC-MK 2 cells in a 6-well plate were infected with the respective virus at an MO] of 1.0. Six days after infection, cells were labeled with [ 35 S]-methionine (60 µCi per well, specific activity 600 Ci/mmole) in methionine-free MEM (Eagle's minimal essential medium) for 4 hr as described by Lai et al., Proc. Natl. Acad. Sci. USA, (1991), supra. Lysates were immunoprecipitated with 1) TBEV-specific HMAF; 2) DEN4-specific HMAF; 3) rabbit serum raised against TBEV E, or 4) rabbit antiserum specific to DEN4 preM, E, NS3, or NS5 protein. Immunoprecipitates were analyzed by PAGE under denaturation conditions using the techniques described by Laemmli, supra. ANALYSIS OF PROTEIN SYNTHESIS OVER TIME LLC-MK 2 or C6/36 cell monolayers in T 25 flasks were infected with vTBE(ME)/DEN4 or DEN4 (v2A(Xhol)) at an MOI of 1. After adsorption at 37°C for 1 hr, the virus inoculum was removed and fresh medium was added to the cells. For analysis of protein synthesis, infected cells at various times (0, 4, 8, 24 and 48 hr) following virus adsorption were incubated with methionine-free MEM for 20 min and labeled with [ 35 S]-methionine in the same medium for 2 hr. The lysate of labeled cells was precipitated with DEN4-specific HMAF and analyzed by PAGE and autoradiography. Example 19 Growth Analysis of Chimeric Construct Parental DEN4 and its chimeric viruses were characterized by plaque assay on simian LLC-MK 2 and mosquito C6/36 cells. The techniques for plaque assay are described by Bancroft, et al., supra. Growth analysis as provided in Figure 19 was performed by quantitating the amount of virus present in DEN4 or TBE(ME)/DEN4 infected LLC-MK 2 and C6/36 cultures at various days postinfection. Following infection and virus adsorption, cultures were harvested and each culture was sampled by plaque assay. The changing virus titer in Log (pfu/ml) is presented as growth curves over time. Example 20 Analysis of Viral RNA Synthesis For analysis of viral RNA synthesis, infected LLC-MK 2 or C6/36 cells (approximately 10 6 cells) were collected at various times (0, 4, 8, 24, 48 hr) following virus adsorption, rinsed with 0.5 ml phosphate-buffered saline (PBS), pH 7.4, and then lysed in buffer containing 0.3M NaAc, pH 5.2, 5 mM EDTA and 1% SDS. Total RNA was isolated from the cell lysate and the medium by phenol extraction using techniques provided Maniatis, et al., supra. RNA samples were denatured by incubation in 6xSSC (1xSSC is 0.15M NaCI and 0.015M sodium citrate) containing 7.5% (wt/vol) formaldehyde at 60°C for 15 min and bound to a nitrocellulose filter BA85 (Schleicher and Schuell). Filters were baked at 80°C for 1 hr and prehybridized in a solution containing 6xSSC, 3xDenhardt solution, 25 mM Na 2 HPO 4 (pH 6.5), 0.1% SDS and salmon sperm DNA 25 µg/ml at 65°C for 1 hr. Hybridization was continued overnight at 65°C in the same solution containing a nick-translated, [ 32 P]-labeled pTBE(ME)/DEN4 DNA probe (50 ng/ml, specific activity 3,4x10 8 cpm/µg). After hybridization, the filters were washed five times in 0.1xSSC containing 0.1% SDS at 65°C, dried and exposed to an X-ray film at 70°C. The radioactivity of [ 32 P]DNA hybridized to RNA was also measured in a liquid scintillation counter. RNA transcripts made in vitro from pTBE(ME)/DEN4 were used as a positive control. Example 21 Testing Protective Efficacy of the Vaccine Preparation and Neurovirulence of Chimeric Viruses VTBE(ME)/DEN4 and parental dengue virus were analyzed for neurovirulence by inoculating mice by intracerebral (IC), intradermal (ID) or intraperitoneal (IP) routes. Three-day-old suckling BALB/c mice were injected IC with a dose of 10 2 pfu of virus in 0.02 ml of MEM/0.25% human serum albumin. Six-week-old BALB/c female mice were (i) inoculated IC with a dose of 10 3 pfu of virus, diluted as above, in a volume of 0.03 ml, or (ii) inoculated ID or IP with 10 3 pfu of a virus, diluted in a volume 0.10 ml. Mice were observed for 21 days for symptoms of encephalitis or death, and surviving adult mice were bled 20 days after infection to evaluate antibody responses to the inoculated virus. Surviving mice were challenged IP at 21 days with 10 3 LD 50 TBEV (strain Sofjin) in a BL-4 containment facility. Example 22 Construction and Sequencing of Chimeric Viral Mutants Mutations were introduced into the TBEV(ME)/DEN4 construct using site-directed mutagenesis according to the standard oligo, PCR-drive procedure well known by those with skill in molecular biology (see oligo sequences below). The resulting constructs were transfected into LLC-MK 2 cells using techniques disclosed in Example 17. Progeny virus that was recovered from the mutagenized constructs was assessed for alterations in plaque morphology using the techniques provided in Example 19. Strategic mutations were introduced into the full-length TBE(ME)/DEN4 cDNA construct in order to systematically achieve modifications in predetermined regions of the chimeric viral genome. Six mutant chimeras were recovered from the transfected LLC-MK 2 cells and the virus progeny were analyzed for their ability to induce mouse neurovirulence. Other viral growth properties were additionally assessed in cultured cells. As provided in Figure 23, four virus mutants that contained an ablated glycosylation site in the PreM, E, or NS1 glycoprotein were predicted to exhibit a glycosylation defect at the site indicated. Mutant *PreM/M - contained a defective cleavage sequence at the PreM/M intergenic junction. Mutant *E contained two amino acid substitutions these substitutions were chosen because these amino acids are found only in mosquito-borne flaviviruses. Mutations were introduced into the TBE(ME)/DEN4 construct by site-directed mutagenesis according to the standard oligo, PCR-driven procedure. Briefly, a positive strand oligo and its complementary negative strand oligo, each containing the mutation sequence and a unique restriction enzyme site, were used together with the oligo of the respective downstream or upstream primer pair in a PCR. Both PCP products were joined at the shared unique restriction enzyme site. In this manner, the resulting DNA fragment containing the mutated sequence was made to substitute for the corresponding wildtype DNA fragment in full-length TBE(ME)/DEN4 cDNA. Following is a list of specific mutant chimeric DNA's and oligonucleotides used in the construction. (1) Mutant PreM/M-GIc: (2) Mutant E-Glc: (3) Mutant NS1(1)-Glc: (4) Mutant NS1(2)-Glc: (5) Mutant PreM/M: (6) Mutant E: Also, the upstream primer was (SEQ ID NO: 44) and the downstream primer was (SEQ ID NO: 45) Example 23 Analysis of Neurovirulence in Mice In addition the mutated chimeric virus preparations were inoculated IC into mice and compared with DEN4, TBE(ME)/DEN4 and TBE(CME)/DEN4 using the techniques provided in Example 21. The lethal dose required to kill one half of the animals tested was compared for each of the mutated chimeric viruses. Six-week old BALB/c mice in groups of 8 were inoculated intracerebrally with various mutant chimeras, TBE(ME)/DEN4, TBE(CME)/DEN4, or DEN4 at a dose of 1000, 100, 10, 1, and 0.1 pfu. Infected mice were observed for signs of encephalitis and death for 31 days. The 50% lethaTdose (LD50) was calculated for each virus and the value was used as the basis for mouse neurovirulence. As can be seen in Figure 24, the LD50 of the parental TBE(ME)/DEN4 virus was approximately 10 pfu. This level of LD50 was also observed for several mutant viruses. At least two mutants, *NS1(1)-Glc - and *PreM/M - , showed an LD50 greater than 100 pfu indicating that these mutants exhibited a greater than 100-fold reduction of mouse neurovirulence. This finding indicates that the NS1(1)-Glc - or PreM/M - mutation in the chimeric viral genome can each confer attenuation of mouse neurovirulence. These mutant viruses separately or together should be valuable for further evaluation of their usefulness as candidate vaccines. Example 24 Testing Optimized Chimeric Constructs in Primates It has been established that subhuman primates, but not other animals, are readily infected with dengue virus by the peripheral route (Simmons, et al., Philipp. J . Sci. 44:1-247, 1931 and Rosen, Am. J. Trop. Med . Hyg. 7:406-410, 1958). Infection of monkeys represents the closest experimental system to dengue virus infection of humans. The response of rhesus monkeys to dengue infection is similar to that of humans in that there is a four to six day viremia, although lower primates do not develop clinical dengue symptoms. The objectives of dengue or other flavivirus studies in monkeys are: (1) to evaluate the immunogenicity of various candidate vaccines; (2) to evaluate the infectivity and virulence (attenuation phenotype) of candidate live dengue or chimeric dengue virus vaccines as measured by the duration of viremia in days and the peak virus titer in pfu/ml; and (3) to evaluate the protective efficacy of the above-mentioned vaccines against challenge by homotypic dengue virus or other flavivirus of the same chimeric virus specificity. (1) Inoculation: Each rhesus monkey is inoculated with a total of 3x10 6 pfu of virus diluted in Eagle's minimal essential medium/0.25% human serum albumin. Normally, two subcutaneous doses are given to anesthetized animals. (2) Blood collection: Following inoculation of dengue virus or chimeric dengue virus, blood sample of 3.0 ml are taken daily for two weeks and 5.0 ml at 3 weeks, 4 weeks, 6 weeks, and 8 weeks. (3) Challenge dengue virus or other flavivirus: Where virus challenge is deemed appropriate to evaluate the protective efficacy, monkeys are inoculated with nonattenuated virus at 10 5 pfu/dose in a 0.5 ml volume subcutaneously in the upper arm area. (4) Laboratory assays: Serum samples are used to determine: (a) the viremic duration by direct viral plaquing assay; (b) the titer of dengue or other flavivirus specific antibodies by radio-immunoprecipitation and ELISA; and (c) the titer of neutralization antibodies by plaque reduction neutralization test, all tests well known to those skilled in the art of vaccine development. While particular embodiments of the invention have been described in detail, it will be apparent to those skilled in the art that these embodiments are exemplary rather than limiting, and the true scope of the invention is to be interpreted in view of the following claims. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANTS: THE UNITED STATES OF AMERICA AS REPRESENTED BY THE SECRETARY, DEPARTMENT OF HEALTH AND HUMAN SERVICES Lai, Ching-Juh (United States only) Bray, Michael (United States only) Men, Ruhe (United States only) Pethel, Michele (United States only) (ii) TITLE OF INVENTION: CHIMERIC AND/OR GROWTH RESTRICTED FLAVIVIRUSES (iii) NUMBER OF SEQUENCES: 45 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: KNOBBE, MARTENS, OLSON AND BEAR (B) STREET: 620 NEWPORT CENTER DRIVE 16TH FLOOR (C) CITY: NEWPORT BEACH (D) STATE: CA (E) COUNTRY: USA (F) ZIP: 92660 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: PatentIn Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: ALTMAN, DANIEL E (B) REGISTRATION NUMBER: 34,115 (C) REFERENCE/DOCKET NUMBER: NIH024.024HPC (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: 714-760-0404 (B) TELEFAX: 714-760-9502 (2) INFORMATION FOR SEQ ID NO:1: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 18 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1: (2) INFORMATION FOR SEQ ID NO:2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 33 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 33 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: (2) INFORMATION FOR SEQ ID NO:4: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 33 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4: (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 33 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5: (2) INFORMATION FOR SEQ ID NO:6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 33 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6: (2) INFORMATION FOR SEQ ID NO:7: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7: (2) INFORMATION FOR SEQ ID NO:8: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 39 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8: (2) INFORMATION FOR SEQ ID NO:9: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 39 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9: (2) INFORMATION FOR SEQ ID NO:10: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 28 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10: (2) INFORMATION FOR SEQ ID NO:11: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE 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MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15: (2) INFORMATION FOR SEQ ID NO:16: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16: (2) INFORMATION FOR SEQ ID NO:17: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 27 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:17: (2) INFORMATION FOR SEQ ID NO:18: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:18: (2) INFORMATION FOR SEQ ID NO:19: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:19: (2) INFORMATION FOR SEQ ID NO:20: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:20: (2) INFORMATION FOR SEQ ID NO:21: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:21: (2) INFORMATION FOR SEQ ID NO:22: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 37 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:22: (2) INFORMATION FOR SEQ ID NO:23: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 43 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:23: (2) INFORMATION FOR SEQ ID NO:24: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:24: (2) INFORMATION FOR SEQ ID NO:25: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 52 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:25: (2) INFORMATION FOR SEQ ID NO:26: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 41 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:26: (2) INFORMATION FOR SEQ ID NO:27: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 38 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:27: (2) INFORMATION FOR SEQ ID NO:28: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:28: (2) INFORMATION FOR SEQ ID NO:29: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:29: (2) INFORMATION FOR SEQ ID NO:30: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 34 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:30: (2) INFORMATION FOR SEQ ID NO:31: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 42 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:31: (2) INFORMATION FOR SEQ ID NO:32: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 46 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:32: (2) INFORMATION FOR SEQ ID NO:33: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 46 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:33: (2) INFORMATION FOR SEQ ID NO:34: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 43 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:34: (2) INFORMATION FOR SEQ ID NO:35: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 43 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:35: (2) INFORMATION FOR SEQ ID NO:36: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 43 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:36: (2) INFORMATION FOR SEQ ID NO:37: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 43 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID 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HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:41: (2) INFORMATION FOR SEQ ID NO:42: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 51 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:42: (2) INFORMATION FOR SEQ ID NO:43: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 45 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:43: (2) INFORMATION FOR SEQ ID NO:44: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 27 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:44: (2) INFORMATION FOR SEQ ID NO:45: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 49 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:45:

Claims: A nucleic acid molecule comprising a region of nucleic acid operably encoding premembrane protein and envelope protein from tick-borne encephalitis virus (TBEV) operably linked to regions of nucleic acid encoding capsid protein and non-structural proteins from dengue virus. The nucleic acid molecule of claim 1, which is a recombinant DNA construct or an RNA segment. The nucleic acid molecule of claim 1 or 2, wherein said dengue virus is dengue type 4 virus. The nucleic acid molecule according to any one of claims 1 to 3, wherein said molecule contains at least one mutation in said nucleic acid. The nucleic acid molecule according to claim 4, wherein said mutation is one or more mutations that reduce glycosylation of premembrane protein, envelope protein or NS1(1) protein, one or more mutations that reduce cleavage of premembrane protein to membrane protein, one or more substitutions at a site encoding glycine, which site is at position +1 following polyprotein NS1-NS2A cleavage site, or one or more mutations in a sequence encoding one or more of eight amino acids at a C terminus cleavage site of NS1. A chimeric virus having a genome, said genome comprising a nucleic acid molecule according to any one of claims 1 to 5. A host cell stably transformed with the recombinant DNA construct according to claim 6 in a manner allowing expression of said DNA construct. The host cell according to claim 7, wherein said host cell is a prokaryotic host cell. A vaccine against tick-borne encephalitis comprising the chimeric virus of claim 6 and a pharmacologically acceptable carrier, wherein said vaccine is capable of generating a protective immune response in a vertebrate. A process for the preparation of the nucleic acid molecule of any one of claims 1 to 5 which comprises assembling the respective nucleic acid regions and, optionally, introducing the desired mutation(s) according to techniques known in the art. A process for the preparation of the vaccine of claim 9 which comprises combining the virus with a pharmacologically acceptable carrier.

Inventors: BRAY MICHAEL, CHANOCK ROBERT M, LAI CHING-JUH, MEN RUHE, PLETNEV ALEXANDER G, BRAY, MICHAEL, CHANOCK, ROBERT, M., LAI, CHING-JUH, MEN, RUHE, PLETNEV, ALEXANDER, G.
Applicants: US HEALTH, THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY, DEPARTMENT OF HEALTH AND HUMAN SERVICES
Application year: 1992
Main classification: C12N15
IPC codes: A61K39, C07K14, C12N15, C12N7, C12N1, C12R1, A61K31, A61P31