NUCLEIC ACID VACCINES FOR PREVENTION OF FLAVIVIRUS INFECTION

Abstract: The invention encompasses nucleic acid molecules containing transcription units which encode the flavivirus M and E protein antigens. The flaviviruses include Japanese encephalitis virus, dengue, yellow fever virus and St. Louis encephalitis virus. The nucleic acids function to provide the M and E protein antigens when the nucleic acid resides in an appropriate host cell, especially when the host cell is the cell of a subject. The invention also encompasses a vaccine whose active agent is the nucleic acid. The invention further encompasses the cultured host cells when they contain within them nucleic acid molecules containing the transcription units. The invention in addition encompasses a method of immunizing a subject against flavivirus infection by administering to the subject an effective amount of a vaccine containing a nucleic acid molecule containing the transcription unit of the invention.

Description: NUCLEIC ACID VACCINES FOR PREVENTION OF FLAVIVIRUS INFECTION Field of the Invention This invention relates to novel vaccines for flaviviruses. In particular, the vaccines are recombinant nucleic acids which contain genes for structural proteins of flaviviruses, such as Japanese encephalitis virus (JEV). These vaccines serve as a transcriptional unit for the biosynthesis of the virus protein antigens when administered in vivo. Background of the Invention Flaviviruses are members of the genus Fiavivirus, which is classified within the family Flaviviridae. The flaviviruses are largely pathogenic to humans and other mammals. Flaviviruses that inflict disease on humans include yellow fever virus. JE\\ dengue virus (including the four serotypes dengue- 1, dengue-2, dengue-3 and dengue- 4), tick-borne encephalitis virus, St. Louis encephalitis virus (SLEV), and others Altogether there are about 70 species currently identified (Kuno et al., J. of Virol 72. 73-83 (1998)). The flaviviruses generally contain three structural proteins: M, the matrix or membrane protein. E, the envelope protein, and C, the capsid protein. (Monath, in "Virology" (Fields, ed.). Raven Press, New York, 1990, pp. 763-814: Heinz and Roehrig. in "Immunochemistry of Viruses II: The Basis for Serodiagnosis and Vaccines" (van Regenmortel and Neurath, eds.), Elsevier, Amsterdam, 1990, pp 289- 305). M has a molecular weight (MW) of about 7-8 kDa; and E has a MW of about 55-60 kDa. M is synthesized as a larger precursor termed prM. The additional portion of prM is processed in the host cell to form M prior to secretion of mature virions M and E are found in the membrane or envelope of the fiavivirus particle, and so have long been considered to constitute important imrnunogenic components of the viruses. The flaviviruses are RNA viruses whose single stranded RNA has a length, among the various species, of about 10 kb. The C protein, whose MW is 12-1 kDa. complexes with the RNA to form a nucleocapsid complex Several nonstructural proteins are also encoded in the RNA genome; they are termed NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5. The genome is translated within the host cell as a polyprotein, then processed co- or post-translationally into the individual gene products by viral- or host-specific proteases (Figure 1). The nucleotide sequences of the genomes of several flaviviruses are known, as summarized in U. S. Patent 5,494,671. That for JEV is provided by Sumiyoshi et al (Virology 161 :497-510 (1987)) and Hashimoto et al. (Virus Genes 1, 305-317 (1988)) The nucleotide sequences of the virulent strain SA-14 of JEV and the attenuated strain SA- 14- 14-2 used as a vaccine in the People's Republic of China are compared in the work of Nitayaphan et al (Virology 177 541-552 (1990)) Nucleotide sequences encoding the structural proteins of other fiavivirus species are also known In many cases the sequences for the complete genomes have been reported The sequences av ailable include dengue type 1 virus (Mason et al , Virology 161 262-267 ( 1987)), dengue type 2 virus (Deubel et al , Virology 155.365-377 (1986), Gruenberg et al J Gen Virol 69, 1391 -1398 ( 1988), Hahn et al Virology 162. 167-180 ( 1988)), dengue type 3 virus (Osatomi et al , Virus Genes 2 99- 108 (1988)). dengue type 4 virus (Mackow et al., Virology 159 217-228 ( 1987), Zhao et al Virology 155, 77-88 ( 1986)). and yeilow fever virus ( TV)(Rice et al , Science 229 726-733 (1985)) Many flav iviruses including JEV are transmitted to humans and other host animals by mosquitoes They therefore occur over widespread areas, and their transmission is not easily interrupted or prevented JEV affects adults and children, and there is a high mortality rate among infants, children, and the elderly, in areas of tropical and subtropical Asia (Tsai et al., in "Vaccines" (Plotkin, ed.) W B Saunders. Philadelphia. Pa, 1994, pp 671 -713) Among survivors, there are serious neurological consequences, related to the symptoms of encephalitis, that persist after infection In more developed countries of this region such as Japan, the Republic of China, and Korea, JEV has been largely controlled by use of a vaccine of inactivated JEV Nevertheless, it is still prevalent in other countries of the region Dengue virus disease is also mosquito-borne, occurring globally in regions with tropical and sub-tropical climates. Symptoms include fever, rash, severe headache and joint pain, but mortality from dengue is low. Epidemics of dengue virus are sufficiently frequent and widespread that the disease represents a major public health problem Nevertheless, safe and effective vaccines to protect against dengue are not available, despite decades of effort. There thus is a strong need for a vaccine against dengue Yellow fever is prevalent in tropical regions of South America and sub-Saharan Africa, and is transmitted by mosquitos. Infection leads to fever, chills, severe headache and other pains, anorexia, nausea and vomiting, with the emergence of jaundice A live virus vaccine, 17D, grown in infected chicken embryos, is considered safe and effective Nevertheless, there remains a need for a \ accine that avoids the necessity of administering live virus, with its attendant development of mild symptoms and viremia The vaccines available for use against JEV include live virus inactivated b\ such methods as formalin treatment as well as attenuated virus (Tsai et al ) Whole virus vaccines, although effective, do have certain problems and/or disadvantages The viruses are cultivated in mouse brain or in cell culture using mammalian cells as the host Such culture methods are cumbersome and expensive Furthermore, there is the attendant risk of incorporating antigens from the host cells, i e , the brain or other host. into the final vaccine product, potentially leading to unintended and undesired allergic responses in the vaccine recipients There is also the risk of inadvertent infeciion among workers involved in vaccine production Finally, there is the risk that the virus may not be fully or completely inactivated or attenuated, and thus, the vaccine may actually cause disease A recombinant fiavivirus which is a chimera between two flaviviruses is disclosed in WO 93/06214 The chimera is a construct fusing non-structural proteins from one "type", or serotype, of dengue viruses or a fiavivirus, with structural proteins from a different "type", or serotype, of dengue virus or another fiavivirus The second fiavivirus av be JEV Several recombinant subunit and viral vaccines have been devised in recent years. U. S. Patent No. 4,810,492 describes the production of the E glycoprotein of JEV for use as the antigen in a vaccine. The corresponding DNA is cloned into an expression system in order to express the antigen protein in a suitable host cell such as E. coli, yeast, or a higher organism cell culture. U. S. Patent No. 5,229,293 discloses recombinant baculovirus harboring the gene for JEV E protein. The virus is used to infect insect cells in culture such that the E protein is produced and recovered for use as a vaccine U. S. Patent No. 5,021 ,347 discloses a recombinant vaccinia virus into whose genome the gene for JEV E protein has been incorporated The live recombinant vaccinia virus is used as the vaccine to immunize against JEV Recombinant vaccinia and baculoviruses in which the viruses incorporate a gene for a C-terminal truncation of the E protein of dengue type 2, dengue type 4, and JEV are disclosed in U S. Patent 5,494,671 U. S. Patent 5,514,375 discloses various recombinant vaccinia viruses which express portions of the JEV open reading frame extending from prM to NS2B These pox viruses induced formation of extracellular particles that contain the processed M protein and the E protein Two recombiπants encoding these JEV proteins produced high titers of neutralizing and hemagglutinin-inhibiting antibodies, and protective immunity, in mice. The extent of these effects was greater after two immunization treatments than after only one. Recombinant vaccinia virus containing genes for the M and E proteins of JEV conferred protective immunity when administered to mice (Konishi et al., Virology 180: 401 -410 (1991)). HeLa cells infected with recombinant vaccinia virus bearing genes for prM and E from JEV were shown to produce subviral panicles (Konishi et al., Virology 188: 714-720 (1992)) Dmitriev et al report immunization of mice with a recombinant vaccinia virus encoding structural and certain nonstructural proteins from tick-borne encephalitis virus (J. Biotechnol 44.97-103 (1996)). Recombinant virus vectors have also been prepared to serve as virus vaccines for dengue fever Zhao et al. (J. Virol 61, 4019-4022 (1987)) prepared recombinant vaccinia virus bearing structural proteins and NS 1 from dengue type 4 and achieved express i on after infecting mammalian cells with the recombinant. Similar expression was obtained using recombinant baculovirus infecting target insect cells (Zhang et al. J . Virol . 62, 3027-3031(1988)). Bray et al. (J. Virol. 63, 2853-2856 (1989)) also report a recombinant vaccinia dengue vaccine based on the E protein gene that confers 5 protective immunity on mice when challenged to develop dengue encephalitis. Falgout et al . (J . Virol 63, 1852-1860 (1989)) and Falgout et al. J. Virol. 64, 4356-4363 (1990) report similar results. Zhang et al. (J. Virol 62, 3027-3031 (1988)) showed that recombinant baculovirus encoding dengue E and NS1 proteins likewise can protect mice against dengue encephalitis when challenged. Other combinations in which structural and nonstructural genes are incorporated into recombinant virus vaccines fail to produce significant immunity (Bray et al. J. Virol. 63, 2853-2856 (1989)) Also , monkeys failed to develop folly protective immunity to dengue virus challenge when immunized with recombinant baculovirus expressing the E protein (Lai et al ( 1990) pp 1 19-124 in F . Brown, R. M. Chancock, H. S. Ginsberg and R . Lerner (eds ) "Vaccines 90 . Modern approaches to new vaccines including prevention of AIDS" , Cold Spring Harbor Laboratory, Cold Spring Harbor, NY). Immunization using recombinant DNA preparations has been reported for St Louis encephalitis virus (SLEV) and dengue-2 virus using weaniing mice as the model (Phillpotts et al. Arch Virol. 141 . 743-749 (1996). Kochel et al . . Vaccine 15 547-552 ( 1997)) Plasmid DNA encoding the prM and E genes of SLEV provided partial protection against SLEV challenge with a single or double dose of DNA immunization In these experiments control mice exhibited about 25% survival , and no protective antibody was detected in the DNA immunized mice (Phillpotts et al . ) . In mice that rece i ved three intradermal injections of recombinant dengue-2 plasmid DNA containing prM , 100% developed anti-dengue-2 neutralizing antibodies, and 92% of those rece i ving the corresponding E gene likewise developed neutralizing antibodies (Kochel et al ) Challenge experiments using a two-dose schedule, however , failed to protect mice against lethal dengue-2 virus challenge . The vaccines developed to date for immunizing against JEV have a number of disadvantages and problems attending their use. Inactivated virus vaccine is costly and O 99/63 inconvenient to prepare. In addition, it carries the risk of allergic reaction originating from proteins of the host used in preparing the virus. Furthermore, it presents considerable risk to the workers employed in their production. Candidate attenuated JEV vaccines are undergoing clinical trials but as of 1996 have not found wide acceptance outside of the People's Republic of China (Hennessy et al. Lancet 347. 1583-1586 (1996)). Recombinant vaccines based on the biosynthetic expression of only certain of the proteins of the JEV genome appear not to induce high antibody titers, and, as with the whole virus preparations, carry the risk of adverse allergic reaction to antigens from the host organism, or to the vaccinia virus vector, as the case may be. Similar problems attend the preparation of vaccines against YFV Vaccine development against dengue is less advanced, and such virus -based or recombinant protein-based vaccines face similar problems as those just alluded to There is therefore a need for vaccines directed against flaviviruses such as yellow fever, dengue, JEV, and SLEV which are inexpensive to prepare, present little risk to workers involved in their manufacture, carry minimal risk of adverse immunological reactions due to impurities or adventitious immunogenic components, and are highly effective in eliciting neutralizing antibodies and protective immunity There is furthermore a need for a vaccine against JEV and related flaviviruses that minimizes the number of immunizing doses required. Summary of the Invention The present invention provides a nucleic acid molecule which contains a transcriptional unit (TU) for an immunogenic fiavivirus antigen. The TU directs a host cell, after being incorporated within the cell, to synthesize the antigen. In an important aspect of the invention, the fiavivirus is either yellow fever virus (YFV), dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 virus, St. Louis encephalitis virus (SLEV), or Japanese encephalitis virus (JEV) In important embodiments of the present invention, the antigen may be the fiavivirus M protein, the E protein, or both In particular, when the TU is for both the M and E proteins, the host cell secretes subviral particles containing the M and E antigens In a further important aspect of the invention, the nucleic acid is a DNA molecule. In additional significant embodiments, the nucleic acid TU includes a control sequence disposed appropriately such that it operably controls the expression of the M and E antigens; this control sequence may advantageously be the cytomegaJovirus immediate early 5 promoter. In an additional embodiment the transcriptional unit also includes a poly-A terminator. The present invention further provides a host cell harboring a nucleic acid molecule which includes a transcriptional unit for an immunogenic fiavivirus antigen that directs the host cell to synthesize the immunogenic antigen. The fiavivirus may be 1 0 YFV, dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 v i rus , SLEV, or JEV. In important embodiments, the antigen may be the M protein. the E protein, or both the M and the E proteins; in the latter case, the cell secretes subviral particles containing the M and E antigens Additionally the invention provides a composition for vaccinating a subject 15 against a fiavivirus containing a nucleic acid molecule that includes a transcriptional un i t for an immunogenic flaviviral antigen. The transcriptional unit directs a cell within the body of the subject, after being incorporated therein, to synthesize the immunogenic antigen . The composition further includes a pharmaceutically acceptable carrier In significant embodiments, the fiavivirus may be yellow fever virus, dengue type 1 virus. 20 dengue type 2 virus, dengue type 3 virus, dengue type 4 virus. SLEV, or JEV Furthermore, the antigen may be the M protein, the E protein, or both the M and the E proteins; in the latter instance the cell secretes subviral particles comprising the fiavivirus M and E antigens. In important embodiments, the nucleic acid molecule is a DNA molecule. In further significant embodiments, the transcriptional unit additionally -> contains a control sequence disposed appropriately such that it operably controls the expression of the M and E antigens when the nucleic acid is introduced into the cell of the subject; advantageously the control sequence is the cytomegalovirus immediate early promoter. In a still further embodiment, the transcriptional unit also includes a poly-A terminator. The invention provides still further a method of immunizing a subject against infection by a fiavivirus The method involves administering to the subject an effective amount of a vaccinating composition that contains a nucleic acid molecule which includes a transcriptional unit for an immunogenic fiavivirus antigen. The transcriptional unit directs a cell within the body of the subject, after being taken up by the cell, to synthesize the immunogenic antigen. The composition additionally includes a pharmaceutically acceptable carrier. In significant embodiments of the method, the fiavivirus may be yellow fever virus, dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 virus, SLEV, or JEV. In yet other important aspects of the method, the antigen may be the M protein, the E protein, or both the M and the E proteins When the antigen is both the M and the E protein.., the cell within the body of the subject, after incorporating the nucleic acid within it, secretes subviral particles comprising the flaviviral M and E antigens Additionally, in significant embodiments of the method, the vaccinating composition is administered to the subject in a single dose. and is administered via a parenteral route In yet a further aspect of the method, the nucleic acid is a DNA molecule. In yet additional embodiments of the method, the transcriptional unit further includes a control sequence disposed appropriately such that it operably controls the expression of the M and E antigens, in a significant aspect of this embodiment, the control sequence is the cytomegalovirus immediate early promoter Furthermore, the transcriptional unit may further include a poly-A terminator These aspects and embodiments of the invention are the basis for its distinct attributes and advantages Being a nucleic acid construct involving only portions of the fiavivirus genome rather than the sequence encompassing the complete genome, the nucleic acid TU-containing vaccine is completely nonviable It therefore poses no danger of infection by the fiavivirus to those involved in its manufacture, and none to subjects receiving the vaccine The nucleic acid vaccine is easy to prepare and to administer, and is stable to storage prior to use. Unexpectedly it has been found that the nucleic acid vaccine of the invention is essentially 100% successful in conferring protective immunity in mammals after administering only a single dose A further unexpected result is that the nucleic acid TU is able to engender immunity to a fiavivirus in a female mammal which can be transmitted to its progeny through the milk Without wishing to be limited by theory, the inventor believes that a possible mechanism for the success of the nucleic acid in conferring protective immunity is that a host cell harboring the nucleic acid, such as the cell of a subject to whom the vaccine is administered, produces subviral particles containing the flaviviral M and E antigens These particles may closely mimic the immunogenic attributes of virulent flaviviruses themselves Brief Description of the Drawings Figure 1 is a schematic representation of flaviviral pe'yprotein processing The central horizontal region provides a schematic representation of the viral genome The lines denote the 5' and 3' non-translated regions and the boxed regions represent the open reading frame for structural (left and top) and non-structural (right and bottom) proteins Cleavage by host cell signalase occurs simultaneously with translation at the E protein C-terminus separating structural and non-structural regions A subtilase-like cellular enzyme, furin, may be responsible for prM cleavage Potential transmembrane domains of viral polyprotein are indicated by shaded areas Figure 2 presents a map of the JEV genome (top), the DNA sequence of oligonucleotides used in a reverse transcriptase-polymerase chain reaction (PCR) (center) to construct the transcription unit for the expression of prM-E protein coding regions (bottom) Potential transmembrane domains of viral polyprotein are indicated by shaded areas Figure 3 shows a schematic representation of the plasmid vectors, pCDNA3, pCBamp, and pCLBamp, and the relationship between them These plasmids include the CMV (cytomegalovirus) promoter/enhancer element, BGHp(A) (bovine growth hormone polyadenylation signal and transcription termination sequence), ampicillin resistance gene and ColEl origin of replication for selection and maintenance in E. coli The fl origin of replication for single-stranded rescue in E. coli cells. SV40 origin replication (SV40 ORJ), neomycin resistance coding region, and SV40p(A) sequences were deleted from pCDNA3 to generate pCBamp. An intron sequence was inserted in the Ncol-Kpnl site of pCBamp to generate plasmid pCIBamp. The multiple cloning site for the insertion of genes for flaviviral structural proteins, located between the TATA box of the CMV promoter/enhancer and BGHp(A), is shown. Figure 4 shows SDS-PAGE-immunoblot analyses of the sucrose gradient purified subviral particles from JE-4B COS-1 culture fluid (4B, right lane of each pair) The density gradient purified JE virion from JEV infected C6/36 cell culture was used as a positive control (JEV, left lane of each pair) JE HIAF (hyperimmune ascetic fluid). 4G2. anti-E monoclonal antibody, JM01 , anti-M monoclonal antibody, NMAF (normal mouse ascitic fluid) Figure 5 shows a profile of the E antigen in a rate zonal sucrose gradient analysis prepared from the PEG precipitate of JE-4B cell culture medium with or without Triton X-100 treatment Figure 6 is a map of the yellow fever virus (YFV) genome (top) and the DNA sequence of oligonucleotides (center) used in a reverse transcriptase-PCR to construct the transcription unit for the expression of YFV prM-E protein coding regions (bottom) Potential transmembrane domains of viral polyprotein are indicated by shaded areas Figure 7 is a map of the St Louis encephalitis virus (SLEV) genome (top) and the DNA sequence of oligonucleotides (center) used in a reverse transcriptase-PCR to construct the transcription unit for the expression of SLEV prM-E protein coding regions (bottom) Potential transmembrane domains of viral polyprotein are indicated by shaded areas Figure 8 shows photographs of YF or SLE viral proteins detected by an indirect immunofluorescent antibody assay (IFA) using either YFV or SLEV HIAF Viral proteins prM and E were expressed in COS-1 cells transformed by pCDYF2 or pCDSLE4-3. respectively Detailed Description of the Invention The invention encompasses nucleic acid transcriptional units which encode flaviviral antigenic proteins, such as the M and E protein antigens. The nucleic acids function to express the M and E protein antigens when the nucleic acid is taken up by an appropriate host cell, especially when the host cell is the cell of a subject. The invention also encompasses a vaccine whose active agent is the nucleic acid transcriptional unit (TU). The invention further encompasses the cultured host cells when they contain within them a nucleic acid TU. The invention in addition encompasses a method of immunizing a subject against flaviviral infection by administering to the subject an effective amount of a vaccine containing the nucleic acid TU molecules As used herein, "nucleic acid transcriptional unit" or "nucleic acid transcriptional unit molecule" relates to a nucleic acid encoding one or more specified genes The TU has the biological activity that, after having been introduced into a suitable host cell, the nucleic acid induces the biosynthesis of one or more specified gene products encoded by the specified gene or genes The gene product(s) is(are) other biological macromolecules, such as proteins, not chemically related to the TU The nucleic acid TU induces the cell to employ its cellular components to produce the specific gene product or products whose gene or genes are contained in the TU Although any nucleic acid may serve as a TU, in a preferred embodiment, the TU is the DNA of a plasmid or similar vector, wherein the plasmid or vector encompasses in addition coding sequences for marker genes or other sequence constructions that facilitate experimentation and biosynthesis of the TU As used herein, a "control sequence" is a regulatory nucleotide sequence incorporated within a nucleic acid TU which interacts with appropriate cellular components of the host cell and leads to enhanced or activated biosynthesis of the gene products encoded by the TU Thus a suitable control sequence is one with which the components of the host cell have the capability to interact, resulting in stimulated synthesis of the gene product When operably disposed in a nucleic acid with respect to a specified gene, a control sequence effectively controls expression of the specified gene. As used herein, a "promoter" is a nucleotide sequence in a nucleic acid TU which serves as a control sequence. As used herein, a "terminator" is an extended nucleotide sequence which acts to induce polyadenylation at the 3' end of a mature mRNA. A terminator sequence is found after, or downstream from, a particular coding sequence. As used herein, a "host cell" is a prokaryotic or eukaryotic cell harboring a nucleic acid TU coding for one or more gene products, or into which such a TU has been introduced Thus a host cell harbors a foreign or heterologous substance, the TU. which is not naturally or indigenously found in it as a component A suitable host cell is one which has the capability for the biosynthesis of the gene products as a consequence of the introduction of the TU In particular, a suitable host cell is one which responds to a control sequence and to a terminator sequence, if any, that may be included within the TU In important embodiments of the present invention, the host cell is a mammalian cell In particularly important embodiments of this invention, the host cell is a naturally occurring cell in the body of a human or nonhuman subject to whom (which) the TU has been administered as a component of a vaccine Alternatively, in analytical, or diagnostic applications, or for demonstrative purposes, the mammalian cell may be a human or nonhuman cell cultured in vitro As used herein, a "vaccine" or a "composition for vaccinating a subject" specific for a particular pathogen relates to a preparation, which, when administered to a subject, leads to an immunogenic response in a subject. As used herein, an "immunogenic" response is one that confers upon the subject protective immunity against the pathogen Without wishing to be bound by theory, it is believed that an immunogenic response may arise from the generation of neutralizing antibodies, or from cytotoxic cells of the immune system, or both As used herein, an "immunogenic antigen" is an antigen which leads to an immunogenic response when it is introduced into a subject, or. as in the case of the present invention, when it is synthesized within the cells of a host or a subject As used herein, an "effective amount" of a vaccine or vaccinating composition is an amount which, when administered to a subject, is sufficient to confer protective immunity upon the subject. Historically, a vaccine has been understood to contain as an active principle one or more specific molecular components or structures which comprise the pathogen, especially its surface Si structures may include surface components such as proteins, complex carbohydrates, and/or complex lipids which commonly are found in pathogenic organisms As used herein, however, it is to be stressed that the terms "vaccine" or "composition for vaccinating a subject" extend the conventional meaning summarized in the preceding paragraph As used herein, these terms also relate to the nucleic acid TU molecule of the instant invention or to compositions containing the TU The TU induces the biosynthesis of one or more specified gene products encoded by the TU within the cells of the subject, wherein the gene products are specified antigenic proteins of the pathogen The biosynthetic antigens then serve as the immunogen As already noted, the TU, and hence the vaccine, may be any nucleic acid that bears specified genes for the specified immunogenic antigens In a preferred embodiment of this invention, the TU of the vaccine is a DNA The TU may be a plasmid or vector incorporating additional genes or particular sequences for the convenience of the skilled worker in the fields of molecular biology, cell biology, and viral immunology (See "Molecular Cloning A Laboratory Manual", 2nd Ed , Sambrook, Fritsch and Maniatis. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, 1989, and "Current Protocols in Molecular Biology", Ausubel et al , John Wiley and Sons. New York 1987 (updated quarterly), which are incorporated herein by reference) The nucleic acid TU molecules of the instant invention designate nucleic acids, or derivatives of nucleic acids, whose nucleotide sequences encode specific gene products related to antigenic proteins of flaviviruses such as JEV, dengue, yellow fever virus and St Louis encephalitis virus Although any nucleic acid may serve as a TU, in an important embodiment, the TU is a DNA Alternatively, the nucleic acids may be RNA molecules They may also be any one of several derivatives of DNA or RNA whose backbone phosphodiester bonds have been chemically modified to increase the stability of the TU as a pharmaceutical agent Modifications so envisioned include, but are not limited to, phosphorothioate derivatives or phosphonate derivatives; these and other examples of derivatives are well known to persons skilled in the field of nucleic acid chemistry JEV is an RNA virus whose genome has been characterized and sequenced (see Figures 1 and 2) The gene for the M structural gene includes a pre-M sequence (prM) which is translated intracellularly. This sequence allows assembly of JEV particles intracellularly The pre-M sequence is then cleaved from the gene product to yield virus particles containing mature M proteins prior to secretion Related flaviviruses, such as YFV, dengue, and SLEV, have similar genomic structures and functions (see, for example. Figures 6 and 7) An important TU for flaviviral M and E proteins in the instant invention is a DNA In accord with the discussion in the preceding paragraph, this DNA encodes the gene for M comprising the pre-M sequence as well, it also encodes the gene for the E protein In this way the intended gene products are enabled to form subviral particles within the host cell The host cell then may cleave the pre-M sequence in a fashion analogous to that which occurs with respect to replete virions In order to function effectively in vivo as a vaccine, it is advantageous to include within the nucleic acid TU a control sequence that has the effect of enhancing or promoting the translation of the sequences encoding the antigens Use of such promoters is well known to those of skill in the fields of molecular biology, cell biology, and viral immunology (See "Molecular Cloning A Laboratory Manual", 2nd Ed . Sambrook. Fritsch and Maniatis, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY ' , 1989, and "Current Protocols in Molecular Biology", Ausubel et al, John Wiley and Sons, New York 1987 (updated quarterly)) Since the nucleic acid TU is intended for use as a vaccine in a mammalian host, the promoter to be employed is preferably one which operates effectively in mammalian cells Such a promoter is disposed with respect to the genes whose translation is to be promoted, at a position at which it may operably promote such translation In a significant embodiment of the instant in ention, this promoter is the cytomegalovirus early promoter In addition, in a further preferred embodiment of the invention, the genes are followed, in the TU nucleic acid. by a terminator sequence (Sambrook et al.). Particular embodiments of the invention relate to both prokaryotic and eukaryotic host cells. Many promoter sequences are known that are useful in either prokaryotic or eukaryotic host cells. (See Sambrook et al ) Preparation of the nucleic acid TU of the invention is readily accomplished by methods well known to workers of skill in the field of molecular biology Procedures involved are set forth, for example, in Sambrook, Fritsch and Maniatis, Cold Spring Harbor Laboratory', Cold Spring Harbor, NY, 1989, and "Current Protocols in Molecular Biology". Ausubel et al . John Wiley and Sons, New York 1987 (updated quarterly) The flaviviral RNA molecule may be isolated from a sample of live virus bv methods widely known among virologists familiar with flavi * iridae, for example, and with other groups of viruses as well Methods used with JEV are summarized in Kuno et al (1990) The RNA is used as a template for the synthesis of cDNA using reverse transcriptase From the cDNA, a fragment containing the pre-M through E gene (see Figure 2) may be obtained by digestion with restriction nucleases known to cleave the cDNA appropriately to provide such fragments Examples of restriction digestion of JEV, for example, are provided in Nitayaphan et al ( 1990) and Konishi et al ( 1 91 ) Incorporation of promoters, such as the cytomegalovirus promoter, and of the polyadenylation signal, is likewise well known to skilled practitioners in molecular biology and recombinant DNA engineering When a nucleic acid molecule harboring a TU containing the desired genes and control sequences is prepared, it may be obtained in larger quantities by methods that amplify' a nucleic acid fragment Such methods are widely known to workers skilled in molecular biology and recombinant DNA engineering Examples of these methods include incorporation of the nucleic acid fragment into a plasmid for replication by culturing in a cell such as a prokaryotic cell and harvesting the plasmid after completing the culture, as well as amplification of the nucleic acid fragment by methods using the polymerase chain reaction These examples are not intended to limit the ways in which the nucleic acid containing the TU may be obtained The TU-containing nucleic acid molecules of the instant invention may be introduced into appropriate host cells in many ways well known to skilled workers in the fields of molecular biology and viral immunology. By way of example, these include, but are not limited to, incorporation into a plasmid or similar nucleic acid vector which is taken up by the host cells, or encapsulation within vesicular lipid structures such as liposomes, especially liposomes comprising cationic lipids, or adsorption to particles that are incorporated into the host cell by endocytosis In general, a host cell is a prokaryotic or eukaryotic cell harboring a nucleic acid TU, or into which such a TU molecule has been introduced The TU of the present invention induces the intracellular biosynthesis of the encoded E and M antigens A suitable host cell is one which has the capability for the biosynthesis of the gene products as a consequence of the introduction of the nucleic acid In particular embodiments of the invention, a suitable host cell is one which responds to a control sequence and to a terminator sequence, if any, which may be included within the TU In order to respond in this fashion, such a host cell contains within it components which interact with a control sequence and with a terminator and act to carry out the respective promoting and terminating functions When the host cell is cultured in vitro, it may be a prokaryote. a single-celled eukaryote or a mammalian cell In particular embodiments of the present invention, the host cell is a mammalian cell In these cases. the synthesized E and M protein gene products are available for use in analytical, or diagnostic applications, or for demonstrative purposes In fav orable circumstances, such as when the host cell is a cultured mammalian cell, the E and M antigens are secreted in the form of subviral particles These are aggregates of E and M proteins resembling live virus in surface ultrastructural morphology and immunogenic properties Since the nucleic acid TU of the invention does not include the remainder of the flaviviral genome, however, there is no capsid incorporated, and most importantly, no infectious viral RNA. In another important embodiment of this invention, the host cell is a natural cellular component of the subject to whom the TU has been administered as a vaccine The nucleic acid TU. when so administered, is understood to be taken up by the cells of the subject, whereby those cells become host cells as used herein The subject's cells have the capability of responding to any promoter sequences, and terminator, if present In any case, the TU nucleic acid induces the subject's cells to synthesize flaviviral E and M gene products Without wishing to be constrained by theoretical considerations, it is believed that the subject's host cells produce subviral particles in vivo consisting of the M and E antigens, just as has been found to occur with cultured mammalian host cells in vitro Such subviral particles, it is believed, then serve as the in vivo immunogen, stimulating the immune system of the subject to generate immunological responses which confer protective immunity on the subject Again without wishing to be limited b> theory, the resulting protective immunity may arise via either humoral or cellular immunit i e . via either an MHC class II- or class I-restricted mechanism, respectivel . or by both mechanisms According to the invention, subjects may be immunized against infection bv flaviviruses. such as JEV, YFV, dengue, and SLEV, by administering to them an effective amount of a nucleic acid TU encoding genes for the M and E antigens The nucleic acid, after being incorporated into the cells of the subject, leads to the synthesis of the flaviviral M and E antigens In order to administer the nucleic acid TU to the subject, it is incorporated into a composition which comprises as well a pharmaceutically acceptable carrier Such carriers are well known to those of skill in pharmaceutical science They include water for injection, and common physiological buffers (Remington, Pharmaceutical Sciences) They may also include vesicle or liposome structures, especially those containing cationic lipids. as is known to skilled workers in the fields of pharmaceutical science and immunology An effective amount of a vaccinating composition is readily determined by those of skill in the field of viral immunology to be an amount which, when administered to a subject, confers protective immunity upon the subject In order to undertake such a determination, the skilled artisan may assess the ability to induce flaviviral M- and E- specific antibodies and'or flaviviral M- and E-specific cytotoxic T lymphocytes present in the blood of a subject to whom the vaccine has been administered One may in addition determine the level of protective immunity conferred upon an. experimental animal by challenge with live JEV. Such challenge experiments are well known to workers of skill in viral immunology. In general, in order to immunize a subject against infection by JEV, YFV, dengue, or SLEV, according to the present invention, and recognizing that the nucleic acid TU molecules employed in such methods may have differing overall sizes, doses ranging from about 0.1 μg/kg body weight to about 50 μg/kg body weight may be used. It has unexpectedly been found that a TU of the present invention which is a DNA confers protective immunity at a level of effectiveness approximating 100% after administration of only a single effective dose of the TU This is in contrast to many immunization methods carried out using conventional vaccines (as described above), which frequently require one or more booster vaccinations and which may not confer protective immunity to an effectiveness near 100% It has further been found unexpectedly that protective immunity may be transmitted from a vaccinated female subject to the offspring of the subject A significant proportion of neonatal mice was shown to be protected against viral challenge after their mothers were vaccinated using the TU DNA of the invention Without wishing to be limited by theory, it is known that passive immunity may be conferred on neonatal mammals due to the presence in maternal milk of neutralizing antibodies specific for various pathogens It is possible that the protective immunity against JEV found with the neonates was transmitted to them in this way Particular embodiments of the present invention are set forth in the examples which follo . These examples are not intended to limit the scope of the invention as disclosed in this specification. Examples General methods utilizing molecular biology and recombinant DNA techniques related to preparing and expressing the nucleic acid TU molecules of the invention are set forth in. for example. "Current Protocols in Molecular Biology", Ausubel et al . John Wiley and Sons, New York 1987 (updated quarterly), and Molecular Cloning A Laboratory Manual 2nd Ed, Sambrook, Fritsch and Maniatis, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, 1989. Example 1 Preparation of recombinant plasmids containing the transcriptional unit encoding JEV prM and E antigens Genomic RNA was extracted from 150 μL of JEV strain SA 14 virus seed grown from mouse brain using a QIAamp™ Viral RNA Kit (Qiagen. Santa Clarita, CA). RNA, adsorbed on a silica membrane, was eluted in 80 μL of nuclease-free water, and used as a template for the amplification of JEV prM and E gene coding sequences Primer sequences were obtained from the work of Nitayaphan et al (1990) A single cDNA fragment containing the genomic nucleotide region 389-2478 was amplified by the reverse transcriptase-polymerase chain reaction (RT-PCR) Restriction sites Kpnl and Xbal, the consensus Kozak ribosomal binding sequence, and the translation initiation site were engineered at the 5' terminus of the cDNA by amplimer 14DV389 (SEQ ID NO 1 ) An in-frame translation termination codon, followed by a Notl restriction site, was introduced at the 3' terminus of the cDNA by amplimer C14DV2453 (SEQ LD NO 2) (see Figure 2) One-tube RT-PCR was performed using a Titan RT-PCR Kit (Boehringer Mannheim, Indianapolis. IN) 10 μL of viral RNA was mixed with 1 μL each of 14DV389 (50 μM) and cl 4DY2453 (50 μM) and 18 μL of nuclease-free water and the mixture was heated at 85 °C for 5 min and then cooled to 4 C C 75 μL of reaction mix [20 μL 5x buffer. 2 μL of dNTP mixture ( 10 mM each). 5 μL of dithiothreitol (0 1 mM), 0 5 μL of RNasin™ (40 U/μL, Boehringer Mannheim). 2 μL of polymerase mixture, and 45 5 μL of nuclease-free water] was added and RT-PCR performed as follows 1 cycle (50°C for 30 min, 94 C C for 3 min, 50 C C for 30 s, 68°C for 2.5 min), 9 cycles (94°C for 30 s, 50°C for 30 s, 68 °C for 2 5 min), 20 cycles (94 °C for 30 s, 50°C for 30 s, 68 °C for 2.5 min in the first cycle, with an increment of 5 s per cycle thereafter), and a final extension at 68 °C for 15 min The RT-PCR product was purified by a QIAquick™ PCR Purification Kit (Qiagen) and eluted with 50 μL of 1 mM Tris-HCl, pH 7 5 All vector constructions and analyses were carried out by using standard techniques (Sambrook et al , 1989) RT-PCR amplified cDNA, digested with Kpnl and Notl nucleases, was inserted into the Kpnl-Notl site of eukaryotic expression plasmid vector (pCDNA3, Invitrogen, Carlsbad, CA). Electroporation-competent Escherichia coli XLl-Blue cells (Stratagene, La Jolla, CA) were transformed by electroporation (Gene Pulser™, Bio-Rad, Hercules, CA) and plated onto LB agar plates containing 100 μg/mL carbenicillin (Sigma Chemical Co, St. Louis, MO). Clones were picked and inoculated into 3 mL LB broth containing 100 μg mL carbenicillin. Plasmid DNA was extracted from a 14 h culture using a QIAprep™ Spin Miniprep Kit (Qiagen). Automated DNA sequencing was performed as recommended (Applied Biosystems/Perkin Elmer, Foster City, CA) Both strands of the cDNA were sequenced and shown to be identical to the sequence for the original SA14 strain (Nitayaphan et al, 1990). The fragment of plasmid pCDNA3 (Invitrogen, Carlsbad. CA) from nucleotide (nt) 1289 to nt 3455, containing fl ori, SV40 ori, the neomycin resistance gene, and SV40 poly(A) elements was deleted by Pvull digestion and then ligated to generate the pCBamp plasmid The vector pCIBamp, containing a chimeric intron insertion at the Ncol/Kp l site of the pCBamp was constructed by excising the intron sequence from pCI (Promega. Madison, WI) by digestion with Ncol and Kpnl The resulting 566-bp fragment was cloned into pCBamp by digesting with Ncol-Kpnl to replace its 289-bp fragment Figure 3 presents the relationships between the plasmids pCDA3. pCBamp. and pCIBamp Plasmids containing the transcriptional unit encoding JEV prM and E proteins were prepared from these plasmids The cDNA fragment containing the JEV prM and E coding regions in the recombinant plasmid pCDJE2-7, derived from the pCDNA3 vector, was excised by digestion with Notl and Kpnl or Xbal and cloned into the Kpnl- Notl site of pCBamp, pCIBamp, pCEP4 (Invitrogen, Carlsbad. CA), or pREP4 (Invitrogen. Carlsbad, CA), or into the Spel-Notl site of pRc/RSV (Invitrogen. Carlsbad. CA) expression vector to create pCBJEl-14, pCIBJES 14, pCEJE, pREFE, and pRCJE, respectively Both strands of the cDNA from clones of each plasmid were sequenced and recombinant clones with the correct nucleotide sequence were identified Plasmid DNA for use in the in vitro transformation of mammalian cells or mouse immunization experiments was purified by anion exchange chromatography using an EndoFree™ Plasmid Maxi Kit (Qiagen). Example 2 Evaluation of JEV prM and E proteins expressed by various recombinant plasmids using an indirect immunofluorescent antibody assay The expression of JEV specific gene products by the various recombinant expression plasmids was evaluated in transiently transfected cell lines of COS-1, COS-7 and SV- T2 (ATCC, Rockville MD, 1650-CRL, 1651-CRL, and 163 1-CCL, respectively) by indirect immunofluorescent antibody assay (IFA) The SV-T2 cell line was excluded from further testing since a preliminary- result showed only 1-2% of transformed SV-T2 cells were JEV antigen positive For transformation, cells were grown to 75% confluence in 150 cnr culture flasks, trvpsinized, and resuspended at 4°C in phosphate buffered saline (PBS) to a final cell count 5 x 10 6 per mL 10 μg of plasmid DNA was electroporated into 300 μL of cell suspension using a BioRad Gene Pulse™ (Bio-Rad) set at 150 V, 960 μF and 100 Ω resistance Five minutes after electroporation, cells were diluted with 25 mL fresh medium and seeded into a 75 cnr flask 48 h after transformation the medium was removed from the cells, and the cells were trypsinized and resuspended in 5 mL PBS with 3% normal goat serum 10 μL aliquots were spotted onto slides, air dried and fixed with acetone at -20°C for 20 min IFA was performed with acetone-fixed plasmid-transformed cells using fluorescein isothiocyanate-conjugated goat anti-mouse immunoglobulin G (Sigma Chemical Co ) and JEV HIAF To determine the influence of various promoter and poly(A) elements on the JEV prM and E protein expression, COS-1 and COS-7 cell lines were transiently transformed by an equal amount of pCDJE2-7, pCEJE, pREJE, or pRCJE plasmid DNA JEV antigens were expressed in both cell lines transformed by all four recombinant plasmids, thus confirming that the CMV or RSV (rous sarcoma virus) promoter and BGH or SV40 poly(A) elements were functionally active However, the percentage of transformed cells and the level of JEV antigens expressed, as determined by the number of IFA positive cells and IFA intensity, respectively, differed greatlv among the various plasmids (see Table 1 ). A significantly high percentage of COS-1 cells transformed by pCDJE2-7, pCBJEl-14 and pCLBJES14 expressed the JEV antigens, and the level of the expressed proteins was compatible with JEV-infected cells Cells transfected with pCEJE, pREJE, or pRCJE vectors, on the other hand, had a low percentage of antigen-expressing cells, as well as a low intensity of fluorescence, indicating weak expression of the antigens. In order to ascertain whether the enhanced expression of JEV proteins by pCDJE2-7 was influenced by the SV40-encoded eukaryotic origin of replication, the plasmid pCBJEl-14 was constructed so that a 2166-bp fragment, containing Ω ori, SV40 ori, the neomycin resistance gene and SV40 poly(a) elements from pCDJE2-7, was deleted A chimeric intron was then inserted into pCBJE l -14 to generate pCLBJES M The pCIBJES l 4 plasmid was used to determine if the expression of JEV proteins could be enhanced by the intron sequence Following transformation, cells harboring both pCBJEl -14 and pCLBJES14 vectors expressed a level of JEY antigens similar to that observed with pCDJE2-7 (see Table 1) This result indicates that expression of JEV prM and E antigens by recombinant vectors is influenced only by the transcriptional regulators' elements Neither the eukaryotic origin of replication nor the intron sequence enhanced JEV antigen expression in the cells used Vectors containing the CMY promoter and BGH poly(A) (see Figure 3) were selected for further analysis Table 1. Transient expression of .IE prM and E proteins by various recombinant plasmids in two transferred cell lines. ♦ Various cell lines were transformed with pCDNAV AT (negative control) pCDJE2-7, pCBJEl-14. pCIBJESI4, pCEJEm pREJE, orpRCJE, Cells were trvpsmi/cd 48 hours later and tested (IFA) with JE virus-specific HIAF Data are presented as the mtcnsilλ (scale of 1+ to 4+) and the percentage of II- A c cells The pCDNA /CAI transformed cells were used as the negative control Example 3. Selection of an in vitro transformed, stable cell line constitutively expressing JEV specific gene products. COS-1 cells were transformed with 10 μg of pCDJE2-7 DNA by electroporation as described in the previous example. After a 24 hr incubation in non-selective culture medium, cells were treated with neomycin (0 5 mg/mL, Sigma Chemical Co.). Neomycin-resistant colonies, which became visible after 2-3 weeks, were cloned by limited dilution in neomycin-coπtaining medium Expression of vector-encoded JEV gene products was initially screened by IFA using JEV FflAF. One JEV-EFA positive clone (JE-4B) and one negative clone (JE-5A) were selected for further analysis and maintained in medium containing 200 μg/mL neomycin Authenticity of the JEV E protein expressed by the JE-4B clone was demonstrated by epitope mapping by IFA using a panel of JEV E-specific murine monoclonal antibodies (Mab) (Kimura-Kuroda et al., J. Virol. 45, 124-132 ( 1983), Kimura-Kuroda et al., J. Gen Virol. 67, 2663-2672 (1986), Zhang et al., J Med Virol 29, 133-138 ( 1989); and Roehrig et al., Virol. 128, 1 18-126 (1983)). JEV Hl.AF and normal mouse serum were used as positive and negative antibody controls, respectively. Four JEV-specific, six flavivirus-subgroup specific, and two flavivirus- group reactive Mabs reacted similarly with the 4B clone or JEV-infected COS- 1 cells (see Table 2). Table 2. Characterization of proteins expressed by a pCDJ£2-7 stably transformed clone (JE-4B) of COS-1 cells with JE virus-reactive antibodies. Example 4 .Antigenic properties and immunoloeical detection of subviral particles secreted bv the JE-4B COS-1 cell line. a Preparation of subviral particles. JE-4B COS-1 cells were grown and maintained in medium containing 200 μg/mL of neomycin. The cultured medium was routinely harvested and stored at 4°C, and replenished twice weekly, and the cells were split 1 :5 every 7-10 days Culture medium was clarified by centrifugation at 10,000 φm for 30 min in a Sorvall F 16/250 rotor at 4°C, and centrifuged further for 4 hr at 39,000 φ in a Sorvall TH641 rotor at 4°C through a 5% sucrose cushion (w/w, prepared with 10 mM Tris HCl. pH 7.5, 100 mM NaCl (TN buffer)) The pellet containing subviral particles was resuspended in TN buffer and stored at 4°C. Alternatively, 7% or 10% PEG-8000 (w/v) was added to the clarified culture medium The mixture was stirred at 4°C for at least 2 hr, and the precipitated particles were collected by centrifugation at 10,000 m for 30 min. The precipitate was resuspended in TN buffer and stored at 4°C. The subviral particles were purified from both pelleted and PEG-precipitated preparations by rate zonal centrifugation in a 5- 25% continuous sucrose gradient in TN at 38,000 φm at 4 C C for 90 min 1-mL fractions were collected from the top of the gradient, tested by antigen capture ELISA (see below), and the positive fractions loaded onto a 25-50% sucrose gradient in TN This was centrifuged overnight in an equilibrium density centrifugation at 35,000 φm at 4 C C 0 9-mL fractions from the equilibrium gradients were collected from the bottom They were tested by antigen-capture ELISA and assessed for hemagglutination (HA) activity at pH 6 6 An aliquot of 100 μL of each fraction was weighed precisely to determine its density The ELISA-positive fractions were pooled and pelleted at 39,000 φm at 4°C for 3-4 hr and the pellet resuspended in TN buffer Antigen-capture ELISA and HA titers were determined on the pelleted samples JEV- infected COS- 1 cell supernatant was also subjected to similar purification protocols as detailed above and used as a positive control for the gradient analysis JE virions were also purified from infected C6'36 cells 5-6 days postinfection by sedimentation in a glycerol/tartrate equilibrium gradient b Western blots of subviral particles Gradient-purified samples of the subviral particles were mixed with electrophoresis sample buffer and run on 10 or 12 5% sodium dodecyl sulfate-containing polyacrylamide gels (SDS-PAGE) as described bv Laemmli (Nature 277, 680-685 (1970)) Proteins were transferred to a nitrocellulose membrane and immunochemically detected with polyclonal JEV HIAF, fiavivirus cross-reactive anti-E Mab 4G2 (Henchal et al , Amer J Trop Med Hyg 31, 830-836 (1982)), or mouse anti-prM peptide hyperimmune serum (JMOl , Chiueh et al , unpublished results) Figure 4 shows a comparison of the M and E proteins produced by JEV infected C6/36 and JE-4B COS-1 cells Some nonspecific reactivitv, to E protein was observed in the normal mouse ascitic fluid and Jmol anti-peptide serum Proteins identical in size to M and E were secreted in the subviral particles and could be detected by E-specific Mab'4G2 and prM-specific JMOl antiserum. respectivelv c. Density gradient detection of JEV subviral particles in culture medium. For ELISA, antigen-capture antibody (4G2) was diluted in 0.1 M sodium carbonate buffer, pH 9.6, and used to coat 96- well microtiter plates (Immulon II, Dynatech. Chantilly, VA) by overnight incubation at 4° C. After blocking with 3% normal goat serum in PBS, two-fold serially-diluted samples were added to the 4G2-coated plate and incubated 1.5 hours at 37°C. Captured antigen was detected by horseradish peroxidase-conjugated 6B6C-1 Mag, and incubated for 1 hour at 37°C. The enzyme activity on the solid phase was then detected with TMB (3,3',5,5'- tetramethylbenzidine)-ELISA (Life Technologies, Grand Island, NY) Approximately 500 mL of cell culture medium from 15 X 150 cm 2 flasks of JE-4B cells was collected four days after cells were seeded PEG-precipitated subviral particles were resuspended in 2 mL of TN buffer, pH 7 5, a 0 7 mL aliquot of this resuspended pellet was loaded onto a 5-25% sucrose gradient Triton X- 100. which disrupts subviral particles, was added to another 0 7 mL aliquot to a final concentration of 0 1% and this was loaded onto a 5-25% sucrose gradient prepared in TN buffer containing 0 1% Triton X-100 A definite opaque band was observed approximately 2 5 cm from the top of the gradient containing Triton X- 100 but not in the gradient without detergent Fractions (1 mL) were collected from top to bottom for each gradient and were analyzed by antigen capture ELISA (Figure 5) Antigen was detected in fractions 4-6, indicating relatively rapid sedimentation characteristic of subviral particles Treatment of the PEG precipitate from JE-4B culture medium with Triton X- 100 shifted the position of ELISA-reactive material to the top of the gradient Thus treatment with Triton X-100 produces only slow-sedimenting molecules A similar finding was reported by Konishi et al , 1992 (Virol 188 714- 720) These results show that rapidly sedimenting subviral particles containing prM/M and E could be disrupted by detergent treatment HA activity was determined in the pH range from 6 1 to 7 0 by the method of Clarke and Casals (Amer J Trop Med Hyg 7:561 -573 (1958)) The subviral particle secreted by JE-4B cells and the virion particle produced by JEV infected COS- 1 cells had a similar HA profile with the optimum pH determined to be 6 6 Example 5 Comparison of ' the immune response in mice vaccinated with pCDJE2-7 nucleic acid vaccine of the invention and commercial JEV vaccine Groups of five 3-week-old female, ICR outbred mice were injected intramuscularly in the left and right quadriceps with 100 μg of pCDJE2-7 plasmid in 100 μL of dH 2 O or were given doses of JE-VAX (manufactured by the Research Foundation for Microbial Disease of Osaka University and distributed by Connaught Laboratories, Swift water, PA.) subcutaneously that are one-fifth the dose given to humans. The plasmid pCDNA3/CAT, which encodes and expresses an unrelated protein, (Invitrogen), was used as the negative vaccination control. Except for one group of pCDJE2-7- vaccinated mice, all animals were boosted 3 weeks later with an additional dose of plasmid or JE-VAX Mice were bled from the retroorbital sinus at 3, 6, 9, 23, 40 and 60 weeks after inoculation JEV antibody titers were determined by enzyme-linked imunosorbent assay (ELISA) against purified JEV or by plaque reduction neutralization tests (PRNT) (Roehrig et al , Virol 171 49-60 (1989). and Hunt and Cahsher. Amer J Trop Med Hyg 28 740-749 ( 1979)) The pCDJE2-7 nucleic acid vaccine and JE-VAX provided 100% seroconversion three weeks after the first vaccination in all three groups of mice (Table 3) The JEV ELISA and PRNT antibody titers reached the highest level at week 6 and week 9, respectively, after immunization Mice receiving 1 dose of DNA vaccine had similar antibody responses as those receiving 2 doses Comparable ELISA antibody titers were maintained in DNA-vaccinated groups up to 60 weeks. after which the experiment was terminated However, only one of four mice in the JE-VAX group was JEV antibody positive at 60 weeks post-inoculation The pCDNA3/CAT control group did not have any measurable JEV antibodv These results demonstrate that a single dose of JEV-specific nucleic acid vaccine is more effective in maintaining JEV antibody in mice than the commercial. FDA approved JE-VAX vaccine Table 3. Persistence of Ihe immune response in mice immunized with pCDJE2-7 or JE-VEX vaccine. Mice ucrc inoculalcd with I or 2. 100 nμ/dosc plasmid DNA. or !/* human dose of JF-VAX vaccine Sera were collected for testing prior to the second immunization * Individual scrum liters Example 6. Comparison of various nucleic acid vaccine constructs of the invention and commercial JEV vaccine for effectiveness of vaccination at different ages. A similar level of JEV protein was expressed by COS-1 cells transformed by either pCDJE2-7 pCBJEl-14, or pCIBJES14. JEV antibody induction by these nucleic acid constructs was compared to JE-VAX commercial vaccine at two different ages at vaccination Three-day (mixed sex) or 3 -week-old (female) ICR outbred mice, 10 per group, were vaccinated intramuscularly with 50 or 100 μg of plasmid DNA, or subcutaneously with doses of JE-VAX that are one-tenth or one-fifth the dose given to humans. Serum specimens were collected at 3 and 7 weeks after immunization and tested at a 1 1600 dilution by ELISA using purified JEV as an antigen Results are shown in Table 4 Plasmid pCBJEl -14 provided the highest extent of seroconversion. i e , antibody titer greater than 1 1600, achieving 80- 100% at both ages of vaccination Administration of pCDJE2-7 or pCIBJESl4 provided moderate seroconversion by 7 weeks when 3-day old mice were vaccinated (60% for each), but weaker seroconversion (40% and 10%, respectively) when measured 3 weeks after vaccination When these plasmids were administered at the age of 3 weeks, however, seroconversions of 90% or 100% were attained at both 3 weeks and 7 weeks after vaccination In contrast, the commercial vaccine, JE-VAX, conferred no seroconversion when administered at 3 days of age, and 100% when given at 3 weeks of age Thus the nucleic acid TU's for JEV prM and E provided an extent of seroconversion better than a very high dose of the commercial vaccine, and unexpectedly high seroconversion in both young and more mature animals Table 4. The age-dependent percent seropositive rate in mice following vaccination with various JEV vaccines. Example 7 Protective immunity conferred by the nucleic acid vaccine of the invention Three-day old vaccinated groups from Example 6 were challenged 7 weeks after vaccination by intraperitoneal injection of 50,000 pfu/ 100 μL of the mouse- adapted JEV strain SA14 and observed for 3 weeks 100% protection was achieved in 5 groups that received various nucleic acid TU-containing vaccine constructs for up to 21 days (Table 5). In contrast, 60% of the JE-VAX-vaccinated mice, as well as 70% of the pCDNA3/CAT-vaccinated negative controls, did not survive virus challenge by 21 days These results indicate that the nucleic acid TU's of the invention confer unexpectedly effective protection on vaccinated mice This suggests the possibility of 10 employing the nucleic acid vaccine of the invention as an early childhood vaccine for humans In contrast, JE-VAX, the inactivated human vaccine currently used, does not appear to be effective in young animals. Table 5. Protection from JEV challenge in 8 week old mice following vaccination at 3 days old with various JEV vaccines. Example 8 Passive protection of neonatal mice con elated with the maternal antibody titer. Female ICR mice at the age of 3 weeks were vaccinated with either one dose or two doses spaced two days apart of pCDJE2-7 plasmid DNA, at 100 μg 100 μL. or with two doses of JE-VAX that were one-fifth the dose given to 5 humans. The negative control group received two doses of 100 μg/100 μL of pCDNA-3/CAT plasmid Passive protection by maternal antibody was evaluated in pups resulting from matings of experimental females with non-immunized male mice that occurred nine weeks following the first vaccination or 6 weeks following the second vaccination. Pups were challenged between 3-15 days after birth by 10 intraperitoneal administration of 5,000 pfu/ 100 μL of mouse-adapted SA14 virus and observed daily for 3 weeks (see Table 6) The survival rates correlated with the maternal neutralizing antibody titers. 100% of pups nursed by mothers with a PRNT of 1 :80 survived viral infection, whereas none of the pups from the control mother survived (Table 6). Partial protection of 45% and 75% was observed in older pups 15 that were nursed by mothers with a PRNT titer of 1 :20 and 1 :40, respectively. The survival rates also correlated with the length of time that pups were nursed by the immune mother. As just indicated, 13-15 day old pups had high survival rates None of the 3-4 day old pups, however, survived virus challenge when the mother had a PRNT titer of 1 :20 or 1 :40. Thus maternal antibody provides partial to complete 20 protective immunity to the offspring. In addition, JEV antibody was detected by ELISA in the sera of 97% (29/30) of the post-challenge pups Table 6. Evaluation of the ability of maternal antibody from JEV-nucleic acid- vaccinated female mice to protect their pups from fatal JEV encephalitis. Mice were inoculated imramuscularh w ith 1 or 2. 100 μg dose of plasmid DNA. or subcutaneouslv with two. 1/5 human dose of JE-VAX vaccine Sera were collected 9 weeks post-vaccination for PRNT testing pπor to mating with non-immune male 1 No Surviv ors 'total for each litter : Number of JEY ELIS A-antibodv -positiv e animals (titer i 1 400)/No of surv iv ors, sera were collected for testinu 12 weeks after challenge Example 9 Preparation of recombinant plasmids containing coding sequences for yellow fever virus (YFV) or St Louis encephalitis virus (SLEV) prM and E proteins A strategy similar to constructing the pCDJE2-7 recombinant plasmid was used to prepare YFV and SLEV recombinant plasmids Genomic RNA was extracted from 150 μL of YFV strain TRJ-788379 or SLE strain 78V-6507 virus seeds using Q 1 Aamp™ Viral R A Kit (Qiagen, Santa Clarita, C A ) The viral RNA was used as a template for amplification of YFV or SLEV prM and E gene coding regions Primer sequences and structures of the amplified YFV and SLEV DNA products are shown in Figures 6 and 7. respectively RT-PCR amplified cDNA, digested with Kpnl and Notl enzymes, was inserted into the Kpnl-Notl site of a eukaryotic expression plasmid vector, pCDNA3 (Invitrogen). Both strands of the cDNA were sequenced and verified for identity to sequences from YFV strain TRI-788379 or SLEV strain 78V- 6507 (unpublished; Chang, 1998). Recombinant plasmids pCDYF2 and pCDSLE4-3, which contained the nucleotide sequences of the prM and E coding regions for YFV or SLEV, respectively, were purified using an EndoFree™ Plasmid Maxi Kit (Qiagen), and used for in vitro transformation or mouse immunization. YFV or SLEV specific antigens were expressed in COS-1 cells transformed by pCDYF2 or pCDSLE4-3, respectively (Figure 8). The level of expressed proteins was similar to a YFV- or SLEV-infected COS-1 cell control As in the JEV model, COS-1 cell lines transformed by vectors bearing genes for the viral antigens were obtained which constitutively express YFV or SLEV antigenic proteins Epitope mapping by IFA using a panel of YFV or SLEV E-specific Mabs indicated that the authentic E protein was expressed by the pCDYF2- or pCDSLE4-3-transformed COS- 1 cells A preliminary study indicated that 100% of three week-old female, ICR mice seroconverted after intramuscular inoculation with a single dose of 100 μg'100 μL of pCDSLE4-3 plasid in deionized water Example 10 Preparation of plasmids containing coding sequences for dengue type 2 structural proteins Procedures such as those carried out for JEV (see Example 1) are to be followed to prepare vectors including nucleic acid TU's for dengue type 2 antigens A plasmid containing the dengue type 2 gene region from prM to E is to be constructed The dengue type 2 prM and E genes (Deubel et al., Virology 155.365- 377 (1986), Gruenberg et al., J. Gen. Virol 69.1301-1398 (1988), Hahn et al., Virology 162: 167-180 (1988)) are to be ligated into a plasmid such as pCDNA3, and then excised and cloned into vectors such as pCBamp, pCEP4, pREP4, or pRc/RSV (supplied by Invitrogen, Carlsbad, CA) to enable expression. If necessary a dengue type 2 virus-specific sequence encoded in a cDNA sequence may be amplified using a procedure such as the polymerase chain reaction (PCR) Alternatively, if the viral RNA is the source of the gene region, a DNA sequence may be amplified by a reverse transcriptase-PCR procedure A DNA fragment including an initiation codon at the 5' end, and a termination codon at the 3' end is to be cloned into an expression vector at an appropriate restriction nuclease-specific site, in such a way that the cytomegalovirus (CMV) immediate early (IE) promoter, an initiation codon, and a terminator, are operably linked to the dengue type 2 virus sequence. Example 1 1 Vaccination of mice using a dengue type 2 DNA vaccine. The dengue type 2 nucleic TU vaccine encoding the gene region from prM to E prepared in Example 10 is to be suspended in a suitable pharmaceutical carrier, such as water for injection or buffered physiological saline, and injected intramuscularly into groups of weanling mice. Control groups receive a comparable plasmid preparation lacking the dengue type 2 specific genes The generation of dengue type 2-specific antibodies. and/or of dengue type 2-specific immune system cytotoxic cells, is to be assessed at fixed intervals thereafter, for example at weekly intervals At about two to four months after administration of the nucleic acid TU vaccine, mice are to be challenged with dengue type 2 virus Levels of viremia are to be assessed at appropriate intervals thereafter, such as every second dav Passive protection by maternal antibodv is to be assessed as indicated in Example 8

Claims: CLAIMS I claim 1. A nucleic acid molecule which comprises a transcriptional unit for an immunogenic fiavivirus antigen, wherein the transcriptional unit directs a host cell, after being incorporated therein, to synthesize the immunogenic antigen 2. The nucleic acid molecule described in claim 1, wherein the fiavivirus is chosen from the group consisting of yellow fever virus, dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 virus, and Japanese encephalitis virus (JEV) 3 The nucleic acid molecule described in claim 2 wherein the antigen is chosen from the group consisting of the M protein, the E protein, and both the M and the E proteins 4 The nucleic acid molecule described in claim 3 wherein the antigen is both the M and the E proteins, and wherein the host cell secretes subviral particles comprising the fiavivirus M and E antigens 5 The nucleic acid molecule described in claim 1 which is a DNA molecule 6 The nucleic acid molecule described in claim 1 wherein the transcriptional unit further comprises a control sequence disposed appropriately such that it operably controls the expression of the M and E antigens 7 The nucleic acid molecule described in claim 6 wherein the control sequence is the cytomegalovirus immediate early promoter 8. The nucleic acid molecule described in claim 1 wherein the transcriptional unit further comprises a poly-A terminator. 9. A host cell harboring a nucleic acid molecule which comprises a transcriptional unit for an immunogenic fiavivirus antigen, wherein the transcriptional unit directs the host cell to synthesize the immunogenic antigen. 10 The host cell described in claim 9 wherein the fiavivirus is chosen from the group consisting of yellow fever virus, dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 virus, and JEV. 1 1 The host cell described in claim 10 wherein the fiavivirus antigen is chosen from the group consisting of the M protein, the E protein, and both the M and the E proteins 12 The host cell described in claim 1 1 wherein the antigen is both the M and the E proteins, and wherein the cell secretes subviral particles comprising the fiavivirus M and E antigens 13 A composition for vaccinating a subject against a fiavivirus comprising a nucleic acid molecule which comprises a transcriptional unit for an immunogenic antigen of the fiavivirus, wherein the transcriptional unit directs a cell within the body of the subject, after being incorporated therein, to synthesize the immunogenic antigen. and wherein the composition further comprises a pharmaceutically acceptable carrier 14. The vaccinating composition described in claim 13, wherein the fiavivirus is chosen from the group consisting of yellow fever virus, dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 virus, and JEV. 15. The vaccinating composition described in claim 14 wherein the antigen is chosen from the group consisting of the M protein, the E protein, and both the M and the E proteins. 16. The vaccinating composition described in claim 15 wherein the antigen is both the M and the E proteins, and wherein the cell secretes subviral particles comprising the fiavivirus M and E antigens 17. The vaccinating composition described in claim 13 wherein the nucleic acid molecule is a DNA molecule 18 The vaccinating composition described in claim 13 wherein the transcriptional unit further comprises a control sequence disposed appropriately such that it operably controls the expression of the M and E antigens when the nucleic acid is introduced into the cell of the subject 19 The vaccinating composition described in claim 18 wherein the control sequence is the cytomegalovirus immediate early promoter 20 The vaccinating composition described in claim 13 wherein the transcriptional unit further comprises a poly-A terminator 21 A method of immunizing a subject against infection by a fiavivirus comprising administration to the subject of an effective amount of a vaccinating composition comprising a nucleic acid molecule which comprises a transcriptional unit for an immunogenic fiavivirus antigen, wherein the transcriptional unit directs a cell within the body of the subject, after being incorporated therein, to synthesize the immunogenic antigen, the composition further comprising a pharmaceutically acceptable carrier 22. The method of immunizing a subject described in claim 21, wherein the fiavivirus is chosen from the group consisting of yellow fever virus, dengue type 1 virus, dengue type 2 virus, dengue type 3 virus, dengue type 4 virus, and JEV. 23. The method of immunizing a subject described in claim 22 wherein the antigen is chosen from the group consisting of the M protein, the E protein, and both the M and the E proteins. 24. The method of immunizing a subject described in claim 23 wherein the antigen is both the M and the E proteins, and wherein a cell within the body of the subject, after incorporating the nucleic acid within it, secretes subviral particles comprising the fiavivirus M and E antigens 25 The method of immunizing a subject described in claim 21 wherein the method further comprises administering the vaccinating composition to the subject in a single dose 26 The method described in claim 21 wherein the vaccinating composition is administered via a parenteral route 27 The method of immunizing a subject described in claim 21 wherein the nucleic acid is a DNA molecule 28 The method of immunizing a subject described in claim 21 wherein the transcriptional unit further comprises a control sequence disposed appropriately such that it operably controls the expression of the M and E antigens 29 The method of immunizing a subject described in claim 28 wherein the control sequence is the cytomegalovirus immediate early promoter 30 The method of immunizing a subject described in claim 21 wherein the transcriptional unit further comprises a poly-A terminator 31. The nucleic acid molecule described in claim 1, wherein the fiavivirus is St. Louis encephalitis virus. 32. The host cell harboring the nucleic acid progenitor described in claim 9 wherein the fiavivirus is St. Louis encephalitis virus. 33. The vaccinating composition described in claim 13, wherein the fiavivirus is St. Louis encephalitis virus. 34 The method of immunizing a subject against infection by a fiavivirus described in claim 21 , wherein the fiavivirus is St. Louis encephalitis virus

Inventors: CHANG GWONG-JEN J, CHANG, GWONG-JEN, J.
Applicants: CHANG GWONG JEN J, US HEALTH, CHANG, GWONG-JEN, J., THE GOVERNMENT OF THE UNITED STATES OF AMERICA AS REPRESENTED BY THE SECRETARY OF THE DEPARTMENT OF HEALTH AND HUMAN SERVICES
Application year: 1999
Main classification: C12N15
IPC codes: A61K31, A61K39, A61K48, A61P31, C07K14, C07K16, C12N5, C12N15, C12Q1, G01N33