METHOD FOR INTRODUCING AND EXPRESSING RNA IN ANIMAL CELLS

Abstract: The invention pertains to a method for introducing RNA molecules into eukaryotic cells, wherein the RNA molecules are capable of being translated in the eukaryotic cells or is an antisense RNA or a catalytic RNA, as well as to such bacteria, compositions comprising such bacteria, and nucleic acids which can be introduced into bacteria for practicing the method of the invention. Examples of products the RNA molecule may encode include vaccine antigens, therapeutic agents, immunoregulatory agents or anti-sense RNA molecules or catalytic RNA molecules.

Description: METHOD FOR INTRODUCING AND EXPRESSING RNA IN ANIMAL CELLS Background of the Invention The advent of recombinant DNA technology has greatly accelerated the development of vaccines to control epidemic, endemic, and pandemic infectious diseases (Woodrow et al, New Generation Vaccines: The Molecular Approach, Eds , Marcel Dekker, Inc , New York, NY (1989), Cryz, Vaccines and lmmunotherapy, Ed , Pergamon Press, New York, NY (1991), and Levine et al, Ped Ann , 22 719-725 (1993)) In particular, this technology has enabled the growth of nucleic acid vaccines Although the first nucleic acid vaccine was only reported in 1992, the study of nucleic acid vaccines has grown dramatically and examples of this approach have been reported in a wide array of animals using numerous antigens (Tang, D C , et al (1992) Nature 356 152, Fynan, E F et al (1993) PNAS USA 90 11478, Donnelly, J J et al (1995) Nat Med 1 583, Wang, B et al (1993) PNAS USA 90 4156, Davis, H L , et al (1993) Hum Mol Genet 2 1847, Ulmer, J B et al (1993) Science 259 1745, Robinson, H L et al (1993) Vaccine 11 957, Eisenbraun, M D et al (1993) DNA Cell Biol 12 791, Wang, B et al (1994) AIDS Res Hum Retroviruses 10 S35, Coney, L et al (1994) Vaccine 12 1545, Sedegah, M et al (1994) Proc Natl Acad Sci USA 91 9866, Raz, E et al (1994) ProcNatl Acad Sci USA 91 9519, Xiang, Z Q et al (1994) Virology 199 132) (for a more comprehensive list see URL http //www genweb com/ Dnavax/Biblio/articles html) Early murine experiments with HIV-1 DNA vaccines produced impressive immunological responses, including neutralizing antibody responses against HIV-1 and strong CTL responses against several HI V-l antigens (Wang, B et al (1993) supra, Wang, B et al (1994) supra, Coney, L et al (\994) supra, Lu, S et al (1995) Virology 209 147, Shiver, J W et al (1995) Annals of the New York Academy of Sciences 772 198, Wahren, B et al (1995) Ann N Y Acad Sci 772 278) However, an initial attempt to induce protective immunity with an SIV DNA vaccine in Rhesus monkeys was disappointing (Lu, S et al (1996) J Virol 70 3978) In contrast, an HIV-1 \(N Env DNA vaccine induced measurable protection against a chimeric SIV/HIV (SHPV) challenge in vaccinated cynomologous macaques (Boyer, J D et al (1996) Journal of Medical Primatology 25 242) More recently, intramuscular immunization of chimpanzees with this latter vaccine engendered protection against parenteral challenge with HIV-l SF - 2 (Boyer, J.D. et al. (1997) Nature Medicine 3 :526). These differences may have resulted from antigenic differences in the vaccines or differences in the potency of the challenges (Lu, S. et al. (\996) supra; Boyer, J.D. et al. (1996) supra; Boyer, J.D. et al. (1997) supra). It is noteworthy to mention that in the aforementioned chimpanzee study, DNA vaccination only induced modest humoral and cellular responses despite giving 9 doses of vaccine containing a total of 2.9 mg of DNA before challenge (Boyer, J.D. et al. (1997) supra). Thus, although these results are encouraging, the immunogenicity of HIV- 1 DNA vaccines must be improved before this approach achieves practical utility in large scale vaccination programs. The mechanism though which DNA vaccines induce immunity is not fully understood. Muscle cells express low levels of MHC class 1 and do not express detectable levels of co- stimulatory molecules B7-1 and B7-2 (review by Ertl, H.C. and Z.Q. Xiang (1996) Journal of Immunology 156:3579). While it remains conceivable that muscle cells may serve as an antigen depot (Ertl and Xiang (1996) supra), their participation in the induction of MHC class I and II responses may be secondary to other antigen presenting cells (Ertl and Xiang (1996) supra). Xiang and Ertl (Ertl and Xiang (1996) supra; Xiang, Z. and H.C. Ertl (1995) Immunity 2:129) have suggested that resident dendritic cells may be involved in the primary inductive events. They showed that co-expression of GM-CSF, a cytokine know to activate growth of dendritic cells, at the site of inoculation resulted in a more rapid response to DNA vaccine encoded antigens (Xiang and Ertl (1995) supra). In contrast, co-expression of IFN-γ diminished the responses (Xiang and Ertl (1995) supra). In agreement, Manickan et al. ((1997) Journal of Leukocyte Biology 61 : 125) showed that immunization with dendritic cells transfected with a DNA vaccine induced elevated immune responses, compared to the identical DNA vaccine given alone. In addition, dendritic cells have been shown to express antigen following intradermal vaccination with a DNA vaccines (Raz, E. et al. (1994) supra). Although inconclusive, these data strongly suggest that dendritic cells may play a substantial role in the presentation of DNA vaccine-encode antigens. Another new class of vaccines are bacterial vector vaccines (Curtiss, In: New Generation Vaccines: The Molecular Approach, Ed., Marcel Dekker, Inc., New York, NY, pages 161-188 and 269-288 (1989); and Mims et al, In: Medical Microbiology, Eds., Mosby- Year Book Europe Ltd., London (1993)). These vaccines can enter the host, either orally, intranasally or parenterally Once gaining access to the host, the bacterial vector vaccines express an engineered prokaryotic expression cassette contained therein that encodes a foreign antigen(s) Foreign antigens can be any protein (or part of a protein) or combination thereof from a bacterial, viral, or parasitic pathogen that has vaccine properties (New Generation Vaccines: The Molecular Approach, supra, Vaccines and Immunotherapy, supra, Hilleman, Dev Biol Stand, 82 3-20 (1994), Formal et al, Infect Immun 34 746-751 (1981), Gonzalez et al, J Infect Dis , 169 927-931 (1994), Stevenson et al, FEMS Lett , 28 317-320 (1985), Aggarwal et al, J Exp Med, 111 1083-1090 (1990), Hone et al, Microbial Path , 5 407-418 (1988), Flynn et al, Mol Mwrobiol , 4 21 1 1-21 18 (1990), Walker et al, Infect Immun , 60 4260-4268 (1992), Cardenas et al, Vacc , U 126-135 (1993), Curtiss et al, Dev Biol Stand , 82 23-33 (1994), Simonet et al, Infect Immun , 62 863-867 (1994), Charbit et al, Vacc , 11 1221-1228 (1993), Turner et al, Infect Immun , 61 5374-5380 (1993), Schodel et al, Infect Immun , 62 1669-1676 (1994), Schodel et al, J Immunol , 145 4317-4321 (1990), Stabel et al, Infect Immun , 59 2941-2947 (1991), Brown, J Infect Dis , 1_55 86-92 (1987), Doggett et al, Infect Immu , 61 1859-1866 (1993), Brett et al, Immunol , 80 306-312 (1993), Yang et al, J Immunol , 145 2281-2285 (1990), Gao et al, Infect Immun , 60 3780- 3789 (1992), and Chatfield et al, B oXechnology, 10 888-892 (1992)) Delivery of the foreign antigen to the host tissue using bacterial vector vaccines results in host immune responses against the foreign antigen, which provide protection against the pathogen from which the foreign antigen originates (Mims, 77?e Pathogenesis of Infectious Disease, Academic Press, London (1987), and New Generation Vaccines: The Molecular Approach, supra) Of the bacterial vector vaccines, live oral Salmonella vector vaccines have been studied most extensively There are numerous examples showing that Salmonella vectors are capable of eliciting humoral and cellular immunity against bacterial, viral and parasitic antigens (Formal et al, Infect. Immun., 34 746-751 (1981), Gonzalez et al, supra, Stevenson et al, supra, Aggarwal et al, supra, Hone et al, supra, Flynn et al, supra, Walker et al, supra, Cardenas et al, supra, Curtiss et al, supra, Simonet et al, supra, Charbit et al, supra, Turner et al, supra, Schodel et al, supra, Schodel et al (1990), supra, Stabel et al, supra, Brown, supra, Doggett et al, supra, Brett et al, supra, Yang et al, supra, Gao et al, supra, and Chatfield et al, supra) These humoral responses occur in the mucosal (Stevenson et al, supra, Cardenas et al, supra, Walker et al, supra, and Simonet et al, supra) and systemic compartments (Gonzalez et al, supra; Stevenson et al, supra; Aggarwal et al, supra; Hone et al, supra; Flynn et al, supra; Walker et al, supra; Cardenas et al, supra; Curtiss et al, supra; Simonet et al, supra; Charbit et al, supra; Turner et al, supra; Schodel et al, supra, Schodel et al (1990), supra; Stabel et al, supra; Brown, supra; Doggett et al, supra; Brett et al, supra; Yang et al, supra; Gao et al, supra; and Chatfield et al, supra). Live oral Salmonella vector vaccines also elicit T cell responses against foreign antigens (Wick et al, Infect. Immun., 62:4542-4548 (1994)). These include antigen-specific cytotoxic CD8 + T cell responses (Gonzalez et al, supra; Aggarwal et al, supra; Flynn et al, supra; Turner et al, supra; and Gao et al, supra). Ideally, bacterial vector vaccines are genetically defined, attenuated and well-tolerated by the recipient animal or human, and retain immunogenicity (Hone et al, Vaccine, 9:810-816 (1991); Tacket et al, Infect. Immun., 60:536-541 (1992); Hone et al, J. Clin. Invest., 90:412- 420 (1992); Chatfield et al, Vaccine, 10:8-1 1 (1992); Tacket et al, Vaccine, 10:443-446 (1992); and Mims, supra). Recently, the number of potential bacterial vector vaccines for the delivery of prokaryotic expression cassettes has grown. They now include, but are not restricted to Yersinia enterocolitica (van Damme et al, Gastroenterol, 103:520-531 (1992)), Shigella spp. (Noriega et al, Infect. Immun. , 62:5168-5172 (1994)), Vibrio cholerae (Levine et al, In: Vibrio cholerae, Molecular to Global Perspectives, Wachsmuth et al, Eds, ASM Press, Washington, D.C., pages 395-414 (1994)), Mycobacterium strain BCG (Lagranderie et al, Vaccine, 11:1283-1290 (1993); Flynn, Cell. Molec. Biol, 40rSuppl. l :31-36 (1994)), and Listeria monocytogenes (Schafer et al, J. Immunol, 149:53-59 (1992)) vector vaccines. The commercial application of DNA delivery technology to animal cells is broad and includes, in addition to vaccine antigens, delivery of immunotherapeutic agents and therapeutic agents (Darris et al, Cancer, 14(3 Suppl.V.1021-1025 (1994); Magrath, Ann. Oncol, 5fSuρpl L):67-70 (1994); Milligan et al, Ann. NY Acad. Sci., 716:228-241 (1994); Schreier, Pharma. ActaHelv., 68:145-159 (1994); Cech, Biochem. Soc. Trans., 21:229-234 (1993); Cech, Gene, 135:33-36 (1993); Long et al, FASEB J, 7:25-30 (1993); and Rosi et al, Pharm. Therap., 50:245-254 1991)). The delivery of endogenous and foreign genes to animal tissue for gene therapy has shown significant promise in experimental animals and volunteers (Nab el, Circulation, 91 :541- 548 (1995); Coovert et al, Curr. Opin. Neuro., 7:463-470 (1994); Foa, Bill. Clin. Haemal, 7:421-434 (1994); Bowers et al, J Am. Diet. Assoc, 95:53-59 (1995); Perales et al, Ewr. J. Biochem., 226 255-266 (1994), Danko et al, Vacc, U 1499-1502 (1994), Conry et al, Cane. Res., 54 1 164-1 168 (1994), and Smith, J Hemat, I 155-166 (1992)) From the onset nucleic acid vaccine studies focused on the use of DNA vaccines (Tang et al (1 92) supra; Fynan et al (1993) supra, Donnelly et al (1995) supra; Wang et al (1993) supra, Davis et al (1993) supra, Ulmer et al (1993) supra, Robinson et al (1993) supra, Eisenbraun et al (1993) supra, Wang et al (1994) supra, Coney et al. (1994) supra, Sedegah et al (1994) supra, Raz et al (1994) supra, Xiang et al (1994) supra) More recently the use of RNA vaccines has been proposed as an alternative approach to the injection of DNA based nucleic vaccines (Zhou, X et al (1994) Vaccine 12 1510, Conry, R M et al (1995) Cancer Res 55 1397) In support, "naked" RNA vaccines have proven modestly immunogenic in mice (Zhou et al (1994) supra, Conry et al (1995) supra) An RNA vaccine based on a recombinant Semliki Forrest Virus that expressed the SIV-PBJ 14 Env gene engendered protection against SIV-PBJ 14 (Mossman, S P et al (\991) Journal of Virology 70 1953) RNA vaccines would offer two main advantages of over DNA vaccines First, RNA vaccines would avoid placing vaccinees at risk of an integration event, which over a human life span might lead to the development of malignancy Second, RNA vaccines would avoid the barrier function of the nuclear membrane This is particularly relevant given that antigen expression in non-replicating antigen presenting cells is central to the induction of immunity using nucleic acid vaccines (Ertl and Xiang (1996) supra, Xiang and Ertl (1995) supra, Manickan et al (1997) supra) Thus, it is desirable to have an efficient method of delivering RNA to eukaryotic cells, such as mammalian cells, such that, the RNA can be expressed in the eukaryotic cell Furthermore, it is also desirable to have a system permitting efficient delivery of RNA molecules to mucosal tissue in addition to permitting parenteral delivery of RNA molecules Summary of the Invention The invention provides a system for delivery of RNA molecules to eukaryotic cells, e g , cells of mucosal tissue The invention is based at least in part on the discovery that bacteria which are capable of invading eukaryotic cells can deliver RNA molecules to eukaryotic cells and tissues, and where appropriate, the RNA can be translated if the RNA contains the appropriate regulatory elements Accordingly, in one embodiment, the invention provides an isolated bacterium comprising a DNA which is transcribed into a messenger RNA molecule in the bacterium, wherein the RNA is capable of being translated in a eukaryotic cell, or is an antisense RNA or a catalytic RNA The DNA can be operably linked to a prokaryotic promoter, e g , the E coli NirB promoter Alternatively, the DNA can be operably linked to a first promoter, and the bacterium further comprises a gene encoding a polymerase, which is capable of mediating transcπption from the first promoter, wherein the gene encoding the polymerase is operably linked to a second promoter In a preferred embodiment the second promoter is a prokaryotic promoter In a preferred embodiment, the polymerase is a bacteπophage polymerase, e g , T7 polymerase, and the first promoter is a bacteπophage promoter, e g , T7 promoter The DNA which is capable of being transcribed into said RNA and the gene encoding a polymerase can be located on one or more plasmids However, in a preferred embodiment, the DNA is located on the bacterial chromosome In a preferred embodiment, the RNA can be translated in a eukaryotic cell For allowing efficient translation in a eukaryotic cell, the RNA preferably comprises a Cap Independent Translation Enhancer (CITE) sequence The RNA can further comprise additional regulatory elements, which can, e g , affect the stability of the RNA in the eukaryotic cell, e g , polyA tail The RNA can encode one polypeptide Alternatively, the RNA can be polycistronic and encode more than one polypeptide The polypeptide can be, e g , a vaccine antigen or an immunoregulatory molecule The polypeptide can further be an endogenous or a foreign polypeptide Foreign polypeptides include prokaryotic, e g , bacterial or viral polypeptides In another preferred embodiment, the RNA is an antisense RNA or a catalytic RNA, e g , πbozyme Preferred antisense RNAs or catalytic RNAs are capable of hybridizing to a nucleic acid in the eukaryotic cell, to thereby, e g , regulate synthesis of a gene product In another embodiment, the invention provides an isolated bacterium comprising an RNA which is capable of being translated in a eukaryotic cell, or is an antisense RNA, or a catalytic RNA In a preferred embodiment, the RNA is transcπbed in the bacterium, e g , from introduced DNA In another embodiment, the RNA is introduced into the bacterium by, e g , electroporation Preferred bacteria of the invention are invasive, i e , capable of delivering at least one molecule, e g., an RNA molecule, to a target cell, such as by invading the cytoplasm of the cell Even more preferred bacteria are live bacteria, e g , live invasive bacteria More specifically, preferred bacteria of the invention are those capable of invading a vertebrate cell, e g , a mammalian cell, such as a cell selected from the group consisting of a human, cattle, sheep, goat, horse, and primate cell A preferred invasive bacterium is Shigella, which is naturally invasive vis a vis vertebrate cells At least one advantage of Shigella RNA vaccine vectors is their tropism for lymphoid tissue in the colonic mucosal surface In addition, the primary site of Shigella replication is believed to be within dendritic cells and macrophages, which are commonly found at the basal lateral surface of M cells in mucosal lymphoid tissues Thus, Shigella vectors provide a means to express antigens in these professional antigen presenting cells and thereby induce an immune response, e g , a vaccine antigen Other naturally invasive bacteria include Listeria spp., Rickettsia spp. and enteroinvasive Escherichia coli The term "spp. " refers to species of the genus preceding this term In another embodiment, a bacterium can be modified, such as by genetic engineering means, to increase its invasive potential In a preferred embodiment, the bacterium has been genetically engineered to mimic the invasion properties of Shigella spp. , Listeria spp. , Rickettsia spp. and enteroinvasive Escherichia coli Any bacterium can be modified to increase its invasive potential and can be, e g , a bacterium selected from the group consisting of Yersinia spp. , Escherichia spp. , Klebsiella spp. , Bordetella spp. , Neisseria spp. , A eromonas spp. , Franciesella spp. , Coryne bacterium spp. , Citrobacter spp. , Chlamydia spp. , Hemophilus spp., Brucella spp., Mycobacterium spp., Legionella spp., Rhodococcus spp., Pseudomonas spp. , Hehcobacter spp. , Salmonella spp. , Vibrio spp. , Bacillus spp. , Leishmama spp. and Erysipelothrix spp. In another embodiment, the bacterium is modified with an invasion factor, e g , the bacterium is coated with an invasion factor, e g , invasin The bacterium of the invention is preferably non harmful to a subject to whom a bacterium of the invention is administered Accordingly, the bacterium can be a naturally non pathogenic bacterium Alternatively, the bacterium can be an attenuated bacterium The invention further provides a pharmaceutical composition comprising any of the above-described bacteria and a pharmaceutically acceptable carrier, e g , a physiological buffer and a hpoprotectant Such pharmaceutical compositions can be used, e g , for vaccinating an individual Also within the scope of the invention are eukaryotic cells, e g , human cells, comprising any of the above-described bacteria The invention also provides isolated DNA operably linked to a prokaryotic promoter, wherein the DNA encodes RNA which is capable of being translated in a eukaryotic cell, or is an antisense RNA or a catalytic RNA, e g , πbozyme Preferred prokaryotic promoters are the E coli Up promoter and NirB promoter The invention further provides a method for introducing RNA into a eukaryotic cell According to the invention, a eukaryotic cell is contacted with at least one invasive bacterium comprising a DNA molecule which is either capable of being transcribed into RNA in the bacteπum or compπses RNA wherein the RNA is capable of being translated in a eukaryotic cell or is an antisense RNA or catalytic RNA The step of contacting the eukaryotic cell with at least one invasive bacterium can be performed in vitro at a multiplicity of infection ranging from about 0 1 to about 10 6 bacteria per eukaryotic cell The contacting step is preferably performed in vitro at a multiplicity of infection from about 10 2 to about 10 4 bacteria per eukaryotic cell In one embodiment, the contacting step is performed in vitro, and can, e g , further compπse the step of administeπng the eukaryotic cell to a subject In another preferred embodiment, the contacting step is performed in vivo, and comprises, e g , administering to a subject the at least one bacteπum, but preferably no more than about 10" bacteria of the invention In a preferred embodiment, from about 10 5 to about 10 9 bacteria are administered to a subject The bacteria can be administered, e g , orally, intrarectally, or intranasaly to the subject The bacterium can also be administered parenterally The invasive bacteπum can be cell type specific Alternatively, the invasive bacterium can be capable of invading one or more cell types The invasive bacterium can also be modified to change its target specificity, e g , by genetic engineering and/or by linking a specific targeting factor to the bacterium In yet another embodiment, a non-invasive bacterium is be modified to become invasive The bacterium can also be modified by engineering the bacterium to contain a suicide gene The eukaryotic cell to which the bacteπum of the invention is targeted can be any type of cell A preferred cell is from a mucosal tissue In one embodiment, the cell is a natural target of the bacterium In another embodiment, the target cell is modified, e g , genetically to contain a surface receptor necessary for mediating the interaction between the bacterium and the target cell. Thus, the method of the invention retains all the advantages and properties of introducing RNA into a eukaryotic cell and provides a more efficient manner to deliver the RNA to the target eukaryotic cell. The advantages of introducing RNA into a eukaryotic cell instead of a DNA molecule include (i) avoidance of risk of insertion of DNA into the genome of the target eukaryotic cell and thus strongly reduced risk of mutation of the target eukaryotic cell; (ii) absence of need for the nucleic acid introduced in the eukaryotic cell to traverse the nuclear membrane; and (iii) avoidance of the possibility of shedding of plasmid molecules from the bacteria. Delivery of RNA to eukaryotic cells by use of a bacterium, compared to delivery of "naked" RNA e.g., where expression of the RNA is desired, provides at least the advantage that the RNA is protected and less likely to be degraded prior to entering the eukaryotic cell. Furthermore, the RNA can be specifically targeted to certain types of cells, since the bacterium can naturally target or be modified to target specific types of cells, e.g., antigen presenting cells in the mucosal lymphoid tissue. Furthermore, the invention provides methods and compositions for oral vaccines, in particular, for an oral mucosal HIV-1 vaccine. Historically, oral vaccines have proven to be an efficacious means to invoke mucosal immunity. The invention provides oral vaccines using Shigella bacteria which possess specialized adaptations that allow this organism to invade and replicate in the cytoplasm of antigen presenting cells associated with the colonic lymphoid tissue, thus eliciting strong immune responses. Thus, the invention provides efficacious oral vaccines. Other features and advantages of the invention will be apparent from the following detailed description and claims. Brief Description of the Figures Figure 1 shows a nucleotide sequence of a CITE sequence from the encephalomyocarditis virus (ECMV) 5 1 non-coding region. Detailed Description of the Invention The invention is based at least in part on the discovery that bacteria, which are capable of invading eukaryotic cells, can deliver RNA to eukaryotic cells Furthermore, if the RNA contains the necessary eukaryotic translation recognition elements, the RNA can be efficiently translated in the eukaryotic cell to which the RNA was delivered by the bacteπum Accordingly, in one embodiment, the invention provides a bacterium comprising DNA which is capable of being transcribed into RNA in the bacteπum, wherein the RNA is capable of being translated in a eukaryotic cell, or is an antisense RNA or a catalytic RNA In a preferred embodiment, the invention provides a mucosal RNA bacterial vector, capable of efficiently dehveπng RNA to mucosal cells Such RNA bacterial vectors are useful as vaccines since they can be administered orally and will be targeted to sites which are rich in professional antigen presenting cells, e g, macrophages and dendritic cells, thus eliciting a strong immune response to the vaccine antigen Thus, the invention provides live oral bacterial vaccine vectors 1 Bacteria Delivering RNA to Eukaryotic Cells According to the invention, any microorganism which is capable of delivering a molecule, e g , an RNA molecule, into the cytoplasm of a target cell, such as by traversing the membrane and entering the cytoplasm of a cell, can be used to deliver RNA to such cells In a preferred embodiment, the microorganism is a prokaryote In an even more preferred embodiment, the prokaryote is a bacterium Also within the scope of the invention are microorganisms other than bacteria which can be used for delivering RNA to a cell For example, the microorganism can be a fungus, e g , Cryptococcus neoformans, protozoan, e g , Trypanosoma cru∑i, Toxoplasma gondu, Leishmama donovam, and plasmodia In certain embodiments, the mircroorganism is a virus As used herein, the term "invasive" when referring to a microorganism, e g , a bacteπum, refers to a microorganism which is capable of delivering at least one molecule, e g , an RNA molecule, to a target cell An invasive microorganism can be a microorganism which is capable of traversing a cell membrane, thereby entering the cytoplasm of said cell, and deliveπng at least some of its content, e g , RNA, into the target cell The process of delivery of the at least one molecule into the target cell preferably does not significantly modify the mvasion apparatus In a preferred embodiment, the microorganism is a bacterium A preferred invasive bacterium is a bacterium which is capable of delivering at least one molecule, e g , an RNA molecule, to a target cells, such as by entering the cytoplasm of a eukaryotic cell Preferred invasive bacteria are live bacteria, e g , live invasive bacteria Invasive microorganisms include microorganisms that are naturally capable of deliveπng at least one molecule to a target cell, such as by traversing the cell membrane, e g , a eukaryotic cell membrane, and entering the cytoplasm, as well as microorganisms which are not naturally invasive and which have been modified, e g , genetically modified, to be invasive In another preferred embodiment, a microorganism which is not naturally invasive can be modified to become invasive by linking the bacteπum to an "invasion factor", also termed "entry factor" or "cytoplasm-targeting factor" As used herein, an "invasion factor" is a factor, e g , a protein, which, when expressed by a non-invasive bacterium, renders the bacterium invasive As used herein, an "invasion factor" is encoded by a "cytoplasm-targeting gene" Naturally invasive microorganisms, e g , bacteria, may have a certain tropism, i e , preferred target cells Alternatively, microorganisms, e g , bacteria can be modified, e g , genetically, to mimic the tropism of a second microorganism Delivery of at least one molecule into a target cell can be determined according to methods known in the art For example, the presence of the molecule, such as an RNA molecule or polypeptide encoded thereby, can be detected by hybridization or PCR methods, or by immunological methods which may include the use of an antibody Determining whether a microorganism is sufficiently invasive for use in the invention may include determining whether sufficient RNA, or polypeptide encoded thereby, was delivered to host cells, relative to the number of microorganisms contacted with the host cells If the amount of RNA, or polypeptide encoded thereby, is low relative to the number of microorganisms used, it may be desirable to further modify the microorganism to increase its invasive potential Bacterial entry into cells can be measured by various methods Intracellular bacteria survive treatment by aminoglycoside antibiotics, whereas extracellular bacteria are rapidly killed A quantitative estimate of bacterial uptake can be achieved by treating cell monolayers with the antibiotic gentamicin to inactivate extracellular bacteria, then by removing said antibiotic before liberating the surviving intracellular organisms with gentle detergent and determining viable counts on standard bacteriological medium Furthermore, bacterial entry into cells can be directly observed, e g , by thin-section-transmission electron microscopy of cell layers or by lmmunofluorescent techniques (Falkow et al (1 92) Annual Rev Cell Biol 8 333) Thus, various techniques can be used to determine whether a specific bacteria is capable of invading a specific type of cell or to confirm bacterial invasion following modification of the bacteria, such modification of the tropism of the bacteria to mimic that of a second bacterium Bacteπa that can be used for dehveπng RNA according to the method of the invention are preferably non-pathogenic However, pathogenic bacteria can also be used, so long as their pathogenicity has been attenuated, to thereby render the bacteria non-harmful to a subject to which it is administrated As used herein, the term "attenuated bacteπum" refers to a bacterium that has been modified to significantly reduce or eliminate its harmfulness to a subject A pathogenic bacterium can be attenuated by various methods, set forth below Without wanting to be limited to a specific mechanism of action, the bacterium delivering the RNA into the eukaryotic cell can enter various compartments of the cell, depending on the type of bacterium For example, the bacterium can be in a vesicle, e g , a phagocytic vesicle Once inside the cell, the bacterium can be destroyed or lysed and its contents delivered to the eukaryotic cell A bacterium can also be engineered to express a phagosome degrading enyzme to allow leakage of RNA from the phagosome In some embodiments, the bacterium can stay alive for various times in the eukaryotic cell and may continue to produce RNA The RNA can then be released from the bacterium into the cell by, e g , leakage In certain embodiments of the invention, the bacterium can also replicate in the eukaryotic cell In a preferred embodiment, bacterial replication does not kill the host cell The invention is not limited to delivery of RNA by a specific mechanism and is intended to encompass methods and compositions permitting delivery of RNA by a bacterium independently of the mechanism of delivery of the RNA Set forth below are examples of bacteπa which have been described in the literature as being naturally invasive (section 1 1), as well as bacteria which have been described in the literature as being naturally non-invasive bacteπa (section 1 2), as well as bacteria which are naturally non-pathogenic or which are attenuated Although some bacteria have been described as being non-invasive (section 1 2), these may still be sufficiently invasive for use according to the invention Whether traditionally described as naturally invasive or non- invasive, any bacterial strain can be modified to modulate, in particular to increase, its invasive characteristics (e g., as described in section 1 3) 1.1 Naturally Invasive Bacteria The particular naturally invasive bacteria employed in the present invention is not critical thereto Examples of such naturally-occurring invasive bacteria include, but are not limited to, Shigella spp., Salmonella spp., Listeria spp., Rickettsia spp., and enteroinvasive Escherichia coli The particular Shigella strain employed is not critical to the present invention Examples of Shigella strains which can be employed in the present invention include Shigella flexneri 2a (ATCC No 29903), Shigella sonnei (ATCC No 29930), and Shigella disentenae (ATCC No 13313) An attenuated Shigella strain, such as Shigella flexneri 2a 2457T aroA virG mutant CVD 1203 (Noriega et al, supra), Shigella flexneri M90T icsA mutant (Goldberg et al, Infect. Immun., 62 5664-5668 (1994)), Shigella flexneri Y SFL1 14 aroO mutant (Karnell et al, Vacc, VO 167-174 (1992)), and Shigella flexneri aroA aroD mutant (Verma et al, Vacc, 9 6-9 (1991)) are preferably employed in the present invention Alternatively, new attenuated Shigella spp. strains can be constructed by introducing an attenuating mutation either singularly or in conjunction with one or more additional attenuating mutations At least one advantage to Shigella RNA vaccine vectors is their tropism for lymphoid tissue in the colonic mucosal surface In addition, the primary site of Shigella replication is believed to be within dendritic cells and macrophages, which are commonly found at the basal lateral surface of M cells in mucosal lymphoid tissues (reviewed by McGhee, J R et al (1994) Reproduction, Fertility, & Development 6 369, Pascual, D W et al (1994) Immunomethods 5:56) As such, Shigella vectors may provide a means to express antigens in these professional antigen presenting cells Another advantage of Shigella vectors is that attenuated Shigella strains deliver nucleic acid reporter genes in vitro and in vivo (Sizemore, D.R et al (1995) Science 270 299, Courvalin, P et al (1995) Comptes Rendus de I Academie des Sciences Serie Ill-Sciences de la Vie-Life Sciences 318.1207, Powell, R J et al (1996) In: Molecular approaches to the control of infectious diseases F Brown, E Norrby, D Burton and J Mekalanos, eds Cold Spring Harbor Laboratory Press, New York 183, Anderson, R J et al (1997) Abstracts for the 97th General Meeting of the American Society for Microbiology E ) On the practical side, the tightly restricted host specificity of Shigella stands to prevent the spread of Shigella vectors into the food chain via intermediate hosts Furthermore, attenuated strains that are highly attenuated in rodents, primates and volunteers have been developed (Anderson et al (1997) supra, Li, A et al (1992) Vaccine 10 395, Li, A et al (1993) Vaccine 1 1 180, Karnell, A et al (1995) Vaccine 13 88, Sansonetti, P J and J Arondel (1989) Vaccine 7443, Fontaine, A et al (1990) Research in Microbiology 141 907; Sansonetti, P J et al (1991) Vaccine 9 416, Noriega, F R et al. (1994) Infection & Immunity 62 5168, Noriega, F R et al (1996) Infection & Immunity 64 3055, Noriega, F R et al (1996) Infection & Immunity 64 23, Noriega, F R et al (1996) Infection & Immunity 64 3055, Kotlofζ K L et al (1996) Infection & Immunity 64 4542) This latter knowledge will allow the development of well tolerated Shigella vectors for use in humans Attenuating mutations can be introduced into bacterial pathogens using non-specific mutagenesis either chemically, using agents such as N-methyl-N'-nitro-N-nitrosoguanidine, or using recombinant DNA techniques, classic genetic techniques, such as Tn/0 mutagenesis, P22-mediated transduction, λ phage mediated crossover, and conjugational transfer, or site-directed mutagenesis using recombinant DNA techniques Recombinant DNA techniques are preferable since strains constructed by recombinant DNA techniques are far more defined Examples of such attenuating mutations include, but are not limited to (i) auxotrophic mutations, such as aro (Hoiseth et al, Nature, 291 238-239 (1981)), gua (McFarland et al, Microbiol. Path., 3 129-141 (1987)), nad (Park et al, J. Bad, 170 3725-3730 (1988), thy (Nnalue et al, Infect. Immun., 55 955-962 (1987)), and asd (Curtiss, supra) mutations, (ii) mutations that inactivate global regulatory functions, such as cya (Curtiss et al, Infect. Immun., 55 3035-3043 (1987)), crp (Curtiss et al (1987), supra), phoVlphoQ (Groisman et al, Proc Natl. Acad. Sci., USA, 86:7077-7081 (1989), and Miller et al, Proc. Natl. Acad. Sci., USA, 86 5054-5058 (1989)), phoP < (Miller et al, J. Bact. , 172.2485-2490 ( 1990)) or ompR (Dorman et al, Infect. Immun., 57 2136-2140 (1989)) mutations, (iii) mutations that modify the stress response, such as recA (Buchmeier et al, Mol. Micro., 7.933-936 (1993)), htrA (Johnson et al, Mol. Micro, 5 401-407 (1991)), htpR (Neidhardt et al, Biochem. Biophys. Res. Com., 100 894-900 (1981)), hsp (Neidhardt et al, Ann. Rev. Genet, 18 295-329 (1984)) and gr EL (Buchmeier et al, Sci., 248 730-732 (1990)) mutations, (iv) mutations in specific virulence factors, such as IsyA (Libby et al, Proc. Natl. Acad. Sci., USA, 91 489-493 (1994)), pag or prg (Miller et al (1990), supra, and Miller et al (1989), supra), iscA or virG (d'Hauteville et al, Mol. Micro., 6 833-841 (1992)), p cA (Mengaud et al, Mol. Microbiol, 5 367-72 (1991), Camilli et al, J. Exp. Med, 173 751 -754 ( 1991 )), and act (Brundage et al, Proc. Natl. Acad. Sci., USA, 90 11890-1 1894 (1993)) mutations, (v) mutations that affect DNA topology, such as topA (Galan et al, Infect. Immun., 58 1879-1885 (1990)), (vi) mutations that disrupt or modify the cell cycle, such as mm (de Boer et al, Cell, 56.641-649 (1989)) (vii) introduction of a gene encoding a suicide system, such as sacB (Recorbet et al, App. Environ. Micro., 59 1361-1366 (1993), Quandt et al, Gene, 127 15-21 (1993)), n c (Ahrenholtz et al, App. Environ. Micro, 60 3746-3751 (1994)), hok, gefl hi, oτphlA (Molin et al, Ann. Rev. Microbiol , 47 139-166 (1993)), (viii) mutations that alter the biogenesis of lipopolysaccharide and/or lipid A, such as rβ (Raetz in Esherishia coli and Salmonella typhimurium, Neidhardt et al , Ed , ASM Press, Washington DC pp 1035-1063 (1996)), ga/E (Hone et al, J. Infect. Dis., 156 164-167 (1987)) and htrB (Raetz, supra), msbB (Reatz, supra) (ix) introduction of a bacteriophage lysis system, such as lysogens encoded by P22 (Rennell et al, Virol, 143 280-289 (1985)), λ murein transglycosylase (Bienkowska-Szewczyk et al, Mol. Gen. Genet., 184 111-114 (1981)) or S- gene (Reader et al, Virol, 43 623-628 (1971)), and The attenuating mutations can be either constitutively expressed or under the control of inducible promoters, such as the temperature sensitive heat shock family of promoters (Neidhardt et al, supra), or the anaerobically induced nirB promoter (Harborne et al, Mol. Micro., 6-2805-2813 (1992)) or repressible promoters, such as uapA (Gorfinkiel et al, J. Biol. Chem., 268 23376-23381 (1993)) or gcv (Stauffer et al, J Bact. , 176 6159-6164 ( 1994)) The particular Listeria strain employed is not critical to the present invention Examples of Listeria strains which can be employed in the present invention include Listeria monocytogenes (ATCC No 15313) Attenuated Listeria strains, such as L. monocytogenes act A mutant (Brundage et al, supra) or L. monocytogenes pic A (Camilli et al, J Exp. Med., 171 751-754 (1991)) are preferably used in the present invention Alternatively, new attenuated Listeria strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above The particular Salmonella strain employed is not critical to the present invention Examples of Salmonella strains which can be employed in the present invention include Salmonella typhi (ATCC No 7251) and S. fyphimurium (ATCC No 1331 1) Attenuated Salmonella strains are preferably used in the present invention and include S. typhi-aroC-aroD (Hone et al Vacc 9 810 (1991) and S. typhimunum-aroA mutant (Mastroeni et al Micro Pathol 13 477 (1992)) Alternatively, new attenuated Salmonella strains can be constructed by introducing one or more attenuating mutations as described fro Shigella spp above The particular Rickettsia strain employed is not critical to the present invention Examples of Rickettsia strains which can be employed in the present invention include Ricketsia rickettsiae (ATCC Nos VR149 and VR891), Ricketsia prowaseckn (ATCC No VR233), Ricketsia tsutsugamuchi (ATCC Nos VR312, VR150 and VR609), Ricketsia mooseri (ATCC No VR144), Ricketsia sibinca (ATCC No VR151), and Rochahmaea quitana (ATCC No VR358) Attenuated Ricketsia strains are preferably used in the present invention and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular enteroinvasive Escherichia strain employed is not critical to the present invention Examples of enteroinvasive Escherichia strains which can be employed in the present invention include Escherichia coli strains 4608-58, 1184-68, 53638-C-17, 13-80, and 6-81 (Sansonetti et al, Ann. Microbiol. (Inst. Pasteur), 132A 351-355 (1982)) Attenuated enteroinvasive Escherichia strains are preferably used in the present invention and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above Furthermore, since certain microorganisms other than bacteria can also interact with integrin molecules (which are receptors for certain invasion factors) for cellular uptake, such microorganisms can also be used for introducing RNA into target cells For example, viruses, e g , foot-and-mouth disease virus, echovirus, and adenovirus, and eukaryotic pathogens, e g , Histoplasma capsulatum and Leishmania major interact with integrin molecules 1 2 Less Invasive Bacteria Examples of bacteria which can be used in the invention and which have been described in the literature as being non-invasive or at least less invasive than the bacteria listed in the previous section (1 1) include, but are not limited to, Yersinia spp., Escherichia spp., Klebsiella spp., Bordetella spp., Neisseria spp., Aeromonas spp., Franciesella spp., Corynebacterium spp., Citrobacter spp., Chlamydia spp., Hemophilus spp. , Brucella spp. , Mycobacterium spp. , Legionella spp. , Rhodococcus spp., Pseudomonas spp., Helicobacter spp., Vibrio spp , Bacillus spp., and Erysipelothrix spp. It may be necessary to modify these bacteria to increase their invasive potential The particular Yersinia strain employed is not critical to the present invention Examples ofYersinia strains which can be employed in the present invention include Y. enterocohtica (ATCC P\o 9610) or Y. pestis (ATCC Po 19428) Attenuated Yersinia strains, such as Y. enter ocolitica Ye03-R2 (al-Hendy et al, Infect. Immun., 60 870-875 (1992)) or Y. enterocohtica aroA (O'Gaora et al, Micro. Path, 9 105-116 (1990)) are preferably used in the present invention Alternatively, new attenuated Yersinia strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above The particular Escherichia strain employed is not critical to the present invention. Examples of Escherichia strains which can be employed in the present invention include E. coli H10407 (Elinghorst et al, Infect. Immun., 60:2409-2417 (1992)), and E. coli EFC4, CFT325 and CPZ005 (Donnenberg et al, J. Infect. Dis., 169:831-838 (1994)). Attenuated Escherichia strains, such as the attenuated turkey pathogen E. coli 02 car AB mutant (Kwaga et al, Infect. Immun., 62:3766-3772 (1994)) are preferably used in the present invention. Alternatively, new attenuated Escherichia strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Klebsiella strain employed is not critical to the present invention. Examples of Klebsiella strains which can be employed in the present invention included, pneuoniae (ATCC No. 13884). Attenuated Klebsiella strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Bordetella strain employed is not critical to the present invention. Examples of Bordetella strains which can be employed in the present invention include B. bronchiseptica (ATCC No. 19395). Attenuated Bordetella strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Neisseria strain employed is not critical to the present invention. Examples of ' Neisseria strains which can be employed in the present invention include N. meningitidis (ATCC No. 13077) and N. gonorrhoeae (ATCC No. 19424). Attenuated Neisseria strains, such as N. gonorrhoeae MS 11 aro mutant (Chamberlain et al, Micro. Path., 15:51-63 (1993)) are preferably used in the present invention. Alternatively, new attenuated Neisseria strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Aeromonas strain employed is not critical to the present invention. Examples of Aeromonas strains which can be employed in the present invention included, eucrenophila (ATCC No. 23309). Alternatively, new attenuated Aeromonas strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Franciesella strain employed is not critical to the present invention. Examples of Franiesella strains which can be employed in the present invention include F. tularensis (ATCC No. 15482). Attenuated Franciesella strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Coryne bacterium strain employed is not critical to the present invention. Examples of Coryne bacterium strains which can be employed in the present invention include C. pseudotuberculosis (ATCC No. 19410). Attenuated Corynebacterium strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Citrobacter strain employed is not critical to the present invention. Examples of Citrobacter strains which can be employed in the present invention include C. freundii (ATCC No. 8090). Attenuated Citrobacter strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Chlamydia strain employed is not critical to the present invention. Examples of Chlamydia strains which can be employed in the present invention include C. pneumoniae (ATCC No. VR1310). Attenuated Chlamydia strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Hemophilus strain employed is not critical to the present invention. Examples of Hemophilus strains which can be employed in the present invention include H. sornnus (ATCC No. 43625). Attenuated Hemophilus strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Brucella strain employed is not critical to the present invention. Examples of Brucella strains which can be employed in the present invention include B. abortus (ATCC No. 23448). Attenuated Brucella strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Mycobacterium strain employed is not critical to the present invention. Examples of Mycobacterium strains which can be employed in the present invention include M intracellulare (ATCC No. 13950) andM tuberculosis (ATCC No. 27294). Attenuated Mycobacterium strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Legionella strain employed is not critical to the present invention. Examples of Legionella strains which can be employed in the present invention include L. pneumophila (ATCC No. 33156). Attenuated Legionella strains, such as Ά L. pneumophila mip mutant (Ott, FEMS Micro. Rev., 14: 161-176 (1994)) are preferably used in the present invention. Alternatively, new attenuated Legionella strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Rhodococcus strain employed is not critical to the present invention. Examples of Rhodococcus strains which can be employed in the present invention include R. equi (ATCC No. 6939). Attenuated Rhodococcus strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Pseudomonas strain employed is not critical to the present invention. Examples of Pseudomonas strains which can be employed in the present invention include P. aeruginosa (ATCC No. 23267). Attenuated Pseudomonas strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Helicobacter strain employed is not critical to the present invention. Examples of Helicobacter strains which can be employed in the present invention include H. mustelae (ATCC No. 43772). Attenuated Helicobacter strains are preferably used in the present invention, and can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Salmonella strain employed is not critical to the present invention. Examples of Salmonella strains which can be employed in the present invention include Salmonella typhi (ATCC No. 7251) and S. typhimurium (ATCC No. 1331 1 ). Attenuated Salmonella strains are preferably used in the present invention and include S. typhi aroC aro (Hone et al, Vacc, 9:810-816 (1991)) and S. typhimurium aro A mutant (Mastroeni et al, Micro. Pathol, 11:477-491 (1992))). Alternatively, new attenuated Salmonella strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Vibrio strain employed is not critical to the present invention. Examples of Vibrio strains which can be employed in the present invention include Vibrio cholerae (ATCC No. 14035) and Vibrio cincinnatiensis (ATCC No. 35912). Attenuated Vibrio strains are preferably used in the present invention and include V. cholerae RSI virulence mutant (Taylor et al, J Infect. Dis., 170: 1518-1523 (1994)) and V cholerae ctxA, ace, zot, cep mutant (Waldor et al, J. Infect. Dis., 170:278-283 (1994)). Alternatively, new attenuated Vibrio strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above. The particular Bacillus strain employed is not critical to the present invention. Examples of Bacillus strains which can be employed in the present invention include Bacillus subtilis (ATCC No. 6051). Attenuated Bacillus strains are preferably used in the present invention and include B. anthracis mutant pXOl (Welkos et al, Micro. Pathol, 14:381-388 (1993)) and attenuated BCG strains (Stover et al, Nat, 351 :456-460 (1991)). Alternatively, new attenuated Bacillus strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above The particular Erysipelothrix strain employed is not critical to the present invention Examples of Erysipelothrix strains which can be employed in the present invention include Erysipelothrix rhusiopathiae (ATCC No 19414) and Erysipelothrix tonsillarum (ATCC No 43339) Attenuated Erysipelothrix strains are preferably used in the present invention and include E. rhusiopathiae Kg- 1 a and Kg-2 (Watarai et al, J. Vet. Med. Sci., 55 595-600 (1993)) and E. rhusiopathiae ORVAC mutant (Markowska-Daniel et al, Int. J. Med. Microb. Virol. Parisit Infect. Dis. , 277 547- 553 (1992)) Alternatively, new attenuated Erysipelothrix strains can be constructed by introducing one or more attenuating mutations in groups (i) to (vii) as described for Shigella spp. above 1 3 Methods for Increasing the Invasive Properties of a Bacterial Strain Whether organisms have been traditionally described as invasive or non- invasive, these organisms can be engineered to increase their invasive properties, e.g , by mimicking the invasive properties of Shigella spp., Listeria spp., Rickettsia spp., or enteroinvasive E coli spp. For example, one or more genes that enable the microorganism to access the cytoplasm of a cell, e g , a cell in the natural host of said non-invasive bacteria, can be introduced into the microorganism Examples of such genes referred to herein as "cytoplasm-targeting genes" include genes encoding the proteins that enable invasion by Shigella or the analogous invasion genes of entero-invasive Escherichia, or listeriolysin O of Listeria, as such techniques are known to result in rendering a wide array of invasive bacteria capable of invading and entering the cytoplasm of animal cells (Formal et al, Infect. Immun., 46:465 (1984), Bielecke et al, Nature, 145 175-176 (1990), Small et al, In: Mιcrobiology-1986, pages 121-124, Levine et al, Eds , American Society for Microbiology, Washington, D C (1986), Zychlinsky et al, Molec Micro., \ \ 619-627 (1994), Gentschev et al (1995) Infection & Immunity 63 4202, Isberg, R R and S Falkow (1985) N rt/re 317 262, and lsberg, R R et al (1987) Cell 50 769) Methods for transferring the above cytoplasm-targeting genes into a bacterial strain are well known in the art. Another preferred gene which can be introduced into bacteria to increase their invasive character encodes the invasin protein from Yersinia pseudotuberculosis, (Leong et al. EMBO J, 9: 1979 (1990)). Invasin can also be introduced in combination with listeriolysin, thereby further increasing the invasive character of the bacteria relative to the introduction of either of these genes . The above genes have been described for illustrative purposes; however, it will be obvious to those skilled in the art that any gene or combination of genes, from one or more sources, that participates in the delivery of a molecule, in particular an RNA molelecule, from a microorganism into the cytoplasm of a cell, e.g., an animal cell, will suffice. Thus, such genes are not limited to bacterial genes, and include viral genes, such as influenza virus hemagglutinin HA-2 which promotes endosmolysis (Plank et al, J. Biol. Chem., 269: 12918-12924 (1994)). The above cytoplasm-targeting genes can be obtained by, e.g., PCR amplification from DNA isolated from an invasive bacterium carrying the desired cytoplasm-targeting gene. Primers for PCR can be designed from the nucleotide sequences available in the art, e.g., in the above-listed references and/or in GenBank, which is publically available on the internet (http:Wwww.ncbi.nlm.nih.gov/). The PCR primers can be designed to amplify a cytoplasm-targeting gene, a cytoplasm-targeting operon, a cluster of cytoplasm-targeting genes, or a regulon of cytoplasm-targeting genes. The PCR strategy employed will depend on the genetic organization of the cytoplasm-targeting gene or genes in the target invasive bacteria. The PCR primers are designed to contain a sequence that is homologous to DNA sequences at the beginning and end of the target DNA sequence. The cytoplasm-targeting genes can then be introduced into the target bacterial strain, e.g., by using Hfr transfer or plasmid mobilization (Miller, A Short Course in Bacterial Genetics, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1992); Bothwell et al, supra; and Ausubel et al, supra), bacteriophage-mediated transduction (de Boer, supra; Miller, supra; and Ausubel et al, supra), chemical transformation (Bothwell et al, supra; Ausubel et al, supra), electroporation (Bothwel et al, supra; Ausubel et al, supra; and Sambrook, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY) and physical transformation techniques (Johnston et al, supra; and Bothwell, supra). The cytoplasm-targeting genes can be incorporated into lysogenic bacteriophage (de Boer et al, Cell, 5_6:641-649 (1989)), plasmids vectors (Curtiss et al, supra) or spliced into the chromosome (Hone et al, supra) of the target strain. In addition to genetically engineering bacteria to increase their invasive properties, as set forth above, bacteria can also be modified by linking an invasion factor to the bacteria. Accordingly, in one embodiment, a bacterium is rendered more invasive by coating the bacterium, either covalently or non-covalently, with an invasion factor, e.g., the protein invasin, invasin derivatives, or a fragment thereof sufficient for invasiveness. In fact, it has been shown that non-invasive bacterial cells coated with purified invasin from Yersinia pseudotuberculosis or the carboxyl- terminal 192 amino acids of invasin are able to enter mammalian cells (Leong et al. (1990) EMBO J. 9: 1979). Furthermore, latex beads coated with the carboxyl terminal region of invasin are efficiently internalized by mammalian cells, as are strains of Staphylococcus aureus coated with antibody-immobilized invasin (reviewed in Isberg and Tran van Nhieu (1994) Ann. Rev. Genet. 27:395). Alternatively, a bacterium can also be coated with an antibody, variant thereof, or fragment thereof which binds specifically to a surface molecule recognized by a bacterial entry factor. For example, it has been shown that bacteria are internalized if they are coated with a monoclonal antibody directed against an integrin molecule, e.g., 5βl, known to be the surface molecule with which the bacterial invasin protein interacts (Isberg and Tran van Nhieu, supra). Such antibodies can be prepared according to methods known in the art. The antibodies can be tested for efficacy in mediating bacterial invasiveness by, e.g., coating bacteria with the antibody, contacting the bacteria with eukaryotic cells having a surface receptor recognized by the antibody, and monitoring the presence of intracellular bacteria, according to the methods described above. Methods for linking an invasion factor to the surface of a bacterium are known in the art and include cross- linking. Bacteria can also be modified genetically to express an antibody, or variant thereof, or other factor capable of binding specifically to a receptor of an invasion factor, e.g., an integrin molecule. Antibody genes can be isolated according to methods known in the art. Furthermore, to reduce an immune reaction of the host towards the antibody, it may be desirable to use humanized antibodies. Such antibodies can be prepared as described in U.S. Patent No. 5,585,089. Accordingly, the invention encompasses any bacterium modified, either genetically or otherwise, to have on its surface a factor, e.g., antibody, binding specifically to a receptor of an invasion factor, said bacterium being capable of delivering RNA to a eukaryotic cell. 2. Target Cells The invention provides a method for delivering RNA to any type of target cell. As used herein, the term "target cell" refers to a cell which can be invaded by a bacterium, i.e., a cell which has the necessary surface receptor for recognition by the bacterium. Preferred target cells are eukaryotic cells. Even more preferred target cells are animal cells. "Animal cells" are defined as nucleated, non-chloroplast containing cells derived from or present in multicellular organisms whose taxanomic position lies within the kingdom animalia. The cells may be present in the intact animal, a primary cell culture, explant culture or a transformed cell line. The particular tissue source of the cells is not critical to the present invention. The recipient animal cells employed in the present invention are not critical thereto and include cells present in or derived from all organisms within the kingdom animalia, such as those of the families mammalia, pisces, avian, reptilia. Preferred animal cells are mammalian cells, such as humans, bovine, ovine, porcine, feline, canine, goat, equine, and primate cells. The most preferred animal cells are human cells. In a preferred embodiment, the target cell is in a mucosal suiface. Certain enteric pathogens, e.g., E. coli, Shigella, Listeria, and Salmonella, are naturally adapted for this application, as these organisms possess the ability to attach to and invade host mucosal surfaces (Kreig et al, supra). Therefore, in the present invention, such bacteria can deliver RNA molecules to cells in the host mucosal compartment. Although certain types of bacteria may have a certain tropism, i.e., preferred target cells, delivery of RNA to a certain type of cell can be achieved by choosing a bacterium which has a tropism for the desired cell type or which is modified such as to be able to invade the desired cell type. Thus, e.g., a bacterium could be genetically engineered to mimic mucosal tissue tropism and invasive properties, as discussed above, to thereby allow said bacteria to invade mucosal tissue, and deliver RNA to cells in those sites. Bacteria can also be targeted to other types of cells. For example, bacteria can be targeted to erythrocytes of humans and primates by modifying bacteria to express on their surface either, or both of, the Plasmodium vivax reticulocyte binding proteins- 1 and -2, which bind specifically to erythrocytes in humans and primates (Galinski et al, Cell, 69:1213-1226 (1992)). In another embodiment, bacteria are modified to have on their surface asialoorosomucoid, which is a ligand for the asilogycoprotein receptor on hepatocytes (Wu et al, J. Biol. Chem., 261: 14621-14624 (1988)). In yet another embodiment, bacteria are coated with insulin-poly-L-lysine, which has been shown to target plasmid uptake to cells with an insulin receptor (Rosenkranz et al, Expt. Cell Res., 199:323-329 (1992)). Also within the scope of the invention are bacteria modified to have on their surface p60 of Listeria monocytogenes, which allows for tropism for hepatocytes (Hess et al, Infect. Immun., 63:2047-2053 (1995)), or a 60kD surface protein from Trypanosoma cruzi which causes specific binding to the mammalian extra-cellular matrix by binding to heparin, heparin sulfate and collagen (Ortega-Barria et al, Cell, 67:41 1-421 (1991)). In another embodiment, a bacterium is genetically engineered to express an antibody molecule, derivative thereof, or fragment thereof, on its surface, wherein the antibody is specific for a cell surface antigen of the desired target cell. In yet another embodiment, an antibody molecule, derivative thereof or fragment thereof is linked covalently or non-covalently to the suiface of the bacteria, as set forth above. Yet in another embodiment, a cell can be modified to become a target cell of a bacterium for delivery of RNA. Accordingly, a cell can be modified to express a surface antigen which is recognized by a bacterium for its entry into the cell, i.e., a receptor of an invasion factor. The cell can be modified either by introducing into the cell a nucleic acid encoding a receptor of an invasion factor, such that the surface antigen is expressed in the desired conditions. Alternatively, the cell can be coated with a receptor of an invasion factor. Receptors of invasion factors include proteins belonging to the integrin receptor superfamily. A list of the type of integrin receptors recognized by various bacteria and other microorganisms can be found, e.g., in Isberg and Tran Van Nhieu (1994) Ann. Rev. Genet. 27:395. Nucleotide sequences for the integrin subunits can be found, e.g., in GenBank, publically available on the internet. As set forth above, yet other target cells include fish, avian, and reptilian cells. Examples of bacteria which are naturally invasive for fish, avian, and reptilian cells are set forth below. Examples of bacteria which can naturally access the cytoplasm of fish cells include, but are not limited to Aeromonas salminocida (ATCC No. 33658) and Aeromonas schuberii (ATCC No. 43700). Attenuated bacteria are preferably used in the invention, and include A salmonicidia vapA (Gustafson et al, J. Mol. Biol, 217:452-463 (1994)) or salmonicidia aromatic-dependent mutant (Vaughan et al, Infect. Immun. , 61:2172-2181 ( 1993)). Examples of bacteria which can naturally access the cytoplasm of avian cells include, but are not restricted to, Salmonella galinarum (ATCC No. 9184), Salmonella enteriditis (ATCC No. 4931) and Salmonella typhimurium (ATCC No. 6994). Attenuated bacteria are preferred to the invention and include attenuated Salmonella strains such as S. galinarum cya crp mutant (Curtiss et al, (1987) supra) or S. enteritidis aro A aromatic-dependent mutant CVL30 (Cooper et al, Infect. Immun., 62:4739-4746 (1994)). Examples of bacteria which can naturally access the cytoplasm of reptilian cells include, but are not restricted to, Salmonella typhimurium (ATCC No. 6994). Attenuated bacteria are preferable to the invention and include, attenuated strains such as S. typhimuirum aromatic-dependent mutant (Hormaeche et al, supra). The invention also provides for delivery of RNA to other eukaryotic cells, e.g., plant cells, so long as there are microorganisms which are capable of invading such cells, either naturally or after having been modified to become invasive. Examples of microorganisms which can invade plant cells include Agrobacterium tumerfacium, which uses a pilus-like structure which binds to the plant cell via specific receptors, and then through a process that resembles bacterial conjugation, delivers at least some of its content to the plant cell. Set forth below are examples of cell lines to which RNA can be delivered according to the method of this invention. Examples of human cell lines include but are not limited to ATCC Nos. CCL 62, CCL 159, HTB 151, HTB 22, CCL 2, CRL 1634, CRL 8155, HTB 61, and HTB104. Examples of bovine cell lines include ATCC Nos. CRL 6021, CRL 1733, CRL 6033, CRL 6023, CCL 44 and CRL 1390. Examples of ovine cells lines include ATCC Nos. CRL 6540, CRL 6538, CRL 6548 and CRL 6546. Examples of porcine cell lines include ATCC Nos. CL 184, CRL 6492, and CRL 1746. Examples of feline cell lines include CRL 6077, CRL 61 13, CRL 6140, CRL 6164, CCL 94, CCL 150, CRL 6075 and CRL 6123. Examples of buffalo cell lines include CCL 40 and CRL 6072. Examples of canine cells include ATCC Nos. CRL 6213, CCL 34, CRL 6202, CRL 6225, CRL 6215, CRL 6203 and CRL 6575. Examples of goat derived cell lines include ATCC No. CCL 73 and ATCC No. CRL 6270. Examples of horse derived cell lines include ATCC Nos. CCL 57 and CRL 6583. Examples of deer cell lines include ATCC Nos. CRL 6193-6196. Examples of primate derived cell lines include those from chimpanzee's such as ATCC Nos. CRL 6312, CRL 6304, and CRL 1868; monkey cell lines such as ATCC Nos. CRL 1576, CCL 26, and CCL 161; orangautan cell line ATCC No. CRL 1850; and gorilla cell line ATCC No. CRL 1854. 3. RNA That can be Delivered to a Target Cell 3.1 RNA That can be Translated in a Eukaryotic Cell The invention provides a method for delivering RNA to a eukaryotic cell, e.g., an animal cell In a preferred embodiment, the RNA is capable of being translated in the eukaryotic cell As is well known in the art, translation of RNA into a protein in a eukaryotic cell requires RNA modifications and/or regulatory elements which differ from those required for translation of an RNA in a prokaryotic cell RNA which is capable of being translated in a eukaryotic cell is referred to herein as "eukaryotic RNA" RNA which is capable of being translated in a prokaryotic cell is referred to herein as "prokaryotic RNA" In particular, prior to the initiation of translation of eukaryotic RNA, the 5' end of the RNA molecule is "capped" by addition of methylated guanylate to the first RNA nucleotide residue in the nucleus (Darnell et al, supra, Lewin, supra, Watson et al, supra, and Alberts et al, supra) Capping provides a recognition template for eukaryotic ribosomes and/or stabilizes the mRNA in the cytoplasm Capping of RNA does not occur in prokaryotic cells and thus, normally, a prokaryotic RNA is not efficiently translated in a eukaryotic cell, e g , animal cell However, specific modifications to prokaryotic RNA can be effected to render the RNA translatable in eukaryotic cells For example, it is possible for cap-independent translation initiation to occur by including a cap-independent translation enhancer (CITE) sequence, such as those derived from viruses, e g , the cardioviruses including encephalomyocarditis virus (Duke et al, J. Virol, 66 1602-1609 (1992), U S Patent No 5,135,855 by Moss et al and U S Patent No 4,937,190 by Palmenberg et al ), Mengovirus, Mous-Elberfeld virus, MM virus, and Columbia SK virus (U S Patent No 4,937, 190 by Palmenberg et al ), Semliki Forest Virus (SFV), poliovirus (U S Patent No 5,358,856 by Baltimore et al , and, adenovirus Such sequences are usually present in the 5 'untranslated regions of these viruses As used herein, "CITE sequences", also referred to herein as "Internal Ribosome Entry Site" and "IRES sequence", are any nucleotide sequences, which, when present in an RNA molecule, increase the translation efficiency of the RNA molecule in eukaryotic cells Such sequences are preferably present downstream of a promoter, but upstream of the coding region of the RNA Furthermore, it will be obvious to those skilled in the art that it is possible to express multiple products from a single mRNA molecule (i e , polycistronic RNA) by inserting a CITE sequence upstream of each coding region of the RNA CITE sequences are further descπbed in the following references Borman, A M et al (\991) Nucleic Acids Research 25 925, Rijnbrand, R et al (1997) Journal of Virology 71 451, Shiroki, K et al (1997) Journal of Virology 71 l , Agol, V I et al (1996) Journal of Biotechnology 44 1 19, Das, S et al (1996) Journal of Virology 70 1624, Barton, D J et al (1996) Virology 217 459, Schultz, D E et al (1996) Journal of Virology 70 1041, Aldabe, R et al (1995) FEBS Letters 377, Degener, A M et al (1995) Virus Research 37 291 , Ziegler, E et al (1995) Virology 213 549, Vagner, S et al (1995) Journal of Biological Che istiy 270 20376, Sjoberg, E M et al (1994) Bio Technology 12 1127, Yoo, B J et al (1992) Vnology 191 889, Donze, O and P F Spahr (1992) EMBO Journal 1 1 3747, Kwee, L et al (1992) Journal of Virology 66 4382, Forman, B M and H H Samuels (1991) Gene 105 9, Ruiz-Linares, A et al (1989) Nucleic Acids Research 17 2463, Wang et al (1997) EMBO J 16 4 107, Gan et al (1996) J Biol Chem 271 623, Turner et al (1976) Arch Virol 134 321, Danthinne et al (1993) Mol Cell Biol 13 3340, Duke et al (1992) J Virol 66 1602, Davies et al (1991) ./ Biol Chem 266 14714, Dolph et al, (1990) J Virol 64 2669, Ratner (1989) Nucl Acids Res 12 4101 CITE sequences can be linked to an RNA, e g , by transcription of an RNA from a promoter located upstream of a CITE sequence, which in turn is located upstream of the coding region CITE sequences strongly increase translation of RNA both in whole cells and in vitro translation systems, e g , in a reticulocyte lysate (Elroy- Stein et al (1989) Proc Natl Acad Sci USA 86 6126) A CITE sequence can have, e g , the nucleotide sequence shown in Figure 1 and set forth in SEQ ID NO 1 , which corresponds to the nucleotide sequence from nucleotide 2416 to nucleotide 2914 of pCITE-1 from Novagen, Inc (Madison, WI) DNA vectors containing a CITE sequence cloned upstream of a polylinker, which were designed for transcription of RNA that can be translated in eukaryotic cells, are commercially available, and can be obtained, e g , from Novagen, Inc (Madison, WI) (pCITE vectors, which are further descπbed in the Examples) Such vectors can be used for preparing nucleic acids of the invention, l e , nucleic acids which can be transcπbed into RNA in prokaryotic cells and wherein the RNA can subsequently be translated in eukaryotic cells The term "eukaryotic RNA expression cassette" is used herein to define elements which when present in an RNA molecule significantly increase its translation efficiency in a eukaryotic cell. For example, a CITE sequence is a preferred element of a eukaryotic RNA expression cassette. Another sequence which may be included in a eukaryotic RNA expression cassette is a polyadenylate sequence. The presence of a polyadenylate tail on an RNA may improve its stability both in the invasive bacterium and in the target eukaryotic cells, and may therefore be a desirable element. The eukaryotic RNA expression cassette may also contain specific sequences which stabilize the RNA, which destabilize the RNA, which increases its translation efficiency or which decreases its translation efficiency. For example, RNA destabilizing sequences, i.e., nucleotide sequences which when present in an RNA decreases the half-life of the RNA, can be AU-rich sequences, which are found, e.g., in the RNA encoding cytokines, e.g., lymphokines, such as interferon-beta and proto- oncogenes (Shaw and Kamen (1986) Cell 46:659). Alternatively, if one desires to express cytokines or products of genes having such destabilizing elements, it may be desirable to remove or destroy such destabilizing sequences, e.g., by mutagenesis. In a preferred embodiment, the RNA contains upstream of the CITE sequence a nucleotide sequence which can prevent degradation of a CITE sequence by a 5'-3' exonuclease activity. While no such RNAse activity has been identified in enteric bacteria (reviewed in Kushner. "mRNA Decay" in Neidhhart et al. Eds. Escherichia coli and Salmonella. Cellular and Molecular Biology. Second ed. Vol. 1. Washington D.C.: ASM Press, 1996:849), it may be desirable in certain situations to add a 5' nucleotide sequence which would significantly reduce degradation of the CITE sequence. A preferred nucleotide sequence that can be inserted upstream of the CITE sequence is at least a portion of a 5' untranlated region of OmpA, which forms a three stem-loop structure and has been shown to impart stability to RNA (Emory et al. (1992) Genes Dev. 6: 135). In another embodiment of the invention, the level of translation of a specific RNA is increased by modifying the RNA to include nucleotide sequences which improve the translation efficiency of an RNA. Such sequences can be located, e.g., in the 5' untranslated region of a mRNA. For example, the ferritin mRNA comprises an iron regulatory element (IRE) in the 5' untranslated (UT) region of its mRNA, which increases translation of the mRNA in the presence of iron In conditions of iron deprivation, a binding protein, IRE-BP, specifically interacts with the IRE sequence and represses translation (Kuhn et al (1993) J Inorg. Biochem. 47 183) Thus, based at least on the possible presence of specific elements in, 5' or 3' untranslated regions of RNAs which influence stability and/or translation efficiency, constitutively, or in an inducible manner, the stability and/or translation efficiency of specific RNAs will vary depending on whether 5' and/or 3' untranslated regions are present The effect of the presence of 5' and/or 3' untranslated regions or portions thereof in an RNA can be determined experimentally, e g , by transfection experiments For example, RNA stability may be determined by pulse labeling experiments Accordingly, the invention provides methods for delivering RNA to eukaryotic cells, wherein the amount of protein synthesized in the eukaryotic cell from the RNA can be controlled by modifying the RNA such as to effect its translation efficiency and/or half- life The eukaryotic RNA expression cassette can also encode self-amplifying sequences For example, the RNA expression cassette can encode an attenuated alpha virus, such as Venezuela equine encephalitis virus (VEE), semiliki forest virus (SFV), or sindbis virus, that have been modified to express passenger vaccine antigens (Pushko P et al , 1997 Virology 239 389-401 , Tsuji M et al , 1998, J Virol 72 6907-10, Hariharan M j , et al , 1998 J Virol , 72 950-8, Gorrell M D , et al , 1997, J Virol 71 3415-9) Self-amplifying RNA expression cassettes can be expressed in eukaryotic cells by delivering bacterial RNA that are subsequently amplified in the target eukaryotic cells or transcribed directly in the eukaryotic cell by including a T7 promoter upstream RNA expression cassette and simultaneously delivering T7 polymerase, as described (Selby M J , 1997, J. Virol 71 7827-31) Alternatively, expression of the self-amplifying RNA expression cassette can be initiated by including a eukaryotic promoter (Hariharan M j , et al , 1998 J. Virol , 72 950-8) In another embodiment, the invention provides a bacterium comprising a nucleic acid which is capable of being transcribed into RNA in the bacterium, wherein the RNA is capable of being translated in a eukaryotic cell as well as in the bacterium Accordingly, in one embodiment of the invention, the RNA comprises regulatory sequences necessary for translation of the RNA in eukaryotic cells, e.g., a CITE sequence, as well as regulatory elements necessary for translation of the RNA in prokaryotic cells, e.g., a Shine-Dalgarno ribosome binding site. The necessary elements for efficient translation in a prokaryotic cell are referred to herein as "prokaryotic expression cassette". Thus, in one embodiment, a desired gene product can be synthesized both in the bacterium and the eukaryotic target cell. Such an embodiment can be of interest if the desired gene product, i.e., protein, is not heavily posttranslationally modified, when the posttranslational modifications are not required to achieve the desired effects, or when such combinations are found to improve the immunogenicity of a product that is intended to be a vaccine antigen. The RNA may allow expression of either a foreign or an endogenous protein. As used herein, "foreign protein" refers to a protein, which is foreign to the recipient eukaryotic cell or tissue, such as a vaccine antigen, immunoregulatory agent, or therapeutic agent. An "endogenous protein" refers to a protein which is naturally present in the recipient animal cell or tissue. In a preferred embodiment, the RNA delivered to the eukaryotic cell which is translated in the eukaryotic cell is a vaccine antigen. As used herein, a "vaccine antigen" refers to a polypeptide or derivative thereof, which can be used to vaccinate a subject, i.e., to mount an immune response in the subject against the antigen. A vaccine antigen may be a protein or antigenic fragment thereof from viral pathogens, bacterial pathogens, and parasitic pathogens. Alternatively, the vaccine antigen may be a synthetic polypeptide, constructed using recombinant DNA methods, which contains antigens or parts thereof from viral, bacterial, parasitic pathogens. These pathogens can be infectious in humans, domestic animals or wild animals. In addition, multiple RNA molecules can be delivered that express any combination of viral, bacterial, parasitic antigens, or said synthetic antigens. The antigen or synthetic antigen can be any molecule or part thereof that is expressed by any viral, bacterial, or parasitic pathogen prior to or during entry into, colonization of, or replication of, their animal host. Preferred viral pathogens, from which the viral antigens are derived, include, but are not limited to, Orthomyxoviruses, such as influenza virus, Retroviruses, such as Human T Lymphotrophic Virus (HTLVI, e g , HTLV-1, HTLV-2, and HTLV-5), Human Immunodeficiency Virus (HIV, e g , HIV-1, HIV-2), and SIV, Herpesviruses, such as Epstein-Barr Virus (EBV), Cytomegalo virus (CMV) or Herpes Simplex Virus (HSV), Rhabdoviruses, such as rabies virus, Picornoviruses, such as po ovirus, Poxviruses, such as vaccinia, Rotavirus, and Parvoviruses Examples of vaccine antigens of viral pathogens include, but are not limited to, the human immunodeficiency virus antigens Nef, Gag, Env, gpl20, gp41, Tat, Rev, and Pol (Hahn et al, Nαtwre, H 277-280 (1985)) and T cell and B cell epitopes of gpl20 (Palker et al, J. Immunol , 142 3612-3619 (1989)), the hepatitis B surface antigen (Wu et al, Proc. Natl. Acad. Sci., USA, 86 4726-4730 (1989)), rotavirus antigens, such as VP4 (Mackow et al, Proc. Natl. Acad. Sci , USA, 87 518-522 (1990)) and VP7 (Green et al, J Virol, 62 1819-1823 (1988)), influenza virus antigens such as hemagglutinin or nucleoprotein (Robinson et al , Supra, Webster et al, Supra) and herpes simplex virus thymidine kinase (Whitley et al, In: New Generation Vaccines, pages 825-854) Bacterial pathogens, from which the bacterial antigens can be derived, include but are not limited to, Mycobacterium spp. , Helicobacter pylori, Salmonella spp. , Shigella spp. , E coli, Rickettsia spp , Listeria spp. , Legionella pneumoniae, Pseudomonas spp , Vibrio spp. , and Borellia burgdorfen Examples of vaccine antigens of bacterial pathogens include CFA/I fimbrial antigen of enterotoxigenic E. coli (Yamamoto et al, Infect. Immun., 50 925-928 (1985)), cholera toxin of Vibrio cholerae (Mekalanos et al (1992) PΝASUSA 79 151), heat-labile enterotoxin of Escherichia coli (LT, Moseley, L L et al (1983) Infection & Immunity 39 1167, Moseley, L L et al (1983) Journal of Bacteπology 156 441), and the nontoxic B-subunit of CT and LT (Clements et al, 46 564-569 (1984)), pertactin of Bordetella pertussis (Roberts et al, Vacc , 10 43-48 (1992)), adenylate cyclase-hemolysin of B. pertussis (Guiso et al, Micro. Path., j . 423-431 (1991)), and fragment C of tetanus toxin of Clostridium tetani (Fairweather et al, Infect. Immun., 58 1323-1326 (1990)) Parasitic pathogens, from which the parasitic antigens can be derived, include but are not limited to, Plasmodium spp. , Trypanosome spp. , Giardia spp. , Boophilus spp. , Babesia spp. , Entamoeba spp. , Eimeria spp. , Leishmania spp. , Schistosome spp. , Brugia spp., Fascida spp., Dirofi/aria spp., Wuchereria spp., and Onchocerea spp. Examples of vaccine antigens of parasitic pathogens include the circumsporozoite antigens of Plasmodium spp. (Sadoff et al, Science, 240:336-337 (1988)), such as the circumsporozoite antigen of P. falciparum; the merozoite surface antigen of Plasmodium spp. (Spetzler et al, Int. J. Pept Prot Res., 43:351-358 (1994)); the galactose specific lectin of Entamoeba histolytica (Mann et al, Proc. Natl. Acad. Sci., USA, 88:3248-3252 (1991)), gp63 of Leishmania spp. (Russell et al, J Immunol, 140: 1274-1278 (1988)), paramyosin of Brugia malayi (Li et al, Mol. Biochem. Parasitol, 49:315-323 (1991)), the triose-phosphate isomerase of Schistosoma mansoni (Shoemaker et al, Proc. Natl. Acad. Sci., USA, 89: 1842-1846 (1992)); the secreted globin-like protein of Trichostrongylus colubriformis (Frenkel et al, Mol. Biochem. Parasitol, 50:27-36 (1992)); the glutathione-S-transferase of Frasciola hepatica (Hillyer et al, Exp. Parasitol. , 75:176- 186 (1992)), Schistosoma bovis and S. japonicum (Bashir et al, Trop. Geog. Med., 46:255-258 (1994)); and KLH of Schistosoma bovis and S. japonicum (Bashir et al, supra). In the present invention, bacteria can also deliver RNA molecules encoding a therapeutic vaccine. As used herein, "therapeutic vaccine" refers to a vaccine comprising a therapeutic agent, which is a eukaryotic protein or peptide which is present or may be present in a subject, the shielding of which or elimination of which is desired. For example, the RNA molecules can encode a tumor-specific, transplant, or autoimmune antigen or part thereof. Examples of tumor vaccine antigens include prostate specific antigen (Gattuso et al, Human Pathol, 26:123-126 (1995)), TAG-72 and carcinoembryonic antigen (CEA) (Guadagni et al, Int. J Biol. Markers, 9:53-60 (1994)), MAGE-1 and tyrosinase (Coulie et al, J. Immunothera. , 14: 104-109 (1993)). Yet any other tumor antigen can also be used according to the method of the invention, to elicit an immune reaction against tumor cells expressing the antigen. It has in fact been shown in mice that immunization with non-malignant cells expressing a tumor antigen provides a vaccine effect, and also helps the animal mount an immune response to clear malignant tumor cells displaying the same antigen (Koeppen et al, Anal. N Y. Acad. Sci., 690:244-255 (1993)). Accordingly, tumor specific antigens can be delivered to a subject having a tumor or likely to develop a tumor according to the method of the invention to thereby induce an immune reaction against the tumor and either reduce or elimate the tumor or prevent a tumor from developing. Examples of transplant antigens that can be administered to a subject according to the method of the invention include the CD3 receptor on T cells (Alegre et al, Digest. Dis. Sci., 40:58-64 (1995)). In fact, it has been shown that treatment with an antibody to CD3 receptor results in rapid clearance of circulating T cells and reverses most rejection episodes (Alegre et al, supra). The invention provides for administration of any transplant antigen. As used herein, the term "transplant antigen" refers to an antigen which is involved in an immune reaction and includes antigens involved in transplant rejections and antigens involved in establishment of tolerance. Accordingly, transplant antigens include antigens involved in the interaction of B and T cells with other cells, e.g., antigen presenting cells. Transplant antigens within the scope of the invention include costimulatory molecules or receptors thereof, e.g., B7-1, B7-2, CD28, CTLA4; gp39 (also termed CD40 ligand); CD40; CD4; CD8; major histocompatibility complex antigens (MHC), e.g., MHC class I and MHC class II; and adhesion factors, e.g., LFA-1, LFA-3, ICAM-1, VLA4, and CD2. Without wanting to be limited to a specific mechanism of action, administration of a transplant antigen to a subject according to the method of the invention results in stimulation a humoral and/or cell-mediated immune response against the antigen, thereby resulting in inhibiting or at least reducing the interaction of the transplant antigen with its receptor or destroying the cells carrying the antigen, thereby establishing tolerance. The method of the invention can also be used to destroy autoimmune cells. According to the method of the invention, an autoimmune antigen is administered to a subject prophylactically or therapeutically, to thereby result in the production of antibodies against the autoimmune antigen, thereby inducing destruction of autoimmune cells or preventing their development. Examples of autoimmune antigens include IAS β chain (Topham et al, Proc. Natl. Acad. Sci., USA, 91:8005-8009 (1994)). In fact, it has been shown that vaccination of mice with an 18 amino acid peptide from IAS β chain provides protection and treatment to mice with experimental autoimmune encephalomyelitis (Topham et al, supra) Alternatively, in the present invention, bacteria can deliver RNA molecules encoding immunoregulatory molecules, e g , to boost the immune response against said antigens These immunoregulatory molecules include, but are not limited to, growth factors, such as M-CSF, GM-CSF, erythropoietin, and cytokines, such as LL-2, IL-4, IL-5, IL-6, IL-10, IL-12 or IFN-γ, clotting factors, tissue plasminogen activators, recombinant soluble receptors, e g , IL-1 or TNF receptor Furthermore, delivery of cytokines expression cassettes to tumorous tissue has been shown to stimulate potent systemic immunity and enhanced tumor antigen presentation without systemic cytokine toxicity (Golumbek et al, Cane. Res., 5_1 5841-5844 (1993), Golumbek et al, Immun. Res., \2 183-192 (1993), Pardoll, Curr. Opm. Oncol, 4.1124-1 129 (1992), and Pardoll, Curr. Op . Immunol, 4 619-623 (1992)) Yet other proteins or polypeptides can be delivered to eukaryotic cells according to the method of the invention Generally, any peptide that is beneficial to a subject can be administered Thus, the invention can be used in gene therapy in general For example, the peptide can be a peptide which is deficient in a subject and the method of the invention provides a method for compensating for the deficient peptide The peptide can be secreted, membraneous, or cytoplasmic 3 2 Antisense RNAs and Catalytic RNAs In another embodiment, the invention relates to the delivery of RNA in "antisense" therapy. As used herein, "antisense" therapy refers to administration or in situ generation of nucleic acid molecules, e g , RNA, or their derivatives which specifically hybridize (e g , bind) under cellular conditions, with the cellular RNA, e g., mRNA, and/or genomic DNA, so as to, e g , inhibit expression of a specific protein, e.g by inhibiting transcription and/or translation The binding may be by conventional base pair complementarity, or, for example, in the case of binding to DNA duplexes, through specific interactions in the major groove of the double helix Absolute complementarity between the antisense RNA molecule and the target molecule, although preferred, is not required An RNA sequence "complementary" to a portion of a nucleic acid, as referred to herein, means a sequence having sufficient complementarity to be able to hybridize with the nucleic acid, forming a stable duplex or triplex The ability to hybridize will depend on both the degree of complementarity and the length of the antisense RNA Generally, the longer the hybridizing RNA, the more base mismatches with an RNA it may contain and still form a stable duplex (or triplex, as the case may be) One skilled in the art can ascertain a tolerable degree of mismatch by use of standard procedures to determine the melting point of the hybridized complex Antisense RNAs which are complementary to the 5' end of an RNA message, e g , the 5' untranslated sequence up to and including the AUG initiation codon, should work most efficiently at inhibiting translation However, sequences complementary to the 3' untranslated sequences of mRNAs have also been shown to be effective at inhibiting translation of mRNAs as well (Wagner, R (1994) Nature 172 333) Therefore, antisense RNAs complementary to either the 5' or 3' untranslated, non- coding regions of a gene could be used in an antisense approach to inhibit translation of endogenous mRNA Antisense RNAs complementary to the 5' untranslated region of the mRNA should include the complement of the AUG start codon Antisense RNAs complementary to mRNA coding regions may be less efficient inhibitors of translation but could be used in accordance with the invention Whether designed to hybridize to the 5', 3' non coding or coding region of a mRNA, antisense RNAs should be at least six nucleotides in length, and are preferably ranging from 6 to about 50 nucleotides in length. In certain embodiments, the RNA is at least 50 nucleotides, at least 70 nucleotides, at least 90 nucleotides, or at least 100 nucleotides Regardless of the choice of target sequence, it is preferred that in vitro studies are first performed to quantitate the ability of the antisense RNA to inhibit gene expression It is preferred that these studies utilize controls that distinguish between antisense gene inhibition and nonspecific biological effects of antisense RNAs It is also preferred that these studies compare levels of the target RNA or protein with that of an internal control RNA or protein Additionally, it is envisioned that results obtained using the antisense RNA are compared with those obtained using a control RNA, e g , a sense RNA It is preferred that the control RNA is of approximately the same length as the test RNA and that the nucleotide sequence of the antisense RNA differs from the antisense sequence no more than is necessary to prevent specific hybridization to the target sequence In addition to the sequence complementary to a target sequence, the antisense RNA can comprise sequences which are capable of modulating the stability of the RNA Accordingly, the same sequences set forth above for either enhancing or decreasing RNA stability, e g , AU-πch sequences, can be linked to the antisense sequence per se Alternatively, the portion of the antisense RNA which is complementary to the target sequence can be modified to eliminate specific sequences, e g , sequences rendering the RNA unstable, so long as the antisense RNA is still capable of hybπd zing to the target sequence Modification of the antisense RNA can be performed according to methods known in the art, e g , site directed mutagenesis The invention also provides a method for delivering catalytic RNAs to a eukaryotic cell In a preferred embodiment, the catalytic RNA is a ribozyme Ribozyme molecules designed to catalytically cleave specific mRNA transcripts can be used to prevent translation of mRNA and expression of specific proteins in eukaryotic cells (See, e_g_, PCT International Publication WO 94/11364, published October 4, 1990, Sarver et al , 1990, Science 247 1222-1225) Ribozymes are enzymatic RNA molecules capable of catalyzing the specific cleavage of RNA The mechanism of πbozyme action involves sequence specific hybridization of the ribozyme molecule to complementary target RNA, followed by an endonucleolytic cleavage The composition of ribozyme molecules must include one or more sequences complementary to the target gene mRNA, and must include the well known catalytic sequence responsible for mRNA cleavage For this sequence, see U S Pat No 5,093,246, which is incorporated by reference herein in its entirety Specific πbozyme cleavage sites within any potential RNA target are initially identified by scanning the molecule of interest for ribozyme cleavage sites which include the following sequences, GUA, GUU and GUC Once identified, short RNA sequences of between 15 and 20 πbonucleotides corresponding to the region of the target gene containing the cleavage site may be evaluated for predicted structural features, such as secondary structure, that may render the RNA sequence unsuitable The suitability of candidate sequences may also be evaluated by testing their accessibility to hybπdization with complementary ohgonucleotides, using πbonuclease protection assays While πbozymes that cleave mRNA at site specific recognition sequences can be used to destroy specific mRNAs, the use of hammerhead πbozymes is preferred Hammerhead πbozymes cleave mRNAs at locations dictated by flanking regions that form complementary base pairs with the target mRNA The sole requirement is that the target mRNA have the following sequence of two bases 5'-UG-3' The construction and production of hammerhead πbozymes is well known in the art and is descπbed more fully in Haseloff and Geriach, 1988, Nature, 334 585-591 There are usually hundreds of potential hammerhead ribozyme cleavage sites within the nucleotide sequence of a specific mRNA Preferably the ribozyme is engineered so that the cleavage recognition site is located near the 5' end of the mRNA, ι_e_, to increase efficiency and minimize the intracellular accumulation of non-functional mRNA transcπpts The πbozymes of the present invention also include RNA endoπbonucleases (hereinafter "Cech-type πbozymes") such as the one which occurs naturally in Tetrahymena thermophila (known as the IVS, or L-l 9 IVS RNA) and which has been extensively descπbed by Thomas Cech and collaborators (Zaug, et al (1984) Science 224 574-578, Zaug and Cech (1986) Science 23_1 470-475, Zaug, et al (1986) Nature 124 429-433, published International patent application No WO88/04300 by University Patents Inc , Been and Cech (1986) Cell 47 207-216) The Cech-type πbozymes have an eight base pair active site which hybridizes to a target RNA sequence whereafter cleavage of the target RNA takes place As in the antisense approach, the πbozymes can be composed of modified RNAs (e g , for improved stability) Because πbozymes, unlike antisense molecules, are catalytic, a lower intracellular concentration is required for efficiency The antisense RNA and catalytic RNA species delivered to animal cells can be targeted against any molecule present within the recipient cell or likely to be present within the recipient cell These include but are not limited to RNA species encoding cell regulatory molecules, such as ιnterlukιn-6 (Mahieu et al, Blood, 84 3758-3765 (1994)), oncogenes such as ras (Kashani-Sabet et al, Antisen. Res. Devel, 2:3-15 (1992)), causative agents of cancer such as human papillomavirus (Steele et al, Cane Res., 52:4706-4711 (1992)), enzymes, viral RNAs and pathogen derived RNAs such as HIV-1 (Meyer et al, Gene, 129:263-268 (1993); Chatterjee et al, Sci., 258: 1485- 1488 (1992); and Yamada et al, Virol, 205: 121-126 (1994)). The RNAs can also be targeted at non-transcribed DNA sequences, such as promoter or enhancer regions, to form, e.g., triplex molecules, or to any other molecule present in the recipient cells, such as but not limited to, enzymes involved in DNA synthesis or tRNA molecules (Scanlon et al, Proc. Natl. Acad. Sci. USA, 8£: 10591 -10595 (1991); and Baier et al, Mol. Immunol, 11:923-932 (1994)). As a further alternative, single or multiple RNA molecules encoding a vaccine antigen, a therapeutic antigen, or which are antisense or catalytic RNA can be delivered in any combination. 4. Introduction of RNA into a Bacterium for Delivery to a Eukaryotic Cell In a preferred embodiment of the invention, the RNA is synthesized in the bacterium, and the RNA is delivered to the eukaryotic cell upon invasion of the eukaryotic cell by the bacterium. In one embodiment, the RNA which is capable of being translated in a eukaryotic cell or which is an antisense or a catalytic RNA is encoded by DNA that is operably linked to a bacterial promoter, e.g., the anaerobic E. coli, NirB promoter or the E. coli lipoprotein Up promoter, described, e.g., in Inouye et al. (1985) Nucl. Acids Res. 11:3101; Salmonella pagC promoter (Miller et al., supra), Shigella ent promoter (Schmitt and Payne, J. Bacteriol 173:816 (1991)), the tet promoter on TnlO (Miller et al., supra), or the ctx promoter of Vibrio cholera. Any other promoter can be used in the invention. The bacterial promoter can be a constitutive promoter or an inducible promoter. A preferred inducible promoter is a promoter which is inducible by iron or in iron-limiting conditions. In fact, some bacteria, e.g., intracellular organisms, are believed to encounter iron-limiting conditions in the host cytoplasm. Examples of iron-regulated promoters of FepA and TonB. are known in the art and are described, e.g., in the following references: Headley, V. et al. (1997) Infection & Immunity 65:818; Ochsner, U.A. et al. (1995) Journal of Bacteriology 177:7194; Hunt, M.D. et al. (1994) Journal of Bacteriology 176:3944; Svinarich, D.M. and S. Palchaudhuri. (1992) Journal of Diarrhoea! Diseases Research 10: 139; Prince, R.W. et al. (1991) Molecular Microbiology 5:2823; Goldberg, M.B. et al. (1990) Journal of Bacteriology 172:6863; de Lorenzo, V. et al. (1987) Journal of Bacteriology 169:2624; and Hantke, K. (1981) Molecular & General Genetics 182:288. The nucleic acid to be transcribed and the promoter to which it is operably linked are preferably in a vector or plasmid. As used herein, the term "vector" refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. One type of preferred vector is an episome, i.e., a nucleic acid capable of extra-chromosomal replication. Preferred vectors are those capable of autonomous replication and/or expression of nucleic acids contained therein. Vectors capable of directing the expression of genes to which they are operatively linked are referred to herein as "expression vectors". The term "plasmid" as used herein, refers generally to circular double stranded DNA loops which are not bound to the chromosome. In the present specification, "plasmid" and "vector" are used interchangeably as the plasmid is the most commonly used form of vector. However, the invention is intended to include such other forms of expression vectors which serve equivalent functions and which become known in the art subsequently hereto. A plasmid for practicing the invention preferably comprises sequences required for appropriate transcription of the nucleic acid in bacteria, e.g., a transcription termination signal. The vector can further comprise sequences encoding factors allowing for the selection of bacteria comprising the nucleic acid of interest, e.g., gene encoding a protein providing resistance to an antibiotic, sequences required for the amplification of the nucleic acid, e.g., a bacterial origin of replication. In a preferred embodiment, the DNA is operably linked to a first promoter and the bacterium further comprises a second DNA encoding a first polymerase which is capable of mediating transcription from the first promoter, wherein the DNA encoding the first polymerase is operably linked to a second promoter. In a preferred embodiment, the second promoter is a bacterial promoter, such as those delineated above. In an even more preferred embodiment, the polymerase is a bacteriophage polymerase, e.g., SP6, T3, or T7 polymerase and the first promoter is a bacteriophage promoter, e.g., an SP6, T3, or T7 promoter, respectively. Plasmids comprising bacteriophage promoters and plasmids encoding bacteriophage polymerases can be obtained commercially, e.g., from Promega Corp. and InVitrogen, or can be obtained directly from the bacteriophage using standard recombinant DNA techniques (J. Sambrook, E. Fritsch, T. Maniatis, Molecular Cloning: A Laboratory Manual, Cold Spring Laboratory Press, 1989). Bacteriophage polymerases and promoters are further described, e.g., in the following references: Sagawa, H. et al. (1996) Gene 168:37; Cheng, X. et al. (1994) PNAS USA 91 :4034; Dubendorff, J.W. and F.W. Studier (1991) Journal of Molecular Biology 219:45; Bujarski, J.J. and P. Kaesberg (1987) Nucleic Acids Research 15: 1337; and Studier, F.W. et al. (1990) Methods in Enzymology 185:60). Such plasmids can further be modified according to the specific embodiment of the invention. In another preferred embodiment, the bacterium further comprises a DNA encoding a second polymerase which is capable of mediating transcription from the second promoter, wherein the DNA encoding the second polymerase is operably linked to a third promoter. In a preferred embodiment, the third promoter is a bacterial promoter. However, more than two different polymerases and promoters could be introduced in a bacterium to obtain high levels of transcription. The use of one or more polymerase for mediating transcription in the bacterium can provide a significant increase in the production of RNA in the bacterium relative to a bacterium in which the DNA is directly under the control of a bacterial promoter. The selection of the system to adopt will vary depending on the specific use of the invention, e.g., on the amount of RNA that one desires to produce. The above described DNA, i.e., a DNA which is capable of being transcribed into RNA in a bacterium, wherein the RNA is capable of being translated in a eukaryotic cell or is an antisense RNA or a catalytic RNA, and the one or more DNA molecules encoding one or more polymerases can be present on one plasmid or more than one plasmid. Introduction of one or more plasmids in the bacterium can be performed according to methods well known in the art, e.g., electroporation or chemical transformation techniques. In a preferred embodiment, the DNA molecules encoding said RNA are integrated into the bacterial chromosome. A bacterial RNA delivery system in which the DNA encoding the RNA is integrated into the bacterial chromosome is referred to herein as "chromosomal-based RNA delivery system". Such a system is described in the Examples. Methods for integrating a DNA sequence into a bacterial chromosome are also set forth in Hone et al. (1988) Microbiol Pathogenesis 5:407; Strugnell et al. Gene 88:57 (1990); and Hohman et al. (Proc. Natl. Acad. Sci. U.S.A. 92:2904 (1995)). In a preferred embodiment, chromosomal integration is achieved by first introducing a hisOG deletion mutation into the target bacterial strain; the hisOG deletion is then replaced by the complete hisOG region and the segment of heterologous DNA of interest. One advantage of a chromosomal-based RNA delivery system is that recombinant plasmids are not delivered to the target eukaryotic cell, thereby decreasing the risk of forming mutations in the nuclear DNA of the target cell. In yet another embodiment, the RNA is introduced into the bacterium for delivery to the eukaryotic cell. The RNA can be, e.g., synthesized in vitro using, e.g., an in vitro transcription system. Alternatively, the RNA can be isolated from a source, and can be, e.g., a mixture of different RNAs. In one embodiment, the RNA is RNA extracted from a pathogenic organism. In an even more preferred embodiment, a specific population of RNA from a pathogenic organism is selected, preferably a population of RNA which does not encode pathogenic proteins, and introduced in the bacterium. The RNA can be introduced into bacteria using the same methods for introducing DNA into bacteria. For example, RNA can be introduced by electroporation or chemical transformation. 5. Exemplary Uses The bacterial compositions and methods of the invention are useful for delivering RNA to a eukaryotic cell, wherein the RNA is capable of being translated into a protein or peptide, or is an antisense RNA or a catalytic RNA into a cell. In a preferred embodiment, the invention provides an RNA vaccine. In an even more preferred embodiment, the RNA vaccine is a mucosal vaccine which can be delivered via routes other than parenteral, e.g., oral or nasal route. Such methods of dehvery may be more convenient for vaccination than parenteral administration It is known that the mucosal and systemic immune systems are compartmentalized (Mesteky, J Clin Immunol , 7 265-270 (1987), Newby, In Local Immune Response of the Gut, Boca Raton, CRC Press, Newby and Stocks Eds , pages 143-160 (1984), and Pascual et al , Immuno Methods , 5_ 56-72 (1994)) Thus, antigens delivered to mucosal surfaces elicit mucosal and systemic responses, whereas parentally delivered antigens elicit mainly systemic responses but only stimulate poor mucosal responses (Mesteky, supra) Another advantage of delivery of antigen to mucosal surfaces is that mucosal stimulation at one mucosal site (for example the intestine) can result in development of immunity at other mucosal surfaces (for example genital/urinary tract) (Mesteky, supra) This phenomenon is referred to as the common mucosal system and is well documented (Mesteky, supra, and Pascual et al, supra) The development of mucosal vaccines has been hindered by the poor immunogenicity of antigens when delivered by these routes In this context, antigens can be divided into two classes those that bind to intestinal surfaces and those that do not bind, where the former are significantly more lmmunogenic than the latter (De Aizpurua et al, J Exp Med , 176 440-451 (1988)) Similarly, delivery of RNA molecules to mucosal surfaces is inefficient due to the many natural host defenses found at these surfaces, such as the gastric barrier and nucleases in the gastrointestinal tract, and the glycocalyx layer in the respiratory tract Accordingly, the invention provides a method for eliciting mucosal and systemic responses by efficiently delivering RNA encoding an antigen to mucosal surfaces The method of the invention can also be carried out in such a way to limit spreading of the virus from the target site This could be accomplished by any of several methods including delivery of a very limited dose, delivery of a severely attenuated auxotrophic strains, such as an asd mutant (Curtiss et al, supra) that will be rapidly inactivated or die, or delivery of a bacterial strain that contains attenuating lesions In a preferred embodiment, the bacterium is modified to contain a suicide system (Rennell et al, supra, and Reader et al, supra), I e , a gene encoding a toxic product, e g , a toxin, under the control of a strong promoter, such as the anaerobic nir promoter (Harborne et al, supra) or an inducible promoter Accordingly, a bacteπa containing a suicide gene under the control of a nirB promoter is capable of inducing the death of the bacteria once the bacterium has entered the cytoplasm of a target cell A bacterium containing a suicide gene is capable of inducing the death of the bacteπa upon induction with an appropriate stimulatory agent In addition, cytochalasin can be used to inhibit entry of all intracellular bacteria into cells Accordingly, the invention provides methods that allow control of the fate of the bacteria once these are introduced into a subject In a preferred embodiment of the invention, the invasive bacteria containing the RNA molecules, and/or DNA encoding such, are introduced into an animal by intravenous, intramuscular, intradermal, intraperitoneally, peroral, intranasal, intraocular, intrarectal, intravaginal, oral, immersion and intraurethral inoculation routes The amount of the live invasive bacteria of the present invention to be administered to a subject will vary depending on the species of the subject, as well as the disease or condition that is being treated Generally, the dosage employed will be about 10 3 to 10 11 viable organisms, preferably about 10 5 to 10 9 viable organisms per subject The invasive bacteπa of the present invention are generally administered along with a pharmaceutically acceptable carrier and/or diluent The particular pharmaceutically acceptable earner an/or diluent employed is not critical to the present invention Examples of diluents include a phosphate buffered saline, buffer for buffeπng against gastπc acid in the stomach, such as citrate buffer (pH 7 0) containing sucrose, bicarbonate buffer (pH 7 0) alone (Levine et al, J Clin Invest , 79 888-902 (1987), and Black et al J Infect Dis , 155 1260-1265 (1987)), or bicarbonate buffer (pH 7 0) containing ascorbic acid, lactose, and optionally aspartame (Levine et al, Lancet, II 467-470 (1988)) Examples of carriers include proteins, e g , as found in skim milk, sugars, e g , sucrose, or polyvinylpyrrolidone Typically these carriers would be used at a concentration of about 0 1-30% (w/v) but preferably at a range of 1-10% (w/v) Set forth below are other pharmaceutically acceptable carriers or diluents which may be used for delivery specific routes Any such carrier or diluent can be used for administration of the bacteria of the invention, so long as the bacteria are still capable of invading a target cell In vitro or in vivo tests for invasiveness can be performed to determine appropriate diluents and carriers The compositions of the invention can be formulated for a variety of types of administration, including systemic and topical or localized administration Lyophilized forms are also included, so long as the bacteria are invasive upon contact with a target cell or upon administration to the subject Techniques and formulations generally may be found in Remmington's Pharmaceutical Sciences, Meade Publishing Co , Easton, PA For systemic administration, injection is preferred, including intramuscular, intravenous, intraperitoneal, and subcutaneous For injection, the composition, e g , bacteria, of the invention can be formulated in liquid solutions, preferably in physiologically compatible buffers such as Hank's solution or Ringer's solution For oral administration, the pharmaceutical compositions may take the form of, for example, tablets or capsules prepared by conventional means with pharmaceutically acceptable excipients such as binding agents (e g , pregelatinised maize starch, polyvinylpyrrolidone or hydroxypropyl methylcellulose), fillers (e.g , lactose, microcrystalline cellulose or calcium hydrogen phosphate), lubricants (e g., magnesium stearate, talc or silica), disintegrants (e g , potato starch or sodium starch glycolate), or wetting agents (e g , sodium lauryl sulphate) The tablets may be coated by methods well known in the art Liquid preparations for oral administration may take the form of, for example, solutions, syrups or suspensions, or they may be presented as a dry product for constitution with water or other suitable vehicle before use Such liquid preparations may be prepared by conventional means with pharmaceutically acceptable additives such as suspending agents (e g , sorbitol syrup, cellulose derivatives or hydrogenated edible fats), emulsifying agents (e g , lecithin or acacia), non-aqueous vehicles (e g , almond oil, oily esters, ethyl alcohol or fractionated vegetable oils), and preservatives (e g , methyl or propyl-p- hydroxybenzoates or sorbic acid) The preparations may also contain buffer salts, flavoring, coloring and sweetening agents as appropriate Preparations for oral administration may be suitably formulated to give controlled release of the active compound. For buccal administration the compositions may take the form of tablets or lozenges formulated in conventional manner For administration by inhalation, the pharmaceutical compositions for use according to the present invention are conveniently delivered in the form of an aerosol spray presentation from pressurized packs or a nebuliser, with the use of a suitable propellant, e g , dichlorodifluoromethane, trichlorofluoromethane, dichlorotetrafluoroethane, carbon dioxide or other suitable gas In the case of a pressurized aerosol the dosage unit may be determined by providing a valve to deliver a metered amount Capsules and cartridges of e g gelatin for use in an inhaler or insufflator may be formulated containing a powder mix of the composition, e g , bacteria, and a suitable powder base such as lactose or starch The pharmaceutical compositions may be formulated for parenteral administration by injection, e.g , by bolus injection or continuous infusion Formulations for injection may be presented in unit dosage form, e g , in ampoules or in multi-dose containers, with an added preservative The compositions may take such forms as suspensions, solutions or emulsions in oily or aqueous vehicles, and may contain formulatory agents such as suspending, stabilizing and/or dispersing agents Alternatively, the active ingredient may be in powder form for constitution with a suitable vehicle, e g , sterile pyrogen-free water, before use The pharmaceutical compositions may also be formulated in rectal, intravaginal or intraurethral compositions such as suppositories or retention enemas, e.g , containing conventional suppository bases such as cocoa butter or other glycerides Systemic administration can also be by transmucosal or transdermal means For transmucosal or transdermal administration, penetrants appropriate to the barrier to be permeated are used in the formulation Such penetrants are generally known in the art, and include, for example, for transmucosal administration bile salts and fusidic acid derivatives In addition, detergents may be used to facilitate permeation Transmucosal administration may be through nasal sprays or using suppositories For topical administration, the bacteria of the invention are formulated into ointments, salves, gels, or creams as generally known in the art, so long as the bacteria are still invasive upon contact with a target cell. The compositions may, if desired, be presented in a pack or dispenser device and/or a kit which may contain one or more unit dosage forms containing the active ingredient The pack may for example comprise metal or plastic foil, such as a blister pack The pack or dispenser device may be accompanied by instructions for administration The invasive bacteria containing the RNA expression cassette can be used to infect animal cells that are cultured in vitro, such as cells obtained from a subject These in v/troinfected cells can then be introduced into animals, e g , the subject from which the cells were obtained initially, intravenously, intramuscularly, intradermally, or intraperitoneally, or by any inoculation route that allows the cells to enter the host tissue When delivering RNA to individual cells, the dosage of viable organisms to administered will be at a multiplicity of infection ranging from about 0 1 to 10 6 , preferably about 10 2 to 10 4 bacteπa per cell In yet another embodiment of the present invention, bacteria can also deliver RNA molecules encoding proteins to cells, e g , animal cells, from which the proteins can later be harvested or purified For example, a protein can be produced in a tissue culture cell The present invention is further illustrated by the following examples which should not be construed as limiting in any way The contents of all cited references including literature references, issued patents, published patent applications as cited throughout this application are hereby expressly incorporated by reference The practice of the present invention will employ, unless otherwise indicated, conventional techniques of cell biology, cell culture, molecular biology, transgenic biology, microbiology, recombinant DNA, and immunology, which are within the skill of the art Such techniques are explained fully in the literature See, for example, Molecular Cloning A Laboratory Manual, 2nd Ed , ed by Sambrook, Fπtsch and Maniatis (Cold Spring Harbor Laboratory Press 1989), DNA Cloning, Volumes I and II (D N Glover ed , 1985), Ohgonucleotide Synthesis (M J Gait ed , 1984), Mullis et al U S Patent No 4,683,195, Nucleic Acid Hybridization (B D Hames & S J Higgins eds 1984), Transcription And Translation (pi D Hames & S J Higgins eds 1984), Culture Oj Animal Cells (R I Freshney, Alan R Liss, Inc , 1987), Immobilized Cells And Enzymes (LRL Press, 1986), B Perbal, A Practical Guide To Molecular Cloning (1984); the treatise, Methods In Enzymology (Academic Press, Inc., N.Y.); Gene Transfer Vectors For Mammalian Cells (J. H. Miller and M. P. Calos eds., 1987, Cold Spring Harbor Laboratory); Methods In Enzymology, Vols. 154 and 155 (Wu et al. eds.), Immunochemical Methods In Cell And Molecular Biology (Mayer and Walker, eds., Academic Press, London, 1987); Handbook Of Experimental Immunology, Volumes I-IV (D. M. Weir and C. C. Blackwell, eds., 1986); Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1986). Examples Example 1: RNA Delivery Using a Bacterial Vector This Example provides evidence that bacteria can be used as a vehicle for delivering RNA into eukaryotic cells, where the RNA is subsequently translated. Accordingly, this Example describes the modification of Shigella to contain (i) a β- galactosidase gene, containing eukaryotic translation elements, under the control of a T7 promoter; and (ii) a gene encoding a T7 polymerase under the control of a bacterial promoter, and that contacting of Hela cells with such modified Shigella bacteria results in efficient expression of β-galactosidase RNA transcribed in the bacteria in the HeLa cells. A. Modification of Shigella for Use as an RNA Delivery Vehicle For efficient translation of β-galactosidase RNA in eukaryotic cells, the β- galactosidase gene was cloned downstream of the cap-independent translation enhancer (CITE) sequence from encephalomyocarditis virus (Duke et al, J. Virol, 66:1602- 1609 ( 1992)). This sequence allows for efficient translation of an RNA in the absence of a 7 methyl-guanosine at the 5' end of the RNA. The LacZ gene encoding the bacterial β-galactosidase gene was cloned in the pCITE4 vector (Novagen, Inc., Madison, WI). Expression vector pCITE4 contains a T7 promoter sequence juxtaposed and upstream of a CITE sequence which in turn is upstream of several unique cloning sites. Downstream of the cloning sites are sequences encoding a poly-A tail and T7 transcription terminator. Accordingly, the T7 polymerase promoter in pCITE4 allows high-level expression of the mRNA containing the CITE sequence fused to the β-galactosidase gene. pCITE4 vector further comprises a gene providing resistance to ampicillin. This plasmid is referred to herein as pCITE4-LacZ. To supply T7 polymerase in trans, plasmid pGB-T7 was constructed. This plasmid was derivatived from streptomycin-resistant low copy plasmid pGB2 and expresses the T7 polymerase under the control of the in v/ ' voinducible anaerobic E. coli nirB promoter (see, e.g., Chatfield et al. (1992) Biotechnology 10:888). In short, plasmid pGB2 (Churchward, G et al. (1984) Gene 31 : 165) was digested with restriction endonucleases Sail and PstI (Sambrook et al., supra) and ligated to a PCR generated fragment encoding the nirB promoter resulting in plasmid pBA-23 (Wu, S. et al. ( 1997) AIDS Research and Human Retroviruses 13: 1 187). Then the sequences encoding T7 RNA polymerase were excised from plasmid pGPl-2 by digesting with Smal and Sail (Tabor, S. and C.C. Richardson (1992) Biotechnology 24:280) and inserted into Smal digested pBA-23 by blunt digestion (Sambrook et al., supra), resulting in plasmid pGB-T7. The plasmids pCITE4-β-LacZ and pGB-T7 were introduced into attenuated Shigella flexneri strain CVD 1203 by electroporation (Hone et al. (1991) Vaccine • 9:810). This bacterial strain contains deletions in both aroA and virG loci and is further described in Noriega et al. (1994) Infection & Immunity 62:5168. Since pCITE4-LacZ and pGB-T7 contain different antibiotic selection markers and compatible replicons, these plasmids will coexist in a single bacterial clone. Control strains having either pGB-T7 or pCITE4-lacZ alone were also constructed by electroporation. Expression of β-galactosidase-encoding RNA in the transformed bacteria was confirmed by RNA hybridization using the RNeasy™ total RNA isolation system according to the manufacturer's instructions (Qiagen). Prior to infection of eukaryotic cells, the transformed bacteria were seeded from 30% (w/v) glycerol stocks, maintained at -70°C, onto solid medium (Tryptic Soy Agar, DIFCO, Madison, WI) containing 100 μg/ml of ampicillin and 100 μg/ml of streptomycin, to select for bacteria containing the plasmids, and incubated overnight at 37°C. B. Eukaryotic Cells HeLa cells (ATCC No. CCL-2) were grown on plastic tissue culture plates at 37°C in 5% (v/v) CO 2 in RPMI medium supplemented with 10% (v/v) fetal bovine serum, 2.0 mM L-glutamine, 1.0 mM L-pyruvate, 50 U/ml penicillin and 50 μg/ml streptomycin (hereinafter "RPMI/FBS"). 24 to 48 hours prior to RNA delivery, the HeLa cells were trypsinized with 0.25% (w/v) trypsin containing 1.0 mM EDTA, and split by limiting dilution such that they were 70-90% confluent at the time of the experiment. Prior to RNA delivery, the number of HeLa cells present was ascertained by counting in a hemocytometer (Celis, Cell Biology: A Laboratory Manual, Ed. Academic Press, San Diego, CA (1994)). C. Delivery of Bacterial RNA and Expression of the Product of Said RNA in Eukaryotic Cells Semi-confluent HeLa cells in 24 well plates (at 5 x 10 5 cells/well; Costar) were washed once with RPMI media lacking fetal bovine serum and penicillin/streptomycin (hereinafter "SFM"), then overlaid with each bacterial suspension in SFM, and incubated at 37°C in 5% CO 2 . Suspensions of the bacterial RNA delivery vectors were either harvested from overnight agar plates or from a fresh broth cultures as described above, and were inoculated at an infection ratio of approximately 10 1 to 10 3 viable transformed bacteria per HeLa cell. After 3 hours, the SFM containing the extra-cellular bacteria was removed, the HeLa cells were rinsed once with RPMI/FBS, and then fresh RPMI/FBS containing 100 μg/ml gentamicin was added. Following a further 1 hour incubation at 37°C in 5% CO 2 , the gentamicin solution was removed, the HeLa cells were rinsed once with RPMI FBS, fresh RPMI/FBS was added and the cells were returned to 37°C in 5% CO 2 . Survival of the attenuated S. flexneri inside of the HeLa cells was determined by counting viable bacteria present in each of 3 wells at 6 hours, 24 hours, 48 hours, 72 hours and 92 hours, post-bactofection (i.e., post contacting of the HeLa cells with the engineered Shigella bacteria) by standard methods (Celis, Cell Biology: A Laboratory Manual, Ed, Academic Press, San Diego CA (1994)). Briefly, the post-bactofected HeLa cells were lysed with 1.0% (w/v) deoxycholic acid and viable bacteria enumerated on solid media. The results show that the S. flexneri strains were rapidly inactivated inside the HeLa cells. This demonstrates that the delivery strain is attenuated and rapidly inactivated in human cells. At 6 hours, 24 hours, 48 hours, and 72 hours post-inoculation, the HeLa cells were harvested and the β-galactosidase activity was quantitated using standard colorimetric procedures. The results of this experiment demonstrate the presence of a significant amount of β-galactosidase activity in cells infected by the Shigella vector containing both pCITE-lacZ and pGB-T7 (greated than 200 μU β-galactosidase activity/mg protein) . HeLa cells infected with a vector containing either pCITE-LacZ or pGB-T7 alone did not result in any significant β-galactosidase activity (i.e., less than 50 μU β- galactosidase activity/mg protein). Thus, these results indicate that RNA was successfully produced in Shigella and translated in HeLa cells. Example 2: Delivery of RNA Encoding HIV-1 Env Gene Product This example describes a system permitting the delivery of RNA encoding the HTV-1 envelope (HIV env) protein to HeLa cells by an engineered Shigella bacteria and efficient translation of the RNA in HeLa cells. The advantage of expressing HIV- 1 env in human cells compared to bacterial expression is the ability of human cells to glycosylate and produce an oligomeric Env structure that closely resembles Env expressed in HIV-1 infected individuals (Earl et al. (1994) J. Virol. 68:3015). To produce an Env mRNA expression cassette, DNA sequences encoding HIV- I MN env are cloned from plasmid pMN-STl into plasmid pCITE4a (Novagen, Inc., Madison, WI) as follows. The HIV-1 Env-encoding sequences from plasmid pMN-STl (described in Lori et al. (1992) J Virol. 66:5067) are amplified by PCR using Vent polymerase from the second nucleotide of the second codon to the termination codon. pCITE4a is digested with Mscl and Bglϊl (New England Biolabs) to remove unwanted sequences in the cloning region that encode a thrombin digestion site and a His tag The Bglil site is end-filled using T4 DNA polymerase (MscI creates blunt-ends) and the PCR-generated Env sequences are inserted into the resultant linear vector by blunt ligation, resulting in pCITE4-e«v λr This ligation produces a transcriptional fusion between pCITE4a and the Env encoding sequences To construct a vector delivery strain, plasmids pGB-T7 (see Example 1) and pCITE4-e«v MN are co-transformed into attenuated Shigella vector strain CVD 1203 (Noriega et al , supra) by electroporation as described above Following transformation, the plasmids are characterized by restriction endonuclease digestion and PCR analysis as above to verify that the correct configurations are present in each strain In addition, expression of Env-encoding RNA by each construct is confirmed by RNA hybridization using the RNeasy total RNA isolation system according to the manufacturer's instructions (Qiagen) Strains that display the appropπate genotype and RNA profile can be subsequently used to evaluate the efficacy of such a plasmid-based RNA delivery system in vitro and vivo To evaluate the delivery and expression of Shigella-vectored Env RNA vaccines, 70-80% confluent HeLa cells will be treated with a range of doses (multiplicity of infection (moi) from 1 to 1000) of each Shigella vector as described above The first series of experiments will involve Shigella vectors having plasmids pCITE4-e»v MN and pGB-T7 alone or in combination The second series of experiments will involve Shigella vectors having plasmids pnιrB-T7 and pCITE4- eλ?v MN , alone or in combination As controls, lipofectamine-treated pCITE4-env MN DNA and pRc/CMV -env will be transfected into HeLa cells or HeLa cells pretreated with vaccinia T7 (vT7) (provided by the AIDS Repository, NIAID), which expresses T7 in the cytoplasm of infected cells, to provide T7 polymerase for the expression of env Cells treated with the bacterial vectors or the control cells will be harvested following incubation at 37°C in 5% CO 2 for 3, 6, 24, 48 and 72 hrs The cells will be washed twice with PBS and lysed in IX SDS sample buffer and run on SDS-PAGE gels made with 5% to 15% gradients of polyacrylamide The samples will be run under non-reducing and reducing conditions to estimate the yields of oligomeπc forms of gp 160 (Env) The cell samples will also be transferred to PVDF membranes which will be probed with a mixture of monoclonal antibodies specific for defined epitopes of gpl20 and gp41 (Abacioglu et al. (1994) AIDS Res. Hum. Retroviruses 10:371). The extent of glycosylation of Env proteins will be estimated by treatment with Endo-H prior to separation and evidence of glycosylation will be taken as sine qua non that the Env RNA was expressed in the eukaryotic cell. The Env protein will be characterized further for CD4 binding and epitope exposure using capture ELISA methods and published criteria (Abacioglu et al., supra, Moore et al. (1994) J. Virol. 68:6836, Poignard et al. (1996) J Exp. Med. 183:473). Example 3: Chromosomal-Based RNA Delivery and Expression System This example describes an RNA delivery and expression system using an attenuated Shigella vector CVD 1203 with the T7-driven RNA expression cassettes integrated in the bacterial chromosome. This RNA delivery system presents several advantages, including the absence of delivery of plasmid DNA to the eukaryotic cells, thereby preventing random integration of the plasmid DNA in the eukaryotic chromosomal DNA. This Example describes integration of these constructs into sequences immediately upstream of hisOGD. In previous studies it was found that this region could accommodate a 7 kb fragment that expressed an E. coli fimbrial antigen (Hone et al. (1988) Microbial Pathogenesis 5:407). More importantly, the integrated sequences were highly stable both in vitro and in vivo (Hone et al., supra) and did not adversely affect the invasiveness of the bacterial vector. Using this system, integration is achieved in two steps. First, a hisG deletion is introduced into the target strain using plasmid pADE172 (Hone et al., supra). Second, the sequences encoding the integrand are introduced into the mutant allele, resulting in rescue of a his ' phenotype (Hone et al., supra). In short, PCR generated fragments spanning P mrB -pol T7 and P T7 -CITE- em > either in combination or alone will be introduced into PstI digested pADE171 by blunt-end ligation. As before, each construct that displays the appropriate restriction endonuclease digestion pattern and PCR profile will be sequenced to ensure that no errors were introduced as a result of PCR infidelity. The resulting plasmids will then be introduced into CVD 1203, as above. The presence of the integrands will be verified by PCR analysis and DNA hybridization analysis. In addition, expression of Env-encoding RNA by each construct will be confirmed by RNA hybridization as described above. Strains that display the appropriate genotype and RNA profile will be subsequently used to evaluate the efficacy of a chromosome-based RNA expression cassette in vitro and in vivo. Example 4: Delivery of a Reporter Gene in vivo to Animal Tissue This Example describes in vivo assays in mice to demonstrate the immunogenicity of a Shigella RNA vaccine delivery system by monitoring humoral and cellular immune responses in both the mucosal and systemic compartments. Pretreatment of mice with streptomycin alters the intestinal flora and creates a favorable environment for Shigella invasion of the mouse gastrointestinal tract (Cooper (1959) Australian J. Exp. Biol. Med. Sci. 17:193). Thus, this model offers inexpensive means to evaluate RNA vaccine delivery to these tissues. The effectiveness of this delivery system will then be fully investigated in Rhesus monkeys and volunteers. BALB/c mice, 6-8 weeks old, will be housed in sterilized microisolator cages and maintained on sterile food and water containing streptomycin ( 1 mg/ml) (Cooper, supra). Strains bearing pGB-T7 will be streptomycin resistant but the chromosomal constructs will be sensitive to this antibiotic. Therefore, the chromosomal constructs must first be made streptomycin-resistant prior to the murine studies. This will be accomplished by isolating spontaneous streptomycin-resistant derivatives of these constructs, which are selected on solid agar containing 1 mg/ml streptomycin. To vaccinate the animals, each mouse will be given 0.2 ml of 50%> saturated bicarbonate solution by orogastric intubation as described in Hone et al. (1987) J. Infect. Diseases 156: 167. Two separate experiments will be conducted: (1) To evaluate the induction of humoral responses, groups of 5 BALB/c mice will be immunized by orogastric intubation with a single 10 9 cfu dose of the Shigella constructs. (2) To evaluate T cell- mediated responses, groups of 35 BALB/c mice will be immunized by orogastric intubation with a single 10 9 cfu dose of the Shigella constructs These two experimental groups will be boosted 30, 58, and 86 days after the primary dose The first group will be immunized with Shigella strains containing either of both of the plasmids pGB-T7 and pCITE4 env^,, prepared as described above The second group will be immunized with Shigella strains having the plasmids pGB-T7 and pCITE4 env^, or with Shigella strains having either or both of plasmids pwri?-PolT7 and pT7-CITE-envMN, prepared as described above Positive control mice will be immunized intranasally with 10 μg Env mixed with 5 μg cholera toxin, which induces strong Env-specific immunity (i) Characterization of humoral responses Blood will be collected from the tail veins of vaccinated mice 14, 28, 42, 56, 70, 84, 98, and 1 12 days after the primary immunization The level of Env-specific IgG and IgA will be measured in sera separated from these blood samples by ELISA using purified fully glycosylated HIV- l Nf Env as described, e g , in Abacioglu et al (1994) AIDS Res. & Human Retroviruses JO 371 or in Moore et al (1994) J. Virol 68 6836 In each ELISA, Env- specific mAbs will be used as a positive control (Abacioglu et al , supra, Moore et al , supra, and Moore (1990) AIDS 4 297) In addition, these sera will be used to follow the development of HIV- 1 neutralizing antibodies by the quantitative, linear HIV-1 infectivity assay as described, e g., in Nara et al (1990) J. Virol 64 3779, Layne et al ( 1991 ) Virol 189 695, Layne et al ( 1991 ) J. Virol 65 3291 , and Wu et al (1995) J. Virol 69 6054, incorporating HIV-1 I (Gurgo et al (1988) Virol 164 531) to measure homologous neutralization and HIV-l^, HIV- I RP as well as primary HIV-1 isolates (from the AIDS Repository, NIALD) to measure clade-specific and cross-clade neutralization (Nara et al, supra, Layne et al , supra, Wu et al, supra). (ii) Tissue harvesting for T cell assays and Env-specific antibody secreting cells (ASC in the lamina propria Lamina propria antigen-specific antibody secreting cells are a useful and convenient measure of mucosal immunity, since these cells are T cell- dependent and the level of the response correlates with the development of humoral responses at the mucosal surface after immunization Groups of 5 mice will be sacrificed 7, 14, 28, 42, 56, and 70 days after primary immunization and mononuclear cells (MNCs) from the spleen, small intestine and mesenteric lymph nodes will be prepared as descπbed (Wu S et al (1995) Infection & Immunity 6_! 4933-8, Fouts TR et al (1995) Vaccine H 1697-705, Okahashi N et al (1996) Infection & Immunity 64 1516-25, Xu-Amano J et al (1993) ./ Exp Med 178 1309-20) (in) Enumeration of Env-specific ASC in the lamina propria Lamina propria MNCs will be prepared as descπbed (Wu S et al supra, Okahashi N et al supra, Xu-Amano J et al supra ) Standard IgA- and IgG-specific ELISPOT assays will be used to enumerate Env-specific ASC in the lamina propria MNCs, antigens included in this assay included fully-glycosylated Env (Intracel) or diphtheria toxoid (negative control) The results will be expressed as a function of the total IgA or IgG ASCs (iv) Charactenzation of CD4 + T cell responses To measure T cell proliferation, each MNC preparation will be stimulated with purified fully glycosylated HIV-1 1N Env (Intracel Inc ) and proliferation will be quantitated by 3 H-TdR incorporation, as descπbed, e g , in (Wu S et al supra, Fouts TR et al supra, Okahashi N et al supra, Xu-Amano J et al supra) Each proliferative assay will include a mitogen (Con A) control, an ovalbumin control, and a dose-response curve to Env (0 01-10 g/ml) Supernatants will also be collected 72 hr after antigenic stimulation and chemokines MJP-1 and MIP-1 , and cytokines IL-4, IL-5, IL-6, IL-10, IL-12 and IFN-γ will be measured by ELISA using commercially available reagents The amount of cytokines secreted will be relevant, e g , in light of evidence suggesting that TH1 responses correlate with chemokines production (Schrum S et al (1996) ./ Immunol 157 3598- 3604) Specific T cells will be further characterized by chemokine- and cytokine- specific ELISPOT assays for MLP-1 , MLP-1 , IL-2, IL-4, IL-5, IL-6, IL-10, and IFN- γ production after enrichment of CD4" T cells using commercially available monoclonal antibodies (R&D Systems and Pharmingen) (Okahashi N et al supra, Xu- Amano J et al supra) In addition, MNCs will be cultured in complete medium (CM) only, CM containing fully glycosylated HIV-l xr Env at 0 1-10 or CM containing Shigella outer membrane antigen (10 μg/ml) as shown above The culture plates will be incubated then for 24 hr at 37 °C in 5% CO 2 After stimulation, CD4 and CD8" T cells will be isolated by flow cytometry and no fewer than 5x10 5 of the purified cells will be placed directly into Tπzol R reagent and cDNA will be synthesized, the resultant samples then will be used in quantitative-competitive (QC) PCR reactions to evaluate the relative levels of chemokine and cytokine cDNA sequences MIP-1, MIP-1, RANTES, TNF, IL-2, IL-4, IL-5, IL-6, and IFN-γ sequence specific RT-PCR primers are designed based on the known sequences of these molecules, e g , GenBank Each QC PCR reaction will be conducted in parallel with the same reactions containing of control plasmid DNA at a range of defined concentrations, which encode truncated MIP-1, MLP-1 and RANTES cDNA sequences and will serve as competitive sequences (R&D Systems and National Biosciences Inc) The PCR fragments will be separated by agarose gel electrophoresis, strained with ethidium bromide and scanned using a BioRad UV densitometer The results will be expressed as arbitrary mRNA units (v) Characterization of CD8 * T cells CD8+ CTL responses have been shown to correlate with reduced viral burdens in acutely infected individuals (Koup RA et al (1994) J. Virol 68 4650-5, Borrow P et al (1994) J Virol 68 6103-10, Koup RA and Ho DD (1994) Nature 170 416) It has also, enhanced production of HIV- suppressive activity by CD8 + T cells and chemokines by T cells from infected (Walker CM et al (1989) Cellular Immunology JJ9 4' '0-5 , Walker CM et al (1991) J. Virol 65 5921-7, Hsueh FW et al (1994) Cellular Immunology 159 271-9) as well as repeatedly-exposed, uninfected individuals (Paxton W et al (1996) Nature Medicine 2 412-417) correlated with significantly reduced capacity of these cells to support the growth of HIV- 1 Furthermore, protective immunity against a rectal SIV challenge in Rhesus macaques, after direct immunization with SIV Env and p27 into the iliac lymph nodes, correlated with the development of HIV-suppressive, chemokine-secreting CD8 + T cells (Lehner, T. et al (1996) Nature Medicine 2 767-75) in the draining nodes To characterize HlV-specific chemokine-secreting CD8 * T cell responses, a further group of mice will be immunized as above On days 7, 14, 28, 42, 56, and 70 after immunization, groups of 5 mice will be sacrificed and MNCs from the spleens, the intestinal epithelial layer and lamina propria, and mesenteric lymph nodes will be prepared as described above The MNCs will be divided into total, CD4 + T cell- depleted and CD8 " T cell-depleted cells, and will be cultured in (i) CM only, (ii) CM containing PHA (Flynn JL et al (1990) Molecular Microbiology 4 2111-8), (iii) CM containing BC-/αcZ fibroblasts expressing galactosidase (Aggarwal A. et al. (1990) J. Exp. Med. 172:1083-90) or (iv) CM containing C-env fibroblasts expressing gpl60 (Abimiku, AG et al. (1995) AIDS Research & Human Retroviruses 11:383-93). The stimulator cells will be titrated in the range of 1000 to 500000 cells per well. The cells then will incubated for 72 hr at 37 °C in 5% CO 2 and supernatants will be collected every 24 hr and stored at -80 °C. These supernatants will be used to quantitate the levels of MIP-1, MIP-1, TNF-α, LL-2, IL-4, IL-5, IL-6 and IFN-γ by ELISA (see above). In addition, after stimulation CD8 + T cells will be isolated from total MNC cultures by flow cytometry to obtain a minimum of 5x10 5 cells, which will be placed directly into Trizol R reagent and cDNA will be synthesized; QC PCR reactions will be used to evaluate the relative levels of chemokine and cytokine cDNA. (vi) Class I restricted CTL responses: After 5 days of such stimulation HlV-specific CTL activity will be measured at both mucosal and systemic effector sites with methodology appropriate to the site (Chada S. et al. (1993) J. Virol. 67:3409-17; Fujihashi K. et al. (1990) J. Immunol. 145:2010-9): the spleen will serve as the systemic site and intestinal intraepithelial and lamina propria lymphocytes will serve as the mucosal sites. CTL activity will be measured using targets 51 Cr-labeled (i) BC-/αcZ fibroblast or (ii) BC-e«v fibroblast (Chada S. et al., supra; Fujihashi K. et al, supra). When killing is observed, the effector cells will be phenotyped using standard methods applied by our group previously (Aggarwal A. et al., supra). For quantitative comparisons, CTL responses will be expressed in lytic units/10 6 cells. Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims: Claims: 1 An isolated bacteπum compπsing a DNA which is capable of being transcribed into RNA in the bacterium, wherein the RNA is capable of being translated in a eukaryotic cell, or is an antisense RNA or a catalytic RNA 2 The isolated bacteπum of claim 1, wherein the RNA which is capable of being translated in a eukaryotic cell comprises a Cap Independent Translation Enhancer sequence 3 The isolated bacterium of claim 1 , wherein the DNA is operably linked to a prokaryotic promoter 4 The isolated bacterium of claim 1 , wherein the DNA is operably linked to a first promoter, and the bacterium further comprises a gene encoding a polymerase which is operably linked to a second promoter which is prokaryotic and the polymerase is capable of mediating transcription from the first promoter 5 The isolated bacteπum of claim 4, wherein the first promoter is a T7 promoter and the polymerase is a T7 polymerase 6 The isolated bacteπum of claim 4, wherein the second promoter is the bacterial nirB promoter 7 The isolated bacteπum of claim 1, wherein the DNA is located on the bacterial chromosome 8 The isolated bacteπum of claim 4, wherein the DNA and the gene encoding the polymerase are located on the bacterial chromosome The isolated bacterium of claim 1, wherein the DNA encodes a vaccine antigen or an immunoregulatory agent The isolated bacterium of claim 1, which is an invasive bacterium The isolated bacterium of claim 10, which is capable of invading a vertebrate cell The isolated bacterium of claim 11, wherein the vertebrate cell is a mammalian cell The isolated bacterium of claim 12, wherein the mammalian cell is selected from the group consisting of a human, cattle, sheep, goat, horse, donkey, primate, and buffalo cell The isolated bacterium of claim 11, which is selected from the group consisting of Shigella spp., Listeria spp., Rickettsia spp. and enteroinvasive Escherichia coli The isolated bacterium of claim 10, wherein the bacterium has been modified to increase its invasive potential The isolated bacterium of claim 15, wherein the bacterium has been genetically engineered to increase its invasive potential The isolated bacterium of claim 16, wherein the bacterium has been genetically engineered to mimic the invasive properties of Shigella spp., Listeria spp., Rickettsia spp. and enteroinvasive Escherichia coli The isolated bacterium of claim 17, wherein the bacterium is selected from the group consisting of Yersinia spp., Escherichia spp., Klebsiella spp., Bordetella spp., Neisseria spp., Aeromonas spp., Franciesella spp., Corynebacteri urn spp., Citrobacter spp., Chlamydia spp., Hemophilus spp., Brucella spp., Mycobacterium spp., Legionella spp., Rhodococcus spp., Pseudomonas spp., Helicobacter spp. , Salmonella spp. , Vibrio spp. , Bacillus spp. , Leishmania spp. and Erysipelothrix spp. 19. The isolated bacterium of claim 15, wherein the bacterium has been modified with an invasion factor. 20. The isolated bacterium of claim 19, wherein the invasion factor is invasin. 21. The isolated bacterium of claim 1, which is an attenuated bacterium. 22. An isolated bacterium comprising an RNA which is capable of being translated in a eukaryotic cell, or is an antisense RNA, or a catalytic RNA. 23. The isolated bacterium of claim 22, wherein the RNA which is capable of being translated in a eukaryotic cell comprises a Cap Independent Translation Enhancer sequence. 24. The isolated bacterium of claim 22, wherein the catalytic RNA is a ribozyme. 25. The isolated bacterium of claim 22, wherein the RNA encodes a vaccine antigen or an immunoregulatory agent. 26. The isolated bacterium of claim 22, which is an invasive bacterium. 27. The isolated bacterium of claim 26, which is capable of invading a vertebrate cell. 28. The isolated bacterium of claim 27, wherein the vertebrate cell is a mammalian cell The isolated bacterium of claim 28, wherein the mammalian cell is selected from the group consisting of a human, cattle, sheep, goat, horse, donkey, primate, and buffalo cell The isolated bacterium of claim 29, which is selected from the group consisting of Shigella spp., Listeria spp., Rickettsia spp. and enteroinvasive Escherichia The isolated bacterium of claim 26, wherein the bacterium has been modified to increase its invasive potential The isolated bacterium of claim 26, wherein the bacterium has been genetically engineered to increase its invasive potential The isolated bacterium of claim 32, wherein the bacterium has been genetically engineered to mimic the invasion properties of Shigella spp., Listeria spp., Rickettsia spp. and enteroinvasive Escherichia coli The isolated bacterium of claim 33, wherein the bacterium is selected from the group consisting of ' Yersmia spp., Escherichia spp., Klebsiella spp., Bordetella spp., Neisseria spp., Aeromonas spp., Franciesella spp., Corynebacteri um spp. , Citrobacter spp. , Chlamydia spp. , Hemophilus spp. , Brucella spp. , Mycobacterium spp. , Legionella spp. , Rhodococcus spp. , Pseudomonas spp. , Helicobacter spp. , Salmonella spp. , Vibrio spp. , Bacillus spp. , Leishmania spp. and Erysipelothrix spp. The isolated bacterium of claim 31 , wherein the bacterium has been modified with an invasion factor The isolated bacterium of claim 35, wherein the invasion factor is invasin The isolated bacterium of claim 22, which is an attenuated bacterium A pharmaceutical composition comprising the isolated bacterium of claim 1 and a pharmaceutically acceptable carrier A pharmaceutical composition of comprising the isolated bacterium of claim 22 and a pharmaceutically acceptable carrier A eukaryotic cell comprising a bacterium of claim 1 The eukaryotic cell of claim 40, which is a mammalian cell The eukaryotic cell of claim 41, which is a human cell A eukaryotic cell comprising a bacterium of claim 22 The eukaryotic cell of claim 43, which is a mammalian cell The eukaryotic cell of claim 44, which is a human cell An isolated DNA operably linked to a prokaryotic promoter, wherein the DNA encodes an RNA which is capable of being translated in a eukaryotic cell, or is an antisense RNA or a catalytic RNA The isolated DNA of claim 46, wherein the RNA which is capable of being translated in a eukaryotic cell comprises a Cap Independent Translation Enhancer sequence The isolated DNA of claim 46, wherein the catalytic RNA is a ribozyme The isolated DNA of claim 46, wherein the RNA encodes a vaccine antigen or an immunoregulatory agent A method for introducing an RNA into a eukaryotic cell, comprising contacting the eukaryotic cell with at least one bacterium comprising a DNA which is either capable of being transcribed into RNA in the bacterium or comprises RNA, wherein the RNA is capable of being translated in a eukaryotic cell or is an antisense RNA or catalytic RNA The method of claim 50, wherein the contacting is performed at a multiplicity of infection ranging from about 0 1 to about 10 6 bacteria per eukaryotic cell The method of claim 51, wherein the multiplicity of infection ranges from about 10 2 to about 10 4 bacteria per eukaryotic cell The method of claim 50, wherein the contacting is performed in vitro The method of claim 53, further comprising the step of administering the at least one eukaryotic cell to a eukaryotic subject The method of claim 50, wherein the at least one eukaryotic cell is in a subject and the method further comprises administering to the subject the at least one bacterium The method of claim 55, wherein the at least one bacterium is administered orally to the subject The method of claim 55, wherein the at least one bacterium is administered intrarectally to the subject 58. The method of claim 55, wherein the at least one invasive bacterium is administered intranasally to the subject. 59. A method for immunizing a subject, comprising administering to the subject a number of isolated bacteria of any one of claims 1-37, to produce an amount of antigen sufficient for mounting an immune response against the antigen in the subject. 60. The method of claim 59, wherein the bacteria are administered orally. 61. The method of claim 60, wherein the antigen is a polypeptide derived from HIV. 62. The method of claim 61, wherein the bacteria are Shigella bacteria.

Inventors: HONE DAVID M, LEWIS GEORGE, POWELL ROBERT, HONE, DAVID, M., LEWIS, GEORGE, POWELL, ROBERT
Applicants: HONE DAVID M, LEWIS GEORGE, POWELL ROBERT, UNIV MARYLAND BIOTECH INST, HONE, DAVID, M., LEWIS, GEORGE, POWELL, ROBERT, UNIVERSITY OF MARYLAND BIOTECHNOLOGY INSTITUTE
Application year: 1998
Main classification: C12N15
IPC codes: A61K35, A61K39, A61K48, C07K14, C12N1, C12N5, C12N15