RECOMBINANT FLAVIVIRUSES AND METHODS OF USE THEREOF

Abstract: The present invention provides recombinant flaviviruses, particularly live attenuated recombinant flavivirus, which comprise exogenous nucleotide sequences which encode exogenous amino acid sequences. These recombinant flavivirus viruses comprise an exogenous nucleic acid. Infection of a host cell with a recombinant flavivirus provides for expression of the exogenous nucleic acid in a host cell and production of an antigenic polypeptide encoded by the exogenous nucleic acid. Such recombinant flavivirus are useful in eliciting an immune response to the exogenous polypeptide.

Description: RECOMBINANT FLAVIVIRUSES AND METHODS OF USE THEREOF CROSS-REEERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 60/177,449, filed January 21, 2000, which application is incorporated herein by reference. GOVERNMENT RIGHTS The United States Government may have certain rights in this application pursuant to U.S. Public Health Service grant AI44343. FIELD OF THE INVENTION The invention relates generally to the field of recombinant viruses and induction of specific immunity, specifically to induction of tumor-specific immunity. BACKGROUND OF THE INVENTION Tumor-specific cytotoxic T lymphocytes (CTLs) can prevent or eradicate tumors in a number of experimental systems and in patients with cancer (1-3). Clinical trials have demonstrated that 35% of patients with melanoma treated with specific, tumor-reactive lymphocytes can achieve either partial or complete tumor regression (4). The antigens recognized by the T cells have, in some cases, been identified (5, 6). Although cancer cells may express tumor-associated antigens (TAAs), CTLs directed against TAAs are not efficiently elicited by the growing tumor and, therefore, the immune system fails to control tumor growth. Thus, it appears that tumor cells lack either immunogenicity and/or the appropriate co-stimulation required for CTL activation. In contrast to tumor cells, viruses are strong inducers of cellular immune responses. Thus, activation of the tumor-directed CTL response by vaccination with recombinant viruses expressing tumor-associated antigens is a promising approach for the prevention and treatment of malignancies. Viral vaccine vectors that have been successfully used in experimental cancer models include poxviruses, adenoviruses, picornaviruses and influenza viruses (7-10). However, because each vaccine vector may present its own set of beneficial and adverse properties, the search for new vectors continues to be an active area of research. For example, clinical use of some vectors currently under study may be limited by their record of safety, efficacy, potential oncogenicity or induction of immunosuppression. In addition, pre-existing immunity against the vector could hinder the potency of treatment (8, 11), and therefore alternative viral vectors are needed. There is a need in the field for viral vectors that can be used to induce immunity to a wide variety of antigens, including those present on tumors. The present invention addresses this need, and provides related advantages as well. SUMMARY OF THE INVENTION The present invention provides a recombinant flavivirus comprising an exogenous nucleic acid sequence which encodes an exogenous polypeptide. In general, the recombinant flaviviruses of the invention are replication competent, i.e., they are live, attenuated viruses. In some embodiments, the flavivirus is a yellow fever virus. Accordingly, in some embodiments, the invention provides recombinant yellow fever viruses (YFV), particularly live attenuated recombinant YFV, which comprise an exogenous (/. e. , non-YFV) nucleotide sequence which encodes an exogenous (/. e. , non-YFV) polypeptide. These recombinant flaviviruses comprise an exogenous nucleic acid. Infection of a host cell with a recombinant flavivirus provides for expression of the exogenous nucleic acid in a host cell and production of an antigenic polypeptide encoded by the exogenous nucleic acid. Such recombinant flavivirus are useful in eliciting an immune response to the exogenous polypeptide. The recombinant live attenuated flavivirus express an exogenous nucleotide sequence which encodes an exogenous polypeptide incorporated into the flavivirus polyprotein, such as, but not limited to, a polypeptide obtained from a pathogenic agent other than flavivirus , a tumor antigen, and the like. These recombinant flavivirus are useful, when introduced into a mammalian subject, in eliciting an immune response to the exogenous polypeptide in the subject. Thus, the recombinant flavivirus of the invention serve as immunization vehicles. A wide variety of antigenic amino acid sequences can be incorporated into the flavivirus polyprotein, including those of microbial pathogens (e.g., bacteria, parasites, viruses (other than flavivirus ), fungi, and the like) and tumor-associated antigens. In general, following infection of a host cell by the recombinant virus of the invention, the exogenous polypeptide is proteolytically cleaved from the viral polyprotein precursor into which it is incorporated. The exogenous polypeptide may then be exported to the host cell surface, may be presented on the cell surface as a peptide with a major histocompatibility antigen, may be secreted from the cell, or may remain in the cytoplasm of the cell. In the context of tumor immunotherapy, expression of an exogenous polypeptide in a host elicits an immune response to the tumor, with the result that the tumor cell mass and/or tumor cell number is reduced, development of a tumor is prevented or delayed, and/or the probability that a tumor will develop is reduced. The invention provides pharmaceutical compositions comprising recombinant flavivirus of the invention. Such compositions can be used, for example, to reduce the severity of disease, reduce the risk of clinical disease, prevent the onset of a disease and/or to ameliorate the disease via recruitment of the host immune system. The invention also provides methods of eliciting an immune response to an antigen in a mammalian subject. Such methods comprise administering a recombinant flavivirus of the invention to a mammalian subject so as to elicit an immune response to the exogenous polypeptide. The antigen can be a host antigen or an antigen of a non- flavivirus pathogen. The invention further provides methods of reducing or inhibiting tumor cell growth, and methods of reducing tumor cell mass and or tumor cell numbers. Such methods comprise administering a recombinant flavivirus of the invention which comprises exogenous sequences encoding a tumor- associated antigen (TAA)/epitope to a host bearing a tumor, such that the recombinant flavivirus enters a cell of the host and the exogenous TAA polypeptide is expressed on the surface of a host cell, is presented in the context of an MHC molecule, or, alternatively, secreted from the host cell. An immune response is elicited to a tumor which bears on its surface an antigen which comprises the exogenous TAA polypeptide or which resembles the exogenous polypeptide sufficiently to elicit an immune response toward the tumor cell. The immune response to the tumor bearing the tumor-associated antigen on its surface is sufficient to reduce, inhibit, or eliminate the tumor. The invention further provides methods of preventing tumor cell growth, and methods of reducing the probability that a tumor will form, comprising aα iinistering a recombinant flavivirus of the invention, which comprises a tumor-associated antigen/epitope-encoding nucleic acid, to a host not bearing a tumor, such that the recombinant flavivirus enters a host cell, the tumor-associated antigen is expressed on the host cell surface and or presented in the context of an MHC molecule (e.g., MHC Class I), and an immune response is elicited to the tumor-associated antigen. The immune response to the tumor-associated antigen is sufficient to prevent, or reduce the likelihood, of tumor development in the host. A primary object of the invention is to provide a recombinant flavivirus that provides for production of an exogenous polypeptide that is suitable for induction of an immune response to the polypeptide in a host following infection with the recombinant flavivirus. Such exogenous polypeptides include, but are not limited to, an antigen produced by the host (e.g., a tumor antigen), or an antigen from a non- flavivirus pathogen (e.g., a retroviral antigen). An advantage of the invention is that the recombinant flavivirus of the invention are live attenuated virus, which will continue to propagate until the intervention of the host's immune system. Another advantage of the invention is that the recombinant flavivirus exhibits low toxicity in vivo. Yet another advantage of the invention is that the recombinant flavivirus can express a polypeptide inside the host cell, and thus can provide for induction of an immune response, particularly a cellular immune response that involves, for example, antigen-specific cytotoxic T lymphocytes (CTLs). Yet another advantage of the invention is that the immune response elicited using the recombinant flavivirus of the invention is not limited to the infected cells, as the immune system will also recognize cells bearing the antigen expressed by the recombinant flavivirus. For example, the production of a tumor antigen by the recombinant flavivirus can "break immune tolerance" to tumor antigens, and induce an immune response against the tumor effective to, for example, inhibit tumor growth. These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a yellow fever virus vector, YF-pOva, and strategy for expression of chicken ovalbumin. The top bar represents YF vector genomic RNA. Boxes below represent mature viral proteins. White arrows indicate NS2B/NS3 cleavage sites, black triangles indicate cellular signal peptidase cleavage sites. Nucleotide sequences encoding the ovalbumin K b epitope SIINFEKL and flanking viral protease cleavage were inserted at the N-terminus or at the junctions between C-prM and NS2B-NS3 (indicated by black arrows). Figure 2 depicts one-step growth curves of parental yellow fever 17D strain (YF-17D) and three YF virus vectors: YF-pOva-1 (open triangles), YF-pOva-2 (open circles), and YF-pOva-8 (closed squares). SW13 cell monolayers were infected (MOI=5) with YF-17D, YF-pOva-1, YF-ρOva-2, or YF- pOva-8. Virus production (pfu/ml) was determined at each time point by plaque assay. Figures 3A-3C depict in vivo activation of CD8+ T cells to Ova peptide. Tetrameric SIINFEKL/murine MHC class I molecule H-2 K complex bound to CD8 + splenocytes. Flow cytometry histograms illustrating tetramer binding to gated CD8 + T lymphocytes of naϊve mice or mice infected with parental YF-17D or recombinant YF-pOva-8. Splenocytes were restimulated by co-cultivation for five days with EL4 cells expressing SIINFEKL (EL4-SL8). The values indicate the percentage of CD8 + T lymphocytes that bound the tetramer. Figures 4A-D and 5A-D depict the results of immunization with YF vector, which induces protective and antigen-specific immunity to melanoma B16 expressing Ova. C57BL/6 mice were immunized either twice every 2 weeks intravenously (i.v., Figures 4A and 5A), intraperitoneally (i.p. , Figures 4B and 5B), subcutaneously (s.c. , Figures 4C and 5C) or intramuscularly (i.m. , Figures 4D and 5D) with YF-pOva-8 (3x 10 5 pfu mouse) (closed squares). As a control, mice were inoculated with either parental YF-17D (open squares) or saline (naive) (closed circles). Thirty days after first 4 immunization (day 0) animals were challenged with 5x10 B16-Ova. Figures 4A-D: Local tumor growth. The size of the tumor was determined every five days and is plotted as the average tumor area + /- standard deviation in square cm 2 vs. time post-challenge (days). Figures 5A-D: Survival is plotted as the percentage of surviving animals vs. time. All experiments included 10 mice per group and were repeated three times. Figures 6A-F depict the results of inoculating mice having established B 16 with YF-pOva-8. 3 C57BL/6 mice were injected subcutaneously with melanoma B16-Ova (5x10 cells/mouse (Figures 6A- 4 6C) or 5x10 cells/mouse (Figures 6D-6F) at day 0 (tumor implantation). Animals received three subcutaneous inoculations every three days of either PBS (naive) (closed circles), parental 17D (YF- 17D) (open squares) or YF-pOva-8 (4x10 pfu/mouse) (closed squares). Vaccines were administered at the day of tumor implantation (day 0, Figures 6A and 6D), five days (day 5, Figures 6B and 6E) or ten days (day 10, Figures 6C and 6F) after tumor inoculation. Mice were monitored for evidence of tumor growth by palpation and inspection twice a week. Figure 7 depicts the lungs of treated mice evaluated in a coded, blinded manner for pulmonary metastases 30 days after the tumor inoculation. The number of pulmonary metastases is shown for individual mice. Figure 8 depicts pictures of lungs treated with YF-pOva-8 or YF-17D at day 30 after tumor inoculation. Figures 9A and 9B depict survival plotted as the percentage of surviving animals vs. time following implantation of 5 x 10 4 cells (Figure 9A) or 1 x 10 δ cells (Figure 9B). All experiments included 10 mice per group. Naive: closed circles; YF-17D, open squares; YF-pOva-8, closed squares. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. It must be noted that as used herein and in the appended claims, the singular forms "a", "and", and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a recombinant flavivirus" includes a plurality of such viruses and reference to "the epitope" includes reference to one or more epitopes and equivalents thereof known to those skilled in the art, and so forth. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. DEFINITIONS The terms "polynucleotide" and "nucleic acid", used interchangeably herein, refer to a polymeric forms of nucleotides of any length, either ribonucleotides or deoxynucleotides. Thus, this term includes, but is not limited to, single-, double-, or multi-stranded DNA or RNA, genomic DNA, cDNA, DNA-RNA hybrids, or a polymer comprising purine and pyrimidine bases or other natural, chemically or biochemically modified, non-natural, or derivatized nucleotide bases. The terms "peptide," "oligopeptide," "polypeptide," "polyprotein," and "protein" are used interchangeably herein, and refer to a polymeric form of amino acids of any length, which can include coded and non-coded amino acids, chemically or biochemically modified or derivatized amino acids, and polypeptides having modified peptide backbones. "Recombinant," as used herein, means that a particular DNA sequence is the product of various combinations of cloning, restriction, and/or ligation steps resulting in a construct having a structural coding sequence distinguishable from homologous sequences found in natural systems. Generally, DNA sequences encoding the structural coding sequence can be assembled from cDNA fragments and short oligonucleotide linkers, or from a series of oligonucleotides, to provide a synthetic gene which is capable of being expressed in a recombinant transcriptional unit. Such sequences can be provided in the form of an open reading frame uninterrupted by internal nontranslated sequences, or introns, which are typically present in eukaryotic genes. Genomic DNA comprising the relevant sequences could also be used. Sequences of non-translated DNA may be present 5 ' or 3' from the open reading frame, where such sequences do not interfere with manipulation or expression of the coding regions. Thus, e.g., the term "recombinant" polynucleotide or nucleic acid refers to one which is not naturally occurring, or is made by the artificial combination of two otherwise separated segments of sequence. This artificial combination is often accomplished by either chemical synthesis means, or by the artificial manipulation of isolated segments of nucleic acids, e.g., by genetic engineering techniques. Such is usually done to replace a codon with a redundant codon encoding the same or a conservative amino acid, while typically introducing or removing a sequence recognition site. Alternatively, it is performed to join together nucleic acid segments of desired functions to generate a desired combination of functions. By "construct" is meant a recombinant nucleic acid, generally recombinant DNA, that has been generated for the purpose of the expression of a specific nucleotide sequence(s), or is to be used in the construction of other recombinant nucleotide sequences. Similarly, a "recombinant polypeptide" or "recombinant polyprotein" refers to a polypeptide or polyprotein which is not naturally occurring, or is made by the artificial combination of two otherwise separated segments of amino acid sequences. This artificial combination may be accomplished by standard techniques of recombinant DNA technology, such as described above, i.e., a recombinant polypeptide or recombinant polyprotein may be encoded by a recombinant polynucleotide. Thus, a recombinant polypeptide or recombinant polyprotein is an amino acid sequence encoded by all or a portion of a recombinant polynucleotide. As used herein, the term "exogenous polypeptide" refers to a polypeptide that is not normally associated with a parent flavivirus in nature. The term "immunologically active" or "immunogenic" refers to the capability of the natural, recombinant, or synthetic peptide to induce a specific humoral and/or cellular immune response in a mammal. As used herein, "antigenic amino acid sequence," "antigenic polypeptide," or "antigenic peptide" means an amino acid sequence that, either alone or in association with an accessory molecule (e.g., a class I or class II major histocompatibility antigen molecule), can elicit an immune response in a mammal. As used herein, "an immune response" is meant to encompass cellular and/or humoral immune responses that are sufficient to inhibit or prevent infection, or prevent or inhibit onset of disease symptoms caused by a microbial organism, particularly a pathogenic microbial organism, and/or to inhibit, reduce, or prevent proliferation of a tumor cell, and/or to reduce tumor cell numbers or tumor mass, and/or to reduce the likelihood that a tumor will form. The term "tumor-associated antigen" is a term well understood in the art, and refers to surface molecules that are differentially expressed in tumor cells relative to non-cancerous cells of the same cell type. As used herein, "tumor-associated antigen" includes not only complete tumor-associated antigens, but also epitope-comprising portions (fragments) thereof. A tumor-associated antigen (TAA) may be one found in nature, or may be a synthetic version of a TAA found in nature, or may be a variant of a naturally-occurring TAA, e.g., a variant has enhanced immunogenic properties. The terms "antigen" and "epitope" are well understood in the art and refer to the portion of a macromolecule which is specifically recognized by a component of the immune system, e.g., an antibody or a T-cell antigen receptor. Epitopes are recognized by antibodies in solution, e.g., free from other molecules. Epitopes are recognized by T-cell antigen receptor when the epitope is associated with a class I or class II major histocompatibility complex molecule. A "CTL epitope" is an epitope recognized by a cytotoxic T lymphocyte (usually a CD8 + cell) when the epitope is presented on a cell surface in association with an MHC Class I molecule. As used herein the term "isolated" is meant to describe a compound of interest (e.g., a recombinant virus, a peptide, etc.) that is in an environment different from that in which the compound naturally occurs. "Isolated" is meant to include compounds that are within samples that are substantially enriched for the compound of interest and/or in which the compound of interest is partially or substantially purified. As used herein, the term "substantially purified" refers to a compound that is removed from its natural environment and is at least 60% free, preferably 75% free, and most preferably 90% free from other components with which it is naturally associated. By "subject" or "individual" or "patient," which terms are used interchangeably herein, is meant any subject, particularly a mammalian subject, for whom diagnosis or therapy is desired, particularly humans. Other subjects may include cattle, dogs, cats, guinea pigs, rabbits, rats, mice, horses, and so on. Of particular interest are those subjects susceptible to infection by a flavivirus (e.g., yellow fever virus), e.g. , subjects who can support flavivirus replication. A "biological sample" encompasses a variety of sample types obtained from an organism and can be used in a diagnostic or monitoring assay. The term encompasses blood and other liquid samples of biological origin, solid tissue samples, such as a biopsy specimen or tissue cultures or cells derived therefrom and the progeny thereof. The term encompasses samples that have been manipulated in any way after their procurement, such as by treatment with reagents, solubilization, or enrichment for certain components. The term encompasses a clinical sample, and also includes cells in cell culture, cell supernatants, cell lysates, serum, plasma, biological fluids and tissue samples. The terms "treatment," "treating," and the like are used herein to generally refer to obtaining a desired pharmacologic or physiologic effect. The effect may be prophylactic in terms of completely or partially preventing a disease or symptom thereof and/or may be therapeutic in terms of a partial or complete cure for a disease and/or adverse effect attributable to the disease. "Treatment" as used herein covers any treatment of a disease in a mammal, e.g., a human, and includes: (a) preventing the disease from occurring in a subject which may be predisposed to the disease but has not yet been diagnosed as having it, e.g., reducing the risk that an individual will develop the disease, reducing the severity of a disease symptom; (b) inhibiting the disease, i.e., arresting its development; and (c) relieving the disease, i.e., causing regression of the disease. In some embodiments, the invention is directed toward treating patients with cancer. In these embodiments, "treatment" can include reducing or inhibiting tumor cell growth, eliminating a tumor, reducing metastasis, reducing or inhibiting tumor cell proliferation, reducing tumor cell mass, reducing tumor cell number, and reducing the probability that a tumor will form. The terms "cancer", "neoplasm", "tumor", and "carcinoma", are used interchangeably herein to refer to cells which exhibit relatively autonomous growth, so that they exhibit an aberrant growth phenotype characterized by a significant loss of control of cell proliferation. Cancerous cells can be benign or malignant. Recombinant Flaviviruses The present invention provides recombinant flaviviruses, particularly live attenuated recombinant flaviviruses, which comprise exogenous nucleotide sequences (i.e., nucleotide sequence from other than the parent flavivirus) which encode exogenous amino acid sequences (i.e., amino acid sequences from other than the parent flavivirus). Such recombinant flavivirus are useful in eliciting an immune response to the exogenous peptide. For simplicity, "exogenous" is used throughout as exemplary of such sequences, but is not intended to be limiting. Yellow fever virus (YFV) is one non-limiting example of a flavivirus. The following description of YFV is generally applicable to other flaviviruses. Yellow fever virus is an enveloped, positive-stranded RNA virus and a member of the flaviviridae genus. The genome is approximately 1 lkb in length and encodes a single polypeptide (16). This polypeptide precursor is proteolytically processed during and after translation, generating the functional proteins necessary for viral replication. Processing is mediated by cellular and viral proteases that recognize short specific amino acid sequences present at the junctions of the viral proteins. The viral protease NS2B NS3 mediates most of the cleavages of the non-structural proteins in the cytosol of the infected cell (17-19). Recombinant flaviviruses of the invention comprise a nucleic acid encoding an exogenous polypeptide. The encoded exogenous polypeptide is expressed in the context of normal viral protein translation, preferably as a component of a recombinant or fusion precursor polypeptide. The recombinant polyprotein generally comprises the exogenous polypeptide and viral polypeptide, and preferably further comprises an artificial proteolytic recognition site or sites. The proteolytic recognition site(s) can be positioned in recombinant precursor polypeptide so that the precursor polyprotein is proteolytically processed by viral or cellular protease(s) so as to release of the free exogenous protein from the viral proteins. The starting flavivirus , which is subsequently modified to include the exogenous sequences, is referred to as the "parent" flavivirus , which can be a native flavivirus (either pathogenic or, preferably, non-pathogenic), an attenuated flavivirus , a vaccine flavivirus strain or a recombinant flavivirus . Any of a variety of strains of flavivirus can be used in generating recombinant flavivirus as described herein. Flavivirus which are suitable for use in generating recombinant flavivirus of the invention include, but are not limited to, yellow fever virus (YFV); Dengue virus, including Dengue types 1-4; Japanese Encephalitis virus; Murray Valley Encephalitis virus; St. Louis Encephalitis virus; West Nile virus; tick-borne encephalitis virus; Hepatitis C virus; Kunjin virus; Central European encephalitis virus; Russian spring-summer encephalitis virus; Powassan virus; Kyasanur Forest disease virus; and Omsk hemorrhagic fever virus. In many embodiments, the flavirus used as the starting flavivirus to generate recombinant flavivirus is YFV. The nucleotide sequences of a number of YFV strains are available in public databases, including, e.g., GenBank. An exemplary strain is "YFV 17D." The nucleotide sequence of the YFV genome, as well as the amino acid sequence of the encoded viral polyprotein are found under GenBank Accession No. X03700, and are also described in Rice et al. ((1985) Science 229:726-733), both of which areis incorporated herein by reference in their entirety for the nucleotide and protein sequences disclosed therein. Production of yellow fever virions (viral particles) is well known in the art. In many embodiments, the flavivirus nucleotide sequences of the recombinant flavivirus are wild-type, i.e., they are sequences found in nature. In other embodiments, the flavivirus nucleotide sequences contain one or more mutations compared to a wild-type flavivirus. In still other embodiments, the flavivirus portion of the recombinant flavivirus is derived from two or more different flaviviruses, i.e., the flavivirus used to construct a recombinant flavivirus is a chimeric flavivirus. In general, an exogenous nucleic acid(s) inserted into the flavivirus genome comprises a nucleotide sequence encoding an exogenous polypeptide and at least one nucleotide sequence encoding a proteolytic cleavage site. The nucleotide sequence encoding the exogenous polypeptide may be flanked on either side by nucleotide sequences encoding proteolytic cleavage sites. Alternatively, the nucleotide sequence encoding the exogenous polypeptide may be flanked on only one side by a nucleotide sequence encoding a proteolytic cleavage site. In the latter case, the insertion site may be chosen such that, after insertion, the nucleotide sequence encoding the exogenous polypeptide is flanked on either side by nucleotide sequences encoding proteolytic cleavage sites, one of which was present in the parent flavivirus genome immediately adjacent the site of insertion. The exogenous nucleic acid sequence encoding the exogenous polypeptide and the nucleic acid sequence encoding the proteolytic cleavage sites can be positioned various sites within the flavivirus genome. As non-limiting examples, the exogenous nucleic acid sequence may be inserted at one or more of the following locations: (1) the N-terminus of the viral polypeptide; (2) between viral proteins C and prM; (3) between viral proteins NS2A and NS2B; (4) between viral proteins NS2B and NS3; (5) between viral proteins NS3 and NS4A; and (5) NS4A and NS4B. The exogenous nucleic acid can be inserted at other sites in the flavivirus genome. Preferably, insertion of the exogenous nucleic acid does not disrupt flavivirus protein function, and/or proteolytic processing of the viral polypeptide, and/or viral replication. Whether viral replication is adversely affected can be determined using well-established techniques, including, but not limited to, a plaque assay, and a one-step growth curve assay, as described in Example 1. Unlike other vectors which will produce only one cycle of antigen expression and or which will stop expression without the intervention of the host immune system, the active recombinant virus of the invention will continue to propagate until the immune system is sufficiently activated to halt the infection. This produces a stronger immune response against the exogenous antigenic peptide produced from the flavivirus as compared to the immune response that would be elicited using conventional expression vectors (e.g., a viral replicon). The recombinant flavivirus also exhibits low toxicity to a host upon infection. For example, YF-17D is a very safe and effective live viral vaccine, prepared from infected chicken embryos under standards developed by the World Health Organization. After vaccination, immunity is elicited within 10 days in over 95% of vaccines (12) and neutralizing antibodies directed against the virus can be detected for more than 35 years (13). The vaccine safety record is outstanding: serious adverse reactions to YF-17D vaccine are extremely uncommon, and reversion to wild type is virtually non-existent (14, 15). Additional features may be incorporated into the design of replication-competent recombinant flavivirus viruses, such as polylinker sequences (e.g., EcoRl, Notl, BssH2, and Xhol) to facilitate the ease of insertion of desired foreign sequences into the recombinant vector. Also, variants, such as a poly-glycine tract, may be inserted adjacent to the inserted sequence so as to enhance the structural flexibility of the region and potentially increase the efficiency of proteolytic processing. More than one nucleic acid sequence encoding an exogenous protein or polypeptide to be produced can be included in the recombinant replication-competent flavivirus virus which, as a result, produces the corresponding number of exogenous proteins or polypeptides. The two or more nucleic acid sequences can each encode a different product or can encode the same product (e.g., if enhanced production of a protein or polypeptide is desired). Insertion of exogenous nucleic acid can be accomplished by standard techniques of molecular biology, such as described in numerous standard protocol texts, including e.g., Current Protocols in Molecular Biology. (F. M. Ausubel, et al., Eds. 1987, and updates. Example 1 provides further guidance for how particular insertions were accomplished. Using these guidelines, any of a variety of exogenous nucleic acids can be inserted into the flavivirus genome. The exogenous nucleic acid can be from about 12 to about 18, from about 15 to about 24, from about 21 to about 30, from about 30 to about 60, from about 60 to about 90, from about 90 to about 120, from about 120 to about 150, from about 150 to about 180, from about 180 to about 240, from about 240 to about 300, from about 300 to about 600, from about 600 to about 1200, from about 1200 to about 1500, from about 1500 to about 2100, from about 2100 to about 2400, from about 10 to about 1000, from about 20 to about 500, or from about 2400 to about 3000 nucleotides in length. Recombinant flavivirus as described herein can be used to induce an immune response against an antigen in an individual and, e.g., provide protection against challenge or infection by the exogenous pathogen (bacterial, viral, fungal, parasitic) in which the antigen occurs. They are, therefore, useful as vaccines to provide immune protection against such pathogens. Any DNA sequence which encodes a polypeptide or protein which, when expressed, produces protective immunity against, for example, a pathogenic organism or against a condition or disorder associated with the presence of or caused by an antigen, can be considered exogenous nucleic acids for the purpose of the present invention. Nucleic acid sequences encoding one or more exogenous polypeptides (e.g., antigens or epitopes) of interest can be included in a vaccine of the present invention. If more than one exogenous antigen or epitope of interest is encoded by the exogenous nucleic acid sequences, they can be antigens or epitopes of a single pathogen or antigens or epitopes from more than one (different). In a preferred embodiment, such an organism is a pathogenic microorganism. For example, such an exogenous epitope may be found on bacteria, parasites, viruses or fungi which are the causative agents of diseases or disorders. In addition, epitopes of allergens, sperm and cancer cells can be used. Thus, in some embodiments, where more than one exogenous peptide is encoded by the exogenous nucleic acid sequences, the more than one exogenous peptide can be different epitopes found on a single TAA. The exogenous proteins in the vaccine formulations of the invention can also comprise an epitope of an exogenous organism. When the exogenous polypeptide is expressed in a vertebrate host, it elicits an immune response that protects against a condition or disorder caused by or associated with expression of or the presence in the host of, an antigen comprising the epitope. For example, in this embodiment of the invention, exogenous proteins that comprise an epitope(s) or protein(s) of snake venom, bee venom, a hormone, sperm (for contraception), an allergy-inducing antigen or any other antigen to which an immune response is desired, may be used. In another embodiment, a tumor-specific antigen can be expressed as a recombinant exogenous protein, for induction of a protective, or otherwise therapeutic, immune response against cancer. The gene sequences encoding the exogenous protein to be expressed by the recombinant virus according to the present invention, can be obtained by techniques known in the art, including but not limited to, chemical or enzymatic synthesis, purification from genomic DNA of the microorganism, by purification or isolation from a cDNA encoding a TAA, by cDNA synthesis from RNA of the microorganism, or by recombinant DNA methods (Maniatis et al., Molecular Cloning, A Laboratory Manual, 1982, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). When they are used as vaccines, the recombinant flavivirus of the present invention are administered to an individual using known methods. They will generally be administered by the same routes by which conventional (presently-available) vaccines are administered and or by routes which mimic the route by which infection by the pathogen of interest occurs. They can be administered in a vaccine composition which includes, in addition to the replication-competent recombinant virus, a physiologically acceptable carrier. The composition may also include an immunostimulating agent or adjuvant, flavoring agent, or stabilizer. Conventional and pharmaceutically acceptable routes of administration include intranasal, intramuscular, intratracheal, intratumoral, subcutaneous, intradermal, vaginal, intrapulmonary, intravenous, rectal, nasal, oral and other parenteral routes of administration. Routes of administration may be combined, if desired, or adjusted depending upon the antigenic peptide or the disease. The vaccine composition can be administered in a single dose or in multiple doses, and may encompass administration of booster doses, to elicit and or maintain immunity. The recombinant flavivirus vaccine is administered in an "effective amount"that is, an amount of recombinant flavivirus that is effective in a selected route of aα^ninistration to elicit an immune response effective to facilitate protection of the host against infection, or symptoms associated with infection, by a pathogenic organism. In some embodiments, an "effective amount" of a recombinant flavivirus vaccine is an amount of recombinant flavivirus that is effective in a route of administration to elicit an immune response effective to reduce or inhibit tumor cell growth, to reduce tumor cell mass or tumor cell numbers, or to reduce the likelihood that a tumor will form. The amount of recombinant flavivirus in each vaccine dose is selected as an amount which induces an immunoprotective or other immunotherapeutic response without significant, adverse side effects generally associated with typical vaccines. Such amount will vary depending upon which specific immunogen is employed, whether or not the vaccine formulation comprises an adjuvant, and a variety of host-dependent factors. In general, it is expected that each dose of vaccine will be sufficient to generate, upon infection of host cells, about 1-1000 μg of protein, generally from about 1-200 μg, normally from about 10-100 μg. An effective dose of recombinant flavivirus nucleic acid-based vaccine will generally involve administration of from about 1-1000 μg of nucleic acid. Alternatively, an effective dose of recombinant flavivirus vaccine will generally be in a range of from about 10 2 to about 10 7 , from about 10 3 to about 10 6 , or from about 10 4 to about 10 5 plaque forming units (PFU). An optimal amount for a particular vaccine can be ascertained by standard studies involving observation of antibody titers and other responses in subjects. The levels of immunity provided by the vaccine can be monitored to determine the need, if any, for boosters. Following an assessment of antibody titers in the serum, optional booster immunizations may be desired. The immune response to the protein of this invention is enhanced by the use of adjuvant and or an immunostimulant. Recombinant flavivirus viruses of the present can also be used as a system for producing the exogenous polypeptide in host cells, such as mammalian, particularly human, cells or other cell types. The exogenous protein is then isolated using standard methods. In either application (i.e., immunization or tissue culture production) the exogenous nucleic acid sequence introduced into the flavivirus can be one obtained from a source in which it occurs naturally, produced using genetic engineering methods or synthesized chemically or enzymatically. The exogenous nucleic acid sequence introduced into the virus can encode an entire antigen against which an immune response is desired or antigenic epitopes or portions, e.g., an immunogenic fragment of from about 4 to about 1000, from about 10 to about 500, from about 15 to about 250, from about 20 to about 100, from about 25 to about 50 amino acids. Inserted sequences are expected to present to the host immune system, antigenic structures defined both by primary sequence and structural conformation. Compositions comprising recombinant flaviviruses of the invention The present invention further provides compositions, including pharmaceutical compositions, comprising a recombinant flavivirus of the invention. Compositions comprising a recombinant flavivirus of the invention may include a buffer, which is selected according to the desired use of the recombinant flavivirus, and may also include other substances appropriate to the intended use. Those skilled in the art can readily select an appropriate buffer, a wide variety of which are known in the art, suitable for an intended use. In some instances, the composition can comprise a pharmaceutically acceptable excipient, a variety of which are known in the art and need not be discussed in detail herein. Pharmaceutically acceptable excipients have been amply described in a variety of publications, including, for example, "Remington: The Science and Practice of Pharmacy", 19th Ed. (1995) Mack Publishing Co. Pharmaceutical compositions can be prepared in various forms, such as granules, tablets, pills, suppositories, capsules, suspensions, sprays, suppositories, transdermal applications (e.g., patches, etc.), salves, lotions and the like. Pharmaceutical grade organic or inorganic carriers and/or diluents suitable for oral and topical use can be used to make up compositions containing the therapeutically active compounds. Diluents known to the art include aqueous media, vegetable and animal oils and fats. Stabilizing agents, wetting and emulsifying agents, salts for varying the osmotic pressure or buffers for securing an adequate pH value, and skin penetration enhancers can be used as auxiliary agents. When used as a vaccine, a recombinant flavivirus of the invention can be formulated in a variety of ways. In general, the vaccine of the invention is formulated according to methods well known in the art using suitable pharmaceutical carrier(s) and/or vehicle(s). A suitable vehicle is sterile saline. Other aqueous and non-aqueous isotonic sterile injection solutions and aqueous and non-aqueous sterile suspensions known to be pharmaceutically acceptable carriers and well known to those of skill in the art may be employed for this purpose. Optionally, a vaccine composition of the invention may be formulated to contain other components, including, e.g., adjuvants, stabilizers, pH adjusters, preservatives and the like. Such components are well known to those of skill in the vaccine art. Adjuvants include, but are not limited to, aluminum salt adjuvants (Nicklas (1992) Res. Immunol. 143:489-493); saponin adjuvants; Ribi's adjuvants (Ribi ImmunoChem Research Inc., Hamilton, MT); Montanide ISA adjuvants (Seppic, Paris, France); Hunter's TiterMax adjuvants (CytRx Corp., Norcross, GA); Gerbu adjuvants (Gerbu Biotechnik GmbH, Gaiberg, Germany); and nitrocellulose (Nilsson and Larsson (1992) Res. Immunol. 143:553-557). In addition, other components that may modulate an immune response may be included in the formulation, including, but not limited to, cytokines, such as interleukins; colony-stimulating factors (e.g., GM-CSF, CSF, and the like); and tumor necrosis factor. Methods of using the recombinant flaviviruses of the invention The present invention provides methods for eliciting an immune response to an antigen, comprising administering a recombinant flavivirus of the invention to a mammalian subject, wherein the flavivirus enters a cell, the exogenous polypeptide is released by proteolytic cleavage, and an immune response is elicited to the exogenous polypeptide. In some embodiments, a polypeptide antigen expressed on a given tumor cell (e.g., a tumor associated antigen; "TAA") is inserted into a recombinant flavivirus of the invention as described herein. Such recombinant flavivirus can be administered to an individual having, or suspected of having, a tumor. In some cases, such recombinant flavivirus can be administered to an individual who does not have a tumor, but in whom protective immunity is desired. As is often the case, the immune system does not mount an immune response effective to inhibit or suppress tumor growth, or eliminate a tumor altogether. Tumor-associated antigens are often poorly immunogenic; perhaps due to an active and ongoing immunosuppression against them. Furthermore, cancer patients tend to be immunosuppressed, and only respond to certain T-dependent antigens. In these cases, introduction into the host of a recombinant flavivirus of the invention which expresses an exogenous peptide conesponding to an antigen expressed on the tumor cell surface can elicit an immune response to the tumor in the host. As shown in the Examples, a recombinant YFV comprising an insert encoding the sequence SIINFEKL ("Ova") (SEQ ID NO.: 1), which is recognized specifically by CD8 + cytotoxic T cells (CTLs) when presented on the surface of a cell together with a class I MHC molecule, was introduced into mice. When mice were subsequently challenged with a tumor expressing Ova on its surface, a robust CTL response was mounted against the tumor. Any of a variety of known tumor-associated antigens (TAA) can be inserted into flavivirus of the invention. The entire TAA may be, but need not be, inserted. Instead, a portion of a TAA, e.g. , an epitope, particularly an epitope that is recognized by a CTL, may be inserted. Tumor-associated antigens (or epitope-containing fragments thereof) which may be inserted into flavivirus include, but are not limited to, MAGE-2, MAGE-3, MUC-1, MUC-2, HER-2, high molecular weight melanoma- associated antigen MAA, GD2, carcinoembryonic antigen (CEA), TAG-72, ovarian-associated antigens OV-TL3 and MOV18, TUAN, alpha-feto protein (AFP), OFP, CA-125, CA-50, CA-19-9, renal tumor- associated antigen G250, EGP-40 (also known as EpCAM), S100 (malignant melanoma-associated antigen), p53, prostate tumor-associated antigens (e.g., PSA and PSMA), and p21ras. Recombinant flavivirus comprising a TAA can be administered to an individual as described above. Whether an immune response is elicited to a given tumor can be determined by methods standard in the art, including, but not limited to, assaying for the presence and/or amount of TAA-specific antibody in a biological sample derived from the individual, e.g., by enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA), and the like; assaying for the presence and/or numbers of CTLs specific for a TAA; and the like. Examples of how to assay for the presence and/or numbers of TAA-specific CTLs are found in the Examples section herein below. Standard immunological protocols may be used, which can be found in a variety of texts, including, e.g., Current Protocols in Immunology (J. E. Coligan, A. M. Kruisbeek, D. H Margulies, E. M. Shevach and W. Strober Eds. 1991). In other embodiments, the exogenous polypeptide is an antigenic polypeptide of a microbial pathogen. Such recombinant flavivirus can then be administered to a host to prevent or treat infection by the pathogen, or to prevent or treat symptoms of such pathogenic infection. Of particular interest is the prevention or treatment of infection or disease caused by microbial pathogens that, during the course of infection, are present intracellularly, e.g., viruses (e.g., HIV), bacteria (e.g., Shigella, Listeria, and the like), parasites (e.g., malarial parasites (e.g., Falciparum), trypanosomes, and the like), etc. Antigenic polypeptides of such microbial pathogens are well known in the art, and can be readily selected for use in the present recombinant flavivirus vaccine by the ordinarily skilled artisan. In addition, a recombinant flavivirus of the invention can be used as a delivery vehicle to delivery any antigen to an individual, to provoke an immune response to the antigen. In some embodiments, recombinant flavivirus of the invention are used as bivalent or multivalent vaccine to treat human or veterinary diseases caused by infectious pathogens, particularly viruses, bacteria, and parasites. Examples of epitopes which could be delivered to a host in a multivalent flavivirus vaccine of the invention include multiple epitopes from various serotypes of Group B streptococcus, influenza virus, rotavirus, and other pathogenic organisms known to exist in nature in multiple forms or serotypes; epitopes from two or more different pathogenic organisms; and the like. Whether an immune response has been elicited to a pathogenic organism can be determined (quantitatively, e.g., by measuring a parameter, or qualitatively, e.g., by assessing the severity of a symptom, or by detecting the presence of a particular parameter) using known methods. Methods of measuring an immune response are well known in the art and include enzyme-linked immunosorbent assay (ELISA) for detecting and/or measuring antibody specific to a given pathogenic organism; and in vitro assays to measure a cellular immune response (e.g., a CTL assay using labeled, inactivated cells expressing the epitope on their cell surface with MHC Class I molecules). Whether an immune response is effective to facilitate protection of the host against infection, or symptoms associated with infection, by a pathogenic organism can be readily determined by those skilled in the art using standard assays, e.g., determining the number of pathogenic organisms in a host (e.g., measuring viral load, and the like); measuring a symptom caused by the presence of the pathogenic organism in the host (e.g., body temperature, CD4+ T cell counts, and the like). Whether an immune response is effective in reducing the number of tumor cells in an individual can be determined by standard assays, including, but not limited to, measuring tumor cell mass, measuring numbers of tumor cells in an individual, and measuring tumor cell metastasis. Such assays are described in the Examples section herein below. Using the methods and compositions described herein in connection with the subject invention, an immunoprotective response against microbial infection can be induced in any subject, human or non-human, susceptible to infection by a microbial pathogen. Where the recombinant flavivirus comprises an exogenous nucleic acid sequence encoding a TAA, the subject may be one that is known to have cancer, is suspected of having cancer, or does not have cancer, but in whom immunity to cancer is to be induced. EXAMPLES The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the present invention, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all or the only experiments performed. Efforts have been made to ensure accuracy with respect to numbers used (e.g., amounts, temperature, etc.) but some experimental enors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature is in degrees Celcius, and pressure is at or near atmospheric. Example 1: Generating recombinant yellow fever virus Materials and Methods Plasmids and PCR fragments. Plasmids pYF5 '3 ' and pYFM5.2 which encode for the complete YFV-17D sequence were kindly provided by Dr. Charles Rice. A full-length viral RNA can be generated from these plasmids by an in vitro ligation procedure (20). Briefly, we inserted a PCR generated DNA fragment, encoding a 13 amino acid peptide from chicken ovalbumin flanked by BssHII and BstEII or Clal and Ndel restriction enzyme sites, and followed by the viral peptidase (NS2B-NS3 complex) recognition site. We inserted the fragment at the following sites of the viral cDNA: the N- terminus of the viral polypeptide, or between proteins C-prM, NS2A-NS2B, NS2B-NS3, NS3-NS4A and NS4A-NS4B. To insert foreign sequences within the structural region of the genome, PCR fragments containing the foreign sequences were cloned into plasmid p5'3' at each different location. To generate recombinants in the non-structural part of the genome, 8 kb PCR fragments corresponding to the YFV-sequences in pYFM5.2 and containing the inserts of interest were produced. These PCR fragments were used as substitutes for plasmid pYFM5.2 in the in vitro ligation reaction described below. Generation of Viruses from Plasmids. Production of amolecular clone of YFV-17D was carried out following a similar procedure originally published by Rice and co-workers (20). Briefly, 5 μg of each plasmid (or the corresponding sequences generated by PCR) were digested with restriction enzymes Aat II and Apa I. After digestion, the plasmid containing the 5' and 3' ends of the viral genome and the fragment from YFM5.2 corresponding to the middle region were each purified using low-melting agarose gel electrophoresis and ligated in equimolar concentrations for 4 hours at 16° C. Ligase was inactivated by incubation for 20 minutes at 60° C . The ligated DNA was then digested with Xho I and used as the template for in vitro transcription by SP6 RNA polymerase (Promega, Madison, WI) in the presence of m 7 GpppAmp (New England Biolabs, Beverly, MA). Without further purification, synthetic RNA was transfected into BHK-21 cells by electroporation (BTX electro cell manipulator 600, San Diego, CA). Viral Stocks. Cytopathic effect (CPE) was observed 3 to 5 days following transfection. Viruses were cloned from individual plaques produced in BHK-21 cells. To generate viral stocks, cloned viruses were propagated in SW13 cells; supernatants of infected cells were cleared, aliquoted, titered and stored at -70°C. Single-Step Growth Curves. Subconfluent SW13 cell monolayers were washed once with PBS and infected at a multiplicity of infection of 5 pfu cell. After a 2 hr incubation period at 37 ° C, the cells were washed twice with PBS and then covered with L- 15 medium supplemented with 10% FCS . Infected cell cultures were incubated at 37 ° C for several days, and 100 μl aliquots were recovered every six hours for a period of 6 days or until total CPE occurred. Titers were determined by plaque assay. Analysis of Viral RNA by RT-PCR. After subsequent passages of recombinant viruses on SW13 cells, total cytoplasmic RNA was obtained from infected cells following the method of (21). Reverse transcription was carried out with Superscript (Gibco-BRL), using random hexamers and specific primer (ATCGCGGACCGAGTGGTTTTGTGTTTGTCATCCAAAGGTCTGCTTATTCT TGAGC (SEQ ID NO.: 2)) and following the manufacturer's recommended protocol. After 1 hour incubation at 42° C, 2 μl of each reaction product was used as template in a PCR reaction using RtTh (Perkin-Elmer) and specific primers flanking the sequence to be studied (CAATGAGGCACTCGCAGCAGCTGG (SEQ ID NO. : 3) and TGCCCTAGCTCTGTGCGCTGCCC YF-pOva-8 (SEQ ID NO. : 4)). The amplified PCR product was analyzed by restriction enzyme digestion and/or DNA sequencing. Results Generation of YFV 17 D recombinants expressing a chicken ovalbumin T-cell epitope. Foreign sequences (flanked by protease recognition sites) are inserted in-frame at different positions within the YF polyprotein precursor. In this way, the viral protease recognizes and cleaves the flanking proteolytic sites, freeing the exogenous antigenic sequences from the rest of the YF polyprotein, and all of the YF proteins are produced correctly and viral replication proceeds normally (Figure 1). At several positions of the viral genome, we introduced sequences encoding a chicken ovalbumin CTL epitope followed by an eight amino acid cleavage site for the viral protease NS2B/NS3 (Figure 1). The inserted fragment encoded the amino acid sequence SIINFEKL (SEQ ID NO.: 1), an epitope restricted to the murine MHC class I molecule H-2 K (25). Recombinant live yellow fever viruses were recovered by transfection of BHK cells with in vitro synthesized RNA. Insertion of exogenous sequences at three sites in the genome yielded viable recombinant viruses: the amino terminus, C-prM, and NS2B-NS3 junctions (the resulting viruses were named YF-pOva-1, YF-pOva-2 and YF-pOva-8, respectively). The viruses were isolated from individual plaques and viral stocks were generated by two sequential passages in SW13 cells. In a different set of experiments, a number of different antigens were introduced, using the same techniques described above. Among the sequences that were used to generate recombinant flavivirus are sequences derived from influenza virus (e.g., a fragment from the nucleoprotein of influenza virus, e.g., NPι 47- i 55 from strain N1H1); the G protein (144-159) of respiratory syncitial virus (RSV, strain A); and tumor associated antigens from human and murine melanomas (e.g., TRP). Plaque assays and one-step growth curves showed that recombinants YF-pOva-1, YF-pOva-2 and YF-pOva-8, as well as all the viral recombinants that carried other exogenous antigens (e.g., influenza, RSV, or melanoma antigens) replicate at rates remarkably similar to the parental 17D strain (Figure 2). Recombinant YF-pOva-8 and all other recombinants tested bearing the insertion at this site of the genome replicated with identical kinetics to 17D and achieved equivalent titers. Recombinant YF- pOva-1 and YF-pOva-2 replicated slower than the parental 17D, exhibiting a lag in replication, and by three days post-infection achieved only 10-20% the titer of 17D (Figure 2). In view of the better replication properties of YF-pOva-8, this recombinant was used in all subsequent studies to further characterize the immunogenicity of yellow fever vectors. Genetic stability problems have been reported with some positive strand RNA virus vectors (26, 27). Therefore, we confirmed the presence of the inserted sequence by RT-PCR, a diagnostic restriction enzyme digest, and sequencing of the amplified fragment. The original genetic structure of YF-pOva-8 was retained after six passages in BHK-21 cells. Thus, foreign peptides can be introduced between the NS2B and NS3 coding sequences of the yellow fever genome without major compromise of viral replication. Example 2: Cells infected with YF-pOva-8 present the Ova-peptide SIINFEKL in a MHC class I- restricted manner. Materials and Methods Cell lines. In addition to BHK-21 and SW13 cells already described, the following cell lines were used in the antigen presentation assay or in the tumor rejection challenge: B3Z (T-cell hybridoma), EL-4 (thymoma) and EL4-SL8 cells. B3Z cells and EL4-SL8 were kindly provided by Nilabh Shastri, University of California, Berkeley. EL4-SL8 cells stably express and present the Ova CTL epitope (SIINFEKL (SEQ ID NO.: 1)). B3Z is a murine T cell hybridoma specific for K b +SIINFEKL (SEQ ID NO . : 1 ), which is transfected with the LacZ reporter gene under the transcriptional control of the IL-2 enhancer element. Thus, B3Z cell activation can be easily detected by the expression of β-galactosidase (22). The C57BL/6-derived melanoma B16F0 and B16-OVA were a kind gift from Kenneth Rock, University of Massachusetts. B16-Ova (Mo5.20.10) is a cell line that stably expresses ovalbumin constructed by transfection of B16F0 with plasmid pAc-neo-Ova (1). Antigen Presentation Assay. EL4 cells were mock-infected or infected with YF-17D and recombinant viruses at a MOI of 10 pfu/cell. After a 48 hours incubation period at 37 °C, the infected cells or the same number of B16-Ova control cells were co-cultured with 5x10 4 B3Z cells for 16 hrs at 37°C. To determine the expression of β -galactosidase, cultures were washed with PBS and then fixed with 1 % formaldehyde/0.2 % glutaraldehyde for 5 min at 4°C. Cells were washed again and incubated with a solution consisting of 1 mg/ml X-Gal, 5mM potassium ferrocyanide, 5mM potassium ferricyanide, and 2 mM MgCl 2 in PBS. They were then incubated overnight at 37 ° C and examined microscopically for the presence of β -galactosidase activity (blue cells). Results Cells infected with YF-pOva-8 present the Ova-peptide SIINFEKL in a MHC class I-restricted manner. To determine whether cells infected with recombinant yellow fever viruses are able to both express and present antigens in an MHC class I-restricted manner, we infected EL4 cells with YF-pOva- 8 and co-cultivated the infected cells with a T-cell hybridoma that recognizes SIINFEKL presented in the context of K b MHC class I molecules. The antigen is initially expressed in the cytoplasm of the infected cell, but it should be transported and presented on the surface of the cell through the MHC class I pathway. We used the T-cell hybridoma B3Z, carrying a lacZ reporter gene under the transcriptional control of the interleukin-2 enhancer element NF-AT (22). Thus, TCR-specific stimulation of B3Z cells can be measured by the production of β-galactosidase activity. EL4 cells infected with YF-pOva-8 activated 10% of the hybridoma T-cells after 12-16 hours of co-cultivation. In contrast, co-cultivation with uninfected EL4 cells or the hybridoma B3Z by itself did not produce β-galactosidase activity. Co-cultivation with a cell line that stably expresses ovalbumin strongly induced β-galactosidase production. These results demonstrated that polypeptides expressed by yellow fever recombinants are processed and presented in a MHC class I-restricted manner. In addition, western blotting using antibodies directed against viral proteins indicated that all yellow fever virus proteins are correctly produced and processed in YF-pOva-8 infected cells suggesting that the foreign antigen was appropriately cleaved away from the viral polyprotein. Example 3: Induction o Ovα-specific CD8 + T-cells by recombinant YF-pOva-8 Materials and Methods CD8 + T cell responses. Immunizations. Groups of 3 C57BL/6 mice were mock infected or immunized i.v. with 10 7 pfu/mouse of either YFV 17D or recombinant virus YF-pOva-8. All groups were boosted i.p. with the same dose at day 21 post infection. Seven days later, all mice were sacrificed and their spleens removed and dispersed to single cell suspensions. Restimulations and tetramer binding assays. 3xl0 6 splenocytes were restimulated by co- culturing them for 4 days with 10 5 SIINFEKL-expressing EL4 cells (EL4-SL8) irradiated at 6000 rads. SIINFEKL-MHC class I tetramers were the kind gift of Dr. John Airman (Emory University, Atlanta, GA). At day 4, cells were stained as described by Altaian et al (23) and analyzed by flow cytometry. Immunizations and Tumor Challenge. C57BL/6 mice (H-2 b ) were purchased from the Jackson Laboratory and used between 6-8 weeks of age. Groups of 5 or 10 mice were immunized intraperitoneally (i.p.), subcutaneously (s.c), intramuscularly (i.m.) or intravenously (i.v.) with 3xl0 5 pfu/mouse of YF 17D or YF-pOva-8 (Ova-expressing 17D recombinant virus). All groups were boosted with the same dose two weeks later. Non-immunized mice were used as naϊve controls. Melanoma cells were harvested by incubation in Ca ++ , Mg -1-1" free PBS for 5 minutes, and viable cells counted by trypan blue exclusion and 30 days post infection all mice were challenged with a s.c. injection of 5x10 4 B 16-OVA or B 16F0 melanoma cells. The size of tumors was determined twice a week and expressed as tumor area corresponding to the largest perpendicular diameter in cm 2 . Animals that developed tumors greater than 2.0 cm 2 were sacrificed. Immunotherapy of solid tumors. Mice were injected s.c. with 5xl0 4 B16-OVA cells. Treatment was started at day 0, 5 or 10 post tumor implantation and consisted of three s.c. injections of 4xl0 5 pfu YF-pOVA-8 or 17D given in 5 day intervals. A control group was left untreated. Mice were observed for tumor development every 3 days and tumors larger than 0.3 cm 2 were scored as positive. Immunotherapy of experimental pulmonary metastasis. Mice were injected i.v. with either 5xl0 4 or lxlO 5 B16-OVA cells. Immunotherapy was performed as described for solid tumors. On day 30, ten mice of each group were sacrificed, then the lungs were removed, placed for 5 minutes in 3% H 2 0 2 in H 2 0, and then fixed in Bouin's solution (Sigma diagnostics, St. Louis, MO, USA) . The H 2 0 2 treatment facilitates the analysis of metastasis under the dissecting microscope by inflating the lungs and bleaching hemorrhages which otherwise could be mistaken for metastases. Results To determine whether recombinant yellow fever viruses are able to induce specific CD8 + T- lymphocytes, mice were inoculated with either YF-pOva-8, parental YF-17D, or PBS (naϊve). + Splenocytes obtained from immunized mice were monitored for the development of a CD8 T- lymphocyte population that bound MHC class I-SIINFEKL tetramers (Figures 3A-3C). Splenocytes from naive mice, or mice immunized with YF-17D, failed to produce SIINFEKL specific T-cells. Both freshly isolated splenocytes and in vitro restimulated splenocytes from YF-pOva-8 immunized mice were assessed for tetrameric binding. Freshly isolated lymphocytes showed nunimal tetramer binding after one inoculation (0.5 % of CD8 + T-cells). However, after in vitro stimulation a significant percentage of CD8 + T-cells (8.75%) obtained from mice immunized with YF-pOva-8 were specific for SIINFEKL. Protective immunity in vivo. To evaluate whether the vector induces protective CTL immunity, we used an established tumor model in which CTLs play an essential role in protecting the host from challenge with a lethal dose of malignant melanoma cells (1). Mice were immunized twice with YF- pOva-8 or YF-17D and then challenged thirty days later with one of two C57BL/6 derived melanomas: the parental B 16-FO tumor cell line, or B 16-Ova which stably expresses chicken ovalbumin. Subcutaneous inoculation of naive mice with B16-Ova cells or parental B16-F0 melanoma cells yielded tumors that grew with similar kinetics and killed naive animals in a few weeks (Figures 4A-4D and 5A- 5D, data only shown for B16-Ova). Immunization with YF-pOva-8 protected animals against a B16- Ova challenge with a dose 10 times the LD 50 . Immunization protected mice from local tumor growth (Figures 4A-4D) and also from death (Figures 5A-D). We administered the recombinant YF-pOva-8 by four different routes -subcutaneous (s.c), intramuscular (i.m.), intraperitoneal (i.p.) and intravenous (i.v.) - to compare the efficiency of the protective immune response. Subcutaneous and intravenous inoculation elicited potent responses. All of the animals vaccinated with YF-pOva-8 were protected at the time when 100% of the control mice had died. Intraperitoneal and intramuscular inoculations were slightly less efficient; in these groups 10 to 20 % of the mice developed tumors and died. The vaccine effect was specific for SIINFEKL because mice vaccinated with YF-pOva-8 were not protected against challenge with parental B16 melanoma cells, which do not express Ova. However, we observed a slight, although not significant, delay in tumor growth in mice inoculated with YF-17D when the virus was adtninistered intraperitoneally, subcutaneously or intramuscularly. This effect may be due to increased cytokine production or other immunological responses induced by yellow fever replication. This is not inconsistent with the literature since it has been shown previously, that IFN-γ and IFN-α have anti-tumor and anti-cellular activities on B16 melanoma cells (28-30). Active immunotherapy of established tumors. Next, we determined whether vaccination with YF recombinant is able to induce regression of established tumors. Mice were inoculated subcutaneously with either 5xl0 3 or 5xl0 4 B16-Ova tumor cells and subsequently infected with YF- pOva-8 at the day of tumor implantation (day 0), five days post tumor implantation (day 5) or ten days post tumor implantation (day 10). Mice inoculated with the lower number of B16-Ova tumor cells (5x10 3 ), produced tumors in about 60% of unvaccinated mice and were completely protected by vaccination, even if treatment was started 10 days after tumor implantation (Figures 6A-6F). For animals inoculated with the higher doses of tumor cells (5xl0 4 ), eighty percent of the animals vaccinated with YF-pOva-8 at day 0 remained tumor-free 45 days after tumor injection, while those injected with parental YF-17D or saline developed tumors and died within 3-4 weeks (Figures 6A-6F). Vaccination with YF-17D slightly delayed tumor growth relative to saline, but the effect was minimal and 90% of the animals developed tumors 30 days after tumor implantation. Vaccination with recombinant YF-pOva-8 five days post-tumor implantation resulted in a partial protection (60% of the mice remained tumor-free). Vaccination at day ten had little or no effect on tumor growth. These results demonstrate that treatment of established tumors can be achieved by vaccination with yellow fever recombinants. Active immunotherapy of pulmonary metastasis. B16 melanoma cells, when injected into the tail vein of syngenic mice, reproducibly metastasize to the lungs (31). This provides a model to evaluate whether yellow fever recombinants are able to elicit effective anti-metastatic responses. Mice were inoculated intravenously with either 5xl0 4 or lxlO 5 B16-Ova cells, and at day 0, 5 or 10 post-tumor inoculation animals were vaccinated subcutaneously with YF-pOva-8 or control virus. Treatment with the vector prevented death (Figures 9A-9B) and substantially reduced both the size and number of lung metastasis (Figures 7 and 8). Ten weeks after tumor implantation of 5xl0 4 cells, 100% of the animals vaccinated at day 0 were healthy (Figures 9A-9B). Protection dropped to 80% when vaccination was started at day 5, and dropped further to 20 % when animals were vaccinated starting at day 10. Inoculation with YF-17D had no protective effect, and metastases developed with the same kinetics as in animals inoculated with saline (Figures 7 and 8). When mice were inoculated with lxl 0 5 tumor cells, only 40% of mice treated at day 0 were protected. These results underline the importance of starting immunotherapy when the tumor burden is low. Nonetheless, these results indicate that recombinant yellow fever viruses expressing a single antigenic epitope are able to elicit a therapeutic anti-tumor response in mice. While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. BIBLIOGRAPHY 1. Falo L.D., Jr., M. Kovacsovics-Bankowski, K. Thompson, and K.L. Rock. 1995. Targeting antigen into the phagocytic pathway in vivo induces protective tumour immunity. NatMed. 1:649-653. 2. Kawakami, Y, S. Eliyahu, CH. Delgado, P.F. Robbins, K. Sakaguchi, E. Appella, J.R. Yannelli, G.J. Adema, T. Miki, and S.A. Rosenberg. 1994. 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Claims: CLAIMS What is claimed is: 1. A replication-competent recombinant flavivirus comprising an exogenous nucleic acid inserted into a parent flavivirus, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said recombinant flavivirus encodes a recombinant polyprotein precursor that comprises said exogenous polypeptide and that is proteolytically processed, resulting in release of the exogenous polypeptide from the recombinant polyprotein precursor. 2. The recombinant flavivirus of claim 1, wherein the exogenous polypeptide is a tumor- associated antigen. 3. The recombinant flavivirus of claim 1, wherein the exogenous polypeptide is an epitope recognized by a cytotoxic T lymphocyte. 4. The recombinant flavivirus of claim 1, wherein the exogenous polypeptide is a bacterial, viral, fungal, or parasite antigen. 5. The recombinant flavivirus of claim 1 , wherein the parent flavivirus is yellow fever virus. 6. A recombinant flavivirus comprising an exogenous nucleic acid inserted into a parent flavivirus, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said exogenous nucleic acid is flanked by nucleotide sequences encoding a proteolytic cleavage site. 7. A method of eliciting an immune response to an antigen in a mammalian host, comprising administering to a host a recombinant flavivirus comprising an exogenous nucleic acid inserted into a parent flavivirus, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said recombinant flavivirus encodes a recombinant polyprotein precursor that comprises said exogenous polypeptide and that is proteolytically processed, resulting in release of the exogenous polypeptide from the recombinant polyprotein precursor. 8. A method of eliciting an immune response to an antigen in a mammalian host, comprising administering to a host a recombinant flavivirus comprising an exogenous nucleic acid inserted into a parent flavivirus, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said exogenous nucleic acid is flanked by nucleotide sequences encoding a proteolytic cleavage site. 9. A method of reducing the number of tumor cells in a host, comprising administering to a host a recombinant flavivirus comprising an exogenous nucleic acid inserted into a parent flavivirus, said exogenous nucleic acid comprising a nucleotide sequence encoding a tumor associated antigen, wherein said recombinant flavivirus encodes a recombinant polyprotein precursor that comprises said tumor associated antigen and that is proteolytically processed, resulting in release of the tumor associated antigen from the recombinant polyprotein precursor. 10. A method of reducing the number of tumor cells in a host, comprising administering to a host a recombinant flavivirus comprising an exogenous nucleic acid inserted into a parent flavivirus, said exogenous nucleic acid comprising a nucleotide sequence encoding a tumor associated antigen, wherein said exogenous nucleic acid is flanked by nucleotide sequences encoding a proteolytic cleavage site. 11. A composition comprising a recombinant flavivirus according to claim 1 or claim 6. 12. The composition of claim 11, wherein said composition comprises a pharmaceutically acceptable excipient. 13. A replication-competent recombinant yellow fever virus (YFV) comprising an exogenous nucleic acid inserted into a parent YFV, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said recombinant YFV encodes a recombinant polyprotein precursor that comprises said exogenous polypeptide and that is proteolytically processed, resulting in release of the exogenous polypeptide from the recombinant polyprotein precursor. 14. A replication-competent recombinant yellow fever virus (YFV) comprising an exogenous nucleic acid inserted into a parent YFV, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said recombinant YFV encodes a recombinant polyprotein precursor that comprises said exogenous polypeptide and that is proteolytically processed, resulting in release of the exogenous polypeptide from the recombinant polyprotein precursor, wherein infection of a host with the recombinant YFV provides for expression of the exogenous polypeptide by the host cell and induction of an immune response in a host to the exogenous polypeptide. 15. A replication-competent recombinant yellow fever virus (YFV) comprising an exogenous nucleic acid inserted into a parent YFV, said exogenous nucleic acid comprising a nucleotide sequence encoding an exogenous polypeptide, wherein said exogenous nucleic acid is flanked by nucleotide sequences encoding a proteolytic cleavage site positioned to provide for release of the exogenous polypeptide upon proteolytic processing, wherein said recombinant YFV encodes a recombinant polyprotein precursor that comprises said exogenous polypeptide and that is proteolytically processed, resulting in release of the exogenous polypeptide from the recombinant polyprotein precursor. 16. The recombinant yellow fever virus of any one of claims 13-15, wherein the exogenous polypeptide is a tumor-associated antigen. 17. The recombinant yellow fever virus of any one of claims 13-15, wherein the exogenous polypeptide is a polypeptide of a virus other than yellow fever virus. 18. The recombinant yellow fever virus of any one of claims 13-15, wherein the exogenous polypeptide is a polypeptide of a microbial pathogen other than yellow fever virus. 19. The recombinant yellow fever virus of any one of claims 13-15, wherein the virus is live and attenuated. 20. The recombinant yellow fever virus any one of claims 13-15, wherein the exogenous polypeptide is expressed on the surface of the host cell following infection. 21. The recombinant yellow fever virus of any one of claims 13-15, wherein the exogenous polypeptide is secreted from the host cell following infection. 22. An isolated host cell infected with the recombinant yellow fever virus any one of claims 13-15. 23. A composition comprising the recombinant yellow fever virus any one of claims 13-15, and a buffer. 24. The composition of claim 23, further comprising a pharmaceutically acceptable excipient. 25. A method of eliciting an immune response in a mammalian host to an antigenic polypeptide, comprising: aα'ministering a recombinant yellow fever virus any one of claims 13-15 to a mammalian host, wherein the exogenous polypeptide is an antigenic polypeptide, wherein said administering provides for infection of a host cell and expression of the antigenic polypeptide; wherein expression of the exogenous polypeptide results in induction of an immune response in the host to the antigenic polypeptide. 26. The method of claim 25, wherein the antigenic polypeptide is a polypeptide of a tumor- associated antigen. 27. The method of claim 25, wherein the antigenic polypeptide is a polypeptide of a microbial pathogen. 28. A method of reducing the number of tumor cells in a host, comprising: administering the recombinant yellow fever virus any one of claims 13-15 to a host having a tumor, wherein the exogenous polypeptide is a polypeptide of a tumor-associated antigen which is found on a tumor cell of the tumor in the host; wherein expression of the tumor-associated antigen elicits an immune response in the host against the tumor cells effective to reduce the number of tumor cells in the host.

Inventors: ANDINO-PAVLOVSKY RAUL, MCALLISTER-MORENO ANDRES, ANDINO-PAVLOVSKY, RAUL, MCALLISTER-MORENO, ANDRES
Applicants: ANDINO PAVLOVSKY RAUL, MCALLISTER MORENO ANDRES, UNIV CALIFORNIA, ANDINO-PAVLOVSKY, RAUL, MCALLISTER-MORENO, ANDRES, THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
Application year: 2001
Main classification: C12N7
IPC codes: A61K35, A61K38, A61K39, A61K48, A61P35, A61P37, C12N5, C12N7, C12N15