AN ANTI-CYANOVIRIN ANTIBODY WITH AN INTERNAL IMAGE OF gp120, A METHOD OF USE THEREOF, AND A METHOD OF USING A CYANOVIRIN TO INDUCE AN IMMUNE RESPONSE TO gp120

Abstract: The present invention provides an anti-cyanovirin antibody with an internal image of gp120, a method of using an anti-cyanovirin antibody with an internal image of gp120 to induce an immune response to gp120 so as to prevent or treat a viral infection in an animal, and a method of using a cyanovirin to induce an immune response to gp120 so as to prevent or treat a viral infection in an animal.

Description: AN ANΗ-CYANOVERTN ANTIBODY WITH AN INTERNAL IMAGE OF gpl20, A METHOD OF USE THEREOF, AND A METHOD OF USING A CYANO VIRIN TO INDUCE AN IMMUNE RESPONSE TO gp!20 TECHNICAL FIELD OF THE INVENTION This invention relates to an anti-cyanovirin antibody with an internal image of gpl20, a method of using an anti-cyanovirin antibody with an internal image of gpl20 to induce an immune response to gpl20, and a method of using a cyanovirin to induce an immune response to gpl20. BACKGROUND OF THE INVENTION Acquired immune deficiency syndrome (AIDS) is a fatal disease, reported cases ofwhich have increased dramatically within the past two decades. The virus that causes AIDS was first identified in 1983. It has been known by several names and acronyms. It is the third known T-lymphotropic virus (HTLV-III), and it has the capacity to replicate within cells of the immune system, causing profound cell destruction. The AIDS virus is a retrovirus, a virus that uses reverse transcriptase during replication. This particular retrovirus also has been known as lymphadenopathy-associated virus (LAV), ATDS-related virus (ARV) and, presently, as human immunodeficiency virus (HTV). Two distinct families of HTV have been, described to date, namely HTV-l and HTV-2. The acronym HIV is used herein to refer generically to human immunodeficiency viruses. HIV exerts profound cytopathic effects on the CD4 + helper/inducer T-cells, thereby severely compromising the immune system. HIV infection also results in neurological deterioration and, ultimately, in death of infected individuals. Tens of millions of people are infected with HIV worldwide, and, without effective therapy, most of these are doomed to die. During the long latency, the period of time from initial infection to the appearance of symptoms, or death, due to AIDS, infected individuals spread the infection further, by sexual contacts, exchanges of contaminated needles during i.v. drug abuse, transfusions of blood or blood products, or maternal transfer of HIV to a fetus or newborn. Thus, there is not only an urgent need for effective therapeutic agents to inhibit the progression of HIV disease in individuals already infected, but also for methods of prevention of the spread of HIV infection from infected individuals to noninfected individuals. Indeed, the World Health Organization (WHO) has assigned an urgent international priority to the search for an effective anti-HIV prophylactic virucide to help curb the further expansion of the AIDS pandemic (Baiter, Science 266, 1312-1313, 1994; Merson, Science 260, 1266-1268, 1993; Taylor, J. NIH Res. 6, 26-27, 1994; Rosenberg et al., Sex. Transm. DJs, 20, 41-44, 1993; Rosenberg, Am. J. Public Health 82, 1473-1478, 1992). The field of viral therapeutics has developed in response to the need for agents effective against retroviruses, especially HIV. There are many ways in which an agent can exhibit anti-retroviral activity (see, e.g., DeClercq, Adv. Virus Res. 42, 1- 55, 1993: DeClercq. J. Acquir. Immun. Def. Svnd. 4. 207-218. 1991; Mitsuya et al., Science 249, 1533-1544, 1990). Nucleoside derivatives, such as AZT, which inhibit the viral reverse transcriptase, are among the few clinically active agents that are currently available commercially for anti-HIV therapy. Although very useful in some patients, the utility of AZT and related compounds is limited by toxicity and insufficient therapeutic indices for fully adequate therapy. Also, given more recent revelations of the dynamics of HIV infection (Coffm, Science 267, 483-489, 1995; Cohen, Science 267, 179, 1995; Perelson et al., Science 271, 1582-1586, 1996), it is now increasingly apparent that agents acting as early as possible in the viral replicative cycle are needed to inhibit infection of newly produced, uninfected immune cells generated in the body in response to the virus-induced killing of infected cells. Also, it is essential to neutralize or inhibit new infectious virus produced by infected cells. Infection of CD4 + cells by HIV-1 and related primate immunodeficiency viruses begins with interaction of the respective viral envelope glycoproteins (generically termed "gpl20") with the cell-surface receptor CD4, followed by fusion and entry (Sattentau, AIDS 2, 101-105, 1988; Koenig et al., PNAS USA 86, 2443- 2447, 1989). Productively infected, virus-producing cells express gpl20 at the cell surface; interaction of gpl20 of infected cells with CD4 on uninfected cells results in formation of dysfunctional multicellular syncytia and further spread of viral infection (Freed et al, Bull. Inst. Pasteur 88, 73, 1990). Thus, the gpl20/CD4 interaction is a particularly attractive target for interruption of HIV infection and cytopathogenesis, either by prevention of initial virus-to-cell binding or by blockage of cell-to-cell fusion (Capon et al., Ann. Rev. Immunol. 9, 649-678, 1991). Virus-free or "soluble" gpl20 shed from virus or from infected cells in vivo is also an important therapeutic target, since it may otherwise contribute to noninfectious immunopathogenic processes throughout the body, including the central nervous system (Capon et al., 1991, supra; Lipton, Nature 367, 113-114, 1994). Much vaccine research has focused upon gpl20; however, progress has been hampered by hypervariability of the gpl20- neutralizing determinants and the consequent extreme strain-dependence of viral sensitivity to gpl20-directed antibodies (Berzofsky, J. Acq. Immun. Def. Synd. 4, 451-459, 1991). Considerable effort has been devoted to truncated, recombinant "CD4" proteins ("soluble CD4" or "sCD4"), which bind to gpl20 and inhibit HIV infectivity in vitro (Capon et al., 1991, supra: Schooley et al., Ann. Int. Med. 112, 247-253, 1990; Husson et al., J. Pediatr. 121, 627-633, 1992). However, clinical isolates, in contrast to laboratory strains of HIV, have proven highly resistant to neutralization by sCD4 (Orloff et al, AIDS Res. Hum. Retrovir. 11, 335-342, 1995; Moore et al, J. Virol. 66, 235-243, 1992). Initial clinical trials of sCD4 (Schooley et al., 1990, supra; Husson et al., 1992, supra), and of sCD4-coupled immunoglobulins (Langner et al., Arch. Virol. 130, 157-170, 1993), and likewise of sCD4-coupled toxins designed to bind and destroy virus-expressing cells (Davey et al., J. Infect. Pis. 170, 1180-1188, 1994; Ramachandran et al.. J. Infect. Pis. 170. 1009-1013, 1994), have been disappointing. Newer gene-therapy approaches to generating sCD4 directly in vivo (Morgan et al., AIDS Res. Hum. Retrovir. 10, 1507-1515, 1994) will likely suffer similar frustrations. The development of a safe and effective vaccine is now considered to be the single most important long-term goal of current research efforts. However, as pointed out by Varmus and Nathanson (Science 280, 1815, 1998), this is a daunting, if not impossible, task. Many past efforts focused on eliciting a neutralizing antibody response against the envelope complex (reviewed in Burton, PNAS USA, 94, 10018- 10023, 1997; see, also, Haynes, Lancet. 348, 933-937, 1996). Current strategies for development of an HIV-1 vaccine include live, attenuated virus, inactivated virus combined with an adjuvant, a subunit vaccine, e.g., a recombinant monomeric envelope protein or a peptide, a live vector-based vaccine, and the use of DNA plasmids. However, each approach has its limitations. For example, the use of live, attenuated virus poses the risk of eventual pathogenicity in vaccines. Use of inactivated virus combined with an adjuvant results in anti-cellular, rather than anti- viral, antibodies. The absence -of neutralizing antibodies presents a problem for subunit vaccines. In fact, human vaccine trials using monomeric gpl20 as an immunogen were disappointing at best (VanCott et al., J. Immunol.. 155, 4100-4110, 1995; Haynes, Lancet. 348, 933-937, 1996; Bolognesi et al.. Nature. 391, 638-639, 1998; and Connor et al, J. Virol. 72, 1552-1576, 1998). Immunogenicity limits the efficacy of live vector-based vaccines. The use of DNA plasmids is limited by current experience. Anti-idiotypic antibodies, which carry an internal image of an epitope of an antigen and which are designated as Ab2β antibodies, have been shown to induce protective immunity in animals that intentionally have not been exposed to the native antigen epitope (Poskitt et al., Vaccine. 9, 792-796, 1991 ; and Greenspan et al, FASEB J.. 7, 437-444, 1993). Furthermore, anti-idiotypic antibodies have been shown to have potential for immunotherapy of colorectal cancer and ovarian cancer (Denton et al., Int. J. Cancer. 57, 10-14, 1994; Madiyalakan et al, Hybridoma. 14(2), 199-203 (1995); and Wagner et al, Hybridoma. 16(1), 33-40 (1997)). Therefore, new antiviral agents, such as anti-idiotypic antibodies, to be used alone or in combination with AZT and/or other available antiviral agents, are needed for effective antiviral therapy against AIDS. New agents, which may be used to prevent HIV infection, also are important for prophylaxis. In both areas of need, the ideal new agents would act as early as possible in the viral life cycle, be as virus- specific as possible (i.e., attack a molecular target specific to the virus but not to the infected or infectible animal host), render the intact virus noninfectious, prevent the death or dysfunction of virus-infected mammalian cells, prevent further production of virus from infected cells, prevent spread of virus infection to uninfected mammalian cells, be highly potent and active against the broadest possible range of strains and isolates of HIV, be resistant to degradation under physiological and rigorous environmental conditions, and be readily and inexpensively produced on a large-scale. The present invention seeks to provide such antiviral agents as well as methods of using such antiviral agents to prevent or treat a viral infection in an animal. These and other objects and advantages of the present invention, as well as additional inventive features, will become apparent from the description provided herein. BRIEF SUMMARY OF THE INVENTION The present invention provides an anti-cyanovirin antibody, which has an internal image of gpl20 of a primate immunodeficiency virus. Preferably, the antibody can compete with gpl20 of a primate immunodeficiency virus for binding to a cyano virin. In this regard, the primate immunodeficiency virus preferably is HIV-1 or HIV-2 and the cyanovirin preferably comprises SEQ ID NO: 2. The present invention also provides a method of preventing or treating a viral infection in an animal. The method comprises administering to the animal an anti- cyanovirin antibody in an amount sufficient to induce in the animal an immune response to a virus sufficient to prevent or treat an infection of the animal with the virus. The anti-cyanovirin antibody has an internal image of gpl20 of a primate immunodeficiency virus. Preferably, the antibody can compete with gpl20 of a primate immunodeficiency virus for binding to a cyanovirin. In this regard, the primate immunodeficiency virus preferably is HIV-1 or HIV-2 and the cyanovirin preferably comprises SEQ ID NO: 2. Also provided by the present invention is another method of preventing or treating a viral infection in an animal. The method comprises administering to the animal a cyanovirin, which binds gpl20 of a primate immunodeficiency virus, in an amount sufficient to induce in the animal an anti-cyanovirin antibody in an amount sufficient to induce an immune response to a virus sufficient to treat or prevent an infection of the animal with the virus. Preferably, the anti-cyanovirin antibody can compete with gpl20 of a primate immunodeficiency virus for binding to a cyanovirin. In this regard, the primate immunodeficiency virus preferably is HIV-1 or HIV-2 and the cyanovirin preferably comprises SEQ ID NO: 2. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 A is a graph of OD (206 nm) versus time (min), which represents an HPLC chromatogram of nonreduced cyanovirin-N. Figure IB is a bar-graph of maximum dilution for 50% protection versus HPLC fraction, which illustrates the maximum dilution of each HPLC fraction that provided 50% protection from the cytopathic effects of HIV infection for the nonreduced cyanovirin-N HPLC fractions. Figure 1C is an SDS-polyacrylamide gel electrophoretogram of nonreduced cyanovirin-N HPLC fractions. Figure ID is a graph of OD (206 nm) versus time (min), which represents an HPLC chromatogram of reduced cyanovirin-N. Figure IE is a bar graph of maximum dilution for 50% protection versus HPLC dilution, which illustrates the maximum dilution of each fraction that provided 50% protection from the cytopathic effects of HIV infection for the reduced cyanovirin-N HPLC fractions. Figure IF is an SDS-polyacrylamide gel electrophoretogram of reduced cyanovirin-N HPLC fractions. Figure 2 shows an example of a DNA sequence encoding a synthetic cyanovirin gene. Figure 3 illustrates a site-directed mutagenesis maneuver used to eliminate codons for a FLAG octapeptide and a Hind III restriction site from the sequence of Figure 2. Figure 4 shows a typical HPLC chromatogram from the purification of recombinant native cyanovirin-N. Figure 5 A is a graph of % control versus cyanovirin-N concentration (nM), which illustrates the antiviral activity of native cyanovirin-N from Nostoc ellipsosporum. Figure 5B is a graph of % control versus cyanovirin-N concentration (nM), which illustrates the antiviral activity of recombinant cyanovirin-N. Figure 5C is a graph of % control versus cyanovirin-N concentration (nM), which illustrates the antiviral activity of recombinant FLAG-cyanovirin-N. Figure 6A is a graph of % control versus cyanovirin-N concentration (nM), which depicts the relative numbers of viable CEM-SS cells infected with HIV-1 in a BCECF assay. Figure 6B is a graph of % control versus cyanovirin-N concentration (nM), which depicts the relative DNA contents of CEM-SS cell cultures infected with HIV-1. Figure 6C is a graph of % control versus cyanovirin-N concentration (nM), which depicts the relative numbers of viable CEM-SS cells infected with HIV-1 in an XTT assay. Figure 6D is a graph of % control versus cyanovirin-N concentration (nM), which depicts the effect of a range of concentrations of cyanovirin-N upon indices of infectious virus or viral replication. Figure 7 is a graph of % uninfected control versus time-of-addition (hr), which shows the results of delayed-addition studies of cyanovirin-N, showing anti-HIV activity in CEM-SS cells infected with HTV-1 M . Figure 8A is a graph of OD (450 nm) versus cyanovirin-N concentration (μg/ml), which illustrates cyanovirin/gpl20 interactions defining gpl20 as a principal molecular target of cyanovirins. Figure 8B is a dot-blot of the binding of cyanovirin-N and a gpl20-HRP conjugate, which shows that cyanovirin-N specifically bound a horseradish peroxidase conjugate of gpl20 (gpl20-HRP) in a concentration-dependent manner. Figure 9 schematically illustrates a DNA coding sequence comprising a FLAG-cyanovirin-N coding sequence coupled to a Pseudomonas exotoxin coding sequence. Figure 10 is a graph of OD (450 nM) versus PPE concentration (nM), which illustrates selective killing of viral gpl20-expressing (H9/IIIB) cells by a FLAG- cyanoviήn- Pseudomonas exotoxin protein conjugate (PPE). Figure 11 is a westem-blot from an SDS-polyacrylamide gel elctrophoretogram of lysed COS-7 cells which had been engineered and transformed to express a FLAG-cyanovirin-N; detection was by an anti-FLAG antibody. Figure 12 is a westem-blot from an SDS-polyacrylamide gel electrophoretogram of secreted products, digested with peptide-N4-(N-acetyl-β- glucosaminyl) asparagine amidase, from Pichia pastoris engineered and transformed to produce a cyanovirin; detection was by an anti-cyanovirin-N polyclonal antibody. Figure 13 is an SDS-polyacrylamide gel electrophoretogram (A) and a western-blot (B) from a whole-cell lysate from E. coli engineered to produce cyanovirin-N; detection was by an anti-cyanovirin-N polyclonal antibody. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is predicated, at least in part, on the observation that certain extracts from cultured cyanobacteria (blue-green algae) exhibited antiviral activity in an anti-FflV screen. The anti-HIV screen was conceived in 1986 (by M.R. Boyd of the National Institutes of Health) and has been developed and operated at the U.S. National Cancer Institute (NCI) since 1988 (see Boyd, in AIDS. Etiology. Diagnosis. Treatment and Prevention. DeVita et al., eds., Philadelphia: Lippincott, 1988, pp. 305-317). Cyanobacteria (blue-green algae) were specifically chosen for anti-HIV screening because they had been known to produce a wide variety of structurally unique and biologically active non-nitrogenous and amino acid-derived natural products (Faulkner, Nat. Prod. Rep. 11, 355-394, 1994; Glombitza et al, m Algal and Cvanobacterial Biotechnology. Cresswell, R.C., et al. eds., 1989, pp. 211-218). These photosynthetic procaryotic organisms are significant producers of cyclic and linear peptides (molecular weight generally <3 kDa), which often exhibit hepatotoxic or antimicrobial properties (Okino et al, Tetrahedron Lett. 34, 501-504, 1993; Krishnamurthy et al, PNAS USA 86, 770-774, 1989; Sivonen et al, Chem. Res. Toxicol. 5, 464-469, 1992; Carter et al., J. Org. Chem. 49, 236-241, 1984; Frankmolle et al, J. Antibiot. 45, 1451-1457, 1992). Sequencing studies of higher molecular weight cyanobacterial proteins have generally focused on those associated with primary metabolic processes or ones that can serve as phylogenetic markers (Suter et al.. FEBS Lett. 217, 279-282, 1987; Rumbeli et al.. FEBS Lett. 221. 1-2, 1987; Swanson et al., J. Biol. Chem. 267, 16146-16154, 1992; Michalowski et al., Nucleic Acids Res. 18, 2186, 1990; Sherman et al., in The Cyanobacteria, Fay et al., eds., Elsevier: New York, 1987, pp. 1-33; Rogers, m The Cyanobacteria, Fay et al., eds., Elsevier: New York, 1987, pp. 35-67). In general, proteins with antiviral properties have not been associated with cyanobacterial sources. The cyanobacterial extract leading to the present invention was among many thousands of different extracts initially selected randomly and tested blindly in the anti-HIV screen described above. A number of these extracts had been determined preliminarily to show anti-HIV activity in the NCI screen (Patterson et al., J. Phvcol. 29, 125-130, 1993). From this group, an aqueous extract from Nostoc ellipsosporum, which had been prepared as described (Patterson, 1993, supra) and which showed an unusually high anti-HIV potency and in vitro "therapeutic index" in the NCI primary screen, was selected for detailed investigation. A specific bioassay-guided strategy was used to isolate and purify a homogenous protein highly active against HTV. In the bioassay-guided strategy, initial selection of the extract for fractionation, as well as the decisions concerning the overall chemical isolation method to be applied, and the nature of the individual steps therein, were determined by interpretation of biological testing data. The anti-HIV screening assay (see, e.g., Boyd, 1988, supra; Weislow et al, J. Natl. Cancer Inst. 81, 577-586, 1989), which was used to guide the isolation and purification process, measures the degree of protection of human T-lymphoblastoid cells from the cytopathic effects of HIV. Fractions of the extract of interest are prepared using a variety of chemical means and are tested blindly in the primary screen. Active fractions are separated further, and the resulting subfractions are likewise tested blindly in the screen. This process -is repeated as many times as necessary in order to obtain the active compound(s), i.e., antiviral fraction(s) representing pure compound(s), which then can be subjected to detailed chemical analysis and structural elucidation. Using this strategy, aqueous extracts oϊNostoc ellipsosporum were discovered to contain an antiviral protein. It should be noted that the term "protein" as used herein to describe the present invention is not restricted to an amino acid sequence of any particular length and includes molecules comprising 100 or more amino acids, as well as molecules comprising less than 100 amino acids (which are sometimes referred to as "peptides"). In view of the above, an isolated and purified antiviral protein from Nostoc ellipsosporum, specifically an isolated and purified antiviral protein known as cyanovirin-N, is provided. Other cyanovirins are also provided. The term "cyanovirin" is used herein to generically refer to a native antiviral protein isolated from Nostoc ellipsosporum ("native cyanovirin") and any functionally equivalent protein or derivative thereof. Cyanovirins and functionally equivalent proteins and derivatives thereof as described herein can inactivate a wide range of HTV strains and isolates. Such a functionally equivalent protein or derivative thereof (a) contains a sequence of at least nine (preferably at least twenty, more preferably at least thirty, and most preferably at least fifty) amino acids directly homologous with (preferably the same as) any subsequence of nine contiguous amino acids contained within a native cyanovirin (especially cyanovirin-N), and (b) is antiviral, in particular capable of specifically binding to a virus, more specifically a primate immunodeficiency virus, more specifically HIV-1, HIV-2, or SIV, or to an infected host cell expressing one or more viral antigen(s), more specifically an envelope glycoprotein, such as gpl20, of the respective virus. In addition, such a functionally equivalent protein or derivative thereof can comprise the amino acid sequence of a native cyanovirin, particularly cyanovirin-N (see SEQ ID NO:2), in which 1-20, preferably 1-10, more.preferably 1, 2, 3, 4, or 5, and most preferably 1 or 2, amino acids have been removed from one or both ends, preferably from only one end, and most preferably from the amino-terminal end, of the native cyanovirin. The cyanovirin preferably comprises an amino acid sequence that is substantially homologous to that of an antiviral protein from Nostoc ellipsosporum, specifically a native cyanovirin, particularly cyanovirin-N. In the context of the cyanovirins, the term "substantially homologous" means sufficient homology to render the cyanovirin antiviral, preferably with antiviral activity characteristic of an antiviral protein isolated from Nostoc ellipsosporum. There preferably exists at least about 50% homology, more preferably at least about 75% homology, and most preferably at least about 90% homology. Thus, also provided is an isolated and purified protein encoded by a nucleic acid molecule comprising a coding sequence for a cyanovirin, such as particularly an isolated and purified protein encoded by a nucleic acid molecule comprising a sequence of SEQ ID NO:l, a nucleic acid molecule comprising a sequence of SEQ ID NO:3, a nucleic acid molecule encoding an amino acid sequence of SEQ ID NO:2, or a nucleic acid molecule encoding an amino acid sequence of SEQ ID NO:4. Likewise, a cyanovirin conjugate, which comprises a cyanovirin coupled to one or more selected effector molecule(s), such as a toxin or immunological reagent, is provided. The term "immunological reagent" is used herein to refer to an antibody, an immunoglobulin, and an immunological recognition element. An immunological recognition element is an element, such as a peptide, e.g., the FLAG sequence of the recombinant cyanovirin-FLAG fusion protein, which facilitates, through immunological recognition, isolation and/or purification and/or analysis of the protein to which it is attached. A cyanovirin fusion protein is a type of cyanovirin conjugate, wherein a cyanovirin is coupled to one or more other protein(s) having any desired properties or effector functions, such as cytotoxic or immunological properties, or other desired properties, such as to facilitate isolation, purification, or analysis of the fusion protein. Also provided is a method of obtaining a cyanovirin from Nostoc ellipsosporum. The method comprises (a) identifying an extract of Nostoc ellipsosporum containing antiviral activity, (b) optionally removing high molecular weight biopolymers from the extract, (c) antiviral bioassay-guided fractionating the extract to obtain a partially purified extract of cyanovirin, and (d) further purifying the partially purified extract by reverse-phase HPLC to obtain a cyanovirin (see Example 1). The method preferably involves the use of ethanol to remove high molecular weight biopolymers from the extract and the use of an anti-HIV bioassay to guide fractionation of the extract. The isolated and purified cyanovirin, such as cyanovirin-N (CV-N), can be subjected to conventional procedures typically used to determine the amino acid sequence of a given pure protein. Thus, the cyanovirin can be sequenced by N- terminal Edman degradation of intact protein and overlapping peptide fragments generated by endoproteinase digestion. Amino acid analysis desirably will be in agreement with the deduced sequence. Similarly, ESI mass spectrometry of reduced, HPLC-purified cyanovirin-N desirably will show a molecular ion value consistent with the calculated value. These studies indicated that cyanovirin-N from Nostoc ellipsosporum comprises a unique sequence of 101 amino acids having little or no significant homology to previously described proteins or transcription products of known nucleotide sequences. No more than eight contiguous amino acids from cyanovirin are found in any amino acid sequences from known proteins, nor are there any known proteins from any source containing greater than 13% sequence homology with cyanovirin-N. Given the chemically deduced amino acid sequence of cyanovirin-N, a corresponding recombinant cyanovirin-N (r-cyanovirin-N, or r-CV-N) was created and used to definitively establish that the deduced amino acid sequence is, indeed, active against viruses, such as HIV (see Examples 2-5). Also provided are an isolated and purified nucleic acid molecule and a synthetic nucleic acid molecule, which comprise a coding sequence for a cyanovirin (particularly a native cyanovirin, especially cyanovirin-N). Such nucleic acid molecules include an isolated and purified nucleic acid molecule comprising a sequence of SEQ ID NO:l, an isolated and purified nucleic acid molecule comprising a sequence of SEQ ID NO:3, an isolated and purified nucleic acid molecule encoding an amino acid sequence of SEQ ID NO:2, an isolated and purified nucleic acid molecule encoding an amino acid sequence of SEQ ID NO:4, and a nucleic acid molecule that is substantially homologous to any one or more of the aforementioned nucleic acid molecules. In the context of the nucleic acid molecule of the present invention, the term "substantially homologous" means sufficient homology to render the protein encoded by the nucleic acid molecule antiviral, preferably with antiviral activity characteristic of an antiviral protein isolated from Nostoc ellipsosporum. There preferably exists at least about 50% homology, more preferably at least about 75% homology, and most preferably at least about 90% homology. The nucleic acid molecule desirably comprises a nucleic acid sequence encoding at least nine (preferably at least twenty, more preferably at least thirty, and most preferably at least fifty) contiguous amino acids of the amino acid sequence of SEQ ID NO:2. The present inventive nucleic acid molecule also desirably comprises a nucleic acid sequence encoding a protein comprising the amino acid sequence of a native cyanovirin, particularly cyanovirin-N, in which 1-20, preferably 1-10, more preferably 1, 2, 3, 4, or 5, and most preferably 1 or 2, amino acids have been removed from one or both ends, preferably from only one end, and most preferably from the amino-terminal end, of the native cyanovirin. In view of the above, it will be apparent to one skilled in the art that a partial cyanovirin-N gene codon sequence will likely suffice to code for a fully functional, i.e., antiviral, such as anti-HIV, cyanovirin. A minimum essential DNA coding sequence(s) for a functional cyanovirin can readily be determined by one skilled in the art, for example, by synthesis and evaluation of sub-sequences comprising the native cyanovirin, and by site-directed mutagenesis studies of the cyanovirin-N DNA coding sequence. Using an appropriate DNA coding sequence, a recombinant cyanovirin can be made by genetic engineering techniques (see, e.g., for general background, Nicholl, in An Introduction to Genetic Engineering, Cambridge University Press: Cambridge, 1994, pp. 1-5 & 127-130; Steinberg et al.. in Recombinant DNA Technology Concepts and Biomedical Applications. Prentice Hall: Englewood Cliffs, NJ, 1993, pp. 81-124 & 150-162; Sofer in Introduction to Genetic Engineering, Butterworth- Heinemann, Stoneham, MA, 1991, pp. 1-21 & 103-126; Old et al., m Principles of Gene Manipulation, Blackwell Scientific Publishers: London, 1992, pp. 1-13 & 108- 221 ; Emtage, in Delivery Systems for Peptide Drugs, Davis et al., eds., Plenum Press: New York, 1986, pp. 23-33). For example, a Nostoc ellipsosporum gene or cDNA encoding a cyanovirin can be identified and subcloned. The gene or cDNA can then be incorporated into an appropriate expression vector and delivered into an appropriate protein-synthesizing organism (e.g., E. coli, S. cerevisiae, P. pastoris, or other bacterial, yeast, insect, or mammalian cell), where the gene, under the control of an endogenous or exogenous promoter, can be appropriately transcribed and translated. Such expression vectors (including, but not limited to, phage, cosmid, viral, and plasmid vectors) are known to those skilled in the art, as are reagents and techniques appropriate for gene transfer (e.g., transfection, electroporation, transduction, micro-injection, transformation, etc.). Subsequently, the recombinantly produced protein can be isolated and purified using standard techniques known in the art (e.g., chromatography, centrifugation, differential solubility, isoelectric focusing, etc.), and assayed for antiviral activity. Alternatively, a native cyanovirin can be obtained from Nostoc ellipsosporum by non-recombinant methods (see, e.g., Example 1 and foregoing discussion) and sequenced by conventional techniques. The sequence can then be used to synthesize the corresponding DNA, which can be subcloned into an appropriate expression vector and delivered into a protein-producing cell for en mass recombinant production of the desired protein. In this regard, a vector comprising a nucleic acid molecule encoding a cyanovirin, e.g., a DNA sequence such as a Nostoc ellipsosporum gene sequence for cyanovirin, a cDNA encoding a cyanovirin, or a synthetic DNA sequence encoding a cyanovirin, is provided. Also provided is a host cell comprising such a nucleic acid molecule or vector, as well as a method of using such a host cell to produce a cyanovirin. The DNA, whether isolated and purified or synthetic, or cDNA encoding a cyanovirin can encode for either the entire cyanovirin or a portion thereof (desirably an antivirally active portion thereof). Where the DNA or cDNA does not comprise the entire coding sequence of the native cyanovirin, the DNA or cDNA can be subcloned as part of a gene fusion. In a transcriptional gene fusion, the DNA or cDNA will contain its own control sequence directing appropriate production of protein (e.g., ribosome binding site, translation initiation codon, etc.), and the transcriptional control sequences (e.g., promoter elements and/or enhancers) will be provided by the vector. In a translational gene fusion, transcriptional control sequences as well as at least some of the translational control sequences (i.e., the translational initiation codon) will be provided by the vector. In the case of a translational gene fusion, a chimeric protein will be produced. Genes also can be constructed for specific fusion proteins containing a functional cyanovirin component plus a fusion component conferring additional desired attribute(s) to the composite protein. For example, a fusion sequence for a toxin or immunological reagent, as defined above, can be added to facilitate purification and analysis of the functional protein (e.g., such as the FLAG-cyanovirin- N fusion protein described in Examples 2-5). Genes can be specifically constructed to code for fusion proteins, which contain a cyanovirin coupled to an effector protein, such as a toxin or immunological reagent, for specific targeting to viral-infected, e.g., HIV and/or HIV-infected, cells. In these instances, the cyanovirin moiety serves not only as a neutralizing agent but also as a targeting agent to direct the effector activities of these molecules selectively against a given virus, such as HIV. Thus, for example, a therapeutic agent can be obtained by combining the HIV-targeting function of a functional cyanovirin with a toxin aimed at neutralizing infectious virus and/or by destroying cells producing infectious virus, such as HIV. Similarly, a therapeutic agent can be obtained, which combines the viral-targeting function of a cyanovirin with the multivalency and effector functions of various immunoglobulin subclasses. Example 6 fiirther illustrates the viral-targeting, specifically gpl20-targeting, properties of a cyanovirin. Similar rationales underlie extensive developmental therapeutic efforts exploiting the HIV gpl20-targeting properties of sCD4. For example, sCD4-toxin conjugates have been prepared in which sCD4 is coupled to a Pseudomonas exotoxin component (Chaudhary et al., in The Human Retrovirus. Gallo et al., eds., Academic Press: San Diego, 1991, pp. 379-387; Chaudhary et al., Nature 335, 369-372, 1988), a diphtheria toxin component (Aullo et al, EMBO J. 11, 575-583, 1992), or a ricin A- chain component (Till et al., Science 242, 1166-1167, 1988). Likewise, sCD4- immunoglobulin conjugates have been prepared in attempts to decrease the rate of in vivo clearance of functional sCD4 activity, to enhance placental transfer, and to effect a targeted recruitment of immunological mechanisms of pathogen elimination, such as phagocytic engulfment and killing by antibody-dependent cell-mediated cytotoxicity, to kill and/or remove HIV-infected cells and virus (Capon et al., Nature 337, 525-531, 1989; Traunecker et al, Nature 339, 68-70, 1989; Langner et al., 1993, supra). While such CD4-immunoglobulin conjugates (sometimes called "immunoadhesins") have, indeed, shown advantageous pharmacokinetic and distributional attributes in vivo, and anti-HIV effects in vitro, clinical results have been discouraging (Schooley et al., 1990, supra; Husson et al., 1992, supra; Langner et al., 1993, supra . This is not surprising since clinical isolates of HIV, as opposed to laboratory strains, are highly resistant to binding and neutralization by sCD4 (Orloff et al., 1995, supra; Moore et al., 1992, supra). Therefore, the extraordinarily broad antiviral activity and targeting properties of a functional cyanovirin to viruses, e.g., primate retroviruses, in general, and clinical and laboratory strains, in particular (see, e.g., Example 7), are especially advantageous for combining with toxins, immunoglobulins, and other selected effector proteins. Viral-targeted conjugates can be prepared either by genetic engineering techniques (see, for example, Chaudhary et al., 1988, supra) or by chemical coupling of the targeting component with an effector component. The most feasible or appropriate technique to be used to construct a given cyanovirin conjugate or fusion protein will be selected based upon consideration of the characteristics of the particular effector molecule selected for coupling to a cyanovirin. For example, with a selected non-proteinaceous effector molecule, chemical coupling, rather than genetic engineering techniques, represents the most feasible option for creating the desired cyanovirin conjugate. Accordingly, nucleic acid molecules encoding cyanovirin fusion proteins, in addition to the cyanovirin fusion proteins themselves, are provided. In particular, a nucleic acid molecule comprising SEQ ID NO:3 and substantially homologous sequences thereto are provided. A vector comprising a nucleic acid sequence encoding a cyanovirin fusion protein is provided as is a method of obtaining a cyanovirin fusion protein by expression of the vector encoding a cyanovirin fusion protein in a protein-synthesizing organism as described above. Further provided is an isolated and purified nucleic acid molecule comprising a first nucleic acid sequence that encodes a protein of the present invention, e.g., a cyanovirin coding sequence, such as one of the aforementioned nucleic acids, coupled to a second nucleic acid encoding an effector protein, such as a toxin or immunological reagent as described above. Also provided is an isolated and purified protein encoded by such a nucleic acid molecule. The coupled molecule (conjugate) desirably targets a virus, more preferably HIV, and most preferably glycoprotein gpl20. The coupling can be effected at the DNA level or by chemical coupling as described above. For example, a cyanovirin- effector protein conjugate of the present invention can be obtained by (a) selecting a desired effector protein, (b) synthesizing a composite DNA coding sequence comprising a first DNA coding sequence comprising one of the aforementioned nucleic acid sequences, which codes for a functional cyanovirin, coupled to a second DNA coding sequence for an effector protein, e.g., a toxin or immunological reagent, (e) expressing the composite DNA coding sequence in an appropriate protein- synthesizing organism, and (d) purifying the desired fusion protein to substantially pure form. Alternatively, a cyanovirin-effector molecule conjugate can be obtained by (a) selecting a desired effector molecule and a cyanovirin or cyanovirin fusion protein, (b) chemically coupling the cyanovirin or cyanovirin fusion protein to the effector molecule, and (c) isolating the desired cyanovirin-effector molecule conjugate in substantially pure form. Conjugates containing a functional cyanovirin coupled to a desired effector component, such as a toxin, immunological reagent, or other functional reagent, can be designed even more specifically to exploit the unique gpl20-targeting properties of a cyanovirin, in accord with the following observations. Example 6 reveals novel gpl20-directed effects of a cyanovirin. Additional insights can be gained from solid-phase ELISA experiments (Boyd et al., 1996, unpublished). For example, both C-terminal gpl20-epitope-specifιc capture or CD4- receptor capture of gpl20, when detected either with polyclonal HIV-l-Ig or with mouse MAb to the immunodominant, third hypervariable (V3) epitope (Matsushita et al., J. Virol. 62, 2107-2114, 1988), can be shown to be strikingly inhibited by cyanovirin-N. Generally, engagement of the CD4 receptor does not interfere with antibody recognition of the V3 epitope, and vice versa (Moore et al., AIDS Res. Hum. Retrovir. 4, 369-379, 1988; Matsushita et al., 1988, supra). However, cyanovirin-N apparently is capable of more global conformational effects on g l20, as can be demonstrated by loss of immunoreactivity at multiple, distinct, non-overlapping epitopes. The range of antiviral activity (Boyd et al., 1996, supra) of cyanovirin-N against diverse CD4 + -tropic immunodeficiency virus strains in various target cells is remarkable; diverse strains of HIV-1, HIV-2, and SIV can be shown to be similarly sensitive to cyanovirin; clinical isolates and laboratory strains typically will show essentially equivalent sensitivity (for further illustration, see Example 7). Cocultivation of chronically infected and uninfected CEM-SS cells with cyanovirin-N will show that the protein will not inhibit viral replication, but will cause a concentration-dependent inhibition of cell-to-cell fusion and virus transmission; similar results from binding and fusion inhibition assays employing HeLa-CD4-LTR- β-galactosidase cells can be shown consistent with cyanovirin-N inhibition of virus- cell and/or cell-cell binding (Boyd, et al, 1996, supra). Example 8, illustrates the construction of a conjugate DNA coding sequence and expression thereof to provide a cyanovirin-toxin conjugate that selectively targets and kills HIV-infected cells. The antiviral, e.g., anti-HIV, activity of the cyanovirins and conjugates thereof can be further demonstrated in a series of interrelated in vitro antiviral assays (Gulakowski et al., J. Virol. Methods 33, 87-100, 1991), which reasonably predict antiviral activity in humans. These assays measure the ability of compounds to prevent the replication of HIV and/or the cytopathic effects of HIV on human target cells. These measurements directly correlate with the pathogenesis of HIV-induced disease in vivo. The results of the analysis of the antiviral activity of cyanovirins or conjugates, as set forth in Example 5 and as illustrated in Figures 8, 9 and 10, predict antiviral activity of these products in vivo in humans and, therefore, further establish the utility of the cyanovirins and conjugates thereof . Also, ex vivo methods of use of cyanovirins and conjugates (e.g., see results set forth in Example 5, and in Figures 6 and 7), provide even broader utility. The cyanovirins and conjugates thereof can be shown to inhibit a virus, specifically a retrovirus, such as the human immunodeficiency virus, i.e., HIV-1 or HIV-2. The cyanovirins and conjugates of the present invention can be used to inhibit other retroviruses as well as other viruses. Examples of viruses that can be treated include, but are not limited to, Type C and Type D retroviruses, HTLV-1, HTLV-2,' HIV, FLV, SIV, MLV, BLV, BIV, equine infectious virus, anemia virus, avian sarcoma viruses, such as Rous sarcoma virus (RSV), hepatitis type A, B, non-A and non-B viruses, arboviruses, varicella viruses, measles, mumps and rubella viruses. Cyanovirins and conjugates thereof comprise proteins and, as such, are particularly susceptible to hydrolysis of amide bonds (e.g., catalyzed by peptidases) and disruption of essential disulfide bonds or formation of inactivating or unwanted disulfide linkages (Carone et al., J. Lab. Clin. Med. 100, 1-14, 1982). There are various ways to alter molecular structure, if necessary, to provide enhanced stability to the cyanovirin or conjugate thereof (Wunsch, Biopolymers 22, 493-505, 1983; Samanen, in Polymeric Materials in Medication, Gebelein et al., eds., Plenum Press: New York, 1985, pp. 227-242), which, in some circumstances, may be essential for preparation and use of pharmaceutical compositions containing cyanovirins or conjugates thereof for therapeutic or prophylactic applications against viruses, e.g., HIV. Possible options for useful chemical modifications of a cyanovirin or conjugate thereof include, but are not limited to, the following (adapted from Samanen, J.M., 1985, supra): (a) olefin substitution, (b) carbonyl reduction, (c) D-amino acid substitution, (d) N α-methyl substitution, (e) C α-methyl substitution, (f) C α-C- methylene insertion, (g) dehydro amino acid insertion, (h) retro-inverso modification, (i) N-terminal to C-terminal cyclization, and (j) thiomethylene modification. Cyanovirins and conjugates thereof also can be modified by covalent attachment of carbohydrate and polyoxyethylene derivatives, which are expected to enhance stability and resistance to proteolysis (Abuchowski et al, in Enzymes as Drugs, Holcenberg et al., eds., John Wiley: New York, 1981, pp. 367-378). Other important general considerations for design of delivery systems and compositions, and for routes of administration, for protein drugs, such as cyanovirins and conjugates thereof (Eppstein, CRC Crit. Rev. Therapeutic Drug Carrier Systems 5, 99-139, 1988; Siddiqui et al., CRC Crit. Rev. Therapeutic Drug Carrier Systems 3, 195-208, 1987); Banga et al„ Int. J. Pharmaceutics 48. 15-50, 1988: Sanders, Eur. J. Drug Metab. Pharmacokinetics 15, 95-102, 1990; Verhoef, Eur. J. Drug Metab. Pharmacokinetics 15, 83-93, 1990), also apply. The appropriate delivery system for a given cyanovirin or conjugate thereof will depend upon its particular nature, the particular clinical application, and the site of drug action. As with any protein drug, oral delivery of a cyanovirin or a conjugate thereof will likely present special problems, due primarily to instability in the gastrointestinal tract and poor absorption and bioavailability of intact, bioactive drug therefrom. Therefore, especially in the case of oral delivery, but also possibly in conjunction with other routes of delivery, it will be necessary to use an absorption-enhancing agent in combination with a given cyanovirin or conjugate thereof. A wide variety of absorption-enhancing agents have been investigated and/or applied in combination with protein drugs for oral delivery and for delivery by other routes (Verhoef, 1990, supra; van Hoogdalem, Pharmac. Ther. 44, 407-443, 1989; Davis, J. Pharm. Pharmacol. 44(Supρl. 1), 186-190, 1992). Most commonly, typical enhancers fall into the general categories of (a) chelators, such as EDTA, salicylates, and N-acyl derivatives of collagen, (b) surfactants, such as lauryl sulfate and polyoxyethylene-9-lauryl ether, (c) bile salts, such as glycholate and taurocholate, and derivatives, such as taurodihydrofusidate, (d) fatty acids, such as oleic acid and capric acid, and their derivatives, such as acylcarnitines, monoglycerides, and diglycerides, (e) non-surfactants, such as unsaturated cyclic ureas, (f) saponins, (g) cyclodextrins, and (h) phospholipids. Other approaches to enhancing oral delivery of protein drugs, such as the cyanovirins and conjugates thereof , can include the aforementioned chemical modifications to enhance stability to gastrointestinal enzymes and/or increased lipophilicity. Alternatively, the protein drug can be administered in combination with other drugs or substances that directly inhibit proteases and/or other potential sources of enzymatic degradation of proteins. Yet another alternative approach to prevent or delay gastrointestinal absorption of protein drugs, such as cyanovirins or conjugates, is to incorporate them into a delivery system that is designed to protect the protein from contact with the proteolytic enzymes in the intestinal lumen and to release the intact protein only upon reaching an area favorable for its absorption. A more specific example of this strategy is the use of biodegradable microcapsules or microspheres, both to protect vulnerable drugs from degradation, as well as to effect a prolonged release of active drug (Deasy, in Microencapsulation and Related Processes, Swarbrick, ed., Marcell Dekker, Inc.: New York, 1984, pp. 1-60, 88-89, 208-211). Microcapsules also can provide a useful way to effect a prolonged delivery of a protein drug, such as a cyanovirin or conjugate thereof, after injection (Maulding, J. Controlled Release 6. 167-176, 1987). Given the aforementioned potential complexities of successful oral delivery of a protein drug, it is preferred in many situations that the cyanovirins and conjugates thereof be delivered by one of the numerous other potential routes of delivery of a protein drug. These routes include intravenous, intraarterial, intrathecal, intracisternal, buccal, rectal, nasal, pulmonary, transdermal, vaginal, ocular, and the like (Eppstein, 1988, supra: Siddiqui et al., 1987, supra; Banga et al, 1988, supra; Sanders, 1990, supra; Verhoef, 1990, supra; Barry, in Delivery Systems for Peptide Drugs, Davis et al., eds., Plenum Press: New York, 1986, pp. 265-275; Patton et al., Adv. Drug Delivery Rev. 8, 179-196, 1992). With any of these routes, or, indeed, with any other route of administration or application, a protein drug, such as a cyanovirin or conjugate thereof, may initiate an immunogenic reaction. In such situations it may be necessary to modify the molecule in order to mask immunogenic groups. It also can be possible to protect against undesired immune responses by judicious choice of method of formulation and/or administration. For example, site- specific delivery can be employed, as well as masking of recognition sites from the immune system by use or attachment of a so-called tolerogen, such as polyethylene glycol, dextran, albumin, and the like (Abuchowski et al, 1981, supra; Abuchowski et al., J. Biol. Chem. 252, 3578-3581, 1977; Lisi et al., J. Appl. Biochem. 4, 19-33, 1982; Wileman et al, J. Pharm. Pharmacol. 38, 264-271, 1986). Such modifications also can have advantageous effects on stability and half-life both in vivo and ex vivo. Other strategies to avoid untoward immune reactions also can include the induction of tolerance by administration initially of only low doses. In any event, it will be apparent from the present disclosure to one skilled in the art that for any particular desired medical application or use of a cyanovirin or conjugate thereof, the skilled artisan can select from any of a wide variety of possible compositions, routes of administration, or sites of application, whatever is advantageous. Accordingly, the antiviral cyanovirins and conjugates thereof can be formulated into various compositions for use either in therapeutic treatment methods for virally, e.g., HIV, infected individuals, or in prophylactic methods against viral, e.g., HIV, infection of uninfected individuals. Thus, a composition comprising a cyanovirin or cyanovirin conjugate, especially a pharmaceutical composition comprising an antiviral effective amount of an isolated and purified cyanovirir or cyanovirin conjugate and a pharmaceutically acceptable carrier is provided. Instead of, or in addition to, the aforementioned isolated and purified cyanovirin or cyanovirin conjugate, the composition can comprise viable host cells transformed to directly express a cyanovirin or conjugate thereof in vivo. The composition further can comprise an antiviral effective amount of at least one additional antiviral compound other than a cyanovirin or conjugate thereof. Suitable antiviral compounds include AZT, ddl, ddC, gancyclovir, fluorinated dideoxynucleosides, nevirapine, R82913, Ro 31-8959, BI-RJ-70, acyclovir, α-ihterferon, recombinant sCD4, michellamines, calanolides, nonoxynol-9, gossypol and derivatives thereof, and gramicidin. The cyanovirin used in the pharmaceutical composition can be isolated and purified from naturally occurring organisms or from genetically engineered organisms. Similarly, cyanovirin conjugates can be derived from genetically engineered organisms or from chemical coupling. The compositions can be used to treat a virally infected animal, such as a human. The compositions are particularly useful for inhibiting the growth or replication of a virus, such as a retrovirus, in particular a human immunodeficiency virus, specifically HIV-1 and HIV-2. The compositions are useful in the therapeutic or prophylactic treatment of animals, such as humans, who are infected with a virus or who are at risk for viral infection, respectively. The compositions also can be used to treat objects or materials, such as medical equipment, supplies, or fluids, including biological fluids, such as blood, blood products, and tissues, to prevent viral infection of an animal, such as a human. Such compositions also are useful to prevent sexual transmission of viral infections, e.g., HIV, which is the primary way in which the world's AIDS cases are contracted (Merson, 1993, supra). Potential virucides used or being considered for application against sexual transmission of HIV are very limited; present agents in this category include, for example, nonoxynol-9 (Bird, AIDS 5, 791-796, 1991), gossypol and derivatives (Polsky et al., Contraception 39, 579-587, 1989; Lin, Antimicrob. Agents Chemother. 33, 2149-2151, 1989; Royer, Pharmacol. Res. 24, 407-412, 1991), and gramicidin (Bourinbair, Life Sci. Pharmacol. Lett. 54, PL5-9, 1994; Bourinbair et al., Contraception 49, 131-137, 1994). In a novel approach to anti-HIV prophylaxis currently being initiated under the auspices of the U.S. National Institute of Allergy and Infectious Diseases (NIAID) (e.g., as conveyed by Painter, USA Today, February 13, 1996), the vaginal suppository instillation of live cultures of lactobacilli is being evaluated in a 900- woman study. This study is based especially upon observations of anti-HIV effects of certain H 2 O 2 -producing lactobacilli in vitro (e.g., see published abstract by Hilier, from NIAID-sponsored Conference on "Advances in AIDS Vaccine Development", Bethesda, MD, February 11-15, 1996). Lactobacilli readily populate the vagina, and indeed are a predominant bacterial population in most healthy women (Redondo- Lopez et al., Rev. Infect. Pis. 12, 856-872, 1990; Reid et al., Clin. Microbiol. Rev. 3, 335-344, 1990: Bruce and Reid, Can. J. Microbiol. 34. 339-343. 1988; Reu et al., I Infect. Pis. 171, 1237-1243, 1995; Hilier et al.. Clin. Infect. Pis. 16(Suppl 4), S273- S281; Agnew et al., Sex. Transm. Pis. 22, 269-273, 1995). Lactobacilli are also prominent, nonpathogenic inhabitants of other body cavities such as the mouth, nasopharynx, upper and lower gastrointestinal tracts, and rectum. It is well-established that lactobacilli can be readily transformed using available genetic engineering techniques to incorporate a desired foreign PNA coding sequence, and that such lactobacilli can be made to express a corresponding desired foreign protein (see, e.g., Hols et al., Appl. and Environ. Microbiol. 60, 1401-1413, 1994). Therefore, within the context of the present disclosure, it will be appreciated by one skilled in the art that viable host cells containing a PNA sequence or vector as described above , and expressing an above-described protein , can be used directly as the delivery vehicle for a cyanovirin or conjugate thereof to the desired site(s) in vivo. Preferred host cells for such delivery of cyanovirins or conjugates thereof directly to desired site(s), such as, for example, to a selected body cavity, can comprise bacteria. More specifically, such host cells can comprise suitably engineered strain(s) of lactobacilli, enterococci, or other common bacteria, such as E. coli, normal strains of which are known to commonly populate body cavities. More specifically yet, such host cells can comprise one or more selected nonpathogenic strains of lactobacilli, such as those described by Andreu et al. (1995, supra), especially those having high adherence properties to epithelial cells, such as, for example, adherence to vaginal epithelial cells, and suitably transformed using the above-described PNA sequences . As reviewed by McGroarty fFEMS Immunol. Med. Microbiol. 6, 251-264, 1993) the "probiotic" or direct therapeutic application of live bacteria, particularly bacteria that occur normally in nature, more particularly lactobacilli, for treatment or prophylaxis against pathogenic bacterial or yeast infections of the urogenital tract, in particular the female urogenital tract, is a well-established concept. Recently, the use of a conventional probiotic strategy, in particular the use of live lactobacilli, to inhibit sexual transmission of HIV has been suggested, based specifically upon the normal, endogenous production of virucidal levels of H 2 O 2 and/or lactic acid and/or other potentially virucidal substances by certain normal strains of lactobacilli (e.g., Hilier, 1996, supra). However, the present inventive use of non-mammalian cells, particularly bacteria, more particularly lactobacilli, specifically engineered with a foreign gene, more specifically a cyanovirin gene, to express an antiviral substance, more specifically a protein, and even more specifically a cyanovirin, is heretofore unprecedented as a method of treatment of an animal, specifically a human, to prevent infection by a virus, specifically a retrovirus, more specifically HIV-1 or HIV-2. Elmer et al. (JAMA 275, 870-876, 1996) have recently speculated that "genetic engineering offers the possibility of using microbes to deliver specific actions or products to the colon or other mucosal surfaces ... other fertile areas for future study include defining the mechanisms of action of various biotherapeutic agents with the possibility of applying genetic engineering to enhance activities." Elmer et al. (1996, supra) further point out that the terms "probiotic" and "biotherapeutic agent" have been used in the literature to describe microorganisms that have antagonistic activity toward pathogens in vivo; those authors more specifically prefer the term "biotherapeutic agent" to denote "microorganisms having specific therapeutic properties. In view of the present disclosure, one skilled in the art will appreciate that taught herein is an entirely novel type of "probiotic" or "biotherapeutic" treatment using specifically engineered strains of microorganisms that do not occur in nature. Nonetheless, available teachings concerning selection of optimal microbial strains, in particular bacterial strains, for conventional probiotic or biotherapeutic applications can be employed in the context of the present invention. For example, selection of optimal lactobacillus strains for genetic engineering, transformation, direct expression of cyanovirins or conjugates thereof, and direct probiotic or biotherapeutic applications, to treat or prevent HIV infection, can be based upon the same or similar criteria, such as those described by Elmer et al. (1996, supra), typically used to select normal, endogenous or "nonengineered" bacterial strains for conventional probiotic or biotherapeutic therapy. Furthermore, the recommendations and characteristics taught by McGroarty, particularly for selection of optimal lactobacillus strains for conventional probiotic use against female urogenital infections, are pertinent to the present invention: "... lactobacilli chosen for incorporation into probiotic preparations should be easy and, if possible, inexpensive to cultivate ... strains should be stable, retain viability following freeze-drying and, of course, be non-pathogenic to the host ... it is essential that lactobacilli chosen for use in probiotic preparations should adhere well to the vaginal epithelium ... ideally, artificially implanted lactobacilli should adhere to the vaginal epithelium, integrate with the indigenous microorganisms present, and proliferate" (McGroarty, 1993 supra). While McGroarty' s teachings specifically address selections of "normal" lactobacillus strains for probiotic uses against pathogenic bacterial or yeast infections of the female urogenital tract, similar considerations will apply to the selection of optimal bacterial strains for genetic engineering and "probiotic" or "biotherapeutic" application against viral infections as particularly encompassed herein. Accordingly, the method for the prevention of sexual transmission of viral infection, e.g., HIV infection, comprises vaginal, rectal, oral, penile, or other topical, insertional, or instillational treatment with an antiviral effective amount of a cyanovirin and/or cyanovirin conjugate, and/or viable host cells transformed to express a cyanovirin or conjugate thereof, alone or in combination with another antiviral compound (e.g., as described above). The compositions herein for use in the prophylactic or therapeutic treatment methods described herein can comprise one or more cyanovirin(s), conjugate(s) thereof, or host cell(s) transformed to express a cyanovirin or conjugate thereof, and a pharmaceutically acceptable carrier. Pharmaceutically acceptable carriers are well-known to those skilled in the art, as are suitable methods of administration. The choice of carrier will be determined in part by the particular cyanovirin, or conjugate thereof, or host cell(s), as well as by the particular method used to administer the composition. One skilled in the art will appreciate that various routes of administering a drug are available, and, although more than one route may be used to administer a particular drug, a particular route may provide a more immediate and more effective response than ' by another route. Furthermore, one skilled in the art will appreciate that the particular pharmaceutical carrier employed will depend, in part, upon the particular cyanovirin, conjugate thereof, or host cell employed, and the chosen route of administration. Accordingly, there is a wide variety of suitable formulations of the provided composition. Formulations suitable for oral, rectal, or vaginal administration can consist of, for example, (a) liquid solutions or suspensions, such as an effective amount of the pure compound(s), and/or host cell(s) engineered to produce directly a cyanovirin or conjugate thereof, dissolved or suspended in diluents, such as water, culture medium, or saline, (b) capsules, suppositories, sachets, tablets, lozenges, or pastilles, each containing a predetermined amount of the active ingredient(s), as solids, granules, or freeze-dried cells, and (c) oil-in-water emulsions or water-in-oil emulsions. Tablet forms can include one or more of lactose, mannitol, corn starch, potato starch, microcrystalline cellulose, acacia, gelatin, colloidal silicon dioxide, croscarmellose sodium, talc, magnesium stearate, stearic acid, and other excipients, colorants, diluents, buffering agents, moistening agents, preservatives, flavoring agents, and pharmacologically compatible carriers. Lozenges can comprise the active ingredient in a flavor, for example sucrose and acacia or tragacanth, while pastilles can comprise the active ingredient in an inert base, such as gelatin and glycerin, or sucrose and acacia. Suitable formulations for oral or rectal delivery also can be incorporated into synthetic and natural polymeric microspheres, or other means to protect the agents of the present invention from degradation within the gastrointestinal tract (see, for example, Wallace et al., Science 260, 912-915, 1993). Formulations for rectal or vaginal administration can be presented as a suppository with a suitable aqueous or nonaqueous base; the latter can comprise, for example, cocoa butter or a salicylate. Furthermore, formulations suitable for vaginal administration can be presented as pessaries, suppositories, tampons, creams, gels, pastes, foams, or spray formulas containing, in addition to the active ingredient, such as, for example, freeze-dried lactobacilli genetically engineered to directly produce a cyanovirin or conjugate thereof of the present invention, such carriers as are known in the art to be appropriate. Similarly, the active ingredient can be combined with a lubricant as a coating on a condom. The cyanovirins, conjugates thereof, or host cells expressing cyanovirins or conjugates thereof, alone or in combination with other antiviral compounds, can be made into aerosol formulations to be administered via inhalation. These aerosol formulations can be placed into pressurized acceptable propellants, such as dichlorodifluoromethane, propane, nitrogen and the like. The cyanovirins or conjugates thereof, alone or in combinations with other antiviral compounds or absorption modulators, can be made into suitable formulations for dermal application and absorption (Wallace et al., 1993, supra). Transdermal electroporation or iontophoresis also can be used to promote and/or control the systemic delivery of the above-described compounds and/or compositions through the skin (see, e.g., Theiss et al, Meth. Find. Exp. Clin. Pharmacol. 13, 353-359, 1991). Formulations suitable for topical administration include creams, emulsions, gels, and the like containing, in addition to the active ingredient, such carriers as are known in the art, as well as mouthwashes comprising the active ingredient in a suitable liquid carrier. Formulations suitable for parenteral administration include aqueous and nonaqueous, isotonic sterile injection solutions, which can contain anti-oxidants, buffers, bacteriostats, and solutes that render the formulation isotonic with the blood of the intended recipient, and aqueous and nonaqueous sterile suspensions that can include suspending agents, solubilizers, thickening agents, stabilizers, and preservatives. The formulations can be presented in unit-dose or multi-dose sealed containers, such as ampules and vials, and can be stored in a freeze-dried (lyophilized) condition requiring only the addition of the sterile liquid carrier, for example, water, for injections, immediately prior to use. Extemporaneous injection solutions and suspensions can be prepared from sterile powders, granules, and tablets of the kind previously described. Formulations comprising a cyanovirin or cyanovirin conjugate suitable for virucidal (e.g., against HJV) sterilization of inanimate objects, such as medical supplies or equipment, laboratory equipment and supplies, instruments, devices, and the like, can be, for example, selected or adapted as appropriate, by one skilled in the art, from any of the aforementioned compositions or formulations. The cyanovirin or conjugate thereof can be produced by recombinant PNA technology or by chemical coupling of a cyanovirin with an effector molecule as described above. Preferably, the cyanovirin, or conjugate thereof, is produced by recombinant PNA technology. Similarly, formulations of cyanovirins and/or conjugates thereof, suitable for ex vivo virucidal sterilization of blood, blood products, sperm, or other bodily products or tissues, or any other solution, suspension, emulsion, or any other material which can be administered to a patient in a medical procedure, can be selected or adapted as appropriate by one skilled in the art, from any of the aforementioned compositions or formulations. However, suitable formulations for such ex vivo applications or for virucidal treatment of inanimate objects are by no means limited to any of the aforementioned formulations or compositions. One skilled in the art will appreciate that a suitable or appropriate formulation can be selected, adapted, or developed based upon the particular application at hand. For ex vivo uses, such as virucidal treatments of inanimate objects or materials, blood or blood products, or tissues, the amount of cyanovirin, or conjugate or composition thereof, to be employed should be sufficient that any virus or virus- producing cells present will be rendered noninfectious or will be destroyed. For example, for HIV, this would require that the virus and/or the virus-producing cells be exposed to concentrations of cyanovirin-N in the range of 0.1-1000 nM. Similar considerations apply to in vivo applications. Therefore, the phrase "antiviral effective amount" or "virucidal effective amount" is used generally to describe the amount of a particular cyanovirin, conjugate thereof, or composition thereof required for antiviral efficacy in any given application. For in vivo uses, the dose of a cyanovirin, conjugate thereof, host cells producing a cyanovirin or conjugate thereof, or composition thereof, administered to an animal, particularly a human, in the context of the present invention should be sufficient to effect a prophylactic and/or therapeutic response in the individual over a reasonable time-frame. The dose used to achieve a desired virucidal concentration in vivo (e.g., 0.1-1000 nM) will be determined by the potency of the particular cyanovirin or conjugate thereof, or of the cyanovirin and/or conjugate production of the host cells employed, the severity of the disease state of infected individuals, as well as, in the case of systemic administration, the body weight and age of the infected individual. The effective or virucidal dose also will be determined by the existence of any adverse side-effects that may accompany the administration of the particular cyanovirin, conjugate thereof, host cells producing a cyanovirin or conjugate thereof, or composition thereof, employed. It is always desirable, whenever possible, to keep adverse side effects to a minimum. The dosage can be in unit dosage form, such as a tablet or capsule. The term "unit dosage form" as used herein refers to physically discrete units suitable as unitary dosages for human and animal subjects, each unit containing a predetermined quantity of a cyanovirin, conjugate thereof, or amount of host cells producing a cyanovirin or conjugate thereof, alone or in combination with other antiviral agents, calculated in a quantity sufficient to produce the desired effect in association with a pharmaceutically acceptable carrier, diluent, or vehicle. The specifications for the unit dosage forms of the present invention depend on the particular cyanovirin, conjugate, host cells, or composition thereof employed, and the effect to be achieved, as well as the pharmacodynamics associated with each cyanovirin, conjugate, host cells, or composition thereof in the treated animal. The dose administered should be an "antiviral effective amount" or "virucidal effective amount" or an amount necessary to achieve an "effective virucidal level" in the individual animal, e.g., the human patient. Since the "effective virucidal level" is used as the preferred endpoint for dosing, the actual dose and schedule can vary, depending upon interindividual differences in pharmacokinetics, drug distribution, and metabolism. The "effective virucidal level" can be defined, for example, as the blood or tissue level (e.g., 0.1- 1000 nM) desired in the patient that corresponds to a concentration of one or more cyanovirins or conjugates thereof, which inhibits a virus, such as HIV-1 and/or HTV- 2, in an assay known to predict for clinical antiviral activity of chemical compounds and biological agents. The "effective virucidal level" for agents of the present invention also can vary when the cyanovirin, conjugate, or composition thereof, is used in combination with AZT or other known antiviral compounds or combinations thereof. One skilled in the art can easily determine the appropriate dose, schedule, and method of administration for the exact formulation of the composition being used, in order to achieve the desired "effective virucidal level" in the individual patient. One skilled in the art also can readily determine and use an appropriate indicator of the "effector concentration" of the compounds of the present invention by a direct (e.g., analytical chemical analysis) or indirect (e.g., with surrogate indicators such as p24 or RT) analysis of appropriate patient samples (e.g., blood and/or tissues). In the treatment of some virally infected individuals, it may be desirable to utilize a "mega-dosing" regimen, wherein a large dose of a selected cyanovirin or conjugate thereof is administered, and time thereafter is allowed for the drug to act, and then a suitable reagent is administered to the individual to inactivate the drug. The pharmaceutical composition can contain other pharmaceuticals, in conjunction with the cyanovirin, conjugate thereof, or host cells producing a cyanovirin or conjugate thereof, when used to therapeutically treat a viral infection, such as that which causes AIPS. Representative examples of these additional pharmaceuticals include antiviral compounds, virucides, immunomodulators, immunostimulants, antibiotics, and absorption enhancers. Exemplary antiviral compounds include AZT, ddl, ddC, gancylclovir, fluorinated dideoxynucleosides, nonnucleoside analog compounds, such as nevirapine (Shih et al, PNAS 88, 9878- 9882, 1991), TIBO derivatives, such as R82913 (White et al., Antiviral Res. 16, 257- 266, 1991), BI-RJ-70 (Merigan, Am. J. Med. 90 (Supp A), 8S-17S, 1991), michellamines (Boyd et al., J. Med. Chem. 37, 1740-1745, 1994), and calanolides (Kashman et al, J. Med. Chem. 35, 2735-2743, 1992), nonoxynol-9, gossypol and derivatives, and gramicidin (Bourinbair et al, 1994, supra). Exemplary immunomodulators and immunostimulants include various interleukins, sCO4, cytokines, antibody preparations, blood transfusions, and cell transfusions. Exemplary antibiotics include antifungal agents, antibacterial agents, and anti- Pneumocystitis carnii agents. Exemplary absorption enhancers include bile salts and other surfactants, saponins, cyclodextrins, and phospholipids (Pavis, 1992, supra). The administration of a cyanovirin or conjugate thereof with other antiretro viral agents and particularly with known RT inhibitors, such as ddC, AZT, ddl, ddA, or other inhibitors that act against other HIV proteins, such as anti-TAT agents, is expected to inhibit most or all replicative stages of the viral life cycle. The dosages of ddC and AZT used in AIPS or ARC patients have been published. A virustatic range of ddC is generally between 0.05 μM to 1.0 μM. A range of about 0.005-0.25 mg/kg body weight is virustatic in most patients. The preliminary dose ranges for oral administration are somewhat broader, for example 0.001 to 0.25 mg/kg given in one or more doses at intervals of 2, 4, 6, 8, 12, etc. hours. Currently, 0.01 mg/kg body weight ddC given every 8 hrs, is preferred. When given in combined therapy, the other antiviral compound, for example, can be given at the same time as the cyanovirin, or conjugate thereof, or the dosing can be staggered as desired. The different drugs also can be combined in a composition. Poses of each can be less when used in combination than when either is used alone. It also will be appreciated by one skilled in the art that a PNA sequence of a cyanovirin or conjugate thereof can be inserted ex vivo into mammalian cells previously removed from a given animal, in particular a human. Such transformed autologous or homologous host cells, reintroduced into the animal or human, will express directly the corresponding cyanovirin or conjugate in vivo. The feasibility of such a therapeutic strategy to deliver a therapeutic amount of an agent in close proximity to the desired target cells and pathogens (e.g., to the virus, more particularly to the retrovirus, specifically to HIV and its envelope glycoprotein gpl20), has been demonstrated in studies with cells engineered ex vivo to express sCP4 (Morgan et al., 1994, supra). As an alternative to ex vivo insertion of the PNA sequences of the present invention, such sequences can be inserted into cells directly in vivo, such as by use of an appropriate viral or other suitable vector. Such cells transfected in vivo may be expected to produce antiviral amounts of cyanovirin or conjugate thereof directly in vivo. Example 9 illustrates the transformation and expression of a cyanovirin by a mammalian cell. Given the present disclosure, it will be additionally appreciated that a DNA sequence corresponding to a cyanovirin or conjugate thereof can be inserted into suitable nonmammalian host cells, and that such host cells will express therapeutic or prophylactic amounts of a cyanovirin or conjugate thereof directly in vivo within a desired body compartment of an animal, in particular a human. Example 3 illustrates the transformation and expression of effective virucidal amounts of a cyanovirin in a non-mammalian cell, more specifically a bacterial cell. Example 10 illustrates the transformation and expression of a cyanovirin in a non-mammalian cell, specifically a yeast cell. In a preferred embodiment of the present invention, a method of female- controllable prophylaxis against HIV infection comprises the intravaginal administration and/or establishment of, in a female human, a persistent intravaginal population of lactobacilli that have been transformed with a coding sequence of the present invention to produce, over a prolonged time, effective virucidal levels of a cyanovirin or conjugate thereof, directly on or within the vaginal and/or cervical and/or uterine mucosa. It is noteworthy that both the World Health Organization (WHO), as well as the U.S. National Institute of Allergy and Infectious Oiseases, have pointed to the need for development of female-controlled topical microbicides, suitable for blocking the transmission of HIV, as an urgent global priority (Lange et al., Lancet 341, 1356, 1993; Fauci, NIAID News. April 27, 1995). Also provided are antibodies directed to the above-described proteins. More specifically provided by the present invention is an anti-cyanovirin antibody, which has an internal image of gpl20 of a primate immunodeficiency virus. Preferably, the antibody can compete with gpl20 of a primate immunodeficiency virus for binding to a cyanovirin. In this regard, the primate immunodeficiency virus preferably is HIV-1 or HIV-2 and the cyanovirin preferably comprises SEQ IP NO: 2. The availability of antibodies to any given protein is highly advantageous, as it provides the basis for a wide variety of qualitative and quantitative analytical methods, separation and purification methods, and other useful applications directed to the subject proteins. Antibodies, in particular antibodies specifically binding to a protein, i.e., a cyanovirin, such as that of SEQ IP NO: 2, as described above, can be prepared using well-established methodologies (e.g., such as the methodologies described in detail by Harlow and Lane, in Antibodies. A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, 1988, pp. 1-725). Such antibodies can comprise both polyclonal and monoclonal antibodies. Anti-idiotypic antibodies also can be generated in accordance with methods known in the art (see, for example, Benjamin, In Immunology: a short course, Wiley-Liss, NY, 1996, pp. 436-437; Kuby, In Immunology, 3rd. ed., Freeman, NY, 1997, pp. 455-456; Greenspan et al., FASEB _ 7, 437-443, 1993; and Poskitt, Vaccine. 9, 792-796, 1991). Such antibodies can be obtained and employed either in solution-phase or coupled to a desired solid-phase matrix. Having in hand such antibodies, one skilled in the art will further appreciate that such antibodies, using well-established procedures (e.g., such as described by Harlow and Lane (1988, supra), are useful in the detection, quantification, or purification of a cyanovirin, a conjugate thereof, or a host cell transformed to produce a cyanovirin or conjugate thereof. Example 11 further illustrates an antibody that specifically binds to a cyanovirin. Such antibodies are also useful in a method of prevention and treatment of a viral infection in an animal as provided herein. In view of the above, a cyanovirin can be administered to an animal. The animal generates anti-cyanovirin antibodies. Among the anti-cyanovirin antibodies generated or induced in the animal are antibodies that have an internal image of gpl20. In accordance with well-known methods, polyclonal or monoclonal antibodies can be obtained, isolated and selected. Selection of an anti-cyanovirin antibody that has an internal image of gpl20 can be based upon competition between the anti- cyanovirin antibody and gpl20 for binding to a cyanovirin, or upon the ability of the anti-cyanovirin antibody to bind to a free cyanovirin as opposed to a cyanovirin bound to gpl20. Such an anti-cyanovirin antibody can be administered to an animal to prevent or treat a viral infection in accordance with methods provided herein. Although nonhuman anti-idiotypic antibodies, such as an anti-cyanovirin antibody that has an internal image of gpl20 and, therefore, is anti-idiotypic to gpl20, are proving useful as vaccine antigens in humans, their favorable properties might, in certain instances, be further enhanced and/or their adverse properties further diminished, through "humanization" strategies, such as those recently reviewed by Vaughan, Nature Biotech., 16, 535-539, 1998. Alternatively, a cyanovirin can be directly administered to an animal for prevention or treatment of a viral infection in accordance with methods provided herein such that the treated animal, itself, generates an anti-cyanovirin antibody that has an internal image of gp 120. The production of anti-idiotypic antibodies, such as an anti-cyanovirin antibody that has an internal image of gpl20 and, therefore, is anti-idiotypic to gpl20, in an animal to be treated is known as "anti-idiotype induction therapy," and is described by Madiyalakan et al., Hybridoma. 14, 199-203, 1995, for example. In view of the above, the present invention provides a method of preventing or treating a viral infection in an animal. The method comprises administering to the animal an anti-cyanovirin antibody in an amount sufficient to induce in the animal an immune response to a virus sufficient to prevent or treat an infection of the animal with the virus. The anti-cyanovirin antibody has an internal image of gpl20 of a primate immunodeficiency virus. Preferably, the antibody can compete with gpl20 of a primate immunodeficiency virus for binding to a cyanovirin. In this regard, the primate immunodeficiency virus preferably is HIV-1 or HIV-2 and the cyanovirin preferably comprises SEQ IP NO: 2. Also provided by the present invention is another method of preventing or treating a viral infection in an animal. The method comprises administering to the animal a cyanovirin, which binds gpl20 of a primate immunodeficiency virus, in an amount sufficient to induce in the animal an anti-cyanovirin antibody in an amount sufficient to induce an immune response to a virus sufficient to treat or prevent an infection of the animal with the virus. Preferably, the anti-cyanovirin antibody can compete with gpl20 of a primate immunodeficiency virus for binding to a cyanovirin. In this regard, the primate immunodeficiency virus preferably is HIV-1 or HIV-2 and the cyanovirin preferably comprises SEQ IP NO: 2. With respect to the above methods, sufficient amounts can be determined in accordance with methods known in the art. Similarly, the sufficiency of an immune response in the prevention or treatment of a viral infection in an animal also can be assessed in accordance with methods known in the art. Either one of the above methods can further comprise concurrent, pre- or post- treatment with an adjuvant to enhance the immune response (see, for example, Harlow et al., 198%, supra). The above-described nucleic acid sequences, cyanovirins, conjugates, host cells, antibodies, compositions, and methods are further described in the context of the following examples. These examples serve to illustrate further the present invention and are not intended to limit the scope of the invention. Examples Example 1 This example details the anti-HIV bioassay-guided isolation and elucidation of pure cyanovirin from aqueous extracts of the cultured cyanobacterium, Nostoc ellipsosporum. The method described in Weislow et al. (1989, supra) was used to monitor and direct the isolation and purification process. Cyanobacterial culture conditions, media, and classification were as described previously (Patterson, J. Phvcol. 27, 530- 536, 1991). Briefly, the cellular mass from a unialgal strain of Nostoc ellipsosporum (culture Q68P170) was harvested by filtration, freeze-dried, and extracted with MeOH-CH 2 Cl 2 (1:1) followed by H 2 0. Bioassay indicated that only the H,0 extract contained HlV-inhibitory activity. A solution of the aqueous extract (30 mg/ml) was treated by addition of an equal volume of ethanol (EtOH). The resulting 1:1 H 2 0- EtOH solution was kept at -20°C for 15 hrs. Then, the solution was centrifuged to remove precipitated materials (presumably, high molecular weight biopolymers). The resulting HIV-inhibitory supernatant was evaporated, fractionated by reverse-phase vacuum-liquid chromatography (Coll et al., J. Nat. Prod. 49, 934-936, 1986; Pelletier et al., J. Nat. Prod. 49, 892-900, 1986) on wide-pore C 4 packing (300A, BakerBond WP-C 4 ), and eluted with increasing concentrations of methanol (MeOH) in H 2 0. Anti- HIV activity was concentrated in the material eluted with MeOH-H 2 0 (2:1). SOS- PAGE analysis of this fraction showed one main protein band, with a relative molecular mass (Mr) of approximately 10 kPa. Final purification was achieved by repeated reverse-phase HPLC on 1.9 x 15 cm μBondapak C 18 (Waters Associates) columns eluted with a gradient of increasing concentration of acetonitrile in H 2 0. The mobile phase contained 0.05% (v/v) TFA, pH=2. Eluted proteins were detected by UV absorption at 206, 280, and 294 nm with a rapid spectral detector (Pharmacia LKB model 2140). Individual fractions were collected, pooled based on the UV chromatogram, and lyophilized. Pooled HPLC fractions were subjected to SPS- PAGE under reducing conditions (Laemmli, Nature 227, 680-685, 1970), conventional amino acid analysis, and testing for anti-HIV activity. Figure 1A is a graph of OP (206 nm) versus time (min), which shows the μ Bondapak C 18 HPLC chromatogram of nonreduced cyanovirin eluted with a linear CH 3 CN/H 2 0 gradient (buffered with 0.05% TFA) from 28-38% CH 3 CN. Figure IP is a graph of OP (206 nm) versus time (min), which shows the chromatogram of cyanovirin that was first reduced with β-mercaptoethanol and then separated under identical HPLC conditions. HPLC fractions from the two runs were collected as indicated. 10% aliquots of each fraction were lyophilized, made up in 100 μl 3 : 1 H 2 0/PMSO, and assessed for anti-HIV activity in the XTT assay. Figure IB is a bar graph of maximum dilution for 50% protection versus HPLC fraction, which illustrates the maximum dilution of each fraction that provided 50% protection from the cytopathic effects of HIV infection for the nonreduced cyanovirin HPLC fractions. Corresponding anti-HIV results for the HPLC fractions from reduced cyanovirin are shown in Figure IE, which is a bar graph of maximum dilution for 50% protection versus HPLC fraction. 20% aliquots of selected HPLC fractions were analyzed by SPS-PAGE. The results from the nonreduced HPLC fractions are shown in Figure 1C, and those from the reduced HPLC fractions are shown in Figure IF. In the initial HPLC separation, using a linear gradient from 30-50% CH 3 CN, the anti-HIV activity coeluted with the principal UV-absorbing peak at approximately 33% CH 3 CN. Fractions corresponding to the active peak were pooled and split into two aliquots. Re-injection of the first aliquot under similar HPLC conditions, but with a linear gradient from 28-38% CH 3 CN, resolved the active material into two closely eluting peaks at 33.4 and 34.0% CH 3 CN. The anti-HIV activity profile of the fractions collected during this HPLC run (as shown in Figure IB) corresponded with the two UV peaks (as shown in Figure 1 A). SPS-PAGE of fractions collected under the individual peaks showed only a single protein band (as shown in Figure 1C). The second aliquot from the original HPLC separation was reduced with β- mercaptoethanol prior to re-injection on the HPLC. Using an identical 28-38% gradient, the reduced material gave one principal peak (as shown in Figure IP) that eluted later in the run with 36.8% CH 3 CN. Only a trace of anti-HIV activity was detected in the HPLC fractions from the reduced material (as shown in Figure IE). The two closely eluting HPLC peaks of the nonreduced material (Figure 1A) gave only one identical band on SOS-PAGE (run under reducing conditions) (Figure 1 C), and reduction with β-mercaptoethanol resulted in an HPLC peak with a longer retention time than either of the nonreduced peaks (Figure IF). This indicated that disulfides were present in the native protein. Amino acid analysis of the two active peaks showed they had virtually identical compositions. It is possible that the two HPLC peaks resulted from cis/trans isomerism about a proline residue or from microheterogeneity in the protein sample that was not detected in either the amino acid analysis or during sequencing. The material collected as the two FfiV-inhibitory peaks was combined for further analyses and was given the name cyanovirin-N. Example 2 This example illustrates the synthesis of cyanovirin genes. The chemically deduced amino acid sequence of cyanovirin-N was back- translated to obtain a PNA coding sequence. In order to facilitate initial production and purification of recombinant cyanovirin-N, a commercial expression vector (pFLAG-1, from International Biotechnologies, Inc., New Haven, CT), for which reagents were available for affinity purification and detection, was selected. Appropriate restriction sites for ligation to pFLAG-1, and a stop codon, were included in the PNA sequence. Figure 2 is an example of a PNA sequence encoding a synthetic cyanovirin gene. This PNA sequence design couples the cyanovirin-N coding region to codons for a "FLAG" octapeptide at the N-terminal end of cyanovirin, providing for production of a FLAG-cyanovirin fusion protein. A flowchart for synthesis of this PNA sequence is as follows: Design DNA Coding Sequence 327 Synthesize Oligonucleotide Elements bp - Drop Dialysis Against Distilled Purification of Oligos ater/Quantitation <r i Dry 10 nmole of Each Oligo in Resuspend in T4 DNA Ligase Buffer Speedvac -> Heat for 20 min at 65°C to inactivate Treat with T4 Polynucleotide Kinase the Kinase <- for 2 hrs at 37°C Pool Oligos and Boil for 10 min Anneal for 20 min at 70°C Add T4 DNA Ligase and Additional Buffer and Incubate at 16°C <r Ovemisht I Phenol:Chloroform Extract/EtOH Dissolve in Distilled Water Precipitate Run Low-Melting Agarose Gel Preparative PCR ^ Excise 327 bp Band Recover/Purify Dissolve DNA -> The PNA sequence was synthesized as 13 overlapping, complementary oligonucleotides and assembled to form the double-stranded coding sequence. Oligonucleotide elements of the synthetic PNA coding sequence were synthesized using a dual-column nucleic acid synthesizer (Model 392, Applied Biosystems Inc., Foster City, CA). Completed oligonucleotides were cleaved from the columns and deprotected by incubation overnight at 56°C in concentrated ammonium hydroxide. Prior to treatment with T4 polynucleotide kinase, 33-66 mers were drop-dialyzed against distilled water. The 13 oligonucleotide preparations were individually purified by HPLC, and 10 nmole quantities of each were ligated with T4 PNA ligase into a 327 bp double-stranded PNA sequence. PNA was recovered and purified from the reaction buffer by pheno chloroform extraction, ethanol precipitation, and further washing with ethanol. Individual oligonucleotide preparations were pooled and boiled for 10 min to ensure denaturation. The temperature of the mixture was then reduced to 70°C for annealing of the complementary strands. After 20 min, the tube was cooled on ice and 2,000 units of T4 PNA ligase were added together with additional ligase buffer. Ligation was performed overnight at 16°C. PNA was recovered and purified from the ligation reaction mixture by phenol: chloroform extraction and ethanol precipitation and washing. The purified, double-stranded synthetic PNA was then used as a template in a polymerase chain reaction (PCR). One μl of the PNA solution obtained after purification of the ligation reaction mixture was used as a template. Thermal cycling was performed using a Perkin-Elmer instrument. "Vent" thermostable PNA polymerase, restriction enzymes, T4 PNA ligase, and polynucleotide kinase were obtained from New England Biolabs, Beverly, MA. Vent polymerase was selected for this application because of its claimed superiority in fidelity compared to the usual Taq enzyme. The PCR reaction product was run on a 2% agarose gel in TBE buffer. The 327 bp construct was then cut from the gel and purified by electroelution. Because it was found to be relatively resistant to digestion with Hind III and Xho I restriction enzymes, it was initially cloned using the pCR-Script system (Stratagene). Pigestion of a plasmid preparation from one of these clones yielded the coding sequence, which was then ligated into the multicloning site of the pFLAG-1 vector. E. coli were transformed with the pFLAG-construct, and recombinant clones were identified by analysis of restriction digests of plasmid PNA. Sequence analysis of one of these selected clones indicated that four bases deviated from the intended coding sequence. This included deletion of three bases coding for one of four cysteine residues contained in the protein and an alteration of the third base in the preceding codon (indicated by the boxes in Figure 2). In order to correct these "mutations," which presumably arose during the PCR amplification of the synthetic template, a double-stranded "patch" was synthesized, which could be ligated into restriction sites flanking the mutations (these Bst XI and Espl sites are also indicated in Figure 2). The patch was applied and the repair was confirmed by PNA sequence analysis. For preparation of a PNA sequence coding for native cyanovirin, the aforementioned FLAG-cyanovirin construct was subjected to site-directed mutagenesis to eliminate the codons for the FLAG octapeptide and, at the same time, to eliminate a unique Hind III restriction site. This procedure is illustrated in Figure 3, which illustrates a site-directed mutagenesis maneuver used to eliminate codons for a FLAG octapeptide and a Hind III restriction site from the sequence of Figure 2. A mutagenic oligonucleotide primer was synthesized, which included portions of the codons for the Omp secretory peptide and cyanovirin, but lacked the codons for the FLAG peptide. Annealing of this mutagenic primer, with creation of a PNA hairpin in the template strand, and extension by PNA polymerase resulted in the generation of a new plasmid PNA lacking both the FLAG codon sequence and the Hind III site (refer to Figure 2 for details). The digestion of the plasmid PNA with Hind III resulted in linearization of "wild-type" strands but not "mutant" strands. Since transformation of E. coli occurs more efficiently with circular PNA, clones could be readily selected which had the revised coding sequence which specified production of native cyanovirin-N directly behind the Omp secretory peptide. PNA sequencing verified the presence of the intended sequence. Site-directed mutagenesis reactions were carried out using materials (polymerase, buffers, etc.) obtained from Pharmacia Biotech, Inc., Piscataway, NJ. Example 3 This example illustrates the expression of synthetic cyanovirin genes. As indicated in the following flowchart: Recover/Purify/Pissolve PCR Product I Blunt-End Clone (Stratagene Kit) I Plasmid Preparation on Selected Clone I Pigest with Hind III and Xho I and Ligate to pFLAG-1 , Transform E. coli, Isolate, and Identify Recombinant Clones (Experimental and BAP Control) I Seed Small-Scale Shake Cultures i Induce Express with IPTG I Prepare Crude Periplasmic Extract I Anti-HIV Bioassay and FLAG Pot-Blot E. coli (strain PH5α) were transformed (by electroporation) with the pFLAG- 1 vector containing the coding sequence for the FLAG-cyanovirin-N fusion protein (see Figure 2 for details of the PNA sequence). Selected clones were seeded into small-scale shake flasks containing (LB) growth medium with 100 μg/ml ampicillin and expanded by incubation at 37°C. Larger-scale Erlenmeyer flasks (0.5-3.0 liters) were then seeded and allowed to grow to a density of 0.5-0.7 OO 600 units. The expression of the FLAG-cyanovirin-N fusion protein was then induced by adding IPTG to a final concentration of 1.7 mM and continuing incubation at 30°C for 3-6 hrs. For harvesting of periplasmic proteins, bacteria were pelleted, washed, and then osmotically shocked by treatment with sucrose, followed by resuspension in distilled water. Periplasmic proteins were obtained by sedimenting the bacteria and then filtering the aqueous supernatant through Whatman paper. Crude periplasmic extracts showed both anti-HIV activity and presence of a FLAG-cyanovirin-N fusion protein by Western or spot-blotting. The construct for native cyanovirin-N described in Example 2 was used to transform bacteria in the same manner as described above for the FLAG-cyanovirin-N fusion protein. Cloning, expansion, induction with IPTG, and harvesting were performed similarly. Crude periplasmic extracts showed strong anti-HIV activity on bioassay. Example 4 This example illustrates purification of recombinant cyanovirin proteins. Using an affinity column based on an anti-FLAG monoclonal antibody (International Biotechnologies, Inc., New Haven, CT), FLAG-cyanovirin-N fusion protein could be purified as follows: Periplasmic Extract of IPTG Induced Cultures I Load onto Affinity Column I Wash Through E. coli Proteins , Elute Bound Fusion Protein with EPTA , Pialyze Against Water and Lyophilize The respective periplasmic extract, prepared as described in Example 3, was loaded onto 2-20 ml gravity columns containing affinity matrix and washed extensively with PBS containing CA " " to remove contaminating proteins. Since the binding of the FLAG peptide to the antibody is Ca^-dependent, fusion protein could be eluted by passage of EPTA through the column. Column fractions and wash volumes were momtored by spot-blot analysis using the same anti-FLAG antibody. Fractions containing fusion protein were then pooled, dialyzed extensively against distilled water, and lyophilized. For the purification of the recombinant native cyanovirin-N, the corresponding periplasmic extract from Example 3 was subjected to step-gradient C 4 reverse-phase, vacuum-liquid chromatography to give three fractions: (1) eluted with 100% H 2 0, (2) eluted with MeOH-H 2 0 (2:1), and (3) eluted with 100% MeOH. The anti-HIV activity was concentrated in fraction (2). Purification of the recombinant cyanovirin-N was performed by HPLC on a 1.9x15 cm μBondapak (Waters Associates) C 18 column eluted with a gradient of increasing concentration of CH 3 CN in H 2 0 (0.05% TFA, v/v in the mobile phase). A chromatogram of the final HPLC purification on a 1x10 cm (Cohensive Technologies, Inc.) C column monitored at 280 nm is shown in Figure 4, which is typical HPLC chromatogram during the purification of a recombinant native cyanovirin. Gradient elution, 5 ml/min, from 100% H 2 0 to H 2 0-CH 3 CN (7:3) was carried out over 23 min with 0.05% TFA (v/v) in the mobile phase. Example 5 This example illustrates the anti-HIV activities of natural and recombinant cyanovirin-N and FLAG-cyanovirin-N. Pure proteins were initially evaluated for antiviral activity using an XTT- tetrazolium anti-HIV assay described previously (Boyd, in AIPS, Etiology, Piagnosis. Treatment and Prevention. 1988, supra; Gustafson et al., J. Med. Chem. 35, 1978-1986, 1992; Weislow, 1989, supra: Gulakowski, 1991, supra). The CEM-SS human lymphocytic target cell line used in all assays was maintained in RPMI 1650 medium (Gibco, Grand Island, NY), without phenol red, and was supplemented with 5% fetal bovine serum, 2 mM L-glutamine, and 50 μg/ml gentamicin (complete medium). Exponentially growing cells were pelleted and resuspended at a concentration of 2.0x10 5 cells/ml in complete medium. The Haitian variant of HIV, HTLV-III RF (3.54x10 6 SFU/ml), was used throughout. Frozen virus stock solutions were thawed immediately before use and resuspended in complete medium to yield 1.2xl2 5 SFU/ml. The appropriate amounts of the pure proteins for anti-HIV evaluations were dissolved in H 2 0-PMSO (3:1), then diluted in complete medium to the desired initial concentration. All serial drug dilutions, reagent additions, and plate-to-plate transfers were carried out with an automated Biomek 1000 Workstation (Beckman Instruments, Palo Alto; C A). Figures 5A-5C are graphs of % control versus concentration (nM), which illustrate antiviral activities of native cyanovirin from Nostoc ellipsosporum (A), recombinant native (B), and recombinant FLAG-fusion cyanovirins. The graphs show the effects of a range of concentrations of the respective cyanovirins upon CEM-SS cells infected with HIV-1 (•), as determined after 6 days in culture. Pata points represent the percent of the respective uninfected, nondrug-treated control values. All three cyanovirins showed potent anti-HIV activity, with an EC 50 in the low nanomolar range and no significant evidence of direct cytotoxicity to the host cells at the highest tested concentrations (up to 1.2 μM). As an example of a further demonstration of the anti-HIV activity of pure cyanovirin-N, a battery of interrelated anti-HIV assays was performed in individual wells of 96-well microtiter plates, using methods described in detail elsewhere (Gulakowski, 1991, supra). Briefly, the procedure was as follows. Cyanovirin solutions were serially diluted in complete medium and added to 96-well test plates. Uninfected CEM-SS cells were plated at a density of lxl 0 4 cells in 50 μl of complete medium. Piluted HIV-1 was then added to appropriate wells in a volume of 50 μl to yield a multiplicity of infection of 0.6. Appropriate cell, virus, and drug controls were incorporated in each experiment. The final volume in each microtiter well was 200 μl. Quadruplicate wells were used for virus-irifected cells. Plates were incubated at 37°C in an atmosphere containing 5% CO 2 for 4, 5, or 6 days. Subsequently, aliquots of cell-free supernatant were removed from each well using the Biomek, and analyzed for reverse transcriptase activity, p24 antigen production, and synthesis of infectious virions as described (Gulakowski, 1991, supra). Cellular growth or viability then was estimated on the remaining contents of each well using the XTT (Weislow et al., 1989, supra). BCECF (Rink et al., J. Cell Biol. 95, 189-196, 1982) and OAPI (McCaffrey et al, In vitro Cell Pevelop. Biol. 24, 247-252, 1988) assays as described (Gulakowski et al., 1991, supra). To facilitate graphical displays and comparisons of data, the individual experimental assay results (of at least quadruplicate determinations of each) were averaged, and the mean values were used to calculate percentages in reference to the appropriate controls. Standard errors of the mean values used in these calculations typically averaged less than 10% of the respective mean values. Figures 6A-6P are graphs of % control versus concentration (nM), which illustrate anti-HIV activity of a cyanovirin in a multiparameter assay format. Graphs 6A, 6B, and 6C show the effects of a range of concentrations of cyanovirin upon uninfected CEM-SS cells (O), and upon CEM-SS cells infected with HIV-1 (•), as determined after 6 days in culture. Graph 6A depicts the relative numbers of viable CEM-SS cells, as assessed by the BCECF assay. Graph 6B depicts the relative PNA contents of the respective cultures. Graph 6C depicts the relative numbers of viable CEM-SS cells, as assessed by the XTT assay. Graph 6P shows the effects of a range of concentrations of cyanovirin upon indices of infectious virus or viral replication as determined after 4 days in culture. These indices include viral reverse transcriptase (A), viral core protein p24 (♦), and syncytium-forming units (■). In graphs 6A, 6B, and 6C, the data are represented as the percent of the uninfected, nondrug-treated control values. In graph 6P the data are represented as the percent of the infected, nondrug-treated control values. As illustrated in Figure 6, cyanovirin-N was capable of complete inhibition of the cytopathic effects of HIV-1 upon CEM-SS human lymphoblastoid target cells in vitro; direct cytotoxicity of the protein upon the target cells was not observed at the highest tested concentrations. Cyanovirin-N also strikingly inhibited the production of RT, p24, and SFU in HIV-1 -infected CEM-SS cells within these same inhibitory effective concentrations, indicating that the protein halted viral replication. The anti-HIV activity of the cyanovirins is extremely resilient to harsh environmental challenges. For example, unbuffered cyanovirin-N solutions withstood repeated freeze-thaw cycles or dissolution in organic solvents (up to 100% PMSO, MeOH, or CH 3 CN) with no loss of activity. Cyanovirin-N tolerated detergent (0.1% SOS), high salt (6 M guanidine HC1), and heat treatment (boiling, 10 min in H 2 0) with no significant loss of HIV-inhibitory activity. Reduction of the disulfides with β- mercaptoethanol, followed immediately by C 18 HPLC purification, drastically reduced the cytoprotective activity of cyanovirin-N. However, solutions of reduced cyanovirin-N regained anti-HIV inhibitory activity during prolonged storage. When cyanovirin-N was reduced (β-mercaptoethanol, 6 M guanidine HC1, pH 8.0) but not put through C ]8 HPLC, and, instead, simply desalted, reconstituted, and assayed, it retained virtually full activity. Example 6 This example illustrates that the HIV viral envelope gp 120 is a principal molecular target of cyanovirin-N. Initial experiments, employing the XTT-tetrazolium assay (Weislow et al., 1989, supra), revealed that host cells preincubated with cyanovirin (10 nM, 1 hr), then centrifuged free of cyanovirin-N, retained normal susceptibility to HIV infection; in contrast, the infectivity of concentrated virus similarly pretreated, then diluted to yield non-inhibitory concentrations of cyanovirin-N, was essentially abolished. This indicated that cyanovirin-N was acting directly upon the virus itself, i.e., acting as a direct "virucidal" agent to prevent viral infectivity even before it could enter the host cells. This was fA-rther confirmed in time-of-addition experiments, likewise employing the XTT-tetrazolium assay (Weislow et al., 1989, supra), which showed that, to afford maximum antiviral activity, cyanovirin-N had to be added to cells before or as soon as possible after addition of virus as shown in Figure 7, which is a graph of % uninfected control versus time of addition (hrs), which shows results of time-of-addition studies of a cyanovirin, showing anti-HIV activity in CEM-SS cells infected with HIV-l^. Introduction of cyanovirin (•) or ddC (■) (10 nM and 5 μM concentrations, respectively) was delayed by various times after initial incubation, followed by 6 days incubation, then assay of cellular viability (linegraphs) and RT (open bars, inset). Points represent averages (±S.P.) of at least triplicate determinations. In marked contrast to the reverse transcriptase inhibitor ddC, delay of addition of cyanovirin-N by only 3 hrs resulted in little or no antiviral activity (Figure 7). The aforementioned results suggested that cyanovirin-N inhibited FfiV-infectivity by interruption of the initial interaction of the virus with the cell; this would, therefore, likely involve a direct interaction of cyanovirin-N with the viral gpl20. This was confirmed by ultrafiltration experiments and dot-blot assays. Ultrafiltration experiments were performed to determine if soluble gpl20 and cyanovirin-N could bind directly, as assessed by inhibition of passage of cyanovirin-N through a 50 kPa-cutoff ultrafilter. Solutions of cyanovirin (30 μg) in PBS were treated with various concentrations of gpl20 for 1 hr at 37°C, then filtered through a 50 kPa-cutoff centrifugal ultrafilter (Amicon). After washing 3 times with PBS, filtrates were desalted with 3 kPa ultrafilters; retentates were lyophilized, reconstituted in 100 μl H 2 0, and assayed for anti-HIV activity. Figure 8A is a graph of OP (450 nm) versus cyanovirin concentration (μg/ml), which illustrates cyanovirin/gpl20 interactions defining gpl20 as a principal molecular target of cyanovirin. Free cyanovirin-N was readily eluted, as evidenced by complete recovery of cyanovirin-N bioactivity in the filtrate. In contrast, filtrates from cyanovirin-N solutions treated with gpl20 revealed a concentration-dependent loss of filtrate bioactivity; moreover, the 50 kPa filter retentates were all inactive, indicating that cyanovirin-N and soluble gpl20 interacted directly to form a complex incapable of binding to gpl20 of intact virus. There was further evidence of a direct interaction of cyanovirin-N and gpl20 in a PVPF membrane dot-blot assay. A PVPF membrane was spotted with 5 μg CP4 (CP), 10 μg aprotinin (AP), 10 μg bovine globulin (BG), and decreasing amounts of cyanovirin: 6 μg [1], 3 μg [2], 1.5 μg [3], 0.75 μg [4], 0.38 μg [5], 0.19 μg [6], 0.09 μg [7], and 0.05 μg [8], then washed with PBST and visualized per the manufacturer's recommendations. Figure 8B is a dot blot of binding of cyanovirin and a gpl20-HRP conjugate (Invitrogen), which shows that cyanovirin-N specifically bound a horseradish peroxidase conjugate of gpl20 (gpl20-HRP) in a concentration- dependent manner. Example 7 This example further illustrates the extraordinarily broad range of antiretroviral activity against diverse lab-adapted and clinical strains of human and nonhuman primate immunodeficiency retroviruses. Table 1 below shows the comparative ranges of anti-immunodeficiency virus activities of cyanovirin-N and sCP4 tested against a wide range of virus strains in diverse host cells. Particularly noteworthy is the similar potency of cyanovirin-N against both lab-adapted strains as well as clinical isolates of HIV. This was in sharp contrast to the lack of activity of sCP4 against the clinical isolates. The EC 50 values (Table 1) were determined from concentration-response curves from eight dilutions of the test agents (averages from triplicate wells per concentration); G910-6 is an AZT-resistant strain; A17 is a pyridinone-resistant strain; HIV-1 Ba-L was tested in human peripheral blood macrophage (PBM) cultures by supernatant reverse transcriptase activity; all other assays employed XTT-tetrazolium (Gulakowski et al., 1991, supra). Further details of virus strains, cell lines, clinical isolates, and assay procedures are published (Buckheit et al., AIPS Res. Hum. Retrovir. 10, 1497-1506, 1994; Buckheit et al, Antiviral Res. 25, 43-56, 1994; and references contained therein). In Table 1, N.P.=not determined. TABLE 1. Comparative Ranges of Antiviral Activity of CV-N and sCD4 EC 50 (nM) a Virus Target Cells Cyanovirin-N sCD4 HIV-1 Laboratory Strains RF CEM-SS 0,5 0.8 RF U937 0.5 0.1 IIIB CEM-SS 0.4 1.6 IIIB MT-2 0.4 13 MN MT-2 2.3 N.D G-910-6 MT-2 5.8 N.D A17 MT-2 0.8 13 HIV-1 Promonocvtotrorjic Isolates 2 21144 C CEEMM--SSSS 0.4 N.D. SKI CEM-SS 4.8 N.D. HIV-1 LymDhotropic Isolates 2 20055 C CEEMM--SSSS 0.8 N.D. GI CEM-SS 0.9 N.D. HIV-1 Clinical Isolates WEJO PBL 6.7 >100 VIHU PBL 5.5 >100 BAKI PBL 1.5 >100 WOMEPBL 4.3 >100 HIV-2 ROD CEM-SS 7.6 >200 MS CEM-SS 2.3 N.D. SIV Delta B670 174xCEM 11 3.0 The inactivating activity of cyanovirin is conserved across an extraordinarily wide range of strains and isolates of HIV. Therefore, the epitope(s) of gpl20 and/or other envelope component(s) of HIV that are uniquely targeted by cyanovirin must be highly conserved across diverse strains and isolates of HIV. Example 8 This example further illustrates the construction of a conjugate PNA coding sequence, and expression thereof, to provide a cyanovirin-toxin protein conjugate that selectively targets and kills HIV-infected cells. More specifically, this example illustrates construction and expression of a conjugate PNA coding sequence for a cyanovirin/i > 5ew^0772ona^-exotoxin which selectively kills viral gpl20-expressing host cells. A PNA sequence (SEQ IP NO:3) coding for FLAG-cyanovirin-N and a PNA sequence coding for the PE38 fragment of Pseudomonas exotoxin (Kreitman et al., Blood 83, 426-434, 1994) were combined in the pFLAG-1 expression vector. The PE38 coding sequence was excised from a plasmid, adapted, and ligated to the C- terminal position of the FLAG-cyanovirin-N coding sequence using standard recombinant PNA procedures. This construct is illustrated schematically in Figure 9. Transformation of E. coli with this construct, selection of clones, and induction of gene expression with IPTG resulted in production of a conjugate protein with the expected molecular weight and immunoreactivity on western-blot analysis using an anti-FLAG antibody. The chimeric molecule was purified by FLAG-affmity chromatography (e.g., as in Example 4) and evaluated for toxicity to human lymphoblastoid cells infected with HTV (H9/IIIB cells) as well as their uninfected counterparts (H9 and CEM-SS cells). Cells were plated in 96-well microtitre plates and exposed to various concentrations of the conjugate protein (named PPE). After three days, viability was assessed using the XTT assay (Gulakowski et al., 1991, supra). Figure 10 illustrates the results of this testing. As anticipated, the infected H9/IIIB cells expressing cell-surface gpl20 were dramatically more sensitive to the toxic effects of PPE than were the uninfected H9 or CEM-SS cells. The IC50 values determined from the concentration-effect curves were 0.014 nM for H9/IIIB compared to 0.48 and 0.42 nM for H9 and CEM-SS, respectively. Example 9 This example illustrates transformation of a mammalian cell to express a cyanovirin therein. A genetic construct suitable for demonstration of expression of a cyanovirin in mammalian cells was prepared by ligating a PNA sequence coding for FLAG-cyanovirin-N into the pFLAG CMV-1 expression vector (IBI-Kodak, Rochester, NY). The FLAG-cyanovirin-N coding sequence (SEQ IP NO:3) was excised from a previously constructed plasmid and ligated to the pFLAG CMV-1 vector using standard recombinant PNA procedures. African green monkey cells (COS-7 cells, obtained from the American Type Culture Collection, Rockville, MD) were transformed by exposure to the construct in OEAE dextran solution. To assess expression of FLAG-cyanovirin-N, cells were lysed after 72 hours and subjected to PAGE and western-blot analysis. As illustrated in Figure 11, anti-FLAG immunoreactive material was readily detected in transformed COS-7 cells, albeit at an apparent molecular weight substantially greater than native recombinant FLAG- cyanovirin-N produced in E. coli. Oiagnostic analyses of digests, performed in the same manner as in Example 10 which follows, indicated that this increased molecular weight was due to post-translational modification (N-linked oligosaccharides) of the FLAG-cyanovirin-N. Example 10 This example illustrates transformation and expression of a cyanovirin in a non-mammalian cell, more specifically a yeast cell. A genetic construct suitable for demonstration of expression of a cyanovirin in Pichia pastoris was prepared by ligating a PNA sequence coding for cyanovirin-N into the pPIC9 expression vector (Invitrogen Corporation, San Piego, CA). The cyanovirin-N coding sequence (SEQ IP NO:l) was excised from a previously constructed plasmid and ligated to the vector using standard recombinant PNA procedures. Yeast cells were transformed by electroporation and clones were selected for characterization. Several clones were found to express, and to secrete into the culture medium, material reactive with anti-cyanovirin-N polyclonal antibodies (see, e.g., Example 11). Similar to the observations with the mammalian forms described in Example 9, the elevated apparent molecular weight of the yeast-derived product on PAGE and western-blot analysis, suggested that post-translational modification of the cyanovirin- N was occurring in this expression system. To further define this modification, the secreted products from two clones were digested with peptide-N4-(N-acetyl-β-glucosaminyl) asparagine amidase. This enzyme, obtained from New England Biolabs (Beverly, MA), specifically cleaves oligosaccharide moieties attached to asparagine residues. As illustrated in Figure 12, this treatment reduced the apparent molecular weight of the product to that equivalent to native recombinant cyanovirin-N expressed in E. coli. Inspection of the amino acid sequence of cyanovirin revealed a single recognition motif for N-linked modification (linkage to the asparagine located at position 30). To further establish this as the site of glycosylation, a mutation was introduced at this position to change the asparagine residue to glutamine (N30Q). Expression of this mutant form resulted in production of immunoreactive material with a molecular weight consistent with that of native recombinant FLAG-cyanovirin-N. Example 11 This example further illustrates an antibody specifically binding to a cyanovirin. Three 2-month old New Zealand White rabbits (1.8-2.2 kg) were subjected to an immunization protocol as follows: A total of 100 μg of cyanovirin-N was dissolved in 100 μl of a 1:1 suspension of phosphate-buffered saline (PBS) and Freunds incomplete adjuvant and administered by intramuscular injection at 2 sites on each hind leg; 8-16 months from the initial injection, a final boost of 50 μg of cyanovirin-N per rabbit was dissolved in 1000 μl of a 1 :1 suspension of PBS and Freunds incomplete adjuvant and administered by intraperitoneal injection; on days 42, 70, 98 and 122, 10 ml of blood was removed from an ear vein of each rabbit; 14 days after the last intraperitoneal boost, the rabbits were sacrificed and bled out. The IgG fraction of the resultant immune sera from the above rabbits was isolated by protein-A Sepharose affinity chromatography according to the method of Goudswaard et al. (Scand. J. Immunol. 8, 21-28, 1978). The reactivity of this polyclonal antibody preparation for cyanovirin-N was demonstrated by western-blot analysis using a 1 :1000 to 1 :5000 dilution of the rabbit IgG fractions. Figure 13 further illustrates that the antibody prepared according to the aforementioned procedure is an antibody specifically binding to a protein of the present invention. SPS-PAGE of a whole-cell lysate, from E. coli strain PH5α engineered to produce cyanovirin-N, was carried out using 18% poly aery lamide resolving gels and standard discontinuous buffer systems according to Laemmeli (Nature 227, 680-685, 1970). Proteins were visualized by staining with Coomassie brilliant blue (Figure 13 A). For western-blot analyses, proteins were electroeluted from the SPS-PAGE gel onto a nitrocellulose membrane. Non-specific binding sites on the membrane were blocked by washing in a 1% solution of bovine serum albumin (BSA). The membrane was then incubated in a solution of the IgG fraction from the aforementioned rabbit anti-cyanovirin-N immune serum diluted 1 :3000 with phosphate buffered saline (PBS). Subsequently, the membrane was incubated in a secondary antibody solution containing goat-antirabbit-peroxidase conjugate (Sigma) diluted 1 : 10000. The bound secondary antibody complex was visualized by incubating the membrane in a chemiluminescence substrate and then exposing it to x- ray film (Figure 13B). One skilled in the art additionally will appreciate that, likewise by well- established, routine procedures (e.g., see Harlow and Lane, 1988, supra), monoclonal antibodies may be prepared using as the antigen a protein of the present invention, and that such a resulting monoclonal antibody likewise can be shown to be an antibody specifically binding a protein of the present invention. All of the references cited herein are hereby incorporated in their entireties by reference. While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations of the preferred proteins, conjugates, host cells, compositions, methods, and the like can be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the following claims.

Claims: WHAT IS CLAIMED IS: 1. An anti-cyanovirin antibody, wherein said antibody has an internal image of gp 120 of an immunodeficiency virus. 2. The anti-cyanovirin antibody of claim 1, wherein said antibody can compete with gp 120 of an immunodeficiency virus for binding to a cyanovirin. 3. The anti-cyanovirin antibody of claim 2, wherein said immunodeficiency virus is HIV-1 or HIV-2. 4. The anti-cyanovirin antibody of claim 2, wherein said cyanovirin comprises SEQ ID NO: 2. 5. A method of inhibiting a viral infection in an animal, which method comprises administering to said animal an anti-cyanovirin antibody of any of claims 1-4 in an amount sufficient to induce in said animal an immune response to a virus sufficient to inhibit an infection of said animal with said virus. 6. A method of inhibiting a viral infection in an animal, which method comprises administering to said animal a cyanovirin, which binds gp 120 of an immunodeficiency virus, in an amount sufficient to induce in said animal an anti- cyanovirin antibody in an amount sufficient to induce an immune response to a virus sufficient to inhibit an infection of said animal with said virus. 7. The method of claim 6, wherein said anti-cyanovirin antibody can compete with gp 120 of an immunodeficiency virus for binding to a cyanovirin. 8. The method of claim 7, wherein said immunodeficiency virus is HIV-1 or HIV-2. 9. The method of claim 7, wherein said cyanovirin comprises SEQ ID NO: 2. 10. A method of selecting an anti-cyanovirin antibody that has an internal image of gpl20 of an immunodeficiency virus, which method comprises: (a) contacting a sample of anti-cyanovirin antibodies with gpl20 and cyanovirin, and (b) selecting anti-cyanovirin antibodies that compete with gpl20 for binding to cyanovirin. 11. A method of selecting an anti-cyanovirin antibody that has an internal image of gp 120 of an immunodeficiency virus, which method comprises: (a) contacting a sample of anti-cyanovirin antibodies with cyanovirin and cyanovirin to which is bound gpl20; and (b) selecting anti-cyanovirin antibodies that bind to cyanovirin as opposed to cyanovirin to which is bound gpl20. 12. The method of claim 10 or 11, wherein said immunodeficiency virus is HIV-1 or HIV-2. 13. The method of any of claims 10-12, wherein said cyanovirin comprises SEQ ID NO: 2.

Inventors: BOYD MICHAEL R, BOYD, MICHAEL, R.
Applicants: BOYD MICHAEL R, US HEALTH, BOYD, MICHAEL, R., THE GOVERNMENT OF THE UNITED STATES OF AMERICA, REPRESENTED BY THE SECRETARY, DEPARTMENT OF HEALTH AND HUMAN SERVICES
Application year: 1999
Main classification: C07K16
IPC codes: A61K38, A61K39, A61P31, C07K14, C07K16, C12P21, C12R1, G01N33