MODIFIED HIV ENV POLYPEPTIDES

Abstract: Polynucleotide encoding modified HIV Env polypeptides are disclosed. The Env polypeptides are modified so as to expose at least part of the CD4 binding region. Methods of diagnosis, treatment and prevention using the polynucleotides and polypeptides are also provided.

Description: MODIFIED HIV ENV POLYPEPTIDES Technical Field The invention relates generally to modified HIV envelope (Env) polypeptides which are useful as immunizing agents or for generating an immune response in a subject, for example a cellular immune response or a protective immune response. More particularly, the invention relates Env polypeptides such as gpl20, gpl40 or gpl60, wherein at least one of the native β-sheet configurations has been modified. The invention also pertains to methods of using these polypeptides to elicit an immune response against a broad range of HIV subtypes. Background of the Invention The human immunodeficiency virus (HIV-1, also referred to as HTLV-III, LAV or HTLV-III/LAV) is the etiological agent of the acquired immune deficiency syndrome (AIDS) and related disorders, (see, e.g., Barre-Sinoussi, et al., (1983) Science 220:868-871; Gallo et al. (1984) Science 224:500-503; Levy et al., (1984) Science 225:840-842; Siegal et al., (1981) N. Engl. J. Med. 305:1439-1444). AIDS patients usually have a long asymptomatic period followed by the progressive degeneration of the immune system and the central nervous system. Replication of the virus is highly regulated, and both latent and lytic infection of the CD4 positive helper subset of T-lymphocytes occur in tissue culture (Zagury et al., (1986) Science 231:850-853). Molecular studies of HIV-1 show that it encodes a number of genes (Ratner et al., (1985) Nαtwre 313:277-284; Sanchez-Pescador et al., (1985) Science 227:484- 492), including three structural genes — gag, pol and env — that are common to all retroviruses. Nucleotide sequences from viral genomes of other retroviruses, particularly HIV-2 and simian immunodeficiency viruses, SIV (previously referred to as STLV-III), also contain these structural genes. (Guyader et al., (1987) Nature 326:662-669; Chakrabarti et al., (1981) Nature The envelope protein of HIV-1, HIV-2 and SIV is a glycoprotein of about 160 kd (gpl60). During virus infection of the host cell, gpl60 is cleaved by host cell proteases to form gpl20 and the integral membrane protein, gp41. The gp41 portion is anchored in the membrane bilayer of virion, while the gpl20 segment protrudes into the surrounding environment. gpl20 and gp41 are more covalently associated and free gpl20 can be released from the surface of virions and infected cells. As depicted in Figure 1, crystallography studies of the g l20 core polypeptide indicate that this polypeptide is folded into two major domains having certain emanating structures. The inner domain (inner with respect to the N and C terminus) features a two- helix, two-stranded bundle with a small five-stranded β-sandwich at its termini-proximal end and a projection at the distal end from which the VI /V2 stem emanates. The outer domain is a staked double barrel that lies along side the inner domain so that the outer barrel and inner bundle axes are approximately parallel. Between the distal inner domain and the distal outer domain is a four-stranded bridging sheet which holds a peculiar minidomain in contact with, but distinct from, the inner, the outer domain, and the VI /N2 domain. The bridging sheet is composed of four β-strand structures (β-3, β-2, β-21 , β-20, shown in Figure 1). The bridging region can be seen in Figure 1 packing primarily over the inner domain, although some surface residues of the outer domain, such as Phe 382, reach into the bridging sheet to form part of its hydrophobic core. The basic unit of the β-sheet conformation of the bridging sheet region is the β-strand which exists as a less tightly coiled helix, with 2.0 residues per turn. The β-strand conformation is only stable when incorporated into a β-sheet, where hydrogen bonds with close to optimal geometry are formed between the peptide groups on adjacent β-strands; the dipole moments of the strands are also aligned favorably. Side chains from adjacent residues of the same strand protrude from opposite sides of the sheet and do not interact with each other, but have significant interactions with their backbone and with the side chains of neighboring strands. For a general description of β-sheets, see, e.g., T.E. Creighton, Proteins: Structures and Molecular Properties (W.H. Freeman and Company, 1993); and A.L. Lehninger, Biochemistry (Worth Publishers, Inc., 1975). The gpl20 polypeptide is instrumental in mediating entry into the host cell. Recent studies have indicated that binding of CD4 to gpl20 induces a conformational change in Env that allows for binding to a co-receptor (e.g, a chemokine receptor) and subsequent entry of the virus into the cell. (Wyatt, R., et al. (1998) Nature 393:705-711 ; Kwong, P., et al.(1998) Nature 393:648-659). Referring again to Figure 1, CD4 is bound into a depression formed at the interface of the outer domain, the inner domain and the bridging sheet of gpl20. Immunogenicity of the gpl20 polypeptide has also been studied. For example, individuals infected by HIV-1 usually develop antibodies that can neutralize the virus in in vitro assays, and this response is directed primarily against linear neutralizing determinants in the third variable loop of gpl20 glycoprotein (Javaherian, K., et al. (1989) Proc. Natl. Acad. Sci. 86:6786-6772; Matsushita, M., et al. (1988) J. Virol. 62:2107-2144; Putney, S., et al. (1986) Science 234:1392-1395; Rushe, j. R., et al . (1988) Proc. Nat. Acad. Sci. USA 85: 3198-3202.). However, these antibodies generally exhibit the ability to neutralize only a limited number of HIV-1 strains (Matthews, T. (1986) Proc. Natl. Acad. Sci. USA. 83:9709- 9713; Nara, P. L., et al. (1988) J. Virol. 62:2622-2628; Palker, T. j., et al. (1988) Proc. Natl. Acad. Sci. USA. 85:1932-1936). Later in the course of HIV infection in humans, antibodies capable of neutralizing a wider range of HIV-1 isolates appear (Barre-Sinoussi, F., et al. (1983) Science 220:868-871; Robert-Guroff, M., et al. (1985) Nature (London) 316:72-74; Weis, R., et al. (1985) Nature (London) 316:69-72; Weis, R., et al. (1986) Nature (London) 324:572-575). Recent work done by Stamatatos et al (1998) AIDS Res Hum Retroviruses 14(13):! 129-39, shows that a deletion of the variable region 2 from a HIV-1 SF162 virus, which utilizes the CCR-5 co-receptor for virus entry, rendered the virus highly susceptible to serum- mediated neutralization. This N2 deleted virus was also neutralized by sera obtained from patients infected not only with clade B HIV-1 isolates but also with clade A, C, D and F HIV- 1 isolates. However, deletion of the variable region 1 had no effect. Deletion of the variable regions 1 and 2 from a LAI isolate HIV-I I1IB also increased the susceptibility to neutralization by monoclonal antibodies whose epitopes are located within the V3 loop, the CD4-binding site, and conserved gpl20 regions (Wyatt, R., et al. (1995) J Virol. 69:5723-5733). Rabbit immunogenicity studies done with the HIV-1 virus with deletions in the VI /N2 and V3 region from the LAI strain, which uses the CXCR4 co-receptor for virus entry, showed no improvement in the ability of Env to raise neutralizing antibodies (Leu et al. (1998) AIDS Res. and Human Retroviruses. 14:151-155). Further, a subset of the broadly reactive antibodies, found in most infected individuals, interferes with the binding of gpl20 and CD4 (Kang, C.-Y., et al. (1991) Proc. Natl. Acad. Sci. USA. 88:6171-6175; McDougal, j. S., et al. (1986) J. Immunol. 137:2937- 2944). Other antibodies are believed to bind to the chemokine receptor binding region after CD4 has bound to Env (Thali et al. (1993) J. Virol. 67:3978-3988). The fact that neutralizing antibodies generated during the course of HIV infection do not provide permanent antiviral effect may in part be due to the generation of "neutralization escapes" virus mutants and to the general decline in the host immune system associated with pathogenesis. In contrast, the presence of pre-existing neutralizing antibodies upon initial HIV-1 exposure will likely have a protective effect. It is widely thought that a successful vaccine should be able to induce a strong, broadly neutralizing antibody response against diverse HIV-1 strains (Montefiori and Evans (1999) AIDS Res. Hum. Ret. 15(8):689-698; Bolognesi, D.,P., et al. (1994) Ann. Int. Med. 8:603-611 ; Haynes, B„ F., et al. (1996) Science ;271: 324-328.). Neutralizing antibodies, by attaching to the incoming virions, can reduce or even prevent their infectivity for target cells and prevent the cell-to-cell spread of virus in tissue culture (Hu et al. (1992) Science 255:456- 459; Burton, D.,R. and Montefiori, D. (1997) AIDS ll(suppl. A): 587-598). However as described above, antibodies directed against gpl20 do not generally exhibit broad antibody responses against different HIV strains. Currently, the focus of vaccine development, from the perspective of humoral immunity, is on the neutralization of primary isolates that utilize the CCR5 chemokine co- receptor believed to be important in virus entry (Zhu, T., et al. (1993) Science 261:1179- 1181; Fiore, J., et al. (1994) Virology; 204:297-303). These viruses are generally much more resistant to antibody neutralization than T-cell line adapted strains that use the CXCR4 co- receptor, although both can be neutralized in vitro by certain broadly and potent acting monoclonal antibodies, such as IgGlbl2, 2G12 and 2F5 (Trkola, A., et al. (1995) J. Virol. 69:6609-6617; D'Sousa PM., et al (1997) J. Infect. Dis. 175:1062-1075). These monoclonal antibodies are directed to the CD4 binding site, a glycosylation site and to the gp41 fusion domain, respectively. The problem that remains, however, is that it is not known how to induce antibodies of the appropriate specificity by vaccination. Antibodies (Abs) elicited by gpl20 glycoprotein from a given isolate are usually only able to neutralize closely related viruses generally from similar, usually from the same, HIV-1 subtype. Despite the above approaches, there remains a need for Env antigens that can elicit an immunological response (e.g., neutralizing and/or protective antibodies) in a subject against multiple HIV strains and subtypes, for example when administered as a vaccine. The present invention solves these and other problems by providing modified Env polypeptides (e.g., gpl20) to expose epitopes in or near the CD4 binding site. Summary of the Invention In accordance with the present invention, modified HIV Env polypeptides are provided. In particular, deletions and/or mutations are made in one or more of the 4-β antiparallel-bridging sheet in the HIV Env polypeptide. In this way, enough structure is left to allow correct folding of the polypeptide, for example of gpl20, yet enough of the bridging sheet is removed to expose the CD4 groove, allowing an immune response to be generated against epitopes in or near the CD4 binding site of the Env polypeptide (e.g., gpl20). In one aspect, the invention includes a polynucleotide encoding a modified HIV Env polypeptide wherein the polypeptide has at least one modified (e.g., deleted or replaced) amino acid residue deleted in the region corresponding to residues 421 to 436 relative to HXB-2, for example the constructs depicted in Figures 6-29 (SEQ ID NOs:3 to 26). In certain embodiments, the polynucleotide also has the region corresponding to residues 124- 198 of the polypeptide HXB-2 (e.g., V1/V2) deleted and at least one amino acid deleted or replaced in the regions corresponding to the residues 119 to 123 and 199 to 210, relative to HXB-2. In other embodiments, these polynucleotides encode Env polypeptides having at least one amino acid of the small loop of the bridging sheet (e.g., amino acid residues 427 to 429 relative to HXB-2) deleted or replaced. The amino acid sequences of the modified polypeptides encoded by the polynucleotides of the present invention can be based on any HIV variant, for example SF162. In another aspect, the invention includes immunogenic modified HIV Env polypeptides having at least one modified (e.g., deleted or replaced) amino acid residue deleted in the region corresponding to residues 421 to 436 relative to HXB-2, for example a deletion or replacement of one amino acids in the small loop region (e.g., amino acid residues 427 to 429 relative to HXB-2). These polypeptides may have modifications (e.g., a deletion or a replacement) of at least one amino acid between about amino acid residue 420 and amino acid residue 436, relative to HXB-2 and, optionally, may have deletions or truncations of the VI and/or V2 regions. The immunogenic, modified polypeptides of the present invention can be based on any HIV variant, for example SF162. In another aspect, the invention includes a vaccine composition comprising any of the polynucleotides encoding modified Env polypeptides described above. Vaccine compositions comprising the modified Env polypeptides and, optionally, an adjuvant are also included in the invention. In yet another aspect, the invention includes a method of inducing an immune response in subject comprising, administering one or more of the polynucleotides or constructs described above in an amount sufficient to induce an immune response in the subject. In certain embodiments, the method further comprises administering an adjuvant to the subject. In another aspect, the invention includes a method of inducing an immune response in a subject comprising administering a composition comprising any of the modified Env polypeptides described above and an adjuvant. The composition is administered in an amount sufficient to induce an immune response in the subject. In another aspect, the invention includes a method of inducing an immune response in a subject comprising (a) administering a first composition comprising any of the polynucleotides described above in a priming step and (b) administering a second composition comprising any of the modified Env polypeptides described above, as a booster, in an amount sufficient to induce an immune response in the subject. In certain embodiments, the first composition, the second composition or both the first and second compositions further comprise an adjuvant. These and other embodiments of the subject invention will readily occur to those of skill in the art in light of the disclosure herein. Brief Description of the Drawings Figure 1 is a schematic depiction of the tertiary structure of the HIV-1 HXB _, Env gpl20 polypeptide, as determined by crystallography studies. Figures 2A-C depict alignment of the amino acid sequence of wild-type HIV-1 HXB _ 2 Env gpl60 polypeptide (SEQ ID NO:l) with amino acid sequence of HIV variants SF162 (shown as "162") (SEQ ID NO:2), SF2, CM236 and US4. Arrows indicate the regions that are deleted or replaced in the modified polypeptides. Black dots indicate conserved cysteine residues. The star indicates the position of the last amino acid in gρl20. Figures 3A-J depict alignment of nucleotide sequences of polynucleotides encoding modified Env polypeptides having V1/Y2 deletions. The unmodified amino acid residues encoded by these sequences correspond to wildtype SF162 residues but are numbered relative to HXB-2. Figures 4A-M depict alignment of nucleotide sequences of polynucleotides encoding modified Env polypeptides having deletions or replacements in the small loop. The unmodified amino acid residues encoded by these sequences correspond to wildtype SF162 residues but are numbered relative to HXB-2. Figures 5A-N depict alignment of nucleotide sequences of polynucleotides encoding modified Env polypeptides having both VI /V2 deletions and, in addition, deletions or replacements in the small loop. The unmodified amino acid residues encoded by these sequences correspond to wildtype SF162 residues but are numbered relative to HXB-2. Figure 6 depicts the nucleotide sequence of the construct designated Vall20-Ala204 (SEQ ID NO.3). Figure 7 depicts the nucleotide sequence of the construct designated Vall20-Ile201 (SEQ ID NO:4). Figure 8 depicts the nucleotide sequence of the construct designated Vall20-Ile201B (SEQ ID NO:5). Figure 9 depicts the nucleotide sequence of the construct designated Lysl21-Val200 (SEQ ID NO:6). Figure 10 depicts the nucleotide sequence of the construct designated Leul22-Serl99 (SEQ ID NO:7). Figure 11 depicts the nucleotide sequence of the construct designated Vall20-Thr202 (SEQ ID NO:8). Figure 12 depicts the nucleotide sequence of the construct designated Trp427-Gly431 (SEQ ID NO:9). Figure 13 depicts the nucleotide sequence of the construct designated Arg426-Gly431 (SEQ ID NO: 10). Figure 14 depicts the nucleotide sequence of the construct designated Arg426- Gly431B (SEQ ID NO:l l). Figure 15 depicts the nucleotide sequence of the construct designated Arg426-Lys432 (SEQ ID NO: 12). Figure 16 depicts the nucleotide sequence of the construct designated Asn425-Lys432 (SEQ ID NO: 13). Figure 17 depicts the nucleotide sequence of the construct designated Ile424-Ala433 (SEQ ID NO: 14). Figure 18 depicts the nucleotide sequence of the construct designated Ile423-Met434 (SEQ ID NO: 15). Figure 19 depicts the nucleotide sequence of the construct designated Gln422-Tyr435 (SEQ ID NO: 16). Figure 20 depicts the nucleotide sequence of the construct designated Gln422- Tyr435B (SEQ ID NO:17). Figure 21 depicts the nucleotide sequence of the construct designated Leu 122- Serl99;Arg426-Gly431 (SEQ ID NO: 18). Figure 22 depicts the nucleotide sequence of the construct designated Leul22- Serl 99; Arg426-Lys432 (SEQ ID NO: 19). Figure 23 depicts the nucleotide sequence of the construct designated Leul22-Serl99; Trp427-Gly431 (SEQ ID NO:20). Figure 24 depicts the nucleotide sequence of the construct designated Lysl21-Val200; Asn425-Lys432 (SEQ ID NO:21). Figure 25 depicts the nucleotide sequence of the construct designated Vall20-Ile201 ; Ile424-Ala433 (SEQ ID NO:22). Figure 26 depicts the nucleotide sequence of the construct designated Vail 20- Ile201B; Ile424-Ala433 (SEQ ID NO:23). Figure 27 depicts the nucleotide sequence of the construct designated Vall20-Thr202; Ile424-Ala433 (SEQ ID NO:24). Figure 28 depicts the nucleotide sequence of the construct designated Vall27-Asnl95 (SEQ ID NO:25). Figure 29 depicts the nucleotide sequence of the construct designated Vail 27- Asnl95; Arg426-Gly431 (SEQ ID NO:26). Detailed Description of the Invention The practice of the present invention will employ, unless otherwise indicated, conventional methods of protein chemistry, viral immunobiology, molecular biology and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., T.E. Creighton, Proteins: Structures and Molecular Properties (W.H. Freeman and Company, 1993); Nelson L.M. and Jerome H.K. HIV Protocols in Methods in Molecular Medicine, vol. 17, 1999; Sambrook, et al., Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory, 1989); F.M. Ausubel et al. Current Protocols in Molecular Biology. Greene Publishing Associates & Wiley Interscience New York; and Lipkowitz and Boyd, Reviews in Computational Chemistry, volumes 1 -present (Wiley-VCH, New York, New York, 1999). It must be noted that, as used in this specification and the appended claims, the singular forms "a", "an" and "the" include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to "a polypeptide" includes a mixture of two or more polypeptides, and the like. Definitions In describing the present invention, the following terms will be employed, and are intended to be defined as indicated below. The terms "polypeptide," and "protein" are used interchangeably herein to denote any polymer of amino acid residues. The terms encompass peptides, oligopeptides, dimers, multimers, and the like. Such polypeptides can be derived from natural sources or can be synthesized or recombinantly produced. The terms also include postexpression modifications of the polypeptide, for example, glycosylation, acetylation, phosphorylation, etc. A polypeptide as defined herein is generally made up of the 20 natural amino acids Ala (A), Arg (R), Asn (N), Asp (D), Cys (C), Gin (Q), Glu (E), Gly (G), His (H), He (I), Leu (L), Lys (K), Met (M), Phe (F), Pro (P), Ser (S), Thr (T), Tip (W), Tyr (Y) and Val (V) and may also include any of the several known amino acid analogs, both naturally occurring and synthesized analogs, such as but not limited to homoisoleucine, asaleucine, 2- (methylenecyclopropyl)glycine, S-methylcysteine, S-(prop-l-enyl)cysteine, homoserine, ornithine, norleucine, norvaline, homoarginine, 3-(3-carboxyphenyl)alanine, cyclohexylalanine, mimosine, pipecolic acid, 4-methylglutamic acid, canavanine, 2,3- diaminopropionic acid, and the like. Further examples of polypeptide agents which will find use in the present invention are set forth below. By "geometry" or "tertiary structure" of a polypeptide or protein is meant the overall 3-D configuration of the protein. As described herein, the geometry can be determined, for example, by crystallography studies or by using various programs or algorithms which predict the geometry based on interactions between the amino acids making up the primary and secondary structures. By "wild type" polypeptide, polypeptide agent or polypeptide drug, is meant a naturally occurring polypeptide sequence, and its corresponding secondary structure. An "isolated" or "purified" protein or polypeptide is a protein which is separate and discrete from a whole organism with which the protein is normally associated in nature. It is apparent that the term denotes proteins of various levels of purity. Typically, a composition containing a purified protein will be one in which at least about 35%, preferably at least about 40-50%, more preferably, at least about 75-85%, and most preferably at least about 90% or more, of the total protein in the composition will be the protein in question. By "Env polypeptide" is meant a molecule derived from an envelope protein, preferably from HIV Env. The envelope protein of HIV-1 is a glycoprotein of about 160 kd (gplόO). During virus infection of the host cell, gpl60 is cleaved by host cell proteases to form gpl20 and the integral membrane protein, gp41. The gp41 portion is anchored in (and spans) the membrane bilayer of virion, while the gpl20 segment protrudes into the surrounding environment. As there is no covalent attachment between gpl20 and gp41, free gpl20 is released from the surface of virions and infected cells. Env polypeptides may also include gpl40 polypeptides. Env polypeptides can exist as monomers, dimers or multimers. By a "gpl20 polypeptide" is meant a molecule derived from a gpl20 region of the Env polypeptide. Preferably, the gpl20 polypeptide is derived from HIV Env. The primary amino acid sequence of gpl20 is approximately 511 amino acids, with a polypeptide core of about 60,000 daltons. The polypeptide is extensively modified by N-linked glycosylation to increase the apparent molecular weight of the molecule to 120,000 daltons. The amino acid sequence of gpl20 contains five relatively conserved domains interspersed with five hypervariable domains. The positions of the 18 cysteine residues in the gpl20 primary sequence of the HIV-1 HXB _ 2 (hereinafter "HXB-2") strain, and the positions of 13 of the approximately 24 N-linked glycosylation sites in the gρl20 sequence are common to most, if not all, gpl20 sequences. The hypervariable domains contain extensive amino acid substitutions, insertions and deletions. Despite this variation, most, if not all, gpl20 sequences preserve the virus's ability to bind to the viral receptor CD4. A "gpl20 polypeptide" includes both single subunits or multimers. Env polypeptides (e.g., gpl20, gpl40 and gplόO) include a "bridging sheet" comprised of 4 anti-parallel β-strands (β-2, β-3, β-20 and β-21) that form a β-sheet. Extruding from one pair of the β-strands (β-2 and β-3) are two loops, VI and V2. The β-2 sheet occurs at approximately amino acid residue 119 (Cys) to amino acid residue 123 (Thr) while β-3 occurs at approximately amino acid residue 199 (Ser) to amino acid residue 201 (He), relative to HXB-2. The "V1/V2 region" occurs at approximately amino acid positions 126 (Cys) to residue 196 (Cys), relative to HXB-2. (see, e.g., Wyatt et al. (1995) J. Virol. 69:5723-5733; Stamatatos et al. (1998) J. Virol. 72:7840-7845). Extruding from the second pair of β-strands (β-20 and β-21) is a "small-loop" structure, also referred to herein as "the bridging sheet small loop." In HXB-2, β-20 extends from about amino acid residue 422 (Gin) to amino acid residue 426 (Met) while β-21 extends from about amino acid residue 430 (Val) to amino acid residue 435 (Tyr). In variant SF162, the Met-426 is an Arg (R) residue. The "small loop" extends from about amino acid residue 427 (Tip) through 429 (Lys), relative to HXB-2. A representative diagram of gpl20 showing the bridging sheet, the small loop, and VI N2 is shown in Figure 1. In addition, alignment of the amino acid sequences of Env polypeptide gpl60 of selected variants is shown, relative to HXB-2, in Figures 2A-C. Furthermore, an "Env polypeptide" or "gρl20 polypeptide" as defined herein is not limited to a polypeptide having the exact sequence described herein. Indeed, the HIV genome is in a state of constant flux and contains several variable domains which exhibit relatively high degrees of variability between isolates. It is readily apparent that the terms encompass Env (e.g., gpl20) polypeptides from any of the identified HIV isolates, as well as newly identified isolates, and subtypes of these isolates. Descriptions of structural features are given herein with reference to HXB-2. One of ordinary skill in the art in view of the teachings of the present disclosure and the art can determine corresponding regions in other HIV variants (e.g, isolates HIV IIlb , HIV SF2 , HIV-1 SF162 , HIV-1 SF170 , HIV LAV , HIV LA1 , HIV MΝ , HIV-1 CM235 „ HIV-1 US4 , other HIV-1 strains from diverse subtypes(e.g., subtypes, A through G, and O), HIV-2 strains and diverse subtypes (e.g., HIV-2 UC1 and HIV-2 UC2 ), and simian immunodeficiency virus (SIV). (See, e.g., Virology, 3rd Edition (W.K. Joklik ed. 1988); Fundamental Virology, 2nd Edition (B.N. Fields and D.M. Knipe, eds. 1991); Virology, 3rd Edition (Fields, BN, DM Knipe, PM Howley, Editors, 1996, Lippincott-Raven, Philadelphia, PA; for a description of these and other related viruses), using for example, sequence comparison programs (e.g., BLAST and others described herein) or identification and alignment of structural features (e.g., a program such as the "ALB" program described herein that can identify β-sheet regions). The actual amino acid sequences of the modified Env polypeptides can be based on any HIV variant. Additionally, the term "Env polypeptide" (e.g., "gpl20 polypeptide") encompasses proteins which include additional modifications to the native sequence, such as additional internal deletions, additions and substitutions. These modifications may be deliberate, as through site-directed mutagenesis, or may be accidental, such as through naturally occurring mutational events. Thus, for example, if the Env polypeptide is to be used in vaccine compositions, the modifications must be such that immunological activity (i.e., the ability to elicit an antibody response to the polypeptide) is not lost. Similarly, if the polypeptides are to be used for diagnostic purposes, such capability must be retained. Thus, a "modified Env polypeptide" is an Env polypeptide (e.g., gpl20 as defined above), which has been manipulated to delete or replace all or a part of the bridging sheet portion and, optionally, the variable regions VI and V2. Generally, modified Env (e.g., gpl20) polypeptides have enough of the bridging sheet removed to expose the CD4 binding site, but leave enough of the structure to allow correct folding (e.g., correct geometry). Thus, modifications to the β-20 and β-21 regions (between about amino acid residues 420 and 435 relative to HXB-2) are preferred. Additionally, modifications to the β-2 and β-3 regions (between about amino acid residues 119 (Cys) and 201 (He)) and modifications (e.g., truncations) to the VI and V2 loop regions may also be made. Although not all possible β- sheet and V1/N2 modifications have been exemplified herein, it is to be understood that other disrupting modifications are also encompassed by the present invention. Normally, such a modified polypeptide is capable of secretion into growth medium in which an organism expressing the protein is cultured. However, for purposes of the present invention, such polypeptides may also be recovered intracellularly. Secretion into growth media is readily determined using a number of detection techniques, including, e.g., polyacrylamide gel electrophoresis and the like, and immunological techniques such as Western blotting and immunoprecipitation assays as described in, e.g., International Publication No. WO 96/04301, published February 15, 1996. A gpl20 or other Env polypeptide is produced "intracellularly" when it is found within the cell, either associated with components of the cell, such as in association with the endoplasmic reticulum (ER) or the Golgi Apparatus, or when it is present in the soluble cellular fraction. The gpl20 and other Env polypeptides of the present invention may also be secreted into growth medium so long as sufficient amounts of the polypeptides remain present within the cell such that they can be purified from cell lysates using techniques described herein. An "immunogenic" gpl20 or other Env protein is a molecule that includes at least one epitope such that the molecule is capable of either eliciting an immunological reaction in an individual to which the protein is administered or, in the diagnostic context, is capable of reacting with antibodies directed against the HIV in question. By "epitope" is meant a site on an antigen to which specific B cells and/or T cells respond, rendering the molecule including such an epitope capable of eliciting an immunological reaction or capable of reacting with HIV antibodies present in a biological sample. The term is also used interchangeably with "antigenic determinant" or "antigenic determinant site." An epitope can comprise 3 or more amino acids in a spatial conformation unique to the epitope. Generally, an epitope consists of at least 5 such amino acids and, more usually, consists of at least 8-10 such amino acids. Methods of determining spatial conformation of amino acids are known in the art and include, for example, x-ray crystallography and 2-dimensional nuclear magnetic resonance. Furthermore, the identification of epitopes in a given protein is readily accomplished using techniques well known in the art, such as by the use of hydrophobicity studies and by site-directed serology. See, also, Geysen et al., Proc. Natl. Acad. Sci. USA (1984) 81:3998-4002 (general method of rapidly synthesizing peptides to determine the location of immunogenic epitopes in a given antigen); U.S. Patent No. 4,708,871 (procedures for identifying and chemically synthesizing epitopes of antigens); and Geysen et al., Molecular Immunology (1986) 23:709-715 (technique for identifying peptides with high affinity for a given antibody). Antibodies that recognize the same epitope can be identified in a simple immunoassay showing the ability of one antibody to block the binding of another antibody to a target antigen. An "immunological response" or "immune response" as used herein is the development in the subject of a humoral and/or a cellular immune response to the Env (e.g., gpl20) polypeptide when the polypeptide is present in a vaccine composition. These antibodies may also neutralize infectivity, and/or mediate antibody-complement or antibody dependent cell cytotoxicity to provide protection to an immunized host. Immunological reactivity may be determined in standard immunoassays, such as a competition assays, well known in the art. Techniques for determining amino acid sequence "similarity" are well known in the art. In general, "similarity" means the exact amino acid to amino acid comparison of two or more polypeptides at the appropriate place, where amino acids are identical or possess similar chemical and/or physical properties such as charge or hydrophobicity. A so-termed "percent similarity" then can be determined between the compared polypeptide sequences. Techniques for determining nucleic acid and amino acid sequence identity also are well known in the art and include determining the nucleotide sequence of the mRNA for that gene (usually via a cDNA intermediate) and determining the amino acid sequence encoded thereby, and comparing this to a second amino acid sequence. In general, "identity" refers to an exact nucleotide to nucleotide or amino acid to amino acid correspondence of two polynucleotides or polypeptide sequences, respectively. Two or more polynucleotide sequences can be compared by determining their "percent identity." Two or more amino acid sequences likewise can be compared by determining their "percent identity." The percent identity of two sequences, whether nucleic acid or peptide sequences, is generally described as the number of exact matches between two aligned sequences divided by the length of the shorter sequence and multiplied by 100. An approximate alignment for nucleic acid sequences is provided by the local homology algorithm of Smith and Waterman, Advances in Applied Mathematics 2:482-489 (1981). This algorithm can be extended to use with peptide sequences using the scoring matrix developed by Dayhoff, Atlas of Protein Sequences and Structure, M.O. Dayhoff ed., 5 suppl. 3:353-358, National Biomedical Research Foundation, Washington, D.C., USA, and normalized by Gribskov, Nucl. Acids Res. 14(6):6745-6763 (1986). An implementation of this algorithm for nucleic acid and peptide sequences is provided by the Genetics Computer Group (Madison, WI) in their BestFit utility application. The default parameters for this method are described in the Wisconsin Sequence Analysis Package Program Manual, Version 8 (1995) (available from Genetics Computer Group, Madison, WI). Other equally suitable programs for calculating the percent identity or similarity between sequences are generally known in the art. For example, percent identity of a particular nucleotide sequence to a reference sequence can be determined using the homology algorithm of Smith and Waterman with a default scoring table and a gap penalty of six nucleotide positions. Another method of establishing percent identity in the context of the present invention is to use the MPSRCH package of programs copyrighted by the University of Edinburgh, developed by John F. Collins and Shane S. Sturrok, and distributed by IntelliGenetics, Inc. (Mountain View, CA). From this suite of packages, the Smith- Waterman algorithm can be employed where default parameters are used for the scoring table (for example, gap open penalty of 12, gap extension penalty of one, and a gap of six). From the data generated, the "Match" value reflects "sequence identity." Other suitable programs for calculating the percent identity or similarity between sequences are generally known in the art, such as the alignment program BLAST, which can also be used with default parameters. For example, BLASTN and BLASTP can be used with the following default parameters: genetic code = standard; filter = none; strand = both; cutoff = 60; expect = 10; Matrix = BLOSUM62; Descriptions = 50 sequences; sort by = HIGH SCORE; Databases = non-redundant, GenBank + EMBL + DDBJ + PDB + GenBank CDS translations + Swiss protein + Spupdate + PIR. Details of these programs can be found at the following internet address: http://www.ncbi.nlm.gov/cgi-bin/BLAST. One of skill in the art can readily determine the proper search parameters to use for a given sequence in the above programs. For example, the search parameters may vary based on the size of the sequence in question. Thus, for example, a representative embodiment of the present invention would include an isolated polynucleotide having X contiguous nucleotides, wherein (i) the X contiguous nucleotides have at least about 50% identity to Y contiguous nucleotides derived from any of the sequences described herein, (ii) X equals Y, and (iii) X is greater than or equal to 6 nucleotides and up to 5000 nucleotides, preferably greater than or equal to 8 nucleotides and up to 5000 nucleotides, more preferably 10-12 nucleotides and up to 5000 nucleotides, and even more preferably 15-20 nucleotides, up to the number of nucleotides present in the full-length sequences described herein (e.g., see the Sequence Listing and claims), including all integer values falling within the above-described ranges. The synthetic expression cassettes (and purified polynucleotides) of the present invention include related polynucleotide sequences having about 80% to 100%, greater than 80-85%), preferably greater than 90-92%, more preferably greater than 95%, and most preferably greater than 98%> sequence (including all integer values falling within these described ranges) identity to the synthetic expression cassette sequences disclosed herein (for example, to the claimed sequences or other sequences of the present invention) when the sequences of the present invention are used as the query sequence. Computer programs are also available to determine the likelihood of certain polypeptides to form structures such as β-sheets. One such program, described herein, is the "ALB" program for protein and polypeptide secondary structure calculation and predication. In addition, secondary protein structure can be predicted from the primary amino acid sequence, for example using protein crystal structure and aligning the protein sequence related to the crystal structure (e.g., using Molecular Operating Environment (MOE) programs available from the Chemical Computing Group Inc., Montreal, P.Q., Canada). Other methods of predicting secondary structures are described, for example, in Gamier et al. (1996) Methods Enzymol. 266:540-553; Geourjon et al. (1995) Comput. Applic. Biosci. 11 :681-684; Levin (1997) Protein Eng 10:771-776; and Rost et al. (1993) J. Molec. Biol. 232:584-599. Plomology can also be determined by hybridization of polynucleotides under conditions which form stable duplexes between homologous regions, followed by digestion with single-stranded-specific nuclease(s), and size determination of the digested fragments. Two DNA, or two polypeptide sequences are "substantially homologous" to each other when the sequences exhibit at least about 80%-85%, preferably at least about 90%, and most preferably at least about 95%-98% sequence identity over a defined length of the molecules, as determined using the methods above. As used herein, substantially homologous also refers to sequences showing complete identity to the specified DNA or polypeptide sequence. DNA sequences that are substantially homologous can be identified in a Southern hybridization experiment under, for example, stringent conditions, as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Sambrook et al., supra; DNA Cloning, supra; Nucleic Acid Hybridization, supra. A "coding sequence" or a sequence which "encodes" a selected protein, is a nucleic acid sequence which is transcribed (in the case of DNA) and translated (in the case of mRNA) into a polypeptide in vitro or in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5' (amino) terminus and a translation stop codon at the 3' (carboxy) terminus. A coding sequence can include, but is not limited to cDNA from viral nucleotide sequences as well as synthetic and semisynthetic DNA sequences and sequences including base analogs. A transcription termination sequence may be located 3' to the coding sequence. "Control elements" refers collectively to promoter sequences, ribosome binding sites, polyadenylation signals, transcription termination sequences, upstream regulatory domains, enhancers, and the like, which collectively provide for the transcription and translation of a coding sequence in a host cell. Not all of these control elements need always be present so long as the desired gene is capable of being transcribed and translated. A control element "directs the transcription" of a coding sequence in a cell when RNA polymerase will bind the promoter sequence and transcribe the coding sequence into mRNA, which is then translated into the polypeptide encoded by the coding sequence. "Operably linked" refers to an arrangement of elements wherein the components so described are configured so as to perform their usual function. Thus, control elements operably linked to a coding sequence are capable of effecting the expression of the coding sequence when RNA polymerase is present. The control elements need not be contiguous with the coding sequence, so long as they function to direct. the expression thereof. Thus, for example, intervening untranslated yet transcribed sequences can be present between, e.g., a promoter sequence and the coding sequence and the promoter sequence can still be considered "operably linked" to the coding sequence. "Recombinant" as used herein to describe a nucleic acid molecule means a polynucleotide of genomic, cDNA, semisynthetic, or synthetic origin which, by virtue of its origin or manipulation: (1) is not associated with all or a portion of the polynucleotide with which it is associated in nature; and/or (2) is linked to a polynucleotide other than that to which it is linked in nature. The term "recombinant" as used with respect to a protein or polypeptide means a polypeptide produced by expression of a recombinant polynucleotide. "Recombinant host cells," "host cells," "cells," "cell lines," "cell cultures," and other such terms denoting procaryotic microorganisms or eucaryotic cell lines cultured as unicellular entities, are used interchangeably, and refer to cells which can be, or have been, used as recipients for recombinant vectors or other transfer DNA, and include the progeny of the original cell which has been transfected. It is understood that the progeny of a single parental cell may not necessarily be completely identical in morphology or in genomic or total DNA complement to the original parent, due to accidental or deliberate mutation. Progeny of the parental cell which are sufficiently similar to the parent to be characterized by the relevant property, such as the presence of a nucleotide sequence encoding a desired peptide, are included in the progeny intended by this definition, and are covered by the above terms. By "vertebrate subject" is meant any member of the subphylum chordata, including, without limitation, humans and other primates, including non-human primates such as chimpanzees and other apes and monkey species; farm animals such as cattle, sheep, pigs, goats and horses; domestic mammals such as dogs and cats; laboratory animals including rodents such as mice, rats and guinea pigs; birds, including domestic, wild and game birds such as chickens, turkeys and other gallinaceous birds, ducks, geese, and the like. The term does not denote a particular age. Thus, both adult and newborn individuals are intended to be covered. As used herein, a "biological sample" refers to a sample of tissue or fluid isolated from an individual, including but not limited to, for example, blood, plasma, serum, fecal matter, urine, bone marrow, bile, spinal fluid, lymph fluid, samples of the skin, external secretions of the skin, respiratory, intestinal, and genitourinary tracts, samples derived from the gastric epithelium and gastric mucosa, tears, saliva, milk, blood cells, organs, biopsies and also samples of in vitro cell culture constituents including but not limited to conditioned media resulting from the growth of cells and tissues in culture medium, e.g., recombinant cells, and cell components. The terms "label" and "detectable label" refer to a molecule capable of detection, including, but not limited to, radioactive isotopes, fluorescers, chemiluminescers, enzymes, enzyme substrates, enzyme cofactors, enzyme inhibitors, chromophores, dyes, metal ions, metal sols, ligands (e.g., biotin or haptens) and the like. The term "fluorescer" refers to a substance or a portion thereof which is capable of exhibiting fluorescence in the detectable range. Particular examples of labels which may be used with the invention include, but are not limited to fluorescein, rhodamine, dansyl, umbelliferone, Texas red, luminol, acradimum esters, NADPH, -β-galactosidase, horseradish peroxidase, glucose oxidase, alkaline phosphatase and urease. Overview The present invention concerns modified Env polypeptide molecules (e.g., glycoprotein ("gp") 120). Without being bound by a particular theory, it appears that it has been difficult to generate immunological responses against Env because the CD4 binding site is buried between the outer domain, the inner domain and the VI /V2 domains. Thus, although deletion of the VI V2 domain may render the virus more susceptible to neutralization by monoclonal antibody directed to the CD4 site, the bridging sheet covering most of the CD4 binding domain may prevent an antibody response. Thus, the present invention provides Env polypeptides that maintain their general overall structure yet expose the CD4 binding domain. This allows the generation of an immune response (e.g., an antibody response) to epitopes in or near the CD4 binding site. Various forms of the different embodiments of the invention, described herein, may be combined. β-Sheet Conformations In the present invention, location of the β-sheet structures were identified relative to 3-D (crystal) structure of an HXB-2 crystallized Env protein (see, Example 1A). Based on this structure, constructs encoding polypeptides having replacements and or excisions which maintain overall geometry while exposing the CD4 binding site were designed. In particular, the crystal structure of HXB-2 was downloaded from the Brookhaven Database. Using the default parameters of the Loop Search feature of the Biopolymer module of the Sybyl molecular modeling package, homology and fit of amino acids which could replace the native loops between β-strands yet maintain overall tertiary structure were determined. Constructs encoding the modified Env polypeptides were then designed (Example B.). Thus, the modified Env polypeptides typically have enough of the bridging sheet removed to expose the CD4 groove, but have enough of the structure to allow correct folding of the Env glycoprotein. Exemplary constructs are described below. Polypeptide Production The polypeptides of the present invention can be produced in any number of ways which are well known in the art. In one embodiment, the polypeptides are generated using recombinant techniques, well known in the art. In this regard, ohgonucleotide probes can be devised based on the known sequences of the Env (e.g., gpl20) polypeptide genome and used to probe genomic or cDNA libraries for Env genes. The gene can then be further isolated using standard techniques and, e.g., restriction enzymes employed to truncate the gene at desired portions of the full-length sequence. Similarly, the Env gene(s) can be isolated directly from cells and tissues containing the same, using known techniques, such as phenol extraction and the sequence further manipulated to produce the desired truncations. See, e.g., Sambrook et al., supra, for a description of techniques used to obtain and isolate DNA. The genes encoding the modified (e.g., truncated and/or substituted) polypeptides can be produced synthetically, based on the known sequences. The nucleotide sequence can be designed with the appropriate codons for the particular amino acid sequence desired. The complete sequence is generally assembled from overlapping oligonucleotides prepared by standard methods and assembled into a complete coding sequence. See, e.g., Edge (1981) Nature 292:756; Nambair et al. (1984) Science 223:1299; Jay et al. (1984) J. Biol. Chem. 259:6311; Stemmer et al. (1995) Gene 164:49-53. Recombinant techniques are readily used to clone a gene encoding an Env polypeptide gene which can then be mutagenized in vitro by the replacement of the appropriate base pair(s) to result in the codon for the desired amino acid. Such a change can include as little as one base pair, effecting a change in a single amino acid, or can encompass several base pair changes. Alternatively, the mutations can be effected using a mismatched primer which hybridizes to the parent nucleotide sequence (generally cDNA corresponding to the RNA sequence), at a temperature below the melting temperature of the mismatched duplex. The primer can be made specific by keeping primer length and base composition within relatively narrow limits and by keeping the mutant base centrally located. See, e.g., Innis et al, (1990) PCR Applications: Protocols for Functional Genomics; Zoller and Smith, Methods Enzymol. (1983) 100:468. Primer extension is effected using DNA polymerase, the product cloned and clones containing the mutated DNA, derived by segregation of the primer extended strand, selected. Selection can be accomplished using the mutant primer as a hybridization probe. The technique is also applicable for generating multiple point mutations. See, e.g., Dalbie-McFarland et al. Proc. Natl. Acad. Sci USA (1982) 79:6409. Once coding sequences for the desired proteins have been isolated or synthesized, they can be cloned into any suitable vector or replicon for expression. As will be apparent from the teachings herein, a wide variety of vectors encoding modified polypeptides can be generated by creating expression constructs which operably link, in various combinations, polynucleotides encoding Env polypeptides having deletions or mutation therein. Thus, polynucleotides encoding a particular deleted VI /V2 region can be operably linked with polynucleotides encoding polypeptides having deletions or replacements in the small loop region and the construct introduced into a host cell for polypeptide expression. Non-limiting examples of such combinations are discussed in the Examples. Numerous cloning vectors are known to those of skill in the art, and the selection of an appropriate cloning vector is a matter of choice. Examples of recombinant DNA vectors for cloning and host cells which they can transform include the bacteriophage λ (E. coli), pBR322 (E. coli), pACYC177 (E. coli), pKT230 (gram-negative bacteria), pGVl 106 (gram-negative bacteria), pLAFRl (gram-negative bacteria), pMΕ290 (non-E. coli gram-negative bacteria), pHV14 (E. coli and Bacillus subtilis), pBD9 (Bacillus), pIJ61 (Streptomyces), pUC6 (Streptomyces), YIp5 (Saccharomyces), YCpl9 (Saccharomyces) and bovine papilloma virus (mammalian cells). See, generally, DNA Cloning: Vols. I & II, supra; Sambrook et al, supra; B. Perbal, supra. Insect cell expression systems, such as baculovirus systems, can also be used and are known to those of skill in the art and described in, e.g., Summers and Smith, Texas Agricultural Experiment Station Bulletin No. 1555 (1987). Materials and methods for baculovirus/insect cell expression systems are commercially available in kit form from, ter alia, Invitrogen, San Diego CA ("MaxBac" kit). Plant expression systems can also be used to produce the modified Εnv proteins. Generally, such systems use virus-based vectors to transfect plant cells with heterologous genes. For a description of such systems see, e.g., Porta et al., Mol. Biotech. (1996) 5:209- 221; and Hackland et al., Arch. Virol. (1994) 139:1-22. Viral systems, such as a vaccinia based infection transfection system, as described in Tomei et al., J. Virol. (1993) 67:4017-4026 and Selby et al., J. Gen. Virol. (1993) 74:1103-1113, will also find use with the present invention. In this system, cells are first transfected in vitro with a vaccinia virus recombinant that encodes the bacteriophage T7 RNA polymerase. This polymerase displays exquisite specificity in that it only transcribes templates bearing T7 promoters. Following infection, cells are transfected with the DNA of interest, driven by a T7 promoter. The polymerase expressed in the cytoplasm from the vaccinia virus recombinant transcribes the transfected DNA into RNA which is then translated into protein by the host translational machinery. The method provides for high level, transient, cytoplasmic production of large quantities of RNA and its translation product(s). The gene can be placed under the control of a promoter, ribosome binding site (for bacterial expression) and, optionally, an operator (collectively referred to herein as "control" elements), so that the DNA sequence encoding the desired Env polypeptide is transcribed into RNA in the host cell transformed by a vector containing this expression construction. The coding sequence may or may not contain a signal peptide or leader sequence. With the present invention, both the naturally occurring signal peptides or heterologous sequences can be used. Leader sequences can be removed by the host in post-translational processing. See, e.g., U.S. Patent Nos. 4,431,739; 4,425,437; 4,338,397. Such sequences include, but are not limited to, the TPA leader, as well as the honey bee mellitin signal sequence. Other regulatory sequences may also be desirable which allow for regulation of expression of the protein sequences relative to the growth of the host cell. Such regulatory sequences are known to those of skill in the art, and examples include those which cause the expression of a gene to be turned on or off in response to a chemical or physical stimulus, including the presence of a regulatory compound. Other types of regulatory elements may also be present in the vector, for example, enhancer sequences. The control sequences and other regulatory sequences may be ligated to the coding sequence prior to insertion into a vector. Alternatively, the coding sequence can be cloned directly into an expression vector which already contains the control sequences and an appropriate restriction site. In some cases it may be necessary to modify the coding sequence so that it may be attached to the control sequences with the appropriate orientation; i.e., to maintain the proper reading frame. Mutants or analogs may be prepared by the deletion of a portion of the sequence encoding the protein, by insertion of a sequence, and/or by substitution of one or more nucleotides within the sequence. Techniques for modifying nucleotide sequences, such as site-directed mutagenesis, are well known to those skilled in the art. See, e.g., Sambrook et al., supra; DNA Cloning, Vols. I and II, supra; Nucleic Acid Hybridization, supra. The expression vector is then used to transform an appropriate host cell. A number of mammalian cell lines are known in the art and include immortalized cell lines available from the American Type Culture Collection (ATCC), such as, but not limited to, Chinese hamster ovary (CHO) cells, HeLa cells, baby hamster kidney (BHK) cells, monkey kidney cells (COS), human hepatocellular carcinoma cells (e.g., Hep G2), Vero293 cells, as well as others. Similarly, bacterial hosts such as E. coli, Bacillus subtilis, and Streptococcus spp., will find use with the present expression constructs. Yeast hosts useful in the present invention include inter alia, Saccharomyces cerevisiae, Candida albicans, Candida maltosa, Hansenula polymorpha, Kluyveromyces fragilis, Kluyveromyces lactis, Pichia guillerimondii, Pichia pastoris, Schizosaccharomyces pombe and Yarrowia lipolytica. Insect cells for use with baculovirus expression vectors include, mter alia, Aedes aegypti, Autographa californica, Bombyx mori, Drosophila melanogaster, Spodoptera frugiperda, and Trichoplusia ni. Depending on the expression system and host selected, the proteins of the present invention are produced by growing host cells transformed by an expression vector described above under conditions whereby the protein of interest is expressed. The selection of the appropriate growth conditions is within the skill of the art. In one embodiment, the transformed cells secrete the polypeptide product into the surrounding media. Certain regulatory sequences can be included in the vector to enhance secretion of the protein product, for example using a tissue plasminogen activator (TPA) leader sequence, a γ-interferon signal sequence or other signal peptide sequences from known secretory proteins. The secreted polypeptide product can then be isolated by various techniques described herein, for example, using standard purification techniques such as but not limited to, hydroxyapatite resins, column chromatography, ion-exchange chromatography, size-exclusion chromatography, electrophoresis, HPLC, immunoadsorbent techniques, affinity chromatography, immunoprecipitation, and the like.. Alternatively, the transformed cells are disrupted, using chemical, physical or mechanical means, which lyse the cells yet keep the Env polypeptides substantially intact. Intracellular proteins can also be obtained by removing components from the cell wall or membrane, e.g., by the use of detergents or organic solvents, such that leakage of the Env polypeptides occurs. Such methods are known to those of skill in the art and are described in, e.g., Protein Purification Applications: A Practical Approach, (E.LN. Harris and S. Angal, Eds., 1990) For example, methods of disrupting cells for use with the present invention include but are not limited to: sonication or ultrasonication; agitation; liquid or solid extrusion; heat treatment; freeze-thaw; desiccation; explosive decompression; osmotic shock; treatment with lytic enzymes including proteases such as trypsin, neuraminidase and lysozyme; alkali treatment; and the use of detergents and solvents such as bile salts, sodium dodecylsulphate, Triton, NP40 and CHAPS. The particular technique used to disrupt the cells is largely a matter of choice and will depend on the cell type in which the polypeptide is expressed, culture conditions and any pre-treatment used. Following disruption of the cells, cellular debris is removed, generally by centrifugation, and the intracellularly produced Env polypeptides are further purified, using standard purification techniques such as but not limited to, column chromatography, ion- exchange chromatography, size-exclusion chromatography, electrophoresis, HPLC, immunoadsorbent techniques, affinity chromatography, immunoprecipitation, and the like. For example, one method for obtaining the intracellular Env polypeptides of the present invention involves affinity purification, such as by immunoaffmity chromatography using anti-Env specific antibodies, or by lectin affinity chromatography. Particularly preferred lectin resins are those that recognize mannose moieties such as but not limited to resins derived from Galanthus nivalis agglutinin (GNA), Lens culinaris agglutinin (LCA or lentil lectin), Pisum sativum agglutinin (PSA or pea lectin), Narcissus pseudonarcissus agglutinin (NPA) and Allium ursinum agglutinin (AUA). The choice of a suitable affinity resin is within the skill in the art. After affinity purification, the Env polypeptides can be further purified using conventional techniques well known in the art, such as by any of the techniques described above. It may be desirable to produce Env (e.g., gpl20) complexes, either with itself or other proteins. Such complexes are readily produced by e.g., co-transfecting host cells with constructs encoding for the Env (e.g., gpl20) and/or other polypeptides of the desired complex. Co-transfection can be accomplished either in trans or cis, i.e., by using separate vectors or by using a single vector which bears both of the Env and other gene. If done using a single vector, both genes can be driven by a single set of control elements or, alternatively, the genes can be present on the vector in individual expression cassettes, driven by individual control elements. Following expression, the proteins will spontaneously associate. Alternatively, the complexes can be formed by mixing the individual proteins together which have been produced separately, either in purified or semi-purified form, or even by mixing culture media in which host cells expressing the proteins, have been cultured. See, International Publication No. WO 96/04301, published February 15, 1996, for a description of such complexes. Relatively small polypeptides, i.e., up to about 50 amino acids in length, can be conveniently synthesized chemically, for example by any of several techniques that are known to those skilled in the peptide art. In general, these methods employ the sequential addition of one or more amino acids to a growing peptide chain. Normally, either the amino or carboxyl group of the first amino acid is protected by a suitable protecting group. The protected or derivatized amino acid can then be either attached to an inert solid support or utilized in solution by adding the next amino acid in the sequence having the complementary (amino or carboxyl) group suitably protected, under conditions that allow for the formation of an amide linkage. The protecting group is then removed from the newly added amino acid residue and the next amino acid (suitably protected) is then added, and so forth. After the desired amino acids have been linked in the proper sequence, any remaining protecting groups (and any solid support, if solid phase synthesis techniques are used) are removed sequentially or concurrently, to render the final polypeptide. By simple modification of this general procedure, it is possible to add more than one amino acid at a time to a growing chain, for example, by coupling (under conditions which do not racemize chiral centers) a protected tripeptide with a properly protected dipeptide to form, after deprotection, a pentapeptide. See, e.g., J. M. Stewart and J. D. Young, Solid Phase Peptide Synthesis (Pierce Chemical Co., Rockford, IL 1984) and G. Barany and R. B. Merrifield, The Peptides: Analysis. Synthesis. Biology, editors E. Gross and J. Meienhofer, Vol. 2, (Academic Press, New York, 1980), pp. 3-254, for solid phase peptide synthesis techniques; and M. Bodansky, Principles of Peptide Synthesis, (Springer-Verlag, Berlin 1984) and E. Gross and J. Meienhofer, Eds., The Peptides: Analysis. Synthesis. Biology. Vol. 1, for classical solution synthesis. Typical protecting groups include t-butyloxycarbonyl (Boc), 9- fluorenylmethoxycarbonyl (Fmoc) benzyloxycarbonyl (Cbz); p-toluenesulfonyl (Tx); 2,4- dinitrophenyl; benzyl (Bzl); biphenylisopropyloxycarboxy-carbonyl, t- amyloxycarbonyl, isobornyloxycarbonyl, o-bromobenzyloxycarbonyl, cyclohexyl, isopropyl, acetyl, o-nitrophenylsulfonyl and the like. Typical solid supports are cross-linked polymeric supports. These can include divinylbenzene cross-linked-styrene-based polymers, for example, divinylbenzene- hydroxymethylstyrene copolymers, divinylbenzene-chloromethylstyrene copolymers and divinylbenzene-benzhydrylaminopolystyrene copolymers. The polypeptide analogs of the present invention can also be chemically prepared by other methods such as by the method of simultaneous multiple peptide synthesis. See, e.g., Houghten Proc. Natl. Acad. Sci. USA (1985) 82:5131-5135; U.S. Patent No. 4,631,211. Diagnostic and Vaccine Applications The intracellularly produced Env polypeptides of the present invention, complexes thereof, or the polynucleotides coding therefor, can be used for a number of diagnostic and therapeutic purposes. For example, the proteins and polynucleotides or antibodies generated against the same, can be used in a variety of assays, to determine the presence of reactive antibodies/and or Env proteins in a biological sample to aid in the diagnosis of HIV infection or disease status or as measure of response to immunization. The presence of antibodies reactive with the Env (e.g., gpl 0) polypeptides and, conversely, antigens reactive with antibodies generated thereto, can be detected using standard electrophoretic and immunodiagnostic techniques, including immunoassays such as competition, direct reaction, or sandwich type assays. Such assays include, but are not limited to, western blots; agglutination tests; enzyme-labeled and mediated immunoassays, such as ELISAs; biotin/avidin type assays; radioimmunoassays; immunoelectrophoresis; immunoprecipitation, etc. The reactions generally include revealing labels such as fluorescent, chemi luminescent, radioactive, or enzymatic labels or dye molecules, or other methods for detecting the formation of a complex between the antigen and the antibody or antibodies reacted therewith. Solid supports can be used in the assays such as nitrocellulose, in membrane or microtiter well form; polyvinylchloride, in sheets or microtiter wells; polystyrene latex, in beads or microtiter plates; polyvinylidine fluoride; diazotized paper; nylon membranes; activated beads, and the like. Typically, the solid support is first reacted with the biological sample (or the gpl20 proteins), washed and then the antibodies, (or a sample suspected of containing antibodies), applied. After washing to remove any non-bound ligand, a secondary binder moiety is added under suitable binding conditions, such that the secondary binder is capable of associating selectively with the bound ligand. The presence of the secondary binder can then be detected using techniques well known in the art. Typically, the secondary binder will comprise an antibody directed against the antibody ligands. A number of anti-human immunoglobulin (Ig) molecules are known in the art (e.g., commercially available goat anti-human Ig or rabbit anti-human Ig). Ig molecules for use herein will preferably be of the IgG or IgA type, however, IgM may also be appropriate in some instances. The Ig molecules can be readily conjugated to a detectable enzyme label, such as horseradish peroxidase, glucose oxidase, Beta-galactosidase, alkaline phosphatase and urease, among others, using methods known to those of skill in the art. An appropriate enzyme substrate is then used to generate a detectable signal. Alternatively, a "two antibody sandwich" assay can be used to detect the proteins of the present invention. In this technique, the solid support is reacted first with one or more of the antibodies directed against Env (e.g., gpl20), washed and then exposed to the test sample. Antibodies are again added and the reaction visualized using either a direct color reaction or using a labeled second antibody, such as an anti-immunoglobulin labeled with horseradish peroxidase, alkaline phosphatase or urease. Assays can also be conducted in solution, such that the viral proteins and antibodies thereto form complexes under precipitating conditions. The precipitated complexes can then be separated from the test sample, for example, by centrifugation. The reaction mixture can be analyzed to determine the presence or absence of antibody-antigen complexes using any of a number of standard methods, such as those immunodiagnostic methods described above. The modified Env proteins, produced as described above, or antibodies to the proteins, can be provided in kits, with suitable instructions and other necessary reagents, in order to conduct immunoassays as described above. The kit can also contain, depending on the particular immunoassay used, suitable labels and other packaged reagents and materials (i.e. wash buffers and the like). Standard immunoassays, such as those described above, can be conducted using these kits. The Env polypeptides and polynucleotides encoding the polypeptides can also be used in vaccine compositions, individually or in combination, in e.g., prophylactic (i.e., to prevent infection) or therapeutic (to treat HIV following infection) vaccines. The vaccines can comprise mixtures of one or more of the modified Env proteins (or nucleotide sequences encoding the proteins), such as Env (e.g., gpl20) proteins derived from more than one viral isolate. The vaccine may also be administered in conjunction with other antigens and immunoregulatory agents, for example, immunoglobulins, cytokines, lymphokines, and chemokines, including but not limited to IL-2, modified IL-2 (cysl25→serl25), GM-CSF, IL- 12, γ-interferon, IP-10, MlPlβ and RANTES. The vaccines may be administered as polypeptides or, alternatively, as naked nucleic acid vaccines (e.g., DNA), using viral vectors (e.g., retroviral vectors, adenoviral vectors, adeno-associated viral vectors) or non-viral vectors (e.g., liposomes, particles coated with nucleic acid or protein). The vaccines may also comprise a mixture of protein and nucleic acid, which in turn may be delivered using the same or different vehicles. The vaccine may be given more than once (e.g., a "prime" administration followed by one or more "boosts") to achieve the desired effects. The same composition can be administered as the prime and as the one or more boosts. Alternatively, different compositions can be used for priming and boosting. The vaccines will generally include one or more "pharmaceutically acceptable excipients or vehicles" such as water, saline, glycerol, ethanol, etc. Additionally, auxiliary substances, such as wetting or emulsifying agents, pH buffering substances, and the like, may be present in such vehicles. A carrier is optionally present which is a molecule that does not itself induce the production of antibodies harmful to the individual receiving the composition. Suitable carriers are typically large, slowly metabolized macromolecules such as proteins, polysaccharides, polylactic acids, polyglycollic acids, polymeric amino acids, amino acid copolymers, lipid aggregates (such as oil droplets or liposomes), and inactive vims particles. Such carriers are well known to those of ordinary skill in the art. Furthermore, the Env polypeptide may be conjugated to a bacterial toxoid, such as toxoid from diphtheria, tetanus, cholera, etc. Adjuvants may also be used to enhance the effectiveness of the vaccines. Such adjuvants include, but are not limited to: (1) aluminum salts (alum), such as aluminum hydroxide, aluminum phosphate, aluminum sulfate, etc.; (2) oil-in-water emulsion formulations (with or without other specific immunostimulating agents such as muramyl peptides (see below) or bacterial cell wall components), such as for example (a) MF59 (International Publication No. WO 90/14837), containing 5% Squalene, 0.5% Tween 80, and 0.5%o Span 85 (optionally containing various amounts of MTP-PE (see below), although not required) formulated into submicron particles using a microfluidizer such as Model HOY microfluidizer (Micro fluidics, Newton, MA), (b) S AF, containing 10% Squalane, 0.4%> Tween 80, 5% pluronic-blocked polymer L121, and thr-MDP (see below) either microfluidized into a submicron emulsion or vortexed to generate a larger particle size emulsion, and (c) Ribi™ adjuvant system (RAS), (Ribi Immunochem, Hamilton, MT) containing 2% Squalene, 0.2%> Tween 80, and one or more bacterial cell wall components from the group consisting of monophosphorylipid A (MPL), trehalose dimycolate (TDM), and cell wall skeleton (CWS), preferably MPL + CWS (Detox™); (3) saponin adjuvants, such as Stimulon™ (Cambridge Bioscience, Worcester, MA) may be used or particle generated therefrom such as ISCOMs (immunostimulating complexes); (4) Complete Freunds Adjuvant (CFA) and Incomplete Freunds Adjuvant (IF A); (5) cytokines, such as interleukins (IL-1, IL-2, etc.), macrophage colony stimulating factor (M-CSF), tumor necrosis factor (TNF), etc.; (6) detoxified mutants of a bacterial ADP-ribosylating toxin such as a cholera toxin (CT), a pertussis toxin (PT), or an E. coli heat-labile toxin (LT), particularly LT-K63 (where lysine is substituted for the wild-type amino acid at position 63) LT-R72 (where arginine is substituted for the wild-type amino acid at position 72), CT-S109 (where serine is substituted for the wild-type amino acid at position 109), and PT-K9/G129 (where lysine is substituted for the wild-type amino acid at position 9 and glycine substituted at position 129) (see, e.g., International Publication Nos. W093/13202 and W092/19265); and (7) other substances that act as immunostimulating agents to enhance the effectiveness of the composition. Muramyl peptides include, but are not limited to, N-acetyl-muramyl-L-threonyl-D- isoglutamine (thr-MDP), N-acteyl-normuramyl-L-alanyl-D-isogluatme (nor-MDP), N- acetylmuramyl-L-alanyl-D-isogluatminyl-L-alanine-2-(r-2'-dipalmitoyl-5«-glycero-3- huydroxyphosphoryloxy)-ethylamine (MTP-PΕ), etc. Typically, the vaccine compositions are prepared as injectables, either as liquid solutions or suspensions; solid forms suitable for solution in, or suspension in, liquid vehicles prior to injection may also be prepared. The preparation also may be emulsified or encapsulated in liposomes for enhanced adjuvant effect, as discussed above. The vaccines will comprise a therapeutically effective amount of the modified Εnv proteins, or complexes of the proteins, or nucleotide sequences encoding the same, and any other of the above-mentioned components, as needed. By "therapeutically effective amount" is meant an amount of a modified Εnv (e.g., gpl20) protein which will induce a protective immunological response in the uninfected, infected or unexposed individual to which it is administered. Such a response will generally result in the development in the subject of a secretory, cellular and/or antibody-mediated immune response to the vaccine. Usually, such a response includes but is not limited to one or more of the following effects; the production of antibodies from any of the immunological classes, such as immunoglobulins A, D, E, G or M; the proliferation of B and T lymphocytes; the provision of activation, growth and differentiation signals to immunological cells; expansion of helper T cell, suppressor T cell, and/or cytotoxic T cell. Preferably, the effective amount is sufficient to bring about treatment or prevention of disease symptoms. The exact amount necessary will vary depending on the subject being treated; the age and general condition of the individual to be treated; the capacity of the individual's immune system to synthesize antibodies; the degree of protection desired; the severity of the condition being treated; the particular Env polypeptide selected and its mode of administration, among other factors. An appropriate effective amount can be readily determined by one of skill in the art. A "therapeutically effective amount" will fall in a relatively broad range that can be determined through routine trials. Once formulated, the nucleic acid vaccines may be accomplished with or without viral vectors, as described above, by injection using either a conventional syringe or a gene gun, such as the Accell® gene delivery system (PowderJect Technologies, Inc., Oxford, England). Delivery of DNA into cells of the epidermis is particularly preferred as this mode of administration provides access to skin-associated lymphoid cells and provides for a transient presence of DNA in the recipient. Both nucleic acids and/or peptides can be injected either subcutaneously, epidermally, intradermally, mtramucosally such as nasally, rectally and vaginally, intraperitoneally, intravenously, orally or intramuscularly. Other modes of administration include oral and pulmonary administration, suppositories, needle-less injection, transcutaneous and transdermal applications. Dosage treatment may be a single dose schedule or a multiple dose schedule. Administration of nucleic acids may also be combined with administration of peptides or other substances. While the invention has been described in conjunction with the preferred specific embodiments thereof, it is to be understood that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. Experimental Below are examples of specific embodiments for carrying out the present invention. The examples are offered for illustrative purposes only, and are not intended to limit the scope of the present invention in any way. Efforts have been made to ensure accuracy with respect to numbers used (e.g., amounts, temperatures, etc.), but some experimental error and deviation should, of course, be allowed for. EXAMPLE 1 A.l. Best-Fit and Homology Searches The crystal stmcture of HXB-2 gp 120 was downloaded from the Brookhaven database (COMPLEX (HIV ENVELOPE PROTEIN/CD4/FAB) 15-JUN-98 1GC1 TITLE: HIV-1 GP120 CORE COMPLEXED WITH CD4 AND A NEUTRALIZING HUMAN ANTIBODY). Beta strands 3, 2, 21, and 20 of gp 120 form a sheet near the CD4 binding site. Strands β-3 and β-2 are connected by the VI V2 loop. Strands β-21 and β-20 are connected by another small loop. The H-bonds at the interface between strands β-2 and β-21 are the only connection between domains of the "lower" half of the protein (joining helix alpha 1 to the CD4 binding site). This beta sheet and these loops mask some antigens (e.g., antigens which may generate neutralizing antibodies) that are only exposed during the CD4 binding. Constructs that remove enough of the beta sheet to expose the antigens in the CD4 binding site, but leave enough of the protein to allow correct folding were designed. Specifically targeted were modifications to the small loop and, optional deletion of the N1 N2 loops. Three different types of constructs were designed: (1) constmcts encoding polypeptides that leave the number of residues making up the entire 4-strand beta sheet intact, but replace one or more residues; (2) constmcts that encode polypeptide having at least one residue of at least one beta strand excised or (3) constmcts encoding polypeptides having at least two residues of at least one beta strand excised. Thus, a total of 6 different turns were needed to rejoin the ends of the strands. Initially, residues in the small loop (residues 427-430, relative to HXB-2) and connected beta strands (β-20 and β-21) were modified to contain Gly and Pro (common in beta turns). These sequences were then used as the target to match in each search. The geometry of the target was matched to known proteins in the Brookhaven Protein Data Bank. In particular, 5-residue turns (including an overlapping single residue at the N-terminal, the 2 residue target turn and 2 overlapping residues at the C-terminal) were searched in the databases. In other words, these modified loops add a 2 residue turn that should be able to support a geometry that will maintain the beta-sheet stmcture of the wild type protein. The calculations were performed using the default parameters in the Loop Search feature of the Biopolymer module of the Sybyl molecular modeling package. In each case, the 25 best fits based on geometry alone were reviewed and, of those, several selected for homology and fit. In addition, it was also determined what modifications could be made to remove most of the V1 N2 loop (residues 124-198, relative to HXB-2) yet leave the geometry of the protein intact. As with the small loop, constmcts were also designed which excised one or more residues from the β-2 strand (residues 119-123 of HXB-2), the β-3 strand (residues 199- 201 of HXB-2) or both β-2 and β-3. For these constmcts, known loops were searched to match the geometry of a pentamer (including two remaining residues from the Ν-terminal side, a 2 residue turn and 1 C-terminal residue). For these searches, Gly-Gly was preferred as the insert along with at least one C-terminal substitution. A.2. Small Loop Replacements In one aspect, the native sequence was replaced with residues that expose the CD4 binding site, but leave the overall geometry of the protein relatively unchanged. For the small loop replacements, the target to match was: ASΝ425-MET426-GLY427-GLY428- GLY431. Results of the search are summarized in Table 1. Table 1 : Search of Small Loop (Asn425 through Gly431) Based on these results, constmcts encoding Gly-Gly (#7), Gly-Ser (#12) or Gly-Gly- Asn (#7) were recommended. As N1/N2 and one or more residues of β-2 and β-3 are also optionally deleted in the modified polypeptides of the invention, known loops to match the geometry of the V1/V2 loop were also searched. The N1/N2 loop the target to match was: Lysl21-Leu-122-Glyl23- Glyl24-Serl99. Some notable matches are shown in Table 2: Table 2: Search of V1/V2 loop (Lysl21 through Serl99) Based on these searches, constmcts encoding Gly-Asn in place of V1/V2 were recommended. A.3. One Additional Residue Excisions For a slightly tmncated small loop, one more residue was trimmed from each beta strand to slightly shorten the beta sheet. The target to match was: ILE424-ASN425- GLY426-GLY427-LYS432. Results are shown in Table 3: Table 3: Search of Beta sheet shortened by One residue (Ile424 through Lys432) Although these searches showed more variation and worse fits than the previous tmncation, the Pro-Val or Pro-Leu encoding constmcts were very similar. Accordingly, Ala- Pro encoding constmcts were recommended. Sequences encoding gpl20 polypeptides having N1 N2 deleted and an additional residue from β-2 or β-3 excised were also searched. The N1/V2 loop the target to match was: VAL120-LYS121-GLY122-GLY123-VAL200. Some notable matches are shown in Table 4. Table 4: Search of V1/N2 loop (Vall20 through Val200) The constmct encoding Ala-Pro (e.g., #1) was recommended. A.4. Further Excisions In yet another tmncation, an additional residue was trimmed from the β-20 and β-21 strands to further shorten the beta sheet. The target to match was ILE423-ILE424-GLY425- GLY426-ALA433. Notable matches are shown in Table 5. Table 5: Search of Beta sheet shortened by Two Residues (Ile423 through Ala433) A constmct encoding Gly-Gly (e.g., #3), which has 100% homology, was recommended. Also searched were sequences encoding a deleted VI /V2 region and at least two residues excised from β-2, β-3 or at least one residue excised from β-2 and β-3. The target to match was: CYS119-VAL120-GLY121-GLY122-ILE201. Notable matches are shown in Table 6. Table 6: Search of V1/V2 loop (Cysl 19 through Ile201) It was determined that both constmcts would be used. B.1. Constmcts encoding modified Env polypeptides As described above, the native loops extmding from the 4-β antiparallel-stands were excised and replaced with 1 to 3 residue turns. The loops were replaced so as to leave the entire β-strands or excised by trimming one or more amino acid from each side of the connected strands. The ends of the strands were rejoined with turns that preserve the same backbone geometry (e.g., tertiary stmcture of β-20 and β-21), as determined by searching the Brookhaven Protein Data Bank. Table 7 A is a summary of the tmncations of the variable regions 1 and 2 recommended for this study, as determined in Example 1.A. above. Table 7A As previously noted, the polypeptides encoded by the constmcts of the present invention are numbered relative to HXB-2, but the particular amino acid residue of the polypeptides encoded by these exemplary constmcts is based on SF-162. Thus, for example, although amino acid residue 195 in HXB-2 is a serine (S), constmcts encoding polypeptides having then wild type SF162 sequence will have an asparagine (N) at this position. Table 7B shows just three of the variations in amino acid sequence between strains HXB-2 and SF162. The entire sequences, including differences in residue and amino acid number, of HXB-2 and SF162 are shown in the alignment of Figure 2 (SEQ ID NOs:l and 2). Table 7B Constmcts containing deletions in the β-20 strand, β-21 stand and small loop were also constmcted. Shown in Table 8 are constmcts encoding tmncations in these regions. The constmcts in Table 8 are numbered relative to HXB-2 but the unmodified amino acid sequence is based on SF162. Thus, the constmct encodes an arginine (Arg) as is found in SF162 in the amino acid position numbered 426 relative to HXB-2 (See, also, Table 7B). Changes from wildtype (SF162) are shown in bold in Table 8B. Table 8 The deletion constmcts shown in Tables 7 and 8 for each one of the β-strands and combinations of them are constmcted. These deletions will be tested in the Env forms gpl20, gpl40 and gplόO from different HIV strains like subtype B strains (e.g., SF162, US4, SF2), subtype E strains (e.g., CM235) and subtype C strains (e.g., AF1 10968 or AF110975). Exemplary constmcts for SF162 are shown in the Figures and are summarized in Table 9. As noted above in Figure 2 and Table 7B, in the bridging sheet region, the amino acid sequence of SF162 differs from HXB-2 in that the Met426 of HXB-2 is an Arg in SF162. In Table 9, V1/V2 refers to deletions in the V1/V2 region; # bsm refers to a modification in the bridging sheet small loop. Combinations of V1/V2 deletions and bridging sheet small loop modifications in addition to those specifically shown in Table 9 are also within the scope of the present invention. Various forms of the different embodiments of the invention, described herein, may be combined. The first screening will be done after transient expression in COS-7, RD and/or 293 cells. The proteins that are expressed will be analyzed by immunoblot, ELISA, and for binding to mAbs directed to the CD4 binding site and other important epitopes on gpl20 to determine integrity of stmcture. They will also be tested in a CD4 binding assay and, in addition, the binding of neutralizing antibodies, for example using patient sera or mAb 448D (directed to Glu370 and Tyr384, a region of the CD4 binding groove that is not altered by the deletions). The immunogenicity of these novel Env glycoproteins will be tested in rodents and primates. The stmctures will be administered as DNA vaccines or adjuvanted protein vaccines or in combined modalities. The goal of these vaccinations will be to archive broadly reactive neutralizing antibody responses.

Claims: Claims: What is claimed is: A polynucleotide encoding a modified HIV Env polypeptide wherein the polypeptide has at least one amino acid deleted or replaced in the region corresponding to residues 420 to 436 relative to HXB-2 (SEQ ID NO:l). 2. The polynucleotide of claim 1, wherein the region corresponding to residues 124- 198 relative to HXB-2 is deleted and at least one amino acid is deleted or replaced in the regions corresponding to the residues 119 to 123 and 199 to 210 relative to HXB-2 (SEQ ID NO:l). 3. The polynucleotide of claim 1, wherein at least one amino acid in the region corresponding to residues 427 through 429 relative to HXB-2 (SEQ ID NO: 1) is deleted or replaced. 4. The polynucleotide of claim 2, wherein at least one amino acid of the in the region corresponding to residues 427 through 429 relative to HXB-2 (SEQ ID NO:l) is deleted or replaced. 5. The polynucleotide of claim 1 , wherein the amino acid sequence of the modified HIV Env polypeptide is based on strain SF162. 6. An immunogenic modified HIV Env polypeptide having at least one amino acid deleted or replaced in the region corresponding to residues 420 through 436, relative to HXB- 2 (SEQ ID NO: 1). 7. The polypeptide of claim 6, wherein one amino acid is deleted in the region corresponding to residues 420 through 436, relative to HXB-2 (SEQ ID NO: 1). 8. The polypeptide of claim 6, wherein more than one amino acid is deleted in the region corresponding to residues 420 through 436, relative to HXB-2 (SEQ ID NO:l). 9. The polypeptide of claim 6, wherein at least one amino acid is replaced in the region corresponding to residues 420 through 436, relative to HXB-2 (SEQ ID NO: 1). 10. The polypeptide of claim 6, wherein at least one amino acid residue between about amino acid residue 427 and amino acid residue 429 relative to HXB-2 (SEQ ID NO:l) is deleted or replaced. 11. The polypeptide of claim 6, wherein the VI and V2 regions of the polypeptide are tmncated. 12. The polypeptide of claim 10, wherein the VI and V2 regions of the polypeptide are tmncated. 13. The polypeptide of claim 6, wherein the amino acid sequence of the modified HIV Env polypeptide is based on strain SF162. 14. A constmct comprising the nucleotide sequence depicted in Figure 6 (SEQ ID NO:3). 15. A constmct comprising the nucleotide sequence depicted in Figure 7 (SEQ ID NO:4). 16. A constmct comprising the nucleotide sequence depicted in Figure 8 (SEQ ID NO:5). 17. A constmct comprising the nucleotide sequence depicted in Figure 9 (SEQ ID NO:6). 18. A constmct comprising the nucleotide sequence depicted in Figure 10 (SEQ ID NO:7). 19. A constmct comprising the nucleotide sequence depicted in Figure 11 (SEQ ID NO:8). 20. A constmct comprising the nucleotide sequence depicted in Figure 12 (SEQ ID NO:9). 21. A constmct comprising the nucleotide sequence depicted in Figure 13 (SEQ ID NO: 10). 22. A constmct comprising the nucleotide sequence depicted in Figure 14 (SEQ ID NO:l l). 23. A constmct comprising the nucleotide sequence depicted in Figure 15 (SEQ ID NO: 12). 24. A constmct comprising the nucleotide sequence depicted in Figure 16 (SEQ ID NO: 13). 25. A constmct comprising the nucleotide sequence depicted in Figure 17 (SEQ ID NO:14). 26. A constmct comprising the nucleotide sequence depicted in Figure 18 (SEQ ID NO: 15). 27. A constmct comprising the nucleotide sequence depicted in Figure 19 (SEQ ID NO: 16). 28. A construct comprising the nucleotide sequence depicted in Figure 20 (SEQ ID NO: 17). 29. A constmct comprising the nucleotide sequence depicted in Figure 21 (SEQ ID NO:18). 30. A constmct comprising the nucleotide sequence depicted in Figure 22 (SEQ ID NO: 19). 31. A constmct comprising the nucleotide sequence depicted in Figure 23 (SEQ ID NO:20). 32. A constmct comprising the nucleotide sequence depicted in Figure 24 (SEQ ID NO-21). 33. A constmct comprising the nucleotide sequence depicted in Figure 25 (SEQ ID NO:22). 34. A constmct comprising the nucleotide sequence depicted in Figure 26 (SEQ ID NO:23). 35. A constmct comprising the nucleotide sequence depicted in Figure 27 (SEQ ID NO:24). 36. A constmct comprising the nucleotide sequence depicted in Figure 28 (SEQ ID NO:25). 37. A constmct comprising the nucleotide sequence depicted in Figure 29 (SEQ ID NO:26). 38. A vaccine composition comprising a polynucleotide encoding a modified Env polypeptide according to any one of claims 1-5. 39. A vaccine composition comprising a polynucleotide constmct encoding a modified Env polypeptide according to any of claims 14-37. 40. A vaccine composition comprising a modified Env polypeptide according to any of claims 6-13. 41. The vaccine composition of any of claims 38-40, further comprising an adjuvant. 42. A method of inducing an immune response in subject comprising, administering a polynucleotide according to any one of claims 1-5 in an amount sufficient to induce an immune response in the subject. 43. A method of inducing an immune response in subject comprising, administering a polynucleotide constmct according to any one of claims 14-37 in an amount sufficient to induce an immune response in the subject. 44. A method of inducing an immune response in a subject comprising administering a composition comprising a modified Env polypeptide according to any one of claims 6-13, wherein the composition is administered in an amount sufficient to induce an immune response in the subject 45. The method of any of claims 42-44 further comprising administering an adjuvant to the subject. 46. A method of inducing an immune response in a subject comprising (a) administering a first composition comprising a polynucleotide according to any of claims 1-5 in a priming step and (b) administering a second composition comprising a modified Env polypeptide according to any of claims 6-13, as a booster, in an amount sufficient to induce an immune response in the subject. 47. The method of claim 46 wherein the first composition or second composition further comprise an adjuvant. 48. The method of claim 46 wherein the first and second compositions further comprise an adjuvant.

Inventors: BARNETT SUSAN, HARTOG KARIN, MARTIN ERIC, BARNETT, SUSAN, HARTOG, KARIN, MARTIN, ERIC
Applicants: CHIRON CORP, CHIRON CORPORATION
Application year: 1999
Main classification: C12N15
IPC codes: A61K39, A61K48, A61P31, A61P37, C07K14, C12N7, C12N15