A NOVEL CHIMERIC PROTEIN FOR PREVENTION AND TREATMENT OF HIV INFECTION

Abstract: This invention relates to bispecific fusion proteins effective in viral neutralization. More specifically, such proteins have two different binding domains, an inducing-binding domain and an induced-binding domain, functionally linked by a peptide linker. Such proteins, nucleic acid molecules encoding them, and their production and use in preventing or treating viral infections are provided. One prototypical bispecific fusion protein is sCD4-SCFv(17b), in which a soluble CD4 fragment (containing domains D1 and D2) is fused to a single chain Fv portion of antibody 17b via a linker.

Description: A NOVEL CHIMERIC PROTEIN FOR PREVENTION AND TREATMENT OF HIV INFECTION FIELD OF THE INVENTION This invention relates to proteins useful in the prevention and treatment of human immunodeficiency virus (HIV) infection More specifically, it relates to fusion proteins that bind to two sites on a single target protein, especially when one binding domain of the fusion protein binds to an induced site (on the target protein) that is exposed by the binding ofthe other binding domain ofthe fusion protein BACKGROUND OF THE INVENTION Acquired immune deficiency syndrome (AIDS) is a fatal disease of growing prevalence m the modern world The agent responsible for this disease, human immunodeficiency virus (HIV), was first identified in 1983 HIV is a T-lymphotropic retrovirus that invades and replicates m cells of the immune system, primarily helper T- lymphocytes The consequent dysfunction in T-lymphocyte- mediated immunity results m an immuno-compromised condition Patients usually die of associated opportunistic viral, bacterial or fungal infections A characteπstics laboratory finding in AIDS is the decrease in helper T lymphocytes (CD4), and particularly a steady decrease m the ratio of CD4 to suppressor T lymphocytes CD8 as the disease progresses Virus binding is primarily mediated by interaction of gpl20, the external subunit ofthe HIV envelope glycoprotein (Env) with CD4 protein and various coreceptor molecules (one of several alternative chemokme receptors) These interactions then activate the gρ41 transmembrane subunit ofthe envelope glycoprotein, to cause fusion between the virus and cell membranes See Retroviruses, Coffin et al (eds ) (1997) CSHP, New York, Ch 11 The humoral immune system is triggered by HIV mfection, though it generally does not provide sufficient protection to ward off the mfection Env is the major target of anti-HIV neutralizing antibodies (Wyatt et al Nature 393 705-711, 1998) However, Env has evolved so that its relatively invariant neutralizing determinants are protected from the humoral immune system Antibodies to these regions therefore are generated at a low frequency and their neutralizing activities in vivo are generally weak Certain variable regions (e g , the V3 loop) are targets for potent neutralizing antibodies, but these are typically restricted to a limited number of HlV-strams (in other words, they are not broadly cross-reactive) For a list of several gpl20 antigemc epitopes and consensus definitions ofthe conserved and variable regions of gpl20, see published PCT application PCT US98/02766 (publication number WO 98/36087) and Coffin et al (eds ) (1997) CSHP, New York, Ch 12 A neutralizing monoclonal antibody (MAb) with potent and broadly cross-reactive activity would have great potential value m protocols aimed at preventing HIV infection before or immediately after exposure, for example in neonatal transmission, post-exposure prophylaxis, and as a topical inhibitor. Such a MAb may also be useful in treating chronic infection (D'Souza et al J Infect Dis 175.1056-1062, 1997) However only a handful of MAbs with the desired broadly cross- reactive neutralizing activities have been described Because of limited potency and cross-reactivity of these molecules, even the three most promising candidates have questionable clinical value (D'Souza et al , 1997). Extensive efforts are underway to provide immunological or pharmacological approaches to controlling HIV infection (Coffin et al , 1997, Ch. 12) The specific interaction between gpl20 and CD4 has been exploited in efforts to provide a possible treatment for HIV infection. See, e g , U.S Patent No. 5,817,767; Capon et al , Nature 337:525-531, 1989 A soluble fragment of CD4 (sCD4), comprising the first and second domains of this protein (D1D2) has been generated, and this molecule interacts specifically with gpl20, essentially serving as a molecular decoy sCD4 has been shown to block the spread of HIV between cultured cells (Moore et al , Science 250.1139-1142, 1990). However, clinical trials with sCD4 were inconclusive as to the effects on human viral load (Schooley et al , Ann Internal Med 112 247-253, 1990, Kahn et al , Ann Internal Med. 112:254- 261, 1990). Subsequent studies mdicated that, unlike laboratory-adapted HIV strains, isolates obtained directly from mfected patients (primary isolates) are resistant to neutralization by sCD4 (Dan * et al , Proc Natl Acad Set 87:6574-6578, 1990). In another approach, researchers have generated an antibody-like molecule by fusing the binding portion of CD4 to the constant region (Fc) of a human IgG heavy chain (see, e g , Capon et al , Nature 337:525-531, 1989; and Byrn et al , Nature 344:667-670, 1990) This molecule, termed CD4 lmmunoadhesin, exploits the native functions of lmmunoglobuhn Fc, such as its ability to fix complement, its ability to mediate antibody-dependent cytotoxicity, and its transfer across the placental barrier. There are significant drawbacks to using Fc receptors in association with CD4, because such a construct may be responsible for targeting HIV to Fc-receptor beaπng cells (e g macrophages), and might lead to increased transmission of HIV- 1 across the placental barrier A complementary recombinant molecule has also been made, wherein the binding portion of CD4 is fused to the Fv region of an antibody directed to human CD3, this "Janusm" molecule may be able to re-target cytotoxic T-lymphocytes onto HIV-infected cells (Traunecker et al , Embo J 10 * 3655-3659, 1991 ; Traunecker et al , Int J Cancer Supp 7 51-52, 1992) Janusm has been reported to inhibit HIV-mediated cell fusion when administered in vitro with neutralizing antibody to either gp41 or the V3 loop of gpl20 (Allaway et al , AIDS Res Hum Retroviruses 9 581-587, 1993, U S. Patent No. 5,817,767) This system is inherently complicated and inefficient because multiple molecules must be co-administered to the subject This invention is directed to proteins that address key failures ofthe prior art SUMMARY OF THE INVENTION The present invention takes advantage ofthe finding that the neutralizing activities of MAbs against certain highly conserved determinants ofthe coreceptor-bmdmg region of gpl20 are revealed only when CD4 first bmds to gpl20 (as m an sCD4-actιvated fusion assay) Although some MAbs to CD4-ιnduced epitopes (e g , the human MAbs 17b and 48d, Thah et al , J Virol 67:3978-3988, 1993) are broadly cross-reactive with Envs from diverse HIV genetic subtypes (Clades), these neutralizing epitopes are only briefly exposed in vivo, and therefore have provided poor targets for clinically protective antibody bindmg. The inventors have overcome these difficulties by creating a fusion protein containing a fragment of CD4 attached via a linker to a human single chain Fv directed against an mduced (for example, a CD4-mduced) neutralizing epitope on gpl20, for instance a coreceptor-bmding determinant of gpl20. CD4-bmdmg exposes highly conserved gpl20 determinants involved in binding to coreceptor; therefore the provided fusion protein will have the properties of a highly potent, broadly cross-reactive neutralizing antibody with high in vivo activity and no Fc-mediated undesirable targeting properties. When the fusion protem is substantially derived from human proteins, it has minimal lmmunogenicity and toxicity in humans Such an agent has great value in the prevention of infection during or immediately after HIV exposure (mother/mfant transmission, post-exposure prophylaxis, topical inhibitor), and also m the treatment of chronic infection. Accordmgly, a first embodiment ofthe current invention is a neutralizing bispecific fusion protein capable of binding to two sites on a target protein. This protein has two different binding domams, an mducing-bmdmg domain and an induced-binding domain, functionally linked by a peptide linker Nucleic acid molecules encoding such fusion proteins are further aspects of this invention Also encompassed m the invention are protein analogs, derivatives, or mimetics of such neutralizing bispecific fusion proteins The arrangement of the inducing- and induced-binding domains need not be organized m binding sequence; the ammo-proximal or carboxy-proximal binding domam ofthe fusion protein may be either the induced-binding or the rnducing-bmding domain. In certain embodiments, the linker of this invention is of such length and secondary structure that the linker allows the second binding domam to be m binding proximity to the induced epitope ofthe target protein when the first binding domain is bound to the inducing site ofthe target protein The linker may for instance be substantially flexible Linkers of about 25- 100 angstroms (A), or about 15-100 ammo acid residues in length, are examples of linkers of a sufficient length to maintain the second binding domain m binding proximity to the induced epitope Specific examples of linkers will include one or more occurrences of the ammo acid sequence represented by SEQ ID NO 1 For instance, the invention encompasses bispecific fusion proteins wherein the two binding domains are functionally linked by the amino acid sequence represented by SEQ ID NO 2 Targets for bispecific fusion proteins according to this invention include vnal envelope proteins For instance, viral envelope proteins from the human immunodeficiency virus (HIV) are targets for the disclosed invention In a specific embodiment ofthe invention, the viral envelope protein target is gpl20 In further aspects ofthe invention, the first binding domam is capable of binding to an inducing site on the target protein, thereby exposing an induced epitope For instance, the first bindmg domam can be a ligand such as CD4 or fragments thereof Alternatively, such a first binding domain may be a bindmg portion of a variable region of an antibody heavy or light chain The first bindmg domam may include, for example, an antibody-bmdmg domain, a smgle-chain Fv (SCFv), or bindmg fragments thereof The second binding domam, which is capable of forming a neutralizing complex with an mduced epitope ofthe target protein, may be for example an antibody or fragments thereof, such as the variable region, Fv, Fab or antigen-binding domain of an antibody Another example ofthe second binding domain ofthe fusion protein is an engineered single-chain Fv (SCFv) In some particular examples where HIV gpl20 is the target, and the inducing site is the gpl20 CD4 binding site, the induced epitope may be a coreceptor-bindmg determinant of gpl20 Accordingly, aspects of this invention include proteins in which the first binding domain binds to gpl20 in such a way as to cause a CD4-ιnduced conformational change in the complexed gpl20 that exposes the second bindmg domain The first binding domain may be derived from a CD4 molecule, and include CD4 and soluble fragments thereof (sCD4, eg Dl, D1D2 and other such fragments), and proteins that mimic the biological activity of a CD4 molecule m binding to the inducing site of gpl20 In another embodiment ofthe invention, the first domain ofthe gpl20-targeted bispecific fusion protein is deπved from a CD4 anti-idiotypic antibody, or antibodies that mimic CD4 in exposmg epitopes The second domain ofthe gpl20-targeted bispecific fusion protein, which bmds to an epitope mduced by binding ofthe first fusion domain, may be chosen from domains and fragments of proteins that bind to such CD4 induced epitopes Antibodies directed to the induced epitopes, as well as the HIV coreceptor (e g Ά chemokine receptor), HIV coreceptor mimics, and fragments of HIV coreceptor protems, are examples of sources for the second binding domain of a gpl20-target bispecific fusion protein of this mvention Examples of chemokine receptors with HIV coreceptor activity include CXCR4, CCR5, CCR2B, and CCR3 Neutralizing antibodies, including 17b and 48d, are examples of antibodies Fusion proteins wherein the second domain is an engmeered single chain Fv (SCFv) deπved from such a neutralizing antibody are also encompassed A particular embodiment of this invention is a functional recombmant bispecific fusion protein capable of binding to two sites on gpl20, wherein the inducing-binding domain is sCD4, the induced-binding domain is SCFv(17b), and these two domains are linked by a linker of a length sufficient to maintain the SCFv(17b) in binding proximity an SCFv(17b) epitope when sCD4 is bound to gpl20 A prototypical bispecific fusion protein has the amino acid sequence shown in SEQ ID NO 3 Nucleic acid molecules encoding such a fusion protein are also encompassed, the prototypical nucleic acid molecule has the sequence shown m SEQ ID NO 4 Vectors and cells comprising this nucleic acid molecule are also encompassed m the current invention, as are transgenic plants and animals that express the nucleic acid molecule This invention also provides methods for producing functional recombmant bispecific fusion proteins capable of binding two sites on a target protein Such a protein can be produced in a prokaryotic or eukaryotic cell (e g , yeast, insect and mammalian cells), for instance by transforming or transfectmg such a cell with a recombmant nucleic acid molecule comprising a sequence which encodes a disclosed bispecific fusion protein Such transformed cells can then be cultured under conditions that cause production ofthe fusion protein, which is then recovered through protein purification means The protein can include a molecular tag, such as a six-histidine tag, to facilitate its recovery In particular embodiments, the protein has a hexa-histidme (hexa-his) tag, and a hrombm cleavage site The invention further provides methods for inactivating a target protein, for mstance a gpl20 protem, by contactmg the target protein with a fusion protein accordmg to this invention Where the target protein is gpl20, this method involves contacting gpl20 with a gpl20-targeted bispecific fusion protein, for mstance sCD4-SCFv(17b) Proteins according to the current mvention can also be used to neutralize a human immunodeficiency virus, by contacting the human immunodeficiency virus with a gpl20-targeted fusion protein according to this invention Binding of a viral or recombmant gpl20 protein to soluble CD4 or lymphocyte CD4 can also be blocked and/or prevented by contacting the gp 120 protem with gp 120- targeted fusion protem In any of these methods, a variant protem, analog or mimetic ofthe fusion protein as provided herein may also be used Proteins ofthe current invention can be used to inhibit virus replication or lnfectivity in a subject by administering to the subject an amount ofthe fusion protein (for example the sCD4- SCFv(17b) fusion protem), or a variant protein, analog or mimetic thereof, sufficient to inhibit HIV virus replication or mfectivity The fusion protem can be administered in a pharmaceutical composition, and given therapeutically to a person who is known to be infected with HIV, or prophylactically to help prevent infection in someone who has been exposed to the virus, or is at high risk for exposure Proteins of this invention can also be administered in combmation with another compound for the treatment or prevention of HIV infection, such as an HIV reverse transcπptase (RT), mtegrase, or protease inhibitor, another HIV-1 neutralizing antibody, or an Env- targeted toxin The other drug may be an HIV antiviral agent, an HIV anti-infective agent, and/or an immunomodulator, or combinations thereof BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a bar graph illustrating relative HIV-1 Env-mediated fusion, in the presence (+) or absence (-) of soluble CD4, between effector cells expressing Env (Ba-L) and target cells expressing CCR5 (co-receptor), but no CD4 (primary receptor) Figure 2 is a graph showmg that antibody 17b does not inhibit HIV-1 Env-mediated fusion in the conventional assay (open box CXCR4 and CD4 on target cell), but strongly inhibits cell fusion in the sCD4-actιvated assay (filled circle only CXCR4 on target cell, sCD4 provided) Additional experiments indicate that this phenomenon occurs with diverse Envs using either CXCR4 or CCR5, and that 17b has broad cross-reactive activity with Envs from genetically diverse HIV-1 isolates Figure 3 is a schematic diagram ofthe CD4-SCFv(17b) genetic construct The genetic construct encodes sCD4 (D1D2, plus the native CD4 N-terminal signal sequence), followed by the LI linker (Gly 4 Ser) 7 , which attaches the 17b SCFv (V H attached to V L via the L2 linker (Gly 4 Ser) 3 ), followed by the thrombin cleavage site and hexa-his tag There is a BamH I site m the middle of LI to facilitate production of constructs of different lengths Figure 4 is a drawing of mechanisms of binding of a sCD4-SCFv(17b) to gpl20, and the resulting neutralization of HIV Env function (fusion and infectivity) Figs 4A, 4B, and 4C depict the proposed interaction of HIV (mediated by gpl20) with the cell surface receptor CD4 and co-receptor CCR5, and the beginning of fusion (mediated by gp41) Interaction between gpl20 and CD4 (Fig 4A) causes a change m the conformation of gpl20 (Fig 4B), which enables interaction between gpl20 and CCR5 (Fig 4B) This triggers a conformational change m gp41 (Fig 4C), and leads to fusion Antibody (for instance, MAb 17b) binds poorly to the transiently exposed epitope on gpl20 (Fig 4B), and thus results in only weak neutralization of fusion or infection Figs 4D and 4E depict a proposed mechanism of sCD4-SCFV( 17b) neutralization of fusion In the presence ofthe bispecific chimeπc fusion protem, the sCD4 domain can bind to gpl20 and induce a conformational change in this protein sufficient to permit binding ofthe SCFV(17b) (Fig 4D) This effectively blocks fusion between the HIV and infection and the target cell SEQUENCE LISTING The nucleic and amino acid sequences listed in the accompanying sequence listing are shown using standard letter abbreviations for nucleotide bases, and three letter code for ammo acids Only one strand of each nucleic acid sequence is shown, but the complementary strand is understood as mcluded by any reference to the displayed strand SEQ ID NO 1 shows the basic repeat cassette for a linker polypeptide SEQ ID NO 2 shows a seven-repeat polypeptide linker SEQ ID NO 3 shows the amino acid sequence of the CD4-SCFv(17b) chimeπc protem SEQ ID NO * 4 shows the nucleic acid sequence of CD4-SCFv(17b). SEQ ID NO 5 shows the pair of synthetic oligonucleotides used to form the second half of the Stu I site near the 3' end of CD4 and to produce an Spe I overhang at the 3' end of an intermediate construct (site to be destroyed upon ligation into pCB-3); the oligonucleotide sequences reconstruct the remainder ofthe second domain of CD4 (through ser, 83 ), and encode an ammo acid sequence mcluding ala 182 ser, 83 of CD4 D2 plus an mtermediate 37 residue linker (gly 4 ser) 6 gly 4 thr 2 ser, followed directly by the universal translational termination sequence (UTS). SEQ ID NO: 6 shows the peptide sequence encoded for by the nucleotide sequences m SEQ ID NO: 5. SEQ ID NO- 7 shows the forward (5') primer used to amplify and attach the 17b SCFv sequence to the CD4- linker sequence pCD2. Italics show the region ofthe primer that overlaps SEQ ID NO: 8 shows the ammo acid sequence encoded by the oligonucleotide primer in SEQ ID NO.7. This sequence includes the GlySer residues at the third (Gly 4 Ser) repeat within LI (encoded by the BamH I site, followed by the remaining four (Gly 4 Ser) repeats, followed by the first ten residues ofthe 17b SCFv (shown in italics). SEQ ID NO: 9 shows the 3' primer used to amplify and attach the 17b SCFv sequence plus the hrombin cleavage site and the hexa-his tag to the CD4- linker sequence in pCD2. SEQ ID NO: 10 shows the peptide encoded for by the nucleotide sequence in SEQ ID NO: 9. DETAILED DESCRIPTION OF THE INVENTION I. Abbreviations and Definitions A. Abbreviations HIV. human immunodeficiency virus gpl20: the external subunit ofthe envelope glycoprotein complex of HIV Env: the envelope glycoprotein complex of HIV MAb: monoclonal antibody Fv: antibody "fragment variable", the variable region of an antibody SCFv: single-chain antibody vaπable region B. Definitions Unless otherwise noted, technical terms are used according to conventional understanding Definitions of common terms in molecular biology may be found in Benjamin Lewm, Genes V, Oxford University Press, 1994 (ISBN 0-19-854287-9), Kendrew et al (eds ), The Encyclopedia of Molecular Biology, Blackwell Science Ltd., 1994 (ISBN 0-632-02182-9), and Robert A Meyers (ed.), Molecular Biology and Biotechnology: a Comprehensive Desk Reference, VCH Publishers, Inc., 1995 (ISBN 1-56081-569-8). In order to facilitate review ofthe various embodiments of the invention, the following definitions of terms are provided: Animal: Living multi-cellular vertebrate organisms, a category that includes, for example, mammals and birds. The term mammal includes both human and non-human mammals. Similarly, the term "subject" includes both human and veterinary subjects. Bispecific fusion protein: Proteins that have at least two domains fused together, each domain comprising a binding region capable of forming a specific complex with a target protein. In general, the two domains are genetically fused together, in that nucleic acid molecules that encode each protein domain are functionally linked together, for instance by a linker oligonucleotide, thereby producing a single fusion-encoding nucleic acid molecule. The translated product of such a fusion-encoding nucleic acid molecule is the bispecific fusion protein. The two binding regions of such a bispecific protein may associate with two different binding determinants or epitopes on a single target molecule. One binding domain may bind first to such a target and thereby induce a conformational change in the target such that the binding ofthe second binding domain to the target is enabled, facilitated, or otherwise increased in affinity. In such an instance, the domain that binds first to the target can be referred to as the inducing-binding domain, while the domain that binds second is the induced-binding domain. These fusion protein domains need not be organized in binding sequence; the amino-proximal binding domain ofthe fusion protein may be either the induced-binding or the inducing-binding domain; likewise for the carboxy-proximal binding domain. Bispecific fusion proteins can be further labeled according to the target protein they bind to and neutralize. For instance, a bispecific fusion protein according to the cuπent invention that binds to two specific sites on HIV gpl20 protein may be refeπed to as a gpl20-targeted bispecific fusion protein. CD4: Cluster of differentiation factor 4, a T-cell surface protein that mediates interaction with the MHC class II molecule. CD4 also serves as the primary receptor site for HIV on T-cells during HIV infection. Molecules that are derived from CD4 include fragments of CD4, generated either by chemical (e.g. enzymatic) digestion or genetic engineering means. Such a fragment may be one or more entire CD4 protein domains (for example, extracellular domains Dl, D2, D3, and D4), as defined in the immunological literature, or a portion of one or more of these well-defined domains. For instance, a binding molecule or binding domain derived from CD4 would comprise a sufficient portion of the CD4 protein to mediate specific and functional interaction between the binding fragment and a native or viral binding site of CD4. One such binding fragment includes both the Dl and D2 extracellular domains of CD4 (CD4 D1D2), though smaller fragments may also provide specific and functional CD4-lιke binding The gpl20-bιndιng site has been mapped to Dl of CD4 The term "CD4-deπved molecules" also encompasses analogs (non-protein organic molecules), derivatives (chemically functionalized protein molecules obtained starting with the disclosed protein sequences) or mimetics (three-dimensionally similar chemicals) ofthe native CD4 structure, as well as protems sequence variants or genetic alleles, that maintain the ability to functionally bind to a target molecule. CD4-induced conformational change: A change mduced in the three-dimensional conformation ofthe interacting gpl20 protein when CD4 specifically interacts with gpl20 to form a complex One characteristic of such a change is the exposure of at least one induced epitope on the interacting gpl20 molecule. An epitope induced by such a change is called a CD4-ιnduced epitope. Such a CD4-ιnduced epitope may for instance include gpl20 epitopes at or near the co-receptor- bmding region ofthe protem. In addition to CD4 binding, the binding of other molecules may induce the exposure of induced epitopes on gpl20. Such other mducmg molecules are considered CD4-hke in terms of their epitope-mducmg ability, to the extent that they expose epitopes congruent with or equivalent to those induced epitopes exposed upon the binding of native CD4 These other inducing molecules mclude, but in no way are limited to, fragments of CD4, for mstance sCD4, or a fragment containmg the Dl or Dl and D2 domains of native CD4. A mannose-specifϊc lectin (SC) may also serve to expose a CD4-ιnduced epitope (see U.S. Patent No. 5,843,454), as can certain antι-gpl20 MAbs. Complex (complexed): Two proteins, or fragments or derivatives thereof, are said to form a complex when they measurably associate with each other m a specific manner. Such association can be measured m any of various ways, both direct and indirect Direct methods may mclude co- migration in non-denaturing fractionation conditions, for mstance Indirect measurements of association will depend on secondary effects caused by the association ofthe two proteins or protein domains. For instance, the formation of a complex between a protein and an antibody may be demonstrated by the antibody-specific inhibition of some function ofthe target protem. In the case of gpl20, the formation of a complex between gpl20 and a neutralizing antibody to this protem can be measured by determining the degree to which the antibody inhibits gρl20-dependent cell fusion or HIV lnfectivity. Cell fusion inhibition and mfectivity assays are discussed further below Exposing an induced epitope: The process by which two proteins interact specifically to form a complex (an inducing complex), thereby causing a conformational change m at least one of the two proteins (the target protein) such that at least one previously poorly accessible epitope (an induced epitope) is made accessible to intramolecular interaction The formation of such an inducing complex will generally cause the exposure of more than one induced epitope, each of which may be thereby rendered accessible for intramolecular interaction HIV coreceptor: A cell-surface protem other than CD4 involved in the interaction of HIV virus and its subsequent entry into a target cell. These proteins may also be refeπed to as fusion coreceptors for HIV. Examples of such coreceptor proteins include, for instance, members ofthe chemokine receptor family (e g CXCR4, CCR5, CCR3, and CCR2B) HIV coreceptor proteins interact with coreceptor binding determinants of gp 120. In general, it is believed that some of these determmants are exposed on g l20 only after the specific mteraction of gpl20 with CD4, and the consequent CD4-ιnduced conformational change m the interacting gpl20 Thus certain HIV coreceptor binding determinants are, or overlap with, CD4-mduced epitopes Neutralization of gp 120 can be achieved by the specific binding of neutralizing proteins or protein fragments or domains to one or more coreceptor bindmg determinants of gpl20, thereby blocking interaction between complexed gpl20 and the native coreceptor. HIV neutralizing ability: The measurable ability of a molecule to inhibit lnfectivity of HIV virus, either in vivo or in vitro The art is replete with methods for measuring the neutralizing ability of various molecules. Techniques include in vitro peπpheral blood mononuclear cell (PBMC) based assays (D'Souza et al , 1997); measurement of vinon attachment (Mondor et al , J Virol 72.3623-3634, 1998); neutral red dye uptake and antigen capture assays (U S. Patent No. 5,695,927); vaccmia-based reporter gene cell fusion assay (Nussbaum et al, J Virol 68:5411-5422, 1994) (standard and sCD4 activated assays), productive mfection assays (measurmg gag antigen p24 or RT synthesis) (Karn, HIV a practical approach Oxford Univ. Press, Cambridge, 1995); and mfectivity titer reduction assays (Karn, 1995). In addition, physical interaction between gpl20 and CD4 or other CD4-hke molecules can be examined by various methods See, for instance U.S Patent No. 5,843,454 (measurmg conformational changes of gpl20 on binding of various proteins by virus release and susceptibility of gpl20 to thrombm-mediated cleavage ofthe V3 loop) Alternately, the ability ofthe CD4-hke molecule to compete for binding to gpl20 with either native CD4 or antibody that recognizes the CD4 binding site on gpl20 (CD4BS) can be measured This will allow the calculation of relative binding affinities through standard techniques The invention also includes analogs, derivatives or mimetics of the disclosed fusion proteins, and which have HIV neutralizing ability Such molecules can be screened for HIV neutralizing ability by assaying a protein similar to the disclosed fusion protein, in that it has one or more conservative ammo acid substitutions, or analogs, deπvatives or mimetics thereof, and determining whether the similar protem, analog, derivative or mimetic provides HIV neutralization The HIV neutralization ability and gpl20 binding affinity of these derivative compounds can be measured by any known means, including those discussed m this application Injectable composition: A pharmaceutically acceptable fluid composition comprising at least one active ingredient, e g a bispecific fusion protein The active ingredient is usually dissolved or suspended in a physiologically acceptable earner, and the composition can additionally comprise minor amounts of one or more non-toxic auxiliary substances, such as emulsifying agents, preservatives, and pH buffering agents and the like. Such injectable compositions that are useful for use with the fusion proteins of this invention are conventional; formulations are well known m the art Isolated: An "isolated" biological component (such as a nucleic acid molecule, protein or organelle) is one that has been substantially separated or purified away from other biological components m the cell ofthe organism m which the component naturally occurs, i e , other chromosomal and extra-chromosomal DNA and RNA, proteins and organelles Nucleic acids and proteins that have been "isolated" include nucleic acids and proteins purified by standard purification methods. The term also embraces nucleic acids and proteins prepared by recombmant expression in a host cell as well as chemically synthesized nucleic acids Neutralizing antibodies. An antibody that is able to specifically bind to a target protem such a way as to inhibit the subsequent biological functioning of that target protein is said to be neutralizing of that biological function. In general, any protein that can perform this type of specific blocking activity is considered a neutralizing protem; antibodies are therefore a specific class of neutralizing protein The complex formed by binding of a neutralizing protem to a target protem is called a neutralizing complex. Antibodies that bind to viruses and bacteria and thereby prevent the binding of these pathogens to target host cells are said to neutralize the pathogen. Therefore, antibodies that bind to HIV proteins and measurably reduce the ability ofthe virus to bind to or enter target cells (e g , T-cells or macrophages) are HlV-neutralizmg antibodies In general, HIV neutralizing antibodies can be broken down into several different classes dependent on what region ofthe viral envelope protein the antibody binds to Broad classes of such antibodies include antι-gρ41 and antι-gpl20 antibodies. There are several antigemc regions on the gρl20 protem that provide epitopes for the natural or laboratory generation of HIV neutralizing antibodies (see WO 98/36087). Broadly cross-reactive neutralizing antibodies usually interact with relatively invariant regions of Env A primary source of neutralizing antibodies is the peripheral blood of patients infected with the HIV virus. Such primary isolates can be cloned and/or immortalized using standard techniques. In addition to the isolation of naturally-occurring neutralizing antibodies, procedures specifically directed toward their production are known in the art See U S Patent Nos 5,843,454; 5,695,927, 5,643,756, and 5,013,548 for mstance Linker: A peptide, usually between two and 150 amino acid residues in length that serves to join two protein domains in a multi-domain fusion protem Examples of specific linkers can be found, for mstance, in Hennecke et al (Protein Eng 1 1 -405-410, 1998), and U S Patent Nos. 5,767,260 and 5,856,456 Depending on the domams being joined, and their eventual function m the fusion protem, linkers may be from about two to about 150 ammo acids m length, though these limits are given as general guidance only The tendency of fusion proteins to form specific and non-specific multimenc aggregations is influenced by linker length (Alfthan et al , 1998 Protein Eng 8 725-731 , 1998) Thus, shorter linkers will tend to promote multimeπzation, while longer linkers tend to favor maintenance of monomeπc fusion proteins Aggregation can also be minimized through the use of specific linker sequences, as demonstrated m U S Patent No 5,856,456 Linkers may be repetitive or non-repetitive One classical repetitive linker used m the production of single chain Fvs (SCFvs) is the (Gly 4 Ser) 3 (or (GGGGS) 3 or (G 4 S) 3 ) linker More recently, non-repetitive linkers have been produced, and methods for the random generation of such linkers are known (Hennecke et al , Protein Eng 11 405-410, 1998) In addition, linkers may be chosen to have more or less secondary character (e g helical character, U S Patent No 5,637,481) dependmg on the conformation desired in the final fusion protein The more secondary character a linker possesses, the more constrained the structure ofthe final fusion protein will be Therefore, substantially flexible linkers that are substantially lacking in secondary structure allow flexion ofthe fusion protem at the linker A linker is capable of retaining a binding domain of a protein in binding proximity of a target binding site when the linker is of sufficient length and flexibility to allow specific interaction between the bindmg domain and the target binding site In the case ofthe bispecific fusion protems of this mvention, a linker that maintains binding proximity permits the sequential binding with the target of first the mducing-bmdmg domain ofthe fusion protem, then the induced-binding domain A linker that maintains the domams of a bispecific fusion protem in binding proximity to a target can be considered an operable or functional linker as relates to such a bispecific fusion protein Oligonucleotide: A linear polynucleotide sequence of between six and 300 nucleotide bases m length Operably linked: A first nucleic acid sequence is operably linked with a second nucleic acid sequence when the first nucleic acid sequence is placed in a functional relationship with the second nucleic acid sequence For mstance, a promoter is operably linked to a coding sequence if the promoter affects the transcription or expression ofthe coding sequence Generally, operably linked DNA sequences are contiguous and, where necessary to join two protein-coding regions, in the same readmg frame ORF (open reading frame) A series of nucleotide tπplets (codons) coding for ammo acids without any internal termination codons These sequences are usually translatable into a peptide Parenteral: Administered outside ofthe intestine, e g , not via the alimentary tract Generally, parenteral formulations are those that will be administered through any possible mode except mgestion This term especially refers to injections, whether administered intravenously, mtrathecally, intramuscularly, mtrapeπtoneally, or subcutaneously, and various surface applications including mtranasal, mtradermal, and topical application, for instance Pharmaceutically acceptable carriers: The pharmaceutically acceptable earners useful in this invention are conventional Remington 's Pharmaceutical Sciences, by E W Martin, Mack Publishing Co , Easton, PA, 15th Edition (1975), describes compositions and formulations suitable for pharmaceutical delivery ofthe fusion proteins herein disclosed In general, the nature ofthe carrier will depend on the particular mode of administration being employed For mstance, parenteral formulations usually compπse injectable fluids that include pharmaceutically and physiologically acceptable fluids such as water, physiological salme, balanced salt solutions, aqueous dextrose, glycerol or the like as a vehicle For solid compositions (e g , powder, pill, tablet, or capsule forms), conventional non-toxic solid carriers can include, for example, pharmaceutical grades of mannitol, lactose, starch, or magnesium stearate In addition to biologically-neutral carriers, pharmaceutical compositions to be administered can contain minor amounts of non-toxic auxiliary substances, such as wetting or emulsifying agents, preservatives, and pH buffering agents and the like, for example sodium acetate or sorbitan monolaurate Purified: The term puπfied does not require absolute purity, rather, it is intended as a relative term Thus, for example, a puπfied fusion protein preparation is one in which the fusion protein is more enriched than the protem is in its generative environment, for mstance within a cell or m a biochemical reaction chamber In some embodiments, a preparation of fusion protein is puπfied such that the fusion protem represents at least 50% ofthe total protem content ofthe preparation Recombinant: A recombmant nucleic acid molecule is one that has a sequence that is not naturally occurring or has a sequence that is made by an artificial combination of two otherwise separated segments of sequence This artificial combination can be accomplished by chemical synthesis or, more commonly, by the artificial manipulation of isolated segments of nucleic acids, e g , by genetic engineering techniques Similarly, a recombinant protem is one encoded for by a recombinant nucleic acid molecule Sequence identity: The similarity between two nucleic acid sequences, or two ammo acid sequences is expressed m terms ofthe similarity between the sequences, otherwise referred to as sequence identity Sequence identity is frequently measured in terms of percentage identity (or similarity or homology), the higher the percentage, the more similar the two sequences are Homologs ofthe bispecific fusion protein will possess a relatively high degree of sequence identity when aligned using standard methods Methods of alignment of sequences for comparison are well known in the art Various programs and alignment algorithms are described in Smith and Waterman (Adv Appl Math 2 482, 1981), Needleman and Wunsch (J Mol Biol 48 443-453, 1970), Pearson and Lipman (Proc Natl Acad Sci , USA 85 2444-2448, 1988), Higgins and Sharp (Gene, 73 237-244, 1988), Higgms and Sharp (CABIOS 5 151-153, 1989), Corpet et αl (Nuc Acids Res 16 10881-10890, 1988), Huang et αl (Comp Appls Biosci 8 155-165, 1992), and Pearson et al (Methods in Molecular Biology 24 307- 331, 1994) Altschul et al (Nature Genet , 6 119-129, 1994) presents a detailed consideration of sequence alignment methods and homology calculations The alignment tools ALIGN (Myers and Miller, CABIOS 4 11-17, 1989) or LFASTA (Pearson and Lipman, Proc Natl Acad Sci , USA 85 2444-2448, 1988) may be used to perform sequence comparisons (Internet Program © 1996, W R Pearson and the University of Virginia, "fasta20u63" version 2 0u63, release date December 1996) ALIGN compares entire sequences against one another, while LFASTA compares regions of local similarity These alignment tools and their respective tutorials are available on the Internet at http //biology ncsa uiuc edu Orthologs ofthe disclosed bispecific fusion proteins are typically characterized by possession of greater than 75% sequence identity counted over the full-length alignment with the amino acid sequence of bispecific fusion protem usmg ALIGN set to default parameters The NCBI Basic Local Alignment Search Tool (BLAST) (Altschul et al , J Mol Biol 1990 215 403-410, 1990) is available from several sources, including the National Center for Biotechnology Information (NCBI, Bethesda, MD) and on the Internet, for use m connection with the sequence analysis programs blastp, blastn, blastx, tblastn and tblastx It can be accessed at http//www ncbi nlm nih gov/BLAST/ A descπption of how to determine sequence identity using this program is available at http //www ncbi nlm nih gov/BLAST/blast help html For comparisons of ammo acid sequences of greater than about 30 ammo acids, the "Blast 2 sequences" function is employed using the default BLOSUM62 matrix set to default parameters, (gap existence cost of 11, and a per residue gap cost of 1) When aligning short peptides (fewer than around 30 ammo acids), the alignment should be performed using the Blast 2 sequences function, employing the PAM30 matrix set to default parameters (open gap 9, extension gap 1 penalties) Pro terns with even greater similarity to the reference sequences will show increasing percentage identities when assessed by this method, such as at least 90%, at least 92%, at least 94%, at least 95%, at least 97%, at least 98%, or at least 99% sequence identity In addition, sequence identity can be compared over the full length of one or both binding domains ofthe disclosed fusion proteins In such an instance, percentage identities will be essentially similar to those discussed for full-length sequence identity When significantly less than the entire sequence is being compared for sequence identity, homologs will typically possess at least 80% sequence identity over short windows of 10-20 amino acids, and may possess sequence identities of at least 85%, at least 90%, at least 95%, or at least 99% depending on their similarity to the reference sequence Sequence identity over such short windows can be determined using LFASTA, methods are described at http //biology ncsa uiuc edu One of skill in the art will appreciate that these sequence identity ranges are provided for guidance only, it is entirely possible that strongly significant homologs could be obtained that fall outside ofthe ranges provided The present invention provides not only the peptide homologs that are described above, but also nucleic acid molecules that encode such homologs An alternative indication that two nucleic acid molecules are closely related is that the two molecules hybridize to each other under stringent conditions Stringent conditions are sequence- dependent and are different under different environmental parameters Generally, stringent conditions are selected to be about 5° C to 20° C lower than the thermal meltmg point (T m ) for the specific sequence at a defined ionic strength and pH The T m is the temperature (under defined ionic strength and pH) at which 50% ofthe target sequence hybridizes to a perfectly matched probe. Conditions for nucleic acid hybridization and calculation of stringencies can be found in Sambrook et al (In Molecular Cloning A Laboratory Manual, Cold Spring Harbor, New York, 1989) and Tyssen (Laboratory Techniques in Biochemistry and Molecular Biology Part I, Ch. 2, Elsevier, New York, 1993). Nucleic acid molecules that hybridize under stringent conditions to the disclosed bispecific fusion protein sequences will typically hybridize to a probe based on either the entire fusion protem encoding sequence, an entire bindmg domam, or other selected portions ofthe encoding sequence under wash conditions of 0.2 x SSC, 0.1% SDS at 65° C Nucleic acid sequences that do not show a high degree of identity may nevertheless encode similar ammo acid sequences, due to the degeneracy ofthe genetic code. It is understood that changes m nucleic acid sequence can be made using this degeneracy to produce multiple nucleic acid sequences, each encoding substantially the same protem Specific binding agent: An agent that bmds substantially only to a defined target Thus a gpl20-specιfic binding agent binds substantially only the gpl20 protein As used herein, the term "gpl20-specιfic binding agent" includes antι-gpl20 antibodies and other agents that bind substantially only to a gρl20 protein. Antι-gpl20 antibodies may be produced using standard procedures described in a number of texts, including Harlow and Lane (Using Antibodies, A Laboratory Manual, CSHL, New York, 1999, ISBN 0-87969-544-7). In addition, certain techniques may enhance the production of neutralizing antibodies (U.S. Patents No 5,843,454; 5,695,927, 5,643,756; and 5,013,548). The determination that a particular agent binds substantially only to gpl20 protem may readily be made by using or adapting routine procedures. One suitable in vitro assay makes use ofthe Western blottmg procedure (described in many standard texts, including Harlow and Lane, 1999) Western blottmg may be used to determine that a given protein binding agent, such as an antι-gpl20 monoclonal antibody, binds substantially only to the MSG protein. Antibodies to gpl20 are well known in the art Shorter fragments of antibodies can also serve as specific binding agents. For instance, FAbs, Fvs, and smgle-chain Fvs (SCFvs) that bind to gpl20 would be gpl20-specιfic binding agents Therapeutically effective amount of a bispecific fusion protein: A quantity of bispecific fusion protein sufficient to achieve a desired effect in a subject being treated For instance, this can be the amount necessary to inhibit viral proliferation or to measurably neutralize disease organism binding mechanisms In general, this amount will be sufficient to measurably inhibit virus (e g HIV) replication or lnfectivity An effective amount of bispecific fusion protein may be administered in a single dose, or m several doses, for example daily, during a course of treatment However, the effective amount of fusion protein will be dependent on the fusion protem applied, the subject being treated, the severity and type ofthe affliction, and the manner of administration ofthe fusion protein For example, a therapeutically effective amount of fusion protein can vary from about 0 01 mg/kg body weight to about 1 g/kg body weight The fusion protems disclosed in the present invention have equal application in medical and veterinary settings Therefore, the general term "subject being treated" is understood to include all animals (e g humans, apes, dogs, cats, horses, and cows) that are or may be mfected with a virus or other disease-causing microorganism that is susceptible to bispecific fusion protein-mediated neutralization Transformed: A transformed cell is a cell into which has been introduced a nucleic acid molecule by molecular biology techniques As used herein, the term transformation encompasses all techniques by which a nucleic acid molecule might be introduced into such a cell, including rransfection with viral vectors, transformation with plasmid vectors, and introduction of naked DNA by electroporation, hpofection, and particle gun acceleration Vector: A nucleic acid molecule as introduced into a host cell, thereby producing a transformed host cell A vector may include nucleic acid sequences that permit it to replicate in a host cell, such as an origin of replication A vector may also include one or more selectable marker genes and other genetic elements known in the art II. Construction, Expression, and Purification of Bispecific Fusion Proteins. A. Selection of component domains. This invention provides generally a bispecific fusion protein that binds to two different sites on a target protein As such, any target protein that has two different binding sites is an example of a target for a bispecific fusion protein Particular targets include proteins on which one ofthe two bindmg sites (the mduced-bindmg site) is exposed/induced by the binding ofthe fusion protein to a first binding site (the inducing-binding site) on the target The choice of protein binding domams for incorporation into the disclosed bispecific fusion protem will be dictated by the target protem chosen The choice of linker will also be influenced by the target protem and binding sites chosen In general, the linker used in any bispecific fusion will be of a length and secondary character to hold the mduced-bindmg domain within binding proximity ofthe target protein induced binding site, once the inducing-binding domain of the fusion protein has formed a specific complex with the target In certain embodiments, the target protein is an HIV envelope glycoprotein, for instance HIV-1 gpl20 In certain of these and other embodiments, the mducmg-binding site is the CD4 binding site on gpl20 As such, the mducmg-bmding domain ofthe disclosed bispecific fusion protem can be a binding fragment of CD4, for instance sCD4 Alternately, any other molecule that specifically interacts with gpl20 m such a way as to expose one or more induced epitopes would also serve as the source of an mducing-b dmg protein domain The specific fragments used to construct the fusion protem should be chosen so that the conformation ofthe final fusion provides functional and inducing bmding to gpl20; this can be assayed either directly (e g , affinity measurements) or indirectly (e g , neutralization assays). Non-CD4-deπved CD4 mimics may also be employed as sources for mducing-bindmg domains ofthe disclosed fusion proteins. For instance, a mannose-specific lectm (SC) may serve to induce CD4 induced conformational changes (see U.S. Patent No 5,843,454) Alternatively, antibodies that bind the CD4-bιndmg site or another epitope of gpl20 and thereby mduce a CD4-hke conformational change on the complexed protein can also be used Non-peptide CD4 analogs can also be used in this invention, for instance organic or non- organic structural analog ofthe gpl20-ιnteractmg domaιn(s) ofthe CD4 molecule. Induced-binding domains of a gpl20-targeted fusion protein will include antibodies (or fragments thereof) that recognize induced epitopes of the complexed gpl20 In some embodiments, such antibodies are broadly cross-reactive against diverse HIV-1 isolates Induced epitopes include all of those referred to as CD4-ιnduced (CD4ι) epitopes, and m particular those which overlap coreceptor-bindmg determinants of gpl20. Previously identified neutralizing monoclonal antibodies can be used, and mclude but are not limited to human monoclonal antibodies 17b, 48d, and CG10. Likewise, induced binding domains ofthe disclosed chimeπc molecules can be non-peptide molecules, for instance organic or non-organic structural analogs of SCFv(17b). In addition to antibodies that bind mduced epitopes of gpl20, other sources for induced- binding domains include fragments of coreceptors that specifically interact with a coreceptor binding domaιn(s) of gpl20. The construction of a gpl20-specιfϊc bispecific fusion prote can be aided by review ofthe X-ray crystallographic structure of the ternary complex containing the gpl20 core, a two-domain fragment of CD4 (D1D2), and an FAb from a broadly cross-reactive human MAb (17b) directed against the coreceptor-bmdmg determmants of gρl20 (Kwong et al , Nature 393-648-659, 1998) Computer-based examination of the structural coordinates of this ternary complex, usmg FRODO (Jones et al , Meth Enzymol 115 157-171, 1985, Jones, J Appl Cryst 11.268-272, 1978; Pflugrath et al Methods and Applications in Crystallography, pages 407-420, Clarendon Press, Oxford), has revealed choices for constructing the chimenc protein. The shortest distance between free termini of CD4 and the 17b FAb is 56 A, i e from the free C-terminus ofthe D1D2 sCD4 fragment to the N- terminus ofthe 17b FAb heavy chain A linker connecting these termini would be essentially free of steπc hindrance from CD4 and the N- terminus ofthe 17b light chain Possible connections could also be made between the N-terminus of CD4 and the C-termini of the 17b heavy or light chains, such connections would require linkers of about 65 A and about 86 A, respectively In the latter two connections the linker is required to circumvent other portions ofthe complex, including the bulky variable loops B. Assembly. The construction of chimeπc molecules, m particular fusion proteins, from domains of known protems is well known In general, a nucleic acid molecule that encodes the desired protem domains are joined using standard genetic engineermg techniques to create a single, operably linked fusion oligonucleotide Molecular biological techniques may be found in Sambrook et al (In Molecular Cloning A Laboratory Manual, Cold Sprmg Harbor, New York, 1989) Specific examples of genetically engineered multi-domain proteins, especially those based on molecules of the immunoglobulin superfamily, jomed by various linkers, can be found m the following patent documents U S Patent No 5,856,456 ("Linker for linked fusion polypeptides"), U S Patent No 5,696,237 ("Recombinant antibody-toxin fusion protein"), U S Patent No 5,767,260 ("Antigen-binding fusion protems"), U S Patent No 5,587,455 ("Cytotoxic agent agamst specific virus mfection"), and WO 98/36087 ("Immunological tolerance to HIV epitopes") Non-peptide analogs that serve as inducing-binding or induced binding domains ofthe invention can be linked to the opposite domam ofthe chimeπc molecules using known chemical linking techniques, including chemical cross-lmkmg Cross-linkers are well known, and examples of molecules used for cross-linking can be found, for instance, in U S Patent No 6,027,890 ("Methods and compositions for enhancing sensitivity m the analysis of biological-based assays") C. Expression. One skilled in the art will understand that there are myriad ways to express a recombmant protem such that it can subsequently be purified In general, an expression vector carrying the nucleic acid sequence that encodes the desired protein will be transformed into a microorganism for expression Such microorganisms can be prokaryotic (bacteria) or eukaryotic (e g , yeast) One example species of bacteπa that can be used is Escherichia coh (E coli), which has been used extensively as a laboratory experimental expression system An eukaryotic expression system can be used where the protein of interest requires eukaryote-specific post-translational modifications such as glycosylation Also, protem can be expressed using a viral (e g , vaccinia) based expression system Protein can also be expressed in animal cell tissue culture, and such a system can be used where animal-specific protem modifications are desirable or required in the recombinant protem The expression vector can include a sequence encoding a targeting peptide, positioned in such a way as to be fused to the coding sequence of the bispecific fusion protein This allows the bispecific fusion protem to be targeted to specific sub-cellular or extra-cellular locations Various prokaryotic and eukaryotic targeting peptides, and nucleic acid molecules encoding such, are known In a prokaryotic expression system, a signal sequence can be used to secrete the newly synthesized protein In an eukaryotic expression system, the targetmg peptide would specify targeting ofthe hybrid protem to one or more specific sub-cellular compartments, or to be secreted from the cell, depending on which peptide is chosen Through the use of an eukaryotic secretion signal sequence, the bispecific fusion protein can be expressed in a transgenic animal (for instance a cow, pig, or sheep) m such a manner that the protem is secreted into the milk of the animal Vectors suitable for stable transformation of culturable cells are also well known Typically, such vectors mclude a multiple-clonmg site suitable for inserting a cloned nucleic acid molecule, such that it will be under the transcriptional control of 5' and 3' regulatory sequences In addition, transformation vectors include one or more selectable markers, for bacterial transformation this is often an antibiotic resistance gene Such transformation vectors typically also contam a promoter regulatory region (e g , a regulatory region controlling inducible or constitutive expression), a transcription initiation start site, a πbosome binding site, an RNA processing signal, and a transcription termination site, each functionally arranged m relation to the multiple-clonmg site For production of large amounts of recombinant protems, an inducible promoter can be used This permits selective production ofthe recombmant protem, and allows both higher levels of production than constitutive promoters, and enables the production of recombinant protems that may be toxic to the expressmg cell if expressed constitutively In addition to these general guidelines, protem expression/purification kits are produced commercially See, for instance, the QIAexpress™ expression system from QIAGEN (Chatsworfh, CA) and vaπous expression systems provided by INVITROGEN (Carlsbad, CA) Depending on the details provided by the manufactures, such kits can be used for production and purification ofthe disclosed bispecific fusion proteins D. Purification. One skilled in the art will understand that there are myriad ways to purify recombmant polypeptides, and such typical methods of protein purification may be used to purify the disclosed bispecific fusion proteins Such methods include, for mstance, protem chromatographic methods including ion exchange, gel filtration, HPLC, monoclonal antibody affinity chromatography and isolation of insoluble protein inclusion bodies after over production In addition, purification affinity-tags, for instance a six-histidme sequence, may be recombinanfly fused to the protein and used to facilitate polypeptide purification A specific proteolytic site, for instance a fhrombin- specific digestion site, can be engineered into the protem between the tag and the fusion itself to facilitate removal ofthe tag after punfication Commercially produced protein expression/purification kits provide tailored protocols for the purification of proteins made using each system See, for instance, the QIAexpress™ expression system from QIAGEN (Chatsworth, CA) and various expression systems provided by INVITROGEN (Carlsbad, CA) Where a commercial kit is employed to produce a bispecific fusion protem, the manufacturer's purification protocol is a particularly disclosed protocol for purification of that protein For instance, proteins expressed with an ammo-termmal hexa-his tag can be purified by binding to nickel-nitπlotπacetic acid (Ni-NTA) metal affinity chromatography matrix (The QIAexpresswnist, QIAGEN 1997) Alternately, the bmdmg specificities of either the first or second binding domains, or both, of the disclosed fusion protem may be exploited to facilitate specific purification ofthe protems One method of performing such specific purification would be column chromatography using column resin to which the target molecule, or an epitope or fragment or domain ofthe target molecule, has been attached If the bispecific fusion protem is produced in a secreted form, e g secreted into the milk of a transgenic animal, purification will be from the secreted fluid Alternately, purification may be unnecessary if the fusion protein can be applied directly to the subject in the secreted fluid (e g III. Variation of a Bispecific Fusion Protein A. Sequence Variants The binding characteπstics and therefore neutralizing activity ofthe fusion proteins disclosed herein lies not m the precise ammo acid sequence, but rather in the three-dimensional structure inherent m the ammo acid sequences encoded by the DΝA sequences It is possible to recreate the binding characteπstics of any of these protems or protem domains of this invention by recreating the three-dimensional structure, without necessarily recreating the exact amino acid sequence This can be achieved by designing a nucleic acid sequence that encodes for the three- dimensional structure, but which differs, for mstance by reason of the redundancy ofthe genetic code Similarly, the DΝA sequence may also be varied, while still producing a functional neutralizing protem Variant neutralizing bispecific bmdmg proteins include protems that differ in ammo acid sequence from the disclosed sequence, but that share structurally significant sequence homology with any ofthe provided proteins Variation can occur in any single domain ofthe fusion protem (e g the first or second binding domain, or the linker) Variation can also occur in more than one of such domains in any particular variant protein Such variants may be produced by manipulating the nucleotide sequence of the, for instance a CD4-SCFv(17b)-encodιng sequence, using standard procedures, mcluding site-directed mutagenesis or PCR The simplest modifications involve the substitution of one or more ammo acids for ammo acids having similar biochemical properties These so-called conservative substitutions are likely to have minimal impact on the activity ofthe resultant protein, especially when made outside of the binding site of each domain Table 1 shows ammo acids that may be substituted for an original ammo acid in a protem, and which are regarded as conservative substitutions Table 1 Original Residue Conservative Substitutions Ala ser Arg lys Asn gin, his Asp glu Cys ser Gin asn Glu asp Leu lie, val Phe met, leu, tyr Ser thr Thr ser Trp tyr More substantial changes in protein structure may be obtained by selecting amino acid substitutions that are less conservative than those listed Table 1 Such changes include changmg residues that differ more significantly m their effect on maintaining polypeptide backbone structure (e g , sheet or helical conformation) near the substitution, charge or hydrophobicity of the molecule at the target site, or bulk of a specific side chain The following substitutions are generally expected to produce the greatest changes in protein properties (a) a hydrophihc residue (e g , seryl or threonyl) is substituted for (or by) a hydrophobic residue (e g , leucyl, isoleucyl, phenylalanyl, valyl or alanyl), (b) a cysteme or proline is substituted for (or by) any other residue, (c) a residue havmg an electropositive side chain (e g , lysyl, argmyl, or histadyl) is substituted for (or by) an electronegative residue (e g , glutamyl or aspartyl), or (d) a residue having a bulky side chain (e g , phenylalanrne) is substituted for (or by) one lacking a side chain (e g , glycme) Variant bindmg domain or fusion protein-encoding sequences may be produced by standard DNA mutagenesis techniques, for example, Ml 3 primer mutagenesis Details of these techniques are provided in Sambrook (In Molecular Cloning A Laboratory Manual, Cold Spring Harbor, New York, 1989), Ch 15 By the use of such techniques, variants may be created which differ in minor ways from the bispecific fusion protein-encoding sequences disclosed DNA molecules and nucleotide sequences which are derivatives of those specifically disclosed herein and that differ from those disclosed by the deletion, addition, or substitution of nucleotides while still encoding a protein that binds twice to gpl20, thereby neutralizing HIV virus mfectivity, are comprehended by this invention In their most simple form, such variants may differ from the disclosed sequences by alteration ofthe coding region to fit the codon usage bias ofthe particular organism into which the molecule is to be introduced Alternatively, the coding region may be altered by taking advantage ofthe degeneracy of the genetic code to alter the coding sequence such that, while the nucleotide sequence is substantially altered, it nevertheless encodes a protein having an amino acid sequence substantially similar to the disclosed fusion sequences For example, the 18th ammo acid residue ofthe CD4-SCFv(17b) protein (after cleavage ofthe N- terminal signal sequence) is alanme The nucleotide codon triplet GCT encodes this alanine residue Because of the degeneracy of the genetic code, three other nucleotide codon triplets - (GCG, GCC and GCA) - also code for alanine Thus, the nucleotide sequence ofthe disclosed CD4-SCFv(17b)encodιng sequence could be changed at this position to any of these three alternative codons without affecting the ammo acid composition or characteristics of the encoded protein Based upon the degeneracy ofthe genetic code, variant DNA molecules may be deπved from the cDNA and gene sequences disclosed herein using standard DNA mutagenesis techniques as described above, or by synthesis of DNA sequences Thus, this invention also encompasses nucleic acid sequences which encode a neutralizing bispecific fusion protein, but which vary from the disclosed nucleic acid sequences by virtue ofthe degeneracy ofthe genetic code B. Peptide Modifications The present invention includes biologically active molecules that mimic the action ofthe bispecific fusion protems ofthe present invention, and specifically neutralize HIV Env The protems ofthe invention include synthetic embodiments of naturally-occumng proteins described herein, as well as analogues (non-peptide organic molecules), derivatives (chemically functionalized protein molecules obtained starting with the disclosed peptide sequences) and variants (homologs) of these protems that specifically bind with and neutralize HIV gpl20 Each protein ofthe invention is comprised of a sequence of amino acids, which may be either L- and/or D- ammo acids, naturally occurring and otherwise Proteins may be modified by a variety of chemical techniques to produce derivatives having essentially the same activity as the unmodified proteins, and optionally having other desirable properties For example, carboxyhc acid groups ofthe protein, whether carboxyl-terminal or side chain, may be provided in the form of a salt of a pharmaceutically-acceptable cation or esteπfied to form a C,-C 16 ester, or converted to an amide of formula NR,R 2 wherein R, and R 2 are each independently H or C,-C I6 alkyl, or combined to form a heterocychc ring, such as a 5- or 6- membered πng Ammo groups of the protem, whether amino-terminal or side chain, may be in the form of a pharmaceutically-acceptable acid addition salt, such as the HCl, HBr, acetic, benzoic, toluene sulfomc, maleic, tartaπc and other organic salts, or may be modified to C r C l6 alkyl or dialkyl ammo or further converted to an amide Hydroxyl groups of the protein side chains may be converted to C r C 16 alkoxy or to a C,-C ]6 ester using well-recognized techniques Phenyl and phenolic rings of the protein side chains may be substi ted with one or more halogen atoms, such as fluorine, chlorine, bromine or iodine, or with C r C 16 alkyl, C,-C 16 alkoxy, carboxyhc acids and esters thereof, or amides of such carboxyhc acids Methylene groups ofthe protein side chains can be extended to homologous C 2 -C 4 alkylenes Thiols can be protected with any one of a number of well-recognized protecting groups, such as acetamide groups Those skilled in the art will also recognize methods for mtroducmg cyclic structures into the proteins of this invention to select and provide conformational constraints to the structure that result in enhanced stability It also may be advantageous to introduce one or more disulfide bonds to connect the frameworks ofthe heavy and light chains in the SCFv domain This modification often enhances the stability and affinity of SCFvs (Reiter et al , Protein Engineering 7 697-704, 1994) Here too, the X- ray crystal structure containing the 17 FAb (Kwong et al , Nature 393 648-659, 1998) can be used to assess optimal sites for engineering cysteme residues ofthe heavy and light chains Peptidomimetic and organomimetic embodiments are also within the scope ofthe present invention, whereby the three-dimensional arrangement ofthe chemical constituents of such peptido- and organomimetics mimic the three-dimensional arrangement ofthe protein backbone and component amino acid side chains in the bispecific neutralizing fusion protein, resulting in such peptido- and organomimetics ofthe proteins of this invention having measurable or enhanced neutralizing ability For computer modeling applications, a pharmacophore is an idealized, three- dimensional definition ofthe structural requirements for biological activity Peptido- and organomimetics can be designed to fit each pharmacophore with current computer modeling software (using computer assisted drug design or CADD) See Walters, "Computer-Assisted Modeling of Drugs", in Klegerman & Groves, eds , 1993, Pharmaceutical Biotechnology Interpharm Press Buffalo Grove, IL, pp 165-174 and Principles of Pharmacology Munson (ed ) 1995, Ch 102, for descriptions of techniques used in CADD Also included within the scope ofthe invention are mimetics prepared using such techniques that produce neutralizing fusion protems C. Domain length variation. It will be appreciated that the protem domains of the current invention may be combined to produce fusion protein molecules without necessarily splicing the components in the same place It is believed to be possible to use shorter or longer fragments of each component domain, linked by a functional linker For instance, any component which is spliced within about 10 amino acid residues ofthe residue specified, and which still provides a functional binding fragment, comprises about the same domain However, domains of substantially longer or substantially shorter length can be used For instance, in certain embodiments, the protem can include a leader sequence plus a four-domain CD4 (D1-D4, ammo acid residues 1-372), or just the first domain of CD4 (Dl residues 1-113) IV. Activity of Fusion Proteins It is important to assess the chemical, physical and biological activity ofthe disclosed bispecific fusion proteins Among other uses, such assays permit optimization ofthe domains chosen, as well as optimization ofthe length and conformation ofthe linkers used to connect them Control molecules should be included in each assay, usually such will include each domain alone, as well as the two domains as separate molecules mixed in the reaction, for instance m a l l molar ratio In the case of a CD4-SCFv(17b) bispecific fusion protein, such controls would include sCD4 and SCFv(17b), for instance A. Fusion protein affinity for target protein Fusion protem affinity for the target protem can be determined using various techniques For instance, co-immunoprecipitation analyses with metabohcally labeled protems can be employed to determine binding of sCD4-SCFv proteins, e g sCD4-SCFv(17b) to soluble HIV-1 gpl20, usmg antι-gpl20 MAbs that do not interfere with CD4 binding (e g MAb D47 that binds to V3), or polyclonal antibody to the C-termmus of gpl20 ELISA can also be used to examine the bmdmg characteristics of each domain of the chimera B. Neutralization assays Various assays can be used to measure the ability ofthe disclosed fusion protems to inhibit function ofthe target protem Individual components ofthe fusion protem will serve as controls In general, assays will be specific for the target/fusion protein For instance, many functional analyses can test the ability of sCD4-SCFv fusions to neutralize the HIV Env It is particularly advantageous to use Envs from diverse HIV-1 strains to test the breadth of inhibition (neutralizing ability) of each fusion protein for different HIV-1 genetic subtypes and different phenotypes (i e coreceptor usage) In addition, it is advantageous to test such fusion protems m the standard and sCD4-actιvated assays for Env-mediated cell fusion Known HIV-1 neutralizing MAbs and MAbs against CD4-mduced epitopes on gpl20 are examples of controls for such experiments Possible synergistic inhibition with other known broadly cross-reactive neutralizing MAbs should be tested (e g bl2, 2F5, F105, 2G12) In the case of gpl20-targeted fusion proteins, the vaccinia-based reporter gene cell fusion assay may be used to assess fusion inhibition (Nussbaum et al , J Virol 68 5411-5422, 1994) One population of tissue culture cells (e g BS-C-1, HeLa, or NIH 3T3) uniformly expressing vaccinia virus-encoded binding and fusion-mediating viral envelope glycoprotem(s) is mixed with another population expressing the coπespondmg cellular receptor(s) In the case of sCD4-SCFv fusions, where the target protein is HIV-1 gpl20, one cell population expresses HIV-1 Env, while the other expresses necessary HIV-1 receptors (e g CD4 and a chemokine receptor) The cytoplasm of either cell population also contains vaccinia virus-encoded bacteπophage T7 DNA polymerase, the cytoplasm of the other contains a transfected plasmid with the E coh lacZ gene linked to the T7 promoter Upon mixing of the two populations, cell fusion results in activation ofthe lacZ gene through the introduction ofthe T7 RNA polymerase into proximity with the transfected T7 promoter- lacZ in the cytoplasm of the fused cells The resultant β-galactosidase (β-gal) activity is proportional to the amount of fusion that occurs, and can be measured by coloπmetπc assay of detergent cell lysates or in situ staining Cell- fusion neutralizing activity of bispecific fusion proteins is therefore assessed by measuring their inhibition of β-gal production The gpl20-targeted fusions (e g sCD4-SCFv) can also be tested for ability to block HIV-1 infection using single round assays (e g using mdicator cell lines, Vodicka et al , Virology 233 193- 198, 1997) Target cells expressing CD4 and a specific coreceptor, and containing the lacZ reporter gene linked to the HIV-1 long terminal repeat (LTR), are infected with specific HIV-1 strains (Vodicka, 1997) Integration of an HIV provirus in these cells leads to production ofthe viral transactivator, Tat, which then turns on expression ofthe β-gal gene via interaction with LTR The activity of sCD4-SCFv is assessed by its inhibition of production of β-gal-positive cells (stained blue with X-gal), which is proportional to its ability to block HIV-1 infection V. Incorporation of Bispecific Fusion Proteins into Pharmaceutical Compositions Pharmaceutical compositions that comprise at least one bispecific fusion protein as described herein as an active ingredient will normally be formulated with a solid or liquid carrier, depending upon the particular mode of administration chosen The pharmaceutically acceptable carriers and excipients useful m this invention are conventional For instance, parenteral formulations usually comprise injectable fluids that are pharmaceutically and physiologically acceptable fluid vehicles such as water, physiological saline, other balanced salt solutions, aqueous dextrose, glycerol or the like Excipients that can be included are, for instance, other proteins, such as human serum albumin or plasma preparations If desired, the pharmaceutical composition to be administered may also contain minor amounts of non-toxic auxiliary substances, such as wetting or emulsifying agents, preservatives, and pH buffeπng agents and the like, for example sodium acetate or sorbitan monolaurate Other medicinal and pharmaceutical agents, for instance nucleoside derivatives (e g AZT) or protease inhibitors, also may be included It may also be advantageous to include other fusion inhibitors, for instance one or more neutralizing antibodies The dosage form of the pharmaceutical composition will be determined by the mode of administration chosen For instance, m addition to injectable fluids, topical and oral formulations can be employed Topical preparations can include eye drops, ointments, sprays and the like Oral formulations may be liquid (e g , syrups, solutions or suspensions), or solid (e g , powders, pills, tablets, or capsules) For solid compositions, conventional non-toxic solid carriers can include pharmaceutical grades of manmtol, lactose, starch, or magnesium stearate Actual methods of preparing such dosage forms are known, or will be apparent, to those skilled in the art The pharmaceutical compositions that comprise bispecific fusion protem may be formulated in unit dosage form, suitable for individual administration of precise dosages One possible unit dosage contains approximately 100 μg of protein The amount of active compound administered will be dependent on the subject being treated, the severity ofthe affliction, and the manner of administration, and is best left to the judgment of the prescribing clinician Within these bounds, the formulation to be administered will contain a quantity ofthe active component(s) in an amount effective to achieve the desired effect in the subject being treated VI. Clinical Use of Bispecific Fusion Proteins The potent viral-neutralizmg activity exhibited by the disclosed bispecific fusion proteins makes them useful for treating viral infections in human and other animal subjects Possibly susceptible viruses include the immunodeficiency viruses, such as HIV and similar or related viruses m simians and other animals In addition, other viral or microbial systems that involve the interaction of a first inducing and second induced binding site of a smgle protein will also be susceptible to neutralization using bispecific fusion protems ofthe cuπent invention The bispecific fusion protems disclosed herein can also be used in highly sensitive detection or purification of target protem The bispecific fusion proteins of this invention may be administered to humans, or other animals on whose cells they are effective, in various manners such as topically, orally, intravenously, intramuscularly, mtraperitoneally, mtranasally, intradermally, mtrathecally, and subcutaneously The particular mode of administration and the dosage regimen will be selected by the attending clinician, taking into account the particulars ofthe case (eg , the subject, the disease, and the disease state involved, and whether the treatment is prophylactic or post-infection) Treatment may involve daily or multi-daily doses of bispecific fusion protem(s) over a period of a few days to months, or even years If treatment is through the direct administration of cells expressing the bispecific fusion protein to the subject, such cells (e g transgenic pluπpotent or hematopoietic stem cells or B cells) may be administered at a dose of between about 10 6 and 10 10 cells, on one or several occasions The number of cells will depend on the patient, as well as the fusion protem and cells chosen to express the protem A general strategy for transferring genes mto donor cells is disclosed m U S Patent No 5,529,774, which is incorporated by reference Generally, a gene encodmg a protem havmg therapeutically desired effects is cloned mto a viral expression vector, and that vector is then introduced into the target organism The virus infects the cells, and produces the protem sequence m vivo, where it has its desired therapeutic effect See, for example, Zabner et al , Cell 75 207-216, 1993 As an alternative to addmg the sequences encoding the bispecific fusion protem or a homologous protem to the DNA of a virus, it is also possible to introduce such a gene mto the somatic DNA of mfected or unmfected cells, by methods that are well known m the art (Sambrook et al , In Molecular Cloning A Laboratory Manual, Cold Spring Harbor, New York, 1989) These methods can be used to introduce the here disclosed fusion protems to human cells to provide long-term resistance to HIV-1 mfection or AIDS For example, gene therapy can be used to secrete the protem at mucosal surfaces, such as the vagmal, rectal, or oral mucosa HIV-1 gpl20-targeted bispecific fusion proteins, for mstance sCD4-SCFv(17b), are particularly useful in the prevention of infection during or immediately after HIV exposure (e g , mother/mfant transmission, post-exposure prophylaxis, and as a topical inhibitor) In such instances, one or more doses ofthe bispecific fusion protein are administered before or soon after the triggering event To prevent or ameliorate mother/infant transmission of viral infection, for instance, it may be beneficial to administer the gp 120- targeted bispecific fusion protem to the mother mtermittently throughout pregnancy, and or immediately before or following delivery, and/or directly to the newborn immediately after birth Post-exposure prophylactic treatments may be particularly beneficial where there has been accidental exposure (for mstance, a medically related accidental exposure), including but not limited to a contaminated needle-stick or medical exposure to HIV-1 contaminated blood or other fluid The present mvention also includes combinations of chimeπc bispecific fusion protems with one or more other agents useful in the treatment of disease, e g HIV disease For example, the compounds of this invention may be administered, whether before or after exposure to the virus, in combination with effective doses of other anti-virals, immunomodulators, anti-mfectives, and/or vaccines The term "administration m combination" refers to both concurrent and sequential administration of the active agents Examples of antiviral agents that can be used m combination with the chimeπc bispecific fusion proteins ofthe invention are AL-721 (from Ethigen of Los Angeles, CA), recombmant human mterferon beta (from Triton Biosciences of Alameda, CA), Acemannan (from Carπngton Labs of Irving, TX), gangiclovir (from Syntex of Palo alto, CA), didehydrodeoxyfhyrmdine or d4T (from Bπstol-Myers-Squibb), EL 10 (from Elan Corp of Gainesville, GA), dideoxycytidine or ddC (from Hoffman-LaRoche), Novapren (from Novaferon labs, Inc of Akron, OH), zidovudine or AZT (from Burroughs Wellcome), πbariπn (from Viratek of Costa Mesa, CA), alpha mterferon and acyclovir (from Buπoughs Wellcome), Indmavir (from Merck & Co ), 3TC (from Glaxo Wellcome), Ritonavir (from Abbott), Saquinavir (from Hoffmann-LaRoche), and others Examples of lmmuno-modulators that can be used in combination with the chimeπc bispecific fusion proteins ofthe invention are AS-101 (Wyeth-Ayerst Labs ), bropiπmine (Upjohn), gamma mterferon (Genentech), GM-CSF (Genetics Institute), IL-2 (Cetus or Hoffman-LaRoche), human immune globulin (Cutter Biological), IMREG (from Imreg of New Orleans, La ), SK&F106528, and TNF (Genentech) Examples of some anti-mfectives with which the chimeπc bispecific fusion proteins can be used include clmdamycm with pπmaquine (from Upjohn, for the treatment of Pneumocystis pneumonia), fluconazlone (from Pfizer for the treatment of cryptococcal meningitis or candidiasis), nystatm, pentamidme, tπmethapπm-sulfamethoxazole, and many others The combination therapies are of course not limited to the lists provided in these examples, but includes any composition for the treatment of HIV disease (including treatment of AIDS). VII. Kits The chimeric proteins disclosed herein can be supplied in the form of a kit for use m prevention and/or treatment of diseases (e g , HIV infection and AIDS). In such a kit, a clinically effective amount of one or more ofthe chimeric bispecific fusion proteins is provided in one or more containers. The chimeric bispecific fusion proteins may be provided suspended in an aqueous solution or as a freeze-dπed or lyophilized powder, for instance In certain embodiments, the chimeric proteins will be provided in the form of a pharmaceutical composition. Kits according to this invention can also include instructions, usually written instructions, to assist the user m treating a disease (e , HIV infection or AIDS) with a chimeric bispecific fusion protem Such instructions can optionally be provided on a computer readable medium The contaιner(s) in which the protem(s) are supplied can be any conventional container that is capable of holding the supplied form, for instance, microfuge tubes, ampoules, or bottles. In some applications, chimeπc protems may be provided in pre-measured single use amounts in individual, typically disposable, tubes or equivalent containers The amount of a chimeπc bispecific fusion protem supplied in the kit can be any appropriate amount, depending for instance on the market to which the product is directed For instance, if the kit is adapted for research or clinical use, the amount of each chimeric protem provided would likely be an amount sufficient for several treatments Certain kits according to this invention will also include one or more other agents useful m the treatment of disease, e g HIV disease. For example, such kits may include one or more effective doses of other anti-vrrals, lmmunomodulators, anti-infectives, and/or vaccines EXAMPLE 1 Construction of a CD4-SCFv(17b) encoding sequence A gpl20-targeted fusion protein, sCD4-SCFv(17b), is constructed by linking the C- terminus of CD4 (D1D2, 183 amino acid residues) to the N-termmus of the heavy chain ofthe 17b SCFv, which contains the heavy chain at its N- terminus, linked via its C-terminus to the N-terminus of the light chain (see schematic diagram of the construct, Fig 3) The 17b SCFv DNA was obtained from R Wyatt and J Sodroski, Dana Farber Cancer Institute, Boston, MA The 17b-MAb producmg- hybπdoma was obtained from J Robinson, Tulane University Lmkers were chosen to have sufficient length and flexibility to connect the desired protein segments without inducing unacceptable torsion For the SCFv, the 15 amino acid residue sequence (Gly 4 Ser) 3 (designated L2) was chosen, which has been employed successfully for production of SCFvs This sequence confers excellent flexibility with minimal aggregation The linker between the C- terminus of CD4 and the N-terminus ofthe SCFv (designated LI , SEQ ID NO 2), is seven repeats ofthe same Gly 4 Ser sequence Conservative estimates mdicate that this 35 am o acid residue linker is sufficiently long to allow CD4 and SCFv to bind simultaneously to their respective binding sites on gp 120 A schematic of the genetic construct is shown in Fig 3 A unique BamH I restriction site has been introduced withm LI to enable the production of constructs with shorter or longer linkers, and especially to provide negative controls (linkers too short, thereby not allowing both the CD4 and SCFv moieties of a single molecule to bind simultaneously to their respective binding sites on gpl20) The starting CD4 plasmid is pCB-3, which contains the full-length CD4 cDNA (including its natural 5' signal sequence) in the vaccinia expression plasmid pSC59 (Broder & Berger, J Virol 67 913-926, 1993) This plasmid was digested with Stu I, which cuts near the end ofthe 2nd domam of sCD4, and with Spe I, which cuts withm the vector downstream ofthe CD4 insert and leaves a 5' overhang Synthetic oligonucleotides (SEQ ID NO 11) were annealed together to recapitulate the 5' end ofthe second half of the Stu I site (CCT) and the next two bases (CC) ofthe CD4 cDNA, and to produce an Spe I overhang at the 3 ' end (this site to be destroyed upon ligation mto pCB-3) The oligonucleotide sequence reconstructs the remainder ofthe second domain of CD4 (through ser, 83 ), and encodes the 37 amino acid intermediate linker (gly 4 ser) 5 gly 4 thr 2 ser, followed directly by the universal translational termination sequence (UTS) (SEQ ID NO 6) A BamH I site has been deliberately included withm the linker near the end ofthe third (gly 4 ser) repeat, to enable subsequent linkage to the 17b SCFv with the exact LI sequence, and to enable modification of linker length The resulting intermediate plasmid is designated pCD 1 This construct was confirmed by DNA sequence analysis using standard techniques To facilitate subsequent procedures, the sCD4-hnker sequence was recloned into a ρSC59 derivative lacking a BamH I site, forming intermediate plasmid pCD2 The starting 17b plasmid containing the 17b SCFv cDNA in a plasmid vector (pmt del 0) was donated by Dr Richard Wyatt (Dana Farber Cancer Institute, Boston, MA) The SCFv cDNA is constructed with the heavy chain at the 5' segment and light chain at the 3' segment, attached via DNA encoding the L2 linker (gly 4 ser) 3 The 17b SCFv construct has a TPA signal sequence at the 5' end, and sequences coπesponding to a thrombm cleavage site and a hexa-his tag (to facilitate purification) at the 3' end, followed by a stop codon A comparable construct without the thrombm cleavage site and hexa-his tag can also be produced PCR technology was used to attach the 17b SCFv sequence to the CD4-lιnker sequence in pCD2 Suitable primers are represented in SEQ ID NOs 7 and 9 The forward (5') primer (SEQ ID NO 7) contains a BamH I site near the 5 ' end (preceded by an overhang), followed by nucleotides that reconstruct the third (gly 4 ser) plus four additional (gly 4 ser) repeats, this is followed by nucleotides exactly coπesponding to the start ofthe 17b heavy chain (excludmg the 5' signal sequence, beginning at CAG GTG ) The 3 ' primer (SEQ ID NO 9) begins with convenient restriction sites for cloning mto pCD2 (Spe I and others), followed by nucleotides exactly complementary to the 3' end of the 17b SCFv sequence m pmt del 0 (stop codon, hexa-his tag, and thrombm cleavage site) These primers are used to prime the plasmid vector containing the 17b SCFv sequence in pmt del 0, and the resultant PCR product digested with BamH I plus a restriction enzyme that cleaves at the opposite 3' end (e g , Spe I) This digested fragment is then force-cloned mto pCD2 that has been digested with the same enzymes (BamH I and Spe I) The resulting plasmid (designated herein as pCD3) contains the final sCD4-SCFv(17b) construct (with the thrombm cleavage site and hexa-his tag) downstream from the strong, synthetic early/late vaccinia promoter m pSC59 There are convenient, unique restriction sites on each side ofthe sCD4-SCFv sequence for possible future cloning steps The 17b SCFv cDNA (including the 5' signal sequence) also has been excised from the pmt del 0 vector by restriction enzyme digestion or PCR, and cloned mto the vaccinia expression plasmid pSC59 to provide a control construct EXAMPLE 2 Expression and Purification of CD4-SCFv(17b) fusion protein A. Expression For small amounts of protein expression, vaccinia expression technology can be used to produce the sCD4-SCFv(17b) (as well as the control 17b SCFv protein) The plasmid containing the construct in the vaccmia expression plasmid ρSC59 is used to produce a vaccinia recombinant, using standard technology For such expression, suitable cells (HeLa, BSC-1, etc ) are infected with the recombinant vaccinia virus, after incubation for 24-36 hours at 37° C, the recombinant protem is present in the culture supernatant Initial biochemical and functional studies can be done with unfractionated supernatant, where necessary, the sCD4-SCFv protein may be purified (see below) Small scale, initial experiments can be performed with small amounts of material (5-20 micrograms, obtained from 1 - 5 X 10 7 cells) The preparation can be scaled up, for such large-scale production, it may be advantageous to employ higher yield technologies for expression of the recombinant proteins (e g , baculovirus, yeast, or E coli) Expression ofthe pCDl secreted protem product (the first two domains of CD4 through ser, g3 , plus the 37 ammo acid linker) was analyzed BSC-1 cells were transfected with pCDl and mfected with wild type vaccmia virus, then incubated overnight at 37° C Supernatants were analyzed by Western (immnunoblot) analysis, using antibodies against CD4 As expected, the protem encoded by pCDl migrated slightly more slowly than standard purified two-domain sCD4 (Upjohn- Pharmacia, Kalamazoo, MI) The pCD3 full-length sCD4-SCFv( 17b) (sCD4- 17b) fusion protein has been expressed and tested similarly, and 17b SCFv domain (as cloned into pSC59) can be examined likewise The sCD4-17b fusion protem (at least a portion of which is secreted) has the expected molecular size (approximately 55 kD) when analyzed by SDS PAGE and Western blotting The protein reacted strongly with antibodies against CD4 or the hexa-his tag, confirming the presence of these N- terminal and C-terminal moieties, as well as the coπect reading frame B. Purification Expressed fusion protem as constructed above with an amino-termmal hexa-his tag was purified using this molecular tag The tag enables the specific binding and purification ofthe fusion protem by binding to nickel-nitπlotπacetic acid (Ni-NTA) metal affinity chromatography matrix (see, for instance, The QIAexpressionist, QIAGEN, 1997) A hexa-his tag was used in the present examples Alternative purification methods mclude a combination of HPLC and conventional liquid column chromatography (gel filtration, ion exchanger, lsoelectric focusmg) C. Primary Characterization In order to test gpl20 binding to the 17b domain ofthe sCD4-17b fusion protein, 96-well ELISA plates were first coated with the 13B8 2 antι-CD4 MAb (Beckman Coulter, Chaska, MN, Catalogue no IM0398), whose epitope on CD4 overlaps determinants involved in binding to gpl20 The plates were then incubated with either the purified sCD4-17b or control buffer When the chimeric protein was captured this way, the 17b moiety remained available to bind gpl20 complexed to sCD4, however the sCD4 moiety could not bind free gpl20, since it was captured on the plate by the antι-CD4 MAb that blocks the bmdmg site The plates were incubated with gpl20 (MB isolate, Ratner et al , Nature 313 277-284, 1985) complexed to sCD4 Bmdmg of gpl20 was detected by a polyclonal antι-gpl20 antiserum, followed by anti-rabbit IgG conjugated to horseradish peroxidase The plates were washed and incubated with ABTS substrate, and the oxidized product was quantitated by measuring absorbance at 405 nm The results indicated specific binding absorbance values were 0 15 with the sCD4-17b chimeπc protem, compared to 0 05 with the control buffer For testing functionality ofthe sCD4 region ofthe chimeric protein, the ELISA plates were first coated with an anti-His tag MAb (QIAGEN Inc , Valencia, CA, Catalog no 34670), then incubated with either the purified chimeπc protein or control buffer With the chimeric protem captured in this way, the sCD4 moiety was available to bind free gpl20, however the 17b moiety could not bind gpl20 that was not complexed to sCD4 The plates were incubated with free gpl20, and binding was detected as detailed above The results indicated specific binding absorbance values were 046 with sCD4-17b, compared to 0 05 with the control buffer Thus, the ELISA assays confirmed the expected functional binding properties for each moiety of the chimeric protein 17b moiety bound the gpl20/sCD4 complex, and the CD4 moiety bound free gpl20 EXAMPLE 3 HIV-envelope neutralization measurements A. Vaccinia-based reporter gene cell fusion Env-mediated cell fusion activated by CD4 was measured using the vaccinia-based reporter gene assay (Nussbaum et al , J Virol 68 5411-5422, 1994) For the experiment shown in Table 2 and Fig 1, effector cells (HeLa) were transfected with plasmid pGlNT7- β-gal (lacZ linked to T7 promoter), then infected with vaccinia recombinants encoding either the mutant uncleaved Unc Env or the wildtype (WT) SF162 Env (Broder & Berger, Proc Natl Acad Sci , USA 92 9004-9008, 1995) Target cells were created by transfecting NIH 3T3 cells with plasmid pGA9-CKR5, containing the CCR5 cDNA linked to a vaccinia promoter (Alkhatib et al , Science 272 1955-1958, 1996), then infecting these cells with wild type vaccinia virus WR Target cells also carry and express a bacteπophage T7 RNA polymerase Prior to fusion assays, transfected cells were incubated overnight at 31° C to allow expression of recombinant proteins, then washed For each fusion assay, mixtures of effector and target cells (1 x 10 s of each cell type per well, duplicate wells) were prepared m the absence or presence of sCD4 (100 nM final) After 2 5 hours at 37° C, cells were lysed with NP-40 and β-gal activity was quantitated using standard procedures (Table 2 and Fig 1) Relative fusion (specific β -gal activity) was determined from the mean of duplicate samples, and calculated as WT-Unc Table 2 Vaccinia-based reporter gene cell fusion assay using soluble CD4 Total β-gal Relative fusion (Raw data) (WT - Unc) Unc Env (Control) WT Env (SF162) duplicates mean duplicates mean - sCD4 0 50 0 40 0 45 0 50 0 40 0 45 0 0 + sCD4 0 40 0 50 0 45 6 60 5 20 5 90 5 45 This vaccinia-based fusion assay can be used to assess the neutralizing ability ofthe herein disclosed bispecific fusion proteins The neutralizing ability of MAb 17b was demonstrated to be dependent on the addition of soluble CD4 as follows (see Table 3 and Fig 2) Effector cells were created by co-mfect g HeLa cells with a vaccinia recombinant encoding HIV-1 Env (LAV) (Broder & Berger, Proc Natl Acad Sci USA 92 9004-9008, 1995), and another encoding T7 RNA polymerase Target cells were created by co-transfectmg NIH 3T3 cells with plasmids pYFl-fusm (Feng et al , Science 272 872-877, 1996) encoding CXCR4, and pGlNT7-β-gal (lacZ linked to the T7 promoter). The target cells were then infected with vaccmia viruses vCB-3 (encoding CD4, standard assay) (Broder et al , Virology 193:483-491, 1993), or WR (wild type virus, sCD4 assay). As background controls, target cells were transfected with pGlNT7- β-gal only (i e , no coreceptor) Transfected cells were incubated overnight at 31° C to allow expression of recombinant proteins, then washed. Effector cells were incubated 30 mmutes at 37° C with the indicated concentration of MAb 17b (Table 3). For fusion assays, mixtures were prepared between effector and indicated target cells (2 x 10 5 of each cell type per well, duplicate wells); in the standard assay, target cells expressed CXCR4 and CD4, and no soluble CD4 added; m the sCD4 assay, target cells expressed CXCR4 alone, and soluble CD4 was added (200 nM final). After 2.5 hours at 37° C, cells were lysed and β -gal activity was quantitated. Background control β -gal values (standard assay, 0.6; sCD4 assay, 0.2), obtained with target cells lacking coreceptor, were subtracted to give the data presented m Table 3. Data represent percentage of control (no MAb) for each assay. Table 3 MAb-mediated inhibition of fusion assay [17b] Standard Assay sCD4 Assay (μg/ml) β-gal % control β-gal % control none 42.3 100.0 11.89 100.0 0.1 39.5 93.4 13.55 113.9 0.5 43.9 103.8 4.66 39.2 1 39.8 94.1 1.68 14.1 5 50.5 119.4 0 0 The effectiveness ofthe herein described bispecific fusion protems for neutralizing fusion is tested m a similar manner, by adding varying amounts ofthe bispecific fusion protem, e g sCD4- SCFv(17b), to the above assay. Exogenous sCD4 and SCFv(17b) or other gpl20-bιndιng protems need not be added, though they can be used as controls as above, or to determine relative inhibitory efficiencies compared to the bispecific fusion protein Using this assay, the effects of media from control cells infected with wild-type vaccinia virus WR, were compared with media from cells infected with the recombinant vaccinia virus encoding sCD4-l 7b The relative specific β- galactosidase values were 23 4 with the control media and <1 with sCD4-SCFv media Thus, the chimeric sCD4-17b protein strongly inhibited HIV-1 Env-mediated cell fusion EXAMPLE 4 Large scale production and analysis of sCD4-17b To produce large amounts ofthe sCD4-17b protem, the DNA construct has been transferred to the pETl lb plasmid vector (Novagen, Madison, WI, Catalog no 69437-3), which is suitable for high level inducible expression m E coli This system involves cloning of target genes under contiol of strong bacteriophage T7 transcription signal Once established m a non-expression host bacterial cell, plasmids are then transferred mto expression hosts containing a chromosomal copy of the T7 RNA polymerase gene under lacUV5 control, and expression ofthe recombinant protein of interest (here, sCD4-17b) is induced by the addition of IPTG The expressed protein is produced at a very high level, and may constitute more than 50% ofthe total cell protem in the induced culture within a few hours after induction Western blot results mdicate high level expression ofthe sCD4-17b from the pETl lb plasmid The protein produced can be denatured and renatured from inclusion bodies to provide a large quantity of functional sCD4-17b protein This protem can be used for in vitro studies to test inhibition m assays of both Env-mediated cell fusion and HIV infection (p24 production) In addition, the sCD4- 17b protem can be used for in vivo studies One in vivo model involves SCID mice reconstituted with human thymus plus liver (Pettoello-Montovani et al , J Infect Dis 111 337-346, 1998), this system will be used to test whether sCD4-17b inhibits (and to what extent), or prevents, acute HIV-1 infection This system has been successfully used to demonstrate potent blocking activities of other anti-HIV agents (e g , protease inhibitors and reverse transcπptase inhibitors, and Env-targeted toxins) (Pettoello-Montovani et al , J Infect Dis 111 337- 346, 1998) A second example of an in vivo system for testing sCD4-17b activity involves rhesus macaques challenged with SHIV viruses (recombmant viruses containing SIV gag and pol, plus an HIV envelope, Li et al , J Virol 69 7061-7071, 1995) This system will be used to test whether the sCD4-17b protein inhibits (and to what extent), or prevents, acute SHIV infection The effects of sCD4-17b against chronic infection will also be exammed, again using the SCID-hu HIV-1 mouse system and the macaque/SHIV system Both in vitro and in vivo study systems also will be used to test the potency of sCD4-17b protein when used in combination with other anti-HIV agents (e g , RT and protease inhibitors or other HIV-1 neutralizing MAbs) The foregoing examples are provided by way of illustration only Numerous variations on the biological molecules and methods described above may be employed to make and use bispecific fusion molecules capable of binding to two sites on a single protein, and especially two sites on the HIV envelope protein gpl20, and for their use in detection, treatment, and prevention of HIV mfection We claim all such subject matter that falls withm the scope and spirit ofthe following claims

Claims: We claim 1 A neutralizing bispecific fusion protein capable of binding to two sites on a target protein, comprising a first binding domain capable of binding to an inducing site on the target protein, thereby exposmg an induced epitope, a second binding domain capable of forming a neutralizing complex with an induced epitope ofthe target protein, and a linker connecting the first domain to the second domam 2 A protem according to claim 1, wherein the first binding domain comprises a binding portion of a variable region of an antibody heavy or light cham 3 A protem according to claim 2, wherein the first binding domain comprises an epitope bindmg domain of an antibody 4 A protein according to claim 3, wherein the first domain comprises a smgle-chain Fv (SCFv) 5 A protein according to claim 3, wherein the antibody binding domain mimics a biological activity of a CD4 molecule in binding to the inducing site and exposmg the mducmg epitope 6 A protein according to claim 5, wherem the antibody binding domain is derived from a CD4 anti-idiotypic antibody 7 A protein according to claim 1, wherein the target protein is a viral envelope protem of a virus 8 A protein according to claim 7, wherein the virus is human immunodeficiency virus (HIV) 9 A protem according to claim 8, wherein the viral envelope protem is gpl20 10 A protein according to claim 2, wherein the first binding domain is derived from a CD4 molecule 11 A protem according to claim 10, wherein the first binding domain comprises CD4 Dl or CD4DlD2 12 A protem according to claim 11, wherein the first bmdmg domain is sCD4 13 A protem according to claim 1, wherein the second binding domain comprises a binding portion of a variable region of an antibody heavy or light chain 14 A protein according to claim 13, wherein the second binding domain comprises a binding domain of an antibody 15 A protein according to claim 14, wherein the second binding domain comprises a smgle-chain Fv (SCFv) 16 A protein according to claim 15, wherein the SCFv is selected from the group consisting of SCFv 17b, SCFv48d and SCFvCGlO 17 A protein according to claim 14, wherein the antibody bindmg domain is derived from a neutralizing monoclonal antibody 18 A protein according to claim 17, wherein the neutralizing monoclonal antibody is selected from the group consistmg of 17b, 48d, and CG10 19 A protem according to claim 9, wherein the second bindmg domain mimics a biological activity of an HIV coreceptor molecule m binding to gpl20 20 A protein according to claim 19, wherein the second binding domain comprises a peptide fragment of an HIV coreceptor 21 A protein accordmg to claim 20, wherein the HIV coreceptor is a chemokine receptor 22 A protem according to claim 21, wherein the chemokine receptor is selected from the group consistmg of CXCR4, CCR2B, CCR3, and CCR5, CCR8, CCR9, CX 3 CR1 , US28, or the chemokine receptor related protems including STRL33, GPR15, APJ, ChemR23, and BLTR 23 A protein according to claim 9, wherein the mduced epitope comprises at least one coreceptor binding determinant of gpl20 24 A protein accordmg to claim 9, wherein the inducing site is a gpl20 CD4 binding 25 A protein according to claim 14, wherein the binding domam ofthe antibody is capable of binding to at least one coreceptor bmdmg determinant of gpl20 26 A protein according to claim 1, wherein the linker maintains the second bmdmg domain in binding proximity to the induced epitope when the first binding domain is bound to the inducing site 27 A protem according to claim 26, wherein the linker is substantially flexible 28 A protem according to claim 26, wherein the linker is 15-100 angstroms (A) long 29 A protein according to claim 26, wherein the linker is 10-100 amino acid residues in length 30 A protein according to claim 26, wherem the linker comprises at least one occurrence of an amino acid sequence as represented by SEQ ID NO 1 31 A protein according to claim 1, wherein the linker comprises at least one occuπence of an amino acid sequence represented by SEQ ID NO 1 32 A protein according to claim 31, wherein the linker comprises an ammo acid sequence represented by SEQ ID NO 2 33 A functional recombinant bispecific fusion protein capable of bmdmg to two sites on gpl20, comprising a) sCD4, b) SCFv(17b), and c) a linker of a length sufficient to maintain the SCFv( 17b) in binding proximity an SCFv(17b) epitope when sCD4 is bound to gpl20 34 A protem according to claim 33, wherein the linker has an amino acid sequence as represented by SEQ ID NO 2 35 An isolated nucleic acid molecule encoding a protein according to claim 34 36 A nucleic acid molecule according to claim 35, wherein the nucleic acid sequence is represented by SEQ ID NO 3 37 A protem encoded for by the nucleic acid molecule according to claim 36 38 An isolated nucleic acid molecule encoding a protem according to claim 1 39 The protein according to claim 38, having ammo acid sequence SEQ ID NO 4 40 A transgemc eukaryotic cell comprising the isolated nucleic acid molecule according to claim 38 41 A method for producing m a eukaryotic cell a functional recombmant bispecific fusion protem capable of binding two sites on a target protein, comprising the steps of a) transfecting the eukaryotic cell with a recombinant nucleic acid molecule according to claim 38, b) cultuπng the transfected eukaryotic cell under conditions that cause production ofthe protem, and c) recoveπng the protem produced by the cultured eukaryotic cell 42 The method of claim 41 , wherein the eukaryotic cell is a mammalian cell 43 The method of claim 41 , wherein recovermg the protem comprises a) identifying the protein by the presence of a molecular tag, and b) separating the protem having the molecular tag so identified from molecules without the tag, so as to recover the protem produced by the cultured eukaryotic cell 44 A method for inactivating a gpl20 protein, comprising the step of contacting the gpl20 protem with a protem according to claim 9, or a vaπant protein, analog or mimetic thereof 45 A method for neutralizing a human immunodeficiency virus, comprising the step of contacting the human immunodeficiency virus with a protein according to claim 9, or a variant prote , analog or mimetic thereof 46 A method for blocking and preventing the bmdmg of a viral or recombinant gpl20 protein to soluble CD4 or lymphocyte CD4, comprising the step of contacting the gpl20 protein with a protein according to claim 9, or a variant protem, analog or mimetic thereof 47 A method for inhibiting HIV virus replication or mfectivity m a subject, comprising administering to the subject an amount ofthe protein according to claim 9, or a variant protem, analog or mimetic thereof, sufficient to inhibit HIV virus replication or mfectivity 48. A composition compπsmg the protein according to claim 1, or a variant protem, analog or mimetic thereof 49 A pharmaceutical composition comprising the protein according to claim 1, or a variant protein, analog or mimetic thereof, and a pharmaceutically acceptable carrier. 50. The method according to claim 47, wherein the protein is administered in the form of a pharmaceutical composition. 51. A protem analog, derivative, or mimetic of the protem of claim 1. 52. A kit for treatment and/or prevention of HIV infection, comprising a clinically effective dose ofthe neutralizing bispecific fusion protein of claim 1. 53 The kit of claim 52, further comprising instructions. 54. The kit of claim 53, wherein the instructions include directions for admimsteπng at least one dose ofthe neutralizing bispecific fusion protein to a subject in need of such treatment. 55 The kit of claim 52, wherein the neutralizing bispecific fusion protein is provided in the form of a pharmaceutical composition

Inventors: BERGER EDWARD A, DEL CASTILLO CHRISTIE M, BERGER, EDWARD, A., DEL CASTILLO, CHRISTIE, M.
Applicants: BERGER EDWARD A, DEL CASTILLO CHRISTIE M, US HEALTH, BERGER, EDWARD, A., DEL CASTILLO, CHRISTIE, M., THE GOVERNMENT OF THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY, DEPARTMENT OF HEALTH & HUMAN SERVICES, THE NATIONAL INSTITUTES OF HEALTH
Application year: 2000
Main classification: C07K16
IPC codes: A61K38, A61P31, C07K5, C07K14, C07K16, C07K19, C12N5, C12N15, C12P21, C12R1