IMPROVEMENTS IN OR RELATING TO BINDING ASSAYS

Abstract: Disclosed is a method of testing a substance for the ability to affect the formation or oligomerisation of a complex comprising members of a specific binding pair, a first member of the binding pair being present on the surface of a lipid enveloped particle comprising a transferrable label, said first member of the binding pair being capable of binding to a second member of the specific binding pair present on the surface of a cell, the lipid envelope of the particle being capable of fusing with the membrane of the cell so as to transfer the label to the cell, said transfer being inhibited by formation or oligomerisation of a complex between the first and second members of the binding pair, wherein the method comprises reacting the particle and the cell, in the presence of the substance under test, (under conditions which would allow for binding of the first and second members of the specific binding pair in the absence of the substance under test), and detecting the label transferred, if any; together with a composition for use in the assay method of the invention.

Description: Title : Improvements in or Relating to Binding Assays Field of the Invention This invention relates to a method of performing an assay, and a composition for performing an assay. Background of the Invention Cells use their surface receptors to monitor and respond to subtle changes in the composition of their immediate environment. Drugs which block or activate specific receptors can therefore be used to modify cellular functions for medicinal purposes. During the last two or three decades some of the most profitable drug discoveries have resulted from the detailed analysis of ligand-receptor interactions and the development of simple assays which have been used to screen for compounds that block ligand binding or directly stimulate receptors. Molecular biology has now uncovered a plethora of high molecular weight polypeptide ligands (for examples, see WO 94/27643 [Targeted Genetics Corporation] and references therein) with diverse biological activities and d e Human Genome Project promises to uncover many more. Thus, a major challenge for the pharmaceutical industry is to discover new drugs diat block or mimic the effects of these ligands, or exhibit greater specificity for receptor subtypes (Luyten & Leysen 1993 Trends in Biotechnology 11, p247). The process of drug discovery is relatively simple in principle (Hodgson 1992 Bio/Technology 10, p973). Large numbers of compounds are screened using assays that can detect the desired biological activity and "lead" compounds identified in this way are then optimised to meet development criteria (Bevan et al. , 1995 Trends in Biotechnology 13, pll5; Ecker & Crooke 1995 Bio/Technology 13, p351). The likelihood of identifying (and successfully optimising) a lead compound would be increased by me use of simple, sensitive assays that can tolerate a high throughput of drug candidates per unit time and by screening as many different compounds as possible. There are many types of binding assay available, each with its own advantages and disadvantages. Non-equilibrium assays are suitable for detection of high affinity binding reactions whereas equilibrium assays, such as the scintillation proximity assay (Bosworth & Towers, 1989 Nature 341, pl67) are required for detection of low affinity binding reactions characterised by rapid dissociation of bound ligand. Assays such as these, that require a pure source of receptor protein, are problematic because it remains difficult to purify membrane receptor proteins in large quantity (Schertler 1992, Curr. Opin. Struct. Biol. 2, p534). Cell-based assays are therefore preferred because most receptors can be expressed easily from cloned DNA. Usually die ligand is obtained in pure form and labelled, such that it can be easily detected after it has bound to its cognate receptor on die surface of a mammalian cell. Binding to me receptor is monitored by detection of label on d e target cells and lead compounds are identified by virtue of their ability to reduce the amount of receptor-bound label. However, it can be problematic to obtain a pure source of the ligand, and attaching a label (e.g. radioiodine, fluorescein, biotin) to the ligand can alter its affinity for the receptor. Alternatively, the ligand may be attached to a toxic moiety such diat binding to the receptor causes the target cells to die; the lead compound is men detected by its ability to prevent the death of die target cells. However, this type of assay is unlikely to detect a typical lead compound with weak blocking activity, insufficient to prevent the delivery of at least some toxin to die target cells. In other cell-based assays die binding of a ligand to its receptor is detected by measuring a biochemical or physiological response of the target cell. One of the pitfalls of this type of assay is that a compound may block me response of the target cell in other ways than by interfering with ligand binding, giving rise to a large number of false positives. Ligand-dependent, receptor-mediated retroviral sequestration Retroviral envelope glycoproteins mediate specific viral attachment to cell surface receptors and subsequently trigger fusion between die viral envelope and the target cell membrane. Retroviral envelope glycoproteins consist of an external glycoprotein moiety (SU) noncovalendy attached at its C-terminus to a smaller transmembrane polypeptide moiety (TM). Each surface projection (or spike), visible by electron microscopy on the viral surface, is a trimer of identical envelope glycoprotein subunits. SU comprises two domains connected by a proline-rich hinge, die N-terminal domain conferring receptor specificity and exhibiting a high degree of conservation between murine leukaemia viruses (MLVs) with different host ranges (Battini et al., 1992 J. Virol. 66, ρl468-1475). A general method has been disclosed diat allows the display of a polypeptide ligand (which may be glycosylated) on ie surface of a retroviral vector as a genetically encoded extension of die viral envelope protein (WO94/06920, Medical Research Council). The engineered retroviral vector then adopts die binding specificity of die displayed ligand. To date several different polypeptide ligands have been displayed on murine leukaemia virus (MLV)-based retroviral vectors, including single chain antibodies, cellular growth factors and immunoglobulin binding domains (WO94/06920 Medical Research Council; Cosset et al., 1994 Gene Therapy 1 pSl; Nilson et al., 1994 Gene Therapy 1, pS17). In principle, this technology should allow d e display of many different structural classes of binding domains on retroviral vectors, including glycopolypeptides and glycoproteins. The present inventors, in collaboration wim colleagues, have also recentiy discovered a novel biological phenomenon called ligand-dependent, receptor-mediated viral sequestration (illustrated in example 1). A polypeptide ligand is fused (by genetic engineering) to the envelope protein of an MLV-based retroviral vector such diat die envelope protein to which it has been grafted remains substantially intact and capable of binding to its natural receptor, and die fused non-viral polypeptide ligand is displayed on die viral surface. The virus displaying die fused non- viral polypeptide ligand is dien capable of multivalent attachment to the namral virus receptor and to d e d e cognate receptor for d e non-viral ligand; attachment to the natural virus receptor leads to infection of the target cell, whereas attachment to the cellular receptor for the displayed non- viral ligand may not lead to infection of d e target cell. Where the target cell expresses both species of receptor and attachment through die displayed non-viral ligand does not lead to infection, the two binding reactions (envelope protein to natural receptor and non-viral ligand to its cognate receptor) proceed in competition and die infectivity of the virus for die target cells is reduced in proportion to me efficiency widi which die second binding reaction competes virus away from the natural virus receptor. For example, when epidermal growtii factor (EGF) was displayed on an amphotropic retroviral vector, the engineered vector bound preferentially to EGF receptors present on EGF receptor-positive human cells and gene transfer did not occur. EGF receptor- negative cells were fully susceptible to die engineered retroviral vector but showed reduced susceptibility when diey were genetically modified to express EGF receptors. The reduction in susceptibility was in proportion to e level of EGF receptor expression. Moreover, when soluble EGF was added to competitively inhibit virus capture by die EGF receptors, gene transfer was restored. The loss of infectivity of the EGF- displaying virus on EGF receptor expressing cells is believed to reflect a block to membrane fusion between the viral envelope and die target cell plasma membrane. The degree to which gene transfer is inhibited depends, at least in part, on die relative affinities of die two binding reactions (envelope protein to natural receptor and non- viral ligand to its cognate receptor), the relative densities of die two receptors on die target cell surface, and die relative densities of die non- viral ligand and die intact envelope protein on die viral surface. Inhibition of gene transfer is additionally influenced by intrinsic properties of die receptor for the non-viral ligand, such as die distance it projects from die target cell membrane, its mobility witiiin the target cell membrane and its half life on die cell surface after engagement of ligand. The present invention makes use of these discoveries to provide die basis for a new assay method, ideally suited to die screening of compounds which may affect the binding between a ligand and its receptor. Summary of the invention In a first aspect the invention provides a mediod of testing a substance for the ability to affect the formation or oligomerisation of a complex comprising members of a specific binding pair ("sbp"), a first member of die binding pair being present on the surface of a lipid enveloped particle comprising a transferrable label, said first member of the binding pair being capable of binding to a second member of die sbp present on die surface of a cell, die lipid envelope of die particle being capable of fusing wim die cell so as to transfer the label to the cell, said transfer being inhibited by formation or oligomerisation of a complex comprising die first and second members of e binding pair, wherein die mediod comprises reacting the particle and die cell, in die presence of the substance under test, (under conditions which would allow for binding of die first and second members of the sbp in the absence of the substance under test), and detecting die label transferred, if any. Detection of die transferred label may be qualitative or, more preferably, quantitative. By way of explanation, when performing die assay of die invention lipid enveloped particles displaying a first member of a specific binding pair (typically, a cloned ligand molecule) are mixed with cells expressing a second member of the specific binding pair (the respective receptor for the ligand molecule) in die presence of the test compound. Throughout die specification, where die context so permits, "ligand" may generally be taken as being equivalent to "first member" of the sbp and "receptor" may generally be taken as being equivalent to "second member" of the sbp. Generally speaking, formation of a complex by binding of die ligand to die cell receptor will inhibit fusion of die particle widi die cell, tiius inhibiting transfer of the label from the particle to die cell. If die substance under test blocks die formation of the complex between die ligand and die receptor, (e.g. by binding to one or both diereof), diis will tend to increase die amount of fusion which occurs between me particle and die cell, diereby increasing e amount of label acquired by the cell. Alternatively die test substance may affect die level of expression of die receptor on d e surface of d e cell (up or down regulation), ti us altering die effective concentration of die receptor and diereby altering die likelihood of complex formation between die receptor and die ligand. Substances which affect either die binding reaction or which affect levels of expression of the receptor are considered as having the ability to affect the formation of a complex between the first and second members of die sbp. In addition, die mediod of die invention may be used to test substances for the ability to affect oligomerisation of a complex formed between d e first and second members of the sbp, as explained below. It will be apparent to tiiose skilled in die art diat, in particular, die reverse assay mediod could be used in which binding of die ligand to die cell receptor enhances die amount of fusion, and one would dien screen for compounds which can reduce die amount of label acquired by die cell. It will be noted diat, when a particle fuses widi a cell, die label may not be, strictly speaking, "transferred" to die cell because die particle may cease to have an independent exsistence. Thus die label will be acquired by die cell, and such is to be understood as being within d e meaning of the term "transferrable label". It will be appreciated by diose skilled in die art diat it may not be necessary to add the substance under test at die time the particles are mixed widi die cells. It may be sufficient, for example, to pre-incubate the test substance witii die particles and/or die cells. If such pre-incubation is used, die particles and/or cells will desirably be washed to remove unbound substance prior to performance of the assay. Preferably die lipid enveloped particle is a lipid enveloped virus, conveniently a retrovirus. The lipid enveloped particle may however be a synmetic lipid vesicle, liposome or the like. Fusion between die particle and die cell may be by simple merging of the lipid envelope witii die cell, but desirably will be facilitated by inclusion in the particle of one or more polypeptides which are capable of enhancing lipid membrane fusion. Conveniently die polypeptide will be glycosylated and typically will be of viral origin (i.e. a polypeptide substantially similar to one naturally encoded by a viral gene), conveniently an effective portion of a C type retroviral envelope glycoprotein. Suitable polypeptides are known which can enhance fusion at neutral pH (e.g. retroviral envelope proteins) or at acid pH (e.g. influenza haemagglutinin). It will be appreciated diat an effective portion only of such a polypeptide (i.e, a portion sufficient to retain a membrane fusion enhancement function) may be employed, altiiough, for convenience, the polypeptide will preferably be a substantially intact viral envelope protein. The term "substantially intact" in relation to a viral envelope protein is intended to indicate that the protein retains all its domains and is conserved for post-translational processing, oligomerisation, viral incorporation and fusogenic activity. However, certain alterations (e.g. substitutions, deletions or additions) can be made to die protein which do not substantially affect diese functions. Such polypeptides which enhance fusion frequently require die presence, in the reciprocal membrane, of cognate molecules to allow the fusion enhancing reaction to occur. Such cognate molecules are preferably provided, dierefore, on die surface of the cell used in the assay. Often, when a cell surface receptor binds a ligand, d e resulting receptor/ligand complex may undergo oligomerisation, which may be essential for signalling to occur. It is suspected diat d e retroviral envelope trimer must undergo de-oligomerisation to expose its fusion domain, and without such de-oligomerisation fusion is inhibited. Thus if the ligand displayed on die lipid enveloped particle binds to a receptor to form a receptor/ligand complex, which then undergoes oligomerisation, de-oligomerisation or exposure of die fusion domain on die retroviral envelope protein will be further inhibited, diereby inhibiting fusion and label transfer. Thus in particular embodiments the invention may provide an assay for testing a substance for the ability to affect oligomerisation of a complex comprising members of an sbp. If a substance inhibits such oligomerisation it will tend to promote fusion of die lipid enveloped particle with the cell, so promoting me transfer of label tiiereto. Such substances may be of considerable interest clinically, as offering means of inhibiting "transforming" signals generated by die products of certain oncogenes, which are mutant receptors or receptor-associated polypeptides. Generally the cell used in the mediod of d e invention will advantageously be eukaryotic (e.g. a yeast or insect cell) but could be bacterial. The cell will preferably be living (at least at the start of the assay) but could be dead, or even a synthetic inert cell equivalent (such as a synthetic lipid vesicle, liposome or planar lipid membrane). Most desirably d e cell will be mammalian, convenientiy a cell line. The second member of the sbp will conveniently be a cell surface receptor, and die first member of the sbp will be a ligand tiierefor. Examples of suitable sbps are well known (e.g. from WO94/27643). Advantageously the cell will naturally express the surface receptor of interest, but tiiose skilled in die art will appreciate d at it is equally possible to transfect the cell widi an appropriate expression construct so as to enable die cell to express die receptor of interest. The ligand (i.e. first member of the sbp) may be displayed on die particle as a part of a fusion polypeptide. Such a chimaeric membrane polypeptide may be distinct from the membrane glycoprotein optionally included to facilitate membrane fusion or may form part thereof. As mentioned above, in addition to d e second member of the sbp (i.e. "receptor"), die cell used in die assay may also comprise cognate molecules which interact witii fusion-enhancing polypeptides provided on die surface of die particle. The relative densities of tiiese two types of receptor molecule on the surface of die cell can be varied by die use of standard transfection and gene expression methods known to tiiose skilled in the art. The transferrable label may be radioactive, fluorescent or an otherwise-modified lipid or non-lipid component of the lipid enveloped particle. The function of die transferrable label is to indicate mixing of the constituents of die particle widi die membrane of die cell. The label may be a low molecular weight compound, enzyme or other protein whose transfer to the cell is a consequence of fusion between die carrier particle and die cell, and whose presence in (e.g. die cytoplasm or nucleus of) the cell is readily detectable. In a ' preferred embodiment, die transferrable label is a nucleic acid coding for a readily detectable polypeptide (i.e. a reporter gene) which can be expressed in die cell after the particle has fused tiierewith. Many reporter genes are known and include chloramphenicol acetyl transferase and 3-galactosidase. In a preferred embodiment, die particle comprises a retroviral vector and an effective portion of die 4070A MLV envelope glycoprotein, which molecule enhances fusion between the viral envelope and the cell membrane. Conveniently, the first member of die sbp is displayed as a (genetically encoded) N-terminal extension of the envelope glycoprotein. The term "N-terminal" is intended to apply to fusions at the N-terminal region (i.e. within 30 or so amino acid residues of die N-terminal of the molecule, preferably widiin 20 or so residues, and more preferably witiiin 10 residues), and not solely to fusions at die extreme N-terminal of the molecule. Use of such a viral vector has the advantage, among others, of allowing for die ready incoφoration of different nucleic acid sequences encoding otiier first members of different sbps. The substance under test may be a synthetic compound, or mixture of compounds, or may be a natural product (e.g. plant extract or culture supernatant). The duration of contact between d e particle and d e cell, d e temperature and the biochemical composition (e.g. pH, salt concentration, etc.) of the medium in which die contact takes place can be varied and optimised by routine trial-and-error. After sufficient time has elapsed for transfer of the label from the particle to die cell and, where die label is a gene encoding a marker protein, for its expression in die cell, die presence of die label in d e cell is detected qualitatively or, preferably, quantitatively. Convenientiy, the label is a gene coding for E. coli β-galactosidase tagged widi a nuclear localisation signal and is readily detected by its ability to cleave X-gal, leading to formation of a coloured product. Gene transfer will be dependent on die presence of an active test compound diat blocks die ligand-receptor interaction witiiout damaging me viability of the target cells. Thus, in preferred embodiments, the invention provides an assay which will indicate not only if a test substance can affect the binding between members of an sbp, but will also indicate if me test substance is toxic to the cell used in die assay. The assay has further advantages in diat, typically, the receptor and ligand will be present in their native form (e.g. correctly folded and glycosylated), under physiological conditions (i.e. at body temperamre and pH) and (especially in die case of die receptor) expressed on die surface of, for example, mammalian cells. Thus die results of die assay will be more directly relevant to substances of possible medicinal use dian tiiose results generated by many existing assay formats. It is believed tiiat such ligand-customised assays will offer significant advantages over die assays diat are currently employed in pharmaceutical drug discovery programmes. They will be simple to perform, easy to read and do not require prior purification of ligand or receptor. In a particular embodiment die assay method can be used to screen for substances which affect the level of expression of a receptor on a cell surface. For example, cells expressing a receptor of interest may be incubated under appropriate conditions widi a test substance for a period sufficient to allow the test substance to affect the expression level of the receptor molecule. The method of die invention may dien be performed and label transferred to the cells detected. Thus, for example, where the receptor molecule is one to which a ligand displayed on a lipid enveloped particle will bind so as to inhibit transfer of the label to the cell, incubation of the cells witii a substance which increases expression of the receptor will have the effect of decreasing die amount of label transferred to die cells by the lipid enveloped particle. Conversely, where die test substance decreases expression of die receptor, die effect will be to increase the amount of label transferred. It has been found diat exposure of cells to EGF causes down-regulation of EGF-receptor expression (a negative feedback mechanism) and it is likely diat odier ligands and mimics thereof will have similar effects on the expression of their cognate receptor. In a second aspect, die invention provides a composition for use in a binding assay, comprising: a lipid enveloped particle comprising a first member of a specific binding pair and a transferrable label; and a cell displaying a second member of de specific binding pair, die lipid envelope of die particle being capable of fusing with the membrane of the cell so as tb transfer uie label to the cell, said transfer being inhibited by formation or oligomerisation of a complex between the first and second members of die specific binding pair. The composition may conveniently take the form of a kit, comprising instructions for use according to die mediod of die invention defined previously. The invention will now be further described by way of illustrative example and widi reference to die accompanying drawings, of which: Figure 1 is a schematic representation of various wild type and chimaeric retrovirus envelope constructs; (EGF = epidermal growtii factor, L = Leader signal sequence from Moloney MLV env; all env genes were expressed using die same promoter, die Friend MLV long terminal repeat "LTR", codon numbering refers to distance from N-terminal of mamre SU glycoprotein); Figure 2 is a picture of Western blots, showing detection of envelope SU proteins; immunoblots of lysates (on the left) of TELCeBό cells transfected with the envelopes shown in Figure 1 or of pellets (on the right) of viral particles produced from these cells. Both blots were stained witii an SU anti-serum. The immunoblot of pellets was cut at 46 kD, and die lower part was stained witii a p30-CA anti-serum; Figure 3 shows a series of graphs produced by fluorescence-activated cell sorting (FACS), illustrating the results of EGF receptor binding assays; cells were A431, HT1080, TE671, or K422. The top row shows binding assays performed using EMO envelopes (black histograms) compared to MO envelopes (white histograms), whilst the bottom row shows the results when the cells were stained widi an anti-hEGF receptor (anti-hEGF.R) antibody; Figure 4 shows a series of graphs, produced by FACS, illustrating the specificity of EGF binding; A431 cells were used as target cells. Cells were (+ rEGF) or were not (- rEGF) treated with recombinant EGF (IO -6 M, 30 min, 37°C) prior to binding assays using EMO (top row of graphs) or A envelopes (bottom row); and Figure 5 is a graph showing EGF binding of various fractions (see text) after S-l 000 chromatography. Each fraction was analysed botii for its binding activity using A431 cells as targets and for infectivity on 3T3 cells: (-): no infectivity, (+/-): 1-10 lacZ-EFU, (+): 10-100 lacZ-EFU, (+ +): 100-1000 lacZ-EFU, (+ + +): > 1000 lacZ-EFU. Examples In an attempt to target retroviral gene delivery dirough the human epidermal growth factor receptor (EGFR) to EGFR-positive cells, the inventors and tiieir colleagues constructed a chimaeric envelope construct in which die human EGF coding sequences were cloned, in frame, into a Sfil - NotI cloning site which had been introduced between the 6dι and 7th codons of die env gene of Moloney MLV (Russell et al, 1993 Nucleic Acids Research 21, 1081-1085; Russell et al, WO94/06920). This chimaeric envelope construct (EMO) was then transfected into the complementing cell line TELCeBό which expresses MLV gag and pol functions and a packagable RNA transcript coding for β- galactosidase. The transfected TELCeBό cells released retroviral particles which efficiently incorporated die chimaeric EMO envelope glycoprotein, as shown by western blot analysis of pelleted virus. Viruses incorporating the chimaeric EMO envelope glycoprotein were shown by chromogenic substrate assay to transfer the β-galactosidase gene to mouse NIH3T3 cells, indicating diat die chimaeric protein could still bind to die ecotropic MLV receptor and catalyse membrane fusion. Viruses incorporating the chimaeric EMO envelope glycoprotein were then separated from soluble (shed) envelope glycoprotein by sephacryl column chromatography of 0.45μM-filtered culture supernatant and were shown by immunofluorescence staining to bind efficiently to EGF.R-positive human cell lines (i.e. cell lines expressing EGF receptor). Binding to tiiese cells was competitively inhibited by soluble EGF. However, die bound virus did not transfer a functional /3-galactosidase gene to the human cell lines, whereas control viruses incoφorating the amphotropic 4070A envelope efficientiy transferred d e 3-galactosidase gene to tiiese same human cell lines. The inventors and colleagues next examined die efficiency of gene transfer to EGFR- positive human cells using recombinant retroviruses displaying die EGF polypeptide fused to die amphotropic 4070A envelope glycoprotein. The chimaeric EGF-4070A (EA) expression construct was created by substituting most of the MoMLV env sequence in FBEGFMosA (from codon 7, immediately 3' of the NotI cloning site to the Clal site close to die 3' end of d e env gene) for corresponding 4070 A env sequence (from codon 5 to the Clal site close to the 3' end of d e env gene). Recombinant retroviruses displaying the chimaeric EA envelope protein behaved in an unexpected fashion. They transferred die /3-galactosidase gene efficiently to mouse NIH3T3 cells and to human cells diat were negative for EGF receptor expression, but not to EGF receptor-expressing human cells. The titre reduction on EGF receptor-positive human cells was as much as ten million-fold compared to control viruses incoφorating unmodified 4070 A amphotropic envelopes. Subsequent experiments using various target cells of mouse and human origin, some of which had been transfected witii an EGFR expression plasmid, proved tiiat viruses displaying d e chimaeric EMO and EA envelope glycoproteins had selectively impaired ability to infect ecotropic or amphotropic MLV receptor-positive cells that also expressed EGF receptors. Higher levels of EGF receptor expression were associated widi greater impairment of infectivity. Cells transfected with an EGF receptor expression plasmid became selectively resistant to EMO and EA viruses, and diis resistance was reversible upon treating die cells widi soluble EGF to block/downregulate the EGF receptors. Thus, retroviral host range was selectively restricted to EGF receptor-negative cells by displaying a high affmity ligand for EGF receptors on die viral surface. The present inventors and their colleagues thus discovered a novel biological phenomenon which they have called ligand-dependent, receptor-mediated viral sequestration - a method by which to restrict the host range of a MLV or MLV-based retroviral vector in a ligand- dependent fashion. The first step is to identify a polypeptide ligand which binds specifically to a cell surface marker present on nontarget cells but absent from the target cell population. This polypeptide is then fused (by genetic engineering) to the retroviral envelope protein such that the envelope protein to which it has been grafted remains substantially intact and capable of binding to its namral receptor, and d e fused non-viral polypeptide ligand is displayed on die viral surface. The virus displaying die fused non¬ viral polypeptide ligand is dien capable of multivalent attachment to the namral virus receptor and to die die cognate receptor for the non-viral ligand; attachment to the natural virus receptor leads to infection of the target cell, whereas attachment to the cellular receptor for die displayed non- viral ligand may not lead to infection of the target cell. Where the target cell expresses both species of receptor and attachment through the displayed non-viral ligand does not lead to infection, the two binding reactions (envelope protein to natural receptor and nonviral ligand to its cognate receptor) proceed in competition and the infectivity of the virus for the target cells is reduced in proportion to the efficiency with which the second binding reaction competes virus away from the namral virus receptor. The degree to which gene transfer is inhibited will dierefore be influenced by die relative affinities of the two binding reactions, the relative densities of die two receptors on die target cell surface, and die relative densities of the non-viral ligand and die intact envelope protein on the viral surface. Inhibition of gene transfer is additionally influenced by intrinsic properties of the receptor for die non-viral ligand, such as the distance it projects from the target cell membrane, its tendency to oligomerise upon binding ligand, its mobility within the target cell membrane and its half life on the cell surface after engagement of ligand. This method of host range restriction may be applicable to the membrane spike glycoproteins of other enveloped viruses, and to die attachment proteins of non-enveloped viruses such as the adenovirus fibre protein. Where an enveloped virus has multiple distinct membrane spike glycoproteins with differing binding specificities and fusogenic capabilities (eg Paramyxoviridae, Heφesviridae), die restriction of virus host range by this method may or may not require the modification of more than one of the glycoproteins. The discovery offers a novel strategy for targeting retroviral gene delivery by host range extension (which is the subject of International Patent Application WO 96/00294). After binding to its receptor, EGF is endocytosed and routed to lysosomes, where EGF-EGF receptor complexes are degraded (Carpenter & Cohen, 1990 J. Biol. Chem. 265 p7709-7712). The inventors suspected tiiat viruses bound to EGF receptors might therefore also be rapidly endocytosed and " routed to lysosomes for degradation. The inventors therefore attempted to rescue EMO-carrying viral particles from this degradative patiiway by treating infected human cells with chloroquine phosphate, a lysosomotropic base which inhibits lysosomal acidification. Viruses carrying unmodified Moloney envelopes (which do not bind efficiently to human cells) were unable to infect the human cell lines, irrespective of the presence of chloroquine. In contrast, viruses carrying EMO envelopes which were shown to bind efficiently to EGF receptors on human cells showed significant infectivity on human EGF receptor-positive cells in the presence of chloroquine. In contrast, there was no evidence of infection in the presence of chloroquine on EGF receptor negative K422 B cells, to which die EMO virus did not bind. The present inventors realised tiiat this discovery was not only of importance in the field of gene therapy, but could provide die basis for a method of screening substances for their ability to affect the binding between members of a specific binding pair, as set out previously. Example 1 Construction of chimaeric retroviral envelopes. The sequence coding for EGF (epidermal growtii factor) was inserted in MLV (murine leukemia virus) env gene in a position corresponding to amino-acid +6 in the SU glycoprotein of MoMLV (Fig. 1). This position of insertion was previously shown to allow the functional display of single chain antibodies at die surface of virions (Russell et al, 1993 NAR 21 pl081-1085). The EGF domain was separated from d e wild type receptor binding domain in die envelope by a small linker containing 3 alanine residues. In die chimera EMO, EGF was inserted in me Mo-MLV envelope, whereas chimera EA had an EGF insertion in the MLV amphotropic (4070A) envelope at position +5. Envelopes, including die control envelopes from ecotropic (MO) and amphotropic (A) MLV, were transfected into TELCeBό cells which express MLV gag-pol core particles and a lacZ retroviral vector. Expression and viral incorporation of chimaeric envelopes. Lysates of TELCeBό cells were analysed for envelope expression using antibodies against MLV SU (Fig. 2). For both chimeric envelopes, both a precursor and a processed SU product were detected at ratios similar to wild-type envelopes, suggesting that the mutants were correctly expressed and processed. Cell surface expression of mutant envelopes was examined by FACS analysis of producer cells, using antibodies against die SU or a monoclonal anti-hEGF antibody. All transfected cells were stained widi die anti-SU antibodies, and cells expressing the EGF-fusion envelopes were also stained witii anti-EGF monoclonals (data not shown). To demonstrate incorporation of the chimeric envelope glycoproteins into retroviral particles, supernatants ofthe various TELCeBό-transfected cell lines were ultracentrifuged to pellet viral particles. Pellets were dien analysed on immunoblots for their content of gag (p30-CA) and envelope proteins (Fig. 2). The chimaeric SU glycoproteins were detected at a similar ratio to gag compared to wild-type envelope. Binding of chimaeric envelopes to EGF receptors. Human cell lines expressing different numbers of EGF receptors (Fig. 3 bottom) were used for binding assays. Cells were incubated witii virus supernatants and binding of viral envelopes to the target cell surface was analysed by FACS using antibodies against the MLV SU. MoMLV-derived EGF-fusion envelopes (EMO envelopes) were found to bind to A431 cells (Fig. 3 top) over-expressing EGF.R (Fig. 3 bottom). Less binding was found on TE671 and HT1080 target cells which express less EGF.R (Fig. 3). No binding could be detected on K422 lymphoma cells with no detectable expression of EGF receptor (Fig. 3). The EA envelopes bound to A431 cells as well as EMO (data not shown). EGF receptors on A431 cells were down-regulated by pre-incubation with EGF. This treatment did not affect the binding of amphotropic envelopes (Fig. 4 bottom) but abolished binding of EMO envelopes (Fig. 4 top). SU envelope glycoproteins of MLVs are known to be weakly associated widi tiieir TM protein counteφarts (Gliniak et al, 1991 J. Biol. Chem. 266 p22991-22997) and a very low proportion of SU is retained on virions. Therefore it is likely that binding assays in Fig. 3 are due in part to soluble envelope glycoproteins shed from virions. To determine whether viral particles could also bind, die supernatant of producer cells was separated by gel-filtration and fractions were analysed for binding activity on A431 cells (Fig. 5). As expected, very little binding activity was found in the early fractions confining the viral particles, with most of the binding activity in the late fractions containing soluble envelopes. However when viral particles were produced at 32°C in order to reduce the dissociation between SU and TM a significant binding activity was also found in die fractions containing the virions (Fig. 5), demonstrating that viral particles could bind EGF.R. Host range properties of viruses carrying EMO envelopes. Table IA shows that viruses incoφorating EMO envelopes can infect NIH3T3 mouse fibroblasts. Infection is through the ecotropic MLV receptor because the EMO virus cannot infect NIH3T3 cells expressing the Moloney envelope glycoprotein but can infect those expressing the 4070A envelope glycoprotein. Viruses incoφorating EMO envelopes can not only bind to EGF receptors but can also bind and infect cells through ecotropic MLV receptors. Table IB shows that viruses incoφorating EMO envelopes could not infect human cells expressing various densities of EGF receptors, despite tiieir ability to bind to die EGF receptors on tiiese cells (Fig 3 A). Suφrisingly, the cell line EJ.Al, which stably expresses ecotropic MLV receptors from a transfected plasmid, could not be infected by die EMO virus, but was readily infected by viruses incoφorating unmodified MO envelopes. This result suggested that the EMO virus could be competitively sequestered by EGF receptors on EJ.Al cells, preventing it from binding to die ecotropic viral receptors. Competitive sequestration of viruses carrying EMO envelopes. To test the idea tiiat EMO viruses could be competitively sequestered by EGF receptors at die surface of an otherwise permissive target cell, the inventors titrated viruses incoφorating EMO or MO envelopes on mouse fibroblasts (NRό and NTH3T3) expressing variable numbers of EGF receptors (Table 2). The titers of viruses carrying EMO envelopes were reduced up to 1000-fold by EGF.R expression and diere was a correlation between die density of EGF receptor expression and d e magnitude of reduction in virus titre (Table 2B). When NR6- hEGF.R cells were pre-treated witii rEGF, which down-regulates EGF.R as confirmed by antibody staining (not shown), titers of viruses coated widi EGF-fusion envelopes were greatly enhanced, reaching the range of titres obtained on parental NRό cells (Table 2A). These data confirm that interaction of virions with EGF receptors leads to tiieir sequestration into an abortive entry pathway that does not lead to membrane fusion or cytoplasmic release of the viral cores. Host range and competitive sequestration of viruses carrying EA envelopes. Table IA shows that viruses incoφorating EA envelopes can infect NIH3T3 mouse fibroblasts through the amphotropic MLV receptor. When titrated on a panel of human cell lines expressing varying densities of EGF receptors, die EA virus showed a selective inability to infect all of the EGF receptor-positive human cells in the panel (Table IB). However, tiiey could easily infect human B and T cell lines (K422 and Jurkat) which are devoid of EGF.R and are presumably infected tiirough the amphotropic receptor (Table 1). These data suggested diat die EA viruses were efficientiy sequestered by EGF.R expressed on human cells and to confirm tiieir competitive sequestration, they were tested on parental and EGF receptor-expressing NRό mouse fibroblasts (Table 2A). EGF receptor expression on NRό cells led to a competitive inhibition (100-fold) of viral infection which was reversible when the NRό transfectants were pre-treated witii EGF to block/downregulate their EGF receptors. Some sequestered virus can be rescued into an infectious entry pathway. After binding to receptor, EGF induces receptor dimerisation and its signal transduction, followed by ligand-receptor internalisation and routing to lysosomes, where EGF/EGF receptor complexes are degraded (Caφenter & Cohen, 1990 J. Biol. Chem 265 p7709- 7712). The inventors suspected tiiat viruses bound to EGF receptors might therefore be rapidly internalised into the cell and routed to lysosomes for degradation. When EMO- carrying viral particles were used to infect A431 cells treated witii the inhibitor of lysosomal degradation, chloroquine, a significant increase of infectivity (by approximately 2 logs) was obtained (Table 3). This effect was specific to EGF. receptors as EGF.receptor negative cells, such as K422 cells, did not respond similarly (Table 3). Materials and Methods Cell lines. TELCeBό cell line was derived from the TELac2 line (Takeuchi et al, 1994 J. Virol. 68 p8001-8007) after transfection and clonal selection of cells containing a plasmid expressing MoMLV (Moloney murine leukemia virus) gag and pol proteins. TELCeBό cells produce non-infectious viral core particles, carrying the MGFnlsLacZ reporter retroviral vector (Ferry et al, 1991 Proc. Natl. Acad. Sci. USA 55 ρ8377-8381). A431, TE671 (ATCC CRL8805), HT1080 (ATCC CCL121), EJ (Bubenik et al, 1973 Int. J. Cancer 11 p765-773) and EJ.Al, (an EJ clone that expresses ecotropic MLV receptors, Albritton et al, 1989 Cell 57 p659-666) were grown in DMEM (Gibco-BRL) supplemented with 10% fetal bovine serum (Gibco-BRL). K422 cells (Dyer et al, 1990 Blood 75 p709-714) and Jurkat T cells were grown in RPMI 1640 (Gibco-BRL) supplemented with 10% fetal bovine serum (Gibco-BRL). NRό murine fibroblasts lacking detectable EGF receptors (Schneider et al, 1986 Proc. Natl. Acad. Sci. USA 83 p333-336) and NRό-EGF.R (an NRό subclone obtained after transfection of a plasmid expressing the human epidermal growth factor receptor) cells were kindly provided by G. Gill (La Jolla, USA). psi2 cells (Mann et al, 1983 Cell 33 pl53-159) and GP+EAM12 cells (Markowitz et al, 1988 Virol. 167 p400-406) were derived from NTH- 3T3 cells and express respectively MoMLV (ecotropic) and MLV-amphotropic envelopes which block the corresponding receptors (EcoR-1 and RAM-1) by interference. NIH3T3 clones transfected witii EGF receptor expression constructs and expressing moderate or high levels of EGF receptors were kindly provided by Prof Thierry Velu (Erasmus Hospital, Brussels). NIH-3T3 and NTH-3T3-derived cell lines were grown in DMEM (Gibco-BRL) supplemented widi 10% new born bovine serum (Gibco-BRL). Chimeric envelopes. A PCR-derived DNA fragment encoding die 53 aa of hEGF (Bell et al, 1986 Nucleic Acids Res. 14 p8427-8446) was generated using a cDNA template (ATCC 59957) and two primers: OUEGF 5'- ATGCTCAGAGGGGTCAGTACGGCCCAGCCG GCCATGGCCAATAGTGACTCTGAATGTCCC - 3' (Seq. ID No. 1) with an Sfil restriction site, and OLEGF 5'- ACCTGAAGTGGTGGGAACTGCGCGC GGCCGCATGTGGGGGTCCAGACTCC - 3' (Seq. ID No. 2) witii a NofI site, and cloned after digestion witii Sfil and NotI in either MoMLV SU for " the EMO chimeric envelope or 4070A SU for the EA envelope (Figure 1). All envelope constructs were expressed as Bglϋ-Clal fragments (corresponding to positions 5408 and 7676 in MoMLV), cloned between BamHI and Clal sites of die FBMosALF expression vector (Cosset et al, submitted), in which a phleo selectable marker (Gatignol et al, 1988 FEBS Lett 230 pl71-175) fused to the PGK (phospho¬ glycerate kinase) gene poly-adenylation sequence was introduced downstream to the C57 MLV LTR of FB3 (Battini et al, 1992 J. Virol. 66 pl468-1475). Production of viruses. Envelope expression plasmids were transfected by calcium phosphate precipitation into TELCeBό cells. Transfected cells were selected widi phleomycin (50 mg/ml) and pools of phleomycin-resistant clones were used to harvest viruses from confluent cells after overnight incubation in DMEM and FBS (10%). These supernatants were used for ultracentrifugation to provide Western blot samples, for binding assays and for infection assays. Viruses (in 100 ml of producer cell supernatant) were also purified by gel-filtration on 2 ml columns (Bio-Rad) on a bed of S-1000 Sephacryl (Pharmacia). Fractions were obtained by elution with PBS at 4°C. Immunoblots. Virus producer cells were lysed in a 20 mM Tris-HCl buffer (pH 7.5) containing 1% Triton-XlOO, 0.05% SDS, 5 mg/ml sodium deoxycholate, 150 mM NaCl, and 1 mM PMSF. Lysates were incubated for 10 min at 4°C and were centrifuged for 10 min at 10,000xg to pellet the nuclei. Supernatants were then frozen at -70°C until further analysis. Virus samples were obtained by ultracentrifugation of viral supernatants (10 ml) in a SW41 Beckman Rotor (30,000 RPM, 1 hr, 4°C). Pellets were suspended in 100 μl of PBS (phosphate buffered saline), and frozen at -70°C. Samples (30 mg for cell lysates, or 10 μl for purified viruses) were mixed in a 375 mM Tris-HCl (pH 6.8) buffer containing 6% SDS, 30% b-mercapto-ethanol, 10% glycerol, and 0.06% bromophenol blue, boiled for 3 min, then run on 10% SDS acrylamide gels. After protein transfer onto nitrocellulose filters, immunostaining was performed in TBS (Tris base saline, pH 7.4) with 5% milk powder and 0.1% TWEEN. Antibodies (Quality Biotech Inc, USA) were goat antisera raised against eitiier RLV (Rausher leukemia virus) gp70- SU protein or RLV p30-CA protein, and were diluted 1/1,000 and 1/10,000, respectively. Blots were developed using horseradish peroxidase (HRPO)-conjugated rabbit anti-goat antibodies (DAKO, UK) and an electtochernduπiinescence kit (Amersham Life Science). Binding assays. Target cells were washed in PBS and detached by a 10 min incubation at 37 °C with versene 0.02% in PBS. Cells were washed in PBA (PBS with 2% FCS and 0.1 % sodium azide). 10° cells were mcubated witii viruses for 30 min at 4°C. Cells were then washed widi PBA and incubated in PBA containing 1/200 of RLV gp70 immune serum for 30 min at 4°C. Cells were washed twice widi PBA and incubated witii rabbit anti-goat FITC-conjugated antibodies (DAKO, U.K.). 5 min before die two final washes in PBA, cells were stained with 20 mg/ml propidium iodide. Fluorescence of living cells was analysed witii a fluorescent-activated cell sorter (FACSCan, Beckton Dickinson). For hEGF.R staining, IO 6 cells in 100 ml of PBA were incubated with 10 ml of anti-EGF.R antibodies (M886, DAKO, U.K.) for 30 min at 4°C. Infection assays. Target cells were seeded in 24 multi-well plates at a density of 3xl0 4 cells per well or in 6-multi-well plates at a density of 2x10 s cells per well. Viral supernatant dilutions containing 4 mg/ml polybrene were added and cells were incubated for 3-5 hrs at 37°C. Viral supernatant was then removed and cells were incubated in regular medium for 24-48 hrs. X-Gal staining was performed as previously described (Takeuchi et al, 1994 J. Virol. 68 p8001-8007). To block EGF.R, target cells were incubated 30 min at 37°C in a medium containing 10 "6 M rEGF (236-EG, R&D Systems, U.K.). Cells were then washed and infections were carried out as previously described. To block lysosomal acidification, 100 mM chloroquine phosphate (Sigma, U.K.) was added to the medium for 6 hr from the start of the infection protocol after which the cells were washed and incubated in regular medium. Table 1. Infection by virions expressing targeting envelopes A. On mouse fibroblasts* enV 3T3 3T3/E 3T3/A A Hf Hf Hf MO Hf <1 Hf EMO Hf <1 Hf EA Hf nd Hf B. On human cell lines" enV A431 HT108 TE671 K422 Jurkat EJ EJ.A1 EGFR A Hf Hf Hf Hf Hf Hf Hf MO <1 <1 <1 <1 <1 <1 Hf EMO <1 <1 <1 <1 <1 <1 <1 EA <1 <1 10 1 Hf Hf <1 <1 a: envelope expressed on lacZ virions b: titres as lacZ-EFU/ml. Abbreviations for cell lines: 3T3: NIH3T3; 3T3/E: psi2; 3T3/A: GP+EAM12 Table Z Inhibition of infection by EGF.R A NR6 cells' NR6 NR6-wt hEGF.R - rEGF + rEGF env * titre titre titre MO Kf SxHf 5xHf EMO 5x10* 1xHf Hf A 7xHf 2xHf .txHf EA 2xHf 3xHf 5xHf B. NIH3T3 cells EGFR No. 10.000 80.000 400.000 env * titre titre titre MO Hf Hf Hf EMO Hf Hf Hf a: envelope expressed on lacZ virions b: titres as lacZ-EFU/ml. c: cells were (+) or were not (-) pre-incubated with 10 * M recombinant EGF for 30 min at 37°C Table 3. Effect of chloroquine on infection NIH3T? . A431" . TE671" . K422" . env * + + + + MO Hf 5xHf <1 6 <1 1 <1 <1 EMO Hf 5xHf 1 225 <1 46 <1 <1 a: envelope expressed on lacZ virions b: titres as lacZ EFU/ml. Cells were treated (*) or not (-) with chloroquine. Example 2 The investigators and their colleagues wished to determine whedier they could selectively impair the efficiency of gene delivery by viruses incoφorating chimaeric 4070A envelope proteins displaying a ligand otiier than EGF. To diis end they cloned die coding sequence of human insulin-like growth factor-1 (IGF-1) (Hodgson et αl., 1995 Eur. J. Biochem. 233, 299-309), in frame, into a Sfil-Notl cloning site which had been introduced 5' of the first codon of the 4070A MLV env gene (Nilson et αl. , 1996 Gene Therapy 3, 280-286), producing construct IA. In addition, the IGF-I coding sequence was cloned, in frame, into a Sfil-Notl site 5' of the sequence coding for a Factor Xa protease cleavage site (amino acid residues IEGR), which had been previously introduced 5' of die first codon of the env gene of 4070A MLV (Nilson et αl., 1996 cited above). This construct was designated D A. The chimaeric envelope constructs were then transfected into the complementing cell line TelCeBό. Westem blot analysis of pelleted viruses revealed tiiat both IA and IXA envelope proteins were efficientiy incoφorated into viral particles. The efficiency of β- galactosidase gene transfer to mouse NIH 3T3 cells by viral particles bearing IA and IXA envelope proteins was comparable to that of particles incoφorating wild-type 4070A envelope proteins, (NIH-3T3 cells being IGF-1 receptor negative). However, viral particles incorporating chimaeric IA and IXA envelope proteins did not efficiently transfer the β-galactosidase gene to type-I IGF-receptor positive human MCF-7 cells or mouse 3T3 cells which had been transfected with type-I IGFR (Insulin-like growth factor-1 receptor) (Lammers et al., 1989 EMBO J. 8, 1369-1375). The titres produced by infection of the IGFR-positive cell lines with the IA and IXA viral particles were up to 1000-fold reduced relative to the titres produced by control viruses incorporating unmodified 4070A envelope proteins. Factor Xa protease cleavage of viral particles incoφorating IXA envelope proteins restored die titre produced on MCF-7 and IGFR/3T3 cells to wild-type levels. This demonstrated that the titre reductions for IA and D A viruses on type-I IGFR-positive cell lines had been due to die displayed IGF-I domain. In an attempt to prove that the aforementioned reduction in titre was due to specific binding of the displayed IGF-I to the type-I IGFR, the inventors and tiieir colleagues determmed the efficiency of 3-galactosidase gene transfer to IGFR/3T3 cells by IA- incoφorating viral particles in the presence of increasing concentrations of insulin. Insulin has about a 100-fold reduced affinity for the Type-I IGF-receptor relative to IGF-I (Steele-Perkins et al., 1988 J. Biol. Chem. 263, 11486-11492). The Kd of insulin for the type-I IGF receptor has been reported to be around 300 nm (Hodgson et al., 1995 cited above), which corresponds to a 50% displacement of IGF-I at approximately 1.5 μg/ml insulin. A maximal increase in titre of about 100-fold was obtained in d e presence of 100 μg/ml insulin. A modest increase in titre of about tiiree-fold was observed in the presence of as little as 1 μg/ml insulin. This demonstrates that the proposed assay can be used to detect ligands with at least a 100-fold reduced affinity for die type-I IGF receptor. The infection assays in which insulin was present were performed essentially as described above except that insulin was added at d e indicated concentrations 10 minutes prior to addition of the viral supernatant to cells. Table 4 A. On mouse NIH 3T3 fibroblasts env -Xa +Xa A - IO 6 - IO 6 IA - 10 6 - IO 6 IXA - IO 6 - IO 6 B. On IGFR-positive cell-lines MCF-7 IGFR 3T3 env -Xa +Xa -Xa +Xa A - IO 6 - IO 6 - 10 6 - IO 6 IA 6.4X10 2 4.8xl0 3 5JX10 2 1.6x10 s IXA 9.2X10 2 - 10 6 5.6X10 2 - IO 6 Table 5 inhibition of IGFR 3T3 infection by insulin Insulin concentration env 0 lμg/ml lOμg/ml lOOμg/ml A - 10 6 n.d. n.d. - IO 6 IA 1.5X10 3 4.4X10 3 l.όxlO 4 - 10 s Assay for altered receptor density The assay format described in this invention can be employed for the detection of substances which influence receptor density. Receptor density is modulated by factors which influence die level of receptor gene expression and receptor biosynthesis or by factors which enhance die rate of receptor endocytosis and degradation. For example, growth factor treatment of cells causes rapid capping and endocytosis of receptor-ligand complexes resulting in a substantial decrease in the number of surface receptors for the growth factor in question (A Sorkin and C M Walters, 1993, Bioessays, 15, p375-382). In die majority of cell types studied, EGF receptor downregulation occurs rapidly after exposure to EGF, leading to internalisation of greater than 80% of surface receptors within 2 hours (Carpenter and Cohen, 1976, J Cell Biol, 71 , pl59; Beguinot et al. , 1984, PNAS, 81, p2384; Dunn and Hubbard, 1984, J Cell Biol, 98, p2148; Gilligan et al., 1992, Exp Cell Res, 200, p235). jS-galactosidase-transducing retroviral vectors displaying EA1 chimaeric envelopes (comprising an EGFR-binding polypeptide fused to die N-terminus of the 4070A MLV envelope glycoprotein) are known to be capable of binding either to the 4070A envelope glyoprotein) are known to be capable of binding either to the 4070A envelope glycoprotein receptor, RAM-1, leading to transfer and expression of the 3-galactosidase gene, or to EGFR leading to inhibition of /3-galactosidase gene transfer and expression (Nilson BHK et al. , 1996, Gene Therapy, 3, p280-286). It was shown in the same study that the infectivity of these vectors is strongly inhibited on human EGFR-positive cells (El, HT1080 and A431) and that infectivity, at least on EJ cells, can be restored by adding lμM EGF to the culture medium at the time of infection (i.e. during exposure to the vector particles). Example 3. Assay for a substance causing a reduction in the cell surface density of EGF receptors. EGFR-expressing human cell lines [EJ bladder carcinoma cells and HT1080 fibrosarcoma cells (Nilson BHK etal., 1996, Gene Therapy, 3, p280), and PLC/PRF/5 hepatoma cells (Gilligan et a , 1992, Exp Cell Res, 200, p235)] are seeded at 2 x 10 s cells/well in six- well plates and incubated at 37 °C overnight in regular medium. For varying periods of time (ten minutes up to four hours) before they are due to be infected, the target cells are exposed to varying concentrations (InM up to lμM) of EGF or to other test substances. Control target cells are maintained in regular medium. Immediately prior to infection, the target cells are washed three times with DMEM. Infections are performed using 0.45μM-filtered, serum-free producer cell supernatants containing /3-galactosidase- transducing retroviruses incoφorating EA1 chimaeric envelopes (Nilson BHK et al., 1996, Gene Therapy, 3, 280-286). Target cells are incubated for 2 hours with 1 ml of EA1 viral supernatant diluted in 2.5 ml serum-free medium containing 8 μg/ml polybrene. The retroviral supernatant is then removed and the cells are incubated with regular medium for 48-72 hours. X-Gal staining for detection of /3-galactosidase activity is then performed and viral titre (enzyme forming units/ml) is calculated by counting blue stained colonies microscopically with the use of a grid placed underneath the 6 well plates. Viral titres are low on the control target cells. Higher viral titres are obtained on target cells that have been exposed to EGF prior to infection because their surface density of EGFR is reduced. The enhancement of titre is greater at higher concentrations of EGF and widi longer durations of exposure. Higher viral titres are also obtained on target cells that have been exposed to test substances which downmodulate EGF receptors. SEQUENCE LISTING (1) GENERAL INFORMATION: (1) APPLICANT: (A) NAME: Medical Research Council (B) STREET: 20 Park Crescent (C) CITY: London (E) COUNTRY: United Kingdom (F) POSTAL CODE (ZIP): WIN 4AL (G) TELEPHONE: (0171) 6365422 (H) TELEFAX: (0171) 3231331 (ii) TITLE OF INVENTION: Improvements in or Relating to Binding Assays (iii) NUMBER OF SEQUENCES: 2 (iv) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentin Release #1.0. Version #1.30 (EPO) (2) INFORMATION FOR SEQ ID NO: 1: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 60 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 1: ATGCTCAGAG GGGTCAGTAC GGCCCAGCCG GCCATGGCCA ATAGTGACTC TGAATGTCCC 60 (2) INFORMATION FOR SEQ ID NO: 2: (i) SEQUENCE CHARACTERISTICS: (A) 1ENGTH: 50 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 2: ACCTGAAGTG GTGGGAACTG CGCGCGGCCG CATGTGGGGG TCCAGACTCC 50

Claims: CLAIMS 1. A method of testing a substance for the ability to affect the formation or oligomerisation of a complex comprising members of a specific binding pair, a first member of the binding pair being present on the surface of a lipid enveloped particle comprising a transferrable label, said first member of the binding pair being capable of binding to a second member of the specific binding pair present on die surface of a cell, the lipid envelope of tiie particle being capable of fusing with the membrane of the cell so as to transfer the label to the cell, said transfer being inhibited by formation or oligomerisation of a complex between the first and second members of the binding pair, wherein the method comprises reacting the particle and the cell, in die presence of the substance under test, under conditions which would allow for binding of the first and second members of the specific binding pair in tiie absence of die substance under test, and detecting die label transferred, if any. 2. A method according to claim 1, wherein the lipid enveloped particle is a synthetic lipid vesicle, liposome or the like. 3. A method according to claim 1, wherein the lipid enveloped particle is a lipid enveloped virus. 4. A method according to claim 3, wherein the particle is a retrovirus. 5. A method according to any one of the preceding claims, wherein the particle comprises a polypeptide capable of enhancing fusion between the particle and the cell. 6. A method according to claim 5, wherein the polypeptide is glycosylated. 7. A method according to claim 5 or 6, wherein the polypeptide is of viral origin. 8. A method according to any one of claims 5, 6 or 7, wherein the polypeptide comprises an effective portion of a retroviral envelope glycoprotein or influenza virus haemagglutinin. 9. A method according to any one of claims 5-8, wherein the polypeptide comprises a substantially intact viral envelope glycoprotein. 10. A method according to any one of claims 5 to 9, wherein the cell expresses a cognate surface receptor for the polypeptide. 11. A memod according to any one of the preceding claims, wherein the cell is eukaryotic. 12. A method according to any one of the preceding claims, wherein the cell is mammalian 13. A method according to any one of die preceding claims, wherein the transferrable label is radioactive, fluorescent, a reporter gene, or an enzyme. 14. A method according to claim 13, wherein the reporter gene codes for the expression of j3-galactosidase. 15. A method according to any one of die preceding claims, wherein the first member of the specific binding pair is present on the surface of the particle as a fusion polypeptide. 16. A method according to claim 15, wherein die first member of the specific binding pair is expressed as a fusion with the polypeptide defined in claims 5 to 9. 17. A method according to claim 16, wherein d e first member of the specific binding pair is expressed as an N-terminal fusion with an effective portion of a C type retroviral envelope glycoprotein. 18. A method according to claim 16 or 17, wherein the first member of the specific binding pair is expressed as an N-terminal fusion with an effective portion of 4070A MLV envelope glycoprotein. 19. A method according to any one of the preceding claims for screening substances for the ability to affect the level of expression of the second member of the specific binding pair. 20. A method according to any one of die preceding claims for screening substances for the ability to inhibit oligomerisation of a complex between first and second members of a specific binding pair. 21. A composition for use in a binding assay, comprising: a lipid enveloped particle comprising a first member of a specific binding pair and a transferrable label; and a cell displaying a second member of the specific binding pair, the lipid envelope of tiie particle being capable of fusing with the membrane of the cell so as to transfer the label to the cell, said transfer being inhibited by formation or oligomerisation of a complex between the first and second members of die specific binding pair. 22. A composition according to claim 21, further comprising a substance under test for its ability to affect the binding of the first and second members of die specific binding pair.

Inventors: CHADWICK MARK PHILIP, MORLING FRANCES JOANNE, RUSSELL STEPHEN JAMES, CHADWICK, MARK, PHILIP, MORLING, FRANCES, JOANNE, RUSSELL, STEPHEN, JAMES
Applicants: CHADWICK MARK PHILIP, MEDICAL RES COUNCIL, MORLING FRANCES JOANNE, RUSSELL STEPHEN JAMES, CHADWICK, MARK, PHILIP, MEDICAL RESEARCH COUNCIL, MORLING, FRANCES, JOANNE, RUSSELL, STEPHEN, JAMES
Application year: 1996
Main classification: G01N33
IPC codes: G01N33