System for Isolating and Identifying Eukaryotic Cells Transfected With Genes and Vectors, Host Cells and Methods Thereof

This invention was made with Government support under Grant No. DK48845 with the National Institutes of Health (NIH). The Government may have certain rights in this invention.

The present invention relates generally to the fields of cell biology, molecular biology and immunology and, more specifically, to a novel system of identifying and isolating cells transfected with vectors encoding genes of interest. Use of this novel system allows rapid selection of transfected cells from total populations of cells in culture.

Introduction

Recent advances in molecular biology have allowed the production of recombinant immunoglobulin molecules (rAbs) from existing hybridomas, as described in Morrison, S. L., et al., Clin. Chem. 34:1668 (1988); Orlandi, R., et al., Proc. Natl. Acad. Sci. (1989); Larrick, J. W., et al., Biochem. Biophys. Res. Commun. 160:1250 (1989) and de novo from phage display libraries as described in McCafferty, J., et al., Nature 348:552 (1990); Clackson, T., et al., Nature 352:624 (1991); Marks, J. D., et al., J. Mol. Biol. 222:581 (1991); Hoogenboom, H. R., et al., Nucl. Acids Res. 19:4133 (1991); Winter, G. et al., Annu. Rev. Immunol. 12:433 (1994). Recombinant immunoglobulin molecules (rAbs), including single chain antibodies (sFvs) and Fabs, are able to bind their cognate antigens with high specificity and affinity, as described in Winter, G., et al., Annu. Rev. Immunol. 12:433 (1994). These modular binding regions can be fused with bioactive proteins or drugs and used to direct these molecules to their intended site of action, as described in Siegall, C. B., et al., J. Immunol. 152:2377 (1994). By using phage display technology, rAbs can now be isolated and produced in vitro against molecules, both natural and synthetic, that are either non-immunogenic or of such a high toxicity as to preclude their production in vivo, as described in McCafferty, J., et al., Nature 348:552 (1990); Clackson, T., et al., Nature 352:624 (1991); Hoogenboom, H. R., et al., Nucl. Acid Res. 19:4133 (1991); Marks et al., J. D., J. Mol. Biol. 222:581 (1991); Winter, G., et al., Annu. Rev. Immunol. (1994). The power and versatility of these proteins allows rAbs to be used in ways that conventional antibodies could not.

The present invention uses such recombinant antibody binding units, in conjunction with expression vectors coding for genes of interest, as "molecular hooks" to identify and separate transfected cells from a culture. The present invention allows for identification and selection of transfected cells as early as two hours after transfection, thus allowing study of the acute effects of the expression of the gene of interest.

The use of the invention's "molecular hooks" will assist in the identification and characterization of many cellular signaling factors heretofore not possible with current technology. Such identification and characterization has been possible only as a result of the development of technology enabling the introduction of expression plasmids into mammalian cells. The subsequent examination of the effect (on cellular growth and differentiation) of constitutively expressing an otherwise tightly regulated molecule has permitted the elucidation of many complex signaling pathways. With current technology, not all of the functional characteristics of signaling molecules are readily detectable using these systems. For example, it would be of great value to study the effect of dominant negative mutations of signaling molecules in both transformed and primary cells. Those negative or toxic mutations that result in inhibition of cell growth or cell death may be masked due to the low efficiency of transfection. In addition, it is not possible to increase the population of cells expressing a gene of interest by selecting for stable transformants as negative growth phenotypes are not amenable to this type of selection. This limitation of current technology in expression systems has, to a limited extent, been addressed by the use of inducible promoter systems, see, for example, those described in Levinson, A. D., "Gene Expression Technology," In D. V. Goeddel (Ed.), Methods in Enzymology, Academic Press, p. 497 (1991). However, this approach is not always optimal or applicable and has met with varied success depending on the cell type and origin of the promoter utilized. If cells expressing dominant-negative signaling molecules could be selected from culture soon after, within hours, of transfection, rather than days or weeks later, as is the case with current technology, assessment of the effects of the expression of a potentially negative effector would be possible. Similarly, early enrichment of transfected cells would allow studies of acute expression of transfected genes in homogeneously expressing cell cultures.

Selection of primary cell cultures that do not divide, such as neuronal cell cultures, have been limited to techniques that involve negative selection, such as antibiotic resistance conferred by the transfected vector. Selection of transfected cells by utilizing resistance to antibiotics takes days. In contrast, selection of primary cultures with the vectors of the instant invention allows selection as soon as 2 hours after the transfection event, depending on the primary cell culture.

The present invention is a novel alternative technology, encompassing a new expression system that will enable selection of transfected cells from culture to be carried out soon after, within 2 hours, of the transfection event, along with other advantages that will become apparent below.

The present invention satisfies these needs and provides related advantages as well.

The present invention relates to a eukaryotic expression vector for the identification and separation of transfected cells from a total cell population, comprising: a first DNA sequence encoding an anti-hapten recombinant antibody, said recombinant antibody capable of binding a specific hapten; a second DNA sequence encoding for a transmembrane domain functionally linked to said first DNA sequence; a third DNA sequence encoding for a signal sequence functionally linked to said first DNA sequence; a first promoter operatively linked to said first DNA sequence; a fourth DNA sequence encoding for at least one protein; a promoter operatively linked to said fourth DNA sequence.

The invention also relates to a mixture of eukaryotic expression vectors for the identification and separation of transfected cells from a total cell population comprising a first vector which in turn comprises: a first DNA sequence encoding an anti-hapten recombinant antibody, said recombinant antibody capable of binding a specific hapten; a second DNA sequence encoding for a transmembrane domain functionally linked to said first coding sequence; a third DNA sequence encoding for a signal sequence functionally linked to said first DNA sequence; and a promoter operatively linked to said first DNA sequence.

The invention also relates to a method of identifying and isolating transfected cells from the total cell population, comprising: transfecting a eukaryotic cell with a eukaryotic expression vector; exposing said cell to a hapten conjugated to a cell selection means; separating said cell, bound to said selection means, from the total cell population.

The invention also relates to a kit for the identification and separation of transfected cells from a total cell population, comprising a eukaryotic expression vector and a cell separation means.

The invention also relates to cells transfected with the expression vectors of the invention.

FIGS. 1A-1,1A-2 and 1B demonstrate features and the plasmid map of the eukaryotic expression vector pPhOx.TM, which encodes for an anti-hapten (anti-phOx) sFv.

FIG. 2 demonstrates the in vitro transcription and translation product of pPhOx.TM using an SDS polyacrylamide gel autoradiogram. As seen in lane 3, the transcription/translation reaction produced a protein of the expected molecular weight, which is approximately 30 kD (phOx sFv) plus 7.6 kD (the PDGFR transmembrane domain), totaling approximately 40 kD. Note lane 1 contains the positive control beta-galactosidase encoding DNA and lane 2 contained no exogenous DNA.

FIG. 3A demonstrates microscopic inspection of adenovirus-transformed human kidney cells, ATCC#CRL-1573 (designated "293") transfected with pPhOx.TM. 24 hours after transfection, the cells were incubated with phOx-BSA magnetic beads for 30 at 37.degree. C. with gentle agitation. Cell binding to antigen (phOx-BSA) coated magnetic beads at 24 hours post-transfection is observed in this micrograph.

FIG. 3B demonstrates transfected "293" (ATCC #CRL-1573) and HeLa cells (ATCC #CCL-2) transfected with pPhOx.TM by electroporation. "293" cells can be selected from culture as early as two hours post-transfection with pPhOx.TM, indicating that sFv is displayed on the cell surface at two hours post-transfection. HeLa cell display of pPhox sFv did not occur until eight hours post-electroporation (transfection).

FIG. 3C demonstrates that outer cell membrane expression of sFv can occur in differing cell types. Four cell lines derived from breast tumors and one cell line derived from a malignant melanoma were electroporated with pPhOx.TM and selected with pPhOx-BSA beads at 24 hours. The four breast tumor cell lines, as indicated in Table I, are: (1) MDA-MB-468 (ATCC #HTB-132), a human adenocarcinoma of the breast isolated from pleural effusion, which expresses EGFR; (2) MDA-MB-453 (ATCC #HTB-131), a human adenocarcinoma of the breast isolated from breast effusion, which expresses HER2/neu (3) MCF-7 (ATCC #HTB-22), a human adenocarcinoma of the breast isolated from pleural effusion, which expresses neither EGFR nor HER2/neu; and,(4) SKBR-3 (ATCC #HTB-30), a human adenocarcinoma of the breast isolated from malignant pleural effusion, which expresses both EGFR and HER2/neu. Selected cells were counted and are presented in comparison with the number of cells surviving the electroporation and with the size of the original population (2.times.10.sup.6 cells). Note that selection efficiency varied from cell line to cell line. Increased selection efficiency can be obtained by optimizing transfection conditions for each cell line.

FIGS. 4A and 4B demonstrate that virtually all of the cells that express the sFv fusion protein are efficiently selected from culture using the pPhOx-BSA coated magnetic bead cell selection means. SKBR-3 and MDA-MB-453 cells were transfected and selected with phOx/BSA coated magnetic beads at 24 hours post-transfection. Cellular proteins were then separated by size using an SDS-polyacrylamide gel electrophoresis. The separated proteins were transferred by immunoblot to a nitrocellulose membrane and reacted with radiolabeled antibodies able to bind sFv. Note in the "unselected" lane, meaning cells that did not bind to the magnetic beads, virtually no sFv is detected, indicating that all cells that were transfected were separable from the total cell culture using the cell separation means (the coated magnetic beads).

FIGS. 5A-5D demonstrate the efficiency of coexpression of pPhOx.TM and beta-galactosidase. SKBR-3 cells were co-transfected with pPhOx.TM and a vector expressing the gene for .beta.-galactosidase, named pCMV.beta., (Clontech, Palo Alto, Calif). One third of each transfection reaction was plated in each chamber of a four chamber microscope slide (Nunc, Napierville, Ill.). Details of the experiment are described in Example III (e) below. Panel A shows mock transfected cells; panel B shows cells transfected with pPhOx.TM alone; panel C shows cells transfected with pCMV.beta. (.beta.-galactosidase expressing; and panel D shows cells transfected with both pPhOx.TM and pCMV.beta..

The results demonstrate that most if not all of the cells expressing the functional pPhOx.TM product (cells with silver grains, denoted by arrows) are also expressing .beta.-galactosidase (blue staining, the point of the triangles opposite the stars points towards representative cells staining for .beta.-galactosidase). Greater than 98% of the cells selected with pPhOx-BSA-coated magnetic beads also stained positively for protein product of the experimental gene of interest, in this experiment, the .beta.-galactosidase gene.

FIGS. 6A-6N set forth the DNA sequence (SEQ ID NOS: 1 and 2) of pPhOx.TM.

FIGS. 7A-7N set forth the DNA sequence (SEQ ID NOS: 3) of pCR.TM.3lacZ.

In the following description, reference will be made to various methodologies known to those skilled in the art of molecular genetics, immunology and general biology. Publications and other materials, as cited herein, setting forth such known methodologies to which reference is made, are incorporated herein by reference in their entireties as though set forth in full.

General principles of antibody engineering are set forth in Antibody Engineering, 2nd edition, Ed. C. A. K. Borrebaeck, Oxford Univ. Press (1995). General principles of protein engineering are set forth in Protein Engineering, A Practical Approach, Ed. Rickwood, D., et al., IRL Press at Oxford Univ. Press, Oxford, Eng. (1995). General principles of antibodies and antibody binding to haptens are set forth in: Nisonoff, A., Molecular Immunology, 2nd edition, Sinauer Associates, Sunderland, Mass. (1984); and, Steward, M. W., Antibodies, Their Structure and Function, Chapman and Hall, New York, N.Y. (1984).

The present invention generally relates to a novel system of identifying and separating cells transfected with a gene of interest. Such a system allows the study of experimental genes of interest on cellular events soon after transfection, as described above in the Summary. In a preferred embodiment, cells transfected with the expression system of the invention can be selected and experimented on as soon as 2 hours post-transfection.

This new technology, the present invention, thereby aids in the identification and characterization of genes of experimental interest soon after transfection. Intracellular signaling proteins and dominant-negative signaling molecules are now accessible to study. Early events initiated by dominantly acting oncogenes, negatively acting tumor suppressors, as well as temporal events along differentiated pathways can now be studied.

For example, signaling pathways in cell lines derived from a certain tumor type can be studied with the present invention. The invention can be used to study the role of the HER-2/neu oncogene in breast carcinoma by expressing dominant negative mutations of signaling proteins in breast cancer cell lines. HER-2/neu (c-erbB-2) is overexpressed in 30% of breast tumors and its presence is correlated with lower survival rates of patients with these tumors (Elledge, R. M., et al., Seminars in Oncology 19:244 (1992). The HER-2/neu protein demonstrates close sequence homology with, but is distinct from, the epidermal growth factor receptor (EGFR) (Scheuter, A. L., et al., Science 229:976 (1985). The unregulated growth characteristics of HER-2/neu-positive tumors is hypothesized to arise, at least in part, from the effect of HER-2/neu on intracellular signaling pathways (Kumar, R., et al., Mol. Cell. Biol. 11:979 (1991)). The invention described herein can be used to isolate homogeneous populations of cells expressing dominant negative mutations of cellular signaling proteins known to interact with the EGF receptor such as PI3K, PLC.gamma.1, Grb2, Syp, Nck, Shc, and p91 in several cell lines derived from breast tumors (see Table I).

For another example, efficient study of regulatory proteins, such as early events in the Ras-regulated serine/threonine kinase pathways, requires a transfection system that allows rapid selection of transfected cells. The present invention will allow an analysis of when this pathway diverges into the Ras-MEK-MAPK axis and the Ras-MEKK-SEK-SAPK (JNK) axis (Sanchez, I., et al., Nature 372:794 (1994); Yan, M., et al., Nature 372:798 (1994); Derijard, B., et al., Science 267:682 (1995)).

This expression system of the invention, by giving researchers the ability to select cells expressing genes of interest from culture as soon as 2 hours after transfection, allows the study of the acute effects of expression of a wide variety of experimental systems otherwise not accessible to study. For example, dominant negative or constitutively active mutations of proteins involved in signal transduction can be studied using the present invention. Analyses of early transcription events are now accessible to study. Experimentation on the acute effects of transfection on primary cell cultures, including cells that normally do not divide, such as neurons, is now possible.

The present invention relates to a novel system for rapidly isolating and identifying eukaryotic cells after transfection. The invention employs a vector encoding for a "molecular hook," including an rAb or a receptor-like molecule, that is expressed on the cell's surface. Such expression may occur as early as 2 hours after transfection. The rAb binds to a specific "hapten," which, as defined below, can be any unique, selective epitope. Structurally, the rAb can be in the form of double or single chain antibody (sFv), an Fab fragment, or any functional binding unit.

The invention's use of the rAb binding domain on the transfected cell and the hapten on the cell selection means has advantages over the converse option (the hapten expressed on the transfected cell). First, it is advantageous to have a high density of hapten or epitope on the cell selection means, such as a bead. Second, it is advantageous to have the entity that has a higher level specific binding, i.e. less cross-reactivity with irrelevant molecules, on the cell selection means. The rAb or receptor-like molecule has a greater possibility of cross-reactivity than the hapten or epitope molecule. The cell selection means, with a high hapten density and binding specificity, will yield a relatively pure population of cells transfected with and expressing the requisite rAb or receptor-like molecule.

In another embodiment of the invention, in place of the rAb, the "selective hook" expressed on the cell's surface is a receptor-like or adhesion molecule capable of selectively binding to a specific hapten, epitope or ligand. One skilled in the art would have the means to select receptor-like or adhesion molecule binding domains for purposes of incorporation into the eukaryotic expression vector of the invention. As used herein, the term "receptor-like" molecule means any protein capable of specifically binding a hapten, epitope, or ligand. Examples of protein binding sites, to be expressed on the cell's surface, that can be used to selectively bind epitopes or haptens, include adhesion molecules such as cadherins, selecting, fasciclins, integrins, leukocyte adhesion receptor, neuroglian, VLA family molecules and the like. Examples of protein binding sites that can be used to selectively bind include growth factor receptor binding sites, including growth hormone receptor, insulin receptor, interleukin receptors and the like. Examples of specific protein binding interactions useful in the instant invention are described in Creighton, T. E., in Proteins, Structure and Molecular Principles, W. H. Freeman and Company, New York, N.Y. (1984); and, adhesion molecules are described in Pigott, R., et al., in The Adhesion Molecule, Academic Press, Harcourt Brace & Co., New York, N.Y. (1993). These references, as all references cited herein, are incorporated by reference in their entirety.

The rAb and receptor-like or adhesion molecule are also engineered to include coding sequences for a transmembrane domain or any membrane anchoring sequence and a secretion signal (leader sequence), thus allowing its expression on the transfected cell's outer membrane surface (i.e., extracellular expression). All coding sequences include 3' eukaryotic polyadenylation (poly-A) sequences, for the necessary 3' poly-adenylic acid RNA sequence needed.

Once expressed on the cell's outer membrane surface, the rAb or receptor-like domain is capable of binding to a specific hapten or epitope. This hapten or epitope is bound either directly or indirectly to a cell separation means, such as magnetic beads or sheets, tubes, porous matrices, or any natural or synthetic material including metals, polymers, latex beads, agarose, Sepharose, or any solid surface. The hapten or epitope can also include or be conjugated to a fluorescent or other labeled, selectable hapten or epitope. An example is PhOx-BSA-FITC. This allows for identification and selection of the transfected cell shortly after transfection, which can be as soon as approximately 2 hours after transfection, depending on the experimental system.

The transfected cells can be separated from unbound, untransfected cells by any physical means, such as filtration, isolation, by magnetic field, centrifugation, washing and the like. This rapid enrichment of transfected cells allows studies of the acute expression of the transfected experimental genes of interest.

The eukaryotic expression vector of the invention can use any vector or mixture of vectors capable of transfection and expression of DNA in eukaryotic cells. Such vectors are well known in the art and include, but are not limited to plasmids, viruses (such as adenoviruses, bovine papillomavirus, Epstein Barr virus, papovavirus, and retroviruses) or linear, double-stranded DNA. For example, retrovirus vectors are described in Somia, N. V., et al., Proc. Natl. Acad. Sci. 92:7570 (1995). Additional vectors are described in Catalogue of Recombinant DNA Materials, 2nd Edition, ATCC, Parklawn, Md. (1991); and viral vectors are described in Levinson, A. D., "Expression of Heterologous Genes in Mammalian Cells", In Methods in Enzymology 185:485 (1990). One skilled in the art would know how to choose a vector of choice for a particular eukaryotic cell line or experimental system. Vectors are available to one skilled in the art that, upon transfection, are transient and episomal, stable and episomal, or stable and integrated. The vector containing the experimental gene(s) of interest can be encoded within the same vector as the rAb or can be on another or mixture of other vectors. If a mixture of vectors are used, they are co-transfected.

The rAb is designed to bind to a specific hapten or epitope. As used herein, the term "hapten" or "epitope" means any organic or inorganic molecule capable of being bound by any rAb or recombinant receptor-like molecule, and includes molecule that can serve as a ligand for receptor-like or adhesion molecules. As noted above, by using phage display technology, rAbs can now be isolated and produced in vitro against "hapten" molecules, both natural and synthetic, that are either non-immunogenic or of such a high toxicity as to preclude their production in vivo. If small rigid haptens are used, antibody/hapten affinities as high as 10.sup.12 M-1 can be generated, as described in Searle, S. J., et al., Antibody Structure and Function, In Antibody Engineering, 2nd Ed, Ed. C. A. K. Borrebaeck, Oxford Univ. Press (1995). Thus, for the purpose of this invention, a hapten is defined as not only any molecule that is immunogenic either alone or conjugated to a carrier but any molecule capable of binding to an rAb as described above. Such hapten molecules include aniline derivatives such as: diazonium salts; benzene and derivatives such as dinitro-benzenesulfonate or dinitrobenzene or p-amino-benzenearsonate; phenol and derivatives as dinitrophenol (DNP), DNP-lysine; benzoates and benzoate derivatives such as phenylazobenzoate; acetates and derivatives such as phenylacetate; and the like. Analysis of haptens and Ab-hapten interactions are described in Nisonoff, A., Molecular Immunology, 2nd edition, Sinauer Associates, Sunderland, Mass. (1984); and, Steward, M. W., Antibodies, Their Structure and Function, Chapman and Hall, New York, N.Y. (1984).

As used herein, the term "antibody binding unit" means any functional protein unit which can bind a hapten. Therefore, structurally, the recombinant rAb protein can be designed to take the final form of a double or single chain antibody (designated "sFv"), Fab, Fab' or F(ab').sub.2 fragments, or any functional antigen-antibody binding unit. rAbs, including single chain antibodies (sFvs) and Fabs, are able to bind their cognate antigens with high specificity and affinity, as described in Winter, G., et al., Annu. Rev. Immunol. 12:433 (1994). By using phage display technology, rAbs can now be isolated and produced in vitro against molecules, both natural and synthetic, that are either non-immunogenic or of such a high toxicity as to preclude their production in vivo, as described in: Clackson, T., et al., Nature 352:624 (1991); Figini, M., et al., J. Mol. Biol. 239:68 (1994); Hawkins, R. E., et al., J. Mol Biol. 226:889 (1992); Hoogenboom, H. R., et al., Immunol. Rev. 130:41 (1992); Hoogenboom, H. R., et al., Nucl. Acid Res. 19:4133 (1991); Jespers, L. S., et al., Biotechnology 12:899 (1994); Marks et al., J. D., J. Mol. Biol. 222:581 (1991); McCafferty, J., et al., Nature 348:552 (1990); Winter, G., et al., Annu. Rev. Immunol. 12:433 (1994). The synthesis of single-stranded sFv antibody fragement gene repetoires is also described by Marks, J. D., "Human Monoclonal Antibodies from V-Gene Repertoires Expressed on Bacteriophage," In Antibody Engineering, 2nd Ed, Ed. C. A. K. Borrebaeck, Oxford Univ. Press (1995). Hilyard, K. L. discusses "Protein Engineering of Antibody Combining Sites" In Protein Engineering, edited by Rees, A. R. et al., IRL Press at Oxford Univ. Press, New York, N.Y. (1992). As noted above, all references cited herein are incorporated by reference in their entirety.

In the rAb-containing vectors of the invention, the coding sequence for the rAb is operably linked to a strong constitutive promoter capable of expression immediately upon transfection or soon thereafter. As disclosed herein, this enables selection of cells expressing genes of interest, through the extracellular expression of the rAb, within hours after transfection. Such constitutive promoters are well known in the art and include, but are not limited to viral, bacterial or eukaryotic promoters. One skilled in the art would know how to choose a vector of choice for a particular experimental system. Examples of strong constitutive promoters include cytomegalovirus (CMV) immediate early promoter, Rous sarcoma virus (RSV) promoter, adenovirus major late promoter, the lac-inducible promoter, SV40 early promoter and retroviral long terminal repeats (LTRs).

Alternatively, the rAb can be operatively linked to an inducible promoter, such as interferon beta promoter, heat-shock promoter, glucocorticoid promoter and the like, as generally described in Lewin, B., Genes V, Oxford Univ. Press, New York, N.Y. (1994). In this situation, the rAb is expressed on the cell surface and the transfected cell can be identified and isolated from the total cell population as soon as two hours after induction of the promoter.

One skilled in the art would know how to choose additional genetic elements necessary for an experimental system, such as the need to include enhancers within an expression vector, as discussed by Kriegler, M., "Assembly of Enhancers, Promoters, and Splice Signals to Control Expression of Transferred Genes," In Methods in Enzymology 185:512 (1990).

One or more genes of interest to be expressed in the transfected cell of the instant invention can be contained within a second vector. The second vector can be co-transfected with the rAb encoding vector. Alternatively, it can be spliced within the rAb-encoding vector.

The experimental gene(s) can be operatively linked to the same or a similar type of strong constitutive promoter as the rAb. Alternatively, it can be operatively linked to a different promoter. This promoter can be an inducible promoter, such as interferon beta promoter, heat-shock promoter, glucocorticoid promoter and the like, as described in Lewin, B., Genes V, Oxford Univ. Press, New York, N.Y. (1994). If the gene of interest or the rAb is operatively linked to an inducible promoter, that rAb or gene can be expressed on the cell's surface as soon as two hours after induction. Alternatively, the experimental gene(s) of interest can be operatively linked to the same promoter as the rAb. This can be effected by inserting an Internal Ribosome Entry Site (IRES) between the coding region for the rAb and the second, downstream, gene (Glass, M. J., et al., Virology 193(2):842-852 (1993)).

In designing and synthesizing the promoters, they can be initially placed within the expression vector or genome or can be synthesized in conjunction with the rAb or gene of interest before splicing into their respective vector(s). A polylinker can be designed between the promoter and a poly A sequence for simplified insertion of rAb or gene of interest coding sequences in the expression vector or genome.

In one embodiment of the present invention, the vector of the expression vector is pCR3.1 (Invitrogen, San Diego, Calif). pCR3.1 is a eukaryotic expression vector which includes polylinker sites, cytomegalovirus (CMV) promoter, bovine growth hormone (bGH) poly A signal and the ampicillin and neomycin resistance genes for selection, as described in FIGS. 1A-1, 1A-2 and 1B.

The rAb sequence is linked to a signal, or leader, sequence that is functional in the transfected host cell. Such signal sequences, also called leader sequences, are well known in the art. A signal sequence is composed of 15-30 amino acids that are relatively hydrophobic, thus allowing insertion into microsomal membrane. One skilled in the art would know how to choose an appropriate signal (leader) sequence for a particular eukaryotic cell line or experimental system. For example, the leader sequence can be either homologous or heterologous to the transfected host. The desired rAb coding sequence can be linked to any signal (leader) sequence which will allow insertion of the rAb protein in the membrane of the selected host and its expression as a functional, hapten-binding extracellular protein. In one embodiment of the invention, the rAb sFv coding sequence was combined with the murine kappa chain V-J2-C region signal peptide. This signal peptide is described in Coloma, M. J., et al., J. Immunol. Methods 152:89 (1992) and Kabat, E. A., et al., Sequences of Proteins of Immunological Interest, 4th ed. U.S. Dept. of Health and Human Services. Washington, D.C. (1987).

The rAb and receptor-like coding sequences are also linked to a transmembrane domain, or any membrane anchoring sequence. One skilled in the art would know how to choose an appropriate transmembrane domain sequence for a particular eukaryotic cell line or experimental system. The desired rAb coding sequence can be linked to any transmembrane domain which will allow insertion of the rAb protein in the membrane of the selected host and its expression as a functional, hapten-binding extracellular protein. In one embodiment of the present invention, the rAb coding sequence is combined with the transmembrane domain of the human platelet derived growth factor receptor (PDGFR). The PDGFR transmembrane domain is described in Gronwald, G. M., et al., Proc. Natl. Acad. Sci. U.S.A. 85:3435 (1988).

In one embodiment of the present invention, the expression vector employs a single chain antibody (sFv) directed against a hapten, 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one (phOx), to isolate transiently transfected cells from total populations in culture. The fusion protein, phOx sFv, as described in Hoogenboom, H. R., et al., Nucl. Acids Res. 19:4133 (1991), also contained two epitope tag peptides (for protein identification by anti-tag antibodies), and the transmembrane domain of the human PDGFR. When expressed in transfected cells, this fusion protein is anchored to the membrane via the transmembrane domain of the PDGFR. The functional antibody binding unit, phOx sFv, is therefore exposed to the extracellular environment. Cells were transiently transfected with an expression vector encoding phOx sFv, designated pPhox.TM. The cells were then selected from culture using antigen (phox)-coated magnetic beads (the method for cell separation by magnetic bead is described in detail, see Example III(b) below). Furthermore, when cells were co-transfected with pPhOx.TM and a plasmid containing the gene for .beta.-galactosidase (pCMV.beta., Clontech), greater than 98% of the cells selected from culture using the instant method were found to express .beta.-galactosidase activity.

In this embodiment, use of a single-chained rAb, versus a dimeric rAb, is advantageous because the smaller size of the single chain coding sequence allows other inserted coding sequences to be longer without losing cloning efficiency. Cloning efficiency is inversely .alpha. to vector size. For example, if the gene of interest is cloned into the same vector as the rAb, then use of the smaller single-chained rAb allows for the inclusion (insertion) of a longer genes or multiple genes, of interest without increasing the overall size of the vector.

The cell selection means of the instant invention comprises any molecule or device that can be coupled to the hapten of choice and can be used to physically separate transfected cells from culture. For example, the hapten may be coupled directly or indirectly to any insoluble separation agent, including but not limited to magnetic beads, gelatin, glass, SEPHAROSE macrobeads or dextran microcarriers such as CYTODES.RTM. (Pharmacia, Uppsala, Sweden). The hapten may be coupled, either directly or indirectly, to plates, tubes, bottles, flasks, magnetic beads or sheets, tubes, porous matrices, or any natural or synthetic material including metals, polymers, latex beads, agarose, SEPHAROSE, or any solid surface and the like. Any molecule or reagent may be used to link to hapten of choice to the cell separation means, including lectins, avidin/biotin, inorganic or organic linking molecules and the like. The cell separation means may utilize antibodies specific for any chemical or biological reagent and any form of detection system known in the art. For example, methods of manufacturing antibodies and utilizing antibodies in detection and separation systems are described in Antibodies, A Laboratory Manual, edited by E. Harlow et al., Cold Spring Harbor Labs, Cold Spring Harbor, N.Y. (1989), which incorporated by reference in its entirety. The transfected cells can be separated from unbound, untransfected cells by any physical means, such as filtration, isolation, by magnetic field, centrifugation, washing and the like.

The transfection of any expression system can be effected by any means, physical or biological. Physical means include direct injection, or, DEAE-dextran mediated transfection, electroporation, calcium phosphate mediated or lipid-mediated transfection and the like.

The invention also relates to cells transfected with the expression vector and methods for selection and isolation of cells transfected with the expression system.

The following examples are intended to illustrate, but not limit, the present invention.

This example describes methods for the generation of a vector capable of expressing a single chain antibody directed against a hapten.

a. Construction of pPhOx.TM

The parent vector for pPhOx.TM is pCR3.1 (Invitrogen, San Diego, Calif.), a eukaryotic expression vector containing the cytomegalovirus (CMV) promoter, bovine growth hormone (bGH), poly A signal and the ampicillin and neomycin resistance genes for selection, as described in FIGS. 1A-1 and 1A-2.

A DNA fragment encompassing the nucleotides encoding amino acids 514-562 of the human platelet-derived growth factor receptor (PDGFR) was amplified using nucleotide primers. PDGFR is described in Gronwald et al., Proc. Natl. Acad. Sci. U.S.A. 85:3435 (1988). These primers incorporate restriction sites and the Myc.1 epitope tag EQKLISEEDLN (SEQ ID NO.: 4), recognized by the monoclonal antibody 9E10.2, as described in Evan, G. I., et al., Mol. Cell Biol. 5:3610 (1985). This fragment was cloned into the T/A cloning vector PCRII (Invitrogen, San Diego, Calif.) and sequenced entirely on both strands to verify integrity. The PDGFR transmembrane fragment was constructed to contain a unique Sal I restriction site at the 5' end that is in the same reading frame as a Sal I site introduced at the 3' end of the phOx sFv sequence. This fragment was also constructed to contain a Not I site at its 3' end immediately following a stop codon which follows amino acid 562 of the human PDGFR sequence. The PDGFR DNA fragment was excised from the pCRII vector by digestion with Sal I and Not I, purified by standard procedures, and ligated into Sal I/Not I digested pCR3.1 vector thereby creating the vector pCR3.1.1.

The sequence encoding the murine Ig kappa-chain V-J2-C-region signal peptide (METDTLLLWVLLLWVPGSTGD) (SEQ ID NO: 5) containing an EcorV site at its 5' end, an influenza hemagglutinin (HA) epitope tag (YPYDVPDYA) (SEQ ID NO: 6), and Sfi I and Sal I sites at its 3' end was then subcloned from another sFv-containing vector (pCR3.2) as an EcoRV to Sal I fragment (sFv is a single-stranded antibody specific for 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one, also designated phOx). This fragment was then ligated with EcoRV/Sal I digested pCR3.1.1 creating the vector pCR3.1.2.

The anti-phOx sFv was amplified from the phage display vector pHEN-I (phOx) (Hoogenboom et al., 1991) using primers that encompassed the Sfi I site on the 5' end of the sFv and incorporated a Sal I site on the 3'end of the 3' Myc.1 tag already present in pHEN-I. The PCR product was cloned into pCRII and its sequence integrity determined by dideoxy sequencing. The resulting clone was then digested with Sfi I and Sal I, purified by standard procedures, and ligated with Sfi I/Sal I digested pCR3.1.2 creating pPhOx.TM, as illustrated in FIGS. 1A-1,1A-2 and 1B. As a result of the cloning strategy, the Myc.1 epitope tag was fused to the carboxyl-terminal end of the anti-phOx sFv as a tandem repeat. The HA epitope tag (recognized by the monoclonal antibody 12CA5, Boehringer Mannheim, Indianapolis, Ind.) was fused to the amino terminus immediately after the leader peptide cleavage site such that it is the first sequence in the mature protein. The two epitope tag peptides, one 3' and one 5' to the sFv, were included as controls for complete expression and membrane display of the fusion protein. Expression of the sFv/PDGFR fusion protein from this plasmid is driven by the cytomegalovirus (CMV) promoter, the sequence of which is included in FIGS. 6A-6N.

b. In Vitro Transcription/translation of pPhOx.TM

As an assay for the integrity of the sFv:PDGFR sequence, the fusion protein was expressed from pPhOx.TM in vitro using a rabbit reticulocyte lysate system (Novagen, Inc., Madison, Wisc.), as illustrated in FIG. 2. Production of an RNA transcript in this system relied on the T7 promoter that is found between the CMV promoter and the sFv sequence in pPhOx.TM. The protein translated from the resulting message is approximately 40 kD. The expected molecular weight of the phOx sFv:PDGFRTM fusion protein is approximately 37.6 kD (30 kD (phox sFv)+7.6 kD (PDGFR TM domain, amino acids 514-562)).

This example describes methods for the synthesis of a hapten capturing agent through its coupling to a cell separation means.

a. Coupling of the Hapten phOx to BSA

4-ethoxymethylene-2-phenyl-2-oxazolin-5-one (phox) (Sigma, St. Louis, Mo.) was coupled to bovine serum albumin (BSA) as described previously by Makela et al., J. Exp. Med. 148:1644 (1978). By analysis of the UV absorbance spectra of the product and comparison with the molar extinction coefficient (.epsilon.) of PhOx (where concentration=absorbance at 352 nm/.epsilon.), it was determined that under these conditions a coupling efficiency of 20 moles of phOx per mole of BSA was achieved.

b. Coupling of phOx-BSA a Cell Separation Means, Tosyl-activated Magnetic Beads

The phOx-BSA conjugate described above was coupled to tosyl-activated magnetic beads (DYNABEADS M-450, Dynal, Inc.) using the manufacturer's recommended protocol. Beads were suspended in 50 mM NaHCO.sub.3, pH 9.5 to a concentration of 2.times.10.sup.8 beads/ml. PhOx-BSA was added to a final concentration of 150 .mu.g/ml and the bead/protein mixture was incubated at 4.degree. C. for 24 hours with gentle rotation. The beads were washed extensively and stored at 4.degree. C. in PBS/0.1% BSA/0.01% NaN.sub.3 at a concentration of 2.times.10.sup.8 beads/ml.

2) Alternatively, magnetic beads activated by carboxy groups can be attached to the BSA-phOx conjugate. Thus, 2 ml of 0.01 M sodium acetate buffer (pH 5.0); the phOx-BSA conjugate from above (2 mg), 2 ml of 0.45 micron carboxylpolystyrene-plated magneted beads and 1-ethyl-3-(dimethylaminoprophy) carbodiimide (EDAC, Sigma, St. Louis, Mo.) were combined in a 15 ml glass centrifuge tube. The suspension was vortexed and incubated for two hours at ambient temperature on a rotary mixer. The suspension was subjected to a strong magnetic field and the supernatent was decanted. The beads were resuspended in 4 ml of the sodium acetate buffer and repelleted with the magnetic field twice to wash away contaminants.

This example describes methods for transfection of cells and selection with hapten capturing agent through its coupling to a cell separation means.

a. Eukaryotic Cell Transfection

Following confirmation of the integrity of the phOx sFv:PDGFRTM coding sequences, as described in Example II above, transient expression was carried out in cultured cells.

Cell lines tested include the "293" adenovirus-transformed human kidney cells, the human adenocarcinomas of the breast described in Table I, and HeLa cells, as described in above. Cell lines were grown to approximately 50-70% confluence in either RPMI-1640 or Dulbecco's Modified Eagle's Medium (DMEM, GIBCO, Grand Island, N.Y.) supplemented with 10% fetal calf serum (FCS, Gemini Bioproducts, Inc., Calabasas, Calif.) and the media changed 24 hours prior to electroporation. Cells were harvested by incubation with trypsin or 3 mM EDTA/PBS for 5 minutes at 37.degree. C. and collected by centrifugation (800-1000 g for 5 to 10 minutes at room temperature). The supernatant was decanted. The cell pellet was then resuspended to a concentration of 1.times.10.sup.7 cells per ml in complete medium per 60 mm plate. The cells were pipetted up and down to break up cell clumps and achieve single cell suspension.

The cells, as described above, were transfected by combining 5 .mu.g plasmid DNA with 0.2 ml cell suspension (2.times.10.sup.6 cells) and pulsing the mixture at 500 .mu.F and 250 V in an IBI Gene Zapper. The electroporated cells were added to 5 ml media and incubated at 37.degree. C. in a humidified CO.sub.2 incubator. Adherent cells were harvested by incubation with PBS/3 mM EDTA and combined with cells that remained suspended. Cells were collected by centrifugation and resuspended in 0.5 ml medium to which 1.5.times.10.sup.5 phOx-BSA coated magnetic beads would be added.

b. Cell Separation by Magnetic Bead

Transfected cells were collected by centrifugation and resuspended in 0.5 ml PBS/3 mM EDTA medium, to which 1.5.times.10.sup.5 phOx-BSA coated magnetic beads will be added.

The magnetic beads were washed before use to remove the sodium azide. One microcentrifuge tube for each 60 mm plate of cells was set up. The magnetic bead slurry was vortexed to resuspend beads. 10 ul (1.5.times.10.sup.6 beads) was added into each microcentrifuge tube. The beads were washed by adding 1 ml complete medium to each tube and mixed by inversion 3 times. The beads were pelleted with a strong magnet or magnetic stand and pipet or aspirate off medium.

The cell/bead mixture was rotated for 30 minutes at 37.degree. C. on a Dynal mixer. The bound cells were separated from the mixture by placing the tubes in a Dynal MPC-E magnetic particle concentrator. Unbound cells were drawn off and the bead pellet was washed twice by resuspension in 1 ml complete medium followed by gentle vortexing. Live unbound cells and bead-bound cells were counted by Trypan blue exclusion.

c. Evaluating sFv Produced from pPhOx.TM Displayed on the Cell Surface.

To determine whether the sFv produced from pPhOx.TM was successfully displayed on the cell surface, adenovirus-transformed human kidney cells "293" were transfected with either pPhOx.TM or psFv.MUT (which produces a truncated, inactive sFv) and returned to culture for 24 hours. The transiently transfected cell population was harvested and incubated with phOx-BSA magnetic beads for 30 minutes at 37.degree. C. in complete medium with gentle agitation. At the completion of the incubation, bead-bound cells were selected from culture by magnetic interaction. Upon microscopic inspection of the magnetic bead pellet, each selected cell was observed to have bound to it at least one and in many cases several beads. FIG. 3A shows cells at 24 hours post-transfection by electroporation, cells can be observed binding to phOx-BSA coated magnetic beads from culture. None of the cells that had been transfected with psFv.MUT were bound to beads or were selected from culture.

A time course of selection was performed in order to demonstrate the ability of the instant invention in selecting transfected cells very soon after introduction of exogenous DNA. In these experiments, "293" (adenovirus transformed human kidney) and HeLa cells were transfected with pPhOx.TM by electroporation. Aliquots of the transiently transfected cell population were incubated with phOx-BSA beads for 30 minutes at 1, 2, 4, and 8 hours post-transfection followed by selection and counting as described. These results, seen in FIG. 3B, show that transiently transfected 293 cells (approximately 2.5% of the surviving population) were selected from the total population as early as 2 hours post-electroporation.

When HeLa cells were transfected in parallel reactions, display of phox sFv sufficient for selection under these conditions occurred at 8 hours post-electroporation. From 2.times.10.sup.6 cells in the original population, 1.times.10.sup.4 transfected 293 cells were selected at 2 hours and 1.times.10.sup.4 HeLa cells were selected at 8 hours. This data is also displayed in FIG. 3B.

Cell membrane expression of sFV from pPhOx.TM expression can occur in different cell types. pPhOx.TM was introduced into several cell lines including four lines derived from carcinoma of the breast, as summarized in Table I, and adenovirus-transformed human kidney cells designated "293". Cells were selected at 24 hours post-electroporation on phOx-BSA beads and compared for selection efficiency. Under these transfection conditions, all cell lines tested displayed sFv on their membranes sufficient for selection from culture, as graphically displayed in FIG. 3C and Table II. Selection efficiency varied across the cell lines tested. Increased selection efficiency can be obtained by optimizing transfection conditions for specific cells using techniques known to one skilled in the art.

In parallel reactions, transfected cells were also incubated with magnetic beads coated with BSA alone as a negative control. In each case incubation with BSA beads yielded selection efficiencies of less than 0.03% of the live cells present.

d. Selection Efficiency of Transfected Cells Evaluated by Immunoblot Analysis

As an indication of cell selection efficiency, immunoblot experiments were conducted using samples of transiently transfected cells selected from culture or those that remained unbound to magnetic beads. The presence of sFv in these cell populations was determined using an anti-HA epitope tag antibody 12CA5 (Boehringer Mannheim). MDA-MB-453 and SK-BR-3 cells (see Table I) transfected with pPhOx.TM, described above, were selected from culture at 24 hours post-transfection. Equivalent numbers of untransfected, transfected and selected, or non-selected cells were run on an SDS-polyacrylamide gel (Laemmli, 1970). Separated proteins were transferred to a nitrocellulose membrane and blocked in PBS/0.05% TWEEN-20/5% milk protein (Carnation, Los Angeles, Calif.) for 1 hour at room temperature. Membranes were probed with anti-HA epitope tag antibody, the 12CA5 antibody, by incubating with 12CA5 (Boehringer Mannheim) diluted to 5 .mu.g/ml in blocking buffer for 1 hour at room temperature. The membranes were then washed with PBS/0.05% Tween-20 and incubated with horseradish peroxidase-conjugated goat anti-mouse antibody (BioRad) diluted 1:5000 in blocking buffer for 1 hour at room temperature. Membranes were washed as above, developed using ECL reagents (Amersham) and exposed to film.

As shown in FIGS. 4A and 4B, virtually all of the immunoreactive sFv appears in the cells that were selected from culture and only a trace of activity remained in the unselected cells. This result suggests that in the two cell lines tested, virtually all of the cells that express the sFv fusion protein are efficiently selected from culture.

e. Coexpression of phox.TM and .beta.-galactosidase in Cotransfected Cells

SK-BR-3 cells were co-transfected with pPhOx.TM and pCMV.beta. (Clontech) which carries the gene encoding .beta.-galactosidase. Cells were mock transfected or transfected with either 5 .mu.g pPhOx.TM, 5 .mu.g pCMV.beta., or 5 .mu.g of each. A non-promoter containing plasmid was used as carrier DNA to make a total of 10 .mu.g in each reaction. One third of each transfection reaction was plated in each chamber of a four chamber microscope slide (Nunc). Slides were incubated at 37.degree. C. for 24 hours then 1.times.10.sup.5 cpm of .sup.125 I-phOx-BSA was added to each chamber and allowed to bind for 30 minutes. Slide chambers were then gently washed three times with 1 ml PBS. Cells were then fixed with 1% paraformaldehyde/0.2% glutaraldehyde for 2 minutes and incubated with the colorimetric substrate (5 mM K.sub.4 Fe(CN).sub.6, 5 mM K.sub.3 Fe(CN).sub.6, 1 mM MgCl.sub.2, 0.08% chlorobromo-indolyl .beta.-D galactopyranoside, X-gal, Sigma) for .beta.-galactosidase activity for 15 hours at 27.degree. C. The slides were washed with PBS and the cells dehydrated by successive 5 minute washes in 50%, 75%, and 100% ethanol and air dried. They were then coated with photographic emulsion (NTB-3, Kodak) and dried overnight. Coated slides were exposed at 4.degree. C. for four days and developed using Kodak developing solutions. In addition, 1 ml of each transfection reaction was incubated with phOx-BSA beads as described in Example III(b) above. The selected cells were then stained for .beta.-galactosidase activity.

.sup.125 I-phOx-BSA was prepared by combining 100 .mu.g BSA protein and 500 .mu.Ci Na.sup.125 I (Dupont/NEN, Boston, Mass.) to iodogen-coated tubes using the manufacturer's protocol (Pierce). Free .sup.125 I was removed by applying reactions to an ECONO-PAC 10DG column (BioRad) that had been blocked with BSA and equilibrated in PBS. Labeled protein was eluted in PBS.

The results, depicted in the radiograph/photograph of FIGS. 5 A-D, demonstrate that most if not all of the cells expressing the functional pPhOx.TM product (cells with silver grains, denoted by arrows) are also expressing .beta.-galactosidase (blue staining, the point of the triangles opposite the stars points towards representative cells staining for .beta.-galactosidase). The data demonstrates that greater than 98% of the cells selected with phox-BSA-coated magnetic beads stained positively for .beta.-galactosidase activity.

General Procedure for Co-Transfection with PhOx.TM Vector and Second Plasmid Containing Gene of Interest

In order to insure that the plasmid DNA used in the instant procedure is of high quality and free of contaminants, the PhOx.TM vector and the vector containing the gene of interest was subjected to CsCl gradient ultracentrifugation. Boiled or alkaline lysis miniprep DNA should not be used in this procedure. Further purification methods can be found in Ausubel, F. M., Brent, R., Kingston, R. E., Moore, D. D., Seidman, J. G., Smith, J. A., Struhl, K., eds (1990) Current Protocols in Molecular Biology. Greene Publishing Associates and Wiley-Interscience, New York.

In addition, the PhOx.TM Vector can be amplified prior to use in the instant invention by transforming the plasmid into a recA, endA E. coli (e.g. DH5.alpha.) strain. The lyophilized vector is resuspended in 20 .mu.l of sterile water to make a stock solution. A small portion (1 .mu.l) of the stock solution can be used to transfect the E. coli of choice on LB plates containing 100 .mu.g/ml ampicillin or 50 .mu.g/ml kanamycin.

The pCR.TM.3lacZ (8.1 kb) plasmid used in this procedure as a positive control is constructed by inserting the lacZ gene in the EcoR1 cite of the pCR.TM.3 plasmid (Invitrogen, San Diego, Calif.). The positive control serves to assist in optimizing the transfection conditions for the PhOx.TM and co-tranfected vectors. The pCR.TM.3lacZ contains the E. coli gene encoding .beta.-galactosidase, which gene is expressed in mammalian cells using the immediate-early promoter from cytomegalovirus. A successful cotransfection with the PhOx.TM or the vector bearing the gene of interest will result in positive .beta.-galactosidase expression in selected cells and can be easily monitored with a calorimetric b-galactosidase assay, as described below.

Transfection procedures for the cell line of interest may often be found in articles discussing that particular cell line. Such methods of transfections are well known and may include calcium phosphate, DEAE-dextran, liposome-mediated, or electroporation. The protocol discussed in the art for the cell line of interest should be followed exactly. Particular attention should be paid to medium requirements, when to pass the cells, and at what dilution to split the cells. Further information can be found in Current Protocols in Molecular Biology. supra.

In the event that the art does not teach a transfection method for the cell line of interest, electroporation is the method of choice. For instance, the following electroporation protocol may be used (a "no DNA" negative control should also be used):

1. Prepare Trypsin/versene (EDTA) or PBS/3 mM EDTA. The latter can be prepared as follows:

137 mM NaCl

2.7 mM KCl

10 mM Na.sub.2 HPO.sub.4

1.8 mM KH.sub.2 PO.sub.4

(3 mM EDTA, optional)

a. Dissolve: 8 g NaCl

0.2 g KCl

1.44 g Na.sub.2 HPO.sub.4

0.24 g KH.sub.2 PO.sub.4

(6 ml 0.5 M EDTA, pH 8)

in 800 ml deionized water.

b. Adjust the pH to 7.4 with concentrated HCl.

c. Bring the volume to 1 liter and autoclave for 20 minutes on liquid cycle.

d. Store at +4.degree. C. or room temperature.

2. Change medium on the cells 24 hours prior to electroporation.

3. Harvest the cells at 60-80% confluency using half of the initial culture volume of PBS/3 mM EDTA.

4. Count the cells and resuspend them in complete medium at 1.times.10.sup.7 cells/ml.

5. Mix PhOx.TM and the construct containing the gene of interest (or pCR3lacZ) in a 1:1 molar ratio in a volume of 10 .mu.l or less. Use 1-5 .mu.g of each plasmid.

6. The plasmid mixture is added to 200 .mu.l of the cell suspension (2.times.10.sup.6 cells). The suspension is mixed gently and is transferred to a chilled electroporation cuvette (0.4 cm gap width).

7. The cells are electroporated using the recommended settings of the electroporation device.

8. The electroporated cells are transferred to a 60 mm plate containing 5-7 ml complete medium. The plates are incubated in a 37.degree. C., 5% CO.sub.2 incubator for 2-48 hours.

The transfected cells from the above Section C can be isolated using the following procedure. In general, the procedure employs 1.5.times.10.sup.6 beads per 60 mm plate of transfected cells. These conditions may vary due to the method of transfection and the cell line used. Sterile techniques should be used when performing the following steps.

1. Preparation of Transfected Cells

The PBS/3 mM EDTA buffer described above and complete medium should be prepared before attempting the following steps:

a. PBS/3 mM EDTA (3-5 ml) is added to the cells. The cells are incubated for 5 minutes at 37.degree. C. and then are harvested. Untransfected cells (or the cells from the negative transfection control) may be harvested for use as a negative control when assaying for b-galactosidase activity.

b. The cells are centrifuged at 800-1000 .times.g for 5-10 minutes at room temperature. The supernatant is decanted.

c. The cells are resuspended in 1 ml complete medium per 60 mm plate. The cells are pipetted up and down in order to break up cell clumps and achieve a single-cell suspension.

2. Preparation of Magnetic Beads

The magnetic beads are washed before use to remove any sodium azide present.

d. A microcentrifuge tube is prepared for each 60 mm plate of cells.

e. The magnetic beads slurry is vortexed to resuspend beads and is added (10 .mu.l (1.5.times.10.sup.6 beads)) into each microcentrifuge tube.

f. The beads are washed by adding 1 ml complete medium to each tube and are mixed by inversion 3 times. The beads are pelleted with a strong magnet or magnetic stand and the medium is removed by pipetting or aspiration.

3. Selection of Transfected Cells

g. Cell suspension (1 ml) from Step 1C is added to a tube containing washed beads from Step 2f. The suspension is incubated for 30 minutes.

h. The tubes containing the bead-cell mixture are placed in a magnetic stand and are mixed for 30 seconds to 1 minute with a gentle end over end rotation.

i. While the tube is still in contact with the magnet, the non-selected cells are removed with a pipet. (These cells may be saved for further analysis.)

j. The tubes are removed from the magnetic stand and the beads and cells are resuspended in 1 ml complete medium. The suspension is vortexed gently.

k. The beads (and bound cells) are pelleted using the magnetic stand, the supernatant is removed by pipet.

l. Repeat Steps j and k two more times.

m. Selected cells are resuspended in 100 .mu.l complete medium (for pCR.TM.3lacZ control, use X-gal Reagent, see below) and the cells are counted. The cells are ready to culture or analyze.

The first step in utilizing the method of this invention can be to optimize the transfection conditions for the cell line of interest. Once transfection conditions have been optimized, the cell line can then be cotransfected with the PhOx.TM vector and the vector containing the gene of interest.

The pCR.TM.3lacZ positive control plasmid can be used to check for cotransfection of selected cells and assessing transfection efficiencies. Transfected cells are selected using the above methods. Untransfected cells, selected cells, and non-selected cells are assayed with X-gal and counted. (Cells expressing b-galactosidase will turn blue in the presence of X-gal.) Comparison of the number of blue, non-selected cells versus blue, selected cells will allow the determinination of selection efficiency. (Untransfected cells should not stain with X-gal.) Optimal cotransfection conditions are defined as when the PhOx.TM to pCR.TM.3lacZ ratio gives the greatest enrichment of blue-stained cells in the selected population.

1. Preparation of X-gal Reagent

1 mg/ml X-Gal in DMF

4 mM potassium ferricyanide (K.sub.3 Fe(CN).sub.6)

4 mM potassium ferrocyanide (K.sub.4 Fe(CN).sub.6 -3H.sub.2 O)

2 mM magnesium chloride hexahydrate in PBS, pH 7.4

a. Each of the following stock solutions (10 ml) are prepared. These solutions are stable indefinitely if stored as indicated.

X-gal: (20 mg/ml in dimethylformamide (DMF)): Dissolve 200 mg of X-gal in 10 ml DMF and store at -20.degree. C.

Potassium Ferricyanide and Potassium Ferrocyanide: (0.4 M each in deionized water.): Dissolve 1.32 g of potassium ferricyanide and 1.69 g of potassium ferrocyanide in 10 ml deionized water.

Store at -20.degree. C.

Magnesium Chloride: (200 mM in deionized water.): Dissolve 0.4 g in 10 ml deionized water and store at room temperature or -20.degree. C.

b. For 10 ml of X-gal reagent, mix together:

0.5 ml of 20 mg/ml X-Gal stock solution;

0.1 ml of the potassium ferricyanide/ferrocyanide stock solution;

0.1 ml of the magnesium chloride stock solution; and

9.3 ml of PBS.

2. Colorimetric Assay for .beta.-galactosidase

a. To Assay Selected Cells:

i. The selected cells are resuspended in 100 .mu.l X-gal Reagent:

ii. The cells are incubated overnight at room temperature:

iii. The cells are examined under the microscope for the development of blue color and the number of stained and total cells is counted.

b. To Assay Non-selected Cells:

i. The non-selected cells are centrifuged 5 minutes at 4000 rpm to pellet the cells. The supernatant is decanted.

ii. The cells are resuspended in 1 ml PBS and again pelleted. The supernatant is decanted.

iii. The cells are resuspended in 100 .mu.l of X-gal Reagent and are incubated overnight at room temperature.

iv. The cells are examined under a microscope for the development of blue color. The number of total cells and blue cells are counted.

c. To Assay Untransfected Cells (Negative Control):

i. The untransfected cells are centrifuged for 5 minutes at 4000 rpm to pellet the cells.

ii. The cells are resuspended in 1 ml PBS and recentrifuged in order to pellet the cells.

iii. The cells are resuspended in 100 .mu.l of X-gal Reagent and are incubated overnight at room temperature.

iv. The cells are examined under a microscope for the development of blue color. The number of total cells and blue cells are counted.

In all of the above counting procedures the total cell number is normalized.

The presence of unbound beads after the application of the magnet to the transfection mixture indicates that a proper number of magnetic beads. If no unbound beads are observed, it may mean that not all tranfected cells were selected in the procedure. Should the procedure using those particular conditions be repeated, it is desirable to double the number of beads (e.g., 20 .mu.l or 3.times.10.sup.6 beads) in order to ensure that you isolate all transfected cells.

In the transfection optimization procedure, nearly all selected cells should express .beta.-galactosidase. If there are non-selected cells that are blue, then the relative amount of PhOx.TM to pCR.TM.3lacZ should be increased.

Although the invention has been described with reference to the examples provided above, it should be understood that various modifications can be made by those skilled in the art without departing from the invention. Accordingly, the invention is set out in the following claims.