CELLULAR TOXIC CONJUGATES

Abstract: Novel compositions are provided which comprise recombinantly-produced targeting components capable of binding to cell membrane constituents linked, such as by polypeptide linkers, to toxin components capable of killing cells by damaging their cellular membranes. The conjugates may find use, e.g., in novel methods for selectively killing predetermined cell populations in an individual or in in vitro cell cultures.

Description: CELLULAR TOXIC CONJUGATES FIELD OF THE INVENTION This invention relates generally to the use of site directed toxins for cell killing, either in vitro or in vivo, such as in cancer therapy or the treatment of autoimmune diseases and other human treatment applications, and more particularly, to improving the efficacy of these compositions. BACKGROUND OF THE INVENTION The selective removal of certain cell populations, such as cancer cells or cells responsible for an undesirable immune response (e.g., certain antigen-specific T and B cells) , represents a primary goal of modern medical research. For many years, the most common therapeutic approach to reach this goal entailed treating a patient systemically with agents that were more toxic to the undesirable cells than to "normal" cells. In chemotherapy, the use of antiproliferative agents, such as alkylating agents, antimetabolites, antibiotics, enzymes and others, has become the most prevalent treatment. While certainly somewhat successful, these cytotoxic agents are generally non-specific, causing numerous serious side effects in the patients undergoing the therapy. Some improvement can be achieved by varying dosages and modes of administration, or use in conjunction with other cytotoxic agents, but the problems associated with this type of treatment are legion. In attempts to overcome these difficulties, researchers began to focus on the concept of a "magic bullet." Most often, this magic bullet consisted of a combination of an immunoglobulin, particularly a monoclonal antibody, conjugated to a toxin, thus the term "immunotoxin." It is believed that the antibody or other targeting component can selectively deliver the toxic agent primarily to the predetermined cell populations, such as tumors, in the patient - resulting only in death of the unwanted cells. In theory, then, cancer cells or other diseased or unwanted cellular material can be selectively killed without the non-specific side-effects common with most treatment regimens. In spite of the enormous potential inherent in the use of these compositions, very few successful products have been developed, despite extensive research efforts. In practice, finding the appropriate combination of targeting agent and toxin that can actually improve efficacy of a presently accepted chemotherapeutic or other therapy has proven to be extremely difficult. Perhaps the most widely clinically tested i munotoxins utilize ribosomal inhibiting proteins as the toxic agent, such as ricin and diphtheria toxin. For killing, these toxins require entry into the cytoplasm of the cell, wherein the toxin catalytically inactivates ribosomal protein synthesis, ultimately causing cell death. Indeed, substantially all of the commonly used immunotoxins have this property, i.e., to be effective, these toxins generally must be incorporated into the cytosol. Importantly, the toxins typically also must be cleaved from the monoclonal antibody, or other recognition component of the immunotoxin, before becoming active. Unfortunately, the utility of these toxin conjugates has been generally less than ideal, in part because many cells do not easily transport the immunotoxin, or its toxin component, across the cell membrane and into the cell. Furthermore, the bonds joining the targeting component to the cytotoxic agents often cleave at inappropriate times, i.e., before binding to the desired cells. This results in significant random cell killing, with concomitant complications. Also, these conjugates are often subject to rapid clearance from the patient's blood stream, ultimately reducing the amount of toxin available to interact with the undesirable cell population. Moreover, many of these toxic conjugates are difficult to manufacture. Frequently the toxins are so toxic that they can only be handled by very experienced research personnel. Also, the particular linking systems used are frequently complex and uneconomical, resulting often in low production yields. Finally, quality control problems in the manufacture of such materials are often very high. Thus, there exists a need for site-directed toxin conjugates exhibiting superior properties, particularly i-n vivo. These conjugates should retain high specific cytotoxicity, yet minimize the host's nonspecific toxicity. Ideally, they should be relatively economical to manufacture, and in a reproducible manner. It would also be desirable to be able to modify the structure of the toxic conjugate in a simple and systematic manner, to improve or alter cytotoxic properties of the conjugate on a case by case basis. The present invention fulfills these needs. SUMMARY OF THE INVENTION The present invention provides improved compositions and methods for removing preselected cells from a cell population, such as the in vivo treatment of 5 patients utilizing novel targeting conjugate compositions capable of selectively killing eukaryotic or prokaryotic cells in a patient. The compositions are composed of two components: a polypeptide targeting component linked to a polypeptide toxic component, either joined by one or o more linker amino acids or produced recombinantly as a single polypeptide, wherein the targeting component is capable of binding to a cell membrane constituent marker and the toxic component is capable of killing a cell by damaging its membrane. The targeting component is 5 preferably specific only for the desired cell type, and, for example, can be an immunoglobulin, particularly an immunoglobulin modified to exhibit reduced immunogenicity, or a human growth factor substantially specific for a receptor on the cell population to be o killed can also be utilized. The toxic component can be any of a variety of enzymes or other agents, whether bacterial or mammalian proteins, capable of damaging the cell membrane sufficiently to induce cell death. When the two components are in peptide linkage, simple 5 production procedures and modifications of the conjugate compositions can be performed, such as through standard recombinant DNA techniques and cell culture production. Pharmaceutical compositions of the conjugates are also provided for use in a variety of therapeutic treatments. 0 Also provided are cloned recombinant DNA sequences coding for an exemplary type of toxin, Clostridium Phospholipase C polypeptides. The sequences, when operably linked to an expression control sequence and expressed in a host, produce Clostridium 5 Phospholipase C, or fragments thereof, substantially free from naturally-associated Clostridium gene products. Novel polypeptides having a portion of the primary structure of Clostridium Phospholipase C and/or one or more of the biological properties of the toxin can be readily produced by modifying the DNA sequences encoding the naturally occurring genes. The genes also may be used as probes for isolating related genes and identifying the presence of Clostridium in samples from human patients. BRIEF DESCRIPTION OF THE FIGURES Figure 1 depicts oligonucleotides used in genetic constructions. a) Oligonucleotide used to introduce restriction sites at end of PLC gene. b) Oligonucleotides used to synthesize DNA coding for Gly2Pro5Gly2 positioner. c) Oligonucleotides used to synthesize DNA coding for polyproline positioners. Arrows indicate boundaries between WPS 12 and CNC 25, and between CNC 26 and WPS 13. d) Oligonucleotides used to construct random positioners. Figure 2 depicts the sequence of the junction region between PLC and protein A. The arrows separate the 3' end of the PLC gene from the introduced Apa I site and then from the 5' end of the protein A gene. Figure 3 depicts toxicity of the toxins and toxins linked to anti-Tac antibody. The toxicity was measured by % viable CR2-2 cells (trypan blue dye excluded) . Figure 4 shows the toxicity of toxins and toxins linked to anti-Tac antibody via positioners. The toxicity was measured by % viable CR2-2 cells (trypan blue dye excluded) . Figure 5 shows the toxicity of toxins and toxins linked to anti-Tac antibody via Gly2Pro5Gly2 positioner. The toxicity was measured by % 3 H-leucine incorporation. 100% = incorporation without toxin treatment. Figure 6 shows toxicity of toxin and toxin linked to MA2.1 antibody via Gly2Pro5Gly2 positioner. The toxicity was measured by % 3 H-leucine incorporation. 100% = incorporation without toxin treatment. Figure 7 shows extent of cell killing by toxin linked to anti-Tac antibody via random positioners. Cell killing was measured by % 3 H-thymidine incorporation after addition of two concentrations of toxin conjugate. 100% = incorporation without toxin treatment (antibody alone) . DESCRIPTION OF THE PREFERRED EMBODIMENTS Novel compositions and methods are provided for improving the effectiveness of targeted toxic conjugates, which find use in removing a set of cells from a cell population, e.g., such as in killing a preselected cell type in a patient. The conjugates typically comprise a polypeptide targeting component capable of binding to a cell membrane marker linked, by peptide linkage or through a linker comprising one or more amino acids, to a toxic component capable of killing cells by damaging the cell membrane. Thus, increased killing of a preselected population of cells is achieved, whether i-n vivo or extra-corporeally, without significantly increasing nonspecific toxicity. The improved killing properties can be utilized against a wide range of prokaryotic and eukaryotic cell types, and the conjugate can be readily produced by recombinant DNA technology, which allows simple modifications of the conjugate construct. This permits further adjustment of the conjugate's cytotoxicity or other characteristics. As used herein, the term "targeting conjugate" refers to the combination of a specific binding component complexed with a cytotoxic agent. The specific binding component, or targeting component, of the conjugate is a polypeptide capable of providing the means for delivering the toxic agent to a particular cell type, typically preselected, such as cells forming a carcinoma. The targeting component is linked to a polypeptide toxic component in a manner that is likely to ensure that the toxic agent is not separated from the binding agent even after attachment of the entire conjugate to the desired cell. Typical disease states treatable by the conjugates of the present invention include graft-related conditions, such as organ transplant rejection and graft- versus-host disease; autoimmune diseases, such as type I diabetes, rheumatoid arthritis, systemic lupus erythematosus, myasthenia gravis, and multiple sclerosis; and lymphomas, leukemias and other cancers. A preferred use is the pretreatment of tissue, such as bone marrow, prior to transplantation to remove harmful cells (e.g., donor T-cells; see, U.S. Patent No. 4,489,710, which is incorporated herein by reference) . In general, the conjugates of the present invention will have the formula: SBA - L- T In this depiction, "SBA" represents a specific binding agent, "T" represents a toxic agent and "L" represents the linker between the two agents, which can be a peptide bond, or an oligopeptide or other proteinaceous structure for linking the specific binding agent to the toxic agent without a substantial loss of either agent's authentic activity, i.e., that each actually possessed before conjugation. For example, one embodiment of the present invention is a conjugate comprising the enzyme C. perfringens phospholipase C (PLC) joined through a protein A fragment to a monoclonal antibody specifically reactive with the human interleukin-2 receptor (anti-Tac antibody) . The genes for both of these polypeptides have been isolated, and each can be readily expressed in a variety of cellular hosts. The two proteins are preferably linked after purification from the host cells. Thus, by utilizing a DNA segment encoding two of the five repeating units of Staphylococcus aureus protein A linked downstream of the PLC gene, a fusion protein encoding PLC followed by part of protein A can be obtained. When this is mixed with purified monoclonal anti-Tac antibody and run through an FPLC sizing column a conjugate of the present invention is formed. The ultimate conjugate has the structure "PLC-protein A-anti- Tac", because the protein A units have high affinity for the anti-Tac antibody. This conjugate has been shown to be about 20 times more effective than the PLC-protein A alone in killing cells containing the IL-2 receptor on their surface. Further increases in killing activity were obtained by adding positioner amino acids between the enzyme and protein A. Specifically, five to ten-fold increased killing activity was seen when various positioners were inserted after the enzyme, before the protein A-antibody proteins. A number of positioner combinations of glycine and proline, having 9, 14, 16, and 42 amino acids, exhibited improved killing capability. Importantly, these proteins were easily produced by inserting appropriate DNA sequences into the expression vector encoding for the original conjugates. The specific binding agent, acting as the delivery vehicle for the cytotoxic agent in the conjugate, can be obtained from a number of sources. Preferably, intact immunoglobulins or their fragments, such as Fv, Fab, F(ab) 2 ', half antibody molecules (i.e., a single heavy/light chain pair) , will be used. Most preferably, the immunoglobulins are monoclonal antibodies of the IgM or IgG isotype, of mouse, human, or other mammalian origin, depending upon the ultimate use. Other proteins or agents including growth factors (e.g., insulin-like growth factors, the interleukins, fibroblast growth factor, platelet derived growth factor [PDGF] etc.) capable of binding to receptors (e.g., interleukin receptors, PDGF receptor, insulin receptor, etc.), on selected cell populations may be utilized. Generally, the specific binding agent will be limited only by its capability to bind with a sufficient affinity to an accessible cellular component, whether glycoprotein, lipoprotein, polysaccharide, or the like, to allow the toxic agent to act. Thus, the binding affinity of the specific binding agent to its target will typically be at least about 10 —7, preferably about 10—8 to 10—10 or more. The general immunization, fusion, screening and expansion methods of monoclonal antibody technology, as well as the choice of markers, are well known to those skilled in the art and do not form part of the present invention. Recent technical advances have provided additional forms of immunoglobulins and methods of making them. For example, the utilization of recombinant DNA technology has produced functional, assembled immunoglobulins or hybrid immunoglobulins (e.g., the constant region with or without variable regions from human monoclonal antibodies combined with mouse variable or hypervariable regions; see, e.g., EPA 85305604.2, incorporated herein by reference) , suitable for use in "the conjugates (see, e.g., EPA 84302368.0, which is incorporated herein by reference) . Typically, the antibodies are capable of binding to epitopes of markers that are membrane constituents on selected cell populations, such as neoplastic cells or T-cells. The marker is generally a surface glycoprotein, but a large variety of markers, such as other proteins, lipoproteins, polysaccharides and the like, which are displayed by the cells on their membranes, can be utilized in accordance with the present invention. More specifically, proteins on the membranes of some or all T cells that can serve as targets for the immunotoxins described here include CD2 (Til) , CD3, CD4 (on helper T cells) and the IL-2 receptor (see. Leucocyte Typing- III. A.J. McMichael, ed., Oxford University Press, 1987, which is incorporated herein by reference) . Proteins found predominantly on B cells that might serve as targets include CD10 (CALLA antigen), CD19 and CD20. CD45 is a possible target that occurs broadly on lymphoid cells. Antigens found on cancer cells that might serve as targets include carcinoembryσnic antigen (CEA) , the transferrin receptor, and the antigens recognized by 17- 1A antibody (Herlyn et al., Proc. Nat. Acad. Sci. USA. 76:1438, 1979), L6 antibody (Hellstro et al., Cancer Res. 46:3917-3923, 1986) and B6.2 antibody (Colcher et al., Proc. Nat. Acad. Sci. USA. 78:3199, 1981). Other protein targets on cancer cells are described in the Abstracts of the Third International Conference on Monoclonal Antibody Im unoconjugates for Cancer (San Diego, CA 1988) . The toxin component of the present invention, when oriented adjacent to a cell, will be capable of damaging a cell membrane. As used herein "damaging" shall mean altering the membrane, its normal function, or one or more of the membrane's components, sufficiently to induce cell dysfunction or death. Typically, the toxin will be active at the cell surface, such as cleaving membrane structural components or otherwise forming channels or pores (e.g., thiol-activated cytolytic toxins from bacteria) , ultimately creating holes in the membrane. Also, toxins may damage the cell membrane by destroying any critical structural component, by detergent-like surfactant action, by sequestering some necessary component, or by permanently activating or inactivating a membrane-bound enzyme ( e.g., E. coli heat stable enterotoxin; see generally. Bacterial Toxins. 2nd ed. , J. Stephen and R.A. Peitrowski, American Society for Microbiology, 1986, which is incorporated herein by reference) . In this regard, suitable toxins from bacteria include the following listed in Table I, which are offered by way of example and not limitation (see. "The Specificity and Action of Animal, Bacterial and Plant Toxins," in Receptors and Recognition, series B, Vol. 1, ed. Cuatrecasas, P., Chapman and Hall, London, U.K., 1976, which is incorporated herein by reference) . TABLE I Streptolysin O and S Alouf, -T.E. and Raynaud, M. (1973) Biochimie 55:1187- 1193 Clostridium perfringens Smyth, C. (1975) J. θ-toxin Gen. Micrσbio1. 87:219-238 Clostridium perfringens Smyth, C. and α-tαxin Arbuthnott, J.P. (1974) J. Med. Microbiol. 7:41-65 Bacillus cereus phospholipase C Johansen, T. et al. (1988) Gene 65:293- 304 Pseudomonas aeruginosa Liu, P.V. (1966) J. phospholipase C Infect. Pis. 116:112-116 Staphylococcus aureus Cassidy et al. (1974) α-toxin Bioci . Biophys. Acta 332:413-423 Staphylococcus aureus Bernheimer et β-toxin al. (1974) Ann. N.Y. Acad. Sci. 236:292-305 Staphylococcus aureus Kantor et al. (1972) ό * -toxin Arch. Biochem. Biophvs. 151:142-156 Escherichia coli hemolysin Rennie R.P., and Arbuthnot , J.P. (1974) J. Med. Microbiol. 7:179-188 In accordance with one embodiment of the present invention, DNA sequences encoding polypeptide sequences of the toxin Clostridium Phospholipase C (PLC) enzymes are provided. When placed in expression vectors and suitable eukaryotic and prokaryotic hosts, large quantities of polypeptides displaying one or more of the various biological properties of naturally-occurring Clostridium PLC can be produced. As desired, these also can be utilized in the conjugates of the present invention. Thus, the present invention also contemplates isolating recombinant DNA segments encoding Clostridium PLC polypeptides, the DNA segment comprising a DNA sequence selected from group consisting of: (a) the DNA sequence of Clostridium perfringens PLC as shown in Table II and the DNA sequence of Clostridium bifermentans shown in Table III, or their complementary strands; (b) DNA sequences, or their complementary strands, which on expression code for the polypeptide sequences shown in Tables II and III due, e.g. , due to codon degeneracy; and DNA sequences (e.g.. from other Clostridium species) which hybridize, typically under stringent conditions, to the DNA sequences of (a) or (b) above and which produce a polypeptide exhibiting at least one biological activity of PLC. The hybridizing sequences, including those encoding native or mutated gene sequences, will usually be at least about 50 to 60% homologous, preferably 85 to 90% homologous or more, to similarly-sized portions of the gene sequences in the immediately following Tables. TABLE II 45 ATG AAA AGA AAG ATT TGT AAG GCG CTT ATT TGT GCC GCG CTA GCA Met Lys Arg Lys lie Cys Lys Ala Leu lie Cys Ala Ala Leu Ala 90 ACT ACG CTA TGG GCT GGG GCA TCA ACT AAA GTC TAC GCT TGG GAT Thr Thr Leu Trp Ala Gly Ala Ser Thr Lys Val Tyr Ala Trp Asp 135 GGA AAG ATT GAT GGA ACA GGA ACT CAT GCT ATG ATT GTA ACT CAA Gly Lys He Asp Gly Thr Gly Thr His Ala Met He Val Thr Gin 180 GGG GTT TCA ATC TTA GAA AAT GAT CTG TCC AAA AAT GAA CCA GAA Gly Val Ser He Leu Glu Asn Asp Leu Ser Lys Asn Glu Pro Glu 225 AGT GTA AGA AAA AAC TTA GAG ATT TTA AAA GAG AAC ATG CAT GAG Ser Val Arg Lys Asn Leu Glu He Leu Lys Glu Asn Met His Glu 270 CTT CAA ' TTA GGT TCT ACT TAT CCA GAT TAT GAT AAG AAT GCA TAT Leu Gin Leu Gly Ser Thr Tyr Pro Asp Tyr Asp Lys Asn Ala Tyr 315 GAT CTA TAT CAA GAT CAT TTC TGG GAT CCT GAT ACA GAT AAT AAT Asp Leu Tyr Gin Asp His Phe Trp Asp Pro Asp Thr Asp Asn Asn 360 TTC TCA AAG GAT AAT AGT TGG TAT TTA GCT TAT TCT ATA CCT GAC Phe Ser Lys Asp Asn Ser Trp Tyr Leu Ala Tyr Ser He Pro Asp 405 ACA GGG GAA TCA CAA ATA AGA AAA TTT TCA GCA TTA GCT AGA TAT Thr Gly Glu Ser Gin He Arg Lys Phe Ser Ala Leu Ala Arg Tyr 450 GAA TGG CAA AGA GGA AAC TAT AAA CAA GCT ACA TTC TAT CTT GGA Glu Trp Gin Arg Gly Asn Tyr Lys Gin Ala Thr Phe Tyr Leu Gly 495 GAG GCT ATG CAC TAT TTT GGA GAT ATA GAT ACT CCA TAT CAT CCT Glu Ala Met His Tyr Phe Gly Asp He Asp Thr Pro Tyr His Pro 540 GCT AAT GTT ACT GCC GTT GAT AGC GCA GGA CAT GTT AAG TTT GAG Ala Asn Val Thr Ala Val Asp Ser Ala Gly His Val Lys Phe Glu 585 ACT TTT GCA GAG GAA AGA AAA GAA CAG TAT AAA ATA AAC ACA GCA Thr Phe Ala Glu Glu Arg Lys Glu Gin Tyr Lys He Asn Thr Ala 630 GGT TGC AAA ACT AAT GAG GCT TTT TAT ACT GAT ATC TTA AAA AAC Gly Cys Lys Thr Asn Glu Ala Phe Tyr Thr Asp He Leu Lys Asn TABLE II (continued) 675 AAA GAT TTT AAT GCA TGG TCA AAA GAA TAT GCA AGA GGT TTT GCT Lys Asp Phe Asn Ala Trp Ser Lys Glu Tyr Ala Arg Gly Phe Ala 720 AAA ACA GGA AAA TCA ATA TAC TAT AGT CAT GCT AGC ATG AGT CAT Lys Thr Gly Lys Ser He Tyr Tyr Ser His Ala Ser Met Ser His 765 AGT TGG GAT GAT TGG GAT TAT GCA GCA AAG GTA ACT TTA GCT AAC Ser Trp Asp Asp Trp Asp Tyr Ala Ala Lys Val Thr Leu Ala Asn 810 TCT CAA AAA GGA ACA GCG GGA TAT ATT TAT AGA TTC TTA CAC GAT Ser Gin Lys Gly Thr Ala Gly Tyr He Tyr Arg Phe Leu His Asp 855 GTA TCA GAG GGT AAT GAT CCA TCA GTT GGA AAG AAT GTA AAA GAA Val Ser Glu Gly Asn Asp Pro Ser Val Gly Lys Asn Val Lys Glu 900 CTA GTA GCT TAC ATA TCA ACT AGT GGT GAG AAA GAT GCT GGA ACA Leu Val Ala Tyr He Ser Thr Ser Gly Glu Lys Asp Ala Gly Thr 945 GAT GAC TAC ATG TAT TTT GGA ATC AAA ACA AAG GAT GGA AAA ACT Asp Asp Tyr Met Tyr Phe Gly He Lys Thr Lys Asp Gly Lys Thr 990 CAA GAA TGG GAA ATG GAC AAC CCA GGA AAT GAT TTT ATG ACT GGA Gin Glu Trp Glu Met Asp Asn Pro Gly Asn Asp Phe Met Thr Gly 1035 AGT AAA GAC ACT TAT ACT TTC AAA TTA AAA GAT GAA AAT CTA AAA Ser Lys Asp Thr Tyr Thr Phe Lys Leu Lys Asp Glu Asn Leu Lys 1080 ATT GAT GAT ATA CAA AAT ATG TGG ATT AGA AAA AGA AAA TAT ACA He Asp Asp He Gin Asn Met Trp He Arg Lys Arg Lys Tyr Thr 1125 GCA TTC TCA GAT GCT TAT AAG CCA GAA AAC ATA AAG ATA ATA GCA Ala Phe Ser Asp Ala Tyr Lys Pro Glu Asn He Lys He He Ala 1170 AAT GGA AAA GTT GTA GTG GAC AAA GAT ATA AAC GAG TGG ATT TCA Asn Gly Lys Val Val Val Asp Lys Asp He Asn Glu Trp He Ser GGA AAT TCA ACT TAT AAT ATA AAA TAA Gly Asn Ser Thr Tyr Asn He Lys End TABLE HI 45 ATG AAA GCT TTA AAA AAA GTA TCT AAT ATA CTG TGC GTA TTA GGT Met Lys Ala Leu Lys Lys Val Ser Asn He Leu Cys Val Leu Gly 90 TTA TGT ACT CTT ATG GGG GGT ACT TCT TAT GCA TGG GAT GGT AAA Leu Cys Thr Leu Met Gly Gly Thr Ser Tyr Ala Trp Asp Gly Lys 135 AAA GAT GGA ACA GGG ACA CAT TCA TTA ATA GCA GAA CAT GGG TTA Lys Asp Gly Thr Gly Thr His Ser Leu He Ala Glu His Gly Leu 180 AGT ATG CTA AAC AAT GAT TTA AGT GGA AAT GAA TCT CAG CAA GTT Ser Met Leu Asn Asn Asp Leu Ser Gly Asn Glu Ser Gin Gin Val 225 AAG GAT AAT ATT AAA ATT TTA AAT GAA TAC TTA GGG GAT TTA AAG Lys Asp Asn He Lys He Leu Asn Glu Tyr Leu Gly Asp Leu Lys 270 TTA GGG TCT ACA TAT CCA GAT TAC GAT CCT AAC GCA TAT GAT CTA Leu Gly Ser Thr Tyr Pro Asp Tyr Asp Pro Asn Ala Tyr Asp Leu 315 TAT CAA GAT CAT TTC TAT GAT CCT GAC ACT GGA AAC AAT TTT ACT Tyr Gin Asp His Phe Tyr Asp Pro Asp Thr Gly Asn Asn Phe Thr 360 ATT GAT AAT AGT TGG TAT GCA TCA TAT CCT ATA TAT GAT ACA AGC He Asp Asn Ser Trp Tyr Ala Ser Tyr Pro He Tyr Asp Thr Ser 405 AGA AAC TCA GTA AGA AAG TTT GCT ACA TTA GCT AAA AAT GAA TGG Arg Asn Ser Val Arg Lys Phe Ala Thr Leu Ala Lys Asn Glu Trp 450 GAA AAG GGA AAT TTT AAA GAA GCT ACA TTC CTT TTA GGC CAA GGA Glu Lys Gly Asn Phe Lys Glu Ala Thr Phe Leu Leu Gly Gin Gly 495 TTA CAT TAT CTA GGG GAT TTA AAT ACT CCA TAT CAT GCT TCA AAT Leu His Tyr Leu Gly Asp Leu Asn Thr Pro Tyr His Ala Ser Asn 540 GTA ACT GCA GTG GAT AGT CCT GGA CAT GTT AAA TAT GAA ACT TTT Val Thr Ala Val Asp Ser Pro Gly His Val Lys Tyr Glu Thr Phe 585 GTA GAA GAA AGA AAA GAT AAC TAT GCT TTA AAT ACT TCA GGA AAT Val Glu Glu Arg Lys Asp Asn Tyr Ala Leu Asn Thr Ser Gly Asn 630 GAT ACT ACA TCA GGA GTA TAT AAA GAA GCT ATG GAA AAT CCA AGT Asp Thr Thr Ser Gly Val Tyr Lys Glu Ala Met Glu Asn Pro Ser TABLE HI (continued) 675 TTT AAT AAA TGG ATG ACA CAA AAC TCT ATA AAA TAT GCT AAG ATA Phe Asn Lys Trp Met Thr Gin Asn Ser He Lys Tyr Ala Lys He 720 GCT AAA GAT TTA TAT TAT AGT CAT TCA ACT ATG AGT CAT AGT TGG Ala Lys Asp Leu Tyr Tyr Ser His Ser Thr Met Ser His Ser Trp 765 GAT GAT TGG GAT TAT TCT GGA AGA GAA GCT ATA AAA AAT TCT CAA Asp Asp Trp Asp Tyr Ser Gly Arg Glu Ala He Lys Asn Ser Gin 810 GTA TGT ACT GCT GGA TTT TTA TAT AGA TTC ATG AAT GAA GTT TCT Val Cys Thr Ala Gly Phe Leu Tyr Arg Phe Met Asn Glu Val Ser 855 AAT GGA AAT ACA GGA GAT AAT GAT TCA TTA ACT AAT GAA TTC AAT Asn Gly Asn Thr Gly Asp Asn Asp Ser Leu Thr Asn Glu Phe Asn 900 ATA GTA TTA AAG ACT GCA GAC AAT AAA TAT GCA GGA ACT GAT GAT He Val Leu Lys Thr Ala Asp Asn Lys Tyr Ala Gly Thr Asp Asp 945 AAT GTA TAT TTC GGA TTT GAA ACA AAT GAA GGT AAG AAA TTT GAA Asn Val Tyr Phe Gly Phe Glu Thr Asn Glu Gly Lys Lys Phe Glu 990 TGG AAA TTA GAT AAT GCA GGA AAT GAC TTT GAA AGA AAT CAA GTA Trp Lys Leu Asp Asn Ala Gly Asn Asp Phe Glu Arg Asn Gin Val 1035 GAT AAC TAT ATA TTA AAA ACA AAA GAT GGA GAA GAA GTA GAT ATA Asp Asn Tyr He Leu Lys Thr Lys Asp Gly Glu Glu Val Asp He 1080 AAT AAT ATA TCT AAT TAT TGG ATA AGA AAA GAA AGA TTA ACA TCT Asn Asn He Ser Asn Tyr Trp He Arg Lys Glu Arg Leu Thr Ser 1125 ATA AGT GAT GAT TGG GAG TTA AGC AAC TTC AAA TTA ATA GCA AAT He Ser Asp Asp Trp Glu Leu Ser Asn Phe Lys Leu He Ala Asn 1170 GGA AAA GTA ATA CAA CAA CAA GAT GTA AAT AAA GTT TTT ACA GGT Gly Lys Val He Gin Gin Gin Asp Val Asn Lys Val Phe Thr Gly AAC GAA ACT TAT TAC ATA AAT AAA TAA Asn Glu Thr Tyr Tyr He Asn Lys End The DNA segment will typically further include an expression control DNA sequence operably linked to the PLC coding sequences, including naturally-associated promoter regions. Preferably, the expression control sequences will be prokaryotic promoter systems in vectors capable of transforming or transfecting prokaryotic host cells. Once the vector has been incorporated into the appropriate host, the host is maintained under conditions suitable for high level expression of the sequences, and, as desired, the collection and purification of the Clostridium PLC polypeptides may follow. Two putative amino acid sequences based on isolated and purified nucleotide sequences of genes coding for polypeptides of the present invention are shown in Table II and III. The initial 22 amino acids in Table II and 23 amino acids in Table III are rich in hydrophobic amino acids, as would be expected for a leader or signal sequence of a secreted mature protein (indicated by an arrow) . Tne naturally-occurring PLC Clostridium genes from various Clostridium species may be identified and isolated utilizing well-known hybridization technology. In view of differing homology among the various PLC genes, the stringency of hybridization conditions must be adjusted from species to species. Filter hybridization is preferred, the hybridization buffer and procedures of which are generally described in Nucleic Acid Hybridization: A Practical Approach. Eds. Hames and Haggins, IRL Press, Washington, D.C. (1985), which is incorporated herein by reference. It is well known that native forms of this "mature" Clostridium PLC will vary among Clostridium species in terms of length by deletions, substitutions, insertions or additions of one or more amino acids in the sequences. Common species of Clostridia from which various pertinent DNA sequences can be obtained include C. perfringens, C. bifermentans, C. tertium, and C. sporogenes, C. novyi, C. septicum. and C ^ . histolyticum. Samples of these species can be obtained from a variety of sources, such as the American Tissue culture Collection, ("Catalogue of Bacteria, Phages and rDNA vectors," Sixteenth edition (1985) Rockville, Maryland, U.S.A., which is incorporated herein by reference). In addition to these naturally-occurring forms of Clostridium PLC, other Clostridium PLC peptides can be readily designed and manufactured utilizing various recombinant DNA techniques well known to those skilled in the art. For example, polypeptides can vary at the primary structure level by amino acid substitutions, terminal and intermediate additions and deletions, and the like. Alternatively, polypeptide fragments comprising only a portion of the primary structure may be produced, which fragments possess one or more Clostridium PLC activities (e.g. , enzyme activity) , while exhibiting lower immunogenicity) . In particular, it is noted that like many genes, the Clostridium PLC genes may contain separate functional regions, each having one or more distinct biological activities. In general, modifications of the genes may be readily accomplished by a variety of well-known techniques, such as site-directed utagenesis (see. Gillman and Smith, Gene 8:81-97 (1979), which is incorporated herein by reference) which can be employed to generate various analogues and derivatives of naturally-occurring, mature Clostridium PLC. In addition to bacterial toxins, various animal toxins may be utilized in accordance with the present invention. For therapeutics, a preferred toxin would be mammalian pancreatic phospholipase A- (e.g., Nieuwenhuizen, W. et al., Meths. Enzymol. 328:147-154, 1974) , particularly human. Phospholipase A_ from snake venom (e.g., Crotalus atrox (e.g., Hachi ori, Y. et al., (1971) Biochem. 10:4084-4089) and Naja naja (e.g., Salach, J.I., et al., (1968) Biochem. Biophvs. Res. Com. 33:936-941) and melittin from bee venom (e.g.. Apis mellitera (e.g., Habermann, E. and Jentsch, J. (1967) Biochem. Z. 348:37) are suitable. Also, mammalian complement proteins and cell-killing factors from cytotoxic T lymphocytes (e.g., perforin) may be utilized. The term "linker" as utilized in the specification and claims is meant to include broadly the firm attachment of the specific binding agent to the toxic agent. Such attachment may be covalent or noncovalent, although it is preferably covalent by peptide linkage. The coupling of the two entities may be direct or, most commonly, by means of a polypeptide coupling or conjugating agent, such as Proteins A or G. The covalent coupling should not, however, substantially interfere with each of the agent's authentic activities. Typically, the two components will retain about 80-90%, preferably 95% or more of their desired activity after coupling with a linker. As explained more fully below, it is possible to produce a fused conjugate of the present invention, wherein the specific binding agent is coupled to the toxic agent at the genetic level. For instance, an antibody/toxic agent conjugate could be produced wherein the transformed cell secretes an antibody which has had its Fc portion replaced with a different protein which functions as the toxin (see, e.g.. Neuberger, M.S. et al., Nature 312:604 (1984), which is incorporated herein b Y reference) . As noted previously, the linker may be an oligopeptide or a protein, which can have a variable length and structure to effect increased killing activity (e.g. , by positioning the toxin proximate to the membrane) . The linker will preferably be resistant to cleavage, but cleavage (such as by complement activation; see. U.S. Patent No. 4,671,958, which is incorporated herein by reference) may be desirable to, e.g. , control half-life of the conjugate jln vivo. The oligopeptides will typically be at least about 15 A in length and comprise between about 2 to 50 or more amino acid residues, preferably about 6 to 20 amino acid residues. The linker can contain a rigid section make up to 4 to 6 prolines bounded by flexible regions of other amino acids, such as glycine, serine or threonine. The conjugates of this invention are preferably produced by recombinant DNA techniques and can be 5 expressed in prokaryotic or eukaryotic host cell systems by standard techniques. Polydeoxynucleotides encoding the desired polypeptides can be constructed by conventional techniques of gene synthesis and gene splicing as set forth in more detail below. o Double stranded DNA constructs encoding representative conjugates can be shown, in 5' to 3' orientation, as follows: 5' [DNA encoding SBA]-[DNA encoding L]- [DNA encoding T] 5 or 5' [DNA encoding T]-[DNA encoding L]- [DNA encoding SBA] . The DNA encoding L is optional, for example, if the agents are in peptide linkage. Thus, the construct o can contain a first DNA sequence encoding SBA and a second DNA sequence encoding T, and where desired, an oligodeoxynucleotide encoding a linker oligopeptide L is interposed between the other two DNA sequences. Nucleotides may be added to the 5* and 3* terminals of 5 these constructs to provide restriction enzyme recognition sites for convenient insertion of the construct into DNA vectors. The ultimate product will have the formula SBA(L)T or T(L)SBA, where the parenthesis indicates the linker is optional. 0 When the polypeptide components are naturally- occurring proteins, DNA sequences encoding this portion of the conjugate can be obtained from cell lines harboring the DNA sequence, which can be obtained by standard techniques of gene cloning. When the DNA sequences encoding the desired polypeptides are known (e.g., interleukin-2; U.S. Patent No. 4,738,927,) or can be determined (sequenced) , the polydeoxnucleotide can be synthesized by standard procedures such as the phosphotriester and phosphite triester techniques. DNA encoding immunoglobulins or their fragments can be obtained from a variety of sources (see. EPA 84302368.0, which is incorporated herein by reference) . The genes are bounded by additional short segments of DNA, if necessary, to provide start and stop codons for translation and to provide convenient restriction sites. The preferred DNA construct design, however, permits easy segregation of the DNA into three "cassettes", each cassette corresponding to a different portion of the molecule (i.e., SBA, T and L DNA segments) . By using this cassette-based construction, the resulting protein can be readily manipulated at the genetic level to easily modify the resultant polypeptide product. In addition, during construction, restriction sites unique to individual cassettes are included, thus permitting clean excision of shorter portions of the molecule. The sub-cassette fragments can be readily excised and rearranged to provide additional DNA constructs encoding conjugates of the present invention. Transformation of DNA vectors into prokaryotes or eukaryotes for expression is performed by well known procedures. A DNA construct encoding the desired conjugate is operably linked to (i.e., positioned to ensure the function of) expression control systems compatible with the host and disposed on a suitable DNA expression vector which is capable of replication in the host cell. E. coli is the prokaryotic host for cloning the DNA sequences of the present invention. It and other microbial hosts can be used for expressing some components of the invention, such hosts can include bacilli, such as Bacillus subtilus. and other enterobacteriaceae, such as Salmonella. Serratia. and various Pseudomonas species. In these prokaryotic hosts, the expression vectors will typically contain expression control sequences compatible with the host cell (e.g.. an origin of replication) . In addition, any number of a variety of well-known promoters will be present, such as the lactose promoter system, a tryptophan (trp) promoter system, a beta-lactamase promoter system, or a promoter system from phage lambda. The promoters will typically control expression, optionally with an operator sequence, and have ribosome binding site sequences and the like, for initiating and completing transcription and translation. Other microbes, such as yeast may also be used for expression. Saccharorovces is a preferred host, with suitable vectors having expression control sequences, such as promoters, including 3-phosphoglycerate kinase or other glycolytic enzymes, and an origin of replication, termination sequences and the like as desired. In addition to microorganisms, mammalian tissue cell culture may also be used to produce the polypeptides of the present invention (see, Winnacker, "From Genes to Clones," VCH Publishers, N.Y., N.Y. (1987), which is incorporated herein by reference) . A number of suitable host cell lines have been developed, and include the CHO cell lines, the various COS cell lines, HeLa cells, myeloma cell lines, etc. Expression vectors for these cells can include expression control sequences, such as an origin of replication, a promoter, and necessary processing information sites, such as ribosome binding sites, RNA splice sites, polyadenalation sites, and transcriptional terminator sequences. Preferred expression control sequences are promoters derived from SV40, from Adenovirus, Bovine Papilloma Virus, immunoglulin genes and the like. The vectors containing the DNA segments of interest (e.g.. the PLC encoding sequences and expression control sequences) can be transferred into the host cell by well-known methods, which vary depending on the type of cellular host. For example, calcium chloride transfection is commonly utilized for prokaryotic cells, whereas calcium phosphate treatment may be used for other cellular hosts. (See, generally, Maniatis et al. Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Press, (1982) , which is incorporated herein by reference.) Depending on the sizes of the two conjugate components and the linker, it may be preferable to synthesize the polypeptides by conventional polypeptide techniques (Mangolin and Merrifield, Ann. Rev. Biochem. 39:841, 1970). The conjugate may be isolated and purified in accordance with standard procedures, such as extraction, precipitation, gel filtration chromatography, affinity chromatography, HPLC, electrophoresis, or the like. Ideally, the conjugates will be substantially pure, at least about 85 to 95% homogeneous, preferably 98 to 99% or more homogeneous. The conditions and concentrations useful for producing and using the conjugates of the invention can be readily determined by those of skill in the art by reference to known literature or by no more than routine experimentation. Generally, the conjugates will be utilized in purified form together with pharmacologically appropriate carriers. Typically, these carriers include aqueous or alcoholic/aqueous solutions, emulsions or suspensions, including saline and buffered media. Parenteral vehicles include sodium chloride solution. Ringer's dextrose, dextrose and sodium chloride, lactated Ringer's or fixed oils. Intravenous vehicles include fluid and nutrient replenishers, electrolyte replenishers, such as those based on Ringer's dextrose, and the like. Preservatives and other additives can also be present, such as, for example, antimic obials, antioxidants, chelating agents, inert gases, and the like. See, generally, Remington's Pharmaceutical Sciences, 16th Ed., Mack, ed. 1982, which is incorporated herein by reference. The conjugates of the present invention may be used as separately administered compositions or in conjunction with other cytotoxic agents. These can include various chemotherapeutic drugs, such as anti- neoplastic drugs (e.g., vindesine, methotrexate, adriamycin, cisplatinum) , immunotoxins and the like. Pharmaceutical compositions can include "cocktails" of various cytotoxic agents in conjunction with the conjugates of the present invention. The route of administration of pharmaceutical compositions prepared according to the method of the invention may be any of those commonly known to those of ordinary skill in the art. For therapy, including without limitation chemotherapy, the conjugates of the invention can be administered to any patient in accordance with standard techniques. The administration can be by any appropriate mode, including parenterally, intravenous, intramuscular, intraperitoneal, or also appropriately, by direct infusion with a catheter. The dosage and frequency of administration will depend on the age, sex and condition of the patient, concurrent administration of other drugs, counter indications and other parameters to be taken into account by the clinician. An adequate amount to accomplish at least partial inhibition or killing of a population of selected cells is defined as a "therapeutically-effective dose." Amounts needed to achieve this dosage will depend upon the severity of the disease and the general state of the patient's own immune system, but generally range from about 0.005 to about 5.0 mg of conjugate per kilogram of body weight, with doses of 0.05 to 2.0 mg/kg/dose being more commonly used. For prophylactic applications, compositions containing the present conjugates or cocktails thereof may also be administered in similar or slightly lower dosages. The targeting conjugate composition may be utilized in prophylactic and therapeutic settings to aid in the killing or removal of a wide variety of predetermined cell populations in a mammal, including infectious organisms, depending upon the disease state. By way of example, the specific binding protein of the present conjugates may recognize markers on tumor cells, immune cells, hormone responsive cells and growth factor responsive cells, fungi, bacteria, parasites, or virus infected cells. In another embodiment, the conjugates described herein may be used extracorporeally or in vitro to selectively kill, deplete, or otherwise effectively remove a cell population from a heterogenous collection of cells. Blood from the mammal may be combined extracorporeally with the conjugates, whereby the undesired cells are killed or otherwise removed from the blood for return to the mammal in accordance with standard techniques. Another aspect includes treating an in vitro culture of heterogenous cells to kill or inhibit, via a conjugate described herein, the selected cells, which may be contaminants, etc. In addition to the therapeutic uses, the 0 conjugates will find use in diagnostic assays and for other in vivo purposes. For example, the conjugates can be used for selective cell killing in assays of peripheral blood cells where it is desired to eliminate only certain lymphocytes (e.g., B - or T- cells) or 5 similarly in cell culture to eliminate unwanted cell types. The following examples are offered by way of illustration and not limitation. d 5 EXPERIMENTAL Example 1 CLONING OF PHOSPHOLIPASE GENE FROM C. PERFRINGENS C. perfringens (ATCC 13124) bacterial cells were collected by centrifugation, resuspended in 25 mM Tris, pH 8.0, 10 mM EDTA, 50 mM glucose, 4 mg/ml lysozyme and incubated 10 min at 23 β C. SDS was added to a final concentration of 0.2%, and proteinase K to a final concentration of 100 μg/ml, and the sample incubated at 37°C for 4 hr. The sample was extracted with an equal volume of phenol 2 times, and then 2 volumes of ethanol were added to the aqueous phase without mixing. The DNA was spooled from this solution with a glass rod, incubated with 2 mg/ ml RNAse and extracted with phenol. It was then precipitated with ethanol, dried, and resuspended in TE buffer (lOmM Tris, pH 7.4, 1 nM EDTA). The chromosomal DNA was digested with either Hindlll or EcoRI and cloned into the respective sites in the plasmid pEMBL8+ (Dente, et al, Nucl. Acids Res. 11, 1645-1655, (1983) , which is incorporated herein by reference) as follows. About 5 μg purified Clostridium DNA was digested with Hindlll. The sample was extracted with phenol, precipitated with ethanol, and resuspended in TE. About 2 μg pEMBL8+ DNA was digested with Hindlll, then diluted 1:5 in TE buffer and 2.5 units calf intestinal phosphatase added and incubated for a further hr at 65°C. The sample was extracted with phenol, precipitated with ethanol and resuspended in TE. The Hindlll-digested Clostridium and pEMBL8+ DNAs were then ligated together, using T4 DNA ligase (New England Biolabs) , and the ligated DNA transformed into E^. coli according to standard methods (Maniatis et al., Cold Spring Harbor Laboratory, Cold Spring Harbor NY, 1982, pp. 250-251). The same process was performed with EcoRI. The Hindlll and EcoRI generated plasmids were then used to transform E. coli host DH1 (Hanahan (1983) , J. Mol. Biol. 166, 557-580) and transformants were plated on LB agar plates containing 50 μg/ml ampicillin and 1% (V/V) sheep red blood cells. After screening 2,000 trans ormants from each library, a total of five colonies that created zones of hemolysis were isolated. All five isolates exhibit phospholipase C activity in their periplasmic space as assayed by the method of Krug and Kent (supra) . Restriction analysis of the two kinds of plasmid (Hindlll and EcoRI generated recombinants) in these colonies indicated they shared a common 2.0 kilobase (kb) EcoRI-Hindlll fragment in their inserts. This 2.0 kb EcoRl-Hindlll fragment was then subcloned between the EcoRI and Hindlll site of pEMBL8+ for detailed restriction and DNA sequence analysis. E. coli transformants harboring the resulting plasmid, pδPLC, are hemolytic and exhibit phospholipase C activity • in the periplasmic space, confirming that the gene is intact in this 2.0 kb fragment. Furthermore, a predominant protein with molecular weight of 43,000 KD is present in the osmotic shock fluid (Nossal and Heppel (1966) J. Biol. Chem. 241, 3055-2062) from the periplasmic space of DHl/p8PLC. This protein is absent from the shock fluid of the host cell. Table IV shows the PLC enzymatic activity obtained from PLC polypeptides (non-glycosylated) of the present invention in comparison to naturally-occurring Clostridium PLC. The recombinantly produced polypeptides were obtained from the periplasmic space of the transformed organisms by osmotic shock. A cell pellet was obtained from about 250 ml of the cultures, and the cells washed in 50 mM Tris-HCl, ph7.8, and centrifuged. The pellet was resuspended in 20 ml of the wash buffer containing 40% sucrose, to which was added EDTA to 2mM. After 10 minutes, the solution was centrifuged and the cell pellets resuspended in 10ml of cold H 2 0, which was left standing on ice for about 10 minutes. This solution was centrifuged and the supernatant contained the shock fluid. The assay, a PLC-alkaline phosphatase coupled assay, was performed as follows. Reagents Tris-HCl, 0.4 M, pH 7.3 at 37°C Calcium chloride, 50 mM Bovine serum albumin, 1 mg/ml Phosphatidylcholine, 10 mM in absolute ethanol Ammonium sulfate, 0.2 M Alkaline phosphatase (E_ s _ coli, Sigma Chemical Co., type III-S) , 100 u/0.3 ml of 2.5M (NH 4 ) 2 S0. suspension Sodium dodecyl sulfate, 20% Ascorbate-molybdate reagent: 1 part 10% ascorbic acid and 6 parts 0.42% ammonium olybdate in Procedures Reagents are added in the following order and to the final concentrations indicated (final volume = 40μl) : 50 mM Tris-HCl, 6.3 mM calcium chloride, 0.13 mg/ml bovine serum albumin, 2.5 mM phosphatidylcholine, 70 mM ammonium sulfate (including the amount of ammonium sulfate contributed by the suspension of alkaline phosphatase), 0.15 unit ' of alkaline phosphatase, and 0.1 to 1 X 10 —3 unit of phospholipase C. Routinely, all the assay components except phospholipase C may be combined as one large suspension, and aliquots can be removed for each assay. The assay mixture is incubated at 37°C for 15 min in a shaking water bath. The reaction is terminated by addition of 40 μl of sodium dodecyl sulfate, then 200 μl of ascorbic-molybdate reagent is added, followed by incubation at 45°C for 20 min or 37°C for 60 min. The absorbance is determined at 820 nm. One nano ole of inorganic phosphate corresponds to a net absorbance of about 0.045 above a reaction mixture blank absorbance of 0.040. One unit of phospholipase C activity is defined as that which produces 1 μ ol of inorganic phosphate per minute. TABLE IV Assay for Phospholipase C (PLC) PLC assay Protein Specific A g2 reading concentra- activity (vol/dilution) tion (mg/ml) (units/mg Osmotic shock .542 (20μl/l:100) 0.56 107 fluid from E. coli transfected with p8PLC Purified PLC .520 (20μl/l:100) 0.76 79 from E_ t . coli transfected with p8PLC Purified PLC .440 (20μl/l:100) 0.29 184 from Clostridium perfringens Sequence analysis (Sanger et al, (1977) Proc. Natl. Acad. Sci. USA 74:5463-5467) of the 2.0 kb EcoRl- Hindlll fragment revealed an open-reading frame of 1196 base pairs (bp) located in the Hindlll half of the fragment (Table II) . The sequence codes for a 22 amino acid putative signal peptide and a 377 amino acid mature protein, the amino acid composition of which agrees well with that of prior reports (Krug and Kent, supra) . Example 2 CLONING OF PHOSPHOLIPASE C GENE FROM C BIFERMENTANS C. bifermentans (ATCC 638) cells were prepared and DNA extracted as in Example 1. Then, the chromosomal DNA was digested with EcoRI and cloned into the EcoRI site of GgtlO arms (Stratagene, Inc.) and packaged (Huynh et al, (1984) DNA Cloning Techniques; A practical Approach, David Glover, ed. IRL Press, Oxford 50 - 70, which is incorporated herein by reference) . The genomic library was plated on E. coli host C600 Hfl and screened by plaque hybridization (Benton and Davis (1977) Science 196, 180-182) using the previously cloned C ^ perfringens phospholipase C gene as a probe. Hybridization was performed at 5 x SSC/50% formamide/lX Denhardt/0.2% SDS/0.1 mg/ml calf thymύs DNA at 30°C overnight. After screening 2,000 plaques, a total of 20 plaques that hybridized to the probe were isolated. All of these plaques contain a 2.5 kb EcoRI insert that was later subcloned into pEMBL8+ to form plasmid pCBPLCl. Restriction and Southern analysis was performed by digesting with EcoRI and Hindlll, running the fragments on agarose gel and transferring the dispersed fragments to nitrocellulose (Southern (1975) J. Mol. Biol. 98, 503-518) . DNA on the nitrocellulose was hybridized to a radioactive probe prepared from the previously cloned C. perfringens gene by nick translation (Maniatis et al. , supra at pgs. 387-389 and 109-112). Only the 0.83kb Hindlll - EcoRI fragment hybridized. This indicated that the sequence related to the C-i. perfringens phospholipase C gene is located only in that fragment at the end of the insert. DNA sequence analysis of this fragment confirmed that it indeed contains part of the phospholipase C coding sequence, but is truncated at the EcoRI site. So a second genomic library was created to isolate the missing coding sequence. The second library was created by cloning Hindlll-digested chromosomal DNA from C. bifermentans into the Hindlll site of pUC19 (Norrander et al, (1983) Gene 26:101-110, which is incorporated herein by reference) . E. Coli transformants containing the recombinant plasmids were then screened by colony hybridization (Norrander, J. , et al, supra) using the 0.83 kb Hindlll - EcoRI fragment from pCBPLCl as a probe. From 2,000 transformants, seven colonies that hybridized to the probe were isolated. All plasmids in these colonies, of which a representative one is called pCBPLC2, contain a 5.2 kb Hindlll fragment in their inserts. The 1.0 kb EcoRI fragment that is adjacent to the EcoRI insert of pCBP in the chromosome was subcloned from pCBPLC2 into pCBPLCl to reconstitute the entire phospholipase C gene. The resulting plasmid is called pCBPLC3. The entire coding sequence for phospholipase C in pCBPLC3 is shown in Table III. The gene codes for a protein of 377 amino acid residues, with approximately 20 N-terminal residues corresponding to a signal peptide. The C. perfringens and C. bifermentans phospholipase C proteins have an overall amino acid homology of 51%. The two genes have overall homology at the nucleotide level of 64%. Example 3 CONSTRUCTION OF PLC MUTANTS DNA of the plasmid p8PLC, containing the complete Clostridium perfringens PLC gene, was cut at the unique Accl site, which occurs at nucleotides 76-81 of the PLC sequence (Table II) . The ends were filled in using the Klenow fragment of polymerase 1. Xba linkers (New England Biolabs) were treated with polynucleotide kinase and then ligated to the cut plasmid DNA, using T4 DNA ligase. The ligated DNA was cut with Xbal, and then run on a 1% agarose gel. The linear plasmid DNA was purified from the agarose, ligated again, and transformed into E_ Ϊ _ coli. A colony was picked, plasmid DNA purified (denoted p8PLC-Xba) , and sequenced around the Xbal linker insert. The construction had the effect of deleting amino acid 26, Val, and inserting at the deletion four amino acids: Gly Ser Arg and Ala. The mutant protein was purified and assayed. There was no detectable difference in its activity relative to normal PLC. DNA of the plasmid p8PLC was cut with EcoRI, which cuts about 800 nucleotides upstream of the pLC gene. The cut plasmid was treated with the nuclease Bal31, which progressively removes nucleotides from the free DNA ends produced by the cut. The DNA was filled in with the Klenow fragment, ligated with Xbal linkers and cut with Xbal as well as Hindlll, which cuts at the end of the PLC gene. The DNA was run on an agarose gel, and Xbal - Hindlll fragments were isolated, of a size corresponding to removal of 0-100 nucleotides from the beginning of the PLC gene. These fragments were ligated with the large Xbal - Hindlll fragment of p8PLC-Xba and transformed into E^. coli. DNA was extracted from various colonies and sequenced around the Xbal site. Example 4 MUTANTS' HEMOLYTIC ACTIVITY Two such mutants were selected for further study. In one mutant, RlCHyb-3, the mutated PLC gene in p8PLC-Xba had been further changed by deletion of amino acids 27-30, Tyr Ala Trp Asp. In the other mutant RlCHyb-9, amino acids 27-35, Tyr...Gly, had been deleted. The hemolytic activity of the mutant PLC proteins were respectively reduced approximately 10-fold and 50-fold relative to the normal PLC protein. From the foregoing, it will be appreciated that the isolated DNA sequences provide improved means for the safe and economic production of substantial quantities of Clostridium Phospholipase C. polypeptides. Importantly, the Clostridium Phospholipase C products are produced substantially free from other, naturally-occurring Clostridium proteins. Example 5 CONSTRUCTION OF A GENE FOR A PLC - PROTEIN A FUSION In order to genetically fuse the C. perfringens phospholipase C gene with the S. aureus protein A gene, new restriction sites were introduced at the 3* end of the PLC gene by using the in vitro mutagenesis procedure described in commonly assigned U.S.S.N. 223,037, which is incorporated herein by reference. Specifically, the 950 bp Bam HI - Hindlll fragment of the PLC gene was isolated from the plasmid P8PLC as described, and was subcloned in the phage M13mpl0. A synthetic oligonucleotide primer CNC (Figure 1A) was used to prime DNA synthesis from the phage DNA toward the Bam HI site. 38 bases of the primer were complementary to the 3' end of PLC; the remainder formed a tail containing the Apa I, Sma I, Eco RI, TAG stop codon, and Hindlll-sites to be introduced. After heat-denaturing the DNA, another oligonucleotide primer (New England Biolabs #1201) was used to prime synthesis to the newly synthesized DNA strand. Digestion with BamHI released a 950 bp mutageneized fragment containing the 3• end of the PLC gene followed by the introduced restriction sites. The vector into which this fragment was inserted was constructed as follows: 1) A 2.0 kb Eco RI- Hindlll fragment containing the entire PLC gene, including its promoter and signal sequence, was isolated form P8PLC and subcloned between the Eco RI and Hindlll sites of pBR322 to create the plasmid pBRPLCl, and 2) the EcoRI site in pBRPLC 1 was destroyed by digesting the plasmid with EcoRI followed by a fill-in reaction with Klenow polymerase and ligation. The new plasmid was called pBRPLCl. This plasmid was digested with BamHI and Pvu II and the 950 bp mutagenized fragment was cloned between these sites, replacing the native 3 1 half of the PLC gene. The new plasmid, called pBRPLC2, thus contained the entire PLC gene followed by the introduced restriction sites Apa I, Sma I, Eco RI and Hindlll. The plasmid pEZZ3308 contains a synthetic protein A gene including the E domain followed by two copies of the IgG binding domain Z domain (Nilsson, B. et al., Protein Engineering. (1987) 1:107-113, which is incorporated herein by reference) . The 366 bp Fsp I - Eco RI fragment containing the E and both Z domains of protein A was isolated from pEZZ3308 and ligated between the Sma I and Eco RI sites of pBRPLC2, following the PLC gene. The final construct, which contained the PLC gene fused in-frame to part of protein A, was called pBRPLCZZ. The sequence of the junction region is shown in Figure 2. Example 6 a) CONSTRUCTION OF PROLINE-GLYCINE POSITIONERS Segments of DNA were inserted between the PLC and protein A genes at the Apa I site to code for peptide positioners. The positioner Gly2Pro5Gly2 was made by synthesizing two complementary oligonucleotides CNC 10 and CNC 11 (Figure IB) . These were annealed, digested with Apa I and cloned at the Apa I site between PLC and Protein A. Since the Apa I site itself codes for GlyPro the final sequence at the junction was GlyProGly2Pro5Gly2Pro. In a similar way, positioners were made that had the sequences Gly2ProlOGly2 and Gly2Pro5Gly2Pro5Gly2. b) CONSTRUCTION OF POLYPROLINE POSITIONER The strategy for producing positioners of > 10 proline residues relied on overlapping oligonucleotides that could form tandem repeats of proline codons (Figure 1C) . The alternation of CCT's and CCA's were designed in such a way that the olig's could anneal in only one frame in order to be most stable. CNC 25 and CNC 26 could anneal to each other repeatedly. Once the two outer oligos, WPS 12 and WPS 13 annealed, extension stopped. WPS 12 and WPS 13 contain the Apa I site used for insertion into the plasmid. 200ng each of CNC 25 and CNC 26 were annealed first for 3 minutes at 95 C and allowed to cool slowly. Then 40 ng of WPS 13 and 23 ng WPS 12 were added in the presence of ATP, dNTP's, Klenow polymerase and T4 ligase. This mixture was ligated overnight at 16 C, then digested with Apa I. The DNA was run on a 5% acrylamide gel and bands between 50-200 bp and 200 bp and up were purified. These were ligated into plasmid pBRPLCZZ at the Apa I site, and the colonies were screened for the presence of the fusion protein. The screening procedure was based on Broome, S. and Gilbert, W. , Proc. Nat. Acad. Sci. USA 75:2746-2749 (1978) and relied on the ability of the Protein A portion of the fusion protein to bind alkaline phosphatase conjugated IgG. DNA from positive colonies was sequenced to determine the number of prolines incorporated. One insert coded for Gly2Pro40Gly2. C) CONSTRUCTION OF RANDOM POSITIONERS The oligonucleotide AC1 used to make fixed length, random composition positioners is shown in Figure ID. AC 1 contained 45 random nucleotides preceded and followed by fixed sequences containing Apa I sites. The M13 universal primer was annealed to AC1 and extended with Klenow polymerase to form double stranded DNA. The mixture was digested with Apa I, and the 55 bp fragment was gel purified, ligated into pBRPLCZZ, and transformed into E. coli. The screening procedure described above was used to select only bacterial colonies that produced a PLC-Protein A fusion protein. Due to amino acids contributed by the Apa I site, the junctions in the fusion proteins have the sequence GlyPro(XXX)15GlyPro. Example 7 PREPARATION OF PLC-PROTEIN A FUSION PROTEINS FROM E. COLI E. coli strain DH 1 carrying the appropriate plasmid was grown in 1 liter M9 medium with lOOug/ml ampicillin to an OD600 of 0.8 to 1.0. Cells were harvested by centrifugation, washed with 60ml 50 mM Tris- HCl pH 7-8, and resuspended in 50 ml 40% sucrose 50 mM Tris-HCl 2 mM EDTA pH 7-8. The mixture was incubated for 10 minutes at 20 C with occasional gentle shaking. After centrifugation for 30 minutes at 15,000 x g, pelleted cells were resuspended in 5 ml ice cold H 2 0 and incubated 10 minutes at 0 C. Cells were removed by centrifugation for 10 minutes at 15,000 x g and the supernatant, containing the extracted periplasmic space proteins, was adjusted to 50 mM Tris-HCl pH 7-8. This periplasmic space extract was passed over a 0.5 ml rabbit IgG agarose column (Sigma) equilibrated with 50mM Tris 150 mM NaCl pH 8.0 (Tris-NaCl) . After washing with 5 ml Tris-NaCl, the bound PLC-protein A fusion protein was eluted with lOOmM glycine pH 3.0. 450 ul fractions were collected in tubes containing 225 ul of 400 mM Tris-HCl pH 8.0. Preparations of PLC-protein A were > 90% pure as estimated by coomassie staining following SDS-polyacrylamide gel electrophoresis. Example 8 PREPARATION OF FUSION PROTEIN - ANTIBODY IMMUNOCONJUGATES Two hundred micrograms of antibody (see, e.g., commonly owned U.S.S.N. 181,862, which is incorporated herein by reference) was mixed with a four to ten-fold molar excess of PLC - protein A fusion protein in 700 ul final volume of 50 mM Tris-HCl pH 8.0, 150 mM NaCl. The mixture was incubated for 20 min at 0 C. Fusion protein linked to antibody was separated from free fusion protein by gel filtration chromatography using Superose 12 equilibrated with 50 mM Tris HC1 pH 8.0, 150 mM Nacl and connected to a Pharmacia FPLC system. Separation was confirmed by monitoring the A280 of column fractions. The final concentration of immunoconjugate was determined by hemolytic assay on sheep erythrocytes (see below) and expressed as ug/ l of PLC. Example 9 SMALL-SCALE PREPARATION OF IMMUNOCONJUGATES FOR RANDOM POSITIONER FUSION PROTEINS Overnight LB-amp cultures were used to inoculate 10ml M9-amp media supplemented with caesamino acids, to OD550 = 0.2. Cells were incubated at 37 C and harvested 1 hour after they reached an OD550 = 1.0. To extract fusion protein from the periplasmic space the cell pellet was washed in 1.0 ml 50 mM Tris HCL pH 8.0, centrifuged 8K for 25 minutes, osmotically shocked with 200 ul ice cold dH 2 0 for 10 minutes on ice and centrifuged for 10 minutes at 10K. The PLC - Protein A protein was batch purified from this shock fluid over a 10 ul rabbit IgG agarose column (Sigma) . Shock fluid was allowed to bind to the column for 1 hour at room temperature. The column was washed two times with lOOul 50 mM Tris HC1 pH 8.0, 50 mM NaCl and the bound PLC - Protein A protein was eluted with 50 ul 100 mM Glycine HC1 pH 3.0 for 5 minutes at room temperature. The collected supernatant was equilibrated with 25 ul 200 mM Tris HC1 pH 8.0. Purified proteins were quantitated by visualization on coomassie stained gels and by Bradford assay. 3.5 ug toxin and 2.1 ug anti-Tac antibody were linked on ice for 30 minutes in a 50 mM Tris HC1 pH 8.0, 150 mM NaCl, O.lmg/ml gelatin buffer. Five-fold serial dilutions were made in RPMI media. Example 10 a) ASSAYS OF CYTOTOXICITY BY TRYPAN BLUE STAINING Aliquots of approximately 1 x 10 5 CR2-2 cells in 1 ml Hanks salts were incubated in 24-well plates with dilutions of toxin or toxin linked to antibody for 90 min at 37 C. One hundred microliters of 0.5% trypan blue in PBS was added to each well and cells were scored for dye exclusion (indicating viability) under a microscope. b) ASSAY OF CYTOTOXICITY BY 3 H-LEUCINE INCORPORATION Aliquots of approximately 1 x 10 5 CR2-2 cells in 1 ml RPMI medium supplemented with 5% FCS were incubated in 24-well plates with dilutions of toxin or toxin linked to antibody for 18 hrs at 37 C. Then 5 uCi of 3 H-leucine (Amersham, 171 Ci/mmole) was added to each well and cells were incubated an additional 4 hrs. Cells from individual wells were collected by centrifugation, washed once with Hank's salts, and then lysed by resuspending in 200 ul distilled water. Protein was precipitated by addition of 800 ul cold 10% TCA. Samples were then filtered through glass fiber filters, washed with cold 10% TCA, and counted. c) ASSAY OF CYTOTOXICITY OF RANDOM POSITIONER FUSION PROTEINS LINKED TO ANTIBODY BY 3 H-THYMIDINE INCORPORATION CR2 cells at 10 5 cells/well in a 96 well microtiter dish were treated with 420 ng/ml and 84 ng/ml of fusion protein linked to anti-Tac antibody overnight at 37 C, 7.5% C02 in RPMI media. The following day 1 uCi H-thymidine was added to each well and cells were incubated 4 hours at 37 C. Cells were harvested onto glass fiber filters using a PHD cell harvester (Cambridge Technology) and incorporated label was measured. The percent incorporation was calculated as : (cpm cells plus toxin) / (cpm cells minus toxin) . d) HEMOLYTIC ASSAY OF PLC - POSITIONER - PROTEIN A FUSION PROTEINS Sheep erythrocytes were collected by gentle centrifugation, washed twice with isotonic borate buffer (130 mM NaCl, 3.5 mM H 3 B0 3 , 1.5 mM Na 2 B 4 0 7 , 1.2 mM CaCl 2 , pH 7.6), and resuspended in the same buffer to 1% (v:v) . 300 ul of the 1% erythrocyte suspension were mixed with 300 ul of fusion protein in isotonic borate buffer. The mixture was incubated first for 30 min at 37 C, then for 30 min at 0 C. Cell lysis was determined by removing whole cells by centrifugation and measuring the A550 of the supernatant. Example 11 RESULTS WITH PROLINE-GLYCINE POSITIONER FUSION PROTEINS The toxicity of PLC - protein A, PLC - Gly2Pro5Gly2 - protein A, PLC - protein A - anti-Tac conjugate, and PLC - Gly2Pro5Gly2 - protein A - anti-Tac conjugate was compared by incubating various concentrations of these proteins with CR2-2 cells, to which the anti-Tac antibody binds. After 90 min, the percent viable cells remaining was determined by trypan blue dye exclusion (Figure 3) . Cell killing was more extensive with the linked toxin than with toxin alone, and toxin conjugate containing the Gly2Pro5Gly2 positioner was most toxic. Fifty percent killing was seen with 1.25 ug/ml PLC for PLC - Gly2Pr5Gly2 - protein A alone, while PLC - protein A - anti-Tac and PLC - Gly2pro5Gly2 - protein A - anti-Tac killed 50% of target cells at 33-fold lower (37.5 ng/ml) and 180-fold lower (7ng/ml) PLC concentrations, respectively. Linking of PLC to anti-Tac did not increase toxicity against CEM cells, which do not express the IL-2 receptor. These results show that combining the anti-Tac antibody with PLC increased its toxicity specifically to a line of T cells by targeting it to them. They also show that the Gly2Pro5Gly2 positioner increased the toxicity of the PLC - protein A fusion protein when it was linked to ant-Tac, but not when it was unlinked. Similar toxicity comparisons were made with PLC - protein A - anti-Tac conjugates, where the PLC and protein A were joined by positioners with the sequences Gly2Pro5Gly2Pro5Gly2 (Figure 4) and Gly2ProlOGly2. In both cases, toxicity of the linked toxin was substantially greater than toxin alone and similar to the toxin conjugate with the Gly2Pro5Gly2 positioner. Analogous results were also obtained when cell killing was measured by 3 H-leucine incorporation rather than trypan blue exclusion (Figure 5) . However, the extent of killing was somewhat less when measured by this assay, because some protein synthesis and therefore leucine uptake can continue even after a cell is no longer viable. Example 12 RESULTS WITH THE MA 2.1 ANTIBODY To verify that these results applied to other targeting antibodies in addition to anti-Tac, the MA 2.1 antibody was used. This antibody binds to HLA molecules on the surface of, for example, JY cells (Ways, J. and Parham, P., Bioche . J. 216:423-432, (1983)). The antibody was conjugated with PLC - Gly2Pro5Gly2 - protein A fusion protein as described above. JY cells were aliquoted in wells of a microtiter dish (2 x 10 4 cells/well in RPMI medium with 5% FCS) , and incubated with dilutions of toxin and toxin linked to antibody for 20 hours at 37 C. 1 uCi of 3 H-leucine was added to each well followed by a 4 hour incubation at 37 C. About 50% cell killing, as measured by H-leucine incorporation, was seen with approximately 10 ug/ml PLC -Gly2Pro5Gly2 - protein A and with approximately 28 ng/ml PLC - Gly2Pro5Gly2 -protein A linked to the MA 2.1 antibody (Figure 6) . Hence, the linked toxin is about 350 times more toxic to JY cells than the toxin alone. Example 13 RESULTS WITH RANDOM POSITIONER FUSION PROTEINS The toxicity of conjugates of anti-Tac and different PLC - random positioner - protein A fusion proteins were determined by incubating two concentrations with CR2-2 cells and measuring cell killing by 3 H- thymidine uptake, as described above (Figure 7) . 3 H- thymidine uptake is expressed as a percentage of the value when anti-Tac antibody alone (no toxin) was incubated with the cells. The results show that some positioners produced fusion proteins that were more toxic to cells after linking to anti-Tac (less thymidine incorporation) than Pro5, notably RP7, RP19, RP25 and RP29, while others, for example RP8, were equal or less toxic than Pro5. Although the invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims.

Claims: WE CLAIM: 1. A conjugate composition capable of killing a preselected cell population comprising a polypeptide targeting component conjugated to a polypeptide toxic component, wherein the targeting component is capable of binding to a cell membrane constituent and the toxic component is capable of killing cells by damaging the cell membrane, wherein the components of the conjugate are in peptide linkage or are linked by a linker comprising one or more amino acids. 2. A conjugate composition according to Claim 1, wherein the targeting component is a growth factor. 3. A conjugate composition according to Claim 2, wherein the membrane constituent is a receptor for the growth factor. 4. A conjugate composition according to Claim 1, wherein the membrane constituent is primarily on T- cells and/or B-cells. 5. A conjugate composition according to Claim 1, wherein the targeting component is an immunoglobulin. 6. A conjugate composition according to Claim 5, wherein the immunoglobulin is a chimeric or humanized immunoglobulin. 7. A conjugate composition according to Claim 1, wherein the toxic component is an enzyme. 8. A conjugate composition according to Claim 7, wherein the enzyme is a bacterial or mammalian protein. 9. A conjugate composition according to Claim 7, wherein the enzyme is phospholipase C or phospholipase A 2- 10. A conjugate composition according to Claim 1, wherein the linker is attached to at least one of the components by a peptide bond. 11. A method of treating preselected cells in a patient comprising administering to said patient a therapeutically effective amount of a conjugate composition according to Claims l, 6 or 9. 12. A DNA construct comprising: i) a first DNA segment coding for a targeting component capable of binding to a cell surface constituent; and ii) a second DNA segment coding for a toxic component, wherein the toxic component is capable of killing cells by acting at cell membranes, wherein the DNA segments encode a single polypeptide comprising the two components. 13. A DNA construct according to Claim 12, further comprising a third DNA segment coding for at least one linker amino acid, wherein the third DNA segment is located between the first and second DNA segments. 14. A DNA construct according to Claim 12, wherein the second DNA segment encodes an enzyme. 15. A DNA construct according to Claim 14, wherein the second DNA segment encodes a phospholipase C polypeptide and is selected from the group consisting of: a) the DNA sequences of Tables II and III, or their complementary strands; b) DNA sequences, or their complementary strands, which on expression code for the polypeptide sequences of Tables II and III; and c) DNA sequences which hybridize to the DNA sequences of (a) or (b) . 16. A DNA construct according to Claims 12, 13 or 15, further comprising an expression control DNA segment operably linked to the coding segments. 17. A cell transfected with a DNA construct according to Claim 12, 13 or 15. 18. A conjugate composition capable of killing a preselected cell population which comprises a polypeptide targeting component capable of binding to a cell membrane constituent conjugated by a linker to a polypeptide toxic component capable of killing cells by damaging the cell membrane, wherein the linker comprises one or more amino acids. 19. A conjugate composition according to Claim 18, wherein the linker is in peptide linkage with at least one of the components. 20. A conjugate composition according to Claim 18, wherein the linker molecule is a polypeptide from about five to about twenty amino acids. 21. A conjugate composition according to Claim 18, wherein the linker molecule is protein A or a fragment thereof. 22. A conjugate composition according to Claim 18, wherein the targeting component is an immunoglobulin.

Inventors: CHOVNICK AMY, QUEEN CARY L, SCHNEIDER WILLIAM P, TSO J YUN, CHOVNICK, AMY, QUEEN, CARY, L., SCHNEIDER, WILLIAM, P., TSO, J., YUN
Applicants: PROTEIN DESIGN LABS INC, PROTEIN DESIGN LABS, INC.
Application year: 1988
Main classification: C07H17
IPC codes: A61K38, A61K39, A61K47, A61P35, A61P37, C07H21, C07K1, C07K14, C07K16, C07K19, C12N1, C12N9, C12N15, C12P21, C12R1