BACKGROUND OF THE INVENTION The present invention relates generally to methods for producing soluble platelet-derived growth factor receptor fragments. More particularly, it provides cell lines and methods using those cell lines, allowing for highly efficient production of purified platelet-derived growth factor receptor soluble fragments exhibiting ligand binding functions.

Polypeptide growth factors are mitogens that act upon cells by specifically binding to receptors located on the cell plasma membrane. The platelet-derived growth factor (PDGF) stimulates a diverse group of biochemical responses, e.g., changes in ion fluxes, activation of various inases, alteration of cell shape, transcription of various genes, and modulation of enzymatic activities associated with phospholipid metabolism. See, e.g., Bell et al. (1989) Circulation Research 65:1075-1065. The platelet-derived growth factor is a polypeptide factor which interacts with a membrane bound receptor, the platelet-derived growth factor receptor (PDGF-R) . The receptor has a binding site which binds the PDGF ligands. Particular medical conditions result from abnormal receptor-ligand interactions. The specificity of binding allows the use of one binding partner to determine, qualitatively or quantitatively, the presence of the other, and to detect abnormal interactions. These diagnostic reagents would be useful.

Receptors for platelet-derived growth factor have been expressed in various cells. See, e.g., Orchansky et al. (1988) J. Biol. Chem. 263:15159-15165; Duan et al. (1991) ___. Biol. Chem. 266:413-418; Heidaran et al. (1990) J. Biol. Chem. 265:18741-18744; Claesson-Welsh et al. (1988) Mol. and Cell. Biol. 8:3476-3486; and Escobedo et al. (1988) J. Biol. Chem. 263:1482-1487.

Although others have reported expressing PDGF receptors or fragments thereof in cells, the ligand binding fragments have all been of approximately intact extracellular regions of the receptor. In many instances, a smaller and soluble ligand binding segment would be useful. For example, a soluble ligand binding fragment may serve as an antagonist to modulate the effect of PDGF ligands. Antagonists which are soluble and smaller than the original receptor will be useful. The physiological bioavailability of small soluble antagonists will be better than the natural intact receptor. The intact receptor is a membrane bound protein and would typically not circulate in the blood. Soluble antagonist fragments, e.g., which are shorter than the native receptor binding site, will typically also be produced in greater quantities at lower cost. Moreover, a smaller soluble peptide is more likely to be capable of reaching remote and circulation compromised regions of the body.

Thus, a need exists for a highly efficient means for producing a purified ligand binding region, and for soluble molecules which have PDGF binding activities. Fragments smaller than the intact extracellular region of each receptor are desired. Economical and high efficiency production of PDGF ligand binding proteins, e.g., fragments containing critical ligand binding regions, is greatly desired. The present invention provides these and other needs.

SUMMARY OF THE INVENTION The present invention provides cell lines and methods for high efficiency production of soluble peptides which bind platelet-derived growth factor (PDGF) . Cell lines are described which secrete fragments of an extracellular region of a PDGF receptor (PDGF-R) , or related peptides which exhibit the activity of PDGF ligand binding. Many of these fragments result from deletion of segments of the extracellular region which do not contribute to ligand binding affinity or specificity. Methods for using these cell lines to produce PDGF-binding polypeptides, e.g., from human receptors, are also provided.

BRIEF DESCRIPTION OF THE FIGURES Fig. 1 illustrates a strategy for oligonucleotide directed in vitro deletion mutagenesis of soluble hPDGF-R extracellular domains. Many of these constructs will be soluble peptides, or can be modified to be such. The abbreviations used are:

PR = PDGF-R; intact P = PDGF-R; extracellular region TM - transmembrane K = kinase

S - signal sequence Fig. 2 illustrates the structure of a plas id derived from pcDL-Sα296 used for expressing various deletion polypeptides. Fig. 3 illustrates the structure of a plasmid pBJΔ derived from pcDLα296. See Takabe et al. (1988) Mol. Cell. Biol. 8:466-472, which is incorporated herein by reference.

1. The pcDL-SRα296 is cut with Xhol.

2. A polylinker (XhoI-Xbal-Sfil-Notl-EcoRI- EcoRV-Hindlll-Clal-Sall) is inserted into the Xhol cut vector.

3. Sail is compatible with the Xhol site; and generates both a Sail and an Xhol site.

4. The SV40 16s splice junction is no longer present.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, novel cells for producing soluble polypeptides which bind platelet-derived growth factor (PDGF) ligands and compositions comprising such fragments are provided. The subject cells contain nucleic acid sequences which encode desired polypeptide fragments. The cells will express the nucleic acids, thereby producing large amounts of the fragments. In preferred embodiments, the fragments are usually secreted into the medium and available for purification. The compositions are capable of specifically binding to platelet-derived growth factor (PDGF) ligands with affinity and specificity.

The polypeptide fragments will find use as diagnostic and therapeutic compositions. For example, they will be useful as antagonists of PDGF ligands, or as reagents for qualitative or quantitative assay of a PDGF ligand. The cells may also be used in methods for preparing large quantities of binding polypeptides, e.g., as sources of starting material for purification processes.

Analogues of the PDGF protein are referred to as PDGF ligands. These ligands typically are agonists of the PDGF-R. The PDGF ligands will usually be proteinaceous, but may be other compounds which exhibit structural features important in interaction with the receptors.

I. Cell lines The cells of the present invention are typically capable of stable cultured production of the PDGF ligand binding polypeptide. The cells will usually be eukaryotic, e.g., from a mammal which provides cells that are easily manipulated. Rodent cells, e.g., mouse, hamster or rat cells, are preferred embodiments. Cells which will process or modify the soluble proteins in manners analogous to human proteins will be preferred. Glycosylation and other protein modifications are particularly important.

In some embodiments, the cells and DNA constructs are derived from a human cell line. Especially preferred cell lines include the pΔl-5 line, which contains an expression vector for producing a type B PDGF-R polypeptide fragment, and the pΔQ.RF line, which contains an expression vector for producing a type A PDGF-R polypeptide fragment. Each of these cell lines efficiently express the appropriate polypeptide fragments.

Novel cell lines are provided herein for expressing fragments of the extracellular region of the platelet-derived growth factor receptor. These fragments exhibit unexpected properties, in part, because it has been discovered that deletions of various portions of the extracellular region of the PDGF-R do not affect ligand binding. Particular segments of the extracellular region of the receptor have been

identified which do not significantly contribute to ligand binding affinity or specificity. Deletion of these extraneous segments of the PDGF-R extracellular region increase, the solubility of the shortened polypeptide and decrease its immunogenicity. Moreover, higher efficiency production and lower cost provide significant commercial advantages.

The cell products will typically be soluble fragments of human PDGF receptor polypeptides. Type B or type A receptors fragments will be produced. By virtue of their human origin, the fragments introduced into human subjects should exhibit low immunogenicity and function as more effective therapeutic reagents. Their homology to natural human proteins should decrease the likelihood of adverse reactions and side effects from therapeutic administration. The ligand binding regions (LBRs) are defined, in part, by their effect on the affinity or specificity of binding to PDGF ligands. The natural, native full length PDGF-R binds its natural ligand with a Kd of about 0.2 mM. See, e.g., Duan et al. (1991) J. Biol. Chem. 266:413-418, which is hereby incorporated herein by reference. A ligand binding region is a segment of the polypeptide whose presence significantly affects ligand binding, e.g. , absolute affinity and specificity. Affinity will usually be affected by a factor of at least about two, typically by a least a factor of about four, more typically by at least a factor of about eight, and preferably by at least about a factor of twelve or more. Measures for specificity are more difficult to quantitate, but will typically be evaluated by comparison to comparative affinity to ligands exhibiting similar structural features. Preparation of mammalian cells of the present invention can be accomplished by standard methods of transforming many different types of mammalian cells with appropriate expression vectors. See, e.g., Ausubel et al. (1987 and supplements) Current Protocols in Molecular Biology, Greene Publishing/Wiley-Interscience, New York, which is hereby incorporated herein by reference. Proper selection of a combination of cellular properties and expression vector

properties can lead to improved methods of producing the desired platelet-derived growth factor receptor fragment.

Various methods are available for expressing defined proteins at high level. Amplification methods similar to those using dehydrofolate reductase (DHFR) can be applied. See, e.g., Kaufman et al. (1985) Mol. Cell. Biol. 5:1750-1759. Other well known high expression techniques will also be applicable.

These cells are particularly selected for high level expression of desired receptor fragments. Cell lines resulting from transformation with vectors encoding the desired peptides are provided. Different isolates of transformants will have different copy numbers and integration sites resulting in differential expression levels. Favorable cell strains will have integrated DNA in positions and numbers providing particularly high receptor fragment expression.

With respect to the constructs, PΔl through PΔ9 refer to DNA constructs of the type B receptor inserted into cloning sites in the expression vectors. The PΔI construct inserted into the pBJ vector is designated pBJPΔ. The vector is transferred into a specific cell background and various clonal transformants are isolated and designated, e.g., pΔl-1, pΔl-2, etc. Each clonal isolate differs from others by sequence copy number and integration sites, with resultant variability in expression levels. The pΔl-5 is a particularly useful cell line embodiment expressing high levels of type B ligand binding regions.

The pΔαRF cell line expresses the entire extracellular region of a human type A PDGF receptor. The encoding DNA sequence was introduced into a pBJ-1. CHO cells were transformed with the resulting DNA construct and selected for both expression of the neo plasmid and for production of a type A receptor fragment.

II. Methods

The present invention also provides methods for" producing the described fragments. In particular, cell cultures are available to express the nucleic acids described.

Usually, the fragments are secreted thereby considerably simplifying purification of the receptor fragments. The cells need not be disrupted and cellular contamination is minimized. Thus, the cells will be separable from the secreted products by physical techniques while allowing recovery of the intact cells. See, e.g., Ausubel et al. (1987 and supplements) Current Protocols in Molecular Biology. Greene/Wiley- Interscience, New York, especially section 10:Vii.

Usually, the soluble proteins will be secreted, and will be susceptible to recovery from the medium. Various techniques will be available for separating the soluble proteins in the media from the cells, e.g., filtration or centrifugation. Cell cultures attached to solid substrates will be easily separable from the medium by filtration or centrifugation, while suspension cultures of fragile cells will usually be subjected to centrifugation.

Standard methods for protein purification will be used, e.g., chromatography, centrifugation, precipitations, electrophoresis, immunoaffinity methods, and other techniques well known to protein chemists and enzy ologists. See, e.g., Deutscher et al. (1990) Protein Purification, in Methods in Enzvmology; and Ausubel et al. (1987 and supplements) Current Protocols in Molecular Biology.

Particularly useful purification reagents include affinity reagents, e.g., either PDGF-ligand affinity columns or immunoaffinity columns. A PDGF-ligand affinity column will be readily prepared using a cloned PDGF-ligand sequence or analog for isolation of the protein product, and attachment to a solid substrate. An immunoaffinity column will be readily prepared by attaching immunoglobulins prepared against PDGF-R peptides, either produced by the cells of the invention, or by other methods.

PDGF-receptor specific monoclonal antibodies with binding affinities of 10 ⁵ M ^"1 , preferably 10 ⁷ to 10 ¹⁰ , or stronger, will typically be made by standard procedures as described, e.g., in Harlow and Lane, Antibodies: A Laboratory Manual, CSH Laboratory (1988) ; or Goding, Monoclonal Antibodies: Principles and Practice (2d ed.) Academic Press,

New York (1986) , which are hereby incorporated herein by reference. Briefly, appropriate animals will be immunized. After the appropriate period of time, the spleens of such animals are excised and individual spleen cells fused, typically, to immortalized myeloma cells. The fusion products are subjected to appropriate selection conditions and clonally separated. The supernatants of each clone are tested for antibody specific for binding the desired region of the antigen. Other suitable techniques involve in vitro exposure of lymphocytes to the antigenic polypeptides or alternatively to selection of libraries of antibodies in phage or similar vectors. See. Huse et al. (1989) "Generation of a Large Combinatorial Library of the Immunoglobulin Repertoire in Phage Lambda," Science 246:1275-1281; and Ward et al. (1989) "Binding activities of a repertoire of single immunoglobulin variable domains secreted from Escherichia coli _f " Nature 341:544-546; each of which, is hereby incorporated herein by reference. The polypeptides and antibodies of the present invention may be used with or without modification.

The appropriate affinity reagent, e.g., the PDGF or antibody, will usually be attached to a solid substrate. Typical substrates include chromatography matrices , e.g., CNBr- sepharose, glass beads, and plastics. Usually, the attachment will be covalent, though non-covalent attachment methods may, under certain conditions, be sufficient.

Once the appropriate affinity reagent, or combination of affinity reagents is prepared, solutions containing the soluble fragments will be passed though the affinity reagent for specific binding and elution. These steps may be interspersed with other purification or concentration steps. A particularly useful fragment purification method incorporates an immunoaffinity substrate comprising monoclonal antibodies. The receptor fragments are attached to immobilized antibodies in the column and eluted at high pH. There may also be incorporated additional steps for specific removal of identified contaminating components, e.g., by affinity removal techniques.

Isolated soluble peptide fragments may be used for various purposes, e.g., as diagnostic reagents or for therapeutic reagents. The cells will be useful for transfer into an animal, e.g., into a mouse, or for culturing in an appropriate implantation container, which will allow for diffusion of the product into the body of a host organism. Appropriate cell culturing apparatuses which can be implanted into a subject to secrete therapeutic cell products are described, e.g., in U.S. Pat. No. 4,806,355; 4,402,694; and 3,093,831.

EXPERIMENTAL In general, standard techniques of recombinant DNA technology are described in various publications, e.g., Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory; Ausubel et al. (1987) Current Protocols in Molecular Biology, vols. 1 and 2 and supplements; and Wu and Grossman (eds.) (1987) Methods in Enzymology. Vol. 53 (Recombinant DNA Part D) ; each of which is incorporated herein by reference.

I. Human Extracellular Region

Equivalent techniques for construction, expression, and determination of the physiological effect of truncation or deletion analogues of the soluble extracellular receptor fragments from the human receptor may be performed using the nucleic acid, polypeptide, and other reagents provided herein.

A. Type B Segments Constructs of type B receptor polypeptides were made as follows:

The 3.9 kb EcoRI-Hind III cDNA fragment of the human type B hPDGF-R was subcloned into the EcoRI-Hind III site of M13 Mpl8 to produce a vector Mpl8PR. For techniques, see Maniatis et al. (1982) Molecular Cloning: A Laboratory Manual. Cold Spring Harbor, N.Y., which is incorporated herein by reference. Verification of subcloning was performed by restriction enzyme digestion analysis and dideoxy chain

termination sequencing, as described by Sanger et al. (1977) Proc. Nat'l Acad. Sci. USA 74:5463. Oligonucleotide directed in vitro mutagenesis was performed according to the method described by Kunkel et al. (1987) Methods in Enzvmol.. 154:367. The strategy for oligonucleotide directed in vitro deletion mutagenesis of MplδPR is outlined in Fig. 1.

In brief, a series of oligonucleotides were designed to create a nested set of soluble type B hPDGF receptor extracellular regions by deletion mutagenesis. Intact domains have been deleted. These domains are designated Domain 1 through Domain 5 (D1-D5) , suitable for expression in an appropriate eukaryotic expression system. Exemplary mutagenic oligonucleotides are listed in Table 1. These oligonucleotides are complementary to regions of the human PDGF receptor spanning various defined IgG-like domains. See, e.g., Williams (1989) Science 243:1564-1570, which is hereby incorporated herein by reference.

TABLE 1

HUMAN B-type PDGF-R MUTAGENESIS OLIGOMERS

[note that these are complementary; given 3 '-5']

PΔl

3'-GTG TGA GGA ACG GGA AAT TCA TCG AAG GAC ATC CCC CGA-5' (SEQ ID NO:l)

PΔ2

3 '-GGA AGC TGG ATG TCT AGT TAA TCG AAG GAC ATC CCC CGA C-5 ^• (SEQ ID NO:2)

PΔ3

3 '-TAG TGG CAC CAA CTC TCG CCG ATC GAA GGA CAT CCC CCG AC-5 ^• (SEQ ID

NO:3) PΔ4

3 '-ATG TCT GAG GTC CAC AGT AGG ATC GAA GGA CAT CCC CCG AC-5 ' (SEQ ID NO:4)

P 5 3'-GAG ATG TAG AAA CAC GGT CTA GGG ATC GAA GGA CAT CCC CCG AC-5 • (SE ID NO:5)

PΔ6

3'-GTC TAG AGA GTC CCG GAC CAG TGG CAC CCG AAG GAG GGA TTA GTA-5' (SE ID NO:6)

PΔ7

3 '-GTC TAG AGA GTC CCG GAC CAG TAG TTG CAG AGA CAC TTG CGT CAC GTC-5 • (SEQ ID NO:7)

PΔ8

3'-GTC TAG AGA GTC CCG GAC CAG ATG CAC GCC GAG GAC CCT CTC GAC-5 ^• (SE

ID NO:8) PΔ9

3'-GTC TAG AGA GTC CCG GAC CAG CAG GCT CAC GAC CTC GAT TCA-5 ^• (SEQ ID NO:9)

The resulting constructs are labeled as indicated in Table 2. The antisense strand was used for mutagenesis throughout. Mutagenesis of PΔI, PΔ2, PΔ3, PΔ4, and PΔ5 utilized MplδPR as the template and mutagenesis of PΔ6, PΔ7, PΔ8, and PΔ9 utilized Mpl8 PΔI as the template. PΔI, a 41 bp oligomer, introduced a TAG stop codon after Lysine _4g9 (K ₄₉₉ ) of D5 and removed the transmembrane (TM) as well as entire intracellular kinase domain (K) , producing an Mpl8 PΔI (see Fig. 1) .

TABLE 2 HUMAN TYPE B PDGF-R EXPRESSION CONSTRUCTS

The human PDGF receptor constructs were subsequently subcloned into the EcoRI-Hind III site of pBJl a derivation of pCDL-SRα296, as described in Takabe et al. (1988) Molec. Cell Biol. 8:466, and co-transfected with pSV2NEO, as described by Southern and Berg (1982) J. Mol. Appl. Gen.. l: 327, into Chinese hamster ovary cells (CHO) . See. Figs. 2 and 3.

Function of the purified or semipurified constructs was demonstrated as follows:

A sample of 0.33 nM PDGF BB ligand is preincubated for 1 hr at 4°C under the following conditions:

1. a polyclonal antibody to human PDGF (this antibody recognizes human PDGF AA, PDGF BB and PDGF AB) ;

2. 18 nM (60 fold molar excess to PDGF BB) human type B PDGF receptor; 3. phosphate buffered saline solution that the receptor and antibody are in; or

4. no additions but the ligand itself.

In a duplicate set of experiments, 0.33 nM PDGF AA is incubated with three of the above preincubation conditions, e.g., 2, 3, and 4 above. The human type B PDGF receptor does not appreciably recognize PDGF AA but this ligand will still activate cell-associated human type A PDGF receptor from NIH3T3 cells and so is a control for human type B PDGF receptor specificity and PDGF BB-dependent activation versus non¬ specific general cellular effect, e.g., cytotoxicity. The preincubated materials were in a final volume of

0.5 ml. They were placed in one well each of a six well tissue culture dish containing a confluent layer of serum starved (quiescent) NIH3T3 cells which were chilled to 4°C. The cells and incubation mixtures were agitated, e.g., rocked, at 4°C for 2 h. They were then washed twice with 4°C phosphate buffered saline. Forty μl of 125 mM Tris(hydroxymethyl)amino methane (Tris), pH 6.8, 20% (v/v) glycerol, 2% (w/v) sodium dodecyl sulfate (SDS) , 2% (v/v) 2-mercaptoethanol, and 0.001% bromphenol blue, (known as SDS sample buffer) , was added per microtiter well followed by 40 μl of 100 mM Tris, pH 8.0, 30 mM sodium pyrosphosphate , 50 mM sodium fluoride, 5 mM ethylenediaminetetraacetic acid (EDTA) , 5 mM ethylenebis(oxyethylenenitrilio)tetraacetic acid, 1% (w/v) SDS, 100 mM dithiothreitol, 2 mM phenyl ethylsulfonylfluoride (PMSF) , and 200 μM sodium vanadate was added to the cells. The cells were solubilized and 40 μl additional SDS sample buffer was added to the solubilizate. This material was boiled 5 minutes and loaded onto a single gel sample well of a 7.5% sodium dodecyl sulfate polyacrylamide gel. Cellular proteins were separated by electrophoresis.

The separated proteins were transferred to nitrocellulose by electrotransfer and the resulting "Western blot" was incubated with 3 changes of 0.5% (w/v) sodium chloride, 5 mg/ml bovine serum albumin, 50 mM Tris, pH 7.5, (designated blocking buffer) for 20 minutes each at room temperature. A 1/1000 dilution of PY20 (a commercially available monoclonal antibody to phosphotyrosine [ICN]) in blocking buffer was incubated with the blot overnight at 4°C.

The blot was washed 3 times for 20 minutes each at room temperature in blocking buffer. The blot was incubated with 4 μCi/40 ml of ¹²⁵ I-Protein A [Amersham] in blocking buffer for 1 hour at room temperature and washed 3 times for 20 minutes each at room temperature in blocking buffer. The blot was exposed to X-ray film for 48 h with one intensifying screen at -70°C and developed with standard reagents.

B. Type A Sequence Similar manipulations may be performed using either mutagenic oligonucleotides or PCR primers based upon the sequences presented in Table 3. Appropriate oligonucleotides are used to construct type A constructs, as listed in Table 4, which can be functionally tested by various formats as described in the type B assays detailed above.

TABLE 3 PROPOSED HUMAN A-type PDGF-R MUTAGENESIS OLIGOMERS [note that these are complementary; given 3'-5']

PΔIOI

3 ^» -CGA GGG TGG GAC GCA AGA CTT ATT GAC CGC CTA AGC TCC CC-5' (SEQ ID NO:10)

PΔ102

3'-CTT GAC AAT TGA GTT CAA GGA ATT GAC CGC CTA AGC TCC CC-5' (SEQ ID NO:11)

PΔ103

3 '-TAA AGA CAG GTA CTC TTT CCA ATT GAC CGC CTA AGC TCC CC-5 ¹ (SEQ ID

NO:12) PΔ104

3 '-ATA CGA AAT TTT CGT TGT AGT ATT GAC CGC CTA AGC TCC CC-5' (SEQ ID NO:13)

PΔ105 3 '-TAA ATG TAG ATA CAC GGT CTG GGT ATT GAC CGC CTA AGC TCC CC-5' (SEQ ID NO:14)

PΔ106

3 '-TCG GAT TAG GAG ACG GTC GAA CTA CAT CGG AAA CAT GGA GAT CCT-5 • (SE ID NO:15)

PΔ107

3 '-TCG GAT TAG GAG ACG GTC GAA CTC GAC CTA GAT CTT TAC CTT CGA GAA-5 • (SEQ ID NO:16)

PΔ108

3*-TCG GAT TAG GAG ACG GTC GAA AAG TAA CTT TAG TTT GGG TGG AAG-5 • (SE

ID NO:17) PΔ109

3'-TCG GAT TAG GAG ACG GTC GAA AGT AGG TAA GAC CTG AAC CAG-5' (SEQ ID NO:18)

Table 4

SUGGESTED HUMAN TYPE A PDGF-R EXPRESSION CONSTRUCTS type A

Soluble Membrane Bound pARSR pARSΔl pARSΔ2 pARSΔ3 pARSΔ4 pARSΔ5 pARSΔβ

PARSΔ7 pARSΔδ pARSΔ9

C. PDGF Plate Assay

Polystyrene microtiter plates (Immulon, Dynatech Laboratories) were coated with the extracellular region fragment of the type B human PDGF receptor (described above) by incubating approximately 10-100 ng of this purified protein per well in 100 μl of 25 mM Tris, 75 mM NaCl, pH 7.75 for 12 to 18 h at 4°C.

The protein was. expressed in transfected CHO cells and collected in serum-free media (Gibco MEMα) at a concentration of 0.2 - 1 μg/ml, with a total protein concentration of 150 - 300 μg/ml. The human PDGF type B receptor extracellular region fragment was concentrated and partially purified by passing the media over wheat germ- agglutinin-sepharose at 4°C (at 48 ml/h) in the presence of 1 mM PMSF. After extensive washing, the protein was eluted in 0.3 M N-acetyl-glucosamine, 25 mM Hepes, 100 mM NaCl, 1 mM PMSF, pH 7.4. This fraction was then applied to Sephacryl S-200 HR (Pharmacia) equilibrated in 0.15 M ammonium bicarbonate pH 7.9. The fractions containing receptor (3 - 10 ng/μl) were detected by SDS-PAGE and Western blotting with a polyclonal rabbit antibody, made by standard methods, against a Domain 1 (Dl) segment from the receptor external region. These fractions (3 - 10 ng/μl) were used to coat the microtiter wells as described above. The wells were then drained, rinsed once

with 200 μl each of 0.5% gelatin (Bio-Rad, EIA grade), 25 mM Hepes, 100 mM NaCl, pH 7.4, and incubated for 1-2 h at 24°C with 150 μl of this same solution. The wells were drained and rinsed twice with 0.3% gelatin, 25 mM Hepes, 100 mM NaCl, pH 7.4 (150 μl each). 90 μl of the 0.3% gelatin solution was put in each well (wells used to test nonspecific binding received just 80 μl and then 10 μl of 0.01 mg/ml non-labeled PDGF in the 0.3% gelatin solution). PDGF BB (A gen) was iodinated at 4°C to 52,000 CPM/ng with di-iodo Bolton-Hunter reagent (Amersham) and approximately 40,000 CPM was added per well in 10 μl, containing 0.024% BSA, 0.4% gelatin, 20 mM Hepes, 80 mM NaCl, 70 mM acetic acid, pH 7.4. The plate was incubated for 2-3 h at 24°C, after which wells were washed three times with 150 μl each with 0.3% gelatin, 25 mM Hepes, 100 mM NaCl, pH 7.4. The bound radioactivity remaining was solubilized from the wells in 200 μl 1% SDS, 0.5% BSA, and counted in a gamma-counter. The nonspecific binding was determined in the presence of a 150-fold excess of unlabeled PDGF BB (Amgen) and was about 7% of the total bound ¹²⁵ I-PDGF. Similar assays will be possible using type A receptor fragments. However, the type A receptor fragments are more sensitive to the presence of other proteins than the type B fragments, and appear to require a different well coating reagent from the gelatin. Hemoglobin is substituted for gelatin in the buffers at 1 part per 1000.

The present assays require less than 500 ng/well of receptor soluble form, which was expressed in transfected CHO cells, and partially purified by affinity and gel chromatography. Using iodinated PDGF-BB, the specific binding of less than 10 pg of ligand can be detected in an assay volume of 100 μg/well. At 4°C, the binding of ¹²⁵ I-PDGF BB to immobilized receptor is saturable and of high affinity. The Kd by Scatchard analysis was about 1 nM with 1.8 x lθ ¹⁰ sites per well. The nonspecific binding, determined in the presence of a 100-fold excess of cold PDGF BB, was usually only about 5-10% of the total binding. The binding was also specific for the isoform of the ligand, insofar as excess cold PDGF AA did not inhibit ¹²⁵ I-PDGF BB binding. Furthermore, the external region

of the type B PDGF receptor in solution competes with its immobilized form for binding iodinated PDGF BB (IC ₅₀ = 5nM) . The ¹²⁵ I-PDGF BB bound after 4 h at 4°C is only slowly dissociable in binding buffer (t _1/2 > 6 h) , but is completely displaced by the addition of a 150-fold excess of unlabeled PDGF BB (t _1/2 < 1 h) .

D. Purification of hPDGF-R fragments

Type B human PDGF-R fragment was prepared from cells disclosed herein. Secreted fragments were purified by wheat germ agglutinin affinity chromatography and by S-200 sizing chromatography. Purity was evaluated by SDS-PAGE and estimated to be about 70% pure.

Type A human PDGF-R fragment was also prepared from cells disclosed herein. Secreted fragments were purified by ion exchange chromatography, wheat germ agglutinin affinity chromatography, and S200 sizing chromatography. Purity was evaluated by SDS-PAGE and estimated to be about 70% pure.

E. Preparation of Antibodies

Antibodies are prepared using standard techniques. See, e.g., Harlow and Lane (1990) Antibodies: A Laboratory Manual, Cold Spring Harbor Press, New York; and Goding (1986) Monoclonal Antibodies. Principles and Practice, each of which is hereby incorporated herein by reference. Either polyclonal or monoclonal antibodies will be prepared.

Two Balb/C mice were immunized for preparation of monoclonal antibodies specific for each of type B and type A receptor fragments. Each mouse was injected intraperitoneally with about 100 μg of the appropriate purified receptor fragment. Each mouse was boosted with about 100 μg of the protein. The mouse was sacrificed and its spleen recovered. The spleen was disrupted and cells fused to myeloma line P3X using polyethylene glycol (PEG) . Cell fusion products were distributed into microtiter plates at limiting dilutions to clonally isolate antibody producing hybridomas. Antibody producing clones were identified by an enzyme-linked immunosorbent assay (ELISA) .

Selected antibody producing clones were grown to sufficient numbers and introduced into a mouse peritoneum to produce ascites fluid. Typically about 1-1.5 x 10 ⁶ cells were injected into a mouse in 1 ml. Antibodies from ascites fluid were purified by standard methods.

Resulting antibodies were characterized for their binding specificity and binding affinity. Clone IC705 is a preferred monoclonal antibody with type B receptor fragment binding specificity; and clone 1H-2H8 is a preferred monoclonal antibody with type A receptor fragment binding specificity.

F. Preparation of Fragments using Cells and Antibodies Appropriate receptor fragments will be easily prepared using the cells disclosed herein. As an example, PΔI-5 cells were grown in hollow fiber bioreactors in a 5 liter culture. Supernatant media was removed by centrifugation and remaining cells were removed by filtration. The media was then subjected to ultrafiltration and passed over a 50 cm monoclonal antibody-Tresyl agarose immunoaffinity column. The column was washed and the bound receptor fragments eluted by high pH. The purified soluble receptor fragment was at least about 97% pure with a yield of about 50 mg from 5 liters of cell supernatant. These studies were made possible by the availability of growth factor preparations substantially devoid of contamination with other growth factors and by the use of a receptor expression system in which all of the measured PDGF responses could be attributed to this single transfected receptor cDNA.

All publications and patent applications herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the appended claims.

SEQUENCE LISTING

(1) GENERAL INFORMATION:

(i) APPLICANT: Wolf, David

Tomlinson, James E.

(ii) TITLE OF INVENTION: Methods for Production of Purified Soluble Type B and Type A Human Platelet-Derived Growth

Factor Receptor Fragments

(iii) NUMBER OF SEQUENCES: 18

(iv) CORRESPONDENCE ADDRESS:

(A) ADDRESSEE: William M. Smith

(B) STREET: One Market Plaza, Steuart Tower, Suite 2000

(C) CITY: San Francisco

(D) STATE: California

(E) COUNTRY: USA

(F) ZIP: 94105

(V) COMPUTER READABLE FORM:

(A) MEDIUM TYPE: Floppy disk

(B) COMPUTER: IBM PC compatible

(C) OPERATING SYSTEM: PC-DOS/MS-DOS

(D) SOFTWARE: Patentln Release #1.0, Version #1.25

(vi) CURRENT APPLICATION DATA:

(A) APPLICATION NUMBER: PCT

(B) FILING DATE:

(C) CLASSIFICATION:

( ii) PRIOR APPLICATION DATA:

(A) APPLICATION NUMBER: US 07/801,794

(B) FILING DATE: 02-DEC-1991

(viii) ATTORNEY/ GENT INFORMATION:

(A) NAME: Smith, William M.

(B) REGISTRATION NUMBER: 30,223

(C) REFERENCE/DOCKET NUMBER: 12418-15

(ix) TELECOMMUNICATION INFORMATION:

(A) TELEPHONE: 415-326-2400

(B) TELEFAX: 415-326-2422

(2) INFORMATION FOR SEQ ID NO:l:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 39 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:l:

AGCCCCCTAC AGGAAGCTAC TTAAAGGGCA AGGAGTGTG 39

(2) INFORMATION FOR SEQ ID NO:2:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 39 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:

AGCCCCCTAC AGGAAGCTAA TTGATCTGTA GGTCGAAGG 39

(2) INFORMATION FOR SEQ ID NO:3:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 41 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:

CAGCCCCCTA CAGGAAGCTA GCCGCTCTCA ACCACGGTGA T 41

(2) INFORMATION FOR SEQ ID NO:4:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 41 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(Xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:

CAGCCCCCTA CAGGAAGCTA GGATGACACC TGGAGTCTGT A 41

(2) INFORMATION FOR SEQ ID NO:5:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 44 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(Xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:

GAGCCCCCTA CAGGAAGCTA GGGATCTGGC ACAAAGATGT AGAG 44

(2) INFORMATION FOR SEQ ID NO:6:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 45 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:

ATGATTAGGG AGGAAGCCCA CGGTGACCAG GCCCTGAGAG ATCTG 45

(2) INFORMATION FOR SEQ ID NO:7:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 48 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(Xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:

CTGCACTGCG TTCACAGAGA CGTTGATGAC CAGGCCCTGA GAGATCTG 48

(2) INFORMATION FOR SEQ ID NO:8:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 45 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

( i) SEQUENCE DESCRIPTION: SEQ ID NO:8:

CAGCTCTCCC AGGAGCCGCA CGTAGACCAG GCCCTGAGAG ATCTG 45

(2) INFORMATION FOR SEQ ID NO:9:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 42 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:

ACTTAGCTCC AGCACTCGGA CGACCAGGCC CTGAGAGATC TG 42

30

(2) INFORMATION FOR SEQ ID NO:10:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 41 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

( _X i) SEQUENCE DESCRIPTION: SEQ ID NO:10:

CCCCTCGAAT CCGCCAGTTA TTCAGAACGC AGGGTGGGAG C 41

(2) INFORMATION FOR SEQ ID NO:11:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 41 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:

CCCCTCGAAT CCGCCAGTTA AGGAACTTGA GTTAACAGTT C 41

(2) INFORMATION FOR SEQ ID NO:12:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 41 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:

CCCCTCGAAT CCGCCAGTTA ACCTTTCTCA TGGACAGAAA T 41

(2) INFORMATION FOR SEQ ID NO:13:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 41 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(Xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:

CCCCTCGAAT CCGCCAGTTA TGATGTTGCT TTTAAAGCAT A 41

(2) INFORMATION FOR SEQ ID NO:14:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 44 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(Xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:

CCCCTCGAAT CCGCCAGTTA TGGGTCTGGC ACATAGATGT AAAT 44

(2) INFORMATION FOR SEQ ID NO:15:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 45 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:

TCCTAGAGGT ACAAAGGCTA CATCAAGCTG GCAGAGGATT AGGCT 45

(2) INFORMATION FOR SEQ ID NO:16:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 49 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

( i) SEQUENCE DESCRIPTION: SEQ ID NO:16:

AAGAGCTTCC ATTTCTAGAT CCGACTCCAA GCTGGCAGAG GATTAGGCT 49

(2) INFORMATION FOR SEQ ID NO:17:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 45 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

(xi) SEQUENCE DESCRIPTION: SEQ ID NO:17:

GAAGGTGGGT TTGATTTCAA TGAAAAGCTC GCAGAGGATT AGGCT 45

(2) INFORMATION FOR SEQ ID NO:18:

(i) SEQUENCE CHARACTERISTICS:

(A) LENGTH: 42 base pairs

(B) TYPE: nucleic acid

(C) STRANDEDNESS: single

(D) TOPOLOGY: linear

(ii) MOLECULE TYPE: DNA (genomic)

( i) SEQUENCE DESCR8PTION: SEQ ID NO:18:

GACCAAGTCC AGAATGGATG AAAGCTCGCA GAGGATTAGG CT 42