# Patent

## Title
Antibodies that specifically bind PMS2

## Abstract

Antibodies against PMS2 and PMS2-134 and cells that produce the anti-PMS2 and anti-PMS2-134 antibodies are provided. The antibodies of the invention may be used in methods for detecting a PMS2 protein, including a truncated PMS2, and in methods for detecting an abnormal condition in a patient.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This claims benefit of U.S. Provisional Application Ser. No. 60/528,269, filed Dec. 8, 2003, the entire contents of which is incorporated herein by reference.


FIELD OF THE INVENTION
The invention relates to antibodies against PMS2 and cells that produce the anti-PMS2 antibodies. The invention also relates to methods for detecting a PMS2 protein and methods for detecting an abnormal condition in a patient using the antibodies of the invention.
BACKGROUND OF THE INVENTION
PMS2 is a protein involved in mismatch repair (MMR). The process of MMR, also called mismatch proofreading, is carried out by protein complexes in cells ranging from bacteria to mammalian cells. A MMR gene is a gene that encodes for one of the proteins of such a mismatch repair complex. The MMR complex is believed to detect distortions of the DNA helix resulting from non-complementary pairing of nucleotide bases. The non-complementary base on the newer DNA strand is excised, and the excised base is replaced with the appropriate base, which is complementary to the older DNA strand. In this way, cells eliminate many mutations that occur as a result of mistakes in DNA replication.
Dominant negative alleles of mismatch repair genes have been shown to cause a MMR-defective phenotype even in the presence of a wild-type allele in the same cell. An example of a dominant negative allele of a MMR gene is the human gene hPMS2-134, which carries a truncating mutation at codon 134. The mutation causes the product of this gene to abnormally terminate at the position of the 134th amino acid, resulting in a shortened polypeptide containing the N-terminal 133 amino acids. Such a mutation causes an increase in the rate of mutations, which accumulate in cells after DNA replication. Expression of a dominant negative allele of a mismatch repair gene results in impairment of mismatch repair activity, even in the presence of the wild-type allele. Any allele which produces such effect can be used in this invention. Dominant negative alleles of a MMR gene can be obtained from the cells of humans, animals, yeast, bacteria, or other organisms.
Antibodies to detect PMS2 and truncation mutants thereof would be useful in biological assays for studying mismatch repair, and in diagnostic applications for detecting the presence of a form of PMS2 which may predispose a patient to cancer.
SUMMARY OF THE INVENTION
The invention relates to novel antibodies that specifically bind PMS2. The antibodies specifically recognize a portion of the amino-terminal portion of PMS2, such that truncation mutants of PMS2 may also be detected.
The antibodies of the invention may be used in immunological assays to detect the presence of PMS2 in a sample. The methods may also be used to detect truncated forms of PMS2. Such assays include, but are not limited to radioimmunoassay, Western blot, ELISA, immunoprecipitation, and the like.
The antibodies of the invention may be used in a method for detecting a predisposition to cancer in a patient, wherein a truncated form of PMS2 is detected in a patient sample in a screening assay and correlated to a risk of cancer in the patient.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows reactivity in a Western blot of antibody from a 349-29.5.2 cell supernate and an HRP-conjugated purified antibody from 349-29.5.2 against human PMS2-134 expressed in a human cell line.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The reference works, patents, patent applications, and scientific literature, including accession numbers to GenBank database sequences that are referred to herein establish the knowledge of those with skill in the art and are hereby incorporated by reference in their entirety to the same extent as if each was specifically and individually indicated to be incorporated by reference. Any conflict between any reference cited herein and the specific teachings of this specification shall be resolved in favor of the latter. Likewise, any conflict between an art-understood definition of a word or phrase and a definition of the word or phrase as specifically taught in this specification shall be resolved in favor of the latter.
Standard reference works setting forth the general principles of recombinant DNA technology known to those of skill in the art include Ausubel et al., CURRENT PROTOCOLS IN MOLECULAR BIOLOGY, John Wiley & Sons, New York (1998); Sambrook et al., MOLECULAR CLONING: A LABORATORY MANUAL, 2D ED., Cold Spring Harbor Laboratory Press, Plainview, N.Y. (1989); Kaufman et al., Eds., HANDBOOK OF MOLECULAR AND CELLULAR METHODS IN BIOLOGY AND MEDICINE, CRC Press, Boca Raton (1995); McPherson, Ed., DIRECTED MUTAGENESIS: A PRACTICAL APPROACH, IRL Press, Oxford (1991).
As used herein, the term “epitope” refers to the portion of an antigen to which a monoclonal antibody specifically binds.
As used herein, the term “conformational epitope” refers to a discontinuous epitope formed by a spatial relationship between amino acids of an antigen other than an unbroken series of amino acids.
As used herein, the term “about” refers to an approximation of a stated value within an acceptable range. Preferably the range is +/−5% of the stated value.
The antibodies of the invention specifically bind to PMS2 and truncated fragments thereof. The antibodies include those in which the epitope is found within the amino acid sequence of SEQ ID NO:1 or SEQ ID NO:2. In other embodiments, the epitope comprises the sequence of SEQ ID NO:1 or SEQ ID NO:2. In specific embodiments, the antibody is 349-22.1.3. In other embodiments, the antibody is 349-29.5.2.
In some embodiments the antibody is produced in a host cell other than a hybridoma cell. In these cases the antibody genes are cloned out of the hybridomas 349.22.1.3 and/or 349-29.5.2 and placed in an expression vector, operably linked to expression control sequences such that a functional antibody is produced.
Preferred antibodies and antibodies suitable for use in the methods of the invention include, for example, fully human antibodies, human antibody homologs, humanized antibody homologs, chimeric antibody homologs, Fab, Fab′, F(ab′)2 and F(v) antibody fragments, single chain antibodies, and monomers or dimers of antibody heavy or light chains or mixtures thereof.
The antibodies of the invention may include intact immunoglobulins of any isotype including types IgA, IgG, IgE, IgD, IgM (as well as subtypes thereof). The light chains of the immunoglobulin may be kappa or lambda. Class switching may be induced or may be engineered through recombinant techniques known in the art using the antibodies expressed in the hybridoma cells 349-22.1.3 and 349-29.5.2.
The antibodies of the invention include portions of intact antibodies that retain antigen-binding specificity, for example, Fab fragments, Fab′ fragments, F(ab′)2 fragments, F(v) fragments, heavy chain monomers or dimers, light chain monomers or dimers, dimers consisting of one heavy and one light chain, and the like. Thus, antigen-binding fragments as well as full-length dimeric or trimeric polypeptides derived from the above-described antibodies are themselves useful.
The expression cells of the invention include any insect expression cell line known, such as, for example, Spodoptera frugiperda cells. The expression cell lines may also be bacterial or fungal cell lines. The expression cell lines also may be yeast cell lines, such as, for example, Saccharomyces cerevisiae and Schizosaccharomyces pombe cells. The expression cells may also be mammalian cells such as, for example, Chinese hamster ovary, baby hamster kidney cells, human embryonic kidney line 293, normal dog kidney cell lines, normal cat kidney cell lines, monkey kidney cells, African green monkey kidney cells, COS cells, and non-tumorigenic mouse myoblast G8 cells, fibroblast cell lines, myeloma cell lines, mouse NIH/3T3 cells, LMTK31 cells, mouse sertoli cells, human cervical carcinoma cells, buffalo rat liver cells, human lung cells, human liver cells, mouse mammary tumor cells, TR1 cells, MRC 5 cells, and FS4 cells.
A “chimeric antibody” is an antibody produced by recombinant DNA technology in which all or part of the hinge and constant regions of an immunoglobulin light chain, heavy chain, or both, have been substituted for the corresponding regions from another animal's immunoglobulin light chain or heavy chain. In this way, the antigen-binding portion of the parent monoclonal antibody is grafted onto the backbone of another species' antibody. One approach, described in EP 0239400 to Winter et al., describes the substitution of one species' complementarity determining regions (CDRs) for those of another species, such as substituting the CDRs from human heavy and light chain immunoglobulin variable region domains with CDRs from mouse variable region domains. These altered antibodies may subsequently be combined with human immunoglobulin constant regions to form antibodies that are human except for the substituted murine CDRs which are specific for the antigen. Methods for grafting CDR regions of antibodies may be found, for example in Riechmann et al. (1988) Nature 332:323-327 and Verhoeyen et al. (1988) Science 239:1534-1536.
Chimeric antibodies were thought to circumvent the problem of eliciting an immune response in humans as chimeric antibodies contain less murine amino acid sequence. It was found that the direct use of rodent monoclonal antibodies (MAbs) as human therapeutic agents led to human anti-rodent antibody (“HARA”) responses which occurred in a significant number of patients treated with the rodent-derived antibody (Khazaeli, et al., (1994) Immunother. 15:42-52).
As a non-limiting example, a method of performing CDR grafting may be performed by sequencing the mouse heavy and light chains of the antibody of interest that binds to the target antigen (e.g., PMS2), genetically engineering the CDR DNA sequences, and imposing these amino acid sequences to corresponding human V regions by site-directed mutagenesis. Human constant region gene segments of the desired isotype are added, and the “humanized” heavy and light chain genes are co-expressed in mammalian cells to produce soluble humanized antibody. A typical expression cell is a Chinese Hamster Ovary (CHO) cell. Suitable methods for creating the chimeric antibodies may be found, for example, in Jones et al. (1986) Nature 321:522-525; Riechmann (1988) Nature 332:323-327; Queen et al. (1989) Proc. Nat. Acad. Sci. USA 86:10029; and Orlandi et al. (1989) Proc. Natl. Acad. Sci. USA 86:3833.
Further refinement of antibodies to avoid the problem of HARA responses led to the development of “humanized antibodies.” Humanized antibodies are produced by recombinant DNA technology, in which at least one of the amino acids of a human immunoglobulin light or heavy chain that is not required for antigen binding has been substituted for the corresponding amino acid from a nonhuman mammalian immunoglobulin light or heavy chain. For example, if the immunoglobulin is a mouse monoclonal antibody, at least one amino acid that is not required for antigen binding is substituted using the amino acid that is present on a corresponding human antibody in that position. Without wishing to be bound by any particular theory of operation, it is believed that the “humanization” of the monoclonal antibody inhibits human immunological reactivity against the foreign immunoglobulin molecule.
Queen et al. (1989) Proc. Nat. Acad. Sci. USA 86:10029-10033 and WO 90/07861 describe the preparation of a humanized antibody. Human and mouse variable framework regions were chosen for optimal protein sequence homology. The tertiary structure of the murine variable region was computer-modeled and superimposed on the homologous human framework to show optimal interaction of amino acid residues with the mouse CDRs. This led to the development of antibodies with improved binding affinity for antigen (which is typically decreased upon making CDR-grafted chimeric antibodies). Alternative approaches to making humanized antibodies are known in the art and are described, for example, in Tempest (1991) Biotechnology 9:266-271.
“Single chain antibodies” refer to antibodies formed by recombinant DNA techniques in which immunoglobulin heavy and light chain fragments are linked to the F(v) region via an engineered span of amino acids. Various methods of generating single chain antibodies are known, including those described in U.S. Pat. No. 4,694,778; Bird (1988) Science 242:423-442; Huston et al. (1988) Proc. Natl. Acad. Sci. USA 85:5879-5883; Ward et al. -(1989) Nature 334:54454; and Skerra et al. (1988) Science 242:1038-1041.
The antibodies of the invention may be used alone or as immunoconjugates with a label. Such labels include enzymes, biotin, radionuclides, fluorophores, chemiluminescers, paramagnetic particles, and the like. Suitable labels include, but are not limited to fluorescein, rhodamine, isothiocyanate, phycoerythrin, horseradish peroxidase, and colloidal gold.
The antibodies of the invention include derivatives that are modified, e.g., by the covalent attachment of any type of molecule to the antibody such that covalent attachment does not prevent the antibody from binding to its epitope. Examples of suitable derivatives include, but are not limited to glycosyled antibodies and fragments, acetyled antibodies and fragments, pegylated antibodies and fragments, phosphorylated antibodies and fragments, and amidated antibodies and fragments. The antibodies and derivatives thereof of the invention may themselves by derivatized by known protecting/blocking groups, proteolytic cleavage, linkage to a cellular ligand or other proteins, and the like. Further, the antibodies and derivatives thereof of the invention may contain one or more non-classical amino acids.
The monoclonal antibodies of the invention may be produced by immunizing animals with PMS2, truncated fragments thereof, or peptide fragments thereof. Animals so immunized will produce antibodies against the protein. Standard methods are known for creating monoclonal antibodies including, but are not limited to, the hybridoma technique (see Kohler & Milstein (1975) Nature 256:495-497); the trioma technique; the human B-cell hybridoma technique (see Kozbor et al. (1983) Immunol. Today 4:72) and the EBV hybridoma technique to produce human monoclonal antibodies (see Cole, et al. in MONOCLONAL ANTIBODIES AND CANCER THERAPY, Alan R. Liss, Inc., 1985, pp. 77-96).
Screening for antibodies that specifically bind to PMS2 or truncated fragments thereof may be accomplished using an enzyme-linked immunosorbent assay (ELISA) in which microtiter plates are coated with the PMS2, for example.
Confirmation of reactivity of the antibodies to PMS2, or truncated forms thereof may be accomplished, for example, using a Western Blot assay in which protein from normal patients or a patient with Hereditary Non-Polyposis Colon Cancer (HNPCC) are run on an SDS-PAGE gel under reducing and non-reducing conditions and subsequently are blotted onto a membrane. The membrane may then be probed with the putative anti-PMS2 antibodies. Appropriately-sized bands on Western indicates specificity of the antibodies and the ability to bind both full-length and truncated forms of PMS2.
The antibodies and derivatives thereof of the invention have binding affinities that include a dissociation constant (Kd) of less than 1×10−2. In some embodiments, the Kd is less than 1×10−3. In other embodiments, the Kd is less than 1×10−4. In some embodiments, the Kd is less than 1×10−5. In still other embodiments, the Kd is less than 1×10−6. In other embodiments, the Kd is less than 1×10−7. In other embodiments, the Kd is less than 1×10−8. In other embodiments, the Kd is less than 1×10−9. In other embodiments, the Kd is less than 1×10−10. In still other embodiments, the Kd is less than 1×10−11. In some embodiments, the Kd is less than 1×10−12. In other embodiments, the Kd is less than 1×10−13. In other embodiments, the Kd is less than 1×10−14. In still other embodiments, the Kd is less than 1×10−15.
Antibodies of the invention may be produced in vivo or in vitro. For in vivo antibody production, animals are generally immunized with an immunogenic portion of PMS2 (such as an immunogenic peptide of PMS2). The antigen is generally combined with an adjuvant to promote immunogenicity. Adjuvants vary according to the species used for immunization. Examples of adjuvants include, but are not limited to: Freund's complete adjuvant (“FCA”), Freund's incomplete adjuvant (“FIA”), mineral gels (e.g., aluminum hydroxide), surface active substances (e.g., lysolecithin, pluronic polyols, polyanions), peptides, oil emulsions, keyhole limpet hemocyanin (“KLH”), dinitrophenol (“DNP”), and potentially useful human adjuvants such as Bacille Calmette-Guerin (“BCG”) and corynebacterium parvum. Such adjuvants are also well known in the art.
Immunization may be accomplished using well-known procedures. The dose and immunization regimen will depend on the species of mammal immunized, its immune status, body weight, and/or calculated surface area, etc. Typically, blood serum is sampled from the immunized mammals and assayed for anti-PMS2 antibodies using appropriate screening assays as described below, for example.
Splenocytes from immunized animals may be immortalized by fusing the splenocytes (containing the antibody-producing B cells) with an immortal cell line such as a myeloma line. Typically, myeloma cell line is from the same species as the splenocyte donor. In one embodiment, the immortal cell line is sensitive to culture medium containing hypoxanthine, aminopterin and thymidine (“HAT medium”). In some embodiments, the myeloma cells are negative for Epstein-Barr virus (EBV) infection. In preferred embodiments, the myeloma cells are HAT-sensitive, EBV negative and Ig expression negative. Any suitable myeloma may be used. Murine hybridomas may be generated using mouse myeloma cell lines (e.g., the P3-NS1/1-Ag4-1, P3-x63-Ag8.653 or Sp2/O-Ag14 myeloma lines). These murine myeloma lines are available from the ATCC. These myeloma cells are fused to the donor splenocytes polyethylene glycol (“PEG”), preferably 1500 molecular weight polyethylene glycol (“PEG 1500”). Hybridoma cells resulting from the fusion are selected in HAT medium which kills unfused and unproductively fused myeloma cells. Unfused splenocytes die over a short period of time in culture. In some embodiments, the myeloma cells do not express immunoglobulin genes.
Hybridomas producing a desired antibody which are detected by screening assays, such as those described below, may be used to produce antibodies in culture or in animals. For example, the hybridoma cells may be cultured in a nutrient medium under conditions and for a time sufficient to allow the hybridoma cells to secrete the monoclonal antibodies into the culture medium. These techniques and culture media are well known by those skilled in the art. Alternatively, the hybridoma cells may be injected into the peritoneum of an unimmunized animal. The cells proliferate in the peritoneal cavity and secrete the antibody, which accumulates as ascites fluid. The ascites fluid may be withdrawn from the peritoneal cavity with a syringe as a rich source of the monoclonal antibody.
Another non-limiting method for producing human antibodies is described in U.S. Pat. No. 5,789,650 which describes transgenic mammals that produce antibodies of another species (e.g., humans) with their own endogenous immunoglobulin genes being inactivated. The genes for the heterologous antibodies are encoded by human immunoglobulin genes. The transgenes containing the unrearranged immunoglobulin encoding regions are introduced into a non-human animal. The resulting transgenic animals are capable of functionally rearranging the transgenic immunoglobulin sequences and producing a repertoire of antibodies of various isotypes encoded by human immunoglobulin genes. The B-cells from the transgenic animals are subsequently immortalized by any of a variety of methods, including fusion with an immortalizing cell line (e.g., a myeloma cell).
Antibodies against PMS2 may also be prepared in vitro using a variety of techniques known in the art. For example, but not by way of limitation, fully human monoclonal antibodies against PMS2 may be prepared by using in vitro-primed human splenocytes (Boerner et al. (1991) J. Immunol. 147:86-95).
Alternatively, for example, the antibodies of the invention may be prepared by “repertoire cloning” (Persson et al. (1991) Proc. Nat. Acad. Sci. USA 88:2432-2436; and Huang and Stollar (1991) J. Immunol. Methods 141:227-236). Further, U.S. Pat. No. 5,798,230 describes preparation of human monoclonal antibodies from human B antibody-producing B cells that are immortalized by infection with an Epstein-Barr virus that expresses Epstein-Barr virus nuclear antigen 2 (EBNA2). EBNA2, required for immortalization, is then inactivated resulting in increased antibody titers.
In another embodiment, antibodies against PMS2 are formed by in vitro immunization of peripheral blood mononuclear cells (“PBMCs”). This may be accomplished by any means known in the art, such as, for example, using methods described in the literature (Zafiropoulos et al. (1997) J. Immunological Methods 200:181-190).
In one embodiment of the invention, the procedure for in vitro immunization is supplemented with directed evolution of the hybridoma cells in which a dominant negative allele of a mismatch repair gene such as PMS1, PMS2, PMS2-134, PMSR2, PMSR3, MLH1, MLH2, MLH3, MLH4, MLH5, MLH6, PMSL9, MSH1, and MSH2 is introduced into the hybridoma cells after fusion of the splenocytes, or to the myeloma cells before fusion. Cells containing the dominant negative mutant will become hypermutable and accumulate mutations at a higher rate than untransfected control cells. A pool of the mutating cells may be screened for clones that produce higher affinity antibodies, or that produce higher titers of antibodies, or that simply grow faster or better under certain conditions. The technique for generating hypermutable cells using dominant negative alleles of mismatch repair genes is described in U.S. Pat. No. 6,146,894, issued Nov. 14, 2000. Alternatively, mismatch repair may be inhibited using the chemical inhibitors of mismatch repair described by Nicolaides et al. in WO 02/054856 “Chemical Inhibitors of Mismatch Repair” published Jul. 18, 2002. The technique for enhancing antibodies using the dominant negative alleles of mismatch repair genes or chemical inhibitors of mismatch repair may be applied to mammalian expression cells expressing cloned immunoglobulin genes as well. Cells expressing the dominant negative alleles can be “cured” in that the dominant negative allele can be turned off if inducible, eliminated from the cell, and the like, such that the cells become genetically stable once more and no longer accumulate mutations at the abnormally high rate.
The immunogen may be any PMS2, however, mammalian PMS2 is preferred. Truncated forms of PMS2 may also be used. As the N-terminus of PMS2 is highly conserved across species, in some embodiments, antibodies that recognize one species of PMS2 is expected to also recognize another species. For example, but not by way of limitation, a monoclonal antibody that binds human PMS2 (SEQ ID NO:2) in the N-terminal region may also bind the same region in mouse PMS2 (SEQ ID NO:5) and even Arabidopsis thaliana PMS2 (SEQ ID NO:6) and in the truncated human PMS2-134 (SEQ ID NO:1). The immunogen may also be immunogenic peptides of PMS2 or highly conserved peptides of PMS2. Two such peptides that may be used are: IQEFADLTQVETFGFR (SEQ ID NO:3) and ELVENSLDAGATNIDLK (SEQ ID NO:4).
The invention also provides a method for detecting an abnormal condition in a patient expressing a truncated PMS2. The method comprises contacting a test cell lysate from the patient suspected of having a defect in mismatch repair with a monoclonal antibody secreted by hybridoma cell 349-29.5.2 or 349-22.1.3 and detecting the presence or absence of a truncated form of PMS2. The presence of a truncated form of PMS2 is indicative of an abnormal condition in mismatch repair which predisposes the patient to cancer. Such cancers include, but are not limited to hereditary non-polyposis colon cancer. The presence of the truncated form of PMS2 may be detected by various means including immunoprecipitation, western blot, and ELISA.
The following Examples are provided to illustrate the present invention, and should not be construed as limiting thereof.
EXAMPLE 1
Immunogen Expression
Five milliliters of IPTG-induced (100 mM) culture of E. coli BL21(DE3) cells transformed with plasmid p-ET-k-134 (a plasmid that expresses hPMS2-134 from a T7 promoter, out of frame with His tag, NB37p46) were obtained. Expression was induced by inoculation of 1 ml (OD600=0.5) into 45 ml LB-Kan (50 mg/ml). The cells were lysed by addition of B-PER bacterial protein extraction reagent, and inclusion bodies were purified from lysates as per manufacturer's instructions. The inclusion body pellet was dissolved in 400 μl 2×LDS sample buffer, boiled 5 min, and electrophoresed 125 μl/gel, on 4 gels, of solubilized inclusion bodies in reducing 12% Bis-Tris 2-D gels in MES buffer. The gels were stained with Gelcode Blue colloidal Coomassie Blue (Pierce). Fifteen kilodalton bands were excised and sent to St. Louis University Hybridoma Facility. One gel slice was subjected to amino acid analysis. Amino acid analysis was consistent with hPMS2-134 polypeptide. Another gel slice was processed for MALDI-TOF MS analysis of trpytic peptides (NB37p72). Two peptide matches to hPMS2-134 were found upon database search (IQEFADLTQVETFGFR (SEQ ID NO:3) and ELVENSLDAGATNIDLK (SEQ ID NO:4)). For generation of hybridomas, four mice were immunized. All four were shown to be reactive to the original immunogen by Western blotting using mouse sera. Mouse #464 was chosen for lymphocyte fusion (NB70p3).
EXAMPLE 2
Cloning of a Second Bacterial Expression Construct and IMAC Purification of His-hPMS2-134
A second arabinose-inducible bacterial expression construct was made in plasmid pBAD-HisA, this time with an N-terminal 6×His tag in-frame with hPMS2-134 (NB37p1). This plasmid was designated p0126. His-tagged hPMS2-134 was purified from induced cultures of BL21 carrying p0126 by immobilized metal affinity chromatography over Talon cobalt affinity resin (Clontech, NB37p93). A single hybridoma which reacted specifically with purified hPMS2-134 (clone 349-1) was identified.
EXAMPLE 3
Screening of Murine Hybridomas and Epitope Mapping
Clone 349-1 was further subcloned by limiting dilution and screened again (NB70p8). Twelve subclones from 349-1 were tested for reactivity by Western blotting. All 12 clones were specifically reactive to bacterially produced hPMS2-134 (NB70p12). Only clone 349-1.1 was reactive towards hPMS2-134 expressed from CHO-124 or CHO-125 (CHO transfectants expressing hPMS2-134 or C-terminal V5-tagged hPMS2-134, respectively, NB70p14). A second set of three twice-subcloned hybridomas from 349-1 (349-1.2.1 through 349-1.2.3), were obtained, as well as four twice-subcloned hybridomas from 349-1 (349-1.1.1 through 349-1.1.4) and all were tested against bacterially expressed hPMS2-134. All retained reactivity against hPMS2-134. However, only clone 349-1.2.2 displayed specific reactivity towards CHO-expressed hPMS2-134. This mAb also identified a second band of Mr 120 kD from CHO lysates (putative hamster PMS2). Two hybridomas were retained from this screen (349-1.1.3 and 349-1.2.2). IgG was purified from 35 ml of culture supernate of each by protein G chromatography (NB70p44). Neither purified mAb specifically reacted with hPMS2-134 expressed in CHO.
A second round of fusion, using mouse #480, and screening was initiated (NB70p48). Seventeen hybridomas were selected (based on their reactivity towards bacterially expressed hPMS2-134 by the Yaciuk group) and tested for reactivity towards CHO-expressed hPMS2-134. None displayed specific reactivity towards hPMS2-134. Screening against bacterial hPMS2-134 was repeated. Four hybridomas (349-22, 349-25, 349-29, 349-36) were reactive (NB70p52).
Deletion studies pointed to the originally isolated mAbs (349-1.1.3 and 349-1.2.2) sharing an epitope C-terminal to residue 81, while second generation mAbs shared epitopes located between amino acids 55 and 81. Epitope mapping studies using overlapping 15-mer peptides failed to identify relevant epitopes.
Second generation hybridomas (from mouse #480) were subcloned by limiting dilution twice. Culture supernatants were tested for reactivity towards bacterial hPMS2-134. The majority displayed reactivity by Western blotting (NB71p7). Of these, clones 349-22.1.3 and 349-29.5.2 were selected for expansion. Further validation was performed. Horseradish peroxidase (HRP) conjugation to 349-29.5.2 was conducted, and the results of a Western blot probed with supernatant fluid from clone 349-29.5.2 and with HRP-conjugated 349-29.5.2 antibody is shown in FIG. 1. Each well contained increasing amounts of a human cell line expressing PMS2-134. The wells shown contained 30,000; 60,000; 90,000; and 120,000 cells/well in lanes 1, 2, 3, and 4, respectively.

## Claims

1. An isolated monoclonal antibody that specifically binds to an epitope located between amino acids 55 and 81 of SEQ ID NO:1.

2. The antibody of claim 1 wherein said antibody is conjugated to a detectable label.

3. The antibody of claim 2 wherein said label is an enzyme, biotin, a radionuclide, a fluorophore, a chemiluminescer, or a paramagnetic particle.

4. An isolated antibody-producing cell that produces the antibody of claim 1.

5. An isolated monoclonal antibody that specifically binds to an epitope located between amino acids 81 and 133 of SEQ ID NO:1.

6. The antibody of claim 5 wherein said antibody is conjugated to a detectable label.

7. The antibody of claim 6 wherein said label is an enzyme, biotin, a radionuclide, a fluorophore, a chemiluminescer, or a paramagnetic particle.

8. An isolated antibody-producing cell that produces the antibody of claim 5.