Abstract:
Monoclonal antibodies produced by a family of murine hybrid cell lines are cross-reactive with both toxin A and toxin B of Clostridium difficile. The monoclonal antibodies are useful in an immunoassay for toxin A and toxin B of C. difficile, a casual agent of antibiotic-associated pseudomembranous colitis in humans and animals.

Description:
BACKGROUND OF THE INVENTION 
     Clostridium difficile, the etiological agent of pseudomembranous colitis in humans and animals, produces two toxins, designated toxin A and toxin B, that are cytotoxic for tissue-cultured mammalian cells. Toxin B is approximately 1,000-fold more cytotoxic than toxin A per mg of protein, however. In addition to its cytotoxicity, toxin A also possesses enterotoxin activity and causes a fluid response when injected into ligated rabbit intestinal loops. 
     While toxin A and toxin B can be quantitated by their activity against tissue culture cells, the greater activity against tissue culture cells of toxin B interferes with the detection of toxin A by this assay. As a consequence, the two toxins have to be separated before the assay to determine the cytotoxic titer of toxin A. The different activities of toxins A and B in the rabbit intestinal loop and suckling mouse assays indicate that these assays might be useful in detecting toxin A; however, both assays are tedious and not very sensitive. An enzyme-linked immunosorbent assay (ELISA) using affinity-purified antibody against toxin A has been developed which is said to be specific for toxin A and does not require an initial separation of toxins A and B. The sensitivity of the disclosed ELISA procedure for tissue culture-positive fecal specimens is only 59%, but more sensitive enzyme immunoassays for toxin A and toxin B, respectively, have been reported. Laughon et al, J. Infectious Diseases 149: 781-88 (1984). Aronsson et al also discloses ELISA&#39;s for either toxin A or toxin B. &#34;Enzyme immunoassay for detection of Clostridium difficile toxins A and B in patients with antibiotic-associated diarrhoea and colitis,&#34; Eur. J. Clin. Microbiol. 4: 102-07 (1985). 
     Lyerly et al have compared monoclonal antibody, affinity-purified polyclonal antibody, and monospecific antiserum against toxin A by counterimmunoelectrophoresis, latex agglutination, and ELISA. J. Clin. Microbiol. 21: 12-14 (1985). But a single monoclonal antibody-based assay sensitive to both toxin A and toxin B, without the above-mentioned interference effect, has apparently not been reported. Indeed, the absence of reported polyclonal antibody cross-reactivity between the toxins suggested that a cross-reactive monoclonal antibody might not exist. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide fused somatic cell lines that produce monoclonal antibodies displaying immunological cross-reactivity between toxin A and toxin B. 
     It is another object of the present invention to provide monoclonal antibodies that recognize toxin A and toxin B of C. difficile. 
     It is yet another object of the present invention to provide a single immunoassay that can detect the presence in a sample of toxin A, toxin B, or both toxins together. 
     In accomplishing the foregoing objects, there have been provided, in accordance with one aspect of the present invention, murine monoclonal antibodies that are cross-reactive with both toxin A and toxin B of Clostridium difficile. In a preferred embodiment, the monoclonal antibodies of the present invention have an A/B reactivity ratio, as measured by radioimmunoassay, of between about 0.5 and about 6.9. In another preferred embodiment, the monoclonal antibodies of the present invention are produced by continuous hybrid cell lines which are the product of a process comprising the step of fusing (i) mouse spleen cells from a mouse immunized with a mixture comprising cytotoxically neutralized toxin A and toxin B and (ii) mouse myeloma cells. 
     In accordance with another aspect of the present invention, there have been provided continuous hybrid cell lines that produce murine monoclonal antibodies which are cross-reactive with both toxin A and toxin B of Clostridium difficile. 
     In accordance with yet another aspect of the present invention, there has been provided an immunoassay for the presence of Clostridium difficile which utilizes murine monoclonal antibodies that are cross-reactive with C. difficile toxins A and B. 
     Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are graphs showing the amount of mouse polyclonal antibody that is bound by varying concentrations of C. difficile toxin A and toxin B, respectively. 
     FIGS. 2A, 2B, 2C, and 2D are graphs showing the amounts of varying dilutions of monoclonal antibodies within the present invention that are bound by 380 ng of C. difficile toxin A and toxin B, respectively. 
     FIG. 3 is an autoradiograph generated out of an immunoblot analysis of monoclonal antibodies of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention involves the production of monoclonal antibodies against the purified proteins that are C. difficile toxins A and B, respectively. Purified toxins can be prepared, for example, by the method of Rothman et al, &#34;Differential cytotoxic effects of toxins A and B isolated from Clostridium difficile,&#34; Infect. Immun. 46: 324-31 (1984) (&#34;Rothman, 1984&#34;), the contents of which are hereby incorporated by reference. In particular, toxins A and B can be purified from dialyzed filtrates of a cultured C. difficile strain by hydrophobic interaction chromatography (HIC), followed by ion-exchange chromatography and concentration, e.g., either by dialysis or by ultrafiltration. (The term &#34;dialyzed filtrate&#34; is used herein to refer to the crude filtrate obtained when a C. difficile strain is grown anaerobically in a dialysis bag suspended in a suitable nutrient medium, such as brain heart infusion broth. See Rothman and Brown, &#34;Inhibition of membrane functions in intact HeLa cells by Clostridium difficile cytotoxic culture filtrates,&#34; Curr. Microbiol. 6: 221 (1981), the contents of which are hereby incorporated by reference.) Typically, the C. difficile strain employed as the source for toxin A and toxin B immunogens can be grown in 100 ml phosphate-buffered saline in a dialysis bag suspended in 3 liters of Difco brain-heart infusion broth. Before the culture is inoculated, the flask is gassed with N 2 , H 2  and CO 2 , to replace dissolved O 2 , and sealed to maintain anaerobiosis. After incubation at 35° C. for 5 days to allow maximum expression of toxin in the medium, cells can be sedimented by centrifugation (twice for 10 minutes at 48,000×g has proved suitable), and the supernatant fluids then sterilized by filtration, e.g., through a 0.45-μm Millipore membrane, to obtain the dialyzed filtrate. 
     Generally, substances are separated via HIC on the basis of differing strengths of their hydrophobic interactions with an uncharged bed material which contains hydrophobic groups. The HIC fractionation of the C. difficile dialyzed filtrate initially entails raising the ionic strength of the filtrate, e.g., by the addition of a readily ionized salt like ammonium sulfate, in order to render highly hydrophilic the solvent environment for proteins in the dialyzed filtrate. The filtrate is then passed over a chromatography column which provides a distinctly nonpolar moiety, such as the phenyl group available in a matrix comprised of a phenyl-substituted cross-linked agarose gel filtration medium (PHENYL-SEPHAROSE CL-4B, manufactured by Pharmacia Fine Chemicals, Inc., Piscataway, N.J.). Relatively hydrophobic proteins in the dialyzed filtrate, including both toxins A and B, are retained by such a column. Moreover, the C. difficile toxins can be eluted from the column, essentially together, by imposing a linear gradient of increasing hydrophobicity, for example, a buffered ethylene glycol gradient. Thus, elution is effected by decreasing the strength of the hydrophobic interaction between the nonpolar moiety and the surface-exposed hydrophobic areas of the toxins. 
     The desired eluted fraction, identified by its cytotoxicity as the one containing the C. difficile toxins, is then subjected to ion-exchange chromatography, e.g., employing a bead-formed, agarose gel anion exchanger medium. A weak anion exchanger like DEAE-SEPHAROSE CL-6B (manufactured by Pharmacia Fine Chemicals, Inc., Piscataway, N.J.), which preferentially binds biopolymers of greater than 10,000 molecular weight, can be used in this context. 
     Both toxin A and toxin B are lethal to animals when injected in amounts sufficient to elicit an immune response. To obtain primed mouse lymphocyte cells for subsequent fusion with an immortalizing mouse myeloma cell line, mice must therefore be injected with toxoids prepared from purified toxin A and toxin B, respectively. By definition, a &#34;toxoid&#34; is a toxin of a pathogenic organism treated so as to destroy its toxicity but leave it capable of inducing the formation of antibodies on injection. In accordance with the present invention, it is preferred that toxoids of toxins A and B be produced by incubating each toxin with glutaraldehyde, which is believed to act as a cross-linking agent joining two or more toxin molecules in a toxoid complex. It is particularly preferred that the respective toxoids of the two toxins, however prepared, should be pooled to provide an immunogenic mixture for injection. 
     EXAMPLE 1 
     Preparation of Purified C. difficile Toxins 
     Toxins A and B were purified from dialyzed filtrates of C. difficile strain VPI10463 (made publically available by Dr. N. M. Sullivan, Dept. Anaerobic Microbiology, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061); any C. difficile strain that produces both toxin A and toxin B can be used, however. In subsequent purification steps, Tris buffers were used, and are referred to hereinafter as follows: Buffer &#34;2OT8&#34; consisted of 0.020M Tris hydrochloride (pH 8.0). With 80 and 100 mM NaCl, this buffer was designated &#34;20T8/80&#34; and &#34;20T8/100&#34;, respectively. 
     The initial hydrophobic interaction chromatography step (see Table 1) was performed at 25° C. in an 8- by 7-cm column at a flow rate of 6.3 ml/min. 
     
                                           TABLE 1__________________________________________________________________________Purification of C. difficile toxins from dialyzed filtrates             Cytotoxicity                    Protein                         Sp act                               Purification                                     Recovery.sup.aPrepn        Vol (ml)             (MCD/ml)                    (mg/ml)                         (MCD/mg)                               (fold)                                     (%)__________________________________________________________________________Dialyzed filtrates        2,600             1 × 10.sup.7a                    1.1  9 × 10.sup.6                               1     100Phenyl-Sepharose CL-4B          830             6 × 10.sup.7a                    0.9  7 × 10.sup.7                               8     100DEAE-Sepharose CL-6BToxin A        30 5 × 10.sup.5                    3.9  1 × 10.sup.5                               1Toxin B        120             1 × 10.sup.8                    0.31 3 × 10.sup.8                               33     46ConcentrationToxin A, PEG 20        8.5  4 × 10.sup.6                    25   2 × 10.sup.5                               .sup. 2.sup.bToxin B, Amicon        6.0  2 × 10.sup.9                    5.6  4 × 10.sup.8                               44     46XM-100A__________________________________________________________________________ .sup.a Values for toxin B only. .sup.b Value for toxin A only after DEAE treatment. 
    
     Ultrapure grade ammonium sulfate (manufactured by Schwartz/Mann, Orangeburg, N.Y.) was added to the filtrate to a concentration preferably just below that at which salting out of protein occurs. (Typically, no precipitate formed when the salt concentration was increased to about 15% saturation.) The sample (2,600 ml) was then applied to a PHENYL-SEPHAROSE column (PHENYL-SEPHAROSE CL-4B, manufactured by Pharmacia Fine Chemicals, Inc., Piscataway, N.J.) which had been equilibrated with starting buffer (20T8/100 buffer, 15% saturated with ammonium sulfate). The column was thereafter eluted with starting buffer. No toxin was detected in the initial 1,800-ml eluant of nonbinding protein. Elution with a linear gradient of 0 to 50% ethylene glycol in 20T8/100 buffer yielded a single absorbance peak, with cytotoxic activity being highest at a conductivity of about 20 mmho. The toxin-containing fractions were pooled and dialyzed for 36 h at 4° C. with three changes of 20T8/80 buffer. Total toxin activity was enriched eightfold with 100% recovery. All further steps were at 4°  C. 
     Ion-exchange chromatography was performed in a 6.5- by 7-cm column at a flow rate of 3.2 ml/min. The toxin preparation (830 ml) was applied to a DEAE-SEPHAROSE CL-6B column equilibrated with 20T8/80 buffer, followed by elution with the same buffer. Elution with a linear salt gradient from 80 to 500 mM NaCl in 20T8 buffer yielded two cytotoxic bands, designated toxins A and B in accordance with the nomenclature used by Sullivan et al, &#34;Purification and characterization of toxins A and B of Clostridium difficile,&#34; Infect. Immun. 35: 1032 (1982). Toxin A eluted with the large absorbance peak at 12 mmho. Toxin B was detected at a conductivity of 25 mmho where there was little detectable absorbance. Pools of toxins A and B were concentrated by ultrafiltration (XM-100A membrane, manufactured by Amicon Corp., Danvers, Mass.) or by dialysis against polyethylene glycol (20,000 daltons in dialysis tubing which excluded 12,000 daltons). Purified toxins were stored at 4° C. 
     EXAMPLE 2 
     Preparation and Injection of Toxoids 
     Purified toxin A (19 μg/ml) was incubated in 0.08% glutaraldehyde, and purified toxin B (6.4 μg/ml) in 0.1% glutaraldehyde, in 20 mM Tris HCL, 100 mM NaCl (pH 8) at 37° C. for 2 hours. Lysine at twofold molar excess over glutaraldehyde was added (37° C., 15 minutes) to stop the reactions. After dialysis, the toxoid solutions (toxin A, 13 μg/ml; toxin B, 4.3 μg/ml) were stored at 4° C. To prime lymphocytes, BALB/c mice (Jackson Laboratories, Bar Harbor, Me.) were thereafter injected intramuscularly with a mixture of toxoid A and toxoid B, 0.6 μg each in 0.2 ml. Freund&#39;s complete adjuvant, 0.1 ml, was also injected intraperitoneally. On day 146 after injection, the mice received booster shots intravenously with the same amount of the mixed toxoids. 
     EXAMPLE 3 
     Preparation of Activated Spleen-cell Dispersion 
     On day 149, the mice injected with the toxoid mixture as described above were sacrificed and their spleens removed. In accordance with Gentry et al, &#34;Identification of distinct antigenic determinants on dengue-2 virus using monoclonal antibodies,&#34; Am. J. Trop. Med. Hyg. 31(3): 548-555, the contents of which are hereby incorporated by reference, the mouse spleens were teased through a 60-mesh screen, and the fragments rinsed with Dulbecco&#39;s minimum essential medium with high glucose (DMEM) containing 20% fetal bovine serum (FBS). The resulting cell suspension was allowed to settle for 5 minutes and the supernatant was removed and centrifuged at 1,000 rpm for 5 minutes at room temperature in an IEC-2R centrifuge to collect the spleen cells. The cells were resuspended in 5 ml of 0.17M NH 4  Cl to lyse the red cells, diluted in DMEM with 20% FBS, and the spleen cells were pelleted. The concentration of live cells was determined by trypan blue exclusion. 
     Preferably on the same day the mouse spleen cells are prepared as described above, cell fusion with a murine myeloma cell preparation can be effected, for example, following the technique of Kennett et al, &#34;Hybrid plasmacytoma production: Fusions with adult spleen cells, monoclonal spleen cells, monoclonal spleen fragments, neonatal spleen cells and human spleen cells,&#34; Curr. Top. Microbiol. Immunol. 81: 77-91 (1978), the contents of which are hereby incorporated by reference. The choice of fusion technique is not crucial; protocols for hybridization which use polyethylene glycol are preferred. See Note, &#34;Protocols for hybridization with PEG,&#34; Art Sci. Tissue Culture 4(1): 4 (1985). Selection of an immortalizing myeloma line for fusion purposes is also not critical, although a line that does not itself secrete antibody (a &#34;non-secretor&#34; line) is preferred. The murine myeloma cell line P3×63 Ag8.653 (ATCC CRL 1575), developed by Kearney et al, &#34;A new mouse myeloma cell line that has lost immunoglobulin expression but permits the construction of antibody-secreting hybrid cell lines,&#34; J. Immunol. 123(4): 1548-50 (1979), is suitable for use in producing a fusion product in accordance with the present invention. 
     EXAMPLE 4 
     Illustrative Cell Fusion Procedure 
     Thus, following Kennett et al, myeloma cells of the P3×63 Ag8 line were maintained in growth medium with the addition of 5 μg/ml 8-azaguanine. The day before fusion, the myeloma cells were subcultured by diluting 1:2 in fresh medium. Approximately 10 7  myeloma cells were then mixed with 1.5×10 8  spleen cells, rinsed free of serum, and resuspended in 0.2 ml of 30% polyethylene glycol 1000 (J. T. Baker Chemical Co., Phillipsburg, N.J.) for a total of 8 minutes, during which time they were centrifuged for 6 minutes at room temperature at 1,000 rpm. At 8 minutes, the polyethylene glycol was diluted by the addition of 5 ml DMEM, then with an additional 5 ml of DMEM with 20% FBS. After centrifugation, the cell mixture was suspended in 30 ml of the &#34;HY medium&#34; of Kennett et al and dispensed into 96-well plates in 50 μl aliquots. After 24 hours  100 μl of HY medium with 6×10 -7  M aminopterin were added to each well. Thereafter, aminopterin concentration was maintained at 4×10 -7  M. 
     An immunoassay was developed for use in screening hybrid cell lines for antibody production in accordance with the present invention. Generally, the steps involved in establishing an effective assay include (1) preparing polyclonal mouse antibodies against C. difficile toxins and (2) measuring the binding of antibody by the toxin. To effect step (1), polyclonal hyperimmune mouse ascitic fluid (HMAF) was prepared, following Brandt et al, &#34;Production and characterization of arborvirus antibody in mouse ascitic fluid,&#34; Am. J. Trop. Med. Hyg. 16: 339-47 (1967), by immunizing mice with a crude cytotoxic culture filtrate of C. difficile which had been neutralized by the addition of excess rabbit antiserum (see Rothman, 1984). Outbred WRM: (ICR) BR mice (Charles River Laboratories, Wilmington, Mass.) were injected with the antigen-antibody mixture on days 1 and 3: intramuscularly (0.1 ml), subcutaneously and intraperitoneally (0.2 ml each). On day 3, Freund&#39;s complete adjuvant (0.1 ml) was separately injected intraperitoneally. A separate injection of freshly harvested sarcoma 180 cells (0.3 ml) was given on day 30 to induce ascites. The fluids, which contained polyclonal antibodies to the C. difficile toxins, were harvested on day 45 by gravity flow through a 16-gauge hypodermic needle. 
     To determine optimum binding conditions in a solid-phase radioimmunoassay (RIA), RIA plates were prepared by drying dilutions of C. difficile toxin A and toxin B purified in the manner described above, in 96-well flexible assay plates (manufactured by Falcon Labware, Oxnard, Calif.). Dilutions of primary antibody, i.e., the polyclonal antibody mixture contained in the HMAF, were added and incubated 4-18 hours at room temperature. To reveal the extent of binding between the primary antibody and the plated antigen, a second detecting antibody broadly reactive to mouse antibody is used to attach a &#34;label,&#34; such as a radionuclide in the case of an RIA, a fluorescent molecule in the case of a fluorescent antibody assay, or enzymes such as alkaline phosphatase or horseradish peroxidase in the case of an enzyme linked immunosorbent assay (ELISA), to the antigen-primary antibody complexes. In the present example, the detecting antibody was affinity-purified goat antiserum to mouse IgG, IgA and IgM (obtained from Kirkegaard &amp; Perry Laboratories, Gaithersburg, Md.) which had been labeled with  125  I to approximately 2.5×10 5  cpm/25 μl via the &#34;chloramine T&#34; method described by Greenwood et al, &#34;The preparation of  131  I-labeled human growth hormone of high specific radioactivity,&#34; Biochem. 89: 114-23 (1963), the contents of which are hereby incorporated by reference. The diluent used was Dulbecco&#39;s phosphate buffered saline with 0.5% sodium azide; blocking and rinsing were diluent to which fetal bovine serum (20%) had been added. 
     Antibody-secreting cell lines produced, in accordance with the present invention, by fusion of toxoid-activated mouse lymphocytes and mouse myeloma cells were identified by means of the above-identified RIA developed using HMAF. More specifically, the above-described RIA was first employed to establish effective conditions for detecting polyclonal antibodies in mouse hyperimmune ascitic fluids, particularly the range of toxin concentrations over which the binding antibody could be readily observed. Results of binding of this polyclonal antiserum to toxin A and toxin B are shown in FIG. 1. In FIG. 1A, the concentrations for toxin A (in nanograms (ng) per well) are: Δ, 450;  , 230; □, 110;  , 56; ◯ , 28;  , 14;  , 7.0;  , 3.5. In FIG. 1B, the concentrations for toxin B (ng per well) are: Δ, 180;  , 90; □, 45;  , 23; ◯ , 11;  , 5.6;  , 2.8;  , 1.4. Each data point is the mean of duplicate determinations. Toxin A and toxin B were observed to bind mouse polyclonal antibodies over a range of toxin concentrations. Toxin A readily bound antibody at a concentration of 56 ng/well, toxin B at 45 ng/well. 
     Thereafter, the RIA was used to screen primed lymphocyte/myeloma fusion products; the highest toxin concentrations tested (see FIG. 1) were used in the assay so that low antibody concentrations could be detected. Thus, about 450 ng of purified toxin A or about 180 ng of purified toxin B were added per well to the assay plates prepared as previously described to detect even low levels of antibody present in the supernatant fluids of desired fused-cell cultures. In this way, 124 hybrid cell lines were identified, all producing antibodies that bound both toxin A and toxin B. Twenty hybrids, selected to represent a range of cross-reactivity values, were cloned twice and further characterized, as described below. One cell line of the select group proved somewhat unstable and was ultimately discarded. 
     To ascertain the isotype of the antibodies produced by those cell lines identified by the RIA screening, heavy and light chain classes of the monoclonal antibodies were determined by radioimmunoassay. Clostridium difficile toxin A or goat anti-mouse antibodies (450 ng or 31 ng/well, respectively) were dried onto assay plates. Undiluted culture supernatant fluids (25 μl) from a hybrid cell line were allowed to react overnight and were then removed by rinsing. Thereafter, 2.5 μg (25 μl) of rabbit antisera to mouse immunoglobulin isotype subclasses (produced by Litton Bionetics, Charleston, S.C.) were added. After 2 to 4 hours, anti-isotype antibodies were removed by rinsing, and binding was detected with 25 μl goat antiserum to rabbit immunoglobulins, G, A, and M (produced by Calbiochem, San Diego, Calif.). Before use in this assay, the goat anti-mouse antiserum had been affinity-purified on glutaraldehyde-treated rabbit serum, following Avrameas and Ternynck, &#34;The cross-linking of proteins with glutaraldehyde and its use for the preparation of immunoadsorbents,&#34; Immunochem. 6: 53-66 (1969), and labeled with  125  I to approximately 2.5×10 5  cpm/25 μl using the chloramine T method of Greenwood et al, cited above. 
     The antibodies produced by all of the 124 screened fused-cell lines mentioned above were, with two exceptions, of the IgM isotype. In two cases, the isotype appeared to be mixed IgM and IgG 2a  or IgG 1 . All light chains of the antibodies were kappa, save in one instance of mixed kappa and lambda and two other instances of lambda. All the antibodies produced by the nineteen cloned hybrids were of the IgM isotype. 
     For the nineteen cloned hybrids selected for further characterization, the binding of produced monoclonal antibodies to C. difficile toxin A and toxin B was determined in a solid-phase radioimmunoassay in which equal amounts of toxin A or toxin B (380 ng) were added per well. The antibody preparations were the undiluted tissue culture fluids of the cloned hybrids, respectively. The results of the binding assay are shown in Table 2. Cultures of each of the nineteen hybrids were deposited with the American Type Culture Collection (Rockville, Md.) under accession numbers also shown in Table 2. The date of deposit for the hybridomas listed in TABLE 2 are as follows: 
     (1) For ATCC Numbers HB9028, HB9029, HB9030, HB9031, HB9032 and HB9033 the deposit date was Mar. 11, 1986; 
     (2) For ATCC Numbers HB9055, HB9056, HB9057, HB9058, HB9059, HB9060, HB9061, HB9062, HB9063, HB9064, HB9065, HB9066 and HB9067 the deposit date was Apr. 1, 1986. 
     From the binding assay, it was observed that all of the monoclonal antibodies secreted by the 19 hybrid lines produced in accordance with the present invention reacted with both toxins. More specifically, the monoclonal antibodies of the present invention can be divided into at least two groups: those that bound more strongly to toxin A than to toxin B, and those that bound about equally to each. For the 19 tested hybrids, the ratio of antibody bound to toxin A versus to toxin B (see &#34;ratio A/B&#34; in Table 2) ranged between about 0.5 and about 6.9 for the undiluted tissue culture supernatant fluids. (Background binding ranged from 115 to 270 cpm.) This binding assay was also performed to titrate the binding of a series of concentrations of the 19 monoclonal antibodies. The results of this titration are shown in FIGS. 2A-2D, which demonstrate reaction of some of the antibodies with toxin A (open circles) and toxin B (filled circles) over the whole range of concentrations from undiluted through the 2 -11  dilution. 
     
                       TABLE 2______________________________________Radioimmunoassays of Monoclonal Antibodies vs.C. difficile Toxins                           Ratio   ATCC Deposit              RIA (mean cpm)                           A (cpm)/Hybridoma line     Accession No.                  ToxA    ToxB   B (cpm)______________________________________T3-1C9    HB9033       10393    3211  3.2T3-1C12   HB9030       14861   13008  1.1T3-1D8    HB9032       12095    8192  1.5T3-1E5    HB9029       22497   21126  1.1T3-1E7    HB9055        6315   13396  0.5T3-1E10   HB9060        3878    2561  1.5T3-2A9    HB9028       21167   17124  1.2T3-2B1    HB9057        8037    1168  6.9T3-2D2    HB9061        2495    1390  1.8T3-2G4    HB9058        794     893   0.9T3-2H5    HB9056       25029   19140  1.3T3-2H6    HB9062       14832   16931  0.9T3-3F11   HB9059        6212    2864  2.2T3-3H1    HB9064       18431   18126  1.0T3-4A7    HB9063       19206   16139  1.2T3-4C11   HB9031       18612   21214  0.9T3-8B8    HB9066        2745    3184  0.9T3-8F5    HB9067       14662    6166  2.4T3-9E7    HB9065       12269    3670  3.3______________________________________ 
    
     Recognition of both toxins A and B by the monoclonal antibodies of the present invention was confirmed by immunoblotting using a Western blot technique like that described by Burnette, &#34;`Western blotting`: Electrophoretic transfer of proteins from sodium dodecyl sulfate-polyacrylamide gels to unmodified nitro cellulose and radiographic detection with antibody and radioiodinated Protein A,&#34; Analyt. Biochem. 112: 195-203 (1981), the contents of which are hereby incorporated by reference. Electrophoresis was first carried out in 5.5% slab gels by the method of Davis, &#34;Disc electrophoresis, II. Method and application to human serum proteins,&#34; Ann. N.Y. Acad. Sci. 121: 404-27 (1964). Purified C. difficile toxin A (98 μg) and toxin B (180 μg), prepared in accordance with Rothman, 1984, were mixed and added in a volume of 1.7 ml to a single, long well formed with a blank comb. After electrophoresis, an edge section was cut off and stained with Coomassie blue dye. Proteins from the major portion of the gel were transferred electrophoretically to nitrocellulose and processed as previously described by Brown et al, &#34;Purification and biological characterization of Shiga toxin from Shigella dysenteriae 1.,&#34; Infect. Immun. 36: 996-1005 (1982), the contents of which are hereby incorporated by reference, with the following modifications. After protein transfer, the nitrocellulose was cut into strips to allow separate processing. The nitrocellulose strips were incubated sequentially in buffer containing bovine serum albumin (filler), filler and monoclonal antibody (1:20 dilution of tissue culture supernatant fluids), filler and affinity-purified rabbit antiserum to mouse immunoglobulins (14 μg/ml), and filler and  125  I-labeled protein A (5×10 5  cpm/ml), respectively. Addition of affinity-purified rabbit antiserum amplified the reaction of the antigens with the monoclonal antibodies. To reduce background, rabbit antiserum raised against mouse immunoglobulins G, A and M (Calbiochem, La Jolla, Calif.) was purified by affinity chromatography using cross-linked normal mouse serum as the immunoadsorbent. 
     The autoradiographic pattern generated by the nitro cellulose strips is shown in FIG. 3, and is read as follows: T3-1C9, T3-1C12, T3-1D8, T3-1E5, T3-1E7, T3-1E10, T3-2A9, T3-2B1, (lanes 1 through 8); T3-2D2, T3-2G4, T3-2H5, T3-2H6, T3-3F11, T3-3H1, T3-4A7, T3-4C11, T3-8B8, T3-8F5, T3-9E7 (lanes 10 through 20). In lane 9, incubation was with rabbit antiserum to crude C. difficile toxin. The control was hybridoma growth medium (not shown). The open arrow shows the position of toxin A. The closed arrow points to the major band of toxin B, which separates into a number of cytotoxic bands. From FIG. 3, it is apparent that the toxin B preparation contains a number of cytotoxic species, as reported by Rothman, 1984. A number of the monoclonal antibodies of the present invention were also shown by the abovedescribed blotting technique to recognize both toxin A and toxin B, confirming the RIA results. Four antibodies appeared to react more strongly in the blot with toxin A than with toxin B (see T3-1C9, T3-1E5, T3-2H5, T3-8F5); two (T3-1E7 and T3-4C11) appeared to react more strongly with toxin B than with toxin A. The remaining thirteen reacted equally with both toxins. There was no detection of either antigen in a control, in which medium used for growth of hybrids was substituted for monoclonal antibody. 
     To test for neutralization of cytotoxicity by monoclonal antibodies of the present invention, serial twofold dilutions of tissue culture supernatant fluids from each of the hybrid cell lines listed in Table 2 were mixed with an equal volume of complete minimal essential medium which contained 5 minimum cytotoxic doses of toxin A or toxin B. The toxin-antibody mixture, 100 μl per well, was incubated with HeLa cell monolayers over night at 35° C. The cells were fixed and stained and the dye absorbance at 595 nm was compared with controls, following the techniques of Rothman, 1984. None of the tested antibodies neutralized the cytoxicity of either C. difficile toxin, as determined by the HeLa cell assay. 
     The monoclonal antibodies of the present invention, characterized by an affinity for both toxin A and toxin B of C. difficile, can be employed to advantage in various types of immunoassays for C. difficile in tissue and fecal samples. The antibodies can be used, for example, in &#34;sandwich&#34;-type assays, as described above, with horseradish peroxidase, alkaline phosphatase or some other well-known label material linked to a suitable antimouse antibody. See, e.g., VOLLER et al, THE ENZYME LINKED IMMUNOSORBENT ASSAY (ELISA) (Dynatech Laboratories 1979), the contents of which are hereby incorporated by reference. Particularly preferred is the modified double-antibody sandwich ELISA, wherein a plate provided with a layer comprised of antibody of the present invention is contacted first with a test sample (possibly toxin-containing) and thereafter with immunoglobulin solution containing a specific antibody (B) of a different species, such as goat or sheep. Enzyme-labelled anti-immunoglobulin which is reactive with antibody B, but not with the antibody of the present invention, is added to form the &#34;double sandwich&#34; (antibody/antigen/antibody B/anti-B globulin) for which the assay is named. When substrate for the enzyme is added, the resulting color change is proportional to the amount of C. difficile toxin (antigen) in the test sample. 
     The antibodies can also be used in RIA or other assay methods that are not solid-state assays, as well as ELISA&#39;s and dot-blot assays adapted from conventional protocols. By attaching a monoclonal antibody of the present invention to a substance which &#34;tags&#34; the antibody, one can also use the antibody as a diagnostic probe for C. difficile toxin in tissues of animals or humans. 
     By the same token, the antibodies of the present invention can be used in an immunoassay for antibodies specific for both toxin A and toxin B. Adapting the protocol described above for identifying antibody-secreting cell lines prepared in accordance with the present invention, one can prepare assay plates coated with known amounts of toxin A and toxin B, as extrapolated from the concentration data in FIG. 1. One or more samples prepared from a biological material like blood serum, including human serum, to be tested for the presence of toxin A/toxin B-specific antibody are then prepared in a conventional manner and brought into contact, respectively, with the coated plates. Another set of plates are likewise contacted with samples containing a monoclonal antibody within the present invention. Both sets of plates are then contacted with a labeled detector substance, which can be protein A or an antibody that is broadly cross-reactive with immunoglobulin protein of the appropriate species, i.e., either the species of the test serum or mouse. (Protein A, found on the surface of certain strains of Staphylococcus aureus cells, binds to a specific portion of an antibody molecule. Labeled protein A, as well as labeled detecting antibody, can be prepared by known techniques, and is also commercially available.) The two sets of plates are then compared, with the detectable reactions on the plates containing the monoclonal antibody of the present invention serving as a standard for comparison with the plates containing the tested samples. If the test serum contains antibodies specific for toxin A and toxin B, a similar, detectable reaction should be observed.