Purified DNA of the 170-Kd Surface Gal/GalNAc Adherence Lectin of Pathogenic E. Histolytica

The invention relates to diagnostics and therapeutics for Entamoeba histolytica infection. More specifically, the invention is directed to diagnostic methods which take advantage of the genes encoding the Gal/GalNAc lectin of this amoeba, and specifically cDNA encoding the 170 kd heavy chain subunit.

Entamoeba histolytica infection is extremely common and affects an estimated 480 million individuals annually. However, only about 10% of these persons develop symptoms such as colitis or liver abscess. The low incidence of symptom occurrence is putatively due to the existence of both pathogenic and nonpathogenic forms of the amoeba. As of 1988, it had been established that the subjects who eventually exhibit symptoms harbor "zymodemes" which have been classified as such on the basis of their distinctive hexokinase and phosphoglucomutase isoenzymes. The pathogenic forms are not, however, conveniently distinguishable from the nonpathogenic counterparts using morphogenic criteria.

The distinction between pathogenic and nonpathogenic strains in diagnosis is of great practical importance, because only persons infected with E. histolytica who will develop the disease should be treated. This is bad enough in developed countries where it would at least be possible economically to treat every carrier with a known effective drug (metronidazole); it is, of course, undesirable to administer such drugs unnecessarily. In less developed countries, the cost of these unnecessary administrations is significant enough to have a dramatic negative impact on the resources for overall health care.

There is an almost perfect correlation between infection with a pathogenic zymodeme and development of symptoms and between infection with a nonpathogenic zymodeme and failure to develop these symptoms. As a general proposition, only pathogenic strains can be grown axenically (i.e., in the absence of an associated microorganism) and nonpathogenic strains have been made to grow in this manner only by "training" them to do so in a series of media alterations beginning with attenuated bacteria. The adaptation was accompanied by exhibition of the enzyme pattern characteristic of pathogenic strains (Mirelman, D., et al., Infect Immun (1986) 54:827-832). This work has not been repeatable in other laboratories, and more recent work on genomic differences (see below) indicates that the pathogenic and nonpathogenic forms are separate species.

It is known that E. histolytica infection is mediated at least in part by the "Gal/GalNAc" adherence lectin which was isolated from a pathogenic strain and purified 500 fold by Petri, W. A., et al., J Biol Chem (1989) 264:3007-3012. This successful isolation and purification was preceded by the production of mouse monoclonal antibodies which inhibit the in vitro adherence of the amoebic trophozoites; the antibodies were prepared from immortalized cells from spleens of mice immunized with sonicated trophozoites grown in axenic culture after having originally been isolated as a pathogenic strain from an affected subject. (Ravdin, J. I., et al., Infect Immun (1986) 53:1-5.) The cells were screened by the ability of the supernatants to inhibit adherence of the trophozoites to target tissue. All of these reported monoclonal antibodies, therefore, are presumably immunoreactive with the Gal/GalNAc surface adhesion of the pathogen. The Gal/GalNAc lectin was then prepared by galactose affinity chromatography and reported in 1987. (Petri, W. J., et al., J Clin Invest (1987) 80:1238-1244). Studies of serological cross-reactivity among patients having symptomology characteristic of E. histolytica pathogenic infection, including liver abscess and colitis, showed that the adherence lectin was recognized by all patients' sera tested (Petri, Jr., W. A., et al., Am J Med Sci (1989) 296:163-165).

The purified "Gal/GalNAc" lectin was shown to have a nonreduced molecular weight of 260 kd on SDS-PAGE. After reduction, with beta-mercaptoethanol, the lectin separated into two subunits of 170 and 35 kd MW. Further studies showed that antibodies directed to the 170 kd subunit were capable of blocking surface adhesion to test cells, and therefore, the 170 kd subunit is believed to be of primary importance in mediating adhesion.

Despite the generally interesting and useful results cited above, the ability to diagnose the presence or absence of pathogenic strains of E. histolytica has proved difficult. Since both pathogens and nonpathogens are morphologically similar, microscopic tests are not particularly useful. ELISA techniques have been used to detect the presence or absence of E. histolytica antigen in both stool specimens and in sera, but these tests do not seem to distinguish between the pathogenic and nonpathogenic strains. Root et al., Arch Invest Med (Mex) (1978) 9: Supplement 1:203, pioneered the use of ELISA techniques for the detection of amoebic antigen in stool specimens using rabbit polyclonal antiserum. Various forms of this procedure have been used since, some in correlation with microscopic studies, and all using polyclonal antisera. None of these, apparently, pinpoints the instances of infection with the pathogenic as opposed to nonpathogenic form. See, for example, Palacios et al., Arch Invest Med (Mex) (1978) 9: Supplement 1:203; Randall et al., Trans Roy Soc Trop Med Hyg (1984) 78:593; Grundy, Trans Roy Soc Trop Med Hyg (1982) 76:396; Ungar, Am J Trop Med Hyg (1985) 34:465.

These studies on stool specimens are summarized in Amebiasis: Human Infections by Entamoeba Histolytica, J. Ravdin, ed. (1988) Wiley Medical Publishing, pp. 646-648. Similar methods to detect characteristic E. histolytica antigens in serum and in liver abscess fluid are equally unable to distinguish pathogens from non-pathogens (ibid., pp. 661-663). As summarized in this article, as of 1988, the only known way to distinguish pathogenic from nonpathogenic forms of this amoeba was through characterizing the isoenzyme pattern using electrophoresis.

Recently it has been shown by two different groups that differences between pathogenic and nonpathogenic strains can be demonstrated using comparisons of DNA isolates. Garfinkel, L. I., et al., Infect Immun (1989) 57:926-931 developed DNA probes which hybridize to DNA isolated from E. histolytica and four types of restriction fragment length patterns were obtained. These patterns correlated with pathogenic/nonpathogenic distinctions. Similarly, Tannich, E., et al., Proc Natl Acad Sci (1989) 86:5118-5122 probed cDNA libraries constructed from various strains and showed that pathogenic isolates were genetically distinct from nonpathogenic ones. However, these techniques require the culture of the organisms isolated from patients to obtain sufficient quantities for testing, and are thus time consuming and labor intensive.

Strachan, W. D., et al., Lancet (1988) 561-562, report the production of two monoclonal antibodies designated 22.3 and 22.5 which were members of a large group prepared by standard procedures from mice immunized with axenic cultures of a pathogenic E. histolytica strain NIH200/ATCC 30458. These monoclonal antibodies were tested in an immunofluorescence assay with cultures obtained from both putatively invasive and noninvasive strains, and appeared to immunoreact only with culture samples of invasive strains. There is no indication in this publication as to the manner of screening for antibodies with this characteristic, it is not known to what target these antibodies bond, nor would it be possible, without these specific antibodies, to reproduce this result. The test described requires intact E. histolytica and therefore cannot be applied in serum, urine or liver abscess fluid and can only be applied to stool samples which are freshly collected.

It would be highly desirable to have a relatively simple clinical test which would detect the presence of E. histolytica antigen in samples taken from patients and to be able to distinguish whether these antigens are associated with pathogenic or nonpathogenic strains. The present invention offers such a test, by employing conventional immunoassay procedures using monoclonal antibody reagents which are specifically immunoreactive with pathogenic and/or nonpathogenic Gal/GalNAc adherence lectin.

The invention provides diagnostic tests which permit the assessment of patients for invasive E. histolytica infection. The tests comprise the use of DNA hybridization with respect to cDNA encoding the 170 kd subunit of the adherence lectin. Pathogenic and nonpathogenic strains can be distinguished by use of this test. In addition, the availability of the cDNA encoding the subunit permits the interruption of the production of the adherence lectin by providing an antisense form of the cDNA.

Thus, in one aspect, the invention is directed to a method to detect the presence or absence of pathogenic or nonpathogenic E. histolytica in a biological sample, which method comprises subjecting the sample to polymerase chain reaction (PCR) using primers framing a region of the 170 kd heavy chain of the Gal/GalNAc adherence lectin which is characteristic of the pathogenic or nonpathogenic form as the case may be and probing the amplified DNA under stringent conditions with an oligomer corresponding to said characteristic portion. In another aspect, the invention is directed to a method of detecting the presence or absence of pathogenic or nonpathogenic E. histolytica which comprises subjecting the sample to PCR using primers corresponding to at least one region which is characteristic of either the pathogenic or nonpathogenic cDNA and probing the amplified DNA with an appropriate oligomer.

In still another aspect, the invention is directed to methods to interrupt the production of the 170 kd heavy chain of the adherence lectin by supplying to a subject or an E. histolytica culture the complement to the DNA encoding at least a portion of the 170 kd subunit of the adherence lectin. Other aspects include kits, vectors and pharmaceuticals directed to these purposes.

FIG. 1 shows the nucleotide sequence and deduced amino acid sequence corresponding to the 170 kd heavy chain of the adherence lectin from NIH303.

FIG. 2 shows the peptide sequence of the CNBR derived fragment of NIH303.

FIG. 3 shows a comparison of the nucleotide sequence of a portion of the cDNA encoding the adherence lectin from a pathogenic and a nonpathogenic strain.

The present invention provides cDNA sequences which are useful in diagnostic assays for both pathogenic and nonpathogenic forms of E. histolytica. The diagnostic assays can be conducted on biological samples derived from cell cultures or from subjects at risk for infection. The assays utilize hybridization probes and by the design of the assay can distinguish pathogenic from nonpathogenic forms of the amoeba. In addition, the availability of the cDNA provides an opportunity for preventing the production of the adherence lectin using an "antisense" approach.

As used herein, a nucleotide sequence "corresponding to" a referent nucleic acid refers to a sequence which is substantially the same as or complementary to the referent nucleic acid. Thus, a sequence "corresponding to" a region of the nucleic acid encoding the 170 kd subunit refers to a sequence which is the same as that region or complementary to it. It will be apparent from the context whether this sequence must be as found in the organism or may include degenerate sequences. Where hybridization, PCR amplification processes or antisense effects are the subject matter, degenerate sequences are not workable. For recombinant production of the protein, they are.

By "substantially" is meant homology is sufficient to provide the requisite hybridization to target under the conditions the DNA is employed. Similarly, by "effective fragment" is meant a fragment of sufficient size to provide the requisite hybridization to target under the conditions the DNA is employed.

A nucleotide sequence "characteristic of" a pathogenic or nonpathogenic strain refers to a nucleotide sequence which differentiates between these types of strains. Thus, a nucleotide sequence "characteristic of" a pathogenic E. histolytica refers to a nucleotide sequence wherein the analogous sequence in the nonpathogenic strain is not identical and is characteristically different from that of the sequence in the pathogenic strain.

A "composite" nucleotide sequence refers to a nucleotide sequence which in part corresponds to the sequence as it appears in a pathogenic strain, and in part as it appears in a nonpathogenic strain of E. histolytica.

An "analogous" region of a DNA refers to the sequence which encodes the referent protein or peptide sequence in a different E. histolytica strain.

"Replicon" refers to a DNA vector which is capable of self-replication when transformed into a suitable host, the context most frequently used herein when transformed into E. histolytica.

The diagnostic tests involve hybridization under various levels of hybridization stringency, and these levels are defined as follows:

low stringency corresponds to washing filters in 0.2.times.SSC, 0.1% SDS at 37.degree. C. (1.times.SSC is 0.15M NaCl, 0.015M Na citrate, SDS- Na lauryl sulfate);

high stringency corresponds to washing in 0.2.times.SSC, 0.1% SDS at 65.degree. C.;

moderate stringency corresponds to washing in 0.2.times.SSC, 0.1% SDS at 45.degree. C.

As used herein, "immunospecific" with respect to a specified target means that the antibody thus described binds that target with significantly higher affinity than that with which it binds to alternate haptens. The degree of specificity required may vary with circumstances, but typically an antibody immunospecific for a designated target will bind to that target with an affinity which is at least one or two, or preferably several orders of magnitude greater than that with which it binds alternate haptens.

Furthermore, as used herein, the term "antibody" refers not only to immunoglobulins per se, but also to fragments of immunoglobulins which retain the immunospecificity of the complete molecule. Examples of such fragments are well known in the art, and include, for example, Fab, Fab', and F(ab').sub.2 fragments. The term "antibody" also includes not only native forms of immunoglobulins, but forms of the immunoglobulins which have been modified, as techniques become available in the art, to confer desired properties without altering the immunospecificity. For example, the formation of chimeric antibodies derived from two species is becoming more practical. In short, "antibodies" refers to any component of or derived form of an immunoglobulin which retains the immunospecificity of the immunoglobulin per se.

The term "pathogenic forms" of E. histolytica refers to those forms which are invasive and which result in symptomology in infected subjects. "Nonpathogenic forms" refer to those forms which may be harbored asymptomatically by carriers.

"Gal/GalNAc lectin" refers to a glycoprotein found on the surface of E. histolytica which mediates the adherence of the amoeba to target cells, and which mediation is inhibited by galactose or N-acetylgalactosamine. The Gal/GalNAc lectin refers specifically to the lectin reported and isolated by Petri et al. (supra) from the pathogenic strain HMI-IMSS, and to the corresponding lectin found in other species of E. histolytica. The "170 kd subunit" refers to the large subunit obtained by Petri et al. upon reduction of the Gal/GalNAc lectin and its corresponding counterparts in other species.

The "antibodies" described herein are immunospecific for the 170 kd subunit of the Gal/GalNAC lectin associated with Entamoeba histolytica. Three categories of monoclonal antibodies have been prepared. One category of antibody is immunospecific for epitopes which are found on the 170 kd subunit of Gal/GalNAc lectin which are "unique" to pathogenic forms. These antibodies are capable, therefore, of immunoreaction to a significant extent only with the pathogenic forms of the amoeba or to the 170 kd subunit of lectin isolated from pathogenic forms. Conversely, a second set of monoclonal antibodies is immunoreactive with epitopes which are "unique" to the 170 kd subunit of Gal/GalNAc lectin which is found in nonpathogenic forms. Thus, these antibodies are immunoreactive to a substantial degree only with the nonpathogenic amoeba or their lectins and not to the pathogenic forms. Finally, a third category of monoclonal antibodies is immunoreactive specifically with epitopes common to pathogenic and nonpathogenic forms and contained in the 170 kd subunit of the Gal/GalNAc lectin. These antibodies are capable of immunoreaction with the subunit or with the amoeba regardless of pathogenicity.

With respect to the monoclonal antibodies described herein, those immunoreactive with epitopes 1 and 2 of the 170 kd subunit isolated from the pathogenic strain exemplified are capable of reacting, also, with the corresponding epitopes on nonpathogens. On the other hand, those immunoreactive with epitopes 3-6 are capable of immunoreaction only with the 170 kd subunit of pathogenic strains. By applying the techniques for isolation of the pathogenic 170 kd subunit to amoeba which are nonpathogenic, a 170 kd subunit can be obtained for immunization protocols which permit the analogous preparation of Mabs immunoreactive with counterpart epitopes 3-6 in the nonpathogenic forms.

The monoclonal antibodies reported in the art, as set forth in the Background section above, were prepared using a screening procedure which screens for inhibition of an adherence of the amoeba to target cells. Therefore, the prior art antibodies are distinct from those which bind to epitopes 1, 2 and 3, which are shown hereinbelow either to enhance or to have no effect on the binding of the amoeba to target cells or proteins. Accordingly, those antibodies which immunoreact with epitopes 1, 2 and 3 are not available in the art.

The monoclonal antibodies of the invention are prepared by culturing immortalized cell lines which are capable of secreting them. The culturing of these lines is generally done in two ways--through in vitro culture methods with nutrients as generally understood, or by injection into suitable hosts, such as mice, in order to permit proliferation in vivo, with subsequent recovery of the mabs from ascites fluid. As used herein, "culturing" an immortalized cell line and "recovering the mabs from the culture" includes the procedures using both of these approaches.

The preparation of a highly purified form of the Gal/GalNAc lectin derived from a pathogen of E. histolytica is described in detail in Example 1. The preparation comprises an affinity chromatography step wherein monoclonal antibodies immunospecific to the 170 kd subunit of the reduced form of the Gal/GalNAc lectin are used as an affinity ligand to isolate the lectin from a solubilized preparation of the amoeba. The isolated lectin can be then reduced using standard methods, such as contact with a sulfhydryl reducing agent, for example dithiothreitol or beta-mercaptoethanol, to obtain the 170 kd subunit which is significant in mediating adhesion. Either the isolated 170 kd subunit or the purified lectin itself may be used for immunization to obtain antibody preparations, and in serological testing.

In a manner similar to that set forth in Example 1 as applied to purification of the Gal/GalNAc lectin from pathogenic strains, nonpathogenic lectin may also be purified to a similar level of purity by employing as affinity ligands in the affinity chromatography step monoclonal antibodies which are immunospecific for epitopes which are shared by both pathogenic and nonpathogenic forms. Two such monoclonal preparations are prepared as illustrations herein--those immunoreactive with epitope 1 and those immunoreactive with epitope 2. By use of these monoclonal antibody preparations as affinity ligands, the purified Gal/GalNAc lectin from either pathogenic or nonpathogenic forms may be prepared.

The monoclonal antibodies described below are prepared by immunization protocols using the isolated and purified Gal/GalNAc lectins of the invention or the 170 kd subunits obtained by reduction thereof. Use of these lectins in purified and isolated form as immunogens, as well as their availability for use in screening the monoclonal preparations obtained greatly facilitates the preparation and identification of suitable monoclonal antibodies.

For immunization, standard protocols are employed, and any suitable vertebrate, typically a mammal, such as rats, mice, rabbits, and the like, can be used as the subject. When sufficient titers are obtained, the sera are harvested. The sera prepared as above are useful polyclonal compositions which, it has been found, are required for recovery of recombinant clones which express the genes encoding the 170 kd heavy subunit.

If monoclonals are desired, the antibody-producing cells of the subject, preferably spleen cells, are subjected to immortalization protocols, most conveniently those for the formation of hybridomas as set forth originally by Kohler and Millstein. However, additional techniques for immortalization such as viral infection may also be used.

The immortalized cells are then screened for the production of the desired mabs. Generally, the supernatants of the cultured immortalized cells are tested in standard immunoassays, such as ELISA or RIA, which employ as antigen the purified lectin or subunit used as an immunogen. Positively reacting supernatants are then further tested. It is convenient to verify immunoreactivity with the 170 kd subunit by using, as antigen in the assay or in Western blots, the reduced form of the isolated lectin.

The supernatants are then tested for cross-reactivity with the alternate forms of the lectin or subunit. For example, supernatants of antibody-secreting cells prepared from subjects immunized by pathogenic E. histolytica are tested by immunoassay against the purified lectin, or other lectin-containing antigen composition of nonpathogenic amoeba. Conversely, supernatants of antibody-secreting cells of subjects immunized with the lectin from nonpathogenic forms are checked for cross-reactivity with the lectin or other antigen-containing composition derived from the pathogenic alternatives.

Thus, monoclonal antibody preparations are obtained which are either immunoreactive with epitopes shared by both pathogens and nonpathogens, or with epitopes which are unique to the form from which they are derived.

Expression libraries in .lambda.gt-11 were probed using the above-described monoclonal antibodies without success. Degenerate probes designed based on the amino terminal sequence of the purified pathogenic Gal/GalNAc lectin of Example 1 were also unsuccessful in retrieving the 170 kd subunit-encoding DNA. However, polyclonal antisera prepared from the purified Gal/GalNAc lectin as described above were immunoreactive with an E. coli transformed with a .lambda.gt-11 vector which contained a 2.0 kb cDNA insert. The sequence and deduced amino acid sequence of this insert are shown in FIG. 1.

Verification of the identity of the clone as bearing cDNA encoding the heavy subunit of the pathogenic strain NIH303 of E. histolytica was obtained by comparison of the deduced amino acid sequence with the cyanogen bromide cleavage sequence internal to the peptide. This sequence is shown in FIG. 2.

The availability of this cDNA permits the recovery of analogous cDNA from other strains of E. histolytica, both pathogenic and nonpathogenic using the standard methods described in Maniatis et al. and using the cDNA shown in FIG. 1 as a probe under stringent conditions. Thus, the retrieval of the cDNA shown in FIG. 1 makes available 170 kd subunit-encoding cDNA from all E. histolytica strains.

For the conduct of the assay of the invention, samples are prepared and any amoebae contained therein solubilized according to standard procedures for the type of sample provided. Stool samples are treated as described, for example, by Ungar, et al., Am J Trop Med Hyg (1985) 34:465. Serum or plasma samples are diluted serially in phosphate buffered saline. Stool, serum or plasma samples are preferred, although other biological fluids or biopsy materials can also be used.

The DNA is extracted from the sample by the method of Kawasaki, E. S., in "PCR Protocols," Innis, M. A., et al., Eds., (1990) Academic Press, Ch. 18, p. 153 et seq. Briefly, in this protocol, the sample is lyophilized and resuspended in 100 .mu.l of 50 Mm Tris, pH 8.3, 150 mM NaCl, 0.5% NP40. Proteinase K is added to 100 .mu.g/ml. The sample is incubated at 55.degree. C. for one hour, boiled for 3 minutes, and then placed on ice. After cooling, the sample is pelleted for 5 minutes in a microfuge and the supernatant is saved and either stored at 4.degree. C. or used immediately. About 5 .mu.l of the supernatant is used as a DNA template for further amplification per 50 .mu.l reaction mixture in this procedure.

For preparation of standards, identified trophozoites are used in the initial preparation. Approximately 2.times.10.sup.6 trophozoites are washed once with PBS, pelleted, lyophilized and stored at 200.degree. C. for future use. About 10 mg of lyophilized amoebae are then used in the extraction.

The presence or absence of DNA associated with pathogenic or nonpathogenic strains can then be assessed using a protocol which takes advantage of nucleic acid sequence differences between these types of strains. All protocols involve amplification of the DNA using the polymerase chain reaction.

The PCR reaction is conducted using standard procedures, with primers designed to match specific regions of the target. In one approach, the primers are designed to frame a region of the DNA which is characteristic of the pathogenic or nonpathogenic form as desired. Thus, only a characteristic region will be amplified for detection with probe. The probe is then selected to correspond to that region specifically. Pathogenic strains are then distinguishable from nonpathogenic strains by their ability to hybridize to a probe corresponding to the region as it appears in the pathogenic strains; similarly, nonpathogens are detectable by their ability to hybridize with probes which correspond to sequences characteristic of nonpathogenic strains.

In an alternate approach, the choice of primer is used to distinguish between the pathogens and nonpathogens. In this approach, at least one of the primers used in the PCR reaction will correspond to the sequence which is characteristic of one or the other type of strain. Thus only the desired sequences will be amplified and detected by probe. In this instance, the probe used can be from either pathogenic or nonpathogenic strains or a composite thereof, and the stringency of the hybridization conditions can be adjusted to accommodate the selection of the probe; the stringency will be high, low or moderate depending on the degree of probe homology.

Where probes corresponding to the region to be detected are used, high stringency is generally employed. In the first method set forth above, high stringency is needed in order to distinguish between pathogenic and nonpathogenic strains; in the second approach which relies on characterization through selection of PCR primers, low stringency can be used since the distinction between pathogen/nonpathogen has already taken place. The hybridization to probe is detected using conventional techniques and under the stringency conditions set forth hereinabove.

Kits suitable for the above-described methods of diagnosis are also provided by the invention. These kits include, at a minimum, the appropriate primers, probes, additional reagents if desired, and instructions for conduct of the assay.

The DNAs of the invention can also be used to control the production of the 170 kd Gal/GalNAc binding lectin subunit using antisense technology. The complement to the DNA of FIG. 1, or a significant portion thereof, is supplied to an amoeba culture or to host organism harboring E. histolytica infection using standard methods of administration, such as those set forth in Remington's Pharmaceutical Sciences, Mack Publishing Company, Easton, Pa., latest edition. Preferably, for in vivo treatment of a subject, the complement is provided by injection, and formulated using conventional excipients therefor, such as Ringer's solution, Hank's solution, and the like. Oral administration with proper formulation can also be effected. While most administration is systemic, in the case of localized conditions such as solid tumor growth, administration may be topical or otherwise local. Slow release mechanisms for drug delivery may also be used.

Alternatively, the complementary nucleotide sequence may be generated in situ by providing an expression system which contains the cDNA of FIG. 1 or its analogous DNA from other E. histolytica strains or an effective fragment thereof in a "reverse orientation" expression system. The expression system may either be designed to be operable in the host subject, such as a mammalian subject wherein the reverse oriented E. histolytica encoding sequence is under the control of, for example, an SV-40 promoter, an adenovirus promoter, and the like so that the complement is transcribed in situ. When used in a cell culture of E. histolytica, the expression system will be provided on a replicon compatible with the E. histolytica strain.

The following examples are intended to illustrate but not to limit the invention:

Axenic E. histolytica, pathogenic strain HMI-IMSS were grown in medium TYIS-33 (trypticase yeast extract, iron and serum) with 100 U/ml penicillin and 100 mg/ml streptomycin sulfate at 37.degree. C. in 250 ml plastic tissue culture flasks. The amoebae were harvested after 72 hours of growth by centrifugation at 150.times.g for 5 min at 4.degree. C. and washed twice in ice cold 75 mM Tris, 65 mM NaCl, pH 7.2.

The trophozoites harvested from four 250 ml flasks were preincubated on ice in 5 ml of 75 mM Tris, 65 mM NaCl with a 1:1000 dilution of diisopropylfluorophosphate before solubilization in 10 ml of 150 Mm NaCl, 50 mM Tris, pH 8.3, 0.5% Nonidet P-40, 5 mM EDTA and 2 mM phenylmethylsulfonyl fluoride. The solubilized amoebae were microfuged for 10 min and the supernatant was applied at 4.degree. C. to a monoclonal antibody affinity column consisting of 2 mg each of protein A-purified anti-lectin monoclonal antibodies H85, 7F4, 5B8, 3F4 and 6D2 immobilized on 1-2 ml of Affi-Gel 10. This purification method and these antibodies are described in Petri, Jr., W. A., et al., J Biol Chem (1989) 264:3007-3012 (supra), incorporated herein by reference. The supernatant was recirculated through the column with a peristaltic pump overnight, and the column was then washed with 15 ml of solubilization buffer followed with 15 ml of phosphate-buffered saline, pH 7.5.

The bound lectin was eluted with 10 ml of 0.2N acetic acid, pH 2.5; immediately neutralized by collecting eluted fractions into a 50 ml tube containing 5 ml of 1.5M Tris, pH 8.8; dialyzed overnight against distilled water, frozen and lyophilized.

The purified lectin was used for immunization. Polyclonal antisera were obtained for use in the procedure of Example 4.

Immunized mice spleen cells from Example 1 were used for fusions to produce hybridomas as previously described in detail in Petri, Jr., W. A., et al., J Clin Invest (1987) 80:1238; Petri, Jr., W. A., et al., J Biol Chem (1989) 264:3007; and Ravdin, J. I., et al., Infect Immun (1986) 53:1, all cited above and all incorporated herein by reference.

Monoclonal antibodies were obtained from two fusions using spleen cells from BALB/C mice injected with purified lectin in complete and then in incomplete Freund's adjuvant followed by a final boost intrasplenically. The supernatants were assayed by standard ELISA using the lectin-coated (1 mg/well) microtiter plates and 36 positively-testing supernatants were obtained. The positive supernatants were then tested for immunoreactivity with the 170 kd lectin subunit by Western blot and 10 of the 36 were positive in this assay. None of these Mabs were able to identify clones containing the 170 kd subunit-encoding DNA.

The lectin 170 kd subunit purified to homogeneity in Example 1 was subjected to N-terminal sequencing and fragmented using CNBR to obtain internal sequence information. The results of these sequence determinations appear in FIG. 2. Probes designed on the basis of these sequences were unable to identify clones containing the 170 kd subunit-encoding DNA; however, matching of the DNA obtained as set forth in Example 4 with the sequence of the internal CNBR fragment sequence permitted verification that the correct cDNA had been obtained.

Total RNA was isolated from pathogenic E. histolytica trophozoites, strain NIH303, by the method of Chirgwin et al., Biochemistry (1979) 18:5294-5299. PolyA.sup.+ RNA was isolated by PolyU Sepharose guanidinium isothiocyanate preparation as described by Edman et al., Proc Natl Acad Sci USA (1987) 84:3024-3028, and the cDNA was synthesized using the commercially available kit marketed by Amersham, Arlington Heights, Ill., according to the manufacturer's protocol. EcoRI linkers were added to the double-stranded cDNA and ligated and packaged in vitro into .lambda.gt11 according to the procedure of Maniatis et al., "Molecular Cloning: A Laboratory Cloning Manual" (1982) Cold Spring Harbor Laboratories, Cold Spring Harbor, N.Y., and as described by Davis and Young, Proc Natl Acad Sci USA (1983) 80:1194-1198.

Approximately 100,000 plaques were obtained using E. coli strain Y1090 which permits expression of fusion proteins from the recombinant bacteriophage. Plaques were plated and incubated at 42.degree. C. until plaques were just barely visible (3-4 hours). A nitrocellulose filter that had been soaked in 10 mmol isopropyl .beta.-D-thiogalactopyranoside was placed on top of the plates and the plates were incubated for 3.5 hours. The filters were removed and blocked with 3% BSA and incubated at room temperature with 1:100 dilution of rabbit anti-lectin antisera, prepared as set forth in Example 1, that had been preabsorbed with E. coli proteins. The filters were washed and incubated with alkaline phosphatase conjugated anti-Rabbit IgG.

After several hours at room temperature, the filters were washed and developed with alkaline phosphate substrate. Positive plaques were isolated and the procedure was repeated until a pure plaque was obtained. Reagents used were from a Protoblot kit from Promega (Madison, Wis.).

The purified positive plaque obtained was sequenced using standard techniques and the results showed the sequence set forth in FIG. 1. FIG. 1 provides the nucleotide sequence and deduced amino acid sequence corresponding to the 170 kd heavy chain of the adherence lectin.

A cDNA library in E. coli is prepared from either a pathogenic or nonpathogenic strain of E. histolytica using the methods set forth in Example 4 above. The library is then probed with the cDNA of FIG. 1 or a 17 nucleotide or greater fragment thereof under stringent conditions. Analogous cDNA from the other known E. histolytica strains is thus recovered.

The cDNA encoding the 170 kd heavy subunit of the adherence lectin from a nonpathogenic strain was recovered and partially sequenced. A comparison of this partial sequence with the corresponding sequence in the pathogenic strain cDNA obtained in Example 4 is shown in FIG. 3. As indicated, there is extensive homology in this region.