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J Clin Microbiol. Mar 2004; 42(3): 1069–1074.
PMCID: PMC356887

Evaluation of Recombinant Fragments of Entamoeba histolytica Gal/GalNAc Lectin Intermediate Subunit for Serodiagnosis of Amebiasis


We have recently identified a 150-kDa surface antigen of Entamoeba histolytica as an intermediate subunit (Igl) of galactose- and N-acetyl-d-galactosamine-inhibitable lectin, which is a cysteine-rich protein consisting of 1,101 amino acids (aa) and containing multiple CXXC motifs in amino acid sequences. In the present study, full-length Igl except for the signal sequences (aa 14 to 1088) and three fragments of Igl—the N-terminal part (aa 14 to 382), the middle part (aa 294 to 753), and the C-terminal part (aa 603 to 1088)—were prepared in Escherichia coli, and the reactivity of these recombinant proteins with sera from patients with amebiasis was examined by means of enzyme-linked immunosorbent assay (ELISA). Sera from 57 symptomatic patients with amebic liver abscess or amebic colitis, sera from 15 asymptomatic cyst passers, sera from 40 individuals with other protozoan infections, and sera from 50 healthy controls were used. The sensitivity and specificity of the recombinant full-length Igl in the ELISA were 90 and 94%, respectively. When three fragments were used as antigens in the ELISA, the sensitivities were 56% in the N terminus, 92% in the middle part, and 97% in the C terminus. The specificities of the three antigens were 96% in the N terminus and 99% in both the middle and C-terminal fragments. These results demonstrate that Igl is well recognized in not only symptomatic but also asymptomatic patients with E. histolytica infection and that the carboxyl terminus of Igl is an especially useful antigen for the serodiagnosis of amebiasis.

Amebiasis caused by infection with Entamoeba histolytica is an important parasitic disease in both developing and developed countries. It has been estimated that E. histolytica causes 50 million cases of colitis and liver abscess, resulting in 40,000 to 110,000 deaths annually (27). In cases of liver abscesses, diagnosis and treatment at an early stage are required to prevent fatal infection. Whereas the detection of trophozoites in liver pus is not easy, a serological test is practical as a sensitive and noninvasive means of diagnosis of amebic liver abscess (11, 21). On the other hand, in cases of asymptomatic cyst passers, E. histolytica and E. dispar, which is a nonpathogenic commensal ameba, must be distinguished by PCR analysis or by the detection of E. histolytica-specific antigens (2). However, since positive serology is found in most asymptomatic cases infected with E. histolytica, serological tests are also applicable (24). Thus, serodiagnosis is an important laboratory diagnostic tool for amebiasis, as well as microscopic detection of the pathogen.

Recently, several recombinant E. histolytica antigens were prepared, and their usefulness for serodiagnosis has been reported (12, 14, 18-20, 22, 28). One of the useful antigens is the 170-kDa heavy subunit (Hgl) of galactose- and N-acetyl-d-galactosamine-inhibitable lectin, which is the key factor in amebic adherence and subsequent pathogenesis (16). Hgl is a transmembrane protein that assumes a heterodimeric conformation that conforms with glycosylphosphatidylinositol (GPI)-anchored 31/35-kDa light subunit by disulfide bonds (15). We recently identified a GPI-anchored 150-kDa intermediate subunit (Igl) of lectin, which is noncovalently associated with Hgl (4, 8). A mouse monoclonal antibody specific for Igl significantly inhibits adherence and cytotoxicity of trophozoites to mammalian cells, erythrophagocytosis, and liver abscess formation in hamsters (5, 7, 23). Igl is a cysteine-rich protein that consists of 1,101 amino acids (aa) and contains multiple CXXC motifs in amino acid sequences (4). In the previous study, we examined the reactivity of sera from ameba-infected patients to affinity purified Igl by Western immnoblot analysis. The native Igl was recognized by all sera from not only symptomatic patients but also asymptomatic cyst passers (8). However, the possibility that copurified Hgl might affect the reactivity of Igl could not be excluded. Therefore, in the present study, recombinant Igl was prepared in Esherichia coli, and its reactivity with sera from patients with amebiasis was examined. We also report here on the different reactivity of partial Igl fragments with the sera.


Plasmid constructs for recombinant proteins.

The DNA fragment coding for the full-length Igl, except for the N terminus and C terminus signal sequences (F-Igl, aa 14 to 1088 of E. histolytica Igl1), was obtained by PCR amplification of a plasmid containing the gene encoding Igl1 of E. histolytica HM-1:IMSS strain (4). DNA fragments, coding for three overlapping parts of Igl—the N-terminal part (N-Igl, aa 14 to 382), the middle part (M-Igl, aa 294 to 753), and the C-terminal part (C-Igl, aa 603 to 1088)—were also amplified by PCR. The oligonucleotide primers used are listed in Table Table1.1. Twenty cycles of PCR were performed as follows: denaturation at 94°C for 15 s (135 s in cycle 1), annealing at 55°C for 30 s, and polymerization at 72°C for 60 s (360 s in cycle 20). Each of the amplified DNA fragments were digested with XhoI, purified, and then ligated with the pET19b vector (Novagen, Madison, Wis.). The plasmids were introduced into competent Escherichia coli JM109 cells and then clones containing the right direction of inserts were selected.

List of oligonucleotide primers used in this study

Expression, purification, and refolding of recombinant proteins.

E. coli BL21 Star(DE3)pLysS cells (Invitrogen, Carlsbad, Calif.) were transformed with the cloned plasmids. Each bacterial clone was cultured in 400 ml of Luria-Bertani (LB) medium containing ampicillin until an optical density at 600 nm (OD600) of 0.6 was achieved. Isopropyl-β-d-thiogalactopyranoside was added to the bacterial cultures to a final concentration of 1 mM, and the cultures were then incubated at 37°C for 3 h. Preparation of inclusion bodies and refolding of the proteins were performed by a using the Protein Refolding Kit (Novagen) essentially according to the manufacturer's recommendations. The bacteria were pelleted by centrifugation and suspended in 16 ml of wash buffer (20 mM Tris-HCl [pH 7.5], 10 mM EDTA, 1% Triton X-100). The suspension was sonicated and then centrifuged. This washing step was repeated five times to obtain the inclusion body. The pellet of the inclusion body was suspended in solubilization buffer (500 mM 3-(cyclohexyamino)-1-propanesulfonic acid (CAPS) [pH 11], 0.3% N-lauroylsarcosine) and incubated at room temperature for 15 min. After centrifugation, the supernatant was dialyzed in dialysis buffer (20 mM Tris-HCl [pH 8.5], 0.1 mM dithiothreitol) overnight at 4°C. Dialysis was continued in the buffer without dithiothreitol for 9 h, then in redox refolding buffer (0.2 mM oxidized glutathione, 1 mM reduced glutathione) overnight at 4°C, and finally for 3 h at room temperature.

SDS-PAGE and Western immunoblot analysis.

Recombinant proteins were treated with an equal volume of the sample buffer (13) containing 2 mM phenylmethylsulfonyl fluoride, 2 mM TLCK (Nα-p-tosyl-l-lysine chloromethyl ketone), 2 mM p-hydroxymercuriphenyl sulfonic acid, and 4 μM leupeptin for 5 min at 95°C and then subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). For the Western immunoblot analysis, the protein bands were transferred to polyvinylidene difluoride membranes (26). After a blocking step with 3% skim milk in 10 mM phosphate-buffered saline (PBS; pH 7.4), each strip was incubated with serum samples diluted 1:100 in PBS containing skim milk for 1 h. After being washed with PBS containing 0.05% Tween 20 (PBS-Tween), the strips were treated with horseradish peroxidase (HRP)-conjugated goat antibody to human immunoglobulin G (IgG; whole molecule; ICN Pharmaceuticals, Aurora, Ohio). Development was by the Konica Immunostaining HRP-1000 kit (Konica Co., Tokyo, Japan).

Serum samples.

A total of 162 serum samples were studied. Sera from 23 patients with an amebic liver abscess and sera from 34 patients with amebic colitis but without a liver abscess were used as symptomatic cases. The diagnosis of these patients was based on their clinical symptoms, positive serology in an indirect immunofluorescent antibody test, ultrasound examination (liver abscess), endoscopic or microscopic examination (colitis), and prompt response to treatment with metronidazole except for a liver abscess patient who died. Sera from 15 asymptomatic cyst passers, obtained from children in institutions for the mentally retarded, were also studied. In these asymptomatic cyst passers, cysts were identified as E. histolytica but not E. dispar by PCR (17, 25). In addition, sera from 40 individuals with infection of other protozoa, Blastocystis hominis infection (23 cases), malaria (7 cases), toxoplasmosis (7 cases), and giardiasis (3 cases), were used. Most of the serum samples except for asymptomatic cyst passers were obtained from hospitalized patients and outpatients in Tokai University Hospital, Tokyo Metropolitan Komagome Hospital, and St. Luke's International Hospital in Tokyo. As a negative control, sera obtained from 50 healthy individuals with no known history of amebiasis and no parasites in their stools were also used. All of the serum samples were stored at −30°C or at −80°C before use.


Crude antigen was prepared from trophozoites of E. histolytica HM-1:IMSS axenically grown in BI-S-33 medium (9). After 3 washes with ice-cold PBS, trophozoites were sonicated and centrifuged at 12,000 × g for 30 min. The supernatant was used as crude antigen. Enzyme-linked immunosorbent assay (ELISA) was performed in duplicate on serum samples in 96-well flat-bottom microtiter plates (Coaster, Corning, N.Y.). The wells of the ELISA plates, containing 1 μg of crude antigen or 100 ng of recombinant Igl antigens diluted with 50 mM sodium bicarbonate buffer (pH 9.6), were incubated overnight at 4°C. The plates were washed with PBS-Tween and then treated with PBS containing 1% skim milk for 1 h. A total of 100 μl of the serum samples diluted 1:400 with PBS were added to the wells, followed by incubation for 1 h at room temperature. After being washed, the wells were incubated with 100 μl of HRP-conjugated goat antibody to human IgG (whole molecule; ICN Pharmaceuticals) for 1 h at room temperature. After being washed with PBS-Tween, the wells were incubated with 200 μl of substrate solution (0.4 mg per ml of o-phenylenediamine in citric acid-phosphate buffer [pH 5.0] containing 0.001% hydrogen peroxide). After 30 min of incubation, the reaction was stopped by the addition of 50 μl of 2.5 M H2SO4, and the OD490 was recorded by using a Bio-Rad (Hercules, Calif.) model 550 microplate reader. The cutoff point for a positive result was defined as an ELISA value with >3 standard deviations above the mean of the healthy negative controls. OD values were plotted and analyzed by using computer graphics software Prism version 4.0 (GraphPad, San Diego, Calif.).


Expression of recombinant fragments of E. histolytica Igl.

Igl constructs prepared in the present study were full-length Igl and three partial fragments of Igl: F-Igl (aa 14 to 1088), N-Igl (aa 14 to 382), M-Igl (aa 294 to 753), and C-Igl (aa 603 to 1088). In the design of the constructs, N-Igl and M-Igl, as well as M-Igl and C-Igl, shared overlapping regions to reduce the possible loss of antigenic epitopes in the recombinant fragments. In addition, the region where CXXC motifs were not found in Igl, aa 399 to 524, were included in only M-Igl. All of the four recombinant proteins were expressed in bacteria as inclusion bodies. The purity of the refolded proteins was analyzed by SDS-PAGE (Fig. (Fig.1).1). The apparent molecular masses of these proteins in 7.5% gel were 150 kDa for F-Igl, 53 kDa for N-Igl, 67 kDa for M-Igl, and 85 kDa for C-Igl. Since minor protein bands still existed in F-Igl, Western immunoblot analysis was performed to confirm the antigenicity of the recombinant protein (Fig. (Fig.2).2). Serum samples from four individuals each from among cases of amebic liver abscess, amebic colitis, and asymptomatic cyst passers were used in the analysis. All of these sera were reactive with the 150-kDa F-Igl band. On the other hand, two sera from patients with giardiasis did not react with the band of F-Igl as well as a serum from the healthy control.

FIG. 1.
SDS-PAGE analyses of recombinant Igl. A total of 4 μg of refolded proteins was electrophoresed in a 7.5% gel under reducing conditions. Protein bands were visualized with Coomassie brilliant blue. Lane 1, F-Igl; lane 2, N-Igl; lane 3, M-Igl; lane ...
FIG. 2.
Western immunoblot analysis of human sera reactivity with F-Igl. Purified F-Igl was subjected to SDS-PAGE in a 7.5% gel under reducing conditions and then transferred to polyvinylidene difluoride membranes. Protein bands of lane 1 were stained with Coomassie ...

ELISA reactivity of recombinant fragments with sera from patients with amebiasis.

The reactivities of four recombinant proteins—F-Igl, N-Igl, M-Igl, and C-Igl—with patients' sera were examined by ELISA and compared to that of crude antigen (Fig. (Fig.3).3). When crude antigen was used in the ELISA, all of the sera from individuals with E. histolytica infection were scored as positive (sensitivity, 100%), and 5 of 90 negative control sera were judged as positive (specificity, 94%). By the use of F-Igl as the ELISA antigen, all of the sera from asymptomatic cyst passers were positive, but 2 of 23 sera from patients with liver abscess and 5 of 34 sera from patients with colitis were scored as negative (sensitivity, 90%). In the negative controls, five of 90 (four of B. hominis infections and one of the healthy controls) results were false positives (specificity, 94%). When three fragments were used as antigens, the mean OD values in the sera from patients with amebiasis were as follows: C-Igl > M-Igl > N-Igl. In the ELISA with N-Igl, sera from 9 cyst passers, sera from 6 patients with liver abscess, and sera from 17 patients with colitis were judged to be negative (sensitivity, 56%). Three sera from patients with malaria and a serum from B. hominis-infected individuals were positive (specificity, 96%). When M-Igl was used as an antigen, all of the cyst passers and patients with liver abscess were scored as seropositive, but six of the patients with colitis were seronegative (sensitivity, 92%). In the controls, only one of the B. hominis-infected individuals was seropositive (specificity, 99%). With the use of C-Igl, there were two false-negative cases, both of which were patients with colitis (sensitivity, 97%), whereas there was only one false-positive case, which was from a patient with malaria (specificity, 99%). OD values obtained with the ELISA using C-Igl were compared to those obtained with the crude antigen-based ELISA (Fig. (Fig.4).4). There was a significant correlation between the results of these two ELISA tests (r = 0.8115; P < 0.0001).

FIG. 3.
ELISA reactivities of crude antigen and recombinant Igls from E. histolytica with sera from various patients. ELISA plates were coated with 1 μg per well of crude antigen (A) or 100 ng per well of F-Igl (B), N-Igl (C), M-Igl (D), or C-Igl (E). ...
FIG. 4.
Correlation between ELISA reactivities to C-Igl and crude antigen in sera from E. histolytica-infected individuals and controls. The assay was as described in Fig. Fig.33.


The results of the present study demonstrate that recombinant Igl is recognized well not only by sera from symptomatic patients with amebic liver abscess and amebic colitis but also by sera from asymptomatic patients in both Western immunoblotting and ELISA. This observation is coincident with the previous data in which affinity-purified native protein was recognized by sera from symptomatic and asymptomatic individuals in a Western immunoblot analysis (8).

In the ELISA system reported in the present study, only 100 ng of antigen was used for the coating of each well of the microplates. The OD values in this concentration were almost comparable with those in 1 μg of crude antigen. This fact demonstrates that the recombinant Igls possess high antigenicity. To date, several recombinant proteins of E. histolytica were prepared, and their efficacy in serodiagnosis was examined. When recombinant fragments derived from Hgl were used, sensitivities were 90 to 95% (18, 19, 28). With the use of other antigens, sensitivities were 100% for a 125-kDa surface antigen (14), 88% for 43.5-kDa alcohol dehydrogenase (12), and 82% for serine-rich 46- to 52-kDa antigens (SREHP) (22). In contrast, when a cysteine-rich 29-kDa surface antigen was used, sensitivity of the sera from patients with liver abscess was 76% but only 8% in samples from patients with colitis (20). In comparison with these previous reports, the sensitivity of C-Igl demonstrated in the present study (99%) was considerably higher. Although it is not clear at present how Hgl and Igl associate as a surface lectin complex, the present study demonstrated that Igl, as well as Hgl, is a valuable molecule for diagnostic purposes.

The most interesting observations in the present study were the different reactivities of three Igl fragments with sera from individuals with E. histolytica infections. Although high sensitivity was observed in the use of M-Igl and C-Igl as antigen, N-Igl was recognized by only about a half of the patients with amebic infections. One of the possible explanations is that the antigenic epitope(s) located in N-Igl may not be exposed to the surface in native Igl. Another possibility is that antigenic differences among E. histolytica isolates may exist in the N terminus of Igl. In the previous study, monoclonal antibodies EH3015 and EH3023 were reactive with all of the 47 E. histolytica isolates but with none of the E. dispar isolates, indicating the existence of a common epitope in E. histolytica isolates (23). On the other hand, a difference in the reactivity of monoclonal antibodies EH3056 and EH3126 was observed among E. histolytica isolates, suggesting that qualitative and/or quantitative differences of Igl may exist. In Hgl genes, it has been demonstrated recently that only slight genetic diversity exists even in the isolates showing distinct diversity in the SREHP genes (3). Since the primary structure of Igl has been clarified in only the HM-1:IMSS strain at present, sequence analysis of Igl in various strains of E. histolytica will be required in further studies.

CXXC motifs existing in Igl have also been observed in the variant-specific surface antigen of Giardia intestinalis (1, 10). Therefore, it is possible that common epitopes between E. histolytica Igl and G. intestinalis variant-specific surface antigen may exist. In the present study, however, sera from patients with giardiasis did not react with recombinant Igls in both the Western immunoblotting and ELISA, although the number of sera examined was limited. Whereas several commercial kits using crude antigens are now available for serodiagnosis of amebiasis, an expected merit of recombinant proteins for diagnostic purposes must be their higher specificity. Indeed, specificity of the ELISA with C-Igl (99%) was higher than that of crude antigen-based ELISA (94%) in the present study when the cutoff point was defined as an OD value with three standard deviations above the mean of the healthy negative controls. However, a comparison of Fig. 3A and E suggests that discrimination between amebic cases and controls is more evident with the use of the crude antigen. Therefore, we cannot exclude the interpretation that, if the cutoff point is set above the mean plus three standard deviations of the controls, the specificity of the ELISA with crude antigen increases. Another advantage of the recombinant protein is that the use of defined proteins in serodiagnosis will facilitate standardization of the assays. In addition, production of a recombinant protein in large quantities may be an economically effective method compared to the cultivation of trophozoites (14, 18, 22).

When F-Igl was used as the antigen, whereas two sera from patients with amebic liver abscess and five sera from patients with amebic colitis were negative, none of the sera from asymptomatic cyst passers was negative. In the cases of ELISA with M-Igl and C-Igl as antigens, false-negative results were detected in only the colitis cases. These observations suggest that the antibodies which recognized the epitopes located in M-Igl and C-Igl may function to prevent the invasion of trophozoites into host tissues. Indeed, when hamsters have been immunized with native Igl, liver abscess formation has been significantly inhibited (6). Therefore, a partial fragment of Igl, such as M-Igl and C-Igl, may also be one of the candidate vaccines for amebiasis.

In conclusion, recombinant Igl was well recognized by sera from patients with amebiasis but not by sera from patients with other protozoan infections. In particular, the C terminus fragment (aa 603 to 1088) of Igl was valuable for the serodiagnosis of amebiasis.


This study was supported by a Grant-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science and by grants from the Ministry of Health, Labour, and Welfare of Japan. X.-J.C. is a recipient of the Japanese Society for the Promotion of Science Postdoctoral Fellowship for Foreign Researchers.


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