Protection of gerbils from amebic liver abscess by immunization with the galactose-specific adherence lectin of Entamoeba histolytica

Chitinase secretion by encysting Entamoeba invadens and transfected Entamoeba histolytica trophozoites: localization of secretory vesicles, endoplasmic reticulum, and Golgi apparatus

Entamoeba histolytica, the protozoan parasite that phagocytoses bacteria and host cells, has a vesicle/vacuole-filled cytosol like that of macrophages. In contrast, the infectious cyst form has four nuclei and a chitin wall. Here, anti-chitinase antibodies identified hundreds of small secretory vesicles in encysting E. invadens parasites and in E. histolytica trophozoites overexpressing chitinase under an actin gene promoter. Abundant small secretory vesicles were also identified with antibodies to the surface antigen Ariel and with a fluorescent substrate of cysteine proteinases. Removal of an N-terminal signal sequence directed chitinase to the cytosol. Addition of a C-terminal KDEL peptide, identified on amebic BiP, retained chitinase in a putative endoplasmic reticulum, which was composed of a few vesicles of mixed sizes. A putative Golgi apparatus, which was Brefeldin A sensitive and composed of a few large, perinuclear vesicles, was identified with antibodies to ADP-ribosylating factor and to epsilon-COP. We conclude that the amebic secretory pathway is similar to those of other eukaryotic cells, even if its appearance is somewhat different.

Host-pathogen interaction in amebiasis and progress in vaccine development

Entamoeba histolytica, the causative organism of invasive intestinal and extraintestinal amebiasis, infects approximately 50 million people each year, causing an estimated 40 to 100 thousand deaths annually. Because amebae only infect humans and some higher non-human primates, an anti-amebic vaccine could theoretically eradicate the organism. Uncontrolled epidemiologic studies indicate that acquired immunity to amebic infection probably occurs and that such a vaccine might be feasible. Application of molecular biologic techniques has led to rapid progress towards understanding how Entamoeba histolytica causes disease, and to the identification of several amebic proteins associated with virulence. These proteins are now being evaluated as potential vaccine components. Parenteral and oral vaccine preparations containing recombinant amebic proteins have been effective in preventing disease in a gerbil model of amebic liver abscess. Although systemic and mucosal cellular and humoral immunity both appear to play a role in protection against Entamoeba histolytica, the relative importance of each in the human immune response remains unknown. No animal model of intestinal amebiasis currently exists, moreover, so it has been impossible to evaluate protection against colonization and colitis. Further investigation of the fundamental mechanisms by which Entamoeba histolytica causes disease and of the human immune response to amebic infection is necessary to assess the true feasibility of an anti-amebic vaccine.

Protection against invasive amebiasis by a single monoclonal antibody directed against a lipophosphoglycan antigen localized on the surface of Entamoeba histolytica

A panel of monoclonal antibodies was raised from mice immunized with a membrane preparation from Entamoeba histolytica, the pathogenic species causing invasive amebiasis in humans. Antibody EH5 gave a polydisperse band in immunoblots from membrane preparations from different E. histolytica strains, and a much weaker signal from two strains of the nonpathogenic species Entamoeba dispar. Although the exact chemical structure of the EH5 antigen is not yet known, the ability of the antigen to be metabolically radiolabeled with [32P]phosphate or [3H]glucose, its sensitivity to digestion by mild acid and phosphatidylinositol-specific phospholipase C, and its specific extraction from E. histolytica trophozoites by a method used to prepare lipophosphoglycans from Leishmania showed that it could be classified as an amebal lipophosphoglycan. Confocal immunofluorescence and immunogold labeling of trophozoites localized the antigen on the outer face of the plasma membrane and on the inner face of internal vesicle membranes. Antibody EH5 strongly agglutinated amebas in a similar way to concanavalin A (Con A), and Con A bound to immunoaffinity-purified EH5 antigen. Therefore, surface lipophosphoglycans may play an important role in the preferential agglutination of pathogenic amebas by Con A. The protective ability of antibody EH5 was tested in a passive immunization experiment in a severe combined immunodeficient (SCID) mouse model. Intrahepatic challenge of animals after administration of an isotype-matched control antibody or without treatment led to the development of a liver abscess in all cases, whereas 11 out of 12 animals immunized with the EH5 antibody developed no liver abscess. Our results demonstrate the importance and, for the first time, the protective capacity of glycan antigens on the surface of the amebas.

Progress towards development of a vaccine for amebiasis

The application of molecular biologic techniques over the past decade has seen a tremendous growth in our knowledge of the biology of Entamoeba histolytica, the causative agent of amebic dysentery and amebic liver abscess. This approach has also led to the identification and structural characterization of three amebic antigens, the serine-rich Entamoeba histolytica protein (SREHP), the 170-kDa subunit of the Gal/GalNAc binding lectin, and the 29-kDa cysteine-rich protein, which all show promise as recombinant antigen-based vaccines to prevent amebiasis. In recent studies, an immunogenic dodecapeptide derived from the SREHP molecule has been genetically fused to the B subunit of cholera toxin, to create a recombinant protein capable of inducing both antiamebic and anti-cholera toxin antibodies when administered by the oral route. Continued progress in this area will bring us closer to the goal of a cost-effective oral combination "enteric pathogen" vaccine, capable of inducing protective mucosal immune responses to several clinically important enteric diseases, including amebiasis.

The tumor necrosis factor alpha-stimulating region of galactose-inhibitable lectin of Entamoeba histolytica activates gamma interferon-primed macrophages for amebicidal activity mediated by nitric oxide

Entamoeba histolytica adheres via galactose-lectin (Gal-lectin) to human colonic mucins and intestinal epithelial cells as a prerequisite to amebic invasion. Native Gal-lectin is a protective antigen in the gerbil model of amebiasis. Amino acids 596 to 1082 of Gal-lectin mediate E. histolytica adherence to target cells and stimulate tumor necrosis factor alpha (TNF-alpha) production by naive murine bone marrow macrophages (BMM). Resistance to amebiasis requires an effective cell-mediated immune response against E. histolytica trophozoites mediated by nitric oxide (NO) released from activated macrophages. Herein, we determine whether the TNF-alpha-stimulating region of Gal-lectin can activate gamma interferon (IFN-gamma)-primed BMM for NO production and amebicidal activity. Native Gal-lectin (100 to 500 ng/ml) stimulated TNF-alpha and inducible nitric oxide synthase (iNOS) mRNA expression in IFN-gamma-primed BMM as did lipopolysaccharide (100 ng/ml). Primed BMM produced TNF-alpha and NO in response to Gal-lectin in a dose-dependent manner. Antilectin monoclonal antibody IG7, which recognizes a domain (amino acids 596 to 818) of the TNF-alpha mRNA-stimulating region of Gal-lectin, specifically inhibited TNF-alpha and iNOS mRNA induction and TNF-alpha and NO production by primed BMM in response to Gal-lectin (100 ng/ml). Simultaneous treatment of BMM with IFN-gamma and Gal-lectin (100 ng/ml) activated the cells to kill E. histolytica trophozoites, whereas IFN-gamma treatment alone had no effect. In the presence of monoclonal antibody 1G7 or aminoguanidine (an iNOS inhibitor), NO production and amebicidal activity were inhibited >80%. These results suggest that the TNF-alpha-stimulating region of native Gal-lectin is a potent stimulus of IFN-gamma-primed BMM for NO production, which is essential for host defense against amebiasis.

Identification of an epitope on the Entamoeba histolytica 170-kD lectin conferring antibody-mediated protection against invasive amebiasis

The emergence of multidrug-resistant organisms and the failure to eradicate infection by a number of important pathogens has led to increased efforts to develop vaccines to prevent infectious diseases. However, the nature of the immune response to vaccination with a given antigen can be complex and unpredictable. An example is the galactose- and N-acetylgalactosamine-inhibitable lectin, a surface antigen of Entamoeba histolytica that has been identified as a major candidate in a vaccine to prevent amebiasis. Vaccination with the lectin can induce protective immunity to amebic liver abscess in some animals, but others of the same species exhibit exacerbations of disease after vaccination. To better understand this phenomenon, we used recombinant proteins corresponding to four distinct domains of the molecule, and synthetic peptides to localize both protective and exacerbative epitopes of the heavy chain subunit of the lectin. We show that protective immunity after vaccination can be correlated with the development of an antibody response to a region of 25 amino acid residues of the lectin, and have confirmed the importance of the antibody response to this region by passive immunization studies. In addition, we show that exacerbation of disease can be linked to the development of antibodies that bind to an NH2-terminal domain of the lectin. These findings are clinically relevant, as individuals who are colonized with E. histolytica but are resistant to invasive disease have a high prevalence of antibodies to the protective epitope(s), compared to individuals with a history of invasive amebiasis. These studies should enable us to develop an improved vaccine for amebiasis, and provide a model for the identification of protective and exacerbative epitopes of complex antigens.

Protection in a gerbil model of amebiasis by oral immunization with Salmonella expressing the galactose/N-acetyl D-galactosamine inhibitable lectin of Entamoeba histolytica

Infection with the enteric parasite Entamoeba histolytica can result in colitis and dysentery as well as abscesses at extra-intestinal sites. An effective vaccine must be able to protect against both mucosal and systemic disease. In this study an attenuated Salmonella strain that expressed a portion of the GalNAc lectin of E, histolytica was used to orally immunize gerbils. Animals were challenged by intrahepatic injection of amebic trophozoites. A significant decrease in size of amebic liver abscesses was observed in orally immunized animals. Oral immunization with a Salmonella-based vaccine was as effective as systemic immunization for protection against systemic challenge.

Immunodominant Entamoeba histolytica antigens recognized by serum and intestinal antibodies after local or systemic immunization of mice with glutaraldehyde fixed trophozoites

We performed an immunoblot analysis of the main E. histolytica proteins recognized by immune sera and intestinal fluids of Balb/c mice immunized with glutaraldehyde fixed trophozoites (GFT) by intragastric, rectal and intraperitoneal routes, to determine if there were differences in the amebic antigens immunodominantly recognized at mucosal and systemic levels. The antigen patterns recognized by mice immunized via intraperitoneal and rectal routes were complex and similar suggesting that the immunization route (systemic or local), does not influence the recognition pattern elicited at mucosal or systemic levels. However, the number of amebic bands recognized after intragastric immunization was very low. The molecular weights of the principal amebic proteins recognized by serum antibodies were 150-130, 116, 104, 84, 56, 42, 18, and 16 kDa. The intestinal fluids of mice immunized via intraperitoneal and rectal routes contained antibodies that recognized five bands of 220-200, 150-134, 93-84, 43-41, and 16-14 kDa. These results suggest that there are differences in the number of immunodominant amebic antigens recognized at mucosal and systemic levels. Moreover we found that the bands of 150, 39 and 19 kDa. were mainly recognized by IgG, whereas the bands of 116, 93, and 16 were mainly recognized by IgM, indicating differences between the antigens immunodominantly recognized by serum antibodies from different isotypes.

Recent advances in amebiasis

Advancements in our understanding of amebiasis have been rapid over the decade that I have followed this field. What was identified morphologically for years as Entamoeba histolytica has been redescribed with modern techniques as a complex of two species, the commensal parasite E. dispar and the pathogenic parasite E. histolytica that is the cause of colitis and liver abscess. Antigen detection tests are now available for the rapid detection in stool of the pathogenic species E. histolytica. New understandings of the importance of luminal as well as tissue-active antimebic medications in the treatment of invasive disease have been reached. The groundwork is being laid for an understanding of the protective immune responses to infection, and at the lab bench DNA transfection of the parasite has opened studies of pathogenesis to genetic analysis. While necessarily an incomplete sketch of the field, I have attempted here to highlight some recent and important developments of interest to clinicians and microbiologists.

Progress towards an amebiasis vaccine

Amebiasis (infection by Entamoeba histolytica) remains a major health problem in much of the developing world. Morbidity and mortality from amebic dysentery and amebic liver abscess have persisted despite the availability of effective anti-amebic therapy, suggesting a need for alternative measures of disease control. Through the application of recombinant DNA technology, several E. histolytica antigens have now been expressed in prokaryotic systems and tested in animal models as vaccines to prevent invasive amebiasis. In this review, Sam Stanley Jr discusses why a vaccine for amebiasis may be feasible, and describes the recent development of several promising recombinant E. histolytica antigen-based parenteral and oral vaccine candidates.

Identification of the galactose-adherence lectin epitopes of Entamoeba histolytica that stimulate tumor necrosis factor-alpha production by macrophages

The 170-kDa subunit of the galactose-adherence lectin (Gal-lectin) of Entamoeba histolytica mediates adherence to human colonic mucins and intestinal epithelium as a prerequisite to amebic invasion. The Gal-lectin is an immunodominant molecule and a protective antigen in the gerbil model of amebiasis. Tumor necrosis factor alpha (TNF-alpha) produced by activated macrophages enhances nitric oxide-dependent cytotoxicity in host defense against E. histolytica. The purpose of this study was to identify the Gal-lectin epitopes which stimulate TNF-alpha production by macrophages. Murine bone marrow-derived macrophages (BMMs) exposed to Gal-lectin (100-500 ng/ml) stimulated stable expression of TNF-alpha mRNA (8-fold increase) and TNF-alpha production similar to that of lipopolysaccharide-stimulated cells (100 ng/ml). Polyclonal anti-lectin serum specifically inhibited TNF-alpha mRNA induction in response to the Gal-lectin but not to lipopolysaccharide. Anti-lectin monoclonal antibodies 8C12, H85 and 1G7, which recognize nonoverlapping epitopes of the cysteine-rich region of the 170-kDa heavy subunit, inhibited both amebic adherence to mammalian cells and Gal-lectin-stimulated TNF-alpha mRNA expression by BMMs,but monoclonal antibody 7F4 did neither. As these inhibitory antibodies map to amino acids 596-1082 of the 170-kDa Gal-lectin, our results have identified the functional region that mediates amebic adherence and TNF-alpha mRNA induction in BMMMs; thus, this region of the Gal-lectin is a subunit vaccine candidate.

Identification of immunogenic epitopes of the 170-kDa subunit adhesin of Entamoeba histolytica in patients with invasive amebiasis

Entamoeba histolytica causes amebic dysentery (AD) and liver abscess (ALA). Little is known about protective immunity to amebiasis, and studies in this area have been complicated by the paucity of defined ameba antigens. We examined the proliferative responses of peripheral blood mononuclear cells (PBMC) from patients with AD and ALA to a recombinant protein containing a portion of the 170 kDa adhesin of E. histolytica (170CR), and to two synthetic peptides (1 and 2) derived from the 170 kDa sequence that were predicted to contain T cell epitopes. A significant number of patients with AD and ALA had PBMC that proliferated to 170CR molecule, and several individuals with ALA and AD had T cells that recognized one or both peptides. Contrarily, individuals from a non-endemic region for amebiasis did not respond to 170CR protein, or to both peptides. In regard to antibody response, nine of fifteen patients with ALA showed antibodies to 170CR protein. These same patients had antibodies to peptide 2. We identified peptides from 170-kDa adhesin that may contain both T and B cell epitopes recognized by some patients with invasive amebiasis. These peptides may be valuable reagents in studies of the immune response to amebiasis.

Adherence and cytotoxicity of Entamoeba histolytica or how lectins let parasites stick around

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Galactosyl and N-acetylgalactosaminyl homeostasis: a function for mammalian asialoglycoprotein receptors

Mammalian livers express endocytic cell surface receptors that specifically bind natural or synthetic molecules containing terminal galactosyl or N-acetylgalactosaminyl sugars. One of these hepatocyte receptors is the asialoglycoprotein receptor, which mediates the endocytosis and subsequent lysosomal degradation of these glyco-molecules. Although the receptor was discovered almost 30 years ago, the physiological reason why mammals have this receptor is still unknown. At the cellular level, the basic molecular function of the receptor is to mediate the uptake and ultimate degradation of galactosyl/N-acetylgalactosaminyl-containing molecules (ligands). At the organism level, however, the physiological function is uncertain. The identity of the natural ligands and the reasons for this elaborate receptor system to remove these ligands are both unknown. This article proposes an explanation for the purpose of this asialoglycoprotein receptor and its role in regulating the dynamic flux of galactosyl/N-acetylgalactosaminyl glycoconjugates in mammals.

Protection of gerbils from amebic liver abscess by immunization with a recombinant protein derived from the 170-kilodalton surface adhesin of Entamoeba histolytica

The protozoan parasite Entamoeba histolytica causes extensive morbidity and mortality worldwide through intestinal infection and amebic liver abscess. Here we show that vaccination of gerbils, a standard model for amebic liver abscess, with recombinant proteins derived from the 170-kDa galactose-binding adhesin of E. histolytica and the serine-rich E. histolytica protein or a combination of the two recombinant antigens provides excellent protection against subsequent hepatic challenge with virulent E. histolytica trophozoites.

Antibodies to the serine rich Entamoeba histolytica protein (SREHP) prevent amoebic liver abscess in severe combined immunodeficient (SCID) mice

Amoebic liver abscess caused by Entamoeba histolytica is a major cause of morbidity and mortality worldwide. We used mice with severe combined immunodeficiency (SCID mice) to study the role of antibody in protection from amoebic liver abscess, and to identify protective antigens of E. histolytica. Antisera to recombinant versions of two major surface antigens of E. histolytica, the serine rich E. histolytica protein (SREHP) and the 170 kDa adhesin were used in this study. We found that 100% of SCID mice passively immunized with antiserum to the recombinant SREHP molecule were protected from developing amoebic liver abscess after intrahepatic challenge with virulent E. histolytica trophozoites. In contrast, preimmune serum, antiserum to a portion of the 170 kDa adhesin, and antiserum to the trpE fusion partner of SREHP did not protect SCID mice from amoebic liver abscess. Our study demonstrates that antibodies to a recombinant version of the amoebic SREHP molecule can protect against amoebic liver abscess, and suggest the recombinant SREHP molecule should be considered as a possible vaccine candidate to prevent amoebic liver abscess.

Characterization of monoclonal antibodies to conserved antigens of Entamoeba histolytica and E. dispar and development of a stool ELISA

Eight murine monoclonal antibodies (MAbs) were generated against Entamoeba histolytica NIH:200. All MAbs reacted with three axenic strains of E. histolytica, NIH:200, HM-1: IMMS, and SAW 1734R clAR, but not with enteric bacteria or Giardia lamblia. Five of the MAbs reacted with low-molecular-weight, periodate-sensitive antigens of 14-21 kD, while one (AB31) reacted with high-molecular-weight, 90 to 200-kD protein determinants. MAb PC14 appeared to be specific for antigen in its native state. Another MAb (BB12) agglutinated live trophozoites and caused membrane fluorescence in contrast to the five other MAbs tested. Although BB12 reacted with the same 14 to 21-kD band on Western blot as AC55, the latter reacted with different cytoplasmic epitopes. All the MAbs reacted to five pathogenic (E. histolytica) and six nonpathogenic (E. dispar) clinical isolates. These MAbs may be helpful for studying conserved antigens of E. histolytica and were used to develop a sandwich ELISA for the diagnosis of intestinal amoebiasis. The assay was sensitive to 60 ng of E. histolytica antigen. A comparative study of microscopic examination of stool samples and the sandwich ELISA was conducted on 102 samples from patients with gastrointestinal complaints. The ELISA could detect all microscopically positive samples with a sensitivity of 100% and specificity of 93%. A sandwich ELISA using a monoclonal antibody to conserved antigens of E. histolytica has the potential to be a reliable method for the diagnosis of intestinal amoebiasis.

Protection of gerbils from amebic liver abscess by immunization with a recombinant Entamoeba histolytica antigen

Amebiasis, infection by the intestinal protozoan parasite Entamoeba histolytica, is a leading parasitic cause of death. As a step in the development of a recombinant antigen vaccine to prevent E. histolytica infection, we looked at the ability of a recombinant version of the serine-rich E. histolytica protein (SREHP) to elicit a protective immune response against invasive amebic disease. Gerbils, a standard model for amebic liver abscess, were immunized with either a recombinant SREHP/maltose-binding protein (MBP) fusion, recombinant MBP alone, or phosphate-buffered saline (PBS), all combined with complete Freund's adjuvant. In the first trial (group 1), gerbils received a primary and two booster immunizations intraperitoneally; in the second trial (group 2), gerbils were immunized by a single intradermal injection. SREHP/MBP-immunized gerbils in both groups produced antibody to native SHEHP and developed delayed-type hypersensitivity responses to recombinant SREHP. All gerbils were challenged by an intrahepatic injection with 5 x 10(4) virulent E. histolytica HM1-IMSS trophozoites. Complete protection from amebic liver abscess was seen in 64% of the SHEHP/MBP-immunized gerbils in group 1 and in 100% of the SREHP/MBP-immunized gerbils in group 2. There was no protection observed in MBP- or PBS-immunized gerbils in either group. Our results indicate that the SREHP molecule has potential as a vaccine to prevent amebic infection and demonstrate that successful vaccination of animals with recombinant E. histolytica antigen vaccines is possible.

Serum-independent and serum-dependent cytoadherence in the interaction of Entamoeba histolytica with mammalian target cells

Entamoeba histolytica kills target cells only on direct contact, suggesting that trophozoite-mediated cytolysis is initiated by the contact between trophozoites and target cells. We have shown that adherence between E. histolytica and target cells (polymorphonuclear granulocytes, erythrocytes, Chinese hamster ovary cells, human colon carcinoma cells) was inhibited by specific carbohydrates, and adherence between E. histolytica and polymorphonuclear granulocytes (PMN) was enhanced by preincubation of the trophozoites with serum. Inhibition of adherence clearly paralleled inhibition of cytolysis and phagocytosis of target cells. Cytolysis of PMN, however, was not increased by preincubation of the trophozoites with serum. These results suggest that the effector functions of trophozoites are only dependent on carbohydrate-specific adherence mechanisms mediated by the amoebic Gal/GalNAc-binding lectin. E. histolytica trophozoites themselves can be killed by PMN, depending on the virulence of the trophozoites. PMN could not kill E. histolytica trophozoites more effectively when the adherence was enhanced by preincubation of the trophozoites with serum or when adherence was only mediated by serum-dependent mechanisms.

Neutralizing monoclonal antibody epitopes of the Entamoeba histolytica galactose adhesin map to the cysteine-rich extracellular domain of the 170-kilodalton subunit

Entamoeba histolytica adheres to human colonic mucins and colonic epithelial cells via a galactose-binding adhesin. The adhesin is a heterodimeric glycoprotein composed of 170- and 35-kDa subunits. Fragments of the hgl1 gene encoding the 170-kDa subunit were expressed as recombinant fusion proteins in Escherichia coli and reacted with anti-adhesin monoclonal antibodies (MAbs) or pooled human immune sera. The MAbs tested recognize seven distinct epitopes on the 170-kDa subunit and have distinct effects on the adherence and complement-inhibitory activities of the adhesin. All seven MAbs reacted with a fusion protein containing the cysteine-rich domain of the protein. Pooled human immune sera reacted with the same cysteine-rich domain as the MAbs and also with a construct containing the first 596 amino acids. Reactivity of three MAbs with the surface of intact trophozoites confirmed that the cysteine-rich domain was located extracellularly. The location of individual epitopes was fine mapped by constructing carboxy-terminal deletions in the cysteine-rich region of the fusion protein. The locations of adherence-enhancing and -inhibiting epitopes were partially distinguished, and the epitopes where complement-inhibitory MAbs bound were demonstrated to be near the adhesin's area of sequence identity with the human complement inhibitor CD59.

Biochemical and immunological studies on soluble antigens of Entamoeba histolytica

The soluble antigens of Entamoeba histolytica trophozoites were analysed in detail by biochemical and immunochemical methods. The antigen was highly complex and heterogeneous as revealed by Sephacryl S-300 column chromatography, which showed four distinct fractions. The molecular mass of fractions FI, FII, FIII and FIV was 660, 170, 65 and 13 kDa, respectively. Protein was the major constituent in crude soluble antigen (CSA) and fractions FI and FII (67%, 80% and 90%, respectively). Polysaccharide was predominant in the FIII fraction (59%). Antigenic activity observed after different physico-chemical treatments revealed that CSA and FI antigens were predominantly glycoprotein in nature. However, the antigenicity of FIII antigen was greatly reduced after sodium meta-periodate treatment, whereas no alteration in reactivity was discerned after trypsin treatment. Sodium dodecyl sulphate-polyacryl-amide gel electrophoresis (SDS-PAGE) analysis demonstrated nearly 28 Coomassie blue bands for CSA and 20, 16, 15 and 3 polypeptide bands for the FI, FII, FIII and FIV fractions, respectively. The molecular mass of the polypeptides of these bands ranged from 210 to 20 kDa. Antigenic activity was observed in CSA and in the first three fractions, both in counter immunoelectrophoresis (CIEP) and in enzyme-linked immunosorbent assay (ELISA). However, the highest antigenic activity was noted in fraction FI. Major immunoreactive polypeptides of CSA and FI antigens against whole trophozoite antibody were observed in the 10- to 170-kDa regions. However, major differences in the immunoreactivity of the two antigens were noted at 116 and 14 kDa for FI antigen and at 84, 30 and 20 kDa for CSA.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigenicity, immunogenicity and vaccine efficacy of the galactose-specific adherence protein of Entamoeba histolytica

Entamoeba histolytica is an enteric protozoan that causes amoebic colitis and liver abscess. Human immunity to E. histolytica is apparently mediated by a serum antibody response and amoebicidal cellular mechanisms. The galactose-specific adherence protein of E. histolytica is a 260 kDa glycoprotein which mediates amoebic in vitro adherence to human colonic mucins, epithelium, and inflammatory cells. Amoebic lysis of cells is dependent upon binding by this adherence protein. Serum IgG and salivary IgA antibodies from greater than 90% of subjects with invasive amoebiasis recognize the adherence protein's 170 kDa heavy subunit. Incubation of peripheral blood mononuclear cells from antibody-positive subjects with the purified galactose-specific adherence protein induces in vitro T lymphocyte proliferation, IL-2 and gamma interferon production, and direct lymphocyte amoebicidal activity. The rationale for an adherence protein vaccine includes sIgA blockage of amoebic binding to colonic mucins, humoral IgG prevention of parasitic adherence in tissues, and development of amoebicidal cell-mediated immunity. Immunization of gerbils with purified adherence protein in Freund's adjuvant provides protection against intrahepatic challenge with the trophozoites. In summary, the galactose specific adherence protein of E. histolytica contains highly conserved B- and T-cell epitopes, and has a high degree of vaccine efficacy in the gerbil model of amoebic liver abscess.

Analysis of adhesion and cytotoxicity of Tritrichomonas foetus to mammalian cells by use of monoclonal antibodies

The relationship of Tritrichomonas foetus adhesion to mammalian cells and cytotoxicity to these targets was investigated. High-adherence and low-adherence lines of T. foetus, derived by repeated adhesion to HeLa cells, showed high and low cytotoxicity, respectively, to HeLa cells. When parasites were separated from targets by membranes (0.4-microns pore size), no cytotoxicity was detectable. Monoclonal antibodies elicited against T. foetus that lowered adhesion also lowered parasite-mediated cytotoxicity. Flow cytometry experiments revealed that the levels of an adhesion- and cytotoxicity-blocking antibody bound to the surface of high-adherence clones of T. foetus were higher than those in low-adherence clones. Western blots of parasite extracts separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were probed with an anti-T. foetus antibody. A molecule with a molecular weight of approximately 190,000 composed of subunits with molecular weights of approximately 140,000 to 150,000 and approximately 65,000 was identified. Immunoprecipitation experiments with metabolically labeled T. foetus and the same antibody confirmed that similar subunits were synthesized by the parasite. These results indicate that adhesion of T. foetus to mammalian cells is an important step in cytotoxic damage of these targets and that a surface adhesin on the parasite is involved in the adhesion mechanism.

Amebiasis

Entamoeba histolytica, the causative agent of amebiasis, was first described in 1875. Although a large number of people throughout the world are infected with this organism, only a small percentage will develop clinical symptoms. Morbidity and mortality due to E. histolytica vary from area to area and person to person. Recent findings have suggested that there are pathogenic and nonpathogenic strains of E. histolytica that can be differentiated by isoenzyme (zymodeme) analysis, monoclonal antibodies, and DNA probes. Whether pathogenicity is a genotypic trait or can be changed by environmental influences has not been resolved. Exchange of genetic material between strains of amebae can influence zymodeme patterns. Currently, detection of E. histolytica infections depends on examinations for ova and parasites and on serologic tests; however, the development of monoclonal antibodies and DNA probes specific for pathogenic zymodemes may be beneficial for clinical laboratory testing and therapeutic decisions when approved tests become available. A better understanding of the mechanisms of pathogenicity at the molecular level is evolving and should promote the development of vaccines and better target selection for therapeutic agents.

Inhibition of the complement membrane attack complex by the galactose-specific adhesion of Entamoeba histolytica

The human complement system is an important early host defense against infection. Entamoeba histolytica activates the complement system but is resistant to killing by complement C5b-9 complexes deposited on the membrane surface. Our aim was to identify components of the amebic plasma membrane that mediate resistance to human complement C5b-9 by screening for neutralizing monoclonal antibodies. A monoclonal antibody was identified that abrogated amebic resistance to C5b-9, and the mAb was shown to recognize the parasite's galactose-specific adhesin. The purified adhesin bound to C8 and C9 and conferred C5b-9 resistance to sensitive ameba upon reconstitution; these activities of the adhesin were inhibited by the antiadhesin mAb. The E. histolytica adhesin shared sequence similarities and antigenic cross-reactivity with CD59, a membrane inhibitor of C5b-9 in human blood cells, suggesting both molecular mimicry and shared complement-inhibitory functions.

Human T-lymphocyte proliferation, lymphokine production, and amebicidal activity elicited by the galactose-inhibitable adherence protein of Entamoeba histolytica

We studied human T-lymphocyte responses to the purified Entamoeba histolytica galactose-inhibitable adherence protein. Individuals having serum anti-adherence protein antibodies possess peripheral blood lymphocytes which demonstrate antigen-specific responses to the purified adherence protein (10 micrograms/ml) and whole soluble amebic antigen (100 micrograms/ml). This was determined by incorporation of [3H]thymidine (53,080 and 73,114 dpm, respectively) and by increased production of interleukin-2 and gamma interferon (42.0 and 67.5 U/ml, respectively) (P less than 0.05 for each in comparison with values for control lymphocyte responses). Lymphocytes from antiamebic antibody-positive subjects develop in vitro amebicidal activity only when incubated for 5 days with the purified adherence protein (P = 0.02). In conclusion, the E. histolytica galactose-inhibitable adherence protein elicits an in vitro amebicidal cell-mediated immune response, further supporting the potential for the use of this protein in a subunit amebiasis vaccine.

Role of the galactose lectin of Entamoeba histolytica in adherence-dependent killing of mammalian cells

Entamoeba histolytica extracellular killing of host cells is contact dependent. Adherence to human colonic epithelial cells and mucins is mediated by a galactose-specific lectin. The effect on cytotoxicity of a panel of monoclonal antibodies (MAb) directed against the galactose lectin was tested. As expected, those MAb which inhibited adherence also decreased cytotoxicity. However, one antilectin MAb blocked cytotoxicity after adherence had occurred, indicating that the lectin has a role in cell killing that is distinct from its adherence function.

Cell surface proteins of Entamoeba histolytica

To ask what is new in Entamoeba histolytica research, one need look no further than the surface of this protozoan parasite. In the past year the cloning and partial characterization of five different surface antigens have been reported, a remarkable result of international research efforts against amebiosis. One of these proteins is the first protective immunogen identified in the animal model of amebic liver abscess. Barbara Mann and William Petri review these recent results, propose a nomenclature for the gene family of E. histolytica galactose lectins and discuss the roles of the different surface proteins in adhesion.

Sequence of a cysteine-rich galactose-specific lectin of Entamoeba histolytica

Entamoeba histolytica trophozoites adhere to human colonic mucins and epithelial cells by a cell surface galactose-specific lectin. This lectin, which is composed of two subunits linked by disulfide bonds, has been shown to be a protective antigen in an animal model of amebiasis. We have determined the sequence of the mature form of the 170-kDa heavy subunit from cDNA clones and PCR-amplified fragments. The heavy subunit sequence consisted of a putative extracellular domain containing 1209 amino acids with 16 potential sites for N-linked glycosylation, a 26-amino acid hydrophobic region, and a 41-amino acid cytoplasmic tail. The presence of N-linked oligosaccharides was confirmed by culturing amebae with tunicamycin, which resulted in a decrease in the heavy subunit molecular mass to 160 kDa and a loss of lectin activity. The extracellular domain was remarkable for an extensive cysteine-rich domain that shared identify with similar regions of several other cell surface proteins and appeared to confer protease resistance to the subunit.