The bittersweet interface of parasite and host: lectin-carbohydrate interactions during human invasion by the parasite Entamoeba histolytica

Stress by heat shock induces massive down regulation of genes and allows differential allelic expression of the Gal/GalNAc lectin in Entamoeba histolytica

Gene expression analysis by microarray assay revealed that when exposed to stress, Entamoeba histolytica exhibits a specific heat shock response, together with a dramatic overall reduction in gene transcription as well as differential allelic expression of key genes participating in virulence, such as the galactose/N-acetylgalactosamine (Gal/GalNAc) lectin.

Impact of intestinal colonization and invasion on the Entamoeba histolytica transcriptome

A genome-wide transcriptional analysis of Entamoeba histolytica was performed on trophozoites isolated from the colon of six infected mice and from in vitro culture. An Affymetrix platform gene expression array was designed for this analysis that included probe sets for 9435 open reading frames (ORFs) and 9066 5' and 3' flanking regions. Transcripts were detected for > 80% of all ORFs. A total of 523 transcripts (5.2% of all E. histolytica genes) were significantly changed in amebae isolated from the intestine on Days 1 and 29 after infection: 326 and 109 solely on Days 1 and 29, and 88 on both days. Quantitative real-time reverse transcriptase PCR confirmed these changes in 11/12 genes tested using mRNA isolated from an additional six mice. Adaptation to the intestinal environment was accompanied by increases in a subset of cell signaling genes including transmembrane kinases, ras and rho family GTPases, and calcium binding proteins. Significant decreases in mRNA abundance for genes involved in glycolysis and concomitant increases in lipases were consistent with a change in energy metabolism. Defense against bacteria present in the intestine (but lacking from in vitro culture) was suggested by alterations in mRNA levels of genes similar to the AIG1 plant antibacterial proteins. Decreases in oxygen detoxification pathways were observed as expected in the anaerobic colonic lumen. Of the known virulence factors the most remarkable changes were a 20-35-fold increase in a cysteine proteinase four-like gene, and a 2-3-fold decrease in two members of the Gal/GalNAc lectin light subunit family. Control of the observed changes in mRNA abundance in the intestine might potentially rest with four related proteins with DNA binding domains that were down-regulated 6-16-fold in the intestinal environment. In conclusion, the first genome-wide analysis of the transcriptome of E. histolytica demonstrated that the vast majority of genes are transcribed in trophozoites, and that in the host intestine trophozoites altered the expression of mRNAs for genes implicated in metabolism, oxygen defense, cell signaling, virulence, antibacterial activity, and DNA binding.

New insights into molecular mechanisms of phagocytosis in Entamoeba histolytica by proteomic analysis

The protozoan parasite Entamoeba histolytica ingests microorganisms and mammalian cells. Phagocytosis is essential for cell growth and is implicated in pathogenesis of E. histolytica. Phagocytosis consists of a number of steps including recognition of and binding to ligands on the target cells via a galactose/N-acetylgalactosamine-specific lectin, activation of a signaling pathway leading to cytoskeletal reorganization, and vesicle trafficking, all of which play distinct but coordinated roles in phagocytosis. Recent studies of proteomic analysis of purified phagosomes or affinity-purified Gal/GalNAc-binding proteins using reversed phase capillary liquid chromatography and ion trap tandem mass spectrometry enabled high throughput identification of proteins involved in phagosome biogenesis. These studies provided a list of proteins involved in the pathway and also shed light on the dynamic process of phagosome maturation. These approaches should provide significant insights into molecular mechanisms of phagosome biogenesis and help to elucidate the pathogenesis of this important parasite.

Protozoan parasite-specific carbohydrate structures

The carbohydrate moieties displayed by pathogenic protozoan parasites exhibit many unusual structural features and their expression is often developmentally regulated. These unique structures suggest a specific relationship between such carbohydrates and parasite pathogenicity. Studies of infected humans indicate that immune responses to protozoan parasites are elicited by glycan determinants on cell-surface or secreted molecules. Infections by protozoa are a major worldwide health problem, and no vaccines or efficacious treatments exist to date. Recent progress has been made in elucidating the structure and function of carbohydrates displayed by major protozoan parasites that infect man. These structures can be used as prototypes for the chemical or combined chemo-enzymatic synthesis of new compounds for diagnosis and vaccine development, or as inhibitors specifically designed to target parasite glycan biosynthesis.

Pathogenesis of Acute Experimental Liver Amebiasis

Classical descriptions of the pathology of amebiasis portray the parasite as the cause of tissue damage and destruction, and in recent years a number of amebic molecules have been identified as virulence factors. In this review we describe a series of experiments that suggest a more complex host-parasite relation, at least during the early stages of acute experimental amebic liver abscess in hamsters. The problems of extrapolating experiments in vitro to explain observations in vivo are discussed. The role of amebic cysteine proteases is examined and evidence presented to suggest that they are primarily related not to tissue damage but to amebic survival, which is required for the progression of the lesion. Inflammation is shown to be not only the major cause of tissue damage but also an absolute requirement for amebic survival in the liver, whereas complement and ischemia are not involved in the disappearance of the parasite in the absence of inflammation.

Recent Progress in Vaccines for Amebiasis

The persistence of amebiasis as a global health problem, despite the availability of effective treatment, has led to the search for vaccines to prevent this deadly disease. Recent clinical studies suggest that mucosal immunity could provide some protection against recurrent intestinal infection with E. histolytica, but there is contradictory evidence about protective immunity after amebic liver abscess. Progress in vaccine development has been facilitated by new animal models that allow better testing of potential vaccine candidates and by the application of recombinant technology to vaccine design. Oral vaccines utilizing amebic antigens either co-administered with some form of cholera toxin or expressed in attenuated strains of Salmonella or Vibrio cholera have been developed and tested in animals for mucosal immunogenicity. Although there has been significant progress on a number of fronts, there are unanswered questions regarding the effectiveness of immune responses in preventing disease in man and, as yet, no testing of any of these vaccines in humans has been performed.

Adherence-Blocking Vaccine for Amebiasis

The Gal/GalNAc lectin is a candidate vaccine antigen for an amebiasis vaccine due to its mediation of parasite adherence to the human intestine, because partial immunity in humans is associated with a mucosal IgA response against it, and because it is effective as a vaccine against amebic colitis in the murine model. The LecA domain of the Gal/GalNAc lectin contains neutralizing antibody epitopes. LecA contains the active site of the lectin (the carbohydrate recognition domain or "CRD") and has been an effective vaccine antigen in animal models of amebic colitis and liver abscess. Research needs include production of the LecA domain of the Gal/GalNAc lectin by a process that can be transferred to cGMP and optimization for immunogenicity, using adjuvants such as alum, MF59 or QS-21 adjuvants. Accomplishing this will enable testing of the ability of LecA immunizations to protect from amebic colitis in humans.

Genomic and proteomic approaches highlight phagocytosis of living and apoptotic human cells by the parasite Entamoeba histolytica

Phagocytosis plays a major role during the invasive process of the human intestine by the pathogenic amoeba E. histolytica. This parasite is the etiologic agent causing amoebic dysentery, a worldwide disease causing 50 million of clinical cases leading to about 100,000 deaths annually. The invasive process is characterized by a local acute inflammation and the destruction of the intestinal tissue at the invasion site. The recent sequencing of the E. histolytica genome has opened the way to large-scale approaches to study parasite virulence such as processes involved in human cell phagocytosis. In particular, two different studies have recently described the phagosome proteome, providing new insights into the process of phagocytosis by this pathogenic protozoan. It has been previously described that E. histolytica induces apoptosis and phagocytosis of the human target cells. Induction of apoptosis by the trophozoites is thought to be involved in the close regulation of the inflammatory response occurring during infection. Little is known about the molecular mechanisms responsible for induction of apoptosis or in the recognition of apoptotic cells by E. histolytica. In this review, we comment on the recent data we obtained after isolation of the early phagosomes and the identification of its associated proteins. We focus on the surface molecules potentially involved in human cell recognition. In particular, we propose several parasite molecules, potentially involved in the induction of apoptosis and/or the phagocytosis of human apoptotic cells.

Characterisation of carbohydrate-binding sites in developmental stages of Myxobolus cerebralis

Glycans and lectins (carbohydrate-binding molecules) form a mutual recognition system, which enables parasitic organisms to attach themselves to the host cells and/or take part in the migration of their developmental stages into the target tissue. The aim of the present study was to identify and characterise the potential binding activity of glycoconjugates in different developmental stages of Myxobolus cerebralis, the causative agent of whirling disease in salmonids. The binding patterns of 13 biotinylated neoglycoconjugates were histochemically examined in thin-sections of infected rainbow trout (Oncorhynchus mykiss) and oligochaetes (Tubifex tubifex), as well as isolated waterborne triactinomyxon spores. A distinct structure-selective and developmental stage-regulated expression of certain classes of carbohydrate binding was observed. In triactinomyxon spores, the expression of carbohydrate binding activity for alpha-l-Fuc-BSA-biotin, alpha-d-GalNAc-BSA-biotin, beta-d-GlcNAc-BSA-biotin, Lac-BSA-biotin and ASF-biotin was up-regulated in the polar capsules; the shell valves showed no activity. In the gut of T. tubifex, polar capsules of the parasite showed strong positive reaction only for beta-d-GlcNAc-BSA-biotin. In fish cartilage, polar capsules were negative, but the spore shell valves showed a broad range of carbohydrate binding activity. No activity was detected for either alpha6- or alpha3-linked N-acetyl-d-neuraminic acid to galactose. An adhesion assay was performed on GlycoWell plates and Myxobolus spores were found to specifically adhere to matrices containing residues of lactose, fucose, galactose, N-acetyl-d-galactosamine and N-acetyl-d-glucosamine. This is the first study to identify lectin activity in a myxozoan parasite; activity that is likely to play a role in the recognition systems involved in host specificity and the processes of spore attachment and invasion.

Proteomic analysis of phagocytosis in the enteric protozoan parasite Entamoeba histolytica

Proteomic analysis of phagosomes isolated from Entamoeba histolytica by liquid chromatography and mass spectrometry identified 85 proteins involved in surface recognition, actin cytoskeleton rearrangement, vesicular trafficking, and degradation. Phagosome localization of representative proteins was verified by immunofluorescence assay. This study should provide a basis for molecular identification and characterization of phagosome biogenesis.

Identification and gene expression analysis of a large family of transmembrane kinases related to the Gal/GalNAc lectin in Entamoeba histolytica

We identified in the Entamoeba histolytica genome a family of over 80 putative transmembrane kinases (TMKs). The TMK extracellular domains had significant similarity to the intermediate subunit (Igl) of the parasite Gal/GalNAc lectin. The closest homolog to the E. histolytica TMK kinase domain was a cytoplasmic dual-specificity kinase, SplA, from Dictyostelium discoideum. Sequence analysis of the TMK family demonstrated similarities to both serine/threonine and tyrosine kinases. TMK genes from each of six phylogenetic groups were expressed as mRNA in trophozoites, as assessed by spotted oligoarray and real-time PCR assays, suggesting nonredundant functions of the TMK groups for sensing and responding to extracellular stimuli. Additionally, we observed changes in the expression profile of the TMKs in continuous culture. Antisera produced against the conserved kinase domain identified proteins of the expected molecular masses of the expressed TMKs. Confocal microscopy with anti-TMK kinase antibodies revealed a focal distribution of the TMKs on the cytoplasmic face of the trophozoite plasma membrane. We conclude that E. histolytica expresses members of each subgroup of TMKs. The presence of multiple receptor kinases in the plasma membrane offers for the first time a potential explanation of the ability of the parasite to respond to the changing environment of the host.

Entamoeba histolytica activates host cell caspases during contact-dependent cell killing

Entamoeba histolytica is a human intestinal parasite that causes amoebic colitis as well as liver abscesses. Host tissues are damaged through a three-step process involving adherence, contact-dependent cytolysis, and phagocytosis. These three processes all contribute to the pathogenicity of this parasite. Adherence is provided by the Gal/GalNAc adherence lectin. Host cells are lysed in a contact-dependent fashion. There is evidence that suggests that this contact-dependent killing involves the induction of the host cell's apoptotic machinery. Phagocytosis can then occur, consistent with metazoan apoptotic clearance.

Roles for the galactose-/N-acetylgalactosamine-binding lectin of Entamoeba in parasite virulence and differentiation

Entamoeba histolytica, an intestinal protozoan parasite, is a major cause of morbidity and mortality in developing countries. The pathology of the disease is caused by the colonization of the large intestine by the amoebic trophozoites and the invasion of the intestinal epithelium. Some of the trophozoites will eventually differentiate into the infectious cyst form, allowing them to be transmitted out of the bowel and into water supplies to be passed from person to person. Both the virulence of the organism and the differentiation process relies on a galactose-/N-acetylgalactosamine (GalNAc)-binding lectin that is expressed on the surface of trophozoites. The functional activity of this lectin has been shown to be involved in host cell binding, cytotoxicity, complement resistance, induction of encystation, and generation of the cyst wall. The role of the lectin in both differentiation and virulence suggests that it may be a pivotal molecule that determines the severity of the infection from a commensal state resulting from increased encystation to an invasive state. The lectin-glycan interactions that initiate these diverse processes are discussed with emphasis on comparing the binding of host ligands and the interactions involved in encystation.

Proteomic analysis of Gal/GalNAc lectin-associated proteins in Entamoeba histolytica

Contact-dependent killing and phagocytosis of target cells by Entamoeba histolytica trophozoites is mediated by the galactose (Gal) and N-acetyl-d-galactosamine (GalNAc)-inhibitable lectin. Previous work has suggested that this lectin functions as part of a signal transduction complex. To identify proteins that might be part of this complex, amebic trophozoites were bound to GalNAc-BSA-labeled magnetic beads and lysed. Bound proteins were eluted from the beads and analyzed by tandem mass spectrometry. Along with the Gal/GalNAc lectin subunits, several cytoskeletal proteins, potential signaling proteins, and a novel transmembrane protein, consistently purified with the GalNAc-BSA beads.

NADPH oxidase-derived reactive oxygen species-mediated activation of ERK1/2 is required for apoptosis of human neutrophils induced by Entamoeba histolytica

The extracellular tissue penetrating protozoan parasite Entamoeba histolytica has been known to induce host cell apoptosis. However, the intracellular signaling mechanism used by the parasite to trigger apoptosis is poorly understood. In this study, we investigated the roles of reactive oxygen species (ROS), and of MAPKs in the Entamoeba-induced apoptosis of human neutrophils. The neutrophils incubated with live trophozoites of E. histolytica revealed a marked increase of receptor shedding of CD16 as well as phosphatidylserine (PS) externalization on the cell surface. The Entamoeba-induced apoptosis was effectively blocked by pretreatment of cells with diphenyleneiodonium chloride (DPI), a flavoprotein inhibitor of NADPH oxidase. A large amount of intracellular ROS was detected after exposure to viable trophozoites, and the treatment with DPI strongly inhibited the Entamoeba-induced ROS generation. However, a mitochondrial inhibitor rotenone did not attenuate the Entamoeba-induced ROS generation and apoptosis. Although E. histolytica strongly induced activation of ERK1/2 and p38 MAPK in neutrophils, the activation of ERK1/2 was closely associated with ROS-mediated apoptosis. Pretreatment of neutrophils with MEK1 inhibitor PD98059, but not p38 MAPK inhibitor SB202190, prevented Entamoeba-induced apoptosis. Moreover, DPI almost completely inhibited Entamoeba-induced phosphorylation of ERK1/2, but not phosphorylation of p38 MAPK. These results strongly suggest that NADPH oxidase-derived ROS-mediated activation of ERK1/2 is required for the Entamoeba-induced neutrophil apoptosis.

Improved affinity of a human anti-Entamoeba histolytica Gal/GalNAc lectin Fab fragment by a single amino acid modification of the light chain

We previously produced, in Escherichia coli, a human monoclonal antibody Fab fragment, CP33, specific for the galactose- and N-acetyl-D-galactosamine-inhibitable lectin of Entamoeba histolytica. To prepare antibodies with a higher affinity to the lectin, recombination PCR was used to exchange Ser91 and Arg96 in the third complementarity-determining region of the light chain with other amino acids. The screening of 200 clones of each exchange by an indirect fluorescent antibody test showed that 14 clones for Ser91 and nine clones for Arg96 reacted strongly with E. histolytica trophozoites. Sequence analyses revealed that the substituted amino acids at Ser91 were Ala in five clones, Gly in three clones, Pro in two clones, and Val in two clones, while the amino acid at position 96 was substituted with Leu in three clones. The remaining eight clones exhibited no amino acid change at position 91 or 96. These mutant Fab fragments were purified and subjected to a surface plasmon resonance assay to measure the affinity of these proteins to the cysteine-rich domain of lectin. Pro or Gly substitution for Ser91 caused an increased affinity of the Fab, but substitution with Ala or Val did not. The replacement of Arg96 with Leu did not affect affinity. These results demonstrate that modification of antibody genes by recombination PCR is a useful method for affinity maturation and that amino acid substitution at position 91 yields Fabs with increased affinity for the lectin.

Entamoeba histolytica: intracellular distribution of the sec61alpha subunit of the secretory pathway and down-regulation by antisense peptide nucleic acids

The Sec61alpha protein is defined as a highly conserved essential integral component of the endoplasmic reticulum in eukaryotic cells. We report a detailed immunolocalization of the Entamoeba histolytica homologue of the Sec61alpha subunit (EhSec61alpha), which shows an irregular pattern throughout the cell and is also found on the cell surface, its effective down-regulation by means of antisense peptide nucleic acids and its effects on cell proliferation, subcellular distribution of two virulence factors, and the ability of the trophozoites to cause liver abscess in hamsters. Although Sec61alpha levels are specifically decreased in antisense PNA-treated trophozoites, which proliferate more slowly than the controls, mobilization of the cysteine protease 5 and amoebapore to the cell surface is not significantly impeded and the capacity to induce liver abscess in hamsters is largely unaffected. The implications of these findings are discussed in the context of the peculiar cell biology of E. histolytica.

Genomic DNA microarrays for Entamoeba histolytica: applications for use in expression profiling and strain genotyping

The parasite Entamoeba histolytica is a causative agent of dysentery and liver abscesses. Found predominantly in developing countries, this parasitic infection is responsible for significant morbidity and mortality. We have developed a genomic DNA microarray for E. histolytica. The array composed of 11,328 clones contains >2000 unique genes and was utilized for expression profiling and comparative genomic hybridizations of Entamoeba strains. We present a synopsis of our results to date and potential future applications of microarray technology for the study of Entamoeba biology.

Roles of cell adhesion and cytoskeleton activity in Entamoeba histolytica pathogenesis: a delicate balance

The protozoan parasite Entamoeba histolytica colonizes the human large bowel. Invasion of the intestinal epithelium causes amoebic colitis and opens the route for amoebic liver abscesses. The parasite relies on its dynamic actomyosin cytoskeleton and on surface adhesion molecules for dissemination in the human tissues. Here we show that the galactose/N-acetylgalactosamine (Gal/GalNAc) lectin clusters in focal structures localized in the region of E. histolytica that contacts monolayers of enterocytes. Disruption of myosin II activity impairs the formation of these structures and renders the trophozoites avirulent for liver abscess development. Production of the cytoplasmic domain of the E. histolytica Gal/GalNAc lectin in engineered trophozoites causes reduced adhesion to enterocytes. Intraportal delivery of these parasites to the liver leads to the formation of a large number of small abscesses with disorganized morphology that are localized in the vicinity of blood vessels. The data support a model for invasion in which parasite motility is essential for establishment of infectious foci, while the adhesion to host cells modulates the distribution of trophozoites in the liver and their capacity to migrate in the hepatic tissue.

Enzymatic glycosidation of sugar oxazolines having a carboxylate group catalyzed by chitinase

Enzymatic glycosidation using sugar oxazolines 1-3 having a carboxylate group as glycosyl donors and compounds 4-6 as glycosyl acceptors was performed by employing a chitinase from Bacillus sp. as catalyst. All the glycosidations proceeded with full control in stereochemistry at the anomeric carbon of the donor and regio-selectivity of the acceptor. The N,N'-diacetyl-6'-O-carboxymethylchitobiose oxazoline derivative 1 was effectively glycosidated, under catalysis by the enzyme, with methyl N,N'-diacetyl-beta-chitobioside (4), pent-4-enyl N-acetyl-beta-D-glucosaminide (5), and methyl N-acetyl-beta-D-glucosaminide (6), affording in good yields the corresponding oligosaccharide derivatives having 6-O-carboxymethyl group at the nonreducing GlcNAc residue. The N,N'-diacetyl-6-O-carboxymethylchitobiose oxazoline derivative 2 was subjected to catalysis by the enzyme catalysis; however, no glycosidated products were produced through the reactions with 4, 5, and 6. Glycosidation reactions of the beta-d-glucosyluronic-(1-->4)-N-acetyl-D-glucosamine oxazoline derivative 3 proceeded with each of the glycosyl acceptors, giving rise to the corresponding oligosaccharide derivative having a GlcA residue at their nonreducing termini in good yields.

Entamoeba histolytica: identification of a distinct beta2 integrin-like molecule with a potential role in cellular adherence

Entamoeba histolytica infection causes dysentery, intestinal colitis, and hepatic abscess in an estimated 50 million people worldwide. Attachment of E. histolytica trophozoites to intestinal epithelium and vascular endothelium during liver metastasis results in an inflammatory process. We report the identification of a distinct amebic beta2 integrin (CD18)-like molecule which affords adherence to TNF-alpha-activated endothelial cells. Data from flow cytometry and indirect immunofluorescence assays suggest the amebic beta2 integrin was localized to focal adhesion plates and was present in both E. histolytica and Entamoeba dispar. The amebic beta2 integrin appeared to be distinct from the amebic Gal/GalNAc lectin based on recombinant expression, amebic colocalization, and ELISA studies. Trophozoite adherence to endothelial cells expressing ICAM-1 (CD54) following activation with TNF-alpha or ICAM-1-transfected CHO cells was specifically inhibited with anti-CD18 or anti-CD54 MAbs. In summary, evidence in support of a distinct beta2 integrin-like molecule participating in amebic adherence to TNF-alpha-activated endothelial cells expressing ICAM-1 is presented. The presence of integrin-dependent binding may allow trophozoites to opportunistically adhere to activated intestinal epithelium or vascular endothelium expressing ICAM-1 during amebic colitis or hepatic abscess.

A lysine- and glutamic acid-rich protein, KERP1, from Entamoeba histolytica binds to human enterocytes

Contact-dependent cytolysis of host cells by Entamoeba histolytica is an important hallmark of amoebiasis that points out the importance of molecules involved in the interaction between the parasite and the human cells. To decipher the molecular and cellular mechanisms supporting the invasion of the intestinal epithelium by E. histolytica, we analysed proteins involved in the interaction of the parasite with enterocytes. Affinity chromatography revealed several amoebic proteins interacting with purified brush border of differentiated Caco2 cells. Among them were found the intermediate subunit of the Gal/GalNAc lectin, an alpha-actinin-like protein and two new proteins KERP1 and KERP2 rich in lysine and glutamic acid. In silico analysis revealed the presence of KERP2 in the closely related non-pathogenic amoeba species Entamoeba dispar but not of KERP1. In additon, polymerase chain reaction analysis allowed to suggest the absence of kerp1 homologous gene in E. dispar. Therefore, we concentrated on the cellular analysis of KERP1. Cloning of the KERP1-encoding gene, production of a recombinant protein in Escherichia coli and production of a specific antibody allowed us to show the following properties: (i) purified KERP1 binds to epithelial cell surface, (ii) KERP1 is located on the plasma membrane and in vesicles of trophozoites and (iii) KERP1 is delivered in the interstitial area between the trophozoites and the intestinal cells.

Ultrastructural observation of human neutrophils during apoptotic cell death triggered by Entamoeba histolytica

Neutrophils are important effector cells against protozoan extracellular parasite Entamoeba histolytica, which causes amoebic colitis and liver abscess in human beings. Apoptotic cell death of neutrophils is an important event in the resolution of inflammation and parasite's survival in vivo. This study was undertaken to investigate the ultrastructural aspects of apoptotic cells during neutrophil death triggered by Entamoeba histolytica. Isolated human neutrophils from the peripheral blood were incubated with or without live trophozoites of E. histolytica and examined by transmission electron microscopy (TEM). Neutrophils incubated with E. histolytica were observed to show apoptotic characteristics, such as compaction of the nuclear chromatin and swelling of the nuclear envelop. In contrast, neutrophils incubated in the absence of the amoeba had many protrusions of irregular cell surfaces and heterogenous nuclear chromatin. Therefore, it is suggested that Entamoeba-induced neutrophil apoptosis contribute to prevent unwanted tissue inflammation and damage in the amoeba-invaded lesions in vivo.

The pathogenicity of Entamoeba histolytica is related to the capacity of evading innate immunity

The host and parasite factors that influence susceptibility to Entamoeba histolytica infection and disease are not well understood. Entamoeba histolytica pathogenicity has been considered by focusing principally on parasite rather than host factors. Thus, research has concentrated on explaining the molecular differences between pathogenic E. histolytica and non-pathogenic E. dispar. However, the amoeba molecules considered most important for host tissue destruction (amoebapore, galactose/N-acetyl galactosamine inhibitable lectin, and cysteine proteinases) are present in both pathogenic E. histolytica and non-pathogenic E. dispar. In addition, the genetic differences in pathogenicity among E. histolytica isolates are unlikely to completely explain the different outcomes of infection. Considering that the principal difference between pathogenic and non-pathogenic amoebas lies in their surface coats, we propose that pathogenicity of the amoebas is related to the composition and properties of the surface coat components (or pathogen-associated molecular patterns, PAMPs), and the ability of innate immune response to recognize these components and eliminate the parasite. According to this hypothesis, a key feature that may distinguish pathogenic (E. histolytica) from non-pathogenic (E. dispar) strains is whether or not they can overcome innate immune defences. A corollary of this hypothesis is that in susceptible individuals the PAMPs are either not recognized or they are recognized by a set of Toll-like receptors (TLRs) that leads to an inflammatory response. In both cases, the result is tissue damage. On the contrary, in resistant individuals the innate/inflammatory response, induced through the activation of a different set of TLRs, eliminates the parasite.

Involvement of raft-like plasma membrane domains of Entamoeba histolytica in pinocytosis and adhesion

Lipid rafts are highly ordered, cholesterol-rich, and detergent-resistant microdomains found in the plasma membrane of many eukaryotic cells. These domains play important roles in endocytosis, secretion, and adhesion in a variety of cell types. The parasitic protozoan Entamoeba histolytica, the causative agent of amoebic dysentery, was determined to have raft-like plasma membrane domains by use of fluorescent lipid analogs that specifically partition into raft and nonraft regions of the membrane. Disruption of raft-like membrane domains in Entamoeba with the cholesterol-binding agents filipin and methyl-beta-cyclodextrin resulted in the inhibition of several important virulence functions, fluid-phase pinocytosis, and adhesion to host cell monolayers. However, disruption of raft-like domains did not inhibit constitutive secretion of cysteine proteases, another important virulence function of Entamoeba. Flotation of the cold Triton X-100-insoluble portion of membranes on sucrose gradients revealed that the heavy, intermediate, and light subunits of the galactose-N-acetylgalactosamine-inhibitible lectin, an important cell surface adhesion molecule of Entamoeba, were enriched in cholesterol-rich (raft-like) fractions, whereas EhCP5, another cell surface molecule, was not enriched in these fractions. The subunits of the lectin were also observed in high-density, actin-rich fractions of the sucrose gradient. Together, these data suggest that pinocytosis and adhesion are raft-dependent functions in this pathogen. This is the first report describing the existence and physiological relevance of raft-like membrane domains in E. histolytica.

Amebic liver abscess

Amebic liver abscess is the most common extraintestinal manifestation of infection with Entamoeba histolytica, and it is associated with significant morbidity and mortality. In this article the most recent available information is reviewed relating to epidemiology, pathogenesis, presentation, diagnosis, and treatment. We reviewed thousands of cases of amebic liver abscess in the medical literature and present that information as it pertains to mortality, gender, anatomic location of abscesses, and clinical signs and symptoms.

Myosin II and the Gal-GalNAc lectin play a crucial role in tissue invasion by Entamoeba histolytica

Entamoeba histolytica is the human parasite responsible of amoebiasis, during which highly motile trophozoites invade the intestinal epithelium leading to amoebic colitis, and disseminate via the blood circulation causing liver abscesses. The invasive process, central to the pathogenesis, is known to be driven by parasites motility. To investigate molecules responsible for in vivo motion, we performed a high resolution dynamic imaging analysis using two-photon laser scanning microscopy. Image analysis of the parasites during invasion of Caco-2 cell monolayers, an enterocyte-like model, and hamster liver shows that E. histolytica undergoes non-Brownian motion. However, studies of movements of parasite strains dominant negative for myosin II, a central component of the cytoskeleton, and for Gal-GalNAc lectin, a major adhesion molecule, indicate that myosin II is essential for E. histolytica intercellular motility through intestinal cell monolayers and for its motility in liver. In contrast, the Gal-GalNAc lectin exclusively triggers invasion of the liver. These observations are in agreement with emerging studies that highlight marked differences in the way that cells migrate in vitro in two dimensions versus in vivo in three dimensions. The approach that we have developed should be powerful to identify adhesive complexes required for in vivo cell migration in normal and pathogenic situations and may, thereby, lead to new therapeutic drug, for pathologies based on cell motility and adhesion.

Human amoebic hepatic abscess: in situ interactions between trophozoites, macrophages, neutrophils and T cells

Amoebic liver abscesses (ALA) are the most frequent and severe extraintestinal clinical presentations of amoebiasis. During the early establishment of amoebae in the liver parenchyma, as well as during the extension of the tissue necrosis, parasites interact with the parenchymal liver cells and, as a consequence of these interactions, hepatocytes can be destroyed and host immune cells can become activated. However, little is known about the nature of these interactions in the liver or about the factors involved in the local immune response. In this investigation we studied the localization of Entamoeba histolytica trophozoites, TCD4+, TCD8+ cells, CD68+ macrophages and CD15+ neutrophils in human ALA using immunohistochemical techniques. Trophozoites were found close to undamaged hepatocytes in both lysed and non-lysed areas with either sparse or abundant inflammatory infiltrate. CD8+ cells were more abundant than CD4+ T cells. CD 68+ macrophages and CD15+ neutrophils were also detected, suggesting that neutrophils, macrophages and T cells might be related to the local host immune mechanisms in ALA. We also found that E. histolytica possesses proteins recognized by antibodies raised against inducible nitric oxide synthase.

Signal transduction through the Gal–GalNAc lectin of Entamoeba histolytica involves a spectrin-like protein

Capping followed by uroid formation in Entamoeba histolytica has been implicated in resistance against the host immune response during development of amoebiasis. The amebic actomyosin cytoskeleton is essential for such a process. A protein from the spectrin family co-localizes with the Gal-GalNAc lectin during capping of this surface protein complex. Co-localization is not observed when capping of the Gal-GalNAc lectin is specifically inhibited by production of the carboxyl-terminal region of its heavy chain that includes the lectin cytoplasmic tail. A peptide encompassing the lectin last 77 amino acids fused to glutathione-S-transferase interacts in vitro with purified spectrin. The spectrin-binding site was narrowed down to a stretch of 21 amino acids within the lectin cytoplasmic domain. This is the first report identifying an amino acid sequence involved in the interaction between the Gal-GalNAc lectin and cytoskeletal spectrin.

Entamoeba histolytica: ultrastructure of trophozoites recovered from experimental liver lesions

Ultrastructural studies on Entamoeba histolytica have been carried out mostly with trophozoites cultured for many years. Under these conditions, the availability of nutrients and the absence of environmental stimuli may switch off some phenotypic characteristics of the parasite. As a result, virulence of E. histolytica diminishes with prolonged culture passages, and the ability to form cysts disappears in axenically maintained trophozoites. The present analysis by transmission electron microscopy of trophozoites recovered from experimental amebic liver lesions in hamsters revealed two types of cytoplasmic changes. On the one hand, the number of peripheral electron dense granules significantly increased in amebas obtained from liver lesions 15 min and 6h after inoculation. On the other hand, large cytoplasmic vesicles with a microfibrillar content appeared in trophozoites cultured from 72 or 96 h hepatic lesions. With fluorescence microscopy, a chitin-like material was identified in these vesicles by reactivity with calcofluor M2R. Ultrastructurally, these cytoplasmic components resemble the encystation vesicles of Entamoeba invadens and Giardia lamblia. The release of large amounts of electron dense granules, known to contain collagenase activity, probably contributes to degrade extracellular matrix components during tissue invasion. In addition, under the conditions mentioned above, amebas form encystation-like vesicles in an incomplete process of differentiation into cysts, which are the resistant form of the parasite.

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.

Entamoeba histolytica trophozoites transfer lipophosphopeptidoglycans to enteric cell layers

Transfer of antigens frequently follows adhesion of protozoan parasites to host cells. We were interested in such transfer from the Entamoeba surface to enterocytes following adhesion of trophozoites. Therefore, cocultures of enterocytes in vitro and ex vivo with Entamoeba histolytica (strain HM-1:IMSS) or Entamoeba dispar (strain SAW760) trophozoites were processed for immunocytochemistry. The EH5 monoclonal antibody against amoebic proteophosphoglycans marked a dotted pattern on the apical side of enterocytes in in vitro cocultures with HM-1:IMSS and SAW760 trophozoites. Basolateral staining was present in cocultures following dysfunction of tight junctions, or when trophozoites made direct contact with the basolateral side of enterocytes in in vitro and ex vivo cocultures. Based on the molecular mass in Western blot, the transferred proteophosphoglycan was identified as a lipophosphopeptidoglycan. In conclusion, trophozoites transfer LPPG to the apical side of enterocytes following adhesion and prior to dysfunction of tight junctions.

Solution Structure of the Pore-forming Protein of Entamoeba histolytica

Amoebapore A is a 77-residue protein from the protozoan parasite and human pathogen Entamoeba histolytica. Amoebapores lyse both bacteria and eukaryotic cells by pore formation and play a pivotal role in the destruction of host tissues during amoebiasis, one of the most life-threatening parasitic diseases. Amoebapore A belongs to the superfamily of saposin-like proteins that are characterized by a conserved disulfide bond pattern and a fold consisting of five helices. Membrane-permeabilizing effector molecules of mammalian lymphocytes such as porcine NK-lysin and the human granulysin share these structural attributes. Several mechanisms have been proposed to explain how saposin-like proteins form membrane pores. All mechanisms indicate that the surface charge distribution of these proteins is the basis of their membrane binding capacity and pore formation. Here, we have solved the structure of amoebapore A by NMR spectroscopy. We demonstrate that the specific activation step of amoebapore A depends on a pH-dependent dimerization event and is modulated by a surface-exposed histidine residue. Thus, histidine-mediated dimerization is the molecular switch for pore formation and reveals a novel activation mechanism of pore-forming toxins.

Gastrointestinal parasite and host interactions

PURPOSE OF REVIEW

The mechanisms responsible for the Th2-mediated immune response to enteric nematode parasites are of interest for several reasons. First, intestinal parasites continue to be a major worldwide health issue. Second, the low incidence of parasite infection in industrial nations is cited as a factor in the increased prevalence of proinflammatory-based pathologies. Third, a seemingly paradoxical protection against Th2-mediated allergic reactions is afforded by helminth infection. This review focuses on studies that use enteral parasitic infections as a tool to investigate the functional consequences of upregulation of Th2-mediated immunity and that manipulate host-parasite interactions in an effort to identify mechanisms that can be exploited as potential therapeutic targets.

RECENT FINDINGS

Enteric helminth infection improved indices of inflammatory bowel disease in humans and murine models and diminished the allergy-induced changes in pulmonary function. There are emerging or enlarged roles for interleukin-10, interleukin-18, interleukin-9, chemokines, activation of nuclear factor-kappabeta, and factors that alter host resistance in the development of host immunity, and for interleukin-13Ralpha2 receptor in downregulating Th2 responses. As part of the growing appreciation for the contribution of nonimmune cells to parasite-induced changes in intestinal function, studies show that Th2 cytokines exert Stat6-dependent effects that promote worm expulsion.

SUMMARY

Further insight into the nature of host-parasite interactions, identification of the pathways and critical mediators that contribute to host resistance, identification of the factors that modulate susceptibility to infection, and the impact of enteric parasites on intestinal function hold much promise for development of novel therapeutic interventions.

Prevention of intestinal amebiasis by vaccination with the Entamoeba histolytica Gal/GalNac lectin

Prevention of intestinal infection by Entamoeba histolytica would block both invasive disease and parasite transmission. The amebic Gal/GalNAc lectin mediates parasite adherence to the colonic surface and fecal anti-lectin IgA is associated with protection from intestinal reinfection in children. We tested if vaccination with the E. histolytica Gal/GalNAc lectin could prevent cecal infection in a C3H mouse model of amebic colitis. Two trials using native lectin purified from the parasite and two trials using a 64 kDa recombinant fragment ("LecA") were performed with a combined intranasal and intraperitoneal immunization regimen using cholera toxin and Freund's adjuvants, respectively. Two weeks after immunization mice were challenged intracecally with trophozoites, and 4-12 weeks after challenge mice were sacrificed for histopathologic evaluation of infection. Vaccination prevented intestinal infection with efficacies of 84 and 100% in the two native lectin trials and 91 and 34% in the two LecA trials. Mice with detectable pre-challenge fecal anti-lectin IgA responses were significantly more resistant to infection than mice without fecal anti-lectin IgA responses. These results show for the first time that immunization with the Gal/GalNAc lectin can prevent intestinal amebiasis in mice and suggest a protective role for fecal anti-lectin IgA in vivo.

Entamoeba histolytica: an update

PURPOSE OF REVIEW

Over the past decade, since it was formally recognized that Entamoeba histolytica and Entamoeba dispar were two distinct species, studies in this field have made dramatic in-roads into the understanding of E. histolytica and the pathogenesis of invasive amoebiasis. Over the same period it has also become clear that the true incidence of E. histolytica infection, particularly in vulnerable populations such as low socioeconomic children, is exceedingly high. Understanding the epidemiology, pathophysiology, and the molecular and genetic biology of the organism will not only lead to improved diagnostic and treatment options but, ultimately, to the development of a safe and efficacious vaccine.

RECENT FINDINGS

The recent advances in the genetic and molecular sciences have increased our understanding of the mechanisms that make E. histolytica unique among enteric protozoa in causing invasive disease. In addition, host factors, which predispose individuals or populations to infection or disease, are beginning to be elucidated. New diagnostic tools specific to E. histolytica are being exploited by clinicians and researchers to identify and treat patients as well as to add to the knowledge of the epidemiology and natural history of this infection. The ultimate goal - eradication of disease - is theoretically feasible since humans and primates are the only reservoirs of E. histolytica. Many talented and dedicated individuals are pursuing the development of an effective and safe amoebiasis vaccine.

SUMMARY

E. histolytica remains an important pathogen in many populations of the world and although there has been substantial progress into understanding the disease major challenges still exist.

Amoebic gill disease: sequential pathology in cultured Atlantic salmon, Salmo salar L

Amoebic gill disease (AGD) affects the marine culture phase of Atlantic salmon, Salmo salar L., in Tasmania. Here, we describe histopathological observations of AGD from smolts, sampled weekly, following transfer to estuarine/marine sites. AGD was initially detected histologically at week 13 post-transfer while gross signs were not observed for a further week post-transfer. Significant increases (P < 0.001) in the proportion of affected gill filaments occurred at weeks 18 and 19 post-transfer coinciding with the cessation of a halocline and increased water temperature at the cage sites. The progression of AGD histopathology, during the sampling period, was characterized by three phases. (1) Primary attachment/interaction associated with extremely localized host cellular alterations, juxtaposed to amoebae, including epithelial desquamation and oedema. (2) Innate immune response activation and initial focal hyperplasia of undifferentiated epithelial cells. (3) Finally, lesion expansion, squamation-stratification of epithelia at lesion surfaces and variable recruitment of mucous cells to these regions. A pattern of preferential colonization of amoebae at lesion margins was apparent during stage 3 of disease development. Together, these data suggest that AGD progression was linked to retraction of the estuarine halocline and increases in water temperature. The host response to gill infection with Neoparamoeba sp. is characterized by a focal fortification strategy concurrent with a migration of immunoregulatory cells to lesion-affected regions.

Induction of permeability changes and death of vertebrate cells is modulated by the virulence of Entamoeba spp. isolates

Although Entamoeba histolytica is capable of inducing an apoptotic response in vertebrate cells in vitro (Cell. Microbiol. 2 (2000) 617), it is not known whether vertebrate cell death requires direct amoeba-vertebrate cell contact or simply the presence of amoebae in the area of the vertebrate cells. In addition, Entamoeba spp. vary in their virulence and pathogenicity. The potential effects of these critical parameters also have not been elucidated. We tested the virulent HM-1:IMSS isolate and the non-virulent Rahman isolate of E. histolytica, and the non-virulent E. dispar CYNO16:TPC isolate against two vertebrate cell lines, HeLa and Chinese hamster ovary cells in vitro using ethidium homodimer as a fluorescent indicator of changes in vertebrate cell permeability. Fluorescence appeared in vertebrate cell nuclei within approximately 2-3 min of contact between HM-1 amoebae and vertebrate cells independent of vertebrate cell type. However, vertebrate cells in the immediate vicinity of but not contacted by HM-1 amoebae were not affected. In contrast, although both E. histolytica Rahman and E. dispar CYNO16 amoebae moved freely among and contacted vertebrate cells, the nuclei of the vertebrate cells never fluoresced implying that the cells remained alive and impermeant to the ethidium homodimer. This is the first demonstration that direct contact between virulent amoebae and vertebrate cells is required to kill vertebrate cells and that the process is restricted to virulent Entamoeba isolates. An understanding at the molecular level of the processes involved could help to reduce the pathology associated with this parasite.

Identification of Entamoeba histolytica thiol-specific antioxidant as a GalNAc lectin-associated protein

Entamoeba histolytica is a human intestinal parasite that causes amebic dysentery. A cell surface amebic adhesin, the galactose and N-acetyl-D-galactosamine inhibitable (GalNAc) lectin mediates amebic adherence to and contact-dependent killing of host cells. Previous work has suggested that the GalNAc lectin transduces signals via protein interactions with its short cytoplasmic domain. We used a yeast two-hybrid system to screen an E. histolytica cDNA library for proteins that interact with the GalNAc lectin cytoplasmic domain. One isolate was the E. histolytica thiol-specific antioxidant (TSA). TSA is an enzyme that detoxifies hydrogen peroxide. TSA did not interact in yeast two-hybrid experiments with a mutant version of the lectin cytoplasmic domain, confirming the specificity of the lectin-TSA interaction. Furthermore, mutational analyses of the TSA isolate demonstrated that an in-frame five amino acid sequence introduced between amino acids 61-62 yielded a TSA mutant that did not interact with the lectin cytoplasmic domain upon expression in the yeast two-hybrid system. The association of TSA and GalNAc lectin was further supported by co-immunoaffinity purification. Confocal microscopy demonstrated co-localization of TSA and GalNAc lectin at sites of ameba:host cell contact. Recruitment of TSA by the GalNAc lectin suggests a novel mechanism of parasite defense against reactive oxygen intermediates generated by host peripheral mononuclear cells.

Prospect for an Entamoeba histolytica Gal-lectin-based vaccine

Entamoeba histolytica is the aetiological agent of invasive amoebiasis, the third leading parasitic cause of mortality in the world. The disease can be easily cured by chemotherapy; however, prevention, mainly in the form of vaccination, could greatly decrease the incidence of the disease, and possibly help in its eradication. The parasite's surface galactose and N-acetyl-d-galactosamine-inhibitable adherence lectin (Gal-lectin) is highly antigenic and is the most promising subunit vaccine candidate. We have generated a Gal-lectin-based DNA vaccine and tested its immunogenicity in mice. Although further optimization will probably be required, this vaccine could help in the generation of an amoebiasis DNA vaccine for use in humans.

Entamoeba histolytica expressing a dominant negative N-truncated light subunit of its gal-lectin are less virulent

The 260-kDa heterodimeric Gal/GalNAc-specific Lectin (Gal-lectin) of Entamoeba histolytica dissociates under reducing conditions into a heavy (hgl, 170 kDa) and a light subunit (lgl, 35 kDa). We have previously shown that inhibition of expression of the 35-kDa subunit by antisense RNA causes a decrease in virulence. To further understand the role of the light subunit of the Gal-lectin in pathogenesis, amoebae were transfected with plasmids encoding intact, mutated, and truncated forms of the light subunit lgl1 gene. A transfectant in which the 55 N-terminal amino acids of the lgl were removed, overproduced an N-truncated lgl protein (32 kDa), which replaced most of the native 35-kDa lgl in the formation of the Gal-lectin heterodimeric complex and exerted a dominant negative effect. Amoebae transfected with this construct showed a significant decrease in their ability to adhere to and kill mammalian cells as well as in their capacity to form rosettes with and to phagocytose erythrocytes. In addition, immunofluorescence confocal microscopy of this transfectant with anti-Gal-lectin antibodies showed an impaired ability to cap. These results indicate that the light subunit has a role in enabling the clustering of Gal-lectin complexes and that its N-truncation affects this function, which is required for virulence.

Opportunities and obstacles in developing a vaccine for Entamoeba histolytica

The development of a vaccine against Entamoeba histolytica, the causative agent of amebic colitis and liver abscess, would reduce childhood mortality in countries such as Bangledesh where community-based studies have demonstrated a high prevalence of E. histolytica. Immunological studies from this population have shown that protection from amebiasis is associated with mucosal anti-E. histolytica Gal/GalNAc lectin antibodies, suggesting that a vaccine is an achievable goal. However, garnering resources for vaccine development is a challenge when the vaccine is targeted to poor people living in developing countries.

Entamoeba histolytica: sequence conservation of the Gal/GalNAc lectin from clinical isolates

The Gal/GalNAc lectin gene of Entamoeba histolytica is a major amebic virulence protein responsible for interaction with host tissues. We investigated sequence differences in the Gal/GalNAc lectin heavy subunit in three isolates from Bangladesh and one isolate from Georgia, each of which was determined to be genetically distinct by SREHP AluI digestion. Interestingly, we observed only slight genetic diversity in the lectin gene as compared with the HM1:IMSS laboratory strain, originally a clinical isolate from Mexico. Genetic conservation of the Gal/GalNAc lectin between isolates may reflect that the lectin is under strong functional selection or possibly, that E. histolytica is a clonal population. Sequence conservation of the lectin indicates that immune responses against it should be cross-protective.