Intestinal invasive amebiasis: an experimental model in rodents using axenic or monoxenic strains of Entamoeba histolytica

Intestinal amoebiasis: 160 years of its first detection and still remains as a health problem in developing countries

Amoebiasis is a parasitic disease caused by Entamoeba histolytica (E. histolytica), an extracellular enteric protozoan. This infection mainly affects people from developing countries with limited hygiene conditions, where it is endemic. Infective cysts are transmitted by the fecal-oral route, excysting in the terminal ileum and producing invasive trophozoites (amoebae). E. histolytica mainly lives in the large intestine without causing symptoms; however, possibly as a result of so far unknown signals, the amoebae invade the mucosa and epithelium causing intestinal amoebiasis. E. histolytica possesses different mechanisms of pathogenicity for the adherence to the intestinal epithelium and for degrading extracellular matrix proteins, producing tissue lesions that progress to abscesses and a host acute inflammatory response. Much information has been obtained regarding the virulence factors, metabolism, mechanisms of pathogenicity, and the host immune response against this parasite; in addition, alternative treatments to metronidazole are continually emerging. An accesible and low-cost diagnostic method that can distinguish E. histolytica from the most nonpathogenic amoebae and an effective vaccine are necessary for protecting against amoebiasis. However, research about the disease and its prevention has been a challenge due to the relationship between E. histolytica and the host during the distinct stages of the disease is multifaceted. In this review, we analyze the interaction between the parasite, the human host, and the colon microbiota or pathogenic microorganisms, which together give rise to intestinal amoebiasis.

Host Invasion by Pathogenic Amoebae: Epithelial Disruption by Parasite Proteins

The epithelium represents the first and most extensive line of defence against pathogens, toxins and pollutant agents in humans. In general, pathogens have developed strategies to overcome this barrier and use it as an entrance to the organism. Entamoeba histolytica, Naegleriafowleri and Acanthamoeba spp. are amoebae mainly responsible for intestinal dysentery, meningoencephalitis and keratitis, respectively. These amoebae cause significant morbidity and mortality rates. Thus, the identification, characterization and validation of molecules participating in host-parasite interactions can provide attractive targets to timely intervene disease progress. In this work, we present a compendium of the parasite adhesins, lectins, proteases, hydrolases, kinases, and others, that participate in key pathogenic events. Special focus is made for the analysis of assorted molecules and mechanisms involved in the interaction of the parasites with epithelial surface receptors, changes in epithelial junctional markers, implications on the barrier function, among others. This review allows the assessment of initial host-pathogen interaction, to correlate it to the potential of parasite invasion.

Towards the establishment of a porcine model to study human amebiasis

BACKGROUND

Entamoeba histolytica is an important parasite of the human intestine. Its life cycle is monoxenous with two stages: (i) the trophozoite, growing in the intestine and (ii) the cyst corresponding to the dissemination stage. The trophozoite in the intestine can live as a commensal leading to asymptomatic infection or as a tissue invasive form producing mucosal ulcers and liver abscesses. There is no animal model mimicking the whole disease cycle. Most of the biological information on E. histolytica has been obtained from trophozoite adapted to axenic culture. The reproduction of intestinal amebiasis in an animal model is difficult while for liver amebiasis there are well-described rodent models. During this study, we worked on the assessment of pigs as a new potential model to study amebiasis.

METHODOLOGY/PRINCIPAL FINDINGS

We first co-cultured trophozoites of E. histolytica with porcine colonic fragments and observed a disruption of the mucosal architecture. Then, we showed that outbred pigs can be used to reproduce some lesions associated with human amebiasis. A detailed analysis was performed using a washed closed-jejunal loops model. In loops inoculated with virulent amebas a severe acute ulcerative jejunitis was observed with large hemorrhagic lesions 14 days post-inoculation associated with the presence of the trophozoites in the depth of the mucosa in two out four animals. Furthermore, typical large sized hepatic abscesses were observed in the liver of one animal 7 days post-injection in the portal vein and the liver parenchyma.

CONCLUSIONS

The pig model could help with simultaneously studying intestinal and extraintestinal lesion development.

Mechanisms of adherence, cytotoxicity and phagocytosis modulate the pathogenesis of Entamoeba histolytica

The unicellular parasite Entamoeba histolytica, the causative agent of the human disease amebiasis, has traditionally been distinguished from its nonpathogenic cousin Entamoeba dispar by its propensity for the ingestion of erythrocytes. This classic feature, along with the parasite’s ability to cause extensive host cell death, are critical mechanisms of pathogenesis during human infection. Recent advances have led to a greater understanding of the molecular components that allow E. histolytica to kill and phagocytose extracellular targets during human infection and include detailed studies of the role of the parasite’s cysteine proteinases and other effectors of cytotoxicity, as well as the mechanisms of ligand recognition, signaling and intracellular trafficking during phagocytosis.

A novel ex vivo set-up for dynamic long-term characterization of processes on mucosal interfaces by confocal imaging and simultaneous cytokine measurements

We present a novel organ-explant imaging system for easy and cost-effective extended-time observation of host-pathogen interactions at mucosal interfaces. Data are complemented by parallel cytokine measurements at high temporal resolution. The set-up is based on a custom-built reusable organ chamber compatible with standard microscopes. Luminal and basal side of the explanted mucosa are connected to separate channels for optimized incubation and cytokine measurements, oxygen is provided via membrane oxygenation. Dynamic imaging with confocal microscopy permits a detailed analysis of the dynamics of pathogen-host cell interactions at the mucosal interface and the neighbouring tissue at high resolution. The system can be applied to various hollow organs with few modifications. Here we present first applications to study representative infections such as uropathogenic Escherichia coli (UPEC) infections in the urinary bladder or amoebiasis of the colon by using mouse organs. We show (i) intracellular bacteria in UPEC infections, (ii) phagocytic events on tissue during infection, as well as (iii) tissue invasion of virulent protozoans into epithelia. The versatility of this system and its higher degree of control in comparison with both traditional explant microscopy and in vivo two photon imaging solutions make it a valuable and easy-to-use addition to other current imaging techniques.

Experimental amebiasis: a selected review of some in vivo models

The use of in vivo animal models in amebiasis has contributed significantly to the knowledge of this common human parasitic disease. Although there is no animal model that mimics the whole cycle of the human disease, the use of different susceptible and resistant laboratory animals and the availability for many years of techniques for the axenic culture of trophozoites of Entamoeba histolytica have allowed a better understanding of the parasite and the host-parasite relationship. The recent introduction of frontier methodologies in biology has increased our comprehension of this parasite. New information on the cellular and molecular biology and genetics of this organism has been extensively reported, and much of this has clearly required the more frequent use of animal models to verify specific facts. Based on experimental animals characterized previously, the introduction of new animal models with genetic or surgical modifications, especially in mice, has allowed a more adequate analysis of the mechanisms of pathogenesis. Multiple factors have been considered in the promotion of the invasiveness and virulence of E. histolytica. Additionally, the immunological and physiological responses of the host, depending on the environmental conditions, lead to the establishment or the rejection of the parasite. The role of inflammatory reaction to amebic infection constitutes one of the controversies that has been studied by several authors. In susceptible animals (hamsters and gerbils), inflammatory cell damage seems to be related to target cell lysis, while in resistant animals (mice), inflammatory cells appear to protect the host by lysing the parasite. Presently, the involvement of various substances in the development of lesions including lectins, proteases, amebapores, promoters of apoptosis, cytokines, nitric oxide, etc., is being examined using different in vivo models.

The pathogenesis of amoebiasis

Amoebiasis, the infection of humans with Entamoeba histolytica, has a worldwide distribution; humans are the main reservoir and source of infection(1), although some other primates can also be infected. The motile trophozoite of E. histolytica (Fig. 1) lives in the lumen of the large intestine where it multiplies and eventually differentiates into cysts which are shed in the faeces and are responsible for transmission of infection. Two forms of amoebiasis are recognized: luminal amoebiasis where no clinical signs or symptoms are apparent, and invasive amoebiasis where the trophozoites invade the intestinal mucosa to produce dysentery or amoeboma, and can spread in blood to give extraintestinal lesions such as liver abscess. Isoenzyme markers for pathogenic and non-pathogenic types of E. histolytica are well documented, but there is some debate (see Parasitology Today, vol. 3, 37-43) about whether the two types represent completely separate entities or if they can change from one type to the other under certain circumstances (Box 1). Nonpathogenic types produce no apparent symptoms; in this article Adolfo Martínez-Palomo discusses the pathology associated with pathogenic types.

Entamoeba histolytica: acute granulomatous intestinal lesions in normal and neutrophil-depleted mice

To study the role of neutrophils in the innate resistance to Entamoeba histolytica intestinal infection in mice, animals were treated with anti-neutrophil monoclonal antibodies prior to intracecal parasite inoculation and the resulting lesions were compared with normal mice that had been equally infected. In contrast to our previous finding that neutrophils are critical in eliminating E. histolytica infection in the liver, we show here that neutrophils are not absolutely required to eliminate E. histolytica infection from the intestine. Although the neutrophils are not critical for resolution of the E. histolytica infection, neutrophils do appear to provide some measure of protection as the intestinal amoeba burden was higher at early timepoints after infection in the neutropenic animals. In addition, we found that while both the normal and the neutrophil-depleted mice developed ulcerative lesions in the colon, the neutropenic mice had an increased frequency of granulomas that formed around the amoeba. Thus, our findings appear to be the first evidence showing that granulomatous inflammation can occur after intestinal infection in mice using axenically cultured amoeba.

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.

A comparative study of experimental caecal amoebiasis and the evaluation of amoebicides

Caecal amoebiasis models (mouse, hamster and rat) were compared for amoebic infection and chemotherapeutic evaluation using a pathogenic strain of Entamoeba histolytica of human origin. Caecal infection in the hamster was of the acute type, with a large number of amoebae in the caecal tissue, whereas the amoebae in rats were seen in the lumen, on the mucosal surface. Amoebic lesions in the mouse caecum were of moderate severity. The mouse model responded to both metronidazole (tissue amoebicide) and diloxanide furoate (luminal amoebicide), whereas the hamster model only responded well to metronidazole and the rat model only to diloxanide furoate. Using the mouse as a primary screening model, a new oxadiazole compound, BTI 2405E, was found to have tissue and luminal amoebicidal activity. In rodent caecal models, a single dose of BTI 2405E was found to be more effective than one of metronidazole or diloxanide furoate. The mouse model appears to be the most useful for primary screening of anti-amoebic compounds.

Dose- and time-dependent functional and structural damage to the colon mucosa by Entamoeba histolytica trophozoite lysates

Analysis of the initial interaction between Entamoeba histolytica trophozoites and the large intestine is impossible in humans and very difficult in experimental animals. To circumvent this obstacle we treated the luminal side of full-thickness rabbit colon segments mounted in Ussing-type chambers with trophozoite lysates of the E. histolytica HM1 virulent strain. Exposure to lysates for up to 90 min produced dose- and time-dependent effects on the colon, consisting of (a) increased decay rates for potential difference, short-circuit current, and transmural resistance and (b) mucosal damage ranging from vacuolation at the bases and shortening of epithelial cells to the loss of intercellular junctions, destruction of microvilli, and necrosis of interglandular epithelial zones. This acute model of intestinal amebiasis is sensitive, fast, and reliable.

In vitro activity of nalidixic acid and its iron (III) complex on Entamoeba histolytica

The in vitro activity of the antibacterial agent nalidixic acid (HNal) and its iron (III) complex (FeNal) against Entamoeba histolytica HM1 strain trophozoites in axenic or monoxenic (associated with Clostridium symbiosum) cultures was investigated. Using a dilution test with TYI-S-33 medium, this protozoan was found to be susceptible to both drugs, but FeNal showed amoebicidal activity only at concentrations higher than those used with HNal.

Entamoeba histolytica: virulence potential and sensitivity to metronidazole and emetine of four isolates possessing nonpathogenic zymodemes

The pathogenic potential of four Entamoeba histolytica isolates obtained from asymptomatic carriers and possessing nonpathogenic zymodemes was compared to four E. histolytica strains obtained from invasive cases of amebiasis and having pathogenic zymodemes. Both xenic and axenic cultures of a number of strains were tested. Determinations of cytopathogenicity were done in vitro by measuring the rates of destruction of tissue cultured monolayers of baby hamster kidney cells by intact amebae or by its cell-free extracts. The in vivo virulence was tested by assessing their capacity to form hepatic abscesses in hamsters or cecal ulcerations in rats. The results obtained show that two of the isolates from asymptomatic carriers (strains SAW 1734R clAR and WI:0385:191) were as virulent as three of the invasive ones (HM-1:IMSS, 200:NIH, and SAW 408). Two other isolates from asymptomatic carriers and one from a dysentery case were avirulent. All the E. histolytica isolates tested were similarly sensitive to metronidazole and emetine (IC50 1-10 micrograms/ml). The results indicate that the pathogenic potential of E. histolytica varies between isolates and can be affected by culture conditions and by the presence or absence of bacterial cells. These findings suggest that virulence does not necessarily correlate with a pathogenic zymodeme.

Interaction between pathogenic amebas and fibronectin: substrate degradation and changes in cytoskeleton organization

Invasion of human tissues by the parasitic protozoan Entamoeba histolytica is a multistep process involving, as a first step, the recognition of surface molecules on target tissues by the amebas or trophozoites. This initial contact is followed by the release of proteolytic and other activities that lyse target cells and degrade the extracellular matrix. In other parasitic diseases, as well as in certain cancers, the interaction of invasive organisms or cells with fibronectin (FN) through specific receptors has been shown to be the initial step in target cell recognition. Interaction with FN triggers the release of proteolytic activities necessary for the effector cell migration and invasion. Here, we describe the specific interaction of Entamoeba histolytica trophozoites with FN, and identify a 37-kD membrane peptide as the putative receptor for FN. The interaction between the parasite and FN leads to a response reaction that includes the secretion of proteases that degrade the bound FN and the rearrangement of amebic actin into "adhesion plates" at sites of contact with FN-coated surfaces. The kinetics of the interaction was determined by measuring the binding of soluble 125I-FN to the trophozoites and visualization of the bound protein using specific antibodies. Degradation of FN was measured by gel electrophoresis and the release of radioactivity into the incubation medium. Focal degradation of FN was visualized as black spots under the trophozoites at contact sites with fluorescent FN. We conclude that the interaction of E. histolytica with FN occurs through a specific surface receptor. The interaction promotes amebic cytoskeleton changes and release of proteases from the parasite. The binding and degradation of extracellular matrix components may facilitate the migration and penetration of amebas into tissues, causing the lesions seen in human hosts.

Cytopathogenicity of Entamoeba histolytica: the role of amebic adherence and contact-dependent cytolysis in pathogenesis

The mechanisms by which Entamoeba histolytica trophozoites adhere to and lyse target cells are reviewed from the perspective of pathogenesis. Adherence via the galactose and N-acetyl-galactosamine inhibitable amebic lectin and possible additional amebic adhesin molecules is followed by target cell death. Inhibition of the Gal/GalNAc lectin with GalNAc inhibits amebic cytolysis of target cells. Amebic activities implicated in the cytolytic event include vesicle exocytosis and maintenance of an acid pH, pore forming proteins, phospholipase A and proteases. Increased knowledge of the sequence of events leading to target cell lysis should lead to more effective treatment or prevention of infection by this enteric parasite and add to our basic understanding of eukaryotic cell-cell interactions.