Entamoeba histolytica induced NETosis and the dual role of NETs in amoebiasis

Entamoeba histolytica (Eh), a microaerophilic parasite, causes deadly enteric infections that result in Amoebiasis. Every year, the count of invasive infections reaches 50 million approximately and 40,000 to 1,00,000 deaths occurring due to amoebiasis are reported globally. Profound inflammation is the hallmark of severe amoebiasis which is facilitated by immune first defenders, neutrophils. Due to size incompatibility, neutrophils are unable to phagocytose Eh and thus, came up with the miraculous antiparasitic mechanism of neutrophil extracellular traps (NETs). This review provides an in-depth analysis of NETosis induced by Eh including the antigens involved in the recognition of Eh and the biochemistry of NET formation. Additionally, it underscores its novelty by describing the dual role of NETs in amoebiasis where it acts as a double-edged sword in terms of both clearing and exacerbating amoebiasis. It also provides a comprehensive account of the virulence factors discovered to date that are implicated directly and indirectly in the pathophysiology of Eh infections through the lens of NETs and can be interesting drug targets.

Entamoeba histolytica Trophozoites and Lipopeptidophosphoglycan Trigger Human Neutrophil Extracellular Traps

Neutrophil defense mechanisms include phagocytosis, degranulation and the formation of extracellular traps (NET). These networks of DNA are triggered by several immune and microbial factors, representing a defense strategy to prevent microbial spread by trapping/killing pathogens. This may be important against Entamoeba histolytica, since its large size hinders its phagocytosis. The aim of this study was to determine whether E. histolytica and their lipopeptidophosphoglycan (EhLPPG) induce the formation of NETs and the outcome of their interaction with the parasite. Our data show that live amoebae and EhLPPG, but not fixed trophozoites, induced NET formation in a time and dose dependent manner, starting at 5 min of co-incubation. Although immunofluorescence studies showed that the NETs contain cathelicidin LL-37 in close proximity to amoebae, the trophozoite growth was only affected when ethylene glycol tetra-acetic acid (EGTA) was present during contact with NETs, suggesting that the activity of enzymes requiring calcium, such as DNases, may be important for amoeba survival. In conclusion, E. histolytica trophozoites and EhLPPG induce in vitro formation of human NETs, which did not affect the parasite growth unless a chelating agent was present. These results suggest that NETs may be an important factor of the innate immune response during infection with E. histolytica.

Evidence for a link between parasite genotype and outcome of infection with Entamoeba histolytica

The factors determining whether a person infected with Entamoeba histolytica develops disease remain obscure. To investigate whether the parasite genome contributes to the outcome, we have investigated the distribution of parasite genotypes among E. histolytica-infected individuals in Bangladesh. Samples were obtained from individuals who either were asymptomatic, had diarrhea/dysentery, or had developed a liver abscess. Genotypes were determined by using six tRNA-linked polymorphic markers, and their distributions among the three sample groups were evaluated. A significant population differentiation in the genotype distribution was found for four of the six individual markers as well as for the combined genotypes, suggesting that the parasite genome does contribute in some way to the outcome of infection with E. histolytica. The markers themselves do not indicate the nature of the underlying genetic differences, but they may be linked to loci that do have an impact on the outcome of infection.

Prevalence and species distribution of E. Histolytica and E. Dispar in the Venda region, Limpopo, South Africa

The prevalence and species distribution of Entamoeba histolytica and E. dispar in the Venda region were determined in stool samples collected from public hospitals and primary schools by ELISA and a nested polymerase chain reaction (PCR). E. histolytica was detected in 37/197 (18.8%) and 1/47 (2.1%) samples, whereas 50/197 (25.3%) and 4/47 (8.5%) had E. dispar in the hospitals and schools, respectively. The age groups most infected were 0-2 (33%) years followed by 20-29 years (27%). E. histolytica was significantly associated with diarrhea (77.4% versus 22.6%; chi 2 = 39.48, P < 0.05), and with the presence of lactoferrin (85.7% versus 14.2%) in the stools, indicating intestinal inflammation (chi 2 = 29.605, P < 0.05). E. histolytica was found in 5 (16.12%) of the 31 HIV-positive individuals and in 33 (15.5%) of the 213 HIV-negative individuals. E. histolytica infections are common in the Venda region and are associated with diarrhea and intestinal inflammation.

Cognitive effects of diarrhea, malnutrition, and Entamoeba histolytica infection on school age children in Dhaka, Bangladesh

Cognitive function was assessed in 191 Bangladeshi children 6-9 years of age using verbal and nonverbal tests. These scores were added to a health surveillance database that was compiled over the four previous years that includes incidence of diarrhea and Entamoeba histolytica infection and nutritional status. The associations of diarrhea, malnutrition, and social factors with cognitive scores were analyzed statistically, and associations between diarrhea and test scores were controlled for the influence of social factors. Cognitive scores were negatively associated with stunting during school age, as well as the height-for-age and weight-for-age scores at study enrollment. Incidence of diarrhea was associated with nonverbal test scores before, but not after, controlling for socioeconomic factors. Generally E. histolytica infection was not found to independently influence scores, except that E. histolytica-associated dysentery was associated with lower test scores while dysentery of any etiology was not. Thus, malnutrition during the school age years, but not diarrhea or E. histolytica infection, was associated with a lower level of cognitive functioning. This suggested that intervention during school age years may be able to mitigate the cognitive deficiencies associated with malnutrition.

Dehydroepiandrosterone decreases while cortisol increases in vitro growth and viability of Entamoeba histolytica

In vitro exposure of Entamoeba histolytica trophozoites to the sex steroids 17beta-estradiol, progesterone, and dehydrotestosterone had little effect on parasite viability or proliferation. However, treatment with the adrenal steroid dehydroepiandrosterone (DHEA) markedly inhibited parasite proliferation, adherence and motility, and at a certain dose it induced trophozoite lysis. The opposite effect on proliferation was found when the trophozoites were exposed to cortisol. Moreover, DHEA decreased while cortisol increased the parasite's DNA synthesis determined by 3H-thymidine incorporation. Trophozoite lysis by DHEA appeared to be caused by a necrotic rather than an apoptotic process, as observed in propidium iodide and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling assays. A possible mechanisms of action was derived from experiments demonstrating that the activity of a putative 3-hydroxy-3-methyl glutaryl CoA reductase detected in trophozoite extracts was inhibited in the presence of DHEA. Contrary to its in vitro inhibitory effect, in vivo administration of DHEA to infected hamsters resulted in exacerbation of the amebic liver abscesses. These results demonstrated that androgen steroids act directly upon E. histolytica growth and viability, and may shed new light on some age and gender differences in disease progression, as well as finding application in the drug treatment of human amebiasis.

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.

Cytokine mRNA expressions in symptomatic vs. asymptomatic amoebiasis patients

Infection with Entamoeba histolytica results in high mortality worldwide. Studies on the cytokine response in symptomatic and asymptomatic amoebiasis (caused by E. histolytica, the pathogenic species and E. dispar, the non-pathogenic species) subjects and their correlation with symptomatology are lacking. The present study reports the cytokine response (TNF-alpha, IFN-gamma, IL-4, IL-10 and TGF-beta) in such subjects as measured by RT-PCR. The results showed significantly (< 0.05) higher expressions of IL-10 and TGF-beta in the symptomatic group as compared to the asymptomatic and healthy controls. The cytokine profile indicated the role of suppressive immune response in symptomatic amoebiasis patients.

An experimental model for amoebic abscess production in the cheek pouch of the Syrian golden hamster, Mesocricetus auratus

A new experimental model was developed in hamsters for amoebic abscess caused by Entamoeba histolytica. E. histolytica trophozoites were cultured in a liquid axenic medium, and then injected intradermally into the cheek pouch of the Syrian golden hamster, Mesocricetus auratus. Inoculation consistently resulted in abscess formation at the site in 20 of 22 (91%) study animals. The amoebic nature of the abscesses was confirmed by light microscopy and histopathologic examination. Abscess formation was maximal at day 12 post-inoculation. Potential applications of this simple and reliable model include further elucidation of the pathogenesis of invasive amoebiasis, studies of the host response to amoebae, and in vivo evaluation of chemotherapeutic agents that show in vitro efficacy against E. histolytica.

Expression in fibroblasts and in live animals of Entamoeba histolytica polypeptides EhCP112 and EhADH112

EhCPADH is an immunogenic, heterodimeric protein that is formed by EhCP112 (cysteine protease) and EhADH112 (adhesin), polypeptides involved in Entamoeba histolytica's cytopathic effect, target-cell adherence and phagocytosis. The EhCPADH complex is located in the plasma membrane and cytoplasmic vacuoles. Here, the independent expression of EhCP112 and EhADH112 in fibroblasts and hamsters was analysed. Also investigated was the immunological response in animals independently inoculated with plasmid pcDNA-Ehcp112, which carries the complete cysteine protease-encoding gene, or with plasmid pcDNA-Ehadh112, which carries the C terminus of the adhesin-encoding gene, or with a mixture of both. Both proteins were expressed in the plasma membranes of the transfected fibroblasts. EhCP112 was toxic for the mammalian cells. Proteins were also independently expressed in hamsters after inoculation with the plasmids. Their expression was indirectly evaluated by the presence of antibodies in the inoculated animals. Remarkably, co-immunization of the animals with the two DNA plasmids resulted in an earlier and higher anti-E. histolytica IgG induction than immunization with separate plasmids. In contrast, the cellular immune response was not noticeably improved by the plasmid mixture. Interestingly, protection against liver abscesses was detected only in animals that received the plasmid mixture and no protection was observed in hamsters independently inoculated with plasmid pcDNA-Ehcp112 or pcDNA-Ehadh112.

Pathogenesis of amebiasis

It is an exciting time in the study of Entamoeba histolytica. Over the past two years, the natural history and burden of disease in humans has been redefined, mucosal immune responses associated with protection identified, and the developmental regulation of encystation outlined. The number of genes sequenced has increased from a few hundred to a few thousand, and study of the genome structure is revealing unusual repetitive elements and plasticity. DNA microarrays promise the first ability to examine global patterns of mRNA abundance. The mechanism of transcriptional control via histone modifications and sequence-specific DNA-binding proteins are to be delineated. Advances in cell biology are providing new insights into invasion through the intestinal epithelium.

Isolated amoebic appendicitis

Amoebiasis, a disease of worldwide distribution, is endemic in tropical countries with suboptimal sanitation facilities. Isolated amoebic appendicitis (IAA) is regarded as a rare manifestation of the disease globally. Because there are no defined clinical features that distinguish IAA from bacterial appendicitis, diagnosis is usually dependent on histopathological examination. A 9-year retrospective study was undertaken to investigate the clinicopathological aspects of IAA. The main complaints were fever and abdominal pain. None of the patients had dysentery. The pre-operative clinical diagnosis was acute appendicitis and acute abdomen in 13 and 8 patients, respectively. In all cases the intra-operative diagnosis was acute appendicitis. Gross pathological appraisal revealed peritonitis and perforation in 19 and 17 cases, respectively. Histopathological examination of these appendices demonstrated appendiceal ulceration, transmural mixed inflammation, haematophagous amoebic trophozoites and necrosis in all cases. Vascular pathology comprised venous and capillary luminal plugging (11 cases), necrotising small vessel vasculitis (11 cases), thrombophlebitis of medium sized veins (9 cases) and arteritis with associated thrombosis (1 case). Organising fibrinopurulent peritonitis was present in 19 cases. Two appendices that appeared normal macroscopically demonstrated ulceration and inflammation that were confined to the mucosa and submucosa. All of 18 patients who were treated with metronidazole survived without further surgery, while three patients who were untreated succumbed to the disease. Appendicectomy, accurate histopathological appraisal thereof and optimal, timely management of IAA were critical to the favourable outcome in the present study.

Early interactions of Entamoeba histolytica trophozoites with parenchymal and inflammatory cells in the hamster liver: an immunocytochemical study

We studied the early in situ interactions of live and fixed Entamoeba histolytica trophozoites with hamster hepatic parenchymal and inflammatory cells using immunoperoxidase and immunoelectronmicroscopy. Close contact between trophozoites and endothelial cells and the diffusion of amoebic molecules from trophozoites towards nearby endothelial cells and distant hepatocytes were observed. The inflammatory cells around the amoebae and the remnants of parenchymal cells and hepatocytes located close to the lesion had a positive stain for amoebic molecules. In the amoebae, at the ultrastructural level, molecules were attached to the membranes and inside the vesicles. These molecules were apparently released into the space formed between the parasite and the endothelial cells. The endothelial cells and the nearby and distant hepatocytes captured amoebic molecules, and later they became necrotic. Contrarily, when fixed amoebae were inoculated, amoebic molecules were captured by endothelial cells and polymorphonuclear (PMN) leukocytes, but neither suffered any damage. In this work, we are presenting evidence clearly showing that some molecules of the amoeba can diffuse away long distances causing cytotoxic effects and even necrosis on hepatic cells of hamster liver without the need of the trophozoite being in close contact with the target cells. They also may promote lytic or proinflammatory effects by inducing the secretion of enzymes or cytokines in other nonparenchymal cells, like PMN leukocytes and endothelial cells. Our results suggest that the accepted mechanisms of cytotoxicity by amoebae are not exclusively restricted to the following sequence: adhesion, phagocytosis, and necrosis.

Protozoon infections and intestinal permeability

Intestinal permeability (IP) studies using some macromolecules have been assumed to demonstrate the intactness of intestinal mucosa. The aim of the present study is to determine the changes in IP among patients with protozoan infections. Thirty nine patients with protozoan infections and ten healthy controls were enrolled in the study. Protozoa were diagnosed by Native-lugol, Richie and Trichrome staining of faeces. IP was evaluated by diethyl triamine penta acetic acid labeled with 99m Technetium (99mTc labeled DTPA) assay. The IP was found to have increased in patients with protozoan infections compared with control patients (7.20+/-5.52 vs. 4.47+/-0.65%, P=0.0017). The IP values were 9.91+/-10.05% in Giardia intestinalis group, 6.81+/-2.25% in Blastocystis hominis group, 5.78+/-2.84% in Entamoeba coli group. In comparison with the control group, the IP was significantly higher in G. intestinalis and B. hominis patients (P=0.0025, P=0.00037, respectively), but not in E. coli patients. In conclusion, the IP increases in patients with G. intestinalis and B. hominis but not with E. coli infection. This finding supports the view that IP increases during the course of protozoan infections which cause damage to the intestinal wall while non-pathogenic protozoan infections have no effect on IP. The increase in IP in patients with B. hominis brings forth the idea that B. hominis can be a pathogenic protozoan.

Pathophysiology of amoebiasis

Few organisms are more aptly named than Entamoeba histolytica, an intestinal protozoan parasite that can lyse and destroy human tissue. Within the past four years, new models of E. histolytica infection have begun to illuminate how amoebic trophozoites cause intestinal disease and liver abscess, and have expanded our understanding of the remarkable killing ability of this parasite. Here, I summarize recent work on the interactions between E. histolytica and human intestine, and between E. histolytica and hepatocytes, and discuss what these studies tell us about the role of inflammation and programmed cell death in the pathogenesis of amoebiasis.

Caspase 3-dependent killing of host cells by the parasite Entamoeba histolytica

The parasite Entamoeba histolytica is named for its ability to lyse host tissues. To determine the factors responsible, we have initiated an examination of the contribution of parasite virulence factors and host caspases to cellular destruction by the parasite. Amoebic colitis in C3H/HeJ mice was associated with extensive host apoptosis at sites of E. histolytica invasion. In vitro studies of E. histolytica-Jurkat T-cell interactions demonstrated that apoptosis required contact via the amoebic Gal/GalNAc lectin, but was unaffected by 75% inhibition of the amoebic cysteine proteinases. Parasite-induced DNA fragmentation was unaffected in caspase 8-deficient Jurkat cells treated with the caspase 9 inhibitor Ac-LEHD-fmk. In contrast, caspase 3-like activity was observed within minutes of E. histolytica contact and the caspase 3 inhibitor Ac-DEVD-CHO blocked Jurkat T cell death, as measured by both DNA fragmentation and 51Cr release. These data demonstrate rapid parasite-induced activation of caspase 3-like caspases, independent of the upstream caspases 8 and 9, which is required for host cell death.

Inhibition of Entamoeba histolytica proteolytic activity by human salivary IgA antibodies

Entamoeba histolytica is a protozoan parasite that causes amoebiasis in humans; as the infection occurs mainly in the intestinal epithelium, the secretory immune response of the host could have an influence on the outcome. Secretory IgA antibodies against E. histolytica have been detected in asymptomatic and symptomatic patients, but little is known about their protective role. E. histolytica cysteine proteases seem to be involved in the pathogenesis of amoebiasis; therefore, it is important to evaluate the human IgA response against these proteases and its effect on their enzymatic activity. When human saliva samples with and without antibodies against E. histolytica were tested by Western blot against one purified 70 kDa amoebic cysteine protease, 84% of anti-amoeba-positive samples recognized it. The secretory IgA purified from a pool of anti-protease-positive samples had a strong in vitro inhibitory effect on the E. histolytica proteolytic activity. These results suggest that this effect, if it occurs in vivo, could be an important protective factor against this parasite.

Amebiasis and "nonpathogenic" intestinal protozoa

Infection with single or multiple species of intestinal protozoa is common in humans and can result in either asymptomatic colonization or symptoms of intestinal disease. Entamoeba histolytica serves as a paradigm for invasive colonic protozoal infection. The key to diagnosis and treatment of amebiasis is knowledge of the epidemiologic risk factors and clinical manifestations, a rational approach to diagnosis, and an understanding of the sites of action and uses of anti-amebic drugs. This knowledge of treatment provides a context for consideration of intestinal infection with less common protozoan pathogens such as Dientamoeba fragilis and Balantidium coli and 'nonpathogenic' protozoa such as Blastocystis hominis and Entamoeba coli.

Phagocytosis and proteinase activity are not related to pathogenicity of E. histolytica

To examine the relationship between phagocytosis, proteinase activity and pathogenicity of axenically grown trophozoites of E. histolytica strain HM-1:IMSS four different cultures were used: (1) a culture preserved in our laboratory for over 4 years, which lost its pathogenicity 3 years ago; (2) a culture passaged several times through hamster liver, which lost its pathogenicity recently; (3) a highly virulent culture supplied by another laboratory; and (4) amebas recovered from hamster liver abscesses caused by culture 3. Phagocytosis was measured as erythrophagocytosis. Proteinase activity was determined on azocasein. Pathogenicity was defined as the capacity to cause liver abscesses in hamsters. A negative correlation was found between phagocytic activity and pathogenicity, since amebas unable to cause liver abscesses had the highest phagocytic activity, whereas those recovered from liver abscesses had the lowest phagocytic activity. The percent of phagocytic amebas showed wide variations through a 2-month observation period, with no change in amebic pathogenicity. No correlation was found between the level of proteinase activity and pathogenicity. It is concluded that neither phagocytosis nor proteinase activity is an adequate marker of amebic pathogenicity.

Inflammatory reaction in experimental hepatic amebiasis. An ultrastructural study

One of the hallmarks of tissue necrosis produced by the human protozoan parasite Entamoeba histolytica, the causative agent of human amebiasis, appeared to be the lack of inflammatory reaction to the invading trophozoites. Recent evidence suggests, however, that inflammatory cells do appear during early stages of amebic destructive lesions and that they contribute to the establishment of foci of tissue necrosis in intestinal and liver lesions. The present analysis of the fine-structural changes that take place during early stages of amebic liver abscesses induced in hamsters after the intraportal inoculation of axenic amebas has shown that large numbers of polymorphonuclear leukocytes (PMNs) are recruited around invading amebas. These leukocytes lyse as a consequence of contact-mediated damage induced by the trophozoites. Amebas were also capable of ingesting apparently intact PMNs. Macrophages and eosinophils were also recruited at the foci of inflammation. At all times examined, trophozoites of Entamoeba histolytica survived in spite of being in close contact with PMNs or degranulating eosinophils. The ultrastructural observations have also shown the lack of direct contact between amebas and liver parenchymal cells during the initial stages of the focal liver necrosis induced by the parasite, therefore supporting the view that hepatic damage may be effected indirectly through lysis of inflammatory cells. The results also provide a basis for the understanding of the induction of experimental protective immunity against invasive amebiasis, a process which seems to be mostly dependent on cellular mechanisms.

Experimental infection of rhesus monkeys with Entamoeba histolytica mimics human infection

Experimental infection of Entamoeba histolytica was successfully established in rhesus monkeys (Macaca mulatta). Parasites of proven pathogenicity maintained through liver passage in hamsters were used in this study. Laparotomized monkeys were inoculated intracecally/intrahepatically with trophozoites, or by oral administration of cysts (2000 cysts/ml). The cysts were isolated from an infected human stool sample. Formation of typical amebic lesions in the inoculated tissues along with alterations in hematologic indices were studied in the infected monkeys. All the experimentally inoculated monkeys showed leukocytosis and mild neutrophilia, while the hemoglobin content and levels of blood sugar and blood urea remained unaltered during the post-infection period. Specific antiamebic antibodies were readily detectable in post-infected sera.

Entamoeba histolytica: purification of cathepsin B

A cytotoxic cysteine proteinase with a molecular weight of 16,000 was isolated from axenically grown trophozoites of Entamoeba histolytica. The enzyme was purified from frozen-thawed strain HM-1 by ion-exchange chromatography on DEAE-cellulose, organomercurial agarose affinity chromatography, and size-exclusion chromatography. The purified enzyme had proteinase activity that could be demonstrated on azocasein (pH 5), hemoglobin (pH 5), or carbobenzoxy-L-arginyl--L-arginyl-7-amino-4-trifluoromethylcoumarin++ + (Z-arg-arg-AFC), a substrate specific for cathepsin B. Enzyme activity was stable to high pH, but not to 40 C for 1 hr or 56 C for 0.5 hr. As typical of cysteine proteinases, inhibition of activity on Z-arg-arg-AFC by p-chloromercuribenzoate or mercury was reversed by free sulfhydryl groups. Both the proteinase and cytotoxic activities of the purified amoebal cathepsin B were inhibited by leupeptin and serum and activated by free sulfhydryl groups, supporting the hypothesis that both activities are characteristics of amoebal cathepsin B. Virulent strains of E. histolytica (HM-1 and Rahman) had significantly more cathepsin B activity per milligram protein than less virulent strains (HK-9, Laredo, and Huff). The correlation between higher levels of cathepsin B activity in strains with greater virulence could indicate a role for amoebal cathepsin B in the pathogenesis of amoebiasis.

[Entamoeba histolytica: I. Mechanism of cytotoxic activity]

Cytotoxic action against K562-tissue culture cells was investigated under various conditions with a Chromium-release-assay. When amoebae and target cells were centrifuged together, pathogenic strains of amoebae induced a very fast increase of target cell lysis (up to 50% of maximum lysis after 10 minutes). Only a minor degree of target cell lysis resulted, however, when amoebae and K562 cells were kept in suspension. When amoebae were eliminated selectively by addition of complement 10 minutes after starting the experiment, this fast increase of lysis could not be prevented. These observations suggest that the cytotoxic action might take place in two distinct phases. The first step ("lethal hit") seems to be temperature-independent, whereas a temperature of 37 degrees C is necessary for the second step to occur during which cytoplasmic material is released (chromium release). The presence of amoebae is not necessary for the second step. When amoebae together with and target cells are kept in suspension, amoebae lost their capability of setting the "lethal hit" with increasing time of coincubation. It seems, as if the "lethal hit" cannot be accomplished effectively under the conditions of suspension: cytotoxic substances released by the amoebae cannot be transferred to the target cells and are lost in the fluid phase. Thereby, the amoebae are depleted of such substances. Thus, a stable contact between amoebae and target cells for at least a few minutes seems to be necessary for the expression of cytotoxicity.

Entamoeba histolytica causes intestinal secretion: role of serotonin

Lysates of the protozoan parasite Entamoeba histolytica altered active electrolyte transport when present on the serosal surface of rabbit ileum and rat colon. The lysate-induced effects on electrolyte transport were similar to those caused by serotonin, and were blocked by bufotenine, an analog known to inhibit the action of serotonin. The transport effects were partially inhibited by antibody to serotonin. The amebic lysates were shown to contain serotonin by radioimmunoassay, high-performance liquid chromatography, and thin-layer chromatography. These results suggest that the serotonin present in Entamoeba histolytica may be important in the diarrhea seen in amebiasis.