Susceptibility of Entamoeba histolytica to oxidants

Apoptosis-inducing factor-like protein-mediated stress and metronidazole-responsive programmed cell death pathway in Entamoeba histolytica

Apoptosis-inducing factor (AIF) is the major component of the caspase-independent cell death pathway that is considered to be evolutionarily ancient. Apoptosis is generally evolved with multicellularity as a prerequisite for the elimination of aged, stressed, or infected cells promoting the survival of the organism. Our study reports the presence of a putative AIF-like protein in Entamoeba histolytica, a caspase-deficient primitive protozoan, strengthening the concept of occurrence of apoptosis in unicellular organisms as well. The putative cytoplasmic EhAIF migrates to the nucleus on receiving stresses that precede its binding with DNA, following chromatin degradation and chromatin condensation as evident from both in vitro and in vivo experiments. Down-regulating the EhAIF expression attenuates the apoptotic features of insulted cells and increases the survival potency in terms of cell viability and vitality of the trophozoites, whereas over-expression of the EhAIF effectively enhances the phenomena. Interestingly, metronidazole, the most widely used drug for amoebiasis treatment, is also potent to elicit similar AIF-mediated cell death responses like other stresses indicating the AIF-mediated cell death could be the probable mechanism of trophozoite-death by metronidazole treatment. The occurrence of apoptosis in a unicellular organism is an interesting phenomenon that might signify the altruistic death that overall improves the population health.

Ascorbic Acid Ameriolates Liver Damage by Myeloperoxidase Oxidative Products in a Hamster Model of Amoebic Liver Abscess

Entamoeba histolytica is a protozoan-pathogen-causing amoebic liver abscess (ALA). After amoeba establishment in the liver, it causes abundant infiltrate of neutrophils. Liver tissue damage by neutrophils results in part from anti-amoebic oxidative intermediates, including reactive oxygen species (ROS), reactive nitrogen species (RNS), and hypochlorous acid (HOCl), derived from the myeloperoxidase (MPO) enzyme. Ascorbic acid (ASC) is an antioxidant that acts as a scavenger for ROS and NOS-derived free radicals. No previous information regarding the effect of ASC concerning the participation of MPO in an experimental model of ALA in hamsters has been reported. Thus, the aim of the present work was to analyze the effect of ASC on acute ALA development and to measure the activity and gene expression of the MPO enzyme. Hamsters were treated with ASC (800 mg/kg) and then intrahepatically inoculated with E. histolytica trophozoites. Animals were sacrificed at 3, 6, and 12 h post-inoculation (p.i.), and liver samples were collected. The percentage of lesions, amoeba in situ count, MPO activity, and mpo gene expression were ascertained. Compared to ALA hamsters without ASC treatment as the control group (CT), the ALA group treated with ASC had a significant decrease in liver lesions (all p.i. hours) and viable amoeba count (12 h p.i.) and an increase in MPO activity (12 h p.i.) and mpo gene expression (6 h/12 h p.i.). These data suggest that ASC ameliorated liver damage caused by oxidizing products via modulation of mpo expression and activity.

A review of the proposed role of neutrophils in rodent amebic liver abscess models

Host invasion by Entamoeba histolytica, the pathogenic agent of amebiasis, can lead to the development of amebic liver abscess (ALA). Due to the difficulty of exploring host and amebic factors involved in the pathogenesis of ALA in humans, most studies have been conducted with animal models (e.g., mice, gerbils, and hamsters). Histopathological findings reveal that the chronic phase of ALA in humans corresponds to lytic or liquefactive necrosis, whereas in rodent models there is granulomatous inflammation. However, the use of animal models has provided important information on molecules and mechanisms of the host/parasite interaction. Hence, the present review discusses the possible role of neutrophils in the effector immune response in ALA in rodents. Properly activated neutrophils are probably successful in eliminating amebas through oxidative and non-oxidative mechanisms, including neutrophil degranulation, the generation of free radicals (O₂⁻, H₂O₂, HOCl) and peroxynitrite, the activation of NADPH-oxidase and myeloperoxidase (MPO) enzymes, and the formation of neutrophil extracellular traps (NETs). On the other hand, if amebas are not eliminated in the early stages of infection, they trigger a prolonged and exaggerated inflammatory response that apparently causes ALAs. Genetic differences in animals and humans are likely to be key to a successful host immune response.

The effect of IFN-γ and TGF-β in the functional activity of mononuclear cells in the presence of Entamoeba histolytica

Background

Entamoeba histolytica (E. histolytica) causes amoebiasis, which is a disease with significant morbidity and mortality. Phagocytic cells and cytokines appear to be important in amoebiasis, but very little is known about the influence of these cells and cytokines in protozoan infections. The aim of this study was to analyse the supernatant of cultures of mononuclear (MN) cells with E. histolytica to determine: 1) the levels of the cytokines IFN-γ and TGF-β, and 2) the amoebicidal activity of MN cells after incubation with cytokines.

Methods

Blood samples were collected from 30 volunteer donors. The cytokine concentrations in MN cells culture supernatants, superoxide release, leukophagocytosis, amoebicide activity, intracellular calcium release and apoptosis were analysed.

Results

The IFN-γ concentrations were 6.22 ± 0.36 and TGF-β concentrations were 17.01 ± 2.21 in cells–trophozoite culture supernatants. MN cells, independently of cytokines, in the presence of amoeba increase the superoxide release. In the absence of cytokines, the ingestion of MN cells by amoebae was higher. In the presence of IFN- γ or TGF- β, a lower ingestion of MN cells was observed by amoebae. MN cells treated with cytokines exhibited higher amoebicide and apoptosis indexes. The incubation of cytokines increased the intracellular calcium release by MN cells.

Conclusions

These results suggest that cytokines play a beneficial role for the host by activating MN cells against E. histolytica. The increased death of amoebae during the leukophagocytosis suggests that both cytokines (IFN-γ and TGF-β) can modulate the functional activity of MN cells and that these cytokines probably are important in the control of amoebic infections.

Differential expression and immunolocalization of antioxidant enzymes in Entamoeba histolytica isolates during metronidazole stress

Entamoeba histolytica infections are endemic in the Indian subcontinent. Five to eight percent of urban population residing under poor sanitary conditions suffers from Entamoeba infections. Metronidazole is the most widely prescribed drug used for amoebiasis. In order to understand the impact of metronidazole stress on the parasite, we evaluated the expression of two antioxidant enzymes, peroxiredoxin and FeSOD, in Entamoeba histolytica isolates during metronidazole stress. The results reveal that, under metronidazole stress, the mRNA expression levels of these enzymes did not undergo any significant change. Interestingly, immunolocalization studies with antibodies targeting peroxiredoxin indicate differential localization of the protein in the cell during metronidazole stress. In normal conditions, all the Entamoeba isolates exhibit presence of peroxiredoxin in the nucleus as well as in the membrane; however with metronidazole stress the protein localized mostly to the membrane. The change in the localization pattern was more pronounced when the cells were subjected to short term metronidazole stress compared to cells adapted to metronidazole. The protein localization to the cell membrane could be the stress response mechanism in these isolates. Colocalization pattern of peroxiredoxin with CaBp1, a cytosolic protein, revealed that the membrane and nuclear localization was specific to peroxiredoxin during metronidazole stress.

Peroxynitrite and peroxiredoxin in the pathogenesis of experimental amebic liver abscess

The molecular mechanisms by which Entamoeba histolytica causes amebic liver abscess (ALA) are still not fully understood. Amebic mechanisms of adherence and cytotoxic activity are pivotal for amebic survival but apparently do not directly cause liver abscess. Abundant evidence indicates that chronic inflammation (resulting from an inadequate immune response) is probably the main cause of ALA. Reports referring to inflammatory mechanisms of liver damage mention a repertoire of toxic molecules by the immune response (especially nitric oxide and reactive oxygen intermediates) and cytotoxic substances released by neutrophils and macrophages after being lysed by amoebas (e.g., defensins, complement, and proteases). Nevertheless, recent evidence downplays these mechanisms in abscess formation and emphasizes the importance of peroxynitrite (ONOO⁻). It seems that the defense mechanism of amoebas against ONOO⁻, namely, the amebic thioredoxin system (including peroxiredoxin), is superior to that of mammals. The aim of the present text is to define the importance of ONOO⁻ as the main agent of liver abscess formation during amebic invasion, and to explain the superior capacity of amoebas to defend themselves against this toxic agent through the peroxiredoxin and thioredoxin system.

Hydrogen peroxide induces apoptosis-like death in Entamoeba histolytica trophozoites

Programmed cell death (PCD) is an essential process in the growth and development of multicellular organisms. However, accumulating evidence indicates that unicellular eukaryotes can also undergo PCD with apoptosis-like features. This study demonstrates that after exposure to 0.8 mM H2O2 for 9 h Entamoeba histolytica presents morphological and biochemical evidence of apoptosis-like death. Morphological characteristics of apoptosis-like death including DNA fragmentation, increased vacuolization, nuclear condensation and cell rounding were observed for H2O2-exposed trophozoites with preservation of membrane integrity. Biochemical alteration in ion fluxes is also a key feature in PCD, and H2O2-exposed trophozoites showed overproduction of reactive oxygen species, increased cytosolic Ca2+ and decreased intracellular pH. Phosphatidylserine was also found to be expressed in the outer leaflet of the plasma membrane of the H2O2-treated trophozoites. Pretreatment with the cysteine protease inhibitor E-64d, the extracellular and intracellular Ca2+ chelators EGTA and BAPTA/AM, and the Ca2+ influx inhibitor verapamil prior to H2O2 exposure abolished DNA fragmentation. The oxidatively stressed trophozoites also showed an increased calpain activity, indicating involvement of Ca2+-dependent calpain-like cysteine proteases in PCD of E. histolytica. A homogeneous caspase assay showed no significant caspase activity, and administration of caspase 1 inhibitor also did not prevent the death phenotype for the oxidatively stressed trophozoites, indicating a caspase-independent apoptosis-like death. Our observations clearly demonstrate that there is a distinct calpain-dependent but caspase-independent pathway for apoptosis-like death in oxidatively stressed E. histolytica trophozoites.

The immunopathogenesis of Entamoeba histolytica

Amebiasis is the disease caused by the enteric dwelling protozoan parasite Entamoeba histolytica. The WHO considers amebiasis as one of the major health problems in developing countries; it is surpassed by only malaria and schistosomiasis for death caused by parasitic infection. E. histolytica primarily lives in the colon as a harmless commensal, but is capable of causing devastating dysentery, colitis and liver abscess. What triggers the switch to a pathogenic phenotype and the onset of disease is unknown. We are becoming increasingly aware of the complexity of the host-parasite interaction. During chronic stages of amebiasis, the host develops an immune response that is incapable of eliminating tissue resident parasites, while the parasite actively immunosuppresses the host. However, most individuals with symptomatic infections succumb only to an episode of dysentery. Why most halt invasion and a minority progress to chronic disease remains poorly understood. This review presents a current understanding of the immune processes that shape the outcome of E. histolytica infections during its different stages.

Hydrogen peroxide-induced apoptosis-like cell death in Entamoeba histolytica

The microaerophilic intestinal parasitic protozoan Entamoeba histolytica has been previously shown to be highly susceptible to oxidative stress induced by hydrogen peroxide. However the mechanism of cell death was not investigated. Studies presented in this paper demonstrate several morphological features in the parasite when exposed to H(2)O(2) which are identical to metazoan apoptotic phenotype indicating a possible apoptosis-like cell death exhibited by E. histolytica in response to H(2)O(2) treatment. Trophozoite cell shrinkage, DNA fragmentation, phosphatidyl serine externalization and increased endogenous reactive oxygen species level have been observed in the protozoan parasite when exposed to 2.0mM H(2)O(2) for different time periods. Although the parasite genome is completely devoid of any of the homologues of mammalian caspases it still codes for a huge number of cysteine proteases which may take over the apoptotic function of the caspases. But the present study indicates the existence of a cysteine protease independent programmed cell death in the parasite since E-64 the specific cysteine protease inhibitor could not rescue the cells from H(2)O(2) induced apoptosis-like cell death.

Molecular nature of virulence in Entamoeba histolytica

For many years virulence of pathogenic Entamoeba histolytica has been attributed to the capacity of the parasite to destroy tissues through the expression and/or secretion of various molecules. Such view is supported mainly by in vitro experimentation, whereas data obtained by using animal models of the disease have clearly demonstrated that the host's inflammatory response is primarily responsible for tissue damage. This review analyzes the content and/or activity of some of the presumed toxic amebic molecules present in amebic strains with different degrees of virulence compared to various parasite in vitro functions that are supposed to correlate with in vivo virulence. The analysis suggests that amebic virulence is primarily determined by the parasite's capacity to adapt and survive the aerobic conditions present in animal tissues. This initial episode in the host-parasite relationship is an absolute requirement for the further development of tissue lesions, which result from the concerted action of many molecules derived from both, the host and the parasite.

Innate immunity prevents tissue invasion by Entamoeba histolytica

Although innate and adaptive immunity both play a role in amoebiasis, the mechanisms involved in the elimination of Entamoeba histolytica are poorly understood. To provide more information about the innate immune mechanisms that may confer protection against invasive amoebiasis, we administered inflammatory substances (bacillus Calmette-Guérin, lipopolysaccharide, complete Freund's adjuvant, or mineral oil) into the peritoneum of hamsters. The animals were then challenged with pathogenic trophozoites of E. histolytica and, after 7 days, the protective host response was analysed. We found that the nonspecific inflammatory response induced in the peritoneum was sufficient to prevent liver invasion by E. histolytica. In vitro experiments showed that the killing of trophozoites was mediated by peritoneal macrophages and a protein of 68 kDa with peroxidase activity.

Host-microbe interactions and defense mechanisms in the development of amoebic liver abscesses

SUMMARY

Amoebiasis by Entamoeba histolytica is a major public health problem in developing countries and leads to several thousand deaths per year. The parasite invades the intestine (provoking diarrhea and dysentery) and the liver, where it forms abscesses (amoebic liver abscesses [ALAs]). The liver is the organ responsible for filtering blood coming from the intestinal tract, a task that implies a particular structure and immune features. Amoebae use the portal route and break through the sinusoidal endothelial barrier to reach the hepatic parenchyma. When faced with systemic and cell-mediated defenses, trophozoites adapt to their new environment and modulate host responses, leading to parasite survival and the formation of inflammatory foci. Cytopathogenic effects and the onset of inflammation may be caused by diffusible products originating from parasites and/or immune cells either by their secretion or by their release after cell death. Liver infection thus results from the interplay between E. histolytica and hepatic cells. Despite its importance in terms of public health burden, the lack of integrated data on ALA genesis means that we have only an incomplete description of the initiation and development of hepatic amoebiasis. Here, we review the main steps of ALA development as well as the responses triggered in both the host and the parasite. Transcriptome studies highlighted parasite factors involved in adherence to human cells, cytopathogenic effects, and adaptative and stress responses. An understanding of their role in ALA development will help to unravel the host-pathogen interactions and their evolution throughout the infection.

Entamoeba histolytica: oxygen resistance and virulence

Entamoeba histolytica virulence has been attributed to several amoebic molecules such as adhesins, amoebapores and cysteine proteinases, but supporting evidence is either partial or indirect. In this work we compared several in vitro and in vivo features of both virulent E. histolytica (vEh) and non-virulent E. histolytica (nvEh) axenic HM-1 IMSS strains, such as complement resistance, proteinase activity, haemolytic, phagocytic and cytotoxic capacities, survival in mice caecum, and susceptibility to O(2). The only difference observed was a higher in vitro susceptibility of nvEh to O(2). The molecular mechanism of that difference was analyzed in both groups of amoebae after high O(2) exposure. vEh O(2) resistance correlated with: (i) higher O(2) reduction (O(2)(-) and H(2)O(2) production); (ii) increased H(2)O(2) resistance and thiol peroxidase activity, and (iii) reversible pyruvate: ferredoxin oxidoreductase (PFOR) inhibition. Despite the high level of carbonylated proteins in nvEh after O(2) exposure, membrane oxidation by reactive oxygen species was not observed. These results suggest that the virulent phenotype of E. histolytica is related to the greater ability to reduce O(2) and H(2)O(2) as well as PFOR reactivation, whereas nvEh undergoes irreversible PFOR inhibition resulting in metabolic failure and amoebic death.

An unusual surface peroxiredoxin protects invasive Entamoeba histolytica from oxidant attack

Peroxiredoxins are an important class of antioxidant enzymes found from Archaea to humans, which reduce and thereby detoxify peroxides and peroxynitrites. The major thiol-containing surface antigen of the invasive ameba, Entamoeba histolytica, is a peroxiredoxin and is likely to be important during the transition from the anaerobic environment of the large intestine to human tissues. The closely related species, Entamoeba dispar, is incapable of invasion and more sensitive to hydrogen peroxide, yet also has a peroxiredoxin. We cloned and expressed the two active recombinant enzymes and found that their activity was similar by a fluorometric stopped-flow assay, giving a Km of <10 microM for hydrogen peroxide. Three monoclonal antibodies produced to recombinant E. histolytica peroxiredoxin cross-reacted with Entamoeba dispar.E. histolytica contains as much as 50 times more peroxiredoxin than E. dispar as demonstrated by a sensitive capture ELISA. In addition, the peroxiredoxin is present largely on the outer surface of the cell, in contrast to E. dispar. This unusual peroxiredoxin localizes to the site of parasite-host cell contact where it can effectively counteract oxidants generated by host cells, thus facilitating invasion.

Entamoeba histolytica: Mechanism of decrease of virulence of axenic cultures maintained for prolonged periods

Intraportal injection of non-virulent E. histolytica (derived from prolonged axenic culture of virulent E. histolytica) strain HM1-IMSS in normal hamsters results in no liver lesions and disappearance of the parasites 48-72 h after injection. Viability of non-virulent E. histolytica after 2 h of in vitro incubation in either fresh or decomplemented hamster serum is the same as control virulent E. histolytica (50-90%). In hamsters made leukopenic, or both leukopenic+hypocomplementemic, or hypocomplementemic+sephadex microspheres (to produce focal liver ischemia) intraportally injected non-virulent E. histolytica cause no lesions and disappear after 24 h. In addition, neither hypocomplementemia nor immunosuppression with cyclosporin A prolonged the survival of non-virulent E. histolytica. Methyl prednisolone treatment of hamsters resulted in survival of large numbers of non-virulent E. histolytica in the liver, with little inflammation and minimal tissue damage, for up to 7 days. Inflammatory cells (macrophages) would appear to be chiefly responsible for elimination of non-virulent E. histolytica. Parallel experiments with E. dispar suggest a different mechanism for its non-pathogenicity.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

The protistan parasite Perkinsus marinus is resistant to selected reactive oxygen species

The parasite Perkinsus marinus has devastated natural and farmed oyster populations along the Atlantic and Gulf coasts of North America. When viable P. marinus trophozoites are engulfed by oyster hemocytes, the typical accumulation of reactive oxygen species (ROS) normally associated with phagocyte activity is not observed. One hypothesis to explain this is that the parasite rapidly removes ROS. A manifestation of efficient ROS removal should be a high level of resistance to exogenous ROS. We investigated the in vitro susceptibility of P. marinus to ROS as compared to the estuarine bacterium Vibrio splendidus. We find that P. marinus is markedly less susceptible than V. splendidus to superoxide and hydrogen peroxide (H(2)O(2)), but equally sensitive to hypochlorite. Viable P. marinus trophozoites degrade H(2)O(2) in vitro, but lack detectable catalase activity. However, extracts contain an ascorbate dependent peroxidase activity that may contribute to H(2)O(2) removal in vitro and in vivo.

The PmSOD1 gene of the protistan parasite Perkinsus marinus complements the sod2Delta mutant of Saccharomyces cerevisiae, and directs an iron superoxide dismutase to mitochondria

The facultative intracellular oyster parasite, Perkinsus marinus, taxonomically related to both dinoflagellates and apicomplexan parasites, possesses at least two distinct genes (PmSOD1 and PmSOD2) predicted to encode iron-containing superoxide dismutases (Fe-SOD). The present study demonstrates that PmSOD1 complements a Saccharomyces cerevisiae mutant lacking the mitochondrial manganese-containing SOD (Mn-SOD), whereas PmSOD2 complements an Escherichia coli mutant lacking genes for cytosolic SOD activities. Mitochondria isolated from complemented yeast contain an SOD activity susceptible to inhibition by hydrogen peroxide, but resistant to cyanide, both characteristics of Fe-SODs. In cultured P. marinus trophozoites, indirect immunofluorescence using anti-PmSOD1 antibodies shows colocalization of PmSOD1 product with the mitochondrial marker MitoTracker Red. Further analysis of the leader sequence of the predicted PmSOD1 product revealed similarities to a mitochondrial targeting domain, an unusual observation for Fe-SODs, which are typically localized in the cytoplasm. These results suggest that PmSOD1 encodes a mitochondrial Fe-SOD, which may contribute to P. marinus resistance to exogenous oxidative damage in host phagocytes. The present study constitutes the first report of an endogenous Fe-SOD that is directed to the mitochondria, and suggests that mitochondria targeting sequences have been conserved among diverse branches of the eukaryotes, including the early protista. It also illustrates the potential of complementation-based approaches for further gene discovery and characterization in P. marinus.

Antioxidant defense mechanisms in parasitic protozoa

Many of the parasitic protozoa, such as Entamoeba histolytica, Giardia, Trypanosoma, Leishmania, and Plasmodium, are considered to be anaerobes because they can be grown in vitro only under conditions of reduced oxygen tension. However, these parasitic protozoa have been found to be aerotolerant or microaerophilic, and also to consume oxygen to a certain extent. Furthermore, these organisms are highly susceptible to exogenous reactive oxygen species, such as hydrogen peroxide. They must, therefore, detoxify both oxygen and free radical products of enzymatic reactions. However, they lack some or all of the usual antioxidant defense mechanisms present in aerobic or other aerotolerant cells, such as catalase, superoxide dismutase, reduced glutathione, and the glutathione-recycling enzymes glutathione peroxidase and glutathione reductase. Instead, they possess alternative mechanisms for detoxification similar to those known to exist in certain prokaryotes. Although the functional aspects of these alternative mechanisms are yet to be understood completely, they could provide new insights into the biochemical peculiarities of these enigmatic pathogens.

In vitro effect of recombinant interferon gamma in combination with LPS on amoebicidal activity of murine Kupffer cells

The present study examines the role of liver macrophages (Kupffer cells), of C57BL/6 mice, as effector cells responsible for the killing of Entamoeba histolytica trophozoites in vitro. It was shown that unstimulated Kupffer cells were inefficient in the killing of E. histolytica trophozoites in vitro. Interferon gamma (IFN-gamma) alone was not able to activate Kupffer cells to amoebicidal state. However, Interferon gamma and lipopolysaccharide (LPS) acted synergistically in this phenomenon. It seems that the acquisition of amoebicidal activity is associated with the involvement of hydrogen peroxide, because the addition of catalase partially decreases the killing of this parasite by Kupffer cells. In addition, it appears that the amoebicidal activity of IFN-gamma-treated Kupffer cells is contact-dependent. Our results indicate that the immunologic production of IFN-gamma is important in the activation of Kupffer cells for controlling this parasite and that Kupffer cells are strong effector cells against the amoebae.

Entamoeba histolytica proteins modulate the respiratory burst potential by murine macrophages

Reactive oxygen intermediates are important components of macrophage microbicidal mechanisms and pathogenesis of parasitic disease. The purpose of the present study was to investigate the effect of virulent Entamoeba histolytica (strain HM1-IMSS) on respiratory burst potential of macrophages. Pretreatment of elicited peritoneal macrophages (EPM) with crude soluble amoebic proteins from 1 to 6 hr was found to prime EPM for enhanced O2 and H2O2 release in response to phorbol myristate acetate (PMA) in a dose-dependent manner, whereas pretreatment with the same concentrations of the non-pathogenic E. histolytica-like Laredo strain was without priming effect. Low molecular weight (MW) amoebic proteins (27,000-67,000) purified by Sephacryl-200 column chromatography and subfractionated by diethylaminoethyl cellulose chromatography were 10-fold more potent than crude amoebic proteins in priming EPM for an enhanced respiratory burst potential. Both crude and purified amoebic proteins inhibited the priming effect of lipopolysaccharide (LPS) or interferon-gamma (IFN-gamma) and antagonized the stimulating effect of PMA. Amoebic proteins by themselves were incapable of stimulating EPM respiratory burst. These findings demonstrate that amoebic proteins are capable of modulating the respiratory burst response of macrophages, suggesting an important role for them in the immunoregulation and pathogenesis of amoebiasis.

In vitro effect of human recombinant tumor necrosis factor on Entamoeba histolytica trophozoites

The effect of tumor necrosis factor (TNF) on E. histolytica trophozoites was examined by using three virulent (IP: 0682:1, HM-1: IMSS, 200: NIH) and one nonvirulent (DKB) strain of E. histolytica. Various concentrations of recombinant TNF were added to E. histolytica trophozoites and the total parasite numbers and their viability were periodically assessed by microscopic observation and trypan blue staining after incubation at 37 degrees C in a nonhumidified chamber. In this study, concentrations of 10(1)-10(6) units of TNF were used. Over a concentration range of 10(4)-10(6) units, the number of trophozoites was significantly lowered in the amoebic cultures containing TNF as compared to untreated controls. It was also found that the effect of TNF was dependent on the densities of both virulent and non-virulent strains of E. histolytica trophozoites in axenic conditions. TNF has no significant affect on the growth of amoebae at the lower starting number of amoebae. The amoebae cultured at the higher density were growth-inhibited significantly in comparison with the control groups. When the growth of the virulent and nonvirulent strains of amoebae was compared in TNF treated culture, it was found that TNF has an inhibitory effect on both the virulent and nonvirulent strans of E. histolytica.

Mechanism of tissue damage through free oxygen radicals during hepatic amoebiasis in guinea pigs

The capacity of Kupffer cells and blood monocytes to release free oxygen radicals was studied by chemiluminescence (CL) response during hepatic amebiasis in guinea pigs. Guinea pigs infected with Entamoeba histolytica intramesenterically were sacrificed on days 0, 2, 5 and 8 post-infection. Hepatic lesions were graded I-IV. A significant increase in the CL response was observed from day 2 post-infection and it increased with the progress of infection. Maximum increase was observed on the 8th post infection day. Animals with grade III or IV hepatic lesions had significantly elevated CL response. The degree of hepatic lesions correlated well with the CL response (P less than 0.01). It is postulated that tissue damage during hepatic amoebiasis may be mediated through enhanced release of free oxygen radicals by Kupffer cells and blood monocyte.

Activation of macrophages by Entamoeba histolytica extracts in mice

The effect of Entamoeba histolytica extracts on the production of inflammatory macrophages and the release of hydrogen peroxide (H2O2) and superoxide (O2-) from these cells was examined in C57BL/6 mice. Two different strains of E. histolytica, either virulent (IP:0682:1) or nonvirulent (DKB), were used in this study. The number of macrophages recovered from the peritoneal cavity of mice treated 5 days previously with 150 micrograms of either strain of amoebic extracts was significantly higher than in the saline-treated groups. Macrophages from mice treated with 150 micrograms of the IP:0682:1 strain of amoebic extracts exhibited a powerful burst of oxidative metabolis when triggered with phorbol myristate acetate (PMA). Extract from the non-virulent strain was much less effective in activating macrophages for the production of oxidative metabolites. Thus, a crude extract preparation from E. histolytica, particularly the virulent strain, induces a strong macrophage inflammatory response in which the cells also produce high levels of H2O2 and O2-.

In vitro killing of Entamoeba histolytica trophozoites by interferon-gamma-activated mouse macrophages

The effect of murine interferon gamma (IFN-gamma) on macrophage activation for amoebicidal activity was examined. Peritoneal macrophages were harvested from C57BL/6 mice and preincubated with IFN-gamma and/or lipopolysaccharide (LPS). In vitro amoebicidal activity of these macrophages was determined by trypan blue exclusion test against a virulent strain of E. histolytica (IP:0682:1). It was found that in vitro amoebicidal activity was evident in macrophage monolayers treated with both IFN-gamma and LPS. Macrophages treated with IFN-gamma alone did not develop cytotoxic activity unless they were exposed to LPS as a second triggering signal. The ability of IFN-gamma to prime macrophages to respond to trigger signals of LPS and develop cytotoxicity increased with time of incubation, the highest response being observed after 24 h. There was a dose-dependent relationship between the concentrations of both IFN-gamma and LPS used to activate macrophages and the number of dead trophozoites. These data suggest that macrophages are important in host defense against amoebiasis.