Ischemia in experimental acute amebic liver abscess in hamsters

Mechanisms of natural resistance of Balb/c mice to experimental liver amoebiasis

Entamoeba histolytica is the parasite responsible for human amoebiasis. The analysis of the natural resistance mechanisms of some rodents to amoebic liver abscess (ALA) may reveal alternative pathogenicity mechanisms to those previously discovered in the experimental model of ALA in hamsters. In this work the natural resistance of BALB/c mice to ALA was explored by performing: (i) in vivo chemotaxis analysis with a specifically designed chamber; (ii) in vitro amoebic survival in fresh and decomplemented serum; (iii) histological temporal course analysis of ALA development in mice with different treatments (hypocomplementemic, hyperimmune and treated with iNOS and NADPH oxidase inhibitors) and (iv) mouse liver amoebic infection by both in situ implantation of ALA from hamsters and inoculation of parasites into the peritoneal cavity. The results show that E. histolytica clearance from the mouse liver is related to a low chemotactic activity of complement, which results in poor inflammatory response and parasite inability to cause tissue damage. Also, the absence of amoebic tropism for the mouse liver is correlated with resistance to experimental liver amoebiasis.

Complement is a rat natural resistance factor to amoebic liver infection

Amoebiasis is a parasitic disease caused by Entamoeba histolytica. This illness is prevalent in poor countries causing 100,000 deaths worldwide. Knowledge of the natural resistance mechanisms of rats to amoebic liver abscess (ALA) development may help to discover new pathogenic factors and to design novel therapeutic strategies against amoebiasis. In this work, histologic analyses suggested that the complement system may play a central role in rat natural resistance to ALA. E. histolytica trophozoites disappeared from rat liver within 6 h post-infection with minimal or no inflammatory infiltrate. In vitro findings indicate that rat complement was lethal for the parasite. Furthermore, hamsters became resistant to ALA by intravenous administration of fresh rat serum before infection. The amoebicidal potency of rat complement was 10 times higher than hamster complement and was not related to their respective CH50 levels. The alternative pathway of complement plays a central role in its toxicity to E. histolytica since trypan blue, which is a C3b receptor inhibitor, blocks its amoebicidal activity. These results suggest that amoebic membrane affinity, high for C3b and/or low for Factor H, in comparison with the hamster ones, may result in higher deposition of membrane complex attack on parasite surface and death.

Water-Soluble Ruthenium (II) Chiral Heteroleptic Complexes with Amoebicidal in Vitro and in Vivo Activity

Three water-soluble Ru(II) chiral heteroleptic coordination compounds [Ru(en)(pdto)]Cl₂ (1), [Ru(gly)(pdto)]Cl (2), and [Ru(acac)(pdto)]Cl (3), where pdto = 2,2'-[1,2-ethanediylbis-(sulfanediyl-2,1-ethanediyl)]dipyridine, en = ethylendiamine, gly = glycinate, and acac = acetylacetonate, have been synthezised and fully characterized. The crystal structures of compounds 1-3 are described. The IC₅₀ values for compounds 1-3 are within nanomolar range (14, 12, and 6 nM, respectively). The cytotoxicity for human peripheral blood lymphocytes is extremely low (>100 μM). Selectivity indexes for Ru(II) compounds are in the range 700-1300. Trophozoites exposed to Ru(II) compounds die through an apoptotic pathway triggered by ROS production. The orally administration to infected mice induces a total elimination of the parasite charge in mice faeces 1-2-fold faster than metronidazole. Besides, all compounds inhibit the trophozoite proliferation in amoebic liver abscess induced in hamster. All our results lead us to propose these compounds as promising candidates as antiparasitic agents.

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.

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.

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.

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.

Late experimental amebic liver abscess in hamster is inhibited by cyclosporine and N-acetylcysteine

During early experimental amebic liver abscess in hamsters (EALAH), acute inflammation is primarily responsible for tissue damage. However, during the late stages of this process, the relative contribution to tissue destruction of both parasite factors and host response is unknown. In the present work, the role of the cellular immune response in tissue damage during EALAH is explored by using the immunosuppressor drug cyclosporine A (CsA). CsA treatment inhibits tissue damage after 72 h (but not at 24 h). Also, many well-preserved parasite clusters with minimal or no leukocyte influx and with minimal or no tissue destruction characterize the late stage of the process (7 days). The same results are observed with the immunosuppressor tacrolimus, but not with sirolimus; the latter drug does not cause immunosuppression in hamsters. On the other hand, similar results are observed with the antioxidant and anti-inflammatory N-acetylcysteine, with minimal immunosuppression in hamsters. These results suggest that, as in the early EALAH (24 h), during the late stages of the process (7 days), inflammation is also primarily responsible for tissue damage. However, lysosomal and cationic proteins are responsible for the early lesions, whereas reactive oxygen and nitrogen species are primarily involved in late stages.

Entamoeba histolytica: apoptosis induced in vitro by nitric oxide species

Apoptosis has been described in some parasites like Leishmania, Trypanosoma, and Trichomonas. This phenomenon has not been observed yet in Entamoeba histolytica. This work analyzed the in vitro effect of sodium nitroprusside, sodium nitrite and sodium nitrate (NOs) on E. histolytica apoptosis. Parasites incubated for 1h with NOs revealed apoptosis 6h later (95% viability), demonstrated by YOPRO-1, TUNEL, DNA fragmentation and low ATP levels. The caspase inhibitor Z-VAD-FMK inhibited total intracellular cysteine protease activity (CPA) but had no effect on apoptosis. When treated with NOs some amebic functions like complement resistance and hemolytic activity decreased but CPA and erythrophagocytosis remained unchanged. After treatment in vitro with NOs, parasite death was almost complete at 24h; but when injected into hamster livers they disappeared in less than 6h. These results show that apoptosis is induced in vitro by NOs in E. histolytica and renders them incapable of surviving in hamster's livers.

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.

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.

Cysteine proteinase activity is required for survival of the parasite in experimental acute amoebic liver abscesses in hamsters

Axenic trophozoites of Entamoeba histolytica strain HM1-IMSS grown in vitro in the presence of E-64, a potent irreversible inhibitor of cysteine proteinases, preserved their viability, their rate of replication, their resistance to complement, their haemolytic capacity and their ability to destroy target cells, despite complete inhibition of total cysteine proteinase activity. On the other hand, their erythrophagocytic capacity and their ability to decrease TER of MDCK cells was partially decreased. The same trophozoites injected into the portal vein of hamsters receiving a maintaining dose of E-64 failed to cause tissue damage and were rapidly eliminated. Our results suggest that amoebic cysteine proteinase activity is not required for amoebic functions in in vitro conditions, but that it becomes necessary for survival of trophozoites in in vivo conditions, whatever other role (if any) it may play in the parasite's virulence.

Inflammation, complement, ischemia and amoebic survival in acute experimental amoebic liver abscesses in hamsters

We have examined the role of inflammatory cells, ischemia and serum complement on the development of acute experimental amoebic liver abscess in hamsters (AEALAH). In hamsters made leukopenic by whole body radiation (800 rad) and daily intraperitoneal glycogen injections, the absence of inflammatory cells and liver tissue damage surrounding the parasites resulted in their rapid (24 h) disappearance from the liver, which showed no lesions. Focal liver ischemia, always present in control AEALAH with inflammation and tissue destruction, was reproduced in radiated hamsters by injection of amoebae mixed with Superdex microspheres, but again in the absence of inflammation, amoebae caused no liver damage and disappeared in 24 h. In hamsters made hypocomplementemic by injection of purified cobra venom factor (CVF), amoebae caused AEALA indistinguishable from controls, but in leukopenic + hypocomplementemic hamsters, amoebae were unable to produce lesions and disappeared from the liver in 48 h. We conclude that inflammation and tissue damage are required for the survival of amoebae in AEALAH and for the progression of the experimental disease.

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.

Amebic cysteine proteinase 2 (EhCP2) plays either a minor or no role in tissue damage in acute experimental amebic liver abscess in hamsters

Amebic cysteine protease 2 (EhCP2) was purified from ethyl ether extracts of axenically grown trophozoites of Entamoeba histolytica strain HM1-IMSS. The purification procedure involved molecular filtration and electroelution. Sequence analysis of the purified product revealed EhCP2 and ubiquitin(s). Electrophoretic migration patterns, isoelectric point determination and Western blot studies failed to reveal other EhCP molecules. Polyclonal antibodies against the purified EhCP2 prepared in rabbits either stabilized or enhanced the enzyme activity in a dose-response manner. Purified EhCP2 was enclosed within inert resin microspheres (22-44 microm in diameter) and injected into the portal vein of normal hamsters. In the liver, the microspheres caused mild acute inflammation and occasional minimal necrosis of short duration. Sections of the liver were immunohistochemically stained with the anti-EhCP2 antibody and the microspheres were positive for only a very short period (1 h) after injection. Sections of experimental acute (1 day, 5 days) amebic liver abscess produced in hamsters were also stained with the anti-EhCP2 antibody; and amebas were intensely positive but no staining was observed at any time in the surrounding necrotic structures. It is suggested that EhCP2 plays either a minor or no role in the causation of tissue damage in experimental acute liver amebiasis.