Pathogenesis of Acute Experimental Liver Amebiasis

Neutrophils versus Protozoan Parasites: Plasmodium, Trichomonas, Leishmania, Trypanosoma, and Entameoba

Neutrophils are the most abundant polymorphonuclear granular leukocytes in human blood and are an essential part of the innate immune system. Neutrophils are efficient cells that eliminate pathogenic bacteria and fungi, but their role in dealing with protozoan parasitic infections remains controversial. At sites of protozoan parasite infections, a large number of infiltrating neutrophils is observed, suggesting that neutrophils are important cells for controlling the infection. Yet, in most cases, there is also a strong inflammatory response that can provoke tissue damage. Diseases like malaria, trichomoniasis, leishmaniasis, Chagas disease, and amoebiasis affect millions of people globally. In this review, we summarize these protozoan diseases and describe the novel view on how neutrophils are involved in protection from these parasites. Also, we present recent evidence that neutrophils play a double role in these infections participating both in control of the parasite and in the pathogenesis of the disease.

Immune Response to the Enteric Parasite Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite responsible for amoebiasis, a disease with a high prevalence in developing countries. Establishing an amoebic infection involves interplay between pathogenic factors for invasion and tissue damage, and immune responses for protecting the host. Here, we review the pathogenicity of E. histolytica and summarize the latest knowledge on immune response and immune evasion mechanisms during amoebiasis.

Neutrophils vs. amoebas: Immunity against the protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite with high prevalence in developing countries, and causes amoebiasis. This disease affects the intestine and the liver, and is the third leading cause of human deaths among parasite infections. E. histolytica infection of the intestine or liver is associated with a strong inflammation characterized by a large number of infiltrating neutrophils. Consequently, several reports suggest that neutrophils play a protective role in amoebiasis. However, other reports indicate that amoebas making direct contact with neutrophils provoke lysis of these leukocytes, resulting in the release of their lytic enzymes, which in turn provoke tissue damage. Therefore, the role of neutrophils in this parasitic infection remains controversial. Neutrophils migrate from the circulation to sites of infection, where they display several antimicrobial functions, including phagocytosis, degranulation, and formation of neutrophil extracellular traps (NET). Recently, it was found that E. histolytica trophozoites are capable of inducing NET formation. Neutrophils in touch with amoebas launched NET in an explosive manner around the amoebas and completely covered them in nebulous DNA and cell aggregates where parasites got immobilized and killed. In addition, the phenotype of neutrophils can be modified by the microbiome resulting in protection against amoebas. This review describes the mechanisms of E. histolytica infection and discusses the novel view of how neutrophils are involved in innate immunity defense against amoebiasis. Also, the mechanisms on how the microbiome modulates neutrophil function are described.

Neutrophil extracellular traps and MPO in models of susceptibility and resistance against Entamoeba histolytica

The main effector mechanisms of neutrophils are the release of neutrophil extracellular traps (NETs) and myeloperoxidase (MPO). In this work, we evaluated the role of NETs and the activity of MPO in the interactions of rodent neutrophils with amoebae and in amoebic liver abscess (ALA)-resistant and ALA-susceptible models. We showed with in vitro assays that mice produced greater amounts of NETs and MPO than did hamsters, and the elastase activity was high in both models. However, the inhibition of NETs and MPO promoted an increase in amoeba viability in the mice. The mouse ALAs showed a more profound presence of NETs and MPO than did the hamster ALAs. We concluded that both effector mechanisms were essential for the amoebic damage and could prevent the formation of ALAs in the resistant model.

Comparative Pathobiology of the Intestinal Protozoan Parasites Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum

Protozoan parasites can infect the human intestinal tract causing serious diseases. In the following article, we focused on the three most prominent intestinal protozoan pathogens, namely, Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum. Both C. parvum and G. lamblia colonize the duodenum, jejunum, and ileum and are the most common causative agents of persistent diarrhea (i.e., cryptosporidiosis and giardiasis). Entamoeba histolytica colonizes the colon and, unlike the two former pathogens, may invade the colon wall and disseminate to other organs, mainly the liver, thereby causing life-threatening amebiasis. Here, we present condensed information concerning the pathobiology of these three diseases.

Pathogenic Entamoeba histolytica, but not Entamoeba dispar, induce neutrophil extracellular trap (NET) formation

Amoebiasis is an infection of global importance, caused by the eukaryotic parasite Entamoeba histolytica. Pathogenic E. histolytica is associated worldwide with over a million cases of amoebic dysentery, colitis, and amoebic liver abscess. In contrast, the nonpathogenic Entamoeba dispar does not cause these diseases, although it is commonly found in the same areas as pathogenic amoeba. Entamoeba histolytica infection is usually associated with infiltrating neutrophils. These neutrophils appear to play a defensive role against this parasite, by mechanisms not completely understood. Recently, our group reported that neutrophil extracellular traps (NET) are produced in response to E. histolytica trophozoites. But, there is no information on whether nonpathogenic E. dispar can also induce NET formation. In this report, we explored the possibility that E. dispar leads to NET formation. Neutrophils were stimulated by E. histolytica trophozoites or by E. dispar trophozoites, and NET formation was assessed by video microscopy. NET induced by E. histolytica were important for trapping and killing amoebas. In contrast, E. dispar did not induce NET formation in any condition. Also E. dispar did not induce neutrophil degranulation or reactive oxygen species production. In addition, E. histolytica-induced NET formation required alive amoebas and it was inhibited by galactose, N-acetylgalactosamine, and lactose. These data show that only alive pathogenic E. histolytica activates neutrophils to produce NET, and suggest that recognition of the parasite involves a carbohydrate with an axial HO- group at carbon 4 of a hexose.

Entamoeba histolytica Induce Signaling via Raf/MEK/ERK for Neutrophil Extracellular Trap (NET) Formation

Amoebiasis, the disease caused by Entamoeba histolytica is the third leading cause of human deaths among parasite infections. E. histolytica was reported associated with around 100 million cases of amoebic dysentery, colitis and amoebic liver abscess that lead to almost 50,000 fatalities worldwide in 2010. E. histolytica infection is associated with the induction of inflammation characterized by a large number of infiltrating neutrophils. These neutrophils have been implicated in defense against this parasite, by mechanisms not completely described. The neutrophil antimicrobial mechanisms include phagocytosis, degranulation, and formation of neutrophil extracellular traps (NETs). Recently, our group reported that NETs are also produced in response to E. histolytica trophozoites. But, the mechanism for NETs induction remains unknown. In this report we explored the possibility that E. histolytica leads to NETs formation via a signaling pathway similar to the pathways activated by PMA or the Fc receptor FcγRIIIb. Neutrophils were stimulated by E. histolytica trophozoites and the effect of various pharmacological inhibitors on amoeba-induced NETs formation was assessed. Selective inhibitors of Raf, MEK, and NF-κB prevented E. histolytica-induced NET formation. In contrast, inhibitors of PKC, TAK1, and NADPH-oxidase did not block E. histolytica-induced NETs formation. E. histolytica induced phosphorylation of ERK in a Raf and MEK dependent manner. These data show that E. histolytica activates a signaling pathway to induce NETs formation, that involves Raf/MEK/ERK, but it is independent of PKC, TAK1, and reactive oxygen species (ROS). Thus, amoebas activate neutrophils via a different pathway from the pathways activated by PMA or the IgG receptor FcγRIIIb.

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.

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.

Invasive amebiasis: a microcirculatory disorder?

The two current models of invasive amebiasis both hold that direct contact of toxic molecules and amebas with tissue produces the necrotic areas characteristic of this disorder. Whereas one model characterizes these toxic molecules as amebic products (e.g., lectins, amebapores, cysteine proteinases and other proteolytic enzymes), the other describes them as products of the inflammatory response (e.g., cytokines, nitric oxide, reactive oxygen intermediates and cytotoxic granules). Both these models can account for necrotic areas with many amebas present and with acute inflammation, but not those with few or no amebas present or with scarce inflammation. A new model poses that an inadequate immune response leads to a continuous and prolonged activation of endothelial cells (ECs) by amebas, amebic molecules and cytokines, which triggers the mechanisms leading to necrosis. Other toxic molecules later contribute to EC activation: nitric oxide, reactive oxygen intermediates, the activated complement and proteases. Hyperactivated endothelial cells continuously express adhesion molecules (e.g., ICAM-1 and E-selectin), pro-coagulant molecules (e.g., tissue factor, von Willebrand factor, and the plasminogen activator inhibitor), resulting in ever greater inflammation and thrombosis, which eventually reduces or blocks blood flow in some vessels and starves certain tissue areas of an adequate oxygen and nutrient supply. When necrotic areas first develop, they are surrounded by inflammatory cells due to the acute inflammation at this stage. However, these cells are starved of oxygen and essential nutrients by the same microcirculatory dysfunction. The increasing concentration of nitric oxide during amebiasis eventually has an anti-inflammatory and vasodilating effect, creating a new mechanism for the microcirculatory dysfunction. This local microcirculatory dysfunction can explain necrotic areas in the presence of many, few, or no amebas, with abundant or scarce inflammation.

Use and endocytosis of iron-containing proteins by Entamoeba histolytica trophozoites

Iron is essential for nearly all organisms; in mammals, it is part of proteins such as haemoglobin, and it is captured by transferrin and lactoferrin. Transferrin is present in serum, and lactoferrin is secreted by the mucosa and by neutrophils at infection sites, as a host iron-withholding response, sequestering iron away from invading microorganisms. Additionally, all cells contain ferritin, which sequesters iron when its intracellular levels are increased, detoxifying and preventing damage. Liver ferritin contains 50% of iron corporal reserves. During evolution, pathogens have evolved diverse strategies to obtain iron from their hosts in order to survive. The protozoan Entamoeba histolytica invades the intestinal mucosa, causing dysentery, and the trophozoites often travel to the liver producing hepatic abscesses; thus, intestine and liver proteins could be important iron supplies for E. histolytica. We found that E. histolytica trophozoites can grow in both ferrous and ferric iron, and that they can use haemoglobin, holo-transferrin, holo-lactoferrin, and ferritin as in vitro iron sources. These proteins supported the amoeba growth throughout consecutive passages, similarly to ferric citrate. By confocal microscopy and immunoblotting, iron-binding proteins were observed specifically bound to the amoeba surface, and they were endocytosed, trafficked through the endosomal/lysosomal route, and degraded by neutral and acidic cysteine-proteases. Transferrin and ferritin were mainly internalized through clathrin-coated vesicles, and holo-lactoferrin was mainly internalized by caveola-like structures. In contrast, apo-lactoferrin bound to membrane lipids and cholesterol, inducing cell death. The results suggest that in vivo trophozoites secrete products that can destroy enterocytes, erythrocytes, and hepatocytes, releasing transferrin, haemoglobin, ferritin, and other iron-containing proteins, which, together with lactoferrin derived from neutrophils and acinar cells, could be used as abundant iron supplies by amoebas.

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.