Resistance to lysis by human serum of pathogenic Entamoeba histolytica

Immunization with the Entamoeba histolytica surface metalloprotease EhMSP-1 protects hamsters from amebic liver abscess

Diarrhea and amebic liver abscesses due to invasive Entamoeba histolytica infections are an important cause of morbidity and mortality in the developing world. Entamoeba histolytica adherence and cell migration, two phenotypes linked to virulence, are both aberrant in trophozoites deficient in the metallosurface protease EhMSP-1, which is a homologue of the Leishmania vaccine candidate leishmanolysin (GP63). We examined the potential of EhMSP-1 for use as a vaccine antigen to protect against amebic liver abscesses. First, existing serum samples from South Africans naturally infected with E. histolytica were examined by enzyme-linked immunosorbent assay (ELISA) for the presence of EhMSP-1-specific IgG. Nine of 12 (75%) people with anti-E. histolytica IgG also had EhMSP-1-specific IgG antibodies. We next used a hamster model of amebic liver abscess to determine the effect of immunization with a mixture of four recombinant EhMSP-1 protein fragments. EhMSP-1 immunization stimulated a robust IgG antibody response. Furthermore, EhMSP-1 immunization of hamsters reduced development of severe amebic liver abscesses following intrahepatic injection of E. histolytica by a combined rate of 68% in two independent animal experiments. Purified IgG from immunized compared to control animals bound to the surface of E. histolytica trophozoites and accelerated amebic lysis via activation of the classical complement cascade. We concluded that EhMSP-1 is a promising antigen that warrants further study to determine its full potential as a target for therapy and/or prevention of invasive amebiasis.

Proteases from Entamoeba spp. and Pathogenic Free-Living Amoebae as Virulence Factors

The standard reference for pathogenic and nonpathogenic amoebae is the human parasite Entamoeba histolytica; a direct correlation between virulence and protease expression has been demonstrated for this amoeba. Traditionally, proteases are considered virulence factors, including those that produce cytopathic effects in the host or that have been implicated in manipulating the immune response. Here, we expand the scope to other amoebae, including less-pathogenic Entamoeba species and highly pathogenic free-living amoebae. In this paper, proteases that affect mucin, extracellular matrix, immune system components, and diverse tissues and cells are included, based on studies in amoebic cultures and animal models. We also include proteases used by amoebae to degrade iron-containing proteins because iron scavenger capacity is currently considered a virulence factor for pathogens. In addition, proteases that have a role in adhesion and encystation, which are essential for establishing and transmitting infection, are discussed. The study of proteases and their specific inhibitors is relevant to the search for new therapeutic targets and to increase the power of drugs used to treat the diseases caused by these complex microorganisms.

Host-parasite interaction: parasite-derived and -induced proteases that degrade human extracellular matrix

Parasitic protozoa are among the most important pathogens worldwide. Diseases such as malaria, leishmaniasis, amoebiasis, giardiasis, trichomoniasis, and trypanosomiasis affect millions of people. Humans are constantly threatened by infections caused by these pathogens. Parasites engage a plethora of surface and secreted molecules to attach to and enter mammalian cells. The secretion of lytic enzymes by parasites into host organs mediates critical interactions because of the invasion and destruction of interstitial tissues, enabling parasite migration to other sites within the hosts. Extracellular matrix is a complex, cross-linked structure that holds cells together in an organized assembly and that forms the basement membrane lining (basal lamina). The extracellular matrix represents a major barrier to parasites. Therefore, the evolution of mechanisms for connective-tissue degradation may be of great importance for parasite survival. Recent advances have been achieved in our understanding of the biochemistry and molecular biology of proteases from parasitic protozoa. The focus of this paper is to discuss the role of protozoan parasitic proteases in the degradation of host ECM proteins and the participation of these molecules as virulence factors. We divide the paper into two sections, extracellular and intracellular protozoa.

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.

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.

Host-pathogen interaction in amebiasis and progress in vaccine development

Entamoeba histolytica, the causative organism of invasive intestinal and extraintestinal amebiasis, infects approximately 50 million people each year, causing an estimated 40 to 100 thousand deaths annually. Because amebae only infect humans and some higher non-human primates, an anti-amebic vaccine could theoretically eradicate the organism. Uncontrolled epidemiologic studies indicate that acquired immunity to amebic infection probably occurs and that such a vaccine might be feasible. Application of molecular biologic techniques has led to rapid progress towards understanding how Entamoeba histolytica causes disease, and to the identification of several amebic proteins associated with virulence. These proteins are now being evaluated as potential vaccine components. Parenteral and oral vaccine preparations containing recombinant amebic proteins have been effective in preventing disease in a gerbil model of amebic liver abscess. Although systemic and mucosal cellular and humoral immunity both appear to play a role in protection against Entamoeba histolytica, the relative importance of each in the human immune response remains unknown. No animal model of intestinal amebiasis currently exists, moreover, so it has been impossible to evaluate protection against colonization and colitis. Further investigation of the fundamental mechanisms by which Entamoeba histolytica causes disease and of the human immune response to amebic infection is necessary to assess the true feasibility of an anti-amebic vaccine.

Virulence parameters in the characterization of strains of Entamoeba histolytica

Differences in virulence of strains of Entamoeba histolytica have long been detected by various experimental assays, both in vivo and in vitro. Discrepancies in the strains characterization have been arisen when different biological assays are compared. In order to evaluate different parameters of virulence in the strains characterization, five strains of E. histolytica, kept under axenic culture, were characterized in respect to their, capability to induce hamster liver abscess, erythrophagocytosis rate and cytopathic effect upon VERO cells. It was found significant correlation between in vitro biological assays, but not between in vivo and in vitro assays. Good correlation was found between cytopathic effect and the mean number of uptaken erythrocytes, but not with percentage of phagocytic amoebae, showing that great variability can be observed in the same assay, according to the variable chosen. It was not possible to correlate isoenzyme and restriction fragment pattern with virulence indexes since all studied strains presented pathogenic patterns. The discordant results observed in different virulence assays suggests that virulence itself may not the directly assessed. What is in fact assessed are different biological characteristics or functions of the parasite more than virulence itself. These characteristics or functions may be related or not with pathogenic mechanisms occurring in the development of invasive amoebic disease.

Protozoa. Amebiasis

The intestinal protozoan parasite Entamoeba histolytica causes amebic dysentery and amebic liver abscess, and ranks third worldwide among parasitic causes of death. The application of molecular techniques to the study of this organism have led to major advances in understanding the pathophysiology of amebic infection. This article reviews what is currently known about the pathogenesis, clinical manifestations, diagnosis, and treatment of amebiasis.

Zymodeme alteration of Entamoeba histolytica isolates under varying conditions

Cysts of Entamoeba histolytica obtained from 2 asymptomatic subjects were cloned in vitro and the isoenzyme patterns and virulence of the cultures derived from them were determined. Incubation in Diamond's TYI-S-33 medium with Crithidia sp. for 80 d resulted in a change from zymodeme III to zymodeme II, with consistent virulence. Axenization by replacing Crithidia sp. with crithidial lysate had no further effect on the zymodeme but the virulence of the amoebae was attenuated. A reversal to zymodeme III, together with augmentation of virulence, was observed on the 20th day after transfer of axenic amoebae to Diamond's TYSGM-9 medium containing the original bacterial flora. The possibility that the isoenzyme pattern determining the zymodeme and the virulence behave as 2 independent variables in E. histolytica is discussed.

Induction of complement resistance in cloned pathogenic Entamoeba histolytica

The lytic effect of complement activated through the alternative pathway (AP) was studied on pathogenic and nonpathogenic Entamoeba histolytica recently isolated from stool samples. Recent nonpathogenic isolates were nearly unaffected by exposure to AP whereas recent pathogenic stool isolates were highly susceptible to AP dependent complement-mediated lysis. Complement susceptible pathogenic stool isolates developed complement resistance in vivo during hamster liver passage and in vitro during cultivation in the presence of increasing concentrations of normal human serum (NHS). Since a clone of pathogenic HM-1:IMSS which initially was highly susceptible also acquired complement resistance during cultivation in the presence of NHS, it is concluded that complement resistance was caused by induction rather than by selection alone. Because cultivation in the presence of heat-inactivated NHS did not affect complement susceptibility of the cloned HM-1:IMSS, complement activation itself might induce complement resistance in pathogenic E. histolytica.

Antigenicity, immunogenicity and vaccine efficacy of the galactose-specific adherence protein of Entamoeba histolytica

Entamoeba histolytica is an enteric protozoan that causes amoebic colitis and liver abscess. Human immunity to E. histolytica is apparently mediated by a serum antibody response and amoebicidal cellular mechanisms. The galactose-specific adherence protein of E. histolytica is a 260 kDa glycoprotein which mediates amoebic in vitro adherence to human colonic mucins, epithelium, and inflammatory cells. Amoebic lysis of cells is dependent upon binding by this adherence protein. Serum IgG and salivary IgA antibodies from greater than 90% of subjects with invasive amoebiasis recognize the adherence protein's 170 kDa heavy subunit. Incubation of peripheral blood mononuclear cells from antibody-positive subjects with the purified galactose-specific adherence protein induces in vitro T lymphocyte proliferation, IL-2 and gamma interferon production, and direct lymphocyte amoebicidal activity. The rationale for an adherence protein vaccine includes sIgA blockage of amoebic binding to colonic mucins, humoral IgG prevention of parasitic adherence in tissues, and development of amoebicidal cell-mediated immunity. Immunization of gerbils with purified adherence protein in Freund's adjuvant provides protection against intrahepatic challenge with the trophozoites. In summary, the galactose specific adherence protein of E. histolytica contains highly conserved B- and T-cell epitopes, and has a high degree of vaccine efficacy in the gerbil model of amoebic liver abscess.

The amoeba enigma

After more than 70 years of intermittent debate over the true relationship between the 'pathogenic' and 'non-pathogenic' forms of Entamoeba histolytica, the application of molecular biology has finally yielded an unambiguous answer: these are not interconvertible phenotypes of the same parasite, a kind of unicellular Jekyll and Hyde, but two quite distinct genetic entities that just happen to look the same. But given the overwhelming evidence now available from gene sequences, pointing to an evolutionary divergence some tens of millions of years ago, why is it that certain eminent workers in the field are still claiming that, at least in vitro, conversion between the two phenotypes can take place? In this article Bill Spice and John Ackers review recent developments in the molecular biology of E. histolytica and assess the continuing controversy over the status of this enigmatic parasite.

A severe combined immunodeficient (SCID) mouse model for infection with Entamoeba histolytica

We used severe combined immunodeficient (SCID) mice to study resistance to invasive infection with Entamoeba histolytica. Seven of seven SCID mice developed liver abscesses when challenged intrahepatically with virulent HM1:IMSS strain E. histolytica trophozoites. Only one of seven similarly challenged immunocompetent congenic C.B-17 mice developed an abscess. Adoptive transfer of polyclonal rabbit anti-E. histolytica antiserum, but not preimmune rabbit serum, completely protected 7 of 12 SCID mice from intrahepatic challenge with ameba. These results demonstrate that lymphocyte-based immunity is important in protection against amebic liver abscess, and that anti-E. histolytica antibody can protect against amebic infection in this system. The SCID mouse may provide a powerful model for studying the components of protective immunity to invasive amebiasis.

A monoclonal antibody for distinction of invasive and noninvasive clinical isolates of Entamoeba histolytica

Approximately 10% of the world population is infected with Entamoeba histolytica, but only 10% of the carriers develop symptomatic amebiasis. This discrepancy could be explained by the genotypic differences between the morphologically indistinguishable invasive and noninvasive strains of E. histolytica currently identified by zymodeme analysis, a technique that is unsuitable for routine diagnostic laboratories. Here we report the production of a monoclonal antibody against E. histolytica and its use in an immunofluorescence assay to identify invasive isolates cultured from stool samples of infected patients in several regions where amebiasis is endemic: Bangladesh, Colombia, and Mexico. After testing a total of 88 E. histolytica isolates, the correlation between zymodeme characterization and the immunofluorescence assay with the invasive isolate-specific monoclonal antibody was 100%. The epitope detected by the invasive isolate-specific monoclonal antibody resides in a previously undescribed internal protein with molecular masses of 84 and 81 kDa in axenic and polyxenic E. histolytica strains, respectively.

High cell surface hydrophobicity of virulent Entamoeba histolytica isolates

Thirty isolates of Entamoeba histolytica were examined for their cell surface hydrophobicity. It was observed that increased hydrophobicity of the trophozoites was associated with lesion-forming ability in hamster liver, a higher rate of erythrophagocytosis and greater resistance to complement. The relationship of cell surface hydrophobicity to other virulence markers in E. histolytica is discussed.

Determination of taxonomic status of pathogenic and nonpathogenic Entamoeba histolytica zymodemes using isoenzyme analysis

Entamoeba histolytica infection results in either asymptomatic colonization or invasion of host tissues leading generally to clinical symptoms. Zymodemes studies have demonstrated a correlation between isoenzyme profiles and clinical presentation. Thus, strains have been attributed to pathogenic or nonpathogenic groups according to their zymodeme. To determine the taxonomic relationship of these two groups, the isoenzyme profiles of 14 loci of 38 E. histolytica strains (pathogenic and nonpathogenic) and seven strains of other species of the same genus were analyzed. Genetic distance analysis clearly demonstrates the existence of two separate groups within the species E. histolytica.

Ischemia in experimental acute amebic liver abscess in hamsters

In experimental acute amebic liver abscess, produced in hamsters by the intraportal inoculation of 1 x 10(6) axenic trophozoites of Entamoeba histolytica strain HM-1, we examined the blood perfusion of the lesions 5, 10, 24 and 72 h after injection of the parasites. India ink introduced into the portal circulation filled all liver vessels but was systematically excluded from even the earlier amebic lesions. The absence of serum proteinase inhibitors from the lesions may allow the participation of amebic proteinases in the causation of tissue necrosis.

Lysis of pathogenic and nonpathogenic Entamoeba histolytica by human complement: methodological analysis

The effect of nonimmune human serum on Entamoeba histolytica trophozoites was studied: (a) using whole serum in the presence of Ca and Mg ions allowing complement activation via both the alternative and classical pathways or in the presence of MgEGTA permitting alternative pathway activation only; (b) using different E. histolytica isolates; (c) varying serum and trophozoite concentrations and the time of incubation; and (d) using three different methods to quantify lysis, i.e., microscopic inspection, flow cytometry and 111In release. All three methods yielded similar results, with flow cytometry being most sensitive in identifying membrane damage and 111In release being most valid in determining cell death. Microscopic analysis was reliable only when a chamber was used to calculate the number of complement treated cells in relation to the initial cell count. E. histolytica isolates were classified into three groups according to their susceptibility to lysis by complement: (i) pathogenic isolates after long term cultivation in vitro were susceptible; (ii) pathogenic isolates after recent in vivo passage were less susceptible; and (iii) nonpathogenic isolates were nearly unaffected by exposure to the alternative pathway alone. The extent of lysis of the various isolates correlated with the degree of complement consumption in the serum samples, suggesting that unlysed isolates did not activate complement under the conditions employed. In general, lysis of susceptible trophozoites increased with the serum concentration and with the time of incubation. However, when the trophozoite concentration was 10(6)/ml or higher, lysis no longer reflected complement susceptibility because of exhaustion of the complement supply.

A new in vitro model of Entamoeba histolytica adhesion, using the human colon carcinoma cell line Caco-2: scanning electron microscopic study

The human colon carcinoma cell line Caco-2, which is widely used to study the adhesion and cytotoxicity of enterobacteria, was used to investigate the adhesion of the trophozoites of Entamoeba histolytica. We observed a high percentage of adhesion of amoebae to Caco-2 cells. Scanning electron microscopy showed that amoebial membrane structures were involved in adhesion and the cytolytic action. These differentiated cells should prove to be a useful model system for investigation of the pathogenic action of amoebae.

Enhancement of virulence of Entamoeba histolytica by in vitro liver treatment

Enhancement of the virulence of five strains of Entamoeba histolytica (three xenically maintained and two axenically maintained) was studied after in vitro incubation with normal hamster liver. Increased virulence was shown by the ability of a small number of liver-treated trophozoites to produce liver lesions in hamsters. Enhancement of virulence was positively correlated with increased resistance to normal hamster serum complement in vitro.

Lysis of complement-sensitive Entamoeba histolytica by activated terminal complement components. Initiation of complement activation by an extracellular neutral cysteine proteinase

Activation of complement by Entamoeba histolytica may be initiated by the extracellular 56-kD neutral cysteine proteinase which cleaves the alpha chain of C3. To determine the relationship between the fluid-phase activation of complement and our observation that only strains isolated from patients with invasive disease are resistant to complement-mediated lysis, we investigated the fate of C3 with recent amebic isolates. When 125I-C3 was incubated with trophozoites in serum, C3 in the fluid phase was cleaved to C3b or C3bi, but the alpha chain of the C3 molecules on the cell surface appeared intact. Since the lysis of nonpathogenic strains takes place in the absence of bound C3b, we demonstrated that this reaction occurs by reactive lysis initiated in the fluid phase: (a) the killing of nonpathogenic strains was enhanced when alternative pathway activation was accelerated by the addition of cobra venom factor; (b) non-pathogenic strains were lysed by purified terminal components; and (c) sera incubated with pathogenic E. histolytica produced passive lysis of chicken erythrocytes. These results demonstrate for the first time that complement-sensitive E. histolytica are lysed by activation of the terminal complement components in the fluid phase where the 56-kD neutral cysteine proteinase cleaves C3, and not by the surface deposition of activated C3.

Suppression of T-lymphocyte responses to Entamoeba histolytica antigen by immune sera

Lymphocytes from patients cured of amebic liver abscesses proliferate and produce gamma interferon upon incubation with soluble Entamoeba histolytica antigen: however, amebic liver abscesses exhibit a relentless progression without treatment. To determine whether suppressive factors are present in sera, we studied T-lymphocyte responses to total soluble E. histolytica antigen by using cells from five patients treated for amebic liver abscesses in the presence of 15 different immune sera and 10 control sera. In the presence of immune sera, E. histolytica antigen-induced lymphocyte proliferation decreased by 63% and production of gamma interferon was reduced by 93.2% (P less than 0.01). Immune sera had no effect on the mitogenic responses of patient lymphocytes to phytohemagglutinin or on the proliferative responses of control lymphocytes to phytohemagglutinin or tetanus toxoid. The suppressive activity of immune sera diminished as the time between therapy for amebic liver abscesses and serum collection increased (P less than 0.05). Suppressive activity did not correlate with the titers of serum anti-amebic antibody and was not affected when serum was absorbed with viable amebic trophozoites. In conclusion, soluble factors present in the sera of amebic liver abscess patients suppressed in vitro lymphocyte responses to E. histolytica antigen and may have contributed to the lack of development of effective in vivo cell-mediated immune responses following the onset of amebic liver abscesses.

Immune response in patients with amoebiasis: evaluation of IgG-subclasses

In order to evaluate the immune response with respect to IgG-subclasses (IgG1-IgG4) in patients with extraintestinal amoebiasis, an ELISA technique was established. It was the aim of this pilot study to quantify the IgG subclass response and to compare the resulting pattern with other systemic protozoal infections. Our results give evidence that IgG4 contributes to more than one third of the total immune response, followed by IgG2, IgG3 and IgG1. Regarding the IgG4 response in patients with Plasmodium falciparum malaria or Chagas disease, IgG4 plays only a minor role in these systemic protozoal infections. The interpretation of a prognostic value of the high IgG4 titres in our patients is not possible at present. However, in patients with a prolonged clinical course of extraintestinal amoebiasis, extremely high IgG4 titres were observed.

Interaction of different strains of Entamoeba histolytica with target cells: characterization of electrophysiological and morphological features

Two strains of Entamoeba histolytica with pathogenic zymodemes (SFL3, HK9), one strain with non-pathogenic zymodeme ("Bru") and one non-pathogenic Entamoeba sp. strain ("cold strain"), were investigated with respect to their interaction with target cells. Three test systems were used: 1) direct microscopical observation and qualitative as well as quantitative evaluation of contact and binding events with MDCK cells as targets, 2) kinetics of cytotoxic activity as measured by means of chromium release from 51Cr-labelled K562 cells, and 3) electrophysiological observations with freshly prepared mouse liver cells. We observed that the non-pathogenic cold strain interacted only shortly with target cells (statistical events, interaction type "I"), but did not induce morphological changes, chromium release or depolarization of targets. Non-pathogenic and avirulent strain "Bru" showed, apart from type "I"-binding, the ability to establish tight (type "II") and long-lasting contact (type "III") with targets, but again without cytotoxic effects. The pathogenic but avirulent strain HK9 tightly interacted (type "II") and sometimes long-lasting with target cells, but morphological changes and chromium release were of a moderate degree during the first 20 min, and depolarization was only a rare event. In contrast, strain SFL3 produced tight and long-lasting contacts (type "III" binding), leading to cell death in 83% (type "IV" interaction) within 20 min, substantial chromium release within 10 min and rapid depolarization ("electric collapse") of target cells.

In vitro and in vivo studies of macrophage functions in amebiasis

Experimental intrahepatic inoculation of the gerbil with Entamoeba histolytica trophozoites was used as a model of liver amebiasis to study the cellular immune response elicited by the parasite. It was shown that abscess-derived macrophages (5 to 20 days old) were deficient in their capacity to develop a respiratory burst, to secrete and express membrane-bound interleukin-1-like activity, and to kill E. histolytica trophozoites as well as to respond to lymphokines in vitro. However, macrophages isolated from the spleen and peritoneal cavities from the same infected animals were not significantly down regulated in these functions. Splenocytes from infected gerbils were shown to develop a strong responsiveness to amebic antigen, whereas their response to concanavalin A was suppressed. Crude E. histolytica extracts or conditioned medium down regulated murine BALB/c macrophage accessory and effector cell functions in vitro in a manner similar to abscess-derived macrophages, whereas crude extracts of the nonvirulent E. histolytica-like Laredo strain did not. Our results indicate that intrinsic or secreted products or both from E. histolytica are actively regulating macrophage functions at the abscess site and can possibly mediate other immunoregulatory mechanisms at distant targets.

Ameba-bacterium relationship in amebiasis

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Changes in isoenzyme patterns of a cloned culture of nonpathogenic Entamoeba histolytica during axenization

The axenization of an Entamoeba histolytica isolate with a nonpathogenic isoenzyme electrophoretic pattern (zymodeme) was recently achieved for the first time (15). Forty days after the cells were transferred to the medium used for axenic cultivation, the amebae developed virulence properties, and the zymodeme converted to a pathogenic pattern. To exclude the possibility that the original isolate consisted of two zymodeme populations and that conditions of growth selected for a particular population, the experiment was repeated with a cloned culture of a nonpathogenic (zymodeme III) strain, E. histolytica SAW 1734R clAR, isolated by and obtained from P. G. Sargeaunt. Axenization was accomplished, as before, by transferring trophozoites to TYI-S-33 medium containing a mixture of antibiotics to suppress the growth of the associated bacterial flora and a nutritional supplement consisting of gamma-irradiated bacteria. A change in the hexokinase and phosphoglucomutase isoenzyme pattern was observed 21 days after the amebae had been transferred to the axenic medium but before complete axenization of the amebae had occurred. The change in zymodeme was accompanied by an increase in virulence, as evidenced by the ability of fewer amebae to induce hepatic abscesses in hamsters. A reverse conversion to a nonpathogenic zymodeme was also accomplished by reassociating and subculturing the newly converted pathogenic trophozoites of strain SAW 1734R clAR with the bacterial flora that accompanied this ameba in the original xenic culture. The electromobilities of the hexokinase isoenzymes changed back to their original pattern 7 days after the amebae were returned to xenic growth conditions. Our in vitro results demonstrate that culture conditions and bacterial flora can cause changes in the zymodeme and virulence of a cloned ameba isolate and raise the concern that this could happen also in vivo. Thus, the finding of a particular zymodeme in a culture of E. histolytica isolated from a carrier should not be used to predict a clinical condition or serve as a basis for the recommendation of therapy.

Interaction of human leukocytes and Entamoeba histolytica. Killing of virulent amebae by the activated macrophage

Capable effector mechanisms in the human immune response against the cytolytic, protozoan parasite Entamoeba histolytica have not been described. To identify a competent human effector cell, we studied the in vitro interactions of normal human polymorphonuclear neutrophils, peripheral blood mononuclear cells (PBMC), monocytes (MC), and MC-derived macrophages with virulent axenic amebae (strain HMI-IMSS). Amebae killed neutrophils, PBMC, MC, and MC-derived macrophages (P less than 0.001), without loss of parasite viability. The addition of heat-inactivated immune serum did not enable leukocytes to kill amebae, nor did it protect these host cells from amebae. MC-derived macrophages, activated with lymphokine elicited by the mitogens conconavalin A, phytohemagglutinin, or an amebic soluble protein preparation (strain HK9), killed 55% of amebae by 3 h in a trypan blue exclusion assay (P less than 0.001); during this time, 40% of the activated macrophages died. Lysis of amebae was confirmed using 111Indium oxine radiolabeled parasites and was antibody independent. Macrophage death appeared to be due to the deleterious effect of lysed amebae rather than the contact-dependent effector mechanisms of E. histolytica. Adherence between activated macrophages and amebae was greater than that between other leukocytes and amebae (P less than 0.001). Microscopic observations, kinetic analysis of the killing of amebae by activated macrophages, and suspension of amebae with adherent activated macrophages in a 10% dextran solution indicated that contact by activated macrophages was necessary to initiate the killing of amebae. Catalase but not superoxide dismutase inhibited the amebicidal capacity of activated macrophages (P less than 0.001). However, activated macrophages from an individual with chronic granulomatous disease were able to kill amebae, but not as effectively as normal cells (P less than 0.01). In summary, activated MC-derived macrophages killed virulent E. histolytica trophozoites through a contact-dependent, antibody-independent mechanism involving oxidative-dependent and -independent processes.

Seroepidemiological study of antibody responses to the zymodemes of Entamoeba histolytica

In 303 apparently healthy subjects and 54 patients with confirmed invasive amoebiasis amoebae were isolated where possible, and Entamoeba histolytica was classified into its various zymodems. 20% of the healthy subjects were antibody-positive; 17 healthy subjects proved to be symptomless carriers of pathogenic zymodemes and, like patients with invasive disease, all these were seropositive. Furthermore, of subjects infected with pathogenic zymodemes 94-100% were strongly seropositive, compared with 2-4% of subjects with non-pathogenic zymodemes. Pathogenic zymodemes are therefore believed to be in constant contact with the host's tissues, even in symptom-free individuals; the presence of these potentially invasive organisms is thought to be directly related to the seropositivity in a given population, and seroepidemiological surveys would therefore provide information only on the distribution of this form of the parasite. Quantitative serological data constitute valuable information on the endemicity of an area, and the results will gain reliability from use of two complementary tests instead of one.

Lethal recognition between Entamoeba histolytica and the host tissues

Lethal recognition between Entamoeba histolytica and the tissues and defence systems of the host results in a continuous interplay that determines the development of pathological lesions: (i) we have identified several of the steps and mediators utilized by the trophozoites to destroy host cells by contact-mediated cytolysis; (ii) we have established that the alternative complement system represents the main defence available to the host against the invading parasite. The amoebae recognize target cells by means of a lectin specific for N-acetylgalactosamine-containing surface glycoproteins. This recognition appears to activate the amoeba to release, in the area of contact, an attack complex that induces the host cells to undergo cytolysis. The main component of the attack complex is thought to be amoebapore, an ion-channel forming protein that incorporates spontaneously into target cells leading to their depolarization by creating a pathway for ions to flow down their concentration gradient. The known properties of amoebapore are described. The acquisition of complement resistance by the invading trophozoites is essential for their survival within the host and therefore underlies virulence. The resistance to complement killing is not a permanent property of the amoebae. It is lost during axenization and reappears on passage through the host or when the trophozoites are grown axenically in the presence of active complement.

Influence of geographical factors in the distribution of pathogenic zymodemes of Entamoeba histolytica: identification of zymodeme XIV in India

Stocks of Entamoeba histolytica isolated and maintained in a variety of culture media, both axenic and polyxenic, were compared with each other and with strains previously characterized. The present stocks were isolated from subjects living in various cities in India. Using the enzyme patterns of: E C 5319 glucose phosphate isomerase (GPI); E C 11140 L-malate: NADP+ oxidoreductase (oxaloacetate decarboxylating) (ME): E C 2751 phosphoglucomutase (PGM); and E C 2711 hexakinase (HK), developed after electrophoresis, three zymodemes were identified in 54 individual isolations of E. histolytica. Most importantly only one zymodeme associated with pathogenicity occurred, identified 28 times, among the collection, one of which was from a liver abscess.