Role of adherence in cytopathogenic mechanisms of Entamoeba histolytica. Study with mammalian tissue culture cells and human erythrocytes

Sequence of a cysteine-rich galactose-specific lectin of Entamoeba histolytica

Entamoeba histolytica trophozoites adhere to human colonic mucins and epithelial cells by a cell surface galactose-specific lectin. This lectin, which is composed of two subunits linked by disulfide bonds, has been shown to be a protective antigen in an animal model of amebiasis. We have determined the sequence of the mature form of the 170-kDa heavy subunit from cDNA clones and PCR-amplified fragments. The heavy subunit sequence consisted of a putative extracellular domain containing 1209 amino acids with 16 potential sites for N-linked glycosylation, a 26-amino acid hydrophobic region, and a 41-amino acid cytoplasmic tail. The presence of N-linked oligosaccharides was confirmed by culturing amebae with tunicamycin, which resulted in a decrease in the heavy subunit molecular mass to 160 kDa and a loss of lectin activity. The extracellular domain was remarkable for an extensive cysteine-rich domain that shared identify with similar regions of several other cell surface proteins and appeared to confer protease resistance to the subunit.

Mucin and nonmucin secretagogue activity of Entamoeba histolytica and cholera toxin in rat colon

Depletion of colonic mucus occurs before invasion of the colonic mucosa by Entamoeba histolytica trophozoites. It is hypothesized that E. histolytica releases a mucus secretagogue; this was studied in a rat colonic loop model. In colonic loops exposed to live amebae, mucus secretion was quantitated by release of acid-precipitable [3H]glucosamine-labeled luminal glycoprotein and by specific immunoassay. Mucus secretion increased in dose-dependent fashion in response to greater than or equal to 1 X 10(5) trophozoites; cholera toxin (20 micrograms per loop), a known mucus secretagogue, elicited a similar response. Thin-section histological analysis of amebae and cholera toxin-exposed loops showed increased mucus release and streaming from mucosal goblet cells with cellular cavitation compared with control loops. Sepharose-4B chromatography of amebae and cholera toxin-stimulated glycoproteins demonstrated secretion of mucins and an 80%-90% increase in low-molecular-weight proteins. E. histolytica trophozoites and cholera toxin enhanced the secretion of preformed and newly synthesized mucin glycoproteins and stimulated colonic glycoprotein synthesis. The level of mucus secretion elicited by axenic E. histolytica strains correlated with their virulence in vivo and in vitro. The amebic secretagogue was released into the culture medium and was heat stable. Mucus secretagogue activity of E. histolytica may contribute to depletion or alteration of the protective mucus blanket, facilitating pathogenesis of invasive amebiasis.

Amebiasis: clinical and laboratory perspectives

Entamoeba histolytica, the premier intestinal protozoan, has traversed time in its relentless quest for survival in its dichotomous role of parasite and pathogen. Enigmatic in its transition from human intestinal commensal to invader of human tissue, diverse in its pathogenicity for the human host, and intricate in its bacterial interrelationship in the bowel, E. histolytica has become the focal point of intensive investigation in its basic biology underscoring human pathogenicity. This review will focus on facets of cell biology, pathophysiology, clinical, therapeutic, and epidemiologic, correlates, along with diagnostic modalities and future research trends.

Protection of gerbils from amebic liver abscess by immunization with the galactose-specific adherence lectin of Entamoeba histolytica

No protective antigens from Entamoeba histolytica have been previously defined. We tested the ability of the galactose-specific adherence lectin of E. histolytica to elicit a protective immune response in conjunction with Freund's incomplete and complete adjuvants. The gerbil (Meriones unguiculatus) model of an experimental amebic liver abscess was used. Gerbils were immunized intraperitoneally or subcutaneously with 10 micrograms of the affinity-purified lectin in complete Freund's adjuvant and then at 2 and 4 weeks with 10 micrograms of the lectin in incomplete Freund's adjuvant. All of the immunized animals developed antilectin antibody titers of greater than 1/1,024 as measured by a radioimmunoassay. The gerbil antilectin antibodies were shown by Western immunoblotting to be directed to the heavy subunit but not the light subunit of the lectin. Immune gerbil sera inhibited amebic adherence by 100% at a 1/10 dilution. Immune and control gerbils were challenged at 6 weeks by the intrahepatic injection of 5 x 10(5) E. histolytica trophozoites. Four independent trials demonstrated complete protection from amebic liver abscess formation in 67% of lectin-immunized gerbils. Unexpectedly, liver abscess weights were significantly higher in the gerbils that failed to become immune than in the control animals. Our results demonstrate that the galactose lectin is a protective antigen and provide an immune-animal model to study the mechanisms of protection and potential disease exacerbation conferred by the antilectin immune response.

Antigens in electron-dense granules from Entamoeba histolytica as possible markers for pathogenicity

In vitro interaction of Entamoeba histolytica with collagen induces intracellular formation and release of electron-dense granules (EDG) and stimulation of collagenolytic activity. Purified EDG contain 1.66 U of collagenase per mg of protein. Thus, EDG may participate in tissue destruction during invasive amebiasis. Monoclonal antibodies (MAbs) L1.1 and L7.1 reacted specifically with EDG in enzyme-linked immunosorbent assay (ELISA) and immunofluorescence and immunoelectron microscopy. MAb L7.1 immunoprecipitated three polypeptides with molecular weights of 95,000, 68,000, and 28,000 from lysates of biosynthetically labeled E. histolytica. Both MAbs recognized the pathogenic E. histolytica axenic strains HM1:IMSS, HM38:IMSS, and HK-9 but failed to react in ELISA with Entamoeba moshkovskii, Entamoeba invadens, and E. histolytica-like Laredo. In addition, MAb L7.1 reacted with one E. histolytica isolate from a symptomatic patient but did not react with four of five isolates from asymptomatic patients. EDG antigens were detected by a MAb L7.1-based ELISA in E. histolytica-containing fecal samples from symptomatic, but not asymptomatic, individuals. These results suggest that the EDG antigen detected with MAb L7.1 may be differentially expressed in pathogenic and nonpathogenic E. histolytica.

Recognition of 29 kDa surface-associated adhesive molecule of Entamoeba histolytica by monoclonal antibodies

Monoclonal antibodies have been developed and used as specific probe to locate and identify a 29-kDa molecule of axenic Entamoeba histolytica trophozoites. Monoclonal antibody produced by clone C8 (MoAb C8) strongly agglutinated the amoebic trophozoites. The immunofluorescence of live E. histolytica trophozoites and surface fluorescence of acetone-fixed trophozoites by MoAb C8 indicated existence of a 29-kDa molecule on surface-associated plasma membrane of E. histolytica. The monoclonal antibody belonged to IgG1 isotype. The prior treatment of E. histolytica trophozoites with MoAb C8 resulted in significant (P less than 0.01) reduction in adherence of amoebic trophozoites to cultured Chinese Hamster Ovary cells and significant (P less than 0.01) reduction in cytotoxicity to cultured Baby Hamster Kidney cells. Pretreatment of amoebic trophozoites with MoAb C8 prior to cultivation in TPS-1 medium resulted in significant (P less than 0.01) reduction in growth of the parasite. Thus, the data suggested that the surface-exposed 29-kDa molecule may be one of the receptors involved in E. histolytica host cell interactions and may possibly modulate amoebic disease processes.

Entamoeba histolytica: correlation of the cytopathic effect of virulent trophozoites with secretion of a cysteine proteinase

Work from several laboratories suggests a correlation between expression of cysteine proteinase activity and the cytopathic effect of virulent HM1 strain Entamoeba histolytica trophozoites on cultured cell monolayers. Consistent with this relationship, we find that L-6 trophozoites, mutants cloned from the HM1 parent strain, are deficient in both proteinase expression and cytopathic effect. Three other clones, with proteinase expression equal to or greater than that of the HM1 strain, express the cytopathic effect. Furthermore, a nontoxic specific proteinase inhibitor, Z-phenylalanyl-alanyl-CH2F, inhibits the cytopathic effect of live trophozoites in a dose-dependent manner. These results support the hypothesis that expression and release of the cysteine proteinase is an important factor in producing the cytopathic effect, presumably by its degradation of cell anchoring proteins.

Binding of proteolytically-degraded human colonic mucin glycoproteins to the Gal/GalNAc adherence lectin of Entamoeba histolytica

Rat and human colonic mucin glycoproteins bind to the Gal/GalNAc adherence lectin on the surface of Entamoeba histolytica in vitro, thus inhibiting the organism from adhering to and lysing the target cells. Human colonic mucin glycoproteins were isolated by Sepharose 4B gel filtration chromatography, they were proteolytically degraded with trypsin, pronase, and papain, and the glycoprotein fractions were reisolated by Sephacryl S-200 gel filtration chromatography. Binding of the mucin glycoprotein fractions to amoebae was quantitated by the inhibition of adherence of Chinese hamster ovary cells to the surface of the amoebae. Trypsin and papain digests caused 40 and 20% reductions, respectively, in the excluded fractions (void volume) that contained all the carbohydrates; pronase digests resulted in extensive degradation of the mucin glycoprotein with the carbohydrate fractions eluting over 40% of the gel bed volume. 3H-labelled mucin glycoprotein and sodium dodecylsulfate-polyacrylamide gel electrophoresis confirmed the presence of the high molecular weight carbohydrate-rich glycoproteins with no subunits in the excluded fractions and the absence of sugars in the included peptides. Only the high molecular weight carbohydrate-containing fractions bind amoebae and inhibit amoebic adherence to Chinese hamster ovary cells. The trypsin digested mucins in the excluded volume were more efficient than the native undigested mucins in binding amoebae. The carbohydrate-containing fractions of the pronase digests were the least effective in binding amoebae and inhibiting adherence of Chinese hamster ovary cells. This suggests that proteolytically-degraded colonic mucins that are glycosylated, as well as the undegraded native mucin glycoproteins of the gut, may play a protective role in binding to amoebae, thus preventing contact of amoebae with mucosal epithelial cells and potential invasion.

Roles of target cell membrane carbohydrate and lipid in Entamoeba histolytica interaction with mammalian cells

Latex beads and liposomes carrying glycoproteins with carbohydrate sequences recognized by an Entamoeba histolytica galactose-specific binding protein were assessed for their ability to adhere to trophozoites and to stimulate amoeba actin polymerization. Glycoprotein-conjugated beads bound significantly to amoebae but did not stimulate actin polymerization. Glycoprotein-bearing liposomes bound to amoebae and did enhance actin polymerization, as do recognized glycosphingolipid-bearing liposomes (G. B. Bailey, E. D. Nudelman, D. B. Day, C. F. Harper, and J. R. Gilmour, Infect. Immun. 58:43-47, 1990). Liposome-stimulated actin polymerization occurred only if the vesicle contained negatively charged phospholipid. It was concluded that both glycoprotein and glycosphingolipid glycans on the target cell surface are involved in attachment to E. histolytica but do not themselves induce the transmembrane signals that lead to cytoskeleton activation and target destruction. This requires interaction with lipids of the target membrane bilayer.

Pathogenic and nonpathogenic strains of Entamoeba histolytica can be differentiated by monoclonal antibodies to the galactose-specific adherence lectin

Entamoeba histolytica infection results in either asymptomatic colonization or invasive colitis and liver abscess. E. histolytica isolates from patients with invasive disease have characteristic isoenzyme profiles (pathogenic zymodemes), suggesting a role for parasite factors in determining the severity of infection. A galactose-specific cell surface lectin from a pathogenic zymodeme was shown to mediate in vitro adherence to human colonic mucins and contact-dependent killing of target cells. Six nonoverlapping antigenic determinants were identified on the 170-kilodalton heavy subunit of the pathogenic lectin. Anti-lectin monoclonal antibodies (MAb) directed against epitopes 1 and 2 enhanced adherence whereas MAb to epitopes 3 through 6 either inhibited or had no effect on adherence. We tested 50 pathogenic and nonpathogenic strains for reactivity to these anti-lectin MAb by radioimmunoassay. MAb to epitopes 1 through 6 reacted in the radioimmunoassay with all 16 pathogenic zymodeme strains tested. In contrast, only MAb to epitopes 1 and 2 bound to the lectin from nonpathogenic strains. Western immunoblots with anti-lectin antibodies showed that the 170-kilodalton heavy subunit was present in the nonpathogenic amebae. Adherence of the nonpathogenic SAW 760 strain to human erythrocytes was enhanced by MAb to epitope 1 and blocked by galactose, confirming the presence of a functionally active lectin. A lectin radioimmunoassay based on MAb to epitopes 1 and 3 proved to be a simple and rapid method to distinguish pathogenic from nonpathogenic amebae in culture. Further exploration of the functional consequences of the antigenic differences demonstrated for the lectin may lead to a better understanding of its role in pathogenesis.

Specificity of glycosphingolipid recognition by Entamoeba histolytica trophozoites

The ability of purified glycosphingolipids to enhance liposome-stimulated Entamoeba histolytica actin polymerization was assessed as a means of defining the specificity of mammalian cell membrane lipid glycan recognition by this parasite. Synthetic liposomes containing a variety of individual glycosphingolipids bearing neutral, straight-chain oligomeric glycans with galactose or N-acetylgalactosamine termini stimulated rapid (90-s) polymerization of amoeba actin. Glycans with terminal N-acetylglucosamine residues were not stimulatory at all or were only weakly stimulatory. Glycans with glucose, N-acetylglucosamine, galactose, and N-acetylgalactosamine as the penultimate residue were recognized. Attachment of N-acetylneuraminate to the terminal residue of a stimulatory glycosphingolipid eliminated activity; attachment of fucose to the penultimate sugar reduced activity. Glycans with a terminal beta 1-4 or 1-3 glycosidic bond were most effective; glycans with terminal alpha 1-4 or 1-3 glycosides were less effective. The activity of glycans with both beta- and alpha-linked terminal glycosides was inhibited by lactose, suggesting recognition of both configurations by a single amoeba protein. The ability of liposomes to stimulate actin polymerization reflected the extent of liposome phagocytosis.

Characterization of a cytochalasin D-resistant mutant of Entamoeba histolytica

Characterization of a cytochalasin D-resistant mutant of the human parasite Entamoeba histolytica capable of growing at 10 microM cytochalasin is described. The mutant cells also show resistance to 5 mM colchicine and 100 microM cytochalasin B, drugs proved deleterious for wild type trophozoites. The mutants show increased osmotic fragility and electric mobility but reduced phagocytic activity, and agglutination by Concanavalin A. On the other hand pinocytic activity remains unaltered when compared with the wild type cells. Polymerized actin, seen by staining with phalloidin, often appears polarized to one end of the trophozoites and forms few of the endocytic invaginations found in wild type amebas. An altered distribution of part of the actin could explain the differences in surface properties and motility observed in the mutant amebas.

Adherence and surface properties of Tritrichomonas mobilensis, an intestinal parasite of the squirrel monkey

Adherence properties of the potentially enteropathogenic Tritrichomonas mobilensis were studied in vitro. Axenically cultivated trichomonads readily attached to isolated intestinal epithelial cells and mucus of the squirrel monkey. The kinetics and nature of T. mobilensis cytadherence were microscopically evaluated in cell-suspension assay using Chinese hamster ovary (CHO) cells and in microplate hemagglutination assay with human erythrocytes. Adherence of the parasites to target cells was concentration- and time-dependent; it was inhibited by sialic acid (N-acetylneuraminic or N-glycolylneuraminic acid) and sialyllactose. Neither trypsinization of the flagellates nor their exposure to low temperature (4 degrees C) affected their cytadherence capacities. The data indicate the presence of adhesin(s) with lectin properties on T. mobilensis. Agglutination of live protozoa by animal and plant lectins with various carbohydrate-binding specificities as well as the occurrence of an electron-dense cell coat on plasma membrane suggest marked glycosylation of the parasite surface.

Characterization of cell surface carbohydrate receptors for Entamoeba histolytica adherence lectin

Binding and cytolysis of Chinese hamster ovary (CHO) cells by Entamoeba histolytica trophozoites is inhibitable by galactose (Gal) or N-acetyl-D-galactosamine (GalNAc). To better define the carbohydrate receptor for E. histolytica, we compared the binding and cytolytic target properties of 10 CHO glycosylation mutants. Each mutant expresses a uniquely altered array of N- and/or O-linked cell surface carbohydrates. Amebic adherence was reduced when lactosamine-containing N-linked carbohydrates were essentially absent (Lec1 mutant), almost undetectable when Gal and GalNAc residues were absent on both N- and O-linked carbohydrates (ldlD.Lec1 mutant), and enhanced for mutants with increased terminal Gal residues (Lec2 and Lec3). Parental CHO cells treated with neuraminidase to expose Gal residues behaved like Lec2 mutants. Binding of purified Gal or GalNAc lectin to parental, Lec1, ldlD.Lec1, and Lec2 mutant CHO cells corroborated the adherence results. The suitability of CHO cell mutants as targets for amebic cytolysis correlated with their glycosylation phenotype: the Lec1 mutants were less susceptible than parental CHO cells, the ldlD.Lec1 mutants were highly resistant, and the Lec2 mutants required higher concentrations of Gal for inhibition. The E. histolytica Gal or GalNAc adherence lectin bound preferentially to beta 1-6-branched, N-linked carbohydrates lacking terminal sialic acid or fucose residues. However, amebic lectin binding to either N- or O-linked cell surface carbohydrates was sufficient to initiate parasite cytolytic activity.

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.

Nondysenteric intestinal amebiasis. Colonic morphology and search for Entamoeba histolytica adherence and invasion

There is controversy regarding the presence of colonic mucosal abnormalities or mucosal invasion by Entamoeba histolytica in patients with "nondysenteric intestinal amebiasis." To determine the role of E. histolytica in causing symptoms and mucosal changes and to detect if mucosal invasion by E. histolytica is present in nondynsenteric intestinal amebiasis, we evaluated 24 E. histolytica-infected patients (stool microscopy positive for E. histolytica) and 12 noninfected controls who presented with chronic gastrointestinal symptoms, but without dysentery, to a clinic in Calcutta. The colonic mucosa was evaluated at colonoscopy, and mucosal biopsies obtained from the cecum, sigmoid colon, and rectum were evaluated by light microscopy, indirect immunofluorescence microscopy, and scanning electron microscopy. At colonoscopy mucosal ulcerations were absent in all the controls and all except one of the E. histolytica-infected patients. E. histolytica trophozoites or cysts were not seen in the lamina propria or on the luminal surface in any infected patient by light and immunofluorescence microscopy. On scanning electron microscopy, structures that resembled rounded E. histolytica trophozoites were seen on the luminal surface in two of 19 cecal specimens from the infected patients. Moderate or severe mucosal inflammation was frequent on light microscopy in both the E. histolytica-infected patients and the noninfected controls with the cecum involved in two thirds of both groups. Antibodies to E. histolytica were detected in serum of 25% of study patients and 58% of controls. Mucosal inflammation did not correlate with stool positivity for E. histolytica or seropositivity for ameba antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigenic stability and immunodominance of the Gal/GalNAc adherence lectin of Entamoeba histolytica

Immunoprecipitation of Entamoeba histolytica proteins was performed with the sera of patients recovered from amebic liver abscess and colitis. The patients' amebic infection had been acquired in diverse areas of the world. The amebic galactose and N-acetyl-D-galactosamine-inhibitable adherence lectin was the major amebic antigen immunoprecipitated. The adherence lectin was recognized by all of the patients' sera tested regardless of the site (liver abscess vs. colitis) or geographic region that the amebic infection had occurred.

Chinese hamster ovary cells deficient in N-acetylglucosaminyltransferase I activity are resistant to Entamoeba histolytica-mediated cytotoxicity

To study the relationship between carbohydrate-specific amebic cytoadherence and ameba-mediated cytotoxicity, we measured Entamoeba histolytica trophozoite-mediated cytolysis directed against a panel of four Chinese hamster ovary (CHO) cell lines that have defined alterations in their glycosylation patterns. We recently measured amebic trophozoite adherence to this panel of CHO cells and showed that trophozoites bind variant cells (RICR 15B), which are deficient in Asn-linked N-acetyllactosamine units, at 12% of the level observed for wild-type cells (E. Li, A. Becker, and S. L. Stanley, J. Exp. Med 167:1725-1730, 1988). Using a 51Cr release assay to measure trophozoite-mediated cytolysis, we demonstrate in this study that RICR 15B cells are less susceptible to trophozoite-mediated cytolysis than are wild-type cells. In addition, we found that N-acetyllactosamine, which inhibits trophozoite adherence to CHO cells, also inhibited trophozoite-mediated cytolysis of wild-type cells. These studies indicate that surface carbohydrates on target cells can influence susceptibility to ameba-mediated cytotoxicity. This panel of CHO cells provides a useful model system for investigating the role of glycoconjugates in mediating amebic interactions with mammalian cells.

Maintenance of integrity, viability, and adhesion of Entamoeba histolytica trophozoites in different incubation media

We have determined the integrity, viability and adhesion of Entamoeba histolytica HK9 and HM1 trophozoites during their incubation in two basal culture media (TP and TYI) and three saline media ("maintenance medium" MM-1 and two others buffered with HEPES). In basal culture media, more than 70% of the trophozoites maintained their integrity and adhesion to human red blood cells (RBC) for up to 4 h, and the proportion of those excluding Trypan blue decreased slowly after 2 h. In saline media, the number of ameba-RBC complexes reached a maximum after 20-30 min and then decreased rapidly (and fastest in MM-1), less than 10% of the amebae were intact after 3-4 h, and dye exclusion fell abruptly from the start of incubation. The number of ameba-RBC complexes formed and the rate of adhesion were highest in basal TP medium. Normal nonvacuolated refringent (NVR) trophozoites deteriorated progressively in all media--although much faster in the saline ones--to vacuolated refringent (VR), nonrefringent, and disrupted. Trypan blue was excluded by all NVR and a fraction of the VR trophozoites. Horse serum helped to maintain ameba integrity and viability, but inhibited adhesion in a concentration-dependent manner. We conclude that E. histolytica trophozoite integrity and adhesion are adequately preserved and should be characterized only in basal culture media, that refringence without vacuolization is a more stringent characteristic of ameba quality than Trypan blue exclusion, and that some serum component inhibits ameba adhesion.

Entamoeba histolytica: virulence potential and sensitivity to metronidazole and emetine of four isolates possessing nonpathogenic zymodemes

The pathogenic potential of four Entamoeba histolytica isolates obtained from asymptomatic carriers and possessing nonpathogenic zymodemes was compared to four E. histolytica strains obtained from invasive cases of amebiasis and having pathogenic zymodemes. Both xenic and axenic cultures of a number of strains were tested. Determinations of cytopathogenicity were done in vitro by measuring the rates of destruction of tissue cultured monolayers of baby hamster kidney cells by intact amebae or by its cell-free extracts. The in vivo virulence was tested by assessing their capacity to form hepatic abscesses in hamsters or cecal ulcerations in rats. The results obtained show that two of the isolates from asymptomatic carriers (strains SAW 1734R clAR and WI:0385:191) were as virulent as three of the invasive ones (HM-1:IMSS, 200:NIH, and SAW 408). Two other isolates from asymptomatic carriers and one from a dysentery case were avirulent. All the E. histolytica isolates tested were similarly sensitive to metronidazole and emetine (IC50 1-10 micrograms/ml). The results indicate that the pathogenic potential of E. histolytica varies between isolates and can be affected by culture conditions and by the presence or absence of bacterial cells. These findings suggest that virulence does not necessarily correlate with a pathogenic zymodeme.

Relationship of free intracellular calcium to the cytolytic activity of Entamoeba histolytica

Entamoeba histolytica adherence and destruction of host cells is required for in vivo pathogenicity; amebic in vitro adherence is mediated by a galactose- or N-acetyl-D-galactosamine-inhibitable surface lectin (Gal/GalNAc adherence lectin). Free intracellular Ca2+ concentration [( Ca2+]i) was measured in living amebae and target cells during amebic cytolysis of Chinese hamster ovary (CHO) cells and human polymorphonuclear neutrophils by utilizing the Ca2+ probe Fura-2 and computer-enhanced digitized microscopy. Motile E. histolytica trophozoites had oscillatory increases in [Ca2+]i in head or tail regions; however, there was no increase in regional or total amebic [Ca2+]i upon contact with a target CHO cell. Target CHO cells and polymorphonuclear neutrophils demonstrated marked irreversible increases in [Ca2+]i within 30 to 300 s following contact by an ameba (P less than 0.01); increased [Ca2+]i preceded the occurrence of nonspecific surface membrane permeability and death of the target cell. Target CHO cells contiguous on a monolayer to a cell contacted by an ameba experienced a rapid but reversible rise in [Ca2+]i (P less than 0.01) and were not killed. Galactose (40 mg/ml) totally abrogated the rise in target CHO cell [Ca2+]i that followed contact by amebae (P less than 0.01); immunoaffinity-purified amebic Gal/GalNAc adherence lectin (0.25 micrograms/ml) induced a rapid and reversible rise in CHO cell [Ca2+]i (P less than 0.01) which was inhibited by galactose. Amebic [Ca2+]i was not elevated following parasite adherence to target cells; a rapid and substantial rise in target cell [Ca2+]i occurred which was mediated, at least in part, by the Gal/GalNAc adherence lectin of the parasite and led to the death of target cells.

Attachment of Giardia lamblia to rat intestinal epithelial cells

The human enteric protozoan, Giardia lamblia, has surface membrane lectin activity which mediates parasite adherence to erythrocytes. To determine whether an intestinal binding site exists for this lectin we have studied the interaction in vitro between axenically cultured Giardia trophozoites and isolated rat intestinal epithelial cells. Scanning electron microscopy showed that Giardia attached to the apical microvillus membrane and basolateral membrane of rat enterocytes. Any location on the parasite surface could mediate attachment without predeliction for the ventral disc. Trophozoites attached more avidly to jejunal compared with colonic epithelial cells. Attachment was inhibited at 4 degrees C, by sugars and glycoproteins containing D-mannosyl residues and by subagglutinating concentrations of anti-Giardia rabbit serum and two monoclonal antibodies, all with reactivity to parasite surface membrane determinants. Trypsinisation of trophozoites also reduced attachment but the ability to attach was rapidly restored after returning trophozoites to TYI-S culture medium for 4 h at 37 degrees C. Attachment was unaltered by the presence of the microfilament inhibitor cytochalasin B and in the absence of Ca++ and Mg++ ions. These findings support previous work that Giardia possesses a surface membrane mannose binding lectin and indicate that appropriate binding sites are present on rat intestinal epithelial cells. This lectin may play a part in mediating adherence of Giardia to mammalian intestine and could be a target for host immune defence.

Phorbol esters specifically enhance the cytolytic activity of Entamoeba histolytica

Entamoeba histolytica causes invasive amebiasis by lysis of host tissue and inflammatory cells. The in vitro cytolysis of target Chinese hamster ovary (CHO) cells by axenic E. histolytica trophozoites (strain HM1:IMSS) is a calcium- and phospholipase A-dependent event initiated by the binding to the target cell of the galactose-inhibitable surface lectin of the parasite. We utilized phorbol esters as a probe to determine whether an amebic protein kinase C has a role in the cytolytic event. The addition of phorbol 12-myristate 13-acetate (PMA) at 10(-6) or 10(-7) M resulted in a greater than twofold enhancement of amebic killing of target CHO cells over 30 min (P less than 0.01). Prior exposure of only the amebae, but not the CHO cells, to PMA produced a similar effect (P less than 0.01). The inactive analog 4-alpha-phorbol had no effect on amebic killing of CHO cells. The PMA-mediated enhancement of amebic cytolysis persisted for up to 60 min after a 5-min exposure; however, after a 30-min exposure to PMA (10(-6) M) there was no augmentation of amebic killing of CHO cells. PMA (10(-6) M) did not promote adherence of parasites to CHO cells but did enhance amebic cytolysis of previously adherent target cells (P less than 0.01). Sphingosine, a specific inhibitor of protein kinase C, abolished both the PMA-stimulated and the basal cytolytic activity of E. histolytica. PMA enhanced CHO cell cytolysis by the less virulent wild-type strain H-303:NIH (P less than or equal to 0.02) but did not augment the activity of the less virulent strain H-200:NIH or two avirulent clones of HM1 (L6 and C919). In summary, these experiments with the phorbol esters and sphingosine as probes to modulate the activity of protein kinase C indicate participation of a parasite protein kinase C in the cytolytic activity of virulent, axenic E. histolytica trophozoites and thus in the pathogenesis of amebiasis.

The cellular regulation of vesicle exocytosis by Entamoeba histolytica

We studied the cellular regulation of vesicle exocytosis by Entamoeba histolytica utilizing release of endocytosed 125iodine (125I) labeled tyrosine conjugated dextran; 125I-dextran entered the acid pH vesicles of the amebae and was not degraded during these studies. Exocytosis was temperature dependent with 74%, 36%, 4%, and 0% of 125I-dextran released after 120 min at 37 degrees C, 31 degrees C, 25 degrees C, and 4 degrees C, respectively (P less than 0.01 for each). Exocytosis at 37 degrees C was inhibited by cytochalasin D (10 micrograms/ml), EDTA (10 mM), or the putative intracellular calcium antagonist TMB-8 (250 microM) (P less than 0.01 for each at greater than or equal to 60 min). Calcium ionophore A23187 (1 microM) enhanced exocytosis at 5 and 15 min (P less than 0.01). Elevation of vesicle pH with NH4Cl (10 mM) had no effect on release of 125I-dextran; phorbol myristate acetate (10(-6) M) increased exocytosis by 46% at 30 min (P less than 0.01). Centrifugation of amebae with target Chinese hamster ovary cells resulted in decreased 125I-dextran release into the cell supernatant after 30 and 60 min at 37 degrees C (by 40% and 42%, respectively, P less than 0.01); release of 125I-dextran returned to control values with addition of 1.0 g% galactose or GalNac but not with mannose or N-acetyl-D-glucosamine. Amebic phagocytosis of serum-exposed latex beads had no effect on release of dextran by amebae (n = 16). Exocytosis of acid pH vesicles by E. histolytica is temperature-, microfilament-, and calcium-dependent, and stimulated by phorbol esters.

Isolation of the galactose-binding lectin that mediates the in vitro adherence of Entamoeba histolytica

Entamoeba histolytica adheres to human colonic mucus, colonic epithelial cells, and other target cells via a galactose (Gal) or N-acetyl-D-galactosamine (GalNAc) inhibitable surface lectin. Blockade of this adherence lectin with Gal or GalNAc in vitro prevents amebic killing of target cells. We have identified and purified the adherence lectin by two methods: affinity columns derivatized with galactose monomers or galactose terminal glycoproteins, and affinity columns and immunoblots prepared with monoclonal antibodies that inhibit amebic adherence. By both methods the adherence lectin was identified as a 170-kD secreted and membrane-bound amebic protein. The surface location of the lectin was confirmed by indirect immunofluorescence. Purified lectin competitively inhibited amebic adherence to target cells by binding to receptors on the target Chinese hamster ovary cells in a Gal-inhibitable manner.

Rat and human colonic mucins bind to and inhibit adherence lectin of Entamoeba histolytica

Establishment of adherence by Entamoeba histolytica is mediated by a 170-kD Gal/GalNAc inhibitable lectin and is required for cytolysis and phagocytosis of mammalian target cells. We studied the biochemical mechanisms of the in vitro interaction between rat and human colonic mucins and axenic E. histolytica trophozoites. Crude mucus prevented amebic adherence to Chinese hamster ovary (CHO) cells by up to 70%. Purification of the colonic mucins by Sepharose 4B chromatography, nuclease digestion, and cesium chloride gradient centrifugation resulted in a 1,000-fold enrichment of the inhibitory mucins. Purified rat mucin inhibited amebic adherence to and cytolysis of homologous rat colonic epithelial cells. Oxidation and enzymatic cleavage of rat mucin Gal and GalNAc residues completely abrogated mucin inhibition of amebic adherence. The binding of rat 125I-mucin to amebae was galactose specific, saturable, reversible, and pH dependent. A monoclonal antibody specific for the 170-kD amebic Gal/GalNAc lectin completely inhibited the binding of rat 125I-mucin. Rat mucin bound to Affigel affinity purified the amebic lectin from conditioned medium. Colonic mucin glycoproteins act as an important host defense by binding to the parasite's adherence lectin, thus preventing amebic attachment to and cytolysis of host epithelial cells.

Recognition of the galactose- or N-acetylgalactosamine-binding lectin of Entamoeba histolytica by human immune sera

Cure of amebic liver abscess is associated with resistance to recurrent invasive amebiasis and the development of a humoral and cell-mediated immune response. We determined whether human immune sera contain blocking antibody for the 170-kilodalton (kDa) galactose or N-acetylgalactosamine (Gal/GalNAc)-binding lectin of Entamoeba histolytica. By Western blot (immunoblot) of whole amebae subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, all eight immune sera studied here prominently recognized a 170-kDa amebic protein. Western blot of the purified Gal/GalNAc lectin with pooled human immune sera (PHIS) confirmed that the 170-kDa band was the adherence lectin. Immunoprecipitation of [35S]methionine-metabolically-labeled amebae with the antilectin monoclonal antibody H8-5 and with PHIS demonstrated that the 170-kDa lectin was the major antigen recognized by PHIS. The in vitro adherence of E. histolytica trophozoites to CHO cells at 4 degrees C was inhibited by prior exposure of amebae to greater than or equal to 1.0% PHIS. The humoral response to the Gal/GalNAc-binding lectin of the parasite may contribute to the development of protective immunity against invasive amebiasis.

Entamoeba histolytica: chemoattractant activity for gerbil neutrophils in vivo and in vitro

The kinetics of the host's cellular response in the peritoneal cavity of gerbils toward axenic pathogenic and nonpathogenic Entamoeba histolytica strains were examined. Amebae contained in diffusion chambers or free in the peritoneum elicited a neutrophilic response accompanied by decreased levels of macrophages and lymphocytes. Pathogenic amebae (IP:0682:1 strain) elicited a neutrophilic response greater than the nonpathogenic DKB and "entamoeba-like" Laredo amebae. The neutrophil eliciting factor was found in high levels in disrupted freeze-thawed amebae (53% elicited neutrophils vs 8% for control), glutaraldehyde fixed amebae (45%) and amebic membranes (65%), and low levels in conditioned amebic medium (15%) and the supernatant fraction of amebae (16%). The factor was heat stable to high temperature (100 C for 30 min) and at various pH (6 to 9). The neutrophil eliciting factor in amebic membranes was lowered following pretreatment for 30 min with 1% immune and nonimmune gerbil or human sera (34-48% lowered neutrophil response vs control), acidic pH (less than 3, 69%), proteolytic digestion [trypsin (68%) and alpha-chymotrypsin (72%), 100 micrograms/ml], and 2% Triton X-100 (75%). Peritoneal neutrophils isolated following stimulation with amebic membranes or thioglycollate medium demonstrated higher chemotaxis in vitro toward live pathogenic amebae and amebic membranes (IP:0682:1 strain) compared to either the supernatant fraction or the nonpathogenic DKB or Laredo amebae. The results of this study indicate that membrane bound proteins of pathogenic amebae are chemotactic for gerbil neutrophils which may be important in the pathogenesis and pathology of amebiasis.

Stimulation by target cell membrane lipid of actin polymerization and phagocytosis by Entamoeba histolytica

The identity of molecules of mammalian target cells that stimulate contact-dependent attack by Entamoeba histolytica was sought using human erythrocytes (RBC) as a model. Protein-free liposomes prepared from RBC membrane lipids stimulated the same rapid E. histolytica actin polymerization and phagocytosis as did whole target cells. Liposomes constructed from the major phospholipids of RBC stimulated these responses but only if a negatively charged phospholipid was included. The addition to these liposomes of digalactosyl diglyceride significantly enhanced their stimulatory activity. The results demonstrate that ligands that trigger attack-related responses by E. histolytica reside in the target cell membrane lipid fraction and suggest roles for both glycolipid and phospholipid components.

Cytopathogenicity of Entamoeba histolytica: the role of amebic adherence and contact-dependent cytolysis in pathogenesis

The mechanisms by which Entamoeba histolytica trophozoites adhere to and lyse target cells are reviewed from the perspective of pathogenesis. Adherence via the galactose and N-acetyl-galactosamine inhibitable amebic lectin and possible additional amebic adhesin molecules is followed by target cell death. Inhibition of the Gal/GalNAc lectin with GalNAc inhibits amebic cytolysis of target cells. Amebic activities implicated in the cytolytic event include vesicle exocytosis and maintenance of an acid pH, pore forming proteins, phospholipase A and proteases. Increased knowledge of the sequence of events leading to target cell lysis should lead to more effective treatment or prevention of infection by this enteric parasite and add to our basic understanding of eukaryotic cell-cell interactions.

Ameba-bacterium relationship in amebiasis

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Genomic and cDNA actin sequences from a virulent strain of Entamoeba histolytica

Invasiveness of Entamoeba histolytica strains that cause acute amoebiasis is characterized by aggressive behavior associated with cell motility and actin function. Analysis of actin genes from E. histolytica was initiated by devising methods for the isolation of biologically active nucleic acids, which allowed the preparation of cDNA and genomic DNA libraries. E. histolytica actin-encoding cDNAs and genomic clones have been isolated from libraries prepared from the virulent HM1:IMSS strain using a heterologous actin probe. Nucleotide sequence analysis of three independent cDNA clones and one genomic clone reveals a highly unusual codon bias and the absence of intervening sequences in E. histolytica actin. The coding sequence of the genomic clone is identical to that of two of the three cDNA clones. These represent at least two distinct mRNAs differing only by five silent changes in the protein coding sequence. Multiple genomic copies of the actin gene can be detected by Southern hybridization. E. histolytica actin exhibits a higher degree of homology to cytoplasmic than to muscle actin. Although the protein has been shown not to bind DNase I, the inferred amino acid sequence indicates conservation of all residues implied to participate in this binding.

Entamoeba histolytica: cytopathogenicity and lectin activity of avirulent mutants

Three clones of Entamoeba histolytica (L-6, C93, C919) were isolated by mutagenesis with ethyl methanesulfonate from the axenic strain HM1:IMSS and were studied for adherence, cytolytic, and soluble galactose inhibitable lectin activity. Avirulent clones adhered to and killed fewer Chinese hamster ovary cells than HM1:IMSS (P less than 0.01). However, only C919 was deficient in adherence to red blood cells. Galactose (1.0 g) completely inhibited adherence of all the mutants to Chinese hamster ovary cells; however, adherence to erythrocytes was only partially inhibitable by galactose. Avirulent mutants were more susceptible to being killed by human neutrophils in vitro (P less than 0.01 compared to HM1:IMSS). Soluble protein preparations from all the avirulent mutants were markedly less mitogenic for human lymphocytes and had lower lectin activity for Chinese hamster ovary cells compared to the HM1:IMSS wild type (P less than 0.01 for each activity with each mutant). Indirect immunofluorescence with a monoclonal antibody (F-14) that recognizes the Gal/GalNAc lectin was positive for L-6 and C919. These findings utilizing avirulent mutants of E. histolytica further support a role for the amebic galactose inhibitable lectin in the in vivo pathogenesis of amebiasis.

The interaction of human T-lymphocytes and Entamoeba histolytica: killing of virulent amoebae by lectin-dependent lymphocytes

Clinical and experimental studies indicate that following invasive disease due to Entamoeba histolytica, development of human cell-mediated immune mechanisms may provide protective immunity. Activated, human monocyte-derived macrophages in vitro can kill virulent axaenic amoebic trophozoites. This study describes the interaction of lectin-stimulated T-lymphocytes and E. histolytica trophozoites (virulent strain HM1-IMSS). Amoebae progressively killed unstimulated nonimmune T-lymphocytes over 18 h incubation with no effect on amoebic viability. T-lymphocytes, stimulated with phytohaemagglutinin (PHA), were progressively cytotoxic for virulent HMI amoebae over 18 h incubation, but were also reduced in viability themselves. Lymphocyte cytotoxicity for amoebae was absent if PHA was removed before or added only during the assay. PHA-stimulated T-lymphocytes killed amoebae at cell ratios of lymphocytes to amoebae as low as 50:1 and cytotoxicity was antibody-independent. PHA-stimulated T-lymphocytes, depleted of T8-bearing cells by complement-mediated lysis, were unable to kill amoebae. Adherence of PHA-stimulated T-lymphocytes to amoebae was greater than with unstimulated T-lymphocytes. Inhibition of the amoebic adherence lectin with N-acetyl-D-galactosamine decreased lymphocyte-amoebic adherence and resulted in increased lymphocyte amoebicidal activity and lymphocyte survival. Suspension of amoebae with or without adherent PHA-stimulated T-lymphocytes in a 10% dextran solution indicated that cytotoxicity was contact dependent. In summary, PHA-stimulated T-lymphocytes of the T8-phenotype can kill virulent axaenic E. histolytica trophozoites through a contact-dependent, antibody-independent mechanism.

Inhibition of the adhesion of Entamoeba histolytica trophozoites to human erythrocytes by carbohydrates

We have analyzed the effect of 14 carbohydrates (seven monosaccharides, four disaccharides and three aminosugars) on the adhesion of Entamoeba histolytica HK9 trophozoites to human red blood cells (RBC). Amebal adhesion was significantly inhibited by five of these carbohydrates with the following order of potency: lactose (Lac) greater than N-acetylgalactosamine (GalNac) greater than melibiose (Mel) greater than galactose (Gal) greater than N-acetylglucosamine (GlcNAc). The mean inhibitory concentration of Lac was 2.66 mM. Adhesion increased by 20% in the presence of 5.5 mM glucose (Glc). Inhibition of the adhesion was lower in the absence rather than in the presence of Glc only with Gal-NAc, whereas it was similar with Lac, Mel, Gal, and GlcNAc in both cases. The initial rate of amebal adhesion decreased 27% by RBC fixation, but adhesion to fixed RBC was also inhibited by the same five carbohydrates. Inhibition was higher in mixtures containing Lac, GalNAc, and Mel, than with the same isolated carbohydrates; Lac + Gal-NAc was the most potent mixture. Inhibition decreased when Lac, GalNAc, and Mel were mixed either with Gal or GlcNAc. We conclude that E. histolytica adhesion depends on amebal metabolic energy generated from Glc and on several surface components of RBC, some of which are inactivated with glutaraldehyde whereas others are inhibited by sugars containing Gal, GlcNAc, or Gal-NAc residues.

Acid intracellular vesicles and the cytolysis of mammalian target cells by Entamoeba histolytica trophozoites

Entamoeba histolytica kills mammalian target cells in a multi-step sequential process with separate adherence, cytolytic, and phagocytic events. In the studies reported here, we used fluorescein isothiocyanate linked to dextran to label the endocytic vesicles of the HM1 strain of E. histolytica and measure vesicle pH (5.1 +/- 0.2 by spectrofluorimetry). Concentrations of NH4Cl (1.0-10.0 mM) sufficient to increase vesicle pH to greater than or equal to 5.7 inhibited amebic killing of target Chinese hamster ovary (CHO) cells as assayed by trypan blue staining, by the release of 3H-thymidine previously incorporated into CHO cell monolayers, and by the release of 111indium oxine from radiolabeled CHO cells. Similar effects were also observed with two other weak bases, primaquine and chloroquine (both 50 microM). In contrast, NH4Cl (10 mM) did not affect either the adherence or phagocytic events, as measured by amebic adherence to CHO cells at 4 degrees C and by the binding and ingestion of 3H-leucine-labeled bacteria. In the presence of NH4Cl and the carbohydrate ligand asialofetuin, there was no evidence of intracellular trapping of the amebic galactose-inhibitable lectin; inhibition of adherence by cycloheximide (10 micrograms/ml for 3 h) suggested rapid turnover of the surface lectin. Prolonged exposure to NH4Cl for 48 h (which had no effect on amebic protein synthesis) or shorter exposure to cycloheximide (10 micrograms for 3 h) produced persistent inhibition of cytolysis. These results indicate that an uninterrupted acid pH in intracellular endocytic vesicles is necessary for the cytolysis of target cells by E. histolytica trophozoites.

Production of mouse monoclonal antibodies which inhibit in vitro adherence of Entamoeba histolytica trophozoites

Adherence by axenic Entamoeba histolytica trophozoites to mammalian cells is mediated by an N-acetylgalactosamine (GalNAc)-inhibitable adhesin on the surface of the parasite. We isolated 35 hybridoma cell lines producing antibodies to E. histolytica as indicated by ELISA with sonicated amebic protein or by immunofluorescence assay with fixed whole trophozoites. Tissue culture supernatants were further screened for subcloning by the ability to bind to Chinese hamster ovary (CHO) cells which were first exposed to a partially purified soluble preparation of the amebic GalNAc-inhibitable lectin. Eight tissue culture supernatants were positive in this assay. Antibodies from four subcloned cell lines (D3-14, H8-5, I12-2, and I1-21) inhibited amebic adherence to CHO cells (P less than 0.01). Of the original 35 tissue culture supernatants, 3 also inhibited amebic adherence (P less than 0.01; F1, F14, and J10); monoclonal antibodies in these supernatants did not bind to lectin-exposed CHO cells. Three purified monoclonal antibodies (H8-5, I12-2, and I1-21) inhibited amebic adherence at greater than or equal to 2 micrograms/10(4) amebae (P less than 0.05). None of these inhibitory monoclonal antibodies immunoprecipitated with a soluble amebic protein preparation following sodium dodecyl sulfate-polyacrylamide gel electrophoresis under denaturing conditions. Monoclonal antibodies which inhibit in vitro adherence by E. histolytica will be useful in purification of the GalNAc-inhibitable lectin.

Description and characterization of a surface lectin from Giardia lamblia

The mechanisms by which the human enteric pathogen Giardia lamblia colonizes the proximal small intestine are poorly understood. Although the parasite possesses an attachment organelle on its ventral surface, the "sucking" disk, we considered that like many bacteria and some protozoa, G. lamblia might also have a surface membrane-associated modality for adherence to its host. Using an erythrocyte mixed-agglutination model, we demonstrated a parasite surface lectin with specificities for D-glucosyl and D-mannosyl residues. This lectin is soluble in Triton X-100, is calcium dependent, and is maximally active at pH 5.5 to 6.0. Partial purification was achieved by serial extraction of parasites in Triton X-100 followed by Sephadex G-150 affinity chromatography. The lectin could not be surface radiolabeled with 125I-Bolton-Hunter reagent, but radiolabeling of the hapten eluate from an affinity column produced four bands of 57,000 to 78,000 Mr on sodium dodecyl sulfate-polyacrylamide gels under reducing conditions. The biological function of this lectin is unknown. The presence of mannosyl residues on the luminal surface of human small intestinal epithelial cells suggests that there are receptors for Giardia lectin at the site of colonization.

Contact-dependent cytopathogenic mechanisms of Trichomonas vaginalis

The cytopathogenic mechanisms of Trichomonas vaginalis have been debated since the 1940s. We examined the following three proposed pathogenic mechanisms: contact-dependent extracellular killing, cytophagocytosis, and extracellular cytotoxins. Serial observations of Chinese hamster ovary (CHO) cell monolayers exposed to trichomonads revealed that (i) trichomonads form clumps, (ii) the clumps adhere to cells in culture, and (iii) monolayer destruction occurs only in areas of contact with T. vaginalis. Kinetic analysis of target cell killing by trichomonads revealed that the probability of CHO cell death was related to the probability of contact with T. vaginalis, supporting the observation by microscopy that trichomonads kill cells only by direct contact. Simultaneous studies of 111indium oxine label release from CHO cells and trypan blue dye exclusion demonstrated that T. vaginalis kills target cells without phagocytosis. Filtrates of trichomonad cultures or from media in which trichomonads were killing CHO cells had no effect on CHO cell monolayers, indicating that trichomonads do not kill cells by a cell-free or secreted cytotoxin. The microfilament inhibitor cytochalasin D (10 micrograms/ml) inhibited trichomonad killing of CHO cell monolayers by 80% (P less than 0.0001). In contrast, the microtubule inhibitor vinblastine (10(-6) M) caused only 17% inhibition of trichomonad destruction of CHO cell monolayers (P less than 0.020), whereas colchicine (10(-6) M) had no effect. T. vaginalis kills target cells by direct contact without phagocytosis. This event requires intact trichomonad microfilament function; microtubule function appears not to be essential.

Rapid polymerization of Entamoeba histolytica actin induced by interaction with target cells

Within 5 s of challenge of Entamoeba histolytica trophozoites with red blood cells (RBC), attachment and deformation of target cells occurred at multiple sites on the amoeba surface. Many trophozoite-target interfaces were outlined with a ring of polymerized amoeba actin, revealed by rhodamine-phalloidin staining of glutaraldehyde-fixed and Triton-X 100-extracted cells. The beginnings of phagocytic pseudopods rimmed many targets. The phagocytic membrane and underlying actin network grew uniformly about a target cell, which became dramatically elongated and constricted, sometimes severed, as it entered the amoeba. Total engulfment of RBC targets occurred within 10 s. By methanol extraction and spectrofluorimetric measurement of bound rhodamine-phalloidin we were able to quantitate polymerized actin in amoebae. Interaction with target cells was accompanied by a net increase of up to twofold in the average polymerized actin content of trophozoites. This reached a maximum during the period of most active phagocytosis (4 min after challenge at 25 degrees C), and declined as phagocytic activity diminished (8-16 min). Challenge with latex beads of similar size and number, which E. histolytica phagocytized more slowly than RBC, induced neither a detectable increase in polymerized actin content nor appearance of polymerized actin at the contact interface. RBC inhibited phagocytosis of latex beads, but the reverse did not occur. The results demonstrate a rapid, recognition-specific stimulation of reorganization of the actin cytoskeleton of E. histolytica induced by binding to target cells. Vigorous phagocytic activity is frequently an immediate consequence of cell-cell contact, which emphasizes the importance of this process in the contact-mediated attack mechanism of this pathogen. The quantitative assay of polymerized actin may be useful in further studies of this mechanism.

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.