Lethal recognition between Entamoeba histolytica and the host tissues

Epigenetics in the unicellular parasite Entamoeba histolytica

Amoebiasis is a serious infectious disease that is caused by the unicellular parasite, Entamoeba histolytica. This parasite is mainly found in developing countries, and are named owing to its ability to destroy tissues. The molecular mechanisms that regulate the virulence of this parasite are not well understood. In recent years, an increasing interest in the epigenetic regulation of the parasite's virulence has emerged. In this article, an overview of our current knowledge about the role of DNA methylation, histone modifications and RNA-associated silencing in the biology of E. histolytica is provided. The relevance of some features of the parasite's unique epigenetic machinery to the development of new antiamoebic therapeutic molecules is discussed.

Inhibition of the complement membrane attack complex by the galactose-specific adhesion of Entamoeba histolytica

The human complement system is an important early host defense against infection. Entamoeba histolytica activates the complement system but is resistant to killing by complement C5b-9 complexes deposited on the membrane surface. Our aim was to identify components of the amebic plasma membrane that mediate resistance to human complement C5b-9 by screening for neutralizing monoclonal antibodies. A monoclonal antibody was identified that abrogated amebic resistance to C5b-9, and the mAb was shown to recognize the parasite's galactose-specific adhesin. The purified adhesin bound to C8 and C9 and conferred C5b-9 resistance to sensitive ameba upon reconstitution; these activities of the adhesin were inhibited by the antiadhesin mAb. The E. histolytica adhesin shared sequence similarities and antigenic cross-reactivity with CD59, a membrane inhibitor of C5b-9 in human blood cells, suggesting both molecular mimicry and shared complement-inhibitory functions.

The effect of immunosuppressive and immunostimulatory treatment on experimental amoebiasis

Immunosuppressive treatments consisting of ionizing irradiation or drugs were employed in inbred and outbred mice and in golden hamsters. Following treatment, mice were challenged intracaecally or intrahepatically with virulent, axenically-grown Entamoeba histolytica. Hamsters were challenged by intraperitoneal injection of the amoebae. Many of the experimental animals died of the combined effects of treatment and challenge. Mice remained essentially refractory to infection with E. histolytica regardless of the immunosuppressive means employed. Liver infection rates in treated and control hamsters were largely similar to one another, i.e. immunosuppressive treatment had no effect on resistance to infection. Our inability to alter the susceptibility of mice and hamsters to amoebic infection by suppressing components of the immune system does not enable us to draw any clearcut conclusions as to the effect of immunosuppression on human amoebiasis. Of the various immunostimulatory materials employed in hamsters, including polysaccharides, BCG and muramyl peptides, only glucan displayed protective capacity against infection with E. histolytica, making it an effective protective agent in an extracellular parasitic infection in addition to its published effectiveness in intracellular protozoal infections. Peritoneal cells extracted from hamsters injected intraperitoneally with E. histolytica seemed capable of reducing the infectivity of virulent amoebae after coincubation in vitro, as shown by reduced infection rates in challenged hamsters. Apparently polymorphonuclear cells, which constituted the vast majority of the extracted cells 24 hours after the stimulatory injection, can, under certain conditions, diminish the infectivity of E. histolytica.

Kinetics of the immune responses during the course of hepatic amoebic infection--an experimental study

Following intrahepatic inoculation of axenic Entamoeba histolytica (strain NIH:200V), all hamsters developed amoebic abscesses in the liver by 7 d after inoculation. All the animals cleared the infection spontaneously by 28 d after inoculation. The establishment of amoebae leading to acute liver abscess was accompanied by significantly reduced levels of antibodies to the plasma membrane (PM) protein of E. histolytica, reduced direct macrophage cytotoxicity, and reduced anti-PM antibody-mediated macrophage-dependent cytotoxicity to amoebic trophozoites. This phase of infection was also accompanied by a significant reduction in cellular sensitization to PM proteins and to phytohaemagglutinin. Reduction in size of the liver abscesses and clearance of infection were accompanied by significant development of anti-PM antibodies, cellular sensitization to the PM proteins, enhanced direct macrophage cytotoxicity, and anti-PM antibody-mediated macrophage-dependent cytotoxicity to trophozoites. The data suggest that the amoebic disease process is modulated by immune responses to the PM protein of E. histolytica.

Entamoeba histolytica: role of amebic proteinases and polymorphonuclear leukocytes in acute experimental amebiasis in the rat

The injection of 1 x 10(6) trophozoites of axenically grown Entamoeba histolytica strain HM-1 in the subcutaneous tissue of the rat results in an acute and self-limited inflammatory process, characterized by the early onset of conspicuous tissue necrosis and focal hemorrhage in the vicinity of the parasites, followed by infiltration with polymorphonuclear leukocytes. The process develops for 5-10 hr but during that period amebic trophozoites progressively disappear, leukocytes undergo degenerative changes, and the lesion tends to heal in 72-96 hr. In leukopenic animals (less than 1000 white blood cells/ml) tissue necrosis and hemorrhage are equally conspicuous in the neighborhood of amebas. Inhibition of amebic proteinase activity prior to injection by heat denaturation, p-hydroxy-mercuri-benzoate (PHMB), soybean trypsin inhibitor (STI), and human alpha-2-macroglobulin (alpha 2M), alone or in various combinations, results in absence or notorious decrease in tissue necrosis as well as in clearly diminished inflammatory reaction. This effect is particularly evident when cysteine proteinases are either specifically or generally inhibited. On the other hand, amebic proteinase inhibition with alpha 2M and STI does not interfere with the cell-killing capacity of trophozoites co-incubated in vitro for 2 hr with rat peritoneal cells enriched for macrophages. We conclude that in acute experimental amebiasis produced in the subcutaneous tissue of the rat, amebic cysteine (and perhaps other) proteinases are primarily responsible for necrosis and are also important, but not essential, for inflammation. We also suggest that in this model polymorphonuclear leukocytes are not required for tissue necrosis. Finally, in an in vitro model, the cell-killing capacity of amebas is not influenced by the proteinase activity of the parasite.

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.