Luminal bacteria and proteases together decrease adherence of Entamoeba histolytica trophozoites to Chinese hamster ovary epithelial cells: a novel host defence against an enteric pathogen

The Biological Fight Against Pathogenic Bacteria and Protozoa

The animal gastrointestinal tract is a tube with two open ends; hence, from the microbial point of view it constitutes an open system, as opposed to the circulatory system that must be a tightly closed microbial-free environment. In particular, the human intestine spans ca. 200 m² and represents a massive absorptive surface composed of a layer of epithelial cells as well as a paracellular barrier. The permeability of this paracellular barrier is regulated by transmembrane proteins known as claudins that play a critical role in tight junctions.

Clinical manifestations and endoscopic findings of amebic colitis in a United States-Mexico border city: a case series

Background

Invasive amebiasis is not frequently seen in the United States. It is associated with considerable morbidity in patients residing in or traveling to endemic areas. We report a case series of patients with amebic colitis in a United States-Mexico border city to alert physicians to the varied clinical manifestations.

Case presentation

Nine patients were diagnosed with amebic colitis. Mean age was 56 (38–83), 6 were males, and all were Hispanic. Common symptoms were diarrhea (56 %), hematochezia (33 %) and abdominal bloating (11 %). The diagnosis of amebic colitis was established in the following ways: 8 patients by colonoscopy with biopsy, 1 by surgery for colonic obstruction. The diagnosis of amebic colitis was confirmed in 8 patients (89 %) by amebic trophozoites present in histopathologic sections. One patient was diagnosed with amebic colitis based upon clinical symptoms, colitis on colonoscopy and visualization of amebic trophozoites on stool examination. In the 8 patients in whom colonoscopy was done, 6 (75 %) had inflammation with rectosigmoid involvement and 5 (62.5 %) had ulcerations. Infection resolved after treatment with metronidazole in most patients; however, one patient developed a liver abscess and another had a colonic perforation and later developed a liver abscess.

Conclusion

The occurrence of amebic colitis in this United States-Mexico border city hospital population was low, but in some cases potentially life-threatening. Physicians should be alert to the less common presentations of amebic colitis, such as overt gastrointestinal bleeding, exacerbation of inflammatory bowel disease, and the incidental finding of association with colon cancer, or a surgical abdomen. Rectosigmoid involvement was typically found on colonoscopy.

Molecular nature of virulence in Entamoeba histolytica

For many years virulence of pathogenic Entamoeba histolytica has been attributed to the capacity of the parasite to destroy tissues through the expression and/or secretion of various molecules. Such view is supported mainly by in vitro experimentation, whereas data obtained by using animal models of the disease have clearly demonstrated that the host's inflammatory response is primarily responsible for tissue damage. This review analyzes the content and/or activity of some of the presumed toxic amebic molecules present in amebic strains with different degrees of virulence compared to various parasite in vitro functions that are supposed to correlate with in vivo virulence. The analysis suggests that amebic virulence is primarily determined by the parasite's capacity to adapt and survive the aerobic conditions present in animal tissues. This initial episode in the host-parasite relationship is an absolute requirement for the further development of tissue lesions, which result from the concerted action of many molecules derived from both, the host and the parasite.

Entamoeba histolytica: oxygen resistance and virulence

Entamoeba histolytica virulence has been attributed to several amoebic molecules such as adhesins, amoebapores and cysteine proteinases, but supporting evidence is either partial or indirect. In this work we compared several in vitro and in vivo features of both virulent E. histolytica (vEh) and non-virulent E. histolytica (nvEh) axenic HM-1 IMSS strains, such as complement resistance, proteinase activity, haemolytic, phagocytic and cytotoxic capacities, survival in mice caecum, and susceptibility to O(2). The only difference observed was a higher in vitro susceptibility of nvEh to O(2). The molecular mechanism of that difference was analyzed in both groups of amoebae after high O(2) exposure. vEh O(2) resistance correlated with: (i) higher O(2) reduction (O(2)(-) and H(2)O(2) production); (ii) increased H(2)O(2) resistance and thiol peroxidase activity, and (iii) reversible pyruvate: ferredoxin oxidoreductase (PFOR) inhibition. Despite the high level of carbonylated proteins in nvEh after O(2) exposure, membrane oxidation by reactive oxygen species was not observed. These results suggest that the virulent phenotype of E. histolytica is related to the greater ability to reduce O(2) and H(2)O(2) as well as PFOR reactivation, whereas nvEh undergoes irreversible PFOR inhibition resulting in metabolic failure and amoebic death.

Crosstalk at the initial encounter: interplay between host defense and ameba survival strategies

The host-parasite relationship is based on a series of interplays between host defense mechanisms and parasite survival strategies. Progress has been made in understanding the role of host immune response in amebiasis. While host cells elaborate diverse mechanisms for pathogen expulsion, amebae have also developed complex strategies to modulate host immune response and facilitate their own survival. This paper will give an overview of current research on the mutual interactions between host and Entamoeba histolytica in human and experimental amebiasis. Understanding this crosstalk is crucial for the effective design and implementation of new vaccines and drugs for this leading parasitic disease.

Luminal host-defense mechanisms against invasive amebiasis

Most humans infected with the virulent protozoan parasite Entamoeba histolytica do not develop invasive disease. Available evidence indicates that beneficial bacteria and the mucus gel layer in the colon lumen protect the host mucosa. Glycosidases produced by some normal colonic bacteria and luminal proteases degrade the key adherence lectin on E. histolytica trophozoites and decrease their adherence to epithelial cells. The mucus gel layer prevents those trophozoites that escape the hydrolases from reaching the epithelial cells. Trophozoite mucosal invasion is triggered only when both protective mechanisms are lost, as might occur during an unrelated pathogenic enteric bacterial infection. A newly developed gnotobiotic model of intestinal amebiasis should enable testing of this hypothesis and provide clues to help design practical studies in humans.

The role of mucins in host-parasite interactions. Part I-protozoan parasites

Parasite-derived mucin-like molecules might be involved in parasite attachment to and invasion of host cells. In addition, parasites might secrete mucin-degrading enzymes, enabling the penetration of protective mucus gels that overlie the mucosal surfaces of their potential hosts. Furthermore, they might generate binding ligands on the membrane-bound mucins of host cells by using specific glycosidases. It is possible that host mucins and mucin-like molecules prevent the establishment of parasites or facilitate parasite expulsion. They might also serve as a source of metabolic energy and adhesion ligands for those parasites adapted to exploit them. Sally Hicks and colleagues here review the biochemical properties of mucins and mucin-like molecules in relation to interactions (established and putative) between protozoan parasites and their hosts.

The major surface antigens of Entamoeba histolytica trophozoites are GPI-anchored proteophosphoglycans

Trophozoites of the parasitic protozoa, Entamoeba histolytica, synthesize a cell surface lipoglycoconjugate, termed lipophosphoglycan, which is thought to be an important virulence factor and potential vaccine candidate against invasive amebiasis. Here, we show that the E. histolytica lipophosphoglycans are in fact glycosylphosphatidylinositol (GPI)-anchored proteophosphoglycans (PPGs). These PPGs contain a highly acidic polypeptide component which is rich in Asp, Glu and phosphoserine residues. This polypeptide component is extensively modified with linear glycan chains having the general structure, [Glcalpha1-6](n)Glcbeta1-6Gal (where n=2-23). These glycan chains can be released after mild-acid hydrolysis with trifluoroacetic or hydrofluoric acid and are probably attached to phosphoserine residues in the polypeptide backbone. The PPGs are further modified with a GPI anchor which differs from all other eukaryotic GPI anchors so far characterized in containing a glycan core with the structure, Gal(1)Man(2)GlcN-myo-inositol, and in being heterogeneously modified with chains of alpha-galactose. Trophozoites of the pathogenic HM-1:IMSS strain synthesize two distinct classes of PPG which have polydisperse molecular masses of 50-180 kDa (PPG-1) and 35-60 kDa (PPG-2) and are modified with glucan side-chains of different average lengths. In contrast, the non-pathogenic Rahman strain synthesizes one class of PPG which is only elaborated with short disaccharide side-chains (i.e. Glcbeta1-6Gal). However, the PPGs are abundant in all strains (8x10(7) copies per cell) and are likely to form a protective surface coat.