Recognition of Entamoeba histolytica lipophosphoglycan by a strain-specific monoclonal antibody and human immune sera

Revisiting the isolation and characterisation of Entamoeba histolytica lipopeptidophosphoglycan

The parasite Entamoeba histolytica is the cause of amoebic dysentery and liver abscess in humans. On the protozoan cell surface, a variety of glycosylated molecules are involved in the interaction with the environment, such as attachment to the colonic mucus. One of these molecules is the lipopeptidophosphoglycan (LPPG), a complex surface component with antigenic properties. Its structure is only partly known, it is a glycosylphosphatidylinositol (GPI)-linked glycoprotein with a large amount of O-glycosylation. To date, the sequence of a core protein has not been identified. In this study, we further investigated this complex surface molecule aided by the availability of the monoclonal antibody EH5, which had been raised in our laboratory. We studied the extraction of LPPG in various solvent mixtures and discovered that 2-butanol saturated water was simple and superior to other solvents used in the past. The isolated LPPG was subjected to treatment with several proteases and the Ser/Thr specific cleavage agent scandium (III) trifluoromethanesulfonate (scandium triflate). The products were probed with antibody EH5 and the blots showed that the LPPG preparation was largely resistant to standard proteases, but could be cleaved by the scandium compound. These observations could point to the existence of a Ser- or Thr-rich core protein structure.

The antiretroviral lectin cyanovirin-N targets well-known and novel targets on the surface of Entamoeba histolytica trophozoites

Entamoeba histolytica, the protist that causes amebic dysentery and liver abscess, has a truncated Asn-linked glycan (N-glycan) precursor composed of seven sugars (Man(5)GlcNAc(2)). Here, we show that glycoproteins with unmodified N-glycans are aggregated and capped on the surface of E. histolytica trophozoites by the antiretroviral lectin cyanovirin-N and then replenished from large intracellular pools. Cyanovirin-N cocaps the Gal/GalNAc adherence lectin, as well as glycoproteins containing O-phosphodiester-linked glycans recognized by an anti-proteophosphoglycan monoclonal antibody. Cyanovirin-N inhibits phagocytosis by E. histolytica trophozoites of mucin-coated beads, a surrogate assay for amebic virulence. For technical reasons, we used the plant lectin concanavalin A rather than cyanovirin-N to enrich secreted and membrane proteins for mass spectrometric identification. E. histolytica glycoproteins with occupied N-glycan sites include Gal/GalNAc lectins, proteases, and 17 previously hypothetical proteins. The latter glycoproteins, as well as 50 previously hypothetical proteins enriched by concanavalin A, may be vaccine targets as they are abundant and unique. In summary, the antiretroviral lectin cyanovirin-N binds to well-known and novel targets on the surface of E. histolytica that are rapidly replenished from large intracellular pools.

Invasive amebiasis: a microcirculatory disorder?

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

Natural killer T cells activated by a lipopeptidophosphoglycan from Entamoeba histolytica are critically important to control amebic liver abscess

The innate immune response is supposed to play an essential role in the control of amebic liver abscess (ALA), a severe form of invasive amoebiasis due to infection with the protozoan parasite Entamoeba histolytica. In a mouse model for the disease, we previously demonstrated that Jalpha18(-/-) mice, lacking invariant natural killer T (iNKT) cells, suffer from more severe abscess development. Here we show that the specific activation of iNKT cells using alpha-galactosylceramide (alpha-GalCer) induces a significant reduction in the sizes of ALA lesions, whereas CD1d(-/-) mice develop more severe abscesses. We identified a lipopeptidophosphoglycan from E. histolytica membranes (EhLPPG) as a possible natural NKT cell ligand and show that the purified phosphoinositol (PI) moiety of this molecule induces protective IFN-gamma but not IL-4 production in NKT cells. The main component of EhLPPG responsible for NKT cell activation is a diacylated PI, (1-O-[(28:0)-lyso-glycero-3-phosphatidyl-]2-O-(C16:0)-Ins). IFN-gamma production by NKT cells requires the presence of CD1d and simultaneously TLR receptor signalling through MyD88 and secretion of IL-12. Similar to alpha-GalCer application, EhLPPG treatment significantly reduces the severity of ALA in ameba-infected mice. Our results suggest that EhLPPG is an amebic molecule that is important for the limitation of ALA development and may explain why the majority of E. histolytica-infected individuals do not develop amebic liver abscess.

Lipopopeptidephosphoglycan from Entamoeba histolytica activates human macrophages and dendritic cells and reaches their late endosomes

Lipopopeptidephosphoglycan (LPPG) is a complex macromolecule from the surface of Entamoeba histolytica trophozoites. We analysed the interaction between LPPG and human macrophages and dendritic cells (DCs) and found that LPPG is internalized by these cells and activates them. The internalization process involves intracellular traffic from the cell membrane to late endosomes, as shown by co-localization of LPPG with late endosomes marked with FITC-dextran and LAMP-1. LPPG-activated DCs have increased expression of co-stimulatory molecules CD80, CD86 and CD40 and produce pro-inflammatory cytokines TNF-alpha, IL-8 and IL-12. Taken together, these results show that LPPG activates antigen-presenting cells and reaches intracellular compartments that are involved in antigen presentation.

Reduction of cell surface glycosylphosphatidylinositol conjugates in Entamoeba histolytica by antisense blocking of E. histolytica GlcNAc-phosphatidylinositol deacetylase expression: effect on cell proliferation, endocytosis, and adhesion to target cells

Glycosylphosphatidylinositol (GPI)-anchored molecules such as cell surface Gal/GalNAc lectin and proteophosphoglycans of the protozoan parasite Entamoeba histolytica are thought to be involved in pathogenesis. Here, we report the identification of genes that may be involved in the GPI biosynthetic pathway of E. histolytica by use of bioinformatic tools applied to the recently published genome sequence. Of the genes identified, one of the early genes, GlcNAc-phosphatidylinositol deacetylase (PIG-L), was partially characterized. Cell lines deficient in E. histolytica PIG-L (EhPL-AS) or overproducing it (EhPL-S) were generated by expressing the gene in the antisense or sense orientation, respectively, in a tetracycline-inducible system. The overexpressing cells showed higher EhPIG-L activity and increased production of GlcN-PI. Conversely, cells expressing the antisense RNA displayed reduced GlcN-PI production. The total number of GPI-containing molecules was also reduced in these cells, as demonstrated by Alexa 488 fluorescently labeled proaerolysin labeling. The distribution of GPI-linked PPG and Gal/GalNAc lectin was altered in the tetracycline-induced EhPL-AS cell lines. Further, the antisense-blocked cells showed 36% suppression of cell growth, 50 to 60% inhibition of fluid phase endocytosis, and about 50% inhibition of adhesion to target cells. Therefore, our data suggest the importance of GPI anchors in regulating some of the events in amoebic pathogenesis. They also demonstrated the use of antisense RNA-mediated inhibition of GPI biosynthetic enzymes as an approach to decrease the amount of GPI conjugates in E. histolytica.

The innate immune response to Entamoeba histolytica lipopeptidophosphoglycan is mediated by toll-like receptors 2 and 4

Entamoeba histolytica is a human pathogen that may invade the intestinal mucosa, causing amoebic colitis or hepatic abscesses when the trophozoites travel through the portal circulation to the liver. Lipopeptidophosphoglycan (LPPG) is a molecular pattern of E. histolytica recognized by the human immune system. Here we report that LPPG is exposed on the cell surface of E. histolytica trophozoites, and is recognized by the host through toll-like receptor (TLR) 2 and TLR4. Correspondingly, human embryonic kidney (HEK)-293 cells were rendered LPPG responsive through overexpression of TLR2 or TLR4/MD2. Moreover, co-expression of CD14 enhanced LPPG signal transmission through TLR2 and TLR4. The interaction of LPPG with TLR2 and TLR4 resulted in activation of NF-kappaB and release of interleukin (IL)-10, IL-12p40, tumour necrosis factor (TNF)-alpha, and IL-8 from human monocytes. Consistent with these findings, responsiveness of mouse macrophages lacking TLR2 expression (TLR2-/-) or functional TLR4 (TLR4d/d) to E. histolytica LPPG challenge was impaired while double deficient macrophages were unresponsive. In contrast to wild-type control and TLR2-/- animals succumbing to lethal shock syndrome, TLR4d/d mice were resistant to systemic LPPG challenge-induced pathology.

The pathogenicity of Entamoeba histolytica is related to the capacity of evading innate immunity

The host and parasite factors that influence susceptibility to Entamoeba histolytica infection and disease are not well understood. Entamoeba histolytica pathogenicity has been considered by focusing principally on parasite rather than host factors. Thus, research has concentrated on explaining the molecular differences between pathogenic E. histolytica and non-pathogenic E. dispar. However, the amoeba molecules considered most important for host tissue destruction (amoebapore, galactose/N-acetyl galactosamine inhibitable lectin, and cysteine proteinases) are present in both pathogenic E. histolytica and non-pathogenic E. dispar. In addition, the genetic differences in pathogenicity among E. histolytica isolates are unlikely to completely explain the different outcomes of infection. Considering that the principal difference between pathogenic and non-pathogenic amoebas lies in their surface coats, we propose that pathogenicity of the amoebas is related to the composition and properties of the surface coat components (or pathogen-associated molecular patterns, PAMPs), and the ability of innate immune response to recognize these components and eliminate the parasite. According to this hypothesis, a key feature that may distinguish pathogenic (E. histolytica) from non-pathogenic (E. dispar) strains is whether or not they can overcome innate immune defences. A corollary of this hypothesis is that in susceptible individuals the PAMPs are either not recognized or they are recognized by a set of Toll-like receptors (TLRs) that leads to an inflammatory response. In both cases, the result is tissue damage. On the contrary, in resistant individuals the innate/inflammatory response, induced through the activation of a different set of TLRs, eliminates the parasite.

Biosynthesis of Entamoeba histolytica proteophosphoglycan in vitro

A complex glycoconjugate proteophosphoglycan (PPG) is present on the surface of the pathogenic protozoan parasite Entamoeba histolytica but not in the non-pathogenic Entamoeba dispar. It is thought to be an important molecule involved in pathogenesis. In order to study its biosynthesis, an in vitro cell-free system was developed. The specificity of the system was demonstrated by various criteria including immunoprecipitation by a specific monoclonal antibody. The in vitro synthesized molecule was found to be susceptible to mild acid hydrolysis, digestion by phosphoinositol-specific phospholipase C and nitrous acid deamination, the salient features for a PPG-like molecule. The in vitro product was not synthesized when heat-treated cellular-extract was used in the assay or when the cell extract was prepared from Entamoeba invadens, a species that lacks these glycoconjugates. Analysis of the glycan side chains of the in vitro synthesized product by thin layer chromatography revealed side chains of variable sizes including a fraction greater than six glycan units. The crude membranes used in the cell-free system were further fractionated by sucrose density gradient centrifugation. The fraction containing the PPG synthesizing activity when used in the assay resulted in a 10-fold increase in specific activity. Development of this cell-free system will facilitate further studies on the nature of intracellular organelles and the pathways that are involved in PPG biosynthesis.

Molecular changes in Entamoeba histolytica in response to bacteria

Entamoeba histolytica, the protozoan parasite, is the causative agent of amoebiasis. The degree of virulence, as inferred from invasiveness, of potentially pathogenic strains may be regulated by both host and parasite factors that determine the gut environment. One such factor that plays an important role is the bacterial flora in the gut. Previous studies have clearly shown that bacterial flora is an important determinant of virulence in E. histolytica. However, the exact nature of changes induced in E. histolytica in response to bacteria and their role in virulence is not clear. In this study the levels of a number of molecules potentially important in virulence mechanisms were determined in E. histolytica cells grown with and without normal human bacterial flora, using enzyme-linked immunosorbent assay. Significant changes were observed only after the E. histolytica cells had been adapted to grow with bacterial flora for a number of generations, and not in short term culture.

Protection against invasive amebiasis by a single monoclonal antibody directed against a lipophosphoglycan antigen localized on the surface of Entamoeba histolytica

A panel of monoclonal antibodies was raised from mice immunized with a membrane preparation from Entamoeba histolytica, the pathogenic species causing invasive amebiasis in humans. Antibody EH5 gave a polydisperse band in immunoblots from membrane preparations from different E. histolytica strains, and a much weaker signal from two strains of the nonpathogenic species Entamoeba dispar. Although the exact chemical structure of the EH5 antigen is not yet known, the ability of the antigen to be metabolically radiolabeled with [32P]phosphate or [3H]glucose, its sensitivity to digestion by mild acid and phosphatidylinositol-specific phospholipase C, and its specific extraction from E. histolytica trophozoites by a method used to prepare lipophosphoglycans from Leishmania showed that it could be classified as an amebal lipophosphoglycan. Confocal immunofluorescence and immunogold labeling of trophozoites localized the antigen on the outer face of the plasma membrane and on the inner face of internal vesicle membranes. Antibody EH5 strongly agglutinated amebas in a similar way to concanavalin A (Con A), and Con A bound to immunoaffinity-purified EH5 antigen. Therefore, surface lipophosphoglycans may play an important role in the preferential agglutination of pathogenic amebas by Con A. The protective ability of antibody EH5 was tested in a passive immunization experiment in a severe combined immunodeficient (SCID) mouse model. Intrahepatic challenge of animals after administration of an isotype-matched control antibody or without treatment led to the development of a liver abscess in all cases, whereas 11 out of 12 animals immunized with the EH5 antibody developed no liver abscess. Our results demonstrate the importance and, for the first time, the protective capacity of glycan antigens on the surface of the amebas.

Characterization of EhCaBP, a calcium-binding protein of Entamoeba histolytica and its binding proteins

A novel calcium-binding protein (EhCaBP) has been recently identified and characterized from the protozoan parasite Entamoeba histolytica. In order to decipher the function of this protein, a few basic properties were investigated and compared with the ubiquitous Ca(2+)-signal transducing protein calmodulin (CaM). Indirect immunofluorescence and immunoprecipitation analyses using specific antibodies against EhCaBP suggest that it is a soluble cytoplasmic protein with no major post-translational modification. EhCaBP did not stimulate cAMP-phosphodiesterase activity, differentiating it from all known CaMs. Affinity chromatography of [35S]methionine-labelled proteins of E. histolytica trophozoites using EhCaBP-sepharose column showed Ca(2+)-dependent binding of a group of proteins. Radiolabelled proteins from the same extract also bound to CaM-sepharose. However, the proteins bound to the two columns were different as revealed by sodium dodecyl sulphate polyacrylamide gel electrophoresis. At least one of the EhCaBP-binding proteins became phosphorylated as revealed by in vivo phosphorylation analysis. The binding-proteins could not be detected in E. invadens (a species that is pathogenic in reptiles) and E. moshkovskii (which is found in the human gut but is not pathogenic), two species in which EhCaBP-like protein has not been found. Two distinct Ca(2+)-dependent protein kinases, which get activated by EhCaBP and CaM respectively, were detected in E. histolytica. These kinases require different levels of Ca2+ for their maximal activities. Affinity chromatography also showed the binding of protein kinase(s) to EhCaBP in a Ca(2+)-dependent manner. Our data suggest that there may be novel Ca(2+)-signal transduction pathway in E. histolytica mediated by EhCaBP.

Lipophosphoglycan is present in distinctly different form in different Entamoeba histolytica strains and absent in Entamoeba moshkovskii and Entamoeba invadens

Lipophosphoglycan has recently been demonstrated on the cell surface of Entamoeba histolytica strain HM-1:IMSS. A monoclonal antibody against this molecule had failed to react with some other strains of E. histolytica, including the strain Rahman. To determine if a structurally distinct lipophosphoglycan existed in Rahman, [3H]galactose-labeled glycoconjugates were electrophoresed through sodium dodecyl sulfate polyacrylamide gel electrophoresis. The electrophoretic pattern in Rahman was very different compared to that obtained with strains HM-1:IMSS and 200:NIH. A number of experiments including sensitivity to mild acid, nitrous acid and phosphoinositol-specific phospholipase C suggest that the Rahman glycoconjugate is indeed a lipophosphoglycan-like molecule but distinctly different from that of HM-1:IMSS. Mild acid-treated glycoconjugates from Rahman and HM-1:IMSS revealed the presence of neutral trisaccharides and monosaccharides in Rahman but not in HM-1:IMSS. Human immune sera from amoebiasis patients and a polyclonal antibody against HM-1:IMSS lipophosphoglycan both recognized Rahman glycoconjugate. Thus, while lipophosphoglycan molecules from the two strains share common epitopes, they are clearly distinct from each other. Molecules bearing resemblance to lipophosphoglycan could not be detected in other Entamoeba species, namely Entamoeba invadens and Entamoeba moshkovskii.