Protection of gerbils from amebic liver abscess by immunization with a recombinant Entamoeba histolytica antigen

Evaluation of the PEΔIII-LC3-KDEL3 Chimeric Protein of Entamoeba histolytica-Lectin as a Vaccine Candidate against Amebic Liver Abscess

Entamoeba histolytica is an intestinal parasite that causes dysentery and amebic liver abscess. E. histolytica has the capability to invade host tissue by union of virulence factor Gal/GalNAc lectin; this molecule induces an adherence-inhibitory antibody response as well as to protect against amebic liver abscess (ALA). The present work showed the effect of the immunization with PEΔIII-LC3-KDEL3 recombinant protein. In vitro, this candidate vaccine inhibited adherence of E. histolytica trophozoites to HepG2 cell monolayer, avoiding the cytolysis, and in a hamster model, we observed a vaccine-induced protection against the damage to tissue liver and the inhibition of uncontrolled inflammation. PEΔIII-LC3-KDEL3 reduced the expression of TNF-α, IL-1β, and NF-κB in all immunized groups at 4- and 7-day postinfection. The levels of IL-10, FOXP3, and IFN-γ were elevated at 7 days. The immunohistochemistry assay confirmed this result, revealing an elevated quantity of +IFN-γ cells in the liver tissue. ALA formation in hamsters immunized was minimal, and few trophozoites were identified. Hence, immunization with PEΔIII-LC3-KDEL3 herein prevented invasive amebiasis, avoided an acute proinflammatory response, and activated a protective response within a short time. Finally, this recombinant protein induced an increase of serum IgG.

Genetic Diversity and Gene Family Expansions in Members of the Genus Entamoeba

Amoebiasis is the third-most common cause of mortality worldwide from a parasitic disease. Although the primary etiological agent of amoebiasis is the obligate human parasite Entamoeba histolytica, other members of the genus Entamoeba can infect humans and may be pathogenic. Here, we present the first annotated reference genome for Entamoeba moshkovskii, a species that has been associated with human infections, and compare the genomes of E. moshkovskii, E. histolytica, the human commensal Entamoeba dispar, and the nonhuman pathogen Entamoeba invadens. Gene clustering and phylogenetic analyses show differences in expansion and contraction of families of proteins associated with host or bacterial interactions. They intimate the importance to parasitic Entamoeba species of surface-bound proteins involved in adhesion to extracellular membranes, such as the Gal/GalNAc lectin and members of the BspA and Ariel1 families. Furthermore, E. dispar is the only one of the four species to lack a functional copy of the key virulence factor cysteine protease CP-A5, whereas the gene's presence in E. moshkovskii is consistent with the species' potentially pathogenic nature. Entamoeba moshkovskii was found to be more diverse than E. histolytica across all sequence classes. The former is ∼200 times more diverse than latter, with the four E. moshkovskii strains tested having a most recent common ancestor nearly 500 times more ancient than the tested E. histolytica strains. A four-haplotype test indicates that these E. moshkovskii strains are not the same species and should be regarded as a species complex.

Target identification and intervention strategies against amebiasis

Entamoeba histolytica is the etiological agent of amebiasis, which is an endemic parasitic disease in developing countries and is the cause of approximately 70,000 deaths annually. E. histolytica trophozoites usually reside in the colon as a non-pathogenic commensal in most infected individuals (90% of infected individuals are asymptomatic). For unknown reasons, these trophozoites can become virulent and invasive, cause amebic dysentery, and migrate to the liver where they cause hepatocellular damage. Amebiasis is usually treated either by amebicides which are classified as (a) luminal and are active against the luminal forms of the parasite, (b) tissue and are effective against those parasites that have invaded tissues, and (c) mixed and are effective against the luminal forms of the parasite and those forms which invaded the host's tissues. Of the amebicides, the luminal amebicide, metronidazole (MTZ), is the most widely used drug to treat amebiasis. Although well tolerated, concerns about its adverse effects and the possible emergence of MTZ-resistant strains of E. histolytica have led to the development of new therapeutic strategies against amebiasis. These strategies include improving the potency of existing amebicides, discovering new uses for approved drugs (repurposing of existing drugs), drug rediscovery, vaccination, drug targeting of essential E. histolytica components, and the use of probiotics and bioactive natural products. This review examines each of these strategies in the light of the current knowledge on the gut microbiota of patients with amebiasis.

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

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

The future for vaccine development against Entamoeba histolytica

Entamoeba histolytica is the causative agent of amebiasis, one of the top three parasitic causes of mortality worldwide. In the majority of infected individuals, E. histolytica asymptomatically colonizes the large intestine, while in others, the parasite breaches the mucosal epithelial barrier to cause amebic colitis and can disseminate to soft organs to cause abscesses. Vaccinations using native and recombinant forms of the parasite Gal-lectin have been successful in protecting animals against intestinal amebiasis and amebic liver abscess. Protection against amebic liver abscesses has also been reported by targeting other E. histolytica components including the serine-rich protein and the 29-kDa-reductase antigen. To date, vaccines against the Gal-lectin hold the most promise but clinical trials will be required to validate its efficacy in humans. Here, we review the current strategies and future perspectives involved in the development of a vaccine against E. histolytica.

Obtaining of three recombinant antigens of Entamoeba histolytica and evaluation of their immunogenic ability without adjuvant in a hamster model of immunoprotection

A 30-kDa surface collagen binding protein peroxiredoxin of Entamoeba histolytica (EhCBP30) was evaluated either alone or fused to the chaperone (CHP) or ATPase (ATP) domains of heat shock protein 70 of Trypanosoma cruzi (TcHSP70) as a vaccine candidate in a hamster model of experimental amoebic liver abscess (ALA) development. Three constructs were produced containing the EhCBP30 DNA sequence, one expressing EhCBP30 and two expressing EhCBP30 fused to either CHP or ATP domains of TcHSP70. High purity recombinant proteins rEhCBP30, rEhCBP30-CHP and rEhCBP30-ATP with N-terminal His tag were obtained by single step affinity purification. Hamsters were immunized without adjuvant with the antigenic recombinant proteins and then challenged intrahepatically with E. histolytica trophozoites. A 70% decrease in ALA development was detected in hamsters immunized with rEhCBP30 and rEhCBP30-CHP, while animals immunized with rEhCBP30-ATP did not show a statistically significant decrease in ALA formation compared with non-immunized animals. Histological analysis of liver tissue showed that the inflammatory infiltrate was discrete or moderate in hamsters immunized with rEhCBP30 or rEhCBP30-CHP compared with that observed in control hamsters or hamsters immunized with rEhCBP30-ATP. These results suggest that rEhCBP30 and rEhCBP30-CHP are able to induce an effective immune response that may protect hamsters against ALA development.

Efficacy of a Gal-lectin subunit vaccine against experimental Entamoeba histolytica infection and colitis in baboons (Papio sp.)

To determine the efficacy of a Gal-lectin based intranasal synthetic peptide vaccine, we developed a new experimental primate model of Entamoeba histolytica intestinal infection. Release of xenic E. histolytica trophozoites (5×10(6)) into the small bowel of baboons (Papio sp.) resulted in a rapid intestinal anti-amebic antibody response and a brief infection; however, release of trophozoites directly into the cecum (5 baboons) elicited a sustained E. histolytica infection, as determined by quantitative fecal PCR, and an ulcerative, inflammatory colitis observed on colonoscopy and histopathology. In three controlled experiments, baboons received four immunizations at seven day intervals of 1600 μg of the vaccine/nostril, with Cholera toxin, 20 μg/nostril as adjuvant; vaccinated (n=6) and control baboons (n=6) baboons were then challenged via colonoscopy with xenic trophozoites (5×10(6)). During 90 days of follow up, 250 of 415 (60.24%) fecal samples in control baboons had a (+) PCR for E. histolytica, compared to only 36 of 423 (8.51%) samples from vaccinated baboons (P<0.001). All 6 vaccinated baboons were free of infection by the 51st day after challenge, 5 of 6 controls positive had (+) fecal PCRs for up to 126 days post-challenge (P=0.019). Inflammatory colitis developed in 4 of 6 control baboons post-challenge, with invasive E. histolytica trophozoites present in 2 of the 4 on histopathology. There was no evidence of inflammatory colitis or parasite invasion in any of the vaccinated baboons; there was a strong inverse correlation between positive ELISA OD value indicating the presence of intestinal anti-peptide IgA antibodies and baboons having a positive fecal PCR CT value, P<0.001. In conclusion, we developed a novel primate model of E. histolytica intestinal infection and demonstrated that a Gal-lectin-based intranasal synthetic peptide vaccine was highly efficacious in preventing experimental E. histolytica infection and colitis in baboons.

Mucosal delivery of ACNPV baculovirus driving expression of the Gal-lectin LC3 fragment confers protection against amoebic liver abscess in hamster

Mucosal vaccination against amoebiasis using the Gal-lectin of E. histolytica has been proposed as one of the leading strategies for controlling this human disease. However, most mucosal adjuvants used are toxic and the identification of safe delivery systems is necessary. Here, we evaluate the potential of a recombinant Autographa californica baculovirus driving the expression of the LC3 fragment of the Gal-lectin to confer protection against amoebic liver abscess (ALA) in hamsters following oral or nasal immunization. Hamsters immunized by oral route showed complete absence (57.9%) or partial development (21%) of ALA, resulting in some protection in 78.9% of animals when compared with the wild type baculovirus and sham control groups. In contrast, nasal immunization conferred only 21% of protection efficacy. Levels of ALA protection showed lineal correlation with the development of an anti-amoebic cellular immune response evaluated in spleens, but not with the induction of seric IgG anti-amoeba antibodies. These results suggest that baculovirus driving the expression of E. histolytica vaccine candidate antigens is useful for inducing protective cellular and humoral immune responses following oral immunization, and therefore it could be used as a system for mucosal delivery of an anti-amoebic vaccine.

The Jacob2 lectin of the Entamoeba histolytica cyst wall binds chitin and is polymorphic

Background

The infectious and diagnostic form of Entamoeba histolytica (Eh), cause of amebic dysentery and liver abscess, is the quadranucleate cyst. The cyst wall of Entamoeba invadens (Ei), a model for Eh, is composed of chitin fibrils and three sets of chitin-binding lectins that cross-link chitin fibrils (multivalent Jacob lectins), self-aggregate (Jessie lectins), and remodel chitin (chitinase). The goal here was to determine how well the Ei model applies to Entamoeba cysts from humans.

Methods/Results

An Eh Jacob lectin (EhJacob2) has three predicted chitin-binding domains surrounding a large, Ser-rich spacer. Recombinant EhJacob2 made in transfected Eh trophozoites binds to particulate chitin. Sequences of PCR products using primers flanking the highly polymorphic spacer of EhJacob2 may be used to distinguish Entamoeba isolates. Antibodies to the EhJacob2, EhJessie3, and chitinase each recognize cyst walls of clinical isolates of Entamoeba. While numerous sera from patients with amebic intestinal infections and liver abscess recognize recombinant EhJacob1 and EhJessie3 lectins, few of these sera recognize recombinant EhJacob2.

Conclusions/Significance

The EhJacob2 lectin binds chitin and is polymorphic, and Jacob2, Jessie3, and chitinase are present in cyst walls of clinical isolates of Entamoeba. These results suggest there are substantial similarities between cysts of the human pathogen (Eh) and the in vitro model (Ei), even though there are quantitative and qualitative differences in their chitin-binding lectins.

For many years, we and others have used cysts of Entamoeba invadens (Ei), a reptilian parasite, to model the infectious and diagnostic cysts of the human pathogen Entamoeba histolytica (Eh). The Ei cyst wall is composed of chitin fibrils, as well as Jacob and Jessie lectins that have unique chitin-binding domains. Our recent results suggest a “wattle and daub” model of the Ei cyst wall, where the wattle or sticks (chitin fibrils bound by multivalent Jacob lectins) is constructed prior to the addition of the mortar or daub (self-aggregating Jessie3 lectins). Here we “humanize” the Ei model of the cyst wall with four findings. First, a recombinant Eh Jacob2 lectin, which has three predicted chitin-binding domains surrounding a large spacer domain, binds chitin beads. Second, polymorphisms in the spacer domain of EhJacob2 discriminate clinical isolates of Entamoeba. Third, chitinase, Jacob2 lectin, and Jessie3 lectin are present in cyst walls of clinical isolates of Entamoeba. Finally, numerous sera from patients infected with Entamoeba recognize recombinant Eh Jacob1 and Jessie3 lectins.

Participation of the serine-rich Entamoeba histolytica protein in amebic phagocytosis of apoptotic host cells

Entamoeba histolytica is an intestinal ameba that causes dysentery and liver abscesses. Cytotoxicity and phagocytosis of host cells characterize invasive E. histolytica infection. Prior to phagocytosis of host cells, E. histolytica induces apoptotic host cell death, using a mechanism that requires contact via an amebic galactose-specific lectin. However, lectin inhibition only partially blocks phagocytosis of already dead cells, implicating at least one additional receptor in phagocytosis. To identify receptors for engulfment of apoptotic cells, monoclonal antibodies against E. histolytica membrane antigens were screened for inhibition of phagocytosis. Of 43 antibodies screened, one blocked lectin-independent uptake of apoptotic cells, with >90% inhibition at a dose of 20 microg/ml (P < 0.0003 versus control). The same antibody also inhibited adherence to apoptotic lymphocytes and, to a lesser extent, adherence to and killing of viable lymphocytes. The antigen recognized by the inhibitory antibody was purified by affinity chromatography and identified by liquid chromatography-mass spectrometry as the serine-rich E. histolytica protein (SREHP). Consistent with this, the inhibitory antibody bound to recombinant SREHP present in bacterial lysates on immunoblots. The SREHP is an abundant immunogenic surface protein of unclear function. The results of this unbiased antibody screen strongly implicate the SREHP as a participant in E. histolytica phagocytosis and suggest that it may play an important role in adherence to apoptotic cells.

Adherence-inhibitory intestinal immunoglobulin a antibody response in baboons elicited by use of a synthetic intranasal lectin-based amebiasis subunit vaccine

We designed an amebiasis subunit vaccine that is constructed by using four peptide epitopes of the galactose-inhibitable lectin heavy subunit that were recognized by intestinal secretory immunoglobulin A (IgA) antibodies from immune human subjects. These epitopes are contained in the region encompassing amino acids 758 to 1134 of the lectin heavy subunit, designated LC3. Baboons (Papio anubis) are natural hosts for Entamoeba histolytica; naturally infected baboons raised in captivity possess serum IgA antibodies to the same four LC3 epitopes as humans. Uninfected, seronegative baboons received four intranasal immunizations at 7-day intervals with the synthetic peptide vaccine (400, 800, or 1,600 mug per nostril) with cholera toxin (20 mug) as the adjuvant. As determined by an enzyme-linked immunosorbent assay (ELISA), each dose of the peptide vaccine elicited antipeptide serum IgA and IgG and intestinal IgA antibody responses in all six immunized baboons by day 28, 7 days after the last immunization (P, <0.01 for each dose compared to the cholera toxin control). The peptide vaccine elicited serum IgG and intestinal IgA antibodies that recognized purified recombinant LC3 protein (P, <0.008 and 0.02, respectively) and native lectin protein (P < 0.01). In addition, an indirect immunofluorescence assay with whole trophozoites (P < 0.01) and Western blot analysis confirmed that serum IgG antibodies from vaccinated baboons recognized native lectin protein on the surfaces of axenic E. histolytica trophozoites or from solubilized amebae. All four synthetic peptides were immunogenic; the vaccine elicited dose- and time-dependent responses, as determined by ELISA optical density readings indicating the production of serum and intestinal antibodies (P, <0.02 for antipeptide and antilectin antibodies). As a positive control, intranasal immunization with purified recombinant LC3 protein with cholera toxin as the adjuvant elicited a serum anti-LC3 IgA and IgG antibody response (P, 0.05 and <0.0001, respectively); however, no intestinal anti-LC3 IgA antibody response was observed (P = 0.4). Of interest, serum IgA and IgG antibodies elicited by the recombinant LC3 vaccine did not recognize any of the four putatively protective LC3 peptide epitopes. Both serum and fecal antibodies elicited by the peptide vaccine exhibited neutralizing activity, as determined by their dose-dependent inhibition of the galactose-specific adherence of E. histolytica trophozoites to Chinese hamster ovary cells in vitro (P, <0.001 for each group of antibodies compared to the control). In summary, a lectin-based intranasal polylysine-linked synthetic peptide vaccine was effective in eliciting an adherence-inhibitory, intestinal antilectin IgA antibody response in baboons. Future studies with the baboon model will determine vaccine efficacy against asymptomatic E. histolytica intestinal infection.

Vaccines for amoebiasis: barriers and opportunities

Amoebiasis, infection by the protozoan parasite Entamoeba histolytica, remains a global health problem, despite the availability of effective treatment. While improved sanitation could lead to the eradication of this disease, it is unlikely that this will occur worldwide in the foreseeable future; thus alternative measures must be pursued. One approach is to develop a vaccine to prevent this deadly disease. Clinical studies indicate that mucosal immunity may provide some protection against recurrent intestinal infection with E. histolytica, but there is no clear evidence that protective immunity develops after amoebic liver abscess. Over the past decade, progress in vaccine development has been facilitated by new animal models that allow better testing of potential vaccine candidates and the application of recombinant technology to vaccine design. Oral vaccines and DNA-based vaccines have been successfully tested in animals models for immunogenicity and efficacy. There has been significant progress on a number of fronts, but there are unanswered questions regarding the effectiveness of immune responses in preventing disease in man and, as yet, no testing of any of these vaccines in humans has been performed. In addition, there are strong economic barriers to developing an amoebiasis vaccine and questions about how and where an effective vaccine would be utilized.

Transcriptomic comparison of two Entamoeba histolytica strains with defined virulence phenotypes identifies new virulence factor candidates and key differences in the expression patterns of cysteine proteases, lectin light chains, and calmodulin

The availability of Rahman, and the virulent HM-1:IMSS strain of E. histolytica, provides a powerful tool for identifying virulence factors of E. histolytica. Here we report an attempt to identify potential virulence factors of E. histolytica by comparing the transcriptome of E. histolytica HM-1:IMSS and E. histolytica Rahman. With phenotypically defined strains, we compared the transcriptome of Rahman and HM-1:IMSS using a custom 70mer oligonucleotide based microarray that has essentially full representation of the E. histolytica HM-1:IMSS genome. We find extensive differences between the two strains, including distinct patterns of gene expression of cysteine proteinases, AIG family members, and lectin light chains.

Mucosal immunity to asymptomatic Entamoeba histolytica and Entamoeba dispar infection is associated with a peak intestinal anti-lectin immunoglobulin A antibody response

We monitored 93 subjects cured of amebic liver abscess (ALA) and 963 close associate controls in Durban, South Africa, and determined by enzyme-linked immunosorbent assay that the intestinal immunoglobulin A (IgA) antibody response to the Entamoeba histolytica galactose-inhibitable adherence lectin is most accurately represented by a complex pattern of transitory peaks. One or more intestinal anti-lectin IgA antibody peaks occurred in 85.9% of ALA subjects over 36 months compared to 41.6% of controls (P < 0.0001). ALA subjects exhibited a greater number of anti-lectin IgA antibody peaks (P < 0.0001) than controls. In addition, their peak optical density values were higher (peak numbers 1 to 3, P < 0.003), peaks were of longer duration (for peaks 1 and 2, P </= 0.0054), and there was a shorter time interval between peaks (between 1 and 2 or 2 and 3, P </= 0.0106) than observed for control subjects. A prior E. histolytica infection was associated with the occurrence of an anti-lectin IgA antibody peak (79.1%, P < 0.0001) more so than for Entamoeba dispar infection (57.2%, P < 0.001). The annual number of anti-lectin IgA antibody peaks in ALA subjects was 0.71 per year, compared to just 0.22 in controls (P<0.0001), indicating a higher rate of exposure to the parasite than previously appreciated. Anti-lectin IgA antibody peaks were of higher amplitude following a E. histolytica infection compared to E. dispar (P = 0.01) and, for either, were of greater height in ALA subjects than controls (P < 0.01). ALA subjects demonstrated greater clearance of amebic infection after an anti-lectin IgA antibody peak compared to controls, and only 14.3% remained with a positive culture after the peak, compared to 38.9% in controls (P = 0.035). In summary, this prospective controlled longitudinal study elucidated the dynamic nature of the human intestinal IgA antibody response to E. histolytica and E. dispar infection and revealed that ALA subjects exhibit heightened intestinal anti-lectin IgA antibody peaks that are associated with clearance of E. histolytica and E. dispar infection.

Experimental amebiasis: a selected review of some in vivo models

The use of in vivo animal models in amebiasis has contributed significantly to the knowledge of this common human parasitic disease. Although there is no animal model that mimics the whole cycle of the human disease, the use of different susceptible and resistant laboratory animals and the availability for many years of techniques for the axenic culture of trophozoites of Entamoeba histolytica have allowed a better understanding of the parasite and the host-parasite relationship. The recent introduction of frontier methodologies in biology has increased our comprehension of this parasite. New information on the cellular and molecular biology and genetics of this organism has been extensively reported, and much of this has clearly required the more frequent use of animal models to verify specific facts. Based on experimental animals characterized previously, the introduction of new animal models with genetic or surgical modifications, especially in mice, has allowed a more adequate analysis of the mechanisms of pathogenesis. Multiple factors have been considered in the promotion of the invasiveness and virulence of E. histolytica. Additionally, the immunological and physiological responses of the host, depending on the environmental conditions, lead to the establishment or the rejection of the parasite. The role of inflammatory reaction to amebic infection constitutes one of the controversies that has been studied by several authors. In susceptible animals (hamsters and gerbils), inflammatory cell damage seems to be related to target cell lysis, while in resistant animals (mice), inflammatory cells appear to protect the host by lysing the parasite. Presently, the involvement of various substances in the development of lesions including lectins, proteases, amebapores, promoters of apoptosis, cytokines, nitric oxide, etc., is being examined using different in vivo models.

Epigenetic silencing of gene expression in Entamoeba histolytica

Transcriptional silencing of an amebapore (ap-a) gene occurred in Entamoeba histolytica following the transfection of plasmids containing a DNA segment (473 bp) homologous to the 5' upstream region of the gene. This segment contains the promoter region of the ap-a gene, a T-rich stretch, followed by a truncated SINE1 (short interspersed element) that is transcribed from the opposite strand. The downstream silencing of the ap-a gene did not occur with plasmids containing the entire SINE1 sequence or lacking the entire SINE1 sequences including the T-rich stretch. Such plasmids promoted the overexpression of the ap-a gene. The transcription of the SINE element required both the T-rich stretch as well as sequences from the 5' end of SINE. RNA extracts from gene-silenced cultures showed small amounts of short (approximately 140 nt), single-stranded molecules with homology to SINE1 transcripts but no siRNA. Chromatin immunoprecipitation (ChIP) analysis of silenced G3 trophozoites with an antibody against methylated K4 of histone H3 revealed a demethylation of K4 at the domain of the ap-a gene indicating transcriptional inactivation. These results suggest the involvement of the SINE1 element in triggering the gene silencing and the role of histone modification in its epigenetic maintenance. The avirulent phenotype of the silenced trophozoites was demonstrated in various assays and the results suggest they may have a potential use for vaccination.

The current status of an amebiasis vaccine

Efficient control of infectious diseases requires the development and application of suitable vaccines. Development of vaccines against amebiasis is still in its infancy. However, in recent years progress has been made in the identification of possible vaccine candidates, the route of application and the understanding of the immune response that is required for protection against amebiasis.

Adherence-Blocking Vaccine for Amebiasis

The Gal/GalNAc lectin is a candidate vaccine antigen for an amebiasis vaccine due to its mediation of parasite adherence to the human intestine, because partial immunity in humans is associated with a mucosal IgA response against it, and because it is effective as a vaccine against amebic colitis in the murine model. The LecA domain of the Gal/GalNAc lectin contains neutralizing antibody epitopes. LecA contains the active site of the lectin (the carbohydrate recognition domain or "CRD") and has been an effective vaccine antigen in animal models of amebic colitis and liver abscess. Research needs include production of the LecA domain of the Gal/GalNAc lectin by a process that can be transferred to cGMP and optimization for immunogenicity, using adjuvants such as alum, MF59 or QS-21 adjuvants. Accomplishing this will enable testing of the ability of LecA immunizations to protect from amebic colitis in humans.

Survival strategies of Entamoeba histolytica: Modulation of cell-mediated immune responses

Tissue invasion and disease associated with the protozoan Entamoeba histolytica has long been connected with suppression of host cellular immunity. Dampening of the host's defences may facilitate survival of amoebae in extraintestinal sites and development of the characteristic amoebic abscesses. In recent years, several studies have begun to clarify, at the cellular level, the specific effects E. histolytica has on immune cell accessory and effector cell functions. Here, Darren Campbell and Kris Chadee discuss the parasite's multiple modulatory effects on macrophages and T cells and how this manipulation of immune defences may enable the parasite to remain viable in the host. They suggest the putative amoebic molecules involved and potential modulation by the cytokines: interleukins IL-4 and IL-10 and transforming growth factor-beta.

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.

The EhADH112 recombinant polypeptide inhibits cell destruction and liver abscess formation by Entamoeba histolytica trophozoites

The Entamoeba histolytica EhCPADH complex, formed by a cysteine proteinase (EhCP112) and an adhesin (EhADH112), is involved in adherence, phagocytosis and cytolysis. This makes this complex an attractive candidate as a vaccine against amoebiasis. Here, we produced the recombinant polypeptide EhADH243, which includes the adherence epitope detected by a monoclonal antibody against the EhCPADH complex. EhADH243 was purified, and the effect of the polypeptide on in vitro and in vivo virulence was studied. Antibodies against EhADH243 reacted with the EhCPADH complex and with the recombinant polypeptide. EhADH243 and antibodies against this polypeptide inhibited adherence, phagocytosis and destruction of cell monolayers by live trophozoites, but had little effect on cell monolayer destruction by trophozoite extracts. EhADH243 recognized a 97 kDa protein in the MDCK membrane fraction that could be a putative receptor for E. histolytica trophozoites. Hamsters immunized with EhADH243 developed humoral response against EhCPADH, and animals were partially protected from amoebic liver abscess.

A virulence attenuated amoebapore-less mutant of Entamoeba histolytica and its interaction with host cells

Entamoeba histolytica, the protozoan parasite which causes amoebiasis, is an exclusively human pathogen so developing a vaccine could effectively impact the spread of the disease. Recently we developed a genetically modified avirulent strain, termed G3, from the virulent E. histolytica strain HM-1:IMSS. The new strain lacks the important virulence factor, the amoebapore-A. The objective of our current study was to investigate the avirulence of the attenuated strain as well as to examine the antigenic and immunogenic responses of these trophozoites as potential candidates for a live vaccine. Functional assays were conducted to characterise the virulent behaviour of the G3 strain. This behaviour was compared to the virulent strain HM-1:IMSS and the non-virulent strain Rahman. Western blots were conducted to confirm the lack of amoebapore-A in the E. histolytica G3 strain and to demonstrate that it had no influence on the presence of other virulence factors. Results of these two sets of tests proved the G3 strain to be phenotypically similar to the avirulent Rahman strain while antigenically identical to the virulent HM-1:IMSS, apart from the lack of the amoebapore-A protein. Intraperitoneal immunisation of hamsters with G3 trophozoites compared to sham immunised hamsters resulted in IgG anti-HM-1:IMSS antibodies. The level of humoral response was variable and further testing has to take place before introducing this new strain as a vaccine.

Amebic cysteine proteinase 2 (EhCP2) plays either a minor or no role in tissue damage in acute experimental amebic liver abscess in hamsters

Amebic cysteine protease 2 (EhCP2) was purified from ethyl ether extracts of axenically grown trophozoites of Entamoeba histolytica strain HM1-IMSS. The purification procedure involved molecular filtration and electroelution. Sequence analysis of the purified product revealed EhCP2 and ubiquitin(s). Electrophoretic migration patterns, isoelectric point determination and Western blot studies failed to reveal other EhCP molecules. Polyclonal antibodies against the purified EhCP2 prepared in rabbits either stabilized or enhanced the enzyme activity in a dose-response manner. Purified EhCP2 was enclosed within inert resin microspheres (22-44 microm in diameter) and injected into the portal vein of normal hamsters. In the liver, the microspheres caused mild acute inflammation and occasional minimal necrosis of short duration. Sections of the liver were immunohistochemically stained with the anti-EhCP2 antibody and the microspheres were positive for only a very short period (1 h) after injection. Sections of experimental acute (1 day, 5 days) amebic liver abscess produced in hamsters were also stained with the anti-EhCP2 antibody; and amebas were intensely positive but no staining was observed at any time in the surrounding necrotic structures. It is suggested that EhCP2 plays either a minor or no role in the causation of tissue damage in experimental acute liver amebiasis.

Entamoeba histolytica: genetic diversity of clinical isolates from Bangladesh as demonstrated by polymorphisms in the serine-rich gene

The varied organ tropisms and clinical presentations of infection by Entamoeba histolytica have stimulated interest in the role of parasite genetic diversity in virulence. We investigated genetic diversity among 54 E. histolytica isolates from Bangladesh by analyzing polymorphism in the serine-rich gene by nested PCR on DNA extracted from stool and liver aspirate pus. We detected both size and restriction site polymorphisms among the isolates within this endemic area. A combination of the nested PCR results and the AluI digestion of the PCR products examined yielded 25 distinct DNA banding patterns among the 42 stool isolates and an additional 9 distinct patterns among the 12 liver abscess isolates. Approximately half of the isolates had unique polymorphisms. Interestingly, the majority of E. histolytica from the liver had polymorphisms which were not present in intestinal isolates from the same geographic area. These data are consistent with the existence of genetic differences between E. histolytica which cause intestinal and those which cause hepatic disease. We conclude that there is genetic diversity within E. histolytica isolates from an endemic population as reflected in serine-rich E. histolytica protein gene polymorphism. The correlation of genetic differences with the pathogenic potential of E. histolytica strains and the implications of genetic diversity for the immunoprophylaxis of amebiasis require further study.

Efficacy of intradermal vaccination

Intradermal (ID) inoculation has been investigated as a means of vaccinating laboratory animals, domestic farm animals, and humans. Various forms of viral, bacterial, parasitic, and fungal antigens have been administered ID, with varying results. This review emphasizes results from studies reporting clinically relevant outcomes such as clinical protection and body weight change following experimental challenge. Antibody titers, cytokines, cellular responses are included as supportive data. Based on the reports reviewed, ID vaccination is a promising alternative to more traditional routes of vaccination. ID vaccination has particular appeal to the beef cattle industry based on recently emphasized quality assurance issues. It is evident that the ultimate test of vaccine efficacy is the ability to protect against clinical disease under natural challenge conditions. We propose that the immune response of ID vaccinated cattle, using clinically relevant outcomes such as morbidity, mortality, average daily gain and feed efficiency, needs to be further investigated to define the value of this potentially effective and practical means of antigen delivery, particularly for domesticated farm animals.

Molecular epidemiology of Entamoeba spp.: evidence of a bottleneck (Demographic sweep) and transcontinental spread of diploid parasites

Entamoeba histolytica causes amebic colitis and liver abscess in developing countries such as Mexico and India. Entamoeba dispar is morphologically identical but is not associated with disease. Here we determined the ploidy of E. histolytica and developed PCR-based methods for distinguishing field isolates of E. histolytica or E. dispar. Fluorescence in situ hybridization showed that E. histolytica trophozoites are diploid for five "single-copy" probes tested. Intergenic sequences between superoxide dismutase and actin 3 genes of clinical isolates of E. histolytica from the New and Old Worlds were identical, as were those of E. dispar. These results suggest a bottleneck or demographic sweep in entamoebae which infect humans. In contrast, E. histolytica and E. dispar genes encoding repeat antigens on the surface of trophozoites (Ser-rich protein) or encysting parasites (chitinase) were highly polymorphic. chitinase alleles suggested that the early axenized strains of E. histolytica, HM-1 from Mexico City, Mexico, and NIH-200 from Calcutta, India, are still present and that similar E. dispar parasites can be identified in both the New and Old Worlds. Ser-rich protein alleles, which suggested the presence of the HM-1 strain in Mexico City, included some E. histolytica genes that predicted Ser-rich proteins with very few repeats. These results, which suggest diversifying selection at chitinase and Ser-rich protein loci, demonstrate the usefulness of these alleles for distinguishing clinical isolates of E. histolytica and E. dispar.

Amebic liver abscess

Amebiasis is a widespread parasitic disease caused by Entamoeba histolytica. This protozoan organism is the third leading parasitic cause of death in the developing world and is an important health risk to travelers in endemic areas. Amebiasis most commonly results in asymptomatic colonization of the gastrointestinal tract, but some patients may develop intestinal invasive disease or extraintestinal disease-amebic liver abscess being the most common extraintestinal manifestation. This article reviews epidemiologic features, pathophysiology, clinical features, diagnostic tests, imaging studies, treatment of amebic liver abscess, and prevention measures.

Protection of gerbils from amebic liver abscess by vaccination with a 25-mer peptide derived from the cysteine-rich region of Entamoeba histolytica galactose-specific adherence lectin

The protozoan parasite Entamoeba histolytica causes extensive morbidity and mortality through intestinal infection and amebic liver abscess. Here we show that immunization of gerbils with a single keyhole limpet hemocyanin-coupled 25-mer peptide derived from the 170-kDa subunit of the E. histolytica galactose-binding adhesin is sufficient to confer substantial protection against experimentally induced amebic liver abscesses. Vaccination provided total protection in 5 of 15 immunized gerbils, and abscesses were significantly smaller (P < 0.01) in the remaining vaccinated animals. The degree of protection correlated with the titer of antibodies to the peptide, and results of passive transfer experiments performed with SCID mice were consistent with a role for antibodies in protection. In addition, parenteral or oral vaccination of gerbils with 13-amino-acid subfragments of the peptide N-terminally fused to the B subunit of cholera toxin also significantly inhibited liver abscess formation (P < 0.05). These data indicate that small peptides derived from the galactose-binding adhesin administered by the parenteral or oral route can provide protection against amebic liver abscess and should be considered as components of a subunit vaccine against invasive amoebiasis.

Cysteine proteinases and the pathogenesis of amebiasis

Amebiasis is a major cause of morbidity and mortality throughout the tropical world. Entamoeba histolytica is now recognized as a separate species from the morphologically identical E. dispar, which cannot invade. Cysteine proteinases are a key virulence factor of E. histolytica and play a role in intestinal invasion by degrading the extracellular matrix and circumventing the host immune response through cleavage of secretory immunoglobulin A (sIgA), IgG, and activation of complement. Cysteine proteinases are encoded by at least seven genes, several of which are found in E. histolytica but not E. dispar. A number of new animal models, including the formation of liver abscesses in SCID mice and intestinal infection in human intestinal xenografts, have proven useful to confirm the critical role of cysteine proteinases in invasion. Detailed structural analysis of cysteine proteinases should provide further insights into their biochemical function and may facilitate the design of specific inhibitors which could be used as potential chemotherapeutic agents in the future.

Cysteine proteinases and the pathogenesis of amebiasis

Amebiasis is a major cause of morbidity and mortality throughout the tropical world. Entamoeba histolytica is now recognized as a separate species from the morphologically identical E. dispar, which cannot invade. Cysteine proteinases are a key virulence factor of E. histolytica and play a role in intestinal invasion by degrading the extracellular matrix and circumventing the host immune response through cleavage of secretory immunoglobulin A (sIgA), IgG, and activation of complement. Cysteine proteinases are encoded by at least seven genes, several of which are found in E. histolytica but not E. dispar. A number of new animal models, including the formation of liver abscesses in SCID mice and intestinal infection in human intestinal xenografts, have proven useful to confirm the critical role of cysteine proteinases in invasion. Detailed structural analysis of cysteine proteinases should provide further insights into their biochemical function and may facilitate the design of specific inhibitors which could be used as potential chemotherapeutic agents in the future.

DNA vaccination with the serine rich Entamoeba histolytica protein (SREHP) prevents amebic liver abscess in rodent models of disease

Amebiasis remains one of the leading parasitic causes of death worldwide. A vaccine that prevented amebic liver abscess would significantly reduce mortality from this disease. To test the feasibility of a DNA vaccine to prevent amebic liver abscess, we immunized both mice and gerbils with plasmid DNA encoding the serine rich Entamoeba histolytica protein (SREHP). Animals receiving the SREHP DNA vaccine developed both antibody and cell mediated immune responses that recognized amebic trophozoites. A single dose of the SREHP DNA vaccine protected 80% of vaccinated mice and 60% of vaccinated gerbils from developing amebic liver abscess after direct hepatic inoculation of amebic trophozoites. Our study indicates that DNA vaccination with SREHP can provide high levels of protection against amebic liver abscess in animal models of disease.

Chitinase secretion by encysting Entamoeba invadens and transfected Entamoeba histolytica trophozoites: localization of secretory vesicles, endoplasmic reticulum, and Golgi apparatus

Entamoeba histolytica, the protozoan parasite that phagocytoses bacteria and host cells, has a vesicle/vacuole-filled cytosol like that of macrophages. In contrast, the infectious cyst form has four nuclei and a chitin wall. Here, anti-chitinase antibodies identified hundreds of small secretory vesicles in encysting E. invadens parasites and in E. histolytica trophozoites overexpressing chitinase under an actin gene promoter. Abundant small secretory vesicles were also identified with antibodies to the surface antigen Ariel and with a fluorescent substrate of cysteine proteinases. Removal of an N-terminal signal sequence directed chitinase to the cytosol. Addition of a C-terminal KDEL peptide, identified on amebic BiP, retained chitinase in a putative endoplasmic reticulum, which was composed of a few vesicles of mixed sizes. A putative Golgi apparatus, which was Brefeldin A sensitive and composed of a few large, perinuclear vesicles, was identified with antibodies to ADP-ribosylating factor and to epsilon-COP. We conclude that the amebic secretory pathway is similar to those of other eukaryotic cells, even if its appearance is somewhat different.

Amebiasis

More than 80% of cases of amebic liver abscess can be managed with a 14-day course of intravenous or oral metronidazole. In cases of suspected amebic liver abscess, treatment should be started before diagnostic confirmation. If no clinical improvement is evident by 72 to 96 hours, treatment should be changed to dehydroemetine and chloroquine. Invasive treatment is necessary only in patients in whom medical treatment fails within 5 days or in whom signs of clinically severe disease are present. A 10-day course with a luminal agent such as paromomycin to eliminate intestinal cysts, which are resistant to imidazoles, should always follow treatment of the liver abscess. Percutaneous catheter drainage is indicated in patients with impending rupture, with a lesion 6 cm or more in diameter, with an abscess located in the left lobe or high in the dome of the right lobe, or in whom medical treatment fails. Although sympathetic pleural effusion is not an indication for drainage, direct pulmonary involvement or spread to pleural or lung tissues requires drainage. Intraperitoneal rupture and peritonitis necessitate open surgical drainage. Only a small minority of amebic liver abscesses are secondarily infected by other organisms. Because relapses are possible, feces should be checked for cysts monthly for several months after therapy.

Host-pathogen interaction in amebiasis and progress in vaccine development

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

Preserved epitope-specific T cell activation by recombinant Bet v 1-MBP fusion proteins

BACKGROUND

Allergen-specific T cells play an important role in the allergic immune response, and are thought to be the principal target in specific immunotherapy.

OBJECTIVE

The aim of the present study was to evaluate if fusion proteins of allergens with bacterial proteins can be used to activate and bias allergen-specific T cells, and to characterize T cell epitopes.

METHODS

The complete gene of Bet v 1, the major birch pollen allergen, was amplified by PCR from birch pollen mRNA, and cloned in pKK223-3. The complete gene or truncated sequences were transferred to pMAL-c and expressed in E. coli as fusion proteins with maltose binding protein (MBP). The complete fusion protein, and the truncated fusion proteins were used for studies with Bet v 1-specific T cells.

RESULTS

Bet v 1-specific T cells reacted similarly with purified and crude Bet v 1-MBP proteins. Therefore, crude preparations were used to study the epitope-specificity of 11 Bet v 1-specific T cell clones. Six distinct T cell epitopes were determined in this way. Interestingly, the T cell epitope of three T cell clones, that did not react with synthetic peptides in a previous study, was identified. In addition, the presence of MBP as a fusion partner to Bet v 1 was shown to influence TH2/TH1 cytokine production in T cell lines, but not in established T cell clones.

CONCLUSION

Using crude preparations of recombinant fusion proteins of Bet v 1 with MBP, multiple T cell epitopes were identified in Bet v 1. As T cell activation is preserved in this system, the generation of recombinant allergens with TH1-inducing proteins as fusion partners might be considered as a T-cell targeted approach for specific immunotherapy.

Mucosal immunogenicity of a holotoxin-like molecule containing the serine-rich Entamoeba histolytica protein (SREHP) fused to the A2 domain of cholera toxin

One strategy for the induction of mucosal immune responses by oral immunization is to administer the antigen in conjunction with cholera toxin. Cholera toxin consists of one A polypeptide (CTA) which is noncovalently linked to five B subunits (CTB) via the A2 portion of the A subunit (CTA2). Coupling of antigens to the nontoxic B subunit of cholera toxin may improve the immunogenicity of antigens by targeting them to GM1 ganglioside on M cells and intestinal epithelial cells. Here, we describe the construction of a translational fusion protein containing the serine-rich Entamoeba histolytica protein (SREHP), a protective amebic antigen, fused to a maltose binding protein (MBP) and to CTA2. When coexpressed in Escherichia coli with the CTB gene, these proteins assembled into a holotoxin-like chimera containing MBP-SREHP-CTA2 and CTB. This holotoxin-like chimera (SREHP-H) inhibited the binding of cholera toxin to GM1 ganglioside. Oral vaccination of mice with SREHP-H induced mucosal immunoglobulin A (IgA) and serum IgG antiamebic antibodies and low levels of mucosal anti-CTB antibodies. Our studies confirm that the genetic coupling of antigens to CTA2 and their coexpression in E. coli can produce holotoxin-like molecules that are mucosally immunogenic without the requirement for supplemental cholera toxin, and they establish the SREHP-H protein as a candidate for evaluation as a vaccine to prevent amebiasis.

A new gene family (ariel) encodes asparagine-rich Entamoeba histolytica antigens, which resemble the amebic vaccine candidate serine-rich E. histolytica protein

A family of genes, called ariel, are named for and encode asparagine-rich Entamoeba histolytica antigens containing 2 to 16 octapeptide repeats. Ariel proteins, which are constitutively expressed by trophozoites, belong to a large antigen family that includes the serine-rich E. histolytica protein (SREHP), an amebic vaccine candidate.

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.

Progress towards development of a vaccine for amebiasis

The application of molecular biologic techniques over the past decade has seen a tremendous growth in our knowledge of the biology of Entamoeba histolytica, the causative agent of amebic dysentery and amebic liver abscess. This approach has also led to the identification and structural characterization of three amebic antigens, the serine-rich Entamoeba histolytica protein (SREHP), the 170-kDa subunit of the Gal/GalNAc binding lectin, and the 29-kDa cysteine-rich protein, which all show promise as recombinant antigen-based vaccines to prevent amebiasis. In recent studies, an immunogenic dodecapeptide derived from the SREHP molecule has been genetically fused to the B subunit of cholera toxin, to create a recombinant protein capable of inducing both antiamebic and anti-cholera toxin antibodies when administered by the oral route. Continued progress in this area will bring us closer to the goal of a cost-effective oral combination "enteric pathogen" vaccine, capable of inducing protective mucosal immune responses to several clinically important enteric diseases, including amebiasis.

Expression of the serine rich Entamoeba histolytica protein (SREHP) in the avirulent vaccine strain Salmonella typhi TY2 chi 4297 (delta cya delta crp delta asd): safety and immunogenicity in mice

Infection by the intestinal protozoan parasite Entamoeba histolytica remains a significant threat to health in much of the world. Here we describe the successful expression of the serine rich Entamoeba histolytica protein (SREHP), a protective antigen of ameba, in an attenuated vaccine strain Salmonella typhi TY2 chi 4297 (delta cya delta crp delta asd). The attenuation of S. typhi TY2 chi 4297 was not altered by expression of the SREHP-maltose binding protein (MBP) fusion protein and mice parenterally vaccinated with S. typhi TY2 chi 4297 expressing SREHP-MBP developed serum anti-amebic and anti-LPS antibodies. S. typhi TY2 chi 4297 expressing SREHP-MBP represents a prototype combination vaccine designed to prevent both amebiasis and typhoid fever.

Oral immunization with attenuated vaccine strains of Vibrio cholerae expressing a dodecapeptide repeat of the serine-rich Entamoeba histolytica protein fused to the cholera toxin B subunit induces systemic and mucosal antiamebic and anti-V. cholerae antibody responses in mice

Entamoeba histolytica is a significant cause of morbidity and mortality worldwide. The serine-rich E. histolytica protein (SREHP) is a surface-expressed trophozoite protein that includes multiple hydrophilic tandem repeats. A purified fusion protein between the dodecapeptide repeat of SREHP and cholera toxin B subunit (CTB) has previously been shown to be immunogenic in mice after oral inoculation when cholera toxin is coadministered as an immunoadjuvant. We engineered a live attenuated El Tor Vibrio cholerae vaccine strain, Peru2, to express the SREHP-12-CTB fusion protein to the supernatant from either a plasmid [Peru2 (pETR5.1)] or from a chromosomal insertion (ETR3). Vector strains were administered orally to germfree mice that were subsequently housed under nongermfree conditions; mice received one (day 0) or two (days 0 and 14) inoculations. No immunoadjuvant or cholera holotoxin was administered. Mice that received two inoculations of Peru2(pETR5.1) had the most pronounced antiamebic systemic and mucosal immunologic responses. Less marked, but significant, anti-SREHP serum immunoglobulin G antibody responses were also induced in mice that received either one or two oral inoculations of strain ETR3. Anti-V. cholerae responses were also induced, as measured by the induction of serum vibriocidal antibodies and by serum and mucosal anti-CTB antibody responses. These results suggest that V. cholerae vector strains can be successful delivery vehicles for the SREHP-12-CTB fusion protein, to induce mucosal and systemic antiamebic and anti-V. cholerae immune responses. The magnitude of these responses is proportional to the amount of SREHP-12-CTB produced by the vector strain.

Cloning and expression of chitinases of Entamoebae

Entamoeba histolytica (Eh) and Entamoeba dispar (Ed) are protozoan parasites that infect hundreds of millions of persons. In the colonic lumen, amebae form chitin-walled cysts, the infectious stage of the parasite. Entamoeba invadens (Ei), which infects reptiles and is a model for amebic encystation, produces chitin synthase and chitinase during encystation. Ei cysts formation is blocked by the chitinase-inhibitor allosamidin. Here molecular cloning techniques were used to identify homologous genes of Eh, Ed, and Ei that encode chitinases (EC 3.2.1.14). The Eh gene (Eh cht1) predicts a 507-amino acid (aa) enzyme, which has 93 and 74% positional identities with Ed and Ei chitinases, respectively. The Entamoeba chitinases have signal sequences, followed by acidic and hydrophilic sequences composed of multiple tandemly arranged 7-aa repeats (Eh and Ed) or repeats varying in length (Ei). The aa compositions of the chitinase repeats are similar to those of the repeats of the Eh and Ed Ser-rich proteins. The COOH-terminus of each chitinase has a catalytic domain, which resembles those of Brugia malayi (33% positional identity) and Manduca sexta (29%). Recombinant entamoeba chitinases are precipitated by chitin and show chitinase activity with chitooligosacharide substrates. Consistent with previous biochemical data, chitinase mRNAs are absent in Ei trophozoites and accumulate to maximal levels in Ei encysting for 48 h.

Protozoa. Amebiasis

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

Oral immunization with an attenuated vaccine strain of Salmonella typhimurium expressing the serine-rich Entamoeba histolytica protein induces an antiamebic immune response and protects gerbils from amebic liver abscess

Attenuated salmonellae represent attractive candidates for the delivery of foreign antigens by oral vaccination. In this report, we describe the high-level expression of a recombinant fusion protein containing the serine-rich Entamoeba histolytica protein (SREHP), a protective antigen derived from virulent amebae, and a bacterially derived maltose-binding protein (MBP) in an attenuated strain of Salmonella typhimurium. Mice and gerbils immunized with S. typhimurium expressing SREHP-MBP produced mucosal immunoglobulin A antiamebic antibodies and serum immunoglobulin G antiamebic antibodies. Gerbils vaccinated with S typhimurium SREHP-MBP were protected against amebic liver abscess, the most common extraintestinal complication of amebiasis. Our findings indicate that the induction of mucosal and immune responses to the amebic SREHP antigen is dependent on the level of SREHP-MBP expression in S. typhimurium and establish that oral vaccination with SREHP can produce protective immunity to invasive amebiasis.

Recent advances in amebiasis

Advancements in our understanding of amebiasis have been rapid over the decade that I have followed this field. What was identified morphologically for years as Entamoeba histolytica has been redescribed with modern techniques as a complex of two species, the commensal parasite E. dispar and the pathogenic parasite E. histolytica that is the cause of colitis and liver abscess. Antigen detection tests are now available for the rapid detection in stool of the pathogenic species E. histolytica. New understandings of the importance of luminal as well as tissue-active antimebic medications in the treatment of invasive disease have been reached. The groundwork is being laid for an understanding of the protective immune responses to infection, and at the lab bench DNA transfection of the parasite has opened studies of pathogenesis to genetic analysis. While necessarily an incomplete sketch of the field, I have attempted here to highlight some recent and important developments of interest to clinicians and microbiologists.

Progress towards an amebiasis vaccine

Amebiasis (infection by Entamoeba histolytica) remains a major health problem in much of the developing world. Morbidity and mortality from amebic dysentery and amebic liver abscess have persisted despite the availability of effective anti-amebic therapy, suggesting a need for alternative measures of disease control. Through the application of recombinant DNA technology, several E. histolytica antigens have now been expressed in prokaryotic systems and tested in animal models as vaccines to prevent invasive amebiasis. In this review, Sam Stanley Jr discusses why a vaccine for amebiasis may be feasible, and describes the recent development of several promising recombinant E. histolytica antigen-based parenteral and oral vaccine candidates.

Identification of the galactose-adherence lectin epitopes of Entamoeba histolytica that stimulate tumor necrosis factor-alpha production by macrophages

The 170-kDa subunit of the galactose-adherence lectin (Gal-lectin) of Entamoeba histolytica mediates adherence to human colonic mucins and intestinal epithelium as a prerequisite to amebic invasion. The Gal-lectin is an immunodominant molecule and a protective antigen in the gerbil model of amebiasis. Tumor necrosis factor alpha (TNF-alpha) produced by activated macrophages enhances nitric oxide-dependent cytotoxicity in host defense against E. histolytica. The purpose of this study was to identify the Gal-lectin epitopes which stimulate TNF-alpha production by macrophages. Murine bone marrow-derived macrophages (BMMs) exposed to Gal-lectin (100-500 ng/ml) stimulated stable expression of TNF-alpha mRNA (8-fold increase) and TNF-alpha production similar to that of lipopolysaccharide-stimulated cells (100 ng/ml). Polyclonal anti-lectin serum specifically inhibited TNF-alpha mRNA induction in response to the Gal-lectin but not to lipopolysaccharide. Anti-lectin monoclonal antibodies 8C12, H85 and 1G7, which recognize nonoverlapping epitopes of the cysteine-rich region of the 170-kDa heavy subunit, inhibited both amebic adherence to mammalian cells and Gal-lectin-stimulated TNF-alpha mRNA expression by BMMs,but monoclonal antibody 7F4 did neither. As these inhibitory antibodies map to amino acids 596-1082 of the 170-kDa Gal-lectin, our results have identified the functional region that mediates amebic adherence and TNF-alpha mRNA induction in BMMMs; thus, this region of the Gal-lectin is a subunit vaccine candidate.

Immunogenicity of the recombinant serine rich Entamoeba histolytica protein (SREHP) amebiasis vaccine in the African green monkey

We report the first study in non-human primates of the safety and immunogenicity of a recombinant vaccine designed to prevent amebic liver abscess. In a pilot study, a recombinant vaccine containing the serine rich Entamoeba histolytica protein (SREHP) attached to a maltose binding protein (SREHP/MBP), which has been shown to be effective in preventing amebic liver abscess in rodent models of infection, was used to immunize two African Green Monkeys. Vaccination with SREHP/MBP resulted in no systemic side-effects. The monkeys receiving the SREHP/MBP protein developed antibodies that recognized the recombinant SREHP/MBP molecule, the native SREHP protein, and the surface of amebic trophozoites. Antiserum from SREHP/MBP-vaccinated monkeys could block the adhesion of E. histolytica trophozoites to mammalian cells, a feature that may correlate with vaccine efficacy. Attempts to produce amebic liver abscess in naive African Green Monkeys by direct hepatic inoculation with virulent E. histolytica trophozoites was not successful, suggesting this species is probably not suitable for vaccine efficacy studies.

Oral immunization with the dodecapeptide repeat of the serine-rich Entamoeba histolytica protein (SREHP) fused to the cholera toxin B subunit induces a mucosal and systemic anti-SREHP antibody response

The intestinal protozoan parasite Entamoeba histolytica causes amebic dysentery, a major cause of morbidity worldwide. The induction of a mucosal antibody response capable of blocking amebic adhesion to intestinal cells could represent an approach to preventing E. histolytica infection and disease. Here we describe the expression of a chimeric protein containing an immunogenic dodecapeptide derived from the serine-rich E. histolytica protein (SREHP), fused to the cholera toxin B subunit (CtxB). The CtxB-SREHP-12 chimeric protein was purified from Escherichia coli lysates and retained the critical GM1 ganglioside-binding activity of the CtxB moiety. Mice fed the CtxB-SREHP-12 fusion protein along with a subclinical dose of cholera toxin developed mucosal immunoglobulin A and immunoglobulin G and systemic antibody responses that recognized recombinant and native SREHP. Our study confirms the feasibility of inducing mucosal immune responses to immunogenic peptides by their genetic fusion to the CtxB subunit and identifies the CtxB-SREHP-12 chimeric protein as a candidate oral vaccine to prevent E. histolytica infection.

The serine-rich Entamoeba histolytica protein is a phosphorylated membrane protein containing O-linked terminal N-acetylglucosamine residues

Previously, we described the isolation of a cDNA clone and the gene encoding a protective antigen of the protozoan parasite Entamoeba histolytica, the serine-rich Entamoeba histolytica protein (SREHP). The derived amino acid sequence of the SREHP cDNA clone was remarkable for a high serine content (52/233 amino acids), a putative signal sequence, multiple hydrophilic dodecapeptide and octapeptide tandem repeats, and a hydrophobic C-terminal putative membrane-spanning region. Here, we show that SREHP is modified by the addition of phosphate at serine residues, O-linked terminal N-acetylglucosamine residues, and by acylation. When the SREHP gene is expressed in baculovirus transformed Sf-9 cells, the product is also phosphorylated and glycosylated and is localized to the plasma membrane of the insect cells. The native SREHP molecule also serves as a potent chemoattractant for amebic trophozoites. The data presented here suggest that SREHP is a unique membrane protein with phosphorylation and glycosylation patterns usually associated with nuclear or cytoplasmic proteins.