Echinococcus multilocularis: developmental stage-specific expression of Antigen B 8-kDa-subunits

Expression and serodiagnostic efficacy of a novel echinococcosis-specific recombinant fusion antigen rAgB8/1-Em18-Eg95

Echinococcosis lacks sensitive serological diagnostic tools. The echinococcosis-specific antigens Eg95, AgB8/1 and the Em18 gene sequences were fused and expressed as the novel recombinant antigens rAgB8/1-Em18-Eg95 (T3) and rEm18-Eg95 (T2), used for the diagnosis of hydatid disease, prepared into an enzyme-linked immunosorbent reaction (ELISA) kit, and evaluated for their serological diagnostic value. The relative molecular weight of the T3 protein was 88.1 kDa, the purified concentration was 1.5 mg mL−1, and the purity was 80%. The relative molecular weight of T2 protein was 79.9 kDa, the total protein concentration was 0.5 mg mL−1, and the purity was less than 50%. The overall coincidence rate of T2 protein was low, and it was impossible to distinguish between negative and positive sera. The T3 antigen was coated at 1.0 μg mL−1, the cutoff value was 0.5271, and the serum dilution ratio was 1:400. A T3 ELISA kits (96 tests) was constructed to detect the serum of 272 clinically and pathologically confirmed cases. The sensitivity of T3 was 93.8%, and the specificity was 83.3%. The parasite cross-reaction was 30%. Satisfactorily, the Pearson correlation coefficient between the T3 OD value and lesion diameter was 0.707, showing a strong correlation. T3 exhibits better antigenicity than T2, and the prepared T3 ELISA diagnostic kits reached the laboratory diagnostic level of a commercial kits. T3 can distinguish human cystic echinococcosis (CE) and alveolar echinococcosis (AE) more significantly and predict the diameter of lesions according to the OD value, which provides practical value for drug or surgical efficacy.

Targeted and non-targeted proteomics to characterize the parasite proteins of Echinococcus multilocularis metacestodes

The larval stage of the cestode Echinococcus multilocularis is the causative agent of alveolar echinococcosis. To investigate the biology of these stages and to test novel compounds, metacestode cultures represent a suitable in vitro model system. These metacestodes are vesicles surrounded by an envelope formed by the vesicle tissue (VT), which is formed by the laminated and germinal layer, and filled with vesicle fluid (VF). We analyzed the proteome of VF and VT by liquid chromatography tandem mass spectrometry (LC-MS/MS) and identified a total of 2,954 parasite proteins. The most abundant protein in VT was the expressed conserved protein encoded by EmuJ_000412500, followed by the antigen B subunit AgB8/3a encoded by EmuJ_000381500 and Endophilin B1 (protein p29). In VF, the pattern was different and dominated by AgB subunits. The most abundant protein was the AgB8/3a subunit followed by three other AgB subunits. In total, the AgB subunits detected in VF represented 62.1% of the parasite proteins. In culture media (CM), 63 E. multilocularis proteins were detected, of which AgB subunits made up 93.7% of the detected parasite proteins. All AgB subunits detected in VF (encoded by EmuJ_000381100-700, corresponding to AgB8/2, AgB8/1, AgB8/4, AgB8/3a, AgB8/3b, and AgB8/3c) were also found in CM, except the subunit encoded by EmuJ_000381800 (AgB8/5) that was very rare in VF and not detected in CM. The relative abundance of the AgB subunits in VF and CM followed the same pattern. In VT, only the subunits EmuJ_000381500 (AgB8/3a) and EmuJ_000381200 (AgB8/1) were detected among the 20 most abundant proteins. To see whether this pattern was specific to VF from in vitro cultured metacestodes, we analyzed the proteome of VF from metacestodes grown in a mouse model. Here, the AgB subunits encoded by EmuJ_000381100-700 constituted the most abundant proteins, namely, 81.9% of total protein, with the same order of abundance as in vitro. Immunofluorescence on metacestodes showed that AgB is co-localized to calcareous corpuscles of E. multilocularis. Using targeted proteomics with HA-tagged EmuJ_000381200 (AgB8/1) and EmuJ_000381100 (AgB8/2), we could show that uptake of AgB subunits from CM into VF occurs within hours.

Transcriptomic Analysis of the Early Strobilar Development of Echinococcus granulosus

Echinococcus granulosus has a complex life cycle involving two mammalian hosts. The transition from one host to another is accompanied by changes in gene expression, and the transcriptional events that underlie this transition have not yet been fully characterized. In this study, RNA-seq was used to compare the transcription profiles of samples from E. granulosus protoscoleces induced in vitro to strobilar development at three time points. We identified 818 differentially expressed genes, which were divided into eight expression clusters formed over the entire 24 h period. An enrichment of gene transcripts with molecular functions of signal transduction, enzymes, and protein modifications was observed upon induction and developmental progression. This transcriptomic study provides insights for understanding the complex life cycle of E. granulosus and contributes for searching for the key genes correlating with the strobilar development, which can be used to identify potential candidates for the development of anthelmintic drugs.

Species specificity preliminary evaluation of an IL-4-based test for the differential diagnosis of human echinococcosis

The diagnosis of cystic echinococcosis (CE) is based on imaging, while serology is a complementary test of particular use when imaging is inconclusive. Serology has several limitations. Among them, false‐positive results are often obtained in subjects with alveolar echinococcosis (AE), rendering difficult the differential diagnosis. We set up an immune assay based on IL‐4‐specific production after stimulating whole blood with an antigen B (AgB)‐enriched fraction from E granulosus that associates with CE and CE cysts in active stage. We aimed to evaluate potential cross‐reactivity of this test using samples from patients with AE. Twelve patients with AE were recruited; IL‐4 levels ranged from 0 to 0.07 pg/mL. Based on the previously identified cut‐off of 0.39 pg/mL using samples from patients with CE, none of samples from AE patients scored positive. In contrast, almost 80% of samples from AE patients scored positive in serology tests based on different E granulosus‐derived antigenic preparations. Our preliminary data show that this experimental whole‐blood assay has no cross‐reactivity in our cohort of patients with AE, in turn indicating a high specificity of the assay for CE diagnosis. This result supports further work towards the development of improved diagnostic tests for CE.

Identification of antigen B (AgB) Gene polymorphism in cattle and sheep isolates of Echinococcus granulosus and investigation of effects on serological diagnosis

Cystic Echinococcosis (CE) is a worldwide common helminth disease caused by the larval form of Echinococcus granulosus. The aim of this study is to determine the genetic differences between distinct isolates of E. granulosus obtained from cattle and sheep and determine the polymorphism of the AgB1 gene by DNA sequence analysis, as well as investigating its relationship with serological response using ELISA and Western Blot tests. For this aim, germinal membranes of hydatid cysts of 30 cattle and 30 sheep from the provinces of Elazig and Erzincan in Turkey and serum samples of these animals were collected. Following isolation of the total genomic DNA, the 12S rRNA gene of all isolates was amplified by PCR for genetic characterization, and the mt-CO1 gene region was examined by DNA sequence analysis. The gDNAs were then amplified by PCR using AgB1-specific primers, and genetic variation was investigated by DNA sequence analysis. At the final stage, all serum samples were analyzed by ELISA and Western Blot tests using a partially purified hydatid cyst fluid antigen. As a result, 114 (95%) of the 120 isolates were determined to be E. granulosus sensu stricto by using 12S rRNA-PCR. Subsequently, the DNA sequence analysis of the remaining 6 samples of the mt-CO1 gene revealed that all samples were E. granulosus sensu stricto. According to the DNA sequence analysis that followed, nucleotide changes in the AgB1 gene were observed in 13 (10.8%) of 120 samples. With this study, 9 (69.2%) out of 13 hydatid cysts in which polymorphism was detected by DNA sequence analysis in their AgB1 gene were found to be positive with ELISA, and 6 (46.1%) were found positive by WB. While 80 (74.7%) of 107 non-polymorphic samples in the AgB1 gene were found to be positive with ELISA, and 75 (70.9%) were positive with WB. As a result, variation in different ratios was determined in the AgB1 gene of E. granulosus sensu stricto, and it was determined that this had a partial effect on serological response.

Expression and serodiagnostic potential of antigen B and thioredoxin peroxidase from Taenia multiceps

Coenurosis is a serious parasitic disease of herbivorous animals caused by the metacestode of Taenia multiceps (Coenurus cerebralis). Accordingly, a significant amount of research is currently dedicated to the development of appropriate antigens for use in rapid and accurate coenurosis diagnosis kits. In the present study, antigen B (AgB) and thioredoxin peroxidase (TPx) from T. multiceps were cloned and expressed using a prokaryotic system, molecular characterization of Tm-AgB was determined by bioinformatical analyses. The serological diagnostic potentials of rTm-AgB and rTm-TPx were evaluated by indirect ELISA and compared with those of previously reported rTm-AnxB2, rTm-HSP70, and rTm-GST. The results showed that Tm-AgB is a specific lipoprotein of cestodes with good thermal stability. The ELISA assay showed that rTm-AgB exhibited a sensitivity of 95.8% and a specificity of 87.5%, indicating its strong potential for serological diagnosis of T. multiceps.

Comparative proteomics of hydatid fluids from two Echinococcus multilocularis isolates

The hydatid fluid (HF) that fills Echinococcus multilocularis metacestode vesicles is a complex mixture of proteins from both parasite and host origin. Here, a LC-MS/MS approach was used to compare the HF composition of E. multilocularis H95 and G8065 isolates (EmH95 and EmG8065, respectively), which present differences in terms of growth and fertility. Overall, 446 unique proteins were identified, 392 of which (88%) were from parasite origin and 54 from culture medium. At least 256 of parasite proteins were sample exclusive, and 82 of the 136 shared proteins presented differential abundance between E. multilocularis isolates. The parasite's protein repertoires in EmH95 and EmG8065 HF samples presented qualitative and quantitative differences involving antigens, signaling proteins, proteolytic enzymes, protease inhibitors and chaperones, highlighting intraspecific singularities that could be correlated to biological features of each isolate. The repertoire of medium proteins found in the HF was also differential between isolates, and the relevance of the HF exogenous protein content for the parasite's biology is discussed. The repertoires of identified proteins also provided potential molecular markers for important biological features, such as parasite growth rate and fertility, as well potential protein targets for the development of novel diagnostic and treatment strategies for alveolar echinococcosis.

BIOLOGICAL SIGNIFICANCE

E. multilocularis metacestode infection of mammal hosts involve complex interactions mediated by excretory/secretory (ES) products. The hydatid fluid (HF) that fills the E. multilocularis metacestode vesicles contains complex repertoires of parasite ES products and host proteins that mediate important molecular interactions determinant for parasite survival and development, and, consequently, to the infection outcome. HF has been also extensively reported as the main source of proteins for the immunodiagnosis of echinococcosis. The performed proteomic analysis provided a comprehensive profiling of the HF protein composition of two E. multilocularis isolates. This allowed us to identify proteins of both parasite and exogenous (medium) origin, many of which present significant differential abundances between parasite isolates and may correlate to their differential biological features, including fertility and growth rate.

Laboratory Diagnosis of Echinococcus spp. in Human Patients and Infected Animals

Among the species composing the genus Echinococcus, four species are of human clinical interest. The most prevalent species are Echinococcus granulosus and Echinococcus multilocularis, followed by Echinococcus vogeli and Echinococcus oligarthrus. The first two species cause cystic echinococcosis (CE) and alveolar echinococcosis (AE) respectively. Both diseases have a complex clinical management, in which laboratory diagnosis could be an adjunctive to the imaging techniques. To date, several approaches have been described for the laboratory diagnosis and followup of CE and AE, including antibody, antigen and cytokine detection. All of these approaches are far from being optimal as adjunctive diagnosis particularly for CE, since they do not reach enough sensitivity and/or specificity. A combination of several methods (e.g., antibody and antigen detection) or of several (recombinant) antigens could improve the performance of the adjunctive laboratory methods, although the complexity of echinococcosis and heterogeneity of clinical cases make necessary a deep understanding of the host-parasite relationships and the parasite phenotype at different developmental stages to reach the best diagnostic tool and to make it accepted in clinical practice. Standardization approaches and a deep understanding of the performance of each of the available antigens in the diagnosis of echinococcosis for the different clinical pictures are also needed. The detection of the parasite in definitive hosts is also reviewed in this chapter. Finally, the different methods for the detection of parasite DNA in different analytes and matrices are also reviewed.

Biochemical Characterization of Echinococcus multilocularis Antigen B3 Reveals Insight into Adaptation and Maintenance of Parasitic Homeostasis at the Host-Parasite Interface

Alveolar echinococcosis (AE) caused by Echinococcus multilocularis metacestode is frequently associated with deleterious zoonotic helminthiasis. The growth patterns and morphological features of AE, such as invasion of the liver parenchyme and multiplication into multivesiculated masses, are similar to those of malignant tumors. AE has been increasingly detected in several regions of Europe, North America, Central Asia, and northwestern China. An isoform of E. multilocularis antigen B3 (EmAgB3) shows a specific immunoreactivity against patient sera of active-stage AE, suggesting that EmAgB3 might play important roles during adaptation of the parasite to hosts. However, expression patterns and biochemical properties of EmAgB3 remained elusive. The protein profile and nature of component proteins of E. multilocularis hydatid fluid (EmHF) have never been addressed. In this study, we conducted proteome analysis of EmHF of AE cysts harvested from immunocompetent mice. We observed the molecular and biochemical properties of EmAgB3, including differential transcription patterns of paralogous genes, macromolecular protein status by self-assembly, distinct oligomeric states according to individual anatomical compartments of the worm, and hydrophobic ligand-binding protein activity. We also demonstrated tissue expression patterns of EmAgB3 transcript and protein. EmAgB3 might participate in immune response and recruitment of essential host lipids at the host-parasite interface. Our results might contribute to an in depth understanding of the biophysical and biological features of EmAgB3, thus providing insights into the design of novel targets to control AE.

An Echinococcus multilocularis Antigen B3 Proteoform That Shows Specific Antibody Responses to Active-Stage Alveolar Echinococcosis

Alveolar echinococcosis (AE), caused by the Echinococcus multilocularis metacestode, represents one of the most frequently fatal zoonoses. Early diagnosis significantly reduces morbidity and mortality associated with AE. Diagnosis of AE largely depends on a combination of imaging and serological tests due to its minimal clinical manifestations. Several antigens derived from the whole worm and protoscolex have been targeted for AE serodiagnosis, while the antigenic properties of E. multilocularis hydatid fluid (EmHF) are unclear. We observed two AE-specific 6- and 8-kDa antigen proteoforms through an immunoproteome array of the EmHF. We identified these proteins as representing an E. multilocularis antigen B3 (EmAgB3) isoform, and the proteins were shown to be encoded by the same gene. We cloned the gene and expressed the recombinant EmAgB3 protein (rEmAgB3) in Escherichia coli. rEmAgB3 exhibited sensitivity of 90.9% (80/88 cases) and specificity of 98.5% (597/606 samples) by immunoblotting. The positive and negative predictive values were 89.9% and 98.6%, respectively. The protein did not show antibody responses to 33 AE sera collected during posttreatment follow-up monitoring. Mouse sera experimentally infected with AE protoscoleces began to demonstrate specific antibody responses to native and recombinant EmAgB3 6 months after infection. At that stage, fully mature metacestode vesicles that harbored the brood capsule, primary cell, and protoscolex were observed within an AE mass(es). The response declined along with worm degeneration. Our results demonstrate that the immune responses to this EmAgB3 isoform were highly correlated with worm viability accompanied with AE progression. rEmAgB3 is a promising biomarker for serological assessment of AE patients.

Lipid-free antigen B subunits from echinococcus granulosus: oligomerization, ligand binding, and membrane interaction properties

Background

The hydatid disease parasite Echinococcus granulosus has a restricted lipid metabolism, and needs to harvest essential lipids from the host. Antigen B (EgAgB), an abundant lipoprotein of the larval stage (hydatid cyst), is thought to be important in lipid storage and transport. It contains a wide variety of lipid classes, from highly hydrophobic compounds to phospholipids. Its protein component belongs to the cestode-specific Hydrophobic Ligand Binding Protein family, which includes five 8-kDa isoforms encoded by a multigene family (EgAgB1-EgAgB5). How lipid and protein components are assembled into EgAgB particles remains unknown. EgAgB apolipoproteins self-associate into large oligomers, but the functional contribution of lipids to oligomerization is uncertain. Furthermore, binding of fatty acids to some EgAgB subunits has been reported, but their ability to bind other lipids and transfer them to acceptor membranes has not been studied.

Methodology/Principal Findings

Lipid-free EgAgB subunits obtained by reverse-phase HPLC were used to analyse their oligomerization, ligand binding and membrane interaction properties. Size exclusion chromatography and cross-linking experiments showed that EgAgB8/2 and EgAgB8/3 can self-associate, suggesting that lipids are not required for oligomerization. Furthermore, using fluorescent probes, both subunits were found to bind fatty acids, but not cholesterol analogues. Analysis of fatty acid transfer to phospholipid vesicles demonstrated that EgAgB8/2 and EgAgB8/3 are potentially capable of transferring fatty acids to membranes, and that the efficiency of transfer is dependent on the surface charge of the vesicles.

Conclusions/Significance

We show that EgAgB apolipoproteins can oligomerize in the absence of lipids, and can bind and transfer fatty acids to phospholipid membranes. Since imported fatty acids are essential for Echinococcus granulosus, these findings provide a mechanism whereby EgAgB could engage in lipid acquisition and/or transport between parasite tissues. These results may therefore indicate vulnerabilities open to targeting by new types of drugs for hydatidosis therapy.

Echinococcus granulosus is a causative agent of hydatidosis, a parasitic disease that affects humans and livestock with significant economic and public health impact worldwide. Antigen B (EgAgB), an abundant product of E. granulosus larvae, is a lipoprotein that carries a wide variety of lipids, including fatty acids and cholesterol. As E. granulosus is unable to synthesize these lipids, EgAgB likely plays an important role in parasite metabolism, participating in both the acquisition of host lipids and their distribution between parasite tissues. The protein component of EgAgB consists of 8 kDa subunits encoded by separate genes. However, the biochemical properties of EgAgB subunits, particularly their ability to bind and transfer lipids, are poorly known. Herein, using in vitro assays, we found that EgAgB subunits were capable of oligomerizing in the absence of lipids and to bind fatty acids, but not cholesterol. Moreover, EgAgB subunits showed the ability to transfer fatty acids to artificial phospholipid membranes. These results indicate new points of attack at which the parasite might be vulnerable to drugs.

Validation of suitable reference genes for expression normalization in Echinococcus spp. larval stages

In recent years, a significant amount of sequence data (both genomic and transcriptomic) for Echinococcus spp. has been published, thereby facilitating the analysis of genes expressed during a specific stage or involved in parasite development. To perform a suitable gene expression quantification analysis, the use of validated reference genes is strongly recommended. Thus, the aim of this work was to identify suitable reference genes to allow reliable expression normalization for genes of interest in Echinococcus granulosus sensu stricto (s.s.) (G1) and Echinococcus ortleppi upon induction of the early pre-adult development. Untreated protoscoleces (PS) and pepsin-treated protoscoleces (PSP) from E. granulosus s.s. (G1) and E. ortleppi metacestode were used. The gene expression stability of eleven candidate reference genes (βTUB, NDUFV2, RPL13, TBP, CYP-1, RPII, EF-1α, βACT-1, GAPDH, ETIF4A-III and MAPK3) was assessed using geNorm, Normfinder, and RefFinder. Our qPCR data showed a good correlation with the recently published RNA-seq data. Regarding expression stability, EF-1α and TBP were the most stable genes for both species. Interestingly, βACT-1 (the most commonly used reference gene), and GAPDH and ETIF4A-III (previously identified as housekeeping genes) did not behave stably in our assay conditions. We propose the use of EF-1α as a reference gene for studies involving gene expression analysis in both PS and PSP experimental conditions for E. granulosus s.s. and E. ortleppi. To demonstrate its applicability, EF-1α was used as a normalizer gene in the relative quantification of transcripts from genes coding for antigen B subunits. The same EF-1α reference gene may be used in studies with other Echinococcus sensu lato species. This report validates suitable reference genes for species of class Cestoda, phylum Platyhelminthes, thus providing a foundation for further validation in other epidemiologically important cestode species, such as those from the Taenia genus.

Cystic echinococcoses in Mongolia: molecular identification, serology and risk factors

Background

Cystic echinococcosis (CE) is a globally distributed cestode zoonosis that causes hepatic cysts. Although Echinococcus granulosus sensu stricto (s.s.) is the major causative agent of CE worldwide, recent molecular epidemiological studies have revealed that E. canadensis is common in countries where camels are present. One such country is Mongolia.

Methodology/Principal Findings

Forty-three human hepatic CE cases that were confirmed histopathologically at the National Center of Pathology (NCP) in Ulaanbaatar (UB) were identified by analysis of mitochondrial cox 1 gene as being caused by either E. canadensis (n = 31, 72.1%) or E. granulosus s.s. (n = 12, 27.9%). The majority of the E. canadensis cases were strain G6/7 (29/31, 93.5%). Twenty three haplotypes were identified. Sixteen of 39 CE cases with data on age, sex and province of residence were citizens of UB (41.0%), with 13 of the 16 cases from UB caused by E. canadensis (G6/7) (81.3%). Among these 13 cases, nine were children (69.2%). All pediatric cases (n  =  18) were due to E. canadensis with 17 of the 18 cases (94.4%) due to strain G6/7. Serum samples were available for 31 of the 43 CE cases, with 22 (71.0%) samples positive by ELISA to recombinant Antigen B8/1 (rAgB). Nine of 10 CE cases caused by E. granulosus s.s. (90.0%) and 13 of 20 CE cases by E. canadensis (G6/7) (65.0%) were seropositive. The one CE case caused by E. canadensis (G10) was seronegative. CE cases caused by E. granulosus s.s. showed higher absorbance values (median value 1.131) than those caused by E. canadensis (G6/7) (median value 0.106) (p  =  0.0137).

Conclusion/Significance

The main species/strains in the study population were E. canadenis and E. granulossus s.s. with E. canadensis the predominant species identified in children. The reason why E. canadensis appears to be so common in children is unknown.

Cystic echinococcosis (CE) is a parasitic zoonosis with a cosmopolitan distribution. Molecular analysis was carried out on 43 hepatic CE cysts from 43 cases confirmed histopathologically at the NCP, Mongolia. Molecular analysis revealed two species, Echinococcus canadensis and Echinococcus granulosus s.s. Twenty three haplotypes of the cox1 gene were identified. All pediatric cases (n = 18) were by E. canadensis. Sixteen of 39 CE cases with data on age, sex and province of residence were from UB (41.0%), and 13 of these 16 cases were caused by E. canadensis (81.3%). Among the 13 cases from UB, nine were children (69.2%). A total of 31 serum samples from these 43 cases were analyzed for antibody response to rAgB with 22 (71.0%) samples positive by ELISA to rAgB. Thirteen of 20 E. canadensis (G6/7) (65%) and nine of 10 E. granulosus s.s. (90%) were seropositive. CE cases by E. granulosus s.s. showed a higher absorbance value than cases by E. canadensis (p  =  0.0137). This is the first study to evaluate age distribution of and antibody responses to rAgB in CE cases caused by the two species in Mongolia. It remains unknown why E. canadensis appears to be more common in pediatric cases.

Echinococcus P29 antigen: molecular characterization and implication on post-surgery follow-up of CE patients infected with different species of the Echinococcus granulosus complex

The protein P29 is a potential serological marker for post-treatment monitoring of cystic echinococcosis (CE) especially in young patients. We now have demonstrated that P29 is encoded in the Echinococcus genus by a single gene consisting of 7 exons spanning 1.2 kb of DNA. Variability of the p29 gene at inter- and intra-species level was assessed with 50 cDNA and 280 genomic DNA clones isolated from different E. granulosus s.l. isolates (E. granulosus sensu stricto (G1), E. equinus (G4), E. ortleppi (G5), E. canadensis (G6), E. canadensis (G7) and E. canadensis (G10)) as well as four E. multilocularis isolates. Scarce interspecies polymorphism at the p29 locus was observed and affected predominantly E. granulosus s.s. (G1), where we identified two alleles (A1 and A2) coding for identical P29 proteins and yielding in three genotypes (A1/A1, A2/A2 and A1/A2). Genotypic frequencies expected under Hardy-Weinberg equilibrium revealed a high rate of heterozygosity (47%) that strongly supports the hypothesis that E. granulosus s.s. (G1) is predominantly outbreeding. Comparative sequence analyses of the complete p29 gene showed that phylogenetic relationships within the genus Echinococcus were in agreement with those of previous nuclear gene studies. At the protein level, the deduced P29 amino acid (AA) sequences exhibited a high level of conservation, ranging from 97.9% AA sequence identity among the whole E. granulosus s.l. group to 99.58% identity among E. multilocularis isolates. We showed that P29 proteins of these two species differ by three AA substitutions without implication for antigenicity. In Western-blot analyses, serum antibodies from a human CE patient infected with E. canadensis (G6) strongly reacted with recombinant P29 from E. granulosus s.s. (G1) (recEg(G1)P29). In the same line, human anti-Eg(G1)P29 antibodies bound to recEcnd(G6)P29. Thus, minor AA sequence variations appear not to impair the prognostic serological use of P29.

Serological and molecular characteristics of the first Korean case of Echinococcus multilocularis

In December 2011, we reported an autochthonous case of Echinococcus multilocularis infection in a 42-year-old woman in Korea. The diagnosis was based on histopathological findings of the surgically resected liver cyst. In the present study, we evaluated the serological and molecular characteristics of this Korean E. multilocularis case. The patient's serum strongly reacted with affinity-purified native Em18 and recombinant Em18 antigens (specific for E. multilocularis) but negative for recombinant antigen B8/1 (reactive for Echinococcus granulosus). In immunoaffinity chromatography, the serum also strongly reacted with E. multilocularis and only weakly positive for E. granulosus. We determined the whole nucleotide sequence of cox1 (1,608 bp) using the paraffin-embedded cystic tissue which was compared with E. multilocularis isolates from China, Japan, Kazakhstan, Austria, France, and Slovakia. The Korean case showed 99.8-99.9% similarity with isolates from Asia (the highest similarity with an isolate from Sichuan, China), whereas the similarity with European isolates ranged from 99.5 to 99.6%.

Strategies of Echinococcus species responses to immune attacks: implications for therapeutic tool development

Echinococcus species have been studied as a model to investigate parasite-host interactions. Echinococcus spp. can actively communicate dynamically with a host to facilitate infection, growth and proliferation partially via secretion of molecules, especially in terms of harmonization of host immune attacks. This review systematically outlines our current knowledge of how the Echinococcus species have evolved to adapt to their host's microenvironment. This understanding of parasite-host interplay has implications in profound appreciation of parasite plasticity and is informative in designing novel and effective tools including vaccines and drugs for the treatment of echinococcosis and other diseases.

Priorities for research and control of cestode zoonoses in Asia

Globally, cestode zoonoses cause serious public health problems, particularly in Asia. Among all neglected zoonotic diseases, cestode zoonoses account for over 75% of global disability adjusted life years (DALYs) lost. An international symposium on cestode zoonoses research and control was held in Shanghai, China between 28th and 30th October 2012 in order to establish joint efforts to study and research effective approaches to control these zoonoses. It brought together 96 scientists from the Asian region and beyond to exchange ideas, report on progress, make a gap analysis, and distill prioritizing settings with a focus on the Asian region. Key objectives of this international symposium were to agree on solutions to accelerate progress towards decreasing transmission, and human mortality and morbidity caused by the three major cestode zoonoses (cystic echinococcosis, alveolar echinococcosis, and cysticercosis); to critically assess the potential to control these diseases; to establish a research and validation agenda on existing and new approaches; and to report on novel tools for the study and control of cestode zoonoses.

A transcriptomic analysis of Echinococcus granulosus larval stages: implications for parasite biology and host adaptation

Background

The cestode Echinococcus granulosus - the agent of cystic echinococcosis, a zoonosis affecting humans and domestic animals worldwide - is an excellent model for the study of host-parasite cross-talk that interfaces with two mammalian hosts. To develop the molecular analysis of these interactions, we carried out an EST survey of E. granulosus larval stages. We report the salient features of this study with a focus on genes reflecting physiological adaptations of different parasite stages.

Methodology/Principal Findings

We generated ∼10,000 ESTs from two sets of full-length enriched libraries (derived from oligo-capped and trans-spliced cDNAs) prepared with three parasite materials: hydatid cyst wall, larval worms (protoscoleces), and pepsin/H⁺-activated protoscoleces. The ESTs were clustered into 2700 distinct gene products. In the context of the biology of E. granulosus, our analyses reveal: (i) a diverse group of abundant long non-protein coding transcripts showing homology to a middle repetitive element (EgBRep) that could either be active molecular species or represent precursors of small RNAs (like piRNAs); (ii) an up-regulation of fermentative pathways in the tissue of the cyst wall; (iii) highly expressed thiol- and selenol-dependent antioxidant enzyme targets of thioredoxin glutathione reductase, the functional hub of redox metabolism in parasitic flatworms; (iv) candidate apomucins for the external layer of the tissue-dwelling hydatid cyst, a mucin-rich structure that is critical for survival in the intermediate host; (v) a set of tetraspanins, a protein family that appears to have expanded in the cestode lineage; and (vi) a set of platyhelminth-specific gene products that may offer targets for novel pan-platyhelminth drug development.

Conclusions/Significance

This survey has greatly increased the quality and the quantity of the molecular information on E. granulosus and constitutes a valuable resource for gene prediction on the parasite genome and for further genomic and proteomic analyses focused on cestodes and platyhelminths.

Cestodes are a neglected group of platyhelminth parasites, despite causing chronic infections to humans and domestic animals worldwide. We used Echinococcus granulosus as a model to study the molecular basis of the host-parasite cross-talk during cestode infections. For this purpose, we carried out a survey of the genes expressed by parasite larval stages interfacing with definitive and intermediate hosts. Sequencing from several high quality cDNA libraries provided numerous insights into the expression of genes involved in important aspects of E. granulosus biology, e.g. its metabolism (energy production and antioxidant defences) and the synthesis of key parasite structures (notably, the one exposed to humans and livestock intermediate hosts). Our results also uncovered the existence of an intriguing set of abundant repeat-associated non-protein coding transcripts that may participate in the regulation of gene expression in all surveyed stages. The dataset now generated constitutes a valuable resource for gene prediction on the parasite genome and for further genomic and proteomic studies focused on cestodes and platyhelminths. In particular, the detailed characterization of a range of newly discovered genes will contribute to a better understanding of the biology of cestode infections and, therefore, to the development of products allowing their efficient control.

Analysis on the reactivity of five subunits of antigen B family in serodiagnosis of echinococcosis

In this study, the reactivity and differences of five subunits of echinococcus antigen B (AgB) family, recognizing specific antibodies in echinococcosis patient serum, were analyzed. Eight recombinant subunit antigens from Echinococcus granulosus (EgAgB1-EgAgB4) and Echinococcus multilocularis (EmAgB1-EmAgB3 and EmAgB5) were tested by ELISA using a panel of 243 serum samples collected from cystic echinococcosis (CE), alveolar echinococcosis (AE), cysticercosis (CC) patients and clinically normal individuals (NH). The results showed that the diagnostic sensitivity of the subunits for CE sera were 83.06%, 62.90%, 29.03%, 75.81% and 41.13%, and the specificities were 73.95%, 72.27%, 76.47%, 73.11% and 85.71%, respectively. The reactivity of three paralogous subunits, EgAgB1, EgAgB2 and EgAgB3 from E. granulosus and EmAgB1, EmAgB2 and EmAgB3 from E. multilocularis were compared by serological assay. All of the orthologous subunits showed no statistical difference (P>0.05) in detecting CE and AE sera; it revealed that the reactive epitopes may be similar between the orthologous subunits. In a total of 124 CE sera, the positive recognition rate by EgAgB1 was the highest (103/124), yet cocktail subunit antigens may detect even more positives from 100/124 to 112/124 using different subunit combinations. IgG4 subclass was the predominant antibody in reacting with subunit antigens. To conclude, the epitopes of orthologous AgB subunits from E. granulosus and E. multilocularis that recognize specific antibodies may be similar. The paralogous subunits EgAgB1, EgAgB2 and EgAgB4 were the main reactive subunit in sera detection and may have utility as echinococcosis diagnostics, with EgAgB1 possessing the greatest potential. Cocktail subunits may improve the positive detection rate.

Echinococcus granulosus antigen B structure: subunit composition and oligomeric states

BACKGROUND

Antigen B (AgB) is the major protein secreted by the Echinococcus granulosus metacestode and is involved in key host-parasite interactions during infection. The full comprehension of AgB functions depends on the elucidation of several structural aspects that remain unknown, such as its subunit composition and oligomeric states.

METHODOLOGY/PRINCIPAL FINDINGS

The subunit composition of E. granulosus AgB oligomers from individual bovine and human cysts was assessed by mass spectrometry associated with electrophoretic analysis. AgB8/1, AgB8/2, AgB8/3 and AgB8/4 subunits were identified in all samples analyzed, and an AgB8/2 variant (AgB8/2v8) was found in one bovine sample. The exponentially modified protein abundance index (emPAI) was used to estimate the relative abundance of the AgB subunits, revealing that AgB8/1 subunit was relatively overrepresented in all samples. The abundance of AgB8/3 subunit varied between bovine and human cysts. The oligomeric states formed by E. granulosus AgB and recombinant subunits available, rAgB8/1, rAgB8/2 and rAgB8/3, were characterized by native PAGE, light scattering and microscopy. Recombinant subunits showed markedly distinct oligomerization behaviors, forming oligomers with a maximum size relation of rAgB8/3>rAgB8/2>rAgB8/1. Moreover, the oligomeric states formed by rAgB8/3 subunit were more similar to those observed for AgB purified from hydatid fluid. Pressure-induced dissociation experiments demonstrated that the molecular assemblies formed by the more aggregative subunits, rAgB8/2 and rAgB8/3, also display higher structural stability.

CONCLUSIONS/SIGNIFICANCE

For the first time, AgB subunit composition was analyzed in samples from single hydatid cysts, revealing qualitative and quantitative differences between samples. We showed that AgB oligomers are formed by different subunits, which have distinct abundances and oligomerization properties. Overall, our findings have significantly contributed to increase the current knowledge on AgB expression and structure, highlighting issues that may help to understand the parasite adaptive response during chronic infection.

Morphological and biological characterization of cell line developed from bovine Echinococcus granulosus

The taeniid tapeworm Echinococcus granulosus is the causative agent of echinococcal disease, a major zoonosis with worldwide distribution. Several efforts to establish an in vitro model of E. granulosus have been undertaken; however, many of them have been designed for Echinococcus multilocularis. In the present study, we have described and characterized a stable cell line obtained from E. granulosus bovine protoscoleces maintained 3 yr in vitro. Growth characterization, morphology by light, fluorescent and electronic microscopy, and karyotyping were carried out. Cell culture origin was confirmed by immunofluorescent detection of AgB4 antigen and by PCR for the mitochondrial cytochrome c-oxidase subunit 1 (DCO1) gene. Cells seeded in agarose biphasic culture resembled a cystic structure, similar to the one formed in secondary hosts. This cell line could be a useful tool to research equinococcal behavior, allowing additional physiological and pharmacological studies, such as the effect of growth factors, nutrients, and antiparasitic drugs on cell viability and growth and on cyst formation.

The Echinococcus granulosus antigen B gene family comprises at least 10 unique genes in five subclasses which are differentially expressed

BACKGROUND

Antigen B (EgAgB) is a major protein produced by the metacestode cyst of Echinococcus granulosus, the causative agent of cystic hydatid disease. This protein has been shown to play an important role in modulating host immune responses, although its precise biological function still remains unknown. It is generally accepted that EgAgB is comprised of a gene family of five subfamilies which are highly polymorphic, but the actual number of genes present is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Based on published sequences for the family, we designed specific primers for each subfamily and used PCR to amplify them from genomic DNA isolated from individual mature adult worms (MAW) taken from an experimentally infected dog in China and individual larval protoscoleces (PSC) excised from a single hydatid cyst taken from an Australian kangaroo. We then used real-time PCR to measure expression of each of the genes comprising the five EgAgB subfamilies in all life-cycle stages including the oncosphere (ONC).

CONCLUSIONS/SIGNIFICANCE

Based on sequence alignment analysis, we found that the EgAgB gene family comprises at least ten unique genes. Each of the genes was identical in both larval and adult E. granulosus isolates collected from two geographical areas (different continents). DNA alignment comparisons with EgAgB sequences deposited in GenBank databases showed that each gene in the gene family is highly conserved within E. granulosus, which contradicts previous studies claiming significant variation and polymorphism in EgAgB. Quantitative PCR analysis revealed that the genes were differentially expressed in different life-cycle stages of E. granulosus with EgAgB3 expressed predominantly in all stages. These findings are fundamental for determining the expression and the biological function of antigen B.

Echinococcus multilocularis as an experimental model in stem cell research and molecular host-parasite interaction

Totipotent somatic stem cells (neoblasts) are key players in the biology of flatworms and account for their amazing regenerative capability and developmental plasticity. During recent years, considerable progress has been made in elucidating molecular features of neoblasts from free-living flatworms, whereas their role in parasitic species has so far merely been addressed by descriptive studies. Very recently, however, significant advances have been made in the in vitro culture of neoblasts from the cestode Echinococcus multilocularis. The isolated cells proved capable of generating mature metacestode vesicles under laboratory conditions in a manner that closely resembles the oncosphere-metacestode transition during natural infections. Using the established neoblast cultivation protocols, combined with targeted manipulation of Echinococcus genes by RNA-interference, several fundamental questions of host-dependent parasite development can now be addressed. Here, I give an overview of current cultivation techniques for E. multilocularis neoblasts and present experimental approaches to study their function. Furthermore, I introduce the E. multilocularis genome sequencing project that is presently in an advanced stage. The combined input of data from the E. multilocularis sequencing project, stem cell cultivation, and recently initiated attempts to genetically manipulate Echinococcus will provide an ideal platform for hypothesis-driven research into cestode development in the next years.

Taenia crassiceps cysticerci: Characterization of the 14-kDa glycoprotein with homologies to antigens from Taenia solium cysticerci

Glycoproteins from the total vesicular fluid of Taenia crassiceps (VF-Tc) were prepared using three different purification methods, consisting of ConA-lectin affinity chromatography (ConA-Tc), preparative electrophoresis (SDS-PAGE) (14 gp-Tc), and monoclonal antibody immunoaffinity chromatography (18/14-Tc). The complex composition represented by the VF-Tc and ConA-Tc antigens revealed peptides ranging from 101- to 14-kDa and from 92- to 12-kDa, respectively. Immunoblotting using lectins confirmed glucose/mannose (glc/man) residues in the 18- and 14-kDa peptides, which are considered specific and immunodominant for the diagnosis of cysticercosis, and indicated that these fractions are glycoproteins. Serum antibodies from a patient with neurocysticercosis that reacted to the 14 gp band from T. crassiceps (Tc) were eluted from immunoblotting membranes and showed reactivity to 14 gp from Taenia solium. In order to determine the similar peptide sequence, the N-terminal amino acid was determined and analyzed with sequences available in public databases. This sequence revealed partial homology between T. crassiceps and T. solium peptides. In addition, mass spectrometry along with theoretical M(r) and pI of the 14 gp-Tc point suggested a close relationship to some peptides of a 150-kDa protein complex of the T. solium previously described. The identification of these common immunogenic sites will contribute to future efforts to develop recombinant antigens and synthetic peptides for immunological assays.

Relative expression of antigen B coding gene of bubaline isolates of Echinococcus granulosus in fertile and sterile cysts

This article communicates the relative quantification of five isoforms of antigen B (AgB) of Echinococcus granulosus. Relative expression of the AgB was quantified in active and inactive cysts. Cysts with germinal membrane, clear cyst fluid and protoscoleces showed uniform expression of the five isoforms and were utilized as control. Relative expression of AgB1 was the highest in cysts, where calcification has initiated. AgB2 and AgB4 were expressed more in fertile cysts irrespective of the condition of germinal membrane. The lowest expression of AgB3 was seen in calcified cysts. The relative expression of AgB5 could not be correlated with respect to the condition of the cyst because AgB5 is typically expressed by the adult stage of the parasite.

Genetic variability of antigen B among Echinococcus granulosus Egyptian isolates

Genetic polymorphisms of encoding antigen B2 gene (AgB2) in Echinococcus granulosus were studied using PCR-RFLP and DNA sequencing among 20 Egyptian isolates. Five isolates from different host origins (humans, camels, pigs, and sheep) were collected and used. All examined isolates of each host group gave very similar patterns of PCR-RFLP after restriction enzyme digestion with AluI, with the gene size of approximately 140 bp and 240 bp for sheep and human isolates, and approximately 150 bp and 250 bp for pig and camel isolates. No digestion pattern was obtained after incubation of all studied isolates with EcoRI. These results reveal high intra-group homogeneity. DNA sequence analysis highlighted that human infecting strain showed 100% identity with respect to sheep infecting isolate, 96% and 99% with pig and camel infecting isolates, respectively.

Production of monoclonal antibodies against the 8 kDa subunit of Echinococcus granulosus Antigen B (EgAgB8/2) using DNA immunization

Cystic echinococcosis (CE), an endemic cosmopolitan zoonotic helminthic disease caused by the larval stage of Echinococcus granulosus, lacks reliable diagnostic tools that fulfill the criteria of high sensitivity and specificity. Antigen B (AgB), a thermostable lipoprotein that constitutes a considerable fraction of the cystic hydatid fluid (HF), is being considered as a suitable source for vaccination and immunodiagnosis of CE due to its high specificity. Genetic immunization was used to immunize BALB/c mice with the second subunit of antigen B (EgAgB8/2) for the production of monoclonal antibodies (MAb). Fusion products between the spleen cells and myeloma cells produced six MAbs of the following isotypes: IgG2a (two clones), IgG2b (three clones), and IgM (one clone). The MAbs were tested for their specificity to crude sheep hydatid fluid (CSHF) versus other antigens prepared from other helminthic parasites including Toxocara canis, Acanthocheilonema viteae, Fasciola hepatica, Schistosoma mansoni, and Taenia. Five MAbs reacted with E. granulosus antigens, one showed cross reactivity with S. mansonia antigens, and one showed a high reactivity with E. granulosus but was cross reactive with all helminthic antigens tested. Using SDS-PAGE and immunoblotting under reducing conditions, all MAbs identified the four AgB subunits with molecular weights of 8, 16, 24, and 36 kDa. Further work on the specificity and sensitivity of these MAbs as well as their use in detecting circulating parasite antigens and in antigen purification will be assessed in future studies.

Echinococcus granulosus antigen B gene family: Further studies of strain polymorphism at the genomic and transcriptional levels

Echinococcus granulosus AgB gene family is constituted by five gene loci. In a previous study, we analyzed the strain variation of EgAgB2/B4 sequences. Here, we have analyzed, by SSCP and sequencing, 250 genomic clones of EgAgB1/B3/B5 gene cluster from five E. granulosus strains. Several new EgAgB genomic variants were found. EgAgB1 and EgAgB3 genomic sequences grouped E. granulosus strains by phylogenetic tools in two clusters: one formed by G1/G2 and the other by G5, G6/G7 strains, in accordance with other molecular markers. EgAgB5 genomic and cDNA sequences were only found in G1/G2 cluster. Reverse transcription-PCR analysis using subunit specific primers revealed that all the EgAgB genes were transcribed in G1 and G7 strains with the exception of EgAgB5 transcripts that were not detected in G7 strain. Interestingly, AgB2 transcripts that were probably processed by an aberrant splicing mechanism leading to a non-functional B2 protein were found in G7 strain.

Echinococcosis: serological detection of patients and molecular identification of parasites

Alveolar (AE) and cystic echinococcosis (CE) are two of the most dangerous helminthic zoonoses worldwide, representing chronic hepatic diseases, often with lethal outcome. Since early diagnosis of echinococcosis is essential for effective treatment, an overview of serological methods for the detection of echinococcosis and differentiation between AE and CE is given. Recombinant antigens Em18 and Antigen B8/1 are highly recommended for patient screening and identification of AE and CE, respectively, in combination with imaging techniques. Novel aspects of molecular phylogenetic studies on the genus Echinococcus will also be addressed, including the description of Echinococcus shiquicus as a new sister-species of Echinococcus multilocularis. Both the serological detection of the disease and molecular phylogeny will be discussed in perspective.

Molecular characterization of a novel gene encoding an 8-kDa-subunit of antigen B from Echinococcus granulosus genotypes 1 and 6

Antigen B in hydatid cyst fluid of Echinococcus granulosus is a polymeric lipoprotein of 160 kDa, and is an aggregate of several different but homologous small proteins with approximately 8 kDa which are encoded by a multigene family. Four genes encoding 8-kDa-subunit monomers of the antigen B have been identified from E. granulosus. Recently, we have isolated another novel gene from Echinococcus multilocularis encoding a fifth 8-kDa-subunit of AgB (named EmAgB8/5), predominantly transcribed in the adult worm, but not in vesicles of metacestodes. In this study, we cloned and characterized two EmAgB8/5 homologue genes from E. granulosus genotypes 1 and 6 by PCR, and named as EgG1AgB8/5 and EgG6AgB8/5, respectively. The phylogenetic relationship of these genes with other genes encoding the antigen B 8-kDa-subunit monomers was also discussed.

The immunodiagnosis of Echinococcus multilocularis infection

Alveolar echinococcosis (AE) is a severe zoonotic disease caused by the metacestode stage of Echinococcus multilocularis. The infection can have fatal consequences in humans if treatment is not provided, so early diagnosis is fundamental for initiating treatment and reducing morbidity and mortality. In addition, detection of the parasite in the definitive host plays a central role in epidemiological studies and surveillance programmes for control of AE. This review presents an overview of the present situation regarding the immunodiagnosis of E. multilocularis infection. Special attention is given to the description of the native, partially purified and recombinant antigens available currently for immunodiagnostic purposes. Recent advances in the primary serodiagnosis and follow-up of AE patients are highlighted, including the detection of specific cytokine profiles. Progress in the immunodiagnosis of intestinal E. multilocularis infection in definitive hosts, particularly the detection of excretory-secretory and integument products of the worm in faeces (copro-antigens) by ELISA, is also discussed.

Self-assembly and structural characterization of Echinococcus granulosus antigen B recombinant subunit oligomers

Echinococcus granulosus antigen B is an oligomeric protein of 120-160 kDa composed by 8-kDa (AgB8) subunits. Here, we demonstrated that the AgB8 recombinant subunits AgB8/1, AgB8/2 and AgB8/3 are able to self-associate into high order homo-oligomers, showing similar properties to that of parasite-produced AgB, making them valuable tools to study AgB structure. Dynamic light scattering, size exclusion chromatography and cross-linking assays revealed approximately 120- to 160-kDa recombinant oligomers, with a tendency to form populations with different aggregation states. Recombinant oligomers showed helical circular dichroism spectra and thermostability similar to those of purified AgB. Cross-linking and limited proteolysis experiments indicated different degrees of stability and compactness between the recombinant oligomers, with the AgB8/3 one showing a more stable and compact structure. We have also built AgB8 subunit structural models in order to predict the surfaces possibly involved in electrostatic and hydrophobic interactions during oligomerization.

Expression and diversity of Echinococcus multilocularis AgB genes in secondarily infected mice: evaluating the influence of T-cell immune selection on antigenic variation

The T-cell-mediated immune response exhibits a crucial function in the control of the intrahepatic proliferation of Echinococcus multilocularis larvae in mice and humans, both being natural intermediate hosts of the parasite. Antigen B (AgB), a metabolized Echinococcus spp. lipoprotein, contributes to the modulation of the T-cell immune response, and distinct sites of the corresponding AgB1, AgB3 and AgB4 genes were shown to be under positive selection pressure. Since several AgB gene variants are present in a single Echinococcus metacestode, we used secondary E. multilocularis infections in BALB/c and in athymic nude mice (devoid of T-cell responses) to analyze the effect of the cellular immune response on the expression and diversity of EmAgB1-EmAgB4 genes. We demonstrated hereby that EmAgB transcripts were less abundant in nude mice during the early phase of infection (at one month post-infection), and that EmAgB2 is simultaneously down-regulated when compared to the other three genes. A negative relationship exists between the level of transcription and diversity of EmAgB genes. Moreover, no excess of non-synonymous substitutions was found among the distinct EmAgB alleles from a single host. Together, these results pointed to the effect of purifying selection, which seemed to eliminate the detrimental AgB variants generated during the development of the metacestode within the peritoneal cavity of its intermediate host.

Redundancy and recombination in the Echinococcus AgB multigene family: is there any similarity with protozoan contingency genes?

Numerous genetic variants of the Echinococcus antigen B (AgB) are encountered within a single metacestode. This could be a reflection of gene redundancy or the result of a somatic hypermutation process. We evaluate the complexity of the AgB multigene family by characterizing the upstream promoter regions of the 4 already known genes (EgAgB1-EgAgB4) and evaluating their redundancy in the genome of 3 Echinococcus species (E. granulosus, E. ortleppi and E. multilocularis) using PCR-based approaches. We have ascertained that the number of AgB gene copies is quite variable, both within and between species. The most repetitive gene seems to be AgB3, of which there are more than 110 copies in E. ortleppi. For E. granulosus, we have cloned and characterized 10 distinct upstream promoter regions of AgB3 from a single metacestode. Our sequences suggest that AgB1 and AgB3 are involved in gene conversion. These results are discussed in light of the role of gene redundancy and recombination in parasite evasion mechanisms of host immunity, which at present are known for protozoan organisms, but virtually unknown for multicellular parasites.