Molecular cloning and characterization of an Echinococcus granulosus cDNA encoding malate dehydrogenase

Evaluation of the sensitivity and specificity of GST-tagged recombinant antigens 2B2t, Ag5t and DIPOL in ELISA for the diagnosis and follow up of patients with cystic echinococcosis

Cystic echinococcosis (CE) is a neglected zoonotic disease caused by Echinococcus granulosus sensu lato. Diagnosis and monitoring of CE rely primarily on imaging while serology is used as a confirmatory test. However, imaging is not always conclusive and currently available serological assays have suboptimal sensitivity and specificity, lack standardization, and are not useful for patients´ follow-up. Seroassays for CE are usually based on hydatid fluid (HF), a complex, variable antigenic mixture, and cross-reactivity exists especially with alveolar echinococcosis. Recombinant proteins based on immunogenic antigens most abundant in HF, such as AgB1, AgB2 and Ag5, have been used to overcome these limitations. None of them so far showed potential to replace HF; however, their performance have been largely tested on a limited number of samples, and comparison of different antigens using the same cohort has been rarely performed. The combination of several immunogenic epitopes in a single recombinant protein could enhance test sensitivity. For the diagnosis and follow-up of patients with CE, we compared the performance of the crude HF, previously described recombinant 2B2t antigen, and GST-tagged version of 2B2t, and novel designed recombinants (GST-Ag5t and the GST-DIPOL chimera containing AgB1, AgBB2 and Ag5 epitopes) by IgG-ELISA format. Samples belong to a retrospective cohort of 253 well-characterized patients with CE, previously described for the evaluation of the 2B2t antigen, 92 patients with alveolar echinococcosis, and 82 healthy donors. The reference standard for CE diagnosis was the presence of a CE lesion as diagnosed by ultrasonography. The highest sensitivity was obtained with HF [86.7%, 95% confidence interval (CI): 81.2–91.0], followed by GST-2B2t (70.0%, 95% CI: 63.1–76.2), 2B2t (65.5%, 95% CI: 58.5–72.0), GST-Ag5t (64.5%, 95% CI: 57.5–71.1) and GST-DIPOL (63.1%, 95% CI: 56.0–69.7). The GST-2B2t had the best specificity (95.8%, 95% CI: 88.3–99.1) and the lowest cross-reactivity (38.7%, 95% CI: 27.6–50.6). Good response to treatment also correlated to negative test results in the GST-2B2t ELISA. While none of the tested recombinant antigen appears suitable to replace HF for the diagnosis of CE, GST-2B2t should be further explored as a confirmation test, based on its high specificity and low cross-reactivity, and for the follow-up after treatment in those patients with positive serology for this antigen.

Cystic echinococcosis (CE) is a neglected parasitic zoonosis. Its diagnosis and follow-up require evaluation with imaging. Currently available serological tests are applied to confirm the diagnosis in doubtful cases, although having limitations in diagnostic accuracy, and they are not useful for patients’ follow-up. Seroassays for CE are usually based on hydatid fluid (HF) obtained from infected animals, with consequent problems of heterogeneity and low specificity. The use of semi-purified HF derivatives or recombinant antigens has been attempted to improve these aspects, but with an unacceptable loss in sensitivity. Most newly developed antigens have been tested on a limited number of samples, not always well characterized, and have been rarely compared using the same samples cohort. Here, we tested and compared three recombinant antigens (2B2t, GST-2B2t and GST-Ag5t), and a recombinant chimeric antigen (DIPOL) based on three highly immunogenic components of HF (B1, B2 and Ag5), in an attempt to increase the sensitivity of recombinant antigen-based seroassays for the diagnosis and follow-up of patients with CE. We found that GST-2B2t had higher sensitivity than the other antigenic preparations, but still not as high as HF, and that GST-2B2t and GST-DIPOL had statistically higher specificity than any of the other tested antigens. GST-2B2t also showed potential for the follow-up of patients with CE after drug treatment.

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.

Fructose-bisphosphate aldolase and enolase from Echinococcus granulosus: genes, expression patterns and protein interactions of two potential moonlighting proteins

Glycolytic enzymes, such as fructose-bisphosphate aldolase (FBA) and enolase, have been described as complex multifunctional proteins that may perform non-glycolytic moonlighting functions, but little is known about such functions, especially in parasites. We have carried out in silico genomic searches in order to identify FBA and enolase coding sequences in Echinococcus granulosus, the causative agent of cystic hydatid disease. Four FBA genes and 3 enolase genes were found, and their sequences and exon-intron structures were characterized and compared to those of their orthologs in Echinococcus multilocularis, the causative agent of alveolar hydatid disease. To gather evidence of possible non-glycolytic functions, the expression profile of FBA and enolase isoforms detected in the E. granulosus pathogenic larval form (hydatid cyst) (EgFBA1 and EgEno1) was assessed. Using specific antibodies, EgFBA1 and EgEno1 were detected in protoscolex and germinal layer cells, as expected, but they were also found in the hydatid fluid, which contains parasite's excretory-secretory (ES) products. Besides, both proteins were found in protoscolex tegument and in vitro ES products, further suggesting possible non-glycolytic functions in the host-parasite interface. EgFBA1 modeled 3D structure predicted a F-actin binding site, and the ability of EgFBA1 to bind actin was confirmed experimentally, which was taken as an additional evidence of FBA multifunctionality in E. granulosus. Overall, our results represent the first experimental evidences of alternative functions performed by glycolytic enzymes in E. granulosus and provide relevant information for the understanding of their roles in host-parasite interplay.

Purification, properties, and kinetic studies of cytoplasmic malate dehydrogenase from Taenia solium cysticerci

Malate dehydrogenase (L: -malate: NAD oxidoreductase, EC 1.1.1.37) from the cytoplasm of Taenia solium cysticerci (cMDHTs) was purified 48-fold through a four-step procedure involving salt fractionation, ionic exchange, and dye affinity chromatography. cMDHTs had a native M (r) of 64,000, while the corresponding value per subunit, obtained under denaturing conditions, was 32,000. The enzyme is partially positive, with an isoelectric point of 8.7, and had a specific activity of 2,615 U mg(-1) in the reduction of oxaloacetate. The second to the 21st amino acids from cMDHTs N-terminal group were P G P L R V L I T G A A G Q I A Y N L S. This sequence is 100% identical to that of Echinococcus granulosus. Basic kinetic parameters were determined for this enzyme. The optimum pH for enzyme reaction was at 7.6 for oxaloacetate reduction and at 9.6 for malate oxidation. K (m) values for oxaloacetate, malate, NAD, and NADH were 2.4, 215, 50, and 48 microM, respectively. V (max) values for the substrates and cosubstrates as described above were 1,490, 87.8, 104, and 1,714 micromol min(-1) mg(-1). Several NAD analogs, structurally altered in either the pyridine or purine moiety, were observed to function as coenzymes in the reaction catalyzed by the purified malate dehydrogenase. cMDHTs activity was uncompetitive inhibited by arsenate for both the forward (Ki = 8.2 mM) and reverse (Ki = 77 mM) reactions. The mechanism of the cMDHTs reactivity was investigated kinetically by the product inhibition approach. The results of this study are qualitatively consistent with an Ordered Bi Bi reaction mechanism, in which only the coenzymes can react with the free enzyme.

Comparative analysis of the diagnostic performance of six major Echinococcus granulosus antigens assessed in a double-blind, randomized multicenter study

The serodiagnosis of hydatid disease is a valuable instrument for clinical diagnosis and epidemiological surveillance of high-risk populations. In the past decade a wealth of reports on the diagnostic performance of numerous antigens have been produced. However, their diagnostic value has been estimated under different conditions, using different serum collection, therefore precluding their direct comparison. Here we report an unbiased comparison of the same batch of six major E. granulosus antigens, namely, hydatid cyst fluid (HCF), native antigen B (AgB), two recombinant AgB subunits, an AgB-derived synthetic peptide, and recombinant cytosolic malate dehydrogenase from E. granulosus (EgMDH), against the same serum collection. The double-blind analysis was performed using a standardized protocol and receiver operating characteristic (ROC) data analysis by a network of six South American laboratories. High intercenter reproducibility was attained, and the intralaboratory analysis allowed the comparative ranking of the antigen panel. HCF, AgB, and its AgB8/1 subunit exhibited equivalent diagnostic efficiencies, 81.4% +/- 0.5%, 81.3% +/- 0.6%, and 81.9% +/- 2.0%, respectively; with a more favorable balance toward specificity in the case of the last antigen. The diagnostic efficiencies for the other three antigens were 76.8% +/- 6.8%, 69.1% +/- 2.7%, and 66.8% +/- 2.1%, for the peptide, the AgB8/2 subunit, and the EgMDH, respectively. The study also included an analysis of batch-to-batch variation in the diagnostic performance of different HCF regional preparations. Based on these results, a suggested recommendation on the use of these antigens was drawn.

Purification, cloning, and expression of the mitochondrial malate dehydrogenase (mMDH) from protoscolices of Echinococcus granulosus

Protoscolices of the parasitic helminth Echinococcus granulosus contain two malate dehydrogenases (EC 1.1.1.37), one cytosolic and one mitochondrial. The latter has been separated from the other isoform and purified to protein homogeneity. Sequencing of tryptic peptides by Edman degradation allowed the design of oligonucleotide primers for PCR, leading to the cloning and sequencing of a full length cDNA. The encoding gene is present as a single copy per haploid genome and codes for a protein with high sequence identity (56-58%) with the similar enzymes from mammals, Caenorhabditis elegans and yeast. Active recombinant mitochondrial malate dehydrogenase was expressed in Escherichia coli, as protein fusions with glutathione S-transferase or a poly-His tail. The purified recombinant enzymes had a kinetic behaviour similar to that of the native enzyme, being inhibited by excess of the substrate oxaloacetate and unaffected by excess L-malate. The results indicate that E. granulosus contains two typical eukaryotic malate dehydrogenases, with relative levels quite different from those present in mammalian tissues like heart, in good agreement with the predominantly fermentative metabolism of the protoscolices.

A set of recombinant antigens from Echinococcus granulosus with potential for use in the immunodiagnosis of human cystic hydatid disease

Several recombinant clones expressing antigens from Echinococcus granulosus were isolated previously from a parasite cDNA library using cystic hydatid disease (CHD) patients' sera or rabbit hyperimmune antiserum against a lipoproteic fraction from bovine cyst fluid. Six of these antigens were expressed in Escherichia coli and the purified recombinant proteins were tested in enzyme-linked immunosorbent assay (ELISA) for specific IgG with a panel of sera from patients with surgically confirmed (n = 58) or immunologically diagnosed (n = 71) CHD. Sera from clinically normal individuals (n = 203) and sera from individuals with other helminthic infections (n = 65) were assayed for the assessment of specificity. A cut-off value was determined by receiver-operating-characteristic plots for each antigen. A recombinant antigen B subunit (AgB8/2) presented the highest sensitivity (93.1%), considering the group of sera from patients with CHD surgically confirmed, and specificity (99.5%) and is proposed as the basis for an immunodiagnostic test. The other recombinant antigens tested presented sensitivities between 58.6% and 89.7%, and three of them were considered of complementary value. In subclass-specific ELISA, different IgG isotypes showed dominance in the response for each of the recombinant antigens. There was a clear predominance of IgG4 response for all antigens tested, indicating that this would be the subclass of choice to be assessed for these recombinant proteins.

Identification of a differentially expressed Echinococcus multilocularis protein Em6 potentially related to antigen 5 of Echinococcus granulosus

By a strategy of differential immunological screening of an expression library constructed from adult Echinococcus multilocularis parasites, a partial cDNA sequence encoding a protein termed Em6 was isolated. This molecule displayed high sequence homology to the recombinant antigen 'Eg6' which was previously described as an immunogenic epitope of antigen 5 of E. granulosus. Further Em6 sequences and the corresponding sequences from a cattle isolate of E. granulosus were obtained by a PCR approach. By immunoblot analyses using affinity purified antibodies, expression of Em6 in fertile cysts producing protoscoleces of the E. multilocularis metacestode stage was observed. However, Em6 was absent in non-fertile metacestodes. The demonstration of a protein in E. multilocularis displaying identities to 'antigen 6' of E. granulosus could potentially contribute to the future elucidation of the relationship between antigen 5 and 'antigen 6' in the genus Echinococcus, and shed some lights on the performance of serodiagnostic assays for hydatid disease based on the respective antigens.

Stage-specific expression of the 14-3-3 gene in Echinococcus multilocularis

A cDNA expression library representing the metacestode developmental stage of the tapeworm Echinococcus multilocularis was immunoscreened with monospecific antibodies affinity purified following differential immunoblot analysis. Using this procedure, a metacestode-specific clone was isolated representing a 14-3-3 gene of the parasite, which is present as a single copy in the parasite genome. The identity of this clone was demonstrated by cross-reactivity of the recombinant E. multilocularis 14-3-3 protein with antibodies raised against a heterologous 14-3-3 protein from Saccharomyces cerevisiae. In addition, expression of the E. multilocularis 14-3-3 gene in the mutant S. cerevisiae strain, DS9-22, resulted in complementation of the phenotypic deficiency of this strain, thus demonstrating the functionality of the respective gene product. By reverse transcription-polymerase chain reaction (RT-PCR) we showed that the E. multilocularis 14-3-3 protein is about 10-fold overexpressed in the metacestode stage compared with the expression level in the adult stage. Immunolocalization of the 14-3-3 protein in E. multilocularis metacestodes revealed its predominant presence in the germinal layer of the parasite. The results of this study, taken together with the current knowledge on the 14-3-3 protein family, suggest that this parasite molecule may contribute to the promotion of the progressive, potentially unlimited growth behaviour of the E. multilocularis metacestode within the host tissue.

Characterization of the gene family encoding a host-protective antigen of the tapeworm Taenia ovis

Genomic structure has been determined for a gene encoding a host-protective antigen of the parasitic platyhelminth Taenia ovis. An incomplete cDNA, known as 45W, encodes the protective antigen. Southern hybridisation experiments using 45W cDNA as a probe, revealed that the 45W gene was a member of a multigene family. Differential Southern hybridisation and rapid amplification of cDNA end (RACE) experiments were used to characterise the related genes, allowing the full-length coding region of the 45W encoded antigen to be determined. The gene family comprises a minimum of four members per haploid genome with each member showing varying degrees of 5' and 3' homology with respect to the 45W cDNA. A close homologue of the 45W gene, designated 45S, differed from 45W at 11 of 985 nt comprising the full-length mRNA. Sequencing of several independent RACE products for both 45W and 45S identified a cDNA which may be a product of homologous recombination between these genes, suggesting that the two genes may be alleles. Homologous recombination in genes which encode a host protective antigen such as 45W would provide a mechanism by which antigenic variants could arise.

Comparison of the patterns of codon usage and bias between Brugia, Echinococcus, Onchocerca and Schistosoma species

Patterns of codon usage and bias were compared among taxa of the genera Brugia, Echinococcus, Onchocerca and Schistosoma by metric multidimensional scaling and three commonly used indices of bias: Nc, GC3S and B. The overall codon usage for each taxon was compared, as was the codon usage for each individual gene within the taxa. Differences in the patterns of codon usage observed between taxa were dependent on the overall base composition of the genes analysed. The codon usage of Echinococcus was distinct from that of the other taxa. Furthermore, the pattern of codon usage detected by the average codon usage summed across all genes for each taxon was not shown by all genes from that taxon.

Expression and analysis of the diagnostic value of an Echinococcus granulosus antigen gene clone

A pool of 9 sera from Echinococcus granulosus infected patients (PSP) was used to screen an E. granulosus cDNA library constructed in the expression vector lambda gt11. Ten reactive phage clones were isolated and 8 were confirmed in spot-lysis arrays probed with PSP. The insert of 1 of these clones (lambda AgEg4) previously characterized as an E. granulosus cytosolic malate dehydrogenase encoding gene was subcloned into the plasmid vector pGEX-1 and expressed as a fusion with glutathione S-transferase. The fusion peptide (Ag4-GST) was produced in Escherichia coli and its antigenicity was confirmed in colony immunoassay and in immunoblot using nondenaturing conditions. The lack of antigenicity of Ag4-GST in immunoblot using denaturing conditions suggests that the recognized epitopes are conformational. Ag4-GST was purified by affinity chromatography and tested in ELISA and immunodots to access its sensitivity and specificity in the diagnosis of human cystic hydatid disease. An overall sensitivity of 53.6% was obtained. Cross-reactions were observed with some sera from patients infected with Schistosoma mansoni and Wuchereria bancrofti. Ag4-GST was not recognized by any of the sera from Taenia solium infected patients tested. These preliminary results suggest that Ag4-GST could be useful as an accessory antigen to discriminate some cross-reactions with sera from cysticercosis patients, especially in regions like southern Brazil, where schistosomiasis and filariasis are not prevalent.

Molecular cloning and characterization of actin genes from Echinococcus granulosus

An Echinococcus granulosus genomic library has been screened with a mouse beta-actin cDNA probe. Two clones carrying DNA fragments of about 15 kb, possibly derived from the same genome region, have been isolated. This 15-kb genomic region includes 2 actin-related sequences (EgactI and EgactII) separated by about 4 kb. The nucleotide sequences of both genes were determined. The EgactI sequence presents no introns, but an intron of 591 bp was observed in the EgactII sequence. The genes potentially encode 375 and 376 amino-acid-long actins, respectively, with a homology of 85.3%. The deduced amino acid sequences from both genes were compared to the actin sequences from other organisms, showing similarities ranging from 63.5% to 90.6%. The nucleotide sequence of a partial actin cDNA clone has been determined. The deduced amino acids sequence showed a homology of 90.3% and 88.0% in relation to the EgactI and EgactII sequences respectively, suggesting the existence of at least one more actin gene in E. granulosus. This hypothesis is reinforced by the number of bands detected in the Southern blot analysis. Experiments based on the amplification of DNA segments using 3'-specific actin primers indicate that the EgactI gene is transcribed in protoscoleces.