Molecular characterization of cDNA sequences encoding glutathione S-transferases of Fasciola hepatica

Partial purification and characterization of glutathione S-transferase from the somatic tissue of Gastrothylax crumenifer (Trematoda: Digenea)

AIM

Aim of the present study was to carry out the partial purification and biochemical characterization of glutathione S-transferase (GST) from the somatic tissue of ruminal amphistome parasite, Gastrothylax crumenifer (Gc) infecting Indian water buffalo (Bubalus bubalis).

MATERIALS AND METHODS

The crude somatic homogenate of Gc was subjected to progressive ammonium sulfate precipitation followed by size exclusion chromatography in a Sephacryl S 100-HR column. The partially purified GST was assayed spectrophotometrically, and the corresponding enzyme activity was also recorded in polyacrylamide gel. GST isolated from the amphistome parasite was also exposed to variable changes in temperature and the pH gradient of the assay mixture.

RESULTS

The precipitated amphistome GST molecules showed maximum activity in the sixth elution fraction. The GST subunit appeared as a single band in the reducing polyacrylamide gel electrophoresis with an apparent molecular weight of 26 kDa. The GST proteins were found to be fairly stable up to 37°C, beyond this the activity got heavily impaired. Further, the GST obtained showed a pH optima of 7.5.

CONCLUSION

Present findings showed that GST from Gc could be conveniently purified using gel filtration chromatography. The purified enzyme showed maximum stability and activity at 4°C.

A single amino acid substitution in isozyme GST mu in Triclabendazole resistant Fasciola hepatica (Sligo strain) can substantially influence the manifestation of anthelmintic resistance

The helminth parasite Fasciola hepatica causes fascioliasis in human and domestic ruminants. Economic losses due to this infection are estimated in U$S 2000-3000 million yearly. The most common method of control is the use of anthelmintic drugs. However, there is an increased concern about the growing appearance of F. hepatica resistance to Triclabendazole (TCBZ), an anthelmintic with activity over adult and young flukes. F. hepatica has eight Glutathione S-Transferase (GST) isozymes, which are enzymes involved in the detoxification of a wide range of substrates through chemical conjugation with glutathione. In the present work we identified and characterized the GST mu gene isolated from the TCBZ-susceptible and TCBZ-resistant F. hepatica strains. Total RNA was transcribed into cDNA by reverse transcription and a 657 bp amplicon corresponding to the GST mu gene was obtained. The comparative genetic analysis of the GST mu gene of the TCBZ susceptible strain (Cullompton) and TCBZ resistant strain (Sligo) showed three nucleotide changes and one amino acid change at position 143 in the GST mu isozyme of the TCBZ-resistant strain. These results have potential relevance as they contribute better understand the mechanisms that generate resistance to anthelmintics.

The Sigma class glutathione transferase from the liver fluke Fasciola hepatica

Background

Liver fluke infection of livestock causes economic losses of over US$ 3 billion worldwide per annum. The disease is increasing in livestock worldwide and is a re-emerging human disease. There are currently no commercial vaccines, and only one drug with significant efficacy against adult worms and juveniles. A liver fluke vaccine is deemed essential as short-lived chemotherapy, which is prone to resistance, is an unsustainable option in both developed and developing countries. Protein superfamilies have provided a number of leading liver fluke vaccine candidates. A new form of glutathione transferase (GST) family, Sigma class GST, closely related to a leading Schistosome vaccine candidate (Sm28), has previously been revealed by proteomics in the liver fluke but not functionally characterised.

Methodology/Principal Findings

In this manuscript we show that a purified recombinant form of the F. hepatica Sigma class GST possesses prostaglandin synthase activity and influences activity of host immune cells. Immunocytochemistry and western blotting have shown the protein is present near the surface of the fluke and expressed in eggs and newly excysted juveniles, and present in the excretory/secretory fraction of adults. We have assessed the potential to use F. hepatica Sigma class GST as a vaccine in a goat-based vaccine trial. No significant reduction of worm burden was found but we show significant reduction in the pathology normally associated with liver fluke infection.

Conclusions/Significance

We have shown that F. hepatica Sigma class GST has likely multi-functional roles in the host-parasite interaction from general detoxification and bile acid sequestration to PGD synthase activity.

Combating neglected parasitic diseases is of paramount importance to improve the health of human populations and/or their domestic animals. Uncovering key roles in host-parasite interactions may support the vaccine potential portfolio of a parasite protein. Fasciola hepatica causes global disease in humans and their livestock but no commercial vaccines are available. Members of the Sigma class glutathione transferase (GST) family have long been highlighted as vaccine candidates towards parasitic flatworms. To this end, a Sigma class GST is currently undergoing phase II clinical trials to protect against infection from the schistosomes. In this study we characterise the protein from F. hepatica following four work pathways that 1) confirm its designation as a Sigma class GST using substrate profiling, 2) assess prostaglandin synthase activity and its effect on host immune cells, 3) localise the Sigma GST within adult fluke and between ontogenic stages and 4) measure its potential as a vaccine candidate. The work presented here shows F. hepatica Sigma class GST to have key host-parasite roles and we suggest, warrants further investigation for inclusion into vaccine formulations.

Evaluation of hepatic damage and local immune response in goats immunized with native glutathione S-transferase of Fasciola hepatica

Worm burden, hepatic damage and local cellular and humoral immune responses were assessed in goats immunized with glutathione-S-transferase and challenged with Fasciola hepatica. Infected but unimmunized and uninfected control groups were also studied. Hepatic damage was evaluated grossly and microscopically. Local immune response was evaluated by (1) microscopical examination of hepatic lymph nodes (HLNs); (2) analysis of the distribution of CD2(+), CD4(+), CD8(+), T-cell receptor gammadelta(+) lymphocytes and immunoglobulin (Ig) G(+) plasma cells; and (3) investigation of the distribution of cells expressing interleukin (IL)-4 and interferon (IFN)-gamma in the hepatic inflammatory infiltrates and HLNs. Immunized animals did not have significant reduction in fluke number, but there was significant (P<0.05) reduction of fluke size relative to the control groups. The lesions in the two infected groups were similar and consisted of fibrous perihepatitis and white tortuous tracts, mainly involving the left hepatic lobe. Microscopical lesions were similar in both infected groups and were typical of chronic fascioliosis. These included portal fibrosis, inflammatory infiltration with plasma cells, formation of lymphoid follicles, accumulation of haemosiderin-laden macrophages and granulomatous foci. Both infected groups had a marked local immune response characterized by infiltration of CD2(+), CD4(+) and CD8(+) T lymphocytes, and IgG(+) plasma cells in hepatic lesions and in HLNs. There was no expression of IL-4 or INF-gamma by cells in the hepatic inflammatory infiltrate, but expression of INF-gamma in HLNs was much lower than that of IL-4, suggesting an immune response dominated by T helper 2 cells.

Xenobiotic metabolizing enzymes and metabolism of anthelminthics in helminths

Anthelminthics remain the only accessible means in the struggle against helminth parasites, which cause significant morbidity and mortality in man and farm animals. The treatment of helminthic infections has become problematic because of frequent drug resistance of helminth parasites. The development of drug resistance can be facilitated by the action of xenobiotic metabolizing enzymes (XMEs). In all organisms, XMEs serve as an efficient defense against the potential negative action of xenobiotics. The activities of XMEs determine both desired and undesired effects of drugs, and the knowledge of drug metabolism is necessary for safe, effective pharmacotherapy. While human and mammalian XMEs have been intensively studied for many years, XMEs of helminth parasites have undergone relatively little investigation, so far. However, many types of XMEs, including oxidases, reductases, hydrolases, transferases, and transporters, have been described in several helminth species. XMEs of helminth parasites may protect these organisms from the toxic effects of anthelminthics. In case of certain anthelminthics, metabolic deactivation was reported in helminth larvae and/or adults. Moreover, if a helminth is in the repeated contact with an anthelminthic, it defends itself against the chemical stress by the induction of biotransformation enzymes or transporters. This induction can represent an advantageous defense strategy of the parasites and may facilitate the drug-resistance development.

Fasciola hepatica and Fasciola gigantica: cloning and characterisation of 70 kDa heat-shock proteins reveals variation in HSP70 gene expression between parasite species recovered from sheep

Fasciola hepatica and Fasciola gigantica are trematode parasites responsible for fasciolosis, a disease of ruminant animals which is also increasingly recognised as a disease in humans. By biochemical and in silico methods, we have cloned and characterised the 70 kDa heat-shock proteins (HSP70s) of F. hepatica and F. gigantica. The nucleotide and protein sequences for HSP70 were found to be 98% and 99% identical between liver fluke species, respectively, and to encode conserved amino acid motifs that are of putative functional importance. Western blot analysis demonstrated that HSP70 proteins were expressed at a higher level in F. gigantica recovered from sheep relative to F. hepatica, but HSP70 was not detected in the excretory-secretory products of these liver fluke samples. Real-time reverse-transcriptase PCR analysis of HSP70 expression in parasites from sheep, but not cattle, showed HSP70 expression to be higher in F. gigantica than F. hepatica. These results suggest that hosts refractory to F. gigantica are associated with higher HSP70 expression by this parasite and that HSP70 expression may represent a biochemical marker of the stress response of F. gigantica.

Proteomic analysis of glutathione transferases from the liver fluke parasite,Fasciola hepatica

The parasite Fasciola hepatica causes major global disease of livestock, with increasing reports of human infection. Vaccine candidates with varying protection rates have been identified by pre-genomic approaches. As many candidates are part of protein superfamilies, sub-proteomics offers new possibilities to systematically reveal the relative importance of individual family proteins to vaccine formulations within populations. The superfamily glutathione transferase (GST) from liver fluke has phase II detoxification and housekeeping roles, and has been shown to contain protective vaccine candidates. GST were purified from cytosolic fractions of adult flukes using glutathione- and S-hexylglutathione-agarose, separated by 2-DE, and identified by MS/MS, with the support of a liver fluke EST database. All previously described F. hepatica GST isoforms were identified in 2-DE. Amongst the isoforms mapped by 2-DE, a new GST, closely related to the Sigma class enzymes is described for the first time in the liver fluke. We also describe cDNA encoding putative Omega class GST in F. hepatica.

A mu-class glutathione S-transferase from the marine shrimp Litopenaeus vannamei: molecular cloning and active-site structural modeling

A cDNA clone coding for a mu-class glutathione S-transferase (GST) was isolated from a hepatopancreas cDNA library from the shrimp Litopenaeus vannamei. The deduced amino acid sequence (215 amino acids) has >50% identity to rodents and other mammals mu-class GSTs. Using RT-PCR, the shrimp GST transcript was detected in hepatopancreas, hemocytes, gills, and muscle, but not in pleopods. The shrimp GST sequence was computer modeled and found to fit the classical two-domain GST structure. Domain I, containing the glutathione (GSH) binding site, is more conserved compared to the flexible C-terminal domain II. Residue Q208 appears to be a key to substrate specificity by comparison with mammalian GST mutants. This position is commonly occupied by serine or threonine in mammalian mu-class GSTs, and shrimp Q208 may affect the affinity to substrates like aminochrome or 1,3-dimethyl-2-cyano-1-nitrosoguanidine. This is the first report of molecular cloning and structural modeling of a crustacean GST and provides new insights into the nature of the detoxification response on marine invertebrates.

Functional analysis and localisation of a delta-class glutathione S-transferase from Sarcoptes scabiei

The mite Sarcoptes scabiei causes sarcoptic mange, or scabies, a disease that affects both animals and humans worldwide. Our interest in S. scabiei led us to further characterise a glutathione S-transferase. This multifunctional enzyme is a target for vaccine and drug development in several parasitic diseases. The S. scabiei glutathione S-transferase open reading frame reported here is 684 nucleotides long and yields a protein with a predicted molecular mass of 26 kDa. Through phylogenetic analysis the enzyme was classified as a delta-class glutathione S-transferase, and our paper is the first to report that delta-class glutathione S-transferases occur in organisms other than insects. The recombinant S. scabiei glutathione S-transferase was expressed in Escherichia coli via three different constructs and purified for biochemical analysis. The S. scabiei glutathione S-transferase was active towards the substrate 1-chloro-2,4-dinitrobenzene, though the positioning of fusion partners influenced the kinetic activity of the enzyme. Polyclonal antibodies raised against S. scabiei glutathione S-transferase specifically localised the enzyme to the integument of the epidermis and cavities surrounding internal organs in adult parasites. However, some minor staining of parasite intestines was observed. No staining was seen in host tissues, nor could we detect any antibody response against S. scabiei glutathione S-transferase in sera from naturally S. scabiei infected dogs or pigs. Additionally, the polyclonal sera raised against recombinant S. scabiei glutathione S-transferase readily detected a protein from mites, corresponding to the predicted size of native glutathione S-transferase.

Immunology of the host–parasite relationship in fasciolosis (Fasciola hepaticaandFasciola gigantica)

The protective resolution of liver fluke (Fasciola hepatica and Fasciola gigantica) infection is a dynamic interplay between the host's effector responses and the parasite's defence and immunomodulatory systems. The evidence suggests that the juvenile or immature parasite is the target of protective host immune responses but the effector mechanisms employed vary between hosts. Moreover, F. hepatica and F. gigantica differ in their susceptibility to these killing mechanisms. In the rat, in vitro killing of juvenile F. hepatica involves an antibody-dependent cell cytotoxicity mediated by nitric oxide produced by activated monocytes and (or) macrophages. However, monocytes and (or) macrophages from Indonesian sheep do not produce nitric oxide yet can effectively kill juvenile F. gigantica in vitro and in vivo by a mechanism that is ineffective against F. hepatica. These data show that disease progression or resolution in fasciolosis is determined both by biochemical differences between Fasciola species and by host-dependent factors. Understanding the genetic basis for these differences is a key question for the future. Fasciola hepatica and F. gigantica actively modulate the host immune response, downregulating type 1 responses during infection. It is important to determine whether such modulation of the immune response by Fasciola spp. directly leads to enhanced parasite survival in the various hosts.

Cloning and partial characterization of a Boophilus microplus (Acari: Ixodidae) glutathione S-transferase

A cDNA of glutathione S-transferase (GST) was isolated from a cDNA library of salivary glands of Boophilus microplus. The recombinant protein was purified by glutathione affinity chromatography and assayed upon the chromogenic substrate CDNB. The 864 bp cloned fragment was sequenced and showed an open reading frame coding for a protein of 220 amino acids. Expression of the GST gene was tested by RT-PCR in tick tissues and larvae mRNA. Comparison of the deduced amino acid sequence with GSTs from other species revealed that the enzyme is closely related to the mammalian class mu GSTs.

The contribution of molecular biology to the development of vaccines against nematode and trematode parasites of domestic ruminants

Rapid developments in molecular biology have had an enormous impact on the prospects for the development of vaccines to control the major nematode and trematode infestations of livestock. Vaccine candidates are purified using conventional protein chemistry techniques but the limitations imposed by the scarcity of parasite material provide an insurmountable barrier for commercial vaccine production by this means. The ability to purify mRNA from different parasite life-cycle stages and to prepare cDNA expression libraries from it has proven central to the identification of immunogenic parasite proteins. Potentially, protective parasite antigens can now be produced in recombinant form in a variety of vectors and this represents a key breakthrough on the road to commercial vaccine production. The contribution of molecular biology to this process is discussed using several examples, particularly in vaccine development against the pathogenic abomasal nematode of sheep and goats, Haemonchus contortus, and the liver fluke of sheep and cattle, Fasciola hepatica. The difficulties of producing recombinant proteins in the correct form, with appropriate post-translational modification and conformation, are discussed as well as emerging means of antigen delivery including DNA vaccination. The opportunities offered by genome and expressed sequence tag analyses programmes for antigen targeting are discussed in association with developing microarray and proteomics technologies which offer the prospect of large scale, rapid antigen screening and identification.

Molecular cloning and enzymatic expression of the 28-kDa glutathione S-transferase of Schistosoma japonicum: evidence for sequence variation but lack of consistent vaccine efficacy in the murine host

Glutathione S-transferases (GSTs) have long been regarded as attractive vaccine (and drug) targets in schistosomes due to their suspected role in detoxification processes. Indeed, the 28-kDa GST of Schistosoma mansoni (SmGST28) has proven efficacy as an antigen for protective immunity reducing worm burden, female fecundity and egg viability. In contrast, the vaccinating effects of the bacterial expressed homologue of Philippine S. japonicum (SjpGST28) have proved disappointing, possibly because this recombinant form was an incomplete sequence, lacking five N-terminal amino acids which may have affected its vaccination efficacy. Here we describe the cloning and functional enzymatic expression of a complete cDNA encoding SjpGST28. We report also on the immunogenicity and vaccine efficacy of this molecule as a purified recombinant protein and as a DNA plasmid vaccine in the murine model. We further describe the cloning of several complete cDNAs encoding the Chinese homologue of SjpGST28 and the identification of 3 SjcGST28 sequence variants which are probably encoded by distinct alleles.

Schistosoma mansoni gene GP22 encodes the tegumental antigen sm25: (1) antibodies to a predicted B-cell epitope of Sm25 cross-react with other candidate vaccine worm antigens; (2) characterization of a recombinant product containing tandem-repeats of this peptide as a vaccine

Monospecific antibodies against two putative epitopes of schistosome protein encoded by gene GP22 (182 codons, no introns) were used to probe worm extracts fractionated by lentil-lectin affinity chromatography or by electrophoresis. Anti-peptide-alpha (codons 70-84) exclusively identifies the N-glycanase-sensitive, 25 kDa tegumental glycoprotein Sm25 in the lectin-bound fraction of detergent-solubilized adult worm extract S3. In contrast, antipeptide-delta (codons 151-162) does not react with Sm25 but cross-reacts with other schistosome proteins, including candidate vaccine antigens paramyosin (Sm97) and glutathione-S-transferases (Sm26, Sm28, Sj26). Recombinant protein r4 x 47, constructed to express multiple copies of codon sequence 117-163 (containing delta), reacts with anti-delta and is uniquely recognized by protective Fischer twice-infected (F-2x) rat antiserum. Immunization with r4 x 47 induces antibodies with cross reactivities similar to anti-delta, but which also recognize Sm25. Despite these cross-reactivities with protective antigens, rodents vaccinated with r4 x 47 were not protected against cercarial infection. On the basis of these data, two hypotheses are proposed: (1) antigenic epitopes other than delta are present within the r4 x 47 sequence which induce antibodies reactive with Sm25 and/or (2) peptide-delta assumes alternative antigenic conformations, dependent upon the context of neighbouring sequences, some of which mimic epitopes of proteins encoded by other schistosome genes. These mimotopes are not targets of protective antibodies.

Paragonimus westermani: a cytosolic glutathione S-transferase of a sigma-class in adult stage

We purified cytosolic glutathione S-transferase (GST) of adult Paragonimus westermani monitoring its activity with 1-chloro-2,4-dinitrobenzene (CDNB). The enzyme was purified 18.4-fold to electrophoretic homogeneity with 21% recovery rate through a three-step procedure. The purified enzyme (Pw28GST) has a subunit molecular weight of 28 kDa with an isoelectric point at 4.6. Monoclonal antibody (anti-Pw28GST) against Pw28GST did not cross-react with GSTs from other helminths. cDNA library was constructed in lambdaZAP II bacteriophage and screened with anti-Pw28GST. The corresponding gene containing a single open reading frame of 804 bp encoded 211 amino acids. The predicted amino acid sequence exhibited a higher homology with catalytic domain near N-terminus of class sigma GSTs (58%) than with schistosome 28-kDa GSTs (45-41%) or with class sigma GSTs themselves (33-31%). The sequence contained both Tyr-6 and Tyr-10 that are highly conserved in mammalian and helminth GSTs. The apparent K(m) value of a recombinant enzyme was 0.78 mM. Both native and recombinant enzymes showed the highest activity against CDNB, relatively weak activity against ethacrynic acid and reactive carbonyls, and no activity against epoxy-3-(p-nitrophenoxy)-propane. The activities were inhibited by bromosulfophthalein, cibacron blue, and albendazole, but not by praziquantel. These findings indicate that adult P. westermani has a class sigma GST.

Humoral responses in mice following vaccination with DNA encoding glutathione S-transferase of Fasciola hepatica: effects of mode of vaccination and the cellular compartment of antigen expression

The humoral responses in mice following vaccination with DNA constructs encoding Fasciola hepatica glutathione S-transferase (GST) have been evaluated. GST47 cDNA was subcloned into two DNA vaccine vectors, VR1012 and VR1020, which direct expression to the cytoplasmic and extracellular compartments, respectively. Expression was confirmed by transfection into COS 7 cells. Groups of mice were vaccinated with these constructs, by either intramuscular injection with the VR1012-or VR1020-based constructs, or intradermal vaccination (with a gene gun) with the VR1020-based construct. Vaccination with the construct designed for secretion resulted in an increased humoral response compared to vaccination with the nonsecretory construct. The level of the total humoral response after vaccination with the secretion construct was not dependent on the route of vaccination. However, the isotype profile of the response differed between the groups; intramuscular vaccination with the construct directing cytoplasmic expression yielded an immuoglobulin (Ig)G2a dominant (Th1-type) response, intradermal vaccination with the secretory construct a IgG1/IgE dominant (Th2-type) response, and intramuscular vaccination with the secretory construct a mixed isotype response. These results demonstrate that the immunogenicity of a DNA vaccine based on Fasciola GST, as well as the isotype of the response against GST, is determined by the mode of vaccine administration.

Progress in Development of Liver Fluke Vaccines

Infection of ruminants by Fasciola spp continues to cause large economic losses worldwide. Recent results from several laboratories have demonstrated that animals can be significantly protected against infection by vaccination with defined Fasciola antigens. Apart from reducing fluke burdens, some vaccines can elicit a concurrent reduction in parasite egg production. The expectation of a commercially feasible vaccine that might also reduce parasite transmission in the field is now realistic, although major hurdles still exist. Here, Terry Spithill and John Dalton review the results of several recent vaccine trials and discuss the future prospects for vaccine development.

Fasciola hepatica: stage-specific expression of novel gene sequences as identified by differential display

Differences in gene expression between adult and immature Fasciola hepatica (liver fluke) parasites isolated from the mammalian host were investigated using the technique of differential display. For any given primer combination used to produce these displays there were, on average, 22% apparently adult-specific and 14% apparently immature-specific cDNA products able to be identified, consistent with a high degree of differential gene expression between these two parasite developmental stages. Several cDNA fragments specific to immature parasite RNA were isolated and cloned. An abundant 400- to 500-bp RNA species was identified on a Northern blot by hybridization to the cloned DD2 cDNA fragment and was determined to be expressed at levels at least 10-fold higher in immature parasites relative to adult parasites. mRNA transcripts corresponding to the remaining cDNA fragments (DD14, DD16, DISP10, and DISP2) were apparently expressed at levels below the sensitivity limits of Northern analysis, although differential expression of these transcripts was confirmed by reverse transcriptase PCR (RT-PCR). The identities or functional significance of each of the five differentially expressed cDNAs identified in this study is still unclear due to the lack of any significant sequence similarity to the entries currently held within sequence databases.

Crystallization, structural determination and analysis of a novel parasite vaccine candidate: Fasciola hepatica glutathione S-transferase

Glutathione S-transferases (GSTs) represent the major class of detoxifying enzymes from parasitic helminths. As a result, they are candidates for chemotherapeutic and vaccine design. Indeed, GSTs from Fasciola hepatica have been found to be effective for vaccinating sheep and cattle against fasciolosis. This helminth contains at least seven GST isoforms, of which four have been cloned. The cloned isoforms (Fh51, Fh47, Fh7 and Fh1) all belong to the mu class of GSTs, share greater than 71% sequence identity, yet display distinct substrate specificities. Crystals of Fh47 were obtained using the hanging drop vapour diffusion technique. The crystals belong to space group I4122, with one monomer in the asymmetric unit, which corresponds to a very high solvent content of approximately 75%. The physiological dimer is generated via a crystallographic 2-fold rotation. The three-dimensional structure of Fh47 was solved by molecular replacement using the Schistosoma japonicum glutathione S-transferase (Sj26) crystal structure as a search model. The structure adopts the canonical GST fold comprising two domains: an N-terminal glutathione-binding domain, consisting of a four-stranded beta-sheet and three helices whilst the C-terminal domain is entirely alpha-helical. The presence of Phe19 in Fh47 results in a 6 degrees interdomain rotation in comparison to Sj26, where the equivalent residue is a leucine. Homology models of Fh51, Fh7 and Fh1, based on the Fh47 crystal structure, reveal critical differences in the residues lining the xenobiotic binding site, particularly at residue positions 9, 106 and 204. In addition, differences amongst the isoforms in the non-substrate binding site were noted, which may explain the observed differential binding of large ligands. The major immunogenic epitopes of Fh47 were surprisingly found not to reside on the most solvent-exposed regions of the molecule.

Immunological approaches for the control of fasciolosis

The immunological relationship between liver flukes and their mammalian hosts is being unravelled by in vivo and in vitro studies. Vaccine studies in cattle and sheep with purified antigens (fatty acid binding protein, FABP; glutathione S-transferase, GST; cathepsin L, CatL; hemoglobin) have shown that high reductions in worm burdens (31-72%) and egg production (69-98%) can be achieved, raising the realistic possibility that immunological control of Fasciola infection is a commercially achievable goal. Combination vaccines may also be feasible since a cocktail of CatL and hemoglobin elicits a significant 72% protection in cattle. Analysis of immune responses to Fasciola during infection in ruminants suggests that chronic infection correlates with a type 2 helper T cell response, implying that type 1 helper T cell responses are down-regulated in fasciolosis. Recent results studying the resistance of Indonesian Thin Tail (ITT) sheep to F. gigantica have shown that this breed exhibits high innate (or rapidly acquired) resistance to infection and acquires a higher level of resistance after a primary challenge. Initial studies suggest that the resistance of ITT sheep to F. gigantica may be determined by a major gene. Merino sheep also acquire resistance to F. gigantica. In contrast, ITT and Merino sheep do not exhibit resistance to F. hepatica. These results suggest that there are fundamental differences between these two species of Fasciola in the biology of their interaction with the sheep immune system. In vitro studies on immune mechanisms of killing of juvenile fluke have shown that juvenile larvae of F. hepatica are susceptible to antibody-dependent killing by activated rat macrophages in vitro which is mediated by nitric oxide. Future studies on the immune effector mechanisms expressed by resistant sheep which control infection by F. gigantica will lead to new knowledge which may allow the design of more effective vaccines for fasciolosis.

Vaccination against worm parasites of animals

The 1990s have seen the culmination of decades of painstaking research with the registration and launch of Tickgard (Hoechst), a recombinant vaccine against Boophilus microplus, and the provisional registration of a Taenia ovis vaccine. Research continues to hold promise for immunological control of Echinococcus, Fasciola, Haemonchus, Trichostrongylus and Ostertagia. Blood-sucking parasites (e.g. ticks and H. contortus) are susceptible to control by vaccines containing 'novel' or 'concealed' antigens where serum antibodies in blood meals attack targets in the gut. Antibodies also provide protection in taeniid models, whereas the protective response to be sought in Fasciola remains unclear. More problematic are formulations and delivery strategies to induce expulsion of gastrointestinal nematodes, using vaccines containing recombinant 'conventional' antigens. The use of computer models to simulate vaccine efficacy in worm control and challenges to the concept of 'hypo-responsiveness' of young lambs will encourage cautious optimism and lively debate as to the prospects for integrated worm control using parasite vaccines. This review covers the aspirations, current success and problems faced by researchers in the parasite arena.

Methods for purification of glutathione transferases in the earthworm genus Eisenia, and their characterization

Isoenzymes of glutathione transferase (GST) were partially purified from the earthworm species Eisenia andrei and E. veneta using affinity chromatography followed by ion exchange chromatography and reversed-phase HPLC. In E. veneta, five activity peaks, named EvGST Ia, Ib, II, III and IV, were separated by anion exchange chromatography. The GSTs in E. andrei were resolved by cation exchange chromatography into six groups, named EaGST I-VI. Using reversed-phase HPLC, the affinity-purified GSTs from E. andrei and E. veneta were resolved into 14 subunits, named Ea1-Ea14 and Ev1-Ev14, respectively. EaGST I, II, IV and EvGST Ia were further characterized. These forms displayed different substrate specificity towards the substrates 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene, ethacrynic acid (ETHA) and cumene hydroperoxide, as well as different subunit composition determined by SDS-PAGE and reversed-phase HPLC. EaGST IV and EvGST Ia showed exceptionally high ETHA activity compared with the other forms. EaGST IV consisted of a homodimeric protein involving subunit Ea6 with an apparent molecular weight of 26.5 kDa, whereas EvGST Ia is composed of two different subunits (Ev9 and Ev10). Amino acid composition and N-terminal analysis of the first 33 residues of Ea6 indicated that the enzyme is most related to the pi class. Subunit Ev10 had 67% identity with Ea6, over the region sequenced (12 residues), but up to 90% identity with GSTs from several nematodes. Exposure of both species to trans-stilbene oxide, 3-methylcholanthrene and phenobarbital for three weeks did not elevate the activity of GST measured with CDNB and ETHA.

A PCR strategy for the isolation of glutathione S-transferases (GSTs) from nematodes

Anchor based PCR technology has been used to isolate a GST sequence from the gastro-intestinal nematode Heligmosomoides polygyrus. A 800 base pair product was amplified from first-strand cDNA using primers based on the N-terminal sequence of purified H. polygyrus GST (upstream primer) and a non-specific polyadenylate tail with an anchor sequence (downstream primer). The product was cloned into pUC18 and sequenced. A reading frame of 648 bases in the sequence encoded a protein which has 30% homology with the alpha family of mammalian glutathione S-transferases.

A novel type of glutathione S-transferase in Onchocerca volvulus

Onchocerca volvulus is a pathogenic human filarial parasite which, like other helminth parasites, is capable of evading the host's immune responses by a variety of defense mechanisms which are likely to include the detoxification and repair mechanisms of the enzyme glutathione S-transferase (GST). In this study, we show that one of the previously described GSTs from O. volvulus appears to possess the characteristics of a secreted enzyme. When the complete O. volvulus GST1 (OvGST1) sequence presented here is compared with those of other GSTs, 50 additional residues at the N terminus are observed, the first 25 showing characteristics of a signal peptide. This is consistent with the N-terminal sequence data on the native mature enzyme which begins at amino acid 26, based on the deduced protein sequence from the cDNA. The native protein, without the signal peptide sequence, possesses a 24-amino-acid extension not present in other GSTs. The deduced amino acid sequence of the OvGST1 cDNA clone was shown to possess four potential N-glycosylation sites. Digestion of O. volvulus homogenate with endoglycosidase, followed by detection of OvGST1 with specific antibody, indicated that the enzyme possesses at least two N-linked oligosaccharide chains. Gel filtration of the Escherichia coli-produced recombinant OvGST1 showed that it is enzymatically active as a nonglycosylated dimer. OvGST1 is found in the media surrounding adult worms maintained in culture, indicating that, in vitro, this enzyme is released from the worm. The strongest immunostaining for OvGST1 was observed in the outer cellular covering of the adult worm body, the syncytial hypodermis, especially in the interchordal hypodermis, where the peripheral membrane forms a series of lamellae which run into the outer zone of the hypodermal cytoplasm.

Vaccination against animal parasites

A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.

Molecular variation in fasciolids and Paragonimus

Molecular variation, both intra- and inter-specific, in fasciolids and Paragonimus has been detected in isozymes and other proteins, antigens and nucleic acid sequences. The use of such data is reviewed for studies on population genetics, taxonomy, phylogeny, immunodiagnosis, ontogeny and host-induced variation.