Immunity to helminths: Ready to tip the biochemical balance?

Evaluation of antigens of Fasciola gigantica as vaccines against tropical fasciolosis in cattle

Vaccine trials were conducted in Brahman cross cattle evaluating the efficacy of 4 native antigens purified from adult Fasciola gigantica flukes, and 1 recombinant F. gigantica antigen, as vaccines against tropical fasciolosis. The antigens tested were native glutathione S-transferase, cathepsin L, paramyosin, fatty acid binding protein (FABP), and a recombinant FABP expressed in E. coli, and were formulated in 1 or more of several adjuvants (Quil A, Squalene Montanide 80, MF59-100, Auspharm, NAGO, polylactoglycolide microspheres, Algammulin, DEAE, Freund's). Vaccination induced low, moderate or high antibody titres to the various antigens which were dependent on the adjuvant. Low but significant reductions in fluke burdens (31%, P < 0.026) and fluke wet weight (36%, P < 0.041) were only observed in cattle vaccinated with the native FABP in Freund's adjuvant. There was no correlation between total antibody titres to FABP and protection. The protection observed in cattle vaccinated with native FABP of F. gigantica supports the notion that this class of proteins is a useful target for protection of animals against Fasciola and extends the efficacy of FABPs to the tropical liver fluke. This is the first report of vaccination of cattle against F. gigantica with a purified protein.

Schistosoma mansoni: the developmental regulation and immunolocalization of antioxidant enzymes

Antioxidant enzymes from S. mansoni, cytosolic Cu-Zn superoxide dismutase (CT-SOD), signal-peptide-containing SOD (SP-SOD), glutathione peroxidase (GPX), and glutathione transferase (GST) were compared for their relative levels of transcript expression throughout development in a semiquantitative reverse transcriptase-polymerase chain reaction assay. All of the antioxidant enzymes exhibited a similar pattern of developmental regulation. Adult worms have the highest level of specific mRNA compared with larval stages. GST shows the highest level of expression, being approximately 10-fold more abundant than CT-SOD and SP-SOD and 100-fold more abundant than GPX. This order of expression was nearly consistent for all the developmental stages studied. To localize the antioxidant enzymes, immunofluorescence staining was performed on 3-hr schistosomula and adult worms. GPX, SP-SOD, and CT-SOD were all found to be associated with the adult tegument and gut epithelium. SP-SOD was also associated with organelle and cell membranes of parenchymal cells and interestingly with the spines of adult worms. Schistosomula, on the other hand, showed little immunofluorescence. These studies further demonstrate the developmental regulation of antioxidant enzymes and localize them to the host-parasite interface, supporting the notion that they have a role in allowing adult worms to evade immune attack.

Developmental differences determine larval susceptibility to nitric oxide-mediated killing in a murine model of vaccination against Schistosoma mansoni

A persistent paradox in our understanding of protective immunity against Schistosoma mansoni infection in animals vaccinated with attenuated parasites has been that attrition of challenge parasites occurs during migration through the lungs in vivo, although parasites recovered from the lungs appear to be relatively resistant to cytotoxic effector mechanisms in vitro. We have compared the susceptibilities of different stages of larvae to killing by nitric oxide (NO), which was previously shown to be involved in the larvicidal function of cytokine-activated cytotoxic effector cells. Lung-stage larvae obtained 1 week after infection were not killed in vitro by NO generated either by a chemical NO donor or by activated cells. In contrast, parasites obtained from the portal system of control mice or from the lungs of vaccinated mice 2.5 weeks following challenge infection were killed by NO. As previously shown for mammalian cell targets, the effects of NO in susceptible larval stages may involve enzymes required for aerobic energy metabolism, since similar cytotoxicity was demonstrated by chemical inhibitors of the citric acid cycle or mitochondrial respiration. Taken together with previous observations of enhanced Th1 activity and expression of NO synthase in the lungs of vaccinated mice at 2.5 weeks after challenge infection, these observations elucidate the immune mechanism of vaccine-induced resistance to S. mansoni infection. Moreover, they suggest that conversion to a less metabolically active state may allow pathogens to escape the effects of the important effector molecule NO.

Implications of nutrition for the ability of ruminants to withstand gastrointestinal nematode infections

Resistance and resilience of the ruminant host to gastrointestinal (GI) parasitic nematode infections are influenced by many factors, including nutrition. This review examines the effects of host nutrition on the ability of ruminants to withstand GI nematode infections. Firstly the effects of infection on host metabolism are summarised briefly. An important factor in the pathogenesis is a reduction in feed intake by the host. Gut nematodes also increase endogenous protein losses, which result in net loss of amino acids to the parasitised host, though energy and mineral metabolism are also perturbed. The indications are that the major nutritional change is in protein metabolism. Resilience (the ability of an animal to withstand the effects of infection) can be enhanced markedly by increasing metabolisable protein supply and to a lesser extent metabolisable energy supply. Resistance to GI nematodes (ability of host to prevent establishment and/or development of infection) is also influenced by diet, particularly metabolisable protein supply. While there do not appear to be any effects of host nutrition on establishment of infective larvae, the rate of rejection of adult worms can be enhanced by improved nutrition. The exact nutritional requirements or the mechanisms involved are not known. It appears that the effects of improving nutritional status on host resilience are more clearly defined than effects on host resistance. The implication of changes in host resistance with nutritional state for host productivity need to be better described. Understanding the role of nutrition in improving both resistance and resilience of the host to GI parasites will be important if producers are to make better use of host acquired immunity and reduce dependence on pesticides for prophylaxis.

Expression and characterization of glutathione peroxidase activity in the human blood fluke Schistosoma mansoni

Antioxidants may play an important role in immune evasion by schistosome parasites. Previous studies have focused on the roles of superoxide dismutase and glutathione S-transferase. In the present study, glutathione peroxidase (GPX) activity was measured in different fractions of worm extracts from several developmental stages of Schistosoma mansoni. The enzyme activity was shown to be developmentally regulated, with higher specific activities being found in the tegument-enriched Nonidet P-40 extract of adult worms (the stage least susceptible to immune killing) than in the larval stages (which are most susceptible to immune elimination). In all extracts tested, the activity against cumene hydroperoxide, even when glutathione S-transferase activity was removed, was higher than that for hydrogen peroxide. The expression of GPX cDNA in pGEX-2T by bacteria produced a 50-kDa fusion protein and a 32-kDa truncated protein. The latter was due to termination at the internal UGA codon that codes for selenocysteine. GPX activity was detected in the recombinantly produced GPX but not with Sj26-glutathione S-transferase from the vector. Mutating the TGA codon to TGT produced a full-length product, GPXm (19 kDa), that was used to produce 19 monoclonal antibodies. Anti-GPXm monoclonal antibodies recognized a 19-kDa molecule in adult-worm extract which, upon removal by immunoprecipitation, resulted in the loss of over 90% of the GPX activity, suggesting that a single form of GPX exists in the schistosome.

Biochemical analysis, gene structure and localization of the 24 kDa glutathione S-transferase from Onchocerca volvulus

Survival of Onchocerca volvulus, a pathogenic human filarial parasite, is likely to depend upon the detoxification activities of the glutathione S-transferases (GSTs). The 24 kDa O. volvulus GST, OvGST2, was expressed in a bacterial system and the recombinant protein was purified to homogeneity by affinity chromatography. Specific activities of the recombinant OvGST2 (rOvGST2) with a variety of substrates, and in the presence of inhibitors, were determined. With the universal substrate 1-chloro-2,4-dinitrobenzene, the specific activity of rOvGST2 was 2130 nmol min-1 mg-1. The rOvGST2 showed relatively limited selenium-independent glutathione peroxidase activity, but secondary products of lipid peroxidation, namely members of the trans,trans-alka-2,4-dienal,trans-alk-2-enal and 4-hydroxyalk-2-enal series, were conjugated to glutathione via OvGST2 dependent activity. The gene encoding the OvGST2 was isolated and the nucleotide sequence determined. The ovgst2 gene was found to possess seven exons with six intervening sequences, with all except one having consensus splice-site junctions. This intron/exon organisation of the ovgst2 gene is almost identical with those described for the mammalian Pi class GST genes, consistent with the protein structural evidence that the OvGST2 is related to the Pi class GSTs. Southern blot analysis with total parasite genomic DNA indicated a single copy gene, with a restriction pattern consistent with that of the isolated gene. The tissue distribution of the OvGST2 was examined in O. volvulus by immunohistochemistry and was shown to be distinct from that of the OvGST1. The OvGST2 was located throughout the syncytial hypodermis of male and female adult worms, as well as in the uterine epithelium. Microfilariae, and infective third stage larvae of O. volvulus, isolated from Simulium neavei, were immunopositive for OvGST2.

Glutathione S-transferase (GST) expression in the human hookworm Necator americanus: potential roles for excretory-secretory forms of GST

The difficulty in demonstrating protective immunity to human gastro-intestinal nematodes is thought to be a consequence of the expression of defences by the parasites directed against the toxic metabolites of leukocytes produced during inflammation (Brophy and Pritchard, 1992a). Parasite glutathione S-transferases (GSTs) may provide part of this defence by detoxifying the secondary products of lipid peroxidation produced via immune initiated free-radical attack on host or parasite membranes (Brophy and Pritchard, 1994; Taylor et al., 1988). Neutralisation of parasite immune defence components could tip the molecular balance in favour of the immune response during chronic infections. For example, GSTs have been extensively investigated from the digenean parasites Schistosoma and Fasciola hepatica and provide protection in animal-model systems (Mitchell, 1988; Wijffels et al., 1991). In contrast, although GSTs have been initially characterised in filarial nematodes (Salinas et al., 1994; Leibau et al., 1994; Jaffe and Lambert, 1986), there is limited information on GSTs from human gastro-intestinal nematode parasites. We were particularly interested in analysing the products of hookworms for evidence of the presence of excretory-secretory forms of this putative immune defence protein.

Differential expression of glutathione S-transferase (GST) by adult Heligmosomoides polygyrus during primary infection in fast and slow responding hosts

Glutathione S-transferase (GST) specific enzymatic activity, assayed with the model substrate 1-chloro-2,4-dinitrobenzene, was 45% higher in adult Heligmosomoides polygyrus passaged through a slow responder mouse strain, C57/BL10 compared to worms passaged through a fast-responder strain (SWR x SJL) F1. Western analysis using polyclonal antisera raised to purified H. polygyrus GSTs did not appear to positively correlate the expression of GST protein with functional enzymatic activity. However, western blotting did indicate a sex-linked expression pattern of GST protein, with male worms expressing a higher ratio of the 24 kDa to the 23 kDa GST family than female worms.

Proteinases released in vitro by the parasitic stages of Teladorsagia circumcincta, an ovine abomasal nematode

Proteinases released during in vitro maintenance of third (L3) and fourth larval stage (L4) and adult Teladorsagia circumcincta (formerly Ostertagia circumcincta), an ovine abomasal nematode parasite, were characterized on the basis of pH optima, molecular size and specific proteinase inhibitor sensitivity. Enzyme activity was maximal at alkaline pH and stage-specific release was demonstrated. Proteinases released by the adult parasite degraded a variety of protein substrates including plasminogen, albumin and haemoglobin, in a pH-dependent manner. At alkaline pH fibrinogen degradation was restricted to the alpha and beta peptide chains although the gamma peptide chain was also degraded at acidic pH. Inhibitor sensitivity studies indicated that degradation was predominantly due to metalloproteinases although aspartyl proteinase activity was indicated at acidic pH.

The purification and properties of glutathione reductase from the cestode Moniezia expansa

Glutathione reductase has a central role in glutathione metabolism and as such is a potential target for chemotherapy. The aim of the work was to purify and characterise glutathione reductase from the cestode Moniezia expansa and to compare the properties of the helminth enzyme with its mammalian counterpart. The enzyme was purified by a combination of anion exchange and affinity chromatography and further characterized by chromatofocusing and gel electrophoresis. Analysis revealed a single isoenzyme of glutathione reductase in Moniezia expansa, with a pI of 5.8. The enzyme was a homodimer of native molecular weight 114 kDa, subunit weight 63 kDa. Enzyme activity was affected by buffer concentration and the presence of monovalent sodium salts. The pH optimum was 7.4 with NADPH as cofactor and 5 with NADH. The Kma for oxidized glutathione was 76 microM and for NADPH and NADH, 21 and 350 microM respectively. In addition to oxidized glutathione only the mixed disulphide between CoA and glutathione (CoASSG) showed any significant activity as substrate. The cestode enzyme was inhibited by a variety of compounds including arsonic derivatives, 2,4,6 trinitrobenzene sulfonate 1,3-bis (2-chlorethyl)-1-nitrosourea and oxidized glutathione. In conclusion the glutathione reductase of M. expansa resembles the mammalian enzyme in its general physical properties and its substrate and inhibitor profile. However, the parasite enzyme shows an unusually high activity with the mixed disulphide of coenzyme A and glutathione (CoASSG) and appears to be more sensitive to inhibition by sodium ions.

How schistosomes profit from the stress responses they elicit in their hosts

Results obtained with the model Trichobilharzia ocellata-Lymnaea stagnalis have confirmed the hypothesis that the physiological effects evoked by schistosomes in their snail host--castration and giant growth--are brought about by them interfering with the neuroendocrine systems (NES) regulating the physiological processes concerned. As soon as differentiating cercariae are present in the daughter sporocysts a factor can be detected in the haemolymph of the snail host, called schistosomin, which acts both at the central and the peripheral parts of the NES involved in regulation of reproduction and growth. Schistosomin appears to be a host-derived factor, which is probably released by cells of the internal defence system, the haemocytes, and by connective tissue cells, the telo-glial cells. It meets the criteria of having a cytokine-like function although its molecular structure does not show sequence homology with any of the vertebrate-type cytokines identified to date. Its cytokine nature explains why schistosomin can interfere with different neuroendocrine regulatory systems both at the central and peripheral--target--level, namely after binding to its own receptor. Schistosomin is probably not only responsible for the effects exerted by the parasite on female reproduction but also for those on male reproduction and on growth so that energy and space become available for the continuous production of cercariae. The nature of the humoral cercarial factor, which induces schistosomin release, is as yet unknown. Based on its hydrophobic character and on the fact that it can pass through the wall of the daughter sporocyst, it is supposed to be a diffusible molecule or a protonephridial excretion product. It does not seem to be a vertebrate-type steroid, an ecdysteroid or an eicosanoid. Results obtained in vitro have indicated that schistosomin might have a suppressive effect on haemocyte activity. Plasma from snails 5-6 weeks post-exposure showed a tendency to inhibit phagocytic activity of haemocytes from non-infected snails, that is preparatory to the escape and migration of cercariae. Once shedding has started this effect of schistosomin is overrruled by a strong activation of haemocyte activity coinciding with the tissue damage that the cercariae cause in the host. The cercariae escape from being attacked by masking their surface coat with host molecules. As the physiological effects caused by schistosomes resemble those observed during stress in mammals, experiments were carried out to find out whether schistosomin is also released in non-parasitized snails during stress resulting in an inhibiting effect on reproduction.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutathione S-transferases from the gastrointestinal nematode Heligmosomoides polygyrus and mammalian liver compared

Glutathione S-transferases have been partially characterised from the gastrointestinal nematode Heligmosomoides polygyrus. Two major subunit families were purified (24 and 23 kDa) with N-terminal homology to the mammalian Alpha family. Four dimeric forms of GST were purified from the nematode by glutathione-affinity chromatography, two major enzymes (pI 8.1, 5.0) and two minor forms (pI 5.8, 5.3). The purified GST pool could neutralize model and lipid peroxides via peroxidase activity but not peroxidation derived reactive carbonyls via glutathione transferase activity. Antisera raised to the pooled nematode GSTs appeared to recognize other Strongylida GSTs more strongly on Western blotting compared to mammalian GSTs.

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.

Parasite enzymes and the control of roundworm and fluke infestation in domestic animals

The potential application of parasite enzymes to the serodiagnosis and control of veterinary helminthiases is reviewed. Consideration is given to the use of secreted enzymes as potential antihelminth vaccine components, in the search for novel anthelmintic agents and as serodiagnostic targets. The discussion focuses on recent advances in the definition of the molecular and functional properties of helminth enzymes and the application of this information to the development of novel anthelmintics as well as vaccines. Enzymes included are acetylcholinesterases, enzymes of polyamine and carbohydrate metabolism, proteases and detoxifying activities such as superoxide dismutases and glutathione S-transferases.

Effector functions of activated macrophages against parasites

Evidence in experimental animals indicates a major role for cytokine-activated macrophages as effector cells in protective immunity against parasites. Research on cytokine function during this past year has contributed many insights into the immune mechanisms regulating murine macrophage function as well as the effector molecules employed by these cells to kill both intracellular and extracellular parasites.