Schistosomes: the road from host-parasite interactions to vaccines in clinical trials

Developing vaccines to combat hookworm infection and intestinal schistosomiasis

Hookworm infection and schistosomiasis rank among the most important health problems in developing countries. Both cause anaemia and malnutrition, and schistosomiasis also results in substantial intestinal, liver and genitourinary pathology. In sub-Saharan Africa and Brazil, co-infections with the hookworm, Necator americanus, and the intestinal schistosome, Schistosoma mansoni, are common. The development of vaccines for these infections could substantially reduce the global disability associated with these helminthiases. New genomic, proteomic, immunological and X-ray crystallographic data have led to the discovery of several promising candidate vaccine antigens. Here, we describe recent progress in this field and the rationale for vaccine development.

The nature and combination of subunits used in epitope-based Schistosoma japonicum vaccine formulations affect their efficacy

Background

Schistosomiasis remains a major public health problem in endemic countries and is caused by infections with any one of three primary schistosome species. Although there are no vaccines available to date, this strategy appears feasible since natural immunity develops in individuals suffering from repeated infection during a lifetime. Since vaccinations resulting in both Th1- and Th2-type responses have been shown to contribute to protective immunity, a vaccine formulation with the capacity for stimulating multiple arms of the immune response will likely be the most effective. Previously we developed partially protective, single Th- and B cell-epitope-based peptide-DNA dual vaccines (PDDV) (T3-PDDV and B3-PDDV, respectively) capable of eliciting immune responses against the Schistosoma japonicum 22.6 kDa tegument antigen (Sj22.6) and a 62 kDa fragment of myosin (Sj62), respectively.

Results

In this study, we developed PDDV cocktails containing multiple epitopes of S. japonicum from Sj22.6, Sj62 and Sj97 antigens by predicting cytotoxic, helper, and B-cell epitopes, and evaluated vaccine potential in vivo. Results showed that mice immunized with a single-epitope PDDV elicited either Tc, Th, or B cell responses, respectively, and mice immunized with either the T3- or B3- single-epitope PDDV formulation were partially protected against infection. However, mice immunized with a multicomponent (3 PDDV components) formulation elicited variable immune responses that were less immunoprotective than single-epitope PDDV formulations.

Conclusions

Our data show that combining these different antigens did not result in a more effective vaccine formulation when compared to each component administered individually, and further suggest that immune interference resulting from immunizations with antigenically distinct vaccine targets may be an important consideration in the development of multicomponent vaccine preparations.

Control of important helminthic infections vaccine development as part of the solution

Among the tools available for the control of helminth infections, chemotherapy has come to totally dominate the field. In the veterinary field, development of drug resistance has appeared but this is not (yet) a problem in the control of human diseases. Although there is no vaccine commercially available for any human parasitic infection yet, recent progress in vaccine development is making this a future possibility for several diseases. The goal of chemotherapy is to alleviate infection and morbidity in the definitive host, or reduce transmission, while the effect of available vaccine candidates would mainly be to influence transmission through targeting the intermediate or reservoir host, when the infection is zoonotic. Apart from this general scheme, there are also vaccine candidates targeting the parasites in the definitive host, in particular the early developmental stages, which should reduce the risk of drug failure. Since the biological targets in most cases are different, vaccination would be synergistic with drug therapy. This review covers diseases caused by helminthes in both humans and animals and includes examples of diseases caused by cestodes, nematodes and trematodes. The focus is on infections for which vaccine development has been undertaken for a long time, resulting in products that could realistically become integrated into control strategies in the near future.

Protective effects of Sm-p80 in the presence of resiquimod as an adjuvant against challenge infection with Schistosoma mansoni in mice

OBJECTIVES

To determine the prophylactic efficacy of an Sm-p80-based vaccine formulation against challenge infection with Schistosoma mansoni in mice using an approach comprising of initial priming with DNA and boosting with recombinant protein in the presence of resiquimod (R848) as an adjuvant.

METHODS

In the first experiment (prime-boost approach), mice were primed with Sm-p80-pcDNA3 (week 0) and boosted at weeks 4 and 8 with recombinant Sm-p80 formulated in resiquimod (R848). Each mouse in the control group first received only pcDNA3 and was boosted with R848. In the second set of experiments (recombinant protein approach), mice were immunized (week 0) and boosted (weeks 4 and 8) with rSm-p80 formulated in R848. Animals of the control group in this series of experiments received only R848 at 0, 4, and 8 weeks. All of the animals from both the 'prime-boost' and 'recombinant protein' groups were challenged with cercariae of S. mansoni, 4 weeks after the last immunization. The mice were sacrificed 6 weeks post-challenge and the reductions in worm burden and egg production were determined. Sm-p80-specific antibody titers were estimated in the mice sera by ELISA. Cytokine mRNA and protein production by proliferating splenocytes in response to in vitro stimulation with Sm-p80, were estimated via RT-PCR and ELISA, respectively.

RESULTS

Vaccination with Sm-p80 (prime-boost approach) showed 49% reduction in worm burden; with the recombinant protein approach the protection was found to be 50%. The protection levels were correlated with antibody production. Upon antigenic stimulation with recombinant Sm-p80, splenocytes secreted significant levels of interferon (IFN)-γ and interleukin (IL)-2, indicating that the immune responses were Th1-biased and this was further supported in terms of distribution of antibody isotypes and mRNA expression of cytokines.

CONCLUSIONS

In conclusion the present study clearly demonstrates that Sm-p80 consistently maintained its protective nature, and resiquimod as an immunopotentiating agent slightly boosted the protective effects of Sm-p80 in both 'DNA prime-protein boost' and 'recombinant protein' immunization approaches in a murine model.

Risk factors for Schistosoma haematobium infection and morbidity in two villages with different transmission patterns in Niger

A better control of major neglected tropical diseases such as schistosomiasis is urgently needed to reduce their impact on public health in developing countries. To optimize the efficiency of intervention campaigns, we assessed the influence of individual human factors on the level of Schistosoma haematobium infection and morbidity in a typical Sahelian country (Niger). Random samples of 246 and 257 individuals were selected from general census in two villages with distinct patterns of schistosome transmission. One village (Lossa) is located in an area of perennial transmission whereas transmission is seasonal in the other village (Tara). Despite comparable levels of both egg excretion and lower tract pathology in the two villages, the inhabitants of Lossa had a higher risk (OR: 2.1, 95% CI: 1.1-3.9) of developing upper tract lesions compared to those living in Tara. In both villages, bladder lesions were more serious in males than in females. Children between 7 and 15 years old were the most at risk to experience heavy infections (OR: 3.4, 95% CI: 2.1-5.7), bladder (OR: 4.5, 95% CI: 2.6-7.8) and upper tract (OR: 10.4, 95% CI: 2.4-45.0) lesions, independently of gender and village. These results confirm that targeted intervention campaigns should include foci regardless of their schistosome transmission pattern and focus on the school-aged population.

Sm-p80-based DNA vaccine made in a human use approved vector VR1020 protects against challenge infection with Schistosoma mansoni in mouse

Although there is an effective drug (praziquantel) available for the treatment of schistosomiasis, yet the disease is still spreading unabated and is rampant in 76 countries. Control via praziquantel treatment has so far been insufficient in reducing the disease transmission. Therefore, a vaccine in addition to other strategies, for example, improving sanitation and introduction of new drugs are essential to successfully control and eventually eradicate schistosomiasis. To this effect, we have targeted a functionally important antigen, Sm-p80 as a vaccine candidate. In this study, full length cDNA of Sm-p80 was cloned in VR1020, a FDA approved vector for human use. The protective efficacy of this vaccine formulation was tested in a murine model. Sm-p80-VR1020 vaccine formulation was able to induce 47% reduction in worm burden. Serology on samples obtained from vaccinated animals revealed a strong antibody response which included IgG and all of its subtypes, IgM and IgA. Proliferating splenocytes in response to recombinant Sm-p80 produced a wide spectrum of cytokines representing Th1, Th2 and Th17 types, as ascertained via RT-PCR analysis. These findings further strengthen the importance of Sm-p80 molecule as a vaccine candidate for intestinal schistosomiasis.

Schistosomiasis in the People's Republic of China: the era of the Three Gorges Dam

The potential impact of the Three Gorges Dam (TGD) on schistosomiasis transmission in China has invoked considerable global concern. The TGD will result in changes in the water level and silt deposition downstream, favoring the reproduction of Oncomelania snails. Combined with blockages of the Yangtze River's tributaries, these changes will increase the schistosomiasis transmission season within the marshlands along the middle and lower reaches of the Yangtze River. The changing schistosome transmission dynamics necessitate a comprehensive strategy to control schistosomiasis. This review discusses aspects of the epidemiology and transmission of Schistosoma japonicum in China and considers the pathology, clinical outcomes, diagnosis, treatment, immunobiology, and genetics of schistosomiasis japonica together with an overview of current progress in vaccine development, all of which will have an impact on future control efforts. The use of synchronous praziquantel (PZQ) chemotherapy for humans and domestic animals is only temporarily effective, as schistosome reinfection occurs rapidly. Drug delivery requires a substantial infrastructure to regularly cover all parts of an area of endemicity. This makes chemotherapy expensive and, as compliance is often low, a less than satisfactory control option. There is increasing disquiet about the possibility that PZQ-resistant schistosomes will develop. Consequently, as mathematical modeling predicts, vaccine strategies represent an essential component in the future control of schistosomiasis in China. With the inclusion of focal mollusciciding, improvements in sanitation, and health education into the control scenario, China's target of reducing the level of schistosome infection to less than 1% by 2015 may be achievable.

Schistosoma haematobium: identification of new estrogenic molecules with estradiol antagonistic activity and ability to inactivate estrogen receptor in mammalian cells

We have previously identified the expression of an estradiol (E2)-related molecule by Schistosoma haematobium total antigen (Sh). We now show that this molecule has an antagonistic effect of estradiol in vitro. Our results are consistent with the existence of an estrogenic molecule that antagonizes the activity of estradiol. We found evidence for this molecule as we identified and characterized by mass spectrometry new estrogenic molecules previously unknown, present in schistosome worm extracts and sera of Schistosoma-infected individuals. We also show that Sh is able to interact in vitro with estrogen receptor (ER), explaining how host endocrine system can favor the establishment of schistosomes. These findings highlight the exploitation of the host endocrine system by schistosomes and represent an additional regulatory component of schistosome development that defines a novel paradigm enabling host-parasite interactions. The identification of these molecules opens new ways for the development of alternative drugs to treat schistosomiasis.

Opisthorchis viverrini: evaluation of 28 kDa glutathione S-transferase as diagnostic tool in human opisthorchiasis

The liver fluke Opisthorchis viverrini is the agent of human opisthorchiasis in Thailand with a high prevalence observed in the rural population of north and northeastern regions of the country. A focus of research has therefore been the development of diagnostic tools to indicate infection by this parasite. In the present study, a 28 kDa glutathione S-transferase of O. viverrini (OV28GST), which is found in the excretion/secretion product of the parasite, was evaluated for its application in diagnosis of human opisthorchiasis. Bacterially expressed and functionally active rOV28GST was used in immunoblots and indirect ELISA to detect anti-OV28GST antibody in sera of infected individuals. Crude whole worm extract, sera of uninfected individuals and a rabbit anti-rOV28GST antiserum were used as controls in the assays while positivity for parasite DNA by PCR and egg count in faeces were used as primary indicators of infection. The results showed weak or absent reactivity of the infected sera to immunoblotted rOV28GST and no significant difference in absorbance values when compared to uninfected sera in ELISA. In addition, a glutathione capture ELISA which was performed to test for circulating OV28GST in human and hamster sera showed negative results. In conclusion, OV28GST is not applicable as a diagnostic tool in established infections due to low specific antibody titre and abundance as circulating antigen.

Molecular cloning, biochemical characterization, and partial protective immunity of the heme-binding glutathione S-transferases from the human hookworm Necator americanus

Hookworm glutathione S-transferases (GSTs) are critical for parasite blood feeding and survival and represent potential targets for vaccination. Three cDNAs, each encoding a full-length GST protein from the human hookworm Necator americanus (and designated Na-GST-1, Na-GST-2, and Na-GST-3, respectively) were isolated from cDNA based on their sequence similarity to Ac-GST-1, a GST from the dog hookworm Ancylostoma caninum. The open reading frames of the three N. americanus GSTs each contain 206 amino acids with 51% to 69% sequence identity between each other and Ac-GST-1. Sequence alignment with GSTs from other organisms shows that the three Na-GSTs belong to a nematode-specific nu-class GST family. All three Na-GSTs, when expressed in Pichia pastoris, exhibited low lipid peroxidase and glutathione-conjugating enzymatic activities but high heme-binding capacities, and they may be involved in the detoxification and/or transport of heme. In two separate vaccine trials, recombinant Na-GST-1 formulated with Alhydrogel elicited 32 and 39% reductions in adult hookworm burdens (P < 0.05) following N. americanus larval challenge relative to the results for a group immunized with Alhydrogel alone. In contrast, no protection was observed in vaccine trials with Na-GST-2 or Na-GST-3. On the basis of these and other preclinical data, Na-GST-1 is under possible consideration for further vaccine development.

Schistosomiasis--a rich vein of research

The expansion in the volume of research activity over the last 100 years and the acceleration of technological change that underpins the nature and, to a lesser extent, the quality of the science we do, means that those in science today have little time, or indeed opportunity, for reflection on some of the pioneering efforts within their fields. From the first discovery of schistosome parasites in Egypt in 1851 by Theodor Bilharz, research on schistosomiasis has provided valuable insights into many wider aspects of biology and medicine and has attracted an array of colourful and dynamic scientists who have made major advances through elegant experimentation.

A wake up call for urinary schistosomiasis: reconciling research effort with public health importance

This review considers the current status of urinary schistosomiasis, caused by infection with Schistosoma haematobium, and argues that greater research effort and focus are needed to improve understanding of this neglected tropical disease (NTD). The inappropriateness of relying solely on data concerning the much more extensively studied intestinal form of schistosomiasis caused by S. mansoni is highlighted. The current lack of genome and transcriptome information for S. haematobium is directly hindering further targeted research and must be quickly rectified. Recent molecular phylogenies caution the expectation of similarities between schistosome species and highlight the close relationships of species within the S. haematobium group. Treatment, current and prospective drugs and vaccines, together with diagnosis are considered, highlighting the differences associated with urinary schistosomiasis. This infection has a significant and specific impact on the urino-genital system and has a strong association with bladder cancer, leading to severe and chronic morbidity. There is a clear need for new clinical initiatives in this area to better quantify the disease burden. Furthermore, emerging associations with HIV and other pathogens need to be closely monitored. Research is urgently needed to improve current knowledge in order to develop the next generation of control tools.

Conquering 'snail fever': schistosomiasis and its control in China

Schistosomiasis japonica is a serious parasitic disease and a major health risk for more than 60 million people living in the tropical and subtropical zones of south China. The disease is a zoonosis and its cause, the parasitic trematode Schistosoma japonicum, has a range of mammalian reservoirs, making control efforts difficult. Current control programs are heavily based on community chemotherapy with a single dose of the highly effective drug praziquantel. However, vaccines (for use in bovines and in humans) in combination with other control strategies are needed to eliminate the disease. In this review, we provide an overview of the transmission, clinical features, pathogenesis, diagnosis, treatment, genetics and susceptibility, epidemiology, and prospects for control of schistosomiasis japonica in China. The threat posed by the Three Gorges Dam may undermine control efforts because it will change the local ecology and associated schistosomiasis transmission risks over the next decade and beyond.

Immunopathogenesis of human schistosomiasis

Schistosomiasis continues to be a significant cause of parasitic morbidity and mortality worldwide. This review considers the basic features of the pathology and clinical outcomes of hepatointestinal and genitourinary schistosomiasis, presents an overview of the numerous studies on animal models that have clarified many of the immunopathological features, and provides insight into our current understanding of the immunopathogenesis and genetic control of human schistosomiasis. In murine schistosomiasis, pathology is induced by a CD4(+) Th2 driven granulomatous response directed against schistosome eggs lodged in the host liver. The Th2 cytokines IL-4 and IL-13 drive this response, whereas IL-10, IL13Ralpha2, IFN-gamma and a subset of regulatory T-cells act to limit schistosome induced pathology. A variety of cell types including hepatic stellate cells, alternatively activated macrophages and regulatory T-cells have also been implicated in the pathogenesis of schistosomiasis. Current knowledge suggests the immunopathogenic mechanisms underlying human schistosomiasis are likely to be similar. The review also considers the future development of anti-pathology schistosome vaccines. As fibrosis is an important feature of many other diseases such as Crohn's disease and sarcoidosis, a comprehensive understanding of the cellular and molecular mechanisms involved in schistosomiasis may also ultimately contribute to the development an effective disease intervention strategy for other granulofibrotic diseases.

Enzymatic antioxidant systems in helminth parasites

Parasitic helminths have a coexistence with mammalian hosts whereby they survive for several years in known hostile conditions of their hosts. Many explanations exist describing how these parasitic helminths are able to survive. In the last years, a lot of studies have focused on both enzymatic and non-enzymatic antioxidant systems now shown to exist in these parasites and which may serve as defence tactics against the host-generated oxygen radicals. The relevance of antioxidant enzymes is confirmed by the fact that some of these molecules represent putative protective anti-parasite vaccines (i.e. in schistosomiasis). This review tries to compile what is known to date of the enzymatic antioxidant systems in selected parasitic helminths.

Sm-p80-based DNA vaccine formulation induces potent protective immunity against Schistosoma mansoni

No effective vaccine exists for the human parasitic disease, schistosomiasis. We have targeted a functionally important antigen, Sm-p80 as a vaccine candidate because of its consistent immunogenicity, protective potential and important role in the immune evasion process. In this study we report that a Sm-p80-based DNA vaccine formulation confers 59% reduction in worm burden in mice. Animals immunized with Sm-p80-pcDNA3 exhibited a decrease in egg production by 84%. Sm-p80 DNA elicited strong immune responses that include IgG2A and IgG2B antibody isotypes in vaccinated animals. Splenocytes proliferated in response to Sm-p80 produced appreciably more Th1 response enhancing cytokines (IL-2, IFN-gamma) than Th2 response enhancing cytokines (IL-4, IL-10). These data reinforce the potential of Sm-p80 as an excellent vaccine candidate for schistosomiasis.

Immunoglobulin E (IgE) responses to paramyosin predict resistance to reinfection with Schistosoma japonicum and are attenuated by IgG4

Schistosomiasis remains a public health concern in developing countries, and rapid reinfection fostered by continued exposure to contaminated water sources necessitates a vaccine to augment current mass treatment-based control strategies. We report isotype-specific (immunoglobulin A [IgA], IgE, IgG1, IgG4, and IgG) antibody responses to soluble worm antigen preparation and the recombinant vaccine candidates rSj97, rSj67, and rSj22 from a Schistosoma japonicum-infected cohort in Leyte, the Philippines, where schistosomiasis is endemic. Sera were collected from infected individuals 1 month posttreatment with praziquantel, and antibody responses were measured using a bead-based multiplex platform. Reinfection was monitored by stool sampling every 3 months, and data up to 1 year were included in the analysis (n = 553). In repeated-measures models, individuals with detectible IgE responses to rSj97 had a 26% lower intensity of reinfection at 12 months posttreatment compared to nonresponders after adjusting for age, gender, village, exposure, pretreatment infection intensity, and clustering by household (P = 0.018). In contrast, IgG4 responses to rSj97 as well as rSj67 and rSj22 were associated with markedly increased reinfection intensity. When stratified by IgG4 and IgE responder status, individuals with IgE but not IgG4 responses to rSj97 (n = 16) had a 77% lower intensity of reinfection at 12 months compared to individuals with IgG4 responses but not IgE responses (n = 274), even after adjusting for potential confounders (P = 0.016). Together with our previously described protective cytokine responses, these data further support paramyosin as a leading vaccine candidate for human schistosomiasis japonica and underscore the importance of careful adjuvant selection to avoid the generation of blocking IgG4 antibody responses.

Schistosomes--proteomics studies for potential novel vaccines and drug targets

Schistosomiasis is a major health problem and, despite decades of research, only one effective drug, Praziquantel is currently available. Recent expansion of sequence databases on Schistosoma mansoni and S. japonicum has permitted a wealth of novel proteomic studies on several aspects of the organization and development of the parasite in the human host. This unprecedented accumulation of molecular data is allowing a more rational approach to propose drug targets and vaccine candidates, such as proteins located at the parasite surface. Successful preliminary trials of two vaccine candidates that have been detected at the parasite surface by proteomics give grounds for believing that such an approach may provide a fresh start for the field.

Helminth infections: the great neglected tropical diseases

Helminths are parasitic worms. They are the most common infectious agents of humans in developing countries and produce a global burden of disease that exceeds better-known conditions, including malaria and tuberculosis. As we discuss here, new insights into fundamental helminth biology are accumulating through newly completed genome projects and the nascent application of transgenesis and RNA interference technologies. At the same time, our understanding of the dynamics of the transmission of helminths and the mechanisms of the Th2-type immune responses that are induced by infection with these parasitic worms has increased markedly. Ultimately, these advances in molecular and medical helminth biology should one day translate into a new and robust pipeline of drugs, diagnostics, and vaccines for targeting parasitic worms that infect humans.

Can schistosomiasis really be consigned to history without a vaccine?

Recently, considerable enthusiasm has been expressed for expanding and combining control efforts for neglected tropical diseases (NTDs). While these efforts are laudable, the drugs in question require repeated mass administration for indefinite periods of time, and their use to achieve eradication is fraught with challenges. Mass drug administration is unlikely to be effective in isolation, and should not proceed without concurrent control methods, such as vaccines. Schistosomiasis is one of the most important NTDs, and one whose effective control is unlikely in the absence of improved sanitation and a vaccine. Recent advances in biotechnologies have enhanced antigen discovery and new molecules that show promise as recombinant vaccines are being reported. Funding bodies supporting research into the control of schistosomiasis should invest not only in mass drug administration but also in the development of new control strategies, including the development of vaccines.

Current status of vaccines for schistosomiasis

Schistosomiasis, caused by trematode blood flukes of the genus Schistosoma, is recognized as the most important human helminth infection in terms of morbidity and mortality. Infection follows direct contact with freshwater harboring free-swimming larval (cercaria) forms of the parasite. Despite the existence of the highly effective antischistosome drug praziquantel (PZQ), schistosomiasis is spreading into new areas, and although it is the cornerstone of current control programs, PZQ chemotherapy does have limitations. In particular, mass treatment does not prevent reinfection. Furthermore, there is increasing concern about the development of parasite resistance to PZQ. Consequently, vaccine strategies represent an essential component for the future control of schistosomiasis as an adjunct to chemotherapy. An improved understanding of the immune response to schistosome infection, both in animal models and in humans, suggests that development of a vaccine may be possible. This review considers aspects of antischistosome protective immunity that are important in the context of vaccine development. The current status in the development of vaccines against the African (Schistosoma mansoni and S. haematobium) and Asian (S. japonicum) schistosomes is then discussed, as are new approaches that may improve the efficacy of available vaccines and aid in the identification of new targets for immune attack.

Experimental vaccines in animal models for schistosomiasis

Considerable morbidity and mortality results from the affliction of an estimated 200 million people worldwide by several species of schistosomes; 779 million are exposed to the disease in 74 different countries. Even though anti-parasitic drugs and other control measures, including public hygiene and snail control are available, the advent of an effective vaccine still remains the most potentially powerful means for the control of this disease. The putative vaccine could be administered to small children prior to the time when their contact with infected water is maximal, so as to prevent severe infection in the subsequent years. This review attempts to summarize the status of schistosome vaccine development with special emphasis on functionally important vaccine candidates. The importance of utilizing both murine and nonhuman primate models as a prerequisite for clinical trials is discussed.

Schistosome vaccines: a critical appraisal

An effective schistosome vaccine is a desirable control tool but progress towards that goal has been slow. Protective immunity has been difficult to demonstrate in humans, particularly children, so no routes to a vaccine have emerged from that source. The concept of concomitant immunity appeared to offer a paradigm for a vaccine operating against incoming larvae in the skin but did not yield the expected dividends. The mining of crude parasite extracts, the use of monoclonal antibodies and protein selection based on immunogenicity produced a panel of vaccine candidates, mostly of cytoplasmic origin. However, none of these performed well in independent rodent trials, but glutathione-S-transferase from Schistosoma haematobium is currently undergoing clinical trials as an anti-fecundity vaccine. The sequencing of the S. mansoni transcriptome and genome and the development of proteomic and microarray technologies has dramatically improved the possibilities for identifying novel vaccine candidates, particularly proteins secreted from or exposed at the surface of schistosomula and adult worms. These discoveries are leading to a new round of protein expression and protection experiments that will enable us to evaluate systematically all the major targets available for immune intervention. Only then will we know if schistosomes have an Achilles' heel.

Glutathione transferases from parasites: a biochemical view

The glutathione transferase (GST) system of parasites represents the main detoxification mechanism of hydrophobic and electrophilic compounds. Parasites lack the CYP450 activity, hence part of its function has been taken over by other enzymes including GSTs. Cytosolic GSTs (cGSTs) are found in this system and constitute a versatile and numerous group that in parasites display many peculiarities in contrast to mammalian cGSTs. This review summarizes aspects of the biochemistry of parasite cGSTs such as substrate specificities, inhibitor sensitivities, classification, kinetics and catalysis, as well as some aspects of their protective role.

Protective immune mechanisms in helminth infection

Important insights have recently been gained in our understanding of how host immune responses mediate resistance to parasitic helminths and control associated pathological responses. Although similar cells and cytokines are evoked in response to infection by helminths as diverse as nematodes and schistosomes, the components of the response that mediate protection are dependent on the particular parasite. In this Review, we examine recent findings regarding the mechanisms of protection in helminth infections that have been elucidated in murine models and discuss the implications of these findings in terms of future therapies.

Cloning and characterization of an orphan seven transmembrane receptor from Schistosoma mansoni

A partial cDNA sequence was obtained from the human blood fluke, Schistosoma mansoni using a signal sequence trap approach. The full-length cDNA was cloned and termed Sm-7TM. The corresponding open reading frame had 7 membrane spanning domains and shared identity with a small, novel group of seven transmembrane (7TM) receptors from vertebrates and invertebrates, including the human ee3 receptor - a heptahelical protein implicated in neuronal signalling. Phylogenetic analysis of this novel family showed that the Sm-7TM ORF formed a clade with exclusively invertebrate sequences. Based on topology modelling with ee3, Sm-7TM was predicted to possess an intracellular C-terminal tail, which was expressed as a soluble thioredoxin fusion protein (Sm-7TMC) in Escherichia coli and purified using metal ion-affinity chromatography. A polyclonal antiserum against this domain was used to detect Sm-7TM in detergent-soluble parasite extracts and to immunolocalize the receptor to the tegument of adult S. mansoni.

X-ray structures of Na-GST-1 and Na-GST-2 two glutathione S-transferase from the human hookworm Necator americanus

BACKGROUND

Human hookworm infection is a major cause of anemia and malnutrition of adults and children in the developing world. As part of on-going efforts to control hookworm infection, The Human Hookworm Vaccine Initiative has identified candidate vaccine antigens from the infective L3 larval stages and adult stages of the parasite. Adult stage antigens include the cytosolic glutathione-S-transferases (GSTs). Nematode GSTs facilitate the inactivation and degradation of a variety of electrophilic substrates (drugs) via the nucleophilic addition of reduced glutathione. Parasite GSTs also play significant roles in multi-drug resistance and the modulation of host-immune defense mechanisms.

RESULTS

The crystal structures of Na-GST-1 and Na-GST-2, two major GSTs from Necator americanus the main human hookworm parasite, have been solved at the resolution limits of 2.4 A and 1.9 A respectively. The structure of Na-GST-1 was refined to R-factor 18.9% (R-free 28.3%) while that of Na-GST-2 was refined to R-factor 17.1% (R-free 21.7%). Glutathione usurped during the fermentation process in bound in the glutathione binding site (G-site) of each monomer of Na-GST-2. Na-GST-1 is uncomplexed and its G-site is abrogated by Gln 50. These first structures of human hookworm parasite GSTs could aid the design of novel hookworm drugs.

CONCLUSION

The 3-dimensional structures of Na-GST-1 and Na-GST-2 show two views of human hookworm GSTs. While the GST-complex structure of Na-GST-2 reveals a typical GST G-site that of Na-GST-1 suggests that there is some conformational flexibility required in order to bind the substrate GST. In addition, the overall binding cavities for both are larger, more open, as well as more accessible to diverse ligands than those of GSTs from organisms that have other major detoxifying mechanisms. The results from this study could aid in the design of novel drugs and vaccine antigens.

Gene discovery for the carcinogenic human liver fluke, Opisthorchis viverrini

Background

Cholangiocarcinoma (CCA) – cancer of the bile ducts – is associated with chronic infection with the liver fluke, Opisthorchis viverrini. Despite being the only eukaryote that is designated as a 'class I carcinogen' by the International Agency for Research on Cancer, little is known about its genome.

Results

Approximately 5,000 randomly selected cDNAs from the adult stage of O. viverrini were characterized and accounted for 1,932 contigs, representing ~14% of the entire transcriptome, and, presently, the largest sequence dataset for any species of liver fluke. Twenty percent of contigs were assigned GO classifications. Abundantly represented protein families included those involved in physiological functions that are essential to parasitism, such as anaerobic respiration, reproduction, detoxification, surface maintenance and feeding. GO assignments were well conserved in relation to other parasitic flukes, however, some categories were over-represented in O. viverrini, such as structural and motor proteins. An assessment of evolutionary relationships showed that O. viverrini was more similar to other parasitic (Clonorchis sinensis and Schistosoma japonicum) than to free-living (Schmidtea mediterranea) flatworms, and 105 sequences had close homologues in both parasitic species but not in S. mediterranea. A total of 164 O. viverrini contigs contained ORFs with signal sequences, many of which were platyhelminth-specific. Examples of convergent evolution between host and parasite secreted/membrane proteins were identified as were homologues of vaccine antigens from other helminths. Finally, ORFs representing secreted proteins with known roles in tumorigenesis were identified, and these might play roles in the pathogenesis of O. viverrini-induced CCA.

Conclusion

This gene discovery effort for O. viverrini should expedite molecular studies of cholangiocarcinogenesis and accelerate research focused on developing new interventions, drugs and vaccines, to control O. viverrini and related flukes.

The schistosome in the mammalian host: understanding the mechanisms of adaptation

In this review, we envisage the host environment, not as a hostile one, since the schistosome thrives there, but as one in which the relationship between the two organisms consists of constant communication, through signalling mechanisms involving sense organs, surface glycocalyx, surface membrane and internal organs of the parasite, with host fluids and cells. The surface and secretions of the schistosome egg have very different properties from those of other parasite stages, but adapted for the dispersal of the eggs and for the preservation of host liver function. We draw from studies of mammalian cells and other organisms to indicate how further work might be carried out on the signalling function of the surface glycocalyx, the raft structure of the surface and existence of pores in the surface membrane, the repair of the surface membrane, the role of the membrane structure in ion channel function (including recent work on the actin cytoskeleton and calcium channels) and the possible role of P-glycoproteins in the adaptation of the parasite to its environment. We are speculative in some areas, such as the suggestions that variability in surface properties of schistosomes may relate to the existence of membrane rafts and that parasite communities may exhibit quorum sensing. This speculative approach is adopted with the hope that future work on the whole organisms and their interactions will be encouraged.

Malaria and helminth interactions in humans: an epidemiological viewpoint

In the tropics, helminths are among the most common chronic infections of humans and Plasmodium infections the most deadly. As these two groups of parasites have similar geographical distributions, co-infection is commonplace. It has increasingly been speculated that helminth infections may alter susceptibility to clinical malaria, and there is now increasing interest in investigating the consequences of co-infection, with studies yielding contrasting results. The immunological interactions between helminths and malarial parasites are unclear, although several hypotheses have been proposed. This review provides an epidemiological overview of the possible interactions between helminths and malarial parasites, in relation to geographical distributions and disease patterns, and provides a critical discussion of the results of the epidemiological studies that have so far been conducted to investigate the possible associations. Future studies that might be considered, in order to address the gaps in knowledge, are also considered.

Vaccines against the zoonotic trematodesSchistosoma japonicum,Fasciola hepaticaandFasciola gigantica

Schistosoma japonicum,Fasciola hepaticaandF. giganticaare digenetic trematodes and, therefore, possess similar life cycles. While schistosomiasis japonica has for a long time been recognised as a major disease of both humans and animals, infection with fasciolids has only been considered of relevance to animals. However, a number of recent reports indicate that fasciolosis is becoming a serious public health problem, especially in South America, Egypt and Iran (sporadic cases are also on the increase throughout Europe). Vaccines targeted at animals could play an important role in controlling these three diseases in animals and, by blocking transmission of infection, have a concurrent beneficial effect on disease in humans. Approaches towards identifying and producing vaccines against these parasites are similar and are discussed in this reveiw.

Human schistosomiasis

Schistosomiasis or bilharzia is a tropical disease caused by worms of the genus Schistosoma. The transmission cycle requires contamination of surface water by excreta, specific freshwater snails as intermediate hosts, and human water contact. The main disease-causing species are S haematobium, S mansoni, and S japonicum. According to WHO, 200 million people are infected worldwide, leading to the loss of 1.53 million disability-adjusted life years, although these figures need revision. Schistosomiasis is characterised by focal epidemiology and overdispersed population distribution, with higher infection rates in children than in adults. Complex immune mechanisms lead to the slow acquisition of immune resistance, though innate factors also play a part. Acute schistosomiasis, a feverish syndrome, is mostly seen in travellers after primary infection. Chronic schistosomal disease affects mainly individuals with long-standing infections in poor rural areas. Immunopathological reactions against schistosome eggs trapped in the tissues lead to inflammatory and obstructive disease in the urinary system (S haematobium) or intestinal disease, hepatosplenic inflammation, and liver fibrosis (S mansoni, S japonicum). The diagnostic standard is microscopic demonstration of eggs in the excreta. Praziquantel is the drug treatment of choice. Vaccines are not yet available. Great advances have been made in the control of the disease through population-based chemotherapy but these required political commitment and strong health systems.

Equilibrium in lung schistosomula sphingomyelin breakdown and biosynthesis allows very small molecules, but not antibody, to access proteins at the host-parasite interface

The mechanism by which lung-stage schistosomula expose proteins at the host-parasite interface to nutrient, but not antibody, uptake has been obscure. We have found that Schistosoma mansoni and Schistosoma haematobium larvae emerging from host lung at a pH of around 7.5, and fixed with diluted formaldehyde (HCHO), readily bind specific antibodies in indirect membrane immunofluorescence. Data on inhibitors and activators of parasite tegument-bound, magnesium-dependent, neutral sphingomyelinase (nSMase), and sphingomyelin biosynthesis inhibitors revealed that equilibrium in schistosomular sphingomyelin breakdown and biosynthesis prevents antibody binding, yet permits access of small HO-CH2-OH polymers to interact with and cross-link proteins at the host-parasite interface, allowing for their serological visualization.

The antipoverty vaccines

The neglected tropical diseases represent a group of parasitic and bacterial diseases, occurring primarily in rural areas or impoverished urban areas of developing countries. Because of their chronic and stigmatizing character and their impact on child development, pregnancy outcomes, and worker productivity, the neglected tropical diseases are considered poverty-promoting conditions. Through the activities of public-private partnerships, first or second-generation recombinant vaccines for three of these conditions--hookworm, leishmaniasis, and schistosomiasis, have undergone early development and clinical testing. However, through the acquisition of extensive bioinformatics information or animal model testing for several other neglected tropical diseases pathogens, it is possible to consider new generation vaccines as well for amebiasis, Buruli ulcer, Chagas disease, Chlamydia infections (including trachoma), leprosy, leptospirosis, and the treponematoses. Early development of such antipoverty vaccines will require the establishment of product development public-private partnerships and partnerships with innovative developing countries where these diseases are endemic.

Recombinant tegumental protein Shistosoma japonicum very lowdensity lipoprotein binding protein as a vaccine candidate against Schistosoma japonicum

A polyhistidine-tagged recombinant tegumental protein Schistosoma japonicum very lowdensity lipoprotein binding protein (SVLBP) from adult Schistosoma japonicum was expressed in Escherichia coli. The affinity purified rSVLBP was used to vaccinate mice. The worm numbers and egg deposition recovered from the livers and veins of the immunized mice were 33.5% and 47.6% less than that from control mice, respectively (p<0.05). There was also a marked increase in the antibody response in vaccinated mice: the titer of IgG1 and IgG2a, IgG2b in the vaccinated group was significantly higher than that in the controls (>1:6,400 in total IgG). In a comparison of the reactivity of sera from healthy individuals and patients with rSVLBP, recognition patterns against this parasite tegumental antigen varied among different groups of the individuals. Notably, the average titres of anti-rSVLBP antibody in sera from faecal egg-negative individuals was significantly higher than that in sera from the faecal egg-positives, which may be reflect SVLBP-specific protection. These results suggested that the parasite tegumental protein SVLBP was a promising candidate for further investigation as a vaccine antigen for use against Asian schistosomiasis.

Incorporating a rapid-impact package for neglected tropical diseases with programs for HIV/AIDS, tuberculosis, and malaria

Hotez et al. argue that achieving success in the global fight against HIV/AIDS, tuberculosis, and malaria may well require a concurrent attack on the neglected tropical diseases.

Identification of paramyosin T cell epitopes associated with human resistance to Schistosoma mansoni reinfection

Paramyosin, a Schistosoma mansoni myoprotein associated with human resistance to infection and reinfection, is a candidate antigen to compose a subunit vaccine against schistosomiasis. In this study, 11 paramyosin peptides selected by TEPITOPE algorithm as promiscuous epitopes were produced synthetically and tested in proliferation and in vitro human leucocyte antigen (HLA)-DR binding assays. A differential proliferative response was observed in individuals resistant to reinfection compared to individuals susceptible to reinfection in response to Para (210-226) peptide stimulation. In addition, this peptide was able to bind to all HLA-DR molecules tested in HLA-DR binding assays, confirming its promiscuity. Para (6-22) and Para (355-371) were also shown to be promiscuous peptides, because they were able to bind to the six and eight most prevalent HLA-DR alleles used in HLA-DR binding assays, respectively, and were also recognized by T cells of the individuals studied. These results suggest that these paramyosin peptides are promising antigens to compose an anti-schistosomiasis vaccine.

Biochemical characterization and vaccine potential of a heme-binding glutathione transferase from the adult hookworm Ancylostoma caninum

We report the cloning and expression of Ac-GST-1, a novel glutathione S-transferase from the adult hookworm Ancylostoma caninum, and its possible role in parasite blood feeding and as a vaccine target. The predicted Ac-GST-1 open reading frame contains 207 amino acids (mass, 24 kDa) and exhibited up to 65% amino acid identity with other nematode GSTs. mRNA encoding Ac-GST-1 was detected in adults, eggs, and larval stages, but the protein was detected only in adult hookworm somatic extracts and excretory/secretory products. Using antiserum to the recombinant protein, Ac-GST-1 was immunolocalized to the parasite hypodermis and muscle tissue and weakly to the intestine. Recombinant Ac-GST-1 was enzymatically active, as determined by conjugation of glutathione to a model substrate, and exhibited a novel high-affinity binding site for hematin. The possible role of Ac-GST-1 in parasite heme detoxification during hemoglobin digestion or heme uptake prompted interest in evaluating it as a potential vaccine antigen. Vaccination of dogs with Ac-GST-1 resulted in a 39.4% reduction in the mean worm burden and 32.3% reduction in egg counts compared to control dogs following larval challenge, although the reductions were not statistically significant. However, hamsters vaccinated with Ac-GST-1 exhibited statistically significant worm reduction (53.7%) following challenge with heterologous Necator americanus larvae. These studies suggest that Ac-GST-1 is a possible drug and vaccine target for hookworm infection.

Resistance to re-infection after exposure to normal and attenuated schistosome parasites in the baboon model

The baboon model of schistosomiasis has been used extensively to study parasite biology, immune responses and pathological manifestations after natural and experimental infections. The body of knowledge accumulated so far has placed this animal model at the pinnacle in the continuing search for new interventions and might hold the key to the development of new anti-schistosome vaccines. In this review paper, we highlight previous and recent studies that have elevated the baboon to be the model of choice for schistosomiasis research. In particular, the long-term studies of re-infection after chemotherapy as well as the interaction between vaccination, chemotherapy and infection are highlighted.

Prospects for development of a transmission blocking vaccine against Schistosoma japonicum

Despite intensive long-term control programmes, schistosomiasis japonica remains a serious public health problem in China and the Philippines. The termination of mass praziquantel-treatment has seen a dramatic recent rebound in both its prevalence and associated morbidity. Schistosomiasis japonica is a zoonosis but, despite complicating control efforts, this feature provides a practical method for attacking Schistosoma japonicum through development and deployment of a transmission blocking veterinary vaccine. A recently completed bovine drug intervention trial and mathematical modelling of the transmission of S. japonicum underpin the concept that such a vaccine, targeting water buffalo, would have major implications for future integrated schistosomiasis control in China. A major block to success is the low ceiling efficacy achieved with current vaccine molecules. To solve this challenge, an antigen discovery pipeline needs to be established for identification of new vaccine targets that induce greater potency than the current anti-S. japonicum candidate vaccines. Excretory-secretory products and molecules exposed on epithelial surfaces (including receptors) which interact directly with the host immune system warrant especial attention. Extensive schistosome genomics programmes currently underway coupled with new advances in proteomics and microarray technology provide an unparalleled opportunity to identify new molecules exploitable as vaccine targets. These will then need to be produced in quantity and rigorously tested first in the laboratory and then the field. If a transmission blocking veterinary vaccine developed for bovines can be put into practice in combination with other control strategies such as human chemotherapy, elimination of S. japonicum from China may be achievable.

Immunity induced by the radiation-attenuated schistosome vaccine

As a paradigm for the development of a vaccine against human schistosomiasis, the radiation-attenuated (RA) vaccine has enabled the dissection of different immune responses as putative effector mechanisms. This review considers advances made in the past, and updates our knowledge with reference to recent studies that have provided new information relevant particularly to the early innate events after vaccination, and to the nature of the protective effector mechanism. Priming of a protective response by RA larvae is a highly co-ordinated series of events starting in the skin, draining lymph nodes and lungs, leading to the development of various effector responses, ranging from Th1-associated cell-mediated activity, to anti-parasitic antibodies, all of which contribute to the elimination of challenge larvae to varying extents. In this respect, the RA vaccine elicits a multifaceted immune response, from which we can derive valuable insights relevant to the future design of novel delivery systems and adjuvants for recombinant and subunit vaccines.