Modulation of Innate Antigen-Presenting Cell Function by Pre-patent Schistosome Infection

Schistosomes are intravascular helminths that infect over 200 million people worldwide. Deposition of eggs by adult schistosomes stimulates Th2 responses to egg antigens and induces granulomatous pathology that is a hallmark of schistosome infection. Paradoxically, schistosomes require host immune function for their development and reproduction and for egress of parasite eggs from the host. To identify potential mechanisms by which immune cells might influence parasite development prior to the onset of egg production, we assessed immune function in mice infected with developing schistosomes. We found that pre-patent schistosome infection is associated with a loss of T cell responsiveness to other antigens and is due to a diminution in the ability of innate antigen-presenting cells to stimulate T cells. Diminution of stimulatory capacity by schistosome worms specifically affected CD11b⁺ cells and did not require concomitant adaptive responses. We could not find evidence for production of a diffusible inhibitor of T cells by innate cells from infected mice. Rather, inhibition of T cell responsiveness by accessory cells required cell contact and only occurred when cells from infected mice outnumbered competent APCs by more than 3∶1. Finally, we show that loss of T cell stimulatory capacity may in part be due to suppression of IL-12 expression during pre-patent schistosome infection. Modulation of CD4⁺ T cell and APC function may be an aspect of host immune exploitation by schistosomes, as both cell types influence parasite development during pre-patent schistosome infection.     

Failure of in vitro-cultured schistosomes to produce eggs: how does the parasite meet its needs for host-derived cytokines such as TGF-β?

When adult schistosome worm pairs are transferred from experimental hosts to in vitro culture they cease producing viable eggs within a few days. Female worms in unisexual infections fail to mature, and when mature adult females are separated from male partners they regress sexually. Worms cultured from the larval stage are also permanently reproductively defective. The cytokine transforming growth factor beta derived from the mammalian host is considered important in stimulating schistosome female worm maturation and maintenance of fecundity. The means by which schistosomes acquire TGF-β have not been elucidated, but direct uptake in vivo seems unlikely as the concentration of free, biologically active cytokine in host blood is very low. Here we review the complexities of schistosome development and male–female interactions, and we speculate about two possibilities on how worms obtain the TGF-β they are assumed to need: (i) worms may have mechanisms to free active cytokine from the latency-inducing complex of proteins in which it is associated, and/or (ii) they may obtain the cytokine from alpha 2-macroglobulin, a blood-borne protease inhibitor to which TGF-β can bind. These ideas are experimentally testable.     

Schistosoma mansoni: TGF-beta signaling pathways

Schistosome parasites have co-evolved an intricate relationship with their human and snail hosts as well as a novel interplay between the adult male and female parasites. We review the role of the TGF-beta signaling pathway in parasite development, host-parasite interactions and male-female interactions. The data to date support multiple roles for the TGF-beta signaling pathway throughout schistosome development, in particular, in the tegument which is at the interface with the host and between the male and female schistosome, development of vitelline cells in female worms whose genes and development are regulated by a stimulus from the male schistosome and embryogenesis of the egg. The human ligand TGF-beta1 has been demonstrated to regulate the expression of a schistosome target gene that encodes a gynecophoric canal protein in the schistosome worm itself. Studies on signaling in schistosomes opens a new era for investigation of host-parasite and male-female interactions.     

Schistosoma mansoni, S. japonicum, S. haematobium: glycogen content and glucose uptake in parasites from fasted and control hosts

The glycogen content of male and female Schistosoma mansoni has been measured in flukes from normally fed hosts and those from fasted hosts. In infections from both the mouse and the hamster, a significant reduction in schistosomal glycogen of males is seen hours after food is withdrawn from the host. Reductions in protein content of the schistosomes were only observed in hamster infections fasted at least 72 hr. The livers of infected mice not only decrease in size during fasting, but there is a concomitant reduction in glycogen per unit wet weight. Comparisons of glycogen:protein ratios of mansonian males, females, and host livers indicate that the fasting-induced loss of liver glycogen is also observed in the male schistosome, but not the female. Studies of both S. mansoni and S. haematobium pairs from fed hosts suggest that the ratio of glycogen:protein contents in the male schistosome correlates with the glycogen:protein ratio of the female partner. Measurements of glucose uptake in vitro suggest that greater uptake rates may be observed in flukes perfused from fasted hosts. In S. japonicum from infected mice, a reduction in male glycogen was also detected as early as after a 6-hr fasting period, but changes in the females were not significant. Unmated male S. japonicum also exhibit a reduction in glycogen levels after fasting, but the quantity of worm glycogen present in these males remains higher than comparable mated males. In mice entrained to a regulated pattern of available food, fluctuations in glycogen content of the male schistosomes were observed, but in the female partners fluctuations were of a smaller magnitude.(ABSTRACT TRUNCATED AT 250 WORDS)     

The metabolic control of schistosome egg production

Schistosomiasis is a neglected tropical disease caused by infection with trematode parasites of the genus Schistosoma. Despite ongoing treatment programmes, the prevalence of schistosomiasis has failed to decline and the disease remains a cause of severe morbidity in millions of people. Understanding the biology of egg production by schistosomes is critical since eggs allow transmission of the infection, and when trapped in host tissues induce the immune responses that are responsible for the pathologic changes that underlie disease development. Unusually among trematodes, adult schistosomes exhibit sexual dimorphism and display a fascinating codependency in that the female is dependent on the male to grow and sexually mature. Thus, virgin females are developmentally stunted compared with females from mixed‐sex infections and are unable to lay eggs. Moreover, fecund female schistosomes rapidly lose the ability to produce eggs when placed in tissue culture. Here we discuss the metabolic regulation of egg production in schistosomes, and in particular the critical role played by fatty acid oxidation in this process.     

Protein characterization of sexually mature and immature forms of Schistosoma mansoni

• 1.1. Protein composition of different stages of Schistosoma mansoni was compared using specific antisera, 2D polyacrylamide gel electrophoresis and 14-C-leucine incorporation into proteins.
• 2.2. Major qualitative differences were detected when an anti-membrane antiserum was used.
• 3.3. 2D gel electrophoresis showed that the protein composition varied when mature and immature females were compared, whereas no differences were noted when mature and immature male worms were compared.
• 4.4. Experiments measuring protein synthesis by the different schistosome stages confirmed that upon maturation, only the female schistosomes displayed qualitative differences.
• 5.5. The protein pattern of the male schistosomes did not vary significantly as a function of development.

     

Herbimycin A suppresses mitotic activity and egg production of female Schistosoma mansoni

The eggs of the endoparasite Schistosoma are the causative agent of schistosomiasis, an important disease of humans, which is endemic in (sub-) tropical regions. The absence of a vaccine with sufficient protective qualities and increasing resistance to approved and established drugs like praziquantel, justify the exploration of novel ways to fight schistosomes. Our strategy is based on interference with the sexual maturation of the female. Prerequisites for gonad development in adult females are a continuous pairing contact with the male and significantly increased mitotic activity. In this study we show that the male governs sexual maturation of the female, as the separation of couples causes a clear reduction of female mitotic activity and, consequently, egg production. We demonstrate that treatment of schistosomes with Herbimycin A, an inhibitor of protein tyrosine kinases (PTKs), mimics the separation of couples as the drug blocks mitotic activity and egg production of paired females. However, the synthesis of the eggshell precursor protein p14 is elevated. Furthermore, we show for the first time in invertebrates that Herbimycin A decreases tyrosine phosphorylation and PTK stability in schistosomes. Summarised, our data provide evidence that PTKs have key functions in regulating gonad development, eggshell gene expression and, consequently, egg production. Therefore, we suggest envisaging schistosome PTKs as novel targets for strategies to combat schistosomiasis.     

MicroRNAs Are Involved in the Regulation of Ovary Development in the Pathogenic Blood Fluke Schistosoma japonicum

Schistosomes, blood flukes, are an important global public health concern. Paired adult female schistosomes produce large numbers of eggs that are primarily responsible for the disease pathology and critical for dissemination. Consequently, understanding schistosome sexual maturation and egg production may open novel perspectives for intervening with these processes to prevent clinical symptoms and to interrupt the life-cycle of these blood-flukes. microRNAs (miRNAs) are key regulators of many biological processes including development, cell proliferation, metabolism, and signal transduction. Here, we report on the identification of Schistosoma japonicum miRNAs using small RNA deep sequencing in the key stages of male-female pairing, gametogenesis, and egg production. We identified 38 miRNAs, including 10 previously unknown miRNAs. Eighteen of the miRNAs were differentially expressed between male and female schistosomes and during different stages of sexual maturation. We identified 30 potential target genes for 16 of the S. japonicum miRNAs using antibody-based pull-down assays and bioinformatic analyses. We further validated some of these target genes using either in vitro luciferase assays or in vivo miRNA suppression experiments. Notably, suppression of the female enriched miRNAs bantam and miR-31 led to morphological alteration of ovaries in female schistosomes. These findings uncover key roles for specific miRNAs in schistosome sexual maturation and egg production.     

Helminth-modified pulmonary immune response protects mice from allergen-induced airway hyperresponsiveness

It has been shown that the presence of certain helminth infections in humans, including schistosomes, may reduce the propensity to develop allergies in infected populations. Using a mouse model of schistosome worm vs worm + egg infection, our objective was to dissect the mechanisms underlying the inverse relationship between helminth infections and allergies. We have demonstrated that conventional Schistosoma mansoni egg-laying male and female worm infection of mice exacerbates airway hyperresponsiveness. In contrast, mice infected with only schistosome male worms, precluding egg production, were protected from OVA-induced airway hyperresponsiveness. Worm-infected mice developed a novel modified type 2 cytokine response in the lungs, with elevated allergen-specific IL-4 and IL-13 but reduced IL-5, and increased IL-10. Although schistosome worm-only infection is a laboratory model, these data illustrate the complexity of schistosome modulation of host immunity by the worm vs egg stages of this helminth, with the potential of infections to aggravate or suppress allergic pulmonary inflammation. Thus, infection of mice with a human parasitic worm can result in reduced airway inflammation in response to a model allergen.     

Molecular and enzymatic characterization of Schistosoma mansoni thioredoxin peroxidase

The ability of Schistosoma mansoni to escape oxidative damage from immune system-generated reactive oxygen intermediates has been extensively documented. The limiting step in the parasite's detoxification process appears to be at the level of hydrogen peroxide neutralization. In the present study, the possible role of a novel class of antioxidant enzymes, thioredoxin peroxidase (TPx), in hydrogen peroxide neutralization by schistosomes was investigated. An expressed sequence tag was characterized from the Schistosoma Genome Initiative with high similarity to TPx from other organisms. The gene encodes a polypeptide containing 2 conserved active-site cysteines and flanking amino acids, and 60-70% identity with previously characterized TPx proteins. Recombinant schistosome TPx was enzymatically active and found to have thioredoxin-dependent hydrogen peroxide reducing activity of 4500 nmol hydrogen peroxide/min/mg protein. Native TPx activity was determined to be 48.1 nmol hydrogen peroxide/min/mg protein in adult worm homogenates compared with 46.9 for glutathione peroxidase. TPx activity was precipitated from adult worm homogenates with antibodies prepared against the recombinant protein. Western blotting with antibodies made against recombinant protein showed that TPx was expressed in both male and female adult worms. This is the first demonstration of a TPx activity in schistosomes and our results suggest that TPx plays a significant role in schistosome-host interactions.     

血吸虫基因操作研究进展

近年来,血吸虫基因组、转录组、蛋白质组和分泌组研究广泛开展,迫切需要针对单个分子功能深入研究。开展血吸虫基因操作研究不仅可在虫体内深入研究基因功能,对进一步理解血吸虫生长发育机理和寄生生活特征具有重要意义,还可建立抗血吸虫候选疫苗和药物靶标筛选的重要平台。为此,本文总结基因操作技术在血吸虫学中的应用并分析其现状。     

Schistosome female reproductive development

Schistosome parasites have evolved to produce a number of unique features in their life history; one of these is separate sexes. This has, in turn, led to a novel interplay between the male and female parasite that has been recognized for over 50 years: the growth and reproductive development of the female parasite is in some way regulated by the male schistosome. Early classical and later experimental studies established that the presence of the male schistosome is necessary not only for the initiation of female development but also for the maintenance of her mature state. The male parasite regulates the reproductive development of the female, partly by providing a stimulus that is necessary for the development of the vitelline gland. The cells of the vitelline gland provide nutrients and shell precursors for the egg. Also in this review by Philip LoVerde and Li-ly Chen, it is interesting to note that recent molecular studies have confirmed early work by showing that gene expression in the female parasite is developmentally regulated in a tissue-specific manner and that this gene expression is controlled by the presence of a male parasite.     

Test of pangamy by genetic analysis of Schistosoma mansoni pairs within its natural murine host in Guadeloupe

Mating system plays a determinant role in the maintenance and distribution of genetic variations. It can be assessed indirectly by analyzing the distribution of the genetic variability within populations or directly by considering how mating pairs are formed. In the present study, 71 pairs of adult Schistosoma mansoni worms sampled from naturally infected rats were genotyped to investigate how male and female schistosomes paired according to their genetic relatedness. Among all samples, pangamy, the random association between males and females, could not be rejected. Whereas the schistosome mating system has been intensively studied under experimental conditions, to the best of our knowledge, our study is the first to attempt to understand the way in which males and females pair in natural conditions.     

Pathophysiological responses to a schistosome infection in a wild population of mourning doves (Zenaida macroura)

The blood trematode Gigantobilharzia huronensis typically infects passerine birds and has not been reported in other orders of wild birds. However, in the summer of 2011 in Tempe, Arizona, USA, mourning doves (Zenaida macroura; order: Columbiformes) were collected with infections of G. huronensis. This is the first report of a natural schistosome infection found in wild populations of doves. We sought to determine if G. huronensis infections alter the general body condition and physiology of doves, a seemingly unlikely host for this parasite. Specifically, we hypothesized that birds infected with schistosomes would exhibit reduced weight as well as increased markers of stress and immune system activation. Adult male mourning doves (n = 14) were captured using walk-in style funnel traps. After weighing the birds, blood and mesenteric tissue samples were collected. We measured biomarkers of stress including circulating heat shock proteins (HSPs) 60 and 70, as well as oxidized lipoproteins in schistosome-infected and non-infected birds. Indices of immune system reactivity were assessed using agglutination and lysis assays in addition to determining the leukocyte to erythrocyte ratios and prevalence of hemoparasite infections from blood smears. Schistosome-infected mourning doves had significantly increased oxidative stress and evidence of HSP70 mobilization. There was no evidence for weight loss in schistosome-infected birds nor evidence of significant immune system activation associated with schistosome infection. This may be a reflection of the small sample size available for the study. These findings suggest that schistosome infections have pathological effects in doves, but the lack of mature worms suggests that infected birds in this sampling may not have been suitable hosts for parasite maturation.     

Sexing schistosome larvae

Biological and immunological factors may influence changes in sex ratios at different points of the schistosome life cycle, resulting in the fact that female schistosomes are significantly outnumbered by males in chronic infections of snails and mammalian hosts. Analysis of this phenomenon has long been hampered by shortcomings in the methods used to determine sex ratios. Here, Robin Gasser describes recently developed molecular methods for sexing schistosome larvae. These have opened the way towards understanding why sex ratios become male biased and allow a proper assessment of its consequences in the epidemiology, diagnosis and pathology of infection.     

Signalling pathways and the host-parasite relationship: putative targets for control interventions against schistosomiasis: signalling pathways and future anti-schistosome therapies

A better understanding of how schistosomes exploit host nutrients, neuro-endocrine hormones and signalling pathways for growth, development and maturation may provide new insights for improved interventions in the control of schistosomiasis. This paper describes recent advances in the identification and characterisation of schistosome tyrosine kinase and signalling pathways. It discusses the potential intervention value of insulin signalling, which may play an important role in glucose uptake and carbohydrate metabolism in schistosomes, providing the nutrients essential for parasite growth, development and, notably, female fecundity. Significant progress has also been made in the characterisation of other schistosome growth factor receptors, such as transforming growth factor beta receptor and epidermal growth factor receptor, and in our understanding of their roles in the host-parasite molecular dialogue and parasite development. The use of parasite signal transduction components as novel vaccine or drug targets may prove invaluable in prevention, treatment and control strategies to combat schistosomiasis.     

Effects of schistosomes on host anti-viral immune response and the acquisition,virulence, and prevention of viral infections: A systematic review

Although a growing number of studies suggest interactions between Schistosoma parasites and viral infections, the effects of schistosome infections on the host response to viruses have not been evaluated comprehensively. In this systematic review, we investigated how schistosomes impact incidence, virulence, and prevention of viral infections in humans and animals. We also evaluated immune effects of schistosomes in those coinfected with viruses. We screened 4,730 studies and included 103. Schistosomes may increase susceptibility to some viruses, including HIV and Kaposi’s sarcoma-associated herpesvirus, and virulence of hepatitis B and C viruses. In contrast, schistosome infection may be protective in chronic HIV, Human T-cell Lymphotropic Virus-Type 1, and respiratory viruses, though further research is needed. Schistosome infections were consistently reported to impair immune responses to hepatitis B and possibly measles vaccines. Understanding the interplay between schistosomes and viruses has ramifications for anti-viral vaccination strategies and global control of viral infections.     

Suppression of adaptive immunity to heterologous antigens by SJ16 of Schistosoma japonicum

Despite the great effort that has been given to control the disease, schistosomiasis remains the most important human helminth infection in terms of morbidity and mortality. Natural infection of schistosomes induces very little protective immunity against reinfection. Moreover, effective schistosome vaccines for practical use have not been developed. These parasites appear to have evolved highly effective modulatory mechanisms on their host's immune system that promote the parasites' survival and also hinder the development of effective strategies for treatment of the disease. Understanding of the mechanisms of schistosome-mediated immune modulation would be most helpful in schistosomiasis prevention and control. Previously, we have identified from Schistosoma japonicum an anti-inflammatory protein, Sj16, which suppresses thioglycollate-induced peritoneal inflammation in BALB/c mice, as well as thioglycollate-mediated peritoneal macrophage maturation, while modulating cytokine and chemokine production from peritoneal cells. In the present study, we have further investigated the modulatory effect of Sj16 on the host's adaptive immunity to heterologous antigens with the use of recombinant Sj16 (rSj16) expressed and purified from Escherichia coli . Results from this study indicate that rSj16 significantly suppresses antibody production, in addition to Th1 and Th2 responses to heterologous antigens in the BALB/c mouse model. Our study also reveals that rSj16 suppresses lipopolysaccharide-induced major histocompatibility complex II expression and IL-12 production, while increasing IL-10 production in resident peritoneal macrophages. These results may partially explain why parasite-related antigens cannot mount a protective immunity during early stages of schistosome infection.