Complementary transcriptomic and proteomic analyses reveal the cellular and molecular processes that drive growth and development of Fasciola hepatica in the host liver

Neoblast-like stem cells of Fasciola hepatica

The common liver fluke (Fasciola hepatica) causes the disease fasciolosis, which results in considerable losses within the global agri-food industry. There is a shortfall in the drugs that are effective against both the adult and juvenile life stages within the mammalian host, such that new drug targets are needed. Over the last decade the stem cells of parasitic flatworms have emerged as reservoirs of putative novel targets due to their role in development and homeostasis, including at host-parasite interfaces. Here, we investigate and characterise the proliferating cells that underpin development in F. hepatica. We provide evidence that these cells are capable of self-renewal, differentiation, and are sensitive to ionising radiation- all attributes of neoblasts in other flatworms. Changes in cell proliferation were also noted during the early stages of in vitro juvenile growth/development (around four to seven days post excystment), which coincided with a marked reduction in the nuclear area of proliferating cells. Furthermore, we generated transcriptomes from worms following irradiation-based ablation of neoblasts, identifying 124 significantly downregulated transcripts, including known stem cell markers such as fgfrA and plk1. Sixty-eight of these had homologues associated with neoblast-like cells in Schistosoma mansoni. Finally, RNA interference mediated knockdown of histone h2b (a marker of proliferating cells), ablated neoblast-like cells and impaired worm development in vitro. In summary, this work demonstrates that the proliferating cells of F. hepatica are equivalent to neoblasts of other flatworm species and demonstrate that they may serve as attractive targets for novel anthelmintics.

Thioredoxin (Trx): A redox target and modulator of cellular senescence and aging-related diseases

Thioredoxin (Trx) is a compact redox-regulatory protein that modulates cellular redox state by reducing oxidized proteins. Trx exhibits dual functionality as an antioxidant and a cofactor for diverse enzymes and transcription factors, thereby exerting influence over their activity and function. Trx has emerged as a pivotal biomarker for various diseases, particularly those associated with oxidative stress, inflammation, and aging. Recent clinical investigations have underscored the significance of Trx in disease diagnosis, treatment, and mechanistic elucidation. Despite its paramount importance, the intricate interplay between Trx and cellular senescence-a condition characterized by irreversible growth arrest induced by multiple aging stimuli-remains inadequately understood. In this review, our objective is to present a comprehensive and up-to-date overview of the structure and function of Trx, its involvement in redox signaling pathways and cellular senescence, its association with aging and age-related diseases, as well as its potential as a therapeutic target. Our review aims to elucidate the novel and extensive role of Trx in senescence while highlighting its implications for aging and age-related diseases.

Development of a novel method for diagnosis of fasciolosis based on cathepsin L7 in ruminants

Fasciolosis is a widely distributed zoonosis reported over 81 countries around the world. Good and early diagnostic method is critical in controlling this disease and prevention of injury to the liver and bile ducts. In this study, we identified a novel member (cathepsin L7) of cathepsin family from Fasciola spp.. Firstly, the biological character of CL7 was analyzed according to the information of cathepsin L family, and then rCL7 was expressed and purified, a new iELISA based on CL7 was developed. The results exhibited CL7 iELISA had 100% sensitivity 100% specificity in sheep (cut-off 1.329) and 100% sensitivity 93.75% specificity in cattle (cut-off 0.756). Moreover, anti-Fasciola CL7 antibodies could be detected in early Fasciola gigantica infected buffaloes, as early as 3 week-post-infection (WPI). In conclusion, it is suggested that CL7 with low cost, early detection, good specificity and sensitivity could be used as a candidate antigen for detection of ruminant fasciolosis.

Regulation of immune response against third-stage Gnathostoma spinigerum larvae by human genes

Background

Gnathostomiasis is an important zoonosis in tropical areas that is mainly caused by third-stage Gnathostoma spinigerum larvae (G. spinigerum L3).

Objectives

This study aimed to prove whether G. spinigerum L3 produces extracellular vesicles (EVs) and investigate human gene profiles related to the immune response against the larvae.

Methods

We created an immune cell model using normal human peripheral blood mononuclear cells (PBMCs) co-cultured with the larvae for 1 and 3 days, respectively. The PBMCs were harvested for transcriptome sequencing analysis. The EV ultrastructure was examined in the larvae and the cultured medium.

Results

Extracellular vesicle-like particles were observed under the larval teguments and in the pellets in the medium. RNA-seq analysis revealed that 2,847 and 3,118 genes were significantly expressed on days 1 and 3 after culture, respectively. The downregulated genes on day 1 after culture were involved in pro-inflammatory cytokines, the complement system and apoptosis, whereas those on day 3 were involved in T cell-dependent B cell activation and wound healing. Significantly upregulated genes related to cell proliferation, activation and development, as well as cytotoxicity, were observed on day 1, and genes regulating T cell maturation, granulocyte function, nuclear factor-κB and toll-like receptor pathways were predominantly observed on day 3 after culture.

Conclusion

G. spinigerum L3 produces EV-like particles and releases them into the excretory-secretory products. Overall, genotypic findings during our 3-day observation revealed that most significant gene expressions were related to T and B cell signalling, driving T helper 2 cells related to chronic infection, immune evasion of the larvae, and the pathogenesis of gnathostomiasis. Further in-depth studies are necessary to clarify gene functions in the pathogenesis and immune evasion mechanisms of the infective larvae.

A promising new target to control fasciolosis: Fasciola hepatica leucine aminopeptidase 2

Parasite M17 leucine aminopeptidases (LAPs) have been associated with critical roles in different key functions such as the nutrition, migration, and invasion of the natural host. Native or recombinant LAP used as a vaccine antigen has proved effective to elicit protection against Fasciola hepatica infection in sheep, pointing to potential vaccine candidates against fascioliasis in ruminant species. Previously, the FhLAP1, abundantly secreted in vitro by the mature adult parasite was used as a vaccine antigen obtaining promising protection results against F. hepatica challenge in small ruminants. Here, we report the biochemical characterization of a second recombinant LAP (FhLAP2) associated with the juvenile stage of F. hepatica. FhLAP2 showed aminopeptidase activity using different synthetic substrates, including leucine, arginine, and methionine and was increased in the presence of Mn+ 2 and Mg+ 2. The activity was inhibited by bestatin, 1,10-phenanthroline, and EDTA, specific inhibitors of aminopeptidase and/or metalloproteases. Finally, the recombinant FhLAP2 functional form was tested in combination with Freund's incomplete adjuvant in an immunization trial in mice followed by an experimental challenge with F. hepatica metacercariae. The immunization with FhLAP2/FIA resulted in a significant reduction of parasite recovery compared to control groups. The immunized group elicited total specific IgG and subclasses IgG1 and IgG2 antibody responses. This study highlights the potential of a new candidate vaccine formulation with potential applications in natural ruminant hosts, especially those targeting the juvenile stage.

Molecular Characterization of the Interplay between Fasciola hepatica Juveniles and Laminin as a Mechanism to Adhere to and Break through the Host Intestinal Wall

Fasciola hepatica is the main causative agent of fasciolosis, a zoonotic parasitic disease of growing public health concern. F. hepatica metacercariae are ingested by the host and excyst in the intestine, thereby releasing the newly excysted juveniles (FhNEJ), which traverse the gut wall and migrate towards the biliary ducts. Since blocking F. hepatica development is challenging after crossing of the intestinal wall, targeting this first step of migration might result in increased therapeutic success. The intestinal extracellular matrix (ECM) is constituted by a network of structural proteins, including laminin (LM) and fibronectin (FN), that provide mechanical support while acting as physical barrier against intestinal pathogens. Here, we employed ELISA and immunofluorescent assays to test for the presence of LM- and FN-binding proteins on a tegument-enriched antigenic fraction of FhNEJ, and further determined their identity by two-dimensional electrophoresis coupled to mass spectrometry. Additionally, we performed enzymatic assays that revealed for the first time the capability of the juvenile-specific cathepsin L3 to degrade LM, and that LM degradation by FhNEJ proteins is further potentiated in the presence of host plasminogen. Finally, a proteomic analysis showed that the interaction with LM triggers protein changes in FhNEJ that may be relevant for parasite growth and adaptation inside the mammalian host. Altogether, our study provides valuable insights into the molecular interplay between FhNEJ and the intestinal ECM, which may lead to the identification of targetable candidates for the development of more effective control strategies against fasciolosis.

Development of an enzyme linked immunosorbent assay using recombinant cathepsin B5 antigen for sero-surveillance of bovine tropical fasciolosis

Bovine tropical fasciolosis, caused by Fasciola gigantica, is a major parasitic disease in tropical countries responsible for significant production losses in animal husbandry practices. The disease is transmitted by the Radix sp. snails. In the early developmental stage of the parasite, the juveniles and immature flukes cause considerable damage to the liver parenchyma of the bovine host while migrating through the liver. The cathepsin (cat) B5 is a cysteine protease that is present in the excretory-secretory product of the fluke both in immature and adult stages. The early detection of fasciolosis is very critical in effective disease management. In this study, the cathepsin B5 gene from newly excysted juveniles were cloned, sequenced and analyzed. The phylogenetic analysis revealed existence of two distinct clades. The clade I includes cat B 1 to B3 whereas clade II consist of cat B4 to B7. Further, the present study was aimed to develop an enzyme linked immuno sorbent assay (ELISA) using recombinant cat B5 antigen. The developed enzyme immuno assay showed 95.3 % sensitivity and 92.4 % specificity with a cut-off of 60 % percent positive. It revealed weighted Kappa value as 0.768 (95 % CI 0.648-0.889) when compared with ELISA using native cathepsin protein. Hence, the developed assay can be exploited as a potent tool in the diagnosis and sero-surveillance of bovine tropical fasciolosis.

Genetic manipulations in helminth parasites

Screening of vaccine or drug target in parasitic helminth is hindered by lack of robust tool for functional studies of parasite protein which account for the availability of only a few anti-helminthic vaccines, diagnostic assay and slower pace of development of an anthelmintic drug. With the piling up of parasite transcriptomic and genomic data, in silico screening for possible vaccine/drug target could be validated by functional characterization of proteins by RNA interference or CRISPR/Cas9. These reverse genetic engineering tools have opened up a better avenue and opportunity for screening parasitic proteins in vitro as well as in vivo. RNA interference provides a technique for silencing targeted mRNA transcript for understanding a gene function in helminth as evidence by work in Caenorhabditis elegans. Recent genetic engineering tool, CRISPR/Cas9 allows knock-out/deletion of the desired gene in parasitic helminths and the other provision it provides in terms of gene knock-in/insertion in parasite genome is still to be explored in future. This manuscript discussed the work that has been carried out on RNAi and CRISPR/Cas9 for functional studies of helminth parasitic proteins.

Liver Proteome Alterations in Red Deer (Cervus elaphus) Infected by the Giant Liver Fluke Fascioloides magna

Liver fluke infections are recognised as diseases with worldwide distribution and considerable veterinary and public health importance. The giant liver fluke, Fascioloides magna, is an important non-native parasite which has been introduced to Europe, posing a threat to the survival of local wildlife populations such as red deer (Cervus elaphus). The aim of the study was to analyse differences in liver proteomes between F. magna-infected and control red deer groups using a label-based high-throughput quantitative proteomics approach. The proteomics analysis identified 234 proteins with differential abundance between the control and infected groups. Our findings showed that F. magna infection in this definitive host is associated with changes in the metabolism of proteins and fatty acids, oxidative stress, fibrosis, and signaling pathways. The identified proteins and associated biological pathways represent a valuable contribution to the understanding of host-parasite interactions and the pathogenesis of liver fluke infection.

Omics tools enabling vaccine discovery against fasciolosis

In the past decade significant advances in our understanding of liver fluke biology have been made through in-depth interrogation and analysis of evolving Fasciola hepatica and Fasciola gigantica omics datasets. This information is crucial for developing novel control strategies, particularly vaccines necessitated by the global spread of anthelmintic resistance. Distilling them down to a manageable number of testable vaccines requires combined rational, empirical, and collaborative approaches. Despite a lack of clear outstanding vaccine candidate(s), we must continue to identify salient parasite-host interacting molecules, likely in the secretory products, tegument, or extracellular vesicles, and perform robust trials especially in livestock, using present and emerging vaccinology technologies to discover that elusive liver fluke vaccine. Omics tools are bringing this prospect ever closer.

Transcriptomic analysis supports a role for the nervous system in regulating growth and development of Fasciola hepatica juveniles

Fasciola spp. liver flukes have significant impacts in veterinary and human medicine. The absence of a vaccine and increasing anthelmintic resistance threaten sustainable control and underscore the need for novel flukicides. Functional genomic approaches underpinned by in vitro culture of juvenile Fasciola hepatica facilitate control target validation in the most pathogenic life stage. Comparative transcriptomics of in vitro and in vivo maintained 21 day old F. hepatica finds that 86% of genes are expressed at similar levels across maintenance treatments suggesting commonality in core biological functioning within these juveniles. Phenotypic comparisons revealed higher cell proliferation and growth rates in the in vivo juveniles compared to their in vitro counterparts. These phenotypic differences were consistent with the upregulation of neoblast-like stem cell and cell-cycle associated genes in in vivo maintained worms. The more rapid growth/development of in vivo juveniles was further evidenced by a switch in cathepsin protease expression profiles, dominated by cathepsin B in in vitro juveniles and by cathepsin L in in vivo juveniles. Coincident with more rapid growth/development was the marked downregulation of both classical and peptidergic neuronal signalling components in in vivo maintained juveniles, supporting a role for the nervous system in regulating liver fluke growth and development. Differences in the miRNA complements of in vivo and in vitro juveniles identified 31 differentially expressed miRNAs, including fhe-let-7a-5p, fhe-mir-124-3p and miRNAs predicted to target Wnt-signalling, which supports a key role for miRNAs in driving the growth/developmental differences in the in vitro and in vivo maintained juvenile liver fluke. Widespread differences in the expression of neuronal genes in juvenile fluke grown in vitro and in vivo expose significant interplay between neuronal signalling and the rate of growth/development, encouraging consideration of neuronal targets in efforts to dysregulate growth/development for parasite control.

Exploiting Comparative Omics to Understand the Pathogenic and Virulence-Associated Protease: Anti-Protease Relationships in the Zoonotic Parasites Fasciola hepatica and Fasciola gigantica

The helminth parasites, Fasciola hepatica and Fasciola gigantica, are the causative agents of fasciolosis, a global and economically important disease of people and their livestock. Proteases are pivotal to an array of biological processes related to parasitism (development, feeding, immune evasion, virulence) and therefore their action requires strict regulation by parasite anti-proteases (protease inhibitors). By interrogating the current publicly available Fasciola spp. large sequencing datasets, including several genome assemblies and life cycle stage-specific transcriptome and proteome datasets, we reveal the complex profile and structure of proteases and anti-proteases families operating at various stages of the parasite's life cycle. Moreover, we have discovered distinct profiles of peptidases and their cognate inhibitors expressed by the parasite stages in the intermediate snail host, reflecting the different environmental niches in which they move, develop and extract nutrients. Comparative genomics revealed a similar cohort of peptidase inhibitors in F. hepatica and F. gigantica but a surprisingly reduced number of cathepsin peptidases genes in the F. gigantica genome assemblies. Chromosomal location of the F. gigantica genes provides new insights into the evolution of these gene families, and critical data for the future analysis and interrogation of Fasciola spp. hybrids spreading throughout the Asian and African continents.

Two Distinct Superoxidase Dismutases (SOD) Secreted by the Helminth Parasite Fasciola hepatica Play Roles in Defence against Metabolic and Host Immune Cell-Derived Reactive Oxygen Species (ROS) during Growth and Development

The antioxidant superoxide dismutase (SOD) catalyses the dismutation of superoxide, a dangerous oxygen free radical, into hydrogen peroxide and molecular oxygen. Superoxide generation during the oxidative burst of the innate immune system is considered a key component of the host defence against invading pathogens. We demonstrate the presence and differential expression of two SODs in Fasciola hepatica, a leaderless cytosolic (FhSOD1) and an extracellular (FhSOD3) form containing a secretory signal peptide, suggesting that the parasites exploit these enzymes in distinct ways to counteract reactive oxygen species (ROS) produced by cellular metabolism and immune defences. Both enzymes are highly expressed by the infective newly excysted juvenile (NEJ) stages and are found in abundance in their excretory–secretory products (ES), but only FhSOD1 is present in adult ES, suggesting that the antioxidants have different functions and pathways of secretion, and are under separate temporal expression control during the migration, growth, and development of the parasite. Functionally, the recombinant FhSOD1 and FhSOD3 exhibit similar activity against superoxide to their mammalian counterparts. Confocal immuno-localisation studies demonstrated the presence of FhSOD1 and FhSOD3 on the NEJ tegument and parenchyma, supporting our suggestion that these enzymes are secreted during host invasion to protect the parasites from the harmful oxidative bursts produced by the activated innate immune response. By producing superoxide enzymatically in vitro, we were able to demonstrate robust killing of F. hepatica NEJ within 24 h post-excystment, and that the lethal effect of ROS was nullified with the addition of SOD and catalase (the antioxidant enzyme responsible for the dismutation of hydrogen peroxide, a by-product of the SOD reaction). This study further elucidates the mechanism by which F. hepatica protects against ROS derived from cellular metabolism and how the parasite could mitigate damage caused by the host’s immune response to benefit its survival.

Study of the migration of Fasciola hepatica juveniles across the intestinal barrier of the host by quantitative proteomics in an ex vivo model

Fasciola hepatica is a trematode parasite that infects animals and humans causing fasciolosis, a worldwide-distributed disease responsible for important economic losses and health problems. This disease is of growing public health concern since parasite isolates resistant to the current treatment (triclabendazole) have increasingly been described. F. hepatica infects its vertebrate host after ingestion of the encysted parasite (metacercariae), which are found in the water or attached to plants. Upon ingestion, newly excysted juveniles of F. hepatica (FhNEJ) emerge in the intestinal lumen and cross the intestinal barrier, reach the peritoneum and migrate to the biliary ducts, where adult worms fully develop. Despite the efforts made to develop new therapeutic and preventive tools, to date, protection against F. hepatica obtained in different animal models is far from optimal. Early events of host-FhNEJ interactions are of paramount importance for the infection progress in fasciolosis, especially those occurring at the host-parasite interface. Nevertheless, studies of FhNEJ responses to the changing host environment encountered during migration across host tissues are still scarce. Here, we set-up an ex vivo model coupled with quantitative SWATH-MS proteomics to study early host-parasite interaction events in fasciolosis. After comparing tegument and somatic fractions from control parasites and FhNEJ that managed to cross a mouse intestinal section ex vivo, a set of parasite proteins whose expression was statistically different were found. These included upregulation of cathepsins L3 and L4, proteolytic inhibitor Fh serpin 2, and a number of molecules linked with nutrient uptake and metabolism, including histone H4, H2A and H2B, low density lipoprotein receptor, tetraspanin, fatty acid binding protein a and glutathione-S-transferase. Downregulated proteins in FhNEJ after gut passage were more numerous than the upregulated ones, and included the heath shock proteins HSP90 and alpha crystallin, amongst others. This study brings new insights into early host-parasite interactions in fasciolosis and sheds light on the proteomic changes in FhNEJ triggered upon excystment and intestinal wall crossing, which could serve to define new targets for the prevention and treatment of this widespread parasitic disease.

Fasciolosis caused by the helminth parasite Fasciola hepatica is a serious health and economic problem worldwide. Treatment and prevention of this disease pose several drawbacks that have so far not been solved. The definition of suitable parasite molecular targets to overcome such drawbacks should be based on thoroughly deciphering host-parasite interactions, and in this regard most studies have focused on the adult stages of F. hepatica. Nevertheless, in this context, the study of the transient juvenile stages of this parasite could be of higher utility due to the importance of early interactions with the host for parasite migration and the successful establishment of infection. In this work, we set-up an ex vivo model and performed a quantitative proteomics approach to study the changes in F. hepatica juveniles upon gut passage. We found that the parasite tegument and somatic compartments experienced deep changes in their composition and showed that the host triggers the expression of specific molecules that are important for parasite migration and survival at this stage. The molecules described here could serve to better understand host-parasite interactions and to define new targets against fasciolosis.

Transcriptomic and proteomic profiling of peptidase expression in Fasciola hepatica eggs developing at host's body temperature

Fasciola hepatica is a global parasite of livestock which also causes a neglected zoonosis in humans. The parasite’s communication with the host during its complicated lifecycle is based on an ingenious enzymatic apparatus which includes a variety of peptidases. These enzymes are implicated in parasite migration, pathogenesis of the disease, and modification of host immune response. Although the dynamics of proteolytic machinery produced by intra-mammalian F. hepatica life stages has been previously investigated in great detail, peptidases of the eggs so far received little scientific attention. In this study, we performed a comparative RNA-seq analysis aimed at identification of peptidases expressed in F. hepatica eggs, cultured at 37 °C to represent gall bladder retained eggs, for different time periods and employed mass spectrometry in order to identify and quantify peptidases translated in F. hepatica egg lysates. We demonstrated that F. hepatica eggs undergo significant molecular changes when cultured at the physiological temperature of the definitive host. Egg transcriptome is subject to numerous subtle changes while their proteome is even more variable. The peptidase profile is considerably modified on both transcriptome and proteome level. Finally, we measured and classified proteolytic activities in extracts from F. hepatica eggs using a library of fluorogenic substrates and peptidase class-selective inhibitors. Activities of threonine peptidases were detected constantly, while the cysteine peptidases prevailing in freshly laid eggs are substituted by aspartic peptidase and metallopeptidase activities in the later stages of egg development.

Stage-specific miRNAs regulate gene expression associated with growth, development and parasite-host interaction during the intra-mammalian migration of the zoonotic helminth parasite Fasciola hepatica

Background

MiRNAs are small non-coding RNAs that post-transcriptionally regulate gene expression in organisms ranging from viruses to mammals. There is great relevance in understanding how miRNAs regulate genes involved in the growth, development, and maturation of the many parasitic worms (helminths) that together afflict more than 2 billion people.

Results

Here, we describe the miRNAs expressed by each of the predominant intra-mammalian development stages of Fasciola hepatica, a foodborne flatworm that infects a wide range of mammals worldwide, most importantly humans and their livestock. A total of 124 miRNAs were profiled, 72 of which had been previously reported and three of which were conserved miRNA sequences described here for the first time. The remaining 49 miRNAs were novel sequences of which, 31 were conserved with F. gigantica and the remaining 18 were specific to F. hepatica. The newly excysted juveniles express 22 unique miRNAs while the immature liver and mature bile duct stages each express 16 unique miRNAs. We discovered several sequence variant miRNAs (IsomiRs) as well as miRNA clusters that exhibit strict temporal expression paralleling parasite development. Target analysis revealed the close association between miRNA expression and stage-specific changes in the transcriptome; for example, we identified specific miRNAs that target parasite proteases known to be essential for intestinal wall penetration (cathepsin L3). Moreover, we demonstrate that miRNAs fine-tune the expression of genes involved in the metabolic pathways that allow the parasites to move from an aerobic external environment to the anerobic environment of the host.

Conclusions

These results provide novel insight into the regulation of helminth parasite development and identifies new genes and miRNAs for therapeutic development to limit the virulence and pathogenesis caused by F. hepatica.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08644-z.

Evolutionary Adaptations of Parasitic Flatworms to Different Oxygen Tensions

During the evolution of the Earth, the increase in the atmospheric concentration of oxygen gave rise to the development of organisms with aerobic metabolism, which utilized this molecule as the ultimate electron acceptor, whereas other organisms maintained an anaerobic metabolism. Platyhelminthes exhibit both aerobic and anaerobic metabolism depending on the availability of oxygen in their environment and/or due to differential oxygen tensions during certain stages of their life cycle. As these organisms do not have a circulatory system, gas exchange occurs by the passive diffusion through their body wall. Consequently, the flatworms developed several adaptations related to the oxygen gradient that is established between the aerobic tegument and the cellular parenchyma that is mostly anaerobic. Because of the aerobic metabolism, hydrogen peroxide (H₂O₂) is produced in abundance. Catalase usually scavenges H₂O₂ in mammals; however, this enzyme is absent in parasitic platyhelminths. Thus, the architecture of the antioxidant systems is different, depending primarily on the superoxide dismutase, glutathione peroxidase, and peroxiredoxin enzymes represented mainly in the tegument. Here, we discuss the adaptations that parasitic flatworms have developed to be able to transit from the different metabolic conditions to those they are exposed to during their life cycle.

Developmental Regulation and Functional Prediction of microRNAs in an Expanded Fasciola hepatica miRNome

The liver fluke, Fasciola hepatica, is a global burden on the wellbeing and productivity of farmed ruminants, and a zoonotic threat to human health. Despite the clear need for accelerated discovery of new drug and vaccine treatments for this pathogen, we still have a relatively limited understanding of liver fluke biology and host interactions. Noncoding RNAs, including micro (mi)RNAs, are key to transcriptional regulation in all eukaryotes, such that an understanding of miRNA biology can shed light on organismal function at a systems level. Four previous publications have reported up to 89 mature miRNA sequences from F. hepatica, but our data show that this does not represent a full account of this species miRNome. We have expanded on previous studies by sequencing, for the first time, miRNAs from multiple life stages (adult, newly excysted juvenile (NEJ), metacercariae and adult-derived extracellular vesicles (EVs)). These experiments detected an additional 61 high-confidence miRNAs, most of which have not been described in any other species, expanding the F. hepatica miRNome to 150 mature sequences. We used quantitative (q)PCR assays to provide the first developmental profile of miRNA expression across metacercariae, NEJ, adult and adult-derived Evs. The majority of miRNAs were expressed most highly in metacercariae, with at least six distinct expression clusters apparent across life stages. Intracellular miRNAs were functionally analyzed to identify target mRNAs with inversely correlated expression in F. hepatica tissue transcriptomes, highlighting regulatory interactions with key virulence transcripts including cathepsin proteases, and neuromuscular genes that control parasite growth, development and motility. We also linked 28 adult-derived EV miRNAs with downregulation of 397 host genes in F. hepatica-infected transcriptomes from ruminant lymph node, peripheral blood mononuclear cell (PBMC) and liver tissue transcriptomes. These included genes involved in signal transduction, immune and metabolic pathways, adding to the evidence for miRNA-based immunosuppression during fasciolosis. These data expand our understanding of the F. hepatica miRNome, provide the first data on developmental miRNA regulation in this species, and provide a set of testable hypotheses for functional genomics interrogations of liver fluke miRNA biology.

Targeting Secreted Protease/Anti-Protease Balance as a Vaccine Strategy against the Helminth Fasciola hepatica

The liver fluke Fasciola hepatica is an economically important global pathogen of humans and their livestock. To facilitate host invasion and migration, F. hepatica secretes an abundance of cathepsin peptidases but prevents excessive damage to both parasite and host tissues by co-secreting regulatory peptidase inhibitors, cystatins/stefins and Kunitz-type inhibitors. Here, we report a vaccine strategy aimed at disrupting the parasite’s protease/anti-protease balance by targeting these key inhibitors. Our vaccine cocktail containing three recombinant stefins (rFhStf-1, rFhStf-2, rFhStf-3) and a Kunitz-type inhibitor (rFhKT1) formulated in adjuvant Montanide 61VG was assessed in two independent sheep trials. While fluke burden was not reduced in either trial, in Trial 1 the vaccinated animals showed significantly greater weight gain (p < 0.05) relative to the non-vaccinated control group. In both trials we observed a significant reduction in egg viability (36–42%). Multivariate regression analyses showed vaccination and increased levels of IgG2 antibodies specific for the F. hepatica peptidase inhibitors were positive indicators for increased weight gain and levels of haemoglobin within the normal range at 16 weeks post-infection (wpi; p < 0.05). These studies point to the potential of targeting peptidase inhibitors as vaccine cocktails for fasciolosis control in sheep.

Pathogenicity and virulence of the liver flukes Fasciola hepatica and Fasciola Gigantica that cause the zoonosis Fasciolosis

Fasciolosis caused by the liver flukes Fasciola hepatica and Fasciola gigantica is one of the most important neglected parasitic diseases of humans and animals. The ability of the parasites to infect and multiply in their intermediate snail hosts, and their adaptation to a wide variety of mammalian definitive hosts contribute to their high transmissibility and distribution. Within the mammalian host, the trauma caused by the immature flukes burrowing through the liver parenchyma is associated with most of the pathogenesis. Similarly, the feeding activity and the physical presence of large flukes in the bile ducts can lead to anemia, inflammation, obstruction and cholangitis. The high frequency of non-synonymous polymorphisms found in Fasciola spp. genes allows for adaptation and invasion of a broad range of hosts. This is also facilitated by parasite’s excretory-secretory (ES) molecules that mediate physiological changes that allows their establishment within the host. ES contains cathepsin peptidases that aid parasite invasion by degrading collagen and fibronectin. In the bile ducts, cathepsin-L is critical to hemoglobin digestion during feeding activities. Other molecules (peroxiredoxin, cathepsin-L and Kunitz-type inhibitor) stimulate a strong immune response polarized toward a Treg/Th2 phenotype that favors fluke’s survival. Helminth defense molecule, fatty acid binding proteins, Fasciola-specific glycans and miRNAs modulate host pro-inflammatory responses, while antioxidant scavenger enzymes work in an orchestrated way to deter host oxidant-mediated damage. Combining these strategies Fasciola spp. survive for decades within their mammalian host, where they reproduce and spread to become one of the most widespread zoonotic worm parasites in the world.

Prospects for liver fluke vaccines

Fasciola spp., Opisthorchis spp. and Clonorchis sinensis are common liver flukes that can cause a variety of diseases, mainly cholangiocarcinoma induced by clonorchiasis and liver damage and associated pathology induced by fascioliasis. Because these trematodes are parasites of humans and domestic animals, they have greatly affected the economy of agricultural industries and public health worldwide. Due to the emergence of drug resistance and the living habits of flukes, among other reasons, a possibility of reinfection remains even when antiparasitic drugs are used. Therefore, developing a safe, efficient and cost-effective vaccine against trematodes is an important goal. Here, we briefly describe the progress in the development of vaccines against liver flukes. Related innovations may provide effective protection against these helminths and the diseases that they cause.

Complementary liver and serum protein profile in wild boars infected by the giant liver fluke Fascioloides magna using tandem mass tags quantitative approach

Liver fluke, Fascioloides magna, is an important non-native parasite introduced to Europe, posing a threat to survival of local wildlife populations. The aim of this study was to assess the serum and liver protein profile of control and F. magna infected wild boars, by means of shotgun tandem mass tag - based quantitative high resolution proteomics approach. In serum, 4 differentially abundant proteins were found out of total 1073 identified, while in liver from 3520 identified proteins, 116 were differentially abundant between healthy and F. magna infected wild boars. Pathway analysis revealed that most of the proteins differing in abundance are involved in metabolism, biological oxidations, cellular responses to stimuli, fatty acid metabolism, and others. Validation of proteomic results was performed for paraoxonase-1, ceruloplasmin, glutathione S-transferase and liver enzymes by ELISA and automated assays. Complementary analysis of liver and serum in F. magna infection enabled insight into changes of proteome profile of the host at local and sistemic level. Our findings showed that chronic infection with F. magna is associated with immune response in host, oxidative stress and metabolomic changes in liver. SIGNIFICANCE: Liver fluke infections are recognised as worldwide neglected diseases with considerable veterinary and public health importance. Pathological changes, clinical signs and outcome of F. magna infection are strongly related to the type of final hosts and their different tolerance to infection. In order to gain insight into host-parasite interactions in wild boars, dead-end host for F. magna, we assessed proteomics profile of serum and liver of control animals and those infected with F. magna. Proteomics analysis of serum and liver in parallel showed as advantageous and beneficial, demonstrating protein alterations mainly at local level. Bioinformatics analysis enabled elucidation of molecular pathways associated with F. magna infection. Identification and validation of proteins associated with infection may have added value to current tools for efficient liver fluke control.

Diagnosis of sheep fasciolosis caused by Fasciola hepatica using cathepsin L enzyme-linked immunosorbent assays (ELISA)

Fasciolosis, a global parasitic disease of agricultural livestock, is caused by the liver fluke Fasciola hepatica. Management and strategic control of fasciolosis on farms depends on early assessment of the extent of disease so that control measures can be implemented quickly. Traditionally, this has relied on the detection of eggs in the faeces of animals, a laborious method that lacks sensitivity, especially for sub-clinical infections, and identifies chronic infections only. Enzyme linked immunosorbent assays (ELISA) offer a quicker and more sensitive serological means of diagnosis that could detect early acute infection before significant liver damage occurs. The performance of three functionally-active recombinant forms of the major F. hepatica secreted cathepsins L, rFhCL1, rFhCL2, rFhCL3, and a cathepsin B, rFhCB3, were evaluated as antigens in an indirect ELISA to serologically diagnose liver fluke infection in experimentally and naturally infected sheep. rFhCL1 and rFhCL3 were the most effective of the four antigens detecting fasciolosis in sheep as early as three weeks after experimental infection, at least five weeks earlier than both coproantigen and faecal egg tests. In addition, the rFhCL1 and rFhCL3 ELISAs had a very low detection limit for liver fluke in lambs exposed to natural infection on pastures and thus could play a major role in the surveillance of farms and a 'test and treat' approach to disease management. Finally, antibodies to all three cathepsin L proteases remain high throughout chronic infection but decline rapidly after drug treatment with the flukicide, triclabendazole, implying that the test may be adapted to trace the effectiveness of drug treatment.

Antigen-specific response of CD4+ T cells and hepatic lymph node cells to Fasciola hepatica-derived molecules at the early and late stage of the infection in sheep

The immunomodulatory capacity of F. hepatica antigens is probably one of the main reasons for the development of a driven non-protective Th2 immune response. In this study, we analysed the cellular response of hepatic lymph node cells and CD4⁺ T cells in terms of proliferative response, efficiency of antigen presentation and cytokine production, to F. hepatica-derived molecules, at early and late stages of the infection. Thirty-one sheep were allocated into five groups and were slaughtered at 16 dpi and 23 wpi. In order to analyse antigen-specific response, the following F. hepatica recombinant molecules were used: rFhCL1, rFhCL2, rFhCL3, rFhCB1, rFhCB2, rFhCB3, rFhStf-1, rFhStf-2, rFhStf-3 and rFhKT1. A cell proliferation assay using hepatic lymph node cells and an antigen presentation cell assay using CD4⁺ T cells were performed. At 16 dpi, all molecules but rFhStf-2 and rFhKT1 elicited a significant cell proliferative response on hepatic lymph node cells of infected animals. At both early and late stage of the infection, antigen presentation of rFhCB3 and rFhCL2 resulted in higher stimulation index of CD4⁺ T cells which was IL-2 mediated, although no statistically significant when compared to uninfected animals. Significant cytokine production (IL-4, IL-10 and IFN-γ) was conditioned by the antigen-specific cell stimulation. No CD4⁺ T cell exhaustion was detected in infected sheep at the chronic stage of the infection. This study addressed antigen-specific response to F. hepatica-derived molecules that are involved in key aspects of the parasite survival within the host.

Autonomous Non Antioxidant Roles for Fasciola hepatica Secreted Thioredoxin-1 and Peroxiredoxin-1

Trematode parasites of the genus Fasciola are the cause of liver fluke disease (fasciolosis) in humans and their livestock. Infection of the host involves invasion through the intestinal wall followed by migration in the liver that results in extensive damage, before the parasite settles as a mature egg-laying adult in the bile ducts. Genomic and transcriptomic studies revealed that increased metabolic stress during the rapid growth and development of F. hepatica is balanced with the up-regulation of the thiol-independent antioxidant system. In this cascade system thioredoxin/glutathione reductase (TGR) reduces thioredoxin (Trx), which then reduces and activates peroxiredoxin (Prx), whose major function is to protect cells against the damaging hydrogen peroxide free radicals. F. hepatica expresses a single TGR, three Trx and three Prx genes; however, the transcriptional expression of Trx1 and Prx1 far out-weighs (>50-fold) other members of their family, and both are major components of the parasite secretome. While Prx1 possesses a leader signal peptide that directs its secretion through the classical pathway and explains why this enzyme is found freely soluble in the secretome, Trx1 lacks a leader peptide and is secreted via an alternative pathway that packages the majority of this enzyme into extracellular vesicles (EVs). Here we propose that F. hepatica Prx1 and Trx1 do not function as part of the parasite’s stress-inducible thiol-dependant cascade, but play autonomous roles in defence against the general anti-pathogen oxidative burst by innate immune cells, in the modulation of host immune responses and regulation of inflammation.

Trematode Proteomics: Recent Advances and Future Directions

Trematodes cause disease in millions of people worldwide, but the absence of commercial vaccines has led to an over-reliance on a handful of monotherapies to control infections. Since drug-resistant fluke populations are emerging, a deeper understanding of parasite biology and host interactions is required to identify new drug targets and immunogenic vaccine candidates. Mass spectrometry-based proteomics represents a key tool to that end. Recent studies have capitalised on the wider availability of annotated helminth genomes to achieve greater coverage of trematode proteomes and discover new aspects of the host-parasite relationship. This review focusses on these latest advances. These include how the protein components of fluke extracellular vesicles have given insight into their biogenesis and cellular interactions. In addition, how the integration of transcriptome/proteome datasets has revealed that the expression and secretion of selected families of liver fluke virulence factors and immunomodulators are regulated in accordance with parasite development and migration within the mammalian host. Furthermore, we discuss the use of immunoproteomics as a tool to identify vaccine candidates associated with protective antibody responses. Finally, we highlight how established and emerging technologies, such as laser microdissection and single-cell proteomics, could be exploited to resolve the protein profiles of discrete trematode tissues or cell types which, in combination with functional tools, could pinpoint optimal targets for fluke control.