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

Qualitative analysis of Fasciola gigantica excretory and secretory products coimmunoprecipitated with buffalo secondary infection sera shows dissimilar components from primary infection sera

Buffaloes cannot mount a robust adaptive immune response to secondary infection by Fasciola gigantica. Even if excretory and secretory products (ESPs) exhibit potent immunoregulatory effects during primary infection, research on ESPs in secondary infection is lacking, even though the ESP components that are excreted/secreted during secondary infection are unknown. Therefore, qualitative analysis of ESP during secondary infection was performed and compared with that of primary infection to deepen the recognition of secondary infection and facilitate immunoregulatory molecules screening. Buffaloes were divided into three groups: A (n = 3, noninfected), B (n = 3, primary infection) and C (n = 3, secondary infection). Buffaloes in the primary (0 weeks post infection; wpi) and secondary (-4 and 0 wpi) infection groups were infected with 250 metacercariae by oral administration. Then, sera were collected from groups at different wpi, and interacting proteins were precipitated by coimmunoprecipitation (Co-IP), qualitatively analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and annotated by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses to infer their potential functions. In group C, 324 proteins were identified, of which 76 proteins were consistently identified across 7 time points (1, 3, 6, 8, 10, 13, and 16 wpi). Compared with 87 proteins consistently identified in group B, 22 proteins were identified in group C. Meanwhile, 34 proteins were only identified in group C compared to 200 proteins identified in group B. Protein pathway analysis indicated that these proteins were mainly involved in the cellular processes and metabolism of F. gigantica. Among them, 14-3-3θ was consistently identified in group C and may be involved in various cellular processes and innate immune signalling pathways. Members of the HSP family were identified in both groups B and C and may function in both primary and secondary infection processes. The proteins discovered in the present study will help to deepen the understanding of the molecular interactions between F. gigantica and buffalo during secondary infection and facilitate the identification of new potential immunoregulatory molecules.

Helminth-derived molecules: pathogenic and pharmacopeial roles

Parasitic helminths, taxonomically comprising trematodes, cestodes, and nematodes, are multicellular invertebrates widely disseminated in nature and have afflicted people continuously for a long time. Helminths play potent roles in the host through generating a variety of novel molecules, including some excretory/secretory products and others that are involved in intracellular material exchange and information transfer as well as the initiation or stimulation of immune and metabolic activation. The helminth-derived molecules have developed powerful and diverse immunosuppressive effects to achieve immune evasion for parasite survival and establish chronic infections. However, they also improve autoimmune and allergic inflammatory responses and promote metabolic homeostasis by promoting metabolic reprogramming of various immune functions, and then inducing alternatively activated macrophages, T helper 2 cells, and regulatory T cells-mediated immune responses. Therefore, a deeper exploration of the immunopathogenic mechanism and immune regulatory mechanisms of helminth-derived molecules exerted in the host is crucial for understanding host-helminth interactions as well as the development of therapeutic drugs for infectious or non-infectious diseases. In this review, we focus on the properties of helminth-derived molecules to give an overview of the most recent scientific knowledge about their pathogenic and pharmacopeial roles in immune-metabolic homeostasis.

Flavonoid-Quinoxaline Hybrid Compounds as Cathepsin Inhibitors Against Fascioliasis

Fasciola hepatica is a parasitic trematode that infects livestock animals and humans, causing significant health and economic burdens worldwide. The extensive use of anthelmintic drugs has led to the emergence of resistant parasite strains, posing a threat to treatment success. The complex life cycle of the liver fluke, coupled with limited funding and research interest, have hindered progress in drug discovery. Our group has been working in drug development against this parasite using cathepsin proteases as molecular targets, finding promising compound candidates with in vitro and in vivo efficacy. Here, we evaluated hybrid molecules that combine two chemotypes, chalcones and quinoxaline 1,4-di- N-oxides, previously found to inhibit F. hepatica cathepsin Ls and tested their in vitro activity with the isolated targets and the parasites in culture. These molecules proved to be good cathepsin inhibitors and to kill the juvenile parasites at micromolar concentrations. Also, we performed molecular docking studies to analyze the compounds-cathepsins interface, finding that the best inhibitors interact at the active site cleft and contact the catalytic dyad and residues belonging to the substrate binding pockets. We conclude that the hybrid compounds constitute promising scaffolds for the further development of new fasciolicidal compounds.

Moonlighting on the Fasciola hepatica tegument: Enolase, a glycolytic enzyme, interacts with the extracellular matrix and fibrinolytic system of the host

Enolase is a 47 kDa enzyme that functions within the glycolysis and gluconeogenesis pathways involved in the reversible conversion of D-2-phosphoglycerate (2PGA) to phosphoenolpyruvate (PEP). However, in the context of host-pathogen interactions, enolase from different species of parasites, fungi and bacteria have been shown to contribute to adhesion processes by binding to proteins of the host extracellular matrix (ECM), such as fibronectin (FN) or laminin (LM). In addition, enolase is a plasminogen (PLG)-binding protein and induces its activation to plasmin, the main protease of the host fibrinolytic system. These secondary 'moonlighting' functions of enolase are suggested to facilitate pathogen migration through host tissues. This study aims to uncover the moonlighting role of enolase from the parasite Fasciola hepatica, shedding light on its relevance to host-parasite interactions in fasciolosis, a global zoonotic disease of increasing concern. A purified recombinant form of F. hepatica enolase (rFhENO), functioning as an active homodimeric glycolytic enzyme of ~94 kDa, was successfully obtained, fulfilling its canonical role. Immunoblotting studies on adult worm extracts showed that the enzyme is present in the tegument and the excretory/secretory products of the parasite, which supports its key role at the host-parasite interface. Confocal immunolocalisation studies of the protein in newly excysted juveniles and adult worms also localised its expression within the parasite tegument. Finally, we showed by ELISA that rFhENO can act as a parasitic adhesin by binding host LM, but not FN. rFhENO also binds PLG and enhances its conversion to plasmin in the presence of the tissue-type and urokinase-type PLG activators (t-PA and u-PA). This moonlighting adhesion-like function of the glycolytic protein enolase could contribute to the mechanisms by which F. hepatica efficiently invades and migrates within its host and encourages further research efforts that are designed to impede this function by vaccination or drug design.

Fasciolosis: pathogenesis, host-parasite interactions, and implication in vaccine development

Fasciola hepatica is distributed worldwide, causing substantial economic losses in the animal husbandry industry. Human fasciolosis is an emerging zoonosis in Andean America, Asia, and Africa. The control of the disease, both in humans and animals, is based on using anthelmintic drugs, which has resulted in increased resistance to the most effective anthelmintics, such as triclabendazole, in many countries. This, together with the concerns about drug residues in food and the environment, has increased the interest in preventive measures such as a vaccine to help control the disease in endemic areas. Despite important efforts over the past two decades and the work carried out with numerous vaccine candidates, none of them has demonstrated consistent and reproducible protection in target species. This is at least in part due to the high immunomodulation capacity of the parasite, making ineffective the host response in susceptible species such as ruminants. It is widely accepted that a deeper knowledge of the host-parasite interactions is needed for a more rational design of vaccine candidates. In recent years, the use of emerging technologies has notably increased the amount of data about these interactions. In the present study, current knowledge of host-parasite interactions and their implication in Fasciola hepatica vaccine development is reviewed.

Fasciola hepatica antioxidant and protease-inhibitor cocktail recombinant vaccines administered five times elicit potent and sustained immune responses in sheep but do not confer protection

Our laboratory's vaccine development strategy against the livestock parasite Fasciola hepatica centres around disrupting key biological processes by combining groups of antigens with similar/complementary functional actions into a single vaccine cocktail. In this study the focus was on antioxidant protein vaccines and a protease inhibitor vaccine aimed at disrupting the parasite's ability to defend against oxidative stress and protease-inhibitor balance, respectively. Two combinations of recombinantly expressed antioxidants were assessed, namely peroxiredoxin (rFhPrx), thioredoxin (rFhTrx) and thioredoxin-glutathione reductase (rFhTGR) (Group 1) and rFhPrx, rFhTrx, and two superoxide dismutases (rFhSOD1 and rFhSOD3) (Group 2). The protease inhibitor vaccine cocktail included representatives of each of the key secreted protease inhibitor families, namely a Kunitz-type inhibitor (rFhKT1), a serpin (rFhSrp1) and a stefin, (rFhStf1) (Group 3). The vaccine combinations were formulated in adjuvant Montanide 61VG administered at five timepoints; two before experimental challenge with 60 F. hepatica metacercariae and three after infection. The vaccine combinations did not reduce the liver fluke burden, and only Group 2 displayed a marginal reduction in egg viability (8.2%). Despite previous results showing an effect of liver fluke vaccines on overall weight gain in infected animals, no significant (P value >0.05) impact on weight gain was observed in this study. Antibodies were elicited against all the vaccine antigens within the cocktails and were maintained at high levels to the end of the trial, due to our strategy of continuing vaccine administration after infection. However, these responses were not boosted by the challenge F. hepatica infection. A comparative analysis with previous vaccine data using a protease inhibitor vaccine found no repeat of the promising outcomes associated with this vaccine, indicating that the addition of rFhSrp1 to the vaccine cocktail did not improve vaccine efficacy. Assessment of liver pathology across the two trials using a modified liver enzyme score (glutamate dehydrogenase to platelet ratio) at eight weeks post infection suggests an association with liver fluke burden above 45 flukes, which could be used to predict liver pathology in future trials. The results reported in this study highlight the ambiguousness in liver fluke vaccine development and the difficulty in obtaining consistent and repeatable protection. This work stresses the need for repetition of trials and the use of sufficiently sized groups to assess vaccine efficacy with adequate statistical power.

Study of the cross-talk between Fasciola hepatica juveniles and the intestinal epithelial cells of the host by transcriptomics in an in vitro model

Fasciolosis is a globally widespread trematodiasis with a major economic and veterinary impact. Therefore, this disease is responsible for millions of dollars in losses to the livestock industry, and also constitutes an emerging human health problem in endemic areas. The ubiquitous nature of Fasciola hepatica, the main causative agent, is one of the key factors for the success of fasciolosis. Accordingly, this parasite is able to subsist in a wide variety of ecosystems and hosts, thanks to the development of a plethora of strategies for adaption and immune evasion. Fasciolosis comprises a growing concern due to its high prevalence rates, together with the emergence of strains of the parasite resistant to the treatment of choice (triclabendazole). These facts highlight the importance of developing novel control measures which allow for an effective protection against the disease before F. hepatica settles in a niche inaccessible to the immune system. However, knowledge about the initial phases of the infection, including the migration mechanisms of the parasite and the early innate host response, is still scarce. Recently, our group developed an in vitro host-parasite interaction model that allowed the early events to be unveiled after the first contact between the both actors. This occurs shortly upon ingestion of F. hepatica metacercariae and the emergence of the newly excysted juveniles (FhNEJ) in the host duodenum. Here, we present a transcriptomic analysis of such model using an approach based on RNA sequencing (RNA-Seq), which reveals changes in gene expression related to proteolysis and uptake of metabolites in FhNEJ. Additionally, contact with the parasite triggered changes in host intestinal cells related to pseudogenes expression and host defence mechanisms, including immune response, among others. In sum, these results provide a better understanding of the early stages of fasciolosis at molecular level, and a pool of targets that could be used in future therapeutic strategies against the disease.

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.

Draft genome of the bluefin tuna blood fluke, Cardicola forsteri

The blood fluke Cardicola forsteri (Trematoda: Aporocotylidae) is a pathogen of ranched bluefin tuna in Japan and Australia. Genomics of Cardicola spp. have thus far been limited to molecular phylogenetics of select gene sequences. In this study, sequencing of the C. forsteri genome was performed using Illumina short-read and Oxford Nanopore long-read technologies. The sequences were assembled de novo using a hybrid of short and long reads, which produced a high-quality contig-level assembly (N50 > 430 kb and L50 = 138). The assembly was also relatively complete and unfragmented, comprising 66% and 7.2% complete and fragmented metazoan Benchmarking Universal Single-Copy Orthologs (BUSCOs), respectively. A large portion (> 55%) of the genome was made up of intergenic repetitive elements, primarily long interspersed nuclear elements (LINEs), while protein-coding regions cover > 6%. Gene prediction identified 8,564 hypothetical polypeptides, > 77% of which are homologous to published sequences of other species. The identification of select putative proteins, including cathepsins, calpains, tetraspanins, and glycosyltransferases is discussed. This is the first genome assembly of any aporocotylid, a major step toward understanding of the biology of this family of fish blood flukes and their interactions within hosts.

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.