Immunomodulation by Helminths: Intracellular Pathways and Extracellular Vesicles

Promising Technologies in the Field of Helminth Vaccines

Helminths contribute a larger global burden of disease than both malaria and tuberculosis. These eukaryotes have caused human infections since before our earliest recorded history (i.e.: earlier than 1200 B.C. for Schistosoma spp.). Despite the prevalence and importance of these infections, helminths are considered a neglected tropical disease for which there are no vaccines approved for human use. Similar to other parasites, helminths are complex organisms which employ a plethora of features such as: complex life cycles, chronic infections, and antigenic mimicry to name a few, making them difficult to target by conventional vaccine strategies. With novel vaccine strategies such as viral vectors and genetic elements, numerous constructs are being defined for a wide range of helminth parasites; however, it has yet to be discussed which of these approaches may be the most effective. With human trials being conducted, and a pipeline of potential anti-helminthic antigens, greater understanding of helminth vaccine-induced immunity is necessary for the development of potent vaccine platforms and their optimal design. This review outlines the conventional and the most promising approaches in clinical and preclinical helminth vaccinology.

Harnessing immunomodulatory mechanisms of Trichinella spiralis to design novel nanomedical approaches for restoring self-tolerance in autoimmunity

The rapid increase in the prevalence of autoimmune diseases in recent decades, especially in developed countries, coincided with improved living conditions and healthcare. Part of this increase could be ascribed to the lack of exposure to infectious agents like helminths that co-evolved with us and display potent immune regulatory actions. In this review we discussed many investigations, including our own, showing that Trichinella spiralis via its excretory-secretory products attenuate Th1/Th17 immunopathological response in autoimmunity and potentiate the protective Th2 and or regulatory T cell response, acting as an effective induction of tolerogenic dendritic cells (DCs), and probably mimicking the autoantigen in some diseases. A recent discovery of T. spiralis extracellular vesicles (TsEVs) suggested that inducing a complex regulation of the immune response requires simultaneous delivery of different signals in nano-sized packages. Indeed, different artificial nanomedical approaches discussed here suggested that co-delivery of multiple signals via nanoparticles is the most promising strategy for the treatment of autoimmune diseases. Although a long way is ahead of us before we could completely replicate natural nano-delivery systems which are both safe and potent in restoring self-tolerance, a clear path is being opened from a careful examination of parasite-host interactions.

Parasite worm antigens instruct macrophages to release immunoregulatory extracellular vesicles

Emerging evidence suggests that immune cells not only communicate with each other through cytokines, chemokines, and cell surface receptors, but also by releasing small membranous structures known as extracellular vesicles (EVs). EVs carry a variety of different molecules that can be taken up by recipient cells. Parasitic worms are well known for their immunomodulatory properties, but whether they can affect immune responses by altering EV-driven communication between host immune cells remains unclear. Here we provide evidence that stimulation of bone marrow-derived macrophages (BMDMs) with soluble products of Trichuris suis (TSPs), leads to the release of EVs with anti-inflammatory properties. Specifically, we found that EVs from TSP-pulsed BMDMs, but not those from unstimulated BMDMs can suppress TNFα and IL-6 release in LPS-stimulated BMDMs and BMDCs. However, no polarization toward M1 or M2 was observed in macrophages exposed to EVs. Moreover, EVs enhanced reactive oxygen species (ROS) production in the exposed BMDMs, which was associated with a deregulated redox homeostasis as revealed by pathway analysis of transcriptomic data. Proteomic analysis identified cytochrome p450 (CYP450) as a potential source of ROS in EVs from TSP-pulsed BMDMs. Finally, pharmacological inhibition of CYP450 activity could suppress ROS production in those BMDMs. In summary, we find that TSPs can modulate immune responses not only via direct interactions but also indirectly by eliciting the release of EVs from BMDMs that exert anti-inflammatory effects on recipient cells.

Evidence of Immune Modulators in the Secretome of the Equine Tapeworm Anoplocephala perfoliata

Anoplocephala perfoliata is a neglected gastro-intestinal tapeworm, commonly infecting horses worldwide. Molecular investigation of A. perfoliata is hampered by a lack of tools to better understand the host-parasite interface. This interface is likely influenced by parasite derived immune modulators released in the secretome as free proteins or components of extracellular vesicles (EVs). Therefore, adult RNA was sequenced and de novo assembled to generate the first A. perfoliata transcriptome. In addition, excretory secretory products (ESP) from adult A. perfoliata were collected and EVs isolated using size exclusion chromatography, prior to proteomic analysis of the EVs, the EV surface and EV depleted ESP. Transcriptome analysis revealed 454 sequences homologous to known helminth immune modulators including two novel Sigma class GSTs, five α-HSP90s, and three α-enolases with isoforms of all three observed within the proteomic analysis of the secretome. Furthermore, secretome proteomics identified common helminth proteins across each sample with known EV markers, such as annexins and tetraspanins, observed in EV fractions. Importantly, 49 of the 454 putative immune modulators were identified across the secretome proteomics contained within and on the surface of EVs in addition to those identified in free ESP. This work provides the molecular tools for A. perfoliata to reveal key players in the host-parasite interaction within the horse host.

Parasitic Helminth-Derived microRNAs and Extracellular Vesicle Cargos as Biomarkers for Helminthic Infections

As an adaption to their complex lifecycles, helminth parasites garner a unique repertoire of genes at different developmental stages with subtle regulatory mechanisms. These parasitic worms release differential components such as microRNAs (miRNAs) and extracellular vesicles (EVs) as mediators which participate in the host-parasite interaction, immune regulation/evasion, and in governing processes associated with host infection. MiRNAs are small (~ 22-nucleotides) non-coding RNAs that regulate gene expression at the post-transcriptional level, and can exist in stable form in bodily fluids such as serum/plasma, urine, saliva and bile. In addition to reports focusing on the identification of miRNAs or in the probing of differentially expressed miRNA profiles in different development stages/sexes or in specific tissues, a number of studies have focused on the detection of helminth-derived miRNAs in the mammalian host circulatory system as diagnostic biomarkers. Extracellular vesicles (EVs), small membrane-surrounded structures secreted by a wide variety of cell types, contain rich cargos that are important in cell-cell communication. EVs have attracted wide attention due to their unique functional relevance in host-parasite interactions and for their potential value in translational applications such as biomarker discovery. In the current review, we discuss the status and potential of helminth parasite-derived circulating miRNAs and EV cargos as novel diagnostic tools.

Secretory microRNA Profiles of Third- and Fourth-Stage Dirofilaria immitis Larvae with Different Macrocyclic Lactone Susceptibility: In Search of Biomarkers for Early Detection of Infection

The canine heartworm, Dirofilaria immitis, is among the most important parasites of dogs in the United States and worldwide, and may cause severe and potentially fatal disease. Current diagnostic recommendations rely on serological detection of an adult female antigen, and visualization of microfilariae in the blood. Therefore, a reliable diagnosis can be only performed approximately six months post-infection. There is a growing need to characterize novel diagnostic markers that are capable of detecting the early stages of heartworm infection, in special markers associated with third-stage larvae (L3) and fourth-stage larvae (L4). The early detection of infection would guide medical interventions that could impede the development of patent infections and further parasite transmission. We cultured D. immitis L3 and L4 of two laboratorial strains with different susceptibility statuses to macrocyclic lactone drugs in vitro. Excretory/secretory microRNAs were sequenced and analyzed. We identified two miRNA novel candidates secreted abundantly by both L3 and L4 of both strains. These candidates were previously detected in the secretions of other D. immitis stages and one of them was found in the blood of D. immitis-infected dogs. These miRNAs have not been found in the secretions of other nematodes and could be D. immitis-specific diagnostic biomarkers, which could allow for the early detection of infection.

Helminth derived factors inhibit neutrophil extracellular trap formation and inflammation in bacterial peritonitis

Despite their protective antimicrobial function, neutrophil extracellular traps (NETs) have been implicated in propagation of inflammatory responses in several disease conditions including sepsis. Highly diffusible exogenous ROS produced under such inflammatory conditions, can induce exuberant NETs, thus making inhibition of NETs desirable in inflammatory diseases. Here we report that helminth parasite excretory/secretory factors termed as parasitic ligands (PL) inhibit ROS-induced NETs by blocking the activation of nonselective calcium permeable channel Transient Receptor Potential Melastatin 2 (TRPM2). Therapeutic implication of PL mediated blockage of NET formation was tested in preclinical model of septic peritonitis, where PL treatment regulated neutrophil cell death modalities including NET formation and mitigated neutrophil mediated inflammatory response. This translated into improved survival and reduced systemic and local bacterial load in infected mice. Overall, our results posit PL as an important biological regulator of neutrophil functions with implications to a variety of inflammatory diseases including peritonitis.

Understanding the Pathophysiology of Exosomes in Schistosomiasis: A New Direction for Disease Control and Prevention

Schistosomiasis is a parasitic disease endemic to freshwater areas of Southeast Asia, Africa, and South America that is capable of causing serious damage to the internal organs. Recent studies have linked exosomes to the progression of schistosomiasis. These structures are important mediators for intercellular communication, assist cells to exchange proteins, lipids, and genetic material and have been shown to play critical roles during host-parasite interactions. This review aims to discuss the pathophysiology of exosomes in schistosomiasis and their roles in regulating the host immune response. Understanding how exosomes are involved in the pathogenesis of schistosomiasis may provide new perspectives in diagnosing and treating this neglected disease.

In silico prediction of secretory proteins of Opisthorchis viverrini, Clonorchis sinensis and Fasciola hepatica that target the host cell nucleus

Liver flukes Fasciola hepatica, Opisthorchis viverrini and Clonorchis sinensis are causing agents of liver and hepatobiliary diseases. A remarkable difference between such worms is the fact that O. viverrini and C. sinensis are carcinogenic organisms whereas F. hepatica is not carcinogenic. The release of secretory factors by carcinogenic flukes seems to contribute to cancer development however if some of these target the host cell nuclei is unknown. We investigated the existence of O. viverrini and C. sinensis secretory proteins that target the nucleus of host cells and compared these with the corresponding proteins predicted in F. hepatica. Here we applied an algorithm composed by in silico approaches that screened and analyzed the potential genes predicted from genomes of liver flukes. We found 31 and 22 secretory proteins that target the nucleus of host cells in O. viverrini and C. sinensis, respectively, and that have no homologs in F. hepatica. These polypeptides have enriched the transcription initiation process and nucleic acid binding in O. viverrini and C. sinensis, respectively. In addition, other 11 secretory proteins of O. viverrini and C. sinensis, that target the nucleus of host cells, had F. hepatica homologs, have enriched RNA processing function. In conclusion, O. viverrini and C. sinensis have 31 and 22 genes, respectively, that may be involved in their carcinogenic action through a direct targeting on the host cell nuclei.

Immunometabolism: Towards a Better Understanding the Mechanism of Parasitic Infection and Immunity

As a relatively successful pathogen, several parasites can establish long-term infection in host. This “harmonious symbiosis” status relies on the “precise” manipulation of host immunity and metabolism, however, the underlying mechanism is still largely elusive. Immunometabolism is an emerging crossed subject in recent years. It mainly discusses the regulatory mechanism of metabolic changes on reprogramming the key transcriptional and post-transcriptional events related to immune cell activation and effect, which provides a novel insight for understanding how parasites regulate the infection and immunity in hosts. The present study reviewed the current research progress on metabolic reprogramming mechanism exploited by parasites to modulate the function in various immune cells, highlighting the future exploitation of key metabolites or metabolic events to clarify the underlying mechanism of anti-parasite immunity and design novel intervention strategies against parasitic infection.

In silico identification of excretory/secretory proteins and drug targets in monogenean parasites

The Excretory/Secretory (ES) proteins of parasites are involved in invasion and colonization of their hosts. In addition, since ES proteins circulate in the extracellular space, they can be more accessible to drugs than other proteins, which makes ES proteins optimal targets for the development of new and better pharmacological strategies. Monogeneans are a group of parasitic Platyhelminthes that includes some pathogenic species problematic for finfish aquaculture. In the present study, 8297 putative ES proteins from four monogenean species which genomic resources are publicly available were identified and functionally annotated by bioinformatic tools. Additionally, for comparative purposes, ES proteins in other parasitic and free-living platyhelminths were identified. Based on data from the monogenean Gyrodactylus salaris, 15 ES proteins are considered potential drug targets. One of them showed homology to 10 cathepsins with known 3D structure. A docking molecular analysis uncovered that the anthelmintic emodepside shows good affinity to these cathepsins suggesting that emodepside can be experimentally tested as a monogenean's cathepsin inhibitor.

Helminth virus co-infection: Implications for women's health

In many parts of the world, women are co-infected with intestinal helminths and sexually transmitted pathogens. In this issue of Cell Host & Microbe, Chetty et al. demonstrate that intestinal helminth infection increases epithelial damage and pathology associated with herpes virus infection.

Overview of the interaction of helminth extracellular vesicles with the host and their potential functions and biological applications

Helminth Extracellular Vesicles (EVs) have emerged as important mediators in host-parasite communications, participating in the parasite survival and its pathogenic effects. In the last decade, a growing amount of information reporting the isolation and characterization of EVs from different helminth species has appeared, but unfortunately, few reports have focused on functional studies of helminth EVs in different cell lines, organoids or animal models. We here review these in vitro and in vivo studies, which clearly demonstrate that helminths secrete EVs, which affect their environment. Helminth EVs are actively internalized by different cell lines, modulating cellular functions important for host-parasite communication. We discuss how these lines of investigation should provide potential new biomarkers of infection, and since helminth EVs can modulate the host immune response, we also discuss how they can provide a new landscape for the development of new vaccine tools against helminthiases as well as immunotherapy.

Extracellular Vesicles: Schistosomal Long-Range Precise Weapon to Manipulate the Immune Response

Schistosomiasis (Bilharziasis), a neglected tropical disease that affects more than 240 million people around the world, is caused by infection with the helminth parasite Schistosoma. As part of their secretome, schistosomes release extracellular vesicles (EVs) that modulate the host immune response. The EV-harbored miRNAs upregulate the innate immune response of the M1 pathway and downregulate the differentiation toward the adaptive Th2 immunity. A schistosomal egg-derived miRNA increases the percentage of regulatory T cells. This schistosomal-inducible immunoediting process generates ultimately a parasitic friendly environment that is applied carefully as restrained Th2 response is crucial for the host survival and successful excretion of the eggs. Evidence indicates a selective targeting of schistosomal EVs, however, the underlying mechanisms are unclear yet. The effects of the schistosomes on the host immune system is in accordance with the hygiene hypothesis, attributing the dramatic increase in recent decades in allergy and other diseases associated with imbalanced immune response, to the reduced exposure to infectious agents that co-evolved with humans during evolution. Deciphering the bioactive cargo, function, and selective targeting of the parasite-secreted EVs may facilitate the development of novel tools for diagnostics and delivered therapy to schistosomiasis, as well as to immune-associated disorders.

Immunoinformatics Design of Multi-Epitope Peptide-Based Vaccine Against Schistosoma mansoni Using Transmembrane Proteins as a Target

Schistosomiasis remains a serious health issue nowadays for an estimated one billion people in 79 countries around the world. Great efforts have been made to identify good vaccine candidates during the last decades, but only three molecules reached clinical trials so far. The reverse vaccinology approach has become an attractive option for vaccine design, especially regarding parasites like Schistosoma spp. that present limitations for culture maintenance. This strategy also has prompted the construction of multi-epitope based vaccines, with great immunological foreseen properties as well as being less prone to contamination, autoimmunity, and allergenic responses. Therefore, in this study we applied a robust immunoinformatics approach, targeting S. mansoni transmembrane proteins, in order to construct a chimeric antigen. Initially, the search for all hypothetical transmembrane proteins in GeneDB provided a total of 584 sequences. Using the PSORT II and CCTOP servers we reduced this to 37 plasma membrane proteins, from which extracellular domains were used for epitope prediction. Nineteen common MHC-I and MHC-II binding epitopes, from eight proteins, comprised the final multi-epitope construct, along with suitable adjuvants. The final chimeric multi-epitope vaccine was predicted as prone to induce B-cell and IFN-γ based immunity, as well as presented itself as stable and non-allergenic molecule. Finally, molecular docking and molecular dynamics foresee stable interactions between the putative antigen and the immune receptor TLR 4. Our results indicate that the multi-epitope vaccine might stimulate humoral and cellular immune responses and could be a potential vaccine candidate against schistosomiasis.

Helminth-derived cystatins: the immunomodulatory properties of an Ascaris lumbricoides cystatin

Helminth infections such as ascariasis elicit a type 2 immune response resembling that involved in allergic inflammation, but differing to allergy, they are also accompanied with strong immunomodulation. This has stimulated an increasing number of investigations, not only to better understand the mechanisms of allergy and helminth immunity but to find parasite-derived anti-inflammatory products that could improve the current treatments of chronic non-communicable inflammatory diseases such as asthma. A great number of helminth-derived immunomodulators have been discovered and some of them extensively analysed, showing their potential use as anti-inflammatory drugs in clinical settings. Since Ascaris lumbricoides is one of the most successful parasites, several groups have focused on the immunomodulatory properties of this helminth. As a result, several excretory/secretory components and purified molecules have been analysed, revealing interesting anti-inflammatory activities potentially useful as therapeutic tools. One of these molecules is A. lumbricoides cystatin, whose genomic, cellular, molecular, and immunomodulatory properties are described in this review.

BCG immunomodulation: From the 'hygiene hypothesis' to COVID-19

The century-old tuberculosis vaccine BCG has been the focus of renewed interest due to its well-documented ability to protect against various non-TB pathogens. Much of these broad spectrum protective effects are attributed to trained immunity, the epigenetic and metabolic reprogramming of innate immune cells. As BCG vaccine is safe, cheap, widely available, amendable to use as a recombinant vector, and immunogenic, it has immense potential for use as an immunotherapeutic agent for various conditions including autoimmune, allergic, neurodegenerative, and neoplastic diseases as well as a preventive measure against infectious agents. Of particular interest is the use of BCG vaccination to counteract the increasing prevalence of autoimmune and allergic conditions in industrialized countries attributable to reduced infectious burden as described by the 'hygiene hypothesis.' Furthermore, BCG vaccination has been proposed as a potential therapy to mitigate spread and disease burden of COVID-19 as a bridge to development of a specific vaccine and recombinant BCG expression vectors may prove useful for the introduction of SARS-CoV-2 antigens (rBCG-SARS-CoV-2) to induce long-term immunity. Understanding the immunomodulatory effects of BCG vaccine in these disease contexts is therefore critical. To that end, we review here BCG-induced immunomodulation focusing specifically on BCG-induced trained immunity and how it relates to the 'hygiene hypothesis' and COVID-19.

A putative lysozyme and serine carboxypeptidase from Heterorhabditis bacteriophora show differential virulence capacities in Drosophila melanogaster

Nematode virulence factors are of interest for a variety of applications including biocontrol against insect pests and the alleviation of autoimmune diseases with nematode-derived factors. In silico "omics" techniques have generated a wealth of candidate factors that may be important in the establishment of nematode infections, although the challenge of characterizing these individual factors in vivo remains. Here we provide a fundamental characterization of a putative lysozyme and serine carboxypeptidase from the host-induced transcriptome of Heterorhabditis bacteriophora. Both factors accelerated the mortality rate following Drosophila melanogaster infections with Photorhabdus luminescens, and both factors suppressed phenoloxidase activity in D. melanogaster hemolymph. Furthermore, the serine carboxypeptidase was lethal to a subpopulation of flies and suppressed the upregulation of antimicrobial peptides as well as phagocytosis. Together, our findings suggest that this serine carboxypeptidase possess both toxic and immunomodulatory properties while the lysozyme is likely to confer immunomodulatory, but not toxic effects.

Extracellular vesicles secreted by model tapeworm Hymenolepis diminuta: biogenesis, ultrastructure and protein composition

We provided the first known evidence of the presence and release of extracellular vesicles in adults of important model tapeworm Hymenolepis diminuta. Two different subtypes have been observed on the surface of the worm and among the secretory products confirmed by several microscopical methods. Proteomic analysis revealed the presence of parasite-specific proteins as well as those of the host in purified extracellular vesicles. Among the protein cargo, we identified potential drug targets, vaccine candidates and H. diminuta antigens. Finally, the protein composition further revealed proteins participating in the endosomal complex required for transport-dependent biogenesis pathway.

Beneficial effects of Strongyloides venezuelensis antigen extract in acute experimental toxoplasmosis

BACKGROUND

Toxoplasma gondii is a protozoan with worldwide distribution and triggers a strong Th1 immune response in infected susceptible hosts. On the contrary, most helminth infections are characterized by Th2 immune response and the use of helminth-derived antigens to regulate immune response in inflammatory disorders has been broadly investigated.

OBJECTIVES

The aim of this study was to investigate whether treatment with Strongyloides venezuelensis antigen extract (SvAg) would alter immune response against T gondii.

METHODS

C57BL/6 mice were orally infected with T gondii and treated with SvAg, and parasitological, histological and immunological parameters were investigated.

RESULTS

It was observed that SvAg treatment improved survival rates of T gondii-infected mice. At day 7 post-infection, the parasite load was lower in the lung and small intestine of infected SvAg-treated mice than untreated infected mice. Remarkably, SvAg-treated mice infected with T gondii presented reduced inflammatory lesions in the small intestine than infected untreated mice and decreased intestinal and systemic levels of IFN-γ, TNF-α and IL-6. In contrast, SvAg treatment increased T gondii-specific IgA serum levels in infected mice.

CONCLUSIONS

S venezuelensis antigen extract has anti-parasitic and anti-inflammatory properties during T gondii infection suggesting as a possible alternative to parasite and inflammation control.

A review on the interactions between dendritic cells, filarial parasite and parasite-derived molecules in regulating the host immune responses

Lymphatic filariasis (LF) is the second leading cause of parasitic disabilities that affects millions of people in India and several other tropical countries. The complexity of this disease is endorsed by various immunopathological consequences such as lymphangitis, lymphadenitis and elephantiasis. The immune evasion strategies that a filarial parasite usually follows are chiefly initiated with the communication between the invaded parasites and parasite-derived molecules, with the Toll-like receptors (TLRs) present on the surface of the antigen-presenting cells (APCs). Classically, the filarial parasites interact with the DCs resulting in lowering of CD4⁺ T-cell responses. These CD4⁺ T-cell responses are the key players behind the immune-mediated pathologies associated with LF. In chronic stage, the canonical pro-inflammatory immune responses are shifted towards an anti-inflammatory subtype, which is favouring the parasite survivability within the host. The central theme of this review article is to present the overall immune response elicited when an APC, particularly a DC, encounters a filarial parasite.

The Two Faces of Nematode Infection: Virulence and Immunomodulatory Molecules From Nematode Parasites of Mammals, Insects and Plants

Helminths stage a powerful infection that allows the parasite to damage host tissue through migration and feeding while simultaneously evading the host immune system. This feat is accomplished in part through the release of a diverse set of molecules that contribute to pathogenicity and immune suppression. Many of these molecules have been characterized in terms of their ability to influence the infectious capabilities of helminths across the tree of life. These include nematodes that infect insects, known as entomopathogenic nematodes (EPN) and plants with applications in agriculture and medicine. In this review we will first discuss the nematode virulence factors, which aid parasite colonization or tissue invasion, and cause many of the negative symptoms associated with infection. These include enzymes involved in detoxification, factors essential for parasite development and growth, and highly immunogenic ES proteins. We also explore how these parasites use several classes of molecules (proteins, carbohydrates, and nucleic acids) to evade the host's immune defenses. For example, helminths release immunomodulatory molecules in extracellular vesicles that may be protective in allergy and inflammatory disease. Collectively, these nematode-derived molecules allow parasites to persist for months or even years in a host, avoiding being killed or expelled by the immune system. Here, we evaluate these molecules, for their individual and combined potential as vaccine candidates, targets for anthelminthic drugs, and therapeutics for allergy and inflammatory disease. Last, we evaluate shared virulence and immunomodulatory mechanisms between mammalian and non-mammalian plant parasitic nematodes and EPNs, and discuss the utility of EPNs as a cost-effective model for studying nematode-derived molecules. Better knowledge of the virulence and immunomodulatory molecules from both entomopathogenic nematodes and soil-based helminths will allow for their use as beneficial agents in fighting disease and pests, divorced from their pathogenic consequences.

The Functional Parasitic Worm Secretome: Mapping the Place of Onchocerca volvulus Excretory Secretory Products

Nematodes constitute a very successful phylum, especially in terms of parasitism. Inside their mammalian hosts, parasitic nematodes mainly dwell in the digestive tract (geohelminths) or in the vascular system (filariae). One of their main characteristics is their long sojourn inside the body where they are accessible to the immune system. Several strategies are used by parasites in order to counteract the immune attacks. One of them is the expression of molecules interfering with the function of the immune system. Excretory-secretory products (ESPs) pertain to this category. This is, however, not their only biological function, as they seem also involved in other mechanisms such as pathogenicity or parasitic cycle (molting, for example). We will mainly focus on filariae ESPs with an emphasis on data available regarding Onchocerca volvulus, but we will also refer to a few relevant/illustrative examples related to other worm categories when necessary (geohelminth nematodes, trematodes or cestodes). We first present Onchocerca volvulus, mainly focusing on the aspects of this organism that seem relevant when it comes to ESPs: life cycle, manifestations of the sickness, immunosuppression, diagnosis and treatment. We then elaborate on the function and use of ESPs in these aspects.

Host Immunity and Inflammation to Pulmonary Helminth Infections

Helminths, including nematodes, cestodes and trematodes, are complex parasitic organisms that infect at least one billion people globally living in extreme poverty. Helminthic infections are associated with severe morbidity particularly in young children who often harbor the highest burden of disease. While each helminth species completes a distinct life cycle within the host, several helminths incite significant lung disease. This impact on the lungs occurs either directly from larval migration and host immune activation or indirectly from a systemic inflammatory immune response. The impact of helminths on the pulmonary immune response involves a sophisticated orchestration and activation of the host innate and adaptive immune cells. The consequences of activating pulmonary host immune responses are variable with several helminthic infections leading to severe, pulmonary compromise while others providing immune tolerance and protection against the development of pulmonary diseases. Further delineation of the convoluted interface between helminth infection and the pulmonary host immune responses is critical to the development of novel therapeutics that are critically needed to prevent the significant global morbidity caused by these parasites.

Old friends meet a new foe: A potential role for immune-priming parasites in mitigating COVID-19 morbidity and mortality

The novel virus, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), and the associated Coronavirus Disease 2019 (COVID-19) represent a pathogen to which human beings have limited to no evolved immune response. The most severe symptoms are associated with overactive inflammatory immune responses, leading to a cytokine storm, tissue damage, and death, if not balanced and controlled. Hypotheses within Evolutionary Medicine, including the Hygiene/Old Friends Hypothesis, provide an important lens through which to understand and possibly control this overactive immune response. In this article, we explore the role that infection with soil-transmitted helminths (STHs; i.e. intestinal parasitic worms) may play in dampening SARS-CoV-2 symptoms and mitigating the worst COVID-19 outcomes. Specifically, STHs stimulate the immunosuppressive and regulatory T-helper 2 (TH2) branch of the immune system, which decreases ACE2-receptor expression (i.e. receptors SARS-CoV-2 uses to infect host cells), balances the inflammatory TH1/TH17 branches of the immune system triggered by SARS-CoV-2 infection, and reduces inflammation through the release of anti-inflammatory/regulatory cytokines. Because STHs are common and affect the most vulnerable and marginalized members of society, it is especially important to consider how these parasites may impact COVID-19 outcomes. Areas experiencing endemic STH infections are often characterized by a lack of preventative infrastructure and medical care, which may further exacerbate risk of SARS-CoV-2 infection and COVID-19 development. For this reason, we also explore biocultural factors that contribute to disease outcomes for both SARS-CoV-2 and STH infections. Biocultural and Evolutionary Medicine perspectives on COVID-19 are crucial for understanding the global impact of the disease. Lay summary: An evolutionary perspective is required to understand the global impact and various presentations of COVID-19. We consider how coinfection with soil-transmitted helminths (common parasitic worms that coevolved with humans) may suppress inflammatory immune activity, thereby potentially reducing COVID-19 disease severity. Structural and lifestyle factors shaping coinfection patterns are also discussed.

Helminth Induced Immunoregulation and Novel Therapeutic Avenue of Allergy

Allergic diseases are increasing at an alarming rate worldwide, particularly in developed countries. In contrast, there is a decrease in the prevalence of helminthic infections and other neglected diseases. The hygiene hypothesis elaborates parasitic infection, and allergy-associated diseases have an inverse relationship. Acute helminthic infection and allergic reaction stimulate Type 2 helper cells (Th2) immune response with up-regulation of cytokines IL-4-, IL-5-, and IL-13-mediated IgE and mast cell production, as well as eosinophilia. However, people who chronically suffer from helminthic infections are demarcated through polarized Th2 resulting in alternative macrophage activation and T regulatory response. This regulatory system reduces allergy incidence in individuals that are chronically diseased through helminth. As a result, the excretory-secretory (ES) substance derived from parasites and extracellular vesicular components can be used as a novel therapeutic modality of allergy. Therefore, the aim of this review meticulously explored the link between helminth infection and allergy, and utilization of the helminth secretome for therapeutic immunomodulation.

Proteomic analysis of plasma exosomes from Cystic Echinococcosis patients provides in vivo support for distinct immune response profiles in active vs inactive infection and suggests potential biomarkers

The reference diagnostic method of human abdominal Cystic Echinococcosis (CE) is imaging, particularly ultrasound, supported by serology when imaging is inconclusive. However, current diagnostic tools are neither optimal nor widely available. The availability of a test detecting circulating biomarkers would considerably improve CE diagnosis and cyst staging (active vs inactive), as well as treatments and follow-up of patients. Exosomes are extracellular vesicles involved in intercellular communication, including immune system responses, and are a recognized source of biomarkers. With the aim of identifying potential biomarkers, plasma pools from patients infected by active or inactive CE, as well as from control subjects, were processed to isolate exosomes for proteomic label-free quantitative analysis. Results were statistically processed and subjected to bioinformatics analysis to define distinct features associated with parasite viability. First, a few parasite proteins were identified that were specifically associated with either active or inactive CE, which represent potential biomarkers to be validated in further studies. Second, numerous identified proteins of human origin were common to active and inactive CE, confirming an overlap of several immune response pathways. However, a subset of human proteins specific to either active or inactive CE, and central in the respective protein-protein interaction networks, were identified. These include the Src family kinases Src and Lyn, and the immune-suppressive cytokine TGF-β in active CE, and Cdc42 in inactive CE. The Src and Lyn Kinases were confirmed as potential markers of active CE in totally independent plasma pools. In addition, insights were obtained on immune response profiles: largely consistent with previous evidence, our observations hint to a Th1/Th2/regulatory immune environment in patients with active CE and a Th1/inflammatory environment with a component of the wound healing response in the presence of inactive CE. Of note, our results were obtained for the first time from the analysis of samples obtained in vivo from a well-characterized, large cohort of human subjects.

Schistosoma antigens as activators of inflammasome pathway: from an unexpected stimulus to an intriguing role

Parasites of the genus Schistosoma are organisms capable of living for decades within the definitive host. They interfere with the immune response by interacting with host's receptors. In this review, we discuss from the first reports to the most recent discoveries regarding the ability of Schistosoma antigens in triggering intracellular receptors and inducing inflammasome activation.

Extracellular vesicles: new targets for vaccines against helminth parasites

The hunt for effective vaccines against the major helminth diseases of humans has yet to bear fruit despite much effort over several decades. No individual parasite antigen has proved to elicit full protective immunity, suggesting that combinatorial strategies may be required. Recently it has been discovered that extracellular vesicles released by parasitic helminths contain multiple potential immune modulators, which could together be targeted by a future vaccine. Increasing knowledge of helminth extracellular vesicle components, both enclosed by and exposed on the membrane, will open up a new field of targets for an effective vaccine. This review discusses the interactions between helminth extracellular vesicles and the immune system discovered thus far, and the advantages of targeting these lipid-bound packages with a vaccine. In addition, we also comment upon specific antigens that may be the best targets for an anti-helminth vaccine. In the future, extensive knowledge of the parasites' full arsenal in controlling their host may finally provide us with the ideal target for a fully effective vaccine.

Fluorescent Labeling of Helminth Extracellular Vesicles Using an In Vivo Whole Organism Approach

In the last two decades, extracellular vesicles (EVs) from the three domains of life, Archaea, Bacteria and Eukaryotes, have gained increasing scientific attention. As such, the role of EVs in host-pathogen communication and immune modulation are being intensely investigated. Pivotal to EV research is the determination of how and where EVs are taken up by recipient cells and organs in vivo, which requires suitable tracking strategies including labelling. Labelling of EVs is often performed post-isolation which increases risks of non-specific labelling and the introduction of labelling artefacts. Here we exploited the inability of helminths to de novo synthesise fatty acids to enable labelling of EVs by whole organism uptake of fluorescent lipid analogues and the subsequent incorporation in EVs. We showed uptake of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) (DOPE-Rho) in Anisakis spp. and Trichuris suis larvae. EVs isolated from the supernatant of Anisakis spp. labelled with DOPE-Rho were characterised to assess the effects of labelling on size, structure and fluorescence of EVs. Fluorescent EVs were successfully taken up by the human macrophage cell line THP-1. This study, therefore, presents a novel staining method that can be utilized by the EV field in parasitology and potentially across multiple species.

The protein and microRNA cargo of extracellular vesicles from parasitic helminths - current status and research priorities

Helminth parasites have a remarkable ability to persist within their mammalian hosts, which is largely due to their secretion of molecules with immunomodulatory properties. Although the soluble components of helminth secretions have been extensively studied, the discovery that helminths release extracellular vesicles (EVs) has added further complexity to the host-parasite interaction. Whilst several studies have begun to characterise the molecules carried by helminth EVs, work aimed at investigating their biological functions has been hindered by a lack of helminth-specific EV markers. To begin to address this, we summarised helminth EV literature to date. With a focus on the protein and microRNA (miRNA) cargo, we aimed to detect similarities and differences across those major groups of helminths for which data are available; namely nematodes, trematodes and cestodes. Pfam analysis revealed that although there is no universal EV marker for all helminth species, the EF-hand protein family was present in all EV datasets from cestodes and trematodes, and could serve as a platyhelminth EV biomarker. In contrast, M13 metallopeptidases and actin may have potential as markers for nematode EVs. As with proteins, many miRNA families appeared to be species-, stage-, or dataset-specific. Two miRNA families were common to nematode EVs (mir-10 and let-7); the miRNA cargo of EVs secreted by clade I species appeared somewhat different from species from other clades. Five miRNA families (mir-71, mir-10, mir-190, let-7 and mir-2) were shared by all trematode species examined. Our analysis has identified novel markers that may be used in studies aimed at characterising helminth EVs and interrogating their function at the host-parasite interface. In addition, we discuss the heterogeneity of methods used for helminth EV isolation and emphasise the need for a standardised approach in reporting on helminth EV data.

Diversity of extracellular vesicles from different developmental stages of Fasciola hepatica

The secretion of extracellular vesicles (EVs) in Fasciola hepatica adult worms was described by our group in 2012. Since then, EVs have been found in other helminths, thus providing a new paradigm for the complete understanding of host-parasite communication. However, information was lacking regarding the possible existence and role of EVs from other developmental stages of the parasite. In this study, we confirm the secretion of EVs by F. hepatica eggs and juvenile forms. EVs were isolated by size exclusion chromatography and characterised by nanoparticle tracking analysis and electron microscopy. We observed a large diversity in the morphologies of these EVs, suggesting specific functions for different subpopulations, as has been proposed in other model systems. The identification of these populations of morphologically diverse EVs will facilitate future studies aimed at biochemically characterising the different classes of these vesicles as a first step in deciphering their role in host-parasite communication.

Behind Enemy Lines: Immunomodulatory Armamentarium of the Schistosome Parasite

The deeply rooted, intricate relationship between the Schistosoma parasite and the human host has enabled the parasite to successfully survive within the host and surreptitiously evade the host's immune attacks. The parasite has developed a variety of strategies in its immunomodulatory armamentarium to promote infection without getting harmed or killed in the battlefield of immune responses. These include the production of immunomodulatory molecules, alteration of membranes, and the promotion of granuloma formation. Schistosomiasis thus serves as a paradigm for understanding the Th2 immune responses seen in various helminthiases. This review therefore aims to summarize the immunomodulatory mechanisms of the schistosome parasites to survive inside the host. Understanding these immunomodulatory strategies not only provides information on parasite-host interactions, but also forms the basis in the development of novel drugs and vaccines against the schistosome infection, as well as various types of autoimmune and inflammatory conditions.

Unique glycan and lipid composition of helminth-derived extracellular vesicles may reveal novel roles in host-parasite interactions

Although the study of helminth-derived extracellular vesicles (EVs) is in its infancy, proteomic studies of EVs from representatives of nematodes, cestodes and trematodes have identified homologs of mammalian EV proteins including components of the endosomal sorting complexes required for transport and heat-shock proteins, suggesting conservation of pathways of EV biogenesis and cargo loading between helminths and their hosts. However, parasitic helminth biology is unique and this is likely reflected in helminth EV composition and biological activity. This opinion article highlights two exceptional studies that identified EVs released by Heligmosomoides polygyrus and Fasciola hepatica which display differential lipid and glycan composition, respectively, when compared with EVs derived from mammalian cells. Furthermore, we discuss the potential implications of helminth EV lipid and glycan composition upon helminth infection and host pathology. Future studies, focusing on the unique composition and functional properties of helminth EVs, may prove crucial to the understanding of host-parasite communication.

Isolation and Functions of Extracellular Vesicles Derived from Parasites: The Promise of a New Era in Immunotherapy, Vaccination, and Diagnosis

Experimental and epidemiological evidence shows that parasites, particularly helminths, play a central role in balancing the host immunity. It was demonstrated that parasites can modulate immune responses via their excretory/secretory (ES) and some specific proteins. Extracellular vesicles (EVs) are nano-scale particles that are released from eukaryotic and prokaryotic cells. EVs in parasitological studies have been mostly employed for immunotherapy of autoimmune diseases, vaccination, and diagnosis. EVs can carry virulence factors and play a central role in the development of parasites in host cells. These molecules can manipulate the immune responses through transcriptional changes. Moreover, EVs derived from helminths modulate the immune system via provoking anti-inflammatory cytokines. On the other hand, EVs from parasite protozoa can induce efficient immunity, that makes them useful for probable next-generation vaccines. In addition, it seems that EVs from parasites may provide new diagnostic approaches for parasitic infections. In the current study, we reviewed isolation methods, functions, and applications of parasite's EVs in immunotherapy, vaccination, and diagnosis.

Training the Fetal Immune System Through Maternal Inflammation-A Layered Hygiene Hypothesis

Over the last century, the alarming surge in allergy and autoimmune disease has led to the hypothesis that decreasing exposure to microbes, which has accompanied industrialization and modern life in the Western world, has fundamentally altered the immune response. In its current iteration, the “hygiene hypothesis” suggests that reduced microbial exposures during early life restricts the production and differentiation of immune cells suited for immune regulation. Although it is now well-appreciated that the increase in hypersensitivity disorders represents a “perfect storm” of many contributing factors, we argue here that two important considerations have rarely been explored. First, the window of microbial exposure that impacts immune development is not limited to early childhood, but likely extends into the womb. Second, restricted microbial interactions by an expectant mother will bias the fetal immune system toward hypersensitivity. Here, we extend this discussion to hypothesize that the cell types sensing microbial exposures include fetal hematopoietic stem cells, which drive long-lasting changes to immunity.

Cystatin from Filarial Parasites Suppress the Clinical Symptoms and Pathology of Experimentally Induced Colitis in Mice by Inducing T-Regulatory Cells, B1-Cells, and Alternatively Activated Macrophages

Potential alternative therapeutic strategies for immune-mediated disorders are being increasingly recognized and are studied extensively. We previously reported the therapeutic potential of Brugia malayi derived recombinant cystatin (rBmaCys) in attenuating clinical symptoms of experimental colitis. The aim of this study was to elucidate the mechanisms involved in the rBmaCys-induced suppression of inflammation in the colon. Our results show that, the frequency of CD4⁺CD25⁺FoxP3⁺ regulatory T-cells was elevated in the colon and mesenteric lymph nodes. Similarly, the peritoneal macrophages recovered from the rBmaCys-treated colitis mice were alternatively activated and displayed reduced expression of TNF-α and IL-6. Another finding was significant increases in IgM⁺B1a-cells in the peritoneal cavity of mice following rBmaCys-treatment. These findings suggested that the regulatory cell network promoted by the rBmaCys in the colon and associated lymphoid tissues is important for its anti-inflammatory activity in the dextran sulfate sodium (DSS)-induced colitis mice.

Trichinella spiralis muscle larvae release extracellular vesicles with immunomodulatory properties

AIMS

Extracellular vesicles (EVs) represent a newly discovered but universal communication tool between cells or organisms. However, few data exist on nematode EVs and none for Trichinella spiralis. Here, we aimed to investigate whether T spiralis muscle larvae produce EVs, whether they carry immunomodulatory proteins and whether they have a role in immunomodulation as a component of excretory-secretory muscle larvae products (ES L1).

METHODS AND RESULTS

EVs were enriched from conditioned medium of T spiralis muscle larvae. Transmission electron microscopy images showed T spiralis EVs to be 30-80 nm in size, and Western blot confirmed the presence of two out of three glycoproteins with the immunodominant epitope characteristic for muscle larvae of the genus Trichinella. Using a peripheral blood mononuclear cell (PBMC) stimulation assay, it was shown that these EVs elevated production of IL10 and IL6.

CONCLUSION

T spiralis muscle larvae produce EVs. Those EVs carry immunomodulatory proteins and have the capacity independently to induce regulatory responses in the same way as the T spiralis excretory-secretory muscle larvae products from which they were isolated.

A long-distance relationship: the commensal gut microbiota and systemic viruses

Recent advances defining the role of the commensal gut microbiota in the development, education, induction, function, and maintenance of the mammalian immune system inform our understanding of how immune responses govern the outcome of systemic virus infection. While characterization of the impact of the local oral, respiratory, dermal and genitourinary microbiota on host immune responses and systemic virus infection is in its infancy, the gut microbiota interacts with host immunity systemically and at distal non-gastrointestinal tract sites to modulate the pathogenesis of systemic viruses. Gut microbes, microbe-associated molecular patterns, and microbe-derived metabolites engage receptors expressed on the cell surface, in the endosome, or in the cytoplasm to orchestrate optimal innate and adaptive immune responses important for controlling systemic virus infection.