Vaccination-induced IgG response to Galalpha1-3GalNAc glycan epitopes in lambs protected against Haemonchus contortus challenge infection

Serum antibody screening using glycan arrays

Humans and other animals produce a diverse collection of antibodies, many of which bind to carbohydrate chains, referred to as glycans. These anti-glycan antibodies are a critical part of our immune systems' defenses. Whether induced by vaccination or natural exposure to a pathogen, anti-glycan antibodies can provide protection against infections and cancers. Alternatively, when an immune response goes awry, antibodies that recognize self-glycans can mediate autoimmune diseases. In any case, serum anti-glycan antibodies provide a rich source of information about a patient's overall health, vaccination history, and disease status. Glycan microarrays provide a high-throughput platform to rapidly interrogate serum anti-glycan antibodies and identify new biomarkers for a variety of conditions. In addition, glycan microarrays enable detailed analysis of the immune system's response to vaccines and other treatments. Herein we review applications of glycan microarray technology for serum anti-glycan antibody profiling.

A reverse vaccinology approach identifies putative vaccination targets in the zoonotic nematode Ascaris

Ascariasis is the most prevalent helminthic disease affecting both humans and pigs and is caused by the roundworms Ascaris lumbricoides and Ascaris suum. While preventive chemotherapy continues to be the most common control method, recent reports of anthelminthic resistance highlight the need for development of a vaccine against ascariasis. The aim of this study was to use a reverse vaccinology approach to identify potential vaccine candidates for Ascaris. Three Ascaris proteomes predicted from whole-genome sequences were analyzed. Candidate proteins were identified using open-access bioinformatic tools (e.g., Vacceed, VaxiJen, Bepipred 2.0) which test for different characteristics such as sub-cellular location, T-cell and B-cell molecular binding, antigenicity, allergenicity and phylogenetic relationship with other nematode proteins. From over 100,000 protein sequences analyzed, four transmembrane proteins were predicted to be non-allergen antigens and potential vaccine candidates. The four proteins are a Piezo protein, two voltage-dependent calcium channels and a protocadherin-like protein, are all expressed in either the muscle or ovaries of both Ascaris species, and all contained high affinity epitopes for T-cells and B-cells. The use of a reverse vaccinology approach allowed the prediction of four new potential vaccination targets against ascariasis in humans and pigs. These targets can now be further tested in in vitro and in vivo assays to prove efficacy in both pigs and humans.

Glyco-Engineering Plants to Produce Helminth Glycoproteins as Prospective Biopharmaceuticals: Recent Advances, Challenges and Future Prospects

Glycoproteins are the dominant category among approved biopharmaceuticals, indicating their importance as therapeutic proteins. Glycoproteins are decorated with carbohydrate structures (or glycans) in a process called glycosylation. Glycosylation is a post-translational modification that is present in all kingdoms of life, albeit with differences in core modifications, terminal glycan structures, and incorporation of different sugar residues. Glycans play pivotal roles in many biological processes and can impact the efficacy of therapeutic glycoproteins. The majority of biopharmaceuticals are based on human glycoproteins, but non-human glycoproteins, originating from for instance parasitic worms (helminths), form an untapped pool of potential therapeutics for immune-related diseases and vaccine candidates. The production of sufficient quantities of correctly glycosylated putative therapeutic helminth proteins is often challenging and requires extensive engineering of the glycosylation pathway. Therefore, a flexible glycoprotein production system is required that allows straightforward introduction of heterologous glycosylation machinery composed of glycosyltransferases and glycosidases to obtain desired glycan structures. The glycome of plants creates an ideal starting point for N- and O-glyco-engineering of helminth glycans. Plants are also tolerant toward the introduction of heterologous glycosylation enzymes as well as the obtained glycans. Thus, a potent production platform emerges that enables the production of recombinant helminth proteins with unusual glycans. In this review, we discuss recent advances in plant glyco-engineering of potentially therapeutic helminth glycoproteins, challenges and their future prospects.

Helminth Glycans at the Host-Parasite Interface and Their Potential for Developing Novel Therapeutics

Helminths are parasitic worms that have successfully co-evolved with their host immune system to sustain long-term infections. Their successful parasitism is mainly facilitated by modulation of the host immune system via the release of excretory-secretory (ES) products covered with glycan motifs such as Lewis X, fucosylated LDN, phosphorylcholine and tyvelose. Evidence is accumulating that these glycans play key roles in different aspects of helminth infection including interactions with immune cells for recognition and evasion of host defences. Moreover, antigenic properties of glycans can be exploited for improving the efficacy of anti-helminthic vaccines. Here, we illustrate that glycans have the potential to open new avenues for the development of novel biopharmaceuticals and effective vaccines based on helminth glycoproteins.

The excretory-secretory antigen HcADRM1 to generate protective immunity against Haemonchus contortus

The prevention, treatment and control of Haemonchus contortus have been increasingly problematic due to its widespread occurrence and anthelmintic resistance. There are very few descriptions of recombinant antigens being protective for H. contortus, despite the success of various native antigen preparations, including Barbervax. We recently identified an H. contortus excretory–secretory antigen, H. contortus adhesion-regulating molecule 1 (HcADRM1), that served as an immunomodulator to impair host T-cell functions. Given the prophylactic potential of HcADRM1 protein as a vaccine candidate, we hereby assessed the efficacies of HcADRM1 preparations against H. contortus infection. Parasitological and immunological parameters were evaluated throughout all time points of the trials, including fecal egg counts (FEC), abomasal worm burdens, complete blood counts, cytokine production profiles and antibody responses. Active vaccination with recombinant HcADRM1 (rHcADRM1) protein induced protective immunity in inoculated goats, resulting in reductions of 48.9 and 58.6% in cumulative FEC and worm burdens. Simultaneously, passive administration of anti-HcADRM1 antibodies generated encouraging levels of protection with 46.7 and 56.2% reductions in cumulative FEC and worm burdens in challenged goats. In addition, HcADRM1 preparations-immunized goats showed significant differences in mucosal and serum antigen-specific immunoglobulin G (IgG) levels, total mucosal IgA levels, haemoglobin values and circulating interferon-γ, interleukin (IL)-4 and IL-17A production compared to control goats in both trials. The preliminary data of these laboratory trials validated the immunoprophylactic effects of rHcADRM1 protein. It can be pursued as a potential vaccine antigen to develop an effective recombinant subunit vaccine against H. contortus under field conditions.

Ascaris lumbricoides and ticks associated with sensitization to galactose α1,3-galactose and elicitation of the alpha-gal syndrome

BACKGROUND

IgE to galactose alpha-1,3 galactose (alpha-gal) causes alpha-gal syndrome (delayed anaphylaxis after ingestion of mammalian meat). Development of sensitization has been attributed to tick bites; however, the possible role of other parasites has not been well studied.

OBJECTIVE

Our aims were to assess the presence, relative abundances, and site of localization of alpha-gal-containing proteins in common ectoparasites and endoparasites endemic in an area of high prevalence of alpha-gal syndrome, as well as to investigate the ability of ascaris antigens to elicit a reaction in a humanized rat basophil in vitro sensitization model.

METHODS

Levels of total IgE, Ascaris-specific IgE, and alpha-gal IgE were measured in sera from patients with challenge-proven alpha-gal syndrome and from controls without allergy. The presence, concentration, and localization of alpha-gal in parasites were assessed by ELISA, Western blotting, and immunohistochemistry. The ability of Ascaris lumbricoides antigen to elicit IgE-dependent reactivity was demonstrated by using the RS-ATL8 basophil reporter system.

RESULTS

Alpha-gal IgE level correlated with A lumbricoides-specific IgE level. Alpha-gal protein at 70 to 130 kDa was detected in A lumbricoides at concentrations higher than those found in Rhipicephalus evertsi and Amblyomma hebraeum ticks. Immunohistochemistry was used to localize alpha-gal in tick salivary acini and the helminth gut. Non-alpha-gal-containing A lumbricoides antigens activated RS-ATL8 basophils primed with serum from subjects with alpha-gal syndrome.

CONCLUSION

We demonstrated the presence, relative abundances, and site of localization of alpha-gal-containing proteins in parasites. The activation of RS-ATL8 IgE reporter cells primed with serum from subjects with alpha-gal syndrome on exposure to non-alpha-gal-containing A lumbricoides proteins indicates a possible role of exposure to A lumbricoides in alpha-gal sensitization and clinical reactivity.

Tick-human interactions: from allergic klendusity to the α-Gal syndrome

Ticks and the pathogens they transmit, including bacteria, viruses, protozoa, and helminths, constitute a growing burden for human and animal health worldwide. The ability of some animal species to acquire resistance to blood-feeding by ticks after a single or repeated infestation is known as acquired tick resistance (ATR). This resistance has been associated to tick-specific IgE response, the generation of skin-resident memory CD4+ T cells, basophil recruitment, histamine release, and epidermal hyperplasia. ATR has also been associated with protection to tick-borne tularemia through allergic klendusity, a disease-escaping ability produced by the development of hypersensitivity to an allergen. In addition to pathogen transmission, tick infestation in humans is associated with the α-Gal syndrome (AGS), a type of allergy characterized by an IgE response against the carbohydrate Galα1-3Gal (α-Gal). This glycan is present in tick salivary proteins and on the surface of tick-borne pathogens such as Borrelia burgdorferi and Anaplasma phagocytophilum, the causative agents of Lyme disease and granulocytic anaplasmosis. Most α-Gal-sensitized individuals develop IgE specific against this glycan, but only a small fraction develop the AGS. This review summarizes our current understanding of ATR and its impact on the continuum α-Gal sensitization, allergy, and the AGS. We propose that the α-Gal-specific IgE response in humans is an evolutionary adaptation associated with ATR and allergic klendusity with the trade-off of developing AGS.

N-glycome and N-glycoproteome of a hematophagous parasitic nematode Haemonchus

N-glycosylation is a physiologically vital post-translational modification of proteins in eukaryotic organisms. Initial work on Haemonchus contortus - a blood-sucking nematode of ruminants with a broad geographical distribution - has shown that this parasite harbors N-glycans with exclusive chitobiose modifications. Besides, several immunogenic proteins (e.g., amino- and metallo-peptidases) are known to be N-glycosylated in adult worms. However, an informative atlas of N-glycosylation in H. contortus is not yet available. Herein, we report 291 N-glycosylated proteins with a total of 425 modification sites in the parasite. Among them, many peptidase families (e.g., peptidase C1 and M1) including potential vaccine targets were enriched. Notably, the glycan-rich conjugates are distributed primarily in the intestine and gonads of adult worms, and consequently hidden from the host's immune system. Collectively, these data provide a comprehensive atlas of N-glycosylation in a prevalent parasitic nematode while underlining its significance for infection, immunity and prevention.

Sweet systems: technologies for glycomic analysis and their integration into systems biology

Found in virtually every organism, glycans are essential molecules that play important roles in almost every aspect of biology. The composition of glycome, the repertoire of glycans in an organism or a biological sample, is often found altered in many diseases, including cancer, infectious diseases, metabolic and developmental disorders. Understanding how glycosylation and glycomic changes enriches our knowledge of the mechanisms of disease progression and sheds light on the development of novel therapeutics. However, the inherent diversity of glycan structures imposes challenges on the experimental characterization of glycomes. Advances in high-throughput glycomic technologies enable glycomic analysis in a rapid and comprehensive manner. In this review, we discuss the analytical methods currently used in high-throughput glycomics, including mass spectrometry, liquid chromatography and lectin microarray. Concomitant with the technical advances is the integration of glycomics into systems biology in the recent years. Herein we elaborate on some representative works from this recent trend to underline the important role of glycomics in such integrated approaches to disease.

Protection studies of an excretory-secretory protein HcABHD against Haemonchus contortus infection

Unlike the successful immunization of native H. contortus antigens that contributed to the realization of the first commercial vaccine Barbervax, not many studies revealed the encouraging protective efficacies of recombinant H. contortus antigens in laboratory trials or under field conditions. In our preliminary study, H. contortus α/β-hydrolase domain protein (HcABHD) was demonstrated to be an immunomodulatory excretory-secretory (ES) protein that interacts with goat T cells. We herein evaluated the protective capacities of two HcABHD preparations, recombinant HcABHD (rHcABHD) antigen and anti-rHcABHD IgG, against H. contortus challenge via active and passive immunization trials, respectively. Parasitological parameter, antibody responses, hematological pathology and cytokine profiling in unchallenged and challenged goats were monitored and determined throughout both trials. Subcutaneous administration of rHcABHD with Freund adjuvants elicited protective immune responses in challenged goats, diminishing cumulative fecal egg counts (FEC) and total worm burden by 54.0% and 74.2%, respectively, whereas passive immunization with anti-rHcABHD IgG conferred substantial protection to challenged goats by generating a 51.5% reduction of cumulative FEC and a 73.8% reduction of total worm burden. Additionally, comparable changes of mucosal IgA levels, circulating IgG levels, hemoglobin levels, and serum interleukin (IL)-4 and IL-17A levels were observed in rHcABHD protein/anti-rHcABHD IgG immunized goats in both trials. Taken together, the recombinant version of HcABHD might have further application under field conditions in protecting goats against H. contortus infection, and the integrated immunological pipeline of ES antigen identification, screening and characterization may provide new clues for further development of recombinant subunit vaccines to control H. contortus.

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.

Infection with Toxocara canis Inhibits the Production of IgE Antibodies to α-Gal in Humans: Towards a Conceptual Framework of the Hygiene Hypothesis?

α-Gal syndrome (AGS) is a type of anaphylactic reaction to mammalian meat characterized by an immunoglobulin (Ig)E immune response to the oligosaccharide α-Gal (Galα1-3Galβ1-4GlcNAc-R). Tick bites seems to be a prerequisite for the onset of the allergic disease in humans, but the implication of non-tick parasites in α-Gal sensitization has also been deliberated. In the present study, we therefore evaluated the capacity of helminths (Toxocara canis, Ascaris suum, Schistosoma mansoni), protozoa (Toxoplasma gondii), and parasitic fungi (Aspergillus fumigatus) to induce an immune response to α-Gal. For this, different developmental stages of the infectious agents were tested for the presence of α-Gal. Next, the potential correlation between immune responses to α-Gal and the parasite infections was investigated by testing sera collected from patients with AGS and those infected with the parasites. Our results showed that S. mansoni and A. fumigatus produce the terminal α-Gal moieties, but they were not able to induce the production of specific antibodies. By contrast, T. canis, A. suum and T. gondii lack the α-Gal epitope. Furthermore, the patients with T. canis infection had significantly decreased anti-α-Gal IgE levels when compared to the healthy controls, suggesting the potential role of this nematode parasite in suppressing the allergic response to the glycan molecule. This rather intriguing observation is discussed in the context of the 'hygiene hypothesis'. Taken together, our study provides new insights into the relationships between immune responses to α-Gal and parasitic infections. However, further investigations should be undertaken to identify T. canis components with potent immunomodulatory properties and to assess their potential to be used in immunotherapy and control of AGS.

Tick Bites Induce Anti-α-Gal Antibodies in Dogs

Due to the functional inactivation of the gene encoding for the enzyme that is involved in the oligosaccharide galactose-α-1,3-galactose (α-Gal) synthesis, humans and Old-World primates are able to produce a large amount of antibodies against the glycan epitope. Apart from being involved in the hyperacute organ rejection in humans, anti-α-Gal antibodies have shown a protective effect against some pathogenic agents and an implication in the recently recognized tick-induced mammalian meat allergy. Conversely, non-primate mammals, including dogs, have the ability to synthetize α-Gal and, thus, their immune system is not expected to naturally generate the antibodies toward this self-antigen molecule. However, in the current study, we detected specific IgG, IgM, and IgE antibodies to α-Gal in sera of clinically healthy dogs by an indirect enzyme-linked immunosorbent assay (ELISA) for the first time. Furthermore, in a tick infestation experiment, we showed that bites of Ixodes ricinus induce the immune response to α-Gal in dogs and that the resulting antibodies (IgM) might be protective against Anaplasma phagocytophilum. These findings may help lead to a better understanding of the underlying mechanisms involved in mammalian meat allergy and tick-host-pathogen interactions, but they also open up the question about the possibility that dogs could develop an allergy to mammalian meat after tick bites, similar to that in humans.

Glycans in the roles of parasitological diagnosis and host-parasite interplay

The investigation of the glycan repertoire of several organisms has revealed a wide variation in terms of structures and abundance of glycan moieties. Among the parasites, it is possible to observe different sets of glycoconjugates across taxa and developmental stages within a species. The presence of distinct glycoconjugates throughout the life cycle of a parasite could relate to the ability of that organism to adapt and survive in different hosts and environments. Carbohydrates on the surface, and in excretory-secretory products of parasites, play essential roles in host-parasite interactions. Carbohydrate portions of complex molecules of parasites stimulate and modulate host immune responses, mainly through interactions with specific receptors on the surface of dendritic cells, leading to the generation of a pattern of response that may benefit parasite survival. Available data reviewed here also show the frequent aspect of parasite immunomodulation of mammalian responses through specific glycan interactions, which ultimately makes these molecules promising in the fields of diagnostics and vaccinology.

Comparisons of N-glycans across invertebrate phyla

Many invertebrates are either parasites themselves or vectors involved in parasite transmission; thereby, the interactions of parasites with final or intermediate hosts are often mediated by glycans. Therefore, it is of interest to compare the glycan structures or motifs present across invertebrate species. While a typical vertebrate modification such as sialic acid is rare in lower animals, antennal and core modifications of N-glycans are highly varied and range from core fucose, galactosylated fucose, fucosylated galactose, methyl groups, glucuronic acid and sulphate through to addition of zwitterionic moieties (phosphorylcholine, phosphoethanolamine and aminoethylphosphonate). Only in some cases are the enzymatic bases and the biological function of these modifications known. We are indeed still in the phase of discovering invertebrate glycomes primarily using mass spectrometry, but molecular biology and microarraying techniques are complementary to the determination of novel glycan structures and their functions.

Photorhabdus luminescens lectin A (PllA): A new probe for detecting α-galactoside-terminating glycoconjugates

Lectins play important roles in infections by pathogenic bacteria, for example, in host colonization, persistence, and biofilm formation. The Gram-negative entomopathogenic bacterium Photorhabdus luminescens symbiotically lives in insect-infecting Heterorhabditis nematodes and kills the insect host upon invasion by the nematode. The P. luminescens genome harbors the gene plu2096, coding for a novel lectin that we named PllA. We analyzed the binding properties of purified PllA with a glycan array and a binding assay in solution. Both assays revealed a strict specificity of PllA for α-galactoside-terminating glycoconjugates. The crystal structures of apo PllA and complexes with three different ligands revealed the molecular basis for the strict specificity of this lectin. Furthermore, we found that a 90° twist in subunit orientation leads to a peculiar quaternary structure compared with that of its ortholog LecA from Pseudomonas aeruginosa We also investigated the utility of PllA as a probe for detecting α-galactosides. The α-Gal epitope is present on wild-type pig cells and is the main reason for hyperacute organ rejection in pig to primate xenotransplantation. We noted that PllA specifically recognizes this epitope on the glycan array and demonstrated that PllA can be used as a fluorescent probe to detect this epitope on primary porcine cells in vitro In summary, our biochemical and structural analyses of the P. luminescens lectin PllA have disclosed the structural basis for PllA's high specificity for α-galactoside-containing ligands, and we show that PllA can be used to visualize the α-Gal epitope on porcine tissues.

Helminth glycomics - glycan repertoires and host-parasite interactions

Glycoproteins and glycolipids of parasitic helminths play important roles in biology and host-parasite interaction. This review discusses recent helminth glycomics studies that have been expanding our insights into the glycan repertoire of helminths. Structural data are integrated with biological and immunological observations to highlight how glycomics advances our understanding of the critical roles that glycans and glycan motifs play in helminth infection biology. Prospects and challenges in helminth glycomics and glycobiology are discussed.

Immunity to Haemonchus contortus and Vaccine Development

Sheep are capable of developing protective immunity to Haemonchus contortus through repeated exposure to this parasite, although this immune protection is the result of a complex interaction among age, gender, physiological status, pregnancy, lactation, nutrition and innate and adaptive immunity in the host animal. There are multiple effectors of the protective immune response, which differ depending on the developmental stage of the parasite being targeted, and our understanding of the effector mechanisms has developed considerably in the 2000s. The rational design of vaccines based on 'natural' or 'exposed' antigens depends on an understanding of this exposure-induced immunity. However, the most effective current vaccines rely on protection via the induction of high circulating antibody levels to 'hidden' gut antigens of H. contortus. The success of this latter strategy has resulted in the launch of a vaccine, which is based on extracts of the parasite's gut, to aid in the control of Haemonchus in Australia. The development of recombinant subunit vaccines based on the components of the successful native vaccine has not yet been achieved and most of the recent successes with recombinant subunit vaccines have focussed on antigens unrelated to the gut antigens. The future integration of an understanding of the immunobiology of this parasite with advances in antigen identification, expression (or synthesis) and presentation is likely to be pivotal to the further development of these recombinant subunit vaccines. Recent progress in each of the components underpinning this integrated approach is summarized in this review.

Novel O-linked methylated glycan antigens decorate secreted immunodominant glycoproteins from the intestinal nematode Heligmosomoides polygyrus

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Heligmosomoides polygyrus excretory–secretory (ES) proteins carry diverse N- and O-glycans, and many are O-methylated.

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A methylhexose containing O-glycan of abundant ES glycoproteins is immunodominant.

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This dominant glycan is not the immunomodulatory heat-stable ES component.

Glycan molecules from helminth parasites have been associated with diverse biological functions ranging from interactions with neighbouring host cell populations to down-modulation of specific host immunity. Glycoproteins secreted by the intestinal nematode Heligmosomoides polygyrus are of particular interest as the excretory–secretory products (termed HES) of this parasite contain both heat-labile and heat-stable components with immunomodulatory effects. We used MALDI-TOF-MS and LC–MS/MS to analyse the repertoire of N- and O-linked glycans released from Heligmosomoides polygyrus excretory–secretory products by PNGase A and F, β-elimination and hydrazinolysis revealing a broad range of structures including novel methylhexose- and methylfucose-containing glycans. Monoclonal antibodies to two immunodominant glycans of H. polygyrus, previously designated Glycans A and B, were found to react by glycan array analysis to a methyl-hexose-rich fraction and to a sulphated LacDiNAc (LDN; GalNAcβ1–4GlcNAc) structure, respectively. We also analysed the glycan repertoire of a major glycoprotein in Heligmosomoides polygyrus excretory–secretory products, VAL-2, which contains many glycan structures present in Heligmosomoides polygyrus excretory–secretory products including Glycan A. However, it was found that this set of glycans is not responsible for the heat-stable immunomodulatory properties of Heligmosomoides polygyrus excretory–secretory products, as revealed by the inability of VAL-2 to inhibit allergic lung inflammation. Taken together, these studies reveal that H. polygyrus secretes a diverse range of antigenic glycoconjugates, and provides a framework to explore the biological and immunomodulatory roles they may play within the mammalian host.

Inhibition of Haemonchus contortus larval development by fungal lectins

Background

Lectins are carbohydrate-binding proteins that are involved in fundamental intra- and extracellular biological processes. They occur ubiquitously in nature and are especially abundant in plants and fungi. It has been well established that certain higher fungi produce lectins in their fruiting bodies and/or sclerotia as a part of their natural resistance against free-living fungivorous nematodes and other pests. Despite relatively high diversity of the glycan structures in nature, many of the glycans targeted by fungal lectins are conserved among organisms of the same taxon and sometimes even among different taxa. Such conservation of glycans between free-living and parasitic nematodes is providing us with a useful tool for discovery of novel chemotherapeutic and vaccine targets. In our study, a subset of fungal lectins emanating from toxicity screens on Caenorhabditis elegans was tested for their potential to inhibit larval development of Haemonchus contortus.

Methods

The effect of Coprinopsis cinerea lectins - CCL2, CGL2, CGL3; Aleuria aurantia lectin – AAL; Marasmius oreades agglutinin - MOA; and Laccaria bicolor lectin – Lb-Tec2, on cultivated Haemonchus contortus larval stages was investigated using a larval development test (LDT). To validate the results of the toxicity assay and determine lectin binding capacity to the nematode digestive tract, biotinylated versions of lectins were fed to pre-infective larval stages of H. contortus and visualized by fluorescent microscopy. Lectin histochemistry on fixed adult worms was performed to investigate the presence and localisation of lectin binding sites in the disease-relevant developmental stage.

Results

Using an improved larval development test we found that four of the six tested lectins: AAL, CCL2, MOA and CGL2, exhibited a dose-dependent toxicity in LDT, as measured by the number of larvae developing to the L3 stage. In the case of AAL, CGL2 and MOA lectin, doses as low as 5 μg/ml caused >95 % inhibition of larval development while 40 μg/ml were needed to achieve the same inhibition by CCL2 lectin. MOA was the only lectin tested that caused larval death while other toxic lectins had larvistatic effect manifesting as L1 growth arrest. Using lectin histochemistry we demonstrate that of all lectins tested, only the four toxic ones displayed binding to the larvae’s gut and likewise were found to interact with glycans localized to the gastrodermal tissue of adults.

Conclusion

The results of our study suggest a correlation between the presence of target glycans of lectins in the digestive tract and the lectin-mediated toxicity in Haemonchus contortus. We demonstrate that binding to the structurally conserved glycan structures found in H. contortus gastrodermal tissue by the set of fungal lectins has detrimental effect on larval development. Some of these glycan structures might represent antigens which are not exposed to the host immune system (hidden antigens) and thus have a potential for vaccine or drug development. Nematotoxic fungal lectins prove to be a useful tool to identify such targets in parasitic nematodes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1032-x) contains supplementary material, which is available to authorized users.

Vaccination of sheep and cattle against haemonchosis

Vaccines against gastrointestinal nematodes are one potential option for the control of parasitic gastroenteritis in ruminants. Excretory/secretory (E/S) and hidden antigens are being studied as candidates for vaccines against Haemonchus spp., which is a major parasite in cattle and small ruminants that are raised in warm climates. Protection has been observed after vaccination with some E/S proteases, particularly cysteine proteases and with some glycans that are abundant on the surfaces and in the secretory products of helminths. However, the most promising results are being obtained with glycoprotein antigens extracted from the microvillar surfaces of the Haemonchus contortus intestinal cells. These antigens are called 'hidden' because they are not exposed to the host's immune system during infection. Thus far, recombinant forms of these antigens have not been usefully protective. However, because only 5 μg of antigen is required per dose, production of a native antigen vaccine from adult parasites has been found to be practical and commercially viable. Trials indicate that a vaccine made from one particular isolate will cross-protect against geographically distant isolates.

Immunization with recombinantly expressed glycan antigens from Schistosoma mansoni induces glycan-specific antibodies against the parasite

Schistosomiasis caused by infection with parasitic helminths of Schistosoma spp. is a major global health problem due to inadequate treatment and lack of a vaccine. The immune response to schistosomes includes glycan antigens, which could be valuable diagnostic markers and vaccine targets. However, no precedent exists for how to design vaccines targeting eukaryotic glycoconjugates. The di- and tri-saccharide motifs LacdiNAc (GalNAcβ1,4GlcNAc; LDN) and fucosylated LacdiNAc (GalNAcβ1,4(Fucα1-3)GlcNAc; LDNF) are the basis for several important schistosome glycan antigens. They occur in monomeric form or as repeating units (poly-LDNF) and as part of a variety of different glycoconjugates. Because chemical synthesis and conjugation of such antigens is exceedingly difficult, we sought to develop a recombinant expression system for parasite glycans. We hypothesized that presentation of parasite glycans on the cell surface would induce glycan-specific antibodies. We generated Chinese hamster ovary (CHO) Lec8 cell lines expressing poly-LDN (L8-GT) and poly-LDNF (L8-GTFT) abundantly on their membrane glycoproteins. Sera from Schistosoma mansoni-infected mice were highly cross-reactive with the cells and with cell-surface N-glycans. Immunizing mice with L8-GT and L8-GTFT cells induced glycan-specific antibodies. The L8-GTFT cells induced a sustained booster response, with antibodies that bound to S. mansoni lysates and recapitulated the exquisite specificity of the anti-parasite response for particular presentations of LDNF antigen. In summary, this recombinant expression system promotes successful generation of antibodies to the glycans of S. mansoni, and it can be adapted to study the role of glycan antigens and anti-glycan immune responses in many other infections and pathologies.

Differential expression of anti-glycan antibodies in schistosome-infected humans, rhesus monkeys and mice

Schistosomiasis is a debilitating parasitic disease of humans, endemic in tropical areas, for which no vaccine is available. Evidence points to glycan antigens as being important in immune responses to infection. Here we describe our studies on the comparative humoral immune responses to defined schistosome-type glycan epitopes in Schistosoma mansoni-infected humans, rhesus monkeys and mice. Rhesus anti-glycan responses over the course of infection were screened on a defined glycan microarray comprising semi-synthetic glycopeptides terminating with schistosome-associated or control mammalian-type glycan epitopes, as well as a defined glycan microarray of mammalian-type glycans representing over 400 glycan structures. Infected rhesus monkeys generated a high immunoglobulin G (IgG) antibody response to the core xylose/core α3 fucose epitope of N-glycans, which peaked at 8-11 weeks post infection, coinciding with maximal ability to kill schistosomula in vitro. By contrast, infected humans generated low antibody levels to this epitope. At 18 months following praziquantel therapy to eliminate the parasite, antibody levels were negligible. Mice chronically infected with S. mansoni generated high levels of anti-fucosylated LacdiNAc (GalNAcβ1, 4(Fucα1, 3)GlcNAc) IgM antibodies, but lacked a robust response to the core xylose/core α3 fucose N-glycan antigens compared with other species studied, and their sera demonstrated an intermediate level of schistosomula killing in vitro. These differential responses to parasite glycan antigens may be related to the ability of rhesus monkeys to self-cure in contrast to the chronic infection seen in humans and mice. Our results validate defined glycan microarrays as a useful technology to evaluate diagnostic and vaccine antigens for schistosomiasis and perhaps other infections.

Glycoconjugates in host-helminth interactions

Helminths are multicellular parasitic worms that comprise a major class of human pathogens and cause an immense amount of suffering worldwide. Helminths possess an abundance of complex and unique glycoconjugates that interact with both the innate and adaptive arms of immunity in definitive and intermediate hosts. These glycoconjugates represent a major untapped reservoir of immunomodulatory compounds, which have the potential to treat autoimmune and inflammatory disorders, and antigenic glycans, which could be exploited as vaccines and diagnostics. This review will survey current knowledge of the interactions between helminth glycans and host immunity and highlight the gaps in our understanding which are relevant to advancing therapeutics, vaccine development, and diagnostics.

Intranasal immunization of lambs with serine/threonine phosphatase 2A against gastrointestinal nematodes

Seven 3-month-old, female, helminth-free lambs were immunized intranasally with three doses (1 mg total) of a recombinant part of the catalytic region of the serine/threonine phosphatase 2A (PP2Ar) (group 1 [G1]). In addition, four lambs were used as an adjuvant control group (G2), four as unimmunized, infected controls (G3), and four as unimmunized, uninfected controls (G4). Fifteen days after the last immunization, lambs from G1, G2, and G3 were challenged with 10,000 larval stage 3 (L3) organisms in a plurispecific nematode infection composed of ca. 40% Trichostrongylus colubriformis, 40% Haemonchus contortus, and 20% Teladorsagia circumcincta. All the lambs were clinically monitored throughout the experiment. Parasitological (fecal egg output and immunological response), biopathological (packed-cell volume and leukocyte and eosinophil counts), and zootechnical (live-weight gain) analyses were conducted. On day 105 of the experiment, all the animals were slaughtered and the adult worm population in their abomasa examined. Intranasal administration of PP2Ar with bacterial walls as an adjuvant elicited a strong immune response in the immunized lambs, as evidenced by their humoral immune response. Immunized animals and animals receiving the adjuvant shed significantly (P < 0.001) fewer numbers of parasites' eggs in their feces. The immunization significantly reduced the helminth burden in the abomasa by the end of the experiment (>68%), protection being provided against both Haemonchus and Teladorsagia. Live-weight gain in the immunized lambs was similar to that in the uninfected controls versus the infected or adjuvanted animal groups. Our results suggest that heterologous immunization of ruminants by intranasal administration may be efficacious in the struggle to control gastrointestinal helminths in these livestock.

Glycosyltransferases in chemo-enzymatic synthesis of oligosaccharides

Many oligosaccharides are not commercially available, which limits studies focused on elucidation of glycan functions; therefore chemo-enzymatic methods to synthesize them can be very useful. Here, we describe the procedure to synthesize the Galα1-3GalNAcβ1-4GlcNAcβ-R (Gal-LDN) moiety, containing the Galα1-3GalNAc epitope found on the parasitic helminth Haemonchus contortus. An acceptor substrate providing a terminal N-acetylglucosamine was prepared by coupling the fluorescent hydrophobic aglycon, 2,6-diaminopyridine (DAP), to N,N'-diacetylchitobiose. By the subsequent action of recombinant Caenorhabditis elegans β1,4-N-acetylgalactosaminyltransferase the substrate was efficiently converted to GalNAcβ1-4GlcNAcβ-R (LDN-R). Since no recombinant α1,3-galactosyltransferase has been described that acts on terminal βGalNAc, we used bovine α1,3-galactosyltransferase to obtain a partial conversion of LDN-R to the Gal-LDN antigen. This method can be applied to synthesize any oligosaccharide, provided that specific glycosyltransferases are available, or related enzymes that can be pushed to elongate the selected acceptor.

Carrier molecules for use in veterinary vaccines

The practice of immunization of animals and humans has been carried out for centuries and is generally accepted as the most cost effective and sustainable method of infectious disease control. Over the past 20 years there have been significant changes in our ability to produce antigens by conventional extraction and purification, recombinant DNA and synthesis. However, many of these products need to be combined with carrier molecules to generate optimal immune responses. This review covers selected topics in the development of carrier technologies for use in the veterinary vaccine field, including glycoconjugate and peptide vaccines, microparticle and nanoparticle formulations, and finally virus-like particles.

Novel immunomic technologies for schistosome vaccine development

Schistosomiasis remains one of the most common human helminthiases, despite the availability of an effective drug against the causative parasites. Drug treatment programmes have several limitations, and it is likely that a vaccine is required for effective control. While decades of vaccine development have seen the discovery and testing of several candidate antigens, none have shown consistent and acceptable high levels of protection. The migrating larval stages are susceptible to immunity, however few larval-specific antigens have been discovered. Therefore, there is a need to identify novel larval-specific antigens, which may prove to be more efficacious than existing targets. Immunomics, a relatively new field developed to cope with the recent large influx of biological information, holds promise for the discovery of vaccine targets, and this review highlights some immunomic approaches to schistosome vaccine development. Firstly, a method to focus on the immune response elicited by the important and vulnerable larval stage is described, which allows a targeted study of the immunome at different tissue sites. Then, two high-throughput arrays are discussed for the identification of protein and carbohydrate antigens. It is anticipated that these approaches will progress vaccine development against the schistosomes, as well as other parasites.

Phage-display library biopanning as a novel approach to identifying nematode vaccine antigens

Infections with parasitic nematodes are of significant welfare and economic importance worldwide, and because of the emergence of anthelmintic resistance, this has lead to alternative methods of parasite control being required. Vaccination offers a feasible alternative control, and the majority of research has focused on the production of recombinant versions of native antigens previously identified as protective in vaccinated animals. Attempts at the production of protective recombinant subunit vaccines have been hindered, however, as these antigens have invariably failed to replicate the same level of protective immune response as seen with the native versions. It has been proposed that these failures are owing to the fact that the recombinant proteins do not contain the appropriate post-translational modifications to retain the protective capacity of the native molecules. In this review, we discuss a novel approach to vaccine antigen identification through the application of random peptide phage-display libraries and their use to identify peptide sequences that potentially mimic the structure(s) of antigenic epitopes. This area of research is still relatively novel with respect to parasites, and the current state of the art will be discussed here.

Galactosylated fucose epitopes in nematodes: increased expression in a Caenorhabditis mutant associated with altered lectin sensitivity and occurrence in parasitic species

The modification of α1,6-linked fucose residues attached to the proximal (reducing-terminal) core N-acetylglucosamine residue of N-glycans by β1,4-linked galactose ("GalFuc" epitope) is a feature of a number of invertebrate species including the model nematode Caenorhabditis elegans. A pre-requisite for both core α1,6-fucosylation and β1,4-galactosylation is the presence of a nonreducing terminal N-acetylglucosamine; however, this residue is normally absent from the final glycan structure in invertebrates due to the action of specific hexosaminidases. Previously, we have identified two hexosaminidases (HEX-2 and HEX-3) in C. elegans, which process N-glycans. In the present study, we have prepared a hex-2;hex-3 double mutant, which possesses a radically altered N-glycomic profile. Whereas in the double mutant core α1,3-fucosylation of the proximal N-acetylglucosamine was abolished, the degree of galactosylation of core α1,6-fucose increased, and a novel Galα1,2Fucα1,3 moiety attached to the distal core N-acetylglucosamine residue was detected. Both galactosylated fucose moieties were also found in two parasitic nematodes, Ascaris suum and Oesophagostomum dentatum. As core modifications of N-glycans are known targets for fungal nematotoxic lectins, the sensitivity of the C. elegans double hexosaminidase mutant was assessed. Although this mutant displayed hypersensitivity to the GalFuc-binding lectin CGL2 and the N-acetylglucosamine-binding lectin XCL, the mutant was resistant to CCL2, which binds core α1,3-fucose. Thus, the use of C. elegans mutants aids the identification of novel N-glycan modifications and the definition of in vivo specificities of nematotoxic lectins with potential as anthelmintic agents.