Repeated vaccination against Haemonchus contortus provides continuous protection to young grazing sheep

Haemonchus contortus is the most important gastrointestinal nematode in the tropics and subtropics causing huge economic losses to the small ruminant industry. Vaccination is potentially a sustainable approach to control this parasite and the performance of Barbervax® a vaccine containing integral membrane glycoproteins from H. contortus intestinal cells, was evaluated in naturally infected grazing sheep during their development from sucking lambs to adults. The sheep were randomly assigned to two groups: Vaccine and Control. The Vaccine group were vaccinated 23 times over the course of this two-year trial at intervals of 3-6 weeks. They responded with anti-vaccine specific antibodies, had 80 % lower Haemonchus egg counts and were less anaemic compared with the controls. Packed cell volumes (PCV) were always greater than 25 % in the vaccinated sheep but averaged between 23 % and 24 % in the controls. Total plasma protein values were higher in the vaccinated group from the third vaccination until the end of the trial. Throughout the trial, 88 % of the control sheep were drenched (average of 3.1 drenches per treated animal) but only 57 % of vaccinates, needed a salvage anthelmintic treatment (average of 1.9 drenches per treated animal), however, between group no differences in body weight were observed. In summary, these results indicate that a continuous course of Barbervax® can provide lambs with substantial year-round protection against H. contortus until they reached adulthood.

Structure of the protective nematode protease complex H-gal-GP and its conservation across roundworm parasites

Roundworm parasite infections are a major cause of human and livestock disease worldwide and a threat to global food security. Disease control currently relies on anthelmintic drugs to which roundworms are becoming increasingly resistant. An alternative approach is control by vaccination and 'hidden antigens', components of the worm gut not encountered by the infected host, have been exploited to produce Barbervax, the first commercial vaccine for a gut dwelling nematode of any host. Here we present the structure of H-gal-GP, a hidden antigen from Haemonchus contortus, the Barber's Pole worm, and a major component of Barbervax. We demonstrate its novel architecture, subunit composition and topology, flexibility and heterogeneity using cryo-electron microscopy, mass spectrometry, and modelling. Importantly, we demonstrate that complexes with the same architecture are present in other Strongylid roundworm parasites including human hookworm. This suggests a common ancestry and the potential for development of a unified hidden antigen vaccine.

Transcriptomic profiling of nematode parasites surviving vaccine exposure

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Surviving Haemonchus contortus from vaccinated sheep were compared with control worms.

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There is no evidence for changes in expression of genes encoding Barbervax® antigens.

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There was increased expression of other proteases and regulators of lysosome trafficking.

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Surviving worms displayed up-regulated lipid storage and defecation abilities.

Some nematode species are economically important parasites of livestock, while others are important human pathogens causing some of the most important neglected tropical diseases. In both humans and animals, anthelmintic drug administration is the main control strategy, but the emergence of drug-resistant worms has stimulated the development of alternative control approaches. Among these, vaccination is considered to be a sustainable and cost effective strategy. Currently, Barbervax® for the ruminant strongylid Haemonchus contortus is the only registered subunit vaccine for a nematode parasite, although a vaccine for the human hookworm Necator americanus is undergoing clinical trials (HOOKVAC consortium). As both these vaccines comprise a limited number of proteins, there is potential for selection of nematodes with altered sequences or expression of the vaccine antigens. Here we compared the transcriptome of H. contortus populations from sheep vaccinated with Barbervax® with worms from control animals. Barbervax® antigens are native integral membrane proteins isolated from the brush border of the intestinal cells of the adult parasite and many of those are proteases. Our findings provide no evidence for changes in expression of genes encoding Barbervax® antigens in the surviving parasite populations. However, surviving parasites from vaccinated animals showed increased expression of other proteases and regulators of lysosome trafficking, and displayed up-regulated lipid storage and defecation abilities that may have circumvented the effect of the vaccine. Implications for other potential vaccines for human and veterinary nematodes are discussed.

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.

Attempts to immunize sheep against Haemonchus contortus using a cocktail of recombinant proteases derived from the protective antigen, H-gal-GP

The objective of this study was to determine whether an antigen cocktail containing recombinantly expressed versions of most of the protective proteases of H-gal-GP, a known protective antigen from Haemonchus contortus, would confer any protection to lambs in a vaccine-challenge trial. Haemonchus contortus metalloendopeptidases, MEP1, MEP3 and MEP4, were expressed as soluble recombinant proteins in insect cells, but attempts to express the H. contortus aspartyl proteases, PEP1 and PEP2, by the same techniques were not successful. Recombinant H. contortus PEP1 was therefore expressed in Escherichia coli and refolded. Groups of sheep were immunized thrice with either native H-gal-GP, a cocktail of recombinantly expressed proteins (rMEP1, rMEP3, rMEP4 and rPep1), or adjuvant only (QuilA in PBS). All sheep were challenged with 5000 infective larvae 1 week after the final vaccination. High levels of serum antibodies that recognized H-gal-GP were detected in both the native antigen and recombinant cocktail-immunized groups by the time of challenge, but protective immunity was only observed in the group immunized with native H-gal-GP.

Molecular characterization of a family of metalloendopeptidases from the intestinal brush border of Haemonchus contortus

Substantial protection against the economically important parasitic nematode Haemonchus contortus has been achieved by immunizing sheep with a glycoprotein fraction isolated from the intestinal membranes of the worm (H-gal-GP). Previous studies showed that one of the major components of H-gal-GP is a family of at least 4 zinc metalloendopeptidases, designated MEPs 1–4. This paper describes aspects of the molecular architecture of this protease family, including the proteomic analysis of the MEP fraction of the H-gal-GP complex. These enzymes belong to the M13 zinc metalloendopeptidase family (EC 3.4.24.11), also known as neutral endopeptidases or neprilysins. The sequences of MEPs 1 and 3 suggested a typical Type II integral membrane protein structure, whilst MEPs 2 and 4 had putative cleavable signal peptides, typical of secreted proteins. Proteomic analysis of H-gal-GP indicated that the extracellular domain of all 4 MEPs had been cleaved close to the transmembrane region/signal peptide with additional cleavage sites mid-way along the polypeptide. MEP3 was present as a homo-dimer in H-gal-GP, whereas MEP1 or MEP2 formed hetero-dimers with MEP4. It was found that expression of MEP3 was confined to developing 4th-stage larvae and to adult worms, the stages of Haemonchus which feed on blood. MEP-like activity was detected in the H-gal-GP complex over a broad pH range (5–9). Since all 4 MEPs must share a similar microenvironment in the complex, this suggests that each might have a different substrate specificity.

Presence of the LDNF glycan on the host-protective H-gal-GP fraction from Haemonchus contortus

Immunization of sheep with the gut membrane-associated protein complex H-gal-GP of adult Haemonchus contortus induces high levels of protection against a homologous challenge infection. Protection is correlated with a systemic IgG response against the antigen. Analysis of the antibody response showed that the majority of the antigen-specific IgG was of the IgG2 isotype. A substantial proportion (74%) of this response was directed against the glycan component of H-gal-GP. The high immunogenicity of the H-gal-GP glycans may be due to the presence of the fucosylated LacdiNAc (LDNF) antigen. 2D electrophoresis, Western blotting and mass spectrometry analysis of H-gal-GP showed that this glycan epitope was specifically located on a metalloendopeptidase, MEP3. MEP3 is the most abundant protein in H-gal-GP and has been identified as one of the most likely protective components of the complex. Here, we present evidence that the LDNF glycan does not contribute to the protective capacity of H-gal-GP. Animals vaccinated with reduced and denatured H-gal-GP are not protected against subsequent infection, although the antibody response against the LDNF glycan is very similar to that of animals vaccinated with the native H-gal-GP. In addition, an alternative version of H-gal-GP, H-sialgal-GP, which is equally protective, but isolated by affinity chromatography on jacalin lectin rather than peanut lectin, contains a MEP3 component which has no detectable LDNF glycan.

Metalloendopeptidases from the intestinal brush border of Haemonchus contortus as protective antigens for sheep

Substantial protection against the economically important nematode Haemonchus contortus has been achieved by immunizing sheep with a glycoprotein fraction isolated from the intestinal membranes of this parasite. This fraction has been termed Haemonchus galactose-containing glycoprotein complex (H-gal-GP) since it was originally isolated through its selective binding to lectins with a specificity for N-acetylgalactosamine. A major component of this highly protective antigen complex is a family of four zinc metalloendopeptidases, designated MEPs 1-4. Various combinations of these MEPs were evaluated in immunization-challenge trials in sheep. In two experiments a combination of all four MEPs, separated from the rest of the complex by gel filtration in 8 m urea, significantly reduced H. contortus egg counts by 45 and 50%, an effect not significantly different from that conferred by 8 m urea treatment of H-gal-GP itself. Similarly, MEP3 alone or MEPs 1, 2 and 4 in combination, electroeluted from the complex following SDS gel electrophoresis, each reduced egg counts by some 33%. The MEPs are therefore protective components of H-gal-GP and from previously published findings, it appears that MEP3 is the most effective member of this metalloendopeptidase family. However, there was no significant protection when sheep were immunized with fully reduced and denatured H-gal-GP or with bacterially expressed recombinant forms of MEP 1 or the principal domains of MEP3, suggesting that conformational epitopes on the MEPs are required for immunity.

Aspartyl proteases from the intestinal brush border of Haemonchus contortus as protective antigens for sheep

A novel pepsin-like aspartyl protease was identified as a component of Haemonchus galactose-containing glycoprotein (H-gal-GP), which is an integral membrane glycoprotein complex located on the intestinal cells of Haemonchus contortus, and a highly protective antigen for sheep. This molecule, designated HcPEP2, showed 50% sequence identity with a previously described aspartyl protease from H-gal-GP known as HcPEP1. Fractions of H-gal-GP, either containing both HcPEP1 and 2 or other lower molecular weight components of the complex, were evaluated as protective antigens in immunization - challenge trials in sheep. When separated from the rest of the complex by gel filtration in 8 m urea, the HcPEP1 and 2 fraction significantly reduced H. contortus egg counts by 48% and worm numbers by 36%, but the lower molecular weight components were not significantly protective. However, the HcPEP1 and 2 fraction did not protect if electro-eluted from SDS-dissociated H-gal-GP, nor did bacterially expressed recombinant HcPEP1, suggesting that conformational epitopes are important for inducing immunity.