Vaccination against gastrointestinal nematode parasites of ruminants using gut-expressed antigens

Analysis of Protection and Immune Response against Teladorsagia circumcincta in Goats Immunised with Thiol-Binding Proteins from Adult Worms

In view of the increasing occurrence of anthelmintic-resistant strains of gastrointestinal nematodes in ruminants, various alternative control strategies have been investigated, such as those based on the induction of protective immune responses by immunisation with parasite antigens. In this study, the protective activity of somatic antigens from adult worms of Teladorsagia circumcincta purified by affinity chromatography on thiol-sepharose was analysed in goats. After challenge, the enriched products induced a slight reduction in the cumulative faecal egg counts (21%) and in the number of worms (23.3%), with a greater effect on female worms, which also showed a reduction in parameters related to their fertility. These parasitological findings were associated with a Th2 immune response, with a prominent local humoral response and an eosinophilic infiltrate in the gastric mucosa (negatively associated with the fertility of female worms and the number of worms, respectively), as well as an infiltration of MCHII+, CD4+, IgG+ and IgA+ cells. However, several analyses showed an increase in CD8+ cells in the mucosa, as well as IL-2 expression in the gastric lymph nodes, which may have been associated with inhibition of protective responses or with the development of mixed Th1/Th2 responses, a finding that should be analysed in future studies.

Haemonchosis in Sheep and Goats, Control Strategies and Development of Vaccines against Haemonchus contortus

Simple Summary

Haemonchus contortus is the most pathogenic blood-feeding parasitic nematode in sheep and goats, threatening animal welfare and causing tremendous economic losses to the small ruminant industry. This comprehensive review article sums up current control strategies, worm-derived antigens and recent advances in anti-Haemonchus vaccine development. New insights into antigen engineering and general considerations for clinical trials are discussed here.

Abstract

The evolutionary success of parasitic worms causes significant economic losses and animal health problems, including in the small ruminant industry. The hematophagous nematode Haemonchus contortus is a common endoparasite that infects wild and domestic ruminants worldwide, especially in tropical and subtropical regions. To date, the most commonly applied control strategy is the administration of anthelminthic drugs. The main disadvantages of these chemicals are their ecotoxic effects, the necessary withdrawal period (especially important in dairy animals) and the increasing development of resistance. Vaccines offer an attractive alternative control strategy against Haemonchus infections. In previous years, several potential vaccine antigens prepared from H. contortus using the latest technologies have been assessed in clinical trials using different methods and strategies. This review highlights the current state of knowledge on anti-H. contortus vaccines (covering native, recombinant and DNA-based vaccines), including an evaluation, as well a discussion of the challenges and achievements in developing protective, efficient, and long-lasting vaccines to control H. contortus infection and haemonchosis in small ruminants. This paper also addresses novel developments tackling the challenge of glycosylation of putative candidates in recombinant form.

Hookworm infection: Toward development of safe and effective peptide vaccines

Hookworms are hematophagous nematode parasites that have infected a billion people worldwide. Anthelmintic drugs have limited efficacy and do not prevent reinfection. Therefore, prophylactic vaccines are in high demand. Whole parasite vaccines are allergic and unsafe; thus, research into subunit vaccines has been warranted. A comprehensive overview of protein or peptide subunit vaccines' safety, protective efficacy, and associated immune responses is provided herein. The differences between the immune responses against hookworm infection by patients from epidemic versus nonepidemic areas are discussed in detail. Moreover, the different immunologic mechanisms of protection are discussed, including those that rely on allergic and nonallergic humoral and antibody-dependent cellular responses. The allergic and autoimmune potential of hookworm antigens is also explored, as are the immunoregulatory responses induced by the hookworm secretome. The potential of oral mucosal immunizations has been overlooked. Oral immunity against hookworms is a long-lived and safer immune response that is associated with elimination of infection and protective against reinfections. However, the harsh conditions of the gastrointestinal environment necessitates special oral delivery systems to unlock vaccines' protective potential. The potential for development of safer and more effective peptide- and protein-based anthelmintic vaccines is explored herein.

Approaches to Integrated Parasite Management (IPM) for Theileria orientalis with an Emphasis on Immunity

Integrated parasite management (IPM) for pests, pathogens and parasites involves reducing or breaking transmission to reduce the impact of infection or infestation. For Theileria orientalis, the critical impact of infection is the first wave of parasitaemia from the virulent genotypes, Ikeda and Chitose, associated with the sequelae from the development of anaemia. Therefore, current control measures for T. orientalis advocate excluding the movement of naïve stock from non-endemic regions into infected areas and controlling the tick Haemaphysalislongicornis, the final host. In Australia, treatment of established infection is limited to supportive therapy. To update and expand these options, this review examines progress towards prevention and therapy for T. orientalis, which are key elements for inclusion in IPM measures to control this parasite.

Haemonchus contortus transthyretin domain - containing protein (HcTTR): A promising vaccine candidate against Haemonchus contortus infection

Haemonchus contortus transthyretin domain-containing protein (HcTTR) with 136 amino acids belongs to a transthyretin-like (TTL) family member. In our previous study, it was reported that HcTTR was a novel antagonist of the goat cytokine Interleukin 4 (IL-4), and was involved in the regulation of host immune responses, implying that it might be applied for therapeutic strategies or vaccine development against the infection of H. contortus. Thus, the protective capacities of HcTTR against H. contortus infections via active and passive immunization trials were examined. For the passive protection trials, H. contortus-infected goats were intravenously immunized twice with 5 mg of total IgG containing anti-rHcTTR goat polyclonal antibodies. The results showed that the significant rates of reduction in egg shedding and worm burden was 58.12% and 64.61%, respectively, as compared with the positive control group. For the active protection trials, local goats were vaccinated twice with 500 μg of recombinant HcTTR to generate antigen-specific circulating antibodies, resulting in 63.7% reduction in eggs shedding and 66.4% reduction in worm burdens after H. contortus challenge. In both passive and active trials, the immunized goats displayed higher mucosal IgA levels and less anaemic compared to the challenged positive controls. Pen trials indicated that HcTTR generated partial immune protective effects against H. contortus challenge and it could be a promising vaccine candidate for development of effective strategy to control H. contortus.

Immunization of Goats with Recombinant Protein 14-3-3 Isoform 2(rHcftt-2) Induced Moderate Protection against Haemonchus contortus Challenge

A previous study identified that isoform 2 (Hcftt-2) of the 14-3-3 protein of Haemonchus contortus (H. contortus) could suppress immune functions of goat peripheral blood mononuclear cells (PBMCs) and might be a potential vaccine target, as neutralization of the protein function may enhance anti-parasite immunity. In this research, the recombinant Hcftt-2 was evaluated for its immunoprotective efficacy against H. contortus infection in goats. Five experimental goats were immunized twice with rHcftt-2 along with Freund’s adjuvant. The five immunized goats and five nonimmunized goats (adjuvant only) were challenged with 5000 L3-stage H. contortus larvae after 14 days of second immunization. Five nonimmunized and uninfected goats (adjuvant only) were set as the uninfected group. A significant increase in the serum immunoglobin G(IgG) and serum IgA levels were identified in the rHcftt-2 immunized animals. The mean eggs per gram in feces (EPG) and the worm burdens of rHcftt-2 immunized group were reduced by 26.46% (p < 0.05) and 32.33%, respectively. In brief, immunization of goats with rHcftt-2 induced moderate protection against H. contortus challenge.

Lessons along the Critical Path: Developing Vaccines against Human Helminths

Helminthic parasites are important targets for vaccine research as they infect an estimated 1 billion people worldwide. Despite significant progress in the discovery of defined antigens as candidates for vaccines, the potential of a helminth vaccine advancing to an investigational product to be tested in humans remains as challenging as it did 50 years ago. Candidate helminth vaccines must still advance along a 'critical path' of preclinical research, vaccine process development (which includes 'chemistry, manufacturing, and controls' or CMC), current good manufacturing practice (cGMP) production of the vaccine, and clinical trials. This path is highly targeted towards meeting the safety, immunogenicity, and efficacy criteria of regulatory bodies such as the US Food and Drug Administration (FDA). For nearly 20 years our product development partnership (PDP), the Texas Children's Hospital Center for Vaccine Development (TCH-CVD), has followed the critical paths of several novel subunit vaccines for the human hookworm Necator americanus and the intestinal trematode Schistosoma mansoni. Herein, we describe the critical lessons learned along this critical path.

The Serine/Threonine-Protein Phosphatase 1 From Haemonchus contortus Is Actively Involved in Suppressive Regulatory Roles on Immune Functions of Goat Peripheral Blood Mononuclear Cells

Serine/threonine-protein phosphatases (STPs), as integral constituents of parasitic excretory/secretory proteins, are assumed to be released during the host–parasite interactions. However, knowledge about these phosphatases and their immunoregulatory and immune protective efficiencies with host peripheral blood mononuclear cells (PBMCs) is scant. In this study, an open reading frame of STP from Haemonchus contortus designated as HcSTP-1 was amplified and cloned using reverse-transcription-polymerase chain reaction (RT-PCR) method. The 951-bp nucleotides sequence was encoded to a protein of 316 amino acid residues, conserved in characteristics motifs GDXHG, GDYVDRG, GNHE, HGG, RG, and H. The HcSTP-1 protein was detected at approximately 35 kDa as recombinant protein fused in an expression vector system and resolved on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunohistochemically, HcSTP-1 was found to be localized in both male and female adult worm sections. Using immunofluorescence assay, the binding activity of rHcSTP-1 was confirmed on surface of goat PBMCs, which resulted in expression of multiple cytokines and various immunoregulatory activities in vitro. The RT-PCR results showed that mRNA level of interleukin-2, TGF-β1, IFN-γ, and IL-17 (with 10 µg/ml) was upregulated and IL-10 was decreased. However, IL-6 showed no change after PBMCs incubated with rHcSTP-1 protein. Further functional analysis showed that migratory activity of cells, intracellular nitrite production (NO), and apoptotic efficiency of PBMCs were elevated at significant level, whereas the proliferation of goat PBMCs and monocytes-associated major histocompatibility complex (MHC)-I and MHC-II expressions were decreased significantly at concentration-dependent fashion. Our results showed that the HcSTP-1 protein engaged in vital suppressive regulatory roles on host immune cells, which might represent a potential molecular target for controlling H. contortus infection in future.

Understanding Haemonchus contortus Better Through Genomics and Transcriptomics

Parasitic roundworms (nematodes) cause substantial mortality and morbidity in animals globally. The barber's pole worm, Haemonchus contortus, is one of the most economically significant parasitic nematodes of small ruminants worldwide. Although this and related nematodes can be controlled relatively well using anthelmintics, resistance against most drugs in common use has become a major problem. Until recently, almost nothing was known about the molecular biology of H. contortus on a global scale. This chapter gives a brief background on H. contortus and haemonchosis, immune responses, vaccine research, chemotherapeutics and current problems associated with drug resistance. It also describes progress in transcriptomics before the availability of H. contortus genomes and the challenges associated with such work. It then reviews major progress on the two draft genomes and developmental transcriptomes of H. contortus, and summarizes their implications for the molecular biology of this worm in both the free-living and the parasitic stages of its life cycle. The chapter concludes by considering how genomics and transcriptomics can accelerate research on Haemonchus and related parasites, and can enable the development of new interventions against haemonchosis.

Identification of differentially expressed proteins between free-living and activated third-stage larvae of Haemonchus contortus

The disease caused by Haemonchus contortus, a blood-feeding nematode of small ruminants, is of major economic importance worldwide. The infective third-stage larva (L3) of this nematode is enclosed in a second cuticle. Once the L3 is ingested by the host, the outer cuticle undergoes an exsheathment process that marks the transition from the free-living stage to the parasitic stage. This study explored the changes in protein expression relative to this transition. Proteins extracted from free living L3 and exsheathed L3 (xL3) were analyzed by two dimensional differential gel electrophoresis (2D-DIGE). More than 2200 protein spots were recognized, and 124 of them was found to be differentially expressed (average ratio of xL3/L3>1.5 or xL3/L3<-1.5, p<0.05). Of these, 83 spots were up-regulated and 41 spots were down-regulated in xL3 when compared with L3. These differentially expressed spots were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) or MALDI-TOF-MS/MS and 40 proteins were identified. To predict the functions of these identified proteins, they were assigned for gene ontology (GO) annotation. Results showed that the proteins may be involved in biological processes of reproduction, cellular organization or biogenesis, multi-cellular organismal processes, single-organism processes, metabolic processes, signaling, biological regulation, response to stimulus, cellular processes, biological adhesion, growth, locomotion, localization, developmental processes and multi-organism processes. Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations were also performed, which was useful for exploring the process of metabolism and signal transduction pathways. This study indicated that some key alterations taking place, during the transition from L3 to xL3 may be interesting antiparasite targets, and some of the proteins involved in this process might be candidate antigens for vaccine development.

Role of parasitic vaccines in integrated control of parasitic diseases in livestock

Parasitic infections adversely affect animal’s health and threaten profitable animal production, thus affecting the economy of our country. These infections also play a major role in the spread of zoonotic diseases. Parasitic infections cause severe morbidity and mortality in animals especially those affecting the gastrointestinal system and thus affect the economy of livestock owner by decreasing the ability of the farmer to produce economically useful animal products. Due to all these reasons proper control of parasitic infection is critically important for sustained animal production. The most common and regularly used method to control parasitic infection is chemotherapy, which is very effective but has several disadvantages like drug resistance and drug residues. Integrated approaches to control parasitic infections should be formulated including grazing management, biological control, genetic resistance of hosts, and parasitic vaccines. India ranks first in cattle and buffalo population, but the majority of livestock owners have fewer herds, so other measures like grazing management, biological control, genetic resistance of hosts are not much practical to use. The most sustainable and economical approach to control parasitic infection in our country is to vaccinate animals, although vaccines increase the initial cost, but the immunity offered by the vaccine are long lived. Thus, vaccination of animals for various clinical, chronic, subclinical parasitic infections will be a cheaper and effective alternative to control parasitic infection for long time and improve animal production.

The human hookworm vaccine: recent updates and prospects for success

Approximately 440 million people globally are afflicted by hookworm disease, one of the 17 WHO-recognized neglected tropical diseases (NTDs). The iron-deficiency anaemia attributed to this disease contributes to at least 3.2 million disability-adjusted life years (DALYs) according to the Global Burden of Disease Study 2010. The current WHO-recommended control strategies rely primarily on mass drug administration or preventive chemotherapy. However, evidence is starting to accumulate confirming that preventive chemotherapy alone will not be sufficient to reduce the reinfection rates of hookworm, especially in areas of heavy transmission. The global health and research community is currently building a consensus stressing the need for the advancement of research and innovation to bridge the gaps and identify new public health interventions for diseases such as hookworm and other NTDs. This paper presents the strategies used by the Sabin Vaccine Institute Product Development Partnership (Sabin PDP) in their ongoing endeavour for the development of a human hookworm vaccine. Recent updates and the current prospects for success of an effective human hookworm vaccine, as a new technology to be linked to or combined with drug interventions, are presented.

Impact of next-generation technologies on exploring socioeconomically important parasites and developing new interventions

High-throughput molecular and computer technologies have become instrumental for systems biological explorations of pathogens, including parasites. For instance, investigations of the transcriptomes of different developmental stages of parasitic nematodes give insights into gene expression, regulation and function in a parasite, which is a significant step to understanding their biology, as well as interactions with their host(s) and disease. This chapter (1) gives a background on some key parasitic nematodes of socioeconomic importance, (2) describes sequencing and bioinformatic technologies for large-scale studies of the transcriptomes and genomes of these parasites, (3) provides some recent examples of applications and (4) emphasizes the prospects of fundamental biological explorations of parasites using these technologies for the development of new interventions to combat parasitic diseases.

Expression of Caenorhabditis elegans-expressed Trans-HPS, partial aminopeptidase H11 from Haemonchus contortus

Aminopeptidase H11 present in the surface of intestine microvilli in Haemonchus contortus was identified as the most effective antigen candidate. However, its recombinant forms produced in Escherichiacoli, insect cells and yeast could not provide promising protection against H. contortus challenge, probably due to the inappropriate glycosylation and/or conformational folding. Herein, partial H11 containing the potential zinc-binding domain and two predicted glycosylation sites (nt 1 bp-1710 bp, Trans-HPS) was subcloned downstream of 5' flanking region of Caenorhabditis elegans cpr-1 gene in pPD95.77 vector, with the deletion of GFP gene. The recombinant was expressed in C. elegans and verified by blotting with anti-H11 and anti-Trans-HPS rabbit polyclonal antibodies and anti-His monoclonal antibody. Stably inherited Trans-HPS in worm descendants was achieved by integration using UV irradiation. Immunization with the crude Trans-HPS extracted from transgenic worms resulted in 37.71% reduction in faecal egg counts (FEC) (P<0.05) and 24.91% reduction in worm burden, but an upward curve with moderate rate of daily FEC in goats. These results suggested an apparent delay against H. contortus egg-laying in goats, which differed from that with bacteria-origin form of partial H11 (nt 670 bp-1710 bp, HPS) (26.04% reduction in FEC and 18.46% reduction in worm burden). These findings indicate the feasibility of sufficient C. elegans-expressed H11 for the immunological research and vaccine development.

Proteomic analysis of Mecistocirrus digitatus and Haemonchus contortus intestinal protein extracts and subsequent efficacy testing in a vaccine trial

BACKGROUND

Gastrointestinal nematode infections, such as Haemonchus contortus and Mecistocirrus digitatus, are ranked in the top twenty diseases affecting small-holder farmers' livestock, yet research into M. digitatus, which infects cattle and buffalo in Asia is limited. Intestine-derived native protein vaccines are effective against Haemonchus, yet the protective efficacy of intestine-derived M. digitatus proteins has yet to be determined.

METHODOLOGY/PRINCIPAL FINDINGS

A simplified protein extraction protocol (A) is described and compared to an established method (B) for protein extraction from H. contortus. Proteomic analysis of the H. contortus and M. digitatus protein extracts identified putative vaccine antigens including aminopeptidases (H11), zinc metallopeptidases, glutamate dehydrogenase, and apical gut membrane polyproteins. A vaccine trial compared the ability of the M. digitatus extract and two different H. contortus extracts to protect sheep against H. contortus challenge. Both Haemonchus fractions (A and B) were highly effective, reducing cumulative Faecal Egg Counts (FEC) by 99.19% and 99.89% and total worm burdens by 87.28% and 93.64% respectively, compared to the unvaccinated controls. There was no effect on H. contortus worm burdens following vaccination with the M. digitatus extract and the 28.2% reduction in cumulative FEC was not statistically significant. However, FEC were consistently lower in the M. digitatus extract vaccinates compared to the un-vaccinated controls from 25 days post-infection.

CONCLUSIONS/SIGNIFICANCE

Similar, antigenically cross-reactive proteins are found in H. contortus and M. digitatus; this is the first step towards developing a multivalent native vaccine against Haemonchus species and M. digitatus. The simplified protein extraction method could form the basis for a locally produced vaccine against H. contortus and, possibly M. digitatus, in regions where effective cold chains for vaccine distribution are limited. The application of such a vaccine in these regions would reduce the need for anthelmintic treatment and the resultant selection for anthelmintic resistant parasites.

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.

Vaccination of goats with DNA vaccine encoding Dim-1 induced partial protection against Haemonchus contortus: a preliminary experimental study

Disorganized muscle family member (Dim-1) belongs to immunoglobulin superfamily, and is a structural protein localized to the region of the muscle cell membrane around and between the dense bodies. Strong immunogenicity to host's immune system was induced by Dim-1 from Ascaris suum, which indicated that Dim-1 could be a potential candidate for vaccine. The homologues of Dim-1 were also detected in nematodes Brugia malayi and Trichostrongylus colubriformis. However, information on the complete coding sequence and protection potential of this molecule in Haemonchus contortus is lacking. In this study, full length of Dim-1 cDNA was cloned using a rapid amplification of cDNA ends (RACE) strategy and the DNA vaccine encoding Dim-1 open reading frame (ORF) was tested for protection against experimental H. contortus infections in goats. Fifteen goats were allocated into three trial groups. The animals of Dim-1 group were vaccinated with the DNA vaccine encoding Dim-1 on day 0 and 14, and challenged with 5000 infective H. contortus third larval stage (L3) on day 28. An unvaccinated positive control group was challenged with L3 at the same time. An unvaccinated negative control group was not challenged with L3. The results indicated Dim-1 DNA vaccines were transcribed at local injection sites and expressed in vivo post immunizations respectively. Following L3 challenge, the mean eggs per gram feces (EPG) and worm burdens of Dim-1 group were reduced by 45.7% and 51.1%, respectively. Significantly high levels of serum IgG, serum IgA, mucosal IgA, CD4(+) T lymphocytes and B lymphocytes of Dim-1 group were produced. While compared with the negative control group, increased numbers of blood eosinophils and lymphocytes and declined haemoglobin level were observed in the Dim-1 group after L3 challenge. The preliminary study suggest that recombinant H. contortus Dim-1 DNA vaccine induced partial immune response and has protective potential against goat haemonchosis.

Granule exocytosis of granulysin and granzyme B as a potential key mechanism in vaccine-induced immunity in cattle against the nematode Ostertagia ostertagi

Ostertagia ostertagi is considered one of the most economically important bovine parasites. As an alternative to anthelmintic treatment, an experimental host-protective vaccine was previously developed on the basis of ASP proteins derived from adult worms. Intramuscular injection of this vaccine, combined with QuilA as an adjuvant, significantly reduced fecal egg counts by 59%. However, the immunological mechanisms triggered by the vaccine are still unclear. Therefore, in this study, the differences in immune responses at the site of infection, i.e., the abomasal mucosa, between ASP-QuilA-vaccinated animals and QuilA-vaccinated control animals were investigated on a transcriptomic level by using a whole-genome bovine microarray combined with histological analysis. Sixty-nine genes were significantly impacted in animals protected by the vaccine, 48 of which were upregulated. A correlation study between the parasitological parameters and gene transcription levels showed that the transcription levels of two of the upregulated genes, those for granulysin (GNLY) and granzyme B (GZMB), were negatively correlated with cumulative fecal egg counts and total worm counts, respectively. Both genes were also positively correlated with each other and with another upregulated gene, that for the IgE receptor subunit (FCER1A). Surprisingly, these three genes were also correlated significantly with CMA1, which encodes a mast cell marker, and with counts of mast cells and cells previously described as globule leukocytes. Furthermore, immunohistochemical data showed that GNLY was present in the granules of globule leukocytes and that it was secreted in mucus. Overall, the results suggest a potential role for granule exocytosis by globule leukocytes, potentially IgE mediated, in vaccine-induced protection against O. ostertagi.

Vaccinomics for the major blood feeding helminths of humans

Approximately one billion people are infected with hookworms and/or blood flukes (schistosomes) in developing countries. These two parasites are responsible for more disability adjusted life years lost than most other neglected tropical diseases (NTDs), and together, are second only to malaria. Although anthelmintic drugs are effective and widely available, they do not protect against reinfection, resistant parasites are likely to emerge, and mass drug administration programs are unsustainable. Therefore, there is a pressing need for the development of vaccines against these parasites. In recent years, there have been major advances in our understanding of hookworms and schistosomes at the molecular level through the use of "omics" technologies. The secretomes of these parasites have been characterized using transcriptomics, genomics, proteomics, and newly developed gene manipulation and silencing techniques, and the proteins of interest are now the target of novel antigen discovery approaches, notably immunomics. This research has resulted in the discovery, development, and early stage clinical trials of subunit vaccines against hookworms and schistosomes.

Vaccination of goats with DNA vaccines encoding H11 and IL-2 induces partial protection against Haemonchus contortus infection

DNA vaccines expressing Haemonchus contortus H11 antigen with or without interleukin (IL)-2 were tested for protection against H. contortus infection in goats. Sixteen goats (8-10 months of age) were allocated into four trial groups. On days 0 and 14, group 1 was immunised with a DNA vaccine expressing H11 and IL-2 and group 2 was immunised with a DNA vaccine expressing H11 only. Group 3 was an unvaccinated positive control group challenged with H. contortus third stage larvae (L3). Group 4 was an unvaccinated negative control group that was not challenged with L3. Animals in groups 1-3 were challenged with 5000 infective H. contortus L3 14 days after the second immunisation. Transcription of H11 and IL-2 was demonstrated in muscle by reverse transcriptase-PCR 10 days after primary immunisation and translation of H11 was detected by Western blot analysis 7 days after the second immunisation. Following immunisation with a DNA vaccine expressing H11 and IL-2, high levels of specific serum immunoglobulin (Ig) G, non-specific serum IgA, mucosal IgA, CD4(+) T lymphocytes, CD8(+) T lymphocytes and B lymphocytes were produced. Following challenge with L3, cumulative mean faecal worm egg counts and worm burdens in group 1 were reduced by 56.6% and 46.7%, respectively, while corresponding reductions in group 2 were 44.8% and 38.0%. There was a small but significant difference in abomasal worm burdens in goats in groups 1 (395.3±37.6) and 2 (459.5±101.6) compared to group 3 (741.5±241.5; P<0.05). Use of a DNA vaccine expressing H11 and IL-2 conferred partial protection against Haemonchus contortus infection in goats.

Using lectins to identify hidden antigens in Fasciola hepatica

Fasciola hepatica is the causative agent of fascioliasis, one of the most economically important helminth diseases of livestock worldwide. Traditionally, fascioliasis has been controlled by the strategic use of fasciolicidal drugs, but the emergence of resistant parasites has spurred an interest in developing vaccines as an alternative means of control. Most vaccine studies to date have evaluated conventional antigens, which are exposed to the host's immune system during the course of a natural infection. 'Hidden' antigens have proven to be effective vaccine candidates in other parasite species, most notably the blood-feeding nematode parasite, Haemonchus contortus, and tend to be expressed in the intestine or gut of the parasite. Fasciola hepatica is known to ingest large quantities of blood and may be vulnerable to this approach. Most, if not all, of the candidate antigens identified thus far have been membrane-bound glycoproteins which were solubilized by detergents. Here, we have attempted to employ lectins to select gut-associated glycoproteins from complex mixtures of somatic extracts of adult F. hepatica. We have conducted a comprehensive lectin-binding screen on adult histological sections with a panel of 16 fluorescently labelled lectins. Seven of the lectins bound to molecules within the gastrodermis but also bound to a range of other tissues. Within the gut tissues, jacalin and peanut lectins bound selectively to the gut lamellae and gastrodermal cells, respectively. These lectins were then used to isolate proteins from the integral membrane protein component of the adult fluke. Both lectins showed selectivity for relatively simple subsets of proteins compared to the original crude extracts.

An AC-5 cathepsin B-like protease purified from Haemonchus contortus excretory secretory products shows protective antigen potential for lambs

The immunogenic properties of cysteine proteases obtained from excretory/secretory products (ES) of Haemonchus contortus were investigated with a fraction purified with a recombinant H. contortus cystatin affinity column. The enrichment of H. contortus ES for cysteine protease was confirmed with substrate SDS-PAGE gels since the cystatin-binding fraction activity was three times higher than total ES, despite representing only 3% of total ES. This activity was inhibited by a specific cysteine protease inhibitor (E64) and by recombinant cystatin. The one-dimensional profile of the cystatin-binding fraction displayed a single band with a molecular mass of 43 kDa. Mass spectrometry showed this to be AC-5, a cathepsin B-like cysteine protease which had not been identified in ES products of H. contortus before. The cystatin binding fraction was tested as an immunogen in lambs which were vaccinated three times (week 0, 2.5 and 5), challenged with 10 000 L3 H. contortus (week 6) before necropsy and compared to unvaccinated challenge controls and another group given total ES (n = 10 per group). The group vaccinated with cystatin-binding proteins showed 36% and 32% mean worm burden and eggs per gram of faeces (EPG) reductions, respectively, compared to the controls but total ES was almost without effect. After challenge the cystatin-binding proteins induced significantly higher local and systemic ES specific IgA and IgG responses.

Molecular determinants archetypical to the phylum Nematoda

Background

Nematoda diverged from other animals between 600–1,200 million years ago and has become one of the most diverse animal phyla on earth. Most nematodes are free-living animals, but many are parasites of plants and animals including humans, posing major ecological and economical challenges around the world.

Results

We investigated phylum-specific molecular characteristics in Nematoda by exploring over 214,000 polypeptides from 32 nematode species including 27 parasites. Over 50,000 nematode protein families were identified based on primary sequence, including ~10% with members from at least three different species. Nearly 1,600 of the multi-species families did not share homology to Pfam domains, including a total of 758 restricted to Nematoda. Majority of the 462 families that were conserved among both free-living and parasitic species contained members from multiple nematode clades, yet ~90% of the 296 parasite-specific families originated only from a single clade. Features of these protein families were revealed through extrapolation of essential functions from observed RNAi phenotypes in C. elegans, bioinformatics-based functional annotations, identification of distant homology based on protein folds, and prediction of expression at accessible nematode surfaces. In addition, we identified a group of nematode-restricted sequence features in energy-generating electron transfer complexes as potential targets for new chemicals with minimal or no toxicity to the host.

Conclusion

This study identified and characterized the molecular determinants that help in defining the phylum Nematoda, and therefore improved our understanding of nematode protein evolution and provided novel insights for the development of next generation parasite control strategies.

Advances in the sequencing of the genome of the adenophorean nematode Trichinella spiralis

The adenophorean nematodes are evolutionarily distant from other species in the phylum Nematoda. Interspecific comparisons of predicted proteins have supported such an ancient divergence. Accordingly, Trichinella spiralis represents a basal nematode representative for genome sequencing focused on gaining a deeper insight into the evolutionary biology of nematodes. In addition, molecular characteristics that are conserved across the phylum could be of great value for control strategies with broad application. In this review, we describe and summarize progress that has been made on the sequencing and analysis of the T. spiralis genome. The genome sequence was used in preliminary analyses for the investigation of specific questions relating to the biology of T. spiralis and, more generally, to parasitic nematodes. For instance, we evaluated an unusually large DNase II-like protein family, predicted proteins of prospective interest in the parasite-host muscle cell interaction, anthelmintic targets and prospective intestinal genes, the encoded proteins (potentially) linked to immunological control against other nematodes. The results are discussed in relation to characteristics that are broadly conserved among evolutionary distant nematodes. The results lead to expectations that this genome sequence will contribute to advances in research on T. spiralis and other parasitic nematodes.

Intestinal transcriptomes of nematodes: comparison of the parasites Ascaris suum and Haemonchus contortus with the free-living Caenorhabditis elegans

Background

The nematode intestine is a major organ responsible for nutrient digestion and absorption; it is also involved in many other processes, such as reproduction, innate immunity, stress responses, and aging. The importance of the intestine as a target for the control of parasitic nematodes has been demonstrated. However, the lack of detailed knowledge on the molecular and cellular functions of the intestine and the level of its conservation across nematodes has impeded breakthroughs in this application.

Methods and Findings

As part of an extensive effort to investigate various transcribed genomes from Ascaris suum and Haemonchus contortus, we generated a large collection of intestinal sequences from parasitic nematodes by identifying 3,121 A. suum and 1,755 H. contortus genes expressed in the adult intestine through the generation of expressed sequence tags. Cross-species comparisons to the intestine of the free-living C. elegans revealed substantial diversification in the adult intestinal transcriptomes among these species, suggesting lineage- or species-specific adaptations during nematode evolution. In contrast, significant conservation of the intestinal gene repertories was also evident, despite the evolutionary distance of ∼350 million years separating them. A group of 241 intestinal protein families (IntFam-241), each containing members from all three species, was identified based on sequence similarities. These conserved proteins accounted for ∼20% of the sampled intestinal transcriptomes from the three nematodes and are proposed to represent conserved core functions in the nematode intestine. Functional characterizations of the IntFam-241 suggested important roles in molecular functions such as protein kinases and proteases, and biological pathways of carbohydrate metabolism, energy metabolism, and translation. Conservation in the core protein families was further explored by extrapolating observable RNA interference phenotypes in C. elegans to their parasitic counterparts.

Conclusions

Our study has provided novel insights into the nematode intestine and lays foundations for further comparative studies on biology, parasitism, and evolution within the phylum Nematoda.

Biological properties of the nematode intestine warrant in-depth investigation, the results of which can be utilized in the control of parasitic nematodes that infect humans, livestock, and plants. Both the importance of intestinal antigens from Haemonchus contortus in immunity and the damage to H. contortus intestine by anthelmintic fenbendazole have highlighted the versatility of the intestine as an emerging target. However, biological information regarding fundamental intestinal cell functions and mechanisms is currently limited. Conserved intestinal genes across nematode pathogens could offer molecular targets for broad parasite control. Furthermore, qualitative and quantitative comparisons on intestinal gene expression among species and lineages can identify basic adaptations relative to a critical selective force, the nutrient acquisition. This study begins to identify intestinal cell characteristics that are conserved across representatives of two clades of nematodes (V and III) and further clarifies diversities that likely reflect species- or lineage-specific adaptations. Results consistent with functional data on digestive enzymes from H. contortus and RNAi in Caenorhabditis elegans, as examples, support the potential for the comparative genomics approach to produce practical applications. This study provides a platform on which extensive investigation of intestinal genes and a more comprehensive understanding of the Nematoda can be gained.

Identification of immuno-reactive proteins from a sheep gastrointestinal nematode, Trichostrongylus colubriformis, using two-dimensional electrophoresis and mass spectrometry

Gastrointestinal nematode infections of livestock animals are prevalent and costly problems worldwide. Currently, infections are controlled by anthelmintic chemicals but increasing drug resistance has prompted research interest to shift towards alternative methods of control such as vaccine development and selection of worm-resistant animals. The present study analyses proteins from Trichostrongylus colubriformis infective L3s that are recognised by IgG of immune sheep. Following protein separation via two-dimensional electrophoresis and Western blot probing with plasma from sheep resistant to T. colubriformis, mass spectrometry-based proteomic analyses were used to identify immuno-reactive protein spots. We were able to identify 28 immune targets, including aspartyl protease inhibitor, enolase, chaperone proteins, galectin, glycolytic enzymes, kinase, phosphatase and structural muscle proteins such as myosin, paramyosin, calponin and DIM-1. The data suggest that immune responses to T. colubriformis are dispersed over a relatively large number of parasite antigens, including several cytoplasmically expressed proteins. The results have new implications for understanding the molecular mechanisms that underpin host-parasite interaction during gastrointestinal nematode infections.

Up-regulation of intelectin in sheep after infection with Teladorsagia circumcincta

A novel intelectin molecule designated sheep intelectin 2 (sITLN2) was detected in sheep abomasal mucosa. The full sequence shared 76-83% homology with other mammalian intelectins. Intelectins are mucus-associated proteins that have been shown to be up-regulated in gastrointestinal nematode infections in rodents and in human asthma. Expression of sheep abomasal ITLN2 mRNA was significantly up-regulated on day 10 post-challenge of worm-free sheep with Teladorsagia circumcincta and at day 2 in previously infected, immune sheep. Increased expression of ITLN protein following challenge was confirmed by Western blot and was immunolocalised to the mucous neck cells of the abomasal mucosa. Infection with T. circumcincta was also associated with increased levels of abomasal transcripts encoding sheep mast cell protease-1, ovine galectin-14 and IL4, which collectively suggested a Th2 type response. Intelectin may play an important role in the mucosal response to gastrointestinal nematode infections in ruminants.

Haemonchus contortus: characterization of the baculovirus expressed form of aminopeptidase H11

Recombinant form of Haemonchus contortus aminopeptidase H11, an intestinal membrane glycoprotein considered to be in its native form the most promising vaccine candidate, was produced in insect cells, characterised and tested in pilot vaccination-challenge trial on sheep. The sequence of the cloned gene, obtained by RT PCR isolated from adult worms, showed 97% identity to the highly immunogenic H11 clone, described by Graham et al., (database accession number AJ249941.1). A 1305 bp fragment of H11 was expressed in E. coli and used to raise a specific antiserum, which recognized recombinant forms of H11 and 110 kDa protein from H. contortus extract. H11 was expressed by baculovirus recombinants in insect cells in full length and as a fusion protein with H. contortus glutathione S-transferase (GST). The baculovirus produced recombinant antigens were used without adjuvants to immunize sheep, which resulted in 30% (full length H11) and 20% (GST-H11) reduction of worm burden. These animal experiments indicated that, although the protection induced by in vitro produced protein is lower than in case of H11 isolated from worms, recombinant forms of aminopeptidase may be considered as antigens for the control of haemonchosis.

Haemonchus contortus intestine: a prominent source of mucosal antigens

We sought to identify antigens from Haemonchus contortus, an abomasal nematode of small ruminants, that stimulate local (abomasal lymph node, ALN) CD4+ T lymphocyte responses during a primary infection. Results led to a focus on antigens from the parasite intestine. The H. contortus intestine proved to be a major source of antigens that stimulated ALN CD4+, CD25+ T lymphocyte responses during infections in lambs. When stimulated by intestinal antigens, ALN lymphocytes from these lambs expressed IL-4 and IL-13 transcripts, and, more variably, IFN-gamma. An immunoaffinity-purified fraction, enriched for H. contortus apical intestinal membrane proteins, stimulated similar ALN responses. On further fractionation, antigens from six size classes (ranging from 30 to 200 kDa) also stimulated proliferation of ALN lymphocytes. Mass spectrometry analysis of these size classes identified several known apical intestinal membrane proteins from H. contortus. The results show that H. contortus intestinal antigens warrant investigation in strategies to induce mucosal immunity against this parasite. The specific proteins identified have value for this purpose. The results are in contrast with the now generalized idea that H. contortus intestinal antigens are 'hidden' from the host immune system, and this issue is discussed. The approach also has potential application to other gastrointestinal nematode parasites.

Vaccines against blood-feeding nematodes of humans and livestock

This paper summarises the progress towards vaccine development against the major blood-feeding nematodes of man and livestock, the hookworms and Haemonchus contortus, respectively. The impact of the diseases and the drivers for vaccine development are summarized as well as the anticipated impact of the host immune response on vaccine design. The performance requirements are discussed and progress towards these objectives using defined larval and adult antigens, many of these being shared between species. Specific examples include the Ancylostoma secreted proteins and homologues in Haemonchus as well as proteases used for digestion of the blood meal. This discussion shows that many of the major vaccine candidates are shared between these blood-feeding species, not only those from the blood-feeding stages but also those expressed by infective L3s in the early stages of infection. Challenges for the future include: exploiting the expanding genome information for antigen discovery, use of different recombinant protein expression systems, formulation with new adjuvants, and novel methods of field testing vaccine efficacy.

Anthelmintic resistance and alternative control methods

Anthelmintic resistance has been a problem almost from the first use of the drugs introduced during the past 50 years. Evaluation of anthelmintics on a specific farm is essential to determine which drugs may be used in controlling parasite numbers. Treatment of livestock during the season in which parasite transmission is unfavorable lessens pasture contamination. Targeting treatment places less pressure on susceptible worms, diluting the resistant parasite population. Pasture management and using safe pastures for animals at highest risk lower pasture exposure. Selection for individual animals resistant to the effects of parasites lessens the need for use of anthelmintics. Alternatives to anthelmintics include cryptic antigen vaccines, copper wires, and biologic compounds.

Developments and hurdles in generating vaccines for controlling helminth parasites of grazing ruminants

As a direct consequence of rising drug resistance among common nematodes of grazing animals, efforts toward state-of-the-art vaccine development have clearly intensified in recent years, fuelled primarily by the advent of newer technologies in gene discovery, by advancements in antigen identification, characterisation and production. In this regard, it is appropriate to review progress that has been made in generating helminth vaccines and in particular, vaccines against common nematodes of production animals for consumption. In like manner, it is prudent to evaluate barriers that have hindered progress in the past and continue to present obstacles that must be solved when utilizing and depending on host immunity to attenuate parasitic infections.

Vaccination with Setaria cervi 175 kDa collagenase induces high level of protection against Brugia malayi infection in jirds

A zinc containing metalloprotease, 175 kDa collagenase, purified from adult female Setaria cervi showed strong cross-reactivity with sera from putatively immune (PI) individuals (unpublished observation) and induced cytotoxicity to B. malayi L3 larvae and microfilariae by ADCC mechanism [Srivastava Y, Bhandari YP, Reddy MVR, Harinath BC, Rathaur S. An adult 175 kDa collagenase antigen of Setaria cervi in immunoprophylaxis against Brugia malayi. J Helminth 2004;78:347-52]. These preliminary observations suggested the immunoprotective nature of collagenase. To confirm the vaccine potential of this protease, a vaccine trial was conducted in jirds (Meriones unguiculatus) against human filarial parasite B. malayi. The vaccination resulted into a mean protection level of 75.86% and produced high level of protease neutralizing antibodies. Cytokine analysis in immune jirds sera suggested a mixed Th1/Th2 type cellular immune response whereas ELISA, immunoblotting and enzyme antibody inhibition assay revealed the presence of specific anti-collagenase antibodies. Taken together, all these results suggest that S. cervi 175 kDa collagenase could form the basis of an effective molecular vaccine against human lymphatic filariasis.

Alternative approaches to control--quo vadit?

The increasing prevalence of anthelmintic resistance has provided a spur for research into 'alternative/novel' approaches to the control of helminthoses that are intended to reduce our reliance upon using chemoprophylaxis. The different approaches either target the parasite population in the host or on pasture, but the goal of all of them is to restrict host parasite contact to levels which minimise the impact of helminths on host welfare and/or performance. Infrapopulation regulation can be achieved through methods that enhance immunity such as optimised nutrition (immunonutrition), genetic selection and vaccination, or by an 'anthelmintic' route using bioactive forages, copper oxide wire particles, or use of targeted selective treatment strategies such as FAMACHA, which reduce the selection pressure for the development of resistance by maintaining a population in refugia. Suprapopulation control can be achieved through grazing management, or by using predacious fungi such as Duddingtonia flagrans. All of these approaches have been developed beyond the proof of concept stage and some are capable of being employed currently. However, some still require knowledge transfer, or commercialisation before they can be tested and widely applied in the field. All of the different approaches present unique challenges to the researchers engaged in developing them, and in comparison to simple prescriptive anthelmintic treatments, their use appears complex and requires some expertise on behalf of the advisor and/or end user. At present, most of our data are derived from trials using single approaches, but it is apparent that we need to move towards integrating some of these technologies which again represents a further challenge to the extension/advisory services. Progress in establishing different approaches requires not only the funding to support their scientific development but also to support the development of computer based models which can be used to highlight deficiencies in our understanding of the control mechanisms and to identify impediments to their introduction. It is inevitable that some of the approaches currently under investigation will fail to become widely applied for a variety of reasons that are not solely financial. These include issues concerned with practicability/applicability, affordability/appropriateness, availability/deliverability and above all, the failure to provide a consistent, reliable effect when used under commercial farming conditions.

Sericea lespedeza hay as a natural deworming agent against gastrointestinal nematode infection in goats

Infection with gastrointestinal nematodes (GIN), particularly Haemonchus contortus, is the biggest constraint to profitable goat production in the United States (US). Due to widespread prevalence of anthelmintic resistance in goat GIN, alternative, non-chemical control methodologies are needed to increase profitability of small ruminant industries. A study was designed to test the efficacy of a high condensed tannin (CT) legume, sericea lespedeza [SL, Lespedeza cuneata (Dum.-Cours. G. Don)] against GIN of goats fed in confinement. The goats were given a trickle infection of 500 H. contortus larvae/animal three times per week during the trial to simulate natural infection. Twenty Boer bucks (6-8 months old) were fed bermudagrass [BG, Cynodon dactylon (L.) Pers.] hay plus concentrate for 5 weeks in confinement and then 10 animals were switched to SL hay for an additional 7 weeks. Throughout the trial, feces and blood were collected weekly from individual animals to determine fecal egg count (FEC) and blood packed cell volume (PCV). Fecal cultures were made weekly from pooled samples to determine treatment effects on GIN larval development. All goats were slaughtered at the end of the trial, with adult worms in the abomasum and small intestine of each goat recovered, counted, and identified to species. Feeding SL hay to goats significantly (P<0.01) reduced FEC and increased PCV compared with BG hay. In addition, a lower percentage of ova in feces from SL-fed goats developed into infective (L3) larvae. There was a direct effect of SL hay on adult worms, with significantly (P<0.01) lower numbers of both abomasal (H. contortus, Teladorsagia circumcincta) and small intestinal (Trichostrongylus colubriformis) nematodes compared with goats fed BG hay. Feeding SL hay to goats is an effective means of controlling parasitic nematodes and may be a potential supplement/replacement for chemical anthelmintics.

Gastrointestinal nematode infection in sheep--a review of the alternatives to anthelmintics in parasite control

Efforts to curb production losses caused by nematode parasitism in sheep have led to the development of a number of control methods to complement or replace anthelmintics. The need for alternative control measures stems from the emergence of anthelmintic-resistant parasitic nematodes with reports of multi-class resistance to these drugs now emerging. A number of these control methods such as predacious microfungi, protein supplementation, plant extracts in feed and vaccines have demonstrated potential to control infection but require development and examination under natural conditions. Breeding for natural resistance to nematode infection has already shown success in controlling the disease under natural conditions. Selection for resistance is currently based on fecal egg count measurements but identification of genetic indicators of resistance will provide a more efficient method of selection. Current quantitative trait loci for nematode resistance include the MHC genes, interferon gamma gene, IgE gene and microsatellites on chromosome 1, 5 and 6. This paper reviews the current alternatives to anthelmintics to control infection, with an emphasis on breeding for host resistance and identification of genetic indicators of resistance.

Using Caenorhabditis elegans for functional analysis of genes of parasitic nematodes

Information on the functional genomics of Caenorhabditis elegans has increased significantly in the last few years with the development of RNA interference. In parasitic nematodes, RNA interference has shown some success in gene knockdown but optimisation of this technique will be required before it can be adopted as a reliable functional genomics tool. Comparative studies in C. elegans remain an appropriate alternative for studying the function and regulation of some parasite genes and will be extremely useful for fully exploiting the increasing parasite genome sequence data becoming available.

A survey of the intestinal transcriptomes of the hookworms, Necator americanus and Ancylostoma caninum, using tissues isolated by laser microdissection microscopy

The gastrointestinal tracts of multi-cellular blood-feeding parasites are targets for vaccines and drugs. Recently, recombinant vaccines that interrupt the digestion of blood in the hookworm gut have shown efficacy, so we explored the intestinal transcriptomes of the human and canine hookworms, Necator americanus and Ancylostoma caninum, respectively. We used Laser Microdissection Microscopy to dissect gut tissue from the parasites, extracted the RNA and generated cDNA libraries. A total of 480 expressed sequence tags were sequenced from each library and assembled into contigs, accounting for 268 N. americanus genes and 276 A. caninum genes. Only 17% of N. americanus and 36% of A. caninum contigs were assigned Gene Ontology classifications. Twenty-six (9.8%) N. americanus and 18 (6.5%) A. caninum contigs did not have homologues in any databases including dbEST-of these novel clones, seven N. americanus and three A. caninum contigs had Open Reading Frames with predicted secretory signal peptides. The most abundant transcripts corresponded to mRNAs encoding cholesterol-and fatty acid-binding proteins, C-type lectins, Activation-Associated Secretory Proteins, and proteases of different mechanistic classes, particularly astacin-like metallopeptidases. Expressed sequence tags corresponding to known and potential recombinant vaccines were identified and these included homologues of proteases, anti-clotting factors, defensins and integral membrane proteins involved in cell adhesion.

The end of the line for hookworm? An update on vaccine development

Gut-associated molecules of human hookworms have been the focus of recent interest as potential vaccine candidates.

Vaccination with recombinant aspartic hemoglobinase reduces parasite load and blood loss after hookworm infection in dogs

BACKGROUND

Hookworms infect 730 million people in developing countries where they are a leading cause of intestinal blood loss and iron-deficiency anemia. At the site of attachment to the host, adult hookworms ingest blood and lyse the erythrocytes to release hemoglobin. The parasites subsequently digest hemoglobin in their intestines using a cascade of proteolysis that begins with the Ancylostoma caninum aspartic protease 1, APR-1.

METHODS AND FINDINGS

We show that vaccination of dogs with recombinant Ac-APR-1 induced antibody and cellular responses and resulted in significantly reduced hookworm burdens (p = 0.056) and fecal egg counts (p = 0.018) in vaccinated dogs compared to control dogs after challenge with infective larvae of A. caninum. Most importantly, vaccinated dogs were protected against blood loss (p = 0.049) and most did not develop anemia, the major pathologic sequela of hookworm disease. IgG from vaccinated animals decreased the catalytic activity of the recombinant enzyme in vitro and the antibody bound in situ to the intestines of worms recovered from vaccinated dogs, implying that the vaccine interferes with the parasite's ability to digest blood.

CONCLUSION

To the best of our knowledge, this is the first report of a recombinant vaccine from a hematophagous parasite that significantly reduces both parasite load and blood loss, and it supports the development of APR-1 as a human hookworm vaccine.

Immunobiology of hookworm infection

Hookworms infect almost one billion people and are a major cause of iron-deficiency anaemia in developing countries of the tropics. Despite their prevalence and the morbidity they cause, little is known about the immune response to this complex eukaryotic parasite. Recent publications have shed light on the human cellular immune responses to hookworms, as well as mechanisms that hookworms utilize to skew the immune response in its favour. Unlike most other human helminth infections, neither age- nor exposure-related immunity develops in the majority of infected people. A vaccine is therefore a highly desirable goal. To this end, gene sequencing efforts have resulted in the deposition of more than 10,000 hookworm cDNA sequences in the public domain, providing a molecular snapshot of this intriguing parasite and providing novel tools for the development of new control strategies. Significant progress has been made in the development of anti-hookworm recombinant vaccines, and clinical trials are expected to begin in the near future.