Protective studies in sheep immunized with cuticular collagen proteins and peptides of Haemonchus contortus

Molecular cloning of a cDNA encoding a putative cuticle collagen of Trichinella spiralis

A 5-day-old adult stage-specific cDNA fragment from Trichinella spiralis was identified by suppression subtractive hybridization and was used as a probe to screen the cDNA library. The cDNA sequence coding for a putative T. spiralis cuticle collagen was isolated. The cDNA encoded an open reading frame of 343 amino acid residues with molecular weight of 35.1 k Da. The deduced protein contained an N-terminal signal peptide, a nematode cuticle collagen N-terminal domain and a collagen triple helix repeat domain. Searches in GenBank using BLASTP showed up to 47% identity to cuticle collagens from other nematodes. Southern blot analysis of genomic DNA indicated this gene was present as a single copy in T. spiralis genome.

Vaccination against the nematode Haemonchus contortus with a thiol-binding fraction from the excretory/secretory products (ES)

Fractionated excretory/secretory products (ES) of adult Haemonchus contortus were evaluated as protective antigens. The proteins were successively eluted from a Thiol Sepharose column using 25 mM cysteine followed by 25 mM Dl-dithiothreitol (DTT). Sheep were vaccinated three times and challenged with 5000 third stage infective larvae (L3) of H. contortus. Highest level of protection was found in sheep vaccinated with the DTT-eluted fraction in which egg output and worm burden were reduced by 52 and 50%, respectively, compared to the adjuvant control group. There was a positive correlation between fecundity (number of eggs per female) and the cumulative EPG or worm burden. Serum and mucus antibody levels of ES-specific immunoglobulins increased after immunizations and after challenge for IgG, IgA and IgE. The harvesting of H. contortus from animals clustered per group revealed the presence of cysteine protease activity in the ES of all groups but in addition to that, metalloprotease activity was also detected in the groups vaccinated with the DTT-eluted fraction, total ES and adjuvant only, in contrast to previous batches of ES (completely inhibited by E64) obtained from non vaccinated animals.

Caenorhabditis elegans and the study of gene function in parasites

The free-living nematode Caenorhabditis elegans is a tractable experimental model system for the study of both vertebrate and invertebrate biology. Its most significant advantages are its simplicity, both in anatomy and in genomic organization, and the elaborate methods that have been developed to attribute function to previously uncharacterized genes. Importantly, > 40% of parasitic nematode genes exhibit high levels of homology to genes within the C. elegans genome. Studying such genes using the C. elegans model should yield new insights into key molecules and their possible implications in parasite survival, leading to the discovery of new drug targets and vaccine candidates.

Characterisation of Haemonchus contortus-derived cell populations propagated in vitro in a tissue culture environment and their potential to induce protective immunity in sheep

Cell populations derived from viable Haemonchus contortus L(3) larvae were propagated in vitro in a tissue culture environment for a prolonged period (>48 months). Microscopic evaluation of H. contortus-derived cell populations revealed gross morphological characteristics highly analogous to those described for cell types originating from species of plant nematodes propagated in vitro in a tissue culture environment for a briefer period of time (<6 months). The characterisation of extracts harvested from tissue culture populations of H. contortus-derived cells by SDS-PAGE analysis detected molecular fractions of approximately 29, 45, 55, and 200-kDa that closely correlated with reports for preparations obtained from intact/viable H. contortus larvae. Complementary investigations detected the dual biochemical expression of phosphohydrolase and aminopeptidase-M activities based on the hydrolysis of the synthetic enzyme-specific substrates, para-nitrophenylphosphate and leucine-para-nitroanaline, respectively. The identification of phosphohydrolase and aminopeptidase-M-like biochemical activity in fractions harvested from H. contortus-derived cell populations and propagated in vitro in tissue culture served as evidence validating their parasitic-origin. Further validation of H. contortus-derived cell populations propagated in tissue culture entailed the formulation of Triton X-100 extracts containing potential immunoprotective antigens with SEAM adjuvant and its administration by intramuscular injection (100 microg total protein) to healthy sheep (n=8) on day 0 (left rear-limb) and day +14 (right rear-limb). Animals on day 28 subsequently received a single oral challenge of 10,000 infective L(3)-stage H. contortus larvae. Applying ELISA methodologies, increases in antigen-specific IgM and IgG were detected in ovine serum samples. Interpretation of experimental findings revealed that sheep with the greatest antigen-specific humoral immune responses (IgG titre 1/3125) also demonstrated a degree of reduced abomasal H. contortuslarvae burdens (60% reduction). Polyclonal antibody from immunoprotected sheep was subsequently found to recognise both the: (i), digestive tract; and (ii), antigen extracts associated with intact/viable H. contortus larvae. These experimental findings reveal the potential feasibility of propagating parasite-derived cell populations in an in vitro tissue culture environment in a manner that retains their ability to express immunoprotective antigenic fractions.

Ostertagia ostertagi challenge of calves vaccinated with Haemonchus placei intestinal homogenate

The protective capacity of vaccination with Haemonchus placei whole gut homogenate against challenge with the non-blood-feeding nematode Ostertagia ostertagi was evaluated in calves. Ten helminth-free calves were randomly assigned to two groups. Group 1 received 100microg H. placei intestinal homogenate in the adjuvant 5% dextran sulfate/PBS, while Group 2 received the adjuvant alone. Injections were administered subcutaneously on Days 0 and 28. All calves were challenged with approximately 26,100 O. ostertagi larvae on Day 42. Serum antibody response and counts of nematode eggs per gram of feces (EPG) were determined throughout the study. Calves were necropsied at 5.5 weeks post-challenge for recovery of nematodes. Although significant increases were detected in both serum IgG(1) and IgG(2) of Group 1 calves (p<0.05), there was no significant difference in the total number of O. ostertagi recovered from the two groups (p>0.05). Lengths of adult nematodes were not significantly different between groups nor were the numbers of eggs present in adult females recovered from each group significantly different (p>0.05). There were also no significant differences between groups regarding fecal egg counts (p>0.05). Results suggest either: (1) the antigens targeted by the induced antibodies were not present in O. ostertagi; (2) the antigens targeted by the induced antibodies were present, but not essential to O. ostertagi survival; or (3) the antigen was present and essential, but amount of antibody ingested was insufficient to cause damage to the nematode.

Vaccination of calves with Haemonchus placei intestinal homogenate

The ability of adult Haemonchus placei intestinal homogenate to confer protection against homologous challenge infection was evaluated. Calves were immunized twice with 100 microg H. placei intestinal protein in 5% dextran-sulfate/PBS (vaccinates) or PBS alone (controls) and were challenged with approximately 3300 infective H. placei larvae. There was no significant difference between groups in the total number of nematodes recovered but significantly fewer (p < 0.001) adult females were recovered from vaccinates. The proportion of fourth-stage larvae in vaccinates was significantly greater (p < or = 0.05) than in controls. Lengths of adult male and female nematodes were significantly shorter (p < 0.001) in vaccinated calves, and the numbers of eggs present in the uteri of female nematodes from vaccinates were significantly decreased (p < 0.001). Counts of nematode eggs per gram of feces (EPG) of vaccinates were significantly less than that for controls on Days 29-49 post-challenge (p < or = 0.05). Vaccinates had significant increases in serum IgG1 and IgG2 log(10) titers (p < or = 0.05) but not in serum IgM. EPG, numbers of females, and size of males and females were negatively correlated with increased mean post-challenge IgG1 and IgG2 titers. Reduction in binding of periodate-treated gut homogenate by immune serum indicated a carbohydrate specific component in the immune response.

Integument ultrastructure of Oestrus ovis (L.) (Diptera:Oestridae) larvae: host immune response to various cuticular components

The nasal bot fly, Oestrus ovis, was investigated to establish which specific cuticular component is most immunogenic to infested sheep and how larval cuticle attains a protective role, if any, against the host immune system. To accomplish these goals, larval cuticle was extracted by a variety of agents and tested against immune sera from infested sheep and experimentally immunized rabbits. The cuticle substructure remaining after extraction was examined to localize various immunogenic components. O. ovis larval integument comprises an inner cellular layer, the epidermis, and an overlying cuticle layer. In 3rd instar larvae, the cuticle comprises 2 additional layers: the procuticle with numerous pore canals and the epicuticle which includes the wax canals. Three additional layers, altogether comprising the cuticulin layer, are present external to the epicuticle. The epicuticle is completed by apposition of an amorphous electrondense material extending for up to 1 micron in thickness. When fixed with ruthenium red, cuticle becomes heavily stained all along the epicuticular surface in larvae of all developmental stages. However, in 3rd instar larvae, ruthenium red deposits are restricted to the cuticulin layer alone. By gel electrophoresis, 3rd instar larval cuticle is shown to contain a number of polypeptides ranging in molecular weight from 180 to 4.5 kDa. The number and relative concentration of low molecular weight polypeptides was shown to vary in relation to the extraction media employed. Cuticular fragments examined after extraction exhibit an altered ultrastructure. When tested by immunoblotting, the cuticular polypeptides most reactive against sheep antisera are in the range of 180-56 kDa. A similar reaction was also detected with sera from rabbits infested experimentally with O. ovis larvae. Results are interpreted in relation to differential polypeptide distribution within the larval cuticle and to accessibility of the host immune system.

Vaccination against worm parasites of animals

The 1990s have seen the culmination of decades of painstaking research with the registration and launch of Tickgard (Hoechst), a recombinant vaccine against Boophilus microplus, and the provisional registration of a Taenia ovis vaccine. Research continues to hold promise for immunological control of Echinococcus, Fasciola, Haemonchus, Trichostrongylus and Ostertagia. Blood-sucking parasites (e.g. ticks and H. contortus) are susceptible to control by vaccines containing 'novel' or 'concealed' antigens where serum antibodies in blood meals attack targets in the gut. Antibodies also provide protection in taeniid models, whereas the protective response to be sought in Fasciola remains unclear. More problematic are formulations and delivery strategies to induce expulsion of gastrointestinal nematodes, using vaccines containing recombinant 'conventional' antigens. The use of computer models to simulate vaccine efficacy in worm control and challenges to the concept of 'hypo-responsiveness' of young lambs will encourage cautious optimism and lively debate as to the prospects for integrated worm control using parasite vaccines. This review covers the aspirations, current success and problems faced by researchers in the parasite arena.

Identification and sequence comparison of a cuticular collagen of Brugia pahangi

The cuticle of filarial nematodes is a specialized extracellular matrix that covers the parasite and protects it from adverse conditions of the environment. As a surface structure it is in direct contact with the host defence mechanisms and therefore plays an important role in the molecular host-parasite relationship. Using polyclonal antisera raised against the insoluble components of the cuticle of the adult filarial parasite Brugia pahangi, we have isolated cDNA clones encoding collagen molecules of the cuticle. The protein domain structure of cDNA clone Bpcol-1 was compared with the known structures of cuticular collagens of the nematodes Brugia malayi, Caenorhabditis elegans, Ascaris suum and Haemonchus contortus, confirming interspecies similarities. Using affinity-purified anti-Bpcol-1 antibodies we identified Bpcol-1 antigenic determinants in different nematode extracts, and determined the localization of such epitopes within the cuticle of B. pahangi.

The effect of immunization of pigs with Ascaris suum cuticle components on the development of resistance to parenteral migration during a challenge infection

The development of immunity to Ascaris suum was studied in pigs immunized with isolated cuticle fragments from A. suum second and third stage larvae (L2/L3) and adult worms, and compared with other methods that stimulate a strong protective response in pigs. A significant protective response was seen in animals immunized with isolated cuticle fragments from A. suum L2/L3 and adults, but it was less than that seen in animals inoculated with UV-irradiated eggs or naturally exposed to eggs on a dirt lot. Significant IgG responses to 2-mercaptoethanol (2ME)-soluble cuticle components were seen in all groups, but the level of the antibody response did not relate to protection. Group differences in antibody and lymphocyte blastogenic responses to cuticle proteins indicated quantitative and qualitative stage specific differences in 2ME-soluble and insoluble cuticular proteins. Intestinal immunity was notably absent from cuticle immunized pigs because a marked liver white spot response was observed following the challenge inoculation. Thus, cuticle fragments from larval and adult A. suum are capable of inducing a protective response to larval migration; however, the development of intestinal immunity is not a direct function of exposure to these antigens.

Surface and excretory/secretory antigens of fourth-stage larvae and adult Ostertagia circumcincta

With the use of an enzyme-linked immunosorbent assay (ELISA), it was revealed that surface antigens of both adult and fourth-stage larvae (L4) of Ostertagia circumcincta induced high levels of serum IgG antibodies, while serum IgA antibody levels were low but increased significantly (P < 0.01) after infection. Immunofluorescence studies on the surface of viable L4 and adult nematodes showed that the IgG response was stage-specific only in animals vaccinated with adult surface extracts. The results of Western blot analysis using these antibodies suggested that at least eight polypeptides were shed from the L4 surface to the environment and that infection induced (or boosted) IgG antibody against a further four polypeptides. A comparison of reactivity of pre- and post-infection sera of sheep vaccinated with adult nematode surface antigens suggested that only one of the antigens stripped from the nematode surface was immunogenic and/or present in a concentration sufficient to induce an IgG response following parenteral vaccination. Infection boosted the IgG antibodies to a further four polypeptides. Only one polypeptide of 63 kDa seems to be shed in vivo from the adult nematode surface. Ten to eleven antigens were recognised in adult excretory/secretory products by serum IgG of multiple-infected sheep.

Production of vaccines against gastrointestinal nematodes of livestock

Three international collaborations involving Australian research scientists are currently developing vaccines against Haemonchus, Trichostrongylus and Ostertagia parasites using recombinant DNA technology. The variety of protective antigens identified can be classified as 'conventional' (stimulate naturally acquired immunity) or 'novel'/'convert'/'concealed' (protective once immunity is induced by vaccination). To date, the most gratifying progress has resulted in 60-90% protection against Haemonchus and other blood-sucking parasites (e.g. ticks) using novel antigens, where high titres of serum antibody ingested by feeding worms leads to their demise. A great deal of research effort is unravelling the complexity of naturally acquired immunity so that conventional antigens, which may be the principal means of removing 'mucosal-browsing' parasites, may be formulated and delivered to achieve optimal efficacy. This work reveals that to remove early stages of parasites before they take up residence, deliberate induction of hypersensitivity responses akin to asthma, may be a desirable goal for vaccines and that the two models have much in common.

The potential value of integral membrane proteins in the vaccination of lambs against Haemonchus contortus

An extract of adult Haemonchus contortus enriched in the parasite's intestinal microvillar membrane protein H11 and other integral membrane proteins but free of the protein contortin was evaluated as a potential vaccine in two breeds of sheep. The worm burdens of Clun Forest sheep injected with the extract and challenged with 25,000 infective larvae were reduced 89% by weight compared to the average for the controls. The worm burdens of Dorset sheep (challenged with 10,000 infective larvae) were reduced 72%. In both breeds the reduction in the number of female worms, 92 and 71.8%, respectively, was greater than the reduction in the males (86.5 and 46%). Parasite egg output, determined only for the Dorsets, was reduced 92% protection correlated with serum antibody titre. Most of the antibodies were directed against H11.

Vaccines against gastrointestinal nematode parasites of ruminants

A consequence of intensive livestock production is an increase in the incidence and impact of gastrointestinal (GI) parasites. Farmers have sought to redress this shift in the natural host-parasite relationship by chemotherapy. However, with the widespread development of resistance to anthelmintics and the current impetus for sustainable agricultural practices, alternatives such as vaccines are being sought to maintain animal productivity. In this article, David Emery and Barry Wagland discuss recent advances in immunity to nematode infections of ruminants and the development of vaccines made possible by the dogged persistence and ingenuity of cadres of parasitologists who have done more than 'go through the motions'.