The antibody repertoire in infection and vaccination with Dictyocaulus viviparus: heterogeneity in infected cattle and genetic control in guinea pigs

Genetic variation in resistance to mixed, predominantly Teladorsagia circumcincta nematode infections of sheep: from heritabilities to gene identification

In cool temperate areas, such as Scotland, sheep are infected by a variety of nematodes but the dominant nematode is Teladorsagia circumcincta. Resistant animals have one or more of the following features: fewer adult nematodes, more inhibited larvae, shorter adult nematodes and decreased production of nematode eggs. In lambs at the end of the first grazing season, the heritability of adult worm length is very strong, whereas the heritability of egg production is moderate. The heritability of worm number is low while there is no detectable genetic variation in the number of inhibited larvae. The major mechanisms underlying resistance to T. circumcincta appear to be the IgA mediated suppression of worm growth and the mast cell mediated regulation of worm number. Mast cell responses are slow to develop, possibly because they are responsible for protein loss and reduced growth of the host. Two genes have been repeatedly associated with resistance to T. Circumcincta: the MHC class II DRB1 locus on chromosome 20 and the interferon-gamma locus on chromosome 3. Although the causative mutations are still unknown both genes are plausible candidates.

The recognition of molecules from fourth-stage larvae of Ostertagia circumcincta by IgA from infected sheep

The recognition of antigens from fourth-stage Ostertagia circumcincta by plasma IgA was studied in a group of 30, ten-month-old Scottish Blackface sheep which had been naturally, then deliberately infected. Western blotting revealed 49 bands that were recognized by antibody from one or more of the 30 sheep. There was a heterogeneous pattern of antigen recognition amongst the sheep. No individual recognized all the 49 bands and only one of the 49 bands was recognized by all sheep. Two antigens with approximate molecular weights 87,000 Da and 129,000 Da were significantly associated with a reduction in mean adult worm lengths. The observed variation in recognition of these two antigens on fourth-stage larval preparations accounted, in a statistical sense, for nearly 40% of the total variation in worm length. In addition to the variation in antibody mediated recognition of these two parasite molecules, three other components have been implicated in regulating worm length. They are a 37,000 Da band from adult worms, the amount of fourth-stage larval specific IgA in the abomasal mucosa and the density-dependent influence of adult worm burden. Together, these components and their interactions accounted for over 90% of the observed variation in worm length. These results indicate that the parasite-specific IgA response, or something extremely closely associated with it, is the major immunological mechanism controlling worm length.

Immunisation of guinea pigs against Dictyocaulus viviparus using adult ES products enriched for acetylcholinesterases

The adult ES products of Dictyocaulus viviparus are a source of protective antigens against challenge in the guinea pig laboratory model. High levels of acetylcholinesterase (AChE) activity are present in these products and these enzymes are immunogenic in infected cattle. Here, the potential role of these enzymes in protective immunity was investigated using a fraction enriched for AChE to immunise guinea pigs. The antibody response stimulated by immunisation with AChE-enriched ES products and the worm burdens obtained following challenge with infective larvae were compared with those in animals immunised with whole ES products and challenge controls. The AChE-enriched preparation stimulated high levels of enzyme-specific antibody in immunised animals, which was not the case for those which received unfractionaed ES products. Worm burdens of guinea pigs which received the AChE-enriched fraction were significantly lower than those obtained in adjuvant controls. The animals which received the unfractionated ES products were not significantly protected against challenge. These results suggest that AChEs may be potential candidates for incorporation in a sub-unit vaccine against D. viviparus.

Extensive diversity in repeat unit sequences of the cDNA encoding the polyprotein antigen/allergen from the bovine lungworm Dictyocaulus viviparus

The complete sequence of the cDNA encoding the nematode polyprotein allergen/antigen (NPA) of the bovine lungworm Dictyocaulus viviparus was obtained by immunoscreening of cDNA expression libraries and by 5' RACE (rapid amplification of cDNA ends). The encoded polypeptide is similar in sequence to the ABA-1 allergen of Ascaris, the gp15/400 'ladder' protein of Brugia malayi, Brugia pahangi and Wuchereria bancrofti, and a 15-kDa antigen of Dirofilaria immitis. As with these, the predicted amino-acid sequence comprises a head-to-tail array of similar polypeptides with regularly spaced consensus proteinase cleavage sites. The D. viviparus protein was designated DvA-1 (D. viviparus antigen-1) and the gene dva-1. The deduced amino-acid sequence of DvA-1 showed features not observed before in other NPAs: (i) a hydrophobic leader peptide is present, (ii) none of the 12 units in the array are identical and the sequences diverge to a degree hitherto unseen in the NPAs of other nematode parasites, (iii) the predicted proteinase cleavage sites are also diverse in sequence and, in two instances, no consensus cleavage site was identifiable at the expected position, (iv) a short repeat unit is present, which is the only one containing a consensus N-glycosylation site and (v) a C-terminal extension peptide is encoded which shows no similarity to that from A. suum ABA-1. Comparison of independent cDNAs revealed slight variations in the sequence of the gene within the parasite population. Antisera to recombinant DvA-1 polypeptide identified 14-15-kDa antigens in both parasite somatic and excretory-secretory material. DvA-1 is the only NPA for which the complete coding sequence is available and the new principles which it illustrates may lie unsuspected in the NPA-encoding genes of all nematode parasites.

Protective immunisation of guinea pigs against Dictyocaulus viviparus using excretory/secretory products of adult parasites

Parasite preparations were examined for their ability to induce protective immunity against Dictyocaulus viviparus in guinea pigs. Dunkin-Hartley strain guinea pigs were immunised with somatic extracts of adult parasites, somatic extracts of third stage larvae or excretory/secretory (ES) products from adult parasites. The groups were immunised twice with Freund's adjuvant four weeks apart and challenged with 6000 infective L3. Significant levels of protective immunity were observed only in the adult ES-immunised animals. The antibody responses of the different groups were compared following analysis by ELISA and immunoprecipitation. To examine the protective role of antibody, guinea pigs were passively immunised with serum from animals immunised with adult ES products or serum from guinea pigs exposed to experimental D. viviparus infection. Following challenge with infective L3, lung-worm burdens of these groups were significantly lower than in guinea pigs which received normal sera. The results suggest that D. viviparus adult ES products contain protective antigens and that antibody-mediated mechanisms contribute to immune protection.

Identification and isolation of a 19.7 kDa Ostertagia ostertagi specific antigen and evaluation of its potential for immunodiagnosis

Ostertagia ostertagi adult worm extracts were analysed by Western blotting using sera from calves experimentally infected with O. ostertagi and Cooperia oncophora. Strong differences in antigen recognition were noticed, even between animals from the same group. Two Ostertagia specific antigens with apparent molecular mass of 19.7 (OoA19.7) and 20.7 kDa (OoA20.7) were identified. One of them (OoA19.7) was purified by three subsequent chromatographic steps, i.e. gelfiltration, ion-exchange and reversed phased chromatography. It was demonstrated that this antigen does not show any cross-reactions with heterologous sera from C. oncophora, Dictyocaulus viviparus, Nematodirus helvetianus and Fasciola hepatica-infected animals. It was found that only IgG1 antibodies reacted against OoA19.7. The application of this antigen in an ELISA resulted in a highly species-specific test when compared to crude worm extracts. However, strong individual differences in anti-OoA19.7 response could be noticed between calves which received the same number of O. ostertagi larvae. These individual differences can hinder the application of the ELISA as a diagnostic tool, since the anti-OoA19.7 response does not seem to reflect the level of exposure to L3 larvae. This was supported by the absence of a clear infection-dose-related effect. It was shown that the anti-OoA19.7 response started from week 6 to 8, and reached its highest level at week 15.

The immunogenicity of the acetylcholinesterases of the cattle lungworm Dictyocaulus viviparus

Somatic extracts and excretory/secretory (ES) products of the adult stage of the cattle nematode, Dictyocaulus viviparus, were examined for acetylcholinesterase (AChE) activity. Both were found to contain activity which had an optimum pH of 9.5, however, the adult ES products contained over 200 times more AChE activity per unit protein. Gel electrophoresis and specific enzyme staining revealed 5 migratory isoforms of AChE which were common to adult ES products and adult homogenates. Comparison of L3 with L4 and adult extracts indicated that the AChE were only produced by later developmental stages of this parasite. The antigenicity of D. viviparus AChE was demonstrated by binding to serum IgG from naturally and experimentally infected calves but the enzymes were not recognized by calves vaccinated twice with 400 Gy-irradiated larvae. This is the first report of helminth AChE release by a parasitic nematode in a pulmonary location. The presence of these enzymes in such high amounts in the ES products, along with their immunogenicity, suggests that they might have an important role to play in the immunobiology of D. viviparus in the lungs.

Genetic control of the antibody repertoire against excretory/secretory products and acetylcholinesterases of Dictyocaulus viviparus

Outbred Dunkin-Hartley and inbred strain 2 and strain 13 guinea pigs were immunized with Dictyocaulus viviparus adult ES products prior to challenge with third stage larvae. Antibody responses of the three strains to adult ES products and the acetylcholinesterase (AChE) isoforms which they contain were examined. Using immunoprecipitation and ELISA, it was observed that responses in the three strains to adult ES products were distinct: considerable heterogeneity in the antibody repertoire was observed between outbred Dunkin-Hartley animals, with only slight variation occurring amongst the inbred individuals. Responses to the AChE isoforms were heterogeneous amongst individual outbred guinea pigs but were more consistent in inbred strain 2 and 13 animals in which strain-specific patterns of recognition were observed. Previous studies with nematode infections have indicated a role for the major histocompatibility complex in determining the nature and level of the immune response. As the inbred strains bear different alleles at the Class II region but are identical at the Class I region, the differences observed are likely to be due to genes mapping to the Class II locus. This is therefore the first report of genetic restriction of the antibody repertoire to secreted AChEs of a parasitic nematode.

Characterisation of Ostertagia ostertagi antigens by the different bovine immunoglobulin isotypes

Antigenic differences between the developmental stages of Ostertagia ostertagi were studied by SDS-PAGE and Western blotting. Gel electrophoresis showed a complex protein pattern different for every stage with the O ostertagi fourth stage larvae (L4) showing an intermediate protein pattern between the third stage larvae (L3) and the adult stage. Immunoblotting showed that IgG1, IgG2 and IgM immunoglobulins present in serum from uninfected calves identified several O ostertagi antigens at every stage. When using serum from O ostertagi infected calves, O ostertagi specific IgG1 was the predominant bovine immunoglobulin. Specific IgG2 and IgM responses were also observed, while specific IgA antibodies were hardly detectable. Severe IgG1 cross reactivity was demonstrated when using anti-Cooperia oncophora serum.

Characterization of excretory-secretory products of adult Dictyocaulus viviparus and the antibody response to them in infection and vaccination

In vitro released products of the adult stage of the bovine lungworm. Dictyocaulus viviparus, were characterized according to their SDS-PAGE profile, glycosylation pattern, in vitro synthesis and antigenicity in the context of infection and vaccination with irradiated larvae. Biosynthetic labelling experiments with 35S-methionine indicated active synthesis of ES throughout this time. There was, however, little incorporation of 3H-glucosamine into ES products, and lectin affinity chromatography and glycopeptidase F digestion identified only one glycosylated component. Immunoprecipitation of 125I-labelled ES products with sera from calves patently infected with D. viviparus demonstrated that all of these, with the exception of two components, are antigenic to the bovine host. One of those not immunoprecipitated was shown to be host serum albumin carried over into culture. A limited degree of cross-reactivity between nematode species was observed, with a D. viviparus female-specific antigen of 290 kDa being recognized by serum antibody from calves infected with the gastrointestinal nematodes Cooperia oncophora and Ostertagia ostertagi. Calves vaccinated with irradiated larvae of D. viviparus, despite not being exposed to the adult stage of the parasite, also showed some recognition of adult ES products. This might suggest that vaccination with irradiated larvae operates against both pre-pulmonary and pulmonary stages of the infection.

Identification and purification of Cooperia oncophora-specific antigens to improve serological diagnosis

Cooperia oncophora total adult extracts were examined by Western blotting with sera from C. oncophora- and O. ostertagi-infected calves to determine species-specific antigens. It was shown that two antigens with apparent molecular weights of 14.2 and 14.9 kDa were only recognized by calves which received a Cooperia infection and not by Ostertagia mono-infected calves or parasite-naive animals. The partial purification of these two antigens was achieved by gel filtration and ion-exchange chromatography. An enzyme-linked immunosorbent assay (ELISA) was developed based on the fractions containing these two antigens and no cross-reactivity could be noticed with serum from Ostertagia mono-infected calves. In contrast, the ELISA with total worm extracts showed strong cross-reactivity with heterologous serum. It was concluded that the 14.2 and 14.9 kDa Cooperia adult antigens have diagnostic potential, at least to differentiate C. oncophora and O. ostertagi.