Molecular vaccines against animal parasites

Protective effect of rSm28GST-specific T cells in schistosomiasis: role of gamma interferon

Immunization with a single dose of 50 micrograms of recombinant Schistosoma mansoni 28-kDa glutathione-S-transferase (rSm28GST) was able to induce a reduction in the worm burden, the number of eggs, and the degree of hepatic fibrosis as quantified by the measurement of collagen content in the liver of S. mansoni-infected mice. No relationship was found between anti-Sm28GST immunoglobulin G and immunoglobulin A titers and the levels of protection obtained. Adoptive transfers of Sm28GST-specific total, CD4+, or CD8+ T cells reproduced the protective effect obtained with the recombinant molecule. Moreover, experiments studying in vivo T-cell depletion demonstrated that anti-CD4- or anti-CD8-treated mice showed a significant decrease in the protective effect conferred, suggesting a role of the two T-cell subpopulations in the expression of Sm28GST-mediated protection against hepatic damage. Sm28GST-specific cells produced little interleukin-4 and high levels of gamma interferon. Treatment of immunized mice with anti-gamma interferon antibody totally suppressed the Sm28GST-induced protective effect and led to the rapid death of infected animals, suggesting a role for this cytokine in the expression of the protective immunity obtained after immunization with rSm28GST.

Vaccination of goats against the trematode Schistosoma bovis with a recombinant homologous schistosome-derived glutathione S-transferase

We assayed the vaccine potentialities of a recombinant S. bovis-derived glutathione S-transferase (rSb28GST), member of a molecular family already shown to have protective capacities in the S. mansoni and S. japonicum models. Injection of the rSb28GST in Freund's Complete Adjuvant resulted in good specific IgG responses allowing all the animals to display high antibody titres on the day of experimental challenge with S. bovis cercariae. No statistically significant differences were observed in the faecal egg output. Although tissue egg counts in vaccinated animals were lower than in controls, the difference was not statistically significant, apart from the number of eggs trapped in the liver (P < 0.05). Likewise, PCV values remained parallel between the two groups. However, immunized goats gained 1.4 kg of body weight throughout the experiment whereas controls lost 1.2 kg (P < 0.05). In addition, the mean worm burden, assessed by perfusion 20 weeks after infection, was significantly reduced by 48% in the vaccinated group, the sex ratio being unaffected. It appears that a recombinant homologous protein can affect, in a natural host, the course of an experimental infection with a local strain of S. bovis, by affecting worm viability but not fecundity. These results also point to the striking differences in the effect of vaccination according to animal species. Because it has the capacity to prevent growth impairment due to schistosome pathogenicity, the molecule can be proposed as a valuable tool in the development of vaccine-based control programs in endemic areas.

Towards sustainable nematode parasite control of livestock

Farmers worldwide have come to expect, and rely almost exclusively on, broad-spectrum anthelmintics to effectively control nematode parasites amongst their livestock. However, the threats of resistance, residues and ecotoxicity are of increasing concern to the future of chemotherapy. It is imperative that sustainable parasite control schemes be developed and implemented which will integrate a range of techniques to minimise anthelmintic use and still maintain high levels of profitability of the farming enterprise. At present, these need to focus on the better use of existing drugs to maximise their effectiveness and minimise the selection for resistance and impact on the environment. New drugs should also be used according to these principles. In the future it is expected that other non-chemotherapeutic options will become available, e.g. helminth vaccines, resistant hosts, biological control, nematode growth regulators, which will revolutionize the current thinking on nematode parasite control of livestock.

Purification and evaluation of the integral membrane protein H11 as a protective antigen against Haemonchus contortus

A detergent extract of adult Haemonchus contortus enriched in the integral membrane protein H11, previously shown to give protective immunity against the parasite, was fractionated by lectin and ion-exchange chromatography. The fractions were evaluated for their ability to immunize Clun Forest and Dorset Horn sheep against experimental haemonchosis. Most of the protective activity was associated with H11. Used in an approximately 95% pure form it gave a mean reduction in parasite egg output of 94.6% and reduced male and female worm numbers by 86.5 and 93.5%, respectively. Level of protection correlated with serum antibody titre to H11.

Immunization of mice and baboons with the recombinant Sm28GST affects both worm viability and fecundity after experimental infection with Schistosoma mansoni

A member of the glutathione S-transferase family, Sm28GST has previously demonstrated a good ability to protect rodents against experimental infection with Schistosoma mansoni. In order to evaluate its efficacy in a model closer to man, two different protocols of immunization with recombinant Sm28GST were tested on baboons in a large-scale trial. Three injections in the presence of aluminium hydroxide as adjuvant resulted in a significant 38% reduction in the adult worm burden together with a trend for a lower percentage of inflammatory tissue in the liver. Individual levels of protection, ranging from 0 to 80%, underlined the heterogeneity of the immune response to this purified molecule in outbred primates. On the other hand, two injections of Sm28GST in the presence of aluminium hydroxide and Bordetella pertussis reduced female schistosome fecundity by 33%, with a more pronounced effect (66%) on faecal egg output; there was also a trend, in this protocol, for decrease of the mean granuloma surface in the liver. Individual anti-Sm28GST IgG antibodies were apparently unrelated to levels of immunity, but there was partial evidence that cytophilic IgE might play a role in the immune mechanisms affecting worm viability, but not fecundity. In the mouse model, Sm28GST vaccination resulted in a lower hatching ability of tissue eggs recovered from immunized mice whereas passive transfer of specific anti-Sm28GST T-lymphocytes, one day before infection, significantly reduced the number of eggs in the liver of mice. We propose that different protocols of immunization with a recombinant molecule can impede Schistosoma mansoni worm viability and fecundity, but can also affect miracidium physiology, with important consequences for disease transmission and granuloma-derived pathology.

Live attenuated Salmonella vaccines and their potential as oral combined vaccines carrying heterologous antigens

Live attenuated salmonellae are protective, and are candidate vaccines against invasive salmonella infections in man and animals. Different attenuating mutations have been described, and more than one can be incorporated in a vaccine for added safety. Combined salmonella vaccines express target carbohydrate and protein antigens or epitopes from viruses, bacteria and eukaryotic parasites, either within or on the surface of the cell, as capsules, fimbriae, or in the flagellin. Humoral, secretory and cellular responses to the recombinant antigens has been demonstrated. Experimental protection against diseases including streptococcal infection, tetanus, influenza and malaria has been obtained.