Vaccination against cestode parasites: anti-helminth vaccines that work and why.

Highly effective recombinant vaccines have been developed against the helminth parasites Taenia ovis, Taenia saginata and Echinococcus granulosus. These vaccines indicate that it is possible to achieve a reliable, high level of protection against a complex metazoan parasite using defined recombinant antigens. However, the effectiveness of the vaccines against the taeniid cestodes stands in contrast to the more limited successes which characterise attempts to develop vaccines against other platyhelminth or nematode parasites. This review examines the features of the host-parasite relationships among the taeniid cestodes which have formed the basis for vaccine development. Particular consideration is given to the methodologies that have been used in making the cestode vaccines that might be of interest to researchers working on vaccination against other helminths. In developing the cestode vaccines, antigens from the parasites' infective larval stage contained within the egg (oncosphere) were identified as having the potential to induce high levels of protection in vaccinated hosts. A series of vaccination trials with antigen fractions, and associated immunological analyses, identified individual protective antigens or fractions. These were cloned from cDNA and the recombinant proteins expressed in Escherichia coli. This strategy was independently successful in developing vaccines against T. ovis and E. granulosus. Identification of protective antigens for these species enabled rapid identification, cloning and expression of their homologues in related species and thereby the development of effective vaccines against T. saginata, E. multilocularis and, more recently, T. solium. The T. saginata vaccine provides an excellent example of the use of two antigen components, each of which were not protective when used individually, but when combined they induce a reliable, high level of protection. One important contributing factor to the success of vaccine development for the taeniid cestodes was the concentration on studies seeking to identify native host-protective antigens, before the adoption of recombinant methodologies. The cestode vaccines are being developed towards practical (commercial) application. The high level of efficacy of the vaccines against T. solium cysticercosis and hydatid disease suggests that they would be effective also if used directly in humans.