Progress in recombinant vaccine development against coccidiosis. A review and prospects into the next millennium.

The increasing problems encountered by the poultry industry, despite the extensive use of drugs, have emphasised the need for an immunological solution for the economic damage caused by the Eimeria parasite. Although immunity develops relatively fast following a natural infection, to induce protection by using parasite extracts or single antigens appears more difficult. Nevertheless, the development of a vaccine based on defined antigens seems the best solution in the long run. At the VIth International Coccidiosis Conference in 1993 the first promising results were reported from small-scale experiments using recombinant antigens. This review summarises the advances in this field of research from 1993 onwards. Although since then not many reports have been published about the effects of using recombinant. antigens as a vaccine against coccidiosis, a number of interesting new proteins which could be considered good targets for such a vaccine have been described and are referred to herein. Proteins involved in the process of invasion of the host cell by the extracellular parasite are regarded as key components in the developmental cycle of the parasite. These components possibly bind to receptors on the host cell. Interference with this process could be a target of the protective immune response. Progress has also been made in characterising the immune mechanisms activated by infection with the parasite. From experimental mouse models and from studies in chickens, a better insight has been obtained towards the involvement of CD4- and CD8-type T cells in, respectively, the inductor and the effector branch of the immune response, although not all questions have been answered. Several antigens have been selected using T-cell stimulation and cytokine assays and these are reviewed. In a third section, mostly unpublished results of our own experiments dealing with the use of live vectors to present defined antigens such as Ea1A and EaSC2, a parasite refractile body transhydrogenase and a lactate dehydrogenase, respectively, are summarised. Partial protection could be induced using Salmonella typhimurium as a carrier for these antigens, in that the oocyst output was reduced by up to 50% after challenge and weight gain could be improved by 5-10% over non-vaccinated challenged chickens, when tested in a floor-pen trial. Similar results were obtained when these antigens were presented by viral vectors such as Fowlpox virus or Herpes virus of turkey. These data seem to offer good prospects for the accomplishment of a safe and efficaceous vaccine based on recombinant DNA technology. These expectations are corroborated by recent breakthrough in transfection of related parasites such as Plasmodium and Toxoplasma gondii, and by the increasing amount of genomic information becoming available every day, the impact of which cannot even be estimated yet. These new technologies will allow us to solve the complex problems that we once created ourselves.