Submitted to: International Journal for Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 28, 1998
Publication Date: N/A
Interpretive Summary: Protozoa in the phylum Apicomplexa include parasites which cause debilitating diseases of man and animals. These parasites include agents of malaria, toxoplasmosis, neosporosis, cryptosporidiosis, and coccidiosis. Although many drugs have been produced to combat these diseases, widespread resistance amongst the protozoa has developed. There is an urgent need to develop alternative strategies to combat these protozoa. One such approach, which is the subject of this review, is to develop vaccines for immunizing susceptible individuals against disease. The successful development of attenuated live vaccines or subunit vaccines containing selected antigens of the parasite requires a thorough understanding of host-parasite interactions. The many factors leading to immunity against protozoa include host genetics, developmental stage and antigens of the parasite that elicit immunity, and the immune cells involved in resistance. Based on research conducted over the last ten years on several of these parasites leads to the conclusion that induction of cellular immunity is critical to protection. Also, actively metabolizing parasites are necessary and sufficient for induction of protective immunity. The advent of gene injection techniques and development of live replicating delivery vectors, such as fowlpox virus, should provide reagents for production of effective subunit vaccines against these protozoa.
The past ten years of research aimed at developing subunit vaccines against a number of apicomplexans, including Eimeria, Plasmodium, and Toxoplasma, has, if anything, revealed the complex nature of parasite-host interactions. The knowledge gained from this research has shown why developing a subunit vaccine based on a single recombinant antigen from one developmental stage of the parasite was an overly optimistic approach. Many apicomplexan parasites have acquired unique strategies to evade host immunity. The variable expression of genes encoding erythrocyte membrane protein 1 of P. falciparum exemplifies one such strategy. The particular mechanism for evading immune destruction depends on a number of interrelated factors not least of which is the parasite life cycle and the availability of susceptible hosts. The goal of any vaccine, be it an attenuated organism or a recombinant antigen, is to break the cycle of infection. The development of a recombinant vaccine against apicomplexan parasites will depend on identifying those antigens and intracellular processes that are vital to the parasite survival and those which exist merely as a way of evading immunity. The information that follows is a review of both molecular biology/biochemistry of eimerian parasites and factors that influence host immune responses to coccidia.