Genetic Mapping Identifies Novel Highly Protective Antigens for an Apicomplexan Parasite

Abstract

Apicomplexan parasites are responsible for a myriad of diseases in humans and livestock; yet despite intensive effort, development of effective sub-unit vaccines remains a long-term goal. Antigenic complexity and our inability to identify protective antigens from the pool that induce response are serious challenges in the development of new vaccines. Using a combination of parasite genetics and selective barriers with population-based genetic fingerprinting, we have identified that immunity against the most important apicomplexan parasite of livestock (Eimeria spp.) was targeted against a few discrete regions of the genome. Herein we report the identification of six genomic regions and, within two of those loci, the identification of true protective antigens that confer immunity as sub-unit vaccines. The first of these is an Eimeria maxima homologue of apical membrane antigen-1 (AMA-1) and the second is a previously uncharacterised gene that we have termed ‘immune mapped protein-1’ (IMP-1). Significantly, homologues of the AMA-1 antigen are protective with a range of apicomplexan parasites including Plasmodium spp., which suggest that there may be some characteristic(s) of protective antigens shared across this diverse group of parasites. Interestingly, homologues of the IMP-1 antigen, which is protective against E. maxima infection, can be identified in Toxoplasma gondii and Neospora caninum. Overall, this study documents the discovery of novel protective antigens using a population-based genetic mapping approach allied with a protection-based screen of candidate genes. The identification of AMA-1 and IMP-1 represents a substantial step towards development of an effective anti-eimerian sub-unit vaccine and raises the possibility of identification of novel antigens for other apicomplexan parasites. Moreover, validation of the parasite genetics approach to identify effective antigens supports its adoption in other parasite systems where legitimate protective antigen identification is difficult.

Population, genetic, and antigenic diversity of the apicomplexanEimeria tenellaand their relevance to vaccine development

“…revealed significant strain-specific variation and some lack of heterologous immunoprotection, although the breakthrough in parasite replication was at a level unlikely to lead to occurrence of clinical disease (39). Antigenic diversity has been described elsewhere for E. tenella (40), albeit at a much lower level than reported for species such as E. maxima and Eimeria acervulina (14,41,42). The relevance of these findings to the other Eimeria species that infect poultry is not currently clear.…”

“…As recombinant or vectored subunit anticoccidial vaccines become technically more feasible, the regional occurrence of each Eimeria species and the extent of vaccine-targeted antigenic diversity assume greater relevance (5,14). Under experimental conditions, studies with clonal reference strains of Eimeria have found antigens such as AMA1 to induce good levels of immunoprotection (14,26).…”

“…As recombinant or vectored subunit anticoccidial vaccines become technically more feasible, the regional occurrence of each Eimeria species and the extent of vaccine-targeted antigenic diversity assume greater relevance (5,14). Under experimental conditions, studies with clonal reference strains of Eimeria have found antigens such as AMA1 to induce good levels of immunoprotection (14,26). AMA1 also is an effective vaccine candidate in related apicomplexans including Neospora caninum, T. gondii, and several Plasmodium species (27)(28)(29)(30), but its value for Plasmodium has been undermined by high levels of naturally occurring antigenic diversity (31,32).…”

“…Each Eimeria species expresses between 6,000 and 9,000 proteins throughout its lifecycle (13), exposing the host to a complex portfolio of antigens. Genetic mapping studies with Eimeria maxima indicated that at least six loci associate absolutely with susceptibility to strong natural immune responses, and additional loci under partial selection also were detectable (14). This breadth of antigenic repertoire is likely to have limited the impact of diversifying selection on the efficacy of live vaccines and explains why resistance has not developed.…”

“…Experimental testing of subunit anticoccidial vaccines containing recombinant antigens such as apical membrane antigen-1 (AMA1) has prompted interest in the extent of naturally occurring genetic diversity in these antigens and has raised the question of whether polymorphism might result in vaccine breakthrough by field parasite populations (14). We sequenced and assembled full-length AMA1-coding sequences for 55 E. tenella samples recovered from four continents (accession nos.…”