The molecular basis for the distinct host and tissue tropisms of coccidian parasites

The phylum Apicomplexa is home to a variety of parasites of significant medical and economic relevance, including the coccidian species Toxoplasma gondii, Neospora caninum and Eimeria tenella. In spite of their shared ancestry, the aforementioned coccidian species exhibit highly variable host and tissue tropisms; whilst T. gondii invades a broad spectrum of cell types and host organisms, E. tenella infection is restricted to the caecum in chicken. apicomplexans are obligatory intracellular parasites, and are uniquely adapted for host cell invasion via several conserved features. The process of initial host cell recognition and attachment is governed by the regulated deployment of surface microneme proteins (MICs), which therefore are likely to be major determinants of the host and tissue tropism of each parasite. Structural and functional data are now available for several coccidian MICs, providing insights into their receptor specificities and modes of recognition in atomic detail. Here, detailed analysis of these data has been performed, encouraging rationalization of the marked differences in the host and tissue tropism. We have observed that T. gondii expresses a wide repertoire of MICs, binding a broad range of oligosaccharide epitopes, including a unique preference for a α2,9-disialyl terminated receptor. By contrast, the MIC repertoire of Neospora caninum appears to be more restrictive, and even further so in E. tenella, correlating with the reduced tropisms of these parasites.