Identification of an Apically‐Located Antigen That Is Conserved in Sporozoan Parasites

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Abstract
Sporozoan parasites of the phylum Apicomplexa all possess common apical structures. The current study used a monoclonal antibody (mAb‐E12) to identify a conserved antigen in the apical region of merozoites of seven species of Plasmodium (including rodent, primate and human pathogens), tachyzoites of Toxoplasma gondii, bradyzoites of Sarcocystis bovis, and sporozoites and merozoites of Eimeria tenella and E. acervulina. The antigen was also present in sporozoites of haemosporinid parasites. Immunofluorescence studies showed that the antigen was restricted to the apical 3rd of these invasive stages. Using immunoelectron microscopy, labeling was demonstrated in the region of the polar ring, below the paired inner membranes of the parasite pellicle, and near the subpellicular microtubules radiating from the polar ring of merozoites and sporozoites of E. tenella. The majority of the antigen could be extracted with 1% Triton‐X 100, but a portion remained associated with the cytoskeletal elements. The molecule has a relative rate of migration (Mr) of 47,000 in Plasmodium spp. and 43–46,000 in coccidian species. Since the epitope recognized by mAb‐El 2 is highly conserved, restricted to motile stages, and appears to be associated with microtubules, this antigen could be involved in cellular motility and cellular invasion.