Elongation Factor-1α Is a Novel Protein Associated with Host Cell Invasion and a Potential Protective Antigen of Cryptosporidium parvum

Background: Apicomplexan parasites, including Cryptosporidium, possess organelles associated with host cell invasion. Results: A chicken-derived monoclonal antibody inhibited host cell invasion by C. parvum. The corresponding antigen, which localizes to the apical region of the parasite, was identified as elongation factor-1 (EF-1). Conclusion: EF-1 mediates cryptosporidial cytoskeletal complex critical for host cell invasion. Significance: The findings suggest that cryptosporidial EF-1 could serve as a novel anti-infective target antigen. The phylum Apicomplexa comprises obligate intracellular parasites that infect vertebrates. All invasive forms of Apicomplexa possess an apical complex, a unique assembly of organelles localized to the anterior end of the cell and involved in host cell invasion. Previously, we generated a chicken monoclonal antibody (mAb), 6D-12-G10, with specificity for an antigen located in the apical cytoskeleton of Eimeria acervulina sporozoites. This antigen was highly conserved among Apicomplexan parasites, including other Eimeria spp., Toxoplasma, Neospora, and Cryptosporidium. In the present study, we identified the apical cytoskeletal antigen of Cryptosporidium parvum (C. parvum) and further characterized this antigen in C. parvum to assess its potential as a target molecule against cryptosporidiosis. Indirect immunofluorescence demonstrated that the reactivity of 6D-12-G10 with C. parvum sporozoites was similar to those of anti-- and anti--tubulins antibodies. Immunoelectron microscopy with the 6D-12-G10 mAb detected the antigen both on the sporozoite surface and underneath the inner membrane at the apical region of zoites. The 6D-12-G10 mAb significantly inhibited in vitro host cell invasion by C. parvum. MALDI-TOF/MS and LC-MS/MS analysis of tryptic peptides revealed that the mAb 6D-12-G10 target antigen was elongation factor-1 (EF-1). These results indicate that C. parvum EF-1 plays an essential role in mediating host cell entry by the parasite and, as such, could be a candidate vaccine antigen against cryptosporidiosis.