Evaluation of immunoprotective effects of recombinant proteins and DNA vaccines derived from Eimeria tenella surface antigen 6 and 15 in vivo

Coccidiosis is an intestinal parasitic disease that causes huge economic losses to the poultry industry globally. Eimeria tenella belonging to protozoon is the causative agent of cecal coccidiosis in chicken, and it causes enormous damage to poultry industry. The surface antigens (SAGs) of apicomplexan parasites have functions of attachment and invasion in host-parasite interaction. As a result of parasitic invasion, host immune response is triggered. However, the immunogenicity and potency of E. tenella surface antigen 6 and 15 (EtSAG 6 and 15), as vaccinal candidate antigen, remain largely unknown. Therefore, gene fragments of E. tenella EtSAG 6 and 15 were amplified and transformed to pET28a prokaryotic vector for recombinant protein expression. The pEGFP-N1 eukaryotic vectors with EtSAG 6 and 15 amplification fragments (pEGFP-N1-EtSAG 5 and 16) were transformed into 293 T cell line. The results of reverse transcription-polymerase chain reaction (RT-PCR) and western blot analysis revealed successful expressions of EtSAG 6 and 15 in Escherichia coli and 293 T cells. Subsequently, animal experiments of 49 cobb broilers were performed to evaluate immunoprotection of recombinant proteins and DNA vaccines derived from E. tenella EtSAG 5 and 16 with an immunizing dose of 100 μg, respectively. Chickens vaccinated with rEtSAG 6 protein, rEtSAG 15 protein, pEGFP-N1-EtSAG 6 plasmid, or pEGFP-N1-EtSAG 15 plasmid showed no significant increase in IFN-γor interleukin-4 (IL-4) level compared with control groups. Chickens vaccinated with protein rEtSAG 6, protein rEtSAG 15, pEGFP-N1-EtSAG 6 plasmid, or pEGFP-N1-EtSAG 15 exhibited higher weight gains, lower oocyst output, and lower mean lesion scores, compared with infection control group. Among the four immunized groups, plasmid EGFP-N1-EtSAG 6 (100 μg) group exhibited the highest anticoccidial index (ACI) value (150.20). Overall, plasmids EGFP-N1-EtSAG 6 and 15, as DNA vaccines, provided a more effective immunoprotection for chickens against E. tenella than protein rEtSAG 6 and protein rEtSAG 15 as subunit vaccines. EtSAG 6 and 15 are promising candidate antigen genes for developing coccidiosis vaccine.