Molecular mechanisms of surface antigen suppression by ApiAP2 and its implications for vaccine development.

The apicomplexan parasite Eimeria is the causative agent of coccidiosis, which results in enteritis in animals and imposes a significant economic burden on the farming industry. Apicomplexan Apetala2/ERF (ApiAP2) transcription factors are known to play crucial roles in various processes of Apicomplexa parasites. However, little is understood regarding the role of ApiAP2s in Eimeria species. In this study, we performed a functional characterisation of ApiAP2 through CRISPR-Cas9-mediated knockout. Our results revealed that the parasite's invasion efficiency, total oocyst output, and virulence were significantly impaired after EtAP2-S1 depletion. RNA sequencing (RNA-Seq) and Cleavage Under Targets and Tagmentation (CUT&Tag) analyses showed that EtAP2-S1 targets the promoters of numerous genes, and its knockout resulted in the up-regulation of 59 sag genes. Furthermore, the knockout strain exhibited significantly lower virulence while offering excellent immune protection, making it a promising candidate for a vaccine. This study demonstrates that EtAP2-S1 is a fitness-conferring gene that suppresses the expression of sag genes in E. tenella, which is the first step in developing gene-knockout vaccines to control coccidiosis.