Integrative comparative genomics and transcriptomics reveal key roles of SAG17 and SAG23 in early-stage virulence divergence of Eimeria tenella

Eimeria tenella is one of the major pathogens threatening the poultry industry. However, the molecular mechanisms underlying the divergence of virulence among geographic strains remain poorly understood. In this study, clinical phenotypes, histopathology, cytokine assays, whole-genome resequencing, and transcriptome analyses were integrated to systematically compare the Beijing and Guizhou strains. The results showed that infection with the Guizhou strain caused more severe clinical symptoms and intestinal lesions in chickens, accompanied by stronger interleukin (IL)-17 and tumor necrosis factor (TNF)-α responses. Genomic variation analysis revealed abundant nonsynonymous and promoter mutations in the surface antigen (SAG) membrane protein family of the Guizhou strain, and transcriptomic data further confirmed high sporozoite-stage expression of SAG5, SAG13, SAG17, and SAG23, which strongly correlated with virulence phenotypes. Functional validation demonstrated that the recombinant SAG17 and SAG23 proteins significantly inhibited invasion by the E. tenella sporozoites and enhanced levels of the host's interferon (IFN)-γ and immunoglobulin G (IgG), with SAG17 showed stronger immunostimulatory effects than SAG23. Collectively, this study identified SAG17 and SAG23 as key molecules driving virulence divergence of E. tenella geographic strains, providing molecular evidence for the mechanisms of parasite virulence and highlighting important targets for the development of novel vaccines and anticoccidial drugs.