Roles of SPI-2 T3SS effectors in virulence of Choleraesuis and Construction of a triple-gene mutant vaccine strain.

Effector protein functions of Type III secretion system (T3SS) encoded by pathogenicity islands 2 (SPI-2) have not been fully characterized in serovar Choleraesuis. This study characterized 21 effectors of SPI-2 T3SS of in terms of macrophage survival and virulence in mice via construction of various gene mutant strains. Eight effector genes including , , and contributed to bacterial survival in macrophage cell line RAW264.7; whereas only also promoted bacterial virulence in mice like other three effector genes , and . The mutant strain, Δ , Δ, Δ, or Δ, led to higher mouse survival compared to the wild-type strain post-oral infection, while their bacterial loads in spleen and liver were not reduced except the Δ that was undetectable in mouse tissues. Then, the triple-gene mutant strain ΔΔ Δ was constructed and found to be virulence attenuated with a compromised colonization ability. Finally, immunization of this mutant orally induced robust serum IgG responses and provided 40% protection against lethal challenge. Our study highlights the critical role of four SPI-2 T3SS effectors in pathogenesis.