Deletion of the type IV secretion system promoter in A19 strain attenuated the virulence of the bacteria and promotes autophagy.

is a gram-negative intracellular parasite bacteria that causes serious health hazards in humans and animals. The type IV secretion system (T4SS), encoded by the promoter, has been identified as an important virulence factor for , but its impact on A19 remains unclear. In this study, the T4SS of A19 was inactivated by deletion of the promoter, resulting in a mutant strain A19Δ. Real-time PCR and western blotting analysis demonstrated that T4SS-related proteins were not expressed after promoter deletion. Moreover, the survival rate of A19 in high-salt and strong acidic environments decreased after promoter deletion. Compared to the parental strain A19, the A19Δ mutant strain showed reduced growth rate in TSB, decreased invasion ability to macrophages and dendritic cells, and reduced virulence of the mutant strain in macrophages, dendritic cells, and mice. In addition, the A19Δ mutant strain showed enhanced autophagy in macrophages and dendritic cells compared with A19, and the A19Δ mutant strain was able to upregulate IL-6 and downregulate IL-10 in macrophages. These data help us to better understand the T4SS of the A19 vaccine strain and contribute to our efforts to improve vaccines.