SspH2 as anti-inflammatory candidate effector and its contribution in Salmonella Enteritidis virulence

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a facultative intracellular pathogen deploying the type III secretion system (T3SS) encoded by Salmonella Pathogenicity Island 2 (SPI2) to transfer effector proteins into host cells to modify its functions and accomplish intracellular replication. To study the effect of SspH2 on immune response induced by S. Enteritidis, we generated a deletion mutant of the effector gene sspH2 and a plasmid mediated complementary strain in S. Enteritidis C50336. The results of LD50 showed that SspH2 has no obvious effect on the virulence of S. Enteritidis. However, deletion of sspH2 decreased the invasion and intercellular colonization of the bacteria in Caco2 BBE cells. Using bacteriological counts from tissue homogenates the result of colonization in internal organs showed that in spleen and liver tissues, at 3rd and 4th day p.i. there is a significance decreased number of C50336-ΔsspH2 compared to the C50336-WT and C50336-ΔsspH2-psspH2, respectively. The qRT-PCR analysis results of both in vivo and in vitro experiments clearly showed that the mutant strain C50336ΔsspH2 significantly promoted expression of IL-1β, INF-γ, IL-12, and iNOS cytokines compared to the groups infected with the wild type or complementary strains, while the IL-8 synthesis was decreased in the mutant strain infected group. All of these findings revealed that SspH2 promotes the colonization of S. Enteritidis in host cells, and it is an important anti-inflammatory biased effector in Salmonella.