Systematic analysis of the SsrAB virulon of Salmonella enterica

Salmonella pathogenicity island 2 expression negatively controlled by EIIANtr-SsrB interaction is required for Salmonella virulence

SsrA/SsrB is a primary two-component system that mediates the survival and replication of Salmonella within host cells. When activated, the SsrB response regulator directly promotes the transcription of multiple genes within Salmonella pathogenicity island 2 (SPI-2). As expression of the SsrB protein is promoted by several transcription factors, including SsrB itself, the expression of SPI-2 genes can increase to undesirable levels under activating conditions. Here, we report that Salmonella can avoid the hyperactivation of SPI-2 genes by using ptsN-encoded EIIA(Ntr), a component of the nitrogen-metabolic phosphotransferase system. Under SPI-2-inducing conditions, the levels of SsrB-regulated gene transcription increased abnormally in a ptsN deletion mutant, whereas they decreased in a strain overexpressing EIIA(Ntr). We found that EIIA(Ntr) controls SPI-2 genes by acting on the SsrB protein at the posttranscriptional level. EIIA(Ntr) interacted directly with SsrB, which prevented the SsrB protein from binding to its target promoter. Finally, the Salmonella strain, either lacking the ptsN gene or overexpressing EIIA(Ntr), was unable to replicate within macrophages, and the ptsN deletion mutant was attenuated for virulence in mice. These results indicated that normal SPI-2 gene expression maintained by an EIIA(Ntr)-SsrB interaction is another determinant of Salmonella virulence.

Efficacy of intracellular activated promoters for generation of Salmonella-based vaccines

Salmonella enterica is a versatile vaccine carrier for heterologous antigens. One strategy for vaccine antigen delivery is the use of live attenuated S. enterica strains that translocate heterologous antigens into antigen-presenting cells by means of type III secretion systems (T3SS). The feasibility of this approach has been demonstrated in various experimental vaccination studies. The efficacy of recombinant live vaccines is critically influenced by the optimal level of attenuation and many other factors. For the rational design of approaches involving translocation by T3SS, additional parameters are the level of expression of the heterologous antigens and the selection of carrier proteins for the delivery of antigens to desirable subcellular compartments of the target cell. We deployed the Salmonella pathogenicity island 2 (SPI2)-encoded T3SS for antigen delivery. The SPI2-T3SS and effector proteins are encoded by members of the large SsrAB regulon, including promoters with highly variable strength of expression. We investigated the effect of various in vivo-activated promoters of the SsrAB regulon on the efficacy of recombinant Salmonella vaccines. We observed that the use of promoters with higher strength results in greater synthesis of recombinant antigens and greater stimulation of T-cell responses in cell culture assays for the stimulation of T cells by the model antigen ovalbumin. In contrast, in vaccination experiments, promoters with a low level of expression resulted in the induction of higher amounts of T cells reactive to the model antigen listeriolysin. These results demonstrate that high-level expression of heterologous antigens does not necessarily result in optimal stimulation of immune responses.

The inflammatory cytokine tumor necrosis factor modulates the expression of Salmonella typhimurium effector proteins

Tumor necrosis factor alpha (TNF-alpha)is a host inflammatory factor. Bacteria increase TNF-alpha expression in a variety of human diseases including infectious diseases, inflammatory bowel diseases, and cancer. It is unknown, however, how TNF-alpha directly modulates bacterial protein expression during intestinal infection and chronic inflammation. In the current study, we hypothesize that Salmonella typhimurium senses TNF-alpha and show that TNF-alpha treatment modulates Salmonella virulent proteins (called effectors), thus changing the host-bacterial interaction in intestinal epithelial cells. We investigated the expression of 23 Salmonella effectors after TNF-alpha exposure. We found that TNF-alpha treatment led to differential effector expression: effector SipA was increased by TNF-alpha treatment, whereas the expression levels of other effectors, including gogB and spvB, decreased in the presence of TNF-alpha. We verified the protein expression of Salmonella effectors AvrA and SipA by Western blots. Furthermore, we used intestinal epithelial cells as our experimental model to explore the response of human intestinal cells to TNF-alpha pretreated Salmonella. More bacterial invasion was found in host cells colonized with Salmonella strains pretreated with TNF-alpha compared to Salmonella without TNF-alpha treatment. TNF-alpha pretreated Salmonella induced higher proinflammatory JNK signalling responses compared to the Salmonella strains without TNF-alpha exposure. Exposure to TNF-alpha made Salmonella to induce more inflammatory cytokine IL-8 in intestinal epithelial cells. JNK inhibitor treatment was able to suppress the effects of TNF-pretreated-Salmonella in enhancing expressions of phosphorylated-JNK and c-jun and secretion of IL-8. Overall, our study provides new insights into Salmonella-host interactions in intestinal inflammation.