Salmonella virulence factor SpiC is involved in expression of flagellin protein and mediates activation of the signal transduction pathways in macrophages

SpiC is a virulence factor encoded within Salmonella pathogenicity island 2 (SPI-2). We have previously reported that infection of macrophages with Salmonella enterica serovar Typhimurium results in the SPI-2-dependent activation of the mitogen-activated protein kinase (MAPK) signalling pathways, leading to the expression of suppressor of cytokine signalling (SOCS)-3, which is involved in the inhibition of cytokine signalling. Here, we investigated the mechanism by which SpiC mediates the activation of signal transduction pathways in macrophages. Proteomic analysis showed that the level of FliC protein, a component of the flagellar filaments, was lower in the culture supernatant of a spiC mutant than in the supernatant from wild-type Salmonella. Furthermore, quantitative real-time RT-PCR showed that this mutant had a much lower level of fliC mRNA, indicating that SpiC regulates the transcription of FliC. We also found that the level of SOCS-3 in J774 macrophages was lower when they were infected with the fliC mutant than with wild-type Salmonella. FliC participated in the activation of MAPK pathways in Salmonella-infected macrophages, leading to the upregulation of SOCS-3 expression. Collectively, these results indicate that SpiC is involved in the expression of FliC, which plays a significant role in the SPI-2-dependent activation of MAPK pathways in Salmonella-infected macrophages.