Treponema pallidum flagellin FlaA2 induces IL-6 secretion in THP-1 cells via the Toll-like receptor 2 signaling pathway

Treponema pallidum subsp. pallidum membrane proteins are considered as potent inducers in the initiation and development of inflammation. In the present study, the mechanism that leads to the production of interleukin 6 (IL-6), one of the key proinflammatory cytokines, by human monocytic THP-1 cells when these cells are treated with T. pallidum flagellin FlaA2 was investigated. Stimulation with flagellin FlaA2 can induce IL-6 expression in human monocytes and augment the phosphorylation of ERK, p38, and NF-κB, but has no effect on the phosphorylation of JNK. Likewise, FlaA2-induced IL-6 production was found to be attenuated by inhibitors for ERK, p38, and NF-κB, but not by JNK inhibitor. Immunofluorescence analysis showed that flagellin FlaA2 could stimulate the translocation of IκBα from the cytosol to the nucleus, and this phenomenon could be inhibited by the specific inhibitor BAY11-7082. FlaA2–induced IL-6 expression was also proved to be abrogated by transfection with dominant negative (DN) plasmid of MyD88. We further demonstrated that transfection with DN-TLR2 was sufficient to attenuate IL-6 expression and the phosphorylation of ERK, p38, and IκBα. These results suggest that flagellin FlaA2 induces IL-6 production via signaling pathways involving TLR2, MyD88, ERK, p38, and NF-κB in monocytes, which could contribute to the pathogenesis of T. pallidum.