Innate Immune System Activation by Treponema denticola Periplasmic Flagella

Treponema denticola's increased presence within the periodontium of the host is associated with localized inflammation. Previous studies have indicated that T. denticola can stimulate the innate immune system through Toll-like receptor (TLR) 2, however, the pathogen-associated molecular patterns responsible for T. denticola's activation of the innate immune system are currently unknown. Objectives: In this study we investigated and characterized the role T. denticola periplasmic flagella (PF) play in stimulating an innate immune response. Methods: T. denticola cells were grown anaerobically in NOS media to late logarithmic growth (4 days), washed in phosphate buffer and brought to an OD₆₀₀ = 1.0. Cytokine profiles of monocytes from human peripheral blood mononuclear cells were analyzed in response to T. denticola ATCC 35405 and its isogenic non-motile mutant T. denticola HL51 lacking PF. In addition, sheared PF from T. denticola wild-type cells were isolated, purified, and assessed for TLR-usage using TLR2-, TLR4-, and TLR5-transfected HEK (human embryonic kidney) 293 cell lines. Cytokine responses of human monocytes to purified PF were also determined. Results: We demonstrated that the ability of T. denticola to stimulate the production of inflammatory mediators was largely dependent upon the presence of PF. Moreover, we determined that purified PF stimulated the innate immune system via TLR2, and enhanced cytokine production in human monocytes. Conclusion: These findings demonstrate that T. denticola stimulates innate immune cells in a TLR2-dependent fashion, and that PF are a key bacterial component involved in this process. This study was supported by funding from NIDCR Grant# R03DE016040-02