DNA from Porphyromonas gingivalis and Tannerella forsythia induce cytokine production in human monocytic cell lines

Toll‐like receptor 9 (TLR9) expression is increased in periodontally diseased tissues compared with healthy sites indicating a possible role of TLR9 and its ligand, bacterial DNA (bDNA), in periodontal disease pathology. Here, we determine the immunostimulatory effects of periodontal bDNA in human monocytic cells (THP‐1). THP‐1 cells were stimulated with DNA of two putative periodontal pathogens: Porphyromonas gingivalis and Tannerella forsythia. The role of TLR9 in periodontal bDNA‐initiated cytokine production was determined either by blocking TLR9 signaling in THP‐1 cells with chloroquine or by measuring IL‐8 production and nuclear factor‐κB (NF‐κB) activation in HEK293 cells stably transfected with human TLR9. Cytokine production (IL‐1β, IL‐6, and TNF‐α) was increased significantly in bDNA‐stimulated cells compared with controls. Chloroquine treatment of THP‐1 cells decreased cytokine production, suggesting that TLR9‐mediated signaling pathways are operant in the recognition of DNA from periodontal pathogens. Compared with native HEK293 cells, TLR9‐transfected cells demonstrated significantly increased IL‐8 production (P < 0.001) and NF‐κB activation in response to bDNA, further confirming the role of TLR9 in periodontal bDNA recognition. The results of PCR arrays demonstrated upregulation of proinflammatory cytokine and NF‐κB genes in response to periodontal bDNA in THP‐1 cells, suggesting that cytokine induction is through NF‐κB activation. Hence, immune responses triggered by periodontal bacterial nucleic acids may contribute to periodontal disease pathology by inducing proinflammatory cytokine production through the TLR9 signaling pathway.