Oral bacterial DNA differ in their ability to induce inflammatory responses in human monocytic cell lines

Deoxyribonucleic acids (DNA) of periodontal pathogens, Porphyromonas gingivalis (Pg) and Tannerella forsythia, stimulate cytokine production in human monocytic cells (THP-1) through Toll-like receptor 9 (TLR-9) and nuclear factor-κB signaling. Fusobacterium nucleatum (Fn) is one of the most frequently isolated bacteria in periodontally diseased tissues and is reported to synergize with Pg, enhancing the pathogenicity. We investigate inflammatory mediator production in THP-1 cells challenged with Fn and Streptococcus sanguinis (Ss) DNA, a non-pathogenic oral bacteria, and further assess whether cytokines triggered by whole pathogens or Pg lipopolysaccharide (LPS) are affected by TLR-9 signaling inhibitors (chloroquine).

THP-1 cells were stimulated with Pg-DNA (100 ng/μL), Fn-DNA (100 ng/μL), Ss-DNA (100 ng/μL), Pg-LPS (10 ng/μL), and heat-killed whole bacteria (multiplicity of infection, 1:100) for 16 hours with or without chloroquine pretreatment (10 μg/mL). Interleukin (IL)-1β, IL-6, IL-8, and tumor necrosis factor-α levels were determined using enzyme-linked immunosorbent assay. Statistical analyses included analysis of variance with multiple comparisons using Dunnett or Tukey methods and paired t test. A value of P <0.05 was significant.

Inflammatory mediator levels were increased in response to all the stimuli with the exception of Ss-DNA (P <0.05). Chloroquine pretreatment significantly decreased cytokine production from THP-1 cells with the exception of IL-6 production triggered by whole Fn and Ss (P <0.05).

Differences exist among oral bacterial DNA in inducing immune responses. By altering the conditions in cytosolic compartments, we can interfere with cellular responses triggered by extracellular receptor activation. Thus, alternative treatment approaches targeted to intracellular receptors might be of benefit in controlling periodontal inflammation.