Cytokine Induction by Porphyromonas gingivalis Depends on Epithelial Cell Type

Objectives: Infection of epithelial cells with Porphyromonas gingivalis (Pg) results in production of pro-inflammatory cytokines involved in the initiation and progression of periodontal disease. Interleukin-6 (IL-6) is a pleiotropic cytokine that stimulates immunoglobulin secretion and plays an important role in regulating immune responses to periodontal pathogens. Interleukin-8 (IL-8) is a potent chemoattractant inducing the influx of neutrophils into periodontal lesions. The objective of the study was to compare IL-6 and IL-8 responses of three human oral epithelial cell lines to two Pg strains.

Methods: Pg strains, 2561 and W83, were sub-cultivated on blood agar plates and suspended in Medium 199. Non-tumor-derived immortalized oral epithelial GMSM-K cells, and oral squamous cell carcinoma, HSC-3 and H413 cells, were exposed to live and heat-inactivated Pg at 10⁸ bacteria/well (inoculation ratio ~500 bacteria per cell), and incubated at 37^°C for 6 and 24 h. IL-6 and IL-8 were determined by ELISA.

Results: The levels of IL-8 produced by GMSM-K cells were much lower than that produced by HSC-3 and H413 cells. Live Pg induced significant IL-6 and IL-8 secretion in GMSM-K and HSC-3 cells. Heat-inactivation of Pg enhanced greatly IL-6 and IL-8 stimulation in these cells. Uninfected H413 cells produced higher levels of IL-6 and IL-8 than HSC-3 and GMSM-K cells, but these cells were not responsive to live Pg. However, heat-inactivated Pg-2561 and Pg-W83 resulted in a 2- to 4-fold increase in IL-6 and IL-8 secretion in H413 cells.

Conclusions: The amount of IL-6 and IL-8 secreted by control cells and their response to Pg was strongly dependent on the cell type; thus, conclusions on cytokine responses to Pg should not be based on studies with a single cell type. Heat-inactivated Pg strains stimulated higher IL-6 and IL-8 secretion, suggesting an effect of Pg cysteine proteinases (gingipains) on both cytokines.