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Porphyromonas gingivalis is a major bacterial species implicated in chornic periodontitis, a disease characterized by inflammatory destruction of the tooth supporting tissues. Its main virulence factors are lipopolysaccharide (LPS) and gingipains, a group of cysteine proteinases. Interleukin (IL)-18 is a potent pro-inflammatory cytokine with structural similarities to IL-1beta. This study aimed to investigate if P .gingivalis regulates IL-1beta and IL-18 in monocytic cells. Monomac-6 cells were challenged with P. gingivalis culture supernatants. Quantitative real-time PCR and ELISA were used to investigate IL-1beta and IL-18 mRNA expression and protein secretion, respectively. P. gingivalis enhanced IL-1beta and IL-18 mRNA expression, the former being induced earlier, but transiently. IL-18 up-regulation was not affected by P. gingivalis heat-inactivation or chemical inhibition of its gingipains, whereas both treatments resulted in 50% reduction of IL-1beta expression. Purified P. gingivalis LPS enhanced both IL-1beta and IL-18 expression. However, only IL-1beta, but not IL-18, secretion was detected, and was up-regulated by P. gingivalis. In conclusion, although IL-1beta and IL-18 belong to the same cytokine family, their gene expression and secretion are differentially regulated in human monocytic cells in response to P. gingivalis. Therefore, cytokines of the IL-1 family may participate via different pathways in the complex pathogenesis of periodontitis.
Hamedi et al. (2008) studied this question.