Porphyromonas gingivalis, a major periodontopathogen, is involved in the pathogenesis of periodontitis. Interleukin-1␤ (IL-1␤), a proinflammatory cytokine, regulates innate immune responses and is critical for the host defense against bacterial infection. However, excessive IL-1␤ is linked to periodontal destruction. IL-1␤ synthesis, maturation, and secretion are tightly regulated by Toll-like receptor (TLR) signaling and inflammasome activation. We found much higher levels of inflammasome components in the gingival tissues from patients with chronic periodontitis than in those from healthy controls. To investigate the molecular mechanisms by which P. gingivalis infection causes IL-1␤ secretion, we examined the characteristics of P. gingivalis-induced signaling in differentiated THP-1 cells. We found that P. gingivalis induces IL-1␤ secretion and inflammatory cell death via caspase-1 activation. We also found that P. gingivalis-induced IL-1␤ secretion and pyroptic cell death required both NLRP3 and AIM2 inflammasome activation. The activation of the NLRP3 inflammasome was mediated by ATP release, the P2X 7 receptor, and lysosomal damage. In addition, we found that the priming signal via TLR2 and TLR4 activation precedes P. gingivalis-induced IL-1␤ release. Our study provides novel insight into the innate immune response against P. gingivalis infection which could potentially be used for the prevention and therapy of periodontitis.

Porphyromonas gingivalis is one of the suspected periodontopathic bacteria. The lipopolysaccharide (LPS) of P. gingivalis is a key factor in the development of periodontitis. Inflammatory cytokines play important roles in the gingival tissue destruction that is a characteristic of periodontitis. Macrophages are prominent at chronic inflammatory sites and are considered to contribute to the pathogenesis of periodontitis. Xylitol stands out and is widely believed to possess anticaries properties. However, to date, little is known about the effect of xylitol on periodontitis. The aim of the present study was to determine tumor necrosis factor alpha (TNF-␣) and interleukin-1␤ (IL-1␤) expression when RAW 264.7 cells were stimulated with P. gingivalis LPS (hereafter, LPS refers to P. gingivalis LPS unless stated otherwise) and the effect of xylitol on the LPS-induced TNF-␣ and IL-1␤ expression. The kinetics of TNF-␣ and IL-1␤ levels in culture supernatant after LPS treatment showed peak values at 1 h (TNF-␣) and 2 to 4 h (IL-1␤), respectively. NF-B, a transcription factor, was also activated by LPS treatment. These cytokine expressions and NF-B activation were suppressed by pretreatment with pyrrolidine dithiocarbamate (an inhibitor of NF-B). Pretreatment with xylitol inhibited LPS-induced TNF-␣ and IL-1␤ gene expression and protein synthesis. LPS-induced mobilization of NF-B was also inhibited by pretreatment with xylitol in a dose-dependent manner. Xylitol also showed inhibitory effect on the growth of P. gingivalis. Taken together, these findings suggest that xylitol may have good clinical effect not only for caries but also for periodontitis by its inhibitory effect on the LPS-induced inflammatory cytokine expression.

Autophagy provides a mechanism for the turnover of cellular organelles and proteins through a lysosome-dependent degradation pathway and is a possible mechanism in inflammatory disease. Periodontitis is an inflammatory disease caused by periodontal pathogens. Porphyromonas gingivalis, an important periodontal pathogen, activates cellular autophagy to provide a replicative niche while suppressing apoptosis in endothelial cells. However, the molecular basis for a causal relationship between P. gingivalis and autophagy is unclear. This research examines the involvement of P. gingivalis in autophagy through light chain 3 (LC3) and autophagic proteins, and the role of P. gingivalis-induced autophagy in the clearance of P. gingivalis and inflammation. To investigate the molecular mechanism of autophagy induced by P. gingivalis, PMA-differentiated THP-1-derived macrophages were infected with live P. gingivalis. The P. gingivalis increased the formation of autophagosomes in a multiplicity of infection-dependent manner, as well as autophagolysosomes. Porphyromonas gingivalis activated LC3-I/LC3-II conversion and increased the conjugation of autophagy-related 5 (ATG5) -ATG12 and the expression of Beclin1. The expressions of Beclin1, ATG5-ATG12 conjugate, and LC3-II were significantly inhibited by the presence of 3-methyladenine, an autophagy inhibitor. Interestingly, 3-methyladenine increased the survival of P. gingivalis and proinflammatory cytokine interleukin-1β production. The data indicate that P. gingivalis induces autophagy in PMA-differentiated THP-1-derived macrophages and in turn, macrophages eliminate P. gingivalis through an autophagic response, which can lead to the restriction of an excessive inflammatory response by downregulating interleukin-1β production. The induction of autophagy by P. gingivalis may play an important role in the periodontal inflammatory process and serve as a target for the development of new therapies.