Background: Periodontitis is generally a chronic disorder characterized by the breakdown of tooth-supporting tissues. P. gingivalis, a Gram-negative anaerobic rod, is one of the major pathogens associated with periodontitis. Frequently, P. gingivalis infection leads to cell death. However, the correlation between P. gingivalis–induced cell death and periodontal inflammation remains to be elucidated. Among cell deaths, the death of immune cells appears to play a significant role in inflammatory response. Thus, the aim of this study was to examine P. gingivalis–induced cell death, focusing on autophagy and apoptosis in THP-1 cells. Methods: Human acute monocytic leukemia cell line (THP-1) was used for all experiments. Autophagy induced by P. gingivalis in THP-1 cells was examined by Cyto ID staining. Intracellular autophagic vacuoles were observed by fluorescence microscopy using staining Acridine orange (AO); and 3-methyladenine (3-MA) was used to inhibit autophagy. Total cell death was measured by LDH assay. Cytokine production was measured by an ELISA method. Results: P. gingivalis induced autophagy in an MOI-dependent manner in THP-1 cells, but 3-MA treatment decreased autophagy and increased the apoptotic blebs. P. gingivalis infection did not increase apoptosis compared to the control cells, whereas inhibition of autophagy by 3-MA significantly increased apoptosis in P. gingivalis-infected THP-1 cells. Inhibition of autophagy by 3-MA also increased total cell deaths and inflammatory cytokine production, including IL-1β and TNF-⍺. Conclusion: P. gingivalis induced autophagy in THP-1 cells, but the inhibition of autophagy by 3-MA stimulated apoptosis, leading to increased cell deaths and pro-inflammatory cytokines production. Hence, the modulation of cell deaths may provide a mechanism to fight against invading microorganisms in host cells and could be a promising way to control inflammation.

Introduction
 Materials and Methods
  Cell treatment
  Bacterial culture
  Autophagy determined by flow cytometry
  Quantifying autophagy with AO staining
  Apoptosis analyses with flow cytometry
  Cell death assay
  Cytokine analysis
  Statistics
 Results
  1. P. gingivalis induced autophagosome formation inTHP-1 cells.
  2. Inhibition of autophagy by 3-MA increased P.gingivalis-induced apoptosis in THP-1 cells.
 Discussion
 Acknowledgements
 Conflict of interest
 References

• YuRi Song(Department of Oral Microbiology, School of Dentistry, Pusan National University)
• SeYeon Kim(Department of Oral Microbiology, School of Dentistry, Pusan National University)
• Mee Hee Park(Department of Oral Microbiology, School of Dentistry, Pusan National University)
• Jin Chung(Department of Oral Microbiology, School of Dentistry, Pusan National University) Correspondence to