Involvement of TLR2 in apoptosis of Aggregatibacter actinomycetemcomitans-infected monocytes

Objective: Aggregatibacter actinomycetemcomitans infection induces apoptosis in the human monocyte cell line THP-1. Toll-like receptors (TLRs), which are key components of the innate immune system, recognize conserved sequences on the surface of pathogens and trigger host cell function. However, roles of TLRs in the induction of apoptosis in A. actinomycetemcomitans-infected cells is unclear. The objective of this study was to examine the roles of TLRs in apoptosis of A. actinomycetemcomitans-infected THP-1 cells.

Methods: A. actinomycetemcomitans was added to THP-1 cells suspended in microtubes; subsequently, the tubes were centrifuged (1000 x g, 10 min). Non-adherent bacteria were removed via serial centrifugation. Infected cells were transferred to 6-well culture plates and incubated in the presence of either anti- TLR2 or –TLR4 antibody (10 μg/ ml). TLR mRNA level was confirmed by RT-PCR. Apoptotic cells were detected by APOPercentage™ dye staining. Cellular p38 activity and NFκB activity were assessed with ELISA kits.

Results: A significant increased in TLR2 mRNA level was observed in A. actinomycetemcomitans-infected cells. However, no meaningful change was detected in TLR4 mRNA level. NFκB activity increased following infection. Moreover, percentages of apoptotic cells as well as p38 MAPK and NFκB activities in infected cells decreased upon addition of anti-TLR2 antibody.

Conclusion: These findings indicate that TLR2 play a key role in the apoptotic signal pathway and that both p38 MAPK and NFκB are involved in the pathway. Apoptosis of infected human immune cells may be an important mechanism in the progression of periodontal disease.