Periodontitis is a chronic inflammatory disease characterized by a constant presence of bacteria, alongside a persistent host response. Porphyromonas gingivalis (Pg) is a black-pigmented gram-negative anaerobic coccobacillus that is considered to be one of the major periodontal pathogens. Toll-like receptors (TLRs) are key molecules of the innate immune system, which detect various structures of invading pathogens and trigger inflammation. We discovered that TLR2 plays a major role in the innate response to Pg. Surprisingly, we discovered that TLR2-mediated activation leads to enhanced bacterial survival in a murine model of Pg infection.
Objectives:
1. To determine whether TLR2 activation by live Pg affects the phagocytic response of human neutrophils.
2. To understand the role of monocytes in regulating the phagocytic response of human neutrophils to live Pg.
Methods:
Neutrophils and monocytes were isolated from whole blood of healthy donors by density gradient separation. Pg 381 was grown for 48h and then labeled with fluorescein isothiocyanate (FITC). The cells were exposed to the FITC-labeled Pg and phagocytosis was allowed to proceed.
To asses the role of TLR2 in Pg neutrophil activation, we used blocking antibodies to TLR2 vs. TLR4, and evaluated the following parameters:
1. phagocytosis level
2. apoptosis
3. cytokine secretion
Results:
In the presence of monocytes, there was a decrease in the phagocytosis of Pg by neutrophils. After blocking TLR2, in a mixed population, there was a significant enhancement in the phagocytosis. Exposure of neutrophils to Pg delayed cell apoptosis, but TLR2 blockage increased the apoptosis level, and decreased the cytokine levels.
Conclusions:
1. Neutrophil phagocytosis of Pg is actively down-regulated by monocytes.
2. TLR2 plays a role in neutrophil cytokine secretion and delayed apoptosis in response to live Pg.
These results are consistent with the hypothesis that TLR2-mediated activation benefits Pg by promoting its survival in periodontal lesions.