Pattern recognition receptors in Aggragatibacter actinomycetemcomitans mediated periodontal disease

Objective: A. actinomycetemcomitans (Aa) is a commonly implicated pathogen in severe periodontitis. The host cell internalization of Aa is associated with actin rearrangement and receptor mediated endocytosis. Toll like receptors (TLR) are pattern recognition receptors that respond to microbial invasion and mediate host immune responses. In vitro studies have suggested that the Aa may interact with TLR-2 and TLR-4. Since gingival epithelial cells are the primary cells that encounter periodontal pathogens the objective of this study was to investigate the effect of Aa infection on gingival epithelial expressions of TLR-2, TLR-4 and the co-receptor CD14.

Methods: 8-10 week old C57BL/6 mice were induced Aa mediated periodontal disease (PD) following the method of Garlet et al. Mice were sacrificed 50 days post Aa inoculation. Bone loss was assessed by micro-CT. The degree of inflammation was evaluated by measuring cytokine and lymphocytic infiltration in gingiva. The expression of CD14, TLR-2 and TLR-4 in the gingival epithelial cells was detected by flow cytometry and measured by semi-quantitative RT-PCR.

Results: Increased number of lymphocytes and measurable alveolar bone loss was observed in mice infected with Aa. Gingival lymphocytes from PD induced mice secreted inflammatory cytokines, IL-6, IL-12 and INF-gamma upon restimulation with Aa sonicate. The gingival epithelial cells of mice with PD exhibited significant upregulation of TLR-2 and TLR-4 expression. The nature of the epithelial cells was confirmed by cytokeratin-13 staining. The TLR-4 mRNA was higher in gingival epithelial cells of mice with PD. No significant difference was observed in the epithelial cell CD14/TLR-2 mRNA between groups of mice.

Conclusions: Signaling via TLR stimulates cytokine secretion in mammalian cells. The upregulated expressions of TLR-2 and TLR-4 on gingival epithelial cells and secretion of inflammatory cytokine as observed in this study suggests that both TLRs may contribute to Aa mediated pathology in periodontal disease.