Natural killer T cells mediate alveolar bone resorption and a systemic inflammatory response in response to oral infection of mice with Porphyromonas gingivalis

Background and Objective: T and B cells are known to be involved in the disease process of periodontitis. However, the role of natural killer T cells in the pathogenesis of periodontitis has not been clarified. Materials and Methods: To examine the role of these cells, C57BL/6J (wild-type), CD1d(-/-) and alpha-galactosylceramide (alpha GC)-stimulated wild-type mice were orally infected with Porphyromonas gingivalis strain W83. Results: Apart from CD1d(-/-) mice, the level of alveolar bone resorption was elevated by the infection and was further accelerated in alpha GC-stimulated mice. The infection induced elevated levels of serum amyloid A and P. gingivalis-specific IgG in the sera, although the degree of elevation was much smaller in the CD1d(-/-) mice. Infection-induced RANKL elevation was only observed in aGC-stimulated mice. Although the cytokines produced by splenocytes were mainly T-helper 1 type in wild-type mice, those in alpha GC-stimulated mice were predominantly T-helper 2 type. In the liver, the infection demonstrated no effect on the gene expression for interferon-gamma, interleukin-4 and RANKL except alpha GC-stimulated mice in which the infection upregulated the gene expressions. Conclusion: This study is the first to show that natural killer T cells upregulated systemic and local inflammatory responses induced by oral infection with P. gingivalis, thereby contributing to the progression of alveolar bone resorption.