Figure 2.

The mechanism by which Fusobacterium nucleatum induces host responses. (1) Under healthy conditions, one of the most important factors is that Fusobacterium nucleatum uses FAD-I to stimulate gingival epithelial cells to express human beta-defensin-2 via gingival epithelial cell Toll-like receptors. Stimulated human beta-defensin-2 helps to defend against invading microbes, including Fusobacterium nucleatum itself, and maintains homeostasis in the complex oral environment. (2) In a state of disease, Fusobacterium nucleatum destroys the epithelial barrier and affects human gingival fibroblasts. It not only induces cell autophagy but also stimulates the secretion of several cytokines and chemokines. When challenged by Fusobacterium nucleatum, the immune system is activated to eliminate invasive bacteria. However, Fusobacterium nucleatum could restrain the human immune system to reduce this removal effect. Moreover, the production of TNF-α from activated NK cells may lead to periodontal tissue destruction.