TLR9 and periodontitis: TLR2-/-, TLR4-/-, TLR2&4-/- (Lin et al., 2017), and TLR9-/- (Kim et al., 2015; Crump et al., 2017) mice exhibited the reduced alveolar bone loss in various mouse periodontitis models, suggesting that TLRs indeed played an important role in developing discordant oral barrier immunity.