IL21 and periodontitis: Previous studies in humans have shown that elevated levels of some cytokines, such as IL-1β, IL-6, IL-21, and IL-23, are necessary for promoting Th17 differentiation and maintenance during the destructive phase of periodontitis (Takahashi et al., 2005; Moutsopoulos et al., 2012; Rao et al., 2015).