DPP4 and periodontitis: As the biofilm formation and production of CD26 are correlated in P gingivalis, both being important factors of virulence in periodontitis in the mouse (Clais et al. 2014), and cytokines or bacterial components from P gingivalis, Prevotella intermedia, and Escherichia coli augment the CD26 expression by gingival fibroblasts (Nemoto et al. 1999), we also suggest the possibility of bacterial origin for part of the Anti-CD26 isotypes, at least in some groups.