Mahanonda et al. (2004) discovered the expression of CD40 on Monocyte was dramatically upregulated by LPS derived from Porphyromonas gingivalis, which is the major pathogenic bacterium of periodontitis (Mahanonda et al., 2004; Gao et al., 2018). Qin et al. (2005) also showed that this phenomenon was also present in macrophages and microglia. Consistently, our study demonstrated the elevated expression of CD40 in periodontitis. Interestingly, Conley et al. pointed out that neither CD40 nor CD40 ligand has any extra ligands or receptors (Conley et al., 2009). This evidence concerns the gene CD40 and periodontitis.