TNF and periodontitis: Of the studies included in this systematic review, four studies [21,25,26,27] evaluated the levels of the pro-inflammatory cytokines IL-1β [21,25], IL-6 [26], and TNF-α [21,25,26], the anti-inflammatory cytokine IL-10 [21,27], and the matrix metalloproteinase-8 (MMP8) [27], which is associated with the clinical manifestation of periodontitis [44].