The mediation analyses showed that serum TREM-1 levels were directly mediated by the presence of periodontitis (B = 52.7259, p = 0.0262) and indirectly mediated via PGLYRP1 serum levels in patients with RA (B = 0.1840, p = 0.0005) (Fig. 3). This evidence concerns the gene TREM1 and periodontitis.