2.1. Ligature-Induced Periodontitis Model in Mice

The well-established ligature-induced periodontitis model in specific pathogen-free C57BL/6 mice was used as described earlier [22]. All animal procedures were performed according to protocols approved by the Institutional Animal Care and Use Committee of the University of Pennsylvania, and adequate measures were taken to minimize pain or discomfort. To induce experimental periodontitis, a 5-0 silk ligature was tied around the maxillary left second molar for up to 8 days (n = 4–5 mice/group). The unligated contralateral molar in each mouse was used as baseline control (unligated control). A synthetic peptide derived from the extracellular domain of TREM-1 (LP17; LQVTDSGLYRCVIYHPP, Pepscan, Lelystad, Netherlands) was used as described earlier [7]. The LP17 blocking peptide is considered as a competitive antagonist of membrane-bound TREM-1 for its natural ligand, therefore acting as a decoy receptor for TREM signaling [23]. For the intervention experiments performed in this study, 5 μg of LP17 peptide or PBS were injected into the palatal gingiva of the ligated second maxillary molar 1 days before placing the ligature and every day thereafter until the day before sacrifice (day 5).

The measurements on the alveolar bone height were done on defleshed maxillae under a Nikon SMZ800 microscope (Nikon Instruments, Melville, NY, USA), and images of the maxillae were captured using a Nikon Digital Sight DS-U3 camera controller. The distance between the cemento-enamel junction (CEJ) and alveolar bone crest (ABC) was measured at six predetermined points on the ligated molar and adjacent regions using NIS Elements software (Nikon Instruments, Melville, NY, USA) [22]. To calculate bone loss, the six-site total CEJ–ABC distance for the ligated side of each mouse was subtracted from the six-site total CEJ–ABC distance of the contralateral unligated side. The results are presented in millimeters, and negative values indicate bone loss relative to the unligated control.