Heat-killed Lancefieldella Rimae Induces Bone Resorption by Promoting Osteoclast Differentiation

Apical periodontitis, mainly caused by bacterial infection in the dental pulp, is often accompanied by abscess, periapical inflammation, and alveolar bone loss. Lancefieldella rimae has been detected in the root canals of patients with apical periodontitis. Here, we investigated whether L. rimae is associated with bone resorption.

L. rimae was anaerobically cultured and heat-killed (HKLr). A mouse calvarial implantation model was used to determine the bone resorption in vivo. Committed osteoclasts prepared from C57BL/6 wild-type or Toll-like receptor 2 (TLR2)-deficient mice were differentiated into mature osteoclasts in the presence or absence of HKLr. The mRNA expression of tartrate-resistant acid phosphatase (TRAP), ATPase H⁺ transporting V0 subunit D2, cathepsin K, interleukin-6, tumor necrosis factor-α, and glyceraldehyde 3-phosphate dehydrogenase was quantified using real-time reverse transcription-polymerase chain reaction. The protein levels of c-Fos and NFATc1 were determined by Western blot analysis.

Implantation of HKLr onto the mouse calvaria induced the bone destruction with an increase of TRAP-positive areas. While HKLr enhanced the differentiation of osteoclasts, this effect was not observed in TLR2-deficient osteoclasts. HKLr dose-dependently increased the mRNA expression of genes associated with osteoclast differentiation including TRAP, ATPase H⁺ transporting V0 subunit D2, and cathepsin K. In addition, HKLr enhanced the expression of c-Fos and NFATc1, which are important transcription factors for osteoclast differentiation. Moreover, HKLr increased the expression of interleukin-6 and tumor necrosis factor-α.

L. rimae induces bone resorption by enhancing osteoclast differentiation through the TLR2 signaling pathway, implying that L. rimae is a causative agent responsible for the alveolar bone resorption accompanying apical periodontitis.