Lipoteichoic Acid of Enterococcus faecalis Inhibits the Differentiation of Macrophages into Osteoclasts

Introduction: Enterococcus faecalis is associated with persistent endodontic infection and refractory apical periodontitis. Recently, we have shown that heat killed E. faecalis attenuates osteoclast differentiation. Because lipoteichoic acid (LTA) is a major virulence factor of gram-positive bacteria, we investigated the effect of LTA from E. faecalis (EfLTA) on osteoclast differentiation. Methods: EfLTA was purified through organic solvent extraction, hydrophobic interaction column chromatography, and ion exchange column chromatography. Bone marrow cells from C57BL/6 or Toll-like receptor 2 deficient mice were incubated with macrophage colony-stimulating factor (M-CSF) for 5 days to generate macrophages (bone marrow derived macrophages [BMMs]). The cells were differentiated into osteoclasts with M-CSF and receptor activator of NF-kappa B ligand (RANKL) in the presence or absence of EfLTA. The degree of osteoclast differentiation was determined by tartrate-resistant acid phosphatase staining. The expression of NFATc1 and c-Fos transcription factors was determined by Western blotting. A phagocytosis assay was performed by measuring the uptake of carboxyfluorescein diacetate succinimidyl ester stained E. faecalis. An enzyme-linked immunosorbent assay was used to determine the amount of cytokines and chemokines. Results: When BMMs were treated with EfLTA, osteoclast differentiation was attenuated. EfLTA inhibited the RANKL-induced expression of NFATc1 and c-Fos. EfLTA inhibition of osteoclast differentiation was not observed in TLR2-deficient BMMs. In addition, EfLTA sustained the phagocytic capacity of BMMs even after the differentiation into osteoclasts, whereas it induced the expression of inflammatory cytokines and chemokines. Conclusions: EfLTA inhibits the differentiation of macrophages into osteoclasts and thereby maintains the phagocytic and inflammatory capacities of macrophages, potentially contributing to refractory apical periodontitis.