Enterococcus faecalis attenuates the differentiation of macrophages into osteoclasts

Abstract

Introduction: Enterococcus faecalis is closely associated with refractory apical periodontitis, manifesting periapical lesions and alveolar bone loss. Macrophages playing an important role in the induction of inflammation can differentiate into bone-resorbing osteoclasts. In the present study, we investigated the effect of E. faecalis on the differentiation and function of macrophages as osteoclast precursors.

Methods: Bone marrow-derived macrophages (BMMs) were differentiated into osteoclasts with macrophage colony-stimulating factor and receptor activator of nuclear factor kappa B ligand in the presence or absence of heat-killed E. faecalis (HKEF). Tartrate-resistant acid phosphatase-positive multinucleated giant cells were analyzed to determine osteoclast differentiation. Western blotting was performed to examine the expression of c-Fos and NFATc1 transcription factors. Phagocytic capacity was analyzed by measuring uptake of carboxyfluorescein succinimidyl ester-labeled E. faecalis. Secretion of tumor necrosis factor-α, interleukin-6, keratinocyte-derived chemokine, and monocyte chemotactic protein-1 was determined by enzyme-linked immunosorbent assay.

Results: Differentiation of BMMs into osteoclasts was attenuated in the presence of HKEF, and expression of c-Fos and NFATc1 was inhibited. HKEF exposure also prevented a reduction in the phagocytic capacity of BMMs after differentiation into osteoclasts. Concomitantly, HKEF induced the expression of chemokines monocyte chemotactic protein-1 and keratinocyte-derived chemokine and proinflammatory cytokines tumor necrosis factor-α and interleukin-6.

Conclusions: E. faecalis attenuated macrophages from differentiating into osteoclasts, allowing them to keep their ability to phagocytose and kill pathogens and to induce proinflammatory cytokine and chemokine secretion.

Keywords: Chemokines; Enterococcus faecalis; cytokines; macrophages; osteoclast differentiation; phagocytosis.