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Plast Reconstr Surg. 2017 Aug;140(2):296e-306e. doi: 10.1097/PRS.0000000000003541.

TLR4 Inactivation in Myeloid Cells Accelerates Bone Healing of a Calvarial Defect Model in Mice.

Author information

1
Shanghai, People's Republic of China; Pittsburgh, Pa.; and Baltimore, Md. From the Department of Stomatology, Tenth People's Hospital of Tongji University; the Departments of Plastic Surgery, Surgery, Oral Biology, and Bioengineering, University of Pittsburgh; and the Department of Surgery, The Johns Hopkins University.

Abstract

BACKGROUND:

Toll-like receptor 4 (TLR4) has been implicated in inflammation-induced bone destruction in various chronic bone diseases; however, its direct influence on bone healing is not well understood. The authors' previous study showed accelerated bone healing with higher osteoclastogenesis gene expression in toll-like receptor 4 knockout mice (TLR4). This study aimed to further elucidate the underlying cellular mechanisms during fracture healing by generating a myeloid cell-specific toll-like receptor 4 knockout model (Lyz-TLR4 mice).

METHODS:

Calvarial defects, 1.8 mm in diameter, were created in wild-type, TLR4, and Lyz-TLR4 mice. Bone healing was investigated using micro-computed tomography and histologic, histomorphometric, and immunohistochemistry analyses. Primary bone marrow-derived cells were also isolated from wild-type, TLR4, and Lyz-TLR4 mice to measure their osteoclast differentiation and resorption properties.

RESULTS:

A similar faster bone healing response, with active intramembranous bone formation, intense osteopontin staining, and more osteoblast infiltration, was observed in TLR4 and Lyz-TLR4 mice. Tartrate-resistant acid phosphatase staining showed more osteoclast infiltration in Lyz-TLR4 mice than in wild-type mice at day 7. Primary bone marrow-derived cells isolated from TLR4 and Lyz-TLR4 mice presented enhanced osteoclastogenesis and resorption activity compared with those from wild-type mice. Comparable M0, M1, and M2 macrophage infiltration was found among all groups at days 1, 4, and 7.

CONCLUSIONS:

This study revealed that inactivation of toll-like receptor 4 in myeloid cells enhanced osteoclastogenesis and accelerated healing response during skull repair. Together with the role of toll-like receptor 4 in inflammation-mediated bone destruction, it suggests that toll-like receptor 4 might regulate inflammation-induced osteoclastogenesis under different clinical settings.

PMID:
28746278
PMCID:
PMC5542792
DOI:
10.1097/PRS.0000000000003541
[Indexed for MEDLINE]
Free PMC Article

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