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

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.

• Authors: Dan Wang, James R Gilbert, Gwen M Taylor, Chhinder P Sodhi, David J Hackam, Joseph E Losee, Timothy R Billiar, Gregory M Cooper
• Journal: Plastic and reconstructive surgery (Plast Reconstr Surg) Vol. 140 Issue 2 Pg. 296e-306e (Aug 2017) ISSN: 1529-4242 [Electronic] United States
• PMID: 28746278 (Publication Type: Journal Article)

Chemical References

• Toll-Like Receptor 4

Topics

• Animals
• Female
• Fracture Healing (physiology)
• Mice
• Models, Animal
• Myeloid Cells
• Skull (injuries)
• Toll-Like Receptor 4 (physiology)