Accelerated Calvarial Healing in Mice Lacking Toll-Like Receptor 4

The bone and immune systems are closely interconnected. The immediate inflammatory response after fracture is known to trigger a healing cascade which plays an important role in bone repair. Toll-like receptor 4 (TLR4) is a member of a highly conserved receptor family and is a critical activator of the innate immune response after tissue injury. TLR4 signaling has been shown to regulate the systemic inflammatory response induced by exposed bone components during long-bone fracture. Here we tested the hypothesis that TLR4 activation affects the healing of calvarial defects. A 1.8 mm diameter calvarial defect was created in wild-type (WT) and TLR4 knockout (TLR4-/-) mice. Bone healing was tested using radiographic, histologic and gene expression analyses. Radiographic and histomorphometric analyses revealed that calvarial healing was accelerated in TLR4-/- mice. More bone was observed in TLR4-/- mice compared to WT mice at postoperative days 7 and 14, although comparable healing was achieved in both groups by day 21. Bone remodeling was detected in both groups on postoperative day 28. In TLR4-/- mice compared to WT mice, gene expression analysis revealed that higher expression levels of IL-1β, IL-6, TNF-α,TGF-β1, TGF-β3, PDGF and RANKL and lower expression level of RANK were detected at earlier time points (≤ postoperative 4 days); while higher expression levels of IL-1β and lower expression levels of VEGF, RANK, RANKL and OPG were detected at late time points (> postoperative 4 days). This study provides evidence of accelerated bone healing in TLR4-/- mice with earlier and higher expression of inflammatory cytokines and with increased osteoclastic activity. Further work is required to determine if this is due to inflammation driven by TLR4 activation. © 2012 Wang et al

Graph showing quantitative gene expression on day 0 (untreated control) bone tissue.

<p>Siginificant differences were identified in the expression of IL-1β,VEGF and RANKL between the two groups. IL-1β and RANKL expression were higher in WT mice and VEGF expression was higher in TLR4^−/− mice on day 0 (mean +/−SEM; n = 5 to 7; *<i>p</i><0.05).</p

Graph showing relative fold change expression of growth factors for WT and TLR4^−/− mice (mean fold change over day 0; ND, not detectable).

<p>Similar expression patterns of BMP2, BMP4, TGF-β2, VEGF, and PDGF were observed between the two groups. Higher expression levels of TGF-β1 and TGF-β3 were detected in TLR4^−/− mice than in WT mice at early time points (mean +/−SEM; n = 5 to 7).</p

Graph showing relative fold change expression of inflammatory cytokines for WT and TLR4^−/− mice (mean fold change over day 0; ND, not detectable).

<p>The fold change patterns in IL-1α, IL-1β, and TNF-α expression were similar in both groups. IL-6 had a different expression pattern in TLR4^−/− mice compared to WT mice at early time points (mean +/−SEM; n = 5 to 7).</p

Histophotomicrographs of pentachrome stained tissues in WT and TLR4^−/− mice.

<p>Similar stains were observed between WT and TLR4^−/− mice on day 0 and day 1. An increased amount of newly-formed bone was observed in TLR4^−/− mice at day 7, suggesting accelerated healing compared to WT. Lamellar bone (*), which stains positive for acid fuchsin (red) was observed in both groups on day 28, suggesting maturation and remodeling of the newly formed bone matrix. (scale bar: 100 µm; bolded black arrows: defect margin; LB: lamellar bone; NFB: newly-formed bone).</p

Graph showing similar amounts of radiographically opaque tissue observed on day 28 in both groups.

<p>Independent t-test showed no significant differences between the calvarial healing of WT and TLR4^−/− mice at this time point. Out of 20 mice tested, none showed complete healing during the 28 days of observation. (mean +/−SEM; n = 10 each).</p

Graph showing newly-formed bone areas measured from H&E stained histology slides.

<p>No obvious bone healing was seen before day 7. Significant differences in bone healing areas were observed between WT and TLR4^−/− mice on days 7 and 14 postoperatively. Newly-formed bone areas were not significantly different after day 21 in WT mice or after day 14 in TLR4^−/− mice (mean +/− SEM ; *<i>p</i><0.05).</p

Graph showing relative fold change expression of RANK, RANKL and OPG for WT and TLR4^−/− mice (mean fold change over day 0; ND, not detectable).

<p>Expression levels of RANK, RANKL and OPG were realtively constant in WT at all time points. Greater variation of the three genes was detected in TLR4^−/− (mean +/−SEM; n = 5 to 7).</p

Histophotomicrographs for H&E stained tissues at the defect margins at postoperative time points.

<p>WT and TLR4^−/− mice showed similar histological staining patterns on days 0, 1 and 4, while larger areas of newly-formed bone were seen in TLR4^−/− mice than in WT mice on day 7. Newly-formed cellularized bone matrix was observed on the endocortical (dural) side of the calvarial bone lateral to the defect perimeter in both groups since day 7. Active bone formation was suggested by the presence of large regions of woven bone matrix at the defect margin in both groups on days 14 and 21. Defects in WT and TLR4^−/− mice were histologically similar since day 21. (scale bar: 100 µm; bolded arrows: defect margin; endo: endocortical surface of calvarial bone; ecto: ectocortical surface of calvarial bone; H: hematoma; IC: infiltrating cells; LB: lamellar bone; WB: woven bone; NFB: newly-formed bone; DC: dural cells).</p