Monitoring of macrophage recruitment enhanced by Toll-like receptor 4 activation with MR imaging in nerve injury.

INTRODUCTION: Macrophage recruitment is critical for nerve regeneration after an injury. The aim of this study was to investigate whether ultrasmall superparamagnetic iron oxide (USPIO) nanoparticle-based MRI could be used to monitor the enhanced macrophage recruitment by Toll-like receptor 4 (TLR4) activation in nerve injury. METHODS: Rats received intraperitoneal injections of either lipopolysaccharide (LPS) or phosphate buffered saline (PBS) or no injection (controls) after a sciatic nerve crush injury. After intravenous injection of the USPIOs (LPS and PBS groups) or PBS (control group), MRI was performed and correlated with histological findings. RESULTS: LPS group showed more remarkable hypointense signals on T2*-weighted imaging and lower T2 values in the crushed nerves than PBS group. The hypointense signal areas were associated with an enhanced recruitment of iron-loaded macrophages to the injured nerves. DISCUSSION: USPIO-enhanced MRI can be used to monitor the enhanced macrophage recruitment by means of TLR4 signal pathway activation in nerve injury. Muscle Nerve, 2018.