Dose-dependent Nerve Inflammatory Response to rhBMP-2 in a Rodent Spinal Nerve Model

Abstract

Study Design. We developed a spinal nerve root wrapping rodent model to evaluate the relationship between recombinant human bone morphogenetic protein 2 (rhBMP-2) dosage and the degree of inflammation. Objective. To investigate the direct effects of recombinant human bone morphogenetic protein 2 (rhBMP-2) dosage and the degree of inflammation in rodent spinal nerve roots. Summary of Background Data. rhBMP-2 is commonly used in clinical practice to augment spinal fusion. However, complications such as postoperative leg pain, and a higher rate of postoperative neurologic deficits have been reported. These may be attributable to the exposure of adjacent nerve roots to high doses of rhBMP-2. Methods. Eighteen rats were randomized into three groups as follows: Group 1: absorbable collagen sponge (ACS) + 10μg rhBMP-2, Group 2: ACS + 1μg rhBMP-2, and Group 3 ACS with 20μL saline. The ACS containing rhBMP-2 or saline were then wrapped around the L5 nerve root and secured loosely with nonabsorbable sutures. At 1-week postoperation, the rats were sacrificed, and the L5 nerve root and dorsal root ganglion harvested for reverse transcription polymerase chain reaction (RT-PCR), histology and immunohistochemical staining. Results. In our study, 10μg rhBMP-2 induced a 10-fold increase in seroma compared with 1μg group. Using RT-PCR, macrophage markers MIP3- and CD-68 were upregulated by 8- and 2-fold respectively in comparison with the saline group. Haematoxylin and eosin (H&E) images demonstrated disruption of nerve structures in the high dose 10μg rhBMP-2, but not at 1μg rhBMP-2 and with saline. Conclusion. High doses of rhBMP-2 induced neuroinflammation in a dose dependent manner, resulting in higher seroma volume, macrophage marker gene expressions, and higher proportions of immunohistochemically stained TNF-alpha and more macrophage infiltration.