Genetically Modified Human Derived Bone Marrow Cells for Postero-Lateral Lumbar Spine Fusion in Athymic Rats

Purpose: The purpose of this study was to determine whether human derived bone marrow cells(HBMC) genetically modified to over-express BMP-2 can induce lumbar spine fusion in an athymic rat model. High pseudoarthrosis rates after attempted spinal arthrodesis as well as the morbidity associated with iliac crest bone graft harvest make alternative bone graft materials desirable. Other treatment options include allograft demineralized bone matrix and more recently, recombinant bone morphogenetic proteins. However, high doses of the recombinant protein are required, and a single dose of recombinant protein may not induce a sufficient osteoinductive response in all patients. An alternative strategy is to genetically manipulate cells to overexpress BMP-2 protein. Previously we have created BMP-2 producing bone marrow cells via adenoviral gene transfer to induce intertransverse process fusion of the spine and to heal critical sized femoral defects in syngeneic Lewis rats. The purpose of the present study was to assess the ability of human derived bone marrow cells infected with a BMP-2 adenovirus to induce an intertransverse process fusion of the spine in an athymic rat model.

Methods: HBMC obtained from healthy donors were plated and grown in DMEM containing 10% FBS. 5x10^6 HBMC were infected with an adenovirus containing the cDNA for BMP-2(AdBMP2) or lacZ(AdlacZ). At 24 hours, BMP-2 production was assessed by Western blot. Cells were trypsinized, implanted on a collagen sponge, and were implanted at the L4-L5 level as described in a previously reported posterolateral spine fusion model. Rats were assessed by radiographs at 4 week intervals and were sacrificed at 12 weeks. After sacrifice, the spines were explanted and assessed by manual palpation. Specimens were embedded in MMA and underwent histologic analysis.

Results: 7 female athymic rats were implanted with 5 million HBMC which had been infected with AdBMP2. All(7/7) of the spines from this experimental group were fused by radiographic evaluation and manual palpation at 12 weeks. Control groups consisted of: 4 rats implanted with the collagen sponge alone, 5 rats implanted with a collagen sponge impregnated with 5 million uninfected HBMC, and 5 rats implanted with HBMC infected with AdlacZ. None of the rats in the control groups developed a fusion at L4-L5.

Discussion: This study demonstrates that human derived bone marrow cells can be infected with a BMP-2 containing adenovirus and produce sufficient bone in vivo to fuse the lumbar spine. Further experiments are planned to determine the role of the transduced bone marrow cells in the induction of the fusion mass. Safety issues such as the quantification of systemic spread of the adenovirus will need to be addressed before any adenovirus based bone inducing therapy will be used clinically.