Ex Vivo Priming of Culture Expanded Mesenchymal Stem Cells with Bone Morphogenetic Protein-2 in a Lumbar Spinal Fusion Model: A Comparative Study of Adipose and Bone Marrow Derived Stem Cells

Introduction: In vivo studies have proven that culture-expanded adipose-derived stem cells (ADSCs) enhance spinal fusion, especially when combined with bone morphogenetic protein-2 (BMP-2). Supraphysiologic doses of rhBMP-2, however, can result in complications. It has been reported that short in vitro treatments with rhBMP-2 prior to transplantation enhance bone marrow-derived stem cell (BMSC)-mediated bone regeneration in vivo in a calvarial defect model. This study aimed to examine whether brief in vitro rhBMP-2 priming can enhance MSC-mediated spinal fusion and to compare responses of adipose- and bone marrow-derived cells in a rat model.

Methods: ADSCs and BMSCs were isolated from inguinal fat pads and long bones of 6–8-week-old syngeneic Lewis rats and cultured in vitro. Passage 1 (P1) cells were primed with 1 ng of rhBMP-2 for 24 hours. Passage 2 (P2) MSCs (2x106) were seeded onto Vitoss scaffolds. Female Lewis rats underwent L4-L5 posterolateral intertransverse spinal fusion and were assigned to one of four experimental groups: [1]Vitoss+rhBMP-2-Primed BMSCsP2, [2]Vitoss+non-primed BMSCsP2, [3]Vitoss+rhBMP-2-Primed ADSCsP2, [4]Vitoss+non-primed ADSCsP2. Fusion was assessed eight weeks post-surgery by micro-computed tomography (µCT) (scored as 0=non-fused, 1=unilateral fusion, 2=bilateral fusion), manual palpation (0=non-fused, 1=partial fusion, 2=fused), and histology. µCT images were used to measure fusion mass volume.

Results: µCT data indicated that culture-expanded rhBMP-2-primed ADSCs and BMSCs displayed significantly larger fusion mass volumes than their non-primed counterparts (29.94 mm3 vs. 25.56 mm3, p=0.04; and 23.8 mm3 vs. 17.79 mm3, p=0.06 respectively); no significant difference between primed ADSCsP2 and BMSCsP2 was noted. CT fusion scores were not significant across groups. Primed ADSCsP2 had higher manual palpation scores than non-primed ADSCsP2 (1.9vs1.3, p=0.03). Histology revealed greater osteoid deposition, osteoblast presence and vascularity in primed groups.

Conclusion : rhBMP-2-priming increased fusion mass volume, manual palpation scores, and bone formation on histology in primed ADSCsP2 and enhanced fusion mass volume and bone formation in BMSCsP2 compared to their non-primed counterparts.