Allogeneic mesenchymal stem cells improve indices of lumbar intervertebral disc degeneration without site specificity of injection in an ovine model

Purpose: Low back pain (LBP) is the most common musculoskeletal condition affecting an individual's ability to work and manage activities of daily living. Degenerative intervertebral disc (IVD) disease is one of the most common causes of LBP with up to 10% of these individuals developing chronic disability. Spinal fusion for chronic discogenic LBP is invasive, has mixed long-term results, and appears to have little value over conservative measures such as physiotherapy and rehabilitation. There is increasing interest in cell therapy for degenerative IVD disease that aims to repair and regenerate the IVD. Previous studies assessing the efficacy of stem cell injection into degenerated IVDs have reported positive findings. However studies have been predominantly limited to small animals, targeting the nucleus pulposus (NP), with short term follow-up. The purpose of this study was to utilize an in vivo ovine model of IVD degeneration to determine if allogeneic mesenchymal stem cells (MSCs) delivered to the NP or the annulus fibrosus (AF) of degenerated lumbar IVDs leads to improved indices of disc health. Methods: Bone marrow aspirates were obtained from the iliac crest of 8-week-old sheep. MSCs were isolated and then culture expanded for 4 passages. IVD degeneration was induced by a precise postero-lateral annulotomy at three lumbar levels in eight 2-year-old sheep. Six months later, each degenerated IVD was randomized to one of three treatments: Injection of MSC (1 million cells/0.2ml phosphate buffered saline) into i) the previously incised AF (AFI) or ii) into the NP (NPI); or iii) no injection (negative control, NC). The superior adjacent (normal) IVD received an injection of phosphate buffered saline only (0.2ml) into the NP (positive control, PC). Radiographs and magnetic resonance image (MRI) scans were obtained at baseline, 6, 9, and 12 months to determine disc height index (DHI), disc height (DH) and grade of disc degeneration at each timepoint. Discs were harvested at 12 months for biochemical (water, proteoglycan and hydroxyproline content) and histological analysis (disc degeneration grade). Results: IVD degeneration was consistently observed at 6 months in the 24 discs that underwent postero-lateral annulotomy. This was characterized by reduced DHI (33%, p<0.01), reduced DH (p<0.0001), and increased grade of disc degeneration both on MRI scan and subsequent histological analysis at 12 months (p<0.0001). These results were not observed in the PC discs. Six months following stem cell injection; DHI had recovered by 16% and 33% in the NPI and AFI groups respectively (p<0.01); DH had recovered in the NPI and AFI groups by 18% and 22% respectively (p<0.02); Mean Pfirrmann grade improved from 3.25 to 2.67 (-0.58) in the AFI group and from 2.96 to 2.43 (-0.53) in the NPI group, whilst the NC group improved from 3.10 to 2.81 (-0.29). Mean histopathological grade improved in both the NPI (p<0.02) and the AFI (p<0.002) groups when compared to the NC group. No significant differences were noted in the glycosaminoglycan (GAG) or the hydroxyproline (HyPro) content across all four groups. Conclusions: Intervertebral disc degeneration was reliably induced in 2-year-old sheep by the performance of a postero-lateral annulotomy and allowing 6 months for the degeneration to occur. In this ovine model, the injection of allogeneic MSCs into either the annulus fibrosus or the nucleus pulposus of degenerated IVD resulted in significant improvements over 6 months in disc health (DHI, DH and grade of disc degeneration) when compared to control groups. Thus, improvements in disc health appear to be independent of the site of stem cell injection (annulus fibrosus or nucleus pulposus). No measurable improvements were observed in disc biochemistry. Further study into the efficacy of MSC would appear to bewarranted for the treatment of established disc degeneration.