Intradiscal delivery of mesenchymal stem cells in collagen microspheres in a rabbit disc degeneration model: a preliminary study

Abstract

BACKGROUND: Disc degeneration has been characterized by the breakdown of the extracellular matrix and loss of water content and proteoglycan in intervertebral disc (IVD). Our group has previously demonstrated the potential of mesenchymal stem cells (MSCs) in arresting disc degeneration. Nevertheless, cell leakage has been a major problem during injection even hydrogel was used. We have developed a collagen-microencapsulation platform to make injectable solid microspheres entrapping cells for cell delivery. Here we aim to compare the efficacy of injecting MSCs alone and injecting MSCs entrapped in collagen microspheres in treating disc degeneration in a rabbit model. We also studied whether the cell density of the microsphere affects the regeneration. METHODS: After successfully creating an IVD degeneration model by nucleus pulposus (NP) aspiration, same amount of rMSC in saline or collagen-rMSC microspheres with three different cell densities, were injected into different levels of the IVD using an uninjured level as the internal control. At different time-points, MRI and X-ray were used to evaluate water content and disc height index (DHI) of IVDs. RESULTS: According to MRI data, both time (p=0.005) and treatment (p<0.001) group factors significantly affected water content of the degenerated levels. Difference at 3rd month post-injection was significantly lower than that immediately after degeneration (p=0.006). Cells alone and cells in microsphere carrier did not show any difference at all (p=1.0) but both of them showed significant difference with the uninjured control (p<0.001). . As for X-ray results, both time and treatment group showed insignificant effect on DHI but the result for treatment group was marginal (p=0.062). Post-hoc Bonferroni’s test showed evidence of significant difference between cell alone and uninjured control (p=0.026) but not between cells in collagen microspheres and uninjured control (p=0.603). More samples are being processed before conclusive statement on the effectiveness of the microsphere carrier can be made. Cell density in the microsphere did not affect the outcome at all. CONCLUSION: We developed an injectable collagen-microsphere system for intradiscal cell delivery. The microsphere group did not show advantage over the cell alone group in water content but may show some advantage over the cell alone group in better maintaining the DHI particularly in early time points (1-2 months). Nevertheless, large sample size is needed to confirm the finding. Moreover, imaging data on longer time-point, mechanical and histological evaluations are underway.