Intradiscal Delivery of Mesenchymal Stem Cells in Collagen Microspheres in a Rabbit Disk Degeneration Model: A Preliminary Study

Abstract

IntroductionDegeneration of intervertebral disk (IVD) has been characterized by the breakdown of the extracellular matrix and loss of water content and proteoglycan. Mesenchymal stem cells (MSCs) have been demonstrated as a promising cell source in disk regeneration. Injectable biomaterials as MSCs carriers allow needle-guided graft delivery to minimize trauma of the disc, and thus are preferable for IVD engineering. Our lab has developed a microencapsulation platform to make injectable solid microspheres entrapping cells in fiber meshwork of different extracellular matrices for scaffolding or cell delivery. As a first attempt, this study aims to compare the efficacy of injecting MSCs entrapped in collagen microspheres with injecting MSCs in saline in treating disk degeneration in an established rabbit model. To prevent cell leakage, we used Histoacryl glue system to block the injection path postinjection. We also studied whether the cell density of the microsphere affects the regeneration.Materials and MethodsAfter successfully creating IVD degeneration by nucleus pulposus (NP) aspiration, same amount of rMSC in saline or collagen microspheres with three different cell densities (1.25 × 105, 5 × 105, and 1 × 106 cells/mL), were injected into different levels of the IVD using an uninjured level as the internal control. Histoacryl glue was applied to block the injection portal immediately after injection. At different time-points, MRI and X-ray were used to evaluate water content and disk height index (DHI) of IVDs. After 6 months, all three IVDs of each rabbit were collected for mechanical, biochemical, and histological assays.ResultsPreliminary MRI data showed that both the time factor (p < 0.001) and the treatment group (p < 0.001) significantly affected water content of the IVDs. The differences between the immediately-after-injection group with the 3rd month (p = 0.001) and with the 6th month (p = 0.008) postinjection groups were both statistically significant. However, the MSC-saline group and the MSC-microsphere group did not show any difference (p > 0.05) while both of them showed significant difference with the uninjured control (p < 0.001). As for X-ray results, treatment group showed significant difference with uninjured control on DHI (p = 0.001). Posthoc Bonferroni' s test showed that both the MSC-saline group (p < 0.001) and the MSC-microsphere group (p = 0.021) showed significant difference with the uninjured control. Furthermore, cell density in the microsphere did not affect the results (p > 0.05). Moreover, both treatment groups showed intense staining of type II collagen and glycosaminoglycans (GAGs) at sixth month postinjection, however chondrocyte-like cells were observed in the NP region of the MSC-microsphere group. Furthermore, application of Histoacryl glue was necessary to block the immediate cell leakage after injection. However, its long-term effect was uncertain because large osteophyte-like structure with type I collagen immunopositivity, which was reported to associate with cell leakage, was seen in the MSC-saline group. This also suggests that the collagen microsphere system may help to reduce the extent of cell leakage after injection and hence prevent osteophyte formation.ConclusionPreliminary MRI and X-ray results showed that the MSC-microsphere group did not show advantage over the MSC-saline group. The Histoacryl glue system was able to block immediate cell leakage but may not be effective in long term. Imaging data on longer time-point, mechanical and biochemical tests and histological evaluation are underway. Moreover, a more nature-simulating carrier system with GAG-rich microspheres and a more effective injection-portal-blocking agent namely an injectable annulus plug are being developed.I confirm having declared any potential conflict of interest for all authors listed on this abstractNoDisclosure of InterestNone declared