Feasibility of a stem cell therapy for intervertebral disc degeneration

Different strategies to supplement/replenish the disc cell population have been proposed. Recently, adult stem cells have shown promise as a cell source for a variety of tissue engineering and cell therapy applications. A stem cell can renew itself through cell division and can be induced to develop into many different specialized cell types. Moreover, stem cells have shown ability to migrate and engraft within various tissues, as well as to exert stimulatory effects on other cell types through various mechanisms (eg, paracrine effects, cell-cell interactions). These characteristics make stem cells worthy of investigation as a source of cells for intervertebral disc (IVD) tissue engineering and cell therapy.

To determine feasibility of a stem cell therapy of IVD degeneration.

In vitro studies of adult human cells to examine interactions between nucleus pulposus cells (NPCs) and mesenchymal stem cells (MSCs) at different ratios in 3-D pellet culture. In vivo studies of healthy adult rabbit discs injected with allogenic adult rabbit MSCs to examine stem cell survival and engraftment in living disc tissue.

In vitro study: Human NPCs were cocultured with human MSCs in different ratios (75:25, 50:50, 25:75) for 2 weeks in pellet culture, for comparison with pure NPC (100:0) and pure MSC (0:100) pellet cultures. Proteoglycan synthesis rate and glycosaminoglycan (GAG) content were measured by radioactive sulfate incorporation and dimethylmethylene blue assay, respectively. In vivo study: MSCs were isolated from the bone marrow of a New Zealand White (NZW) rabbit, retrovirally transduced with the lacZ marker gene, and injected into the nucleus pulposi of the L2-3, L3-4, and L4-5 lumbar discs of 12 other NZW rabbits. Three rabbits each were sacrificed at 3, 6, 12, or 24 weeks after cell implantation, and X-Gal staining was done to assess survival and localization of MSCs in the disc tissues.

In vitro study: the 75:25 and 50:50 NPC:MSC cocultures yielded the greatest increases in extracellular matrix (ECM) production. In vivo study: MSCs were detected in histological sections of rabbit discs up to 24 weeks after allogenic stem cell implantation, without evidence of systemic illness in the recipient rabbits. The 24-week results in particular suggested the possibility of stem cell migration and engraftment into the inner annulus fibrosus.

These encouraging results support feasibility of a stem cell therapy approach toward supplementation/replenishment of IVD cells and synthesis/maintenance of a more functional ECM in a degenerated disc. Moreover, the in vivo results demonstrate that transplanted MSCs survive and successfully engraft into the IVD tissue, and are effective vehicles for exogenous gene delivery to the IVD--thus there appear to be multiple mechanisms whereby stem cells might able to confer therapeutic effects in a stem cell therapy of IVD degeneration.