Olfactory Ensheathing Cell Transplantation after a Complete Spinal Cord Transection Mediates Neuroprotective and Immunomodulatory Mechanisms to Facilitate Regeneration

Multiple neural and peripheral cell types rapidly respond to tissue damage after spinal cord injury to form a structurally and chemically inhibitory scar that limits axon regeneration. Astrocytes form an astroglial scar and produce chondroitin sulfate proteoglycans (CSPGs), activate microglia, and recruit blood-derived immune cells to the lesion for debris removal. One beneficial therapy, olfactory ensheathing cell (OEC) transplantation, results in functional improvements and promotes axon regeneration after spinal cord injury. The lack of an OEC-specific marker, however, has limited the investigation of mechanisms underlying their proregenerative effects. We compared the effects of enhanced green fluorescent protein-labeled fibroblast (FB) and OEC transplants acutely after a complete low-thoracic spinal cord transection in adult rats. We assessed the preservation of neurons and serotonergic axons, the levels of inhibitory CSPGs and myelin debris, and the extent of immune cell activation between 1 and 8 weeks postinjury. Our findings indicate that OECs survive longer than FBs post-transplantation, preserve axons and neurons, and reduce inhibitory molecules in the lesion core. Additionally, we show that OECs limit immune-cell activation and infiltration, whereas FBs alter astroglial scar formation and increase immune-cell infiltration and concomitant secondary tissue damage. Administration of cyclosporine-A to enhance graft survival demonstrated that immune suppression can augment OEC contact-mediated protection of axons and neurons during the first 2 weeks postinjury. Collectively, these data suggest that OECs have neuroprotective and immunomodulatory mechanisms that create a supportive environment for neuronal survival and axon regeneration after spinal cord injury.