Stepwise combined cell transplantation using mesenchymal stem cells and induced pluripotent stem cell-derived motor neuron progenitor cells in spinal cord injury

Spinal cord injury (SCI) is an intractable neurological disease in which functions cannot be permanently restored due to nerve damage. Stem cell therapy is a promising strategy for neuroregeneration after SCI. However, experimental evidence of its therapeutic effect in SCI is lacking. This study aimed to investigate the efficacy of transplanted cells using stepwise combined cell therapy with human mesenchymal stem cells (hMSC) and induced pluripotent stem cell (iPSC)-derived motor neuron progenitor cells (iMNP) in a rat model of SCI. A contusive SCI model was developed in Sprague-Dawley rats using multicenter animal spinal cord injury study (MASCIS) impactor. Three protocols were designed and conducted as follows: (Subtopic 1) chronic SCI + iMNP, (Subtopic 2) acute SCI + multiple hMSC injections, and (Main topic) chronic SCI + stepwise combined cell therapy using multiple preemptive hMSC and iMNP. Neurite outgrowth was induced by coculturing hMSC and iPSC-derived motor neuron (iMN) on both two-dimensional (2D) and three-dimensional (3D) spheroid platforms during mature iMN differentiation in vitro. Stepwise combined cell therapy promoted mature motor neuron differentiation and axonal regeneration at the lesional site. In addition, stepwise combined cell therapy improved behavioral recovery and was more effective than single cell therapy alone. In vitro results showed that hMSC and iMN act synergistically and play a critical role in the induction of neurite outgrowth during iMN differentiation and maturation. Our findings show that stepwise combined cell therapy can induce alterations in the microenvironment for effective cell therapy in SCI. The in vitro results suggest that co-culturing hMSC and iMN can synergistically promote induction of MN neurite outgrowth.