Therapeutic potential of induced pluripotent stem cells for spinal cord injury

Once the safety issue has been overcome, induced pluripotent stem cells (iPSCs), which do not entail ethical or immunological concerns, may become the preferred cell source for regenerative medicine. Various types of iPSCs have been established by different methods, and each type exhibits different biological properties. Before iPSC-based clinical applications can be initiated, detailed evaluations of the cells, including their differentiation potentials and tumorigenic activities in different contexts, should be investigated to establish their safety and effectiveness for cell transplantation therapies. Recently, we demonstrated the directed neural differentiation of mouse iPSCs and examined their therapeutic potential in a mouse spinal cord injury (SCI) model. Mouse iPSC-derived neural stem/progenitor cells (NS/PCs), which had been pre-evaluated as non-tumorigenic by their transplantation into nonobese diabetic-severe combined immunodeficiency (NOD-scid) mouse brain, were transplanted into the spinal cord 9 days after SCI. Mouse iPSC-derived NS/PCs differentiated into all three neural lineages without forming teratomas or other tumors. They also participated in re-myelination and induced the axonal re-growth, promoting motor functional recovery. Nevertheless, our results constitute only the first step toward clinical application. The safety and effectiveness of human iPSC-derived NS/PCs need to be more intensively investigated in future preclinical studies, for example, using non-human primate SCI models. In particular, human iPSCs established by delivering reprogramming factors using a safer method than retrovirus system, such as an integration-free virus system, virus-free system, or transgene-free system should be evaluated.