Few subjects in contemporary medicine have generated more excitement and controversy than stem cells. The potential of stem cells to generate different cell types opens up the possibility of new treatments for regenerating damaged tissue. Stem cells have attracted particular interest for their potential to treat neurological disease because tissues within the central nervous system cannot regenerate.

Multiple sclerosis is an autoimmune disease where the immune response targets myelinating cells (oligodendrocytes) within the central nervous system. It is characterised by focal inflammation, demyelination, and axonal damage. Multiple sclerosis differs fundamentally from the “classic” neurodegenerative diseases—Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease—in which neurones are irreversibly destroyed, because lost oligodendrocytes and thus myelin can be replaced spontaneously by adult brain stem cells.1 This natural regeneration (called remyelination) can be widespread during the early stages of multiple sclerosis,2 but it is not sustained, and the increasing failure of remyelination underlies the loss of axons that characterises disease progression.

Effective treatments for multiple sclerosis must therefore be able to limit damage by inhibiting the immune response and repair damage by replacing lost …