The transplantation of well defined populations of precursor cells offers a means of repairing damaged tissue and of delivering therapeutic compounds to sites of injury or degeneration. For example, a functional immune system can be reconstituted by transplantation of purified haematopoietic stem cells, and transplanted skeletal myoblasts and keratinocytes can participate in the formation of normal tissue in host animals. Cell transplantation in the central nervous system (CNS) has been proposed as a means of correcting neuronal dysfunction in diseases associated with neuronal loss; it might also rectify glial cell dysfunction, with transplanted oligodendrocyte precursor cells eventually allowing repair of demyelinating damage in the CNS. Here we use co-operating growth factors to expand purified populations of oligodendrocyte type-2 astrocyte (O-2A) progenitor cells for several weeks in vitro. When injected into demyelinating lesions in spinal cords of adult rats, created in such a way as to preclude host-mediated remyelination, these expanded populations are capable of producing extensive remyelination. In addition, transplantation of O-2A progenitor cells genetically modified to express the bacterial beta-galactosidase gene gives rise to beta-galactosidase-positive oligodendrocytes which remyelinate demyelinated axons within the lesion. These results offer a viable strategy for the manipulation of neural precursor cells which is compatible with attempts to repair damaged CNS tissue by precursor transplantation.

The ethidium bromide model of demyelination/remyelination provides a system for studying the interactions between demyelinated axons, host glia and transplanted glia. The injection of 0.1% ethidium bromide in isotonic saline into the white matter of the spinal cord produces a glia-free demyelinating lesion which is subsequently remyelinated by Schwann cells and, to a lesser extent, oligodendrocytes. the in vitro description of an oligodendrocyte progenitor isolated from the adult CNS, together with the recognized role of type-1 astroctyes in controlling the developmental programme of perinatal O-2A progenitors, suggested the possibility that transplanted type-1 astrocytes may potentiate oligodendrocyte remyelination of the ethidium bromide lesion. Purified type-1 astrocyte cultures were prepared by removing cells of the oligodendrocyte lineage using a combination of exposure to cytosine arabinoside and complement-mediated immunocytolysis. Following transplantation of purified type-1 astrocyte cultures into ethidium bromide lesions, a significant increase in the extent of oligodendrocyte remyelination was achieved. Because the purified type-1 astrocyte cultures had no demonstrable oligodendrocyte-generating potential it was concluded that the additional oligodendrocytes appearing in the type-1 astrocytes transplanted lesion were of host origin. These results indicate that type-1 astrocytes can facilitate repair of demyelinating lesions by host oligodendrocytes. The possible mechanisms whereby this facilitation occurs are discussed.

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Transplantation of myelin-forming glial cells may provide a means of achieving remyelination in situations in which endogenous remyelination fails. For this type of cell therapy to be successful, cells will have to migrate long distances in normal tissue and within areas of demyelination. In this study, 40 Gy of X-irradiation was used to deplete tissue of endogenous oligodendrocyte progenitors (OPCs). By transplanting neonatal OPCs into OPC-depleted tissue, we were able to examine the speed with which neonatal OPCs repopulate OPC-depleted tissue. Using antibodies to NG-2 proteoglycan and in situ hybridisation to detect platelet-derived growth factor alpha-receptor Ralpha (PDGFRalpha) mRNA to visualise OPCs, we were able to show that neonatal OPCs repopulate OPC-depleted normal tissue 3-5 times more rapidly than endogenous OPCs. Transplanted neonatal OPCs restore OPC densities to near-normal values and when demyelinating lesions were made in tissue into which transplanted OPCs had been incorporated 1 month previously, we were able to show that the transplanted cells retain a robust ability to remyelinate axons after their integration into host tissue. In order to model the situation that would exist in a large OPC-depleted area of demyelination such as may occur in humans; we depleted tissue of its endogenous OPC population and placed focal demyelinating lesions at a distance (< or =1 cm) from a source of neonatal OPCs. In this situation, cells would have to repopulate depleted tissue in order to reach the area of demyelination. As the repopulation process would take time, this model allowed us to examine the consequences of delaying the interaction between OPCs and demyelinated axons on remyelination. Using this approach, we have obtained data that suggest that delaying the time of the interaction between OPCs and demyelinated axons restricts the expression of the remyelinating potential of transplanted OPCs.