In mammals, myeloid progenitors infiltrate the developing central nervous system (CNS), through the immature blood-brain barrier (BBB), the ventricular layer or the pial surface migrate and give rise to resident microglia. In the mature brain, however, the BBB hampers such recruitment from the blood-stream and long-term establishment of blood borne myeloid cells in the CNS thus appears at best limited. Hematopoietic stem cell-derived microglia, nevertheless, represents a promising tool for the correction of genetic deficits in the brain. We thus investigated the fate of primary human monocytes, and monocyte-derived macrophages, following transplantation into the adult mouse brain overpassing the BBB. Furthermore, we documented the ability of such cells to deliver a lysosomal enzyme into the brain following genetic modification with a recombinant adenoviral vector carrying the human β-glucuronidase cDNA. When implanted into the mouse striatum, the engineered primary cells survived and expressed the transgene for as much as 8 months. Moreover, the donor cells could migrate out of the grafting site and settle along blood vessels or myelin tracts although at limited distance. Migrating donor cells down-regulated the expression of CD14 andHLA DR, suggesting the adoption of a deactivated microglia-like phenotype. Our observations establish the ability of circulating mononuclear phagocytes to integrate into the brain after transplantation and express a transgene on the long term. These cells might thus be employed for autologous transplantation for the delivery of secreted therapeutic proteins in the context of a wide range of brain affections.
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