Genetically modified CD34+ hematopoietic stem cells contribute to turnover of brain perivascular macrophages in long-term repopulated primates.

Studies in rodents have shown that brain perivascular macrophages are derived from bone marrow precursors. Less is known about the origin and turnover of perivascular cells in the human central nervous system. We took advantage of non-human primates reconstituted with autologous CD34+ hematopoietic stem cells that had been transduced with a lentiviral vector expressing the enhanced green fluorescent protein (EGFP) to study the ontogeny of brain macrophages of rhesus macaques. Flow cytometry and immunohistochemistry/fluorescence microscopy showed long-term reconstitution of monocytes/macrophages in the blood, lymphoid, and brain tissues 4 years post-transplant. In the brain, EGFP+ cells were detected in the choroid plexus, cerebellum, and cerebrum, where the percent engraftment between animals reflected the percentage of EGFP+ monocytes in the blood. Morphology and location of brain EGFP+ cells exclusively in the vicinity of blood vessels were consistent with perivascular macrophages. Up to 85% of brain EGFP+ cells expressed CD163, a marker of perivascular macrophages, and greater than 70% were CD68+ macrophages. These findings clearly demonstrate that a subpopulation of CD163+/CD68+ brain perivascular macrophages in rhesus macaques are renewed by CD34+ hematopoietic stem cell-derived precursors and exhibit a continuous long-lasting turnover. Because perivascular macrophages are significant targets of productive HIV/simian immunodeficiency virus infection in the brain, these observations point to hematopoietic stem cells as targets of both HIV/simian immunodeficiency virus infection and potential gene therapy.