MICROGLIAL PROGENITORS WITH A HIGH PROLIFERATIVE POTENTIAL IN THE EMBRYONIC AND ADULT-MOUSE BRAIN

Single cell suspensions, prepared from brain stem, cerebellum, and forebrain parenchyma of embryonic and adult mice, were plated on monolayers of an astroglial cell line derived from a spontaneously immortalized mouse cerebellar culture, the D19 clone. A few of the brain cells adhering to the D19 monolayers were immunoreactive to the Mac-1 antibody, which labels all cells of the monocytic and granulocytic lineages. The Mac-1-positive cells proliferated vigorously and later most of them acquired the F4/80 epitope specific for macrophages and microglia cells. Studies in clonal conditions allowed development of large colonies of about 2 x 10(5) cells that expressed typical microglia markers. Bone marrow Mac-1-positive cells cocultured on D19 monolayers were also induced to proliferate, whereas peritoneal macrophages were not. D19 astrocytes express macrophage colony-stimulating factor (CSF-1) activity at a high level, and their conditioned media induced the proliferation of brain and bone marrow Mac-1-positive cells. A specific anti-CSF-1 antiserum completely blocked bone marrow macrophage progenitor proliferation and significantly reduced the multiplication of microglial precursors induced by the D19-conditioned medium. These data indicate that the embryonic and adult mouse brain parenchyma contains potential progenitors for microglial cells.