Separate precursor cells for macrophages and microglia in mouse brain: immunophenotypic and immunoregulatory properties of the progeny.

The brain contains two populations of macrophages: the microglia of brain parenchyma, and the central nervous system (CNS) macrophages located in the perivascular spaces, the leptomeninges and the choroid plexus. The microglia are characterized, in part, by their paucity of major histocompatibility complex (MHC) molecules and lack of constitutive antigen (Ag)-presenting activity for naïve CD4+ T-cells. Some CNS macrophages, on the other hand, constitutively express MHC molecules and present Ag to naïve CD4+ T-cells. We have reported that mouse brain contains precursor cells that, in the presence of colony-stimulating factor-1, the macrophage growth factor, give rise to clones of cells that differ in their ability to constitutively present Ag to naive CD4+ T cells. Here we report that this population of precursor cells can be separated into two discrete subpopulations based on differences in cell density and that the two cell populations give rise to progeny that differ in their content of cells constitutively expressing MHC class II and CD86 molecules, and the ability to present Ag to naïve CD4+ T-cells. A comparison of the level of CD45 staining of the progeny, an indication of a microglial or a CNS macrophage origin, suggests that one population of precursor cells yields immunologically immature microglia and the other CNS macrophages.