Novel monocyte-like properties of microglial/astroglial cells. Constitutive secretion of lysozyme and cystatin-C.

Evidence implicates cells belonging to the mononuclear phagocytic system (MPS) in the development of some forms of amyloidosis (10, 22). Whether or not the MPS is involved in central nervous system amyloidosis is not known. As a first step to address this issue, microglial and astroglial cells isolated from mouse brains were cultured and characterized as to the properties they may share with other members of the MPS. It was shown by light and electron microscopy that both cell types phagocytose latex particles, but that only microglial cells engulf immunoglobulin sensitized erythrocytes. By means of immunohistochemical, immunofluorescence, and immunoblotting techniques, it was established that the cells contain and secrete lysozyme as well as the proteinase inhibitor cystatin-C (-gamma trace). Cystatin-C was distributed in the cytoplasm and the nucleus and was strikingly associated with filaments and bundles of fibrils. Another enzyme, commonly used to distinguish cells belonging to the MPS, is alpha-naphthyl butyrate esterase. Shortly after their isolation, only the microglial cells were positive, but on continued culturing, increasing numbers of astroglial cells became positive for alpha-naphthyl butyrate esterase. By day 22, almost all of the cells were positive. Freshly isolated cells were negative for the monocyte-specific antigen Mac-1. However, after 4 days, cells with the morphology of microglia had become positive, whereas astroglia failed to exhibit this antigen with up to 22 days in culture. Thus, both astroglia and microglia have properties in common with cells of the MPS which may be useful for future studies. However, on fresh isolation only microglia were indistinguishable from monocytes for all features tested.