IMMUNOCYTOCHEMICAL AND ELECTRON-MICROSCOPIC CHARACTERIZATION OF MACROPHAGE MICROGLIA CELLS AND EXPRESSION OF CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX IN THE PINEAL-GLAND OF THE RAT

Interstitial cells in the pineal gland of the rat were characterized i mmunocytochemically using the monoclonal antibodies MRC OX-42 and ED1 for macrophages/microglia, and MRC OX-6, which recognizes major histoc ompatibility complex (MHC) class II antigen. A polyclonal antibody aga inst GFAP was used to identify astrocytes. Cells immunopositive for OX -42 and/or ED1 were distributed throughout the gland; they extended pr ocesses primarily along the perivascular spaces and occasionally withi n the parenchyma of the gland. Ultrastructurally, these OX-42-positive cells were characterized by a nucleus with sparse heterochromatin and cytoplasmic vacuoles/lysosomes. Cells expressing MHC class II antigen had a distribution and morphology similar to OX-42-immunopositive cel ls, suggesting that pineal macrophages/microglia play a role as antige n-presenting cells. GFAP-positive astrocytes were concentrated at the proximal end of the pineal where the pineal stalk enters the gland. Th e occurrence of antigen-presenting cells in this circumventricular neu roendocrine gland has important functional implications as these cells may be mediators of neuroimmunomodulatory mechanisms, and involved in certain disease states such as autoimmune pinealitis.