Human retinal microglia express phenotypic characteristics in common with dendritic antigen-presenting cells

Neural tissue has been considered to be immunologically privileged and major histocompatibility complex (MHC) class II antigens not expressed in normal human brain grey matter and retina. In the present study we compare phenotypic characteristics of human retinal microglia and dendritic Langerhans cells, including their morphologies and distribution, MHC class II and CD45 antigen expression and nucleotidase reactivity. Levels of class II expression were measured using optical densitometry in combination with standard immunohistochemical techniques applied to retinal flatmounts. The results indicate that ramified retinal microglia have features in common with dendritic antigen presenting cells of cornea and conjunctivum, including the constitutive expression of MHC class II antigens.