Morphologic and ultrastructural examination of I-A+ cells in the murine iris

The surface membrane expression of major histocompatibility (MHC) class II antigens is an important prerequisite for presentation of foreign antigens to the immune system. Because particular antigens that are placed within the anterior chamber of the eye elicit a deviant form of immunity in which effector delayed-type hypersensitivity responses are suppressed, it has been proposed that novel MHC class II antigen-bearing cells exist in the tissues that line the anterior chamber. Class II MHC antigen expression has been identified within the iris, but the detailed morphologic description of these cells is incomplete. With the use of in situ immunoperoxidase and immunoelectron microscopic techniques, we examined the morphologic and ultrastructural characteristics of resident MHC-positive class II (I-A+) cells in murine irises. A significant number of these cells was found in the connective tissue of BALB/c irises. The majority showed extensive dendritic morphologic characteristics and formed a network throughout the iris that did not overlap. Ultrastructurally, I-A+ cells had an indented nucleus, some vacuoles, lysosomes, mitochondria, and an occasional phagosome within their cytoplasm and an absence of desmosomes or other intercellular junctions. Based on these features, it is unlikely that these cells are epithelial or endothelial in origin, but rather are similar to cells of the monocyte/macrophage/dendritic cell lineage. These results show the presence of an I-A+ dendritic cell population, within the murine iris, distributed in a pattern that is similar to that of Langerhans cells in the skin. Due to their compartmentalization within the eye, this cell population may represent a novel antigen-presenting cell that contributes to the immunologic privilege of the anterior chamber.