LOCALIZATION AND CHARACTERIZATION OF MAJOR HISTOCOMPATIBILITY COMPLEXCLASS II-POSITIVE CELLS IN THE POSTERIOR SEGMENT OF THE EYE - IMPLICATIONS FOR INDUCTION OF AUTOIMMUNE UVEORETINITIS

Purpose. To identify potential antigen-presenting cells in the choroid and retina of the normal rat eye, with a view to proposing a role for such cells in the induction and perpetuation of experimental autoimmu ne uveoretinitis, a model of human uveoretinal inflammation. Methods. Immunohistochemical and electron microscopic studies using a panel of monoclonal antibodies were performed on frozen sections of the perfuse d-fixed normal Lewis rat eye, choroid whole mounts, and cytospin prepa rations of cells harvested from choroid/ciliary body explant cultures. In addition, time-lapse video recordings of migratory uveal tract cel ls in culture were taken. Results. No major histocompatibility complex class II-positive cells were found in the normal Lewis rat retina. Ho wever, at least three populations of potential antigen-presenting cell s were found in the uveal tissues of the eye: classical dendritic cell s expressing high levels of major histocompatibility complex class II antigen; resident dendritiform macrophages, which were negative for ma jor histocompatibility complex class II antigen, but expressed specifi c macrophage markers (ED2); and blood-borne macrophages (ED1) that had emigrated from the vasculature into the tissue compartment. In additi on there were small numbers of cells expressing novel markers such as markers usually found only on macrophage subsets in splenic tissue (ED 3) and a recently described marker for veiled dendritic cells (OX62). Dendritic cells and resident dendritiform macrophages closely interact ed with each other and with tissue cells, particularly retinal pigment epithelial cells. Conclusions. The posterior uveal tract is richly po pulated with classical dendritic cells expressing constitutive high le vels of major histocompatibility complex class II antigen. There are a lso several types of macrophages with the potential to modulate immune responses in the posterior segment. Interactions among these cells an d with resident tissue cells such as retinal pigment epithelial cells are probably central to the initiation of (auto)immune responses in th e posterior segment of the eye.