Ocular immune privilege: the eye takes a dim but practical view of immunity and inflammation.

The delicate visual axis that makes precise vision possible is highly vulnerable to the destructive potential of immunogenic inflammation. Immune privilege of the eye is the experimental expression of the way in which evolution has coped with the countermanding threats to vision of ocular infections and ocular immunity and inflammation. Ocular immune privilege has five primary features that account for its existence: blood:ocular barriers, absent lymphatic drainage pathways, soluble immunomodulatory factors in aqueous humor, immunomodulatory ligands on the surface of ocular parenchymal cells, and indigenous, tolerance-promoting antigen-presenting cells (APCs). Three manifestations of ocular immune privilege that have received the most extensive study are the intraocular microenvironment, which is selectively anti-inflammatory and immunosuppressive; the prolonged acceptance of solid tissue and tumor allografts in the anterior chamber; and the induction of systemic tolerance to eye-derived antigens. Anterior chamber-associated immune deviation is known to arise when indigenous, ocular APCs capture eye-derived antigens and deliver them to the spleen where multicellular clusters of these cells, natural killer T cells, marginal zone B cells, and gammadelta T cells create an antigen-presentation environment that leads to CD4(+) and CD8(+) alpha/beta T cells, which as regulators, suppress induction and expression of T helper cell type 1 (Th1) and Th2 immune expression systems. The ways the eye influences local and systemic immune responses to ocular antigens and pathogens carry risks to and benefits for mammalian organisms. As loss of sight is a powerful, negative-selecting force, the benefits of ocular immune privilege outweigh the risks.