Immunosuppressive properties of the pigmented epithelial cells and the subretinal space.

The immune privilege of the anterior chamber of the eye has been recognized for over 100 years. However, the unique immunological properties of the pigmented epithelial (PE) cells of the eye and the subretinal space (SRS) have only recently been appreciated. The PE cells of the iris, ciliary body, and retina reside in anatomically disparate locations and serve distinctly different functions, yet share interesting immunomodulatory properties that contribute to ocular immune privilege. PE cells in the ciliary body and retina elaborate a variety of soluble factors that either directly or indirectly dampen immune-mediated inflammation; these include transforming growth factor-Beta, somatostatin, thrombospondin and pigment epithelial derived factor (PEDF). The constitutive expression of the immune co-stimulatory molecule, CD86, on iris PE cells not only inhibits T cell proliferation, but also promotes the generation of regulatory T cells. The SRS is now recognized as an immune-privileged site that shares many, but not all, of the properties ascribed to the anterior chamber, including the induction of systemic immune deviation. The prospect of therapeutic retinal transplantation and the possible immunologic etiology for some forms of age-related macular degeneration provides new impetus for gaining a better understanding of ocular immune privilege in the posterior regions of the eye.