Does innate immune privilege exist?

Immune privilege in the eye is believed to originate from the eye's need to avoid the sight-destroying consequences of inflammation. Over the past 25 years, many of the anatomical, cellular, and molecular mechanisms by which the eye avoids inflammation secondary to adaptive immune responses have been elucidated. In the recent past, it has become increasingly clear that innate immune responses play a critical role in activating the adaptive immune response. Moreover, innate immunity itself carries a heavy burden of inflammation, thereby posing a threat to vision if it should occur intraocularly. Ocular immunologists have now begun to inquire into the extent to which the eye regulates the expression of innate immunity in oculi. Evidence is presented which indicates that factors found in normal aqueous humor (1) prevent NK cells from lysing their targets, (2) inhibit neutrophil activation by CD95 ligand, (3) suppress nitric oxide production by activated macrophages, and (4) interfere with complement activation via the alternative pathway. These factors include transforming growth factor-beta2, alpha-melanocyte-stimulating hormone, calcitonin gene-related peptide, and migration inhibitory factor. The ability of the eye to prevent intraocular activation of innate immune effectors spares the corneal endothelium (which expresses CD95 ligand constitutively, but low levels of MHC class I molecules) from destruction by NK cells and neutrophils, and protects the visual axis from distortion by macrophage and complement-mediated inflammation. Thus, privilege exists in the eye for both adaptive and innate immunity.