Isolation and characterization of a unique natural killer cell inhibitory factor present in the anterior chamber of the eye

The eye is a classic example of an immunologically privileged organ. Immune privilege in the anterior chamber is a multifaceted phenomenon that may have evolved to protect delicate ocular tissues from immune-mediated damage. The anterior chamber of the eye is lined with corneal endothelial cells, which are terminally differentiated cells that are incapable of regeneration. Since corneal endothelial cells are crucial for maintaining corneal clarity, damage to these cells leads to impairment of vision and, in some cases, blindness. The absence of constitutive expression of MHC class I molecules on corneal endothelial cells makes them potential targets for NK cell-mediated cytolysis. The aqueous humor (AH) that fills the anterior chamber of the eye and bathes the corneal endothelial surface contains numerous immunomodulatory molecules, such as TGF-β, which may protect the corneal endothelium from NK cell-mediated damage. Functionally significant levels of TGF-β are present in AH. In this study, we demonstrate that AH inhibits NK cell-mediated cytotoxicity in vitro, but does not affect cytotoxic T lymphocytemediated lysis. The inhibitory effect was immediate and occurred without preincubation of NK cells with aqueous humor. The inhibitory factor could not be neutralized by Abs to TGF-β, but was abrogated by treating AH with proteases. Inhibition was not simply due to lysis of NK cells, since AH did not alter NK cell viability. Initial purification using gel filtration chromatography showed that the activity was restricted to a single elution peak. Analysis of the active fractions by gel electrophoresis indicated that the inhibitory factor was a single protein of approximately 10 kilodaltons. The NK inhibitory factor might be an important modulator for protecting the corneal endothelium from unbridled NK cell-mediated injury and, thus, might be instrumental in preserving vision.