Immune privilege: keeping an eye on natural killer T cells.

Abstract

The concept of immune privilege refers to the observation that tissue grafts placed in certain anatomical sites, including the brain and eye, can survive for extended periods of time 1. Immune privilege is thought to reflect an evolutionary adaptation to protect vital structures from damage by inflammatory responses directed against pathogens. It was originally believed that antigens in immune-privileged sites are concealed from the immune system by physical barriers and therefore ignored. However, subsequent studies have shown that antigens do leave immune-privileged sites, that these antigens can induce immune responses, and that immune effector cells can have access to immune-privileged sites. It is now clear that immune privilege is maintained by an active rather than passive process 234. The eye is an excellent example of an immune-privileged site. This organ enjoys immune privilege to protect it from destructive inflammation that may impair vision. Immune privilege in the eye is attributed to a variety of mechanisms (for reviews, see references 2–4), including lack of lymphatic drainage, low expression of MHC molecules, increased expression of surface molecules (CD59, membrane cofactor protein, and decay-accelerating factor) that inhibit complement activation, local production of immunosuppressive cytokines such as TGF-β, presence of neuropeptides, and constitutive expression of Fas ligand (FasL). The latter mechanism controls the entry of Fas-expressing lymphoid cells into the eye 5. Additionally, antigen presentation in the eye can elicit regulatory T (Tr) cells that induce immune deviation and suppress inflammatory responses, both locally and systemically (for a review, see reference 2). In this issue, Sonoda et al. 6 demonstrate that immune deviation induced by antigens placed in the anterior chamber (AC) of the eye requires CD1d-reactive natural killer T (NKT) cells. These findings provide insights into the maintenance of immune privilege and indicate that NKT cells play a critical role in the regulation of immune responses.