A Current Understanding of Ocular Immune Privilege

Immune privileged mechanisms allow the eye to be protected from the pathological consequences of inflammation by expressing immune responses that do not elicit inflammation. These mechanisms are established with rigor and very few experimental events have been capable of aborting immune privilege in the ocular environment. The multiple overlapping mechanisms that contribute to the totality of ocular immune privilege are reviewed here, in the light of contemporary knowledge of immune homeostasis throughout the organisms. The review considers the regulatory mechanisms in terms of 1) physical and structural barriers that lessen the availability of immune cells to reach the eye; 2) activities that prevent the immune response from being activated in the ocular tissue; 3) events that actively induce apoptosis or anergy of the immune cells; 4) the protective system of the pigmented cells that line the border of the eye and prevent immune activation or actually modulate the function of the activated immune cells; and 5) mechanisms of anterior chamber immune deviation, ACAID, a paradigm of mechanisms that induces Treg cells in the periphery to seed the local area and is proposed to contribute to self tolerance of ocular antigens. The consequences of losing immune privilege are considered.