Ocular immune privilege in the immunosuppressive intraocular microenvironment

Immune privilege exists in the normal eye (anterior chamber, vitreous cavity, subretinal space): when foreign tissue/cellular grafts are placed in the eye, rejection is largely, if not completely, avoided. The ineffectiveness of the immune response in this circumstance results from eye-dependent alterations in (a) the induction of systemic immunity to graft antigens, and (b) the expression of cell-mediated immunity within the eye. Recent evidence indicates that eye-derived factors, present in the intraocular microenvironment, modify the afferent and efferent limbs of the immune response. Studies of aqueous humor as a prototype of the intraocular microenvironment have demonstrated that this fluid profoundly inhibits antigen-driven T cell activation such that proliferation and lymphokine production are curtailed. Moreover, aqueous humor modifies the functional properties of conventional antigen presenting cells (APC) such that exogenous antigens are processed uniquely. When antigen-pulsed APC exposed to aqueous humor are injected intravenously, they induce Anterior Chamber Associated Immune Deviation (ACAID). Several factors in aqueous humor have been implicated as immunosuppressants: transforming growth factorbeta, alpha-melanocyte stimulating hormone, vasoactive intestinal peptide, calcitonin gene related peptide, and hydrocortisone (because Cortisol binding globulin is virtually absent). In certain experimentally-induced ocular disorders, resident ocular cells fail to secrete immunosuppressive factors constitutively, and this may explain why immune privilege is also lost.