SIGNIFICANCE OF IMMUNOLOGICAL SEQUESTRATION IN THE EYE

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DESCRIPTION: (from abstract). The eye contains immune privileged tissues. Immune privilege refers to the attenuated nature of immune responses fund in certain tissues, such as the eye. Immune privilege is thought to enable the eye to respond to antigenic challenges in ways that preserve delicate structures upon which vision depends. Immune privilege of the eye was observed long ago, but is not yet understood. A long standing question concerning mechanisms of ocular immune privilege is the role of sequestration, the passive immune tolerance attributed to localization of antigens behind anatomic barriers. In the eye, these barriers include the retinal vascular endothelium and retinal pigment epithelium, which comprise the "blood/retinal barrier." Sequestration was postulated to reduce lymphatic perusal of retina and minimize leakage of antigens from this tissue. Absence of interaction between the immune system and antigens can lead to ignorance of those antigens. Most current studies of ocular immune privilege concentrate on elucidating active mechanisms of tolerance, termed immune deviation, calling the significance of sequestration into question. Assessment of the contribution of sequestration to retinal immune privilege in normal animals has been difficult because adequate negative controls were not available, analysis requires comparison of immune responses of animals that express an antigen in sequestered versus non-sequestered sites, especially in the absence of mechanical compromises to the blood-retinal barrier. Using genetic approaches, our two experimental strategies for achieving these conditions have been fruitful. The PI reported that transgenic (Tg) expression of beta-galactosidase (b-gal) in mouse retina created a target for b-gal-mediated EAU by C04 T cells, but the mice were not tolerant to b-gal, appealing ignorant of it unless antigen-specific, activated C04 T cells were raised. His other approach showed UM retroviral transduction of the rat retinal S-Ag (A/K/A arrestin) gene into rat bone marrow-derived cells led to loss of susceptibility to EAU induction by rat S-Ag peptides. The straight-forward conclusion is that sequestration inhibits induction of active tolerance by retinal antigens, and contributes substantially to retinal immune privilege in normal eyes. Important questions remain, and he proposes to continue using the Tg mice. First, our observation of a lack of tolerance could be explained by proposing that adjuvant-driven immunization, or adoptive transfer of activated CD4 T cells, overwhelmed existing, active mechanisms of tolerance. Using T cell receptor Tg mice, we will ask if naive CD4 T cells specific for b-gal become exposed to retinal b-gal in normal or damaged, or inflamed eyes. If so, what is the outcome? The PI will extend these questions to CD8 T cells. Second, since potent active ocular immune deviation mechanisms have been experimentally induced, do they apply to normal eyes? Third since our studies of sequestration to date show it to be important, do active mechanisms provide "back-up' for sequestration in damaged or inflamed eyes?