Thrombospondin Plays a Vital Role in the Immune Privilege of the Eye

Abstract

Purpose: The role of thrombospondin (TSP)-1 in TGF-β activation and T-cell suppression was studied in the retinal pigment epithelial (RPE) cells, a monolayer of pigmented cells that line the subretinal space, an immune-privileged site in the eye. Methods: Posterior eyecups were prepared by excising the anterior segment, lens, and retina from enucleated eyes of C57BL/6, thrombospondin-1 knockout (TSP-1KO), and TGF-β2 receptor II double-negative (TGF-β2 RII DN) mice, leaving behind a healthy monolayer of RPE resting on choroid and sclera. Serum-free medium was added to these RPE eyecups, and, after various time intervals, supernatants (SNs) were removed and tested. Results: SNs of an ex vivo culture of RPE cells from C57BL/6 mice were shown to inhibit both antigen and anti-CD3 activation of T cells, partially due to constitutive production of TGF-β and to the ability of RPE to activate the latent form of TGF-β. Activation of TGF-β was entirely dependent on TSP-1, also produced by RPE. SNs of RPE from TSP-1KO mice failed to inhibit T-cell activation. Ovalbumin (OVA)-specific delayed hypersensitivity (DH) was not impaired when OVA was injected either into the subretinal space or into the anterior chamber of TSP-1KO mice before OVA immunization. Moreover, experimental autoimmune uveoretinitis was significantly more intense in eyes of TSP-1KO mice and failed to undergo spontaneous resolution unlike wild-type mice. Conclusions: Production of both TSP-1 and active TGF-β by RPE is essential to the creation and maintenance of immune privilege in the subretinal space and that the immune privilege limits the severity and duration of retinal inflammation due to autoimmunity.