Immunosuppressive properties of the pigmented epithelial cells and the subretinal space.

The immune privilege of the anterior chamber of the eye has been recognized for over 100 years. However, the unique immunological properties of the pigmented epithelial (PE) cells of the eye and the subretinal space (SRS) have only recently been appreciated. The PE cells of the iris, ciliary body, and retina reside in anatomically disparate locations and serve distinctly different functions, yet share interesting immunomodulatory properties that contribute to ocular immune privilege. PE cells in the ciliary body and retina elaborate a variety of soluble factors that either directly or indirectly dampen immune-mediated inflammation; these include transforming growth factor-Beta, somatostatin, thrombospondin and pigment epithelial derived factor (PEDF). The constitutive expression of the immune co-stimulatory molecule, CD86, on iris PE cells not only inhibits T cell proliferation, but also promotes the generation of regulatory T cells. The SRS is now recognized as an immune-privileged site that shares many, but not all, of the properties ascribed to the anterior chamber, including the induction of systemic immune deviation. The prospect of therapeutic retinal transplantation and the possible immunologic etiology for some forms of age-related macular degeneration provides new impetus for gaining a better understanding of ocular immune privilege in the posterior regions of the eye.

B7+ iris pigment epithelial cells convert T cells into CTLA-4+, B7-expressing CD8+ regulatory T cells.

PURPOSE: To determine whether iris PE (IPE) promotes the generation of regulatory T-cells (Tregs) with cell contact via B7-2/CTLA-4 interactions. METHODS: T cells were cocultured with IPE cells obtained from eyes of normal and B7-deficient mice, x-irradiated, and used as regulators. IPE T regulator cells (IPE Tregs) of normal and CD28- or CTLA-4-deficient mice were established. Target bystander T cells were established from normal splenic T cells with anti-CD3 antibodies. T-cell activation was assessed for proliferation by [3H]-thymidine incorporation. Neutralizing anti-B7-1 and/or B7-2 antibodies, anti-CTLA-4 antibodies, CTLA-4-Ig fusion proteins were used to abolish regulatory function. IPE-exposed CD8+ T cells were evaluated for expression of B7, CTLA-4, and Foxp3 by using RT-PCR and flow cytometry. CD8+ IPE Tregs were depleted of B7-2+ and CTLA-4+ T cells and assayed for suppressive activity by adding them to bystander T cells. RESULTS: T cells acquired T regulatory activity when exposed to cultured IPE. Ciliary body PE cells did not promote conversion of T cells into Tregs. IPE converted CD8+, but not CD4+, T cells into Tregs by direct cell contact. In the conversion, IPE and responding T cells must both express endogenously synthesized B7-1 and B7-2, and the T cells must also express CTLA-4. Expression of CD28 molecules was not necessary for Treg generation. In addition, the CD8+ Tregs that fully suppress activation of bystander T cells expressed Foxp3. CONCLUSIONS: IPE cells promote conversion of T cells into Tregs solely through a contact-dependent mechanism. T cells exposed to IPE cells acquire full regulatory capacity.

Pigment epithelial growth factor suppresses inflammation by modulating macrophage activation.

PURPOSE: To study the contribution of murine retinal pigment epithelial (RPE) cells to the innate immune-privilege status of the subretinal space as determined by the ability of pigment epithelial-derived factor (PEDF) and somatostatin (SOM), produced by RPE, to regulate macrophage-mediated inflammation. METHODS: Serum-free medium was added to RPE eyecups (a healthy monolayer of RPE resting on choroid and sclera) and the supernatants were removed after 24 hours (RPE SN). The RPE SN was assayed for the presence of PEDF and SOM and for its ability to regulate interleukin (IL)-12, IL-10, and nitric oxide (NO) production by resting and activated macrophages. A group of mice received intradermal injection of lipopolysaccharide (LPS) and PEDF in one ear and LPS alone in the other ear. Ear thickness was measured before- and 24 hours after ear injections. RESULTS: Soluble factors present in the RPE SN inhibited IL-12 production and substantially increased IL-10 while having minimal effects on NO production by activated macrophages. The message for PEDF, SOM, and IL-10 was detected in RPE cells, and the protein for these factors was found in the RPE SN. The stimulation of IL-10 and suppression of IL-12 production by RPE-SN-treated macrophages was neutralized by anti-PEDF antibodies. Neutralization of SOM in the RPE SN, suppressed NO production by activated macrophages. Intradermal injection of PEDF substantially inhibited LPS-induced inflammatory response. CONCLUSIONS: PEDF inhibits LPS-driven macrophage activation in vitro and in vivo. By producing PEDF, the RPE contributes to innate immune privilege of the eye.

Nonvascular VEGF receptor 3 expression by corneal epithelium maintains avascularity and vision.

Transparency of the cornea, the window of the eye, is a prerequisite for vision. Angiogenesis into the normally avascular cornea is incompatible with good vision and, therefore, the cornea is one of the few tissues in the human body where avascularity is actively maintained. Here, we provide evidence for a critical mechanism contributing to corneal avascularity. VEGF receptor 3, normally present on lymphatic and proliferating blood vascular endothelium, is strongly constitutively expressed by corneal epithelium and is mechanistically responsible for suppressing inflammatory corneal angiogenesis.

Time course of angiogenesis and lymphangiogenesis after brief corneal inflammation.

PURPOSE: To study the time course of angiogenesis and lymphangiogenesis in the cornea after a short inflammatory insult. This might be helpful for the timing of corneal transplantation in high-risk eyes. METHODS: The mouse model of suture-induced inflammatory corneal neovascularization was used. After placement of 3 interrupted 11-0 sutures into the corneal stroma of BALB/c mice (left in place for 14 days), corneas were excised 2, 3, 5, 7, 14, and 21 days as well as 1, 2, 3, 6, and 8 months after surgery. Hem- and lymphangiogenesis were evaluated using double immunohistochemistry of corneas with CD31/PECAM1 as panendothelial and LYVE-1 as lymphatic endothelial marker. RESULTS: Both blood and lymphatic vessels grew into the cornea as early as day 2 after suture placement. The outgrowth was initially parallel. Hem- and lymphangiogenesis peaked around day 14. Thereafter, both vessel types started to regress. Regression of lymphatic vessels started earlier and was more pronounced than that of blood vessels. Whereas at 6 and 8 months (partly) perfused CD31+++/LYVE-1(-) blood vessels and (nonperfused) ghost vessels could still be observed, there were no CD31+/LYVE-1+++ lymphatic vessels detectable beyond 6 months after this short inflammation. CONCLUSIONS: After a temporary inflammatory insult to the cornea, there is initially parallel outgrowth of both blood and lymphatic vessels. But thereafter, lymphatic vessels regress earlier than blood vessels and are completely regressed by 6 months. Earlier regression of pathologic corneal lymph versus blood vessels suggests that corneal graft survival in high-risk eyes might best be delayed for a prolonged interval following an inflammatory insult.

CD8+ T regulatory cells use a novel genetic program that includes CD103 to suppress Th1 immunity in eye-derived tolerance.

PURPOSE: The peripheral tolerance that arises after injection of antigen into the anterior chamber (anterior chamber-associated immune deviation; ACAID) is associated in part with CD8+ T cells that suppress the expression of Th1 and Th2 immunity. The purpose of these studies was to determine the genes and molecules that are critical for CD8+ T regulatory cell (T reg) functions in ACAID. METHODS: Ovalbumin (OVA)-specific CD8+ T cells from T-cell receptor (TCR) transgenic OT-1 mice acquire efferent regulatory properties similar to in vivo-generated CD8+ T regs after stimulation with OVA-pulsed TGF-beta2-treated APCs. Changes in the genetic program associated with acquisition of efferent regulatory function in OT-1 CD8+ T cells in vitro were determined by DNA microarray analyses and confirmed by RT-PCR analyses and biological assays. RESULTS: T regulatory OT-1 T cells acquired a novel transcriptional pattern indicative of their function. Genes for molecules associated with TGF-beta function, resistance to TCR-triggered apoptosis, and localization of cells to antigen deposition in peripheral tissues were upregulated, and genes related to cytolytic function were downregulated. Further study showed that CD103, a cell-adhesion molecule that binds E-cadherin, was highly upregulated in in vivo-generated ACAID T regs and was necessary for their suppression of T-cell activation in vitro. CONCLUSIONS: OT-1 CD8 T cells modulated in vitro by exposure to antigen-pulsed, TGF-beta2-treated APCs expressed genes related to immune suppression. Thus, the necessity for CD103 emerges in the efferent CD8+ T-cell regulatory mechanisms in eye-derived tolerance.

Thrombospondin orchestrates the tolerance-promoting properties of TGFbeta-treated antigen-presenting cells.

Eye-derived antigen-presenting cells (APCs) are known to contribute to the immune privilege status of the eye by inducing a form of peripheral tolerance that deviates T(h)1 type of pro-inflammatory immune responses. Similar systemic tolerance can also be induced by non-ocular APCs exposed to transforming growth factor beta (TGFbeta) in vitro. Such APCs were found to express enhanced levels of thrombospondin (TSP)-1, an extracellular matrix (ECM) protein. In this report, we analyzed the significance of TSP-1 in conferring tolerance-inducing properties on APCs. While TSP-treated APCs matched TGFbeta-treated APCs in their functional ability to induce systemic tolerance, a deficiency of TSP-1 or its receptor CD36 prevented APCs from becoming tolerogenic in response to TGFbeta. Exogenous TSP-1 restored tolerogenic ability of TGFbeta-treated TSP-1 null APCs. Both TGFbeta-treated TSP-1 null and CD36 knockout APCs failed to inhibit IL-12 secretion. Furthermore, TGFbeta-treated TSP-1 null APCs, unlike similarly treated wild-type APCs, failed to increase secretion of active TGFbeta. Similar to TGFbeta, TSP could also up-regulate expression of MIP-2, TGFbeta2 and tumor necrosis factor alpha-all of which are required for tolerance induced by TGFbeta-treated APCs. We conclude that TSP-1, an ECM protein induced by TGFbeta treatment, orchestrates the changes in APC functional programs that equip these cells to promote tolerance of the eye-derived type.

Induction of eye-derived tolerance does not depend on naturally occurring CD4+CD25+ T regulatory cells.

PURPOSE: Regulatory CD4+ T cells (T regs) arise in the spleens of mice with anterior chamber-associated immune deviation (ACAID), an eye-derived tolerance evoked by injection of antigen into the ocular anterior chamber (AC). The current study was conducted to investigate the possibility that these T regs express CD25 and are derived from natural CD4+CD25+ T cells. METHODS: Naïve T cells from DO11.10 mice were activated in vitro by ovalbumin (OVA)-pulsed, TGFbeta-treated antigen-presenting cells (APCs), and the expression of CD25 assayed by flow cytometry. OVA-specific ACAID T regs were obtained from the spleens of DO11.10 mice with ACAID to OVA. Immunomagnetic enrichment was used to sort out CD4+CD25+, and CD4+CD25- ACAID T cells before they were injected into OVA-immunized mice or examined for mRNA expression of the regulatory T-cell transcription factor Foxp3. In addition, before AC injection of OVA, systemic depletion of CD25+ T cells was performed with injections of anti-IL-2 receptor antibody into the mice. RESULTS: OVA-specific T cells from DO11.10 mice expressed CD25 when exposed to OVA-pulsed, TGFbeta-treated APCs, even when the DO11.10 T cells were depleted of CD25+ cells before their in vitro stimulation. In addition, DH was suppressed in naïve mice that were injected with CD4+CD25+ or CD4+CD25- ACAID T cells. The CD4+CD25+, but not the CD4+CD25-, ACAID T regs expressed Foxp3. Finally, OVA induced ACAID in mice depleted of CD25+ cells. CONCLUSIONS: Some of the CD4+ T regs of ACAID arise from CD25- precursors, and the induction of ACAID is not dependent on the presence of natural CD4+CD25+ T regs.