Retinal laser burn disrupts immune privilege in the eye

Homeostatic plasticity in the retina

Vision begins in the retina, whose intricate neural circuits extract salient features of the environment from the light entering our eyes. Neurodegenerative diseases of the retina (e.g., inherited retinal degenerations, age-related macular degeneration, and glaucoma) impair vision and cause blindness in a growing number of people worldwide. Increasing evidence indicates that homeostatic plasticity (i.e., the drive of a neural system to stabilize its function) can, in principle, preserve retinal function in the face of major perturbations, including neurodegeneration. Here, we review the circumstances and events that trigger homeostatic plasticity in the retina during development, sensory experience, and disease. We discuss the diverse mechanisms that cooperate to compensate and the set points and outcomes that homeostatic retinal plasticity stabilizes. Finally, we summarize the opportunities and challenges for unlocking the therapeutic potential of homeostatic plasticity. Homeostatic plasticity is fundamental to understanding retinal development and function and could be an important tool in the fight to preserve and restore vision.

The Role of Retinal Pigment Epithelial Cells in Regulation of Macrophages/Microglial Cells in Retinal Immunobiology

The ocular tissue microenvironment is immune privileged and uses several mechanisms of immunosuppression to prevent the induction of inflammation. Besides being a blood-barrier and source of photoreceptor nutrients, the retinal pigment epithelial cells (RPE) regulate the activity of immune cells within the retina. These mechanisms involve the expression of immunomodulating molecules that make macrophages and microglial cells suppress inflammation and promote immune tolerance. The RPE have an important role in ocular immune privilege to regulate the behavior of immune cells within the retina. Reviewed is the current understanding of how RPE mediate this regulation and the changes seen under pathological conditions.

Immune regulation in the aging retina

The retina is an immune privileged tissue, which is protected from external and internal insults by its blood-retina barriers and immune suppressive microenvironment. Apart from the avoidance and tolerance strategies, the retina is also protected by its own defense system, i.e., microglia and the complement system. The immune privilege and defense mechanisms work together to maintain retinal homeostasis. During aging, the retina is at an increased risk of developing various degenerative diseases such as age-related macular degeneration, diabetic retinopathy, and glaucomatous retinopathy. Previously, we have shown that aging induces a para-inflammatory response in the retina. In this review, we explore the impact of aging on retinal immune regulation and the connection between homeostatic control of retinal immune privilege and para-inflammation under aging conditions and present a view that may explain why aging puts the retina at risk of developing degenerative diseases.

Effect of Corneal Nerve Ablation on Immune Tolerance Induced by Corneal Allografts, Oral Immunization, or Anterior Chamber Injection of Antigens

Purpose

Severing corneal nerves during corneal transplantation does not affect first corneal transplants, but abolishes immune privilege of subsequent corneal allografts. This abrogation of immune privilege is attributable to the disabling of T regulatory cells (T regs) induced by corneal transplantation. The goal of this study was to determine if severing corneal nerves induces the development of contrasuppressor (CS) cells, which disable T regs that impair other forms of immune tolerance.

Methods

Effect of corneal nerve ablation on immune tolerance was assessed in four forms of immune tolerance: anterior chamber-associated immune deviation (ACAID); oral tolerance; corneal transplantation, and intravenously (IV) induced immune tolerance. T regulatory cell activity was assessed by adoptive transfer and by local adoptive transfer (LAT) of suppression assays.

Results

Corneal nerve ablation prevented ACAID and oral tolerance, but did not affect IV-induced immune tolerance. Contrasuppressor cells blocked the action of T regs that were generated by anterior chamber injection, oral tolerance, or orthotopic corneal transplantation. The neuropeptide substance P (SP) was crucial for contrasuppressor activity as CS cells could not be induced in SP-/- mice and the SP receptor inhibitor, Spantide II, prevented the expression of CS cell activity in vivo. Contrasuppressor cells expressed CD11c surface marker that identifies dendritic cells (DC).

Conclusions

The loss of immune privilege produced by corneal nerve ablation following corneal transplantation extends beyond the eye and also affects immune tolerance induced through mucosal surfaces and appears to be mediated by a novel cell population of CD11c+ CS cells that disables T regs.

Ocular Stem Cell Research from Basic Science to Clinical Application: A Report from Zhongshan Ophthalmic Center Ocular Stem Cell Symposium

Stem cells hold promise for treating a wide variety of diseases, including degenerative disorders of the eye. The eye is an ideal organ for stem cell therapy because of its relative immunological privilege, surgical accessibility, and its being a self-contained system. The eye also has many potential target diseases amenable to stem cell-based treatment, such as corneal limbal stem cell deficiency, glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa (RP). Among them, AMD and glaucoma are the two most common diseases, affecting over 200 million people worldwide. Recent results on the clinical trial of retinal pigment epithelial (RPE) cells from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) in treating dry AMD and Stargardt’s disease in the US, Japan, England, and China have generated great excitement and hope. This marks the beginning of the ocular stem cell therapy era. The recent Zhongshan Ophthalmic Center Ocular Stem Cell Symposium discussed the potential applications of various stem cell types in stem cell-based therapies, drug discoveries and tissue engineering for treating ocular diseases.

Ocular Immune Privilege and Transplantation

Allografts are afforded a level of protection from rejection within immune-privileged tissues. Immune-privileged tissues involve mechanisms that suppress inflammation and promote immune tolerance. There are anatomical features, soluble factors, membrane-associated proteins, and alternative antigen-presenting cells (APC) that contribute to allograft survival in the immune-privileged tissue. This review presents the current understanding of how the mechanism of ocular immune privilege promotes tolerogenic activity by APC, and T cells in response to the placement of foreign antigen within the ocular microenvironment. Discussed will be the unique anatomical, cellular, and molecular mechanisms that lessen the chance for graft destroying immune responses within the eye. As more is understood about the molecular mechanisms of ocular immune privilege greater is the potential for using these molecular mechanisms in therapies to prevent allograft rejection.

Macro- and microglial responses in the fellow eyes contralateral to glaucomatous eyes

Most studies employing experimental models of unilateral glaucoma have used the normotensive contralateral eye as the normal control. However, some studies have recently reported the activation of the retinal macroglia and microglia in the uninjured eye, suggesting that the eye contralateral to experimental glaucoma should not be used as a control. This review analyzes the studies describing the contralateral findings and discusses some of the routes through which the signals can reach the contralateral eye to initiate the glial reactivation.

Rod-like microglia are restricted to eyes with laser-induced ocular hypertension but absent from the microglial changes in the contralateral untreated eye

In the mouse model of unilateral laser-induced ocular hypertension (OHT) the microglia in both the treated and the normotensive untreated contralateral eye have morphological signs of activation and up-regulation of MHC-II expression in comparison with naïve. In the brain, rod-like microglia align to less-injured neurons in an effort to limit damage. We investigate whether: i) microglial activation is secondary to laser injury or to a higher IOP and; ii) the presence of rod-like microglia is related to OHT. Three groups of mice were used: age-matched control (naïve, n=15); and two lasered: limbal (OHT, n=15); and non-draining portion of the sclera (scleral, n=3). In the lasered animals, treated eyes as well as contralateral eyes were analysed. Retinal whole-mounts were immunostained with antibodies against, Iba-1, NF-200, MHC-II, CD86, CD68 and Ym1. In the scleral group (normal ocular pressure) no microglial signs of activation were found. Similarly to naïve eyes, OHT-eyes and their contralateral eyes had ramified microglia in the nerve-fibre layer related to the blood vessel. However, only eyes with OHT had rod-like microglia that aligned end-to-end, coupling to form trains of multiple cells running parallel to axons in the retinal surface. Rod-like microglia were CD68+ and were related to retinal ganglion cells (RGCs) showing signs of degeneration (NF-200+RGCs). Although MHC-II expression was up-regulated in the microglia of the NFL both in OHT-eyes and their contralateral eyes, no expression of CD86 and Ym1 was detected in ramified or in rod-like microglia. After 15 days of unilateral lasering of the limbal and the non-draining portion of the sclera, activated microglia was restricted to OHT-eyes and their contralateral eyes. However, rod-like microglia were restricted to eyes with OHT and degenerated NF-200+RGCs and were absent from their contralateral eyes. Thus, rod-like microglia seem be related to the neurodegeneration associated with HTO.

Mechanisms of immune privilege in the posterior eye

Immune privilege protects vital organs and their functions from the destructive interference of inflammation. Because the eye is easily accessible for surgical manipulation and for assessing and imaging the outcomes, the eye has been a major tissue for the study of immune privilege. Here, we focus on the immune regulatory mechanisms in the posterior eye, in part, because loss of immune privilege may contribute to development of certain retinal diseases in the aging population. We begin with a background in immune privilege and then focus on the select regulatory mechanisms that have been studied in the posterior eye. The review includes a description of the immunosuppressive environment, regulatory surface molecules expressed by cells in the eye, types of cells that participate in immune regulation and finally, discusses animal models of retinal laser injury in the context of mechanisms that overcome immune privilege.

Marked Effects of Tachykinin in Myositis Both in the Experimental Side and Contralaterally: Studies on NK-1 Receptor Expressions in an Animal Model

Muscle injury and inflammation (myositis) in a rabbit model of an unilateral muscle overuse were examined. It is unknown if the tachykinin system has a functional role in this situation. In this study, therefore, the neurokinin-1 receptor (NK-1R) expression patterns were evaluated. White blood cells, nerve fascicles, fine nerve fibers, and blood vessel walls in myositis areas showed NK-1R immunoreaction. NK-1R mRNA reactions were observable for white blood cells and blood vessel walls of these areas. NK-1R immunoreaction and NK-1R mRNA reactions were also seen for muscle fibers showing degenerative and regenerative features. There were almost no NK-1R immunoreactions in normal muscle tissue. Interestingly, marked NK-1R expressions were seen for myositis areas of both the experimental side and the contralateral nonexperimental side. EIA analyses showed that the concentration of substance P in the muscle tissue was clearly increased bilaterally at the experimental end stage, as compared to the situation for normal muscle tissue. These observations show that the tachykinin system is very much involved in the processes that occur in muscle injury/myositis. The effects can be related to proinflammatory effects and/or tissue repair. The fact that there are also marked NK-1R expressions contralaterally indicate that the tachykinin system has crossover effects.

Lack of thrombospondin 1 and exacerbation of choroidal neovascularization

OBJECTIVES

To assess the impact of thrombospondin 1(TSP1) deficiency on choroidal neovascularization (CNV)and to determine whether administration of a TSP1 antiangiogenic mimetic peptide attenuates CNV.

METHODS

The impact of TSP1 deficiency on laser induced CNV was assessed using wild-type (TSP1 +/+) and TSP1-deficient (TSP1 −/−) mice. Three laser burns were placed in each eye of TSP1 +/+ and TSP1 −/− mice to induce CNV. Intravitreal injection of the TSP1 mimetic peptide was performed on days 1 and 7 postlaser in the mice.For quantitative measurements of neovascularization, intercellular adhesion molecule 2 staining was performed at 14 days postlaser of the choroidal-sclera flat mounts. The recruitment of macrophages to the sites of damage was investigated by immunohistochemistry. The CNV area was measured by intercellular adhesion molecule 2 staining and use of ImageJ software.

RESULTS

The TSP1 −/− mice exhibited significantly larger areas of neovascularization on choroidal flat mounts compared with TSP1 +/ mice. This was consistent with enhanced recruitment of macrophages in TSP1 −/− mice compared with TSP1 +/+ mice 3 days postlaser. The development of CNV was significantly attenuated in mice receiving the TSP1 antiangiogenic mimetic peptide compared with those receiving vehicle alone.

CONCLUSIONS

Deficiency of TSP1 contributes to enhanced choroidal neovascularization. This is consistent with the anti-inflammatory and antiangiogenic activity of TSP1. The TSP1 antiangiogenic peptide was effective in attenuation of CNV.

CLINICAL RELEVANCE

Intravitreal injection of TSP1 antiangiogenic mimetic peptides may provide alternative treatment for CNV.

Retinal laser burn-induced neuropathy leads to substance P-dependent loss of ocular immune privilege

Inflammation in the eye is tightly regulated by multiple mechanisms that together contribute to ocular immune privilege. Many studies have shown that it is very difficult to abrogate the immune privileged mechanism called anterior chamber-associated immune deviation (ACAID). Previously, we showed that retinal laser burn (RLB) to one eye abrogated immune privilege (ACAID) bilaterally for an extended period of time. In an effort to explain the inflammation in the nonburned eye, we postulated that neuronal signals initiated inflammation in the contralateral eye. In this study, we test the role of substance P, a neuroinflamatory peptide, in RLB-induced loss of ACAID. Histological examination of the retina with and without RLB revealed an increase of the substance P-inducible neurokinin 1 receptor (NK1-R) in the retina of first, the burned eye, and then the contralateral eye. Specific antagonists for NK1-R, given locally with Ag within 24 h, but not 3, 5, or 7 d post-RLB treatment, prevented the bilateral loss of ACAID. Substance P knockout (KO) mice retained their ability to develop ACAID post-RLB. These data support the postulate that substance P transmits early inflammatory signals from the RLB eye to the contralateral eye to induce changes to ocular immune privilege and has a central role in the bilateral loss of ACAID. The possibility is raised that blocking of the substance P pathway with NK1-R antagonists postocular trauma may prevent unwanted and perhaps extended consequences of trauma-induced inflammation in the eye.

The Induction of Circulating, ACAID-Inducing Monocytes Requires CCR2/CCL2-Dependent Migration of Circulating F4/80(+) Cells into the Anterior Chamber

To determine the origin of peripheral blood mononulclear cells (PBMC) that activate regulatory T cells in anterior chamber-associated immune deviation (ACAID), fluorescein-labeled PBMC were intravenously injected into mice before the mice received an intracameral injection of antigen. Six-24 hr after intracameral injection, fluorescein-labeled PBMC increased in the iris. Twenty-four-48 hr labeled cells decreased in the iris and increased in the thymus and spleen. The entry of the labeled PBMC into the anterior chamber and subsequent production of PBMC that transfer ACAID required the expression of CCR2 by the PBMC and the production of the chemokine CCL2 by the recipient of the PBMC. The results suggest that the intracameral injection of antigen induces i) the infiltration of F4/80⁺ PBMC into the AC, ii) where these PBMC are converted to a regulatory phenotype, and iii) recirculate to activate T cells that suppress cell-mediated immunity.

Dendritic cell physiology and function in the eye

The eye and the brain are immunologically privileged sites, a property previously attributed to the lack of a lymphatic circulation. However, recent tracking studies confirm that these organs have good communication through classical site-specific lymph nodes, as well as direct connection through the blood circulation with the spleen. In addition, like all tissues, they contain resident myeloid cell populations that play important roles in tissue homeostasis and the response to foreign antigens. Most of the macrophage and dendritic cell (DC) populations in the eye are restricted to the supporting connective tissues, including the cornea, while the neural tissue (the retina) contains almost no DCs, occasional macrophages (perivascularly distributed), and a specialized myeloid cell type, the microglial cell. Resident microglial cells are normally programmed for immunological tolerance. The privileged status of the eye, however, is relative, as it is susceptible to immune-mediated inflammatory disease, both infectious and autoimmune. Intraocular inflammation (uveitis and uveoretinitis) and corneal graft rejection constitute two of the more common inflammatory conditions affecting the eye leading to considerable morbidity (blindness). As corneal graft rejection occurs almost exclusively by indirect allorecognition, host DCs play a major role in this process and are likely to be modified in their behavior by the ocular microenvironment. Ocular surface disease, including allergy and atopy, also comprise a significant group of immune-mediated eye disorders in which DCs participate, while infectious disease such as herpes simplex keratitis is thought to be initiated via corneal DCs. Intriguingly, some more common conditions previously thought to be degenerative (e.g. age-related macular degeneration) may have an autoimmune component in which ocular DCs and macrophages are critically involved. Recently, the possibility of harnessing the tolerizing potential of DCs has been applied to experimental models of autoimmune uveoretinitis with good effect. This approach has considerable potential for use in translational clinical therapy to prevent sight-threatening disease caused by ocular inflammation.