Retinal Pigment Epithelial Cells Suppress Phagolysosome Activation in Macrophages

Ocular immune privilege and retinal pigment epithelial cells

The ocular tissue microenvironment is immune-privileged and uses multiple immunosuppressive mechanisms to prevent the induction of inflammation. The retinal pigment epithelium plays an essential role in ocular immune privilege. In addition to serving as a blood barrier separating the fenestrated choriocapillaris from the retina, the retinal pigment epithelium is a source of immunosuppressive cytokines and membrane-bound negative regulators that modulate the activity of immune cells within the retina. This article reviews the current understanding of how retinal pigment epithelium cells mediate immune regulation, focusing on the changes under pathologic conditions.

The innate immune receptor Nlrp12 suppresses autoimmunity to the retina

Background

Nod-like receptors (NLRs) are critical to innate immune activation and induction of adaptive T cell responses. Yet, their role in autoinflammatory diseases of the central nervous system (CNS) remains incompletely defined. The NLR, Nlrp12, has been reported to both inhibit and promote neuroinflammation in an animal model of multiple sclerosis (experimental autoimmune encephalomyelitis, EAE), where its T cell-specific role has been investigated. Uveitis resulting from autoimmunity of the neuroretina, an extension of the CNS, involves a breach in immune privilege and entry of T cells into the eye. Here, we examined the contribution of Nlrp12 in a T cell-mediated model of uveitis, experimental autoimmune uveitis (EAU).

Methods

Mice were immunized with interphotoreceptor retinoid-binding protein peptide 1–20 (IRBP_1–20) emulsified in Complete Freund’s adjuvant, CFA. Uveitis was evaluated by clinical and histopathological scoring, and comparisons were made in WT vs. Nlrp12^−/− mice, lymphopenic Rag1^−/− mice reconstituted with WT vs. Nlrp12^−/− CD4⁺ T cells, or among bone marrow (BM) chimeric mice. Antigen-specific Th-effector responses were evaluated by ELISA and intracellular cytokine staining. Cellular composition of uveitic eyes from WT or Nlrp12^−/− mice was compared using flow cytometry. Expression of Nlrp12 and of cytokines/chemokines within the neuroretina was evaluated by immunoblotting and quantitative PCR.

Results

Nlrp12^−/− mice developed exacerbated uveitis characterized by extensive vasculitis, chorioretinal infiltrates and photoreceptor damage. Nlrp12 was dispensable for T cell priming and differentiation of peripheral Th1 or Th17 cells, and uveitis in immunodeficient mice reconstituted with either Nlrp12^−/− or WT T cells was similar. Collectively, this ruled out T cells as the source of Nlrp12-mediated protection to EAU. Uveitic Nlrp12^−/− eyes had more pronounced myeloid cell accumulation than uveitic WT eyes. Transplantation of Nlrp12^−/− BM resulted in increased susceptibility to EAU regardless of host genotype, but interestingly, a non-hematopoietic origin for Nlrp12 function was also observed. Indeed, Nlrp12 was found to be constitutively expressed in the neuroretina, where it suppressed chemokine/cytokine induction.

Conclusions

Our data identify a combinatorial role for Nlrp12 in dampening autoimmunity of the neuroretina. These findings could provide a pathway for development of therapies for uveitis and potentially other autoinflammatory/autoimmune diseases of the CNS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02425-x.

Melanocortin receptor agonists suppress experimental autoimmune uveitis

The melanocortin system plays an essential role in the regulation of immune activity. The anti-inflammatory microenvironment of the eye is dependent on the expression of the melanocortin-neuropeptide alpha-melanocyte stimulating hormone (α-MSH). In addition, the melanocortin system may have a role in retinal development and retinal cell survival under conditions of retinal degeneration. We have found that treating experimental autoimmune uveitis (EAU) with α-MSH suppresses retinal inflammation. Also, this augmentation of the melanocortin system promotes immune tolerance and protection of the retinal structure. The benefit of α-MSH-therapy appears to be dependent on different melanocortin receptors. Therefore, we treated EAU mice with α-MSH-analogs with different melanocortin-receptor targets. This approach demonstrated which melanocortin-receptors suppress inflammation, preserve retinal structure, and induce immune tolerance in uveitis. At the chronic stage of EAU the mice were injected twice 1 day apart with 50 μg of α-MSH or an α-MSH-analog. The α-MSH-analogs were a pan-agonist PL8331, PL8177 (potent MC1r-only agonist), PL5000 (a pan-agonist with no MC5r functional activity), MT-II (same as PL5000) and PG901 (MC5r agonist, but also an antagonist to MC3r, and MC4r). Clinical EAU scores were measured until resolution in the α-MSH-treated mice, when the eyes were collected for histology, and spleen cells collected for retinal-antigen-stimulated cytokine production. Significant suppression of EAU was seen with α-MSH or PL8331 treatment. This was accompanied with significant preservation of retinal structure. A similar effect was seen in EAU-mice that were treated with PL8177, except the suppression of EAU was temporary. In EAU mice treated with PL5000, MTII, or PG901, there was no suppression of EAU with a significant loss in whole retina and outer-nuclear layer thickness. There was significant suppression of IL-17 with induction of IL-10 by retinal-antigen stimulated spleen T cells from EAU mice treated with α-MSH, PL8331, PL8177, or PL5000, but not from EAU mice treated with MT-II, or PG901. Our previous studies show the melanocortin system's importance in maintaining ocular immune privilege and that α-MSH-treatment accelerates recovery and induces retinal-antigen-specific regulatory immunity in EAU. Our current results show that this activity is centered around MC1r and MC5r. In addition, the results suggest that a therapeutic potential to target MC1r and MC5r together to suppress uveitis induces regulatory immunity with potentially maintaining a normal retinal structure.

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.

Extracellular Soluble Membranes from Retinal Pigment Epithelial Cells Mediate Apoptosis in Macrophages

A central characterization of retinal immunobiology is the prevention of proinflammatory activity by macrophages. The retinal pigment epithelial cells (RPEs) are a major source of soluble anti-inflammatory factors. This includes a soluble factor that induces macrophage apoptosis when the activity of the immunomodulating neuropeptide alpha-melanocyte-stimulating hormone (α-MSH) is neutralized. In this manuscript, isolated extracellular soluble membranes (ESMs) from primary RPE were assayed to see if they could be the soluble mediator of apoptosis. Our results demonstrated that RPE ESMs mediated the induction of macrophage apoptosis that was suppressed by α-MSH. In contrast, the RPE line ARPE-19, cultured under conditions that induce similar anti-inflammatory activity to primary RPEs, did not activate apoptosis in the macrophages. Moreover, only the ESMs from primary RPE cultures, and not those from the ARPE-19 cell cultures, expressed mFasL. The results demonstrate that RPE ESMs are a soluble mediator of apoptosis and that this may be a mechanism by which the RPEs select for the survival of α-MSH-induced suppressor cells.

Melanocortin 5 Receptor Expression and Recovery of Ocular Immune Privilege after Uveitis

Purpose: The therapeutic use of the RPE-neuropeptide α-MSH suppresses experimental autoimmune uveitis (EAU). This suppression is partially through the α-MSH melanocortin 5 receptor (MC5r). Therefore, we examined the possible role of MC5r-expression in the recovery of RPE suppression of phagolysosome-activation in macrophages following α-MSH-treatment of EAUMethods: The conditioned media of cultured in situ RPE-eyecup from α-MSH-treated EAU wild-type and MC5r(-/-) mice were used to treat macrophages to assay for phagolysosome activation.Results: MC5r(-/-) mice treated with α-MSH recovered from EAU, but with greater retinal damage, and the RPE suppressed phagolysosome activation in wild type but not in MC5r(-/-) macrophages. In addition, α-MSH did not suppress phagolysosome activation in MC5r(-/-) macrophages, and resting-MC5r(-/-) macrophages had augmented phagocytic activity.Conclusion: α-MSH treatment of EAU mediates a MC5r-dependent recovery of RPE suppression of phagolysosome activation in macrophages possibly altering antigen processing and presentation. Also, MC5r-expression helps protect the retina from inflammatory damage. In addition, MC5r-expression is important in the homeostatic maintenance of phagosome-maturation within macrophages.

Neuropeptide Y Is an Immunomodulatory Factor: Direct and Indirect

Neuropeptide Y (NPY), which is widely distributed in the nervous system, is involved in regulating a variety of biological processes, including food intake, energy metabolism, and emotional expression. However, emerging evidence points to NPY also as a critical transmitter between the nervous system and immune system, as well as a mediator produced and released by immune cells. In vivo and in vitro studies based on gene-editing techniques and specific NPY receptor agonists and antagonists have demonstrated that NPY is responsible for multifarious direct modulations on immune cells by acting on NPY receptors. Moreover, via the central or peripheral nervous system, NPY is closely connected to body temperature regulation, obesity development, glucose metabolism, and emotional expression, which are all immunomodulatory factors for the immune system. In this review, we focus on the direct role of NPY in immune cells and particularly discuss its indirect impact on the immune response.

Oxidative stress-induced KLF4 activates inflammatory response through IL17RA and its downstream targets in retinal pigment epithelial cells

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness worldwide. Oxidative stress (OS), inflammation and genetics are considered the key pathogenic factors contributing to AMD development. Recent evidence shows the pro-inflammatory interleukin 17 (IL17) signaling is activated in AMD patients and promotes disease pathogenesis. However, the interplay between OS and IL17 signaling, and the regulatory mechanism of IL17 pathway are largely unknown. OS-induced retinal pigment epithelial cell (RPE) damage causes both the initial pathogenesis of AMD and secondary degeneration of rods and cones. Healthy RPE is essential for ocular immune privilege, however, damaged RPE cells can activate inflammatory response. In the present study, we identified IL17RA, the principle receptor of IL17 signaling, is one of the most upregulated inflammatory genes in human RPE cells upon OS exposure. The prominent increase of IL17RA was also observed in RPE and retina of an AMD-like mouse model. Knockdown of IL17RA in RPE cells prevented OS-induced RPE cell apoptosis and reduced the inflammatory response in both RPE and macrophages. Furthermore, we found that transcription factor KLF4 directly activates IL17RA expression, therefore, promotes the production of IL1β and IL8 in an IL17RA-dependent manner. In addition, the mRNA level of KLF4 isoform 2 was positively correlated with that of IL17RA in AMD patients. Together, our study demonstrates an unrevealed relationship between IL17RA and OS, and a new regulatory mechanism of IL17RA by KLF4 in RPE cells. These findings suggest that inhibition of IL17RA as a new potential therapeutic target for AMD through RPE protection and inflammatory suppression upon OS exposure.

Chitinase-3-Like-1 Promotes M2 Macrophage Differentiation and Induces Choroidal Neovascularization in Neovascular Age-Related Macular Degeneration

Purpose

Choroidal neovascularization (CNV) is the principal pathological factor contributing to blindness in neovascular age-related macular degeneration (nAMD). Infiltration of M2 macrophage is thought to contribute to CNV progress, although the way that regulates its differentiation remains unclear. Here, we investigate the role of CHI3L1 in M2 differentiation and angiogenesis in CNV.

Methods

Serums from nAMD patients were tested for CHI3L1 expression. Mice were subjected to laser injury to induce CNV, and lesion expansion were tracked using fundus fluorescence angiography (FFA) and immunofluorescence analysis. Several strategies were taken to verify the contribution of M2 macrophage and CHI3L1: macrophage depletion by clodrosome, local CHI3L1 inhibition using intravitreally injection neutralize antibody (mAY), and depletion of CHI3L1 receptor (IL13-Ra2) by small-interfering RNA (siRNA). Tuber analysis was used to further determine angiogenetic effect of CHI3L1. Anti-VEGFA was used as positive control for mAY.

Results

Serum levels of CHI3L1 were highly elevated in nAMD patients. CHI3L1 was expressed by infiltrating M2 macrophages and was elevated as CNV progress in a mice model. System macrophage depletion and local suppression of CHI3L1 alleviated CNV formation while enhancing anti-VEGFA therapeutic effect. Stimulation of macrophage with recombinant CHI3L1 activated MAPK signaling cascade and induced transition to M2, while siRNA knockdown of IL13-Ra2 abolished it. In an in vitro coculture system, supernatants from CHI3L1-stimulated M2 macrophages and promoted tube vascularization.

Conclusions

These results unveil novel angiogenic regulation of CHI3L1 and M2 polarized macrophages in CNV development. These mechanistic insights may point to CHI3L1 as a new therapeutic target for treatment for nAMD.

Peripheral retinal avascularity and capillary leakage in central serous chorioretinopathy

Purpose

To report a novel finding of peripheral retinal avascularity (PRA) and peripheral capillary leakage (PCL) on wide-field fluorescein angiography (WFA) in non-dependent quadrants, in eyes with bilateral chronic central serous chorioretinopathy (CSCR).

Methods

Forty six patients with bilateral CSCR were studied. Four patients had PRA and PCL, and 42 patients did not. The demographic profile, clinical findings, and imaging characteristics of the two groups were compared.

Results

There was no significant difference between those patients with and without PRA and PCL with respect to the demographic profile, clinical findings, and imaging characteristics. Laser photocoagulation to extrafoveal points of leakage seen on fluorescein angiography (FA) was sufficient to cause complete resolution of CSCR in these 4 patients. PRA areas were not treated.

Conclusion

The PRA and PCL in CSCR are novel findings, which have not been previously described.

The Neuropeptides of Ocular Immune Privilege, α-MSH and NPY, Suppress Phagosome Maturation in Macrophages

The ocular microenvironment has evolutionarily adapted several mechanisms of immunosuppression to minimize the induction of inflammation. Neuropeptides produced by the retinal pigment epithelial cells regulate macrophage activity. Two neuropeptides, α-melanocyte–stimulating hormone (α -MSH) and neuropeptide Y (NPY), are constitutively expressed by the retinal pigment epithelial cells. Together these two neuropeptides induce anti-inflammatory cytokine production in endotoxin-stimulated macrophages and suppress phagocytosis of unopsonized bioparticles. These neuropeptides do not suppress the phagocytosis of opsonized bioparticles; however, they do suppress phagolysosome activation or formation. In this report, we studied the possibility that α-MSH with NPY suppress phagosome maturation within macrophages using opsonized OVA-coated magnetic beads to isolate and analyze the phagosomes. The magnetic bead–containing intercellular vesicles were isolated and assayed for Rab5, Rab7, LAMP1, Ia^d, and OVA. The macrophages cotreated with α-MSH and NPY were suppressed in Rab7 recruitment to the phagosome with suppression in LAMP1 expression but not in Ia^d expression. The results demonstrated that the α-MSH/NPY cotreatment suppressed phagosome maturation. In addition, the a-MSH/NPY–cotreated macrophages were suppressed in their ability to Ag stimulate CD4⁺ T cell proliferation. These results imply a potential mechanism of ocular immune privilege to divert Ag processing to prevent autoreactive effector T cells from binding their target cognate Ag within the ocular microenvironment.

Negative regulators that mediate ocular immune privilege

The ocular microenvironment has adapted several negative regulators of inflammation to maintain immune privilege and health of the visual axis. Several constitutively produced negative regulators within the eye TGF-β2, α-melanocyte stimulating hormone (α-MSH), Fas ligand (FasL), and PD-L1 standout because of their capacity to influence multiple pathways of inflammation, and that they are part of promoting immune tolerance. These regulators demonstrate the capacity of immune privilege to prevent the activation of inflammation, and to suppress activation of effector immune cells even under conditions of ocular inflammation induced by endotoxin and autoimmune disease. In addition, these negative regulators promote and expand immune cells that mediate regulatory and tolerogenic immunity. This in turn makes the immune cells themselves negative regulators of inflammation. This provides for a greater understanding of immune privilege in that it includes both molecular and cellular negative regulators of inflammation. This would mean that potentially new approaches to the treatment of autoimmune disease can be developed through the use of molecules and cells as negative regulators of inflammation.