Myeloid suppressor cells induced by retinal pigment epithelial cells inhibit autoreactive T-cell responses that lead to experimental autoimmune uveitis

Dual roles of myeloid-derived suppressor cells in various diseases: a review

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that originate from bone marrow stem cells. In pathological conditions, such as autoimmune disorders, allergies, infections, and cancer, normal myelopoiesis is altered to facilitate the formation of MDSCs. MDSCs were first shown to promote cancer initiation and progression by immunosuppression with the assistance of various chemokines and cytokines. Recently, various studies have demonstrated that MDSCs play two distinct roles depending on the physiological and pathological conditions. MDSCs have protective roles in autoimmune disorders (such as uveoretinitis, multiple sclerosis, rheumatoid arthritis, ankylosing spondylitis, type 1 diabetes, autoimmune hepatitis, inflammatory bowel disease, alopecia areata, and systemic lupus erythematosus), allergies, and organ transplantation. However, they play negative roles in infections and various cancers. Several immunosuppressive functions and mechanisms of MDSCs have been determined in different disease conditions. This review comprehensively discusses the associations between MDSCs and various pathological conditions and briefly describes therapeutic approaches.

Local administration of myeloid-derived suppressor cells prevents progression of immune-mediated dry eye disease

Myeloid derived suppressor cells (MDSCs) are a heterogenous population of immature hematopoietic precursors with known immunoregulatory functions. The immunosuppressive role of MDSCs has been highlighted in several inflammatory ophthalmic disorders; however, their therapeutic application in suppressing the immune-mediated changes in dry eye disease (DED) has not been studied. We observed significant reduction in antigen presenting cell (APC) frequencies and their maturation in the presence of MDSCs. Moreover, co-culturing MDSCs with T helper 17 cells (Th17) resulted in reduced Th17 frequencies and their IL-17 expression. On the contrary, MDSCs maintained regulatory T cell frequencies and enhanced their function in-vitro. Furthermore, we delineated the role of interleukin-10 (IL-10) secreted by MDSCs in their immunoregulatory functions. We confirmed these results by flow cytometry analysis and observed that treatment with MDSCs in DED mice effectively suppressed the maturation of APCs, pathogenic Th17 response, and maintained Treg function and significantly ameliorated the disease. The results in this study highlight the potential therapeutic application of MDSCs in treating refractory DED.

A novel combination approach to effectively reduce inflammation and neurodegeneration in multiple sclerosis models

Multiple sclerosis (MS) is an autoimmune disease characterized by immune-mediated attacks on the central nervous system (CNS), resulting in demyelination and recurring T-cell responses. Unfortunately, there is no cure for it. Current therapies that target immunomodulation and/or immunosuppression show only modest beneficial effects, have many side effects, and do not block neurodegeneration or progression of the disease. Since neurodegeneration and in particular axonal degeneration is implicated in disability in progressive MS, development of novel therapeutic strategies to attenuate the neurodegenerative processes is imperative. This study aims to develop new safe and efficacious treatments that address both the inflammatory and neurodegenerative aspects of MS using its animal model, experimental allergic encephalomyelitis (EAE). In EAE, the cysteine protease calpain is upregulated in CNS tissue, and its activity correlates with neurodegeneration. Our immunologic studies on MS have indicated that increased calpain activity promotes pro-inflammatory T helper (Th)1 cells and the severity of the disease in EAE, suggesting that calpain inhibition could be a novel target to combat neurodegeneration in MS/EAE. While calpain inhibition by SNJ1945 reduced disease severity, treatment of EAE animals with a novel protease-resistant altered small peptide ligand (3aza-APL) that mimic myelin basic protein (MBP), also decreased the incidence of EAE, disease severity, infiltration of inflammatory cells, and protected myelin. A reduction in inflammatory T-cells with an increase in Tregs and myeloid suppressor cells is also found in EAE mice treated with SNJ1945 and 3aza-APL. Thus, a novel combination strategy was tested in chronic EAE mouse model in B10 mice which showed multiple pathological mechanisms could be addressed by simultaneous treatment with calpain inhibitor SNJ1945 and protease-resistant 3aza-APL to achieve a stronger therapeutic effect.

Short-chain fatty acids ameliorate allergic airway inflammation via sequential induction of PMN-MDSCs and Treg cells

Background

Reinforcement of the immune-regulatory pathway is a feasible strategy for prevention and therapy of allergic asthma. The short-chain fatty acids (SCFAs) acetate, propionate, and butyrate are pleiotropic microbial fermentation products known to induce regulatory T (Treg) cells and exert an immune-regulatory effect. The cellular mechanism underlying SCFA immune regulation in asthma is not fully understood.

Objective

We investigated the role of myeloid-derived suppressor cells (MDSCs) and Treg cells, the immune-regulatory cells of innate and adaptive origin, respectively, in SCFA-elicited protection against allergic airway inflammation.

Methods

BALB/c mice were given SCFA-containing drinking water before being rendered asthmatic in response to ovalbumen. When indicated, mice were given a GR1-depleting antibody to investigate the function of MDSCs in allergic inflammation of the airways. MDSCs were sorted to examine their immunosuppressive function and interaction with T cells.

Results

The mice receiving SCFAs developed less severe asthma that was accompanied by expansion of PMN-MDSCs and Treg cells. Mice depleted of PMN-MDSCs exhibited aggravated asthma, and the protective effect of SCFAs was abrogated after PMN-MDSC depletion. SCFAs were able to directly induce T-cell differentiation toward Treg cells. Additionally, we found that PMN-MDSCs enhanced Treg cell expansion in a cell contact-dependent manner. Whilst membrane-bound TGF-β has been shown to induce Treg cell differentiation, we found that MDSCs upregulated surface expression of TGF-β after coculture with T-cells and that MDSC-induced Treg cell differentiation was partially inhibited by TGF-β blockage.

Conclusions

Although previous studies revealed Treg cells as the effector mechanism of SCFA immune regulation, we found that SCFAs ameliorate allergic airway inflammation by relaying immune regulation, with sequential induction of PMN-MDSCs and Treg cells.

Local Myeloid-Derived Suppressor Cells Impair Progression of Experimental Autoimmune Uveitis by Alleviating Oxidative Stress and Inflammation

Purpose

To evaluate the immune regulatory effect of human cord blood myeloid-derived suppressor cells (MDSCs) in experimental autoimmune uveitis (EAU) models.

Methods

MDSCs (1 × 106) or PBS were injected into established C57BL/6 EAU mice via the subconjunctival route on days 0 and 7. The severity of intraocular inflammation was evaluated for up to 3 weeks. Tissue injury and inflammation were analyzed using immunolabelled staining, real-time PCR, and ELISA. In addition, immune cells in draining lymph nodes (LNs) were quantified using flow cytometry.

Results

After 21 days, the clinical scores and histopathological grades of EAU were lower in the MDSCs group compared with the PBS group. Local administration of MDSCs suppressed the oxidative stress and the expression of TNF-α and IL-1β in the retinal tissues. In addition, it inhibited the activation of pathogenic T helper 1 (Th1) and Th17 cells in draining LNs. MDSCs increased the frequency of CD25+ Foxp3+ regulatory T cells and the mRNA expression of IL-10, as an immune modulator.

Conclusions

MDSCs suppressed inflammation and oxidative stress in the retina and inhibited pathogenic T cells in the LNs in EAU. Therefore, ocular administration of MDSCs has therapeutic potential for uveitis.

The emerging role of regulatory cell-based therapy in autoimmune disease

Autoimmune disease, caused by unwanted immune responses to self-antigens, affects millions of people each year and poses a great social and economic burden to individuals and communities. In the course of autoimmune disorders, including rheumatoid arthritis, systemic lupus erythematosus, type 1 diabetes mellitus, and multiple sclerosis, disturbances in the balance between the immune response against harmful agents and tolerance towards self-antigens lead to an immune response against self-tissues. In recent years, various regulatory immune cells have been identified. Disruptions in the quality, quantity, and function of these cells have been implicated in autoimmune disease development. Therefore, targeting or engineering these cells is a promising therapeutic for different autoimmune diseases. Regulatory T cells, regulatory B cells, regulatory dendritic cells, myeloid suppressor cells, and some subsets of innate lymphoid cells are arising as important players among this class of cells. Here, we review the roles of each suppressive cell type in the immune system during homeostasis and in the development of autoimmunity. Moreover, we discuss the current and future therapeutic potential of each one of these cell types for autoimmune diseases.

Uveitis-mediated immune cell invasion through the extracellular matrix of the lens capsule

While the eye is considered an immune privileged site, its privilege is abrogated when immune cells are recruited from the surrounding vasculature in response to trauma, infection, aging, and autoimmune diseases like uveitis. Here, we investigate whether in uveitis immune cells become associated with the lens capsule and compromise its privilege in studies of C57BL/6J mice with experimental autoimmune uveitis. These studies show that at D14, the peak of uveitis in these mice, T cells, macrophages, and Ly6G/Ly6C+ immune cells associate with the lens basement membrane capsule, burrow into the capsule matrix, and remain integrated with the capsule as immune resolution is occurring at D26. 3D surface rendering image analytics of confocal z‐stacks and scanning electron microscopy imaging of the lens surface show the degradation of the lens capsule as these lens‐associated immune cells integrate with and invade the lens capsule, with a subset infiltrating both epithelial and fiber cell regions of lens tissue, abrogating its immune privilege. Those immune cells that remain on the surface often become entwined with a fibrillar net‐like structure. Immune cell invasion of the lens capsule in uveitis has not been described previously and may play a role in induction of lens and other eye pathologies associated with autoimmunity.

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.

Mechanisms Underlying the Role of Myeloid-Derived Suppressor Cells in Clinical Diseases: Good or Bad

Myeloid-derived suppressor cells (MDSCs) have strong immunosuppressive activity and are morphologically similar to conventional monocytes and granulocytes. The development and classification of these cells have, however, been controversial. The activation network of MDSCs is relatively complex, and their mechanism of action is poorly understood, creating an avenue for further research. In recent years, MDSCs have been found to play an important role in immune regulation and in effectively inhibiting the activity of effector lymphocytes. Under certain conditions, particularly in the case of tissue damage or inflammation, MDSCs play a leading role in the immune response of the central nervous system. In cancer, however, this can lead to tumor immune evasion and the development of related diseases. Under cancerous conditions, tumors often alter bone marrow formation, thus affecting progenitor cell differentiation, and ultimately, MDSC accumulation. MDSCs are important contributors to tumor progression and play a key role in promoting tumor growth and metastasis, and even reduce the efficacy of immunotherapy. Currently, a number of studies have demonstrated that MDSCs play a key regulatory role in many clinical diseases. In light of these studies, this review discusses the origin of MDSCs, the mechanisms underlying their activation, their role in a variety of clinical diseases, and their function in immune response regulation.

Programmed Death-Ligand 1 Expression Potentiates the Immune Modulatory Function Of Myeloid-Derived Suppressor Cells in Systemic Lupus Erythematosus

Multiple studies have explored the potential role of programmed death-ligand 1 (PD-L1) as a mediator of Myeloid-derived suppressor cells (MDSCs) effects in various cancers. However, the role PD-L1 expression in MDSCs on autoimmune disease is still largely unknown.This study was undertaken to whether MDSC expressing PD-L1 have more potent immunoregulatory activity and control autoimmunity more effectively in two murine models of lupus (MRL/lpr mice and Roquin^san/san mice). The populations of MDSC were increased in peripheral blood of lupus patients. The mRNA levels of immunosuppressive molecules were profoundly decreased in MDSCs from lupus patients and mice. Co-culture with splenocytes showed that PD-L1 expressing MDSCs from control mice expand both Treg cells and regulatory B cells more potently. Infusion of PD-L1 expressing MDSCs reduced autoantibody levels and degree of proteinuria and improved renal pathology of two animal models of lupus. Moreover, PD-L1 expressing MDSCs therapy can suppress double negative (CD4-CD8-CD3+) T cells, the major pathogenic immune cells and follicular helper T cells in MRL/lpr mice, and podocyte damage. Our results indicate PD-L1 expressing MDSCs have more potent immunoregualtory activity and ameliorate autoimmunity more profoundly. These findings suggest PD-L1 expressing MDSCs be a promising therapeutic strategy targeting systemic autoimmune diseases.

Mesenchymal stromal cells for the treatment of ocular autoimmune diseases

Mesenchymal stromal cells, commonly referred to as MSCs, have emerged as a promising cell-based therapy for a range of autoimmune diseases thanks to several therapeutic advantages. Key among these are: 1) the ability to modulate innate and adaptive immune responses and to promote tissue regeneration, 2) the ease of their isolation from readily accessible tissues and expansion at scale in culture, 3) their low immunogenicity enabling use as an allogeneic "off-the-shelf" product, and 4) MSC therapy's safety and feasibility in humans, as demonstrated in more than one thousand clinical trials. Evidence from preclinical studies and early clinical trials indicate the therapeutic potential of MSCs and their derivatives for efficacy in ocular autoimmune diseases such as autoimmune uveoretinitis and Sjögren's syndrome-related dry eye disease. In this review, we provide an overview of the current understanding of the therapeutic mechanisms of MSCs, and summarize the results from preclinical and clinical studies that have used MSCs or their derivatives for the treatment of ocular autoimmune diseases. We also discuss the challenges to the successful clinical application of MSC therapy, and suggest strategies for overcoming them.

Myeloid-derived suppressor cells as cellular immunotherapy in transplantation and autoimmune diseases

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells, which have been characterized for their immunosuppressive capacity through multiple mechanisms. These cells have been extensively studied in the field of tumor immunity. Emerging evidence has highlighted its essential role in maintaining immune tolerance in transplantation and autoimmunity. Because of their robust immune inhibitory activities, there has been growing interest in MDSC-based cellular therapy. Various pre-clinical studies have demonstrated that the adoptive transfer of MDCS represented a promising therapeutic strategy for immune-related disorders. In this review, we summarize relevant studies of MDSC-based cell therapy in transplantation and autoimmune diseases and discuss the challenges and future directions for clinical application of MDSC-based cell therapy.

Potential Role of Myeloid-Derived Suppressor Cells (MDSCs) in Age-Related Macular Degeneration (AMD)

Myeloid cells, such as granulocytes/neutrophils and macrophages, have responsibilities that include pathogen destruction, waste material degradation, or antigen presentation upon inflammation. During persistent stress, myeloid cells can remain partially differentiated and adopt immunosuppressive functions. Myeloid-derived suppressor cells (MDSCs) are primarily beneficial upon restoring homeostasis after inflammation. Because of their ability to suppress adaptive immunity, MDSCs can also ameliorate autoimmune diseases and semi-allogenic responses, e.g., in pregnancy or transplantation. However, immunosuppression is not always desirable. In certain conditions, such as cancer or chronically inflamed tissue, MDSCs prevent restorative immune responses and thereby aggravate disease progression. Age-related macular degeneration (AMD) is the most common disease in Western countries that severely threatens the central vision of aged people. The pathogenesis of this multifactorial disease is not fully elucidated, but inflammation is known to participate in both dry and wet AMD. In this paper, we provide an overview about the potential role of MDSCs in the pathogenesis of AMD.

Innate immune response in retinal homeostasis and inflammatory disorders

Innate immune cells such as neutrophils, monocyte-macrophages and microglial cells are pivotal for the health and disease of the retina. For the maintenance of retinal homeostasis, these cells and immunosuppressive molecules in the eye actively regulate the induction and the expression of inflammation in order to prevent excessive activation and subsequent tissue damage. In the disease context, these regulatory mechanisms are modulated genetically and/or by environmental stimuli such as damage-associated molecular patterns (DAMPs), and a chronic innate immune response regulates or contributes to the formation of diverse retinal disorders such as uveitis, retinitis pigmentosa, retinal vascular diseases and retinal fibrosis. Here we summarize the recent knowledge regarding the innate immune response in both ocular immune regulation and inflammatory retinal diseases, and we describe the potential of the innate immune response as a biomarker and therapeutic target.

Phenotypic and Functional Diversities of Myeloid-Derived Suppressor Cells in Autoimmune Diseases

Myeloid-derived suppressor cells (MDSCs) are identified as a heterogeneous population of cells with the function to suppress innate as well as adaptive immune responses. The initial studies of MDSCs were primarily focused on the field of animal tumor models or cancer patients. In cancer, MDSCs play the deleterious role to inhibit tumor immunity and to promote tumor development. Over the past few years, an increasing number of studies have investigated the role of MDSCs in autoimmune diseases. The beneficial effects of MDSCs in autoimmunity have been reported by some studies, and thus, immunosuppressive MDSCs may be a novel therapeutic target in autoimmune diseases. There are some controversial findings as well. Many questions such as the activation, differentiation, and suppressive functions of MDSCs and their roles in autoimmune diseases remain unclear. In this review, we have discussed the current understanding of MDSCs in autoimmune diseases.

Tolerance Induction in Relation to the Eye

Inflammatory intraocular eye diseases, grouped under the term uveitis are blinding conditions, believed to be mediated by pathogenic autoimmune processes that overcome the protective mechanisms of the immune privilege status of the eye. An animal model for these diseases, named experimental autoimmune uveitis (EAU), is induced by initiation of immunity against ocular-specific antigens, or it develops spontaneously in mice with T-cells that transgenically express TCR specific to the target eye antigen(s). T-Cells specific to ocular antigens are generated in the thymus and their majority are eliminated by exposure to their target antigen expressed in this organ. T-cells that escape this negative selection acquire pathogenicity by their activation with the target antigen. In spontaneous EAU, the microbiota play crucial roles in the acquisition of pathogenicity by providing both antigenic stimulation, by molecules that mimic the target ocular antigen, and an additional stimulation that allows invasion of tissues that harbor the target antigen. The pathogenic process is physiologically inhibited by the peripheral tolerance, composed of antigen-specific T-regulatory (Treg) lymphocytes. Deleting the Tregs enhances the ocular inflammation, whereas adoptively transferring them suppresses the pathogenic response. Potential usage of Treg cells for suppression of autoimmune diseases in humans is under intensive investigation.

Induced pluripotent stem cells-derived myeloid-derived suppressor cells regulate the CD8+ T cell response

Myeloid-derived suppressor cells (MDSCs) are markedly increased in cancer patients and tumor-bearing mice and promote tumor growth and survival by inhibiting host innate and adaptive immunity. In this study, we generated and characterized MDSCs from murine-induced pluripotent stem cells (iPSCs). The iPSCs were co-cultured with OP9 cells, stimulated with GM-CSF, and became morphologically heterologous under co-culturing with hepatic stellate cells. Allogeneic and OVA-specific antigen stimulation demonstrated that iPS-MDSCs have a T-cell regulatory function. Furthermore, a popliteal lymph node assay and autoimmune hepatitis model showed that iPS-MDSCs also regulate immune responsiveness in vivo and have a therapeutic effect against hepatitis. Taken together, our results demonstrated a method of generating functional MDSCs from iPSCs and highlighted the potential of iPS-MDSCs as a key cell therapy resource for transplantation and autoimmune diseases.

Re-programming immunosurveillance in persistent non-infectious ocular inflammation

Ocular function depends on a high level of anatomical integrity. This is threatened by inflammation, which alters the local tissue over short and long time-scales. Uveitis due to autoimmune disease, especially when it involves the retina, leads to persistent changes in how the eye interacts with the immune system. The normal pattern of immune surveillance, which for immune privileged tissues is limited, is re-programmed. Many cell types, that are not usually present in the eye, become detectable. There are changes in the tissue homeostasis and integrity. In both human disease and mouse models, in the most extreme cases, immunopathological findings consistent with development of ectopic lymphoid-like structures and disrupted angiogenesis accompany severely impaired eye function. Understanding how the ocular environment is shaped by persistent inflammation is crucial to developing novel approaches to treatment.

Myeloid-Derived Suppressor Cells Mediate Inflammation Resolution in Humans and Mice with Autoimmune Uveoretinitis

Resolution of inflammation is an active process that leads to tissue homeostasis and involves multiple cellular and molecular mechanisms. Myeloid-derived suppressor cells (MDSCs) have recently emerged as important cellular components in the resolution of inflammation because of their activities to suppress T cell activation. In this article, we show that HLA-DR^-CD11b⁺CD33⁺CD14⁺ human MDSCs and CD11b⁺Ly6G^-Ly6C⁺ mouse MDSCs markedly increased in patients and mice during and before the resolution phase of autoimmune uveoretinitis. CD11b⁺Ly6C⁺ monocytes isolated from autoimmune uveoretinitis mice were able to suppress T cell proliferation in culture, and adoptive transfer of the cells accelerated the remission of autoimmune uveoretinitis in mice. Alternatively, depletion of CD11b⁺Ly6C⁺ monocytes at the resolution phase, but not CD11b⁺Ly6G⁺ granulocytes, exacerbated the disease. These findings collectively indicate that monocytic MDSCs serve as regulatory cells mediating the resolution of autoimmune uveoretinitis.

Myeloid suppressor cells in cancer and autoimmunity

A bottleneck for immunotherapy of cancer is the immunosuppressive microenvironment in which the tumor cells proliferate. Cancers harness the immune regulatory mechanism that prevents autoimmunity from evading immunosurveillance and promoting immune destruction. Regulatory T cells, myeloid suppressor cells, inhibitory cytokines and immune checkpoint receptors are the major components of the immune system acting in concert with cancer cells and causing the subversion of anti-tumor immunity. This redundant immunosuppressive network poses an impediment to efficacious immunotherapy by facilitating tumor progression. Tumor-associated myeloid cells comprise heterogeneous populations acting systemically (myeloid-derived suppressor cells/MDSCs) and/or locally in the tumor microenvironment (MDSCs and tumor-associated macrophages/TAMs). Both populations promote cancer cell proliferation and survival, angiogenesis and lymphangiogenesis and elicit immunosuppression through different pathways, including the expression of immunosuppressive cytokines and checkpoint inhibitors. Several evidences have demonstrated that myeloid cells can express different functional programs in response to different microenvironmental signals, a property defined as functional plasticity. The opposed extremes of this functional flexibility are generally represented by the classical macrophage activation, which identifies inflammatory and cytotoxic M1 polarized macrophages, and the alternative state of macrophage activation, which identifies M2 polarized anti-inflammatory and immunosuppressive macrophages. Functional skewing of myeloid cells occurs in vivo under physiological and pathological conditions, including cancer and autoimmunity. Here we discuss how myeloid suppressor cells can on one hand support tumor growth and, on the other, limit autoimmune responses, indicating that their therapeutic reprogramming can generate opportunities in relieving immunosuppression in the tumor microenvironment or reinstating tolerance in autoimmune conditions.

Retinal Pigment Epithelial Cells Suppress Phagolysosome Activation in Macrophages

Purpose

The eye is an immune-privileged microenvironment that has adapted several mechanisms of immune regulation to prevent inflammation. One of these potential mechanisms is retinal pigment epithelial cells (RPE) altering phagocytosis in macrophages.

Methods

The conditioned media of RPE eyecups from eyes of healthy mice and mice with experimental autoimmune uveitis (EAU) were used to treat primary macrophage phagocytizing pHrodo bacterial bioparticles. In addition, the neuropeptides were depleted from the conditioned media of healthy RPE eyecups and used to treat phagocytizing macrophages. The conditioned media from healthy and EAU RPE eyecups were assayed for IL-6, and IL-6 was added to the healthy conditioned media, and neutralized in the EAU conditioned media. The macrophages were treated with the conditioned media and assayed for fluorescence. The macrophages were imaged, and the fluorescence intensity, relative to active phagolysosomes, was measured. Also, the macrophages were assayed using fluorescent viability dye staining.

Results

The conditioned media from healthy, but not from EAU RPE eyecups suppressed phagolysosome activation. Depletion of the neuropeptides alpha-melanocyte–stimulating hormone and neuropeptide Y from the healthy RPE eyecup conditioned media resulted in macrophage death. In the EAU RPE eyecup conditioned media was 0.96 ± 0.18 ng/mL of IL-6, and when neutralized the conditioned media suppressed phagolysosome activation.

Conclusions

The healthy RPE through soluble molecules, including alpha-melanocyte–stimulating hormone and neuropeptide Y, suppresses the activation of the phagolysosome in macrophages. In EAU, the IL-6 produced by the RPE promotes the activation of phagolysosomes in macrophages. These results demonstrate that under healthy conditions, RPE promotes an altered pathway of phagocytized material in macrophages with implications on antigen processing and clearance.

Hypoxia inducible factors are dispensable for myeloid cell migration into the inflamed mouse eye

Hypoxia inducible factors (HIFs) are ubiquitously expressed transcription factors important for cell homeostasis during dynamic oxygen levels. Myeloid specific HIFs are crucial for aspects of myeloid cell function, including their ability to migrate into inflamed tissues during autoimmune disease. This contrasts with the concept that accumulation of myeloid cells at ischemic and hypoxic sites results from a lack of chemotactic responsiveness. Here we seek to address the role of HIFs in myeloid trafficking during inflammation in a mouse model of human uveitis. We show using mice with myeloid-specific Cre-deletion of HIFs that myeloid HIFs are dispensable for leukocyte migration into the inflamed eye. Myeloid-specific deletion of Hif1a, Epas1, or both together, had no impact on the number of myeloid cells migrating into the eye. Additionally, stabilization of HIF pathways via deletion of Vhl in myeloid cells had no impact on myeloid trafficking into the inflamed eye. Finally, we chemically induce hypoxemia via hemolytic anemia resulting in HIF stabilization within circulating leukocytes to demonstrate the dispensable role of HIFs in myeloid cell migration into the inflamed eye. These data suggest, contrary to previous reports, that HIF pathways in myeloid cells during inflammation and hypoxia are dispensable for myeloid cell tissue trafficking.

Painting factor H onto mesenchymal stem cells protects the cells from complement- and neutrophil-mediated damage

Mesenchymal stem cells (MSCs) are undergoing intensive testing in clinical trials as a promising new therapy for many inflammatory diseases and for regenerative medicine, but further optimization of current MSC-based therapies is required. In this study, we found that in addition to direct complement-mediated attack through the assembly of membrane attack complexes (MACs) that we and others have recently reported, of the released complement activation products, C5a, but not C3a, activates neutrophils in the blood to further damage MSCs through oxidative burst. In addition, we have developed a simple method for painting factor H, a native complement inhibitor, onto MSCs to locally inhibit complement activation on MSCs. MSCs painted with factor H are protected from both MAC- and neutrophil-mediated attack and are significantly more effective in inhibiting antigen-specific T cell responses than the mock-painted MSCs both in vitro and in vivo.

The retinal pigment epithelium as a gateway for monocyte trafficking into the eye

The choroid plexus epithelium within the brain ventricles orchestrates blood-derived monocyte entry to the central nervous system under injurious conditions, including when the primary injury site is remote from the brain. Here, we hypothesized that the retinal pigment epithelium (RPE) serves a parallel role, as a gateway for monocyte trafficking to the retina following direct or remote injury. We found elevated expression of genes encoding leukocyte trafficking determinants in mouse RPE as a consequence of retinal glutamate intoxication or optic nerve crush (ONC). Blocking VCAM-1 after ONC interfered with monocyte infiltration into the retina and resulted in a local pro-inflammatory cytokine bias. Live imaging of the injured eye showed monocyte accumulation first in the RPE, and subsequently in the retina, and peripheral leukocytes formed close contact with the RPE Our findings further implied that the ocular milieu can confer monocytes a phenotype advantageous for neuroprotection. These results suggest that the eye utilizes a mechanism of crosstalk with the immune system similar to that of the brain, whereby epithelial barriers serve as gateways for leukocyte entry.

Arginase-1-dependent promotion of TH17 differentiation and disease progression by MDSCs in systemic lupus erythematosus

Expansion of myeloid-derived suppressor cells (MDSCs) has been documented in some murine models and patients with autoimmune diseases, but the exact role of MDSCs in this process remains largely unknown. The current study investigates this question in patients with systemic lupus erythematosus (SLE). Patients with active SLE showed a significant increase in HLA-DR(-)CD11b(+)CD33(+)MDSCs, including both CD14(+)CD66b(-)monocytic and CD14(-)CD66b(+)granulocytic MDSCs, in the peripheral blood compared to healthy controls (HCs). The frequency of MDSCs was positively correlated with the levels of serum arginase-1 (Arg-1) activity, T helper 17 (TH17) responses, and disease severity in SLE patients. Consistently, in comparison with MDSCs from HCs, MDSCs from SLE patients exhibited significantly elevated Arg-1 production and increased potential to promote TH17 differentiation in vitro in an Arg-1-dependent manner. Moreover, in a humanized SLE model, MDSCs were essential for the induction of TH17 responses and the associated renal injuries, and the effect of MDSCs was Arg-1-dependent. Our data provide direct evidence demonstrating a pathogenic role for MDSCs in human SLE. This study also provides a molecular mechanism of the pathogenesis of SLE by demonstrating an Arg-1-dependent effect of MDSCs in the development of TH17 cell-associated autoimmunity, and suggests that targeting MDSCs or Arg-1 may offer potential therapeutic strategies for the treatment of SLE and other TH17 cell-mediated autoimmune diseases.

CD33(+) /HLA-DR(neg) and CD33(+) /HLA-DR(+/-) Cells: Rare Populations in the Human Decidua with Characteristics of MDSC

PROBLEM

Human pregnancy needs a remarkable local immune tolerance toward the conceptus. Myeloid-derived suppressor cells (MDSC) are important players promoting cancer initiation and progression by suppressing T-cell functions and thus inducing immune tolerance. Therefore, MDSC were expected within decidua.

METHODS

Subpopulations of CD33(+) immune cells were isolated from human early pregnancy decidua and characterized phenotypically and functionally by microscopy, FACS analysis, RT-PCR, Western blotting and in the coculture with T cells.

RESULTS

Decidua harbors CD33(+) /HLA-DR(neg) and CD33(+) /HLA-DR(+/-) cells which both express arginase, iNOS and IDO and a typical cytokine profile. Both subtypes potently suppress T-cell proliferation and therefore fulfill the criteria of MDSC.

CONCLUSION

We characterized a new population of CD33(+) /HLA-DR(neg) and CD33(+) /HLA-DR(+/-) cells in human early pregnancy decidua with properties of classical MDSC and thus potentially being an important player in immune tolerance in pregnancy.

Myeloid derived suppressor cells in inflammatory conditions of the central nervous system

The knowledge of the immune system elements and their relationship with other tissues, organs and systems are key approximations for the resolution of many immune-related disorders. The control of the immune response and/or its modulation from the pro-inflammatory to the anti-inflammatory response is being deeply studied in the field. In the last years, the study of myeloid-derived suppressor cells (MDSCs), a group of immature myeloid cells with a high suppressive activity on T cells has been extensively addressed in cancer. In contrast, their role in neuroimmune diseases is far from being totally understood. In this review, we will summarize data about MDSCs coming from the study of neuroinflammatory diseases in general and their potential role in multiple sclerosis, in order to introduce the putative use of this extraordinary promising cell type for future cell-based therapies. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Immunopathogenic Background of Pars Planitis

Pars planitis is defined as an intermediate uveitis of unknown background of systemic disease with characteristic formations such as vitreous snowballs, snowbanks and changes in peripheral retina. The incidence of pars planitis varies 2.4–15.4 % of the uveitis patients. The pathogenesis of the disease is to be determined in future. Clinical and histopathological findings suggest an autoimmune etiology, most likely as a reaction to endogenous antigen of unknown source, with T cells predominant in both vitreous and pars plana infiltrations. T cells subsets play an important role as a memory-effector peripheral cell. Snowbanks are formed as an effect of post inflammatory glial proliferation of fibrous astrocytes. There is also a genetic predisposition for pars planitis by human leukocyte antigen and several other genes. A coexistence of multiple sclerosis and optic neuritis has been described in numerous studies. Epiretinal membrane, cataract, cystoid macular edema, retinal detachment, retinal vasculitis, neovascularization, vitreous peripheral traction, peripheral hole formation, vitreous hemorrhage, disc edema are common complications observed in pars planitis. There is a need to expand the knowledge of the pathogenic and immunologic background of the pars planitis to create an accurate pharmacological treatment.

The Role and Potential Therapeutic Application of Myeloid-Derived Suppressor Cells in Allo- and Autoimmunity

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that consists of myeloid progenitor cells and immature myeloid cells. They have been identified as a cell population that may affect the activation of CD4(+) and CD8(+) T-cells to regulate the immune response negatively, which makes them attractive targets for the treatment of transplantation and autoimmune diseases. Several studies have suggested the potential suppressive effect of MDSCs on allo- and autoimmune responses. Conversely, MDSCs have also been found at various stages of differentiation, accumulating during pathological situations, not only during tumor development but also in a variety of inflammatory immune responses, bone marrow transplantation, and some autoimmune diseases. These findings appear to be contradictory. In this review, we summarize the roles of MDSCs in different transplantation and autoimmune diseases models as well as the potential to target these cells for therapeutic benefit.

Retinal Pigment Epithelial Cell Line Suppression of Phagolysosome Activation

The eye is an immune privileged tissue with multiple mechanisms of immunosuppression to protect the light gathering tissues from the damage of inflammation. One of theses mechanisms involves retinal pigment epithelial cell suppression of phagosome activation in macrophages. The objective of this work is to determine if the human RPE cell line ARPE-19 is capable of suppressing the activation of the phagolysosome in macrophages in a manner similar to primary RPE. The conditioned media of RPE eyecups, sub-confluent, just confluent cultures, or established confluent cultures of human ARPE-19 cells were generated. These condition media were used to treat macrophages phagocytizing pHrodo bioparticles. After 24 hours incubation the macrophages were imaged by fluorescent microscopy, and fluorescence was measured. The fluorescent intensity is proportional to the amount of bioparticles phagocytized and are in an activated phagolysosome. The conditioned media of in situ mouse RPE eyecups significantly suppressed the activation of phagolysosome. The conditioned media from cultures of human ARPE-19 cells, grown to sub-confluence (50%) or grown to confluence had no effect on phagolysosome activation. In contrast, the conditioned media from established confluent cultures significantly suppressed phagolysosome activation. The neuropeptides alpha-MSH and NPY were depleted from the conditioned media of established confluent ARPE-19 cell cultures. This depleted conditioned media had diminished suppression of phagolysosome activation while promoting macrophage cell death. In addition, the condition media from cultures of ARPE-19 monolayers wounded with a bisecting scrape was diminished in suppressing phagolysosome activation. This technical report suggests that like primary RPE monolayers, established confluent cultures of ARPE-19 cells produce soluble factors that suppress the activation of macrophages, and can be used to study the molecular mechanisms of retinal immunobiology. In addition, the results further demonstrate the importance of an intact monolayer of RPE cells to modulate immune cell activity within the eye.

Autoimmune and autoinflammatory mechanisms in uveitis

The eye, as currently viewed, is neither immunologically ignorant nor sequestered from the systemic environment. The eye utilises distinct immunoregulatory mechanisms to preserve tissue and cellular function in the face of immune-mediated insult; clinically, inflammation following such an insult is termed uveitis. The intra-ocular inflammation in uveitis may be clinically obvious as a result of infection (e.g. toxoplasma, herpes), but in the main infection, if any, remains covert. We now recognise that healthy tissues including the retina have regulatory mechanisms imparted by control of myeloid cells through receptors (e.g. CD200R) and soluble inhibitory factors (e.g. alpha-MSH), regulation of the blood retinal barrier, and active immune surveillance. Once homoeostasis has been disrupted and inflammation ensues, the mechanisms to regulate inflammation, including T cell apoptosis, generation of T_reg cells, and myeloid cell suppression in situ, are less successful. Why inflammation becomes persistent remains unknown, but extrapolating from animal models, possibilities include differential trafficking of T cells from the retina, residency of CD8⁺ T cells, and alterations of myeloid cell phenotype and function. Translating lessons learned from animal models to humans has been helped by system biology approaches and informatics, which suggest that diseased animals and people share similar changes in T cell phenotypes and monocyte function to date. Together the data infer a possible cryptic infectious drive in uveitis that unlocks and drives persistent autoimmune responses, or promotes further innate immune responses. Thus there may be many mechanisms in common with those observed in autoinflammatory disorders.

Role of myeloid-derived suppressor cells in autoimmune disease

Myeloid-derived suppressor cells (MDSCs) represent an important class of immunoregulatory cells that can be activated to suppress T cell functions. These MDSCs can inhibit T cell functions through cell surface interactions and the release of soluble mediators. MDSCs accumulate in the inflamed tissues and lymphoid organs of patients with autoimmune diseases. Much of our knowledge of MDSC function has come from studies involving cancer models, however many recent studies have helped to characterize MDSC involvement in autoimmune diseases. MDSCs are a heterogeneous group of immature myeloid cells with a number of different functions for the suppression of T cell responses. However, we have yet to fully understand their contributions to the development and regulation of autoimmune diseases. A number of studies have described beneficial functions of MDSCs during autoimmune diseases, and thus there appears to be a potential role for MDSCs in the treatment of these diseases. Nevertheless, many questions remain as to the activation, differentiation, and inhibitory functions of MDSCs. This review aims to summarize our current knowledge of MDSC subsets and suppressive functions in tissue-specific autoimmune disorders. We also describe the potential of MDSC-based cell therapy for the treatment of autoimmune diseases and note some of hurdles facing the implementation of this therapy.

Uveitis in mouse and man

Uveitis is underappreciated as a sight-threatening cause of blindness. There are two broad causative classes of uveitis: infectious and non-infectious. Non-infectious uveitis is considered a prototypical autoimmune disorder based mainly on data from experimental models in the mouse. Several different experimental models exist that reflect the different types of uveitis in man (anterior, intermediate, and posterior uveitis). These models have demonstrated that uveitis is predominantly a Th1/Th17 mediated disease, although innate immune cells play a significant role both in induction of disease and in tissue damage. Most experimental models of uveitis rely on activation of the innate immune system by use of adjuvants that activate a range of pathogen recognition receptors (PRRs). This begs the question of the underlying role of initial and/or persistent infection, including latent infection, in immune-mediated uveitis in which active infection cannot be demonstrated. This further raises the possibility of pathogenic mechanisms such as antigenic cross-reactivity and molecular mimicry. Alternatively, residual/latent antigen from infectious agents may act as "endogenous" adjuvants for induction of immune reactions to damaged/altered self antigen, suggesting a commonality in pathogenesis for both infectious and non-infectious uveitis in man.

Cell-based therapies for ocular inflammation

Since the plasticity and the potential for re-programming cells has become widely accepted, there has been great interest in cell-based therapies. These are being applied to a range of diseases, not least ocular diseases, where it is assumed that there is a reduced risk of immune rejection although this may be more perceived than real. There are two broad classes of cell-based therapies: those aimed at restoring structure and function of specific tissues and cells; and those directed towards restoring immunological homeostasis by controlling the damaging effects of inflammatory disease. Stem cells of all types represent the first group and prototypically have been used with the aim of regenerating failing cells. In contrast, immune cells have been suggested as potential modulators of inflammation. However, there is functional overlap in these two applications, with some types of stem cells, such as mesenchymal stem cells, demonstrating a potent immunomodulatory effect. This review summarises recent information on cell based therapies for ocular disease, with special emphasis on ocular inflammatory disease, and explores current uses, potential and limitations.

Bowman lecture on the role of inflammation in degenerative disease of the eye

Inflammation, in the pathogenesis of many diseases previously thought to be strictly genetic, degenerative, metabolic, or endocrinologic in aetiology, has gradually entered the framework of a general mechanism of disease. This is exemplified by conditions such as Parkinson's disease, Alzheimer's disease, atherosclerosis, diabetes, and the more recently described Metabolic Syndrome. Chronic inflammatory processes have a significant, if not primary role, in ophthalmic diseases, particularly in retinal degenerative diseases. However, inflammation itself is not easy to define, and some aspects of inflammation may be beneficial, in a process described as 'para-inflammation' by Medhzitov. In contrast, the damaging effects of inflammation, mediated by pro-inflammatory macrophages through activation of the intracellular protein-signalling complexes, termed inflammasomes, are well recognised and are important therapeutic targets. In this review, the range of inflammatory processes in the eye is evaluated in the context of how these processes impact upon retinal degenerative disease, particularly diabetic retinopathy and age-related macular degeneration.