Expression of GITR ligand abrogates immunosuppressive function of ocular tissue and differentially modulates inflammatory cytokines and chemokines

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.

Pathogenic Effects of Mineralocorticoid Pathway Activation in Retinal Pigment Epithelium

Glucocorticoids are amongst the most used drugs to treat retinal diseases of various origins. Yet, the transcriptional regulations induced by glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation in retinal pigment epithelium cells (RPE) that form the outer blood-retina barrier are unknown. Levels of endogenous corticoids, ligands for MR and GR, were measured in human ocular media. Human RPE cells derived from induced pluripotent stem cells (iRPE) were used to analyze the pan-transcriptional regulations induced by aldosterone-an MR-specific agonist, or cortisol or cortisol + RU486-a GR antagonist. The retinal phenotype of transgenic mice that overexpress the human MR (P1.hMR) was analyzed. In the human eye, the main ligand for GR and MR is cortisol. The iRPE cells express functional GR and MR. The subset of genes regulated by aldosterone and by cortisol + RU-486, and not by cortisol alone, mimics an imbalance toward MR activation. They are involved in extracellular matrix remodeling (CNN1, MGP, AMTN), epithelial-mesenchymal transition, RPE cell proliferation and migration (ITGB3, PLAUR and FOSL1) and immune balance (TNFSF18 and PTX3). The P1.hMR mice showed choroidal vasodilation, focal alteration of the RPE/choroid interface and migration of RPE cells together with RPE barrier function alteration, similar to human retinal diseases within the pachychoroid spectrum. RPE is a corticosteroid-sensitive epithelium. MR pathway activation in the RPE regulates genes involved in barrier function, extracellular matrix, neural regulation and epithelial differentiation, which could contribute to retinal pathology.

The Role of GITR/GITRL Interaction in Autoimmune Diseases

Glucocorticoid-induced TNFR-related protein (GITR) is a member of the TNFR superfamily which is expressed in various cells, including T cells, natural killer cells and some myeloid cells. GITR is activated by its ligand, GITRL, mainly expressed on antigen presenting cells and endothelial cells. It has been acknowledged that the engagement of GITR can modulate both innate and adaptive immune responses. Accumulated evidence suggests GITR/GITRL interaction is involved in the pathogenesis of tumor, inflammation and autoimmune diseases. In this review, we describe the effects of GITR/GITRL activation on effector T cells, regulatory T cells (Tregs) and myeloid cells; summarize its role and the underlying mechanisms in modulating autoimmune diseases.

Glucocorticoid-induced tumour necrosis factor receptor-related protein: a key marker of functional regulatory T cells

Glucocorticoid-induced tumour necrosis factor receptor-related protein (GITR, TNFRSF18, and CD357) is expressed at high levels in activated T cells and regulatory T cells (Tregs). In this review, we present data from mouse and human studies suggesting that GITR is a crucial player in the differentiation of thymic Tregs (tTregs), and expansion of both tTregs and peripheral Tregs (pTregs). The role of GITR in Treg expansion is confirmed by the association of GITR expression with markers of memory T cells. In this context, it is not surprising that GITR appears to be a marker of active Tregs, as suggested by the association of GITR expression with other markers of Treg activation or cytokines with suppressive activity (e.g., IL-10 and TGF-β), the presence of GITR(+) cells in tissues where Tregs are active (e.g., solid tumours), or functional studies on Tregs. Furthermore, some Treg subsets including Tr1 cells express either low or no classical Treg markers (e.g., FoxP3 and CD25) and do express GITR. Therefore, when evaluating changes in the number of Tregs in human diseases, GITR expression must be evaluated. Moreover, GITR should be considered as a marker for isolating Tregs.

OX40 ligand expression abrogates the immunosuppressive function of retinal pigment epithelium

Background

This study aims to investigate the role of OX40 ligand (OX40L) in ocular inflammation via abrogation of retinal pigment epithelium (RPE)-mediated immunosuppression using an in vitro expression approach. OX40L cDNA was polymerase chain reaction-amplified and cloned into an eYFP fusion vector. Cultured retinal pigment epithelial cells (ARPE-19) were transfected with the vector. Total RNA from unstimulated or inflammatory cytokine-stimulated ARPE cells were isolated and analyzed for OX40L expression by reverse transcription-polymerase chain reaction. Peripheral blood mononuclear cells (PBMCs) were isolated from healthy human donors. Human ARPE cells (±OX40L ± GITR ligand (GITRL) expression) and PBMCs were co-cultured for in vitro proliferation studies.

Results

Polymerase chain reaction confirmed the insertion of the OX40L gene into the fusion vector. Flow cytometry and fluorescence microscopy further confirmed surface expression of OX40L on ARPE cells after transfection. OX40L expression was induced in the RPE cells stimulated with pro-inflammatory cytokines. In the co-culture studies, there was a significant reversal (20% to 30%) of the RPE-induced suppression of activated PBMCs when the ARPE cells were transfected with OX40L. When both OX40L and GITRL were concomitantly transfected into ARPE cells, there was an additive reversal of RPE-mediated T cell suppression, when compared to the reversal caused by RPE cells expressing either OX40L alone or GITRL alone.

Conclusions

Using an in vitro approach, we found that OX40L causes an abrogation of the RPE-mediated immunosuppression. OX40L appears to be regulated in the ARPE-19 cell line and may play an important role in the pathogenesis of various ocular inflammatory conditions.

IL-4 attenuates pulmonary epithelial cell-mediated suppression of T cell priming

We have previously shown that Th2-polarized airway inflammation facilitates sensitization towards new, protein antigens. In this context, we could demonstrate that IL-4 needs to act on cells of the hematopoetic and the structural compartment in order to facilitate sensitization towards new antigens. We thus aimed to elucidate possible mechanisms of action of IL-4 on structural cells choosing to analyze pulmonary epithelial cells as an important part of the lung's structural system. We used a co-culture system of DC- or APC-dependent in vitro priming of T cells, co-cultivated on a layer of cells of a murine pulmonary epithelial cell line (LA-4) pretreated with or without IL-4. Effects on T cell priming were analyzed via CFSE-dilution and flow cytometric assessment of activation status. Pulmonary epithelial cells suppressed T cell proliferation in vitro but this effect was attenuated by pre-treatment of the epithelial cells with IL-4. Transwell experiments suggest that epithelial-mediated suppression of T cell activation is mostly cell-contact dependent and leads to attenuation in an early naive T cell phenotype. Secretion of soluble factors like TARC, TSLP, GM-CSF and CCL20 by epithelial cells did not change after IL-4 treatment. However, analysis of co-stimulatory expression on pulmonary epithelial cells revealed that pre-treatment of epithelial cells with IL-4 changed expression GITR-L, suggesting a possible mechanism for the effects observed. Our studies provide new insight into the role of IL-4 during the early phases of pulmonary sensitization: The inhibitory activity of pulmonary epithelial cells in homeostasis is reversed in the presence of IL-4, which is secreted in the context of Th2-dominated allergic airway inflammation. This mechanism might serve to explain facilitated sensitization in the clinical context of polysensitization where due to a pre-existing sensitization increased levels of IL-4 in the airways might facilitate T cell priming towards new antigens.

Pharmacological modulation of GITRL/GITR system: therapeutic perspectives

Glucocorticoid-induced TNFR-related (gitr) is a gene coding for a member of the TNF receptor superfamily. GITR activation by its ligand (GITRL) influences the activity of effector and regulatory T cells, thus participating in the development of immune response against tumours and infectious agents, as well as in autoimmune and inflammatory diseases. Notably, treating animals with GITR-Fc fusion protein ameliorates autoimmune/inflammatory diseases while GITR triggering, by treatment with anti-GITR mAb, is effective in treating viral, bacterial and parasitic infections, as well in boosting immune response against tumours. GITR modulation has been indicated as one of the top 25 most promising research areas by the American National Cancer Institute, and a clinical trial testing the efficacy of an anti-GITR mAb in melanoma patients has been started. In this review, we summarize results regarding: (i) the mechanisms by which GITRL/GITR system modulates immune response; (ii) the structural and functional studies clearly demonstrating differences between GITRL/GITR systems of mice and humans; (iii) the molecules with pharmacological activities including anti-GITR mAbs, GITR-Fc and GITRL-Fc fusion proteins, GITRL in monomer or multimer conformation; and (iv) the possible risks deriving from GITRL/GITR system pharmacological modulation. In conclusion, GITR triggering and inhibition could be useful in treating tumours, infectious diseases, as well as autoimmune and inflammatory diseases. However, differences between mouse and human GITRL/GITR systems suggest that further preclinical studies are needed to better understand how safe therapeutic results can be obtained and to design appropriate clinical trials.

Induction of GITRL expression in human keratinocytes by Th2 cytokines and TNF-α: implications for atopic dermatitis

BACKGROUND

Glucocorticoid-induced TNF receptor-related protein ligand (GITRL), a ligand for the T cell co-stimulatory molecule GITR, is expressed by keratinocytes and involved in chemokine production. The expression of GITRL in skin inflammation remains unknown.

OBJECTIVES

This study investigated cytokine regulation of keratinocyte GITRL expression.

METHODS

Glucocorticoid-induced TNF receptor expression was evaluated in cytokine-treated human epidermal keratinocytes (HEK)s, murine PAM 212 cell line, murine and human skin explants by real time PCR, flow cytometry and immunostaining. Functional responses to GITR fusion protein were examined by real time PCR and ELISA. GITRL expression in AD and psoriasis was studied by immunohistochemistry.

RESULTS

Skin biopsies from STAT6VT transgenic mice, which develop spontaneous atopic skin inflammation, were found by immunofluoresence, to have increased keratinocyte GITRL expression. Exposure to Th2 cytokines augmented GITRL mRNA expression in the murine PAM 212 keratinocytic cell line and murine skin explants. In contrast, GITRL mRNA and protein expression was only increased in HEKs and human skin explants in the presence of the combination of TNF-α and Th2 cytokines. A synergistic effect of Th2 cytokines and GITR fusion protein on production of CCL17, the Th2 chemokine, by murine keratinocytes was demonstrated. Immunohistochemical staining showed that acute AD lesions have increased expression of GITRL compared with normal skin, chronic AD lesions and psoriatic plaques.

CONCLUSIONS AND CLINICAL RELEVANCE

Our studies demonstrate that GITRL expression is augmented by Th2 cytokines and TNF-α in keratinocytes. Increased GITRL expression in acute AD skin lesions is shown. This observation suggests a link between cytokine-regulated keratinocyte GITRL expression and its role in inflammatory responses in AD.

Retinal pigment epithelial cells upregulate expression of complement factors after co-culture with activated T cells

In this study we examined the effect of T cell-derived cytokines on retinal pigment epithelial (RPE) cells with respect to expression of complement components. We used an in vitro co-culture system in which CD3/CD28-activated human T cells were separated from the human RPE cell line (ARPE-19) by a membrane. Differential gene expression in the RPE cells of complement factor genes was identified using gene arrays, and selected gene transcripts were validated by q-RT-PCR. Protein expression was determined by ELISA and immunoblotting. Co-culture with activated T cells increased RPE mRNA and/or protein expression of complement components C3, factors B, H, H-like 1, CD46, CD55, CD59, and clusterin, in a dose-dependent manner. Soluble factors derived from activated T cells are capable of increasing expression of complement components in RPE cells. This is important for the further understanding of inflammatory ocular diseases such as uveitis and age-related macular degeneration.

Glucocorticoid-induced TNFR-related (GITR) protein and its ligand in antitumor immunity: functional role and therapeutic modulation

The ability of the tumor necrosis factor receptor (TNFR) family member GITR to modulate immune responses has been the subject of multiple studies. Initially thought to be critically involved in governing functions of regulatory T cells, GITR and its ligand GITRL have meanwhile been found to modulate the reactivity of various different cell types and to influence a broad variety of immunological conditions including the immune response against tumors. Not only GITR, but also GITRL is capable of transducing signals, and the consequences of GITR-GITRL interaction may vary among different effector cell types, differ upon signal transduction via the receptor, the ligand, or both, depend on the level of an ongoing immune response, and even differ among mice and men. In this paper, we address available data on GITR and its ligand in immune responses and discuss the role and potential therapeutic modulation of this molecule system in antitumor immunity.

Enhancement of T-cell-mediated anti-tumour immunity via the ectopically expressed glucocorticoid-induced tumour necrosis factor receptor-related receptor ligand (GITRL) on tumours

Glucocorticoid-induced tumour necrosis factor receptor-related receptor (GITR) costimulates functions of both effector and regulatory T cells. The administration of agonistic anti-GITR monoclonal antibodies efficiently enhances various T-cell-mediated immune responses; however, it is unknown to what extent the ligand of GITR (GITRL) contributes to T-cell responses. We investigated the involvement of endogenously expressed GITRL on dendritic cells and ectopically expressed GITRL on tumours in T-cell-mediated immunity. Expression of GITRL on dendritic cells in secondary lymphoid organs was limited, and treatment with anti-GITRL monoclonal antibodies did not substantially affect T-cell-mediated immunity to alloantigens, a specific protein antigen (ovalbumin), or tumour antigens. The introduction of GITRL promoted anti-tumour immunity in four tumour models. Tumour-associated GITRL greatly augmented the effector function of CD8(+) T cells and enhanced the contribution of CD8(+) T cells. These events reduced the crucial contribution of CD25(+) CD4(+) regulatory T cells, which were found to inhibit immunity against tumours lacking GITRL. Peritumoral injection of GITRL tumour vaccine efficiently inhibited the growth of established tumours. Our results suggest that the ectopic expression of GITRL in tumour cells enhances anti-tumour immunity at peripheral tumour sites. Consequently, the combined use of a GITRL tumour vaccine with methods aimed at enhancing the activation of host antigen-presenting cells in secondary lymphoid tissues may be a promising strategy for tumour immunotherapy.

Identification of glucocorticoid-induced TNF receptor-related protein ligand on keratinocytes: ligation by GITR induces keratinocyte chemokine production and augments T-cell proliferation

Glucocorticoid-induced tumor necrosis factor (TNF) receptor-related protein ligand (GITRL) is a recently described co-stimulatory molecule expressed by antigen-presenting cells (APCs). Activated keratinocytes are known to engage intraepithelial T cells through co-stimulatory molecules. This study investigated the expression and function of GITRL in resting keratinocytes. We showed by immunofluorescence and flow cytometry that keratinocytes from Balb/C and C57Bl/6 mice, as well as PAM 212 murine cell line keratinocytes and human epidermal keratinocytes (HEK), express cell-surface GITRL. Stimulation of murine skin biopsies and HEK with GITR fusion protein (GITR: Fc FP) resulted in mRNA induction for chemoattractants: cutaneous T-cell-attracting chemokine (CTACK), thymus and activation-regulated chemokine (TARC), IL-8, monocyte chemoattractant protein-1 (MCP-1), and murine beta-defensin 3 (MBD3). Immunofluorescent studies on mouse biopsies treated with GITR: Fc FP confirmed corresponding TARC and MCP-1 protein production by keratinocytes. Chemokine induction was shown to be NF-kappaB-mediated. T-cell proliferation was enhanced by the addition of keratinocytes. This was reversed by pretreatment with an anti-GITRL antibody. We conclude that keratinocytes express GITRL, and that through this important co-stimulatory molecule, they have the potential to influence T-cell numbers in the skin through chemokine production and through a direct cell-cell effect on T-cell proliferation.

GITR: a modulator of immune response and inflammation

Glucocorticoid-Induced TNFR-Related (GITR) protein belongs to Tumor Necrosis Factor Receptor Superfamily (TNFRSF) and stimulates both the acquired and innate immunity. It is expressed in several cells and tissues, including T and Natural Killer (NK) cells and is activated by its ligand, GITRL, mainly expressed on Antigen Presenting Cells (APCs) and endothelial cells. GITR/GITRL system participates in the development of autoimmune/inflammatory responses and graft vs. host disease and potentiates response to infection and tumors. These effects are due to several concurrent mechanisms including: co-activation of effector T-cells, inhibition of regulatory T (Treg) cells, NK-cell co-activation, activation of macrophages, modulation of DC function and regulation of the extravasation process. In this chapter we describe: 1) the main structural features of GITR and GITRL, 2) the transduction pathways activated by GITR triggering, 3) the effects derived from GITR/GITRL system interaction, considering the interplay between the different cells of the immune system. Moreover, the potential use of GITR/GITRL modulators in disease treatment is discussed.

Human glucocorticoid-induced TNF receptor ligand regulates its signaling activity through multiple oligomerization states

Ligation between glucocorticoid-induced tumor necrosis factor receptor (GITR) and its ligand (GITRL) provides an undefined signal that renders CD4(+)CD25(-) effector T cells resistant to the inhibitory effects of CD4(+)CD25(+) regulatory T cells. To understand the structural basis of GITRL function, we have expressed and purified the extracellular domain of human GITR ligand in Escherichia coli. Chromotography and cross-linking studies indicate that human GITRL (hGITRL) exists as dimers and trimers in solution and also can form a supercluster. To gain insight into the nature of GITRL oligomerization, we determined the crystallographic structures of hGITRL, which revealed a loosely associated open trimer with a deep cavity at the molecular center and a flexible C-terminal tail bent for trimerization. Moreover, a tetramer of trimers (i.e., supercluster) has also been observed in the crystal, consistent with the cross-linking analysis. Deletion of the C-terminal distal three residues disrupts the loosely assembled trimer and favors the formation of a dimer that has compromised receptor binding and signaling activity. Collectively, our studies identify multiple oligomeric species of hGITRL that possess distinct kinetics of ERK activation. The studies address the functional implications and structural models for a process by which hGITRL utilizes multiple oligomerization states to regulate GITR-mediated signaling during T cell costimulation.

Glucocorticoid-induced TNFR-related protein lowers the threshold of CD28 costimulation in CD8+ T cells

CD28 is well characterized as a costimulatory molecule in T cell activation. Recent evidences indicate that TNFR superfamily members, including glucocorticoid-induced TNFR-related protein (GITR), act as costimulatory molecules. In this study, the relationship between GITR and CD28 has been investigated in murine CD8(+) T cells. When suboptimal doses of anti-CD3 Ab were used, the absence of GITR lowered CD28-induced activation in these cells whereas the lack of CD28 did not affect the response of CD8(+) T cells to GITR costimulus. In fact, costimulation of CD28 in anti-CD3-activated GITR(-/-) CD8(+) T cells resulted in an impaired increase of proliferation, impaired protection from apoptosis, and an impaired rise of activation molecules such as IL-2R, IL-2, and IFN-gamma. Most notably, CD28-costimulated GITR(-/-) CD8(+) T cells revealed lower NF-kappaB activation. As a consequence, up-regulation of Bcl-x(L), one of the major target proteins of CD28-dependent NF-kappaB activation, was defective in costimulated GITR(-/-) CD8(+) T cells. What contributed to the response to CD28 ligation in CD8(+) T cells was the early up-regulation of GITR ligand on the same cells, the effect of which was blocked by the addition of a recombinant GITR-Fc protein. Our results indicate that GITR influences CD8(+) T cell response to CD28 costimulation, lowering the threshold of CD8(+) T cell activation.

Age-related macular degeneration and the immune response: implications for therapy

PURPOSE

To review the available information concerning the immune mediation of age-related macular degeneration (AMD) and to speculate on proposed mechanisms and immunotherapy.

DESIGN

Interpretative essay.

METHODS

Literature review and interpretation.

RESULTS

An ever-growing body of evidence is gathering concerning the role of the immune system in AMD. Evidence to date suggests that the underlying mechanism leading to AMD is the decline of the ocular downregulatory immune environment. The subsequent activation of the immune system would lead to T-cell sensitization. When combined with local antiangiogenic therapy, several existing immunotherapies may be used to downregulate the immune response, potentially leading to a more efficient inhibition of choroidal neovascularization.

CONCLUSIONS

The loss of the downregulatory immune environment is central to the development of AMD, permitting activation of the immune system. If so, immunotherapy could positively alter the course of the disease.

Modulation of acute and chronic inflammation of the lung by GITR and its ligand

Glucocorticoid-induced TNFR-related (GITR) protein, a member of the tumor necrosis factor receptor superfamily, is expressed in many components of the innate and adaptive immune system and modulates their activation following interaction with its ligand (GITRL). Here we review and discuss results described in previous publications where the role of the GITR/GITRL system in lung inflammation was evaluated using two experimental systems. We also discuss the proinflammatory role played by the GITR/GITRL system and the potential use of GITR fusion protein in inhibiting inflammation.

GITR/GITRL: more than an effector T cell co-stimulatory system

Glucocorticoid-induced TNFR-related protein (GITR) is a member of the TNFR superfamily, expressed in several cells and tissues including T lymphocytes, NK cells and antigen-presenting cells (APC). GITR activation, upon interaction with its ligand (GITRL), functions as a co-activating signal. GITRL is mainly expressed on APC and GITR/GITRL interaction is important for the development of immune response. This review summarizes recent results about the GITR/GITRL system, focusing on the interplay between APC, effector and regulatory T cells.