Genetic and pharmacological inhibition of GITR‐GITRL interaction reduces chronic lung injury induced by bleomycin instillation

TNF superfamily control of tissue remodeling and fibrosis

Fibrosis is the result of extracellular matrix protein deposition and remains a leading cause of death in USA. Despite major advances in recent years, there remains an unmet need to develop therapeutic options that can effectively degrade or reverse fibrosis. The tumor necrosis super family (TNFSF) members, previously studied for their roles in inflammation and cell death, now represent attractive therapeutic targets for fibrotic diseases. In this review, we will summarize select TNFSF and their involvement in fibrosis of the lungs, the heart, the skin, the gastrointestinal tract, the kidney, and the liver. We will emphasize their direct activity on epithelial cells, fibroblasts, and smooth muscle cells. We will further report on major clinical trials targeting these ligands. Whether in isolation or in combination with other anti-TNFSF member or treatment, targeting this superfamily remains key to improve efficacy and selectivity of currently available therapies for fibrosis.

Blockade of GITRL/GITR signaling pathway attenuates house dust mite-induced allergic asthma in mice through inhibition of MAPKs and NF-κB signaling

GITRL/GITR signaling pathway plays an important role in allergy, inflammation, transplantation and autoimmunity. However, its role in asthma remains unclear. Thus, the present study aimed to investigate changes in this pathway and observe the therapeutic effect of its blocking on asthma. By using house dust mite-induced asthma model, changes of GITRL/GITR and its downstream molecules MAPKs (e.g., p38 MAPK, JNK and Erk) and NF-κB were observed. After that, GITRL in lung of mice was knocked down by recombinant adeno-associated virus to observe the impact on its downstream molecules and assess the therapeutic effect on asthma. These results showed that GITRL/GITR and its downstream molecules MAPKs/NF-κB were activated in asthmatic mice. This activation was suppressed after GITRL knockdown, and allergic airway inflammation and airway hyperresponsiveness were alleviated. These results demonstrate that GITRL/GITR-MAPKs/NF-κB signaling pathway participates in the pathogenesis of asthma. Blockade of GITRL/GITR signaling pathway exhibits protective effects in a mouse model of house dust mite-induced allergic asthma.

Reverse Signaling of Tumor Necrosis Factor Superfamily Proteins in Macrophages and Microglia: Superfamily Portrait in the Neuroimmune Interface

The tumor necrosis factor (TNF) superfamily (TNFSF) is a protein superfamily of type II transmembrane proteins commonly containing the TNF homology domain. The superfamily contains more than 20 protein members, which can be released from the cell membrane by proteolytic cleavage. Members of the TNFSF function as cytokines and regulate diverse biological processes, including immune responses, proliferation, differentiation, apoptosis, and embryogenesis, by binding to TNFSF receptors. Many TNFSF proteins are also known to be responsible for the regulation of innate immunity and inflammation. Both receptor-mediated forward signaling and ligand-mediated reverse signaling play important roles in these processes. In this review, we discuss the functional expression and roles of various reverse signaling molecules and pathways of TNFSF members in macrophages and microglia in the central nervous system (CNS). A thorough understanding of the roles of TNFSF ligands and receptors in the activation of macrophages and microglia may improve the treatment of inflammatory diseases in the brain and periphery. In particular, TNFSF reverse signaling in microglia can be exploited to gain further insights into the functions of the neuroimmune interface in physiological and pathological processes in the CNS.

Design and Synthesis of N-Arylphthalimides as Inhibitors of Glucocorticoid-Induced TNF Receptor-Related Protein, Proinflammatory Mediators, and Cytokines in Carrageenan-Induced Lung Inflammation

N-Arylphthalimides (1-10P) derived from thalidomide by insertion of hydrophobic groups were evaluated for anti-inflammatory activity, and (4-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)-N'-[(4-ethoxyphenyl)methylidene]benzohydrazide 6P was identified as a promising anti-inflammatory agent. Further testing confirmed that compared with the control, 6P treatment resulted in a considerable decrease in CD4(+), NF-κB p65(+), TNF-α(+), IL-6(+), GITR(+), and IL-17(+) cell populations and an increase in the Foxp3(+), CD4(+)Foxp3(+), and IκBα(+) populations in whole blood and pleural fluid of a mouse model of lung inflammation. Moreover, treatment with compound 6P decreased the proteins associated with inflammation including TNF-α, IL-6, IL-17, GITR, NF-κB, COX-2, STAT-3, and iNOS and increased the anti-inflammatory mediators such as IL-10 and IL-4. Further, histopathological examination confirmed the potent anti-inflammatory effects of compound 6P. Thus, the N-arylphthalimide derivative 6P acts as a potent anti-inflammatory agent in the carrageenan-induced lung inflammation model, suggesting that this compound may be useful for the treatment of inflammation in a clinical setting.

GITR promoter polymorphism contributes to risk of coal workers' pneumoconiosis: a case-control study from China

BACKGROUND

Glucocorticoid-induced tumor necrosis factor (TNF) receptor-related protein (GITR) mainly affects the functions of effector T cells and regulatory T cells thus it may influence various diseases. Coal workers' pneumoconiosis (CWP) is a serious occupational disease worldwide. In the present study, we examined the association between the functional polymorphisms in GITR and risk of CWP in a Chinese population.

METHODS

An association study analyzing three polymorphisms (rs3753348, rs2298213, and rs11466668) in GITR were performed in a case-control study including 693 patients with CWP and 690 controls. Genotyping was carried out by Taqman method.

RESULTS

The GITR rs3753348 GG/GC genotypes significantly enhanced the risk of CWP (adjusted OR=1.32, 95%CI=1.02-1.71), compared with the CC genotype, particularly among subgroups of long exposure years (adjusted OR=1.47, 95%CI=1.06-2.04) and non-smokers (adjusted OR=1.45, 95%CI=1.01-2.09). Moreover, the polymorphism was significantly associated with risk for CWP cases with stage II.

CONCLUSIONS

This is the first report revealing an association between the GITR rs3753348 polymorphism and CWP, and our results suggest that the GITR rs3753348 polymorphism may be involved in the development and susceptibility of CWP.

Transferrin and the transferrin receptor for the targeted delivery of therapeutic agents to the brain and cancer cells

The potential use of many promising novel drugs is limited by their inability to specifically reach their site of action after intravenous administration, without secondary effects on healthy tissues. In order to remediate this problem, the protein transferrin (Tf) has been extensively studied as a targeting molecule for the transport of drug and gene delivery systems to the brain and cancer cells. A wide range of delivery approaches have been developed to target the Tf receptor and they have already improved the specific delivery of Tf-bearing therapeutic agents to their site of action. This review provides a summary of the numerous delivery strategies used to target the Tf receptor and focuses on recent therapeutic advances.

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.

GITR-expressing regulatory T-cell subsets are increased in tumor-positive lymph nodes from advanced breast cancer patients as compared to tumor-negative lymph nodes

Lymph node (LN) infiltration by neoplastic process involves important changes in lymph node immune microenvironment. In particular, regulatory T cells (Treg) seem to have a key role in altering the immunoediting function of the immune system which leads to the elusion of the tumor from immune surveillance. In this study, we evaluated the expression of T-cell markers in CD4+ and CD8+ subsets from tumor-positive and tumor-negative lymph nodes from the same, advanced stage breast cancer patient. The study was carried out on 3 patients and similar results were obtained. Flow cytometric analysis of CD8+ cells demonstrated a significant difference in the expression of CD25, CD45RA, CD45RO, and GITRL (Glucocorticoid-Induced TNF receptor-Related ligand). Flowcytometric analysis of CD4+ cells demonstrated a significant difference in the expression of GITR (Glucocorticoid-Induced TNF receptor-Related), CD25, FoxP3 (Forkhead box P3), CD28, and CD45RA. Multiple staining allowed the identification of two Treg subpopulations, CD4+ CD25 highGITR+ CD127-/low and CD4+ CD25 low GITR+ CD127+ cells, proving that both are increased in the positive nodes in comparison with the negative nodes from the same patient. We identified for the first time the CD4+ CD25 low GITR+ CD127+ Treg subpopulation in cancer, and the 2.6 fold increase in positive LN suggests that this Treg subpopulation could be a key player in metastasis. We also found GITRL expression in the CD8 lymphocytes, which may also contribute to the changes of metastatic lymph node microenvironment. These findings make both GITR and GITRL good possible co-candidates for future therapeutical intervention against metastasis and perhaps also as disease evolution biomarkers.

CD8+ T cells: GITR matters

As many members of the tumor necrosis factor receptor superfamily, glucocorticoid-induced TNFR-related gene (GITR) plays multiple roles mostly in the cells of immune system. CD8⁺ T cells are key players in the immunity against viruses and tumors, and GITR has been demonstrated to be an essential molecule for these cells to mount an immune response. The aim of this paper is to focus on GITR function in CD8⁺ cells, paying particular attention to numerous and recent studies that suggest its crucial role in mouse disease models.

GITR gene deletion and GITR-FC soluble protein administration inhibit multiple organ failure induced by zymosan

Multiple organ dysfunction syndrome (MODS) is a systemic inflammatory event that can result in organ damage, failure, and high risk of mortality. The aim of this study was to evaluate the possible role of glucocorticoid-induced TNFR-related (GITR) on zymosan-induced MODS. Mice were allocated into one GITR knockout (GITR-KO) and two GITR wild-type (GITR-WT) experimental groups. All the animals were treated with zymosan (500 mg/kg, suspended in saline solution, i.p.), and animals of one GITR-WT group received GITR-Fc (6.25 μg/mouse; 3 h after zymosan injection) by mini-osmotic pump. Moreover, three control groups were performed (one GITR-KO and two GITR-WT experimental groups), administering saline instead of zymosan and treating one of the GITR-WT group with GITR-Fc (6.25 μg/mouse; 3 h after saline injection) by mini-osmotic pump. A number of inflammatory parameters such as edema formation, histological damage, adhesion molecules expression, neutrophil infiltration, proinflammatory cytokines, nitrotyrosine, and iNOS production are significantly reduced in GITR-KO as compared with GITR-WT mice as well as in GITR-WT mice treated with GITR-Fc. We here show that GITR plays a role in the modulation of experimental MODS. In particular, we show that genetic inhibition of GITR expression, in GITR-KO mice, or administration of soluble GITR-Fc receptor in GITR-WT mice, reduces inflammation, organ tissue damage, and mortality. Results, while confirming the proinflammatory role of GITR, extend our observations indicating that GITR plays a role in zymosan-induced inflammation and MODS.

Modulation of GITR for cancer immunotherapy

Modulation of co-inhibitory and co-stimulatory receptors of the immune system has become a promising new approach for immunotherapy of cancer. With the recent FDA approval of CTLA-4 blockade serving as an important proof of principal, many new targets are now being translated into the clinic. Preclinical research has demonstrated that targeting glucocorticoid-induced tumor necrosis factor (TNF) receptor related gene (GITR), a member of TNF receptor superfamily, by agonist antibodies or natural ligand, can serve as an effective anti-tumor therapy. In this review, we will cover this research and the rationale that has led to initiation of two phase 1 clinical trials targeting GITR as a new immunotherapeutic approach for cancer.

Dexamethasone attenuates lipopolysaccharide-induced liver injury by downregulating glucocorticoid-induced tumor necrosis factor receptor ligand in Kupffer cells

AIM

  Glucocorticoid-induced tumor necrosis factor receptor ligand (GITRL) plays pro-inflammatory roles in immune response. Thus, our aim was to assess if dexamethasone attenuates lipopolysaccharide (LPS)-induced liver injury by affecting GITRL in Kupffer cells (KC).

METHODS

  A BALB/c mouse model of liver injury was established by i.p. injecting with LPS (10 mg/kg) co-treated with or without dexamethasone (3 mg/kg). Blood and liver samples were obtained for analysis of liver morphology, GITRL expression, hepatocellular function and cytokine levels at 24 h after injection. KC were isolated and challenged by LPS (1 µg/mL), with or without dexamethasone (10 µM) co-treatment, or with GITRL siRNA pre-transfection. The GITRL expression and cytokine levels were assayed at 24 h after challenge.

RESULTS

  Dexamethasone treatment significantly improved the survival rate of endotoxemic mice (P < 0.05), whereas serum alanine aminotransferase, aspartate aminotransferase, tumor necrosis factor (TNF)-α, interleukin (IL)-6 and γ-interferon levels were significantly decreased (P < 0.05, respectively). Concurrently, LPS-induced hepatic tissue injury was attenuated as indicated by morphological analysis; and expression of GITRL in liver tissue and KC was downregulated (P < 0.05). Consistent with these in vivo experiments, inhibited expression of GITRL, TNF-α and IL-6 caused by dexamethasone treatment were also observed in LPS-stimulated KC. The GITRL, TNF-α and IL-6 expression was also significantly inhibited by GITRL gene silencing.

CONCLUSION

  The TNF-α and IL-6 expression of LPS-stimulated KC was inhibited by GITRL gene silencing. Dexamethasone attenuates LPS-induced liver injury, at least proportionately, by downregulating GITRL in KC.

Boswellic acids extract attenuates pulmonary fibrosis induced by bleomycin and oxidative stress from gamma irradiation in rats

BACKGROUND

Interstitial pulmonary fibrosis is characterized by an altered cellular composition of the alveolar region with excessive deposition of collagen. Lung inflammation is also common in pulmonary fibrosis. This study aims to test the inhibition of 5-lipooxygenase (5-LOX) by boswellic acid (BA) extract in an experimental model of pulmonary fibrosis using bleomycin (BL).

METHODS

Boswellic acid extract (1 g/kg) was force-fed to rats seven days prior to administration of BL or gamma irradiation or both. BL (0.15 U/rat) in 25 μl of 0.9% normal saline (NS) or 0.9% NS alone was administered intratracheally. Rats were exposed to two fractionated doses of gamma irradiation (0.5 Gy/dose/week) with a gamma cell-40 (Cesium-137 irradiation units, Canada) during the last two weeks of the experiment. BA was administered during BL or irradiation treatment or both. After the animals were sacrificed, bronchoalveolar lavage was performed; lungs were weighed and processed separately for biochemical and histological studies.

RESULTS

In rats treated with BL, levels of transforming growth factor-β1 (TGF-β1) and tumor necrosis factor-α (TNF-α) were significantly elevated (P = 0.05 and P = 0.005). Hydroxyproline was highly and extensively expressed. Immunoreactive compounds were abundantly expressed, represented in the levels of macrophages infiltrate, accumulation of eosinophils and neutrophils in the lung as well as the aggregation of fibroblasts in the fibrotic area. The levels of lipoxygenase enzyme activity were significantly increased (P = 0.005). Antioxidant activities measured in BL-treated rats deteriorated, coupled with the elevation of both levels of plasma lipid peroxide (LP) content and bronchoalveolar lavage lactate dehydrogenase activity. BA-treated rats had reduced number of macrophages, (P = 0.01), neutrophils in bronchoalveolar lavage (P = 0.01) and protein (P = 0.0001). Moreover, the hydroxyproline content was significantly lowered in BA-treated rats (P = 0.005). BA extract inhibited the TGF-ß induced fibrosis (P = 0.01) and 5-LOX activity levels (P = 0.005).Histologically, BA reduced the number of infiltrating cells, ameliorated the destruction of lung architecture and attenuated lung fibrosis.

CONCLUSION

BA attenuates the BL-induced injury response in rats, such as collagen accumulation, airway dysfunction and injury. This study suggests that the blocking of 5-LOX may prevent the progression of fibrosis.

CD4(+) CD25(low) GITR(+) cells: a novel human CD4(+) T-cell population with regulatory activity

Treg subsets play a role in sustaining peripheral tolerance, are characterized by markers such as forkhead winged-helix transcription factor (FOXP3) and CD25, and produce suppressive cytokines, such as IL-10 and TGF-β. Glucocorticoid-induced TNF receptor family-related (GITR) protein has been suggested to regulate Treg activity in mice. The aim of our study was to investigate GITR expression in human CD4(+) T lymphocytes and its possible role in Treg function. Results indicate that a subset of CD4(+) T cells in the peripheral blood expresses GITR and low levels of CD25 (CD4(+) CD25(low) GITR(+) ). These cells show Treg features as they express FOXP3, IL-10, TGF-β and are anergic but, as opposed to natural Tregs, express low levels of CTLA-4 and are CD127(high) . CD4(+) CD25(low) GITR(+) cells represent a low percentage of the CD4(+) T-cell population (0.32-1.74%) and are mostly memory cells. Functional experiments demonstrated that CD4(+) CD25(low) GITR(+) cells have relevant suppressive activity that depends on TGF-β. Moreover, an anti-GITR Ab inhibited their suppressive activity, as observed in CD4(+) CD25(+) murine Tregs. Taken together, these data indicate that human CD4(+) CD25(low) GITR(+) cells represent a distinct Treg subpopulation.

The glucocorticoid-induced TNF receptor family-related protein (GITR) is critical to the development of acute pancreatitis in mice

BACKGROUND AND PURPOSE

Pancreatitis represents a life-threatening inflammatory condition where leucocytes, cytokines and vascular endothelium contribute to the development of the inflammatory disease. The glucocorticoid-induced tumour necrosis factor (TNF) receptor family-related protein (GITR) is a costimulatory molecule for T lymphocytes, modulates innate and adaptive immune system and has been found to participate in a variety of immune responses and inflammatory processes. Our purpose was to verify whether inhibition of GITR triggering results in a better outcome in experimental pancreatitis.

EXPERIMENTAL APPROACH

In male GITR knock-out (GITR(-/-)) and GITR(+/+) mice on Sv129 background, acute pancreatitis was induced after i.p. administration of cerulein. Other experimental groups of GITR(+/+) mice were also treated with different doses of Fc-GITR fusion protein (up to 6.25 µg·mouse⁻¹), given by implanted mini-osmotic pump. Clinical score and pro-inflammatory parameters were evaluated.

KEY RESULTS

A less acute pancreatitis was found in GITR(-/-) mice than in GITR(+/+) mice, with marked differences in oedema, neutrophil infiltration, pancreatic dysfunction and injury. Co-treatment of GITR(+/+) mice with cerulein and Fc-GITR fusion protein (6.25 µg·mouse⁻¹) decreased the inflammatory response and tissue injury, compared with treatment with cerulein alone. Inhibition of GITR triggering was found to modulate activation of nuclear factor κB as well as the production of TNF-α, interleukin-1β, inducible nitric oxide synthase, nitrotyrosine, poly-ADP-ribose, intercellular adhesion molecule-1 and P-selectin.

CONCLUSIONS AND IMPLICATIONS

The GITR-GITR ligand system is crucial to the development of acute pancreatitis in mice. Our results also suggest that the Fc-GITR fusion protein could be useful in the treatment of acute pancreatitis.

Glucocorticoid-Induced TNFR family Related gene (GITR) enhances dendritic cell activity

Glucocorticoid-Induced TNFR family Related gene (GITR), a Tumor Necrosis Factor Receptor Superfamily (TNFRSF) member involved in immune/inflammatory processes, has been previously shown to regulate T cell activation. To study GITR role in antigen presenting cells, we evaluated the capability of bone marrow derived dendritic cells (BMDC) from GITR(-/-) mice to stimulate the activation of CD4(+)CD25(-) T lymphocytes. We found that GITR(-/-) BMDC are weaker stimulators of T cell proliferation than GITR(+/+) BMDC, either in syngenic or allogenic BMDC/T cell co-cultures. Expression of GITR in GITR(-/-) BMDC restored their ability to activate T cells while GITR silencing in GITR(+/+) BMDC inhibited the capability to stimulate T cells. GITR(-/-) BMDC showed a reduced production of the pro-inflammatory cytokine IL-6 and an increased production of the anti-inflammatory cytokine IL-10. Notably, co-culture of CD4(+)CD25(-) cells with GITR(-/-) BMDC originated FoxP3(+) cells, secreting IL-10 and TGF-β. Finally, in vivo injection of GITR(-/-) OVA-loaded BMDC led to a lower cell number and a lower activated cell number in draining lymph nodes than in GITR(+/+) OVA-loaded BMDC injected mice. Together, these results indicate that GITR plays a role in regulating BMDC activity.

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.

Forty-eight hours of unloading and 24 h of reloading lead to changes in global gene expression patterns related to ubiquitination and oxidative stress in humans

Although short-term disuse does not result in measurable muscle atrophy, studies suggest that molecular changes associated with protein degradation may be initiated within days of the onset of a disuse stimulus. We examined the global gene expression patterns in sedentary men (n = 7, mean age ± SD = 22.1 ± 3.7 yr) following 48 h unloading (UL) via unilateral lower limb suspension and 24 h reloading (RL). Biopsy samples of the left vastus lateralis muscle were collected at baseline, 48 h UL, and 24 h RL. Expression changes were measured by microarray and gene clustering; identification of enriched functions and canonical pathways were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA). Four genes were validated with quantitative RT-PCR (qRT-PCR), and protein levels were measured with Western blot. Of the upregulated genes after UL, the most enriched functional group and highest ranked canonical pathway were related to protein ubiquitination. The oxidative stress response pathway was the second highest ranked canonical pathway. Of the downregulated genes, functions related to mitochondrial metabolism were the most highly enriched. In general, gene expression patterns following UL persisted following RL. qRT-PCR confirmed increases in mRNA for ubiquitin proteasome pathway-related E3 ligase Atrogin1 (but not accompanying increases in protein products) and stress response gene heme oxygenase-1 (HMOX, which showed a trend toward increases in protein products at 48 h UL) as well as extracellular matrix (ECM) component COL4A3. The gene expression patterns were not reversed on RL, suggesting that molecular responses to short-term periods of skeletal muscle inactivity may persist after activity resumes.

GITR Blockade Facilitates Treg Mediated Allograft Survival

BACKGROUND

Many models of transplant tolerance have been found to depend on the induction of regulatory T cells (Tregs). Innate immune signals are known to suppress Tregs thereby augmenting immunity by abrogating Treg function. Such signals may also provide a barrier to transplantation tolerance mediated by Tregs. A number of cell surface molecules expressed by Tregs have been found to inhibit Treg activity, the best characterized of which is the glucocorticoid-induced tumor necrosis factor receptor-related (GITR) protein.

METHODS

By using an adoptive transfer model of allograft rejection, we can study the effects of inflammation and antigen-specific Tregs on graft survival. Inflammation resulting from the transplant procedure counter-regulates the suppressor activity of Tregs. To assess whether Treg activity could be enhanced by blocking GITR signaling, we compared the capacity of Tregs to prolong the survival of grafts in the presence or absence of activation-inducible TNF receptor (AITRL)-Fc, a novel construct that binds GITR.

RESULTS

We report that interruption of GITR-GITR ligand (GITRL) binding by AITRL-Fc resulted in long-term Treg-dependent acceptance of skin grafts in the setting of innate immune signals that otherwise interfere with Treg activity.

CONCLUSIONS

Inflammation and other innate immune signals may activate antigen presenting cells to upregulate GITRL. GITR-GITRL interaction is one pathway by which antigen presenting cells may enhance the adaptive response to foreign antigen by counter-regulating Tregs and by costimulating effector T cells. By blocking this interaction with AITRL-Fc, one can sustain the benefit conferred by graft-protective Tregs.

Key role of the GITR/GITRLigand pathway in the development of murine autoimmune diabetes: a potential therapeutic target

Background

The cross-talk between pathogenic T lymphocytes and regulatory T cells (Tregs) plays a major role in the progression of autoimmune diseases. Our objective is to identify molecules and/or pathways involved in this interaction and representing potential targets for innovative therapies. Glucocorticoid-induced tumor necrosis factor receptor (GITR) and its ligand are key players in the T effector/Treg interaction. GITR is expressed at low levels on resting T cells and is significantly up-regulated upon activation. Constitutive high expression of GITR is detected only on Tregs. GITR interacts with its ligand mainly expressed on antigen presenting cells and endothelial cells. It has been suggested that GITR triggering activates effector T lymphocytes while inhibiting Tregs thus contributing to the amplification of immune responses. In this study, we examined the role of GITR/GITRLigand interaction in the progression of autoimmune diabetes.

Methods and Findings

Treatment of 10-day-old non-obese diabetic (NOD) mice, which spontaneously develop diabetes, with an agonistic GITR-specific antibody induced a significant acceleration of disease onset (80% at 12 weeks of age). This activity was not due to a decline in the numbers or functional capacity of CD4⁺CD25⁺Foxp3⁺ Tregs but rather to a major activation of ‘diabetogenic’ T cells. This conclusion was supported by results showing that anti-GITR antibody exacerbates diabetes also in CD28^−/− NOD mice, which lack Tregs. In addition, treatment of NOD mice, infused with the diabetogenic CD4⁺BDC2.5 T cell clone, with GITR-specific antibody substantially increased their migration, proliferation and activation within the pancreatic islets and draining lymph nodes. As a mirror image, blockade of the GITR/GITRLigand pathway using a neutralizing GITRLigand-specific antibody significantly protected from diabetes even at late stages of disease progression. Experiments using the BDC2.5 T cell transfer model suggested that the GITRLigand antibody acted by limiting the homing and proliferation of pathogenic T cells in pancreatic lymph nodes.

Conclusion

GITR triggering plays an important costimulatory role on diabetogenic T cells contributing to the development of autoimmune responses. Therefore, blockade of the GITR/GITRLigand pathway appears as a novel promising clinically oriented strategy as GITRLigand-specific antibody applied at an advanced stage of disease progression can prevent overt diabetes.

The glucocorticoid-induced TNF receptor-related protein (GITR)-GITR ligand pathway acts as a mediator of cutaneous dendritic cell migration and promotes T cell-mediated acquired immunity

Glucocorticoid-induced TNFR-related protein (GITR) has various roles in the activation of T cells and inflammation. In this study, we investigated the roles of the GITR-GITR ligand (GITRL) pathway in contact hypersensitivity (CH). Treatment with anti-GITRL mAb at sensitization inhibited CH responses. Depletion studies using an anti-CD25 or anti-PDCA-1 mAb revealed that regulatory T cells and plasmacytoid dendritic cells (DCs), known to express high levels of GITR and GITRL, respectively, were not apparently involved in GITRL-mediated CH responses. Treatment with/addition of anti-GITRL mAb in the experiments for hapten-specific T cell proliferation and IFN-gamma production showed a minor contribution of the GITRL, which was weakly expressed on DCs in draining lymph nodes (dLNs). Interestingly, anti-GITRL mAb treatment inhibited the migration of cutaneous DCs to the dLNs. Epidermal keratinocytes (KCs) constitutively express GITR, whereas Langerhans cells (LCs) express higher levels of GITRL compared with DCs in dLNs. GITR ligation, by an anti-GITR mAb, in KCs promoted expression of multiple proinflammatory cytokines and blockade of GITRL-inhibited IL-1beta and CCR7 expression in sensitized skin. These results suggest that the GITR-GITRL pathway promotes epidermal inflammatory cytokine production by KCs and LCs, resulting in migration of cutaneous DCs from the skin to the dLNs. This is the first report demonstrating the involvement of the GITR-GTRL pathway in interactions with KCs and LCs and the migration of DCs. Our findings provide important implications for understanding the molecular bases of KC-LC interactions and for developing new therapeutic strategies in skin disease.

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.

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.

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.