The soluble glucocorticoid-induced tumor necrosis factor receptor causes cell cycle arrest and apoptosis in murine macrophages

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.

An isoleucine-zipper motif enhances costimulation of human soluble trimeric GITR ligand

Glucocorticoid-induced tumor-necrosis factor receptor (GITR) and its ligand, GITRL, play significant roles in regulating immune responses. It is clear that human soluble GITRL (hsGITRL) transduces signal activity through multiple oligomerization states. To develop human soluble trimeric GITRL protein as a potential therapeutic target, we explored the link of the isoleucine-zipper (ILZ) motif to the N-terminus of the human soluble GITRL with two leucine sequences. hsGITRL, with the ILZ motif (ILZ-hsGITRL), was firstly expressed in Escherichia coli, which exhibited a predominant trimer when identified by Sephadex G-100 filtration and non-reducing SDS-polyacrylamide gel electrophoresis (SDS-PAGE). The significantly higher biological activity of the ILZ-hsGITRL compared with hsGITRL was confirmed by CD4(+) T proliferation, interferon-gamma (IFN-gamma) secretion and binding activity assay. To reveal and compare the underlying mechanisms, the level of extracellular signal-regulated kinase-1/2 (ERK1/2) phosphorylation was examined, indicating that ILZ-hsGITRL induced more persistent and stronger ERK1/2 activation than hsGITRL. In conclusion, the incorporation of an ILZ motif could markedly improve the costimulation of hsGITRL.

Sequence, structure, function, immunity: structural genomics of costimulation

SUMMARY

Costimulatory receptors and ligands trigger the signaling pathways that are responsible for modulating the strength, course, and duration of an immune response. High-resolution structures have provided invaluable mechanistic insights by defining the chemical and physical features underlying costimulatory receptor:ligand specificity, affinity, oligomeric state, and valency. Furthermore, these structures revealed general architectural features that are important for the integration of these interactions and their associated signaling pathways into overall cellular physiology. Recent technological advances in structural biology promise unprecedented opportunities for furthering our understanding of the structural features and mechanisms that govern costimulation. In this review, we highlight unique insights that have been revealed by structures of costimulatory molecules from the immunoglobulin and tumor necrosis factor superfamilies and describe a vision for future structural and mechanistic analysis of costimulation. This vision includes simple strategies for the selection of candidate molecules for structure determination and highlights the critical role of structure in the design of mutant costimulatory molecules for the generation of in vivo structure-function correlations in a mammalian model system. This integrated 'atoms-to-animals' paradigm provides a comprehensive approach for defining atomic and molecular mechanisms.

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.

Cyclin-dependent kinase 4 hyperactivity promotes autoreactivity in the immune system but protects pancreatic cell mass from autoimmune destruction in the nonobese diabetic mouse model

Cyclin-dependent kinase 4 (Cdk4) plays a central role in perinatal pancreatic beta cell replication, thus becoming a potential target for therapeutics in autoimmune diabetes. Its hyperactive form, Cdk4R24C, causes beta cell hyperplasia without promoting hypoglycemia in a nonautoimmune-prone mouse strain. In this study, we explore whether beta cell hyperproliferation induced by the Cdk4R24C mutation balances the autoimmune attack against beta cells inherent to the NOD genetic background. To this end, we backcrossed the Cdk4R24C knockin mice, which have the Cdk4 gene replaced by the Cdk4R24C mutated form, onto the NOD genetic background. In this study, we show that NOD/Cdk4R24C knockin mice exhibit exacerbated diabetes and insulitis, and that this exacerbated diabetic phenotype is solely due to the hyperactivity of the NOD/Cdk4R24C immune repertoire. Thus, NOD/Cdk4R24C splenocytes confer exacerbated diabetes when adoptively transferred into NOD/SCID recipients, compared with NOD/wild-type (WT) donor splenocytes. Accordingly, NOD/Cdk4R24C splenocytes show increased basal proliferation and higher activation markers expression compared with NOD/WT splenocytes. However, to eliminate the effect of the Cdk4R24C mutation specifically in the lymphocyte compartment, we introduced this mutation into NOD/SCID mice. NOD/SCID/Cdk4R24C knockin mice develop beta cell hyperplasia spontaneously. Furthermore, NOD/SCID/Cdk4R24C knockin females that have been adoptively transferred with NOD/WT splenocytes are more resistant to autoimmunity than NOD/SCID WT female. Thus, the Cdk4R24C mutation opens two avenues in the NOD model: when expressed specifically in beta cells, it provides a new potential strategy for beta cell regeneration in autoimmune diabetes, but its expression in the immune repertoire exacerbates autoimmunity.

Assembly and structural properties of glucocorticoid-induced TNF receptor ligand: Implications for function

Glucocorticoid-induced TNF receptor ligand (GITRL), a recently identified member of the TNF family, binds to its receptor GITR on both effector and regulatory T cells and generates positive costimulatory signals implicated in a wide range of T cell functions. Structural analysis reveals that the human GITRL (hGITRL) ectodomain self-assembles into an atypical expanded homotrimer with sparse monomer-monomer interfaces. Consistent with the small intersubunit interfaces, hGITRL exhibits a relatively weak tendency to trimerize in solution and displays a monomer-trimer equilibrium not reported for other TNF family members. This unique assembly behavior has direct implications for hGITRL-GITR signaling, because enforced trimerization of soluble hGITRL ectodomain results in an approximately 100-fold increase in its receptor binding affinity and also in enhanced costimulatory activity. The apparent reduction in affinity that is the consequence of this dynamic equilibrium may represent a mechanism to realize the biologically optimal level of signaling through the hGITRL-GITR pathway, as opposed to the maximal achievable level.

Stimulation of osteoclastogenesis by enhanced levels of MIP-1alpha in BALB/c mice in vitro

OBJECTIVES

We compared osteoclast (OC) formation in bone marrow-derived macrophages (BMM) from C57BL/6 (B/6) and BALB/c (B/c) mice. After stimulation of receptor activator of nuclear factor-kappaB ligand (RANKL), enhanced OC formation and higher level of macrophage inflammatory protein-1alpha (MIP-1alpha) were observed in the BMM from B/c mice. In this study, we determined whether MIP-1alpha is responsible for stimulated OC formation in the BMM.

MATERIALS AND METHODS

OC formation was evaluated in BMM. Expression of MIP-1alpha during OC formation was analyzed at the mRNA and protein levels. Apoptosis of mature OCs was evaluated by observing the degradation of DNA. Activation of nuclear factor-kappaB (NF-kappaB) was measured by electrophoretic mobility shift assay.

RESULTS

After stimulation by RANKL expression of MIP-1alpha at the mRNA and protein levels was much higher in BMM from B/c mice than in BMM from B/6 mice. Transcripts of the MIP-1alpha receptors, CCR1 and CCR5, were present at similar levels in unstimulated BMM of the two strains. Blockade of MIP-1alpha inhibited OC formation, and exogenously added MIP-1alpha stimulated it in RANKL-stimulated BMM. MIP-1alpha affected not only the early precursors but also mature OCs. It prevented apoptosis of mature OCs by activating NF-kappaB, and the effect of RANKL on survival was dependent on its ability to induce MIP-1alpha.

CONCLUSIONS

MIP-1alpha, induced by RANKL during OC differentiation, increases OC formation by acting on OC progenitor cells, and prolongs survival of mature OC via signaling through NF-kappaB. The enhanced OC formation in BMM from B/c mice could be due to, at least in part, to their higher levels of MIP-1alpha.

Cancer immunoediting by GITR (glucocorticoid-induced TNF-related protein) ligand in humans: NK cell/tumor cell interactions

Glucocorticoid-induced TNF-related protein (GITR) has been shown to stimulate T cell-mediated antitumor immunity in mice. However, the functional relevance of GITR and its ligand (GITRL) for non-T cells has yet to be fully explored. In addition, recent evidence suggests that GITR plays different roles in mice and humans. We studied the role of GITR-GITRL interaction in human tumor immunology and report for the first time that primary gastrointestinal cancers and tumor cell lines of different histological origin express substantial levels of GITRL. Signaling through GITRL down-regulated the expression of the immunostimulatory molecules CD40 and CD54 and the adhesion molecule EpCAM, and induced production of the immunosuppressive cytokine TGF-beta by tumor cells. On NK cells, GITR is constitutively expressed and up-regulated following activation. Blocking GITR-GITRL interaction in cocultures of tumor cells and NK cells substantially increased cytotoxicity and IFN-gamma production of NK cells demonstrating that constitutive expression of GITRL by tumor cells diminishes NK cell antitumor immunity. GITRL-Ig fusion protein or cell surface-expressed GITRL did not induce apoptosis in NK cells, but diminished nuclear localized c-Rel and RelB, indicating that GITR might negatively modulate NK cell NF-kappaB activity. Taken together, our data indicate that tumor-expressed GITRL mediates immunosubversion in humans.

Soluble glucocorticoid-induced tumor necrosis factor receptor stimulates osteoclastogenesis by down-regulation of osteoprotegerin in bone marrow stromal cells

Soluble glucocorticoid-induced tumor necrosis factor receptor (sGITR) is a potent stimulator of osteoclastogenesis. The mechanism by which it induces osteoclastogenesis was studied by culturing bone-marrow-derived macrophages (BMM) with conditioned medium from mouse bone marrow stromal cells. GITR and GITR ligand (GITRL) were expressed on the surface of bone marrow stromal cells, and sGITR-induced osteoclastogenesis was inhibited by anti-GITRL Ab, indicating that stimulatory effect of osteoclastogenesis by sGITR involved signaling via GITRL. Bone marrow stromal cells up-regulated cyclooxygenase-2 (COX-2) and produced prostaglandin E(2) (PGE(2)) early in their response to sGITR, and the stimulation of osteoclastogenesis was markedly inhibited by NS398, a COX-2 inhibitor. Later, sGITR markedly reduced the steady-state level of osteoprotegerin (OPG) mRNA and increased receptor activator of nuclear factor-kappaB ligand (RANKL) mRNA. NS398 blocked the sGITR-induced reduction of OPG mRNA but did not significantly affect the sGITR-induced rise in RANKL mRNA. This suggests that down-regulation of OPG by PGE(2) is involved in sGITR-induced osteoclast (OC) formation in the presence of conditioned medium from mouse bone marrow stromal cells.

Proinflammatory role of glucocorticoid-induced TNF receptor-related gene in acute lung inflammation

Glucocorticoid-induced TNFR-related gene (GITR) participates in the immune/inflammatory response. Because GITR expression has been described in cells other than T lymphocytes, we investigated whether it also modulates acute inflammatory response. Using GITR-deficient (GITR(-/-)) mice, we analyzed the role of GITR in the development of carrageenan-induced lung inflammation (pleurisy) by studying several proinflammatory markers 2-8 h after carrageenan injection. When compared with GITR(+/+), GITR(-/-) mice exhibited decreased production of turbid exudate containing a lower number of leukocytes. This was correlated with the reduction of inflammatory markers (including TNF-alpha, IL-1beta, myeloperoxidase, inducible NO synthase, and cyclooxygenase 2) in the pleural exudate and/or in the lung. Moreover, endothelial cells expressed lower levels of adhesion molecules. In lungs of GITR(+/+) mice, GITR ligand expression was not modulated during pleurisy, while that of GITR increased, as a consequence of increased infiltration by GITR-expressing cells and of GITR up-regulation in macrophages and endothelial cells. Finally, cotreatment of GITR(+/+) mice with carrageenan and Fc-GITR fusion protein decreased the number of inflammatory cells (pleural macrophages and lung neutrophils) as compared with carrageenan treatment alone, confirming that GITR plays a role in the modulation of pleurisy.

A signal through 4-1BB ligand inhibits receptor for activation of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis by increasing interferon (IFN)-beta production

We tested whether any intracellular signals are transmitted through 4-1BB/CD137 ligand (4-1BBL), using a 4-1BB-Fc fusion protein and 4-1BB-deficient mice. Immobilized 4-1BB-Fc fusion protein strongly inhibited osteoclastogenesis induced by macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappaB ligand (RANKL) derived from bone marrow macrophages (BMM). Incubation of BMM with M-CSF increased 4-1BBL mRNA and surface expression of 4-1BBL protein. Cross-linking 4-1BBL with immobilized 4-1BB-Fc also dramatically reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells (MNC) derived from the BMM from 4-1BB-deficient mice, suggesting that the inhibitory effect of immobilized 4-1BB on osteoclastogenesis is due to a signal through 4-1BBL. Reverse signaling by 4-1BB-Fc increased the level of interferon (IFN)-beta in BMM and neutralization of IFN-beta reversed the inhibitory effect of immobilized 4-1BB-Fc. Inhibition of osteoclastogenesis by immobilized 4-1BB-Fc is, therefore, at least in part, due to elevation of the level of the negative regulator, IFN-beta in BMM.

Induction of MMP-13 expression by soluble human glucocorticoid-induced tumor necrosis factor receptor in fibroblast-like synovial cells

OBJECTIVE

We tested the hypothesis that human glucocorticoid-induced tumor necrosis factor receptor (hGITR/TR11) expressed on the surface of activated CD4(+) T cells is responsible for up-regulating the production of matrix metalloproteinase (MMP)-13 by fibroblast-like synoviocytes (FLSs).

METHODS

The level of MMP-13 was measured by Western blot and reverse transcriptase polymerase chain reaction (RT-PCR). Expressions of hGITR ligand (hGITRL) on the surface of FLSs and hGITR on the surface of human CD4(+) T cells were analyzed by flow cytometry and RT-PCR. Neutralizing antibodies (Abs) were used to block hGITRL and hGITR on the surface of FLSs and human CD4(+) T cells, respectively. Human CD4(+) T cells were cocultured with FLSs to facilitate interaction between hGITR on CD4(+) T cells and hGITRL on FLSs.

RESULTS

Soluble hGITR (shGITR) stimulated FLSs to produce MMP-13, and blockade of hGITRL reduced this effect. Direct contact between activated CD4(+) T and FLSs also induced the production of MMP-13, and neutralization of hGITR on activated CD4(+) T cells during coculture decreased the amount of MMP-13 produced by FLSs.

CONCLUSION

shGITR stimulated FLSs to produce MMP-13 via a signal through hGITRL. Direct contact between activated CD4(+) T cells and FLSs facilitated hGITR-hGITRL interaction, and resulted in inducing MMP-13. This effect may increase tissue destruction in chronic inflammation such as rheumatoid arthritis (RA).

The glucocorticoid-induced tumor necrosis factor receptor-related gene modulates the response to Candida albicans infection

The glucocorticoid-induced tumor necrosis factor (TNF) receptor-related gene (GITR; TNFRSF18) modulates immune response activating coaccessory signals in T cells and is expressed at high levels in CD4+CD25+ cells. Its ligand (GITRL) is expressed in antigen-presenting cells, where it is capable of promoting signaling. We investigated the role of GITR/GITRL interaction during disseminated candidiasis in GITR knockout (GITR-/-) mice. GITR-/- mice survived longer and had a significantly decreased yeast load in kidneys and brain compared to GITR+/+ mice. Since protective immunity to the fungus is mediated by antigen-specific T helper (Th) 1 cells, we studied in vitro cytokine production following infection. CD4+ T cells of GITR-/- mice demonstrated a more efficient Th1 polarization as suggested by a two- to threefold decreased production of interleukin- (IL-)4 and IL-10 and a four- to fivefold increased production of gamma interferon compared to GITR+/+ mice. This effect was not due to differences in lymphocyte and dendritic cell (DC) subpopulations in infected mice as demonstrated by flow cytometric studies. To verify whether DC activity was differently modulated, DCs were cocultured with CD4+ T cells in the presence of heat-inactivated Candida albicans. DCs, cocultured with GITR+/+ CD4+CD25+ cells produced a lower amount of IL-12 than DCs cocultured with GITR-/- CD4+CD25+ T cells. These results suggest that GITR regulates susceptibility to systemic candidiasis by negatively modulating IL-12 production and promoting polarization of CD4+ T cells towards Th2 by analogy with OX40, another TNF receptor superfamily member.

GITR: a multifaceted regulator of immunity belonging to the tumor necrosis factor receptor superfamily

Glucocorticoid-induced TNFR-related gene (GITR; TNFRSF18), a receptor belonging to the TNFR superfamily (TNFRSF), is activated by GITRL. GITR is expressed at low levels on resting responder T lymphocytes and is up-regulated in T regulatory cells (Treg cells) and in activated T cells. GITRL is expressed in endothelial and antigen-presenting cells. The cytoplasmic region of GITR has a striking homology with other TNFRSF members (4-1BB, CD27, OX40) and binds TRAF molecules and Siva. Over recent years, the role of GITR in the development and in the pathophysiology of the immune system has been actively explored by several groups. GITR triggering induces both pro- and anti-apoptotic effects, abrogates the suppressive activity of Treg cells and co-stimulates responder T cells, with the latter activities over-stimulating the immune system. In vivo, GITR activation causes development of autoimmune diseases and restores immune responses in a persistent retroviral infection model and in a tumor model. Intriguingly, GITR knockout mice demonstrate lower mortality in an ischemia model. The GITR-GITRL system appears crucial in regulating immunity and warrants further study.