The interaction of glucocorticoid-induced tumor necrosis factor receptor-family related (GITR) protein with its ligand (GITRL) modulates different functions, including immune/inflammatory response. These effects are consequent to intracellular signals activated by both GITR and GITRL. Previous results have suggested that lack of GITR expression in GITR−/− mice decreases the number of leukocytes within inflamed tissues. We performed experiments to analyze whether the GITRL/GITR system modulates leukocyte adhesion and extravasation. For that purpose, we first evaluated the capability of murine splenocytes to adhere to endothelial cells (EC). Our results indicated that adhesion of GITR−/− splenocytes to EC was reduced as compared with wild-type cells, suggesting that GITR plays a role in adhesion and that this effect may be due to GITRL-GITR interaction. Moreover, adhesion was increased when EC were pretreated with an agonist GITR-Fc fusion protein, thus indicating that triggering of GITRL plays a role in adhesion by EC regulation. In a human in vitro model, the adhesion to human EC of HL-60 cells differentiated toward the monocytic lineage was increased by EC pretreatment with agonist GITR-Fc. Conversely, antagonistic anti-GITR and anti-GITRL Ab decreased adhesion, thus further indicating that GITRL triggering increases the EC capability to support leukocyte adhesion. EC treatment with GITR-Fc favored extravasation, as demonstrated by a transmigration assay. Notably, GITRL triggering increased intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression and anti–ICAM-1 and anti-VCAM-1 Abs reversed GITR-Fc effects. Our study demonstrates that GITRL triggering in EC increases leukocyte adhesion and transmigration, suggesting new anti-inflammatory therapeutic approaches based on inhibition of GITRL-GITR interaction.
Glucocorticoid-induced tumor necrosis factor (TNF) receptor family-related (GITR, also named TNFRSF18) protein, is a member of the TNF receptor superfamily that interacts with its specific ligand (GITRL, also named TNFSF18). After interaction, both GITRL and GITR are reciprocally stimulated and activate intracellular signals regulating immune functions (Nocentini and Riccardi, 2009; Azuma, 2010; Schaer et al., 2012). In particular, GITR-driven T-cell costimulation was found to be the main mechanism by which the GITRL-GITR system contributes to tumor rejection and the development of autoimmune/inflammatory diseases (Kohm et al., 2004; Cuzzocrea et al., 2005; Lee et al., 2006; Morris and Kong, 2006; Santucci et al., 2007; Ronchetti et al., 2012; Schaer et al., 2012). This was also demonstrated by the mild inflammatory response of GITR knockout (GITR−/−) mice (Nocentini and Riccardi, 2009).
Despite the likelihood that GITR-triggered T cells are involved in exacerbation of chronic inflammatory responses (Cuzzocrea et al., 2007; Santucci et al., 2007; Liao et al., 2012), the development of acute inflammation (Cuzzocrea et al., 2004, 2006; Galuppo et al., 2011a,b) is very likely favored by GITR activation in cells other than T cells. Indeed, GITR is expressed in B cells and in cells of the innate immune system (Krausz et al., 2007; Azuma, 2010; Ronchetti et al., 2011).
Interestingly, in the pleurisy model, we detected differences in the number of proinflammatory cells in the pleura of GITR−/− compared with wild-type (WT) mice soon after the proinflammatory stimulus. The number of all leukocyte subsets was reduced, suggesting that the extravasation process is affected in GITR−/− mice (Cuzzocrea et al., 2006). Of note, during the inflammatory response, the expression level of adhesion molecules was reduced in GITR−/− as compared with WT mice (Cuzzocrea et al., 2004, 2006, 2007; Galuppo et al., 2011b). Other studies demonstrated that GITR triggering in T cells upregulates adhesion molecules (Mahesh et al., 2006) and favors migration of cutaneous dendritic cells (DC) from the skin to the draining lymph nodes (Kamimura et al., 2009).
GITRL is expressed in DC, monocytes/macrophages, and other cells of the immune system where it activates nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and extracellular signal-regulated kinase (ERK) and increases expression of chemoattractants such as interleukin-8 (IL-8) and monocyte chemotactic protein-1 (Grohmann et al., 2007; Bae et al., 2008; Baessler et al., 2009; Byrne et al., 2009; Nocentini and Riccardi, 2009; Azuma, 2010). Therefore, it stands to reason that the phenotypic differences of GITR−/− mice may also be due to the lack of GITR interaction with GITRL and the consequent lack of GITRL signaling.
It is noteworthy that GITRL is expressed at high levels in endothelial cells (EC), particularly when activated (Gurney et al., 1999; Nardelli et al., 2006). For this reason, it has been hypothesized to be an adhesion molecule (Gurney et al., 1999; Kwon et al., 1999; Krausz et al., 2007), and a direct role of the GITRL-GITR system in the regulation of extravasation cannot be ruled out. However, to date the issue has yet to be investigated.
We examined the role of the GITRL-GITR system in leukocyte adhesion to EC and extravasation. We demonstrate that GITRL triggering by an agonist GITR-Fc fusion protein increases signal transducers and activators of transcription 1 (STAT1) phosphorylation, vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) expression as well as leukocyte adhesion. In contrast, GITRL inhibition by antagonist anti-GITRL and anti-GITR Abs decreases adhesion and extravasation. Our results suggest that GITRL in EC is a regulator of the inflammatory process and that therapeutic approaches based on inhibition of GITR interaction with GITRL may represent a new line of investigation for the treatment of inflammatory diseases.
Materials and Methods
Male mice with a targeted disruption of the GITR gene (GITR−/−) and corresponding WT mice were obtained on a Sv129 background as previously described elsewhere (Ronchetti et al., 2004). Animal care was in compliance with regulations in Italy (D.M. 116192), Europe (O.J. of E.U. L276/33 20/10/2010), and the United States (Animal Welfare Assurance No. A5594-01; U.S. Department of Health and Human Services).
Cell Lines and Cell Culture Reagents.
The murine EC line HYKO6P (a gift from Dr. J. Yélamos, IMIM-Hospital del Mar, Barcelona, Spain) was maintained in 10% fetal bovine serum (FBS)/endothelial basal medium-2 (EBM-2). Splenocytes were isolated from the spleen of WT and GITR−/− mice and cultured in RPMI 1640 medium supplemented with 10% FBS, 100 μg/ml streptomycin, 10 mM HEPES, 0.1% nonessential amino acids, 1 mM sodium pyruvate, and 50 μM 2-mercaptoethanol. The immortalized human EC line HUV-ST was maintained in culture in EGM-2 supplemented with 0.4 mg/ml Geneticin (gentamycin) and 5 µg/ml puromycin (Tentori et al., 2005). HL-60 cells (American Type Culture Collection [ATCC], Manassas, VA) were cultured in 10% FBS/RPMI 1640 medium.
EBM-2 and EGM-2 were obtained from Clonetics (BioWhittaker, Walkersville, MD), FBS was from Euroclone (Milan, Italy), and RPMI 1640 medium and other cell culture reagents were from Lonza (Verviers, Belgium). The fatty acid-free bovine serum albumin (BSA) and gentamycin were from Invitrogen (Groningen, The Netherlands); puromycin, heparin, phorbol 12-myristate 13-acetate (PMA), lipopolysaccharide (LPS), and other chemicals were from Sigma-Aldrich (St. Louis, MO). The recombinant human TNF-α was from ImmunoTools (Friesoythe, Germany).
The EC monolayers were prepared by seeding in 96-well plates 5 × 104 HYKO6P or HUV-ST cells/well in 100 µl of 0.1% FBS/EBM-2. After overnight incubation at 37°C, cells were activated with TNF-α (100 ng/ml, 18 hours, 37°C), washed with prewarmed medium, and tested.
The HL-60 cells, differentiated toward the monocytic lineage (10 ng/ml PMA, 24 hours, 37°C), and murine splenocytes (activated with 50 μg/ml LPS for 72 hours at 37°C) were resuspended in complete medium at 2 × 106 cells/ml, and were labeled and protected from light with calcein acetoxymethylester (calcein-AM; Molecular Probes/Invitrogen, Monza, Italy) (1 µM, 15 minutes at 37°C). The cells were then washed three times and resuspended at 2 × 106 cells/ml in 0.5% BSA/RPMI 1640 medium.
Before adhesion, EC and/or leukocytes were pretreated as indicated in the text with recombinant murine GITR-human Fc, human GITR-human Fc, or Thy-1-human Fc proteins (all from Alexis Biochemicals, Postfach, Switzerland), and/or with anti-human GITR Ab (clone 110416; R&D Systems, Minneapolis, MN), anti-human GITRL (clone 109114; R&D Systems), anti-human ICAM-1 (clone 1H4; ImmunoTools), anti-human VCAM-1 (clone STA; ImmunoTools), or control Abs (mouse IgG1 control, clone 11711; R&D Systems; or mouse anti-human CD19 Ab, clone LT19; Serotec, Kidlington, UK). When we tested the effect of anti-human GITR Ab in comparison with control Ab, we added Fc protein (2 µg/ml) to differentiated HL-60 cells soon before Ab to saturate the Fc receptors and inhibit binding of anti-GITR and control Ab to the Fc receptor.
Adhesion of leukocytes to EC was tested by placing 50 µl of calcein-AM-labeled cells into the appropriate wells of the 96-well plate and allowing them to adsorb to the EC monolayer for 30 minutes at 37°C. Afterward wells were washed three times with prewarmed medium, and remaining cells were lysed with 100 µl of 1% Triton X-100/PBS. Cell extracts were transferred to 96-well Opti-Plates (PerkinElmer, Zaventem, Belgium) and fluorescence measured in a LS 55 Luminescence Spectrometer (PerkinElmer) set at absorption/emission of 494/517 nm.
HUV-ST cells (2 × 105 in 300 µl of complete medium) were plated in gelatin-coated (20 µg/ml) 24-well transwell inserts with 8 µm diameter pore (Becton Dickinson, San Jose, CA), and allowed to form a cell monolayer at 37°C. Cells were starved in 0.1% FBS/EBM-2 for 6 hours and then activated with TNF-α (100 ng/ml, 18 hours, 37°C). Cell monolayer was then washed with prewarmed medium and treated with GITR-Fc or control-Fc (4 µg/ml, 30 minutes, RT) in 200 µl of transmigration medium (0.5% BSA, 2 mM CaCl2, 2 mM MgCl2 RPMI 1640 medium). HL-60 cells (5 × 105 in 200 µl of transmigration medium), differentiated with PMA and labeled with calcein-AM, were loaded on EC monolayer, and the plates containing Transwell inserts were incubated at 37°C for 6 hours. We transferred 400 µl of the bottom medium comprising migrated cells to a clean well, filled with 100 µl of 5% Triton X-100/PBS, incubated for 10 minutes at room temperature with shaking and 200 µl of the mixture transferred to 96-well Opti-Plates (PerkinElmer, Zaventem, Belgium) to measure fluorescence as indicated above. Cell number in the samples was determined using a reference curve fluorescence versus number of HL-60–labeled cells.
Real-Time Polymerase Chain Reaction.
RNA was purified and cDNA was prepared as previously described elsewhere (Rinaldi et al., 2011). The analysis of GITR (Hs00188345_m1) and GITRL (Hs00183225_m1) expression was performed as previously described elsewhere (Bianchini et al., 2006) using a FAM-labeled TaqMan probe (investigated gene); a VIC-labeled TaqMan probe (HPRT-1) (Applied Biosystems, Foster City, CA) was used as the housekeeping gene for data normalization. For each experimental group, amplification was performed in quadruplicate.
Analysis of cell surface molecule expression was performed using standard procedures. For murine cell staining, the following Abs were used: anti-mouse GITR-PE (clone DTA-1; Biolegend, San Diego, CA), anti-mouse GITRL-PE (clone YGL386; eBioscience, Hatfield, UK), anti-mouse B220-FITC (clone RA3-6B2; Miltenyi Biotech, Auburn, CA), anti-mouse GR-1-FITC (clone RB6-8C5; BD Pharmingen, Erembodegem, Belgium), anti-mouse CD3-FITC (clone 17A2; BD Pharmingen).
For human cell staining, HUV-ST cells were detached from the culture flasks by gentle trypsin digestion and allowed to recover in complete medium for 2 hours at room temperature in a rotating wheel in the presence of 100 ng/ml TNF-α. Differentiated HL-60 cells were gently scraped from the culture flask and directly stained with Abs. For each sample (0.5 × 106 cells) 2.5 μg of an unlabeled anti-human GITRL mouse Ab (clone 109114; R&D Systems), a secondary goat anti-mouse IgG (Fc specific)-FITC Ab (1:100; Sigma-Aldrich), or 5 μl of a PE-conjugated anti-human CD357 (GITR) (clone 621; Biolegend, Aachen, Germany) were used. In other experiments, anti-human GITRL-FITC (clone 10910; R&D Systems) was used. GITRL detection in PMA-treated HL-60 cells required the presence of protease inhibitors in the culture medium during differentiation. For staining of adhesion molecules, anti-human ICAM-1-FITC (clone 1H4; ImmunoTools) and anti-human VCAM-1-FITC (clone STA; ImmunoTools) were used. All isotype controls were purchased from Dako (Glostrup, Denmark).
Analysis of Protein Phosphorylation.
EC monolayers were prepared by seeding in 96-well plates 5 × 104 HUV-ST cells/well in 100 µl of 0.1% FBS/EBM-2. After overnight incubation at 37°C, cells were activated with TNF-α (100 ng/ml, 18 hours, 37°C), washed with prewarmed medium, and treated with control-Fc or GITR-Fc (4 μg/ml in 0.5% BSA/PhosStop/RPMI 1640 medium). Samples were run in an 8% SDS-polyacrylamide gel, Western blot analysis was performed using anti-phospho-STAT1, and the goat anti–phospho-STAT1 (sc-7988) (Tyr701, 1:200) from Santa Cruz Biotechnology (Santa Cruz, CA) and anti–β-tubulin (H-286, 1:500) were used as as loading control. The phosphatase inhibitors PhosStop and the protease inhibitors cocktails from Roche (Mannheim, Germany) were used during the procedure (Lacal et al., 2009).
For each experimental group, adhesion was performed six times. Data are reported as the mean ± S.D. of at least three independent experiments. Statistical significance was evaluated according to Student’s t test: *P < 0.05; **P < 0.01; ***P < 0.001.
GITRL/GITR System Favors Adhesion of Murine Splenocytes to EC.
To evaluate the effect of the GITRL-GITR system on cell adhesion, we first set up an in vitro assay in which labeled LPS-activated splenocytes were seeded on a monolayer of murine activated EC. LPS-activated WT splenocytes expressed both GITRL and GITR whereas activated EC displayed high GITRL but very low GITR expression levels (Fig. 1, D and E).
When we evaluated the number of adherent cells by a fluorometric method, we found (Fig. 2A) that activated splenocytes of WT mice adhered to murine EC better than the activated splenocytes of GITR−/− mice, suggesting that GITRL-GITR interaction favors the adhesion process.
We then used an agonist GITR-Fc fusion protein to trigger GITRL in EC. The results, as shown in Fig. 2B, were that GITR-Fc pretreatment of murine EC statistically significantly enhanced the adhesion of activated GITR−/− splenocytes as compared with control-Fc in a dose-dependent manner (P < 0.01), demonstrating that GITRL triggering in EC increases leukocyte adhesion to EC.
GITRL and GITR Are Expressed in Human Cells That Participate in the Cell Adhesion Process.
Previous studies had shown that mouse and human GITRL and GITR have different tridimensional structures and different functions (Chattopadhyay et al., 2009; Nocentini et al., 2012). We investigated whether GITRL plays a similar role in humans and in mice. For that purpose, we used a human promyelocytic leukemia cell line (HL-60), differentiated in vitro toward the monocytic lineage by treatment with PMA and labeled with calcein-AM. HL-60 cells were cocultured with immortalized human EC (HUV-ST cells), and the number of adherent cells was evaluated after washing out nonadherent cells by a fluorometric method. As shown in Fig. 3A, differentiated HL-60 cells adhered only to activated EC. Therefore, studies were performed using PMA differentiated HL-60 cells and TNF-α activated HUV-ST cells.
Interestingly, TNF-α treatment markedly upregulated GITRL in HUV-ST cells, as shown by real-time polymerase chain reaction (PCR) and flow cytometry analysis (Fig. 3, B and D). In addition, GITR was upregulated in TNF-α–activated HUV-ST cells (Fig. 3C), although at very low level (~30 times lower than in Treg cells) so that it was undetectable by flow cytometry analysis (Fig. 3D). On the other hand, both GITR and GITRL were upregulated in PMA-differentiated HL-60 cells (Fig. 3, B–D).
GITRL Triggering Favors Cell Adhesion and Increases Transmigration.
The expression of GITRL in activated EC and that of GITR and GITRL in differentiated HL-60 cells suggest that the GITRL-GITR system is involved in up-modulation of human cell adhesion. Pretreatment of HUV-ST cells with agonist GITR-Fc favored the adhesion of HL-60 cells to human EC, whereas pretreatment of HL-60 cells with GITR-Fc had no effect on cell adhesion (Fig. 4A). The effect of HUV-ST pretreatment was dose-dependent, being irrelevant at 0.5 µg/ml and significant at 1 and 2 µg/ml (Fig. 4A). At the highest concentration, adhesion of HL-60 cells to GITR-Fc-treated EC increased, and difference was statistically significant (60% increase, P < 0.01), compared with adhesion of control-Fc–treated cells. Results suggest that similar to the mouse system the GITR-Fc effect on cell adhesion is attributed to GITRL activation in EC cells.
To confirm this conclusion, EC or HL-60 cells were pretreated with antagonistic anti-GITRL Abs (Satoguina et al., 2008; Kamimura et al., 2009) (referred to as pretreatment 1 in Fig. 4B) before treatment with GITR-Fc or control Fc (referred to as pretreatment 2 in Fig. 4B). Results in Fig. 4B indicate that the increased adhesion induced by agonist GITR-Fc was counteracted when HUV-ST cells were pretreated with antagonist anti-GITRL Abs (column 6 versus column 5). On the other hand, no inhibitory effect was observed when HL-60 cells were pretreated with the Abs (column 9 versus column 8). These data provide evidence for a predominant role of GITRL triggering in HUV-ST cells.
Figure 4B also shows that the anti-GITRL Abs statistically significantly (P < 0.05) inhibited basal adhesion (i.e., adhesion to untreated or control-Abs treated EC; column 3 versus columns 1 and 2) by inhibiting the physiologic interaction between GITR in HL-60 and GITRL in EC. These data suggest that up-modulation of leukocyte adhesion to EC occurs, at least in part, upon interaction of endothelial GITRL with GITR physiologically expressed in leukocytes and that it can be further increased by pharmacological treatment with GITR-Fc fusion protein.
To test this hypothesis, we verified whether inhibition of GITRL triggering by GITR expressed in differentiated HL-60 cells influences adhesion. To this end, we pretreated HL-60 cells with a GITR-blocking Ab. Indeed, the anti-GITR Ab significantly inhibited adhesion of HL-60 cells to EC, confirming that GITRL triggering in EC by GITR expressed in leukocytes favors cell adhesion during the inflammatory response. Results of the effects of GITR-Fc and anti-GITR on cell adhesion are shown in Fig. 4C.
Improved adhesion after GITR-Fc treatment may cause an increased transmigration. Therefore, we tested the influence of GITR-Fc on the ability of leukocytes to migrate through an EC monolayer in a transmigration assay. PMA-treated HL-60 cells were loaded on a TNF-α–activated EC monolayer grown into a Transwell insert and were allowed to migrate through the monolayer. The transmigration of differentiated HL-60 cells was 3-fold higher than the control-Fc pretreated EC when the EC monolayer was pretreated with GITR-Fc (Fig. 4D), suggesting that GITRL triggering in EC favors extravasation. This increased extravasation is likely favored by the increased adhesion promoted by GITRL triggering.
GITRL Triggering Upregulates Adhesion Molecules in HUV-ST Cells and Increases Phosphorylation of STAT1 Proteins.
To investigate how GITRL triggering in EC favors cell adhesion, we treated activated HUV-ST cells with GITR-Fc and evaluated the expression of adhesion molecules known to be upregulated in EC upon TNF-α activation and to be involved in cell-to-cell interactions. Flow cytometry studies showed that ICAM-1 and VCAM-1 expression on the cell surface was upregulated by GITR-Fc treatment (Fig. 5A): the mean fluorescence intensity of ICAM-1 and VCAM-1 staining increased by 34% (P < 0.01) and 296% (P < 0.001), respectively. Conversely, GITR-Fc did not increase VCAM-1 and ICAM-1 expression when HUV-ST cells were not activated (data not shown).
The GITR-Fc–dependent upregulation of adhesion molecules in activated HUV-ST cells has a functional meaning, because blocking Abs against the adhesion molecules inhibited the effect of GITR-Fc treatment (Fig. 5B). This inhibition was not due to the downregulation of GITRL expression as a consequence of blocking Abs treatment (data not shown).
In an attempt to investigate how GITRL triggering upregulates ICAM-1 and VCAM-1, we evaluated the phosphorylation level of STAT family members known to be involved in the modulation of adhesion molecules (Duff et al., 1997; Naik et al., 1997). Western blotting analysis demonstrated that GITRL triggering increased phosphorylation of STAT1 (Fig. 6A). Actually, the densitometric analysis of the immunoreactive bands showed that the phosphorylation level of both STAT1α and STAT1β increased within 5 minutes after GITR-Fc treatment (Fig. 6B).
It is well known that the GITRL-GITR system plays a proinflammatory role in murine disease models and that GITR triggering in cells of the adaptive and innate immune system has an activating function (Krausz et al., 2007; Nocentini and Riccardi, 2009; Azuma, 2010; Ronchetti et al., 2011). Our study demonstrates for the first time that GITRL triggering in EC promotes adhesion in both mice and humans, suggesting that the proinflammatory activity of GITRL/GITR system is due, at least in part, to increased expression of adhesion molecules in EC that may then favor extravasation during the inflammatory response. We also demonstrate that increased adhesion is accompanied by increased STAT-1 phosphorylation and the consequent augmented expression of adhesion molecules such as VCAM-1 and ICAM-1.
Previous studies indicate that extravasation is promoted by activation of immune cells, and some data suggest that GITR-stimulated cells have the same behavior. A recent study performed on mice demonstrated that GITR triggering favors migration of cutaneous DC by increasing inflammatory cytokine production (Kamimura et al., 2009), and another study showed that GITR triggering in T cells upregulates expression of adhesion molecules (Mahesh et al., 2006). Our data, while demonstrating a role for GITRL signaling in EC-activation, do not exclude the possibility that GITR triggering in peripheral blood leukocytes plays a role in increasing their adhesion to EC. Indeed, the inhibition of adhesion caused by antagonist anti-GITR Abs and shown in Fig. 4C may be due not only to inhibition of GITRL triggering on EC but also to inhibition of GITR triggering on leukocytes. In this respect, data from our and other laboratories suggest that the anti-human GITR mAbs currently available are antagonistic (Bianchini et al., 2011; Buechele et al., 2012; Nocentini et al., 2012). If the antagonistic properties of anti-human GITR Abs are confirmed by in vivo studies, such Abs will deserve to be tested as anti-inflammatory drugs.
The evaluation of anti-GITR Abs as potential anti-inflammatory drugs is hampered by the structural complexity of the ligands belonging to the TNF superfamily and of their cognate receptors. In fact, both ligands and receptors activate intracellular signaling. Moreover, the differences between human and mouse GITRL-GITR systems clearly suggest that the effects of anti-GITR Abs in mouse experiments cannot be easily extrapolated to predict the activity in human models. For example, GITR-Fc fusion protein has been demonstrated to have various effects subjected to the experimental settings. In some murine in vivo models, GITR-Fc shows an anti-inflammatory effect, and its main activity appears to be the inhibition of GITR triggering (Cuzzocrea et al., 2004, 2006; Galuppo et al., 2011a,b). In in vitro models, particularly in human cells, GITR-Fc and soluble GITR exert proinflammatory activity, and their main activity appears to be GITRL triggering (Kim et al., 2006). Moreover, while anti-human GITR mAbs are antagonistic, all anti-murine GITR Abs available possess agonistic properties (Nishioka et al., 2008; Bianchini et al., 2011; Buechele et al., 2012; Nocentini et al., 2012). Such differences may depend on 1) the different primary sequence of the extracellular domain of GITR giving rise also to different alternatively spliced products (Nocentini et al., 2000; Krausz et al., 2007); 2) the tridimensional structures of GITR and GITRL in mice and humans that may cause different triggering/inhibition potentials of GITR-Fc and of Abs or even different functions in mouse and humans (Chattopadhyay et al., 2009; Nocentini et al., 2012); 3) their partially different tissue distribution (Nocentini et al., 2012). Therefore, future studies on the potential use of anti-GITR Abs should consider these differences.
Our study demonstrates that GITRL triggering enhances STAT1 phosphorylation and suggests that this increment could be responsible, at least in part, for VCAM-1 and ICAM-1 upregulation. Indeed, induction of adhesion molecule expression and cell adhesion through a STAT1-dependent pathway has been previously described in response to interferon-γ (Naik et al., 1997; Lee et al., 2011). Moreover, the STAT1 pathway can upregulate VCAM-1 and ICAM-1 independently of interferon-γ (Duff et al., 1997; Stojanovic et al., 2009). There is evidence that STAT1 synergizes with NF-κΒ in inducing VCAM-1 and ICAM-1 (Jahnke and Johnson, 1994; Ohmori et al., 1997; Nizamutdinova et al., 2009; Lee et al., 2011) and that GITRL triggering activates the noncanonical pathway leading to NF-κΒ activation (Grohmann et al., 2007). In our experimental model, it is likely that NF-κΒ is already activated due to treatment with TNF-α (Mackay et al., 1993) and GITRL-dependent-NF-κΒ–activation might synergize with TNF-α in potentiating the upregulation of adhesion molecules.
In conclusion, we have demonstrated that the GITRL-GITR system promotes leukocyte adhesion to EC. This effect is consequent to intracellular signaling activated by GITRL triggering in EC. Future studies will evaluate the in vivo relevance of such mechanisms and the possible anti-inflammatory efficacy of therapeutic use of GITRL and GITR signaling inhibitors.
The authors thank Dr. J. Yélamos who generously provided us with the murine EC line HYKO6P.
Participated in research design: Lacal, Riccardi, Graziani, Nocentini.
Conducted experiments: Lacal, Petrillo, Ruffini, Muzi, Bianchini, Ronchetti.
Performed data analysis: Lacal, Petrillo, Bianchini, Migliorati, Nocentini.
Wrote or contributed to the writing of the manuscript: Lacal, Migliorati, Riccardi, Graziani, Nocentini.
- Received June 29, 2013.
- Accepted July 24, 2013.
↵1 Current affiliation: Clinical Research Center Salzburg GmbH (CRCS), Landeskrankenhaus Salzburg (SALK) and Universitätsklinikum der Paracelsus Medizinischen Privatuniversität (PMU), Salzburg, Austria.
G.G. and G.N. contributed equally to this work (joint senior investigators).
This work was supported by a research grant from the Italian Association for Cancer Research (AIRC) in Milan, Italy, and by the Italian Ministry of Health.
- bovine serum albumin
- calcein acetoxymethylester
- dendritic cells
- endothelial basal medium-2
- endothelial cells
- endothelial growth factor medium
- fetal bovine serum
- glucocorticoid-induced TNF receptor family-related
- GITR knockout
- GITR-Fc fusion protein
- GITR ligand
- intercellular adhesion molecule-1
- nuclear factor κ-light-chain-enhancer of activated B cells
- polymerase chain reaction (PCR)
- phorbol 12-myristate 13-acetate
- signal transducers and activators of transcription
- tumor necrosis factor
- vascular cell adhesion molecule-1
- Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics