Inhibitors of mitogen-activated protein kinases differentially regulate costimulated T cell cytokine production and mouse airway eosinophilia
© Chialda et al. 2005
Received: 28 July 2004
Accepted: 15 April 2005
Published: 15 April 2005
T cells play a dominant role in the pathogenesis of asthma. Costimulation of T cells is necessary to fully activate them. An inducible costimulator (ICOS) of T cells is predominantly expressed on Th2 cells. Therefore, interference of signaling pathways precipitated by ICOS may present new therapeutic options for Th2 dominated diseases such as asthma. However, these signaling pathways are poorly characterized in vitro and in vivo.
Human primary CD4+ T cells from blood were activated by beads with defined combinations of surface receptor stimulating antibodies and costimulatory receptor ligands. Real-time RT-PCR was used for measuring the production of cytokines from activated T cells. Activation of mitogen activated protein kinase (MAPK) signaling pathways leading to cytokine synthesis were investigated by western blot analysis and by specific inhibitors. The effect of inhibitors in vivo was tested in a murine asthma model of late phase eosinophilia. Lung inflammation was assessed by differential cell count of the bronchoalveolar lavage, determination of serum IgE and lung histology.
We showed in vitro that ICOS and CD28 are stimulatory members of an expanding family of co-receptors, whereas PD1 ligands failed to co-stimulate T cells. ICOS and CD28 activated different MAPK signaling cascades necessary for cytokine activation. By means of specific inhibitors we showed that p38 and ERK act downstream of CD28 and that ERK and JNK act downstream of ICOS leading to the induction of various T cell derived cytokines. Using a murine asthma model of late phase eosinophilia, we demonstrated that the ERK inhibitor U0126 and the JNK inhibitor SP600125 inhibited lung inflammation in vivo. This inhibition correlated with the inhibition of Th2 cytokines in the BAL fluid. Despite acting on different signaling cascades, we could not detect synergistic action of any combination of MAPK inhibitors. In contrast, we found that the p38 inhibitor SB203580 antagonizes the action of the ERK inhibitor U0126 in vitro and in vivo.
These results demonstrate that the MAPKs ERK and JNK may be suitable targets for anti-inflammatory therapy of asthma, whereas inhibition of p38 seems to be an unlikely target.
Asthma is a chronic inflammatory disease of the airways. The inflammation characterizing asthma is complex and involves multiple cells and mediators. The cells involved include well-recognized immune and inflammatory cells, lymphocytes, macrophages, eosinophils, mast cells and neutrophils as well as resident lung cells [1, 2]. The importance of allergen-specific CD4+ Th2 cells has been demonstrated. Th2-associated cytokines such as IL-4, IL-5, IL-9 and IL-13 are known to be involved in IgE production, airway eosinophilia, and airway hyperresponsiveness . Consequently, the inhibition or modulation of allergen-specific Th2 cells and their cytokines has become an attractive target for novel therapeutic intervention strategies [4–6]. Downregulation of cytokine production has been achieved by glucocorticoids and immunosuppressants both in vitro and in vivo. As these agents suppressed a broad spectrum of immune function, more specific regulatory pathways of T cell need to be addressed.
In recent years, considerable effort has been mounted to dissect the signaling events in T cells. Full activation of T cells requires signaling through both the TCR/CD3 complex and the CD28 costimulatory receptor [7–10]. CD28 engagement by B7-1 and B7-2 on resting T cells provides a primary costimulatory signal critical for initial cell cycle progression, interleukin 2 production and clonal expansion . Engagement of CTLA-4 by the same B7-1 or B7-2 ligands results in attenuation of T cell responses. Recently, molecular homologues of CD28 and CTLA-4 coreceptors and their B7-like ligands have been identified. These homologues presumably play an essential role in the acquisition of effector function and/or tolerance induction. One of the CD28-like molecules is induced during activation of T cells, thus it is referred as an inducible costimulator (ICOS) and has a unique B7-like ligand (B7-H2, B7 h or B7RP-1). PD-1 is an inhibitory CD28-like receptor, with two B7-like ligands (PD-L1 and PD-L2) [12, 13]. It has been shown that the ICOS/B7-H2 pathway controls T cell dependent immune responses [14–17].
Several protein kinases such as the family of MAPKs are involved in the transmission of extracellular signals into the nucleus. . MAPKs are serine/threonine kinases that include extracellular signal-regulated kinases (ERKs) , Jun NH2-terminal kinases (JNKs)  and p38 MAPK . CD3 signaling alone has been shown to activate ERK, and the combination of CD3 and CD28 signaling can synergistically activate JNK in T cell lines and clones [22, 23]. In these studies, CD28 signaling alone was shown to minimally activate ERK and JNK, while others have reported much more significant ERK activation . Activation of p38 has also been observed in Jurkat cells and in mouse T cells treated with CD3/CD28 [25, 26]. While it has become clear that productive T cell activation requires the cooperation of multiple signaling pathways, the composition and the relative contribution of these pathways to the activation process and cytokine production remain yet to be fully deciphered.
The predominant expression of ICOS on Th2 cells and its effect on the development of Th2 cells has been reported [15, 27]. Therefore, interference of signaling pathways precipitated by ICOS may present new therapeutic options for Th2 dominated diseases such as asthma. However, these signaling pathways are poorly characterized. Recently, we reported that the ERK pathway plays a pivotal role in regulating IL-13 production in human CD4+ T cells after CD28 costimulation . In the present study, we studied the activation of different MAP kinases in human CD4+ T cells stimulated by α-CD3/α-ICOS, their function in the production of cytokines from these cells and compared these results to α-CD3/α-CD28 stimulated T cells. Our results demonstrate, that in human CD4+ T cells, p42/p44 ERK, p38 and JNK differentially regulate the production of IL-2, IL-4, IL-13 and IFN-γ in response to α-CD3/α-CD28 or α-CD3/α-ICOS. Furthermore, inhibitors able to inhibit ICOS induced signaling proved to be effective in the murine asthma model of late phase eosinophilia.
Oligonucleotides were synthesized by TIB Molbiol (Berlin, Germany). DMSO, TPA, ionomycin, dexamethasone, Histopaque-1077 was from Sigma Chemical Co (Deisenhofen, Germany). U0126, SP600125 and SB203580 were purchased from Biotrend Chemikalien GmbH (Köln, Germany). Purified anti-human CD3 and purified anti-human CD28 were from PharMingen Becton Dickinson Co (Heidelberg, Germany). RPMI 1640 medium was from Life Technologies (Heidelberg, Germany). Unless otherwise indicated, all other chemicals were purchased from the Sigma Chemical Co (Deisenhofen, Germany).
Preparation of CD4+ T cells
Buffy coats from healthy human volunteers were obtained from the Erlangen Blood Bank. PBMCs were isolated by density gradient centrifugation over Histopaque 1077 (Sigma, Deisenhofen, Germany), washed twice in Hanks buffer (Life Technologies, Heidelberg, Germany) and resuspended in RPMI 1640 medium (Roche Diagnostics, Penzberg, Germany). PBMCs were incubated with a hapten-antibody cocktail (containing monoclonal hapten-conjugated CD8, CD11b, CD16, CD19, CD36 and CD56 antibodies, Miltenyi Biotec, Bergisch Gladbach, Germany) and MACS anti-hapten microbeads (CD4+ T cell isolation kit, Miltenyi Biotec, Bergisch Gladbach, Germany). CD4+ T cells were isolated by negative selection on LS+ columns using a high gradient magnetic cell separation system MACS (Miltenyi Biotec, Bergisch Gladbach, Germany) according to the manufacturer's instructions. Purity of CD4+ T cells was assessed by flow cytometry and was 90–95% (FACScan, Becton Dickinson, Heidelberg, Germany). Additional cells were monocytes (4–8%) and B cells (1–2%), which did not respond to α-CD3 antibodies. Purified CD4+ T cells were resuspended in RPMI 1640 medium.
For cytokine production, CD4+ T cells were resuspended at 0.25 × 106 cells/ml and incubated in 500 μl aliquots in 24-well tissue culture plates (Falcon Becton Dickinson Labware, Heidelberg, Germany) at 37°C, 5%CO2. After preincubation with test substances for 30 min, cells were stimulated with α-CD3/α-CD28 Dynal beads (8*104/2 μl/well). B7-H1, B7-H2 and B7-H3 binding by their receptors was assessed by stimulation of the cultured cells with self coated Dynabeads M-450 Epoxy (Dynal Biotech GmbH, Hamburg, Germany). α-CD3 mAb (0.1 μg/μl) (Pharmingen, Heidelberg, Germany) together with 0.04 μg/μl recombinant fusion proteins of B7-H1, B7-H2 and B7-H3 respectively, (R&D, Wiesbaden-Nordenstadt, Germany), were added to the prewashed beads according to the manufacturer's recommendations. The beads were added at a ratio of 3:1 to the CD4+ T cells. At the indicated times (presented in figure legends), cells were sedimented by centrifugation, the supernatants were harvested and kept frozen at -80°C until cytokine protein determination; the cell pellet was lysed by RLT lysis buffer (Qiagen, Hilden, Germany) and frozen at -80°C until RNA isolation.
Enzyme-linked immunosorbent assay
Cytokine measurements in culture supernatants were done by sandwich ELISA using matched antibody pairs (BD Pharmingen, Heidelberg, Germany). ELISA plates (Maxisorb, Nunc) were coated overnight with anti-cytokine mAb in 0.1 M carbonate buffer, pH 9.5. After being washed, plates were blocked with Assay Diluent (Pharmingen, Heidelberg, Germany) for 1 h and washed again. Appropriately diluted supernatant samples and standards were distributed in duplicates and the plates were incubated for 2 h at room temperature. Plates were washed, incubated for 1 h with working detector (biotinylated anti-cytokine Ab and Avidin-horseradish peroxidase conjugate). After washing, substrate (TMB and hydrogen peroxide) was added. The reaction was stopped by addition of 1 M H3PO4. Plates were read at 450 nm (reference 570 nm) in a microplate reader (Dynatech). The results were expressed as a percentage of the control level of cytokines production by cells stimulated in the presence of the vehicle of the corresponding compound.
Determination of serum levels of total and OVA-specific IgE
Serum levels of OVA-specific IgE were measured by IgE ELISA (OptEIA, BD Pharmingen). Briefly, 96-well plates (Maxisorb, Nunc) were coated with OVA (100 μg/ml). After addition of serum samples, a sheep anti-IgE Ab was added to individual wells and its binding was detected with peroxidase-conjugated anti-sheep IgG. IgE concentrations were calculated by comparison with commercial mouse IgE standards (BD Pharmingen).
Analysis of cytokine mRNA expression by real-time RT-PCR
RNA was prepared from frozen lysates using Rneasy (QIAGEN, Hilden, Germany). One-tube RT-PCR was performed using Quantitect Probe RT-PCR Kit from QIAGEN (Hilden, Germany). Expression of cytokines were determined in relation to beta-actin by real time RT-PCR using TaqMan assay on a ABI Prism 7900. Human primers and probes have been described . Murine primers and probes were Assay-On-Demand purchased from Applied Biosystems (Darmstadt, Germany). Quantity of mRNA was calculated using the ΔΔCT method (PE Applied Biosystems User Bulletin #2; ABI PRISM 7700 Sequence Detection System, 1997). For each RT-PCR the threshold cycle (CT) was determined, being defined as the cycle at which the fluorescence exceeds 10 times the standard deviation of the mean baseline emission for cycles 3 to 10. Cytokine mRNA levels were normalized to the housekeeping gene β-actin according to the following formula: ΔCT = CT β-actin - CT cytokine. Subsequently, respective cytokine mRNA levels were calculated using the ΔΔCT method, i.e., ΔCT values representing mRNA from cells treated with stimulus in combination with a test compound were set in relation to the ΔCT value representing mRNA levels from cells treated with stimulus alone according to the following formula: ΔΔCT = ΔCT(drug) - ΔCT(vehicle). The relative mRNA level for the respective test compound was calculated as 2-ΔΔC T * 100% based on the results of control experiments with an efficiency of the PCR reaction of approximately 100%.
Western Blotting analysis of the MAPKs
Human CD4+ T cells were stimulated with Dynabeads CD3/CD28 T Cell Expander (Dynal Biotech GmbH, Hamburg, Germany) or with Dynal beads coated with α-CD3/B7-H2 Fc for 0, 5, 15, 60 or 120 min. After stimulation, cells were washed with ice-cold PBS and lysed in cell lysis buffer (Tris 50 mM, pH 7.2, containing 10 mM EDTA, 150 mM NaCl, 1% NaDOC, 0.1% SDS, 1% Triton X-100) supplemented with protease inhibitors (1 mM PMSF, 5 μg/μl Aprotinin and 5 μg/μl Leupeptin). Cells were left on ice for 20 min and then centrifuged at 14.000 rpm for 10 min. The supernatants were stored at -80°C until measurement of the MAPKs. Equal amounts of protein were resolved in 10% SDS-polyacrylamid gel electrophoresis and transferred onto PVDF membrane. Membranes were blocked overnight at 4°C with 0.2 % I-block (Applied Biosystems, Darmstadt, Germany) in TBS/T buffer and probed with rabbit primary antibodies specific for the dually phosphorylated active forms of ERK, p38 and JNK (Promega GmbH, Mannheim, Germany) for 2 h at room temperature. Blots were washed, incubated 1 h at room temperature with second antibody (donkey anti-rabbit horseradish peroxidase conjugated) (Promega GmbH, Mannheim, Germany). Immunoreactive bands were visualized by enhanced chemiluminiscence (ECL) according to the manufacturer's recommendations (Amersham Pharmacia Biotech, Freiburg, Germany). After detection of the active form of the MAPK, membranes were stripped and reprobed with the corresponding antibody that recognize both active and inactive forms of p38, JNK (Santa Cruz Biotechnology, Inc., Heidelberg, Germany) and ERK (Promega GmbH, Mannheim, Germany).
ELISA detection of activated JNK
The FACE (Fast Activated Cell-based ELISA) kit (Active Motif, Rixensart, Belgium) was used according to the provider's protocol. CD4+ T cells were cultured in precoated (poly-L-lysine) (Sigma, Deisenhofen, Germany) 96-well plates at a concentration of 1 × 106 /ml and were stimulated with α-CD3/α-CD28 and α-CD3/B7-H2 Fc for different time points (1 min, 5 min, 15 min, 60 min and 120 min) to induce the JNK pathway. Following stimulation the cells were fixed in order to preserve protein modifications, including phosphorylation. Corresponding wells were then incubated with a primary antibody specific for pJNK and total JNK respectively. Subsequent incubation with secondary HRP-conjugated antibody and developing solution was followed by a colorimetric detection. The signals were then normalized for cell number using Crystal Violet.
Murine model of late phase eosinophilia
Male Balb/c mice weighing 22–25 g were used. Animals were purchased from Harlan (Borchen, Germany). The animals were kept under constant environmental conditions (temperature: 18 ± 2°C, humidity: 40–60 %, light cycle: 7 am – 7 pm). They had free access to standardized food pellets (purchased from Altromin, Lage, Germany) and tap water. All animal studies were performed in accordance with the national animal protection rules and permitted by the local governmental authority (Regierung von Mittelfranken, Germany). The sensitization strategy was adapted from previously described mouse model . Briefly, all Balb/c mice were actively immunized and boosted by intraperitoneal injections with 200 μl of a solution containing 100 μg ovalbumin (grade V, Sigma, Taufkirchen, Germany) adsorbed on 4 mg of an aqueous solution of aluminium hydroxide and magnesium hydroxide (Alum) (Perbio Science, Bonn, Germany) on days 1, 14 and 20. On day 26, the animals were used for experiments. Compounds, as suspension in 5% DMSO, were given intraperitoneally two hours prior to challenge once a day. Control animals (negative and positive) received only vehicle solution. Mice were challenged twice a day for 4 days by intranasal application of 50 μl of 2 μg/μl ovalbumin solution in NaCl to provoke an influx of inflammatory cells into the airways. Negative control mice were sensitized intraperitoneally with OVA/Alum and were challenged intranasally with saline solution. Positive control animals were sensitized as mentioned above and challenged intranasally with ovalbumin. On day 30 animals were sacrificed by putting them into a CO2 rich atmosphere and a bronchoalveolar lavage (BAL) was performed by flushing the lungs and the airways six times with 0.5 ml Hank's balanced solution (Gibco, Karlsruhe, Germany) supplemented with Na-EDTA and HEPES (Sigma, Taufkirchen, Germany). The number of eosinophils as well as the total cell number from the pooled BAL samples of one animal was counted using a haemocytometer (Sysmex microcellcounter F-300, Norderstedt, Germany). BAL cells were spun onto glass slides using a cytospin centrifuge and stained with Diff-Quik (Dade-Behring, Marburg, Germany). The percentage and number of eosinophils were determined microscopically using standard cytological (morphological and staining) criteria by counting 400 cells/slide. Each group of animals treated with compounds was compared with saline challenged (negative control) and vehicle-treated ovalbumin challenged (positive control) groups. For RNA analysis cells recovered from broncheoalveolar lavage were sedimented by centrifugation and RNA was prepared from the cell pellet.
Histological examination of lungs
Lungs removed from the chest were instilled intratracheally with 0.8 ml 4% buffered formalin solution and immersed in this fixative for 48 hours. Tissues were embedded in paraffin and sections of 5 μm were cut. Slices were stained with May-Grünwald Giemsa for examining inflammation and eosinophilic infiltration and periodic acid-Schiffs stain was done for measuring mucus production under the light microscope. To determine the severity of peribronchial inflammation a semiquantitatively score described by Myou et al.  was determined. Briefly, this score describes followingcategories: 0, no inflammation; 1, few inflammatory cells; 2, a ring of inflammatorycells 1 cell layer deep; 3, a ring of inflammatory cells 2–4 cells deep; 4, a ring of inflammatory cells of >4 cells deep. The numerical scores for the abundance of PAS-positive mucus-containingcell in each airway were adapted from the same reference as follows: 0, <0.5%PAS-positive cells; 1, 5–25%; 2, 25–50%; 3, 50–75%;4, >75%.
Data are expressed as means ± s.e.mean. Significant differences were statistically analyzed by the unpaired Student's t-test and by ANOVA. IC50 and ED50 values were calculated using the computer program PRISM 3.0 (GraphPad Software Inc., San Diego, CA, U.S.A).
Influence of coreceptors on TCR induced cytokine expression in CD4+ T cells
Time course of cytokine induction in CD4+ T cells
Activation of MAPKs by costimulatory receptors
Effect of MAPK inhibitors on costimulated CD4+ T cells
IC50 values for the inhibition of cytokine mRNA synthesis by different MAPK inhibitors in CD28 or ICOS costimulated human CD4+ T cells. Human CD4+ T cells were preincubated with increasing concentrations of SP600125, U0126 and SB203580. The cells were then stimulated with beads coated with α-CD3/B7-2 fusion protein or α-CD3/B7-H2 fusion protein for 8 hours. Cytokine mRNA level were determined in triplicates using real time RT-PCR and were normalized to β-actin. Stimulated cells treated with DMSO were used as 100% control. IC50values were calculated for the different cytokines. Results represent mean ± s.e.m. for three different donors.
1.17 ± 0.97
0.18 ± 0.14
0.72 ± 0.39
6.99 ± 4.57
3.02 ± 1.14
4.15 ± 1.38
0.11 ± 0.03
1.29 ± 0.96
0.31 ± 0.09
0.14 ± 0.08
6.93 ± 2.47
1.25 ± 0.18
1.33 ± 0.36
0.79 ± 0.35
Effects of MAPK inhibitors in vivo
Effects of MAPK inhibitors on serum OVA-specific IgE levels
Effects of MAPK inhibitors on lung inflammation in vivo
Effects of combined administration of MAPK inhibitors
IC50 values for the inhibition of cytokine mRNA synthesis by combined administration of MAPK inhibitors in vitro. IC50values were calculated from data shown in Figure 8.
Costimulation is essential for the full activation of CD4+ T cells. In this study, we analyzed the induction and regulation of Th1 and Th2 cytokines in CD4+ T cells upon engagement of different costimulatory receptors. Beads with defined combinations of surface receptor stimulating antibodies and costimulatory receptor ligands were applied to induce cytokine synthesis. Ligation of two costimulatory receptors, CD28 and ICOS, augmented TCR activation and this in part is due to MAPK activation. The effects of pharmacological inhibitors of three different MAPKs on cytokines induced by α-CD3/B7-2 or α-CD3/B7-2 were investigated. Using these compounds, we were able to elucidate different MAPK signaling pathways leading to cytokines synthesis in dependence on the costimulatory receptor engaged. We tested these compounds in an animal model of asthma in which the ERK inhibitor U0126 and the JNK inhibitor SP600125 were able to reduce the influx of eosinophils into the lungs of sensitized and challenged mice. We found no synergistic effect of any combination of these MAPK inhibitors. Unexpectedly, SB203580 antagonized the in vitro and in vivo action of U0126.
For optimal activation, CD4+ T cells require specific antigen recognition by the TCR and additional signals (collectively called costimulatory signals), delivered by the same antigen-presenting cell . In the absence of costimulation, lymphocytes fail to respond effectively and are rendered anergic . CD28 is the best-characterized costimulatory receptor and constitutively present on the surface of T cells (reviewed in ). CD28 is activated by B7-1 and B7-2 counter-receptors on antigen-presenting cells. Its signaling contributes to the overall strength of T cell activation . However, new B7- and CD28-like molecules have recently been discovered and new pathways have been delineated that seem to be important for regulating the responses of previously activated T cells . Some B7 homologues have unknown receptors, indicating that other immunoregulatory pathways remain to be described. We established an in vitro test system to analyze to role of each coreceptor. To this end, we conjugated beads with defined combinations of α-CD3 and B7 counterreceptors. In line with previous reports described above we found that CD28 ligation augments the synthesis of all T cell derived cytokines. Similar results were obtained by ligating ICOS via its counter receptor ICOS-L (B7-H2). The effects of ICOS signals on T cell proliferation and IL-2 production were reported to be modest in comparison with those of CD28 . However, ICOS costimulation is equivalent to that mediated by CD28 costimulation for the production of effector cytokines like IFNγ, IL-4, IL-10 and IL-13. IL-5 was not detected. It has been shown, that efficient in vitro IL-5 production needs elevated cAMP level in addition to TCR stimulation . There was no difference in the strength of induction of Th2 compared with Th1 cytokines for CD28 and ICOS costimulation. Conflicting data has been reported about the function of PD-1. The engagement of PD-1 by its specific ligands, B7-H1 (PD-L1) or B7-DC (PD-L2) was reported to inhibit T and B cell proliferation and cytokine production [39, 40]. In our test system engagement of PD-1 by ligation with B7-H1 neither augmented nor inhibited cytokine synthesis.
Activation of MAPK family members by CD28 and ICOS may contribute to their costimulatory activity. To analyze whether different signaling cascades are precipitated by the different coreceptors, their ability to activate different MAPK pathways was determined. Since only CD28 and ICOS activation augmented cytokine synthesis we focused our analyses on these receptors. We analyzed the signaling pathways of ERK1/2, JNK, and p38 MAP kinase, during the primary activation of human T-cells, costimulated with ICOS or CD28. ERK1/2 and p38 kinase were markedly activated by both CD28- and ICOS-mediated costimulation. In our cellular assay system the activation of ERK has a role in T-cell activation via CD28 and ICOS. The activation of ERK by CD28 is mediated by binding of SOS to the YMNM motif of the intracellular domain of CD28 . The YMNM motif is not conserved in ICOS, the sequence being YMFM, and amino-acid substitutions in this motif results in a failure to associate with Grb2 [16, 42]. Therefore it is currently unclear, how ICOS is able to activate ERK.
Previous studies of p38 MAP kinase in human purified T cells  and in the CD4+ subset  clearly demonstrated the involvement of p38 MAP kinase in the cell activation through TCR and CD28 costimulation signal pathways. However, little is known about this MAP kinase in ICOS costimulated T cells. In our cellular assay activation of p38 MAPK by ICOS was detected. This is in line with recent reports, in which ICOS ligation synergized with TCR signals for activation of the ERK and p38 MAP kinases [44, 45].
While enhanced activityof JNK is an absolute necessity for regulation of IL-2 geneexpression in T cell lines , a defect in JNK signalingwas claimed to be involved in T cell differentiation, but notin T cell activation in vivo, i.e. in JNK-deficient animals . This corresponds to our results with human T cells, where JNK was found not to be activated after CD28 costimulation. Conflicting reports appeared about the ability of ICOS to activate the JNK pathway. Parry et al. reported that only CD28 but not ICOS costimulation activated c-jun N-terminal kinase . Arimura et al. reported that the cross-linking of ICOS induced much less phosphorylation of JNK than did the cross-linking of CD28 . In our cellular assay system we found that ICOS activated the JNK pathway. However, this was only detected by using a sensitive ELISA based assay indicating a very low expression of JNK in primary human T cells. Alltogether, whereas p38 and ERK are consistently found to be activated after CD28 and ICOS costimulation, the activation of JNK appeared only after ICOS costimulation.
In order to elucidate which of the activated MAP kinases are important for cytokine induction after T cell activation and costimulation we employed specific inhibitors of the different MAPKs in our cellular assay system. These compounds were chosen based on their pharmacological profile (high potency and specificity) for these three kinases [31–33]. The p38 inhibitor SB203580 and the ERK inhibitor U0126 inhibited IL-2, IL-13 and IFNγ in CD28 costimulated T-cells. IL-4 induction, in contrast, was not affected by these inhibitors. In line with our observations, that JNK does not become activated by CD28 costimulation, the JNK inhibitor did not affect cytokine induction in CD28 costimulated T cells. Only one report analyzed the effect of a p38 inhibitor in primary human CD4+ T cells. There SB203580 blocked human CD4 T cell production of IL-4, IL-5, TNF-, and IFN-, but not IL-2, in response to CD3 and CD28 stimulation . This difference is most likely due to different experimental protocols and the use of a different T cell population. While U0126 was never tested to inhibit human T cell cytokine production, in the first description of the JNK inhibitor SP600125, its effect on polarized Th1 and Th2 cells was described. There Th1 cytokines and some Th2 cytokines but not IL-4 was inhibited by this compound . It is tempting to speculate, that the polarization of the T cells rendered them susceptible to JNK inhibition. Different observations were made in ICOS costimulated T cells. For human T cells, it has only been reported, that U0126 blocked the proliferation of T-cells and the transcription of IL-2 following costimulation by ICOS in human T cell . We found, that the p38 inhibitor was not effective, but the JNK inhibitor inhibited IL-2, IL-13 and IFNγ. The ERK inhibitor U0126 inhibited all cytokines with similar efficiency as in CD28 costimulated T cells. Together these data indicates that in human T cells ICOS precipitates MAPK signaling cascades differentially compared to murine T cells. More interestingly, our inhibitor studies revealed that not every MAPK signaling cascade activated by costimulation is necessary for cytokine synthesis.
The sensitivity of CD4+ T cells to various MAPK inhibitors prompted us to analyze these compounds in an animal model of asthma. The p38 inhibitor SB203580 did not affect airway inflammation in our model. Conflicting data about the action of p38 inhibitors in asthma models have been reported in the literature. Similar to our results, one study found no effect of SB203580 in rats . In contrast, Underwood et al. demonstrated anti-allergic activity using a novel second-generation p38 MAPK inhibitor . Either increased specificity of this new inhibitor or better bioavailability of this compound may explain its effect in vivo. In contrast to the p38 inhibitor, the ERK as well as the JNK inhibitor dose dependently decreased allergen induced airway eosinophilia in our model. This anti-inflammatory effect of the ERK and the JNK inhibitor was also detected by measuring peribronchial inflammation and mucus production. Elevated serum IgE is a hallmark of a Th2 immune response. Our data showed that serum levels of OVA-specific IgE were also substantially reduced by U0126 and SP600125, whereas no significant inhibition was observed by SB203580. This extends our previous findings, where 10 mg/kg of U0126 were found to be effective in a rat asthma model . During the preparation of the manuscript it was reported that 30 mg/kg SP600125 inhibited airway eosinophilia in rats . This supports our finding, that targeting JNK reduces allergic airway inflammation.
The inhibitory effect could also be observed by the reduction of cytokine mRNA level. However, the inhibition was not selective for Th2 cytokines. The induction of IFNγ in BAL cells was also inhibited by U0126 and SP600125. This is in line with our findings described above, that costimulation of T cells induces Th1 as well as Th2 cytokines and that MAPK inhibitors are also effective in inhibiting IFNγ. This may be beneficial for the anti-inflammatory effect in the asthma model. It has been shown, that in IFNγ knockout mice less pronounced allergic symptoms such as antigen-specificIgE, eosinophilic infiltration, and airway hyperresponsiveness occur . Therefore, it seems likely that endogenous IFNγ is necessaryfor optimal IgE production also during a secondary response.
The in vivo profile of MAPK inhibitors reflects the in vitro profile of ICOS costimulated T cells: ERK and JNK inhibitors are effective whereas the p38 inhibitor is not. In contrast, the inhibitory profile of CD28 costimulated T cells is different. There p38 is effective, while the JNK inhibitor is not. In line with this, it has been shown, that ICOS plays an important role in the Th2 effector responses in the lungs, in that inhibition of ICOS suppresses lung inflammation and Th2 cytokine production . Similarly, ICOS-L-Fc augments lung inflammation and the production of cytokines . Together these data indicate the importance of ICOS costimulation in the allergic lung response. Therefore, targeting signaling pathways precipitated by ICOS should dampen the allergic lung inflammation.
Since the three MAPKs are part of different signaling cascades with different upstream and downstream mediators, we asked whether the combination of two MAPK inhibitors may have synergistic effects on the induction of cytokines in CD4+T cells. Our in vitro studies did not reveal any such synergistic effect regardless of the combination used. However, we could observe that the simultaneous administration of increasing concentrations of U0126 and 1 μM SB203580 to α-CD3/α-CD28 and α-CD3/α-ICOS stimulated CD4+ T cells led to a right-shift of the dose-response curve for U0126 compared to the administration of U0126 alone. We analyzed this combination also in vivo. Whereas treatment of the animals with U0126 alone inhibited late phase eosinophilia markedly as in previous experiments the concomitant treatment with SB203580 diminished the effect of U0126 almost completely. To our knowledge no in vitro or in vivo studies reported the simultaneous addition of two MAPK inhibitors in T cell activation studies or asthma models. Currently, we investigate the mechanism for this antagonistic action.
Different MAPK signaling pathways are activated in T cells dependent on the costimulatory receptor engaged. The MAP kinase ERK is important for both stimuli. p38 MAPK activation is important for CD28 induced cytokine synthesis, whereas JNK is important for ICOS induced cytokine synthesis. The different regulation can be exploited to find new specifically targeted drugs aimed for diseases where different costimulatory molecules play an important pathophysiological role. We could demonstrate for the first time that inhibition of the JNK cascade is a therapeutic option for asthma. The specific JNK inhibitor SP 600125 reduced the influx of eosinophils in an animal model of asthma. The development of more specific MEK-ERK and JNK-targeted drugs would support this approach to treat T cell dominated diseases such as asthma.
List of abbreviations
bronchoalveolar lavage fluid
extracellular signal-regulated kinase
jun NH2-terminal kinase
mitogen-activated protein kinase
T cell receptor
The authors thank Evi Hoier and Brigitte Neupert for her expert technical assistance and Alexandra Fischer for animal care. This work was supported by the Deutsche Forschungsgemeinschaft (Grant Pa 463/1-1, SFB 643) and a DAAD scholarship to Ligia Chialda.
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