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Laboratory of Immunology, National Eye Institute, National Institutes of Health, Bethesda, MD 20892
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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is compromised in STAT1-deficient primary Th cells. Surprisingly, IL-4 is a potent inducer of STAT1 activation in Th2 but not Th1 cells, and SOCS1 or SOCS3 expression is dramatically reduced in STAT1–/– Th2 cells. To our knowledge, this is the first report of IL-4-induced STAT1 activation in Th cells, and suggests that its induction of SOCS, may in part, regulate IL-4 functions in Th2 cells. In fact, overexpression of SOCS1 in Th2 cells represses STAT6 activation and profoundly inhibits IL-4-induced proliferation, while depletion of SOCS1 by an anti-sense SOCS1 cDNA construct enhances cell proliferation and induces constitutive activation of STAT6 in Th2 cells. These results are consistent with a model where IL-4 has dual effects on differentiating T cells: it simulates proliferation/differentiation through STAT6 and autoregulates its effects on Th2 growth and effector functions via STAT1-dependent up-regulation of SOCS proteins. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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, IL-4, and IL-12, have all been shown to induce SOCS expression in Th cells, and results of recent studies suggest that IFN-/STAT1, IL-4/STAT6, and IL-12/STAT4 signaling pathways in differentiating Th cells may be under feedback regulation by SOCS (8, 9, 10). The SOCS1 promoter contains functional STAT1 and STAT3 sites, and SOCS1 induction or promoter activity is drastically reduced in STAT1 null cells and by mutation of STAT1 site, respectively (11, 12, 13). Although these studies have established the molecular basis for feedback regulation of IFN- signaling in Th cells by SOCS1 (13), functional STAT6 binding sites have not been identified in promoters of SOCS genes, and this has led to significant interest in understanding mechanisms that underlie induction of SOCS1 by IL-4. In addition, much less is known about other members of the SOCS family and how these SOCS genes are regulated in Th cells.
In this study, we have used a hen egg lysozyme (HEL) TCR transgenic (Tg) mouse model to study mechanisms that regulate expression of SOCS genes at very early stages of differentiation of naive Th cells to effector Th1 or Th2 cells. We show that naive Th cells constitutively express SOCS1, SOCS2, SOCS3, and CIS (cytokine-induced SH2 protein) genes, and that these genes are differentially regulated by cytokines and Ags. We further show that the STAT1 signaling pathway is commonly activated by each of the three Th-polarizing cytokines (IL-4, IL-12, and IFN-) in early differentiating Th cells, and that optimum induction of SOCS expression by these cytokines requires STAT1. In addition, IL-4 is found to be a potent inducer of SOCS1, SOCS3, and STAT1 activation in Th2 cells, and we provide evidence that IL-4-induced proliferation or STAT6 is under feedback regulation by SOCS proteins.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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CD4+ Th lymphocytes were isolated and purified from spleens and lymph nodes of HEL-specific TCR Tg mice designated "3A9" (a generous gift from Dr. Mark Davis, Stanford University, Stanford, CA), C57BL/6 (wild-type (WT)) or C57BL/6-STAT1–/– mice as described (14, 15, 16). Purified (>98%) CD4+ 3A9 Th cells were cultured at 2.5 x 105/ml in RPMI 1640 medium supplemented with 50 µM 2-ME, antibiotics, and 10% FBS ("complete medium") with 10x irradiated syngeneic splenocytes (as APC) in the presence of 1 µg/ml HEL (Sigma-Aldrich, St. Louis, MO). Purified Th cells from WT or STAT1–/– C57BL/6 mice (14) (2.5 x 105/ml; kindly provided by Dr. D. E. Levy, New York University, New York, NY) were activated in plate-bound anti-CD3 Ab (10 µg/ml; BD Biosciences, San Jose, CA) and anti-CD28 Ab (5 µg/ml) in complete medium. Stimulation of the cells in the absence of exogenous polarizing cytokines and their Abs is referred to as Tho condition. For propagation under neutral condition (Thn), the Tho condition was supplemented with anti-IL-4 Ab (10 µg/ml), anti-IFN- Ab (10 µg/ml), and 10 µg/ml anti-IL-12 Ab (BD Pharmingen, San Diego, CA). For propagation under Th1 condition, the Tho condition was supplemented with anti-IL-4 Ab (10 µg/ml) and IL-12 (10 ng/ml) (PeproTech, Rocky Hill, NJ), and for propagation under Th2 condition the Tho condition was supplemented with IL-4 (10 ng/ml), anti-IFN- Ab (10 µg/ml), and anti-IL-12 Ab (10 µg/ml). FACS analysis using anti-CD4, L-selectin (CD62L), CD25 mAbs, and corresponding isotype control Abs (BD Pharmingen) was performed on the BD FACSCalibur (BD Biosciences) as previously described (16). For signal transduction studies, cells were washed twice in RPMI 1640 medium, and cultured for 2 h under starvation condition (0.5% BSA, RPMI 1640 medium) before stimulation.
Stimulation of D10.G4.1 Th2 and AE7 Th1 cell lines
D10.G4.1 Th2 cell line (American Type Culture Collection (ATCC), Manassas, VA) was cultured in RPMI 1640 medium supplemented with 2 mM L-glutamine, 10 mM HEPES, 1 mM sodium pyruvate, 0.05 mM 2-ME, 10 pg/ml IL-1
(R&D Systems, Minneapolis, MN), and 10% rat T-STIM factor with Con A (BD Biosciences) as recommended. Lymphocyte proliferation assay was performed as detailed elsewhere (15). The AE7 Th1 cell line (17) (kindly provided by J. Ragheb, National Institutes of Health (NIH), Bethesda, MD) was cultured by repeated stimulation with syngeneic irradiated splenocytes and Ag (100 µg/ml pigeon cytochrome c (Sigma-Aldrich) in ER10 medium supplemented with penicillin/streptomycin and L-glutamine, 10% FBS, 2 mM L-glutamine, 50 µM 2-ME, and mouse rIL-2 (10 U/ml).
Lymphocyte proliferation assay and cytokine measurements
Lymphocyte proliferation assay was performed as detailed elsewhere (18). Briefly, cells (2.5 x 105 cells/ml) were propagated in complete medium for 24 or 48 h. The cultures were pulsed with [3H]thymidine (0.5 µCi/10 µl per well) for 4 additional hours. The presented data are mean change in counts per minute ± SEM of responses of five replicate cultures. Supernatants were collected from cultures after 48 or 96 h stimulation with HEL and syngeneic APC and analyzed for cytokine secretion by ELISA, using kits from Endogen (Woburn, MA).
Confocal microscopy
D10.G4.1 Th2 cell line, as well as primary Th2 cells established from Ag-primed naive Th cells after two cycles of polarization, were serum-starved 2 h before treatment with cytokine. Duplicate cultures were treated with IL-4 (10 ng/ml) for 30 min and reactions were terminated by washing with PBS. Cells were fixed and processed as recommended for the Cytofix/Cytoperm Plus kit (BD Pharmingen). The cells were stained with a 1/1000 dilution of anti-STAT-1 Ab (BD BioSciences, Signal Transduction Laboratories), followed by Alexa Fluor 488 goat anti-mouse IgG (Molecular Probes, Eugene, OR). Cell nuclei were stained with 4',6'-diamidino-2-phenol-indol dihydrochloride (Molecular Probes). Images were collected on a Leica SP2 laser scanning confocal microscope (Leica Microsystems, Exton, PA) using a Leica x63 1.32NA UV-corrected planapo objective. Relative fluorescence intensity in cytoplasm or nuclei was determined and all fluorescent dyes were imaged using sequential scan mode to prevent bleed-through artifacts.
RT-PCR and quantitative real-time PCR analysis
RNA was isolated, digested with RNase-free DNase (Promega, Madison, WI) and first strand cDNA synthesis was performed with RNA (5 µg), SuperScript II Reverse Transcriptase (RT) (Invitrogen Life Technologies, Gaithersburg, MD), and oligo(dT)12–16 as previously described (19). Samples were subjected to hot-start PCR using AmpliTaq Gold DNA polymerase (Applied Biosystems, Foster City, CA) and primers used for amplification are: T-bet, 5'-TGCCTGCAGTGCTTCTAACA-3' and 5'-TGCCCCGCTTCCTCTCCAACCAA-3'; GATA-3, 5'-GAAGGCATCCAGACCCGAAAC-3' and 5'-ACCCATGGCGGTGACCATGC-3'; 
-actin, 5'-GTGGGCCGCTCTAGGCACCAA-3' and 5'-TCTT TGCCAATAGTGATGACTTGGC-3'; p21WAF1, 5'-ATGTCCAATCCTGGTGATGT CCGAC-3' and 5'-GAAATCTGTCAGGCTGGTCTGCCT-3'; and p27kip1, 5'-ATGTCAAACGTGCGGGTGTCTAACG-3' and 5'-ACCGTCTGAGACATTTTC-3'. Amplification was conducted for 30–35 cycles of denaturation for 30 s at 95°C, annealing for 45 s at 60°C, and extension at 72°C for 45 s. After the last amplification cycle, the samples were subjected to a final 10-min extension at 72°C. A negative control reaction without RT was performed for each RNA sample. For quantitative RT-PCR analysis, RNA samples were normalized to 18S rRNA using TaqMan Ribosomal RNA Control Reagents kit (Applied Biosystems). Real-time 5'-nuclease fluorogenic RT-PCR analysis was performed on an ABI 7700 (Applied Biosystems) or ICycler iQ Real-Time PCR Sequence Detection System (Bio-Rad, Hercules, CA) with the following primers: SOCS1, 5'-ACCTTCTTGGTGCGCGAC-3' and 5'-AAGCCATCTTCACGCTGAGC-3'; SOCS2, 5'-GGTTGCCGGAGGAACAGTC-3' and 5'-GAGCCTCTTTTAATTTCTCTTTGGC-3'; SOCS3, 5'-CCTTCAGCTCCAAAAGCGAG-3' and 5'-GCTCTCCTGCAGCTTGCG-3'; CIS, 5'-CCAGCCATGCAGCCCTTA-3' and 5'-CGTCTTGGCTATGCACAGCA-3'; and -actin, 5'-CAAGTCATCACTATTGGCAACGA-3' and 5'-CCCAAGAAGGAAGGCTGGA-3'. The hybridization probes are: SOCS1, 6FAM-TCGCCAACGGAACTGCTTCTTCG-TAMRA; SOCS2, 6FAM-CGCGTCTGGCGAAAGCCCTG-TAMRA; SOCS3, 6FAM-CCAGCTGGTGGTGAACGCCGT-TAMRA; CIS, 6FAM-CCCAGAGGAAGTGACAGAGGAGACCCC-TAMRA; and -actin, 6FAM-CGGTTCCGATGCCCTGAGGCTC-TAMRA. PCR parameters are as recommended for the TaqMan Universal PCR master mix kit (Applied Biosystems). SOCS copies per cell were calculated by extrapolation from standard curves generated using SOCS plasmid cDNAs as previously described (16). Standard curves generated for each SOCS cDNA showed excellent linearity and indicated precise quantitative relationship between cDNA copy number and fluorescence signal intensity within the dynamic range of the assay (data not shown).
Western blot analyses
Preparation of whole cell lysates and immunodetection were as described (19). Briefly, samples (40 µg/lane) were fractionated on 4–20% gradient SDS/PAGE, and blots were probed with STAT1, pSTAT6-specific Abs (Cell Signaling Tech. Beverly, MA), anti-phosphotyrosine (P-Tyr) IgG clone 4G10 (Upstate Biotechnology, Lake Placid, NY), STAT1, pSTAT1, SOCS1, p27kip1, or -actin Abs (Santa Cruz Biotechnology, Santa Cruz, CA). Preimmune serum was used in parallel as controls and signals were detected with HRP conjugated-secondary F(ab')2 Ab (Zymed Laboratories, San Francisco, CA) using the ECL system (Amersham Biosciences, Arlington Heights, IL).
EMSA
Nuclear extracts were prepared using buffer containing the following protease inhibitors: 2 µM leupeptin, 2 µM pepstatin, 0.1 µM aprotinin, 1 mM [4-(2-Aminoethyl)benzenesulfonyl fluoride, hydrochloride], 0.5 mM phenylmethyl-sulfonyl fluoride, and 1 µM E-64 [N-(N-L-trans-carboxyoxiran-2-carbonyl)-L-leucyl]agmatine as described (20). Protein levels were determined by the BCA method as recommended, and extracts were stored at –70°C until use. Nuclear extract (5 µg) in binding buffer (20 mM HEPES, pH 7.9, 50 mM KCl, 10% glycerol, 0.5 mM DTT, 0.1 mM EDTA) containing 0.14 µg/µl poly(dI:dC) was incubated on ice for 20 min. Labeled dsDNA probe was then added and incubated for an additional 15 min at room temperature. The probes used are: the c-fos promoter sis-inducible element (SIE) m67 SIE, 5'AGCTTGTCGACATTTCCCGTAAATCGTCGG-3 (21); activation site (GAS) motif from the FcRI promoter, 5'-AGCTTGTATTTCCCAGAAAAGGGATC-3' (22, 23); GAS motif from the human C
regulatory region, 5'-AGTCAAGACCTTTTCCCAAGAAATCTATC-3' (24, 25). The double-stranded oligonucleotides were labeled by fill-in reaction using Klenow polymerase (Invitrogen Life Technologies) with either [-32P]dATP or [-32P]dGTP (3000 Ci/mmol; Amersham Biosciences). Samples were electrophoresed in 5% polyacrylamide gel in x0.25 Tris-borate-EDTA buffer. For supershift assays, the indicated Ab was added to the binding buffer containing nuclear extract mixture and preincubated on ice for 10 min. 32P-labeled probe was then added and the entire mixture was incubated for an additional 20 min on ice before electrophoresis. Gel shift grade anti-mouse STAT1, STAT3, STAT4, STAT5a, STAT5b, or STAT6 polyclonal Abs (Santa Cruz Biotechnology) were used.
Generation of Th2 cells overexpressing sense or anti-sense SOCS1 cDNA
Sense and anti-sense mouse SOCS1 cDNA fragments flanked by BamH1 and EcoRI restriction sites were generated by PCR amplification from pEF-flag-1/SOCS1 plasmid kindly provided by Drs. D. Hilton (Walter and Eliza Hall Institute, Melbourne, Australia) and H. Young (NIH). Primer set 1 (5'-CCCGGATCCATGG TAGCACGCAACCAGGTGGCA-3' and 5'-CCCGAATTCTCAGATCAGGAAGGGGAAGG AACTCA-3') was used to generate the SOCS1 (sense) fragment, and primer set 2 (5'-CCCGA ATTCATGGTAGCACGCAACCAGGTGGCA-3' and 5'-CCCGGATCCTCAGATCAGGAAGG GGAAGGAACTCA-3') was used to generate the anti-sense SOCS1 fragment. The fragments were directionally cloned into the BamH1 and EcoRI sites of pBabe-Puro vector kindly provided by K. W. C. Peden (Laboratory of Retrovirus Research, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, MD) and the sequence was verified by dsDNA sequencing (ABI 310 Genetic Analyzer; Applied Biosystems). For transfection, plasmid cDNAs were prepared by two cycles of CsCl banding. Fifty micrograms of pBabe (empty vector control), pBabe/SOCS1, or pBabe/anti-sense SOCS1 plasmid DNA in culture medium (400 µl) containing 2 x 107/ml D10.G4.1 Th2 cells (ATCC) were electroporated at 120 V, 0.2 cm gap for 20 ms using a square wave electroporator (BTX 600; Genotronics, San Diego, Ca). Stable transfectants were established by selection in 2 µg/ml puromycin (Sigma-Aldrich).
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Results |
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Naive Th cells isolated from the spleen and lymph nodes of HEL-specific TCR Tg mice were used to examine whether transcription of SOCS genes is induced following Th cell activation or in response to stimulation by Th1/Th2-polarizing cytokines. More than 98% of the freshly isolated naive Th cells express cell surface CD4, high levels of CD62L (>95%), and low levels of CD25 (<6%), features associated with the naive cell phenotype (Fig. 1A). Following antigenic stimulation of the naive Th cells under nonpolarizing conditions (26, 27), expression of CD62L is down-regulated (Fig. 1A) while CD25 and IL-2 secretion (Fig. 1C) are up-regulated, indicating that the HEL concentration used induces Th activation and not anergy. Consistent with published reports (28), we do not detect significant expression of T-bet in naive cells (Fig. 1B), but the level of this transcription factor, a marker of the Th1 lineage, increased significantly by day 2 of Ag stimulation (Fig. 1B). Expression of GATA-3 mRNA, a transcription factor that characterizes cells of the Th2 lineage (29), is detectable in naive cells and its level increases with time in culture (Fig. 1B) as previously reported (30). We also detect significant secretion of Th1 (IFN-) and Th2 (IL-5, IL-10) cytokines in the culture supernatants, and increases in the levels of these cytokines are accompanied by a sharp decline in IL-2 secretion (Fig. 1C). Together, these results establish that the early differentiating Th cells used in our analysis contain significant numbers of both Th1 and Th2 progenitor cells. We then assessed the effects of TCR signaling on regulation of SOCS expression by real-time RT-PCR. In Fig. 1D, we show that SOCS1, SOCS2, SOCS3, and CIS mRNAs are constitutively expressed in naive Th cells, with SOCS3 being the most abundant. We further show that stimulation of naive Th cells for 48 h (day 2 cells) induces significant up-regulation of SOCS1, SOCS2, and CIS expression, and a sharp decline in SOCS3 expression (Fig. 1D). Although the decrease in SOCS3 is most dramatic at the 48-h time point, we also observe a slight decrease after 24 h (data not shown). Consistent with previous reports (18, 31), the decrease in SOCS3 is transient as it is followed by steady increase in its steady-state levels thereafter (Fig. 1D). Dynamic changes in the pattern of SOCS gene expression that accompany initiation of naive Th cell differentiation suggest that cellular pathways that regulate T cell activation and lineage commitment may be under feedback regulation by SOCS proteins. These results further suggest that increases in SOCS expression following Ag-priming may serve as a phenotypic marker of activated Th cells.
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The data presented above clearly suggest that transcription of SOCS genes during early stages of naive Th activation is regulated in part by signals emanating from TCR ligation and costimulation. As significant cytokine secretion occurs following antigenic stimulation of naive Th cells (Fig. 1C), we examined the contribution of Th1- and Th2-polarizing cytokines to the induction of SOCS expression in these early differentiating cells. Naive Th cells primed with Ag/APC for 4 days were washed to remove the Ag, starved for 2 h before stimulation with IFN-, IL-4, or IL-12, and then analyzed for induction of SOCS expression by real-time RT-PCR. Across the board, SOCS expression is induced by all three cytokines, although the effects on individual SOCS genes vary and depend on the cytokine used for stimulation (Fig. 2). For example, IL-4 or IFN- induces relatively higher levels of SOCS1 compared with IL-12, while much higher levels of SOCS3 is induced by IL-4 or IL-12. Interestingly, IL-4 appears to be the most potent inducer of all SOCS members analyzed and, among SOCS members, SOCS1 is most responsive to cytokine stimulation. Similar pattern of cytokine-induced SOCS expression was observed in naive Th cells stimulated for 48 h (data not shown). It is of note that the day 2 or day 4 Ag-primed cells induce SOCS3 expression in response to cytokine stimulation, suggesting that the decline in SOCS3 mRNA observed after 48 h of antigenic stimulation may derive from TCR ligation and costimulatory signals
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-GAS and m67 SIE probe that bind with high affinity to most activated STATs, and the C-GAS probe with an N4 GAS site that allows for high affinity binding to the STAT6 site (32, 33). In line with previous studies, we show that in day 4 differentiating Th cells, IL-12 activates STAT1, STAT3, and STAT4 (Fig. 3A), while IFN- exclusively signals through STAT1 homodimers (Fig. 3B) (34, 35). Analysis of STAT activation by IL-4 using the C-GAS motifs shows specific activation of STAT6 (Fig. 3C), consistent with previous reports. However, similar analysis with the Fc probe reveals that IL-4 activates both STAT1 and STAT6 in differentiating Th cells (Fig. 3D). Supershift analysis indicates that the activated STAT1 interacts with the GAS motif independent of STAT6 and does not for a heterodimer with STAT6 (Fig. 3D, lane 5). Thus, in uncommitted early differentiating Th cells, IL-4 signals are transduced by two signaling pathways, one that is mediated by STAT6 homodimers and another involving STAT1 homodimers.
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Activation of STAT1 signaling pathway by each of the three cytokines (IL-4, IL-12, IFN-) that influence commitment to the Th1 or Th2 phenotype (Fig. 3) suggests that this critical signal transducer may be an important regulator of SOCS genes during Th cell differentiation. To assess the contribution of STAT1, if any, in regulating expression of SOCS genes in differentiating Th cells, purified naive CD4+ Th cells from WT or STAT1–/– mice were activated in plate-bound anti-CD3/anti-CD28 Abs and analyzed for SOCS expression by real-time RT-PCR. Propagation of WT Th cells under nonpolarizing or Tho condition (no Abs or exogenous cytokine) induces significant expression of SOCS1 (
200 copies/cell), while culturing the cells under neutral or Thn condition (medium contains anti-IL-4, anti-IL-12, and anti-IFN- Abs) induces much lower levels of SOCS1 expression (80 copies/cell). Propagation of the cells under Th1 or Th2 condition induces similar levels of SOCS1 expression, suggesting that in differentiating and uncommitted Th cells, IL-4 induces SOCS1 expression to the same extent as IL-12 and IFN- combined (Fig. 4, left panel). In terms of SOCS3, we consistently observe much higher levels of induction in differentiating Th cells stimulated under Th2 condition (Fig. 4, right panel) and this further supports studies showing that fully differentiated Th2 cells have high constitutive expression of SOCS3 (16, 36). These results further underscore the role of Th-polarizing cytokines in transcriptional activation of SOCS genes and indicate that IL-4 is a more potent inducer of SOCS1 and SOCS3 in differentiating Th cells than IFN- or IL-12.
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and IL-12, thus provides a mechanistic explanation for high constitutive expression of SOCS3 in mature Th2 but not Th1 cells. The very low levels of SOCS1 and SOCS3 in STAT1 null Th cells stimulated under Th2 condition indicate that induction of these SOCS members by IL-4 is not mediated by STAT6. Similarly, lower levels of SOCS induction under Th1 condition indicate that STAT4 signaling is not a major SOCS inducer and further underscores the central role of STAT1 in transcriptional regulation of SOCS genes. Taken together, these results underscore the role of Ag and cytokines in induction of SOCS expression and suggest that both TCR and cytokine signaling are under feedback regulation by SOCS in differentiating Th cells. IL-4 is a potent inducer of STAT1 activation in Th2 but not Th1 cells
In view of the role of STAT1 in SOCS induction and the dramatic reduction in SOCS1 and SOCS3 expression in naive STAT1–/– Th cells propagated under Th2 condition, it was of interest to determine the Th cell subtypes that activate STAT1 in response to IL-4 signaling. Therefore, we analyzed STAT1 activation in D10.G4.1 Th2, AE7 Th1 cell lines, as well as in primary Th1 and Th2 cells established from Ag-primed naive Th cells after two cycles of polarization. Western blotting using pSTAT1 Abs shows that STAT1 is activated by IL-4 in primary Th2 cells (Fig. 5A) and the D10.G4.1 Th2 cell line (Fig. 5B), and the effect on STAT activation is concentration dependent. However, STAT1 activation is barely detectable in primary Th1 cells stimulated with the higher IL-4 concentration (Fig. 5A) and is not detected in the AE7 Th1 cells (Fig. 5B). These results have also been confirmed by EMSA/supershift analyses using the SIE GAS probe and STAT1 Ab (data not shown). Two groups have previously shown that IL-4, which typically activates STAT6, activates STAT5 in activated T cells by unknown mechanism (37, 38). Because the evidence for IL-4-induced STAT1 activation in Th2 cells is based mainly on results from supershift assay and Western blot analysis with a pSTAT1 Ab that may cross-react with STAT5 and/or STAT6, we verified these using a STAT1-specific Ab. We performed immunoprecipitation (IP)/Western blot analyses on cell extracts from the IL-4 stimulated Th2 cells. The IP is performed with STAT1 Ab, followed by Western blot analysis with anti-P-Tyr Ab, and reveals STAT1 phosphorylation (Fig. 5Ci). Verification of equal protein loading is demonstrated by blotting with STAT1 (Fig. 5Cii). After IP with the STAT1 Ab, the diluted supernatant was subjected to IP/Western blot analysis with either STAT6 Ab (Fig. 5Ciii) or STAT5 (Fig. 5Civ). Similar IP/Western blot analysis was performed using a P-Tyr-specific Ab and STAT1 Ab (Fig. 5D, i and ii). We also performed IP with the NH2-terminal STAT1 Ab, followed by Western blotting using pSTAT1 Ab (Fig. 5Diii). Sequential blot analysis using STAT1 Ab detects equivalent amounts of the STAT1 protein in control and IL-4-stimulated cells (Fig. 5Div), suggesting that our pSTAT1 Ab specifically detects tyrosine-phosphorylated STAT1 in the cells treated with IL-4. Collectively, these results obtained with a STAT1 Ab that does not cross-react with either STAT5 or STAT6 are consistent with our gel shift (Fig. 3B) and Western blot (Fig. 5, A and B) data presented above. We have also demonstrated by immunofluorescence analysis that stimulation of Th2 cells with IL-4 induces STAT1 nuclear localization (Fig. 5, E and F). The IP/Western (Fig. 5D, iii and iv) and immunofluorescence (data not shown) studies were also repeated with established Th2 D10.1 cells with similar results. To our knowledge, this is the first demonstration that IL-4 activates STAT1 in Ag-activated differentiating Th, Th2 cells, and to a lesser extent in Th1 cells. This may represent a form of negative-feedback mechanism regulating IL-4 signaling in Th2 cells and may reflect a fundamental difference in the effects of IL-4 on Th1 and Th2 cells.
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STAT6 signaling pathway plays a central role in IL-4-induced transcriptional activation of Th2 effector genes. Therefore, we used the D10.G4.1 Th2 cell line to directly examine whether SOCS proteins regulate this important pathway of Th2 cells (39, 40). This cell line provides a stringent test system because the D10.G4.1 cell secretes and responds to IL-4, constitutively expresses both SOCS1 and SOCS3 (18), and STAT6 is constitutively phosphorylated under normal culture condition in medium containing IL-2 (40). We transfected the D10.G4.1 cells with a SOCS1 expression vector, and analysis of cells with stable overexpression of SOCS1 (Fig. 6A) shows that STAT6 phosphorylation is inhibited in clones with overexpression of SOCS1 but not in the control clone (Fig. 6B). Thus, inhibition of STAT6 activation of Th2 cells is effected by inducing a net increase in the steady-state levels of SOCS molecules in the cells. A logical implication of this assertion is that a significant reduction of endogenous SOCS levels would enhance STAT6 activation. To test this prediction, we depleted SOCS1 in D10.G4.1 Th2 cells by overexpressing an anti-sense SOCS1 cDNA construct. Control cells transfected with the empty vector or the SOCS1-depleted D10.G4.1 cells were washed, starved for 2 h, and stimulated with various concentrations of IL-4. Results of Western blot analysis using pSTAT6 are consistent with our prediction: whereas significant phosphorylation of STAT6 is detected in control cells only after IL-4 stimulation, STAT6 is constitutively activated in SOCS1-depleted cells (Fig. 6C, upper band).
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We next examined the effect of SOCS1 on the proliferation of D10.G4.1 Th2 cells. Control cells (Vector) and cells overexpressing SOCS1 (sense) or anti-sense SOCS1 cDNA were cultured for 48 h in medium containing exogenous IL-2 and secreted IL-4. Proliferation of the cells was measured by [3H]thymidine incorporation assay. Robust proliferative responses are observed in all three clones after 24 h in culture, although a slight increase in [3H]thymidine incorporation is noted for cells expressing the anti-sense SOCS1 cDNA construct (Fig. 7A). Twenty-four hours later (day 2), proliferation of the vector control (Fig. 7A, left panel) and cells overexpressing SOCS1 (Fig. 7A, center panel) declines, consistent with progressive consumption of IL-2 in the culture (40). However, reduction in proliferation is most drastic in cells with forced expression of SOCS1 on day 2 of stimulation (Fig. 7A, center panel). In contrast, depletion of SOCS1 levels by overexpression of the anti-sense SOCS1 cDNA enhances proliferation of the cells (3-fold), suggesting that proliferation of Th cells is under negative feedback regulation by SOCS1. Similar results have been obtained with D10.G4.1 Th2 cells expressing sense and anti-sense SOCS3 cDNA (18). To explore the mechanistic basis of the growth inhibitory effects of SOCS on these cells, we analyzed expression of several genes that have been implicated in the regulation of the cell cycle. This analysis reveals that the growth regulatory effects of the SOCS1 is mediated in part by its effects on cyclin-dependent kinase protein, p27kip1 (Fig. 7, B and C), a critical modulator of Th cell proliferation (41). The effect is specific, as the level of another cyclin-dependent kinase protein, p21WAF1, is unaffected. However, as expression of SOCS1 or other members of the SOCS family are under stringent regulation to prevent inappropriate inhibition of diverse signaling pathways, we cannot rule out the possibility that overexpression of SOCS1 may have influenced other signaling pathways in a nonspecific manner and contributed to the inhibition of proliferation.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Although TCR ligation and costimulatory signals play central roles in regulating SOCS expression at initial stages of Th cell activation, cytokines are the major inducers of SOCS expression in Th cells. The level of SOCS expression increases tremendously in cells propagated under Th1 or Th2 cell condition compared with cells maintained without exogenous cytokines or under neutral condition (Fig. 4) and SOCS levels are severalfold higher in resting Th cells than naive or day 2 differentiating Th cells. Moreover, we show that Th1- (IFN- and IL-12) and Th2-polarizing (IL-4) cytokines induce SOCS expression in differentiating Th cells. IL-4 appears to be the most potent inducer of SOCS1 and SOCS3 (Fig. 4), and this is particularly surprising considering that IL-4 signals through STAT6 (42, 43) in Th2 cells, and functional STAT6 binding sites have not been positively identified in promoters of SOCS genes. However, analysis of STAT signaling in day 4 differentiating Th cells reveals that IL-4 activates both STAT1 and STAT6 pathways (Figs. 3 and 5), implying that induction of SOCS expression by IL-4 in these cells may be mediated through STAT1. A role for STAT1 in the induction of SOCS expression by IL-4 is supported by a recent study showing that a STAT1 GAS site in the SOCS3 promoter is required for induction of SOCS3 expression by leukemia inhibitory factor in pituitary corticotroph cells (12). Although STAT1 activation by IL-4 has previously been reported in a colon carcinoma cell line (44), to our knowledge this is the first report of STAT1 activation by IL-4 in Th cells. The activation of STAT1 by IL-4, IL-12, or IFN- in early differentiating Th cells and the requirement of STAT1 for optimum induction of SOCS1 and SOCS3 by Th-polarizing cytokines (Fig. 4) underscore the potentially pivotal role of STAT1-induced SOCS expression during Th cell activation, differentiation, and lineage commitment. It is remarkable that the decrease in SOCS expression in STAT1–/–-deficient Th cells is most prominent in response to IL-4, and although STAT1 activation by IL-4 has previously been reported in a colon carcinoma cell line (44), to our knowledge, this is the first report of STAT1 activation by IL-4 in Th cells. Taken together, these results thus reveal a previously unrecognized role of IL-4 in the induction of STAT1 activation or SOCS expression.
Interestingly, IL-4 preferentially induces STAT1 activation in Th2 cells but not Th1 cells (Fig. 5), and the levels of SOCS1 and SOCS3 are dramatically reduced in STAT1–/– Th2 cells (Fig. 4), suggesting that STAT1 signaling pathway may negatively regulate intensity and duration of cytokine signaling in Th2 cells by inducing SOCS expression. We show that one of the possible mechanisms by which SOCS proteins can regulate host immunity is by inhibiting IL-4-induced STAT6 phosphorylation in Th2 cells (Fig. 6). Assuming that development to Th2 phenotype is the default and dominant pathway of differentiating naive Th cells as has been proposed (45), repression of IL-4-induced STAT6 phosphorylation by SOCS proteins may provide a feedback autoregulatory loop that attenuates Th2 effector functions in situations where host immunity requires Th1-dominated responses. In fact, inhibition of IL-4-induced STAT6 phosphorylation by SOCS proteins has been described in B lymphocytes and monocytes, although the mechanism of SOCS induction differs from that of Th cells (46, 47). Whereas IL-4 does not regulate SOCS1 expression in B lymphocytes (46), we find that it is an inducer of SOCS1 and SOCS3 expression in Th cells (Fig. 4). However, it is still not clear whether regulation of SOCS expression by IL-4 in Th but not B cells indicates intrinsic differences between Th cells and B cells, or if this is due to analysis of primary mouse Th cells vs long-term human B lymphoma BJAB cell lines (46). Nonetheless, the biological relevance of regulating the levels of SOCS proteins in Th2 cells is underscored by the profound effects exerted on cell proliferation by changes in the endogenous levels of SOCS proteins in Th2 cells (Fig. 7).
In current models of IL-4 signal transduction, STAT6 mediates IL-4-induced changes in gene transcription and differentiation events while growth signals derive from insulin receptor substrate-2 (IRS-2) signaling pathway (48, 49, 50). As activated STAT6 and IRS-2 proteins are recruited to distinct regions of the IL-4R, it has been proposed that growth and gene expression in Th2 cells are regulated by distinct mechanisms (50). In this report, we have provided evidence suggesting that the two limbs of IL-4 signaling in Th2 cells are under feedback regulation by SOCS proteins. In addition to the inhibition of STAT6 signaling by SOCS1 and SOCS3, we show that overexpression of SOCS1 or SOCS3 (18) profoundly inhibits proliferation of Th2 cells (Fig. 7). Although we did not examine whether the growth inhibitory effects result from inhibition of IRS-2 signaling, a recent study, showing that SOCS1 or SOCS3 binds to IRS-2 in pancreatic cells and targets it for degradation by ubiquitin-mediated proteolysis, is highly suggestive (51). Remarkably, depletion of SOCS1 by forced overexpression of anti-sense SOCS1 construct induces significant proliferation of Th2 cells by regulating the expression of the cyclin-dependent kinase inhibitor p27kip1 (Fig. 7, B and C). These observations are in concert with reports showing that cytokine-stimulated Th cell proliferation is regulated by p27kip1 (41, 52). Thus, SOCS proteins may be important regulators of growth signals in Th cells as they can inhibit IRS-2 signaling (50) and also modulate proliferation of Th cells through their regulatory effects on cyclin-dependent kinase inhibitors.
In summary, results presented in this study are consistent with a model where antagonistic cytokines that influence developmental fate of early differentiating Th cells are under feedback regulation by SOCS proteins, and homeostatic levels of these polarizing cytokines in the Th cell are sensitive and responsive to slight changes in steady levels of SOCS proteins. Skewing of the immune response toward a Th1- or Th2-dominated response may therefore be the result of cross-talk between Th1- and Th2-polarizing cytokines and integration of their antagonistic signals by SOCS proteins. The opposing effects of STAT1 and STAT6 signaling pathways noted in this study further suggests that IL-4 has dual effects on differentiating Th cells and plays a central role in homeostatic regulation of the relative abundance of Th1 and Th2 cells. On one hand, it simulates growth and activates cytokine pathways through activation of STAT6 and IRS-2 signaling pathways, while on the other hand, it inhibits growth and attenuates cytokine signaling via STAT1-dependent up-regulation of SOCS proteins.
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2 Abbreviations used in this paper: SOCS, suppressor of cytokine signaling; SH2, Src homology 2; CIS, cytokine-induced SH2 protein; HEL, hen egg lysozyme; Tg, transgenic; WT, wild type; CD62L, L-selectin; P-Tyr, phosphotyrosine; SIE, sis-inducible element; GAS, activation site; IP, immunoprecipitation; IRS-2, insulin receptor substrate-2; RT, reverse transcriptase.
Received for publication September 4, 2003. Accepted for publication April 22, 2004.
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) or STAT1–/– mice (
) were activated in plate-bound anti-CD3/anti-CD28 Abs for 4 days under nonpolarizing (Tho), neutral (Thn), Th1, or Th2 condition as described in Materials and Methods. SOCS1 or SOCS3 mRNA transcripts expressed in the cells were quantitated by real-time RT-PCR analysis.
