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CUTTING EDGE |



*
Institute of Medical Microbiology and Immunology, and
Institute for Medical Anatomy, Section A, University of Copenhagen, Copenhagen, Denmark;
Wallenberg Laboratory, Lund University, Lund, Sweden; and
§
Tissue Typing Laboratory, Department of Clinical Immunology, National University Hospital, Copenhagen, Denmark
| Abstract |
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promoter. CD3-mediated activation of Stat3
is almost completely inhibited by a Src kinase inhibitor (PP1), whereas
IL-2-induced Stat3 activation is unaffected. In conclusion, we show
that CD3 ligation by mAb and SE triggers a rapid, PP1-sensitive
tyrosine and serine phosphorylation of Stat3 in human
CD4+ T cells. Moreover, we provide evidence that TCR/CD3
and IL-2 induce Stat3 activation via distinct signaling
pathways. | Introduction |
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Stat proteins play a key role in the regulation of T cell growth and differentiation. Stat protein knock-out mice are either embryonic lethal (Stat3) (6) or exhibit various defects in proliferation and signal transduction (4, 7).
In contrast to the well-established role of Stat proteins in cytokine receptor signaling, little is known about the function of Stat proteins in TCR signaling. In a recent study, Welte et al. (8) reported that TCR/CD3 ligation triggered tyrosine phosphorylation and activation of Stat5 in murine T cells. CD3-mediated Stat5 tyrosine phosphorylation was mediated via the Src-family tyrosine kinase Lck and involved a transient interaction between the TCR and Stat5 in murine T cells.
Here, we show that staphylococcal enterotoxin A (SEA) and anti-CD3 mAb induced a rapid and accumulating tyrosine phosphorylation and DNA-binding of Stat3 (but not Stat5) in allogen-specific, human CD4+ T cell lines. Thus, our data support the hypothesis that TCR signaling involves Src kinase-sensitive activation of Stat proteins and suggest that murine and human T lymphocytes differ in their usage of Stat proteins in TCR signaling.
| Materials and Methods |
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Allogen-specific CD4+ human T cell lines (9) selected for this study produced no IL-2 after TCR ligation, as judged by ELISA and RNase protection assay.
Abs and other reagents
Anti-phosphotyrosine mAb (4G10) was purchased from Upstate
Biotechnology (Lake Placid, NY); anti-Stat3 and anti-Stat5 Abs
were from Transduction Laboratories (Lexington, KY); anti-CD3 Ab
(F101.01) was a kind gift from Dr. C. Geisler (Institute of Medical
Microbiology and Immunology, University of Copenhagen, Denmark);
anti-phosphoserine-Stat3 (S727), anti-phosphotyrosine Stat3
(Y705), and anti-phosphotyrosine Stat5 (Y694) were from New England
Biolabs (Beverley, MA). Biotinylated DNA oligonucleotides (ICAM-1,
IL-2R
GASd/EBSd) were from DNA Technology (Aarhus, Denmark):
ICAM-1a, 5'AGCTTAGGTTTCCGGGAAAGCAC and ICAM-1b,
5'GTGCTTTCCCGGAAACCTAAGCT; IL-2R
a, 5'TTTCTTCTGGGAAGTACC; and
IL-2R
b, 5'GGTACTT CCTAGAAGAAA.
Cell surface Ag analysis
Cell surface expression was analyzed by flow cytometry (FACScan; Becton Dickinson, Mountain View, CA).
Preincubation and stimulation of cells
T cells (9) were incubated in medium containing anti-CD3 Ab (5 µg/ml), IL-2 (500 U/ml), SEA, and SEE (1 µg/ml), wortmannin (5 µM), rapamycin (25 ng/ml), or PP1 (10 µM) for the indicated periods of time. Cells were preincubated with inhibitors 1 h before incubation with cytokine, anti-CD3 Ab, or SE. Before stimulation, cells were starved for 4 h in medium containing 5% FCS.
Protein extraction and Western blotting
Immunoblotting and immunoprecipitation were conducted as described earlier (9). Immunoblots were evaluated by chemiluminescence (ECL; Amersham, Arlington Heights, IL).
Oligonucleotide affinity purification
Precipitation with biotinylated oligonucleotides was conducted as described for immunoprecipitations. Purified oligonucleotide-binding proteins were boiled in reducing SDS sample buffer and analyzed by SDS-PAGE and immunoblotting, as described earlier (9).
| Results |
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We utilized biotinylated DNA oligonucleotides from different
Stat3-binding promoter elements to investigate whether SEA induced
binding of Stat3 to relevant DNA sequences. SEA induced binding of
Stat3 to DNA oligonucleotides from the ICAM-1 and the IL-2R
promoter
within 14 h. In contrast, IL-2 induced Stat3 binding to these probes
within 3060 min (Fig. 3
, A
and B). Moreover, IL-2 also induced Stat5 binding to a
biotinylated c-fos probe, whereas SEA induced only Stat3 activation
(data not shown), supporting our conclusion that SEA and IL-2 induce
activation of different Stat proteins.
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probes, an
increase in the surface expression of ICAM-1 and IL-2R
could be
detected by flow cytometry, which peaked within 1624 h (Fig. 3
and CD38 (data not
shown).
Recent evidence suggests that Lck, an Src family tyrosine kinase, plays
a critical role in TCR-mediated activation of Stat5 in murine T cells
(8). Here, we utilized an Src kinase inhibitor (PP1) to
address whether Src kinases also play a role in CD3-mediated activation
of Stat3. At concentrations above 5 µM, PP1 almost completely blocked
SE-induced tyrosine phosphorylation of Stat3, but had
no inhibitory effect on IL-2-induced Stat3 activation (Fig. 4
). SEA-induced serine
phosphorylation of Stat3 was also inhibited by PP1
(data not shown). SEA-induced binding of Stat3 to the ICAM-1-probe was
also profoundly inhibited by PP1, whereas rapamycin had only a weak
inhibitory effect (Fig. 4
B). Wortmannin (wort), an inhibitor
of phosphatidyl inositol-3 (PI-3) kinase, had no effect on SEA- and
IL-2-induced DNA binding of Stat3 (Fig. 4
B). SEA-induced
up-regulation of ICAM-1 and IL-2R
expression was profoundly
inhibited by PP1, whereas rapamycin had only a weak inhibitory effect
(Fig. 4
, C and D).
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| Discussion |
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blocking mAb, whereas CD3-induced Stat3 activation was
not (data not shown). In a recent study, Welte et al. (8) reported that anti-CD3 mAb induced tyrosine phosphorylation of Stat5 in murine T cells. However, we did not detect any Stat5 tyrosine phosphorylation following CD3 stimulation in human T cells. Yet, IL-2 induced tyrosine phosphorylation of Stat5 in these T cells. It is possible that TCR stimulation activates different Stat proteins in man and mouse. An alternative explanation might be that different T cell populations (e.g., naive vs memory T cells) induce different sets of Stat proteins after TCR ligation.
Src tyrosine kinases, such as Lck, play a crucial role in the early steps of TCR-mediated signal transduction. Lck mediates tyrosine phosphorylation of immune receptor tyrosine-based activation motifs in the TCR, which resemble and might function as Stat protein docking sites. Welte et al. (8) provided strong evidence that Lck plays a critical role in TCR-mediated Stat5 activation in murine T cells. Here, we show that PP1, an Src kinase inhibitor, blocked CD3-induced Stat3 tyrosine phosphorylation and DNA binding. It remains unknown whether Stat3 becomes tyrosine phosphorylated directly or indirectly upon TCR stimulation. IL-2R ligation triggers tyrosine phosphorylation of Stat proteins through an activation of Jak3 and Jak1 (3). We were unable to detect activated Jak kinases (Jak1, Jak2, Jak3, and Tyk2) following TCR stimulation in human T cells (data not shown). This observation was in agreement with other reports that TCR stimulation did not trigger activation of Jak kinase activity (10). Differences between the data presented here and previous studies on tyrosine phosphorylation of Stat3 following TCR stimulation (11) may lie partly in the cells and detection systems used. Here, we used Abs against tyrosine phosphorylated Stat3 and Stat5, which are highly sensitive, to detect tyrosine phosphorylation.
Recent evidence suggest that TCR stimulation triggers Stat3 serine phosphorylation via an activation of mitogen-activated protein kinases in T cells (12). Using Abs against serine phosphorylated Stat3, Cantrell and colleague (5) observed serine phosphorylation of Stat3 following TCR stimulation. Our data confirm and extend these findings. Since PP1 inhibited TCR-mediated serine phosphorylation, our results suggest Src family kinases to be involved in both serine and tyrosine phosphorylation of Stat3 in human T cells.
As IL-2 induces Stat3 activation in T cells (9), we investigated the possibility that TCR-induced Stat3 activation was due to an autocrine loop involving IL-2. First, TCR ligation triggers Stat3 activation in T cells that do not produce IL-2 (as judged by RNase protection assay and ELISA). Second, TCR triggers a rapid phosphorylation of Stat3, but not Stat5, whereas IL-2 triggers a rapid phosphorylation of Stat3 and Stat5 in parallel. Third, an IL-2R blocking mAb inhibits IL-2-induced Stat3 activation without affecting TCR-mediated Stat3 phosphorylation. However, our data do not exclude the possibility that the accumulated increase in phosphorylated Stat3 was due to an autocrine, cytokine-mediated activation loop.
In conclusion, we provide the first evidence that CD3 ligation triggers rapid Stat3 tyrosine and serine phosphorylation via a PP1-sensitive pathway in human CD4+ T cell lines.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Niels Ødum, Cell Cybernetics Laboratory, Institute of Medical Microbiology and Immunology, Panum Institute 22.5.22, University of Copenhagen, Blegdamsvej 3c, DK-2200 Copenhagen N, Denmark. E-mail address: ![]()
3 Abbreviations used in this paper: Jak, Janus kinase; SE, staphylococcal enterotoxin. ![]()
Received for publication February 22, 1999. Accepted for publication June 7, 1999.
| References |
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, but not the T cell antigen receptor, in human T lymphocytes. EMBO J. 13:5605.[Medline]
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