Increased expression of TCR vbeta5.1 and 8 in mucosal T-cell lines cultured from patients with Crohn disease.

BACKGROUND: Characterization of the T-cell receptor variable beta chain (Vbeta) repertoire in inflamed mucosa has been used to identify disease-relevant T-cell populations and antigens in Crohn disease (CD). In vitro expansion of mucosal T cells may reveal changes in Vbeta repertoire not apparent in fresh isolates and we aimed to identify Vbeta subpopulations implicated in Crohn disease. METHODS: In vivo activated mucosal T cells were cultivated using IL-2 and IL-4 from biopsies of whole colonic mucosa without use of Vbeta-modifying exogenous antigen or feeder cells. The Vbeta gene expression in mucosal T-cell cultures was determined in 30 patients with CD and 12 healthy controls using reverse transcriptase polymerase reaction (RT-PCR) covering all 23 functional Vbeta families and the Vbeta receptor prevalence was evaluated by flow cytometry in selected cultures. RESULTS: Early T-cell cultures from both CD patients and healthy controls showed a polyclonal Vbeta gene expression that narrowed during culture, which in CD cultures led to a significant over-expression of the Vbeta5.1 (P = 0.04) and Vbeta8 gene segments (P = 0.03). Together with Vbeta6 and Vbeta18, these Vbeta chains form a pattern of staphylococcal enterotoxin type E (SEE) responsive Vbeta chains, also over-expressed in CD cultures (P = 0.02). Further in vitro stimulation of CD cultures with SEE caused expansion of Vbeta8 receptor positive cells together with a proinflammatory cytokine response. CONCLUSIONS: CD may be associated with (super)antigen-specific Vbeta subpopulations selected during long-term cultivation of mucosal biopsies from inflamed colon.

Infliximab downregulates interferon-gamma production in activated gut T-lymphocytes from patients with Crohn's disease.

The tumour necrosis factor-alpha (TNF-alpha) neutralizing antibody, Infliximab (Ifx), reduces disease activity in patients with active steroid-dependent or fistulizing Crohn's disease. The mechanisms underlying the effects of Ifx are not fully understood. This study aims to investigate if and how Ifx regulates the interferon-gamma (IFN-gamma) production in human intestinal T-cells. Colonic T cells were expanded from 25 patients with Crohn's disease and ten healthy controls in an in vitro system, using medium supplemented with interleukin-2 and interleukin-4 but without exogenous antigen. The effect of Ifx was investigated in these in situ activated T cell cultures regarding the IFN-gamma production, proliferation, transmembrane TNF-alpha expression, cytolysis and apoptosis. T cell cultures from patients with Crohn's disease produced significantly higher levels of IFN-gamma (<0.001) and TNF-alpha (P=0.04) than T cell cultures from healthy controls. The production of IFN-gamma was downregulated by Ifx in early T cell cultures (P=0.002). Ifx bound to transmembrane TNF-alpha of activated T cells without inducing complement-mediated cytolysis, apoptosis and without affecting proliferation. Besides its known TNF-alpha neutralizing property, Ifx downregulates INF-gamma production in colonic T cell cultures. Colonic T cells express transmembrane TNF-alpha that binds Ifx. The data suggest that Ifx reduces the level of at least two pro-inflammatory cytokines leading to lower disease activity.

Gab2 is phosphorylated on tyrosine upon interleukin-2/interleukin-15 stimulation in mycosis-fungoides-derived tumor T cells and associates inducibly with SHP-2 and Stat5a.

Cutaneous T cell lymphomas (CTCLs) often show abnormal interleukin-2 (IL-2) receptor signaling. In this study, we investigated the role of Gab2, a recently identified adaptor molecule involved in IL-2 receptor signaling in CTCLs. We show that Gab2 was transiently phosphorylated by tyrosine in human mycosis fungoides (MF) tumor T cells upon IL-2 stimulation and that SHP2 as well as Stat5a associated inducibly with Gab2. IL-15, but not IL-4, also induced tyrosine phosphorylation of Gab2, suggesting that the IL-2 receptor beta-chain is important for IL-2-induced Gab2 phosphorylation. Preincubation of cells with the Src family kinase inhibitor, PP1, surprisingly increased the IL-2- and IL-15-induced tyrosine phosphorylation of Gab2, indicating that an Src family kinase member negatively regulates IL-2 receptor signaling in MF T cells. Thus, although Gab2 seems to function normally in MF T cells compared to normal T cells, Gab2 itself might be abnormally regulated by an Src family kinase.

Constitutive STAT3-activation in Sezary syndrome: tyrphostin AG490 inhibits STAT3-activation, interleukin-2 receptor expression and growth of leukemic Sezary cells.

Interleukin-2 (IL-2) is a growth factor which upon binding to high-affinity receptors (IL-2Ralphabetagamma) triggers mitogenesis in T cells. IL-2Ralpha expression is restricted to T cells which have recently encountered antigen, and in healthy individuals the majority (>95%) of peripheral T cells are IL-2Ralpha negative. An aberrant expression of IL-2Ralpha has recently been described in cutaneous T-cell lymphoma (CTCL). Here, we study the regulation of IL-2Ralpha expression and STATs in a tumor cell line obtained from peripheral blood from a patient with Sezary syndrome (SS), a leukemic variant of CTCL. We show that (1) STAT3 (a transcription factor known to regulate IL-2Ralpha transcription) is constitutively tyrosine-phosphorylated in SS tumor cells, but not in non-malignant T cells; (2) STAT3 binds constitutively to a STAT-binding sequence in the promotor of the IL-2Ralpha gene; (3) the Janus kinase inhibitor, tyrphostine AG490, inhibits STAT3 activation, STAT3 DNA binding, and IL-2Ralpha mRNA and protein expression in parallel; and (4) tyrphostine AG490 inhibits IL-2 driven mitogenesis and triggers apoptosis in SS tumor cells. In conclusion, we provide the first example of a constitutive STAT3 activation in SS tumor cells. Moreover, our findings suggest that STAT3 activation might play an important role in the constitutive IL-2Ralpha expression, survival, and growth of malignant SS cells.

Microdissection and reverse painting in a melanoma cell line: a detailed description of structurally abnormal chromosomes.

Seven structurally abnormal chromosomes from a newly established melanoma cell line were microdissected. The DNA from these chromosomes was DOP amplified and labeled with Biotin or Digoxigenin for FISH analysis. The complex nature of these markers is described.

Identification of sample-specific sequences in mammalian cDNA and genomic DNA by the novel ligation-mediated subtraction (Limes).

The representational difference analysis (RDA) and other subtraction techniques are used to enrich sample-specific sequences by elimination of ubiquitous sequences existing in both the sample of interest (tester) and the subtraction partner (driver). While applying the RDA to genomic DNA of cutaneous lymphoma cells in order to identify tumor relevant alterations, we predominantly isolated repetitive sequences and artificial repeat-mediated fusion products of otherwise independent PCR fragments (PCR hybrids). Since these products severely interfered with the isolation of tester-specific fragments, we developed a considerably more robust and efficient approach, termed ligation-mediated subtraction (Limes). In first applications of Limes, genomic sequences and/or transcripts of genes involved in the regulation of transcription, such as transforming growth factor beta stimulated clone 22 related gene (TSC-22R), cell death and cytokine production (caspase-1) or antigen presentation (HLA class II sequences), were found to be completely absent in a cutaneous lymphoma line. On the assumption that mutations in tumor-relevant genes can affect their transcription pattern, a protocol was developed and successfully applied that allows the identification of such sequences. Due to these results, Limes may substitute/supplement other subtraction/comparison techniques such as RDA or DNA microarray techniques in a variety of different research fields.

STAT3-mediated constitutive expression of SOCS-3 in cutaneous T-cell lymphoma.

A characteristic feature of neoplastic transformation is the loss of external control by cytokines and extracellular matrix of cellular differentiation, migration, and mitogenesis. Because suppressors of cytokine signaling (SOCS) proteins are negative regulators of cytokine-induced signaling, it has been hypothesized that an aberrant SOCS expression plays a role in neoplastic transformation. This study reports on a constitutive SOCS-3 expression in cutaneous T-cell lymphoma (CTCL) cell lines. SOCS-3 protein is constitutively expressed in tumor cell lines (but not in nonmalignant T cells) obtained from affected skin from a patient with mycosis fungoides (MF) and from peripheral blood from a patient with Sezary syndrome (SS). In contrast, constitutive SOCS-3 expression is not found in the leukemic Jurkat T-cell line, the MOLT-4 acute lymphoblastic leukemia cell line, and the monocytic leukemic cell line U937. Expression of SOCS-3 coincides with a constitutive activation of STAT3 in CTCL tumor cells, and stable transfection of CTCL tumor cells with a dominant negative STAT3 strongly inhibits SOCS-3 expression, whereas transfection with wild-type STAT3 does not. Moreover, the reduced SOCS-3 expression in cells transfected with the dominant negative STAT3 is associated with an increased sensitivity to interferon-alpha (IFN-alpha). In conclusion, evidence is provided for a constitutive SOCS-3 expression in cancer cells obtained from patients with CTCL. Moreover, the findings indicate that the aberrant expression of SOCS-3 is mediated by a constitutive activation of STAT3 in CTCL cells and affects the IFN-alpha sensitivity of these cells. (Blood. 2001;97:1056-1062)

Mimotopes for tumor-specific T lymphocytes in human cancer determined with combinatorial peptide libraries.

Mimotopes of a tumor-associated T cell epitope were determined using randomized and combinatorial peptide libraries and a CD8(+) T cell clone specific for the cutaneous T cell lymphoma cell line MyLa. Antigen recognition by this clone was found to be HLA-B8 restricted. More than 80 % of HLA-matched patients with cutaneous T cell lymphoma had mimotope-specific CD8(+) T cells in their peripheral blood. Mimotope-specific T cells isolated and expanded from a patient lysed MyLa cells in in vitro assays thus demonstrating their cytolytic capacity and tumor specificity. Mimotope vaccination of a patient without detectable mimotope-specific T cells induced frequencies of these cells of up to 1.82 % of the peripheral blood CD8(+) T cells.

In situ activated intestinal T cells expanded in vitro--without addition of antigen--produce IFN-gamma and IL-10 and preserve their function during growth.

OBJECTIVE: The balance between mucosal CD4+ T cells producing IFN-gamma or IL-10 is essential in the maintenance of intestinal homeostasis. We aimed to investigate how in situ activated T cells were expanded in vitro according to a new cell culture protocol and if it selected for continuous clonal CD4+ T cells capable of producing mainly IFN-gamma or IL-10. METHODS: T cell cultures were established from colonic biopsy specimens of 27 patients with Crohn's disease and from 10 healthy controls in a medium supplemented with IL-2 and IL-4 but without addition of exogenous antigen or mitogen. Cytokine production was measured after stimulation (IL-12, super antigen) and inhibition (cyclosporin and methylprednisolone). RESULTS: Cytokine production was increased in cultures from patients with Crohn's disease compared to controls (IFN-gamma, p = 0.005; IL-10, p = 0.003; TNF-alpha, p = 0.03). Early cultures were highly responsive to IL-12 stimulation. We established CD4+ T-cell clones escaping cellular senescence with an inflammatory or regulatory cytokine profile. DISCUSSION: The data indicate that cultures of in situ activated inflammatory and regulatory subpopulations of intestinal T lymphocytes with pathogenic importance in Crohn's disease can be established preserving their functional properties during growth.

Oligonucleotide fishing for STAT6: cross-talk between IL-4 and chemokines.

Signal transducer and activator of transcription 6 (STAT6) is essential for the biological activities of interleukin-4 (IL-4) and the development of allergic responses in mice. Here we report on a sensitive and specific assay for STAT6 activation in response to IL-4. We took advantage of double-stranded oligonucleotide probes containing a STAT6-binding gene-sequence from the promotor of the immunoglobulin heavy chain germline epsilon transcript to study the IL-4-induced DNA binding of STAT6. Using these probes, we show that repeated adjacent STAT6-binding sites result in enhanced STAT6-DNA binding. Moreover, the distance between the binding sites is critical for STAT-DNA binding, i.e. STAT6 binding is decreased at distances above 20 nucleotides between neighbouring binding sites. Using this assay to study cross-talk between IL-4 and chemokines, we provide evidence that MIP-1beta and MIG inhibit IL-4-induced STAT6 activation, whereas other chemokines and cytokines do not. In conclusion, our data show that oligonucleotide fishing is a supplementary tool for studying cytokine cross-talk at a genomic level.

Lck is involved in interleukin-2 induced proliferation but not cell survival in human T cells through a MAP kinase-independent pathway.

The role of Lck in IL-2-induced proliferation and cell survival is still controversial. Here, we show that the Src family kinase inhibitor, PP1, reduced the IL-2-induced proliferation of human T cells significantly without inhibiting the anti-apoptotic effect of IL-2. As Lck is the only Src family kinase activated upon IL-2 stimulation in T cells, this indicates that Lck is involved in IL-2-induced proliferation but not survival. IL-2-induced MAP kinase activation was only slightly inhibited by PP1, suggesting that Lck is not essential for IL-2-induced MAP kinase activation in human T cells. We found that an IL-2-sensitive, human mycosis fungoides-derived tumor T cell line is Lck negative, and that the IL-2-induced MAP kinase activation is comparable to non-cancerous T cells, although a little delayed in kinetics. An Lck expressing clone was established by transfecting Lck into mycosis fungoides tumor T cells, but Lck had no influence on the delayed kinetics of MAP kinase activation, indicating that Lck is not essential for MAP kinase activation in mycosis fungoides tumor T cells or in non-cancerous T cells. Taken together, this indicates that Lck is involved in IL-2-induced proliferation, but not cell survival, through a pathway not involving MAP kinase.

A T-cell epitope determined with random peptide libraries and combinatorial peptide chemistry stimulates T cells specific for cutaneous T-cell lymphoma.

BACKGROUND: Mycosis fungoides is the most frequent T-cell lymphoma of the skin. Despite numerous attempts, no tumour antigens have yet been identified. Only in one case has an idiotype-derived peptide been found to trigger CTL of the respective patient. The identification of natural antigens requires the cultivation of large amounts of tumour cells in vitro, which has been possible in two exceptional cases. The identification of synthetic epitopes for tumour-specific CTL with random peptide libraries can overcome this limitation and is a powerful tool for application in the development of immune therapies for a wide range of patients. MATERIALS AND METHODS: The critical amino acids for the construction of epitopes for the CTCL-specific CTL clone My-La CTL were determined with synthetic peptide libraries in positional scanning OX8 format in a standard 61chromium release assay. Sixteen different peptides could be synthesized from the combinatoric of these amino acids with the canonical anchor amino acids for MHC binding. These peptides were tested for their capacity to stimulate My-La CTL and PBMC of an HLA-matched CTCL patient. RESULTS: A synthetic epitope could be identified for My-La CTL, which was recognized in a HLA-restricted manner. The response towards this epitope was comparable to the response towards their natural target My-La. Using these synthetic epitopes, T cells of a HLA-matched patient could be induced in vitro and led to the establishment of different cell lines and clones. Some of these lines recognized the peptides as well as the allogenic but HLA-matched tumour cell line My-La, indicating that they are specific for a naturally expressed tumour antigen. CONCLUSIONS: The identification of synthetic epitopes for tumour-specific CTL clones can be used for the development of vaccines for immune therapies of cancer; such peptides can be applied inter-individually. Synthetic epitopes must not correspond to the natural ones, but they can be even more potent as stimulation of specific T cells and can be fine-tuned to increase the success of the therapy.

Inhibition of protein phosphatase 2A induces serine/threonine phosphorylation, subcellular redistribution, and functional inhibition of STAT3.

Signal transducers and activators of transcription (STATs) are rapidly phosphorylated on tyrosine residues in response to cytokine and growth factor stimulation of cell surface receptors. STATs hereafter are translocated to the nucleus where they act as transcription factors. Recent reports suggest that serine phosphorylation of STATs also is involved in the regulation of STAT-mediated gene transcription. Here, we studied the role of serine/threonine phosphatases in STAT3 signaling in human antigen-specific CD4(+) T cell lines and cutaneous T cell lymphoma lines, expressing a constitutively activated STAT3. We show that an inhibitor of protein phosphatases (PPs) PP1/PP2A, calyculin A, induces (i) phosphorylation of STAT3 on serine and threonine residues, (ii) inhibition of STAT3 tyrosine phosphorylation and DNA binding activity, and (iii) relocation of STAT3 from the nucleus to the cytoplasm. Similar results were obtained with other PP2A inhibitors (okadaic acid, endothall thioanhydride) but not with inhibitors of PP1 (tautomycin) or PP2B (cyclosporine A). Pretreatment with the broad serine/threonine kinase inhibitor staurosporine partly blocked the calyculin A-induced STAT3 phosphorylation, whereas inhibitors of serine/threonine kinases, such as mitogen-activated protein kinase-1 extracellular-regulated kinase-kinase, mitogen-activated protein p38 kinase, and phosphatidylinositol 3-kinase, did not. In conclusion, we provide evidence that PP2A plays a crucial role in the regulation of STAT3 phosphorylation and subcellular distribution in T cells. Moreover, our findings suggest that the level of STAT3 phosphorylation is balanced between a staurosporine-sensitive kinase(s) and PP2A.

Inhibition of constitutively activated Stat3 correlates with altered Bcl-2/Bax expression and induction of apoptosis in mycosis fungoides tumor cells.

The Jak/Stat signaling pathway transmits signals from many cytokine and growth factor receptors to target genes in the nucleus. Constitutive activation of Stat3 has recently been observed in many tumor cells and dysregulation of the Stat signaling pathway has been proposed to be implicated in malignant transformation. In a previous study, we found constitutively tyrosine phosphorylated Stat3 in mycosis fungoides tumor cells. Here, we show that the Jak kinase inhibitor, Ag490, inhibits the constitutive binding of Stat3 to an oligonucleotide representing the Stat-binding sequence from the ICAM promotor. The decreased ability of Stat3 to bind DNA precedes dynamic alterations in the expression of anti-apoptotic Bcl-2 and pro-apoptotic Bax proteins (decreased Bcl-2 expression and increased Bax expression) and induction of apoptosis. Thus, our data suggest that the involvement of Stat3 in oncogenic transformation could be mediated through regulation of survival signals.

Cytokine-driven immortalization of in vitro activated human T lymphocytes. CD28 expression correlates inversely with cell population doublings.

Like other normal human somatic cells, T lymphocytes are believed to have a finite in vitro life span. However, continuous T lymphocyte cell lines can often be established from chronic inflammatory skin diseases when the culture medium is supplemented with IL-2 and IL-4 but without antigen and accessory cells added. Based on the assumption that these continuous T lymphocyte cell lines were activated by antigen during the chronic inflammation taking place in vivo, I investigated whether peripheral blood T lymphocytes could be induced to cytokine-dependent continuous growth following antigen activation. Upon allostimulation, peripheral blood CD4+ T lymphocytes reproducibly escape from cellular senescence. These IL-2- and IL-4-dependent continuous T cell lines show high telomerase activity. Withdrawal of either IL-2 or IL-4 results in cell growth arrest concomitant with down-regulation of telomerase activity. When cultured continuously, these CD4+ human T lymphocytes gradually lose expression of CD28.

Staphylococcus enterotoxin A modulates interleukin 15-induced signaling and mitogenesis in human T cells.

T cells expressing the appropriate T-cell receptor Vbeta chain proliferate in response to Staphylococcus enterotoxin A (SEA) pulsed antigen-presenting cells (APC), whereas other T cells do not (SEA "non-responders"). Activated human T cells express MHC class II molecules that are high affinity receptors for SEA. Here we show that, in the absence of APC, SEA induces a profound inhibition of IL-15-driven proliferation in MHC class II+, human SEA-"responder" T-cell lines. In contrast, proliferation induced by phorbol esther (PMA) was enhanced by SEA. The inhibitory effect on cytokine-mediated mitogenesis correlates with an inhibition of IL-2Rbeta expression and ligand-induced tyrosine phosphorylation of IL-2R. Cyclosporin A (CyA), an inhibitor of the protein phosphatase (PP2B) calcineurin, strongly inhibits the SEA-induced modulations of cytokine receptor expression. Moreover, CyA inhibits both the anti-mitogenic effect of SEA on cytokine-induced proliferation and the pro-mitogenic effect of PMA. In contrast, inhibitors of PP1, PP2A, protein kinase C (PKC), phosphatidyl-inositol-3-kinase (PI-3K) and mammalian target of rapamycin (mTOR) are unable to inhibit the effects of SEA. In a SEA "non-responder" T-cell clone obtained from the affected skin of a patient with psoriasis vulgaris, SEA does not inhibit IL-2Rbeta expression and IL-15-driven proliferation. On the contrary, SEA enhances IL-15- and IL-2-induced proliferation via a CyA-sensitive pathway in this T-cell clone. In conclusion, the present data show that (i) SEA selectively inhibits IL-15- (but not PMA-) mediated proliferation in SEA "responder" T cells, (ii) SEA enhances cytokine-driven growth in psoriasis T cells with a "non-responder" phenotype, and (iii) crosstalk between SEA receptors and the IL-15R (and IL-2R) pathway is mediated via a PP2B-dependent and PP1/PP2A-, PKC-, PI-3 kinase- and mTOR-independent pathway in human T-cell lines.

Staphylococcal enterotoxin-A directly stimulates signal transduction and interferon-gamma production in psoriatic T-cell lines.

Bacterial superantigens such as staphylococcal enterotoxin-A (SEA) have been implicated in the pathogenesis of psoriasis vulgaris. Major histocompatibility complex (MHC) class II molecules are high affinity receptors for SEA, and T cells found in psoriatic skin lesions express high levels of MHC class II. Here we address the question of whether SEA can directly activate psoriatic T cells in the absence of professional antigen-presenting cells. We show that SEA induces i) tyrosine phosphorylation of several proteins, ii) downregulation of the T-cell receptor (TCR), and iii) production of interferon-gamma (IFN-gamma), but not autocrine mitogenesis in CD8-positive T clones obtained from skin lesions of a patient with psoriasis vulgaris. Psoriatic T cells do not respond to SEA molecules if mutations are introduced in the TCRbeta- or in both the two MHC class II alpha- and beta-binding sites of SEA. Mutations in only one of the two MHC class II binding sites of SEA has different effects on T-cell activation. Thus, SEA molecules with a mutation in the MHC class II beta-binding site induce protein tyrosine phosphorylation, but not IFN-gamma production or co-stimulation of cytokine-mediated proliferation. In contrast, SEA with a mutation in the MHC class II alpha-binding site induces IFN-gamma and a qualitatively changed tyrosine phosphorylation profile. Both mutations delete the co-stimulatory effect on cytokine-mediated proliferation. This suggests that both MHC class II binding sites are involved in the autopresentation of SEA by psoriatic T cells. In conclusion, we provide evidence that SEA directly activates MVHC class H-positive psoriatic T-cell lines to produce IFN-gamma, a key cytokine in the pathogenesis of psoriasis vulgaris.

Reversal of human allergic T helper 2 responses by engagement of signaling lymphocytic activation molecule.

Allergen-specific Th2 cells accumulate at high frequencies in the skin of patients with atopic dermatitis (AD), where they contribute to the induction and maintenance of the lesions that are characteristic for the disease. Attenuation of these lesions in response to successful therapy is associated with a reduction in IL-4-producing Th2 cells and the appearance of IFN-gamma-producing Th cells. In this study, we demonstrate that engagement of the signaling lymphocytic activation molecule (SLAM) by an agonistic mAb, during allergen-specific expansion of highly polarized Th2 cell populations derived from skin biopsies of AD patients, results in the generation of stable populations of IFN-gamma-producing cells. SLAM-mediated reversal of Th cell phenotype has important biologic consequences, because supernatants of these activated, allergen-specific Th cells fail to induce IgE synthesis by purified B cells costimulated by anti-CD40 mAbs. Thus, highly polarized, allergen-specific Th2 cell populations derived from the skin of AD patients can be reversed into Th cell populations that contain IFN-gamma-producing cells and that do not support IgE synthesis. These results define a new mechanism to promote Th0/Th1 differentiation and suggest a potential role for anti-SLAM mAbs in the treatment of Th2-mediated allergic diseases.

Constitutive activation of a slowly migrating isoform of Stat3 in mycosis fungoides: tyrphostin AG490 inhibits Stat3 activation and growth of mycosis fungoides tumor cell lines.

Mycosis fungoides (MF) is a low-grade cutaneous T cell lymphoma of unknown etiology. In this report, the Jak/Stat (Janus kinase/signal transducer and activator of transcription) signaling pathway was investigated in tumor cell lines established from skin biopsy specimens from a patient with MF. Jaks link cytokine receptors to Stats, and abnormal Jak/Stat signaling has been observed in some hemopoietic cancers. In MF tumor cells, a slowly migrating isoform of Stat3, Stat3(sm), was found to be constitutively activated, i.e., (i) Stat3(sm) was constitutively phosphorylated on tyrosine residues, and tyrosine phosphorylation was not enhanced by growth factor stimulation; (ii) band shift assays and immunoprecipitations of DNA/Stat complexes showed constitutive DNA-binding properties of Stat3(sm); and (iii) Stat3(sm) was constitutively associated with Jak3. The abnormal activation of Stat3(sm) was highly specific. Thus, neither the fast migrating isoform of Stat3 (Stat3(fm)) nor other Stats (Stat1, Stat2, and Stat4 through Stat6) were constitutively activated. The Jak kinase inhibitor, tyrphostin AG490, blocked the constitutive activation of Stat3(sm) and inhibited spontaneous as well as interleukin 2-induced growth of MF tumor cells. In conclusion, we have provided evidence for an abnormal Jak/Stat signaling and growth regulation in tumor cells obtained from affected skin of an MF patient.