Antigen activation and impaired Fas-induced death-inducing signaling complex formation in T-large-granular lymphocyte leukemia

Tipifarnib-mediated suppression of T-bet-dependent signaling pathways

Large granular lymphocyte (LGL) leukemia is a chronic lymphoproliferative disease in which T-bet [T-box transcription factor 21 gene (tbx21)] overexpression may play a pathogenic role. T-bet orchestrates the differentiation of mature peripheral T-cells into interferon-γ (IFN-γ) and tumor necrosis factor-α producing CD4+ T-helper type I (Th1) and CD8+ T cytotoxic cells that are necessary for antiviral responses. When IL-12 is produced by antigen-presenting cells, T-bet expression is induced, causing direct stimulation of ifng gene transcription while simultaneously acting as a transcriptional repressor of the IL4 gene, which then leads to Th1 dominance and T-helper type 2 differentiation blockade. Additionally, T-bet has been shown to regulate histone acetylation of the ifng promoter and enhancer to loosen condensed DNA, creating greater accessibility for other transcription factor binding, which further amplifies IFNγ production. We found that treatment with a farnesyltransferase inhibitor tipifarnib reduced Th1 cytokines in LGL leukemia patient T-cells and blocked T-bet protein expression and IL-12 responsiveness in T-cells from healthy donors. The mechanism of suppression was based on modulation of histone acetylation of the ifng gene, which culminated in Th1 blockade.

Effects of Toll-like receptor signals in T-cell neoplasms

T-cell neoplasms have poor prognosis and few effective therapeutic options. Therefore, identification of factors in T-cell leukemia/lymphoma that are associated with cancer progression may represent novel therapeutic targets. Recent studies have highlighted a previously unappreciated role for the expression of Toll-like receptors (TLRs) on T cells and their effects on cell survival and proliferation. TLRs can bind exogenous molecules derived from pathogens as well as endogenous self-ligands released from damaged cells. Recent reports demonstrate that TLR engagement on primary mouse or human T cells enhances proliferation and/or cell survival. The mechanisms by which TLR stimulation on T cells influences these parameters and the different T-cell subsets that are affected by TLR stimulation are currently under investigation. Furthermore, neither the biological importance of stimulating TLRs on neoplastic T cells nor the prevalence of TLR expression in T-cell malignancies have yet to be characterized. Based on published reports and compelling preliminary data, we propose that the activation of the TLR-MyD88 signaling pathway in neoplastic T cells contributes to disease progression by reducing cell death and enhancing cell division. In this article, we present both theoretical arguments and experimental data in support of this hypothesis.

Clonal drift demonstrates unexpected dynamics of the T-cell repertoire in T-large granular lymphocyte leukemia

T-cell large granular lymphocyte leukemia (T-LGLL) is characterized by chronic lymphoproliferation of cytotoxic T lymphocytes (CTLs) and is associated with lineage-restricted cytopenias. Introduction of T-cell receptor (TCR) variable β-chain (Vβ) monoclonal antibodies has facilitated identification and enumeration of clonal CTLs by flow cytometry. A highly skewed TCR Vβ repertoire identified by flow cytometry is strongly associated with monoclonal CDR3 regions by quantitative sequencing and positive TCRγ rearrangement assays. Therefore, Vβ expansions can serve as surrogate markers of CTL clonality to assess clonal kinetics in T-LGLL. We analyzed the TCR repertoire in 143 patients, 71 of which were available for serial measurements over 6 to 96 months. Although the majority (38/71, 54%) maintained a consistent monoclonal expansion, many (26/71, 37%) unexpectedly displayed a change in the dominant clone, whereby the original CTL clone contracted and another emerged as demonstrated by Vβ typing. Our results demonstrate that the T-cell repertoire is more dynamic in T-LGLL than recognized previously, illustrating the heterogeneity of disorders under this categorization.

Soluble Fas and Fas ligand and prognosis in children with acute lymphoblastic leukemia

The soluble forms of Fas and its ligand (sFas and sFasL) correlate with disease progression in various malignancies. We compared serum levels of sFas and sFasL in children with acute lymphoblastic leukemia and healthy children to determine the prognostic significance of these molecules. Serum levels of sFas and sFasL were measured with an enzyme-linked immunosorbent assay in 48 patients with newly diagnosed childhood acute lymphoblastic leukemia and 38 healthy children. Cut-off values of sFas and sFasL levels were based on their levels in controls. Clinical and laboratory characteristics were recorded on admission. The mean serum concentration of sFas was 243 ± 40 pg/mL in patients and 238 ± 29 pg/mL in controls. Serum levels of sFasL were 4.33 ± 0.25 ng/mL in patients and 4.27 ± 0.11 ng/mL in controls. Neither difference was significant. Based on the cut-off value, 12.5% of the patients were positive for sFas, and 16.6% were positive for sFasL. Survival was significantly longer in sFasL-positive patients (394 ± 69.6 vs. 254 ± 24.3 days) and the duration of complete remission was also longer (380 ± 65.0 vs. 246 ± 26.0 days) than in sFasL-negative patients (P < 0.02), indicating the important role of this molecule in the response to therapy. Higher sFas levels were associated with hepatosplenomegaly (P < 0.047). In conclusion, sFasL positivity was associated with a favorable outcome in ALL patients.

How I treat LGL leukemia

Large granular lymphocyte (LGL) leukemia is characterized by a clonal expansion of either CD3(+) cytotoxic T or CD3(-) NK cells. Prominent clinical features of T-LGL leukemia include neutropenia, anemia and rheumatoid arthritis (RA). The terminal effector memory phenotype (CD3(+)/CD45RA(+)/CD62L(-)CD57(+)) of T-LGL suggests a pivotal chronic antigen-driven immune response. LGL survival is then promoted by platelet-derived growth factor and interleukin-15, resulting in global dysregulation of apoptosis and resistance to normal pathways of activation-induced cell death. These pathogenic features explain why treatment of T-LGL leukemia is based on immunosuppressive therapy. The majority of these patients eventually need treatment because of severe or symptomatic neutropenia, anemia, or RA. No standard therapy has been established because of the absence of large prospective trials. The authors use low-dose methotrexate initially for T-LGL leukemia patients with neutropenia and/or RA. We recommend either methotrexate or oral cyclophosphamide as initial therapy for anemia. If treatment is not successful, patients are switched to either the other agent or cyclosporine. The majority of patients experience an indolent clinical course. Deaths infrequently occur because of infections related to severe neutropenia. As there are no curative therapeutic modalities for T-LGL leukemia, new treatment options are needed.

The root of many evils: indolent large granular lymphocyte leukaemia and associated disorders

Large granular lymphocytes (LGL) leukaemia can arise from either natural killer (NK) cells or cytotoxic T lymphocytes (CTL). The T-cell form of LGL leukaemia has significant overlap with other haematological disorders and autoimmune diseases. Here we provide an overview of LGL biology. We also focus discussion on the indolent LGL leukaemia related disorders and their causal relationships. We then discuss the potential relationships and distinctions between indolent LGL leukaemia and non-malignant clonal lymphocyte expansion that occur in otherwise healthy individuals, especially elder people.

MICA polymorphism identified by whole genome array associated with NKG2D-mediated cytotoxicity in T-cell large granular lymphocyte leukemia

BACKGROUND

Large granular lymphocyte leukemia is a semi-autonomous clonal proliferation of cytotoxic T cells accompanied by immune cytopenias and various autoimmune conditions. Due to the rarity of this disease and its association with autoimmune diseases, a theoretical germline or somatic mutation might have significant penetrance, thus enabling detection, even from samples of suboptimal size, through genome-wide association studies.

DESIGN AND METHODS

To investigate a non-mendelian genetic predisposition to large granular lymphocyte leukemia, we used a step-wise method for gene discovery. First, a modified 'random forests' technique was used for candidate gene identification: this was followed by traditional allele-specific polymerase chain reaction, sequencing modalities, and mechanistic assays.

RESULTS

Our analysis found an association with MICA, a non-peptide-presenting, tightly regulated, stress-induced MHC-like molecule and cognate receptor for NKG2D, found abundantly on large granular lymphocyte leukemia cells. Sequencing of germline DNA revealed a higher frequency of MICA*00801/A5.1 in patients with large granular lymphocyte leukemia than in matched controls (64% versus 41%, P<0.001, homozygous 40% versus 15%, P<0.001). Flow cytometry was employed to determine the expression of MICA within hematologic compartments, showing that the signal intensity of MICA was increased in granulocytes from neutropenic patients with large granular lymphocyte leukemia in comparison with that in controls (P=0.033). Furthermore, neutrophil counts were inversely correlated with MICA expression (R(2)=0.50, P=0.035). Finally, large granular lymphocyte leukemia cells were able to selectively kill MICA(+) Ba/F3 lymphocytes transfected with human MICA*019 in a dose-dependent manner compared to naïve cells (P<0.001), an effect mitigated by administration of an anti-NKG2D antibody (P=0.033).

CONCLUSIONS

Our results illustrate that MICA-NKG2D played a role in disease pathogenesis in the majority of patients in our cohort of cases of large granular lymphocyte leukemia and further investigation into this signaling axis may provide potent therapeutic targets.

Never say die: survival signaling in large granular lymphocyte leukemia

Large granular lymphocyte (LGL) leukemia is a rare disorder of mature cytotoxic T or natural killer cells. Large granular lymphocyte leukemia is characterized by the accumulation of cytotoxic cells in blood and infiltration in the bone marrow, liver, and spleen. Herein, we review clinical features of LGL leukemia. We focus our discussion on known survival signals believed to play a role in the pathogenesis of LGL leukemia and their potential therapeutic implications.

Platelet-derived growth factor mediates survival of leukemic large granular lymphocytes via an autocrine regulatory pathway

Large granular lymphocyte (LGL) leukemia results from chronic expansion of cytotoxic T cells or natural killer (NK) cells. Apoptotic resistance resulting from constitutive activation of survival signaling pathways is a fundamental pathogenic mechanism. Recent network modeling analyses identified platelet-derived growth factor (PDGF) as a key master switch in controlling these survival pathways in T-cell LGL leukemia. Here we show that an autocrine PDGF regulatory loop mediates survival of leukemic LGLs of both T- and NK-cell origin. We found high levels of circulating PDGF-BB in platelet-poor plasma samples from LGL leukemia patients. Production of PDGF-BB by leukemic LGLs was demonstrated by immunocytochemical staining. Leukemic cells expressed much higher levels of PDGFR-beta transcripts than purified normal CD8(+) T cells or NK cells. We observed that phosphatidylinositol-3-kinase (PI3 kinase), Src family kinase (SFK), and downstream protein kinase B (PKB)/AKT pathways were constitutively activated in both T- and NK-LGL leukemia. Pharmacologic blockade of these pathways led to apoptosis of leukemic LGLs. Neutralizing antibody to PDGF-BB inhibited PKB/AKT phosphorylation induced by LGL leukemia sera. These results suggest that targeting of PDGF-BB, a pivotal regulator for the long-term survival of leukemic LGLs, may be an important therapeutic strategy.

A novel mouse model for the aggressive variant of NK cell and T cell large granular lymphocyte leukemia

Murine models of disease are vital to the understanding of pathogenesis and the development of novel therapeutics. We have previously established interleukin (IL)-15 transgenic (tg) mice that demonstrate rapid proliferation of natural killer (NK) and T cells, followed by spontaneous transformation to lethal leukemia. Herein, we have characterized this model, which has many features in common with the aggressive variants of NK and T large granular lymphocyte leukemia (LGLL) in humans. The LGLL blasts are cytolytic and produce IFN-gammaex vivo. Cytogenetic analysis revealed trisomy of chromosome 17 and/or 15. This model should provide opportunities to develop effective standard therapies for this fatal disease.

Regulation of Fas-mediated immune homeostasis by an activation-induced protein, Cyclon

Activation-induced cell death (AICD) plays an essential role in the contraction of activated T cells after eradication of pathogen. Fas (APO-1/CD95) is one of the key cell surface proteins that mediate AICD in CD4(+) and CD8(+) T cells. Despite its prime importance in cell death, regulation of Fas expression in T cells is poorly understood. Here we show that Cyclon, a newly identified cytokine-inducible protein, is induced in T cells on T-cell receptor ligation and important for immune homeostasis. Transgenic expression of Cyclon ameliorated autoimmune phenotype in mice lacking subunits of IL-2R. Transgenic expression of Cyclon markedly enhanced AICD through increased expression of Fas whose expression is essential for Cyclon action. Finally, we demonstrated that activated but not resting CD4(+) T cells with targeted deletion of a Cyclon allele show reduced AICD and expression of Fas, indicating a critical role of Cyclon in Fas expression in activated T cells. We think that our data provide insight into expression regulation of Fas in T cells.

Network model of survival signaling in large granular lymphocyte leukemia

T cell large granular lymphocyte (T-LGL) leukemia features a clonal expansion of antigen-primed, competent, cytotoxic T lymphocytes (CTL). To systematically understand signaling components that determine the survival of CTL in T-LGL leukemia, we constructed a T-LGL survival signaling network by integrating the signaling pathways involved in normal CTL activation and the known deregulations of survival signaling in leukemic T-LGL. This network was subsequently translated into a predictive, discrete, dynamic model. Our model suggests that the persistence of IL-15 and PDGF is sufficient to reproduce all known deregulations in leukemic T-LGL. This finding leads to the following predictions: (i) Inhibiting PDGF signaling induces apoptosis in leukemic T-LGL. (ii) Sphingosine kinase 1 and NFkappaB are essential for the long-term survival of CTL in T-LGL leukemia. (iii) NFkappaB functions downstream of PI3K and prevents apoptosis through maintaining the expression of myeloid cell leukemia sequence 1. (iv) T box expressed in T cells (T-bet) should be constitutively activated concurrently with NFkappaB activation to reproduce the leukemic T-LGL phenotype. We validated these predictions experimentally. Our study provides a model describing the signaling network involved in maintaining the long-term survival of competent CTL in humans. The model will be useful in identifying potential therapeutic targets for T-LGL leukemia and generating long-term competent CTL necessary for tumor and cancer vaccine development.

Molecular profiling of LGL leukemia reveals role of sphingolipid signaling in survival of cytotoxic lymphocytes

T-cell large granular lymphocyte (LGL) leukemia is characterized by clonal expansion of CD3(+)CD8(+) cells. Leukemic LGLs correspond to terminally differentiated effector-memory cytotoxic T lymphocytes (CTLs) that escape Fas-mediated activation-induced cell death (AICD) in vivo. The gene expression signature of peripheral blood mononuclear cells from 30 LGL leukemia patients showed profound dysregulation of expression of apoptotic genes and suggested uncoupling of activation and apoptotic pathways as a mechanism for failure of AICD in leukemic LGLs. Pathway-based microarray analysis indicated that balance of proapoptotic and antiapoptotic sphingolipid-mediated signaling was deregulated in leukemic LGLs. We further investigated sphingolipid pathways and found that acid ceramidase was constitutively overexpressed in leukemic LGLs and that its inhibition induced apoptosis of leukemic LGLs. We also showed that S1P(5) is the predominant S1P receptor in leukemic LGLs, whereas S1P(1) is down-regulated. FTY720, a functional antagonist of S1P-mediated signaling, induced apoptosis in leukemic LGLs and also sensitized leukemic LGLs to Fas-mediated death. Collectively, these results show a role for sphingolipid-mediated signaling as a mechanism for long-term survival of CTLs. Therapeutic targeting of this pathway, such as use of FTY720, may have efficacy in LGL leukemia.