Transplantable myeloproliferative disease induced in mice by an interleukin 6 retrovirus

Retroviral gene therapy in Germany with a view on previous experience and future perspectives

Gene therapy can be used to restore cell function in monogenic disorders or to endow cells with new capabilities, such as improved killing of cancer cells, expression of suicide genes for controlled elimination of cell populations, or protection against chemotherapy or viral infection. While gene therapies were originally most often used to treat monogenic diseases and to improve hematopoietic stem cell transplantation outcome, the advent of genetically modified immune cell therapies, such as chimeric antigen receptor modified T cells, has contributed to the increased numbers of patients treated with gene and cell therapies. The advancement of gene therapy with integrating retroviral vectors continues to depend upon world-wide efforts. As the topic of this special issue is "Spotlight on Germany," the goal of this review is to provide an overview of contributions to this field made by German clinical and research institutions. Research groups in Germany made, and continue to make, important contributions to the development of gene therapy, including design of vectors and transduction protocols for improved cell modification, methods to assess gene therapy vector efficacy and safety (e.g., clonal imbalance, insertion sites), as well as in the design and conduction of clinical gene therapy trials.

Learning from mouse models of MLL fusion gene-driven acute leukemia

5-10% of human acute leukemias carry chromosomal translocations involving the mixed lineage leukemia (MLL) gene that result in the expression of chimeric protein fusing MLL to >80 different partners of which AF4, ENL and AF9 are the most prevalent. In contrast to many other leukemia-associated mutations, several MLL-fusions are powerful oncogenes that transform hematopoietic stem cells but also more committed progenitor cells. Here, I review different approaches that were used to express MLL fusions in the murine hematopoietic system which often, but not always, resulted in highly penetrant and transplantable leukemias that closely phenocopied the human disease. Due to its simple and reliable nature, reconstitution of irradiated mice with bone marrow cells retrovirally expressing the MLL-AF9 fusion became the most frequently in vivo model to study the biology of acute myeloid leukemia (AML). I review some of the most influential studies that used this model to dissect critical protein interactions, the impact of epigenetic regulators, microRNAs and microenvironment-dependent signals for MLL fusion-driven leukemia. In addition, I highlight studies that used this model for shRNA- or genome editing-based screens for cellular vulnerabilities that allowed to identify novel therapeutic targets of which some entered clinical trials. Finally, I discuss some inherent characteristics of the widely used mouse model based on retroviral expression of the MLL-AF9 fusion that can limit general conclusions for the biology of AML. This article is part of a Special Issue entitled: The MLL family of proteins in normal development and disease edited by Thomas A Milne.

Retroviral Gene Transduction into T Cell Progenitors for Analysis of T Cell Development in the Thymus

The thymus is an organ where T cells develop throughout life. Using mice as a model animal, molecular mechanisms of intrathymic T cell development have been studied. Fetal thymus organ culture technique enables ex vivo reconstitution of fetal-specific T cell development, while bone marrow chimera technique allows in vivo reconstitution of T cell development in adult thymus. These techniques can be combined with retroviral gene transduction into the T cell progenitors to evaluate the function of genes of interest in developing T cells. Here, we describe the basic protocols for retrovirus gene transduction into fetal or adult T cell progenitors and reconstitution of thymic T cell development including experimental tips such as using cryopreserved fetal liver or bone marrow cells as sources of T cell progenitors.

In vivo selection with lentiviral expression of Bcl2T69A/S70A/S87A mutant in hematopoietic stem cell-transplanted mice

Current in vivo selections for hematopoietic stem cell (HSC)-based gene therapy are drug dependent and not without risk of cytotoxicity or tumorigenesis. We developed a new in vivo selection system with the non-phosphorylatable Bcl2 mutant Bcl2^{T69A/S70A/S87A} (Bcl2^AAA), which makes in vivo selection drug independent and without risk of cytotoxicity or tumorigenesis. We demonstrated in HSC-transplanted mice that Bcl2^AAA facilitated efficient in vivo selection in the absence of any exogenously applied drugs under both myeloablative and non-myeloablative conditioning. In mice transplanted with retrovirally transduced sca-1-positive bone marrow cells, the marked cell level increased from 26.38% of input transduced cells to 92.61 ± 0.95% of peripheral blood cells for myeloablative transplantation or to 37.82 ± 6.35% for non-myeloablative transplantation 6 months after transplantation. Bcl2^AAA did not induce tumorigenesis and does not influence hematopoiesis and the function of the reconstituted blood system. However, the high-level constitutive expression of Bcl2^AAA mediated by retroviral vector induced exhaustion of the marked cells after tertiary transplantation. Fortunately, low-level constitutive expression of Bcl2^AAA driven by an internal promoter in lentiviral vector could both maintain the marked cell level (24.13 ± 5.27%, 27.17 ± 5.51%, 24.33 ± 5.08%, and 22.07 ± 4.44% for primary, secondary, tertiary, and quaternary recipients) and avoid the exhaustion of the marked cells even in quaternary recipients. Importantly, the low-level constitutive expression of Bcl2^AAA did not induce tumorigenesis. Thus, the in vivo selection employing the low-level constitutive expression of Bcl2^AAA provides a general platform which is relevant for widespread applications of gene therapy.

Circuit Design Features of a Stable Two-Cell System

Cell communication within tissues is mediated by multiple paracrine signals including growth factors, which control cell survival and proliferation. Cells and the growth factors they produce and receive constitute a circuit with specific properties that ensure homeostasis. Here, we used computational and experimental approaches to characterize the features of cell circuits based on growth factor exchange between macrophages and fibroblasts, two cell types found in most mammalian tissues. We found that the macrophage-fibroblast cell circuit is stable and robust to perturbations. Analytical screening of all possible two-cell circuit topologies revealed the circuit features sufficient for stability, including environmental constraint and negative-feedback regulation. Moreover, we found that cell-cell contact is essential for the stability of the macrophage-fibroblast circuit. These findings illustrate principles of cell circuit design and provide a quantitative perspective on cell interactions.

Retroviral Transduction of T Cells and T Cell Precursors

Transduction of lymphoid progenitors with retroviral or lentiviral vectors is a powerful experimental strategy to tease out the role of a gene or pathway in T cell development via gain-of-function or loss-of-function strategies. Here we discuss different approaches to use this powerful technology, and present some protocols that we use to transduce murine HSCs, thymocytes, and lymphoid cell lines with these viral vectors.

Practical murine hematopathology: a comparative review and implications for research

Hematologic parameters are important markers of disease in human and veterinary medicine. Biomedical research has benefited from mouse models that recapitulate such disease, thus expanding knowledge of pathogenetic mechanisms and investigative therapies that translate across species. Mice in health have many notable hematologic differences from humans and other veterinary species, including smaller erythrocytes, higher percentage of circulating reticulocytes or polychromasia, lower peripheral blood neutrophil and higher peripheral blood and bone marrow lymphocyte percentages, variable leukocyte morphologies, physiologic splenic hematopoiesis and iron storage, and more numerous and shorter-lived erythrocytes and platelets. For accurate and complete hematologic analyses of disease and response to investigative therapeutic interventions, these differences and the unique features of murine hematopathology must be understood. Here we review murine hematology and hematopathology for practical application to translational investigation.

Deregulation of innate immune and inflammatory signaling in myelodysplastic syndromes

Myelodysplastic syndromes (MDSs) are a group of heterogeneous clonal hematologic malignancies that are characterized by defective bone marrow (BM) hematopoiesis and by the occurrence of intramedullary apoptosis. During the past decade, the identification of key genetic and epigenetic alterations in patients has improved our understanding of the pathophysiology of this disease. However, the specific molecular mechanisms leading to the pathogenesis of MDS have largely remained obscure. Recently, essential evidence supporting the direct role of innate immune abnormalities in MDS has been obtained, including the identification of multiple key regulators that are overexpressed or constitutively activated in BM hematopoietic stem and progenitor cells. Mounting experimental results indicate that the dysregulation of these molecules leads to abnormal hematopoiesis, unbalanced cell death and proliferation in patients' BM, and has an important role in the pathogenesis of MDS. Furthermore, there is compelling evidence that the deregulation of innate immune and inflammatory signaling also affects other cells from the immune system and the BM microenvironment, which establish aberrant associations with hematopoietic precursors and contribute to the MDS phenotype. Therefore, the deregulation of innate immune and inflammatory signaling should be considered as one of the driving forces in the pathogenesis of MDS. In this article, we review and update the advances in this field, summarizing the results from the most recent studies and discussing their clinical implications.

SHIP-1 deficiency in the myeloid compartment is insufficient to induce myeloid expansion or chronic inflammation

SHIP-1 has an important role in controlling immune cell function through its ability to downmodulate PI3K signaling pathways that regulate cell survival and responses to stimulation. Mice deficient in SHIP-1 display several chronic inflammatory phenotypes including antibody-mediated autoimmune disease, Crohn's disease-like ileitis and a lung disease reminiscent of chronic obstructive pulmonary disease. The ileum and lungs of SHIP-1-deficient mice are infiltrated at an early age with abundant myeloid cells and the mice have a limited lifespan primarily thought to be due to the consolidation of lungs with spontaneously activated macrophages. To determine whether the myeloid compartment is the key initiator of inflammatory disease in SHIP-1-deficient mice, we examined two independent strains of mice harboring myeloid-restricted deletion of SHIP-1. Contrary to expectations, conditional deletion of SHIP-1 in myeloid cells did not result in consolidating pneumonia or segmental ileitis typical of germline SHIP-1 deficiency. In addition, other myeloid cell abnormalities characteristic of germline loss of SHIP-1, including flagrant splenomegaly and enhanced myelopoiesis, were absent in mice lacking SHIP-1 in myeloid cells. This study indicates that the spontaneous inflammatory disease characteristic of germline SHIP-1 deficiency is not initiated solely by LysM-positive myeloid cells but requires the simultaneous loss of SHIP-1 in other hematolymphoid lineages.

MicroRNA-146a acts as a guardian of the quality and longevity of hematopoietic stem cells in mice

During inflammation and infection, hematopoietic stem and progenitor cells are stimulated to proliferate and differentiate into mature immune cells, especially of the myeloid lineage. MicroRNA-146a (miR-146a) is a critical negative regulator of inflammation. Deletion of miR-146a produces effects that appear as dysregulated inflammatory hematopoiesis, leading to a decline in the number and quality of hematopoietic stem cells (HSCs), excessive myeloproliferation, and, ultimately, to HSC exhaustion and hematopoietic neoplasms. At the cellular level, the defects are attributable to both an intrinsic problem in the miR-146a-deficient HSCs and extrinsic effects of lymphocytes and nonhematopoietic cells. At the molecular level, this involves a molecular axis consisting of miR-146a, signaling protein TRAF6, transcriptional factor NF-κB, and cytokine IL-6. This study has identified miR-146a to be a critical regulator of HSC homeostasis during chronic inflammation in mice and provided a molecular connection between chronic inflammation and the development of bone marrow failure and myeloproliferative neoplasms. DOI:http://dx.doi.org/10.7554/eLife.00537.001.

PML-RARα co-operates with Sox4 in acute myeloid leukemia development in mice

Acute promyelocytic leukemia is characterized by a chromosomal translocation involving the retinoic acid receptor alpha gene. To identify co-operating pathways to leukemogenesis, we crossed MRP8-PML/RARA transgenic mice with BXH-2 mice which harbor an endogenous murine leukemia virus that causes acute myeloid leukemia. Approximately half of the leukemias that arose in this cross showed features of acute promyelocytic leukemia. We identified 22 proviral insertion sites in acute promyelocytic-like leukemias and focused our analysis on insertion at Sox4, a HMG box transcription factor. Using a transplant model, co-operation between PML-RARα and Sox4 was confirmed with increased penetrance and reduced latency of disease. Interestingly, karyotypic analysis revealed cytogenetic changes suggesting that the factors combined to initiate but not complete leukemic transformation. The cooperation between these transcription factors is consistent with the paradigm of multiple routes to the disease and reinforces the concept that transcription factor networks are important therapeutic targets in myeloid leukemias.

C-terminal domain of MEIS1 converts PKNOX1 (PREP1) into a HOXA9-collaborating oncoprotein

The three-amino-acid loop extension (TALE) class homeodomain proteins MEIS1 and PKNOX1 (PREP1) share the ability to interact with PBX and HOX family members and bind similar DNA sequences but appear to play opposing roles in tumor development. Elevated levels of MEIS1 accelerate development of HOX- and MLL-induced leukemias, and this pro-tumorigenic property has been associated with transcriptional activity of MEIS1. In contrast, reduction of PKNOX1 levels has been linked with cancer development despite the absence of an identifiable transactivating domain. In this report, we show that a chimeric protein generated by fusion of the MEIS1 C-terminal region encompassing the transactivating domain with the full-length PKNOX1 (PKNOX1-MC) acquired the ability to accelerate the onset of Hoxa9-induced leukemia in the mouse bone marrow transduction/transplantation model. Gene expression profiling of primary bone marrow cells transduced with Hoxa9 plus Meis1, or Hoxa9 plus Pknox1-MC revealed perturbations in overlapping functional gene subsets implicated in DNA packaging, chromosome organization, and in cell cycle regulation. Together, results presented in this report suggest that the C-terminal domain of MEIS1 confers to PKNOX1 an ectopic transactivating function that promotes leukemogenesis by regulating expression of genes involved in chromatin accessibility and cell cycle progression.

Prdm14 initiates lymphoblastic leukemia after expanding a population of cells resembling common lymphoid progenitors

Understanding the heterogeneous genetic mechanisms of tumor initiation in lymphoid leukemias (LL) will lead to improvements in prognostic classification and treatment regimens. In previous studies of mouse leukemias, we showed that retroviral insertion at the ecotropic viral insertion site 32 locus leads to increased expression of Prdm14, a pluripotency gene implicated in the self-renewal capacity of embryonic stem cells and the early stages of breast cancer. Here, we show that PRDM14 is also overexpressed in ∼25% of human lymphoid neoplasms, with increased frequencies in T-cell acute LL and hyperdiploid precursor B-cell acute LL. To test if Prdm14 overexpression could initiate leukemia, mice were transduced with bone marrow cells transfected with a Prdm14 expression vector. LLs developed in 96% of female mice and 42% of male mice. Before the onset of leukemia, differentiation of transduced cells was biased up to 1000-fold toward cells with features of common lymphoid progenitors (CLPs), and lymphoid differentiation showed a relative block at the pro-B stage. Microarray gene expression analysis of expanded CLP-like cells before the onset of leukemia demonstrated upregulation of genes involved in pluripotency, tumor initiation, early B-lineage commitment, Wnt/Ras signaling and the epithelial-to-mesenchymal transition. Among the dysregulated genes were imprinted genes and non-coding RNAs including Dlk1 and Meg3, which are also key pluripotency mediators. Heightened expression of the estrogen-dependent oncogene, Myb, in tumors suggests a basis for the increased frequency of cancer in female mice. These data provide the first direct evidence for the association of Prdm14 with cancer initiation in an in vivo mouse model and in human lymphoid malignancies, while suggesting mechanisms for Prdm14's mode of action.

Genetic segregation of inflammatory lung disease and autoimmune disease severity in SHIP-1-/- mice

Alternatively activated M2 macrophages are implicated as both regulators and agents of lung disease, but their control is poorly understood. SHIP-1 is a 5' inositol phosphatase that negatively regulates the PI3K signaling pathway implicated in inflammation. SHIP-1-deficient mice have defects in hematopoiesis and B cell development, and die prematurely due to consolidation of lungs with M2-skewed macrophages. SHIP-1 is thought to restrain M2 macrophage polarization, with deregulated M2 skewing coinciding with severe lung disease in SHIP-1-deficient mice. To determine the influence of genetic background on the lung phenotype in SHIP-1(-/-) mice, we backcrossed the SHIP-1 null mutation onto C57BL/6 (Th2-resistant) and BALB/c (Th2-prone) backgrounds. Remarkably, we found that inflammatory lung disease was severe in C57.SHIP-1(-/-) mice, but absent in BALB.SHIP-1(-/-) mice. C57.SHIP-1(-/-), but not BALB.SHIP-1(-/-) mice had greatly increased myeloid progenitors, myeloid hyperplasia, markedly enhanced numbers of activated alveolar macrophages, and elevated amounts of Th2 and proinflammatory cytokines in bronchoalveolar lavage fluid and serum, suggesting that deregulated cytokine production induced disease. C57.SHIP-1(-/-) mice also developed severe B cell-dependent autoimmune disease, which was markedly attenuated on the BALB/c background. These data demonstrate that, contrary to current concepts, loss of SHIP-1 alone is not sufficient to cause lung inflammation, with disease only manifest on a permissive genetic background. This finding questions the nature of the lung disease in SHIP-1(-/-) mice, suggesting that its M2 classification is not strictly correct. Future identification of disease-promoting loci might reveal determinants of comorbid lung disease and autoimmunity and uncover potentially useful therapeutic targets.

Harnessing transposons for cancer gene discovery

Recently, it has become possible to mobilize the Tc1/mariner transposon, Sleeping Beauty (SB), in mouse somatic cells at frequencies high enough to induce cancer. Tumours result from SB insertional mutagenesis of cancer genes, thus facilitating the identification of the genes and signalling pathways that drive tumour formation. A conditional SB transposition system has also been developed that makes it possible to limit where SB mutagenesis occurs, providing a means to selectively model many types of human cancer. SB mutagenesis has already identified a large collection of known cancer genes in addition to a plethora of new candidate cancer genes and potential drug targets.

Innate immune signaling in the myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are heterogeneous clonal hematologic malignancies characterized by cytopenias caused by ineffective hematopoiesis and propensity to progress to acute myeloid leukemia. Innate immunity provides immediate protection against pathogens by coordinating activation of signaling pathways in immune cells. Given the prominent role of the innate immune pathway in regulating hematopoiesis, it is not surprising that aberrant signaling of this pathway is associated with hematologic malignancies. Increased activation of the innate immune pathway may contribute to dysregulated hematopoiesis, dysplasia, and clonal expansion in myelodysplastic syndromes.

Differences between newborn and adult mice in their response to immune thrombocytopenia

BACKGROUND

Sick neonates frequently develop severe thrombocytopenia.

OBJECTIVE AND METHODS

In order to test the ability of fetal mice to increase their megakaryocyte size and ploidy in response to thrombocytopenia, we injected an antiplatelet antibody (MWReg30) into pregnant mice daily for 7 days, and into nonpregnant adult mice to serve as controls. After that time, platelet counts were obtained and megakaryocytes in the bone marrow, liver, and spleen were stained with anti-von Willebrand factor antibody, individually measured, and quantified.

RESULTS

Our study demonstrated that megakaryocytopoiesis in newborn mice shares many features of human fetal/neonatal megakaryocytopoiesis, including the small size of megakaryocytes. In response to thrombocytopenia, adult mice increased megakaryocyte volume and concentration, primarily in the spleen. Newborn mice, in contrast, increased the megakaryocyte concentration in the spleen, but exhibited no increase in megakaryocyte volume in any of the organs studied. In fact, the megakaryocyte mass was significantly lower in the bone marrow of thrombocytopenic neonates than in age-matched controls.

CONCLUSIONS

We concluded that fetuses have a limited ability to increase their megakaryocyte mass in response to consumptive thrombocytopenia, compared to adult mice. These observations provide further evidence for the existence of biological differences between fetal/neonatal and adult megakaryocytopoiesis.

Identification of miR-145 and miR-146a as mediators of the 5q- syndrome phenotype

5q- syndrome is a subtype of myelodysplastic syndrome characterized by severe anemia and variable neutropenia but normal or high platelet counts with dysplastic megakaryocytes. We examined expression of microRNAs (miRNAs) encoded on chromosome 5q as a possible cause of haploinsufficiency. We show that deletion of chromosome 5q correlates with loss of two miRNAs that are abundant in hematopoietic stem/progenitor cells (HSPCs), miR-145 and miR-146a, and we identify Toll-interleukin-1 receptor domain-containing adaptor protein (TIRAP) and tumor necrosis factor receptor-associated factor-6 (TRAF6) as respective targets of these miRNAs. TIRAP is known to lie upstream of TRAF6 in innate immune signaling. Knockdown of miR-145 and miR-146a together or enforced expression of TRAF6 in mouse HSPCs resulted in thrombocytosis, mild neutropenia and megakaryocytic dysplasia. A subset of mice transplanted with TRAF6-expressing marrow progressed either to marrow failure or acute myeloid leukemia. Thus, inappropriate activation of innate immune signals in HSPCs phenocopies several clinical features of 5q- syndrome.

Autoimmune disease in Lyn-deficient mice is dependent on an inflammatory environment established by IL-6

Lyn-deficient mice develop Ab-mediated autoimmune disease resembling systemic lupus erythematosus where hyperactive B cells are major contributors to pathology. In this study, we show that an inflammatory environment is established in Lyn(-/-) mice that perturbs several immune cell compartments and drives autoimmune disease. Lyn(-/-) leukocytes, notably B cells, are able to produce IL-6, which facilitates hyperactivation of B and T cells, enhanced myelopoiesis, splenomegaly, and, ultimately, generation of pathogenic autoreactive Abs. Lyn(-/-) dendritic cells show increased maturation, but this phenotype is independent of autoimmunity as it is reiterated in B cell-deficient Lyn(-/-) mice. Genetic deletion of IL-6 on a Lyn-deficient background does not alter B cell development, plasma cell accumulation, or dendritic cell hypermaturation, suggesting that these characteristics are intrinsic to the loss of Lyn. However, hyperactivation of B and T cell compartments, extramedullary hematopoiesis, expansion of the myeloid lineage and autoimmune disease are all ameliorated in Lyn(-/-)IL-6(-/-) mice. Importantly, our studies show that although Lyn(-/-) B cells may be autoreactive, it is the IL-6-dependent inflammatory environment they engender that dictates their disease-causing potential. These findings improve our understanding of the mode of action of anti-IL-6 and B cell-directed therapies in autoimmune and inflammatory disease treatment.

A modified sleeping beauty transposon system that can be used to model a wide variety of human cancers in mice

Recent advances in cancer therapeutics stress the need for a better understanding of the molecular mechanisms driving tumor formation. This can be accomplished by obtaining a more complete description of the genes that contribute to cancer. We previously described an approach using the Sleeping Beauty (SB) transposon system to model hematopoietic malignancies in mice. Here, we describe modifications of the SB system that provide additional flexibility in generating mouse models of cancer. First, we describe a Cre-inducible SBase allele, RosaSBase(LsL), that allows the restriction of transposon mutagenesis to a specific tissue of interest. This allele was used to generate a model of germinal center B-cell lymphoma by activating SBase expression with an Aid-Cre allele. In a second approach, a novel transposon was generated, T2/Onc3, in which the CMV enhancer/chicken beta-actin promoter drives oncogene expression. When combined with ubiquitous SBase expression, the T2/Onc3 transposon produced nearly 200 independent tumors of more than 20 different types in a cohort of 62 mice. Analysis of transposon insertion sites identified novel candidate genes, including Zmiz1 and Rian, involved in squamous cell carcinoma and hepatocellular carcinoma, respectively. These novel alleles provide additional tools for the SB system and provide some insight into how this mutagenesis system can be manipulated to model cancer in mice.

Monitoring the autophagy pathway in cancer

Autophagy is an ancient cell survival pathway that is induced by metabolic stress and that helps prevent bioenergetic failure. This pathway has emerged as a promising new target in cancer treatment, where agents that inhibit autophagic degradation have efficacy in preventing cancer and in treating resistant disease when combined with conventional chemotherapeutics, which generally activate the pathway. However, agents that specifically target the autophagy pathway are currently lacking, and monitoring the effects of therapeutics on the autophagy pathway raises several challenges. Here we review the potential roles of the autophagy pathway in tumor progression and in maintenance of the malignant state, and introduce novel methods that we have developed that allow one to monitor autophagic activity ex vivo and in vivo.

The Lim-only protein LMO2 acts as a positive regulator of erythroid differentiation

LMO2, a member of the LIM-only protein family, is essential for the regulation of hematopoietic stem cells and formation of erythroid cells. It is found in a transcriptional complex comprising LMO2, TAL1, E47, GATA-1, and LDB1 which regulates erythroid genes. While TAL1 has been shown to induce erythroid differentiation, LMO2 appears to suppress fetal erythropoiesis. In addition to LMO2, the closely related LMO4 gene is expressed in hematopoietic cells, but has unknown functions. Here we demonstrate that LMO2 and LMO4 are expressed at the same level in erythroid colonies from mouse bone marrow, implying a function in erythroid differentiation. However, while LMO2 induced erythroid differentiation, LMO4 had no such effect. Interestingly, both LMO2 and TAL1 were able to partially suppress myeloid differentiation, implying that they activate erythroid differentiation in uncommitted bone marrow progenitors. Both LMO2 and LMO4 interacted strongly to LDB1, which was required for their localization to the nucleus.

The tyrosine phosphatase SHP-2 regulates differentiation and apoptosis of individual primary T lymphocytes

Although phosphatases are key players of intracellular processes, not much is known about the phosphatase SHP-2 during T cell differentiation. Here we show that ectopic over-expression of SHP-2 in primary T helper cells directly reduced the frequency of individual lymphocytes expressing pro-inflammatory cytokines after antigen-specific stimulation by a mechanism impairing activation of protein kinase C. In addition we demonstrate that SHP-2 mediates enhanced migration upon CXCR4 signaling in a G-protein-dependent manner. Most strikingly, SHP-2 mediated a dramatic increase in apoptosis by highly enhanced activation of caspases. Co-immunoprecipitations of SHP-2 and c-Cbl from primary T helper cells demonstrated that SHP-2 strongly interacts with the ubiquitin ligase c-Cbl, indicating that c-Cbl could mediate the negative signals of SHP-2. Our results show that SHP-2 signal transduction regulates central checkpoints of T cell differentiation by the activation of distinct signaling cascades.

Phosphorylation of Bcl10 negatively regulates T-cell receptor-mediated NF-kappaB activation

Bcl10 (B-cell lymphoma 10) is an adaptor protein comprised of an N-terminal caspase recruitment domain and a C-terminal serine/threonine-rich domain. Bcl10 plays a critical role in antigen receptor-mediated NF-kappaB activation and lymphocyte development and functions. Our current study has discovered that T-cell activation induced monophosphorylation and biphosphorylation of Bcl10 and has identified S138 within Bcl10 as one of the T-cell receptor-induced phosphorylation sites. Alteration of S138 to an alanine residue impaired T-cell activation-induced ubiquitination and subsequent degradation of Bcl10, ultimately resulting in prolongation of TCR-mediated NF-kappaB activation and enhancement of interleukin-2 production. Taken together, our findings demonstrate that phosphorylation of Bcl10 at S138 down-regulates Bcl10 protein levels and thus negatively regulates T-cell receptor-mediated NF-kappaB activation.

Early growth response transcriptional regulators are dispensable for macrophage differentiation

Early growth response (Egr) proteins comprise a family of transcriptional regulators (Egr1-4) that modulate gene expression involved in the growth and differentiation of many cell types. In particular, Egr1 is widely believed to have an essential role in regulating monocyte/macrophage differentiation. However, Egr1-deficient mice have normal numbers of functional macrophages, an observation that has led to the hypothesis that other Egr proteins may compensate for Egr1 function in vivo. We examined whether other Egr transcription factors have a functionally redundant role in monocyte/macrophage differentiation. Egr1 and Egr3 expression was found to be induced in myeloid cells when they were differentiated into macrophages by treatment with M-CSF, whereas Egr2 was minimally induced and Egr4 was not detected. In either Egr1/Egr3 or Egr1/Egr2 double homozygous mutant mice, macrophage differentiation and function remained unimpaired. Additionally, the expression of molecules that broadly inhibit Egr function failed to block commitment to the monocytic lineage or inhibit the maturation of monocyte precursors. Finally, several hemopoietic growth factors were found to induce Egr gene expression, indicating that Egr gene expression is not cell lineage specific. Taken together, these results demonstrate that Egr transcription factors are neither essential for nor specific to monocyte/macrophage differentiation.

Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine X-linked severe combined immunodeficiency model

A retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) was used to mark and dynamically follow vector-expressing cells in the peripheral blood of bone marrow transplanted X-linked severe combined immunodeficient dogs. CD34(+) cells isolated from young normal dogs were transduced, using a 2 day protocol, with an amphotropic retroviral vector that expressed enhanced green fluorescent protein (EGFP) and the canine common gamma chain (gammac) cDNAs. Following transplantation of the transduced cells, normal donor peripheral blood lymphocytes (PBL) appeared by 1 month post-bone marrow transplant (BMT) and rescued three of five treated dogs from their lethal immunodeficiency. PCR and flow cytometric analysis of post-BMT PBL documented the peripheral EGFP expressing cells as CD3(+) T cells, which varied from 0% to 28%. Sorting of EGFP(+) and EGFP(-) peripheral blood T cells from two dogs, followed by vector PCR analysis, showed no evidence of vector shutdown. EGFP expression in B cells or monocytes was not detected. These marking experiments demonstrate that the transduction protocol did not abolish the lymphoid engraftment capability of ex vivo transduced canine CD34(+) cells and supports the potential utility of the MSCV retroviral vector for gene transfer to XSCID affected canine hematopoietic progenitor cells (HPC).

Phosphatidylinositol 3-kinase/Akt signaling mediates interleukin-6 protection against p53-induced apoptosis in M1 myeloid leukemic cells

M1 myeloid leukemic cells were used to dissect the molecular mechanisms of myeloid cell survival and apoptosis. A salient feature of M1 cells is that they respond to the physiological survival factor interleukin-6 (IL-6), yet lack the tumor suppressor gene p53. Functional wild-type activation of temperature-sensitive p53 protein (p53 val) at permissive temperature in M1-t-p53 cells results in rapid apoptosis, which is blocked by IL-6. How p53 induces M1 apoptosis and how IL-6 protects against p53-induced apoptosis are not fully understood. Here it is shown that p53-mediated apoptosis of M1 cells involves rapid activation of the proapoptotic Fas/CD95 death pathway, which activates caspases 8 and 10. Functional p53 also targets the mitochondria, causing upregulation of proapoptotic Bax, downregulation of prosurvival Bcl-2 and activation of caspase 9. IL-6 was found to protect against p53-induced apoptosis via activation of the PI3K/Akt survival pathway, which in turn counters both the Fas/CD95 and mitochondrial apoptotic pathways and activates the prosurvival transcription factor nuclear factor-kappaB (NF-kappaB). Taken together, this work supports a novel model for leukemic progression where cells that acquire the ability to produce an autocrine survival factor, such as IL-6, can bypass normal p53 surveillance function by targeting Akt, which in turn can exert effects on the regulators of apoptosis, such as the Fas/CD95 pathway, the mitochondria and NF-kappaB.

Stable gammaretroviral vector expression during embryonic stem cell-derived in vitro hematopoietic development

Unlike conventional gammaretroviral vectors, the murine stem cell virus (MSCV) can efficiently express transgenes in undifferentiated embryonic stem cells (ESCs). However, a dramatic extinction of expression is observed when ESCs are subjected to in vitro hematopoietic differentiation. Here we report the construction of a self-inactivating vector from MSCV, MSinSB, which transmits an intron embedded within the internal transgene cassette to transduced cells. The internal transgene transcriptional unit in MSinSB comprises the composite cytomegalovirus immediate early enhancer-chicken beta-actin promoter and associated 5' splice site positioned upstream of the natural 3' splice site of the gammaretroviral envelope gene, and is configured such that the transgene translational initiation sequence is coincident with the envelope ATG. MSinSB could be produced at titers approaching 10(6) transducing units/ml and directed higher levels of transgene expression in ESCs than a splicing-optimized MSCV-derived vector, MSGV1. Moreover, when transduced ESCs were differentiated into hematopoietic cells in vitro, MSinSB remained transcriptionally active in greater than 90% of the cells, whereas MSGV1 expression was almost completely shut off. Persistent high-level expression of the MSinSB gammaretroviral vector was also demonstrated in murine bone marrow transplant recipients and following in vitro myelomonocytic differentiation of human CD34(+) cord blood stem/progenitor cells.

IAN family critically regulates survival and development of T lymphocytes

The IAN (immune-associated nucleotide-binding protein) family is a family of functionally uncharacterized GTP-binding proteins expressed in vertebrate immune cells and in plant cells during antibacterial responses. Here we show that all eight IAN family genes encoded in a single cluster of mouse genome are predominantly expressed in lymphocytes, and that the expression of IAN1, IAN4, and IAN5 is significantly elevated upon thymic selection of T lymphocytes. Gain-of-function experiments show that the premature overexpression of IAN1 kills immature thymocytes, whereas short hairpin RNA-mediated loss-of-function studies show that IAN4 supports positive selection. The knockdown of IAN5 perturbs the optimal generation of CD4/CD8 double-positive thymocytes and reduces the survival of mature T lymphocytes. We also show evidence suggesting that IAN4 and IAN5 are associated with anti-apoptotic proteins Bcl-2 and Bcl-xL, whereas IAN1 is associated with pro-apoptotic Bax. Thus, the IAN family is a novel family of T cell-receptor-responsive proteins that critically regulate thymic development and survival of T lymphocytes and that potentially exert regulatory functions through the association with Bcl-2 family proteins.

Transgene Expression and RNA Interference in Embryonic Stem Cells

Over the last 30 years of biomedical research, technologies for transgene expression and gene loss of function have been developed from animal model systems to human gene therapy, and, increasingly, embryonic stem cells are a key target for these genetic engineering strategies. In this chapter, we describe how retrovirus/lentivirus vectors can be used to transfer and stably express genes or small interfering RNAs in mouse and human embryonic stem cells and their progeny, thereby allowing genetic gain-of-function or loss-of-function analysis and cell lineage selection in a multitude of experimental settings.

Insertional mutagenesis identifies genes that promote the immortalization of primary bone marrow progenitor cells

Retroviruses can induce hematopoietic disease via insertional mutagenesis of cancer genes and provide valuable molecular tags for cancer gene discovery. Here we show that insertional mutagenesis can also identify genes that promote the immortalization of hematopoietic cells, which normally have only limited self-renewal. Transduction of mouse bone marrow cells with replication-incompetent murine stem cell virus (MSCV) expressing only neo, followed by serial passage in liquid culture containing stem cell factor (SCF) and interleukin-3 (IL-3), produced immortalized immature myeloid cell lines with neutrophil and macrophage differentiation potential in about 50% of the infected cultures. More than half of the lines have MSCV insertions at Evi1 or Prdm16. These loci encode transcription factor homologs and are validated human myeloid leukemia genes. Integrations are located in intron 1 or 2, where they promote expression of truncated proteins lacking the PRDI-BF1-RIZ1 homologous (PR) domain, similar to what is observed in human leukemias with EVI1 or PRDM16 mutations. Evi1 overexpression alone appears sufficient to immortalize immature myeloid cells and does not seem to require any other cooperating mutations. Genes identified by insertional mutagenesis by their nature could also be involved in immortalization of leukemic stem cells, and thus represent attractive drug targets for treating cancer.

HOXB4 enforces equivalent fates of ES-cell-derived and adult hematopoietic cells

Genetic manipulation of hematopoietic stem and progenitor cells is an important tool for experimental and clinical applied hematology. However, techniques that allow for gene targeting, subsequent in vitro selection, and expansion of genetically defined clones are available only for ES cells. Such molecularly defined and, hence, "safe" clones would be highly desirable for somatic gene therapy. Here, we demonstrate that in vitro differentiated ES cells completely recapitulate the growth and differentiation properties of adult bone marrow cells, in vitro and in vivo, when ectopically expressing HOXB4. Myeloid development was enforced and (T) lymphoid development suppressed over a wide range of expression levels, whereas only high expression levels of the transcription factor were detrimental for erythroid development. This indicates a close association between the amounts of ectopic HOXB4 present within a progenitor cell and and the decision to self renew or differentiate. Because HOXB4 mediates similar fates of ES-derived and bone marrow hematopoietic stem cells, the primitive embryonic cells can be considered a promising alternative for investigating hematopoietic reconstitution, in vivo, based on well defined clones. Provided that HOXB4 levels are kept within a certain therapeutic window, ES cells also carry the potential of efficient and safe somatic gene therapy.

The therapeutic potential of interleukin-6 hyperagonists and antagonists

Interleukin-6 (IL-6) is a 4-helical protein that binds to a specific IL-6 receptor on target cells and to two molecules of the promiscuous signal transducing protein, glycoprotein 130 (gp130). Structure-function analysis has led to the definition of molecular contacts between IL-6 and its receptor subunits. This knowledge has led to the design of competitive antagonistic proteins that retain their receptor binding capability, but fail to stimulate one or both gp130 proteins; the properties of such recombinant antagonistic proteins are compared with traditional neutralising monoclonal antibodies targeted at IL-6 or receptor subunits. Furthermore, several strategies have been employed to construct molecules with increased bioactivity. Possible therapeutic applications in putative IL-6 dependent haematologic disorders, e.g., Castleman's disease (CD), POEMS syndrome, multiple myeloma, and bone diseases, e.g., Paget's disease, osteoporosis, are outlined. IL-6 antagonists could also, in theory, suppress inflammatory activity in rheumatic and autoimmune diseases and could prevent secondary amyloidosis. This principle may prove advantageous in myocardial infarction (MI) and unstable angina pectoris. More generally, IL-6 antagonists could improve the wasting and microcytic anaemia of chronic diseases. IL-6 antagonists might slow down development of mesangio-proliferative glomerulonephritis (MPGN). Hyperagonistic variants of IL-6 have a potential use in the ex vivo expansion of haematopoietic progenitor cells and as thrombopoietic agents. They might well be the first drugs to aid liver regeneration in vivo.

IL-6 levels predict disease variant and extent of organ involvement in patients with mastocytosis

Mastocytosis is often associated with organ involvement and hematological disorders. Patients may also exhibit elevated levels of plasma IL-6. To gain insight into the relevance of this observation, we correlated plasma levels of IL-6 and soluble IL-6 receptor (sIL-6R) with multiple disease parameters in 29 patients with mastocytosis. Mean plasma IL-6 levels were elevated in patients compared to healthy controls (P < 0.0001). Disease category significantly correlated with plasma IL-6 levels, as did severity of bone marrow pathology, organomegaly, and extent of skin involvement. In plasma, there was a positive correlation of IL-6 to total tryptase, alkaline phosphatase, IgM, white blood cell count, prothrombin time, partial thromboplastin time, and neutrophil numbers. There was an inverse correlation to hemoglobin. sIL-6R levels were not elevated. These observations demonstrate that IL-6 is a useful surrogate marker of severity of hematologic disease and suggest that IL-6 contributes to pathology.

Overexpression of protein kinase C-{epsilon} in the mouse epidermis leads to a spontaneous myeloproliferative-like disease

Protein kinase C (PKC)-epsilon, a Ca(2+)-independent, phospholipid-dependent serine/threonine kinase, is among the PKC isoforms expressed in mouse epidermis. We reported that FVB/N transgenic mouse lines that overexpress (8- or 18-fold) PKC-epsilon protein in basal epidermal cells and cells of the hair follicle develop papilloma-independent squamous cell carcinoma (SCC) elicited by 7,12-dimethylbenz(a)anthracene initiation and 12-O-tetradecanoylphorbol-13-acetate-promotion or by repeated ultraviolet radiation exposures. The susceptibility to the development of SCC was proportional to the level of expression of the PKC-epsilon transgene. We now report that PKC-epsilon FVB/N transgenic mice (line 215) that overexpress in epidermis approximately 18-fold PKC-epsilon protein more than their wild-type littermates spontaneously develop a myeloproliferative-like disease (MPD) in 100% of PKC-epsilon transgenic mice. The MPD was characterized by an excess of neutrophils and eosinophils, resulting in invasion of almost all vital organs of the mouse by 6 months of age. On gross examination these mice present with splenomegaly, hepatomegaly, and severe lymphadenopathy. Examination of the bone marrow revealed almost complete effacement by neutrophils, eosinophils, and their precursors. Furthermore, the spleen and lymph nodes were enlarged and exhibited marked extramedullary hematopoiesis. Complete pathological analysis of the second PKC-epsilon transgenic mouse (line 224) that expresses approximately eightfold PKC-epsilon protein more than their wild-type littermates revealed no remarkable findings in any of the affected organs as seen in line 215. However, peripheral blood analyses of PKC-epsilon transgenic mice indicated significant increases of neutrophils in the circulating blood in both PKC-epsilon transgenic lines. To determine whether there was an imbalance of cytokines in PKC-epsilon transgenic mice (line 215), resulting in aberrant myelopoiesis, we analyzed 17 cytokines in the peripheral blood. This analysis indicated that interleukin-5, interleukin-6, and granulocyte-colony stimulating factor were up-regulated as a function of age. The transgene PKC-epsilon was not detected in any of the affected organs (bone marrow, liver, spleen, lung) We suggest that overexpression of PKC-epsilon in the epidermis may lead to the induction of specific cytokines that may, in a paracrine mechanism, perturb normal hematopoiesis in bone marrow resulting in a granulocytic skew toward that of neutrophils and eosinophils. The susceptibility of PKC-epsilon transgenic mice to the induction of SCC and the spontaneous development of MPD are unrelated.

A direct binding site for Grb2 contributes to transformation and leukemogenesis by the Tel-Abl (ETV6-Abl) tyrosine kinase

A direct binding site for the Grb2 adapter protein is required for the induction of fatal chronic myeloid leukemia (CML)-like disease in mice by Bcr-Abl. Here, we demonstrate direct binding of Grb2 to the Tel-Abl (ETV6-Abl) fusion protein, the product of complex (9;12) chromosomal translocations in human leukemia, via tyrosine 314 encoded by TEL exon 5. A Tel-Abl point mutant (Y314F) and a splice variant without TEL exon 5 sequences (Deltae5) lacked Grb2 interaction and exhibited decreased binding and phosphorylation of the scaffolding protein Gab2 and impaired activation of phosphatidylinositol 3-kinase, Akt, and extracellular signal-regulated kinase/mitogen-activated protein kinase in hematopoietic cells. Tel-Abl Y314F and Deltae5 were unable to transform fibroblasts to anchorage-independent growth and were defective for B-lymphoid transformation in vitro and lymphoid leukemogenesis in vivo. Previously, we demonstrated that full-length Tel-Abl induced two distinct myeloproliferative diseases in mice: CML-like leukemia similar to that induced by Bcr-Abl and a novel syndrome of small-bowel myeloid infiltration endotoxemia and hepatic and renal failure. Lack of the Grb2 binding site had no effect on development of small bowel syndrome but significantly attenuated the induction of CML-like disease by Tel-Abl. These results suggest that direct binding of Grb2 is a common mechanism contributing to leukemogenesis by oncogenic Abl fusion proteins.

A congenital mutation of the novel gene LRRC8 causes agammaglobulinemia in humans

A girl with congenital agammaglobulinemia and minor facial anomalies lacked B cells in peripheral blood: karyotypic analysis of white blood cells showed balanced translocation, t(9;20)(q33.2;q12). In the current study, we isolated a novel gene, leucine-rich repeat-containing 8 (LRRC8), at the translocation site on chromosome 9. It has four transmembrane helices with one isolated and eight sequentially located leucine-rich repeats (LRRs) and constitutes a new protein family. It is expressed on T cells as well as on B-lineage cells. Translocation truncates the LRRC8 gene, resulting in deletion of the eighth, ninth, and half of the seventh LRR domains located close to the C-terminal. The truncated form of the LRRC8 gene is transcribed with sequences from the noncoding region adjacent to the truncated seventh LRR. Protein products derived from the truncated gene are coexpressed on white blood cells with the intact LRRC8 protein from the untranslocated allele. Transplantation experiments with murine bone marrow cells that were forced to express the truncated LRRC8 show that expression of the truncated protein inhibited B cell development. These results indicate that LRRC8 is responsible for the B cell deficiency in this patient and is required for B cell development.

Itk phosphorylation sites are required for functional activity in primary T cells

The Tec family kinase Itk plays a critical role in signal transduction downstream of the T cell antigen receptor and has been implicated in the activation of phospholipase C-gamma1, a key regulator of calcium mobilization and extracellular signal-regulated kinase (ERK) activation. We have shown previously that Itk is regulated by an activating transphosphorylation event in which Tyr-511 in the kinase domain is phosphorylated by Lck (Heyeck, S. D., Wilcox, H. M., Bunnell, S. C., and Berg, L. J. (1997) J. Biol. Chem. 272, 25401-25408). In this study, we present evidence for another mode of regulation for Itk, the autophosphorylation of Tyr-180 in the Src homology 3 (SH3) domain. To investigate the role of Itk trans- and autophosphorylation in T cell signaling, a retroviral transduction system was used to introduce different versions of Itk into Itk-deficient primary T cells. We report that Itk mutated at either the trans- or the autophosphorylation site is unable to fully restore cytokine production and ERK activation in the Itk-deficient cells; Itk-Y511F is severely defective, whereas Itk-Y180F has partial activity. Because phosphorylation at Tyr-180 is predicted to interfere with ligand binding by the SH3 domain, an SH3 point mutant that cannot bind ligand was also examined and found to be unable to restore function to the Itk-/- cells. These data provide new insights into the complex regulation of Itk in primary T cells.

An activated receptor tyrosine kinase, TEL/PDGFbetaR, cooperates with AML1/ETO to induce acute myeloid leukemia in mice

The t(8;21)(q22;q22) translocation, occurring in 40% of patients with acute myeloid leukemia (AML) of the FAB-M2 subtype (AML with maturation), results in expression of the RUNX1-CBF2T1 [AML1-ETO (AE)] fusion oncogene. AML/ETO may contribute to leukemogenesis by interacting with nuclear corepressor complexes that include histone deacetylases, which mediate the repression of target genes. However, expression of AE is not sufficient to transform primary hematopoietic cells or cause disease in animals, suggesting that additional mutations are required. Activating mutations in receptor tyrosine kinases (RTK) are present in at least 30% of patients with AML. To test the hypothesis that activating RTK mutations cooperate with AE to cause leukemia, we transplanted retrovirally transduced murine bone marrow coexpressing TEL-PDGFRB and AE into lethally irradiated syngeneic mice. These mice (19/19, 100%) developed AML resembling M2-AML that was transplantable in secondary recipients. In contrast, control mice coexpressing with TEL-PDGFRB and a DNA-binding-mutant of AE developed a nontransplantable myeloproliferative disease identical to that induced by TEL-PDGFRB alone. We used this unique model of AML to test the efficacy of pharmacological inhibition of histone deacetylase activity by using trichostatin A and suberoylanilide hydroxamic acid alone or in combination with the tyrosine kinase inhibitor, imatinib mesylate. We found that although imatinib prolonged the survival of treated mice, histone deacetylase inhibitors provided no additional survival benefit. These data demonstrate that an activated RTK can cooperate with AE to cause AML in mice, and that this system can be used to evaluate novel therapeutic strategies.

Myeloproliferative disease in transgenic mice expressing P230 Bcr/Abl: longer disease latency, thrombocytosis, and mild leukocytosis

P230 Bcr/Abl has been associated with indolent myeloproliferative disease (MPD). We generated transgenic mice expressing P230Bcr/Abl driven by the promoter of the long terminal repeat of the murine stem cell virus of the MSCV neo P230 BCR/ABL vector. Two founder mice exhibited mild granulocytosis and marked thrombocytosis and developed MPD. The disease of one founder mouse, no. 13, progressed to extramedullary myeloblastic crisis in the liver at 12 months old. The other founder mouse, no. 22, was found to have chronic-phase MPD with large populations of megakaryocytes and granulocytes in an enlarged spleen. The transgenic progeny of no. 22 clearly exhibited MPD at 15 months old. These results showed that P230Bcr/Abl had leukemogenic properties and induced MPD. The phenotype of the MPD caused by P230Bcr/Abl was characterized by mild granulocytosis, a high platelet count, infiltration of megakaryocytes in some organs, and a longer disease latency compared with the MPD caused by P210Bcr/Abl.

A role for Wnt signalling in self-renewal of haematopoietic stem cells

Haematopoietic stem cells (HSCs) have the ability to renew themselves and to give rise to all lineages of the blood; however, the signals that regulate HSC self-renewal remain unclear. Here we show that the Wnt signalling pathway has an important role in this process. Overexpression of activated beta-catenin expands the pool of HSCs in long-term cultures by both phenotype and function. Furthermore, HSCs in their normal microenvironment activate a LEF-1/TCF reporter, which indicates that HCSs respond to Wnt signalling in vivo. To demonstrate the physiological significance of this pathway for HSC proliferation we show that the ectopic expression of axin or a frizzled ligand-binding domain, inhibitors of the Wnt signalling pathway, leads to inhibition of HSC growth in vitro and reduced reconstitution in vivo. Furthermore, activation of Wnt signalling in HSCs induces increased expression of HoxB4 and Notch1, genes previously implicated in self-renewal of HSCs. We conclude that the Wnt signalling pathway is critical for normal HSC homeostasis in vitro and in vivo, and provide insight into a potential molecular hierarchy of regulation of HSC development.

A model of APL with FLT3 mutation is responsive to retinoic acid and a receptor tyrosine kinase inhibitor, SU11657

The PML-RAR alpha fusion protein is central to the pathogenesis of acute promyelocytic leukemia (APL). Expression of this protein in transgenic mice initiates myeloid leukemias with features of human APL, but only after a long latency (8.5 months in MRP8 PML-RARA mice). Thus, additional changes contribute to leukemic transformation. Activating mutations of the FLT3 receptor tyrosine kinase are common in human acute myeloid leukemias and are frequent in human APL. To assess how activating mutations of FLT3 contribute to APL pathogenesis and impact therapy, we used retroviral transduction to introduce an activated allele of FLT3 into control and MRP8 PML-RARA transgenic bone marrow. Activated FLT3 cooperated with PML-RAR alpha to induce leukemias in 62 to 299 days (median latency, 105 days). In contrast to the leukemias that arose spontaneously in MRP8 PML-RARA mice, the activated FLT3/PML-RAR alpha leukemias were characterized by leukocytosis, similar to human APL with FLT3 mutations. Cytogenetic analysis revealed clonal karyotypic abnormalities, which may contribute to pathogenesis or progression. SU11657, a selective, oral, multitargeted tyrosine kinase inhibitor that targets FLT3, cooperated with all-trans retinoic acid to rapidly cause regression of leukemia. Our results suggest that the acquisition of FLT3 mutations by cells with a pre-existing t(15;17) is a frequent pathway to the development of APL. Our findings also indicate that APL patients with FLT3 mutations may benefit from combination therapy with all-trans retinoic acid plus an FLT3 inhibitor.

The spleen is a major site of megakaryopoiesis following transplantation of murine hematopoietic stem cells

The stem cell pool can be fractionated by using the mitochondrial dye, rhodamine-123, into Rho(low) hematopoietic stem cells and Rho(high) progenitors. Rho(low) stem cells permanently engraft all lineages, whereas Rho(high) progenitors transiently produce erythrocytes, without substantial platelet or granulocyte production. We hypothesized that the inability of the Rho(high) cells to produce platelets in vivo was due to the fact that these cells preferentially engraft in the spleen and lack marrow engraftment. Initially, we demonstrated that Rho(high) progenitors produced more megakaryocytes in vitro than Rho(low) stem cells did. To study the activity of the Rho(low) and Rho(high) subsets in vivo, we used mice allelic at the hemoglobin and glucose phosphate isomerase loci to track donor-derived erythropoiesis and thrombopoiesis. Rho(low) stem cells contributed to robust and long-term erythroid and platelet engraftment, whereas Rho(high) progenitors contributed only to transient erythroid engraftment and produced very low numbers of platelets in vivo. Donor-derived megakaryopoiesis occurred at higher densities in the spleen than in the bone marrow in animals receiving Rho(low) stem cells and peaked around day 28. Blockade of splenic engraftment using pertussis toxin did not affect the peak of splenic megakaryopoiesis, supporting the hypothesis that these megakaryocytes were derived from progenitors that originated in the bone marrow. These data emphasize that in vitro behavior of hematopoietic progenitor cell subsets does not always predict their behavior following transplantation. This study supports a major role for the spleen in thrombopoiesis following engraftment of transplanted stem cells in irradiated mice.

The role of transsignalling via the agonistic soluble IL-6 receptor in human diseases

The activation of cells that do not express the membrane bound interleukin-6 6 receptor (IL-6R) by IL-6 and the soluble IL-6 receptor (sIL-6R) is termed transsignalling. Transsignalling may be an pathogenetic factor in human diseases as diverse as multiple myeloma (MM), Castleman's disease, prostate carcinoma, Crohn's disease, systemic sclerosis, Still's disease, osteoporosis and cardiovascular diseases. IL-6 and sIL-6R may directly or indirectly enhance their own production on endothelial or bone marrow stromal cells. Positive feedback autocrine loops thus created in affected organs may either cause or maintain disease progression. In autoimmune or vasculitic disease, the ability of the IL-6/sIL-6R complex to inhibit apoptosis of autoreactive T-cells may be central to the development of tissue specific autoimmunity. The anti-apoptotic effect of the IL-6/sIL-6R complex may be involved in tumour genesis and resistance to chemotherapy. Only in rare cases, where counterregulation has failed, there is a notable systemic effect of IL-6/sIL-6R. Appropriate animal models are necessary to establish the pathogenetic role of the IL-6/sIL-6R complex. A specific treatment option for diseases influenced by the sIL-6R could be based on gp130-Fc, a soluble gp130 (sgp130) linked to the Fc-fragment of IgG1. gp130-Fc has shown efficacy in vivo in animal models of Crohn's disease.

Increased glucocorticoid receptor Beta expression converts mouse hybridoma cells to a corticosteroid-insensitive phenotype

Glucocorticoid (GC) insensitivity is a challenging clinical problem associated with many chronic inflammatory disorders and life-threatening disease progression. The molecular basis of GC insensitivity, however, is unknown. Alternative splicing of the GC receptor (GCR) pre-mRNA generates a second GCR, termed GCRbeta, which does not bind GC but antagonizes the transactivating activity of the classic GCR, termed GCRalpha. GC-insensitive conditions have been associated with increased GCRbeta expression. Whether or not increased GCRbeta expression can contribute to GC insensitivity, however, remains controversial. To more precisely demonstrate the effect of GCRbeta on steroid responsiveness, we virally transduced GCRbeta cDNA into mouse DO-11.10 hybridoma cells, as mice are known to be deficient in the GCRbeta gene. We demonstrate that viral transduction of GCRbeta cDNA into mouse hybridoma cells to induce stable expression of GCRbeta results in GC insensitivity of these cells. Furthermore, in such cells GCRalpha is complexed with GCRbeta. Such heterodimer formation may account for the reduced effectiveness of GC action in cells overexpressing GCRbeta.

Kinetics of fluorescence expression in nonhuman primates transplanted with GFP retrovirus-modified CD34 cells

Downregulation and loss of proviral expression have been demonstrated to occur in a variety of in vitro studies and in mouse models. Here we evaluated the kinetics of proviral expression after transplantation in a competitive repopulating model in the baboon. Transgene persistence and green fluorescent protein (GFP) expression in peripheral blood leukocytes (PBL) were analyzed in four animals by semiquantitative PCR and flow cytometry for up to 80 weeks (range 17-80). All animals were transplanted with cells transduced with EGFP or EYFP reporters driven by Moloney murine leukemia virus (MoMuLV) or a modified promoter/enhancer, (MND) respectively. Simultaneous dual-color analysis of fluorescence levels in granulocyte and lymphocyte subsets following hematopoietic reconstitution demonstrated progressive loss of fluorescence intensity occurring predominantly early after transplant in cells transduced with both retrovirus backbones and at serial time points. In addition, we carried out PCR analysis of DNA extracted from sorted EGFP(-)/EYFP(-) cells and confirmed the presence of cells genetically marked by either vector in this population, indicating the persistence of cells that have downregulated or lost retroviral gene expression. In comparison to mouse studies, however, we did not detect substantial differences between MND and MoMuLV backbones.

Notch1 activation increases hematopoietic stem cell self-renewal in vivo and favors lymphoid over myeloid lineage outcome

Hematopoietic stem cells sequentially pass through a series of decision points affecting self-renewal or lineage-specific differentiation. Notch1 receptor is a known modulator of lineage-specific events in hematopoiesis that we assessed in the context of in vivo stem cell kinetics. Using RAG-1(-/-) mouse stems cells, we documented increased stem cell numbers due to decreased differentiation and enhanced stem cell self-renewal induced by Notch1. Unexpectedly, preferential lymphoid over myeloid lineage commitment was noted when differentiation occurred. Therefore, Notch1 affects 2 decision points in stem cell regulation, favoring self-renewal over differentiation and lymphoid over myeloid lineage outcome. Notch1 offers an attractive target for stem cell manipulation strategies, particularly in the context of immunodeficiency and acquired immunodeficiency syndrome.

Mutations in the gene encoding the transcription factor CCAAT/enhancer binding protein alpha in myelodysplastic syndromes and acute myeloid leukemias

The CCAAT/enhancer binding protein alpha (C/EBPalpha) protein is essential for proper lung and liver function and granulocytic and adipose tissue differentation. It was hypothesized that abnormalties in C/EBPalpha function contribute to the development of malignancies in a variety of tissues. To test this, genomic DNA from 408 patient samples and 5 cell lines representing 11 different cancers was screened for mutations in the C/EBPalpha gene. Two silent polymorphisms termed P1 and P2 were present at frequencies of 13.5% and 2.2%, respectively. Of the 12 mutations detected in 10 patients, silent changes were identified in one nonsmall cell lung cancer, one prostate cancer, and one acute myelogenous leukemia (AML) subtype M4. The 9 remaining mutations were detected in 1 of 92 (1.1%) myelodysplastic syndrome (MDS) samples and 6 of 78 (7.7%) AML (AML-M2 and AML-M4) samples. Some mutations truncated the predicted protein with loss of the DNA-binding (basic region) and dimerization (leucine zipper [ZIP]) domains by either deletions or nonsense codons. Also, inframe deletions or insertions in the fork region located between the leucine zipper and basic region, or within the leucine zipper, disrupted the alpha-helical phase of the bZIP domain. The inframe deletion and insertion mutations abrogated the transcriptional activation function of C/EBPalpha on the granulocyte colony-stimulating factor receptor promoter. These mutants localized properly to the nucleus, but were unable to bind to the C/EBP site in the promoter and did not possess dominant-negative activity. The mutations in the MDS patient and one AML-M2 patient were biallelic, indicating a loss of C/EBPalpha function. These results suggest that mutation of C/EBPalpha is involved in specific subtypes of AML and in MDS, but may occur rarely in other types of leukemias or nonhematologic malignancies.