B-lymphoid to granulocytic switch during hematopoiesis in a transgenic mouse strain

Mis-expression of GATA6 re-programs cell fate during early hematopoiesis

The traditional view of hematopoiesis is that myeloid cells derive from a common myeloid progenitor (CMP), whereas all lymphoid cell populations, including B, T, and natural killer (NK) cells and possibly plasmacytoid dendritic cells (pDCs), arise from a common lymphoid progenitor (CLP). In Max41 transgenic mice, nearly all B cells seem to be diverted into the granulocyte lineage. Here, we show that these mice have an excess of myeloid progenitors, but their CLP compartment is ablated, and they have few pDCs. Nevertheless, T cell and NK cell development proceeds relatively normally. These hematopoietic abnormalities result from aberrant expression of Gata6 due to serendipitous insertion of the transgene enhancer (Eμ) in its proximity. Gata6 mis-expression in Max41 transgenic progenitors promoted the gene-regulatory networks that drive myelopoiesis through increasing expression of key transcription factors, including PU.1 and C/EBPa. Thus, mis-expression of a single key regulator like GATA6 can dramatically re-program multiple aspects of hematopoiesis.

Transcriptional and Microenvironmental Regulation of Lineage Ambiguity in Leukemia

Leukemia is characterized by the uncontrolled production of leukemic cells and impaired normal hematopoiesis. Although the combination of chemotherapies and hematopoietic stem cell transplantation has significantly improved the outcome of leukemia patients, a proportion of patients still suffer from relapse after treatment. Upon relapse, a phenomenon termed “lineage switch” is observed in a subset of leukemia patients, in which conversion of lymphoblastic leukemia to myeloid leukemia or vice versa is observed. A rare entity of leukemia called mixed-phenotype acute leukemia exhibits co-expression of markers representing two or three lineages. These two phenotypes regarding the lineage ambiguity suggest that the fate of some leukemia retain or acquire a certain degree of plasticity. Studies using animal models provide insight into how lineage specifying transcription factors can enforce or convert a fate in hematopoietic cells. Modeling lineage conversion in normal hematopoietic progenitor cells may improve our current understanding of how lineage switch occurs in leukemia. In this review, we will summarize the role of transcription factors and microenvironmental signals that confer fate plasticity to normal hematopoietic progenitor cells, and their potential to regulate lineage switching in leukemias. Future efforts to uncover the mechanisms contributing to lineage conversion in both normal hematopoiesis and leukemia may pave the way to improve current therapeutic strategies.

DNA-binding of the Tet-transactivator curtails antigen-induced lymphocyte activation in mice

The Tet-On/Off system for conditional transgene expression constitutes state-of-the-art technology to study gene function by facilitating inducible expression in a timed and reversible manner. Several studies documented the suitability and versatility of this system to trace lymphocyte fate and to conditionally express oncogenes or silence tumour suppressor genes in vivo. Here, we show that expression of the tetracycline/doxycycline-controlled Tet-transactivator, while tolerated well during development and in immunologically unchallenged animals, impairs the expansion of antigen-stimulated T and B cells and thereby curtails adaptive immune responses in vivo. Transactivator-mediated cytotoxicity depends on DNA binding, but can be overcome by BCL2 overexpression, suggesting that apoptosis induction upon lymphocyte activation limits cellular and humoral immune responses. Our findings suggest a possible system-intrinsic biological bias of the Tet-On/Off system in vivo that will favour the outgrowth of apoptosis resistant clones, thus possibly confounding data published using such systems.

Tet-transactivators are used for direct regulation of gene expression, RNA interference and for CRISPR/Cas9-based systems. Here the authors show that DNA-bound Tet-transactivators can induce cell death in antigen-activated lymphocytes in vivo, putting into question the use of, and in vivo data generated with, these molecular tools.

Evidence for bone marrow adult stem cell plasticity: properties, molecular mechanisms, negative aspects, and clinical applications of hematopoietic and mesenchymal stem cells transdifferentiation

In contrast to the pluripotent embryonic stem cells (ESCs) which are able to give rise to all cell types of the body, mammalian adult stem cells (ASCs) appeared to be more limited in their differentiation potential and to be committed to their tissue of origin. Recently, surprising new findings have contradicted central dogmas of commitment of ASCs by showing their plasticity to differentiate across tissue lineage boundaries, irrespective of classical germ layer designations. The present paper supports the plasticity of the bone marrow stem cells (BMSCs), bringing the most striking and the latest evidences of the transdifferentiation properties of the bone marrow hematopoietic and mesenchymal stem cells (BMHSCs, and BMMSCs), the two BM populations of ASCs better characterized. In addition, we report the possible mechanisms that may explain these events, outlining the clinical importance of these phenomena and the relative problems.

Context-specific roles for paracrine IL-6 in lymphomagenesis

A basic requirement for the development of complex organ systems is that the cellular response to identical environmental cues can vary significantly between distinct cell types and developmental stages. While it is well established that paracrine signaling can similarly elicit diverse responses in distinct tumor types, the relevance of developmental stage-specific signaling responses to tumor development remains unclear. Here, we show that the same microenvironmental factor, IL-6, can both promote and prevent lymphoma development by acting on cells at distinct stages of hematopoietic development. Specifically, paracrine IL-6 signaling promotes the survival of transplanted hematopoietic stem cells following lethal irradiation, allowing for the persistence and expansion of progenitor cells bearing a cancer-promoting alteration. Conversely, IL-6 signaling also initiates a paracrine secretory program in the bone marrow that promotes B-cell differentiation and inhibits the development of B-cell malignancies. Thus, stage-specific responses to cytokines may promote progenitor cell expansion while also inhibiting neoplastic development within a single developmental lineage. Once transformed, the resulting B-cell lymphomas again use paracrine IL-6 signaling as a survival signal, highlighting the ability of tumor cells to co-opt pathways used for stem cell protection. These data not only suggest a complex regulation of tumor development by the preneoplastic microenvironment, but also that this regulation can decisively impact the outcome of well-established tumor modeling approaches.

A role for E2F activities in determining the fate of Myc-induced lymphomagenesis

The phenotypic heterogeneity that characterizes human cancers reflects the enormous genetic complexity of the oncogenic process. This complexity can also be seen in mouse models where it is frequently observed that in addition to the initiating genetic alteration, the resulting tumor harbors additional, somatically acquired mutations that affect the tumor phenotype. To investigate the role of genetic interactions in the development of tumors, we have made use of the Emu-myc model of pre-B and B cell lymphoma. Since various studies point to a functional interaction between Myc and the Rb/E2F pathway, we have investigated the role of E2F activities in the process of Myc-induced lymphomagenesis. Whereas the absence of E2F1 and E2F3 function has no impact on Myc-mediated tumor development, the absence of E2F2 substantially accelerates the time of tumor onset. Conversely, tumor development is delayed by the absence of E2F4. The enhanced early onset of tumors seen in the absence of E2F2 coincides with an expansion of immature B lineage cells that are likely to be the target for Myc oncogenesis. In contrast, the absence of E2F4 mutes the response of the lineage to Myc and there is no expansion of immature B lineage cells. We also find that distinct types of tumors emerge from the Emu-myc mice, distinguished by different patterns of gene expression, and that the relative proportions of these tumor types are affected by the absence of either E2F2 or E2F4. From these results, we conclude that there are several populations of tumors that arise from the Emu-myc model, reflecting distinct populations of cells that are susceptible to Myc-mediated oncogenesis and that the proportion of these cell populations is affected by the presence or absence of E2F activities.

Endogenous bcl-2 is not required for the development of Emu-myc-induced B-cell lymphoma

Although myc and bcl-2 synergize in tumor development, particularly lymphomagenesis, it is not known whether endogenous bcl-2 is required for myc-induced tumorigenesis. To investigate the role of endogenous Bcl-2 in myc-induced lymphomagenesis, we bypassed the early death of Bcl-2-deficient mice by reconstituting lethally irradiated wild-type (wt) mice with a hematopoietic system from fetal liver-derived stem cells of Emu-myc/bcl-2(-/-) or control Emu-myc transgenic embryos. In premalignant (healthy) recipients, loss of Bcl-2 caused a moderate decrease in pre-B and immature B cells, and a dramatic reduction of mature B lymphocytes expressing the Emu-myc transgene. Furthermore, cultured preneoplastic Emu-myc/bcl-2(-/-) mature B cells displayed accelerated apoptosis compared with Emu-myc B cells. However, despite the striking reduction in B-cell numbers in vivo, ablation of endogenous Bcl-2 did not prevent or even delay development of Emu-myc lymphoma. Moribund mice presented with similar degrees of splenomegaly, blood leukocyte numbers, and tumor dissemination at death. These findings demonstrate that the initiation, development, continued growth, and severity of Emu-myc lymphoma do not depend upon endogenous Bcl-2, nor upon the total number of B lymphoid cells driven by the Emu-myc transgene. These results have implications for the treatment of hematopoietic tumors, particularly those that are not caused by Bcl-2 overexpression.

Stem cell plasticity: the debate begins to clarify

The stem cell story begins with the recognition of the regenerative powers of the head of the Lernean Hydra and the human liver (Prometheus) by the ancient Greeks. In modern times, the adult human stem cell has been epitomized by the hematopoietic stem cell in the bone marrow. More recently, bone marrow derived cells were reported to contribute to nonhematopoietic organs, suggesting a level of plasticity not previously expected. However, other reports failed to repeat some of these results, resulting in a heated debate on the plasticity of adult stem cells that has crossed over into the public domain and become a matter of political impact on the use of embryonic vs adult stem cells for organ regeneration or gene therapy. This review discusses the current status of the "plasticity" debate and presents existing data on detection methodology, underlying mechanisms, physiological implications, and clinical significance.

Determinants of lymphoid-myeloid lineage diversification

In recent years, investigators have made great progress in delineating developmental pathways of several lymphoid and myeloid lineages and in identifying transcription factors that establish and maintain their fate. However, the developmental branching points between these two large cell compartments are still controversial, and little is known about how their diversification is induced. Here, we give an overview of determinants that play a role at lymphoid-myeloid junctures, in particular transcription factors and cytokine receptors. Experiments showing that myeloid lineages can be reversibly reprogrammed into one another by transcription factor network perturbations are used to highlight key principles of lineage commitment. We also discuss experiments showing that lymphoid-to-myeloid but not myeloid-to-lymphoid conversions can be induced by the enforced expression of a single transcription factor. We close by proposing that this asymmetry is related to a higher complexity of transcription factor networks in lymphoid cells compared with myeloid cells, and we suggest that this feature must be considered when searching for mechanisms by which hematopoietic stem cells become committed to lymphoid lineages.

HLS5, a Novel RBCC (Ring Finger, B Box, Coiled-coil) Family Member Isolated from a Hemopoietic Lineage Switch, Is a Candidate Tumor Suppressor

Hemopoietic cells, apparently committed to one lineage, can be reprogrammed to display the phenotype of another lineage. The J2E erythroleukemic cell line has on rare occasions developed the features of monocytic cells. Subtractive hybridization was used in an attempt to identify genes that were up-regulated during this erythroid to myeloid transition. We report here on the isolation of hemopoietic lineage switch 5 (Hls5), a gene expressed by the monocytoid variant cells, but not the parental J2E cells. Hls5 is a novel member of the RBCC (Ring finger, B box, coiled-coil) family of genes, which includes Pml, Herf1, Tif-1alpha, and Rfp. Hls5 was expressed in a wide range of adult tissues; however, at different stages during embryogenesis, Hls5 was detected in the branchial arches, spinal cord, dorsal root ganglia, limb buds, and brain. The protein was present in cytoplasmic granules and punctate nuclear bodies. Isolation of the human cDNA and genomic DNA revealed that the gene was located on chromosome 8p21, a region implicated in numerous leukemias and solid tumors. Enforced expression of Hls5 in HeLa cells inhibited cell growth, clonogenicity, and tumorigenicity. It is conceivable that HLS5 is one of the tumor suppressor genes thought to reside at the 8p21 locus.

Reprogramming of human postmitotic neutrophils into macrophages by growth factors

It is generally recognized that postmitotic neutrophils give rise to polymorphonuclear neutrophils alone. We obtained evidence for a lineage switch of human postmitotic neutrophils into macrophages in culture. When the CD15+CD14- cell population, which predominantly consists of band neutrophils, was cultured with granulocyte macrophage-colony-stimulating factor, tumor necrosis factor-alpha, interferon-gamma, and interleukin-4, and subsequently with macrophage colony-stimulating factor alone, the resultant cells had morphologic, cytochemical, and phenotypic features of macrophages. In contrast to the starting population, they were negative for myeloperoxidase, specific esterase, and lactoferrin, and they up-regulated nonspecific esterase activity and the expression of macrophage colony-stimulating factor receptor, mannose receptor, and HLA-DR. CD15+CD14- cells proceeded to macrophages through the CD15-CD14- cell population. Microarray analysis of gene expression also disclosed the lineage conversion from neutrophils to macrophages. Macrophages derived from CD15+CD14- neutrophils had phagocytic function. Data obtained using 3 different techniques, including Ki-67 staining, bromodeoxyuridine incorporation, and cytoplasmic dye labeling, together with the yield of cells, indicated that the generation of macrophages from CD15+CD14- neutrophils did not result from a contamination of progenitors for macrophages. Our data show that in response to cytokines, postmitotic neutrophils can become macrophages. This may represent another differentiation pathway toward macrophages in human postnatal hematopoiesis.

GATA-1 Converts Lymphoid and Myelomonocytic Progenitors into the Megakaryocyte/Erythrocyte Lineages

GATA-1 is an essential transcription factor for megakaryocyte and erythrocyte (MegE) development. Here we show that hematopoietic progenitors can be reprogrammed by the instructive action of GATA-1. Enforced expression of GATA-1 in hematopoietic stem cells led to loss of self-renewal activity and the exclusive generation of MegE lineages. Strikingly, ectopic GATA-1 reprogrammed common lymphoid progenitors as well as granulocyte/monocyte (GM) progenitors to differentiate into MegE lineages, while inhibiting normal lymphoid or GM differentiation. GATA-1 upregulated critical MegE-related transcription factors such as FOG-1 and GATA-2 in lymphoid and GM progenitors, and their MegE development did not require "permissive" erythropoietin signals. Furthermore, GATA-1 induced apoptosis of proB and myelomonocytic cells, which could not be prevented by enforced permissive Bcl-2 or myeloid cytokine signals. Thus, GATA-1 specifically instructs MegE commitment while excluding other fate outcomes in stem and progenitor cells, suggesting that regulation of GATA-1 is critical in maintaining multilineage homeostasis.

Transdifferentiation and nuclear reprogramming in hematopoietic development and neoplasia

Cell transplantation and tissue regeneration studies indicate a surprisingly broad developmental potential for lineage-committed hematopoietic stem cells (HSCs). Under these conditions HSCs transition into myocytes, neurons, hepatocytes or other types of nonhematopoietic effector cells. Equally impressive is the progression of committed neuronal stem cells (NSCs) to functional blood elements. Although critical cell-of-origin issues remain unresolved, the possibility of lineage switching is strengthened by a few well-controlled examples of cell-type conversion. At the molecular level, switching probably initiates from environmental signals that induce epigenetic modifications, resulting in changes in chromatin configuration. In turn, these changes affect patterns of gene expression that mediate divergent developmental programs. This review examines recent findings in nuclear reprogramming and cell fusion as potential causative mechanisms for transdifferentiation during normal and malignant hematopoiesis.

Lineage commitment and maturation induction in normal and leukemic preprogenitor cells

Analysis of the critical cellular processes of self-generation, commitment, and maturation induction ideally requires the use of clonal cultures using cells with a capacity to undergo all three processes. Preprogenitor cells from normal mouse marrow are proving useful cells for such studies. Cells of a newly established cloned leukemic cell line, the GB2, are providing a useful analogous leukemic system because GB2 cells form stratified subpopulations of clonogenic cells able to be clonally analyzed in vitro in which self-renewal is demonstrable, but in which near-normal maturation can be induced by a wide range of hematopoietic regulators.

Differentiation commitment and regulator-specific granulocyte-macrophage maturation in a novel pro-B murine leukemic cell line

The cloned pro-B-lymphocyte murine leukemic cell line GB2, was established from a leukemic Max41 x Emu-myc double transgenic mouse. Its Igh alleles are rearranged and its surface markers are primarily B-lymphoid, but a small proportion of the cells also express surface Gr-1 and some cells develop the morphology of maturing granulocytes. The cell line grows continuously in suspension culture without the addition of growth factors, but expresses mRNA for M-CSF, TPO and Flt-3-ligand. When stimulated in agar cultures by GM-CSF, G-CSF, M-CSF, IL-3, SCF, IL-6, leukemia inhibitory factor (LIF), IL-5 or IFNgamma, GB2 cells generated blast colonies or colonies of maturing granulocytes and macrophages. There was a striking similarity in colony types, relative colony numbers and maturation of colony cells to those formed by normal bone marrow cells in response to the same stimuli. GB2 blast colony-forming cells exhibited self-renewal as well as an ability to form granulocyte-macrophage colony-forming progeny, with evidence that a hierarchical sequence of clonogenic cells is generated in the cell line even after subcloning. Factor-specific maturation was clearly initiated by the action of the added growth factors. In contrast, FACS-sorting experiments showed that commitment to various types of colony-forming cell occurs in maintenance suspension cultures in the apparent absence of potentially relevant growth factors.

Karyotypic abnormalities associated with haemopoietic lineage switching are not linked with mutations to p53

Leukemic cells can undergo lineage switching to display the phenotypic features of another haemopoietic pathway, as exemplified by B lymphoma and erythroleukemic cell lines generating variants with a monocytic appearance. Unlike the diploid parental lines, the vast majority of myeloid derivative lines examined (12 of 13 lines) were aneuploid. As p53 is involved in the maintenance of chromosomal stability, we investigated the role of p53 in the emergence of abnormal karyotypes in cells which had undergone lineage switching. Single strand conformation polymorphism and sequence analysis of cDNA, together with protein immunoprecipitations, were used to assess the p53 status of parental and variant cell lines. Unexpectedly, four or five monocytic lines with chromosomal alterations contained wild type p53. Conversely, a p53 point mutation found in one aneuploid monocytic line was also present in the diploid parental pre-B cell. These results provide strong evidence that mechanisms other than p53 mutations are responsible for karyotypic abnormalities seen in cells that have undergone lineage switching.

Regulation of chicken haemopoiesis by cytokines

The continuous production, control and functional activation of blood cells involves a complex series of cellular events in which a small population of stem cells generates large numbers of mature cells. The survival, proliferation and development of these cells is strictly dependent on extracellular signals, among these are polypeptide regulators generally known as cytokines. While a large number of mammalian cytokines with proliferative and inhibitory effects have been described in detail, it is surprising that comparatively little is known of the avian system. Given the success of human cytokines as a model, the ability to manipulate the chicken haemopoietic and lymphopoietic systems by precise application of purified cytokines provides a rational approach to defence against disease. As a general caveat, an increased awareness of the existence of regulatory networks and the likelihood that these regulators were designed to function most effectively when acting in combination, will provide an understanding into the regulation of haemopoiesis and hence find application in both clinical and agricultural research.

Max protein expression is associated with survival of children with lymphoblastic lymphoma

Overexpression of c-Myc in murine B lineage cells is associated with a polyclonal pre-B cell expansion as well as development of pre-B or B lymphoblastic lymphoma (LL) accompanied by leukemia, which mirrors the clinical features of childhood LL. Of interest, Max overexpression attenuates aberrant growth of B cells triggered by c-Myc. However, the clinical significance of Max in human lymphoid tissue remains to be clarified. In the present studies, we studied the expression of the c-Myc and Max proteins in normal lymph nodes and in childhood LL. In normal lymph nodes, c-Myc protein was expressed by the majority of cells in germinal center and marginal zone, but Max protein was expressed only by some of them. In contrast, c-Myc and Max were absent in mantle zone. Cells positive for c-Myc and Max expression in LL were 70.6 +/- 19.8% and 47.3 +/- 32.4%, respectively, determined by immunohistochemical staining using paraffin blocks from 23 cases of childhood LL. The survival of children with LL showing higher Max expression (> or = 30%) was significantly greater than that of lower expression (< 30%; P = 0.0027 using the Mantel-Cox test). These results suggest that Max protein may affect prognosis of childhood LL.

The combined absence of the transcription factors Rel and RelA leads to multiple hemopoietic cell defects

Individual Rel/NF-kappaB transcription factors, although dispensable for the development and maturation of most hemopoietic cells, are critical regulators of normal immune function. Redundancy among these proteins prompted us to examine the role of Rel and RelA in hemopoiesis by using mice that lack both subunits. Because of the death of double-mutant fetuses at day 13.5 of gestation (E13.5), E12 fetal liver hemopoietic progenitors were used for in vitro cultures and for repopulating stem cell studies in lethally irradiated normal recipient mice. Most striking, Rel/RelA-deficient hemopoietic precursors failed to promote the survival of myeloablated mice. This phenotype was associated with several defects including a reduction of spleen colony-forming unit progenitors, impaired erythropoiesis, and a deregulated expansion of granulocytes. In vitro progenitor assays also revealed that Rel or RelA serves an antiapoptotic role during monocyte differentiation. Despite the combined loss of Rel and RelA leading to these hemopoietic defects, c-rel(-/-)rela(-/-) stem cells contributed to the development of all lineages in mice engrafted with double-mutant fetal liver cells and normal bone marrow cells, albeit in a reduced fashion compared with controls. Collectively, these data indicate the loss of Rel and RelA does not appear to affect pluripotent stem cells; rather, Rel and RelA serve redundant functions in regulating differentiation and survival of committed progenitors in multiple hemopoietic lineages.

Genetic Influences Determining Progenitor Cell Mobilization and Leukocytosis Induced by Granulocyte Colony-Stimulating Factor

The mechanisms involved in the mobilization of progenitor cells into the blood by granulocyte colony-stimulating factor (G-CSF ) and other cytokines are poorly understood. To identify important influences on this complex process, in vivo murine models were used. Granulocyte-macrophage colony-stimulating factor (GM-CSF ) transgenic, Max41 transgenic, W/WV, Mpl-null, GM-CSF receptor (β chain)-null mice, wild-type littermate controls, and six inbred strains of mice were injected with 200 μg/kg/d G-CSF for 5 days. Three parameters of response were monitored: white blood cell count (WCC), peripheral blood progenitor cell (PBPC) numbers, and spleen weight. In all genotypes studied, G-CSF induced increases in these three parameters. However, PBPC mobilization in W/WV and Mpl-null mice was only 30% and 9%, respectively, of that observed in wild-type mice. In contrast, perturbations of GM-CSF signalling had no demonstrable effect on in vivo responses to G-CSF. Broad variability was evident between inbred strains for each parameter of the response to G-CSF. A 10-fold range in response was observed for circulating progenitor cell numbers, similar to that observed for normal human subjects receiving G-CSF. The interstrain differences were in the distribution of mature and progenitor cells between peripheral blood, bone marrow, and spleen rather than in the total numbers of these cells in the body. Results of an F2 intercross of low-responding C57BL/6 and intermediate-responding SJL mice indicated that regulation of progenitor cell mobilization is a complex genetic trait, that there is a correlation between this trait and WCC response (r2 = .5), and that this approach may serve as a useful model for the identification of genes involved in the mobilization process.