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B lymphoid development is initiated by the differentiation of hematopoietic stem cells into lineage committed progenitors, ultimately generating mature B cells. This highly regulated process generates clonal immunological diversity via recombination of immunoglobulin V, D and J gene segments. While several transcription factors that control B cell development and V(D)J recombination have been defined, how these processes are initiated and coordinated into a precise regulatory network remains poorly understood. Here, we show that the transcription factor ETS Related Gene (Erg) is essential for early B lymphoid differentiation. Erg initiates a transcriptional network involving the B cell lineage defining genes, Ebf1 and Pax5, which directly promotes expression of key genes involved in V(D)J recombination and formation of the B cell receptor. Complementation of Erg deficiency with a productively rearranged immunoglobulin gene rescued B lineage development, demonstrating that Erg is an essential and stage-specific regulator of the gene regulatory network controlling B lymphopoiesis., B cell development is tightly regulated in a stepwise manner to ensure proper generation of repertoire diversity via somatic gene rearrangements. Here, the authors show that a transcription factor, Erg, functions at the earliest stage to critically control two downstream factors, Ebf1 and Pax5, for modulating this gene rearrangement process.

Summary/AbstractB-cell development is initiated by the stepwise differentiation of hematopoietic stem cells into lineage committed progenitors, ultimately generating the mature B-cells that mediate protective immunity. This highly regulated process also generates clonal immunological diversity via recombination of immunoglobulin genes. While several transcription factors that control B-cell development and V(D)J recombination have been defined, how these processes are initiated and coordinated into a precise regulatory network remains poorly understood. Here, we show that the transcription factor ETS Related Gene (Erg) is essential for the earliest steps in B-cell differentiation. Erg initiates a transcriptional network involving the B-cell lineage defining genes, Ebf1 and Pax5, that directly promotes the expression of key genes involved in V(D)J recombination and formation of the B-cell receptor. Complementation of the Erg-deficiency with a productively rearranged immunoglobulin gene rescued B-cell development, demonstrating that Erg is an essential and exquisitely stage specific regulator of the gene regulatory network controlling B-lymphopoiesis.

Upon ligand binding, RIPK1 is recruited to tumor necrosis factor receptor superfamily (TNFRSF) and Toll-like receptor (TLR) complexes promoting prosurvival and inflammatory signaling. RIPK1 also directly regulates caspase-8-mediated apoptosis or, if caspase-8 activity is blocked, RIPK3-MLKL-dependent necroptosis. We show that C57BL/6 Ripk1(-/-) mice die at birth of systemic inflammation that was not transferable by the hematopoietic compartment. However, Ripk1(-/-) progenitors failed to engraft lethally irradiated hosts properly. Blocking TNF reversed this defect in emergency hematopoiesis but, surprisingly, Tnfr1 deficiency did not prevent inflammation in Ripk1(-/-) neonates. Deletion of Ripk3 or Mlkl, but not Casp8, prevented extracellular release of the necroptotic DAMP, IL-33, and reduced Myd88-dependent inflammation. Reduced inflammation in the Ripk1(-/-)Ripk3(-/-), Ripk1(-/-)Mlkl(-/-), and Ripk1(-/-)Myd88(-/-) mice prevented neonatal lethality, but only Ripk1(-/-)Ripk3(-/-)Casp8(-/-) mice survived past weaning. These results reveal a key function for RIPK1 in inhibiting necroptosis and, thereby, a role in limiting, not only promoting, inflammation.

When murine fetal liver cells were transduced with either of the human acute myeloid leukemia fusion oncogenes MLL-ENL or MLL-AF9 and then transplanted to irradiated recipient mice, myelomonocyte leukemias rapidly developed from the transplanted cells. Analysis of initial events following transduction showed that both oncogenes immediately induced a wide range of enhanced proliferative states, the most extreme of which could generate continuous lines of cells. Maturation defects accompanied the enhanced proliferative states. At all times, the transformed cells exhibited complete dependency on hematopoietic growth factors, particularly GM-CSF and IL-3. Myelomonocytic leukemic cells from primary or transplanted mice formed colonies in semisolid agar. The large majority were dependent on hematopoietic growth factors, but a low frequency of autonomous colonies was also detected. Unexpectedly, reculture of autonomous leukemic colonies generated large numbers of growth factor-dependent clonogenic progeny. Similarly, transplanted clonal autonomous leukemic cells produced leukemias containing a majority of factor-dependent cells. Conversely, recultures of factor-dependent colonies in vitro always produced small numbers of autonomous colonies among the dependent progeny. The reversible relationship between factor dependency and autonomy is surprising because autonomy would have been presumed to represent the final, irreversible, leukemic state.

Previous studies have shown that mouse bone marrow cells can produce mast cells when stimulated in vitro by stem cell factor (SCF) and interleukin-3 (IL-3). Experiments to define the marrow cells able to generate mast cells showed that the most active subpopulations were the Kit + Sca1 – progenitor cell fraction and the more ancestral Kit + Sca1 + blast colony-forming cell fraction. In clonal cultures, up to 64% of blast colony-forming cells were able to generate mast cells when stimulated by SCF and IL-3, and, of these, the most active were those in the CD34 – Flt3R – long-term repopulating cell fraction. Basophils, identified by the monoclonal antibody mMCP-8 to mouse mast cell serine protease-8, were also produced by 50% of blast colony-forming cells with a strong concordance in the production of both cell types by individual blast colony-forming cells. Enriched populations of marrow-derived basophils were shown to generate variable numbers of mast cells after a further incubation with SCF and IL-3. The data extend the repertoire of lineage-committed cells able to be produced by multipotential hematopoietic blast colony-forming cells and show that basophils and mast cells can have common ancestral cells and that basophils can probably generate mast cells at least under defined in vitro conditions.

Murine hematopoietic blast colony-forming cells (BL-CFCs) are able to generate up to 30,000 progeny blast cells within 10 d in agar cultures. Contained in these populations are large numbers of lineage-committed progenitor cells in the granulocytic and macrophage lineages. Sequential analyses of blast colonies revealed that self-generation of BL-CFCs occurs but is surprisingly late in clonal expansion, as is the emergence of progenitor cells committed to megakaryocytic and eosinophil lineages. Self-generating BL-CFCs were highly enriched in lineage − Kit + Sca1 + CD34 − Flt3R − populations, and colonies generated by such cells contained colony-forming units–spleen and formed erythroid and lymphoid progeny in vivo. The data suggest the existence of a hierarchical structure in BL-CFC populations with at least a subset being cells assayable as colony-forming units–spleen. Because BL-CFCs can self-generate and are able to generate lymphoid and myeloid populations, BL-CFCs appear to be ideal cells in which to analyze the processes of self-generation and lineage commitment in clonal in vitro cultures.

Two types of blast colonies can be stimulated to develop in semisolid agar cultures of murine bone marrow cells. Typically, these are either multicentric colonies stimulated by stem cell factor (SCF) plus interleukin-6 (IL-6) or dispersed colonies stimulated by Flt3 ligand (FL) plus IL-6. Both types of blast colony-forming cells (BL-CFCs) can generate large numbers of lineage-committed granulocyte-macrophage progenitor cells and exhibit some capacity for self-generation and the formation of eosinophil and megakaryocyte progenitor cells. However, the two populations of BL-CFCs are largely distinct and partially separable by fluorescence-activated cell sorting and are distinguished by differing capacity to form granulocyte-committed progeny. Both types of BL-CFCs can generate dendritic cells and small numbers of lymphocytes but the FL-responsive BL-CFCs have a greater capacity to form both B and T lymphocytes. Both types of blast colonies offer remarkable opportunities to analyze multilineage commitment at a clonal level in vitro .

We have generated mice from a N -ethyl- N -nitrosourea mutagenesis screen that carry a mutation in the translation initiation codon of Gata-1 , termed Plt13 , which is equivalent to mutations found in patients with acute megakaryoblastic leukemia and Down syndrome. The Gata-1 locus is present on the X chromosome in humans and in mice. Male mice hemizygous for the mutation ( Gata-1 Plt13 /Y) failed to produce red blood cells and died during embryogenesis at a similar stage to Gata-1 -null animals. Female mice that carry the Plt13 mutation are mosaic because of random inactivation of the X chromosome. Adult Gata-1 Plt13 /+ females were not anemic, but they were thrombocytopenic and accumulated abnormal megakaryocytes without a concomitant increase in megakaryocyte progenitor cells. Gata-1 Plt13 /+ mice contained large numbers of blast-like colony-forming cells, particularly in the fetal liver, but also in adult spleen and bone marrow, from which continuous mast cells lines were readily derived. Although the equivalent mutation to Gata-1 Plt13 in humans results in production of GATA-1s, a short protein isoform initiated from a start codon downstream of the mutated initiation codon, Gata-1s was not detected in Gata-1 Plt13 /+ mice.

Genetically primed adult C57BL mice were deleted of exon 5 of the gene encoding the transcription factor PU.1 by IFN activation of Cre recombinase. After a 13-week delay, conditionally deleted ( PU.1 -/- ) mice began dying of myeloid leukemia, and 95% of the mice surviving from early postinduction death developed transplantable myeloid leukemia whose cells were deleted of PU.1 and uniformly Gr-1 positive. The leukemic cells formed autonomous colonies in semisolid culture with varying clonal efficiency, but colony formation was enhanced by IL-3 and sometimes by granulocyte-macrophage colony-stimulating factor. Nine of 13 tumors analyzed had developed a capacity for autocrine IL-3 or granulocyte-macrophage colony-stimulating factor production, and there was evidence of rearrangement of the IL-3 gene. Acquisition of autocrine growth-factor production and autonomous growth appeared to be major events in the transformation of conditionally deleted PU.1 -/- cells to fully developed myeloid leukemic populations.

Mpl -/- mice bearing the Plt3 or Plt4 mutations in the c-Myb gene exhibit thrombopoietin (TPO)–independent supraphysiological platelet production accompanied by excessive megakaryocytopoiesis and defective erythroid and lymphoid cell production. To better define the cellular basis for the thrombocytosis in these mice, we analyzed the production and characteristics of megakaryocytes and their progenitors. Consistent with thrombocytosis arising from hyperactive production, the high platelet counts in mice carrying the c-MybPlt4 allele were not accompanied by any significant alteration in platelet half-life. Megakaryocytes in c-Myb mutant mice displayed reduced modal DNA ploidy and, among the excessive numbers of megakaryocyte progenitor cells, more mature precursors were particularly evident. Megakaryocyte progenitor cells carrying the Plt3 or Plt4 c-Myb mutations, but not granulocyte-macrophage progenitors, exhibited 200-fold enhanced responsiveness to granulocyte-macrophage colony-stimulating factor (GM-CSF), suggesting that altered responses to cytokines may contribute to expanded megakaryocytopoiesis. Mutant preprogenitor (blast colony-forming) cells appeared to have little capacity to form megakaryocyte progenitor cells. In contrast, the spleens of irradiated mice 12 days after transplantation with mutant bone marrow contained abundant megakaryocyte progenitor cells, suggesting that altered c-Myb activity skews differentiation commitment in spleen colony-forming units (CFU-S) in favor of excess megakaryocytopoiesis.

Although the transcription factor PU.1 is essential for fetal lymphomyelopoiesis, we unexpectedly found that elimination of the gene in adult mice allowed disturbed hematopoiesis, dominated by granulocyte production. Impaired production of lymphocytes was evident in PU.1-deficient bone marrow (BM), but myelocytes and clonogenic granulocytic progenitors that are responsive to granulocyte colony-stimulating factor or interleukin-3 increased dramatically. No identifiable common lymphoid or myeloid progenitor populations were discernable by flow cytometry; however, clonogenic assays suggested an overall increased frequency of blast colony-forming cells and BM chimeras revealed existence of long-term self-renewing PU.1-deficient cells that required PU.1 for lymphoid, but not granulocyte, generation. PU.1 deletion in granulocyte-macrophage progenitors, but not in common myeloid progenitors, resulted in excess granulocyte production; this suggested specific roles of PU.1 at different stages of myeloid development. These findings emphasize the distinct nature of adult hematopoiesis and reveal that PU.1 regulates the specification of the multipotent lymphoid and myeloid compartments and restrains, rather than promotes, granulopoiesis.

Leukemia inhibitory factor (LIF)-induced cell signaling occurs following sequential binding to the LIF receptor alpha-chain (LIFR), then to the gp130 co-receptor used by all members of the interleukin-6 family of cytokines. By monovalently displaying human LIF on the surface of M13 phage and randomizing clusters of residues in regions predicted to be important for human LIFR binding, we have identified mutations, which lead to significant increases in affinity for binding to LIFR. Six libraries were constructed in which regions of 4-6 amino acids were randomized then panned against LIFR. Mutations identified in three distinct clusters, residues 53-57, 102-103, and 150-155, gave rise to proteins with significantly increased affinity for binding to both human and mouse LIFR. Combining the mutations for each of these regions further increased the affinity, such that the best mutants bound to human LIFR with1000-fold higher affinity than wild-type human LIF. NMR analysis indicated that the mutations did not alter the overall structure of the molecule relative to the native protein, although some local changes occurred in the vicinity of the substituted residues. Despite increases in LIFR binding affinity, these mutants did not show any increase in activity as agonists of LIF-induced proliferation of Ba/F3 cells expressing human LIFR and gp130 compared with wild-type LIF. Incorporation of two additional mutations (Q29A and G124R), which were found to abrogate cell signaling, led to the generation of highly potent antagonists of both human and murine LIF-induced bioactivity.

Injection of neonatal bone marrow cells from mice lacking the gene encoding suppressor of cytokine signaling 1 (SOCS1) into irradiated syngeneic 129/Sv or C57BL/6 mice led to a decreased survival, more rapidly occurring in 129/Sv than in C57BL/6 mice. Moribund mice did not exhibit the acute or chronic diseases developed by Socs1 - / - mice but developed a pathology characteristic of graft-versus-host disease with typical chronic inflammatory lesions in the liver, skin, lungs, and gut. The results indicate that cells derived from the Socs1 - / - bone marrow are autoaggressive but did not identify the cell types involved. Failure of the engrafted Socs1 - / - marrow cells to reproduce the tissue damage typical of Socs1 - / - disease indicates that loss of SOCS1 from target tissues may also be required for the development of the Socs1 -/- diseases, such as fatty degeneration of the liver, polymyositis, or corneal inflammation.

The formation of megakaryocyte colonies in agar cultures of murine bone marrow or spleen cells can be stimulated by the addition of interleukin-3 (IL-3), erythropoietin (EPO), thrombopoietin (TPO), or IL-6. However, greater numbers of colonies developed if combinations of two or more of these stimuli were used, particularly combinations including stem cell factor, with maximal numbers of colonies developing with the combination of stem cell factor plus IL-3 plus EPO. The data indicate that most committed progenitor cells in the megakaryocyte lineage were unusual in that they required stimulation by two or more hematopoietic growth factors. In tests using a range of growth factors, G-CSF was exceptional in that it consistently specifically inhibited megakaryocyte colony formation stimulated by EPO, TPO, or IL-6 but not that stimulated by IL-3. The mechanisms involved in this inhibitory action of G-CSF are unknown, but the inhibitory action could be of relevance for the dose-dependent lowering of platelet levels observed in some subjects injected with G-CSF.

Mice with inactivation of the gene encoding the suppressor of cytokine signaling-1 (SOCS-1) die in neonatal life with an IFN-γ-dependent inflammatory disease dominated by fatty degeneration and necrosis of the liver. To establish the long-term pathological consequences of loss of SOCS-1 in mice, where initial survival was made possible by also deleting the IFN-γ gene, a comparison was made of the lifespan of groups of SOCS-1−/−IFN-γ−/−, SOCS-1+/+IFN-γ−/−and SOCS-1+/+IFN-γ+/+mice. Mice lacking the genes for both SOCS-1 and IFN-γ exhibited an accelerated death rate compared with control groups. Disease states developing selectively in SOCS-1−/−IFN-γ−/−mice were polycystic kidneys, pneumonia, chronic skin ulcers, and chronic granulomas in the gut and various other organs. Mice of all three groups developed cataracts, but disease development was accelerated in the groups lacking IFN-γ. SOCS-1−/−IFN-γ−/−mice exhibited a slightly increased predisposition to the development of T lymphoid leukemia, either spontaneous or radiation-induced. The development of polycystic kidneys may be caused by a developmental defect in renal-tubule organization noted in neonatal SOCS-1−/−mice. The chronic infections and granulomas of SOCS-1−/−IFN-γ−/−mice may be based on autoaggression of SOCS-1−/−T lymphoid and related cells or a functional deficiency of these cells when lacking SOCS-1.

Organs from neonatal mice dying from IFN-γ-dependent inflammatory disease initiated by loss of the gene encoding the suppressor of cytokine signaling-1 (SOCS-1) had a normal capacity to produce G-CSF in vitro but a reduced capacity to produce GM-CSF, most evident with the lung, and some reduction in the production of M-CSF by muscle tissue. In contrast, organs from mice lacking the genes for both SOCS-1 and IFN-γ had a normal capacity to produce CSFs. Organs from young adult mice dying with polymyositis and myocarditis that lacked SOCS-1 but were heterozygous for IFN-γ had a normal capacity to produce GM-CSF and M-CSF, but muscle tissue produced significantly increased amounts of G-CSF and IL-5 with IL-5 production also being elevated for the salivary gland, thymus, and heart. Loss of the IFN-γ gene alone had no impact on organ production of these cytokines in vitro. In none of the inflammatory disease models was IL-3 production detected. The SOCS-1 protein appears to have no direct influence on the cellular production of these cytokines and the abnormalities observed either depend on the coaction of IFN-γ, or more likely, are linked with the invasion and destruction of tissue by T lymphocytes, macrophages, eosinophils, and neutrophils. The ability of local organs to produce these proinflammatory cytokines could contribute to the development and progression of these inflammatory lesions.

Mice lacking the gene encoding the suppressor of cytokine signaling-1 (SOCS-1 -/-) and heterozygous for the IFN-gamma gene (IFN-gamma +/-) avoided the IFN-gamma-dependent preweaning death of SOCS-1 -/- IFN-gamma +/+ mice but did not exhibit the good health of young adult SOCS-1 -/- IFN-gamma -/- mice. SOCS-1 -/- IFN-gamma +/- mice died within 160 days of birth with massive T lymphocyte, macrophage, and eosinophil infiltration of all skeletal muscles and a similar severe myocarditis. The cornea also developed inflammatory infiltration and often a corneal ulcer. The mice exhibited evidence of selective CD8 T lymphocyte activation in populations in the thymus, spleen, and lymph nodes and focal T- and B-lymphoid infiltrates developed in the lung and salivary gland without apparent tissue damage. Comparison of SOCS-1 -/- IFN-gamma +/- mice with various control mice indicated that the development of tissue-damaging T lymphocyte, macrophage, and eosinophil infiltrates required loss of SOCS-1 and the presence of some IFN-gamma, but that the lung lymphoid infiltrates required only loss of SOCS-1 to develop.

GM-CSF transgenic mice were crossed with mice with homozygous inactivation of the gene encoding the common beta chain (beta c) of the GM-CSF receptor to produce mice with constitutively elevated GM-CSF levels but no high-affinity GM-CSF receptors. GM-CSF transgenic beta c -/- mice had exceptionally elevated serum GM-CSF levels but failed to develop the abnormal peritoneal cell population, eye destruction or tissue lesions characteristic of GM-CSF transgenic beta c +/+ mice. The alveolar proteinosis of beta c -/- mice was not altered in GM-CSF transgenic beta c -/- mice. Levels of GM-CSF mRNA in transgenic GM-CSF beta c -/- were elevated but lower than in transgenic beta +/+ mice and the higher serum GM-CSF levels were traced in part to the longer serum half-life of GM-CSF in beta c -/- than in beta c +/+ mice although urinary loss of GM-CSF was higher in beta c -/- than in +/+ mice. The data indicate that the transgenic phenotype was due to stimulation by GM-CSF and not an insertional effect, that low-affinity receptors are not capable of initiating tissue pathology even in the presence of excess GM-CSF levels and that autocrine production of GM-CSF by GM-CSF-responsive cells also fails to induce changes in these cells. The results support current dogma that the action of polypeptide regulators is mediated exclusively by activation of high-affinity membrane receptors.

The transcription factor encoded by the E-twenty-six (ETS)-related gene, ERG , is an essential regulator of hematopoietic stem cell function and a potent human oncoprotein. Enforced expression of ERG in murine hematopoietic cells leads to the development of a well-characterized lymphoid leukemia and a less well-defined non lymphoid disease. To clarify the latter, we generated murine bone marrow chimeras with enforced Erg expression in engrafted hematopoietic progenitor cells. As expected, these mice developed lymphoid leukemia. However, the previously reported non lymphoid disease that developed was shown to be a uniform, transplantable leukemia with both erythroid and megakaryocytic characteristics. In vivo, this disease had the overall appearance of an erythroleukemia, with an accumulation of immature erythroblasts that infiltrated the bone marrow, spleen, liver, and lung. However, when stimulated in vitro, leukemic cell clones exhibited both erythroid and megakaryocytic differentiation, suggesting that transformation occurred in a bipotential progenitor. Thus, in mice, Erg overexpression induces the development of not only lymphoid leukemia but also erythro-megakaryocytic leukemia.

Two distinct bone marrow-derived blast colony-forming cells can generate colonies of lineage-restricted progenitor cells in agar cultures of murine bone marrow. Both cell types selectively had a Kit + ScaI + phenotype distinguishing them from most lineage-restricted progenitor cells. Multicentric blast colony-forming cells stimulated by stem cell factor plus interleukin-6 (IL-6) (BL-CFC-S) were separable from most dispersed blast colony-forming cells stimulated by Flt3 ligand and IL-6 (BL-CFC-F) using CD34 and Flt3R probes. Multicentric BL-CFC-S cofractionated with colony-forming units, spleen (CFU-S) supporting the possibility that the 2 cells may be identical. The colony populations generated by BL-CFC-S were similar in their phenotype and proliferative capacity to progenitor cells in whole bone marrow but the progeny of BL-CFC-F were skewed with an abnormally high proportion of Kit − Flt3R + cells whose clonogenic cells tended to generate only macrophage progeny. Both blast colony populations had a high percentage of GR1 + and Mac1 + cells but BL-CFC-F colonies also contained a significant population of B220 + and IL-7R + cells relevant to the superior ability of BL-CFC-F colony cells to generate B lymphocytes and the known dependency of this process on Flt3 ligand and IL-7. The commitment events and phenotypic changes during the generation of differing progenitor cells in blast colonies can now be clonally analyzed in a convenient in vitro culture system.

Murine granulocytic cells, in becoming leukemic, need to acquire enhanced self-generation and a capacity for autocrine growth stimulation. Mice transplanted with bone marrow cells transduced with the Mixl1 homeobox gene develop a very high frequency of myeloid leukemia derived from the transduced cells. Preleukemic mice contained a high frequency of transduced clonogenic granulocytic cells. They exhibited an abnormally high capacity for self-replication and could generate immortalized granulocytic cell lines that remained absolutely dependent on either GM-CSF or IL-3 and were not leukemic. Organs from mice repopulated by marrow cells transduced either with Mixl1 or the control murine stem cell virus vector exhibited a capacity to produce IL-3 in vitro , activity being highest with the lungs, marrow, bladder, and thymus. Supporting evidence for the in vivo production of IL-3 was the frequent development of mast cells in the marrow. Overexpression of Mixl1 appears capable of inducing an abnormal self-renewal capacity in granulocytic precursors. Aberrant production of IL-3 was not present in the continuous Mixl cell lines and was therefore not in itself likely to be a leukemogenic change but it could support the enhanced survival and proliferation of the Mixl1 granulocytic populations until a final leukemogenic mutation occurs in them.

In agar cultures of marrow cells from adult female BALB/c chimeric GATA-1 Plt13/+ mice, a high frequency of unusual dispersed colonies was noted. Analysis showed that these were colonies of mast cells and that mast cell colony-forming cells (progenitors) could be detected in clonal cultures of adult marrow, neonatal marrow, or fetal liver if the combined stimulus of stem cell factor and interleukin-3 was used. Mast cell progenitors were in active cell cycle and showed an extensive capacity for self-generation. Mast cell colonies both from control GATA-1 +/+ mice and GATA-1 Plt13/+ mice could generate growth factor-dependent cloned cell lines that grew for >18 months. Surprisingly, the majority of the excessive numbers of mast cell progenitors in chimeric GATA-1 Plt13/+ mice were transcribing the inactive Plt13 allele of GATA-1, suggesting that GATA-1 normally acts to restrict the emergence of committed mast cell progenitors. In sharp contrast, all eosinophil progenitors in these mice were transcribing the normal GATA-1 allele. No excess tissue mast cells were observed in GATA-1 Plt13/+ mice, suggesting that the excess mast cell progenitors in these mice might be generating mast cells with a defective in vivo proliferative or tissue homing capacity.

Mixl1 , the sole murine homologue of the Xenopus Mix/Bix family of homeobox transcription factors, is essential for the patterning of axial mesendodermal structures during early embryogenesis. Gene targeting and overexpression studies have implicated Mixl1 as a regulator of hematopoiesis arising in differentiating embryonic stem cells. To assess the role of Mixl1 in the regulation of adult hematopoiesis, we overexpressed Mixl1 in murine bone marrow using a retroviral transduction/transplantation model. Enforced expression of Mixl1 profoundly perturbed hematopoietic lineage commitment and differentiation, giving rise to abnormal myeloid progenitors and impairing erythroid and lymphoid differentiation. Moreover, all mice reconstituted with Mixl1 -transduced bone marrow developed fatal, transplantable acute myeloid leukemia with a mean latency period of 200 days. These observations establish a link between enforced Mixl1 expression and leukemogenesis in the mouse.

In agar cultures of mouse bone marrow cells, mega-karyocyte colony-forming cells exhibited shorter survival times than granulocyte-macrophage progenitor cells when initially cultured in the absence of stimulating factors. Initiation of cultures with G-CSF improved the survival times of granulocyte-macrophage progenitor cells and those of megakaryocyte progenitor cells. Paradoxically, G-CSF was found to consistently inhibit megakaryocyte colony formation stimulated by erythropoietin or by stem cell factor plus interleukin-3 (IL-3) plus erythropoietin. G-CSF was a less-consistent inhibitor of megakaryocyte colonies stimulated by thrombopoietin or IL-3. Analysis of the response of marrow cells from mice with the deletion of the genes encoding CIS, SOCS-1, SOCS-2, SOCS-3, SOCS-5, SOCS-6, or SOCS-7 indicated that the inhibitory SOCS proteins, with the possible exception of SOCS-3, were not involved in the G-CSF–initiated suppression of megakaryocyte colony formation.

Genetic screens in lower organisms, particularly those that identify modifiers of preexisting genetic defects, have been used successfully to order components of complex signaling pathways. To date, similar suppressor screens have not been used in vertebrates. To define the molecular pathways regulating platelet production, we have executed a large-scale modifier screen with genetically thrombocytopenic Mpl - / - mice by using N -ethyl- N -nitrosourea mutagenesis. Here we show that mutations in the c-Myb gene cause a myeloproliferative syndrome and supraphysiological expansion of megakaryocyte and platelet production in the absence of thrombopoietin signaling. This screen demonstrates the utility of large-scale N -ethyl- N -nitrosourea mutagenesis suppressor screens in mice for the simultaneous discovery and in vivo validation of targets for therapeutic discovery in diseases for which mouse models are available.

Mitogenic stimulation in vitro of mouse T lymphocytes induces the production of the hematopoietic cytokines granulocyte–macrophage colony-stimulating factor and IL-3. The present experiments showed that simple crowding of murine spleen or lymph node cells was a sufficient inducing stimulus. Crowding did not have this consequence for thymus or marrow cells or spleen cells from nu/nu or Rag-1 −/− mice lacking T lymphocytes. Crowding for as short a period as 24 h was sufficient to allow subsequent cytokine production in sparse cultures. Purified T lymphocytes also exhibited low levels of crowding induction of cytokine production and cytokine production was enhanced by IL-2 and IFN-γ. However, IFN-γ−/− spleen cells exhibited similar crowding-induced colony-stimulating factor production to that of control spleen cells. Excess cell crowding inhibited cytokine production. This inhibition was not caused by receptor internalization of cytokines but may contribute to the failure to observe IL-3 production in lymphoid organs in vivo . Coculture of allogeneic spleen or peritoneal cells was a strong inducing signal for colony-stimulating factor production but this parameter was unable to detect autoreactivity of T lymphocytes in mice that lack suppressor of cytokine signaling 1 and exhibit T lymphocyte activation.

There are only three known stimuli for eosinophil formation-GM-CSF, interleukin-5 (IL-5), and IL-3. Because mice with inactivation of the gene encoding the common beta receptor chain for GM-CSF, IL-5, and IL-3 (betac(-/-) mice) cannot respond to GM-CSF or IL-5 and do not produce IL-3, they should lack eosinophils. However, they produce reduced numbers of eosinophils, indicating the existence of at least one additional stimulatory factor. Use of betac(-/-) mouse marrow cells failed to detect a novel eosinophil-stimulating factor in cell line- or organ-conditioned media, but stem cell factor (SCF) was found to have a previously unrecognized ability to stimulate the formation of eosinophil colonies or mixed neutrophil-eosinophil colonies. This action of SCF was also observable with marrow cells from other mouse strains and was enhanced by the addition of G-CSF but no other factor tested. Recloning of SCF-stimulated blast colonies showed that progenitors forming pure eosinophil or mixed neutrophil-eosinophil colonies can have a common ancestor but many appear to arise independently from different preprogenitor cells. The observed activity of SCF in marrow cultures was relatively weak, but its action may be stronger in vivo, and SCF needs to be added to GM-CSF, IL-5, and IL-3 in the list of cytokines able to stimulate eosinophil formation.

The Asbs are a family of ankyrin repeat proteins that, along with four other protein families, contain a C-terminal SOCS box motif, which was first identified in the suppressor of cytokine signaling (SOCS) proteins. While it is clear that the SOCS proteins are involved in the negative regulation of cytokine signaling, the biological roles of the other SOCS box-containing families are unknown. We have investigated Asb-1 function by generating mice that lack this protein, as well as mice that overexpress full-length or truncated Asb-1 in a wide range of tissues. Although Asb-1 is expressed in multiple organs, including the hematopoietic compartment in wild-type mice, Asb-1(-/-) mice develop normally and exhibit no anomalies of mature blood cells or their progenitors. While most organs in these mice appear normal, the testes of Asb-1(-/-) mice display a diminution of spermatogenesis with less complete filling of seminiferous tubules. In contrast, the widespread overexpression of Asb-1 in the mouse has no apparent deleterious effects.

Murine Ba/F3 cells were transfected with cDNA for the alpha-chain of the murine interleukin-5 (IL-5) receptor and cloned lines of these cells were able to proliferate in response to as little as 2.5 pg/ml of IL-5. The bioassay was demonstrated to be specific for IL-5 and was able to measure IL-5 produced in culture by organs from adult C57BL/6 and BALB/c mice. The highest levels of IL-5 were produced by lung tissue but thymus and bladder consistently produced IL-5 and more variable production was observed by the heart, spleen, muscle, bone shaft, uterus and testes. Bone marrow cells produced no detectable IL-5. Observed levels of production of IL-5 were similar when using organs from mice lacking high-affinity receptors for IL-5 and from nu/nu, RAG-1-/- and NOD/SCID mice lacking T lymphocytes. In inflammatory peritoneal exudates induced by the injection of casein plus bacteria, levels of induced IL-5 were higher if the mice lacked high-affinity receptors for IL-5. The data indicate that T lymphocytes are not the dominant cellular source of IL-5 in organ-conditioned media and that local IL-5 production can occur with a wide range of normal murine organs.

The intraperitoneal injection into mice of casein preparations containing bacteria induced a rapid accumulation of neutrophils within 3 hours due to selective release of mature cells from the bone marrow. Significant increases in the concentrations of granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) occurred in the peritoneal cavity during the process, but the intraperitoneal injection of neither CSF induced a significant accumulation of neutrophils and the coinjection of G-CSF and casein failed to enhance the neutrophil response. The lack of involvement of either CSF in the neutrophil migration was confirmed by the development of typical neutrophil exudates when casein was injected into mice with inactivation of the genes encoding GM-CSF, G-CSF, or the beta-common chain of the GM-CSF receptor. However, preinjection of G-CSF increased the number of marrow neutrophils available for migration and did result in increased numbers of neutrophils in the peritoneal cavity after casein injection. Typical eosinophil inflammatory responses to the injection of casein or thioglycollate occurred in GM-CSF -/-mice but not in beta c -/-mice, suggesting that interleukin-5 was necessary for this response.

Double transgenic mice were produced by mating granulocyte-macrophage colony-stimulation factor (GM-CSF) transgenic mice with max 41 transgenic mice that exhibit excess granulopoiesis and a predisposition to thymic lymphoma development. Although only two-thirds of the double transgenic mice had elevated circulating GM-CSF levels, double transgenic mice maintained significantly higher blood granulocytes and monocytes and more extreme granulopoietic hypercellularity in the marrow and spleen than max 41 transgenic mice. In double transgenic mice, early death occurred from the GM-CSF transgenic syndrome. Because of these early deaths, the incidence of thymic and generalized lymphomas was artificially lower than in max 41 mice but those lymphomas that did develop occurred earlier than in max 41 mice. While the excess GM-CSF levels in double transgenic mice stimulated increased granulocyte and monocyte formation and peritoneal dendritic cells were excessive, this failed to prevent the spontaneous development of T lymphomas, suggesting that dendritic cell-initiated suppression of tumor development may not be effective with this type of tumor.

Abstract 2432 Introduction Acute myeloid leukemia (AML) is a genetically and morphologically heterogeneous disease characterized by the accumulation of immature myeloid lineage cells in the bone marrow and blood. It results from genetic alterations that cause increased self-renewal of myeloid progenitors, accompanied by a block in their normal differentiation programs. Studies in mice and humans have shown that loss of expression of PU.1, a master transcription factor that is critical for lymphoid and myeloid lineage development, is a recurrent feature of AML1. Restoring the PU.1 differentiation program in AML is an attractive therapeutic strategy, but remains elusive due to a poor understanding of PU.1 target genes and tumor suppressive mechanisms. In a novel approach to understanding PU.1 function, we have used in vivo RNA interference to inducibly inhibit and restore PU.1 expression in normal hematopoietic cells and leukemias. Results PU.1 knockdown promotes leukemia in mice We identified several short hairpin RNAs that can effectively knockdown PU.1 (Fig 1A). We infected primary fetal liver cells with the most effective LMP-shPU.1 retroviruses and performed in vitro and in vivo assays to assess the effect of PU.1 knockdown (Fig 1B). We found that PU.1 knockdown drives 1) an increased frequency of blast colony-forming cells and self-generation of granulocytic progenitors in vitro (Fig 1C) and 2) a GFP+ myeloid leukemia after several months characterized by accumulation of cKit+Gr1+Mac1+ cells (Fig 1D, E). These findings verify that shRNA-mediated PU.1 knockdown can effectively disable its tumor suppressive functions. Inducible restoration of PU.1 in leukemia in vivo To identify transcriptional targets of PU.1 in vivo, we utilized a recently generated reversible RNAi strategy that allows acute restoration of endogenous PU.1 expression upon Dox treatment in leukemias driven by PU.1 knockdown2. This TRMPV vector strategy allows tet-regulated co-expression of an shRNA and the fluorescent marker dsRed, with stable expression of GFP to mark infected cells. We transduced fetal liver cells derived from Vav-tTA transgenic mice with TRMPV-shPU.1 to drive reversible PU.1 knockdown across the hematopoietic system of reconstituted recipient mice. In contrast to the myeloid leukemia generated earlier using LMP-shPU.1, these mice developed pre-B cell (CD19+CD25+) leukemia with a latency of several months. To acutely restore endogenous PU.1 expression in leukemia, primary tumor cells were transplanted into several recipient mice to generate a cohort for analysis of Dox responses (Figure 2A). We found that dsRed intensity decreased incrementally upon Dox treatment of leukemic transplant recipient mice allowing FACS sorting of leukemia cells from triplicate untreated mice (dsRedhigh, minimal PU.1 expression) or after three days of Dox treatment (dsRedmid, partially restored PU.1 expression). We identified gene expression changes associated with PU.1 restoration using RNA sequencing (RNA-seq). Development of a transgenic mouse allowing inducible PU.1 knockdown in vivo To further investigate PU.1 target genes in vivo, we have recently generated TRE-GFP-shPU.1 transgenic mice allowing inducible knockdown and restoration of PU.1 in adult mice. To test this strain we crossed it to CAGs-rtTA3 mice and treated bitransgenic mice with Dox. Western blot analysis of GFP+ Gr1+Mac1+ sorted myeloid cells showed effective PU.1 knockdown in vivo. We are currently using these mice to identify PU.1 regulated genes in normal myeloblasts in vivo. Conclusions These studies have identified several new candidate PU.1-regulated genes. Further experiments may shed light on whether there is a common novel tumor suppressive mechanism for PU.1 in myeloid and lymphoid leukemias driven by loss of PU.1. Disclosures: No relevant conflicts of interest to declare.

Abstract 1283 ETS-related gene, ERG, is a key regulator of hematopoietic stem cell (HSC) function, and a potent oncogene. It is involved in chromosomal translocations with the EWS gene in Ewing's sarcoma, the TLS gene in Acute Myeloid Leukemia (AML) and the TMPRSS2 gene in more than half of all prostate cancers. In addition, increased ERG levels are associated with poor prognosis in cytogenetically normal AML and T-cell Acute Lymphoblastic Leukemia; and our recent data suggests that trisomy of ERG is important for development of myeloproliferative disease and Acute Megakaryocytic Leukemia (AMKL) in Down syndrome individuals. The role ERG, and the oncogenic fusion protein TLS-ERG, play during hematopoietic transformation remains unclear. Hematopoietic overexpression of ERG has been shown to induce T-cell leukemia in mice. Development of a non-lymphoid disease has also been described, however this disease was reported to be an AMKL by one group, and a non-malignant erythroid hyperplasia by another. Hematopoietic overexpression of TLS-ERG in mice has not been described. This fusion has been shown to perturb differentiation and increase self-renewal of human myeloid progenitor cells in vitro, and enable the IL-3 dependent L-G murine myeloid progenitor cell line to induce a leukemia-like disease in vivo. In order to clarify and compare the role of wild-type and rearranged forms of ERG in leukemia development, we injected lethally irradiated mice with fetal liver cells (FLCs) transduced with retrovirus carrying either Erg or TLS-ERG. These mice succumbed to disease with a median latency of 80 days after receiving Erg-transduced FLCs, or 44 days after receiving TLS-ERG-transduced FLCs. Consistent with published data, 30% of Erg mice developed T-cell leukemias. Interestingly, no TLS-ERG mice developed this disease. Strikingly, 100% of Erg and TLS-ERG mice developed an identical non-lymphoid disease characterised by hepatosplenomegaly, anemia and leukocytosis. Histopathological and flow cytometric analysis revealed infiltration of the bone marrow, spleen, lung and liver by nucleated erythroblasts, expressing a high level of CD71 and varying levels of Ter119. Interestingly, in some mice a subset of these cells also expressed the megakaryocytic marker CD41. Primary spleen cells were capable of transplanting disease in non-irradiated mice, demonstrating that this disease was malignant. Spleen cells from Erg and TLS-ERG leukemic mice, but not controls, were capable of generating large numbers of small colonies when cultured in methylcellulose stimulated with IL-3/SCF/EPO. These colonies primarily contained erythroblasts (CD71+Ter119+/−), however some cells also expressed CD41 and were acetylcholinesterase positive. Most acetylcholinesterase positive cells were of small size, indicating either incomplete or early megakaryocyte maturation. Combined, these data suggest that hematopoietic overexpression of Erg or TLS-ERG in mice leads to transformation of a bi-potential erythroid-megakaryocyte progenitor. Following transformation, this cell appears to retain some bi-potentiality in vitro, however in vivo it primarily develops along the erythroid lineage. Thus, Erg- and TLS-ERG- induced non-lymphoid disease may be best described as an erythro-megakaryocytic leukemia. Finally, the data also indicate that truncation and fusion to TLS abrogates ERG's ability to transform lymphoid progenitors, however TLS-ERG retains the ability to transform myeloid progenitors in a manner that strongly resembles that of wild-type Erg. Disclosures: No relevant conflicts of interest to declare.

To determine the importance of suppressor of cytokine signaling-3 (SOCS3) in the regulation of hematopoietic growth factor signaling generally, and of G-CSF-induced cellular responses specifically, we created mice in which the Socs3 gene was deleted in all hematopoietic cells. Although normal until young adulthood, these mice then developed neutrophilia and a spectrum of inflammatory pathologies. When stimulated with G-CSF in vitro, SOCS3-deficient cells of the neutrophilic granulocyte lineage exhibited prolonged STAT3 activation and enhanced cellular responses to G-CSF, including an increase in cloning frequency, survival, and proliferative capacity. Consistent with the in vitro findings, mutant mice injected with G-CSF displayed enhanced neutrophilia, progenitor cell mobilization, and splenomegaly, but unexpectedly also developed inflammatory neutrophil infiltration into multiple tissues and consequent hind-leg paresis. We conclude that SOCS3 is a key negative regulator of G-CSF signaling in myeloid cells and that this is of particular significance during G-CSF-driven emergency granulopoiesis.

SOCS-1 was originally identified as an inhibitor of interleukin-6 signal transduction and is a member of a family of proteins (SOCS-1 to SOCS-7 and CIS) that contain an SH2 domain and a conserved carboxyl-terminal SOCS box motif. Mutation studies have established that critical contributions from both the amino-terminal and SH2 domains are essential for SOCS-1 and SOCS-3 to inhibit cytokine signaling. Inhibition of cytokine-dependent activation of STAT3 occurred in cells expressing either SOCS-1 or SOCS-3, but unlike SOCS-1, SOCS-3 did not directly interact with or inhibit the activity of JAK kinases. Although the conserved SOCS box motif appeared to be dispensable for SOCS-1 and SOCS-3 action when over-expressed, this domain interacts with elongin proteins and may be important in regulating protein turnover. In gene knockout studies, SOCS-1−/− mice were born but failed to thrive and died within 3 weeks of age with fatty degeneration of the liver and hemopoietic infiltration of several organs. The thymus in SOCS-1−/− mice was small, the animals were lymphopenic, and deficiencies in B lymphocytes were evident within hemopoietic organs. We propose that the absence of SOCS-1 in these mice prevents lymphocytes and liver cells from appropriately controlling signals from cytokines with cytotoxic side effects. J. Leukoc. Biol. 66: 588–592; 1999.

Factor-specific cell line bioassays were used to monitor the production in vitro by adult and fetal mouse organs of GM-CSF, G-CSF, M-CSF, Multi-CSF (IL-3), IL-6 and leukemia inhibitory factor (LIF). No tissue was observed to produce Multi-CSF. Highest producers of the other regulators were lung, muscle, thymus, heart and bone shaft and all tissues producing detectable growth factors produced all five with the same rank order of activity. Adult tissues produced more GM-CSF than G-CSF and less M-CSF than either, no differences being observed between male, female and pregnant female tissues. In contrast, the pregnant uterus produced high levels of M-CSF as did the fetal membranes and tissues with only low GM-CSF and no G-CSF production. Pre-irradiation did not alter the pattern of regulator production by adult tissues. The intravenous injection of endotoxin elevated serum levels of GM-CSF, G-CSF, M-CSF and IL-6 but the dominant rise was in G-CSF levels. The data indicating that multiple organs produce the regulators monitored in a common rank order suggest some overall linkage in their production with major differences between adult and fetal tissues.

Marrow cells from mice lacking high-affinity receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF; βc−/− mice) were shown to bind and internalize much less GM-CSF than cells from normal (βc+/+) mice. βc−/− mice were used to determine the effect of negligible receptor-mediated clearance on detectible GM-CSF responses to the intravenous injection of endotoxin or the intraperitoneal injection of casein plus microorganisms. Unlike the minor serum GM-CSF responses to endotoxin seen in βc+/+ mice, serum GM-CSF levels rose 30-fold to 9 ng/mL in βc−/− mice even though loss of GM-CSF in the urine was greater than in βc+/+ mice. Organs from βc−/− and βc+/+ mice had a similar capacity to produce GM-CSF in vitro, as did peritoneal cells from both types of mice when challenged in vitro by casein. However, when casein was injected intraperitoneally, βc−/− mice developed higher and more sustained levels of GM-CSF than did βc+/+ mice. The data indicated that receptor-dependent removal of GM-CSF masks the magnitude of GM-CSF responses to endotoxin and local infections. Because of this phenomenon, serum GM-CSF concentrations can be a misleading index of the occurrence or nonoccurrence of GM-CSF responses to infections.