Two distinct types of murine blast colony-forming cells are multipotential hematopoietic precursors.

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.

Defective gp130-mediated signal transducer and activator of transcription (STAT) signaling results in degenerative joint disease, gastrointestinal ulceration, and failure of uterine implantation.

The receptor subunit gp130 transduces multiple cell type-specific activities of the leukemia inhibitory factor (LIF)/interleukin (IL)-6 family of cytokines through the signal transducer and activator of transcription (STAT) and src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-2/ras/Erk pathways. To define STAT-dependent physiological responses, we generated mice with a COOH-terminal gp130(DeltaSTAT) "knock-in" mutation which deleted all STAT-binding sites. gp130(DeltaSTAT) mice phenocopyed mice deficient for IL-6 (impaired humoral and mucosal immune and hepatic acute phase responses) and LIF (failure of blastocyst implantation). However, unlike mice with null mutations in any of the components in the gp130 signaling pathway, gp130(DeltaSTAT) mice also displayed gastrointestinal ulceration and a severe joint disease with features of chronic synovitis, cartilaginous metaplasia, and degradation of the articular cartilage. Mitogenic hyperresponsiveness of synovial cells to the LIF/IL-6 family of cyto-kines was caused by sustained gp130-mediated SHP-2/ras/Erk activation due to impaired STAT-mediated induction of suppressor of cytokine signaling (SOCS) proteins which normally limits gp130 signaling. Therefore, the joint pathology in gp130(DeltaSTAT) mice is likely to arise from the disturbance of the otherwise balanced activation of the SHP-2/ras/Erk and STAT signaling cascades emanating from gp130.

Suppressor of cytokine signalling 1 (SOCS1) is a physiological regulator of the asthma response.

BACKGROUND: The molecular determinants of the severity and persistence of allergic asthma remain poorly understood. Suppressor of cytokine signalling 1 (SOCS1) is a negative regulator of IL-4-dependent pathways in vitro and might therefore control T-helper type 2 (Th2) immunity associated traits, such as IgE levels, mucin production, IL-5 and IL-13 induction, and eosinophilic mucosal inflammation, which are implicated in allergic asthma. OBJECTIVE: To investigate the role of SOCS1 in regulating Th2-associated disease traits in a murine sub-chronic aeroallergen-driven asthma model. METHODS: Following sensitization and challenge with ovalbumin (OVA), bronchoalveolar lavage and serum were collected from mice lacking the Socs1 gene on an IFN-gamma null background (Socs1(-/-)Ifngamma(-/-)). The composition of infiltrating cells in the lung, serum IgE and IgG1 levels and cytokine levels were analysed. RESULTS: Serum IgE levels and infiltrating eosinophils were considerably increased in the lungs of OVA-treated Socs1(-/-)Ifngamma(-/-) mice compared with Ifngamma(-/-) and C57BL/6 controls. Expression of the Th2 cytokines, IL-4, IL-5 and IL-13 was increased in CD4+ cells and lung tissue from OVA-treated Socs1(-/-)Ifngamma(-/-) mice. IgE, IL-5 levels and infiltrating eosinophils were also elevated in saline-treated Socs1(-/-)Ifngamma(-/-) mice, suggesting that in the absence of SOCS1, mice are already biased towards a Th2 response. It is at present unclear whether the elevated cytokine levels are sufficient to result in the exacerbated Th2 response to OVA challenge or whether enhanced intra-cellular signalling also contributes. Surprisingly, of the various IL-4/IL-13 responsive genes tested, only Arginase I appeared to be modestly up-regulated in the lungs of OVA-treated Socs1(-/-)Ifngamma(-/-) mice, suggesting that regulation by SOCS1 occurs primarily in haematopoietic cells and not in the airway epithelium. CONCLUSIONS: Together these results indicate that SOCS1 is an important regulator of the Th2 response.

Suppressor of cytokine signaling 3 (SOCS3) limits damage-induced crypt hyper-proliferation and inflammation-associated tumorigenesis in the colon.

Intestinal injury or chronic inflammation induce cytokines that promote crypt regeneration and mucosal repair. If excessive or prolonged, such mechanisms may increase colon cancer risk. Factors that terminate or limit cytokine action in intestinal epithelial cells (IEC) may protect against crypt hyperplasia and neoplasia. We hypothesized that suppressor of cytokine signaling-3 (SOCS3) is such a factor. Mice with Vilin-promoter/Cre-recombinase (VC)-mediated IEC-specific SOCS3 gene disruption (VC/HO), WT/HO littermates with floxed but intact SOCS3 genes and VC/WT mice were studied. Colon was examined after acute dextran sodium sulfate (DSS)-induced mucosal injury or after azoxymethane (AOM) and chronic DSS. Signaling pathways were examined in colon, cultured IEC or colon cancer cell lines. VC/HO mice showed no basal phenotype. After acute DSS, VC/HO exhibited enhanced crypt proliferation and crypt hyperplasia and reduced transforming growth factor (TGF) beta expression in colon. Inflammation and mucosal damage were similar across genotypes. Following AOM/DSS, VC/HO mice had increased size, number and load of colonic tumors and increased STAT3 and nuclear factor-kappa B (NF-kappaB) activation in colon. In vitro, SOCS3 overexpression reduced proliferation, IL-6-mediated STAT3 activation and tumor necrosis factor (TNF) alpha-mediated NF-kappaB activation. We conclude that cytokine induction of SOCS3 normally provides an intrinsic mechanism to limit injury-induced crypt hyperproliferation and inflammation-associated colon cancer by regulating both STAT3 and NF-kappaB pathways.

Genetic deletion of murine SPRY domain-containing SOCS box protein 2 (SSB-2) results in very mild thrombocytopenia.

The SSB family is comprised of four highly homologous proteins containing a C-terminal SOCS box motif and a central SPRY domain. No function has yet been ascribed to any member of this family in mammalian species despite a clear role for other SOCS proteins in negative regulation of cytokine signaling. To investigate its physiological role, the murine Ssb-2 gene was deleted by homologous recombination. SSB-2-deficient mice were shown to have a reduced rate of platelet production, resulting in very mild thrombocytopenia (25% decrease in circulating platelets). Tissue histology and other hematological parameters were normal, as was the majority of serum biochemistry, with the exception that blood urea nitrogen (BUN) levels were decreased in mice lacking SSB-2. Quantitative analysis of SSB mRNA levels indicated that SSB-1, -2, and -3 were ubiquitously expressed; however, SSB-4 was only expressed at very low levels. SSB-2 expression was observed in the kidney and in megakaryocytes, a finding consistent with the phenotype of mice lacking this gene. Deletion of SSB-2 thus perturbs the steady-state level of two tightly controlled homeostatic parameters and identifies a critical role for SSB-2 in regulating platelet production and BUN levels.

Suckling defect in mice lacking the soluble haemopoietin receptor NR6.

Cytokines control a variety of cellular responses including proliferation, differentiation, survival and functional activation, via binding to specific receptors expressed on the surface of target cells [1]. The cytokine receptors of the haemopoietin family are defined by the presence of a conserved 200 amino acid extracellular domain known as the haemopoietin domain [2]. We report here the isolation of NR6, a haemopoietin receptor that, like the p40 subunit of interleukin-12 (IL-12) [3] and the EBI3 gene induced by Epstein-Barr virus infection in lymphocytes [4], contains a typical haemopoietin domain but lacks transmembrane and cytoplasmic domains. Although in situ hybridisation revealed NR6 expression at multiple sites in the developing embryo, mice lacking NR6 did not display obvious abnormalities and were born in the expected numbers. Neonatal NR6(-/-) mice failed to suckle, however, and died within 24 hours of birth, suggesting that NR6 is necessary for the recognition or processing of pheromonal signals or for the mechanics of suckling itself. In addition, NR6(-/-) mice had reduced numbers of haemopoietic progenitor cells, suggesting a potential role in the regulation of primitive haemopoiesis.

Expression of the c-myc oncogene under control of an immunoglobulin enhancer in E mu-myc transgenic mice.

Transgenic mice bearing a cellular myc oncogene coupled to the immunoglobulin heavy-chain enhancer (E mu) exhibit perturbed B-lymphocyte development and succumb to B lymphoid tumors. To investigate how the enhancer has affected myc expression, we analyzed the structure and abundance of myc transcripts in tissues of prelymphomatous mice and in the lymphomas. Expression of the E mu-myc transgene appeared to be confined largely to B lymphoid cells, being dominant in bone marrow, spleen, and lymph nodes, with no detectable expression in T cells or other hematopoietic lineages examined. The myc transcripts initiated very predominantly at the normal myc promoters, although use of the more upstream myc promoter was accentuated and an enhancer-associated promoter may be used infrequently. The level of E mu-myc transcripts in the preneoplastic lymphoid tissues and in the E mu-myc tumors was not markedly higher than myc RNA levels in proliferating normal lymphocytes. Thus, enforced expression of structurally normal myc transcripts at only a modestly elevated level has profound biological consequences. The absence of detectable endogenous c-myc RNA in any tumor, or in preneoplastic bone marrow, supports a negative feedback model for normal c-myc regulation.

Oncogene cooperation in lymphocyte transformation: malignant conversion of E mu-myc transgenic pre-B cells in vitro is enhanced by v-H-ras or v-raf but not v-abl.

Although transgenic mice bearing a c-myc gene controlled by the immunoglobulin heavy-chain enhancer (E mu) eventually develop B-lymphoid tumors, B-lineage cells from preneoplastic bone marrow express the transgene but do not grow autonomously or produce tumors in mice. To determine whether other oncogenes can cooperate with myc to transform B-lineage cells, we compared the in vitro growth and tumorigenicity of normal and E mu-myc bone marrow cells infected with retroviruses bearing the v-H-ras, v-raf, or v-abl oncogene. The v-H-ras and v-raf viruses both generated a rapid polyclonal expansion of E mu-myc pre-B bone marrow cells in liquid culture and 10- to 100-fold more pre-B lymphoid colonies than normal in soft agar. The infected transgenic cells were autonomous, cloned efficiently in agar, and grew as tumors in nude mice. While many pre-B cells from normal marrow could also be induced to proliferate by the v-raf virus, these cells required a stromal feeder layer, did not clone in agar, and were not malignant. Most normal cells stimulated to grow by v-H-ras also cloned poorly in agar, and only rare cells were tumorigenic. With the v-abl virus, no more cells were transformed from E mu-myc than normal marrow and the proportion of tumorigenic pre-B clones was not elevated. These results suggest that both v-H-ras and v-raf, but apparently not v-abl, collaborate with constitutive myc expression to promote autonomous proliferation and tumorigenicity of pre-B lymphoid cells.

Functional analysis of Asb-1 using genetic modification in mice.

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.

Primary meningococcal conjunctivitis: an unusual case of transmission by saliva.

A 49-year-old diabetic man presented with a 2-day history of a painful right eye associated with a purulent discharge. Prior to becoming symptomatic, he reported that someone spat at him, resulting in direct contact between the saliva and his affected eye. Gram stain revealed numerous leucocytes with Gram-negative diplococci, and culture yielded Neisseria meningitidis (serogroup C). There was no evidence of any systemic infection, and blood cultures were negative for any growth. He was treated for primary meningococcal conjunctivitis (PMC) with intensive topical antibiotic eyedrops as well as systemic antibiotics. One week after commencing treatment he remained systemically well and his symptoms had fully resolved.

Regulation of platelet lifespan in the presence and absence of thrombopoietin signaling.

UNLABELLED: Essentials We examined platelet survival in models of absent or enhanced thrombopoietin (TPO) signaling. Platelet lifespan is normal in transgenic mice with chronically enhanced TPO signaling. Mpl deficiency does not negatively affect platelet lifespan in the absence of thrombocytopenia. We conclude that TPO and its receptor Mpl are dispensable for platelet survival in adult mice. SUMMARY: Background It is well established that thrombopoietin (TPO), acting via its receptor Mpl, is the major cytokine regulator of platelet biogenesis. The primary mechanism by which TPO signaling stimulates thrombopoiesis is via stimulation of Mpl-expressing hematopoietic progenitors; Mpl on megakaryocytes and platelets acts to control the amount of TPO available. TPO could potentially reduce platelet and/or megakaryocyte apoptosis, and therefore increase the platelet count. However, the effect of TPO receptor signaling on platelet survival is unresolved. Methods and results Here, we investigated platelet survival in mouse models of absent or enhanced TPO signaling. In the absence of thrombocytopenia, Mpl deficiency did not negatively influence platelet lifespan, and nor was platelet survival affected in transgenic mice with chronically increased TPO signaling. Conclusions We conclude that TPO and its receptor Mpl are dispensable for platelet survival in adult mice.

BET inhibition represses miR17-92 to drive BIM-initiated apoptosis of normal and transformed hematopoietic cells.

The BET (bromodomain and extraterminal domain) bromodomain-containing proteins, such as BRD4, are highly promising targets for treating lymphoid and myeloid malignancies. They act to modulate the expression of multiple genes that control diverse cellular processes including proliferation, survival and differentiation that are consequentially disrupted by small-molecule BET bromodomain inhibitors such as JQ1. By assessing the impact of these inhibitors on normal mouse hematopoietic cells or their transformed counterparts, we establish definitively that their cytotoxic action in vitro and in vivo relies predominantly on the activation of BAX/BAK-dependent mitochondrial (intrinsic) apoptosis. In large part, this is triggered by marked upregulation of the BH3-only protein BIM when the BET inhibitors suppress miR-17-92, a key post-transcriptional repressor of BIM expression. Thus, our study strongly suggests that mutations that permit the evasion of apoptosis (for example, BCL2 overexpression, BIM inactivation) are likely to blunt the activity of the BET bromodomain inhibitors and should be anticipated when therapy resistance develops. Strikingly, we also found that certain normal hematopoietic cells, especially those of lymphoid origin, are as prone to apoptosis induced by the BET inhibitors as their transformed counterparts, indicating that their susceptibility to BET inhibitors did not arise from oncogenic transformation.

Structure and transcription of the genomic locus encoding murine c-Mpl, a receptor for thrombopoietin.

We have cloned and characterised the murine gene encoding c-Mpl, a receptor for thrombopoietin and member of the hematopoietin receptor superfamily. The gene encompasses 15 kb of the mouse genome and the organisation of its 12 exons conforms closely to the pattern observed for the genes of other hematopoietin receptor family members. A site for initiation of c-mpl transcription was identified 13 nucleotides upstream of the proposed translation initiation codon. The murine mpl promoter sequence lacks conventional TATA and CAAT motifs although the transcription initiation site shares homology with the initiator sequence that specifies transcription initiation in the terminal deoxynucleotidyl transferase gene. The promoter contains consensus binding sequences for several transcriptional regulators including Ets and GATA factors, which have been implicated in control of transcription in megakaryocytes, a cell type that expresses mpl. The generation of multiple transcripts is a feature of the mpl locus. Two distinct mpl transcripts differing by an in-frame insertion of 24 nucleotides were detected in mouse spleen cells. Genomic sequence analysis identified differential splicing of alternative exon 4 sequences as the likely basis for these transcripts, which are predicted to encode receptors which differ within the first Mpl hematopoietin receptor domain.

Lymphomagenesis in E mu-myc transgenic mice can involve ras mutations.

Transgenic mice bearing c-myc driven by the immunoglobulin heavy chain enhancer (E mu) initially exhibit a preneoplastic lymphoproliferative syndrome from which clonal pre-B or B lymphomas develop at random. To investigate whether this transition involves the activation of oncogenes capable of transforming fibroblasts, we transfected DNA from 14 E mu-myc lymphomas into NIH3T3 cells and tested the tumorigenicity of the transfectants in nude mice. By this assay and subsequent direct analysis of the lymphoma mRNA by cDNA cloning or the polymerase chain reaction, two independent E mu-myc lymphomas were shown to contain an N-ras or a K-ras oncogene mutated at codon 61. When incorporated into a recombinant retrovirus, the mutant N-ras allele could collaborate with myc to transform preneoplastic E mu-myc bone marrow pre-B cells. These results indicate that spontaneous mutation of ras genes is one pathway to lymphomagenesis in E mu-myc mice but that many of the lymphomas arise in response to changes that do not register in fibroblasts.

The multi-organ origin of interleukin-5 in the mouse.

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.

Aberrant hematopoiesis in mice with inactivation of the gene encoding SOCS-1.

Mice with homozygous inactivation of the gene encoding the suppressor of cytokine signaling-1 (SOCS-1) protein die within 21 days of birth with low body weight, fatty degeneration and necrosis of the liver, infiltration of the lung, pancreas, heart and skin by macrophages and granulocytes and a profound depletion of T- and B-lymphocytes. In the present study, SOCS-1 -/- mice were found to have a moderate neutrophilia, and reduced platelet and hematocrit levels. Replacement of the SOCS-1 gene by a lac-Z reporter gene allowed documentation by FACS sorting that at least a proportion of granulocyte-macrophage progenitor cells transcribe SOCS-1. Most hematopoietic progenitor cell frequencies were normal in -/- marrow as were the size and cellular content of colonies formed by -/- progenitor cells in response to various stimulating factors. However, there was an increased frequency of macrophage progenitor cells in -/- mice and, abnormally, one quarter of all progenitor cells were located in the liver. Progenitor cells from -/- mice were hyper-responsive to stimulation by GM-CSF but not by M-CSF or Multi-CSF (IL-3). Progenitor cells from -/- mice were also hypersensitive to inhibition by interferon-gamma (IFN-gamma), the degree of inhibition varying markedly with the stimulating factor used. The suppressive effects of IFN-gamma therefore appear to involve interactions with particular growth factor-initiated signals in -/- cells--interactions that are strongly modulated by the action of the SOCS-1 protein.

Suppressors of cytokine signaling (SOCS): negative regulators of signal transduction.

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 overexpressed, 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.

Granulocyte-macrophage colony-stimulating factor is not responsible for residual thrombopoiesis in mpl null mice.

OBJECTIVE: To examine the role of granulocyte-macrophage colony-stimulating factor (GM-CSF) in thrombopoiesis. MATERIALS AND METHODS: Thrombopoietin-unresponsi ve mice (mpl null mice), which have a profound reduction in platelets and mature megakaryocytes, were interbred with mice that do not respond to GM-CSF or interleukin 5 (betac null mice), and hematopoiesis was examined. In initial experiments on a mixed genetic background, double mutant mice (betac/mpl null mice) showed an unexpected amelioration of the thrombocytopenia seen in mpl null mice. Platelet counts were elevated approximately twofold in betac/mpl null mice compared with mpl null mice (mpl null 73+/-31; betac/mpl null 164+/-70; n = 10 to 29 mice per genotype, p<0.00001). This was associated with lessening of the deficit of megakaryocytes, progenitor cells, and colony-forming units spleen seen in mpl null mice. This amelioration of the mpl null phenotype in betac/mpl null mice on a mixed genetic background was highly statistically significant. To determine whether this amelioration of phenotype was solely the consequence of loss of betac signaling, progeny of a second intercross on a C57BL/6 background (B6betac/mpl null mice) were examined. When the resulting B6betac/mpl null mice were analyzed and compared with B6mpl null littermates, the increase in platelet count, hematopoietic progenitor cell number, and colony-forming units spleen number was no longer observed. CONCLUSION: There was no additional effect seen as a result of loss of betac signaling. GM-CSF did not play a significant role in thrombopoiesis, even in combination with the absence of thrombopoietin signaling. These results highlight problems that can be encountered when studying introduced mutations in mice. They exemplify the importance of eliminating the influence of modifying genes when attributing biologic differences to specific introduced genetic alterations.

Thrombopoietin signaling is required for in vivo expansion of IL-11--responsive hematopoietic progenitor cells in the steady state.

OBJECTIVE: mpl(-/-) mice have a profound defect in platelets and megakaryocytes and a defect in hematopoietic progenitor cells and stem cells. However, no specific subset of the progenitor/stem cell compartment has been shown to be particularly affected by this deficiency in mpl(-/-) mice. In this article, we identified a specific subset of bone marrow progenitor/stem cells that was altered in mpl(-/-) mice. MATERIALS AND METHODS: In vitro and in vivo hematopoietic assays were utilized to examine the response to interleukin-11 in mice lacking the receptor for thrombopoietin (TPO) (mpl(-/-) mice). RESULTS: The interleukin (IL)-11-responsive subset of progenitor cells was not detected in clonal cultures of bone marrow cells from mpl(-/-) mice. However, mpl(-/-) mice responded to IL-11 in vivo as evidenced by a rise in platelet count and an increase in spleen weight. Experiments were performed to address this paradox: administration of 5-fluorouracil with consequent "expansion" of early hematopoietic cells resulted in the appearance of IL-11-responsive cells in mpl(-/-) mice when assayed in in vitro cultures. CONCLUSIONS: Thus, although mpl(-/-) mice have the capacity to produce IL-11-responsive progenitor cells, under steady state conditions their expansion is dependent on TPO. This is the first evidence that a specific subset of bone marrow progenitor/stem cells is altered in mpl(-/-) mice.

Hhex regulates Kit to promote radioresistance of self-renewing thymocytes in Lmo2-transgenic mice.

Lmo2 is an oncogenic transcription factor that is frequently overexpressed in T-cell acute leukemias, in particular poor prognosis early T-cell precursor-like (ETP-) acute lymphoblastic leukemia (ALL). The primary effect of Lmo2 is to cause self-renewal of developing CD4(-)CD8(-) (double negative, DN) T cells in the thymus, leading to serially transplantable thymocytes that eventually give rise to leukemia. These self-renewing thymocytes are intrinsically radioresistant implying that they may be a source of leukemia relapse after therapy. The homeobox transcription factor, Hhex, is highly upregulated in Lmo2-transgenic thymocytes and can phenocopy Lmo2 in inducing thymocyte self-renewal, implying that Hhex may be a key component of the Lmo2-induced self-renewal program. To test this, we conditionally deleted Hhex in the thymi of Lmo2-transgenic mice. Surprisingly, this did not prevent accumulation of DN thymocytes, nor alter the rate of overt leukemia development. However, deletion of Hhex abolished the transplantation capacity of Lmo2-transgenic thymocytes and overcame their radioresistance. We found that Hhex regulates Kit expression in Lmo2-transgenic thymocytes and that abrogation of Kit signaling phenocopied loss of Hhex in abolishing the transplantation capacity and radioresistance of these cells. Thus, targeting the Kit signaling pathway may facilitate the eradication of leukemia-initiating cells in immature T-cell leukemias in which it is expressed.