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Supplementary Methods, Figure Legends, Table Legend

Supplementary Methods

Acute leukemias are systemic malignancies associated with a dire outcome. Because of low immunogenicity, leukemias display a remarkable ability to evade immune control and are often resistant to checkpoint blockade. Here, we discover that leukemia cells actively establish a suppressive environment to prevent immune attacks by co-opting a signaling axis that skews macrophages toward a tumor-promoting tissue repair phenotype, namely the GAS6/AXL axis. Using aggressive leukemia models, we demonstrate that ablation of the AXL receptor specifically in macrophages, or its ligand GAS6 in the environment, stimulates antileukemic immunity and elicits effective and lasting natural killer cell– and T cell–dependent immune response against naïve and treatment-resistant leukemia. Remarkably, AXL deficiency in macrophages also enables PD-1 checkpoint blockade in PD-1–refractory leukemias. Finally, we provide proof-of-concept that a clinical-grade AXL inhibitor can be used in combination with standard-of-care therapy to cure established leukemia, regardless of AXL expression in malignant cells.Significance:Alternatively primed myeloid cells predict negative outcome in leukemia. By demonstrating that leukemia cells actively evade immune control by engaging AXL receptor tyrosine kinase in macrophages and promoting their alternative priming, we identified a target which blockade, using a clinical-grade inhibitor, is vital to unleashing the therapeutic potential of myeloid-centered immunotherapy.This article is highlighted in the In This Issue feature, p. 2659

Supplementary Figures and Figure legends

Supplementary Figure 1. Anti-CD3 stimulation of primary human T-ALLs. Supplementary Figure 2. TCR stimulation by in vivo administration of agonistic monoclonal antibody in a preventive setting inhibits human TCR+ T-ALL development. Supplementary Figure 3. TCR signaling is essential to the anti-leukemic effect of OKT3. Supplementary Table 1. Immunophenotypic and oncogenic characteristics of T-ALL samples.

Supplementary Methods, Tables, figures and references.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy that accounts for ∼20% of ALL cases. Intensive chemotherapy regimens result in cure rates >85% in children and

International audience; Adult "T cell" acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that is associated with poor outcomes, requiring additional therapeutic options. The DNA methylation landscapes of adult T-ALL remain undercharacterized. Here, we systematically analyzed the DNA methylation profiles of normal thymic-sorted T cell subpopulations and 143 primary adult T-ALLs as part of the French GRAALL 2003–2005 trial. Our results indicated that T-ALL is epigenetically heterogeneous consisting of five subtypes (C1-C5), which were either associated with co-occurring DNA methyltransferase 3 alpha (DNMT3A)/isocitrate dehydrogenase [NADP(+)] 2 (IDH2) mutations (C1), TAL bHLH transcription factor 1, erythroid differentiation factor (TAL1) deregulation (C2), T cell leukemia homeobox 3 (TLX3) (C3), TLX1/in cis-homeobox A9 (HOXA9) (C4), or in trans-HOXA9 overexpression (C5). Integrative analysis of DNA methylation and gene expression identified potential cluster-specific oncogenes and tumor suppressor genes. In addition to an aggressive hypomethylated subgroup (C1), our data identified an unexpected subset of hypermethylated T-ALL (C5) associated with poor outcome and primary therapeutic response. Using mouse xenografts, we demonstrated that hypermethylated T-ALL samples exhibited therapeutic responses to the DNA hypomethylating agent 5-azacytidine, which significantly (survival probability; P = 0.001 for C3, 0.01 for C4, and 0.0253 for C5) delayed tumor progression. These findings suggest that epigenetic-based therapies may provide an alternative treatment option in hypermethylated T-ALL.

T-cell acute lymphoblastic leukemia (T-ALL) arises from T cell precursors and is characterized by expression of many lineage-specific proteins. While T-cell antigen receptor (TCR) signaling and its strength are central for thymocyte development, mature T cell homeostasis and immune responses, their roles in T-ALL remain undetermined. Indeed, in contrast to mouse models, in which absence of TCR or major histocompatibility complex binding does not impact on leukemogenesis, other mouse models suggest that basal or weak signaling drives leukemia development. However, recent reports indicate that strong TCR signaling can be detrimental to leukemic cells. Indeed, sustained/high level TCR signaling, stimulated by antigen or CD3 antibody, is strongly anti-leukemic in both murine T-ALL expressing endogenous or transgenic TCR and diagnostic T-ALL cases. As discussed, further work should address the efficacy of T-ALL therapeutic targeting with either TCR/CD3 antibodies or TCR-directed chimeric antigen receptor T cells.

International audience; Leukemias are frequently characterized by the expression of oncogenic fusion chimeras that normally a rise due to chromosomal rearrangements. Cis-splicing of adjacent genes (cis-SAGe) results in transcription of intergenically-spliced chimeric RNAs (ISCs) in the absence of structural genomic changes, and aberrant ISC expression is now recognized as a potential cancer driver. We performed high-throughput RNA-sequencing of human T-acute lymphoblastic leukemia (T-ALL)samples, and used targeted analysis pipelines to detect fusion chimeras. We identified 55 candidate T-ALL-related ISCs, with a median of 4 per patient. We performed additional in-depth characterization of the NFATC3-PLA2G15 chimera, which was expressed at variable levels in primary T-ALL cases. Experimental analysis revealed that the fusion had lower activity than wild-type NFATC3 in vitro, and that T-ALL blasts with elevated NFATC3-PLA2G15 levels had reducedtranscription of canonical NFAT pathway genes in vivo. Strikingly, we found that high expression of the NFATC3-PLA2G15 chimera in leukemic blasts correlated with aggressive disease biology in murine patient-derived T-ALL xenografts,andpoor prognosis in human T-ALLpatients treated as part of the Francophone multinational GRAALL-2003 and -2005 studies. Our results suggest that ISCs are common in T-ALL, and that expression of specific ISCs may correlate with patient outcome.

Leukemias are frequently characterized by the expression of oncogenic fusion chimeras that normally arise due to chromosomal rearrangements. Intergenically spliced chimeric RNAs (ISC) are transcribed in the absence of structural genomic changes, and aberrant ISC expression is now recognized as a potential driver of cancer. To better understand these potential oncogenic drivers, high-throughput RNA sequencing was performed on T-acute lymphoblastic leukemia (T-ALL) patient specimens (

Background: TAM receptor tyrosine kinases—Tyro3, Axl and Mertk—and their ligands, Gas6 and Pros1, have been extensively studied for their cell-intrinsic pro-oncogenic function in cancer cells, including leukemia. However, much less is known about their indirect impact on tumor growth through their function as modulators of the immune system. In particular, no study has yet explored this aspect in the context of hematological malignancies. Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL) is the most aggressive human ALL subtype, in particular in adults, where it represents 30% of all ALL cases. The disease responds poorly to standard chemotherapy and has a very high risk of recurrence. Treatment relies on the use of a BCR-ABL1 tyrosine kinase inhibitor (TKI) with or without chemotherapy followed by an allogenic-SCT. Despite this intensive regimen a significant fraction (40%) of adult patients fails to achieve long-term disease-free survival, clearly pointing to an unmet clinical need. Methods: To explore the importance of TAM receptors and their ligands in the immune response against leukemia, we developed a very aggressive syngeneic model of Ph+ B-ALL. In this model, leukemia is driven by the expression of the fusion oncoprotein BCR-ABL1 and the loss of the Arf gene, often altered in human Ph+ ALL. Functional studies were carried out using small-molecule inhibitors and genetic mouse models. The latter allowed us to interrogate the importance of specific TAM receptors in defined immune cellular subsets. Lastly, we used pharmacologic inhibitors to explore the potential therapeutic benefit of targeting TAMR signaling alone or in combination with standard of care treatment (TKI) and checkpoint inhibitors. Results: Our study for the first time shows that Gas6, a high-affinity Axl ligand, promotes the establishment of an immune-suppressive milieu that contributes to the aggressive phenotype associated with Ph+ B-ALL. Importantly, Gas6 is not expressed by leukemic cells but rather produced by bone marrow associated stromal cells. Using genetic approaches, we demonstrate that Gas6 primarily acts through its high-affinity receptor, Axl, specifically on myeloid cells, to inhibit the anti-leukemic immune response. This immune-suppressive effect can be effectively blocked using an orally available selective Axl inhibitor or the genetic deletion of Axl in Csf1r expressing myeloid cells, leading to reduced leukemic burden and significantly prolonged survival of leukemia-challenged mice. In a subset of long-term survivors, progressive increase in PD1 expression limited the antileukemic effects associated with Axl deficiency, an effect that can be efficiently reverted by combination treatment with anti-PD1 checkpoint inhibitor.Mechanistically, the antileukemic effects promoted by Gas6/Axl blockade are mediated by an enhanced inflammatory response, followed by a potent adaptive immune response. This can be further potentiated by combination with standard of care therapy (i.e., TKI and chemotherapy), leading to an unprecedented cure rate of over 70%. This synergistic effect was completely abolished when the combination was evaluated in immune-compromised hosts, further emphasizing the strict immune-dependent nature of this phenotype. Conclusion: Our work uncovers a novel mechanism through which leukemic cells maintain an immune-suppressive environment by co-opting the ability of stromal cells to produce Gas6, a secreted TAM receptor ligand that dampens the ability of innate immune cells to fight leukemia. Therapeutic targeting of the Gas6/Axl axis effectively unleashes the innate immune system by promoting an enhanced proinflammatory response and engagement of a productive adaptive immune response. When combined with TKI or anti-PD1 in vivo, this treatment significantly improves the outcome of leukemia-challenged mice, leading to a remarkable cure rate of over 70%, unprecedented in this agressive model of Ph+ B-ALL. Citation Format: Irene Tirado-Gonzalez, Aleksandra Nevmerzhitskaya, Arnaud Descot, Devona Soetopo, Ewelina Czlonka, Maresa Weitmann, Carolin Wachtel, Julia Slotta-Huspenina, Christine Tran-Quang, Katharina Götze, Emily Alberto, Carla Vanina Rothlin, Jacques Ghysdael, Hind Medyouf. TAM receptors targeting unleashes antileukemic immunity and enables checkpoint blockade leading to eradication of leukemic cells [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A092.

Cancer onset and progression involves the accumulation of multiple oncogenic hits, which are thought to dominate or bypass the physiologic regulatory mechanisms in tissue development and homeostasis. We demonstrate in T-cell acute lymphoblastic leukemia (T-ALL) that, irrespective of the complex oncogenic abnormalities underlying tumor progression, experimentally induced, persistent T-cell receptor (TCR) signaling has antileukemic properties and enforces a molecular program resembling thymic negative selection, a major developmental event in normal T-cell development. Using mouse models of T-ALL, we show that induction of TCR signaling by high-affinity self-peptide/MHC or treatment with monoclonal antibodies to the CD3ϵ chain (anti-CD3) causes massive leukemic cell death. Importantly, anti-CD3 treatment hampered leukemogenesis in mice transplanted with either mouse- or patient-derived T-ALLs. These data provide a strong rationale for targeted therapy based on anti-CD3 treatment of patients with TCR-expressing T-ALL and demonstrate that endogenous developmental checkpoint pathways are amenable to therapeutic intervention in cancer cells. Significance: T-ALLs are aggressive malignant lymphoid proliferations of T-cell precursors characterized by high relapse rates and poor prognosis, calling for the search for novel therapeutic options. Here, we report that the lineage-specific TCR/CD3 developmental checkpoint controlling cell death in normal T-cell progenitors remains switchable to induce massive tumor cell apoptosis in T-ALL and is amenable to preclinical therapeutic intervention. Cancer Discov; 6(9); 972–85. ©2016 AACR. See related commentary by Lemonnier and Mak, p. 946. This article is highlighted in the In This Issue feature, p. 932

Intensive chemotherapy regimens have led to a substantial improvement in the cure rate of patients suffering from T-cell acute lymphoblastic leukemia (T-ALL). Despite this progress, about 15% and 50% of pediatric and adult cases, respectively, show resistance to treatment or relapse with dismal prognosis, calling for further therapeutic investigations. T-ALL is an heterogeneous disease, which presents intrinsic alterations leading to aberrant expression of transcription factors normally involved in hematopoietic stem/progenitor cell development and mutations in genes implicated in the regulation of cell cycle progression, apoptosis, and T-cell development. Gene expression profiling allowed the classification of T-ALL into defined molecular subgroups that mostly reflects the stage of their differentiation arrest. So far this knowledge has not translated into novel, targeted therapy. Recent evidence points to the importance of extrinsic signaling cues in controlling the ability of T-ALL to home, survive, and proliferate, thus offering the perspective of new therapeutic options. This review summarizes the present understanding of the interactions between hematopoietic cells and bone marrow/thymic niches during normal hematopoiesis, describes the main signaling pathways implicated in this dialog, and finally highlights how malignant T cells rely on specific niches to maintain their ability to sustain and propagate leukemia.

Rearrangement of the FLI-1 locus and ensuing overexpression of FLI-1 protein is an early event in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. When overexpressed in primary erythroblasts, FLI-1 converts erythropoietin (Epo)-induced terminal differentiation into a proliferative response. We found that SLAP, a gene encoding a recently described negative regulator of T-cell antigen receptor function during thymocyte development, is up-regulated both at the RNA and protein levels in FLI-1-transformed erythroblasts. Src-like adaptor protein (SLAP) was found in a specific complex with erythropoietin receptor (EpoR), a cytokine receptor essential to erythroid differentiation. Constitutive expression of SLAP severely impairs hemoglobinization and late survival during Epo-induced terminal differentiation of erythroblasts. This impairment is associated with the specific inhibition of several critical Epo-dependent signaling events, including signal transducer and activator of transcription 5 (STAT5) activation and up-regulation of the expression of the antiapoptotic BCL-X gene. Our data support a model by which FLI-1 inhibits normal erythroid differentiation through the deregulation of genes encoding adaptors/effectors that modify the signaling output of cytokine receptors normally required for terminal differentiation. (Blood. 2003; 102:4555-4562)

SummaryImpaired cell migration has been demonstrated in T cell acute lymphoblastic leukemia (T-ALL) cells upon calcineurin inactivation, among other phenotypic traits including increased apoptosis, inhibition of cell proliferation, and ultimately inhibition of leukemia-initiating cell (LIC) activity. Herein we demonstrate that the chemokine receptor CXCR4 is essential to the LIC activity of T-ALL leukemic cells both in NOTCH-induced mouse T-ALL and human T-ALL xenograft models. We further demonstrate that calcineurin regulates CXCR4 cell-surface expression in a cortactin-dependent manner, a mechanism essential to the migratory properties of T-ALL cells. Because 20%–25% of pediatric and over 50% of adult patients with T-ALL do not achieve complete remission and relapse, our results call for clinical trials incorporating CXCR4 antagonists in T-ALL treatment.

The calcineurin/NFAT signaling pathway is implicated in a wide variety of biological processes, acting as a bridge pathway between calcium signals and gene expression. Although its role as an effector of immune responses figured prominently in early studies, it forms just one part of a larger picture. Indeed calcineurin has been shown to participate in the development and function of e.g. the immune, cardiovascular, nervous and musculoskeletal systems, and dysregulation of calcineurin/NFAT signaling contributes to pathologies affecting these tissues, in particular cancer [1]. Activation of the calcineurin/NFAT pathway was first observed in human lymphoma, as well as in mouse models of T cell acute lymphoblastic leukemia (T-ALL) and xenotransplanted human T-ALL [2]. In T-ALL, calcineurin activation is independent of preTCR/TCR signaling (the major calcineurin activator in normal T cell progenitors), but strongly depends upon micro-environmental signals. The fundamental, intrinsic role of calcineurin in T-ALL was demonstrated in several mouse models, in which conditional calcineurin genetic deletion was restricted to leukemic cells. Calcineurin was found essential to the physical and functional interactions that leukemic cells establish with supportive stromal cells, with its deletion resulting in impaired leukemia propagation, reduced cell survival, proliferation, migration and homing [3]. The therapeutic relevance of these findings was highlighted by preclinical studies showing strong anti-leukemic effects of calcineurin inhibitors (namely cyclosporin A or tacrolimus [FK506]) and long-term leukemia remission in a mouse T-ALL model when vincristine treatment was combined with calcineurin genetic inactivation [2, 3]. However, available calcineurin inhibitors appear suboptimal as potential therapeutic agents since they are associated with a number of toxic side effects [4], show clear off-target effects in T-ALL cells [3] and are expected to interfere with the anti-tumor immune response. A recently developed, alternative option is to identify and target molecular pathways acting downstream of calcineurin and critical to T-ALL maintenance [5]. Our global transcriptomic analysis identified a large number of calcineurin-dependent genes in T-ALL, involved in an array of biological function, including the de-repression of known tumor suppressive pathways (e.g. CDKN1A) [3]. Although of high biological interest, these deregulations are not easily accessible for targeted therapy. In contrast, genes/proteins implicated in the adhesion/migration to the bone marrow microenvironment are promising candidates (i) for a thorough understanding of the factors that contribute to microenvironment-mediated support of leukemia progression and (ii) for the design of niche-targeted therapies. Along these lines, we linked calcineurin-dependent regulation of the adhesive/migratory properties of T-ALL cells to a boost of CXCR4 surface expression and the subsequent ability of the leukemic cells to respond to CXCL12 [5]. Upregulation of CXCR4 cell surface expression was also demonstrated in diagnostic T-ALL cases and primary xenograft in NSG mice [5][6]. The mechanism by which calcineurin affects CXCR4 trafficking is partially explained by the Cn-dependent up-regulation of cortactin [5], an actin-binding protein implicated in the regulation of endosomal trafficking [7]. Because actin polymerization is required for CXCR4 and other chemokine receptors trafficking to recycling vesicles, inhibition of cortactin expression in calcineurin-deficient T-ALL cells likely results in impaired actin dynamics in this endosomal compartment. Further investigation of the intrinsic function of CXCR4 in murine and human T-ALL revealed an important role of CXCL12/CXCR4 signaling in both survival/proliferation and homing/migration of leukemic cells to the supportive bone marrow niche [5][6]. Intravital multiphoton imaging and genetic studies revealed a strong interaction between T-ALL cells and CXCL12-expressing niche(s), and an essential supportive function of CXCL12 produced by vascular endothelial cells [6]. Local CXCL12 production, in addition to induction of CXCR4-dependent signaling cues will result in activation of T-ALL cells specific integrins, further stabilizing adhesion to integrin ligands expressing niche(s) and induction of additional pro-survival signals. In this scenario, the nature of the niche cells expressing the integrin ligands requires further characterization. Strikingly, CXCR4 is also critical to leukemia initiating cell activity (LIC) in murine T-ALL and human xenografts [5][6], highlighting an unexpected, fundamental function of microenvironmental signals for T-ALL maintenance and progression. Many inhibitors of CXCL12 or CXCR4 have been developed and are tested in clinical studies in other pathological contexts, in particular other hematological malignancies [8]. However, only in T-ALL anti-CXCR4 monotherapy shows strong efficacy, suggesting a strong dependence of these tumor cells on CXCR4 signaling [6]. As relapse in T-ALL remains a challenging issue, these new data call for clinical trials to incorporate CXCR4 antagonists either as single agents following induction therapy, or as part of the first induction therapy regimen or later, during the consolidation phase. In conclusion, the calcineurin/NFAT pathway acts as a fundamental bridge between microenvironmental-derived signals and T-ALL cells, mediating a complex crosstalk that is so far only partially dissected, but that already lead to the identification of novel targets of therapeutic relevance to T-ALL treatment.

Spi-1/PU.1 is a myeloid- and B-cell specific transcription factor which is also involved in Friend virus-induced murine erythroleukemia. The pre-leukemic phase of Friend erythroleukemia results from activation of the erythropoietin receptor (EpoR) by the spleen focus forming virus (SFFV) envelope glycoprotein, followed by the emergence of leukemic clones characterized by overexpression of Spi-1 and mutation of the p53 tumor suppressor gene. We developed a heterologous system to analyze the contribution of these alterations to the induction of primary erythroblast transformation. Avian erythroblasts expressing the activated mouse EpoR(R129C) differentiated into erythrocytes in response to hEpo. Expression of Spi-1 in these cells inhibited this ability to differentiate and rescued the cells from the apoptotic cell death program normally induced upon hEpo withdrawal. Although devoid of any effect by itself, a mutant p53 cooperated with Spi-1 and EpoR(R129C) to reinforce both phenotypes. Analysis of erythroblasts co-expressing Spi-1 and the wild-type mouse EpoR showed that differentiation arrest and inhibition of apoptosis depended on specific cooperation between Spi-1 and EpoR(R129C). This cooperation was also required to induce the sustained proliferation of differentiation-blocked erythroblasts in response to ligand activation of the endogenous tyrosine kinase receptor c-Kit. These results show that Spi-1/PU.1 requires signals emanating from specific cytokine and growth factor receptors to affect the survival, proliferation and differentiation control of primary erythroblasts. They also suggest that the function of Spi-1/PU.1 in the late phase of Friend leukemia requires specific signaling from the gp55-modified EpoR generated during the early phase of the disease.

Activation of erythropoietin receptor allows erythroblasts to generate erythrocytes. In a search for genes that are up-regulated during this differentiation process, we have identified ISG15 as being induced during late erythroid differentiation. ISG15 belongs to the ubiquitin-like protein family and is covalently linked to target proteins by the enzymes of the ISGylation machinery. Using both in vivo and in vitro differentiating erythroblasts, we show that expression of ISG15 as well as the ISGylation process related enzymes Ube1L, UbcM8 and Herc6 are induced during erythroid differentiation. Loss of ISG15 in mice results in decreased number of BFU-E/CFU-E in bone marrow, concomitant with an increased number of these cells in the spleen of these animals. ISG15(-/-) bone marrow and spleen-derived erythroblasts show a less differentiated phenotype both in vivo and in vitro, and over-expression of ISG15 in erythroblasts is found to facilitate erythroid differentiation. Furthermore, we have shown that important players of erythroid development, such as STAT5, Globin, PLC γ and ERK2 are ISGylated in erythroid cells. This establishes a new role for ISG15, besides its well-characterized anti-viral functions, during erythroid differentiation.

FLI-1 is a transcriptional regulator of the ETS family of proteins. Insertional activation at the FLI-1 locus is an early event in F-murine leukemia virus-induced erythroleukemia. Consistent with its essential role in erythroid transformation, enforced expression of FLI-1 in primary erythroblasts strongly impairs the response of these cells to erythropoietin (Epo), a cytokine essential to erythropoiesis. We show here that point mutations in the ETS domain that abolished FLI-1 binding to specific DNA elements (ETS-binding sites) suppressed the ability of FLI-1 to transform erythroblasts. The exchange of the entire ETS domain (DNA binding domain) of FLI-1 for that of PU.1 changed the DNA binding specificity of FLI-1 for that of PU.1 and impaired FLI-1 transforming properties. In contrast, ETS domain swapping mutants that maintained the DNA binding specificity of FLI-1 did not affect the ability of FLI-1 to transform erythroblasts. Deletion and swapping mutants that failed to inhibit the DNA binding activity of FLI-1 but impaired its transcriptional activation properties were also transformation-defective. Taken together, these results show that both the ability of FLI-1 to inhibit Epo-induced differentiation of erythroblasts and to confer enhanced cell survival in the absence of Epo critically depend upon FLI-1 ETS-binding site-dependent transcriptional activation properties.

Friend erythroleukemia induced in mice by the spleen focus forming virus (SFFV) is a multi-step process. The pre-leukemic phase of the disease results from the abnormal activation of the Erythropoietin (Epo) receptor by the gp55 env gene product of SFFV. Later in disease progression, the emergence of leukemic clones is associated with recurrent genetic events, in particular the activation of the expression of SPI-1, an ETS family transcriptional regulator. We show here that the expression of either SPI-1 or GP55 with the mouse EPOR in avian primary erythroblasts only marginally affects their normal Epo-induced terminal differentiation. In contrast, the co-expression of GP55 and SPI-1 resulted in inhibition of Epo-induced differentiation of EPOR-expressing erythroblasts, promoting instead their proliferation. Co-expression of SPI-1 and GP55 also inhibited the apoptotic cell death program normally induced in response to Epo withdrawal. This cooperation between SPI-1 and GP55 to induce primary erythroblast transformation suggests that progression of Friend erythroleukemia critically depends upon inter-dependent interactions between the molecular events specific of the early and late phase of the disease.

Friend virus-induced erythroleukemia involves two members of the ETS family of transcriptional regulators, both activated via proviral insertion in the corresponding loci. Spi-1/PU.1 is expressed in the disease induced by the original Friend virus SFFV(F-MuLV) complex in adult mice. In contrast, FLI-1 is overexpressed in about 75% of the erythroleukemias induced by the F-MuLV helper virus in newborn mice. To analyse the consequences of the enforced expression of FLI-1 on erythroblast differentiation and proliferation and to compare its activity to that of PU.1/Spi-1, we used a heterologous system of avian primary erythroblasts previously described to study the cooperation between Spi-1/PU.1 and the other molecular alterations observed in SFFV-induced disease. FLI-1 was found: (i) to inhibit the apoptotic cell death program normally activated in erythroblasts following Epo deprivation; (ii) to inhibit the terminal differentiation program induced in these cells in response to Epo and; (iii) to induce their proliferation. However, in contrast to Spi-1/PU.1, the effects of FLI-1 on erythroblast, differentiation and proliferation did not require its cooperation with an abnormally activated form of the EpoR. Enhanced survival of FLI-1 expressing erythroblasts correlated with the upregulation of bcl2 expression. FLI-1 also prevented the rapid downregulation of cyclin D2 and D3 expression normally observed during Epo-induced differentiation and delayed the downregulation of several other genes involved in cell cycle or cell proliferation control. Our results show that overexpression of FLI-1 profoundly deregulates the normal balance between differentiation and proliferation in primary erythroblasts. Thus, the activation of FLI-1 expression observed at the onset of F-MuLV-induced erythroleukemia may provide a proliferative advantage to virus infected cells that would otherwise undergo terminal differentiation or cell death.

Red blood cells are of vital importance for oxygen transport in vertebrates. Thus, their formation during development and homeostasis requires tight control of both progenitor proliferation and terminal red cell differentiation. Self renewal (i.e. long-term proliferation without differentiation) of committed erythroid progenitors has recently been shown to contribute to this regulation. Avian erythroid progenitors expressing the EGF receptor/c-ErbB (SCF/TGFα progenitors) can be induced to long-term proliferation by the c-ErbB ligand transforming growth factor α and the steroids estradiol and dexamethasone. These progenitors have not yet been described in mammals and their factor requirements are untypical for adult erythroid progenitors. Here we describe a second, distinct type of erythroid progenitor (EpoR progenitors) which can be established from freshly isolated bone marrow and is induced to self renew by ligands relevant for erythropoiesis, i.e. erythropoietin, stem cell factor, the ligand for c-Kit and the glucocorticoid receptor ligand dexamethasone. Limiting dilution cloning indicates that these EpoR progenitors are derived from normal BFU-E/CFU-E. For a detailed study, mEpoR progenitors were generated by retroviral expression of the murine Epo receptor in bone marrow erythroblasts. These progenitors carry out the normal erythroid differentiation program in recombinant differentiation factors only. We show that mEpoR progenitors are more mature than SCF/TGFα progenitors and also do no longer respond to transforming growth factor α and estradiol. In contrast they are now highly sensitive to low levels of thyroid hormone, facilitating their terminal maturation into erythrocytes.

The Janus family of tyrosine kinases (JAK) plays an essential role in development and in coupling cytokine receptors to downstream intracellular signaling events. A t(9;12)(p24;p13) chromosomal translocation in a T cell childhood acute lymphoblastic leukemia patient was characterized and shown to fuse the 3′ portion of JAK2 to the 5′ region of TEL , a gene encoding a member of the ETS transcription factor family. The TEL-JAK2 fusion protein includes the catalytic domain of JAK2 and the TEL-specific oligomerization domain. TEL-induced oligomerization of TEL-JAK2 resulted in the constitutive activation of its tyrosine kinase activity and conferred cytokine-independent proliferation to the interleukin-3–dependent Ba/F3 hematopoietic cell line.

International audience; TEL is a novel member of the ETS family of transcriptional regulators which is frequently involved in human leukemias as the result of specific chromosomal translocations. We show here by co-immunoprecipitation and GST chromatography analyses that TEL and TEL-derived fusion proteins form homotypic oligomers in vitro and in vivo. Deletion mutagenesis identifies the TEL oligomerization domain as a 65 amino acid region which is conserved in a subset of the ETS proteins including ETS-1, ETS-2, FLI-1, ERG-2 and GABP alpha in vertebrates and PNTP2, YAN and ELG in Drosophila. TEL-induced oligomerization is shown to be essential for the constitutive activation of the protein kinase activity and mitogenic properties of TEL-platelet derived growth factor receptor beta (PDGFR beta), a fusion oncoprotein characteristic of the leukemic cells of chronic myelomonocytic leukemia harboring a t(5;12) chromosomal translocation. Swapping experiments in which the TEL oligomerization domain was exchanged by the homologous domains of representative vertebrate ETS proteins including ETS-1, ERG-2 and GABP alpha show that oligomerization is a specific property of the TEL amino-terminal conserved domain. These results indicate that the amino-terminal domain conserved in a subset of the ETS proteins has evolved to generate a specialized protein-protein interaction interface which is likely to be an important determinant of their specificity as transcriptional regulators.

The small G protein RhoA and its GDP/GTP exchange factors (GEFs) Net and Dbl can transform NIH 3T3 fibroblasts, dependent on the activity of the RhoA effector kinase ROCK. We investigated the role of the cytoskeletal linker protein ezrin in this process. RhoA effector loop mutants which can bind ROCK induce relocalization of ezrin to dorsal actin-containing cell surface protrusions, as do Net and Dbl. Both processes are inhibited by the ROCK inhibitor Y27632, which also inhibits association of ezrin with the cytoskeleton, and phosphorylation of T567, conserved between ezrin and its relatives radixin and moesin. ROCK can phosphorylate the ezrin C-terminus in vitro. The ezrin mutant T567A cannot be relocalized by activated RhoA, Net or Dbl or by ROCK itself, and also inhibits RhoA-mediated contractility and focal adhesion formation. Moreover, ezrin T567A, but not wild-type ezrin, restores contact inhibition to Net- and Dbl-transformed cells, and inhibits the activity of Net and Ras in focus formation assays. These results implicate ROCK-mediated ezrin C-terminal phosphorylation in transformation by RhoGEFs.

Acute leukemias are systemic malignancies associated with a dire outcome. Because of low immunogenicity, leukemias display a remarkable ability to evade immune control and are often resistant to checkpoint blockade. Here, we discover that leukemia cells actively establish a suppressive environment to prevent immune attacks by co-opting a signaling axis that skews macrophages toward a tumor-promoting tissue repair phenotype, namely the GAS6/AXL axis. Using aggressive leukemia models, we demonstrate that ablation of the AXL receptor specifically in macrophages, or its ligand GAS6 in the environment, stimulates antileukemic immunity and elicits effective and lasting natural killer cell– and T cell–dependent immune response against naïve and treatment-resistant leukemia. Remarkably, AXL deficiency in macrophages also enables PD-1 checkpoint blockade in PD-1–refractory leukemias. Finally, we provide proof-of-concept that a clinical-grade AXL inhibitor can be used in combination with standard-of-care therapy to cure established leukemia, regardless of AXL expression in malignant cells. Significance: Alternatively primed myeloid cells predict negative outcome in leukemia. By demonstrating that leukemia cells actively evade immune control by engaging AXL receptor tyrosine kinase in macrophages and promoting their alternative priming, we identified a target which blockade, using a clinical-grade inhibitor, is vital to unleashing the therapeutic potential of myeloid-centered immunotherapy. This article is highlighted in the In This Issue feature, p. 2659