Assessing the role of intrinsic disorder in RNA-binding protein function: hnRNP K as a case study.

RNA-binding proteins (RBPs) typically bind to RNA in a sequence-specific manner, resulting in post-transcriptional gene regulation. While the various classes of RNA-binding domains are largely structured, flexible linkers are frequently observed between them. Emerging evidence suggests that these unstructured regions may help spatially position the RNA-binding domains allowing for RNA binding and/or may contribute directly to RNA association via certain sequence motifs contained within them. The importance of these unstructured regions is widely appreciated; however, understanding their contribution to RNA binding, protein stability, and function has been difficult to ascertain. Thus, it is crucial to have a set of rapid and economical assays that do not require specialized instrumentation to study their impact on RBP function. Herein, we discuss the use of plate-based and cell-based thermal shift assays to study the impact of the intrinsically disordered region on the function of a highly conserved RBP, hnRNP K.

AXL/MERTK inhibitor ONO-7475 potently synergizes with venetoclax and overcomes venetoclax resistance to kill FLT3-ITD acute myeloid leukemia.

FMS-like Tyrosine Kinase 3 (FLT3) mutation is associated with poor survival in acute myeloid leukemia (AML). The specific Anexelekto/MER Tyrosine Kinase (AXL) inhibitor, ONO-7475, kills FLT3-mutant AML cells with targets including Extracellular- signal Regulated Kinase (ERK) and Myeloid Cell Leukemia 1 (MCL1). ERK and MCL1 are known resistance factors for Venetoclax (ABT-199), a popular drug for AML therapy, prompting the investigation of the efficacy of ONO-7475 in combination with ABT-199 in vitro and in vivo. ONO-7475 synergizes with ABT-199 to potently kill FLT3-mutant acute myeloid leukemia cell lines and primary cells. ONO-7475 is effective against ABT-199-resistant cells including cells that overexpress MCL1. Proteomic analyses revealed that ABT-199-resistant cells expressed elevated levels of pro-growth and anti-apoptotic proteins compared to parental cells, and that ONO-7475 reduced the expression of these proteins in both the parental and ABT-199-resistant cells. ONO-7475 treatment significantly extended survival as a single in vivo agent using acute myeloid leukemia cell lines and PDX models. Compared to ONO-7474 monotherapy, the combination of ONO-7475/ABT-199 was even more potent in reducing leukemic burden and prolonging the survival of mice in both model systems. These results suggest that the ONO-7475/ABT-199 combination may be effective for AML therapy.

The p53-Mdm2 feedback loop protects against DNA damage by inhibiting p53 activity but is dispensable for p53 stability, development, and longevity.

The p53-Mdm2 feedback loop is perceived to be critical for regulating stress-induced p53 activity and levels. However, this has never been tested in vivo. Using a genetically engineered mouse with mutated p53 response elements in the Mdm2 P2 promoter, we show that feedback loop-deficient Mdm2(P2/P2) mice are viable and aphenotypic and age normally. p53 degradation kinetics after DNA damage in radiosensitive tissues remains similar to wild-type controls. Nonetheless, DNA damage response is elevated in Mdm2(P2/P2) mice. Enhanced p53-dependent apoptosis sensitizes hematopoietic stem cells (HSCs), causing drastic myeloablation and lethality. These results suggest that while basal Mdm2 levels are sufficient to regulate p53 in most tissues under homeostatic conditions, the p53-Mdm2 feedback loop is critical for regulating p53 activity and sustaining HSC function after DNA damage. Therefore, transient disruption of p53-Mdm2 interaction could be explored as a potential adjuvant/therapeutic strategy for targeting stem cells in hematological malignancies.

Role of MSC-derived galectin 3 in the AML microenvironment.

In acute myeloid leukemia (AML), high Galectin 3 (LGALS3) expression is associated with poor prognosis. The role of LGALS3 derived from mesenchymal stromal cells (MSC) in the AML microenvironment is unclear; however, we have recently found high LGALS3 expression in MSC derived from AML patients is associated with relapse. In this study, we used reverse phase protein analysis (RPPA) to correlate LGALS3 expression in AML MSC with 119 other proteins including variants of these proteins such as phosphorylated forms or cleaved forms to identify biologically relevant pathways. RPPA revealed that LGALS3 protein was positively correlated with expression of thirteen proteins including MYC, phosphorylated beta-Catenin (p-CTNNB1), and AKT2 and negatively correlated with expression of six proteins including integrin beta 3 (ITGB3). String analysis revealed that proteins positively correlated with LGALS3 showed strong interconnectivity. Consistent with the RPPA results, LGALS3 suppression by shRNA in MSC resulted in decreased MYC and AKT expression while ITGB3 was induced. In co-culture, the ability of AML cell to adhere to MSC LGALS3 shRNA transductants was reduced compared to AML cell adhesion to MSC control shRNA transductants. Finally, use of novel specific LGALS3 inhibitor CBP.001 in co-culture of AML cells with MSC reduced viable leukemia cell populations with induced apoptosis and augmented the chemotherapeutic effect of AraC. In summary, the current study demonstrates that MSC-derived LGALS3 may be critical for important biological pathways for MSC homeostasis and for regulating AML cell localization and survival in the leukemia microenvironmental niche.

Anexelekto/MER tyrosine kinase inhibitor ONO-7475 arrests growth and kills FMS-like tyrosine kinase 3-internal tandem duplication mutant acute myeloid leukemia cells by diverse mechanisms.

Nearly one-third of patients with acute myeloid leukemia have FMS-like tyrosine kinase 3 mutations and thus have poor survival prospects. Receptor tyrosine kinase anexelekto is critical for FMS-like tyrosine kinase 3 signaling and participates in FMS-like tyrosine kinase 3 inhibitor resistance mechanisms. Thus, strategies targeting anexelekto could prove useful for acute myeloid leukemia therapy. ONO-7475 is an inhibitor with high specificity for anexelekto and MER tyrosine kinase. Herein, we report that ONO-7475 potently arrested growth and induced apoptosis in acute myeloid leukemia with internal tandem duplication mutation of FMS-like tyrosine kinase 3. MER tyrosine kinase-lacking MOLM13 cells were sensitive to ONO-7475, while MER tyrosine kinase expressing OCI-AML3 cells were resistant, suggesting that the drug acts via anexelekto in acute myeloid leukemia cells. Reverse phase protein analysis of ONO-7475 treated cells revealed that cell cycle regulators like cyclin dependent kinase 1, cyclin B1, polo-like kinase 1, and retinoblastoma were suppressed. ONO-7475 suppressed cyclin dependent kinase 1, cyclin B1, polo-like kinase 1 gene expression suggesting that anexelekto may regulate the cell cycle, at least in part, via transcriptional mechanisms. Importantly, ONO-7475 was effective in a human FMS-like tyrosine kinase 3 with internal tandem duplication mutant murine xenograft model. Mice fed a diet containing ONO-7475 exhibited significantly longer survival and, interestingly, blocked leukemia cell infiltration in the liver. In summary, ONO-7475 effectively kills acute myeloid leukemia cells in vitro and in vivo by mechanisms that involve disruption of diverse survival and proliferation pathways.

Loss of the novel tumour suppressor and polarity gene Trim62 (Dear1) synergizes with oncogenic Ras in invasive lung cancer.

Deregulation of cell polarity proteins has been linked to the processes of invasion and metastasis. TRIM62 is a regulator of cell polarity and a tumour suppressor in breast cancer. Here, we demonstrate that human non-small cell lung cancer lesions show a step-wise loss of TRIM62 levels during disease progression, which was associated with poor clinical outcomes. To directly examine the role of Trim62 in development of lung cancer, we deleted Trim62 in a mutant K-Ras mouse model of lung cancer. In this context, haploinsufficiency of Trim62 synergized with a K-RasG12D mutation to promote invasiveness and disrupt three-dimensional morphogenesis, both of which are associated with epithelial-mesenchymal transitions. Re-expression of Trim62 reverted these phenotypes in tumour cell lines. Thus, Trim62 loss cooperates with K-Ras mutation in tumourigenesis and metastasis in vivo, indicating that decreased levels of TRIM62 may play an important role in the evolution of lung cancer.

Insight into the mechanism of AML del(9q) progression: hnRNP K targets the myeloid master regulators CEBPA (C/EBPα) and SPI1 (PU.1).

Deletions on the long arm of chromosome 9 (del(9q)) are recurrent abnormalities in about 2 % of acute myeloid leukemia cases, which usually involve HNRNPK and are frequently associated with other known aberrations. Based on an Hnrnpk haploinsufficient mouse model, a recent study demonstrated a function of hnRNP K in pathogenesis of myeloid malignancies via the regulation of cellular proliferation and myeloid differentiation programs. Here, we provide evidence that reduced hnRNP K expression results in the dysregulated expression of C/EBPα and additional transcription factors. CyTOF analysis revealed monocytic skewing with increased levels of mature myeloid cells. To explore the role of hnRNP K during normal and pathological myeloid differentiation in humans, we characterized hnRNP K-interacting RNAs in human AML cell lines. Notably, RNA-sequencing revealed several mRNAs encoding key transcription factors involved in the regulation of myeloid differentiation as targets of hnRNP K. We showed that specific sequence motifs confer the interaction of SPI1 and CEBPA 5' and 3'UTRs with hnRNP K. The siRNA mediated reduction of hnRNP K in human AML cells resulted in an increase of PU.1 and C/EBPα that is most pronounced for the p30 isoform. The combinatorial treatment with the inducer of myeloid differentiation valproic acid resulted in increased C/EBPα expression and myeloid differentiation. Together, our results indicate that hnRNP K post-transcriptionally regulates the expression of myeloid master transcription factors. These novel findings can inaugurate novel options for targeted treatment of AML del(9q) by modulation of hnRNP K function.

Comprehensive characterization of IFNγ signaling in acute myeloid leukemia reveals prognostic and therapeutic strategies.

Interferon gamma (IFNγ) is a critical cytokine known for its diverse roles in immune regulation, inflammation, and tumor surveillance. However, while IFNγ levels were elevated in sera of most newly diagnosed acute myeloid leukemia (AML) patients, its complex interplay in AML remains insufficiently understood. We aim to characterize these complex interactions through comprehensive bulk and single-cell approaches in bone marrow of newly diagnosed AML patients. We identify monocytic AML as having a unique microenvironment characterized by IFNγ producing T and NK cells, high IFNγ signaling, and immunosuppressive features. IFNγ signaling score strongly correlates with venetoclax resistance in primary AML patient cells. Additionally, IFNγ treatment of primary AML patient cells increased venetoclax resistance. Lastly, a parsimonious 47-gene IFNγ score demonstrates robust prognostic value. In summary, our findings suggest that inhibiting IFNγ is a potential treatment strategy to overcoming venetoclax resistance and immune evasion in AML patients.

Single-Cell Profiling of CD8+ T Cells in Acute Myeloid Leukemia Reveals a Continuous Spectrum of Differentiation and Clonal Hyperexpansion.

Comprehensive investigation of CD8+ T cells in acute myeloid leukemia (AML) is essential for developing immunotherapeutic strategies beyond immune checkpoint blockade. Herein, we performed single-cell RNA profiling of CD8+ T cells from 3 healthy bone marrow donors and 23 newly diagnosed (NewlyDx) and 8 relapsed/refractory (RelRef) patients with AML. Cells coexpressing canonical exhaustion markers formed a cluster constituting <1% of all CD8+ T cells. We identified two effector CD8+ T-cell subsets characterized by distinct cytokine and metabolic profiles that were differentially enriched in NewlyDx and RelRef patients. We refined a 25-gene CD8-derived signature correlating with therapy resistance, including genes associated with activation, chemoresistance, and terminal differentiation. Pseudotemporal trajectory analysis supported enrichment of a terminally differentiated state in CD8+ T cells with high CD8-derived signature expression at relapse or refractory disease. Higher expression of the 25-gene CD8 AML signature correlated with poorer outcomes in previously untreated patients with AML, suggesting that the bona fide state of CD8+ T cells and their degree of differentiation are clinically relevant. Immune clonotype tracking revealed more phenotypic transitions in CD8 clonotypes in NewlyDx than in RelRef patients. Furthermore, CD8+ T cells from RelRef patients had a higher degree of clonal hyperexpansion associated with terminal differentiation and higher CD8-derived signature expression. Clonotype-derived antigen prediction revealed that most previously unreported clonotypes were patient-specific, suggesting significant heterogeneity in AML immunogenicity. Thus, immunologic reconstitution in AML is likely to be most successful at earlier disease stages when CD8+ T cells are less differentiated and have greater capacity for clonotype transitions.

Single-Cell Profiling of CD8+ T Cells in Acute Myeloid Leukemia Reveals a Continuous Spectrum of Differentiation and Clonal Hyperexpansion.

Comprehensive investigation of CD8+ T cells in acute myeloid leukemia (AML) is essential for developing immunotherapeutic strategies beyond immune checkpoint blockade. Herein, we performed single-cell RNA profiling of CD8+ T cells from 3 healthy bone marrow donors and 23 newly diagnosed (NewlyDx) and 8 relapsed/refractory (RelRef) AML patients. Cells co-expressing canonical exhaustion markers formed a cluster constituting <1% of all CD8+ T cells. We identified two effector CD8+ T cell subsets characterized by distinct cytokine and metabolic profiles that were differentially enriched in NewlyDx and RelRef patients. We refined a 25-gene CD8-derived signature correlating with therapy resistance, including genes associated with activation, chemoresistance, and terminal differentiation. Pseudotemporal trajectory analysis supported enrichment of a terminally differentiated state in CD8+ T cells with high CD8-derived signature expression at relapse or refractory disease. Higher expression of the 25-gene CD8 AML signature correlated with poorer outcomes in previously untreated AML patients, suggesting that the bona fide state of CD8+ T cells and their degree of differentiation are clinically relevant. Immune clonotype tracking revealed more phenotypic transitions in CD8 clonotypes in NewlyDx than in RelRef patients. Furthermore, CD8+ T cells from RelRef patients had a higher degree of clonal hyperexpansion associated with terminal differentiation and higher CD8-derived signature expression. Clonotype-derived antigen prediction revealed that most previously unreported clonotypes were patient-specific, suggesting significant heterogeneity in AML immunogenicity. Thus, immunologic reconstitution in AML is likely to be most successful at earlier disease stages when CD8+ T cells are less differentiated and have greater capacity for clonotype transitions.

Reverse phase protein array profiling reveals distinct proteomic signatures associated with chronic myeloid leukemia progression and with chronic phase in the CD34-positive compartment.

BACKGROUND: Chronic myeloid leukemia (CML) is a clonal stem cell malignancy whose pathogenesis is driven by constitutive activation of the breakpoint cluster region-v-abl Abelson murine leukemia viral oncogene homolog 1 (BCR-ABL1) kinase. Although BCR-ABL1 activation is present in all patients with CML, patients can present in 3 different phases characterized by an increasingly worse prognosis and diminished responsiveness to tyrosine kinase inhibitors: chronic phase, accelerated phase, or blastic phase. The biologic basis for progression from chronic phase to blastic phase and for regulating the homeostasis of tyrosine kinase inhibitor-resistant CML stem cells is not entirely understood. METHODS: To shed some light into these aspects of CML biology, the authors used reverse phase protein arrays probed with 112 individual monoclonal antibodies to compare protein expression patterns in 40 samples of leukemia-enriched fractions from patients with CML (25 in chronic phase, 5 in accelerated phase, and 10 in phase). RESULTS: An analysis of variance (significance cutoff, P < .01) unveiled a set of proteins that were overexpressed in blastic phase, including heat-shock protein 90 (hsp90); retinoblastoma (Rb); apoptosis-inducing factor (AIF); serine/threonine-protein phosphatase 2A (PP2A); B-cell leukemia 2 (Bcl-2); X-linked inhibitor of apoptosis protein (Xiap); human homolog of Drosophila Mad (mothers against decapentaplegic) and related Caenorhabditis elegans gene Sma, family member 1 (Smad1); single-stranded DNA binding protein 2 alpha (SSBP2α); poly(adenosine diphosphate-ribose) polymerase (PARP); GRB2-associated binding protein 2 (Gab2); and tripartite motif containing 24 (Trim24). It is noteworthy that several of these proteins also were overexpressed in the CD34-positive compartment, which putatively contains the CML stem cell population. CONCLUSIONS: The results from this study indicated that reverse phase protein array analysis can unveil differentially expressed proteins in advanced phase CML that can be exploited therapeutically with targeted approaches.

Regulation of tissue- and stimulus-specific cell fate decisions by p53 in vivo.

The tumour suppressor p53 pathway is often inactivated by multiple mechanisms in the genesis of human cancers. Aberrant cellular proliferation, DNA damage, hypoxia, and ribosomal stress cause activation of the p53 tumour suppressor with multiple possible consequences to the cell: cell death, cell cycle arrest, or senescence. These mechanisms ultimately ensure that the cell does not replicate, and are thus potent tumour suppressor mechanisms. An important question that has eluded the field is how p53 makes these cell fate decisions. This review summarizes the current status of knowledge regarding p53-mediated stress and tissue-dependent cell fate decisions in mouse models and human tumours.

GLI1 activates pro-fibrotic pathways in myelofibrosis fibrocytes.

Bone marrow (BM) fibrosis was thought to be induced exclusively by mesenchymal stromal cells (MSCs). However, we and others found that neoplastic fibrocytes induce BM fibrosis in myelofibrosis (MF). Because glioma-associated oncogene-1 (GLI1), an effector of the Hedgehog pathway, plays a role in the induction of BM fibrosis, we wondered whether GLI1 affects fibrocyte-induced BM fibrosis in MF. Multiplexed fluorescence immunohistochemistry analysis of MF patients' BM detected high levels of GLI1 in MF fibrocytes compared to MSCs or normal fibrocytes. Immunostaining, RNA in situ hybridization, gene expression analysis, and western immunoblotting detected high levels of GLI1 and GLI1-induced matrix metalloproteases (MMP) 2 and 9 in MF patients BM-derived cultured fibrocytes. Similarly, MF patients' BM-derived GLI1+ fibrocytes were found in BMs and spleens of MF xenograft mice. GLI1 silencing reduced the levels of MMP2/9, phosphorylated SMAD2/3, and procollagen-I, and knockdown or inhibition of GLI1 decreased fibrocyte formation and induced apoptosis of both fibrocytes and fibrocyte progenitors. Because Janus kinase (JAK)2-induced STAT3 is constitutively activated in MF and because STAT3 induces GLI1 expression, we sought to determine whether STAT3 activates GLI1 in MF fibrocytes. Imaging analysis detected phosphotyrosine STAT3 in MF patients' BM fibrocytes, and transfection of fibrocytes with STAT3-siRNA or treatment with a JAK1/2 inhibitor ruxolitinib reduced GLI1 and MMP2/9 levels. Chromatin immunoprecipitation and a luciferase assay revealed that STAT3 induced the expression of the GLI1 gene in both MF BM fibrocytes and fibrocyte progenitors. Together, our data suggest that STAT3-activated GLI1 contributes to the induction of BM fibrosis in MF.

Heterogeneous nuclear ribonucleoprotein K is overexpressed in acute myeloid leukemia and causes myeloproliferation in mice via altered Runx1 splicing.

Acute myeloid leukemia (AML) is driven by numerous molecular events that contribute to disease progression. Herein, we identify hnRNP K overexpression as a recurrent abnormality in AML that negatively correlates with patient survival. Overexpression of hnRNP K in murine fetal liver cells results in altered self-renewal and differentiation potential. Further, murine transplantation models reveal that hnRNP K overexpression results in myeloproliferation in vivo. Mechanistic studies expose a direct functional relationship between hnRNP K and RUNX1-a master transcriptional regulator of hematopoiesis often dysregulated in leukemia. Molecular analyses show that overexpression of hnRNP K results in an enrichment of an alternatively spliced isoform of RUNX1 lacking exon 4. Our work establishes hnRNP K's oncogenic potential in influencing myelogenesis through its regulation of RUNX1 splicing and subsequent transcriptional activity.

High-sensitivity next-generation sequencing MRD assessment in ALL identifies patients at very low risk of relapse.

Measurable residual disease (MRD) is highly prognostic for relapse and overall survival (OS) in acute lymphoblastic leukemia (ALL), although many patients with apparent "MRD negativity" by standard assays still relapse. We evaluated the clinical impact of a highly sensitive next-generation sequencing (NGS) MRD assay in 74 adults with ALL undergoing frontline therapy. Among remission samples that were MRD negative by multiparameter flow cytometry (MFC), 46% were MRD+ by the NGS assay. After 1 cycle of induction chemotherapy, MRD negativity by MFC at a sensitivity of 1 × 10-4 and NGS at a sensitivity of 1 × 10-6 was achieved in 66% and 23% of patients, respectively. The 5-year cumulative incidence of relapse (CIR) among patients who achieved MRD negativity by MFC at complete remission (CR) was 29%; in contrast, no patients who achieved early MRD negativity by NGS relapsed, and their 5-year OS was 90%. NGS MRD negativity at CR was associated with significantly decreased risk of relapse compared with MRD positivity (5-year CIR, 0% vs 45%, respectively; P = .04). Among patients who were MRD negative by MFC, detection of low levels of MRD by NGS identified patients who still had a significant risk of relapse (5-year CIR, 39%). Early assessment of MRD using a highly sensitive NGS assay adds clinically relevant prognostic information to standard MFC-based approaches and can identify patients with ALL undergoing frontline therapy who have a very low risk of relapse and excellent long-term survival.

You can win by losing: p53 mutations in rhabdomyosarcomas.

Rhabdomyosarcomas are soft tissue sarcomas of mesenchymal origin. Unlike rhabdomyosarcomas observed in paediatric patients which typically respond well to chemotherapeutic treatment, adults generally present with pleomorphic rhabdomyosarcomas that are typically associated with poor prognosis. Therefore, understanding the molecular biology that gives rise to pleomorphic rhabdomyosarcomas is critical. In this issue of The Journal of Pathology, Doyle and colleagues have generated elegant tissue-specific Cre/loxP-dependent mouse models that mimic pleomorphic rhabdomyosarcoma development in humans. In this report, the authors employed KRas(G12V)-expressing mouse models that concomitantly either express mutant p53 (p53R172H) or have deleted the p53 gene. Mice that express mutant p53 have decreased survival with development of aggressive metastases as compared to mice that have simply lost wild-type p53. The data presented herein provide the first in vivo evidence that in rhabdomyosarcomas, expression of mutant p53 results in a more aggressive p53R172H-dependent gain-of-function phenotype.

TAM kinases as regulators of cell death.

Receptor Tyrosine Kinases are critical regulators of signal transduction that support cell survival, proliferation, and differentiation. Dysregulation of normal Receptor Tyrosine Kinase function by mutation or other activity-altering event can be oncogenic or can impact the transformed malignant cell so it becomes particularly resistant to stress challenge, have increased proliferation, become evasive to immune surveillance, and may be more prone to metastasis of the tumor to other organ sites. The TAM family of Receptor Tyrosine Kinases (TYRO3, AXL, MERTK) is emerging as important components of malignant cell survival in many cancers. The TAM kinases are important regulators of cellular homeostasis and proper cell differentiation in normal cells as receptors for their ligands GAS6 and Protein S. They also are critical to immune and inflammatory processes. In malignant cells, the TAM kinases can act as ligand independent co-receptors to mutant Receptor Tyrosine Kinases and in some cases (e.g. FLT3-ITD mutant) are required for their function. They also have a role in immune checkpoint surveillance. At the time of this review, the Covid-19 pandemic poses a global threat to world health. TAM kinases play an important role in host response to many viruses and it is suggested the TAM kinases may be important in aspects of Covid-19 biology. This review will cover the TAM kinases and their role in these processes.

The Eµ-hnRNP K Murine Model of Lymphoma: Novel Insights into the Role of hnRNP K in B-Cell Malignancies.

B-cell lymphomas are one of the most biologically and molecularly heterogeneous group of malignancies. The inherent complexity of this cancer subtype necessitates the development of appropriate animal model systems to characterize the disease with the ultimate objective of identifying effective therapies. In this article, we discuss a new driver of B-cell lymphomas - hnRNP K (heterogenous nuclear ribonucleoprotein K)-an RNA-binding protein. We introduce the Eµ-Hnrnpk mouse model, a murine model characterized by hnRNP K overexpression in B cells, which develops B-cell lymphomas with high penetrance. Molecular analysis of the disease developed in this model reveals an upregulation of the c-Myc oncogene via post-transcriptional and translational mechanisms underscoring the impact of non-genomic MYC activation in B-cell lymphomas. Finally, the transplantability of the disease developed in Eµ-Hnrnpk mice makes it a valuable pre-clinical platform for the assessment of novel therapeutics.

Overexpression of CD200 is a Stem Cell-Specific Mechanism of Immune Evasion in AML.

BACKGROUND: Acute myeloid leukemia (AML) stem cells (LSCs) are capable of surviving current standard chemotherapy and are the likely source of deadly, relapsed disease. While stem cell transplant serves as proof-of-principle that AML LSCs can be eliminated by the immune system, the translation of existing immunotherapies to AML has been met with limited success. Consequently, understanding and exploiting the unique immune-evasive mechanisms of AML LSCs is critical. METHODS: Analysis of stem cell datasets and primary patient samples revealed CD200 as a putative stem cell-specific immune checkpoint overexpressed in AML LSCs. Isogenic cell line models of CD200 expression were employed to characterize the interaction of CD200+ AML with various immune cell subsets both in vitro and in peripheral blood mononuclear cell (PBMC)-humanized mouse models. CyTOF and RNA-sequencing were performed on humanized mice to identify novel mechanisms of CD200-mediated immunosuppression. To clinically translate these findings, we developed a fully humanized CD200 antibody (IgG1) that removed the immunosuppressive signal by blocking interaction with the CD200 receptor while also inducing a potent Fc-mediated response. Therapeutic efficacy of the CD200 antibody was evaluated using both humanized mice and patient-derived xenograft models. RESULTS: Our results demonstrate that CD200 is selectively overexpressed in AML LSCs and is broadly immunosuppressive by impairing cytokine secretion in both innate and adaptive immune cell subsets. In a PBMC-humanized mouse model, CD200+ leukemia progressed rapidly, escaping elimination by T cells, compared with CD200- AML. T cells from mice with CD200+ AML were characterized by an abundance of metabolically quiescent CD8+ central and effector memory cells. Mechanistically, CD200 expression on AML cells significantly impaired OXPHOS metabolic activity in T cells from healthy donors. Importantly, CD200 antibody therapy could eliminate disease in the presence of graft-versus-leukemia in immune competent mice and could significantly improve the efficacy of low-intensity azacitidine/venetoclax chemotherapy in immunodeficient hosts. CONCLUSIONS: Overexpression of CD200 is a stem cell-specific marker that contributes to immunosuppression in AML by impairing effector cell metabolism and function. CD200 antibody therapy is capable of simultaneously reducing CD200-mediated suppression while also engaging macrophage activity. This study lays the groundwork for CD200-targeted therapeutic strategies to eliminate LSCs and prevent AML relapse.