Increased Vascular Permeability in the Bone Marrow Microenvironment Contributes to Disease Progression and Drug Response in Acute Myeloid Leukemia

Temporal Single Cell Analysis of Leukemia Microenvironment Identifies Taurine-Taurine Transporter Axis as a Key Regulator of Myeloid Leukemia

Signals from the microenvironment are known to be critical for development, sustaining adult stem cells, and for oncogenic progression. While candidate niche-driven signals that can promote cancer progression have been identified 1-6 , concerted efforts to comprehensively map microenvironmental ligands for cancer stem cell specific surface receptors have been lacking. Here, we use temporal single cell RNA-sequencing to identify molecular cues from the bone marrow stromal niche that engage leukemia stem cells (LSC) during oncogenic progression. We integrate these data with our RNA-seq analysis of human LSCs from distinct aggressive myeloid cancer subtypes and our CRISPR based in vivo LSC dependency map 7 to develop a temporal receptor-ligand interactome essential for disease progression. These analyses identify the taurine transporter (TauT)-taurine axis as a critical dependency of myeloid malignancies. We show that taurine production is restricted to the osteolineage population during cancer initiation and expansion. Inhibiting taurine synthesis in osteolineage cells impairs LSC growth and survival. Our experiments with the TauT genetic loss of function murine model indicate that its loss significantly impairs the progression of aggressive myeloid leukemias in vivo by downregulating glycolysis. Further, TauT inhibition using a small molecule strongly impairs the growth and survival of patient derived myeloid leukemia cells. Finally, we show that TauT inhibition can synergize with the clinically approved oxidative phosphorylation inhibitor venetoclax 8, 9 to block the growth of primary human leukemia cells. Given that aggressive myeloid leukemias continue to be refractory to current therapies and have poor prognosis, our work indicates targeting the taurine transporter may be of therapeutic significance. Collectively, our data establishes a temporal landscape of stromal signals during cancer progression and identifies taurine-taurine transporter signaling as an important new regulator of myeloid malignancies.

Leukemia circulation kinetics revealed through blood exchange method

Leukemias and their bone marrow microenvironments undergo dynamic changes over the course of disease. However, little is known about the circulation kinetics of leukemia cells, nor the impact of specific factors on the clearance of circulating leukemia cells (CLCs) from the blood. To gain a basic understanding of CLC dynamics over the course of disease progression and therapeutic response, we apply a blood exchange method to mouse models of acute leukemia. We find that CLCs circulate in the blood for 1-2 orders of magnitude longer than solid tumor circulating tumor cells. We further observe that: (i) leukemia presence in the marrow can limit the clearance of CLCs in a model of acute lymphocytic leukemia (ALL), and (ii) CLCs in a model of relapsed acute myeloid leukemia (AML) can clear faster than their untreated counterparts. Our approach can also directly quantify the impact of microenvironmental factors on CLC clearance properties. For example, data from two leukemia models suggest that E-selectin, a vascular adhesion molecule, alters CLC clearance. Our research highlights that clearance rates of CLCs can vary in response to tumor and treatment status and provides a strategy for identifying basic processes and factors that govern the kinetics of circulating cells.

Transforming the Niche: The Emerging Role of Extracellular Vesicles in Acute Myeloid Leukaemia Progression

Acute myeloid leukaemia (AML) management remains a significant challenge in oncology due to its low survival rates and high post-treatment relapse rates, mainly attributed to treatment-resistant leukaemic stem cells (LSCs) residing in bone marrow (BM) niches. This review offers an in-depth analysis of AML progression, highlighting the pivotal role of extracellular vesicles (EVs) in the dynamic remodelling of BM niche intercellular communication. We explore recent advancements elucidating the mechanisms through which EVs facilitate complex crosstalk, effectively promoting AML hallmarks and drug resistance. Adopting a temporal view, we chart the evolving landscape of EV-mediated interactions within the AML niche, underscoring the transformative potential of these insights for therapeutic intervention. Furthermore, the review discusses the emerging understanding of endothelial cell subsets' impact across BM niches in shaping AML disease progression, adding another layer of complexity to the disease progression and treatment resistance. We highlight the potential of cutting-edge methodologies, such as organ-on-chip (OoC) and single-EV analysis technologies, to provide unprecedented insights into AML-niche interactions in a human setting. Leveraging accumulated insights into AML EV signalling to reconfigure BM niches and pioneer novel approaches to decipher the EV signalling networks that fuel AML within the human context could revolutionise the development of niche-targeted therapy for leukaemia eradication.

Ablation of Wnt signaling in bone marrow stromal cells overcomes microenvironment-mediated drug resistance in acute myeloid leukemia

The survival of leukemic cells is significantly influenced by the bone marrow microenvironment, where stromal cells play a crucial role. While there has been substantial progress in understanding the mechanisms and pathways involved in this crosstalk, limited data exist regarding the impact of leukemic cells on bone marrow stromal cells and their potential role in drug resistance. In this study, we identify that leukemic cells prime bone marrow stromal cells towards osteoblast lineage and promote drug resistance. This biased differentiation of stroma is accompanied by dysregulation of the canonical Wnt signaling pathway. Inhibition of Wnt signaling in stroma reversed the drug resistance in leukemic cells, which was further validated in leukemic mice models. This study evaluates the critical role of leukemic cells in establishing a drug-resistant niche by influencing the bone marrow stromal cells. Additionally, it highlights the potential of targeting Wnt signaling in the stroma by repurposing an anthelmintic drug to overcome the microenvironment-mediated drug resistance.

Harnessing upregulated E-selectin while enhancing SDF-1α sensing redirects infused NK cells to the AML-perturbed bone marrow

Increased bone marrow (BM) homing of NK cells is associated with positive outcome in patients with acute myeloid leukemia (AML) treated within adoptive NK cell transfer trials. While most efforts to further improve the efficacy focus on augmenting NK cell persistence and cytotoxicity, few address their ability to home to the tumor. Here, we decipher how AML growth alters the BM niche to impair NK cell infiltration and how insights can be utilized to resolve this issue. We show that AML development gradually impairs the BM homing capacity of infused NK cells, which was tightly linked to loss of SDF-1α in this environment. AML development also triggered up-regulation of E-selectin on BM endothelial cells. Given the poor E-selectin-binding capacity of NK cells, introduction of fucosyltransferase-7 (FUT7) to the NK cells per mRNA transfection resulted in potent E-selectin binding and stronger adhesion to E-selectin+ endothelial cells. Co-introduction of FUT7 and gain-of-function CXCR4 (CXCR4R334X) redirected NK cell homing to the BM of AML-bearing mice nearly to the levels in AML-free mice. This work shows how impaired NK cell homing caused by AML-induced microenvironmental changes can be overcome by genetic engineering. We speculate our insights can help further advance future NK cell immunotherapies.

Understanding the interaction between leukaemia stem cells and their microenvironment to improve therapeutic approaches

Although chemotherapeutic regimens can eliminate blasts in leukaemia patients, such therapies are associated with toxicity and often fail to eliminate all malignant cells resulting in disease relapse. Disease relapse has been attributed to the persistence of leukaemia cells in the bone marrow (BM) with the capacity to recapitulate disease; these cells are often referred to as leukaemia stem cells (LSCs). Although LSCs have distinct characteristics in terms of pathobiology and immunophenotype, they are still regulated by their interactions with the surrounding microenvironment. Thus, understanding the interaction between LSCs and their microenvironment is critical to identify effective therapies. To this end, there are numerous efforts to develop models to study such interactions. In this review, we will focus on the reciprocal interactions between LSCs and their milieu in the BM. Furthermore, we will highlight relevant therapies targeting these interactions and discuss some of the promising in vitro models designed to mimic such relationship. LINKED ARTICLES: This article is part of a themed issue on Cancer Microenvironment and Pharmacological Interventions. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.2/issuetoc.

Micro-environment alterations through time leading to myeloid malignancies

The micro-environment plays a critical role in haematopoietic stem cell (HSC) development, self-renewal, differentiation and maintenance by providing a supportive cellular framework and essential molecular cues to sustain homeostasis. In ageing and development of age-related clonal haematopoiesis, the combined contribution of intrinsic alterations in haematopoietic stem cells and their surrounding micro-environment can promote myeloid skewing and release of pro-inflammatory cytokines. A pro-inflammatory micro-environment is a common feature in the initiation and sustenance of several myeloid malignancies. Furthermore, remodelling of the micro-environment is recognized to potentiate the survival of malignant over normal cells. This review explores micro-environmental interactions in the haematopoietic system of adults, especially how the bone marrow micro-environment is impacted by ageing, the onset of age-related clonal haematopoiesis and the development of myeloid malignancies. In addition, we also discuss the possible role age-related clonal haematopoiesis and chronic inflammatory conditions play in altering the bone marrow micro-environment dynamics. Finally, we explore the importance of in vitro models that accurately mimic different aspects of the bone marrow micro-environment in order to study normal and malignant haematopoiesis. LINKED ARTICLES: This article is part of a themed issue on Cancer Microenvironment and Pharmacological Interventions. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.2/issuetoc.

Microenvironment in acute myeloid leukemia: focus on senescence mechanisms, therapeutic interactions, and future directions

Acute myeloid leukemia (AML) is a disease with a dismal prognosis, mainly affecting the elderly. In recent years, new drugs have improved life expectancy and quality of life, and a better understanding of the genetic-molecular nature of the disease has shed light on previously unknown aspects of leukemogenesis. In parallel, increasing attention has been attracted to the complex interactions between cells and soluble factors in the bone marrow (BM) environment, collectively known as the microenvironment. In this review, we discuss the central role of the microenvironment in physiologic and pathologic hematopoiesis and the mechanisms of senescence, considered a fundamental protective mechanism against the proliferation of damaged and pretumoral cells. The microenvironment also represents a fertile ground for the development of myeloid malignancies, and the leukemic niche significantly interacts with drugs commonly used in AML treatment. Finally, we focus on the role of the microenvironment in the engraftment and complications of allogeneic hematopoietic stem cell transplantation, the only curative option in a conspicuous proportion of patients.

m5 C regulator-mediated methylation modification patterns and tumor microenvironment infiltration characteristics in acute myeloid leukemia

BACKGROUND

Recently, many studies have been conducted to examine immune response modification at epigenetic level, but the candidate effect of RNA 5-methylcytosine (m5 C) modification on tumor microenvironment (TME) of acute myeloid leukemia (AML) is still unknown at present.

METHODS

We assessed the patterns of m5 C modification among 417 AML cases by using nine m5 C regulators. Thereafter, we associated those identified modification patterns with TME cell infiltration features. Additionally, stepwise regression and LASSO Cox regression analyses were conducted for quantifying patterns of m5 C modification among AML cases to establish the m5 C-score. Meanwhile, we validated the expression of genes in the m5C-score model by qRT-PCR. Finally, the present work analyzed the association between m5 C-score and AML clinical characteristics and prognostic outcomes.

RESULTS

In total, three different patterns of m5 C modification (m5 C-clusters) were identified, and highly differentiated TME cell infiltration features were also identified. On this basis, evaluating patterns of m5 C modification in single cancer samples was important for evaluating the immune/stromal activities in TME and for predicting prognosis. In addition, the m5 C-score was established, which showed a close relation with the overall survival (OS) of test and training set samples. Moreover, multivariate Cox analysis suggested that our constructed m5 C-score served as the independent predicting factor for the prognosis of AML (hazard ratio = 1.57, 95% confidence interval = 1.38-1.79, p < 1e-5 ).

CONCLUSIONS

This study shows that m5 C modification may be one of the key roles in the formation of diversity and complexity of TME. Meanwhile, assessing the patterns of m5 C modification among individual cancer samples is of great importance, which provides insights into cell infiltration features within TME, thereby helping to develop relevant immunotherapy and predict patient prognostic outcomes.

Imaging Techniques and Clinical Application of the Marrow-Blood Barrier in Hematological Malignancies

The pathways through which mature blood cells in the bone marrow (BM) enter the blood stream and exit the BM, hematopoietic stem cells in the peripheral blood return to the BM, and other substances exit the BM are referred to as the marrow-blood barrier (MBB). This barrier plays an important role in the restrictive sequestration of blood cells, the release of mature blood cells, and the entry and exit of particulate matter. In some blood diseases and tumors, the presence of immature cells in the blood suggests that the MBB is damaged, mainly manifesting as increased permeability, especially in angiogenesis. Some imaging methods have been used to monitor the integrity and permeability of the MBB, such as DCE-MRI, IVIM, ASL, BOLD-MRI, and microfluidic devices, which contribute to understanding the process of related diseases and developing appropriate treatment options. In this review, we briefly introduce the theory of MBB imaging modalities along with their clinical applications.

Novel treatment from a botanical formulation Si-Miao-Yong-an decoction inhibits vasa vasorum angiogenesis and stabilizes atherosclerosis plaques via the Wnt1/β-catenin signalling pathway

CONTEXT

Si-Miao-Yong-An (SMYA) has been widely used for the clinical treatment of atherosclerosis (AS). Yet, its complete mechanism of action is not fully understood.

OBJECTIVE

To investigate the mechanism by which SMYA stabilizes AS plaques from the perspective of inhibiting vasa vasorum (VV) angiogenesis.

MATERIALS AND METHODS

We used male ApoE-/- mice to establish an AS model. The mice were divided into model, SMYA (11.7 mg/kg/d), and simvastatin (SVTT) (2.6 mg/kg/d) groups. Mice were given SMYA or SVTT by daily gavage for 8 weeks. HE staining, immunofluorescence double-labelling staining, and immunohistochemical staining were used to observe the pathological changes in the plaques. Finally, the protein and mRNA expression levels of the Wnt1/β-catenin signalling pathway were detected by Western blot and qRT-PCR, respectively.

RESULTS

SMYA significantly attenuated cholesterol crystallization, and lipid accumulation in AS plaques, resulting in smaller plaque size (0.25 mm2 vs. 0.46 mm2), and lowering ratio of plaque to lumen area (20.04% vs. 38.33%) and VV density (50.64/mm2 vs. 98.02/mm2). Meanwhile, SMYA suppressed both the positive area percentage of Wnt1 (2.53 vs. 3.56), β-catenin (3.33 vs. 5.65) and Cyclin D1 (2.10 vs. 3.27) proteins in the aortic root plaques, and mRNA expression of Wnt1 (1.38 vs. 2.09), β-catenin (2.05 vs. 3.25) and Cyclin D1 (1.39 vs. 2.57).

DISCUSSION AND CONCLUSIONS

SMYA has a protective effect against AS, which may be related to its anti-VV angiogenesis in plaques, suggesting that SMYA has the potential as a novel botanical formulation in the treatment of AS.

Relapse of acute myeloid leukemia after allogeneic stem cell transplantation: immune escape mechanisms and current implications for therapy

Acute myeloid leukemia (AML) is a heterogeneous disease characterized by the expansion of immature myeloid cells in the bone marrow (BM) and peripheral blood (PB) resulting in failure of normal hematopoiesis and life-threating cytopenia. Allogeneic hematopoietic stem cell transplantation (allo-HCT) is an established therapy with curative potential. Nevertheless, post-transplant relapse is common and associated with poor prognosis, representing the major cause of death after allo-HCT. The occurrence of relapse after initially successful allo-HCT indicates that the donor immune system is first able to control the leukemia, which at a later stage develops evasion strategies to escape from immune surveillance. In this review we first provide a comprehensive overview of current knowledge regarding immune escape in AML after allo-HCT, including dysregulated HLA, alterations in immune checkpoints and changes leading to an immunosuppressive tumor microenvironment. In the second part, we draw the line from bench to bedside and elucidate to what extend immune escape mechanisms of relapsed AML are yet exploited in treatment strategies. Finally, we give an outlook how new emerging technologies could help to improve the therapy for these patients, and elucidate potential new treatment options.

Identifying Bone Marrow Microenvironmental Populations in Myelodysplastic Syndrome and Acute Myeloid Leukemia

The bone marrow microenvironment consists of distinct cell populations, such as mesenchymal stromal cells, endothelial cells, osteolineage cells, and fibroblasts, which provide support for hematopoietic stem cells (HSCs). In addition to supporting normal HSCs, the bone marrow microenvironment also plays a role in the development of hematopoietic stem cell disorders, such as myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). MDS-associated mutations in HSCs lead to a block in differentiation and progressive bone marrow failure, especially in the elderly. MDS can often progress to therapy-resistant AML, a disease characterized by a rapid accumulation of immature myeloid blasts. The bone marrow microenvironment is known to be altered in patients with these myeloid neoplasms. Here, a comprehensive protocol to isolate and phenotypically characterize bone marrow microenvironmental cells from murine models of myelodysplastic syndrome and acute myeloid leukemia is described. Isolating and characterizing changes in the bone marrow niche populations can help determine their role in disease initiation and progression and may lead to the development of novel therapeutics targeting cancer-promoting alterations in the bone marrow stromal populations.

A comprehensive analysis of cell-autonomous and non-cell-autonomous regulation of myeloid leukemic cells: The prospect of developing novel niche-targeting therapies

Leukemic cells (LCs) arise from the hematopoietic stem/and progenitor cells (HSCs/HSPCs) and utilize cues from the bone marrow microenvironment (BMM) for their regulation in the same way as their normal HSC counterparts. Mesenchymal stromal cells (MSCs), a vital component of the BMM promote leukemogenesis by creating a protective and immune-tolerant microenvironment that can support the survival of LCs, helping them escape chemotherapy, thereby resulting in the relapse of leukemia. Conversely, MSCs also induce apoptosis in the LCs and inhibit their proliferation by interfering with their self-renewal potential. This review discusses the work done so far on cell-autonomous (intrinsic) and MSCs-mediated non-cell-autonomous (extrinsic) regulation of myeloid leukemia with a special focus on the need to investigate the extrinsic regulation of myeloid leukemia to understand the contrasting role of MSCs in leukemogenesis. These mechanisms could be exploited to formulate novel therapeutic strategies that specifically target the leukemic microenvironment.

Visualizing vasculature and its response to therapy in the tumor microenvironment

Tumor vasculature plays a critical role in the progression and metastasis of tumors, antitumor immunity, drug delivery, and resistance to therapies. The morphological and functional changes of tumor vasculature in response to therapy take place in a spatiotemporal-dependent manner, which can be predictive of treatment outcomes. Dynamic monitoring of intratumor vasculature contributes to an improved understanding of the mechanisms of action of specific therapies or reasons for treatment failure, leading to therapy optimization. There is a rich history of methods used to image the vasculature. This review describes recent advances in imaging technologies to visualize the tumor vasculature, with a focus on enhanced intravital imaging techniques and tumor window models. We summarize new insights on spatial-temporal vascular responses to various therapies, including changes in vascular perfusion and permeability and immune-vascular crosstalk, obtained from intravital imaging. Finally, we briefly discuss the clinical applications of intravital imaging techniques.

Leukemia circulation kinetics revealed through blood exchange method

Leukemias and their bone marrow microenvironment are known to undergo dynamic changes over the course of disease. However, relatively little is known about the circulation kinetics of leukemia cells, nor the impact of specific factors on the clearance of circulating leukemia cells (CLCs) from the blood. To gain a basic understanding of leukemia cell dynamics over the course of disease progression and therapeutic response, we apply a blood exchange method to mouse models of acute leukemia. We find that CLCs circulate in the blood for 1-2 orders of magnitude longer than solid tumor circulating tumor cells. We further observe that: i) leukemia presence in the marrow can limit the clearance of CLCs in a model of acute lymphocytic leukemia (ALL), and ii) CLCs in a model of relapsed acute myeloid leukemia (AML) can clear faster than their untreated counterparts. Our approach can also directly quantify the impact of microenvironmental factors on CLC clearance properties. For example, data from two leukemia models suggest that E-selectin, a vascular adhesion molecule, alters CLC clearance. Our research highlights that clearance rates of CLCs can vary in response to tumor and treatment status and provides a strategy for identifying basic processes and factors that govern the kinetics of circulating cells.

Doxorubicin Activity Is Modulated by Traditional Herbal Extracts in a 2D and 3D Multicellular Sphere Model of Leukemia

The modulation of the tumor microenvironment by natural products may play a significant role in the response of tumor cells to chemotherapy. In this study, we evaluated the effect of extracts derived from P2Et (Caesalpinia spinosa) and Anamú-SC (Petiveria alliacea) plants, previously studied by our group, on the viability and ROS levels in the K562 cell line (Pgp- and Pgp+), endothelial cells (ECs, Eahy.926 cell line) and mesenchymal stem cells (MSC) cultured in 2D and 3D. The results show that: (a) the two botanical extracts are selective on tumor cells compared to doxorubicin (DX), (b) cytotoxicity is independent of the modulation of intracellular ROS for plant extracts, unlike DX, (c) the interaction with DX can be influenced by chemical complexity and the expression of Pgp, (d) the 3D culture shows a greater sensitivity of the tumor cells to chemotherapy, in co-treatment with the extracts. In conclusion, the effect of the extracts on the viability of leukemia cells was modified in multicellular spheroids with MSC and EC, suggesting that the in vitro evaluation of these interactions can contribute to the comprehension of the pharmacodynamics of the botanical drugs.

A Question of Frame: The Role of the Bone Marrow Stromal Niche in Myeloid Malignancies

Until a few years ago, the onset of acute myeloid leukemia (AML) was entirely ascribed to genetic lesions in hematopoietic stem cells. These mutations generate leukemic stem cells, which are known to be the main ones responsible for chemoresistance and relapse. However, in the last years, increasing evidence demonstrated that dynamic interplay between leukemic cells and bone marrow (BM) niche is of paramount relevance in the pathogenesis of myeloid malignancies, including AML. Specifically, BM stromal niche components, such as mesenchymal stromal cells (MSCs) and their osteoblastic cell derivatives, play a key role not only in supporting normal hematopoiesis but also in the manifestation and progression of myeloid malignancies. Here, we reviewed recent clinical and experimental findings about how genetic and functional alterations in MSCs and osteolineage progeny can contribute to leukemogenesis and how leukemic cells in turn generate a corrupted niche able to support myeloid neoplasms. Moreover, we discussed how the newest single-cell technologies may help dissect the interactions between BM stromal cells and malignant hematopoiesis. The deep comprehension of the tangled relationship between stroma and AML blasts and their modulation during disease progression may have a valuable impact on the development of new microenvironment-directed therapeutic strategies, potentially useful for a wide cohort of patients.

Metabolic crosstalk between stromal and malignant cells in the bone marrow niche

Bone marrow is the primary site of blood cell production in adults and serves as the source of osteoblasts and osteoclasts that maintain bone homeostasis. The medullary microenvironment is also involved in malignancy, providing a fertile soil for the growth of blood cancers or solid tumors metastasizing to bone. The cellular composition of the bone marrow is highly complex, consisting of hematopoietic stem and progenitor cells, maturing blood cells, skeletal stem cells, osteoblasts, mesenchymal stromal cells, adipocytes, endothelial cells, lymphatic endothelial cells, perivascular cells, and nerve cells. Intercellular communication at different levels is essential to ensure proper skeletal and hematopoietic tissue function, but it is altered when malignant cells colonize the bone marrow niche. While communication often involves soluble factors such as cytokines, chemokines, and growth factors, as well as their respective cell-surface receptors, cells can also communicate by exchanging metabolic information. In this review, we discuss the importance of metabolic crosstalk between different cells in the bone marrow microenvironment, particularly concerning the malignant setting.

Targeting N-cadherin (CDH2) and the malignant bone marrow microenvironment in acute leukaemia

This review discusses current research on acute paediatric leukaemia, the leukaemic bone marrow (BM) microenvironment and recently discovered therapeutic opportunities to target leukaemia-niche interactions. The tumour microenvironment plays an integral role in conferring treatment resistance to leukaemia cells, this poses as a key clinical challenge that hinders management of this disease. Here we focus on the role of the cell adhesion molecule N-cadherin (CDH2) within the malignant BM microenvironment and associated signalling pathways that may bear promise as therapeutic targets. Additionally, we discuss microenvironment-driven treatment resistance and relapse, and elaborate the role of CDH2-mediated cancer cell protection from chemotherapy. Finally, we review emerging therapeutic approaches that directly target CDH2-mediated adhesive interactions between the BM cells and leukaemia cells.

Role of the bone marrow vascular niche in chemotherapy for MLL-AF9-induced acute myeloid leukemia

Leukemia stem cells in acute myeloid leukemia (AML) can persist within unique bone marrow niches similar to those of healthy hematopoietic stem cells and resist chemotherapy. In the context of AML, endothelial cells (ECs) are crucial components of these niches that appear to promote malignant expansion despite treatment. To better understand these interactions, we developed a real-time cell cycle-tracking mouse model of AML (Fucci-MA9) with an aim of unraveling why quiescent leukemia cells are more resistant to chemotherapy than cycling cells and proliferate during disease relapse. We found that quiescent leukemia cells were more prone to escape chemotherapy than cycling cells, leading to relapse and proliferation. Importantly, post-chemotherapy resting leukemia cells tended to localize closer to blood vessels. Mechanistically, after chemotherapy, resting leukemia cells interacted with ECs, promoting their adhesion and anti-apoptotic capacity. Further, expression analysis of ECs and leukemia cells during AML, after chemotherapy, and after relapse revealed the potential of suppressing the post-chemotherapy inflammatory response to regulate the functions of leukemia cells and ECs. These findings highlight the role of leukemia cells in evading chemotherapy by seeking refuge near blood vessels and provide important insights and directions for future AML research and treatment.

Epigenetic Modification of Mesenchymal Stromal Cells Derived from Bone Marrow and Embryonal Tumors to Facilitate Immunotherapeutic Approaches in Pediatric Malignancies

Mesenchymal stromal cells (MSC) are part of the bone marrow architecture and contribute to the homeostasis of hematopoietic stem cells. Moreover, they are known to regulate immune effector cells. These properties of MSC are pivotal under physiologic conditions, and they may aberrantly also protect malignant cells. MSCs are also found in the leukemic stem cell niche of the bone marrow and as part of the tumor microenvironment. Here, they protect malignant cells from chemotherapeutic drugs and from immune effector cells in immunotherapeutic approaches. Modulation of these mechanisms may improve the efficacy of therapeutic regimens. We investigated the effect of the histone deacetylase inhibitor (HDACi) suberoylanilide hydroxamic acid (SAHA, Vorinostat™) on the immunomodulatory effect and cytokine profile of MSC derived from bone marrow and pediatric tumors. The immune phenotype of MSC was not markedly affected. SAHA-treated MSC showed reduced immunomodulatory effects on T cell proliferation and NK cell cytotoxicity. This effect was accompanied by an altered cytokine profile of MSC. While untreated MSC inhibited the production of certain pro-inflammatory cytokines, SAHA treatment led to a partial increase in IFNγ and TNFα secretion. These alterations of the immunosuppressive milieu might be beneficial for immunotherapeutic approaches.

Hydrogel-Based Pre-Clinical Evaluation of Repurposed FDA-Approved Drugs for AML

In vivo models of acute myeloid leukemia (AML) are low throughput, and standard liquid culture models fail to recapitulate the mechanical and biochemical properties of the extracellular matrix-rich protective bone marrow niche that contributes to drug resistance. Candidate drug discovery in AML requires advanced synthetic platforms to improve our understanding of the impact of mechanical cues on drug sensitivity in AML. By use of a synthetic, self-assembling peptide hydrogel (SAPH) of modifiable stiffness and composition, a 3D model of the bone marrow niche to screen repurposed FDA-approved drugs has been developed and utilized. AML cell proliferation was dependent on SAPH stiffness, which was optimized to facilitate colony growth. Three candidate FDA-approved drugs were initially screened against the THP-1 cell line and mAF9 primary cells in liquid culture, and EC50 values were used to inform drug sensitivity assays in the peptide hydrogel models. Salinomycin demonstrated efficacy in both an 'early-stage' model in which treatment was added shortly after initiation of AML cell encapsulation, and an 'established' model in which time-encapsulated cells had started to form colonies. Sensitivity to Vidofludimus treatment was not observed in the hydrogel models, and Atorvastatin demonstrated increased sensitivity in the 'established' compared to the 'early-stage' model. AML patient samples were equally sensitive to Salinomycin in the 3D hydrogels and partially sensitive to Atorvastatin. Together, this confirms that AML cell sensitivity is drug- and context-specific and that advanced synthetic platforms for higher throughput are valuable tools for pre-clinical evaluation of candidate anti-AML drugs.

Dynamics of Changes in the Properties of Multipotent Mesenchymal Stromal Cells in Patients with Acute Leukemia

In acute leukemia, the stromal microenvironment of the bone marrow that regulates hematopoiesis is modified under the influence of malignant cells. Chemotherapy also adversely affects stromal cells. Multipotent mesenchymal stromal cells (MSC) are involved in the formation of the stromal microenvironment and in the regulation of normal and tumor hematopoietic cells. The properties of MSC from the bone marrow of patients with acute myeloid and lymphoid leukemia were studied at the onset of the disease and after achieving remission. The immunophenotype and the level of gene expression were analyzed in MSC of 34 patients. In MSC from patients with acute leukemia, the expression of CD105 and CD274 was significantly reduced in comparison with MSC from healthy donors. At the onset of the disease, the expression of IL6, JAG1, PPARG, IGF1, and PDGFRA was enhanced, while the expression of IL1B, IL8, SOX9, ANG1, and TGFB was reduced. All these changes affect the course of the disease in patients and can be the targets of therapeutic intervention.

Hematopoietic Stem Cells and Their Bone Marrow Niches

Hematopoietic stem cells (HSCs) are maintained in the bone marrow microenvironment, also known as the niche, that regulates their proliferation, self-renewal, and differentiation. In this chapter, we will introduce the history of HSC niche research and review the interdependencies between HSCs and their niches. We will further highlight recent advances in our understanding of HSC heterogeneity with regard to HSC subpopulations and their interacting cellular and molecular bone marrow niche constituents.

Diversity in the bone marrow niche: Classic and novel strategies to uncover niche composition

Our view on the role and composition of the bone marrow (BM) has dramatically changed over time from a simple nutrient for the bone to a highly complex multicellular tissue that sustains haematopoiesis. Among these cells, multipotent haematopoietic stem cells (HSCs), which are predominantly quiescent, possess unique self-renewal capacity and multilineage differentiation potential and replenish all blood lineages to maintain lifelong haematopoiesis. Adult HSCs reside in specialised BM niches, which support their functions. Much effort has been put into deciphering HSC niches due to their potential clinical relevance. Multiple cell types have been implicated as HSC-niche components including sinusoidal endothelium, perivascular stromal cells, macrophages, megakaryocytes, osteoblasts and sympathetic nerves. In this review we provide a historical perspective on how technical advances, from genetic mouse models to imaging and high-throughput sequencing techniques, are unveiling the plethora of molecular cues and cellular components that shape the niche and regulate HSC functions.

Hematopoietic Stem Cells in Wound Healing Response

Significance: Emerging evidence has shown a link between the status of hematopoietic stem cells (HSCs) and wound healing responses. Thus, better understanding HSCs will contribute to further advances in wound healing research. Recent Advances: Myeloid cells such as neutrophils and monocyte-derived macrophages are critical players in the process of wound healing. HSCs actively respond to wound injury and other tissue insults, including infection and produce the effector myeloid cells, and a failing of the HSC response can result in impaired wound healing. Technological advances such as transcriptome at single-cell resolution, epigenetics, three-dimensional imaging, transgenic animals, and animal models, have provided novel concepts of myeloid generation (myelopoiesis) from HSCs, and have revealed cell-intrinsic and -extrinsic mechanisms that can impact HSC functions in the context of health conditions. Critical Issues: The newer concepts include-the programmed cellular fate at a differentiation stage that is used to be considered as the multilineage, the signaling pathways that can activate HSCs directly and indirectly, the mechanisms that can deteriorate HSCs, the roles and remodeling of the surrounding environment for HSCs and their progenitors (the niche). Future Directions: The researches on HSCs, which produce blood cells, should contribute to the development of blood biomarkers predicting a risk of chronic wounds, which may transform clinical practice of wound care with precision medicine for patients at high risk of poor healing.

The genesis and evolution of acute myeloid leukemia stem cells in the microenvironment: From biology to therapeutic targeting

Acute myeloid leukemia (AML) is a hematological malignancy characterized by cytogenetic and genomic alterations. Up to now, combination chemotherapy remains the standard treatment for leukemia. However, many individuals diagnosed with AML develop chemotherapeutic resistance and relapse. Recently, it has been pointed out that leukemic stem cells (LSCs) are the fundamental cause of drug resistance and AML relapse. LSCs only account for a small subpopulation of all leukemic cells, but possess stem cell properties, including a self-renewal capacity and a multi-directional differentiation potential. LSCs reside in a mostly quiescent state and are insensitive to chemotherapeutic agents. When LSCs reside in a bone marrow microenvironment (BMM) favorable to their survival, they engage into a steady, continuous clonal evolution to better adapt to the action of chemotherapy. Most chemotherapeutic drugs can only eliminate LSC-derived clones, reducing the number of leukemic cells in the BM to a normal range in order to achieve complete remission (CR). LSCs hidden in the BM niche can hardly be targeted or eradicated, leading to drug resistance and AML relapse. Understanding the relationship between LSCs, the BMM, and the generation and evolution laws of LSCs can facilitate the development of effective therapeutic targets and increase the efficiency of LSCs elimination in AML.

Crosstalk between AML and stromal cells triggers acetate secretion through the metabolic rewiring of stromal cells

Acute myeloid leukaemia (AML) cells interact and modulate components of their surrounding microenvironment into their own benefit. Stromal cells have been shown to support AML survival and progression through various mechanisms. Nonetheless, whether AML cells could establish beneficial metabolic interactions with stromal cells is underexplored. By using a combination of human AML cell lines and AML patient samples together with mouse stromal cells and a MLL-AF9 mouse model, here we identify a novel metabolic crosstalk between AML and stromal cells where AML cells prompt stromal cells to secrete acetate for their own consumption to feed the tricarboxylic acid cycle (TCA) and lipid biosynthesis. By performing transcriptome analysis and tracer-based metabolic NMR analysis, we observe that stromal cells present a higher rate of glycolysis when co-cultured with AML cells. We also find that acetate in stromal cells is derived from pyruvate via chemical conversion under the influence of reactive oxygen species (ROS) following ROS transfer from AML to stromal cells via gap junctions. Overall, we present a unique metabolic communication between AML and stromal cells and propose two different molecular targets, ACSS2 and gap junctions, that could potentially be exploited for adjuvant therapy.

Synergistic Lethality Effects of Apatinib and Homoharringtonine in Acute Myeloid Leukemia

Purpose

The significance of vascular endothelial growth factor receptor (VEGFR)-2 in numerous solid tumors and acute myeloid leukemia (AML) has been demonstrated, but Apatinib remains largely unexplored. In this study, whether Apatinib combined with homoharringtonine (HHT) kills AML cell lines and its possible mechanisms have been explored.

Methods

AML cell lines were treated with Apatinib and HHT in different concentrations with control, Apatinib alone, HHT alone, and Apatinib combined with HHT. The changes of IC₅₀ were measured by CCK8 assay, and apoptosis rate, cell cycle, and the mitochondrial membrane potential in each group were measured by flow cytometry. Finally, the possible cytotoxicity mechanism was analyzed by Western blotting.

Results

Our results noted that Apatinib combined with HHT remarkably inhibited cell proliferation, reduced the capacity of colony-forming, and induced apoptosis and cell cycle arrest in AML cells. Mechanistically, Apatinib and HHT play a role as a suppressor in the expression of VEGFR-2 and the downstream signaling cascades, such as the PI3K, MAPK, and STAT3 pathways.

Conclusion

Our preclinical data demonstrate that Apatinib combined with HHT exerts a better antileukemia effect than Apatinib alone by inhibiting the VEGFR-2 signaling pathway, suggesting the potential role of Apatinib and HHT in the treatment of AML. This study provides clinicians with innovative combination therapies and new therapeutic targets for the treatment of AML.

Anlotinib exerts potent antileukemic activities in Ph chromosome negative and positive B-cell acute lymphoblastic leukemia via perturbation of PI3K/AKT/mTOR pathway

OBJECTIVES

Despite advances in the development of novel targeted therapies, the need for B-ALL alternative treatments has not been met. Anlotinib could blunt the proangiogenic activity of VEGFR, PDGFR, and FGFR, and has shown strong antitumor activities across multiple tumors. However, anlotinib cytotoxicity against B-ALL has not ever been evaluated, thus prompting us to initiate this study.

METHODS

Expression2Kinases program was used to identify potential treatment targets. Cell viability and apoptosis were determined by CCK-8 and Annexin V/PI staining kit, respectively. qRT-PCR and Western blotting were utilized to investigate the molecular mechanisms. In vivo antileukemia activity of Anlotinib was evaluated in a Ph⁺ B-ALL patient-Derived Xenograft (PDX) model.

RESULTS

Compared with treatment-naive B-ALL cases, RR B-ALL patients had higher activities in the VEGF/VEGFR signaling and the PI3K/AKT/mTOR pathway. Exposure of Ph^- and Ph⁺ B-ALL cells to anlotinib resulted in significant cell viability reduction, apoptosis enhancement, and cell cycle arrest at G2/M phase. Importantly, anlotinib treatment led to remarkably decreased leukemia burdens and extended the survival period in a Ph⁺ B-ALL PDX model. Blockade of the role of the proangiogenic mediators, comprising VEGFR2, PDGFR-beta, and FGFR3, played a critical role in the cytotoxicity of anlotinib against Ph^- and Ph⁺ B-ALL. Moreover, anlotinib dampened the activity of PI3K/AKT/mTOR pathway that resides in the convergence of the three mentioned proangiogenic signals.

CONCLUSION

This work provides impressive preclinical evidence of anlotinib against Ph^- and Ph⁺ B-ALL and raises a rationale for future clinical evaluation of this drug in the management of Ph^- and Ph⁺ B-ALL.

The Mesenchymal Niche in Myelodysplastic Syndromes

Myelodysplastic syndromes (MDSs) are clonal disorders characterized by ineffective hematopoiesis, resulting in cytopenias and a risk of developing acute myeloid leukemia. In addition to mutations affecting hematopoietic stem cells (HSCs), numerous studies have highlighted the role of the bone marrow microenvironment (BMME) in the development of MDSs. The mesenchymal niche represents a key component of the BMME. In this review, we discuss the role of the mesenchymal niche in the pathophysiology of MDS and provide an overview of currently available in vitro and in vivo models that can be used to study the effects of the mesenchymal niche on HSCs.

Circular RNAs Activity in the Leukemic Bone Marrow Microenvironment

Acute myeloid leukemia (AML) is a hematological malignancy originating from defective hematopoietic stem cells in the bone marrow. In spite of the recent approval of several molecular targeted therapies for AML treatment, disease recurrence remains an issue. Interestingly, increasing evidence has pointed out the relevance of bone marrow (BM) niche remodeling during leukemia onset and progression. Complex crosstalk between AML cells and microenvironment components shapes the leukemic BM niche, consequently affecting therapy responsiveness. Notably, circular RNAs are a new class of RNAs found to be relevant in AML progression and chemoresistance. In this review, we provided an overview of AML-driven niche remodeling. In particular, we analyzed the role of circRNAs and their possible contribution to cell–cell communication within the leukemic BM microenvironment. Understanding these mechanisms will help develop a more effective treatment for AML.

Long non-coding RNA LOC644135 is a potential prognostic indicator in cytogenetically normal acute myeloid leukemia

OBJECTIVES

Acute myeloid leukemia (AML) is a hematological malignancy with highly clinical heterogeneity resulting in poor outcomes. We aim to identify novel prognostic lncRNA in AML expecting to provide new clues for therapy in AML.

METHODS

Three cohorts were enrolled in this study. Differentially expressed lncRNAs between TCGA-AML cohort and GTEx cohort was identified by DESeq2. The relationship between expression level of LOC644135 and prognosis in AML was analyzed by multiple methods.

RESULTS

Pan-cancer analysis indicated that LOC644135 was most highly expressed in AML across 33 types of cancer. Patients with high expression of LOC644135 had poor overall prognosis in both TCGA-AML cohort and the TARGET-AML cohort. Especially, high expression of LOC644135 indicated inferior overall survival and event-free survival in CN-AML patients in the TCGA-AML cohort. Besides, CN-AML patients had higher expression of LOC644135 than normal samples. Multivariable analysis suggested that LOC644135 was an independent prognostic factor in AML. GSEA analysis showed that LOC644135 was associated with some immune-related pathways. Besides, high expression of LOC644135 was associated with less infiltration of CD8⁺ T cell.

CONCLUSION

Our findings indicated that LOC644135 was an independent prognostic factor in AML and provided a new idea in the development of therapy in AML.

Microenvironmental Features Driving Immune Evasion in Myelodysplastic Syndromes and Acute Myeloid Leukemia

Bone marrow, besides the known functions of hematopoiesis, is an active organ of the immune system, functioning as a sanctuary for several mature immune cells. Moreover, evidence suggests that hematopoietic stem cells (the bone marrow’s functional unit) are capable of directly sensing and responding to an array of exogenous stimuli. This chronic immune stimulation is harmful to normal hematopoietic stem cells, while essential for the propagation of myeloid diseases, which show a dysregulated immune microenvironment. The bone marrow microenvironment in myelodysplastic syndromes (MDS) is characterized by chronic inflammatory activity and immune dysfunction, that drive excessive cellular death and through immune evasion assist in cancer cell expansion. Acute myeloid leukemia (AML) is another example of immune response failure, with features that augment immune evasion and suppression. In this review, we will outline some of the functions of the bone marrow with immunological significance and describe the alterations in the immune landscape of MDS and AML that drive disease progression.

Cell energy metabolism and bone formation

Energy metabolism plays an important role in cell and tissue ability to effectively function, maintain homeostasis, and perform repair. Yet, the role of energy metabolism in skeletal tissues in general and in bone, in particular, remains understudied. We, here, review the aspects of cell energy metabolism relevant to bone tissue, such as: i) availability of substrates and oxygen; ii) metabolism regulatory mechanisms most active in bone tissue, e.g. HIF and BMP; iii) crosstalk of cell bioenergetics with other cell functions, e.g. proliferation and differentiation; iv) role of glycolysis and mitochondrial oxidative phosphorylation in osteogenic lineage; and v) most significant changes in bone energy metabolism observed in aging and other pathologies. In addition, we review available methods to study energy metabolism on a subcellular, cellular, tissue, and live animal levels.

Metalloproteinase inhibition reduces AML growth, prevents stem cell loss, and improves chemotherapy effectiveness

Acute myeloid leukemia (AML) is a blood cancer of the myeloid lineage. Its prognosis remains poor, highlighting the need for new therapeutic and precision medicine approaches. AML symptoms often include cytopenias linked to loss of healthy hematopoietic stem and progenitor cells (HSPCs). The mechanisms behind HSPC decline are complex and still poorly understood. Here, intravital microscopy (IVM) of a well-established experimental model of AML allows direct observation of the interactions between healthy and malignant cells in the bone marrow (BM), suggesting that physical dislodgment of healthy cells by AML through damaged vasculature may play an important role. Multiple matrix metalloproteinases (MMPs), known to remodel extracellular matrix, are expressed by AML cells and the BM microenvironment. We reason MMPs could be involved in cell displacement and vascular leakiness; therefore, we evaluate the therapeutic potential of MMP pharmacological inhibition using the broad-spectrum inhibitor prinomastat. IVM analyses of prinomastat-treated mice reveal reduced vascular permeability and healthy cell clusters in circulation and lower AML infiltration, proliferation, and cell migration. Furthermore, treated mice have increased retention of healthy HSPCs in the BM and increased survival following chemotherapy. Analysis of a human AML transcriptomic database reveals widespread MMP deregulation, and human AML cells show susceptibility to MMP inhibition. Overall, our results suggest that MMP inhibition could be a promising complementary therapy to reduce AML growth and limit HSPC loss and BM vascular damage caused by MLL-AF9 and possibly other AML subtypes.

One-Chip Isolation of Drug-Resistant Acute Myeloid Leukemia Cells with CXCR4-Targeted Magnetic Fluorescent Nanoprobes

Drug resistance and relapse lead to high mortality in acute myeloid leukemia, and studies have shown that CXCR4 overexpression is highly correlated with poor prognosis and drug resistance in leukemia cells. Isolation and detection of AML cells with CXCR4 overexpression will be crucial to the treatment of AML. In this paper, magnetic nanoparticles were firstly prepared successfully by high-temperature thermal decomposition method, and then characterized by TEM, VSM and DLS. Subsequently CXCR4-targeted magnetic fluorescent nanoprobes conjugated with antibody 12G5 were constructed by stepwise coupling. In cell experiments, the obtained probes demonstrated excellent targeting efficacy to CXCR4 overexpressed AML cells HL-60. In addition, HL-60 cells labelled with the magnetic probes can be magnetic isolated successfully in one microfluidics chip, with efficiency of 82.92 ± 7.03%. Overall, this method utilizes the superiority of superparamagnetic nanomaterials and microfluidic technology to achieve the enrichment and capture of drug-resistant cells in a microfluidic chip, providing a new idea for the isolation and detective of drug-resistant acute myeloid leukemia cells.

Longitudinal Preclinical Imaging Characterizes Extracellular Drug Accumulation After Radiation Therapy in the Healthy and Leukemic Bone Marrow Vascular Microenvironment

PURPOSE

Recent initial findings suggest that radiation therapy improves blood perfusion and cellular chemotherapy uptake in mice with leukemia. However, the ability of radiation therapy to influence drug accumulation in the extracellular bone marrow tissue is unknown, due in part to a lack of methodology. This study developed longitudinal quantitative multiphoton microscopy (L-QMPM) to characterize the bone marrow vasculature (BMV) and drug accumulation in the extracellular bone marrow tissue before and after radiation therapy in mice bearing leukemia.

METHODS AND MATERIALS

We developed a longitudinal window implant for L-QMPM imaging of the calvarium BMV before, 2 days after, and 5 days after total body irradiation (TBI). Live time-lapsed images of a fluorescent drug surrogate were used to obtain measurements, including tissue wash-in slope (WIS_tissue) to measure extracellular drug accumulation. We performed L-QMPM imaging on healthy C57BL/6 (WT) mice, as well as mice bearing acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).

RESULTS

Implants had no effects on calvarium dose, and parameters for wild-type untreated mice were stable during imaging. We observed decreased vessel diameter, vessel blood flow, and WIS_tissue with the onset of AML and ALL. Two to 10 Gy TBI increased WIS_tissue and vessel diameter 2 days after radiation therapy in all 3 groups of mice and increased single-vessel blood flow in mice bearing ALL and AML. Increased WIS_tissue was observed 5 days after 10 Gy TBI or 4 Gy split-dose TBI (2 treatments of 2 Gy spaced 3 days apart).

CONCLUSIONS

L-QMPM provides stable functional assessments of the BMV. Nonmyeloablative and myeloablative TBI increases extracellular drug accumulation in the leukemic bone marrow 2 to 5 days posttreatment, likely through improved blood perfusion and drug exchange from the BMV to the extravascular tissue. Our data show that neo-adjuvant TBI at doses from 2 Gy to 10 Gy conditions the BMV to improve drug transport to the bone marrow.

Molecular Systems Architecture of Interactome in the Acute Myeloid Leukemia Microenvironment

A molecular systems architecture is presented for acute myeloid leukemia (AML) to provide a framework for organizing the complexity of biomolecular interactions. AML is a multifactorial disease resulting from impaired differentiation and increased proliferation of hematopoietic precursor cells involving genetic mutations, signaling pathways related to the cancer cell genetics, and molecular interactions between the cancer cell and the tumor microenvironment, including endothelial cells, fibroblasts, myeloid-derived suppressor cells, bone marrow stromal cells, and immune cells (e.g., T-regs, T-helper 1 cells, T-helper 17 cells, T-effector cells, natural killer cells, and dendritic cells). This molecular systems architecture provides a layered understanding of intra- and inter-cellular interactions in the AML cancer cell and the cells in the stromal microenvironment. The molecular systems architecture may be utilized for target identification and the discovery of single and combination therapeutics and strategies to treat AML.

Catch me if you can: how AML and its niche escape immunotherapy

In spite of the remarkable progress in basic and preclinical studies of acute myeloid leukemia (AML), the five-year survival rate of AML patients remains poor, highlighting the urgent need for novel and synergistic therapies. Over the past decade, increased attention has been focused on identifying suitable immunotherapeutic strategies for AML, and in particular on targeting leukemic cells and their progenitors. However, recent studies have also underlined the important contribution of the leukemic microenvironment in facilitating tumor escape mechanisms leading to disease recurrence. Here, we describe the immunological features of the AML niche, with particular attention to the crosstalk between the AML blasts and the cellular components of the altered tumor microenvironment (TME) and the mechanisms of immune escape that hamper the therapeutic effects of the most advanced treatments. Considering the AML complexity, immunotherapy approaches may benefit from a rational combination of complementary strategies aimed at preventing escape mechanisms without increasing toxicity.

Microenvironment and drug resistance in acute myeloid leukemia: Do we know enough?

Acute myeloid leukemia (AMLs), as the name suggests, often develop suddenly and are very progressive forms of cancer. Unlike in acute promyelocytic leukemia, a subtype of AML, the outcomes in most other AMLs remain poor. This is mainly attributed to the acquired drug resistance and lack of targeted therapy. Different studies across laboratories suggest that the cellular mechanisms to impart therapy resistance are often very dynamic and should be identified in a context-specific manner. Our review highlights the progress made so far in identifying the different cellular mechanisms of mutation-independent therapy resistance in AML. It reiterates that for more effective outcomes cancer therapies should acquire a more tailored approach where the protective interactions between the cancer cells and their niches are identified and targeted.

Deciphering Tumor Niches: Lessons From Solid and Hematological Malignancies

Knowledge about the hematopoietic niche has evolved considerably in recent years, in particular through in vitro analyzes, mouse models and the use of xenografts. Its complexity in the human bone marrow, in particular in a context of hematological malignancy, is more difficult to decipher by these strategies and could benefit from the knowledge acquired on the niches of solid tumors. Indeed, some common features can be suspected, since the bone marrow is a frequent site of solid tumor metastases. Recent research on solid tumors has provided very interesting information on the interactions between tumoral cells and their microenvironment, composed notably of mesenchymal, endothelial and immune cells. This review thus focuses on recent discoveries on tumor niches that could help in understanding hematopoietic niches, with special attention to 4 particular points: i) the heterogeneity of carcinoma/cancer-associated fibroblasts (CAFs) and mesenchymal stem/stromal cells (MSCs), ii) niche cytokines and chemokines, iii) the energy/oxidative metabolism and communication, especially mitochondrial transfer, and iv) the vascular niche through angiogenesis and endothelial plasticity. This review highlights actors and/or pathways of the microenvironment broadly involved in cancer processes. This opens avenues for innovative therapeutic opportunities targeting not only cancer stem cells but also their regulatory tumor niche(s), in order to improve current antitumor therapies.

Bone marrow niches in myelodysplastic syndromes

Genetic and epigenetic lesions within hematopoietic cell populations drive diverse hematological malignancies. Myelodysplastic syndromes (MDS) are a group of myeloid neoplasms affecting the hematopoietic stem cells characterized by recurrent genetic abnormalities, myelodysplasia (a pathological definition of abnormal bone marrow structure), ineffective hematopoiesis resulting in blood cytopenia, and a propensity to evolve into acute myelogenous leukemia. Although there is evidence that the accumulation of a set of genetic mutations is an essential event in MDS, there is an increased appreciation of the contribution of specific microenvironments, niches, in the pathogenesis of MDS and response to treatment. In physiologic hematopoiesis, niches are critical functional units that maintain hematopoietic stem and progenitor cells and regulate their maturation into mature blood cells. In MDS and other hematological malignancies, altered bone marrow niches can promote the survival and expansion of mutant hematopoietic clones and provide a shield from therapy. In this review, we focus on our understanding of the composition and function of hematopoietic niches and their role in the evolution of myeloid malignancies, with an emphasis on MDS.

Leukemic Stem Cells: From Leukemic Niche Biology to Treatment Opportunities

Acute myeloid leukemia (AML) is one of the most common types of leukemia in adults. While complete remission can be obtained with intensive chemotherapy in young and fit patients, relapse is frequent and prognosis remains poor. Leukemic cells are thought to arise from a pool of leukemic stem cells (LSCs) which sit at the top of the hierarchy. Since their discovery, more than 30 years ago, LSCs have been a topic of intense research and their identification paved the way for cancer stem cell research. LSCs are defined by their ability to self-renew, to engraft into recipient mice and to give rise to leukemia. Compared to healthy hematopoietic stem cells (HSCs), LSCs display specific mutations, epigenetic modifications, and a specific metabolic profile. LSCs are usually considered resistant to chemotherapy and are therefore the drivers of relapse. Similar to their HSC counterpart, LSCs reside in a highly specialized microenvironment referred to as the “niche”. Bidirectional interactions between leukemic cells and the microenvironment favor leukemic progression at the expense of healthy hematopoiesis. Within the niche, LSCs are thought to be protected from genotoxic insults. Improvement in our understanding of LSC gene expression profile and phenotype has led to the development of prognosis signatures and the identification of potential therapeutic targets. In this review, we will discuss LSC biology in the context of their specific microenvironment and how a better understanding of LSC niche biology could pave the way for new therapies that target AML.

Hematopoietic Stem Cell Mobilization: Current Collection Approaches, Stem Cell Heterogeneity, and a Proposed New Method for Stem Cell Transplant Conditioning

Hematopoietic stem cells naturally traffic out of their bone marrow niches into the peripheral blood. This natural trafficking process can be enhanced with numerous pharmacologic agents - a process termed "mobilization" - and the mobilized stem cells can be collected for transplantation. We review the current state of mobilization with an update on recent clinical trials and new biologic mechanisms regulating stem cell trafficking. We propose that hematopoietic mobilization can be used to answer questions regarding hematopoietic stem cell heterogeneity, can be used for non-toxic conditioning of patients receiving stem cell transplants, and can enhance gene editing and gene therapy strategies to cure genetic diseases.

Culturing patient-derived malignant hematopoietic stem cells in engineered and fully humanized 3D niches

Human malignant hematopoietic stem and progenitor cells (HSPCs) reside in bone marrow (BM) niches, which remain challenging to explore due to limited in vivo accessibility and constraints with humanized animal models. Several in vitro systems have been established to culture patient-derived HSPCs in specific microenvironments, but they do not fully recapitulate the complex features of native bone marrow. Our group previously reported that human osteoblastic BM niches (O-N), engineered by culturing mesenchymal stromal cells within three-dimensional (3D) porous scaffolds under perfusion flow in a bioreactor system, are capable of maintaining, expanding, and functionally regulating healthy human cord blood-derived HSPCs. Here, we first demonstrate that this 3D O-N can sustain malignant CD34⁺ cells from acute myeloid leukemia (AML) and myeloproliferative neoplasm patients for up to 3 wk. Human malignant cells distributed in the bioreactor system mimicking the spatial distribution found in native BM tissue, where most HSPCs remain linked to the niches and mature cells are released to the circulation. Using human adipose tissue-derived stromal vascular fraction cells, we then generated a stromal-vascular niche and demonstrated that O-N and stromal-vascular niche differentially regulate leukemic UCSD-AML1 cell expansion, immunophenotype, and response to chemotherapy. The developed system offers a unique platform to investigate human leukemogenesis and response to drugs in customized environments, mimicking defined features of native hematopoietic niches and compatible with the establishment of personalized settings.