Impact of bone marrow nucleated cell subfractions on transplant outcomes in patients with acute lymphoblastic leukemia.

OBJECTIVES: Our previous study showed that a high pre-transplant nucleated cell count in the bone marrow is associated with increased non-relapse mortality (NRM) and decreased overall survival (OS) in patients with acute lymphoblastic leukemia (ALL) in remission. In this retrospective multicenter study, we aimed to examine the association between nucleated cell subfractions and transplant outcomes using the same patient cohort as our previous study. METHODS: This study included patients with ALL who underwent their first allogeneic hematopoietic stem cell transplantation (allo-HSCT) between 2010 and 2022. The patients were stratified into high and low cell group levels to compare transplant outcomes using cutoff values for predicting OS in each subfraction determined using receiver operating curves. RESULTS: In the cohort of 134 patients, the median values for myeloid, erythroid, monocyte, and lymphocyte series were 16,860/µL (468-229,296), 15,584/µL (34-246,992), 1,446/µL (70-25,296), and 4,215/µL (90-33,856), respectively. DISCUSSION: The univariate analysis showed that the groups with high levels of myeloid cells (≥38,000/µL, n = 48), erythroid cells (≥25,000/µL, n = 45), and monocyte cells (≥4,200/µL, n = 44) were all associated with worse 3-year OS and higher NRM than the low-level groups. These findings were confirmed by using multivariate analysis. The high cell count group showed a higher incidence of NRM associated with acute graft-versus-host disease or immunological disorders. CONCLUSION: High myeloid, erythroid, and monocytic cell levels in the bone marrow before allo-HSCT may independently increase the risk of NRM and reduce OS.

Bone marrow stromal cells enhance differentiation of acute myeloid leukemia induced by pyrimidine synthesis inhibitors.

Acute myeloid leukemia (AML) is a heterogeneous group of hematological malignancies characterized by differentiation arrest, high relapse rates, and poor survival. The bone marrow (BM) microenvironment is recognized as a critical mediator of drug resistance and a primary site responsible for AML relapse. Our previous study reported that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr) induces AML cell differentiation by inhibiting pyrimidine synthesis and activating Checkpoint kinase 1. Although the protective effect of BM stroma on leukemia cells in response to cytotoxic drugs is well-documented, its effect on AML differentiation remains less explored. In this study, we investigated the impact of stromal cell lines and primary mesenchymal stromal cells (MSCs) on AML cell line differentiation triggered by AICAr and brequinar, a known dihydroorotate dehydrogenase (DHODH) inhibitor. Our findings indicate that the mouse MS-5 stromal cell line, known for its cytoprotective effects, does not inhibit AML cell differentiation induced by pyrimidine synthesis inhibitors. Interestingly, AICAr caused morphological changes and growth arrest in MS-5 stromal cells via an AMP-activated protein kinase (AMPK)-dependent pathway. Human stromal cell lines HS-5 and HS-27, as well as primary MSCs isolated from patient bone marrow, were superior in promoting AML differentiation compared with mouse cells in response to AICAr and brequinar, with the inhibitors not significantly affecting the stromal cells themselves. In conclusion, our study highlights the supportive role of human BM MSCs in enhancing the differentiation effects of pyrimidine synthesis inhibitors on AML cells, suggesting that AML treatment strategies focusing on differentiation rather than cell killing may be successful in clinical settings.NEW & NOTEWORTHY This study is the first to demonstrate that human stromal cell lines and primary mesenchymal stromal cells from patients enhance the in vitro differentiation of acute myeloid leukemia (AML) cells induced by pyrimidine synthesis inhibitors, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr), and brequinar. Furthermore, this is the first report to show that AICAr affects mouse bone marrow stromal cells by activating AMP-activated protein kinase (AMPK) and that human stromal cells are superior to mouse cells for testing the effects of drugs on AML differentiation.

Mutual regulation between histone methyltransferase Suv39h1 and the Wnt/ß-catenin signaling pathway promoted cell proliferation and inhibited apoptosis in bone marrow mesenchymal stem cells exposed to hydroquinone.

Hydroquinone (HQ), a metabolite of benzene, is frequently utilized as a surrogate for benzene in in vitro studies and is associated with the development of acute myeloid leukemia (AML). In the hemotoxicity caused by benzene and HQ, cell apoptosis plays a key role. However, the molecular mechanisms underlying HQ are unknown. Studies have indicated that Suv39h1 is involved in regulating cell division and proliferation by regulating histone H3K9me3. Meanwhile, the Wnt/ß-catenin signaling pathway also plays a significant role in cell proliferation and apoptosis. Therefore, this study was aimed at exploring the regulatory role of Suv39h1 and the Wnt/ß-catenin signaling pathway in the effects of HQ on bone marrow mesenchymal stem cells (BMSCs), as well as its influence on cell proliferation and apoptosis. The results demonstrated that HQ elevated the levels of Suv39h1 and H3K9me3 and activated the Wnt/ß-catenin signaling pathway by upregulating ß-catenin, Wnt2b, C-myc, and Cyclin D1 and downregulating Wnt5a, resulting in an increase in cell growth and a decrease in apoptosis. Suv39h1 knockdown inhibited the Wnt/ß-catenin signaling pathway. Meanwhile, inhibition of the Wnt/ß-catenin signaling pathway resulted in the down-regulation of Suv39h1 and H3K9me3 in BMSCs. They both promoted cell proliferation and inhibited apoptosis in the effects of HQ on BMSCs by downregulating the expression of Cyt-C, Bax, Caspase 3, and Caspase 9 and upregulating the expression of Bcl-xl. Therefore, we concluded that Suv39h1 and the Wnt/ß-catenin signaling pathway may mutually regulate each other in the effects of HQ on BMSCs in order to ameliorate the altered function of BMSCs.

Mechanobiology and Primary Cilium in the Pathophysiology of Bone Marrow Myeloproliferative Diseases.

Philadelphia-Negative Myeloproliferative neoplasms (MPNs) are a diverse group of blood cancers leading to excessive production of mature blood cells. These chronic diseases, including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), can significantly impact patient quality of life and are still incurable in the vast majority of the cases. This review examines the mechanobiology within a bone marrow niche, emphasizing the role of mechanical cues and the primary cilium in the pathophysiology of MPNs. It discusses the influence of extracellular matrix components, cell-cell and cell-matrix interactions, and mechanosensitive structures on hematopoietic stem cell (HSC) behavior and disease progression. Additionally, the potential implications of the primary cilium as a chemo- and mechanosensory organelle in bone marrow cells are explored, highlighting its involvement in signaling pathways crucial for hematopoietic regulation. This review proposes future research directions to better understand the dysregulated bone marrow niche in MPNs and to identify novel therapeutic targets.

The long-chain polyfluorinated alkyl substance perfluorohexane sulfonate (PFHxS) promotes bone marrow adipogenesis.

Per- and polyfluoroalkyl substances (PFAS) bioaccumulate in different organ systems, including bone. While existing research highlights the adverse impact of PFAS on bone density, a critical gap remains in understanding the specific effects on the bone marrow microenvironment, especially the bone marrow adipose tissue (BMAT). Changes in BMAT have been linked to various health consequences, such as the development of osteoporosis and the progression of metastatic tumors in bone. Studies presented herein demonstrate that exposure to a mixture of five environmentally relevant PFAS compounds promotes marrow adipogenesis in vitro and in vivo. We show that among the components of the mixture, PFHxS, an alternative to PFOS, has the highest propensity to accumulate in bone and effectively promote marrow adipogenesis. Utilizing RNAseq approaches, we identified the peroxisome proliferator-activated receptor (PPAR) signaling as a top pathway modulated by PFHxS exposure. Furthermore, we provide results suggesting the activation and involvement of PPAR-gamma (PPARγ) in PFHxS-mediated bone marrow adipogenesis, especially in combination with high-fat diet. In conclusion, our findings demonstrate the potential impact of elevated PFHxS levels, particularly in occupational settings, on bone health, and specifically bone marrow adiposity. This study contributes new insights into the health risks of PFHxS exposure, urging further research on the relationship between environmental factors, diet, and adipose tissue dynamics.

The impact of ischemic stroke on bone marrow microenvironment and extracellular vesicles: A study on inflammatory and molecular changes.

An ischemic stroke (IS) is caused due to the lack of blood flow to cerebral tissue. Most of the studies have focused on how stroke affects the localized tissue, but it has been observed that a stroke can cause secondary complications in distant organs, such as Bone Marrow (BM). Our study focused on the effect of ischemic strokes on the bone marrow microenvironment. Bone marrow (BM) is a vital organ that maintains inflammatory homeostasis and aids in the repair of damaged tissue after injury/IS. We used the middle cerebral artery occlusion (MCAO) model of ischemic stroke on adult mice (6 months) and investigated the changes in the BM environment. BM cells were used for western blot and RT-PCR, and the BM supernatant was used for cytokine analysis and extracellular vesicle (EVs) isolation. We observed a significant increase in the total cell number within the BM and an increase in TNF-alpha and MCP-1, which are known for inducing a pro-inflammatory environment. Western blots analysis on the whole BM cell lysate demonstrated elevated levels of inflammatory factors (IL-6, TNF-alpha, and TLR-4) and senescence markers (p21 p16). EVs isolated from the BM supernatant showed no change in size or concentration; however, we found that the EVs carried increased miRNA-141-3p and miRNA-34a. Proteomic analysis on BM-derived EVs showed an alteration in the protein cargo of IS. We observed an increase in FgB, C3, Fn1, and Tra2b levels. The signaling pathway analysis showed mitochondrial function is most affected within the bone marrow. Our study demonstrated that IS induces changes in the BM environment and EVs secreted in the BM.

Marrow Adipocyte Senescence in the Pathogenesis of Bone Loss.

PURPOSE OF REVIEW: Beyond aging, senescent cells accumulate during multiple pathological conditions, including chemotherapy, radiation, glucocorticoids, obesity, and diabetes, even earlier in life. Therefore, cellular senescence represents a unifying pathogenic mechanism driving skeletal and metabolic disorders. However, whether senescent bone marrow adipocytes (BMAds) are causal in mediating skeletal dysfunction has only recently been evaluated. RECENT FINDINGS: Despite evidence of BMAd senescence following glucocorticoid therapy, additional evidence for BMAd senescence in other conditions has thus far been limited. Because the study of BMAds presents unique challenges making these cells difficult to isolate and image, here we review issues and approaches to overcome such challenges, and present advancements in isolation and histological techniques that may help with the future study of senescent BMAds. Further insights into the roles of BMAd senescence in the pathogenesis of skeletal dysfunction may have important basic science and clinical implications for human physiology and disease.

Mechanism of the apoptosis of bone marrow erythroblasts in rats under hypobaric hypoxia.

This study aimed to investigate the mechanism of the apoptosis of erythroblasts in rat bone marrow after the exposure to hypobaric hypoxia. Male SD rats were randomly divided into three groups. The hypoxic group was kept in a hypobaric hypoxia chamber at a simulated altitude of 5000 m for 7 and 28 days, respectively. The control group was kept at an altitude of 2260 m. We found that myeloid: erythroid (M:E) ratio was significantly lower after hypoxia exposure and the proportions of polychromatic erythroblasts and orthochromatic erythroblasts significantly increased compared to control group, along with significant increase in the proportion of CD71+ cells and apoptosis rate. The expression levels of caspase-3, Bax, and Cyt-C in CD71+ cells were higher after hypoxia exposure than those in control group, while there was no significant difference in the expression levels of TNFR and Fas. In conclusion, after exposure to hypobaric hypoxia the proliferation of peripheral blood and bone marrow erythroblasts in rats increased, and apoptosis also increased, indicating that bone marrow erythroblasts in rats is regulated by both proliferation and apoptosis, and the mitochondrial pathway is one of the important pathways for apoptosis.

Clinical Characteristics and Diagnosis of Ph-Positive Mixed Phenotype Acute Leukemia.

BACKGROUND: The goal was to improve the clinical cognition of Ph-positive mixed phenotype acute leukemia and avoid misdiagnosis or delayed diagnosis. METHODS: The clinical manifestations and laboratory results (bone marrow cell morphology, multiparameter flow cytometry, and cytogenetics) of a case of Ph-positive mixed phenotype acute leukemia were analyzed, and related literature was reviewed. RESULTS: Blood routine: WBC 386.35 x 109/L, HGB 117.00 g/L, PLT 31 x 109/L; 80% of the original cells can be seen by artificial classification. Morphological examination of bone marrow cells showed that the proliferation of nucleated cells was obviously active, and the original cells accounted for 76%. The size of the original cells was somewhat uniform, most of the cells had less mass, were stained light grayish blue, the cytoplasm particles were not obvious, the nuclei were mostly round or quasi-round, some of them showed distortion and nuclear notch, and the chromatin was coarse. Some of the cells were rich in mass, small azurin granules were seen, the nuclei were regular, most of them were round, the chromatin was fine, the myeloperoxidase and esterase staining were negative, the eosinophils accounted for 2.5%, and the basophils accounted for 0.5%. Flow cytometry immunotyping: Two groups of abnormal cells were seen in the bone marrow. 1. A group included 12.32% of nuclear cells and showed abnormal myeloid primitive cell phenotype. Main expression: CD117, CD34, CD38, HLA-DR, CD33, CD64, CD123, weak expression: CD13, CD19. 2. The other group included 45.61% of the nuclear cells and had a B-lymphoblastic phenotype. Main expression: CD34, CD38, HLA-DR, CD123, CD19, CD10, CD9, cCD79a, TDT, weak expression of CD13, CD22. Mixed phenotype acute leukemia (M/B) immunophenotype was considered. Chromosome: 46,XY,t(9; 22)(q34;q11.2) [20]. BCR-ABL (P210) fusion gene was positive. CONCLUSIONS: Mixed phenotype acute leukemia (MPAL) is a rare type of malignant hematologic disease. Its diagnosis is based on the comprehensive evaluation of bone marrow cell morphology, immunophenotype, molecular and cytogenetic features.

Decreasing circ_0014614 promotes the differentiation of bone marrow flineage cells into megakaryocytes in essential thrombocythemia via activiation of miR-138-5p/caspase3 axis.

BACKGROUND: Circular RNAs (circRNA) are pivotal in hematological diseases. Previous study showed that circ_0014614 (circDAP3) was significantly underexpressed in bone marrow-derived exosomes from essential thrombocythemia (ET) patients, affecting the differentiation of bone marrow lineage cells into megakaryocytes. METHODS: Fluorescence in situ hybridization (FISH) was used to display circ_0014614's primary cytoplasmic location in K562 cells. Cytoscape software was used to predict the circRNA-miRNA-mRNA networks, and their expression at the cellular level was detected by Quantitative reverse transcription-polymerase chain reaction (qRT-PCR). qRT-PCR was utilized to detect the expression levels of circ_0014614，miR-138-5p and caspase3 mRNA. Western blot was used to determine the protein levels of GATA-1, RUNX-1, NF-E2, CD41 and caspase3. The proliferation of K562 cells was assessed using the Cell Counting Kit-8 (CCK-8) Assay. Furthermore, the interplay between miR-138-5p and circ_0014614 or caspase3 was elucidated through a Dual-luciferase reporter assay. RESULTS: FISH assay indicated circ_0014614's primary cytoplasmic location in K562 cells. In ET bone marrow and K562 cells, circ_0014614 and caspase3 were down-regulated, whereas miR-138-5p saw a significant surge. Overexpressing circ_0014614 curtailed K562 cells' proliferation and differentiation. Further, circ_0014614 targeted miR-138-5p, with heightened miR-138-5p levels counteracting circ_0014614's inhibition. MiR-138-5p further targeted caspase3, and caspase3 silencing neutralized suppressed miR-138-5p's effects on K562 cell differentiation. CONCLUSION: Circ_0014614 was down-regulated in ET bone marrow and bone marrow lineage cells, and upregulating circ_0014614 can inhibit bone marrow lineage cells' proliferation and differentiation into megakaryocytes. Mechanistically, circ_0014614 functioned as ceRNA via sponging miR-138-5p and alleviated the inhibitory effect of miR-138-5p on its target caspase3, which potentially deters tumor activity in ET.

Prevalence of massively diluted bone marrow cell samples aspirated from patients with myelodysplastic syndromes (MDS) or suspected of MDS: A retrospective analysis of nationwide samples in Japan.

Bone marrow (BM) examination is a key element in the diagnosis and prognostic grading of myelodysplastic syndromes (MDSs), and obtaining adequate BM cell samples is critical for accurate test results. Massive haemodilution of aspirated BM samples is a well-known problem; however, its incidence in patients with MDS has not been well studied. We report the first study to examine the incidence of massive haemodilution in nationwide BM samples aspirated from patients diagnosed with or suspected of MDS in Japan. Among 283 cases available for analysis, BM smears from 92 cases (32.5%) were hypospicular (massively haemodiluted) and, particularly, no BM particles were observed in 52 cases (18.4%). Regarding hypospicular cases, we examined how the doctors in charge interpreted the BM smears of their patients. In only 19 of 92 cases (20.7%), doctors realised that the BM smears were haemodiluted. Furthermore, the BM biopsy, which can help diagnose hypospicular cases, was oftentimes not performed when the haemodilution was overlooked by doctors (not performed in 50 of 73 such cases). These real-world data highlight that not only researchers who are working to improve diagnostic tests but also clinicians who perform and use diagnostic tests must realise this common and potentially critical problem.

Revisiting the most often used item in the haematological tool box-The extent of haemodilution in bone marrow aspirates.

In this issue, a nationwide retrospective Japanese study finds that, in a second opinion setting, one-third of bone marrow aspirates from patients suspected of myelodysplastic syndromes are heavily haemodiluted. Moreover, in four-fifths of such cases, the failure to obtain the correct material for diagnosis went undetected by the referring institution. These data are intriguing, but given their special set-up, caution should be exerted in transposing them to other countries. Commentary on: Ogata et al. Prevalence of massively diluted bone marrow cell samples aspirated from patients with myelodysplastic syndromes (MDS) or suspected MDS: A retrospective analysis of nationwide samples in Japan. Br J Haematol 2024;204:1856-1861.

Flow cytometric immunophenotypic differentiation patterns of bone marrow eosinophilopoiesis.

BACKGROUND: Flow cytometry has been widely used to study immunophenotypic patterns of maturation of most hematopoietic lineages in normal human bone marrow aspirates, thus allowing identification of changes in patterns in many myeloid malignancies. Eosinophils play an important role in a wide variety of disorders, including some myeloid neoplasms. However, changes in flow cytometric immunophenotypic patterns during normal and abnormal bone marrow eosinophilopoiesis have not been well studied. METHODS: Fresh bone marrow aspirates from 15 healthy donors, 19 patients with hypereosinophilic syndromes (HES), and 11 patients with systemic mastocytosis (SM) were analyzed for candidate markers that included EMR-1, Siglec-8, CCR3, CD9, CD11a, CD11b, CD11c, CD13, CD16, CD29, CD34, CD38, CD45, CD44, CD49d, CD49f, CD54, CD62L, CD69, CD117, CD125 (IL-5Rα), HLA-DR, using 10 parameter flow cytometry. Putative CD34-negative immature and mature normal eosinophil populations were first identified based on changes in expression of the above markers in healthy donors, then confirmed using fluorescence-based cell sorting and morphological evaluation of cytospin preparations. The normal immunophenotypic patterns were then compared to immunophenotypic patterns of eosinophilopoiesis in patients with HES and SM. RESULTS: The eosinophilic lineage was first verified using the human eosinophil-specific antibody EMR-1 in combination with anti-IL-5Rα antibody. Then, a combination of Siglec-8, CD9, CD11b, CCR3, CD49d, and CD49f antibodies was used to delineate normal eosinophilic maturational patterns. Early stages (eosinophilic promyelocytes/myelocytes) were identified as Siglec-8 dim/CD11b dim to moderate/CD9 dim/CCR3 dim/CD49d bright/CD49f dim, intermediate stages (eosinophilic myelocytes/metamyelocytes) as Siglec-8 moderate/CD11b moderate to bright/CD9 moderate/CCR3 moderate/CD49d moderate/CD49f moderate and mature bands/segmented eosinophils as Siglec-8 bright/CD11b bright/CD9 bright/CCR3 bright/CD49d dim/CD49f bright. Overall maturational patterns were also similar in patients with HES and SM; however, the expression levels of several surface markers were altered compared to normal eosinophils. CONCLUSION: A novel flow cytometric antibody panel was devised to detect alterations in immunophenotypic patterns of bone marrow eosinophil maturation and evaluated in normal, HES and SM samples. This approach will allow us to elucidate changes in immunophenotypic patterns of bone marrow eosinophilopoiesis in other hematological diseases.

Tumor microenvironment restricts IL-10 induced multipotent progenitors to myeloid-lymphatic phenotype.

Lymphangiogenesis is induced by local pro-lymphatic growth factors and bone marrow (BM)-derived myeloid-lymphatic endothelial cell progenitors (M-LECP). We previously showed that M-LECP play a significant role in lymphangiogenesis and lymph node metastasis in clinical breast cancer (BC) and experimental BC models. We also showed that differentiation of mouse and human M-LECP can be induced through sequential activation of colony stimulating factor-1 (CSF-1) and Toll-like receptor-4 (TLR4) pathways. This treatment activates the autocrine interleukin-10 (IL-10) pathway that, in turn, induces myeloid immunosuppressive M2 phenotype along with lymphatic-specific proteins. Because IL-10 is implicated in differentiation of numerous lineages, we sought to determine whether this pathway specifically promotes the lymphatic phenotype or multipotent progenitors that can give rise to M-LECP among other lineages. Analyses of BM cells activated either by CSF-1/TLR4 ligands in vitro or orthotopic breast tumors in vivo showed expansion of stem/progenitor population and coincident upregulation of markers for at least four lineages including M2-macrophage, lymphatic endothelial, erythroid, and T-cells. Induction of cell plasticity and multipotency was IL-10 dependent as indicated by significant reduction of stem cell markers and those for multiple lineages in differentiated cells treated with anti-IL-10 receptor (IL-10R) antibody or derived from IL-10R knockout mice. However, multipotent CD11b+/Lyve-1+/Ter-119+/CD3e+ progenitors detected in BM appeared to split into a predominant myeloid-lymphatic fraction and minor subsets expressing erythroid and T-cell markers upon establishing tumor residence. Each sub-population was detected at a distinct intratumoral site. This study provides direct evidence for differences in maturation status between the BM progenitors and those reaching tumor destination. The study results suggest preferential tumor bias towards expansion of myeloid-lymphatic cells while underscoring the role of IL-10 in early BM production of multipotent progenitors that give rise to both hematopoietic and endothelial lineages.

Diet-induced obesity reduces bone marrow T and B cells and promotes tumor progression in a transplantable Vk*MYC model of multiple myeloma.

Obesity is associated with an increased risk of developing multiple myeloma (MM). The molecular mechanisms causing this association is complex and incompletely understood. Whether obesity affects bone marrow immune cell composition in multiple myeloma is not characterized. Here, we examined the effect of diet-induced obesity on bone marrow immune cell composition and tumor growth in a Vk*MYC (Vk12653) transplant model of multiple myeloma. We find that diet-induced obesity promoted tumor growth in the bone marrow and spleen and reduced the relative number of T and B cells in the bone marrow. Our results suggest that obesity may reduce MM immune surveillance and thus may contribute to increased risk of developing MM.

Digital morphology compared to the optical microscope: A validation study on reporting bone marrow aspirates.

INTRODUCTION: This study aims to evaluate the effectiveness and reliability of the utilization for clinical reporting of the evaluation of digital images of bone marrow aspirates by morphologists and their comparability with the classic microscopic morphological evaluation. METHODS: We scanned 180 consecutive bone marrow needle aspirates smears using the "Metafer4 VSlide" whole slide imaging (WSI) digital scanning system. We evaluated the statistical comparability and the risk of bias of the microscopic readings with those performed on the screen on the digitized medullary images. RESULTS: The evaluation of cellularity on the screen was equivalent, with a higher frequency of "normal" than the analysis of digital preparations. The means and medians of the percentage values obtained on the different cell populations with the microscopic and digital reading were comparable as the main categories are concerned, with an average difference equal to 0 for the neutrophilic and eosinophilic granulocytic series, at -0.2% for the total myeloid cells, at 1.2% for the erythroid series, at -0.4% for the lymphocytes and at -0.4% for the blasts. Dysplastic features were consistently identified in 69/71 cell lineages. CONCLUSION: Our study demonstrated that screen evaluation of digitized bone marrow needle aspirates provides quantitative and qualitative results comparable to traditional microscopic analysis of the corresponding slide smears. Digital images offer significant benefits in reducing the workload of experienced operators, reproducibility and sharing of observations, and image preservation. Even in routine diagnostic activities, their use does not alter the quality of the results obtained in evaluating bone marrow needle aspirates.

Stromal bone marrow fibroblasts and mesenchymal stem cells support acute myeloid leukaemia cells and promote therapy resistance.

The bone marrow (BM) is the primary site of adult haematopoiesis, where stromal elements (e.g. fibroblasts and mesenchymal stem cells [MSCs]) work in concert to support blood cell development. However, the establishment of an abnormal clone can lead to a blood malignancy, such as acute myeloid leukaemia (AML). Despite our increased understanding of the pathophysiology of the disease, patient survival remains suboptimal, mainly driven by the development of therapy resistance. In this review, we highlight the importance of bone marrow fibroblasts and MSCs in health and acute myeloid leukaemia and their impact on patient prognosis. We discuss how stromal elements reduce the killing effects of therapies via a combination of contact-dependent (e.g. integrins) and contact-independent (i.e. secreted factors) mechanisms, accompanied by the establishment of an immunosuppressive microenvironment. Importantly, we underline the challenges of therapeutically targeting the bone marrow stroma to improve acute myeloid leukaemia patient outcomes, due to the inherent heterogeneity of stromal cell populations. 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.