Mesenchymal stromal cell density is increased in higher grade myelodysplastic syndromes and independently predicts survival

Mesenchymal stromal cells in myeloid malignancies: Immunotherapeutic opportunities

Myeloid malignancies are clonal disorders of the progenitor cells or hematopoietic stem cells, including acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative malignancies, and chronic myelomonocytic leukemia. Myeloid neoplastic cells affect the proliferation and differentiation of other hematopoietic lineages in the bone marrow and peripheral blood, leading to severe and life-threatening complications. Mesenchymal stromal cells (MSCs) residing in the bone marrow exert immunosuppressive functions by suppressing innate and adaptive immune systems, thus creating a supportive and tolerant microenvironment for myeloid malignancy progression. This review summarizes the significant features of MSCs in myeloid malignancies, including their role in regulating cell growth, cell death, and antineoplastic resistance, in addition to their immunosuppressive contributions. Understanding the implications of MSCs in myeloid malignancies could pave the path for potential use in immunotherapy.

The immunological role of mesenchymal stromal cells in patients with myelodysplastic syndrome

Myelodysplastic syndrome (MDS) is a common hematological malignant disease, characterized by malignant hematopoietic stem cell proliferation in the bone marrow (BM); clinically, it mainly manifests clinically mainly by as pathological hematopoiesis, hemocytopenia, and high-risk transformation to acute leukemia. Several studies have shown that the BM microenvironment plays a critical role in the progression of MDS. In this study, we specifically evaluated mesenchymal stromal cells (MSCs) that exert immunomodulatory effects in the BM microenvironment. This immunomodulatory effect occurs through direct cell-cell contact and the secretion of soluble cytokines or micro vesicles. Several researchers have compared MSCs derived from healthy donors to low-risk MDS-associated bone mesenchymal stem cells (BM-MSCs) and have found no significant abnormalities in the MDS-MSC phenotype; however, these cells have been observed to exhibit altered function, including a decline in osteoblastic function. This altered function may promote MDS progression. In patients with MDS, especially high-risk patients, MSCs in the BM microenvironment regulate immune cell function, such as that of T cells, B cells, natural killer cells, dendritic cells, neutrophils, myeloid-derived suppressor cells (MDSCs), macrophages, and Treg cells, thereby enabling MDS-associated malignant cells to evade immune cell surveillance. Alterations in MDS-MSC function include genomic instability, microRNA production, histone modification, DNA methylation, and abnormal signal transduction and cytokine secretion.

Relationship of HIF‑1α expression with apoptosis and cell cycle in bone marrow mesenchymal stem cells from patients with myelodysplastic syndrome

Myelodysplastic syndrome (MDS) is a group of abnormal clonal disorders with ineffective hematopoiesis, which are incurable with conventional therapy. Of note, MDS features an abnormal bone marrow microenvironment, which is related to its incidence. The hypoxia-inducible factor-1α (HIF-1α) transcriptional signature is generally activated in bone marrow stem/progenitor cells of patients with MDS. To analyze the expression of HIF-1α in bone marrow mesenchymal stem cells (BM-MSCs) and the apoptosis and cell cycle features associated with the disease, BM-MSCs were obtained from 40 patients with a definitive diagnosis of MDS and 20 subjects with hemocytopenia but a negative diagnosis of MDS as a control group. Reverse transcription-quantitative PCR and western blot analyses were used to measure HIF-1α expression in cells from the two groups and apoptosis and cell cycle were also analyzed and compared between the groups using flow cytometry assays. BM-MSCs from both the control group and the MDS group exhibited a fibroblast-like morphology, had similar growth cycles and were difficult to passage stably. It was observed that BM-MSCs from the MDS group had significantly higher HIF-1α expression levels than the control group (P<0.05). Furthermore, the BM-MSCs from the MDS group had a higher proportion of cells in early apoptosis (5.22±1.34 vs. 2.04±0.08%; P<0.0001) and late apoptosis (3.38±0.43 vs. 1.23±0.11%; P<0.01) and exhibited cell cycle arrest. This may be a noteworthy aspect of the pathogenesis of MDS and may be related to high HIF-1α expression under a hypoxic state in the bone marrow microenvironment. Furthermore, the expression of HIF-1α in bone marrow tissue sections from patients with MDS in the International Prognostic Scoring System (IPSS) lower-risk group was higher than that from patients with MDS in the IPSS high-risk group. These results revealed the role of HIF-1α as a central pathobiology mediator of MDS and an effective therapeutic target for a broad spectrum of patients with MDS, particularly for patients in the lower-risk group.

Human, mouse, and dog bone marrow show similar mesenchymal stromal cells within a distinctive microenvironment

Bone marrow stromal cells (BMSCs) are a key part of the hematopoietic niche. Mouse and human BMSCs are recognized by different markers (LepR and NGFR/CD271, respectively). However, there has not been a detailed in situ comparison of both populations within the hematopoietic microenvironment. Moreover, dog BMSCs have not been characterized in situ by any of those markers. We conducted a systematic histopathological comparison of mouse, human, and dog BMSCs within their bone marrow architecture and microenvironment. Human and dog CD271+ BMSCs had a morphology, frequency, and distribution within trabecular bone marrow similar to those of mouse LepR+ BMSCs. However, mouse bone marrow had higher cellularity and megakaryocyte content. In conclusion, highly comparable bone marrow mesenchymal stromal cell distribution among the three species establishes the validity of using mouse and dog as a surrogate experimental model of hematopoietic stem cell-BMSC interactions. However, the distinct differences in adipocyte and megakaryocyte microenvironment content of mouse bone marrow and how they might influence hematopoietic stem cell interactions as compared with humans require further study.

Myeloid-Derived Suppressor Cells and Mesenchymal Stem/Stromal Cells in Myeloid Malignancies

Myeloid malignancies arise from an altered hematopoietic stem cell and mainly comprise acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative malignancies, and chronic myelomonocytic leukemia. Myeloid neoplastic leukemic cells may influence the growth and differentiation of other hematopoietic cell lineages in peripheral blood and bone marrow. Myeloid-derived suppressor cells (MDSCs) and mesenchymal stromal cells (MSCs) display immunoregulatory properties by controlling the innate and adaptive immune systems that may induce a tolerant and supportive microenvironment for neoplasm development. This review analyzes the main features of MDSCs and MSCs in myeloid malignancies. The number of MDSCs is elevated in myeloid malignancies exhibiting high immunosuppressive capacities, whereas MSCs, in addition to their immunosuppression contribution, regulate myeloid leukemia cell proliferation, apoptosis, and chemotherapy resistance. Moreover, MSCs may promote MDSC expansion, which may mutually contribute to the creation of an immuno-tolerant neoplasm microenvironment. Understanding the implication of MDSCs and MSCs in myeloid malignancies may favor their potential use in immunotherapeutic strategies.

Myeloid-Derived Suppressor Cells and Mesenchymal Stem/Stromal Cells in Myeloid Malignancies

Myeloid malignancies arise from an altered hematopoietic stem cell and mainly comprise acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative malignancies, and chronic myelomonocytic leukemia. Myeloid neoplastic leukemic cells may influence the growth and differentiation of other hematopoietic cell lineages in peripheral blood and bone marrow. Myeloid-derived suppressor cells (MDSCs) and mesenchymal stromal cells (MSCs) display immunoregulatory properties by controlling the innate and adaptive immune systems that may induce a tolerant and supportive microenvironment for neoplasm development. This review analyzes the main features of MDSCs and MSCs in myeloid malignancies. The number of MDSCs is elevated in myeloid malignancies exhibiting high immunosuppressive capacities, whereas MSCs, in addition to their immunosuppression contribution, regulate myeloid leukemia cell proliferation, apoptosis, and chemotherapy resistance. Moreover, MSCs may promote MDSC expansion, which may mutually contribute to the creation of an immuno-tolerant neoplasm microenvironment. Understanding the implication of MDSCs and MSCs in myeloid malignancies may favor their potential use in immunotherapeutic strategies.

Identification of CXCL12-abundant reticular cells in human adult bone marrow

A population of mesenchymal stem cells, termed CXC chemokine ligand (CXCL)12‐abundant reticular (CAR) cells or leptin receptor‐expressing cells, are the major cellular component of niches for haematopoietic stem cells (HSCs) in murine bone marrow. CAR cells are characterized by several salient features, including much higher expression of CXCL12, stem cell factor (SCF), forkhead box C1 (FOXC1) and early B‐cell factor 3 (EBF3), which are essential for HSC maintenance, than other cells. However, the human counterpart of CAR cells has not been fully described. Here, we show the presence of cells expressing much higher CXCL12 than other cells in human adult bone marrow using a flow cytometry‐based in situ technique that enables high‐throughput detection of mRNA at single‐cell resolution. Most CXCL12^hi cells expressed high levels of SCF, FOXC1 and EBF3 and had the potential to differentiate into adipocytes and osteoblasts. Histologically, the nuclei of CXCL12^hi cells were identified and quantified by EBF3 expression in fixed marrow sections. CXCL12^hi cells sorted from residual bone marrow aspirates of chronic myeloid leukaemia patients expressed reduced levels of CXCL12, SCF, FOXC1 and EBF3 in correlation with increased leukaemic burden. Together, we identified the human counterpart of CAR cells, enabling the evaluation of their alterations in various haematological disorders by flow cytometric and histological analyses.

Human Aging Alters the Spatial Organization between CD34+ Hematopoietic Cells and Adipocytes in Bone Marrow

Age-related clonal hematopoiesis is a major risk factor for myeloid malignancy and myeloid skewing is a hallmark of aging. However, while it is known that non-cell-autonomous components of the microenvironment can also influence this risk, there have been few studies of how the spatial architecture of human bone marrow (BM) changes with aging. Here, we show that BM adiposity increases with age, which correlates with increased density of maturing myeloid cells and CD34+ hematopoietic stem/progenitor cells (HSPCs) and an increased proportion of HSPCs adjacent to adipocytes. However, NGFR+ bone marrow stromal cell (NGFR+ BMSC) density and distance to HSPCs and vessels remained stable. Interestingly, we found that, upon aging, maturing myeloid cell density increases in hematopoietic areas surrounding adipocytes. We propose that increased adjacency to adipocytes in the BM microenvironment may influence myeloid skewing of aging HSPCs, contributing to age-related risk of myeloid malignancies.

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Aging increases adipose, myeloid, and CD34+ HSPC density in the human bone marrow

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Human CD34+ HSPC niche is reticular, perivascular, and periadipocytic in aging

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Aging increases maturing myeloid cell density surrounding adipocytes

Flaming and fanning: The Spectrum of inflammatory influences in myelodysplastic syndromes

The myelodysplastic syndromes (MDS) represent neoplasms derived from the expansion of mutated clonal hematopoietic cells which often demonstrate aberrant differentiation potential with resultant cytopenias and a propensity to evolve into acute myelogenous leukemia. While multiple mutations have been identified which may serve as drivers of the MDS clone, there is accumulating evidence that MDS clones and subclones are subject to modulation by the marrow microenvironment and its inflammatory milieu. There is also a strong link between autoimmune disorders and MDS. In this review, we examine the role of inflammatory cytokines, toll like receptors, pyroptosis, stromal cells, and cellular inflammatory mediators in MDS initiation, propagation, and progression. These contributions in a background of mutational, epigenetic, and aging changes in the marrow are also reviewed. Such inflammatory mediators may be subject to therapeutic agents which will enhance suppression of the MDS clone with potential to improve therapeutic outcomes in this disease which is usually incurable in aged patients not eligible for stem cell transplantation.

Imaging methods used to study mouse and human HSC niches: Current and emerging technologies

Bone marrow contains numerous different cell types arising from hematopoietic stem cells (HSCs) and non-hematopoietic mesenchymal/skeletal stem cells, in addition to other cell types such as endothelial cells- these non-hematopoietic cells are commonly referred to as stromal cells or microenvironment cells. HSC function is intimately linked to complex signals integrated by their niches, formed by combinations of hematopoietic and stromal cells. Studies of hematopoietic cells have been significantly advanced by flow cytometry methods, enabling the quantitation of each cell type in normal and perturbed situations, in addition to the isolation of these cells for molecular and functional studies. Less is known, however, about the specific niches for distinct developing hematopoietic lineages, or the changes occurring in the niche size and function in these distinct anatomical sites in the bone marrow under stress situations and ageing. Significant advances in imaging technology during the last decade have permitted studies of HSC niches in mice. Additional imaging technologies are emerging that will facilitate the study of human HSC niches in trephine BM biopsies. Here we provide an overview of imaging technologies used to study HSC niches, in addition to highlighting emerging technology that will help us to more precisely identify and characterize HSC niches in normal and diseased states.

KB004, a first in class monoclonal antibody targeting the receptor tyrosine kinase EphA3, in patients with advanced hematologic malignancies: Results from a phase 1 study

EphA3 is an Ephrin receptor tyrosine kinase that is overexpressed in most hematologic malignancies. We performed a first-in-human multicenter phase I study of the anti-EphA3 monoclonal antibody KB004 in refractory hematologic malignancies in order to determine safety and tolerability, along with the secondary objectives of pharmacokinetics (PK) and pharmacodynamics (PD) assessments, as well as preliminary assessment of efficacy. Patients were enrolled on a dose escalation phase (DEP) initially, followed by a cohort expansion phase (CEP). KB004 was administered by intravenous infusion on days 1, 8, and 15 of each 21-day cycle in escalating doses. A total of 50 patients (AML 39, MDS/MPN 3, MDS 4, DLBCL 1, MF 3) received KB004 in the DEP; an additional 14 patients were treated on the CEP (AML 8, MDS 6). The most common toxicities were transient grade 1 and grade 2 infusion reactions (IRs) in 79% of patients. IRs were dose limiting above 250mg. Sustained exposure exceeding the predicted effective concentration (1ug/mL) and covering the 7-day interval between doses was achieved above 190mg. Responses were observed in patients with AML, MF, MDS/MPN and MDS. In this study, KB004 was well tolerated and clinically active when given as a weekly infusion.

Mesenchymal Stem and Progenitor Cells in Normal and Dysplastic Hematopoiesis-Masters of Survival and Clonality?

Myelodysplastic syndromes (MDS) are malignant hematopoietic stem cell disorders that have the capacity to progress to acute myeloid leukemia (AML). Accumulating evidence suggests that the altered bone marrow (BM) microenvironment in general, and in particular the components of the stem cell niche, including mesenchymal stem cells (MSCs) and their progeny, play a pivotal role in the evolution and propagation of MDS. We here present an overview of the role of MSCs in the pathogenesis of MDS, with emphasis on cellular interactions in the BM microenvironment and related stem cell niche concepts. MSCs have potent immunomodulatory capacities and communicate with diverse immune cells, but also interact with various other cellular components of the microenvironment as well as with normal and leukemic stem and progenitor cells. Moreover, compared to normal MSCs, MSCs in MDS and AML often exhibit altered gene expression profiles, an aberrant phenotype, and abnormal functional properties. These alterations supposedly contribute to the “reprogramming” of the stem cell niche into a disease-permissive microenvironment where an altered immune system, abnormal stem cell niche interactions, and an impaired growth control lead to disease progression. The current article also reviews molecular targets that play a role in such cellular interactions and possibilities to interfere with abnormal stem cell niche interactions by using specific targeted drugs.

Beyond the Niche: Myelodysplastic Syndrome Topobiology in the Laboratory and in the Clinic

We review the murine and human microenvironment and hematopoietic stem cell niche in the context of intact bone marrow architecture in man and mouse, both in normal and in myelodysplastic syndrome marrow. We propose that the complexity of the hematopoietic stem cell niche can usefully be approached in the context of its topobiology, and we provide a model that incorporates in vitro and in vivo models as well as in situ findings from intact human marrow to explain the changes seen in myelodysplastic syndrome patients. We highlight the clinical application of the study of the bone marrow microenvironment and its topobiology in myelodysplastic syndromes.