Stem cell regulatory niches and their role in normal and malignant hematopoiesis:

Emerging drugs targeting cellular redox homeostasis to eliminate acute myeloid leukemia stem cells

Acute myeloid leukemia (AML) is a very heterogeneous group of disorders with large differences in the percentage of immature blasts that presently are classified according to the specific mutations that trigger malignant proliferation among thousands of mutations reported thus far. It is an aggressive disease for which few targeted therapies are available and still has a high recurrence rate and low overall survival. The main reason for AML relapse is believed to be due to leukemic stem cells (LSCs) that have unlimited self-renewal capacity and long residence in a quiescent state, which promote greater resistance to traditional therapies for this cancer. AML LSCs have low oxidative stress levels, which appear to be caused by a combination of low mitochondrial activity and high activity of ROS-removing pathways. In this sense, oxidative stress has been thought to be an important new potential target for the treatment of AML patients, targeting the eradication of AML LSCs. The aim of this review is to discuss some drugs that induce oxidative stress to direct new goals for future research focusing on redox imbalance as an effective strategy to eliminate AML LSCs.

Dexamethasone-mediated inhibition of Notch signalling blocks the interaction of leukaemia and mesenchymal stromal cells

Acute myeloid leukaemia (AML) is a haematological malignancy characterized by a poor prognosis. Bone marrow mesenchymal stromal cells (BM MSCs) support leukaemic cells in preventing chemotherapy-induced apoptosis. This encouraged us to investigate leukaemia-BM niche-associated signalling and to identify signalling cascades supporting the interaction of leukaemic cells and BM MSC. Our study demonstrated functional differences between MSCs originating from leukaemic (AML MSCs) and healthy donors (HD MSCs). The direct interaction of leukaemic and AML MSCs was indispensable in influencing AML cell proliferation. We further identified an important role for Notch expression and its activation in AML MSCs contributing to the enhanced proliferation of AML cells. Supporting this observation, overexpression of the intracellular Notch domain (Notch ICN) in AML MSCs enhanced AML cells' proliferation. From a therapeutic point of view, dexamethasone treatment impeded Notch signalling in AML MSCs resulting in reduced AML cell proliferation. Concurrent with our data, Notch inhibitors had only a marginal effect on leukaemic cells alone but strongly influenced Notch signalling in AML MSCs and abrogated their cytoprotective function on AML cells. In vivo, dexamethasone treatment impeded Notch signalling in AML MSCs leading to a reduced number of AML MSCs and improved survival of leukaemic mice. In summary, targeting the interaction of leukaemic cells and AML MSCs using dexamethasone or Notch inhibitors might further improve treatment outcomes in AML patients.

Normal and leukemic stem cell niche interactions

PURPOSE OF REVIEW

Normal hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) interact with the stem cell niche bone marrow in different ways. Understanding the potentially unique microenvironmental regulation of LSCs is key to understanding in-vivo leukemogenic mechanisms and developing novel antileukemic therapies.

RECENT FINDINGS

When leukemic cells are engrafted in the stem cell niche, the cellular nature of the niche - including mesenchymal stromal cells - is reprogramed. Altered mesenchymal cells selectively support leukemic cells and reinforce the pro-leukemic environment. As the niche plays an active role in leukemogenesis, its remodeling may significantly influence the leukemogenic pattern, and cause differences in clinical prognosis. Notably, niche cells could be stimulated to revert to a pronormal/antileukemic state, creating potential for niche-based antileukemic therapy.

SUMMARY

Bone marrow microenvironments are under dynamic regulation for normal and leukemic cells, and there is bi-directional control of leukemic cells in the niche. Leukemic cells are both protected by stroma and able to reprogram stromal cells to transform the niche to a state, which reinforces leukemogenesis. Because of its dynamic nature, the niche could be converted to an environment with antileukemic properties, making it an attractive target for therapy.

Oxidative resistance of leukemic stem cells and oxidative damage to hematopoietic stem cells under pro-oxidative therapy

Leukemic stem cells (LSCs) and hematopoietic stem cells (HSCs) are both dependent on the hypoxic bone marrow (BM) microenvironment (also known as the BM niche). There is always fierce competition between the two types of cells, and the former exhibits a greater competitive advantage than the latter via multiple mechanisms. Under hypoxia, the dynamic balance between the generation and clearing of intracellular reactive oxygen species (ROS) is conducive to maintaining a quiescent state of cells. Quiescent LSCs can reside well in the BM niche, avoiding attack by chemotherapeutic agents, which is the cause of chemotherapeutic resistance and relapse in leukemia. HSCs acquire energy mainly through anaerobic glycolysis, whereas LSCs achieve energy metabolism largely through mitochondrial oxidative respiration. Mitochondria are the primary site of ROS generation. Thus, in theory, mitochondria-mediated respiration will cause an increase in ROS generation in LSCs and a higher intracellular oxidative stress level. The sensitivity of the cells to pro-oxidant drugs increases as well, which allows for the selective clearing of LSCs by pro-oxidative therapy. However, HSCs are also highly sensitive to changes in ROS levels, and the toxic effects of pro-oxidant drugs on HSCs poses a major challenge to pro-oxidative therapy in leukemia. Given the above facts, we reviewed studies on the oxidative resistance of LSCs and the oxidative damage to HSCs under pro-oxidative therapy. An in-depth investigation into the oxidative stress status and regulatory mechanisms of LSCs and HSCs in hypoxic environments will promote our understanding of the survival strategy employed by LSCs and the mechanism of the oxidative damage to HSCs in the BM niche, thus facilitating individualized treatment of leukemia patients and helping eliminate LSCs without disturbing normal hematopoietic cells.

Biomaterials for stem cell engineering and biomanufacturing

Recent years have witnessed the expansion of tissue failures and diseases. The uprising of regenerative medicine converges the sight onto stem cell-biomaterial based therapy. Tissue engineering and regenerative medicine proposes the strategy of constructing spatially, mechanically, chemically and biologically designed biomaterials for stem cells to grow and differentiate. Therefore, this paper summarized the basic properties of embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs) and adult stem cells. The properties of frequently used biomaterials were also described in terms of natural and synthetic origins. Particularly, the combination of stem cells and biomaterials for tissue repair applications was reviewed in terms of nervous, cardiovascular, pancreatic, hematopoietic and musculoskeletal system. Finally, stem-cell-related biomanufacturing was envisioned and the novel biofabrication technologies were discussed, enlightening a promising route for the future advancement of large-scale stem cell-biomaterial based therapeutic manufacturing.

Romiplostim in patients with refractory aplastic anaemia previously treated with immunosuppressive therapy: a dose-finding and long-term treatment phase 2 trial

BACKGROUND

Aplastic anaemia is a rare, life-threatening condition, characterised by pancytopenia with hypocellular bone marrow. Haematopoietic stem cells and most progenitor cells express thrombopoietin receptor (c-MPL). Romiplostim is a peptibody with c-MPL agonist activity that stimulates endogenous thrombopoietin production and leads to promoting the proliferation and differentiation of megakaryocytes in the bone marrow. In this phase 2 trial we aimed to assess the activity and safety of romiplostim in patients with aplastic anaemia who were previously treated with immunosuppressive therapy.

METHODS

We did an open-label, phase 2 study including a randomised, parallel, dose-finding part followed by an extension part to evaluate long-term treatment at two clinical centres in Seoul, South Korea. Eligible patients were aged 19 years or older, and had aplastic anaemia confirmed by bone marrow and cytogenetic studies and thrombocytopenia (platelet count ≤30 × 10⁹/L), an Eastern Cooperative Oncology Group performance status score of 2 or lower, and were previously treated with immunosuppressive therapy, including at least one course of antithymocyte globulin plus cyclosporin. In the dose-finding part, patients were randomly assigned to fixed dose cohorts (1, 3, 6, or 10 μg/kg) of subcutaneous romiplostim once weekly for 8 weeks, according to a static allocation procedure after stratification by platelet count. In the extension part of the study, patients continued romiplostim titrated every 4 weeks in single steps (1, 3, 6, 10, 13, 16, and 20 μg/kg once weekly), depending on platelet response and safety up to 1 year (weeks 9-52). Patients who had a platelet response during weeks 46-53 continued dose titration in single steps (3, 6, 10, 13, 16, and 20 μg/kg once weekly) for an additional 2 years (weeks 53-156). The primary endpoint was the proportion of patients achieving a platelet response at week 9 (after completion of the dose-finding part). Activity was assessed per-protocol in all patients evaluable for response at week 9 and safety was assessed in all patients who received at least one dose of romiplostim. This trial is registered with ClinicalTrials.gov, NCT02094417.

FINDINGS

Between April 14 and Nov 24, 2014, 35 patients were enrolled and randomly assigned to one of four dose cohorts: romiplostim 1 μg/kg (n=7), 3 μg/kg (n=9), 6 μg/kg (n=9), and 10 μg/kg (n=10). Data cutoff for this final analysis was on April 14, 2018. The median duration of treatment for all patients was 53 weeks (IQR 35-155). Ten (30%) of 33 evaluable patients achieved a platelet response at week 9, including seven (70%) of ten patients in the 10 μg/kg cohort, three (33%) of nine patients in the 6 μg/kg cohort, and no patients in both the 3 μg/kg and 1 μg/kg cohorts. During the extension study, 18 (55%) of 33 evaluable patients had a platelet response during weeks 46-53 and were eligible for continued treatment. Ten (30%) patients maintained a platelet response at 2 and 3 years, of whom nine had an erythroid response and five a neutrophil response, and completed protocol treatment. Treatment-related adverse events occurred in three (9%) of 35 patients, including grade 1 or 2 myalgia, fatigue, and dizziness. 17 (49%) of 35 patients had adverse events of grade 3 or higher; seven (20%) had serious adverse events (one event of febrile neutropenia, cataract, retinal detachment, macular fibrosis, inguinal hernia, appendicitis, cellulitis, tendon injury, and transfusion reaction); and one patient died from sepsis during treatment; none of these events were related to treatment. No patients developed clonal evolution.

INTERPRETATION

Romiplostim seems to be active and has a favourable safety profile in patients with refractory aplastic anaemia. 10 μg/kg once weekly might be used as a recommended starting dose in future studies. These findings warrant further investigation.

FUNDING

Kyowa Hakko Kirin Korea.

Autophagy-deficient mice are more susceptible to engrafted leukemogenesis

Autophagy is primarily considered as an important survival mechanism for both normal cells and cancer cells in response to metabolic stress or chemotherapy; but the role of autophagy in leukemogenesis is not fully understood. The aim of this study is to explore the role of intrinsic autophagy in the leukemogenesis of B-cell acute lymphoblastic leukemia (B-ALL). In this study, conditional knockout mice Atg7^f/f;Ubc-Cre, in which an autophagy-essential gene Atg7 is universally deleted, were used as recipients, B-ALL cell line 697 was used as donor cells to generate leukemia mouse model. Compared to wild-type mice, Atg7 knockout mice were more susceptible to engrafted leukemogenesis, shown by increase in white blood cells, lymphocytes, and platelets, decrease in HSPC number and its colony-forming unit (CFU). The liver and spleen displayed hepatosplenomegaly and inflammatory cell infiltration. Furthermore, second competitive transplantation revealed dysfunction of the HSPC in Atg7-knockout leukemia mice represented by destructive self-renew ability (CFU) and reconstitution ability including decreased B220, Ter 119 cells, and increased Gr-1 cell percentage. In summary, Mice with universal deletion of Atg7 are more inclined to the occurrence of engrafted human leukemia, which is largely attributed to the deterioration of the function of HSPC in autophagy deficient mice.

Age-related differences in the bone marrow stem cell niche generate specialized microenvironments for the distinct regulation of normal hematopoietic and leukemia stem cells

The bone marrow (BM) microenvironment serves as a stem cell niche regulating the in vivo cell fate of normal hematopoietic stem cells (HSC) as well as leukemia stem cells (LSCs). Accumulating studies have indicated that the regeneration of normal HSCs and the process of leukemogenesis change with advancing age. However, the role of microenvironmental factors in these age-related effects are unclear. Here, we compared the stem cell niche in neonatal and adult BM to investigate potential differences in their microenvironmental regulation of both normal and leukemic stem cells. We found that the mesenchymal niche in neonatal BM, compared to adult BM, was characterized by a higher frequency of primitive subsets of mesenchymal stroma expressing both platelet-derived growth factor receptor and Sca-1, and higher expression levels of the niche cross-talk molecules, Jagged-1 and CXCL-12. Accordingly, normal HSCs transplanted into neonatal mice exhibited higher levels of regeneration in BM, with no difference in homing efficiency or splenic engraftment compared to adult BM. In contrast, in vivo self-renewal of LSCs was higher in adult BM than in neonatal BM, with increased frequencies of leukemia-initiating cells as well as higher lympho-myeloid differentiation potential towards biphenotypic leukemic cells. These differences in LSC self-renewal capacity between neonates and adults was abrogated by switching of recipients, confirming their microenvironmental origin. Our study provides insight into the differences in leukemic diseases observed in childhood and adults, and is important for interpretation of many transplantation studies involving neonatal animal models.

Adipose Stem Cell Translational Applications: From Bench-to-Bedside

During the last five years, there has been a significantly increasing interest in adult adipose stem cells (ASCs) as a suitable tool for translational medicine applications. The abundant and renewable source of ASCs and the relatively simple procedure for cell isolation are only some of the reasons for this success. Here, we document the advances in the biology and in the innovative biotechnological applications of ASCs. We discuss how the multipotential property boosts ASCs toward mesenchymal and non-mesenchymal differentiation cell lineages and how their character is maintained even if they are combined with gene delivery systems and/or biomaterials, both in vitro and in vivo.

The Adaptive Remodeling of Stem Cell Niche in Stimulated Bone Marrow Counteracts the Leukemic Niche

Accumulating studies have shown the cellular nature of hematopoietic stem cell (HSC) niche in bone marrow (BM) and their degenerative changes under leukemic conditions. However, the dynamic adaptation of niche cells to changes in physiological stimulatory signals remains largely uncharacterized. Here, we have established a niche stimulation model induced by 5-fluorouracil. This model reveals a rapid and reversible conversion of mesenchymal cells into niche-like stromal cells, which exhibit a platelet-derived growth factor receptor-alpha⁺ /leptin receptor⁺ (PL) phenotype. These cells selectively induce the niche signaling molecule, Jagged-1, but not CXCL12, to initiate a stimulation-induced regeneration of HSCs in a Jagged-1 dependent manner. Conversion of mesenchymal cells into niche-like cells occurred independently of mitotic activation. The conversion was accompanied by the acquisition of primitive mesenchymal cell characteristics, including the rapid induction of stage specific embryonic antigen-3 and the acquisition of clonogenic potential. The stimulation-induced remodeling of the BM niche resulted in a positive stimulatory effect on the regeneration of normal HSC, but exerted inhibitory effects on leukemic cells, leading to a competitive advantage for normal HSCs in the BM niche and prolonged survival of mice engrafted with leukemic cells. Thus, the reactive conversion of mesenchymal stroma into niche-like cells reveals the adaptive changes of the BM microenvironment to stimuli, and provides insight on the remodeling of niche toward pronormal/antileukemic microenvironment, which can counteract the progressive proleukemic changes driven by the leukemic niche. Our study raises the potential for antileukemic niche targeting therapy. Stem Cells 2018;36:1617-1629.

Implications of infiltrating immune cells within bone marrow of patients with diffuse large B-cell lymphoma

The implications of infiltrating immune cells, especially T cells and macrophages, in the bone marrow (BM) microenvironment of patients with diffuse large B-cell lymphoma (DLBCL) have rarely been studied. We aimed to investigate the significance of infiltrating immune cells in the BM microenvironment as a prognostic factor for DLBCL patients. Using the initial pretreatment BM biopsy obtained from 198 DLBCL patients, we semiquantitatively evaluated CD3+ T cells, CD8+ T cells, and CD163+ macrophages that infiltrate into the paratrabecular and interstitial areas of BM by immunohistochemistry and analyzed their clinicopathological and prognostic implications. Levels of infiltrating CD3+ T cells, CD8+ T cells, and CD163+ macrophages were significantly higher in BM with DLBCL involvement (BMI-positive group) than in that without DLBCL involvement (BMI-negative group). Infiltration of CD8+ T cells significantly increased in cases with advanced Ann Arbor stage, elevated lactate dehydrogenase level, extranodal site involvement ≥2 sites, higher Eastern Cooperative Oncology Group performance status, and higher International Prognostic Index (IPI) risk. High levels of CD3+ T cells were significantly associated with age ≤60, and high levels of CD163+ macrophages were associated with advanced Ann Arbor stage and higher IPI risk. High infiltration of CD8+ T cells was significantly related to inferior overall and recurrence-free survival rate, even in the BMI-negative group. High infiltration of CD8+ T cells within the pretreatment BM was related to poor prognosis, and might be a useful prognostic factor of DLBCL patients. Therefore, evaluation of CD8+ T cells is helpful for predicting prognosis in initial pretreatment BM biopsy of DLBCL patients.

Combining Intravital Fluorescent Microscopy (IVFM) with Genetic Models to Study Engraftment Dynamics of Hematopoietic Cells to Bone Marrow Niches

Increasing evidence indicates that normal hematopoiesis is regulated by distinct microenvironmental cues in the BM, which include specialized cellular niches modulating critical hematopoietic stem cell (HSC) functions^1,2. Indeed, a more detailed picture of the hematopoietic microenvironment is now emerging, in which the endosteal and the endothelial niches form functional units for the regulation of normal HSC and their progeny^3,4,5. New studies have revealed the importance of perivascular cells, adipocytes and neuronal cells in maintaining and regulating HSC function^6,7,8. Furthermore, there is evidence that cells from different lineages, i.e. myeloid and lymphoid cells, home and reside in specific niches within the BM microenvironment. However, a complete mapping of the BM microenvironment and its occupants is still in progress. Transgenic mouse strains expressing lineage specific fluorescent markers or mice genetically engineered to lack selected molecules in specific cells of the BM niche are now available. Knock-out and lineage tracking models, in combination with transplantation approaches, provide the opportunity to refine the knowledge on the role of specific "niche" cells for defined hematopoietic populations, such as HSC, B-cells, T-cells, myeloid cells and erythroid cells. This strategy can be further potentiated by merging the use of two-photon microscopy of the calvarium. By providing in vivo high resolution imaging and 3-D rendering of the BM calvarium, we can now determine precisely the location where specific hematopoietic subsets home in the BM and evaluate the kinetics of their expansion over time. Here, Lys-GFP transgenic mice (marking myeloid cells)⁹ and RBPJ knock-out mice (lacking canonical Notch signaling)¹⁰ are used in combination with IVFM to determine the engraftment of myeloid cells to a Notch defective BM microenvironment.

Emerging Non-Canonical Functions and Regulation by p53: p53 and Stemness

Since its discovery nearly 40 years ago, p53 has ascended to the forefront of investigated genes and proteins across diverse research disciplines and is recognized most exclusively for its role in cancer as a tumor suppressor. Levine and Oren (2009) reviewed the evolution of p53 detailing the significant discoveries of each decade since its first report in 1979. In this review, we will highlight the emerging non-canonical functions and regulation of p53 in stem cells. We will focus on general themes shared among p53's functions in non-malignant stem cells and cancer stem-like cells (CSCs) and the influence of p53 on the microenvironment and CSC niche. We will also examine p53 gain of function (GOF) roles in stemness. Mutant p53 (mutp53) GOFs that lead to survival, drug resistance and colonization are reviewed in the context of the acquisition of advantageous transformation processes, such as differentiation and dedifferentiation, epithelial-to-mesenchymal transition (EMT) and stem cell senescence and quiescence. Finally, we will conclude with therapeutic strategies that restore wild-type p53 (wtp53) function in cancer and CSCs, including RING finger E3 ligases and CSC maintenance. The mechanisms by which wtp53 and mutp53 influence stemness in non-malignant stem cells and CSCs or tumor-initiating cells (TICs) are poorly understood thus far. Further elucidation of p53's effects on stemness could lead to novel therapeutic strategies in cancer research.

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.

Activation of the polycomb repressive complex pathway in the bone marrow resident cells of diffuse large B-cell lymphoma patients

The present study investigated the activation of polycomb repressive complex 2 (PRC2) pathway proteins in the resident cells within the bone marrow hematopoietic microenvironment of diffuse large B-cell lymphoma (DLBCL) patients. PRC2 proteins (enhancer of zeste homolog 2, suppressor of zeste 12 homolog, and embryonic ectoderm development), histone methylation mark (H3K27me3), and c-MYC activation were evaluated in pretreatment bone marrow from 208 DLBLC patients. Positive expression of the PRC2, H3K27me3, and c-MYC in the bone marrow resident cells was more frequent in cases with bone marrow involvement of tumor. The expression among PRC2, H3K27me3 mark, and c-MYC was closely correlated. Positive PRC2 expression in bone marrow resident cells was significantly associated with inferior progression-free survival (PFS) and overall survival (OS) and determined to be an independent prognostic factor of inferior PFS and OS. In conclusion, the PRC pathway was frequently activated in bone marrow resident cells of DLBCL patients, and PRC activation was tumor-related and associated with poor clinical outcomes.

Microenvironmental remodeling as a parameter and prognostic factor of heterogeneous leukemogenesis in acute myelogenous leukemia

Acute myelogenous leukemia (AML) is a heterogeneous disorder characterized by clonal proliferation of stem cell-like blasts in bone marrow (BM); however, their unique cellular interaction within the BM microenvironment and its functional significance remain unclear. Here, we assessed the BM microenvironment of AML patients and demonstrate that the leukemia stem cells induce a change in the transcriptional programming of the normal mesenchymal stromal cells (MSC). The modified leukemic niche alters the expressions of cross-talk molecules (i.e., CXCL12 and JAG1) in MSCs to provide a distinct cross-talk between normal and leukemia cells, selectively suppressing normal primitive hematopoietic cells while supporting leukemogenesis and chemoresistance. Of note, AML patients exhibited distinct heterogeneity in the alteration of mesenchymal stroma in BM. The distinct pattern of stromal changes in leukemic BM at initial diagnosis was associated with a heterogeneous posttreatment clinical course with respect to the maintenance of complete remission for 5 to 8 years and early or late relapse. Thus, remodeling of mesenchymal niche by leukemia cells is an intrinsic self-reinforcing process of leukemogenesis that can be a parameter for the heterogeneity in the clinical course of leukemia and hence serve as a potential prognostic factor.

Microenvironment and multiple myeloma spread

In patients with multiple myeloma (MM), the bone marrow (BM) contains hematopoietic stem cells (HSCs) and non-hematopoietic cells. HSCs are able to give rise to all types of mature blood cells, while the non hematopoietic component includes mesenchymal stem cells (MSCs), fibroblasts, osteoblasts, osteoclasts, chondroclasts, endothelial cells, endothelial progenitor cells (EPCs), B and T lymphocytes, NK cells, erythrocytes, megakaryocytes, platelets, macrophages and mast cells. All of these cells form specialized "niches" in the BM microenvironment which are close to the vasculature ("vascular niche") or to the endosteum ("osteoblast niche"). The "vascular niche" is rich in blood vessels where endothelial cells and mural cells (pericytes and smooth muscle cells) create a microenvironment that affects the behavior of several stem and progenitor cells. The vessel wall serves as an independent niche for the recruitment of endothelial progenitor cells, MSCs and HSCs. The activation by angiogenic factors and inflammatory cytokines switch the "vascular niche" to promote MM tumor growth and spread. This review will focus on the mechanisms involved in the generation of signals released by endothelial cells in the "vascular niche" that promote tumor growth and spread in MM.

The inherent metastasis of leukaemia and its exploitation by sonodynamic therapy

Nearly all cancers are linked by the inexorable phenotype of metastasis as malignant growths have the capability to spread from their place of origin to distant sites throughout the body. While different cancers may have various propensities to migrate towards specific locations, they are all linked by this unifying principal. Unlike most neoplasms, leukaemia has inherent cell motility as leukocytes are required to move throughout the vascular system, suggesting that no mutations are required for anchorage independent growth. As such, it seems likely that leukaemias are inherently metastatic, endowed with the deadliest phenotype of cancer simply due to cell of origin. This article presents the biology of metastasis development and how leukaemia cells are inherently provided these phenotypic characteristics. It is then proposed how clinicians may be able to exploit the motility of leukaemia and metastatic emboli of other cancer types through an approach known as sonodynamic therapy (SDT), a treatment modality that combines chemotherapeutic agents with ultrasound to preferentially damage malignant cells. As experimental evidence has indicated, SDT is a promising therapeutic approach in need of clinical testing for further validation.

Etiology and treatment of hematological neoplasms: stochastic mathematical models

Leukemias are driven by stemlike cancer cells (SLCC), whose initiation, growth, response to treatment, and posttreatment behavior are often "stochastic", i.e., differ substantially even among very similar patients for reasons not observable with present techniques. We review the probabilistic mathematical methods used to analyze stochastics and give two specific examples. The first example concerns a treatment protocol, e.g., for acute myeloid leukemia (AML), where intermittent cytotoxic drug dosing (e.g., once each weekday) is used with intent to cure. We argue mathematically that, if independent SLCC are growing stochastically during prolonged treatment, then, other things being equal, front-loading doses are more effective for tumor eradication than back loading. We also argue that the interacting SLCC dynamics during treatment is often best modeled by considering SLCC in microenvironmental niches, with SLCC-SLCC interactions occurring only among SLCC within the same niche, and we present a stochastic dynamics formalism, involving "Poissonization," applicable in such situations. Interactions at a distance due to partial control of total cell numbers are also considered. The second half of this chapter concerns chromosomal aberrations, lesions known to cause some leukemias. A specific example is the induction of a Philadelphia chromosome by ionizing radiation, subsequent development of chronic myeloid leukemia (CML), CML treatment, and treatment outcome. This time evolution involves a coordinated sequence of > 10 steps, each stochastic in its own way, at the subatomic, molecular, macromolecular, cellular, tissue, and population scales, with corresponding time scales ranging from picoseconds to decades. We discuss models of these steps and progress in integrating models across scales.

Multiple myeloma as a model for the role of bone marrow niches in the control of angiogenesis

Bone marrow (BM) contains hematopoietic stem cells (HSCs) and nonhematopoietic cells. HSCs give rise to all types of mature blood cells, while the nonhematopoietic component includes osteoblasts/osteoclasts, endothelial cells (ECs), endothelial progenitor cells (EPCs), and mesenchymal stem cells (MSCs). These cells form specialized "niches" which are close to the vasculature ("vascular niche") or to the endosteum ("osteoblast niche"). The "vascular niche", rich in blood vessels where ECs and mural cells (pericytes and smooth muscle cells), create a microenvironment affecting the behavior of several stem and progenitor cells. The vessel wall acts as an independent niche for the recruitment of EPCs and MSCs. This chapter will focus on the description of the role of BM niches in the control of angiogenesis occurring during multiple myeloma progression.

Neutropenia during HIV infection: adverse consequences and remedies

Neutropenia frequently occurs in patients with Human immunodeficiency virus (HIV) infection. Causes for neutropenia during HIV infection are multifactoral, including the viral toxicity to hematopoietic tissue, the use of myelotoxic agents for treatment, complication with secondary infections and malignancies, as well as the patient's association with confounding factors which impair myelopoiesis. An increased prevalence and severity of neutropenia is commonly seen in advanced stages of HIV disease. Decline of neutrophil phagocytic defense in combination with the failure of adaptive immunity renders the host highly susceptible to developing fatal secondary infections. Neutropenia and myelosuppression also restrict the use of many antimicrobial agents for treatment of infections caused by HIV and opportunistic pathogens. In recent years, HIV infection has increasingly become a chronic disease because of progress in antiretroviral therapy (ART). Prevention and treatment of severe neutropenia becomes critical for improving the survival of HIV-infected patients.

CD133(+)/CD44(+)/Oct4(+)/Nestin(+) stem-like cells isolated from Panc-1 cell line may contribute to multi-resistance and metastasis of pancreatic cancer

Pancreatic cancer is an aggressive malignant disease. Owing to the lack of early symptoms, accompanied by extensive metastasis and high resistance to chemotherapy, pancreatic adenocarcinoma becomes the fourth leading cause of cancer-related deaths. In this study, we identified a subpopulation of cells isolated from the Panc-1 cell line and named pancreatic cancer stem-like cells. These Panc-1 stem-like cells expressed high levels of CD133/CD44/Oct4/Nestin. Compared to Panc-1 cells, Panc-1 stem-like cells were resistant to gemcitabine and expressed high levels of MDR1; furthermore, Panc-1 stem-like cells have high anti-apoptotic, but weak proliferative potential. These results indicated that Panc-1 stem-like cells, as a novel group, may be a potential major cause of pancreatic cancer multidrug resistance and extensive metastasis.

MiR-24 promotes the survival of hematopoietic cells

The microRNA, miR-24, inhibits B cell development and promotes myeloid development of hematopoietic progenitors. Differential regulation of cell survival in myeloid and lymphoid cells by miR-24 may explain how miR-24's affects hematopoietic progenitors. MiR-24 is reported to regulate apoptosis, either positively or negatively depending on cell context. However, no role for miR-24 in regulating cell death has been previously described in blood cells. To examine miR-24's effect on survival, we expressed miR-24 via retrovirus in hematopoietic cells and induced cell death with cytokine or serum withdrawal. We observed that miR-24 enhanced survival of myeloid and B cell lines as well as primary hematopoietic cells. Additionally, antagonizing miR-24 with shRNA in hematopoietic cells made them more sensitive to apoptotic stimuli, suggesting miR-24 functions normally to promote blood cell survival. Since we did not observe preferential protection of myeloid over B cells, miR-24's pro-survival effect does not explain its promotion of myelopoiesis. Moreover, expression of pro-survival protein, Bcl-xL, did not mimic miR-24's impact on cellular differentiation, further supporting this conclusion. Our results indicate that miR-24 is a critical regulator of hematopoietic cell survival. This observation has implications for leukemogenesis. Several miRNAs that regulate apoptosis have been shown to function as either tumor suppressors or oncogenes during leukemogenesis. MiR-24 is expressed highly in primary acute myelogenous leukemia, suggesting that its pro-survival activity could contribute to the transformation of hematopoietic cells.

The hematopoietic stem cell niche--home for friend and foe?

The hematopoietic stem cell (HSC) niche is involved in the maintainance and regulation of quiescence, self-renewal and differentiation of hematopoietic stem cells and the fate of their progeny in mammals dealing with the daily stresses to the hematopoietic system. From the discovery that perturbations of the HSC niche can lead to hematopoietic disorders, we have now arrived at the prospect that the HSC niche may play a role in hematological malignancies and that this HSC niche may be a target for therapy. This review attempts to capture the discoveries of the last few years regarding the normal and malignant hematopoietic stem cell niche and possible ways to target this niche.

Of blood cells and the nervous system: hematopoiesis in the Drosophila larva

Hematopoiesis is well-conserved between Drosophila and vertebrates. Similar as in vertebrates, the sites of hematopoiesis shift during Drosophila development. Blood cells (hemocytes) originate de novo during hematopoietic waves in the embryo and in the Drosophila lymph gland. In contrast, the hematopoietic wave in the larva is based on the colonization of resident hematopoietic sites by differentiated hemocytes that arise in the embryo, much like in vertebrates the colonization of peripheral tissues by primitive macrophages of the yolk sac, or the seeding of fetal liver, spleen and bone marrow by hematopoietic stem and progenitor cells. At the transition to the larval stage, Drosophila embryonic hemocytes retreat to hematopoietic "niches," i.e., segmentally repeated hematopoietic pockets of the larval body wall that are jointly shared with sensory neurons and other cells of the peripheral nervous system (PNS). Hemocytes rely on the PNS for their localization and survival, and are induced to proliferate in these microenvironments, expanding to form the larval hematopoietic system. In this process, differentiated hemocytes from the embryo resume proliferation and self-renew, omitting the need for an undifferentiated prohemocyte progenitor. Larval hematopoiesis is the first Drosophila model for blood cell colonization and niche support by the PNS. It suggests an interface where innocuous or noxious sensory inputs regulate blood cell homeostasis or immune responses. The system adds to the growing concept of nervous system dependence of hematopoietic microenvironments and organ stem cell niches, which is being uncovered across phyla.

Aging of the microenvironment influences clonality in hematopoiesis

The mechanisms of the age-associated exponential increase in the incidence of leukemia are not known in detail. Leukemia as well as aging are initiated and regulated in multi-factorial fashion by cell-intrinsic and extrinsic factors. The role of aging of the microenvironment for leukemia initiation/progression has not been investigated in great detail so far. Clonality in hematopoiesis is tightly linked to the initiation of leukemia. Based on a retroviral-insertion mutagenesis approach to generate primitive hematopoietic cells with an intrinsic potential for clonal expansion, we determined clonality of transduced hematopoietic progenitor cells (HPCs) exposed to a young or aged microenvironment in vivo. While HPCs displayed primarily oligo-clonality within a young microenvironment, aged animals transplanted with identical pool of cells displayed reduced clonality within transduced HPCs. Our data show that an aged niche exerts a distinct selection pressure on dominant HPC-clones thus facilitating the transition to mono-clonality, which might be one underlying cause for the increased age-associated incidence of leukemia.

Reconstructing the human hematopoietic niche in immunodeficient mice: opportunities for studying primary multiple myeloma

Interactions within the hematopoietic niche in the BM microenvironment are essential for maintenance of the stem cell pool. In addition, this niche is thought to serve as a sanctuary site for malignant progenitors during chemotherapy. Therapy resistance induced by interactions with the BM microenvironment is a major drawback in the treatment of hematologic malignancies and bone-metastasizing solid tumors. To date, studying these interactions was hampered by the lack of adequate in vivo models that simulate the human situation. In the present study, we describe a unique human-mouse hybrid model that allows engraftment and outgrowth of normal and malignant hematopoietic progenitors by implementing a technology for generating a human bone environment. Using luciferase gene marking of patient-derived multiple myeloma cells and bioluminescent imaging, we were able to follow pMM cells outgrowth and to visualize the effect of treatment. Therapeutic interventions in this model resulted in equivalent drug responses as observed in the corresponding patients. This novel human-mouse hybrid model creates unprecedented opportunities to investigate species-specific microenvironmental influences on normal and malignant hematopoietic development, and to develop and personalize cancer treatment strategies.

Quantitative modeling of chronic myeloid leukemia: insights from radiobiology

Mathematical models of chronic myeloid leukemia (CML) cell population dynamics are being developed to improve CML understanding and treatment. We review such models in light of relevant findings from radiobiology, emphasizing 3 points. First, the CML models almost all assert that the latency time, from CML initiation to diagnosis, is at most ∼10 years. Meanwhile, current radiobiologic estimates, based on Japanese atomic bomb survivor data, indicate a substantially higher maximum, suggesting longer-term relapses and extra resistance mutations. Second, different CML models assume different numbers, between 400 and 10(6), of normal HSCs. Radiobiologic estimates favor values>10(6) for the number of normal cells (often assumed to be the HSCs) that are at risk for a CML-initiating BCR-ABL translocation. Moreover, there is some evidence for an HSC dead-band hypothesis, consistent with HSC numbers being very different across different healthy adults. Third, radiobiologists have found that sporadic (background, age-driven) chromosome translocation incidence increases with age during adulthood. BCR-ABL translocation incidence increasing with age would provide a hitherto underanalyzed contribution to observed background adult-onset CML incidence acceleration with age, and would cast some doubt on stage-number inferences from multistage carcinogenesis models in general.

Mechanisms of defective erythropoiesis and anemia in pediatric acute lymphoblastic leukemia (ALL)

Anemia frequently accompanies the diagnosis of acute lymphoblastic leukemia (ALL) in children and is considered to be one of the most common clinical complications of the disease. In addition, a low hemoglobin (Hb) level is often responsible for fatigue and other associated symptoms that cause a decline in the quality of life of these children. Traditionally, a number of contributing factors such as overcrowding of the marrow, coexisting infections, and nutritional deficits have been used to explain this phenomenon. However, recent advances in in vivo modeling and real-time ultrastructural analytical techniques have enabled researchers to examine leukemic bone marrow (BM) microenvironment more closely and helped to build mechanistic models of this process. Importantly, data from these studies show that in the majority of cases, the required stem cell populations and the erythropoietic growth mechanisms remain intact in leukemia. In this report, we aim to review the current state of knowledge regarding the cellular and molecular mechanisms implicated in the altered erythropoiesis at the time of diagnosis of leukemia. We propose that further understanding of the mechanisms of anemia in leukemia may help to manage some of its clinical consequences more effectively as well as to yield key insight into the process of leukemogenesis itself.

Impaired expression of DICER, DROSHA, SBDS and some microRNAs in mesenchymal stromal cells from myelodysplastic syndrome patients

Background Recent findings suggest that a specific deletion of Dicer1 in mesenchymal stromal cell-derived osteoprogenitors triggers several features of myelodysplastic syndrome in a murine model. Our aim was to analyze DICER1 and DROSHA gene and protein expression in mesenchymal stromal cells (the osteoblastic progenitors) obtained from bone marrow of myelodysplastic syndrome patients, in addition to microRNA expression profile and other target genes such as SBDS, a DICER1-related gene that promotes bone marrow dysfunction and myelodysplasia when repressed in a murine model.

DESIGN AND METHODS

Mesenchymal stromal cells from 33 bone marrow samples were evaluated. DICER, DROSHA and SBDS gene expression levels were assessed by real-time PCR and protein expression by Western blot. MicroRNA expresion profile was analyzed by commercial low-density arrays and some of these results were confirmed by individual real-time PCR.

RESULTS

Mesenchymal stromal cells from myelodysplastic syndrome patients showed lower DICER1 (0.65±0.08 vs. 1.91±0.57; P=0.011) and DROSHA (0.62±0.06 vs. 1.38±0.29; P=0.009) gene expression levels, two relevant endonucleases associated to microRNA biogenesis, in comparison to normal myelodysplastic syndrome. These findings were confirmed at protein levels by Western blot. Strikingly, no differences were observed between paired mononuclear cells from myelodysplastic syndrome and controls. In addition, mesenchymal stromal cells from myelodysplastic syndrome patients showed significant lower SBDS (0.63±0.06 vs. 1.15±0.28; P=0.021) gene expression levels than mesenchymal stromal cells from healthy controls. Furthermore, mesenchymal stromal cells from myelodysplastic syndrome patients showed an underlying microRNA repression compared to healthy controls. Real-time PCR approach confirmed that mir-155, miR-181a and miR-222 were down-expressed in mesenchymal stromal cells from myelodysplastic syndrome patients. Conclusions This is the first description of an impaired microRNA biogenesis in human mesenchymal stromal cells from myelodysplastic syndrome patients, where DICER1 and DROSHA gene and protein downregulation correlated to a gene and microRNA abnormal expression profile, validating the animal model results previously described.

Contribution of an aged microenvironment to aging-associated myeloproliferative disease

The molecular and cellular mechanisms of the age-associated increase in the incidence of acute myeloid leukemia (AML) remain poorly understood. Multiple studies support that the bone marrow (BM) microenvironment has an important influence on leukemia progression. Given that the BM niche itself undergoes extensive functional changes during lifetime, we hypothesized that one mechanism for the age-associated increase in leukemia incidence might be that an aged niche promotes leukemia progression. The most frequent genetic alteration in AML is the t(8;21) translocation, resulting in the expression of the AML1-ETO fusion protein. Expression of the fusion protein in hematopoietic cells results in mice in a myeloproliferative disorder. Testing the role of the age of the niche on leukemia progression, we performed both transplantation and in vitro co-culture experiments. Aged animals transplanted with AML1-ETO positive HSCs presented with a significant increase in the frequency of AML-ETO positive early progenitor cells in BM as well as an increased immature myeloid cell load in blood compared to young recipients. These findings suggest that an aged BM microenvironment allows a relative better expansion of pre-leukemic stem and immature myeloid cells and thus imply that the aged microenvironment plays a role in the elevated incidence of age-associated leukemia.

Mesenchymal niches of bone marrow in cancer

Over the last decade, genetic and cell biology studies have indicated that tumour growth is not only determined by malignant cancer cells themselves, but also by the tumour microenvironment. Cells present in the tumour microenvironment include fibroblasts, vascular, smooth muscle, adipocytes, immune cells and mesenchymal stem cells (MSC). The nature of the relationship between MSC and tumour cells appears dual and whether MSC are pro- or anti-tumorigenic is a subject of controversial reports. This review is focused on the role of MSC and bone marrow (BM) niches in cancer.

Inflamm-aging of the stem cell niche: breast cancer as a paradigmatic example: breakdown of the multi-shell cytokine network fuels cancer in aged people

Inflamm-aging is a relatively new terminology used to describe the age-related increase in the systemic pro-inflammatory status of humans. Here, we represent inflamm-aging as a breakdown in the multi-shell cytokine network, in which stem cells and stromal fibroblasts (referred to as the stem cell niche) become pro-inflammatory cytokine over-expressing cells due to the accumulation of DNA damage. Inflamm-aging self-propagates owing to the capability of pro-inflammatory cytokines to ignite the DNA-damage response in other cells surrounding DNA-damaged cells. Macrophages, the major cellular player in inflamm-aging, amplify the phenomenon, by broadcasting pro-inflammatory signals at both local and systemic levels. On the basis of this, we propose that inflamm-aging is a major contributor to the increase in cancer incidence and progression in aged people. Breast cancer will be presented as a paradigmatic example for this relationship.

Bone marrow mesenchymal stem cells: biological properties and their role in hematopoiesis and hematopoietic stem cell transplantation

Mesenchymal stem cells (MSCs) are multipotent adult stem cells that are present in practically all tissues as a specialized population of mural cells/pericytes that lie on the abluminal side of blood vessels. Originally identified within the bone marrow (BM) stroma, not only do they provide microenvironmental support for hematopoietic stem cells (HSCs), but can also differentiate into various mesodermal lineages. MSCs can easily be isolated from the BM and subsequently expand in vitro and in addition they exhibit intriguing immunomodulatory properties, thereby emerging as attractive candidates for various therapeutic applications. This review addresses the concept of BM MSCs via a hematologist's point of view. In this context it discusses the stem cell properties that have been attributed to BM MSCs, as compared to those of the prototypic hematopoietic stem cell model and then gives a brief overview of the in vitro and vivo features of the former, emphasizing on their immunoregulatory properties and their hematopoiesis-supporting role. In addition, the qualitative and quantitative characteristics of BM MSCs within the context of a defective microenvironment, such as the one characterizing Myelodysplastic Syndromes are described and the potential involvement of these cells in the pathophysiology of the disease is discussed. Finally, emerging clinical applications of BM MSCs in the field of hematopoietic stem cell transplantation are reviewed and potential hazards from MSC use are outlined.

Leptin reverts pro-apoptotic and antiproliferative effects of α-linolenic acids in BCR-ABL positive leukemic cells: involvement of PI3K pathway

It is suspected that bone marrow (BM) microenvironmental factors may influence the evolution of chronic myeloid leukaemia (CML). In this study, we postulated that adipocytes and lipids could be involved in the progression of CML. To test this hypothesis, adipocytes were co-cultured with two BCR-ABL positive cell lines (PCMDS and K562). T cell (Jurkat) and stroma cell (HS-5) lines were used as controls. In the second set of experiments, leukemic cell lines were treated with stearic, oleic, linoleic or α-linolenic acids in presence or absence of leptin. Survival, proliferation, leptin production, OB-R isoforms (OB-Ra and OB-Rb), phosphoinositide 3-kinase (PI3k) and BCL-2 expression have been tested after 24h, 48h and 72h of treatment. Our results showed that adipocytes induced a decrease of CML proliferation and an increase in lipid accumulation in leukemic cells. In addition, CML cell lines induced adipocytes cell death. Chromatography analysis showed that BM microenvironment cells were full of saturated (SFA) and monounsaturated (MUFA) fatty acids, fatty acids that protect tumor cells against external agents. Stearic acid increased Bcl-2 expression in PCMDS, whereas oleic and linoleic acids had no effects. In contrast, α-linolenic acid decreased the proliferation and the survival of CML cell lines as well as BCL-2 and OB-R expression. The effect of α-linolenic acids seemed to be due to PI3K pathway and Bcl-2 inhibition. Leptin production was detected in the co-culture medium. In the presence of leptin, the effect of α-linolenic acid on proliferation, survival, OB-R and BCl-2 expression was reduced.

Is prolonged stem cell mobilization detrimental for hematopoiesis?

Multiple hematological side effects have been reported to result from treatment with psychoactive phenothiazines. These reported toxicities include leucopenia, granulocytopenia, thrombocytopenia, agranulocytosis, and bone marrow aplasia. The physiological mechanism causing these potentially life-threatening blood dyscrasias is unknown. Recently, we discovered that phenothiazines exhibit antagonistic properties towards the VLA-4 integrin, an adhesion molecule that is responsible for homing and retention of hematological stem/progenitor cells (HSPCs) in the bone marrow. After administration of thioridazine we detected rapid mobilization of HSPCs into the peripheral blood. We propose that in patients receiving phenothiazines over a prolonged time period, continuous mobilization of stem cells out of the stem cell niche, results in a disorder of hematopoiesis. Furthermore, we also postulate that such cytopenias are caused by a loss of the niche environment, which is known to be essential for stem cell maintenance.

Stem Cell Transplantation for Hematological Malignancies: Prospects for Personalized Medicine and Co-therapy with Mesenchymal Stem Cells

Bone marrow transplantation is a form of cell therapy that has been in practice for decades for the treatment of hematological disorders and solid tumors. Immunosuppressive therapy has been a mainstay for treatment, but the severity of the adverse effects has made it an undesirable choice. Mesenchymal stem cells (MSCs), which reside in the vascular regions of the bone marrow, have been shown to serve as cellular support for the hematopoietic stem cell (HSC) niche. Furthermore, the immune suppressive properties of MSCs have been explored in the treatment of inflammatory and autoimmune disorders. Thus, co-therapy with MSCs has been shown to facilitate engraftment of hematopoietic cells by suppressive graft versus host disease (GvHD). Although the mechanism by which MSCs suppress GvHD is unclear, the experimental evidence suggests that this partly occurs by modulation of immune response such as the induction of regulatory T cells. This paper discusses the role of MSCs as co-therapy for the future of stem cell transplantation, with the overarching theme of personalized medicine for cell-based health interventions.

Stem cell-biomaterial interactions for regenerative medicine

The synergism of stem cell biology and biomaterial technology promises to have a profound impact on stem-cell-based clinical applications for tissue regeneration. Biomaterials development is rapidly advancing to display properties that, in a precise and physiological fashion, could drive stem-cell fate both in vitro and in vivo. Thus, the design of novel materials is trying to recapitulate the molecular events involved in the production, clearance and interaction of molecules within tissue in pathologic conditions and regeneration of tissue/organs. In this review we will report on the challenges behind translating stem cell biology and biomaterial innovations into novel clinical therapeutic applications for tissue and organ replacements (graphical abstract).

Could metabolic syndrome, lipodystrophy, and aging be mesenchymal stem cell exhaustion syndromes?

One of the most important and complex diseases of modern society is metabolic syndrome. This syndrome has not been completely understood, and therefore an effective treatment is not available yet. We propose a possible stem cell mechanism involved in the development of metabolic syndrome. This way of thinking lets us consider also other significant pathologies that could have similar etiopathogenic pathways, like lipodystrophic syndromes, progeria, and aging. All these clinical situations could be the consequence of a progressive and persistent stem cell exhaustion syndrome (SCES). The main outcome of this SCES would be an irreversible loss of the effective regenerative mesenchymal stem cells (MSCs) pools. In this way, the normal repairing capacities of the organism could become inefficient. Our point of view could open the possibility for a new strategy of treatment in metabolic syndrome, lipodystrophic syndromes, progeria, and even aging: stem cell therapies.

A multicellular basis for the origination of blast crisis in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is characterized by a specific chromosome translocation, and its pathobiology is considered comparatively well understood. Thus, quantitative analysis of CML and its progression to blast crisis may help elucidate general mechanisms of carcinogenesis and cancer progression. Hitherto, it has been widely postulated that CML blast crisis originates mainly via cell-autonomous mechanisms such as secondary mutations or genomic instability. However, recent results suggest that carcinogenic transformation may be an inherently multicellular event, in departure from the classic unicellular paradigm. We investigate this possibility in the case of blast crisis origination in CML. A quantitative, mechanistic cell population dynamics model was employed. This model used recent data on imatinib-treated CML; it also used earlier clinical data, not previously incorporated into current mathematical CML/imatinib models. With the pre-imatinib data, which include results on many more blast crises, we obtained evidence that the driving mechanism for blast crisis origination is a cooperation between specific cell types. Assuming leukemic-normal interactions resulted in a statistically significant improvement over assuming either cell-autonomous mechanisms or interactions between leukemic cells. This conclusion was robust with regard to changes in the model's adjustable parameters. Application of the results to patients treated with imatinib suggests that imatinib may act not only on malignant blast precursors, but also, to a limited degree, on the malignant blasts themselves.