The Bone Marrow Immune Microenvironment in CML: Treatment Responses, Treatment-Free Remission, and Therapeutic Vulnerabilities

The dynamic interaction of pediatric ALL cells and MSCs: influencing leukemic cell survival and modulating MSC β-catenin expression

Bone marrow mesenchymal stromal cells (BM-MSCs) are integral components of the bone marrow microenvironment, playing a crucial role in supporting hematopoiesis. Recent studies have investigated the potential involvement of BM-MSCs in the pathophysiology of acute lymphoblastic leukemia (ALL). However, the exact contribution of BM-MSCs to leukemia progression remains unclear because of conflicting findings and limited characterization. In this study, we compared BM-MSCs derived from pediatric ALL patients with those from matched healthy donors (HDs). Our results indicate that while both ALL-MSCs and HD-MSCs meet the criteria established by the International Society for Cellular Therapy, they exhibit significant differences in proliferation and differentiation capacity. ALL-MSCs displayed markedly lower proliferation rates and reduced osteogenic/adipogenic differentiation potential compared to HD-MSCs. Furthermore, co-culture experiments revealed that MSCs enhance the survival of leukemic blasts through both soluble factors and direct cell-cell interactions, underscoring their anti-apoptotic properties. Importantly, our findings demonstrate that interactions with leukemic cells activate the Wnt/β-catenin signaling pathway in MSCs, suggesting a potential target for therapeutic intervention. Overall, this study enhances our understanding of the role of BM-MSCs in leukemia and highlights β-catenin as a promising target for future therapies.

High-throughput formulation of reproducible 3D cancer microenvironments for drug testing in myeloid leukemia

Leukemic microenvironment has been recognized as a factor that strongly supports the mechanisms of resistance. Therefore, targeting the microenvironment is currently one of the major directions in drug development and preclinical studies in leukemia. Despite the variety of available leukemia 3D culture models, the reproducible generation of miniaturized leukemic microenvironments, suitable for high-throughput drug testing, has remained a challenge. Here, we use droplet microfluidics to generate tens of thousands of highly monodisperse leukemic-bone marrow microenvironments within minutes. We employ gelatin methacryloyl (GelMA) as a model extracellular matrix (ECM) and tune the concentration of the biopolymer, check the impact of other components of the ECM (hyaluronic acid), cell concentration and the ratio of leukemic cells to bone marrow cells within the microbeads to establish the optimal conditions for microtissue formation. We administer model kinase inhibitor, imatinib, at various concentrations to the encapsulated leukemic microtissues, and, via comparing mono- and co-culture conditions (cancer alone vs cancer-stroma), we find that the stroma-leukemia crosstalk systematically protects the encapsulated cells against the drug-induced cytotoxicity. With that we demonstrate that our system mimics the physiological stroma-dependent protection. We discuss applicability of our model to (i) studying the role of direct- or close-contact interactions between the leukemia and bone marrow cells embedded in microscale 3D ECM on the stroma-mediated protection, and (ii) high-throughput screening of anti-cancer therapeutics in personalized leukemia therapies.

An Overview of Myeloid Blast-Phase Chronic Myeloid Leukemia

Myeloid blast-phase chronic myeloid leukemia (MBP-CML) is a rare disease with a dismal prognosis. It is twice as common as lymphoid blast-phase CML, and its prognosis is poorer. Despite the success with tyrosine kinase inhibitors in the treatment of chronic-phase CML, the same does not hold true for MBP-CML. In addition to the Philadelphia chromosome, other chromosomal and molecular changes characterize rapid progression. Although some progress in elucidating the biology of MBP-CML has been made, there is need to discover more in order to develop more satisfactory treatment options. Currently, most common treatment options include tyrosine kinase inhibitors (TKIs) as monotherapy or in combination with acute myeloid leukemia-based intensive chemotherapy regimens. Some patients may develop resistance to TKIs via BCR-ABL1-dependent or BCR-ABL1-independent mechanisms. In this paper, we provide an overview of the biology of MBP-CML, the current treatment approaches, and mechanisms of resistance to TKIs. In order to improve treatment responses in these patients, more emphasis should be placed on understanding the biology of myeloid blastic transformation in CML and mechanisms of resistance to TKIs. Although patient numbers are small, randomized clinical trials should be considered.

Intricate relationship between cancer stemness, metastasis, and drug resistance

Cancer stem cells (CSCs) are widely acknowledged as the drivers of tumor initiation, epithelial-mesenchymal transition (EMT) progression, and metastasis. Originating from both hematologic and solid malignancies, CSCs exhibit quiescence, pluripotency, and self-renewal akin to normal stem cells, thus orchestrating tumor heterogeneity and growth. Through a dynamic interplay with the tumor microenvironment (TME) and intricate signaling cascades, CSCs undergo transitions from differentiated cancer cells, culminating in therapy resistance and disease recurrence. This review undertakes an in-depth analysis of the multifaceted mechanisms underlying cancer stemness and CSC-mediated resistance to therapy. Intrinsic factors encompassing the TME, hypoxic conditions, and oxidative stress, alongside extrinsic processes such as drug efflux mechanisms, collectively contribute to therapeutic resistance. An exploration into key signaling pathways, including JAK/STAT, WNT, NOTCH, and HEDGEHOG, sheds light on their pivotal roles in sustaining CSCs phenotypes. Insights gleaned from preclinical and clinical studies hold promise in refining drug discovery efforts and optimizing therapeutic interventions, especially chimeric antigen receptor (CAR)-T cell therapy, cytokine-induced killer (CIK) cell therapy, natural killer (NK) cell-mediated CSC-targeting and others. Ultimately use of cell sorting and single cell sequencing approaches for elucidating the fundamental characteristics and resistance mechanisms inherent in CSCs will enhance our comprehension of CSC and intratumor heterogeneity, which ultimately would inform about tailored and personalized interventions.

The initial molecular response predicts the deep molecular response but not treatment-free remission maintenance in a real-world chronic myeloid leukemia cohort

In chronic myeloid leukemia, the identification of early molecular predictors of stable treatment-free remission (TFR) after tyrosine kinase inhibitor (TKI) discontinuation is challenging. The predictive values of residual disease (BCR::ABL1 quantification) at month 3 and 6 and more recently, BCR::ABL1 transcript halving time (HT) have been described, but no study compared the predictive value of different early parameters. Using a real-world cohort of 408 patients, we compared the performance of the EUTOS long-term survival (ELTS) score, BCR::ABL1 HT, and residual disease at month 3 and 6 to predict the molecular response, achievement of the TKI discontinuation criteria, and TFR maintenance. The performances of BCR::ABL1 HT and residual disease at month 3 were similar. Residual disease at month 6 displayed the best performance for predicting the optimal response (area under the ROC curve between 0.81 and 0.92; cut-off values: 0.11% for MR4 at month 24 and 0.12% for MR4.5 at month 48). Conversely, no early parameter predicted reaching the TKI discontinuation criteria and TFR maintenance. We obtained similar results when patients were divided in subgroups by first-line treatment (imatinib vs. second-generation TKI [2G-TKI]). We identified a relationship between ELTS score, earlier milestones and TFR maintenance only in the 2G-TKI group. In conclusion, this first comparative study of early therapeutic response parameters showed that they are excellent indicators of TKI efficacy (BCR::ABL1 transcript reduction) and best responders. Conversely, they did not predict the achievement of the TKI discontinuation criteria and TFR maintenance, suggesting that other parameters are involved in TFR maintenance.

Leukemic Stem Cells and Hematological Malignancies

The association between leukemic stem cells (LSCs) and leukemia development has been widely established in the context of genetic alterations, epigenetic pathways, and signaling pathway regulation. Hematopoietic stem cells are at the top of the bone marrow hierarchy and can self-renew and progressively generate blood and immune cells. The microenvironment, niche cells, and complex signaling pathways that regulate them acquire genetic mutations and epigenetic alterations due to aging, a chronic inflammatory environment, stress, and cancer, resulting in hematopoietic stem cell dysregulation and the production of abnormal blood and immune cells, leading to hematological malignancies and blood cancer. Cells that acquire these mutations grow at a faster rate than other cells and induce clone expansion. Excessive growth leads to the development of blood cancers. Standard therapy targets blast cells, which proliferate rapidly; however, LSCs that can induce disease recurrence remain after treatment, leading to recurrence and poor prognosis. To overcome these limitations, researchers have focused on the characteristics and signaling systems of LSCs and therapies that target them to block LSCs. This review aims to provide a comprehensive understanding of the types of hematopoietic malignancies, the characteristics of leukemic stem cells that cause them, the mechanisms by which these cells acquire chemotherapy resistance, and the therapies targeting these mechanisms.

Tyrosine Kinase Inhibitor Discontinuation in Chronic Myeloid Leukemia: Strategies to Optimize Success and New Directions

PURPOSE OF REVIEW

The discovery that patients suffering from chronic myeloid leukemia who obtain deep and long-lasting molecular responses upon treatment with tyrosine kinase inhibitors may maintain their disease silent for many years after therapy discontinuation launched the era of treatment-free remission as a key management goal in clinical practice. The purpose of this review on treatment-free remission is to discuss clinical advances, highlight knowledge gaps, and describe areas of research.

RECENT FINDINGS

Patients in treatment-free remission are a minority, and it is believed that some may still retain a reservoir of leukemic stem cells; thus, whether they can be considered as truly cured is uncertain. Strengthening BCR::ABL1 inhibition increases deep molecular responses but is not sufficient to improve treatment-free remission, and we lack biomarkers to identify and specifically target residual cells with aggressive potential. Another level of complexity resides in the intra- and inter-patient clonal heterogeneity of minimal residual disease and characteristics of the bone marrow environment. Finding determinants of deep molecular responses achievement and elucidating varying biological mechanisms enabling either post-tyrosine kinase inhibitor chronic myeloid leukemia control or relapse may help develop innovative and safe therapies. In the light of the increasing prevalence of CML, targeting the residual leukemic stem cell pool is thought to be the key.

The critical role of the bone marrow stromal microenvironment for the development of drug screening platforms in leukemia

Extensive research over the past 50 years has resulted in significant improvements in survival for patients diagnosed with leukemia. Despite this, a subgroup of patients harboring high-risk genetic alterations still suffer from poor outcomes. There is a desperate need for new treatments to improve survival, yet consistent failure exists in the translation of in vitro drug development to clinical application. Preclinical screening conventionally utilizes tumor cell monocultures to assess drug activity; however, emerging research has acknowledged the vital role of the tumor microenvironment in treatment resistance and disease relapse. Current co-culture drug screening methods frequently employ fibroblasts as the designated stromal cell component. Alternative stromal cell types that are known to contribute to chemoresistance are often absent in preclinical evaluations of drug efficacy. This review highlights mechanisms of chemoresistance by a range of different stromal constituents present in the bone marrow microenvironment. Utilizing an array of stromal cell types at the early stages of drug screening may enhance the translation of in vitro drug development to clinical use. Ultimately, we highlight the need to consider the bone marrow microenvironment in drug screening platforms for leukemia to develop superior therapies for the treatment of high-risk patients with poor prognostic outcomes.

Immortalised chronic myeloid leukemia (CML) derived mesenchymal stromal cells (MSCs) line retains the immunomodulatory and chemoprotective properties of CML patient-derived MSCs

Despite the success of Tyrosine kinase inhibitors (TKIs) in treating chronic myeloid leukemia (CML), leukemic stem cells (LSCs) persist, contributing to relapse and resistance. CML Mesenchymal Stromal Cells (MSCs) help in LSC maintenance and protection from TKIs. However, the limited passage and self-differentiation abilities of primary CML MSCs hinder extensive research. To overcome this, we generated and characterized an immortalised CML patient-derived MSC (iCML MSC) line and assessed its role in LSC maintenance. We also compared the immunophenotype and differentiation potential between primary CML MSCs at diagnosis, post-treatment, and with normal bone marrow MSCs. Notably, CML MSCs exhibited enhanced chondrogenic differentiation potential compared to normal MSCs. The iCML MSC line retained the trilineage differentiation potential and was genetically stable, enabling long-term investigations. Functional studies demonstrated that iCML MSCs protected CML CD34+ cells from imatinib-induced apoptosis, recapitulating the bone marrow microenvironment-mediated resistance observed in patients. iCML MSC-conditioned media enabled CML CD34+ and AML blast cells to proliferate rapidly, with no impact on healthy donor CD34+ cells. Gene expression profiling revealed dysregulated genes associated with calcium metabolism in CML CD34+ cells cocultured with iCML MSCs, providing insights into potential therapeutic targets. Further, cytokine profiling revealed that the primary CML MSC lines abundantly secreted 25 cytokines involved in immune regulation, supporting the hypothesis that CML MSCs create an immune modulatory microenvironment that promotes growth and protects against TKIs. Our study establishes the utility of iCML MSCs as a valuable model to investigate leukemic-stromal interactions and study candidate genes involved in mediating TKI resistance in CML LSCs.