Transforming growth factor inhibits erythropoiesis by blocking proliferation and accelerating differentiation of erythroid progenitors

Creg1 Regulates Erythroid Development via TGF-β/Smad2-Klf1 Axis in Zebrafish

Understanding the regulation of normal erythroid development will help to develop new potential therapeutic strategies for disorders of the erythroid lineage. Cellular repressor of E1A-stimulated genes 1 (CREG1) is a glycoprotein that has been implicated in the regulation of tissue homeostasis. However, its role in erythropoiesis remains largely undefined. In this study, it is found that CREG1 expression increases progressively during erythroid differentiation. In zebrafish, creg1 mRNA is preferentially expressed within the intermediate cell mass (ICM)/peripheral blood island (PBI) region where primitive erythropoiesis occurs. Loss of creg1 leads to anemia caused by defective erythroid differentiation and excessive apoptosis of erythroid progenitors. Mechanistically, creg1 deficiency results in reduced activation of TGF-β/Smad2 signaling pathway. Treatment with an agonist of the Smad2 pathway (IDE2) could significantly restore the defective erythroid development in creg1-/- mutants. Further, Klf1, identified as a key target gene downstream of the TGF-β/Smad2 signaling pathway, is involved in creg1 deficiency-induced aberrant erythropoiesis. Thus, this study reveals a previously unrecognized role for Creg1 as a critical regulator of erythropoiesis, mediated at least in part by the TGF-β/Smad2-Klf1 axis. This finding may contribute to the understanding of normal erythropoiesis and the pathogenesis of erythroid disorders.

Treatment of Anemia in Lower-Risk Myelodysplastic Syndrome

OPINION STATEMENT

A majority of patients with lower-risk myelodysplastic syndrome (MDS) will present with or develop anemia. Anemia in MDS is associated with decreased quality of life and may correlate with decreased progression-free survival and overall survival. In this state of the art review we summarize current risk stratification approaches to identify lower-risk MDS (LR-MDS), the natural history of the disease, and meaningful clinical endpoints. The treatment landscape of LR-MDS with anemia is also rapidly evolving; we review the role of supportive care, erythropoietin stimulating agents, lenalidomide, luspatercept, hypomethylating agents (HMAs), and immunosuppressive therapy (IST) in the management of LR-MDS with anemia. In patients with deletion 5q (del5q) syndrome lenalidomide has both efficacy and durability of response. For patients without del5q who need treatment, the management approach is impacted by serum erythropoietin (EPO) level, SF3B1 mutation status, and ring sideroblast status. Given the data from the Phase III COMMANDS trial, we utilize luspatercept in those with SF3B1 mutation or ring sideroblasts that have an EPO level < 500 U/L; in patients without an SF3B1 mutation or ring sideroblasts there is equipoise between luspatercept and use of an erythropoietin stimulating agent (ESA). For patients who have an EPO level ≥ 500 U/L or have been previously treated there is not a clear standard of care. For those without previous luspatercept exposure it can be considered particularly if there is an SF3B1 mutation or the presence of ring sideroblasts. Other options include HMAs or IST; the Phase III IMERGE trial supports the efficacy of the telomerase inhibitor imetelstat in this setting and this may become a standard option in the future as well.

A Review of Key Regulators of Steady-State and Ineffective Erythropoiesis

Erythropoiesis is initiated with the transformation of multipotent hematopoietic stem cells into committed erythroid progenitor cells in the erythroblastic islands of the bone marrow in adults. These cells undergo several stages of differentiation, including erythroblast formation, normoblast formation, and finally, the expulsion of the nucleus to form mature red blood cells. The erythropoietin (EPO) pathway, which is activated by hypoxia, induces stimulation of the erythroid progenitor cells and the promotion of their proliferation and survival as well as maturation and hemoglobin synthesis. The regulation of erythropoiesis is a complex and dynamic interaction of a myriad of factors, such as transcription factors (GATA-1, STAT5), cytokines (IL-3, IL-6, IL-11), iron metabolism and cell cycle regulators. Multiple microRNAs are involved in erythropoiesis, mediating cell growth and development, regulating oxidative stress, erythrocyte maturation and differentiation, hemoglobin synthesis, transferrin function and iron homeostasis. This review aims to explore the physiology of steady-state erythropoiesis and to outline key mechanisms involved in ineffective erythropoiesis linked to anemia, chronic inflammation, stress, and hematological malignancies. Studying aberrations in erythropoiesis in various diseases allows a more in-depth understanding of the heterogeneity within erythroid populations and the development of gene therapies to treat hematological disorders.

Luspatercept in low-risk myelodysplastic syndromes: a paradigm shift in treatment strategies

INTRODUCTION

In patients with myelodysplastic syndromes (MDS), anemia is prevalent affecting 80%-85% of low-risk (LR-MDS) patients, with 40% eventually requiring red blood cell (RBC) transfusions. Except forlenalidomide, exclusively approved for those with deletion of chromosome 5q,erythropoiesis-stimulating agents (ESAs) are the primary treatment choice for low-risk patients. Those unresponsive to ESAs face limited alternatives, eventually necessitating long-term RBC transfusions, leading to secondary iron overload and adversely affecting quality of life (QoL).

AREA COVERED

Luspatercept is a pioneering erythroid maturation agent. It received approval by both the European Medicines Agency (EMA) and the Food and Drug Administration (FDA) for treating adults experiencing transfusion-dependent anemia associated with LR-MDS or β-thalassemia. Recently, the FDA approved luspatercept as first- line therapy in patients with very low- to intermediate-risk MDS who require RBC transfusions and have not previously received ESAs. This review summarizes the historical impact of luspatercept intreating LR-MDS unresponsive to ESAs and illustrates its potential benefit asfrontline therapy in MDS and its employment in patients with myelofibrosis-induced anemia.

EXPERT OPINION

Luspatercept has revolutionized the therapeutic paradigm of LR-MDS, for which there was a limited therapeutic arsenal, especially in the setting of patients who did not respond or fail after ESA treatment.

Beyond the horizon: emerging therapeutic approaches in myelodysplastic neoplasms

Management of myelodysplastic neoplasms (MDS) requires a personalized approach, with a focus on improving quality of life and extending lifespan. The International Prognostic Scoring System-Revised and the molecular International Prognostic Scoring System are key tools for risk stratification and management of MDS. They provide a framework for predicting survival and the risk of transformation to acute myeloid leukemia. However, a major challenge in MDS management remains the limited therapeutic options available, especially after the failure of first-line therapies. In lower-risk MDS, the failure of erythropoietin-stimulating agents often leaves few alternatives, although in higher-risk MDS, the prognosis after hypomethylating agent failure is dismal. This highlights the urgent need for novel, more personalized therapeutic approaches. In this review, we discuss emerging novel therapeutic approaches in the treatment of MDS. Several new therapeutic targets are currently being evaluated, offering hope for improved management of MDS in the future.

Extramedullary hematopoiesis contributes to enhanced erythropoiesis during pregnancy via TGF-β signaling

Red blood cells are the predominant cellular component in human body, and their numbers increase significantly during pregnancy due to heightened erythropoiesis. CD71+ erythroid cells (CECs) are immature red blood cells, encompassing erythroblasts and reticulocytes, constitute a rare cell population primarily found in the bone marrow, although they are physiologically enriched in the neonatal mouse spleen and human cord blood. Presently, the mechanisms underlying the CECs expansion during pregnancy remain largely unexplored. Additionally, the mechanisms and roles associated with extramedullary hematopoiesis (EMH) of erythroid cells during pregnancy have yet to be fully elucidated. In this study, our objective was to examine the underlying mechanisms of erythroid-biased hematopoiesis during pregnancy. Our findings revealed heightened erythropoiesis and elevated CECs in both human and mouse pregnancies. The increased presence of transforming growth factor (TGF)-β during pregnancy facilitated the differentiation of CD34+ hematopoietic stem and progenitor cells (HSPCs) into CECs, without impacting HSPCs proliferation, ultimately leading to enhanced erythropoiesis. The observed increase in CECs during pregnancy was primarily attributed to EMH occurring in the spleen. During mouse pregnancy, splenic stromal cells were found to have a significant impact on splenic erythropoiesis through the activation of TGF-β signaling. Conversely, splenic macrophages were observed to contribute to extramedullary erythropoiesis in a TGF-β-independent manner. Our results suggest that splenic stromal cells play a crucial role in promoting extramedullary erythropoiesis and the production of CECs during pregnancy, primarily through TGF-β-dependent mechanisms.

From bench to bedside: The promise of sotatercept in hematologic disorders

Sotatercept (ACE-011) is an activin receptor IIA-Fc (ActRIIA-Fc) fusion protein currently under investigation for its potential in the treatment of hematologic diseases. By impeding the activities of the overexpressed growth and differentiation factor 11 (GDF11), activin A, and other members of the transforming growth factor-β (TGF-β) superfamily, commonly found in hematologic disorders, sotatercept aims to restore the normal functioning of red blood cell maturation and osteoblast differentiation. This action is anticipated to enhance anemia management and hinder the progression of myeloma. Simultaneously, comprehensive research is ongoing to investigate sotatercept's pharmacokinetics and potential adverse reactions, thus laying a robust foundation for its prospective clinical use. In this review, we provide a detailed overview of TGF-β pathways in physiological and hematologic disorder contexts, outline the potential mechanism of sotatercept, and delve into its pharmacokinetics and clinical research advancements in various hematologic diseases. A particular emphasis is given to the relationship between sotatercept dosage and its efficacy or associated adverse reactions.

Synergistic immunotherapy targeting cancer-associated anemia: prospects of a combination strategy

Cancer-associated anemia promotes tumor progression, leads to poor quality of life in patients with cancer, and even obstructs the efficacy of immune checkpoint inhibitors therapy. However, the precise mechanism for cancer-associated anemia remains unknown and the feasible strategy to target cancer-associated anemia synergizing immunotherapy needs to be clarified. Here, we review the possible mechanisms of cancer-induced anemia regarding decreased erythropoiesis and increased erythrocyte destruction, and cancer treatment-induced anemia. Moreover, we summarize the current paradigm for cancer-associated anemia treatment. Finally, we propose some prospective paradigms to slow down cancer-associated anemia and synergistic the efficacy of immunotherapy. Video Abstract.

Recent updates of stem cell-based erythropoiesis

Blood transfusions are now an essential part of modern medicine. Transfusable red blood cells (RBCs) are employed in various therapeutic strategies; however, the processes of blood donation, collection, and administration still involve many limitations. Notably, a lack of donors, the risk of transfusion-transmitted disease, and recent pandemics such as COVID-19 have prompted us to search for alternative therapeutics to replace this resource. Originally, RBC production was attempted via the ex vivo differentiation of stem cells. However, a more approachable and effective cell source is now required for broader applications. As a viable alternative, pluripotent stem cells have been actively used in recent research. In this review, we discuss the basic concepts related to erythropoiesis, as well as early research using hematopoietic stem cells ex vivo, and discuss the current trend of in vitro erythropoiesis using human-induced pluripotent stem cells.

Improved benefit of continuing luspatercept therapy: sub-analysis of patients with lower-risk MDS in the MEDALIST study

Red blood cell transfusion independence (RBC-TI) is an important goal in treating lower-risk myelodysplastic syndromes with ring sideroblasts. In the phase 3 MEDALIST study, RBC-TI of ≥ 8 weeks was achieved by significantly more luspatercept- versus placebo-treated patients in the first 24 weeks of treatment. In this post hoc analysis, we evaluated RBC transfusion units and visits based on patients' baseline transfusion burden level and the clinical benefit of luspatercept treatment beyond week 25 in initial luspatercept nonresponders (patients who did not achieve RBC-TI ≥ 8 weeks by week 25) but continued luspatercept up to 144 weeks. RBC transfusion burden, erythroid response, serum ferritin levels, and hemoglobin levels relative to baseline were evaluated. Through week 25, fewer RBC transfusion units and visits were observed in luspatercept-treated patients versus placebo, regardless of baseline transfusion burden. This continued through 144 weeks of luspatercept treatment, particularly in patients with low baseline transfusion burden. Sixty-eight patients were initial nonresponders at week 25 but continued treatment; most (81%) received the maximum dose of luspatercept (1.75 mg/kg). Sixteen percent achieved RBC-TI for ≥ 8 weeks during weeks 25-48, 26% had reduced RBC transfusion burden, 10% achieved an erythroid response, 44% had reduced serum ferritin, and hemoglobin levels increased an average of 1.3 g/dL from baseline. These data have implications for clinical practice, as transfusion units and visits are less in luspatercept-treated patients through week 25 regardless of baseline transfusion burden, and continuing luspatercept beyond week 25 can potentially provide additional clinical benefits for initial nonresponders. Trial registration: NCT02631070.

Erythropoiesis in lower-risk myelodysplastic syndromes and beta-thalassemia

The hematologic disorders myelodysplastic syndromes and beta-thalassemia are characterized by ineffective erythropoiesis and anemia, often managed with regular blood transfusions. Erythropoiesis, the process by which sufficient numbers of functional erythrocytes are produced from hematopoietic stem cells, is highly regulated, and defects can negatively affect the proliferation, differentiation, and survival of erythroid precursors. Treatments that directly target the underlying mechanisms of ineffective erythropoiesis are limited, and management of anemia with regular blood transfusions imposes a significant burden on patients, caregivers, and health care systems. There is therefore a strong unmet need for treatments that can restore effective erythropoiesis. Novel therapies are beginning to address this need by targeting a variety of mechanisms underlying erythropoiesis. Herein, we provide an overview of the role of ineffective erythropoiesis in myelodysplastic syndromes and beta-thalassemia, discuss unmet needs in targeting ineffective erythropoiesis, and describe current management strategies and emerging treatments for these disorders.

TGF-beta signal transduction: biology, function and therapy for diseases

The transforming growth factor beta (TGF-β) is a crucial cytokine that get increasing concern in recent years to treat human diseases. This signal controls multiple cellular responses during embryonic development and tissue homeostasis through canonical and/or noncanonical signaling pathways. Dysregulated TGF-β signal plays an essential role in contributing to fibrosis via promoting the extracellular matrix deposition, and tumor progression via inducing the epithelial-to-mesenchymal transition, immunosuppression, and neovascularization at the advanced stage of cancer. Besides, the dysregulation of TGF-beta signal also involves in other human diseases including anemia, inflammatory disease, wound healing and cardiovascular disease et al. Therefore, this signal is proposed to be a promising therapeutic target in these diseases. Recently, multiple strategies targeting TGF-β signals including neutralizing antibodies, ligand traps, small-molecule receptor kinase inhibitors targeting ligand-receptor signaling pathways, antisense oligonucleotides to disrupt the production of TGF-β at the transcriptional level, and vaccine are under evaluation of safety and efficacy for the forementioned diseases in clinical trials. Here, in this review, we firstly summarized the biology and function of TGF-β in physiological and pathological conditions, elaborated TGF-β associated signal transduction. And then, we analyzed the current advances in preclinical studies and clinical strategies targeting TGF-β signal transduction to treat diseases.

The role of erythrocytes and erythroid progenitor cells in tumors

In the current research context of precision treatment of malignant tumors, the advantages of immunotherapy are unmatched by conventional antitumor therapy, which can prolong progression-free survival and overall survival. The search for new targets and novel combination therapies can improve the efficacy of immunotherapy and reduce adverse effects. Since current research targets for immunotherapy mainly focus on lymphocytes, little research has been done on erythrocytes. Nucleated erythroid precursor stem cells have been discovered to play an essential role in tumor progression. Researchers are exploring new targets and therapeutic approaches for immunotherapy from the perspective of erythroid progenitor cells (EPCs). Recent studies have shown that different subtypes of EPCs have specific surface markers and distinct biological roles in tumor immunity. CD45⁺ EPCs are potent myeloid-derived suppressor cell-like immunosuppressants that reduce the patient's antitumor immune response. CD45^- EPCs promote tumor invasion and metastasis by secreting artemin. A specific type of EPC also promotes angiogenesis and provides radiation protection. Therefore, EPCs may be involved in tumor growth, infiltration, and metastasis. It may also be an important cause of anti-angiogenesis and immunotherapy resistance. This review summarizes recent research advances in erythropoiesis, EPC features, and their impacts and processes on tumors.

Luspatercept (RAP-536) modulates oxidative stress without affecting mutation burden in myelodysplastic syndromes

In low-risk myelodysplastic syndrome (LR-MDS), erythropoietin (EPO) is widely used for the treatment of chronic anemia. However, initial response to EPO has time-limited effects. Luspatercept reduces red blood cell transfusion dependence in LR-MDS patients. Here, we investigated the molecular action of luspatercept (RAP-536) in an in vitro model of erythroid differentiation of MDS, and also in a in vivo PDX murine model with primary samples of MDS patients carrying or not SF3B1 mutation. In our in vitro model, RAP-536 promotes erythroid proliferation by increasing the number of cycling cells without any impact on apoptosis rates. RAP-536 promoted late erythroid precursor maturation while decreasing intracellular reactive oxygen species level. RNA sequencing of erythroid progenitors obtained under RAP-536 treatment showed an enrichment of genes implicated in positive regulation of response to oxidative stress and erythroid differentiation. In our PDX model, RAP-536 induces a higher hemoglobin level. RAP-536 did not modify variant allele frequencies in vitro and did not have any effect against leukemic burden in our PDX model. These results suggest that RAP-536 promotes in vivo and in vitro erythroid cell differentiation by decreasing ROS level without any remarkable impact on iron homeostasis and on mutated allele burden.

Developmental changes in iron metabolism and erythropoiesis in mice with human gain-of-function erythropoietin receptor

Iron availability for erythropoiesis is controlled by the iron-regulatory hormone hepcidin. Increased erythropoiesis negatively regulates hepcidin synthesis by erythroferrone (ERFE), a hormone produced by erythroid precursors in response to erythropoietin (EPO). The mechanisms coordinating erythropoietic activity with iron homeostasis in erythrocytosis with low EPO are not well defined as exemplified by dominantly inherited (heterozygous) gain-of-function mutation of human EPO receptor (mtHEPOR) with low EPO characterized by postnatal erythrocytosis. We previously created a mouse model of this mtHEPOR that develops fetal erythrocytosis with a transient perinatal amelioration of erythrocytosis and its reappearance at 3-6 weeks of age. Prenatally and perinatally, mtHEPOR heterozygous and homozygous mice (differing in erythrocytosis severity) had increased Erfe transcripts, reduced hepcidin, and iron deficiency. Epo was transiently normal in the prenatal life; then decreased at postnatal day 7, and remained reduced in adulthood. Postnatally, hepcidin increased in mtHEPOR heterozygotes and homozygotes, accompanied by low Erfe induction and iron accumulation. With aging, the old, especially mtHEPOR homozygotes had a decline of erythropoiesis, myeloid expansion, and local bone marrow inflammatory stress. In addition, mtHEPOR erythrocytes had a reduced lifespan. This, together with reduced iron demand for erythropoiesis, due to its age-related attenuation, likely contributes to increased iron deposition in the aged mtHEPOR mice. In conclusion, the erythroid drive-mediated inhibition of hepcidin production in mtHEPOR mice in the prenatal/perinatal period is postnatally abrogated by increasing iron stores promoting hepcidin synthesis. The differences observed in studied characteristics between mtHEPOR heterozygotes and homozygotes suggest dose-dependent alterations of downstream EPOR stimulation.

Erythroid Cell Research: 3D Chromatin, Transcription Factors and Beyond

Studies of the regulatory networks and signals controlling erythropoiesis have brought important insights in several research fields of biology and have been a rich source of discoveries with far-reaching implications beyond erythroid cells biology. The aim of this review is to highlight key recent discoveries and show how studies of erythroid cells bring forward novel concepts and refine current models related to genome and 3D chromatin organization, signaling and disease, with broad interest in life sciences.

Molecular and cellular mechanisms that regulate human erythropoiesis

To enable effective oxygen transport, ∼200 billion red blood cells (RBCs) need to be produced every day in the bone marrow through the fine-tuned process of erythropoiesis. Erythropoiesis is regulated at multiple levels to ensure that defective RBC maturation or overproduction can be avoided. Here, we provide an overview of different layers of this control, ranging from cytokine signaling mechanisms that enable extrinsic regulation of RBC production to intrinsic transcriptional pathways necessary for effective erythropoiesis. Recent studies have also elucidated the importance of posttranscriptional regulation and highlighted additional gatekeeping mechanisms necessary for effective erythropoiesis. We additionally discuss the insights gained by studying human genetic variation affecting erythropoiesis and highlight the discovery of BCL11A as a regulator of hemoglobin switching through genetic studies. Finally, we provide an outlook of how our ability to measure multiple facets of this process at single-cell resolution, while accounting for the impact of human variation, will continue to refine our knowledge of erythropoiesis and how this process is perturbed in disease. As we learn more about this intricate and important process, additional opportunities to modulate erythropoiesis for therapeutic purposes will undoubtedly emerge.

Transcriptomic Changes Within Human Bone Marrow After Severe Trauma

BACKGROUND

Severe trauma is associated with severe systemic inflammation and neuroendocrine activation that is associated with erythroid progenitor growth suppression and refractory anemia. Although distinct transcriptional profiles have been detected in numerous tissue types after trauma, no study has yet characterized this within the bone marrow. This study sought to identify a unique bone marrow transcriptomic response following trauma.

METHODS

In a prospective observational cohort study, bone marrow was obtained from severely injured trauma patients with a hip or femur fracture (n = 52), elective hip replacement patients (n = 33), and healthy controls (n = 11). RNA was isolated from bone marrow using a Purelink RNA mini kit. Direct quantification of mRNA copies was performed by NanoString Technologies on a custom gene panel.

RESULTS

Trauma patients displayed an upregulation of genes encoding receptors known to have inhibitory downstream effects on erythropoiesis, including ferroportin, interleukin-6 (IL-6) receptor, transforming growth factor-beta (TGF-β) receptor, and IL-10, as well as genes involved in innate immunity including toll-like receptor 4 (TLR4)-mediated signaling factors. In contrast, hip replacement patients had downregulated transcription of IL-1β, IL-6, TGF-β, tumor necrosis factor alpha, and the HAMP gene with no change in TLR4-mediated signaling factors.

CONCLUSIONS

A unique transcriptomic response within the bone marrow was identified following severe trauma compared to elective hip replacement. These transcriptomic differences were related to the innate immune response as well as known inhibitors of erythropoiesis. Although confined to just one time point, this differential transcriptional response may be linked to refractory anemia and inflammation after injury.

The Role of TGF-β in Bone Metastases

Complications associated with advanced cancer are a major clinical challenge and, if associated with bone metastases, worsen the prognosis and compromise the survival of the patients. Breast and prostate cancer cells exhibit a high propensity to metastasize to bone. The bone microenvironment is unique, providing fertile soil for cancer cell propagation, while mineralized bone matrices store potent growth factors and cytokines. Biologically active transforming growth factor β (TGF-β), one of the most abundant growth factors, is released following tumor-induced osteoclastic bone resorption. TGF-β promotes tumor cell secretion of factors that accelerate bone loss and fuel tumor cells to colonize. Thus, TGF-β is critical for driving the feed-forward vicious cycle of tumor growth in bone. Further, TGF-β promotes epithelial-mesenchymal transition (EMT), increasing cell invasiveness, angiogenesis, and metastatic progression. Emerging evidence shows TGF-β suppresses immune responses, enabling opportunistic cancer cells to escape immune checkpoints and promote bone metastases. Blocking TGF-β signaling pathways could disrupt the vicious cycle, revert EMT, and enhance immune response. However, TGF-β's dual role as both tumor suppressor and enhancer presents a significant challenge in developing therapeutics that target TGF-β signaling. This review presents TGF-β's role in cancer progression and bone metastases, while highlighting current perspectives on the therapeutic potential of targeting TGF-β pathways.

Association of the Degree of Erythroid Expansion and Maturation Arrest with the Clinical Severity of β0-Thalassemia/Hemoglobin E Patients

INTRODUCTION

β-Thalassemia/hemoglobin E represents one-half of all the clinically severe β-thalassemias worldwide. Despite similar genetic backgrounds, patients show clinical heterogeneity ranging from nearly asymptomatic to transfusion-dependent thalassemia. The underlying disease modifying factors remain largely obscure.

METHODS

To elucidate the correlation between ineffective erythropoiesis and β0-thalassemia/hemoglobin E (HbE) disease severity, in vitro culture of erythroid cells derived from patients with different clinical symptoms was established. Cell proliferation, viability, and differentiation were investigated. To identify potential molecular mechanisms leading to the arrested erythroid maturation, the expression levels of erythropoiesis modifying factors were measured.

RESULTS

The β0-thalassemia/HbE cells exhibited enhanced proliferation, limited differentiation, and impaired erythroid terminal maturation but did not show accelerated erythroblast differentiation and increased cell death. Erythroblasts derived from mild patients showed the highest proliferation rate with a faster cell division time, while erythroblasts derived from severe patients displayed extremely delayed erythroid maturation. Downregulation of growth differentiation factor 11 and FOXO3a was observed in mild β0-thalassemia/HbE erythroblasts, while upregulation of heat shock protein 70 and activin receptor 2A was revealed in severe erythroblasts.

DISCUSSION/CONCLUSION

The degree of erythroid expansion and maturation arrest contributes to the severity of β0-thalassemia/HbE patients, accounting for the disease heterogeneity. The findings suggest a restoration of erythroid maturation as a promising targeted therapy for severe patients.

Luspatercept as a therapy for myelodysplastic syndromes with ring sideroblasts

INTRODUCTION

Myelodysplastic syndromes (MDS) are a heterogeneous group of hematopoietic stem cell diseases characterized by cell dysplasia, ineffective hematopoiesis and risk of transformation to acute myeloid leukemia (AML). The median age of 75 years at diagnosis is associated with the presence of comorbidities, which preclude intensive therapies like allogeneic hematopoietic stem cell transplantation in most MDS patients. Risk stratification using the (Revised) International Prognostic Scoring System (IPSS/IPSS-R) is necessary to plan individualized treatment.

AREAS COVERED

Luspatercept (ACE-536), a specific activin receptor fusion protein, promotes late-stage erythropoiesis. Two clinical trials, PACE-MDS (phase 2) and MEDALIST (phase 3), yielded positive results in terms of improved hemoglobin levels and loss of transfusion dependence, with hardly any side effects. A phase 3 trial to compare luspatercept to ESAs (COMMANDS study) is ongoing.

EXPERT OPINION

Luspatercept is a promising alternative to ESAs for a subset of transfusion-dependent patients with lower risk MDS, namely those with a sideroblastic phenotype who are either not suitable for or have already failed erythropoietin-based treatment. The favorable safety profile and convenient subcutaneous administration every 3 weeks are more conducive to patients' quality of life than chronic red blood cell transfusion therapy.

The role of CD71+ erythroid cells in the regulation of the immune response

Complex regulation of the immune response is necessary to support effective defense of an organism against hostile invaders and to maintain tolerance to harmless microorganisms and autoantigens. Recent studies revealed previously unappreciated roles of CD71⁺ erythroid cells (CECs) in regulation of the immune response. CECs physiologically reside in the bone marrow where erythropoiesis takes place. Under stress conditions, CECs are enriched in some organs outside of the bone marrow as a result of extramedullary erythropoiesis. However, the role of CECs goes well beyond the production of erythrocytes. In neonates, increased numbers of CECs contribute to their vulnerability to infectious diseases. On the other side, neonatal CECs suppress activation of immune cells in response to abrupt colonization with commensal microorganisms after delivery. CECs are also enriched in the peripheral blood of pregnant women as well as in the placenta and are responsible for the regulation of feto-maternal tolerance. In patients with cancer, anemia leads to increased frequency of CECs in the peripheral blood contributing to diminished antiviral and antibacterial immunity, as well as to accelerated cancer progression. Moreover, recent studies revealed the role of CECs in HIV and SARS-CoV-2 infections. CECs use a full arsenal of mechanisms to regulate immune response. These cells suppress proinflammatory responses of myeloid cells and T-cell proliferation by the depletion of ʟ-arginine by arginase. Moreover, CECs produce reactive oxygen species to decrease T-cell proliferation. CECs also secrete cytokines, including transforming growth factor β (TGF-β), which promotes T-cell differentiation into regulatory T-cells. Here, we comprehensively describe the role of CECs in orchestrating immune response and indicate some therapeutic approaches that might be used to regulate their effector functions in the treatment of human conditions.

Development of luspatercept to treat ineffective erythropoiesis

Luspatercept (Reblozyl) was recently approved for treating patients with transfusion-dependent lower-risk myelodysplastic syndrome (MDS) with ring sideroblasts (RS) and/or SF3B1 mutation who were not eligible for erythropoiesis-stimulating agents (ESAs) or patients for whom those agents failed. Luspatercept acts as an activin receptor type IIB fusion protein ligand trap that targets the altered transforming growth factor beta pathway in MDS, which is associated with impaired terminal erythroid maturation. Treatment with luspatercept results in decreased SMAD signaling, which enables erythroid maturation by means of late-stage erythroblast differentiation and thus improves anemia. ESAs, the current standard first-line therapeutic option for anemic lower-risk patients with MDS, also improve red cell parameters mainly by expanding proliferation of early erythroid progenitor cells. However, erythropoietin (EPO) and its receptor (EPO-R) are also required for survival of late-stage definitive erythroid cells, and they play an essential role in promoting proliferation, survival, and appropriate timing of terminal maturation of primitive erythroid precursors. Thus, luspatercept joins the mechanism of ESAs in promoting erythroid maturation. Especially in the subgroup of MDS patients with RS, luspatercept showed high clinical activity for the treatment of anemia in the phase 2 (PACE-MDS) trial and subsequently in the phase 3 (MEDALIST) trial, which resulted in approval by both the US Food and Drug Administration and the European Medicines Agency in April 2020. Additional studies are needed to better understand the mechanism of action and pharmacodynamics of this novel agent in MDS.

Iron-loaded transferrin potentiates erythropoietin effects on erythroblast proliferation and survival: a novel role through transferrin receptors

Red blood cell production, or erythropoiesis, is a proliferative process that requires tight regulation. Erythropoietin (Epo) is a glycoprotein cytokine that plays a major role in erythropoiesis by triggering erythroid progenitors/precursors of varying sensitivity. The concentration of Epo in bone marrow is hypothesized to be suboptimal, and the survival of erythroid cells has been suggested to depend on Epo sensitivity. However, the key factors that control Epo sensitivity remain unknown. Two types of transferrin receptors (TfRs), TfR1 and TfR2, are known to play a role in iron uptake in erythroid cells. Here, we hypothesized that TfRs may additionally modulate Epo sensitivity during erythropoiesis by modulating Epo receptor (EpoR) signaling. Using an Epo-sensitive UT-7 (UT7/Epo) erythroid cell and human erythroid progenitor cell models, we report that iron-loaded transferrin, that is, holo-transferrin (holo-Tf), synergizes with suboptimal Epo levels to improve erythroid cell survival, proliferation, and differentiation. This is accomplished via the major signaling pathways of erythropoiesis, which include signal transducer and activator of transcription 5 (STAT5), mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), and phosphoinositide-3-kinase (PI3K)/AKT. Furthermore, we found that this cooperation is improved by, but does not require, the internalization of TfR1. Interestingly, we observed that loss of TfR2 stabilizes EpoR levels and abolishes the beneficial effects of holo-Tf. Overall, these data reveal novel signaling properties of TfRs, which involve the regulation of erythropoiesis through EpoR signaling.

Next Generation Therapeutics for the Treatment of Myelofibrosis

Myelofibrosis is a myeloproliferative neoplasm characterized by splenomegaly, constitutional symptoms, bone marrow fibrosis, and a propensity towards transformation to acute leukemia. JAK inhibitors are the only approved therapy for myelofibrosis and have been successful in reducing spleen and symptom burden. However, they do not significantly impact disease progression and many patients are ineligible due to coexisting cytopenias. Patients who are refractory to JAK inhibition also have a dismal survival. Therefore, non-JAK inhibitor-based therapies are being explored in pre-clinical and clinical settings. In this review, we discuss novel treatments in development for myelofibrosis with targets outside of the JAK-STAT pathway. We focus on the mechanism, preclinical rationale, and available clinical efficacy and safety information of relevant agents including those that target apoptosis (navitoclax, KRT-232, LCL-161, imetelstat), epigenetic modulation (CPI-0610, bomedemstat), the bone marrow microenvironment (PRM-151, AVID-200, alisertib), signal transduction pathways (parsaclisib), and miscellaneous agents (tagraxofusp. luspatercept). We also provide commentary on the future of therapeutic development in myelofibrosis.

Tumor Immune Evasion Induced by Dysregulation of Erythroid Progenitor Cells Development

Cancer cells harness normal cells to facilitate tumor growth and metastasis. Within this complex network of interactions, the establishment and maintenance of immune evasion mechanisms are crucial for cancer progression. The escape from the immune surveillance results from multiple independent mechanisms. Recent studies revealed that besides well-described myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) or regulatory T-cells (Tregs), erythroid progenitor cells (EPCs) play an important role in the regulation of immune response and tumor progression. EPCs are immature erythroid cells that differentiate into oxygen-transporting red blood cells. They expand in the extramedullary sites, including the spleen, as well as infiltrate tumors. EPCs in cancer produce reactive oxygen species (ROS), transforming growth factor β (TGF-β), interleukin-10 (IL-10) and express programmed death-ligand 1 (PD-L1) and potently suppress T-cells. Thus, EPCs regulate antitumor, antiviral, and antimicrobial immunity, leading to immune suppression. Moreover, EPCs promote tumor growth by the secretion of growth factors, including artemin. The expansion of EPCs in cancer is an effect of the dysregulation of erythropoiesis, leading to the differentiation arrest and enrichment of early-stage EPCs. Therefore, anemia treatment, targeting ineffective erythropoiesis, and the promotion of EPC differentiation are promising strategies to reduce cancer-induced immunosuppression and the tumor-promoting effects of EPCs.

Clinical and Molecular Insights in Erythropoiesis Regulation of Signal Transduction Pathways in Myelodysplastic Syndromes and β-Thalassemia

Erythropoiesis regulation is essential in normal physiology and pathology, particularly in myelodysplastic syndromes (MDS) and β-thalassemia. Several signaling transduction processes, including those regulated by inositides, are implicated in erythropoiesis, and the latest MDS or β-thalassemia preclinical and clinical studies are now based on their regulation. Among others, the main pathways involved are those regulated by transforming growth factor (TGF)-β, which negatively regulates erythrocyte differentiation and maturation, and erythropoietin (EPO), which acts on the early-stage erythropoiesis. Also small mother against decapentaplegic (SMAD) signaling molecules play a role in pathology, and activin receptor ligand traps are being investigated for future clinical applications. Even inositide-dependent signaling, which is important in the regulation of cell proliferation and differentiation, is specifically associated with erythropoiesis, with phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI3K) as key players that are becoming increasingly important as new promising therapeutic targets. Additionally, Roxadustat, a new erythropoiesis stimulating agent targeting hypoxia inducible factor (HIF), is under clinical development. Here, we review the role and function of the above-mentioned signaling pathways, and we describe the state of the art and new perspectives of erythropoiesis regulation in MDS and β-thalassemia.

When basic science reaches into rational therapeutic design: from historical to novel leads for the treatment of β-globinopathies

PURPOSE OF REVIEW

β-hemoglobinopathies, such as β-Thalassemias (β-Thal) and sickle cell disease (SCD) are among the most common inherited genetic disorders in humans worldwide. These disorders are characterized by a quantitative (β-Thal) or qualitative (SCD) defects in adult hemoglobin production, leading to anemia, ineffective erythropoiesis and severe secondary complications. Reactivation of the fetal globin genes (γ-globin), making-up fetal hemoglobin (HbF), which are normally silenced in adults, represents a major strategy to ameliorate anemia and disease severity.

RECENT FINDINGS

Following the identification of the first 'switching factors' for the reactivation of fetal globin gene expression more than 10 years ago, a multitude of novel leads have recently been uncovered.

SUMMARY

Recent findings provided invaluable functional insights into the genetic and molecular networks controlling globin genes expression, revealing that complex repression systems evolved in erythroid cells to maintain HbF silencing in adults. This review summarizes these unique and exciting discoveries of the regulatory factors controlling the globin switch. New insights and novel leads for therapeutic strategies based on the pharmacological induction of HbF are discussed. This represents a major breakthrough for rational drug design in the treatment of β-Thal and SCD.

Generation of an immortalised erythroid cell line from haematopoietic stem cells of a haemoglobin E/β-thalassemia patient

The β-thalassemia syndromes are the most prevalent genetic disorder globally, characterised by reduced or absent β-globin chain synthesis. HbE/β-thalassemia is a subtype of β-thalassemia with extremely high frequency in Asia. Studying molecular defects behind β-thalassemia is severely impeded by paucity of material from patients and lack of suitable cell lines. Approaches to derive erythroid cells from induced pluripotent stem cells (iPSCs) created from patients are confounded by poor levels of erythroid cell expansion, aberrant or incomplete erythroid differentiation and foetal/embryonic rather than adult globin expression. In this study we generate an immortalised erythroid cell line from peripheral blood stem cells of a HbE/β-thalassemia patient. Morphological analysis shows the cells are proerythroblasts with some early basophilic erythroblasts, with no change in morphology over time in culture. The line differentiates along the erythroid pathway to orthochromatic erythroblasts and reticulocytes. Importantly, unlike iPSCs, the line maintains the haemoglobin profile of the patient's red blood cells. This is the first human cellular model for β-thalassemia providing a sustainable source of disease cells for studying underlying disease mechanisms and for use as drug screening platform, particularly for reagents designed to increase foetal haemoglobin expression as we have additionally demonstrated with hydroxyurea.

TGFBR3 Polymorphisms (rs1805110 and rs7526590) Are Associated with Laboratory Biomarkers and Clinical Manifestations in Sickle Cell Anemia

Individuals with sickle cell anemia (SCA) present chronic anemia, hemolysis, an exacerbated inflammatory response, and heterogeneous clinical complications, which may be modulated by the transforming growth factor beta (TGF-β) pathway. Thus, we aimed to investigate polymorphisms (rs1805110 and rs7526590) of the transforming growth factor beta receptor III gene (TGFBR3) with regard to laboratory biomarkers and clinical manifestations in individuals with SCA. Hematological, biochemical, immunological, and genetic analyses were carried out, as well as serum endothelin-1 measurements. The minor allele (A) of the TGFBR3 rs1805110 polymorphism was associated with increased hemoglobin, hematocrit, reticulocyte counts, total cholesterol, low-density lipoprotein, uric acid, and endothelin levels, as well as decreased platelet distribution width (PDW) and the occurrence of bone alterations. The minor allele (T) of TGFBR3 rs7526590 was associated with increased red cell distribution width, PDW, alkaline phosphatase, aspartate aminotransferase, total and indirect bilirubin, and lactate dehydrogenase levels, as well as lower ferritin levels and the occurrence of leg ulcers. Our data suggest that the minor allele (A) of TGFBR3 rs1805110 is associated with inflammation and bone alterations, while the minor allele (T) of TGFBR3 rs7526590 is related to hemolysis and the occurrence of leg ulcers.

Transforming growth factor-β boosts the functionality of human bone marrow-derived mesenchymal stromal cells

Transforming growth factor β1 (TGFβ1) is a negative regulator of hematopoiesis, and yet, it is frequently found at the active sites of hematopoiesis. Here, we show for the first time that bone marrow-derived mononuclear cells (BM MNCs) secrete TGFβ1 in response to erythropoietin (EPO). We further show that human bone marrow-derived mesenchymal stromal cells (BMSCs) briefly exposed to the conditioned medium of EPO-primed MNCs, or purified TGFβ1, gain significantly increased hematopoiesis-supportive ability. Mechanistically, we show that this phenomenon involves TGFβ1-mediated activation of nitric oxide (NO) signalling pathway in the BMSCs. The data suggest that EPO-MNC-TGFβ1 could be one of the regulatory axes operative in the bone marrow microenvironment involved in maintaining the functionality of the resident BMSCs.

Enhanced Renewal of Erythroid Progenitors in Myelodysplastic Anemia by Peripheral Serotonin

Tryptophan as the precursor of several active compounds, including kynurenine and serotonin, is critical for numerous important metabolic functions. Enhanced tryptophan metabolism toward the kynurenine pathway has been associated with myelodysplastic syndromes (MDSs), which are preleukemic clonal diseases characterized by dysplastic bone marrow and cytopenias. Here, we reveal a fundamental role for tryptophan metabolized along the serotonin pathway in normal erythropoiesis and in the physiopathology of MDS-related anemia. We identify, both in human and murine erythroid progenitors, a functional cell-autonomous serotonergic network with pro-survival and proliferative functions. In vivo studies demonstrate that pharmacological increase of serotonin levels using fluoxetine, a common antidepressant, has the potential to become an important therapeutic strategy in low-risk MDS anemia refractory to erythropoietin.

Transforming growth factor β1 accelerates and enhances in vitro red blood cell formation from hematopoietic stem cells by stimulating mitophagy

Background

Generation of red blood cells (RBCs) from hematopoietic stem cells (HSCs) in vitro takes about 21 days, making it unaffordable for clinical applications. Acceleration of the in vitro erythropoiesis process by using small molecules could eventually make the large-scale production of these cells commercially viable. Transforming Growth Factor β1 (TGF-β1) has been shown to have a dose-dependent activity on the HSCs: at high concentration it inhibits, whereas at low concentration it stimulates the HSCs growth. At high concentration, it also inhibits erythropoiesis but accelerates terminal erythroid differentiation of cell lines and erythroid progenitors. Here we examined whether the use of low concentration of TGF-β1 would be beneficial for increasing RBC production by stimulating HSC growth and also supporting erythroid differentiation. Such a strategy could make RBC production in vitro more efficient and cost-effective for clinical applications.

Methods

HSCs isolated from Apheresis samples were differentiated into mature RBCs by the sequential addition of specific combinations of growth factors for 21 days. In the control set, only EPO (3 IU/ml) was added whereas, in the test set, TGF-β1 at a concentration of 10 pg/ml was added along with EPO (3 IU/ml) from day 0.

Results

We found that a low concentration of TGF-β1 has no inhibitory effect on the proliferation of the early stages of erythropoiesis. Additionally, it significantly accelerates terminal stages of erythroid differentiation by promoting BNIP3L/NIX-mediated mitophagy.

Conclusions

Incorporation of TGF-β1 at 10 pg/ml concentration in the differentiation medium accelerates the in vitro erythropoiesis process by 3 days. This finding could have potential applications in transfusion medicine.

Electronic supplementary material

The online version of this article (10.1186/s13287-020-01603-z) contains supplementary material, which is available to authorized users.

A three-dimensional in vitro model of erythropoiesis recapitulates erythroid failure in myelodysplastic syndromes

Established cell culture systems have failed to accurately recapitulate key features of terminal erythroid maturation, hampering our ability to in vitro model and treat diseases with impaired erythropoiesis such as myelodysplastic syndromes with ring sideroblasts (MDS-RS). We developed an efficient and robust three-dimensional (3D) scaffold culture model supporting terminal erythroid differentiation from both mononuclear (MNC) or CD34+-enriched primary bone marrow cells from healthy donors and MDS-RS patients. While CD34+ cells did not proliferate beyond two weeks in 2D suspension cultures, the 3D scaffolds supported CD34+ and MNC erythroid proliferation over four weeks demonstrating the importance of the 3D environment. CD34+ cells cultured in 3D facilitated the highest expansion and maturation of erythroid cells, including generation of erythroblastic islands and enucleated erythrocytes, while MNCs supported multi-lineage hemopoietic differentiation and cytokine secretion relevant for MDS-RS. Importantly, MDS-RS 3D-cultures supported de novo generation of ring sideroblasts and maintenance of the mutated clone. The 3D cultures effectively model a clonal disease characterized by terminal erythroid failure and can be used to assess therapeutic compounds.

Shared and Tissue-Specific Expression Signatures between Bone Marrow from Primary Myelofibrosis and Essential Thrombocythemia

Megakaryocytes have been implicated in the micro-environmental abnormalities associated with fibrosis and hematopoietic failure in the bone marrow (BM) of primary myelofibrosis (PMF) patients, the Philadelphia-negative myeloproliferative neoplasm (MPN) associated with the poorest prognosis. To identify possible therapeutic targets for restoring BM functions in PMF, we compared the expression profiling of PMF BM with that of BM from essential thrombocytopenia (ET), a fibrosis-free MPN also associated with BM megakaryocyte hyperplasia. The signature of PMF BM was also compared with published signatures associated with liver and lung fibrosis. Gene set enrichment analysis (GSEA) identified distinctive differences between the expression profiles of PMF and ET. Notch, K-Ras, IL-8, and apoptosis pathways were altered the most in PMF as compared with controls. By contrast, cholesterol homeostasis, unfolded protein response, and hypoxia were the pathways found altered to the greatest degree in ET compared with control specimens. BM from PMF expressed a noncanonical transforming growth factor β (TGF-β) signature, which included activation of ID1, JUN, GADD45b, and genes with binding motifs for the JUN transcriptional complex AP1. By contrast, the expression of ID1 and GADD45b was not altered and there was a modest signal for JUN activation in ET. The similarities among PMF, liver fibrosis, and lung fibrosis were modest and included activation of integrin-α9 and tropomyosin-α1 between PMF and liver fibrosis, and of ectoderm-neural cortex protein 1 and FRAS1-related extracellular matrix protein 1 between PMF and lung fibrosis, but not TGF-β. These data identify TGF-β as a potential target for micro-environmental therapy in PMF.

Interferon-γ exerts dual functions on human erythropoiesis via interferon regulatory factor 1 signal pathway

Hematopoiesis is systematically regulated by microenvironmental factors. The positive and negative factors coordinated together to yield a complicated blood system. Interferon-γ (IFNγ) has been identified as a common cause of various hematopoietic abnormalities, such as aplastic anemia. However, its impact on monolineage development, especially erythropoiesis, has not been fully elucidated from the cellular angle. In this study, we investigated the behavior of IFNγ and found that IFNγ plays dual functions on erythropoiesis; it not only blocks the erythroid lineage commitment but also accelerates the erythroid differentiation process, ultimately leading to the erythropoietic window clearance. IFNγ can even powerfully initiate early differentiation without the existence of erythropoietin (EPO). Interferon regulatory factor 1 (IRF1) was confirmed as the essential downstream effector, and its ectopic overexpression can also have the same effect as that of IFNγ. These results reveal that the IFNγ-IRF1 axis plays a bidirectional role on erythropoiesis, impeding the access to erythroid lineage and driving the coming cells toward the differentiation endpoint. This model may place an innovative implication for IFNγ-IRF1 axis to understand its in-depth mechanism on normal hematopoiesis and abnormal blood disorders, especially aplastic anemia.

Setting Fire to ESA and EMA Resistance: New Targeted Treatment Options in Lower Risk Myelodysplastic Syndromes

During the last decade, substantial advances have been made in the understanding of the complex molecular, immunological and cellular disturbances involved in the initiation as well as evolution of myelodysplastic syndromes (MDS). In 85% of the mainly frail and older patient population, anemia is present at the time of diagnosis and is thus a major therapeutic challenge. High rates of primary resistance to erythropoiesis-stimulating agents (ESAs), the currently only approved standard therapy to treat anemia in lower-risk MDS, demand the development of novel and efficient drugs with a good safety profile. Luspatercept, a ligand trap of activin receptor II, is able to promote late stage erythropoiesis even in patients failing prior ESA treatment. The presence of ring sideroblastic phenotype defines a subgroup of patients with higher response rates. Additionally, recent developments in clinical research using HIF-1 or telomerase modulation by roxadustat or imetelstat are promising. Other areas of translational research involve targeting the inflammasome by anti-inflammatory drugs in order to improve anemia. These efforts will hopefully pave the way for new targeted treatment options for anemic low-risk MDS patients.

The splicing factor Sf3b1 regulates erythroid maturation and proliferation via TGFβ signaling in zebrafish

The spliceosomal component Splicing Factor 3B, subunit 1 (SF3B1) is one of the most prevalently mutated factors in the bone marrow failure disorder myelodysplastic syndrome. There is a strong clinical correlation between SF3B1 mutations and erythroid defects, such as refractory anemia with ringed sideroblasts, but the role of SF3B1 in normal erythroid development is largely unknown. Loss-of-function zebrafish mutants for sf3b1 develop a macrocytic anemia. Here, we explore the underlying mechanism for anemia associated with sf3b1 deficiency in vivo. We found that sf3b1 mutant erythroid progenitors display a G0/G1 cell-cycle arrest with mutant erythrocytes showing signs of immaturity. RNA-sequencing analysis of sf3b1 mutant erythroid progenitors revealed normal expression of red blood cell regulators such as gata1, globin genes, and heme biosynthetic factors, but upregulation of genes in the transforming growth factor β (TGFβ) pathway. As TGFβ signaling is a known inducer of quiescence, the data suggest that activation of the pathway could trigger sf3b1 deficiency-induced anemia via cell-cycle arrest. Indeed, we found that inhibition of TGFβ signaling released the G0/G1 block in erythroid progenitors. Surprisingly, removal of this checkpoint enhanced rather than suppressed the anemia, indicating that the TGFβ-mediated cell-cycle arrest is protective for sf3b1-mutant erythrocytes. Together, these data suggest that macrocytic anemia arising from Sf3b1 deficiency is likely due to pleiotropic and distinct effects on cell-cycle progression and maturation.

Establishment of an erythroid progenitor cell line capable of enucleation achieved with an inducible c-Myc vector

BACKGROUND

A robust scalable method for producing enucleated red blood cells (RBCs) is not only a process to produce packed RBC units for transfusion but a potential platform to produce modified RBCs with applications in advanced cellular therapy. Current strategies for producing RBCs have shortcomings in the limited self-renewal capacity of progenitor cells, or difficulties in effectively enucleating erythroid cell lines. We explored a new method to produce RBCs by inducibly expressing c-Myc in primary erythroid progenitor cells and evaluated the proliferative and maturation potential of these modified cells.

RESULTS

Primary erythroid progenitor cells were genetically modified with an inducible gene transfer vector expressing a single transcription factor, c-Myc, and all the gene elements required to achieve dox-inducible expression. Genetically modified cells had enhanced proliferative potential compared to control cells, resulting in exponential growth for at least 6 weeks. Inducibly proliferating erythroid (IPE) cells were isolated with surface receptors similar to colony forming unit-erythroid (CFU-Es), and after removal of ectopic c-Myc expression cells hemoglobinized, decreased in cell size to that of native RBCs, and enucleated achieving cultures with 17% enucleated cells. Experiments with IPE cells at various levels of ectopic c-Myc expression provided insight into differentiation dynamics of the modified cells, and an optimized two-stage differentiation strategy was shown to promote greater expansion and maturation.

CONCLUSIONS

Genetic engineering of adult erythroid progenitor cells with an inducible c-Myc vector established an erythroid progenitor cell line that could produce RBCs, demonstrating the potential of this approach to produce large quantities of RBCs and modified RBC products.

Transforming growth factor (TGF)-β pathway as a therapeutic target in lower risk myelodysplastic syndromes

The transforming growth factor (TGF)-β superfamily comprises more than 30 soluble growth factors that play a central role in erythropoiesis and are part of a tightly regulated myelosuppressive negative feedback loop under physiologic conditions. TGF-β receptor activation and phosphorylation trigger a regulatory circuit of activating and inhibitory SMAD proteins and increased activation of the TGF-β signaling pathway either by a loss of negative feedback or constitutive activation has been associated with the myelosuppression and ineffective erythropoiesis in myelodysplastic syndromes (MDS). Anemia is the predominant cause of morbidity and quality of life impairment in patients with lower-risk (LR)-MDS, and there are very limited therapy options for these patients after failure of erythropoiesis stimulating agents (ESAs). Targeting the aberrant TGF-ß signaling pathway has therefore been investigated as a promising therapeutic approach to resolve the ineffective erythropoiesis in LR-MDS. In this article, we provide a brief overview of the TGF-β signaling cascade in hematopoiesis under physiologic conditions and its role in MDS pathogenesis. We also review preclinical and clinical data for the activin receptor type IIA ligand traps sotatercept and luspatercept that have recently shown promising results in overcoming the myelosuppressive effects of TGF-β signaling alterations to improve hematopoiesis in transfusion-dependent, non-del(5q) LR-MDS patients. Additional potential targets within the TGF-β pathway have also been identified in preclinical experiments and may provide further therapeutic options. Finally, combining different TGF-β pathway inhibitors or using them in combination with ESAs or the immunomodulator lenalidomide might have synergistic effects as well.

The role of TGFβ in hematopoiesis and myeloid disorders

The role of transforming growth factor-β (TGFβ) signaling in embryological development and tissue homeostasis has been thoroughly characterized. Its canonical downstream cascade is well known, even though its true complexity and other non-canonical pathways are still being explored. TGFβ signaling has been described as an important pathway involved in carcinogenesis and cancer progression. In the hematopoietic compartment, the TGFβ pathway is an important regulator of proliferation and differentiation of different cell types and has been implicated in the pathogenesis of a diverse variety of bone marrow disorders. Due to its importance in hematological diseases, novel inhibitors of this pathway are being developed against a number of hematopoietic disorders, including myelodysplastic syndromes (MDS). In this review, we provide an overview of the TGFβ pathway, focusing on its role in hematopoiesis and impact on myeloid disorders. We will discuss therapeutic interventions with promising results against MDS.

Luspatercept for the treatment of anemia in myelodysplastic syndromes and primary myelofibrosis

Anemia of lower-risk myelodysplastic syndromes (MDSs) and primary myelofibrosis (PMF) generally becomes resistant to available treatments, leading to red blood cell (RBC) transfusions, iron overload, shortened survival, and poor quality of life. The transforming growth factor-β superfamily, including activins and growth differentiation factors (GDFs), is aberrantly expressed in lower-risk MDSs and PMF. Luspatercept (and sotatercept), ligand traps that particularly inhibit GDF11, lead to RBC transfusion independence in 10% to 50% of lower-risk MDSs resistant to available treatments, and have started to be used in PMF.

Serial transplantation reveals a critical role for endoglin in hematopoietic stem cell quiescence

Transforming growth factor β (TGF-β) is well known for its important function in hematopoietic stem cell (HSC) quiescence. However, the molecular mechanism underlining this function remains obscure. Endoglin (Eng), a type III receptor for the TGF-β superfamily, has been shown to selectively mark long-term HSCs; however, its necessity in adult HSCs is unknown due to embryonic lethality. Using conditional deletion of Eng combined with serial transplantation, we show that this TGF-β receptor is critical to maintain the HSC pool. Transplantation of Eng-deleted whole bone marrow or purified HSCs into lethally irradiated mice results in a profound engraftment defect in tertiary and quaternary recipients. Cell cycle analysis of primary grafts revealed decreased frequency of HSCs in G₀, suggesting that lack of Eng impairs reentry of HSCs to quiescence. Using cytometry by time of flight (CyTOF) to evaluate the activity of signaling pathways in individual HSCs, we find that Eng is required within the Lin^-Sca⁺Kit⁺-CD48^- CD150⁺ fraction for canonical and noncanonical TGF-β signaling, as indicated by decreased phosphorylation of SMAD2/3 and the p38 MAPK-activated protein kinase 2, respectively. These findings support an essential role for Eng in positively modulating TGF-β signaling to ensure maintenance of HSC quiescence.

Iron Regulation: Macrophages in Control

Macrophages are sentinel cells of the innate immune system and have important functions in development, tissue homeostasis, and immunity. These phylogenetically ancient cells also developed a variety of mechanisms to control erythropoiesis and the handling of iron. Red pulp macrophages in the spleen, Kupffer cells in the liver, and central nurse macrophages in the bone marrow ensure a coordinated metabolism of iron to support erythropoiesis. Phagocytosis of senescent red blood cells by macrophages in the spleen and the liver provide a continuous delivery of recycled iron under steady-state conditions and during anemic stress. Central nurse macrophages in the bone marrow utilize this iron and provide a cellular scaffold and niche to promote differentiation of erythroblasts. This review focuses on the role of the distinct macrophage populations that contribute to efficient iron metabolism and highlight important cellular and systemic mechanisms involved in iron-regulating processes.

Bone marrow hematopoietic dysfunction in untreated chronic lymphocytic leukemia patients

The consequences of immune dysfunction in B-chronic lymphocytic leukemia (CLL) likely relate to the incidence of serious recurrent infections and second malignancies that plague CLL patients. The well-described immune abnormalities are not able to consistently explain these complications. Here, we report bone marrow (BM) hematopoietic dysfunction in early and late stage untreated CLL patients. Numbers of CD34⁺ BM hematopoietic progenitors responsive in standard colony-forming unit (CFU) assays, including CFU-GM/GEMM and CFU-E, were significantly reduced. Flow cytometry revealed corresponding reductions in frequencies of all hematopoietic stem and progenitor cell (HSPC) subsets assessed in CLL patient marrow. Consistent with the reduction in HSPCs, BM resident monocytes and natural killer cells were reduced, a deficiency recapitulated in blood. Finally, we report increases in protein levels of the transcriptional regulators HIF-1α, GATA-1, PU.1, and GATA-2 in CLL patient BM, providing molecular insight into the basis of HSPC dysfunction. Importantly, PU.1 and GATA-2 were rapidly increased when healthy HSPCs were exposed in vitro to TNFα, a cytokine constitutively produced by CLL B cells. Together, these findings reveal BM hematopoietic dysfunction in untreated CLL patients that provides new insight into the etiology of the complex immunodeficiency state in CLL.

The Shape Shifting Story of Reticulocyte Maturation

The final steps of erythropoiesis involve unique cellular processes including enucleation and reorganization of membrane proteins and the cytoskeleton to produce biconcave erythrocytes. Surprisingly this process is still poorly understood. In vitro erythropoiesis protocols currently produce reticulocytes rather than biconcave erythrocytes. In addition, immortalized lines and iPSC-derived erythroid cell suffer from low enucleation and suboptimal final maturation potential. In light of the increasing prospect to use in vitro produced erythrocytes as (personalized) transfusion products or as therapeutic delivery agents, the mechanisms driving this last step of erythropoiesis are in dire need of resolving. Here we review the elusive last steps of reticulocyte maturation with an emphasis on protein sorting during the defining steps of reticulocyte formation during enucleation and maturation.