Hematopoietic stem cell mobilization for gene therapy of adult patients with severe β-thalassemia: results of clinical trials using G-CSF or plerixafor in splenectomized and nonsplenectomized subjects

In vivo base editing by a single i.v. vector injection for treatment of hemoglobinopathies

Individuals with β-thalassemia or sickle cell disease and hereditary persistence of fetal hemoglobin (HPFH) possessing 30% fetal hemoglobin (HbF) appear to be symptom free. Here, we used a nonintegrating HDAd5/35++ vector expressing a highly efficient and accurate version of an adenine base editor (ABE8e) to install, in vivo, a -113 A>G HPFH mutation in the γ-globin promoters in healthy CD46/β-YAC mice carrying the human β-globin locus. Our in vivo hematopoietic stem cell (HSC) editing/selection strategy involves only s.c. and i.v. injections and does not require myeloablation and HSC transplantation. In vivo HSC base editing in CD46/β-YAC mice resulted in > 60% -113 A>G conversion, with 30% γ-globin of β-globin expressed in 70% of erythrocytes. Importantly, no off-target editing at sites predicted by CIRCLE-Seq or in silico was detected. Furthermore, no critical alterations in the transcriptome of in vivo edited mice were found by RNA-Seq. In vitro, in HSCs from β-thalassemia and patients with sickle cell disease, transduction with the base editor vector mediated efficient -113 A>G conversion and reactivation of γ-globin expression with subsequent phenotypic correction of erythroid cells. Because our in vivo base editing strategy is safe and technically simple, it has the potential for clinical application in developing countries where hemoglobinopathies are prevalent.

Enhanced HbF reactivation by multiplex mutagenesis of thalassemic CD34+ cells in vitro and in vivo

Thalassemia or sickle cell patients with hereditary persistence of fetal hemoglobin (HbF) have an ameliorated clinical phenotype and, in some cases, can achieve transfusion independence. Inactivation via genome editing of γ-globin developmental suppressors, such as BCL11A or LRF/ZBTB7A, or of their binding sites, have been shown to significantly increase expression of endogenous HbF. To broaden the therapeutic window beyond a single-editing approach, we have explored combinations of cis- and trans-editing targets to enhance HbF reactivation. Multiplex mutagenesis in adult CD34+ cells was well tolerated and did not lead to any detectable defect in the cells' proliferation and differentiation, either in vitro or in vivo. The combination of 1 trans and 1 cis mutation resulted in high editing retention in vivo, coupled with almost pancellular HbF expression in NBSGW mice. The greater in vivo performance of this combination was also recapitulated using a novel helper-dependent adenoviral-CRISPR vector (HD-Ad-dualCRISPR) in CD34+ cells from β-thalassemia patients transplanted to NBSGW mice. A pronounced increase in HbF expression was observed in human red blood cells in mice with established predominant β0/β0-thalassemic hemopoiesis after in vivo injection of the HD-Ad-dualCRISPR vector. Collectively, our data suggest that the combination of cis and trans fetal globin reactivation mutations has the potential to significantly increase HbF both totally and on a per cell basis over single editing and could thus provide significant clinical benefit to patients with severe β-globin phenotype.

Therapy Development by Genome Editing of Hematopoietic Stem Cells

Accessibility of hematopoietic stem cells (HSCs) for the manipulation and repopulation of the blood and immune systems has placed them at the forefront of cell and gene therapy development. Recent advances in genome-editing tools, in particular for clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) and CRISPR/Cas-derived editing systems, have transformed the gene therapy landscape. Their versatility and the ability to edit genomic sequences and facilitate gene disruption, correction or insertion, have broadened the spectrum of potential gene therapy targets and accelerated the development of potential curative therapies for many rare diseases treatable by transplantation or modification of HSCs. Ongoing developments seek to address efficiency and precision of HSC modification, tolerability of treatment and the distribution and affordability of corresponding therapies. Here, we give an overview of recent progress in the field of HSC genome editing as treatment for inherited disorders and summarize the most significant findings from corresponding preclinical and clinical studies. With emphasis on HSC-based therapies, we also discuss technical hurdles that need to be overcome en route to clinical translation of genome editing and indicate advances that may facilitate routine application beyond the most common disorders.

Disease severity impacts plerixafor-mobilized stem cell collection in patients with sickle cell disease

Recent studies suggest that plerixafor mobilization and apheresis in patients with sickle cell disease (SCD) is safe and can allow collection of sufficient CD34+ hematopoietic stem cell (HSC) collection for clinical gene therapy applications. However, the quantities of plerixafor-mobilized CD34+ cells vary between different SCD patients for unknown reasons. Twenty-three participants with SCD underwent plerixafor mobilization followed by apheresis, processing, and HSC enrichment under a phase 1 safety and efficacy study conducted at 2 institutions. Linear regression or Spearman's correlation test was used to assess the relationships between various hematologic and clinical parameters with total CD34+ cells/kg collected. Median CD34+ cells/kg after 2 or fewer mobilization and apheresis cycles was 4.0 × 106 (range, 1.5-12.0). Similar to what is observed generally, CD34+ yield correlated negatively with age (P < .001) and positively with baseline (P = .003) and preapheresis blood CD34+ cells/µL (P < .001), and baseline white blood cell (P = .01) and platelet counts (P = .03). Uniquely for SCD, CD34+ cell yields correlated positively with the number of days hydroxyurea was held (for up to 5 weeks, P = .01) and negatively with markers of disease severity, including hospitalization frequency within the preceding year (P = .01) and the number of medications taken for chronic pain (P = .002). Unique SCD-specific technical challenges in apheresis were also associated with reduced CD34+ cell collection efficiency and purification. Here, we describe factors that impact plerixafor mobilization success in patients with SCD, confirming known factors as described in other populations in addition to reporting previously unknown disease specific factors in patients with SCD. This trial was registered at www.clinicaltrials.gov as #NCT03226691.

Parameters affecting successful stem cell collections for genetic therapies in sickle cell disease

Emerging cellular therapies require the collection of peripheral blood hematopoietic stem cells (HSC) by apheresis for in vitro manipulation to accomplish gene addition or gene editing. These therapies require relatively large numbers of HSCs within a short time frame to generate an efficacious therapeutic product. This review focuses on the principal factors that affect collection outcomes, especially relevant to gene therapy for sickle cell disease.

Transfusion and Cellular Therapy in Pediatric Sickle Cell Disease

Red blood cell (RBC) transfusion is critical in managing acute and chronic complications of sickle cell disease. Alloimmunization and iron overload remain significant complications of transfusion therapy and are minimized with prophylactic Rh and K antigen RBC matching and iron chelation. Matched sibling donor hematopoietic stem cell transplant (HSCT) is a curative therapeutic option. Autologous hematopoietic stem cell (HSC)-based gene therapy has recently shown great promise, for which obtaining sufficient HSCs is essential for success. This article discusses RBC transfusion indications and complications, transfusion support during HSCT, and HSC mobilization and collection for autologous HSCT with gene therapy.

Gene therapies for transfusion dependent β-thalassemia: Current status and critical criteria for success

Thalassemia is one of the most prevalent monogenic diseases usually caused by quantitative defects in the production of β-globin leading to severe anemia. Technological advances in genome sequencing, stem cell selection, viral vector development, transduction and gene editing strategies now allow for efficient exvivo genetic manipulation of human stem cells that can lead to production of hemoglobin, leading to a meaningful clinical benefit in thalassemia patients. In this review, the status of the gene-therapy approaches available for transfusion dependent thalassemia are discussed, along with the critical criteria that affect efficacy and lessons that have been learned from the early phase clinical trials. Salient steps necessary for the clinical development, manufacturing, and regulatory approvals of gene therapies for thalassemia are also highlighted, so that the potential of these therapies can be realized. It is highly anticipated that gene therapies will soon become a treatment option for patients lacking compatible donors for hematopoietic stem cell transplant and will offer an alternative for definitive treatment of β-thalassemia.

Relevance of the CXCR4/CXCR7-CXCL12 axis and its effect in pathophysiological conditions

The impact of the C-X-C receptor (CXCR) 7 and its close co-player CXCR4 in different physiological and pathophysiological processes has been extensively investigated within the last decades. Following activation by their shared ligand C-X-C ligand (CXCL) 12, both chemokine receptors can induce various routes of cell signaling and/or scavenge CXCL12 from the extracellular environment. This contributes to organ development and maintenance of homeostasis. Alterations of the CXCR4/CXCR7-CXCL12 axis have been detected in diseases such as cancer, central nervous system and cardiac disorders, and autoimmune diseases. These alterations include changes of the expression pattern, distribution, or downstream effects. The progression of the diseases can be regulated in preclinical models by the use of various modulators suggesting that this axis serves as a promising therapeutic target. It is therefore of great interest to investigate CXCR4/CXCR7/CXCL12 modulators in clinical development, with several CXCR4 and CXCL12 modulators such as plerixafor, ulocuplumab, balixafortide, and olaptesed pegol having already reached this stage. An overview is presented of the most important diseases whose outcomes can be positively or negatively regulated by the CXCR4/CXCR7-CXCL12 axis and summarizes preclinical and clinical data of modulators of that axis. Contrary to CXCR4 and CXCL12 modulators, CXCR7 modulators have, thus far, not been extensively studied. Therefore, more (pre)clinical investigations are needed.

Clinical significance of chemokine receptor antagonists

Introduction: Chemokine receptors are important therapeutic targets for the treatment of many human diseases. This study will provide an overview of approved chemokine receptor antagonists and promising candidates in advanced clinical trials.Areas covered: We will describe clinical aspects of chemokine receptor antagonists regarding their clinical efficacy, mechanisms of action, and re-purposed applications.Expert opinion: Three chemokine antagonists have been approved: (i) plerixafor is a small-molecule CXCR4 antagonist that mobilizes hematopoietic stem cells; (ii) maraviroc is a small-molecule CCR5 antagonist for anti-HIV treatment; and (iii) mogamulizumab is a monoclonal-antibody CCR4 antagonist for the treatment of mycosis fungoides or Sézary syndrome. Moreover, phase 3 trials are ongoing to evaluate many potent candidates, including CCR5 antagonists (e.g. leronlimab), dual CCR2/CCR5 antagonists (e.g. cenicriviroc), and CXCR4 antagonists (e.g. balixafortide, mavorixafor, motixafortide). The success of chemokine receptor antagonists depends on the selective blockage of disease-relevant chemokine receptors which are indispensable for disease progression. Although clinical translation has been slow, antagonists targeting chemokine receptors with multifaced functions offer the potential to treat a broad spectrum of human diseases.

Mobilized peripheral blood: an updated perspective

Enforced egress of hematopoietic stem cells (HSCs) out of the bone marrow (BM) into the peripheral circulation, termed mobilization, has come a long way since its discovery over four decades ago. Mobilization research continues to be driven by the need to optimize the regimen currently available in the clinic with regard to pharmacokinetic and pharmacodynamic profile, costs, and donor convenience. In this review, we describe the most recent findings in the field and how we anticipate them to affect the development of mobilization strategies in the future. Furthermore, the significance of mobilization beyond HSC collection, i.e. for chemosensitization, conditioning, and gene therapy as well as a means to study the interactions between HSCs and their BM microenvironment, is reviewed. Open questions, controversies, and the potential impact of recent technical progress on mobilization research are also highlighted.

CRISPR/Cas9 gene correction of HbH-CS thalassemia-induced pluripotent stem cells

Haemoglobin (Hb) H-constant spring (CS) alpha thalassaemia (- -/-αCS) is the most common type of nondeletional Hb H disease in southern China. The CRISPR/Cas9-based gene correction of patient-specific induced pluripotent stem cells (iPSCs) and cell transplantation now represent a therapeutic solution for this genetic disease. We designed primers for the target sites using CRISPR/Cas9 to specifically edit the HBA2 gene with an Hb-CS mutation. After applying a correction-specific PCR assay to purify the corrected clones followed by sequencing to confirm the mutation correction, we verified that the purified clones retained full pluripotency and exhibited a normal karyotype. This strategy may be promising in the future, although it is far from representing a solution for the treatment of HbH-CS thalassemia now.

Gene Therapy For Beta-Thalassemia: Updated Perspectives

Allogeneic hematopoietic stem cell transplantation was until very recently, the only permanent curative option available for patients suffering from transfusion-dependent beta-thalassemia. Gene therapy, by autologous transplantation of genetically modified hematopoietic stem cells, currently represents a novel therapeutic promise, after many years of extensive preclinical research for the optimization of gene transfer protocols. Nowadays, clinical trials being held on a worldwide setting, have demonstrated that, by re-establishing effective hemoglobin production, patients may be rendered transfusion- and chelation-independent and evade the immunological complications that normally accompany allogeneic hematopoietic stem cell transplantation. The present review will offer a retrospective scope of the long way paved towards successful implementation of gene therapy for beta-thalassemia, and will pinpoint the latest strategies employed to increase globin expression that extend beyond the classic transgene addition perspective. A thorough search was performed using Pubmed in order to identify studies that provide a proof of principle on the aforementioned topic at a preclinical and clinical level. Inclusion criteria also regarded gene transfer technologies of the past two decades, as well as publications outlining the pitfalls that precluded earlier successful implementation of gene therapy for beta-thalassemia. Overall, after decades of research, that included both successes and pitfalls, the path towards a permanent, donor-irrespective cure for beta-thalassemia patients is steadily becoming a realistic approach.

Successful hematopoietic stem cell mobilization and apheresis collection using plerixafor alone in sickle cell patients

Novel therapies for sickle cell disease (SCD) based on genetically engineered autologous hematopoietic stem and progenitor cells (HSPCs) are critically dependent on a safe and effective strategy for cell procurement. We sought to assess the safety and efficacy of plerixafor when used in transfused patients with SCD for HSC mobilization. Six adult patients with SCD were recruited to receive a single dose of plerixafor, tested at lower than standard (180 µg/kg) and standard (240 µg/kg) doses, followed by CD34⁺ cell monitoring in peripheral blood and apheresis collection. The procedures were safe and well-tolerated. Mobilization was successful, with higher peripheral CD34⁺ cell counts in the standard vs the low-dose group. Among our 6 donors, we improved apheresis cell collection results by using a deep collection interface and starting apheresis within 4 hours after plerixafor administration. In the subjects who received a single standard dose of plerixafor and followed the optimized collection protocol, yields of up to 24.5 × 10⁶ CD34⁺ cells/kg were achieved. Interestingly, the collected CD34⁺ cells were enriched in immunophenotypically defined long-term HSCs and early progenitors. Thus, we demonstrate that plerixafor can be employed safely in patients with SCD to obtain sufficient HSCs for potential use in gene therapy.

Gene Therapy for Hemoglobinopathies

Gene therapy for β-thalassemia and sickle-cell disease is based on transplantation of genetically corrected, autologous hematopoietic stem cells. Preclinical and clinical studies have shown the safety and efficacy of this therapeutic approach, currently based on lentiviral vectors to transfer a β-globin gene under the transcriptional control of regulatory elements of the β-globin locus. Nevertheless, a number of factors are still limiting its efficacy, such as limited stem-cell dose and quality, suboptimal gene transfer efficiency and gene expression levels, and toxicity of myeloablative regimens. In addition, the cost and complexity of the current vector and cell manufacturing clearly limits its application to patients living in less favored countries, where hemoglobinopathies may reach endemic proportions. Gene-editing technology may provide a therapeutic alternative overcoming some of these limitations, though proving its safety and efficacy will most likely require extensive clinical investigation.

Gene Therapy for Beta-Hemoglobinopathies: Milestones, New Therapies and Challenges

Inherited monogenic disorders such as beta-hemoglobinopathies (BH) are fitting candidates for treatment via gene therapy by gene transfer or gene editing. The reported safety and efficacy of lentiviral vectors in preclinical studies have led to the development of several clinical trials for the addition of a functional beta-globin gene. Across trials, dozens of transfusion-dependent patients with sickle cell disease (SCD) and transfusion-dependent beta-thalassemia (TDT) have been treated via gene therapy and have achieved reduced transfusion requirements. While overall results are encouraging, the outcomes appear to be strongly influenced by the level of lentiviral integration in transduced cells after engraftment, as well as the underlying genotype resulting in thalassemia. In addition, the method of procurement of hematopoietic stem cells can affect their quality and thus the outcome of gene therapy both in SCD and TDT. This suggests that new studies aimed at maximizing the number of corrected cells with long-term self-renewal potential are crucial to ensure successful treatment for every patient. Recent advancements in gene transfer and bone marrow transplantation have improved the success of this approach, and the results obtained by using these strategies demonstrated significant improvement of gene transfer outcome in patients. The advent of new gene-editing technologies has suggested additional therapeutic options. These are primarily focused on correcting the defective beta-globin gene or editing the expression of genes or genomic segments that regulate fetal hemoglobin synthesis. In this review, we aim to establish the potential benefits of gene therapy for BH, to summarize the status of the ongoing trials, and to discuss the possible improvement or direction for future treatments.

The Functional Effect of Repeated Cryopreservation on Transduced CD34+ Cells from Patients with Thalassemia

Stable gene marking and effective engraftment of gene-modified CD34⁺ hematopoietic stem cells is a prerequisite for gene therapy success but may be challenged by the inevitable cryopreservation of the final product prior to extensive quality assurance testing. We investigated the β-globin gene transfer potency in fresh and cryopreserved CD34⁺ cells from mobilized patients with β-thalassemia, as well as the qualitative impact of repeated freeze/thaw cycles on the functionality of cultured and unmanipulated CD34⁺ cells in terms of engrafting capacity in a xenotransplantation model, under partial myeloablation. Cells transduced fresh or after one freeze-thaw cycle yielded similar clonogenic and gene transfer frequencies. Repeated cryopreservation cycles did not affect the transduction rates whereas either one or two freeze-thaw cycles of cultured-but not of unmanipulated-cells significantly reduced their clonogenicity. No differences in the engrafting potential of gene-corrected cells subjected to either none or up to two cryopreservation cycles, were encountered post xenotransplantation. Overall, we assessed the gene transfer efficiency, clonogenicity and engrafting capacity of cryopreserved CD34⁺ cells and the impact of repeated freeze/thaw cycles in their performance. These observations may prove essential in the design of gene therapy trials, considerably facilitating their logistics.

Disruption of the BCL11A Erythroid Enhancer Reactivates Fetal Hemoglobin in Erythroid Cells of Patients with β-Thalassemia Major

In the present report, we carried out clinical-scale editing in adult mobilized CD34+ hematopoietic stem and progenitor cells (HSPCs) using zinc-finger nuclease-mediated disruption of BCL11a to upregulate the expression of γ-globin (fetal hemoglobin). In these cells, disruption of the erythroid-specific enhancer of the BCL11A gene increased endogenous γ-globin expression to levels that reached or exceeded those observed following knockout of the BCL11A coding region without negatively affecting survival or in vivo long-term proliferation of edited HSPCs and other lineages. In addition, BCL11A enhancer modification in mobilized CD34+ cells from patients with β-thalassemia major resulted in a readily detectable γ-globin increase with a preferential increase in G-gamma, leading to an improved phenotype and, likely, a survival advantage for maturing erythroid cells after editing. Furthermore, we documented that both normal and β-thalassemia HSPCs not only can be efficiently expanded ex vivo after editing but can also be successfully edited post-expansion, resulting in enhanced early in vivo engraftment compared with unexpanded cells. Overall, this work highlights a novel and effective treatment strategy for correcting the β-thalassemia phenotype by genome editing.

Hemoglobin disorders: lentiviral gene therapy in the starting blocks to enter clinical practice

The β-hemoglobinopathies, transfusion-dependent β-thalassemia and sickle cell disease, are the most prevalent inherited disorders worldwide and affect millions of people. Many of these patients have a shortened life expectancy and suffer from severe morbidity despite supportive therapies, which impose an enormous financial burden to societies. The only available curative therapy is allogeneic hematopoietic stem cell transplantation, although most patients do not have an HLA-matched sibling donor, and those who do still risk life-threatening complications. Therefore, gene therapy by one-time ex vivo modification of hematopoietic stem cells followed by autologous engraftment is an attractive new therapeutic modality. The first proof-of-principle of conversion to transfusion independence by means of a lentiviral vector expressing a marked and anti-sickling β^T87Q-globin gene variant was reported a decade ago in a patient with transfusion-dependent β-thalassemia. In follow-up multicenter Phase II trials with an essentially identical vector (termed LentiGlobin BB305) and protocol, 12 of the 13 patients with a non-β⁰/β⁰ genotype, representing more than half of all transfusion-dependent β-thalassemia cases worldwide, stopped red blood cell transfusions with total hemoglobin levels in blood approaching normal values. Correction of biological markers of dyserythropoiesis was achieved in evaluated patients. In nine patients with β⁰/β⁰ transfusion-dependent β-thalassemia or equivalent severity (β^IVS1-110), median annualized transfusion volume decreased by 73% and red blood cell transfusions were stopped in three patients. Proof-of-principle of therapeutic efficacy in the first patient with sickle cell disease was also reported with LentiGlobin BB305. Encouraging results were presented in children with transfusion-dependent β-thalassemia in another trial with the GLOBE lentiviral vector and several other gene therapy trials are currently open for both transfusion-dependent β-thalassemia and sickle cell disease. Phase III trials are now under way and should help to determine benefit/risk/cost ratios to move gene therapy toward clinical practice.

Plerixafor enables safe, rapid, efficient mobilization of hematopoietic stem cells in sickle cell disease patients after exchange transfusion

Sickle cell disease is characterized by chronic anemia and vaso-occlusive crises, which eventually lead to multi-organ damage and premature death. Hematopoietic stem cell transplantation is the only curative treatment but it is limited by toxicity and poor availability of HLA-compatible donors. A gene therapy approach based on the autologous transplantation of lentiviral-corrected hematopoietic stem and progenitor cells was shown to be efficacious in one patient. However, alterations of the bone marrow environment and properties of the red blood cells hamper the harvesting and immunoselection of patients' stem cells from bone marrow. The use of Filgrastim to mobilize large numbers of hematopoietic stem and progenitor cells into the circulation has been associated with severe adverse events in sickle cell patients. Thus, broader application of the gene therapy approach requires the development of alternative mobilization methods. We set up a phase I/II clinical trial whose primary objective was to assess the safety of a single injection of Plerixafor in sickle cell patients undergoing red blood cell exchange to decrease the hemoglobin S level to below 30%. The secondary objective was to measure the efficiency of mobilization and isolation of hematopoietic stem and progenitor cells. No adverse events were observed. Large numbers of CD34⁺ cells were mobilized extremely quickly. Importantly, the mobilized cells contained high numbers of hematopoietic stem cells, expressed high levels of stemness genes, and engrafted very efficiently in immunodeficient mice. Thus, Plerixafor can be safely used to mobilize hematopoietic stem cells in sickle cell patients; this finding opens up new avenues for treatment approaches based on gene addition and genome editing. Clinicaltrials.gov identifier: NCT02212535.

Safety and efficacy of plerixafor dose escalation for the mobilization of CD34+ hematopoietic progenitor cells in patients with sickle cell disease: interim results

Gene therapy for sickle cell disease is limited by the yield of hematopoietic progenitor cells that can be harvested for transduction or gene editing. We therefore performed a phase I dose-escalation study of the hematopoietic progenitor cell mobilizing agent plerixafor to evaluate the efficacy and safety of standard dosing on peripheral blood CD34⁺ cell mobilization. Of 15 patients enrolled to date, only one was chronically transfused and ten were on hydroxyurea. Of eight patients who achieved a CD34⁺ cell concentration >30 cells/μL, six were on hydroxyurea. There was no clear dose response to increasing plerixafor dosage. There was a low rate of serious adverse events; two patients developed vaso-occlusive crises, at the doses of 80 μg/kg and 240 μg/kg. Hydroxyurea may have contributed to the limited CD34⁺ mobilization by affecting baseline peripheral blood CD34 counts, which correlated strongly with peak peripheral blood CD34 counts. Plerixafor administration did not induce significant increases in the fraction of activated neutrophils, monocytes, or platelets. However, increased neutrophils positive for activated β2 integrin and Mac-1 were associated with serious adverse events. In summary, plerixafor was well tolerated but did not achieve consistent CD34⁺ cell mobilization in this cohort of patients, most of whom were being actively treated with hydroxyurea and only one was chronically transfused. The study will continue with escalation of the dose of plerixafor and modification of hydroxyurea administration. Clinicaltrials.gov identifier: NCT02193191.

Gene Therapy and Genome Editing

The β-thalassemias are inherited blood disorders that result from insufficient production of the β-chain of hemoglobin. More than 200 different mutations have been identified. β-Thalassemia major requires life-long transfusions. The only cure for severe β-thalassemia is to provide patients with hematopoietic stem cells. Globin gene therapy promises a curative autologous stem cell transplantation without the immunologic complications of allogeneic transplantation. The future directions of gene therapy include enhancement of lentiviral vector-based approaches, fine tuning of the conditioning regimen, and the design of safer vectors. Progress in genetic engineering bodes well for finding a cure for severe globin disorders.

Reloadable multidrug capturing delivery system for targeted ischemic disease treatment

Human clinical trials of protein therapy for ischemic diseases have shown disappointing outcomes so far, mainly because of the poor circulatory half-life of growth factors in circulation and their low uptake and retention by the targeted injury site. The attachment of polyethylene glycol (PEG) extends the circulatory half-lives of protein drugs but reduces their extravasation and retention at the target site. To address this issue, we have developed a drug capture system using a mixture of hyaluronic acid (HA) hydrogel and anti-PEG immunoglobulin M antibodies, which, when injected at a target body site, can capture and retain a variety of systemically injected PEGylated therapeutics at that site. Furthermore, repeated systemic injections permit "reloading" of the capture depot, allowing the use of complex multistage therapies. This study demonstrates this capture system in both murine and porcine models of critical limb ischemia. The results show that the reloadable HA/anti-PEG system has the potential to be clinically applied to patients with ischemic diseases, who require sequential administration of protein drugs for optimal outcomes.

Gene Therapy Approaches to Hemoglobinopathies

Gene therapy for hemoglobinopathies is currently based on transplantation of autologous hematopoietic stem cells genetically modified with a lentiviral vector expressing a globin gene under the control of globin transcriptional regulatory elements. Preclinical and early clinical studies showed the safety and potential efficacy of this therapeutic approach as well as the hurdles still limiting its general application. In addition, for both beta-thalassemia and sickle cell disease, an altered bone marrow microenvironment reduces the efficiency of stem cell harvesting as well as engraftment. These hurdles need be addressed for gene therapy for hemoglobinopathies to become a clinical reality.

Granulocyte–Colony Stimulating Factor plus Plerixafor in Patients with β-thalassemia Major Results in the Effective Mobilization of Primitive CD34+ Cells with Specific Gene Expression Profile

Successful gene therapy for β-thalassemia requires optimal numbers of autologous gene-transduced hematopoietic stem and progenitor cells (HSPCs) with high repopulating capacity. Previous studies suggested superior mobilization in these patients by the combination of granulocyte–colony stimulating factor (G-CSF) plus plerixafor over single agents. We mobilized four adult patients using G-CSF+plerixafor to assess the intra-individual variation of the circulating CD34+ cells number and subtypes preand post-plerixafor administration. The procedure was well-tolerated and the target cell dose of ≥8 × 106 CD34+ cells/kg was achieved in three of them with one apheresis procedure. The addition of plerixafor unanimously increased the number of circulating CD34+ cells, and the frequency of the most primitive CD34+ subtypes: CD34+/38− and CD34+/133+/38− as well as the in vitro clonogenic potency. Microarray analyses of CD34+ cells purified from the leukapheresis of one patient mobilized twice, with G-CSF and with G-CSF+plerixafor, highlighted in G-CSF+plerixafor-mobilized CD34+ cells, higher levels of expression genes involved in HSPC motility, homing, and cell cycles. In conclusion, G-CSF+plerixafor in β-thalassemia patients mobilizes optimal numbers of HSPCs with characteristics that suggest high capacity of engraftment after transplantation.

Cyclophosphamide improves engraftment in patients with SCD and severe organ damage who undergo haploidentical PBSCT

Peripheral blood stem cell transplantation (PBSCT) offers a curative option for sickle cell disease (SCD). Although HLA-matched sibling transplantation is promising, the vast majority of patients lack such a donor. We sought to develop a novel nonmyeloablative HLA-haploidentical PBSCT approach that could safely be used for patients with severe organ damage. Based on findings in our preclinical model, we developed a phase 1/2 trial using alemtuzumab, 400 cGy total body irradiation, and escalating doses of posttransplant cyclophosphamide (PT-Cy): 0 mg/kg in cohort 1, 50 mg/kg in cohort 2, and 100 mg/kg in cohort 3. A total of 21 patients with SCD and 2 with β-thalassemia received a transplant. The mean hematopoietic cell transplant-specific comorbidity index of 6 reflected patients with cirrhosis, heart failure, and end-stage renal disease. The engraftment rate improved from 1 (33%) of 3 in cohort 1 to 5 (63%) of 8 in cohort 2 and 10 (83%) of 12 in cohort 3. Percentage of donor myeloid and CD3 chimerism also improved with subsequent cohorts. There was no transplant-related mortality, and overall survival was 87%. At present, 0% in cohort 1, 25% in cohort 2, and 50% in cohort 3 remain free of their disease. There was no grade 2 to 4 acute or extensive chronic graft-versus-host disease (GVHD). Therefore, PT-Cy improves engraftment and successfully prevents severe GVHD after nonmyeloablative conditioning in patients with SCD who are at high risk for early mortality. Additional strategies are necessary to decrease the graft rejection rate and achieve a widely available cure for all patients with SCD. This trial was registered at www.clinicaltrials.gov as #NCT00977691.

Bone Marrow as a Hematopoietic Stem Cell Source for Gene Therapy in Sickle Cell Disease: Evidence from Rhesus and SCD Patients

Steady state bone marrow (BM) is the preferred hematopoietic stem cell (HSC) source for gene therapy in sickle cell disease (SCD) due to the recognized risk of vaso-occlusive crisis during granulocyte colony-stimulating factor mobilization. We previously established clinically relevant HSC gene transfer in the rhesus model following transplantation of mobilized peripheral blood (PB) CD34⁺ cells transduced with lentiviral vectors. In this study, we examined steady state bone marrow (BM) in the rhesus competitive repopulation model and demonstrate similar gene marking in vitro and in vivo, as compared with mobilized PB CD34⁺ cells. We then evaluated PB and steady state BM in subjects with SCD and observed a higher frequency of CD34⁺ cells when compared with controls, likely due to enhanced hematopoiesis. However, CD34⁺ cell counts were reduced in both the PB and BM in patients treated with hydroxyurea, and hydroxyurea treatment strongly inhibited iPS cell generation from SCD subjects. Our data support that steady state BM is a useful HSC source for SCD gene therapy with similar transduction. The lower CD34⁺ percentages observed with hydroxyurea treatment warrants withholding hydroxyurea temporarily prior to harvesting HSCs. Our results are important for the design of gene targeting strategies for SCD.

Poor stem cell harvest may not always be related to poor mobilization: lessons gained from a mobilization study in patients with β-thalassemia major

BACKGROUND

Hematopoietic stem cell mobilization and leukapheresis in adult patients with β-thalassemia have recently been optimized in the context of clinical trials for obtaining hematopoietic stem cells for thalassemia gene therapy. In some patients, however, the yield of cluster of differentiation 34-positive (CD34+) cells was poor despite successful mobilization, and a modification of apheresis settings was mandatory for harvest rescue.

STUDY DESIGN AND METHODS

Data were analyzed from 20 adult patients with β-thalassemia who were enrolled in a clinical trial of optimizing mobilization strategies for stem cell gene therapy. The aim of this post-hoc analysis was to assess how certain hematological and/or clinical parameters may correlate with low collection efficiency in the presence of adequate numbers of circulating stem cells after pharmacological mobilization and standard leukapheresis procedures.

RESULTS

Among 19 patients who achieved optimal mobilization with Plerixafor, four who underwent splenectomy demonstrated disproportionately poor CD34+ cell harvests, as determined by their circulating CD34+ cell counts after mobilization. All four patients who underwent splenectomy presented at baseline and before first apheresis with lymphocytosis resulting in lymphocyte/neutrophil ratios well above 1 and marked reticulocytosis compared with patients who achieved optimal mobilization/CD34+ cell harvest. Such unexpected expansion of specific cell populations disrupted the normal cell layer separation and necessitated modification of the apheresis settings to rescue the harvests.

CONCLUSIONS

By close examination of certain hematological and/or clinical parameters before leukapheresis, patients who, despite adequate mobilization, are at risk for poor CD34+ cell harvests may be identified, and harvest failure can be prevented by adjusting the apheresis settings.

Gene Addition Strategies for β-Thalassemia and Sickle Cell Anemia

Beta-thalassemia and sickle cell anemia are two of the most common diseases related to the hemoglobin protein. In these diseases, the beta-globin gene is mutated, causing severe anemia and ineffective erythropoiesis. Patients can additionally present with a number of life-threatening co-morbidities, such as stroke or spontaneous fractures. Current treatment involves transfusion and iron chelation; allogeneic bone marrow transplant is the only curative option, but is limited by the availability of matching donors and graft-versus-host disease. As these two diseases are monogenic diseases, they make an attractive setting for gene therapy. Gene therapy aims to correct the mutated beta-globin gene or add back a functional copy of beta- or gamma-globin. Initial gene therapy work was done with oncoretroviral vectors, but has since shifted to lentiviral vectors. Currently, there are a few clinical trials underway to test the curative potential of some of these lentiviral vectors. This review will highlight the work done thus far, and present the challenges still facing gene therapy, such as genome toxicity concerns and achieving sufficient transgene expression to cure those with the most severe forms of thalassemia.

Cell and Gene Therapy for the Beta-Thalassemias: Advances and Prospects

The beta-thalassemias are inherited anemias caused by mutations that severely reduce or abolish expression of the beta-globin gene. Like sickle cell disease, a related beta-globin gene disorder, they are ideal candidates for performing a genetic correction in patient hematopoietic stem cells (HSCs). The most advanced approach utilizes complex lentiviral vectors encoding the human β-globin gene, as first reported by May et al. in 2000. Considerable progress toward the clinical implementation of this approach has been made in the past five years, based on effective CD34+ cell mobilization and improved lentiviral vector manufacturing. Four trials have been initiated in the United States and Europe. Of 16 evaluable subjects, 6 have achieved transfusion independence. One of them developed a durable clonal expansion, which regressed after several years without transformation. Although globin lentiviral vectors have so far proven to be safe, this occurrence suggests that powerful insulators with robust enhancer-blocking activity will further enhance this approach. The combined discovery of Bcl11a-mediated γ-globin gene silencing and advances in gene editing are the foundations for another gene therapy approach, which aims to reactivate fetal hemoglobin (HbF) production. Its clinical translation will hinge on the safety and efficiency of gene targeting in true HSCs and the induction of sufficient levels of HbF to achieve transfusion independence. Altogether, the progress achieved over the past 15 years bodes well for finding a genetic cure for severe globin disorders in the next decade.

Customizing the genome as therapy for the β-hemoglobinopathies

Despite nearly complete understanding of the genetics of the β-hemoglobinopathies for several decades, definitive treatment options have lagged behind. Recent developments in technologies for facile manipulation of the genome (zinc finger nucleases, transcription activator-like effector nucleases, or clustered regularly interspaced short palindromic repeats-based nucleases) raise prospects for their clinical application. The use of genome-editing technologies in autologous CD34(+) hematopoietic stem and progenitor cells represents a promising therapeutic avenue for the β-globin disorders. Genetic correction strategies relying on the homology-directed repair pathway may repair genetic defects, whereas genetic disruption strategies relying on the nonhomologous end joining pathway may induce compensatory fetal hemoglobin expression. Harnessing the power of genome editing may usher in a second-generation form of gene therapy for the β-globin disorders.

Hematopoietic peripheral circulating blood stem cells as an independent marker of good transfusion management in patients with β-thalassemia: results from a preliminary study

BACKGROUND

Beyond hemoglobin (Hb) levels and performance status, further surrogate markers of appropriate transfusion management should improve the quality of thalassemia care. We investigated the levels of peripheral circulating CD34+ stem cells as an independent marker of appropriate hematopoietic balance in patients with thalassemia.

STUDY DESIGN AND METHODS

Peripheral circulating CD34+ stem cells, colony-forming unitgranulocyte, erythrocyte, macrophage, magakaryocyte (CF-GEMM), colony-forming unitgranulocyte/macrophage (CFU-GM), and erythroidburst-forming units (BFU-E) were assayed, according to standard procedures. Patients with thalassemia major (TM) and thalassemia intermedia (TI) were tested and compared to healthy controls. Demographic and clinical data were recorded.

RESULTS

Overall, 56 patients with TM (median age, 35 years; range, 13-52 years) and 13 with TI (median age, 44 years; range, 27-67 years) were evaluated. Annual red blood cell (RBC) transfusion requirements ranged from 10 to 65 units in all patients except four nontransfused cases. A significant increase in peripheral circulating stem cells was observed in patients, in comparison with healthy controls. Nontransfused patients showed the mean highest levels of stem cells (CD34, 32.5 ± 14.8/μL; BFU-E, 41.3 ± 22.8/mL; CFU-GM, 19.6 ± 5.6/mL; CFU-GEMM, 9.0 ± 6.1/mL). CD34+ cell count was 6.9 ± 4.5/μL in TM (p = 0.014) and 11.8 ± 14.8/μL (p = 0.051) in TI. Furthermore, only in patients with TI was a significant increase in CFU-GEMM (3.0 ± 4.8 vs. 0.75 ± 2.05/mL, p = 0.0001) observed. At multivariate analysis, peripheral circulating CD34+ stem cells did not correlate with age, sex, smoking habit, number of RBCs units transfused, Hb levels, iron chelation therapy, history of splenectomy, and hypothyroidism.

CONCLUSION

Circulating peripheral CD34 + stem cells are increased in β-thalassemia, in particular in nontransfused patients, compared to healthy controls.

No evidence for cell activation or brain vaso-occlusion with plerixafor mobilization in sickle cell mice

Gene therapy for sickle cell disease is currently in active trials. Collecting hematopoietic progenitor cells safely and effectively is challenging, however, because granulocyte colony stimulating factor, the drug used most commonly for mobilization, can cause life-threatening vaso-occlusion in patients with sickle cell disease, and bone marrow harvest requires general anesthesia and multiple hip bone punctures. Plerixafor is an inhibitor of the CXCR4 chemokine receptor on hematopoietic progenitor cells, blocking its binding to SDF-1 (CXCL12) on bone marrow stroma. In support of a clinical trial in patients with sickle cell disease of plerixafor mobilization (NCT02193191), we administered plerixafor to sickle cell mice and found that it mobilizes hematopoietic progenitor cells without evidence of concomitant cell activation or brain vaso-occlusion.

Superior long-term repopulating capacity of G-CSF+plerixafor-mobilized blood: implications for stem cell gene therapy by studies in the Hbb(th-3) mouse model

High numbers of genetically modified hematopoietic stem cells (HSCs) equipped with enhanced engrafting potential are required for successful stem cell gene therapy. By using thalassemia as a model, we investigated the functional properties of hematopoietic stem and progenitor cells (HSPCs) from Hbb(th3)/45.2(+) mice after mobilization with G-CSF, plerixafor, or G-CSF+plerixafor and the engraftment kinetics of primed cells after competitive primary and noncompetitive secondary transplantation. G-CSF+plerixafor yielded the highest numbers of HSPCs, while G-CSF+plerixafor-mobilized Hbb(th3)/45.2(+) cells, either unmanipulated or transduced with a reporter vector, achieved faster hematologic reconstitution and higher levels of donor chimerism over all other types of mobilized cells, after competitive transplantation to B6.BoyJ/45.1(+) recipients. The engraftment benefit observed in the G-CSF+plerixafor group was attributed to the more primitive stem cell phenotype of G-CSF+plerixafor-LSK cells, characterized by higher CD150(+)/CD48 expression. Moreover, secondary G-CSF+plerixafor recipients displayed stable or even higher chimerism levels as compared with primary engrafted mice, thus maintaining or further improving engraftment levels over G-CSF- or plerixafor-secondary recipients. Plerixafor-primed cells displayed the lowest competiveness over all other mobilized cells after primary or secondary transplantation, probably because of the higher frequency of more actively proliferating LK cells. Overall, the higher HSC yields, the faster hematological recovery, and the superiority in long-term engraftment indicate G-CSF+plerixafor-mobilized blood as an optimal graft source, not only for thalassemia gene therapy, but also for stem cell gene therapy applications in general.

Plerixafor+G-CSF-mobilized CD34+ cells represent an optimal graft source for thalassemia gene therapy

Globin gene therapy requires abundant numbers of highly engraftable, autologous hematopoietic stem cells expressing curative levels of β-globin on differentiation. In this study, CD34+ cells from 31 thalassemic patients mobilized with hydroxyurea+granulocyte colony-stimulating factor (G-CSF), G-CSF, Plerixafor, or Plerixafor+G-CSF were transduced with the TNS9.3.55 β-globin lentivector and compared for transducibility and globin expression in vitro, as well as engraftment potential in a xenogeneic model after partial myeloablation. Transduction efficiency and vector copy number (VCN) averaged 48.4 ± 2.8% and 1.91 ± 0.04, respectively, whereas expression approximated the one-copy normal β-globin output. Plerixafor+G-CSF cells produced the highest β-globin expression/VCN. Long-term multilineage engraftment and persistent VCN and vector expression was encountered in all xenografted groups, with Plerixafor+G-CSF-mobilized cells achieving superior short-term engraftment rates, with similar numbers of CD34+ cells transplanted. Overall, Plerixafor+G-CSF not only allows high CD34+ cell yields but also provides increased β-globin expression/VCN and enhanced early human chimerism under nonmyeloablative conditions, thus representing an optimal graft for thalassemia gene therapy.

Current Status and Developments in Gene Therapy for Thalassemia and Sickle Cell Disease

β-thalassemias and sickle cell anemia (SCA) are the most common monogenic diseases worldwide for which curative treatments remain a desired goal. Allogeneic hematopoietic stem cell transplantation (allo-HCT), - the only curative treatment currently available for hemoglobinopaties-, has a narrow application window whereas it incurs several immunological risks. Gene therapy (GT), that is the autologous transplantation of genetically modified hematopoietic stem cells (CD34+), represents a promising new therapeutic strategy which is anticipated to reestablish effective hemoglobin production and render patients transfusion- and drug- independent without the immunological complications that normally accompany allo-HCT. Prior to the application of GT for hemoglobinopathies in the clinic, many years of extensive preclinical research were spent for the optimization of the gene transfer tools and conditions. To date, three GT clinical trials for β-thalassemia and sickle cell disease (SCD) have been conducted or are in progress and 3 cases of transfusion independence in thalassemic β0/βΕ patients have been reported. In the present review, the prerequisites for successful implementation of GT, the tough pathway of GT for hemoglobinopathies towards the clinic and the knowledge gained from the first clinical trials as well as the remaining questions and challenges, will be discussed. Overall, after decades of research including achievements but pitfalls as well, the path to GT of human patients with hemoglobinopathies is currently open and highly promising...

Gene therapy for hemoglobinopathies: the state of the field and the future

After nearly two decades of struggle, gene therapy for hemoglobinopathies using vectors carrying β or γ-globin gene has finally reached the clinical doorsteps. This was made possible by advances made in our understanding of critical regulatory elements required for high level of globin gene expression and improved gene transfer vectors and methodologies. Development of gene editing technologies and reprogramming somatic cells for regenerative medicine holds the promise of genetic correction of hemoglobinopathies in the future. This article will review the state of the field and the upcoming technologies that will allow genetic therapeutic correction of hemoglobinopathies.

Recent trends for novel options in experimental biological therapy of β-thalassemia

INTRODUCTION

β-thalassemias are caused by nearly 300 mutations of the β-globin gene, leading to low or absent production of adult hemoglobin. Achievements have been recently obtained on innovative therapeutic strategies for β-thalassemias, based on studies focusing on the transcriptional regulation of the γ-globin genes, epigenetic mechanisms governing erythroid differentiation, gene therapy and genetic correction of the mutations.

AREAS COVERED

The objective of this review is to describe recently published approaches (the review covers the years 2011 - 2014) useful for the development of novel therapeutic strategies for the treatment of β-thalassemia.

EXPERT OPINION

Modification of β-globin gene expression in β-thalassemia cells was achieved by gene therapy (eventually in combination with induction of fetal hemoglobin [HbF]) and correction of the mutated β-globin gene. Based on recent areas of progress in understanding the control of γ-globin gene expression, novel strategies for inducing HbF have been proposed. Furthermore, the identification of microRNAs involved in erythroid differentiation and HbF production opens novel options for developing therapeutic approaches for β-thalassemia and sickle-cell anemia.

Hematopoietic stem cell mobilization for gene therapy: superior mobilization by the combination of granulocyte-colony stimulating factor plus plerixafor in patients with β-thalassemia major

Successful stem cell gene therapy requires high numbers of genetically engineered hematopoietic stem cells collected using optimal mobilization strategies. Here we focus on stem cell mobilization strategies for thalassemia and present the results of a plerixafor-based mobilization trial with emphasis on the remobilization with granulocyte-colony stimulating factor (G-CSF)+plerixafor in those patients who had previously failed mobilization. Plerixafor rapidly mobilized CD34(+) cells without inducing hyperleukocytosis; however, 35% of patients failed to reach the target cell dose of ≥6×10(6) CD34(+) cells/kg. Four subjects who failed on either plerixafor or G-CSF were remobilized with G-CSF+plerixafor. The combination proved highly synergistic; the target cell dose was readily reached and the per-apheresis yield was significantly increased over initial mobilization, ultimately resulting in single-apheresis collections, despite a more than 50% reduction of the dose of G-CSF in splenectomized patients to avoid hyperleukocytosis. The total stem and progenitor cells mobilized in G-CSF+plerixafor patients were higher than in patients treated by plerixafor alone. Importantly, the G-CSF+plerixafor-mobilized cells displayed a primitive stem cell phenotype and higher clonogenic capacity over plerixafor-mobilized cells. G-CSF+plerixafor represents the optimal strategy when very high yields of stem cells or a single apheresis is required. The high yields and the favorable transplantation features render the G-CSF+plerixafor-mobilized cells the optimal CD34(+) cell source for stem cell gene therapy applications.

Targeting the molecular and cellular interactions of the bone marrow niche in immunologic disease

Recent investigations have expanded our knowledge of the regulatory bone marrow (BM) niche, which is critical in maintaining and directing hematopoietic stem cell (HSC) self-renewal and differentiation. Osteoblasts, mesenchymal stem cells (MSCs), and CXCL12-abundant reticular (CAR) cells are niche components in close association with HSCs and have been more clearly defined in immune cell function and homeostasis. Importantly, cellular inhabitants of the BM niche signal through G protein-coupled surface receptors (GPCRs) for various appropriate immune functions. In this article, recent literature on BM niche inhabitants (HSCs, osteoblasts, MSCs, CAR cells) and their GPCR mechanistic interactions are reviewed for better understanding of the BM cells involved in immune development, immunologic disease, and current immune reconstitution therapies.

Safe mobilization of CD34+ cells in adults with β-thalassemia and validation of effective globin gene transfer for clinical investigation

We conducted a pilot trial to investigate the safety and effectiveness of mobilizing CD34(+) hematopoietic progenitor cells (HPCs) in adults with β-thalassemia major. We further assessed whether thalassemia patient CD34(+) HPCs could be transduced with a globin lentiviral vector under clinical conditions at levels sufficient for therapeutic implementation. All patients tolerated granulocyte colony-stimulating factor well with minimal side effects. All cell collections exceeded 8 × 10(6) CD34(+) cells/kg. Using clinical grade TNS9.3.55 vector, we demonstrated globin gene transfer averaging 0.53 in 3 validation runs performed under current good manufacturing practice conditions. Normalized to vector copy, the vector-encoded β-chain was expressed at a level approximating normal hemizygous protein output. Importantly, stable vector copy number (0.2-0.6) and undiminished vector expression were obtained in NSG mice 6 months posttransplant. Thus, we validated a safe and effective procedure for β-globin gene transfer in thalassemia patient CD34(+) HPCs, which we will implement in the first US trial in patients with severe inherited globin disorders. This trial is registered at www.clinicaltrials.gov as #NCT01639690.

Current clinical indications for plerixafor

Autologous and allogeneic hematopoietic stem cell (HSC) transplantation are considered the standard of care for many malignancies including lymphoma, multiple myeloma, and some leukemias. In many cases, mobilized peripheral blood has become the preferred source for HSCs. Plerixafor, an inhibitor of the interaction between CX chemokine receptor 4 (CXCR4) and stromal derived factor-1 alpha (SDF-1), has been evaluated in clinical trials and approved by the FDA and EMA. This agent has very modest toxicity and appears to be quite potent at HSC mobilization. Current clinical indications for the use of plerixafor are the subject of this review.

Regulatory systems in bone marrow for hematopoietic stem/progenitor cells mobilization and homing

Regulation of hematopoietic stem cell release, migration, and homing from the bone marrow (BM) and of the mobilization pathway involves a complex interaction among adhesion molecules, cytokines, proteolytic enzymes, stromal cells, and hematopoietic cells. The identification of new mechanisms that regulate the trafficking of hematopoietic stem/progenitor cells (HSPCs) cells has important implications, not only for hematopoietic transplantation but also for cell therapies in regenerative medicine for patients with acute myocardial infarction, spinal cord injury, and stroke, among others. This paper reviews the regulation mechanisms underlying the homing and mobilization of BM hematopoietic stem/progenitor cells, investigating the following issues: (a) the role of different factors, such as stromal cell derived factor-1 (SDF-1), granulocyte colony-stimulating factor (G-CSF), and vascular cell adhesion molecule-1 (VCAM-1), among other ligands; (b) the stem cell count in peripheral blood and BM and influential factors; (c) the therapeutic utilization of this phenomenon in lesions in different tissues, examining the agents involved in HSPCs mobilization, such as the different forms of G-CSF, plerixafor, and natalizumab; and (d) the effects of this mobilization on BM-derived stem/progenitor cells in clinical trials of patients with different diseases.

Differential effects of the type of iron chelator on the absolute number of hematopoietic peripheral progenitors in patients with β-thalassemia major

Several studies have established an association between iron chelation therapy with deferasirox and hematopoietic improvement in patients with myelodysplastic syndromes. There are no data from patients with β-thalassemia major. In a cross-sectional study, we evaluated the absolute number of several hematopoietic peripheral progenitors (colony-forming unit-granulocyte/macrophage, erythroid burst-forming units, colony-forming unit-granulocyte/erythrocyte/macrophage/megakaryocyte, and long-term culture-initiating cells) in 30 patients with β-thalassemia major (median age 29.5 years, 40% males) and 12 age-matched controls. For the β-thalassemia major patients, data on splenectomy status, the type of iron chelator used, and serum ferritin levels reflecting changes in iron status on the chelator were also retrieved. All patients had to be using the same iron chelator for at least 6 months with >80% compliance. The absolute number of all hematopoietic peripheral progenitors was higher in β-thalassemia major patients than in controls, and varied between splenectomized and non-splenectomized patients (lower number of erythroid burst-forming units and higher numbers of colony-forming unit-granulocyte/macrophage, colony-forming unit-granulocyte/erythrocyte/macrophage/megakaryocyte, and long-term culture-initiating cells). The number of erythroid burst-forming units was significantly higher in patients taking deferasirox (n=10) than in those taking either deferoxamine (n=10) or deferiprone (n=10) (P<0.05). After adjusting for age, sex, splenectomy status, and serum ferritin changes, the association between a higher absolute number of erythroid burst-forming units in deferasirox-treated patients than in patients taking deferoxamine or deferiprone remained statistically significant (P=0.011). In conclusion, in β-thalassemia major patients, compared with other iron chelators, deferasirox therapy is associated with higher levels of circulating erythroid burst-forming units. This variation is independent of iron status changes and is more likely to be due to the type of chelator.

Development of gene therapy for thalassemia

Retroviral vector-mediated gene transfer into hematopoietic stem cells provides a potentially curative therapy for severe β-thalassemia. Lentiviral vectors based on human immunodeficiency virus have been developed for this purpose and have been shown to be effective in curing thalassemia in mouse models. One participant in an ongoing clinical trial has achieved transfusion independence after gene transfer into bone marrow stem cells owing, in part, to a genetically modified, dominant clone. Ongoing efforts are focused on improving the efficiency of lentiviral vector-mediated gene transfer into stem cells so that the curative potential of gene transfer can be consistently achieved.