Erythroid cells in vitro: from developmental biology to blood transfusion products. Curr Opin Hematol. 2009;16:259-268. [PMID: 19444099 DOI: 10.1097/moh.0b013e32832bcaa2]

Generation of Red Blood Cells from Human Pluripotent Stem Cells-An Update

Red blood cell (RBC) transfusion is a lifesaving medical procedure that can treat patients with anemia and hemoglobin disorders. However, the shortage of blood supply and risks of transfusion-transmitted infection and immune incompatibility present a challenge for transfusion. The in vitro generation of RBCs or erythrocytes holds great promise for transfusion medicine and novel cell-based therapies. While hematopoietic stem cells and progenitors derived from peripheral blood, cord blood, and bone marrow can give rise to erythrocytes, the use of human pluripotent stem cells (hPSCs) has also provided an important opportunity to obtain erythrocytes. These hPSCs include both human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). As hESCs carry ethical and political controversies, hiPSCs can be a more universal source for RBC generation. In this review, we first discuss the key concepts and mechanisms of erythropoiesis. Thereafter, we summarize different methodologies to differentiate hPSCs into erythrocytes with an emphasis on the key features of human definitive erythroid lineage cells. Finally, we address the current limitations and future directions of clinical applications using hiPSC-derived erythrocytes.

The Future of Red Cell Transfusion Lies in Cultured Red Cells

Blood is a very important resource for healthcare-based services and there has been a consistently increasing demand for it in most parts of the world. Poor volunteer-based collection system, high-risk of transfusion-transmitted infections, and emergence of new pathogens as evident from the ongoing Coronavirus Disease 2019 (COVID-19) pandemic are potential challenges to the global healthcare systems. It is imperative to explore safe and reliable alternatives to red cell transfusions. Ex vivo culture of red cells (cRBCs) from different sources such as hematopoietic stem cells (HSCs), pluripotent stem cells, and immortalized progenitors (e.g., BELA-2 cells) could revolutionize transfusion medicine. cRBC could be of great diagnostic and therapeutic utility. It may provide a backup in times of acute shortages in patients with rare blood groups, and in cases with multiple antibodies or sickle cell anemia. The CRISP-Cas9 system has been used to develop personalized, multi-compatible RBCs for diagnostic reagents and patients with multiple allo-antibodies. cRBC could be practically feasible for pediatric patients, who require small quantities of red cell transfusions. cRBC produced under good manufacturing practice (GMP) conditions has been reported to survive in human blood circulation for more than 26 days. Recently, a phase I randomized controlled clinical trial called RESTORE was initiated to assess the survival and recovery of cRBCs. However, feasible technological advancement is required to produce enough cRBCs for clinical use. It is crucial to identify sustainable sources for large-scale production of clinically useful cRBCs. Although the potential cost of one unit of cRBC is extrapolated to be around US$ 8000, it is a life-saving product for patients having rare blood groups and is a “ready to use” source of phenotype-matched, homogenous young red cells in emergency situations.

Towards manufactured red blood cells for the treatment of inherited anemia

Patients with inherited anemia and hemoglobinopathies (such as sickle cell disease and β-thalassemia) are treated with red blood cell (RBC) transfusions to alleviate their symptoms. Some of these patients may have rare blood group types or go on to develop alloimmune reactions, which can make it difficult to source compatible blood in the donor population. Laboratory-grown RBC represent a particularly attractive alternative which could satisfy an unmet clinical need. The challenge, however, is to produce - from a limited number of stem cells - the 2x1012 RBC required for a standard adult therapeutic dose. Encouraging progress has been made in RBC production from adult stem cells under good manufacturing practice. In 2011, the Douay group conducted a successful proof-of-principle mini-transfusion of autologous manufactured RBC in a single volunteer. In the UK, a trial is planned to assess whether manufactured RBC are equivalent to RBC produced naturally in donors, by testing an allogeneic mini-dose of laboratory-grown manufactured RBC in multiple volunteers. This review discusses recent progress in the erythroid culture field as well as opportunities for further scaling up of manufactured RBC production for transfusion practice.

Trends and targets of various types of stem cell derived transfusable RBC substitution therapy: Obstacles that need to be converted to opportunity

A shortage of blood during the pandemic outbreak of COVID-19 is a typical example in which the maintenance of a safe and adequate blood supply becomes difficult and highly demanding. So far, human RBCs have been produced in vitro using diverse sources: hematopoietic stem cells (SCs), embryonic SCs and induced pluripotent SCs. The existing, even safest core of conventional cellular bioproducts destined for transfusion have some shortcoming in respects to: donor -dependency variability in terms of hematological /immunological and process/ storage period issues. SCs-derived transfusable RBC bioproducts, as one blood group type for all, were highly complex to work out. Moreover, the strategies for their successful production are often dependent upon the right selection of starting source materials and the composition and the stability of the right expansion media and the strict compliance to GMP regulatory processes. In this mini-review we highlight some model studies, which showed that the efficiency and the functionality of RBCs that could be produced by the various types of SCs, in relation to the in-vitro culture procedures are such that they may, potentially, be used at an industrial level. However, all cultured products do not have an unlimited life due to the critical metabolic pathways or the metabolites produced. New bioreactors are needed to remove these shortcomings and the development of a new mouse model is required. Modern clinical trials based on the employment of regenerative medicine approaches in combination with novel large-scale bioengineering tools, could overcome the current obstacles in artificial RBC substitution, possibly allowing an efficient RBC industrial production.

[The induction and cryopreservation of erythroid progenitor cells derived from umbilical cord blood mononuclear cells]

OBJECTIVE

To discover the techniques for ex vivo generation and cryopreservation of erythroid progenitor cells (EPCs)derived from umbilical cord blood (UCB)mononuclear cells (MNCs).

METHODS

UCB was chosen as the source of EPCs. Erythrocytes were precipitated by hydroxyethyl starch (HES). MNCs were separated by Ficoll density gradient centrifugation. Erythroid progenitor cell were generated from MNC ex vivo in suspension culture supplemented with stem cell growth factor, insulin growth factor, erythropoietin, Fms- liketyrosinekinase ligand, transferrin and dexamethasone. Cell maturation was evaluated by morphologic analysis and CD71/CD235a expression profiling. In vitro induced cells were cryopreserved using different cryopreservation media. The cell survival rate, phenotype and proliferation curves were detected after cell thawing.

RESULTS

With the extension of culture time, the total number of cells increased significantly accompanied with the elevation of CD71 and CD235 positive populations. After 14- day inducing, the cells reached to approximately 110 times of the starting number with the cell viability as (88.92±0.95)%. The percentages of cell surface markers were (86.77±9.11)% for CD71 and (64.47±16.67)% for CD71/CD235, respectively. With the extension of inducing time, wright- Giemsa staining showed that the middle erythroblasts appeared mostly at day 10, and the late erythroblasts were seen at day 14. The red pellets were present at day 14, which indicated the more production of hemoglobin. Colony forming assay showed that erythroid colonies at induction day 7 were higher than that for non-induced cells (326.00±97.96vs 61.60±20.03 per 2 000 cells). With the extension of culture time, the number of erythroid colonies decreased. Induced EPCs were preserved with different cryopreservation solutions, in which 10% DMSO were better than 5% DMSO. Additionally, 10% DMSO + 2% HSA showed no different with 10% DMSO + 5% HSA. Combined 50% plasma with 2% HSA was more effective.

CONCLUSIONS

This non- serum culture media could effectively induced and expanded EPCs, and 10% DMSO + 2% HSA + 50% plasma appeared to be a desirable cryopreservation solution for EPCs from UCB.

Dynamic changes in replication timing and gene expression during lineage specification of human pluripotent stem cells

Duplication of the genome in mammalian cells occurs in a defined temporal order referred to as its replication-timing (RT) program. RT changes dynamically during development, regulated in units of 400-800 kb referred to as replication domains (RDs). Changes in RT are generally coordinated with transcriptional competence and changes in subnuclear position. We generated genome-wide RT profiles for 26 distinct human cell types, including embryonic stem cell (hESC)-derived, primary cells and established cell lines representing intermediate stages of endoderm, mesoderm, ectoderm, and neural crest (NC) development. We identified clusters of RDs that replicate at unique times in each stage (RT signatures) and confirmed global consolidation of the genome into larger synchronously replicating segments during differentiation. Surprisingly, transcriptome data revealed that the well-accepted correlation between early replication and transcriptional activity was restricted to RT-constitutive genes, whereas two-thirds of the genes that switched RT during differentiation were strongly expressed when late replicating in one or more cell types. Closer inspection revealed that transcription of this class of genes was frequently restricted to the lineage in which the RT switch occurred, but was induced prior to a late-to-early RT switch and/or down-regulated after an early-to-late RT switch. Analysis of transcriptional regulatory networks showed that this class of genes contains strong regulators of genes that were only expressed when early replicating. These results provide intriguing new insight into the complex relationship between transcription and RT regulation during human development.

Anemia and Transfusion in Critical Care

Objective:

The objective of this report is to review the physiology and management of anemia in critical care. Selected publications on physiology and transfusion related to anemia and critical care, including the modern randomized trials of conservative versus liberal transfusion policy, were used. Anemia is compensated and tolerated in most critically ill patients as long as oxygen delivery is at least twice oxygen consumption. There are risks to blood transfusion which can be minimized by blood banking practice. The availability of cultured red cells may allow correction of anemia without significant risk. The benefit of transfusion in anemia must be weighted against the risk in any specific patient.

Conclusion and Recommendation:

In a criticially ill patient, anemia should be managed to avoid oxygen supply dependency (oxygen delivery less than twice comsumption) and to maintain moderate oxygen delivery reserve (DO2/VO2 > 3).

Advances in cellular technology in the hematology field: What have we learned so far?

Despite the advances in the hematology field, blood transfusion-related iatrogenesis is still a major issue to be considered during such procedures due to blood antigenic incompatibility. This places pluripotent stem cells as a possible ally in the production of more suitable blood products. The present review article aims to provide a comprehensive summary of the state-of-the-art concerning the differentiation of both embryonic stem cells and induced pluripotent stem cells to hematopoietic cell lines. Here, we review the most recently published protocols to achieve the production of blood cells for future application in hemotherapy, cancer therapy and basic research.

Dexamethasone targeted directly to macrophages induces macrophage niches that promote erythroid expansion

Cultures of human CD34(pos) cells stimulated with erythroid growth factors plus dexamethasone, a model for stress erythropoiesis, generate numerous erythroid cells plus a few macrophages (approx. 3%; 3:1 positive and negative for CD169). Interactions occurring between erythroblasts and macrophages in these cultures and the biological effects associated with these interactions were documented by live phase-contrast videomicroscopy. Macrophages expressed high motility interacting with hundreds/thousands of erythroblasts per hour. CD169(pos) macrophages established multiple rapid 'loose' interactions with proerythroblasts leading to formation of transient erythroblastic island-like structures. By contrast, CD169(neg) macrophages established 'tight' interactions with mature erythroblasts and phagocytosed these cells. 'Loose' interactions of CD169(pos) macrophages were associated with proerythroblast cytokinesis (the M phase of the cell cycle) suggesting that these interactions may promote proerythroblast duplication. This hypothesis was tested by experiments that showed that as few as 103 macrophages significantly increased levels of 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide incorporation frequency in S/G2/M and cytokinesis expressed by proerythroblasts over 24 h of culture. These effects were observed also when macrophages were co-cultured with dexamethasone directly conjugated to a macrophage-specific CD163 antibody. In conclusion, in addition to promoting proerythroblast proliferation directly, dexamethasone stimulates expansion of these cells indirectly by stimulating maturation and cytokinesis supporting activity of macrophages.

Early exposure of murine embryonic stem cells to hematopoietic cytokines differentially directs definitive erythropoiesis and cardiomyogenesis in alginate hydrogel three-dimensional cultures

HepG2-conditioned medium (CM) facilitates early differentiation of murine embryonic stem cells (mESCs) into hematopoietic cells in two-dimensional cultures through formation of embryoid-like colonies (ELCs), bypassing embryoid body (EB) formation. We now demonstrate that three-dimensional (3D) cultures of alginate-encapsulated mESCs cultured in a rotating wall vessel bioreactor can be differentially driven toward definitive erythropoiesis and cardiomyogenesis in the absence of ELC formation. Three groups were evaluated: mESCs in maintenance medium with leukemia inhibitory factor (LIF, control) and mESCs cultured with HepG2 CM (CM1 and CM2). Control and CM1 groups were cultivated for 8 days in early differentiation medium with murine stem cell factor (mSCF) followed by 10 days in hematopoietic differentiation medium (HDM) containing human erythropoietin, m-interleukin (mIL)-3, and mSCF. CM2 cells were cultured for 18 days in HDM, bypassing early differentiation. In CM1, a fivefold expansion of hematopoietic colonies was observed at day 14, with enhancement of erythroid progenitors, hematopoietic genes (Gata-2 and SCL), erythroid genes (EKLF and β-major globin), and proteins (Gata-1 and β-globin), although ζ-globin was not expressed. In contrast, CM2 primarily produced beating colonies in standard hematopoietic colony assay and expressed early cardiomyogenic markers, anti-sarcomeric α-actinin and Gata-4. In conclusion, a scalable, automatable, integrated, 3D bioprocess for the differentiation of mESC toward definitive erythroblasts has been established. Interestingly, cardiomyogenesis was also directed in a specific protocol with HepG2 CM and hematopoietic cytokines making this platform a useful tool for the study of erythroid and cardiomyogenic development.

Multicolor staining of globin subtypes reveals impaired globin switching during erythropoiesis in human pluripotent stem cells

Adult hemoglobin composed of α- and β-globin reflects a change from expression of embryonic ε- and fetal γ-globin to adult β-globin in human erythroid cells, so-called globin switching. Human pluripotent stem cells (hPSCs) are a potential source for in vitro erythrocyte production, but they show prominent expression of γ-globin with little β-globin expression, which indicates incomplete globin switching. To examine the mechanism of this impaired globin switching, we optimized multicolor flow cytometry to simultaneously follow expression of different globin subtypes using different immunofluorescent probes. This enabled us to detect upregulation of β-globin and the corresponding silencing of γ-globin at the single-cell level during cord blood CD34(+) cell-derived erythropoiesis, examined as an endogenous control. Using this approach, we initially characterized the heterogeneous β-globin expression in erythroblasts from several hPSC clones and confirmed the predominant expression of γ-globin. These hPSC-derived erythroid cells also displayed reduced expression of BCL11A-L. However, doxycycline-induced overexpression of BCL11A-L in selected hPSCs promoted γ-globin silencing. These results strongly suggest that impaired γ-globin silencing is associated with downregulated BCL11A-L in hPSC-derived erythroblasts and that multicolor staining of globin subtypes is an effective approach to studying globin switching in vitro.

Concise review: stem cell-based approaches to red blood cell production for transfusion

Blood transfusion is a common procedure in modern medicine, and it is practiced throughout the world; however, many countries report a less than sufficient blood supply. Even in developed countries where the supply is currently adequate, projected demographics predict an insufficient supply as early as 2050. The blood supply is also strained during occasional widespread disasters and crises. Transfusion of blood components such as red blood cells (RBCs), platelets, or neutrophils is increasingly used from the same blood unit for multiple purposes and to reduce alloimmune responses. Even for RBCs and platelets lacking nuclei and many antigenic cell-surface molecules, alloimmunity could occur, especially in patients with chronic transfusion requirements. Once alloimmunization occurs, such patients require RBCs from donors with a different blood group antigen combination, making it a challenge to find donors after every successive episode of alloimmunization. Alternative blood substitutes such as synthetic oxygen carriers have so far proven unsuccessful. In this review, we focus on current research and technologies that permit RBC production ex vivo from hematopoietic stem cells, pluripotent stem cells, and immortalized erythroid precursors.

Erythropoietin-stimulated endothelial cells support erythroid cell differentiation of CD34(+) haematopoietic progenitors

BACKGROUND AND OBJECTIVES

Endothelial cells provide a unique medium for the proliferation and white lineage differentiation of haematopoietic progenitor cells (HPC). Whether this quality can be exploited to facilitate the differentiation of erythroid precursors is not yet known.

MATERIALS AND METHODS

Haematopoietic progenitor cells derived from cord blood were cultured for 3 weeks in erythropoietin-stimulated supernatants with (n = 6) or without cyclosporine A (CSA, n = 6). Cell count, phenotype and morphology were assessed on a weekly basis, and the haemoglobin content was analysed. These cultures were compared with erythroid differentiation induced by cytokines only (n = 6).

RESULTS

Endothelial supernatants combined with CSA led to equivalent numbers of CD71(+) erythroblasts after 1 week as cytokines only. The purity of glycophorin-positive, CD45-negative cells was higher in cells generated in endothelial supernatants than in cytokine-based media. Additional prostaglandin E2 induced a change from fetal to adult haemoglobin.

CONCLUSION

For the generation of erythroblasts from HPC, endothelial supernatants are a simple and cost-effective alternative to culture conditions based on cytokines.

Stem cell-derived erythrocytes as upcoming players in blood transfusion

BACKGROUND

Blood transfusion is current standard-of-care for genetic forms of anemia that would be otherwise lethal and allows implementation of aggressive cytotoxic/surgical therapies developed for numerous types of cancer. In developed countries the blood supply is adequate and sporadically even in excess. However, difficulties exist in finding blood with rare phenotypes to treat alloimmunized patients and the progressive ageing of the human population predicts that blood will become scarce by 2050. These considerations establish the need for the development of techniques to generate cultured red blood cell (cRBCs) as transfusion products.

MATERIALS AND METHODS

Recent progress in cell culture techniques is revolutionizing organ replacement therapies. Two new disciplines, cell therapy and tissue engineering, have been developed to generate in vitro therapeutic products for a variety of applications ranging from skin grafts to organ-function repairs. It is currently believed that these advances will eventually allow ex-vivo production of various cell types in numbers so great that, in the case of red cells, would be clinically adequate for transfusion.

RESULTS

Proof-of-principle in animal models indicate that cRBCs generated from murine embryonic stem cells protect mice from lethal anemia. Conditions to generate small amounts of clinical grade cRBCs have been established and the first-in-man administration of autologous cRBCs perfomed. The results of this trial indicate that cRBCs survive in vivo at least as long as their natural counterpart.

DISCUSSION

These ground-breaking reports have raised great excitement for clinical evaluation of cRBCs for transfusion. However, skepticism still persist that production of cRBCs in numbers sufficient for transfusion will ever be possible. This paper will discuss diagnostic and clinical goals pursuable with numbers of cRBCs that may be generated with current technology.

CONCLUSION

We are confident that development of relevant clinical goals achievable with current technologies will not only improve clinical care in transfusion medicine but will also foster studies to overcome scientific and technical barriers that render transfusion with cRBCs of the general population impractical today.

Growth of erythroid cells from thawed unseparated cord blood in vitro without exogenous erythropoietin

INTRODUCTION

Previous erythroid cell cultures have depended on added serum or erythropoietin. In this paper, the growth of erythroid cells from thawed unseparated cord blood units in vitro without serum or exogenous erythropoietin is reported.

METHODS

Thawed volume-reduced cord blood was cultured in conditions designed to support the megakaryocytic lineage, with thrombopoietin and interleukins 3 and 6. Erythroid cells were detected with glycophorin A (GlyA), CD71, and benzidine (flow cytometry and immunocytochemistry).

RESULTS

Nucleated and anucleated GlyA-positive, as well as benzidine-positive cells were observed from day 9. In flow cytometry, at days 0 and 9, 5.9% and 14% of all events were GlyA+, and 14% and 53% were CD71+, respectively. At days 0 and 9, 4.5% and 12% of the events were double-positive for GlyA and CD71, respectively. By day 14, the percentages of GlyA+, CD71+ and double-positive events had started to decrease (9.7%, 35%, and 5.3%, respectively).

CONCLUSIONS

Erythroid cells were generated from thawed unseparated cord blood units without exogenous erythropoietin. Thawed cord blood possesses the potential for erythroid growth in vitro in a culture medium designed for other cell types.

Exploring the use of expanded erythroid cells for autologous transfusion for anemia of prematurity

BACKGROUND

Autologous cord blood (CB) red blood cells (RBCs) can partly substitute transfusion needs in premature infants suffering from anemia. To explore whether expanded CB cells could provide additional autologous cells suitable for transfusion, we set up a simple one-step protocol to expand premature CB cells.

STUDY DESIGN AND METHODS

CB buffy coat cells and isolated CD34-positive (CD34(pos) ) cells from premature and full-term CB and adult blood were tested with several combinations of growth factors while omitting xenogeneic proteins from the culture medium. Cell differentiation was analyzed serially during 21 days using flow cytometry, progenitor assays, and high-performance liquid chromatography.

RESULTS

Expanded CB buffy coat cells resulted in a threefold higher number of erythroblasts than the isolated CD34(pos) cells. However, the RBCs contaminating the buffy coat remained present during the culture with uncertain quality. Premature and full-term CB CD34(pos) cells had similar fold expansion capacity and erythroid differentiation. With the use of interleukin-3, stem cell factor, and erythropoietin, the fold increases of all CD34(pos) cell sources were similar: CB 3942 ± 1554, adult peripheral mobilized blood 4702 ± 1826, and bone marrow (BM) 4143 ± 1908. The proportion of CD235a expression indicating erythroblast presence on Day 21 was slightly higher in the adult CD34(pos) cell sources: peripheral blood stem cells (96.7 ± 0.8%) and BM (98.9 ± 0.5%) compared to CB (87.7 ± 2.7%; p = 0.002). We were not able to induce further erythroid maturation in vitro.

CONCLUSION

This explorative study showed that fairly pure autologous erythroid-expanded cell populations could be obtained by a simple culture method, which should be optimized. Future challenges comprise obtaining ex vivo enucleation of RBCs with the use of a minimal manipulating approach, which can add up to autologous RBCs derived from CB in the treatment of anemia of prematurity.

Establishment of immortalized human erythroid progenitor cell lines able to produce enucleated red blood cells

Transfusion of red blood cells (RBCs) is a standard and indispensable therapy in current clinical practice. In vitro production of RBCs offers a potential means to overcome a shortage of transfusable RBCs in some clinical situations and also to provide a source of cells free from possible infection or contamination by microorganisms. Thus, in vitro production of RBCs may become a standard procedure in the future. We previously reported the successful establishment of immortalized mouse erythroid progenitor cell lines that were able to produce mature RBCs very efficiently. Here, we have developed a reliable protocol for establishing immortalized human erythroid progenitor cell lines that are able to produce enucleated RBCs. These immortalized cell lines produce functional hemoglobin and express erythroid-specific markers, and these markers are upregulated following induction of differentiation in vitro. Most importantly, these immortalized cell lines all produce enucleated RBCs after induction of differentiation in vitro, although the efficiency of producing enucleated RBCs remains to be improved further. To the best of our knowledge, this is the first demonstration of the feasibility of using immortalized human erythroid progenitor cell lines as an ex vivo source for production of enucleated RBCs.

Concise review: stem cell-derived erythrocytes as upcoming players in blood transfusion

Blood transfusions have become indispensable to treat the anemia associated with a variety of medical conditions ranging from genetic disorders and cancer to extensive surgical procedures. In developed countries, the blood supply is generally adequate. However, the projected decline in blood donor availability due to population ageing and the difficulty in finding rare blood types for alloimmunized patients indicate a need for alternative red blood cell (RBC) transfusion products. Increasing knowledge of processes that govern erythropoiesis has been translated into efficient procedures to produce RBC ex vivo using primary hematopoietic stem cells, embryonic stem cells, or induced pluripotent stem cells. Although in vitro-generated RBCs have recently entered clinical evaluation, several issues related to ex vivo RBC production are still under intense scrutiny: among those are the identification of stem cell sources more suitable for ex vivo RBC generation, the translation of RBC culture methods into clinical grade production processes, and the development of protocols to achieve maximal RBC quality, quantity, and maturation. Data on size, hemoglobin, and blood group antigen expression and phosphoproteomic profiling obtained on erythroid cells expanded ex vivo from a limited number of donors are presented as examples of the type of measurements that should be performed as part of the quality control to assess the suitability of these cells for transfusion. New technologies for ex vivo erythroid cell generation will hopefully provide alternative transfusion products to meet present and future clinical requirements.

Concise review: production of cultured red blood cells from stem cells

In the Western world, the volunteer-based collection system covers most transfusion needs, but transient shortages regularly develop and blood supplies are vulnerable to potentially major disruptions. The production of cultured red blood cells from stem cells is slowly emerging as a potential alternative. The various cell sources, the niche applications most likely to reach the clinic first, and some of the remaining technical issues are reviewed here.

Combinatorial assembly of developmental stage-specific enhancers controls gene expression programs during human erythropoiesis

Gene-distal enhancers are critical for tissue-specific gene expression, but their genomic determinants within a specific lineage at different stages of development are unknown. Here we profile chromatin state maps, transcription factor occupancy, and gene expression profiles during human erythroid development at fetal and adult stages. Comparative analyses of human erythropoiesis identify developmental stage-specific enhancers as primary determinants of stage-specific gene expression programs. We find that erythroid master regulators GATA1 and TAL1 act cooperatively within active enhancers but confer little predictive value for stage specificity. Instead, a set of stage-specific coregulators collaborates with master regulators and contributes to differential gene expression. We further identify and validate IRF2, IRF6, and MYB as effectors of an adult-stage expression program. Thus, the combinatorial assembly of lineage-specific master regulators and transcriptional coregulators within developmental stage-specific enhancers determines gene expression programs and temporal regulation of transcriptional networks in a mammalian genome.

Biological validation of bio-engineered red blood cell productions

The generation in vitro of cultured red blood cells (cRBC) could become an alternative to classical transfusion products. However, even when derived from healthy donors, the cRBC generated in vitro from hematopoietic stem cells may display alterations resulting from a poor controlled production process. In this context, we attempted to monitor the quality of the transfusion products arising from new biotechnologies. For that purpose, we developed an in vitro erythrophagocytosis (EP) test with the murine fibroblast cell line MS-5 and human macrophages (reference method). We evaluated 38 batches of cRBC, at the stage of reticulocyte, generated from CD34(+) cells isolated from placental blood or by leukapheresis. We showed that (i) the EP test performed with the MS-5 cell line was sensitive and can replace human macrophages for the evaluation of cultured cells. (ii) The EP tests revealed disparities among the batches of cRBC. (iii) The viability of the cells (determined by calcein-AM test), the expression of CD47 (antiphagocytosis receptor) and the externalization of phosphatidylserine (PS, marker of phagocytosis) were not critical parameters for the validation of the cRBC. (iv) Conversely, the cell deformability determined by ektacytometry was inversely correlated with the intensity of the phagocytic index. Assuming that the culture conditions directly influence the quality of the cell products generated, optimization of the production mode could benefit from the erythrophagocytosis test.

Novel, high-yield red blood cell production methods from CD34-positive cells derived from human embryonic stem, yolk sac, fetal liver, cord blood, and peripheral blood

The current supply of red blood cells expressing rare blood groups is not sufficient to cover all the existing transfusion needs for chronically transfused patients, such as sickle cell disease homozygous carriers, because of alloimmunization. In vitro production of cultured red blood cells is slowly emerging as a possible complement to the existing collection-based red blood cell procurement system. The yield of cultured red blood cells can theoretically be maximized by amplifying the stem, progenitor, or precursor compartment. Here, we combined methods designed to expand these three compartments to optimize the yield of cultured red blood cells and found that exposing CD34(+) cells to a short pulse of cytokines favorable for erythroid differentiation prior to stem cell expansion followed by progenitor expansion produced the highest yield of erythroid cells. This novel serum-free red blood cell production protocol was efficient on CD34(+) cells derived from human embryonic stem cells, 6-8-week yolk sacs, 16-18-week fetal livers, cord blood, and peripheral blood. The yields of cells obtained with these new protocols were larger by an order of magnitude than the yields observed previously. Globin expression analysis by high-performance liquid chromatography revealed that these expansion protocols generally yielded red blood cells that expressed a globin profile similar to that expected for the developmental age of the CD34(+) cells.

Functional characterization of TPO-expanded CD34+ cord blood cells identifies CD34- CD61- cells as platelet-producing cells early after transplantation in NOD/SCID mice and rCD34+ cells as CAFC colony-forming cells

Transplantation of thrombopoietin (TPO)-expanded cord blood CD34(+) cells accelerates human platelet recovery in NOD/SCID mice. It is unknown which subpopulations of the TPO-expanded cells mediate accelerated platelet recovery and bone marrow (BM) engraftment. In this study, the contribution of these subpopulations to human platelet appearance in the blood and BM engraftment was studied in NOD/SCID mice. Following transplantation of CD34(-) /CD61(-)/lineage(-) cells (Lin(-)), human platelets were detected in the blood of recipient mice from day 4. Both time to platelet recovery and blood platelet counts at 6 weeks after transplantation showed Lin(-) dose dependence. The Lin(-) population was virtually negative for lineage marker expression and lacked CD42b expression but was heterogeneous with regard to CD36 and CD38 expression, reflecting a population in transit but not fully committed toward the megakaryocyte (MK) lineage. Although no definitive phenotype could be established of the cells generating prompt platelet production and cells generating platelets 6 weeks after transplantation, this relatively heterogeneous Lin(-) population is prerequisite to accelerate platelet recovery in vivo. The interval to platelet recovery after transplantation of the CD34(+) cells remaining after expansion (rCD34(+)) was similar to mice transplanted with nonexpanded CD34(+) cells, although the total platelet counts and the engraftment levels in the BM were lower. Cobblestone area-forming cell colony-forming cells resided mostly in the rCD34(+) population. The pro-MK CD61(+) cells did not contribute to human platelet recovery or engraftment in the BM. Our study shows that not all expanded cells appear critical for transplantation. These data support that functional characterization of the expanded cell populations is warranted to make future expansion protocols suitable for clinical application.

Alternative blood products and clinical needs in transfusion medicine

The primary focus of national blood programs is the provision of a safe and adequate blood supply. This goal is dependent on regular voluntary donations and a regulatory infrastructure that establishes and enforces standards for blood safety. Progress in ex vivo expansion of blood cells from cell sources including peripheral blood, cord blood, induced pluripotent stem cells, and human embryonic stem cell lines will likely make alternative transfusion products available for clinical use in the near future. Initially, alloimmunized patients and individuals with rare blood types are most likely to benefit from alternative products. However, in developed nations voluntary blood donations are projected to be inadequate in the future as blood usage by individuals 60 years and older increases. In developing nations economic and political challenges may impede progress in attaining self-sufficiency. Under these circumstances, ex vivo generated red cells may be needed to supplement the general blood supply.

The potential of stem cells as an in vitro source of red blood cells for transfusion

Recent advances have increased excitement about the potential for therapeutic production of red blood cells (RBCs) in vitro. However, generation of RBCs in the large numbers required for transfusion remains a significant challenge. In this article, we summarize recent progress in producing RBCs from various cell sources, and discuss the hurdles that remain for translation into the clinical arena.

Ex-vivo expansion of red blood cells: how real for transfusion in humans?

Blood transfusion is indispensable for modern medicine. In developed countries, the blood supply is adequate and safe but blood for alloimmunized patients is often unavailable. Concerns are increasing that donations may become inadequate in the future as the population ages prompting a search for alternative transfusion products. Improvements in culture conditions and proof-of-principle studies in animal models have suggested that ex-vivo expanded red cells may represent such a product. Compared to other cell therapies transfusion poses the unique challenge of requiring great cell doses (2.5×10(12) cells vs 10(7) cells). Although production of such cell numbers is theoretically possible, current technologies generate red cells in numbers sufficient only for safety studies. It is conceived that by the time these studies will be completed, technical barriers to mass cell production will have been eliminated making transfusion with ex-vivo generated red cells a reality.

Proof of principle for transfusion of in vitro-generated red blood cells

In vitro RBC production from stem cells could represent an alternative to classic transfusion products. Until now the clinical feasibility of this concept has not been demonstrated. We addressed the question of the capacity of cultured RBCs (cRBCs) to survive in humans. By using a culture protocol permitting erythroid differentiation from peripheral CD34(+) HSC, we generated a homogeneous population of cRBC functional in terms of their deformability, enzyme content, capacity of their hemoglobin to fix/release oxygen, and expression of blood group antigens. We then demonstrated in the nonobese diabetes/severe combined immunodeficiency mouse that cRBC encountered in vivo the conditions necessary for their complete maturation. These data provided the rationale for injecting into one human a homogeneous sample of 10(10) cRBCs generated under good manufacturing practice conditions and labeled with (51)Cr. The level of these cells in the circulation 26 days after injection was between 41% and 63%, which compares favorably with the reported half-life of 28 ± 2 days for native RBCs. Their survival in vivo testifies globally to their quality and functionality. These data establish the proof of principle for transfusion of in vitro-generated RBCs and path the way toward new developments in transfusion medicine. This study is registered at http://www.clinicaltrials.gov as NCT0929266.

Erythroblast enucleation

Even though the production of orthochromatic erythroblasts can be scaled up to fulfill clinical requirements, enucleation remains one of the critical rate-limiting steps in the production of transfusable red blood cells. Mammalian erythrocytes extrude their nucleus prior to entering circulation, likely to impart flexibility and improve the ability to traverse through capillaries that are half the size of erythrocytes. Recently, there have been many advances in our understanding of the mechanisms underlying mammalian erythrocyte enucleation. This review summarizes these advances, discusses the possible future directions in the field, and evaluates the prospects for improved ex vivo production of red blood cells.

Phenotypic definition of the progenitor cells with erythroid differentiation potential present in human adult blood

In Human Erythroid Massive Amplification (HEMA) cultures, AB mononuclear cells (MNC) generate 1-log more erythroid cells (EBs) than the corresponding CD34^pos cells, suggesting that MNC may also contain CD34^neg HPC. To clarify the phenotype of AB HPC which generate EBs in these cultures, flow cytometric profiling for CD34/CD36 expression, followed by isolation and functional characterization (colony-forming-ability in semisolid-media and fold-increase in HEMA) were performed. Four populations with erythroid differentiation potential were identified: CD34^posCD36^neg (0.1%); CD34^posCD36^pos (barely detectable-0.1%); CD34^negCD36^low (2%) and CD34^negCD36^neg (75%). In semisolid-media, CD34^posCD36^neg cells generated BFU-E and CFU-GM (in a 1 : 1 ratio), CD34^negCD36^neg cells mostly BFU-E (87%) and CD34^posCD36^pos and CD34^negCD36^low cells were not tested due to low numbers. Under HEMA conditions, CD34^posCD36^neg, CD34^posCD36^pos, CD34^negCD36^low and CD34^negCD36^neg cells generated EBs with fold-increases of ≈9,000, 100, 60 and 1, respectively, and maturation times (day with >10% CD36^highCD235a^high cells) of 10–7 days. Pyrenocytes were generated only by CD34^neg/CD36^neg cells by day 15. These results confirm that the majority of HPC in AB express CD34 but identify additional CD34^neg populations with erythroid differentiation potential which, based on differences in fold-increase and maturation times, may represent a hierarchy of HPC present in AB.

Recovery and Biodistribution of Ex Vivo Expanded Human Erythroblasts Injected into NOD/SCID/IL2Rγ mice

Ex vivo expanded erythroblasts (EBs) may serve as advanced transfusion products provided that lodgment occurs in the macrophage-niche of the marrow permitting maturation. EBs expanded from adult and cord blood expressed the receptors (CXCR4, VLA-4, and P-selectin ligand 1) necessary for interaction with macrophages. However, 4-days following transfusion to intact NOD/SCID/IL2Rγ(null) mice, CD235a(pos) EBs were observed inside CD235a(neg) splenic cells suggesting that they underwent phagocytosis. When splenectomized and intact NOD/SCID/IL2Rγ(null) mice were transfused using retrovirally labeled human EBs, human cells were visualized by bioluminescence imaging only in splenectomized animals. Four days after injection, human CD235a(pos) cells were detected in marrow and liver of splenectomized mice but only in spleen of controls. Human CD235a(pos) erythrocytes in blood remained low in all cases. These studies establish splenectomized NOD/SCID/IL2Rγ(null) mice as a suitable model for tracking and quantification of human EBs in vivo.

Under HEMA conditions, self-replication of human erythroblasts is limited by autophagic death

The number of erythroblasts generated ex-vivo under human-erythroid massive-amplification conditions by mononuclear cells from one unit of adult blood (~10(10)) are insufficient for transfusion (~10(12) red cells), emphasizing the need for studies to characterize cellular interactions during culture to increase erythroblast production. To identify the cell populations which generate erythroblasts under human-erythroid-massive-amplification conditions and the factors that limit proliferation, day 10 non-erythroblasts and immature- and mature-erythroblasts were separated by sorting, labelled with carboxyfluorescein-diacetate-succinimidyl-ester and re-cultured either under these conditions (for proliferation, maturation and/or apoptosis/autophagy determinations) or in semisolid media (for progenitor cell determination). Non-erythroblasts contained 54% of the progenitor cells but did not grow under human-erythroid-massive-amplification conditions. Immature-erythroblasts contained 25% of the progenitor cells and generated erythroblasts under human-erythroid-massive-amplification conditions (FI at 48 h=2.57±1.15). Mature-erythroblasts did not generate colonies and died in human-erythroid-massive-amplification conditions. In sequential sorting/re-culture experiments, immature-erythroblasts retained the ability to generate erythroblasts for 6 days and generated 2-5-fold more cells than the corresponding unfractionated population, suggesting that mature-erythroblasts may limit erythroblast expansion. In co-cultures of carboxyfluorescein-diacetate-succinimidyl-ester-labelled-immature-erythroblasts with mature-erythroblasts at increasing ratios, cell numbers did not increase and proliferation, maturation and apoptotic rates were unchanged. However, Acridine Orange staining (a marker for autophagic death) increased from ~3.2% in cultures with immature-erythroblasts alone to 14-22% in cultures of mature-erythroblasts with and without immature-erythroblasts. In conclusion, these data identify immature-erythroblasts as the cells that generate additional erythroblasts in human-erythroid-massive-amplification cultures and autophagy as the leading cause of death limiting the final cellular output of these cultures.

The rise of cellular therapy

In 1938, the field of Transfusion Medicine began as the simpler entity - Blood Banking. It was a discipline that focused on collecting, processing, storing and distributing end stage blood cells, plasma and plasma fractions to patients. Over the years, the field progressed to include clinical patient services such as apheresis technology and with the development of stem cell transplantation as a standard of care, Cell Therapy. Now the discipline is also finding a niche in the area of Regenerative Medicine. The role played by Transfusion Medicine practitioners in Cell Therapy and Regenerative Medicine was predicated on many factors: (1) pre-existing, established protocols for therapeutic leukapheresis, (2) prior experience with mononuclear cell collection and processing, (3) long term familiarity with, and a clear understanding of, cGMP and cGLP guidelines, Federal regulations, and the voluntary standards established by various organizations, (4) close relationships with practitioners in clinical departments of medicine, pediatrics, oncology, surgery, and their subspecialty areas. While the initial Cell Therapy programs related primarily to hematopoietic stem cell transplantation, as Regenerative Medicine programs developed, transfusion specialists found it to be a related field that would also benefit from their input. Cell Therapy and Regenerative Medicine, now provide fertile soil for the seeds of Transfusion Medicine to grow. The once narrowly defined field of Blood Banking now encompasses involvement in major new Cellular Therapy/Regenerative Medicine research protocols related to treatment of patients with cancer as well as renal, hepatic and cardiac illnesses. This in turn provides opportunities for residents and fellows to consider robust careers in the field of Transfusion Medicine. In this manner we will move forward with one eye on the past and another on the promising future.

Banking of pluripotent adult stem cells as an unlimited source for red blood cell production: potential applications for alloimmunized patients and rare blood challenges

The transfusion of red blood cells (RBCs) is now considered a well-settled and essential therapy. However, some difficulties and constraints still occur, such as long-term blood product shortage, blood donor population aging, known and yet unknown transfusion-transmitted infectious agents, growing cost of the transfusion supply chain management, and the inescapable blood group polymorphism barrier. Red blood cells can be now cultured in vitro from human hematopoietic, human embryonic, or human-induced pluripotent stem cells (hiPSCs). The highly promising hiPSC technology represents a potentially unlimited source of RBCs and opens the door to the revolutionary development of a new generation of allogeneic transfusion products. Assuming that in vitro large-scale cultured RBC production efficiently operates in the near future, we draw here some futuristic but realistic scenarios regarding potential applications for alloimmunized patients and those with a rare blood group. We retrospectively studied a cohort of 16,486 consecutive alloimmunized patients (10-year period), showing 1 to 7 alloantibodies with 361 different antibody combinations. We showed that only 3 hiPSC clones would be sufficient to match more than 99% of the 16,486 patients in need of RBC transfusions. The study of the French National Registry of People with a Rare Blood Phenotype/Genotype (10-year period) shows that 15 hiPSC clones would cover 100% of the needs in patients of white ancestry. In addition, one single hiPSC clone would meet 73% of the needs in alloimmunized patients with sickle cell disease for whom rare cryopreserved RBC units were required. As a result, we consider that a very limited number of RBC clones would be able to not only provide for the need for most alloimmunized patients and those with a rare blood group but also efficiently allow for a policy for alloimmunization prevention in multiply transfused patients.

Chromatin-modifying agents promote the ex vivo production of functional human erythroid progenitor cells

Presently, blood transfusion products (TPs) are composed of terminally differentiated cells with a finite life span. We have developed an ex vivo–generated TP composed of erythroid progenitor cells (EPCs) and precursors cells. Several histone deacetylase inhibitors (HDACIs) were used in vitro to promote the preferential differentiation of cord blood (CB) CD34+ cells to EPCs. A combination of cytokines and valproic acid (VPA): (1) promoted the greatest degree of EPC expansion, (2) led to the generation of EPCs which were capable of differentiating into the various stages of erythroid development, (3) led to epigenetic modifications (increased H3 acetylation) of promoters for erythroid-specific genes, which resulted in the acquisition of a gene expression pattern characteristic of primitive erythroid cells, and (4) promoted the generation of a TP that when infused into NOD/SCID mice produced mature RBCs containing both human adult and fetal globins as well Rh blood group Ag which persisted for 3 weeks and the retention of human EPCs and erythroid precursor cells within the BM of recipient mice. This ex vivo–generated EPC-TP likely represents a paradigm shift in transfusion medicine because of its potential to continue to generate additional RBCs after its infusion.

The dominant negative β isoform of the glucocorticoid receptor is uniquely expressed in erythroid cells expanded from polycythemia vera patients

Glucocorticoid receptor (GR) agonists increase erythropoiesis in vivo and in vitro. To clarify the effect of the dominant negative GRβ isoform (unable to bind STAT-5) on erythropoiesis, erythroblast (EB) expansion cultures of mononuclear cells from 18 healthy (nondiseased) donors (NDs) and 16 patients with polycythemia vera (PV) were studied. GRβ was expressed in all PV EBs but only in EBs from 1 ND. The A3669G polymorphism, which stabilizes GRβ mRNA, had greater frequency in PV (55%; n = 22; P = .0028) and myelofibrosis (35%; n = 20) patients than in NDs (9%; n = 22) or patients with essential thrombocythemia (6%; n = 15). Dexamethasone stimulation of ND cultures increased the number of immature EBs characterized by low GATA1 and β-globin expression, but PV cultures generated great numbers of immature EBs with low levels of GATA1 and β-globin irrespective of dexamethasone stimulation. In ND EBs, STAT-5 was not phosphorylated after dexamethasone and erythropoietin treatment and did not form transcriptionally active complexes with GRα, whereas in PV EBs, STAT-5 was constitutively phosphorylated, but the formation of GR/STAT-5 complexes was prevented by expression of GRβ. These data indicate that GRβ expression and the presence of A3669G likely contribute to development of erythrocytosis in PV and provide a potential target for identification of novel therapeutic agents.

Red blood cells from pluripotent stem cells for use in transfusion

The use of donated red blood cells in transfusion is a well-established cellular therapy. However, problems including insufficient supply, transfusion-transmitted infections and the need for immunological matching hamper even in the best services. These issues may be eliminated by using pluripotent stem cells to generate universal donor group O, Rhesus D-negative red blood cells. Human embryonic stem cells can be maintained and expanded indefinitely and can, therefore, produce the very large cell numbers required for this application. Red blood cell production is also an attractive goal for pluripotent stem cell-derived therapeutics because it is a well-characterized single cell suspension, lacking nucleated cells and with a low expression of HLA molecules. Much progress has been made; however, a number of challenges remain including scale-up, clinical effectiveness and product safety.

Humanized culture medium for clinical expansion of human erythroblasts

Ex vivo-generated erythroblasts represent alternative transfusion products. However, inclusion of bovine components in media used for their growth precludes clinical use, highlighting the importance of developing culture media based on pharmaceutical grade reagents. In addition, because adult blood generates ex vivo lower numbers of erythroblasts than cord blood, cord blood has been proposed as the source of choice for ex vivo erythroblast production. To clarify the potential of adult blood to generate erythroblasts ex vivo, experiments were designed to identify growth factors [stem cell factor (SCF), interleukin-3 (IL-3), erythropoietin (EPO), and/or thrombopoietin (TPO)] and the optimal concentration and addition schedule of hormones (dexamethasone and estradiol) sustaining maximal erythroid amplification from adult blood mononuclear cells (MNC) using media with serum previously defined as human erythroid massive amplification culture (HEMA(ser)). Adult MNC stimulated with SCF and IL-3 in combination with EPO generated a 6-12-fold increase in erythroid cells while TPO was ineffective. Dexamethasone and estradiol (both at 10(-6) M) exerted partially overlapping but nonredundant functions. Dexamethasone was indispensable in the first 10 days of culture while estradiol was required from day 10 on. The growth factor and hormone combinations identified in HEMA(ser) were then used to formulate a media composed of dialyzed pharmaceutical grade human albumin, human albumin-lipid liposomes, and iron-saturated recombinant human tranferrin (HEMA(def)). HEMA(def) sustained erythroid amplification as efficiently as HEMA(ser) for cord blood MNC and 10-fold higher than HEMA(ser) for adult blood MNC. In fact, the numbers of erythroblasts generated in HEMA(def) by adult MNC were similar to those generated by cord blood MNC. In conclusion, this study identifies growth factors, hormone combinations, and human protein-based media that allow similar levels of ex vivo erythroid expansion from adult and cord blood MNC, paving the way to evaluate adult blood as a source of ex vivo-expanded erythroblasts for transfusion.

Umbilical cord transplantation: epilogue

The field of cord blood transplantation has come a long way since the first transplant more than 20 years ago. Advancements in the field will require continuing efforts to better understand hematopoietic stem and progenitor cell function and engraftment. Cautious optimism is inherent in the potential relevance and applicability of nonhematopoietic stem and progenitor cell types found in cord blood, and induced pluripotent stem cells generated from cord blood cells. Rigorous investigations and close interactions between scientific and clinical investigators are required to translate human in vitro and animal in vivo findings into clinical utility.

Compensated variability in the expression of globin-related genes in erythroblasts generated ex vivo from different donors

BACKGROUND

Ex vivo generated erythroblasts are being evaluated for transfusion. Expression of balanced levels of globin mRNA is essential for normal red blood cell function and survival but it is unknown whether the expression of the globin genes in ex vivo expanded cells is balanced.

STUDY DESIGN AND METHODS

Immature erythroblasts (IEs) were expanded in human erythroid massive amplification cultures from blood mononuclear cells of 19 normal donors and four beta(0)-thalassemia patients (for comparison) and induced to mature for 4 days in the presence of erythropoietin. mRNA was prepared from IEs and mature erythroblasts to evaluate the expression of alpha-, beta-, and gamma-globin genes and of adult hemoglobin-stabilizing protein (AHSP) and BCL11A, two proteins directly controlling globin function and/or production. Results were analyzed using Pearson's correlation coefficient, the Wilcoxon signed rank, and the Mann-Whitney rank sum tests.

RESULTS

The absolute levels of globin, AHSP, and BCL11A mRNA expressed by erythroblasts generated ex vivo from normal donors were distributed along a 2-log range. With maturation, the levels of gamma-globin and BCL11A mRNA did not decrease while those of alpha-globin, gamma + beta-globins, and AHSP mRNA greatly increased. In normal cells, the modest imbalance (two- to fourfold) observed between alpha- and gamma + beta-globin mRNA was fully compensated by AHSP expression. Thus, the levels of alpha-globin mRNA were correlated with those of gamma + beta-globin (R(2) = 0.93, p < 0.0001) and AHSP (R(2) = 0.86, p < 0.0001).

CONCLUSIONS

Ex vivo expanded erythroblasts from normal donors express modestly imbalanced levels of alpha-globin and gamma + beta-globin fully compensated by AHSP expression, likely ensuring normal function and survival.

Supplying clotting factors from hematopoietic stem cell-derived erythroid and megakaryocytic lineage cells

Systemically distributed proteins such as clotting factors have been traditionally expressed from muscle or liver to achieve therapeutic, long-term expression. As long-lived cell capable of generating an abundant progeny, hematopoietic stem cells (HSCs) also merit consideration for this purpose. To be clinically relevant, this approach would require that hematopoietic cells be capable of expressing high levels of functional, secreted proteins, that the risk of insertional oncogenesis be minimized, and that sufficient stem cell engraftment be achieved following minimally invasive conditioning. Recent reports demonstrate the feasibility of expressing either factor IX (FIX) or factor VIII (FVIII) in erythroblasts and platelets using lineage-restricted vectors, resulting in effective treatments in mouse models of hemophilia. The erythroid system is especially powerful in providing high protein output, yielding FIX levels approaching 1 micro g/ml per vector copy in the plasma of long-term hematopoietic chimeras, a secretion level that vastly outperforms any other current mammalian constitutive or long-terminal repeat (LTR)-driven vector system. These early but promising studies raise the prospect of further developing these strategies for clinical investigation.