Development of Hematopoietic Repopulating Cells from Embryonic Stem Cells

Enforced Expression of HOXB4 in Human Embryonic Stem Cells Enhances the Production of Hematopoietic Progenitors but Has No Effect on the Maturation of Red Blood Cells

: We have developed a robust, Good Manufacturing Practice-compatible differentiation protocol capable of producing scalable quantities of red blood cells (RBCs) from human pluripotent stem cells (hPSCs). However, translation of this protocol to the clinic has been compromised because the RBCs produced are not fully mature; thus, they express embryonic and fetal, rather than adult globins, and they do not enucleate efficiently. Based on previous studies, we predicted that activation of exogenous HOXB4 would increase the production of hematopoietic progenitor cells (HPCs) from hPSCs and hypothesized that it might also promote the production of more mature, definitive RBCs. Using a tamoxifen-inducible HOXB4-ER(T2) expression system, we first demonstrated that activation of HOXB4 does increase the production of HPCs from hPSCs as determined by colony-forming unit culture activity and the presence of CD43(+)CD34(+) progenitors. Activation of HOXB4 caused a modest, but significant, increase in the proportion of immature CD235a(+)/CD71(+) erythroid cells. However, this did not result in a significant increase in more mature CD235a(+)/CD71(-) cells. RBCs produced in the presence of enhanced HOXB4 activity expressed embryonic (ε) and fetal (γ) but not adult (β) globins, and the proportion of enucleated cells was comparable to that of the control cultures. We conclude that programming with the transcription factor HOXB4 increases the production of hematopoietic progenitors and immature erythroid cells but does not resolve the inherent challenges associated with the production of mature adult-like enucleated RBCs.

SIGNIFICANCE

As worldwide blood donations decrease and transfusable transmitted infections increase, intense interest has ensued in deriving red blood cells (RBCs) in vitro from alternative sources such as pluripotent stem cells. A translatable protocol was developed to generate RBCs; however, these RBCs have an immature phenotype. It was hypothesized that the transcription factor HOXB4 could enhance their production and maturation. Although HOXB4 increased the production of erythroid progenitors, it did not promote their maturation. Despite the remaining challenges, a robust system has been established to test other candidates and add to the knowledge base in this field.

Portrayal of the Notch system in embryonic stem cell-derived embryoid bodies

We portrayed the Notch system in embryonic stem cell (ESC)-derived embryoid bodies (EBs) differentiating under the standard protocols used to assess yolk sac (YS) hematopoiesis in vitro. Notch receptors and Notch ligands were detected in virtually all cells throughout EB development. Notch 1 and Notch 2, but not Notch 4, were visualized in the nucleus of EB cells, and all these receptors were also observed as patent cytoplasmic foci. Notch ligands (Delta-like 1 and 4, Jagged 1 and 2) were immunodetected mostly as cytoplasmic foci. Widespread Notch 1 activation was evident at days 2-4 of EB differentiation, the time window of hemangioblast generation in this in vitro system. EBs experienced major spatial remodeling beyond culture day 4, the time point coincident with the transition between primitive and multilineage waves of YS hematopoiesis in vitro. At day 6, where definitive YS hematopoiesis is established in EBs, these exhibit an immature densely packed cellular region (DCR) surrounded by a territory of mesodermal-like cells and an outer layer of endodermal cells. Immunolabeling of Notch receptors and ligands was usually higher in the DCR. Our results show that Notch system components are continuously and abundantly expressed in the multicellular environments arising in differentiating EBs. In such an active Notch system, receptors and ligands do not accumulate extensively at the cell surface but instead localize at cytoplasmic foci, an observation that fits current knowledge on endocytic modulation of Notch signaling. Our data thus suggest that Notch may function as a territorial modulator during early development, where it may eventually influence YS hematopoiesis.

Hematopoietic reconstitution with androgenetic and gynogenetic stem cells

Parthenogenetic embryonic stem (ES) cells with two oocyte-derived genomes (uniparental) have been proposed as a source of autologous tissue for transplantation. The therapeutic applicability of any uniparental cell type is uncertain due to the consequences of genomic imprinting that in mammalian uniparental tissues causes unbalanced expression of imprinted genes. We transplanted uniparental fetal liver cells into lethally irradiated adult mice to test their capacity to replace adult hematopoietic tissue. Both maternal (gynogenetic) and paternal (androgenetic) derived cells conveyed long-term, multilineage reconstitution of hematopoiesis in recipients, with no associated pathologies. We also establish that uniparental ES cells can differentiate into transplantable hematopoietic progenitors in vitro that contribute to long-term hematopoiesis in recipients. Hematopoietic tissue in recipients maintained fidelity of parent-of-origin methylation marks at the Igf2/H19 locus; however, variability occurred in the maintenance of parental-specific methylation marks at other loci. In summary, despite genomic imprinting and its consequences on development that are particularly evident in the androgenetic phenotype, uniparental cells of both parental origins can form adult-transplantable stem cells and can repopulate an adult organ.

Transgene Expression and RNA Interference in Embryonic Stem Cells

Over the last 30 years of biomedical research, technologies for transgene expression and gene loss of function have been developed from animal model systems to human gene therapy, and, increasingly, embryonic stem cells are a key target for these genetic engineering strategies. In this chapter, we describe how retrovirus/lentivirus vectors can be used to transfer and stably express genes or small interfering RNAs in mouse and human embryonic stem cells and their progeny, thereby allowing genetic gain-of-function or loss-of-function analysis and cell lineage selection in a multitude of experimental settings.

Hematopoietic Cells

Murine embryonic stem cells (mESC) readily form embryoid bodies (EBs) that exhibit hematopoietic differentiation. Methods based on EB formation or ESC coculture with murine bone marrow stromal cell lines have revealed pathways of both primitive and definitive hematopoietic differentiation progressing from primitive mesoderm via hemangioblasts to endothelium and hematopoietic stem and progenitor cells. The addition of specific hematopoietic growth factors and morphogens to these cultures enhances the generation of neutrophils, macrophages, megakaryocyte/platelets, and hemoglobinized mature red cells. In addition, selective culture systems have been developed to support differentiation into mature T lymphocytes, natural killer cells, B cells, and dendritic cells. In most cases, culture systems have been developed that support equivalent differentiation of various human ESC (hESC). The major obstacle to translation of ESC hematopoietic cultures to clinical relevance has been the general inability to produce hematopoietic stem cells (HSC) that can engraft adult, irradiated recipients. In this context, the pattern of ES hematopoietic development mirrors the yolk sac phase of hematopoiesis that precedes the appearance of engraftable HSC in the aorta-gonad-mesonephros region. Genetic manipulation of mESC hematopoietic progeny by upregulation of HOXB4 or STAT5 has led to greatly enhanced long- or short-term multilineage hematopoietic engraftment, suggesting that genetic or epigenetic manipulation of these pathways may lead to functional HSC generation from hESC.

HOXB4 enforces equivalent fates of ES-cell-derived and adult hematopoietic cells

Genetic manipulation of hematopoietic stem and progenitor cells is an important tool for experimental and clinical applied hematology. However, techniques that allow for gene targeting, subsequent in vitro selection, and expansion of genetically defined clones are available only for ES cells. Such molecularly defined and, hence, "safe" clones would be highly desirable for somatic gene therapy. Here, we demonstrate that in vitro differentiated ES cells completely recapitulate the growth and differentiation properties of adult bone marrow cells, in vitro and in vivo, when ectopically expressing HOXB4. Myeloid development was enforced and (T) lymphoid development suppressed over a wide range of expression levels, whereas only high expression levels of the transcription factor were detrimental for erythroid development. This indicates a close association between the amounts of ectopic HOXB4 present within a progenitor cell and and the decision to self renew or differentiate. Because HOXB4 mediates similar fates of ES-derived and bone marrow hematopoietic stem cells, the primitive embryonic cells can be considered a promising alternative for investigating hematopoietic reconstitution, in vivo, based on well defined clones. Provided that HOXB4 levels are kept within a certain therapeutic window, ES cells also carry the potential of efficient and safe somatic gene therapy.