Identification of the hemogenic endothelial progenitor and its direct precursor in human pluripotent stem cell differentiation cultures

Hematopoietic differentiation of pluripotent stem cells in culture

This chapter describes a two-dimensional "monolayer" system for differentiating human pluripotent stem cells (PSCs) into "primitive" hematopoietic progenitor cells (HPCs) resembling those produced in vivo by the early embryonic yolk sac. This experimental system utilizes defined conditions without serum or feeder cells. Cytokines are added sequentially to stimulate the formation of mesoderm and its subsequent patterning to hematopoietic progenitors. The HPCs produced by this protocol have multi-lineage potential (erythroid, megakaryocyte, and myeloid) and can be isolated as a homogeneous population for use in standard hematopoietic studies including liquid expansion to mature lineages and colony assays. In addition, the HPCs can be cryopreserved for distribution or analysis at later times. The HPCs generated by this protocol have been used successfully to better define intrinsic variation in hematopoietic potential between different PSC lines and to model human hematopoietic diseases using patient-derived induced pluripotent stem cells.

Vascular progenitors from cord blood-derived induced pluripotent stem cells possess augmented capacity for regenerating ischemic retinal vasculature

BACKGROUND

The generation of vascular progenitors (VPs) from human induced pluripotent stem cells (hiPSCs) has great potential for treating vascular disorders such as ischemic retinopathies. However, long-term in vivo engraftment of hiPSC-derived VPs into the retina has not yet been reported. This goal may be limited by the low differentiation yield, greater senescence, and poor proliferation of hiPSC-derived vascular cells. To evaluate the potential of hiPSCs for treating ischemic retinopathies, we generated VPs from a repertoire of viral-integrated and nonintegrated fibroblast and cord blood (CB)-derived hiPSC lines and tested their capacity for homing and engrafting into murine retina in an ischemia-reperfusion model.

METHODS AND RESULTS

VPs from human embryonic stem cells and hiPSCs were generated with an optimized vascular differentiation system. Fluorescence-activated cell sorting purification of human embryoid body cells differentially expressing endothelial/pericytic markers identified a CD31(+)CD146(+) VP population with high vascular potency. Episomal CB-induced pluripotent stem cells (iPSCs) generated these VPs with higher efficiencies than fibroblast-iPSC. Moreover, in contrast to fibroblast-iPSC-VPs, CB-iPSC-VPs maintained expression signatures more comparable to human embryonic stem cell VPs, expressed higher levels of immature vascular markers, demonstrated less culture senescence and sensitivity to DNA damage, and possessed fewer transmitted reprogramming errors. Luciferase transgene-marked VPs from human embryonic stem cells, CB-iPSCs, and fibroblast-iPSCs were injected systemically or directly into the vitreous of retinal ischemia-reperfusion-injured adult nonobese diabetic-severe combined immunodeficient mice. Only human embryonic stem cell- and CB-iPSC-derived VPs reliably homed and engrafted into injured retinal capillaries, with incorporation into damaged vessels for up to 45 days.

CONCLUSIONS

VPs generated from CB-iPSCs possessed augmented capacity to home, integrate into, and repair damaged retinal vasculature.

Hematopoietic specification from human pluripotent stem cells: current advances and challenges toward de novo generation of hematopoietic stem cells

Significant advances in cellular reprogramming technologies and hematopoietic differentiation from human pluripotent stem cells (hPSCs) have already enabled the routine production of multiple lineages of blood cells in vitro and opened novel opportunities to study hematopoietic development, model genetic blood diseases, and manufacture immunologically matched cells for transfusion and cancer immunotherapy. However, the generation of hematopoietic cells with robust and sustained multilineage engraftment has not been achieved. Here, we highlight the recent advances in understanding the molecular and cellular pathways leading to blood development from hPSCs and discuss potential approaches that can be taken to facilitate the development of technologies for de novo production of hematopoietic stem cells.

A short history of hemogenic endothelium

Definitive hematopoietic cells are generated de novo during ontogeny from a specialized subset of endothelium, the so-called hemogenic endothelium. In this review we give a brief overview of the identification of hemogenic endothelium, explore its links with the HSC lineage, and summarize recent insights into the nature of hemogenic endothelium and the microenvironmental and intrinsic regulators contributing to its transition into blood. Ultimately, a better understanding of the processes controlling the transition of endothelium into blood will advance the generation and expansion of hematopoietic stem cells for therapeutic purposes.

Development of a xeno-free autologous culture system for endothelial progenitor cells derived from human umbilical cord blood

Despite promising preclinical outcomes in animal models, a number of challenges remain for human clinical use. In particular, expanding a large number of endothelial progenitor cells (EPCs) in vitro in the absence of animal-derived products is the most critical hurdle remaining to be overcome to ensure the safety and efficiency of human therapy. To develop in vitro culture conditions for EPCs derived from human cord blood (hCB-EPCs), we isolated extracts (UCE) and collagen (UC-collagen) from umbilical cord tissue to replace their animal-derived counterparts. UC-collagen and UCE efficiently supported the attachment and proliferation of hCB-EPCs in a manner comparable to that of animal-derived collagen in the conventional culture system. Our developed autologous culture system maintained the typical characteristics of hCB-EPCs, as represented by the expression of EPC-associated surface markers. In addition, the therapeutic potential of hCB-EPCs was confirmed when the transplantation of hCB-EPCs cultured in this autologous culture system promoted limb salvage in a mouse model of hindlimb ischemia and was shown to contribute to attenuating muscle degeneration and fibrosis. We suggest that the umbilical cord represents a source for autologous biomaterials for the in vitro culture of hCB-EPCs. The main characteristics and therapeutic potential of hCB-EPCs were not compromised in developed autologous culture system. The absence of animal-derived products in our newly developed in vitro culture removes concerns associated with secondary contamination. Thus, we hope that this culture system accelerates the realization of therapeutic applications of autologous hCB-EPCs for human vascular diseases.

Isolation method and xeno-free culture conditions influence multipotent differentiation capacity of human Wharton's jelly-derived mesenchymal stem cells

Introduction

Human Wharton’s jelly (WJ) has become a preferred source of mesenchymal stem cells (MSCs) whose clinical applications are limited by the use of adequate xeno-free (XF), in vitro manipulation conditions. Therefore, the objective of our study was to characterize WJ-derived MSCs (WJ-MSCs), isolated by different methods and cultured in a commercially available, MSC XF medium, not least of all by investigating their endothelial differentiation capacity.

Methods

WJ explants and enzymatically dissociated WJ cells were cultured in a defined, XF medium for MSCs. Adherent cells at passages 2 and 5 were characterized as MSCs by flow cytometry, MTT, real-time quantitative reverse transcription PCR, and functional multipotent differentiation assays. The endothelial differentiation capacity of MSCs isolated and expanded until passage 2 in the MSC XF medium, and then subcultured for five passages in a commercially available endothelial growth medium (group A), was assessed over serial passages, as compared to adherent WJ-derived cells isolated and expanded for five consecutive passages in the endothelial medium (group B).

Results

The MSC phenotype of WJ explant- and pellet-derived cells, isolated and expanded in the MSC XF medium, was proven based on the expression of CD44/CD73/CD90/CD105 surface markers and osteo-/adipo-/chondrogenic multipotent differentiation potential, which differed according to the isolation method and/or passage number. Upon exposure to endothelial differentiation cues, cells belonging to group A did not exhibit endothelial cell characteristics over serial passages; by contrast, WJ pellet-derived cells belonging to group B expressed endothelial characteristics at gene, protein and functional levels, potentially due to culture conditions favoring the isolation of other stem/progenitor cell types than MSCs, able to give rise to an endothelial progeny.

Conclusions

The use of defined, MSC XF media for isolation and expansion of human WJ-MSCs is a prerequisite for the establishment of their real endothelial differentiation capacity, as candidates for clinical therapy applications. Thus, the standardization of WJ-MSCs isolation and culture expansion techniques in defined, MSC XF media, for their accurate characterization, would be a priority in the stem cell research field.

Signalling pathways that control vertebrate haematopoietic stem cell specification

Haematopoietic stem cells (HSCs) are tissue-specific stem cells that replenish all mature blood lineages during the lifetime of an individual. Clinically, HSCs form the foundation of transplantation-based therapies for leukaemias and congenital blood disorders. Researchers have long been interested in understanding the normal signalling mechanisms that specify HSCs in the embryo, in part because recapitulating these requirements in vitro might provide a means to generate immune-compatible HSCs for transplantation. Recent embryological work has demonstrated the existence of previously unknown signalling requirements. Moreover, it is now clear that gene expression in the nearby somite is integrally involved in regulating the transition of the embryonic endothelium to a haemogenic fate. Here, we review current knowledge of the intraembryonic signals required for the specification of HSCs in vertebrates.

Deciphering the hierarchy of angiohematopoietic progenitors from human pluripotent stem cells

Identification of sequential progenitors leading to blood formation from pluripotent stem cells (PSCs) will be essential for understanding the molecular mechanisms of hematopoietic lineage specification and for development of technologies for in vitro production of hematopoietic stem cells (HSCs). It is well established that during development, blood and endothelial cells in the extraembryonic and embryonic compartments are formed in parallel from precursors with angiogenic and hematopoietic potentials. However, the identity and hierarchy of these precursors in human PSC (hPSC) cultures remain obscure. Using developmental stage-specific mesodermal and endothelial markers and functional assays, we recently identified discrete populations of angiohematopoietic progenitors from hPSCs, including mesodermal precursors and hemogenic endothelial cells with primitive and definitive hematopoietic potentials. In addition, we discovered a novel population of multipotent hematopoietic progenitors with an erythroid phenotype, which retain angiogenic potential. Here we introduce our recent findings and discuss their implication for defining putative HSC precursor and factors required for activation of self-renewal potential in hematopoietic cells emerging from endothelium.

T lymphocyte potential marks the emergence of definitive hematopoietic progenitors in human pluripotent stem cell differentiation cultures

The efficient generation of hematopoietic stem cells from human pluripotent stem cells is dependent on the appropriate specification of the definitive hematopoietic program during differentiation. In this study, we used T lymphocyte potential to track the onset of definitive hematopoiesis from human embryonic and induced pluripotent stem cells differentiated with specific morphogens in serum- and stromal-free cultures. We show that this program develops from a progenitor population with characteristics of hemogenic endothelium, including the expression of CD34, VE-cadherin, GATA2, LMO2, and RUNX1. Along with T cells, these progenitors display the capacity to generate myeloid and erythroid cells. Manipulation of Activin/Nodal signaling during early stages of differentiation revealed that development of the definitive hematopoietic progenitor population is not dependent on this pathway, distinguishing it from primitive hematopoiesis. Collectively, these findings demonstrate that it is possible to generate T lymphoid progenitors from pluripotent stem cells and that this lineage develops from a population whose emergence marks the onset of human definitive hematopoiesis.

Human ESC-derived hemogenic endothelial cells undergo distinct waves of endothelial to hematopoietic transition

Several studies have demonstrated that hematopoietic cells originate from endotheliumin early development; however, the phenotypic progression of progenitor cells during human embryonic hemogenesis is not well described. Here, we define the developmental hierarchy among intermediate populations of hematopoietic progenitor cells (HPCs) derived from human embryonic stem cells (hESCs). We genetically modified hESCs to specifically demarcate acquisition of vascular (VE-cadherin) and hematopoietic (CD41a) cell fate and used this dual-reporting transgenic hESC line to observe endothelial to hematopoietic transition by real-time confocal microscopy. Live imaging and clonal analyses revealed a temporal bias in commitment of HPCs that recapitulates discrete waves of lineage differentiation noted during mammalian hemogenesis. Specifically, HPCs isolated at later time points showed reduced capacity to form erythroid/ megakaryocytic cells and exhibited a tendency toward myeloid fate that was enabled by expression of the Notch ligand Dll4 on hESC-derived vascular feeder cells. These data provide a framework for defining HPC lineage potential, elucidate a molecular contribution from the vascular niche in promoting hematopoietic lineage progression, and distinguish unique subpopulations of hemogenic endothelium during hESC differentiation.

KEY POINTS

Live imaging of endothelial to hematopoietic conversion identifies distinct subpopulations of hESC-derived hemogenic endothelium. Expression of the Notch ligand DII4 on vascular ECs drives induction of myeloid fate from hESC-derived hematopoietic progenitors.