Andrea Ditadi

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.

Wnt signaling controls the specification of definitive and primitive hematopoiesis from human pluripotent stem cells

Efforts to derive hematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) are complicated by the fact that embryonic hematopoiesis consists of two programs, primitive and definitive, that differ in developmental potential. As only definitive hematopoiesis generates HSCs, understanding how this program develops is essential for being able to produce this cell population in vitro. Here we show that both hematopoietic programs transition through hemogenic endothelial intermediates and develop from KDR+CD34−CD144− progenitors that are distinguished by CD235a expression. Generation of primitive progenitors (KDR+CD235a+) depends on stage-specific activin-nodal signaling and inhibition of the Wnt–β-catenin pathway, whereas specification of definitive progenitors (KDR+CD235a−) requires Wnt–β-catenin signaling during this same time frame. Together, these findings establish simple selective differentiation strategies for the generation of primitive or definitive hematopoietic progenitors by Wnt–β-catenin manipulation, and in doing so provide access to enriched populations for future studies on hPSC-derived hematopoietic development.

Human definitive haemogenic endothelium and arterial vascular endothelium represent distinct lineages

The generation of haematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) will depend on the accurate recapitulation of embryonic haematopoiesis. In the early embryo, HSCs develop from the haemogenic endothelium (HE) and are specified in a Notch-dependent manner through a process named endothelial-to-haematopoietic transition (EHT). As HE is associated with arteries, it is assumed that it represents a subpopulation of arterial vascular endothelium (VE). Here we demonstrate at a clonal level that hPSC-derived HE and VE represent separate lineages. HE is restricted to the CD34+CD73−CD184− fraction of day 8 embryoid bodies and it undergoes a NOTCH-dependent EHT to generate RUNX1C+ cells with multilineage potential. Arterial and venous VE progenitors, in contrast, segregate to the CD34+CD73medCD184+ and CD34+CD73hiCD184− fractions, respectively. Together, these findings identify HE as distinct from VE and provide a platform for defining the signalling pathways that regulate their specification to functional HSCs.

Haematopoietic stem and progenitor cells from human pluripotent stem cells

A variety of tissue lineages can be differentiated from pluripotent stem cells by mimicking embryonic development through stepwise exposure to morphogens, or by conversion of one differentiated cell type into another by enforced expression of master transcription factors. Here, to yield functional human haematopoietic stem cells, we perform morphogen-directed differentiation of human pluripotent stem cells into haemogenic endothelium followed by screening of 26 candidate haematopoietic stem-cell-specifying transcription factors for their capacity to promote multi-lineage haematopoietic engraftment in mouse hosts. We recover seven transcription factors (ERG, HOXA5, HOXA9, HOXA10, LCOR, RUNX1 and SPI1) that are sufficient to convert haemogenic endothelium into haematopoietic stem and progenitor cells that engraft myeloid, B and T cells in primary and secondary mouse recipients. Our combined approach of morphogen-driven differentiation and transcription-factor-mediated cell fate conversion produces haematopoietic stem and progenitor cells from pluripotent stem cells and holds promise for modelling haematopoietic disease in humanized mice and for therapeutic strategies in genetic blood disorders.

Defining the path to hematopoietic stem cells.

New insights into hemogenic endothelium will facilitate efforts to produce hematopoietic stem cells in vitro.

A view of human haematopoietic development from the Petri dish

Human pluripotent stem cells (hPSCs) provide an unparalleled opportunity to establish in vitro differentiation models that will transform our approach to the study of human development. In the case of the blood system, these models will enable investigation of the earliest stages of human embryonic haematopoiesis that was previously not possible. In addition, they will provide platforms for studying the origins of human blood cell diseases and for generating de novo haematopoietic stem and progenitor cell populations for cell-based regenerative therapies.

Directed differentiation of definitive hemogenic endothelium and hematopoietic progenitors from human pluripotent stem cells

The generation of hematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) remains a major goal for regenerative medicine and disease modeling. However, hPSC differentiation cultures produce mostly hematopoietic progenitors belonging to the embryonic HSC-independent hematopoietic program, which may not be relevant or accurate for modeling normal and disease-state adult hematopoietic processes. Through a stage-specific directed differentiation approach, it is now possible to generate exclusively definitive hematopoietic progenitors from hPSCs showing characteristics of the more developmentally advanced fetal hematopoiesis. Here, we summarize recent efforts at generating hPSC-derived definitive hematopoiesis through embryoid body differentiation under defined conditions. Embryoid bodies are generated through enzymatic dissociation of hPSCs from matrigel-coated plasticware, followed by recombinant BMP4, driving mesoderm specification. Definitive hematopoiesis is specified by a GSK3β-inhibitor, followed by recombinant VEGF and supportive hematopoietic cytokines. The CD34+ cells obtained using this method are then suitable for hematopoietic assays for definitive hematopoietic potential.