Concise review: programming human pluripotent stem cells into blood

Engineering T Cell Development for the Next Generation of Stem Cell-Derived Immunotherapies

Engineered T cells are at the leading edge of clinical cell therapy. T cell therapies have had a remarkable impact on patient care for a subset of hematological malignancies. This foundation has motivated the development of off-the-shelf engineered cell therapies for a broad range of devastating indications. Achieving this vision will require cost-effective manufacturing of precision cell products capable of addressing multiple process and clinical-design challenges. Pluripotent stem cell (PSC)-derived engineered T cells are emerging as a solution of choice. To unleash the full potential of PSC-derived T cell therapies, the field will require technologies capable of robustly orchestrating the complex series of time- and dose-dependent signaling events needed to recreate functional T cell development in the laboratory. In this article, we review the current state of allogenic T cell therapies, focusing on strategies to generate engineered lymphoid cells from PSCs. We highlight exciting recent progress in this field and outline timely opportunities for advancement with an emphasis on niche engineering and synthetic biology.

Progress in the production of haematopoietic stem and progenitor cells from human pluripotent stem cells

Cell therapies are currently used to treat many haematological diseases. These treatments range from the long-term reconstitution of the entire haematopoietic system using the most potent haematopoietic stem cells (HSCs) to the short-term rescue with mature functional end cells such as oxygen-carrying red blood cells and cells of the immune system that can fight infection and repair tissue. Limitations in supply and the risk of transmitting infection has prompted the design of protocols to produce some of these cell types from human pluripotent stem cells (hPSCs). Although it has proven challenging to generate the most potent HSCs directly from hPSCs, significant progress has been made in the development of differentiation protocols that can successfully produce haematopoietic progenitor cells and most of the mature cell lineages. We review the key steps used in the production of haematopoietic stem and progenitor cells (HSPCs) from hPSCs and the cell surface markers and reporter strategies that have been used to define specific transitions. Most studies have relied on the use of known markers that define HSPC production in vivo but more recently single cell RNA sequencing has allowed a less biased approach to their characterisation. Transcriptional profiling has identified new markers for naïve and committed hPSC-derived HSPC populations and trajectory analyses has provided novel insights into their lineage potential. Direct comparison of in vitro- and in vivo-derived RNA single cell sequencing datasets has highlights similarities and differences between the two systems and this deeper understanding will be key to the design and the tracking of improved and more efficient differentiation protocols.

LGR4, Not LGR5, Enhances hPSC Hematopoiesis by Facilitating Mesoderm Induction via TGF-Beta Signaling Activation

Attempts to generate functional blood cells from human pluripotent stem cells (hPSCs) remain largely unsuccessful, mainly due to the lack of understanding of the regulatory network of human hematopoiesis. In this study, we identified leucine-rich-repeat-containing G-protein-coupled receptor 4 (LGR4) as an essential regulator of early hematopoietic differentiation of hPSCs. The deletion of LGR4 severely impairs mesoderm development, thereby limiting hematopoietic differentiation both in vitro and in vivo. In contrast, LGR5 is dispensable for hPSC hematopoiesis. The four R-spondin proteins show differential activities and dependencies on LGR4 in hematopoietic differentiation. The deletion of LGR4 almost entirely abolishes the enhancement induced by R-spondin1 and R-spondin3, but not R-spondin2. In addition, ZNRF3 is required for the response of R-spondin1-R-spondin3. At the mechanistic level, LGR4 regulates transforming growth factor beta (TGF-beta) signaling to control hematopoietic differentiation. Together, our results reveal vital roles of LGR4 in hematopoietic development and uncover distinct functions and underlying mechanisms for R-spondins.

First blood: the endothelial origins of hematopoietic progenitors

Hematopoiesis in vertebrate embryos occurs in temporally and spatially overlapping waves in close proximity to blood vascular endothelial cells. Initially, yolk sac hematopoiesis produces primitive erythrocytes, megakaryocytes, and macrophages. Thereafter, sequential waves of definitive hematopoiesis arise from yolk sac and intraembryonic hemogenic endothelia through an endothelial-to-hematopoietic transition (EHT). During EHT, the endothelial and hematopoietic transcriptional programs are tightly co-regulated to orchestrate a shift in cell identity. In the yolk sac, EHT generates erythro-myeloid progenitors, which upon migration to the liver differentiate into fetal blood cells, including erythrocytes and tissue-resident macrophages. In the dorsal aorta, EHT produces hematopoietic stem cells, which engraft the fetal liver and then the bone marrow to sustain adult hematopoiesis. Recent studies have defined the relationship between the developing vascular and hematopoietic systems in animal models, including molecular mechanisms that drive the hemato-endothelial transcription program for EHT. Moreover, human pluripotent stem cells have enabled modeling of fetal human hematopoiesis and have begun to generate cell types of clinical interest for regenerative medicine.

Single-cell analyses and machine learning define hematopoietic progenitor and HSC-like cells derived from human PSCs

Hematopoietic stem and progenitor cells (HSPCs) develop in distinct waves at various anatomical sites during embryonic development. The in vitro differentiation of human pluripotent stem cells (hPSCs) recapitulates some of these processes; however, it has proven difficult to generate functional hematopoietic stem cells (HSCs). To define the dynamics and heterogeneity of HSPCs that can be generated in vitro from hPSCs, we explored single-cell RNA sequencing (scRNAseq) in combination with single-cell protein expression analysis. Bioinformatics analyses and functional validation defined the transcriptomes of naïve progenitors and erythroid-, megakaryocyte-, and leukocyte-committed progenitors, and we identified CD44, CD326, ICAM2/CD9, and CD18, respectively, as markers of these progenitors. Using an artificial neural network that we trained on scRNAseq derived from human fetal liver, we identified a wide range of hPSC-derived HSPCs phenotypes, including a small group classified as HSCs. This transient HSC-like population decreased as differentiation proceeded, and was completely missing in the data set that had been generated using cells selected on the basis of CD43 expression. By comparing the single-cell transcriptome of in vitro-generated HSC-like cells with those generated within the fetal liver, we identified transcription factors and molecular pathways that can be explored in the future to improve the in vitro production of HSCs.

Human induced pluripotent stem cell line banking for the production of rare blood type erythrocytes

Background

The in vitro production of mature human red blood cells (RBCs) from induced pluripotent stem cells (iPSCs) has been the focus of research to meet the high demand for blood transfusions. However, limitations like high costs and technological requirements restrict the use of RBCs produced by iPSC differentiation to specific circumstances, such as for patients with rare blood types or alloimmunized patients. In this study, we developed a detailed protocol for the generation of iPSC lines derived from peripheral blood of donors with O D-positive blood and rare blood types (D–and Jr(a-)) and subsequent erythroid differentiation.

Methods

Mononuclear cells separated from the peripheral blood of O D-positive and rare blood type donors were cultured to produce and expand erythroid progenitors and reprogrammed into iPSCs. A 31-day serum-free, xeno-free erythroid differentiation protocol was used to generate reticulocytes. The stability of iPSC lines was confirmed with chromosomal analysis and RT-PCR. Morphology and cell counts were determined by microscopy observations and flow cytometry.

Results

Cells from all donors were successfully used to generate iPSC lines, which were differentiated into erythroid precursors without any apparent chromosomal mutations. This differentiation protocol resulted in moderate erythrocyte yield per iPSC.

Conclusions

It has previously only been hypothesized that erythroid differentiation from iPSCs could be used to produce RBCs for transfusion to patients with rare blood types or who have been alloimmunized. Our results demonstrate the feasibility of producing autologous iPSC-differentiated RBCs for clinical transfusions in patients without alternative options.

R-spondin2 promotes hematopoietic differentiation of human pluripotent stem cells by activating TGF beta signaling

BACKGROUND

Human pluripotent stem cells (hPSCs) provide supplies of potential functional blood cells to suffice the clinical needs. However, the underlying mechanism of generating genuine hematopoietic stem cells (HSCs) and functional blood cells from hPSCs remains largely elusive.

METHOD

In this study, we supplied R-spondin2 exogenously during hematopoietic differentiation of hPSCs under various culture conditions and analyzed the production of hematopoietic progenitor cells (HPCs). We further added R-spondin2 at different temporal window to pin down the stage at which R-spondin2 conferred its effects. RNA-SEQ-based gene profiling was applied to analyze genes with significantly altered expression and altered signaling pathways. Finally, megakaryocytic differentiation and platelet generation were determined using HPCs with R-spondin2 treatment.

RESULTS

We found that R-spondin2 generated by hematopoiesis-supporting stromal cells significantly enhances hematopoietic differentiation of hPSCs. Supply of R-spondin2 exogenously at the early stage of mesoderm differentiation elevates the generation of APLNR⁺ cells. Furthermore, early treatment of cells with R-spondin2 enables us to increase the output of hPSC-derived platelet-like particles (PLPs) with intact function. At the mechanistic level, R-spondin2 activates TGF-β signaling to promote the hematopoietic differentiation.

CONCLUSIONS

Our results demonstrate that a transient supply of R-spondin2 can efficiently promote hematopoietic development by activating both WNT and TGF-β signaling. R-spondin2 can be therefore used as a powerful tool for large-scale generation of functional hematopoietic progenitors and platelets for translational medicine.

MicroRNAs as regulators and effectors of hematopoietic transcription factors

Hematopoiesis is a highly-regulated development process orchestrated by lineage-specific transcription factors that direct the generation of all mature blood cells types, including red blood cells, megakaryocytes, granulocytes, monocytes, and lymphocytes. Under homeostatic conditions, the hematopoietic system of the typical adult generates over 10¹¹ blood cells daily throughout life. In addition, hematopoiesis must be responsive to acute challenges due to blood loss or infection. MicroRNAs (miRs) cooperate with transcription factors to regulate all aspects of hematopoiesis, including stem cell maintenance, lineage selection, cell expansion, and terminal differentiation. Distinct miR expression patterns are associated with specific hematopoietic lineages and stages of differentiation and functional analyses have elucidated essential roles for miRs in regulating cell transitions, lineage selection, maturation, and function. MiRs function as downstream effectors of hematopoietic transcription factors and as upstream regulators to control transcription factor levels. Multiple miRs have been shown to play essential roles. Regulatory networks comprised of differentially expressed lineage-specific miRs and hematopoietic transcription factors are involved in controlling the quiescence and self-renewal of hematopoietic stem cells as well as proliferation and differentiation of lineage-specific progenitor cells during erythropoiesis, myelopoiesis, and lymphopoiesis. This review focuses on hematopoietic miRs that function as upstream regulators of central hematopoietic transcription factors required for normal hematopoiesis. This article is categorized under: RNA in Disease and Development > RNA in Development Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs.

Activation of KLF1 Enhances the Differentiation and Maturation of Red Blood Cells from Human Pluripotent Stem Cells

Blood transfusion is widely used in the clinic but the source of red blood cells (RBCs) is dependent on donors, procedures are susceptible to transfusion-transmitted infections and complications can arise from immunological incompatibility. Clinically-compatible and scalable protocols that allow the production of RBCs from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have been described but progress to translation has been hampered by poor maturation and fragility of the resultant cells. Genetic programming using transcription factors has been used to drive lineage determination and differentiation so we used this approach to assess whether exogenous expression of the Erythroid Krüppel-like factor 1 (EKLF/KLF1) could augment the differentiation and stability of iPSC-derived RBCs. To activate KLF1 at defined time points during later stages of the differentiation process and to avoid transgene silencing that is commonly observed in differentiating pluripotent stem cells, we targeted a tamoxifen-inducible KLF1-ERT2 expression cassette into the AAVS1 locus. Activation of KLF1 at day 10 of the differentiation process when hematopoietic progenitor cells were present, enhanced erythroid commitment and differentiation. Continued culture resulted the appearance of more enucleated cells when KLF1 was activated which is possibly due to their more robust morphology. Globin profiling indicated that these conditions produced embryonic-like erythroid cells. This study demonstrates the successful use of an inducible genetic programing strategy that could be applied to the production of many other cell lineages from human induced pluripotent stem cells with the integration of programming factors into the AAVS1 locus providing a safer and more reproducible route to the clinic. Stem Cells 2017;35:886-897.