Development of hematopoietic stem and progenitor cells from mouse embryonic stem cells, in vitro, supported by ectopic human HOXB4 expression.

Differentiation of pluripotent embryonic stem (ES) cells can recapitulate many aspects of hematopoiesis, in vitro, and can even generate cells capable of long-term multilineage repopulation after transplantation into recipient mice, when the homeodomain transcription factor HOXB4 is ectopically expressed. Thus, the ES-cell differentiation system is of great value for a detailed understanding of the process of blood formation. Furthermore, it is also promising for future application in hematopoietic cell and gene therapy. Since the arrival of techniques which allow the reprogramming of somatic cells back to an ES cell-like state, the generation of hematopoietic stem cells from patient-specific so-called induced pluripotent stem cells shows great promise for future therapeutic applications. In this chapter, we describe how to cultivate a certain feeder cell-independent mouse embryonic stem cell line, to manipulate these cells by retroviral gene transfer to ectopically express HOXB4, to differentiate these ES cells via embryoid body formation, and to selectively expand the arising, HOXB4-expressing hematopoietic stem and progenitor cells.

Effective adjunctive therapy by an innate defense regulatory peptide in a preclinical model of severe malaria.

Case fatality rates for severe malaria remain high even in the best clinical settings because antimalarial drugs act against the parasite without alleviating life-threatening inflammation. We assessed the potential for host-directed therapy of severe malaria of a new class of anti-inflammatory drugs, the innate defense regulator (IDR) peptides, based on host defense peptides. The Plasmodium berghei ANKA model of experimental cerebral malaria was adapted to use as a preclinical screen by combining late-stage intervention in established infections with advanced bioinformatic analysis of early transcriptional changes in co-regulated gene sets. Coadministration of IDR-1018 with standard first-line antimalarials increased survival of infected mice while down-regulating key inflammatory networks associated with fatality. Thus, IDR peptides provided host-directed adjunctive therapy for severe disease in combination with antimalarial treatment.

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.

Directed differentiation and mass cultivation of pure erythroid progenitors from mouse embryonic stem cells.

Differentiating embryonic stem (ES) cells are an increasingly important source of hematopoietic progenitors, useful for both basic research and clinical applications. Besides their characterization in colony assays, protocols exist for the cultivation of lymphoid, myeloid, and erythroid cells. With the possible exception of mast cells, however, long-term expansion of pure hematopoietic progenitors from ES cells has not been possible without immortalization caused by overexpression of exogenous genes. Here, we describe for the first time an efficient yet easy strategy to generate mass cultures of pure, immature erythroid progenitors from mouse ES cells (ES-EPs), using serum-free medium plus recombinant cytokines and hormones. ES-EPs represent long-lived, adult, definitive erythroid progenitors that resemble immature erythroid cells expanding in vivo during stress erythropoiesis. When exposed to terminal differentiation conditions, ES-EPs differentiated into mature, enucleated erythrocytes. Importantly, ES-EPs injected into mice did not exhibit tumorigenic potential but differentiated into normal erythrocytes. Both the virtually unlimited supply of cells and the defined culture conditions render our system a valuable tool for the analysis of factors influencing proliferation and maturation of erythroid progenitors. In addition, the system allows detailed characterization of processes during erythroid proliferation and differentiation using wild-type (wt) and genetically modified ES cells.

Raf-1 antagonizes erythroid differentiation by restraining caspase activation.

The Raf kinases are key signal transducers activated by mitogens or oncogenes. The best studied Raf isoform, Raf-1, was identified as an inhibitor of apoptosis by conventional and conditional gene ablation in mice. c-raf-1(-)(/)(-) embryos are growth retarded and anemic, and die at midgestation with anomalies in the placenta and fetal liver. Here, we show that Raf-1-deficient primary erythroblasts cannot be expanded in culture due to their accelerated differentiation into mature erythrocytes. In addition, Raf-1 expression is down-regulated in differentiating wild-type cells, whereas overexpression of activated Raf-1 delays differentiation. As recently described for human erythroid precursors, we find that caspase activation is necessary for the differentiation of murine fetal liver erythroblasts. Differentiation-associated caspase activation is accelerated in erythroid progenitors lacking Raf-1 and delayed by overexpression of the activated kinase. These results reveal an essential function of Raf-1 in erythropoiesis and demonstrate that the ability of Raf-1 to restrict caspase activation is biologically relevant in a context distinct from apoptosis.