Origin of hematopoietic progenitors during embryogenesis

It has been widely accepted that hematopoietic and endothelial cell lineages diverge from a common progenitor referred to as the hemangioblast. Recently, analyses of the potential of progenitor cells purified from mouse embryos as well as embryonic stem cells differentiating in vitro resolved intermediate stages between mesodermal cells and committed precursors for hematopoietic and endothelial cell lineages. There are two distinct hematopoietic cell lineages which have different origins, i.e., primitive hematopoietic lineage derived from mesoderm or hemangioblasts and definitive hematopoietic lineage derived from endothelial cells. The endothelium is suggested to provide a milieu in which the definitive hematopoietic lineage acquires multiple potentials.     

In vitro proliferation of pluripotent hematopoietic progenitors

Summary Different ratios of normal human plasma concentrations to fetal bovine serum concentrations have been tested on the in vitro proliferation of pluripotent hematopoietic progenitors. The slight modifications of the culture procedure described here resulted in significant enhancement of BFU-e formation whereas no significant differences in the formation of CFU-e, CFU-meg, CFU-gm, and CFU-mix were observed.     

Circulating hematopoietic progenitors with T lineage potential

The thymus is seeded via the blood, but the identity of hematopoietic progenitors with access to the circulation in adult mice is unknown. We report here that the only progenitors in blood with efficient T lineage potential were lineage negative with high expression of stem cell antigen 1 and c-Kit (LSK cells). The blood LSK population, like its counterpart in the bone marrow, contained hematopoietic stem cells and nonrenewing, multipotent progenitors, including early lymphoid progenitors and CD62L(+) cells previously described as efficient T lineage progenitors. Common lymphoid progenitors could not be identified in the circulation, suggesting they are not physiological T lineage precursors. We conclude that blood LSK cells are the principal circulating progenitors with T lineage potential.     

Lymphoid and myeloid lineage commitment in multipotent hematopoietic progenitors

Hematopoietic stem cells (HSCs) continuously replenish all classes of blood cells through a series of lineage restriction steps that results in the progressive loss of differentiation potential to other cell lineages. This review focuses on the recent advances in understanding one of the earliest differentiation steps in HSC maturation, which involves the diversification of the lymphoid and myeloid cell lineages, the two major branches of hematopoietic cells. We discuss progress in the identification and characterization of progenitor populations downstream of HSCs, which has been a key to understanding the sequential biological events that take place along the course of differentiation into a certain hematopoietic cell type. We also discuss the importance of bone marrow microenvironment in lymphoid and myeloid lineage choice.     

Correlation between hematopoietic progenitors and erythroblasts in cord blood

Erythroid and granulocyte-monocyte progenitors in cord blood were studied, using the methyl cellulose assay. The mean number of progenitors was 12-16 times higher than that of normal adult blood. The number of both erythroid and granulocyte-monocyte progenitors correlated positively with the number of erythroblasts present in cord blood. The possible origin of hematopoietic progenitors in cord blood was discussed.     

Evidence for histamine uptake by murine hematopoietic progenitors

Based on previous evidence for a role of exogenous and endogenous histamine, we have examined whether this amine can effectively interact with hematopoietic progenitors. We show that tritiated histamine is retained preferentially by bone marrow cells as compared with peritoneal, thymic or spleen cells. Cells interacting with histamine copurify with progenitors in the low density bone marrow fraction. Among this cell population, 5% are labeled as assessed by autoradiography. The characteristics of histamine retention by these cells are consistent with active uptake rather than binding to a known receptor since (a) it is almost completely abrogated at 4°C, by sodium azide or chloroquine, (b) histamine is internalized, and (c) relatively high concentrations of classical receptor antagonists are required to inhibit histamine retention by bone marrow cells.

     

Evidence for histamine uptake by murine hematopoietic progenitors

Based on previous evidence for a role of exogenous and endogenous histamine, we have examined whether this amine can effectively interact with hematopoietic progenitors. We show that tritiated histamine is retained preferentially by bone marrow cells as compared with peritoneal, thymic or spleen cells. Cells interacting with histamine copurify with progenitors in the low density bone marrow fraction. Among this cell population, 5% are labeled as assessed by autoradiography. The characteristics of histamine retention by these cells are consistent with active uptake rather than binding to a known receptor since (a) it is almost completely abrogated at 4°C, by sodium azide or chloroquine, (b) histamine is internalized, and (c) relatively high concentrations of classical receptor antagonists are required to inhibit histamine retention by bone marrow cells.     

Interaction between normal and CML hematopoietic progenitors and stroma influences abnormal hematopoietic development

Several studies have shown defective progenitor-stromal interactions in chronic myeloid leukemia (CML), and adhesive defects induced by BCR/ABL have been described. However, controversial results have been reported, and the role of the stroma in abnormal development of the hematopoietic system is not clear. In this study, CML hematopoietic and irradiated stromal cells were co-cultured in different combinations for 10 or 21 days. Maintenance of viable cells was dependent both on the sources of hematopoietic progenitors and stromal adherent layers, with normal cells performing better than their leukemic counterparts. The frequency of CD34(+) CD38(-) cells in the non-adherent fraction was more related to the source of hematopoietic cells than of stroma, and hematopoietic cells from normal subjects showed better performance. The simultaneous analysis of different combinations of normal and leukemic precursor cells and stromal layers, as done in the present work, suggests that the outcome of the interaction depends on characteristics of both compartments. This hematopoietic system development is influenced by intrinsic qualities of both hematopoietic stem cells and the supportive stroma.     

Fas transduces dual apoptotic and trophic signals in hematopoietic progenitors

Stem cells and progenitors are often required to realize their differentiation potential in hostile microenvironments. The Fas/Fas ligand (FasL) interaction is a major effector pathway of apoptosis, which negatively regulates the expansion of differentiated hematopoietic cells. The involvement of this molecular interaction in the function of hematopoietic stem and progenitor cells is not well understood. In the murine syngeneic transplant setting, both Fas and FasL are acutely upregulated in bone marrow-homed donor cells; however, the Fas(+) cells are largely insensitive to FasL-induced apoptosis. In heterogeneous populations of lineage-negative (lin(-)) bone marrow cells and progenitors isolated by counterflow centrifugal elutriation, trimerization of the Fas receptor enhanced the clonogenic activity. Inhibition of caspases 3 and 8 did not affect the trophic signals mediated by Fas, yet it efficiently blocked the apoptotic pathways. Fas-mediated tropism appears to be of physiological significance, as pre-exposure of donor cells to FasL improved the radioprotective qualities of hematopoietic progenitors, resulting in superior survival of myeloablated hosts. Under these conditions, the activity of long-term reconstituting cells was not affected, as determined in sequential secondary and tertiary transplants. Dual caspase-independent tropic and caspase-dependent apoptotic signaling place the Fas receptor at an important junction of activation and death. This regulatory mechanism of hematopoietic homeostasis activates progenitors to promote the recovery from aplasia and converts into a negative regulator in distal stages of cell differentiation. Disclosure of potential conflicts of interest is found at the end of this article.     

Green fluorescent protein as a selectable marker of retrovirally transduced hematopoietic progenitors.

Recombinant retroviruses are most commonly used in hematopoietic stem cell gene therapy trials, but gene transfer efficiency is still inadequate with the present vectors. One approach for overcoming this problem is to develop methods of selecting and enriching the successfully transduced cells. We investigated the feasibility of using the green fluorescent protein (GFP) gene as a selectable marker of hematopoietic cells. When M1 murine leukemia cells were electroporated with GFP expression vectors, a red-shifted mutant (S65T) GFP showed several-fold greater fluorescence than the wild-type GFP and generated readily detectable green light under control of SRalpha or CAG promoter. We then inserted an SRalpha-S65T GFP cassette into the MSCV retrovirus vector and established virus producer cells. Infection of primary murine bone marrow cells resulted in a distinct population with green fluorescence, which was separated by fluorescence-activated cell sorting. The fractionated bright cells gave rise to fluorescent spleen colonies in lethally irradiated mice, while the fluorescence-negative cells yielded only dark colonies. These results indicated that GFP is a faithful marker in gene transfer into hematopoietic progenitor/stem cells, facilitating selection of the transduced cells and tracking of their progeny in vivo.     

Game theoretic analysis of interferons effect on hematopoietic progenitors growth

The effects of recombinant alpha, beta and gamma interferons (IFN), alone or in combinations were studied in a case of essential thrombocythemia, using the mixed colony formation assay. This assay allows growth of multipotent (CFU-Mix), and unipotent granulocytic-macrophage (CFU-GM), erythroid (BFU-E) and megakaryocytic (CFU-Meg) progenitors. The bone marrow precursors were cultured in the presence of each type of IFN at 100 U/ml and all possible combinations performed. The results were analysed using the Shapley formula, a game theoretic approach. It is concluded that IFN alpha would be the best candidate for reducing megakaryocytic progenitors while growth of other hematopoietic precursors would be stimulated. These types of analytical biological experiments controlled by using six permutations and calculation of the Shapley value for the three types of IFNs are suggested as a fair approach for a rational choice.     

The role of c-Mpl ligands in the expansion of cord blood hematopoietic progenitors

The major limitations to the widespread use of high-dose chemotherapy or radiotherapy followed by autologous or allogeneic transplantation are the scarcity of stem cell donors and the depletion of the autologous stem cell reservoir. Cord blood is a readily available source of stem cells, which, however, might be limited in number. For this reason, up to now, cord blood transplantation has been restricted to children. Therefore, a major goal for experimental and clinical hematology is the identification of mechanisms and conditions that support the expansion of transplantable hematopoietic stem cells. Two systems have been described to identify in vitro these progenitor cell populations in both mice and humans: A) long-term culture-initiating cells (LTC-IC), so named because of their ability to support the growth of hemopoietic colonies (colony-forming cell [CFC]) for five to six weeks when cocultured on stromal layers, and B) the generation of hematopoietic progenitors CFC from stroma-free liquid cultures for extended periods of time, which is another indirect evidence for the presence of primitive stem cells. The two systems detect largely overlapping but not identical cell populations of progenitor cells; thus, the identification of the growth factor requirements for the maintenance and amplification of both systems is relevant. The studies presented here demonstrate that CD34+ cord blood cells can be grown in stroma-free liquid cultures for extremely prolonged periods of time (up to six months). During such a period, hemopoietic precursors and committed progenitors belonging to all of the hematopoietic lineages are continuously and massively generated. Such a massive expansion is sustained by an increasingly larger expansion of primitive stem cells (CFU-BI and LTC-IC). The presence of both FL and thrombopoietin (TPO) was necessary and sufficient to support this phenomenon. The addition of KL +/- interleukin 6 (IL-6) does not appear to substantially modify the extent of LTC-IC expansion. FL and TPO appear to be two unique growth factors that preferentially support the self-renewal of primitive stem cells; the additional presence of KL and IL-6 seems to enhance the proliferative potential of at least a subpopulation of daughter stem cells which can undergo at least three differentiation pathways.     

阵发性睡眠性血红蛋白尿病人骨髓红系和粒－单系祖细胞的特点

本文采用造血祖细胞体外培养技术观察了阵发性睡眠性血红蛋白尿（ＰＮＨ）病人骨髓红系祖细胞（ＢＦＵ一Ｅ和ＣＦＵ一Ｅ）和粒一单系祖细胞（ＣＦＵ一ＧＭ）的增殖能力；骨髓细胞经酸化ＡＢ型血清处理后的ＢＦＵ一Ｅ，ＣＦＵ一Ｅ和ＣＦＵ一ＧＭ的增殖能力；以及ＢＦＵ一Ｅ、ＣＦＵ一Ｅ对红细胞生成素（Ｅｐｏ）和（ＣＦＵ一ＧＭ对粒一单系集落刺激因子（ＧＭ一ＣＳＦ）的反应能力，发现ＰＮＨ病人骨髓ＢＦＵ一Ｅ，ＣＦＵ一Ｅ和ＣＦＵ一ＧＭ集落数明显低于正常；骨髓细胞经新鲜酸化ＡＢ型血清处理后培养的ＢＦＵ一Ｅ、ＣＦＵ一Ｅ和ＣＦＵ一ＧＭ集落数明显低于经热灭活酸化ＡＢ型血清处理后培养的集落数；以及ＢＦＵ一Ｅ，ＣＦＵ一Ｅ对Ｅｐｏ和ＣＦＵ一ＧＭ对ＧＭ一ＣＳＦ的剂量反应曲线低平。因此认为ＰＮＨ病人骨髓红系和粒一单系祖细胞有以下特点：１．增殖能力降低：２．在酸性条件下对补体的敏感性增加；３．对造血因子的敏感性降低。     

A conserved role for Hox paralog group 4 in regulation of hematopoietic progenitors

Regulatory circuits that control stem cell fate decisions can be identified and understood by manipulating individual regulatory elements genetically. While impractical in the rare somatic stem cells of primary tissue, this approach is feasible in embryonic stem cells differentiated in vitro into the somatic stem cell type of interest. We present an improved highly efficient targeting system allowing genes to be integrated into a predetermined, doxycycline-inducible locus, and corresponding inducible embryonic stem cell lines to be generated rapidly. We apply this system to evaluate a key hematopoietic progenitor cell regulatory element, HoxB4, and its mammalian paralogs, whose effects have not yet been tested in this context. We show that all Hox paralog group 4 members, A4, B4, C4, and D4, have similar effects on hematopoietic stem and progenitor self-renewal in vitro, and thus classify Hox paralog group 4 as promoting self-renewal. Each paralog group 4 member both promotes proliferation and inhibits differentiation, enabling the exponential expansion of hematopoietic progenitors from the c-kit(+)/CD41(+) cell fraction of day 6 murine embryoid bodies. By evaluating a set of deletion mutants we show that sequences in addition to the homeodomain and hexapeptide motif are required for this activity. These results highlight the utility of this expression system to perform functional and structural analyses of genetic regulators of cell fate decisions.     

Kinetics of hematopoietic clones in reconstituted mice

In mice reconstituted with genetically marked bone marrow, hematopoiesis is shown to be effected, over a period of 14 months after transplantation, by numerous locally existing and short-lived cell clones that succeed one another as the potential of the clonogenic precursor cell is depleted. In each reconstituted mouse, several dozens of hematopoietic clones are functional in the bone marrow. Immortal self-maintaining stem cells with unlimited proliferative potential are not detected in this system. Translated fromByulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 123, No. 2, pp. 180–183, February, 1997