Ontogeny of human hematopoietic cells: analysis utilizing monoclonal antibodies

Lymphoid and myeloid cells isolated from second trimester fetal lymphoid organs were characterized by utilizing a panel of monoclonal antibodies that define human lineage-restricted, differentiation, histocompatibility, and activation antigens. At distinct gestational stages, the appearance of morphologically identifiable lymphoid and myeloid cells paralleled the appearance of cells expressing definable lymphoid and myeloid antigens. The proportion of cells in fetal liver, bone marrow, and spleen that expressed histocompatibility, myeloid, and B cell antigens increased with fetal maturation. In contrast, even the earliest fetal thymuses studied were of a phenotype no different than that seen during later stages of ontogeny. Although the cellular lineage of most fetal hematopoietic cells could be identified by this panel of reagents, a considerable number of fetal liver and bone marrow cells did not express any of these antigens, suggesting the possibility that they might represent early hematopoietic progenitor cells. These studies support the notion that the adult cellular phenotype is the result of both an orderly acquisition of differentiation antigens and the migration of these primitive cellular populations to specific fetal organs. Identification of hematopoietic progenitors in fetal tissues may facilitate the identification and isolation of early lymphoid and myeloid progenitor cells in adults.     

HIV-1 utilizes the CXCR4 chemokine receptor to infect multipotent hematopoietic stem and progenitor cells

HIV infection is characterized by gradual immune system collapse and hematopoietic dysfunction. We recently showed that HIV enters multipotent hematopoietic progenitor cells and establishes both active cytotoxic and latent infections that can be reactivated by myeloid differentiation. However, whether these multipotent progenitors include long-lived hematopoietic stem cells (HSCs) that could establish viral reservoirs for the life of the infected person remains unknown. Here we provide direct evidence that HIV targets long-lived HSCs and show that infected HSCs yield stable, multilineage engraftment in a xenograft model. Furthermore, we establish that the capacity to use the chemokine receptor CXCR4 for entry determines whether a virus will enter multipotent versus differentiated progenitor cells. Because HSCs live for the life span of the infected person and are crucial for hematopoietic health, these data may explain the poor prognosis associated with CXCR4-tropic HIV infection and suggest HSCs as long-lived cellular reservoirs of latent HIV.     

Bone marrow stromal cells selectively stimulate the rapid expansion of lineage-restricted myeloid progenitors

Bone marrow stromal cells serve hematopoietic microenvironments where different blood cells are controlled in their growth and differentiation. To characterize functions of stromal cells, 33 bone marrow stromal cells including preadipocytes, endothelial cells, and fibroblasts were established from transgenic mice harboring temperature-sensitive SV40 T-antigen gene and their selective stimulatory abilities to support large colony formation of lineage-specific hematopoietic progenitor cells (erythroid, monocyte/macrophage, granulocyte, and monocyte-granulocyte) were examined. Among established stromal cells, 27 clones showed erythropoietic stimulatory activity in the presence of erythropoietin. On myeloid progenitors, the stromal cells showed lineage-restricted stimulatory activity and a reciprocal relationship was observed between granulocyte formation and macrophage formation, but these activities were not dependent on the amount of produced colony-stimulating factors (CSFs). Our present study with many stromal cells established from bone marrow indicated that each stromal cell in the bone marrow may provide the preferable microenvironment for a rapid expansion of the lineage-restricted progenitor cells in combination with CSFs. © 1995 Wiley-Liss, Inc.     

New evidence supporting megakaryocyte-erythrocyte potential of flk2/flt3+ multipotent hematopoietic progenitors

A model of hematopoietic development wherein multipotentiality is conserved until segregation of myeloid and lymphoid potential has recently been challenged, proposing that megakaryocyte/erythrocyte (MegE) potential is lost in Flk2/Flt3-expressing early progenitors. Here, we used sensitive in vivo approaches to quantitatively and kinetically assess the MegE potential of hematopoietic stem cells and various Flk2(+) early progenitors and compared it with the MegE potential of downstream committed myeloid and lymphoid progenitors and with their ability to give rise to mature myelomonocytic and lymphoid cells. We demonstrate that Flk2(+) early progenitors retain MegE potential in vivo both at the population and clonal levels. These results indicate that Flk2 expression by early progenitors is not at the expense of full multipotency and support the current model of hematopoietic development with segregation of myeloid and lymphoid lineages from multipotent progenitors.     

Human Flt3 is expressed at the hematopoietic stem cell and the granulocyte/macrophage progenitor stages to maintain cell survival

FLT3/FLK2, a member of the receptor tyrosine kinase family, plays a critical role in maintenance of hematopoietic homeostasis, and the constitutively active form of the FLT3 mutation is one of the most common genetic abnormalities in acute myelogenous leukemia. In murine hematopoiesis, Flt3 is not expressed in self-renewing hematopoietic stem cells, but its expression is restricted to the multipotent and the lymphoid progenitor stages at which cells are incapable of self-renewal. We extensively analyzed the expression of Flt3 in human (h) hematopoiesis. Strikingly, in both the bone marrow and the cord blood, the human hematopoietic stem cell population capable of long-term reconstitution in xenogeneic hosts uniformly expressed Flt3. Furthermore, human Flt3 is expressed not only in early lymphoid progenitors, but also in progenitors continuously along the granulocyte/macrophage pathway, including the common myeloid progenitor and the granulocyte/macrophage progenitor. We further found that human Flt3 signaling prevents stem and progenitors from spontaneous apoptotic cell death at least through up-regulating Mcl-1, an indispensable survival factor for hematopoiesis. Thus, the distribution of Flt3 expression is considerably different in human and mouse hematopoiesis, and human FLT3 signaling might play an important role in cell survival, especially at stem and progenitor cells that are critical cellular targets for acute myelogenous leukemia transformation.     

Enrichment of functionally distinct mouse hematopoietic progenitor cell populations using CD62L

The details of the bifurcation of the lymphoid and myeloid lineages following commitment by multipotent progenitor cells (MPP) remain a topic of controversy. We report that the surface glycoprotein CD62L can be characterized as a novel marker of this and other stages of early hematopoietic differentiation. Cell isolation and transplant studies demonstrated CD62L(neg/low) long-term hematopoietic stem cells and CD62L(high) MPP within the traditionally defined c-kit(pos)Lin(neg/low)Sca-1(pos) stem/progenitor cell population. Within the MPP population, previously defined as c-kit(pos)Lin(neg/low)Sca-1(pos)-Thy-1.1(neg)Flt3(pos), Sca-1 and CD62L resolved four populations and segregated Sca-1(high)CD62L(neg/low) MPP from Sca-1(high)CD62L(high) leukocyte-biased progenitors. Using a novel transplantation method that allows tracking of erythroid and platelet engraftment as an alternative to the classical method of in vitro colony formation, we characterized Sca-1(high)CD62L(neg/low) cells as MPP, based on transient engraftment of these lineages. These data establish CD62L as a useful tool in the study of early hematopoiesis and emphasize the power of trilineage-engraftment studies in establishing the lineage potential of MPP subsets.     

Mitochondrial respiration defects modulate differentiation but not proliferation of hematopoietic stem and progenitor cells

Mitochondrial energy production is involved in various cellular processes. Here we show that ATP content is significantly increased in lineage-restricted progenitor cells compared with hematopoietic stem and progenitor cells (HSPCs) or more differentiated cells. Transplantation analysis using a mouse model of mitochondrial disease revealed that mitochondrial respiration defects resulted in a significant decrease in the total number and repopulating activity of bone marrow cells, although the number of HSPCs increased. The proliferative activity of HSPCs and lineage-restricted progenitor cells was not impaired by reduction of ATP content and there seems to be no associated increase in reactive oxygen species levels and apoptosis. Our findings indicate that mitochondrial respiration defects modulate HSPC commitment/differentiation into lineage-restricted progenitor cells.     

Proteomic cornerstones of hematopoietic stem cell differentiation: distinct signatures of multipotent progenitors and myeloid committed cells

Regenerative tissues such as the skin epidermis, the intestinal mucosa or the hematopoietic system are organized in a hierarchical manner with stem cells building the top of this hierarchy. Somatic stem cells harbor the highest self-renewal activity and generate a series of multipotent progenitors which differentiate into lineage committed progenitors and subsequently mature cells. In this report, we applied an in-depth quantitative proteomic approach to analyze and compare the full proteomes of ex vivo isolated and FACS-sorted populations highly enriched for either multipotent hematopoietic stem/progenitor cells (HSPCs, Lin(neg)Sca-1(+)c-Kit(+)) or myeloid committed precursors (Lin(neg)Sca-1(-)c-Kit(+)). By employing stable isotope dimethyl labeling and high-resolution mass spectrometry, more than 5000 proteins were quantified. From biological triplicate experiments subjected to rigorous statistical evaluation, 893 proteins were found differentially expressed between multipotent and myeloid committed cells. The differential protein content in these cell populations points to a distinct structural organization of the cytoskeleton including remodeling activity. In addition, we found a marked difference in the expression of metabolic enzymes, including a clear shift of specific protein isoforms of the glycolytic pathway. Proteins involved in translation showed a collective higher expression in myeloid progenitors, indicating an increased translational activity. Strikingly, the data uncover a unique signature related to immune defense mechanisms, centering on the RIG-I and type-1 interferon response systems, which are installed in multipotent progenitors but not evident in myeloid committed cells. This suggests that specific, and so far unrecognized, mechanisms protect these immature cells before they mature. In conclusion, this study indicates that the transition of hematopoietic stem/progenitors toward myeloid commitment is accompanied by a profound change in processing of cellular resources, adding novel insights into the molecular mechanisms at the interface between multipotency and lineage commitment.     

Characterization, chromosomal localization, and expression during hematopoietic differentiation of the gene encoding Arl6ip, ADP-ribosylation-like factor-6 interacting protein (ARL6)

Proliferation and differentiation of hematopoietic stem cells and progenitors are regulated by signals from the microenvironment, involving both secreted cytokines and adhesion molecules. The exact mechanisms by which cytokines act on hematopoietic development are still not well understood. To extend the molecular characterization of gene regulation during cytokine-induced hematopoiesis, we applied mRNA differential display to identify genes regulated when multipotent progenitor cells are allowed to differentiate into monocytes and neutrophils. Here we report the isolation and characterization of a gene that is downregulated during myeloid differentiation and encodes a 23-kDa protein with four putative transmembrane segments. The gene, which we named Arl6ip, is identical to a mouse gene recently identified by its physical interaction with ADP-ribosylation-like factor-6 (ARL6), belonging to the Ras superfamily. We add information on its full-length characterization as well as its regulation during hematopoiesis. It is expressed in all hematopoietic cell lineages, but the highest level of expression is found in early myeloid progenitor cells. Preliminary studies by immunofluorescence microscopy revealed that the ARL6IP protein is predominantly localized to intracytoplasmic membranes. This suggests an involvement of the Arl6ip gene in protein transport, membrane trafficking, or cell signaling during hematopoietic maturation.     

Identification and purification of human erythroid progenitor cells by monoclonal antibody to the transferrin receptor (TU 67)

Anti-TU 67 is a murine monoclonal antibody that recognizes the transferrin receptor. With respect to hematopoietic cells TU 67 is expressed by human multipotent colony-forming cells (CFU-Mix), erythroid progenitor cells (BFU-E and CFU-E) and a fraction of granulocyte/monocyte colony forming cells, but is not expressed by mature hematopoietic cells including erythrocytes, platelets, lymphocytes, and peripheral blood myeloid cells. The TU 67-positive fraction of normal bone marrow, separated by fluorescence-activated cell sorting (FACS) or immune rosettes, contained 87% of the erythroid progenitor cells. Erythroid progenitor cells were enriched up to 50-fold by using a combination of monoclonal antibodies to deplete mature hematopoietic cells, followed by positive selection of BFU-E and CFU-E by TU 67 antibody.     

Age-related changes in human hematopoietic stem/progenitor cells

Adult stem cells are critical for maintaining cellular homeostasis throughout life, yet the effects of age on their regenerative capacity are poorly understood. All lymphoid and myeloid blood cell lineages are continuously generated from hematopoietic stem cells present in human bone marrow. With age, significant changes in the function and composition of mature blood cells are observed. In this study, we report that age-related changes also occur in the human hematopoietic stem cell compartment. We find that the proportion of multipotent CD34(+) CD38(-) cells increases in the bone marrow of elderly (>70 years) individuals. CD34(+) CD38(+) CD90(-) CD45RA(+/-) CD10(-) and CD34(+) CD33(+) myeloid progenitors persist at the same level in the bone marrow, while the frequency of early CD34(+) CD38(+) CD90(-) CD45RA(+) CD10(+) and committed CD34(+) CD19(+) B-lymphoid progenitors decreases with age. In contrast to mice models of aging, transplantation experiments with immunodeficient NOD/SCID/IL-2Rγ null (NSG) mice showed that the frequency of NSG repopulating cells does not change significantly with age, and there is a decrease in myeloid lineage reconstitution. An age-related decrease in the capacity of CD34(+) cells to generate myeloid cells was also seen in colony-forming assays in vitro. Thus, with increasing age, human hematopoietic stem/progenitor cells undergo quantitative changes as well as functional modifications.     

Rapid Lymphocyte Reconstitution of Unconditioned Immunodeficient Mice with Non-Self-Renewing Multipotent Hematopoietic Progenitors

The replacement of abnormal hematopoietic stem cells (HSCs) with normal transplanted HSCs can correct a wide range of hematologic disorders. Here, we provide evidence that transplantation of more differentiated progenitor cells can be used to more rapidly correct lymphoid deficiencies in unconditioned immunocompromised mice. Transplantation of flk2+ multipotent progenitors led to robust B and T cell reconstitution that was maintained for at least 16 weeks. Antigenic challenge at 16 weeks post-transplantation revealed that reconstituted lymphocytes maintained a functional repertoire. In contrast to the persistent lymphocytic engraftment, myeloid chimerism was lost by 12 weeks post-transplantation consistent with the fact that flk2+ progenitors are non-self-renewing. Thus, while more differentiated progenitors are capable of rescuing lymphoid deficiencies, transplantation of HSCs must be used for the correction of non-lymphoid disorders, and, we propose, very long-term immune reconstitution. Based on recent evidence, we discuss novel strategies to achieve the replacement of abnormal HSCs without the use of cytotoxic conditioning regimens.