Upregulation of Flt3 expression within the bone marrow Lin(-)Sca1(+)c-kit(+) stem cell compartment is accompanied by loss of self-renewal capacity

Thymopoiesis independent of common lymphoid progenitors

Early T lineage progenitors (ETPs) in the thymus are thought to develop from common lymphoid progenitors (CLPs) in the bone marrow (BM). We compared thymic ETPs to BM CLPs in mice and found that they differed in several respects. Thymic ETPs were not interleukin 7 (IL-7)-responsive and generated B lineage progeny with delayed kinetics, whereas BM CLPs were IL-7-responsive and rapidly generated B cells. ETPs sustained production of T lineage progeny for longer periods of time than BM CLPs. Analysis of Ikaros-deficient mice that exhibit ongoing thymopoiesis without B lymphopoeisis revealed near-normal frequencies of thymic ETPs, yet undetectable numbers of BM CLPs. We conclude that ETPs can develop via a CLP-independent pathway.

Developmental plasticity of lymphoid progenitors

The identification of the common lymphoid progenitors in mouse bone marrow allows us to directly assess the regulatory mechanisms of lymphoid lineage commitment. The unexpected finding of a latent myeloid differentiation potential in lymphoid progenitors sheds light on the importance of cytokine receptor expression at this stage. We will discuss the biological nature of common lymphoid progenitors as a model of differentiation from multipotent to lineage committed progenitors. Elucidation of this hidden differentiation potential in progenitors will help further our understanding of the molecular mechanisms that control the cell fate determination of not only common lymphoid progenitors, but also their ancestors, hematopoietic stem cells, and their descendents such as committed T and B cell progenitors.

Lymphoid lineage cells in adult murine bone marrow diverge from those of other blood cells at an early, hormone-sensitive stage

Advances in cell sorting and GFP knock-in technology have made it possible to identify rare hematopoietic cells in murine bone marrow that are undergoing lymphocyte fate specification. Steroid hormones also represent important research tools for investigating relationships between different categories of lympho-hematopoietic precursors. By selectively blocking entry into and progression within lymphoid lineages, the hormones probably have a major influence on numbers of lymphocytes that are produced under normal circumstances. These issues are discussed within the context of developmental age-dependent changes that occur in the lymphopoietic process.

Common and distinct features of cytokine effects on hematopoietic stem and progenitor cells revealed by dose-response surface analysis

Recent studies have identified thrombopoietin (TPO), flt-3 ligand (FL), Steel factor (SF), and interleukin-11 (IL-11) as cytokines able to stimulate amplification of the most primitive murine hematopoietic cells in vitro. However, dose-response and interaction parameters that predict how to optimize mixtures of these cytokines have not been previously defined. To obtain this information, Sca-1(+)lin(-) and c-kit(+)Sca-1(+)lin(-) adult mouse bone marrow cells were cultured for 10 and 14 days, respectively, in serum-free medium with varying concentrations of these cytokines. Quantitative assays were performed to determine the influences of the cytokine combinations tested on changes in long-term repopulating hematopoietic stem cells (HSCs), in vitro colony-forming cells (CFCs), and total cell numbers. A two-level factorial design was first used to screen the effects of TPO, SF, FL, and IL-11 as well as two different incubation temperatures. IL-11 and SF were found to be the most significant stimulators of murine HSC expansion. More detailed analyses of the effects on c-kit(+)Sca-1(+)lin(-) cells of IL-11, SF, and FL concentrations and their interactions using response surface methodology showed IL-11 to have a maximal stimulatory effect on HSC expansion at 20 ng/mL with higher concentrations being inhibitory. In contrast, not even high concentration saturation of the effects of either SF or FL was observed as the stimulatory effect of both SF and FL increased beyond 300 ng/mL. A negative interaction between SF and FL on HSCs was discovered. Interestingly, a generally similar pattern of cytokine effects was found to influence the 14-day output of CFCs and total cells from the same c-kit(+)Sca-1(+)lin(-) starting cell population. However, compared with HSCs, the cytokine requirements for maximizing the generation of CFCs and total cells were at much lower cytokine doses. From the information provided by the factorial analysis, mathematical models based on Monod kinetics for inhibitory substrates were developed that allow total cell, CFC, and HSC expansion to be predicted as a function of the IL-11, SF, and FL concentrations in terms of more widely recognized parameters. Overall, these methods should also serve as a guide for the future design and testing of other ex vivo stem cell expansion systems.

Relationships between early B- and NK-lineage lymphocyte precursors in bone marrow

Recent studies have demonstrated that lineage marker-negative (Lin(-)) c-kit(Lo) Flk-2/Flt3(+) IL-7R(+) Sca-1(Lo) CD27(+) Ly-6C(-) Thy-1(-)CD43(+) CD16/32(Lo/-) terminal deoxynucleotidyl transferase (TdT)(+) cells in murine bone marrow are functional lymphocyte precursors. However, it has not been clear if this is an obligate intermediate step for transit of multipotential hematopoietic stem cells to natural killer (NK) cells. We have now used serum-free, stromal cell-free cultures to determine that NK progenitors are enriched among an estrogen-regulated, c-kit(Lo) subset of the Lin(-) fraction. However, several experimental approaches suggested that this population is heterogeneous and likely represents a stage where B and NK lineages diverge. Although most B-cell precursors were directly sensitive to estrogen in culture, much of the NK-cell precursor activity in that fraction was hormone resistant. B-lineage potential was largely associated with interleukin 7 receptor alpha (IL-7R(alpha)) expression and was selectively driven in culture by IL-7. In contrast, many NK precursors did not display detectable amounts of this receptor and their maturation was selectively supported by IL-15. Finally, single-cell experiments showed that the Lin(-) c-kit(Lo) fraction contains a mixture of B/NK, B-restricted, and NK-restricted progenitors. Two-step culture experiments revealed that NK precursors become hormone resistant on or before acquisition of CD122, signaling commitment to the NK lineage. CD45R is preferentially, but not exclusively, expressed on maturing B-lineage cells. Production of these 2 blood cell types is regulated in bone marrow by common and then independent mechanisms that can now be studied with greater precision.

A developing picture of lymphopoiesis in bone marrow

The earliest progenitors of lymphocytes are extremely rare and typically present among very complex populations of hematopoietic cells. Additionally, it is difficult to know how cells with any given set of characteristics are developmentally related to stem cells and maturing lymphoid precursors. However, it is now possible to divide bone marrow into progressively smaller fractions and exploit well-defined culture systems to determine which ones contain cells that can turn into lymphocytes. Analysis of steroid hormone sensitive cells and use of two-step cultures is providing additional information about the most likely differentiation pathways for B and natural killer cell lineage lymphocytes. A newly identified category of early lymphoid progenitors can now be sorted to high purity from RAG1/GFP knock in mice. Furthermore, the same experimental model makes it possible to image lymphoid progenitors in fetal and adult hematopoietic tissues.

A positive look at double-negative thymocytes

In some respects, our understanding of the cellular and molecular aspects of early T-cell differentiation is lagging behind that of B cells. Papers describing gene-knockout and reporter-transgenic mice in which thymocyte development is affected are often difficult to interpret. Progress in this field will be hampered unless a more detailed phenotypic and molecular analysis of progenitor thymocytes at the single-cell level is carried out.

Reconstitution of human haematopoiesis in non-obese diabetic/severe combined immunodeficient mice by clonal cells expanded from single CD34+CD38- cells expressing Flk2/Flt3

In the present study, we examined the expression of Flk2/Flt3, a tyrosine kinase receptor, on human cord blood CD34+ haematopoietic progenitor/stem cells. In flow cytometric analysis, Flk2/Flt3 was expressed on 80% of CD34+ cells and their immature subpopulations, CD34+CD33- and CD34+CD38- cells. Methycellulose clonal culture of sorted CD34+Flk2/Flt3+ and CD34+Flk2/Flt3- cells showed that most of myelocytic progenitors expressed Flk2/Flt3, but erythroid and haematopoietic multipotential progenitors were shared by both fractions. When 1 x 10(4) lineage marker-negative (Lin-)CD34+Flk2/Flt3- cells were transplanted into non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice, none of the recipients possessed human CD45+ cells in bone marrow 11-12 weeks after the transplantation. In contrast, all recipients transplanted with 1 x 10(4) Lin-CD34+Flk2/Flt3+ cells showed successful engraftment. Furthermore, clonal cells expanded from single Lin-CD34+CD38-Flk2/Flt3+ cells in the culture with Flk2/Flt3 ligand, stem cell factor, thrombopoietin, and a complex of interleukin 6/soluble interleukin 6 receptor were individually transplanted into NOD/SCID mice. At 20 to 21 weeks after the transplantation, three out of 10 clones harvested at d 7 of culture, and three out of six clones at d 14 could reconstitute human haematopoiesis in recipient marrow. These results demonstrated that Flk2/Flt3 was expressed on a wide variety of human haematopoietic cells including long-term-repopulating haematopoietic stem cells.

Key role of flt3 ligand in regulation of the common lymphoid progenitor but not in maintenance of the hematopoietic stem cell pool

The first lineage commitment step of hematopoietic stem cells (HSC) results in separation into distinct lymphoid and myeloid differentiation pathways, reflected in the generation of common lymphoid and myeloid progenitors (CLP and CMP, respectively). In this report we present the first evidence for a nonredundant regulator of this process, in that adult mice deficient in expression of the flt3 ligand (FL) have severely (10-fold) reduced levels of the CLP, accompanied by reductions in the earliest identifiable B and T cell progenitors. In contrast, CMP and HSC are unaffected in FL-deficient mice. Noteworthy, CLP express high levels of both the flt3 receptor and ligand, indicating a potential autocrine role of FL in regulation of the earliest lymphoid commitment step from HSC.

Notch signaling in hematopoiesis and early lymphocyte development

Notch signals regulate multiple cell fate decisions during metazoan development. During hematopoiesis, Notch affects both hematopoietic stem cells and committed progenitors. In hematopoietic stem cells, Notch signaling has the propensity to expand the stem cells, promote their self-renewal, and influence their survival. In committed progenitors, Notch signaling plays a key role in determining lymphoid cell fates. This review focuses on recent developments to understand the role of Notch signaling in early events in hematopoiesis.

Nature or nurture? Steady-state lymphocyte formation in adults does not recapitulate ontogeny

Substantial progress has been made in determining developmental relationships between lymphocyte precursors and those corresponding to other blood cell lineages. Indeed, exploitation of RAG1/GFP knock-in mice has recently made it possible to chart the entire sequence of lymphocyte differentiation events in adult bone marrow and thymus. However, the differentiation pathways proposed for fetal life are very different from this model. We review many examples where the results of gene targeting experiments are substantially dependent on developmental age. In mice, adult patterns of gene expression and corresponding properties of lymphocyte precursors are not fully established until several weeks after birth, and the same might be true for humans. Furthermore, examples are cited where fetal hematopoietic cells did not efficiently acquire those properties when transplanted to an adult environment. There are several important implications of these findings. Cognizance of developmental age-related changes might resolve apparent conflicts in the literature. Hematopoietic stem cells and their lymphoid lineage progeny appear in waves, and a direct connection is yet to be established between fetal stem cells and ones that sustain adult blood cell formation. There is the possibility that adult stem cells derive from founders with an unknown origin.

Transcription from the RAG1 locus marks the earliest lymphocyte progenitors in bone marrow

Viable Lin(-) CD27(+) c-kit(Hi) Sca-1(Hi) GFP(+) cells recovered from heterozygous RAG1/GFP knockin mice progressed through previously defined stages of B, T, and NK cell lineage differentiation. In contrast to the GFP(-) cohort, there was minimal myeloid or erythroid potential in cells with an active RAG1 locus. Partial overlap with TdT(+) cells suggested that distinctive early lymphocyte characteristics are not synchronously acquired. Rearrangement of Ig genes initiates before typical lymphoid lineage patterns of gene expression are established, and activation of the RAG1 locus transiently occurs in a large fraction of cells destined to become NK cells. These early lymphocyte progenitors (ELP) are distinct from stem cells, previously described prolymphocytes, or progenitors corresponding to other blood cell lineages.

Myeloerythroid-restricted progenitors are sufficient to confer radioprotection and provide the majority of day 8 CFU-S

Whole-body irradiation at the minimal lethal dose causes bone marrow failure and death within 12-18 days. To identify the principal components of the hematopoietic system that are radioprotective, we transplanted lethally irradiated mice with purified progenitors: common myeloid progenitors (CMPs), megakaryocyte/erythrocyte-restricted progenitors (MEPs), or granulocyte/monocyte-restricted progenitors (GMPs). Transplanted CMPs gave rise to cells both of the granulocyte/monocyte (GM) series and the megakaryocyte/erythrocyte series, whereas GMPs or MEPs showed reconstitution of only GM or ME cells, respectively. CMPs and MEPs but not GMPs protected mice in a dose-dependent manner, suggesting that erythrocytes, platelets, or both are the critical effectors of radioprotection. Accordingly, CMPs and MEPs formed robust colonies in recipient bone marrow and spleen, whereas GMPs formed small colonies that rapidly disappeared. Direct comparisons of spleen CFU (CFU-S) potentials among each progenitor subset showed that MEPs contain the vast majority of day 8 CFU-S activity, suggesting that day 8 CFU-S are the precursors of radioprotective cell subsets. All animals radioprotected for 30 days subsequently survived for at least 6 months post-transplant, and showed only host-derived hematopoiesis after 30 days. These findings suggest that rare hematopoietic stem cells survive myeloablation that can eventually repopulate irradiated hosts if myeloerythroid-restricted progenitors transiently rescue ablated animals through the critical window of bone marrow failure.

Tracing lymphopoiesis with the aid of a pTalpha-controlled reporter gene

A transgenic reporter mouse strain, which expressed the human CD25 (hCD25) surface marker as a reporter under the control of the pre-T cell receptor alpha(pTalpha) promoter, was used to identify lymphoid precursors that expressed pTalpha intracellularly. The hCD25 reporter marked intra- and extrathymic precursors of lymphocytes but not myeloid cells. The earliest intrathymic precursors were CD4(lo)CD8(-)CD25(-)CD44(+)c-Kit(+) cells that expressed elevated levels of Notch-1 mRNA. Clonogenic assays showed that the extrathymic precursors were common lymphoid progenitors (CLPs) that included CD19(-), B220(+), Thy1(+) and CD4(+) cells. Thus, the pTalpha reporter can be used to trace lymphopoiesis between CLPs and alphabeta T cells. The slower extinction of the hCD25 reporter compared to pTalpha enabled us to define points at which pTalpha(-) lineages branched off.

On the brink of becoming a T cell

Recent studies provide fresh insight into the mechanisms by which precursor cells are committed to and develop within the T-lymphocyte lineage. Precursor/product studies have identified developmental stages between that of the pluripotent hematopoietic stem cell and thymocytes committed to the T lineage. Specific ligands and signaling pathways interacting with the Notch-1 receptor and its ability to influence commitment within the lymphoid lineage have been described. Although the structural features or putative ligands endowing the pre-TCR with constitutive signaling capacity remain elusive, numerous distal mediators of pre-TCR signaling have been identified. It remains for the future to determine what roles they may have in survival, proliferation, lineage commitment and allelic exclusion of TCR genes. Receptor editing and lineage commitment of alphabeta T cells still represent controversial topics that need further study.

Flk-2 is a marker in hematopoietic stem cell differentiation: a simple method to isolate long-term stem cells

Clonogenic multipotent mouse hematopoietic stem cells (HSCs) and progenitor cells are contained within the c-kit(+) (K) lineage(-/lo) (L) Sca-1(+) (S) population of hematopoietic cells; long-term (LT) and short-term (ST) HSCs are Thy-1.1(lo). c-kit is a member of the receptor tyrosine kinase family, a class of receptors that are important in the proliferation and differentiation of hematopoietic cells. To establish whether the Flk-2/Flt3 receptor tyrosine kinase was expressed on the most primitive LT-HSCs, we sorted highly purified multipotent stem and progenitor cells on the basis of Flk-2 surface expression and used them in competitive reconstitution assays. Low numbers of Flk-2(-) HSCs gave rise to long-term multilineage reconstitution in the majority of recipients, whereas the transfer of Flk-2(+) multipotent cells resulted in mostly short-term multilineage reconstitution. The KLS subset of adult mouse bone marrow was analyzed for Flk-2 and Thy-1.1 expression. Three phenotypically and functionally distinct populations were isolated: Thy(lo) Flk-2(-) (LT-HSCs), Thy(lo) Flk-2(+) (ST-HSCs), and Thy(-) Flk-2(+) multipotent progenitors. The loss of Thy-1.1 and gain of Flk-2 expression marks the loss of self-renewal in HSC maturation. The addition of Flk-2 antibody to the lineage mix allows direct isolation of LT-HSC from adult bone marrow as c-kit(+) lin(-) Sca-1(+) Flk-2(-) from many strains of mice. Fetal liver HSCs are contained within Flk-2(-) and Flk-2(+) KTLS cells.