All blood cell lineages derive from a common hematopoietic stem cell (HSC). The current model implicates that the first lineage commitment step of adult pluripotent HSCs results in a strict separation into common lymphoid and common myeloid precursors. We present evidence for a population of cells which, although sustaining a high proliferative and combined lympho-myeloid differentiation potential, have lost the ability to adopt erythroid and megakaryocyte lineage fates. Cells in the Lin-Sca-1+c-kit+ HSC compartment coexpressing high levels of the tyrosine kinase receptor Flt3 sustain granulocyte, monocyte, and B and T cell potentials but in contrast to Lin-Sca-1+c-kit+Flt3- HSCs fail to produce significant erythroid and megakaryocytic progeny. This distinct lineage restriction site is accompanied by downregulation of genes for regulators of erythroid and megakaryocyte development. In agreement with representing a lymphoid primed progenitor, Lin-Sca-1+c-kit+CD34+Flt3+ cells display upregulated IL-7 receptor gene expression. Based on these observations, we propose a revised road map for adult blood lineage development.

Flt3 has emerged as a potential regulator of hematopoietic stem cells (HSC). Sixty percent of cells in the mouse marrow Lin(-)Sca1(+)c-kit(+) HSC pool expressed flt3. Although single cell cloning showed comparable high proliferative, myeloid, B, and T cell potentials of Lin(-)Sca1(+)c-kit(+)flt3(+) and Lin(-)Sca1(+)c-kit(+)flt3(-) cells, only Lin(-)Sca1(+)c-kit(+)flt3(-) cells supported sustained multilineage reconstitution. In striking contrast, Lin(-)Sca1(+)c-kit(+)flt3(+) cells rapidly and efficiently reconstituted B and T lymphopoiesis, whereas myeloid reconstitution was exclusively short term. Unlike c-kit, activation of flt3 failed to support survival of HSC, whereas only flt3 mediated survival of Lin(-)Sca1(+)c-kit(+)flt3(+) reconstituting cells. Phenotypic and functional analysis support that Lin(-)Sca1(+)c-kit(+)flt3(+) cells are progenitors for the common lymphoid progenitor. Thus, upregulation of flt3 expression on Lin(-)Sca1(+)c-kit(+) HSC cells is accompanied by loss of self-renewal capacity but sustained lymphoid-restricted reconstitution potential.

Recent studies implicated the existence of adult lymphoid-primed multipotent progenitors (LMPPs) with little or no megakaryocyte-erythroid potential, questioning common myeloid and lymphoid progenitors as obligate intermediates in hematopoietic stem cell (HSC) lineage commitment. However, the existence of LMPPs remains contentious. Herein, global and single-cell analyses revealed a hierarchical organization of transcriptional lineage programs, with downregulation of megakaryocyte-erythroid genes from HSCs to LMPPs, sustained granulocyte-monocyte priming, and upregulation of common lymphoid (but not B and T cell-specific) genes. These biological and molecular relationships, implicating almost mutual exclusion of megakaryocyte-erythroid and lymphoid pathways, are established already in fetal hematopoiesis, as evidenced by existence of LMPPs in fetal liver. The identification of LMPPs and hierarchically ordered transcriptional activation and downregulation of distinct lineage programs is compatible with a model for HSC lineage commitment in which the probability for undergoing different lineage commitment fates changes gradually when progressing from HSCs to LMPPs.

In clinical bone marrow transplantation, the severe cytopenias induced by bone marrow ablation translate into high risks of developing fatal infections and bleedings, until transplanted hematopoietic stem and progenitor cells have replaced sufficient myeloerythroid offspring. Although adult long-term hematopoietic stem cells (LT-HSCs) are absolutely required and at the single-cell level sufficient for sustained reconstitution of all blood cell lineages, they have been suggested to be less efficient at rapidly reconstituting the hematopoietic system and rescuing myeloablated recipients. Such a function has been proposed to rather be mediated by less well-defined short-term hematopoietic stem cells (ST-HSCs). Herein, we demonstrate that Lin ؊ Sca1 ؉ kit hi CD34 ؉ short-term reconstituting cells contain 2 phenotypically and functionally distinct subpopulations: Lin ؊ Sca1 ؉ kit hi CD34 ؉ flt3 ؊ cells fulfilling all criteria of ST-HSCs, capable of rapidly reconstituting myelopoiesis, rescuing myeloablated mice, and generating IntroductionHematopoietic stem cells (HSCs) are defined by their unique capacity to self-renew as well as to multilineage differentiate into all blood cell lineages and are therefore crucial for reconstitution of hematopoiesis on transplantation into recipients with bone marrow (BM) ablation. 1,2 Although representing only approximately 0.05% to 0.1% of total murine BM cells, virtually all HSC activity has been shown to be contained within the lineage Ϫ/lo (Lin Ϫ/lo ) Sca1 ϩ kit hi (LSK) HSC compartment. [3][4][5] However, the LSK HSC compartment is heterogeneous and the prevailing model of hematopoiesis suggests that the HSC pool consists of at least 2 functionally distinct HSC subpopulations, long-term and short-term repopulating HSCs (LT-HSCs and ST-HSCs, respectively). LT-HSCs have extensive (life-long) self-renewing potential and on commitment give rise to ST-HSCs with more restricted self-renewing capacity. 2,6 As a consequence, following transplantation procedures in lethally myeloablated recipients, LT-HSCs are both required and sufficient to secure life-long reconstitution of the entire hematopoietic system. However, because LT-HSCs can reconstitute hematopoiesis long-term at very low numbers (one cell in mice), 7,8 the lack of sufficient LT-HSCs is unlikely to represent a significant problem in clinical BM transplantation. Rather, rapid and efficient replacement of short-lived myeloerythroid cell lineages is critical to overcome life-threatening cytopenia following severe insults to the hematopoietic system, as well as in the first weeks after transplantation. 9 Multiple studies have suggested that LT-HSCs, in addition to being extremely rare, might also not be the most efficient cells at rapidly reconstituting the hematopoietic system and thereby protecting lethally irradiated (myeloablated) mice, probably reflecting their quiescent nature and their delayed production of mature progeny, 7,10-12 although higher doses of LT-HSCs might also be able to rapidly reconstitute ablated recip...

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.

Multipotent self-renewing hematopoietic stem cells (HSCs) are responsible for reconstitution of all blood cell lineages. Whereas growth stimulatory cytokines have been demonstrated to promote HSC self-renewal, the potential role of negative regulators remains elusive. Receptors for tumor necrosis factor (TNF) and Fas ligand have been implicated as regulators of steady-state hematopoiesis, and if overexpressed mediate bone marrow failure. However, it has been proposed that hematopoietic progenitors rather than stem cells might be targeted by Fas activation. Here, murine Lin−Sca1+c-kit+ stem cells revealed little or no constitutive expression of Fas and failed to respond to an agonistic anti-Fas antibody. However, if induced to undergo self-renewal in the presence of TNF-α, the entire short and long-term repopulating HSC pool acquired Fas expression at high levels and concomitant activation of Fas suppressed in vitro growth of Lin−Sca1+c-kit+ cells cultured at the single cell level. Moreover, Lin−Sca1+c-kit+ stem cells undergoing self-renewal divisions in vitro were severely and irreversibly compromised in their short- and long-term multilineage reconstituting ability if activated by TNF-α or through Fas, providing the first evidence for negative regulators of HSC self-renewal.

• Molecular characterization of myeloma requires isolation of malignant plasma cells, which is currently hampered by the instability of CD138.• We identified CD319 and CD269 as robust replacements for CD138, facilitating molecular diagnostics in myeloma.Molecular characterization of malignant plasma cells is increasingly important for diagnostic and therapeutic stratification in multiple myeloma. However, the malignant plasma cells represent a relatively small subset of bone marrow cells, and need to be enriched prior to analysis. Currently, the cell surface marker CD138 (SDC1) is used for this enrichment, but has an important limitation in that its expression decreases rapidly after sampling. Seeking alternatives to CD138, we performed a computational screen for myeloma plasma cell markers and systematically evaluated 7 candidates. Our results conclusively show that the markers CD319 (SLAMF7/CS1) and CD269 (TNFRSF17/BCMA) are considerably more robust than CD138 and enable isolation of myeloma plasma cells under more diverse conditions, including the samples that have been delayed or frozen.Our results form the basis of improved procedures for characterizing cases of multiple myeloma in clinical practice. (Blood. 2014;123(9):1336-1340 IntroductionMultiple myeloma (MM) is characterized by the uncontrolled, clonal growth of plasma cells in the bone marrow. As our knowledge increases about the genetic basis of MM, and new therapeutic options are being developed, molecular characterization of MM plasma cells (MMPCs) is becoming increasingly important for subclassification, prognostication, and treatment stratification.

Myeloid-derived suppressor cells (MDSCs) are known to contribute to immune evasion in cancer. However, the function of the human granulocytic (G)-MDSC subset during tumor progression is largely unknown, and there are no established markers for their identification in human tumor specimens. Using gene expression profiling, mass cytometry, and tumor microarrays, we here demonstrate that human G-MDSCs occur as neutrophils at distinct maturation stages, with a disease-specific profile. G-MDSCs derived from patients with metastatic breast cancer and malignant melanoma display a unique immature neutrophil profile, that is more similar to healthy donor neutrophils than to G-MDSCs from sepsis patients. Finally, we show that primary G-MDSCs from metastatic breast cancer patients co-transplanted with breast cancer cells, promote tumor growth, and affect vessel formation, leading to myeloid immune cell exclusion. Our findings reveal a role for human G-MDSC in tumor progression and have clinical implications also for targeted immunotherapy.