Summary Current dogma asserts that mammalian life-long blood production is established by a small number of blood progenitors. However, this model is based on assays that require the disruption, transplantation and/or culture of embryonic tissues. Here, we used the sample-to-sample variance (SSV) of a multi-colored lineage trace reporter to assess the frequency of emerging life-long blood progenitors while avoiding the disruption, culture or transplantation of embryos. We find that approximately 719 Flk-1+ mesodermal precursors, 633 VE-Cadherin+ endothelial precursors and 545 Vav1+ nascent blood stem and progenitor cells emerge to establish the hematopoietic system at embryonic days (E) 7-E8.5, E8.5-E11.5 and E11.5-E14.5, respectively. We also determined that the spatio-temporal recruitment of endothelial blood precursors begins at E8.5 and ends by E10.5 and that many c-Kit+ clusters of newly specified blood progenitors in the aorta are polyclonal in origin. Our work illuminates the dynamics of the developing mammalian blood system during homeostasis.

Although many recent studies describe the emergence and prevalence of “clonal hematopoiesis of indeterminate potential” in aged human populations, a systematic analysis of the numbers of clones supporting steady-state hematopoiesis throughout mammalian life is lacking. Previous efforts relied on transplantation of “barcoded” hematopoietic stem cells (HSCs) to track the contribution of HSC clones to reconstituted blood. However, ex vivo manipulation and transplantation alter HSC function and thus may not reflect the biology of steady-state hematopoiesis. Using a noninvasive in vivo color-labeling system, we report the first comprehensive analysis of the changing global clonal complexity of steady-state hematopoiesis during the natural murine lifespan. We observed that the number of clones (ie, clonal complexity) supporting the major blood and bone marrow hematopoietic compartments decline with age by ∼30% and ∼60%, respectively. Aging dramatically reduced HSC in vivo–repopulating activity and lymphoid potential while increasing functional heterogeneity. Continuous challenge of the hematopoietic system by serial transplantation provoked the clonal collapse of both young and aged hematopoietic systems. Whole-exome sequencing of serially transplanted aged and young hematopoietic clones confirmed oligoclonal hematopoiesis and revealed mutations in at least 27 genes, including nonsense, missense, and deletion mutations in Bcl11b, Hist1h2ac, Npy2r, Notch3, Ptprr, and Top2b.

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The outcome for metastatic pediatric osteosarcoma (OS) remains poor. Thus, there is an urgent need to develop novel therapies, and immunotherapy with CAR T cells has the potential to meet this challenge. However, there is a lack of preclinical models that mimic salient features of human disease including reliable development of metastatic disease post orthotopic OS cell injection. To overcome this roadblock, and also enable real-time imaging of metastatic disease, we took advantage of LM7 OS cells expressing firefly luciferase (LM7.ffLuc). LM7.ffLuc were implanted in a collagen mesh into the tibia of mice, and mice reliably developed orthotopic tumors and lung metastases as judged by bioluminescence imaging and histopathological analysis. Intratibial implantation also enabled surgical removal by lower leg amputation and monitoring for metastases development post-surgery. We then used this model to evaluate the antitumor activity of CAR T cells targeting B7-H3, an antigen that is expressed in a broad range of solid tumors including OS. B7-H3-CAR T cells had potent antitumor activity in a dose-dependent manner and inhibited the development of pulmonary metastases resulting in a significant survival advantage. In contrast T cells expressing an inactive B7-H3-CAR had no antitumor activity. Using unmodified LM7 cells also enabled us to demonstrate that B7-H3-CAR T cells traffic to orthotopic tumor sites. Hence, we have developed an orthotopic, spontaneously metastasizing OS model. This model may improve our ability not only to predict the safety and efficacy of current and next generation CAR T cell therapies but also other treatment modalities for metastatic OS.

The embryonic site of definitive hematopoietic stem cell (dHSC) origination has been debated for decades. Although an intra-embryonic origin is well supported, the yolk sac (YS) contribution to adult hematopoiesis remains controversial. The same developmental origin makes it difficult to identify specific markers that discern between an intraembryonic versus YS-origin using a lineage trace approach. Additionally, the highly migratory nature of blood cells and the inability of pre-circulatory embryonic cells (i.e., 5–7 somite pairs (sp)) to robustly engraft in transplantation, even after culture, has precluded scientists from properly answering these questions. Here we report robust, multi-lineage and serially transplantable dHSC activity from cultured 2–7sp murine embryonic explants (Em-Ex). dHSC are undetectable in 2–7sp YS explants. Additionally, the engraftment from Em-Ex is confined to an emerging CD31+CD45+c-Kit+CD41− population. In sum, our work supports a model in which the embryo, not the YS, is the major source of lifelong definitive hematopoiesis.

SummaryC57BL/6N (N) and C57BL/6J (J) mice possess key genetic differences, including a deletion in the Nicotinamide nucleotide transhydrogenase (Nnt) gene that results in a non-functional protein in J mice. NNT regulates mitochondrial oxidative stress. Although elevated oxidative stress can compromise hematopoietic stem and progenitor cell (HSPC) function, it is unknown whether N- and J-HSPCs are functionally equivalent. Here, we report that J-HSPCs display compromised short-term hematopoietic repopulating activity relative to N-HSPCs that is defined by a delay in lymphoid reconstitution and impaired function of specific multi-potent progenitor populations post transplant. J-HSPCs also displayed elevated reactive oxygen species (ROS) relative to N-HSPCs post transplant and upregulate ROS levels more in response to hematopoietic stress. Nnt knockdown in N-HSPCs recapitulated J-HSPCs’ short-term repopulating defect, indicating that NNT loss contributes to this defect. In summary, C57BL/6N and C57BL/6J HSPCs are not functionally equivalent, which should be considered when determining the substrain most appropriate for investigations of HSPC biology.

Hematopoietic Stem and Progenitor Cells (HSPCs) sustain life-long hematopoiesis and are first detected in the embryo by transplantation at embryonic day 10.5. (E10.5). HSPCs are mesodermal in origin and ultimately emerge from a subset of arterial endothelium (i.e. hemogenic endothelium), which is highly concentrated in the aorta-gonad-mesonephros region (AGM) of the mid-gestation embryo. Here, we employ clonal ex vivo assays in which endothelial cells isolated from the mid-gestation aorta and vitelline and umbilical arteries are co-cultured on supportive stroma to show that only about 0.1%, 1.3% and 0.29% of E9.5, E10.5 and E11.5 endothelium are functional hemogenic endothelium, respectively. We further show high phenotypic and functional variability in the hematopoietic potential of individual hemogenic endothelial precursors. Notably, by employing unique niche stroma capable of providing the signals necessary for definitive hematopoietic stem cell (dHSC) induction, we demonstrate this variability in hemogenic endothelium includes their potential to support phenotypic dHSCs. These data suggest the presence of either a continuum of maturing hemogenic endothelium with distinct hematopoietic potential or that hemogenic endothelium represents a heterogeneous pool of precursors that give rise to HSPCs with disparate hematopoietic potential.

Hematopoietic stem cell transplantation (HSCT) is often exploited to treat hematologic disease. Donor HSCs must survive, proliferate and differentiate in the damaged environment of the reconstituting niche. Illuminating molecular mechanisms regulating the activity of transplanted HSCs will inform efforts to improve HSCT. Here, we report that GPRASP proteins function as negative regulators of HSCT. Silencing of Gprasp1 or Gprasp2 increased the survival, quiescence, migration, niche retention and hematopoietic repopulating activity of hematopoietic stem and progenitor cells (HSPCs) post-transplant. We further show that GPRASP1 and GPRASP2 promote the degradation of CXCR4, a master regulator of HSC function during transplantation. CXCR4 accumulates in Gprasp-deficient HSPCs, boosting their function post-transplant. Thus, GPRASPs negatively regulate CXCR4 stability in HSCs. Our work reveals GPRASP proteins as negative regulators of HSCT and CXCR4 activity. Disruption of GPRASP/CXCR4 interactions could be exploited in the future to enhance the efficiency of HSCT.