According to current models for hematopoiesis, lymphoid-primed multi-potent progenitors (LMPPs; Lin−Sca-1+c-Kit+CD34+Flt3hi) and common myeloid progenitors (CMPs; Lin−Sca-1+c-Kit+CD34+CD41hi) establish an early branch point for separate lineage commitment pathways from hematopoietic stem cells, with the notable exception that both pathways are proposed to generate all myeloid innate immune cell types through the same myeloid-restricted pre-granulocyte-macrophage progenitor (pre-GM; Lin−Sca-1−c-Kit+CD41−FcγRII/III−CD150−CD105−). By single cell transcriptome profiling of pre-GMs we identify distinct myeloid differentiation pathways: a Gata1-expressing pathway generates mast cells, eosinophils, megakaryocytes and erythroid cells, and a Gata1-negative pathway that generates monocytes, neutrophils and lymphocytes. These results identify an early hematopoietic lineage bifurcation, separating the myeloid lineages prior to their segregation from other hematopoietic lineage potentials.

Hematopoietic stem cells (HSCs) undergo a functional switch in neonatal mice hallmarked by a decrease in self-renewing divisions and entry into quiescence. Here, we investigated whether the developmental attenuation of B-1a cell output is a consequence of a shift in stem cell state during ontogeny. Using cellular barcoding for in vivo single-cell fate analyses, we found that fetal liver definitive HSCs gave rise to both B-1a and B-2 cells. Whereas B-1a potential diminished in all HSCs with time, B-2 output was maintained. B-1a and B-2 plasticity could be reinitiated in a subset of adult HSCs by ectopic expression of the RNA binding protein LIN28B, a key regulator of fetal hematopoiesis, and this coincided with the clonal reversal to fetal-like elevated self-renewal and repopulation potential. These results anchor the attenuation of B-1a cell output to fetal HSC behavior and demonstrate that the developmental decline in regenerative potential represents a reversible HSC state.

Natural killer (NK) cells are cytotoxic lymphocytes and play a vital role in controlling viral infections and cancer. In contrast to B and T lymphopoiesis where cellular and regulatory pathways have been extensively characterized, the cellular stages of early human NK cell commitment remain poorly understood. Here we demonstrate that a Lin(-)CD34(+)CD38(+)CD123(-)CD45RA(+)CD7(+)CD10(+)CD127(-) population represents a NK lineage-restricted progenitor (NKP) in fetal development, umbilical cord blood, and adult tissues. The newly identified NKP has robust NK cell potential both in vitro and in vivo, generates functionally cytotoxic NK cells, and lacks the ability to produce T cells, B cells, myeloid cells, and innate lymphoid-like cells (ILCs). Our findings identify an early step to human NK cell commitment and provide new insights into the human hematopoietic hierarchy.

SummaryETV6-RUNX1 is associated with childhood acute B-lymphoblastic leukemia (cALL) functioning as a first-hit mutation that initiates a clinically silent pre-leukemia in utero. Because lineage commitment hierarchies differ between embryo and adult, and the impact of oncogenes is cell-context dependent, we hypothesized that the childhood affiliation of ETV6-RUNX1 cALL reflects its origins in a progenitor unique to embryonic life. We characterize the first emerging B cells in first-trimester human embryos, identifying a developmentally restricted CD19−IL-7R+ progenitor compartment, which transitions from a myeloid to lymphoid program during ontogeny. This developmental series is recapitulated in differentiating human pluripotent stem cells (hPSCs), thereby providing a model for the initiation of cALL. Genome-engineered hPSCs expressing ETV6-RUNX1 from the endogenous ETV6 locus show expansion of the CD19−IL-7R+ compartment, show a partial block in B lineage commitment, and produce proB cells with aberrant myeloid gene expression signatures and potential: features (collectively) consistent with a pre-leukemic state.

MicroRNAs are important regulators of gene expression in normal development and disease. miR-9 is overexpressed in several cancer forms, including brain tumours, hepatocellular carcinomas, breast cancer and Hodgkin lymphoma (HL). Here we demonstrated a relevance for miR-9 in HL pathogenesis and identified two new targets Dicer1 and HuR. HL is characterized by a massive infiltration of immune cells and fibroblasts in the tumour, whereas malignant cells represent only 1% of the tumour mass. These infiltrates provide important survival and growth signals to the tumour cells, and several lines of evidence indicate that they are essential for the persistence of HL. We show that inhibition of miR-9 leads to derepression of DICER and HuR, which in turn results in a decrease in cytokine production by HL cells followed by an impaired ability to attract normal inflammatory cells. Finally, inhibition of miR-9 by a systemically delivered antimiR-9 in a xenograft model of HL increases the protein levels of HuR and DICER1 and results in decreased tumour outgrowth, confirming that miR-9 actively participates in HL pathogenesis and points to miR-9 as a potential therapeutic target.Oncogene ( Keywords: miR-9; cytokines; Hodgkin lymphoma; HuR; DICER1 INTRODUCTIONChronic inflammation is a well-recognized cause of cancer and up to 25% of all cancers are thought to be induced by either chronic infections or autoimmune diseases. 1 Inflammation not only works as a tumour-promoting agent but also influences other steps of tumorigenesis by inducing DNA damage, angiogenesis, invasion and metastasis. 2 Hodgkin lymphoma (HL) is one of the most frequent lymphomas in the western world 3 and it can be divided in several subtypes based on the morphology, composition of the infiltrates and the phenotype of the immune cells. The nodular sclerosis subtype is the most common subtype of classical HL (cHL) and accounts for about 60--70% of the cHL cases, followed by mixed cellularity cHL accounting for 20--25%.cHL is an unusual B-cell malignancy in which the tumour cells represent only 1% of the tumour bulk, whereas the vast majority of the cells are a mixture of infiltrating immune cells and fibroblasts actively attracted via chemokine secretion by the malignant cells, the so-called Hodgkin and Reed --Sternberg cells. 3 Several lines of evidence indicate that the infiltrating cells are necessary for the survival of the HL cells by providing growth and survival signals, and protection from NK cells. 3,4 MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the posttranscriptional level by base-pairing to target mRNAs and promoting transcript instability and/or translational repression. 5 Mature miRNAs derive from long hairpin precursors that are sequentially processed by the RNase-III-type enzymes DROSHA and DICER1, respectively. 5 miRNAs have been implicated in several physiological and pathological events and can act as both tumour suppressors and oncogenes. 6

Key Points CSF1R is expressed on the earliest fetal B-cell progenitors, and CSF1R deficiency impairs fetal B-cell development. CSF1R+ fetal ProB cells have a B-myeloid gene signature and possess B and myeloid potential.

Within the hematopoietic system, the Notch pathway is critical for promoting thymic T cell development and suppressing the B and myeloid lineage fates; however, its impact on NK lymphopoiesis is less understood. To study the role of Notch during NK cell development in vivo, we investigated different NK cell compartments and function in Rbp-Jk fl/fl Vav-Cre tg/+ mice, in which Rbp-Jk, the major transcriptional effector of canonical Notch signaling, was specifically deleted in all hematopoietic cells. Peripheral conventional cytotoxic NK cells in Rbp-Jk-deleted mice were significantly reduced and had an activated phenotype. Furthermore, the pool of early NK cell progenitors in the bone marrow was decreased, whereas immature NK cells were increased, leading to a block in NK cell maturation. These changes were cell intrinsic as the hematopoietic chimeras generated after transplantation of Rbp-Jk-deficient bone marrow cells had the same NK cell phenotype as the Rbp-Jk-deleted donor mice, whereas the wild-type competitors did not. The expression of several crucial NK cell regulatory pathways was significantly altered after Rbp-Jk deletion. Together, these results demonstrate the involvement of canonical Notch signaling in regulation of multiple stages of NK cell development.

Summary Given that FLT 3 expression is highly restricted on lymphoid progenitors, it is possible that the established role of FLT 3 in the regulation of B and T lymphopoiesis reflects its high expression and role in regulation of lymphoid‐primed multipotent progenitors ( LMPP s) or common lymphoid progenitors ( CLP s). We generated a Flt3 conditional knock‐out ( Flt3 fl/fl ) mouse model to address the direct role of FLT 3 in regulation of lymphoid‐restricted progenitors, subsequent to turning on Rag1 expression, as well as potentially ontogeny‐specific roles in B and T lymphopoiesis. Our studies establish a prominent and direct role of FLT 3, independently of the established role of FLT 3 in regulation of LMPP s and CLP s, in regulation of fetal as well as adult early B cell progenitors, and the early thymic progenitors ( ETP s) in adult mice but not in the fetus. Our findings highlight the potential benefit of targeting poor prognosis acute B‐cell progenitor leukaemia and ETP leukaemia with recurrent FLT 3 mutations using clinical FLT 3 inhibitors.