Chronic myeloid leukemia (CML) is currently treated with tyrosine kinase inhibitors, but these do not effectively eliminate the CML stem cells. As a consequence, CML stem cells persist and cause relapse in most patients upon drug discontinuation. Furthermore, no effective therapy exists for the advanced stages of the disease. Interleukin-1 receptor accessory protein (IL1RAP; IL1R3) is a coreceptor of interleukin-1 receptor type 1 and has been found upregulated on CML stem cells. Here, we show that primitive (CD34CD38) CML cells, in contrast to corresponding normal cells, express a functional interleukin-1 (IL-1) receptor complex and respond with NF-κB activation and marked proliferation in response to IL-1. IL1RAP antibodies that inhibit IL-1 signaling could block these effects. In vivo administration of IL1RAP antibodies in mice transplanted with chronic and blast phase CML cells resulted in therapeutic effects mediated by murine effector cells. These results provide novel insights into the role of IL1RAP in CML and a strong rationale for the development of an IL1RAP antibody therapy to target residual CML stem cells.

Hypoxia is emerging as an important characteristic of the hematopoietic stem cell (HSC) niche, but the molecular mechanisms contributing to quiescence, self-renewal, and survival remain elusive. Vascular endothelial growth factor A (VEGFA) is a key regulator of angiogenesis and hematopoiesis. Its expression is commonly regulated by hypoxiainducible factors (HIF) that are functionally induced in low-oxygen conditions and that activate transcription by binding to hypoxia-response elements (HRE).Vegfa is indispensable for HSC survival, mediated by a cell-intrinsic, autocrine mechanism. We hypothesized that a hypoxic HSC microenvironment is required for maintenance or up-regulation of Vegfa expression in HSCs and therefore crucial for HSC survival. We have tested this hypothesis in the mouse model Vegfa ␦/␦ , where the HRE in the Vegfa promoter is mutated, preventing HIF binding. Vegfa expression was reduced in highly purified HSCs from Vegfa ␦/␦ mice, showing that HSCs reside in hypoxic areas. Loss of hypoxiaregulated Vegfa expression increases the numbers of phenotypically defined hematopoietic stem and progenitor cells. However, HSC function was clearly impaired when assessed in competitive transplantation assays. Our data provide further evidence that HSCs reside in a hypoxic microenvironment and demonstrate a novel way in which the hypoxic niche affects HSC fate, via the hypoxia-VEGFA axis. (Blood. 2011; 118(6):1534-1543) IntroductionThe process of blood formation originates with the hematopoietic stem cell (HSC) which is responsible for the life-long supply of mature blood cells through the unique ability to combine selfrenewal and differentiation. During the later stages of mammal development, HSCs are present in the fetal liver (FL) from where they seed the bone marrow (BM), which remains the main site of hematopoiesis throughout adult life. The HSC niche is a term used to describe the location and regulatory microenvironment of HSCs. In the BM, cellular HSC niche components include boneforming osteoblasts 1-2 and perivascular cells, [3][4] that through production of various secreted factors as well as through direct cell-cell interactions can affect HSC behavior. As for location it is thought that HSCs are present at a higher frequency at the endosteum close to the bone surface, because ex vivo labeled HSCs tend to localize to the endosteal areas of the BM. [5][6] However, within these areas HSCs are likely to associate with both osteoblastic and sinusoidal endothelial cells. 7 The FL HSC niche is much less defined than that in the BM, but a candidate FL niche component is a CD3 ϩ Ter119 Ϫ population that can provide HSC support. 8 Apart from HSC regulatory cues such as cell-cell contact and production of soluble factors, it is possible that variations in oxygen availability can affect HSCs, and it has been hypothesized that hypoxia is a characteristic of the HSC niche. 9 Oxygen levels are generally lower in BM compared with peripheral blood (PB). 10 Two studies have demonstrated that fractions of BM that w...

Prospective isolation is critical for understanding the cellular and molecular aspects of stem cell heterogeneity. Here, we identify the cell surface antigen CD9 as a positive marker that provides a simple alternative for hematopoietic stem cell isolation at high purity. Crucially, CD9 affords the capture of all hematopoietic stem cells in murine bone marrow in the absence of contaminating populations that lack authentic stem cell function. Using CD9 as a tool to subdivide hematopoietic stem-cell-containing populations, we provide evidence for heterogeneity at the cellular, functional, and molecular levels.

The key myeloid transcription factor (TF), CEBPA, is frequently mutated in acute myeloid leukemia (AML), but the direct molecular effects of this leukemic driver mutation remain elusive. To investigateCEBPAmutant AML, we performed microscale, in vivo chromatin immunoprecipitation sequencing and identified a set of aberrantly activated enhancers, exclusively occupied by the leukemia-associated CEBPA-p30 isoform. Comparing gene expression changes in humanCEBPAmutant AML and the correspondingCebpaLp30mouse model, we identifiedNt5e, encoding CD73, as a cross-species AML gene with an upstream leukemic enhancer physically and functionally linked to the gene. Increased expression of CD73, mediated by the CEBPA-p30 isoform, sustained leukemic growth via the CD73/A2AR axis. Notably, targeting of this pathway enhanced survival of AML-transplanted mice. Our data thus indicate a first-in-class link between a cancer driver mutation in a TF and a druggable, direct transcriptional target.