Although recent advances have enabled hematopoietic stem cells (HSCs) to be enriched to near purity, more information about their characteristics will improve our understanding of their development and stage-related functions. Here, using microarray technology, we identified endothelial cell-selective adhesion molecule (ESAM) as a novel marker for murine HSCs in fetal liver. Esam was expressed at high levels within a Rag1⁻ c-kit^Hi Sca1⁺ HSC-enriched fraction, but sharply down-regulated with activation of the Rag1 locus, a valid marker for the most primitive lymphoid progenitors in E14.5 liver. The HSC-enriched fraction could be subdivided into 2 on the basis of ESAM levels. Among endothelial antigens on hematopoietic progenitors, ESAM expression showed intimate correlation with HSC activity. The ESAM^Hi population was highly enriched for multipotent myeloid-erythroid progenitors and primitive progenitors with lymphopoietic activity, and exclusively reconstituted long-term lymphohematopoiesis in lethally irradiated recipients. Tie2⁺ c-kit⁺ lymphohematopoietic cells in the E9.5–10.5 aorta-gonad-mesonephros region also expressed high levels of ESAM. Furthermore, ESAM was detected on primitive hematopoietic progenitors in adult bone marrow. Interestingly, ESAM expression in the HSC-enriched fraction was up-regulated in aged mice. We conclude that ESAM marks HSC in murine fetal liver and will facilitate studies of hematopoiesis throughout life.

The cell-cycle status of hematopoietic stem and progenitor cells (HSPCs) becomes activated following chemotherapy-induced stress, promoting bone marrow (BM) regeneration; however, the underlying molecular mechanism remains elusive. Here we show that BM-resident group 2 innate lymphoid cells (ILC2s) support the recovery of HSPCs from 5-fluorouracil (5-FU)–induced stress by secreting granulocyte-macrophage colony-stimulating factor (GM-CSF). Mechanistically, IL-33 released from chemo-sensitive B cell progenitors activates MyD88-mediated secretion of GM-CSF in ILC2, suggesting the existence of a B cell–ILC2 axis for maintaining hematopoietic homeostasis. GM-CSF knockout mice treated with 5-FU showed severe loss of myeloid lineage cells, causing lethality, which was rescued by transferring BM ILC2s from wild-type mice. Further, the adoptive transfer of ILC2s to 5-FU–treated mice accelerates hematopoietic recovery, while the reduction of ILC2s results in the opposite effect. Thus, ILC2s may function by “sensing” the damaged BM spaces and subsequently support hematopoietic recovery under stress conditions.

It has long been known that lymphopoiesis is transiently suppressed during pregnancy, which can be experimentally simulated by estrogen treatment. We now confirm with Rag1/GFP reporter mice that early lymphoid progenitors in the lineage marker⁻ c-kit^highScaI⁺, hematopoietic stem cell-enriched fraction of bone marrow are particularly depressed in these circumstances. Hematopoietic and environmental cells are both potential hormone targets and, because of this complexity, very little is known regarding mechanisms. We have now identified soluble Frizzled-related protein (sFRP)1 as an estrogen-inducible gene in stromal cells, whose expression corresponded to inability to support lymphopoiesis. Bone-lining stromal cells express sFRP1, and the transcripts were elevated by pregnancy or estrogen injection. Estrogen receptor-α was essential for both lymphoid suppression and induction of the sFRP family. SFRP1 has been mainly described as an antagonist for complex Wnt signals. However, we found that sFRP1, like Wnt3a, stabilized β-catenin and blocked early lymphoid progression. Myeloerythroid progenitors were less affected by sFRP1 in culture, which was similar to estrogen with respect to lineage specificity. Hematopoietic stem cells expressed various Frizzled receptors, which markedly declined as they differentiated to lymphoid lineage. Thus, hormonal control of early lymphopoiesis in adults might partly relate to sFRP1 levels.

Whereas most hematopoietic stem cells (HSC) are quiescent in homeostasis, they actively proliferate in response to bone marrow (BM) injury. Signals from the BM microenvironment are thought to promote entry of HSC into the cell cycle. However, it has been cumbersome to assess cycle status of viable HSC and thus explore unique features associated with division. In this study, we show that expression of endothelial cell-selective adhesion molecule (ESAM) can be a powerful indicator of HSC activation. ESAM levels clearly mirrored the shift of HSC between quiescence and activation, and it was prominent in comparison with other HSC-related Ags. ESAM^hi HSC were actively dividing, but had surprisingly high long-term reconstituting capacity. Immunohistochemical analyses showed that most ESAM^hi HSC were located near vascular endothelium in the BM after 5-fluorouracil treatment. To determine the importance of ESAM in the process of BM recovery, ESAM knockout mice were treated with 5-fluorouracil and their hematopoietic reconstruction was examined. The ESAM deficiency caused severe and prolonged BM suppression, suggesting that ESAM is functionally indispensable for HSC to re-establish homeostatic hematopoiesis. With respect to intracellular regulators, NF-κB and topoisomerase II levels correlated with the ESAM upregulation. Thus, our data demonstrate that the intensity of ESAM expression is useful to trace activated HSC and to understand molecular events involved in stem cell states.