SummaryLymphocytes and neutrophils are rapidly depleted by systemic infection1. Progenitor cells of the hematopoietic system, such as common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs), increase the production of immune cells to restore and maintain homeostasis during chronic infection, but the contribution of hematopoietic stem cells (HSCs) to this process is largely unknown2. Using an in vivo mouse model of Mycobacterium avium infection, we show that an increased proportion of long-term repopulating HSCs (LT-HSCs) proliferate during M. avium infection, and that this response requires interferon-gamma (IFNγ) but not interferon-alpha (IFNα) signaling. Thus, the hematopoietic response to chronic bacterial infection involves the activation not only of intermediate blood progenitors but of LT-HSCs as well. IFNγ is sufficient to promote LT-HSC proliferation in vivo; furthermore, HSCs from mice deficient in IFNγ have a lower proliferative rate, indicating that baseline IFNγ tone regulates HSC activity. These findings are the first to implicate IFNγ both as a regulator of HSCs during homeostasis and under conditions of infectious stress. Our studies contribute to a deeper understanding of hematologic responses in patients with chronic infections such as HIV/AIDS or tuberculosis3-5.

Summary The traditional view of hematopoiesis has been that all the cells of the peripheral blood are the progeny of a unitary homogeneous pool of hematopoietic stem cells (HSCs). Recent evidence suggests that the hematopoietic system is actually maintained by a consortium of HSC subtypes with distinct functional characteristics. We show here that myeloid-biased HSCs (My-HSCs) and lymphoid-biased (Ly-HSCs) can be purified according to their capacity for Hoechst dye efflux in combination with canonical HSC markers. These phenotypes are stable under natural (aging) or artificial (serial transplantation) stress and are exacerbated in the presence of competing HSCs. My- and Ly-HSCs respond differently to TGFβ1, presenting a possible mechanism for differential regulation of HSC subtype activation. This study demonstrates definitive isolation of lineage-biased HSC subtypes and contributes to the fundamental change in view that the hematopoietic system is maintained by a continuum of HSC subtypes, rather than a functionally uniform pool.

Hematopoietic stem cells (HSCs) continuously regenerate the hematologic system, yet few genes regulating this process have been defined. To identify candidate factors involved in differentiation and self-renewal, we have generated an expression database of hematopoietic stem cells and their differentiated progeny, including erythrocytes, granulocytes, monocytes, NK cells, activated and naive T cells, and B cells. Bioinformatic analysis revealed HSCs were more transcriptionally active than their progeny and shared a common activation mechanism with T cells. Each cell type also displayed unique biases in the regulation of particular genetic pathways, with Wnt signaling particularly enhanced in HSCs. We identified approximately 100-400 genes uniquely expressed in each cell type, termed lineage "fingerprints." In overexpression studies, two of these genes, Zfp 105 from the NK cell lineage, and Ets2 from the monocyte lineage, were able to significantly influence differentiation toward their respective lineages, demonstrating the utility of the fingerprints for identifying genes that regulate differentiation.

Recent outbreaks of Zika virus (ZIKV) highlight an urgent need for therapeutics.The protease complex NS2B-NS3 plays essential roles during flaviviral polyprotein processing, and thus represents an attractive drug target. Here, we developed a split luciferase complementation-based high-throughput screening assay to identify orthosteric inhibitors that directly target flavivirus NS2B-NS3 interactions. By screening a total of 2 816 approved and investigational drugs, we identified three potent candidates, temoporfin, niclosamide, and nitazoxanide, as flavivirus NS2B-NS3 interaction inhibitors with nanomolar potencies. Significantly, the most potent compound, temoporfin, not only inhibited ZIKV replication in human placental and neural progenitor cells, but also prevented ZIKV-induced viremia and mortality in mouse models. Structural docking suggests that temoporfin potentially binds NS3 pockets that hold critical NS2B residues, thus inhibiting flaviviral polyprotein processing in a non-competitive manner. As these drugs have already been approved for clinical use in other indications either in the USA or other countries, they represent promising and easily developed therapies for the management of infections by ZIKV and other flaviviruses.

Hematopoietic stem cells (HSCs) remain by far the most well-characterized adult stem cell population both in terms of markers for purification and assays to assess functional potential. However, despite over 40 years of research, working with HSCs in the mouse remains difficult because of the relative abundance (or lack thereof) of these cells in the bone marrow. The frequency of HSCs in bone marrow is about 0.01% of total nucleated cells and $5,000 can be isolated from an individual mouse depending on the age, sex, and strain of mice as well as purification scheme utilized. This prohibits the study of processes in HSCs, which require large amounts of starting material. Adding to the challenge is the continual reporting of new markers for HSC purification, which makes it difficult for the uninitiated in the field to know which purification strategies yield the highest proportion of long-term, multilineage HSCs. This report will review different hematopoietic stem and progenitor purification strategies and compare flow cytometry profiles for HSC sorting and analysis on different instruments. We will also discuss methods for rapid flow cytometric analysis of peripheral blood cell types, and novel strategies for working with rare cell populations such as HSCs in the analysis of cell cycle status by BrdU, Ki-67, and Pyronin Y staining. The purpose of this review is to provide insight into some of the recent experimental and technical advances in mouse hematopoietic stem cell biology. ' International Society for Advancement of CytometryKey terms hematopoietic stem cell; flow cytometry; peripheral blood; cell cycle HEMATOPOIETIC stem cells have tremendous therapeutic potential and have been harnessed in the clinic for more than 40 years in the context of bone marrow transplantation. Multipotent long-term HSCs (LT-HSCs) reside in the bone marrow and through a process of asymmetric cell division, can self-renew to sustain the stem cell pool or differentiate into short-term HSCs (ST-HSCs) or lineage-restricted progenitors that undergo extensive proliferation and differentiation to produce terminally differentiated, functional hematopoietic cells. ST-HSCs or multipotent progenitors (MPPs) are only able to sustain hematopoiesis in the short term, whereas the LTHSCs must persist for the lifespan of the organism to perpetually replenish the hematopoietic system. HSCs can be isolated from bone marrow or peripheral blood using enrichment (magnetic cell separation-MACS) and/or single-cell sorting (fluorescence-activated cell sorting-FACS) based on cell surface markers and/or vital dye staining. The HSC has served as the paradigm for adult stem cell populations by virtue of a well-defined differentiation cascade with distinct intermediaries connecting the differentiation of LT-HSCs into mature, functional hematopoietic cells. Each of the cell stages of HSC differentiation can be purified from the bone marrow or peripheral blood using characteristic cell surface markers, which has greatly facilitated the study of hemato...