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Research suggests that about a quarter of bullying incidences occur within friendships. Yet little attention is given to the underlying social processes and wider macro-system forces that shape fri...

The present study seeks to contribute to qualitative research on students’ perspectives on bystander behaviors to better understand their behaviors in bullying situations. Researchers have found th ...

Hematopoietic stem cells (HSCs) have long been considered the continuous source of all hematopoietic cells for the life of an individual. Recent findings have questioned multiple aspects of this view, including the ability of lifelong HSCs to contribute to tissue-resident immune cells. Here we discuss the most recent findings on the source of B1a cells, innatelike lymphocytes that primarily reside in serous cavities. Powerful experimental approaches including bar coding, single cell transplantation, in vivo lineage tracing, and HSC-specific pulse-chase labeling have provided novel insights on B1a-cell generation during ontogeny. We evaluate the evidence for fetal vs adult B1a-cell production capacity and the identity of putative cells of origin. Integrating these most recent findings with previous work, we propose a working model that encapsulates our current understanding of waves of immune development.

The generation of distinct hematopoietic cell types, including tissue-resident immune cells, distinguishes fetal from adult hematopoiesis. However, the mechanisms underlying differential cell production to generate a layered immune system during hematopoietic development are unclear. Using an irreversible lineage-tracing model, we identify a definitive hematopoietic stem cell (HSC) that supports long-term multilineage reconstitution upon transplantation into adult recipients but does not persist into adulthood in situ. These HSCs are fully multipotent, yet they display both higher lymphoid cell production and greater capacity to generate innate-like B and T lymphocytes as compared to coexisting fetal HSCs and adult HSCs. Thus, these developmentally restricted HSCs (drHSCs) define the origin and generation of early lymphoid cells that play essential roles in establishing self-recognition and tolerance, with important implications for understanding autoimmune disease, allergy, and rejection of transplanted organs.

High-content phenotypic screening has become the approach of choice for drug discovery due to its ability to extract drug-specific multi-layered data. In the field of epigenetics, such screening methods have suffered from a lack of tools sensitive to selective epigenetic perturbations. Here we describe a novel approach, Microscopic Imaging of Epigenetic Landscapes (MIEL), which captures the nuclear staining patterns of epigenetic marks and employs machine learning to accurately distinguish between such patterns. We validated the MIEL platform across multiple cells lines and using dose-response curves, to insure the fidelity and robustness of this approach for high content high throughput drug discovery. Focusing on noncytotoxic glioblastoma treatments, we demonstrated that MIEL can identify and classify epigenetically active drugs. Furthermore, we show MIEL was able to accurately rank candidate drugs by their ability to produce desired epigenetic alterations consistent with increased sensitivity to chemotherapeutic agents or with induction of glioblastoma differentiation., eLife digest Each cell contains a complete copy of a person’s genes coded in their DNA. However, for a cell to perform its specific role, it only needs a small fraction of this genetic information. The mechanisms that control which genes a cell is using fall under the umbrella of ‘epigenetics’ (meaning beyond genetics). These mechanisms involve changes in the way that DNA is organized inside the cell nucleus and changes in how accessible different parts of the genome are to various cellular components. DNA is long and fragile so, to maintain its integrity, it is wrapped around protein complexes called histones. Adding chemical modifications to histones is one of the main epigenetic mechanisms that cells use to regulate which genes are turned on and off. Several methods allow researchers to read patterns of histone modification and use this information to derive what state a cell is in and how it might behave. Improving these methods is of particular interest in drug development, where this information could reveal the effects, and side-effects, of new treatments. Unfortunately, existing techniques are costly in both time and money, and they are not well suited to analyzing epigenetic changes caused by the large numbers of compounds tested during drug development. To overcome this barrier, Farhy et al. have developed a new system called ‘Microscopic Imaging of the Epigenetic Landscape’ (MIEL for short). The system allows them to quickly analyze the epigenetic changes caused by each of a large number of different chemical compounds when they are used on cells. MIEL tags different histone modifications by staining each with a different color, and then uses automated microscopy to produce images. It then extracts information from these images using advanced image analysis tools. The changes induced by different drugs can then be compared and categorized using machine learning algorithms. To test the MIEL system, Farhy et al. grew brain cancer cells (derived from human tumors) in the lab, and treated them with compounds that target proteins involved in histone modifications. Using their newly created pipeline, Farhy et al. were able to identify the unique epigenetic changes caused by these compounds, and train the system to correctly predict which type of drug the cells had been treated with. In a different set of experiments Farhy et al. demonstrate the utility of their new pipeline in identifying drugs that induce a set of epigenetic changes associated with a reduced ability to regrow tumors. This new system could help screen thousands of compounds for their epigenetic effects, which could aid the design of new treatments for many diseases.

Hematopoiesis is the process by which mature blood and immune cells are produced from hematopoietic stem and progenitor cells (HSCs and HSPCs). The last several decades of research have shed light on the origin of HSCs, as well as the heterogeneous pools of fetal progenitors that contribute to lifelong hematopoiesis. The overarching concept that hematopoiesis occurs in dynamic, overlapping waves throughout development, with each wave contributing to both continuous and developmentally limited cell types, has been solidified over the years. However, recent advances in our ability to track the production of hematopoietic cells in vivo have challenged several long-held dogmas on the origin and persistence of distinct hematopoietic cell types. In this review, we highlight emerging concepts in hematopoietic development and identify unanswered questions.

With the advent of automatic cell imaging and machine learning, high-content phenotypic screening has become the approach of choice for drug discovery because it can extract drug-specific multi-layered data, which could be compared to known profiles. In the field of epigenetics, such screening approaches have suffered from a lack of tools sensitive to selective epigenetic perturbations. Here we describe a novel approach, Microscopic Imaging of Epigenetic Landscapes (MIEL), which captures the nuclear staining patterns of epigenetic marks (e.g., acetylated and methylated histones) and employs machine learning to accurately distinguish between such patterns. We validated the fidelity and robustness of the MIEL platform across multiple cells lines using dose-response curves. We employed MIEL to uncover the mechanism by which bromodomain inhibitors synergize with temozolomide-mediated killing of human glioblastoma lines. To explore alternative, non-cytotoxic, glioblastoma treatment, we screen the Prestwick chemical library and documented the power of MIEL platform to identify epigenetically active drugs and accurately rank them according to their ability to produce epigenetic and transcriptional alterations consistent with the induction of glioblastoma differentiation.

The discovery of a fetal origin for tissue-resident macrophages (trMacs) has inspired an intense search for the mechanisms underlying their development. Here, we performed in vivo lineage tracing of cells with an expression history of IL-7Rα, a marker exclusively associated with the lymphoid lineage in adult hematopoiesis. Surprisingly, we found that IL7R-Cre labeled fetal-derived, adult trMacs. Labeling was almost complete in some tissues and partial in other organs. The putative progenitors of trMacs, yolk sac (YS) erythromyeloid progenitors (EMPs), did not express IL-7R, and YS hematopoiesis was unperturbed in IL-7R-deficient mice. In contrast, tracking of IL-7Rα message levels, surface protein expression, and IL7R-Cre-mediated labeling across fetal development revealed dynamic regulation of IL-7Rα mRNA expression and rapid upregulation of IL-7Rα surface protein upon transition from monocyte to macrophage within fetal tissues. Fetal liver monocyte differentiation in vitro produced IL-7R+ macrophages, supporting a direct progenitor-progeny relationship. Additionally, blockade of IL-7R function during late gestation specifically impaired the establishment of fetal-derived tissue macrophages in vivo. These data provide evidence for a distinct function of IL-7Rα in fetal myelopoiesis and identify IL-7R as a novel regulator of tissue-resident macrophage development.

Bullying remains a problem in schools, affecting the health of many young people. In this study, the focus is on exploring how 11- to 15-year-olds talk about their social worlds and social incidents such as bullying. Through semi-structured interviews, analyzed with constructivist grounded theory, the conceptualization of the participants perspectives reveals that three types of incidents take place in their social worlds: Diffuse incidents, Quarrel incidents and Bullying. Incidents are framed differently, which reveals how the social context plays an integral part in how different incidents and interactions were defined and considered as harmful bullying or not. Four contextual aspects are taken into consideration: (1) Iteration, (2) Type of target, (3) Social and emotional harm for the target, (4) Social relationship to target. Even if not all type of incidents are framed as harmful bullying, they interact by being grounded in normative identity constructions that use both social categories such as gender and sexuality and locally produced social categories.

In this chapter, we address the juridification of school violence in the Swedish context by focusing in particular on the issue of school bullying (skolmobbning) and gendered perceptions of differe ...

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The aim with this study was to listen to how children themselves discuss, reason on and make sense of how and why bullying emerges to extend our knowledge of what social processes that are made imp ...

During development, transient fetal hematopoietic stem cells (HSCs) are responsible for the production of "unconventional" innate-like immune cells that contribute to adult immunity. Whereas dysregulation of fetal-derived immune cells contributes to pathogenesis in a variety of immune tolerance disorders, the cellular and molecular drivers of pathogenesis are unknown. We have previously identified a transient developmentally-restricted HSC (drHSC) that specifically gives rise to innate-like lymphocytes during the perinatal period1. Under homeostatic conditions, the drHSC disappears postnatally, but its innate-like lymphocyte progeny persist into adulthood. Our discovery of a transient cell-of-origin for a specialized component of adult immunity underscores a “critical window” of immune development, during which phenotype of the adult immune system can be shaped by extrinsic inputs in early life. Here, we tested how developmental perturbation induced in utero by a single low-dose injection of poly(I:C) at mid-gestation drives both immediate and lasting changes to hematopoiesis and immunity in offspring. In utero perturbation disrupted the entrance of fetal HSCs into quiescence, causing disproportionate expansion of drHSCs, and a resultant shift in multipotent progenitor output. Postnatally, we observed sustained expansion and inappropriate persistence of the drHSC population into adulthood. These fundamental changes to the postnatal HSC compartment resulted in parallel expansion and hyperactivation of innate-like lymphocytes, thereby altering immune landscape and function in offspring. Our work validates the existence of a critical window of immune development by revealing how early perturbation of distinct fetal HSCs can drive long-term changes to immunity and disease susceptibility in offspring.

Our goal is to define the cellular and molecular mechanisms that control aging of the hematopoietic system. Given the increasing global life expectancy of people and the prevalence of age-related hematological diseases, understanding alterations to stem and progenitor cell function during aging is critical for developing preventative and restorative therapies. Previous reports have suggested that age-related morbidity is determined by functional decline of hematopoietic stem cells (HSCs). While HSCs themselves have been associated with hematopoietic aging, the role of progenitor cells during aging remains a mystery. Progenitor cells carry varied capacity to proliferate and differentiate in order to meet the demands of the blood system. Therefore, we hypothesize that progenitor cells play a significant role in the homeostatic decline associated with aging. Indeed, Megakaryocyte Progenitors (MkPs) have been shown to be consistently enriched in the aging bone marrow. Therefore, we specifically tackled the question – how might aging alter the functional and molecular regulation of MkPs? Surprisingly, and in stark contrast to the functional decline of aging HSCs, we found that old MkPs displayed greater capacity to generate platelets compared to young MkPs. These functional differences were apparent by both in vivo reconstitution and in vitro proliferation assays. To gain further insight into the changes in molecular regulation of MkP populations, we performed RNA sequencing (RNA-seq) analysis of young and old MkPs. Interestingly, their functional differences are paralleled by unique molecular regulators of old MkPs. These data demonstrate that megakaryopoiesis and MkP maintenance is fundamentally different between young and old mice, and that old MkPs gain expansion capacity whereas old HSCs functionally decline. Understanding these cellular and molecular events provides a promise for development of effective therapeutic strategies aimed at guiding differentiation and maturation toward a more youthful megakaryopoiesis.

The resistance of Glioblastoma (GBM) to conventional cytotoxic drugs has prompted novel therapeutic strategies, including differentiating tumor propagating cells (TPCs) into less tumorigenic cells using small molecule inducers of TPC differentiation. However, high-throughput screening for such molecules is hampered by the lack of robust markers of GBM differentiation. To obtain a signature of differentiated TPCs, we developed “Microscopic Imaging of Epigenetic Landscapes” (MIEL), which captures patterns of nuclear staining for epigenetic marks to derive feature-fingerprints of individual cells. We confirmed MIEL’s ability to accurately distinguish multiple cell fates and identified a multiparametric epigenetic signature of differentiated TPCs. Critically, we validated epigenetic imaging-based signature using global gene expression thus providing the proof of principle for the MIEL’s ability to select and prioritize small molecules, which induce TPC differentiation.

RNA-seq is a powerful technique to investigate and quantify entire transcriptomes. Recent advances in the field have made it possible to explore the transcriptomes of single cells. However, most widely used RNA-seq protocols fail to provide crucial information regarding transcription start sites. Here we present a protocol, Tn5Prime, that takes advantage of the Tn5 transposase based Smartseq2 protocol to create RNA-seq libraries that capture the 5’ end of transcripts. The Tn5Prime method dramatically streamlines the 5’ capture process and is both cost effective and reliable. By applying Tn5Prime to bulk RNA and single cell samples we were able to define transcription start sites as well as quantify transcriptomes at high accuracy and reproducibility. Additionally, similar to 3’ end based high-throughput methods like Drop-Seq and 10X Genomics Chromium, the 5’ capture Tn5Prime method allows the introduction of cellular identifiers during reverse transcription, simplifying the analysis of large numbers of single cells. In contrast to 3’ end based methods, Tn5Prime also enables the assembly of the variable 5’ ends of antibody sequences present in single B-cell data. Therefore, Tn5Prime presents a robust tool for both basic and applied research into the adaptive immune system and beyond.

Summary Epigenetic regulation serves as the basis for stem cell differentiation into distinct cell types, but it is unclear how global epigenetic changes are regulated during this process. Here, we tested the hypothesis that global chromatin organization affects the lineage potential of stem cells and that manipulation of chromatin dynamics influences stem cell function. Using nuclease sensitivity assays, we found a progressive decrease in chromatin digestion among pluripotent embryonic stem cells (ESCs), multipotent hematopoietic stem cells (HSCs), and mature hematopoietic cells. Quantitative high-resolution microscopy revealed that ESCs contain significantly more euchromatin than HSCs, with a further reduction in mature cells. Increased cellular maturation also led to heterochromatin localization to the nuclear periphery. Functionally, prevention of heterochromatin formation by inhibition of the histone methyltransferase G9A resulted in delayed HSC differentiation. Our results demonstrate global chromatin rearrangements during stem cell differentiation and that heterochromatin formation by H3K9 methylation regulates HSC differentiation., Graphical Abstract, Highlights • Nuclease sensitivity decreases progressively with stem cell differentiation • Distinct chromatin architecture is apparent in cells of different lineage potential • Stem cell differentiation leads to perinuclear heterochromatin localization • G9a-mediated heterochromatin formation facilitates stem cell differentiation, Forsberg and colleagues use a variety of approaches to investigate global chromatin structure and architecture in pluripotent, multipotent, and differentiated cells. They show that progressive changes in nuclease sensitivity, chromatin condensation, and heterochromatin localization correlate with the lineage potential of embryonic and adult stem cells and committed cells. Functionally, inhibition of heterochromatin formation delayed hematopoietic stem cell differentiation. These data provide new insights on the epigenetic regulation of stem cell lineage potential and differentiation.

In February 2015, over 200 scientists gathered for the Keystone Hematopoiesis meeting, which was held at the scenic Keystone Resort in Keystone, Colorado, USA. The meeting organizers, Patricia Ernst, Hanna Mikkola and Timm Schroeder, put together an exciting program, during which field leaders and new investigators presented discoveries that spanned developmental and adult hematopoiesis within both physiologic and pathologic contexts. Collectively, the program highlighted the increasing pace of new discoveries and the substantial progress made in the hematopoiesis field since the last Keystone meeting two years ago. In this Meeting Review, we highlight the main concepts discussed at the conference, with an emphasis on topics relevant to developmental biology.

SummaryAlthough classified as hematopoietic cells, tissue-resident macrophages (MFs) arise from embryonic precursors that seed the tissues prior to birth to generate a self-renewing population, which is maintained independently of adult hematopoiesis. Here we reveal the identity of these embryonic precursors using an in utero MF-depletion strategy and fate-mapping of yolk sac (YS) and fetal liver (FL) hematopoiesis. We show that YS MFs are the main precursors of microglia, while most other MFs derive from fetal monocytes (MOs). Both YS MFs and fetal MOs arise from erythro-myeloid progenitors (EMPs) generated in the YS. In the YS, EMPs gave rise to MFs without monocytic intermediates, while EMP seeding the FL upon the establishment of blood circulation acquired c-Myb expression and gave rise to fetal MOs that then seeded embryonic tissues and differentiated into MFs. Thus, adult tissue-resident MFs established from hematopoietic stem cell-independent embryonic precursors arise from two distinct developmental programs.

Understanding gene regulation and function requires a genome-wide method capable of capturing both gene expression levels and isoform diversity at the single-cell level. Short-read RNAseq is limited in its ability to resolve complex isoforms because it fails to sequence full-length cDNA copies of RNA molecules. Here, we investigate whether RNAseq using the long-read single-molecule Oxford Nanopore MinION sequencer is able to identify and quantify complex isoforms without sacrificing accurate gene expression quantification. After benchmarking our approach, we analyse individual murine B1a cells using a custom multiplexing strategy. We identify thousands of unannotated transcription start and end sites, as well as hundreds of alternative splicing events in these B1a cells. We also identify hundreds of genes expressed across B1a cells that display multiple complex isoforms, including several B cell-specific surface receptors. Our results show that we can identify and quantify complex isoforms at the single cell level., Short-read RNA-seq is limited in its ability to resolve complex transcript isoforms since it cannot sequence full-length cDNA. Here the authors use Oxford Nanopore MinION and their Mandalorion analysis pipeline to measure complex isoforms in B1a cells.

När elever i fjärde till sjunde klass intervjuas om hur de reagerar som åskådare till mobbning framkommer att sociala processer såsom vad som räknas som mobbning, sociala relationer och skuldbeläggande av den utsatta påverkar om de hjälper till eller inte.

The aim of this study is to explore 40 Swedish 7th and 8th grade girls perspectives on bullying by listening to how they discuss and understand bullying. Pair and group interviews were conducted and analysed using grounded theory. Symbolic interactionism was used as a theoretical perspective focusing on social processes and interaction. The participants constructed bullying as an identity process involving gendered identities, victim identities and socially-valuable identities where bullying was located within a gendered order. These identities were negotiated with the concept of self-confidence, where the girls both aligned with and distanced themselves from the gendered order. (C) 2017 John Wiley amp; Sons Ltd and National Childrens Bureau

Tissue-resident macrophages (TRMs) play critical roles in tissue homeostasis and disease. Many populations of TRMs derive from fetal progenitors and independently self-maintain across the lifespan through in situ proliferation. Here, we have identified the interleukin-7 receptor (IL7R) as a novel regulator of TRM development. Using an IL7R-Cre lineage tracing model, we observed that adult TRMs in the brain, epidermis, liver, and lung were highly labeled by IL7R-cre, in the absence of IL7Ra mRNA or protein expression. To gain insight into developmental expression of IL7Ra, we profiled surface expression, mRNA expression, and IL7R-cre driven labeling across fetal development. Erythromyeloid progenitors, putative TRM precursors, were barely labeled by IL7Ra-cre, and IL7R deletion did not affect YS hematopoiesis. In contrast, we observed IL7Ra mRNA expression in fetal monocytes, and robust IL7Ra surface expression on developing TRMs during late gestation. Sorted Ly6chi fetal liver monocytes cultured ex vivo with M-CSF differentiated into macrophages expressing IL7R, suggesting a precursor-product relationship. Blockade of the IL7R with a monoclonal antibody during gestation impaired liver, lung, and epidermal TRM cellularity at birth, with a concomitant increase in cellularity of liver monocytes, suggesting that IL7Ra regulates TRM differentiation from fetal monocytes during fetal development. These data reveal dynamic regulation of IL7Ra expression in TRMs and TRM precursors during late gestation, and provide evidence that IL7R signaling regulates fetal TRM development. Ongoing work addresses downstream signaling and the specific developmental processes regulated by IL7R signaling during fetal TRM development.

Haematopoietic stem cell (HSC) niches provide an environment essential for life-long HSC function. Intense investigation of HSC niches both feed off and drive technology development to increase our capability to assay functionally defined cells with high resolution. A major driving force behind the desire to understand the basic biology of HSC niches is the clear implications for clinical therapies. Here, with particular emphasis on cell type-specific deletion of SCL and CXCL12, we focus on unresolved issues on HSC niches, framed around some very recent advances and novel discoveries on the extrinsic regulation of HSC maintenance. We also provide ideas for possible paths forward, some of which are clearly within reach while others will require both novel tools and vision.

Selective labeling of specific cell types by expression of green fluorescent protein (GFP) within the hematopoietic system would have great utility in identifying, localizing, and tracking different cell populations in flow cytometry, microscopy, lineage tracing, and transplantation assays. In this report, we describe the generation and characterization of a new transgenic mouse line with specific GFP labeling of all nucleated hematopoietic cells, as well as platelets. This new “Vav-GFP” mouse line labels the vast majority of hematopoietic cells with GFP during both embryonic development and adulthood, with particularly high expression in hematopoietic stem and progenitor cells. With the exception of transient labeling of fetal endothelial cells, GFP expression is highly selective for hematopoietic cells, and persists in donor-derived progeny after transplantation of hematopoietic progenitor cells. Finally, we also demonstrate that the loxP-flanked reporter allows for specific GFP labeling of different hematopoietic cell subsets when crossed to various Cre reporter lines. Notably, by crossing Vav-GFP mice to Flk2-Cre mice, we obtained robust and highly selective GFP expression in HSCs. Together, these data describe a new mouse model capable of directing GFP labeling exclusively of hematopoietic cells or exclusively of HSCs.

The aim of the present thesis was to listen to, examine and conceptualise students’ perspectives on bullying. Students’ perspectives have not been commonly heard in research and less qualitative re ...

Genetic fate-mapping approaches provide a unique opportunity to assess differentiation pathways under physiological conditions. We have recently employed a lineage tracing approach to define hematopoietic differentiation pathways in relation to expression of the tyrosine kinase receptor Flk2.1 Based on our examination of reporter activity across all stem, progenitor and mature populations in our Flk2-Cre lineage model, we concluded that all mature blood lineages are derived through a Flk2+ intermediate, both at steady-state and under stress conditions. Here, we re-examine in depth our initial conclusions and perform additional experiments to test alternative options of lineage specification. Our data unequivocally support the conclusion that onset of Flk2 expression results in loss of self-renewal but preservation of multilineage differentiation potential. We discuss the implications of these data for defining stem cell identity and lineage potential among hematopoietic populations.

Tobacco use is associated with an increase in the white blood cell (WBC) count. This association has been attributed to bronchopulmonary inflammation and/or infection. It is not known if nicotine itself may play a role. The objective of this study was to determine whether nicotine itself could affect the WBC count, and to determine whether this was due to a direct effect on hematopoietic stem cells (HSC). C57Bl6J mice received nicotine orally, and measurements of the WBC count, bone marrow and spleen cellularity, and HSC count were made. To determine the functionality of HSCs, irradiated animals received bone marrow transplants from vehicle or nicotine-treated mice. Nicotine increased leukocytes in the peripheral blood, bone marrow and spleen. The peripheral red cell and platelet count were unaffected. Nicotine increased the frequency of HSC in the bone marrow. Isolated long-term HSCs from nicotine-treated mice transplanted into irradiated mice regenerated all hematopoietic cell lineages, demonstrating the functional competence of those HSCs. HSCs expressed nicotinic acetylcholine receptors (nAChRs), as documented by FITC-conjugated alpha-bungarotoxin binding. Nicotine increased soluble Kit ligand, consistent with stem cell activation. In conclusion, the data suggest a new mechanism for the increased WBC associated with tobacco use. The effect of nicotine to activate hematopoiesis may contribute to tobacco-related diseases.

Tissue-resident macrophages play critical roles in tissue homeostasis and disease. Many populations of tissue-resident macrophages derive from fetal progenitors and self-maintain across the lifespan through in situ proliferation, independent of bone marrow hematopoiesis. However, the developmental mechanisms that specify fetal-derived tissue-resident macrophages are poorly understood. Here, we have identified a novel cytokine regulating tissue-resident macrophage development using an IL7Ra-cre lineage tracing model. Adult tissue resident macrophages in the brain, skin, liver, and lung were extensively labeled by IL7ra-cre, in the absence of IL7ra message or protein expression. To gain insight into developmental expression of IL7ra, we profiled IL7Ra surface expression, mRNA expression, and IL7ra-cre-driven labeling across fetal myeloid development. We observed rapid upregulation of IL7ra surface expression during a limited window of tissue macrophage establishment, and dynamic regulation of IL7ra message and protein levels in monocyte precursors, suggesting that IL7ra regulated the transition from fetal liver monocytes into tissue macrophages. Blockade of the IL7R using a monoclonal antibody decreased cellularity of liver, lung and skin tissue resident macrophages at birth, and increased cellularity of fetal liver monocytes. These data suggest that late in gestation, IL7Ra is upregulated as fetal monocytes exit the fetal liver and differentiate into macrophages, and provides evidence that IL-7R signaling regulates fetal tissue-resident macrophage development. Ongoing work addresses the function of IL-7 signaling in myeloid development, and the function of IL-7R marked macrophages in adult immunity.

Hematopoietic stem cells (HSC) produce all blood cell lineages by virtue of their capacity to self-renew and differentiate into progenitors with decreasing cellular potential. Recent studies suggest that epigenetic mechanisms play an important role in controlling stem cell potency and cell fate decisions. To investigate this hypothesis in HSC, we have modified the conventional chromatin immunoprecipitation assay allowing for the analysis of 50,000 prospectively purified stem and progenitor cells. Together with bisulfite sequencing analysis, we found that methylated H3K4 and AcH3 and unmethylated CpG dinucleotides colocalize across defined regulatory regions of lineage-affiliated genes in HSC. These active epigenetic histone modifications either accumulated or were replaced by increased DNA methylation and H3K27 trimethylation in committed progenitors consistent with gene expression. We also observed bivalent histone modifications at a lymphoid-affiliated gene in HSC and downstream transit-amplifying progenitors. Together, these data support a model in which epigenetic modifications serve as an important mechanism to control HSC multipotency.

SummaryA model of hematopoietic development wherein multipotentiality is conserved until segregation of myeloid and lymphoid potential has recently been challenged, proposing that megakaryocyte/erythrocyte (MegE) potential is lost in Flk2/Flt3-expressing early progenitors. Here, we used sensitive in vivo approaches to quantitatively and kinetically assess the MegE potential of hematopoietic stem cells and various Flk2+ early progenitors and compared it with the MegE potential of downstream committed myeloid and lymphoid progenitors and with their ability to give rise to mature myelomonocytic and lymphoid cells. We demonstrate that Flk2+ early progenitors retain MegE potential in vivo both at the population and clonal levels. These results indicate that Flk2 expression by early progenitors is not at the expense of full multipotency and support the current model of hematopoietic development with segregation of myeloid and lymphoid lineages from multipotent progenitors.

Haematopoietic stem cells (HSCs) self-renew for life, thereby making them one of the few blood cells that truly age(1,2). Paradoxically, although HSCs numerically expand with age, their functional activity declines over time, resulting in degraded blood production and impaired engraftment following transplantation(2). While many drivers of HSC ageing have been proposed(2-5), the reason why HSC function degrades with age remains unknown. Here we show that cycling old HSCs in mice have heightened levels of replication stress associated with cell cycle defects and chromosome gaps or breaks, which are due to decreased expression of mini-chromosome maintenance (MCM) helicase components and altered dynamics of DNA replication forks. Nonetheless, old HSCs survive replication unless confronted with a strong replication challenge, such as transplantation. Moreover, once old HSCs re-establish quiescence, residual replication stress on ribosomal DNA (rDNA) genes leads to the formation of nucleolar-associated gamma H2AX signals, which persist owing to ineffective H2AX dephosphorylation by mislocalized PP4c phosphatase rather than ongoing DNA damage. Persistent nucleolar gamma H2AX also acts as a histone modification marking the transcriptional silencing of rDNA genes and decreased ribosome biogenesis in quiescent old HSCs. Our results identify replication stress as a potent driver of functional decline in old HSCs, and highlight the MCM DNA helicase as a potential molecular target for rejuvenation therapies.

The aim with the present study was to investigate bystander actions in bullying situations as well as reasons behind these actions as they are articulated by Swedish students from fourth to seventh grade. Forty-three semi-structured individual interviews were conducted with students. Qualitative analysis of data was performed by methods from grounded theory. The analysis of the student voices of being a bystander in bullying reveals a complexity in which different definition-of-situation processes are evoked (a) relations (friends and social hierarchy), (b) defining seriousness, (c) victim’s contribution to the situation, (d) social roles and intervention responsibilities, and (e) distressing emotions. There are often conflicted motives in how to act as a bystander, which could evoke moral distress among the students. Our analysis is unique in that it introduces the concept of moral distress as a process that has to be considered in order to better understand bystander actions among children The findings also indicate bystander reactions that could be associated with moral disengagement, such as not perceiving a moral obligation to intervene if the victim is defined as a non-friend (‘none of my business’), protecting the friendship with the bully, and blaming the victim.

Defining differentiation pathways is central to understanding the pathogenesis of hematopoietic disorders, including leukemia. The function of the receptor tyrosine kinase Flk2 (Flt3) in promoting myeloid development remains poorly defined, despite being commonly mutated in acute myeloid leukemia. We investigated the effect of Flk2 deficiency on myelopoiesis, focusing on specification of progenitors between HSC and mature cells. We provide evidence that Flk2 is critical for proliferative expansion of multipotent progenitors that are common precursors for all lymphoid and myeloid lineages, including megakaryocyte/erythroid (MegE) cells. Flk2 deficiency impaired the generation of both lymphoid and myeloid progenitors by abrogating propagation of their common upstream precursor. At steady state, downstream compensatory mechanisms masked the effect of Flk2 deficiency on mature myeloid output, whereas transplantation of purified progenitors revealed impaired generation of all mature lineages. Flk2 deficiency did not affect lineage choice, thus dissociating the role of Flk2 in promoting cell expansion and regulating cell fate. Surprisingly, despite impairing myeloid development, Flk2 deficiency afforded protection against myeloablative insult. This survival advantage was attributed to reduced cell cycling and proliferation of progenitors in Flk2-deficient mice. Our data support the existence of a common Flk2(+) intermediate for all hematopoietic lineages and provide insight into how activating Flk2 mutations promote hematopoietic malignancy by non-Flk2-expressing myeloid cells.

Transcription of the β-globin genes is regulated by four erythroid-cell-specific DNaseI-hypersensitive sites (HSs) at the 5′ end of the β-globin locus (12, 46), termed the locus control region (LCR) (16). Although the HSs function together to confer long-range transactivation in transfection assays (5) and transgenic mice (7), HS2 and HS3, individually, have strong erythroid-cell-specific enhancer activity when positioned near a promoter (9, 18, 20, 27, 29, 38, 41, 44). Recently, we reported that LCR-mediated transactivation in stable transfection assays increased upon activation of the mitogen-activated protein kinase (MAPK) signaling pathway (48). The MAPK-dependent stimulation required HS2 and was independent of the distance between HS2 and the promoter. Tandem recognition sites within HS2 for the erythroid-cell- and megakaryocyte-specific transcription factor NF-E2 (1, 33, 37), which are critical for strong enhancer activity, were necessary for induction. Besides the requirement of NF-E2 sites, expression of tethered NF-E2, with covalently linked p45 and p18 subunits, conferred induction in CB3 erythroleukemia cells (30) that lack NF-E2. We postulated that phosphorylation of NF-E2 facilitates the recruitment of a coactivator that partially overcomes a limitation to long-range activation (48). Enhancement of NF-E2-mediated transactivation by MAPK with synthetic promoter constructs in transient expression assays has also been described (36). Two simple mechanisms could explain the MAPK-dependent stimulation of HS2 enhancer activity. First, MAPK may enhance the formation of transcription complexes on inactive promoters. Alternatively, MAPK may augment the activity of preassembled complexes. To distinguish between these possibilities, we established a system for assaying gene expression in single, living cells using cell lines containing integrated constructs with the enhanced green fluorescent protein (EGFP) reporter gene. Gene expression can be quantitatively assayed in single cells with β-galactosidase or GFP reporter genes and flow cytometric analysis. Studies on the mechanism of enhancer action in single cells have found that enhancers act via a stochastic or binary mechanism to increase the probability of gene expression from a given template (11, 15, 24, 32, 40, 43, 50, 53). Accordingly, the enhancer determines whether the promoter will be active or inactive rather than augmenting the efficacy of an active promoter. Thus, there are three possible activity states in a diploid cell, each characterized by either two inactive alleles, one active allele, or two active alleles. Stochastic behavior can be explained by the enhancer-dependent, all-or-none recruitment of the RNA polymerase II holoenzyme (45) to an inactive promoter. As the positioning of a nucleosome on a promoter can occlude cis-acting elements (26, 49), the chromatin could prevent initiation complex assembly and thus maintain the silent state. A signaling pathway that targets an enhancer binding factor could regulate its ability to engage in protein-protein interactions necessary for the assembly of the enhancer complex. Only when the complete complex is assembled, which can be an all-or-none event (4), would the chromatin be disrupted, allowing for holoenzyme recruitment. The alternative graded mechanism (23), involving the augmented efficacy of active promoters, would result from a factor modification that modulates the activity of the enhancer complex rather than the all-or-none complex assembly. Based on the importance of coactivators in transactivation (19), the modification may increase the efficiency of coactivator recruitment and thus increase promoter activity. Although activation may involve exclusively stochastic or graded responses, certain mechanisms may elicit both responses. For example, coactivators that modify chromatin structure to control DNA accessibility could influence both initiation complex assembly and the activity of preassembled complexes. Here, we describe the influence of MAPK on HS2 enhancer activity in single, living cells. Increased enhancer activity is manifested as both graded and stochastic responses, and the implications of this are discussed with respect to a model for HS2-mediated long-range transactivation.

SummaryDespite accumulating evidence suggesting local self-maintenance of tissue macrophages in the steady state, the dogma remains that tissue macrophages derive from monocytes. Using parabiosis and fate-mapping approaches, we confirmed that monocytes do not show significant contribution to tissue macrophages in the steady state. Similarly, we found that after depletion of lung macrophages, the majority of repopulation occurred by stochastic cellular proliferation in situ in a macrophage colony-stimulating factor (M-Csf)- and granulocyte macrophage (GM)-CSF-dependent manner but independently of interleukin-4. We also found that after bone marrow transplantation, host macrophages retained the capacity to expand when the development of donor macrophages was compromised. Expansion of host macrophages was functional and prevented the development of alveolar proteinosis in mice transplanted with GM-Csf-receptor-deficient progenitors. Collectively, these results indicate that tissue-resident macrophages and circulating monocytes should be classified as mononuclear phagocyte lineages that are independently maintained in the steady state.