Lineage-committed Progenitors Research Articles

Identification and Isolation of Oligopotent and Lineage-committed Myeloid Progenitors from Mouse Bone Marrow.

Myeloid progenitors that yield neutrophils, monocytes and dendritic cells (DCs) can be identified in and isolated from the bone marrow of mice for hematological and immunological analyses. For example, studies of the cellular and molecular properties of myeloid progenitor populations can reveal mechanisms underlying leukemic transformation, or demonstrate how the immune system responds to pathogen exposure. Previously described flow cytometry strategies for myeloid progenitor identification have enabled significant advances in many fields, but the fractions they identify are very heterogeneous. The most commonly used gating strategies define bone marrow fractions that are enriched for the desired populations, but also contain large numbers of "contaminating" progenitors. Our recent studies have resolved much of this heterogeneity, and the protocol we present here permits the isolation of 6 subpopulations of oligopotent and lineage-committed myeloid progenitors from 2 previously described bone marrow fractions. The protocol describes 3 stages: 1) isolation of bone marrow cells, 2) enrichment for hematopoietic progenitors by magnetic-activated cell sorting (lineage depletion by MACS), and 3) identification of myeloid progenitor subsets by flow cytometry (including fluorescence-activated cell sorting, FACS, if desired). This approach permits progenitor quantification and isolation for a variety of in vitro and in vivo applications, and has already yielded novel insight into pathways and mechanisms of neutrophil, monocyte, and DC differentiation.

Identification and Isolation of Oligopotent and Lineage-committed Myeloid Progenitors from Mouse Bone Marrow

Identification and Isolation of Oligopotent and Lineage-committed Myeloid Progenitors from Mouse Bone Marrow

Hematopoietic stem cells can differentiate into restricted myeloid progenitors before cell division in mice

Hematopoietic stem cells (HSCs) continuously replenish all blood cell types through a series of differentiation steps and repeated cell divisions that involve the generation of lineage-committed progenitors. However, whether cell division in HSCs precedes differentiation is unclear. To this end, we used an HSC cell-tracing approach and Ki67RFP knock-in mice, in a non-conditioned transplantation model, to assess divisional history, cell cycle progression, and differentiation of adult HSCs. Our results reveal that HSCs are able to differentiate into restricted progenitors, especially common myeloid, megakaryocyte-erythroid and pre-megakaryocyte progenitors, without undergoing cell division and even before entering the S phase of the cell cycle. Additionally, the phenotype of the undivided but differentiated progenitors correlated with the expression of lineage-specific genes and loss of multipotency. Thus HSC fate decisions can be uncoupled from physical cell division. These results facilitate a better understanding of the mechanisms that control fate decisions in hematopoietic cells.

Visualizing Intrapopulation Hematopoietic Cell Heterogeneity with Self-Organizing Maps of SIMS Data.

Characterization of the heterogeneity within stem cell populations, which affects their differentiation potential, is necessary for the design of artificial cultures for stem cell expansion. In this study, we assessed whether self-organizing maps (SOMs) of single-cell time-of-flight secondary ion mass spectrometry (TOF-SIMS) data provide insight into the spectral, and thus the related functional heterogeneity between and within three hematopoietic cell populations. SOMs were created of TOF-SIMS data from individual hematopoietic stem and progenitor cells (HSPCs), lineage-committed common lymphoid progenitors (CLPs), and fully differentiated B cells that had been isolated from murine bone marrow via conventional flow cytometry. The positions of these cells on the SOMs and the spectral variation between adjacent map units, shown on the corresponding unified distance matrix (U-matrix), indicated the CLPs exhibited the highest intrapopulation spectral variation, regardless of the age of the donor mice. SOMs of HSPCs, CLPs, and B cells isolated from young and old mice using the same surface antigen profiles revealed the HSPCs exhibited the most age-related spectral variation, whereas B cells exhibited the least. These results demonstrate that SOMs of single-cell spectra enable characterizing the heterogeneity between and within cell populations that lie along distinct differentiation pathways.

MTOR complexes differentially orchestrates eosinophil development in allergy

Eosinophil infiltration is considered a hallmark in allergic airway inflammation, and the blockade of eosinophil differentiation may be an effective approach for treating eosinophil-related disorders. Mammalian target of rapamycin (mTOR) is a vital modulator in cell growth control and related diseases, and we have recently demonstrated that rapamycin can suppress eosinophil differentiation in allergic airway inflammation. Considering its critical role in haematopoiesis, we further investigated the role of mTOR in eosinophil differentiation in the context of asthmatic pathogenesis. Intriguingly, the inhibition of mTOR, either by genetic deletion or by another pharmacological inhibitor torin-1, accelerated the eosinophil development in the presence of IL-5. However, this was not observed to have any considerable effect on eosinophil apoptosis. The effect of mTOR in eosinophil differentiation was mediated by Erk signalling. Moreover, myeloid specific knockout of mTOR or Rheb further augmented allergic airway inflammation in mice after allergen exposure. Ablation of mTOR in myeloid cells also resulted in an increased number of eosinophil lineage-committed progenitors (Eops) in allergic mice. Collectively, our data uncovered the differential effects of mTOR in the regulation of eosinophil development, likely due to the distinct functions of mTOR complex 1 or 2, which thus exerts a pivotal implication in eosinophil-associated diseases.

Attenuated DNA damage responses and increased apoptosis characterize human hematopoietic stem cells exposed to irradiation

Failure to precisely repair DNA damage in self-renewing Hematopoietic Stem and early Progenitor Cells (HSPCs) can disrupt normal hematopoiesis and promote leukemogenesis. Although HSPCs are widely considered a target of ionizing radiation (IR)-induced hematopoietic injury, definitive data regarding cell death, DNA repair, and genomic stability in these rare quiescent cells are scarce. We found that irradiated HSPCs, but not lineage-committed progenitors (CPs), undergo rapid ATM-dependent apoptosis, which is suppressed upon interaction with bone-marrow stroma cells. Using DNA repair reporters to quantify mutagenic Non-Homologous End Joining (NHEJ) processes, we found that HSPCs exhibit reduced NHEJ activities in comparison with CPs. HSPC-stroma interactions did not affect the NHEJ capacity of HSPCs, emphasizing its cell autonomous regulation. We noted diminished expression of multiple double strand break (DSB) repair transcripts along with more persistent 53BP1 foci in irradiated HSPCs in comparison with CPs, which can account for low NHEJ activity and its distinct control in HSPCs. Finally, we documented clonal chromosomal aberrations in 10% of IR-surviving HSPCs. Taken together, our results revealed potential mechanisms contributing to the inherent susceptibility of human HSPC to the cytotoxic and mutagenic effects of DNA damage.

Direct pharmacologic regulation of the ETS transcription factor PU.1

The ETS‐family transcription factor PU.1 is essential for the self‐renewal of hematopoietic stem cells (HSC) as well as their differentiation to various lineage‐committed hematopoietic progenitors. Specific to the bone marrow niche, PU.1 regulates the expression of several key cytokines (e.g. CXCR4) and their receptors (e.g. GM‐CSF receptor, FLT3) involved in HSC mobilization. Extensive studies have shown that incomplete loss of PU.1 activity, to ~20% wildtype levels, leads to lineage arrest and aggressive leukemic disease. Conversely, lentiviral overexpression of PU.1 resolves the differentiation block in leukemic blasts, and suggests the activation of PU.1 as a potential therapeutic approach. To this end, we have identified by phage display technology a panel of selective PU.1‐binding oligopeptides. Several of these peptides promote PU.1 binding to cognate DNA sites in a dose‐dependent manner and confer resistance to PU.1 against inhibition by netropsin. To determine the essential residues responsible for their activity, we screened a targeted library of truncated hit peptides and found that as few as four residues were sufficient to enhance site‐specific DNA binding by PU.1. These peptide sequences represent leads for developing direct‐acting PU.1‐specific modulators with respect to target DNA binding and interactions with major binding partners. The resulting low‐MW reagents could significantly expand the pharmacology of transcription factors and help overcome barriers in hematopoietic research and therapy that are currently imposed by genetic approaches.Support or Funding InformationSupported by NIH HL129063.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

EFFECTS OF N-ACETYL-CYSTEINE SUPPLEMENTATION ON EX-VIVO CLONOGENICITY AND OXIDATIVE PROFILE OF LINEAGE-COMMITTED HEMATOPOIETIC STEM/PROGENITOR CELLS

Hematopoietic stem and progenitor cells (HSPCs) are exposed to oxidative damage acquired during ex vivo expansion which affects their therapeutic potency. Efforts to overcome this limitation includes the use of antioxidants. The effects of N-Acetyl-Cysteine (NAC) supplementation for 48 hours on maintenance of ex vivo HSPCs was investigated by examining the cell viability at concentrations of 0.125 µM, 0.25 µM, 0.5 µM, 1.0 µM and 2.0 µM, followed by clonogenicity and oxidative status assessments of lineage-committed progenitors (myeloid, erythroid and pre-B lymphoid) at selected NAC concentrations (0.25 µM, 0.5 µM, 2.0 µM). NAC supplementation significantly (p< 0.05) enhanced viability of HSPC at 0.25 µM, 0.5 µM, 2.0 µM. The clonogenicity of each progenitor was not affected as no significant changes of Colony Forming Units (CFUs) counts was noted between NAC-supplemented group than control. NAC showed no significant effects on reactive oxygen species (ROS), glutathione (GSH) and superoxide dismutase (SOD) levels of respective progenitors as compared to control. Conclusively, NAC shows potential property as antioxidant supplement for ex vivo maintenance of HSPCs by promoting survivability and maintaining clonogenicity.

Elevated Eosinophil Lineage-Committed Progenitors In Skin Lesions Of Patients With Moderate-To-Severe Atopic Dermatitis

Elevated Eosinophil Lineage-Committed Progenitors In Skin Lesions Of Patients With Moderate-To-Severe Atopic Dermatitis

Rebuilding a planarian: from early signaling to final shape.

Why some animals can regenerate and others not has fascinated biologists since the first examples of regeneration were reported. Although many animal phyla include species with some regenerative ability, mainly restricted to particular cell types or tissues, there are some other species capable of regenerating complex structures, such as the vertebrate limb and heart. More remarkably, there are some examples of animals that can regenerate the whole body from a tiny piece of them. Understanding how regeneration is triggered and achieved in these animals is fundamental not only to understand this fascinating primary biological question, but also because of its implications for the field of regenerative medicine. Here, we discuss one of the models with higher regenerative capabilities: the freshwater planarians. Two key features make planarians an attractive model to study regeneration: the presence of adult pluripotent stem cells and the permanent activation of the morphogenetic mechanisms that instruct cell fate. Here, we revise our current knowledge of key events that lead to successful regeneration including: how heterogeneous is the stem cell population; what are the immediate changes at the gene level after amputation and what triggers the regenerative response; how is axial polarity re-established; how do the different cell types differentiate from lineage-committed progenitors and how is size and organ proportionality controlled. Finally, we point out some open questions that the field needs to address in the near future.

Engraftment of Reprogramed Human Bile Duct Cells Transiently Rescues Diabetes in Mice

Differentiation of adult stem/progenitor cells into functional beta cells to provide a successful autologous cell therapy for Type 1 diabetes patients has not yet been achieved. Progenitor cells in the adult pancreas or liver have been considered potential sources of endocrine beta cells based on their ability to repopulate the organ following injury. Here we describe the isolation and reprogramming of a lineage-committed progenitor population of cells in the human bile duct towards a beta cell fate. These cells, which possess SOX9 as a marker, were able to manifest beta cell characteristics upon ectopic expression of pancreatic transcription factors. The beta cells derived from SOX9+ progenitor cells were also able to ameliorate high blood glucose in diabetic mice. The insulin+ islet-like clusters which developed from this progenitor cell type demonstrate the potential of this approach to generate functional, autologous beta cells.

Identification of MS4A3 as a reliable marker for early myeloid differentiation in human hematopoiesis

Information of myeloid lineage-related antigen on hematopoietic stem/progenitor cells (HSPCs) is important to clarify the mechanisms regulating hematopoiesis, as well as for the diagnosis and treatment of myeloid malignancies. We previously reported that special AT-rich sequence binding protein 1 (SATB1), a global chromatin organizer, promotes lymphoid differentiation from HSPCs. To search a novel cell surface molecule discriminating early myeloid and lymphoid differentiation, we performed microarray analyses comparing SATB1-overexpressed HSPCs with mock-transduced HSPCs. The results drew our attention to membrane-spanning 4-domains, subfamily A, member 3 (Ms4a3) as the most downregulated molecule in HSPCs with forced overexpression of SATB1. Ms4a3 expression was undetectable in hematopoietic stem cells, but showed a concomitant increase with progressive myeloid differentiation, whereas not only lymphoid but also megakaryocytic-erythrocytic progenitors were entirely devoid of Ms4a3 expression. Further analysis revealed that a subset of CD34+CD38+CD33+ progenitor population in human adult bone marrow expressed MS4A3, and those MS4A3+ progenitors only produced granulocyte/macrophage colonies, losing erythroid colony- and mixed colony-forming capacity. These results suggest that cell surface expression of MS4A3 is useful to distinguish granulocyte/macrophage lineage-committed progenitors from other lineage-related ones in early human hematopoiesis. In conclusion, MS4A3 is useful to monitor early stage of myeloid differentiation in human hematopoiesis.

Distinguishing three subtypes of hematopoietic cells based on gene expression profiles using a support vector machine

Hematopoiesis is a complicated process involving a series of biological sub-processes that lead to the formation of various blood components. A widely accepted model of early hematopoiesis proceeds from long-term hematopoietic stem cells (LT-HSCs) to multipotent progenitors (MPPs) and then to lineage-committed progenitors. However, the molecular mechanisms of early hematopoiesis have not been fully characterized. In this study, we applied a computational strategy to identify the gene expression signatures distinguishing three types of closely related hematopoietic cells collected in recent studies: (1) hematopoietic stem cell/multipotent progenitor cells; (2) LT-HSCs; and (3) hematopoietic progenitor cells. Each cell in these cell types was represented by its gene expression profile among a total number of 20,475 genes. The expression features were analyzed by a Monte-Carlo Feature Selection (MCFS) method, resulting in a feature list. Then, the incremental feature selection (IFS) and a support vector machine (SVM) optimized with a sequential minimum optimization (SMO) algorithm were employed to access the optimal classifier with the highest Matthews correlation coefficient (MCC) value of 0.889, in which 6698 features were used to represent cells. In addition, through an updated program of MCFS method, seventeen decision rules can be obtained, which can classify the three cell types with an overall accuracy of 0.812. Using a literature review, both the rules and the top features used for building the optimal classifier were confirmed to be commonly used or potential biological markers for distinguishing the three cell types of HSPCs. This article is part of a Special Issue entitled: Accelerating Precision Medicine through Genetic and Genomic Big Data Analysis edited by Yudong Cai & Tao Huang.

In Situ Capture of the Molecular Composition of Erythroid Transcriptional Enhancers

Elucidating mechanisms that regulate gene transcription during erythropoiesis is critical for identifying fundamental principles of cellular differentiation, and for developing new therapies for blood disorders. Transcriptional enhancers determine cell identity by directing spatiotemporal gene expression, yet the molecular processes controlling enhancer activation and deactivation during lineage differentiation remain largely unknown. Recently, we and others have identified human erythroid cell-specific enhancers by mapping histone modifications, transcription factor binding and transcriptomic changes in primary fetal and adult hematopoietic stem/progenitor cells and lineage-committed erythroid progenitors. While these studies establish a comprehensive catalog of erythroid transcriptional enhancers, the molecular composition and in vivo function of the vast majority of these enhancers remain unknown. This is a major impediment for understanding the coordinated control of gene transcription in normal and diseased erythroid cells.Given that enhancers are frequently targeted by disease-associated genetic variations, there is a critical need to correct this gap in knowledge. Identifying the complete composition of a specific enhancer within its native chromatin can provide unprecedented insight into the molecular mechanisms regulating its activity. To facilitate the molecular characterization of enhancers within their native chromatin environment, we have developed a new method, CAPTURE (CRISPR Affinity Purification in situ of Regulatory Elements), to isolate enhancer-associated molecular interactions by repurposing the CRISPR/Cas9 system. By using the endonuclease-deficient Cas9 (dCas9) and enhancer-targeting guide RNA (sgRNA), the enhancer-associated protein, DNA and RNA complexes are isolated by affinity purification and identified by high-throughput sequencing and proteomics, respectively. Using these methods, we isolated chromatin-regulating protein complexes associated with several critical cis -regulatory elements within the human β-globin cluster such as the locus control region (LCR) in human erythroid cells, and identified many known factors such as GATA1, TAL1, NFE2, LDB1 and the SWI/SNF chromatin remodeling complexes involved in globin gene regulation. More importantly, we also identified several new factors, such as the nuclear pore proteins NUP98 and NUP153, which have not previously been implicated in globin gene regulation. Subsequent studies of these factors established their roles in the developmental regulation of enhancer activities for globin gene transcription. In situ capture of individual constituents of the enhancer cluster controlling human β-globin genes also establishes evidence for composition-based hierarchical organization of enhancer structure. Furthermore, unbiased analysis of cis -element-mediated long-range chromatin interactions at disease-associated non-coding elements and developmentally controlled super-enhancers reveals spatial features causally regulate gene transcription. Therefore, the dCas9-mediated affinity purification has the potential to advance the characterization of chromatin-templated hierarchical events by providing a method to examine the complete molecular composition of transcriptional enhancers and how composition changes in cellular differentiation. Our studies not only help elucidate the underlying principles regulating lineage-defining enhancer elements, but also establish new approaches for in situ molecular dissection of disease-associated cis -regulatory elements in globin gene transcription and erythropoiesis. Disclosures No relevant conflicts of interest to declare.

Dye-Independent Methods Reveal Elevated Mitochondrial Mass in Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) produce most cellular energy through glycolysis rather than through mitochondrial respiration. Consistent with this notion, mitochondrial mass has been reported to be low in HSCs. However, we found that staining with MitoTracker Green, a commonly used dye to measure mitochondrial content, leads to artefactually low fluorescence specifically in HSCs because of dye efflux. Using mtDNA quantification, enumeration of mitochondrial nucleoids, and fluorescence intensity of a genetically encoded mitochondrial reporter, we unequivocally show here that HSCs and multipotential progenitors (MPPs) have higher mitochondrial mass than lineage-committed progenitors and mature cells. Despite similar mitochondrial mass, respiratory capacity of MPPs exceeds that of HSCs. Furthermore, although elevated mitophagy has been invoked to explain low mitochondrial mass in HSCs, we observed that mitochondrial turnover capacity is comparatively low in HSCs. We propose that the role of mitochondria in HSC biology may have to be revisited in light of these findings.

Neuroinflammation and physical exercise as modulators of adult hippocampal neural precursor cell behavior.

The dentate gyrus of the hippocampus is a plastic structure where adult neurogenesis constitutively occurs. Cell components of the neurogenic niche are source of paracrine as well as membrane-bound factors such as Notch, Bone Morphogenetic Proteins, Wnts, Sonic Hedgehog, cytokines, and growth factors that regulate adult hippocampal neurogenesis and cell fate decision. The integration and coordinated action of multiple extrinsic and intrinsic cues drive a continuous decision process: if adult neural stem cells remain quiescent or proliferate, if they take a neuronal or a glial lineage, and if new cells proliferate, undergo apoptotic death, or survive. The proper balance in the molecular milieu of this neurogenic niche leads to the production of neurons in a higher rate as that of astrocytes. But this rate changes in face of microenvironment modifications as those driven by physical exercise or with neuroinflammation. In this work, we first review the cellular and molecular components of the subgranular zone, focusing on the molecules, active signaling pathways and genetic programs that maintain quiescence, induce proliferation, or promote differentiation. We then summarize the evidence regarding the role of neuroinflammation and physical exercise in the modulation of adult hippocampal neurogenesis with emphasis on the activation of progression from adult neural stem cells to lineage-committed progenitors to their progeny mainly in murine models.

PD-1 blockade alters fate commitment of myeloid progenitors during tumor-mediated “emergency” myelopoiesis

Abstract PD-1 is an immune inhibitory receptor for which blocking agents have achieved significant success as anti-cancer therapeutics. The mechanism(s) of the how PD-1 compromises anti-tumor function and how such effect is reversed by PD-1 blockade remain poorly understood. The rapid change in hematopoietic output that occurs in response to immunologic stress is known as “emergency” myelopoiesis. This process is co-opted by tumors to enhance the generation of tumor-promoting myeloid cells, mostly undifferentiated myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs). We analyzed the myeloid compartment of tumor-bearing mice and determined that myeloid cells express PD-1 and PD-L1. Using PD-1 KO mice or WT mice treated with PD-1 blocking antibody in cancer-driven myelopoiesis experiments we determined a cell-switch fate from MDSCs to effector myeloid cells. PD-1 deletion or blockade, which impaired tumor growth and metastases, stimulated the differentiation of hematopoietic progenitor cells and resulted in increase of Ly6Chimonocytes and monocyte-derived DC (mo-DC). Abrogation of PD-1 signaling enhanced the maturation of MDSCs by inducing the lineage commitment transcription factors IRF8, IRF4 and RORC1. IRF8 and IRF4 promote monocytes/macrophage and hinder granulocyte differentiation, whereas RORC1 is required for expansion of Ly6Chi monocytes and resolution of inflammation. These results reveal a role of the PD-1:PD-L1 pathway in the differentiation of lineage-committed myeloid progenitors to promote undifferentiated MDSCs and suppress effector monocytes/DCs. Switch of myeloid progenitor fate commitment might be a key mechanism by which PD-1 blockade mediates anti-tumor function.

Higher-Level Pathway Objectives of Epigenetic Therapy: A Solution to the p53 Problem in Cancer.

Searches for effective yet nontoxic oncotherapies are searches for exploitable differences between cancer and normal cells. In its core of cell division, cancer resembles normal life, coordinated by the master transcription factor MYC. Outside of this core, apoptosis and differentiation programs, which dominantly antagonize MYC to terminate cell division, necessarily differ between cancer and normal cells, as apoptosis is suppressed by biallelic inactivation of the master regulator of apoptosis, p53, or its cofactor p16/CDKN2A in approximately 80% of cancers. These genetic alterations impact therapy: conventional oncotherapy applies stress upstream of p53 to upregulate it and causes apoptosis (cytotoxicity)-a toxic, futile intent when it is absent or nonfunctional. Differentiation, on the other hand, cannot be completely suppressed because it is a continuum along which all cells exist. Neoplastic evolution stalls advances along this continuum at its most proliferative points-in lineage-committed progenitors that have division times measured in hours compared with weeks for tissue stem cells. This differentiation arrest is by mutations/deletions in differentiation-driving transcription factors or their coactivators that shift balances of gene-regulating protein complexes toward corepressors that repress instead of activate hundreds of terminal differentiation genes. That is, malignant proliferation without differentiation, also referred to as cancer "stem" cell self-renewal, hinges on druggable corepressors. Inhibiting these corepressors (e.g., DNMT1) releases p53-independent terminal differentiation in cancer stem cells but preserves self-renewal of normal stem cells that express stem cell transcription factors. Thus, epigenetic-differentiation therapies exploit a fundamental distinction between cancer and normal stem cell self-renewal and have a pathway of action downstream of genetic defects in cancer, affording favorable therapeutic indices needed for clinical progress.

Human umbilical cord blood-borne fibroblasts contain marrow niche precursors that form a bone/marrow organoid in vivo

Human umbilical cord blood (CB) has attracted much attention as a reservoir for functional hematopoietic stem and progenitor cells, and, recently, as a source of blood-borne fibroblasts (CB-BFs). Previously, we demonstrated that bone marrow stromal cell (BMSC) and CB-BF pellet cultures make cartilage in vitro Furthermore, upon in vivo transplantation, BMSC pellets remodelled into miniature bone/marrow organoids. Using this in vivo model, we asked whether CB-BF populations that express characteristics of the hematopoietic stem cell (HSC) niche contain precursors that reform the niche. CB ossicles were regularly observed upon transplantation. Compared with BM ossicles, CB ossicles showed a predominance of red marrow over yellow marrow, as demonstrated by histomorphological analyses and the number of hematopoietic cells isolated within ossicles. Marrow cavities from CB and BM ossicles included donor-derived CD146-expressing osteoprogenitors and host-derived mature hematopoietic cells, clonogenic lineage-committed progenitors and HSCs. Furthermore, human CD34+ cells transplanted into ossicle-bearing mice engrafted and maintained human HSCs in the niche. Our data indicate that CB-BFs are able to recapitulate the conditions by which the bone marrow microenvironment is formed and establish complete HSC niches, which are functionally supportive of hematopoietic tissue.

Development of platelets during steady state and inflammation.

Megakaryocytes (MK) are the sole source of platelets in the body. They develop from lineage-committed hematopoietic stem and progenitor cells (HSPCs) via intermediate cells, which differ in morphology, size, ploidy, and surface phenotype. Development and maturation of MKs is governed by different transcription factors, including GATA-1, E26 transformation-specific transcription factor (ETS) family members, nuclear factor erythroid 2 transcription factor (NF-E2), and STAT3. During such challenges as acute inflammation, platelets are consumed in high numbers and must be replenished to secure survival of the host. This is achieved by integration of inflammatory signals into early MK development and depends on the STAT1-mediated enhanced translation of transcripts in stem cell-like megakaryocyte progenitors. Here, we review recent developments, which highlight the impact of inflammation on the development of platelets from HSPCs.