Interferon gamma signaling positively regulates hematopoietic stem cell emergence

Functional characterization of irf3 against viral hemorrhagic septicemia virus infection using a CRISPR/Cas9-mediated zebrafish knockout model

Interferon regulatory factors (IRFs) are transcription factors involved in immune responses, such as pathogen response regulation, immune cell growth, and differentiation. IRFs are necessary for the synthesis of type I interferons through a signaling cascade when pathogen recognition receptors identify viral DNA or RNA. We discovered that irf3 is expressed in the early embryonic stages and in all immune organs of adult zebrafish. We demonstrated the antiviral immune mechanism of Irf3 against viral hemorrhagic septicemia virus (VHSV) using CRISPR/Cas9-mediated knockout zebrafish (irf3-KO). In this study, we used a truncated Irf3 protein, encoded by irf3 with a 10 bp deletion, for further investigation. Upon VHSV injection, irf3-KO zebrafish showed dose-dependent high and early mortality compared with zebrafish with the wild-type Irf3 protein (WT), confirming the antiviral activity of Irf3. Based on the results of expression analysis of downstream genes upon VHSV challenge, we inferred that Irf3 deficiency substantially affects the expression of ifnphi1 and ifnphi2. However, after 5 days post infection (dpi), ifnphi3 expression was not significantly altered in irf3-KO compared to that in WT, and irf7 transcription showed a considerable increase in irf3-KO after 5 dpi, indicating irf7's control over ifnphi3 expression. The significantly reduced expression of isg15, viperin, mxa, and mxb at 3 dpi also supported the effect of Irf3 deficiency on the antiviral activity in the early stage of infection. The higher mortality in irf3-KO zebrafish than in WT might be due to an increased inflammation and tissue damage that occurs in irf3-KO because of delayed immune response. Our results suggest that Irf3 plays a role in antiviral immunity of zebrafish by modulating critical immune signaling molecules and regulating antiviral immune genes.

IκBα controls dormancy in hematopoietic stem cells via retinoic acid during embryonic development

Recent findings suggest that Hematopoietic Stem Cells (HSC) and progenitors arise simultaneously and independently of each other already in the embryonic aorta-gonad mesonephros region, but it is still unknown how their different features are established. Here, we uncover IκBα (Nfkbia, the inhibitor of NF-κB) as a critical regulator of HSC proliferation throughout development. IκBα balances retinoic acid signaling levels together with the epigenetic silencer, PRC2, specifically in HSCs. Loss of IκBα decreases proliferation of HSC and induces a dormancy related gene expression signature instead. Also, IκBα deficient HSCs respond with superior activation to in vitro culture and in serial transplantation. At the molecular level, chromatin regions harboring binding motifs for retinoic acid signaling are hypo-methylated for the PRC2 dependent H3K27me3 mark in IκBα deficient HSCs. Overall, we show that the proliferation index in the developing HSCs is regulated by a IκBα-PRC2 axis, which controls retinoic acid signaling.

Single-cell omics identifies inflammatory signaling as a trans-differentiation trigger in mouse embryos

Trans-differentiation represents a direct lineage conversion; however, insufficient characterization of this process hinders its potential applications. Here, to explore a potential universal principal for trans-differentiation, we performed single-cell transcriptomic analysis of endothelial-to-hematopoietic transition (EHT), endothelial-to-mesenchymal transition, and epithelial-to-mesenchymal transition in mouse embryos. We applied three scoring indexes of entropies, cell-type signature transcription factor expression, and critical transition signals to show common features underpinning the fate plasticity of transition states. Cross-model comparison identified inflammatory-featured transition states and a common trigger role of interleukin-33 in promoting fate conversions. Multimodal profiling (integrative transcriptomic and chromatin accessibility analysis) demonstrated the inflammatory regulation of hematopoietic specification. Furthermore, multimodal omics and fate-mapping analyses showed that endothelium-specific Spi1, as an inflammatory effector, governs appropriate chromatin accessibility and transcriptional programs to safeguard EHT. Overall, our study employs single-cell omics to identify critical transition states/signals and the common trigger role of inflammatory signaling in developmental-stress-induced fate conversions.

A multistep computational approach reveals a neuro-mesenchymal cell population in the embryonic hematopoietic stem cell niche

The first hematopoietic stem and progenitor cells (HSPCs) emerge in the Aorta-Gonad-Mesonephros (AGM) region of the mid-gestation mouse embryo. However, the precise nature of their supportive mesenchymal microenvironment remains largely unexplored. Here, we profiled transcriptomes of laser micro-dissected aortic tissues at three developmental stages and individual AGM cells. Computational analyses allowed the identification of several cell subpopulations within the E11.5 AGM mesenchyme, with the presence of a yet unidentified subpopulation characterized by the dual expression of genes implicated in adhesive or neuronal functions. We confirmed the identity of this cell subset as a neuro-mesenchymal population, through morphological and lineage tracing assays. Loss of function in the zebrafish confirmed that Decorin, a characteristic extracellular matrix component of the neuro-mesenchyme, is essential for HSPC development. We further demonstrated that this cell population is not merely derived from the neural crest, and hence, is a bona fide novel subpopulation of the AGM mesenchyme.

Towards an integrated understanding of inflammatory pathway influence on hematopoietic stem and progenitor cell differentiation

Recent research highlights that inflammatory signaling pathways such as pattern recognition receptor (PRR) signaling and inflammatory cytokine signaling play an important role in both on-demand hematopoiesis as well as steady-state hematopoiesis. Knockout studies have demonstrated the necessity of several distinct pathways in these processes, but often lack information about the contribution of specific cell types to the phenotypes in question. Transplantation studies have increased the resolution to the level of specific cell types by testing the necessity of inflammatory pathways specifically in donor hematopoietic stem and progenitor cells (HSPCs) or in recipient niche cells. Here, we argue that for an integrated understanding of how these processes occur in vivo and to inform the development of therapies that modulate hematopoietic responses, we need studies that knockout inflammatory signaling receptors in a cell-specific manner and compare the phenotypes caused by knockout in individual niche cells versus HSPCs.

Nucleic acid-induced inflammation on hematopoietic stem cells

Hematopoiesis, the process of generating blood cells, starts during development with the primitive, pro-definitive, and definitive hematopoietic waves. The first two waves will generate erythrocytes and myeloid cells, although the definitive wave will give rise to hematopoietic stem cells (HSCs) that are multipotent and can produce most of the blood cells in an adult. Although HSCs are highly proliferative during development, during adulthood they remain quiescent in the bone marrow. Inflammatory signaling in the form of interferons, interleukins, tumor necrosis factors, and others is well-established to influence both developmental and adult hematopoiesis. Here we discuss the role of specific inflammatory pathways that are induced by sensing nucleic acids. We discuss the role of RNA-sensing members of the Toll-like, Rig-I-like, nucleotide-binding oligomerization domain (NOD)-like, and AIM2-like protein kinase receptors and the DNA-sensing receptors, DEAD-Box helicase 41 (DDX41) and cGAS. The main downstream pathways of these receptors are discussed, as well as their influence on developmental and adult hematopoiesis, including hematopoietic pathologies.

Inflammation in Development and Aging: Insights from the Zebrafish Model

Zebrafish are an emergent animal model to study human diseases due to their significant genetic similarity to humans, swift development, and genetic manipulability. Their utility extends to the exploration of the involvement of inflammation in host defense, immune responses, and tissue regeneration. Additionally, the zebrafish model system facilitates prompt screening of chemical compounds that affect inflammation. This study explored the diverse roles of inflammatory pathways in zebrafish development and aging. Serving as a crucial model, zebrafish provides insights into the intricate interplay of inflammation in both developmental and aging contexts. The evidence presented suggests that the same inflammatory signaling pathways often play instructive or beneficial roles during embryogenesis and are associated with malignancies in adults.

Nod1-dependent NF-kB activation initiates hematopoietic stem cell specification in response to small Rho GTPases

Uncovering the mechanisms regulating hematopoietic specification not only would overcome current limitations related to hematopoietic stem and progenitor cell (HSPC) transplantation, but also advance cellular immunotherapies. However, generating functional human induced pluripotent stem cell (hiPSC)-derived HSPCs and their derivatives has been elusive, necessitating a better understanding of the developmental mechanisms that trigger HSPC specification. Here, we reveal that early activation of the Nod1-Ripk2-NF-kB inflammatory pathway in endothelial cells (ECs) primes them to switch fate towards definitive hemogenic endothelium, a pre-requisite to specify HSPCs. Our genetic and chemical embryonic models show that HSPCs fail to specify in the absence of Nod1 and its downstream kinase Ripk2 due to a failure on hemogenic endothelial (HE) programming, and that small Rho GTPases coordinate the activation of this pathway. Manipulation of NOD1 in a human system of definitive hematopoietic differentiation indicates functional conservation. This work establishes the RAC1-NOD1-RIPK2-NF-kB axis as a critical intrinsic inductor that primes ECs prior to HE fate switch and HSPC specification. Manipulation of this pathway could help derive a competent HE amenable to specify functional patient specific HSPCs and their derivatives for the treatment of blood disorders.

Hematopoietic stem cells through the ages: A lifetime of adaptation to organismal demands

Hematopoietic stem cells (HSCs), which govern the production of all blood lineages, transition through a series of functional states characterized by expansion during fetal development, functional quiescence in adulthood, and decline upon aging. We describe central features of HSC regulation during ontogeny to contextualize how adaptive responses over the life of the organism ultimately form the basis for HSC functional degradation with age. We particularly focus on the role of cell cycle regulation, inflammatory response pathways, epigenetic changes, and metabolic regulation. We then explore how the knowledge of age-related changes in HSC regulation can inform strategies for the rejuvenation of old HSCs.

Single-cell RNA sequencing of human non-hematopoietic bone marrow cells reveals a unique set of inter-species conserved biomarkers for native mesenchymal stromal cells

BACKGROUND

Native bone marrow (BM) mesenchymal stem/stromal cells (BM-MSCs) participate in generating and shaping the skeleton and BM throughout the lifespan. Moreover, BM-MSCs regulate hematopoiesis by contributing to the hematopoietic stem cell niche in providing critical cytokines, chemokines and extracellular matrix components. However, BM-MSCs contain a heterogeneous cell population that remains ill-defined. Although studies on the taxonomy of native BM-MSCs in mice have just started to emerge, the taxonomy of native human BM-MSCs remains unelucidated.

METHODS

By using single-cell RNA sequencing (scRNA-seq), we aimed to define a proper taxonomy for native human BM non-hematopoietic subsets including endothelial cells (ECs) and mural cells (MCs) but with a focal point on MSCs. To this end, transcriptomic scRNA-seq data were generated from 5 distinct BM donors and were analyzed together with other transcriptomic data and with computational biology analyses at different levels to identify, characterize and classify distinct native cell subsets with relevant biomarkers.

RESULTS

We could ascribe novel specific biomarkers to ECs, MCs and MSCs. Unlike ECs and MCs, MSCs exhibited an adipogenic transcriptomic pattern while co-expressing genes related to hematopoiesis support and multilineage commitment potential. Furthermore, by a comparative analysis of scRNA-seq of BM cells from humans and mice, we identified core genes conserved in both species. Notably, we identified MARCKS, CXCL12, PDGFRA, and LEPR together with adipogenic factors as archetypal biomarkers of native MSCs within BM. In addition, our data suggest some complex gene nodes regulating critical biological functions of native BM-MSCs together with a preferential commitment toward an adipocyte lineage.

CONCLUSIONS

Overall, our taxonomy for native BM non-hematopoietic compartment provides an explicit depiction of gene expression in human ECs, MCs and MSCs at single-cell resolution. This analysis helps enhance our understanding of the phenotype and the complexity of biological functions of native human BM-MSCs.

Activation of lineage competence in hemogenic endothelium precedes the formation of hematopoietic stem cell heterogeneity

Hematopoietic stem and progenitor cells (HSPCs) are considered as a heterogeneous population, but precisely when, where and how HSPC heterogeneity arises remain largely unclear. Here, using a combination of single-cell multi-omics, lineage tracing and functional assays, we show that embryonic HSPCs originate from heterogeneous hemogenic endothelial cells (HECs) during zebrafish embryogenesis. Integrated single-cell transcriptome and chromatin accessibility analysis demonstrates transcriptional heterogeneity and regulatory programs that prime lymphoid/myeloid fates at the HEC level. Importantly, spi2+ HECs give rise to lymphoid/myeloid-primed HSPCs (L/M-HSPCs) and display a stress-responsive function under acute inflammation. Moreover, we uncover that Spi2 is required for the formation of L/M-HSPCs through tightly controlling the endothelial-to-hematopoietic transition program. Finally, single-cell transcriptional comparison of zebrafish and human HECs and human induced pluripotent stem cell-based hematopoietic differentiation results support the evolutionary conservation of L/M-HECs and a conserved role of SPI1 (spi2 homolog in mammals) in humans. These results unveil the lineage origin, biological function and molecular determinant of HSPC heterogeneity and lay the foundation for new strategies for induction of transplantable lineage-primed HSPCs in vitro.

The microbiota regulates hematopoietic stem and progenitor cell development by mediating inflammatory signals in the niche

The commensal microbiota regulates the self-renewal and differentiation of hematopoietic stem and progenitor cells (HSPCs) in bone marrow. Whether and how the microbiota influences HSPC development during embryogenesis is unclear. Using gnotobiotic zebrafish, we show that the microbiota is necessary for HSPC development and differentiation. Individual bacterial strains differentially affect HSPC formation, independent of their effects on myeloid cells. Early-life dysbiosis in chd8-/- zebrafish impairs HSPC development. Wild-type microbiota promote HSPC development by controlling basal inflammatory cytokine expression in kidney niche, and chd8-/- commensals elicit elevated inflammatory cytokines that reduce HSPCs and enhance myeloid differentiation. We identify an Aeromonas veronii strain with immuno-modulatory activities that fails to induce HSPC development in wild-type fish but selectively inhibits kidney cytokine expression and rebalances HSPC development in chd8-/- zebrafish. Our studies highlight the important roles of a balanced microbiome during early HSPC development that ensure proper establishment of lineal precursor for adult hematopoietic system.

Nupr1 Negatively Regulates Endothelial to Hematopoietic Transition in the Aorta-Gonad-Mesonephros Region

In the aorta of mid-gestational mouse embryos, a specialized endothelial subpopulation termed hemogenic endothelial cells (HECs) develops into hematopoietic stem and progenitor cells (HSPCs), through a conserved process of endothelial-to-hematopoietic transition (EHT). EHT is tightly controlled by multiple intrinsic and extrinsic mechanisms. Nevertheless, the molecular regulators restraining this process remain poorly understood. Here, it is uncovered that, one of the previously identified HEC signature genes, Nupr1, negatively regulates the EHT process. Nupr1 deletion in endothelial cells results in increased HSPC generation in the aorta-gonad-mesonephros region. Furthermore, single-cell transcriptomics combined with serial functional assays reveals that loss of Nupr1 promotes the EHT process by promoting the specification of hematopoiesis-primed functional HECs and strengthening their subsequent hematopoietic differentiation potential toward HSPCs. This study further finds that the proinflammatory cytokine, tumor necrosis factor α (TNF-α), is significantly upregulated in Nupr1-deficient HECs, and the use of a specific TNF-α neutralizing antibody partially reduces excessive HSPC generation in the explant cultures from Nupr1-deficient embryos. This study identifies a novel negative regulator of EHT and the findings indicate that Nupr1 is a new potential target for future hematopoietic stem cell regeneration research.

Identification of type II interferons and receptors in an osteoglossiform fish, the arapaima Arapaima gigas

In mammals, type II interferon (IFN; i.e. IFN-γ) signalling transduces through its specific receptors IFN-γR1 and IFN-γR2. In an osteoglossiform fish, the arapaima Arapaima gigas, three type II IFNs, IFN-γ-like, IFN-γ and IFN-γrel, and their four possible receptor subunits IFN-γR1-1, IFN-γR1-2, IFN-γR2-1 and IFN-γR2-2 were identified in this study. The three type II IFN genes are composed of four exons and three introns, and they all contain IFN-γ signature motif and signal peptide, with the presence of potential nuclear localization signal (NLS) in IFN-γ-like and IFN-γ. The IFN-γR1-1, IFN-γR1-2, IFN-γR2-1 and IFN-γR2-2 are composed of seven exons and six introns, with predicted IFN-γR1-1 and IFN-γR1-2 proteins containing JAK1 and STAT1 binding sites, and IFN-γR2-1 and IFN-γR2-2 containing JAK2 binding sites. Gene synteny analysis showed that the type II IFN and their receptor loci are duplicated in arapaima. All these genes were expressed constitutively in all organs/tissues examined, and responded to the stimulation of polyI:C. The prokaryotic recombinant IFN-γ-like, IFN-γ and IFN-γrel proteins can significantly induce the upregulation of immune-related genes in trunk kidney leucocytes. The ligand-receptor relationship analyses revealed that recombinant IFN-γ-like, IFN-γ, and IFN-γrel transduce downstream signalling through IFN-γR1-1/IFN-γR2-1, IFN-γR1-2/IFN-γR2-2, and IFN-γR1-1, respectively, in xenogeneic cells with the overexpression of original or chimeric receptors. In addition, tyrosine (Y) 366 and Y377 in the intracellular region may be essential for the function of IFN-γR1-2 and IFN-γR1-1, respectively. The finding of type II IFN system in A. gigas thus provides different knowledge in understanding the diversity and evolution of type II IFN ligand-receptor relationships in vertebrates.

De Novo Generation of Human Hematopoietic Stem Cells from Pluripotent Stem Cells for Cellular Therapy

The ability to manufacture human hematopoietic stem cells (HSCs) in the laboratory holds enormous promise for cellular therapy of human blood diseases. Several differentiation protocols have been developed to facilitate the emergence of HSCs from human pluripotent stem cells (PSCs). Most approaches employ a stepwise addition of cytokines and morphogens to recapitulate the natural developmental process. However, these protocols globally lack clinical relevance and uniformly induce PSCs to produce hematopoietic progenitors with embryonic features and limited engraftment and differentiation capabilities. This review examines how key intrinsic cues and extrinsic environmental inputs have been integrated within human PSC differentiation protocols to enhance the emergence of definitive hematopoiesis and how advances in genomics set the stage for imminent breakthroughs in this field.

Repeat Element Activation-Driven Inflammation: Role of NFκB and Implications in Normal Development and Cancer?

The human genome is composed of unique DNA sequences that encode proteins and unique sequence noncoding RNAs that are essential for normal development and cellular differentiation. The human genome also contains over 50% of genome sequences that are repeat in nature (tandem and interspersed repeats) that are now known to contribute dynamically to genetic diversity in populations, to be transcriptionally active under certain physiological conditions, and to be aberrantly active in disease states including cancer, where consequences are pleiotropic with impact on cancer cell phenotypes and on the tumor immune microenvironment. Repeat element-derived RNAs play unique roles in exogenous and endogenous cell signaling under normal and disease conditions. A key component of repeat element-derived transcript-dependent signaling occurs via triggering of innate immune receptor signaling that then feeds forward to inflammatory responses through interferon and NFκB signaling. It has recently been shown that cancer cells display abnormal transcriptional activity of repeat elements and that this is linked to either aggressive disease and treatment failure or to improved prognosis/treatment response, depending on cell context and the amplitude of the so-called 'viral mimicry' response that is engaged. 'Viral mimicry' refers to a cellular state of active antiviral response triggered by endogenous nucleic acids often derived from aberrantly transcribed endogenous retrotransposons and other repeat elements. In this paper, the literature regarding transcriptional activation of repeat elements and engagement of inflammatory signaling in normal (focusing on hematopoiesis) and cancer is reviewed with an emphasis on the role of innate immune receptor signaling, in particular by dsRNA receptors of the RIG-1 like receptor family and interferons/NFκB. How repeat element-derived RNA reprograms cell identity through RNA-guided chromatin state modulation is also discussed.

DLL4 and VCAM1 enhance the emergence of T cell-competent hematopoietic progenitors from human pluripotent stem cells

T cells show tremendous efficacy as cellular therapeutics. However, obtaining primary T cells from human donors is expensive and variable. Pluripotent stem cells (PSCs) have the potential to provide a renewable source of T cells, but differentiating PSCs into hematopoietic progenitors with T cell potential remains an important challenge. Here, we report an efficient serum- and feeder-free system for differentiating human PSCs into hematopoietic progenitors and T cells. This fully defined approach allowed us to study the impact of individual proteins on blood emergence and differentiation. Providing DLL4 and VCAM1 during the endothelial-to-hematopoietic transition enhanced downstream progenitor T cell output by ~80-fold. These two proteins synergized to activate notch signaling in nascent hematopoietic stem and progenitor cells, and VCAM1 additionally promoted an inflammatory transcriptional program. We also established optimized medium formulations that enabled efficient and chemically defined maturation of functional CD8αβ+, CD4-, CD3+, TCRαβ+ T cells with a diverse TCR repertoire.

The role of inflammation in hematopoiesis and bone marrow failure: What can we learn from mouse models?

Hematopoiesis is a remarkable system that plays an important role in not only immune cell function, but also in nutrient transport, hemostasis and wound healing among other functions. Under inflammatory conditions, steady-state hematopoiesis switches to emergency myelopoiesis to give rise to the effector cell types necessary to fight the acute insult. Sustained or aberrant exposure to inflammatory signals has detrimental effects on the hematopoietic system, leading to increased proliferation, DNA damage, different forms of cell death (i.e., apoptosis, pyroptosis and necroptosis) and bone marrow microenvironment modifications. Together, all these changes can cause premature loss of hematopoiesis function. Especially in individuals with inherited bone marrow failure syndromes or immune-mediated aplastic anemia, chronic inflammatory signals may thus aggravate cytopenias and accelerate disease progression. However, the understanding of the inflammation roles in bone marrow failure remains limited. In this review, we summarize the different mechanisms found in mouse models regarding to inflammatory bone marrow failure and discuss implications for future research and clinical practice.

AATF/Che-1 localizes to paraspeckles and suppresses R-loops accumulation and interferon activation in Multiple Myeloma

Several kinds of stress promote the formation of three-stranded RNA:DNA hybrids called R-loops. Insufficient clearance of these structures promotes genomic instability and DNA damage, which ultimately contribute to the establishment of cancer phenotypes. Paraspeckle assemblies participate in R-loop resolution and preserve genome stability, however, the main determinants of this mechanism are still unknown. This study finds that in Multiple Myeloma (MM), AATF/Che-1 (Che-1), an RNA-binding protein fundamental to transcription regulation, interacts with paraspeckles via the lncRNA NEAT1_2 (NEAT1) and directly localizes on R-loops. We systematically show that depletion of Che-1 produces a marked accumulation of RNA:DNA hybrids. We provide evidence that such failure to resolve R-loops causes sustained activation of a systemic inflammatory response characterized by an interferon (IFN) gene expression signature. Furthermore, elevated levels of R-loops and of mRNA for paraspeckle genes in patient cells are linearly correlated with Multiple Myeloma progression. Moreover, increased interferon gene expression signature in patients is associated with markedly poor prognosis. Taken together, our study indicates that Che-1/NEAT1 cooperation prevents excessive inflammatory signaling in Multiple Myeloma by facilitating the clearance of R-loops. Further studies on different cancer types are needed to test if this mechanism is ubiquitously conserved and fundamental for cell homeostasis.

An interferon-γ/FLT3 axis positively regulates hemopoietic progenitor cell expansion from human pluripotent stem cells

Since it became possible to differentiate human pluripotent stem cells (hPSCs) into hematopoietic cells in vitro, great efforts have been made to obtain highly potent hematopoietic stem/progenitor cells (HSPCs) from hPSCs. Immunophenotypical HSPCs can be obtained from hPSCs, but their repopulating potential in vivo is low. Here, we developed a novel hematopoietic differentiation method for human induced pluripotent stem cells (hiPSCs) to determine why the existing hPSC differentiation systems are inadequate. hiPSC-derived CD45 +CD34 + cells in our system were mostly CD38 - immunophenotypical HSPCs. The vast majority of human CD45 +CD34 + cells in umbilical cord blood, fetal liver, and bone marrow are CD38 + hematopoietic progenitor cells (HPCs); therefore, the poor production of CD38 + HPCs was indicative of a systematic problem. hiPSC-derived CD45 +CD34 + cells did not express FLT3, a receptor tyrosine kinase. Exogenous FLT3 activity significantly enhanced production of CD38 + HPCs from hiPSCs. Thus, poor production of CD38 + HPCs was due to a lack of FLT3 expression. Interferon-γ upregulated expression of FLT3 and increased the number of CD38 + HPCs among hiPSC-derived CD45 +CD34 + cells. These results suggest that poor production of CD38 + HPCs with hPSC differentiation systems is due to a lack of FLT3 expression, and that addition of interferon-γ can solve this problem.

Zebrafish models of inflammation in hematopoietic development and disease

Zebrafish offer an excellent tool for studying the vertebrate hematopoietic system thanks to a highly conserved and rapidly developing hematopoietic program, genetic amenability, optical transparency, and experimental accessibility. Zebrafish studies have contributed to our understanding of hematopoiesis, a complex process regulated by signaling cues, inflammation being crucial among them. Hematopoietic stem cells (HSCs) are multipotent cells producing all the functional blood cells, including immune cells. HSCs respond to inflammation during infection and malignancy by proliferating and producing the blood cells in demand for a specific scenario. We first focus on how inflammation plays a crucial part in steady-state HSC development and describe the critical role of the inflammasome complex in regulating HSC expansion and balanced lineage production. Next, we review zebrafish studies of inflammatory innate immune mechanisms focusing on interferon signaling and the downstream JAK-STAT pathway. We also highlight insights gained from zebrafish models harbouring genetic perturbations in the role of inflammation in hematopoietic disorders such as bone marrow failure, myelodysplastic syndrome, and myeloid leukemia. Indeed, inflammation has been recently identified as a potential driver of clonal hematopoiesis and leukemogenesis, where cells acquire somatic mutations that provide a proliferative advantage in the presence of inflammation. Important insights in this area come from mutant zebrafish studies showing that hematopoietic differentiation can be compromised by epigenetic dysregulation and the aberrant induction of signaling pathways.

Zebrafish: an underutilized tool for discovery in host-microbe interactions

Zebrafish are relatively new to the field of host-pathogen interactions, although they have been a valuable vertebrate model for decades in developmental biology and neuroscience. Transparent zebrafish larvae have most components of the human innate immune system, and adult zebrafish also produce cells of the adaptive immune system. Recent discoveries using zebrafish infection models include mechanisms of pathogen survival and host cell sensing of microbes. These discoveries were enabled by zebrafish technology, which is constantly evolving and providing new opportunities for immunobiology research. Recent tools include CRISPR/Cas9 mutagenesis, in vivo biotinylation, and genetically encoded biosensors. We argue that the zebrafish model - which remains underutilized in immunology - provides fertile ground for a new understanding of host-microbe interactions in a transparent host.

Blocking UBE2N abrogates oncogenic immune signaling in acute myeloid leukemia

Dysregulation of innate immune signaling pathways is implicated in various hematologic malignancies. However, these pathways have not been systematically examined in acute myeloid leukemia (AML). We report that AML hematopoietic stem and progenitor cells (HSPCs) exhibit a high frequency of dysregulated innate immune-related and inflammatory pathways, referred to as oncogenic immune signaling states. Through gene expression analyses and functional studies in human AML cell lines and patient-derived samples, we found that the ubiquitin-conjugating enzyme UBE2N is required for leukemic cell function in vitro and in vivo by maintaining oncogenic immune signaling states. It is known that the enzyme function of UBE2N can be inhibited by interfering with thioester formation between ubiquitin and the active site. We performed in silico structure-based and cellular-based screens and identified two related small-molecule inhibitors UC-764864/65 that targeted UBE2N at its active site. Using these small-molecule inhibitors as chemical probes, we further revealed the therapeutic efficacy of interfering with UBE2N function. This resulted in the blocking of ubiquitination of innate immune- and inflammatory-related substrates in human AML cell lines. Inhibition of UBE2N function disrupted oncogenic immune signaling by promoting cell death of leukemic HSPCs while sparing normal HSPCs in vitro. Moreover, baseline oncogenic immune signaling states in leukemic cells derived from discrete subsets of patients with AML exhibited a selective dependency on UBE2N function in vitro and in vivo. Our study reveals that interfering with UBE2N abrogates leukemic HSPC function and underscores the dependency of AML cells on UBE2N-dependent oncogenic immune signaling states.

Endothelial MEKK3-KLF2/4 signaling integrates inflammatory and hemodynamic signals during definitive hematopoiesis

The hematopoietic stem cells (HSCs) that produce blood for the lifetime of an animal arise from RUNX1+ hemogenic endothelial cells (HECs) in the embryonic vasculature through a process of endothelial-to-hematopoietic transition (EHT). Studies have identified inflammatory mediators and fluid shear forces as critical environmental stimuli for EHT, raising the question of how such diverse inputs are integrated to drive HEC specification. Endothelial cell MEKK3-KLF2/4 signaling can be activated by both fluid shear forces and inflammatory mediators, and plays roles in cardiovascular development and disease that have been linked to both stimuli. Here we demonstrate that MEKK3 and KLF2/4 are required in endothelial cells for the specification of RUNX1+ HECs in both the yolk sac and dorsal aorta of the mouse embryo and for their transition to intra-aortic hematopoietic cluster cells (IAHCs). The inflammatory mediators lipopolysaccharide and interferon gamma increase RUNX1+ HECs in an MEKK3-dependent manner. Maternal administration of catecholamines that stimulate embryo cardiac function and accelerate yolk sac vascular remodeling increases EHT by wild-type but not MEKK3-deficient endothelium. These findings identify MEKK-KLF2/4 signaling as an essential pathway for EHT and provide a molecular basis for the integration of diverse environmental inputs, such as inflammatory mediators and hemodynamic forces, during definitive hematopoiesis.

MyD88-dependent TLR signaling oppositely regulates hematopoietic progenitor and stem cell formation in the embryo

Hemogenic endothelial (HE) cells in the dorsal aorta undergo an endothelial-to-hematopoietic transition (EHT) to form multipotent progenitors, lympho-myeloid biased progenitors (LMPs), pre-hematopoietic stem cells (pre-HSCs) and adult-repopulating HSCs. These briefly accumulate in intra-arterial hematopoietic clusters (IAHCs) before being released into the circulation. It is generally assumed that the number of IAHC cells correlates with the number of HSCs. Here, we show that changes in the number of IAHC cells, LMPs and HSCs can be uncoupled. Mutations impairing MyD88-dependent toll-like receptor (TLR) signaling decreased the number of IAHC cells and LMPs, but increased the number of HSCs in the aorta-gonad-mesonephros region of mouse embryos. TLR4-deficient embryos generated normal numbers of HE cells, but IAHC cell proliferation decreased. Loss of MyD88-dependent TLR signaling in innate immune myeloid cells had no effect on IAHC cell numbers. Instead, TLR4 deletion in endothelial cells (ECs) recapitulated the phenotype observed with germline deletion, demonstrating that MyD88-dependent TLR signaling in ECs and/or in IAHCs regulates the numbers of LMPs and HSCs.

The Yin and Yang of Immunity in Stem Cell Decision Guidance in Tissue Ecologies: An Infection Independent Perspective

The impact of immune system and inflammation on organ homeostasis and tissue stem cell niches in the absence of pathogen invasion has long remained a conundrum in the field of regenerative medicine. The paradoxical role of immune components in promoting tissue injury as well as resolving tissue damage has complicated therapeutic targeting of inflammation as a means to attain tissue homeostasis in degenerative disease contexts. This confound could be resolved by an integrated intricate assessment of cross-talk between inflammatory components and micro- and macro-environmental factors existing in tissues during health and disease. Prudent fate choice decisions of stem cells and their differentiated progeny are key to maintain tissue integrity and function. Stem cells have to exercise this fate choice in consultation with other tissue components. With this respect tissue immune components, danger/damage sensing molecules driving sterile inflammatory signaling cascades and barrier cells having immune-surveillance functions play pivotal roles in supervising stem cell decisions in their niches. Stem cells learn from their previous damage encounters, either endogenous or exogenous, or adapt to persistent micro-environmental changes to orchestrate their decisions. Thus understanding the communication networks between stem cells and immune system components is essential to comprehend stem cell decisions in endogenous tissue niches. Further the systemic interactions between tissue niches integrated through immune networks serve as patrolling systems to establish communication links and orchestrate micro-immune ecologies to better organismal response to injury and promote regeneration. Understanding these communication links is key to devise immune-centric regenerative therapies. Thus the present review is an integrated attempt to provide a unified purview of how inflammation and immune cells provide guidance to stem cells for tissue sculpting during development, organismal aging and tissue crisis based on the current knowledge in the field.

Hemgn Protects Hematopoietic Stem and Progenitor Cells Against Transplantation Stress Through Negatively Regulating IFN-γ Signaling

Hematopoietic stem and progenitor cells (HSPCs) possess the remarkable ability to regenerate the whole blood system in response to ablated stress demands. Delineating the mechanisms that maintain HSPCs during regenerative stresses is increasingly important. Here, it is shown that Hemgn is significantly induced by hematopoietic stresses including irradiation and bone marrow transplantation (BMT). Hemgn deficiency does not disturb steady-state hematopoiesis in young mice. Hemgn^-/- HSPCs display defective engraftment activity during BMT with reduced homing and survival and increased apoptosis. Transcriptome profiling analysis reveals that upregulated genes in transplanted Hemgn^-/- HSPCs are enriched for gene sets related to interferon gamma (IFN-γ) signaling. Hemgn^-/- HSPCs show enhanced responses to IFN-γ treatment and increased aging over time. Blocking IFN-γ signaling in irradiated recipients either pharmacologically or genetically rescues Hemgn^-/- HSPCs engraftment defect. Mechanistical studies reveal that Hemgn deficiency sustain nuclear Stat1 tyrosine phosphorylation via suppressing T-cell protein tyrosine phosphatase TC45 activity. Spermidine, a selective activator of TC45, rescues exacerbated phenotype of HSPCs in IFN-γ-treated Hemgn^-/- mice. Collectively, these results identify that Hemgn is a critical regulator for successful engraftment and reconstitution of HSPCs in mice through negatively regulating IFN-γ signaling. Targeted Hemgn may be used to improve conditioning regimens and engraftment during HSPCs transplantation.

Making Blood from the Vessel: Extrinsic and Environmental Cues Guiding the Endothelial-to-Hematopoietic Transition

It is increasingly recognized that specialized subsets of endothelial cells carry out unique functions in specific organs and regions of the vascular tree. Perhaps the most striking example of this specialization is the ability to contribute to the generation of the blood system, in which a distinct population of “hemogenic” endothelial cells in the embryo transforms irreversibly into hematopoietic stem and progenitor cells that produce circulating erythroid, myeloid and lymphoid cells for the lifetime of an animal. This review will focus on recent advances made in the zebrafish model organism uncovering the extrinsic and environmental factors that facilitate hemogenic commitment and the process of endothelial-to-hematopoietic transition that produces blood stem cells. We highlight in particular biomechanical influences of hemodynamic forces and the extracellular matrix, metabolic and sterile inflammatory cues present during this developmental stage, and outline new avenues opened by transcriptomic-based approaches to decipher cell–cell communication mechanisms as examples of key signals in the embryonic niche that regulate hematopoiesis.

Hapln1b, a central organizer of the extracellular matrix, modulates kit signalling to control developmental haematopoiesis

During early vertebrate development, hematopoietic stem and progenitor cells (HSPCs) are produced from hemogenic endothelium located in the dorsal aorta, before they migrate to a transient niche where they expand, the fetal liver and the caudal hematopoietic tissue (CHT), in mammals and zebrafish, respectively. In zebrafish, previous studies have shown that the extracellular matrix (ECM) around the aorta needs to be degraded to allow HSPCs to leave the aortic floor and reach blood circulation. However, the role of the ECM components in HSPC specification has never been addressed. We show here that hapln1b, a key component of the ECM is specifically expressed in hematopoietic sites in the zebrafish embryo. Gain- and loss-of-function experiments all resulted in the absence of HSPCs in the early embryo, showing that hapln1b is required, at the correct level, to specify HSPCs in the hemogenic endothelium. Furthermore, we show that the expression of hapln1b is necessary to maintain the integrity of the ECM through its link domain. By combining functional analyses and computer modelling, we show that kitlgb interacts with the ECM to specify HSPCs. We demonstrate that the ECM is an integral component of the microenvironment and mediates cytokine signalling that is required for HSPC specification.

Isoflucypram cardiovascular toxicity in zebrafish (Danio rerio)

Isoflucypram belongs to the new generation of succinate dehydrogenase inhibitor (SDHI) fungicides that are commonly used in crop fungal disease control. Evidence indicates that isoflucypram poses a potential risk to aquatic organisms. However, the effects of isoflucypram during early embryogenesis are not fully understood. In the present study, zebrafish embryos were exposed to 0.025, 0.25, or 2.5 μM isoflucypram for three days. Isoflucypram caused severe developmental abnormalities (yolk sac edema, pericardial edema, and blood clotting clustering), hatching delay, and decreased heart rates in zebrafish. The expression levels of cardiac-specific genes (nkx2.5, myh7, myl7, and myh6) and erythropoiesis-related genes (gata1a, hbbe1, hbbe2, and alas2) were disrupted after isoflucypram exposure. Furthermore, enrichment analysis indicated that most of the differentially expressed genes (DEGs) were enriched in heart development or hemopoiesis processes. Overall, these findings suggest that exposure to isoflucypram is associated with developmental and cardiovascular toxicity in zebrafish.

Zebrafish larvae as experimental model to expedite the search for new biomarkers and treatments for neonatal sepsis

Neonatal sepsis is a major cause of death and disability in newborns. Commonly used biomarkers for diagnosis and evaluation of treatment response lack sufficient sensitivity or specificity. Additionally, new targets to treat the dysregulated immune response are needed, as are methods to effectively screen drugs for these targets. Available research methods have hitherto not yielded the breakthroughs required to significantly improve disease outcomes, we therefore describe the potential of zebrafish (Danio rerio) larvae as preclinical model for neonatal sepsis. In biomedical research, zebrafish larvae combine the complexity of a whole organism with the convenience and high-throughput potential of in vitro methods. This paper illustrates that zebrafish exhibit an immune system that is remarkably similar to humans, both in terms of types of immune cells and signaling pathways. Moreover, the developmental state of the larval immune system is highly similar to human neonates. We provide examples of zebrafish larvae being used to study infections with pathogens commonly causing neonatal sepsis and discuss known limitations. We believe this species could expedite research into immune regulation during neonatal sepsis and may hold keys for the discovery of new biomarkers and novel treatment targets as well as for screening of targeted drug therapies.

The Fetal-to-Adult Hematopoietic Stem Cell Transition and its Role in Childhood Hematopoietic Malignancies

As with most organ systems that undergo continuous generation and maturation during the transition from fetal to adult life, the hematopoietic and immune systems also experience dynamic changes. Such changes lead to many unique features in blood cell function and immune responses in early childhood. The blood cells and immune cells in neonates are a mixture of fetal and adult origin due to the co-existence of both fetal and adult types of hematopoietic stem cells (HSCs) and progenitor cells (HPCs). Fetal blood and immune cells gradually diminish during maturation of the infant and are almost completely replaced by adult types of cells by 3 to 4 weeks after birth in mice. Such features in early childhood are associated with unique features of hematopoietic and immune diseases, such as leukemia, at these developmental stages. Therefore, understanding the cellular and molecular mechanisms by which hematopoietic and immune changes occur throughout ontogeny will provide useful information for the study and treatment of pediatric blood and immune diseases. In this review, we summarize the most recent studies on hematopoietic initiation during early embryonic development, the expansion of both fetal and adult types of HSCs and HPCs in the fetal liver and fetal bone marrow stages, and the shift from fetal to adult hematopoiesis/immunity during neonatal/infant development. We also discuss the contributions of fetal types of HSCs/HPCs to childhood leukemias.

De novo generation of macrophage from placenta-derived hemogenic endothelium

Macrophages play pivotal roles in immunity, hematopoiesis, and tissue homeostasis. In mammals, macrophages have been shown to originate from yolk-sac-derived erythro-myeloid progenitors and aorta-gonad-mesonephros (AGM)-derived hematopoietic stem cells. However, whether macrophages can arise from other embryonic sites remains unclear. Here, using single-cell RNA sequencing, we profile the transcriptional landscape of mouse fetal placental hematopoiesis. We uncover and experimentally validate that a CD44⁺ subpopulation of placental endothelial cells (ECs) exhibits hemogenic potential. Importantly, lineage tracing using the newly generated Hoxa13 reporter line shows that Hoxa13-labeled ECs can produce placental macrophages, named Hofbauer cell (HBC)-like cells. Furthermore, we identify two subtypes of HBC-like cells, and cell-cell interaction analysis identifies their potential roles in angiogenesis and antigen presentation, separately. Our study provides a comprehensive understanding of placental hematopoiesis and highlights the placenta as a source of macrophages, which has important implications for both basic and translational research.

A connexin/ifi30 pathway bridges HSCs with their niche to dampen oxidative stress

Reactive oxygen species (ROS) represent a by-product of metabolism and their excess is toxic for hematopoietic stem and progenitor cells (HSPCs). During embryogenesis, a small number of HSPCs are produced from the hemogenic endothelium, before they colonize a transient organ where they expand, for example the fetal liver in mammals. In this study, we use zebrafish to understand the molecular mechanisms that are important in the caudal hematopoietic tissue (equivalent to the mammalian fetal liver) to promote HSPC expansion. High levels of ROS are deleterious for HSPCs in this niche, however this is rescued by addition of antioxidants. We show that Cx41.8 is important to lower ROS levels in HSPCs. We also demonstrate a new role for ifi30, known to be involved in the immune response. In the hematopoietic niche, Ifi30 can recycle oxidized glutathione to allow HSPCs to dampen their levels of ROS, a role that could be conserved in human fetal liver.

Reactive oxygen species (ROS) are metabolic by-products which in excess can be toxic for hematopoietic stem and progenitor cells (HSPCs). Here the authors show that toxic ROS are transferred by expanding HSPCs to the zebrafish developmental niche via connexin Cx41.8, where Ifi30 promotes their detoxification.

Inflammatory signaling regulates hematopoietic stem and progenitor cell development and homeostasis

Inflammation exerts multiple effects on the early hematopoietic compartment. Best studied is the role of proinflammatory cytokines in activating adult hematopoietic stem and progenitor cells to dynamically replenish myeloid lineage cells in a process known as emergency myelopoiesis. However, it is increasingly appreciated that the same proinflammatory signaling pathways are used in diverse hematopoietic scenarios. This review focuses on inflammatory signaling in the emergence of the definitive hematopoietic compartment during embryonic life, and tonic inflammatory signals derived from commensal microbiota in shaping the adult hematopoietic compartment in the absence of pathogenic insults. Insights into the unique and shared aspects of inflammatory signaling that regulate hematopoietic stem and progenitor cell function across the lifespan and health span of an individual will enable better diagnostic and therapeutic approaches to hematopoietic dysregulation and malignancies.

Inflammation, Aging and Hematopoiesis: A Complex Relationship

All vertebrate blood cells descend from multipotent hematopoietic stem cells (HSCs), whose activity and differentiation depend on a complex and incompletely understood relationship with inflammatory signals. Although homeostatic levels of inflammatory signaling play an intricate role in HSC maintenance, activation, proliferation, and differentiation, acute or chronic exposure to inflammation can have deleterious effects on HSC function and self-renewal capacity, and bias their differentiation program. Increased levels of inflammatory signaling are observed during aging, affecting HSCs either directly or indirectly via the bone marrow niche and contributing to their loss of self-renewal capacity, diminished overall functionality, and myeloid differentiation skewing. These changes can have significant pathological consequences. Here, we provide an overview of the current literature on the complex interplay between HSCs and inflammatory signaling, and how this relationship contributes to age-related phenotypes. Understanding the mechanisms and outcomes of this interaction during different life stages will have significant implications in the modulation and restoration of the hematopoietic system in human disease, recovery from cancer and chemotherapeutic treatments, stem cell transplantation, and aging.

Current Perspectives on the Role of TNF in Hematopoiesis Using Mice With Humanization of TNF/LT System

TNF is a multifunctional cytokine with its key functions attributed to inflammation, secondary lymphoid tissue organogenesis and immune regulation. However, it is also a physiological regulator of hematopoiesis and is involved in development and homeostatic maintenance of various organs and tissues. Somewhat unexpectedly, the most important practical application of TNF biology in medicine is anti-TNF therapy in several autoimmune diseases. With increased number of patients undergoing treatment with TNF inhibitors and concerns regarding possible adverse effects of systemic cytokine blockade, the interest in using humanized mouse models to study the efficacy and safety of TNF-targeting biologics in vivo is justified. This Perspective discusses the main functions of TNF and its two receptors, TNFR1 and TNFR2, in steady state, as well as in emergency hematopoiesis. It also provides a comparative overview of existing mouse lines with humanization of TNF/TNFR system. These genetically engineered mice allow us to study TNF signaling cascades in the hematopoietic compartment in the context of various experimental disease models and for evaluating the effects of various human TNF inhibitors on hematopoiesis and other physiological processes.

DNA methylation and histone modifications are essential for regulation of stem cell formation and differentiation in zebrafish development

The complex processes necessary for embryogenesis require a gene regulatory network that is complex and systematic. Gene expression regulates development and organogenesis, but this process is altered and fine-tuned by epigenetic regulators that facilitate changes in the chromatin landscape. Epigenetic regulation of embryogenesis adjusts the chromatin structure by modifying both DNA through methylation and nucleosomes through posttranslational modifications of histone tails. The zebrafish is a well-characterized model organism that is a quintessential tool for studying developmental biology. With external fertilization, low cost and high fecundity, the zebrafish are an efficient tool for studying early developmental stages. Genetic manipulation can be performed in vivo resulting in quick identification of gene function. Large-scale genome analyses including RNA sequencing, chromatin immunoprecipitation and chromatin structure all are feasible in the zebrafish. In this review, we highlight the key events in zebrafish development where epigenetic regulation plays a critical role from the early stem cell stages through differentiation and organogenesis.

Neurogenic Heterotopic Ossifications Recapitulate Hematopoietic Stem Cell Niche Development Within an Adult Osteogenic Muscle Environment

Hematopoiesis and bone interact in various developmental and pathological processes. Neurogenic heterotopic ossifications (NHO) are the formation of ectopic hematopoietic bones in peri-articular muscles that develop following severe lesions of the central nervous system such as traumatic cerebral or spinal injuries or strokes. This review will focus on the hematopoietic facet of NHO. The characterization of NHO demonstrates the presence of hematopoietic marrow in which quiescent hematopoietic stem cells (HSC) are maintained by a functional stromal microenvironment, thus documenting that NHOs are neo-formed ectopic HSC niches. Similarly to adult bone marrow, the NHO permissive environment supports HSC maintenance, proliferation and differentiation through bidirectional signaling with mesenchymal stromal cells and endothelial cells, involving cell adhesion molecules, membrane-bound growth factors, hormones, and secreted matrix proteins. The participation of the nervous system, macrophages and inflammatory cytokines including oncostatin M and transforming growth factor (TGF)-β in this process, reveals how neural circuitry fine-tunes the inflammatory response to generate hematopoietic bones in injured muscles. The localization of NHOs in the peri-articular muscle environment also suggests a role of muscle mesenchymal cells and bone metabolism in development of hematopoiesis in adults. Little is known about the establishment of bone marrow niches and the regulation of HSC cycling during fetal development. Similarities between NHO and development of fetal bones make NHOs an interesting model to study the establishment of bone marrow hematopoiesis during development. Conversely, identification of stage-specific factors that specify HSC developmental state during fetal bone development will give more mechanistic insights into NHO.

Sugar Rush: Supercharging Blood Cell Specification via the Inflammasome

The generation of adult hematopoietic stem and progenitor cells (HSPCs) from embryonic and induced pluripotent stem cells (iPSCs) will provide therapeutic benefits but is currently elusive. In this issue of Developmental Cell, Frame et al. reveal that the inflammasome has a key role in HSPCs specification during endothelial-to-hematopoietic transition (EHT).

Excessive R-loops trigger an inflammatory cascade leading to increased HSPC production

Hematopoietic stem and progenitor cells (HSPCs) arise during embryonic development and are essential for sustaining the blood and immune systems throughout life. Tight regulation of HSPC numbers is critical for hematopoietic homeostasis. Here, we identified DEAD-box helicase 41 (Ddx41) as a gatekeeper of HSPC production. Using zebrafish ddx41 mutants, we unveiled a critical role for this helicase in regulating HSPC production at the endothelial-to-hematopoietic transition. We determined that Ddx41 suppresses the accumulation of R-loops, nucleic acid structures consisting of RNA:DNA hybrids and ssDNAs whose equilibrium is essential for cellular fitness. Excess R-loop levels in ddx41 mutants triggered the cGAS-STING inflammatory pathway leading to increased numbers of hemogenic endothelium and HSPCs. Elevated R-loop accumulation and inflammatory signaling were observed in human cells with decreased DDX41, suggesting possible conservation of mechanism. These findings delineate that precise regulation of R-loop levels during development is critical for limiting cGAS-STING activity and HSPC numbers.

Role for Leptin and Leptin Receptors in Stem Cells During Health and Diseases

Hematopoietic stem cells (HSCs) give rise to all blood and immune cells in the body. These rare cells reside in the hypoxic niche of the bone marrow (BM) where they are subjected to a complex network of regulatory factors including cellular and molecular components. To sustain hematopoiesis over the lifetime of an individual, HSCs maintain distinctive metabolic programs, and in recent years nutritional factors have been increasingly recognized as critical regulators of HSC numbers and functions. Leptin (LEP), a neuroendocrine messenger, and its receptor (LEPR) are well-known for their immunomodulatory and energy balancing effects; yet, how LEP/LEPR signaling plays a role in hematopoiesis is under-appreciated. In this review, we summarize and highlight recent work that demonstrated involvement of LEP/LEPR in hematopoiesis under steady state or stress-associated situations as well as in pathological conditions such as cardiovascular diseases and malignancies. Although the field is only in its infancy, these studies suggest evidence of potential clinical applications and proof-of-principle for more in-depth future research.

Yin and Yang: The dual effects of interferons on hematopoiesis

Interferons are an ancient and well-conserved group of inflammatory cytokines most famous for their role in viral immunity. A decade ago, we discovered that interferons also play an important role in the biology of hematopoietic stem cells (HSCs), which are responsible for lifelong blood production. Though we have learned a great deal about the role of interferons on HSC quiescence, differentiation, and self-renewal, there remains some controversy regarding how interferons impact these stem cells, with differing conclusions depending on experimental models and clinical context. Here, we review the contradictory roles of Type 1 and 2 interferons in hematopoiesis. Specifically, we highlight the roles of interferons in embryonic and adult hematopoiesis, along with short-term and long-term adaptive and maladaptive responses to inflammation. We discuss experimental challenges in the study of these powerful yet short-lived cytokines and strategies to address those challenges. We further review the contribution by interferons to disease states including bone marrow failure and aplastic anemia as well as their therapeutic use to treat myeloproliferative neoplasms and viral infections, including SARS-CoV2. Understanding the opposing effects of interferons on hematopoiesis will elucidate immune responses and bone marrow failure syndromes, and future therapeutic approaches for patients undergoing HSC transplantation or fighting infectious diseases and cancer.

Aging and immunotherapies: New horizons for the golden ages

The life expectancy of the world’s elderly population (65 and older) continues to reach new milestones with older individuals currently comprising greater than 8.5% (617 million) of the world’s population. This percentage is predicted to approach 20% of the world’s population by 2050 (representing 1.6 billion people). Despite this amazing feat, many healthcare systems are not equipped to handle the multitude of diseases that commonly manifest with age, including most types of cancers. As the world’s aging population grows, cancer treatments continue to evolve. Immunotherapies are a new drug class that has revolutionized our ability to treat previously intractable cancers; however, their efficacy in patients with compromised immune systems remains unclear. In this review, we will discuss how aging-associated losses in immune homeostasis impact the efficacy and safety of immunotherapy treatment in preclinical models of aging. We will also discuss how these findings translate to elderly patients receiving immunotherapy treatment for refractory and relapsed cancers, as well as, strategies that could be explored to improve the efficacy of immunotherapies in aged patients.

Early-Life Stress Regulates Cardiac Development through an IL-4-Glucocorticoid Signaling Balance

Stressful experiences early in life can increase the risk of cardiovascular diseases. However, it remains largely unknown how stress influences susceptibility to the disease onset. Here, we show that exposure to brain-processed stress disrupts myocardial growth by reducing cardiomyocyte mitotic activity. Activation of the glucocorticoid receptor (GR), the primary stress response pathway, reduces cardiomyocyte numbers, disrupts trabecular formation, and leads to contractile dysfunction of the developing myocardium. However, a physiological level of GR signaling is required to prevent cardiomyocyte hyperproliferation. Mechanistically, we identify an antagonistic interaction between the GR and the cytokine interleukin-4 (IL-4) as a key player in cardiac development. IL-4 signals transcription of key regulators of cell-cycle progression in cardiomyocytes via signal transducer and activator of transcription 3 (Stat3). GR, on the contrary, inhibits this signaling system. Thus, our findings uncover an interplay between stress and immune signaling pathways critical to orchestrating physiological growth of the heart.

Leptin receptor, a surface marker for a subset of highly engrafting long-term functional hematopoietic stem cells

The hematopoietic system is sustained by a rare population of hematopoietic stem cells (HSCs), which emerge during early embryonic development and then reside in the hypoxic niche of the adult bone marrow microenvironment. Although leptin receptor (Lepr)-expressing stromal cells are well-studied as critical regulators of murine hematopoiesis, the biological implications of Lepr expression on HSCs remain largely unexplored. We hypothesized that Lepr⁺HSCs are functionally different from other HSCs. Using in vitro and in vivo experimental approaches, we demonstrated that Lepr further differentiates SLAM HSCs into two distinct populations; Lepr⁺HSCs engrafted better than Lepr^-HSCs in primary transplant. Compared to Lepr^-LSK cells, Lepr⁺LSK cells were highly enriched for extensively repopulating and self-renewing HSCs. Molecularly, Lepr⁺HSCs were characterized by a pro-inflammatory transcriptomic profile enriched for Type-I Interferon and Interferon-gamma (IFN-γ) response pathways, which are known to be critical for the emergence of HSCs in the embryo. We conclude that although Lepr⁺HSCs represent a minor subset of HSCs, they are highly engrafting cells that possess embryonic-like transcriptomic characteristics, and that Lepr can serve as a reliable marker for functional long-term HSCs, which may have potential clinical applicability.

Regulation of Hemogenic Endothelial Cell Development and Function

Embryonic definitive hematopoiesis generates hematopoietic stem and progenitor cells (HSPCs) essential for establishment and maintenance of the adult blood system. This process requires the specification of a subset of vascular endothelial cells to become blood-forming, or hemogenic, and the subsequent endothelial-to-hematopoietic transition to generate HSPCs therefrom. The mechanisms that regulate these processes are under intensive investigation, as their recapitulation in vitro from human pluripotent stem cells has the potential to generate autologous HSPCs for clinical applications. In this review, we provide an overview of hemogenic endothelial cell development and highlight the molecular events that govern hemogenic specification of vascular endothelial cells and the generation of multilineage HSPCs from hemogenic endothelium. We also discuss the impact of hemogenic endothelial cell development on adult hematopoiesis.

Notch signaling and the emergence of hematopoietic stem cells

The hematopoietic stem cell (HSC) is able to give rise to all blood cell lineages in vertebrates. HSCs are generated in the early embryo after two precedent waves of primitive hematopoiesis. Canonical Notch signaling is at the center of the complex mechanism that controls the development of the definitive HSC. The successful in vitro generation of hematopoietic cells from pluripotent stem cells with the capacity for multilineage hematopoietic reconstitution after transplantation requires the recapitulation of the most important process that takes place in the hemogenic endothelium during definitive hematopoiesis, that is the endothelial-to-hematopoietic transition (EHT). To meet this challenge, it is necessary to thoroughly understand the molecular mechanisms that modulate Notch signaling during the HSC differentiation process considering different temporal and spatial dimensions. In recent years, there have been important advances in this field. Here, we review relevant contributions describing different genes, factors, environmental cues, and signaling cascades that regulate the EHT through Notch interactions at multiple levels. The evolutionary conservation of the hematopoietic program has made possible the use of diverse model systems. We describe the contributions of the zebrafish model and the most relevant ones from transgenic mouse studies and from in vitro differentiated pluripotent cells.