Quiescent haematopoietic stem cells are activated by IFN-gamma in response to chronic infection

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.

C/EBPβ isoforms sequentially regulate regenerating mouse hematopoietic stem/progenitor cells

The transcription factor CCAAT enhancer-binding protein β (C/EBPβ) is required for stress-induced granulopoiesis at the level of hematopoietic stem/progenitor cells (HSPCs); however, its role and mechanisms of action in HSPCs are unknown. In this study, we assessed the regulation and functions of C/EBPβ in HSPCs, especially under stress conditions. After 5-fluorouracil treatment or bone marrow transplantation, Cebpb-/- HSPCs exhibited impaired cell-cycle activation and myeloid differentiation at the early and late phases of regeneration, respectively, whereas at steady state, Cebpb deficiency did not affect HSPCs. C/EBPβ was upregulated in response to hematopoietic stress, especially in CD150high long term-hematopoietic stem cells (LT-HSCs). Intracellular flow cytometric analysis that detected distinct domains of C/EBPβ revealed that, among the 3 isoforms of C/EBPβ, liver-enriched inhibitory protein (LIP) was upregulated in LT-HSCs prior to liver-enriched activating protein (LAP)/LAP* during regeneration. Early upregulation of LIP promoted cell-cycle entry of LT-HSCs by positively regulating Myc and expanded the HSPCs pool. Subsequent myeloid differentiation of amplified HSPCs was mediated by LAP/LAP*, which were upregulated at a later phase of regeneration. Collectively, our findings show that stress-induced sequential upregulation of C/EBPβ isoforms is critical for fine-tuning the proliferation and differentiation of regenerating HSPCs.

Cytokine combinations for human blood stem cell expansion induce cell type- and cytokine-specific signaling dynamics

How hematopoietic stem cells (HSCs) integrate signals from their environment to make fate decisions remains incompletely understood. Current knowledge is based on either averages of heterogeneous populations or snapshot analyses, both missing important information about the dynamics of intracellular signaling activity. By combining fluorescent biosensors with time-lapse imaging and microfluidics, we measured the activity of the extracellular signal-regulated kinase (ERK) pathway over time (i.e. dynamics) in live single human umbilical cord blood HSCs and multipotent progenitor cells (MPPs). In single cells, ERK signaling dynamics were highly heterogeneous and depended on the cytokines, their combinations, and cell types. ERK signaling was activated by SCF and FLT3L in HSCs, but by SCF, IL3 and GCSF in MPPs. Different cytokines and their combinations led to distinct ERK signaling dynamics frequencies, and ERK dynamics in HSCs were more transient than those in MPPs. A combination of 5 cytokines recently shown to maintain HSCs in long-term culture, had a more-than-additive effect in eliciting sustained ERK dynamics in HSCs. ERK signaling dynamics also predicted future cell fates. E.g. CD45RA expression increased more in HSC daughters with intermediate than with transient or sustained ERK signaling. We demonstrate heterogeneous, cytokine- and cell type- specific ERK signaling dynamics, illustrating their relevance in regulating HSPC fates.

Bioprinting of Human Cord Blood-Derived CD34+ Cells and Exploration of the Multilineage Differentiation Ability in Vitro

The three-dimensional (3D) marrow microenvironment plays an essential role in regulating human cord blood-derived CD34+ cells (hCB-CD34+) migration, proliferation, and differentiation. Extensive in vitro and in vivo studies have aimed to recapitulate the main components of the bone marrow (BM) niche. Nonetheless, the models are limited by a lack of heterogeneity and compound structure. Here, we fabricated coaxial extruded core-shell tubular scaffolds and extrusion-based bioprinted cell-laden mesh scaffolds to mimic the functional niche in vitro. A multicellular mesh scaffold and two different core-shell tubular scaffolds were developed with human bone marrow-derived mesenchymal stromal cells (BMSCs) in comparison with a conventional 2D coculture system. A clear cell-cell connection was established in all three bioprinted constructs. Cell distribution and morphology were observed in different systems with scanning electron microscopy (SEM). Collected hCB-CD34+ cells were characterized by various stem cell-specific and lineage-specific phenotypic parameters. The results showed that compared with hCB-CD34+ cells cocultured with BMSCs in Petri dishes, the self-renewal potential of hCB-CD34+ cells was stronger in the tubular scaffolds after 14 days. Besides, cells in these core-shell constructs tended to obtain stronger differentiation potential of lymphoid and megakaryocytes, while cells encapsulated in mesh scaffolds obtained stronger differentiation tendency into erythroid cells. Consequently, 3D bioprinting technology could partially simulate the niche of human hematopoietic stem cells. The three models have their potential in stemness maintenance and multilineage differentiation. This study can provide initial effective guidance in the directed differentiation research and related screening of drug models for hematological diseases.

Intestinal eosinophils, homeostasis and response to bacterial intrusion

Eosinophils are traditionally considered as end-stage effector cells involved in the pathogenesis of Th2 immune-mediated disorders as well as in the protection against parasite infection. However, this restricted view has recently been challenged by a series of studies revealing the highly plastic nature of these cells and implication in various homeostatic processes. Large numbers of eosinophils reside in the lamina propria of the gastrointestinal tract, at the front line of host defence, where they contribute to maintain the intestinal epithelial barrier function in the face of inflammation-associated epithelial cell damage. Eosinophils confer active protection against bacterial pathogens capable of penetrating the mucosal barrier through the release of cytotoxic compounds and the generation of extracellular DNA traps. Eosinophils also integrate tissue-specific cytokine signals such as IFN-γ, which synergise with bacterial recognition pathways to enforce different context-dependent functional responses, thereby ensuring a rapid adaptation to the ever-changing intestinal environment. The ability of eosinophils to regulate local immune responses and respond to microbial stimuli further supports the pivotal role of these cells in the maintenance of tissue homeostasis at the intestinal interface.

JAK2-V617F and interferon-α induce megakaryocyte-biased stem cells characterized by decreased long-term functionality

We studied a subset of hematopoietic stem cells (HSCs) that are defined by elevated expression of CD41 (CD41hi) and showed bias for differentiation toward megakaryocytes (Mks). Mouse models of myeloproliferative neoplasms (MPNs) expressing JAK2-V617F (VF) displayed increased frequencies and percentages of the CD41hi vs CD41lo HSCs compared with wild-type controls. An increase in CD41hi HSCs that correlated with JAK2-V617F mutant allele burden was also found in bone marrow from patients with MPN. CD41hi HSCs produced a higher number of Mk-colonies of HSCs in single-cell cultures in vitro, but showed reduced long-term reconstitution potential compared with CD41lo HSCs in competitive transplantations in vivo. RNA expression profiling showed an upregulated cell cycle, Myc, and oxidative phosphorylation gene signatures in CD41hi HSCs, whereas CD41lo HSCs showed higher gene expression of interferon and the JAK/STAT and TNFα/NFκB signaling pathways. Higher cell cycle activity and elevated levels of reactive oxygen species were confirmed in CD41hi HSCs by flow cytometry. Expression of Epcr, a marker for quiescent HSCs inversely correlated with expression of CD41 in mice, but did not show such reciprocal expression pattern in patients with MPN. Treatment with interferon-α further increased the frequency and percentage of CD41hi HSCs and reduced the number of JAK2-V617F+ HSCs in mice and patients with MPN. The shift toward the CD41hi subset of HSCs by interferon-α provides a possible mechanism of how interferon-α preferentially targets the JAK2 mutant clone.

Mitochondrial Ca2+ and cell cycle regulation

It has been demonstrated for more than 40 years that intracellular calcium (Ca²⁺) controls a variety of cellular functions, including mitochondrial metabolism and cell proliferation. Cytosolic Ca²⁺ fluctuation during key stages of the cell cycle can lead to mitochondrial Ca²⁺ uptake and subsequent activation of mitochondrial oxidative phosphorylation and a range of signaling. However, the relationship between mitochondrial Ca²⁺ and cell cycle progression has long been neglected because the molecule responsible for Ca²⁺ uptake has been unknown. Recently, the identification of the mitochondrial Ca²⁺ uniporter (MCU) has led to key advances. With improved Ca²⁺ imaging and detection, effects of MCU-mediated mitochondrial Ca²⁺ have been observed at different stages of the cell cycle. Elevated Ca²⁺ signaling boosts ATP and ROS production, remodels cytosolic Ca²⁺ pathways and reprograms cell fate-determining networks. These findings suggest that manipulating mitochondrial Ca²⁺ signaling may serve as a potential strategy in the control of many crucial biological events, such as tumor development and cell division in hematopoietic stem cells (HSCs). In this review, we summarize the current understanding of the role of mitochondrial Ca²⁺ signaling during different stages of the cell cycle and highlight the potential physiological and pathological significance of mitochondrial Ca²⁺ signaling.

Hematopoietic stem and progenitor cells are present in healthy gingiva tissue

Hematopoietic stem cells reside in the bone marrow, where they generate the effector cells that drive immune responses. However, in response to inflammation, some hematopoietic stem and progenitor cells (HSPCs) are recruited to tissue sites and undergo extramedullary hematopoiesis. Contrasting with this paradigm, here we show residence and differentiation of HSPCs in healthy gingiva, a key oral barrier in the absence of overt inflammation. We initially defined a population of gingiva monocytes that could be locally maintained; we subsequently identified not only monocyte progenitors but also diverse HSPCs within the gingiva that could give rise to multiple myeloid lineages. Gingiva HSPCs possessed similar differentiation potentials, reconstitution capabilities, and heterogeneity to bone marrow HSPCs. However, gingival HSPCs responded differently to inflammatory insults, responding to oral but not systemic inflammation. Combined, we highlight a novel pathway of myeloid cell development at a healthy barrier, defining a gingiva-specific HSPC network that supports generation of a proportion of the innate immune cells that police this barrier.

Seven interferon gamma response genes serve as a prognostic risk signature that correlates with immune infiltration in lung adenocarcinoma

Interferon-gamma (IFN-γ) plays a complex role in modulating tumor microenvironment during lung adenocarcinoma (LUAD) development. In order to define the role of IFN-γ response genes in LUAD progression, we characterized the gene expression, mutation profile, protein-protein interaction of 24 IFN-γ response genes, which exhibited significant hazard ratio in overall survival. Two subgroups of LUAD from the TCGA cohort, which showed significant difference in the survival rate, were identified based on the expression of these genes. Furthermore, LASSO penalized cox regression model was used to derive a risk signature comprising seven IFN-γ response genes, including CD74, CSF2RB, PTPN6, MT2A, NMI, LATS2, and PFKP, which can serve as an independent prognostic predictor of LUAD. The risk signature was validated in an independent LUAD cohort. The high risk group is enriched with genes regulating cell cycle and DNA replication, as well as a high level of pro-tumor immune cells. In addition, the risk score is negatively correlated with the expression of immune metagenes, but positively correlated with DNA damage repair genes. Our findings reveal that seven-gene risk signature can be a valuable prognostic predictor for LUAD, and they are crucial participants in tumor microenvironment of LUAD.

TLR7/8 agonist treatment induces an increase in bone marrow resident dendritic cells and hematopoietic progenitor expansion and mobilization

There is accumulating evidence suggesting that toll-like receptor (TLR) signals play an important role in the regulation of hematopoietic stem/progenitor cells (HSPCs). TLR7/8 stimulation induces the myeloid differentiation of normal HSPCs and acute myeloid leukemia cells. However, the in vivo effect of TLR7/8 agonists on hematopoiesis is largely unknown. Here, we show that, similar to TLR4 and TLR2, treatment with the TLR7/8 agonist R848 induces an expansion of phenotypic hematopoietic stem cells (HSCs) with reduced repopulating potential and HSPC mobilization. In contrast to chronic TLR4 stimulation, treatment with R848 for 5 days did not induce a significant increase in myeloid-biased HSCs. Treatment with R848 results in a significant increase in classic dendritic cells (DCs) in the bone marrow, but a decrease in common dendritic cell progenitors and pre-DCs. Phenotypic analysis of DCs revealed that R848 treatment is associated with altered expression of certain chemokines, activation markers, and migratory receptors. Together, these data indicate that systemic administration of a TLR7/8 agonist has unique effects on hematopoiesis, including the expansion of DCs in the bone marrow, that might have clinical relevance to augment responses to certain immunotherapies, such as cancer vaccines and immune checkpoint blockade.

Innate Immune Memory in Hematopoietic Stem/Progenitor Cells: Myeloid-Biased Differentiation and the Role of Interferon

Innate immune memory was first described for monocytes and other myeloid cells. This memory is designated Immune Training, in which the host animals that had experienced pathogen infection earlier acquire improved resistance to a second infection. Innate immune memory is mediated by an epigenetic mechanism traced to transcriptional memory that is conserved throughout evolution and has been selected for the ability to mount an adaptive response to shifting environments. Accumulating evidence shows that not only peripheral myeloid cells but hematopoietic stem/progenitor cells (HSCs/HSPCs) can acquire epigenetic memory upon pathogen exposure. Systemic pathogen infection causes HSCs to exit from quiescence and facilitate myeloid-biased differentiation that leads to efficient host defense. This sequence of events is common in HSC memory generation, which is triggered by different stimuli. Recent studies show that not only pathogens but other stimuli such as metabolic stress can generate memory in HSCs. This review summarizes recent publications relevant to HSC memory. We discuss the current understanding of initial sensors, soluble mediators/cytokines involved in memory formation, including Type I and Type II interferons along with future implications.

Liver type 1 innate lymphoid cells develop locally via an interferon-γ-dependent loop

The pathways that lead to the development of tissue-resident lymphocytes, including liver type 1 innate lymphoid cells (ILC1s), remain unclear. We show here that the adult mouse liver contains Lin-Sca-1+Mac-1+ hematopoietic stem cells derived from the fetal liver. This population includes Lin-CD122+CD49a+ progenitors that can generate liver ILC1s but not conventional natural killer cells. Interferon-γ (IFN-γ) production by the liver ILC1s themselves promotes the development of these cells in situ, through effects on their IFN-γR+ liver progenitors. Thus, an IFN-γ-dependent loop drives liver ILC1 development in situ, highlighting the contribution of extramedullary hematopoiesis to regional immune composition within the liver.

Clonal hematopoiesis: mechanisms driving dominance of stem cell clones

The discovery of clonal hematopoiesis (CH) in older individuals has changed the way hematologists and stem cell biologists view aging. Somatic mutations accumulate in stem cells over time. While most mutations have no impact, some result in subtle functional differences that ultimately manifest in distinct stem cell behaviors. With a large pool of stem cells and many decades to compete, some of these differences confer advantages under specific contexts. Approximately 20 genes are recurrently found as mutated in CH, indicating they confer some advantage. The impact of these mutations has begun to be analyzed at a molecular level by modeling in cell lines and in mice. Mutations in epigenetic regulators such as DNMT3A and TET2 confer an advantage by enhancing self-renewal of stem and progenitor cells and inhibiting their differentiation. Mutations in other genes involved in the DNA damage response may simply enhance cell survival. Here, we review proposed mechanisms that lead to CH, specifically in the context of stem cell biology, based on our current understanding of the function of some of the CH-associated genes.

CCR5 maintains macrophages in the bone marrow and drives hematopoietic failure in a mouse model of severe aplastic anemia

Severe aplastic anemia (SAA) is an acquired, T cell-driven bone marrow (BM) failure disease characterized by elevated interferon gamma (IFNγ), loss of hematopoietic stem cells (HSCs), and altered BM microenvironment, including dysfunctional macrophages (MΦs). T lymphocytes are therapeutic targets for treating SAA, however, the underlying mechanisms driving SAA development and how innate immune cells contribute to disease remain poorly understood. In a murine model of SAA, increased beta-chemokines correlated with disease and were partially dependent on IFNγ. IFNγ was required for increased expression of the chemokine receptor CCR5 on MΦs. CCR5 antagonism in murine SAA improved survival, correlating with increased platelets and significantly increased platelet-biased CD41^hi HSCs. T cells are key drivers of disease, however, T cell-specific CCR5 expression and T cell-derived CCL5 were not necessary for disease. CCR5 antagonism reduced BM MΦs and diminished their expression of Tnf and Ccl5, correlating with reduced frequencies of IFNγ-secreting BM T cells. Mechanistically, CCR5 was intrinsically required for maintaining BM MΦs during SAA. Ccr5 expression was significantly increased in MΦs from aged mice and humans, relative to young counterparts. Our data identify CCR5 signaling as a key axis promoting the development of IFNγ-dependent BM failure, particularly relevant in aging where Ccr5 expression is elevated.

Constructing and deconstructing GATA2-regulated cell fate programs to establish developmental trajectories

Stem and progenitor cell fate transitions constitute key decision points in organismal development that enable access to a developmental path or actively preclude others. Using the hematopoietic system, we analyzed the relative importance of cell fate–promoting mechanisms versus negating fate-suppressing mechanisms to engineer progenitor cells with multilineage differentiation potential. Deletion of the murine Gata2−77 enhancer, with a human equivalent that causes leukemia, downregulates the transcription factor GATA2 and blocks progenitor differentiation into erythrocytes, megakaryocytes, basophils, and granulocytes, but not macrophages. Using multiomics and single-cell analyses, we demonstrated that the enhancer orchestrates a balance between pro- and anti-fate circuitry in single cells. By increasing GATA2 expression, the enhancer instigates a fate-promoting mechanism while abrogating an innate immunity–linked, fate-suppressing mechanism. During embryogenesis, the suppressing mechanism dominated in enhancer mutant progenitors, thus yielding progenitors with a predominant monocytic differentiation potential. Coordinating fate-promoting and -suppressing circuits therefore averts deconstruction of a multifate system into a monopotent system and maintains critical progenitor heterogeneity and functionality.

Aging-induced IL27Ra signaling impairs hematopoietic stem cells

Hematopoietic stem cell (HSC) aging correlates with an increasing risk of myeloproliferative disease and immunosenescence. In this study, we show that aging-related inflammation promotes HSC aging through tumor necrosis factor-α (TNF-α)→ERK→ETS1→interleukin27Ra (IL27Ra) pathway. TNF-α, a well-known biomarker of inflammation, increases during aging and induces the expression of IL27Ra on HSCs via ERK-ETS1 signaling. Deletion of IL27Ra rescues the functional decline and myeloid bias of HSCs and also reverses the inhibitory effect of TNF-α on HSCs. Aged IL27Ra-/- mice had a reduced proportion of myeloid-biased HSCs and did not display the biased myeloid differentiation that occurs in aged wild-type mice. IL27Ra+ HSCs exhibit impaired reconstitution capacity and myeloid-bias compared with IL27Ra- HSCs and serve as a myeloid-recovery pool upon inflammatory insult. Inflammation-related genes were enriched in IL27Ra+ HSCs and this enrichment increases with aging. Our study demonstrates that age-induced IL27Ra signaling impairs HSCs and raises the possibility that interfering with IL27Ra signaling can counter the physiologically deleterious effect of aging on hematopoietic capacity.

Janus Kinases in Leukemia

Janus kinases (JAKs) transduce signals from dozens of extracellular cytokines and function as critical regulators of cell growth, differentiation, gene expression, and immune responses. Deregulation of JAK/STAT signaling is a central component in several human diseases including various types of leukemia and other malignancies and autoimmune diseases. Different types of leukemia harbor genomic aberrations in all four JAKs (JAK1, JAK2, JAK3, and TYK2), most of which are activating somatic mutations and less frequently translocations resulting in constitutively active JAK fusion proteins. JAKs have become important therapeutic targets and currently, six JAK inhibitors have been approved by the FDA for the treatment of both autoimmune diseases and hematological malignancies. However, the efficacy of the current drugs is not optimal and the full potential of JAK modulators in leukemia is yet to be harnessed. This review discusses the deregulation of JAK-STAT signaling that underlie the pathogenesis of leukemia, i.e., mutations and other mechanisms causing hyperactive cytokine signaling, as well as JAK inhibitors used in clinic and under clinical development.

Determinants, mechanisms, and functional outcomes of myeloid cell diversity in cancer

Most, if not all, aspects of carcinogenesis are influenced by the tumor microenvironment (TME), a complex architecture of cells, matrix components, soluble signals, and their dynamic interactions in the context of physical traits of the tissue. Expanding application of technologies for high-dimensional analyses with single-cell resolution has begun to decipher the contributions of the immune system to cancer progression and its implications for therapy. In this review, we will discuss the multifaceted roles of tumor-associated macrophages and neutrophils, focusing on factors that subvert tissue immune homeostasis and offer therapeutic opportunities for TME reprogramming. By performing a critical analysis of available datasets, we elaborate on diversification mechanisms and unifying principles of myeloid cell heterogeneity in human tumors.

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.

Niche derived netrin-1 regulates hematopoietic stem cell dormancy via its receptor neogenin-1

Haematopoietic stem cells (HSCs) are characterized by their self-renewal potential associated to dormancy. Here we identify the cell surface receptor neogenin-1 as specifically expressed in dormant HSCs. Loss of neogenin-1 initially leads to increased HSC expansion but subsequently to loss of self-renewal and premature exhaustion in vivo. Its ligand netrin-1 induces Egr1 expression and maintains quiescence and function of cultured HSCs in a Neo1 dependent manner. Produced by arteriolar endothelial and periarteriolar stromal cells, conditional netrin-1 deletion in the bone marrow niche reduces HSC numbers, quiescence and self-renewal, while overexpression increases quiescence in vivo. Ageing associated bone marrow remodelling leads to the decline of netrin-1 expression in niches and a compensatory but reversible upregulation of neogenin-1 on HSCs. Our study suggests that niche produced netrin-1 preserves HSC quiescence and self-renewal via neogenin-1 function. Decline of netrin-1 production during ageing leads to the gradual decrease of Neo1 mediated HSC self-renewal.

Initiation of Post-Primary Tuberculosis of the Lungs: Exploring the Secret Role of Bone Marrow Derived Stem Cells

Mycobacterium tuberculosis (Mtb), the causative organism of pulmonary tuberculosis (PTB) now infects more than half of the world population. The efficient transmission strategy of the pathogen includes first remaining dormant inside the infected host, next undergoing reactivation to cause post-primary tuberculosis of the lungs (PPTBL) and then transmit via aerosol to the community. In this review, we are exploring recent findings on the role of bone marrow (BM) stem cell niche in Mtb dormancy and reactivation that may underlie the mechanisms of PPTBL development. We suggest that pathogen's interaction with the stem cell niche may be relevant in potential inflammation induced PPTBL reactivation, which need significant research attention for the future development of novel preventive and therapeutic strategies for PPTBL, especially in a post COVID-19 pandemic world. Finally, we put forward potential animal models to study the stem cell basis of Mtb dormancy and reactivation.

Dynamic single-cell RNA sequencing identifies immunotherapy persister cells following PD-1 blockade

Resistance to oncogene-targeted therapies involves discrete drug-tolerant persister cells, originally discovered through in vitro assays. Whether a similar phenomenon limits efficacy of programmed cell death 1 (PD-1) blockade is poorly understood. Here, we performed dynamic single-cell RNA-Seq of murine organotypic tumor spheroids undergoing PD-1 blockade, identifying a discrete subpopulation of immunotherapy persister cells (IPCs) that resisted CD8+ T cell-mediated killing. These cells expressed Snai1 and stem cell antigen 1 (Sca-1) and exhibited hybrid epithelial-mesenchymal features characteristic of a stem cell-like state. IPCs were expanded by IL-6 but were vulnerable to TNF-α-induced cytotoxicity, relying on baculoviral IAP repeat-containing protein 2 (Birc2) and Birc3 as survival factors. Combining PD-1 blockade with Birc2/3 antagonism in mice reduced IPCs and enhanced tumor cell killing in vivo, resulting in durable responsiveness that matched TNF cytotoxicity thresholds in vitro. Together, these data demonstrate the power of high-resolution functional ex vivo profiling to uncover fundamental mechanisms of immune escape from durable anti-PD-1 responses, while identifying IPCs as a cancer cell subpopulation targetable by specific therapeutic combinations.

Zfp521 is essential for the quiescence and maintenance of adult hematopoietic stem cells under stress

Zinc finger protein 521 (Zfp521), a quiescent hematopoietic stem cell (HSC)-enriched transcription factor, is involved in the self-renewal and differentiation of fetal liver HSC. However, its role in adult hematopoiesis remains elusive. Here, we found that Zfp521 deletion did not inhibit adult hematopoiesis under homeostatic conditions. In contrast, Zfp521-null chimeric mice showed significantly reduced pool size of HSC and hematopoietic progenitor cells associated with increased apoptosis and loss of quiescence. Competitive serial transplantation assays revealed that Zfp521 regulates HSC self-renewal and differentiation under regenerative stress. Mechanistically, Zfp521 transcriptionally repressed Rela expression by increasing H3K9ac and decreasing H3K9me3 levels in its promoter. Knockdown of Rela inhibited the hyper-activated NF-κB pathway and reversed the loss of quiescence in Zfp521-null HSC under stress. Thus, our results reveal a previously unrecognized role for Zfp521 as critical regulator of quiescence and self-renewal of HSC in adult hematopoiesis mediated at least partly by controlling Rela expression.

Protection of hematopoietic stem cells from stress-induced exhaustion and aging

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) are defined by their ability to self-renew and differentiate to replenish all blood lineages throughout adult life. Under homeostasis, the majority of HSCs are quiescent, and few stem cells are cycling to sustain hematopoiesis. However, HSCs can be induced to proliferate and differentiate in response to stress signals produced during infection, inflammation, chemotherapy, radiation, bone marrow transplantation, and aging. Recent evidence suggests that acute and chronic stress impact the number and function of HSCs including their ability to repopulate and produce mature cells. This review will focus on how chronic stress affects HSC biology and methods to mitigate HSC loss during chronic hematopoietic stress.

RECENT FINDINGS

Quiescent HSCs exit dormancy, divide, and differentiate to maintain steady-state hematopoiesis. Under conditions of acute stress including infection or blood loss some HSCs are pushed into division by cytokines and proinflammatory stimuli to differentiate and provide needed myeloid and erythroid cells to protect and reconstitute the host; after which, hematopoiesis returns to steady-state with minimal loss of HSC function. However, under conditions of chronic stress including serial bone marrow transplantation (BMT), chronic inflammation, and genotoxic stress (chemotherapy) and aging, HSCs are continuously induced to proliferate and undergo accelerated exhaustion. Recent evidence demonstrates that ablation of inhibitor of DNA binding 1 (Id1) gene can protect HSCs from exhaustion during chronic proliferative stress by promoting HSC quiescence.

SUMMARY

Increasing our understanding of the molecular processes that protect HSCs from chronic proliferative stress could lead to therapeutic opportunities to prevent accelerated HSC exhaustion during physiological stress, genotoxic stress, BMT, and aging.

PPE2 protein of Mycobacterium tuberculosis affects myeloid hematopoiesis in mice

Irregularity in hematopoiesis is noted in humans during tuberculosis. However, influence of mycobacterial protein(s) on bone marrow hematopoiesis is not fully understood. In this study, we have demonstrated the role of a mycobacterial protein, PPE2 (Rv0256c) in suppressing hematopoiesis during infection. PPE2 belongs to PPE (proline-proline-glutamine) family of mycobacterial proteins which are well known for hijacking host machineries for better survival inside host. In the present study, we have shown that mice infected with Mycobacterium smegmatis expressing PPE2 (M. smeg-PPE2) had a marked reduction in cells of myeloid lineage in bone marrow and peripheral blood along with altered bone marrow phenotype. Bone marrow of M. smeg-PPE2-infected mice showed an overall hypo-cellularity with an increase in population of immature cells, along with reduction in mature cell population. Higher number of M. smeg-PPE2 bacilli was observed in bone-marrow, lung, liver and spleen of mice as compared to the control mycobacteria (M. smeg-pVV16). M. smeg-PPE2-infected mice also showed higher expression of IFN-γ than those infected with M. smeg-pVV16. We conclude that PPE2 affects bone-marrow hematopoiesis of myeloid cells, probably by increasing IFN-γ levels, both locally and systemically, thus favoring the bacilli to establish a positive infection.

Manipulating niche composition limits damage to haematopoietic stem cells during Plasmodium infection

Severe infections are a major stress on haematopoiesis, where the consequences for haematopoietic stem cells (HSCs) have only recently started to emerge. HSC function critically depends on the integrity of complex bone marrow (BM) niches; however, what role the BM microenvironment plays in mediating the effects of infection on HSCs remains an open question. Here, using a murine model of malaria and combining single-cell RNA sequencing, mathematical modelling, transplantation assays and intravital microscopy, we show that haematopoiesis is reprogrammed upon infection, whereby the HSC compartment turns over substantially faster than at steady-state and HSC function is drastically affected. Interferon is found to affect both haematopoietic and mesenchymal BM cells and we specifically identify a dramatic loss of osteoblasts and alterations in endothelial cell function. Osteo-active parathyroid hormone treatment abolishes infection-triggered HSC proliferation and-coupled with reactive oxygen species quenching-enables partial rescuing of HSC function.

Fate of Hematopoiesis During Aging. What Do We Really Know, and What are its Implications?

There is an ongoing shift in demographics such that older persons will outnumber young persons in the coming years, and with it age-associated tissue attrition and increased diseases and disorders. There has been increased information on the association of the aging process with dysregulation of hematopoietic stem (HSC) and progenitor (HPC) cells, and hematopoiesis. This review provides an extensive up-to date summary on the literature of aged hematopoiesis and HSCs placed in context of potential artifacts of the collection and processing procedure, that may not be totally representative of the status of HSCs in their in vivo bone marrow microenvironment, and what the implications of this are for understanding aged hematopoiesis. This review covers a number of interactive areas, many of which have not been adequately explored. There are still many unknowns and mechanistic insights to be elucidated to better understand effects of aging on the hematopoietic system, efforts that will take multidisciplinary approaches, and that could lead to means to ameliorate at least some of the dysregulation of HSCs and HPCs associated with the aging process. Graphical Abstract.

Interferon Gamma Mediates Hematopoietic Stem Cell Activation and Niche Relocalization through BST2

During chronic infection, the inflammatory cytokine interferon gamma (IFNγ) damages hematopoietic stem cells (HSCs) by disrupting quiescence and promoting excessive terminal differentiation. However, the mechanism by which IFNγ hinders HSC quiescence remains undefined. Using intravital 3-dimensional microscopy, we find that IFNγ disrupts the normally close interaction between HSCs and CXCL12-abundant reticular (CAR) cells in the HSC niche. IFNγ stimulation increases expression of the cell surface protein BST2, which we find is required for IFNγ-dependent HSC relocalization and activation. IFNγ stimulation of HSCs increases their E-selectin binding by BST2 and homing to the bone marrow, which depends on E-selectin binding. Upon chronic infection, HSCs from mice lacking BST2 are more quiescent and more resistant to depletion than HSCs from wild-type mice. Overall, this study defines a critical mechanism by which IFNγ promotes niche relocalization and activation in response to inflammatory stimulation and identifies BST2 as a key regulator of HSC quiescence. VIDEO ABSTRACT.

Short-term ex-vivo exposure to hydrogen sulfide enhances murine hematopoietic stem and progenitor cell migration, homing, and proliferation

For successful transplantation of Hematopoietic Stem cells (HSCs), it is quite necessary that efficient homing, engraftment and retention of HSC self-renewal capacity takes place, which is often restricted due to inadequate number of adult HSCs. Here, we report that short-term ex-vivo treatment of mouse bone marrow mononuclear cells (BMMNCs) to Sodium Hydrogen Sulfide (NaHS, hydrogen sulfide-H2S donor) can be used as a possible strategy to overcome such hurdle. H2S increases the expression of CXCR4 on HSPCs, enhancing their migration toward SDF-1α in-vitro and thus homing to BM niche. . Additionally, in-vitro studies revealed that H2S has a role in activating mitochondria, thus, pushing quiescent HSCs into division. These results suggest a readily available and cost-effective method to facilitate efficient HSC transplantation.

Normal Hematopoiesis Is a Balancing Act of Self-Renewal and Regeneration

The hematopoietic system is highly organized to maintain its functional integrity and to meet lifelong organismal demands. Hematopoietic stem cells (HSCs) must balance self-renewal with differentiation and the regeneration of the blood system. It is a complex balancing act between these competing HSC functions. Although highly quiescent at steady state, HSCs become activated in response to inflammatory cytokines and regenerative challenges. This activation phase leads to many intrinsic stresses such as replicative, metabolic, and oxidative stress, which can cause functional decline, impaired self-renewal, and exhaustion of HSCs. To cope with these insults, HSCs use both built-in and emergency-triggered stress-response mechanisms to maintain homeostasis and to defend against disease development. In this review, we discuss how the hematopoietic system operates in steady state and stress conditions, what strategies are used to maintain functional integrity, and how deregulation in the balance between self-renewal and regeneration can drive malignant transformation.

Acute Intestinal Inflammation Depletes/Recruits Histamine-Expressing Myeloid Cells From the Bone Marrow Leading to Exhaustion of MB-HSCs

BACKGROUND & AIMS

Histidine decarboxylase (HDC), the histamine-synthesizing enzyme, is expressed in a subset of myeloid cells but also marks quiescent myeloid-biased hematopoietic stem cells (MB-HSCs) that are activated upon myeloid demand injury. However, the role of MB-HSCs in dextran sulfate sodium (DSS)-induced acute colitis has not been addressed.

METHODS

We investigated HDC+ MB-HSCs and myeloid cells by flow cytometry in acute intestinal inflammation by treating HDC-green fluorescent protein (GFP) male mice with 5% DSS at various time points. HDC+ myeloid cells in the colon also were analyzed by flow cytometry and immunofluorescence staining. Knockout of the HDC gene by using HDC-/-; HDC-GFP and ablation of HDC+ myeloid cells by using HDC-GFP; HDC-tamoxifen-inducible recombinase Cre system; diphtheria toxin receptor (DTR) mice was performed. The role of H2-receptor signaling in acute colitis was addressed by treatment of DSS-treated mice with the H2 agonist dimaprit dihydrochloride. Kaplan-Meier survival analysis was performed to assess the effect on survival.

RESULTS

In acute colitis, rapid activation and expansion of MB-HSC from bone marrow was evident early on, followed by a gradual depletion, resulting in profound HSC exhaustion, accompanied by infiltration of the colon by increased HDC+ myeloid cells. Knockout of the HDC gene and ablation of HDC+ myeloid cells enhance the early depletion of HDC+ MB-HSC, and treatment with H2-receptor agonist ameliorates the depletion of MB-HSCs and resulted in significantly increased survival of HDC-GFP mice with acute colitis.

CONCLUSIONS

Exhaustion of bone marrow MB-HSCs contributes to the progression of DSS-induced acute colitis, and preservation of quiescence of MB-HSCs by the H2-receptor agonist significantly enhances survival, suggesting the potential for therapeutic utility.

The Role of the Bone Marrow Microenvironment in the Response to Infection

Hematopoiesis in the bone marrow (BM) is the primary source of immune cells. Hematopoiesis is regulated by a diverse cellular microenvironment that supports stepwise differentiation of multipotent stem cells and progenitors into mature blood cells. Blood cell production is not static and the bone marrow has evolved to sense and respond to infection by rapidly generating immune cells that are quickly released into the circulation to replenish those that are consumed in the periphery. Unfortunately, infection also has deleterious effects injuring hematopoietic stem cells (HSC), inefficient hematopoiesis, and remodeling and destruction of the microenvironment. Despite its central role in immunity, the role of the microenvironment in the response to infection has not been systematically investigated. Here we summarize the key experimental evidence demonstrating a critical role of the bone marrow microenvironment in orchestrating the bone marrow response to infection and discuss areas of future research.

Immuno-Modulation of Hematopoietic Stem and Progenitor Cells in Inflammation

Lifelong blood production is maintained by bone marrow (BM)-residing hematopoietic stem cells (HSCs) that are defined by two special properties: multipotency and self-renewal. Since dysregulation of either may lead to a differentiation block or extensive proliferation causing dysplasia or neoplasia, the genomic integrity and cellular function of HSCs must be tightly controlled and preserved by cell-intrinsic programs and cell-extrinsic environmental factors of the BM. The BM had been long regarded an immune-privileged organ shielded from immune insults and inflammation, and was thereby assumed to provide HSCs and immune cells with a protective environment to ensure blood and immune homeostasis. Recently, accumulating evidence suggests that hemato-immune challenges such as autoimmunity, inflammation or infection elicit a broad spectrum of immunological reactions in the BM, and in turn, influence the function of HSCs and BM environmental cells. Moreover, in analogy with the emerging concept of “trained immunity”, certain infection-associated stimuli are able to train HSCs and progenitors to produce mature immune cells with enhanced responsiveness to subsequent challenges, and in some cases, form an inflammatory or infectious memory in HSCs themselves. In this review, we will introduce recent findings on HSC and hematopoietic regulation upon exposure to various hemato-immune stimuli and discuss how these challenges can elicit either beneficial or detrimental outcomes on HSCs and the hemato-immune system, as well as their relevance to aging and hematologic malignancies.

Trained Immunity and Cardiometabolic Disease: The Role of Bone Marrow

Until recently, immunologic memory was considered an exclusive characteristic of adaptive immunity. However, recent advances suggest that the innate arm of the immune system can also mount a type of nonspecific memory responses. Innate immune cells can elicit a robust response to subsequent inflammatory challenges after initial activation by certain stimuli, such as fungal-derived agents or vaccines. This type of memory, termed trained innate immunity (also named innate immune memory), is associated with epigenetic and metabolic alterations. Hematopoietic progenitor cells, which are the cells responsible for the generation of mature myeloid cells at steady-state and during inflammation, have a critical contribution to the induction of innate immune memory. Inflammation-triggered alterations in cellular metabolism, the epigenome and transcriptome of hematopoietic progenitor cells in the bone marrow promote long-lasting functional changes, resulting in increased myelopoiesis and consequent generation of trained innate immune cells. In the present brief review, we focus on the involvement of hematopoietic progenitors in the process of trained innate immunity and its possible role in cardiometabolic disease.

ATF3 Prevents Stress-Induced Hematopoietic Stem Cell Exhaustion

Protection of hematopoietic stem cells (HSCs) from exhaustion and effective regeneration of the HSC pool after bone marrow transplantation or irradiation therapy is an urgent clinical need. Here, we investigated the role of activating transcription factor 3 (ATF3) in steady-state and stress hematopoiesis using conditional knockout mice (Atf3^fl/flVav1Cre mice). Deficiency of ATF3 in the hematopoietic system displayed no noticeable effects on hematopoiesis under steady-state conditions. Expression of ATF3 was significantly down-regulated in long-term HSCs (LT-HSCs) after exposure to stresses such as 5-fluorouracil challenge or irradiation. Atf3^fl/flVav1Cre mice displayed enhanced proliferation and expansion of LT-HSCs upon short-term chemotherapy or irradiation compared with those in Atf3^fl/fl littermate controls; however, the long-term reconstitution capability of LT-HSCs from Atf3^fl/flVav1Cre mice was dramatically impaired after a series of bone marrow transplantations. These observations suggest that ATF3 plays an important role in preventing stress-induced exhaustion of HSCs.

Inflammation and hematopoietic stem cells aging

Hematopoietic stem cells (HSCs) replenish all lineages of blood cells throughout the lifespan. During aging, the repopulation capacity of HSCs declined, and aged HSCs display a tendency for myeloid differentiation. Several intrinsic and extrinsic factors have been identified to promote HSCs aging. In this review, we focus on the contribution of aging-associated inflammation in provoking HSCs aging and discuss the future research direction of inflammation and HSC aging.

The Crossroads between Infection and Bone Loss

Bone homeostasis, based on a tight balance between bone formation and bone degradation, is affected by infection. On one hand, some invading pathogens are capable of directly colonizing the bone, leading to its destruction. On the other hand, immune mediators produced in response to infection may dysregulate the deposition of mineral matrix by osteoblasts and/or the resorption of bone by osteoclasts. Therefore, bone loss pathologies may develop in response to infection, and their detection and treatment are challenging. Possible biomarkers of impaired bone metabolism during chronic infection need to be identified to improve the diagnosis and management of infection-associated osteopenia. Further understanding of the impact of infections on bone metabolism is imperative for the early detection, prevention, and/or reversion of bone loss. Here, we review the mechanisms responsible for bone loss as a direct and/or indirect consequence of infection.

M. tuberculosis Reprograms Hematopoietic Stem Cells to Limit Myelopoiesis and Impair Trained Immunity

A greater understanding of hematopoietic stem cell (HSC) regulation is required for dissecting protective versus detrimental immunity to pathogens that cause chronic infections such as Mycobacterium tuberculosis (Mtb). We have shown that systemic administration of Bacille Calmette-Guérin (BCG) or β-glucan reprograms HSCs in the bone marrow (BM) via a type II interferon (IFN-II) or interleukin-1 (IL1) response, respectively, which confers protective trained immunity against Mtb. Here, we demonstrate that, unlike BCG or β-glucan, Mtb reprograms HSCs via an IFN-I response that suppresses myelopoiesis and impairs development of protective trained immunity to Mtb. Mechanistically, IFN-I signaling dysregulates iron metabolism, depolarizes mitochondrial membrane potential, and induces cell death specifically in myeloid progenitors. Additionally, activation of the IFN-I/iron axis in HSCs impairs trained immunity to Mtb infection. These results identify an unanticipated immune evasion strategy of Mtb in the BM that controls the magnitude and intrinsic anti-microbial capacity of innate immunity to infection.

Notch signaling mitigates chemotherapy toxicity by accelerating hematopoietic stem cells proliferation via c-Myc

The mechanisms that regulate hematopoietic stem cell (HSC) regeneration after myelosuppressive injury are not well understood. Here, we showed that disruption of Notch signaling aggravated chemotherapy-induced myelosuppression in inducible genetic mice. Conversely, Notch activation correlated positively with clinical HSC engraftment. We used endothelial-targeted chimeric Notch ligand Delta-like 1 (D1R) to activate Notch signaling in hematopoietic stem/progenitor cells through micro-environmental cellular contact. Recombinant protein D1R contributed to the recovery of the HSC pool and sustained HSC vitality in response to various chemotherapeutic agents in vivo. Mechanistically, D1R treatment promoted HSC proliferation transiently, prevented HSC exhaustion, correlated with activation of the downstream phosphoinositide 3-kinase (PI3K)/extracellular-signal-regulated kinase (ERK)/BCL2 associated agonist of cell death (BAD) signaling axis during regeneration, and partially mediated upregulation of c-Myc in HSCs. These data reveal an unrecognized role for Notch signaling in promoting HSC repopulation after myelosuppressive chemotherapy and offer a new therapeutic approach to mitigate chemotherapy-induced injury.

PTEN in Regulating Hematopoiesis and Leukemogenesis

PTEN is one of the most frequently mutated tumor suppressor genes in human cancers. By counteracting the PI3K/AKT/mTOR pathway, PTEN plays an essential role in regulating hematopoietic stem cells (HSCs) self-renewal, migration, lineage commitment, and differentiation. PTEN also plays important roles in suppressing leukemogenesis, especially T-cell acute lymphoblastic leukemia (T-ALL). Herein, we will review the function of PTEN in regulating hematopoiesis and leukemogenesis and discuss potential therapeutic approaches against leukemia with PTEN mutations.

Determinants and role of chromatin organization in acute leukemia

DNA is compacted into higher order structures that have major implications in gene regulation. These structures allow for long-range interactions of DNA elements, such as the association of promoters with their cognate enhancers. In recent years, mutations in genes that control these structures, including the cohesin-complex and the insulator-binding protein CTCF, have been found in a spectrum of hematologic disorders, and especially in acute leukemias. Cohesin and CTCF are critical for mediating looping and establishing boundaries within chromatin. Cells that harbor mutations in these genes display aberrant chromatin architecture and resulting differences in gene expression that contribute to leukemia initiation and progression. Here, we provide detailed discussion of the nature of 3D interactions and the way that they are disrupted in acute leukemia. Continued research in this area will provide new insights into the mechanisms of leukemogenesis and may shed light on novel treatment strategies.

Ablation of IL-17A leads to severe colitis in IL-10-deficient mice: implications of myeloid-derived suppressor cells and NO production

IL-10 is an immune regulatory cytokine and its genetic defect leads to gastrointestinal inflammation in humans and mice. Moreover, the IL-23/T_h17 axis is known to be involved in these inflammatory disorders. IL-17A, a representative cytokine produced by T_h17 cells, has an important role for the pathological process of inflammatory diseases. However, the precise function of IL-17A in inflammatory bowel disease (IBD) remains controversial. In this study, we evaluated the effect of IL-17A on colitis in IL-10-deficient (Il10^−/−) mice. Mice lacking both IL-10 and IL-17A (Il10^−/−Il17a^−/−) suffered from fatal wasting and manifested more severe colitis compared with Il10^−/−Il17a^+/− mice. Moreover, we found that CD11b⁺Gr-1⁺ myeloid-derived suppressor cells (MDSCs) accumulated in the bone marrow, spleen and peripheral blood of Il10^−/−Il17a^−/− mice. These MDSCs highly expressed inducible nitric oxide synthase (iNOS) (Nos2) and suppressed the T-cell response in vitro in a NOS-dependent manner. In correlation with these effects, the concentration of nitric oxide was elevated in the serum of Il10^−/−Il17a^−/− mice. Surprisingly, the severe colitis observed in Il10^−/−Il17a^−/− mice was ameliorated in Il10^−/−Il17a^−/−Nos2^−/− mice. Our findings suggest that IL-17A plays suppressive roles against spontaneous colitis in Il10^−/− mice in an iNOS-dependent manner and inhibits MDSC differentiation and/or proliferation.

Single-cell and spatial transcriptomics approaches of the bone marrow microenvironment

PURPOSE OF REVIEW

The bone marrow is home to hematopoietic stem cells responsible for lifelong blood production, alongside mesenchymal stem cells required for skeletal regeneration. In the bone marrow, a unique combination of signals derived from a multitude of cell types results in the establishment of so-called niches that regulate stem-cell maintenance and differentiation. Recently, single-cell and spatially resolved transcriptomics technologies have been utilized to characterize the murine bone marrow microenvironment during homeostasis, stress and upon cancer-induced remodeling. In this review, we summarize the major findings of these studies.

RECENT FINDINGS

Single-cell technologies applied to bone marrow provided the first systematic and label-free identification of bone marrow cell types, enabled their molecular and spatial characterization, and clarified the cellular sources of key prohematopoietic factors. Large transcriptional heterogeneity and novel subpopulations were observed in compartments previously thought to be homogenous. For example, Lepr Cxcl12-abundant reticular cells were shown to constitute the major source of prohematopoietic factors, but consist of subpopulations differing in their adipogenic versus osteogenic priming, morphology and localization. These subpopulations were suggested to act as professional cytokine secreting cells, thereby establishing distinct bone marrow niches.

SUMMARY

Single-cell and spatially resolved transcriptomics approaches have clarified the molecular identity and localization of bone marrow-resident cell types, paving the road for a deeper exploration of bone marrow niches in the mouse and humans.

Conventional Co-Housing Modulates Murine Gut Microbiota and Hematopoietic Gene Expression

Specific-pathogen-free (SPF) mice have improved hematopoietic characteristics relative to germ-free mice, however, it is not clear whether improvements in hematopoietic traits will continue when the level of microorganism exposure is further increased. We co-housed SPF C57BL/6 mice in a conventional facility (CVT) and found a significant increase in gut microbiota diversity along with increased levels of myeloid cells and T cells, especially effector memory T cells. Through single cell RNA sequencing of sorted KL (c-Kit⁺Lin⁻) cells, we imputed a decline in long-term hematopoietic stem cells and an increase in granulocyte-monocyte progenitors in CVT mice with up-regulation of genes associated with cell survival. Bone marrow transplantation through competitive repopulation revealed a significant increase in KSL (c-Kit⁺Sca-1⁺Lin⁻) cell reconstitution in recipients of CVT donor cells which occurred when donors were co-housed for both one and twelve months. However, there was minimal to no gain in mature blood cell engraftment in recipients of CVT donor cells relative to those receiving SPF donor cells. We conclude that co-housing SPF mice with mice born in a conventional facility increased gut microbiota diversity, augmented myeloid cell production and T cell activation, stimulated KSL cell reconstitution, and altered hematopoietic gene expression.

Metabolic Regulation of Inflammasome Activity Controls Embryonic Hematopoietic Stem and Progenitor Cell Production

Embryonic hematopoietic stem and progenitor cells (HSPCs) robustly proliferate while maintaining multilineage potential in vivo; however, an incomplete understanding of spatiotemporal cues governing their generation has impeded robust production from human induced pluripotent stem cells (iPSCs) in vitro. Using the zebrafish model, we demonstrate that NLRP3 inflammasome-mediated interleukin-1-beta (IL1β) signaling drives HSPC production in response to metabolic activity. Genetic induction of active IL1β or pharmacologic inflammasome stimulation increased HSPC number as assessed by in situ hybridization for runx1/cmyb and flow cytometry. Loss of inflammasome components, including il1b, reduced CD41⁺ HSPCs and prevented their expansion in response to metabolic cues. Cell ablation studies indicated that macrophages were essential for initial inflammasome stimulation of Il1rl1⁺ HSPCs. Significantly, in human iPSC-derived hemogenic precursors, transient inflammasome stimulation increased multilineage hematopoietic colony-forming units and T cell progenitors. This work establishes the inflammasome as a conserved metabolic sensor that expands HSPC production in vivo and in vitro.

Common Sources of Inflammation and Their Impact on Hematopoietic Stem Cell Biology

Purpose of Review

Inflammatory signals have emerged as critical regulators of hematopoietic stem cell (HSC) function. Specifically, HSCs are highly responsive to acute changes in systemic inflammation and this influences not only their division rate but also their lineage fate. Identifying how inflammation regulates HSCs and shapes the blood system is crucial to understanding the mechanisms underpinning these processes, as well as potential links between them.

Recent Findings

A widening array of physiologic and pathologic processes involving heightened inflammation are now recognized to critically affect HSC biology and blood lineage production. Conditions documented to affect HSC function include not only acute and chronic infections but also autoinflammatory conditions, irradiation injury, and physiologic states such as aging and obesity.

Summary

Recognizing the contexts during which inflammation affects primitive hematopoiesis is essential to improving our understanding of HSC biology and informing new therapeutic interventions against maladaptive hematopoiesis that occurs during inflammatory diseases, infections, and cancer-related disorders.

Targeting Unconventional Host Components for Vaccination-Induced Protection Against TB

The current tuberculosis (TB) vaccine, Bacille Calmette-Guerin (BCG), is effective in preventing TB in young children but was developed without a basic understanding of human immunology. Most modern TB vaccine candidates have targeted CD4⁺ T cell responses, thought to be important for protection against TB disease, but not known to be sufficient or critical for protection. Advances in knowledge of host responses to TB afford opportunities for developing TB vaccines that target immune components not conventionally considered. Here, we describe the potential of targeting NK cells, innate immune training, B cells and antibodies, and Th17 cells in novel TB vaccine development. We also discuss attempts to target vaccine immunity specifically to the lung, the primary disease site in humans.