Functionally Distinct Subsets of Lineage-Biased Multipotent Progenitors Control Blood Production in Normal and Regenerative Conditions

CXCR4 signaling determines the fate of hematopoietic multipotent progenitors by stimulating mTOR activity and mitochondrial metabolism

Both cell-intrinsic and niche-derived, cell-extrinsic cues drive the specification of hematopoietic multipotent progenitors (MPPs) in the bone marrow, which comprise multipotent MPP1 cells and lineage-restricted MPP2, MPP3, and MPP4 subsets. Patients with WHIM syndrome, a rare congenital immunodeficiency caused by mutations that prevent desensitization of the chemokine receptor CXCR4, have an excess of myeloid cells in the bone marrow. Here, we investigated the effects of increased CXCR4 signaling on the localization and fate of MPPs. Knock-in mice bearing a WHIM syndrome-associated CXCR4 mutation (CXCR41013) phenocopied the myeloid skewing of bone marrow in patients. Whereas MPP4 cells in wild-type mice differentiated into lymphoid cells, MPP4s in CXCR41013 knock-in mice differentiated into myeloid cells. This myeloid rewiring of MPP4s in CXCR41013 knock-in mice was associated with enhanced signaling mediated by the kinase mTOR and increased oxidative phosphorylation (OXPHOS). MPP4s also localized further from arterioles in the bone marrow of knock-in mice compared with wild-type mice, suggesting that the loss of extrinsic cues from the perivascular niche may also contribute to their myeloid skewing. Chronic treatment with the CXCR4 antagonist AMD3100 or the mTOR inhibitor rapamycin restored the lymphoid potential of MPP4s in knock-in mice. Thus, CXCR4 desensitization drives the lymphoid potential of MPP4 cells by dampening the mTOR-dependent metabolic changes that promote myeloid differentiation.

Ever-evolving insights into the cellular and molecular drivers of lymphoid cell development

Lymphocytes play a critical role in adaptive immunity and defense mechanisms, but the molecular mechanisms by which hematopoietic stem and progenitor cells differentiate into T and B lymphocytes are not fully established. Pioneer studies identify several transcription factors essential for lymphoid lineage determination. Yet, many questions remain unanswered about how these transcription factors interact with each other and with chromatin at different developmental stages. This interaction regulates a network of genes and proteins, promoting lymphoid lineage differentiation while suppressing other lineages. Throughout this intricate biological process, any genetic or epigenetic interruptions can derail normal differentiation trajectories, potentially leading to various human pathologic conditions. Here, we summarize recent advances in understanding lymphoid cell development, which was the focus of the Winter 2024 International Society for Experimental Hematology webinar.

Interleukin-1β modulates lymphoid differentiation of Flt3-positive multipotent progenitors after transplantation

Myeloablative pre-conditioning facilitates the differentiation of transplanted hematopoietic stem and progenitor cells (HSPCs). However, the factors in the stress environment that regulate HSPC behavior remain elusive. Here, we investigated the mechanisms that shaped the cell fates of transplanted murine multipotent progenitors (MPPs) expressing the Fms-related receptor tyrosine kinase 3 gene (Flt3). Using lineage tracing, clonal analysis, and single-cell RNA sequencing (RNA-seq), we showed that the myeloablative environment increased lymphoid priming of Flt3+ MPPs and that their efficient B cell output required intact interleukin 1 (IL-1) signaling. The Flt3+ MPPs with short-term exposure to IL-1β underwent a myeloid-biased to lymphoid-biased cell fate switch and produced more lymphoid-biased progeny with a stronger B lymphopoiesis capacity in vitro. Correspondingly, a brief exposure to IL-1β facilitated the B cell output of transplanted Flt3+ MPPs in vivo. Together, our study demonstrated an unrecognized function of IL-1β in promoting B lymphopoiesis and highlighted a latent effect of IL-1β in regulating MPP cell fate dynamics.

Vasopressin drives aberrant myeloid differentiation of hematopoietic stem cells, contributing to depression in mice

Psychological stress is often linked to depression and can also impact the immune system, illustrating the interconnectedness of mental health and immune function. Hematopoietic stem cells (HSCs) can directly sense neuroendocrine signals in bone marrow and play a fundamental role in the maintenance of immune homeostasis. However, it is unclear how psychological stress impacts HSCs in depression. Here, we report that neuroendocrine factor arginine vasopressin (AVP) promotes myeloid-biased HSC differentiation by activating neutrophils. AVP administration increases neutrophil and Ly6Chi monocyte production by triggering HSCs that rely on intrinsic S100A9 in mice. When stimulated with AVP, neutrophils return to the bone marrow and release interleukin 36G (IL-36G), which interacts with interleukin 1 receptor-like 2 (IL-1RL2) on HSCs to produce neutrophils with high Elane expression that infiltrate the brain and induce neuroinflammation. Together, these findings define HSCs as a relay between psychological stress and myelopoiesis and identify the IL-36G-IL-1RL2 axis as a potential target for depression therapy.

The role of immune cells settled in the bone marrow on adult hematopoietic stem cells

Certain immune cells, including neutrophils, macrophages, dendritic cells, B cells, Breg cells, CD4+ T cells, CD8+ T cells, and Treg cells, establish enduring residency within the bone marrow. Their distinctive interactions with hematopoiesis and the bone marrow microenvironment are becoming increasingly recognized alongside their multifaceted immune functions. These cells play a dual role in shaping hematopoiesis. They directly influence the quiescence, self-renewal, and multi-lineage differentiation of hematopoietic stem and progenitor cells through either direct cell-to-cell interactions or the secretion of various factors known for their immunological functions. Additionally, they actively engage with the cellular constituents of the bone marrow niche, particularly mesenchymal stem cells, endothelial cells, osteoblasts, and osteoclasts, to promote their survival and contribute to tissue repair, thereby fostering a supportive environment for hematopoietic stem and progenitor cells. Importantly, these bone marrow immune cells function synergistically, both locally and functionally, rather than in isolation. In summary, immune cells residing in the bone marrow are pivotal components of a sophisticated network of regulating hematopoiesis.

Mitochondrial serine catabolism safeguards maintenance of the hematopoietic stem cell pool in homeostasis and injury

Hematopoietic stem cells (HSCs) employ a very unique metabolic pattern to maintain themselves, while the spectrum of their metabolic adaptations remains incompletely understood. Here, we uncover a distinct and heterogeneous serine metabolism within HSCs and identify mouse HSCs as a serine auxotroph whose maintenance relies on exogenous serine and the ensuing mitochondrial serine catabolism driven by the hydroxymethyltransferase 2 (SHMT2)-methylene-tetrahydrofolate dehydrogenase 2 (MTHFD2) axis. Mitochondrial serine catabolism primarily feeds NAD(P)H generation to maintain redox balance and thereby diminishes ferroptosis susceptibility of HSCs. Dietary serine deficiency, or genetic or pharmacological inhibition of the SHMT2-MTHFD2 axis, increases ferroptosis susceptibility of HSCs, leading to impaired maintenance of the HSC pool. Moreover, exogenous serine protects HSCs from irradiation-induced myelosuppressive injury by fueling mitochondrial serine catabolism to mitigate ferroptosis. These findings reframe the canonical view of serine from a nonessential amino acid to an essential niche metabolite for HSC pool maintenance.

AMPK-mediated regulation of endogenous cholesterol synthesis does not affect atherosclerosis in a murine Pcsk9-AAV model

BACKGROUND AND AIMS

Dysregulated cholesterol metabolism is a hallmark of atherosclerotic cardiovascular diseases, yet our understanding of how endogenous cholesterol synthesis affects atherosclerosis is not clear. The energy sensor AMP-activated protein kinase (AMPK) phosphorylates and inhibits the rate-limiting enzyme in the mevalonate pathway HMG-CoA reductase (HMGCR). Recent work demonstrated that when AMPK-HMGCR signaling was compromised in an Apoe-/- model of hypercholesterolemia, atherosclerosis was exacerbated due to elevated hematopoietic stem and progenitor cell mobilization and myelopoiesis. We sought to validate the significance of the AMPK-HMGCR signaling axis in atherosclerosis using a non-germline hypercholesterolemia model with functional ApoE.

METHODS

Male and female HMGCR S871A knock-in (KI) mice and wild-type (WT) littermate controls were made atherosclerotic by intravenous injection of a gain-of-function Pcsk9D374Y-adeno-associated virus followed by high-fat and high-cholesterol atherogenic western diet feeding for 16 weeks.

RESULTS

AMPK activation suppressed endogenous cholesterol synthesis in primary bone marrow-derived macrophages from WT but not HMGCR KI mice, without changing other parameters of cholesterol regulation. Atherosclerotic plaque area was unchanged between WT and HMGCR KI mice, independent of sex. Correspondingly, there were no phenotypic differences observed in hematopoietic progenitors or differentiated immune cells in the bone marrow, blood, or spleen, and no significant changes in systemic markers of inflammation. When lethally irradiated female mice were transplanted with KI bone marrow, there was similar plaque content relative to WT.

CONCLUSIONS

Given previous work, our study demonstrates the importance of preclinical atherosclerosis model comparison and brings into question the importance of AMPK-mediated control of cholesterol synthesis in atherosclerosis.

Clonal dynamics and somatic evolution of haematopoiesis in mouse

Haematopoietic stem cells maintain blood production throughout life. While extensively characterised using the laboratory mouse, little is known about how the population is sustained and evolves with age. We isolated stem cells and progenitors from young and old mice, identifying 221,890 somatic mutations genome-wide in 1845 single cell-derived colonies, and used phylogenetic analysis to infer the ontogeny and population dynamics of the stem cell pool. Mouse stem cells and progenitors accrue ~45 somatic mutations per year, a rate only about 2-fold greater than human progenitors despite the vastly different organismal sizes and lifespans. Phylogenetic patterns reveal that stem and multipotent progenitor cell pools are both established during embryogenesis, after which they independently self-renew in parallel over life. The stem cell pool grows steadily over the mouse lifespan to approximately 70,000 cells, self-renewing about every six weeks. Aged mice did not display the profound loss of stem cell clonal diversity characteristic of human haematopoietic ageing. However, targeted sequencing revealed small, expanded clones in the context of murine ageing, which were larger and more numerous following haematological perturbations and exhibited a selection landscape similar to humans. Our data illustrate both conserved features of population dynamics of blood and distinct patterns of age-associated somatic evolution in the short-lived mouse.

Dynamically adjusted cell fate decisions and resilience to mutant invasion during steady-state hematopoiesis revealed by an experimentally parameterized mathematical model

A major next step in hematopoietic stem cell (HSC) biology is to enhance our quantitative understanding of cellular and evolutionary dynamics involved in undisturbed hematopoiesis. Mathematical models have been and continue to be key in this respect, and are most powerful when parameterized experimentally and containing sufficient biological complexity. In this paper, we use data from label propagation experiments in mice to parameterize a mathematical model of hematopoiesis that includes homeostatic control mechanisms as well as clonal evolution. We find that nonlinear feedback control can drastically change the interpretation of kinetic estimates at homeostasis. This suggests that short-term HSC and multipotent progenitors can dynamically adjust to sustain themselves temporarily in the absence of long-term HSCs, even if they differentiate more often than they self-renew in undisturbed homeostasis. Additionally, the presence of feedback control in the model renders the system resilient against mutant invasion. Invasion barriers, however, can be overcome by a combination of age-related changes in stem cell differentiation and evolutionary niche construction dynamics based on a mutant-associated inflammatory environment. This helps us understand the evolution of e.g., TET2 or DNMT3A mutants, and how to potentially reduce mutant burden.

Acute effects of TLR3 agonist Poly(I:C) on bone marrow hematopoietic progenitor cells in mice

Hematopoietic progenitor cells (HPCs) in bone marrow with limited abilities for self-renewal and differentiation continuously supply hematopoietic cells through life. When suffering infection or inflammation, HPCs will actively proliferate to provide differentiated hematopoietic cells to maintain hematopoietic homeostasis. Poly(I:C), an agonist of TLR3, can specifically activate Type I interferon (IFN-I) signaling which exerts anti-inflammatory effects and influence hematopoiesis after infection. However, the effects of Poly(I:C)-induced IFN-I on the bone marrow hematopoietic system still deserve attention. In this study, our results revealed the efficacy of the IFN-I model, with a remarkably decrease in HPCs and a sharp elevation in LSKs numbers after single dose of Poly(I:C) injection. Apoptotic ratios of HPCs and LSKs significantly increased 48 h after Poly(I:C) treatment. Application of Poly(I:C) prompted the transition of HPCs and LSKs from G0 to G1 phases, potentially leading to the accelerated exhaustion of HPCs. From the cobblestone area-forming cell (CAFC) assay, we speculate that Poly(I:C) impairs the differentiation capacity of HPCs as well as their colony-forming ability. RT-qPCR and immunohistochemistry revealed significant upregulation of IFN-I associated genes and proteins following Poly(I:C) treatment. In conclusion, a single dose of Poly(I:C) induced an acute detrimental effect on HPCs within 48 h potentially due to TLR3 engagement. This activation cascaded into a robust IFN-I response emanating from the bone marrow, underscoring the intricate immunological dynamics at play following Poly(I:C) intervention.

Genetic deletion of JAM-C in preleukemic cells rewires leukemic stem cell gene expression program in AML

ABSTRACT

The leukemic stem cell (LSC) score LSC-17 based on a stemness-related gene expression signature is an indicator of poor disease outcome in acute myeloid leukemia (AML). However, it is not known whether "niche anchoring" of LSC affects disease evolution. To address this issue, we conditionally inactivated the adhesion molecule JAM-C (Junctional Adhesion Molecule-C) expressed by hematopoietic stem cells (HSCs) and LSCs in an inducible mixed-lineage leukemia (iMLL)-AF9-driven AML mouse model. Deletion of Jam3 (encoding JAM-C) before induction of the leukemia-initiating iMLL-AF9 fusion resulted in a shift from long-term to short-term HSC expansion, without affecting disease initiation and progression. In vitro experiments showed that JAM-C controlled leukemic cell nesting irrespective of the bone marrow stromal cells used. RNA sequencing performed on leukemic HSCs isolated from diseased mice revealed that genes upregulated in Jam3-deficient animals belonged to activation protein-1 (AP-1) and tumor necrosis factor α (TNF-α)/NF-κB pathways. Human orthologs of dysregulated genes allowed to identify a score that was distinct from, and complementary to, the LSC-17 score. Substratification of patients with AML using LSC-17 and AP-1/TNF-α genes signature defined 4 groups with median survival ranging from <1 year to a median of "not reached" after 8 years. Finally, coculture experiments showed that AP-1 activation in leukemic cells was dependent on the nature of stromal cells. Altogether, our results identify the AP-1/TNF-α gene signature as a proxy of LSC anchoring in bone marrow niches, which improves the prognostic value of the LSC-17 score. This trial was registered at www.ClinicalTrials.gov as #NCT02320656.

Smarcd1 subunit of SWI/SNF chromatin-remodeling complexes collaborates with E2a to promote murine lymphoid specification

Lymphocyte development from murine hematopoietic stem cells (HSCs) entails a loss of self-renewal capacity and a progressive restriction of developmental potential. Previous research from our laboratory suggests that specialized assemblies of ATP-dependent SWI/SNF chromatin-remodeling complexes play lineage-specific roles during murine hematopoiesis. Here, we demonstrate that the Smarcd1 subunit is essential for specification of lymphoid cell fate from multipotent progenitors. Acute deletion of Smarcd1 in murine adult hematopoiesis leads to lymphopenia, characterized by a near-complete absence of early lymphoid progenitors and mature B and T cells, while the myeloid and erythroid lineages remain unaffected. Mechanistically, we demonstrate that Smarcd1 is essential for the coordinated activation of a lymphoid gene signature in murine multipotent progenitors. This is achieved by interacting with the E2a transcription factor at proximal promoters and by regulating the activity of distal enhancers. Globally, these findings identify Smarcd1 as an essential chromatin remodeler that governs lymphoid cell fate.

Unravelling monocyte functions: from the guardians of health to the regulators of disease

Monocytes are a key component of the innate immune system. They undergo intricate developmental processes within the bone marrow, leading to diverse monocyte subsets in the circulation. In a state of healthy homeostasis, monocytes are continuously released into the bloodstream, destined to repopulate specific tissue-resident macrophage pools where they fulfil tissue-specific functions. However, under pathological conditions monocytes adopt various phenotypes to resolve inflammation and return to a healthy physiological state. This review explores the nuanced developmental pathways and functional roles that monocytes perform, shedding light on their significance in both physiological and pathological contexts.

Kmt2c restricts G-CSF-driven HSC mobilization and granulocyte production in a methyltransferase-independent manner

Granulocyte colony-stimulating factor (G-CSF) is widely used to enhance myeloid recovery after chemotherapy and to mobilize hematopoietic stem cells (HSCs) for transplantation. Unfortunately, through the course of chemotherapy, cancer patients can acquire leukemogenic mutations that cause therapy-related myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). This raises the question of whether therapeutic G-CSF might potentiate therapy-related MDS/AML by disproportionately stimulating mutant HSCs and other myeloid progenitors. A common mutation in therapy-related MDS/AML involves chromosome 7 deletions that inactivate many tumor suppressor genes, including KMT2C. Here, we show that Kmt2c deletions hypersensitize murine HSCs and myeloid progenitors to G-CSF, as evidenced by increased HSC mobilization and enhanced granulocyte production from granulocyte-monocyte progenitors (GMPs). Furthermore, Kmt2c attenuates the G-CSF response independently from its SET methyltransferase function. Altogether, the data raise concerns that monosomy 7 can hypersensitize progenitors to G-CSF, such that clinical use of G-CSF may amplify the risk of therapy-related MDS/AML.

Baf155 controls hematopoietic differentiation and regeneration through chromatin priming

Chromatin priming promotes cell-type-specific gene expression, lineage differentiation, and development. The mechanism of chromatin priming has not been fully understood. Here, we report that mouse hematopoietic stem and progenitor cells (HSPCs) lacking the Baf155 subunit of the BAF (BRG1/BRM-associated factor) chromatin remodeling complex produce a significantly reduced number of mature blood cells, leading to a failure of hematopoietic regeneration upon transplantation and 5-fluorouracil (5-FU) injury. Baf155-deficient HSPCs generate particularly fewer neutrophils, B cells, and CD8+ T cells at homeostasis, supporting a more immune-suppressive tumor microenvironment and enhanced tumor growth. Single-nucleus multiomics analysis reveals that Baf155-deficient HSPCs fail to establish accessible chromatin in selected regions that are enriched for putative enhancers and binding motifs of hematopoietic lineage transcription factors. Our study provides a fundamental mechanistic understanding of the role of Baf155 in hematopoietic lineage chromatin priming and the functional consequences of Baf155 deficiency in regeneration and tumor immunity.

Prenatal vitamin D deficiency exposure leads to long-term changes in immune cell proportions

Vitamin D deficiency is a common deficiency worldwide, particularly among women of reproductive age. During pregnancy, it increases the risk of immune-related diseases in offspring later in life. However, how the body remembers exposure to an adverse environment during development is poorly understood. Herein, we explore the effects of prenatal vitamin D deficiency on immune cell proportions in offspring using vitamin D deficient mice established by dietary manipulation. We found that prenatal vitamin D deficiency alters immune cell proportions in offspring by changing the transcriptional properties of genes downstream of vitamin D receptor signaling in hematopoietic stem and progenitor cells of both the fetus and adults. Moreover, further investigations of the associations between maternal vitamin D levels and cord blood immune cell profiles from 75 healthy pregnant women and their term offspring also confirm that maternal vitamin D levels in the second trimester significantly affect immune cell proportions in the offspring. These findings imply that the differentiation properties of hematopoiesis act as long-term memories of prenatal vitamin D deficiency exposure in later life.

Hematopoietic stem cell heterogeneity and age-associated platelet bias are evolutionarily conserved

Hematopoietic stem cells (HSCs) reconstitute multilineage human hematopoiesis after clinical bone marrow (BM) transplantation and are the cells of origin of some hematological malignancies. Although HSCs provide multilineage engraftment, individual murine HSCs are lineage biased and contribute unequally to blood cell lineages. Here, we performed high-throughput single-cell RNA sequencing in mice after xenograft with molecularly barcoded adult human BM HSCs. We demonstrated that human individual BM HSCs are also functionally and transcriptionally lineage biased. Specifically, we identified platelet-biased and multilineage human HSCs. Quantitative comparison of transcriptomes from single HSCs from young and aged BM showed that both the proportion of platelet-biased HSCs and their level of transcriptional platelet priming increase with age. Therefore, platelet-biased HSCs and their increased prevalence and transcriptional platelet priming during aging are conserved features of mammalian evolution.

Ing4-deficiency promotes a quiescent yet transcriptionally poised state in hematopoietic stem cells

Defining the mechanisms that regulate stem cell maintenance, proliferation, and differentiation is critical for identifying therapies for improving stem cell function under stress. Here, we have identified the tumor suppressor, inhibitor of growth 4 (Ing4), as a critical regulator of hematopoietic stem cell (HSC) homeostasis. Cancer cell line models with Ing4 deficiency have shown that Ing4 functions as a tumor suppressor, in part, due to Ing4-mediated regulation of several major signaling pathways, including c-Myc. In HSCs, we show Ing4 deficiency promotes gene expression signatures associated with activation, yet HSCs are arrested in G0, expressing several markers of quiescence. Functionally, Ing4-deficient HSCs demonstrate robust regenerative capacity following transplantation. Our findings suggest Ing4 deficiency promotes a poised state in HSCs, where they appear transcriptionally primed for activation but remain in a resting state. Our model provides key tools for further identification and characterization of pathways that control quiescence and self-renewal in HSCs.

Clonal analysis of fetal hematopoietic stem/progenitor cells reveals how post-transplantation capabilities are distributed

It has been proposed that adult hematopoiesis is sustained by multipotent progenitors (MPPs) specified during embryogenesis. Adult-like hematopoietic stem cell (HSC) and MPP immunophenotypes are present in the fetus, but knowledge of their functional capacity is incomplete. We found that fetal MPP populations were functionally similar to adult cells, albeit with some differences in lymphoid output. Clonal assessment revealed that lineage biases arose from differences in patterns of single-/bi-lineage differentiation. Long-term (LT)- and short-term (ST)-HSC populations were distinguished from MPPs according to capacity for clonal multilineage differentiation. We discovered that a large cohort of long-term repopulating units (LT-RUs) resides within the ST-HSC population; a significant portion of these were labeled using Flt3-cre. This finding has two implications: (1) use of the CD150+ LT-HSC immunophenotype alone will significantly underestimate the size and diversity of the LT-RU pool and (2) LT-RUs in the ST-HSC population have the attributes required to persist into adulthood.

DNA fork remodeling proteins, Zranb3 and Smarcal1, are uniquely essential for aging hematopoiesis

Over a lifetime, hematopoietic stem and progenitor cells (HSPCs) are forced to repeatedly proliferate to maintain hematopoiesis, increasing their susceptibility to DNA damaging replication stress. However, the proteins that mitigate this stress, protect HSPC replication, and prevent aging-driven dysregulation are unknown. We report two evolutionarily conserved, ubiquitously expressed chromatin remodeling enzymes with similar DNA replication fork reversal biochemical functions, Zranb3 and Smarcal1, have surprisingly specialized roles in distinct HSPC populations. While both proteins actively mitigate replication stress and prevent DNA damage and breaks during lifelong hematopoiesis, the loss of either resulted in distinct biochemical and biological consequences. Notably, defective long-term HSC function, revealed with bone marrow transplantation, caused hematopoiesis abnormalities in young mice lacking Zranb3. Aging significantly worsened these hematopoiesis defects in Zranb3-deficient mice, including accelerating the onset of myeloid-biased hematopoietic dysregulation to early in life. Such Zranb3-deficient HSPC abnormalities with age were driven by accumulated DNA damage and replication stress. Conversely, Smarcal1 loss primarily negatively affected progenitor cell functions that were exacerbated with aging, resulting in a lymphoid bias. Simultaneous loss of both Zranb3 and Smarcal1 compounded HSPC defects. Additionally, HSPC DNA replication fork dynamics had unanticipated HSPC type and age plasticity that depended on the stress and Zranb3 and/or Smarcal1. Our data reveal both Zranb3 and Smarcal1 have essential HSPC cell intrinsic functions in lifelong hematopoiesis that protect HSPCs from replication stress and DNA damage in unexpected, unique ways.

G2 arrest primes hematopoietic stem cells for megakaryopoiesis

In contrast to most hematopoietic lineages, megakaryocytes (MKs) can derive rapidly and directly from hematopoietic stem cells (HSCs). The underlying mechanism is unclear, however. Here, we show that DNA damage induces MK markers in HSCs and that G2 arrest, an integral part of the DNA damage response, suffices for MK priming followed by irreversible MK differentiation in HSCs, but not in progenitors. We also show that replication stress causes DNA damage in HSCs and is at least in part due to uracil misincorporation in vitro and in vivo. Consistent with this notion, thymidine attenuated DNA damage, improved HSC maintenance, and reduced the generation of CD41+ MK-committed HSCs. Replication stress and concomitant MK differentiation is therefore one of the barriers to HSC maintenance. DNA damage-induced MK priming may allow rapid generation of a lineage essential to immediate organismal survival, while also removing damaged cells from the HSC pool.

Sex-specific role of high-fat diet and stress on behavior, energy metabolism, and the ventromedial hypothalamus

BACKGROUND

Scientific evidence highlights the influence of biological sex on the relationship between stress and metabolic dysfunctions. However, there is limited understanding of how diet and stress concurrently contribute to metabolic dysregulation in both males and females. Our study aimed to investigate the combined effects of high-fat diet (HFD) induced obesity and repeated stress on fear-related behaviors, metabolic, immune, and hypothalamic outcomes in male and female mice.

METHODS

To investigate this, we used a highly reliable rodent behavioral model that faithfully recapitulates key aspects of post-traumatic stress disorder (PTSD)-like fear. We subjected mice to footshock stressor followed by a weekly singular footshock stressor or no stressor for 14 weeks while on either an HFD or chow diet. At weeks 10 and 14 we conducted glucose tolerance and insulin sensitivity measurements. Additionally, we placed the mice in metabolic chambers to perform indirect calorimetric measurements. Finally, we collected brain and peripheral tissues for cellular analysis.

RESULTS

We observed that HFD-induced obesity disrupted fear memory extinction, increased glucose intolerance, and affected energy expenditure specifically in male mice. Conversely, female mice on HFD exhibited reduced respiratory exchange ratio (RER), and a significant defect in glucose tolerance only when subjected to repeated stress. Furthermore, the combination of repeated stress and HFD led to sex-specific alterations in proinflammatory markers and hematopoietic stem cells across various peripheral metabolic tissues. Single-nuclei RNA sequencing (snRNAseq) analysis of the ventromedial hypothalamus (VMH) revealed microglial activation in female mice on HFD, while male mice on HFD exhibited astrocytic activation under repeated stress.

CONCLUSIONS

Overall, our findings provide insights into complex interplay between repeated stress, high-fat diet regimen, and their cumulative effects on health, including their potential contribution to the development of PTSD-like stress and metabolic dysfunctions, emphasizing the need for further research to fully understand these interconnected pathways and their implications for health.

Terminal deoxynucleotidyl transferase and CD84 identify human multi-potent lymphoid progenitors

Lymphoid specification in human hematopoietic progenitors is not fully understood. To better associate lymphoid identity with protein-level cell features, we conduct a highly multiplexed single-cell proteomic screen on human bone marrow progenitors. This screen identifies terminal deoxynucleotidyl transferase (TdT), a specialized DNA polymerase intrinsic to VDJ recombination, broadly expressed within CD34+ progenitors prior to B/T cell emergence. While these TdT+ cells coincide with granulocyte-monocyte progenitor (GMP) immunophenotype, their accessible chromatin regions show enrichment for lymphoid-associated transcription factor (TF) motifs. TdT expression on GMPs is inversely related to the SLAM family member CD84. Prospective isolation of CD84lo GMPs demonstrates robust lymphoid potentials ex vivo, while still retaining significant myeloid differentiation capacity, akin to LMPPs. This multi-omic study identifies human bone marrow lymphoid-primed progenitors, further defining the lympho-myeloid axis in human hematopoiesis.

Role of Neurotransmitters in Steady State Hematopoiesis, Aging, and Leukemia

Haematopoiesis within the bone marrow (BM) represents a complex and dynamic process intricately regulated by neural signaling pathways. This delicate orchestration is susceptible to disruption by factors such as aging, diabetes, and obesity, which can impair the BM niche and consequently affect haematopoiesis. Genetic mutations in Tet2, Dnmt3a, Asxl1, and Jak2 are known to give rise to clonal haematopoiesis of intermediate potential (CHIP), a condition linked to age-related haematological malignancies. Despite these insights, the exact roles of circadian rhythms, sphingosine-1-phosphate (S1P), stromal cell-derived factor-1 (SDF-1), sterile inflammation, and the complement cascade on various BM niche cells remain inadequately understood. Further research is needed to elucidate how BM niche cells contribute to these malignancies through neural regulation and their potential in the development of gene-corrected stem cells. This literature review describes the updated functional aspects of BM niche cells in haematopoiesis within the context of haematological malignancies, with a particular focus on neural signaling and the potential of radiomitigators in acute radiation syndrome. Additionally, it underscores the pressing need for technological advancements in stem cell-based therapies to alleviate the impacts of immunological stressors. Recent studies have illuminated the microheterogeneity and temporal stochasticity of niche cells within the BM during haematopoiesis, emphasizing the updated roles of neural signaling and immunosurveillance. The development of gene-corrected stem cells capable of producing blood, immune cells, and tissue-resident progeny is essential for combating age-related haematological malignancies and overcoming immunological challenges. This review aims to provide a comprehensive overview of these evolving insights and their implications for future therapeutic strategies.

A new model of human lymphopoiesis across development and aging

Over the past decade our research has implemented a multimodal approach to human lymphopoiesis, combining clonal-scale mapping of lymphoid developmental architecture with the monitoring of dynamic changes in the pattern of lymphocyte generation across ontogeny. We propose that lymphopoiesis stems from founder populations of CD127/interleukin (IL)7R- or CD127/IL7R+ early lymphoid progenitors (ELPs) polarized respectively toward the T-natural killer (NK)/innate lymphoid cell (ILC) or B lineages, arising from newly characterized CD117lo multi-lymphoid progenitors (MLPs). Recent data on the lifelong lymphocyte dynamics of healthy donors suggest that, after birth, lymphopoiesis may become increasingly oriented toward the production of B lymphocytes. Stemming from this, we posit that there are three major developmental transitions, the first occurring during the neonatal period, the next at puberty, and the last during aging.

GM-CSF receptor expression determines opposing innate memory phenotypes at different stages of myelopoiesis

ABSTRACT

Inflammatory responses must be tightly coordinated with the activation of emergency myelopoiesis to produce potent myeloid cells that fight infection without causing excessive host damage. Here, we show that granulocyte-macrophage colony-stimulating factor (GM-CSF) programs myeloid-committed progenitors to produce trained macrophages (increased cytokine response), but programs the upstream noncommitted LKS+ progenitors (defined as Lin- c-Kit+ Sca-1+ cells) to produce tolerized macrophages (decreased cytokine response). In myeloid progenitors, GM-CSF strongly activates signal transducer and activator of transcription 5 (STAT5), Ras-Raf-extracellular signal regulated kinase (ERK), and Akt-mTOR signaling pathways, which are essential to establish a training program, whereas in LKS+ progenitors, GM-CSF induces NF-κB translocation to the nucleus to establish a tolerization program. These differences arise from higher GM-CSF receptor expression in myeloid progenitors compared with LKS+ cells. We demonstrate that β-catenin regulation of NF-κB nuclear translocation is central in this process. In myeloid progenitors, glycogen synthase kinase 3 (GSK3) inactivation by strong ERK and phosphatidylinositol 3 kinase (PI3K)-Akt signaling increases cytoplasmic β-catenin levels to block NF-κB nuclear translocation. In contrast, when ERK and PI3K-Akt signaling are weak, active GSK3 causes a decrease in β-catenin, allowing NF-κB nuclear translocation in LKS+ progenitors. Finally, GM-CSF-induced LKS+ tolerization takes place in several murine models of trained immunity and in human CD34+ CD38- progenitors. Our study reveals that in addition to activating myelopoiesis, GM-CSF also programs early and immediate myeloid progenitors to produce opposing immune memory phenotypes. We propose that the inflammatory response from immediate myeloid progenitors may be balanced by the tolerized phenotype of early progenitors, thus providing a mechanism for appropriate resolution of inflammation and protection against a prolonged cytokine storm.

An Investigation of the Inflammatory Landscape in the Brain and Bone Marrow of the APP/PS1 Mouse

Background

The APP/PS1 mouse model recapitulates pathology of human Alzheimer's disease (AD). While amyloid-β peptide deposition and neurodegeneration are features of AD, the pathology may involve inflammation and impaired vascular regeneration.

Objective

This study evaluated inflammatory environments in the brain and bone marrow (BM), and the impact on brain microvascular density.

Methods

BM and frontal cortex from male nine-month-old APP/PS1 or the control C57Bl6/j mice were studied. Vascular density and inflammatory cells were evaluated in the sections of frontal cortex by immunohistochemistry. Different subsets of hematopoietic stem/progenitor cells (BM) and monocyte-macrophages were characterized by flow cytometry and by clonogenic assays. Myelopoietic or inflammatory factors were evaluated by real-time RT-PCR or by western blotting.

Results

CD34+ or CD31+ vascular structures were lower (p < 0.01, n = 6) in the frontal cortex that was associated with decreased number of Lin-Sca-1+cKit+ vasculogenic progenitor cells in the BM and circulation (p < 0.02, n = 6) compared to the control. Multipotent progenitor cells MPP4, common lymphoid, common myeloid and myeloid progenitor cells were higher in the APP/PS1-BM compared to the control, which agreed with increased numbers of monocytes and pro-inflammatory macrophages. The expression of pro-myelopoietic factors and alarmins was higher in the APP/PS1 BM-HSPCs or in the BM-supernatants compared to the control. Frontal cortices of APP/PS1 mice showed higher number of pro-inflammatory macrophages (CD11b+F4/80+ or CD80+) and microglia (OX42+Iba1+).

Conclusions

These findings show that AD pathology in APP/PS1 mice is associated with upregulated myelopoiesis, which contributes to the brain inflammation and decreased vascularity.

Acute malaria suppresses the B lymphocytic niche in the bone marrow through the alteration of CXCL12-abundant reticular cells

Bone marrow is a dynamic organ composed of stem cells that constantly receive signals from stromal cells and other hematopoietic cells in the niches of the bone marrow to maintain hematopoiesis and generate immune cells. Perturbation of the bone marrow microenvironment by infection and inflammation affects hematopoiesis and may affect immune cell development. Little is known about the effect of malaria on the bone marrow stromal cells that govern the hematopoietic stem cell (HSC) niche. In this study, we demonstrate that the mesenchymal stromal CXCL12-abundant reticular (CAR) cell population is reduced during acute malaria infection. The reduction of CXCL12 and interleukin-7 signals in the bone marrow impairs the lymphopoietic niche, leading to the depletion of common lymphoid progenitors, B cell progenitors, and mature B cells, including plasma cells in the bone marrow. We found that interferon-γ (IFNγ) is responsible for the upregulation of Sca1 on CAR cells, yet the decline in CAR cell and B cell populations in the bone marrow is IFNγ-independent. In contrast to the decline in B cell populations, HSCs and multipotent progenitors increased with the expansion of myelopoiesis and erythropoiesis, indicating a bias in the differentiation of multipotent progenitors during malaria infection. These findings suggest that malaria may affect host immunity by modulating the bone marrow niche.

Nucleoporin Nup358 drives the differentiation of myeloid-biased multipotent progenitors by modulating HDAC3 nuclear translocation

Nucleoporins, the components of nuclear pore complexes (NPCs), can play cell type- and tissue-specific functions. Yet, the physiological roles and mechanisms of action for most NPC components have not yet been established. We report that Nup358, a nucleoporin linked to several myeloid disorders, is required for the developmental progression of early myeloid progenitors. We found that Nup358 ablation in mice results in the loss of myeloid-committed progenitors and mature myeloid cells and the accumulation of myeloid-primed multipotent progenitors (MPPs) in bone marrow. Accumulated MPPs in Nup358 knockout mice are greatly restricted to megakaryocyte/erythrocyte-biased MPP2, which fail to progress into committed myeloid progenitors. Mechanistically, we found that Nup358 is required for histone deacetylase 3 (HDAC3) nuclear import and function in MPP2 cells and established that this nucleoporin regulates HDAC3 nuclear translocation in a SUMOylation-independent manner. Our study identifies a critical function for Nup358 in myeloid-primed MPP2 differentiation and uncovers an unexpected role for NPCs in the early steps of myelopoiesis.

An age-progressive platelet differentiation path from hematopoietic stem cells causes exacerbated thrombosis

Platelet dysregulation is drastically increased with advanced age and contributes to making cardiovascular disorders the leading cause of death of elderly humans. Here, we reveal a direct differentiation pathway from hematopoietic stem cells into platelets that is progressively propagated upon aging. Remarkably, the aging-enriched platelet path is decoupled from all other hematopoietic lineages, including erythropoiesis, and operates as an additional layer in parallel with canonical platelet production. This results in two molecularly and functionally distinct populations of megakaryocyte progenitors. The age-induced megakaryocyte progenitors have a profoundly enhanced capacity to engraft, expand, restore, and reconstitute platelets in situ and upon transplantation and produce an additional platelet population in old mice. The two pools of co-existing platelets cause age-related thrombocytosis and dramatically increased thrombosis in vivo. Strikingly, aging-enriched platelets are functionally hyper-reactive compared with the canonical platelet populations. These findings reveal stem cell-based aging as a mechanism for platelet dysregulation and age-induced thrombosis.

Prenatal vitamin D deficiency alters immune cell proportions of young adult offspring through alteration of long-term stem cell fates

Vitamin D deficiency is a common deficiency worldwide, particularly among women of reproductive age. During pregnancy, it increases the risk of immune-related diseases in offspring later in life. However, exactly how the body remembers exposure to an adverse environment during development is poorly understood. Herein, we explore the effects of prenatal vitamin D deficiency on immune cell proportions in offspring using vitamin D deficient mice established by dietary manipulation. We found that prenatal vitamin D deficiency alters immune cell proportions in offspring by changing the transcriptional properties of genes downstream of vitamin D receptor signaling in hematopoietic stem and progenitor cells of both the fetus and adults. Our results suggest the role of cellular differentiation properties of the hematopoiesis as the long-term memories of prenatal exposure at the adult stage. Moreover, further investigations of the associations between maternal vitamin D levels and cord blood immune cell profiles from 75 healthy pregnant women and their term babies also confirm that maternal vitamin D levels in the second trimester significantly affect immune cell proportions in the babies. This highlights the importance of providing vitamin D supplementation at specific stages of pregnancy.

Alternative platelet differentiation pathways initiated by nonhierarchically related hematopoietic stem cells

Rare multipotent stem cells replenish millions of blood cells per second through a time-consuming process, passing through multiple stages of increasingly lineage-restricted progenitors. Although insults to the blood-forming system highlight the need for more rapid blood replenishment from stem cells, established models of hematopoiesis implicate only one mandatory differentiation pathway for each blood cell lineage. Here, we establish a nonhierarchical relationship between distinct stem cells that replenish all blood cell lineages and stem cells that replenish almost exclusively platelets, a lineage essential for hemostasis and with important roles in both the innate and adaptive immune systems. These distinct stem cells use cellularly, molecularly and functionally separate pathways for the replenishment of molecularly distinct megakaryocyte-restricted progenitors: a slower steady-state multipotent pathway and a fast-track emergency-activated platelet-restricted pathway. These findings provide a framework for enhancing platelet replenishment in settings in which slow recovery of platelets remains a major clinical challenge.

Zeb1 maintains long-term adult hematopoietic stem cell function and extramedullary hematopoiesis

Emerging evidence implicates the epithelial-mesenchymal transition transcription factor Zeb1 as a critical regulator of hematopoietic stem cell (HSC) differentiation. Whether Zeb1 regulates long-term maintenance of HSC function remains an open question. Using an inducible Mx-1-Cre mouse model that deletes conditional Zeb1 alleles in the adult hematopoietic system, we found that mice engineered to be deficient in Zeb1 for 32 weeks displayed expanded immunophenotypically defined adult HSCs and multipotent progenitors associated with increased abundance of lineage-biased/balanced HSC subsets and augmented cell survival characteristics. During hematopoietic differentiation, persistent Zeb1 loss increased B cells in the bone marrow and spleen and decreased monocyte generation in the peripheral blood. In competitive transplantation experiments, we found that HSCs from adult mice with long-term Zeb1 deletion displayed a cell autonomous defect in multilineage differentiation capacity. Long-term Zeb1 loss perturbed extramedullary hematopoiesis characterized by increased splenic weight and a paradoxical reduction in splenic cellularity that was accompanied by HSC exhaustion, lineage-specific defects, and an accumulation of aberrant, preleukemic like c-kit+CD16/32+ progenitors. Loss of Zeb1 for up to 42 weeks can lead to progressive splenomegaly and an accumulation of Gr-1+Mac-1+ cells, further supporting the notion that long-term expression of Zeb1 suppresses preleukemic activity. Thus, sustained Zeb1 deletion disrupts HSC functionality in vivo and impairs regulation of extramedullary hematopoiesis with potential implications for tumor suppressor functions of Zeb1 in myeloid neoplasms.

Heart failure promotes multimorbidity through innate immune memory

Patients with heart failure (HF) often experience repeated acute decompensation and develop comorbidities such as chronic kidney disease and frailty syndrome. Although this suggests pathological interaction among comorbidities, the mechanisms linking them are poorly understood. Here, we identified alterations in hematopoietic stem cells (HSCs) as a critical driver of recurrent HF and associated comorbidities. Bone marrow transplantation from HF-experienced mice resulted in spontaneous cardiac dysfunction and fibrosis in recipient mice, as well as increased vulnerability to kidney and skeletal muscle insults. HF enhanced the capacity of HSCs to generate proinflammatory macrophages. In HF mice, global chromatin accessibility analysis and single-cell RNA-seq showed that transforming growth factor-β (TGF-β) signaling was suppressed in HSCs, which corresponded with repressed sympathetic nervous activity in bone marrow. Transplantation of bone marrow from mice in which TGF-β signaling was inhibited similarly exacerbated cardiac dysfunction. Collectively, these results suggest that cardiac stress modulates the epigenome of HSCs, which in turn alters their capacity to generate cardiac macrophage subpopulations. This change in HSCs may be a common driver of repeated HF events and comorbidity by serving as a key carrier of "stress memory."

Cell of origin epigenetic priming determines susceptibility to Tet2 mutation

Hematopoietic stem cell (HSC) mutations can result in clonal hematopoiesis (CH) with heterogeneous clinical outcomes. Here, we investigate how the cell state preceding Tet2 mutation impacts the pre-malignant phenotype. Using an inducible system for clonal analysis of myeloid progenitors, we find that the epigenetic features of clones at similar differentiation status are highly heterogeneous and functionally respond differently to Tet2 mutation. Cell differentiation stage also influences Tet2 mutation response indicating that the cell of origin's epigenome modulates clone-specific behaviors in CH. Molecular features associated with higher risk outcomes include Sox4 that sensitizes cells to Tet2 inactivation, inducing dedifferentiation, altered metabolism and increasing the in vivo clonal output of mutant cells, as confirmed in primary GMP and HSC models. Our findings validate the hypothesis that epigenetic features can predispose specific clones for dominance, explaining why identical genetic mutations can result in different phenotypes.

Bahcc1 is critical for the aberrant epigenetic program in a mouse model of MLL-ENL-mediated leukemia

ABSTRACT

In leukemogenesis, genotoxic stress in hematopoietic stem and progenitor cells (HSPCs) drives individual context-dependent programs of malignant transformation. In light of the various differentiation stages of HSPCs based on a recently revised definition using CD150/CD48, our analyses showed that a subpopulation of long-term repopulating HSCs was most susceptible to MLL-ENL-mediated transformation. An analysis of the molecular mechanism identified Bromo-adjacent homology domain and coiled-coil containing 1 (Bahcc1), which encodes a reader molecule of trimethylated histone H3 lysine 27 (H3K27me3), as a candidate gene involved in distinct susceptibility to leukemic transformation. Interestingly, Bahcc1 was previously reported to be highly expressed in acute myeloid leukemia (AML) with an unfavorable prognosis, including some cases of MLL-rearranged AML. We found that MLL-ENL upregulated Bahcc1 through binding to its promoter, and that Bahcc1 was involved in MLL-ENL-mediated immortalization at least partly through repression of H3K27me3-marked Cdkn1c. Analyses using bone marrow transplantation in mice showed that depletion of Bahcc1 suppressed the leukemogenic activity of MLL-ENL. In a public database, high BAHCC1 expression was found to be associated with a poor prognosis in pediatric AML, in which BAHCC1 expression was significantly lower in MLL-AF9-AML than in other MLL-fusion-AML. These findings shed light on the distinct immortalization potential of HSPCs and suggest a novel MLL-fusion-Bahcc1 axis, which may lead to development of molecular targeted therapy against MLL-fusion-mediated leukemia.

A single cell framework identifies functionally and molecularly distinct multipotent progenitors in adult human hematopoiesis

Hematopoietic multipotent progenitors (MPPs) regulate blood cell production to appropriately meet the biological demands of the human body. Human MPPs remain ill-defined whereas mouse MPPs have been well characterized with distinct immunophenotypes and lineage potencies. Using multiomic single cell analyses and complementary functional assays, we identified new human MPPs and oligopotent progenitor populations within Lin-CD34+CD38dim/lo adult bone marrow with distinct biomolecular and functional properties. These populations were prospectively isolated based on expression of CD69, CLL1, and CD2 in addition to classical markers like CD90 and CD45RA. We show that within the canonical Lin-CD34+CD38dim/loCD90CD45RA-MPP population, there is a CD69+ MPP with long-term engraftment and multilineage differentiation potential, a CLL1+ myeloid-biased MPP, and a CLL1-CD69-erythroid-biased MPP. We also show that the canonical Lin-CD34+CD38dim/loCD90-CD45RA+ LMPP population can be separated into a CD2+ LMPP with lymphoid and myeloid potential, a CD2-LMPP with high lymphoid potential, and a CLL1+ GMP with minimal lymphoid potential. We used these new HSPC profiles to study human and mouse bone marrow cells and observe limited cell type specific homology between humans and mice and cell type specific changes associated with aging. By identifying and functionally characterizing new adult MPP sub-populations, we provide an updated reference and framework for future studies in human hematopoiesis.

From Hematopoietic Stem Cells to Platelets: Unifying Differentiation Pathways Identified by Lineage Tracing Mouse Models

Platelets are the terminal progeny of megakaryocytes, primarily produced in the bone marrow, and play critical roles in blood homeostasis, clotting, and wound healing. Traditionally, megakaryocytes and platelets are thought to arise from multipotent hematopoietic stem cells (HSCs) via multiple discrete progenitor populations with successive, lineage-restricting differentiation steps. However, this view has recently been challenged by studies suggesting that (1) some HSC clones are biased and/or restricted to the platelet lineage, (2) not all platelet generation follows the "canonical" megakaryocytic differentiation path of hematopoiesis, and (3) platelet output is the default program of steady-state hematopoiesis. Here, we specifically investigate the evidence that in vivo lineage tracing studies provide for the route(s) of platelet generation and investigate the involvement of various intermediate progenitor cell populations. We further identify the challenges that need to be overcome that are required to determine the presence, role, and kinetics of these possible alternate pathways.

Hematopoietic stem cell quiescence and DNA replication dynamics maintained by the resilient β-catenin/Hoxa9/Prmt1 axis

ABSTRACT

Maintenance of quiescence and DNA replication dynamics are 2 paradoxical requirements for the distinct states of dormant and active hematopoietic stem cells (HSCs), which are required to preserve the stem cell reservoir and replenish the blood cell system in response to hematopoietic stress, respectively. Here, we show that key self-renewal factors, β-catenin or Hoxa9, largely dispensable for HSC integrity, in fact, have dual functions in maintaining quiescence and enabling efficient DNA replication fork dynamics to preserve the functionality of hematopoietic stem and progenitor cells (HSPCs). Although β-catenin or Hoxa9 single knockout (KO) exhibited mostly normal hematopoiesis, their coinactivation led to severe hematopoietic defects stemmed from aberrant cell cycle, DNA replication, and damage in HSPCs. Mechanistically, β-catenin and Hoxa9 function in a compensatory manner to sustain key transcriptional programs that converge on the pivotal downstream target and epigenetic modifying enzyme, Prmt1, which protects the quiescent state and ensures an adequate supply of DNA replication and repair factors to maintain robust replication fork dynamics. Inactivation of Prmt1 phenocopied both cellular and molecular phenotypes of β-catenin/Hoxa9 combined KO, which at the same time could also be partially rescued by Prmt1 expression. The discovery of the highly resilient β-catenin/Hoxa9/Prmt1 axis in protecting both quiescence and DNA replication dynamics essential for HSCs at different key states provides not only novel mechanistic insights into their intricate regulation but also a potential tractable target for therapeutic intervention.

Ascorbate depletion increases quiescence and self-renewal potential in hematopoietic stem cells and multipotent progenitors

Ascorbate (vitamin C) limits hematopoietic stem cell (HSC) function and suppresses leukemia development by promoting the function of the Tet2 tumor suppressor. In humans, ascorbate is obtained from the diet while in mice it is synthesized in the liver. In this study, we show that deletion of the Slc23a2 ascorbate transporter severely depleted ascorbate from hematopoietic cells. Slc23a2 deficiency increased HSC reconstituting potential and self-renewal potential upon transplantation into irradiated mice. Slc23a2 deficiency also increased the reconstituting and self-renewal potential of multipotent hematopoietic progenitors (MPPs), conferring the ability to long-term reconstitute irradiated mice. Slc23a2-deficient HSCs and MPPs divided much less frequently than control HSCs and MPPs. Increased self-renewal and reconstituting potential were observed particularly in quiescent Slc23a2-deficient HSCs and MPPs. The effect of Slc23a2 deficiency on MPP self-renewal was not mediated by reduced Tet2 function. Ascorbate thus regulates quiescence and restricts self-renewal potential in HSCs and MPPs such that ascorbate depletion confers MPPs with long-term self-renewal potential.

Inflammatory recruitment of healthy hematopoietic stem and progenitor cells in the acute myeloid leukemia niche

Inflammation in the bone marrow (BM) microenvironment is a constitutive component of leukemogenesis in acute myeloid leukemia (AML). Current evidence suggests that both leukemic blasts and stroma secrete proinflammatory factors that actively suppress the function of healthy hematopoietic stem and progenitor cells (HSPCs). HSPCs are also cellular components of the innate immune system, and we reasoned that they may actively propagate the inflammation in the leukemic niche. In two separate congenic models of AML we confirm by evaluation of the BM plasma secretome and HSPC-selective single-cell RNA sequencing (scRNA-Seq) that multipotent progenitors and long-lived stem cells adopt inflammatory gene expression programs, even at low leukemic infiltration of the BM. In particular, we observe interferon gamma (IFN-γ) pathway activation, along with secretion of its chemokine target, CXCL10. We show that AML-derived nanometer-sized extracellular vesicles (EVAML) are sufficient to trigger this inflammatory HSPC response, both in vitro and in vivo. Altogether, our studies indicate that HSPCs are an unrecognized component of the inflammatory adaptation of the BM by leukemic cells. The pro-inflammatory conversion and long-lived presence of HSPCs in the BM along with their regenerative re-expansion during remission may impact clonal selection and disease evolution.

Fetal liver macrophages contribute to the hematopoietic stem cell niche by controlling granulopoiesis

During embryogenesis, the fetal liver becomes the main hematopoietic organ, where stem and progenitor cells as well as immature and mature immune cells form an intricate cellular network. Hematopoietic stem cells (HSCs) reside in a specialized niche, which is essential for their proliferation and differentiation. However, the cellular and molecular determinants contributing to this fetal HSC niche remain largely unknown. Macrophages are the first differentiated hematopoietic cells found in the developing liver, where they are important for fetal erythropoiesis by promoting erythrocyte maturation and phagocytosing expelled nuclei. Yet, whether macrophages play a role in fetal hematopoiesis beyond serving as a niche for maturing erythroblasts remains elusive. Here, we investigate the heterogeneity of macrophage populations in the murine fetal liver to define their specific roles during hematopoiesis. Using a single-cell omics approach combined with spatial proteomics and genetic fate-mapping models, we found that fetal liver macrophages cluster into distinct yolk sac-derived subpopulations and that long-term HSCs are interacting preferentially with one of the macrophage subpopulations. Fetal livers lacking macrophages show a delay in erythropoiesis and have an increased number of granulocytes, which can be attributed to transcriptional reprogramming and altered differentiation potential of long-term HSCs. Together, our data provide a detailed map of fetal liver macrophage subpopulations and implicate macrophages as part of the fetal HSC niche.

Maternal inflammation regulates fetal emergency myelopoiesis

Neonates are highly susceptible to inflammation and infection. Here, we investigate how late fetal liver (FL) mouse hematopoietic stem and progenitor cells (HSPCs) respond to inflammation, testing the hypothesis that deficits in the engagement of emergency myelopoiesis (EM) pathways limit neutrophil output and contribute to perinatal neutropenia. We show that fetal HSPCs have limited production of myeloid cells at steady state and fail to activate a classical adult-like EM transcriptional program. Moreover, we find that fetal HSPCs can respond to EM-inducing inflammatory stimuli in vitro but are restricted by maternal anti-inflammatory factors, primarily interleukin-10 (IL-10), from activating EM pathways in utero. Accordingly, we demonstrate that the loss of maternal IL-10 restores EM activation in fetal HSPCs but at the cost of fetal demise. These results reveal the evolutionary trade-off inherent in maternal anti-inflammatory responses that maintain pregnancy but render the fetus unresponsive to EM activation signals and susceptible to infection.

Deep learning-based predictive classification of functional subpopulations of hematopoietic stem cells and multipotent progenitors

BACKGROUND

Hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs) play a pivotal role in maintaining lifelong hematopoiesis. The distinction between stem cells and other progenitors, as well as the assessment of their functions, has long been a central focus in stem cell research. In recent years, deep learning has emerged as a powerful tool for cell image analysis and classification/prediction.

METHODS

In this study, we explored the feasibility of employing deep learning techniques to differentiate murine HSCs and MPPs based solely on their morphology, as observed through light microscopy (DIC) images.

RESULTS

After rigorous training and validation using extensive image datasets, we successfully developed a three-class classifier, referred to as the LSM model, capable of reliably distinguishing long-term HSCs, short-term HSCs, and MPPs. The LSM model extracts intrinsic morphological features unique to different cell types, irrespective of the methods used for cell identification and isolation, such as surface markers or intracellular GFP markers. Furthermore, employing the same deep learning framework, we created a two-class classifier that effectively discriminates between aged HSCs and young HSCs. This discovery is particularly significant as both cell types share identical surface markers yet serve distinct functions. This classifier holds the potential to offer a novel, rapid, and efficient means of assessing the functional states of HSCs, thus obviating the need for time-consuming transplantation experiments.

CONCLUSION

Our study represents the pioneering use of deep learning to differentiate HSCs and MPPs under steady-state conditions. This novel and robust deep learning-based platform will provide a basis for the future development of a new generation stem cell identification and separation system. It may also provide new insight into the molecular mechanisms underlying stem cell self-renewal.

Made to order: emergency myelopoiesis and demand-adapted innate immune cell production

Definitive haematopoiesis is the process by which haematopoietic stem cells, located in the bone marrow, generate all haematopoietic cell lineages in healthy adults. Although highly regulated to maintain a stable output of blood cells in health, the haematopoietic system is capable of extensive remodelling in response to external challenges, prioritizing the production of certain cell types at the expense of others. In this Review, we consider how acute insults, such as infections and cytotoxic drug-induced myeloablation, cause molecular, cellular and metabolic changes in haematopoietic stem and progenitor cells at multiple levels of the haematopoietic hierarchy to drive accelerated production of the mature myeloid cells needed to resolve the initiating insult. Moreover, we discuss how dysregulation or subversion of these emergency myelopoiesis mechanisms contributes to the progression of chronic inflammatory diseases and cancer.

A mouse model of Zhu-Tokita-Takenouchi-Kim syndrome reveals indispensable SON functions in organ development and hematopoiesis

Rare diseases are underrepresented in biomedical research, leading to insufficient awareness. Zhu-Tokita-Takenouchi-Kim (ZTTK) syndrome is a rare disease caused by genetic alterations that result in heterozygous loss of function of SON. While patients with ZTTK syndrome live with numerous symptoms, the lack of model organisms hampers our understanding of SON and this complex syndrome. Here, we developed Son haploinsufficiency (Son+/-) mice as a model of ZTTK syndrome and identified the indispensable roles of Son in organ development and hematopoiesis. Son+/- mice recapitulated clinical symptoms of ZTTK syndrome, including growth retardation, cognitive impairment, skeletal abnormalities, and kidney agenesis. Furthermore, we identified hematopoietic abnormalities in Son+/- mice, including leukopenia and immunoglobulin deficiency, similar to those observed in human patients. Surface marker analyses and single-cell transcriptome profiling of hematopoietic stem and progenitor cells revealed that Son haploinsufficiency shifted cell fate more toward the myeloid lineage but compromised lymphoid lineage development by reducing genes required for lymphoid and B cell lineage specification. Additionally, Son haploinsufficiency caused inappropriate activation of erythroid genes and impaired erythropoiesis. These findings highlight the importance of the full gene expression of Son in multiple organs. Our model serves as an invaluable research tool for this rare disease and related disorders associated with SON dysfunction.

Resilient anatomy and local plasticity of naive and stress haematopoiesis

The bone marrow adjusts blood cell production to meet physiological demands in response to insults. The spatial organization of normal and stress responses are unknown owing to the lack of methods to visualize most steps of blood production. Here we develop strategies to image multipotent haematopoiesis, erythropoiesis and lymphopoiesis in mice. We combine these with imaging of myelopoiesis1 to define the anatomy of normal and stress haematopoiesis. In the steady state, across the skeleton, single stem cells and multipotent progenitors distribute through the marrow enriched near megakaryocytes. Lineage-committed progenitors are recruited to blood vessels, where they contribute to lineage-specific microanatomical structures composed of progenitors and immature cells, which function as the production sites for each major blood lineage. This overall anatomy is resilient to insults, as it was maintained after haemorrhage, systemic bacterial infection and granulocyte colony-stimulating factor (G-CSF) treatment, and during ageing. Production sites enable haematopoietic plasticity as they differentially and selectively modulate their numbers and output in response to insults. We found that stress responses are variable across the skeleton: the tibia and the sternum respond in opposite ways to G-CSF, and the skull does not increase erythropoiesis after haemorrhage. Our studies enable in situ analyses of haematopoiesis, define the anatomy of normal and stress responses, identify discrete microanatomical production sites that confer plasticity to haematopoiesis, and uncover unprecedented heterogeneity of stress responses across the skeleton.

Aryl hydrocarbon receptor activation alters immune cell populations in the lung and bone marrow during coronavirus infection

Respiratory viral infections are one of the major causes of illness and death worldwide. Symptoms associated with respiratory infections can range from mild to severe, and there is limited understanding of why there is large variation in severity. Environmental exposures are a potential causative factor. The aryl hydrocarbon receptor (AHR) is an environment-sensing molecule expressed in all immune cells. Although there is considerable evidence that AHR signaling influences immune responses to other immune challenges, including respiratory pathogens, less is known about the impact of AHR signaling on immune responses during coronavirus (CoV) infection. In this study, we report that AHR activation significantly altered immune cells in the lungs and bone marrow of mice infected with a mouse CoV. AHR activation transiently reduced the frequency of multiple cells in the mononuclear phagocyte system, including monocytes, interstitial macrophages, and dendritic cells in the lung. In the bone marrow, AHR activation altered myelopoiesis, as evidenced by a reduction in granulocyte-monocyte progenitor cells and an increased frequency of myeloid-biased progenitor cells. Moreover, AHR activation significantly affected multiple stages of the megakaryocyte lineage. Overall, these findings indicate that AHR activation modulates multiple aspects of the immune response to a CoV infection. Given the significant burden of respiratory viruses on human health, understanding how environmental exposures shape immune responses to infection advances our knowledge of factors that contribute to variability in disease severity and provides insight into novel approaches to prevent or treat disease.NEW & NOTEWORTHY Our study reveals a multifaceted role for aryl hydrocarbon receptor (AHR) signaling in the immune response to coronavirus (CoV) infection. Sustained AHR activation during in vivo mouse CoV infection altered the frequency of mature immune cells in the lung and modulated emergency hematopoiesis, specifically myelopoiesis and megakaryopoiesis, in bone marrow. This provides new insight into immunoregulation by the AHR and extends our understanding of how environmental exposures can impact host responses to respiratory viral infections.

Durable CD4+ T cell immunity: cherchez la stem

Mammalian stem cells govern development, tissue homeostasis, and regeneration. Following years of study, their functions have been delineated with increasing precision. The past decade has witnessed heightened widespread use of stem cell terminology in association with durable T cell responses to infection, antitumor immunity, and autoimmunity. Interpreting this literature is complicated by the fact that descriptions are diverse and criteria for labeling 'stem-like' T cells are evolving. Working under the hypothesis that conceptual frameworks developed for actual stem cells can be used to better evaluate and organize T cells described to have stem-like features, we outline widely accepted properties of stem cells and compare these to different 'stem-like' CD4+ T cell populations.