Hematopoietic stem and progenitor cells (HSPCs) are localized within specialized niches of the bone marrow (BM). However, during hematological disorders or infections, the functionality of these cells in the BM is compromised, leading to extramedullary hematopoiesis (EMH). Chronic inflammation drives EMH, yet its impact on HSPCs outside the BM is poorly understood. Using a mouse model of chronic autoinflammatory disease, we demonstrated the presence of extramedullary HSPCs in blood, spleen, and inflamed tails and paws. Single-cell transcriptomics revealed a unique expression profile in extramedullary HSCs, with significant up-regulation of Cd53, MHCII-associated, and immunosuppressive genes. We further demonstrated that extramedullary CD53+ HSPCs act as antigen-presenting cells, promoting the development of regulatory T cells (Treg cells) to control chronic inflammation at extramedullary sites. Conversely, Treg cells exert a protective role on extramedullary HSPCs. Together, our findings revealed a mutually supportive relationship between a unique subset of HSPCs and T cells in inflamed tissues during chronic inflammation.

Cytosolic redox balance is tightly coupled to aspartate synthesis through the malate-aspartate shuttle, and limiting the malate-aspartate shuttle has been proposed to constrain tumor growth by restricting aspartate availability. Here we show that tumors derived from cancer cells lacking GOT1 and GOT2, the cytosolic and mitochondrial aspartate aminotransferases essential for as-partate production and malate-aspartate shuttle function, grow despite impaired canonical as-partate synthesis. This is because cytosolic redox state, not aspartate supply, is the primary metabolic bottleneck in GOT1/GOT2 knockout cells. Using single-cell transcriptomics, metabo-lite tracing, and a loss-of-function CRISPR screen, we find that these tumors engage an adaptive bypass in which availability of asparagine, a product of aspartate, enables serine- and methio-nine-dependent transsulfuration to generate α-ketobutyrate, whose reduction regenerates cy-tosolic NAD⁺ and restores redox homeostasis. Pharmacological inhibition or genetic ablation of transsulfuration abrogates this asparagine-driven rescue. These findings define asparagine as a regulator of cytosolic NAD⁺/NADH balance and reveal a link between amino acid metabolism and redox control that suggests transsulfuration as a targetable vulnerability in tumor redox maintenance. Significance statement:Aspartate synthesis and cytosolic redox balance are both coupled through the malate-aspartate shuttle. We show that the cytosolic NAD⁺/NADH ratio, not aspartate supply, is a critical output of the malate-aspartate shuttle for tumor growth. Availability of asparagine, a product of aspar-tate, enables serine- and methionine-dependent transsulfuration to restore cytosolic NAD⁺/NADH balance, proliferation and tumor growth independently of canonical aspartate pro-duction by the malate-aspartate shuttle. This defines asparagine as a regulator of cytosolic re-dox and identifies transsulfuration as a targetable vulnerability in tumor redox maintenance.

Bone lengthening and fracture repair depend on the anabolic properties of chondrocytes that function in an avascular milieu. The limited supply of oxygen and nutrients calls into question how biosynthesis and redox homeostasis are guaranteed. Here we show that glucose metabolism by the pentose phosphate pathway (PPP) is essential for endochondral ossification. Loss of glucose-6-phosphate dehydrogenase in chondrocytes does not affect cell proliferation because reversal of the non-oxidative PPP produces ribose-5-phosphate. However, the decreased NADPH production reduces glutathione recycling, resulting in decreased protection against the reactive oxygen species (ROS) produced during oxidative protein folding. The disturbed proteostasis activates the unfolded protein response and protein degradation. Moreover, the oxidative stress induces ferroptosis, which, together with altered matrix properties, results in a chondrodysplasia phenotype. Collectively, these data show that in hypoxia, the PPP is crucial to produce reducing power that confines ROS generated by oxidative protein folding and thereby controls proteostasis and prevents ferroptosis. Loopmans et al. show that the pentose phosphate pathway in chondrocytes provides reducing power to ensure proteostasis necessary for bone lengthening.

Emergency granulopoiesis is a critical process by which hematopoietic progenitors and stem cells facilitate enhanced granulocytic production during severe infections. However, the role of distinct multipotent progenitors (MPPs) at early stages of this process remains underexplored. Here, we investigated the contribution of MPPs to granulocytic production following lipopolysaccharide (LPS) administration in wild-type mice, simulating a bacterial infection. Transplantation assays demonstrated that LPS exposure reduces the engraftment capacity of lymphoid-biased MPP4 and enhances lymphoid production, rather than supporting myeloid lineage output. Further, single-cell RNA sequencing (scRNA-seq) of MPPs isolated from control and LPS-challenged mice revealed transcriptional reprogramming of nonlineage committed MPPs toward myeloid- and erythroid-biased progenitors. Notably, inflammatory progenitor populations emerged on activation of LPS-induced emergency granulopoiesis, displaying chromatin accessibility changes that align with a commitment to myeloid and erythroid fates. Pseudotime analysis elucidated cellular trajectories that suggest a developmental pathway where unbiased progenitors, present under nonstress conditions, transition toward myeloid and erythroid lineage outputs on LPS administration. In line with our functional MPP4 assessment, scRNA-seq suggested that lymphoid-biased progenitors do not transcriptionally rewire during early stages of emergency granulopoiesis. Collectively, our data highlight the critical role of specific MPP subsets in responding to LPS-induced inflammatory signals and underscore the dynamic adaptations that occur during granulocyte production in response to infection.

Hematopoietic stem cells (HSCs) are localized within specialized niches of the bone marrow (BM). However, during hematological disorders or infections, the functionality of HSCs in the BM is compromised, leading to extramedullary hematopoiesis (EMH). Chronic inflammation drives EMH, yet its impact on HSCs outside the BM is poorly understood. Using a mouse model of chronic autoinflammatory disease, we demonstrated the presence of extramedullary HSCs in blood, spleen, and inflamed tails and paws. Single-cell transcriptomics revealed a unique expression profile in extramedullary HSCs, with significant upregulation of Cd53, MHCII-associated, and immunosuppressive genes. We further demonstrated that extramedullary CD53+HSCs act as antigen-presenting cells, promoting the development of regulatory T cells (Tregs) to control chronic inflammation at extramedullary sites. Conversely, Tregs exert a protective role on extramedullary HSCs. Altogether, our findings revealed a mutually supportive relationship between a unique subset of HSCs and T cells in inflamed tissues during chronic inflammation. ### Competing Interest Statement The authors have declared no competing interest.

Increasing evidence highlights macrophages as critical players in tumorigenesis and therapeutic resistance in both solid and liquid tumors. While macrophages in solid cancers are largely pro-tumor and associated with poor prognosis, their contribution to acute myeloid leukemia (AML) pathogenesis—a deadly cancer of myeloid origin—remains controversial. To elucidate precise role of macrophages in AML, we analyzed AML patient dataset (BeatAML) and revealed that high macrophage gene signatures correlated with poor survival outcomes (n=57). We demonstrated that macrophages promote chemoresistance to standard-of-care chemotherapy drug cytarabine (AraC) in AML, as evidenced by the increased sensitivity to AraC following macrophage depletion in AML patient-derived xenograft (PDX) models and mouse syngeneic leukemia models. To identify direct contribution of macrophages in mediating chemoresistance, we conducted in vitro coculture experiments and drug screen (n=31 drugs) with bone-marrow derived macrophage conditioned media (BMDM-CM). In vitro BMDM-CM fractionation experiments revealed that the resistance is primarily mediated by soluble factors (<3kDa) secreted by macrophages. Functional genetic screening using genome-wide CRISPR Knock-out studies identified deoxycytidine kinase (DCK) as a critical target of soluble factors. Metabolite profiling pinpointed deoxycytidine (dC), a pyrimidine metabolite, as the primary soluble factor secreted by macrophages that inhibited DCK activity in AML cells to drive AraC resistance. Intriguingly, single-cell sequencing of healthy and AML samples, and cancer-wide Dependency Map (DepMap) analysis identified that SAM and HD domain-containing protein 1 (SAMHD1) causes dC accumulation. Inhibition of SAMHD1 and dihydroorotate dehydrogenase (DHODH) blocked dC biogenesis and reversed AraC resistance mediated by macrophages in vitro. Further, single cell RNA sequencing analysis of paired AML patient samples revealed an enrichment of SAMHD1high macrophages/monocytes in patient bone marrow after AraC-based chemotherapies. Since SAMHD1 is an interferon-stimulated gene, we hypothesize that the inflammatory state in the bone marrow microenvironment could modulate SAMHD1 expression. Consistent with this hypothesis, the inflammatory level is indeed correlated with SAMHD1 expression in primary AML samples according to BeatAML patient database, implying that inflammation might give rise to AraC resistance by modulating SAMHD1 in immune microenvironment. Abstract Collectively, our findings uncover a novel mechanism of AraC resistance in AML mediated by macrophage-derived dC. We propose that targeting dC metabolism in macrophages could be a potential strategy to overcome chemoresistance and improve outcomes for AML patients. Chuqi Wang, Yuhan Wang, Camillo Benetti, Karanpreet Bhatia, Xiao Xian Lin, Petra Hyroššová, Edward Ayoub, Jakub Rohlena, Katerina Rohlenova, Michael Andreeff, Shruti Bhatt. SAMHD1high macrophages drive resistance to cytarabine in acute myeloid leukemia (AML) by pyrimidine metabolite deoxycytidine (dC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6381.

The tumor microenvironment (TME) programs cancer cells to influence therapeutic responses. Macrophages residing in TME switch from pro-phagocytic to tumor-promoting and immunosuppressive phenotypes as cancer develops. While these pro-tumor functions of macrophages are associated with poor outcomes, the underlying mechanisms by which bone-marrow (BM)-associated macrophages fuel myeloid malignancy and their precise contribution to relapse remain undissected. Here, we show expansion of monocyte/macrophage population in leukemia patients post-chemotherapy relapse, and spatial proximity of macrophages to leukemia blasts in the BM niche. This proximity proved functionally consequential—depletion of macrophages delayed leukemia relapse post cytarabine (AraC), a frontline chemotherapy, in patient-derived xenografts (PDX) and syngeneic leukemia models. Mechanistically, a pyrimidine metabolite, deoxycytidine (dC), secreted by BM macrophages, is taken up by leukemia cells to directly inhibit deoxycytidine kinase (DCK) to hamper AraC activation and subsequent resistance in a cell non-autonomous manner. Diagnosis AML patients exhibited significantly higher circulating dC levels than healthy donors, and dC levels further increased following chemotherapy. SAMHD1, which catalyzes deoxynucleoside triphosphates (dNTPs) into deoxynucleoside, was highly abundant in macrophages and mediated dC accumulation. Blockade of dC production in mouse and human macrophages via genetic and pharmacological inhibition of SAMHD1 or DHODH , a critical enzyme in pyrimidine synthesis, restored AraC sensitivity. Combination with DHODH inhibitors significantly delayed AraC relapse in human PDX and mouse syngeneic AML models. Collectively, we identify a metabolic immune–leukemia crosstalk in which SAMHD1 high macrophages mediates chemoresistance by secreting pyrimidine metabolites and propose macrophage metabolic reprogramming as a tractable strategy to overcome TME-driven chemoresistance in myeloid leukemia. ### Competing Interest Statement The authors have declared no competing interest.

Aspartate is a proteinogenic non-essential amino acid with several essential functions in proliferating cells. It is mostly produced in a cell autonomous manner from oxalacetate via glutamate oxalacetate transaminases 1 or 2 (GOT1 or GOT2), but in some cases it can also be salvaged from the microenvironment via transporters such as SLC1A3 or by macropinocytosis. In this review we provide an overview of biosynthetic pathways that produce aspartate endogenously during proliferation. We discuss conditions that favor aspartate uptake as well as possible sources of exogenous aspartate in the microenvironment of tumors and bone marrow, where most available data have been generated. We highlight metabolic fates of aspartate, its various functions, and possible approaches to target aspartate metabolism for cancer therapy.

β-catenin-TCF/LEF-mediated transcription in hematopoietic stem and progenitor cells (HSPCs) is critical for myeloid differentiation during bacterial infections, where increased numbers of granulocytes are rapidly generated in a process known as emergency granulopoiesis (EG). The β-catenin-TCF/LEF pathway is activated by Wnt ligands, a family of secreted glycoproteins accounting for 19 members, both in human and mice. Nevertheless, whether some of these ligands are secreted during infection and regulate emergency granulopoiesis is largely unknown. Here, we employed Wls conditional KO mice, in which the secretion of all Wnt ligands is blocked upon tamoxifen administration. We observed a severe impairment of EG upon lipopolysaccharide (LPS) treatment at the level of multipotent progenitors, which were not able to execute the lymphoid-to-myeloid bias switch. scRNAseq analysis of HSCs isolated from mice challenged with LPS or PBS control revealed that Wnt10b was the only Wnt ligand upregulated in a myeloid-biased HSC population exclusive of the LPS-treated mice. Next, we confirmed that Wnt10b is released in the serum and in the bone marrow following LPS treatment in vivo using ELISA and whole mount microscopy. We generated Wnt10b KO mice and observed impaired myeloid differentiation in steady-state conditions. Accordingly, in vitro stimulation of WT HSPCs with recombinant Wnt10b led to decrease in proliferation and increased myeloid differentiation. Finally, EG response in these mice was impaired in a similar manner to the Wls mice, albeit to a lesser extent. Altogether, our data shows that proper secretion of Wnt ligands is crucial for EG and that at least part of the response is mediated by Wnt10b, which acts by blocking proliferation and inducing myeloid differentiation. This work was supported by GACR 22-18300S, GAUK 327722, and IMG institutional funding RVO68378050.

BACKGROUND:Amplification of HER2, a receptor tyrosine kinase and a breast cancer-linked oncogene, is associated with aggressive disease. HER2 protein is localised mostly at the cell membrane, but a fraction translocates to mitochondria. Whether and how mitochondrial HER2 contributes to tumorigenicity is currently unknown.METHODS:We enriched the mitochondrial (mt-)HER2 fraction in breast cancer cells using an N-terminal mitochondrial targeting sequence and analysed how this manipulation impacts bioenergetics and tumorigenic properties. The role of the tyrosine kinase activity of mt-HER2 was assessed in wild type, kinase-dead (K753M) and kinase-enhanced (V659E) mtHER2 constructs.RESULTS:We document that mt-HER2 associates with the oxidative phosphorylation system, stimulates bioenergetics and promotes larger respiratory supercomplexes. mt-HER2 enhances proliferation and invasiveness in vitro and tumour growth and metastatic potential in vivo, in a kinase activity-dependent manner. On the other hand, constitutively active mt-HER2 provokes excessive mitochondria ROS generation, sensitises to cell death, and restricts growth of primary tumours, suggesting that regulation of HER2 activity in mitochondria is required for the maximal pro-tumorigenic effect.CONCLUSIONS:mt-HER2 promotes tumorigenicity by supporting bioenergetics and optimal redox balance.

In steady-state hematopoiesis, hematopoietic stem cells (HSCs) reside in specialized niches of the bone marrow (BM). Nevertheless, during hematological disorders or infections, HSC function in the BM is diminished, and there is a need to create new sites of hematopoiesis. This compensatory mechanism is known as extramedullary hematopoiesis (EMH). Remarkably, chronic inflammation promotes EMH, but the impact of chronic inflammation on HSCs outside the BM is poorly understood. Here, using mice suffering from a progressive chronic autoinflammatory disease described as chronic multifocal osteomyelitis (CMO), we observed increased numbers of functional HSCs in blood, spleen, and inflamed paws of CMO mice compared to WT controls. Single cell transcriptomics revealed that HSCs in CMO EMH sites have a unique expression profile, characterized by upregulated Cd53 that distinguishes HSCs from BM HSCs in WT or CMO mice. This upregulation correlates positively with MHCII-associated and immunosuppressive genes such as Cd274 (PD-L1) and Icosl. We observed that CD53+ HSCs isolated from CMO EMH sites displayed lower proliferation but enhanced myeloid colony-forming capacity in comparison to CD53- HSCs. Further, CD53+Lin-c-kit+ cells isolated from CMO EMH sites strongly induced proliferation and survival of T cells in comparison to CD53-Lin-c-kit+ cells. Additionally, CD53+Lin-c-kit+ also promoted the development of Tregs in vitro. Altogether, our findings revealed a unique type of EMH HSCs characterized by enhanced expression of CD53, MHCII, and immunosuppressive genes. Notably, under chronic inflammation, EMH CD53+ HSCs might act as antigen presenting cells inducing naive T cell proliferation and the development of Tregs, suggesting an immunoregulatory role at the periphery. This work was supported by GACR24-10938S and IMG institutional funding RVO68378050.

Abstract The tumor microenvironment plays a crucial role in cancer progression and treatment response. Systemically administered metabolic cancer therapies target not only malignant but also stromal cells, including immune cells - the only stromal population previously addressed in this context. However, how such treatments impact other non-malignant cell types in tumors remains poorly understood. Here we show that inhibition of de novo pyrimidine synthesis in endothelial cells accelerates tumor growth and alters tumor immune repertoire. We found that whole-body de novo pyrimidine synthesis deficiency in mice, caused by the inducible whole-body ablation of DHODH, accelerates the growth rate of orthotopic lung tumors. Single-cell transcriptomic analysis of tumor-bearing lungs revealed that DHODH deficiency in the stroma impacts multiple cell populations, including immune and, surprisingly, endothelial cells. To explore the endothelial-specific effects, we generated a mouse model with inducible DHODH deficiency restricted to endothelial cells. The endothelium-specific model recapitulates the accelerated lung tumor growth observed in the whole-body DHODH deficiency model. Single-cell transcriptomics analysis of tumor-bearing lungs in the endothelium-specific model pointed to changes in the immune repertoire, particularly an enrichment of monocytes. We confirmed these results on the protein level using spectral flow cytometry, and we are currently in the process of uncovering the mechanism by which endothelial deficiency of pyrimidine synthesis affects the immune landscape of tumors. Inhibitors of pyrimidine de novo synthesis have been tested in clinical trials but failed due to their low efficiency. Our findings indicate that systemic treatment targeting pyrimidine synthesis may be hampered by its pro-tumorigenic effects in the endothelium, highlighting the unexpected role of endothelial metabolism in this context. Citation Format: Petra Hyrossova, Isidora Milisav, Silvia Novais, Mirko Milosevic, Jakub Rohlena, Katerina Rohlenova. Endothelial pyrimidine synthesis deficiency promotes tumor growth [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor-body Interactions: The Roles of Micro- and Macroenvironment in Cancer; 2024 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(22_Suppl):Abstract nr A018.

Emergency granulopoiesis (EG) is the process responsible for the rapid and enhanced production of granulocytes during acute infections. Traditionally, EG was understood as a cellular mechanism mostly initiated and supported by myeloid progenitors, however, recent data suggest that hematopoietic stem cells (HSCs) might also participate in the process. To investigate whether and how HSCs contribute to the initial steps of EG, we performed scRNA-seq and ATAC-seq analysis of sorted murine HSCs 4 hours after in vivo lipopolysaccharide (LPS) administration, mimicking an acute bacterial infection. Strikingly, we observed radical transcriptional changes between HSCs isolated from PBS control and LPS treated mice, mostly marked by alterations in HSC lineage bias. We identified a steady state lymphoid-biased subpopulation of HSCs, marked by expression of Procr (CD201). Following LPS stimulation, the CD201 expression was lost and this population was transcriptionally rewired to a myeloid-biased HSC population. Accordingly, ATACseq data corroborated opening of myeloid-bias loci upon LPS administration rather than steady-state lymphoid-biased loci. Further, we confirmed the loss of CD201 expression in HSCs by flow cytometry in mice after LPS and G-CSF treatment, as well as Candida albicans infection, suggesting that the loss of CD201 expression is a general event during EG induced by different stimuli. Interestingly, the CD201 downregulation in HSCs was independent of the master regulator of EG C/EBPβ, while we observed the contribution of the TLR4-MyD88 signaling axis. Next, we functionally validated our scRNA-seq results in vitro and in vivo. When cultivated under myeloid differentiation conditions, the CD201- HSCs gave rise to mature granulocytes, whereas CD201+ HSCs remained rather immature. In transplantation settings, CD201+ HSCs showed increased engraftment and preferential bias to produce lymphoid cells, while CD201- HSC showed decreased engraftment ability and a bias towards myeloid production. Surprisingly, LPS challenge of BM chimeras transplanted with lymphoid-biased CD201+ HSCs led to EG response marked by rapid loss of lymphoid-biased multipotent progenitors (MPPs) and the expansion of myeloid-biased MPPs, thus confirming the lympho-myeloid switch observed in the scRNA-seq data. Interestingly, the treatment of WT mice with CD201 blocking antibody was able to partially impair the lympho-myeloid switch, suggesting that CD201 plays an active role in HSCs during EG. Mechanistically, we observed that both CD201+ and CD201- HSCs rely on different signaling pathways under EG. While CD201+ HSCs express higher levels of TLR4 and have higher activation of NF-κB signaling upon in vitro LPS stimulation, the CD201- HSCs express higher levels of G-CSF-R and have higher activation of pSTAT3 signaling upon G-CSF-stimulation. Moreover, while the LIP isoform of C/EBPβ, important for cell proliferation, is present in both CD201+ and CD201- HSCs, the LAP/LAP* isoform, important for myeloid differentiation, is present only in the CD201- HSCs fraction. Altogether, our data suggest that EG is supported by a population of HSCs which upon pathogen sensing undergo a radical transcriptional rewiring that promotes their myeloid output. Initially, the pathogen is directly sensed by TLR4 on the surface of a steady state lymphoid-biased CD201+ HSCs, causing a rapid activation of the downstream NF-κB signaling pathway. Subsequently, the lymphoid-myeloid transcriptional switch, marked by the loss of CD201 expression, occurs and EG is then supported by myeloid-biased CD201- HSCs. CD201- HSCs respond to the infection in an indirect manner through G-CSF-R on their surface and exhibit enhanced pSTAT3 activation and elevated LAP/LAP* C/EBPβ isoform, cellular mechanisms known to promote myeloid differentiation and granulocytic production. In conclusion, the switch from CD201+ to CD201- HSCs facilitates both fast and sustained EG leading to the supply of new granulocytes to fight the infection. This detailed understanding of the distinct cellular, transcriptional, and mechanistic properties that determine HSC fate during emergency granulopoiesis opens new venues to potentially modulate granulocytic production in certain clinical conditions such as sepsis. This work was partially supported by a GACR 22-18300S, GAUK 327722, and IMG institutional funding RVO 68378050.

AIMS:Severe acute respiratory syndrome coronavirus-2 infection causes COVID-19, which in severe cases evokes life-threatening acute respiratory distress syndrome (ARDS). Transcriptome signatures and the functional relevance of non-vascular cell types (e.g. immune and epithelial cells) in COVID-19 are becoming increasingly evident. However, despite its known contribution to vascular inflammation, recruitment/invasion of immune cells, vascular leakage, and perturbed haemostasis in the lungs of severe COVID-19 patients, an in-depth interrogation of the endothelial cell (EC) compartment in lethal COVID-19 is lacking. Moreover, progressive fibrotic lung disease represents one of the complications of COVID-19 pneumonia and ARDS. Analogous features between idiopathic pulmonary fibrosis (IPF) and COVID-19 suggest partial similarities in their pathophysiology, yet, a head-to-head comparison of pulmonary cell transcriptomes between both conditions has not been implemented to date. METHODS AND RESULTS:We performed single-nucleus RNA-sequencing on frozen lungs from 7 deceased COVID-19 patients, 6 IPF explant lungs, and 12 controls. The vascular fraction, comprising 38 794 nuclei, could be subclustered into 14 distinct EC subtypes. Non-vascular cell types, comprising 137 746 nuclei, were subclustered and used for EC-interactome analyses. Pulmonary ECs of deceased COVID-19 patients showed an enrichment of genes involved in cellular stress, as well as signatures suggestive of dampened immunomodulation and impaired vessel wall integrity. In addition, increased abundance of a population of systemic capillary and venous ECs was identified in COVID-19 and IPF. COVID-19 systemic ECs closely resembled their IPF counterparts, and a set of 30 genes was found congruently enriched in systemic ECs across studies. Receptor-ligand interaction analysis of ECs with non-vascular cell types in the pulmonary micro-environment revealed numerous previously unknown interactions specifically enriched/depleted in COVID-19 and/or IPF. CONCLUSIONS:This study uncovered novel insights into the abundance, expression patterns, and interactomes of EC subtypes in COVID-19 and IPF, relevant for future investigations into the progression and treatment of both lethal conditions.

Emergency granulopoiesis is the enhanced and accelerated production of granulocytes that occurs during acute infection. The contribution of hematopoietic stem cells (HSCs) to this process was reported; however, how HSCs participate in emergency granulopoiesis remains elusive. Here, using a mouse model of emergency granulopoiesis we observe transcriptional changes in HSCs as early as 4 h after lipopolysaccharide (LPS) administration. We observe that the HSC identity is changed towards a myeloid‐biased HSC and show that CD201 is enriched in lymphoid‐biased HSCs. While CD201 expression under steady‐state conditions reveals a lymphoid bias, under emergency granulopoiesis loss of CD201 marks the lymphoid‐to‐myeloid transcriptional switch. Mechanistically, we determine that lymphoid‐biased CD201+ HSCs act as a first response during emergency granulopoiesis due to direct sensing of LPS by TLR4 and downstream activation of NF‐κΒ signaling. The myeloid‐biased CD201− HSC population responds indirectly during an acute infection by sensing G‐CSF, increasing STAT3 phosphorylation, and upregulating LAP/LAP* C/EBPβ isoforms. In conclusion, HSC subpopulations support early phases of emergency granulopoiesis due to their transcriptional rewiring from a lymphoid‐biased to myeloid‐biased population and thus establishing alternative paths to supply elevated numbers of granulocytes.