Hematopoietic progenitor cells as integrative hubs for adaptation to and fine-tuning of inflammation

Adaptation of hematopoietic progenitor cells to inflammation

Similar to the mature immune cells and committed myeloid progenitor cells in the BM, HSPCs can also become directly responsive to acute infection or chronic inflammatory conditions^1,2,27. This HSPC adaptation is tightly orchestrated by a combination of cell-intrinsic (transcriptional, epigenetic and metabolic) and cell-extrinsic (soluble growth factors, cytokines, microbial ligands and adhesive interactions) mechanisms^2,28. HSPCs are more sensitive to exogenous stimuli than traditionally thought, owing to their expression of receptors for microbial products, such as the Toll-like receptors (TLRs) and receptors for inflammatory cytokines and growth factors, such as IL-1β, IL-6, M-CSF, type 1 and 2 interferons (IFN), the abundance of which massively increases upon systemic infection, thereby enabling HSPCs to sense a systemic infectious or inflammatory challenge directly or indirectly^1,2. While such HSPC responses can be beneficial in promoting elimination of an infection, they may also cause impairment of HSPC function and exhaustion if HSPC activation is chronically sustained² or may contribute to chronicity of inflammatory pathologies²⁷. The underlying mechanisms and functional outcomes of HSPC adaptation to inflammatory stimuli have only now begun to be understood. Below we discuss the complex regulatory interactions at the interface between inflammation and HSCs.

Effects of TLR signaling on HSC fate decision and regulation of myelopoiesis

The seminal observation that hematopoietic progenitors express TLRs and that TLR ligation induces myeloid differentiation in a manner dependent on the adaptor MyD88 provided new insight in the role of HSPCs in the host defense against pathogens²⁹. Subsequent studies have identified indirect, cytokine-mediated, and direct effects of TLR ligands on HSPCs (Fig.1). Upon activation of the TLRs, ST-HSCs and MPPs produce substantial amounts of cytokines in an manner dependent on the transcription factor NF-κB³⁰. These cytokines, particularly IL-6, can promote, in a paracrine fashion, the proliferation and myeloid differentiation of HSPCs³⁰. Additionally, systemic LPS administration stimulates enhanced HSC proliferation³¹, while chronic low-dose LPS treatment results in expansion of HSCs and their myeloid skewing in a cell-intrinsic, TLR4-dependent manner, as shown in BM chimera experiments using cells from TLR4-deficient and -proficient mice³². However, chronic low-dose murine endotoxemia induces the functional impairment of HSCs and loss of quiescence, as shown by their reduced repopulation capacity in serial transplantation experiments³³. Moreover, high-dose LPS results in increased BM cell death and dysfunction of HSCs, despite their initial expansion³⁴.

Hence, TLR4 activation on HSCs stimulates their proliferation, but may exert several detrimental actions on their function due to proliferative stress in HSCs upon TLR4 ligation^33,35. Specifically, TLR4 activation stimulates the proliferation of dormant HSCs, while diminishing their self-renewal and repopulation capacity³⁶. These effects are mediated by signaling dependent on the adaptor TRIF and subsequent downstream production of reactive oxygen species (ROS) and activation of the mitogen-activated protein kinase p38, leading to replication stress and activation of the DNA repair machinery³⁶. Scavenging ROS or blockade of p38 prevents this endotoxemia-related HSC exhaustion³⁶. Consistently, TRIF mediates the expansion and functional exhaustion of HSPCs in sepsis³⁵.

Different TLR ligands may lead to different emergency myelopoiesis responses by engaging distinct pathways leading to the generation of Ly6C^hi monocytes³⁷. Specifically, stimulation of TLR4 by LPS causes an increase in neutrophils and neutrophil-like monocytes derived from GMPs, while stimulation of TLR9 by CpG results in expansion of dendritic cells and of a functionally distinct monocyte subtype derived from monocyte-dendritic cell progenitors³⁷. Besides the direct effects of TLR agonists on hematopoietic progenitors, endotoxemia also induces the production of myeloid-lineage growth factors (discussed in detail below), thereby supporting the replenishment of blood neutrophils³⁸. Importantly, the TLR4-MyD88 signaling pathway also mediates LPS-related emergency granulopoiesis through a paracrine pathway, which does not involve direct effects on HSPCs, but requires the production of the growth factor G-CSF by endothelial cells³⁸ (Fig.1), as discussed below.

Effect of growth factors on the regulation of myelopoiesis

G-CSF is a major driver of granulopoiesis under both steady-state and emergency conditions, such as infection, by regulating the expression of myeloid lineage-specific transcription factors and of receptors for myeloid lineage-specific growth factors^2,39. Other growth factors contributing to emergency myelopoiesis are M-CSF and GM-CSF². However, mice deficient in these factors (G-MCF, GM-CSF and M-CSF) can still generate modest numbers of myeloid cells, at least in response to sterile peritoneal inflammation, highlighting a functional redundancy in myelopoiesis⁴⁰. GM-CSF may act alone or together with other cytokines, such as IL-3, in the regulation of myelopoiesis, specifically during chronic inflammation or bacterial infection^2,41.

During emergency granulopoiesis, G-CSF induces proliferative and lineage-specification signals by acting primarily on CMPs and GMPs, which expand and form well-defined clusters that differentiate into granulocytes^39,42,43. Besides affecting the differentiation of myeloid progenitor cells, G-CSF may also induce the proliferation of HSCs⁴⁴. Moreover, G-CSF stimulates the mobilization of HSCs from the BM in an indirect fashion, by acting on monocytes in the HSC niche^45,46. G-CSF-induced activation of its receptor, G-CSFR, leads to signaling through the transcription factor STAT3, which directly stimulates the expression of the master transcriptional regulator of emergency granulopoiesis, C/EBPβ^39,42; contrastingly, C/EBPα is the major driver of steady-state granulopoiesis². The effects of STAT3 are counteracted by the regulator of cytokine signal transduction SOCS3, which downregulates G-CSF-induced emergency granulopoiesis; indeed, hematopoietic cell-specific deficiency of SOCS3 results in sustained G-CSF-induced activation of STAT3, enhanced neutrophilia and mobilization of hematopoietic progenitor cells⁴⁷. Consistently, the epigenetic modifier TET2, which is mutated in patients with myeloid malignancies⁴⁸, promotes infection-induced emergency myelopoiesis by suppressing the expression of SOCS3⁴⁹.

The generation of monocytes in the BM occurs in a manner dependent on the receptor CSFR1, which binds M-CSF⁵⁰. M-CSF acts directly on hematopoietic and myeloid progenitor cells and induces their differentiation into monocytes (Fig.1). In vitro treatment of GMPs with M-CSF promotes their differentiation to mature monocytes⁵¹. The M-CSF-instructed monocyte lineage choice in GMPs is mediated by Src family kinase signaling⁵². M-CSF can instruct myeloid lineage identity in single HSCs, independently of HSC survival or proliferation, by inducing a PU.1-dependent molecular signature⁵³. This mechanism of M-CSF-dependent myelopoiesis may protect against opportunistic infections post-HSC transplantation⁵⁴.

Effect of IFN on HSCs fate decision and the regulation of myelopoiesis

Type 1 IFN, especially IFN-α, and type 2 IFN (IFN-γ) are central cytokines in the adaptation of HSPCs to inflammation. The type 1 IFN inducer polyinosinic:polycytidylic acid (polyI:C), or IFN-α itself, can drive proliferation of dormant HSCs (Fig.1) through a pathway dependent on the IFN-α/β receptor IFNAR and STAT1; however, chronic administration of IFN-α results in impaired HSC repopulation capacity⁵⁵. The transcription factor Irf2, a negative regulator of type 1 IFN signaling, counteracts the effects of type 1 IFN on HSCs and protects quiescent HSCs from type 1 IFN-induced proliferation-associated exhaustion⁵⁶. This type 1 IFN-dependent functional impairment and attrition of HSCs is due to induction of DNA damage in HSCs entering the cell cycle, which is associated with enhanced mitochondrial membrane potential and mitochondrial production of ROS⁵⁷. However, type 1 IFN-triggered proliferation of HSCs may be brief, due to the transient reduction in the expression of genes enforcing quiescence (e.g., p27, p57, Foxo1, Foxo3a or Pten), while reestablishment of HSC quiescence can protect them from the pro-apoptotic effects of IFN-α⁵⁸. Hematopoietic cells may engage additional homeostatic protective mechanisms against IFN-α-induced dysfunction. For example, all-trans retinoic acid-induced signaling protects dormant HSCs against polyI:C-induced replicative stress⁵⁹. The cyclic GMP-AMP (cGAMP) synthase cGAS is an intracellular sensor of pathogen-derived DNAs, mostly from viruses. Upon DNA binding, cGAS generates cGAMP, which – together with the adapter STING – stimulates the production of type 1 IFN⁶⁰. Interestingly, a circular RNA antagonizing cGAS synthase activity, named cia-cGAS, is highly enriched in the nucleus of LT-HSCs and promotes maintenance of their dormancy by protecting HSCs against cGAS-type 1 IFN-mediated exhaustion⁶¹. The complexity of the IFN-α effects on the BM is further exemplified by the observations that acute IFN-α-mediated inflammation acts on HSC-niche endothelial cells, affecting vascularity and vessel leakage in the BM⁶², whereas type 1 IFN can additionally stimulate proliferation and post-trancriptional protein synthesis in a primed subpopulation of stem cell-like megakaryocyte-committed progenitor cells phenotypically residing within the HSCs, leading to rapid platelet replenishment in acute inflammation⁶³.

IFN-γ also modulates the maintenance and proliferation of HSCs. Infection with Mycobacterium avium triggers the proliferation of HSCs in an IFN-γ, rather than IFN-α-dependent manner⁶⁴ (Fig.1). Consistently, HSC proliferation is abrogated in mice with genetic deficiency of either the receptor for IFN-γ or its downstream signal transducer STAT1⁶⁴, whereas administration of recombinant IFN-γ reverses the effect, suggesting that IFN-γ stimulates HSC cell cycle entry⁶⁴. However, the effects of IFN-γ on HSCs are context-dependent, as IFN-γ inhibits the proliferation of HSCs, impairs their maintenance and negatively affects their recovery during infection with the lymphocytic choriomeningitis virus (LCMV)⁶⁵. In the context of LCMV infection, IFN-γ induces myeloid differentiation by acting either directly on myeloid-biased HSC⁶⁶ or indirectly; specifically, IFN-γ from cytotoxic T cells stimulates a paracrine pathway leading to enhanced production of IL-6 from BM-MSCs, resulting in heightened proliferation and myelopoiesis of MPPs⁶⁷. Additionally, IFN-γ is responsible for the attrition of HSCs due to their differentiation into myeloid cells and loss of their self-renewal capacity during chronic mycobacterial infection in mice⁶⁶. Furthermore, intravenously administered mycobacterial Bacille Calmette-Guérin (BCG) triggers the IFN-γ-dependent expansion of HSPCs and their myeloid differentiation through a myelopoiesis-related transcriptional program⁶⁸ (Fig.1).

IL-1β and other cytokines effects on HSCs fate decision

IL-1β, a central mediator in innate immunity⁶⁹, acts directly on HSCs in vitro, promoting their proliferation and myeloid differentiation through activation of the transcription factor PU.1¹⁸ (Fig.1). Administration of IL-1β to mice results in increased numbers of myeloid-biased HSPCs, while BM recovery after chemotherapeutic injury is delayed in IL-1 receptor-deficient mice¹⁸. Chronic administration of IL-1β diminishes the self-renewal capacity of HSCs, although this effect is reversed upon IL-1β withdrawal¹⁸. IL-1β, in fact, may account for the hematologic abnormalities in patients with X-linked chronic granulomatous disease (X-CGD) and in the respective mouse model, both of which are characterized by IL-1β-associated hyperinflammation⁷⁰. Patients with X-CGD have decreased HSC counts, whereas mice with targeted disruption in the X-linked gene gp91^phox have increased HSC proliferation and impaired reconstitution capacity, as well as boosted myelopoiesis, as assessed by enhanced numbers of GMPs⁷⁰. An IL-1 receptor antagonist partially restores hematopoiesis in mice with X-CGD⁷⁰, suggesting IL-1 signaling is in part responsible for this phenotype. Short-term or low-dose administration of IL-1α or IL-1β prevents chemotherapy-induced myelosuppression^71,72 and protects cyclophosphamide-induced neutropenic mice against sepsis⁷², indicating timing and dosing may be critical regulators of the beneficial or detrimental effects of IL-1 on HSCs. While IL-1 may promote restoration of myelopoiesis, persistent IL-1-mediated inflammation negatively impacts HSC functionality.

Tumor necrosis factor (TNF) induces proliferation and instructs myeloid lineage decisions in HSCs and may thus compromise their repopulation capacity⁷³. TNF directly upregulates PU.1 in vitro and mediates its activation in LPS-administered mice, because PU.1 is not affected by LPS in TNF-deficient mice⁷⁴. IL-3, derived from innate response activator B cells, amplifies myelopoiesis and inflammation in sepsis. Specifically, the expansion of HSPC and myeloid progenitor cells is hindered by IL-3 deficiency in murine sepsis, while high concentrations of IL-3 in plasma correlate with higher mortality in human sepsis⁷⁵. IL-6 regulates HSPC proliferation and differentiation during emergency myelopoiesis^2,30. In this context, IL-6 may be derived either from the HSPCs following TLR stimulation³⁰ (Fig.1) or from BM-MSCs upon stimulation with cytotoxic T cell-derived IFN-γ⁶⁷. IL-6, through activation of signaling though SHP2 and STAT3, can also support survival of TET2-deficient HSPCs and myeloid cells⁷⁶; as TET2 mutations are linked with clonal hematopoiesis and a higher risk for leukemia development, these observations may indicate how inflammation may promote survival of preleukemic HSPCs⁷⁶. IL-27, a member of the IL-6-IL-12 cytokine family, also acts on HSPCs and promotes emergency myelopoiesis⁷⁷. In a model of malarial infection, deletion of the IL-27R subunit WSX-1 suppressed the expansion of progenitor cells and their differentiation into myeloid cells, which resulted in increased parasitemia⁷⁷.

Modulation of inflammation-induced HSPC adaptation by the BM niche

Emergency myelopoiesis is also regulated by the HSC niche microenvironment in the BM. The niche modulates HSC quiescence, self-renewal, proliferation and differentiation into committed progenitor cells^5,28,78,79 (Fig.2). Niche cells such as endothelial cells, nestin⁺ MSCs, perivascular stromal cells, including CXCL12-abundant reticular (CAR) cells, and osteolineage cells regulate HSC function through paracrine signals (growth factors or chemokines) or through adhesive interactions that may also involve extracellular matrix proteins^5,28,78,79. DEL-1, an adhesive protein secreted by niche endothelial cells, CAR cells and osteolineage cells in the BM interacts with β3 integrin on HSCs and enhances the proliferation of LT-HSCs and their myeloid differentiation, predominantly in the context of LPS- or G-CSF-induced stress myelopoiesis⁸⁰. Another matrix-associated protein, osteopontin, restrains emergency myelopoiesis in favor of lymphopoiesis through a dual mechanism mediated by two distinct isoforms: intracellular osteopontin is pro-apoptotic in myeloid progenitors, whereas secreted osteopontin is anti-apoptotic in differentiated lymphoid cells⁸¹. Furthermore, BM innervation and sympathoadrenergic activity are important HSC niche regulators and control the circadian HSC release into the circulation; conversely, loss of sympathoadrenergic nerves promotes aging of the niche and impairs regeneration of hematopoiesis^82–84.

The progeny of HSCs in the BM may be important mediators for the adaptation of HSPC to inflammatory signals. Monocytes are a major cellular target of G-CSF-induced egress of HSPCs from the BM niche and emergency granulopoiesis⁴⁶. G-CSF-induced mobilization of HSPCs is linked with depletion of trophic endosteal macrophages in the BM⁸⁵. CD169⁺ macrophages in the BM may interact with osteoblasts or nestin⁺ cells in the niche to control expression of trophic factors such as angiopoietin-1, CXCL12 and Kit ligand on HSC and retention of HSPC in the niche^85,86. Consistently, depletion of BM macrophages is sufficient to induce HSPC mobilization⁸⁵. α-SMA⁺ monocytes and macrophages may protect HSPCs from exhaustion in stress conditions such as sublethal irradiation, in part because PGE2 produced from these macrophages upregulates nestin⁺ stromal cell-derived CXCL12, which is critical for stem-cell quiescence⁸⁷. BM macrophages can also regulate the HSC niche through the phagocytosis of apoptotic cells (known as efferocytosis). Senescent neutrophils in the circulation upregulate their expression of CXCR4 and home back to the BM, where they are efferocytosed by BM-resident CD169⁺ macrophages, a process that in turn promotes the circadian egress of hematopoietic progenitor cells into the circulation⁸⁸ (Fig.2).

Neutrophils and their precursors in the BM also contribute to demand-adapted myelopoiesis. In response to systemic inflammation, Gr1⁺ myeloid cells in the BM release ROS, which stimulates the proliferation and differentiation of myeloid progenitor cells, thus enhancing myeloid cell output⁸⁹. Granulocyte-derived TNF promotes vessel growth and thereby HSPC regeneration⁹⁰. Megakaryocytes regulate HSC quiescence through TGFβ1 or CXCL4 and may promote post-injury regeneration of HSCs through FGF-1^91,92. Megakaryocytes preferentially regulate the proliferation of vWF⁺ platelet- and myeloid-biased HSCs⁹³. In vivo high-resolution imaging revealed the co-localization of HSPCs with CD4⁺CD25⁺FoxP3⁺ regulatory T cells (T_reg cells) that accumulate on the endosteal BM niche⁹⁴. These T_reg cells may confer immune privilege to the HSPC niche, thus preventing allogeneic rejection of transplanted HSPCs or protecting endogenous HSPCs from excessive inflammation. In support of this notion, a niche-specific subpopulation of CD150^hi T_reg cells promotes HSC quiescence and engraftment of HSCs through a mechanism dependent on adenosine generated by the ectoenzyme CD39, which protects HSCs from oxidative stress⁹⁵ (Fig.2). Together, different cells in the HSC niche contribute to context-dependent HSPC adaptation to diverse inflammatory challenges.

The effect of trained immunity on HSPCs

Trained immunity is a form of adaptation that enhances the response of innate immune cells to secondary challenges independently of adaptive immunity^96,97. Trained immunity can be triggered by certain microbial components, such as the fungal cell-wall constituent β-glucan or vaccines like BCG, and involves immunometabolic and epigenetic alterations in target cells^96,97. Trained innate immunity operates in hematopoietic progenitor cells^68,98, resolving an earlier paradox regarding the long-term effect of trained immunity in myeloid cells with a relatively short lifespan. Trained immunity induced by injection of β-glucan in mice leads to the expansion of myeloid-biased HSPCs, such as CD41⁺LT-HSCs and MPP3 cells, in the BM⁹⁸. β-glucan-induced myelopoiesis requires signaling through IL-1β and is associated with enhanced glycolysis and cholesterol biosynthesis in HSPCs. Increased cholesterol amounts in HSPCs from β-glucan-trained mice results in enhanced signaling via CD131, the common β-subunit of the IL-3/GM-CSF receptor (IL-3Rβ), in LT-HSCs and MPPs⁹⁸. Consistently, elevated expression of IL-3Rβ and enhanced myeloid bias are observed in HSPCs upon blockade of cholesterol efflux⁹⁹. Importantly, β-glucan-induced training of HSPCs and enhancement of myelopoiesis confers a favorable outcome to secondary systemic LPS challenge and protection against chemotherapy-induced myelosuppression⁹⁸ (Fig.1). BCG administration in mice also stimulates the expansion of HSPCs and promotes myelopoiesis in a manner requiring IFN-γ signaling. Importantly, macrophages derived from BCG-trained progenitor cells mediate host protection against Mycobacterium tuberculosis infection independently of adaptive immunity⁶⁸ (Fig.1). Hence, innate immune training of hematopoietic progenitor cells^68,98 shares common features with emergency myelopoiesis in response to systemic infection or inflammation².

HSC expansion due to inflammatory stimuli such as LPS or IFN-α leads to exhaustion of HSCs, which is associated with compromised self-renewal and capacity for competitive repopulation and is mediated by DNA damage associated with replication stress^35,36,57,100. Interestingly, β-glucan-triggered training of HSPCs is associated with attenuated LPS- or chemotherapy-induced replication stress in HSCs⁹⁸. The beneficial effect of trained immunity in this case may lie in changes in cellular metabolism. While mitochrondrial ROS and activation of oxidative phosphorylation are associated with DNA damage in HSCs and lead to their functional impairment, LT-HSC maintenance depends on glycolysis^7,57,101,102. HSPCs in β-glucan-trained mice retain a glycolytic signature even after chemotherapy, which may be linked with resistance to replication stress-related DNA damage⁹⁸. This could suggest that certain agonists of trained immunity may act favorably on HSCs and myelopoiesis by counteracting inflammation-dependent stress and impaired function in HSC⁹⁸. Genetic targeting of the pathways implicated in trained immunity, such as IL-1 signalling or cell metabolism in hematopoietic progenitor cells, mature myeloid cells and components of the HSC niche are required to clarify the mechanisms underlying the modulation of myelopoiesis by trained immunity.

Adaptation of hematopoietic progenitors in inflammatory diseases

There is increasing evidence that chronic metabolic-inflammatory conditions, such as obesity and diabetes, or cardiovascular-inflammatory conditions, such as atherosclerosis can alter hematopoiesis. Reciprocally, adaptation of hematopoietic progenitor cells leading to chronic leukocytosis may contribute to the perpetuation and chronicity of these disorders^27,103,104 (Fig.3). Diabetes-associated hyperglycemia and obesity result in monocytosis and neutrophilia in mice^105,106. In hyperglycemia, the neutrophil-derived damage-associated molecular pattern S100A8/A9 interacts with the receptor RAGE on CMPs and triggers the production of M-CSF and GM-CSF, which stimulate the proliferation of GMPs and in turn, monocytosis, neutrophilia and atherosclerosis¹⁰⁵. Similarly, genetic or diet-induced obesity in mice stimulates the proliferation of myeloid progenitor cells in the BM, in a manner dependent on the NLRP3-inflammasome-dependent production of IL-1β in adipose tissue (AT) macrophages¹⁰⁶ (Fig.3). Weight loss in humans following bariatric surgery significantly decreases the number of circulating monocytes and neutrophils¹⁰⁶. The myeloid bias of HSPCs from mice with diet-induced obesity is maintained upon serial transplantation to recipient mice, and the obesity-related myeloid reprogramming is associated with increased numbers of LT-HSCs and MPPs in the BM^107,108. The myeloid-biased reprogramming of HSPCs requires signaling through TLR4, MyD88 and TRIF and contributes to enhanced generation of proinflammatory macrophages in the AT, thereby exacerbating obesity-related AT inflammation and dysfunction^107,108. Experiments with germ-free mice have also indicated an important role of the gut microbiota in myelopoiesis¹⁰⁹. In this context, altered gut microbiota in mice due to high-fat diet promotes HSPC differentiation toward myelopoiesis at the expense of lymphopoiesis through modulation of niche MSCs¹¹⁰.

Altered hematopoiesis is also integral to atherosclerosis and cardiovascular disease^27,103. Myocardial infarction (MI) results in enhanced myelopoiesis, which may involve several complementary and likely synergistic mechanisms²⁷ (Fig.3). For instance, a myeloid-biased subset of HSPCs (CCR2⁺CD150⁺CD48⁻LSKs) with a higher proliferative and lower self-renewal capacity than HSCs contributes to the enhanced myelopoiesis post-MI¹¹¹. Increased sympathoadrenergic activity causes enhanced mobilization of HSPCs from the BM to the spleen, which becomes a site for increased extramedullary myelopoiesis¹¹². Interestingly, enhanced sympathoadrenergic activity mediates the increased, stress-induced, proliferation of LT-HSCs and myelopoiesis, thus likely explaining how psychosocial stress promotes atherosclerosis¹¹³. In a similar context, a disturbed sleep pattern contributes to atherosclerosis by dysregulating a neuro-immune axis linking hypothalamic hypocretin to regulation of myelopoiesis¹¹⁴. IL-1β is a central player in enhanced myelopoiesis post-MI, as it can promote HSPC proliferation in a direct or indirect, niche cell-dependent fashion, as well as stimulate splenic monocytopoiesis^115,116. Detrimental effects of innate immune training may also contribute to western diet-associated atherosclerosis¹¹⁷. Western diet induces NLRP3- and IL-1-dependent transcriptomic and epigenetic reprogramming in GMPs in atherosclerosis-prone LDL-receptor (Ldlr)-deficient mice, driving the GMPs towards enhanced proliferation and inflammatory responses; consistent with the long-term effects of trained immunity, this progenitor cell reprogramming is sustained even after mice are switched back to chow diet¹¹⁷. Moreover, GM-CSF from the infarcted myocardium stimulates expansion of myeloid-biased CD131⁺ MPP3 in the BM¹¹⁸.

Cholesterol biosynthesis and/or mevalonate, the first metabolite of the cholesterol biosynthetic pathway, are important mediators of innate immune training in mature myeloid cells or hematopoietic progenitor cells^98,119,120. The cholesterol-dependent immunometabolic crosstalk during trained immunity may also be intimately linked with the dyslipidemia- or hypercholesterolemia-associated altered hematopoiesis that leads to enhanced myelopoiesis during atherosclerosis^97,103. Disrupted cholesterol efflux pathways and enhanced cholesterol accumulation in HSPCs drive myelopoiesis in a cell-intrinsic fashion^99,121. Major players in the cellular cholesterol efflux are apolipoprotein E (ApoE) and the ATP-binding cassette transporters ABCA1 and ABCG1¹²¹. The latter are upregulated in pro-resolving macrophages upon efferocytosis dependent on transcription factor LXR and contribute to inflammation resolution programs^122,123. Cholesterol accumulation in HSPCs due to double deficiency of ABCA1 and ABCG1, or due to ApoE deficiency, results in HSPC expansion and myeloid differentiation, and hence peripheral monocytosis and neutrophilia^99,124. The underlying mechanism involves membrane alterations associated with enhanced intracellular cholesterol and elevated cell-surface expression of the lipid raft-associated IL-3Rβ (CD131), which facilitates enhanced IL-3- or GM-CSF-dependent signaling for inducing myelopoiesis^99,124 (Fig.3). A similar mechanism, involving enhanced cholesterol accumulation and lipidomic membrane remodeling that augment CD131-dependent signaling in HSPCs leading to their myeloid differentiation operates in β-glucan-induced trained immunity in HSPCs⁹⁸. Together, immunometabolic adaptations of hematopoietic progenitor cells to inflammatory signals may be the common denominator between the modes of action in trained innate immunity and cardio-metabolic disease. In addition, (mal)adaptive training of hematopoietic progenitor cells may provide a mechanistic basis for comorbidities, such as the enhanced risk of cardiovascular complications in patients with rheumatoid arthritis, periodontitis or other autoimmune or inflammatory disorders^125,126.

The age-associated impairment in HSC function is associated with replication stress¹⁰⁰, while decreased DNA damage response and repair pathways in aged HSCs may promote accumulation of mutations leading to hematological malignancies¹²⁷. Autophagy counteracts the age-associated dysfunction of HSCs by clearing mitochondria, thereby facilitating maintenance of HSC quiescence and regenerative capacity¹²⁸. Aged HSCs display diminished self-renewal and a myeloid bias^129–131. CD61^hi LT-HSCs are highly responsive to inflammatory stimuli and predominate amongst aged LT-HSCs as a myeloid-biased subset regulated by the transcription factors KLF5, IKZF1 and STAT3¹³¹.

The propensity of aged HSCs to accrue mutations and have a myeloid bias and enhanced responsiveness to inflammation may provide a mechanism for clonal hematopoiesis (also known as clonal hematopoiesis of indeterminate potential)^132,133, whose prevalence strongly increases with age and is defined by mutations associated with myeloid malignancies, such as mutations in TET2, DNMT3A and ASXL1, with normal blood counts. People with clonal hematopoiesis have higher risk for hematologic malignancies and cardiovascular disease^132–134. Consistently, murine deficiency of the epigenetic regulator TET2 results in the expansion of HSPCs with myeloid bias, leading to myeloproliferation¹³⁵. Chimeric Ldlr-deficient mice partially or completely reconstituted with TET2-deficient BM cells have exacerbated atherosclerosis typified by the presence of pro-inflammatory macrophages with enhanced NLRP3-dependent production of IL-1β^133,134. Clonal hematopoiesis may also accelerate heart failure in an IL-1β-dependent manner¹³⁶. The concept of clonal hematopoiesis could, therefore, integrate aging- and inflammation-related adaptations in myelopoiesis progenitor cells with cardiometabolic inflammation. NLRP3 inflammasome activation is linked to clonal HSPC expansion and progression of the myelodysplastic syndrome (MDS)¹³⁷, while IL-6-related inflammation promotes survival of TET2-deficient HSPCs⁷⁶, implicating inflammation as a driver of clonal hematopoiesis and MDS.

IL-1β appears as the common denominator of innate immune training in hematopoietic progenitor cells, enhanced myelopoiesis in cardiometabolic disease and clonal hematopoiesis-related exacerbated vascular inflammation. The success of IL-1β blockade (CANTOS trial) in atherosclerosis¹³⁸ further supports a major role of IL-1β in these conditions. Moreover, inflammation induced by TLR2 agonists or disseminated bacteria resulting from a disrupted intestinal barrier may promote the progression to pre-leukemic myeloproliferation in TET2-deficiency¹³⁹. Further investigations could elucidate the potential link(s) amongst the aforementioned inflammatory (IL-1β-related) processes, especially trained innate immunity and clonal hematopoiesis, as they appear to participate in a self-sustained vicious cycle, wherein inflammatory disease and inflammatory adaptations of hematopoietic progenitors engage in reciprocally reinforced interactions.

Conclusions

The ability of hematopoietic progenitor cells to sense and adapt to inflammatory stimuli renders them central players in the context of acute peripheral inflammation or infection^1,2. During infection, HSPCs react to TLR ligands, inflammatory cytokines, including IL-1 or IFNs, and growth factors. Such inflammatory signaling in HSPCs can drive their expansion and myelopoiesis, but may also cause HSPC exhaustion, if chronically sustained². With the exception of chronic cardio-metabolic diseases^27,103, the role of hematopoietic progenitors cells as central determinants of the host response in chronic diseases has been underappreciated. In cardio-metabolic disease (and possibly other chronic disorders), the adaptation to inflammation that drives myelopoiesis in HSPCs generates a destructive feed-forward loop (Fig.4), in which increased numbers of inflammatory myeloid cells enhance inflammation, which in turn perpetuates HSPC-mediated myelopoiesis^{27,103,111,112,116,121}. Importantly, previous infectious or inflammatory challenges can epigenetically imprint an “inflammatory memory” in hematopoietic progenitor cells^97,98,117, thereby contributing to trained immunity. As innate immune training facilitates cardiovascular inflammation¹¹⁷, it is intriguing to hypothesize that the development of clonal hematopoiesis, which is associated with higher cardio-metabolic inflammation^132–134, might be linked with selective pressure for inflammatory memory^96–98. A major question is whether adaptation of hematopoietic progenitors to inflammation can provide a unifying concept where inflammatory stimuli (e.g., infections) and multiple risk factors (e.g., obesity, dyslipidemia, psychosocial stress) can be integrated in a meaningful manner to better understand the pathogenesis of different forms of chronic inflammatory diseases or even malignant disorders. This challenging concept needs to be addressed by interdisciplinary studies bringing together experts in immunology, hematopoiesis, metabolism, cardiovascular medicine, as well as mathematical modeling, which could potentially lead to new risk assessment scores for chronic inflammatory diseases.