Haematopoietic stem cells (HSC) maintain lifelong blood production and increase blood cell numbers in response to chronic and acute injury. However, the mechanism(s) by which inflammatory insults are communicated to HSCs and their consequences for HSC activity remain largely unknown. Here, we demonstrate that interleukin-1 (IL-1), which functions as a key pro-inflammatory ‘emergency’ signal, directly accelerates cell division and myeloid differentiation of HSCs via precocious activation of a PU.1-dependent gene program. While this effect is essential for rapid myeloid recovery following acute injury to the bone marrow (BM), chronic IL-1 exposure restricts HSC lineage output, severely erodes HSC self-renewal capacity, and primes IL-1-exposed HSCs to fail massive replicative challenges like transplantation. Importantly, these damaging effects are transient and fully reversible upon IL-1 withdrawal. Our results identify a critical regulatory circuit that tailors HSC responses to acute needs, and likely underlies deregulated blood homeostasis in chronic inflammation conditions.

All hematopoietic and immune cells are continuously generated by hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) through highly organized process of stepwise lineage commitment. In the steady state, HSCs are mostly quiescent, while HPCs are actively proliferating and contributing to daily hematopoiesis. In response to hematopoietic challenges, e.g., life-threatening blood loss, infection, and inflammation, HSCs can be activated to proliferate and engage in blood formation. The HSC activation induced by hematopoietic demand is mediated by direct or indirect sensing mechanisms involving pattern recognition receptors or cytokine/chemokine receptors. In contrast to the hematopoietic challenges with obvious clinical symptoms, how the aging process, which involves low-grade chronic inflammation, impacts hematopoiesis remains undefined. Herein, we summarize recent findings pertaining to functional alternations of hematopoiesis, HSCs, and the bone marrow (BM) microenvironment during the processes of aging and inflammation and highlight some common cellular and molecular changes during the processes that influence hematopoiesis and its cells of origin, HSCs and HPCs, as well as the BM microenvironment. We also discuss how age-dependent alterations of the immune system lead to subclinical inflammatory states and how inflammatory signaling might be involved in hematopoietic aging. Our aim is to present evidence supporting the concept of “Inflamm-Aging,” or inflammation-associated aging of hematopoiesis.

Bacterial infection leads to consumption of short-lived innate immune effector cells, which then need to be replenished from hematopoietic stem and progenitor cells (HSPCs). HSPCs express pattern recognition receptors, such as Toll-like receptors (TLRs), and ligation of these receptors induces HSPC mobilization, cytokine production, and myeloid differentiation. The underlying mechanisms involved in pathogen signal transduction in HSCs and the resulting biological consequences remain poorly defined. Here, we show that in vivo lipopolysaccharide (LPS) application induces proliferation of dormant HSCs directly via TLR4 and that sustained LPS exposure impairs HSC self-renewal and competitive repopulation activity. This process is mediated via TLR4-TRIF-ROS-p38, but not MyD88 signaling, and can be inhibited pharmacologically without preventing emergency granulopoiesis. Live Salmonella Typhimurium infection similarly induces proliferative stress in HSCs, in part via TLR4-TRIF signals. Thus, while direct TLR4 activation in HSCs might be beneficial for controlling systemic infection, prolonged TLR4 signaling has detrimental effects and may contribute to inflammation-associated HSPC dysfunction.

Neutropenia is probably the strongest known predisposition to infection with otherwise harmless environmental or microbiota-derived species. Because initial swarming of neutrophils at the site of infection occurs within minutes, rather than the hours required to induce “emergency granulopoiesis,” the relevance of having high numbers of these cells available at any one time is obvious. We observed that germ-free (GF) animals show delayed clearance of an apathogenic bacterium after systemic challenge. In this article, we show that the size of the bone marrow myeloid cell pool correlates strongly with the complexity of the intestinal microbiota. The effect of colonization can be recapitulated by transferring sterile heat-treated serum from colonized mice into GF wild-type mice. TLR signaling was essential for microbiota-driven myelopoiesis, as microbiota colonization or transferring serum from colonized animals had no effect in GF MyD88−/−TICAM1−/− mice. Amplification of myelopoiesis occurred in the absence of microbiota-specific IgG production. Thus, very low concentrations of microbial Ags and TLR ligands, well below the threshold required for induction of adaptive immunity, sets the bone marrow myeloid cell pool size. Coevolution of mammals with their microbiota has probably led to a reliance on microbiota-derived signals to provide tonic stimulation to the systemic innate immune system and to maintain vigilance to infection. This suggests that microbiota changes observed in dysbiosis, obesity, or antibiotic therapy may affect the cross talk between hematopoiesis and the microbiota, potentially exacerbating inflammatory or infectious states in the host.

Ageing is associated with impaired hematopoietic and immune function. This is caused in part by decreased hematopoietic stem cell (HSC) population fitness and an increased myeloid differentiation bias. The reasons for this aging-associated HSC impairment are incompletely understood. We here demonstrate that aged specific pathogen free (SPF) wild-type mice in contrast to young SPF mice produce more IL-1a/b in steady-state bone marrow (BM), with most of IL-1a/b being derived from myeloid BM cells. Further, blood of steady-state aged SPF wild-type mice contains higher levels of microbe associated molecular patterns (MAMPs), specifically TLR4 and TLR8 ligands. Also, BM myeloid cells from aged mice produce more IL-1b in vitro, and aged mice show higher and more durable IL-1a/b responses upon LPS stimulation in vivo. To test if HSC ageing is driven via IL-1a/b, we evaluated HSCs from IL-1 receptor 1 (IL-1R1) knock-out mice. Indeed, aged HSCs from IL-1R1 knock-out mice show significantly mitigated ageing-associated inflammatory signatures. Moreover, HSCs from aged IL-1R1KO and also from germ-free mice maintain unbiased lympho-myeloid hematopoietic differentiation upon transplantation, thus resembling this functionality of young HSCs. Importantly, in vivo antibiotic suppression of microbiota or pharmacologic blockade of IL-1 signaling in aged wild-type mice was similarly sufficient to reverse myeloid biased output of their HSC populations. Collectively, our data defines the microbiome-IL-1/IL-1R1 axis as a key, self-sustaining, but also therapeutically partially reversible driver of HSC inflamm-ageing.

Key Points• Mpl agonist, but not granulocyte colonystimulating factor, induces self-renewing HSC divisions and expansions.In steady-state adult hematopoiesis, most hematopoietic stem cells (HSCs) are in the resting phase of the cell cycle. Upon enhanced hematopoietic demand, HSCs can be induced to divide and self-renew or differentiate. However, the cell-extrinsic signals inducing HSC cycling remain to be elucidated. Using in vivo high-resolution single HSC divisional tracking, we directly demonstrate that clinically applied thrombopoietin receptor but not granulocyte colony-stimulating factor (G-CSF) receptor agonists drive HSCs into self-renewing divisions leading to quantitative expansion of functional HSC as defined by their in vivo serial multilineage and long-term repopulating potential. These results suggest that thrombopoietin mimetics might be applicable to expand HSCs in vivo and to sensitize thrombopoietin receptor-expressing HSCs to cell cycle-dependent cytotoxic agents. (Blood. 2016;127(25):3175-3179)

Favorable-risk human acute myeloid leukemia (AML) engrafts poorly in currently used immunodeficient mice, possibly because of insufficient environmental support of these leukemic entities. To address this limitation, we here transplanted primary human AML with isolated nucleophosmin (NPM1) mutation and AML with inv(16) in mice in which human versions of genes encoding cytokines important for myelopoiesis (macrophage colony-stimulating factor [M-CSF], interleukin-3, granulocyte-macrophage colony-stimulating factor, and thrombopoietin) were knocked into their respective mouse loci. NPM1 AML engrafted with higher efficacy in cytokine knock-in (KI) mice and showed a trend toward higher bone marrow engraftment levels in comparison with NSG mice. inv(16) AML engrafted with high efficacy and was serially transplantable in cytokine KI mice but, in contrast, exhibited virtually no engraftment in NSG mice. Selected use of cytokine KI mice revealed that human M-CSF was required for inv(16) AML engraftment. Subsequent transcriptome profiling in an independent AML patient study cohort demonstrated high expression of M-CSF receptor and enrichment of M-CSF inducible genes in inv(16) AML cases. This study thus provides a first xenotransplantation mouse model for and informs on the disease biology of inv(16) AML.

Clonal hematopoiesis of indeterminate potential (CHIP), also referred to as aging-related clonal hematopoiesis (ARCH), is defined as an asymptomatic clonal expansion of mutant mature hematopoietic cells over 4% of blood leukocytes. CHIP associates with advanced age and increased risk for hematological malignancy, cardiovascular disease and all-cause mortality. Loss-of-function somatic mutations in TET2 are frequent drivers of CHIP. However, the contribution of aging-associated cooperating cell-extrinsic drivers, like inflammation, remains under-explored. Using bone marrow (BM) transplantation and newly developed genetic mosaicism (HSC-SCL-Cre-ERT; Tet2+/flox; R26+/tm6(CAG-ZsGreen1)Hze) mouse models of Tet2+/--driven CHIP, we observed an association between increased Tet2+/- clonal expansion and higher BM levels of the inflammatory cytokine IL-1 upon aging. Administration of IL-1 to mice carrying CHIP led to an IL-1R1-dependent expansion of Tet2+/- hematopoietic stem and progenitor cells (HSPCs) and mature blood cells. This expansion was caused by increased Tet2+/- HSPC cell-cycle progression, increased multilineage differentiation and higher repopulation capacity compared to their WT counterparts. In agreement, IL-1α-treated Tet2+/- HSCs showed increased DNA replication and repair transcriptomic signatures and reduced susceptibility to IL-1α-mediated downregulation of self-renewal genes. Importantly, genetic deletion of IL-1R1 in Tet2+/- HPSC or pharmacological inhibition of IL-1 signaling impaired Tet2+/- clonal expansion, establishing the IL-1 pathway as a relevant and therapeutically targetable driver of Tet2+/- CHIP progression during aging.