A complex proinflammatory cascade mediates the activation of HSCs upon LPS exposure in vivo.

Infections are a key source of stress to the hematopoietic system. While infections consume short-lived innate immune cells, their recovery depends on quiescent hematopoietic stem cells (HSCs) with long-term self-renewal capacity. Both chronic inflammatory stress and bacterial infections compromise competitive HSC capacity and cause bone marrow (BM) failure. However, our understanding of how HSCs act during acute and contained infections remains incomplete. Here, we used advanced chimeric and genetic mouse models in combination with pharmacological interventions to dissect the complex nature of the acute systemic response of HSCs to lipopolysaccharide (LPS), a well-established model for inducing inflammatory stress. Acute LPS challenge transiently induced proliferation of quiescent HSCs in vivo. This response was not only mediated via direct LPS-TLR4 conjugation on HSCs but also involved indirect TLR4 signaling in CD115+ monocytic cells, inducing a complex proinflammatory cytokine cascade in BM. Downstream of LPS-TLR4 signaling, the combined action of proinflammatory cytokines such as interferon (IFN)α, IFNγ, tumor necrosis factor-α, interleukin (IL)-1α, IL-1ß, and many others is required to mediate full HSC activation in vivo. Together, our study reveals detailed mechanistic insights into the interplay of proinflammatory cytokine-induced molecular pathways and cell types that jointly orchestrate the complex process of emergency hematopoiesis and HSC activation upon LPS exposure in vivo.

Human haematopoietic stem cell lineage commitment is a continuous process.

Blood formation is believed to occur through stepwise progression of haematopoietic stem cells (HSCs) following a tree-like hierarchy of oligo-, bi- and unipotent progenitors. However, this model is based on the analysis of predefined flow-sorted cell populations. Here we integrated flow cytometric, transcriptomic and functional data at single-cell resolution to quantitatively map early differentiation of human HSCs towards lineage commitment. During homeostasis, individual HSCs gradually acquire lineage biases along multiple directions without passing through discrete hierarchically organized progenitor populations. Instead, unilineage-restricted cells emerge directly from a 'continuum of low-primed undifferentiated haematopoietic stem and progenitor cells' (CLOUD-HSPCs). Distinct gene expression modules operate in a combinatorial manner to control stemness, early lineage priming and the subsequent progression into all major branches of haematopoiesis. These data reveal a continuous landscape of human steady-state haematopoiesis downstream of HSCs and provide a basis for the understanding of haematopoietic malignancies.

IFNα-mediated remodeling of endothelial cells in the bone marrow niche.

In the bone marrow, endothelial cells are a major component of the hematopoietic stem cell vascular niche and are a first line of defense against inflammatory stress and infection. The primary response of an organism to infection involves the synthesis of immune-modulatory cytokines, including interferon alpha. In the bone marrow, interferon alpha induces rapid cell cycle entry of hematopoietic stem cells in vivo However, the effect of interferon alpha on bone marrow endothelial cells has not been described. Here, we demonstrate that acute interferon alpha treatment leads to rapid stimulation of bone marrow endothelial cells in vivo, resulting in increased bone marrow vascularity and vascular leakage. We find that activation of bone marrow endothelial cells involves the expression of key inflammatory and endothelial cell-stimulatory markers. This interferon alpha-mediated activation of bone marrow endothelial cells is dependent in part on vascular endothelial growth factor signaling in bone marrow hematopoietic cell types, including hematopoietic stem cells. Thus, this implies a role for hematopoietic stem cells in remodeling of the bone marrow niche in vivo following inflammatory stress. These data increase our current understanding of the relationship between hematopoietic stem cells and the bone marrow niche under inflammatory stress and also clarify the response of bone marrow niche endothelial cells to acute interferon alpha treatment in vivo.

Extracellular matrix protein Matrilin-4 regulates stress-induced HSC proliferation via CXCR4.

During homeostasis, hematopoietic stem cells (HSCs) are mostly kept in quiescence with only minor contribution to steady-state hematopoiesis. However, in stress situations such as infection, chemotherapy, or transplantation, HSCs are forced to proliferate and rapidly regenerate compromised hematopoietic cells. Little is known about the processes regulating this stress-induced proliferation and expansion of HSCs and progenitors. In this study, we identified the extracellular matrix (ECM) adaptor protein Matrilin-4 (Matn4) as an important negative regulator of the HSC stress response. Matn4 is highly expressed in long-term HSCs; however, it is not required for HSC maintenance under homeostasis. In contrast, Matn4 is strongly down-regulated in HSCs in response to proliferative stress, and Matn4 deficiency results in increased proliferation and expansion of HSCs and progenitors after myelosuppressive chemotherapy, inflammatory stress, and transplantation. This enhanced proliferation is mediated by a transient down-regulation of CXCR4 in Matn4(-/-) HSCs upon stress, allowing for a more efficient expansion of HSCs. Thus, we have uncovered a novel link between the ECM protein Matn4 and cytokine receptor CXCR4 involved in the regulation of HSC proliferation and expansion under acute stress.

Inflammation-Induced Emergency Megakaryopoiesis Driven by Hematopoietic Stem Cell-like Megakaryocyte Progenitors.

Infections are associated with extensive platelet consumption, representing a high risk for health. However, the mechanism coordinating the rapid regeneration of the platelet pool during such stress conditions remains unclear. Here, we report that the phenotypic hematopoietic stem cell (HSC) compartment contains stem-like megakaryocyte-committed progenitors (SL-MkPs), a cell population that shares many features with multipotent HSCs and serves as a lineage-restricted emergency pool for inflammatory insults. During homeostasis, SL-MkPs are maintained in a primed but quiescent state, thus contributing little to steady-state megakaryopoiesis. Even though lineage-specific megakaryocyte transcripts are expressed, protein synthesis is suppressed. In response to acute inflammation, SL-MkPs become activated, resulting in megakaryocyte protein production from pre-existing transcripts and a maturation of SL-MkPs and other megakaryocyte progenitors. This results in an efficient replenishment of platelets that are lost during inflammatory insult. Thus, our study reveals an emergency machinery that counteracts life-threatening platelet depletions during acute inflammation.