Different niches for stem cells carrying the same oncogenic driver affect pathogenesis and therapy response in myeloproliferative neoplasms.

Aging facilitates the expansion of hematopoietic stem cells (HSCs) carrying clonal hematopoiesis-related somatic mutations and the development of myeloid malignancies, such as myeloproliferative neoplasms (MPNs). While cooperating mutations can cause transformation, it is unclear whether distinct bone marrow (BM) HSC-niches can influence the growth and therapy response of HSCs carrying the same oncogenic driver. Here we found different BM niches for HSCs in MPN subtypes. JAK-STAT signaling differentially regulates CDC42-dependent HSC polarity, niche interaction and mutant cell expansion. Asymmetric HSC distribution causes differential BM niche remodeling: sinusoidal dilation in polycythemia vera and endosteal niche expansion in essential thrombocythemia. MPN development accelerates in a prematurely aged BM microenvironment, suggesting that the specialized niche can modulate mutant cell expansion. Finally, dissimilar HSC-niche interactions underpin variable clinical response to JAK inhibitor. Therefore, HSC-niche interactions influence the expansion rate and therapy response of cells carrying the same clonal hematopoiesis oncogenic driver.

Megakaryocyte Diversity in Ontogeny, Functions and Cell-Cell Interactions.

Hematopoietic stem cells (HSCs) rely on local interactions in the bone marrow (BM) microenvironment with stromal cells and other hematopoietic cells that facilitate their survival and proliferation, and also regulate their functions. HSCs and multipotent progenitor cells differentiate into lineage-specific progenitors that generate all blood and immune cells. Megakaryocytes (Mks) are hematopoietic cells responsible for producing blood platelets, which are essential for normal hemostasis and blood coagulation. Although the most prominent function of Mks is platelet production (thrombopoiesis), other increasingly recognized functions include HSC maintenance and host immune response. However, whether and how these diverse programs are executed by different Mk subpopulations remains poorly understood. This Perspective summarizes our current understanding of diversity in ontogeny, functions and cell-cell interactions. Cumulative evidence suggests that BM microenvironment dysfunction, partly caused by mutated Mks, can induce or alter the progression of a variety of hematologic malignancies, including myeloproliferative neoplasms (MPNs) and other disorders associated with tissue scarring (fibrosis). Therefore, as an example of the heterogeneous functions of Mks in malignant hematopoiesis, we will discuss the role of Mks in the onset and progression of BM fibrosis. In this regard, abnormal interactions between of Mks and other immune cells might directly contribute to fibrotic diseases. Overall, further understanding of megakaryopoiesis and how Mks interact with HSCs and immune cells has potential clinical implications for stem cell transplantation and other therapies for hematologic malignancies, as well as for treatments to stimulate platelet production and prevent thrombocytopenia.

Bacterial infection disrupts established germinal center reactions through monocyte recruitment and impaired metabolic adaptation.

Consecutive exposures to different pathogens are highly prevalent and often alter the host immune response. However, it remains unknown how a secondary bacterial infection affects an ongoing adaptive immune response elicited against primary invading pathogens. We demonstrated that recruitment of Sca-1+ monocytes into lymphoid organs during Salmonella Typhimurium (STm) infection disrupted pre-existing germinal center (GC) reactions. GC responses induced by influenza, plasmodium, or commensals deteriorated following STm infection. GC disruption was independent of the direct bacterial interactions with B cells and instead was induced through recruitment of CCR2-dependent Sca-1+ monocytes into the lymphoid organs. GC collapse was associated with impaired cellular respiration and was dependent on TNFα and IFNγ, the latter of which was essential for Sca-1+ monocyte differentiation. Monocyte recruitment and GC disruption also occurred during LPS-supplemented vaccination and Listeria monocytogenes infection. Thus, systemic activation of the innate immune response upon severe bacterial infection is induced at the expense of antibody-mediated immunity.

Bone marrow regeneration requires mitochondrial transfer from donor Cx43-expressing hematopoietic progenitors to stroma.

The fate of hematopoietic stem and progenitor cells (HSPC) is tightly regulated by their bone marrow (BM) microenvironment (ME). BM transplantation (BMT) frequently requires irradiation preconditioning to ablate endogenous hematopoietic cells. Whether the stromal ME is damaged and how it recovers after irradiation is unknown. We report that BM mesenchymal stromal cells (MSC) undergo massive damage to their mitochondrial function after irradiation. Donor healthy HSPC transfer functional mitochondria to the stromal ME, thus improving mitochondria activity in recipient MSC. Mitochondrial transfer to MSC is cell-contact dependent and mediated by HSPC connexin-43 (Cx43). Hematopoietic Cx43-deficient chimeric mice show reduced mitochondria transfer, which was rescued upon re-expression of Cx43 in HSPC or culture with isolated mitochondria from Cx43 deficient HSPCs. Increased intracellular adenosine triphosphate levels activate the purinergic receptor P2RX7 and lead to reduced activity of adenosine 5'-monophosphate-activated protein kinase (AMPK) in HSPC, dramatically increasing mitochondria transfer to BM MSC. Host stromal ME recovery and donor HSPC engraftment were augmented after mitochondria transfer. Deficiency of Cx43 delayed mesenchymal and osteogenic regeneration while in vivo AMPK inhibition increased stromal recovery. As a consequence, the hematopoietic compartment reconstitution was improved because of the recovery of the supportive stromal ME. Our findings demonstrate that healthy donor HSPC not only reconstitute the hematopoietic system after transplantation, but also support and induce the metabolic recovery of their irradiated, damaged ME via mitochondria transfer. Understanding the mechanisms regulating stromal recovery after myeloablative stress are of high clinical interest to optimize BMT procedures and underscore the importance of accessory, non-HSC to accelerate hematopoietic engraftment.

Lactate released by inflammatory bone marrow neutrophils induces their mobilization via endothelial GPR81 signaling.

Neutrophils provide first line of host defense against bacterial infections utilizing glycolysis for their effector functions. How glycolysis and its major byproduct lactate are triggered in bone marrow (BM) neutrophils and their contribution to neutrophil mobilization in acute inflammation is not clear. Here we report that bacterial lipopolysaccharides (LPS) or Salmonella Typhimurium triggers lactate release by increasing glycolysis, NADPH-oxidase-mediated reactive oxygen species and HIF-1α levels in BM neutrophils. Increased release of BM lactate preferentially promotes neutrophil mobilization by reducing endothelial VE-Cadherin expression, increasing BM vascular permeability via endothelial lactate-receptor GPR81 signaling. GPR81-/- mice mobilize reduced levels of neutrophils in response to LPS, unless rescued by VE-Cadherin disrupting antibodies. Lactate administration also induces release of the BM neutrophil mobilizers G-CSF, CXCL1 and CXCL2, indicating that this metabolite drives neutrophil mobilization via multiple pathways. Our study reveals a metabolic crosstalk between lactate-producing neutrophils and BM endothelium, which controls neutrophil mobilization under bacterial infection.

Daily Onset of Light and Darkness Differentially Controls Hematopoietic Stem Cell Differentiation and Maintenance.

Hematopoietic stem and progenitor cells (HSPCs) tightly couple maintenance of the bone marrow (BM) reservoir, including undifferentiated long-term repopulating hematopoietic stem cells (LT-HSCs), with intensive daily production of mature leukocytes and blood replenishment. We found two daily peaks of BM HSPC activity that are initiated by onset of light and darkness providing this coupling. Both peaks follow transient elevation of BM norepinephrine and TNF secretion, which temporarily increase HSPC reactive oxygen species (ROS) levels. Light-induced norepinephrine and TNF secretion augments HSPC differentiation and increases vascular permeability to replenish the blood. In contrast, darkness-induced TNF increases melatonin secretion to drive renewal of HSPCs and LT-HSC potential through modulating surface CD150 and c-Kit expression, increasing COX-2/αSMA+ macrophages, diminishing vascular permeability, and reducing HSPC ROS levels. These findings reveal that light- and darkness-induced daily bursts of norepinephrine, TNF, and melatonin within the BM are essential for synchronized mature blood cell production and HSPC pool repopulation.

Free circulating active elastase contributes to chronic inflammation in patients on hemodialysis.

Atherosclerosis and cardiovascular complications are prevalent among patients undergoing chronic hemodialysis (HD). In this population, peripheral polymorphonuclear leukocytes (PMNLs) are primed, releasing proinflammatory mediators such as elastase. Elastase is normally inhibited by a specific inhibitor, avoiding undesirable degradation of cellular and extracellular components. This study tested the hypothesis that in states of noninfectious inflammation, elastase is released by PMNLs and acts in an uncontrolled manner to inflict vascular damage. Blood was collected from patients undergoing HD and healthy controls (HC). PMNL intracellular and surface expressions of elastase were determined by quantitative real-time PCR, Western blotting, and flow cytometry. The elastase activity was evaluated using a fluorescent substrate. The levels of serum α1-antitrypsin (α1-AT), the natural elastase inhibitor, were determined by Western blot. Free active elastase was elevated in HD sera, whereas the levels of α1-AT were decreased compared with HC. The levels of the intracellular elastase enzyme and its activity were lower in HD PMNLs despite similar expression levels of elastase mRNA. Elastase binding to PMNL cell surface was higher in HD compared with HC. The increased circulating levels of free active elastase released from primed HD PMNLs together with the higher cell surface-bound enzymes and the lower levels of α1-AT result in the higher elastase activity in HD sera. This exacerbated elastase activity could lead to the endothelial dysfunction, as hypothesized. In addition, it suggests that free circulating elastase can serve as a new biomarker and therapeutic target to reduce inflammation and vascular complications in patients on hemodialysis.