Yuya Kunisaki

Arteriolar niches maintain haematopoietic stem cell quiescence

Cell cycle quiescence is a critical feature contributing to haematopoietic stem cell (HSC) maintenance. Although various candidate stromal cells have been identified as potential HSC niches, the spatial localization of quiescent HSCs in the bone marrow remains unclear. Here, using a novel approach that combines whole-mount confocal immunofluorescence imaging techniques and computational modelling to analyse significant three-dimensional associations in the mouse bone marrow among vascular structures, stromal cells and HSCs, we show that quiescent HSCs associate specifically with small arterioles that are preferentially found in endosteal bone marrow. These arterioles are ensheathed exclusively by rare NG2 (also known as CSPG4)+ pericytes, distinct from sinusoid-associated leptin receptor (LEPR)+ cells. Pharmacological or genetic activation of the HSC cell cycle alters the distribution of HSCs from NG2+ periarteriolar niches to LEPR+ perisinusoidal niches. Conditional depletion of NG2+ cells induces HSC cycling and reduces functional long-term repopulating HSCs in the bone marrow. These results thus indicate that arteriolar niches are indispensable for maintaining HSC quiescence.

Circadian control of the immune system

Circadian rhythms, which have long been known to play crucial roles in physiology, are emerging as important regulators of specific immune functions. Circadian oscillations of immune mediators coincide with the activity of the immune system, possibly allowing the host to anticipate and handle microbial threats more efficiently. These oscillations may also help to promote tissue recovery and the clearance of potentially harmful cellular elements from the circulation. This Review summarizes the current knowledge of circadian rhythms in the immune system and provides an outlook on potential future implications.

PDGFRα and CD51 mark human Nestin+ sphere-forming mesenchymal stem cells capable of hematopoietic progenitor cell expansion

A subset of human Nestin+ mesenchymal stem cells expresses PDGFRα and CD51, and these markers can be used for prospective isolation of these cells.

Megakaryocytes regulate hematopoietic stem cell quiescence through CXCL4 secretion

In the bone marrow, hematopoietic stem cells (HSCs) lodge in specialized microenvironments that tightly control the proliferative state of HSCs to adapt to the varying needs for replenishment of blood cells while also preventing HSC exhaustion. All putative niche cells suggested thus far have a nonhematopoietic origin. Thus, it remains unclear how feedback from mature cells is conveyed to HSCs to adjust their proliferation. Here we show that megakaryocytes (MKs) can directly regulate HSC pool size in mice. Three-dimensional whole-mount imaging revealed that endogenous HSCs are frequently located adjacent to MKs in a nonrandom fashion. Selective in vivo depletion of MKs resulted in specific loss of HSC quiescence and led to a marked expansion of functional HSCs. Gene expression analyses revealed that MKs are the source of chemokine C-X-C motif ligand 4 (CXCL4, also named platelet factor 4 or PF4) in the bone marrow, and we found that CXCL4 regulates HSC cell cycle activity. CXCL4 injection into mice resulted in a reduced number of HSCs because of their increased quiescence. By contrast, Cxcl4−/− mice exhibited an increased number of HSCs and increased HSC proliferation. Combined use of whole-mount imaging and computational modeling was highly suggestive of a megakaryocytic niche capable of independently influencing HSC maintenance by regulating quiescence. These results indicate that a terminally differentiated cell type derived from HSCs contributes to the HSC niche, directly regulating HSC behavior.

CD169+ macrophages provide a niche promoting erythropoiesis under homeostasis and stress

The role of macrophages in erythropoiesis was suggested several decades ago with the description of “erythroblastic islands” in the bone marrow (BM) composed of a central macrophage surrounded by developing erythroblasts. However, the in vivo role of macrophages in erythropoiesis under homeostasis or disease remains unclear. Specific depletion of CD169+ macrophages markedly reduced erythroblasts in the BM but did not result in overt anemia under homeostasis likely due to concomitant alterations in RBC clearance. However, CD169+ macrophage depletion significantly impaired erythropoietic recovery from hemolytic anemia, acute blood loss and myeloablation. Furthermore, macrophage depletion normalized the erythroid compartment in a JAK2V617F-driven murine model of polycythemia vera (PV), suggesting that erythropoiesis in PV, unexpectedly, remains under the control of macrophages in the BM and splenic microenvironments. These data indicate that CD169+ macrophages promote late erythroid maturation and that modulation of the macrophage compartment represents a novel strategy to treat erythropoietic disorders.A role for macrophages in erythropoiesis was suggested several decades ago when erythroblastic islands in the bone marrow, composed of a central macrophage surrounded by developing erythroblasts, were described. However, the in vivo role of macrophages in erythropoiesis under homeostatic conditions or in disease remains unclear. We found that specific depletion of CD169+ macrophages markedly reduced the number of erythroblasts in the bone marrow but did not result in overt anemia under homeostatic conditions, probably because of concomitant alterations in red blood cell clearance. However, CD169+ macrophage depletion significantly impaired erythropoietic recovery from hemolytic anemia, acute blood loss and myeloablation. Furthermore, macrophage depletion normalized the erythroid compartment in a JAK2V617F-driven mouse model of polycythemia vera, suggesting that erythropoiesis in polycythemia vera remains under the control of macrophages in the bone marrow and splenic microenvironments. These results indicate that CD169+ macrophages promote late erythroid maturation and that modulation of the macrophage compartment may be a new strategy to treat erythropoietic disorders.

Osterix Marks Distinct Waves of Primitive and Definitive Stromal Progenitors during Bone Marrow Development

Mesenchymal stem and progenitor cells (MSPCs) contribute to bone marrow (BM) homeostasis by generating multiple types of stromal cells. MSPCs can be labeled in the adult BM by Nestin-GFP, whereas committed osteoblast progenitors are marked by Osterix expression. However, the developmental origin and hierarchical relationship of stromal cells remain largely unknown. Here, by using a lineage-tracing system, we describe three distinct waves of contributions of Osterix(+) cells in the BM. First, Osterix(+) progenitors in the fetal BM contribute to nascent bone tissues and transient stromal cells that are replaced in the adult marrow. Second, Osterix-expressing cells perinatally contribute to osteolineages and long-lived BM stroma, which have characteristics of Nestin-GFP(+) MSPCs. Third, Osterix labeling in the adult marrow is osteolineage-restricted, devoid of stromal contribution. These results uncover a broad expression profile of Osterix and raise the intriguing possibility that distinct waves of stromal cells, primitive and definitive, may organize the developing BM.

Acute Myelogenous Leukemia-Induced Sympathetic Neuropathy Promotes Malignancy in an Altered Hematopoietic Stem Cell Niche

Perivascular mesenchymal stem and progenitor cells (MSPCs) are critical for forming a healthy hematopoietic stem cell (HSC) niche. However, the interactions and influence of acute myelogenous leukemia (AML) stem cells with the microenvironment remain largely unexplored. We have unexpectedly found that neuropathy of the sympathetic nervous system (SNS) promotes leukemic bone marrow infiltration in an MLL-AF9 AML model. Development of AML disrupts SNS nerves and the quiescence of Nestin(+) niche cells, leading to an expansion of phenotypic MSPCs primed for osteoblastic differentiation at the expense of HSC-maintaining NG2(+) periarteriolar niche cells. Adrenergic signaling promoting leukemogenesis is transduced by the β2, but not β3, adrenergic receptor expressed on stromal cells of leukemic bone marrow. These results indicate that sympathetic neuropathy may represent a mechanism for the malignancy in order to co-opt the microenvironment and suggest separate mesenchymal niche activities for malignant and healthy hematopoietic stem cells in the bone marrow.

Chemotherapy-induced bone marrow nerve injury impairs hematopoietic regeneration

Anticancer chemotherapy drugs challenge hematopoietic tissues to regenerate but commonly produce long-term sequelae. Chemotherapy-induced deficits in hematopoietic stem or stromal cell function have been described, but the mechanisms mediating hematopoietic dysfunction remain unclear. Administration of multiple cycles of cisplatin chemotherapy causes substantial sensory neuropathy. Here we demonstrate that chemotherapy-induced nerve injury in the bone marrow of mice is a crucial lesion impairing hematopoietic regeneration. Using pharmacological and genetic models, we show that the selective loss of adrenergic innervation in the bone marrow alters its regeneration after genotoxic insult. Sympathetic nerves in the marrow promote the survival of constituents of the stem cell niche that initiate recovery. Neuroprotection by deletion of Trp53 in sympathetic neurons or neuroregeneration by administration of 4-methylcatechol or glial-derived neurotrophic factor (GDNF) promotes hematopoietic recovery. These results demonstrate the potential benefit of adrenergic nerve protection for shielding hematopoietic niches from injury.

Pretransplant CSF-1 therapy expands recipient macrophages and ameliorates GVHD after allogeneic hematopoietic cell transplantation

Host macrophages protect against graft-versus-host disease in part by engulfing donor T cells and inhibiting their proliferation.

T helper type 2 differentiation and intracellular trafficking of the interleukin 4 receptor-|[alpha]| subunit controlled by the Rac activator Dock2

The lineage commitment of CD4+ T cells is coordinately regulated by signals through the T cell receptor and cytokine receptors, yet how these signals are integrated remains elusive. Here we find that mice lacking Dock2, a Rac activator in lymphocytes, developed allergic disease through a mechanism dependent on CD4+ T cells and the interleukin 4 receptor (IL-4R). Dock2-deficient CD4+ T cells showed impaired antigen-driven downregulation of IL-4Rα surface expression, resulting in sustained IL-4R signaling and excessive T helper type 2 responses. Dock2 was required for T cell receptor–mediated phosphorylation of the microtubule-destabilizing protein stathmin and for lysosomal trafficking and the degradation of IL-4Rα. Thus, Dock2 links T cell receptor signals to downregulation of IL-4Rα to control the lineage commitment of CD4+ T cells.