The bone marrow (BM) niche comprises multiple cell types that regulate hematopoietic stem/progenitor cell (HSPC) migration out of the niche and into the circulation. Here, we demonstrate that osteocytes, the major cellular component of mature bone, are regulators of HSPC egress. Granulocyte colony-stimulating factor (G-CSF), used clinically to mobilize HSPCs, induces changes in the morphology and gene expression of the osteocytic network that precedes changes in osteoblasts. This rapid response is likely under control of the sympathetic nervous system, since osteocytes express the β2-adrenergic receptor and surgical sympathectomy prevents it. Mice with targeted ablation of osteocytes or a disrupted osteocyte network have comparable numbers of HSPCs in the BM but fail to mobilize HSPCs in response to G-CSF. Taken together, these results indicate that the BM/bone niche interface is critically controlled from inside of the bone matrix and establish an important physiological role for skeletal tissues in hematopoietic function.

Osteocytes act as mechanosensors to control local bone volume. However, their roles in the homeostasis of remote organs are largely unknown. We show that ablation of osteocytes in mice (osteocyte-less [OL] mice) leads to severe lymphopenia, due to lack of lymphoid-supporting stroma in both the bone marrow and thymus, and complete loss of white adipose tissues. These effects were reversed when osteocytes were replenished within the bone. In contrast, neither in vivo supply of T cell progenitors and humoral factors via shared circulation with a normal parabiotic partner nor ablation of specific hypothalamic nuclei rescued thymic atrophy and fat loss in OL mice. Furthermore, ablation of the hypothalamus in OL mice led to hepatic steatosis, which was rescued by parabiosis with normal mice. Our results define a role for osteocytes as critical regulators of lymphopoiesis and fat metabolism and suggest that bone acts as a central regulator of multiple organs.

Posttranscriptional machinery regulates inflammation and is associated with autoimmunity as well as tumorigenesis in collaboration with transcription factors. We previously identified the tumor suppressor gene transformed follicular lymphoma (TFL) on 6q25 in a patient with follicular lymphoma, which transformed into diffuse large B cell lymphoma. TFL families have a common RNase domain that governs macrophage-mediated inflammation. In human peripheral blood, TFL is dominantly expressed at the glycine- and tryptophan-rich cytoplasmic processing bodies of T lymphocytes, and it is persistently upregulated in activated T cells. To address its physiological role, we established TFL−/− mice in which TFL−/− lymphocytes proliferated more rapidly than TFL+/+ upon stimulation with inappropriate cytokine secretion, including IL-2, IL-6, and IL-10. Moreover, TFL inhibited the synthesis of cytokines such as IL-2, IL-6, IL-10, TNF-α, and IL-17a by 3′ untranslated region RNA degradation. Experimental autoimmune encephalitis induced in TFL−/− mice demonstrated persistent severe paralysis. CNS-infiltrated CD4+ T cells in TFL−/− mice contained a higher proportion of Th17 cells than did those in TFL+/+ mice during the resolution phase, and IL-17a mRNA levels were markedly increased in TFL−/− cells. These results suggest that TFL may play an important role in attenuating local inflammation by suppressing the infiltration of Th17 cells in the CNS during the resolution phase of experimental autoimmune encephalitis. TFL is a novel gradual and persistent posttranscriptional regulator, and the TFL-driven attenuation of excessive inflammation could contribute to recovery from T cell–mediated autoimmune diseases.

Granulocyte colony-stimulating factor (G-CSF) is widely used for peripheral blood stem/progenitor mobilization. G-CSF causes low-grade fever that is ameliorated by nonsteroidal anti-inflammatory drugs (NSAIDs), suggesting the activation of arachidonic acid (AA) cascade. How G-CSF regulated this reaction was assessed. G-CSF treatment in mice resulted in fever, which was canceled in prostaglandin E synthase (mPGES-1)-deficient mice. Mobilization efficiency was twice as high in chimeric mice lacking mPGES-1, specifically in hematopoietic cells, suggesting that prostaglandin E (PGE) from hematopoietic cells modulated the bone marrow (BM) microenvironment. Neutrophils from steady-state BM constitutively expressed mPGES-1 and significantly enhanced PGE production in vitro by β-adrenergic stimulation, but not by G-CSF, which was inhibited by an NSAID. Although neutrophils expressed all β-adrenergic receptors, only β3-agonist induced this phenomenon. Liquid chromatography-tandem mass spectrometry traced β-agonist-induced PGE synthesis from exogenous deuterium-labeled AA. Spontaneous PGE production was highly efficient in Gr-1 neutrophils among BM cells from G-CSF-treated mice. In addition to these in vitro data, the in vivo depletion of Gr-1 neutrophils disrupted G-CSF-induced fever. Furthermore, sympathetic denervation eliminated both neutrophil priming for PGE production and fever during G-CSF treatment. Thus, sympathetic tone-primed BM neutrophils were identified as one of the major PGE producers. PGE upregulated osteopontin, specifically in preosteoblasts, to retain progenitors in the BM via EP4 receptor. Thus, the sympathetic nervous system regulated neutrophils as an indispensable PGE source to modulate BM microenvironment and body temperature. This study provided a novel mechanistic insight into the communication of the nervous system, BM niche components, and hematopoietic cells.

Mycophenolate mofetil (MMF) has been widely used for prophylaxis against graft-versus-host disease (GVHD) following allogeneic hematopoietic stem cell transplantation (allo-SCT). However, no clear advantage over methotrexate has been reported, other than reduced incidence of mucositis. We speculated that the wide inter-individual variation of plasma mycophenolic acid (MPA) levels veiled the benefits of MMF. Data from 36 unrelated allogeneic bone marrow (allo-BMT) and cord blood transplantation (CBT) were analyzed retrospectively based on MPA area under the curve (AUC(0-24h)). In allo-BMT, high AUC(0-24h) (>30 μg h/ml) resulted in no incidence of grade II-IV acute/extensive chronic GVHD and tended to show higher overall and disease-free survival, lower relapse rates, and non-relapse mortality. In CBT, AUC(0-24h) less than 30 μg h/ml was sufficient for low incidence of acute/chronic GVHD and high survival. Strong correlation between AUC(0-24h) and C(2h), plasma MPA concentration at 2 h after administration was observed. Single point assessment of C(2h) was shown to provide a useful surrogate of AUC(0-24h) to predict GVHD incidence. The results of this study suggest that individualized MMF dosing in a donor source-dependent fashion may be important for maximizing the benefit of MMF in allo-SCT.

Fibroblast growth factor-23 (FGF23) hormone is produced by bone-embedded osteocytes and regulates phosphate homeostasis in kidneys. We found that granulocyte colony-stimulating factor (G-CSF) administration in mice induced a rapid and tremendous increase in FGF23 mRNA in bone marrow (BM) cells. This increase mainly originated from CD45-Ter119+CD71+ erythroblasts. FGF23 protein in BM extracellular fluid was markedly increased during G-CSF-induced hematopoietic progenitor cell (HPC) mobilization but remained stable in the blood with no change in the phosphate level. Consistent with the BM hypoxia induced by G-CSF, low oxygen concentration induced FGF23 release from human erythroblast HUDEP-2 cells in vitro. The mobilization efficiency by G-CSF was drastically decreased in both FGF23-/- and chimeric mice with FGF23 deficiency only in hematopoietic cells but increased in osteocyte-specific FGF23-/- mice. This suggests that erythroblast-derived, but not bone-derived, FGF23 is required to release HPCs from BM to circulation. Mechanistically, FGF23 did not influence CXCL12 binding to CXCR4 on progenitors but interfered with their transwell migration toward CXCL12, which was canceled by FGF receptor inhibitors. These results suggest that BM erythroblasts facilitate G-CSF-induced HPC mobilization via FGF23 production as an intrinsic suppressor of chemoattraction.

Background and Purpose Inflammation has been associated with stress‐related mental disturbances. Rodent studies have reported that blood‐borne cytokines are crucial for stress‐induced changes in emotional behaviours. However, the roles and regulation of leukocytes in chronic stress remain unclear. Experimental Approach Adult male C57BL/6N mice were subjected to repeated social defeat stress (R‐SDS) with two protocols which differed in stress durations, stress cycles, and housing conditions, followed by the social interaction test. The numbers of leukocyte subsets in the bone marrow, spleen, and blood were determined by flow cytometry shortly after or several days after R‐SDS. These leukocyte changes were studied in two strains of mice with different stress susceptibility, C57BL/6N and BALB/c mice. Key Results R‐SDS with both protocols similarly induced social avoidance in C57BL/6N mice. In the bone marrow, neutrophils and monocytes were increased, and T cells, B cells, NK cells, and dendritic cells were decreased with both protocols. In the blood, neutrophils and monocytes were increased with both protocols, whereas T cells, B cells, NK cells, and dendritic cells were decreased with one of these. Neutrophils and monocytes were also increased in the spleen. Changes in the bone marrow and increased levels of circulating neutrophils were maintained for 6 days after R‐SDS. BALB/c mice showed greater social avoidance and increase in circulating neutrophils than C57BL/6N mice. Conclusion and Implications In two strains of mice, chronic stress induced neutrophil mobilization and its maintenance. These effects were strain‐related and may contribute to the pathology of mental illness. LINKED ARTICLES This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc

Myelofibrosis in myeloproliferative neoplasms (MPNs) with mutations such as JAK2V617F is an unfavorable sign for uncontrollable disease progression in the clinic and is complicated with osteosclerosis whose pathogenesis is largely unknown. Because several studies have revealed that macrophages are an indispensable supporter for bone-forming osteoblasts, we speculated that macrophages might play a significant role in the proliferation of collagen-producing myofibroblasts in marrow fibrotic tissues. Here, we show that myelofibrosis critically depends on macrophages whose differentiation is skewed by vitamin D receptor (VDR) signaling. In our novel myelofibrosis model established by transplantation of VDR+/+ hematopoietic stem/progenitor cells into VDR−/− mice, donor-derived F4/80+ macrophages proliferated together with recipient-derived α-smooth muscle actin–positive myofibroblasts, both of which comprised fibrotic tissues with an indistinguishable spindle-shaped morphology. Interfering VDR signals, such as low vitamin D diet and VDR deficiency in donor cells as well as macrophage depletion prevented myelofibrosis in this model. These interventions also ameliorated myelofibrosis in JAK2V617F-driven murine MPNs likely in a transforming growth factor-β1– or megakaryocyte-independent manner. These results suggest that VDR and macrophages may be novel therapeutic targets for MPNs with myelofibrosis.