Hematological defects in the oc/oc mouse, a model of infantile malignant osteopetrosis

Molecular Mechanisms of Craniofacial and Dental Abnormalities in Osteopetrosis

Osteopetrosis is a group of genetic bone disorders characterized by increased bone density and defective bone resorption. Osteopetrosis presents a series of clinical manifestations, including craniofacial deformities and dental problems. However, few previous reports have focused on the features of craniofacial and dental problems in osteopetrosis. In this review, we go through the clinical features, types, and related pathogenic genes of osteopetrosis. Then we summarize and describe the characteristics of craniofacial and dental abnormalities in osteopetrosis that have been published in PubMed from 1965 to the present. We found that all 13 types of osteopetrosis have craniomaxillofacial and dental phenotypes. The main pathogenic genes, such as chloride channel 7 gene (CLCN7), T cell immune regulator 1 (TCIRG1), osteopetrosis-associated transmembrane protein 1 (OSTM1), pleckstrin homology domain-containing protein family member 1 (PLEKHM1), and carbonic anhydrase II (CA2), and their molecular mechanisms involved in craniofacial and dental phenotypes, are discussed. We conclude that the telltale craniofacial and dental abnormalities are important for dentists and other clinicians in the diagnosis of osteopetrosis and other genetic bone diseases.

The road of NSCLC stem cells toward bone metastases

Despite advancement in therapeutic options, Non-Small Cell lung cancer (NSCLC) remains a lethal disease mostly due to late diagnosis at metastatic phase and drug resistance. Bone is one of the more frequent sites for NSCLC metastatization. A defined subset of cancer stem cells (CSCs) that possess motile properties, mesenchymal features and tumor initiation potential are defined as metastasis initiating cells (MICs). A better understanding of the mechanisms supporting MIC dissemination and interaction with bone microenvironment is fundamental to design novel rational therapeutic option for long lasting efficient treatment of NSCLC. In this review we will summarize findings about bone metastatic process initiated by NSCLC MICs. We will review how MICs can reach bone and interact with its microenvironment that supports their extravasation, seeding, dormancy/proliferation. The role of different cell types inside the bone metastatic niche, such as endothelial cells, bone cells, hematopoietic stem cells and immune cells will be discussed in regards of their impact in dictating the success of metastasis establishment by MICs. Finally, novel therapeutic options to target NSCLC MIC-induced bone metastases, increasing the survival of patients, will be presented.

Osteoclast rich osteopetrosis due to defects in the TCIRG1 gene

Discovery that mutations in TCIRG1 (also known as Atp6i) gene are responsible for most instances of autosomal recessive osteopetrosis (ARO) heralded a new era for comprehension and treatment of this phenotypically heterogeneous rare bone disease. TCIRG1 encodes the a3 subunit, an essential isoform of the vacuolar ATPase proton pump involved in acidification of the osteoclast resorption lacuna and in secretory lysosome trafficking. TCIRG1 defects lead to inefficient bone resorption by nonfunctional osteoclasts seen in abundance on bone marrow biopsy, delineating this ARO as 'osteoclast-rich'. Presentation is usually in early childhood and features of extramedullary haematopoiesis (hepatosplenomegaly, anaemia, thrombocytopenia) due to bone marrow fibrosis, and cranial nerve impingement (blindness in particular). Impaired dietary calcium uptake due to high pH causes the co-occurrence of rickets, described as "osteopetrorickets". Osteoclast dysfunction leads to early death if untreated, and allogeneic haematopoietic stem cell transplantation is currently the treatment of choice. Studies of patients as well as of mouse models carrying spontaneous (the oc/oc mouse) or targeted disruption of Atp6i (TCIRG1) gene have been instrumental providing insight into disease pathogenesis and development of novel cellular therapies that exploit gene correction.

Progressive skeletal defects caused by Kindlin3 deficiency, a model of autosomal recessive osteopetrosis in humans

The cellular and molecular mechanisms of bone development and homeostasis are clinically important, but not fully understood. Mutations in integrins and Kindlin3 in humans known as Leukocyte adhesion deficiencies (LAD) cause a wide spectrum of complications, including osteopetrosis. Yet, the rarity, frequent misdiagnosis, and lethality of LAD preclude mechanistic analysis of skeletal abnormalities in these patients. Here, using inducible and constitutive tissue-specific Kindlin3 knockout (K3KO) mice, we show that the constitutive lack of embryonic-Kindlin3 in myeloid lineage cells causes growth retardation, edentulism, and skull deformity indicative of hydrocephaly. Micro-CT analysis revealed craniosynostosis, choanal stenosis, and micrognathia along with other skeletal abnormalities characteristic of osteopetrosis. A marked progression of osteosclerosis occurs in mature to middle-aged adults, resulting in the narrowing of cranial nerve foramina and bone marrow cavities of long bones. However, postnatal-Kindlin3 is less critical for bone remodeling and architecture. Thus, myeloid Kindlin3 is essential for skeletal development and its deficiency leads to autosomal recessive osteopetrosis (ARO). The study will aid in the diagnosis, management, and treatment choices for patients with LAD-III and ARO.

The Bone Marrow Microenvironment in B-Cell Development and Malignancy

B lymphopoiesis is characterized by progressive loss of multipotent potential in hematopoietic stem cells, followed by commitment to differentiate into B cells, which mediate the humoral response of the adaptive immune system. This process is tightly regulated by spatially distinct bone marrow niches where cells, including mesenchymal stem and progenitor cells, endothelial cells, osteoblasts, osteoclasts, and adipocytes, interact with B-cell progenitors to direct their proliferation and differentiation. Recently, the B-cell niche has been implicated in initiating and facilitating B-cell precursor acute lymphoblastic leukemia. Leukemic cells are also capable of remodeling the B-cell niche to promote their growth and survival and evade treatment. Here, we discuss the major cellular components of bone marrow niches for B lymphopoiesis and the role of the malignant B-cell niche in disease development, treatment resistance and relapse. Further understanding of the crosstalk between leukemic cells and bone marrow niche cells will enable development of additional therapeutic strategies that target the niches in order to hinder leukemia progression.

A Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis is a critical mediator of Ostm1 regulatory network in T lymphopoiesis

Ostm1 mutations cause the severe form of osteopetrosis with bone marrow deficiency in humans and mice, yet a role in T cell ontogeny remains to be determined. Herein, we show that thymi of the Ostm1-null mice (gl/gl) from P8-to-P15 become markedly hypocellular with disturbed architecture. Analysis of gl/gl early T cell program determined a major decrease of 3-fold in bone marrow common lymphoid precursors (CLP), 35-fold in early thymic precursors (ETPs) and 100-fold in T cell double positive subpopulations. Ostm1 ablation in T cell double negative (DN) also appears to induce fast-paced differentiation kinetics with a transitory intermediate CD44⁺CD25^int subpopulation. Transgenic targeting Ostm1 expression from the gl/gl DN1 population partially rescued T cell subpopulations from ETP onwards and normalized the accelerated DN differentiation, indicating a cell-autonomous role for Ostm1. Transcriptome of early DN1 population identified an Ostm1 crosstalk with a Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis. Our findings establish that Ostm1 is an essential regulator of T cell ontogeny.

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Loss of Ostm1 causes severe thymus hypocellularity

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Ostm1 is a modulator of the T cell differentiation program from the CLPs onwards

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Targeted CD2-Ostm1 in Ostm1 null mice leads to partial rescue of DN differentiation

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Ostm1 null DN1 transcriptome identifies a Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis

Ruxolitinib ameliorates progressive anemia and improves survival during episodes of emergency granulopoiesis in Fanconi C−/− mice

Fanconi anemia (FA) is an inherited disorder of DNA repair with hematologic manifestations that range from anemia to bone marrow failure to acute myeloid leukemia. In a murine model of FA (Fancc^-/- mice), we found bone marrow failure was accelerated by repeated attempts to induce emergency (stress) granulopoiesis, the process for granulocyte production during the innate immune response. Fancc^-/- mice exhibited an impaired granulocytosis response and died with profound anemia during repeated challenge. In the current study, we found erythropoiesis and serum erythropoietin decreased in Fancc^-/- and wild-type (Wt) mice as emergency granulopoiesis peaked. Serum erythropoietin returned to baseline during steady-state resumption, and compensatory proliferation of erythroid progenitors was associated with DNA damage and apoptosis in Fancc^-/- mice, but not Wt mice. The erythropoietin receptor activates Janus kinase 2 (Jak2), and we found treatment of Fancc^-/- mice with ruxolitinib (Jak1/2-inhibitor) decreased anemia, enhanced granulocytosis, delayed clonal progression and prolonged survival during repeated emergency granulopoiesis episodes. This was associated with a decrease in DNA damage and apoptosis in Fancc^-/- erythroid progenitors during this process. Transcriptome analysis of these cells identified enhanced activity of pathways for metabolism of reactive oxygen species, and decreased apoptosis- and autophagy-related pathways, as major ruxolitinib-effects in Fancc^-/- mice. In contrast, ruxolitinib influenced primarily pathways involved in proliferation and differentiation in Wt mice. Ruxolitinib is approved for treatment of myeloproliferative disorders and graft-versus-host disease, suggesting the possibility of translational use as a bone marrow protectant in FA.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

OUP accepted manuscript

Osteopetrosis is a rare inherited disease characterized by impaired osteoclast activity causing defective bone resorption and bone marrow aplasia. It is fatal in early childhood unless hematopoietic stem cell transplantation is performed. But, the transplant course is complicated with engraftment failure. Recently, osteoclasts have been described as the potential regulators of hematopoietic stem cell (HSC) niche. Here we investigated the alterations in the HSC and mesenchymal stromal cell (MSC) components of osteopetrotic niche and their interactions to mimic the stem cell dynamics/trafficking in the BM niche after HSC transplantation. Induced pluripotent stem cells were generated from peripheral blood mononuclear cells of patients with osteopetrosis carrying TCIRG1 mutation. iPSC lines were differentiated into hematopoietic and myeloid progenitors, then into osteoclasts using a step-wise protocol. We first demonstrated a shift toward monocyte-macrophages lineage regarding hematopoietic differentiation potential of osteopetrotic iPSC-derived hematopoietic progenitors (HPCs) and phenotypically normal and functionally defective osteoclast formation. The expression of the genes involved in HSC homing and maintenance (Sdf-1, Jagged-1, Kit-L, and Opn) in osteopetrotic MSCs recovered significantly after coculture with healthy HPCs. Similarly, the restoration of phenotype, impaired differentiation, and migratory potential of osteopetrotic iHPCs were observed upon interaction with healthy MSCs. Our results establish significant alterations in both MSC and HPC compartments of the osteopetrotic niche, and support the impact of functionally impaired osteoclasts in defective niche formation.

Immune Checkpoint Inhibitor Therapy for Bone Metastases: Specific Microenvironment and Current Situation

The treatment of bone metastases is a thorny issue. Immunotherapy may be one of the few hopes for patients with unresectable bone metastases. Immune checkpoint inhibitors are the most commonly used immunotherapy drugs currently. In this review, the characteristics and interaction of bone metastases and their immune microenvironment were systematically discussed, and the relevant research progress of the immunological mechanism of tumor bone metastasis was reviewed. On this basis, we expounded the clinical application of immune checkpoint inhibitors for bone metastasis of common tumors, including non-small-cell lung cancer, renal cell carcinoma, prostate cancer, melanoma, and breast cancer. Then, the deficiencies and limitations in current researches were summarized. In-depth basic research on bone metastases and optimization of clinical treatment is needed.

Gene therapy for infantile malignant osteopetrosis: review of pre-clinical research and proof-of-concept for phenotypic reversal

Infantile malignant osteopetrosis is a devastating disorder of early childhood that is frequently fatal and for which there are only limited therapeutic options. Gene therapy utilizing autologous hematopoietic stem and progenitor cells represents a potentially advantageous therapeutic alternative for this multisystemic disease. Gene therapy can be performed relatively rapidly following diagnosis, will not result in graft versus host disease, and may also have potential for reduced incidences of other transplant-related complications. In this review, we have summarized the past sixteen years of research aimed at developing a gene therapy for infantile malignant osteopetrosis; these efforts have culminated in the first clinical trial employing lentiviral-mediated delivery of TCIRG1 in autologous hematopoietic stem and progenitor cells.

The Bone's Role in Myeloid Neoplasia

The interaction of hematopoietic stem and progenitor cells with their direct neighboring cells in the bone marrow (the so called hematopoietic niche) evolves as a key principle for understanding physiological and malignant hematopoiesis. Significant progress in this matter has recently been achieved making use of emerging high-throughput techniques that allow characterization of the bone marrow microenvironment at single cell resolution. This review aims to discuss these single cell findings in the light of other conventional niche studies that together define the current notion of the niche's implication in i) normal hematopoiesis, ii) myeloid neoplasms and iii) disease-driving pathways that can be exploited to establish novel therapeutic strategies in the future.

Disruption of the preB Cell Receptor Complex Leads to Decreased Bone Mass

In the bone marrow, preB cells are found adjacent to the bone endosteum where bone synthesizing osteoblast and bone resorbing osteoclasts reside. Although there is evidence of interactions between preB and bone cells, the factors that contribute to such interactions are poorly understood. A critical checkpoint for preB cell development assesses the integrity of the nascent immunoglobulin μ heavy chain (HC) by testing whether it can participate in the formation of a preB cell receptor (preBCR), composed of the μ HC and surrogate light chain (LC). In this work, we tested whether loss of preBCR components can affect bone synthesis. A panel of gene targeted mice with sequential blocks in preBCR formation or function [surrogate light chain component lambda 5 deleted (λ5^−/−), transmembrane domain of μHC deleted (IgM-mem^−/−), and CD19 preBCR co-receptor deleted (CD19^−/−)] were evaluated for effects on postnatal bone synthesis. Postnatal bone mass was analyzed in 6 month old mice using μ-CT, histomorphometry and double calcein labeling. Both cortical and trabecular bone mass were significantly decreased in the femurs of the λ5 and IgM-mem deficient mice. Histomorphometric analysis showed a decrease in the numbers of osteoblasts and osteoclasts in all three mutant strains. Double calcein labeling revealed a significant decrease in dynamic synthesis and mineralization of bone in λ5^−/− mice. Our data strongly suggest that interference with preBCR formation or function affects bone homeostasis independent of the presence or absence of mature B cells, and that components of the preBCR play important, and potentially distinct, roles in regulating adult bone mass.

Hematopoietic Stem Cell-Targeted Neonatal Gene Therapy with a Clinically Applicable Lentiviral Vector Corrects Osteopetrosis in oc/oc Mice

Infantile malignant osteopetrosis (IMO) is an autosomal recessive disorder characterized by nonfunctional osteoclasts. Approximately 50% of the patients have mutations in the TCIRG1 gene, encoding for a subunit of the osteoclast proton pump. Gene therapy represents a potential alternative treatment to allogeneic stem cell transplantation for IMO. The oc/oc mouse is a model of IMO characterized by a 1,500 bp deletion in the TCIRG1 gene, severe osteopetrosis, and a life span of only 3 weeks. Here we show that the osteopetrotic phenotype in oc/oc mice can be reversed by hematopoietic stem cell-targeted gene therapy with a clinically applicable lentiviral vector expressing a wild-type form of human TCIRG1 under the mammalian promoter elongation factor 1α short (EFS-hT). oc/oc c-kit⁺ fetal liver cells transduced with EFS-hT were transplanted into sublethally irradiated oc/oc mice by temporal vein injection 1 day after birth. A total of 9 of 12 mice survived long term (19-25 weeks) with evidence of tooth eruption, uncharacteristic of oc/oc mice. Splenocytes were harvested 19-25 weeks after transplantation and differentiated into osteoclasts on bone slices to assess resorption and on plastic to assess TCIRG1 protein expression. Vector-corrected osteoclasts showed human TCIRG1 expression by Western blot. CTX-I release relative to that mediated by oc/oc-derived osteoclasts increased 8-239-fold. Resorption pits on bone slices were observed for osteoclasts derived from 7/9 surviving transplanted oc/oc mice. Histopathology of the bones of surviving animals showed varying degrees of rescued phenotype, the majority with almost full correction. The average vector copy number per cell in the bone marrow was 1.8 ± 0.5. Overall, 75% of transplanted mice exhibited long-term survival and marked reversal of the osteopetrotic bone phenotype. These findings represent a significant step toward the clinical application of gene therapy for IMO.

Immune Function and Diversity of Osteoclasts in Normal and Pathological Conditions

Osteoclasts (OCLs) are key players in controlling bone remodeling. Modifications in their differentiation or bone resorbing activity are associated with a number of pathologies ranging from osteopetrosis to osteoporosis, chronic inflammation and cancer, that are all characterized by immunological alterations. Therefore, the 2000s were marked by the emergence of osteoimmunology and by a growing number of studies focused on the control of OCL differentiation and function by the immune system. At the same time, it was discovered that OCLs are much more than bone resorbing cells. As monocytic lineage-derived cells, they belong to a family of cells that displays a wide heterogeneity and plasticity and that is involved in phagocytosis and innate immune responses. However, while OCLs have been extensively studied for their bone resorption capacity, their implication as immune cells was neglected for a long time. In recent years, new evidence pointed out that OCLs play important roles in the modulation of immune responses toward immune suppression or inflammation. They unlocked their capacity to modulate T cell activation, to efficiently process and present antigens as well as their ability to activate T cell responses in an antigen-dependent manner. Moreover, similar to other monocytic lineage cells such as macrophages, monocytes and dendritic cells, OCLs display a phenotypic and functional plasticity participating to their anti-inflammatory or pro-inflammatory effect depending on their cell origin and environment. This review will address this novel vision of the OCL, not only as a phagocyte specialized in bone resorption, but also as innate immune cell participating in the control of immune responses.

Updates on Osteoimmunology: What's New on the Cross-Talk Between Bone and Immune System

The term osteoimmunology was coined many years ago to describe the research field that deals with the cross-regulation between bone cells and the immune system. As a matter of fact, many factors that are classically considered immune-related, such as InterLeukins (i.e., IL-6, -11, -17, and -23), Tumor Necrosis Factor (TNF)-α, Receptor-Activator of Nuclear factor Kappa B (RANK), and its Ligand (RANKL), Nuclear Factor of Activated T-cell, cytoplasmatic-1 (NFATc1), and others have all been found to be crucial in osteoclast and osteoblast biology. Conversely, bone cells, which we used to think would only regulate each other and take care of remodeling bone, actually regulate immune cells, by creating the so-called "endosteal niche." Both osteoblasts and osteoclasts participate to this niche, either by favoring engraftment, or mobilization of Hematopoietic Stem Cells (HSCs). In this review, we will describe the main milestones at the base of the osteoimmunology and present the key cellular players of the bone-immune system cross-talk, including HSCs, osteoblasts, osteoclasts, bone marrow macrophages, osteomacs, T- and B-lymphocytes, dendritic cells, and neutrophils. We will also briefly describe some pathological conditions in which the bone-immune system cross-talk plays a crucial role, with the final aim to portray the state of the art in the mechanisms regulating the bone-immune system interplay, and some of the latest molecular players in the field. This is important to encourage investigation in this field, to identify new targets in the treatment of bone and immune diseases.

Retinoic Acid Receptor γ Activity in Mesenchymal Stem Cells Regulates Endochondral Bone, Angiogenesis, and B Lymphopoiesis

Retinoic acid receptor (RAR) signaling regulates bone structure and hematopoiesis through intrinsic and extrinsic mechanisms. This study aimed to establish how early in the osteoblast lineage loss of RARγ (Rarg) disrupts the bone marrow microenvironment. Bone structure was analyzed by micro-computed tomography (μCT) in Rarg-/- mice and mice with Rarg conditional deletion in Osterix-Cre-targeted osteoblast progenitors or Prrx1-Cre-targeted mesenchymal stem cells. Rarg-/- tibias exhibited less trabecular and cortical bone and impaired longitudinal and radial growth. The trabecular bone and longitudinal, but not radial, growth defects were recapitulated in Prrx1:RargΔ/Δ mice but not Osx1:RargΔ/Δ mice. Although both male and female Prrx1:RargΔ/Δ mice had low trabecular bone mass, males exhibited increased numbers of trabecular osteoclasts and Prrx1:RargΔ/Δ females had impaired mineral deposition. Both male and female Prrx1:RargΔ/Δ growth plates were narrower than controls and their epiphyses contained hypertrophic chondrocyte islands. Flow cytometry revealed that male Prrx1:RargΔ/Δ bone marrow exhibited elevated pro-B and pre-B lymphocyte numbers, accompanied by increased Cxcl12 expression in bone marrow cells. Prrx1:RargΔ/Δ bone marrow also had elevated megakaryocyte-derived Vegfa expression accompanied by smaller sinusoidal vessels. Thus, RARγ expression by Prrx1-Cre-targeted cells directly regulates endochondral bone formation and indirectly regulates tibial vascularization. Furthermore, RARγ expression by Prrx1-Cre-targeted cells extrinsically regulates osteoclastogenesis and B lymphopoiesis in male mice. © 2018 American Society for Bone and Mineral Research.

A Novel Reliable and Efficient Procedure for Purification of Mature Osteoclasts Allowing Functional Assays in Mouse Cells

Osteoclasts (OCLs) are multinucleated phagocytes of monocytic origin responsible for physiological and pathological bone resorption including aging processes, chronic inflammation and cancer. Besides bone resorption, they are also involved in the modulation of immune responses and the regulation of hematopoietic niches. Accordingly, OCLs are the subject of an increasing number of studies. Due to their rarity and the difficulty to isolate them directly ex vivo, analyses on OCLs are usually performed on in vitro differentiated cells. In this state, however, OCLs represent a minority of differentiated cells. Since up to date a reliable purification procedure is still lacking for mature OCLs, all cells present in the culture are analyzed collectively to answer OCL-specific questions. With the development of in-depth transcriptomic and proteomic analyses, such global analyses on unsorted cells can induce severe bias effects in further results. In addition, for instance, analysis on OCL immune function requires working on purified OCLs to avoid contamination effects of monocytic precursors that may persist during the culture. This clearly highlights the need for a reliable OCL purification procedure. Here, we describe a novel and reliable method to sort OCLs based on cell multinucleation while preserving cell viability. Using this method, we successfully purified multinucleated murine cells. We showed that they expressed high levels of OCL markers and retained a high capacity of bone resorption, demonstrating that these are mature OCLs. The same approach was equally applied for the purification of human mature OCLs. Comparison of purified OCLs with mononucleated cells or unsorted cells revealed significant differences in the expression of OCL-specific markers at RNA and/or protein level. This exemplifies that substantially better outcomes for OCLs are achieved after the exclusion of mononucleated cells. Our results clearly demonstrate that the in here presented procedure for the analysis and sorting of pure OCLs represents a novel, robust and reliable method for the detailed examination of bona fide mature OCLs in a range that was previously impossible. Noteworthy, this procedure will open new perspectives into the biology of osteoclasts and osteoclast-related diseases.

Inactivation of mTORC1 Signaling in Osterix-Expressing Cells Impairs B-cell Differentiation

Osteoblasts provide a microenvironmental niche for B-cell commitment and maturation in the bone marrow (BM). Any abnormity of osteoblasts function may result in the defect of B lymphopoiesis. Signaling from mechanistic target of rapamycin complex 1 (mTORC1) has been implicated in regulating the expansion and differentiation of osteoblasts. Thus, we raise a hypothesis that mTORC1 signaling in osteoblasts plays a vital role in B-cell development. Inactivation of mTORC1 in osterix-expressing cells (mainly osteoblast lineage) through Osx-Cre-directed deletion of Raptor (an mTORC1-specific component) resulted in a reduction in the total B-cell population in the BM, which was due to a block in early B-cell development from the pro-B to pre-B cell stage. Further mechanistic studies revealed that this defect was the result of reduction of interleukin-7 (IL-7) expression in osterix-expressing immature osteoblasts, which caused the abnormality of IL-7/Stat5 signaling in early B lymphocytes, leading to an increased apoptosis of pre-B plus immature B cells. In vitro and in vivo studies demonstrated that the addition of exogenous IL-7 partially restored B lymphopoiesis in the BM of Raptor mutant mice. Furthermore, total BM cells cultured in conditioned media from Raptor null immature osteoblasts or media with anti-IL-7 neutralizing antibody failed to differentiate into pre-B and immature B cells, indicating that inactivation of mTORC1 in immature osteoblast cannot fully support normal B-cell development. Taken together, these findings demonstrate a novel role for mTORC1 in the regulation of bone marrow environments that support B-cell differentiation via regulating IL-7 expression. © 2017 American Society for Bone and Mineral Research.

Inflammatory Osteoclasts Prime TNFα-Producing CD4+ T Cells and Express CX3 CR1

Bone destruction is a hallmark of chronic rheumatic diseases. Although the role of osteoclasts in bone loss is clearly established, their implication in the inflammatory response has not been investigated despite their monocytic origin. Moreover, specific markers are lacking to characterize osteoclasts generated in inflammatory conditions. Here, we have explored the phenotype of inflammatory osteoclasts and their effect on CD4⁺ T cell responses in the context of bone destruction associated with inflammatory bowel disease. We used the well-characterized model of colitis induced by transfer of naive CD4⁺ T cells into Rag1^-/- mice, which is associated with severe bone destruction. We set up a novel procedure to sort pure osteoclasts generated in vitro to analyze their phenotype and specific immune responses by FACS and qPCR. We demonstrated that osteoclasts generated from colitic mice induced the emergence of TNFα-producing CD4⁺ T cells, whereas those generated from healthy mice induced CD4⁺ FoxP3⁺ regulatory T cells, in an antigen-dependent manner. This difference is related to the osteoclast origin from monocytes or dendritic cells, to their cytokine expression pattern, and their environment. We identified CX₃ CR1 as a marker of inflammatory osteoclasts and we demonstrated that the differentiation of CX₃ CR1⁺ osteoclasts is controlled by IL-17 in vitro. This work is the first demonstration that, in addition to participating to bone destruction, osteoclasts also induce immunogenic CD4⁺ T cell responses upon inflammation. They highlight CX₃ CR1 as a novel dual target for antiresorptive and anti-inflammatory treatment in inflammatory chronic diseases. © 2016 American Society for Bone and Mineral Research.

Osteopetrosis and its relevance for the discovery of new functions associated with the skeleton

Osteopetrosis is a rare genetic disorder characterized by an increase of bone mass due to defective osteoclast function. Patients typically displayed spontaneous fractures, anemia, and in the most severe forms hepatosplenomegaly and compression of cranial facial nerves leading to deafness and blindness. Osteopetrosis comprises a heterogeneous group of diseases as several forms are known with different models of inheritance and severity from asymptomatic to lethal. This review summarizes the genetic and clinical features of osteopetrosis, emphasizing how recent studies of this disease have contributed to understanding the central role of the skeleton in the whole body physiology. In particular, the interplay of bone with the stomach, insulin metabolism, male fertility, the immune system, bone marrow, and fat is described.

Osteopetrosis mimicking juvenile myelomonocytic leukemia

A 5-month-old boy developed splenomegaly, anemia, thrombocytopenia with elevated white cells, monocytosis and immature granulocytes in the peripheral blood. Bone marrow showed dysplasia without blastosis. Increased colony-forming unit-granulocyte-macrophage was found in the peripheral blood, mimicking granulocyte-macrophage colony-stimulating factor hypersensitivity. These findings fulfilled the diagnosis criteria for juvenile myelomonocytic leukemia (JMML), but no mutations in the CBL, NRAS, KRAS, or PTPN11 genes were detected. In addition to these findings severe hypogammaglobulinemia and elevated alkaline phosphatase were present. Bone X-ray showed dense and radiopaque bones with a bone-in-bone appearance characteristic of infantile malignant osteopetrosis (IMO). Genetic mutation in T-cell, immune regulator 1 (TCIRG1) was identified, confirming the diagnosis of IMO. Careful differential diagnosis including osteopetrosis, is therefore recommended in patients with clinical features and hematologic findings consistent with JMML.

Roles of osteoclasts in the control of medullary hematopoietic niches

Bone marrow is the major site of hematopoiesis in mammals. The bone marrow environment plays an essential role in the regulation of hematopoietic stem and progenitor cells by providing specialized niches in which these cells are maintained. Many cell types participate to the composition and regulation of hematopoietic stem cell (HSC) niches, integrating complex signals from the bone, immune and nervous systems. Among these cells, the bone-resorbing osteoclasts (OCLs) have been described as main regulators of HSC niches. They are not limited to carving space for HSCs, but they also provide signals that affect the molecular and cellular niche components. However, their exact role in HSC niches remains unclear because of the variety of models, signals and conditions used to address the question. The present review will discuss the importance of the implication of OCLs focusing on the formation of HSC niches, the maintenance of HSCs in these niches and the mobilization of HSCs from the bone marrow. It will underline the importance of OCLs in HSC niches.

Mesenchymal progenitors and the osteoblast lineage in bone marrow hematopoietic niches

The bone marrow cavity is essential for the proper development of the hematopoietic system. In the last few decades, it has become clear that mesenchymal stem/progenitor cells as well as cells of the osteoblast lineage, besides maintaining bone homeostasis, are also fundamental regulators of bone marrow hematopoiesis. Several studies have demonstrated the direct involvement of mesenchymal and osteoblast lineage cells in the maintenance and regulation of supportive microenvironments necessary for quiescence, self-renewal and differentiation of hematopoietic stem cells. In addition, specific niches have also been identified within the bone marrow for maturing hematopoietic cells. Here we will review recent findings that have highlighted the roles of mesenchymal progenitors and cells of the osteoblast lineage in regulating distinct stages of hematopoiesis.

Osteoclasts are not crucial for hematopoietic stem cell maintenance in adult mice

The osteoclast is vital for establishment of normal hematopoiesis in the developing animal. However, its role for maintenance of hematopoiesis in adulthood is more controversial. To shed more light on this process, we transplanted hematopoietic stem cells from two osteopetrotic mouse models, with lack of osteoclasts or defective osteoclast function, to normal adult mice and examined the bone phenotype and hematopoiesis in the recipients. B6SJL mice were lethally irradiated and subsequently transplanted with oc/oc, Receptor Activator of Nuclear Factor Kappa B knockout or control fetal liver cells. Osteoclasts derived from the recipient animals were tested in vitro for osteoclastogenesis and resorptive function. Bone remodeling changes were assessed using biomarkers of bone turnover and micro-CT. Hematopoiesis was assessed by flow cytometry and colony formation, and hematopoietic stem cell function by secondary competitive transplantations and cell cycle analysis. After transplantation, a donor chimerism of 97-98% was obtained, and by 15 weeks mild osteopetrosis had developed in recipients of cells from osteopetrotic mice. There were no alterations in the number of bone marrow cells. Colony formation was slightly reduced in Receptor Activator of Nuclear Factor Kappa B knockout recipients but unchanged in oc/oc recipients. Phenotypically, stem cells were marginally reduced in recipients of cells from osteopetrotic mice, but no significant difference was seen in cell cycle status and in competitive secondary transplantations all three groups performed equally well. Our results indicate that osteoclast function is not crucial for hematopoietic stem cell maintenance in adult mice.

Interactions between B lymphocytes and the osteoblast lineage in bone marrow

The regulatory effects of the immune system on the skeleton during homeostasis and activation have been appreciated for years. In the past decade it has become evident that bone tissue can also regulate immune cell development. In the bone marrow, the differentiation of hematopoietic progenitors requires specific microenvironments, called "niches," provided by various subsets of stromal cells, many of which are of mesenchymal origin. Among these stromal cell populations, cells of the osteoblast lineage serve a supportive function in the maintenance of normal hematopoiesis, and B lymphopoiesis in particular. Within the osteoblast lineage, distinct differentiation stages exert differential regulatory effects on hematopoietic development. In this review we will highlight the critical role of osteoblast progenitors in the perivascular B lymphocyte niche.

Reproductive toxicity of denosumab in cynomolgus monkeys

Denosumab is a monoclonal antibody that inhibits bone resorption by targeting RANKL, an essential mediator of osteoclast formation, function, and survival. Reproductive toxicity of denosumab was assessed in cynomolgus monkeys in an embryofetal development study (dosing GD20-50) and a pre-postnatal toxicity study (dosing GD20-parturition). In the embryofetal toxicity study, denosumab did not elicit maternal toxicity, fetal harm or teratogenicity. In the pre-postnatal toxicity study, there were increased stillbirths, and one maternal death due to dystocia. There was no effect on maternal mammary gland histomorphology, lactation, or fetal growth. In infants exposed in utero, there was increased postnatal mortality, decreased body weight gain, and decreased growth/development. Denosumab-related effects in infants were present in bones and lymph nodes. There was full recovery at 6 months of age from most bone-related changes observed earlier postpartum. The effects observed in mothers and infants were consistent with the pharmacological action of denosumab.

High bone mass in the STR/ort mouse results from increased bone formation and impaired bone resorption and is associated with extramedullary hematopoiesis

We here describe the novel high bone mass phenotype in STR/ort mice that leads to increased bone masses of cortical and trabecular bone and is associated with elevated osteoblast activity and impaired osteoclast function alike. Comparison of STR/ort and C57BL/6 mice reveals an increase in trabecular bone volumes of the vertebrae and at femoral metaphysis. In the females, this difference is significant as early as 2 months of age and at 9 months the females by far exceed their age matched males in all parameters measured. The increase in cortical bone mass at femoral diaphysis results from an apposition to the endosteal surface, it is significant for both sexes as early as 1 month of age and leads to bone marrow compression and extramedullary hematopoiesis. Altered activities of both, the osteoblast and the osteoclast contribute to the high bone mass and collectively this phenotype supports a multifactorial pathogenesis. Moreover, the spontaneous development of osteoarthritis in male STR/ort mice is suggestive of a tight correlation between trabecular bone mass and the development of degenerative changes of the articular cartilage.

A novel TCIRG1 gene mutation leads to severe osteopetrosis with altered content of monocytes/macrophages in several organs

Osteopetrosis (OP) is a clinically and genetically heterogeneous disease. Defects in the TCIRG1 gene are most frequently implicated in the osteoclast-rich form of OP. Little is known about the content and/or function of monocytes and macrophages of various organs rich in those cells in patients with OP. We report a patient with a novel TCIRG1 gene mutation that led to an osteoclast-rich OP. A bone marrow transplant failed to engraft, and the patient developed pulmonary hypertension. At autopsy he was found to have abnormal remodeling of the pulmonary vasculature and alveolar proteinosis. Alveolar macrophages were decreased. Pulmonary findings in this patient could be at least partially explained by abnormal surfactant metabolism due to depleted or defective alveolar macrophages.

Osteoclasts promote the formation of hematopoietic stem cell niches in the bone marrow

Osteoclasts promote the formation of the HSC niche by inducing the differentiation of osteoblastic cells from mesenchymal stem cells.

Formation of the hematopoietic stem cell (HSC) niche in bone marrow (BM) is tightly associated with endochondral ossification, but little is known about the mechanisms involved. We used the oc/oc mouse, a mouse model with impaired endochondral ossification caused by a loss of osteoclast (OCL) activity, to investigate the role of osteoblasts (OBLs) and OCLs in the HSC niche formation. The absence of OCL activity resulted in a defective HSC niche associated with an increased proportion of mesenchymal progenitors but reduced osteoblastic differentiation, leading to impaired HSC homing to the BM. Restoration of OCL activity reversed the defect in HSC niche formation. Our data demonstrate that OBLs are required for establishing HSC niches and that osteoblastic development is induced by OCLs. These findings broaden our knowledge of the HSC niche formation, which is critical for understanding normal and pathological hematopoiesis.

Meox2Cre-mediated disruption of CSF-1 leads to osteopetrosis and osteocyte defects

CSF-1, a key regulator of mononuclear phagocyte production, is highly expressed in the skeleton by osteoblasts/osteocytes and in a number of nonskeletal tissues such as uterus, kidney and brain. The spontaneous mutant op/op mouse has been the conventional model of CSF-1 deficiency and exhibits a pleiotropic phenotype characterized by osteopetrosis, and defects in hematopoiesis, fertility and neural function. Studies to further delineate the biologic effect of CSF-1 within various tissues have been hampered by the lack of suitable models. To address this issue, we generated CSF-1 floxed/floxed mice and demonstrate that Cre-mediated recombination using Meox2Cre, a Cre line expressed in epiblast during early embryogenesis, results in mice with ubiquitous CSF-1 deficiency (CSF-1KO). Homozygous CSF-1KO mice lacked CSF-1 in all tissues and displayed, in part, a similar phenotype to op/op mice that included: failure of tooth eruption, osteopetrosis, reduced macrophage densities in reproductive and other organs and altered hematopoiesis with decreased marrow cellularity, circulating monocytes and B cell lymphopoiesis. In contrast to op/op mice, CSF-1KO mice showed elevated circulating and splenic T cells. A striking feature in CSF-1KO mice was defective osteocyte maturation, bone mineralization and osteocyte-lacunar system that was associated with reduced dentin matrix protein 1 (DMP1) expression in osteocytes. CSF-1KO mice also showed a dramatic reduction in osteomacs along the endosteal surface that may have contributed to the hematopoietic and cortical bone defects. Thus, our findings show that ubiquitous CSF-1 gene deletion using a Cre-based system recapitulates the expected osteopetrotic phenotype. Moreover, results point to a novel link between CSF-1 and osteocyte survival/function that is essential for maintaining bone mass and strength during skeletal development.

Dissociation of bone resorption and bone formation in adult mice with a non-functional V-ATPase in osteoclasts leads to increased bone strength

Osteopetrosis caused by defective acid secretion by the osteoclast, is characterized by defective bone resorption, increased osteoclast numbers, while bone formation is normal or increased. In contrast the bones are of poor quality, despite this uncoupling of formation from resorption.To shed light on the effect of uncoupling in adult mice with respect to bone strength, we transplanted irradiated three-month old normal mice with hematopoietic stem cells from control or oc/oc mice, which have defective acid secretion, and followed them for 12 to 28 weeks.Engraftment levels were assessed by flow cytometry of peripheral blood. Serum samples were collected every six weeks for measurement of bone turnover markers. At termination bones were collected for µCT and mechanical testing. An engraftment level of 98% was obtained. From week 6 until termination bone resorption was significantly reduced, while the osteoclast number was increased when comparing oc/oc to controls. Bone formation was elevated at week 6, normalized at week 12, and reduced onwards. µCT and mechanical analyses of femurs and vertebrae showed increased bone volume and bone strength of cortical and trabecular bone.In conclusion, these data show that attenuation of acid secretion in adult mice leads to uncoupling and improves bone strength.

The V-ATPase a3 subunit mutation R740S is dominant negative and results in osteopetrosis in mice

A mouse founder with high bone mineral density and an osteopetrotic phenotype was identified in an N-ethyl-N-nitrosourea (ENU) screen. It was found to carry a dominant missense mutation in the Tcirg1 gene that encodes the a3 subunit of the vacuolar type H(+)-ATPase (V-ATPase), resulting in replacement of a highly conserved amino acid (R740S). The +/R740S mice have normal appearance, size, and weight but exhibit high bone density. Osteoblast parameters are unaffected in bones of +/R740S mice, whereas osteoclast number and marker expression are increased, concomitant with a decrease in the number of apoptotic osteoclasts. Consistent with reduced osteoclast apoptosis, expression of Rankl and Bcl2 is elevated, whereas Casp3 is reduced. Transmission electron microscopy revealed that unlike other known mutations in the a3 subunit of V-ATPase, polarization and ruffled border formation appear normal in +/R740S osteoclasts. However, V-ATPases from +/R740S osteoclast membranes have severely reduced proton transport, whereas ATP hydrolysis is not significantly affected. We show for the first time that a point mutation within the a3 subunit, R740S, which is dominant negative for proton pumping and bone resorption, also uncouples proton pumping from ATP hydrolysis but has no effect on ruffled border formation or polarization of osteoclasts. These results suggest that the V(0) complex has proton-pumping-independent functions in mammalian cells.

Osteoclast activity modulates B-cell development in the bone marrow

B-cell development is dependent on the interactions between B-cell precursors and bone marrow stromal cells, but the role of osteoclasts (OCLs) in this process remains unknown. B lymphocytopenia is a characteristic of osteopetrosis, suggesting a modulation of B lymphopoiesis by OCL activity. To address this question, we first rescued OCL function in osteopetrotic oc/oc mice by dendritic cell transfer, leading to a restoration of both bone phenotype and B-cell development. To further explore the link between OCL activity and B lymphopoiesis, we induced osteopetrosis in normal mice by injections of zoledronic acid (ZA), an inhibitor of bone resorption. B-cell number decreased specifically in the bone marrow of ZA-treated mice. ZA did not directly affect B-cell differentiation, proliferation and apoptosis, but induced a decrease in the expression of CXCL12 and IL-7 by stromal cells, associated with reduced osteoblastic engagement. Equivalent low osteoblastic engagement in oc/oc mice confirmed that it resulted from the reduced OCL activity rather than from a direct effect of ZA on osteoblasts. These dramatic alterations of the bone microenvironment were disadvantageous for B lymphopoiesis, leading to retention of B-cell progenitors outside of their bone marrow niches in the ZA-induced osteopetrotic model. Altogether, our data revealed that OCLs modulate B-cell development in the bone marrow by controlling the bone microenvironment and the fate of osteoblasts. They provide novel basis for the regulation of the retention of B cells in their niche by OCL activity.

Nonablative neonatal bone marrow transplantation rapidly reverses severe murine osteopetrosis despite low-level engraftment and lack of selective expansion of the osteoclastic lineage

Infantile malignant osteopetrosis (IMO) is caused by lack of functional osteoclasts leading to skeletal abnormalities, blindness owing to compression of the optic nerves, bone marrow (BM) failure, and early death. In most patients, TCIRG1, a proton pump subunit essential for bone resorption, is mutated. oc/oc mice represent a model for IMO owing to a deletion in Tcirg1 and die around 4 weeks of age. To determine if hematopoietic stem cell transplantation without prior conditioning can reverse osteopetrosis, neonatal mice were transplanted intravenously with lineage-depleted BM cells. More than 85% of oc/oc mice transplanted with 5 × 10(6) cells survived long term with an engraftment of 3% to 5% in peripheral blood (PB). At 3 weeks, engraftment in the BM was 1% to 2%, but the cellularity had increased 60-fold compared with untreated oc/oc mice, and RANKL and macrophage colony-stimulating factor (M-CSF) expression in the BM was normalized. Histopathology and micro-computed tomography revealed almost complete reversal of osteopetrosis after 4 weeks. In vitro studies showed that bone resorption by osteoclasts from transplanted oc/oc mice was 14% of transplanted controls, and immunofluorescence microscopy revealed that resorption was mainly associated with osteoclasts of donor origin. Lineage analysis of BM, PB, and spleen did not provide any evidence for selective recruitment of cells to the osteoclastic lineage. The vision also was preserved in transplanted oc/oc mice, as determined by a visual tracking drum test. In summary, nonablative neonatal transplantation leading to engraftment of only a small fraction of normal cells rapidly reverses severe osteopetrosis in the oc/oc mouse model.

Dlx homeobox gene family expression in osteoclasts

Skeletal growth and homeostasis require the finely orchestrated secretion of mineralized tissue matrices by highly specialized cells, balanced with their degradation by osteoclasts. Time- and site-specific expression of Dlx and Msx homeobox genes in the cells secreting these matrices have been identified as important elements in the regulation of skeletal morphology. Such specific expression patterns have also been reported in osteoclasts for Msx genes. The aim of the present study was to establish the expression patterns of Dlx genes in osteoclasts and identify their function in regulating skeletal morphology. The expression patterns of all Dlx genes were examined during the whole osteoclastogenesis using different in vitro models. The results revealed that Dlx1 and Dlx2 are the only Dlx family members with a possible function in osteoclastogenesis as well as in mature osteoclasts. Dlx5 and Dlx6 were detected in the cultures but appear to be markers of monocytes and their derivatives. In vivo, Dlx2 expression in osteoclasts was examined using a Dlx2/LacZ transgenic mouse. Dlx2 is expressed in a subpopulation of osteoclasts in association with tooth, brain, nerve, and bone marrow volumetric growths. Altogether the present data suggest a role for Dlx2 in regulation of skeletal morphogenesis via functions within osteoclasts.

Clinical Use of Interferon-gamma

Interferon gamma (IFN-gamma), a pleotropic cytokine, has been shown to be important to the function of virtually all immune cells and both innate and adaptive immune responses. In 1986, early clinical trials of this cytokine began to evaluate its therapeutic potential. The initial studies focused on the tolerability and pharmacology of IFN-gamma and systematically determined its antitumor and anti-infection activities. In the 20-plus years since those first trials, IFN-gamma has been used in a wide variety of clinical indications, which are reviewed in this article.

A single-center experience in 20 patients with infantile malignant osteopetrosis

Infantile malignant osteopetrosis (IMO) includes various genetic disorders that affect osteoclast development and/or function. Genotype-phenotype correlation studies in IMO have been hampered by the rarity and heterogeneity of the disease and by the severity of the clinical course, which often leads to death early in life. We report on the clinical and molecular findings and treatment in 20 consecutive patients (11 males, nine females) with IMO, diagnosed at a single center in the period 1991-2008. Mean age at diagnosis was 3.9 months, and mean follow-up was 66.75 months. Mutations in TCIRG1, OSTM1, ClCN7, and TNFRSF11A genes were detected in nine, three, one, and one patients, respectively. Six patients remain genetically undefined. OSTM1 and ClCN7 mutations were associated with poor neurologic outcome. Among nine patients with TCIRG1 defects, six presented with hypogammaglobulinemia, and one showed primary pulmonary hypertension. Fourteen patients received hematopoietic cell transplantation; of these, nine are alive and eight of them have evidence of osteoclast function. These data may provide a basis for informed decisions regarding the care of patients with IMO.

Direct recruitment of H+-ATPase from lysosomes for phagosomal acidification

The nascent phagosome progressively establishes an acidic milieu by acquiring a proton pump, the vacuolar-type ATPase (V-ATPase). However, the origin of phagosomal V-ATPase remains poorly understood. We found that phagosomes were enriched with the V-ATPase a3 subunit, which also accumulated in late endosomes and lysosomes. We modified the mouse Tcirg1 locus encoding subunit a3, to express an a3-GFP fusion protein. Live-cell imaging and immunofluorescence microscopy revealed that nascent phagosomes received the a3-GFP from tubular structures extending from lysosomes located in the perinuclear region. Macrophages from a3-deficient mice exhibited impaired acidification of phagosomes and delayed digestion of bacteria. These results show that lysosomal V-ATPase is recruited directly to the phagosomes via tubular lysosomes to establish the acidic environment hostile to pathogens.

Low-dose busulphan conditioning and neonatal stem cell transplantation preserves vision and restores hematopoiesis in severe murine osteopetrosis

OBJECTIVE

Infantile malignant osteopetrosis is a fatal disease caused by lack of functional osteoclasts. In most of patients, TCIRG1, encoding a subunit of a proton pump essential for bone resorption, is mutated. Osteopetrosis leads to bone marrow failure and blindness due to optic nerve compression. Oc/oc mice have a deletion in Tcirg1 and die around 3 to 4 weeks, but can be rescued by neonatal stem cell transplantation (SCT) after irradiation conditioning. However, as irradiation of neonatal mice results in retinal degeneration, we wanted to investigate whether conditioning with busulphan prior to SCT can lead to preservation of vision and reversal of osteopetrosis in the oc/oc mouse model.

MATERIALS AND METHODS

Pregnant dams were conditioned with busulphan and their litters transplanted with 1 x 10(6) normal lineage-depleted bone marrow cells intravenously or intraperitoneally. Mice were followed in terms of survival and engraftment level, as well as with peripheral blood lineage analysis, bone and eye histopathology and a visual-tracking drum test to assess vision.

RESULTS

Busulphan at 15 mg/kg was toxic to oc/oc mice. However, six of seven oc/oc mice conditioned with busulphan 7.5 mg/kg survived past the normal lifespan with 10% engraftment, correction of the skeletal phenotype, and normalization of peripheral blood lineages. Busulphan, in contrast to irradiation, did not have adverse effects on the retina as determined by histopathology, and 8 weeks after transplantation control and oc/oc mice retained their vision.

CONCLUSION

Low-dose busulphan conditioning and neonatal SCT leads to prolonged survival of oc/oc mice, reverses osteopetrosis and prevents blindness even at low engraftment levels.

Bone marrow microenvironment controls the in vivo differentiation of murine dendritic cells into osteoclasts

Finding that activated T cells control osteoclast (OCL) differentiation has revealed the importance of the interactions between immune and bone cells. Dendritic cells (DCs) are responsible for T-cell activation and share common precursors with OCLs. Here we show that DCs participate in bone resorption more directly than simply through T-cell activation. We show that, among the splenic DC subsets, the conventional DCs have the higher osteoclastogenic potential in vitro. We demonstrate that conventional DCs differentiate into functional OCLs in vivo when injected into osteopetrotic oc/oc mice defective in OCL resorptive function. Moreover, this differentiation involves the presence of activated CD4+ T cells controlling a high RANK-L expression by bone marrow stromal cells. Our results open new insights in the differentiation of OCLs and DCs and offer new basis for analyzing the relations between bone and immune systems.

OSTM1 bone defect reveals an intercellular hematopoietic crosstalk

The most severe form of bone autosomal recessive osteopetrosis both in humans and in the gray-lethal (gl/gl) mouse is caused by mutations in the Ostm1 gene. Although osteopetrosis is usually associated with a defect in the hematopoietic-derived osteoclast cells, this study determined that Ostm1 is expressed in many hematopoietic cells of the myeloid and lymphoid B- and T-lineages. Hematopoiesis in gl/gl mice is characterized by a marked expansion of the osteoclast lineage but also by deregulation of the lymphoid lineages with a decrease in B-lymphoid cell populations and altered distribution in T-lymphoid double and single CD4 CD8-positive cells. In committed gl/gl osteoclasts, specific Ostm1 transgene targeting showed a requirement of additional factors and/or cells for normal osteoclast function, and importantly, defined the gl osteopetrotic defect as non-cell autonomous. By contrast, gl/gl osteoclast, B- and T-lymphoid lineage phenotypes were rescued when Ostm1 is expressed under PU.1 regulation from a bacterial artificial chromosome transgene, which established an essential role for Ostm1 in hematopoietic cells in addition to osteoclasts. Together these experiments are the first to demonstrate the existence of hematopoietic crosstalk for the production of functional and active osteoclasts.

Osteoblastic regulation of B lymphopoiesis is mediated by Gs{alpha}-dependent signaling pathways

Osteoblasts play an increasingly recognized role in supporting hematopoietic development and recently have been implicated in the regulation of B lymphopoiesis. Here we demonstrate that the heterotrimeric G protein alpha subunit G(s)alpha is required in cells of the osteoblast lineage for normal postnatal B lymphocyte production. Deletion of G(s)alpha early in the osteoblast lineage results in a 59% decrease in the percentage of B cell precursors in the bone marrow. Analysis of peripheral blood from mutant mice revealed a 67% decrease in the number of circulating B lymphocytes by 10 days of age. Strikingly, other mature hematopoietic lineages are not decreased significantly. Mice lacking G(s)alpha in cells of the osteoblast lineage exhibit a reduction in pro-B and pre-B cells. Furthermore, interleukin (IL)-7 expression is attenuated in G(s)alpha-deficient osteoblasts, and exogenous IL-7 is able to restore B cell precursor populations in the bone marrow of mutant mice. Finally, the defect in B lymphopoiesis can be rescued by transplantation into a WT microenvironment. These findings confirm that osteoblasts are an important component of the B lymphocyte niche and demonstrate in vivo that G(s)alpha-dependent signaling pathways in cells of the osteoblast lineage extrinsically regulate bone marrow B lymphopoiesis, at least partially in an IL-7-dependent manner.

IL-7 induces myelopoiesis and erythropoiesis

IL-7 administration to mice was previously reported to increase the mobilization of progenitor cells from marrow to peripheral sites. We now report that IL-7 increases the number of mature myeloid and monocytic cells in spleen and peripheral blood. This effect required T cells, and we show that IL-7 treatment in vivo induced GM-CSF and IL-3 production by T cells with memory phenotype. However, additional myelopoietic cytokines were shown to be involved because mice deficient in both GM-CSF and IL-3 also responded to IL-7 with increased myelopoiesis. Candidate cytokines included IFN-gamma and Flt3 ligand, which were also produced in response to IL-7. Because IFN-gamma-deficient mice also increased myelopoiesis, it was suggested that IL-7 induced production of redundant myelopoietic cytokines. In support of this hypothesis, we found that the supernatant from IL-7-treated, purified T cells contained myelopoietic activity that required a combination of Abs against GM-CSF, IL-3, and anti-Flt3 ligand to achieve maximum neutralization. IL-7 administration increased the number of splenic erythroid cells in either normal, Rag1 or GM-CSF-IL-3-deficient mice, suggesting that IL-7 might directly act on erythroid progenitors. In support of this theory, we detected a percentage of TER-119(+) erythroid cells that expressed the IL-7Ralpha-chain and common gamma-chain. Bone marrow cells expressing IL-7R and B220 generated erythroid colonies in vitro in response to IL-7, erythropoietin, and stem cell factor. This study demonstrates that IL-7 can promote nonlymphoid hemopoiesis and production of cytokines active in the host defense system in vivo, supporting its possible clinical utility.

Hematopoietic stem cell-targeted neonatal gene therapy reverses lethally progressive osteopetrosis in oc/oc mice

Infantile malignant osteopetrosis (IMO) is a fatal disease caused by lack of functional osteoclasts, and the only available treatment is hematopoietic stem cell (HSC) transplantation. In the majority of patients, the TCIRG1 gene, coding for a subunit of a proton pump essential for bone resorption, is mutated. Oc/oc mice have a deletion in the homologue gene (tcirg1) and die at 3 to 4 weeks, but can be rescued by neonatal transplantation of HSCs. Here, HSC-targeted gene therapy of osteopetrosis in the oc/oc mouse model was developed. Oc/oc fetal liver cells depleted of Ter119-expressing erythroid cells were transduced with a retroviral vector expressing tcirg1 and GFP, and subsequently transplanted intraperitoneally to irradiated neonatal oc/oc mice. Eight of 15 mice survived past the normal life span of oc/oc mice. In vitro osteoclastogenesis revealed formation of GFP-positive osteoclasts and bone resorption, albeit at a lower level than from wild-type cells. The skeletal phenotype was analyzed by X-ray and histopathology and showed partial correction at 8 weeks and almost normalization after 18 weeks. In summary, osteopetrosis in oc/oc mice can be reversed by neonatal transplantation of gene-modified HSCs leading to long-term survival. This represents a significant step toward the development of gene therapy for osteopetrosis.