Intramedullary and extramedullary B lymphopoiesis in osteopetrotic mice

Good Cop, Bad Cop: Profiling the Immune Landscape in Multiple Myeloma

Multiple myeloma (MM) is a dyscrasia of plasma cells (PCs) characterized by abnormal immunoglobulin (Ig) production. The disease remains incurable due to a multitude of mutations and structural abnormalities in MM cells, coupled with a favorable microenvironment and immune suppression that eventually contribute to the development of drug resistance. The bone marrow microenvironment (BMME) is composed of a cellular component comprising stromal cells, endothelial cells, osteoclasts, osteoblasts, and immune cells, and a non-cellular component made of the extracellular matrix (ECM) and the liquid milieu, which contains cytokines, growth factors, and chemokines. The bone marrow stromal cells (BMSCs) are involved in the adhesion of MM cells, promote the growth, proliferation, invasion, and drug resistance of MM cells, and are also crucial in angiogenesis and the formation of lytic bone lesions. Classical immunophenotyping in combination with advanced immune profiling using single-cell sequencing technologies has enabled immune cell-specific gene expression analysis in MM to further elucidate the roles of specific immune cell fractions from peripheral blood and bone marrow (BM) in myelomagenesis and progression, immune evasion and exhaustion mechanisms, and development of drug resistance and relapse. The review describes the role of BMME components in MM development and ongoing clinical trials using immunotherapeutic approaches.

BATF sustains homeostasis and functionality of bone marrow Treg cells to preserve homeostatic regulation of hematopoiesis and development of B cells

Bone marrow Treg cells (BM Tregs) orchestrate stem cell niches crucial for hematopoiesis. Yet little is known about the molecular mechanisms governing BM Treg homeostasis and function. Here we report that the transcription factor BATF maintains homeostasis and functionality of BM Tregs to facilitate homeostatic regulation of hematopoiesis and B cell development. Treg-specific ablation of BATF profoundly compromised proportions of BM Tregs associated with reduced expression of Treg effector molecules, including CD44, ICOS, KLRG1, and TIGIT. Moreover, BATF deficiency in Tregs led to increased numbers of hematopoietic stem cells (HSCs), multipotent progenitors (MPPs), and granulocyte-macrophage progenitors (GMPs), while reducing the functionality of myeloid progenitors and the generation of common lymphoid progenitors. Furthermore, Tregs lacking BATF failed to support the development of B cells in the BM. Mechanistically, BATF mediated IL-7 signaling to promote expression of effector molecules on BM Tregs and their homeostasis. Our studies reveal a previously unappreciated role for BATF in sustaining BM Treg homeostasis and function to ensure hematopoiesis.

Bone Marrow Cell Trafficking Analyzed by 89Zr-oxine Positron Emission Tomography in a Murine Transplantation Model

Purpose: The success of hematopoietic stem cell transplantation (HSCT) depends on donor cell homing to the bone marrow. However, there is no reliable method of noninvasively monitoring the kinetics and distribution of transferred cells. Using zirconium-89 (89Zr)-oxine cell labeling combined with PET imaging, we sought to visualize and quantify donor cell homing in a mouse bone marrow transplantation model.Experimental Design: The effect of 89Zr-oxine labeling on bone marrow cell viability and differentiation was evaluated in vitro89Zr-labeled bone marrow cells (2 × 107 cells, 16.6 kBq/106 cells) were transferred intravenously, and serial microPET images were obtained (n = 5). The effect of a CXCR4 inhibitor, plerixafor (5 mg/kg) and G-CSF (2.5 μg) on bone marrow homing and mobilization were examined (n = 4). Engraftment of the transferred 89Zr-labeled cells was evaluated (n = 3).Results:89Zr-oxine-labeled bone marrow cells showed delayed proliferation, but differentiated normally. Transferred bone marrow cells rapidly migrated to the bone marrow, spleen, and liver (n = 5). Approximately 36% of donor cells homed to the bone marrow within 4 hours, irrespective of prior bone marrow ablation. Inhibition of CXCR4 by plerixafor alone or with G-CSF significantly blocked the bone marrow homing (P < 0.0001, vs. nontreated, at 2 hours), confirming a crucial role of the CXCR4-CXCL12 system. Mobilization of approximately 0.64% of pretransplanted bone marrow cells induced a 3.8-fold increase of circulating bone marrow cells. 89Zr-labeled donor cells engrafted as well as nonlabeled cells.Conclusions:89Zr-oxine PET imaging reveals rapid bone marrow homing of transferred bone marrow cells without impairment of their stem cell functions, and thus, could provide useful information for optimizing HSCT. Clin Cancer Res; 23(11); 2759-68. ©2016 AACR.

Macrophage colony-stimulating factor receptor marks and regulates a fetal myeloid-primed B-cell progenitor in mice

Although it is well established that unique B-cell lineages develop through distinct regulatory mechanisms during embryonic development, much less is understood about the differences between embryonic and adult B-cell progenitor cells, likely to underpin the genetics and biology of infant and childhood PreB acute lymphoblastic leukemia (PreB-ALL), initiated by distinct leukemia-initiating translocations during embryonic development. Herein, we establish that a distinct subset of the earliest CD19(+) B-cell progenitors emerging in the E13.5 mouse fetal liver express the colony-stimulating factor-1 receptor (CSF1R), previously thought to be expressed, and play a lineage-restricted role in development of myeloid lineages, and macrophages in particular. These early embryonic CSF1R(+)CD19(+) ProB cells also express multiple other myeloid genes and, in line with this, possess residual myeloid as well as B-cell, but not T-cell lineage potential. Notably, these CSF1R(+) myeloid-primed ProB cells are uniquely present in a narrow window of embryonic fetal liver hematopoiesis and do not persist in adult bone marrow. Moreover, analysis of CSF1R-deficient mice establishes a distinct role of CSF1R in fetal B-lymphopoiesis. CSF1R(+) myeloid-primed embryonic ProB cells are relevant for infant and childhood PreB-ALLs, which frequently have a bi-phenotypic B-myeloid phenotype, and in which CSF1R-rearrangements have recently been reported.

Altered hematopoietic stem cell and osteoclast precursor frequency in cathepsin K null mice

Cathepsin K (CatK) is a lysosomal cysteine protease necessary for bone resorption by osteoclasts (OCs), which originate from myeloid hematopoietic precursors. CatK-deficient (CatK(-/-) ) mice show osteopetrosis due to defective resorption by OCs, which are increased in number in these mice. We investigated whether genetic ablation of CatK altered the number of hematopoietic stem cells (HSCs) and OC precursor cells (OCPs) using two mouse models: CatK(-/-) mice and a knock-in mouse model in which the CatK gene (ctsk) is replaced by cre recombinase. We found that CatK deletion in mice significantly increased the number of HSCs in the spleen and decreased their number in bone marrow. In contrast, the number of early OCPs was unchanged in the bone marrow. However, the number of committed CD11b(+) OCPs was increased in the bone marrow of CatK(-/-) compared to wild-type (WT) mice. In addition, the percentage but not the number of OCPs was decreased in the spleen of CatK(-/-) mice relative to WT. To understand whether increased commitment to OC lineage in CatK(-/-) mice is influenced by the bone marrow microenvironment, CatK(Cre/+) or CatK(Cre/Cre) red fluorescently labeled OCPs were injected into WT mice, which were also subjected to a mid-diaphyseal femoral fracture. The number of OCs derived from the intravenously injected CatK(Cre/Cre) OCPs was lower in the fracture callus compared to mice injected with CatK(+/Cre) OCPs. Hence, in addition to its other effects, the absence of CatK in OCP limits their ability to engraft in a repairing fracture callus compared to WT OCP.

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.

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.

Origins and properties of dental, thymic, and bone marrow mesenchymal cells and their stem cells

Mesenchymal cells arise from the neural crest (NC) or mesoderm. However, it is difficult to distinguish NC-derived cells from mesoderm-derived cells. Using double-transgenic mouse systems encoding P0-Cre, Wnt1-Cre, Mesp1-Cre, and Rosa26EYFP, which enabled us to trace NC-derived or mesoderm-derived cells as YFP-expressing cells, we demonstrated for the first time that both NC-derived (P0- or Wnt1-labeled) and mesoderm-derived (Mesp1-labeled) cells contribute to the development of dental, thymic, and bone marrow (BM) mesenchyme from the fetal stage to the adult stage. Irrespective of the tissues involved, NC-derived and mesoderm-derived cells contributed mainly to perivascular cells and endothelial cells, respectively. Dental and thymic mesenchyme were composed of either NC-derived or mesoderm-derived cells, whereas half of the BM mesenchyme was composed of cells that were not derived from the NC or mesoderm. However, a colony-forming unit-fibroblast (CFU-F) assay indicated that CFU-Fs in the dental pulp, thymus, and BM were composed of NC-derived and mesoderm-derived cells. Secondary CFU-F assays were used to estimate the self-renewal potential, which showed that CFU-Fs in the teeth, thymus, and BM were entirely NC-derived cells, entirely mesoderm-derived cells, and mostly NC-derived cells, respectively. Colony formation was inhibited drastically by the addition of anti-platelet-derived growth factor receptor-β antibody, regardless of the tissue and its origin. Furthermore, dental mesenchyme expressed genes encoding critical hematopoietic factors, such as interleukin-7, stem cell factor, and cysteine-X-cysteine (CXC) chemokine ligand 12, which supports the differentiation of B lymphocytes and osteoclasts. Therefore, the mesenchymal stem cells found in these tissues had different origins, but similar properties in each organ.

Absence of sclerostin adversely affects B-cell survival

Increased osteoblast activity in sclerostin-knockout (Sost(-/-)) mice results in generalized hyperostosis and bones with small bone marrow cavities resulting from hyperactive mineralizing osteoblast populations. Hematopoietic cell fate decisions are dependent on their local microenvironment, which contains osteoblast and stromal cell populations that support both hematopoietic stem cell quiescence and facilitate B-cell development. In this study, we investigated whether high bone mass environments affect B-cell development via the utilization of Sost(-/-) mice, a model of sclerosteosis. We found the bone marrow of Sost(-/-) mice to be specifically depleted of B cells because of elevated apoptosis at all B-cell developmental stages. In contrast, B-cell function in the spleen was normal. Sost expression analysis confirmed that Sost is primarily expressed in osteocytes and is not expressed in any hematopoietic lineage, which indicated that the B-cell defects in Sost(-/-) mice are non-cell autonomous, and this was confirmed by transplantation of wild-type (WT) bone marrow into lethally irradiated Sost(-/-) recipients. WT→Sost(-/-) chimeras displayed a reduction in B cells, whereas reciprocal Sost(-/-) →WT chimeras did not, supporting the idea that the Sost(-/-) bone environment cannot fully support normal B-cell development. Expression of the pre-B-cell growth stimulating factor, Cxcl12, was significantly lower in bone marrow stromal cells of Sost(-/-) mice, whereas the Wnt target genes Lef-1 and Ccnd1 remained unchanged in B cells. Taken together, these results demonstrate a novel role for Sost in the regulation of bone marrow environments that support B cells.

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.

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.

Methods for investigation of osteoclastogenesis using mouse embryonic stem cells

Investigation of osteoclastogenesis in vivo, especially in early development, has proven difficult because of the accessibility of these early embryonic stages. Our ability to culture embryonic stem cells (ESCs) in vitro has overcome this difficulty as these versatile cells can be expanded endlessly. Thus, the whole process of osteoclastogenesis can be monitored in these cultures through the microscope and with the help of molecular biology techniques. We have developed two methods to induce osteoclasts, the bone matrix remodeling cells, from murine ESCs. Surprisingly, one of these induction methods produces osteoclasts, osteoblasts, and also endothelial cells in the same culture dish. Hence, it is likely that ESCs in culture mimic the in vivo development of osteoclasts.

Liver sinusoidal endothelial cells promote B lymphopoiesis from primitive hematopoietic cells

Although the bone marrow (BM) microenvironment is the main inducer niche of early B lymphopoiesis during the adult life, other extramedullar microenvironments, such as the liver, may also have potential for supporting B-cell development. Previously, we reported that murine liver sinusoidal endothelial cells (LSECs) support in vitro and in vivo hematopoietic stem cell (HSC) proliferation and myeloid differentiation. In the present study, we investigated the capacity of LSEC to promote B lymphopoiesis from BM progenitor lineage-negative (Lin(-)) cells. Murine BM Lin(-) cells were co-cultured with LSEC, in the absence of exogenous cytokines. B cells were characterized by flow cytometry and cytokine expression by RT-PCR. We show that BM Lin(-) cells differentiated to early B-lymphoid progenitors (B220(+)) and subsequently to mature (CD19(+)) B cells. Functional studies showed the presence of a high number of non-adherent cells (NACs), collected from lipopolysaccharide (LPS)-treated Lin(-)/LSEC co-cultures, expressing IgM on their surface (sIgM). Colony formation from NAC was observed in the presence of IL-7 (CFU-IL-7). LSEC constitutively express IL-7, Flt-3L, and SCF at the mRNA level, and VCAM-1 on their surface, which may explain the capacity of these cells to promote B lymphopoiesis. These data demonstrate that LSEC promote all stages of B lymphopoiesis. To our knowledge, this is the first report that LSEC constitute an in vitro microenvironment for B lymphopoiesis. Further studies will establish whether LSEC can serve in vivo as a B-lymphopoietic niche under physiological or pathological condition, or when HSC are mobilized.

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.

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.

Mice lacking three myeloid colony-stimulating factors (G-CSF, GM-CSF, and M-CSF) still produce macrophages and granulocytes and mount an inflammatory response in a sterile model of peritonitis

To assess the combined role of G-CSF, GM-CSF, and M-CSF in myeloid cell production, mice deficient in all three myeloid CSFs were generated (G-/-GM-/-M-/- mice). G-/-GM-/-M-/- mice share characteristics found in mice lacking individual cytokines: they are toothless and osteopetrotic and furthermore acquire alveolar proteinosis that is more severe than that found in either GM-/- or G-/-GM-/- mice. G-/-GM-/-M-/- mice have a significantly reduced lifespan, which is prolonged by antibiotic administration, suggesting compromised ability to control bacterial infection. G-/-GM-/-M-/- mice have circulating neutrophils and monocytes, albeit at significantly reduced numbers compared with wild-type mice, but surprisingly, have more circulating monocytes than M-/- mice and more circulating neutrophils than G-/-GM-/- mice. Due to severe osteopetrosis, G-/-GM-/-M-/- mice show diminished numbers of myeloid cells, myeloid progenitors, and B lymphocytes in the bone marrow, but have significantly enhanced compensatory splenic hemopoiesis. Although G-/-GM-/-M-/- mice have a profound deficiency of myeloid cells in the resting peritoneal cavity, the animals mount a moderate cellular response in a model of sterile peritonitis. These data establish that in the absence of G-CSF, GM-CSF, and M-CSF, additional growth factor(s) can stimulate myelopoiesis and acute inflammatory responses.

The evolutionary history of lymphoid organs

Lymphoid organs are important regulators of lymphocyte development and immune responses. During vertebrate evolution, primary lymphoid organs appeared earlier than secondary lymphoid organs. Among the sites of primary lymphopoiesis during evolution and ontogeny, those for B cell differentiation have differed considerably, although they often have had myelolymphatic characteristics. In contrast, only a single site for T cell differentiation has occurred, exclusively the thymus. Based on those observations and the known features of variable-diversity-joining gene recombination, we propose a model for the successive specification of different lymphocyte lineages during vertebrate evolution. According to our model, T cells were the first lymphocytes to acquire variable-diversity-joining-type receptors, and the thymus was the first lymphoid organ to evolve in vertebrates to deal with potentially autoreactive, somatically diversified T cell receptors.

B cells and osteoblast and osteoclast development

The molecules that regulate bone cell development, particularly at the early stages of development, are only partially known. Data are accumulating that indicate a complex relationship exists between B cells and bone cell differentiation. Although the exact nature of this relationship is still evolving, it takes at least two forms. First, factors that regulate B-cell growth and development have striking effects on osteoclast and osteoblast lineage cells. Similarly, factors that regulate bone cell development influence B-cell maturation. Second, a series of transcription factors required for B-cell differentiation have been identified, and these factors function in a developmentally ordered circuit. These transcription factors have unpredicted, pronounced, and non-overlapping effects on osteoblast and/or osteoclast development. These data indicate that at least a regulatory relationship exists between B lymphopoiesis, osteoclastogenesis, and osteoblastogenesis.

Negative role of colony-stimulating factor-1 in macrophage, T cell, and B cell mediated autoimmune disease in MRL-Fas(lpr) mice

Inflammation in the kidney and other tissues (lung, and salivary and lacrimal glands) is characteristic of MRL-Fas(lpr) mice with features of lupus. Macrophages (Mphi) are prominent in these tissues. Given that 1) Mphi survival, recruitment, proliferation, and activation during inflammation is dependent on CSF-1, 2) Mphi mediate renal resident cell apoptosis, and 3) CSF-1 is up-regulated in MRL-Fas(lpr) mice before, and during nephritis, we hypothesized that CSF-1-deficient MRL-Fas(lpr) mice would be protected from Mphi-mediated nephritis, and the systemic illness. To test this hypothesis, we compared CSF-1-deficient MRL-Fas(lpr) with wild-type strains. Renal pathology is suppressed and function improved in CSF-1-deficient MRL-Fas(lpr) mice. There are far fewer intrarenal Mphi and T cells in CSF-1-deficient MRL-Fas(lpr) vs wild-type kidneys. This leukocytic reduction results from suppressed infiltration, and intrarenal proliferation, but not enhanced apoptosis. The CSF-1-deficient MRL-Fas(lpr) kidneys remain preserved as indicated by greatly reduced indices of injury (nephritogenic cytokines, tubular apoptosis, and proliferation). The renal protective mechanism in CSF-1-deficient mice is not limited to reduced intrarenal leukocytes; circulating Igs and autoantibodies, and renal Ig deposits are decreased. This may result from enhanced B cell apoptosis and fewer B cells in CSF-1-deficient MRL-Fas(lpr) mice. Furthermore, the systemic illness including, skin, lung, and lacrimal and salivary glands pathology, lymphadenopathy, and splenomegaly are dramatically suppressed in CSF-1-deficient MRL-Fas(lpr) as compared with wild-type mice. These results indicate that CSF-1 is an attractive therapeutic target to combat Mphi-, T cell-, and B cell-mediated autoimmune lupus.

Enhanced bone formation in lipodystrophic PPARgamma(hyp/hyp) mice relocates haematopoiesis to the spleen

The peroxisome proliferator-activated receptor gamma (PPARgamma) controls adipogenesis and metabolism. We demonstrate here that the absence of PPARgamma in fat has potent osteogenic activities, which affect haematopoiesis. The congenital absence of PPARgamma in fat of lipodystrophic PPARgamma(hyp/hyp) mice, strongly enhanced bone mass and consequentially reduced the bone-marrow cavity. Consistent with this, PPARgamma(hyp/hyp) mice had a significant decrease in bone marrow cellularity and resorted to extramedullary haematopoiesis in the spleen to maintain haematopoiesis. Our data indicate that antagonizing PPARgamma activity in fat could be an effective way to combat osteoporosis and suggest that haematopoietic function should be scrutinized in lipodystrophic subjects.

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

Infantile malignant osteopetrosis (IMO) is a rare and lethal disease characterized by an absence of bone resorption due to inactive OCLs. Affected patients display an increased bone mass and hematological defects. The osteopetrotic oc/oc mouse displays a bone phenotype similar to the one observed in IMO patients, and the same gene, Tcirg1, is mutated in this model and in the majority of these patients. Therefore, we explored in oc/oc mice the consequences of the perturbed bone microenvironment on hematopoiesis. We show that the myelomonocytic differentiation is increased, leading to an elevated number of OCLs and dendritic cells. B lymphopoiesis is blocked at the pro-B stage in the bone marrow of oc/oc mouse, leading to a low mature B-cell number. T-cell activation is also affected, with a reduction of IFNgamma secretion by splenic CD4(+) T cells. These alterations are associated with a low IL-7 expression in bone marrow. All these data indicate that the lack of bone resorption in oc/oc mice has important consequences in both myelopoiesis and lymphopoiesis, leading to a form of immunodeficiency. The oc/oc mouse is therefore an appropriate model to understand the hematological defects described in IMO patients, and to derive new therapeutic strategies.

The origins of osteoclasts

PURPOSE OF REVIEW

It is now dogma that osteoclasts (OCs) arise from cells of the monocyte/macrophage lineage. However, data are accumulating suggesting that a relationship exists between B lymphocytes (B cells) and OC differentiation. Although the exact nature of this relation is unknown, it takes at least two forms. First, molecules that regulate B-cell growth and development have striking effects on OC lineage cells particularly at early stages of differentiation. Second, the possibility exists that pro-B cells can give rise to osteoclast-like cells (OCLs) in vitro and in vivo. Recent data indicate, at the least, that a regulatory relation exists between B lymphopoiesis and osteoclastogenesis.

RECENT FINDINGS

Pax5 is a member of the multigene family that encodes the paired box transcription factors. Pax5 is expressed exclusively in B-lymphocyte lineage cells extending from early B220 pro-B cells to mature B cells. Mice made deficient in Pax5 have a developmental arrest of the B-cell lineage at the pro-B-cell stage. Pax5-/- pro-B cells could be induced to form OCLs by treatment with macrophage colony-stimulating factor and receptor activator of nuclear factor-kappaB ligand (RANKL). Importantly, Pax5-/- mice are severely osteopenic, missing more than 60% of their bone mass. This is the result of a three- to fivefold increase in the number of OCs in bone, whereas the number of osteoblasts is indistinguishable from controls.

SUMMARY

The analysis of a variety of mutations in mice supports the hypothesis that B cells and OCs develop in parallel; that their development is regulated in a reciprocal manner; and that in the Pax5-deficient state, OCs arise from pro-B cells.

Distinct osteoclast precursors in the bone marrow and extramedullary organs characterized by responsiveness to Toll-like receptor ligands and TNF-alpha

Osteoclasts are derived from hemopoietic stem cells and play critical roles in bone resorption and remodeling. Multinucleated osteoclasts are attached tightly to bone matrix, whereas precursor cells with the potential to differentiate into osteoclasts in culture are widely distributed. In this study, we assessed the characteristics of osteoclast precursors in bone marrow (BM) and in extramedullary organs as indicated by their responsiveness to ligands for Toll-like receptors (TLRs) and to TNF-alpha. Development of osteoclasts from precursor cells in the BM was inhibited by CpG oligonucleotides, a ligand for TLR9, but not by LPS, a ligand for TLR4. BM osteoclasts were induced by TNF-alpha as well as receptor activator of NF-kappaB ligand in the presence of M-CSF. Splenic osteoclast precursors, even in osteoclast-deficient osteopetrotic mice, differentiated into mature osteoclasts following exposure to TNF-alpha or receptor activator of NF-kappaB ligand. However, splenic osteoclastogenesis was inhibited by both LPS and CpG. Osteoclastogenesis from peritoneal precursors was inhibited by not only these TLR ligands but also TNF-alpha. The effects of peptidoglycan, a ligand for TLR2, were similar to those of LPS. BM cells precultured with M-CSF were characterized with intermediate characteristics between those of splenic and peritoneal cavity precursors. Taken together, these findings demonstrate that osteoclast precursors are not identical in the tissues examined. To address the question of why mature osteoclasts occur only in association with bone, we may characterize not only the microenvironment for osteoclastogenesis, but also the osteoclast precursor itself in intramedullary and extramedullary tissues.

Incomplete restoration of colony-stimulating factor 1 (CSF-1) function in CSF-1-deficient Csf1op/Csf1op mice by transgenic expression of cell surface CSF-1

The primary macrophage growth factor, colony-stimulating factor 1 (CSF-1), is expressed as a secreted glycoprotein or proteoglycan found in the circulation or as a biologically active cell surface glycoprotein (csCSF-1). To investigate the in vivo roles of csCSF-1, we created mice that exclusively express csCSF-1, in a normal tissue-specific and developmental manner, by transgenic expression of csCSF-1 in the CSF-1-deficient osteopetrotic (Csf1(op)/Csf1(op)) background. The gross defects of Csf1(op)/Csf1(op) mice, including growth retardation, failure of tooth eruption, and abnormal male and female reproductive functions were corrected. Macrophage densities in perinatal liver, bladder, sublinguinal salivary gland, kidney cortex, dermis, and synovial membrane were completely restored, whereas only partial or no restoration was achieved in adult liver, adrenal gland, kidney medulla, spleen, peritoneal cavity, and intestine. Residual osteopetrosis, significantly delayed trabecular bone resorption in the subepiphyseal region of the long bone, and incomplete correction of the hematologic abnormalities in the peripheral blood, bone marrow, and spleens of CSF-1-deficient mice were also found in mice exclusively expressing csCSF-1. These data suggest that although csCSF-1 alone is able to normalize several aspects of development in Csf1(op)/Csf1(op) mice, it cannot fully restore in vivo CSF-1 function, which requires the presence of the secreted glycoprotein and/or proteoglycan forms.

RANK-Fc: a therapeutic antagonist for RANK-L in myeloma

BACKGROUND

Severe bone destruction due to inappropriate osteoclastogenesis is a prominent feature of multiple myeloma (MM). MM increases bone loss by disrupting the checks that normally control signaling by receptor activator of nuclear factor kappaB ligand (RANK-L, also called TRANCE [tumor necrosis factor-related, activation-induced cytokine], osteoprotegerin ligand [OPG-L], osteoclast differentiation factor [ODF], and tumor necrosis factor superfamily member 11 [TNFSF11]), a TNF-family cytokine required for osteoclast differentiation and activation. RANK-L binds to its functional receptor RANK (TNF receptor superfamily member 11a [TNF RSF11a]) to stimulate osteoclastogenesis. Osteotropic cytokines regulate this process by controlling bone marrow stromal expression of RANK-L. Further control over osteoclastogenesis is maintained by regulated expression of osteoprotegerin (OPG, also called osteoclastogenesis inhibitory factor and TNFRSF11b), a soluble decoy receptor for RANK-L. In normal bone marrow, abundant stores of OPG in stroma, megakaryocytes, and myeloid cells provide a natural buffer against increased RANK-L. MM disrupts these controls by increasing expression of RANK-L and decreasing expression of OPG. Concurrent deregulation of RANK-L and OPG expression is found in bone marrow biopsies from patients with MM but not in specimens from patients with non-MM hematologic malignancies.

METHODS

RANK-Fc is a recombinant RANK-L antagonist that is formed by fusing the extracellular domain of RANK to the Fc portion of human immunoglobulin G(1) (hIgG(1)). In vitro, addition of RANK-Fc virtually eliminates the formation of osteoclasts in cocultures of MM with bone marrow and osteoblast/stromal cells. The severe combined immunodeficiency (SCID)/ARH77 mouse model and the SCID-hu-MM mouse model of human MM were used to assess the ability of RANK-Fc to block the development of MM-induced bone disease in vivo. Mice received either RANK-Fc or hIgG(1) 200 microg intravenously three times per week.

RESULTS

RANK-Fc limited bone destruction in both the SCID/ARH-77 model and the SCID-hu-MM model. Administration of RANK-Fc also caused a marked reduction in tumor burden and serum paraprotein in SCID-hu-MM mice that was associated with the restoration of OPG and a reduction in RANK-L expression in the xenograft.

CONCLUSIONS

MM-induced bone destruction requires increased RANK-L expression and is facilitated by a concurrent reduction in OPG, a natural decoy receptor for RANK-L. Administration of the RANK-L antagonist RANK-Fc limits MM-induced osteoclastogenesis, development of bone disease, and MM tumor progression.

Overexpression of RANKL implicates IFN-beta-mediated elimination of B-cell precursors in the osteopetrotic bone of microphthalmic mice

The microphthalmic (mi) mouse possesses a dominant negative mutation in the microphthalmia-associated transcript factor (MITF) transcription factor. These animals are characterized by reduced numbers of peripheral mast and natural killer (NK) cells, are osteopetrotic because of osteoclast reduction and malfunction, lack functional melanocytes, and are deficient for maturing B-cells within the bone marrow. Granulocyte precursor cells, however, are functionally maintained within the mi bone marrow. A central question has been whether the B-cell deficiency of the mi mouse marrow is caused by the absence of an MITF-controlled gene product or because of the compromised, osteopetrotic environment. In this report, we examined mi marrow by performing transcriptional mapping analyses of candidate genes whose products are instrumental for functional osteoclast and B-cell development. Surprisingly, the expression of a subset of such genes including RANKL, stromal-derived factor (SDF-1), B-cell lymphotactin chemokine (BLC), and RANK was dramatically enhanced in the mi marrow. Normal and mutant marrow were also analyzed by subtractive transcript cloning, which identified a number of known and unknown genes with altered transcriptional activity. One such unknown mouse gene possesses a human counterpart that is interferon-beta (IFN-beta) inducible, suggesting the osteopetrotic marrow is enriched for IFN-beta, a cytokine that is known to eliminate B-cell precursors. A model is proposed suggesting excess RANKL sets off a cascade of cytokine production including IFN-beta that leads to the preferential elimination of B-cell precursors in the marrow of osteopetrotic marrow.

Marginal zone macrophages and immune responses against viruses

The effective establishment of antiviral protection requires a coordinated interplay between the innate and adaptive immune system. Using osteopetrotic (op(-/-)) mice, this study investigated the influence of marginal zone macrophages in controlling and initiating a protective immune response against a cytopathic vs a non- or low-cytopathic virus. Despite the generation of potent adaptive immune responses, antiviral protection against cytopathic vesicular stomatitis virus critically depended on the presence of marginal zone macrophages. Infection with low doses (100 PFU) of non- or low-cytopathic lymphocytic choriomeningitis virus was rarely cleared and usually resulted in a carrier state in the majority of mice. This shows that the early innate immune system provides an important preparatory phase to the adaptive immune system and is particularly important for antiviral protection.

Targeted disruption of the mouse colony-stimulating factor 1 receptor gene results in osteopetrosis, mononuclear phagocyte deficiency, increased primitive progenitor cell frequencies, and reproductive defects

The effects of colony-stimulating factor 1 (CSF-1), the primary regulator of mononuclear phagocyte production, are thought to be mediated by the CSF-1 receptor (CSF-1R), encoded by the c-fms proto-oncogene. To investigate the in vivo specificity of CSF-1 for the CSF-1R, the mouse Csf1r gene was inactivated. The phenotype of Csf1(-)/Csf1r(-) mice closely resembled the phenotype of CSF-1-nullizygous (Csf1(op)/Csf1(op)) mice, including the osteopetrotic, hematopoietic, tissue macrophage, and reproductive phenotypes. Compared with their wild-type littermates, splenic erythroid burst-forming unit and high-proliferative potential colony-forming cell levels in both Csf1(op)/Csf1(op) and Csf1(-)/Csf1r(-) mice were significantly elevated, consistent with a negative regulatory role of CSF-1 in erythropoiesis and the maintenance of primitive hematopoietic progenitor cells. The circulating CSF-1 concentration in Csf1r(-)/Csf1r(-) mice was elevated 20-fold, in agreement with the previously reported clearance of circulating CSF-1 by CSF-1R-mediated endocytosis and intracellular destruction. Despite their overall similarity, several phenotypic characteristics of the Csf1r(-)/Csf1r(-) mice were more severe than those of the Csf1(op)/Csf1(op) mice. The results indicate that all of the effects of CSF-1 are mediated via the CSF-1R, but that subtle effects of the CSF-1R could result from its CSF-1-independent activation.

Multiple myeloma disrupts the TRANCE/ osteoprotegerin cytokine axis to trigger bone destruction and promote tumor progression

Bone destruction, caused by aberrant production and activation of osteoclasts, is a prominent feature of multiple myeloma. We demonstrate that myeloma stimulates osteoclastogenesis by triggering a coordinated increase in the tumor necrosis factor-related activation-induced cytokine (TRANCE) and decrease in its decoy receptor, osteoprotegerin (OPG). Immunohistochemistry and in situ hybridization studies of bone marrow specimens indicate that in vivo, deregulation of the TRANCE-OPG cytokine axis occurs in myeloma, but not in the limited plasma cell disorder monoclonal gammopathy of unknown significance or in nonmyeloma hematologic malignancies. In coculture, myeloma cell lines stimulate expression of TRANCE and inhibit expression of OPG by stromal cells. Osteoclastogenesis, the functional consequence of increased TRANCE expression, is counteracted by addition of a recombinant TRANCE inhibitor, RANK-Fc, to marrow/myeloma cocultures. Myeloma-stroma interaction also has been postulated to support progression of the malignant clone. In the SCID-hu murine model of human myeloma, administration of RANK-Fc both prevents myeloma-induced bone destruction and interferes with myeloma progression. Our data identify TRANCE and OPG as key cytokines whose deregulation promotes bone destruction and supports myeloma growth.

Wnt signaling regulates hemopoiesis through stromal cells

Hemopoietic cells develop in a complex milieu that is made up of diverse components, including stromal cells. Wnt genes, which are known to regulate the fate of the cells in a variety of tissues, are expressed in hemopoietic organs. However, their roles in hemopoiesis are not well characterized. In this study, we examined the roles of Wnt proteins in hemopoiesis using conditioned medium containing Wnt-3a. This conditioned medium dramatically reduced the production of B lineage cells and myeloid lineage cells, except for macrophages in the long-term bone marrow cultures grown on stromal cells, although the sensitivity to the conditioned medium differed, depending on the hemopoietic lineage. In contrast, the same conditioned medium did not affect the generation of B lineage or myeloid lineage cells in stromal cell-free conditions. These results suggested that Wnt proteins exert their effects through stromal cells. Indeed, these effects were mimicked by the expression of a stabilized form of beta-catenin in stromal cells. In this study, we demonstrated that Wnt signaling regulates hemopoiesis through stromal cells with selectivity and different degrees of the effect, depending on the hemopoietic lineage in the hemopoietic microenvironment.

Regulation of cell survival during B lymphopoiesis in mouse bone marrow: enhanced pre-B-cell apoptosis in CSF-1-deficient op/op mutant mice

OBJECTIVE

Osteopetrotic (op/op) mice are deficient in macrophages and osteoclasts due to a CSF-1 gene mutation. The aim of this study was to evaluate the effect of these deficiencies and of CSF-1-dependent mechanisms on B lymphopoiesis in bone marrow, with special reference to the apoptotic activity of precursor B cells.

MATERIALS AND METHODS

B-cell development and apoptosis were examined in the bone marrow of op/op mice using immunofluorescence labeling and flow cytometry. Short-term cultures of bone marrow were used to evaluate the effect of recombinant CSF-1 on the rate of B-cell apoptosis.

RESULTS

Bone marrow cellularity was greatly reduced in op/op mice compared with normal littermates. However, precursor B cells were disproportionately decreased, most markedly at the pre-B-cell stage. Precursor B cells, particularly pre-B cells, displayed elevated apoptotic incidences both ex vivo and in short-term culture. Addition of recombinant CSF-1 reduced the incidence of apoptosis among precursor B cells in short-term cultures of whole bone marrow suspensions from normal mice but not in cultures of sorted B220+ B-lineage cells.

CONCLUSIONS

The finding of increased pre-B-cell apoptosis in op/op mice provides evidence that CSF-1-dependent mechanisms can strongly influence the survival of precursor B cells in mouse bone marrow, particularly at the pro-B/pre-B cell transition. It is proposed that the local or systemic levels of CSF-1 during ontogeny may thus play a role in regulating B-cell production within the bone marrow microenvironment.

Mimicry between neurokinin-1 and fibronectin may explain the transport and stability of increased substance P immunoreactivity in patients with bone marrow fibrosis

Bone marrow (BM) fibrosis may occur in myeloproliferative diseases, lymphoma, myelodysplastic syndrome, myeloma, and infectious diseases. In this study, the role of substance P (SP), a peptide with pleiotropic functions, was examined. Some of its functions-angiogenesis, fibroblast proliferation, and stimulation of BM progenitors-are amenable to inducing BM fibrosis. Indeed, a significant increase was found in SP-immunoreactivity (SP-IR) in the sera of patients with BM fibrosis (n = 44) compared with the sera of patients with hematologic disorders and no histologic evidence of fibrosis (n = 46) (140 +/-12 vs 18 +/-3; P <.01). Immunoprecipitation of sera SP indicated that this peptide exists in the form of a complex with other molecule(s). It was, therefore, hypothesized that SP might be complexed with NK-1, its natural receptor, or with a molecule homologous to NK-1. To address this, 3 cDNA libraries were screened that were constructed from pooled BM stroma or mononuclear cells with an NK-1 cDNA probe. A partial clone (clone 1) was retrieved that was 97% homologous to the ED-A region of fibronectin (FN). Furthermore, sequence analyses indicated that clone 1 shared significant homology with exon 5 of NK-1. Immunoprecipitation and Western blot analysis indicated co-migration of SP and FN in 27 of 31 patients with BM fibrosis. Computer-assisted molecular modeling suggested that similar secondary structural features between FN and NK-1 and the relative electrostatic charge might explain a complex formed between FN (negative) and SP (positive). This study suggests that SP may be implicated in the pathophysiology of myelofibrosis, though its role would have to be substantiated in future research. (Blood. 2001;97:3025-3031)