Anna Mansour

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.

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.

Impact of gene dosage, loss of wild-type allele, and FLT3 ligand on Flt3-ITD-induced myeloproliferation

Acquisition of homozygous activating growth factor receptor mutations might accelerate cancer progression through a simple gene-dosage effect. Internal tandem duplications (ITDs) of FLT3 occur in approximately 25% cases of acute myeloid leukemia and induce ligand-independent constitutive signaling. Homozygous FLT3-ITDs confer an adverse prognosis and are frequently detected at relapse. Using a mouse knockin model of Flt3-internal tandem duplication (Flt3-ITD)-induced myeloproliferation, we herein demonstrate that the enhanced myeloid phenotype and expansion of granulocyte-monocyte and primitive Lin(-)Sca1(+)c-Kit(+) progenitors in Flt3-ITD homozygous mice can in part be mediated through the loss of the second wild-type allele. Further, whereas autocrine FLT3 ligand production has been implicated in FLT3-ITD myeloid malignancies and resistance to FLT3 inhibitors, we demonstrate here that the mouse Flt3(ITD/ITD) myeloid phenotype is FLT3 ligand-independent.

Role of osteoclasts in the hematopoietic stem cell niche formation

In mammals, the bone marrow (BM) is the major site for hematopoiesis, and its colonization by hematopoietic stem cells (HSCs) is preceded by and requires endochondral ossification and invasion of blood vessels. There, HSCs are found in niches located in the trabecular regions of the bone in association with bone-lining osteoblasts (OBLs) (endosteal niche) or with vascular area (perivascular niche). Transplantation and homing assays in genetic mutant mice and in vitro assays have identified OBLs as the main components of the endosteal niche and mesenchymal progenitors as components of the perivascular niche (reviewed in ref. 1). The exact function of these niches is still a matter of debate, but it has been suggested that the endosteal niche could be involved in HSC maintenance by providing a quiescent environment, whereas the perivascular niche could harbor mobilized HSCs and could allow proliferation and differentiation. Osteoblasts are tightly coupled with the bone-resorbing osteoclasts (OCLs) in terms of differentiation and activity. Given the role of OBLs in the establishment and the regulation of the HSC niches, the question arose of a possible role of OCLs in these processes. Modulation of OCL activity in adult mice has been shown to favor HSC mobilization in response to mobilizing signals, indicating that OCLs are important regulators of the HSC niche. Moreover, extramedullary hematopoiesis is a hallmark of the loss of OCL activity in murine osteopetrotic models. However, until very recently, no data were available about the precise role of OCLs in the initial establishment of the HSC niche. Using newborn osteopetrotic oc/ oc mice in which OCLs are present but inactive, we reported very recently that Role of osteoclasts in the hematopoietic stem cell niche formation

Inflammatory osteoclasts prime TNFα‐producing CD4+ T cells and express CX3CR1

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 CX3CR1 as a marker of inflammatory osteoclasts and we demonstrated that the differentiation of CX3CR1+ 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 CX3CR1 as a novel dual target for antiresorptive and anti‐inflammatory treatment in inflammatory chronic diseases.

Emerging Roles of Osteoclasts in the Modulation of Bone Microenvironment and Immune Suppression in Multiple Myeloma

Multiple myeloma (MM) is one of the most common forms of hematologic malignancy resulting from cancerous proliferation of mature malignant plasma cells (MPCs). But despite the real improvement in therapeutics in the past years, it remains largely incurable. MM is the most frequent cancer to involve bone due to the stimulation of osteoclast (OCL) differentiation and activity. OCLs have a unique capacity to resorb bone. However, recent studies reveal that they are not restrained to this sole function. They participate in the control of angiogenesis, medullary niches, and immune responses, including in MM. Therefore, therapeutic approaches targeting OCLs probably affect not only bone resorption but also many other functions, and OCLs should not be considered anymore only as targets to improve the bone phenotype but also to modulate bone microenvironment. In this review, we explore these novel contributions of OCLs to MM which reveal their strong implication in the MM physiopathology. We also underline the therapeutic interest of targeting OCLs not only to overcome bone lesions, but also to improve bone microenvironment and anti-tumoral immune responses.