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

Abstract

IntroductionAn increasing number of studies underline the interactions between the bone and immune systems and have led to the emergence of osteoimmunology. 1,2 Excessive bone resorption is frequently associated with chronic infections and autoimmune and inflammatory diseases. [3][4][5][6] The immune system plays a major role in this process, in particular through activated T cells, which secrete proinflammatory cytokines involved in osteoclastogenesis. 7 However, less is known about the involvement of other immune cells in the control of bone resorption. Dendritic cells (DCs) also play an important role in autoimmune and inflammatory diseases. 8 These cells derive from the same myeloid precursor as osteoclasts (OCLs), and both cell types are modulated by common factors, mainly by receptor activator of NF-B ligand (RANK-L). RANK-L is essential for the differentiation of OCLs, 9 the activity and survival of DCs. 10 These data highlight a potential link between DCs and OCLs.Cells from the myelomonocytic lineage, including DCs, display a high developmental and functional plasticity depending on local factors and stimuli experienced during their differentiation and maturation. 11,12 Although they were considered to be terminally differentiated cells, recent studies have suggested that mature splenic DCs can be influenced by their microenvironment to undergo further differentiation. Splenic stromal cells induce mature DCs to differentiate into regulatory DCs, which differ from mature DCs by their phenotype, their cytokine secretion pattern, and their ability to inhibit T-cell proliferation. 13 Moreover, DCs generated in vitro transdifferentiate into endothelial cells when cultured with tumor-conditioned media 14 or into OCLs when cultured with osteoclastogenic factors. 15,16 Although these in vitro studies revealed the capacity of DCs to transdifferentiate into other cell types under specific conditions, it is not clear yet whether this plasticity takes place in vivo.Osteopetrosis is characterized by an impaired bone resorption because of the absence of OCL formation or activity. 17 In the osteopetrotic oc/oc mouse, differentiated OCLs are present but are unable to resorb bone because of a deletion in the Tcirg1 gene encoding the a3 subunit of the vacuolar ATPase. 18 The a3 protein is responsible for the acidification process necessary for the dissolution of the bone matrix leading to the formation of resorption lacunae. In the absence of a3 expression, the bone marrow of oc/oc mice is filled with numerous and disorganized trabeculae, and osteoclastogenesis is highly increased. 19,20 The consequence of this severe osteopetrotic phenotype is a life span less than 3 weeks. Therefore, the oc/oc mouse provides an appropriate model to assess the in vivo capacity of wild-type precursor cells to give rise to functional OCLs.To assess whether DCs have an osteoclastogenic potential, we purified them from normal mice and cultured them with RANK-L and macrophage-colony stimulating factor (M-CSF). We showed that this treatment a...

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

“…(14) Others have shown that RANKL and M-CSF are highly expressed in the BM of oc/oc mice. (20) By qRT-PCR we confirmed this but also saw a normalization of the expression levels of these important cytokines in the BM of oc/oc mice 18 to 20 days after transplantation (Fig. 5A, B).…”

“…Previously, oc/oc mice have been transplanted without prior conditioning with dendritic cells, but only a partial reversal of osteopetrosis and short prolongation of the lifespan were seen. (20) In another recent study, oc/oc mice were injected every 3 days with in vitro-derived osteoclast progenitors and a low number of c-kit þ bone marrow cells. (23) This leads to only a few weeks of increased survival despite the repeated administration of cells.…”

The R740S mutation in the V-ATPase a3 subunit increases lysosomal pH, impairs NFATc1 translocation, and decreases in vitro osteoclastogenesis

“…This increase in osteoclast numbers in vivo is most likely a compensatory response to the increased bone density or decreased resorption capacity of the osteoclasts, and because of the higher expression of Rankl and Csf1 (M-CSF) in þ/R740S bones, (10) consistent with what is seen in oc/oc mice. (27) In contrast, we show that in vitro osteoclastogenesis is impaired, resulting in smaller and fewer osteoclasts in þ/R740S cells compared with þ/þ cells, a phenomenon not observed in the other murine models. In addition, we found that the presence of both a wild-type and an R740S a3 protein, which does not detectably alter expression levels or targeting of the a3 protein to appropriate cellular sites, impairs lysosomal proton pumping and acidification (Fig.…”

Ostéoimmunologie : une vision globale et intégrée du tissu squelettique et du système immunitaire

“…[8][9][10] (Figure 2). Une étude récente vient de démontrer que l'injection de DC purifiées de la rate pouvait partiellement restaurer le phénotype ostéopétrotique de souris oc/oc dépourvues d'ostéo-clastes [11]. Ces résultats mettent donc en évidence l'existence d'un nouveau précurseur ostéoclastique tardif, au même titre que les monocytes et les macrophages.…”