Severe Heterotopic Ossification in the Skeletal Muscle and Endothelial Cells Recruitment to Chondrogenesis Are Enhanced by Monocyte/Macrophage Depletion

Abstract

Altered macrophage infiltration upon tissue damage results in inadequate healing due to inappropriate remodeling and stem cell recruitment and differentiation. We investigated in vivo whether cells of endothelial origin phenotypically change upon heterotopic ossification induction and whether infiltration of innate immunity cells influences their commitment and alters the ectopic bone formation. Liposome-encapsulated clodronate was used to assess macrophage impact on endothelial cells in the skeletal muscle upon acute damage in the ECs specific lineage-tracing Cdh5CreER T2 :R26REYFP/dtTomato transgenic mice. Macrophage depletion in the injured skeletal muscle partially shifts the fate of ECs toward endochondral differentiation. Upon ectopic stimulation of BMP signaling, monocyte depletion leads to an enhanced contribution of ECs chondrogenesis and to ectopic bone formation, with increased bone volume and density, that is reversed by ACVR1/SMAD pathway inhibitor dipyridamole. This suggests that macrophages contribute to preserve endothelial fate and to limit the bone lesion in a BMP/injury-induced mouse model of heterotopic ossification. Therefore, alterations of the macrophage-endothelial axis may represent a novel target for molecular intervention in heterotopic ossification.

Murine Tissue-Resident PDGFRα+ Fibro-Adipogenic Progenitors Spontaneously Acquire Osteogenic Phenotype in an Altered Inflammatory Environment

“…In the CCR2KO model, only infiltrating macrophages are inhibited, and other inflammatory cell populations such as eosinophils and neutrophils may be increased, facilitating the activation of FAPs . Our previous work has shown that clearance of these activated stromal cells is crucial for normal muscle regeneration and that delay or impairment of their clearance can result in muscle fibrosis or as we show here HO, and is consistent with recent work by Tirone and colleagues, who demonstrated that depletion of macrophages leads to increased HO volume . Further, previous studies have suggested that after local traumatic injuries, osteogenic BMPs may be present in the affected tissues .…”

Macrophages Switch to an Osteo‐Modulatory Profile Upon RANKL Induction in a Medaka (Oryzias latipes) Osteoporosis Model

“…Macrophages are highly dynamic and have a variety of functions ranging from the clearance of foreign effectors to mediating communication across different cell types under developmental and pathological conditions. ( 45, 46 ) By time‐lapse analysis, we found that all ectopically induced ctsk ‐expressing osteoclasts in the vertebral column originate from recruited mpeg1 macrophages, suggesting that these macrophages are the exclusive cellular source for osteoclasts. Consistently, the transient depletion of mpeg1 cells by Lipo‐Clo significantly reduced osteoclast numbers upon RANKL induction (Fig.…”

Mechanism of traumatic heterotopic ossification: In search of injury‐induced osteogenic factors

“…43 One study that reported increased HO volume after clodronate liposome treatment showed significant reduction in M2 macrophages, accompanied by a shift in endothelial cell fate towards endochondral ossification. 87 Another study using a burn injury and Achilles tenotomy HO model confirms that clodronate liposome treatment specifically reduces local Ly6C low macrophage number; however, this time decreased HO volume has been found. 80 Thus far, no consensus has been reached regarding the relationship between macrophage phenotype and HO pathogenesis.…”

“…85,86 While a number of previous studies have demonstrated that depletion of macrophages prevents HO formation, 20,21,40 some recent studies have reported that depletion of macrophages or inhibition of macrophage infiltration into damaged tissue can promote HO. 73,87,88 The reported discrepancy may be due to altered macrophage phenotype resulting from non-identical clodronate liposome-based macrophage depletion scheme being used. Indeed, monocyte depletion by clodronate liposome increases local proliferation of macrophage subsets after skeletal muscle injury.…”