Osteoblast cell death triggers a pro-osteogenic inflammatory response regulated by reactive oxygen species and glucocorticoid signaling in zebrafish

In zebrafish, transgenic labeling approaches, robust regenerative responses and excellent in vivo imaging conditions enable precise characterization of immune cell behavior in response to injury. Here, we monitored osteoblast-immune cell interactions in bone, a tissue which is particularly difficult to in vivo image in tetrapod species. Ablation of individual osteoblasts leads to recruitment of neutrophils and macrophages in varying numbers, depending on the extent of the initial insult, and initiates generation of cathepsinK+ osteoclasts from macrophages. Induced osteoblast death triggers the production of pro-inflammatory cytokines and reactive oxygen species, which are needed for successful macrophage recruitment. Excess glucocorticoid signaling as it occurs during the stress response inhibits macrophage recruitment, maximum speed and changes the macrophages phenotype. While osteoblast loss is compensated for within a day by contribution of committed osteoblasts, macrophages continue to populate the region. Their presence is required for osteoblasts to fill the lesion site. Our model enables visualization of homeostatic bone repair after microlesions at single cell resolution and demonstrates a pro-osteogenic function of tissue-resident macrophages in non-mammalian vertebrates. Summary statementLaser-mediated osteoblast ablation induces recruitment of tissue-resident macrophages by a release of reactive oxygen species. The presence of macrophages is required for osteoblasts to repopulate the lesion site and can be modulated by glucocorticoids.