Impaired efferocytosis enables apoptotic osteoblasts to escape osteoimmune surveillance during aging (mRNA)

Macrophages efferocytosis of apoptotic osteoblasts (apoOBs) is a key osteoimmune process for bone homeostasis that is highly regulated in osteoimmune surveillance. However, apoOBs accumulate in aged bone marrow, where they might mount proinflammatory responses and progressive bone loss. Why apoOBs were not cleared during aging remains unknown. Here, we show that apoOBs upregulate immune checkpoint molecule CD47 with age to evade macrophages clearance. Using osteoblast- and myeloid-specific gene knockout mice, we identify histone deacetylase SIRT6 as an essential regulator of the CD47–SIRPa checkpoint. Further, apoOBs that require SIRT6-mediated chemotaxis signals can recruit macrophages by releasing apoptotic extracellular vesicles. Therapeutically, we develop two targeting delivery strategies to enhance SIRT6 activity, showing increased apoOBs clearance and delayed age-related bone loss. Collectively, our data revealed a previously unknown linkage between immune surveillance and bone homeostasis and targeting the mechanism of SIRT6-regulated apoOBs clearance could be a promising therapeutic strategy for age-related bone diseases. Overall design: Comparative gene expression profiling analysis of RNA-seq data for apoptotic osteoblasts (apoOBs) from young and old mice