CD11B+CD36+ cells are bone anabolic macrophages that limit age-associated bone loss

Disruptions in the bone remodeling cycle that occur with increasing age lead to degeneration of the skeleton and increased risk of fragility fractures. Our understanding of how the bone remodeling process within cortical bone is controlled and altered with age in males and females is limited. Here, we generated bone marrow chimeric mice to understand the impacts of age and sex on the bone remodeling process. We demonstrate that transplantation of aged male or female bone marrow into young lethally irradiated male hosts unexpectedly enhances cortical bone mass without an impacting cancellous bone. Our single cell RNA-sequencing data show that mice reconstituted with aged bone marrow exhibited subsets of cells marked by CD11B/CD36 expression that demonstrate enhanced production of anabolic cytokines as compared to young counterparts, and that these myeloid subsets exist under conditions of normal physiology in aged mice. Importantly, CD11B+CD36+ cells do not differentiate into osteoclasts in vitro, and CD36 does not mark TRAP+ cells in vivo. Instead, CD36+ cells localize to resorption sites, including within cortical bone defects, suggesting their involvement in cortical bone remodeling and healing. CD11B+CD36+ cells also express elevated levels of bone anabolic WNT ligands, especially Wnt6. In functional assays, we demonstrate that soluble factors produced by CD11B+CD36+ cells enhance osteoblast progenitor commitment, mineralization, and activation of WNT signaling in vitro. Moreover, CD11B/CD36 exquisitely mark a subset of anabolic myeloid cells within human bone marrow. In conclusion, our studies identified a novel population of aged macrophages that limit cortical bone loss.