Periosteal CD68+F4/80+ Macrophages Are Mechanosensitive for Cortical Bone Formation by Secretion and Activation of TGF-β1

Mechanical force regulates bone density, modeling, and homeostasis. Substantial periosteal bone formation is generated by external mechanical stimuli, yet its mechanism is poorly understood. Here, it is shown that myeloid-lineage cells differentiate into subgroups and regulate periosteal bone formation in response to mechanical loading. Mechanical loading on tibiae significantly increases the number of periosteal myeloid-lineage cells and the levels of active transforming growth factor beta (TGF-beta), resulting in cortical bone formation. Knockout of Tgfb1 in myeloid-lineage cells attenuates mechanical loading-induced periosteal bone formation in mice. Moreover, CD68(+)F4/80(+) macrophages, a subtype of myeloid-lineage cells, express and activate TGF-beta 1 for recruitment of osteoprogenitors. Particularly, mechanical loading induces the differentiation of periosteal CD68(+)F4/80(-) myeloid-lineage cells to the CD68(+)F4/80(+) macrophages via signaling of piezo-type mechanosensitive ion channel component 1 (Piezo1) for TGF-beta 1 secretion. Importantly, CD68(+)F4/80(+) macrophages activate TGF-beta 1 by expression and secretion of thrombospondin-1 (Thbs1). Administration of Thbs1 inhibitor significantly impairs loading-induced TGF-beta activation and recruitment of osteoprogenitors in the periosteum. The results suggest that periosteal myeloid-lineage cells respond to mechanical forces and consequently produce and activate TGF-beta 1 for periosteal bone formation.