The bone is an essential organ for locomotion and protection of the body, as well as hematopoiesis and mineral homeostasis. In order to exert these functions throughout life, bone tissue undergoes a repeating cycle of osteoclastic bone resorption and osteoblastic bone formation. The osteoclast is a large, multinucleated cell that is differentiated from monocyte/macrophage lineage cells by macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). RANKL transduces its signal through the signaling receptor, RANK. RANKL/RANK signaling activates NFATc1, the master regulator of osteoclastogenesis, to induce osteoclastogenic gene expression. Many types of cells express RANKL to support osteoclastogenesis depending on the biological context and the dysregulation of RANKL signaling leads to bone diseases such as osteoporosis and osteopetrosis. This review outlines the findings on osteoclast and RANKL/RANK signaling that have accumulated to date.

Immune responses are crucial not only for host defence against pathogens but also for tissue maintenance and repair after injury. Lymphocytes are involved in the healing process after tissue injury, including bone fracture and muscle damage. However, the specific immune cell subsets and mediators of healing are not entirely clear. Here we show that γδ T cells produce IL-17A, which promotes bone formation and facilitates bone fracture healing. Repair is impaired in IL-17A-deficient mice due to a defect in osteoblastic bone formation. IL-17A accelerates bone formation by stimulating the proliferation and osteoblastic differentiation of mesenchymal progenitor cells. This study identifies a novel role for IL-17-producing γδ T cells in skeletal tissue regeneration.

Receptor activator of NF-κB (RANK) ligand (RANKL) induces the differentiation of monocyte/macrophage-lineage cells into the bone-resorbing cells called osteoclasts. Because abnormalities in RANKL, its signaling receptor RANK, or decoy receptor osteoprotegerin (OPG) lead to bone diseases such as osteopetrosis, the RANKL/RANK/OPG system is essential for bone resorption. RANKL was first discovered as a T cell-derived activator of dendritic cells (DCs) and has many functions in the immune system, including organogenesis, cellular development. The essentiality of RANKL in the bone and the immune systems lies at the root of the field of "osteoimmunology." Furthermore, this cytokine functions beyond the domains of bone metabolism and the immune system, e.g., mammary gland and hair follicle formation, body temperature regulation, muscle metabolism, and tumor development. In this review, we will summarize the current understanding of the functions of the RANKL/RANK/ OPG system in biological processes.