Bone is a component of the skeletal-locomotor system but also functions as an immunological organ that harbors hematopoietic stem and progenitor cells. Since the immune and skeletal systems are closely related through a number of shared regulatory molecules including cytokines and receptors, bone can be affected in various immune disorders. Rheumatoid arthritis is a typical disease in which the immune system affects the bone metabolism. The enhanced activity of osteoclasts by the activation of Th17 cells causes the joint destruction in rheumatoid arthritis. Studies on bone destruction associated rheumatoid arthritis have highlighted the importance of the interplay between the immune and bone systems, and promoted the new interdisciplinary field of "osteoimmunology". Furthermore, recent studies have suggested that regulation of bone tissues by IL-17 is more complicated than we had expected. IL-17-prodcuing cells contribute to new bone formation at the enthesis in ankylosing spondylitis, and IL-17-producing γδ T cells promote bone regeneration by acting on the mesenchymal stem cells in bone fracture healing. It would be necessary to comprehensively understand the interplay between the immune and bone systems for elucidation of the molecular mechanisms underlying the pathogenesis of various diseases that involves the two systems.
Keywords: IL-17; Th17; osteoblast; osteoclast; rheumatoid arthritis.