The immune system and bone metabolism influence each other. An imbalance in the immune system, resulting in inflammatory stimuli may induce an imbalance in bone turnover via induction of osteoclast differentiation and inhibition of osteoblast differentiation, leading to various pathological conditions including osteoporosis. T-cell subsets, helper T (Th)1 and Th17, which activate the immune system, induce osteoclasts, whereas regulatory T (Treg) cells, responsible for immunosuppression, inhibit osteoclastic differentiation. In addition, inflammatory cytokines, such as the tumor necrosis factor (TNF), also cause an imbalance in bone turnover, induction of osteoclasts and inhibition of osteoblasts. Treatments targeting the immune system may regulate abnormalities in bone metabolism, while also controlling immune abnormalities. In rheumatoid arthritis (RA), a representative autoimmune disease, immune abnormality and accompanying prolongation of synovial inflammation cause bone and cartilage destruction, periarticular osteoporosis, and systemic osteoporosis. Joint damage and osteoporosis in RA occur through totally different mechanisms. Stimulation by inflammatory cytokines induces the expression of the receptor activator for nuclear factor-κB ligand (RANKL) in T cells and synovial cells, thereby inducing bone destruction due to osteoblast-independent osteoclast maturation. However, biological products targeting TNF or interleukin-6 not only control disease activity, but also inhibit joint destruction. However, these biological products are not effective for osteoporosis. Conversely, anti-RANKL antibody inhibits osteoporosis and bone destruction, but exerts no influence on RA disease activity. Such differences in therapeutic efficacy may indicate the necessity for rethinking current theories on the mechanism of bone metabolism abnormality and joint destruction. Understanding the mechanisms underlying these pathologies via commonalities existing between the immune system and the metabolic system may lead to the development of new treatments.
Keywords: Bone; Immunity; Joint; Osteoporosis; Rheumatoid arthritis.