“Osteoimmunology” is one of the newest areas of scientific investigation. The term osteo-immunology specifically refers to the study of the regulation of osteoclastogenesis through the Receptor Activator of Nuclear Factor κB Ligand (RANKL). RANKL has been described as the “master switch regulator” of osteoclastogenesis. Since bone homeostasis is a balance between bone formation (osteoblastogenesis) and bone resorption (osteoclastogenesis), understanding the regulatory mechanisms between these phases may provide greater insight into bone diseases where the scale is tipped in favor of resorption. Since periodontitis is an inflammatory disease characterized by alveolar bone loss, controlling the expression of RANKL in the periodontal lesion may be a useful treatment approach.
Osteoblasts express RANKL on their cell membrane. When this ligand binds to the RANKL receptor on a pre-osteoclast, it signals the cell to differentiate into an active osteoclast. The decoy receptor for RANKL, called osteoprotegerin, blocks this activation mechanism, thus helping to maintain bone homeostasis. In periodontitis, the ratio of RANKL to osteoprotegerin increases, whereas in health, the ratio is decreased. This ratio appears to be more important in identifying bone resorbing sites than the concentration of either RANKL or osteoprotegerin alone. What is known about RANKL and the periodontal lesion?
The concentrations of osteoprotegerin and RANKL are controlled not only by the cytokines listed above but also by oseotrophic hormones such as glucocortcoids and parathyroid hormone which inhibit osteoprotegerin and increase RANKL production. Estrogen appears to inhibit RANL and RANKL-stimulated osteoclastogenesis.