'Osteoimmunology' Offers New Perspectives for the Treatment of Pathological Bone Loss

New evidence of the interactions between the immune system and bone has accumulated in bone diseases, including osteoporosis, periodontitis and rheumatoid arthritis. A marked imbalance between bone resorption and formation is central to the onset of pathological bone loss. Osteoimmunology has revealed that the immune system, including T cells, B cells and inflammatory cytokines, is a key regulator of both osteoclasts and osteoblasts. Th1 cells, which differentiate from CD4(+)T cells, are thought to play a major function during bone loss. Moreover, the correlated expression of Th1 cytokines (interleukin-12 (IL-12), interferon-gamma (IFN-gamma)) and bone-resorbing cytokines (tumor necrosis factor-alpha (TNF-alpha), IL-1) also plays a key role during inflammatory induced bone resorption. Furthermore, a relatively new member of the CD4(+)T cell family Th17 displays the ability to promote osteoclast activity. The effect of IFN-gamma and IL-17 released by Th 17 cells on pre-osteoclast proliferation, differentiation and apoptosis provides the preliminary basis for the immune mechanism of pathological bone loss. The role of B cells in osteoimmunological interactions has long been suspected based on findings of B cells as active regulators of the RANK/RANKL/OPG axis. Pathological bone loss, including osteoporosis and human immunodeficiency virus-associated bone loss, are related to the altered RANKL/OPG through modified production by B cells, supporting this assumption. All of the above evidence may provide new theoretical explanations for the relationship between bone metabolism and the immune system as well as offer perspectives for the prevention and treatment of pathological bone loss.