Immune signals in the context of secondary osteoporosis.

Bone homeostasis is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts, and alterations in bone metabolism can lead to diseases such as osteoporosis. Inter-cellular and intra-cellular signaling, originating from the immune system, the largest source of cell-derived regulatory signals, are involved in these processes. Immune-competent cells such as macrophages and lymphocytes deliver cell-cell signaling through soluble factors such as cytokines and through direct contact with the cells. Such immunological signals to the bone are transmitted primarily through osteoblasts or direct stimulation of osteoclasts to induce osteoclast maturation or bone resorption, which may in turn lead to the disequilibrium of bone metabolism. Inflammatory diseases such as rheumatoid arthritis are good examples of such a process, in which immunological signals play a central role in the pathogenesis of the accompanying secondary osteoporosis. We will achieve a better understanding of the pathogenesis of bone metabolism in osteoporosis through immune signaling, and thereby develop improved therapeutic strategies for these conditions.