Mediators of inflammation and bone remodeling in rheumatic disease

Remodeling of bone is a continuous process that occurs throughout life. Under normal physiologic conditions, bone-resorbing osteoclasts and bone-forming osteoblasts are tightly coupled and regulated to ensure proper balance, such that there is no net change in bone mass. However, inflammation perturbs normal bone homeostasis. The impact of inflammation on bone is dependent upon the anatomic site affected, cell types, factors and cytokines present in the local microenvironment, and local mechanical forces. Cytokines are central to the pathogenesis of inflammation-induced bone loss and contribute to the uncoupling of osteoclast-mediated bone resorption and osteoblast-mediated bone formation, thereby disrupting normal remodeling. In this review, we will discuss the effects of cytokines on bone in two settings, rheumatoid arthritis and spondyloarthritis, a disease category that includes ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease, and juvenile onset spondyloarthropathy. The outcome for bone in these disease settings is quite different, and an understanding of the pathogenic mechanisms leading to the net impact on bone has been essential in developing new therapeutic approaches to bone health in these diseases.