Background
Osteoprotegerin (OPG) is a soluble decoy receptor that acts as a receptor antagonist, blocking OPG ligand (OPGL) induced osteoclastogenesis. In vitro, proinflammatory cytokines such as tumour necrosis factor (TNF) upregulate endothelial OPG expression. However, low levels of endothelial OPG expression are seen in synovial biopsies from patients with active rheumatoid arthritis (RA) associated with high levels of TNF. In order to elucidate the in vivo interaction between TNF and the OPG/OPGL system in RA, we investigated the effect of anti-TNF therapy on synovial expression of OPG and OPGL.