Effects of osteoprotegerin (OPG) on histopathological manifestations of arthritis. Human tumor necrosis factor (TNF)-transgenic mice remained untreated or were treated with OPG or anti-TNF. Treatment started at a stage of early arthritis, and effects on synovial inflammation, on bone erosion and on cartilage damage are shown. OPG significantly affects TNF-mediated bone erosion, but not inflammation or cartilage damage. * Significant (P < 0.05) reduction in severity.