Blood-induced bone loss in murine hemophilic arthropathy is prevented by blocking the iRhom2/ADAM17/TNF-a pathway

Hemophilic arthropathy (HA) is a debilitating degenerative joint disease that is a major manifestation of the bleeding disorder hemophilia A. HA typically begins with hemophilic synovitis that resembles inflammatory arthritides, such as rheumatoid arthritis, and frequently results in bone loss in patients. A major cause of rheumatoid arthritis is inappropriate release of the proinflammatory cytokine tumor necrosis factor-a (TNF-a) by the TNF-a convertase (TACE; also referred to as ADAM17) and its regulator, iRhom2. Therefore, we hypothesized that iRhom2/ADAM17-dependent shedding of TNF-a also has a pivotal role in mediating HA. Here, we show that addition of blood or its components to macrophages activates iRhom2/ADAM17-dependent TNF-a shedding, providing the premise to study the activation of this pathway by blood in the joint in vivo. For this, we turned to hemophilic FVIII-deficient mice (F82/² mice), which develop a hemarthrosis following needle puncture injury with synovial inflammation and significant osteopenia adjacent to the affected joint. We found that needle puncture–induced bleeding leads to increased TNF-a levels in the affected joint of F82/2 mice. Moreover, inactivation of TNF-a or iRhom2 in F82/2 mice reduced the osteopenia and synovial inflammation that develops in this mouse model for HA. Taken together, our results suggest that blood entering the joint activates the iRhom2/ ADAM17/TNF-a pathway, thereby contributing to osteopenia and synovitis in mice. Therefore, this proinflammatory signaling pathway could emerge as an attractive new target to prevent osteoporosis and joint damage in HA patients.