PRG4 and rheumatoid arthritis: They are essential for maintaining homeostatic functions under noninflammatory conditions, such as the synthesis of the synovial fluid components proteoglycans and lubricin.22 In contrast, FLSs are active contributors to synovial inflammation and cartilage damage in the context of rheumatoid arthritis.23 Our observations of TMJ cartilage alterations in K/BxN mice prompted us to analyze the functional properties of TMJ FLSs in more detail.