“…A healthy synovium consists of 1–2 linings of synoviocytes [ 14 , 15 , 16 ]. During inflammation, various mononuclear cells, such as T cells, B cells, dendritic cells, plasma cells, mast cells, and macrophages, infiltrate the site of inflammation ( Figure 1 ) [ 17 , 18 , 19 , 20 ]; the synovial lining becomes hyperplastic, resulting in expansion of the synovial membrane and the formation of villi [ 21 , 22 , 23 ]. Macrophage-like synoviocytes produce various proinflammatory cytokines, such as Interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF).…”