PDPN and rheumatoid arthritis: Podoplanin-positive human synovial fibroblasts exhibit characteristics of highly invasive myofibroblasts [139,146] and are able to attach to, invade, and degrade cartilage in a severe combined immunodeficiency mouse model of rheumatoid arthritis [141], suggesting that podoplanin-positive synoviocytes are actively involved in joint destruction.