THY1 and rheumatoid arthritis: Herein, we systemically investigated the role of PDGF-BB and TNF-α/LT-β in FDC development and pathotype differentiation of RA synovitis; and we report that: (1) Early FDCs in the RA synovial perivascular space display the pericyte and fibroblast markers PDGFR-β, NG2, and Thy-1 which are lost in mature FDCs supporting well-developed ELSs.