PRTN3 and rheumatoid arthritis: In the RA synovium, monocyte–macrophages are the main source of this enzyme.11, 12 As a result, citrullination of fibrinogen, vimentin and α-enolase, among others, has been observed within the RA joints and associated with circulating ACPA.13–15 Accordingly, monoclonal antibodies generated from synovial fluid B cells frequently react against citrullinated antigens.16