PC and rheumatoid arthritis: Peptides presented by HLA-DR on cells from the peripheral blood, synovial tissue, and synovial fluid from individuals with RA revealed sequence homology between the S. copri protein Pc-p27 and self-antigens, including N-acetylglucosamine-6-sulfatase (GNS) and filamin A (FLNA), both of which are expressed in RA synovial tissues.