Expression of fibroblast activation protein (FAP) in RA. (A) Immunohistochemical staining of FAP (red) in synovial tissues of subjects with osteoarthritis (OA; n = 4) and rheumatoid arthritis (RA; n = 4). Counterstaining was methyl green. Scale bars, 200 μm. (B) Quantification of FAP positivity presented as percentage of positive stained area of synovial tissue. (C) Expression of FAP in OA synovial fibroblasts (OASF, n = 3) and RA synovial fibroblasts (RASF, n = 3) by immunofluorescence staining; green: FAP, red: actin cytoskeleton, blue: nucleus. (D) Quantification of fluorescence intensity per cell (n = 12, three independent patients). (E) Magnified section from (C) showing localization of FAP at membrane protrusions of RASF (white arrows). (F) Identification of human FAP mRNA transcripts in OASF and RASF by RT-PCR; #1-4 indicate independent patients. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) served as control.