Fig 1: Plasmin contributes to the initiation and progression of OA through multiple mechanisms: the degradation of lubricin and cartilage proteoglycan, activation of pro-MMPs, and induction of inflammatory and degradative mediators from synovial cells.(A) Representative images from immunofluorescence staining of plasmin (green, left), staining of nuclei (blue, middle), and merging (right) in the undamaged articular cartilage area from individuals with knee OA who underwent total knee replacement. (B) Representative images from immunohistochemical staining of plasmin in the damaged articular cartilage area (upper left), the synovium (upper right), and the isotype controls (bottom, respectively) from individuals with OA. The arrowhead indicates binding of plasmin on the surface of chondrocytes (upper left) and cells of the synovial lining (upper right). (A and B) Scale bar, 200 μm; cartilage and synovial tissues from n = 5 individuals were analyzed. (C) Degradation of recombinant lubricin, shown on SDS-PAGE gel stained with Coomassie blue, by plasmin, but not activated tPA or uPA after 4 hours’ 37°C incubation. Red arrowhead shows the lubricin stained with Coomassie blue in different conditions: vehicle, plasmin, tPA, and uPA. (D) ELISA quantification of soluble sGAG released from cartilage explants from individuals with OA, treated with vehicle, pro–MMP-13, plasmin, or plasmin + pro–MMP-13. (E) Quantitative PCR (qPCR) analysis of OA-related inflammatory and degradative mediators as well as VEGFα in human primary synoviocytes, derived from the knee joints of individuals with OA, with or without plasmin stimulation. (F and G) qPCR analysis of relative gene expression levels of OA-related inflammatory and degradative mediators in synovial tissue (F) or articular cartilage from Plg+/+ (n = 5) and Plg–/– (n = 5) mice 20 weeks after DMM. All data are the mean ± SEM of triplicates and are representative of 3 independent experiments. **P < 0.01, and ***P < 0.001. The test in panel D is 1-way ANOVA. The test in panels E–G is 2-tailed t test.