Floating anchorage-independent fibroblast-like cells mediate cartilage destruction independently from the hyperplastic synovial tissue

We characterized the morphological and immunological features of adherent synovial fluid cells derived from patients with rheumatoid arthritis (RA), and explored their potential function in vitro and in vivo by focusing on cartilage destruction. Synovial fluid adherent cells were obtained from patients with RA and from controls, and were characterized by immunohistochemistry and flow cytometry. In vitro, these cells were cultured in the presence of cartilage particles. Cartilage destruction was monitored by the release of sulphated glycosaminoglycans (sGAG) into the medium, and the level of MMP-1 in the cell culture supernatant was measured by ELISA. To inhibit cartilage destruction in vitro, the matrix metalloproteinase (MMP) inhibitor Marimastat was tested in this system. In vivo, in the SCID mouse coimplantation model, RA synovial fluid adherent cells and RA synovial fibroblasts (as positive controls) were coimplanted with human cartilage under the kidney capsule, and were maintained there for 60 days. Synovial fluid adherent cells in vitro consist of two subpopulations, large round-shaped macrophage-like cells(CD68⁺) and spindle-shaped fibroblast-like cells (Thy-1⁺). When passaged, the latter cells proliferate and organize themselves in three-dimensional formations. The majority (>90%) of passaged RA synovial fluid adherent cells expressed the Thy-1⁺, CD68^-, CD86^-phenotype. Compared with synovial tissue fibroblasts, synovial fluid adherent cells expressed lower levels of adhesion molecules, including CD54 and galectin-3. Using RA synovial fluid adherent cells, the in vitro release of sGAG associated with cell activity was 2.5-fold higher compared with negative controls. The release of sGAG correlated with the concentration of MMP-1 and was inhibited by the broad range MMP inhibitor Marimastat in a dose-dependent manner. RA synovial fluid adherent cells coimplanted with cartilage in SCID mice showed the same invasive behaviour as tissue-derived RA synovial fibroblasts. Taken together, our data showed that RA synovial fluid adherent contains 'floating' anchorage-independent fibroblast-like cells similar to tissue-derived RA synovial fibroblasts, contributing to cartilage destruction independently of the process mediated by the hyperplastic synovial tissue.