Functional characterization of adherent synovial fluid cells in rheumatoid arthritis: destructive potential in vitro and in vivo

To characterize the morphologic and immunologic features of adherent synovial fluid cells derived from patients with rheumatoid arthritis (RA), and to explore their potential function in vitro and in vivo by focusing on cartilage destruction.

Synovial fluid adherent cells obtained from patients with RA and from control subjects were characterized by immunohistochemistry, flow cytometry, and electron microscopy. In vitro, these cells were cultured in the presence of cartilage particles. Cartilage destruction was monitored by the release of sulfated glycosaminoglycans (sGAG) into the medium, and the level of matrix metalloproteinase 1 (MMP-1) in the cell culture supernatant was measured by enzyme-linked immunosorbent assay. To inhibit cartilage destruction in vitro, the 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 maintained there for 60 days.

In vitro, the synovial fluid adherent cells consisted of 2 subpopulations, large round-shaped macrophage-like cells (CD68+) and spindle-shaped fibroblast-like cells (Thy-1+). When passaged, the latter cells proliferated and organized themselves into 3-dimensional formations. This allowed them to reach collagen particles fixed with agarose. Fibroblasts derived from synovial tissues could not be used in this assay because they grew only in monolayers and not on agarose. The majority (>90%) of passaged RA synovial fluid adherent cells expressed the Thy-1+,CD45-,CD68-,CD86- phenotype. Electron microscopy did not reveal important morphologic differences between the 2 types of fibroblasts, those from synovial tissue or those from synovial fluid. However, synovial fluid adherent cells expressed lower levels of adhesion molecules, including CD54 and galectin 3, as well as the complement-regulatory molecule CD55. The in vitro release of sGAG associated with cell activity was 2.5-fold higher from RA synovial fluid adherent cells in comparison with that from negative control cells. 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 behavior as that displayed by tissue-derived RA synovial fibroblasts.

Similar to tissue-derived RA synovial fibroblasts, RA synovial fluid adherent cells, which contain "floating" anchorage-independent fibroblast-like cells, mediate cartilage destruction independent of the hyperplastic synovial tissue.