A three-dimensional model to study human synovial pathology.

Therapeutic agents that are used by patients with rheumatic and musculoskeletal diseases were originally developed and tested in animal models, and although retrospective studies show a limited predictive value, it could be explained by the fact that there are no good in vitro alternatives. In this study, we developed a 3-dimensional synovial membrane model made of either human primary synovial cell suspensions or a mix of primary fibroblast-like synoviocytes and CD14+ mononuclear cells. We analyzed the composition of the mature micromasses by immunohistochemical staining and flow cytometry and show that the outer surface forms a lining layer consisting out of fibroblast-like and macrophage-like cells, reflecting the in vivo naïve synovial membrane. To recreate the affected synovial membrane in rheumatoid arthritis (RA), the micromasses were exposed to the pro-inflammatory cytokine Tumor Necrosis Factor Alpha (TNF-α). This led to increased pro-inflammatory cytokine expression and production and to hyperplasia of the membrane. To recreate the synovial membrane in osteoarthritis (OA), the micromasses were exposed to Transforming Growth Factor Beta (TGF-β). This led to fibrosis-like changes of the membrane, including increased Alpha Smooth Muscle Actin and increased expression of fibrosis-related genes PLOD2 and COL1A1. Interestingly, the macrophages in the micromass showed phenotypic plasticity as prolonged TNF-α or TGF-β stimulation strongly reduced the occurrence of Cluster of Differentiation 163-positive M2-like macrophages. We showed the plasticity of the micromasses as a synovial model for studying RA and OA pathology and propose that the synovial lining micromass system can be a good alternative for testing drugs.