Stefani, Robert M., Halder, Saiti S., Estell, Eben G., Lee, Andy J., Silverstein, Amy M., Sobczak, Evie, Chahine, Nadeen O., Ateshian, Gerard A., Shah, Roshan P., and Hung, Clark T.
Little is known about the critical role of the synovium in joint homeostasis and osteoarthritis (OA). We describe a novel in vitrotissue-engineered (TE) model to investigate the structure–function of synovium through quantitative solute transport measures. This TE synovium model was developed using healthy bovine, CD14−fibroblast-like synoviocytes (FLS, or Type B synoviocytes) encapsulated in a Matrigel scaffold. The bovine system is well established in musculoskeletal research, allowing comparisons to be made to previous work. Sheet-like TE constructs were precultured to attain native protein composition and polarized structure, as determined by immunohistochemistry and confocal microscopy, and subsequently exposed to interleukin-1α (IL) or dexamethasone (DEX). The biological responses, including nitric oxide and hyaluronic acid (HA) secretion, of engineered synovium paralleled that of native synovium. While HA media content increased in response to both IL and DEX, a higher proportion of HA was low molecular weight (<460 kDa) in IL compared to CTL or DEX. Meanwhile, lower permeability of 70 kDa dextran was strongly correlated (r= 0.9736, p= 0.0264) with a lower ratio of collagen to DNA in TE synovium, a trend that was qualitatively similar to explants (EXPs). Histological staining confirmed similar structural changes to TE and EXP specimens in response to DEX or IL, including intimal hyperplasia and matrix compaction. This suggests that, in addition to inflammation leading to HA breakdown and increased joint clearance, competing factors such as changes in synovium matrix content and permeability to a given solute size are also at play. Moreover, FLS-only engineered tissues, similar in cell composition to healthy native synovium, grew to contain CD14+macrophage-like synoviocytes (MLS, or Type A synoviocytes) in culture with IL, suggesting the potential role for cell transdifferentiation in the inflammatory response of synovium. Co-culturing FLS with MLS in this model also demonstrated the versatility to reverse engineer healthy and diseased synovium. Through the development of increasingly biofidelic synovium models, key gaps can be filled in our understanding of synovium function in health and OA.Impact StatementThe synovium envelops the diarthrodial joint and plays a key regulatory role in defining the composition of the synovial fluid through filtration and biosynthesis of critical boundary lubricants. Synovium changes often precede cartilage damage in osteoarthritis. We describe a novel in vitrotissue engineered model, validated against native synovium explants, to investigate the structure–function of synovium through quantitative solute transport measures. Synovium was evaluated in the presence of a proinflammatory cytokine, interleukin-1, or the clinically relevant corticosteroid, dexamethasone. We anticipate that a better understanding of synovium transport would support efforts to develop more effective strategies aimed at restoring joint health.