Osteoarthritis (OA) is the most prevalent degenerative joint disorder, but no reversing therapies are currently available . This is mainly due to the disease complexity, that involves a failure of the entire joint, and to the disease multifactorial etiology . Taking all this into account, a gap of knowledge still exists on initial disease mechanisms, linked to the unavailability of reliable human preclinical OA models . In this scenario, organs-on-chip are promising candidates to deeply investigate tissues crosstalk in early OA stages. To this end, we developed a compartmentalized joint-on-chip model for the co-culture of cartilage and synovium, aiming at evaluating the disruption of the physiological cross-talk between these tissues that contributes to the pathogenesis of OA .
The microfluidic platform consists of two separate culture areas, intended for synovium and cartilage 3D cultures, whose communication is controlled through valves that can be opened through vacuum application. An actuation layer allows to apply a mechanical compression to the cartilage compartment upon pressurization . Human chondrocytes, and synovial fibroblasts and macrophages were separately cultured in the two compartments, respectively. To assess the effect of cartilage degeneration on triggering synovial inflammation, a hyperphysiological compression was first applied to the cartilage compartment to induce an OA phenotype . Induction of synovial inflammation was then quantified upon valves opening through FACS analysis.
Culture conditions were first optimized to obtain mature synovium and cartilage microtissues separately, as demonstrated by deposition of extracellular matrix rich in Collagen type-I and lubricin, and Collagen type-II and Aggrecan, respectively. Then, an OA phenotype was successfully induced on cartilageupon application of a hyperphysiological compression, as proven by gene expression analysis, showing up-regulation of inflammatory markers (e.g. MMP13, IL8). Cartilage inflammation exerted a detrimental effect on synovial constructs, as the FACS analysis showed an increase in CD86+ and CD80+ macrophages upon valve aperture, indicating polarization towards M1 pro-inflammatory state.
The proposed compartmentalized microfluidic platform offers a solution to mature cartilage and synovial constructs and/or to induce OA traits in only one of the two compartments, by enabling a temporal control over chambers communications. The device was here exploited to prove that mechanically-damaged cartilage triggers inflammatory changes in the synovium, suggesting the possible role of cartilage as effector responsible for OA initiation. Furthermore, the platform is currently being used to elucidate the role of an inflamed synovium on cartilage degradation, and this will eventually allow to understand which of the two tissues has a predominant role in early OA stages.
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This work was supported by Fondazione Cariplo-uKNEEque - Rif. 2018/0551 and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 841975."