Inflammatory Treatment Used to Mimic Osteoarthritis and Patients' Synovial Fluid Have Divergent Molecular Impact on Chondrocytes In Vitro.

Osteoarthritis (OA) is a chronic disease characterized by joint tissue disruption and inflammation with a paucity of therapeutic options. Chondrocyte in vitro models are commonly used as the first step in evaluating new approaches and rely on the stimulation of an OA-like phenotype with inflammation often the method of choice. Inflammatory priming is frequently based on cytokines used at concentrations very far from the reality in the patients' synovial fluid (SF). The aim of this work was to compare the transcriptional response of chondrocytes to different inflammatory conditions: the high levels of IL1β that are used for standardized inflammation protocols, OA-SF, IL1β, IL6 and IFNγ at SF-like concentrations both individually and simultaneously to mimic a simplified "in vitro" SF. Both high IL1β and OA-SF strongly influenced chondrocytes, while SF-like concentrations of cytokines gave weak (IL1β alone or in combination) or no (IL6 and IFNγ alone) outcomes. Chondrocytes under the two most powerful polarizing conditions had a clearly distinct fingerprint, with only a shared albeit molecularly divergent effect on ECM stability, with IL1β mainly acting on ECM degrading enzymes and OA-SF accounting for a higher turnover in favor of fibrous collagens. Moreover, OA-SF did not induce the inflammatory response observed with IL1β. In conclusion, although partially similar in the endpoint phenotype, this work intends to encourage reflection on the robustness of inflammation-based in vitro OA models for molecular studies on chondrocytes.