The Interplay Between Early Chondrocyte Spreading and Inflammatory Responsivity

Joint injuries are increasingly common and initiate a degenerative cascade in the cartilage extracellular matrix. Chondrocytes experience both intra- and extra-cellular changes during the initial phases of this process, including inflammatory activation and morphological change, initiating a catabolic feedback cycle that progresses toward osteoarthritis (OA). However, the link between this early morphological spreading and susceptibility to future inflammatory events is unclear. Thus, the objective of this study was to explore the implications of cellular spreading on early inflammatory activation. First, we treated bovine cartilage explants with control or degenerative media for 2 weeks and established early chondrocyte spreading and extracellular matrix loss around chondrocytes. Next, we either seeded chondrocytes on or encapsulated them within gelatin hydrogels of different stiffnesses to allow different degrees of spreading, followed by a short (2 h) inflammatory stimulus to measure inflammatory activation (NF-κB). We found in 2D that stiffer substrates led to greater chondrocyte spreading and NF-κB nuclear localization; however, in 3D, this trend was reversed, with the greatest spreading and activation in cells in the softest hydrogels. Finally, we investigated how hyaluronic acid hydrogel incorporation into these environments could impact this spreading-inflammtory activation relationship, showing that augmentation with HA reduced both facets. In conclusion, chondrocyte spreading, especially in 3D, is linked with reduced matrix stiffness, and this can make chondrocytes more susceptible to inflammation. Thus, future therapies should seek to address not only the inflammation in the joint but also to restore chondrocyte morphology and microenvironmental properties.