Association between cytokines and exosomes in synovial fluid of individuals with knee osteoarthritis

Background: Cytokines in synovial fluid (SF) play a crucial role in knee osteoarthritis (KOA). Exosomes are nanovesicles that are abundant in SF and carry a large quantity of signaling molecules. The purpose of this study was to evaluate the cytokine profiles of SF-derived exosomes and try to explore its biological function.Methods: Twenty-four KOA patients who were scheduled for their first intra-articular injection or knee replacement surgery were enrolled and divided into the KL1-2 group and the KL3-4 group according to the Kellgren and Lawrence (KL) classification. SF was collected from the patient's knee for the isolation of exosomes. A multiplex cytokine assay was performed to detect the 21 cytokines in the exosomes. The SF derived-exosomes were exposed to PBMCs and chondrocytes to assess their immunomodulatory potential.Results: Exosomes were successfully extracted from the SF, with an average diameter of 92 nm. Most cytokines were detectable in the SF-derived exosomes. Twelve inflammatory cytokines and eight chemokines were elevated in the exosomes of the KL3-4 group compared to that of the KL1-2 group (p p Conclusion: Our data indicated that most cytokines in SF are not only in a free form but also associated with and enriched in exosomes. Exosomes from end-stage KOA patients have a higher level of cytokines, especially chemokines, in comparison with the cytokine profiles of the soluble SF. SF-derived exosomes recruit inflammatory cells and inhibit cartilage proliferation, thus promoting joint degeneration. These data provide a new perspective for understanding the changes in the inner environment of KOA.