Chondroprotective functions of neutrophil-derived extracellular vesicles by promoting the production of secreted frizzled-related protein 5 in cartilage.

Background: Osteoarthritis (OA) is the most common degenerative joint disease characterized by cartilage degradation and various degrees of inflammation in the synovium. Growing evidence highlights that neutrophil extracellular vesicles (EVs) play a protective role in arthritic joints by promoting the resolution of inflammation and the synthesis of proteoglycans in cartilage. However, this homeostatic function is dependent on the activation state of neutrophils and the surrounding environment/tissues. Hence, we explored the chondroprotective functions of neutrophil-derived EVs under different stimulation conditions and the underlying molecular mechanism.

Methods: Human blood-derived neutrophils, murine bone marrow-derived neutrophils, C-28I2 cells and primary chondrocytes were used. Neutrophils were stimulated with different cytokines, and their EVs were isolated for chondrocyte stimulation and further subjected to RNA-sequencing analysis. Two experimental murine OA models were used, and the treatment was performed by intraarticular injections.

Results: Conditioned medium from neutrophils stimulated with TGF-β (N-β) had the greatest inhibitory effect on the expression of catabolic factors in stimulated chondrocytes. These protective effects were not impaired when conditioned medium of N-β from AnxA1-deficient mice was used. Consistent with these results, EVs isolated from N-β significantly reduced the expression of catabolic factors in stimulated chondrocytes. Bulk RNA-seq analysis revealed that secreted frizzled-related protein 5 (SFRP5) is upregulated in N-β-EV-stimulated chondrocytes. Furthermore, recombinant SFRP5 treatment significantly reduced the expression of catabolic factors in vitro and catabolic process in experimental murine OA models.

Conclusions: The current study emphasizes the potential therapeutic application of neutrophils in OA and provides new knowledge on the molecular mechanisms underlying their function.