Mesenchymal stem cell exosomes alleviate osteoarthritis by inhibiting complement activation via a CD59-dependent pathway

Mesenchymal stromal/stem cell (MSC) exosomes were previously shown to be effective in alleviating joint pain and degeneration in a rat model of temporomandibular joint osteoarthritis (TMJ-OA). However, the role of MSC exosomes in regulating complement activity implicated in OA inflammation remains to be elucidated. Here, we investigate the effects of MSC exosomes on the assembly of terminal complement complex, C5b-9, and its associated inflammation and matrix degradation in TMJ-OA. Following TMJ-OA induction, the rats received 3 weekly intra-articular injections of MSC exosomes or phosphate-buffered saline (PBS). The sham animals were given needle pricks. At 4-weeks post-treatment, tissue samples were harvested for transcriptomic profiling, micro-computed tomography (micro-CT), histology, immunohistochemistry, histomorphometry, C5b-9 enzyme-linked immunosorbent assay (ELISA), and multiplex cytokine analyses. Chondrocyte cultures were used to assess the effects of MSC exosomes on complement serum-induced chondrocyte inflammation and matrix degradation. The role of exosomal CD59 on complement assembly was also evaluated through neutralization with an anti-CD59 antibody. We observed that MSC exosomes mediated joint repair by suppressing pain and inflammation, reducing fibrosis and matrix degradation, and restoring the condylar cartilage and subchondral bone. Concurrently, there was suppressed complement activity with exosome treatment, as evidenced by the reduced levels of C5b-9 in the condylar cartilage and overlying synovium, and preferential synovial mRNA expression of complement inhibitors over effectors. These were accompanied by the decreased levels of pro-inflammatory cytokines such as IL-12 (p70), IL-17 A, IFN-γ, IL-18, and MIP-3α, and increased levels of anti-inflammatory cytokines including IL-2, IL-4, and IL-10 in the synovium with exosome treatment. Using chondrocyte cultures, we attributed the suppression of OA inflammation and matrix degradation to exosomal CD59, which inhibits complement activation. Specifically, MSC exosomes reduced C5b-9 formation and complement-induced inflammatory gene expression and MMP13 secretion in chondrocytes, and these effects were abrogated upon neutralization with an anti-CD59 antibody. These findings demonstrated that MSC exosomes alleviate OA inflammation and matrix degradation by inhibiting complement activity via a CD59-dependent pathway.