Human Mesenchymal Stem Cell-Derived Exosomes Promote Orderly Cartilage Regeneration in an Immunocompetent Rat Osteochondral Defect Model

Mesenchymal stem cell (MSC) therapy is currently evaluated in clinical trials for treatment of cartilage injuries. While cell therapy has demonstrated therapeutic efficacy, logistical and operational challenges associated with shipping constraint, storage and proper handling remain. Exosomes are nano-sized, cell-secreted bi-lipid membrane vesicles present in MSC secretome that have been found to mediate the regenerative and immunomodulatory functions of MSCs in treatment of various diseases in animal models. Here, we hypothesize that human MSC exosomes may represent a novel cell-free therapeutic to promote cartilage regeneration. Accordingly, exosomes were purified from conditioned medium of human embryonic MSCs and evaluated in their ability to heal a critical-sized osteochondral defect in an immunocompetent rat model over a 12-week period. Our results showed that weekly intra-articular injections of MSC exosomes promoted early cellular infiltration and proliferation that facilitated orderly cartilage and subchondral bone regeneration. Analysis of proliferative cell nuclear antigen (PCNA) immunoreactivity showed significantly higher numbers of PCNA positive cells in both the synovium and reparative tissue in animals treated with MSC exosomes than in animals treated with saline (p < 0.001). Concomitantly, we detected reduced numbers of apoptotic cells in the reparative tissue in animals treated with MSC exosomes. By end of 12 weeks, MSC exosome-treated rats showed a smooth continuous hyaline neocartilage layer and regenerated subchondral bone. On contrary, saline-treated defects showed severe surface irregularity and mostly fibrous/non-cartilaginous tissues with minimal matrix deposition. We also demonstrated that MSC exosomes induced polarization of the synovial macrophages with a regenerative M2 phenotype. Importantly, no adverse reactions were observed in all animals. Taken together, our results show that MSC exosomes are safe and effective, and likely mediate cartilage regeneration through multiple mechanisms. This study provides the basis for future investigation of human MSC exosomes as a novel off-the-shelf and cell-free therapeutic for cartilage repair in patients.