Mesenchymal Stem Cell Exosome and Fibrin Sealant Composite Enhances Rabbit Anterior Cruciate Ligament Repair

Background: The anterior cruciate ligament (ACL) fails to heal after rupture, leading to joint instability and an increased risk of osteoarthritis. Mesenchymal stem/stromal cell (MSC) exosomes have reported wide-ranging therapeutic efficacy; however, their potential for augmenting ACL repair remains to be investigated. Purpose: To evaluate the use of MSC exosomes with fibrin sealant on biological augmentation of ACL healing after suture repair and their effects on ACL fibroblast functions. Study Design: Controlled laboratory study. Methods: Twelve rabbit knees underwent ACL transection and suture repair. MSC exosome and fibrin composite (Exosome+Fibrin) or fibrin (Fibrin) alone was used to supplement the suture repair in 6 knees. ACL repair was assessed by magnetic resonance imaging at 6 and 12 weeks postoperatively and by histologic and immunohistochemical analyses at 12 weeks. To investigate the mechanisms through which MSC exosomes augment ACL repair, metabolic activity, proliferation, migration, and matrix synthesis assays were performed using the primary ACL fibroblasts. RNA sequencing was also performed to assess global gene expression changes in exosome-treated ACL fibroblasts. Results: Based on magnetic resonance imaging findings, 5 of 6 Exosome+Fibrin–treated ACLs were completely or partially healed, as opposed to 5 of 6 Fibrin-treated ACLs appearing torn at 6 and 12 weeks postoperatively. Additionally, 4 of 6 Exosome+Fibrin–treated ACLs were isointense, as compared with 5 of 6 Fibrin-treated ACLs that were hyperintense, indicating improved remodeling and maturation of the repaired ACLs with Exosome+Fibrin treatment. Histologically, Exosome+Fibrin–treated ACLs showed more organized collagen fibers and abundant collagen deposition, with a high amount of collagen I and relatively lower amount of collagen III, which are consistent with the matrix structure and composition of the normal ACL. Cell culture studies using ACL fibroblasts showed that MSC exosomes enhanced proliferation, migration, and collagen synthesis and deposition, which are cellular processes relevant to ACL repair. Further gene set enrichment analysis revealed key pathways mediated by MSC exosomes in enhancing proliferation and migration while reducing matrix degradation of ACL fibroblasts. Conclusion: The combination of MSC exosomes and fibrin sealant (Exosome+Fibrin) applied to a suture repair enhanced the morphologic and histologic properties of the ACL in a rabbit model, and these improvements could be attributed to the augmented functions of ACL fibroblasts with exosome treatment. Clinical Relevance: This work supports the use of MSC exosomes in biological augmentation of ACL healing after suture repair.