Intra-Articular Injections of Mesenchymal Stem Cell Exosomes and Hyaluronic Acid Improve Structural and Mechanical Properties of Repaired Cartilage in a Rabbit Model.

Purpose: To compare the efficacy of mesenchymal stem cell (MSC) exosomes with hyaluronic acid (HA) against HA alone for functional cartilage regeneration in a rabbit osteochondral defect model.

Methods: Critical-size osteochondral defects (4.5-mm diameter and 1.5-mm depth) were created on the trochlear grooves in the knees of 18 rabbits and were randomly allocated to 2 treatment groups: (1) exosomes and HA combination and (2) HA alone. Three 1-mL injections of either exosomes and HA or HA alone were administered intra-articularly immediately after surgery and thereafter at 7 and 14 days after surgery. At 6 and 12 weeks, gross evaluation, histologic and immunohistochemical analysis, and scoring were performed. The functional biomechanical competence of the repaired cartilage also was evaluated.

Results: Compared with defects treated with HA, defects treated with exosomes and HA showed significant improvements in macroscopic scores (P = .032; P = .001) and histologic scores (P = .005; P < .001) at 6 and 12 weeks, respectively. Defects treated with exosomes and HA also demonstrated improvements in mechanical properties compared with HA-treated defects, with significantly greater Young's moduli (P < .05) and stiffness (P < .05) at 6 and 12 weeks. By 12 weeks, the newly-repaired tissues in defects treated with exosomes and HA composed mainly of hyaline cartilage that are mechanically and structurally superior to that of HA-treated defects and demonstrated mechanical properties that approximated that of adjacent native cartilage (P > .05). In contrast, HA-treated defects showed some repair at 6 weeks, but this was not sustained, as evidenced by significant deterioration of histologic scores (P = .002) and a plateau in mechanical properties from 6 to 12 weeks.

Conclusions: This study shows that the combination of MSC exosomes and HA administered at a clinically acceptable frequency of 3 intra-articular injections can promote sustained and functional cartilage repair in a rabbit post-traumatic cartilage defect model, when compared with HA alone.

Clinical Relevance: Human MSC exosomes and HA administered in combination promote functional cartilage repair and may represent a promising cell-free therapy for cartilage repair in patients.