Evaluation of Cartilage Repair After Implantation of Labeled Human Chondrocytes as Free Cells or Spheroids in Rabbit Knees.

Articular cartilage repair remains a clinical challenge due to the tissue's limited regenerative capacity. This study aimed to perform in vivo tracking of human chondrocytes after implantation in femoral trochlear defects. Moreover, we tested the rabbit xenogeneic model to assess the efficacy of human chondrocyte implantation, either in the form of cell suspension or in the form of spheroids, for cartilage regeneration. Chondrocytes were isolated from osteoarthritic human knees, expanded in vitro, and labeled with gold nanoparticles (AuNPs) to enable their in vivo tracking. AuNP labeling did not impair chondrocyte viability and spheroid formation. Knee osteochondral defects were then treated with the labeled cells, either injected intra-articularly as single cells in suspension or aggregated into scaffold-free spheroids and further implanted. Two weeks post-transplantation, macroscopic and histological evaluations were performed to assess cartilage repair and cell engraftment. The presence of labeled cells was detected only in spheroid-treated lesions that exhibited significantly greater tissue filling as reflected by O'Driscoll and ICRS scores. Moreover, localized expression of type II collagen was observed in some of the spheroid-treated defects. In contrast, single cell intra-articular injection resulted in poor cartilage repair and the absence of labeled cells at the defect site. Altogether, our results indicate that human chondrocyte delivery in the form of scaffold-free spheroids in rabbit knees enhances their retention at the defect site and supports early stages of cartilage regeneration. Moreover, our results suggest that the xenogeneic rabbit model could be a useful platform for evaluating human-derived cell therapies.