Persistence of fluorescent nanoparticle-labeled bone marrow mesenchymal stem cells in vitro and after intra-articular injection.

Mesenchymal stem cells (MSCs) improve the osteoarthritis condition, but the fate of MSCs after intra-articular injection is unclear. We used fluorescent nanoparticles (quantum dots; QDs) to track equine MSCs (QD-MSCs) in vivo after intra-articular injection to normal and osteoarthritic joints. One week after injection of QD-MSCs, unlabeled MSCs, or vehicle we determined the presence of QD-MSCs in synovium and articular cartilage histologically. In vitro, we evaluated the persistence of QDs in MSCs and whether QDs affected proliferation, immunophenotype or differentiation. In joints injected with QDMSCs, labeled cells were identified on the synovial membrane, and significantly less often on articular cartilage, without differences between normal and osteoarthritic joints. Joints injected with QD-MSCs and MSCs had increased synovial total nucleated cell count and protein compared to vehicle-injected joints. In vitro QDs persisted in non-proliferating cells for up to 8 weeks (length of the study) but fluorescence was essentially absent from proliferating cells within 2 passages (approximately 3 to 5 days). QD-labeling did not affect MSC differentiation into chondrocytes, adipocytes and osteocytes. QD-MSCs had slightly different immunophenotype from control cells, but whether this was due to an effect of the QDs or to drift during culture is unknown. QD-MSCs can be visualized in histological sections one week after intra-articular injection, and are more frequently found in the synovial membrane versus cartilage in both normal and osteoarthritic joints. QDs do not alter MSC viability and differentiation potential in vitro. However, QDs are not optimal markers for long-term tracking of MSCs, especially under proliferative conditions.