Ultrasound imaging tracking of mesenchymal stem cells intracellularly labeled with biosynthetic gas vesicles for treatment of rheumatoid arthritis

Rationale: Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation and damage to articular tissues that can lead to irreversible joint damage and progressive disability. The multipotent mesenchymal stem cells (MSCs) play an important role in immune disorders and tissue regeneration. However, their immunosuppressive effects and the underlying mechanisms are largely unclear due to the lack of tools for real-time imaging of MSCs in vivo. Gas vesicles (GVs) are biosynthetic nanobubbles that are ejected from aquatic microbes, such as bacteria and archaea, and have an excellent ultrasound imaging capacity. Methods: We harvested MSCs from the bone marrow of Sprague Dawley (SD) rats. Then, GVs were synthesized and incubated with MSCs to obtain intracellularly labeled MSCs. We firstly tested the ultrasound imaging of GV@MSCs in vitro and in vivo and then explored the therapeutic effect of GV@MSCs combined with methotrexate (MTX) in RA rats. Results: These GV@MSCs showed significant contrast-enhanced ultrasound signals without a loss of viability and differentiation capacity. In addition, the GV@MSCs could be imaged in real-time for 5 days using ultrasound both in vitro and in vivo, making it possible to visually track their migration and homing to the joint cavity from the subcutaneous layer of lateral malleolus joints in the injected RA rats. Furthermore, GV@MSCs significantly enhanced the curative effect of methotrexate (MTX) against RA, resulting in decreased paw thickness, lower arthritis index score, reduced bone erosion and cartilage destruction, compared to the PBS, free MTX, and GV@MSCs groups. Conclusion: We developed a novel therapeutic strategy against RA using GVs-loaded MSCs that can be tracked in vivo in real-time.