Mesenchymal stem cells (MSCs), derived from nearly all adult organs, can differentiate into multiple lineages including osteoblasts, chondrocytes and adipocytes under different local micro-environments for tissue repair. MSCs have also shown to ameliorate graft versus host disease (GVHD) caused by allogeneic hematopoietic stem cell transplantation, via suppressing the activation and proliferation of the alloreactive lymphocytes. Therefore, MSCs have great potential in bone tissue regeneration and stem cell-based therapies. However, previous techniques to track stem cells in vivo are usually associated with frequent animal sacrifice followed by subsequent histology analysis or RT-PCR verification. A continuous, non-invasive, real time and longitudinal imaging technique is lacking.
In this study, we aimed to establish such a platform technology to image stem cell biodistribution in a disease model (GVHD) in vivo. A triple-reporter gene containing luc-mrfp–ttk was incorporated into hMSCs via lentiviral transduction. The gene of luc was imaged with BLI and ttk imaged with PET. The proliferation rate of the transduced stem cells was reduced compared to the wild type cells although the stem cell differentiation potential was reserved. The transcriptional changes between transduced and wild-type stem cells were examined via gene microarrays to identify the key genes related to proliferation and mesenchymal differentiation to determine the transduction effect. Stem cells that were loaded into ceramic cubes and implanted into NOD/SCID mice s.c. were longitudinally imaged in vivo by BLI for more than 3 months. PET was also capable of imaging the implanted stem cells in the ceramic cubes. Histology studies confirmed the formation of bone from the transduced hMSCs in vivo. These indicate that the imaging template was successfully developed.
In an animal model of GVHD developed in mice, hMSCs were injected intravenously for the treatment of this disease. It was shown that hMSCs enhanced the survival of the GVHD mice compared to the untreated mice. BLI results showed primary entrapment of hMSCs in lung, while only a small fraction of cells transiently migrated to the intestine. Cells in the mice were undetectable 7 days after their transplantation. Although arterial injection led to enhanced cell distribution into different organs such as intestines, and prolonged hMSC retention in allogeneic mice, no efficacy was observed compared with the control group possibly due to the injury associated with the surgery procedure that allowed arterial injection. These data suggest that timely cell injection and repeated injection may be necessary to effectively prevent or alleviate this disease.
In summary, this imaging technology plays an important role in tracking stem cells, in revealing disease development mechanisms, as well as in optimizing stem cell treatment strategy.