Scientists need to understand how stem cells and progenitor cells operate in the body, and what happens to them in the long term, if these cells are to be useful for treating various diseases. So far, much research has involved transplanting labeled stem cells into animals and then euthanizing the animals at certain points to look for histologic evidence of what happened to the cellsAs published in the December issue of the Journal of Nuclear Medicine, researchers at Case Western Reserve University in Cleveland and at the Thomas Jefferson National Accelerator Facility in Newport News, Va., investigated a noninvasive in vivo approach to monitoring human mesenchymal stem cell transplants over time. Although others have studied stem cells in vivo, the researchers used a reporter that consisted of a fusion protein containing functional components of firefly luciferase (fluc), monomeric red fluorescent protein (mrfp) and a truncated thymidine kinase (ttk). The triple-fusion reporter could be examined with bioluminescence imaging (fluc), fluorescence imaging (mrfp), and PET and CT (ttk). The researchers used this fusion protein to examine the effects of the reporters on the primary human mesenchymal stem cell phenotype. They specifically chose the truncated thymidine kinase because of its potential for clinical use.Fluorescence-activated cell sorting using the red fluorescent protein label showed an 83.2 percent transduction efficiency of the fusion protein. A second-generation lentivirus vector delivered the fusion protein to stem cells, which were seeded onto ceramic cubes and implanted into mice. Then the researchers performed bioluminescence imaging with either a Xenogen system or a Princeton Instruments cooled CCD camera. The bioluminescence images were aligned with planar x-ray images of the mice to make sure that the signal originated from the cubes.Because bioluminescence imaging is easier to perform than CT and PET, only the mice with the brightest bioluminescence signals were selected for imaging with the CT component of a Gamma Medica X-SPECT system as well as a Siemens microPET scanner.The method proved reliable for labeling stem cells for investigation in small animals. It produced repeatable real-time images and could be translated into human clinical PET studies.