Noninvasive Imaging in Stem Cell Therapies

Abstract

Determination of the fate of putative stem cells after transplantation requires development of novel cell-labeling and tracing techniques that permit noninvasive whole-body monitoring. Previous methods used in vitro radiolabeling of cells followed by imaging the transplanted radiolabeled cells in vivo, or labeling of cells with supermagnetic agents followed by magnetic resonance imaging with near microscopic resolution. The ex vivo labeling methods provide excellent shortterm results but are not suitable for long-term repetitive imaging because of loss of label owing to radiolabel decay or biological clearance of supermagnetic label. This chapter describes approaches for labeling cells with bioluminescent, fluorescent, and positron emission tomography (PET)-reporter genes for imaging adoptively transplanted cells in vivo. Genetic labeling of cells with different reporter genes allows for long-term, repetitive in vivo imaging using different imaging platforms. These imaging platforms include whole-body fluorescence imaging of green fluorescent protein (GFP) reporter gene expression (in rodents), bioluminescence imaging of the firefly luciferase (Luc) reporter gene expression using luciferin as reporter probe (in rodents), PET imaging of HSV1-tk or human TK2 reporter gene using different radiolabeled nucleoside analogs as reporter probes (in rodents and humans), and PET imaging of human D2 receptor with radiolabeled fluoroethylspiperone as reporter probe (in rodents and humans). The most developed approaches utilize herpes virus thymidine kinase (HSV 1-tk) as a reporter gene, which produces a gene product (an enzyme) that can be identified by phosphorylation of a radiolabeled reporter probe, which is trapped within the gene-labeled cell and can be visualized by PET scanning. This process can be repeated within a short time because of the short half-life of the radiolabeled substrate. This approach has been further developed using a fusion gene between the thymidine kinase gene and a GFP or luciferase genes for multiplatform imaging. Linking the reporter gene to specific promoters allows imaging of different cell types because only the cells activating the promoter will express the reporter gene. Such systems have been used successfully to trace cells expressing p53, T-cells activated to express nuclear factor of activated T-cells, and cells activated to express transforming growth factor-β. Cells with weak promoters may be detected using a two-step amplification system in which promoter strength is increased by linking the weak promoter to a transcription transactivator. Transplanted stem cells may be followed using a dual-reporter system, which uses a constitutive promoter to image the localization and viability of transplanted cells and an inducible promoter, which is activated when the stem cell commits to a certain differentiation lineage. PET imaging has been used successfully for in vivo monitoring of transplanted T-cells and EMV-specific cytotoxic lympocytes in mice, and the first successful PET imaging of HSV 1-tk gene expression in human gene therapy trials has been reported. Noninvasive whole-body imaging of various reporter genes is now being validated in different preclinical models and is expected to play an increasing role in monitoring the fate of transplanted stem cells in human stem cell therapy.