Noninvasive cell tracking.

Cell-based therapies may gain future importance in defeating different kinds of diseases, including cancer, immunological disorders, neurodegenerative diseases, cardiac infarction and stroke. In this context, the noninvasive localization of the transplanted cells and the monitoring of their migration can facilitate basic research on the underlying mechanism and improve clinical translation. In this chapter, different ways to label and track cells in vivo are described. The oldest and only clinically established method is leukocyte scintigraphy, which enables a (semi)quantitative assessment of cell assemblies and, thus, the localization of inflammation foci. Noninvasive imaging of fewer or even single cells succeeds with MRI after labeling of the cells with (ultrasmall) superparamagentic iron oxide particles (SPIO and USPIO). However, in order to gain an acceptable signal-to-noise ratio, at a sufficiently high spatial resolution of the MR sequence to visualize a small amount of cells, experimental MR scanners working at high magnetic fields are usually required. Nevertheless, feasibility of clinical translation has been achieved by showing the localization of USPIO-labeled dendritic cells in cervical lymph nodes of patients by clinical MRI.Cell-tracking approaches using optical methods are important for preclinical research. Here, cells are labeled either with fluorescent dyes or quantum dots, or transfected with plasmids coding for fluorescent proteins such as green fluorescent protein (GFP) or red fluorescent protein (RFP). The advantage of the latter approach is that the label does not get lost during cell division and, thus, makes imaging of proliferating transplanted cells (e.g., tumor cells) possible. In summary, there are several promising options for noninvasive cell tracking, which have different strengths and limitations that should be considered when planning cell-tracking experiments.