Abstract 1108: Ultrasound Facilitates Stem/Progenitor Cell Labeling and Tracking with Perfluorocarbon Nanobeacons via Fundamental Cell Transport Mechanisms
Stem/progenitor cells participate in many pathological and regenerative processes, which can be studied in vivo with molecular imaging approaches. Labeling cells with contrast agents for non-invasive imaging and tracking typically requires long exposure times or adjunctive methods, such as electroporation or transfection agents that can compromise cell membrane integrity and viability. Previously, we demonstrated that perfluorocarbon (PFC) nanoparticles (~200 nm) can serve as intracellular labels that provide a unique spectral signature (fluorine, or 19F) for cell tracking with MRI that entails 12 hour incubations for efficient cellular labeling. In an effort to minimize labeling time, ultrasound (US) enhancement of the labeling process was attempted with stem/progenitor (CD34+CD133+CD31+) cells derived from human umbilical cord mononuclear cells. The cells were incubated with PFC nanoparticles and exposed to clinical levels of US energy with a standard medical imaging system at typical imaging frequencies (2MHz) and power levels (1.9MI). For cell exposures of only 5 minutes, US increased labeling efficiency, as measured by PFC content of cells (an index of nanoparticle internalization), by 6-fold (274 ± 63 vs. 47 ± 15 μg PFC/mg protein, with and without ultrasound, respectively, p<0.001). By comparison, cell PFC levels for 12 hr incubations without US exposure (413 ± 39 μg PFC/mg protein) were only slightly higher. US-enhanced cell labeling was dependent on energy utilization (i.e., ATP) and active, actin-mediated cell transport mechanisms as determined by cellular depletion of ATP energy (20mM sodium azide, 50mM 2-deoxyglucose) and by disruption of the actin cytoskeleton (25μM cytochalasin D) prior to cell labeling. Enhancement of labeling with US did not alter membrane integrity or affect cell viability according to calcein and trypan blue dye exclusion tests, respectively. This unique and rapid approach should facilitate investigation of regenerative therapeutics by providing a safe adjunctive method to label cells for real-time MRI tracking.