In Vivo Micro-CT Imaging of Human Mesenchymal Stem Cells Labeled with Gold-Poly-l-Lysine Nanocomplexes

Developing in vivo cell tracking is an important prerequisite for further development of cell-based therapy. So far, few computed tomography (CT) cell tracking studies have been described due to its notoriously low sensitivity and lack of efficient labeling protocols. A simple method is presented to render human mesenchymal stem cells (hMSCs) sufficiently radiopaque by complexing 40 nm citrate-stabilized gold nanoparticles (AuNPs) with poly-l-lysine (PLL) and rhodamine B isothiocyanate (RITC). AuNP-PLL-RITC labeling does not affect cellular viability, proliferation, or downstream cell differentiation into adipocytes and osteocytes. Labeled hMSCs can be clearly visualized in vitro and in vivo with a micro-CT scanner, with a detection limit of approximate to 2 x 10(4) cells per mu L in vivo. Calculated Hounsfield unit values are 2.27 per pg of intracellular Au, as measured with inductively coupled plasma mass spectrophotometry, and are linear over a wide range of cell concentrations. This linear CT attenuation is observed for both naked AuNPs and those that were taken up by hMSCs, indicating that the number of labeled cells can be quantified similar to the use of radioactive or fluorine tracers. This approach for CT cell tracking may find applications in CT image-guided interventions and fluoroscopic procedures commonly used for the injection of cellular therapeutics.