Towards whole-body imaging at the single cell level using ultra-sensitive stem cell labeling with oligo-arginine...

Mesenchymal stem cells (MSCs) have shown great potential in regenerative medicine. Sensitive and reliable methods for stem cell labeling and in vivo tracking are thus of great importance. Herein, we report the use of upconversion nanoparticles (UCNPs) as an exogenous contrast agent to track mouse MSCs (mMSCs) in vivo. To improve the labeling efficiency, oligo-arginine is conjugated to polyethylene glycol (PEG) coated UCNPs to enhance the nanoparticles uptake by mMSCs. Systematic in vitro tests reveal that the proliferation and differentiation of mMSCs are not notably affected by UCNP-labeling, suggesting that the labeled cells are able to maintain their stem cell potency. No apparent exocytosis is found in our in vitro labeling experiment by using a transwell culture system over a course of 10 days, indicating the potential capability of using our UCNP-labeling method for long-term stem cell tracking. To demonstrate the tracking sensitivity of our stem cell labeling approach, UCNP-labeled mMSCs are subcutaneously transplanted into mice and imaged using an in vivo upconversion luminescence (UCL) imaging system. As few as ~10 cells labeled with UCNPs are detected in vivo, evidencing a remarkable improvement in detection sensitivity of our UCNP-labeled hMSCs compared with other stem cell labeling techniques using conventional exogenous agents. We further track UCNP-labeled mMSCs after intravenous injection, and observe the translocation of mMSCs from lung where they initially accumulate, to liver, a phenomenon consistent to previous reports. Our results highlight the promise of using UCNPs as a new type of ultra-sensitive probes for labeling and in vivo tracking of stem cells at nearly the single cell level.