Abstract 16631: Long Term in vivo Stem Cell Tracking Using Contrast Ultrasound
Background: Human mesenchymal stem cell (hMSC) therapy is promising for cardiac repair. Clinical translation requires a method to serially track cell distribution in vivo. We have previously shown that polylactide microbubbles (MB) are internalized by hMSC, persist in the cytoplasm, and allow ultrasound (US) imaging of intramuscularly injected stem cells in mice, but only up to 4 hr at 37°C. We sought to design a more durable MB for prolonged US cell tracking and validated our method against in vivo bioluminescence imaging (BLI).
Methods: Nitrogen gas polymer MB with a polycaprolactone and albumin double layer shell were synthesized. MB acoustic activity was measured in a water tank (Contrast Pulse Sequencing, 7MHz) after synthesis (Day 0) and 7 day storage at 37°C. Cultured hMSC expressing luciferase gene were incubated with MB. MB uptake was studied by confocal microscopy. MB-hMSC complexes or control hMSC were lifted, washed, and re-suspended for in vitro US imaging or in vivoinjection (INJ) (0.5-1.0 x106 cells) into nude mouse thigh muscle. Mice had serial US and concurrent BLI up to 7 days post INJ.
Results: In vitro videointensity (VI) of MB alone (27±1 dB, Day 0) was stable up to 7 days at 37°C (p=0.50). MB internalization occurred in 67% hMSC and did not impair cell viability by trypan blue exclusion. In vitro VI of MB-hMSC on Day 0 (17±1 dB, n=3) was higher than hMSC alone (0±0 dB, n=2, p<0.01), unchanged at Day 1 (15±2 dB, p=0.18), and decreased but was still easily seen at Day 7 post-labeling (9±1 dB, p<0.05). Mouse thigh US visualized MB-hMSC (n=8) but not hMSC-only (n=4) immediately post INJ (0hr) (7±3 dB vs 0±0 dB, p <0.01). MB-hMSC were still seen by thigh US at 2 days, but VI was decreased (p< 0.05 vs. 0 hr). In the 2 mice receiving 106 MB-hMSC, US signal persisted up to 7 days. BLI showed hMSC luminescence co-localizing with the thigh muscle US signal.
Conclusion: hMSC labeled with a new polymer MB can be serially detected in vivo by US over a time period relevant to clinical cell therapy. US imaging of MB-hMSC identifies viable hMSC as confirmed by concurrent BLI, offers non-invasive serial in vivo tracking of hMSC fate, and may facilitate implementation of cell therapy strategies. Optimization of MB formulation or dose to further extend US persistence in vivo should be possible.
- © 2011 by American Heart Association, Inc.