Background
: Mesenchymal stem cells (MSCs) are a promising new cardiac reparative therapy. The distribution of delivered MSCs is poorly understood due to limited methods to track them
in vivo
. To test the hypothesis that ultrasound can detect MSCs, gas-filled microbubbles (MBs) were developed for MSC uptake.
Methods
: Perfluorocarbon gas-filled MBs were synthesized from a biodegradable polymer. Cultured rat and human bone-marrow derived MSCs were dwelled with MBs (12 hrs), washed, and suspended in medium. MSC uptake of MBs and MB retention of gas were confirmed with transmission electron microscopy (TEM) and density centrifugation. Spectral analysis of free MB response to up to 10MHz ultrasound was performed. 2nd harmonic and contrast pulse sequence (CPS) 2D echo imaging (7MHz) of MB-labeled and unlabeled MSCs was performed at a mechanical index (MI) 0.3 and 1.9. Videointensity was measured in 200 frames/capture.
Results:
Frequency spectra of free MBs contained 2nd and 3rd harmonics, indicating MB non-linearity. TEM showed MBs within MSCs (Figure
). By density centrifugation, MSCs with MBs were less dense than MSCs alone, confirming intracellular MB gas retention. Videointensity (gray scale units) of MSCs containing MBs vs MSCs alone was higher at both low MI (35±2 vs 0±0 harmonic, p<0.005; 21±2 vs 15±0 CPS, p<0.005), and high MI (14±2 vs 6±1 harmonic, p<0.005; 6±2 vs 0±0 CPS, p<0.005).
Conclusions
: Our acoustically active polymer MBs are internalized by MSCs and render them detectable with ultrasound. Because the polymer degrades slowly, this approach may permit serial non-invasive
in vivo
ultrasound imaging of MSC fate, thus facilitating optimization of cell therapy strategies.
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