Directing stem cells to the infarcted heart using targeted microbubbles: development of a new molecular therapeutic technique

Abstract

Stem cell therapy is a promising tool to restore contractile function after myocar-dial infarction. Unfortunately, clinical trials still show disappointing results with only minor improvements in cardiac function. The major problem of cellular therapy is lack of persistence of sufficient stem cells at the site of injury, independent of administration route. We designed a novel technique to overcome this problem by directing stem cells to the infarcted area using targeted microbubbles (MB) and ultrasound (US). For this we coupled adipose-derived stem cells (ASC) to MB using an antibody against CD90 via biotin-avidin bridging. This stem cell-bubble complex was named «StemBell». StemBells were targeted to the infarcted area via a second antibody on the MB: anti-CD54. US (1 MHz) was applied to exert acoustic radiation force on the StemBells. In vitro we demonstrated that the procedure to create StemBells, as well as exposure to US had no negative effect on cell viability, using flow cytometry. Binding of MB to ASC did not affect their ability to attach to a culture dish, demonstrated by light microscopy. In a flow system we showed the ability of US to push StemBells from the main flow (0.2 dyne/cm2) to the side. In vivo, acute myocardial infarction was mimicked in a rat by ligating the left anterior descending coronary artery for 40 min, followed by reperfusion. 7 Days post-infarction 1 million DAPI-labeled StemBells («StB») or ASC were injected intravenously. 3 Hours post-injection hearts were excised, stored in liquid N2 and cryo-sections were made. By performing fluorescence microscopy we found significantly more cells (6-fold increase) specifically in the infarcted area in the StB group compared with «ASC alone» group (n=6; p<0.01). Applying US («StB+US») lead to an 8-fold increase (n=6; p<0.01 vs «ASC alone», p<0.05 vs «StB»). Notably, retrieved stem cells coincided with CD54 positive areas. In conclusion, we successfully demonstrated proof-of-principle of a novel technique to increase the number of stem cells at the site of injury. This holds great promise for stem cell therapies in general.