Ultrasound imaging of acute cardiac transplant rejection with microbubbles targeted to intercellular adhesion molecule-1

Background - Noninvasive techniques for detecting acute cardiac transplant rejection are limited. We hypothesized that ultrasound contrast microbubbles targeted to the endothelial cell (EC) inflammatory marker intercellular adhesion molecule-1 (ICAM-1) would selectively bind to rejecting versus nonrejecting myocardium and that myocardial contrast echocardiography can therefore detect acute rejection. Methods and Results - Lipid-based microbubbles were conjugated to anti-rat ICAM-1 (MB_ICAM) or isotype control antibody (MB_Control). In vitro MB_ICAM adhesion to cultured rat ECs, as assessed in a parallel plate flow apparatus, was greater to inflammatory versus normal ECs (11±versus 3±2 microbubbles/EC, P<0.005). In vivo abdominal heterotopic heart transplantation was performed in rats (rejection group: Brown Norway to Lewis strain; control group: Lewis to Lewis or Brown Norway to Brown Norway). Triggered myocardial contrast echocardiography was performed during intravenous MB_ICAM or MB_Control (2.5×10⁶) injection on postoperative day 5. Myocardial videointensity from adhered MB_ICAM was significantly higher in rejecting (n=8) versus control (n=7) rats (10±4 versus 1±4 U, P=0.01). Postmortem histology showed normal myocardium in control rats, whereas allograft myocardium demonstrated grade III to IV rejection and strong immunohistochemical ICAM-1 staining. Conclusions - Preferential adherence of ICAM-1-targeted microbubbles to rejecting versus nonrejecting rat cardiac transplant myocardium can be detected ultrasonically. Targeted microbubbles may thus offer a noninvasive ultrasound imaging technique for the detection of acute cardiac transplant rejection and other processes characterized by endothelial dysfunction.