Background —Preclinical atherosclerosis is associated with increased endothelial cell (EC) expression of leukocyte adhesion molecules (LAMs), which mediate monocyte adhesion during atherogenesis. Identification of cell-surface LAMs may uniquely allow assessment of endothelial function, but there are no in vivo methods for detecting LAMs. We tested a new microbubble designed to bind to and allow specific ultrasound detection of intercellular adhesion molecule-1 (ICAM-1). Methods and Results —A perfluorobutane gas–filled lipid-derived microsphere with monoclonal antibody to ICAM-1 covalently bound to the bubble shell was synthesized. Bubbles with either nonspecific IgG or no protein on the shell were synthesized as controls. Coverslips of cultured human coronary artery ECs were placed in a parallel-plate perfusion chamber and exposed to 1 of the 3 microbubble species, followed by perfusion with culture medium. Experiments were performed with either normal or interleukin-1β–activated ECs overexpressing ICAM-1, and bubble adherence was quantified with epifluorescent videomicroscopy. There was limited adherence of control bubbles to normal or activated ECs, whereas a 40-fold increase in adhesion occurred when anti–ICAM-1–conjugated bubbles were exposed to activated ECs compared with normal ECs (8.1±3.5 versus 0.21±0.09 bubbles per cell, respectively, P <0.001). Although diminished, this difference persisted even after perfusion at higher wall shear rates. Conclusions —A gas-filled microbubble with anti–ICAM-1 antibody on its shell specifically binds to activated ECs overexpressing ICAM-1. Diagnostic ultrasound in conjunction with targeted contrast agents has the unique potential to characterize cell phenotype in vivo.

Vascular delivery of mesenchymal stem cells (MSCs) following myocardial infarction is under clinical investigation. Little is known about the microvascular fate of MSCs. We used intravital microscopy of rat cremaster muscle microcirculation to track intraarterially delivered MSCs. Rat MSCs (average diameter, 23 μm) were bolused into the ipsilateral common iliac artery. Interrogation of an arteriole–venule pair revealed that 92±7% (n=6) of MSCs arrest and interrupt flow during first pass at the precapillary level, resulting in decreased flow in the feeding arteriole (velocity decreased from 6.3±1.0 to 4.6±1.3 mm/sec; P <0.001). MSC deformability evaluated using filtration through polycarbonate membranes revealed that the cortical tension of MSCs (0.49±0.07 dyne/cm, n=9) was not different from that of circulating mononuclear cells (0.50±0.05 dyne/cm, n=7). When intravital microscopy was performed 3 days following injection, the number of MSCs in the cremaster further decreased to 14% of the initial number, because of cell death in situ. In vivo labeling of the basement membrane revealed that at 1 day, the surviving cells were spread out on the luminal side of the microvessel, whereas at 3 days, they integrated in the microvascular wall. Despite their deformability, intraarterially delivered MSCs entrap at the precapillary level because of their large size, with a small proportion of surviving MSCs integrating in a perivascular niche.