Fate of culture-expanded mesenchymal stem cells in the microvasculature: in vivo observations of cell kinetics.

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 microm) 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.

Myocardial ischemic memory imaging with molecular echocardiography.

BACKGROUND: Diagnosing acute coronary syndrome in patients presenting with chest discomfort is a challenge. Because acute myocardial ischemia/reperfusion is associated with endothelial upregulation of leukocyte adhesion molecules, which persist even after ischemia has resolved, we hypothesized that microbubbles designed to adhere to endothelial selectins would permit echocardiographic identification of recently ischemic myocardium. METHODS AND RESULTS: Lipid microbubbles (diameter, 3.3+/-1.7 microm) were synthesized. The selectin ligand sialyl Lewis(x) was conjugated to the microbubble surface (MB(sLex)). Control bubbles (MB(CTL)) bore surface Lewis(x) or sialyl Lewis(c). Intravital microscopy of mouse cremaster muscle was performed after intravenous injection of MB(sLex) (n=11) or MB(CTL) (n=9) with or without prior intrascrotal tumor necrosis factor-alpha. There was greater adhesion of MB(sLex) to inflamed versus noninflamed endothelium (P = 0.0081). Rats (n=12) underwent 15 minutes of anterior descending coronary artery occlusion. After 30 minutes and 1 hour of reperfusion, high-mechanical-index nonlinear echocardiographic imaging was performed in which single frames were acquired at 3.5 and 4 minutes after intravenous injection of MB(sLex) or MB(CTL). Video intensity at 4 minutes was subtracted from that at 3.5 minutes to derive target-specific acoustic signal. MB(sLex) caused greater opacification in postischemic versus nonischemic myocardium at both time points (P < or = 0.002). Immunostaining confirmed endothelial P-selectin expression in the ischemic bed. CONCLUSIONS: Echocardiographic identification of recently ischemic myocardium is possible using ultrasound contrast agents targeted to selectins. This may offer a new approach to the more timely and precise diagnosis of acute coronary syndrome in patients presenting with chest pain of uncertain cardiac origin.

A novel hydrodynamic method for microvascular flow enhancement.

We have shown that drag-reducing polymers (DRP) restore perfusion to a stenotic bed by lowering microvascular resistance. We studied whether resistance-lowering by DRP are due to changes in hydrodynamics or vasodilation. During intravital microscopy of rat cremaster muscle (n=18), DRP infusion increased aortic flow (p<0.002), decreased vascular resistance (p<0.01), increased arteriolar diameter (p=0.023), and increased RBC velocity in the arterioles (p<0.04), venules (p<0.003) and capillaries (p<0.02). To investigate whether DRP lowers resistance without involvement of shear (nitric oxide [NO])-mediated vasodilation, L-NAME was infused in 19 rats, but failed to abolish DRP resistance-lowering. To further investigate whether DRP resistance-lowering depends on vasodilation, adenosine was infused into rabbit femoral arteries (n=19) prior to DRP to achieve marked vasodilation. DRP caused an additional 14% decrease in femoral vascular resistance (p=0.022). DRP enhance microcirculatory perfusion by lowering vascular resistance. This involves not only some degree of shear-induced vasodilation, but also tone-independent resistance lowering mechanisms, suggesting that DRP favorably alter blood flow hydrodynamics. Modulation of blood flow hydrodynamics to enhance perfusion is unique, and may be of therapeutic value for any condition of compromised blood flow.