Microbubble persistence in the microcirculation during ischemia/reperfusion and inflammation is caused by integrin- and complement-mediated adherenceto activated leukocytes

Background-Albumin microbubbles that are used for contrast echocardiography persist within the myocardial microcirculation after ischemia/reperfusion (I-R). The mechanism responsible for this phenomenon is unknown, Methods and Results-Intravital microscopy of the microcirculation of exteri orized cremaster muscle was performed in 12 wild-type mice during intraveno us injections of fluorescein-labeled microbubbles composed of albumin, anio nic lipids, or cationic lipids, Injections were performed at baseline and a fter 30 to 90 minutes of I-R in 8 mice and 2 hours after intrascrotal tumor necrosis factor-alpha (TNF-alpha) in 4 mice. Microbubble adherence at base line was uncommon (<2/50 high-power fields). After I-R, adherence increased (P<0.05) to 9+/-5 and 5+/-4 per 50 high-power fields for albumin and anion ic lipid microbubbles, respectively, due to their attachment to leukocytes adherent to the venular endothelium. TNF-alpha produced even greater microb ubble binding, regardless of the microbubble shell composition, The degree of microbubble attachment correlated (r=0.84 to 0.91) with the number of ad hered leukocytes, Flow cytometry revealed that microbubbles preferentially attached to activated leukocytes, Albumin microbubble attachment was inhibi ted by blocking the leukocyte beta(2)-integrin Mac-1, whereas lipid microbu bble binding was inhibited when incubations were performed in complement-de pleted or heat-inactivated serum rather than control serum, Conclusions-Microvascular attachment of albumin and lipid microbubbles in t he setting of I-R and TNF-alpha-induced inflammation is due to their beta(2 )-integrin- and complement-mediated binding to activated leukocytes adheren t to the venular wall, Thus, microbubble persistence on contrast ultrasonog raphy may be useful for the detection and monitoring of leukocyte adhesion in inflammatory diseases.