Ultrasound assessment of inflammation and renal tissue injury with microbubbles targeted to P-selectin

Background-Routine methods capable of assessing tissue inflammation noninva sively are currently not available. We hypothesized that tissue retention o f microbubbles targeted to the endothelial cell adhesion molecule P-selecti n would provide a means to assess inflammation with ultrasound imaging. Methods and Results-Phospholipid microbubbles targeted to P-selectin (MBp) were created by conjugating monoclonal antibodies against murine P-selectin to the lipid shell. The microvascular behaviors of MBp and control microbu bbles without antibody (MB) or with isotype control antibody (MBiso) were a ssessed by intravital microscopy of cremasteric venules of control and tumo r necrosis factor (TNF)-alpha -stimulated wild-type mice. Retention of all microbubbles increased (P <0.05) with TNF-alpha treatment because of increa sed attachment to activated leukocytes. Extensive attachment of MBp directl y to the venular endothelium or to adherent platelet-leukocyte aggregates w as observed in TNF-alpha -stimulated mice, resulting in 4-fold greater (P < 0.01) retention of MBp than either MBiso or MB. Enhanced retention of MBp w as completely abolished in TNF-alpha -stimulated P-selectin-deficient mice. The ultrasound signal from microbubbles retained in inflamed tissue was as sessed by contrast-enhanced renal ultrasound imaging of the kidneys of mice undergoing ischemia-reperfusion injury. In wild-type mice, this signal was significantly higher (P<0.05) for MBp (12<plus/minus>2 U) than either MBis o (6 +/-3 U) or MB (5 +/-3 U). In P-selectin-deficient mice, the signal for MBp was equivalent to that from control microbubbles. Conclusions-Microvascular retention of microbubbles targeted to P-selectin produces strong signal enhancement on ultrasound imaging of inflamed tissue . These results suggest that site-targeted microbubbles may be used to asse ss inflammation, tissue injury, and other endothelial responses noninvasive ly with ultrasound.